Secondary Steel Making Notes

April 03, 2025

Secondary Steel Making & Continuous Casting

Master the processes that refine molten steel and transform it into solid products

Table of Contents

• 1. Secondary Steel Making
• 2. Stainless Steel Basics
• 3. Continuous Casting
• 4. Process Diagram
• 5. Knowledge Test

1. Secondary Steel Making: Ladle Processes

Secondary steel making refines molten steel through various ladle treatments to achieve precise chemical composition, temperature control, and cleanliness.

1.1 Deoxidation

Purpose

Remove dissolved oxygen to prevent gas porosity and improve mechanical properties

Methods

Precipitation Deoxidation: Add elements with higher oxygen affinity than iron

• Aluminum (most common): 0.02-0.06%
• Silicon: Often used with manganese (Si-Mn)
• Calcium: For inclusion modification

Vacuum Deoxidation: Carbon deoxidation under reduced pressure

Reactions

2[Al] + 3[O] → Al₂O₃(s) ΔG° = -1,200,000 + 386T J/mol
[Si] + 2[O] → SiO₂(s) ΔG° = -576,000 + 218T J/mol
[C] + [O] → CO(g) ΔG° = -22,400 - 39.6T J/mol

1.2 Argon Stirring

Parameter	Porous Plug	Top Lance	Induction Stirring
Gas Flow Rate	50-200 NL/min	100-500 NL/min	N/A
Stirring Energy	50-200 W/ton	100-500 W/ton	100-400 W/ton
Mixing Time	2-5 min	1-3 min	3-8 min
Applications	Homogenization	Slag-metal reaction	Clean steel production

Benefits of Argon Stirring

• Homogenizes temperature and composition (ΔT < 5°C)
• Promotes inclusion floatation (removes 50-70% of inclusions)
• Accelerates slag-metal reactions
• Reduces hydrogen and nitrogen content

1.3 Desulphurization

Reduces sulfur content to <0.005% for critical applications:

• Slag-Metal Reaction: High basicity slag (CaO/SiO₂ >3) with Al reduction
• Calcium Treatment: Ca injection (wire or powder) forms CaS
• Optimal Conditions:
  • Temperature: 1550-1650°C
  • Oxygen activity: <10ppm
  • Stirring intensity: 100-300 W/ton

[S] + (CaO) + [Al] → (CaS) + (Al₂O₃)
K = (a_CaS·a_Al2O3)/(a_S·a_CaO·a_Al)

1.4 Inclusion Shape Control

Objective

Modify harmful alumina and sulfide inclusions into less detrimental forms

Methods

Calcium Treatment: Converts Al₂O₃ to liquid calcium aluminates
Rare Earth Metals: Form globular oxysulfides
Magnesium: For sulfide shape control in resulfurized steels

Target Inclusion Types

Soft: Liquid calcium aluminates (12CaO·7Al₂O₃)
Deformable: MnS in low melting point silicate matrix
Non-deformable: Avoided (alumina, spinel)

1.5 Degassing Principles

Vacuum Degassing Methods

Method	Vacuum Level	Treatment Time	H Reduction	N Reduction
RH (Ruhrstahl-Heraeus)	0.1-1 mbar	15-25 min	60-80%	20-40%
VD (Vacuum Arc Degassing)	0.5-2 mbar	20-30 min	50-70%	15-30%
DH (Dortmund-Hörder)	0.1-1 mbar	10-20 min	70-90%	25-45%

Degassing Reactions

2[H] → H₂(g) ΔG° = 364,800 - 104.6T J/mol
2[N] → N₂(g) ΔG° = 864,800 - 157.3T J/mol
[C] + [O] → CO(g) ΔG° = -22,400 - 39.6T J/mol

What is the primary purpose of argon stirring in ladle metallurgy?

To increase steel temperature

To homogenize composition and promote inclusion removal

To reduce carbon content

To increase nitrogen pickup

Correct Answer: To homogenize composition and promote inclusion removal

Argon stirring creates a recirculating flow pattern that equalizes temperature and composition throughout the ladle while promoting the floatation of non-metallic inclusions to the slag layer. Typical stirring energies range from 50-500 W/ton.

Which element is most commonly used for deoxidation in aluminum-killed steels?

Silicon

Aluminum

Calcium

Magnesium

Correct Answer: Aluminum

Aluminum is the most powerful common deoxidizer, typically added at 0.02-0.06%. It forms alumina (Al₂O₃) inclusions which must be controlled through calcium treatment or flotation.

2. Basics of Stainless Steel Manufacturing

2.1 Stainless Steel Types

Type	Main Alloys	Cr (%)	Ni (%)	Key Properties
Ferritic (400 series)	Cr, Mo	10.5-30	0-4	Magnetic, moderate corrosion resistance
Martensitic (400 series)	Cr, C	11.5-18	0-2	Hardenable, high strength
Austenitic (300 series)	Cr, Ni, Mn	16-26	6-22	Non-magnetic, excellent corrosion resistance
Duplex (2205 etc.)	Cr, Ni, Mo, N	21-26	4.5-8	High strength, stress corrosion resistance

2.2 Production Routes

EAF → AOD Route

Electric Arc Furnace: Melts scrap and ferroalloys (1600-1650°C)
Argon Oxygen Decarburization: Key refining step for Cr retention

AOD Process

Stage 1: High O₂ blowing (Cr oxidation)
Stage 2: Mixed Ar/O₂ blowing (controlled decarburization)
Stage 3: Argon stirring (final reduction)

Cr₂O₃ + 3[C] → 2[Cr] + 3CO(g) ΔG° = 546,000 - 344T J/mol

Alternative Processes

VOD: Vacuum Oxygen Decarburization for ultra-low carbon grades
CLU: Creusot-Loire Uddeholm process (similar to AOD)
Converter: K-OBM-S process with bottom tuyeres

2.3 Special Considerations

• Chromium Recovery: Must maintain Cr >10.5% while reducing carbon
• Nitrogen Control: Added in duplex steels (0.1-0.3%), removed in others
• Low Carbon: <0.03% C prevents sensitization (Cr carbide precipitation)
• Slag Chemistry: CaO-SiO₂-MgO-Al₂O₃ system with basicity 1.5-2.5

What is the primary purpose of the AOD process in stainless steel production?

To increase chromium content

To decarburize while minimizing chromium oxidation

To remove sulfur completely

To increase nitrogen content

Correct Answer: To decarburize while minimizing chromium oxidation

The AOD (Argon Oxygen Decarburization) process uses argon dilution to lower CO partial pressure, allowing carbon removal at lower temperatures without excessive chromium oxidation. This enables production of low-carbon stainless steels while maintaining high chromium content.

3. Continuous Casting Processes

3.1 Fluid Flow in Tundish

Tundish serves as a buffer and refining vessel between ladle and mold:

• Flow Control: Impact pads, dams, weirs to optimize flow patterns
• Residence Time: Typically 5-10 minutes (minimum 3 min for inclusion floatation)
• Flow Models:
  • Plug flow (ideal)
  • Mixed flow (actual)
  • Dead zones (undesirable)

Residence Time Distribution (RTD): E(t) = C(t)/∫C(t)dt
Mean Residence Time: τ = V/Q (V = volume, Q = flow rate)

3.2 Fluid Flow in Mold

Meniscus Flow

Importance: Affects slag entrapment and surface quality
Control: Submerged Entry Nozzle (SEN) design and casting speed

Flow Patterns

Double Roll: Standard for slabs (2 recirculation zones)
Single Roll: For high-speed casting
Unstable Flow: Causes level fluctuations (>±3mm problematic)

Electromagnetic Control

EMBr: Electromagnetic braking reduces jet velocity
EMLS: Level stabilization
EMS: Stirring for equiaxed zone enlargement

3.3 Heat Transfer in Mold

Mechanism	Heat Flux (MW/m²)	Importance
Initial Solidification	2.0-3.5	Shell formation (10-30mm)
Steady-State	1.0-2.0	Shell growth
Air Gap Formation	0.5-1.5	Reduces heat transfer

Heat Transfer Equation

q = h × (T_steel - T_water)
h = 1/(1/h_gap + d_cu/k_cu + 1/h_water)

Where h_gap ≈ 1000-3000 W/m²K, h_water ≈ 30,000-50,000 W/m²K

3.4 Segregation

Types of Segregation

• Microsegregation: Within dendrites (C, P, S enriched in interdendritic regions)
• Macrosegregation: Across product (centerline segregation of C, S, P)
• Inverse Segregation: Near surface (enriched in alloying elements)

Segregation Ratio: SR = C_max/C₀
Typical SR values: C (1.5-3), S (2-5), P (1.5-3)

3.5 Inclusion Control

Sources

Endogenous: Deoxidation products (Al₂O₃, SiO₂, etc.)
Exogenous: Refractory erosion, slag entrapment

Control Methods

Ladle Treatment: Argon stirring, calcium treatment
Tundish: Flow control, ceramic filters
Mold: SEN design, mold powder absorption

Target Levels

Clean Steels: <10ppm total oxygen
Ultra-Clean Steels: <5ppm total oxygen

What is the primary purpose of the tundish in continuous casting?

To superheat the steel

To ensure steady flow to mold and allow inclusion floatation

To reduce carbon content

To add alloying elements

Correct Answer: To ensure steady flow to mold and allow inclusion floatation

The tundish maintains constant metal head pressure for even flow to the mold while providing residence time (5-10 min) for inclusions to float out. Proper tundish design with flow modifiers can remove 30-50% of remaining inclusions.

Which factor most significantly affects heat transfer in the mold during continuous casting?

Steel carbon content

Air gap formation between shell and mold

Mold material

Tundish temperature

Correct Answer: Air gap formation between shell and mold

As the shell solidifies and contracts, an air gap forms which dramatically reduces heat transfer (h_gap ≈ 1000-3000 W/m²K vs. h_water ≈ 30,000-50,000 W/m²K). This is the dominant resistance in the heat transfer path.

4. Interactive Process Diagram

Ladle Treatment

Tundish

Mold

Secondary Cooling

Cutting

Ladle Treatment

Key processes:

• Temperature homogenization
• Chemical composition adjustment
• Inclusion removal
• Degassing (if required)

Tundish

Functions:

• Steady metal flow to mold
• Inclusion floatation
• Last chance for composition adjustment
• Flow control with dams/weirs

Mold

Critical parameters:

• Heat flux: 1-3 MW/m²
• Meniscus control (±2mm)
• Oscillation: 100-200 cpm
• Strand shell thickness: 10-30mm

Secondary Cooling

Zones:

• Spray cooling (water/air mist)
• Target surface temperature
• Avoid overcooling/reheating
• Final solidification point control

Cutting

Methods:

• Torch cutting (most common)
• Shearing (for small sections)
• Sawing (special applications)

5. Knowledge Test

Test your understanding with these interactive questions. Earn 2 points for each correct answer!

Which degassing method typically achieves the highest hydrogen removal efficiency?

VD (Vacuum Arc Degassing)

RH (Ruhrstahl-Heraeus)

Ladle furnace treatment

Argon stirring alone

Correct Answer: RH (Ruhrstahl-Heraeus)

The RH process achieves 70-90% hydrogen removal due to its efficient recirculation under high vacuum (0.1-1 mbar), compared to 50-70% for VD and minimal reduction with argon stirring alone.

What is the primary mechanism for inclusion removal in the tundish?

Chemical dissolution

Floatation to the slag layer

Filtration through ceramic materials

Electromagnetic separation

Correct Answer: Floatation to the slag layer

Inclusions are removed in the tundish primarily by Stokes floatation - buoyant forces cause them to rise to the slag layer. Proper tundish design provides sufficient residence time (5-10 min) for this process.

Which stainless steel type contains both austenite and ferrite phases?

Martensitic

Duplex

Ferritic

Precipitation-hardened

Correct Answer: Duplex

Duplex stainless steels have a mixed microstructure of approximately equal parts austenite and ferrite, combining benefits of both phases - strength from ferrite and corrosion resistance from austenite.

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