Strip Processing Glossary
Strip Processing Plants
A strip processing plant (or strip line) executes at least one process step with a coiled metal strip. The strip is unwound from a coil at the entry section, processed through one or more treatment zones, and rewound into a coil at the exit section.
Types of Strip Processing Plants
| Plant Type | Function |
|---|---|
| Rewinding Lines | Transfer strip from one coil to another, often with edge trimming or inspection |
| Stretch Levelling Lines | Improve strip flatness through controlled elongation |
| Slitting Lines | Cut wide strip into narrower widths |
| Annealing Lines | Heat treatment to soften metal and restore ductility |
| Heat Treatment Lines | Hardening, tempering, or other thermal processes |
| Coating Lines | Apply protective or functional coatings |
| Pickling Lines | Remove oxide scale through chemical treatment |
→ View WSP Heat Treatment Lines
Operating Modes
Continuous Operation
In continuous operation, the strip moves constantly through the processing line. New coils are connected to the strip end in the decoiling area while the line continues to run. This requires:
- Coil joining equipment (welders or stitchers)
- Strip accumulators to buffer speed differences
- Consistent process conditions throughout
Discontinuous Operation
In discontinuous operation, the strip halts during coil changes. This mode is simpler but results in lower productivity and potential quality variations at coil heads and tails.
Endless Strip Processing
Endless processing connects coils continuously through the system, creating an “endless” strip. This approach maximizes productivity and ensures consistent treatment across the entire coil length.
Single-Feed Mode
In single-feed mode, strips process independently without connection. Each coil passes through the line separately. This mode is common for:
- Short production runs
- Varying strip dimensions
- Special quality requirements
→ WSP supplies lines for both continuous and single-feed operation
Heat Treatment Processes
Annealing
Annealing is a heat treatment process that softens metal by heating it above its recrystallization temperature, holding at temperature, and then cooling. Types include:
| Annealing Type | Purpose | Temperature Range |
|---|---|---|
| Soft Annealing | Maximum softness for forming | Below Ac1 |
| Recrystallization Annealing | Restore ductility after cold working | 550–700 °C (steel) |
| Stress Relief Annealing | Remove internal stresses | 450–650 °C |
| Normalizing | Refine grain structure | Above Ac3 |
→ WSP Strip Flotation Furnaces for Annealing
Austenitizing
Austenitizing heats steel above the Ac3 temperature (typically 800–950 °C for carbon steels) to transform the microstructure into austenite. This is the essential first step before hardening. Critical factors include:
- Heating rate and uniformity
- Holding time at temperature
- Protective atmosphere to prevent oxidation and decarburization
→ WSP Muffle Furnaces for Austenitizing
Hardening (Quenching)
Hardening rapidly cools austenitized steel to form martensite, a hard microstructure. Traditional quenching media include:
- Water – fastest cooling, risk of distortion
- Oil – moderate cooling rate
- Molten salt or lead baths – controlled cooling, environmental concerns
- Gas (hydrogen/nitrogen) – clean, controllable, modern alternative
→ WSP ACQ Hydrogen Quenching Technology
Tempering
Tempering reheats hardened steel to a temperature below Ac1 (typically 150–650 °C) to reduce brittleness and achieve the desired balance of hardness and toughness. Higher tempering temperatures result in lower hardness but greater toughness.
Furnace Technology
Flotation Furnaces
Flotation furnaces use air or gas jets to support the strip without mechanical contact. Benefits include:
- Scratch-free surfaces – no roller marks or contact damage
- Uniform heating – consistent heat transfer across strip width
- Flexible atmosphere – protective, reducing, or oxidizing conditions
- Wide speed range – stable flotation at varying line speeds
→ WSP Strip Flotation Furnaces
Muffle Furnaces
Muffle furnaces separate the heating zone from the process atmosphere using a sealed muffle (typically made from heat-resistant alloys). Advantages:
- Pure atmosphere – no combustion products contact the strip
- Precise control – defined gas composition throughout
- Surface quality – prevents oxidation and decarburization
- High temperatures – up to 1,200 °C for austenitizing
HEXAR High-Temperature Furnaces
HEXAR is WSP’s advanced furnace concept for non-ferrous metals featuring:
- Hex-shaped radiant tube arrangement for uniform heating
- Optimized for copper and copper alloys
- Temperatures up to 950 °C
- Minimal surface discoloration
Direct-Fired vs. Indirect Heating
| Heating Method | Description | Typical Applications |
|---|---|---|
| Direct-fired | Burner flames directly contact the strip atmosphere | Non-critical atmosphere requirements |
| Radiant tubes | Combustion inside tubes, heat radiated to strip | Protective atmospheres |
| Electric heating | Resistance or induction heating elements | Hydrogen atmospheres, precise control |
| Muffle heating | Strip in sealed muffle, heated externally | Highest atmosphere purity |
Quenching Technology
Conventional Quenching Media
Lead Baths Traditionally used for carbon steel strip hardening due to excellent heat transfer. Disadvantages:
- Environmental and health concerns (lead exposure)
- Disposal costs and regulations
- Strip contamination risks
Salt Baths Molten salt provides controlled quenching rates. Issues include:
- Salt drag-out and disposal
- Maintenance requirements
- Cleaning after quenching
WSP Advanced Convection Quench (ACQ)
The WSP ACQ system uses hydrogen gas for convective quenching as a modern alternative to lead baths:
- Segmented nozzle fields for locally adjustable cooling
- Wide heat transfer range adaptable to different steels
- Clean process – no contamination or residues
- Industrial proven – in operation since 2017
→ Learn more about WSP ACQ Technology
Strip Treatment Processes
Degreasing
Degreasing removes oils, greases, and other organic contaminants from the strip surface. Methods include:
- Alkaline cleaning – aqueous solutions with surfactants
- Solvent cleaning – organic solvents (decreasing due to environmental concerns)
- Electrolytic cleaning – enhanced cleaning through electrolysis
Pickling
Pickling removes oxide scale from the strip surface using acid solutions:
- Sulfuric acid (H₂SO₄) – common for carbon steel, operates at elevated temperatures
- Hydrochloric acid (HCl) – faster pickling, better surface quality
- Nitric/hydrofluoric acid – for stainless steels
- Citric or phosphoric acid – for non-ferrous metals
Brushing
Mechanical brushing cleans and conditions strip surfaces:
- Removes loose scale and debris
- Creates defined surface roughness
- Prepares surface for coating or further processing
Protective Atmospheres
Atmosphere Types
| Atmosphere | Composition | Application |
|---|---|---|
| Hydrogen (H₂) | Up to 100% H₂ | Bright annealing, reducing atmosphere |
| Nitrogen (N₂) | Inert | General protection, carrier gas |
| HNX | H₂ + N₂ mixture | Annealing with controlled reduction |
| Exogas | CO, CO₂, H₂, N₂ | Cost-effective protection |
| Vacuum | < 10⁻² mbar | Highest purity requirements |
Atmosphere Control
Critical parameters for protective atmospheres:
- Dew point – moisture content affects oxidation/reduction
- Oxygen content – must be minimized for bright surfaces
- Gas flow – ensures consistent atmosphere throughout furnace
- Sealing – prevents air ingress at entry/exit
Strip Materials
Carbon Steel Strip
Carbon steels contain 0.05–1.5% carbon and are the most common strip materials for heat treatment. Applications include:
- Saw blades and band saws
- Springs and spring steel
- Tools and knives
- Packaging (strapping)
Stainless Steel Strip
Stainless steels contain chromium (>10.5%) for corrosion resistance:
- Austenitic (304, 316) – non-magnetic, excellent corrosion resistance
- Ferritic (430) – magnetic, good formability
- Martensitic (420, 440) – hardenable, wear resistant
Non-Ferrous Metals
Copper and copper alloys require special consideration due to:
- High thermal conductivity
- Low absorption of radiation (reflective surfaces)
- Sensitivity to surface oxidation
- Lower processing temperatures
→ WSP solutions for copper strip processing
Process Parameters
Key Specifications
| Parameter | Description | Typical Range |
|---|---|---|
| Strip width | Maximum processable width | up to 1,200 mm |
| Strip thickness | Material gauge | 0.02–4 mm |
| Line speed | Strip velocity through furnace | 1–200 m/min |
| Throughput | Mass flow rate | up to 20,000 kg/h |
| Temperature | Maximum furnace temperature | up to 1,200 °C |
Heat Transfer
Heat transfer to the strip occurs through:
- Convection – gas flow transferring heat (dominant in flotation and quenching)
- Radiation – infrared energy from furnace walls and heating elements
- Conduction – through contact with rolls or support structures
Engineering Services
CFD Simulation
Computational Fluid Dynamics (CFD) simulates gas flows, temperature distributions, and heat transfer in furnace design:
- Optimize nozzle configurations
- Predict temperature uniformity
- Analyze atmosphere distribution
FEM Analysis
Finite Element Method (FEM) analysis ensures structural integrity:
- Thermal stress analysis
- Mechanical strength calculations
- Fatigue life prediction
Process Optimization
Process optimization improves the performance of existing and new plants through systematic analysis:
- Analysis and optimization of control loops
- Simulation with MATLAB/Simulink
- Improvement of control quality and system stability
- Energy savings through optimized process control
Related Standards
- DIN EN 10052 – Heat treatment terminology for ferrous products
- DIN EN ISO 4885 – Heat treatment vocabulary
- DIN EN 10027 – Steel designation systems
- AMS 2750 – Pyrometry requirements (aerospace)
This glossary is provided by WSP GmbH, specialists in thermal process plants and strip processing lines since 1989. Contact us for technical consultation on your specific application.