Rectangular Stainless Steel Coil Tube (Coil Tube) — Technical Product Specification
1. Product Overview
The Rectangular Stainless Steel Coil Tube (also designated as Coiltube or Coil Tubing) is a precision-engineered product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product category encompasses cold-formed rectangular and square cross-section coil tubes supplied in continuous coil form, designed for high-efficiency downstream processing including bending, welding, stamping, and forming. Unlike conventional round coil tubes, rectangular coil tubes offer superior structural rigidity, increased surface area for heat exchange, and enhanced aesthetic qualities, making them indispensable in advanced engineering applications.
The manufacturing process for rectangular coil tubes represents a significant technological challenge due to the complex geometry involved. The transition from round to rectangular cross-section requires precise control of material deformation, wall thickness uniformity, and dimensional accuracy throughout the continuous coil length. Wenzhou Jixing Metal Materials Co., Ltd. employs state-of-the-art multi-stage rolling and drawing processes to achieve consistent quality across production runs.
1.1 Unique Selling Points
- Superior Structural Performance: Rectangular cross-section provides higher moment of inertia and bending resistance compared to equivalent round tubes of similar cross-sectional area
- Enhanced Surface-to-Volume Ratio: Ideal for heat exchange applications where maximum thermal transfer efficiency is required
- Continuous Coil Supply: Delivered in coil form to minimize material waste and enable automated downstream processing
- Precision Dimensional Tolerances: Advanced multi-stage forming technology ensures tight dimensional control throughout the coil length
- Material Versatility: Available in multiple stainless steel grades including austenitic, ferritic, and duplex families
- Corrosion Resistance: Inherent stainless steel properties provide long-term durability in aggressive environments
2. Material & Properties
2.1 Available Material Grades
Rectangular coil tubes are manufactured from a range of stainless steel grades, each selected to meet specific application requirements:
- 304 / 1.4301 (Austenitic): The most widely used grade, offering excellent corrosion resistance, formability, and weldability. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni). Suitable for general industrial and food-grade applications.
- 304L / 1.4307 (Low-Carbon Austenitic): With carbon content reduced to ≤0.03%, this grade exhibits superior resistance to intergranular corrosion following welding operations. Preferred for welded assemblies.
- 316 / 1.4401 (Austenitic, Molybdenum-Alloyed): Contains 2-3% Molybdenum (Mo), providing enhanced resistance to pitting and crevice corrosion in chloride-containing environments. Ideal for chemical processing and marine applications.
- 316L / 1.4404 (Low-Carbon, Molybdenum-Alloyed): Combines the corrosion advantages of 316 with improved weldability due to reduced carbon content.
- 321 / 1.4541 (Stabilized Austenitic): Stabilized with Titanium (Ti) to prevent sensitization at elevated temperatures (450–850°C). Suitable for high-temperature applications.
- 430 / 1.4016 (Ferritic): Chromium-only stainless steel with good resistance to atmospheric and mild acid corrosion. Cost-effective for less demanding applications.
- 2205 / 1.4462 (Duplex): Dual-phase austenitic-ferritic structure providing approximately twice the yield strength of 304/316 grades with excellent resistance to chloride stress corrosion cracking (SCC).
2.2 Mechanical Properties (Typical Values for 304 Grade)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 520 MPa (75,400 psi) | ASTM A270 / EN 10216-5 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29,700 psi) | ASTM A270 / EN 10216-5 |
| Elongation at Break | ≥ 35% (minimum) | ASTM A270 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM E18 |
| Density | 8.0 g/cm³ (0.289 lb/in³) | — |
| Thermal Conductivity (at 20°C) | 16.3 W/(m·K) | ASTM E153 |
| Modulus of Elasticity | 193 GPa (28,000,000 psi) | ASTM E8 |
| Thermal Expansion Coefficient | 17.3 × 10⁻⁶ /°C (20–100°C) | ASTM E228 |
2.3 Surface Condition Options
- 2B Finish: Cold-rolled and annealed in a protective atmosphere; bright, smooth surface suitable for general use
- No. 4 Brushed Finish: Linear brushed texture; aesthetically pleasing for visible applications
- Mirror / 8K Finish: Highly polished surface with reflectivity ≥ 90%; for decorative and optical applications
- Satin Finish: Uniform matte appearance with Ra 0.2–0.8 μm surface roughness
- Pickled & Passivated: Chemical cleaning to remove scale and form protective chromium oxide layer
3. Specifications
3.1 Dimensional Parameters
| Parameter | Range / Specification | Tolerance |
|---|---|---|
| Width (W) | 10 mm – 150 mm (0.39 in – 5.91 in) | ±0.5 mm (±0.020 in) |
| Height (H) | 10 mm – 150 mm (0.39 in – 5.91 in) | ±0.5 mm (±0.020 in) |
| Wall Thickness (t) | 0.5 mm – 3.0 mm (0.020 in – 0.118 in) | ±10% of nominal |
| Coil Inner Diameter | 200 mm – 600 mm (7.87 in – 23.62 in) | ±10 mm |
| Coil Length | 50 m – 500 m (164 ft – 1,640 ft) per coil | — |
| Corner Radius (R) | 0.5t – 2.0t (where t = wall thickness) | ±0.1 mm |
| Straightness | ≤ 1.5 mm/m (≤ 0.047 in/ft) | — |
| Twist | ≤ 3 mm/m (≤ 0.118 in/ft) | — |
3.2 Common Cross-Section Configurations
| Configuration | W × H (mm) | Wall Thickness (mm) | Typical Application |
|---|---|---|---|
| Square | 12 × 12 | 0.8 – 1.5 | Structural frames, furniture |
| Square | 15 × 15 | 1.0 – 2.0 | Automotive trim, appliance frames |
| Rectangular | 20 × 10 | 0.8 – 1.5 | Heat exchanger fins, HVAC ducts |
| Rectangular | 25 × 15 | 1.0 – 2.0 | Industrial piping, conveyor systems |
| Rectangular | 30 × 20 | 1.2 – 2.5 | Structural supports, architectural railings |
| Rectangular | 40 × 20 | 1.5 – 3.0 | Heavy-duty framing, machinery guards |
| Rectangular | 50 × 25 | 1.5 – 3.0 | Process piping, chemical transfer lines |
3.3 Applicable Standards
- ASTM A270: Standard Specification for Stainless Steel, Nickel Alloy, and Copper Nickel Alloy Tubing
- ASTM A554: Standard Specification for Seamless and Welded Austenitic Stainless Steel Sheet, Strip, and Plate
- EN 10216-5: Thermomechanically Controlled Austenitic Stainless Steel Tubes for Pressure Purposes
- GB/T 14976: Stainless Steel Seamless Tubes for Fluid Transport (Chinese National Standard)
- GB/T 4237: Cold-Rolled Stainless Steel Flat Products (Chinese National Standard)
- JIS G3446: Stainless Steel Tubes (Japanese Industrial Standard)
- ISO 13970: General Product Specification for Seamless and Welded Tubes of Stainless Steel for General Purposes
4. Manufacturing Process
4.1 Process Flow Overview
The manufacture of rectangular stainless steel coil tubes involves a multi-stage process chain requiring precise control at each step to ensure dimensional accuracy, mechanical integrity, and surface quality:
- Raw Material Selection & Inspection: High-quality stainless steel strip (coil) is sourced from certified mills. Incoming material undergoes chemical composition analysis (via Optical Emission Spectrometry, OES), mechanical property testing, and visual inspection for surface defects.
- Strip Preparation: The stainless steel strip is slit to the required width, edge-trimmed to remove burrs, and leveled to ensure flatness and consistent thickness. Surface preparation includes cleaning and degreasing to remove oils and contaminants.
- Forming / Rolling: The flat strip passes through a series of progressively shaped rollers (roll forming) that gradually deform the material into the target rectangular cross-section. Typically, 8–15 forming stations are employed to minimize localized stress concentrations and prevent material cracking.
- Welding (if applicable): For welded rectangular coil tubes, the formed strip is joined along the longitudinal seam using one of the following methods:
- TIG Welding (GTAW): Gas Tungsten Arc Welding with argon shielding for high-quality, clean welds
- MIG Welding (GMAW): Gas Metal Arc Welding for higher production rates
- Resistance Welding: For thinner wall sections where high-speed production is required
- Post-Forming Heat Treatment: Annealing at 1050–1150°C in a controlled atmosphere (nitrogen or vacuum) relieves residual stresses, restores ductility, and eliminates sensitization effects. This step is critical for ensuring the final mechanical properties meet specification requirements.
- Pickling & Passivation: The annealed tube is immersed in a mixed acid bath (typically nitric-hydrofluoric acid solution) to remove oxide scale formed during heat treatment. Subsequent passivation with dilute nitric acid or citric acid promotes the formation of a protective chromium oxide (Cr₂O₃) surface layer.
- Final Dimensional Verification: Automated laser measurement systems verify cross-sectional dimensions, wall thickness uniformity, straightness, and twist along the entire coil length. Non-conforming sections are identified and removed.
- Coiling & Packaging: The finished coil tube is wound onto a steel reel with appropriate protective wrapping (VCI paper, PE film, or kraft paper) to prevent surface damage and corrosion during storage and transportation.
4.2 Key Process Parameters
| Process Stage | Key Parameter | Typical Value | Critical Control Point |
|---|---|---|---|
| Roll Forming | Reduction per pass | ≤ 15% per station | Prevents material cracking and springback |
| Roll Forming | Forming speed | 5 – 30 m/min | Depends on material grade and cross-section |
| Welding | Welding current (TIG) | 100 – 250 A | Ensures full penetration without burn-through |
| Welding | Shielding gas flow rate | 10 – 15 L/min (Ar) | Prevents weld oxidation and porosity |
| Annealing | Temperature | 1050 – 1150°C | Ensures complete stress relief and grain refinement |
| Annealing | Soak time | 15 – 45 min | Uniform temperature distribution throughout cross-section |
| Pickling | Bath temperature | 60 – 80°C | Optimal acid activity without excessive metal loss |
| Pickling | Acid concentration | HNO₃ 15–20% + HF 1–3% | Effective scale removal with minimal surface attack |
4.3 Quality Control Measures
- In-Process Inspection: Real-time monitoring of forming dimensions via laser sensors at each roll station; automatic rejection of out-of-tolerance material
- Weld Quality Assessment: