What Is Coil Slitting?
Coil slitting is the process of cutting a wide master coil of galvanized steel into narrower strips (slits) of specified widths. This is a critical step in the steel service center workflow, as most end-use applications require specific strip widths that are smaller than the standard master coil widths (1000mm, 1219mm, 1250mm, 1500mm).
Galvanized steel coil slitting requires special attention to maintain the integrity of the zinc coating, control edge quality, and achieve precise width tolerances.
The Slitting Process: Step by Step
Step 1: Coil Loading and Uncoiling
- Master coil is loaded onto the uncoiler mandrel using an overhead crane or coil car
- Coil band is cut and the leading edge is fed through a straightener/leveler
- Material is threaded through the entry looper, guide tables, and into the slitter head
Step 2: Slitting
The steel strip passes between paired rotary knives (upper and lower circular blades) that shear the material into multiple narrower strips simultaneously.
- Knife Setup: Upper and lower knives are positioned on arbors at the required strip widths
- Overlap: The knives overlap by 30-50% of the material thickness (typical for GI coil 0.3-2.0mm thick)
- Side Clearance: Set at 5-12% of material thickness depending on material strength
- Speed: 50-300 meters per minute depending on material thickness and width
Step 3: Edge Trimming
- Uneven edges of the master coil are trimmed to produce clean, uniform strip edges
- Trim scrap is collected separately and recycled
Step 4: Tension Control
- Entry and exit tension is carefully controlled to prevent coil set, scratches, or edge wave
- Typical tension: 10-40 MPa depending on material thickness and grade
Step 5: Recoiling
- Individual slit strips are separated by a tension stand or separator unit
- Each strip is recoiled onto its own mandrel, or multiple strips are wound together on a single mandrel with separators
Slitting Methods
Rotary Shear Slitting (Standard)
- Most common method for galvanized steel
- Upper and lower circular knives rotate with the strip movement
- Produces clean, burr-free edges when properly set up
- Knife material: Tool steel (D2, D3) or tungsten carbide for high-volume production
Crush Cut Slitting
- Upper knife is driven; lower knife is idle and contacts the rubber-covered anvil roll
- Used for thin materials (less than 0.5mm) and non-metallic materials
- Not recommended for galvanized steel due to potential coating damage
Rag Cut (Score and Snap)
- Knives score the strip surface; the strip is then snapped along the score line
- Used for very thick materials where standard slitting is impractical
Width Tolerances
| Strip Width (mm) | Standard Tolerance (mm) | Precision Tolerance (mm) |
|---|---|---|
| Less than 25 | plus/minus 0.10 | plus/minus 0.05 |
| 25-75 | plus/minus 0.15 | plus/minus 0.08 |
| 75-150 | plus/minus 0.20 | plus/minus 0.10 |
| 150-300 | plus/minus 0.30 | plus/minus 0.15 |
| Over 300 | plus/minus 0.40 | plus/minus 0.20 |
Burr Control
Burr formation on slit edges is a common quality issue. Burr height must be controlled for downstream processing:
| Application | Maximum Burr Height | Notes |
|---|---|---|
| Standard stamping | Less than 10% of material thickness | Typical for most applications |
| Precision forming | Less than 5% of material thickness | Automotive, electronics |
| Exposed edge applications | Less than 3% or deburred | Appliances, architectural panels |
Factors Affecting Burr Formation
- Knife Clearance: Too much clearance increases burr; too little causes knife wear
- Knife Overlap: Insufficient overlap results in incomplete cutting and burr formation
- Knife Sharpness: Dull knives produce larger burrs; regular re-sharpening required
- Material Hardness: Harder materials (higher-strength grades) produce more burr
- Material Thickness: Thicker materials naturally produce more burr
Zinc Coating Protection During Slitting
Maintaining the integrity of the galvanized coating during slitting is critical:
- Knife Condition: Sharp, well-maintained knives minimize coating flaking at the cut edges
- Lubrication: Light rolling oil or emulsion is applied to reduce friction and protect the coating
- Surface Protection: Polyethylene film or paper interleaving for sensitive applications
- Handling: Avoid metal-to-metal contact between strips during recoiling
- Edge Quality: Minimize zinc pickup on knife edges; clean knives regularly
Quality Inspection
- Width Measurement: Laser measurement or manual caliper verification per strip
- Burr Measurement: Microscope or profilometer measurement of edge burr height
- Coating Thickness: X-ray fluorescence or magnetic gauge to verify zinc coating weight (Z60-Z600)
- Surface Inspection: Visual inspection for scratches, zinc flaking, or edge wave
- Coil Dimensions: OD, ID, and width verification against order specifications
- Edge Condition: Classification per customer requirements (mill edge, slit edge, deburred)
Relevant Standards
- ASTM A653/A653M: Standard Specification for Steel Sheet, Zinc-Coated (Galvanized)
- ASTM A924/A924M: General Requirements for Steel Sheet, Metallic-Coated
- EN 10142: Cold reduced mild steel flat products coated electrolytically with zinc
- EN 10346: Continuously hot-dip coated steel flat products
- ISO 3575: Cold-reduced carbon steel sheet of commercial and drawing qualities
Common Galvanized Coil Slitting Specifications
| Parameter | Range |
|---|---|
| Input Coil Width | 600-1800mm |
| Material Thickness | 0.15-3.0mm |
| Zinc Coating | Z60-Z600 (60-600 g/m2) |
| Slit Width (output) | 6-600mm |
| Line Speed | 50-300 m/min |
| Maximum Coil Weight | Up to 30 tons |
Xi’an Coremetal Steel Co., Ltd. is a leading supplier and exporter of steel and metal products, serving clients in over 60 countries with ISO 9001 certification. Our product range includes carbon steel, stainless steel, galvanized steel, aluminum, copper alloys, and specialty metals.
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