Introduction to Rolling
Rolling is a metal forming process where metal is passed between two rotating rolls to reduce thickness and increase length.
Key Features
- Most widely used metal forming process
- ~90% of all metals produced undergo rolling
- High production rates
- Good surface finish
- Economical for large quantities
Rolling Process Mechanics
Basic Principle
- Metal stock passes between two rotating cylindrical rolls
- Rolls rotate in opposite directions
- Friction pulls metal into the roll gap
- Compressive forces reduce thickness
- Length increases (volume constant)
Roll Gap Geometry
Draft (Δh) = h₀ - hf
- h₀ = Initial thickness
- hf = Final thickness
Reduction (r) = (h₀ - hf)/h₀ × 100%
Roll Radius (R) = Radius of the rolls
Contact Length (L) = √(R × Δh)
- Approximate length of contact between roll and workpiece
Neutral Point/No-slip Point:
- Point where roll surface velocity = workpiece velocity
- Before this point: workpiece slower than roll (pulled in)
- After this point: workpiece faster than roll (pushed out)
Types of Rolling Mills
1. Two-High Rolling Mill
- Two rolls rotating in opposite directions
- Simplest configuration
- Non-reversing: material passes in one direction only
- Reversing: rolls can reverse direction, material passes back and forth
2. Three-High Rolling Mill
- Three rolls stacked vertically
- Material passes forward through bottom gap
- Returns backward through top gap
- No need to reverse roll direction
- Higher productivity than two-high
3. Four-High Rolling Mill
- Two small work rolls (contact with workpiece)
- Two large backup rolls (support work rolls)
- Prevents deflection of work rolls
- Allows smaller diameter work rolls → less force required
- Better for thin sheets
4. Cluster Rolling Mill
- Multiple backup rolls supporting small work rolls
- Used for very thin sheets
- Sendzimir mill (20-high) - for stainless steel and other hard materials
5. Tandem Rolling Mill
- Series of rolling stands in sequence
- Continuous rolling operation
- High production rate
- Used in hot strip mills
Hot Rolling vs Cold Rolling
Hot Rolling
- Performed above recrystallization temperature (typically 1000-1300°C for steel)
- Large reductions possible (up to 99%)
- Lower forces required
- Poor surface finish (scale formation)
- Less dimensional accuracy
- Products: slabs, blooms, billets, plates, structural shapes
Cold Rolling
- Performed at room temperature
- Limited reduction per pass (typically 50% max)
- Higher forces required
- Excellent surface finish
- Better dimensional accuracy
- Strain hardening occurs
- Products: sheets, strips, foils
Rolling Force and Power
Rolling Force (F)
Simplified Formula: F = Yf × L × w
Where:
- Yf = Average flow stress of material
- L = Contact length = √(R × Δh)
- w = Width of workpiece
More Accurate Formula: F = Yf × L × w × Qp
Where Qp is a factor accounting for friction and geometry
Rolling Torque (T)
T = F × a
Where:
- a = Moment arm ≈ 0.5 × L
Rolling Power (P)
P = 2πNT/60
Where:
- N = Roll speed (rpm)
- T = Torque per roll
Maximum Draft
Maximum Possible Draft: Limited by friction
Condition for Rolling: μ ≥ tan α
Where:
- μ = Coefficient of friction
- α = Angle of contact (bite angle)
Maximum Draft: Δh(max) = μ² × R
Implications:
- Higher friction → larger draft possible
- Larger roll radius → larger draft possible
- For cold rolling: μ ≈ 0.1, limited draft
- For hot rolling: μ ≈ 0.4, larger draft possible
Rolling Defects
1. Surface Defects
- Scale: Oxide layer on hot-rolled surface
- Scratches: From roll surface defects
- Pits: From scale particles pressed into surface
2. Structural Defects
- Edge cracks: Due to tensile stresses at edges
- Alligatoring: Horizontal splitting due to non-uniform deformation
- Wavy edges: Edges longer than center
3. Shape Defects
- Camber: Curvature along length (banana shape)
- Warping: Twisting or bending
- Roll deflection effects: Thicker at edges than center
Rolling Products
Primary Rolling (Hot)
- Ingots → Blooms (square cross-section, >150mm)
- Ingots → Slabs (rectangular, width > 2× thickness)
- Ingots → Billets (square, <150mm)
Secondary Rolling
- Blooms → Structural shapes (I-beams, rails, channels)
- Slabs → Plates, sheets, strips
- Billets → Bars, rods, wire (after wire drawing)
Specialized Products
- Seamless tubes: Piercing and rolling
- Thread rolling: For screws and bolts
- Ring rolling: For large rings and bearings
- Shape rolling: Rails, I-beams, channels
Advantages of Rolling
- High production rates
- Good mechanical properties (grain refinement)
- Economical for large quantities
- Wide range of products
- Good dimensional control (especially cold rolling)
- Continuous operation possible
Limitations of Rolling
- High capital cost (expensive equipment)
- Limited to simple cross-sections
- Residual stresses in cold rolling
- Surface defects in hot rolling
- Roll wear and maintenance costs