Rolling of Metals Rolling Processes---------------------------------------------Dr.Adel Fathy 1
Flat-Rolling and Shape-Rolling Processes Rolling Processes---------------------------------------------Dr.Adel Fathy
The rolling process (specifically, flat rolling). Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls The rolling process (specifically, flat rolling). Rolling Processes---------------------------------------------Dr.Adel Fathy 3
Types of Rolling Based on workpiece geometry : Flat rolling - used to reduce thickness of a rectangular cross section Shape rolling - square cross section is formed into a shape such as an I‑beam Based on work temperature : Hot Rolling – most common due to the large amount of deformation required Cold rolling – produces finished sheet and plate stock Rolling Processes---------------------------------------------Dr.Adel Fathy 4
Rolled Products Made of Steel Some of the steel products made in a rolling mill. Rolling Processes---------------------------------------------Dr.Adel Fathy 5
Hot Rolling for I‑beam Product Rolling Processes---------------------------------------------Dr.Adel Fathy 6
Steps in the shape rolling of an I–section part. Rolling Processes---------------------------------------------Dr.Adel Fathy
Blooms, Billets, Slaps After casting, ingots are rolled into one of three intermediate shapes called blooms, billets, and slaps: Blooms have square cross section 6” x 6” or larger. They are rolled into structural shapes. Billets have square cross section 1.5” x 1.5” or larger. they are rolled into bars and rods. Slabs have rectangular cross section 10” x 1.5” or larger. They are rolled into plates, sheets and strips. Rolling Processes---------------------------------------------Dr.Adel Fathy 8 8
Production Line Rolling Processes---------------------------------------------Dr.Adel Fathy
Flat rolling and its analysis R = roller radius p = roll pressure L = contact length θ = contact angle vr = roll speed to = initial plate thickness tf = final plate thickness vo = plate entry speed vf = plate exit speed Side view of flat rolling, indicating before and after thicknesses, work velocities, angle of contact with rolls, and other features. Rolling Processes---------------------------------------------Dr.Adel Fathy 10
Roll bite condition For the work piece to enter the throat of the roll, the component of the friction force must be equal to or greater than the horizontal component of the normal force. But we know Therefore Rolling Processes---------------------------------------------Dr.Adel Fathy
Flat rolling and its analysis The work enters the gap between the rolls at a velocity vo and exits at a velocity vf. Because the volume flow rate is constant and the thickness is decreasing, vf should be larger than vo. The roll surface velocity vr is larger than vo and smaller than vf. This means that slipping occurs between the work and the rolls. Only at one point along the contact length, there is no slipping (relative motion) between the work and the roll. This point is called the “Neutral Point” or the “No Slip Point”. Rolling Processes---------------------------------------------Dr.Adel Fathy
Flat rolling and its analysis 1. Constant material volume where Lo = initial plate length Lf = final plate length 2. Continuity of volume flow rate 3. Forward slip Rolling Processes---------------------------------------------Dr.Adel Fathy 13
After three passes, tf = (0.75)(0.75)(0.75)(2.0) = 0.844 in. Example:- A 2.0-in-thick slab is 10.0 in. wide and 12.0 ft long. Thickness is to be reduced in three steps in a hot rolling operations. Each step will reduce the slap to 75% of its pervious thickness. It is expected that for this metal and reduction the slap will widen by 3% in each step. If the entry speed of the slap in the first step is 40 ft/min. and roll speed is the same for the three steps, determine (a) length and (b) exit velocity of the slap after the final reduction. Solution After three passes, tf = (0.75)(0.75)(0.75)(2.0) = 0.844 in. wf = (1.03)(1.03)(1.03)(10.0) = 10.927 in. (2.0)(10.0)(12) = (0.844)(10.927)Lf Lf = (2.0)(10.0)(12)/(0.844)(10.927) = 26.025 ft Rolling Processes---------------------------------------------Dr.Adel Fathy 14
(b) Given that roll speed is the same at all three stands and that (2.0)(10.0)(40) = (0.844)(10.927)Vf Vf = (2.0)(10.0)(40)/(0.844)(10.927) = 51.78 ft/min. Rolling Processes---------------------------------------------Dr.Adel Fathy
Flat rolling and its analysis In flat rolling, the work is squeezed between two rolls so that its thickness is reduced by an amount called the draft: where d: draft to: starting thickness tf : final thickness As a reduction of the starting thickness (r): Rolling Processes---------------------------------------------Dr.Adel Fathy 16
From triangle ABC, we have The maximum draft From triangle ABC, we have As a is much smaller than R, we can then ignore a2. Where Δh = ho – hf = 2a Rolling Processes---------------------------------------------Dr.Adel Fathy
Flat rolling and its analysis Rolling may not be possible (the sheet will not be pulled) if the draft is large. The maximum draft for successful rolling per pass is: Where: dmax : maximum draft successful rolling per pass μ : coefficient of friction R : roll radius As can be seen from the equation, if μ is zero, then dmax is also zero (rolling is not possible) Rolling Processes---------------------------------------------Dr.Adel Fathy 18
2. If reduction = constant Number of passes 1. If draft = constant 2. If reduction = constant Rolling Processes---------------------------------------------Dr.Adel Fathy
Example:- A series of cold rolling operations is to be used to reduce the thickness of a plate from 50 mm down to 25 mm in a reversing twohigh mill. Roll diameter =700 mm and coefficient of friction between rolls and work = 0.15. The specification is that the draft is to be equal on each pass. Determine (a) minimum number of passes required, and (b) draft for each pass. Solution Rolling Processes---------------------------------------------Dr.Adel Fathy 20
Flat rolling and its analysis Rolling force, F Where the deformation strain the average flow stress and Contact length Rolling Processes---------------------------------------------Dr.Adel Fathy 21
Flat rolling and its analysis The torque in rolling can be estimated by: T = 0.5 F L Where: T: Torque (lb.in or N.m) F: Roll Force L: Contact length The Power required to drive the two rolls is calculated as follows: P = 2πNFL P: Power (in J/s =Watt or in-lb/min) N: Rolls rotational speed (RPM) Rolling Processes---------------------------------------------Dr.Adel Fathy 22
Flat rolling and its analysis From the previous equations we can conclude the following: The contact length decreases by decreasing the roll radius. The roll force depends on the contact length, and therefore, reducing the roll radius will reduce the roll force. The torque and power depend on the roll force and contact length, and therefore, reducing the roll radius will reduce both the torque and power. The power also depends on the rotational speed of the rolls, and therefore, reducing the rolls RPM will reduce the power. Rolling Processes---------------------------------------------Dr.Adel Fathy 23
Example:- A 10 in wide, 1.0 in thick plate is to be reduced in a single pass in a two-high rolling mill to a thickness of 0.8 in . The roll has a radius = 20 in., and its speed = 50 ft/min. The work material has a strength coefficient = 35,00 Ib/in2 and a strain hardening exponent = 0.2. Determine (a) roll force, (b) roll torque, and (c) power required to accomplish this operation. Rolling Processes---------------------------------------------Dr.Adel Fathy 24
Rolling Mills Equipment is massive and expensive Rolling mill configurations: Two-high – two opposing rolls Three-high – work passes through rolls in both directions Four-high – backing rolls support smaller work rolls Cluster mill – multiple backing rolls on smaller rolls Tandem rolling mill – sequence of two-high mills Rolling Processes---------------------------------------------Dr.Adel Fathy 25
Two-High Rolling Mill Various configurations of rolling mills: (a) 2‑high rolling mill. Rolling Processes---------------------------------------------Dr.Adel Fathy 26
Three-High Rolling Mill Various configurations of rolling mills: (b) 3‑high rolling mill. Rolling Processes---------------------------------------------Dr.Adel Fathy 27
Four-High Rolling Mill Various configurations of rolling mills: (c) four‑high rolling mill. Rolling Processes---------------------------------------------Dr.Adel Fathy 28
Various configurations of rolling mills: (d) cluster mill Multiple backing rolls allow even smaller roll diameters Various configurations of rolling mills: (d) cluster mill Rolling Processes---------------------------------------------Dr.Adel Fathy 29
A series of rolling stands in sequence Tandem Rolling Mill A series of rolling stands in sequence Rolling Processes---------------------------------------------Dr.Adel Fathy 30
Ring Rolling Rolling Processes---------------------------------------------Dr.Adel Fathy 31
Actual ring rolling mill Rolling Processes---------------------------------------------Dr.Adel Fathy
Steps in the ring rolling Rolling Processes---------------------------------------------Dr.Adel Fathy
Ring Rolling Deformation process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter As thick-walled ring is compressed, deformed metal elongates, causing diameter of ring to be enlarged Hot working process for large rings and cold working process for smaller rings Applications: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery Advantages: material savings, ideal grain orientation, strengthening through cold working Rolling Processes---------------------------------------------Dr.Adel Fathy 34
Thread Rolling (1) start of cycle (2) end of cycle Thread rolling with flat dies: Rolling Processes---------------------------------------------Dr.Adel Fathy 35
Thread Rolling Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies Important commercial process for mass producing bolts and screws Performed by cold working in thread rolling machines Advantages over thread cutting (machining): Higher production rates Better material utilization Stronger threads and better fatigue resistance due to work hardening Rolling Processes---------------------------------------------Dr.Adel Fathy 36
Effect of hot rolling on the grain structure Rolling Processes---------------------------------------------Dr.Adel Fathy 37
Bending of Rolls Bending of straight cylindrical rolls caused by roll forces. (b) Bending of rolls ground with camber, producing a strip with uniform thickness through the strip width.
Defects in Rolled Plates & Sheets • a) Wavy edges • c) Edge cracks • b) Cracks • d) Alligatoring Rolling Processes---------------------------------------------Dr.Adel Fathy 39