Copyright 2002 - 2009 IFORGEIRON, All rights reserved.
A0009 Cold Rolling and Finishing
Part 2 of 2
by Terry Smith
In this second article on cold roll finishing, I will be discussing the process used to finish steel strip after it has undergone the pickling process. This article will cover, cold roll reduction, annealing, temper rolling, slitting and shearing of steel strip. All steel strip that is run through the cold roll reduction mill must be pickled first. Some of the strip that is annealed and/or rolled in the tempering mill does not have to be pickled first but are merely run through either or both processes to achieve final characteristics as per the customer orders. Some of the strip can also be sent directly to the slitter and/or shear lines as well either from the pickle line or not pickled, or annealed, at all but rather processed as is from the hot strip mills.
Cold Roll Reduction
At the plant where I worked, the hot strip mills rolled steel in thicknesses ranging from 0.047 to 0.625 inches thick in general increments. Some customers require a steel to be of a particular thickness other than the general increment sizes rolled in the hot mill or thinner than the minimum thickness rolled in the mills. These steels are processed in the cold roll reduction mill. These mills are capable of rolling steel to the precise thickness that the customer orders and are a major part of the steel strip production process. The reduction mill in the plant I worked had four rolls in the mill that were stacked upon each other. This arrangement is known as a two high mill. There are two working rolls between which the strip is passed and two large back-up rolls, one on top of the working rolls and one on the bottom. The back-up rolls apply the tremendous pressures required to cold roll (reduce) the strip between the working rolls. The working rolls are usually about two to three feet in diameter while the back-up rolls are about seven to eight feet in diameter. The rolls are made of high alloy steel so they can withstand the tremendous pressure they are under while rolling without deforming. Because of this the rolls are ground in a large lathe using a very large grinding wheel on a movable carriage. Depending on the surface finish required of the strip the working rolls will either have a highly polished (mirror like) finish or a dull finish on them. All working rolls are ground on the lathe in the mill to a highly polished surface periodically. The rolls that have a dull finish on them are shot blasted after grinding to produce the desired surface.
After grinding to a polished surface the rolls that need a dull finish are placed on a large carriage which has a set of rubber rolls on it. The carriage then travels on a small rail track into a large enclosure and the door is closed down. On top of this enclosure is a large hopper filled with fine steel balls called shot. This shot is very small in diameter (about half the size of a BB or smaller) and is very hard. It is fed down a chute and using either compressed air or a impeller type system it is accelerated to high speed (in excess of a hundred miles per hour) and blasted against the surface of the roll. The rubber rolls on the carriage rotate causing the steel roll to rotate so all its surface is exposed to the shot blast. The shot comes in a variety of sizes and hardness grades and different types are used depending on the type of surface finish required on the rolls. After a predetermined cycle time the roll is removed from the Wheelabrator, as it is called and is ready to be used in the mill.
A saddle type conveyor runs along the side of the reduction mill. Steel coils are place on this conveyor by overhead cranes using the same ‘C’ hook as at the entry and exit ends of the pickle lines. This saddle conveyor moves the coils along to the reduction mill where they are lowered onto a frame at the entry side of the mill. A transfer saddle operated by the mill operator moves out to the frame and picks up the coil and moves it back into the feed mandrel on the entry side. The operator cuts the strap, freeing up the loose end of the coil. He opens a space between the work rolls and feeds the end of the exit side. On the exit side is another expandable mandrel the same as the catcher mandrels of the hot mills and pickle line. The entry operator feed the strip until the exit operator can catch the end in the open segment of his mandrel, expanding it and trapping the end of the strip. The entry operator then closes the gap in the working rolls down on the strip. Pressure (thousands of tons) is applied by the back-up rolls by means of hydraulically operated screws, to the working rolls and the reduction rolling process begins. If the thickness of the steel needs to be greatly reduced, the strip will be passed back and forth between the rolls a number of times with the rolls adjusted for each pass. Due to the great amount of pressure exerted in the reduction process the steel strip becomes very hot. In order to prevent the steel from becoming too hot and sticking to the work rolls, the rolls are flooded with a coolant consisting of 95% water and the other 5% water soluble oil. The end of the strip that is in the exit mandrel is not released in the multiple pass process nor is it completely unwound from the entry mandrel. In the final pass through the reduction mill, the portion that was not reduced from the entry end is trimmed off in a set shears just before they enter the work rolls to the exit side. A transfer saddle on the exit side then moves the coil back onto the conveyor that runs beside the mill. Cold rolled steel is sensitive to the presence of cracks and is subject to brittle fracture. In the case of ‘full hard’, steel is reduced by 50% or more of its’ original thickness, the steel is sent to the annealing dept.
Because the steel was reduced in thickness cold, the grains of the steel become compacted and crushed. The steel never reaches a critical temperature to permit re-crystallization of the grain structure and the steel becomes harder and somewhat more brittle. It has become work-hardened.
For some customers this type of steel is what they need for their process and the steel coil may be shipped as is. However the majority of the strip coming from the reduction mill is far too hard to suit customer needs and therefore must be annealed. In the annealing dept. of the cold mill are several furnaces especially designed to anneal coils of steel. The conveyor from the reduction mill feeds into the annealing department and the coils are turned from being on edge to laying flat on their sides.
The base for each annealing furnace has a large flat ring, with a large air circulating fan in the center of it. The coils are picked up from the floor by an overhead crane that has a large set of scissor activated gripper tongs. One half of the tongs goes inside the center of the coil and the other half goes outside. As the tongs are lifted the scissor action of the tongs clamps them together on the coil so it can be lifted while still laying flat. The coil is laid on the ring in the base of the furnace and the tongs are released. Several coils are stacked, one on top of the other. After the coils are stacked a second overhead crane lifts a special high temperature stainless steel cover that fits over the stack of coils and into a groove in the furnace base. This forms an airtight seal to keep the gasses and flame used in the furnace to heat the steel for annealing from reaching the steel. After the inner liner is in place, the annealing furnace itself is lowered down over the inner liner. The furnace portion is brick lined but there is sufficient space between the inside layer of brick and the inner liner to provide a combustion space for the gas to burn.
After the furnace is in place, gas lines are connected to the feeds on various burners surrounding the furnace. These burners are designed such that when they are working the burning gas circulates around the outside of the inner liner to evenly distribute the heat. The furnace is brought up to the appropriate temperature required to anneal the coils in the liner (approx. 950 to 1200 deg. F). The anneal operator monitors the correct temperature and soak time at annealing temperature for each of the individual furnaces. If a bright surface is to be maintained on the steel, even after annealing the inner liner can be purged and filled with nitrogen. The large circulating fan in the base keeps the air/nitrogen moving inside the inner liner to help improve even heating of the coils themselves. Depending on the alloy of the steel, after it has been heated to and soaked for a sufficient time at temperature the coils are cooled at a controlled rate inside the annealing furnace. If the coils are just mild steel the furnaces are quite often removed fairly soon after they are shut down and the inner liner is removed. The hot coils are then removed and set out on racks on the floor to cool completely. Again, after annealing some coils are shipped straight to customers and others are sent to a tempering mill and then the final process line for slitting and shearing, or straight to the slitter or shears.
Some customers require a certain degree of hardness in the steel for their processes. Some of this steel may come directly from the reduction mill if the thickness of the steel has not been cold reduced a significant amount. The steel has become work hardened in the rolling process and can be used as is. If the steel was reduced by a great amount or by 50% or more it would have been annealed. After being annealed it would then be sent to the temper mill for further processing. The temper mill is the same type as a cold reduction mill, with a set of work and a set of back-up rolls about the same size as the reduction mill. The temper mill does not use the extreme pressure that is used in the reduction mill. It has the basically the same set up for the entry and exit sides of the mill with mandrels the same as the cold mill. The exit mandrel however is not segmented but has a type of curved chute that directs the end of the strip coming out of the mill around the mandrel so the finished end just gets trapped in itself on the mandrel for coiling. The chute can be raised into position to start the strip in a tight roll and slowly backs down as the coil winds up. At the entry side, there is a set of shears that is used to trim off the end of the strip that was not reduced by the reduction mill, before it passes through the work rolls of the temper mill.
The temper mill is set so that it reduces the over-all thickness of the steel from 0.5 to 1% maximum. By doing so it only compresses the outer surfaces of the steel and not the center. This is known as ‘skin rolling’ steel. This imparts a certain degree of hardness to the steel to meet customer needs. The minimum reduction is 0.5% and greater hardness is classified as ¼ hard, half hard, and full hard, which is a 1% reduction of over-all thickness.
Steel strip from the hot mill is sometimes temper rolled as well without being reduced and often steel which is not cold reduced first is run through the temper mill without having been pickled first.
From the temper mill it can either be direct shipped or sent to the shears and or slitters for final processing.
Slitters as they are called are simply a set of circular knives mounted on a wide set of rolls. There are always at least two sets of knives set apart from one another that trim off the edges of a coil of steel to ensure there is a straight even edge. There may be three or more sets of knives that will cut a coil of steel into several strips of steel each the width required by the customer. The edges that are trimmed off are fed down into a chute to a another small set of rolls that have several small knives mounted on them that chop the continuous strip of steel cut from the edge into small pieces that are returned to the steelmaking dept as scrap for the next batch of steel. After being cut to width the steel may be recoiled again for shipment or on another slitter line sent to the shears to be cut to length as sheets.
Steel to be sheared into lengths as light gauge ‘plate’ first passes through a slitter that cuts it to the proper width. As the steel comes from the slitter it passes down a roller line and between the blades of a large set of shears. These are called flying shears as they cut steel ‘on the fly’. These shears have a fixed upper blade and a movable bottom blade. The shears move in a rotational manner in the same direction as the steel moves. The bottom blade is controlled by a concentric shaft which is adjustable. The blade cycles up and down with the rotation of the shears but at a different speed so that every so many revolutions the shears are moving down as the blade is coming up and they close shearing off the sheet of steel. As the shears are moving in rotation they match the speed of the steel passing through them. The speed of the concentric shaft for the bottom blades is then adjusted so that the shears close at the precise point needed to shear the strip into the correct length. The cut off sheet then travel down another conveyor where they are stacked in piles to be strapped and shipped.
This concludes this series of articles. Most of what I have written about has come from personal experience working in a steel mill. All the processes I discussed in these articles were done in that manner at the time I worked there. It has been a number of years and technology is changing things every year and a lot of steel making and such has changed but the basic principles remain the same. I also want to reiterate that these articles were not written with any true authority and complete explanations of the whole process but merely as an informational and perhaps entertaining bit of writing.
A0009 Cold Rolling and Finishing