Historical Development of Rolling Mills
Historical Development of Rolling Mills
In metalworking, rolling is a metal forming process in which metal is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, to give it a desired shape, and / or to impart a desired mechanical property. In rolling, work material is deformed by compressive forces between constantly spinning rolls. In other words, metal is passed between spinning rolls and the force applied reduces the thickness of the material. Both the metals’ shapes and internal structures are changed. Metal rolling process can be compared to the rolling of dough with a pin. Rolling is one of the most important manufacturing processes in the modern world. The large majority of all metal products produced today are subject to rolling at one point in their manufacture.
Modern rolling mills are fully automated, high speed, and high capacity mills frequently include metallurgical treatment of materials being rolled in order to achieve the desired microstructure and /of mechanical properties of the rolled material. This level of perfection in the process of rolling and in the rolling mills has not been achieved by a single invention but it is the fruit of da large number of small continuous improvements carried out over the time.
Rolling has existed for hundreds of years. The first rolls were small, hand driven and they were used to flatten gold and silver in the manufacture of jewelry and art. Rolling mills were found in Middle East and South Asia as early as 600 BCE (Before Common Era). These mills were of crude form but they used the same basic principles which are associated with the rolling process. The first known design of a rolling mill in Europe dates back to 1485 and is attributed to Leonardo da Vinci. In one of his drawings (Fig 1), he had described for the first time the possibility of ‘making a material pass’ between two cylindrical rollers with parallel axes to modify its thickness. Also in this case it was provided for the cold rolling of ductile materials, especially for lead, and tin etc. This design was very advanced for the time and it is not certain whether any mill based on this design was ever built.
Fig 1 Sketch of rolling mill by Leonardo da Vinci
The power being used for driving the rolling mills has passed through four phases. The first phase was the manual phase. During this phase, the power source needed for early rolling was small and hand-driven rolls were used to flatten the metals for jewelry and artwork. In later machines, men turned an adjustable cross or crank, which were attached to the rolls. Manual power could only roll softer metals like gold, silver, tin, and lead. Fig 2 shows an early type of rolling mills being operated by the manual power.
Fig 2 Early rolling mill and a rolling mill operated by manual power
In the second phase the rolling mills used hydraulic power (Fig 3). Harnessing the energy of moving water from a river provided free power, strong enough to roll harder metals like iron. Water power had serious drawbacks. Droughts and cold temperatures could reduce flow of water. Further, the rolls utilizing the hydraulic power moved in only in one direction.
Fig 3 Rolling mills with hydraulic power
In the third phase the rolling mills used steam power. Steam power first appeared in the late 1700s because of the invention of James Watt. The steam engine created the physical power which replaced the manual / animal power. The rolling mills of this era harnessed the power created by the steam engines. Steam engines were commonly used for driving the mills in the 1800s, and by the late 1800s, steam engines were the most popular source for power. Despite its strength in supplying power, steam engines were largely inefficient. Fig 4 gives a steam driven sheet rolling mill in the late nineteenth century.
Fig 4 A steam driven sheet mill in the late 19th century
In the fourth phase, the rolling mills used electric power. The advantage was that the power generation could be delinked from the rolling mills, since the electric power was conveniently transmitted from remote generators to motors directly attached to mills. The electric powered rolling mill first appeared around 1900. Electric power remains the choice for operations today.
The earlier production of plate iron in Europe had been by forging, and not by rolling mills. The real turning point in the sheet metal production took place with the introduction of the rolling mill. By the 1600s, rolling mills, rather than small, hand-driven rolls, were known to have been in operation and iron was just being introduced as a metal which was capable of rolling. There are reports of two rolling mills around 1501. One used to obtain gold sheets with uniform thickness from which to draw coins, the second to cut previously formed sheets into strips. Both were more used as ‘finishing devices’ rather than for the reduction of the thickness.
Earliest rolling mills were slitting mills, which were introduced in 1590. These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron. In 1590, Leonardo da Vinci’s rolling mill is brought to life utilizing two heavy cylinders to press different types of metal altering their thickness. The first industrial plant for which there is certain evidence was there in 1615, to obtain lead and tin plates. Others followed, driven by manual or hydraulic force.
Earlier rolling mills were also used to roll black plates for the production of tinplates. Tinplates were made by rolling the steel (formerly iron) in a rolling mill, removing any mill scale by pickling it in acid and then coating it with a thin layer of tin. Black plates were once produced individually or in small groups, in what became known as a ‘pack rolling mill’. The first experiments at rolling iron for tinplate took place around 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll ‘Pontypool plates’ also called the black plates. Later these plates began to be rerolled and tinned to make tinplate.
By the late 1700s, the first hot rolling mills appeared, allowing iron to become a more popularly rolled material. The Industrial Revolution (around 1760s to around 1840s) created an unprecedented demand for iron. New technology was invented to manufacture iron and eventually steel and the metals were used in new products like machinery, railroads, and steam ships. Due to the increased possibility of obtaining ferrous material, the cold rolling of steel is simultaneously started. In 1682 a cold rolling mill of notable sizes was present in Newcastle in England. The first detailed description dates back to few years later. It is a plant in Galles which processed 700 mm long bars with 100 mm width, which could obtain sheets of 1500 mm × 700 mm size. It is the first certain evidence of the steel rolling process to produce sheet metal, the driving force was provided by water wheels. Galles remained the main European producer of thin sheets until the end of 1700.
Already at that time, it was understood that the rolling was a very productive process which was able to satisfy the enormous demand of the semi-finished products, among which obviously was the metal sheet. However, the rolling mills still needed at that time a substantial amount of manual work (Fig 5).
Fig 5 Manual work involved in earlier rolling mills
The product sizes continuously increased as the time passed, and at the ‘British Great Exposition’ in 1851 a sheet metal with over 6 metre length, 1 metre width and 11 mm thickness, weighing over 500 kg was on show.
The progress in the rolling mills complied with the demands of that age for the production of the required materials. In 1600, lead sheets for the roof covers were highly required and this possibility is then developed, at the end of 1700, in the middle of the industrial revolution, the rails and semi-finished steel products were needed, and hence, the rolling mills were developed to satisfy this demand. At the beginning of the nineteenth century the rolling mill structure was essentially the current one i.e. a sturdy cast iron roll stand with two steel rollers and the possibility of adjusting, through a screw, the roller distance (Fig 6).
Fig 6 Rolling mill structure in the beginning of 19th century
Modern rolling practice can be attributed to the pioneering efforts of Henry Cort of Funtley Iron Mills, near Fareham in Hampshire, England. In 1783, a patent was issued to Henry Cort for his use of grooved rolls for rolling iron of bars. In the grooved rolls, a groove of the required dimensions made it possible to roll bars of rounds, squares and flats, or other sections. Although Cort was not the first to use grooved rolls, he was the first to combine the use of many of the best features of various ironmaking and shaping processes known at that time. Thus modern writers have called him ‘father of modern rolling’. Since Cort’s early rolling mill, there has been a continuous development of the process and of the size of mills. In the eighteenth century the rolling of more complex shapes such as rounds, squares, rails, double-T beams etc started
With this new design, rolling mills were able to produce 15 times more output per day than what was being produced with a forge hammer. With the invention of Henry Cort, the little mill at Funtley was able to produce more than 10,000 tons of bars per annum (equivalent to 200 tons per week). From there on, these outputs from the rolling mills have mounted up until, in the USA alone, the quantity of the material rolled in the year 1890 (excluding all flat roll products) was around 5 million tons of iron and steel (equivalent to 100,000 tons per week), and, in 1899, over 8 million tons (equivalent to 167,000 tons per week) were rolled with every pound of which passed through, grooved rolls like those first used in the ‘little mill at Funtley’.
The year 1810 witnessed the establishment of the Brandywine Iron Works and Nail Factory in USA, which operated a rolling mill. Brandywine Iron Works became a success due to its production of the highly demanded boiler plate, and its quality product. In 1867, George Fritz, then chief engineer of the Cambria Iron Company, Johnson, Pennsylvania, USA invented the blooming mill on which steel ingots were are rolled instead of reducing them by forging, as had been the practice. Fig 7 shows the rolling of a steel ingot in a rolling mill of around that time.
Fig 7 Rolling of steel ingot
The mill used by Cort, of- course, was of the ordinary 2-high rolling mill. Modification after modification succeeded Cort’s 2-high mill, calling forth all sorts of types, kinds, and classes, among them the familiar reversing and 3-high mills, all evolved by the demands of advancing civilization in ever varying and countless combinations, for something bigger and better, longer and stronger. Mr. Fritz built his first regular three-high mill in 1871. This departure from the old practice greatly helped to increase the production. The three rolls configuration mills came into existence in which the intermediate roll was with much smaller diameter to achieve the reduction in the required power and to allow the rolling in the two directions without reversing the motion of the motor. Upto the four roll configuration (already hypothesized by the Leonardo da Vinci) with the two external rolls acting as support to the two inner ones for the rolling of flats were introduced. Further mills with higher number of rolls (with even 20 rolls) were also developed for rolling.
Another improvement in the rolling process was the use the mechanical appliances added to the rolls. Earlier the rolling stock was fed manually by the use of hooks and tongs. A number of inventors up to that time had sought to accomplish this work by machinery which would be automatic in its action, but none had been actually built. In 1884, driven roller tables were introduced in front of the finishing rolls of the rail mill of the Albany and Rensselaer Iron and Steel Company of Troy, New York. This worked so well that an automatic arrangement was put in front of the roughing rolls.
A patent was granted in 1766 to Richard Ford of England for the tandem mill. A tandem mill is one in which the metal is rolled in successive stands. Ford’s tandem mill was for hot rolling of wire rods.
The further evolution in the rolling mills was the development of a continuous mill, and the large family of auxiliaries which have grown up with it. Like all inventions, it had its day of small beginnings. There are footprints of tentative efforts made in France to use it. The first recorded patent and first actual use for a continuous mill seems to have been that of an American, J. E. Serrell, in 1842-43. Its use was, however, restricted by him to the rolling of lead pipe and copper. Another American, Henry B. Comer, in 1859, secured a patent for a continuous mill, but this type of the mill was never constructed. By every consideration, the credit for the first continuous rolling mill goes to Mr. George Bedson of Manchester, England for putting into the service such mill for the rolling of iron and steel, for successfully combining its essential elements, and for gaining of it the wide application which it enjoyed.
This mill, as its name implies, works continuously upon the metal. The rolls are placed closely in tandem and the metal is led by means of guides directly from pass to pass, without the intervention of any human labour. There are no interruptions, no catching or looping, but a continuous onward transit of the metal from the bite of the roughing rolls to the discharge of the finishing roll stand. Each pair of rolls, by carefully arranged gearing, is being speeded according to the reduction of each successive pass, to take up the corresponding elongation. Phenomena peculiar to this mill had to be combated, but the door was at once opened for vastly increased lengths, weights, and speeds. Europe, at that time, was rolling easily 50-pound billets by the Belgian or Looping System against the United States mills were rolling 15 pounds on ordinary 2-high and 3-high mills. The Bedson Mill at once handled from 70 pound to 80 pound billets, which was rapidly increased until to the 300 pound billets are normally rolled. In the continuous mills invented by George Bedson, coils of wire rod, weighing 281 pounds, in length 530 yards, were rolled from a single billet.
‘The first of these continuous mills in the USA which was designed and constructed by Bedson and erected and put in operation in the spring of 1869 is shown in Fig 8. The mill consisted of 16 stands of rolls placed in tandem, and rolled 1 and 1/8 inch billets, weighing from 70 pounds to 80 pounds. The continuous rolling of a billet in a plane always at the same angle to its axis, produces flattening of the mass only. To obtain elongation, compression in planes varying in angle with that of the axis is necessary, similar to the rotation of the metal by the blacksmith upon the horn of the anvil between each successive blow. In the Bedson’s mill this was accomplished by placing each pair of rolls (after the manner of the universal mill) at an angle of 90 degrees to its predecessor. This secured the desired end perfectly, but entailed vast annoyance in the care and management of the rolling mill. This had necessitated the use of one system of gearing above the floor line and another in a pit below it. Accessibility to these, as also to the rolls themselves, in changing and adjusting, was seriously impeded. In the next continuous mill which was designed in 1878, all rolls were placed in a horizontal plane, all the gearing was in a single plane, and the very essential requisite of varying the plane of compression secured by the use of twist guides, whereby the metal was turned 90 degrees in travelling between each successive pass which was a device got universally adopted in these mills.
Fig 8 Continuous rod mill of Bedson
It is quite impossible to trace each minute development. As in all new things, surprises constantly had to be met and overcome. Adjustment became inseparably a feature of the continuous rolling mills. Exact matching of the corresponding speed and passes of the rolls was difficult. Without it one of two things was inevitable, delivery to a pass faster than its capacity to receive it, producing lateral looping, or slower than its capacity to receive it, producing stretching. Of the two alternatives, the latter was preferable, for the relief by looping was interfered with by the guides. It was cumulative in effect, and caused serious danger to the safety of the mill and its operators. Stretching, however, threw a demand upon the heated metal greater than many grades of good iron could withstand, by reason of their low tensile strengths and lack of homogeneity. For this reason the use of the continuous mill, as originally installed, was greatly lessened in rolling common grades of iron so much so, that the next mill built by the Washburn & Moen Company, in 1877, was a 3-high mill adapted to the Belgian system. However, with the improvements in Bessemer and Open Hearth steels, with their greater tensile strength and homogeneity, ultimately brought a product for rolling for which the continuous mill was well adapted.
With increase in weight and decrease in the section of the finished product there arose the practice of continuous or simultaneous rolling of the metal in 3-high mills by serpentine looping on each side of the mill. This was distinguished from the ‘back and forth’ rolling. In this case the metal was discharged free from the rolls at each pass. This type of mill was used by Europe, presumably first in Belgium, and hence, the name ’Belgian’ having come to be understood as the meaning of a looping mill. It was the use of these mills which for some time enabled the European manufacturers to roll the 30 pound and 50 pound billets against 15 pounds to 20 pounds billets on the ‘back and forth’ 3-high mills. A plan of the continuous rolling mill of this type is shown in Fig 9. In such a mill, the roughing was done on a 2-high mill capable of handling 4-inch billets, reducing to 1 and 1/8 inches. From this size it went to the looping or Belgian mill, and rolled to the ordinary wire rod. This system was used in a number of mills for rolling wire rods and merchant products.
Fig 8 A Belgian rod mill
The combination or joint continuous-Belgian mill (Fig 10) was the efforts made during the time to secure a consolidation of the merits of both types of the mills. A mill of this type was put in operation in 1881 at the works of the Washburn and Moen Company. It roughed billets of around 100 pounds in weight from 1 and 3/16 inches to 3/8 inches on the continuous mill, and finished from 3/8 inches on the Belgian mill to the ordinary wire rod. This was the first mill of this type built. One of the main merits of this mill was that it rolled common iron equally as well as did the looping or Belgian system, and that, too, with less labour. With increase in size and weight of the billets the accessories of the mill began to grow, although the need for something new at the delivery end of the mill soon followed. The leap from billets and coils of 15 pounds and 20 pounds to those of 70 pounds and 80 pounds forced a ‘policy of expansion’. The mill itself had undeveloped capacity with the limitation lay in the heating and reeling, two collateral functions of the mill which have had a rapid and interesting growth.
Fig 10 A joint continuous Belgian mill
In the 1880s and 1890s, two developments changed the national rolling mill landscapes in the USA. First, there was a growing demand for steel, rather than iron. Steel was preferred since it was stronger than iron. Second, the emerging preference for wider plates, which reduced the number of riveted seams in vessels. Rivets created weak points, sources of failures for boilers and ships. Companies adopted larger rolling mills to make wider plates, which reduced the number of riveted seams.
In 1882, Worth Brothers Company began operating a 90 inch rolling mill in Coatesville, greatly increasing the competition in the USA. For the next three decades, Worth Brothers and Lukens Iron and Steel Company competed for the widest rolling mill. In the year 1890 Lukens commissioned 120 inch plate mill, Worth brothers commissioned 132 inch plate mill in 1896. Lukens commissioned 134 inch plate mill in 1900 and a 140 inch plate mill in 1903. This was followed by Worth brothers commissioning 152 plate mill also in 1903. The competition ended in 1915, when Midvale Steel and Ordnance Company acquired Worth Brothers. Then in 1918, Lukens put into operation the world’s largest plate mill.
Lukens’ original plans were to build a 180 inch three-high mill, just large enough to claim the title of the world’s widest mill. However, no roll manufacturer could produce the required size and weight of the rolls necessary for that mill. Instead, Lukens collaborated with the United Engineering and Foundry Company (UE&F) of Pittsburgh. Lukens and UE&F engineers worked together to produce a 204 inch wide, four-high rolling mill. This mill rolled its first plates on May 22, 1918 and was enlarged to 206 inch wide in 1919. This mill kept the title of the world’s largest plate mill for more than forty years. The mill had 34 inch diameter work rolls (3 tons each) and 50 inch diameter backup rolls (60 tons each). It was capable of rolling ingots upto 90,000 pounds and plates upto 192 inch in width. It had a weekly rolling capacity of upto 4,000 tons. The power to the mill was provided by a 20,000 horse power twin tandem compound steam engine. The mill housing was made of steel which was built in four parts, and was over 42 feet tall and it used transfer tables reduced the amount of manual labour. The mill is shown in Fig 11.
Fig 11 The 206 inch plate mill at Lukens iron and steel company
During this period, rock salt and burlap bags were tossed onto the ingot to break the scale off the exterior as it passes through the rolls of the mill (Fig 12).
Fig 12 Rock salt and burlap bags being tossed on the ingot surface
The first rail rolling mill was established by John Birkenshaw at Bedlington ironworks in Northumberland, England in 1820, where he produced fish-bellied wrought iron rails in lengths of 15 feet to 18 feet. Three high mills for rolling heavy sections were introduced in 1853.
The method of producing an I-beam, as rolled from a single piece of steel, was patented by Alphonse Halbou T of the company Forges de la Providence in 1849. Bethlehem Steel was a leading supplier of rolled structural steel of various cross-sections in American bridge and skyscraper work of the mid-twentieth century.
Hot strip mill were developed in the first half of the twentieth century. The hot strip mill was a major innovation in steel rolling, with the first being erected at Ashland, Kentucky in 1923. This provided a continuous process, cutting out the need to pass the plates over the rolls and to double them, as in a pack mill. At the end the strip was cut with a guillotine shear or rolled into a coil. Early hot strip mills did not produce strip suitable for tinplate, but in 1929 cold rolling began to be used to reduce the gauge further. The first hot strip mill in the United Kingdom was opened at Ebbw Vale in 1938 with an annual output of 200,000 tons. By 1961, the early hot strip mills had been upgraded or replaced by larger capacity semi-continuous and continuous hot strip mills with capacities of around 400,000 tons per year to 3 million tons per year.
With the commercialization of the continuous casting during the 1950s, the slabbing mills and the blooming mills slowly disappeared from the rolling mill scene. Now slabs, blooms, and billets are being continuously cast for further rolling into finished rolled products (Fig 13). In October 1989, thin slab casting and rolling process was commercialized. This process has even eliminated the roughing train of the hot strip mill.
Fig 13 Rolling mill products
Rolling mills continued to develop and the development s are still continuing. Present day mills are continuous mill which are having high capacities and which are running at high speeds. The mills are able to roll materials which are heavier and are having large dimensions. The mills are capable of rolling thinner sections. Further modern mills not only roll but also carry out the metallurgical functions through the on-line heat treatment of the rolled materials. Present day mills are fully automated. The mills have high productivity and high yield. The manpower required to run the mills have also reduced. The rolled products also have better size tolerances. However, it is worth mentioning that the rolling, as it exists today, cannot be attributed to a single inventor but it is fruit of several of small continuous improvements which have led it to be the most used process in metal working process of rolling. Presently, around 90 % of the metals used in industry, have, sooner or later, undergone a rolling process.