diff mill layouts

7/24/2019 Diff Mill Layouts

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Layout of Roll ing Mil ls

Mill Layout

Rolling mills are designed using input parameters such as production rate, incoming bil-let size, product type, and product sizes. The starting point is the production plan, shownin a rolling diagram that maps the rolling process from billet to product. Figure 3 - 1shows a rolling diagram starting with an oval at stand 7, finishing various thicknesses of2 x 2 angles at stand 14. Figure 3 - 2 shows a complete production diagram forrounds, squares, hexes and flats.

Figure 3 - 1

Chapter 3 Mill Layouts

Page 3 - 1 Schweitzer Rolling Technology, Inc.


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Figure 3 - 2

From the pass progression, bar orientation, and section area, the mill layout can be de-termined to provide the most efficient and cost effective production of the proposed pro-duction.

For existing mills, the product types and sizes can generally be deducted from the milllayout. The mill layout will also limit the product type and production rate that can prof-itably be produced.

The rolls in the mill stand can have many different arrangements to make the stand suit-able for the production of different products. Figure 3 - 3 shows twelve different roll ar-rangement in mill housings. The stand shown in arrangement 1 if the most common ar-rangement for continuous rod, bar, and section mills. The stand shown in arrangement2, is a two high reversing mill common in breakdown mills and in special section and al-loy mills. The stand shown in arrangement 3 is called a double duo mill that functions insame way as the stand shown in arrangement 4 only with four rolls so that the passesdo not share a middle roll. Both of these arrangement allow for the reversing of the bardirection without reversing the mill drive motor. The stand shown in arrangements 5 to 9are for the production of strip and plate. The small diameter work rolls reduce the spreadbut bend easily. The larger diameter backup rolls provide the bending strength to keep




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the strip as flat as possible. The stand shown in arrangements 10 to 12 are used in theproduction of beams, channels, and rails.

Figure 3 - 3

Figure 3 - 4




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Early rolling mills used grooved rolls for slitting iron sheets for the production of nails.Figure 3 - 4 shows one such mill, powered by water wheels through several sets ofbevel gears. This mill dates from the early to middle 18th century.

Figure 3 - 5 shows the mill designed by Henry Cort and was issued a patent in 1783 forrolling on grooved rolls for producing half rounds for file stock. Widely acknowledged asthe father of pass design, he had built his mill on money borrowed from a lender whohad been embezzled it from the Royal Navy. The effect of Cort"s work on the iron tradewere incomparable. Large plants, after adapting Cort;s processes, increased productionby 10 times, using fewer workers. In 1797 England became an exporter of iron insteadof an importer and soon led the world in iron production.

Figure 3 - 5

The mill shown in figure 3 - 6, powered by a steam engine, included a hammer to forgethe iron, and many stands including stands for plate, bars, and slit plate.




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Figure 3 - 6

The blooming mill shown in figure 3 - 7, takes ingots and rolls them into semi-finishedbillets, for later processing. The two high reversing mill is one of the most flexible mills tooperate. The powered screw down provides the versatility to take a variety of ingot sizesand produce a wide range of billet sizes.




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Figure 3 - 7




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Another type of blooming mill, or bloomer, is the two high reversing mill with a long rollbody. The driven end is used to break down the ingot in the same way as other boom-ers, after which the rolls are locked in position. The bar is then started through the firstof a series of passes on the opposite end of the roll, using repeaters to take the bararound the end of the rolls. As shown in figure 3 - 8, these mills are called merry-go-

round or ring-around-a-rosie mills.

Figure 3 - 8




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Another design that can reverse the direction of the bar without using a reversing motoris the Lamberton mill. As shown in figure 3 - 9, the housing and rolls are rotated 180 de-grees to reverse the direction of the roll rotation relative to the bar.

Figure 3 - 9

Early bar mills were designed to have few steam engines due to their cost and operating

expense. A bar mill with a continuous roughing mill and a cross-country finishing train isshown in figure 3 - 10. One of the difficulties of operating such a mill is the roughing trainhas constant relative speed from stand to stand. Once set up, this mill is very difficult toadjust. Any variance in bar size result in either compression or tension in the bar usuallycobbling. Any change in the bar size delivered from the last continuous stand requiredproportionate changes in all of the proceeding stands. Some adjustment to roll diametercould compensate for the fixed drive ratios and provide the ability to modify the setup.

A similar mill with a semi-continuous roughing train,shown in figure 3 - 11, eliminatessome of these issues with enough space between the beginning stands for a free bar.These mills were developed about the time that steel billets became widely available.

The speeds of these mills were limited to about 1500 ft/min as this is the fastest bar thata catcher can grab the bar and put in the next pass. Figure 3 - 12 shows a looping barmill for flats, rounds, and squares. The loopers and floor guides made it possible to havethe bar in several stands at once, resulting in larger tonnage production. Figures 3 - 13and 14 show modern looper tables in service.




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Figure 3 - 10




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Figure 3 - 11




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Figure 3 - 12




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Figure 3 - 13

Figure 3 - 14




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The semi-continuous mill in figure 3 - 15 could be used to roll a wide range of finishedsizes due to the ability to but bars on a hot bed out of stands 8, 10, or 12. This mill couldproduce squares from 1/2 to 5, flats from 1/4 x 1 to 2 1/2 x 10 and other products inproportionate sizes. Previous to the time this mill was built, such a size range would re-quire three different mills. This mill layout would also allow stands 9 through 12 to be

changed while finishing out of stand 8.

Figure 3 - 15




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The speed limiting factors of catchers twisting the bar was overcome by the mill shownin figure 3 - 16. A continuous mill with alternating horizontal and vertical stands elimi-nated the need to twist the bar between passes. The in line stands with individual drivesprovided the flexibility for a wide product range. Labor costs on such a mill are low, but itis only suitable for rolling large tonnage of the same section.

Figure 3 - 16




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The mill layout shown in figure 3 - 17 is designed to provide flexibility in producing awide range of large bars. The independent roughing mills and transfer tables providespace for free bar run-out and the ability to finish out of a variety of positions.

Figure 3 - 17




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Scrap rails are converted into a variety of useful products in rail slitting mills such as il-lustrated in figure 3 - 18. After heating to rolling temperature, the rail is slit in the firstpass into three parts, head, web, and flange. Each section moves to a separate rolltrain. This type of mill is very successful in rolling rebar, fence post, bed frame angles,and other small sections.

Figure 3 - 18




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Another method of utilising scrap rail was to re-roll them into small billets for rod mills.Figure 3 - 19 shows two methods for reducing a rail to a small square.

Figure 3 - 19

Figure 3 - 20 shows an early rod mill installed in Worcester, Massachusetts by theWashburn and Moen Company. the mill used billets as large as 4 x 4 and finished 1 1/8 squares from the 12 mill and #6 rod from the 8 mill. The production rate was two tofour tons an hour. This is cited as the first instance of a mill with four loops of the samerod between two stands.

The Bedson rod mill, patented in 1862, shown in figure 3 -21, was the first to roll rodscontinuously. A Bedson mill was introduced into the United States by the Washburn andMoen Company in 1869.




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Figure 3 - 20

Figure 3 - 21




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Rod mills progressed in productivity by using continuous mill layouts and by rolling tworods at the same time. A mill patented by F. H. Daniels in 1884 is shown in figure 3 - 22.It rolled two 4 square billets in the continuous roughing mill feeding two rod finishingmills to reels.

Figure 3 - 22




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The use of roller element bearings throughout the mill allowed the finishing speeds toincrease to 3500 ft/min on the mill shown in figure 3 -23.

Figure 3 - 23

Structural mills have also developed through an evolution of layouts. The mill in figure 3- 24 has a cross country arrangement with large distances between stands. The higherspeed finishing mills creates enough space to have several bars rolling at the same timegiving good tonnage rates.

Figure 3 - 25 shows the layout of a modern structural mill with two reversing breakdownstands and a continuous finishing train with universal stands for rolling beams andchannels.

The roll housing arrangements in figure 3 - 26 are the four roll universal stands for beamrolling and two roll stands for edging the flanges. These stand types are utilised in themodern beam mill in figure 3 - 25.




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Figure 3 -24

Figure 3 - 25




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Figure 3 - 26

Mill Equipment

Reheat Furnaces

A long product rolling mill typically uses a gas-fired reheat furnace like that shown in fig-ure 3 - 27. The furnace shown has three heating zones with a bottom fired zone as well.This furnace is a pusher type where the billets enter the furnace lined up side by sidebeing pushed by the next billet entering. After reaching rolling temperature in the lastzone, the last billet in the furnace is pushed out and slide down onto the mill entry rollertable.




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Figure 3 - 27

Another method for moving the billets through the reheat furnace is by lifting and movingthe billets, thereby leaving space between the billets. This is necessary for certaingrades such as high carbon steel grades that have the tendency to stick together. TOfacilitate this motion there is a set of stationary skids and a set of movable skids that liftand move the billets forward, then retract, to repeat the cycle.

There is also a rotary hearth type of furnace that is circular in design, holding the billets

for heating as they rotate. This type of furnace is typically used for very special gradesof steels, other metals, and pipe.

The above described gas-fired furnaces heat the rolling stock by a series of gas burnersthat heat the refractories. The refractories then radiate heat into the billets. Once thistype of reheat furnace is at working temperature, any changes to its steady state opera-tion requires some time to raise or lower the temperature in any heating zone. It acts asa heating flywheel. The billet is heated in stages, gradually brought up to rolling tem-perature in the different heating zones. It is then held at rolling temperature for theshortest time possible to use the least amount of fuel and to limit the scale loss.

There are other methods for heating rolling stock. Induction heating is the process ofheating an electrically conducting material, the rolling stock in this case, by electromag-netic induction, where eddy currents are generated within the metal and resistanceleads to Joule heating of the metal. An induction heater Has the advantage of heating ondemand. That is, single cold billet can be heated to rolling temperature as needed.There is no large mass of material that needs to be maintained at an elevated tempera-ture.




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Pinch Rolls

A pair of rolls that close on the billet and drive it into the first stand can help overcomebite angle limitations. The force exerted on the billet can also help break up the scale tohelp remove it before the surface of the bar is damaged by the hard shell. An example of

a pinch roll is shown in figure 3 -28.

Figure 3 - 28

Mill Stands

Once the rolling stock has been heated to rolling temperature, it will pass between tworotating rolls held in a mill stand. An example of a two high stand is shown below in fig-ure 3 - 29. This is an open-top style of stand. The cap is removed to allow removal ofthe rolls with chocks and bearings attached. The stand shown in figure 3 - 30 is an open




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widow stand where the rolls and chocks slide into an opening in the housing. The standshown in elevation view in figure 3 - 31 is a housing-less stand that splits with both sidesof the stand with chocks and bearings is removed from the roll necks in a stand buildingdevice.

Figure 3 - 29

The guiding equipment is typically mounted on a restbar that is attached with bolts in aT-slot either on the outside or inside face of each stand half. The roll gap adjustmentmechanism can be seen in the cut view on the left of the figure. Both rolls move verti-cally and only the top roll move horizontally.

An example of a vertical two high stand is shown below in figure 3 - 32. This is anclosed-top style of stand. The rolls with chocks and bearings attached are removedthrough the open window in the housing. The guiding equipment is also typically

mounted on a restbar that is attached with bolts in a T-slot either on the outside or insideface of each stand half. The roll gap adjustment mechanism can be seen in the cut viewon the left of the figure. Both rolls move horizontally. A picture of closed top convertiblestand in the vertical position is shown in figure 3 - 34.

An elevation view of a housing-less stand in the vertical position is shown in figure 3 -33.




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Figure 3 - 30

Figure 3 - 31




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Figure 3 - 32

Figure 3 - 33




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Figure 3 - 34

Figure 3 - 35




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An example of a three high stand (with lifting tables) is shown above in figure 3 - 34.Three high mills are used to reverse the direction of the bar without reversing the direc-tion of motor and gear drive rotation. One gap (between the bottom and middle rolls)take the bar in one direction. The other gap (between the top and middle rolls) take thebar in the other direction. To move the bar from the elevation of the lower gap to the up-

per gap a tilt table (as shown above) can be used. Other methods of moving the bar arealso used, such as, a lift table that moves the whole table up and down. An example of auniversal stand is shown above in figure 3 - 35. This stand is basically a horizontal standwith the addition of vertical rolls held in a cassette on each side. The vertical rolls arenot powered. For rail products, the head side vertical roll would be shaped to form thehead radius. Channels and other products can also be rolled in a universal mill. Othernon-symmetrical products can also be rolled in a universal mill such as, J-bars andTees.

Mill Stand Bearings

One of the most important parts of the stand, no matter the type, are the mill bearings.Sized to resist to the separating force at rated rotation speeds the bearing type and styledetermine the stand"s capacity.

Figure 3 - 36

There are three basic types of neck bearings. Old mills used a fibre bearing lubricatedwith water. A radial and axial fibre bearing are shown in figure 3 - 36, and installed in a

mill stand in figure 3 - 37.

Fibre bearings wear quickly, can easily cause roll neck damage due to the difficulty ofkeeping contaminants from entering the interface between the roll and the bearing. If thelubricating water flow is interrupted the bearing and roll heat up quickly causing roll neckdamage. Fibre bearings are inexpensive and easily replaced.




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Modern stands use roller element bearing such as shown installed in figure 3 - 38 and39. These bearings are lubricated using either grease or a flow of oil propelled by pres-surized air (air - oil lubrication systems).

Figure 3 - 37

Figure 3 - 38




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Although roller bearings are more difficult to install, they are sealed against contamina-tion and have a much lower rolling friction than fibre bearings. The bearings shownabove are mounted at an angle allowing each row of rollers to absorb some force of anyside thrust of the rolls. Most newer mills stands are now constructed with straight necks,flat roller bearings, and a separate thrust bearing to handle any side loading.

Figure 3 - 39

An oil film bearing can also be used. As shown in figure 3 - 40, the neck rotates in a thinfilm of oil, at high pressure, providing a very low friction interface. This is common inhigh speed applications such as rod rolling blocks. the geometry of the interface sur-faces, oil pressure, and oil quality are critical to making this type of bearing work reliably.A dedicated high pressure oil supply system with very fine filters is required.




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Figure 3 - 40

As the bar progresses through a continuous mill where the bar is rolled in multiplestands simultaneously, the stand speed must be controlled to ensure tension free rolling.The main methods of controlling the tension in the mill is with a tension free rollingspeed control system and manually monitoring the current drawn by each motor as thebar proceeds through the mill. A looper table can be used to aid in keeping the bar ten-sion free between stands. The looper table shown in figure 3 - 41 uses a persuader rollfrom under the bar to create a loop. The loop height is monitored by a hot metal detectorin the slots on the backside of the table. This is a vertical looper, horizontal loopers alsoexist where the loop forms on a flat table off to the side of the rolling line.




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Figure 3 - 41Post Rolling Equipment

Once the product has been rolled, it must be cut to a length that fits onto a cooling bed,cooled, possibly straightened, and cold sheared.

Shears

Several types of shears can be employed by a mill to cut the product as it rolls, as it ex-its the finishing stand, and cold shearing before stacking or bundling. Depending on theproduct shape and material grade, shears may be used to cut the front of the bar as itproceeds through the mill. These are typically flying shears of the type shown in figure 3- 42. The blades of this shear move parallel to the bar during the cut. A diagram of theshear blade movement and is shown in figure 3 - 43. Certain rolling practices, such asmulti-strand rolling of rebar, require a shear in the mill to provide a clean front end of thebar to avoid cobbles at the slitting stand. Certain grades, such as leaded steels, requirefront end trimming to prevent cracks at the front end from splitting open and the barwrapping the rolls.




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Figure 3 - 42

Figure 3 - 43




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A drum type shear, shown in figure 3 - 44, can be used for product with a simple shapesuch as flats or rounds. The blades are mounted on a rotating cylinder (or drum) and areset at a lead speed to minimize the kinking of the bar.

Figure 3 - 44

After the shearing to length and cooling to ambient temperature on the cooling bed, thebar needs to be cut to selling lengths. For most products this occurs at a cold shear afterthe cooling bed. Smaller products exit the cooling bed in multiples so that a row of prod-uct is cut at the cold shear. A cold shear set up for cutting multiple angles is shown infigure 3 - 45.

The size of cross section cut by a shear depends on its rating of the maximum cutting

force. The stroke of the blade must be large enough for the largest height product. Theproduct hold down must also clear this height, and then move into place to hold theproduct steady. For structural sections, shaped shear blades are used as well asshaped entry rolls or guide plates to align the product to the shape of the blades.

Referring to figure 3 - 46, the cross section of the blades is usually based on the ratio of:

h/ b= 2.5 to 3




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Figure 3 - 45

Figure 3 - 46




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To select the size of shear required, the maximum cutting force is determined by theformula:

P= k1 k2#bF / 1000$0.8#bF / 1000 tons

where: k1 = a coefficient accounting for the increase in the cutting force due to the dulling

of the blades and an increase in the clearance between them; k2 = ratio of the shear strength to the tensile strength ;

#b= tensile strength of the material at the cutting temperature, kgforce/mm2; F = cross sectional area of the bars being cut.Based on empirical data: k1 = 1.3 and k2 = 0.6.

Some products such as beams require cold sawing as their shape does not cut cleanlyin a cold shear. An arrangement of a single or double abrasive saws can be installed tocut one or both ends of the selling length at one time. These saws are usually enclosed

to catch the kerf chips resulting from cutting.

Cooling Bed

Figure 3 - 47




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Several types of cooling beds are used for long products. A typical one is shown in fig-ure 3 - 47. The bar enters the cooling bed at (9), slides onto the first notch on the rakes.The first 4 to 6 notches will provide continuous support for the bar on a casting called agrid casting. Long plates with notches set at some distance apart support the after itmoves beyond the grid castings. The bar moves across the cooling bed (in this figure,

from right to left) by the movement of alternative plates moving in a cycle of lift, themove, and retract, by the action of eccentric cams. Repeating this cycle to move thebars as they are delivered from the mill. The length of the cooling bed is determined bythe maximum runout bar length, optimized by the selling lengths to minimize croplosses. The width of a cooling bed is determined on the basis of mill productivity (tons/hour) and the time required for cooling.

A cooling bed filled with large angles is shown in figure 3 - 48.

Figure 3 - 48

After cooling structural sections are typically straightened in a roller straightener (figure3 - 49, below) and cut to selling length by a cold shear (as shown above in figure 3 - 45)and either stacked or bundled.

Stacking of a large angle section is shown in figures 3 - 50, 51 and 52. The angle isstacked in a two down, one up arrangement. After the bundle is stacked it is banded andmoved to the shipping bay.




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Figure 3 - 49

Figure 3 - 50




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Figure 3 - 51


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