Hot & Cold Rolling of Sheets and Plates | Metals | Metallurgy

In this article we will discuss about the setup for hot and cold rolling of sheets and plates.

Setup for Hot Rolling of Sheets and Plates:

The overall view of the set up for hot rolling of sheets, strips and plates is shown in Fig. 8.19. The starting point is rolling of slabs out of cast ingot in a slab rolling mill. Slab rolling mill is also a two high reversing mill having roll diameter of 1150 mm or more and barrel length of 3000 mm. The ingot is mainly rolled flat to reduce its thickness. The edging passes are given to even out the non-uniformity in width direction.

In a two high slab mill the upper roll is to be lifted more than 1000 mm to give edging pass. Such a large lift requires a larger rolling stand as well as higher lifting power so that it can be done faster but still it is a time consuming process.

Therefore, now a days, universal slab rolling mills which have both vertical and horizontal rolls are preferred. In these mills the horizontal rolls have diameter of 1100 to 1150 mm and barrel length of 2100 mm. The vertical rolls have diameter of 900 mm. The ingot is rolled simultaneously through both the pairs of rolls.

The ingot in slab rolling is much bigger than that in bloom rolling. It may weigh 7 T to 24 T while in bloom rolling the ingot weight is 5 to 10 T. In modern plants minimum slab thickness is limited to 250 mm because reduction in thickness of slab reduces the throughput of the mill. The width of slab varies from 700 to 1600 mm.

This also means that the mills following the slab mill must have capacity to roll the entire output of the slab mill. Also the soaking pit furnaces are designed to supply the quantity of heated metal that matches the capacity of the mill. A modern slab rolling mill may roll as much as 3 million tons of steel a year. The rolled ingot is cut into suitable slab lengths by a shearing machine. The slabs are individually inspected and any surface defects observed are removed.

The defective slabs are cooled down, defects are removed and slabs are reheated again. This process of defect removal wastes lot of heat energy. Now a days, the slabs are scrapped by oxy-acetylene flames which remove about 3 mm of metal from slab surfaces in the hot state, so there is no loss of heat and the slab can be rolled without re­heating. The process removes small surface cracks as well.

Hot Sheet Rolling Mill:

The hot sheet rolling mill is subdivided into two sets of rolling stands (i) roughing hot strip rolling stands which roll the slab down to 25 mm thickness and (ii) finishing hot strip rolling stands which roll it to 1.5 mm to 12 mm thickness. The sheet width may vary from 700 mm to 2350 mm.

The first set of rolling stands consists of a two high scale breaker stand followed by a broad side rolling stand. A small reduction up to 10% is given in the scale breaker to loosen the scale which is then washed away by high pressure water jets.

The slab is generally 1050-1100 mm wide. Therefore, slab is first rolled broadside to increase width to match the width of sheet desired. This is done by rotating it by 90 degrees on a turntable provided for the purpose. After rolling on broad side till it attains the required width it is again rotated by 90 degrees and rolled through the following stands to the required thickness.

Generally in the first group of hot sheet rolling stands, there are 3 to 4 horizontal 4-high stands and two stands with vertical rolls to square up the edges of thick sheet being rolled. After rolling in the first set of stands, there is a long feed table.

The entire length of sheet can be accommodated on it. It is necessary to regulate the temperature of strip before final rolling so that the finished sheet gets the desired micro-structure and grain size. A fly-shear cuts the ends of sheet.

In the finishing hot sheet rolling mill the first rolling stand is again a two high scale breaker followed by five 4-high stands placed in tandem. The stands are individually speed controlled. The rolling speed is 6 to 14 meter per second and the stands have automatic control of reductions to get the desired gauge. The layout is illustrated in Fig. 8.19.

Hot Plate Rolling Mill:

Plates are 6mm or higher in thickness. Plates are rolled in a wide variety of widths. Very wide plates up to 5m width, are generally used in ship building. The layout of plate rolling mill is shown in Fig. 8.19(b). Besides the furnaces the equipment consists of a two high scale breaker and two 4-high reversing stands, edge cutting circular shears and end shears.

The cooling beds for plates are of special design made of pulleys on which the plate rests so that there is just a line contact. Such a cooling bed promotes uniform cooling on both sides of plate. For plate rolling, hot slabs are transferred from slab rolling mill and reheated to the required temperature. A small reduction is given in the two high scale breaker to loosen the scale which is then removed by high pressure water jets.

First the broadening passes are given in the first 4-high stand till its width is equal to the desired width. It is then rotated by 90 degrees on a turntable to roll it lengthwise. After a few passes in the first 4-high stand the plate is shifted to the finishing stand. After finish rolling the edges and ends are cut and plate is cooled on the cooling beds.

Setup for Cold Rolling of Sheets and Plates:

Hot rolled sheet has oxide layer on its surface, which must be removed before cold rolling otherwise the oxide film will get rolled into the sheet surface and spoil the surface quality. The oxide layer is removed by chemical treatment with dilute sulfuric acid and then by rinsing the sheet twice or thrice in water tanks. The process is carried out continuously.

The sheet is brought from hot rolling mill in the form of coils. The equipment for acid treatment consists of decoiling unit followed by end shears and butt welder to weld tail end of one coil to front end of next coil. It is followed by sheet bank and acid tanks. The strip after passing through acid is washed in rinsing tanks. The strip is then dried in drying unit. The rough edges are then sheared. The strip is then oiled and coiled. The coiled sheet is transported to cold rolling bay.

Cold sheet rolling mill generally consists of five 4-high rolling stands arranged in tandem. The sheet is rolled simultaneously through all the stands at a high speed. Most of the modern mills are computer controlled so that the desired tolerances on sheet thickness are achieved along with high production. The thickness of cold rolled sheet may vary along the length as well as along the width.

The variation of thickness along length may be due to one or more of the following reasons:

(i) The roll surface may be eccentric with respect to its bearing. This will result in periodic variation in thickness.

(ii) Due to change in speed of rolling at the start and towards the end of rolling the co­efficient of friction between rolls and the sheet may change because of hydrodynamic action at the sheet surface. This would result in lowering of co-efficient of friction and hence the rolling load, which will change the elastic deformation of rolling stand resulting a change in sheet thickness.

(iii) The hydrodynamic effect in roll neck bearing (if it is journal bearing) will change the fluid film thickness with change in speed of rolling. This will change the roll gap and hence the sheet thickness.

(iv) Change in material characteristics of sheet and variations in its incoming thickness, which might have occurred during hot rolling can cause variations in cold rolled sheet thickness. Large variations in hot rolled sheet thickness would result in defective cold rolled sheet.

(v) Wear of roll surface may create non-circularity in the work rolls.

Temper Rolling:

After cold rolling the sheet is so hard that it is not suitable for further forming operations. Therefore, it is annealed, which makes it very soft. The material in this condition is also not suitable for many applications. In order to make it stiff, it is again rolled.

This is called temper rolling. By adjusting the reductions in temper rolling, sheets of different stiffness may be produced in order to cater for different applications. Generally 6 tempers (T1 to T6) are produced. T1 being the softest. Sheets of lower tempers are used for deep drawing and those with higher tempers are used for bus body building and for manufacturing pressure cans.