Gautam Ganguli, B.Tech(Hons).IIT Kharagpur
Ex-President,BRG Group of Industries
1.0 Introduction
Rolling is the process by which the shape of a piece of metal is changed by passing it between a pair of rotating rolls. When the process takes place at high temperatures(i.e. above the recrystallization temperature), it is called hot rolling. At temperatures below the recrystallization temperature ( typically at room temperature), the process is called cold rolling.
Hot rolling has two types of products: sections and flat products. Cold rolling is restricted to flat products only.
The rationale for hot rolling is the fact that metals are have a far lower yield strength at elevated temperatures; the deformation processes therefore require far less energy, and in many cases are only practicable at these temperatures.
Cold rolling is exclusively used to produce thin gauges of flat products of superior surface quality . The surface texture being free from the influence of mill scale, and being burnished by deformation by polished steel rolls is far superior to that producible by hot rolling.Very thin gauges(which can go down to down to 100 microns for steel and 10 microns for aluminium) are in fact only producible by cold rolling on on precisely engineered cold rolling mill equipment, and not by hot rolling.
As far as steel is concerned, the bulk of the production of steel for cold rolling is in the low and very low carbon steel ( carbon content of 0.05% C or less) segment. The steels are divided into categories such as commercial , deep drawing(DD), extra-deep drawing(EDD), interstitial free(IF) and so on. Guaranteed properties of formability, drawability, paintability, surface texture , flatness ,uniformity of gauge etc. are to be provided to the customer.
Since the ductility of cold rolled strip is impaired by the production process, annealing ( under a controlled furnace atmosphere to prevent scaling and thus preserve the surface) is very frequently carried out to improve the physical properties for downstream use.
2.0 Rolling loads.
Metal under deformation generates forces on the rolls; the housing retaining the rolls resist them.
To understand these forces we use the concept of the neutral section, and the friction hill.
Let \(v_0\) be the velocity of the strip entering the rolls, and \(v_1\) be the velocity exiting it.Obviously \(v_1\)>\(v_0\).
Experimentally it has been established that the horizontal component of the roll velocity \(v_n\)is such that \(v_1\)>\(v_n\)>\(v_0\). The section of the strip where the strip velocity is equal to the horizontal component of the roll velocity is called the neutral section.
From the strip entry to the neutral section the strip is moving slower than the roll surface; hence frictional forces are dragging the strip towards the neutral section.This is called the region of 'backward slip". From the neutral section to the strip exit the strip is moving faster than the roll surface;the frictional forces are also dragging the strip towards the neutral section.This is called the region of "forward slip."
Experimentally, the curve of the specific rolling load, which is analogous to yield stress -except that the resistance to deformation is quite different from that which occurs in a tensile test, hence the values are different - in Pa vs. the distance from the strip exit is of the form shown in figure 1. The shape of the curve, which is the result of friction, is called the friction hill.
The rolling load will be in the form of
Load=Specific load X contact area X tension factor, which translates into
\(P=P_m\sqrt{R\Delta H} n{'''}\)
where P=Rolling Load in N/m width of strip.
\(P_m\)=Specific rolling load, in Pa
\(R\Delta H\)=Area along the arc of contact/ m width of strip, sq m
\(n{'''} \) = tension factor
For practical use this is inadequate; the expression for contact area has to consider roll flattening which is important for thin strip, and the determination of Pm, which is influenced by the coefficient of friction, strip yield strengths and work hardening, and the strip front and back tension is not simple. Qualitatively we understand that rolling load goes down when:
a) the coefficient of friction is reduced.
b) the work roll diameter is reduced.
c)the front and back tensions are increased.
d) the yield strengths of the metal, both before as well as after the reduction, are reduced.
The accurate prediction of rolling loads,torques,power etc. Is a complex task, and has been studied over decades by several authorities,each with different models,formulae and sets of experimental results, and a deviation of around 15% among them.Engineering companies have each selected their particular model, and are constantly validating and tweaking their formulae based on statistical mill data to present results that satisfy the users.
Figure 1
I am presenting below some formulae to calculate the specific load based on those developed by A.S.Tselikov, and published in his book Stress and Strain in Metal Rolling( Mir Publishers) :
1.\(\delta=\mu.\frac{2l}{\Delta H}\)
where
\(\mu\)= coefficient of friction
l=horizontal projection of the length of the arc of contact.
\(\Delta H=h_0-h\)
\(h_0\)= strip thickness at entry
\(h\)= strip thickness at exit.
2. \(n^{'''}=1-(\tau_0+\tau_1)/2P_m\)
\(\tau_0\)=entry strip tension/ entry strip cross-sectional area
\(\tau_1\)=exit strip tension/ exit strip cross-sectional area
\(P_m\)=Specific rolling load in Pa.
3.\(h_n/h_1=( 1+\sqrt{1+(\delta^2-1)(h_0/h_1)^{\delta}}/(\delta+1))^{1/\delta}\)
\(h_n\)= strip thickness at neutral point.
4.\(k=1.15(\sigma_0+\sigma_1)/2\)
\(\sigma_0\)=yield strength of strip at entry.
\(\sigma_1\)=yield strength of strip at exit
N.B. The two yield strengths are different owing to the different amounts of work hardening.
5.\( P_m=2k(2h_1/(\Delta h (\delta-1))(h_n/h_1)((h_0/h_1)^{\delta}-1). n^{'''} \)
There is an additional complexity regarding the calculation of the contact area; that has to do with roll flattening.
Figure 2
The rolling of thin gauges is characterized by relatively low contact areas and very high specific loads. Under these conditions, work rolls flatten significantly in addition to deflecting parabolically( see para 3.0, crown and shape) .In figure 2, the contact area follows the arc AO rather than BO, increasing the contact area.
Along the length of the roll, therefore, there is the working length which is in contact with the strip and experiences roll flattening, and the non-working length not in contact with the strip where the roll section is circular once more.Between the two, at each end, there is a zone of transition which starts towards the ends of the working length and continues beyond.The roll bite changes from its parabolic shape; this phenomenon is called the edge drop, and is troublesome particularly in the rolling of thin gauge strip.
The following formula is applicable:\(l=\sqrt{R\Delta h+x^2}+x\)
where l is the contact area /mm width of strip, sq mm.
and
\(x=P_m/9500\) mm for steel work rolls.
\(x=P_m/28700\) mm for tungsten carbide work rolls.
This is the formula to calculate the power consumed by the mill motor :
\(N={P_m}^{'}\nu_1 Q_1\ln \lambda\)
Where \({P_m}^{'}=P_m+(\tau_0+\tau_1)/2\)
\(\nu_1\)=exit velocity of strip
\(Q_1\)= cross sectional area of strip as it emerges from the rolls
\(\lambda\)= the reduction ratio
Rolling torque is given by
\(M_{rol}=({P_m}^{'}\ln \lambda+\tau_0-\tau_1)Q_1D/2(1+s)\)
where
D= work roll diameter
s= forward slip.
To reiterate: a number of other researchers have also given their formulae for the calculation of these parameters; some of which are extensively used.
3.0 Crown and shape.
The art of cold rolling lies in keeping the rolled strip flat during and after production.
Figure 3
Figure 3 shows the direction of forces on the work rolls during rolling.There is an upward force on the top roll( and an equal and opposite downward force on the bottom one) exerted by the metal being deformed. Counterbalancing and opposite forces are exerted by the rolling mill housings through the journals at the roll necks.Each roll can therefore be considered to be a simply supported beam which deflects along the arc of a parabola; the roll gap is no longer rectangular but parabolic in shape.
Figure 4
In case the strip shape follows the contours of the roll shape the rolling process is stable.Otherwise, there is a horizontal component of the forces exerted by the work rolls on the strip which tends to make the strip crumple laterally, and rolling is disrupted.
These considerations have strongly influenced both the design and operational practice in cold rolling mills.
3.1 Influence on cold rolling mill design.
One way to reduce the parabolic deflection of the work rolls is to reduce the rolling load, which means that the work rolls should be as small in diameter as possible.This led to the concept of 4-hi mills, where the work rolls are supported by backup rolls of much larger diameter which lends rigidity without increasing the rolling load.
Figure 5
In frequent use are single stand reversing mills with tension reels at each end like the mill shown in figure 5, and tandem mills in which the strip passes from start to finish only once,like the mills shown in figure 6:
Figure 6
A related development is of the Sendzimir ( or Z) Mills, where not only are work roll diameters considerably reduced, but the backup consists of tiers of support rolls, ultimately ending up in large roller bearings assembled on shafts and supported at intermediate points along the depth of an integrated housing.
Figure 7
The mechanical complexity of the Z Mills , and the difficulty of producing absolutely flat products have stood in the way of the general popularity of these mills.However for specialty products like austenitic stainless steels (which work-harden rapidly), these are the mills of choice.
Figure 8
The development of the 6-hi Mill has been to counter edge drop and is relatively recent.A pair of intermediate rolls between the work and backup rolls, each tapered at one end, laterally movable inwards or outwards through hydraulic cylinders , allows the possibility of producing very flat coils of very thin gauges.They have virtually taken over as the mills of choice for the cold rolling of steel.
Figure 9
Figure 8 describes the design principle of 6-hi mills, and figure 9 shows the picture of an installed single stand reversing 6-hi mill.
3.2 Influence on operating practice.
We introduce the term crown to mean the difference between roll diameter at the center and the roll diameter at the roll ends.This is usually a fraction of a millimeter.
3.2.1 External crown. By grinding the work rolls to have a slight positive parabolic crown to offset the negative crown caused by deflection.All roll grinders are equipped with mechanisms to generate a positive( usually) and negative(in some special cases) crown.
3.2.2 Thermal crown. Heat of rolling is generated at the work roll center, and the heat is mostly dissipated through the journals.This is despite a flood of roll coolant which constantly circulates during rolling.There is a temperature difference between the roll center and the roll ends which translates to a positive crown.
3.2.2 Roll wear. The work roll wears during rolling; the center wears more than the ends.This translates to a gradually increasing negative crown. This leads to a necessity of roll changes after a limited amount of production.
3.2.3 Drafting practice.By altering the percentage reduction in a given pass, the rolling load can be increased( leading to an increased negative crown) or reduced( the negative crown is reduced) .
3.2.4 Differential cooling.For reasons that will be discussed later, cold rolling is almost always carried out with roll coolants flooding the rolls. By dividing the cooling sprays into individually controllable sections, and programming the flows to the sprays to be computer controlled, it is possible to control the shape of the roll bite quite closely.By taking the input signals from special shape measuring rolls installed at the exit of the work rolls an automatic shape controlling device is created .This is a fine control, installed in high-end mills,as against the ones described earlier which are coarse controls, and are used in all mills.
3.2.5 Modern mills are equipped with a "work roll bending" feature.Small bore hydraulic cylinders built into the work roll chocks generate an additional controlled force tending to bend the work rolls at the roll necks.
Although the bending force is usually around one-tenth of the rolling load, this has a significant effect on the edge drop, and is an important operating control to maintain strip shape.
Mills have been designed to operate, and are operating at maximum speeds of 1600 meters per minute or higher(Mach 5). The tension reels at either end, and the mill motor or motors are all variable speed motors ( either DC or AC), with an integrated DC( or AC) Drive system that couples accelerations, velocities and decelerations to work seamlessly together.
The concept of base mill speed is important.DC motors are designed with the concept of a maximum base speed, where speeds from zero to base speeds are increased by increasing the armature current, and from base to maximum speeds by weakening the field current. The speed-power and speed-torque characteristics are different in the two ranges. Variable speed AC motors have similar characteristics. These are conceptually shown in Figure 10 below:
Figure 10
The ratio between maximum motor speed and base motor speed depends on motor design.
As can be surmised, this ratio is an important consideration for tension reel motors, which must operate with constant motor power at the desired rolling speed to ensure constant tension.The diameter of the expanded tension reel drum is frequently around 500 mm; while the maximum coil dia demanded is 2000 mm.This calls for a ratio of 4:1.
In practice, motors with ratios above 3.5:1 are scarce.This places a constraint on the maximum coil diameter; alternatively, some wraps must be rolled with a lower tension than the rest of the coil.The practice required must be carefully selected at the mill design stage, as it cannot be reset during operations.
5.0 Strip thickness measurement and Automatic Gauge Control.
X-ray Gauges or Isotope Gauges are used to measure the thickness of the strip .The principle used is that the strip thickness is proportional to the fraction of the incident radiation ( X-rays for X-ray gauges, and gamma rays for isotope gauges) absorbed ; the remaining radiation being measured by the detector.
X-ray gauges were the first to be used.Mechanical unreliability and the fact that the radio-active sources generate gamma rays spontaneously caused isotope gauges to be preferred for a long time.Americium 241 is the isotope commonly used for steel.
Recent improvements in technology, and the safety and regulatory issues with isotope gauges have brought X-ray gauges back into popular use.
Signals from the thickness gauges are linked to hydraulic cylinders in the mill which allow the mill draft to be altered continuously on the fly based on the gauge signals.
Three modes of feedback are used: feedback, feed-forward andf constant load.
6.0 Preparation of hot rolled coils for cold rolling.
The surface of the coil must be completely free of scale. In addition, for metals that have a large resistance to deformation, the hot rolled coils must be annealed as well.
For mild steel, hot rolled coils are pickled in a Pickling Line, with hydrochloric acid being the most common pickling medium in use today. Stainless steels and high silicon electrical steels are processed in Annealing and Pickling Lines where the steel is annealed , and then pickled in a single Processing Line.
Figure 11
Picture of a medium production push-pull Pickling Line for steel.
7.0 Measurement and specification of surface roughness
Figure 12
Surface roughness is measured in microns and is given by the formula: \(R_a=1/L\times\int_0^L h_i dx \)
where L is the sampling length(usually 0.8 mm)
The reference level is at the maximum peak
The datum level is at the deepest valley
The mean is chosen so that the arithmetic sum of areas of the peaks and valleys =0
\(h_i\) is the positive value of the distance from the mean to the point of measurement.
An instrument is used to read out the \(R_a\) value from a spot on the selected surface in microns ,\(\mu\).
8.0 Roll coolants
Given the considerable amount of slip tht takes place during rolling and the demands of a high and consistent surface quality, the maintenance of a consistent and low coefficient of friction between the strip and the work rolls is a factor of considerable importance. The removal of the heat and the impurities generated by cold working and friction is another important consideration.The impurities are usually iron( metallic particles as well as oxides), and oxidation products of the lubricant used.This is achieved through the application of a roll coolant (usually an emulsion) specially engineered for the purpose. In addition to stabilizing the cold rolling process, roll coolants produce uniform surfaces of low surface roughness and high brightness.
Oil-in-water emulsions are used.Good emulsions plate out much of the oil to form a film of neat oil on the roll and strip surfaces at the roll bite.Low emulsion stability, large emulsion droplet size and a high SAP value are desirable properties of a good emulsion.
Note:
The saponification number or SAP value is defined as the number of milligrams of potassium hydroxide required to neutralize the fatty acids resulting from the complete hydrolysis of 1 gram of the sample.
The control over the lubricant required( maintenance of flow,temperature, oil content in the emulsion, scum removal, sludge removal,iron build-up) requires the installation of a Roll Coolant System.
Variation of the coefficient of friction with mill speed
Figure 13
The coefficient of friction for a particular roll coolant is not constant, but varies with mill speed. The coefficient is highest at very low speeds, gradually falls with an increase in mill speed till around 7.5 mps and rises slowly at higher speeds.
9.0The metallurgy of work rolls
The high specific pressures, high roll wear and the demand for a high quality surface have driven development of work rolls for cold rolling. Hardened forged steel rolls with a Chromium content of 3-5 %, and a carbon content of 0.7 to 1.0% are the materials commonly used for the work rolls, with an as-delivered hardness of 92-95 Shore C.
For backup rolls and intermediate rolls also, forged steel rolls are used.The hardnesses are lower for intermediate rolls, and lower still for backup rolls.
Sendzimir Mills use work rolls of much smaller diameter (70-90 mm for mills rolling conventional steel strip, against the 350-400 mm in common use for 4-hi and 6-hi mills. Specialty sendzimir mills can have work roll diameters as low as 10 mm). They allow tungsten carbide to be used as an alternative material of construction.These rolls are much more expensive, and show a greater tendency to spall. However, wear as well as roll flattening are improved.
10.0 Roll Grinding.
The set of work rolls for a mill need to be changed after every 150-300 tins of finished steel rolling.(These figures are for single stand reversing mills.The figures for tandem mills will be higher, and different). The used rolls are sent for grinding to renew their surfaces.
The roll grinder is an important piece of capital equipment in a Cold Rolling Mill, and it contributes significantly to the success of operations.The cost of work rolls/ ton of finish rolled metal is a significant cost element.
Only the top 10 mm or so of a work roll can be used before its hardness falls significantly enough to render it unfit for use. The amount of metal removed at each grinding operation needs therefore to be closely controlled.
Surface fatigue that occurs during rolling causes sub-surface micro-cracks that tend to lengthen if not removed and develop into spalls, which remove a substantial( sometimes the entire) amount of residual life from a roll.Consistent grinding practice(which translates into the removal of visually evident defects, and a minimum amount if no defect is visually evident ) is of cardinal importance. Modern mills have NDT equipment mounted on the Roll Grinder so that the roll can be checked before it is unloaded from the machine. Of the different types of non-destructive tests available, the Eddy Current Test is preferred for its ability to detect cracks just beneath the surface.
Other than an ability top generate a preset parabolic crown, roll grinders must deliver rolls which are capable of imprinting a bright surface on the strip free of optical defects. Roll grinders are therefore installed on foundations which absorb and isolate them from any ground vibrations from moving machinery, overhead cranes,passing vehicles etc.Neglect in this area, improper selection of grinding wheels, improper feeds and depths of cut, lead to the defect on the Strip known as "Chatter marks" ,visible on the strip as a series of alternately dark and bright bands when viewed at a certain angle.
11.0 Post production processes
11.1Annealing.
As mentioned in para 1.0, most cold rolled steel strip is annealed before use.
The high end component of the production is used for deep drawing and forming. Process annealing is done below the phase transformation temperature , with temperatures between\(\;550\; \text{and}\; 600^{\circ}C\)
Figure 14
is the preferred treatment, with box annealing of coils under a protective atmosphere being the preferred process ( producing what are known as CRCA or Cold Rolled Close Annealed coils). The protective atmosphere of for many installations is 96% nitrogen, 4% hydrogen and a dew point of \(-40^{\circ}C\)This is a non- explosive mixture affording a commercial quality surface brightness.The picture of a typical installation is shown in figure 14.
Modern installations use pure hydrogen with a dew point of\( -70^{\circ}C\), as the heat transfer rate is increased from burners to coils, thereby markedly increasing the productivity of the installation, in addition to providing a brighter surface on the coils.This is despite the more expensive protective atmosphere generators as well as the more expensive furnace equipment which can deal with the potentially explosive atmosphere.
11.2 Skin Passing
A cold reduction of 0.5 to 2.0% is given to the annealed coils to prevent the appearnce of "orange peeling" or " Luder Lines" in the forming process.
Figure 15
The diagram of a typical stress-strain curve as obtained from the tensile test of a specimen of low carbon cold rolled steel is shown at figure 15.
For the annealed sample the curve follows the trajectory of a straight line OA, a kinky line AB followed by a smooth curve BCD. The skin passed sample follows a straught line YB followed by the smooth curve BCD.The kinky line AB has been removed- and it is the kinky AB portion that is responsible for the Orange Peel and Luder Lines.Interstitial C and N atoms are responsible ; they pin the dislocations, which are unpinned on deformation; causing the discontinuous behavior of the yield point in the section AB.
Skin Passing is done on dedicated mills which are either 2-hi or 4-hi, the larger work roll diameters of 2-hi mills being favored by some as they promote better flatness. Hardened forged steel work rolls with a Shore C hardness of around 100 are selected for these mills.In addition to the improvement of forming qualities, the process adds to the luster on the coil surface.
12.0 Finishing
Levelling,trimming,slitting and cutting to length are the standard processes applied before the material is packed for the customer.
Strip needs to be sufficiently flat for use.There are different standards which are applied,but all of them involve the cutting of a fixed length of sample (typically 2 meters)from the coil , placing it on a surface plate and measuring the maximum deviation in the sample from the datum level with appropriate measuring instruments.
If the deviation is and the sample length is then the function or some expression involving it is used as a measure of flatness.
Figure 16
Shown above is the basic principle of roller levelling in a strip. The actual levelling machine has seen recent developments designed to get the maximum possible flatness in annealed thin strip. The addition of tension in the line intensifies levelling action; this has created the new generation of tension levellers. An example is shown below:
Slitters separate wide strip into narrower coils using multiple pairs of rotary cutters mounted on two arbors precisely set up with separators.An example of a typical installation is shown below.
Figure 18
A trimmer uses two pairs of cutters only mounted on stub shafts, and the distance between each pair is freely adjustable between cuts. It is designed to remove any damaged edges; this is usually an intermediate step between processes rather than a finishing one.
A cut-to-length line cuts sheets into accurate lengths for stacking in packets. Shears are of the guillotine type; the cutting sequence integrated with the line speed to allow sheets to be cut accurately on the fly. The diagram of a typical installation is shown below:
Figure 19
13 Quality and Use
The quality of cold rolled steel sheet lies mainly in its surface, uniformity( of gauge, flatness and physical properties) , and formability . Surface is important because sheets are frequently used for panels and other enclosing surfaces of equipment and appliances which need to be visually appealing; uniformity because it is frequently the raw material for high speed production lines which need feed of a uniform quality, and formability wherever the application so demands.
Cold rolled sheet steel has demand for use in the many segments of the manufacturing industry, particularly those which are consumer-driven.
A major use is in the manufacture of galvanized , galvalume and color coated steel, which has not been covered in this article.Galvanized corrugated sheet for roofing is a major raw material for the informal housing sector.
Auto body sheet is the archetypal high end application, which imposes stringent quality parameters on all these, and other parameters. There are many other uses which call for less stringent quality.
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