Work Roll Consumption

Reduction in Work Roll Consumption of Skin Pass Mill Using six sigma-A case study A Thesis Submitted to the Ranchi University in partial fulfilment of the requirement for the award of the degree Of Master of Science Engg. In PRODUCTION MANAGEMENT By CHANDRA PRAKASH ROLL NO: 20/M/PM/2002 Under the guidance of Mr K. D. P. Singh Lecturer Department of Mechanical Engineering [pic] MECHANICAL ENGINEERING DEPARTMENT NATIONAL INSTITUTE OF TECHNOLOGY, JAMSHEDPUR September -2008 MECHANICAL ENGINEERING DEPARTMENT NATIONAL INSTITUTE OF TECHNOLOGY, JAMSHEDPUR RANCHI UNIVERSITY CANDIDATE’S DECLARATION
I here certify that the work which is being presented in this thesis entitled “Reduction in Work Roll Consumption of Skin Pass Mill using six sigma-A case study” in partial fulfilments for the award of degree of Master of Science Engg. In Production Management, & Submitted in the Department of Mechanical Engineering NIT, JAMSHEDPUR is an authentic record of my own work carried out under the Supervision of Mr K. D. P Singh Lecturer, Department of Mechanical Engineering. National Institute of Technology, Jamshedpur. The matter embodied in this thesis has not been submitted by me for the award of any other degree. Signature of candidate
This is to certify that above statement made by the candidate is correct to the best of my knowledge. Mr K. D. P Singh Lecturer Mechanical Engineering Department NIT, Jamshedpur Dr J. N. Yadav Professor & HOD Mechanical Engineering Department Signature of internal examiner Signature of external examiner ACKNOWLEDGEMENT First of all I would like to express my gratitude to Mr. C. M. Verma Head, BAF/SPM/ECL Cold Rolling Mill, Tata Steel for allowing me to be a owner of this case study matching to my area of work where I could really apply and develop my technical skills in practical aspect. I am extremely grateful to Mr K.

D. P. Singh (Lecturer NIT, JSR), Mr Vivek (Sr Manager, BAF & Six sigma Co-ordinator CRM), and Mr. Dharmendra Kumar (Manager Roll shop) for guiding this case study. They have guided me at every step, clearly explained the objectives, the problems statements, technical concepts and terminology to make my case study a success. They always extended a helping hand and spent their valuable time to explain the problems that ever obstructed my path. Working in Tata Steel has not only been a learning experience on technical aspects but there were many other things, which could only be experienced in organization of such repute.
Their work culture, discipline, employee’s devotion towards their job and company are extraordinary. Thanks a lot to all those who helped me directly or indirectly during the completion of this case study and special thanks to the associates of Skin Pass Mill and Roll Shop of cold rolling mill Tata steel. Chandra Prakash ABSTRACT This case study deals in reducing Work Roll consumption of Skin Pass Mill of cold rolling Mill. Cold Rolling is a Process by which hot rolled strip or stock is introduced between rollers and squeezed or compressed to the desired thickness.
The quality of work rolls that come into direct contact with the steel product has a direct effect on product quality and mill operation. At the time of taking up this case study, Roll cost was one of the major operational cost element for Skin pass Mill & due to increase in global Roll prices its contribution increases from 15 % of total conversion cost to 22 % of total conversion cost . In the mean time there were some additional problem associated with work roll grinding & operational practices at Skin pass mill which showing variability in the process of Roll grinding & Skin passing.
This case study deals in bringing improvement in the work roll consumption of Skin pass Mill by using Six Sigma techniques (Define-Measure-Analyse-Improve & Control). Statistical methods are used to analyse the data and pin point the vital causes impacting the work roll consumption of Skin Pass Mill. Regression analysis & trial plan conducted during finalizing optimal and feasible solution, as this case study deals in improving standard operating practices & reducing variability within the process of roll grinding & skin passing. CONTENTS PAGE NO ABSTRACT 4 CONTENTS 5
LIST OF FIGURES 8 LIST OF TABLES 10 NOMENCLATURE 11 CHAPTER 1 INTRODUCTION AND LITERATURE SURVEY 1. 1 INTRODUCTION 12 1. 2 BASIC OF COLD ROLLING 15 1. 3 SKIN PASS MILL 16 1. 3. 1 OVERVIEW OF SKIN PASS MILL 18 1. 3. 2 PURPOSE OF SKIN PASSING 19 1. 4 ROLLS & THEIR REQUIREMENTS 25 1. 5 ROLL GRINDING PROCESS 31 1. 6 ROLL TEXTURING PRACTICES 35 . 7 SIX SIGMA APPROACH 41 1. 7. 2 SIX SIGMA IMPLEMENTATION 41 1. 8 PROBLEM DEFINITION 44 1. 9 OBJECTIVE OF CASE STUDY 45 CHAPTER 2: ANALYSIS OF THE PROBLEM PAGE NO 2. 1 INTRODUCTION 46 2. 2 BRAIN STORMING 46 2. 3 PARETO ANALYSIS 47 2. 4 INDIVIDUAL AND MOVING RANGE GRAPH 50 2. 5 MOODS MEDIAN TEST 52 2. 6 CAUSE EFFECT DIAGRAM 57 2. 7 OUTCOME OF ANALYSIS 60 CHAPTER – 3 METHODOLOGY TO ACHIEVE OBJECTIVES 3. 1INTRODUCTION 61 3. 2 PREPERATION OF ACTION PLAN 62 . 3 NORMAL GRINDING TRIALS & ACTION 62 3. 4 CRACK GRINDING TRIALS & ACTIONS 63 3. 5 SKIN PANEL ROLL GRINDING 63 3. 6 OPERATOR VARIABILITY 64 3. 7 ROLL CHANGE DUE TO ROLL ROUGHNESS 64 3. 8 SCHEDULING MODIFICATION 65 3. 9 ROUGHNESS PREDICTION MODEL 66 CHAPTER – 4 RESULTS AND DISCUSSIONS PAGE NO 4. 1 INTRODUCTION 68 4. 2 BEFORE AND AFTER COMPARISSION 68 4. 3 DISCUSSION ON RESULTS 71 4. 4 RESULTS 72 4. 5 CONTROL PLAN 73 . 6 SAVINGSCALCULATION 74 CHAPTER – 5 CONCLUSION & FUTURE SCOPE 75 REFERENCES 76 List of Figures Page No 1. 1 Skin Pass Mill & Tandem Cold Mill Photograph 12 1. 2 Line layout of cold rolling mill 14 1. 3 Skin Pass Mill Tata steel 16 1. 4 Tensile test graph before skin Passing 20 1. 5 Tensile test graph after skin Passing 22 1. 6 Luder band 24 1. 7Correlation between Roughness & peak count28 1. 8 Skewness of Surface28 1. 9 Surface measured at EDT m/c Tata steel 29 2. 0 Waviness Roughness & form of surface30 2. Roll Grinding M/c at Cold Rolling Mill 32 2. 2 EDT Vs Shot Blast Roll 37 2. 3 Effect of time & temp on surface texture38 2. 4 Effect of –ve polarity on surface texture38 2. 5 Roll texturing m/c at Cold rolling mill40 2. 6 Work roll consumption trend at skin pass mill 44 2. 7 Pareto chart for SPM Work roll grinding 49 2. 8 X bar & moving range chart for normal grinding 50 2. 9 Box plot for different operator at Roll Shop 54 List of Figures Page No 3. 0 Capability histogram for normal grinding 55 3. 1 Roll tonnage trend at skin pass Mill 56 . 2 Abnormal roll change trend at skin pass mill 57 3. 3 Cause & effect diagram for roll change due to low Ra 58 3. 4 SPM Work roll consumption trend before case study 68 3. 5 SPM Work roll consumption trend before case study 69 3. 6 Capability histogram before case study 69 3. 7 Capability histogram after case study 70 3. 8 Trend of abnormal roll change at skin pass mill 71 List of Tables Page No 1. 1 Roll roughness detail of tandem & skin pass mill 26 1. 2 Roughness Accuracy detail of texturing m/c 27 1. Selection of peripheral speed 31 1. 4 Standard Stock removal specification 33 1. 5 Grit size for different Roll 34 1. 6 Action on Grinding wheel on various condition 34 1. 7 Structure of grinding based upon requirement 35 1. 8 Grinding data for skin pass mill work roll 49 1. 9 Data capturing sheet51 2. 0 Operator variability matrix52 2. 1 Regression test result53 2. 2 Abnormal Roll Change detail at Skin Pass mill56 2. 3 Prioritization matrix for roll change reasons 57 2. 4 Scheduling Example of skin pass mill59 2. Normal Grinding trials & findings62 2. 6 Crack Grinding trials & findings63 2. 7 Modified Scheduling Example 65 2. 8 list of parameter effecting roughness 66 2. 9 Results of case study 72 3. 0 Control Plan for reducing work roll consumption 73 3. 1 Saving of the case study 74 NOMENCLATURE TCM – Tandem Cold Mill SPM – Skin Pass Mill M/c, m/c – Machine EDT – Electro Discharge Texturing Ra – Average Roughness CRM – Cold Rolling Mill IMR – Individual Moving Range TDC – Technical Delivery Condition ECT – Eddy current testing CRMIS: Cold Rolling Mill Information system PLTCM: Pickling line tandem cold Mill
YPE: Yield Point Elongation SPM: Skin Pass Mill BAF: Batch Annealing Furnace ECL: Electrolytic Cleaning line RCL: Recoiling line NSC: Nippon Steel Corporation CRCA: Cold Rolled Closed Annealed CHAPTER 1 INTRODUCTION AND LITERATURE SURVEY 1. INTRODUCTION This chapter gives information about cold rolling mill, Skin Pass mill, Roll grinding, and Roll texturing process . It also describes about six sigma tool & its implementation at the end it describes objective of the case study. Cold Rolling is a Process by which hot rolled strip or stock is introduced between rollers and squeezed or compressed to the desired thickness.
Amount of strain introduce determines the properties of the finished product. Following are Purpose of Cold Rolling ? Good formability ? Superior surface finish ? Reasonable strength ? Close dimensional tolerance Fig 1. 1 Tandem Cold Mill Tata steel Skin Pass Mill Tata steel Cold Rolling Mill complex has been commissioned at TATA STEEL works in the year 2000. The total output of the Cold Rolling Mill complex consists of 0. 96 mt of cold rolled and annealed products and 0. 5 mtpa (Million Tonne Per Annum) of cold rolled and galvanized products.
Hence, the total installed capacity of this unit is approximately 1. 5 million tons per annum. The range of thickness and width of these cold rolled products are 0. 3 to 3. 2 mm and 800 to1560 mm respectively. The primary input material to the cold rolling complex is a hot rolled coil. The cold rolled products are broadly under the categories: ? Annealed coils ? Galvanized coils ? Cold rolled full hard coils. The cold rolled products from the Cold Rolling Mill complex are designed to cater to various market segments such as construction, general Eng. , automobile, white goods, packaging and others.
CRM Process flow at Tata Steel Pickling (to remove oxides and scales) Cold rolling in tandem mill to achieve desired thickness Electrolytic cleaning line to clean the surface dirt. Batch annealing furnace for internal stress relieving Skin passing to remove luder bands, develop mechanical properties, Impart desired surface finish; improve flatness, Inspection, finishing, dispatch Sub Section of Cold Rolling Mill ? Pickling Line &Tandem Cold Mill (PL-TCM) ? Batch Annealing Furnace (BAF) ? Electrolytic cleaning Line (ECL) ? Skin Pass Mill (SPM) ? Galvanising Line ? Recoiling Line (RCL) ? Coil Packaging Line (CPL) pic] Fig 1. 2 Line lay out of Cold Rolling Mill Tata steel 1. 2 Basic of Cold Rolling Mill Following are the basic Processes ? The Hot Strip Mill sends the hot rolled coils (thickness 2 to 6 mm width and 800-1560 mm) to the Cold Rolling Mill Complex for processing. ? First the hot rolled coils are passed through the Pickling section containing Hydrochloric acid, in order to clean the surface of rust & scales, making them ready for cold rolling. ? The Trimming Section where the edges of pickled hot rolled coils are trimmed( if necessary) ? The coil is then fed into the main mill, viz.
Tandem Cold Mill with five mill stands, each having three pairs of rolls in the five stands which bring down the strip thickness in a controlled manner to the desired target value of (0. 3mm to 3. 2 mm). ? This completes the process of cold rolling or rolling at ambient temperatures. ? From here the two-third of the product goes to Electrolytic cleaning line, where generally two process takes place and they are Predegressing and electrolytic cleaning with the help of NaOH, after this the sheet is washed with high pressure steam to remove the bubbles of NaOH from the surface.
Then the coil is dried in the hot drier. ? The coil comes to Batch Annealing furnace directly from Electrolytic cleaning line where they are stacked covered and heated in a closed hood in a 100% hydrogen atmosphere. This process improves the mechanical properties of the strip. ? The Skin Pass Mill takes care of the coils annealed in Batch Annealing furnace by passing them through a single stand high-speed mill with two pairs of rolls. The objective is to impart the correct surface texture and to control the mechanical properties as per customer requirement. The coils are properly oiled for rust protection and recoiled in the Recoiling Lines (RCL 1, 2 & 3) for inspecting the surface. ? The remaining one- third part of the production from PLTCM goes to the Galvanising Lines (1 & 2) where coils are again cleaned, rinsed, dried, L-annealed/heated and taken through a Molten zinc bath for a continuous uniform coating of zinc. This zinc coating helps give a sacrificial layer on the cold rolled strip for corrosion protection. The Continuous packaging line takes care of the packing requirement of the coils as per the customer specifications. 1. 3 Skin Pass Mill [pic] Fig 1. 3 Skin Pass Mill Tata steel front view Skin pass mill: Annealed coils are given a small cold reduction (typically around 1-3 %) in the skin pass mill. This operation results in the right surface roughness imparted on the strip surface as per the customer specifications. In addition, a metallurgical defects known as stretcher strains are eliminated, and also the flatness of the strip is improved.
The basic operation done in the cold rolling mill is the wet temper rolling as a cold rolling finishing which is the final process in the integrated steel production, where all materials received from the cold rolling process are processed into the final products with required properties through cleaning, heat treatment and then temper rolling. As this process is closely related to user requirement for mechanical properties, surface properties, size etc. many detailed operation standards are required (annealing surface, size change). This process is quickly adaptable to shape correction reprocessing etc. ut there are many operations which require human hands, as compared with cold rolling. As skin passing is the final process of the integrated steel making operation, the information obtained from this process must be completely fed back to the processes on the down stream side of the steel making furnace. This process is located closest to users and achieving in line quality to meet the user requirements must guaranty the quality. The feed back of information to the preceding processes to be reflected in production is very important.
The temper rolling operation falls into three types as shown below: Operation using water-soluble rolling oil Operation using oil-soluble rolling oil Operation in with no rolling oil is used (Dry rolling). Each type of rolling operation has both advantages and disadvantages. The type of rolling operation must be selected in due consideration of the ease of temper rolling, the ease of operation and rust preventive at downstream process at customers end. The surface of roll to be used for temper rolling is mat-finished by shot blasting of steel grit or Elector Discharge Texturing (EDT).
This finish is widely as it ensures good paint ability. When the working rolls are ground, the roll surface is bright- finished to about Ra (0. 05µm) by using a grinding wheel of small grain size. The surface roughness of the strip rolled by bright-finished rolls is below Ra(0. 35µm), which is suitable for prime coating Generally, the surface finish condition of strip in the temper rolling process is controlled in terms of the surface finished of work rolls only. For confirmation of this condition, the roughness and look of sheet surface after temper rolling is
Checked at regular intervals. Temper rolling oil used is mainly applied to thick products, using dull-finished work rolls. The majority of rolling oils used for this rolling are sodium nitrite-based oils. The concentration of sodium nitrite is 5 ( 10% oil-soluble temper rolling oils higher rust preventive power to meet required uses have been developed and put into practical use. 1. 3. 1 OVERVIEW OF SKIN PASS MILL Skin pass an overview ? Single stand mill ? 4- High wet skin pass non-reversible mill ? Capacity: 1mtpa ? Line speed: 900mpm ? Thk range: 0. 3-3. 2 mm ? Width range: 900-1580 mm SPM Equipment ? Main drive-3 ? Mill stand rolls-4 ? Auxiliary roll-8 ? Oiling system ? Fume exhaust system ? Hydraulic gap control system ? Elongation control ? Low pressure hydraulic system ? Quick roll changing car ? Auto tempered car ? Back up rolls polishing ? Unique Features of SPM ? Higher productivity. ? High degree of accuracy- elongation control ? Surface cleanliness- wet skin pass ? Eco friendly fume exhaust ? Automatic quick work rolls change & pass line 1. 3. 2 PURPOSE OF SKIN PASSING ? Improvement of mechanical properties of material ? Shape correction Adjustment of surface properties (roughness) ? Apply rust preventive oil (optional) ? Improvement of mechanical properties of material ? Elimination of yield point elongation ? Improvement of formability by decreasing the yield point ? Improvement of other mechanical properties The skin passing of material has to be done with optimum parameters such that the purpose of skin passing is met. Ideally the skin passing has to be done in such a way that alternate grains are strained by which we will get 50% strained surface grains and 50% strain free surface grains. Upper yield point
Stress Yield point elongation Lower yield point Strain Fig 1. 4 tensile test graphs before skin passing Yield point elongation is a well-known phenomenon in low carbon steel. After the elastic portion of the stress strain curve (a schematic engineering stress-strain curve is shown in the above figure) the load drops at upper yield point. At lower yield point this drop becomes steady, but a continuous series of fluctuation appears in the stress strain curve. This is commonly known as yield point elongation. After this stage, the curve becomes smooth again.
Reason: The reason behind this phenomenon is the alternate locking and unlocking of dislocations by the interstitial atoms (C and N) in steel matrix. C and N atoms form interstitial solid solution and these have natural affinity for locking the dislocations. The locked dislocations cannot move freely, which restricts deformation of the material. The deformation of the material is actually caused by movement and multiplication of dislocations. The deformation stops when the dislocations are not free enough to continue their movements, and further application of load in this situation causes crack generation and failure.
After cold rolling and annealing, a low carbon steel strip is supposed to undergo a forming operation. However, this forming becomes difficult if the dislocations are pinned down by the interstitial solute atoms. The annealing treatment provides ample opportunity for the dislocations to move freely and sit at the thermodynamically favourable sites, where the solute atoms pin the dislocations and kill their mobility. This is commonly known as Cottrell atmosphere. Now, if the material is subjected to a tensile load, the stress strain curve will show a serration, i. . alternate load drop and load jump, just after the yield point. Load drop indicates that the dislocations are pulled off from the solute atmosphere, coupled with generation of fresh dislocations under the external force, and load jump indicates that the momentarily free dislocations are again encountering with the solute atoms. This actually constitutes the stage of yield point elongation. Due to the pinning effect of the solute atoms, the dislocation multiplication sources also become active, which generates fresh dislocations.
After this stage, when sufficient fresh dislocations are available for continuing deformation, the stress-strain curve becomes smooth again. This yield point elongation (YPE) is absolutely detrimental as far as the formability of the material is concerned. It creates Luder bands or stretcher strain marks, which finally leads to failure of the component. These bands are visible on the strip surface. When a test specimen exhibits YPE during its tensile testing, these bands appear on the specimen surface, starting from middle (where necking starts) and spreading towards the ends, at an angle of approximately 450 to the tensile axis.
YPE elongation continues till the entire specimen surface is covered by the Luder band formation, then smooth plastic deformation starts. Here comes the role of skin passing. Since YPE, after batch annealing, cannot be avoided, a skin depth deformation is given to the just annealed steel strip. This skin depth deformation actually overcomes this region of the stress-strain curve. Sufficient number of dislocations is pulled off from the solute (C, N) atmosphere, at the same time fresh dislocations are generated, which is sufficient for facilitating the forming operation at the next stage.
If the material, in skin passed (or temper rolled) condition, is subjected to tensile testing, the stress strain curve will not show any YPE and the plastic deformation will take place without a sharp yield point, as shown in the figure below. That is what precisely desired for drawing or deep drawing grade material. If this skin passed material is left unused for a sufficiently long time, or subjected to a brief heat treatment at a low temperature, the YPE reappears once again. The YS value also goes up and ductility of the material drops. This phenomenon is known as strain aging. UTS YS Stress Strain Fig 1. tensile test graphs after skin passing From the discussion made so far, it is clear that the locking of dislocations are related to the two important factors, one is movement of dislocations, the other is movement of interstitial solute atoms. Therefore diffusion has a very important role to play. If the testing is carried out at room temperature, the mobility of dislocations under the action of external load is more than the mobility of solute atoms. If the similar test is carried out at a higher temperature, the mobility of the solute atoms increases, and movements of dislocation and solute atoms may be comparable.
Such a situation would give rise to an interesting phenomenon called dynamic strain aging, where the solute atoms keep on interacting with the dislocations and the entire stress strain curve (after elastic limit) shows serration. Since YPE is directly related to the concentrations of C and N atoms in steel, the extent of deformation (known as temper elongation) to be given at skin pass mill (SPM), which is a critical factor, varies with steel composition. The magnitude of temper elongation should be high for higher C content. For instance, the temper elongation in case of CQ material should be higher than that in case of EDD grade.
If the temper elongation is less than the required amount, the material will show stretcher strain marks during forming. If temper elongation is higher than the required amount, the strength of the material will increase. This is not desirable, particularly for the softer grades like IF and EDD, because the strain hardening exponent value is higher for these grades, compared to that for ordinary CQ material. Theoretically speaking, IF or interstitial free deep drawing grade steel should not require any skin passing. The reason is that the C and N concentrations are kept very low in this grade (of the order of 30 ppm).
In addition, presence of Ti in this steel promotes the fixing of C and N atoms in form of carbide and carbonitiride precipitates, thereby creating a condition so that the Fe matrix becomes virtually free of interstitial solute atoms. Such a condition favours the easy movement of dislocations without any hindrance, and this steel has been established as the highest formable grade, with maximum deep drawability and ductility. In practice, IF grade steel is subjected to skin passing with a small magnitude of elongation, and, of all grades, it requires minimum temper elongation.
The skin pass depends on Yield strength of the material in the following way: Lower the skin pass (roughly less than 0. 6 %), the material will have the tendency to show Bauschinger’s effect. Higher the skin passing (above 1. 5%) the material will be over strained. That’s the reason why the skin passing for a given YS, has to be done with the optimum reduction such that the material does not get into either of the problems stated above. Also percentage reduction increases with increasing YS to get the optimum properties. Parameters on which Skin pass Load depends:
Grain Size: Higher the ASTM grain size number (finer the grain), higher is the skin pass load. Speed: Increasing the speed of skin pass mill will require higher load for the same reduction Diameter of work roll: Larger the diameter of the work rolls, higher is the roll force required to remove stretcher strain. Roughness of the strip from Tandem Cold Mill: The incoming coil has got some roughness values because of the final finishing in stand number (5) of tandem Cold Mill. Many times to high roughness of the incoming strip to Skin Pass Mill and the requirement of Average roughness values on the surface in the ranger of 0. -1. 2 microns for most applications, the peaks are knocked off during skin passing which is detrimental from forming and image clarity point of view. The best practice for this should be keeping as low roughness as possible on the strip surface after tandem cold mill (of course sticker formation during annealing in Batch annealing furnace has to be kept in mind), and imparting higher roughness on the work rolls in the skin pass mill. 1. 3. 3 THEORY OF SKIN PASSING When the annealed mild steel sheet is preformed, surface markings, called stretcher strains markings, appear on deformed parts.
Stretcher strains are also called as Luder bands. The formation of these markings can be prevented by Skin passing the sheet by giving the sheet elongation of 1-2 % before Performing. LUDER BAND OR STRECHER STRAIN This band is formed with an angle of about 45 deg – 50 deg with respect to the tensile axis the markings formed between Upper and lower are called as “Luder Lines” or “Stretcher Strains” as shown in fig 1. 6 Tensile load Luder band Tensile load Fig 1. 6 Luder band 1. 4 Rolls & their requirement for Cold Rolling
The performance characteristics of rolls used in cold rolling mill, both in Tandem Cold Mill(TCM) and Skin Pass Mill, are critical to mill productivity and to the quality and acceptance of the cold rolled products. With the rapid change in roll technology, roll management in cold rolling has become an area of utmost importance. The increasing requirements of critical surface finish and texture of flat rolled product has necessitated application of the state of art technology in roll preparation and roll inspection. Rolls also represent a significant investment and input to a value analysis of cost per ton rolled.
The quality of work rolls that come into direct contact with the steel product has a direct effect on product quality and mill operation. A forged steel with a chromium content of 5 mass% has been conventionally used to meet the requirement of metallurgical structure homogeneity and high hardness for work rolls in cold rolling. Rolls having improved performance are strongly demanded. 1. 4. 1 Requirement from textured Rolls: 1. 4. 1. 1 Surface finish: Surface roughness is imparted to Work Rolls which are used in 5th stand of Tandem cold mill and to the work rolls of Skin Pass Mill.
The primary requirement of surface roughness for tandem mill rolls is to prevent stickers in the next process i. e. batch annealing. The surface roughness on Skin pass mill is guided by the requirement of surface roughness on Cold rolled strip which is based on its end use. Ra is the universally recognized and most used international parameter of roughness. It is the arithmetic mean of the departures of the profile from the mean line. Ra = 1/L {y (x)}dx For a typical application of auto grade the Ra value in strip ranges from 0. 8 to 1. 2 micron.
The final roughness on SPM roll is decided based on the transfer ratio of roughness from roll to strip (ranges from 45-60% based on mill parameters). A typical transfer plot and the values of roughness is shown in table 1. 1 Table 1. 1 Roll Roughness detail of Tandem cold mill & skin pass mill Work Roll |Tandem Cold MILL Work Rolls |Skin Pass Mill Work Rolls | |Average roughness |PPC |Average roughness |PPC | |3. |75 |3 |120 | |4. 0 |70 |3. 0 |96, 118 | |4. 5 |65 |3. 5 |80 | |5. 0 |60 |4. 0 |70 |
The distribution of surface roughness over the roll body is also of importance to ensure consistency of surface roughness over the strip widths produced in a campaign. The ROLLTEX Electro discharge texturing process of Sarclad machine produces a texture to the capability as mentioned in table 1. 2. Roughness Definition: Roughness is defined as the finer irregularities of the surface texture that usually result from the inherent action of some production process such as machining or wear. Roughness features are typically in the sub micron range.
Continuously recurring, irregular depressions and elevations on the surface of the coil are known as roughness. Rough coil surface is usually caused by severe roll groove wears surface roughness can also be caused due to corrosion if the rod is stored for lengthy periods in damp or corrosive atmospheres. The degree of roughness can be determined by microscopic examination or with Ra meter. Surface roughness has two main attributes: Roughness height or depth, and Lateral dimension. Roughness heights of the structure on polishing or machining surface are frequently measured as a root mean square roughness.
The units of roughness are angstroms or nanometres for smoothers surface ‘lim’ and micrometers “µm” for rougher surface. Lateral dimensions frequently and called surface spatial wave lengths are measured in micrometers. A rough surface is usually described in terms of its deviation from a smooth reference surface. Some conventional methods for surface measurement are optical microscope, scanning electron microscope and transmission electron microscope. These can be used to produce topographic maps of surfaces. Today laser scattering technique has become more common. Ra value:
Average/mean height of surface peaks and troughs over a reference length indicates an overall profile of the sheet surface, dullness or brightness. Roughness is imparted to the rolls by Electro discharge texturing method Table 1. 2 Roughness accuracy detail of texturing m/c |Sno |Range of roughness value |Accuracy of surface produced (Ra) |Accuracy of surface produced (PPC-peaks per | | | | |centimetre) | |1 |0. to 6. 0 micron Ra |+/- 4 % of mean Ra |+/- 4 % of mean PPC | |2 |6. 1 to 10. 0 micron Ra |+/- 5 % of mean Ra |+/- 5 % of mean PPC | |3 |>10. 1 micron Ra |+/- 6 % of mean Ra |+/- 6 % of mean PPC | 1. 4. 1. 2 Peak Count: It is the measurement of number of peaks in the specified length over a particular bandwidth (normally 1 micron).
A profile peak is the highest part of the profile between an upwards and downwards crossing of the mean line. The exposed auto body panels typically require 100 ppi on the Cold rolled sheet. The transfer ratio of peak counts from roll to the strip ranges between 60-70%, based on again the rolling conditions. Figure 1. 7 shows the correlation between the roughness of the surface & the peak counts. Fig 1. 7 Correlation between roughness & peak counts Stability of the surface profile: The textured roll is required to give a consistent transfer of roughness and peak count on the strip while rolling.
During rolling the surface experiences wear of the peaks and the roll is Fig 1. 8 Skewness of surface discarded after a certain tonnage, determined based on the cut-off point of surface requirement on the strip. To assess this requirement of surface stability, metrology experts and certain European instrument manufacturers have devised surface texture height parameters, which can be analysed by a Data Processing Module (DPM), supplied separately by the surface finish tester manufacturers. Out of various parameters used in this analysis, the most commonly used is Rsk (Skewness) and tp % (known as bearing ratio).
Rsk is the measure of the symmetry of the amplitude distribution curve about the mean line. As shown in figure 1. 8 if Rsk is negative the surface peaks are higher, which is prone to a large drop in surface finish during the initial rolling. Based on the practical experience of cold rollers over the world, a slight positive value is preferred. A typical surface plot after texturing a surface to roughness value of 2. 93 micron in Sarclad EDT machine and measured by DPM is shown in Figure 1. 9 Fig 1. 9 Surface measurements done on Tata EDT-Skin Pass Mill roll.
The Bearing ratio (tp%) is a measure of the length of bearing surface (expressed as a % of the assessment length), where the profile peaks have been cut off at a line which runs parallel to the mean line of the profile. The line defining the bearing surface can be set at a selected depth below the highest peak or at a selected distance above or below the mean line of the profile. When this line is set to the depth of the largest profile valley, the tp is 100% because the entire profile is above the bearing line.
By plotting the tp value against depth below the highest profile peak between the 0% and 100 % limits, the bearing ratio (known as Abott- Firestone curve) curve is obtained. Figure 1. 9 shows the bearing ratio curve against a particular value of Rsk. 1. 4. 1. 3Waviness: Most surface profile results from the combined effects of roughness, waviness and form as shown in figure 2 Waviness parameters are produced by passing the data of the surface measurement through a low pass filter, so that longer wavelengths than the cut-off are included. The waviness, Wa is calculated from the resulting profile.
Fig 2 Waviness, Roughness and Form of a surface Wavelengths in the roughness category < 800 micron are covered or filtered out by painting, in the end application of the CR strip. Wavelengths >800 micron defined as “Waviness” remain or are enhanced after painting and contribute to poor Distinctness of image or image clarity. If Wa is held below 0. 6 micron, irrespective of the Ra, then those wavelengths >800 micron have only a marginally adverse effect on Distinctness of image. Samples of sheet produced by tandem/ temper mill rolls textured by the Rolltex EDT process consistently show levels of Wa