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Cold Rolling fundamentals.PPT
- 1. ColdRollingTechnology 1.TYPES OF COLDROLLING MILLS 2. Different Components of PL-TCM 3. Edge Drop, Crown,Wedge 4.Roll Force 5.A.S.C 6.A.G.C 7.Defects
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- 3. ColdRollingTechnology What is ColdRolling ? Process by which hot rolled strip or stock is introduced between rollers and then squeezed or compressed to the desired thickness Amount of strain introduced determines the properties of the finished product Why is Cold Rolling done ? Good Formability Superior Surface Finish Reasonable Strength Close Dimensional Tolerance
- 4. ColdRollingTechnology Commonly used Gradesof Cold Rolled Steel D/DD - Drawing/Deep Drawing Simple forming/bending applications EDD - Extra Deep Drawing Deep Drawn components
- 5. ColdRollingTechnology Commonly used Gradesof Cold Rolled Steel IF (Interstitial Free) Critical Draw and StretchComponents in terms of geometry of the component, type and depth of draw and press parameters IFHS (Interstitial Free High Strength) For components involving draw and requiring resisitance against dent and impact
- 6. ColdRollingTechnology Chemical Composition Typical Ranges %C%M n %Si %S %P %Al N, ppm %Ti %Nb D/DD 0.05 0.3 0.02 0.01 0 0.05 70 EDD </=.04 0.2 0.02 0.01 0 0.03 50 IF 0.003 0.2 0.015 0.01 0 0.04 40 0.06 0.01 IFHS 0.004 0.4 0.007 0.01 0 0.03 40 0.03 0.04
- 7. ColdRollingTechnology Cold Rolling Processand Tata Steel Capability
- 8. ColdRollingTechnology Pickling (to remove oxides andscales) Cold Rolling in reversing or tandem mill to desired thickness Annealing (batch or continuous) for stress relieving Skinpassing to remove Luder lines, develop mech prop,impart desired surface finish improve flatness Process Flow Inspection, Finishing Despatch
- 9. ColdRollingTechnology 2 Hi 4Hi Cluster - Hi 6 - Hi
- 10. ColdRollingTechnology Fig 4. Profileand Flatness Controlled Mills Six Hi Mill Four Hi Mill Pair Cross mill
- 11. ColdRollingTechnology Pickling Why is Picklingdone ? To remove scales and oxide layers present on the surface of hot rolled steel Make the steel surface suitable for cold rolling Scales and oxide layers are harder than steel and hence hinder the process of cold rolling of low carbon steel
- 12. ColdRollingTechnology Pickling How’s scale andoxide removal are achieved? Hot rolled strip is passed through tanks containing hydrochloric acid wherein the acid dissolves the oxide layer The strip is then rinsed with water to remove any excess acid. dried and the strip is ready for cold rolling
- 13. ColdRollingTechnology Pickling Process Variables : Temperature: Strip temperature of 50 to 80degrees centigrade is maintained. This ensures faster rate of reaction with the acid Acid Concentration: Acid concentration in tanks influence the dissolution of scales and oxides Lower concentration is maintained in the first tank and increases in the subsequent tanks Line Speed : Controls the residence time of the strip in the bath
- 14. ColdRollingTechnology Cold Rolling Hot rolledPickled Coils are reduced to the desired thickness Achieved either in Reversing Cold Rolling Mills : Thickness achieved by repeated passes through the same rolling stand back and forth or Tandem Cold Rolling Mills : Thickness achieved through series of rolling stands in tandem
- 15. ColdRollingTechnology Cold Rolling (Reversingvis a vis Tandem Mill) Criteria Reversing Mills Tandem Mills (PLTCM) Productivity Lower Higher Efficiency Threading and tailling off are required for every coil. Threading and tailling off not required for every coil. Increase in efficiency by 50 to 60% Yield Lower yield - Off gauge portion from head and tail end needs to be removed. Higher yield - Off gauge removal not required end to end rolling achieved. Use as combination mill (reduction & temper rolling) Can be used. Can be used only for reduction rolling
- 16. ColdRollingTechnology Process Control Parametersand Variables Dimensional Control Flatness or Shape Control : % Cold Reduction
- 17. ColdRollingTechnology Tata Steel Capability 5Stand 6Hi Tandem Cold Mill coupled to a Continuous Pickling Line (PLTCM) Features Automated Shape and Gauge Control Thickness tol achievable - +/- 0.5% of thickness along centre line with edge drop of 30 microns Mill Clean System
- 18. ColdRollingTechnology Dimensional Tolerances Thickness Tolerance: 0.5% of thickness on centre line Width Tolerance : -0 / +7 mm (Mill Edge) : -0 / +0.4 mm (Trim Edge) : -0 / +0.3 mm (Slit Coils)
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- 20. ColdRollingTechnology Flatness Flatness is abasic requirement of cold rolled sheets Affects • Assembly Accuracy • Efficiency of the assembly operation • Straightness and rigidity for panel applications • Material movement in high speed transfer press lines
- 21. ColdRollingTechnology Te1 Te2 Tc Te1Te2 Tc CROWN = Tc-(Te1+Te2) 2 WEDGE = Te1-Te2
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- 24. ColdRollingTechnology Measurement of Flatness Flatness ofcold rolled strip is measured in terms of height of wave and wavelength and denoted in I units as I = (/2 x h/L)2 x 105 where h is the wave height and L is the length of the wave
- 25. ColdRollingTechnology IMR Shifting Work RollBending IMR Bending
- 26. ColdRollingTechnology Flatness Tata Steel Capability Flatnessachievable at PLTCM - is 10 I units
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- 28. ColdRollingTechnology #5 #4 #3#2 #1 Mill E. Shear Rotary Shear No. 1 Tension Reel No. 2 Tension Reel Process Computer Coolant Control Bending Control THICKNESS GAUGE, VELOCITY METER, ROLL GAP MEASUREMENT SHAPE METER, LOAD CELL #1 #1
- 29. ColdRollingTechnology NEW AGC MODECONVENTIONAL AGC MODE #1 FF #1 FF #1 BISRA #1 BISRA #1 REC #1 REC #1 GMS #1 GMS #1 ABS. BISRA #1 ABS. BISRA #1 FB #1 FB #2 MF #2 FF #3 MF #5 FF #4 MF #5 FB #5 MF ACC/DEC COMPEN ACC/DEC COMPEN
- 30. ColdRollingTechnology Surface Texture Pattern impartedon the steel surface by the grit on the roll Affects the Paint film quality of the steel surface. Image clarity after painting Forming Operations Two important criterion of Surface Texture Surface Roughness : Surface Characteristic with wavelength <800 m Surface Waviness : Surface Characteristic with wavelength >800 m
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- 32. ColdRollingTechnology Surface Texture Parametrs usedas index of surface roughness 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
- 33. ColdRollingTechnology Rmax: Distance betweenthe highest of profile peak and bottom most of profile valley line over a reference length
- 34. ColdRollingTechnology Surface Texture Rz Value: Sum of the average value of the peak heights and deepest valley measured over 5 peaks and 5 crests along the reference length. Rz and Rmax indicate the distribution of matte/roughness on the strip surface.
- 35. ColdRollingTechnology WCA Value :Measure of surface waviness Lower the WCA value , better is the image clarity after painting Preferred WCA value for auto panels should be < or = 0.6 m
- 36. ColdRollingTechnology Surface Texture Afterpainting. Roughness gets covered by the paint Waviness remains after painting. Bright Finish - (Ra value < 0.6 m) Desirable for - Any kind of plating operations which require smooth surface Matte Finish - Coating - Helps in drawing due to oil retaining capacity Extent of desirable roughness varies with the type of painting technology used.
- 37. ColdRollingTechnology Surface Texture Tata Steel’sCapability Electro Dip Texturizing facility Features Uniform distribution of Roughness Control Surface waviness (WCA < 0.6 m) for better image clarity of auto panels
- 38. ColdRollingTechnology The Specifications (CR) ASTMA366/ A619 / A620 JIS G3141 SPCC / SPCD / SPCE / SPCEN EN 10130 FEP01 / FEP02 / FEP03 / FEP04 / FEP05 DIN1623 St 12 / St 13 / St 14 BS 1449 CR4 / CR3 / CR2 / CR1
- 39. ColdRollingTechnology Defects in ColdRolled Steel
- 40. ColdRollingTechnology Shape Defects Shape defectsin cold rolled strip generate due to types of strain developed in cold rolled sheets. Flat steel sheets are the one with minimum strain Fundamental cause of strain is non uniform elongation of steel sheet Higher elongation at the edges with respect to the centre of the sheet generates edge waviness
- 41. ColdRollingTechnology Shape Defects Higher Elongationat centre than at the edges gives rise to centre buckle or a quarter buckle depending on the distance from the edge and centre of the sheet
- 42. ColdRollingTechnology Shape Defects Camber/Cross bow Measureof straightness of edge Higher thickness variation in transverse direction, inacurracy during slitting gives rise to camber or cross bow Normal limit is 2mm deviation in 2000mm length
- 43. ColdRollingTechnology Shape Defects Coil Setor Longitudinal Bow Skin Passed Coils tend to retain its shape when uncoiled due to the coiling tension. Corrected by applying load in straightner during Cut to Length operations
- 44. ColdRollingTechnology Surface Defects Scales Left overoxides formed during hot rolling Rolled into the sheet surface remaining unremoved even after pickling Slivers Defect mainly caused by alumina inclusions entrapped during steel making/casting operations Gets elongated linearly by rolling Leads to cracking/failure during drawing operation
- 45. ColdRollingTechnology Surface Defects Roll Marks Defecttransferred to the strip from the rolls in either convex or concave shape Appears at regular intervals in the rolling directions Caused due to scratches on rolls , foreign substance sticking to rolls,metal pick up on rolls during strip breakage
- 46. ColdRollingTechnology Surface Defects Sticker Marks Semicircular marks on the surface of the strip. Generates due to sticking tendency of subsequent coil wraps Sticking tendency is due to any of the following reasons Higher tension of the coils being taken for annealing Higher soaking time/temperature Poor shape of the strip
- 47. ColdRollingTechnology Surface Defects Orange Peel Appearsas wrinkles on steel surface Occurs - Due to coarse grain size - coarser than ASTM 6 High and Low Spots Observed on formed components either as dents or depression on the surface Occurs due to dirt from atmosphere or from rust preventive oil used High Rmax value also results in high spot/low spot
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- 49. ColdRollingTechnology Annealing Why annealing isdone? Steel Coils are hard and brittle after Cold Rolling Has Elongated grain Structure Not suitable for end use
- 50. ColdRollingTechnology Annealing ensures Softening ofthe cold rolled steel- Relieving the internal stress developed during cold rolling Develop desired mechanical properties suitable for intended applications Bright Surface Finish
- 51. ColdRollingTechnology Annealing Stages of Annealing Processof relieving internal stresses induced during cold rolling takes place in three stages 1. Recovery Physical and mechanical properties ( Hardness, ductility elect prop.)which suffered changes as a result of cold working tend to recover to their original values. Occurs at the initial phase of annealing at lower temperatures
- 52. ColdRollingTechnology Annealing Stages of Annealing 2.Recrystallization Process of nucleation of new strainfree crystals (grains) which replaces all deformed crystals of the cold worked material Takes place at a temperature known as recrystallization temperature
- 53. ColdRollingTechnology Annealing 3 Grain Growth Increasein average size of crystals/grains on further annealing subsequent to the completion of recrystallization Grain size has direct effect on the mechanical properties. Hence it is controlled to achieve optimum properties
- 54. ColdRollingTechnology Annealing Annealing requires Heating thecold rolled steel to the recrystallization temperature in an inert atmosphere. Soaking at this temperature for sufficient time for homogenization and necessary changes to occur Cooling at a predetermined rate Process Variables Temperature, soaking time , cooling rate depending on the desired mechanical properties
- 55. ColdRollingTechnology Comparison of Batchand Continuous Annealing Parameters Batch Annealing Continuous Annealing Chemistry Suitable for low Carbon low alloy steel Best suited for IF Steels Uniformity of Mechanical Properties Variations between outer/ inner wraps to centre of the coil Uniform over the length of the coil Productivity Lower Higher Properties with identical chemistry other than IF Lower hardness and YS Higher hardness and YS Higher r Bar value Lower r Bar value No ageing Ageing IF Steel Properties Good properties achievable with longer cycle time. Better properties achieved with much lower cycle
- 56. ColdRollingTechnology Annealing Tata Steel’s Capability 100%Hydrogen Batch Annealing Unique Features Higher Productivity Uniformity of Properties Surface Cleanliness- Carbon residue <5mg/m2/side Iron Fines < 15mg/m2/side Coil Cooling and Storage (CCSU) in dehumidified chamber- Prevents rust formation in annealed coils
- 57. ColdRollingTechnology Annealing Tata Steel’s Capability AnnealingCycle Times (approx) for various grades Grades Approx. Cycle Time (Hrs) CQ 34 D/DDQ 36 EDD 49 IF 52
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- 59. ColdRollingTechnology Skin Passing Why isSkin Passing Done ? To achieve the optimum mechanical properties (Hardness, YS, ) To prevent occurrence of stretcher strains in annealed steel To impart desired surface finish - “matte” on the strip surface To improve strip shape
- 60. ColdRollingTechnology Skin Passing What areStretcher Strains? Generated due to the yield point phenomenon exhibited by cold rolled and annealed low carbon steel sheet/strip Appears as discrete bands on the strip surface
- 61. ColdRollingTechnology Skin Passing Mechanical Propertiesare controlled by regulating the % elongation during skinpasing % Elongation depends on the grade of the CR steel. Range - Stretcher Strain removal is achieved through suppression of the yield point elongation during skinpassing Before Skin Pass with Yield Point Elongation or Stretcher Strain After Skin Pass Stretcher Strain Suppressed
- 62. ColdRollingTechnology Skin Passing Tata Steel’sCapability Single Stand 4 Hi SkinPass Mill Unique features SkinPassing with temper fluid -Wet skin passing - Ensures freedom from dirt/dents, any foreign particles on strip surface.
- 63. ColdRollingTechnology SPM Constraint: Minimum Widthis 930mm. Coil ID is 508mm. Changeover Time to 608mm is 4 hrs. Bright Rolling is to be taken only in campaign (Min 300T)
- 64. ColdRollingTechnology Properties and SurfaceCharacteristics of Cold Rolled Steel (CRCA)
- 65. ColdRollingTechnology Properties of ColdRolled Steel Mechanical Properties Hardness Yield Strength Ultimate Tensile Strength % Elongation Plastic Strain Ratio
- 66. ColdRollingTechnology Yield Strength (YS) Stressrequired to produce a small specified amount of plastic/permanent deformation. Indicative of ease of forming operation. Higher YS - Higher Load for onset of plastic or permanent deformation Lower YS - Onset of permanent deformation at lower load. Hence suitable for critical draw applications
- 67. ColdRollingTechnology Ultimate Tensile Strength(UTS) Maximum load which the steel can withstand before fracture/ breaking Measure of the strength of the material YS/UTS ratio Indicates the workability of steel Lower ratio (< or =0.5) indicates better workability
- 68. ColdRollingTechnology Elongation Measure of thestrain before fracture. Indicative of the extent to which metal can be deformed without fracture
- 69. ColdRollingTechnology n- Value :Modulus of Work hardening or strain hardening exponent Indicates better stretch formability Calculated from the formula = Kn where is the true stress, is true strain and K is the material constant For good stretchability n value should be >=0.22
- 70. ColdRollingTechnology r & rbar - Value Quantitative index of the anisotropical property- Directionality Ratio of strain along width to strain along thickness Measured along the rolling direction, transverse to the rolling direction and at 45degrees to the rolling direction indicated as r1, r90 and r45. Measured in tensile testing machine or in an instrument module r r bar value is calculated as r1 + r90 + 2 r45 ----------------- 4
- 71. ColdRollingTechnology Indicative of drawabilityand resistance to thinning during drawing operation. Higher the value, less susceptibility to fracture, hence better drawability r & r bar - Value
- 72. ColdRollingTechnology Typical Mechanical Propertiesof Cold Rolled Close Annealed Steel (CRCA) Hardness in Rockwell B scale YS in Kg/mm2 UTS in Kg/mm2 % Elongation on 50mm GL r bar value min n-value min Grades D/DD 48 to 55 >17, < 24 28 to 34 35 min 1.3 0.18 EDD 35 to 45 15 to 18 27 to 32 45 min 1.6 0.2 IF <38 13 to 16 27 to 30 48 min 1.8 >0.22
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- 74. ColdRollingTechnology Application of ColdRolled Steel (low Carbon) Automobile components Panels for White Goods/Appliances Packaging (Drums ,Barrels and Containers) Furniture Electrical Panels General Engineering
- 75. ColdRollingTechnology Automobile Components Excellent drawability-r bar >1.7 YS/UTS ratio < or = 0.5 Surface finish and texture amenable to modern painting technology - Ra , Rz and Rmax values Good Weldability Corrosion Resistance Dent Resistance
- 76. ColdRollingTechnology Panels for Appliances DeadFlatness Close Dimensional Tolerance Thk +/- 0.01mm Optimum Hardness to ensure rigidity , dent resistance and ease of bending. Hardness range 105 to 115VPN Optimum Oiling levels compatible with degreasing facilities Surface free from any blemishes- free from dents/scratches/ rust spots, rolled inscales/pittings Surface finish amenable to modern painting technology
- 77. ColdRollingTechnology Packaging - Drums,Barrels and Containers Close thickness tolerance to maximise yield in terms of no of drums per ton maintaining the min drum wt. Ease of welding - Resistance welding Optimum hardness to ensure rigidity, strength and ease of bending , while forming of drum ends 50 to 60 Rb for Drum body to ensure rigidity and stiffness 45 t0 55 Rb for Drum ends to ensure against cracking during seaming operations
- 78. ColdRollingTechnology Applications Parameters tobe essentially captured in TDC and Order Attributes Automobile Components Grade, Ref Specn, Mechanical Properties , Surface Characteristics (Ra Rz,Rmax), Flatness, Oil levels, Type of Oil,Packing Norms Appliances (White Goods) and Panels Grade, Hardness, Flatness, Surface Characteristics, Oil Levels,Thickness, Length Tolerances,Diagonal Difference,Packing Norms Packaging Drums and Barrels Thickness Tolerance, Hardness, Oil type and Oil level