SURFACE QUALITY IMPROVEMENT OF HOT-DIP GALVANIZED STEEL SHEET, USING HARD CHROME PLATED SKIN PASS MILL ROLL Ms. Nattha Panthong (Production Engineer) Ms. Amika Saenkhum (Maintenance Engineer) Mr. Kim Kyoung Beom (GM / Production Department)
Outline ut Introduction Background Operation Test Result POSCO-TCS and CGL Layout Skin Pass Mill Facility Roll Maintenance Process Chrome coating technology Electro discharge texturing (EDT) Technology Cr coated and non-cr coated work roll life time The effect of SPM roll force to the surface properties The constant surface quality of hard chrome coating roll SEAISI 2018 2
Introduction of POSCO-TCS POSCO Coated Steel (Thailand) Co., Ltd. 7/448 Moo 6 Amata City Industrial Estate, Mabyangporn, Pluakdaeng, Rayong THAILAND 21140 Products : Hot Dip Galvanizing Steel (GI) Hot Dip Galvannealing Steel (GA) Capacity : 450,000 Ton/year Customers : Automotive (Toyota, Nissan etc.) Home Appliance (Samsung, LG etc.) Color Coating (PPT, Dong bu etc.) SEAISI 2018 3
Introduction of CGL (Continuous Galvanizing Line) POSCO-TCS Continuous Galvanizing Line (CGL) Size : 0.3~2.3mmt, width 1,860mm Grade : CQ, IF Steel (DQ, DDQ, EDDQ), AHSS (~60kg) Entry Section Coating Section Exit Section Two Pay Off Reels Seam Welder machine Pre-Cleaning section (Electrolytic & Brushing) Entry Looper Vertical Annealing Furnace Molten Zinc Pot Galvannealing Furnace Air/Mist Cooling Tower Skin Pass Mill & Leveller Chemical Passivation Exit Looper Turret-style side trimmer Vertical/Horizontal Inspection Electrostatic Oiling machine On-line Defect Detector Two Tension Reels SEAISI 2018 4
Skin Pass Mill Facility Purpose To Improve Mechanical Properties (Yp-El) Shape correction (Ondulation, Flatness) Roughness and Surface Appearance Control Specification Type : 4-Hi 1-Stand Work Roll Roll Force Max(Ton): 1000 Elongation Max (%): 3.0 Roll Change : Shift Car SPM Roll Change Process Old Roll traverse (Roll out) Side shift frame is moving Work Roll traverse (Roll in) SEAISI 2018 5
Skin Pass Mill Facility Skin Pass Mill Work Roll Non-coated roll: In 2016, POSCO-TCS started operation with non-coated SPM Work Rolls. At the time, the work roll worn down after ~200 km. The quality of the product is out of standard and required a new work roll frequently. The work roll changed 2-3 times a day which means that 10 Tons of product is loss per day. Chrome-coated roll: Then the chrome-coated roll is introduced due to its durability and wear resistance. However, there are limitation to sourcing the local suppliers. Cr-Coating Shop SEAISI 2018 6
Roll Maintenance Process Roll Receiving Inspection Inspection roll surface after receiving roll from production line. Check Point : Diameter, Roughness, Condition of surface Grinding Modify roll surface and control roll profile to prevent roll crack. Check Point : Roll crown, roughness EDT EDT process is process to provide roughness on roll surface. GA product: Low Ra, GI product: High Ra * control roughness higher than standard 0.2-0.3 µm Hard Chrome Coating Hard chrome plating is an electroplating process in which chromium is deposited from a chromic acid solution. * Hard Chrome thickness : 10±2 µm. Inspection Roll Surface Final for inspection roll surface by check roughness value. follow standard of GA and GI product. Thickness of surface coating by use CT3 machine (acid analysis) Visual inspection : roll surface 100 % no defect SEAISI 2018 7
EDT(Electro Discharge Texturing) Electro Discharge Texturing Formation of the small craters on the rolls surface is the result from the discharge between two electrodes with a gap control. Advantages Able to control height and number of peaks by controlling parameters. EDT (Electro Discharge Texturing) Machine SEAISI 2018 8
Hard Chrome Coating Process Hard Chrome Coating Advantages High hardness, Wear resistance Corrosion resistance Low friction, temp. treatment Maintain the surface texture when applied thin layer SEAISI 2018 9
Operation Test Overview Operation Test Cr coated and non-cr coated work roll life time The effect of SPM roll force to the surface properties The constant surface quality of hard chrome coating work roll SEAISI 2018 10
Operation Test Result Strip Surface Appearance Non-chrome coated roll on GI Product 8.5km 69km 200km Flat area/ Worn down Flat area/ Worn down Chrome coated roll on GI Product 15km 306km 1392km 11 SEAISI 2018 11
Operation Test Result Strip Roughness & Printed Area Roughness (Ra, um) & Printed Area Ratio (%) 1.60 1.40 1.20 1.04 1.00 0.80 0.60 44.6% 0.40 0.20 Non-Chrome Coated roll 0.76 0.73 37.0% Chrome Coated roll 44.8% * Printed Area Ratio : Area of the roll printing mark / the total area 61.1% Ra Roughness(Ra,um) Printed area Area Ratio(%) ratio R/F Roll Force(Ton) 46.8% 0.87 800 700 600 500 400 300 42.5% 200 100 SPM Roll Force (ton) 0.00 100 200 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 SPM Rolling Distance (km) 0 SEAISI 2018 12
Operation Test Result The effect of SPM roll force to the surface properties Optical Microscopic(OM) of GI Product Surface RF 100 Ton RF 200 Ton RF 500 Ton SEM of GI Product Surface RF 100 Ton RF 200 Ton RF 500 Ton SEAISI 2018 13
Operation Test Result The effect of SPM roll force to the surface roughness 3D Roughness of GI Product RF 0 Ton RF 100 Ton RF 200 Ton Ra : 0.39, PPI 154.5, Rmax 6.77, Rz 2.89, RPc 24 Ra : 0.75, PPI 222.3, Rmax 7.36, Rz 4.85, RPc 35 Ra : 0.86, PPI 230.7, Rmax 6.39, Rz 4.59, RPc 36 RF 300 Ton RF 400 Ton RF 500 Ton Ra : 0.84, PPI 226.5, Rmax 6.28, Rz 4.69, RPc 36 Ra : 0.99, PPI 232.8, Rmax 6.74, Rz 5.24, RPc 37 Ra : 1.30, PPI 260.4, Rmax 7.70, Rz 6.54, RPc 41 SEAISI 2018 14
Operation Test Result The effect of SPM roll force to the surface properties SEM of GA Product Surface RF 0 Ton RF 200 Ton RF 500 Ton 100x RF 0 Ton RF 200 Ton RF 500 Ton 300x SEAISI 2018 15
Operation Test Result Strip Roughness & Printed Area - Roughness increased as the roll force is increased. GI Product GA Product 1.4 Ra Printed area ratio 70 1.0 Ra Printed area ratio 60 Roughness (Ra, um) 1.2 1.0 0.8 0.6 0.4 0.2 60 50 40 30 20 10 0.9 0.8 0.7 0.6 50 40 30 20 10 Printed Area Ratio (%) 0.0 0-100 0 100 200 300 400 500 600 SPM Roll Force (ton) 0.5 0-100 0 100 200 300 400 500 600 SPM Roll Force (ton) SEAISI 2018 16
Operation Test Result Strip Surface Friction Property - Roll force showed direct relation to friction of GI product GI Product GA Product SEAISI 2018 17
Operation Test Result Strip Surface Appearance - The surface appearance is constant throughout the life of the work roll. GI Product Surface using Cr-Coated Roll 5km 214km 630km GA Product Surface using Cr-Coated Roll 107km 263km 360km SEAISI 2018 18
Operation Test Result Strip Roughness & Printed Area - The surface quality is constant throughout the life of the work roll. GI Product GA Product Roughness (Ra, um) & Printed Area Ratio (%) 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 Ra Printed area ratio RF 0 200 400 600 500 450 400 350 300 250 200 150 100 50 0 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 Ra Printed area ratio RF 500 450 400 350 300 250 200 150 100 50 0 0 100 200 300 400 SPM Roll Force (ton) SPM Rolling Distance (km) SPM Rolling Distance (km) SEAISI 2018 19
Conclusion The effect of Roll Force Improve the surface appearance - The higher roll force provide the better appearance and higher strip roughness. Control the friction property - The direct relation with friction of GI product. - The higher the roll force provide higher friction property. Control the constant strip quality - the constant roll force must be controlled. The effect of Hard Chrome Coating Control the constant product quality - The product quality is constant from the beginning until work roll is changed - Constant roughness condition - Sharper, deeper texture because the weak peaks is protected. Increase the work roll life. - Reduce work roll changing time and scrap. SEAISI 2018 20