COGNEX®Surface Inspection Systems DivisionHot Strip MillDefect CatalogIndex of contentsGeneral survey of Hot Strip Mill (3)Unique Features (Gray Bins) (4)Lamination (5)Edge cracks (8)Folded strip edge (10)Roll marks (12)Mould powder (13)Shell (15)Scale Grain similar to Red Scale (19)Rolled in secondary scale (21)Divot (Mechanical damage) (22)Gall (Other rolled - in extraneous matter) (24)Roll imprint (Bruise) (27)Hot Strip Scratch (29)Scale pits (31)General survey of Hot Strip MillIntroduction:Below is a rough diagram of a conventional non-reversing Hot Strip Mill for reference.The rolling mill is the process, which reduces a hot slab into a coil of specified thickness; the whole processing is done at a relatively high temperature (when the steel is still "red").Process:FurnaceThe furnace heats the steel “slab” and pushes it onto the roller line. Slabs are approximately 240mm thick and vary in width and length.Roughing MillOnce the slab reaches the roughing mill, it is reduced down to as much as 1/10th its original thickness and is called a “transfer bar”. Width is not changed. The process of reducing the slab to a transfer bar is achieved either through single passes of multiple roughing stands or by multiple passes on reversing roughing stands.Finishing StandsThe finishing stands further reduce the transfer bar into a steel “strip” whose thickness ranges from app. 1mm to as much as 13mm. By the time the strip reaches the surface inspection point (for top inspection), the strip temperature can range anywhere upwards of 950°C.Cooling SectionThe cooling section uses water to cool the strip. By the time the strip reaches the surface inspection point (for bottom inspection), the strip temperature can range anywhere from 300°C to upwards of 700°C.Down Coilers Cooling Section Finishing Stands Roughing Mill Furnace Hot Strip Mill Example BottomBased on Cognex line scan technology with all special developed detection algorithm, there is the unique gray bin feature which enables to use the intensity of the defect as the major information for classification. How does this work?Unique Features (Gray Bins)More classification information is possible with gray bin features than with intensity features alone.Gray bin features provide information about the distribution of gray within a defect. Intensity features only report the maximum and minimum grayscale values in a defect.Gray bin values can be customized to match the application.Gray bin values should be chosen based on differentiation of defect classes.Defect Image Basic IntensityFeaturesWith Gray BinFeaturesBright BinsDark BinsLaminationGeneral description:Laminations take the form of areas of overlapping material that come in various shapes and sizes. These defects are irregularly distributed over the surface of the rolled material and may only partially bonded to the underlying surface.Reason for defect:Laminations occur when the incoming steel slab contains either a non-metallic inclusion, or where the incoming slab has suffered some mechanical surface damage. These are broken open during the rolling process and then folded over in a lamination like form.Edge cracksGeneral description:Material discontinuations extending vertically down from the surface and running through the entire plate or strip thickness at the longitudinal edges and, in the case of sheet and plate, also at the front and rear edges of the rolled material.Reason for defect:Edge cracks can occur during reduction of the continuously cast bars or starting slabs, and also during cooling. A further cause of this defect lies in the breaking strength of the material being locally exceeded in the edge zone.Folded strip edgeGeneral description:Folded strip edges take the form of edges upsets, beaded strip edges or mechanical damage at individual, protruding turns in the coil.Reason for defect:Folded strip edges in the area of the upcoiler/downcoiler or during handling operations and result from heavy contact with final mill stand delivery equipment, ROT (run out table) side plates, or faulty strip winding leaving turns exposed and susceptible to contact damage transfer and storage.Roll marksGeneral description:Roll marks by definition are defects imparted on the strip by a work roll.Reason for defect:These defects are always repeating. The period depends on the size of the work roll that created the defect and on the amount of subsequent reduction that occurs after the mark is created.Mould powderGeneral description:Mould powder is a non-metallic powder used to coat surfaces in contact with molten metal in the casting process.Reason for defect:This defect is caused when the above-mentioned powder is rolled into the surface of the slab in the roughing process. A similar defect is also caused when other fine powders or dirt fall onto the bar or strip during the rolling process. This often produces a distinctive "Christmas tree" shape.ShellGeneral description:Shell takes the form of areas of overlapping material and comes in various shapes and sizes. These defects are irregularly distributed over the surface of the rolled material and may only be partially bonded to the underlying surface.Reason for defect:Shell occurs when rolling a starting material exhibiting high levels of non-metallic inclusions in its surface zone. These inclusions are broken open during the rolling process and then folded over in a shell-like form. Shell also occurs as a result of surface damage in the starting material.Dark Bins Bright BinsDark Bins Bright BinsScale Grain similar to Red ScaleGeneral description:Red scale or scale grain is scale, which is typical for Si-steel grades. Red scale looks dark often surrounded by bright areas. It can be both single spots but also stripes going irregularly over bigger areas.Reason for defect:May be caused by the worn surface of the finishing stand work rolls. This scale remains on the surface of the strip and can be rolled into the surface by successive stands. This defect will vary greatly in shape and concentration.Rolled in secondary scaleGeneral description:Rolled in secondary scale is the term given to those scale particles rolled into the surface, the distribution of which resembles a “salt and pepper” scattering.Reason for defect:Caused by the rolling – in of secondary scale within the finishing train. Thin hot rolled strip is more susceptible to this defect owing to the higher specific loads involved.Divot (Mechanical damage)General description:A Hot Strip Mill divot is a surface defect with the following characteristics:• Always on the bottom.• Peeling areas are many times associated with this defect. • The width of the defect is approx. 10 – 100mm. • The length can be up to 1.4m.• The general shape is that of a “W” or a “V”.Reason for defect:A divot is the surface defect resulting from the hot rolling of holes or impressions on the bottom of a slab. The impressions are a result of a build-up of scale and mold powder on the rider buttons (furnace skids) inside the reheat furnace.Gall (Other rolled - in extraneous matter)General description:Gall is rolled in foreign material.Reason for defect:Typically it is from buildup on the edge guides that falls onto the strip and gets rolled in.Roll imprint (Bruise)General description:Roll imprint is a periodical defect and comes from a slightly damaged working roll. Reason for defect:It is a simple version of the more severe defect " Roll Mark ". The defect can be barely felt - in contradiction to the Roll Mark.Hot Strip ScratchGeneral description:Hot strip scratches are grooves, gouges and deep scratches of various dimensions which occur in the hot rolling mill up to the entry of the pickling line, run in the rolling direction, sometimes undergo a small degree of overrolling and rarely contain scale inclusions.Reason for defect:Sometimes a metal chip is cut out of the surface as a hot strip scratch occurs, or the material may be pushed away to the side or to the end of the scratch. The location of occurrence and flaw shapes vary considerably.Scale pitsGeneral description:Scale pits are caused by scale being embedded into the surface of the material during the hot rolling. The scale may be removed again by pickling, but the defect may not be completely eradicated during the cold rolling process. The appearance of the flaws ranges from punctiform and linear to wide - area defects.Reason for defect:Scale pits arise as a result of insufficient pressurized water descaling of the hot strip. This leads to scale being rolled in during the hot rolling process.��������������������������������������������������������������������������������������������������������������������������������������������������������������information for further please contact us!One Vision Drive, Natick MA 01760-2059Phone:+ 508 650-4141Fax: + 508 650-3344Internet: 。