Mill Scale

Mill Scale Mill scale is the product of oxidation which takes place during hot rolling. The oxidation and scale formation of steel is an unavoidable phenomenon during the process of hot rolling which involve reheating of steel in a reheating furnace, multi-pass hot rolling and air-cooling in the inter-pass delay times and after rolling.  Mill scale is usually removed by process water used for descaling, roll and material cooling, and by other methods. It is subsequently separated by gravity separation techniques. The formation of oxide scale not only results in a significant loss of yield of steel, but also deteriorates the surface quality of the steel product caused by rolled-in scale defects or roughened surface. In addition, the presence of a hard scale layer on the steel can have an adverse effect on roll wear and working life. The amount of mill scale generated in a rolling mill depends on the type of the reheating furnace and on the practice of rolling adopted in the mill. It is generally in the range of 1 % to 3 % of the weight of the steel rolled. Mill scale mill scale is a layered and brittle material, composed of iron oxides with wustite as a predominant phase. It is normally considered as waste material. From the chemical and physical analysis performed on the mill scale, and with respect to the environmental concerns, mill scale is considered to be non-dangerous waste and normally considered as a green waste. Scale formed during the heating of steel to rolling temperatures in the reheating furnace is known as primary scale. This primary scale is removed generally by hydraulic descaling before hot rolling. The removal of the primary scale formed during the reheating operation before hot rolling is usually done for...

Rolling of hot strip and strip defects Jan14

Rolling of hot strip and strip defects...

Rolling of hot strip and strip defects Rolling is one of the most important industrial metal forming operations. Hot Rolling is employed for rolling of steel slabs to hot strips in hot strip mills.  Like any other hot rolling operations, rolling of hot strip is also a plastic deformation of the material of the slab caused by compressive force applied through a set of rolls. The cross section of the slab is reduced by the hot rolling process. The material gets squeezed between a pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Rolling of hot strip is done at high temperature because of requirement of large deformations. Hot rolling results in residual stress free strip. Normally, oxide scaling is accompanied with the hot rolling, due to which dimensional accuracy is inferior when compared with the dimensional accuracy of the cold rolled strips. For rolling of hot strip, the slabs are heated initially at 1100 deg C to 1300 deg C. The temperature in the last finishing stand of the hot strip mill varies from 700 deg C to 900 deg C. It is always to be above the upper critical temperature to produce uniform equiaxed ferrite grains. The objective during the rolling of hot strip is to decrease the thickness of the slab with an increase in the length and with very little increase in the width. The material in the centre of the strip is constrained in the direction across the width of the strip and the constraints of undeformed shoulders of material on each side of the rolls prevent extension of the strip in the direction of the width. This condition is known as plane strain. The material therefore gets longer and not...

Metal Forming Processes...

Metal Forming Processes Metal forming processes consists of deformation processes in which a metal work piece (billet, bloom, or blank) is shaped by tools or dies. The design and control of such processes depend on the characteristics of the material of the work piece, the requirements of the finished product, the conditions at the interface of the tool and the work piece, the mechanics of plastic deformation (metal flow), and the equipment used. These factors influence the selection of geometry and material of the tool as well as processing conditions (examples are temperatures of die and work piece and lubrication). Since many of the metalworking operations are rather complex, models of various types, such as analytical, physical, or numerical models, are often used to design these processes. A brief historical view, a classification of metalworking processes and equipment, and a summary of some of the more recent developments in the field are described below. Historical view Metalworking technology is one of three major technologies used for the fabrication of the metal products. The other two are casting process and powder metallurgy (P/M) technology. It is possibly the oldest and most established of the three technologies. The earliest records of metalworking show that the simple hammering of gold (Au) and copper (Cu) was practiced in various regions of the Middle East around 8000 BCE. The forming of these metals was crude since the skill of refining by smelting was not known and since the ability to work the material was limited by impurities that remained after the metal had been separated from its ore. With the start of Cu smelting around 4000 BCE, a useful method became available for purifying metals through chemical reactions in the liquid state. Later, in the Cu age, it was...

Basics of Rolling of Steel Nov21

Basics of Rolling of Steel...

Basics of Rolling of Steel  Liquid steel is usually cast in continuous casting machines in the shape of billets, blooms, or slabs. In some plants, it is also being cast in continuous casting machines in the shape of thin slabs or dog bone sections. These shapes are processed by hot rolling by passing them through plain or grooved cylindrical rotating rolls to produce plates, sheets, rods, structural sections, and tubes etc. Rolling process is one of the most important and widely used industrial metal forming operations. It provides high production and close control of the final product. It was developed in late 1500s. It accounts for 90 % of all metals produced by metal working processes. Rolling of steel is a metal forming process in which steel is passed through a pair of rotating rolls for plastic deformation of the steel. Plastic deformation is caused by the compressive forces applied through the rotating rolls. High compressive stresses are as a result of the friction between the rolls and the steel stock surface. The steel material gets squeezed between the pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Rolling is classified according to the temperature of the steel rolled. If the temperature of the steel is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the steel is below its recrystallization temperature, the process is termed as cold rolling. The rolls run on massive neck bearings mounted in housings of enormous strength and driven by powerful electric motors. These are known as mill stands. A rolling mill stand contains two or more rolls for plastic deformation of steel between rotating rolls. It basically consists of (i) rolls, (ii) bearings,...

Rolling Process for Steel Mar27

Rolling Process for Steel...

Rolling Process for Steel Rolling is the process of plastically deforming steel by passing it between rolls. Rolling is defined as the reduction of the cross sectional area of the steel piece being rolled, or the general shaping of the steel products, through the use of the rotating rolls. Rolling of steel is one of the most important manufacturing processes for steel. It is usually the first step in the processing of steel after it is made and cast either in Ingot or continuous cast product in a steel melting shop. The initial rolling of steel is done in a hot rolling mill where blooms and slabs are rolled down to various rolled products such as plate, sheet, strip, coil, billet, structures, rails, bars and rods. Cold rolling of steel is also carried out for some products. Many of these rolled products such as rails and reinforcement bars etc. are directly used by the consumers while the other rolled products are the starting raw materials for subsequent manufacturing operations such as forging, sheet metal working, wire drawing, extrusion, machining, and fabrication industry. Steel rolling can produce a wide range of products. The width of a rolled product can vary from a few millimeters to several meters while the thickness can vary from 0.1 mm to more than 200 mm. The rolled section can be square, rectangular, round or shaped sections. Different rolling processes for steel are shown in Fig 1 Fig 1 Rolling processes of steels Principle of rolling steel During rolling, steel work piece is subjected to high compressive stresses as a result of the friction between the rolls and the surface of work piece being rolled. The work piece is plastically deformed by the compressive forces between two constantly rotating rolls. These...