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...

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,...

Cold Rolling of Steels Oct08

Cold Rolling of Steels...

Cold Rolling of Steels  The primary purpose of cold rolling of steels is to reduce the thickness of the hot rolled steel strips (normally in the range of 1.5 mm to 5 mm) into thinner thicknesses (usually in the range of 0.12 mm to 2.5 mm) which cannot be normally achieved during hot rolling in a hot strip mill. Besides reduction in thickness cold rolling is done for improving the surface finish of steels, for improving the thickness tolerances, for offering a range of ‘tempers’, for improving the physical characteristics, and for preparing the strip for surface coating. Cold rolling makes the cold rolled sheets a much improved product. Cold rolled steel products offer good control of thickness, shape, width, surface finish, and other special quality features that compliment the need for highly engineered end user applications.  To meet the various end user requirements, cold rolled sheets are metallurgically designed to provide specific attributes such as high formability, deep drawability, high strength, high dent resistance, good magnetic properties, weldability, enamelability, and paintability etc. Cold rolling of hot rolled steel strips is done below the recrystallization temperature normally at room temperature. In cold rolling process, usually no heat is applied to the hot rolled strip before rolling.  However, frictional energy at the contact surfaces of the strip being rolled gets converted into heat. This heat may increase temperature of the strip being rolled in rapid adiabatic process to a level of 50 deg C to around 250 deg C. During cold rolling process the reduction in thickness is due to the plastic deformation which occurs by means of dislocation movement. Steel gets hardened because of the buildup of these dislocations. This increases strength and strain hardening upto 20 %. These dislocations reduce the ductility of the...