Rolling of Steel in small and medium sized Rolling Mills Jun09

Rolling of Steel in small and medium sized Rolling Mills...

Rolling of Steel in small and medium sized Rolling Mills Rolling of steel consists of passing the material, usually termed as rolling stock, between two rolls driven at the same peripheral speed in opposite directions (i.e. one clockwise and the second anti-clockwise) and so spaced that the distance between them is somewhat less than the thickness of the steel section entering them. In these conditions, the rolls grip the material and deliver it reduced in thickness, increased in length and probably somewhat increased in width. This is one of the most widely used processes among all the metal forming processes, because of its higher productivity, close control of the rolled product, and lower operating cost. Rolling is able to produce a product which is having constant cross section throughout its length. Many shapes and sections are possible to roll by the steel rolling process. Rolling of steel is a metal forming process used 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 surface of the steel material. The steel material gets squeezed between the pair of rolls mounted in a roll stand, as a result of which the thickness of the steel being rolled gets reduced and the length is increased. Steel sections are generally rolled in several passes, whose number is determined by the ratio of initial input material and final cross section of finished product. The cross section area is reduced in each pass and form and the size of the rolling stock gradually approach to the desired profile. Mostly, rolling is done at high temperature, which is called hot rolling, because of requirement of large deformations....

Refractories for a Reheating Furnace...

Refractories for a Reheating Furnace Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. They are the materials which are resistant to heat and exposure to different degrees of mechanical stress and strain, thermal stress and strain, corrosion/erosion from solids, liquids and gases, gas diffusion, and mechanical abrasion at various temperatures. In simplified language, refractories are considered to be materials of construction which are able to withstand high temperatures. The general requirements from the refractories for are (i) ability to withstand high temperatures and trap heat within a limited area such as a reheating furnace, (ii) ability to withstand sudden changes of temperature, (iii) ability to withstand load at service conditions, (iv) ability to withstand chemical and abrasive action of the materials such as liquid metal, liquid slag, and hot gases etc. coming in contact with the refractories, (v) ability to resist contamination of the material such as scale etc. with which it comes into contact, (vi) ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling, (vii) ability to conserve heat, (viii) ability to withstand load and abrasive forces, and (ix) to have low coefficient of thermal expansion. Properties of the refractories can be classified to resist four types of service stresses namely (i) chemical, (ii) mechanical, (iii) thermal, and (iv) thermo-technical. A suitable selection of the refractories for the lining of the reheating furnace can only be made with an accurate knowledge of the refractory properties and the stresses on the refractories during service. The relationship between service stresses and important properties of the refractories are at Tab 1.  Tab 1 Relationship between type of stress and refractory property Sl.No. Type of stress Important refractory property 1 Chemical...

Important Issues related to Re-rolling mills in SME Sector...

Important Issues related to Re-rolling mills in SME Sector The steel sector in India consists of four branches namely (i) multi-million tons integrated steel plants, (ii) mini integrated steel plants, (iii) individual units of steel melting producing pencil ingots, and (iv) re-rolling mills which are rolling steel from billets, pencil ingots and scrap steel. Most of the units under last two categories are in small and medium enterprise (SME) sector. All the four branches which constitute the steel sector play a very important role in the steel economy of the country. Re-rolling mills has a considerable contribution to the steel production of the country.  There are around 1800 re-rolling mills in SME sector in India. The rolling capacities of these mills are mostly in the range of 8,000 tons per year to 80,000 tons per year. On a very conservative estimate, these mills contribute more than 20 % of the total production of finished steel in the country. Hence, these mills play a very important role in supporting the steel economy of the country. The conditions of the most of the re-rolling mills can be termed as not healthy. They operate with very old technologies and operate at low level of productivities and high level of energy consumptions. There are many problems and challenges associated with these mills. Several agencies have carried out detailed studies of the steel and re-rolling units of SME sector. Some of these studies have identified certain barriers associated with these mills (Fig 1) which include (i) high energy consumption due to the use of obsolete technologies, (ii) lack of awareness and knowledge on several fronts, (iii) lack of proper operational procedures, (iv) a negative mind-set among owners of the mills, and (v) lack of finance to effect necessary changes...

Heating of Steel in Reheating Furnace Jun01

Heating of Steel in Reheating Furnace...

Heating of Steel in Reheating Furnace Reheating furnace is important equipment in the process of hot rolling. It is the heart of any hot rolling mill. Reheating of steel is a continuous process. The steel material to be rolled is charged at the entrance of the reheating furnace. The steel material is pushed forward on the hearth of the furnace by means of a pusher machine whose ram is in direct contact with the steel material. The steel material is pre-heated, heated and soaked as it passes through pre-heating, heating and soaking zone of the reheating furnace. At the end of the soaking zone of the furnace, the steel material is discharged from the furnace by ejector for rolling in the rolling mill. The temperature of the heated steel material at the time of discharged depends on several factors and it can vary in the range of 1100 deg C to 1250 deg C. The size of the reheating furnace is normally expressed as the capacity to supply the rolling mill with sufficiently hot steel, in tons per hour. Steel materials with different material compositions, dimensions, and charging temperatures can reside in the furnace simultaneously. The reheating furnace used for heating the steel materials is normally considered to be having high energy consumption. It also emits good amount pollutants in the atmosphere because the process used for heat generation is the combustion process. Reheating process has considerable influence on the economics of the working of the rolling mill. There are usually three types of continuous reheating furnaces used in the rolling mills. These are (i) pusher type furnace, (ii) walking hearth furnace, and (iii) walking beam furnace. Pusher type furnaces have some disadvantages which includes (i) frequent damage of refractory hearth, (ii) skid marks...

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...