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 forces act to reduce the thickness of the steel and affect its grain structure. The reduction in thickness which is the difference in the thickness before and after the reduction is known as draft. In addition to reducing the thickness, the rolls cause feeding of the material as they rotate in the opposite direction to each other. Friction is therefore a necessary part of the rolling process, but too much friction can be detrimental for a variety of the reasons. Since level of friction is to be controlled in the rolling process, lubrication is an important factor during rolling. For the work piece to enter the throat of the roll, the component of the friction force must be equal to or greater than the horizontal component of the normal force.
During the rolling operation, the geometric shape of the work piece is changed but its volume remains essentially the same. The roll zone is the area over which the rolls act on the material. It is here the plastic deformation of the work piece takes place. An important factor in the rolling process is that due to the conservation of the volume of the steel material with the reduction of the thickness, the material coming out of the roll zone will be moving faster than the steel material entering the roll zone. The rolls themselves rotate at a constant speed, hence at some point in the roll zone the surface velocity of the rolls and that of the steel material are exactly the same. This is termed as the no slip point. Before this point, the rolls are moving faster than the material, and after this point the material is moving faster than the rolls.
Torque and power are the two important component of rolling. Torque is the measure of the force applied to the rolls to produce rotational motion while power is applied to a rolling mill by applying a torque to the rolls and by means of work piece tension. In a rolling mill the power is spent principally in the following four ways.
- The energy needed to deform the steel.
- The energy needed to overcome the frictional force.
- The power lost in the pinions and power transmission system.
- Electrical losses in the various motors.
Sometimes during rolling of steel, tension (force) is applied to the work piece as it is being rolled. The tension may be applied to the front (front tension), may be applied to the back (back tension), or may be applied at both the ends. This technique helps the forces necessary for rolling of the steel.
During process of rolling, the plastic deformation, which is causing the reduction in the thickness of steel piece, also causes an increase in the width of the work piece. This phenomenon is known as spreading. When the work piece being processed has a high width to thickness ratio, then the spreading is not of much concern since it is relatively small. On the other hand in case of low width to thickness ratio, the increase in width can be an issue. Vertical rolls (edge rolls) are used to control the spreading and for maintenance of constant width during rolling.
Rolling can be done either by hot rolling or by cold rolling. Cold rolling usually follows the hot rolling.
During hot rolling of steels, the cast grain structure of steel achieved during the steelmaking process (usually large grains grown in the direction of solidification) is converted into a wrought grain steel structure. Cast structure has weak grain boundaries which makes the steel brittle. Cast structures are also associated with many defects like porosity, shrinkage cavities and inclusions. During hot rolling of steel which takes place above the recrystallization temperature, the cast grain structure is broken. Old grain boundaries are destroyed and new tougher grain boundaries are formed along with a more uniform grain structure. Rolling of steel also closes the vacancies and shrinkage cavities within the steel and breaks the inclusions and distributes it uniformly throughout the work piece. The distinctive mark of hot rolling is not only a crystallized structure, but the simultaneous occurrence of dislocation propagation and softening processes. Advantages of hot rolling are as follows.
- Since flow stresses are low, forces and power requirements are relatively lesser. Even very large work pieces can be deformed with equipment of reasonable size.
- Since ductility is high, large deformation levels are possible.
- Complex shapes can be rolled’
Cold roling is done at room temperature, although the work of deformation can raise the temperature of the work piecce to 100 -200 deg C. During cold rolling of steels, good surface finishes and increased mechanical strength with close control of product dimensions can be achieved. The advantages of cold rolling are given below.
- In the absence of cooling and oxidation, tighter tolerance and better surface finish can be obtained..
- Thinner sections can be rolled.
- The final properties of the work piece can be closely controlled. If desired the high strength obtained during cold rolling can be retained. or if high ductility is needed, grain size can be controlled before annealing.
- Lubrication is easier in general.
Rolling of steel is done not only to achieve the desired cross section but also to get the desired properties of steel. Rolling of steels imparts strength and favourable grain orientation. Further heat treatment processes incorporated during controlled rolling helps in modification of steel microstructure to give the steel desired properties.
Controlled rolling is a type of thermo mechanical processing which integrates controlled deformation and heat treating. The heat which brings the work piece above the recrystallization temperature is also used to perform the heat treatments so that any subsequent heat treating is unnecessary. Types of heat treatments include the production of a fine grain structure; controlling the nature, size, and distribution of various transformation products (such as ferrite, austenite, pearlite, bainite and martensite in steel), inducing precipitation hardening, and controlling the toughness. In order to achieve, the entire process must be closely monitored and controlled. Common variables in controlled rolling include the starting material composition and structure, deformation levels, temperatures at various stages, and cool-down conditions. The benefits of controlled rolling include better mechanical properties and energy savings.
Rolling process allows a high degree of closed loop automation and very high speeds, and is thus capable of providing high quality, close tolerance starting material for various down stream industries.
Rolls used in rolling mills are of various sizes and geometries. Rolls used for rolling undergo extreme operating conditions during the rolling process. These conditions include tremendous forces, bending moments, thermal stresses and wear. Roll materials are selected for strength, rigidity, and wear resistance. Roll materials vary and are dependent upon the specific rolling process. Common roll materials used are cast iron, ductile iron, cast steel, and forged steel. Forged steel rolls are stronger and more rigid than the cast iron rolls but have complicated manufacturing process. The composition of iron and steel is selected to suit the rolling process. Nickel steels or molybdenum steel alloys are used as material for rolls for certain rolling processes. In some other rolling processes, rolls are made of tungsten carbide which can provide extreme resistance to deflection.
Maintaining a uniform gap between the rolls is difficult because the rolls deflect under the load required to deform the work piece. Strength and rigidity are important characteristics of the rolls used for steel rolling. During the process of rolling, large forces act on the rolls. Due to these forces rolls are subjected to different degrees of deflection. In case of flat rolling where the widths are larger the effect of deflection is more. The rolls initially are flat. During the rolling operation, the work piece exerts greater force on the rolls towards the centre of the work piece than at the edges. This causes rolls to deflect more at the centre, and hence gives the work piece greater thickness at the centre. To overcome this issue, the rolls are ground so that they are thicker towards the centre in such a way so as to offset the deflection that will occur during the process. This extra thickness is called the camber. Camber that must be ground into a roll is very specific to a particular width and material of the steel work piece and force load. A roll with a camber is also called a crowned roll (parabolic crown). The crowned roller only compensates for one set of conditions, specifically the material, temperature, and amount of deformation.
Other methods of compensating for roll deformation include continual varying crown (CVC), pair cross rolling, and work roll bending. CVC involves grinding a third order polynomial curve into the work rolls and then shifting the work rolls laterally, equally, and opposite to each other. The effect is that the rolls will have a gap between them that is parabolic in shape, and will vary with lateral shift, thus allowing for control of the crown of the rolls dynamically. Pair cross rolling involves using either flat or parabolically crowned rolls, but shifting the ends at an angle so that the gap between the edges of the rolls will increase or decrease, thus allowing for dynamic crown control. Work roll bending involves using hydraulic cylinders at the ends of the rolls to counteract roll deflection.
Another way to overcome deflection issues is by decreasing the load on the rolls, which can be done by applying a longitudinal force; this is essentially drawing. Other method of decreasing roll deflection includes increasing the elastic modulus of the roll material and adding back-up supports to the rolls.