Rolling Mill Stands
Rolling Mill Stands
The purpose of rolling is to convert material of large cross-sections of long work pieces into smaller sections of various shapes. This deformation is accomplished by applying compressive forces through a set of two rolls which rotate in opposite directions, the space between the rolls being less than the thickness of the entering material. In the rolling process, material passed between rolls is plastically deformed.
Rolling mills for rolling of steel differ in many aspects with each other. The rolling mills are of different sizes and capacities. The mills roll steel materials of different cross-sections, sizes and qualities and in material conditions which are either hot or cold. The mills have different configurations and speeds of rolling. The configurations of the mills can vary from cross country, reversing, semi continuous to continuous. The equipments of rolling mills can have manual operations, mechanical operations, electro-mechanical operations, pneumatic operations, hydraulic operations, or a combination of all of these. The controls provided in the mills can be manual controls, remote controls, instrumented controls, or fully automated controls. Further in many types of mills even heat treatment processes are integrated. In spite of the so many differences, all the rolling mills have in common some basic technologies and equipments.
Mill stands are the most essential equipment in the rolling mill process. A mill stand is normally fitted with a number of rolls through which the work piece bar is passed giving it the required cross-section. There are various designs of mill stands each having different applications to the rolling process.
All the rolling mills have rolls for the rolling of materials which are fitted in the roll stands. Roll stand configurations for the most common types of hot rolling and cold rolling mills are described below and shown in Fig 1. These stand configurations are representative of mills widely employed for large-scale production of hot rolled and cold rolled carbon steels. Specialty mills, used to cold roll higher strength and / or thinner gauge materials, such as stainless steels, frequently use cluster mill configurations which are also shown in Fig 1. Roll clustering is necessary to prevent excessive deflections of the rolls during the conditions of relatively higher loading, as is the case when cold rolling higher strength or thinner gauge materials.
Fig 1 Rolls configurations in the stands of different mills
Two-high mill stand – Two-high mill stand consists of two work rolls. The work rolls are pressed together with great force. It is the most commonly used rolling mill stand. The two- high mill stand has non-reversing rolls rotating in same direction. Hence, the work piece bar cannot be fed from the other side. Two-high stand can be either a horizontal stand or a vertical stand. Horizontal stand has both the rolls horizontally mounted while the vertical stand has both the rolls vertically mounted.
The two-high mill stand can be a two-high pullover stand. In the pull-over mill stand, the rolls rotate in only one direction. The work piece is returned over the top of the mill for further rolling. Two-high stand can also be a reversing mill stand. In this case the work piece bar can be passed back and forth through the rolls by reversing the direction of rotation.
Three-high mill stand – The three high mill stand is mainly used in hot rolling reversing mills without changing the direction of rotation of the rolls. There are three working rolls in this type of stand. In three-high mill stands, the top and bottom rolls rotate in the same direction, while the middle roll rotates in the opposite direction. This allows the work piece bar to be passed back and forth alternately through the bottom and middle rolls and then through the top and middle rolls without reversing the direction of roll rotation. First rolling takes place in one direction between lower and middle pair of rolls. Next the work is reversed in direction and fed through the middle and top pair of rolls. The roller tables on both the sides of the roll stand are raised or lowered depending on the rolling direction. This type of mill stand is used for the rolling of the shaped rolled products in grooved rolls.
Four-high mill stand – The four-high mill stand is mainly used in flat rolling mills. This roll stand accommodates two small diameter work rolls which are supported by two larger diameter backup rolls. The larger diameter backup rolls are used for supporting the smaller diameter work rolls. In fact, the backup rolls enable the possibility to reduce the diameter of the work rolls and thus lower the roll separating force, which is dependent on the contact area between the rolls and the work piece bar. The backup rolls are around square, which means that the diameter is as big as the roll face.
The alignment of the rolls in the horizontal direction depends on the type of rolling process. If it is a reversing mill stand the rolls are normally aligned with a common vertical centre point, but in a non-reversing mill stand the rolls have a small offset towards the exit side of the roll-bite. The reason for this is that for work rolls with a small diameter, and thus a low rigidity, the high torque in the roll bite causes them to deflect towards the entry side of the rolling mill. The backup rolls are normally made of softer material than the work rolls to prevent work roll markings. The four-high mill stand is the most popular type of mill stand in use since it can be used for many production steps such as breakdown rolling, finish rolling, and temper rolling. Typical 4-high stands showing various details are shown in Fig 2.
Fig 2 Typical 4-high stands showing various details
Pre-stressed mill stand – In a conventional rolling mill, the roller separating force which arises when the work piece bar is passed through the rolls is transferred through the backup rolls and their bearings to the mill house. In a pre-stressed mill stand, the two backup rolls are connected to each other by an element, pre-stressed to a value which is greater than the maximum roll separating force. The advantages of this type of mill stand are greater accessibility to the rolls and the roll bite, more rigid work piece guides and so on.
Universal mill stand –The universal rolling mill stand is a four way rolling stand used in the rolling of section normally beams, where in the section is shaped from all four sides. It consists of two vertically mounted rolls and two horizontally mounted rolls. The vertical rolls are idle and are arranged between the bearing chocks of the horizontal rolls in the vertical plane. Vertical rolls are used to control the width of the work piece bar simultaneously during rolling.
Six-high mill stand – Six-high mill stand has three rolls on either side of the work piece bar. The intermediate roll allows for a reduction of the work roll diameter further than can be achieved with the four-high mill, since the ratio of two adjacent roll diameters are kept low The intermediate roll can be equipped with a side-shift mechanism with which the rolls can be moved laterally in and out over the strip edge during rolling. Six-high mill stand eliminates most of the roll bending at the edges by lateral movement of the intermediate rolls to let the edge coincide with the edge of the work piece.
Y-mill stand – The Y-mill stand is not a popular mill stand. In this mill stand, there is a small work roll on the top-side of the work piece, supported by two levels of backup rollers which form a 90 degree angle with respect to each other and 45 degree angle to the work piece. On the bottom-side of the work piece bar there is a work roll which is comparable to the intermediate backup roll in size, supported by one backup roll. The purpose of this mill is to get as small a work roll as possible with rigid backup both in the vertical and the horizontal direction.
Cluster mill stand – Roll clustering is necessary to prevent excessive deflections of the rolls during the conditions of relatively higher loading, as is the case when cold rolling higher strength or thinner gauge materials. The cluster mill stand has two work rolls and four or more backup rolls. The number of backup or supporting-rolls depends upon the amount of support needed for the work (small diameter) rolls. Cluster mill is normally used in cold-rolling operation. The cluster mill stand can have six rolls, 12 rolls, or 20 rolls. The 20 roll configuration is also popularly known as Sendzimir mill.
In the cluster mill stand, the small and of equal size work rolls are supported in both the horizontal and vertical direction by two or more backup rolls. In a six high cluster mill stand, the each of the two work rolls has two backup rolls placed in 45 degree angle with respect to the work piece and 90 degree angle with respect to each other. There are no intermediate rolls. In this mill, the smallest diameter of the work rolls which can be used is reached when the horizontal tangent of the work rolls coincides with the horizontal tangent of the backup rolls. A method to decrease the diameter even further is to insert intermediate rolls and organize the mill in a pyramid fashion.
Z-high mill stand – The Z-high mill stand roll configuration is a compromise between the roll configurations of four-high mill and the 20-high cluster-mill. In the four-high mill, the work rolls can be too big for some applications so that the roll separating force can reach too high values. On the other hand, in the 20-high mill, the work rolls for a mill of the same size can be too small, which introduces skidding. In a Z-high mill, the diameter of the work roll is somewhere in-between the diameters of its four-high and 20-high counterpart. The Z-mill can be described as a six-high mill with side-support to prevent deflections due to torque. As with the case of the 20-high cluster mill, no spot cooling is possible in the Z-high mill, which means that there is no control of the thermal crown. On the other hand, the stiffer mill-modulus of the Z-high offers improved gauge accuracy compared to the four-high mill.
Planetary mill stand – Planetary mill stand has a pair of heavy backup rolls surrounded by a large number of planetary rolls. Each planetary roll gives an almost constant reduction to the work piece as it sweeps out of a circular path between the backup roll and the work piece bar. As each pair of planetary rolls ceases to have contact with the work piece bar, another pair of rolls makes contact and repeat the reduction.
Some types of rolling mill stands are shown in Fig 3.
Fig 3 Types of rolling mill stands
Roll stand housings
Roll stand housings are needed for the setting of the rolls in correct vertical and horizontal position and are important elements of the roll stand. These are elements in a rolling mill, which house the chock assemblies, the adjusting and other mechanisms, and retain them in their proper positions. Their construction and dimensions thus have to take into account the sizes of various other elements. There are screw down mechanism and the roll counterbalancing device along with their drives in the roll stand housings. The housings need high rigidity, sufficient strength for taking the loads, simplicity of design and minimum cost of production. Also, there need to be easy access to all parts of the housing and other details of the roll stand.
Roll stand housings create a framework of the rolling mill stand and for absorbing the total metal pressure on rolls during the process of rolling. Hence, they are to be solid and their structure is to enable easy and fast roll changing. The forces, which act on the rolls during rolling, are completely transferred on to them through the nut of the adjusting mechanism. In addition, there exists a tendency for the stand to return as a result of the torques acting on the rolls, which get transmitted to the frame in case of bearing seizures or when rolls are unable to pass the metal due to lack of sufficient power . Hence, the housings are to be adequately clamped to the foundation to withstand the overturning moments. The frame is also to be checked for stresses as well as rigidity.
From the structural view point, the housings can be classified into three types. These are (i) closed type housings where the whole housing is made of one piece, and (ii) open type housings which have the separated cap.
Closed type housings are one piece cast housings of simple form (rectangular section) are used for heavy roughing mills. These housings are either cast housings or welded housings fabricated from forged steel. Fabricated housings can be made quickly without waiting for patterns and castings. Many old cast iron housings are extremely heavy since they are not forged, and due to inherent imperfections during the casting process, had to be twice as large as a superior forged housing. Such housings are to be made very heavy because of the possibility of cracking. This is very common with large cast housing.
Open type housings have a removable top connected to the housing by screws for easier rolls changing. Open type housings facilitate easy removal of rolls. When the housings have to withstand large horizontal forces, the pillars are made with an I-section. The integrally cast construction presents many difficulties in manufacturing. In such case the housing can be made up of two forged pillars bolted to two cast cross beams.
The conventional rolling mill stand besides the roll stand consists of mill spindles, a pinion stand, a gear reducer and a motor. In case of constant pass line, the roll stand slides on the surface of the sole plate.
Housing less mill stands are a pre-stressed stand which is more rigid than the conventional stands. These stands have rigid roll chocks held together solid and pre-stressed joints. The housing-less roll stand has limited stress relaxation (spring-back) of rolls. The stand housings are smaller in size and have lighter structure. The roll chocks are free floating on two tension screws on each side of the mill stand, one is right hand threaded and another one is left hand threaded. This mechanism ensures mathematical opening and closing of the roll gap related to the pass line. The roll changing is easy and much quicker with a roll changing device which pulls out the complete roll changing assembly and replaces the cartridge. The housing less stands are normally arranged in horizontal-vertical no twist arrangement which allows no twist rolling which permits larger reduction and smoother rolling as it eliminates twisting oval into round passes as in the conventional oval – to – round sequence which results in less guide wear and simpler guide design.
Cantilever compact mill stands are employed in a wide range of sizes for a vast variety of applications. Preferred fields of application are namely (i) single strand mill lines in horizontal/vertical arrangement, (ii) In split intermediate trains of two or more strand mill lines, (iii) as replacement stand for mill train modifications (e.g. in the case of space problems), (iv) as refinisher stands in wire rod delivery sections, and (v) in finishing mill stand.