Main Features of a Modern Bar and Light Section Mill
Main Features of a Modern Bar and Light Section Mill
The objective of a bar and light section mill is to reheat and roll steel billets into bars and light sections. The production of bar and light sections in these mills is subject to constant change. There is growing demands on the quality of these products as well as on the flexibility and cost effectiveness of these mills. This has necessitated the development of new and innovative technologies and processes. Modern bar and light section mills are high speed mills capable of rolling bars and light sections of special bar quality grades and engineering steels at high production rates, while keeping investments and operating costs at the reasonable levels.
Modern bar and light section mills are expected to meet the following requirements.
- High mill availability coupled with high productivity and high yields.
- Meeting the need of low maintenance.
- Meeting the need of lower energy consumption.
- Close dimensional tolerances.
- Negative tolerances (In sectional weight).
- No variation in dimensions throughout the length.
- Uniform physical properties.
For achieving these demanding requirements, many important features are incorporated in the modern bar and light section mills. Some of these are described below.
- Reheating furnace – Modern bar and light section mills are equipped with energy efficient walking beam furnaces which are normally computerized controlled. These reheating furnaces uniformly heat the billets to the target temperatures at the required production rates and without skid marks and without cold spots. These furnaces are capable of receiving cold or hot billets as the charge material in the furnace. The modern reheating furnaces have the features of (i) superior heated billet quality, (ii) better heating efficiency, (iii) very low fuel consumption, (iv) minimum scale loss, contributing to achieving high material yield, (v) low decarburization and hence suitable for higher quality steel grade, and (vi) maximum operation flexibility and good working conditions even at low productivity.
- Housingless roll stand – The housing less (HL) roll stands are used normally in roughing and intermediate group of stands in modern bar and light section mills. The modular design permits the use of HL stand cassettes in all possible configurations such as horizontal, vertical, tiltable and universal configuration. The stand sizes differ, depending on the necessary dimensions of the rolls and roll journals, pass schedule, pass form as well as the gearbox and motor characteristics. The main features of the HL stands are compactness and rigidity of components, low roll bending modulus, durable multi row roller bearing with self aligning chocks under load, backlash free balancing of chocks, roller beams designed for simple and exact adjustment of guides and guards etc. The advantages of theses stands include (i) saving in the depth and size of the foundation (ii) the rolled product meets the required form and dimensional tolerances, (iii) there is time savings for stand changes as the roll changing takes place outside the rolling line, (iv) there is significant reduction in time for maintenance due to lesser number of components and easier accessibility, (v) automated roll gap adjustment, and (vi) operational flexibility since the same stand unit can be used in any position.
- Reducing sizing mill (RSM) – It is a versatile sought after rolling technology. It is also known as precision sizing mill. From existing conventional rolling mills, it is difficult to meet the requirements of close tolerances. This can only be met with difficulty and at the expense of loss of efficiency, especially with regard to the loss of mill utilization time and lower yields. At times this is not feasible or cost prohibitive. In conventional roughing and intermediate mills, the tolerance of the finished product is influenced mainly by the variations in the cross section of the feed material into the finishing section of the mill. RSM takes advantage of the special features of the 3-roll technology, in which the spread during deformation is low and the deformation efficiency is high. The advantages of the reducing sizing mill are many. RSM is installed in the mill line for the purpose of rolling any desired finish size to very close tolerances. It is possible to adjust each roll gap under load and it can be fully automated.
- Thermo-mechanical rolling – It is also known as low temperature rolling and is basically a method for on line control of the final material properties during the rolling process. It involves material deformation applied at the last passes of the mill, within the temperature ranges corresponding to partial recrystallization or to suppression of recrystallization. Due to it superior quality product with improved metallurgical and mechanical properties can be obtained directly at the mill itself just by operating at lower rolling temperature. As soon as recrystallization is suppressed, grain refining phenomena occurs, resulting in improved technological properties of the final product. In addition, the surface quality improves considerably. The advantages of thermo-mechanical rolling are fine grain size, avoidance of off line normalizing, improved low temperature toughness, better properties after heat treatment for case hardening steels, shorter annealing time for spring steel, improved fatigue strength on the final component, higher tensile strength for micro-alloyed steels achieved directly in-line, and reduced decarburizing depth etc.
- Walking rack type cooling bed – The purpose of the cooling bed of a movable rack design is to uniformly air-cool the rolled bars or light sections and transport the same in a phased manner from the entry of the cooling bed to discharge side. The front ends of the bars and light sections are also leveled at the discharge side and a fixed number of rolled pieces sent for final length cutting by cold shear and bundling or stacking. The movable rack type cooling bed is normally of a walking beam design. The mechanism ensures that the bars and light sections are uniformly positioned over the toothed racks. The cooling bed is usually designed considering the smallest and the maximum size of the bars and light sections being rolled, delivered from the finishing stand of the mill, and the cooling time required for the various sizes of the bars and light sections. Rack type cooling bed design depends on bars cut previously to given lengths, to slow them down, to transport them crosswise over a cooling surface ensuring that the rolled bars or light sections in very wide range of lengths, are kept as straight as possible, to collect the bars or light sections at the end of the cooling surface to predetermined packs matched to the requirements of the cold shear, and to discharge finally same onto a roller table which conveys the packs to the cold shear. A typical cooling bed is shown in Fig 1
Fig 1 A typical cooling bed
- A number of solutions are available for the finishing of bars and sections in the finishing area of the mill. A typical bar mill would include a cold shear with gauge beam while a section mill would have a straightening machine feeding to cold shear. Correct layer preparation is the key to productivity and this is achieved by profile feeding system. Flying type cold shears are also used when the rate of productions are high from the mill. Multi line straighteners are used at high productivity rates. The concept is to straighten cooling bed lengths in order to have less feeding operations and better utilization of the straightening roll drives. Proper alignment and centering of the bars under the rolls is essential. The recent improvements in this area are (i) use of automatic section feeding to the straighteners, (ii) quick change of roll sets mounted on a stand by carriage, motorized roll gap arrangement, and (iv) the whole unit is mounted on a platform that can be shifted out of the line for maintenance without stopping mill production. Further in order to pre-align the bar layer on the cooling bed run out roller table, a chain transfer and a carriage type extraction system is usually provided so that the bars are inched out of the bed racks at the required centerline distance between the bars and kept this way by gentle depositing on the run out roller table with the carriages.
- The bundling and stacking section of the mill can also have many solutions. A typical solution consists of a simple bundling machine while for sections magnetic stackers are the norm. All the operations are to be mechanized and automated including the removal of the short bars or the labeling in ideal scanning position of tags. Special care is normally given to the final shape of the bundles, with optimum arrangement of the bars and sections. Stackers can have different designs depending of the requirement. For precise stacking of the light sections the overhead pendulum system is used while for light medium sections, the stacking system with magnets underneath is used.
- The bar counting system consists of automatic bar counting which operates on an optical principle and separation system for forming bundles. The separation system is composed of three fixed chain transfer devices between which the counting system is installed. The optical device together with a pulse generator installed on the chain transfer drive performs the counting and recording of Each single bar in transit without overlapping or double reading.
- Bar and section mills have tying and strapping machines for bundles and piles. These machines are designed for continuous operation, Tying machines use commercial size wires cfor tying and the machine head is hydraulically operated. The strapping machines are pneumaticaly operated and use commercial steel straps of different available width. Strapping can be carried out either by clamping or welding.