Rolls in Hot Strip Mills
Rolls in hot strip mills are very different in design and concept. The knowledge of the specific rolling conditions and the appropriate roll materials are critical for achieving the longest service life of the rolls. Hot strip mills consist of several sequential stand with each stand having two working rolls and one backup roll.
The most important factor responsible for determining the quality of the product rolling is the quality of the rolls. The rolling influence is mostly on the surface roughness, shape accuracy, flatness and profile of the strip which are in turn very much dependent on the surface finish of the working rolls and backup rolls. The rolls are subject to wear in the form of eruptions, flaking and plastic deformation. This wear increases with increasing rolling loads, increasing percentage of scale on the material, the presence of sulfur in the material and hot rolling of the strip at higher temperature.
Rolls need to be chosen in an appropriate size and quality to be able to withstand the rolling forces and the related wear on their surface. Rolls should also be able to cope-up with the changes in the rolling conditions like temperature, rolling speed, roll force, etc. In other words, they should perform with a good combination of rigidity and toughness which are affected by the metallurgical microstructure present in them.
The modern hot strip mills use different kinds of rolls for their operation depending upon the product rolling and the rolling conditions. They include backup rolls, work rolls, intermediate rolls, profile and edging rolls, tension leveler rolls, etc. These rolls are made from different kinds of materials like high chrome iron, cast steel, forged steel, wear resistant alloys, etc. Each have their own advantages and disadvantages and various rolling mills have adopted suitable pair of working and backup rolls.
The quality of these rolls depends on the quality of the material used for manufacturing them and rest on the thermal treatment given to them. These rolls are usually subject to mechanical treatment like sand-blasting, grinding, and polishing. Other processes like hard-facing and nitriding can also be used to improve the surface finish. The clearances between the rolls are also one of the key factors that affect the product rolling.
High-pressure water jets are used to cool down the many parts like roll-sleeve, Anti-wrinkle bars, etc. inside the housing during their operation. This cooling system helps in reducing the wear and tear on the rolls and improves the service life.
To summarize, rolls play a very critical role in controlling the performance of the hot strip mill and thus the quality of the product rolling. Their selection, design, and metallurgical quality are key factors which should be taken care of. Through proper selection and maintenance of rolls, it is possible to improve the reliability and service life of the rolls which in turn makes the life of the strips longer and also contributes to the overall efficiency of the hot strip mill.
