Rolls for Cold Rolling
One of the most important parts of a cold rolling mill is the roll. A typical example of a modern, large-scale cold rolling mill is illustrated in Figure 1. In terms of its design, a particular difficulty lies in the selection of the appropriate roll, which requires the consideration of two critical points:
1. The roll has to be sufficiently wear resistant so as to ensure long-term reliability during operating cycles and
2. The roll has to be sufficiently ductile so as to not fracture during use and to ensure the required finish of the rolled product.
The function of the roll is to reduce the thickness of the metal and to develop compressive stresses on the surface of the metal, in order to improve the mechanical properties. This is achieved by mechanically squeezing or wearing down the metal during the rolling process. The efficacy of the process and the quality of the end product are determined by the characteristics of the roll.
Rolls for cold rolling mills are generally manufactured from materials that offer excellent wear resistance and toughness, such as alloy steels or high-chrome alloy steels. The rolls are generally heat-treated so as to develop a hard surface and high wear resistance. As the rolls are subject to considerable abrasion during their operating life, it is critical that the most suitable material is chosen for the purpose.
Rolls suited to cold rolling operations are usually of a semi-steel or ‘semi-solid’ type. Such rolls consist of a center core with a hard surface layer(s) applied to it that functions as the wear-resistant layer. The core may be a solid steel, or it may be a hollow steel block, with a filling material such as a bronze alloy, inserted in order to provide additional stability and mask minor manufacturing irregularities.
The surface layer of semi-steel rolls is usually applied by thermal spraying, a process by which a special electrode is first melted in an electrically heated up torch and then projected at high speed over the surface of the roll where it builds up and forms a homogenous, hard crust. The most common thermal spray materials for semi-steel rolls are high-chromium alloy or iron–chromium alloys. Depending upon the material used, this layer can range from several hundred micrometers to several millimeters in thickness. Furthermore, layer thickness and microstructure can be altered by post-thermal-spray heat treatment to further improve the performance of the rolls.
In addition to the semi-steel rolls, other types of cold rolling rolls may also be used. These include pearlitic-malleable cast rolls, simple castions and wrought iron middles, and blister steel rolls. Each of these rolls come with some drawbacks and benefits, so the selection of the most suitable roll must be made depending upon the specific applications and requirements.
To conclude, rolls for cold rolling mills play a crucial role in the manufacturing process. The selection of the appropriate materials and the process of their thermal treatment determine the performance and quality of the end product. It is important that the most suitable solution is used for each particular application in order to ensure optimal performance and reliability.
