Abstract:
In bar and wire rod mills, rolling procedures featuring three- and four-roll technologies are state-of-the-art to produce high-quality long products.
Therefore, the development of pass sequences for these processes plays an important role in the rolling industry. The assessment of a rolling process is bound to a reliable rolling model, which is suitable for the calculation of the force and power demands of the rolling process to ensure that limiting conditions of the rolling mill are not overridden. For rolling of long products, a suitable pass sequence has to be designed in order to meet the requirements of dimensional tolerance, as well as surface quality.
While a rolling model for the three-roll process including an equivalent pass method was already presented by the author, pass design methods are dealt with in the current work. The geometry of the rolled product and roll grooves are described mathematically using functional contours, allowing geometric-numerical methods to be used for manipulation and deformation simulation of the rolled product. For a given pass, the output section is constructed using these methods.
The very important spread calculation is based on the two-roll procedure and is adopted for the three- and four-roll processes with suitable correction functions. Here, also the temperature and material dependency of the lateral spread for a given material database is taken into account.
The geometric calculation of each pass is carried out iteratively, as to satisfy the given boundary conditions of perfect contact conditions between rolls and rolled material, as well as the desired output material shape.
Roll force, torque and rolling power are calculated using the mathematical rolling model presented before.
Interstand tensions are discussed as influencing parameters on the force, torque and power demands, as well as on the section shape. A mathematical model for the influence of interstand tensions on lateral spread is incorporated in the model.
To complete the pass design model, elastic mill stand feedback is considered in terms of a fixed elastic modulus for each rolling stand, the elastic feedback and therefore cross-sectional variation due to elastic mill spring being calculated iteratively from the roll force by application of the Gagemeter equation.
The interaction of section height and width faults due to elastic mill spring and spread variations is analysed in detail and the consequences for the roll pass design are shown.
The calculated results are presented in terms of an interactive software, where the roll pass designer or rolling mill engineer can check the calculated geometries and change the calculation parameters before recalculating the pass sequence, if necessary. Additionally, numerical output is given for all interesting deformation characteristics of the rolling procedure under consideration.
The model is used to discuss different philosophies of pass design for the three- and four-roll rolling procedures. The different pass designs are compared to one another, dealing with different rolling conditions as varying rolling temperatures, rolling speeds and cross-sectional variations along the strand length.
Pass designs for different purposes are compared, i.e. high reduction versus low reduction pass sequences for sizing purposes. The sizing possibilities of the considered processes are analysed by the mathematical model and detailed numerical results are presented.
