Development Of The Analytical Model Of Three-roll Continuous Retained Mandrel Rolling Process

The three-roll continuous retained mandrel rolling has been widely used in the industry to manufacture high quality seamless pipes and tubes.Due to the complexity of the three-dimensional characteristics of the deformation,the rolling process control parameters of the mill,such as the roll speed and the roll gap,are normally set by using a rolling model.As a result of the increasing competition in product markets,the flexible manufacturing,which is aiming for maximizing the productivity of any given mill,has been increasingly applied.In the flexible manufacturing,maintaining the product consistency and quality is a widespread concern.These increasing demands on higher adaptability and flexibility of the tube rolling control have led to great effort to improve the rolling model which is used for the control and automation of the mill.However,the analytical model of the process,i.e.,the modelling of the tube profile and tube stress distribution,the tube-roll contact geometry and the metal slip distribution in the roll bite,have not been carried out yet,which has thus hindered the development of high efficiency process control technology in tube manufacturing.To minimize the adjustments to the initial roll speed setting in tube manufacturing,both the tube profile and the tube stress distribution during the rolling are needed to be predicted before the actual rolling.While predicting the tube profile is to make the rolling reach to the metal flow balance state efficiently,predicting the tube stress distribution is to decrease the potential formation of the rolled defects caused by the non-ideal stress distribution in tube at each pass.This requires the rolling model to combine the prediction of tube profile with the prediction of stress distribution of tube in its formulation.Along with the modelling of the tube profile and tube stress distribution in the rolling,model of the roll speed setting and the roll separating force prediction are also the essential parts of the rolling model.Firstly,to adapt to the flexible distribution of deformation in the passes in field operation,the roll speed setting model has to be an analytical one rather than an empirical one.Secondly,since the roll stand is not perfectly rigid and its elastic deformation during the tube rolling can lead to variation in the roll gap,prediction of the roll separating force for online application is needed to determine the roll gap compensations in the initial mill settings before the actual rolling.It is quite necessary to develop a new analytical model of the three-roll continuous retained mandrel rolling process.In this study,an original analytical modeling framework is presented,which enables analytical solution of tube profile,tube stress distribution,roll speed setting,as well as roll separating force in tube rolling with various tube specifications and materials.Validity of the model has been examined by hot rolling experiments at the plant.According to the experimental results,the predicted tube profiles are in good agreement with the measured,with the prediction errors of cross-sectional areas being in the range of 0.8～2.5%.The theoretical initial roll speed setting calculated based on the proposed model can be applied directly to tube manufacturing,with further adjustments at the mill being less than 4%.The average error of the predicted forces was approximately 5.5%.In general,the hot rolling experimental results at the plant have verified the validity of the proposed model.Besides,the non-negligible influence of the friction conditions between tube and rolling tools on the roll pitch diameter has been theoretically identified.Having overcome the shortcomings of the conventional empirical model,the proposed model has the required prediction accuracy and flexibility for being applied to the flexible tube rolling.By building all the key algorithms in the predictions of tube profile,tube stress distribution,metal slip distribution and roll separating force around physical models of the process,the modelling not only can advance the rolling control at the plant,but also has advanced our scientific understanding of the mechanics of three-roll continuous retained mandrel rolling process.