Modeling And Instability Mechanism Of Self-excited Vibration During Continuous Cold Rolling Process

The self-excited vibration during high-speed continuous cold rolling process is an important factor which affects the rolled product quality,limits the production efficiency and endangers the rolling equipment safety.Especially for cold-rolled high-strength sheet,the rolling mill vibration and instability during rolling process are more prominent.Considering the continuous rolling relationship between stands,establishing a self-excited vibration model of the continuous cold rolling system and exploring its triggering,induction and spreading mechanism in cold rolling system is the key point to realize vibration control in cold rolling process of high-speed high-strength thin strip.Aiming at the modeling and instability mechanism of self-excited vibration during continuous cold rolling process,the following research works were carried out.Considering the rolled piece hardening effect and the work roll flattening effect,based on the Coulomb friction model and Kalman differential equation,a dynamic rolling process model was established.The dynamic rolling process model was coupled together with rolling mill structure model through the dynamic rolling mechanical parameters and the rolling gap fluctuation.Taking into account the tension coupling effect,the rolled piece thickness fluctuation passed on to next stand after a delay time,and the continuous rolling relationship between stands,a self-excited vibration model of multi-stand continuous cold rolling system was established.With the general criterion,rolling critical velocity calculation models for continuous rolling system which considers the tension coupling effect and the rolled piece thickness fluctuation passed on to next stand were constructed.With the Mikhailov criterion,the critical velocity of continuous rolling system vibration model which considers the delay time of rolled piece thickness fluctuation passed on to next stand was calculated.The above three models were compared to each other.The effects of coupling effects between stands on critical velocity of continuous rolling system were discussed and the regenerative chatter instability mechanism was revealed.The results show that self-excited vibration in continuous rolling system first occurs in the worst-case stand,and then extends to the whole rolling system due to the coupling effect between stands.The tension coupling effect is the main factor which couples the adjacent stands together,but its influence on critical velocity is small.The rolled piece thickness fluctuation passing on with time delay to next stand decreases the critical velocity of continuous rolling system rapidly.The effects of rolling interface friction,tension and reduction on the critical rolling velocity of continuous rolling system were discussed.Based on the process parameter stability domain of self-excited vibration during continuous rolling process,the rolling schedule was optimized to enhance the productivity under the condition that the rolled piece exit velocity at the end stand is maximized and the instability does not happen for any stand.Considering the rolling emulsion characteristics,with the friction coefficient mechanism model,a self-excited vibration model for continuous rolling system was established.The effect of complex friction and lubrication state in rolling gap on the self-excited vibration instability mechanism was discussed.With the friction coefficient mechanism model,the inter-cross influence of rolling process parameters on friction coefficient of the rolling interface and critical speed of the continuous rolling system can be avoided.And the coupling between the micro complex friction lubrication state and the macro rolling mill vibration was realized.Time-frequency analysis was conducted to the 1420 tandem rolling mill vibration signal.It shows that that the vibration coupling effect between the upstream and downstream stand does exit.The critical velocities of tandem rolling mill were calculated and compared to the rolling velocity under different working conditions.Combining with tested vibration phenomenon,it shows that the rolling velocity exceeding the critical velocity is the key reason for self-excited vibration instability in rolling mill.With different specification and rolling process conditions,the continuous rolling system critical velocities are different.So,the suppression of self-excited vibration can be achieved by optimizing the rolling process parameters.