Research On Mechanism And Control Of Quartic Buckle Of Hot-rolled High-strength Strip In Temper Rolling

Replacing the original materials of vehicle structural components with higher strength and thinner strip is an important way to the lightweight of vehicles,and hot-rolled high-strength strip is a vital choice for automobile structure materials.With the fierce competition in the hot-rolled strip market,customers have put forward higher requirements for the flatness of hot-rolled high-strength strips.Temper rolling is the last step to eliminate waves and improve flatness quality before the hot-rolled strip is delivered to users,which is particularly important to those products directly supplied.A single four-high rolling mill is usually used for hot-rolled temper rolling,and the shape control methods are not rich enough.Although it has the ability to solve low-order flatness problems such as centre buckle,double edge wave,it is difficult to effectively control quartic buckle.In the temper rolling of the hot-rolled high-strength strip,quartic buckle frequently occurs and its control lacks theoretical support.So it is an urgent challenge to study the generation mechanism of the quartic buckle of the hot-rolled high-strength steel,the change of the quartic buckle in the process,and the methods to improve the quartic buckle.In this paper,taking the advanced hot-rolled wide-strip temper mill with small-diameter work roll in a domestic 2250 mm hot rolling line is taken as the research object,in this paper,taking the advanced hot-rolled wide-strip temper mill with small-diameter work roll in a domestic 2250 mm hot rolling line is taken as the research object,the finite element model is established by using the research method combining numerical simulation and industrial test to reveal the deformation of the rolls and the stirp,grasp the change law of the quartic buckle in the process,and propose the proper control method.The main results are as follows:(1)Based on the large deformation elastoplastic implicit finite element method,a three-dimensional finite element model of the roller system-rolled piece in the process of the temper rolling is established.In order to simulate the deformation behavior of the hot-rolled high-strength strip in temper rolling with high precision,the non-coordinating mode element is replaced by the reduced integral element,the contact area mesh is refined,the wedge-shaped element is replaced by the hexahedral element,and the loading method with rigid sheets is replaced by the coupling.The re-development of Abaqus through Python realizes one-click parametric modeling.The average relative error of the strip thickness between the calculated value by the model and the measured value of the industrial rolling experiment is 0.11%,and the average deviation of the calculation crown is 2.23?m,which indicate that the model is of high calculation accuracy.(2)Based on the finite element model,the generation mechanism of the quartic buckle of hot-rolled high-strength strip is revealed.The simulations show that the secondary form of the bending deformation of the rollers is inconsistent with the cross-section at the rib of the strip,which makes the contact flattening between the work roll and the strip with a local high-order change at the rib of the strip and forms a high order distribution of the loaded roll gap profile.Therefore,a quartic buckle is generated.The high-strength strip is more prone to quartic buckle than the low-strength strip.Compared with the conventional work roll with a length-to-diameter ratio of 3.5 to 4.5,the work roll with small diameter and large length-to-diameter ratios(5.0-5.5)will increase the local compressive stress difference of the quartic buckle by about 50%.So the high-strength strip in the process of the wide temper mill with small diameter is more prone to the quartic buckle.(3)Based on the finite element model,the change law of the quartic buckle of the hot-rolled high-strength strip in the process of the temper rolling is revealed.A large number of calculation results show that the contour of the work roll and the cross-section of the incoming strip have a fundamental effect on the position and value of the quartic buckle.In the process of the bending force changing from negative bending to positive bending,when the variation distribution curve of the strip cross-section before and after rolling is convex in the middle of the strip and concave at the edge of the strip,the high-strength strip has a quartic buckle.With the rolling force increasing,the bending force range for the quartic buckle increases,and the effect of the tension is small.When the width-to-thickness ratio of the strip is between 568 and 818,the quartic buckle is easy to appear.(4)Based on the generation mechanism of the quartic buckle,combined with the basic effect of the cross-section of the incoming strip and the work roll contour on the quartic buckle,the control method of the quartic buckle is studied.First,the optimization strategy for the cross-section quality of the incoming strip is proposed,and the calculation method of the control range of the equivalent contour is established to realize the accurate design of the continuously variable crown.At the same time,an online automatic monitoring system was developed to accurately monitor the quality of the cross-section of the raw strip for temper rolling,which provides a good basis for the improvement of the quartic buckle for the high-strength strip.Further,combining with design method of the trigonometric function contour,the work roll contour of locally variable crown is proposed.Finally,through industrial application,it is verified that the above methods can effectively improve the quartic buckle of the high-strength strip.The proportion of strip that is downgraded to second level due to the quartic buckle has dropped by 83.53%.