Study On Spiral Wall Thickness Unevenness Of Thin-walled Tubes Rolled By Assel 3-roll Skew Rolling Mill

At present,Assel three-roll skew rolling process is one of the main methods to produce high precision seamless steel pipe.Although the metal utilization rate and the product specification has been improved and extended using Assel mill,there exists serious problems in the actual producing thin-walled tubes process,such as the unevenness of spiral wall thickness and the depth of internal thread etc.As the exploited oil well is deeper and deeper,the oil and gas wells in complex geological environment put forward higher requirements for casing collapse resistance and corrosion resistance.Due to the unevenness wall thickness and the depth of the internal thread of the thin-walled tube,it is readily to lead to the spiral casing crack under high load(internal pressure),resulting in huge economic loss of the oil well.Therefore,it is of great theoretical and practical significance to study on controlling the occurrence of internal thread or reducing the depth of internal thread of rolled tube to improve its collapsing performance.Based on Jiangsu Changbao Steel Pipe Co.Ltd.’s Φ140mm Assel three-roll skew rolling mill equipment and technological conditions,with the aid of 3D solid modeling software UG and commercial finite element software Simufact,numerical simulations of Assel skew rolling process of the 5Cr steel pipe with a typical specification of Φ160×6.5mm was conducted.The depth of the internal thread,unevenness of transverse wall thickness and unit pitch as well as the longitudinal distribution of wall thickness of rolled tube were systematically analyzed with different mandrel movement modes,feeding angles and rolling stripes.The main conclusions are as following:(1)The unit pitches of workpiece increases with the axial velocity increase in the deformation zone.Meanwhile,from the resistance of the mandrel into the pulling of the mandrel and increase the angle of the feed can increase the unit pitch.And it is significantly influenced by mandrel movement modes.(2)In the process of three-roll skew rolling,the shape of rolled tube cross-sectionis changed from round shape to triangular "peach tip" shape,and finally to circular shape.(3)By comparing the depth of the internal thread of the rolled tube,it can be seen that rolled tube using the full floating mandrel has the smallest depth of internal thread,while that using the pull mandrel has the largest one;and it will be gradually increased with the feed angle increases.Meanwhile,compared the depth of the internal thread with the different types of rolling strip we can see the depth of the internal thread of the hyperbolic type rolling strip is shallow.(4)When the mandrel movement changes from the fixed mandrel movement to the pull mandrel movement,the average relative value of the wall thickness unevenness is gradually increased,and the average relative value of the wall thickness unevenness is increased from 2.756% to 3.385%;meanwhile the average relative value of wall thickness unevenness increases from 3.796% to 4.196% with the feed angle increases from 5 ° to 7 °.