New Cross Rolling Method For Ultrafine Bar

In the context of rapid economic development,bars,as an important product of the metallurgical industry,can be used in almost all structures.With the increasing social net demand for bars,the toughness and ductility of bars have become the sensitive issues hindering their high-strength development.And the ultrafine crystal bar material has special superiority compared with ordinary bar material due to the sharp refinement of its internal grain size.Therefore,in order to meet the ever-increasing performance requirements of high-precision pointed structural parts for bars,the preparation and application prospects of ultra-fine crystal bars are broad.At present,one of the most promising methods for preparing ultrafine-crystalline bars is the severe plastic deformation(SPD)technology.Five major classes and hundreds of preparation methods have been derived,but there are common problems such as small effective severe deformation area and huge loads The volume of ultrafine crystals in the block is very small,which cannot meet the needs of industrial production.In this paper,the following main results have been achieved around the preparation of 45 steel ultrafine grain bars:(1)A new SPD forming method called 3D-SPD is proposed.Based on the principle of cross rolling,it uses special curved tapered rolls and guides,and feeds from the large end of the roll diameter.By using large feed angle and roller cone angle,a severe torsional compression plastic deformation of 50% diameter reduction rate was achieved,and an ultrafine crystal rod was prepared.(2)A 3D-SPD damage prediction model based on the oyane criterion was determined.Based on the rigid-plastic finite element method for the calculation of damage,a deformation region capable of strictly suppressing the Mannesmann effect was established,and the phenomenon of crack initiation inside the bar was realized control.(3)The comparative analysis of 3D-SPD,conventional cross rolling and longitudinal rolling shows that compared with conventional cross rolling,3D-SPD significantly weakens the Mannesmann effect,and can achieve severe plastic deformation with equivalent strain exceeding 6 and the degree of torsional deformation.And the degree of torsional deformation exceeds 60%,and the diameter reduction rate exceeds 50%.Compared with longitudinal rolling,the load required for 3D-SPD forming is only 20%.(4)The influence of process parameters on the 3D-SPD plastic deformation was revealed: with feed angle,ovality factor gradually decreasing,and cross angle,roller cone angle and diameter reduction rate gradually increasing,the equivalent strain gradually increased,the change in deformation uniform is not significant;as feed angle gradually increasing,roller cone angle,ovality factor and diameter reduction rate gradually decreasing,and the damage gradually decreased.The significant influencing factors include cross angle,feed angle,roller cone angle,diameter reduction rate and ovality factor.(5)The orthogonal test method is used to optimize the optimal process parameters:feeding angle of 24 °,cross angle of 15 °,roller cone angle of 5 °,diameter reduction rate of 50%,ovality factor of 1.02,temperature of 700℃ and roll speed 40 rpm.45 steel ultrafine grain bars of Φ25mm × 480 mm were successfully prepared on a 3D-SPD testing machine.(6)After rolling,the grain size of the bar was 0.95 μm to 1.1 μm,with an average of0.99 μm.The grain refinement was significant and the degree of refinement reached 95%;Average yield strength increased by 46%,and average tensile strength increased by 42%.