Research And Application On Local Heating Roll Forming Technology Of High Strength Steel

With global energy crisis,serious environmental pressures and improving design of safety performance of steel structure,the development and application of advanced high strength steel(HSS)is the reform trend of steel industry,and is also a common concern of automobile industry and the field of bridge engineering.The development of advanced HSS and its application in lightweight structure meet the requirement for the concept of low carbon,energy saving,safety and sustainable development in contemporary society.The three key technologies to realize this idea are:the development and application of HSS,the lightweight design and optimization as well as the innovation of new manufacturing and forming technology.Despite many advantages of HSS have been popular in the automotive industry and the field of bridge engineering,but the HSS is different from the traditional carbon steel.At room temperature,the strength of HSS is high,the forming is difficult.And the change of mechanical behavior and microstructure are more complicated in the forming process.Besides,the higher requirements on the forming equipment and technology are put forward.Therefore,it is difficult or impossible to complete to produce the high strength parts using the traditional technology process.To solve the difficult problems of introducing HSS into the traditional roll-forming process,hot roll-forming process using local induction heating is proposed in this paper.That is,before the traditional rolling deformation,the plastic deformation zone is heating to a certain temperature to reduce its forming resistance and improve its forming performance.And it must be ensured the strength of parts without loss and without significant hardening of deformation area.In this study,the characteristics of hot rolling process of HSS are analyzed systematically from the basic theory of induction heating,the mechanism of roll-forming,microstructure evolution as well as the finite element method in its process.Taking the HSS tube for car and the new UTU-rib for bridge as an example,a comprehensive finite element simulation analysis is carried out.The influence of different parameters on the hot rolling process is analyzed and the characteristics of traditional forming process are compared.Then the equipment development and experimental study of these two examples are carried out to illustrate the practical application of hot roll-forming process of HSS.Through the finite element analysis and experimental study of the squaretube rolling process,the ideal temperature distribution is obtained in the temperature field,and the self-designed linear composite coil is well satisfied with the local heating requirement of roll process.In the "strip-to-square" and"circle-to-square" process,the stress at square corners is more than 1000 MPa,while it is only about 100 MPa in the "square-to-square" hot rolling process.Moreover,the accuracy of cross section is higher and the roll-forming force is smaller.With the increase of the forming temperature,the forming performance at square corners is improved obviously in experimental study.When the forming temperature is 650 ℃,the hardening effect of square tube corner is obviously weakened,and there is no crack in the flattening test and the corner stress concentration is also released.By means of the observation of microstructure,the microstructure of corners is developed by lath bainite to granular bainite at 650 ℃.And a large number of parabolic dimples in the tensile fracture appeare,which indicates that the plastic deformation occurre.In the numerical simulation of hot roll-forming process of UTU-rib,two kinds of pass design schemes are given,which are compared with the characteristics of metal flowing,the size of section and the force of rollers.According to the comprehensive analysis,the "near end" forming process is considered more suitable for roll-forming process.From the simulation results of different process parameters,when the forming temperature is 900 ℃ and the single pass reduction is 9 mm,the results of hot roll-forming process are relatively good.The experimental results show that the fatigue strength of the UTU-rib after hot roll-formed is greatly improved compared with that of formed by room temperature.When the forming temperature is between 900 ℃ and 1000 ℃,the mechanical properties of UTU-rib have been improved.The microstructure changes from the initial coarse ferrite-pearlite structure to equiaxial ultrafine grains and the precipitates such as(Nb,Ti)(C,N)are uniformly distributed in the matrix.With the temperature rising,the dislocation density drops sharply.Furthermore,a typical characteristic of ductile fracture mechanism with small-size tearing edge are more convinced that the specimens deformed above 900 ℃ have obtained an optimal combination of strength and toughness.