Numerical Simulation And Experimental Study On Hot Roll Forming Of High Strength Steel Square Tube

The application of high strength roll-formed steel in automobile body structure is the best way to realize the lightweight design and improve crash safety performance.However,due to the low ductility of high strength steel,quality defects are easy to appear during the roll forming process.The corner area of high strength roll-formed steel is characterized by high yield strength,low product of strength and elongation,large outer corner radius,and reduced corner thickness.Thus,it is more difficult to assemble and weld the corner area in the engineering application,and the risk of cracking under external forces is higher.In order to improve the mechanical properties and geometric morphology of the corner area in high strength square steel tubes,the hot roll forming technology was studied by combining numerical simulation with experimental research.Based on the APDL command,the indirect coupling of electromagnetic and temperature fields in the local induction heating process of high strength steel square tubes was carried out in ANSYS software.The numerical simulation results show that the physical environment method is much more efficient than the multi-field solver method.The magnetizer can greatly enhance the electromagnetic field intensity in the outer corner area of the square tube,thus improving the induction heating efficiency.The magnetic induction intensity and heating temperature of high strength steel square tubes have the skin effect.According to the target heating temperature and heating range,the optimal values of heating frequency,heating power,and heating time were obtained.The high temperature compression test of high strength steel was carried out by using a thermal simulation test machine,and the John-Cook phenomenological constitutive models of the ferrite and austenite regions of high strength steel were established.With the help of Fortran language,the J-C constitutive model was successfully embedded into the explicit analysis module of ABAQUS finite element software in the form of a VUMAT user subroutine.The direct coupling of temperature and structure fields in the hot roll forming process of high strength steel square tubes was studied in the ABAQUS software.The numerical simulation results show that the decrease in the outer corner radius is due to the transformation from the outer corner edge area to the plane area,while the increase in the corner thickness is due to the accumulation of metal in the inner corner center area.Since the heating temperature in the corner area is higher than that in the plane area,it is difficult to have stress concentration in the corner area,while the strain concentration is easy to occur in the inner corner center area due to the restriction of the metal flow space.The optimal ranges of linear velocity,heating power,and total deformation were obtained according to the analysis results of the geometric morphology of the square tube corner area.Based on the numerical simulation results and field debugging experience,the existing test equipment was upgraded.Through the introduction of the automatic feeding and blanking mechanism,the automatic production process from feeding,heating,roll forming to blanking of a single square tube was realized.The structure of the induction heating coil was optimized to improve the heating uniformity and heating temperature peak.The pass structure was designed flexibly,which greatly enhanced the universality of the roll die.An experimental study on hot roll forming of high strength steel square tubes was carried out by using the automatic production line.The heating temperature range of high strength steel was subdivided into non-austenitizing zone,partial austenitizing zone,and austenitizing homogenizing zone.The experimental results show that with the increase in heating temperature,the longitudinal flaring performance and radial flattening performance of the square tube is improved.The matrix structure in the corner area is gradually dominated by granular bainite,and the ferrite grains change from quasi-polygon to strip shape.The dislocation density in the outer corner area is gradually decreased,and the precipitates appear aggregation and growth phenomenon.Based on the response surface method,response surface models for yield strength,outer corner radius,and microcrack length of high strength steel square tubes were established.The results of variance analysis show that the linear velocity and heating power have significant effects on all three responses,while the total deformation only has significant effects on the outer corner radius and microcrack length.The optimal combination of process parameters satisfying the conditions was obtained according to the optimal target values.Compared with the cold roll-formed square tube,the mechanical properties and geometric morphology of the corner area in the optimized cold and hot composite rollformed square tube have been significantly improved.