Research On Spot Welding Process And Forming Performance Of Grade 980MPa Quenching And Partition Steel

At present,the automotive advanced high-strength steel has developed to the third generation,and quenching and partitioning(Q&P)steel is a typical representative of them.In the automotive industry,the formability and spot welding performance of the steel plate also affect the use of plates in addition to strength,plasticity and toughness.The Q&P steel of 980MPa has been carried in various forming tests on a sheet forming machine.The formability of the experimental steel has been tested in terms of bulging,flanging,bending and cupping,and compared with those of the same grade.The resistance spot welding experiment was carried out by AC resistance spot welder,and the influence of welding parameters on spot welding performance was analyzed by means of TEM,EBSD,XRD and EPMA.The main conclusions are as follows:(1)The Acl of experiment steel by Thermo-calc software is 688?,while Ac3 is 842?.With the decrease of temperature,the content of carbon and manganese in austenite increased gradually,indicating that the carbon and manganese elements in ferrite diffused into austenite.The continuous cooling transition(CCT)curves were simulated by JmatPro software.The phase transition temperature and phase volume fraction at different cooling rates were determined.(2)The resistance spot welding experiment is carried out in AC resistance spot welder.With the increase of heat input,the spot welding joints are divided into three parts,namely nugget zone,heat affected zone(includng coarse grain zone,fine crystal zone,dual phase zone)and base metal zone while the organization gradually get coarser and coarser from inside to outside.The fine grain zone within the minimum grain size,the largest number of sub grain boundaries,residual the maximum stress,hard is the highest,up to 600HV.(3)As the welding current increases from 5kA to 6.5kA,the depth of penetration increases slowly with the increase of current,while the binding force of the spot welding joint,the nugget diameter and the pulling shear force is low.As the welding current continues to increase,the depth of penetration,the nugget diameter and the tensile shear increase gradually,and the peak value is reached at 8.5kA.When the welding current exceeds 9kA,the spatter occurs and the welding performance is deteriorated rapidly.The effect of welding time is similar to the welding current.When welding parameters are 8,5kA-14cyc,the welding quality is the best.At this time the nugget diameter is 6.76mm,and the tensile shear force is 27.4kN.(4)With the addition of temper current,temper martensite appears,the supersaturated carbon precipitates in the form of carbide and dislocation occur recovery in martensite.The hardness of matrix martensite declines,which decreases the difference of hardness between martensite and ferrite,so that the workpiece always have better performance in tensile shear test.When the tempering process is 30cyc-3kA-30cyc,the welding performance is the best,the nugget diameter is 7mm,and the tensile shear force is 30.45kN.(5)The forming experimentthe is carried out in sheet forming machine,the optimum IE value of 1.6mm thick experimental steel is up to 10.34.The limit expansion rate is up to 22%;the minimum relative bending radius of Rmin/t is 1.5.The springback angle is 14.67 degrees when the springback is 90 degrees,the springback angle is 14.67 degrees when the springback is 90 degrees,while 24.67 degrees when the springback is 120 degrees.The limit drawing ratio LDR 2.05.Above all,the experimental steel has good simulation formability.(6)The steel FLD0 is 25%,the forming limit of plane deformation and tensioncompression state stress is excellent and has reached the Baosteel level,while the forming limit under double stress small because of the low yield strength.In the forming process,the crack propagates along the grain boundary,and it can cross directly when it meets the soft ferrite structure.When it meets the martensite and the retained austenite structure,the kink and deflection occur.In this paper,sheet forming test machine is used to carry out the forming simulation experiments such as cupping,reaming,drawing and bending.The drawing and bending properties have reached Baosteel level.