Microstructure And Mechanical Properties Of Automotive Ultra-highstrength QP Steel Laser Welded Joints

The application of advanced automotive high-strength steel is an important approach to achieve automobile lightweight.As the third generation of advanced automotive high-strength steel,quenching and partitioning steel(QP steel)has high strength and plasticity.At present,there are few researches about laser welding of QP steel at home and abroad,therefore it has great engineering application significance to study the laser welding of QP steel and performance evaluation of QP steel welded joints.In this paper,the microstructure,microhardness,tensile properties and limiting dome height(LDH)test properties of QP1180 and QP1180/DP590 joints are analyzed.Besides,the strain dynamic behavior during deformation is further explored.First,the microstructure and crystal morphology of the joints are investigated.In the laser welded joints of QP1180 steel,the microstructure of fusion zone(FZ),coarse grain heat affected zone(CGHAZ)and fine grain heat affected zone(FGHAZ)all consist of martensite.The inter-critical heat affected zone(ICHAZ)has a mixed structure of martensite and ferrite.In the sub-critical heat affected zone(SCHAZ),part of the martensite is tempered and carbide is precipitated,leading to the formation of tempered martensite,besides,the retained austenite is also decomposed.An obvious softening occurs in SCHAZ,which is also called soft zone,where the hardness is 40 HV~60 HV lower than base metal.The softening becomes more serious with the increasing of heat input.In the joint of QP1180/DP590,it has the same microstructure with QP1180 joint.The hardness of HAZ in DP590 side is lower than that in QP1180 side and no obvious softening is found in SCHAZ of DP590 side.There is a very narrow partial molten zone in QP1180 joint.From the partial molten zone to the weld center,the crystal morphology is cell crystal,dendrite and equiaxed crystal respectively,and there are also some plane crystals in local area.With the heat input increasing,the surface nucleation area expands and columnar width increases.The tensile properties and the strain behavior of the joints are studied.QP1180 joints under various heat input all fracture in base metal,indicating good tensile properties.The strain in the weld is always the minimum in plastic deformation,and the tensile strain behavior is similar for different joints.3%~4% plastic deformation occurs at the weld.QP1180/DP590 joint fractures in DP590 base metal in tensile test,and the deformation mainly occurs in DP590 side.Besides,the deformation in the weld of DP590 side is obviously higher than that of QP1180 side.The formability and strain dynamic behavior of the laser welded sheets are investigated.There are two fracture modes for QP1180 laser welded sheets.The softening is more serious under high heat input and the strain concentrates in soft zone,leading to the fracture along the soft zone and parallel to the weld line with low LDH value.The softening degree becomes much lower under low heat input and the strain distribution is more uniform,leading to the fracture perpendicular to the weld with high LDH value.As the weld line offset increases,the strain distribution gradually becomes uniform and the LDH value increases.For QP1180/DP590 welded sheets,the LDH value obviously increases as the weld line goes away from the blank center.Because the strength of the two kind of base metal has a great difference,and the weak material will deform to the strong material,leading to the weld line movement phenomenon.The strain tends to concentrate on DP590 base metal near to the weld.The fracture position gradually goes away from the weld as the weld line offset increases.