Research On Microstructure And Mechanical Property Of Laser Weld Joint Of Hot Rolled DP590 For Cool Rolling

With good stamping property, such as low yield point, high work hardening rate and continuous yielding behaviour, Dual-phase(DP) steel has been extensively accepted by automobile manufacturing factories.Laser welding offers good features such as narrow heat affected zone, low possibility of cold cracking, and high productivity, so high-quality welded joints can be achieved. Therefore, dual phase steels have been widely applied in automotive manufacture, and it is becoming more important to research microstructures and properties of laser-weld dual phase steels.The microstructures and mechanical properties of a weld joint of DP590 commercial dual-phase steel were investigated under different heat treatments through optical microscopy, scanning electron microscopy, energy disperse spectroscopy, microhardness tests, and tensile tests. The following experimental results were achieved.Three regions can be distinguished in the weld joint DP590 steelplate: fusion znoe, heat affected zone, and the matrix steel. The microstructure of the matrix mainly include ferrite and martensite with an average hardness of 206 HV0.1. The heat affected zone consists of ferrite, martensite, tempered martensite and bainite. Softening zones exist in the heat affected zone. The fusion zone has a microstructure of martensite and a few bainite with significantly higher average hardness, up to 2.3 times, than the matrix. The tensile tests in longitudinal orientation show that the the weld steel has slightly higher tensile strength and significantly lower elongation than does the matrix. The significant drop in elongationof the weld specimen is due to, on the one hand, a large amount of martensite in the weld line, andon the other hand, the uncoordinated deformation in the weld joint, where the tensile deformation mainly occur in the matrix,, while the welding fusion zoneand the heat affected zone hardly deform.Aging treatments were conducted on theweld joints of the DP590 steel at 250 ℃, 300 ℃, and 350 ℃. Hardness of the weld joints decreases with the increase of aging temperature. The precipitated carbide increases due to the gradual dissolution of martensite. The transverse tensile tests show that tensile strength of the weld sample slightly decreases, while plasticity and product of tensile strength and elongation increases with a maximum increase of 7% for the product of tensile strength and ductility. The fracture analysis using SEM show that the fracture of the fusion zonetransforms from brittle to ductile with the increase of aging temperature. The ideal aging temperature considering both of the microstructure and mechanical properties for the weld joints is 300 ℃.The weld joints of DP590 steel after aging were annealed at 750 ℃, 800 ℃, and 850 ℃. Both microstructure and mechanical behavior of the weld joints can be improved significantly. At the annealing temperature 800 ℃, the weld joints get the best microstructure and mechanical properties.