Rare Earth Inclusions Control And Its Effect On Welding Performance Of 700MPa High Strength Steel

700 MPa high strength plate steel is widely used in lifting machinery,mining equipment and other engineering fields.It is mainly processed by welding,which usually leads to the deterioration of heat affected zone(HAZ)toughness.Oxide metallurgy is a new idea in the field of metallurgy and materials.In order to improve the welding toughness of steel,the"beneficial"inclusions with small size,dispersed distribution and controllable composition generated in molten steel smelting process,which are used to affect the microstructure and grain size of HAZ during welding thermal cycle.Taking Q690D plate steel as the research object,Rare earth inclusions control technology was proposed in this paper.The morphology,size and type of inclusions in high strength steel were analyzed.And the effect of Ce on microstructure and properties of HAZ was studied by combining welding thermal simulation with actual welding by OM,SEM,EDS and high temperature confocal microscope.The samples were simulated welded in Gleeble3500 and were taken actual welded by MAG.The effect of improving welding properties by inclusions of Ce was further revealed.This study has important research significance and promotion value for enriching the oxide metallurgy theory,improving the weldability of steel and expanding the application of Rare Earth(RE)in steel.High strength steels with 0 ppm,5 ppm,18 ppm and 23 ppm Ce content were simulated welded in Gleeble 3500 with heat input of 25 k J/cm,50 k J/cm,75 k J/cm and 100k J/cm.It was found that Ce can delay the formation of upper bainite in HAZ.Ce inclusions were pinned on the original austenite grain boundary during welding thermal simulation,which inhibited grain boundary migration and prevented grain growth.The impact toughness of HAZ with Ce was higher.In the actual MAG welding,the low temperature impact energy of HAZ with 18 ppm Ce content steel was 45.5%higher than that of conventional high strength steel when the welding heat input was 25 k J/cm.There were fine and dispersed rare earth compound inclusions on the impact fracture surface of HAZ of RE high strength steel,which played a role in inhibiting grain growth during welding,which was consistent with the results of welding thermal simulation test.After adding Ce the proportion of inclusions less than 2?m increases.The inclusions change from irregular large size MgAl₂O₄to spherical or ellipsoidal small size CeAl O₃and Ce₂O₂S.With the increase of heat input,the type of inclusions in samples with different Ce content did not change compared with base metal.The microstructure of HAZ changed from martensite and lower bainite to upper bainite and granular bainite.The impact fracture dimples of samples with 18ppm and 23 ppm Ce were obvious,while the impact fracture of samples without or containing 5ppm Ce showed cleavage fracture.In order to improve the welding toughness of 700 MPa high strength steel,the rare earth inclusions control techniques with industrial application value was developed.Two effects of improving the welding toughness of high strength steel by Ce are obtained,including inhibiting the growth of austenite grains and delaying the formation of bainite in HAZ.