Study On HAZ Toughening Mechanism And Beneficial Inclusions Evolution Of High Strength Steel Welded By High Heat Input

In this paper,the relationship between inclusions and nucleation of intragranular ferrite in high strength steel 12 MnNiVR welded by high heat input was studied by welding thermal simulation experiment,and the types of inclusions inducing nucleation of intragranular ferrite were determined.By means of carbon extraction replica experiment,transmission electron microscopy and Image Pro Plus analysis software,the variation of second phase particles in different welding thermal cycles and their effects on austenite grain growth of experimental steel were studied under 400 kJ/cm and 800kJ/cm conditions.Aiming at the inclusions that can induce nucleation of intragranular ferrite,the existence of manganese-poor zone around these inclusions was studied by scanning electron microscopy,and the mechanism of manganese-poor zone was used to explain the induced nucleation of these inclusions.The evolution of inclusions which can induce nucleation of intragranular ferrite in experimental steel during smelting was studied by ASPEX and scanning electron microscopy,providing reference for industrial production.Through the above research,the following research results were obtained.(1)Al-Ti+MnS and Mg-Al-Ti+MnS composite inclusions could induce IGF nucleation and induce sympathetic nucleation,of which Mg-Al-Ti+MnS composite inclusions accounted for the majority in steel.(2)There were a large number of TiN particles in CGHAZ after welding thermal cycle.With the prolongation of thermal simulation cooling time,the particles dissolved and grew,which weakens the pinning effect of particles on austenite grains.However,even tss=730s under welding thermal cycle conditions,there are still a large number of fine particles in CGHAZ,which can maintain the pinning effect on austenite grains.(3)There were manganese-poor zones around Al-Ti+MnS and Mg-Al-Ti+MnS composite inclusions.The mechanism of manganese-poor zones can be used to explain the induction nucleation mechanism of these two types of inclusions.(4)Mg-Al-Ti composite inclusions were formed after adding titanium in LF refining,and the number was small.Most of the oxide inclusions in molten steel were firstconverted from silicon manganese oxide to Al-Ti(Mg-Al-Ti)composite inclusion,then to Mg-Ti(Mg-Al-Ti)composite inclusion,and finally to Mg-Al-Ti composite inclusion.The inclusions in the continuous casting sample mainly exiseed in the form of Mg-Al-Ti composite inclusions,and the inclusions less than 5um accounted for more than 95%,which indicated that the smelting process not only increased the cleanliness of molten steel and also generated a large amount of inclusions favorable for nucleation of intragranular ferrite.