Effect Of Vanadium And Input On The Microstructure And Impact Toughness Of CGHAZ In High Heat Input Welding Of Low Carbon V-Ti-B Steel

The construction of steel structure bridges in China is gradually moving in the direction of wide-body,long-span,high-speed,and heavy-load bridges,and the need for shorter construction times is becoming more pressing.As a result,the proportion of exceptionally thick steel plates used for some essential load-bearing components is gradually increasing.The standard multi-layer and multi-pass welding process is insufficient to meet the engineering requirements.Welding with a high heat input can considerably enhance efficiency.However,for existing Ti-B low alloy steels with high heat input resistance,when the heat input increased,the coarse grain heat affected zone（CGHAZ）structure is easily coarsened,resulting in local embrittlement that fails to meet the technical criteria.Therefore,improving the composition system of bridge steel for high heat input welding is critical.Ti-B and V-Ti-B test steels were designed and smelted in this study.Gleeble-3500 was used to characterize the CGHAZ microstructure by simulating the welding thermal cycle process with high heat input.The effects of V element addition on the CGHAZ microstructure and low-temperature toughness of test steel,as well as the effects of series heat input on the CGHAZ microstructure and low-temperature toughness of V-Ti-B test steel,were compared using Thermo-Calc thermodynamic calculations and impact toughness tests.V microalloying’s effect on high heat input welding of steel plates,as well as the evolution law of microstructure and low temperature toughness of the V-Ti-B test steel series under heat input,are investigated.Under the typical heat input of 100 k J/cm,the addition of the V element improves the microstructure uniformity of V-Ti-B test steel,and the microstructure type changes from granular bainite（GB）,acicular ferrite（AF）,massive ferrite（PF）and pearlite（P）to AF+PF+P.And submicron（Ti,V）（C,N）particles are precipitated to promote the nucleation of AF.With the increase of AF content,the austenite grains are segmented,the microstructure is refined,the average equivalent grain size（MED）decreases,and the proportion of large-angle grain boundary（HAGB）increases.The impact energy increased from 84 J to 245 J,the secondary crack propagation path changed from crack initiation and kink propagation to micropore aggregation,and the low-temperature impact toughness was significantly improved.With the increase of welding heat input from 30 k J/cm to 100 k J/cm,the lath bainite（LB）in CGHAZ structure of V-Ti-B test steel gradually decreases until it disappears,the granular bainite（GB）first increases and then decreases,AF gradually increases,the massive ferrite（PF）gradually increases,even a small amount of pearlite（P）appears,the size of M/A component decreases.The average size of precipitated particles increased from 32.41 nm to89.53 nm.Ti-rich（Ti,V）（C,N）precipitated particles promoted the nucleation of AF,and the proportion of AF gradually increased from 0%to 46%.The interlocking structure of AF increased the proportion of HAGB,cut austenite grains,and reduced For MED(MTA_θ≥15°).Therefore,with the increase of heat input,the section fiber rate increased from 6%to 54%,the low-temperature impact toughness increases,the crack propagation path gradually shortens.While the fracture morphology changes from the initiation linear propagation of cleavage crack to the initiation kink propagation of cleavage microcrack,and then the aggregation and propagation of micropores are observed.