The Effect Of N/B Addtion On Microstructure And Impact Toughness Of Coarse-grained Heat-affected Zone Of Low-carbon Mo-V-Ti Microalloyed Steel Subjected High Heat Input Welding

With the upsizing of bridge,oil pipeline,ship and other steel structures,the steel plate also increases and thickens.To improve the welding manufacturing efficiency of steel structure,double wire,three wire and even multi wire submerged arc welding has been applied in engineering.But at present,ordinary alloy steel cannot meet the requirements of high heat input welding steel.Low-carbon V-N-Ti steel is a kind of promising construction steel of new generation potential for high heat input welding.The toughness of coarse grain heat affected zone(CGHAZ)in Low-carbon V-N-Ti steel can be improved by enhancing nucleation of fine-grained intergranular ferrite(IGF)on(V,Ti)(C,N)complex precipitates.However,the microstructure refinement of CGHAZ is still insufficient and accordingly the toughness is inadequate owing to the formation of coarse-grain intragranular polygonal ferrite(IGPF),which prohibits its further development and applications.In this paper,a new low-carbon Mo-V-N-Ti-B steel was designed for regulating the IGPF to intragranular acicular ferrite(IGAF)by simultaneous addition of nitrogen and boron.The effects of N/B on the solution-precipitation behavior of precipitated particles and B segregation at prior austenite grain(PAG)boundaries during welding thermal cycles will be investigated for further estimating their effects on thermodynamics/kinetics of IGF transformation.The effect of B content on the continuous cooling transition behavior of CGHAZ in low carbon Mo-V-Ti-B-N steel was studied.The addition of B decreases the initial phase transition temperature of CGHAZ and enlarges the bainite phase region.When the content of B in the steel is 13 ppm,the solid solution B in the steel tend to segregate at prior austenite grain boundaries during the cooling process of welding,which inhibits the formation of grain boundary ferrite(GBF)and lower the phase transition point,thus promoting the transformation of acicular ferrite(AF)and granular bainite(GB).The effect of B content on microstructure and impact toughness of CGHAZ in low carbon Mo-V-N-Ti-B steel was studied at the heat input of 75 k J/cm.GBF appears in CGHAZ without B,which worsens the impact toughness of CGHAZ;When the content of B is 13 ppm,the solid solution B segregate at the prior austenite grain boundary causes the GBF to disappear.At the same time,B combine with N in the steel and precipitates on Ti(C,N)to form the complex(B,Ti)(C,N)particles,which promotes the nucleation of acicular ferrite and increases the proportion of the intracrystalline acicular ferrite in CGHAZ,resulting in the refinement of microstructure and improvement of toughness of the CGHAZ;When B content is 19 ppm,microstructure of CGHAZ is composed of granular bainite with large size due to the increase of solution B content,resulting in the decrease of impact energy.The effect of N content on CGHAZ continuous cooling transformation behavior of low carbon Mo-V-N-Ti-B steel was studied.The increase of N increases the content of fine Ti(C,N)particles in CGHAZ,refines the prior austenite grains,and increases the phase transition temperature Ar3.At the same time,the increase of N promotes the precipitation of V to form more surface V-rich precipitated particles in the CGHAZ.As a result,the nucleation ability of precipitated particles was enhanced,which promote the formation of polygonal ferrite and acicular ferrite,and reduce the bainite phase region,resulting in the refinement of the microstructure.The effect of N content on CGHAZ microstructure and impact toughness of low carbon Mo-V-N-Ti-B was studied when the heat input was 75 k J/cm.The addition of N content raises the content of micron and submicron particles(200 nm-)formed in liquid and nanoprecipitated particles(-100 nm)precipitated from austenite.The nano-precipitated particles nail the prior austenite boundaries,inhibit the growth of austenite and reduce the austenite grain size.The micron and submicron particles promote the nucleation of IGF.Thus,the microstructure of CGHAZ is refined and the impact toughness of CGHAZ is improved.The effect of heat input on CGHAZ microstructure and impact toughness of low carbon Mo-V-N-Ti-B was studied.With the increase of heat input,the impact energy of CGHAZ steel with N content of 0.0144 wt.% increases first and then decreases.When the heat input increases from 25 k J / cm to 75 k J / cm,the main reason for the increase of CGHAZ impact energy is the increase of acicular ferrite content and the refinement of effective grain size.When the heat input is 100 k J/cm,coarse pearlite appears,and a "carbon" film appears at the interface between ferrite and ferrite,resulting in the decrease of impact toughness.The effect of N content on the solid solution and precipitation behavior of(Ti,V)(C,N)particles during welding thermal cycle and the mechanism of(Ti,V)(C,N)particles promoting the formation of intragranular ferrite(IGF)in CGHAZ were studied.At the high temperature of welding thermal cycle,the fine V(C,N)particles with low solid solution precipitation temperature dissolve.In the subsequent cooling process,the dissolved V(C,N)particles precipitate on the insolubled Ti(C,N),resulting in the formation of a large number of surface V-rich composite(Ti,V)(C,N)particles.The mismatch between VC,N)and ferrite is smaller than that between Ti(C,N)and ferrite.In addition,V(C,N)precipitates attached to insoluble Ti(C,N)particles,which increases the particle size,makes more precipitated particles reach the critical nucleation size and improves the nucleation ability of precipitated particles.