On The Role Of N Addtion In Controlling Intragranular Ferrite Formation And Strength/Toughness Combination In Welding Heat-Affected Zone Of Low-Carbon V-Ti Steel

V element, as one of the major alloying elements, coupled with N has been regarded as an effective way to enhance precipitation behavior and promote grain refinement of steel, however there still exists plenty of controversies about the influence of V and N element to the CGHAZ toughness. Therefore, it is necessary to investigate the effect of vanadium microalloyed steel and nitrogen content on the microstructure and mechanical of CGHAZ. The welding thermal simulation method on a Gleeble-3800, SEM, TEM and mechanical property have been performed in the work, aiming to provide an evidence for further understanding the effect of heat welding heat inpt and nitrogen content on microstructures andmechanical properties in CGHAZ of low-carbon V-Ti steel under different welding heat input.Effect of nitrogen contents on the continuous cooling transformation curve(SHCCT) of CGHAZ in low-carbon V-Ti steel were investigated. First, increasing nitrogen markedly raised phase transformation temperature, with the t8/5 increasing, the increment of began temperature was raised; Secondly, an increase in nitrogen content decreased the critical cooling time. Thrid, increasing nitrogen could change the type of microstructure and promote polygonal ferrite and acicular ferrite forming. Finally, increasing nitrogen markedly decreased the CGHAZ hardness.Effect of nitrogen contents on the microstructures and mechanical properties of simulated CGHAZ in low-carbon V-Ti microalloyed steel were investigated. With the total nitrogen increasing from 44 ppm to 94 ppm the effective grain size defined with the high boundary tolerance angle 15 o were refined, decreasing the 50 % fracture appearance transition temperature(50 % FATT) of simulated CGHAZ. However, further increase of total nitrogen would gradually increase the soluble nitrogen content and precipitation strength, and then deteriorated CGHAZ toughness. And therefore, the 50 % FATT of simulated CGHAZ was the result of a combination of effective grain size, soluble nitrogen and precipitation strength factors, the optimum total nitrogen content was 94 ppm with excellent matching strength and toughness of simulated CGHAZ.The effect of welding heat input on low-carbon V-Ti steel had been investigated. The yield strength of simulated CGHAZ decreases with increasing heat input, meanwhile the 50 % FATT of simulated CGHAZ in 91 N steel first decreased with effective grain sizedecreasing, and then increased with effective grain size increasing. 50 % FATT of simulated CGAHZ of 32 N steel increased with the increasing effective grain size. The 91 N steel has a better combination of strength and toughness of simulated CGHAZ compared with 32 N steel. Increasing nitrogen refined the austenite grain size, resulting in refine the effective grain size and raising cleavage fracture stress of simulated CGHAZ. The detrimental effect of the soluble nitrogen on toughness can be remedied by austenite grain and effective grain size refinement.Effect of nitrogen on the second phase particles in low-carbon V–Ti steel during weld thermal cycling were investigated. In the case of the low- nitrogen steel, the base plate consistsmainly of Ti-rich(Ti,V)(C,N) particles which were coarsened after welding thermalcycling, thereby leading to coarsening of the austenite grains. In the case of the high-nitrogen steels, the base plate consistsmainly of Ti-rich(Ti,V)(C,N), V-rich(V,Ti)(C,N), and V(C,N) particles. During welding heating, the Ti-rich(Ti,V)(C,N) particles were only partially dissolved, whereas the V-rich(V,Ti)(C,N) and V(C,N) were completely dissolved.The sub-50-nm Ti-rich(Ti,V)(C,N) particles presented at the peaktemperature and during welding cooling, were effective in refining the austenite grain size, by preventing the migration of theaustenite grainboundaries. The soluble titanium content of austenite decreased withincreasing nitrogen content of 44 ppm-190 ppm, therebyresulting in a low coarsening rate of the Ti-rich(Ti,V)(C,N) particles.The effect of(Ti,V)(C,N) particles on the intragranular ferrite formed in CGHAZ of low-carbon V-Ti microalloyed steel had been studied. During the weld cooling process, the undissolved Ti-rich core acted as a nucleation site for the V-rich cap; the cap reached the critical size at which nucleation of intragranular ferrite occurs nucleation size of intragranularferrite with the aid of the core. The intragranular ferritehave formed on the Ti-rich(Ti,V)(C,N) precipitates, owing to a cap-induced increasingin the nucleation potency, increasing the began transformation of ferrite transformation.