Mechanism Of Intragranular Ferrite Nucleation To Boron In Low Carbon V-N-Ti Steel In The Simulated CGHAZ

With the development of large-scale ship,it is required that the steel used in hull structure have high strength,high toughness and large thickness specifications.The welding time accounts for 30%-40%of the total construction time of ship.Therefore,how to improve the welding efficiency of thick gauge steel plate becomes one of the important issues in hull construction.Large linear energy welding methods,such as gas-electric vertical welding and multi-wire submerged arc welding,can realize one-pass welding of steel plate and significantly improve the welding efficiency.However,under the condition of large linear energy welding,the high temperature residence time of CGHAZ is long,the cooling rate after welding is slow,and the toughness of CGHAZ after phase transformation is easy to form unfavorable structure,which makes the toughness of CGHAZ decrease significantly,which is the technical bottleneck restricting the application of large linear energy welding method.Based on the study of the structure and toughness of low carbon V-N-Ti steel simulated by CGHAZ,the effect of Gleeble-3800 thermal simulation tester B on the welding structure and mechanical properties of V-N-Ti steel under 100 k J/cm welding heat input was simulated.The results show that the ductile-brittle transition temperature of V-N-Ti steel simulated CGHAZ decreases from-20?to-47?,and the number of bainite and polygonal ferrite in V-N-Ti steel simulated CGHAZ decreases with increasing B.Therefore,the increase in the number of simulated CGHAZ polygonal ferrite after adding B is the reason for the increase of low temperature toughness.Secondly,the continuous cooling behavior of V-N-Ti steel after adding B was studied.The results show that the simulated transformation temperature of CGHAZ ferrite in V-N-Ti steel after adding B ranges from 594 to 800?.Compared with V-N-Ti steel,adding B promotes ferrite transformation and increases the starting temperature of ferrite transformation by 25?.The results of intermittent cooling experiments show that the grain boundary ferrite appears at 725?and intragranular ferrite appears at 700?.The nucleation core of?Ti,V??C,N?-Mn S-BN particles is acicular and polygonal ferrite.In addition,TEM,SEM and Thermo-Calc methods were used to investigate the mechanism of B-promoted simulation of intragranular ferrite in CGHAZ.The results show that B exists in the form of BN in the thermodynamic equilibrium state.TEM and SEM observations show that B separately precipitates in the form of BN or forms Al₂O₃-Mn S-BN compound precipitated particles in the base metal.When heated to1350?,BN particles dissolve completely.During the welding cooling process,BN can precipitate on the surface of Al₂O₃,Mn S or?Ti,V??C,N?particles.As the nucleation core of intragranular polygonal ferrite?IGPF?and intragranular acicular ferrite?IGAF?,the effect of intragranular ferrite is promoted.