Study On The Precipitation Behavior And Microstructure Evolution Of The Second Phase During The High-heat Input Welding Of V-N Series Ship Plate

China's Marine area accounts for one third of the land area.In recent years,with the deepening of Marine research,the development of offshore platforms and shipbuilding industry has become particularly important.Welding process occupies a large number of man-hours in production,so it is of great significance to improve welding efficiency for the production of large Marine equipment.The welding condition of large line energy has a very high heat source density,which can reduce the number of welding passes,reduce the defects and adverse effects of multi-pass welding,and improve the production efficiency of large Marine equipment.V and Ti in steel as important alloy element can improve steel group,in the form of the second phase in form to improve the performance of the steel in recent years,some researchers have tried to use Nb,V,Ti as alloying elements to improve steel welding performance,in this paper,based on existing research through the summary and the numerical simulation to study the precipitation of V in the welding heat source under the influence of law;The components of V-N series hot-rolled ship board steel which can bear higher heat input are designed.At the same time,through reasonable ratio of V,Ti and N,the compound precipitation of the second phase particles was used to realize the hierarchical control of the structure under the condition of large line energy welding.LSCM and thermal simulation experiments were used to study the evolution rules of the melting zone and the welding heat affected zone of the welding base material respectively,and the following results were obtained:(1)In V steel can form small and dispersed second phase particles so as to facilitate the formation of acicular ferrite,improve the performance of can be welded on the parent metal,V there exsits a critical value for reinforcement,however,that when the content of 0.1 C,N content is 0.005,content is less than 0.05 V,V when content is more than the critical value,easy to appear too much carbon in steel,makes the material brittleness increased.(2)Under the condition of large line energy welding,there are differences in microstructure between the melting zone and the coarse-grain heat-affected zone.Overall,the performance of the coarse-grain heat-affected zone is still the weakest in the weld zone.The structure is composed of a large number of acicular ferrite and a small number of grain boundary ferrite.With the increase of heat input,the length of acicular ferrite and the width of slab both increase,the number decreases,the width of grain boundary ferrite increases,and gradually grows from strips along the grain boundary to blocks.The precipitates at grain boundary increased,the carbides grew gradually,and the structures such as sostenite,pearlite and granular bainite appeared.(3)The addition of Ti can be realized in heating and cooling two stages classification of solid solution and precipitation,effectively improve the weldingprocess under the condition of unidirectional heat transfer component uniformity,and forms of the second phase particles pinning on the grain boundary,refining the original austenite grain effectively,improve the acicular iron ratio,increase the dislocation density,to improve the toughness of the coarse-grain heat-affected zone,improve weld toughness,low temperature performance,greatly improving the weld area implements the use of precipitation strengthening improve classification of large line energy welding using the planking steel regulation.