Precipitation Behavior Of V-N Microalloyed Steel And Its Effect On Microstructure And Properties Of Steel

With the development of the times,in order to meet the requirements of industrialization and urbanization,the demand for construction machinery and construction steel has increased significantly,and the role of high-strength steel has become increasingly prominent.Under the general environment of industrial development and sustainable development,new requirements have been placed on steel materials.Taking all aspects into consideration,microalloying has become an inevitable trend for high-strength steels.Among them,the strengthening effect of microalloying elements is more significant.Nb,V,Ti micro-alloying can be used to obtain highperformance low-alloy high-strength steels.These steels can achieve good comprehensive performance at relatively low cost.They are widely used in ship bridges,pressure vessels,petroleum casing and other fields.Among them,V-N microalloyed steel has outstanding performance advantages and has been widely used in many fields.In this paper,the influence of precipitation on high temperature thermoplasticity and heat affected zone of welding is studied.The isothermal precipitation of precipitated phase and its action and evolution during welding thermal cycle are analyzed.The microstructure evolution and mechanical properties of welded joints under given linear energy are also studied.The main work content and research results are as follows:(1)The high temperature thermoplasticity of the experimental steel was determined by the high temperature tensile test.The results show that under the same conditions,the greater the strain rate,the better the thermoplastic,and the more the Al content,the worse the thermoplastic.The Al element is combined with N to form AIN,and as the attachment precipitation point of V(C,N),the composite precipitation with a larger precipitation size promotes brittle fracture along the crystal and greatly reduces the thermoplasticity.There is less precipitation in the better thermoplastic structure,and there are fewer sources of stress concentration near the grain boundaries.The proportion of pre-eutectoid ferrite in the structure with poor thermoplasticity is larger,and the pre-eutectoid ferrite film is thicker.(2)The isothermal precipitation process in the austenite region was simulated by a single pass pressure experiment.The results show that in addition to the typical deformation induced V(C,N)during precipitation,the second phase of V will also precipitate in austenite with the promotion of trace Ti element.TiC and TiN precipitated at a high temperature will become the attachment points of carbonitrides of V later,thereby forming coarse(Ti,V)(C,N)precipitation,and MnS will also become the attachment point of V(C,N)precipitation.The precipitation amount increases with the extension of the isothermal time,and the precipitation coarsening phenomenon is more obvious when the isothermal time reaches 1500 s.(3)The microstructure and performance of heat affected zone in single and double passes was simulated by welding thermal simulation technology.The results show that the precipitated phase of V undergoes a high temperature remelting process during the welding thermal cycle,and dissolves and disappears after the welding thermal cycle.The shape of the precipitated phase of Ti will gradually be regularized with the progress of the welding thermal cycle.As the V(C,N)attached to the precipitate of TiN also gradually decreases,the Ti-containing precipitates finally show the square morphology of TiN.And with the increase of the linear energy,the length of the thermal cycle time,the size of the precipitated phase tends to grow.(4)The microstructure and properties of single-pass welding coarse grain heat affected zone were analyzed.The results show that V(C,N)promotes the nucleation of acicular ferrite in the grains in the heat affected zone(HAZ),and has a significant improvement effect on the impact toughness.Its strengthening effect is more significant than that of polygonal ferrite and pearlite.Optimization of the microstructure can compensate for the deterioration of the toughness of the interstitial N atoms.With the increase of the linear energy,the size of the original austenite grains and the proportion of the coarse grain boundary ferrite increases significantly,the strengthening effect decreased,and the tissue hardness decreased.(5)The microstructure and properties of the double-pass coarse-grained heat-affected zone were analyzed.The results show that the small M-A islands in the critically affected coarsegrained heat affected zone are uniformly distributed inside the ferrite grains,and some M-A islands are distributed along the original austenite grains.Because V(C,N)refines the M-A islands,making them smaller than 1 ?m,and absorbing C elements,reducing the C content in the M-A islands,which reduces the hardness and improves the overall toughness.(6)Welding experiments were performed for Q550D plates of different thickness and composition without treatment before and after welding.The results show that the yield strength,tensile strength,and impact energy at-20? were?563 MPa,688 MPa and 62 J,respectively.All meet the national standard requirements.V(C,N)and acicular ferrite in the welded joint structure improve the mechanical properties of the coarse grain region,especially the acicular ferrite with high density dislocations has a great inhibitory effect on crack growth.A large amount of fine equiaxed polygonal ferrite in the fine grain region ensures the low temperature toughness of the fine grain region.