The Evolution Of V(C,N) Particle And Its Effect On Microstructure And Low Temperature Toughness Of Heat Affected Zone Of High Nitrogen And Vanadium Normalized Steel

P460 steel is a high-strength high-nitrogen high-vanadium design normalized steel.The key to its toughening is the precipitation of the second phase particles V?C,N?in the matrix.The precipitation of V?C,N?particles plays the role in precipitation strengthening on the one hand,and grain refinement on the other hand.Grain refinement can improve its strength and toughness at the same time,and precipitation strengthening can improve the strength of the material.In order to improve the strength of the steel for mobile containers,and at the same time achieve the purpose of reducing the weight of the container and saving energy and reducing emissions,it is urgent to promote the use of P460 steel.After the welding process,how the V?C,N?in P460 steel evolves and what effect it has on the structure and low temperature toughness still needs in-depth research.In this paper,through thermal simulation experiments,the continuous cooling phase transition?SHCCT?curve of P460 steel welding simulation heat affected zone is obtained,which provides theoretical guidance for the subsequent actual welding process.And the t_8/5?the time from 800°C to 500°C cooling?of reprecipitation of V?C,N?particles measured by transmission electron microscopy?TEM?is 100s.It can be seen from the SHCCT curve that when t_8/5>100s,the content of proeutectoid ferrite is greatly increased,which shows that the occurrence of reprecipitation reduces the C and N element concentrations in the matrix to promote the formation of proeutectoid ferrite.The thermal-Calc software was used to calculate the precipitation-temperature curve of V?C,N?particles,according to the result of the curve:V?C,N?was completely dissolved in 1160°C,and the precipitation amount reached the maximum at 700°C,which was 0.41326wt.%.A confocal laser scanning microscope was employed to observe the V?C,N?particles on the growth of austenite grains,it is found that when the peak temperature reaches 750°C and 950°C,even after 6 minutes of heat preservation,the growth of austenite grains is not obvious,due to the pinning effect of V?C,N?.At 1200°C,the austenite grains increase sharply because V?C,N?is completely dissolved and the pinning effect on the austenite grain boundaries disappears.Simulate the structure of welding heat affected zone of P460steel under different welding thermal cycles,test its low temperature?-40°C?impact toughness,the results show that the peak temperature is 1350°C,1200°C,t_8/5=15s?80s,and the impact toughness of P460 steel at-40°C is maintained at 13-19J,at these two peak temperatures,V?C,N?is completely dissolved and unable to complete reprecipitation,nitrogen element exists in free state,free nitrogen plays a leading role in the reduction of impact toughness.For t_8/5=45s,the impact toughness of P460 steel at-40°C decreases with the increase of peak temperature,but it will decrease sharply after exceeding 1200°C.This is because V?C,N?particles completely dissolved exceed1200°C and failed to re-precipitate,free nitrogen will greatly reduce the low temperature impact toughness.When the peak temperature is lower than 1160°C,the partially dissolved V?C,N?particles can play a role in refining grains,and can increase the ferrite nucleation rate,which has a positive impact on the improvement of impact toughness significance.The actual submerged arc welding verification experiment is carried out according to the line energy of 25kJ/cm.The fusion line and the coarse grain heat affected zone?CGHAZ?in the joint are granular bainite and a small amount of proeutectoid ferrite,and the fine grain heat affected zone is fine massive iron the ferrite,pearlite and bainite,the inter-critically heated affect zone is the ferrite and pearlite.The impact test shows that the actual joint heat affected zone-40°C impact energy is in the range of 90J-98J,the actual joint low temperature toughness is good.