Study On Microstructure Evolution And Properties Of Ultralow Carbon 5Mn Steel During Welding

With the continuous improvement of requirements for structural materials for hulls,bridges,buildings,pressure vessels,and offshore platforms,the development of steel plates with high strength,high plasticity,and high-low temperature toughness has received extensive attention.As the third generation advanced high-strength steel,medium manganese steel has excellent strength-plasticity matching,which meets the requirements for the above-mentioned structural materials.Studies have shown that its excellent performance is mainly attributed to Mn-rich austenite secondary phase formed on the quenched martensite structure after annealing in the ferrite-austenite two-phase region,which has certain stability at room temperature,and results in a transformation induced plasticity(TRIP)effect.Medium manganese steel reduces its welding performance due to its high manganese content and poor thermal conductivity.Welding is the main way of connecting steel materials,in the welding process of medium manganese steel,the welding thermal cycle will affect the microstructure and properties of the heat-affected zone(HAZ),and then affect the use of medium manganese steel.Therefore,finding out the effect of the microstructure evolution on the properties of the HAZ under the welding thermal cycle to obtain the best welding process for medium manganese steel will play a positive role in the application and promotion of medium manganese steel.However,there are few researches in this field at home and abroad.So,in this paper,the microstructure and properties of ultra-low carbon 5Mn steel during the welding process were studied by thermal simulation experiments,microstructure observation and mechanical properties testing.The results show:(1)In the continuous cooling process,starting temperature of the martensite transformation(Ms)decreased with the increase of cooling rate;the Ms at the peak temperature(P_T)of 850°C is higher than the Ms at the peak temperature of 1320°C.The rate of martensitic transformation increases as the cooling rate increases.(2)The impact absorbed energy at-40°C for the simulated coarse-grained heat affected zone(CGHAZ),fine-grained heat affected zone(FGHAZ)and intercritical heat affected zone(ICHAZ)were 31-42 J,36-56 J,137-165 J.The coarse grains and the low retained austenite(RA)content were the root causes of the poor impact toughness in simulated CGHAZ.The content and stability of RA in the simulated FGHAZ and ICHAZ were comparable,the content of tempered martensite in simulated ICHAZ was the reason why the impact toughness is obviously superior to simulated FGHAZ.(3)The reversed austenite grows in the way of merger and grain boundary migration,martensite not only nucleates within the crystal,but also nucleates at the prior austenite grain boundary.(4)Under both welding methods,simulated ICHAZ had the best impact toughness.Considering the impact toughness of-40°C,the welding heat input range of 5Mn steel plate is recommended to be 15~25 kJ/cm.