Kinetics Of Continuous Cooling Transformation For High Niobium Low Molybdenum Q420E Steel And Toughness Mechanism Of Welding Heat-affected Zone

Nb-containing steels are widely used in steel materials with good strength and toughness,corrosion resistance,heat resistance and other properties.Microstructure evolution of Nb-containing steel,especially granular bainite(GB)transformation during the welding thermal cycle is lack of in-depth study.Toughness mechanism of Nb-containing steel welded heat affected-zone needs further research.In this paper,Granular bainite transformation kinetics of high niobium Q420E steel was investigated.In situ observation of grain e volution and phase transformation during thermal cycle.The microstructure and low temperature toughness of welding heat affected-zone(HAZ)were analyzed.Toughness mechanism of high niobium Q420E steel HAZ was revealed.High niobium Q420E steel welding thermal cycling was simulated.Austenite transformed into GB at a large range of cooling rate.Transformation activation energy decreases with increase of transformed volume fraction(f_b).Avrami exponent(n)of GB transformation kinetics is decreased with increase of f_b and cooling rate.The morphology of GB at different cooling rate is different.At moderate cooling rate,Austenite decomposed into MA constituent.Film-like residual austenite(RA)was found at fast and low cooling rate.Film-like residual austenite is beneficial to the low temperature toughness of welding heat affected-zone(HAZ).In situ observed grain e volution of Q420E steel with different heating and cooling rate.Small grains merged is the dominated pattern of growth during heating process,and grain growth is by grain boundary migration during cooling process.Rapid cooling makes granular bainite to be not coarsen.New grain nucleation and growth constantly happened during slow heating and cooling process,larger size grain is segmented.Thus,the effective grain size decreases after thermal cycle.Without large grain merged and nucleation and growth constantly happened in large grain are two key factors to refine grain during welding thermal cycle of high niobium Q420E steel.The microstructure observation and low temperature toughness test of different welding heat input HAZ indicate that e volution of granular bainite affects HAZ low temperature toughness of Q420E steel significantly.It shows that HAZ microstructure is stability with 15~75 kJ/cm heat input.The welding heat-affected zone shows excellent low temperature toughness.Small size of GB and film-like RA distribution in HAZ impro ve low temperature toughness.The high-Nb,lo w-C and low-Mo microalloying design is adopted of Q420E steel makes stable microstructure and coarsened hard with large-scale welding heat input.