Microstructure Evolution And Low Temperature Toughness Of High Heat Input Welding HAZ Of DH36 Ship Plate Steel

Since February 2022,the Russian Ukrainian conflict has continued and the international situation has become increasingly serious;In response to the changing international situation,the demand for energy reserves has increased dramatically,promoting the further large-scale development of energy carriers.Large heat input welding technology can significantly increase efficiency and reduce cost for thick plate steel.High single heat input leads to coarse austenite and poor toughness heat affected zone（HAZ）,which becomes the weak point of welded joint.Low carbon equivalent technology,microalloying and oxide metallurgy technology are the main ways to improve the toughness of HAZ.In this thesis,Ti-treated 60mm thick DH36ship plate steel was used as experimental material to study the low temperature toughness,microstructure evolution and the effect of inclusions on microstructure transformation in HAZ by thermal simulation cycle curve.This thesis draws the following conclusions:（1）Under the heat input of 350kJ/cm～550kJ/cm,the low temperature toughness of heat affected zone is the best at 400 kJ/cm.The impact energy at the surface is 240J,and the impact energy at 1/2 thickness is 196J.With the increase of heat input,the low temperature toughness of test steel first increases and then decreases rapidly.（2）The microstructure of HAZ is composed of polygonal ferrite,granular bainite and a small amount of pearlite.With the increase of heat input,the volume fraction of polygonal ferrite and grain size in HAZ gradually increase,while the volume fraction of granular bainite gradually decreases.The volume ratio of polygonal ferrite at the surface is larger than that at 1/2 thickness,and the grain size is smaller than that at 1/2thickness.（3）When the heat input is 550kJ/cm,the austenite grain size is larger than400kJ/cm at the surface or 1/2 thickness.Under the same heat input,the maximum austenite grain size at 1/2 thickness is larger than that at the surface,and the heterogeneity of austenitization at 1/2 thickness is more serious,showing mixed crystal phenomenon.（4）The transition temperature of bainitic ferrite（BF）is medium temperature,and the transition temperature ranges from 607℃to 669℃.The nucleation location of BF is grain boundary nucleation inclusion nucleation intragranular nucleation and BF/A phase interface nucleation and the nucleation temperature decreases in turn.At550kJ/cm heat input,the amount of BF nucleated in grain is less than that at 400kJ/cm heat input.（5）The number of inclusions on the surface of the tested steel is much larger than 1/2 thickness in the rolled microstructure;The size distribution of inclusions is concentrated in the range of 0～3-4μm.In-situ dissolution,precipitation and growth of inclusions were observed during heat input,and inclusions pinning austenite was also observed.In addition,MgAl₂O₄induced acicular ferrite（IAF）nucleation.