Microstructure Optimization Of Low-Carbon Ultra-High Strength Steel And The Evolution Of Microstructure And Properties Of HAZ

GPa grade ultra-high strength steel（1 GPa=1000 MPa）has a broad application prospect in large-tonnage construction machinery,ship deck and military equipment and other high-load facilities.However,for a long time,the high carbon content of traditional ultra-high strength steels has led to the deterioration of weldability,and HAZ become the weak point of welded joint with the improvement of strength.The welding application of traditional ultra-high strength steel is greatly limited.How to improve the weldability of ultra-high strength steel and improve the properties of HAZ has become a major problem to be solved.Therefore,the development of ultra-high strength steel with good weldability to improve the properties of HAZ has important engineering value.In this background,low carbon design was adopted to improve weldability,nano Cu-rich precipitates were introduced to compensate the strength loss caused by low carbon design,and by optimizing alloy composition and preparation process,low carbon nano-Cu-rich precipitates strengthen ultra-high strength steel with excellent strength and toughness were finally obtained.On this basis,the evolution of microstructure and properties of HAZ steel was studied in detail through welding thermal simulation experiment.The following research results have been obtained:In this paper,by adding Cr and Mo elements to low carbon nano-precipitate reinforced ultra-high strength steel,the microstructure is optimized and granular bainite with fine grains is obtained.The addition of Cr and Mo elements also reduces the diffusion rate of Cu-rich precipitates and forms the B₂+9R transition state nanostructured Cu-rich precipitates.The yield strength of granular bainitic steel is1155 MPa,the contribution of precipitation strengthening is 596 MPa.The fine grain size of granular bainite effectively compensates for the deterioration of toughness caused by the precipitated phase,and the impact energy at-40°C is 55.3 J.Based on the excellent strength and toughness properties of granular bainite microstructure,the steel preparation process was optimized.The cooling rate after hot rolling was accelerated,and the better matched strength and toughness properties of lath bainitic steel were obtained.The yield strength of the lath bainitic steel is 1334 MPa,the precipitation strengthening contribution was 699 MPa,and the impact energy at-40°C was 63.3 J.The rapid cooling process further inhibits the diffusion rate of Cu rich phase and forms the single structure B₂Cu-rich precipitates with better strengthening effect.The substructure of lath bainite can inhibit the crack propagation and has good impact toughness while greatly increasing the strength.It is observed that CGHAZ is composed of lath martensite and lath bainite by welding thermal simulation experiment on lath bainite steel.FGHAZ consists of fine bainite and martensite.ICHAZ consists of bainite and martensite with distinct size differences.The decrease of the peak temperature of welding thermal simulation results in the slow phase transition rate of HAZ during cooling phase,and the structure of Cu-rich precipitates gradually begins to change.CGHAZ,FGHAZ and ICHAZ contain nanometer Cu rich precipitates with B₂,B₂+9R and B₂+FCC structures,respectively.Due to insufficient precipitation of Cu-rich precipitates in welding thermal simulation,HAZ strength decreased and toughness significantly increased.