Study On Process And Fracture Mechanism Of High Strength And Toughness Anti-seismic Structural Steel Q550GJ

High strength & toughness anti-seismic structural steel Q550 GJ has been studied in the paper with design of low carbon content,appropriate content of alloy elements Mo,Cr,Ni and micro-alloy elements Nb & Ti.Through using TMCP process,microstructure of duplex phase bainite + ferrite can be obtained,and then high strength,excellent toughness and low yield ratio can be achieved accordingly.According to the Gleeble-1500 thermo-simulation test,mechanical test,OM/SEM inspection and CT analysis on voids initiation &growth,the conclusion can be made as below:The CCT curves of steel Q550 GJ indicate that microstructure of ferrite,pearlite and bainite can be obtained when the cooling rate is 0.1 to 5 deg.C/S;microstructure ferrite,bainite and martensite can be obtained when the cooling rate is 5 to 50 deg.C/S.The start cooling temperature and final cooling temperature of control cooling process has big influence to microstructure and yield ratio of testing steel: the lower the srart cooling temperature,the more the ferrite structure and the less the bainite,which leads to lower strength & yield ratio;the lower the final cooling temperature,the thinner the bainite structure,the more the M/A Island,which leads to higher strength and yield ratio accordingly.Through adopting appropriate start cooling temperature and final cooling temperature,the anti-seismic steel can get high strength,high toughness and low yield ratio.According to CT analysis on void initiation & growth,it's found the ductile fracture mechanism of high strength & toughness anti-seismic steel Q550 GJ is that single large void resulted from crack & inclusion connects to each other gradually at first,and then the connections coalescence with large amount of little voids initiated from M/A Island in the end,which leads to final ductile fracture.