Study On High Temperature Mechanical Properties And Crack Generation Mechanism Of Nb Bearing High Strength Steel

The application of Nb microalloying technology in Q355 GJ steel has greatly improved strength properties of steel,making it the most widely used high-strength structural steel.However,Nb can improve crack sensitivity of slab,which seriously affects continuity of production and properties of rolled products.In order to solve the problem of transverse crack in slab,it is necessary to understand high temperature properties of Q355 GJ steel and temperature distribution in secondary cooling zone.The mechanical properties of ingot at high temperature were tested by Gleeble-3500 thermal simulation test machine;precipitation of the two-phase particles was observed by transmission electron microscopy(TEM);the temperature drop process of slab in secondary cooling zone is calculated by numerical model.The results show that microstructure of Q355 GJ steel is mainly ferrite and pearlite.When Nb content in steel is 0.031 %,the grain size of steel is small and distribution of structures is uniform.The third brittle zone of sample is 900 ??800 ?,and the increase of Nb content does not widen low temperature brittle zone of steel obviously.When Nb content is 0.031 %,the tensile and yield strength of steel are relatively ideal;the second phase are mainly square and star(Ti,Nb)(C,N)composite precipitates.Increasing Nb content can raise the precipitation of second phase.Most of second phase(84 %)in ingots with 0.031 % Nb content are less than 80 nm.The continuous increase of Nb content can not improve precipitation of two-phase particles in steel,but increase the size of two-phase particles.In order to solve the problem of transverse crack in corner of Q355 GJ slab,the secondary cooling water distribution system was optimized.The results show that when water quantity in 5 zone of second cooling system is reduced by 20 %,and that in 6,7 and 8 zones of second cooling system is reduced by 25 %,the corner temperature of slab in straightening zone can be increased from 889 ? ?859 ? to 929 ? ? 904 ?.On the premise of ensuring the continuous casting running smoothly,the straightening of low temperature brittle zone at the corner of slab is effectively avoided,and the incidence of corner crack of slab is reduced.Figure47;Table19;Reference 93...