Study On Multiphase Microstructure And Nano Precipitation Regulation Of High Strength Anti-seismic And Fire Resistant Steel

Compared with reinforced concrete structure,steel structure has the advantages of high strength,light weight,excellent anti-seismic resistance,convenient construction,energy saving and environmental protection,and is gradually expanding its application in the construction field.With more and more attention paid to the reduction of fire resistant coating and further lightweight application of building steel structures,the steel for building steel structures is developing towards higher strength,anti-seismic resistance and fire resistance at the same time.In this paper,we developed a new 550 MPa grade hot rolled high strength anti-seismic and fire resistant steel,based on the composite microalloying design characterized by low C,low Mo,high Nb and the hot rolling process route,focusing on its mechanical properties at room and elevated temperatures,the microstructure evolution and nano precipitation fire resistant mechanism during the heating process.Based on the composite microalloying design with high Mn-Ni-Mo alloying,high Nb-V and the hot rolling direct quenching+intercritical heat treatment process route,a new 690 MPa grade heat treatment high strength anti-seismic and fire resistant steel was developed,focusing on the evolution of multiphase,metastable phase and nano precipitation phases during the intercritical heat treatment process and their effects on room temperature properties,low temperature and high temperature properties.Room temperature yield strength of 599 MPa,yield ratio of 0.75,elongation after fracture of 22.0%and-40℃ KV2 impact toughness of 280J was achieved for the 550MPa grade hot rolled high strength anti-seismic and fire resistant steel.After 600℃ for 180 min,the yield strength sill remined at 419MPa.The microstructure of the hot rolled sample was granular bainite;with the increase of heating temperature,the lath structure merged in the bainite ferrite matrix,but the grain size did not grow significantly;The M/A island constituents decomposed,and supersaturated C was precipitated in the form of cementite and microalloying M(C,N)carbonitride(mainly MC phase).There were only a small amount of nano-sized M(C,N)precipitates in the hot rolled samples with an average particle size of 29.2 nm.A large number of fine M(C,N)were precipitated in the matrix,and their average particle size decreased to 19.4 nm at 600℃,significantly improving the precipitation strengthening effect.With the heating time at 600℃increasing to 180 min,the nano-sized M(C,N)precipitation continued to increased,and the mass fraction of Nb,V and Mo in the precipitates approximated the thermodynamic equilibrium calculation results.On the one hand,the nano-sized M(C,N)precipitation particles after heating directly improved the high temperature yield strength in the form of precipitation strengthening.On the other hand,they could pin the interface and dislocation,stabilizing the high temperature microstructure,and indirectly improving the fire resistance.Typical comprehensive mechanical properties of 690 MPa heat treatment high strength anti-seismic and fire resistant steel:the yield strength was 805 MPa at room temperature,the yield ratio was 0.84,the elongation reached 21.0%,the KV2 value was 101 J at-20℃ and the yield strength was 480 MPa at 600℃.The microstructure in the hot rolled state was directly quenched martensite.After intercritical quenching+intercritical tempering heat treatment,a multiphase microstructure consisting of intercritical ferrite+newly formed martensite+retained austenite+nano-sized M(C,N)carbonitride(mainly MC phase)were obtained.The excellent ductility and low temperature toughness of the steel were related to the low yield ratio and high volume fraction of retained austenite(10%)after the intercritical heat treatment.The matrix still maintained the lath morphology and had good high temperature stability after intercritical heat treatment and 600℃.The nano-sized M(C,N)precipitates in the sample after intercritical heat treatment compensated for the loss of strength due to the softening of tempered microstructure.The amount of M(C,N)precipitates smaller than 10 nm continued to increase at 600℃ tensile test,further ensuring the experimental steel had good fire resistant properties of the experimental.