Investigation On The Microstructure And Performance Of Steels For Aseismatic And Fire-Resistant Building

The earthquake,fire and other disasters resulted from the deterioration of the natural environment on the earth and man-made damage to the natural environment in the course of human existence will cause a serious,even fatal damage to buildings.Such damage to buildings not only causes tremendous economic losses,but also affects the human survival.Up to present,people have found that the core of building materials by which such destruction may be reduced or resisted and with sufficient quantity and good economical effiency are ferrous materials.So,people put forward higher requirements for the constructional steel.The shock resistance and refractoriness are two major problems which must be urgently solved for constructional steel.Therefore,the investigational on constructional steels with goog shock resistance and refractoriness become the hot spots of studies in recent years.In the present paper,based on the analyses of the effect of various alloying elements in the traditional constructional steels and the principle of no obvious increase in cost on the premise of guaranteeing or improving the performance of the constructional steels,a series of different microalloyed constructional steels with good shock resistance and refractoriness were designed.The microstructures and mechanical properties of different building steels with different components and subjected to different heat treatment processes were studied by means of metallographic microscope,scanning electron microscope,transmission electron microscopy,thermal simulation and mechanical property test and the relationship among composition,process,microstructure and performance was analyzed,which provides theoretical and technical supports for the development and industrial production of high strength seismic and fire resistant steels.The obtained main conclusions are as follows:Microstructures were varied by three different heat treatments(or quenching paths),namely step quenching(SQ),intermediate quenching(ImQ),and intercritical quenching(IcQ).For the SQ specimen,the microstructure showed two phase aggregates of large,blocky shaped martensite islands surrounded by the coarse ferrite matrix.On the other hand,the ImQ specimen revealed fiber dual phase structure,consisting of lath martensite,lath-like ferrite,and small amount of polygonal ferrite.The IcQ specimen had the microstructure of martensite islands dispersed in polygonal ferrite grain boundaries.The ferrite grain size of IcQ specimen was fine compared to that of SQ specimen.The yield strength and tensile strength of test steel could respectively reach 400-490 MPa and 750y950 MPa by adopting proper intercritical quenching.The maximum elongation and breaking elongation of the experimental steel can reach 12?14%and 18?24%respectively,and the plasticity increases slowly with the increase of IcQ temperature.By studying the effect of trace sulfur and boron on the mechanical properties under high temperature,boron added to the high-purity iron sulfur-containing significantly inhibited high temperature intergranular cracking sulfur-induced and increased high temperature plasticity.This is due to boron segregation at the grain boundary of iron,which significantly reduced the critical segregation of sulfur as a critical role in the high temperature intergranular cracking and its own grain boundary bonding strength.Four kinds of low carbon building steels with different content of Mo,Nb and V were designed.The microstructures of four kinds of steels are mainly composed of ferrite with degenerated pearlite and granular bainite.The effect of Mo+Nb alloying on improving the properties of refractory steel is better than Mo+V alloying.The ratios of high temperature and room temperature yield strength are lower than that of 0.5 in four kinds of hot rolled steels.The decrease of yield strength under high temperature is mainly due to grain coarsening dislocation annihilation and the growth of cellular microstructure.(C,N)and the interface between Nb(C,N)and the substrate,the diffusion of and can be improved obviously High temperature yield strength of experimental steel.Strengthening the grain boundary of Mo segregated in grain,hindering Nb(C,N)growth and the inhibition of the diffusion of C and N atoms of Mo segregated in the interface between Nb(C,N)and the substrate can improve the high temperature yield strength of experiment steel.The effects of deformation and temperature on the formation of acicular ferrite in Nb-V alloyed fire-resistant steels with low Mo content were studied.The results show that with the increase of deformation,the size of ferrite nucleation at grain boundaries of the original austenite as well as its fraction increase,while the lath width of intragranular acicular ferrite gradually reduces and its fraction decreases slightly.The deformation with a true strain of 0.2 is a benefit to the formation of fine acicular ferrite.Dislocation and precipitates generated during the compression deformation in the austenite non-recrystallization region provide more favorable nucleation position,which plays an important role in grain refinement of ferrites.The microstructure and mechanical properties of Nb-V microalloyed fire-resistant steel after thermo-mechanical control process(TMCP)and simulated welding were investigated.It was found that the microstructure after TMCP was composed of a small amount of polygonal ferrite + acicular ferrite + bainite,containing the precipitates with a size of 5 to 8nm.The yield strength of the experimental steel at 600? is 415 MPa,and higher than 166 MPa under hot-rolling state.Study on Microstructure and properties of simulated welding heat affected zone shows that the microstructure is mainly composed of lath martensite and bainite structure and hardness has great deviation compared with that of the matrix when the energy input is low.With the energy input increasing,the microstructures are polygonal ferrite of fine grain,granular bainite,acicular ferrite and internal V(C,N)precipitates of high volume fractionwith the size of 25?30 nm.The fine VN precipitates promote the intragranular nucleation of the acicular ferrite and improve the toughness of the steel.