Research On Microstructure And Second Phase Regulation Of Ultra Low Yield Point Steel

Low yield point steel,which has a excellent combination of lower yield strength and nice deformability,it can quickly enter the yield and consume seismic energy through reciprocating plastic deformation during the earthquake.Low yield point steel is one of the important materials that in energy dissipation and seismic technology,and it will have a great application prospect in construction industry.First of all,the ultra low yield point steel was prepared through two composition design of ultra pure industrial pure iron plus single Ti and single Nb,the microstructures and properties under hot rolling and heat treatment were analyzed,and the softening mechanism was explained.Secondly,the effects of S content on the structure and properties of ultra low yield point steel was studied by thermal simulation experiment and hot rolling in laboratory.Finally,the low cycle fatigue properties of 100 MPa and 160 MPa low yield point steels are compared and analyzed,as to provide data reference for their practical application.The main contents and conclusions involved include three aspects:The microstructure,properties and softening mechanism of three test steels were studied by means of room temperature tension,low temperature impact,OM,EBSD,TEM and SEM.The results show that composition design of ultra-low carbon single Ti or Nb combined with post-rolling softening annealing can realize the preparation of100 Mpa ultra low yield point steel.The annealing temperature in 750⁸50?,grain coarsening reduces the fine grain strengthening increment,precipitates coarsening reduced precipitation strengthening increment.The annealing temperature in850⁹50?,the grain has a significant coarsening to more than 90?m makes the fine grain strengthening increment is further reduced,but the carbon nitrogen atoms redissolve increases the solid solution strengthening increment.The feasible softening annealing temperature of 2#steel?single Ti?is 750⁸00?,and the best comprehensive properties are obtained at 800?.The feasible softening annealing temperature of 3#steel?single Nb?is 750⁹00?,and the best comprehensive properties are obtained at 850?.The effect of S content on the structure and properties of ultra low yield point steel was studied by Gleeble 3800,Vickers hardness tester,OM,TEM and EDS.The results show that the ferrite grain size of the test steel containing 0.0077%S is larger than the test steel containing 0.0018%S,but the Vickers hardness is lower.The type,size and distribution of precipitated particles in steel are affected by different S content.Elliptical Ti₄C₂S₂ and TiS precipitated particles with sizes ranging from 100to 600 nm are dominant in high S steel,while TiC precipitated particles with sizes less than 100 nm are dominant in low S steel.The number of precipitated particles in high S steel is less than low S steel.Contrasting the properties of hot rolled sheets in laboratory,the yield and tensile strength of high S steel is lower than low S steel,but there is no obvious difference in plastic toughness.It showed that increasing S content from 0.0018%to 0.0077%is more conducive to the production of 100 MPa ultra low yield point steel.The low cycle fatigue properties of 100 MPa and 160 MPa low yield point steels were studied by means of axial constant amplitude low cycle fatigue test and SEM observation of fatigue fracture.The results show that the low cycle fatigue property of100 MPa test steel is better than 160 MPa test steel.With the increase of strain amplitude from±1%to±2%,the cycle number of 100 MPa test steel is gradually greater than 160 MPa test steel.Comparing 100 MPa test steel with 160 MPa test steel under 2%strain amplitude,the former's fatigue fracture extension area is wider,the tire pattern is more,and the concave and convex degree is more obvious.