Study On 3D Morphology And Induction Mechanism Of High-rise Building Steel Intragranular Ferrite

High performance building structural steel is referred to as high-performance building steel,which is mainly used in super high-rise buildings,large-span airports,and large-scale buildings such as workshops with steel as the main structure.In order to adapt to the trend of building structure to large-scale,weight reduction,and reduce the difficulty of welding,it is necessary to improve the toughness and plasticity of steel on the premise of ensuring that the steel has sufficient strength.Aiming at the research of improving the toughness and plasticity of high-performance building steel with this idea,taking Q420 high-performance building steel as the object,through oxide metallurgy technology,optimizing steel composition and heat treatment system,improving the precipitation rate of acicular ferrite in the crystal,achieving the purpose of improving the toughness and plasticity of Q420 high-performance building steel,and providing theoretical basis for industrial production.1)Optimizing chemical composition of Gaojian steel,smelting in laboratory by vacuum electromagnetic induction furnace,and observing metallurgical structure of smelting sample,it is found that the increase of Nb and Mg element content has obvious effect on refining grain and promoting acicular ferrite precipitation in grain.2)The CCT curve of the experimental steel was drawn.It was found that the temperature range of acicular ferrite precipitation was 739?696?,the cooling rate was 10?15?/s,and the best cooling rate was 12?/s.3)The austenite structure of experimental steel was observed by high temperature confocal microscope.The change of austenite grain size of experimental steel was analyzed.It was found that the suitable temperature for the precipitation of acicular ferrite was 1250? and 10 min.4)By reconstructing the three-dimensional ferrite morphology of experimental steel with FIB,it is found that the surface of intragranular acicular ferrite and intragranular equiaxed ferrite is smooth,with fine grains and no sub grains,and the formation of interlocking structure is conducive to improving the toughness and plasticity of steel;there are sub grains in the shape of grain boundary ferrite,with coarse structure and pits on the surface,which may be the origin of cracks and affect the quality of steel.5)It is found that the growth rate of grain boundary ferrite is significantly higher than that of intragranular ferrite by in-situ observation.After isothermal treatment at 700?,the precipitation of grain boundary ferrite is reduced,and the proportion of intragranular ferrite is significantly increased,which is conducive to improving the toughness and plasticity of steel.Figure 54;Table 10;Reference 88.