Study On Control Of Microstructure And Properties Of Ti Microalloyed Low Carbon Steel

Since the last century 60's,micro-alloying technology has been developing rapidly.Adding micro-alloying elements can significantly improve the mechanical properties of steel,Like Nb,Ti is the most widely used microalloying element in modern steel products,and the precipitation effect is remarkable.Combined with TMCP(Thermal Mechanical Control Process)technology can greatly improve the strength,impact toughness and welding performance of steel.At present,microalloyed steels are mainly composed of Nb and Ti,while Nb has less reserves in the crust and it's expensive.With the progress of smelting technology,containing only Ti alloying method has been proposed and gradually promoted.Although the research on Ti microalloyed steel is increasing day by day,there are few studies on the effect of Ti composition on the microstructure and properties of alloy steel of C-Mn-Ti system.Based on the above reasons,In this paper,five different Ti content experimental steels are designed to study the influence of Ti on Microstructure and Properties.And the reason of the change of mechanical properties is analyzed,which provides some theoretical guidance for the composition and process development of titanium microalloyed steel.At the same time,based on the 2150 hot rolling production line of Anshan Iron and Steel Company,the mechanical performance prediction model of Ti microalloyed steel for 2150 hot rolling production line is developed on the basis of the prediction model of mechanical properties of traditional C-Mn steel.The main contents of this paper are as follows:(1)The deformation behavior of high temperature austenite of titanium microalloyed steel is studied by thermal simulation experiment,and The results show that the recrystallization activation energies of five different Ti content experimental steels are 190,238,233,296 and 304 kJ/mol respectively.Experimental results show:With the increase of Ti content in low carbon steel,the recrystallization activation energy increases and the recrystallization difficulty increases,and its growth trend was parabolic growth.When the Ti content is high,the effect of Ti-refined initial austenite grains on the recrystallization is to counteract the inhibition of recrystallization by some of the solid solution Ti and precipitated particles.In addition,the dynamic recrystallizat ion model of low carbon Ti microalloyed steel was obtained by means of regression.(2)The phase transition law of high temperature austenite in titanium microalloyed steel is studied by thermal simulation experiment.The static CCT curves are drawn and the influence of other process parameters such as Ti content and cold speed on phase transition is analyzed.It was found that the effect of Ti addition on the static continuous cooling is low,the grain size of ferrite and Ar3,Ar1 don't change significantly with the increase of Ti content.At the same time,increasing the cooling rate can form more non-diffusion phase,refine the grain,reduce Ar3,Ar1.(3)The Optical micrographs of titanium microalloyed steel after laminar cooling and coiling are investigated by thermal simulation,and the precipitation behavior under different cooling rates is analyzed.The experimental results show that the control of the cooling rate and the final cooling temperature of the crimping process have a great influence on the steel properties.Increased cooling rate can refine the grain,change the final organization,improve the hardness and strength.While Ti content has little effect on it.Hardness value and strength value are affected by Ti content similarly.(4)The effect of Ti content on microstructure and mechanical properties of low carbon steel was investigated by laboratory hot rolling experiment.Experiments show that with the increase of Ti content in steel,the yield strength,tensile strength and yield ratio of steel are improved and the elongation decreases.When Ti>0.042%,the content of available titanium increases significantly.The dispersion of TiC in ferrite leads to the yield strength and the tensile strength of Ti steel increases with the Ti content showing a great difference around Ti=0.042%.When Ti content is higher than 0.042%,the increase of Ti content has a more obvious effect on grain refinement.(5)The mechanical properties of the experimental steels are analyzed by Hall-Petch fine grain strengthening formula and Ashby-Orowan precipitation strengthening model.The results show that the yield strength of the model is in good agreement with the measured value,and the contribution of fine grain strengthening to the strength increment is the largest,and the precipitation strengthening is the main reason for the difference of the mechanical properties of the five kinds of experimental steel.(6)Based on Anshan Iron and Steel 2150 hot rolling production line,the whole process temperature field is simulated.Based on the above research on the recrystallization,phase transition and precipitation behavior of Ti microalloyed low carbon steel,the mechanical performance prediction model of Ti micro-alloy steel for 2150 hot rolling production line is developed on the basis of the prediction model of mechanical properties of traditional C-Mn steel.The mechanical properties of industrial steel Q345B are predicted by this model,and the accuracy is achieved.