| At present,the development of structural steel is becoming more and more high strengthened and toughened,and the microalloying technology coupled with the thermal mechanical controlled process is one of the effective methods to obtain a good combination of strength and toughness.In this work,the Nb-V-Ti microalloy system was analyzed,the equilibrium precipitats and their chemical compostions were calculated by Thermo-Calc thermodynamics software,as well as investigated by the field emission scanning electron microscope and transmission electron microscope.The static and dynamic continuous cooling transformation rules of the undercooled austenite of the experimental steel were studied by the quenching dilatometer and thermo mechanical simulator.The effect of increasing Nb content and recrystallized deformation on the microstructure evolution were comparative analyzed.The thermodynamic calculation using Thermo-Calc show that the precipitated carbonitrides in the experimental steel are Ti-rich high temperature particles,Nb-rich medium temperature particles and V-rich low temperature particles,respectively.The electron microscopy analysis shows that the precipitate type is consistent with the abovementioned calculation results.The submicron particles precipitated at the high temperature austenite region can pin grain boundaries and induce the formation of intragranular ferrite,resulting the increase of the strength and toughness by grain refinement strengthening.The nano-sized particles precipitated at the medium temperature of two-phase region and the low temperature ferrite region can effectively hinder the dislocation movement,and enhance the strength of structural steel through precipitation strengthening.During the static continuous cooling process of undercooled austenite,the transformation of ferrite,pearlite,bainite and martensite is happened,and the corresponding range of cooling rate is 0.1-20°C/s,0.1-5°C/s,1-30°C/s and 10-30°C/s,respectively.When the cooling rate increases from 0.1°C/s to 30°C/s,the ferrite content decreases,combined with the average grain size.The content of pearlite and bainite first increases and then decreases.Meanwhile,the martensite content is increased.With Nb content increasing from 0.05wt% to 0.10wt%,the content of ferrite and martensite increases by 3-11% and 2-25%,respectively,and the content of pearlite and bainite decreases by 1-6% and 1-25%,respectively.Moreover,the ferrite grain size decreases by0.15-1.38?m and the microstructure is refined.In the dynamic continuous cooling process of undercooled austenite,the transformation of ferrite,pearlite,bainite and martensite is also happened,and the corresponding range of cooling rate is 0.1-30°C/s,0.1-8°C/s,3-30°C/s and 15-30°C/s,respectively.With the increase of cooling rate,the average grain size of ferrite in Nb-1decreases from 14.0±0.59?m to 2.3±0.27?m.The effective grain size first decreases from12.06?m to 5.98?m and then increases to 6.77?m.The proportion of large angle grain boundary decreases from 0.726 to 0.618.The average grain size of ferrite in Nb-2decreases from 13.3±1.31?m to 2.3±0.45?m.The effective grain size first decreases from10.62?m to 7.46?m and then increases to 9.61?m.The proportion of large angle grain boundary decreases from 0.734 to 0.509.As Nb content increases,the effective grain size is refined and the large angle grain boundaries are increased,which improves impact toughness of structural steel.By comparing the static and dynamic continuous cooling transformation curves of Nb-1 and Nb-2,the transformation regions of ferrite,pearlite and bainite do not change obviously,but the beginning temperature of martensite transformation decreases with Nb content increasing from 0.05wt% to 0.10wt%.With the recrystallized deformation,the continuous cooling transformation curve of structural steel moves up,namely,the transformation regions of ferrite,pearlite and bainite expanding,and the beginning temperature of martensite transformation increases.The effect of recrystallized deformation on microstructure evolution is more significant than the effect of increasing Nb content.As the cooling rate increases,the microhardness of Nb-1 and Nb-2 increases from174±15.4HV0.1 and 167±13.4HV0.1 to 332±14.4HV0.1 and 320±9.8HV0.1,respectively.With the recrystallized deformation,the microhardness increases from 198±35.4HV0.1 and197±23.3HV0.1 to 321±9.2HV0.1 and 316±11.8HV0.1,respectively.Based on the above analysis,the increased addition of Nb can be replaced by the recrystallized deformation to obtain good strength and toughness of structural steel and to save cost at the same time.Furthermore,the suitable range of cooling rate is 5-8°C/s,which can not only ensure the proportion of hardening phases in micro-constituent to increase strength,but also improve toughness by the refined microstructure. |
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