Deformation Of Rare Earth And Austenite Region On Niobium Precipitation In Ferrite Region Impact

Nb-containing steels are widely used microalloyed steels and the influence of rare earths on the dissolution and precipitation behavior of Nb in steel has received attention.The effect of rare earths on the precipitation behavior of niobium in the ferrite region has been less studied,especially the effect of the solid solution amount in austenite on the experimental results has not been taken into account,and no in-depth studies have been carried out on the size and distribution of the precipitated second phase particles,which hinders the effective use of rare earths in niobium-containing steels.In this thesis,three Nb-bearing microalloyed steels without and with 6 ppm and 48 ppm of rare earth La were investigated by using salt bath holding experiments in the ferrite zone and holding experiments after rolling deformation to study the variation of niobium precipitation behavior in the ferrite zone by measuring the solid solution amount of niobium in steel under different process conditions.The growth process of particles precipitated from the second phase in the ferrite zone was studied;a theoretical model of the kinetics of NbC precipitation in ferrite in steels containing rare earths was established to investigate the influence of rare earth La and austenite zone deformation on the kinetic behavior of niobium precipitation in the ferrite zone.The experimental results showed that the rare earth La promoted the solid solution of niobium at high temperature,and the percentage of solid solution of Nb increased from 83.94%to 95.48%under the condition of holding at 1250℃for 1h;the rare earth promoted the precipitation of NbC during the salt bath holding in the ferrite zone,and the amount of NbC precipitation decreased with the decrease of the salt bath temperature.At the salt bath temperature of 700℃and holding time of 10^4.25s,the percentages of Nb precipitation in 1#,2#and 3#steels were 82.69%,85.45%and 89.08%,respectively,and the average sizes of the precipitated second phase were 18.3nm,15.1nm and 9.6nm,respectively;while at 550℃and holding time of 10^4.25s,the percentages of Nb precipitation were 9.94%,10.36%,12.57%,and the average sizes were 12.7nm,10.5nm,6.4nm,respectively.after rolling deformation at 650℃holding time of 10^4.25s,rare earth La was able to refine the ferrite grains and promote the precipitation of NbC,and the ferrite grains of the experimental steels were 183.3μm,137.2μm,63.5μm,respectively.The percentages of NbC precipitation were 51.83%,55.9%,and 59.94%,and the average sizes of the precipitated phases were 18.4 nm,16.6 nm,and 12.8 nm,respectively.Based on the classical nucleation growth theory,the kinetic theoretical model of NbC precipitation in ferrite was established;based on the experimental results,the precipitation precipitation kinetic curves were plotted using the Avrami equation to obtain the onset time of NbC precipitation in ferrite at different salt bath temperatures,and the PTT curves of precipitation precipitation under the action of rare earth La were plotted.The addition of rare earth elements does not change the shape of the precipitation curve,but can shorten the precipitation onset time and shift the precipitation precipitation kinetic curve to the left;the precipitation onset times of NbC in 2#and 3#steels added with rare earths at 650℃without deformation are 426s and 367s,respectively,and the precipitation onset times of NbC in 2#and3#steels added with rare earths after rolling are 343s and 215s,respectively,and the rolling The deformation caused the precipitation start time to advance by 83s and 152s respectively,and the effect was more obvious with the increase of rare earth content.