| In the age of low carbon development,technological advances in energy,construction,transport and other industries are increasing the demand for low-cost,lightweight structural steels.Low carbon microalloyed steels produced by controlled rolling and cooling technology do not rely on large and expensive alloying elements,but rather improve the strength and toughness through the combination of fine grain strengthening and precipitation strengthening,which is of strategic development value.Amongst the various microalloy elements,Ti has significant cost effectiveness and excellent precipitation strengthening effect,based on which the Ti microalloyed steel has a promising development prospect.At present,the main problem that restricts the development of Ti microalloyed steel is the control of the precipitation process of nano-scale Ti C.Under the conventional controlled rolling and cooling process,the strain-induced precipitation of Ti C after rolling has the effect of inhibiting austenite recrystallization and promoting ferrite grain refinement,while the interphase precipitation and supersaturated precipitation of Ti C during continuous cooling and coiling are the main sources of incremental precipitation strengthening.Therefore,control of the Ti C precipitation process is necessary to combine the effects of fine grain strengthening and precipitation strengthening and to improve the stability of the properties of Ti microalloyed steel.Compared to single Ti microalloying,the nanoscale(Ti,Mo)C precipitated in Ti-Mo complex microalloyed steel has a stronger thermal stability and more diffuse distribution,which can further enhance the control of precipitates.For low carbon Ti/Mo steels,most of the previous works have been devoted to maximising the precipitation strengthening effect of nanoscale carbides,lacking an in-depth research on strain-induced precipitation,with no systematic investigation of the relationship between strain-induced precipitation and recrystallization as well as phase transformation being reported.In this paper,the strain-induced precipitation behavior of low-carbon Ti and Ti-Mo steels with the same basic composition was studied in comparison,moreover,the correlation between strain-induced precipitation and austenite recrystallization,as well as the effect of strain-induced precipitation on the continuous cooling phase transformation of supercooled austenite were investigated in depth.The following conclusions were obtained:(1)Research on the strain-induced precipitation of nanocarbons during stress relaxation in deformed austenite of low-carbon Ti/Mo steels at 860-960°C revealed: Strain-induced precipitation of nanocarbides occurred during stress relaxation in deformed austenite,leading to the appearance of a plateau period in the relaxation curve.The PTT curves for the experimental steels were plotted by combining the onset of the plateau period with the stress relaxation temperature,both of which showed an inverted “S” shape.Unlike the common “C” shaped PTT curves for microalloyed steels,the preferential occurrence of recrystallization above 940℃delayed subsequent strain-induced precipitation.Below 940°C,both PTT curves returned to the typical “C” shape,with the fastest precipitation nose temperature at 900°C.Comparing the PTT curves of the two experimental steels,the addition of Mo overall extended the strain-induced precipitation plateau period(150s→500s).HRTEM combined with EDS characterization showed that the strain-induced precipitation particles in both experimental steels were Na Cl-type facecentred cubic structures,with Mo replacing some of the Ti in the(Ti,Mo)C particles by replacement solid solution.At a stress relaxation temperature of 900°C and a stress relaxation time of 150 s there was little difference in the size of the precipitated particles between the two experimental steels,and at a stress relaxation time of 500 s there was a refinement in the mean particle size of(Ti,Mo)C compared to Ti C(21.4±5.7nm→17.0±5.5nm),demonstrating that the greater thermal stability of(Ti,Mo)C inhibited the Ostwald ripening of the precipitated particles at this stage.of Ostwald ripening.At 900°C,the difference in precipitate size between the two experimental steels at 150 s of stress relaxation was not significant,while at 500s(Ti,Mo)C showed a marked refinement in precipitate size compared to Ti C(21.4±5.7nm→17.0±5.5nm),demonstrating that(Ti,Mo)C had greater thermal stability and thus inhibited Ostwald ripening of the precipitated particles at this stage.(2)Research on the interaction between strain-induced precipitation and austenite recrystallization during stress relaxation in low-carbon Ti/Mo steels revealed: Based on the experimental data of stress relaxation and original austenite metallisation,combined with the theoretical recrystallization kinetic model and the comparative model of recrystallization driving force v.s.precipitation particle pinning force,the correlation between strain-induced precipitation and recrystallization during stress relaxation is summarised as a process of competing dominance and inhibiting each other.Strain temperature and isothermal holding time have been found to significantly influence this correlation.In terms of strain temperature,the nose temperature at which SIP occurred the fastest was 900°C,whereas recrystallization occurred more rapidly with increasing holding temperature.In terms of isothermal time,the preferential onset of recrystallization or strain-induced precipitation substantially inhibited and delayed each other,and this inhibition diminishes with increasing isothermal time until its effect is negligible.(3)Research on the role of strain-induced precipitation of low-carbon Ti/Mo steels on the continuous cooling phase transformation of supercooled austenite revealed: During continuous cooling,the phase transformation temperature of the experimental steels showed a single decreasing trend with increasing cooling rate,accompanied by the refinement of average ferrite grain size and the increase in microhardness.The reduction in phase transformation temperature meant the increase in austenite subcooling,which promoted the nucleation and grain refinement of the ferrite phase transformation and provided the fine grain strengthening effect.With the gradual increase of the subcooling degree,the diffusion ability of the replacement solid solution elements Ti & Mo and interstitial solid solution element C in austenite decreased continuously,thus the microstructure after phase transformation changed from uniformly diffused polygonal ferrite to locally diffused quasi-polygonal ferrite and non-diffused bainite ferrite.The microalloying element Mo solidly dissolved in the matrix reduced the continuous cooling phase transformation temperature of Ti steel as a whole,therefore resulting in the refinement of ferrite grains as well as the enhancement of microhardness.On this basis,the effect of strain-induced precipitation on the continuous cooling phase transformation temperature was controlled by the cooling rate.At a cooling rate of 0.1°C/s,the occurrence of strain-induced precipitation during stress relaxation lowered the phase transformation temperature.This is because strain-induced precipitation also occurred during the continuous cooling process prior to phase transformation at this low cooling rate.Excessive strain-induced precipitation caused significant depletion of the deformation energy storage,thereby reduced the ferrite phase transformation driving force.As the continuous cooling rate gradually increased to 5°C/s,the phase transformation temperature gradually increased after strain-induced precipitation occurred.This phenomenon can be attributed to the fact that strain-induced precipitation consumed the microalloy elements solidly dissolved in the matrix,making the role of solid solution elements in delaying the phase transformation weaker;at the same time,the precipitated particles can be used as nucleation sites for the ferrite phase transformation,thus increasing the total phase transformation nucleation rate.In addition,the ferrite grains were refined by strain-induced precipitation at all cooling rates,confirming that strain-induced precipitation contributed to the overall ferrite phase transformation. |
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