| Microalloyed steels based on the precipitation strengthening of second phase particles have been widely used and concerned because of its excellent comprehensive properties.To ensure the slab quality during the continuous casting process is an important cornerstone to realize the efficient production of microalloyed steels.However,a large number of surface crack defects appear frequently at the surface of slab,which seriously restrict the improvement of production efficiency of microalloyed billets,especially the microalloyed hypo-peritectic steel.The reason is that the solidification peritectic transformation process will seriously affect the thermal state,microstructure and stress state of the microalloyed slab,and act together with a large number of secondary phase particles precipitated in the solidification cooling process to deteriorate the high-temperature performance of the slab,and significantly increase the risk of crack formation and even breakout in the weak links of the billet.Therefore,in-depth study on solidification microstructure evolution and second phase precipitation behavior of microalloyed hypo-peritectic steel can provide theoretical and technical guidances for minimizing surface crack defects of billets.Thus,the high temperature microstructure evolution and peritectic transformation process of vanadium microalloyed hypo-peritectic steel(Fe-0.11 wt.%C)and titanium microalloyed hypo-peritectic steel(reducing alloy content,and increasing C to 0.16 wt.%C)with high crack sensitivity were quantitatively studied.Secondly,combined with in-situ observation and theoretical calculation,the precipitation behavior of second phase particles of the titanium microalloyed steel during the solidification and cooling process was studied,and the precipitation and growth mechanism of second phase particles in the slab under the non-equilibrium condition was clarified.Finally,based on the high temperature performance test and microstructure analysis,a non isothermal kinetic model of austenite decomposition was established for predicting phase volume fraction,and the migration behavior of each element during the phase transition process was clarified.The main results of this paper can be summarized as follows:(1)The microstructure evolution and peritectic transition of microalloyed steels with different chemical compositions and cooling rates were studied by ultra-high temperature laser confocal microscope.The peritectic transtion is composed by two events which are peritectic reaction(L+???)and peritectic transformation(L??,???).The peritectic reaction takes place through the local remelting mechanism of?phase to realize the nucleation of?phase,then the liquid phase flows into the remelting notch and reacts with the?phase which is below the?phase to continue the growth of the?phase.The solidification path of microalloyed steel(Fe-0.16 wt.%C)transforms into hyperperitectic solidification sequence.With the increase of cooling rate,the morphology of L/?interface changes from planar to cellular.Higher cooling rate results to a greater initial rate of the peritectic reaction and peritectic transformation,and a less amount of remelted?phase.The critical cooling rate of???massive transformation in Fe-0.11 wt.%C microalloyed steel is 50?/min,and that of Fe-0.16 wt.%C microalloyed steel is 60?/min.(2)The formation mechanism of peritectic reaction and peritectic transformation under different conditions and the effects of C content and alloy content on peritectic solidification behavior were analyzed based on in situ observation results and Thermo-Calc and DICTRA theoretical calculations.In situ observed results show the composition adjustment don't change the mechanisms of the peritectic reaction and the peritectic transformation of microalloyed steel.But the driving force of the peritectic reaction decreases with the increase of C content,which slows down the initial rate of peritectic reaction.When the cooling rate is 10?/min,the initial rate of the peritectic reaction is632?m/sec and 210?m/sec for investigated steels,respectively.DICTRA calculations show the growth of?phase at a lower cooling rate is mainly affected by the temperature and the solute diffusion rate.The remelting zone of?phase is essentially caused by the solute diffusion,and interfacial migration behavior during the peritectic transformation is mainly controlled by the diffusion of C atom.Thermo-Calc calculations show the increase of carbon content will lead to the decrease of?phase formation and the decrease of volume shrinkage amount caused by the peritectic transformation.In addition,the carbon contents of H,J and B points decrease with the increase of Ti and Mn contents,while the carbon contents of H,J and B points increase with the increase of Si content.(3)The precipitation kinetics and growth kinetics of the second phase in the solidification process of Ti microalloyed steel were studied by the microscopic observation and theoretical calculation.The observed results of OM,SEM and TEM show that there are a large number of micron-level Ti N and(Ti0.9-0.93Nb0.01Fe0.06)(C0.01-0.25N0.75-0.99),and nano-level Ti N,Ti4C2S2,Ti(Cx,N1-x)and Ti C.The theoretical calculations show that micron-level Ti N and Ti(Cx,N1-x)precipitate from the liquid phase,and earlier than peritectic reaction.The starting time of Ti(Cx,N1-x)nucleation in the austenite phase is in the order of grain boundary nucleation,dislocation nucleation,and homogeneous nucleation.With the decrease of Ti and N contents,the initial temperatures of Ti N and Ti(Cx,N1-x)precipitation decrease significantly.The initial temperatures of Ti N and Ti(Cx,N1-x)precipitation decrease,but the solid rates fs decrease with the increase of C content.The formed maximum sizes of Ti N and Ti(Cx,N1-x)at the end of solidification are significantly reduced by decreasing Ti and N contents and increasing the cooling rate.(4)The precipitation and growth mecha nism of the second phase in the solidification process of Ti microalloyed steel was clarified by means of ultra-high temperature laser confocal microscope and theoretical calculation.In situ observed results showed that the coarsening rate of Ti N and Ti(Cx,N1-x)in liquid phase is higher than that in solid phase.Ti N and Ti(Cx,N1-x)precipitated from the liquid phase grow in the form of spheroid,and then transform into cube.The critical solid rate of spheroid coarsening zone and cube coarsening zone is 0.897.When the cooling rate is 40?/min,it only takes about1.34-1.45 sec for the size of Ti N sphere to larger than 1?m.Microstructure observation and theoretical calculation results show the growth size of Ti N at the end of spheroid coarsening zone is DS2=(2171.71342±4.43718)/CR,while the growth size of cube Ti N is DC2=(2525.88681±5.20465)/CR at the end of cube coarsening zone?(5)The kinetics of austenite decomposition in Ti microalloyed steel billet was studied using the peak separation and theoretical calculation,and the migration behavior of solute elements was investigated by atom probe tomography.The separation of overlapped peaks shows that the initial temperature of pearlite transformation Ar1 and the end temperature of ferrite transformation Aff are different from Tx.Compared with the results of the lever method and JMat Pro prediction,the phase volume fraction calculated by the peak area method is more consistent with actual microstructure results.The DICTRA calculated results show that the migration behavior of ferrite/austenite interface is controlled by the diffusion of carbon atom in austenite phase.The functional expression of activation energy of ferrite transformation and cooling rate is Q=(143.585±5.63402)+(1.9776±0.41145)ŚCR.After the austenite decomposition,elements Fe,Si,P and Ni are enriched in ferrite phase,while elements Mn,C and Cr are enriched in cementite phase.There are no nonmetallic elements N,S and microalloying elements Ti,Nb and V in the matrix at room temperature. |
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