Study On The Effect Of Ferrite On The Austenite Grain Refinement For Hypoeutectoid Microalloyed Steel During Continuous Casting

This paper focused on the refining of the prior austenite grain in the surface of micro-alloyed slab produced by Pansteel and Baosteel.The process of initial solidification,the transformation of austenite to ferrite,and the thermal cycle of slab surface during the second cooling section of continuous casting were simulated by using confocal scanning laser microscope(CSLM).The effect of ferrite on prior austenite grain during solidification,the transformation of austenite from ferrite,and the refinement mechanism of ferrite austenite grain during double phase transformation process for the micro-alloyed steel were deeply investigated.In addition,considering the limitations of the tensile test in characterizing the formation tendency of the surface transverse crack for the micro-alloyed steel,an improved model was established to estimate the susceptibility of transverse cracking on the slab corner of the micro-alloyed steel based on the features of high-temperature microstructure.Furthermore,a criterion was proposed to evaluate the sensitivity of the transverse crack on slab surface of the micro-alloyed steel.The main results and conclusions of this work are listed as follows:(1)The solidification cooling process was simulated by CLSM,and the growth behavior of austenite was investigated.To characterize the initial austenite grain diameter,the dendrite spacing was measured by in situ observation.The variation of austenite grain diameter with cooling rate is consistent with the austenite initial grain size.Based on the analysis of austenite grain growth,the relationship between the grain boundary migration rate and the cooling rate was established.When the cooling rate is higher than 2 oC /s,the grain boundary mobility has an exponential relationship with the cooling rate.Based on the above results,the existing austenite growth model was modified.The prior austenite grain size is predicted by the large cooling rate,and the prediction error is less than 10%,which makes up the deficiency of the original model.(2)The transformation of austenite to ferrite during the cooling process of slab was simulated.The volume fraction and the morphology of proeutectoid ferrite were analyzed at different cooling conditions.With the increase of cooling rate,the volume fraction of ferrite increased firstly and then decreased.When the cooling rate is 4oC/s,the volume fraction of ferrite is up to 80.17%.The temperature during cooling process is an important factor to control the volume fraction of ferrite.The ferrite volume fraction increases with the decrease of cooling temperature.When the cooling temperature is controlled at 640?,the proeutectoid ferrite volume fraction is close to the theoretical amount of change.The proeutectoid ferrite was studied by electron backscatter diffraction.The cooling rate can change the ferrite nucleation and grain boundary misorientation angle,which leads to the precipitation of proeutectoid ferrite formed different morphology.The proeutectoid ferrite iron with different morphology will lead to the original austenite ferrite grain boundary near the size and density of small angle grain boundaries.It is the fundamental reason of the initiation and propagation of micro cracks.(3)The influencing factors of austenite grain refinement were studied by simulating the thermal process of double phase transformation of microalloyed steel.In addition to the proeutectoid ferrite as the two phase transition source,the morphology of austenite nucleation also have different effects.The ferrite grain orientation is less than that of the grain boundary [1 1 0] plane,which is beneficial to the nucleation of austenite grain in the process of slab heating.In the double phase transition process,the final size of the newly formed austenite grain depends on the temperature and the rate of heat recovery determine,and the essence of the rate control is to control the heating time.When the heating temperature is high,the heat recovery time should be reduced so as to avoid the grain coarsening.On the other hand,when the heat recovery temperature is low,the heating time should be prolonged to avoid the accumulation of the unstable austenite.In addition,the distribution of the second phase precipitates in steel not only inhibits the ferrite to austenite phase transformation,but also plays a role in pinning the new austenite grain growth in the heat recovery process.Based on the analysis of the influencing factors of austenite grain refinement,the existing dual phase change process parameters were optimized.According to the experimental steel,using double phase change process optimization(cooling conditions: 5oC/s cooling to 900 oC,then 1 oC/s cooling to 640 oC;heat condition: 3 oC/s to heat to 1000 oC),the average grain size of austenite can be refined to below 0.3mm.For the prior double phase change working art,refinement rate of the austenite grain is increased by 31%.(4)According to the analysis of the microstructure of billet surface temperature stiffness,the concept of the original austenite equivalent diameter(D?P)was proposed,Considering that the film like proeutectoid ferrite and the precipitation of micro alloying elements weakens the original austenite grain boundaries after the correction of the original austenite grain size,its essence is in the calculation of casting billet surface under temperature tissue stiffness.When the original austenite grain size can be measured,the sensitivity criterion of the transverse crack on the surface of the micro alloyed steel slab can be determined by the size of D?P.The critical D?P diameter on the slab surface is 0.69 mm.When the austenite grain size cannot be obtained,the casting microstructure,ferrite grain size(DF),ferrite mischcrystal index((35)E)and film like proeutectoid ferrite volume fraction(fMF)can be used to determine the transverse crack sensitivity.When DF<35?m,(35)E <1.0,fMF <3%,precipitation distributed evenly,and the crack index is close to 0.In this case,the transverse cracks are difficult to form on the slab surface.When 35?m <DF<50?m,1<(35)E <2,3%< fMF<5% E,chain distribution of the precipitates is observed,and the crack index is less than 35.In this case,the crack sensitivity is weak.When DF >50 ?m;(35)E >3,fMF >5%,chain distribution of the precipitates is observed,and the crack index is higher than 50,meanwhile,the transverse crack sensitivity of slab surface is strong.