The Nucleation, Three Dimensional Morphology And Growth Kinetics Of Ferrite In Low Carbon High Strength Micro-alloyed Steels

In the new generation steel, the microstructure control in the solid-state phasetransformation process has become the most effective way to improve the performances of thesteels. Austenite to ferrite transformation is usually the first solid-state phase transformation insteels during cooling. The grain size and morphology of the ferrite significantly influence thehardenability and mechanical properties of the steels. Based on its industrial application valueand important theoretical significance, the diffusion phase transformation from austenite toferrite has been intensely studied in the last nearly60years. Hence, the deeper understanding inthe3-dimentional morphology and growth kinetics of ferrite in the low carbon high strengthmicro-alloyed steels will significantly contribute to the accurate prediction in the microstructureevolution, and to the designs of the most suitable chemical composition and heat treatmentprocessto obtain the steels with the desired microstructures.In this paper, the serial-sectioning and computer-aided three-dimensional reconstructiontechnique, high temperature confocal laser microscopy, electron back-scattering diffractiontechnology et.al have been utilized to systematically investigate nucleation and growthmechanisms, three dimensional morphology, and growth kinetics of the ferrite formed in a lowcarbon micro-alloyed steel. Main results for this study are as follows：(1) Grain boundary ferrite is the first decomposition product of the high temperatureaustenite during cooling. The morphology of ferrite allotriomorphs nucleated in the grainboundary face varied considerably from one grain to another, even on the same grain boundaryface. Most of face-nucleated ferrite allotriomorphs appeared to be large in one dimension andsmaller in other dimensions. They are better to be described as prolate ellipsoids rather thanexhibiting an equiaxed pancake shape along the grain boundary faces as usually assumed. Ferriteallotriomorphs nucleated at austenite grain boundary edges show the triangular-line shape on thetwo dimensional sections and take the form of triangular pyramids as the three dimensionalmorphology. Ferrite allotriomorphs nucleated at austenite grain boundary corners exhibitirregular shapes. During growth grain boundary ferrite starts to impinge against each other,coarsen and finally cover the austenite grain boundary.(2) Intragranular ferrite idiomorph can be formed at both low and high undercoolings. Atlow undercooling the number and size of intragranular ferrite idiomorphs increased withincreasing holding time. At high undercooling, intragranular ferrite idiomorph formed prior toacicular ferrite. However, they gradually lost its morphological characteristics with increasingholding time and the acicular ferrite began to dominate the microstructure. Intragranular ferriteidiomorph was nucleated on inclusions. The non-specific orientation relationship of intragranularferrite idiomorph to austenite was probably the underlying reason for the formation of theequiaxed shape.(3) Acicular ferrite can be formed on the inclusions under a larger under cooling; thethree-dimensional morphology of acicular ferrite is of a lath rather than a needle as traditionallyassumed; Acicular ferrite lath has special growth directions and its broad face is also parallel tosome special lattice planes of the austenite, which is resulted by its the special orientation relationship to the austenite. The grain refinement mechanism of acicular ferrite is that thepre-formed ferrite lath can partition austenite grains into compartments. The growth ofintragranular ferrite grains formed at later stages during isothermal holding or at lowertransformation temperatures during continuous cooling are thus confined to the smaller zonesand thus have smaller sizes. Intragranular acicular ferrite shows similar growth kinetics withWidmanst tten sideplates in in-situ observation.(4)The growth rate constant of ferrite in nucleated grain boundary edge is larger than thosenucleated in grain boundary face and intragranular ferrite idiomorph. Growth kinetics ofintragranular ferrite idiomorph was similar to that of grain boundary ferrite allotriomorph andfell between paraequilibrium and local equilibrium prediction limits. A transition in proeutectoidferrite growth kinetics from para-equlibrium to negligible partion local equilibrium was observedfrom650°C to750°C below the partion local equilibrium/negligible partition local equilibriumtransition temperature in the investigated steel. This transition is explained in terms of the drageffect of the substitutional alloying elements.(5) Widmanst tten sideplates directly emanated from austenite grain boundaries or from theexisting ferrite allotriomorph through the mechanism of edge-to-face sympathetic nucleation.Face-to-face sympathetic nucleation also occurred on the broad faces of the pre-formed ferritesideplates. The growth of plate on the free surface did not occur at a constant rate but ischaracterized by the acceleration and deceleration. The average lengthening rate ofWidmanst tten sideplates measured in-situ was one order of magnitude larger than the bulk value.The former is close to the prediction assuming paraequilibrium and the later is closer to theprediction assuming the negligible partition local equilibrium.