Theoretical Studies On Acicular Ferrite Transformation In Micro-alloyed Steels

Acicular ferrite (AF) microstructure in micro-alloyed steels, which nucleates around non-metallic inclusions, especially oxide inclusions, has high strength and good toughness and possesses strong grain-refining ability by itself. The ultra-fine grained steel with excellent weldability is likely to be produced by forming in materials a great number of fine and dispersive micro-inclusions which is helpful to induce AF to nucleate around them through oxide metallurgy technology. However, the theoretical studies on AF transformation are incomplete at present. In this paper, AF phase transition was studied using bainite transformation theory for reference. Based on the fact that carbon-depleted zones come into being during the incubation period, using Kaufman, Radcliffe, Cohen (KRC) activity model, Lacher, Fowler, Guggenheim (LFG) activity model, McLellan, Dunn (MD) activity model, respectively, and super-element algorithm, thermodynamic models of AF transformation in carbon-depleted zones of austenite in terms of proeutectoid diffusion and martensite-type shear were established respectively, meanwhile, thermodynamic models of AF nucleation and growth in carbon-depleted zones of austenite were established, and used in the numerical simulation of a certain micro-alloyed steel. The total driving force of AF transformation and driving force of AF nucleation and growth were calculated, and the intermediate transformation mechanism was analyzed. The activation energy to form an AF crystal nucleus in the form of diffusion incoherent way and shear coherent way were calculated respectively, besides, the activation energy to form an AF crystal nucleus between prior austenite interfaces and on intragranular inclusions were calculated respectively, and the influence of carbon-depleted zone on AF nucleation was investigated. The results show that, in aspect of thermodynamics, AF is most likely to transform in both proeutectoid diffusion way and martensite-type shear way in carbon-depleted zones of austenite, and with the variation of carbon content in the carbon-depleted zone, the transformation mechanism of AF tends to be different. The tie up between diffusion mechanism and shear mechanism is unveiled from thermodynamics for the first time, which provides a thermodynamic basis for explaining the complicated characteristics of AF intermediate transformation as well as enriches and develops the AF transformation theory. The total driving forces of AF transformation and driving forces of AF nucleation and growth show the similar tendency, increasing with the decreasing of carbon content in carbon depleted zones and AF transformation temperature, however, the driving forces of AF transformation are smaller than that of AF nucleation and growth. At 943K, the absolute value range of driving force of AF transformation in proeutectoid diffusion way is from 417 to 610 J/mol, while in martensite-type shear way is from 394 to 610 J/mol. The activation energy to form an AF crystal nucleus in the form of shear coherent way is much higher than that in diffusion incoherent way. The forming of carbon-depleted zones reduces the activation energy to form an AF crystal nucleus and increases the nucleation potential of AF on intragranular inclusions. The phenomenon that AF is induced to nucleate around intragranular inclusions firstly can be well explained by carbon-depleted model.