An integrated computer model with applications for austenite-to-ferrite transformation during hot deformation of Nb-micr
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I. INTRODUCTION
HOT working of low-carbon steels is generally carried out over a wide range of conditions in which the deformation-temperature range of the single-phase austenite is stable. However, it is well established that a new and attractive combination of properties can be obtained when the final product has a structure that comes from deformation performed in the two-phase (austenite ⫹ ferrite) region. In steels hot worked in the two-phase region, the inhomogeneity of the microstructure and the resulting variations in mechanical behavior constitute major difficulties. It is well known that the most effective and powerful tool to control such complicated deformation conditions is computer modeling. One of the most basic elements of the thermomechanical treatment of microalloyed steels is the transformation from deformed, unrecrystallized austenite.[1] Such austenite contains an enhanced number of ferrite nucleation sites; i.e., dislocations, deformation bands, and deformation twins. The increase in nucleation density leads to significant ferrite grain refinement. Usually, the ferrite nucleation process starts on the austenite grain-boundary sites. When ferrite nucleation sites change from grain boundaries to intragranular sites during accelerated cooling, for example, the resulting microstructure begins to be refined, but is also more inhomogeneous. To analyze such a complex transformation behavior, a versatile model of the transformation kinetics is needed. The modeling of austenite decomposition under the previously mentioned conditions has been discussed in several JANUSZ MAJTA, Professor, and MACIEJ PIETRZYK, Professor, are with the Metallurgy and Materials Science Department, University of Mining and Metallurgy, 30-059 Krakow, Poland. ANNA K. ZUREK, MARK COLA, and PAT HOCHANADEL, Staff Members, are with Materials Science and Technology, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted November 5, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
recent publications.[2–10] In the present work, the austenite decomposition is described based on the assumption that nucleation of transformed ferrite is a probabilistic phenomenon and occurs mostly on the surface of grain boundaries. Because the steel investigated is a microalloyed grade, attention was mostly focused on the austenite-to-ferrite transformation. Assuming that for low-carbon steels the ferrite phase is the major element of the final structure, the formation of pearlite was not studied in this model. In general, a proper ␥ /␣ transformation model should be constructed in terms of three basic parameters: the transformation start temperature (Ts), the kinetics of austenite decomposition (dX /dt), and the latent heat generated during transformation (Q). Therefore, these parameters should be contained in the complete model. Because of the difficulties in determining the isothermal data for low-carbon microalloyed steels that have very short transformation times, the value of a well-verified computer model cannot be overemphasi
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