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Austenite decomposition of low carbon high strength steels during continuous cooling Petkov, Plamen

Abstract

The process of transformation on the run-out table of a hot strip mill critically determines the final micro structure and thus, the hot rolled steel properties. The austenite decomposition kinetics of a high strength, low alloy steel (HSLA-65) microalloyed with niobium and a molybdenum bearing dual phase steel (DP 600) have been investigated and quantified with dilatometry during continuous cooling using a Gleeble 3500 thermomechanical simulator. The effects of different variables, such as austenite grain size and cooling rate, were quantified. It has been shown that increasing the cooling rate and the austenite grain size results in a decrease of transformation start temperatures. Evolution of the resulting microstructure has been investigated; polygonal ferrite fraction and ferrite grain size were determined. Accelerated cooling and smaller austenite grain size refine the final ferrite grain size. Further, austenite decomposition from pancaked austenite has been investigated. Deformation affects austenite decomposition by introducing additional nucleation sites and thus accelerating the nucleation rate. The retained strain showed a marked influence on the increase in transformation start temperature, as well as the increase in ferrite fraction and the ferrite grain refinement. Two models were applied to describe the process of austenite-to-polygonal ferrite transformation. The first model for the transformation kinetics is divided into two submodels. The transformation start temperature is described with a combined nucleation and early growth model. Subsequent growth can be described by the Avrami equation (or JMAK model) which is adapted to non-isothermal transformation by using the Scheil equation of additivity with an Avrami exponent n=0.9 and rate constant b which depends exponentially on temperature; the effect of austenite grain size on transformation kinetics can be captured with a suitable grain size exponent m. The second model is related to the ferrite grain size which can be expressed as a function of initial austenite grain size and transformation start temperature. An effective grain size, d[sub eff] = d[sub γ] exp(-ε), was used to incorporate the combined effect of austenite grain size and retained strain on the transformation start, transformation kinetics and ferrite grain size models.

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