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Austenite decomposition of a HSLA-Nb/Ti steel and an A1-TRIP steel during continuous cooling Lottey, Kulwinder Kaur

Abstract

The ability to control and predict the mechanical properties of hot rolled steel depends strongly on the thermomechanical processes which the steel undergoes. The phase transformation that occurs on the run-out table of a hot strip mill is a critical processing step which significantly influences the final microstructure, and thus, the properties of the hot rolled steel. This work examines the austenite-to-ferrite phase transformation of a high-strength low-alloy steel (HSLA-90) microalloyed with niobium and titanium and a transformation-induced plasticity steel alloyed with aluminium (Al- TRIP). The austenite decomposition kinetics have been investigated using a Gleeble 3500 thermomechanical simulator equipped with a dilatometer. The effect of cooling rate on the austenite decomposition kinetics were quantified for both steels with initial microstructures comprised of austenite. For the Al-TRIP an additional initial microstructure consisting of a mixture of austenite (67%) and ferrite (33%) phases was also investigated. It was shown that accelerated cooling lowers the transformation temperatures in addition to refining the resulting ferrite grains. Microstructural analyses revealed a decrease in polygonal ferrite fraction with accelerated cooling and the formation of acicular products for the HSLA-90 steel. A transition from high temperature products such as, polygonal ferrite and pearlite, to low temperature products such as, bainite and martensite, was seen for the Al-TRIP with accelerated cooling where there was an increase in bainite and martensite fractions. The effect of initial austenite grain size and a pancaked austenite microstructure was investigated for the HSLA-90 steel. Increasing the austenite grain size resulted in a shift to lower transformation start temperatures and an associated decrease in the polygonal ferrite fraction. However, accelerated cooling and smaller austenite grain sizes resulted in refining the ferrite grains. Additional ferrite grain refinement was obtained with the prior deformation of the initial austenite microstructure which increased the ferrite nucleation rate by introducing additional nucleation sites both on the austenite grain boundary and within the deformed grains at crystallographic defects. The transformation start temperatures and polygonal ferrite fraction were significantly increased by the retained strain. A previously developed sequential transformation model was applied to describe the austenite-to-polygonal ferrite transformation which consisted of sub-models to predict the transformation start temperature, ferrite growth and ferrite grain size under continuous cooling conditions. The first sub-model predicted transformation start temperature by combining corner nucleation of ferrite with early growth. The subsequent ferrite growth was described using a model that employed the Avrami equation (or JMAK model) which was adapted to non-isothermal transformations, i.e. continuous cooling, by using the Scheil equation of additivity with an Avrami exponent of n = 0.85 and a rate constant b which depends exponentially on temperature; the effect of austenite grain size on the transformation kinetics was captured with a suitable grain size exponent m. The ferrite grain size was predicted by employing a model that expressed ferrite grain size as a function of initial austenite grain size and transformation start temperature. The combined effect of austenite grain size and retained strain was incorporated into the transformation start, transformation kinetics and ferrite grain size models by employing an effective grain size, i.e. d[sub eff] = d[sub γ] exp(- ε).

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