A Microstructure Evolution Model for Hot Rolling of a Mo-TRIP Steel

Abstract

A comprehensive study of microstructure evolution for a Mo-TRIP (transformation-inducedplasticity) steel under hot strip rolling conditions has been conducted. This investigation includes austenite grain growth during reheating, deformation behavior, and static recrystallization kinetics of austenite as well as the effect of cooling rate and austenite conditioning on the continuous cooling transformation (CCT) behavior. The physically based Kocks-Mecking model has been employed to describe the deformation behavior of austenite, while the JohnsonMehl-Avrami-Kolmogorov (JMAK) approach has been used to predict static recrystallization, and an empirical equation has been formulated for the recrystallized austenite grain size. Ferrite transformation start is described by a model considering early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous cooling is predicted by the semiempirical JMAK approach in combination with Scheil's equation of additivity. The effect of carbon enrichment on ferrite transformation kinetics is explicitly included in the model. In addition, a phenomenological model for the bainite formation has been proposed. Martensite transformation start is described by an empirical equation taking into account carbon enrichment of remaining austenite. Finally, the entire hot strip rolling and controlled cooling process have been simulated by hot torsion tests, and the optimum coiling temperatures for the formation of TRIP microstructures have been determined.

Modeling of Microstructure Evolution during Hot Strip Rolling of Dual Phase Steels

“…As a result, a pancaked austenite microstructure will enter the run-out table and constitute the initial austenite microstructure for the decomposition of austenite. This behaviour is similar to that recently observed for a TRIP steel 38) and confirms a general trend for AHSS where, because of the increased alloying levels, recrystallization is sufficiently delayed such that its completion during finish rolling in a strip mill may not occur.…”

“…Standard procedures were employed in mounting, grinding and polishing these sections to be examined. 38) For austenite grain growth tests, the as-quenched samples were tempered at 550°C for 20 h and then sectioned and polished. Saturated aqueous picric acid was used to reveal the austenite grain boundaries.…”

“…As described in detail elsewhere, 38) the Kocks-Mecking (KM) approach offers a well-established model to predict the flow stress evolution during deformation of austenite. 41) The advantage of the Kocks-Mecking model is its physical basis such that a reliable extrapolation can be made from laboratory tests with lower strain rates to the higher strain rates of finish rolling.…”

“…As indicated in the Table the values for gЈ 0 and g 0 , respectively, are 1 for both steels, as also observed previously for a TRIP steel. 38) This appears to suggest that gЈ 0 ϭ g 0 ϭ1 may be assumed as a universal relationship independent of steel chemistry such that only the extrapolations of yield and scaling stresses to 0 K remain as chemistry dependent parameters.…”

“…[20][21][22][23][24][25][26][27][28][29][30][31][32] Model development for AHSS steels is still in its infancy; some initial model concepts have recently been proposed for DP steels [33][34][35][36] and first generation models have also been proposed for TRIP steels. 37,38) The development of microstructure process models for advanced high strength steels is particularly significant since these steels are more sensitive to process conditions than conventional steels. Consequently, the availability of process models with predictive capabilities will be of tremendous help to establish robust processing routes for these new steels.…”