Effect of intercritical annealing on retained austenite characterization in textured TRIP-assisted steel sheet

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.

Section: Experimental Methodologymentioning

confidence: 99%

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

Fazeli

Militzer

et al. 2007

“…Hence the topic has been the subject of continued scientific interest as well as a key factor in the industrialization of multiphase steels. [1][2][3][4][5][6][7][8] It is, therefore, important to develop models for the kinetics and the final phase fractions during intercritical annealing. The models should be capable to handle different initial microstructures corresponding to different process routes and to capture the effects of key alloying elements such as C, Si, and Mn.…”

Section: Introductionmentioning

confidence: 99%

Prediction and Validation of the Austenite Phase Fraction upon Intercritical Annealing of Medium Mn Steels

Farahani

Zwaag

2015

In this research, the effects of Mn and Si concentration and that of the isothermal intercritical holding temperature on the austenite-to-ferrite (c fi a) and the martensite-to-austenite (a¢ fi c) phase transformations are studied for a series of Fe-C-Mn-Si steels with up to 7 wt pct Mn. The model is based on the local equilibrium (LE) concept. The model predictions are compared to experimental observations. It is found that the austenite volume fraction at the end of intercritical annealing depends significantly on the initial microstructure. For Mn concentrations between 3 and 7 wt pct, the LE model is qualitatively correct. However, at higher Mn levels the discrepancy between the predicted austenite fractions and the experimental values increases, in particular for the a¢ fi c transformation. Intragrain nucleation is held responsible for the higher austenite fractions observed experimentally. Silicon is found have a much smaller effect on the kinetics of the intercritical annealing than Mn.

“…[9] Little or no information can be found in the literature about the sole effect of aluminum content on the phase transformation and mechanical properties of the TRIPaided steels. De-Meyer et al [8] and E. Emadoddin et al [10] presented some results about Si-Al-alloyed TRIP steels. However, their comparison of the number of phases and the mechanical properties is misleading, because the carbon content was different in their alloys.…”

Section: Introductionmentioning

confidence: 99%

On Factors Affecting the Phase Transformation and Mechanical Properties of Cold-Rolled Transformation-Induced-Plasticity–Aided Steel

Soliman

Palkowski

2008

Two Mo-Nb microalloyed transformation-induced-plasticity (TRIP) steels, with Al contents of 0.23 and 0.65, were subjected to several hot-rolling conditions designed to generate different ferrite morphologies and grain sizes. These structures were then cold rolled and TRIP annealed under different heat-treatment conditions. To further develop TRIP steel in terms of strength and ductility, stabilizing retained austenite by isothermal bainitic transformation was studied in detail. Microstructure observation and tensile tests were conducted, and volume fractions of retained austenite were measured. It was observed that increasing the aluminum content enhances the transformation rate and increases the total amount of bainite fraction at the expense of retained austenite. The latter effect enhances formability by increasing ductility. Furthermore, it was observed that the hot-rolling schedule, prior to cold rolling and heat treatment, has a decisive effect on structure refinement, which enhances the strength-ductility balance of the final product. To study the transformation behavior, dilatometer testing was conducted under conditions similar to that of the heat treatment. Thermodynamic calculations were used to verify the results.