Retransformation Behavior of Dynamically Transformed Ferrite during the Simulated Plate Rolling of a Low C and an X70 Nb Steel

Sun

et al. 2018

steel research int.

When austenite is deformed within the austenite phase field, that is, above the equilibrium transformation temperature Ae 3 , it partially transforms dynamically into ferrite. Here, rough rolling simulations are conducted at 1100 C on an X70 steel to study the influence of grain size and residual strain on this type of transformation. Three simulated roughing passes are applied with pass strains of 0.4 at a strain rate of 1.0 s À1 . The resulting stress-strain curves are recorded and analyzed. The volume fractions of ferrite and martensite (prior austenite) are determined on quenched samples by means of metallographic techniques. It is observed that dynamic transformation took place during the first roughing pass and that the volume fraction of transformed ferrite increases with the cumulative strain. Both the critical strain to the onset of dynamic transformation as well as the grain size decrease with pass number.

Section: Resultsmentioning

confidence: 99%

Effect of Grain Size and Residual Strain on the Dynamic Transformation of Austenite under Plate Rolling Conditions

Sun

et al. 2018

steel research int.

“…There is approximately 60%, 50% and 40% of DT ferrite at deformation temperatures of 900 • C (Figure 1a), 930 • C (Figure 1b) and 960 • C (Figure 1c), respectively. The DT ferrite tends to re-transform back into the more thermodynamically stable austenite upon isothermal holding [17,18]. that the microstructure will be completely martensite after quenching.…”

Section: Resultsmentioning

confidence: 99%

“…The DT ferrite tends to re-transform back into the more thermodynamically stable austenite upon isothermal holding [17,18]. The driving force for transformation (EDF) is the flow stress difference between the DT critical stress (σc−γ) and the yield stress of the fresh ferrite (σYS−α) that takes its place.…”

Section: Resultsmentioning

confidence: 99%

Determination of the Critical Stress Associated with Dynamic Phase Transformation in Steels by Means of Free Energy Method

Fall

et al. 2018

Metals

Abstract:The double differentiation method overestimates the critical stress associated with the initiation of dynamic transformation (DT) because significant amounts of the dynamic phase must be present in order for its effect on the work hardening rate to be detectable. In this work, an alternative method (referred to here as the free energy method) is presented based on the thermodynamic condition that the driving force is equal to the total energy obstacle during the exact moment of transformation. The driving force is defined as the difference between the DT critical stress (measured in the single-phase austenite region) and the yield stress of the fresh ferrite that takes its place. On the other hand, the energy obstacle consists of the free energy difference between austenite and ferrite, and the work of shear accommodation and dilatation associated with the phase transformation. Here, the DT critical stresses in a C-Mn steel were calculated using the free energy method at temperatures ranging from 870 • C to 1070 • C. The results show that the calculated critical stress using the present approach appears to be more accurate than the values measured by the double differentiation method.

“…Note that the samples were strained to 0.3 during each pass applied at a strain rate of 1 s −1 . A strain higher than the critical stains for the onset of DT [9][10][11][12][13][14][15] was selected so as to allow for DT to take place. Water spray quenching after R1, R3, and R5 were performed at minimum rate of 500 • C/s using the Gleeble high flow quenching system.…”

Section: Methodsmentioning

confidence: 99%

“…More recently, Rodrigues et al [13][14][15], investigated the phenomenon of DT under various industrial plate rolling simulation conditions. They found that the application of roughing passes under isothermal conditions can lead to the presence of around 8% of transformed ferrite after deformation.…”

Section: Introductionmentioning

confidence: 99%

Dynamic Phase Transformation Behavior of a Nb-microalloyed Steel during Roughing Passes at Temperatures above the Ae3

Siciliano

et al. 2019

Metals

A five-pass torsion simulation of the roughing passes applied during hot plate rolling was performed in the single-phase austenite region of a Nb-microalloyed steel under continuous cooling conditions. The deformation temperatures were approximately half-way between the Ae3 and the delta ferrite formation temperature (i.e., 250 °C above the Ae3) in which the free energy difference of austenite and ferrite is at maximum. The microstructures in-between passes were analyzed to characterize and quantify the occurrence of deformation-induced dynamic phase transformation. It was observed that about 7% of austenite transforms into ferrite right after the final pass. The results are consistent with the calculated critical strains and driving forces which indicate that dynamic transformation (DT) can take place at any temperature above the Ae3. This mechanism occurs even with the presence of high Nb in the material, which is known to retard and hinder the occurrence of DT by means of pinning and solute drag effects. The calculated cooling rate during quenching and the time–temperature–transformation curves of the present material further verified the existence of dynamically transformed ferrite.