Dynamic Transformation during the Simulated Plate Rolling of a 0.09% Nb Steel

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.

Section: Critical Stresses and Strainssupporting

confidence: 93%

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

Siciliano

et al. 2019

Metals

“…The increase in stress level for the second pass is associated with strain accumulation and the later decrease to the initiation and occurrence of dynamic transformation (DT). Static recrystallization (SRX) between passes and dynamic recrystallization (DRX) during straining also contribute to the flow stress decrease …”

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.

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.

“…The material investigated in this research was an API-X70 microalloyed steel; its complete chemical composition (in wt%) is presented in Table 1. Note that the methods and materials in this work have similarities to the published papers of the present authors [13,14,16,17]. The corresponding paraequilibrium and orthoequilibrium Ae 3 temperatures are also presented in this table.…”

Section: Methodsmentioning

confidence: 67%

“…The 12.5 mm hot-rolled plate material was machined into cylindrical specimens with the axes aligned with the rolling direction. The sample dimensions can be seen in references [13,14,16,17]. The torsion experiments were carried out on a modified MTS machine with a radiation furnace connected to a temperature controller.…”

Section: Methodsmentioning

confidence: 99%

“…In order to show the industrial application of DT, Rodrigues et al [13][14][15][16][17][18] conducted simulations of plate rolling to investigate the occurrence of DT, and they found that the application of roughing pass strains lead to the presence of around 9% and 6.5% of dynamically transformed ferrite after deformation under isothermal and cooling conditions, respectively [16,18]. It is known that the formation of less dense ferrite (as the bar passes through a rolling mill) above the Ae 3 temperature generates lower rolling loads and mean flow stresses (MFS) [19].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Strain-Induced Ferrite Formation During Steckel Mill Simulations with Varying Roughing Pass Schedules

Palhano

et al. 2019

Metals

It has been previously demonstrated that austenite may undergo partial dynamic transformation (DT) during the plate rolling process. Austenite dynamically transforms into unstable ferrite during hot deformation even at very high temperatures. In this work, the plate rolling simulations, with emphasis on Steckel mill operations, through torsion testing under isothermal conditions were performed on an X70 steel. Four different roughing schedules were tested followed by five finishing passes with pass strains of 0.3 applied at 900 °C. The roughing schedules had zero, one, two and three roughing passes at a temperature of 1100 °C, strain of 0.4 and strain rate of 1 s−1. The stress–strain curves as well as the mean flow stress (MFS) behaviors indicated that both dynamic transformation (DT) and dynamic recrystallization (DRX) occurred during straining. The critical strains for the onset of DT and DRX were determined by means of the double differentiation method and the critical strain values decreased with the number of roughing and finishing strains from the first going to the last pass. It was observed that the volume fraction of the dynamically formed ferrite increased sharply during the finishing stage as the number of previous roughing passes increased, which can be attributed to higher strain accumulation. The results presented here indicate that improved models are needed to control the microstructure of the material during subsequent cooling.