Ultrafine Grain Ferrite Transformed from Fine Austenite Grains Produced by Dynamic Reversal Transformation

Abstract

The medium carbon steel warm deformation was carried out in a Gleeble-3500 simulator, and the microstructure was observed on a scan electron microscopy (SEM) and optical microscope (OM). The results show that the dynamic reversal transformation (DRT) of austenite occurred during the multipass deformation at a temperature of 675 °C. The austenite grain size is about 3.4 μm at the stain of 2.67. The thermodynamics was discussed based on the stress activation model. The critical stress of DRT is in the range of 265.94–294.28 MPa, which is related to the Schmit factor, without considering the distortion energy. Meanwhile, the submicron ferrite was obtained after the air cooling stage. The texture of the ultrafine ferrite possessed the characteristics of good, deep drawing properties.

Microstructural Evolution in a 0.09% Niobium Low Carbon Steel during Controlled Hot Deformation

“…The first observations of the dynamic transformation were reported in the patent registered in the 1980s [1], who performed large deformations in common C-Mn steels at approximately the Ar 3 temperature and then rapidly quenched, the final microstructure consisted of 70% or more of equiaxed ferrite grains with a size of 4 µm or less. In later works [2][3][4][5][6][7][8], the dynamic transformation was also observed at temperatures above the Ae 3 in plain carbon steels, the combination of large strains and multiple passes conducted to ultrafine grains of ferrite of around 2 µm. Extensive research has been carried out after the pioneering work of Yada and co-workers and numerous papers have been published mainly in the last two decades.…”

“…One of the most polemic topics has been the transformation mechanism of the dynamic ferrite, at least four different approaches [4][5][6][7][8][9][10][11][12][13][14][15] can be found in the literature attempting to explain the transformation path. One of the most accepted interpretations of the transformation mechanisms was proposed by [16], who carried out torsion tests above the Ae 3 temperature at strains from 0.5 to 2 in a steel containing 0.09%C-1.3%Mn-0.02%Si-0.036%Nb.…”

Microstructural Evolution in a 0.09% Niobium Low Carbon Steel During Controlled Hot Deformation

“…One of the most polemic topics has been the transformation mechanism of the dynamic ferrite, at least four different approaches [4][5][6][7][8][9][10][11][12][13][14][15] can be found in the literature attempting to explain the transformation path. One of the most accepted interpretations of the transformation mechanisms was proposed by [16], who carried out torsion tests above the Ae 3 Metals 2024, 14, 283 2 of 15 temperature at strains from 0.5 to 2 in a steel containing 0.09%C-1.3%Mn-0.02%Si-0.036%Nb.…”