Ferrite grain refinement by controlled rolling of lovv-carbon and microalloyed steel

A phenomenological model is proposed to describe the conditions under which an ultrafine ferrite (UFF) microstructure forms in low-alloyed steels as a result of deformation-induced ferrite transformation (DIFT). Assuming that corner points in the dislocation cell substructure of microshear bands provide suitable nucleation sites for ferrite, the model predicts that UFF forms in low-carbon steels independent of the steel chemistry and is promoted by increasing the strain rate. Analyzing ferrite growth rates suggests that the temperature range for UFF formation depends primarily on a minimum interfacial velocity criterion that must be fulfilled. In addition to these general trends, a more quantitative comparison of the model with experimental results for a Mo-containing steel is provided. This comparison suggests ferrite growth rates during DIFT that are somewhat larger than during conventional cooling transformation. These enhanced growth rates can be attributed to the presence of fast diffusion paths.

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Phenomenological Model for Deformation-Induced Ferrite Transformation

Militzer

Bréchet

2009

“…[13] However, the idea of a (2) thermal cycling through the austenite-to-ferrite tempera-"dynamic" strain-induced austenite-to-ferrite transformation ture, in which deformation heating of ferrite, deformed (i.e., transformation during deformation) to refine ferrite just below the transformation temperature, causes it to grain size has only recently been proposed. transform to austenite briefly before reverting to ferrite Priestner and de los Rios [14] were among the first to recog-(when this process is repeated a number of times, ferrite nize that extra grain refinement may be possible if the transgrain sizes as fine as 1 m result after nucleation on formation could be induced during the deformation process. the boundaries of very fine austenite grains [6,7,8] );…”

Section: Introductionmentioning

confidence: 98%

The production of ultrafine ferrite during hot torsion testing of a 0.11 wt pct C steel

Hurley

Muddle

Hodgson

2002

Ultrafine ferrite grain sizes were produced in a 0.11C-1.6Mn-0.2Si steel by torsion testing isothermally at 675 ЊC after air cooling from 1250 ЊC. The ferrite was observed to form intragranularly beyond a von Mises equivalent tensile strain of approximately 0.7 to 0.8 and the number fraction of intragranular ferrite grains continued to increase as the strain level increased. Ferrite nucleated to form parallel and closely spaced linear arrays or "rafts" of many discrete ultrafine ferrite grains. It is shown that ferrite nucleates during deformation on defects developed within the austenite parallel to the macroscopic shear direction (i.e., dynamic strain-induced transformation). A model austenitic Ni-30Fe alloy was used to study the substructure developed in the austenite under similar test conditions as that used to induce intragranular ferrite in the steel. It is shown that the most prevalent features developed during testing are microbands. It is proposed that high-energy jogged regions surrounding intersecting microbands provide potential sites for ferrite nucleation at lower strains, while at higher strains, the walls of the microbands may also act as nucleation sites.

“…[1] It has been demonsubstructure of an austenitic Ni-30 wt pct Fe alloy strip strated, for example, that it is possible to produce ultrafine processed in similar fashion. The latter alloy has a stacking-(Ͻ1 to 2 m) ferrite grains in the surface regions of rolled fault energy similar to that of low-carbon steel at temperastrip in a wide range of steel compositions, [2] by using a tures where the material is austenitic and has been shown novel approach to processing.…”

Section: Introductionmentioning

confidence: 98%

Nucleation sites for ultrafine ferrite produced by deformation of austenite during single-pass strip rolling

Hurley

Muddle

Hodgson

2001

An austenitic Ni-30 wt pct Fe alloy, with a stacking-fault energy and deformation characteristics similar to those of austenitic low-carbon steel at elevated temperatures, has been used to examine the defect substructure within austenite deformed by single-pass strip rolling and to identify those features most likely to provide sites for intragranular nucleation of ultrafine ferrite in steels. Samples of this alloy and a 0.095 wt pct C-1.58Mn-0.22Si-0.27Mo steel have been hot rolled and cooled under similar conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction, and X-ray diffraction. Following a single rolling pass of ϳ40 pct reduction of a 2 mm strip at 800 ЊC, three microstructural zones were identified throughout its thickness. The surface zone (of 0.1 to 0.4 mm in depth) within the steel comprised a uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation cells, with an average diameter of 0.5 to 1.0 m. The scale and distribution and, thus, nucleation density of the ferrite grains formed in the steel were consistent with the formation of individual ferrite nuclei on cell boundaries within the austenite. In the transition zone, 0.3 to 0.5 mm below the surface of the steel strip, discrete polygonal ferrite grains were observed to form in parallel, and closely spaced "rafts" traversing individual grains of austenite. Based on observations of the equivalent zone of the rolled Ni-30Fe alloy, the ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within the deformed austenite during rolling. Within the central zone of the steel strip, a bainitic microstructure, typical of that observed after conventional hot rolling of this steel, was observed following air cooling. In this region of the rolled Ni-30Fe alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions.