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

Abstract

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.

EFEITOS DE PARÂMETROS TERMOMECÂNICOS EM MECANISMOS DINÂMICOS DE REFINO DE GRÃO EM AÇOS C-Mn-Nb E C-Mn

“…Esses dois fenômenos de amaciamento dinâmico proporcionam um elevado refino dos grãos ferríticos obtidos após transformação. (4,6,7) A ocorrência da TAFID e da RDA é controlada pelas respectivas deformações críticas para esses fenômenos: ε c-TAFID e ε c-RDA , que, para um dado material, são governadas pela temperatura e pela taxa de deformação. Deformações realizadas ligeiramente abaixo ou acima da temperatura de transformação de fase em equilíbrio termodinâmico (Ae 3 ) favorecem a RDA, devido à ε c-RDA , nessas condições, ser menor do que a ε c-TAFID .…”

Effect of Initial Grain Orientation on Evolution of Deformed Microstructure in Hot Compressed Ni-30Fe Alloy

“…The deformation temperature of 1023 K is known to be in the non-recrystal- lized region. 13) The specimen was quenched to room temperature by water jet immediately after the compression to freeze the deformed microstructure.…”

Bainite Formation in Low Carbon Steel by Thermomechanical Control Process

“…Grain refinement is an effective way of increasing strength and ductility of metallic materials simultaneously [1][2][3][4][5] . Deformation induced ferrite transformation (DIFT) theory has been accepted to produce ultrafine grained low carbon steels [6][7][8][9] .…”

“…high yield ratio (YR) which means poor formability [3,4] . Furthermore, it is noted that the heavy deformation must be proposed and the deformation temperature is relatively lower (austenite region near the critical temperature Ar3) [2][3][4] . The preferred orientation has also been found in ultrafine grained low carbon steels, which means the deformed structure is dominant [3] .…”