The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered
diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and
a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the
recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized
{111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify
crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations
higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered
as potential nuclei. However, among all, only some of them become effective due to an important selection. The {111} recrystallized
grains have a significant size and number advantage as compared with other texture components, and a hard impingement between
clusters of {111} grains is produced during grain growth. The effect of grain growth behind the recrystallization front seems
to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored
energy. 相似文献
In the present work, the mechanical properties and the microstructural evolution of a duplex stainless steel in the as-cast
and wrought conditions during deformation under hot-working conditions have been studied. Hot torsion tests, at strain rates
of
= 1 s−1, have been carried out using prepolished samples on which surface parallel scratches have been practiced. The observation
of the surface of the samples shows a large displacement of the scratches produced by two different mechanisms, sliding on
the δ/γ interface, and shearing of the ferrite. The displacements in the as-cast condition have been found to concentrate in a reduced
set of ferrite/austenite interfaces leading to the formation of cracks along them. In the wrought material, the distribution
of the sliding is more homogeneous over all the ferrite/austenite interfaces, and no damage has been produced. These behavioral
differences between both materials have been related in the present work, to the characteristics of the corresponding microstructures,
to the spatial phase distribution, and to the nature of the ferrite/austenite interface, among others. 相似文献
The degree of ferrite grain refinement that can be reached in low-carbon microalloyed steels by thermomechanical processing
is limited, to a certain extent, by the grain coarsening which can take place behind the transformation front. The coarsening
of ferrite grains is the result of two different mechanisms: elimination of ferrite grains produced by normal grain growth
after full impingement on the austenite grain boundary plane and/or coalescence between different ferrite grains with close
orientation formed from the same crystallographic variant. The lack of experimental data to support either process is due
to the experimental difficulties encountered when analyzing the phenomenon. Some transmission electron microscope (TEM) observations
reveal that the ferrite grains formed along a prior grain boundary in deformed austenite are separated by a mixture of low
and high angle grain boundaries upon impingement. In the present work, the electron backscattered diffraction (EBSD) technique
has been applied to investigate the microstructural evolution during transformation, with special emphasis placed on the α-α grain boundary character as a means of investigating the contribution of coalescence/grain growth to coarsening. 相似文献
Often, Nb contributes to the strength of a microalloyed steel beyond the expected level because of the grain size strengthening resulting from thermomechanical processing. Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in solution after hot rolling. The first of them is the increase of the hardenability of the steel as a result of Nb, and the second one is the fine precipitation of NbC in ferrite. Three Nb microalloyed steels were thermomechanically processed in the laboratory and coiled at different temperatures to investigate the effect of Nb content on the tensile properties. The extra strength was linearly related to the Nb remaining in solution after the hot working. The maximum contribution from Nb was reached for a coiling temperature of 873?K (600?°C). 相似文献
In the hot deformation of the duplex stainless steels, the complexity of the microstructure evolution and mechanical response
is increased as compared with those of single-phase ferritic or austenitic stainless steels. In the present work, plane strain
compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior
of a duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of
interface, and the relative mechanical properties of both phases. The law of mixtures has been used to explain the different
flow curves obtained when changing the phase distribution and/or the deformation mode. On deforming as-cast microstructures,
the deformation partitions vary heterogeneously between both phases and some austenite areas act as hard nondeforming particles.
Cracks have been observed to occur at the interface of such regions, from relatively low strains, for which the initial Kurdjumov-Sachs
orientation relationship between ferrite and austenite is still present. 相似文献
Very often Nb contributes to the strength of a microalloyed steel beyond the expected level due to the grain size strengthening resulting from thermomechanical processing. Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in the solution after hot rolling. The first of them is the increase of the hardenability of the steel due to Nb, and the second one is the fine precipitation of NbC in ferrite. The contribution of the precipitates to the work hardening of two thermally and thermomechanically processed microalloyed steels is addressed in this work and this contribution has been integrated into previously developed models by the authors for ferrite–pearlite microstructures. An Leff is considered through the effective spacing associated to the different obstacles and their interactions with the moving dislocations. The model obtained shows good agreement with the experimental tensile curves from the end of yield point elongation to the onset of necking.
Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for
achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the
subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends
on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density.
In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels
with different active particle types and densities. The morphology of the obtained intragranular microstructures has been
found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular
nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures. 相似文献
The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered
diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and
a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the
recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized
{111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify
crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations
higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free
from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become
effective due to an improtant selection. The {111} recrystallized grains have a significant size and number advantage as compared
with other texture components, and a hard impingement between clusters of {111} grians is produced during grain growth. The
effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced
by the migration of this front, driven by the cold-work stored energy.
J.L. Bocos, formerly Researcher with CEIT 相似文献
While the role of Nb during the processing of austenite is quite clear, what happens in subsequent stages to the concentration of this element left in solution is subject to some debate. In particular, some uncertainty still subsists concerning the eventual homogeneous precipitation in Nb supersaturated polygonal ferrite. The present work was aimed at clarifying the precipitation sequence of Nb during coiling, through a systematic work and a careful selection of the processing conditions in order to produce different scenarios concerning the initial state of Nb. A Nb-microalloyed steel was thermomechanically processed in the laboratory followed by simulated coiling at different temperatures in the 873?K to 1023?K (600?°C to 750?°C) range. Transmission electron microscopy (TEM) showed interphase precipitation of NbC at high coiling temperatures, while at 873?K (600?°C), homogeneous general precipitation took place in ferrite and followed a Baker?CNutting orientation relationship. 相似文献