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The coarsest grain was obtained in steel 3(plain carbon steel, 15μm), while the finest grain size was obtained in steel2(10μm).
But compression temperature had an important effect on the ferrite grain size too.
Fig.5 Optical micrographs of steel 2 It can be seen from Fig.5 that the ferrite grains nucleated within the austenite grains besides austenite grain boundary which results to grain refinement in V-N steel.
Low temperature increased the nucleation driving force and activated a number of difficult nucleation sites such as crystal face, consequently refined microstructure.
Had V-N steel plus controlled cooling same grain size or smaller grain size than Nb-bearing steel？

The higher the prior strain level, the higher the velocity of grain boundary migration and the higher the annealing twin density in the recrystallized grains.
A "twin" refers to a grain that has one or two twin boundaries with a larger grain in which it is embedded.
Annealing twins are quantified by their density, which is defined as the number of intercepts of annealing twin boundaries per unit length [7].
The average grain boundary migration velocity is estimated as the change of average grain size of recrystallized grains per unit time.
(a) All grains (b) Recrystallized grains 3.

Modeling of Kinetics and Grain Density Evolution during Isothermal Phase Transformation W.
Introduction The well-known Kolmogorov–Johnson–Mehl–Avrami (KJMA) model considers the relation of transformation fraction and experimental variables, such as time t, temperature T or heating rate Φ, rather than the actual grain number [1].
However, grain density is important for mechanical properties and solute segregation.
KJMA model uses the extended fraction due to extended grain to express actual fraction, while in fact the formation of new grain is determined by the untransformed volume [2-4].
Theory and calculation During isothermal phase transformation, the number of supercritical nuclei in a unit volume, at time during a time lapse, is given by, with as nucleation rate.

The results show that the grain size decreases with strain.
The ultra-fine grained structure formation process was discussed in detail.
It was interesting that, for such a level of strain, the structure was characterized by sharp grain boundaries and there were few dislocation tangles in the grain interiors.
The main idea is the breaking-up of the original grains into cellular substructures and then the cellular substructures transform into equiaxed grains.
Acknowledgements The author would like to acknowledge the financial support of National Natural Science Foundation of China under granted number 50471017.

Moreover, a lower tensile strength was observed at a larger prior austenite grain size.
The prior austenite grain size was calculated from the planar mean area using Eq.1.
R = ( ( 3 A ) / ( 2 N π ) )1/2 (1) Here, R is the mean radius, A is the area of observed cross section and N is the number of grains in observed cross section. 3.
The prior austenite grain size increases with increasing solution treatment time.
These values are shown in Figs.7 to 10 in terms of the prior austenite grain size.

Meanwhile, the grain size can be further reduced.
A number of studies have already been reported concerning the impact of rotating magnetic fields in solidifying metals [11-14].
The comparison of these photographs demonstrates that complete conversion of coarse columnar grain structure to fine equiaxed grains occurred within 5 min after inoculation with grain refiner Al-5Ti-1B.
When applying RMF to the Al without Al-5Ti-1B until solidification, big columnar grains change into fine equiaxed grains.
In the grain refining practice, TiB2 plays an indirect role in grain nucleation.

However, because the overall grain size is so fine and all grains are almost equiaxed in shape, also the dotted grains must be recrystallised, not only recovered.
In the Nb steel (0.15%C, 0.033%Nb) distinctly a higher number of cementite particles were found.
Here, it seemed that two grain growth modes, normal and abnormal, can occur in the UFF grained microstructures.
Anyhow, the present results demonstrate that the UFF microstructure in steels can be thermally stable up to temperatures about 0.5Tm, obviously due to the dispersion of carbide particles present, but in very reasonable numbers and sizes, hardly affecting detrimentally the mechanical properties.
Refining the prior austenite grain size enhances the ferrite grain size refinement.

AGG, also referred to as secondary recrystallization[5], typically results in microstructures of bimodal grain sizes, containing a small number of very large grains called abnormal grains.
The point 3 has 0.9% small angle grain boundary, but point 4 has 0.6% small angle grain boundary.
Abnormal grain growth [J].
Abnormal grain growth and grain boundary faceting in a model Ni-base superalloy [J].
Influence of the primary recrystallization texture on abnormal grain growth of goss grains in grain oriented electrical steel [J].

The aim of adding yttrium was to control b grain growth above the b transus by grain boundary pinning.
In the present case, strengthening of the b texture, occurring during b grain coarsening resulted in strengthening of particular b texture components, which increases the likelihood of a texture modification by selective growth of a variants on the common (110) b grain boundaries into unoccupied large b grains. 1.
While EBSD provides the ability to combine macroscopic texture information with information on the microstructural scale, the requirement of very large EBSD maps to capture a sufficient number of b grains in combination with a small step size in order to capture microstructure information, makes this methodology very time consuming.
It seems that the extensive b grain growth observed in conventional Ti-6Al-4V has resulted in preferential growth of favourably oriented b grains belonging to potentially the original cast texture.
As discussed in [1], large b grains allow relatively free growth of a variants from b grain boundaries with two adjacent b grains having a common (110) normal.

For the bilayer films containing larger Al grains, the nucleation rate of fractal patterns (Ge clusters) is faster and the number of patterns is larger.
Top left numbers represent the annealing times.
From Fig. 5, it is apparent that the number of the patterns increases when the grain size becomes larger.
However, we cannot see any essential difference between Fig. 6 and Fig. 8: that is, the similar influence of the grain size of polycrystalline Al layer can be seen and the number of the patterns increases when the grain size becomes larger.
(a) 19.0ks 22.4ks 24.2ks (b) 7.4ks 9.0ks 10.8ks 10µµµµm 103 104 5x104 103 104 105 Annealing time, t/s Number density, N/mm -2 � � � � 0 1.5x104 3.0x104 0 3 6 Mean radius, r/µµµµm Annealing time, t/s � � large�grain small�grain large�grain small�grain 2x103 10 4 3x104 103 104 2x104 Annealing time, t/s Number density, N/mm -2 � � 0 1.5x104 3.0x104 0 4 8 Annealing time, t/s Mean radius, r/µµµµm � � large�grain small�grain large�grain small�grain (a) (b) (a) (b) Fig. 9 Superimposed SEM images showing the evolutions of the same patterns in the course of in-situ annealing at 423K: (a) Al(50nm)/Ge(50nm)/SiO2 and (b) Al(50nm)/Ge(25nm)/SiO2 bilayer films.