HSLA 강 내의 TiC 조대화에 미치는 Mo 와 W 효과
- HSLA 강 내의 TiC 조대화에 미치는 Mo 와 W 효과
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- There is a new vigour in the development of low-alloy steels containing
a fine dispersion of substitutionally alloyed carbides. The steels have a
microstructure which is essentially ferritic, but with TiC or (Ti,Mo)C particles
which are less than 10 nm in size, generated at the austenite-ferrite
interface during the course of phase transformation. This interphase precipitation
mechanism has been known for a long time, but its application
to automotive steels which compete with dual phase and transformation
plasticity based alloys is much more recent.
The steels are mass produced and in the final stages are coiled at temperatures
in the vicinity of 600C. A key feature of alloy design, therefore,
is the use of the complex (Ti,Mo)C carbide, which is found to coarsen at
a much slower rate than the pure TiC, during the cooling of the coil to
ambient temperature. The mechanism for the effect of molybdenum is
not understood, but it is the fine dispersion of carbides that permits the
otherwise weak ferrite to gain sufficient strength to be of use in a variety
of engineering applications.
Many of the variants that have been developed also contain niobium
as a microalloying addition. High-resolution transmission electron microscopy
has been used to characterise the precipitates in Ti-Nb and
Ti-Nb-Mo bearing steels, using both thin foil and extracted carbides. In this way, precipitation and coarsening kinetics have been characterised
and form the basis for comparison against mathematical models later in
The role of molybdenum has been investigated using wide ranging
first-principles calculations for a variety of (Ti,M)C precipitates, where
'M' stands for niobium, vanadium, molybdenum or tungsten. The purpose
of this was to see whether the molybdenum acts to reduce coarsening by a
thermodynamic effect or whether the phenomenon is principally kinetic.
In fact, molybdenum has the effect of reducing the stability of TiC, but
it at the same time reduces the crystallographic misfit between ferrite
and the carbide, and as a consequence, the interfacial energy per unit
area. It is this latter parameter which controls coarsening and explains
why molybdenum leads to a more stable dispersion. Furthermore, it is
found that molybdenum incorporated into the carbide at the early stages
of precipitation, is rejected as the carbide grows beyond the nucleation
stage, confirming the first principles estimates that its presence in the
TiC is not favoured.
In an interesting the results from the ab-initio calculations suggest
a new alloy system based on (Ti,W)C precipitates which should be as
effective as (Ti,Mo)C by the same mechanism, in resisting coarsening.
Finally, a detailed analysis is reported on three different models for
representing the observed coarsening behaviours. The first is based on
classical Ostwald ripening theory due to Lifshitz, Slyozov and Wagner,
which essentially assigns the problem to the diffusion of a 'controlling'
solute (i.e., a binary alloy), and leads to a result in which the normalised
size distribution is invariant with time, even though the small particles
dissolve and larger ones grow. A model due to Kampmann and Wagner
avoids the assumption of a particular form of particle size distribution, but
still treats the problem as if the system concerned is binary. A computational
model based on the LSW particle size distribution, but which properly
treats multicomponent diffusion has also been studied
this model also has the advantage of revealing concentration profies within the matrix
as the particles evolve. Naturally, the different models give similar results
except for the Kampmann-Wagner method, where the particle size
distribution is not invariant with time.
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