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Although conventional carbon steels have adequate properties for general applications, they
have one big drawback.
That is, they have low strength, and when one tries to increase the strength, then the
fraction of pearlite should be increased resulting in a significant drop in toughness.
To tackle the issue, a new type of steels has been developed called, high strength low
alloy steels, in short HSLA steels.
The basic idea is very simple.
They try to obtain high strength with minimal use of alloying element as a name HSLA suggests.
So this steels actually have somewhat higher strength than the low carbon steels but slightly
lower ductility than low carbon steels.
So how can these steels obtain high strength with so low alloy content?
The answer is using small amount, between 0.05 and 0.15 percent of strong carbide formers
such as nobium, vanadium, and titanium.
This carbide will provide strengthening effect such as precipitation hardening and they restrict
grain growth during high temperature thermomechanical treatment, so that fine grain size can be
obtained.
Although these steels show somewhat poor formability as compared to conventional low carbon steels,
they show excellent toughness at low temperatures due to the refined grain size and therefore
find applications where low temperature toughness is required such as pipelines, bridges and
offshore products.
One thing to note is they are not really readily formable at room temperature as compared to
low carbon steels.
The most important process step of HSLA steel is thermomechanical processing, by which the
steel’s grain size can be refined.
This schematic drawing shows what's happening during thermomechanical treatment.
Firstly, during heating in the austenite field, all of the niobium and vanadium carbide dissolve
so that niobium and vanadium are in solid solution.
And during the hot rolling process, precipitation of carbides occurs due to a decrease in solubility
with a decrease in temperatures and this precipitate will hinder the growth of austenite and inhibit
the recrystallization of austenite depending on rolling temperatures.
At high temperatures, there will be a recrystallization of austenite to form fine recrystallized austenite
grains and at the low temperature, recrystallizaton will not occur so there will be formation
of deformation bands inside the unrecrystallized austenite grains.
This deformation band are the ones playing as nucleation site for the ferrite formation
during cooling.
So during cooling, ferrite will form around these deformation bands forming fine ferrite
structures.
If you continue to cool at slow cooling rate, all of the remaining austenite will transform
into fine ferrite so the net result is, we will obtain very ferrite grain size.
However, if you cool the material quickly, this austenite may not transform into ferrite.
Instead they may transform into some harder phases such as bainite or martensite and sometimes
you can also have retained austenite.
So in this case the structure becomes very similar to TRIP aided steels.
This slide schematically shows how microstructure changes during rolling and cooling, so basically
if you understand this figure you will understand what's happening during thermomechanical treatment
of HSLA steels.
When rolling at the high temperature in the austenite recrystallization region such as
the case of capital A, there will be a recrystallization, but since it is at the high temperatures there
will be a formation of coarse recrystallized austenite.
If the steel is cooled slowly from this state, then relatively coarse ferrite will form,
because only the available nucleation site is austenite grain boundaries.
But if you cool the material very quickly, all of this remaining austenite will transform
into harder phases such as bainite or acicular ferrite and at this time the grain size of
bainite and acicular ferrite will be same as the prior austenite grain size.
Now let's look at the other cases that you finish roll the material at slightly low temperature
in the austenite recrystallization region.
Now there will be a recrystallization and in this case, austenite grain size will be
finer because you are deforming the material at low temperatures.
Phase transformation behavior of the steels during cooling is basically similar to the
case of A. Only difference is in the grain size.
If the steel is cooled slowly from this state, all of these austenite grain boundaries will
be effective nucleation sites for ferrite, so we will obtain very, very fine ferrite
grain size.
But if you cool the material very quickly, then all of this austenite will transform
into hard phases such as bainite or acicular ferrite.
Once again the grain size of bainitic ferrite or acicular ferrite is same as the prior austenite
grain size.
Now the totally different microstructure would evolve when finished rolling is done in non-recrystallization
region such as the case of capital C. In this case there will be no recrystallization.
So you have very large amount of deformation formed inside all such grains.
Also, the austenite grains will be elongated.
So if you cool the material from this state with a slow cooling rate, then all of this
deformation band will be the nucleation site for ferrite formation and also you will have
another nucleation site which is the austenite grain boundaries.
So the final result is, you will obtain very, very fine ferrite grain size.
Actually the grain size you can obtain in the case of C is much finer than the case
of B. But if you cool the material quickly from this state after you deform the material
in the austenite non-recrystallization region, then all of this phase will transform into
hard second phases such as bainite and acicular ferrite and once again the grain shape is
basically same as the one of the prior austenite grains.
In some cases, you can finish roll the material in the two-phase region, which is the ferrite
and austenite.
In such case, now you already formed the ferrite because temperature has cooled down below
Ar3 temperature, but this is rolled, so those ferrite will be deformed.
So eventually the structure is elongated austenite with the deformed ferrite.
If you cool the material slowly from these temperatures, all of this austenite will transform
into fine ferrite grains.
But the remaining one, the remaining ferrite, will still remain as ferrite.
So the final result is, now you are having the mixture of fine ferrite and coarse ferrite.
But sometimes you have to cool the material quickly from this state, so in such case,
all of the remaining austenite which was deformed will transform into bainite or acicular ferrite.
On the other hand the ferrite you already formed will remain as ferrite so you have
a mixture of elongated bainite and acicular ferrite along with coarse ferrite.
So if you concern about the good toughness, the most important factor is grain size.
Then the choice should be, among this four routes, the choice should be either B’ or
C’ because this one will give you very fine ferrite grain size.
But if you want higher strength then the choice should be D, this D process, because of the
presence of strong hard bainite.
But in this case although you can obtain very high strength, this will give you worse toughness
than the other processes.
Sung-Mo JungProfessor
Kim, Sung-JoonProfessor
Kang, Youn-BaeProfessor
Suh, Dong WooAssociate Professor
Kim, Nack JoonProfessor
