Now let's move on to multiphase steels.
Multiphase steels can be defined as the one containing two or more phases in significant
quantities.
One of the typical example is dual phase steel which consists of soft ferrite and hard martensite.
Another example is TRIP aided steel which contain retained austenite beside ferrite
and martensite, and this retained austenite will transform into martensite during the
deformation.
known as TRIP phenomena.
If you look at the compositions of these two kinds of multiphase steels, you can notice,
in the case of dual phase steels, it basically consists of carbon and manganese.
But in the case of TRIP aided steels, it contains a little bit amount of silicon.
Actually silicon plays a very important role in the case of TRIP aided steels because silicon
reject the carbon to other areas, That means, any silicon remaining in the ferrite will
try to reject carbon into remaining austenite so austenite will be enriched with carbon.
That is, so you can obtain retained austenite in the final microstructures.
One thing to notice, there is some other type of TRIP steel which is known for a long time
ago.
Actually this TRIP steel consists of 100% austenite.
This steel was developed something like 50 years ago, and has been known as the original
TRIP steel.
But nowadays TRIP aided steel, which I'm mentioning right now, is also sometimes called as TRIP
steel because it has more importance in the technology areas.
One thing you can see if you look at the strength and elongation chart in the banana plant,
you can see the properties of this multiphase steel is much better than conventional low
strength steels and HSLA steels.
At the same strength level, this multiphase steel will show higher elongation, that means
actually better formability than HSLA steel at the same strength level.
So the application area of this multiphase is actually diverse, one thing is because
of improved formability the material will always be subject to severe deformation or
severe forming process and also sometimes, if you apply this kind of multiphase steel
it will give you better shock absorption during the crash.
So actually it's been good for the crash worthiness for the safety of passengers.
Now, this multiphase steels can be fabricated by several ways.
Actually now this figure does not show the intercritical annealing process and rather
it shows the hot rolling process.
Hot rolling is kind of a continuous process, so if you do the hot rolling in the austenite
region and cool down, the steel will pass through the ferrite region and now after that,
it will pass through a bainite or sometimes if the material has very good hardenability,
it will pass through martensite.
So in such case you can easily obtain the mixture of ferrite and martensite, dual phase
structure.
But if you do the intercritical annealing, intercritical means between the A1 and A3
critical temperatures, so intercritical annealing is usually done in the two phase field, consisting
of ferrite and austenite.
So if you heat treat the material in the ferrite and austenite region and cool quickly, ferrite
remain as ferrite, but austenite will transform into martensite.
So the final result is same as before.
You will obtain ferrite and martensite dual phase structures.
One thing is, as I mentioned previously, you can actually hold the material at isothermal
temperatures for certain period of time.
In such case, now ferrite, you are starting from the ferrite and austenite and ferrite
remain as a ferrite but austenite will transfer into bainite.
So when you form the bainite, carbon will be rejected from the bainite to the surrounding
austenite so austenite will have enriched carbon content, that means austenite will
be retained as austenite even at room temperatures.
So final result is, now you can obtain the mixture of ferrite, bainite, and retained
austenite, which is known as TRIP aided steels.
I already explained this figure before, but you can see, the bright area, ferrite matrix,
dark area either can be martensite or bainite, now you see some yellow region, this is retained
austenite.
Dual phase steels actually have very important characteristics as compared to other type
of steels.
For example, tensile properties can be easily changed by changing the volume fraction of
martensite or hard phases.
How can you change the volume fraction of hard phase?
Very simple.
Simply by changing the annealing temperatures if you are doing the intercritical annealing.
And also this material, this dual phase steel, will show very important characteristics in
terms of mechanical behavior.
That is, it will show low yield strength continuous yielding.
If you look at this stress-strain stress, the steel shows low yield strengths, continuously
yielding and most of the time the steel will show high work hardening rate, of course,
initial.
The reason is when you quench the austenite to form the ferrite and martensite, now austenite
will transform into martensite, and there will be a volume expansion.
This volume expansion will induce the formation of fresh mobile dislocation in the surrounding
ferrite.
So because of fresh mobile dislocations in the ferrite, the dual phase steel will show
low strength, low yield strength and continues yielding behavior.
Then why the dual phase show high work hardening rate, higher work hardening rate than the
others?
Because when you apply the deformation, now don't forget this is the mixture of soft ferrite
and hard martensite, so if you apply the deformation soft ferrite will deform first.
Because it is surrounded by hard martensite, all the deformation will stop at the ferrite
and martensite causing strain localization around the ferrite and martensite interphase.
That is why you have very, very high initial work hardening rate so because of these characteristics
dual phase steel will show high energy absorption during crash, as I mentioned before.
So this is actually essential part of the automobiles.
One thing to note if you look at this figure, is you can increase the uniform elongation
and work hardening rate by having retained austenite besides ferrite and martensite.
So this is, the red one is a stress-strain curve of dual phase, conventional dual phase
steel, black curve is the stress-strain curve of TRIP aided or TRIP steels, and you can
see TRIP aided steel will show higher work hardening rate and the larger value of uniform
elongation than the dual phase steels.
So I want to talk about the importance of TRIP aided steels or TRIP steels, so what
is unique about TRIP aided steels?
I have been telling this many times, but the most important thing is TRIP aided steels
contain retained austenite.
Actually this retained austenite is still metastable, that means mechanically metastable,
so if you apply stress or strain, this austenite will transform into martensite.
That is the key part of this TRIP aided steels.
And because such deformation induced martensitic transformation of austenite will increase
work hardening rate as I’ve shown previously and that means there will be also an increase
in flow strength.
In such case, now uniform, you also have an increase in uniform elongation.
If you recall your basic knowledge on the mechanical behavior materials such as N value,
work hardening exponent is same as uniform elongation.
So now because of high work hardening rate, material will show larger uniform elongation.
That means basically martensitic transformation will inhibit necking during tensile deformation.
So there is the increase in ductility, so this one will always show better combination
of strength and ductility than conventional dual phase steels.
So key part, how can you make a better TRIP steels?
The key part is mechanical stability of austenite.
There are several factors which affect the mechanical stability of austenite such as
grain size, orientation and composition of austenite.
So this is very important, but if you obtain too fine grain size, austenite to martensite
transformation will may not occur.
What about the orientation?
If the orientation is such that it will give you less resolved shear stress, austenite
will not transform into martensite.
What about the composition?
If the austenite has too much carbon or too much solutes then austenite will not transform
into martensite.
So the key point is how can you control the stability of austenite?
That is to obtain optimum combination of mechanical properties.
Simply we really cannot say in the simple way, it’s all depending on these three factors.
Right combination of grain size, orientation, and composition of austenite.
Now this figure basically show what happens during that the deformation of TRIP steels.
All the colored area are austenite here, you apply strain and some of them, not all, some
of them will transform into martensite remaining ones which are not shown in the black color
here in this figure are remaining as austenite.
The thing is, because this austenite ones probably do not have the right orientation.
They probably had a less Schmid factor.
So they couldn't deform into martensite, but this one shown in this figure as colored,
these are the one which was subject to high resolved shear stress, that means high Schmid
factors so they could transform into martensite.
