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So far, we have covered various types of steels that have different characteristics.
These steels find their applications in various sectors depending on their properties.
If you take an example of automobiles, you can see, the different part requires different
combination of properties.
Automobile parts can be divided into three: closure, body-in-white, and chassis.
The closure consists of hood, door and trunk lid, etc and its main function is to cover
the outside of automobiles.
So the materials for the closure should have good surface quality, high formability, excellent
dent resistance and they also should have low yield strength and yield ratio.
Therefore, low strength steels with good formability are ideal for these applications.
Body-in-white is the mainframe structures consisting of bumpers, subframes, cross members,
side panel, and impact beam and pillars.
There, the materials main function is to protect the passengers, so the material
for the panel should have, of course, good surface quality, drawability, and lower DBTT-
ductile brittle transition temperature.
And for the impact absorption, material should have crashworthiness, bendability and low strength.
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For the part, using for the support, the material should have high yield ratio, and bendability.
In general, high strength steels are used to make body-in-white structures.
Chassis consists of steering, suspension, wheel, and etc.
The main function should be supporting the mainframe.
So for the suspension system, the material, actually steel, should have very good, high,
hole expansion ratio, very good fatigue strength, press formability, bendability, and weldability.
Another important part of the chassis, which is steel, the steel should have whole expansion
ratio, fatigue strength and press formability.
So similar to body-in-white, for the chassis application, high strengths are usually needed.
Beside the automotive application, steels are also very important for the applications
that require good toughness, sometimes at low temperatures.
The typical examples are pipelines, ships, and offshore structures.
Particularly for pipeline applications, the steel should have several important mechanical
properties such as high strength and ductility of course, and the material also should have
very good low temperature toughness.
And if the gases or the oils also contain sulfur, the pipeline should have good resistance
to sulfide stress corrosion cracking.
If you look at this figure, there are various type of steels being used for low temperature
application such as the API, American Petroleum Institute, steels.
And depending on the customer requirement and depending on technology level, you can
divide this type of steels based on their microstructures or based on their strength
levels.
For example, if the application requires yield strength less than 60 ksi, 60 ksi is about
420 MPa, if the yield strength is lower than 60 we call it X60, then typical microstructure
is ferrite and pearlite.
If you need the higher strength above 60, then the structure should be changed to acicular
ferrite, because ferrite-pearlite will not give you such high strength and high toughness.
But if you want to go for X80 level, the structure should change to something like acicular ferrite
along with bainite.
But now the costumer requirement becomes much severe, so that means they sometimes need
material which has yield strength higher than 100 ksi which is around the 700 MPa.
In such case, acicular ferrite and bainite mixed structures cannot provide such high
strength along with a good toughness.
So you have to go to the mixture of bainite and martensite.
And if the strength requires higher than 110 ksi such as X120 level, the structure should
be based on martensite, of course this should be tempered one because you also need high
toughness.
Also, the technical level is different depending on the environment, such as, if you have lots
of gas and oil which contains sulphur, then you need higher technology level than the
one which has non-sour pipe.
So basically it shows, depending on the application the material should have adequate properties,
and that can be done by controlling the microstructures, also by alloy composition and processes.
Sung-Mo JungProfessor
Kim, Sung-JoonProfessor
Kang, Youn-BaeProfessor
Suh, Dong WooAssociate Professor
Kim, Nack JoonProfessor
