Now let's look at the details of steels.
Starting with low strength grade steels called conventional low strength steels.
These include interstitial free steels, mild steels, bake hardenable steels, high strength
interstitial free steels and carbon manganese steels.
As you can see in this figure, their tensile strength ranges from about 200 MPa to about
600 MPa, which is on the low side.
However these steels show very good ductility up to 50 percent.
So these steels usually show very good formability which is ideal for deep drawing process.
One of them, the mild steel, is based on ferrite with a small amount of pearlite and have good
drawing capability with low strength.
So this is the typical microstructure of mild steel showing the ferrite matrix with some,
small amount of pearlite.
However to have better formability than mild steels, a new type of steels called interstitial
free steels have been developed.
This steel is called interstitial free because they contain very small amount of interstitial
atom such as carbon and nitrogen.
As I just mentioned, interstitial free steel shows better formability and also higher work
hardening rate, which means n value, than mild steels.
However, the steels have somewhat lower strength, therefore higher strength version has been
developed, which utilize solid solution strengthening, precipitation of carbide and nitride and grain
refinement to increase strength.
So in this case, the control of alloy composition is needed.
That is microalloy concentration must exceed the stoichiometric ratio required to form
carbide and nitride, for example for 0.003 wt% each of C and N, you need 0.01 % titanium
and 0.02 % niobium.
Another type of steel that belongs to conventional low steels is the bake hardenable steels.
As you can expect from its name, strength of steel is increased during bake hardening
of the formed part.
So when the steel is subject to the forming process, for example from a to b, the strength
is increased by work hardening.
During the baking process, carbon atoms diffuse into dislocations making dislocation difficult
to move, which is known as strain aging phenomena.
Hence the formed part will have higher strength after baking and this steel, we have much
better dent resistance, which are ideal for outer panel of automobiles.
The last example of low strength steel is carbon manganese steel.
It's basically similar to mild steel, but they try to obtain higher strength so they
use higher solute content and therefore they can have higher strength, but lower ductility.
Kim, Sung-JoonProfessor
Kang, Youn-BaeProfessor
Suh, Dong WooAssociate Professor
Kim, Nack JoonProfessor
