After continued casting process,
we obtain different types of same final products.
These are Slab, Bloom or Plate.
Let's talk with slab.
Slab is sent to rolling plant and the purpose of rolling process is divided into two.
The first purpose of rolling process is changing dimension of the slab.
So typically it's thickness is decreased.
Then it becomes toned for a final product.
For example, it becomes plate,
this is thick product or the slab maybe load to either sheet or strip.
This is thin product.
But difference between sheet and strip is width.
It changes its shape.
And second, during the rolling process
the internal microstructure of slab is late developed.
Shown in right side is the schematic figure of rolling process.
Now we have slab produced from casting process,
it passes through two rolls layer.
Slab which is moving to right hand side.
This is called rolling friction.
After passing these two rolls,
you find a product is formed.
On the left hand side, the Slab contains
its own old grain structure which was developed during solidification.
It contains certain amount of defects inside.
When slab is pressed by these two rolls,
this old microstructure is destroyed.
And new microstructure is developed.
This process is called recrystallisation.
recrystallization is a process of replacing
already formed grains to a new set of defect free grains.
Therefore, the physical property are changing.
Strength and hardness are decreased and the ductility is increased.
Shown in the right hand side is the schematic figure of
recrystallization from its initial defect containing
microstructure to final recrystalised microstructure to nucleation of new grains.
Depending on temperature wear rolling process,
we divide rolling process into two.
The first one is hot rolling,
the second one is cold rolling.
It's divided due to temperature called recrystallization temperature.
Recrystallization temperature is the lowest temperature
where recrystallization can take place.
So in case of hot rolling,
this process are both recrystallized temperature of the slab.
Other than cold rolling process below recrystallized temperature.
This development of new microstructure recrystallization takes place during hot rolling,
because hot rolling process is above recrystallization temperature.
On the other hand during cold rolling,
recrystallization does not take place because the temperature is low.
But once the final product is annealed at
lots of high temperature then recrystallization takes place even after cold rolling.
Shown in the right hand side is photos of two product.
Hot rolled product, and below is cold rolled product.
In case of hot rolled, the temperature of the product is above
recrystallized temperature and the product
after hot rolling is relatively large and thick.
And since rolling is carried out relatively high temperature,
deformation of this product is rather easy.
The product is rather soft and since the product is still at
high temperature the surface of this product is easily oxidized
so the surface of the product is quite dirty.
And the product from hard rolling process is
sometimes used as raw material for cold rolling process.
So it's final dimension is determined after cold rolling.
So tolerance for the shape at the hot rolling process is rather high.
On the other hand.
The cold rolling process is carried out at low recrystallization temperature
and it produces relevant small and thin pieces and temperature is low,
therefore deformation becomes relatively difficult than the hot rolling process.
And oxidation is less occured so surface of product is much clean.
And as this cold rolling process that demanded protocol should be determined so
tolerance limit is relatively lower than that of hot rolling process.
Now in the next slides, we'll discuss about hot rolling process.
This slide shows the schematic operation loot for hot rolling process.
From the caster, We deliver slab. But first it should be heated.
It ought to have enough heat enough to be soft enough to be lowered.
During the heating, surface of the slab is oxidized and scales are formed.
Then a thick scale formed on the slab should be
removed before rolling process by scale breaker.
Then the slab is sent forth to roughing mill.
The scale removed slab is deformed into appropriate shape and thickness and width.
But this is just be resized to finer dimensions.
So first rolling is done in roughing mill then it is sent to finishing mill.
During the finishing mill rolling,
the slab is transformed into the finer predetermined thickness width and
shape then it is for sent to cooler and the finer coil is produced.
This becomes the final product of the steel.
Bloom and there was a similar rolling process.
So product are sold as a hot coil and the other product
may be sent to cold rolling plant to process for the process cold rolling.
As a result we obtained coil-plate or wire rod.
This is a final product which are produced from the steel plant.
Kim, Nack JoonProfessor
Suh, Dong WooAssociate Professor
Kang, Youn-BaeProfessor
Kim, Sung-JoonProfessor
