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A stainless steel is one of important steel products which contains high amount of chrome
and nickel.
The stainless steel has very good corrosion resistance due to chrome.
The stainless steel is typical produced not from blast furnace-BOF route but it is produced
from electric arc furnace process by collecting the scrap which was stainless steel.
This melts in electric arc furnace, however contrary to the conventional electric arc
furnace process for normal carbon steel, the stainless steel contains high amount of chromium
which is easily oxidized.
Therefore the stainless steel making process after EAF process is a bit different to normal
carbon steel production route.
At the EAF process there are two typical refining called AOD (Argon Oxygen Decarburization)
or VOD (Vacuum Oxygen Decarburization).
Once steel is produced then it is delivered to casting plant.
Raw materials for stainless steel making process is of course stainless steel scrap itself.
Which was stainless steel so it is very important source of chromium and nickel of course iron
and normal carbon steel scrap is also used as iron source and carbon source and in order
to adjust the conversion of stainless steel some other types of alloying materials may
be added and deoxidized like silicon, aluminum, titanium can be used.
The other part is for making slag add lime dolomite or phosphorus.
Once these raw materials are prepared then they are charged it into electric arc furnace,
they melt this shows some typical the mixture of the melt.
This is just an example such as 50 percent of stainless steel scrap, 30 percent of normal
steel scrap and in order to adjust chromic content inside we may use ferro chromium alloys
and add nickel ferromaganese alloys.
Once these scraps are all melt then this steel is decarburized but this decarburization is
a little bit different to normal carbon steel.
But let's look at AOD process, once steel scrap are all melt then like normal carbon
steel, decarburization is necessary.
However, this stainless steel contains high amount of chromium so just direct blowing
of oxygen will cause oxidation of chromium, and it is close to slag phase
This increases production cost of stainless steel a lot.
So it is one of important issues.
How we can keep chromium in liquid steel?
Therefore the oxygen gas is not blown directly, but this is diluted with the argon gas or
nitrogen gas so therefore oxidation power of the gas is lowered so let's oxidation of
chromium is done and once chromium oxide enter into slag then we use carbon in liquid steel
to reduce chromium oxide and we get back chromium in the liquid steel and CO gas is forming.
By blowing oxygen gas diluted with argon or nitrogen gas this CO gas is also diluted.
Therefore, this reduction reaction for chromium oxide is also accelerated and through this
lance oxygen is also blown on to liquid steel.
Purpose of blowing this oxygen gas is to oxidize the CO gas produced from the previous reaction
to form CO2 gas which delivers heat inside this furnace.
Then why is decarburization so difficult in stainless steel making process compared to
normal carbon steel?
Let's look at these chemical reactions.
Carbon dissolve in liquid steel should meet oxygen to form CO gas.
Suppose this K is equivalent constant for this reaction then produced CO has its own
partial pressure this partial pressure is proportional to equal constant and activity
of this carbon and oxygen, activity to carbon and oxygen may be composed of activity coefficients
of oxygen and carbon and concentration of oxygen and carbon.
These activity coefficients are actual function of composition of the liquid steel.
In case of normal common steel it just contains 0.1 percent carbon for example activity coefficient
of oxygen and carbon very close to 1 so it's product is also close to 1.
This is normal however, in case of stainless steel which contains either chromium or chromium
liquid together at the same amount of carbon.
Activity coefficient of oxygen and carbon are much smaller than normal carbon steel
so their product becomes approximately 0.1 or even lower, it means lower activity coefficient.
Lowers partial pressure of CO gas.
This partial pressure of CO gas is low, this reaction does not take place.
This is why decarburization in stainless steel is a little difficult than in normal carbon
steel.
Nevertheless, this decarburization for stainless steel making is done by two different routes:
AOD and VOD.
Now this slide shows VOD process, decarburization is carried out in a vacuum chamber by lowering
pressure inside, so carbon and oxygen from CO gas this is easily evaporated.
The advantage of this VOD process compared to AOD process is, it uses less amount of
argon.
And argon gas with oxygen is not lown in liquid steel so oxidation rate of chromium is less
and it also used less amount of the oxidziable elements.
However, VOD process only depends the relies on the pressure inside chamber to decrease
carbon level, this is less flexible then AOD in terms of lowering carbon concentration,
so it requires the long process time to decrease carbon level up to desired carbon concentration
and if initial concentration carbon in liquid steel is low then this is quite difficult
to further lower in carbon concentration in liquid steel.
So in normal stainless steel making process, the main decarburization is done by AOD process,
then the final adjustment for the coverages is done using VOD process.
But how much portion of AOD is used how much portion of VOD is used?
It partially depends on the argon price and the carbon nitrogen level into the product.
Once liquid steel is made then the steel is sent to casting plant.
Kim, Nack JoonProfessor
Suh, Dong WooAssociate Professor
Kang, Youn-BaeProfessor
Sung-Mo JungProfessor
Kim, Sung-JoonProfessor
