AN IMPROVED METHOD OF DEOXIDATION OF HIGH CHROMIUM
STEEL DURING PROCESSING IN EAF (ELECTRIC ARC FURNACE)
INTRODUCTION TO THE FIELD OF INVENTION.
This invention relates to an unproved method of deoxidation of high chromium steel
during processing in EAF (Electric Arc Furnace)
PRIOR ART & DRAW BACKS.
It is well known that Electric Arc Furnace (EAF) is employed primarily for melting
and preliminary refining whereas final refining is carried out in secondary refining
units like LF (Ladle Furnace), VAD (Vacuum Arc Degassing), VOD (Vacuum
Oxygen Decarburisation) etc for improved productivity and steel quality.
It is also known that in the Electric Arc Furnace (EAF) the charge mix for high
chromium (12-26 %) steel is selected from steel scrap, ferro-chrome and other ferro-
alloys.
In the practice now being carried out during melting, some chromium, along with
iron and manganese is oxidized during melting. After complete melting oxygen is
blown in to liquid steel through oxygen lance to remove carbon. This is called
decarburisation period. During this period also some chromium, iron and manganese
are oxidized along with carbon and these oxides namely Cr2O3, FeO and MnO are
dissolved in to slag. After removal of carbon to desired levels the steel is deoxidized
to remove dissolved oxygen and slag deoxidation is done to recover chromium iron
and manganese from slag.
It is also known to use deoxidisers like aluminium (Bar and shots) and ferro-silicon
(Lump and fine) to deoxidise both steel and slag.
It is also the practice to start arcing after addition of these for melting of deoxidisers
and efficient reduction. After completion of reduction stage the slag is removed by
raking. This stage in EAF is known as reducing period. Steel is tapped in the ladle
after temperature and composition adjustment.
BACKGROUND OF THE INVESTIGATION
However by continuous research and behavioural characteristics to several steps and
experimentation, we have found that efficient deoxidation of slag plays an important
role in the overall economics of the process.
We have also found that an efficient deoxidation of the slag reduces the contents of
above oxides lost in to slag.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose an improved method of
deoxidation of high chromium steel during processing in EAF.
It is another object to propose a special fluxing and deoxidation technique and with a
view to reduce loss of chromium, iron and manganese to slag in EAF.
In our method we have provided a technique which enhances the capability of
deoxidiser like aluminium and ferrosilicon to react preferentially with slag, instead of
burning in atmosphere or getting lost by evaporation during arcing thereby reducing
the loss of Chromium, Iron and Manganese to the slag in the form of oxides.
BRIEF STATEMENT OF THE INVENTION
Thus according to this invention, an improved method of deoxidation of high
chromium steel during processing in EAF is proposed which comprise the following
steps.
(a) Preparing a melt of steel in an Electric Arc Furnace (EAF) using steel scrap,
ferro alloys and lime, then followed by arcing to form melt;
(b) subjecting the melt to oxidation blowing;
(c) adding aluminium and ferro-silicon in EAF during reduction period for
deoxidation through charging bucket to reduce the dissolved oxygen of the
steel and reduces the oxide content of slag;
(d) carrying out the Arcing thereafter followed by ;
(e) tapping the steel in to ladle after deslagging and attaining the required
temperature and composition.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 illustrates the average content of slag constituents for both conventional and
experimental heats in the form of bar chart derived from the data, which is presented
in table in detailed description.
DETAILED DESCRIPTION
It has been our main aim and purpose of the deoxidation to reduce the dissolved
oxygen of the steel and reduce the oxide content of the slag. Some of these
deoxidisers get lost due to evaporation by arcing/ burning in atmosphere instead of
reacting with slag. Thus inefficient deoxidation leads to increased content of oxides
which are lost in to slag. The steel is tapped into ladle after deslagging and attaining
the required temperature and composition.
The advantages of the invention are as follows:
The invention envisages change in methodology of lime addition to facilitate early
slag formation. Also during reducing period, a technique for improved deoxidation for
slag and metal has been developed to give protecting cover to the deoxidisers. In this
technique, both aluminium and ferro-silicon are covered with lime during additions in
EAF, which protects aluminium and ferro-silicon from arc and atmosphere for longer
period. Thus reaction time of aluminium and ferro-silicon is prolonged by few
minutes (more than one minute) and they are able to react preferentially with slag and
metal thereby reducing the oxide content (chromium, iron and manganese) of EAF
final slag.
Several experiments were carried out and comparison was made between
conventional process and improved process in 50T EAF. Steps involved in one such
experiment are given below:
(a) addition of lime (-20 Kg/t) along with charge of steel scrap and ferro alloys
to facilitate early slag formatioa
(b) arcing is started for melting of charge
(c) after the charge mix is melted in EAF, oxygen is blown in to liquid steel to
remove carbon through a consumable lance for duration of 20-30 minutes at a
pressure of 8-10 Kg/Cm2. During this period some iron, manganese and
chromium are also oxidised along with carbon and these oxides are dissolved
in slag. Carbon is brought down to 0.6-1.0% from melt down carbon of 1.2-
1.4%.
(d) after removal of carbon to desired levels the steel is deoxidized to remove
oxygen and slag deoxidation is done to recover iron, manganese and
chromium. Aluminium bar and lump ferro-silicon is used to deoxidise liquid
steel whereas aluminium shots and ferro-silicon is used to deoxidise slag.
(e) during reducing period, bags of lime are first placed in the charging bucket
and aluminium bar/shots along with ferro-silicon is kept over this. During
discharging in EAF the bucket is overturned and both aluminium and ferro-
silicon are covered with lime which protects aluminium from arc and
atmosphere for longer period which enhances the efficiency of slag
reduction. 200-250 Kg of aluminium along with 50 Kg of ferro-silicon and
300 Kg of lime is used during this period. Analysis of ferro-silicon and Al
used is given below;
Fe-Si: Silicon -70% minimum, C- 0.15% max, P & S - 0.05%max and Al- 2% max
Aluminum : Al >99%
Arcing is started after addition of these for melting of deoxidisers and efficient
reduction. Duration of reduction period is 15-30 minutes. Here heat required is
provided by electric arcs created between the electrode and steel. During
arcing electric power is supplied from a three phase multivoltage tap
transformer. The electrodes are connected by heavy flexible cables to the
transformer through electrode holders.
(0 after completion of reduction stage the slag is removed by raking and the steel
is tapped in the ladle. The tap temperature is generally in the range of 1670-
1700 deg. C.
Following conclusions can be drawn from this table
• Average Cr2O3 content of EAF final slag has reduced from 27.9% to 22.2%,
which indicates that loss of chromium to slag in the form of Cr2O3 has reduced
by 5.7%. Accordingly chromium recovery in EAF of experimental heats has
improved by 3.8 %( From 82.6 to 86.4).
• Average combined FeO & MnO content of EAF final slag has also reduced
from 24.1% to 9.8%. This reduction of FeO and MnO content establishes the
fact the modified practice was effective in reducing the oxide (Cr2O3, FeO and
MnO) content of slag.
• AI2O3 content of EAF final slag increased to 11.2% from earlier level of 5.4%
and silica content has increased to 28% from 25.2 %. This increased content of
Al2O3 and silica of the slag indicates that both aluminium and silicon were
able to react more efficiently with oxygen of metal and oxide (Cr2O3, FeO and
MnO)of slag.
• Average Slag basicity also improved from 0.36 to 0.6 due to increased lime
addition
• Reduction in oxides oxide (Cr2O3, FeO and MnO) content by 20.0% (from
52.0%to 32.0%) and increased alumina and silica content in experimental
heats compared to conventional heat indicate the effectiveness of technology
for improved deoxidation.
WE CLAIM
(1) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF which comprises the following steps
(a) preparing a melt of steel in an Electric Arc Furnace (EAF) using steel
scrap, ferro alloys and lime, then followed by arcing to form melt;
(b) subjecting the melt to oxygen blowing;
(c) adding aluminium and ferro-silicon in EAF during reduction period
for deoxidation through charging bucket to reduce the dissolved
oxygen of the steel and reduces the oxide content of slag ;
(d) carrying out the arcing thereafter followed by ;
(e) tapping the steel in to ladle after deslagging and attaining the required
temperature and composition.
(2) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF as claimed in claim 1, wherein addition of lime (-20 Kg/t)
along with charge of steel scrap and ferro alloys to facilitate early slag formation
(3) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF as claimed in claim 1, wherein charge mix is melted in EAF by
blowing oxygen in to liquid steel to remove carbon through a consumable lance for
duration of 20-30 minutes at a pressure of 8-10 Kg/Cm2.
(4) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF as claimed in claim 1, wherein during reduction period for
deoxidation 200-250 Kg of aluminium along with 50 Kg of ferro-silicon and 300
Kg of lime is added in the charging basket.
(5) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF as claimed in claim 1 wherein duration of reduction period in
arcing is 15-30 minutes.
(6) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF as claimed in claim 1, wherein the tap temperature is in the
range of 1670-1700°C.
7) An improved method of deoxidation of high chromium (12-26%) steel during
processing in EAF substantially as herein described.

This invention relates to an improved method of deoxidation of high chromium (12-
26%) steel during processing in EAF (Electric Arc Furnace) which comprises of the
following steps:
(a) preparing a melt of steel in an Electric Arc Furnace (EAF) using steel
scrap, ferro alloys and lime, then followed by arcing to form melt;
(b) subjecting the melt to oxygen blowing;
(c) adding aluminium and ferro-silicon in EAF during reduction period for
deoxidation through charging bucket to reduce the dissolved oxygen of the
steel and reduces the oxide content of slag;
(d) carrying out the arcing thereafter followed by;
(e) tapping the steel in to ladle after deslagging and attaining the required
temperature and composition.