Abstract: The electromagnetic stirrer (ArcSave⑧) installed under the bottom of the electric furnace can effectively mix the molten steel in the entire molten pool, thereby accelerating the homogenization of the molten steel temperature and chemical composition during the melting process. The influence of electromagnetic stirring on the smelting process of stainless steel in a 90t electric furnace at Outokumpu Stainless Steel Plant in Sweden was studied. The thermal test results show that ArcSave can accelerate the melting of scrap steel and ferrochrome alloys, and accurately control the tapping volume;
Read more »the temperature of the molten pool is also more uniform, and the tapping temperature can be accurately controlled; stable electric furnace tapping quality and tapping temperature control Conducive to the smooth operation of subsequent AOD. Stirring the molten steel can reduce the superheat of the molten pool surface and increase the heat conduction in the molten pool, so that the heat loss of the furnace wall and the furnace cover is reduced, thus reducing the power consumption and electrode consumption, and the total energy consumption can be reduced by 3 % ~ 4%. At the same time, stirring can also strengthen the interface reaction between steel and slag, reduce the content of Cr2O3 in the slag, increase the alloy yield and reduce the consumption of FeSi. It can also shorten the smelting cycle, and continuous and stable smelting operations can increase the output of electric furnace smelting by about 6% to 8%. Electromagnetic stirring technology is a safe, reliable and effective solution to improve the level of electric furnace smelting stainless steel technology.Today’s electric arc furnaces generally use
ultra-high input power and short smelting time, which requires strong bath
stirring to strengthen heat and mass transfer, reduce burning loss, reduce
energy consumption, increase yield, and evenly melt Pool temperature and
composition. At present, domestic electric furnaces generally use
bottom-blowing stirring, which has disadvantages such as uneven stirring and
risk of steel leakage at the air-permeable bricks. The electromagnetic stirring
technology stirs the molten steel very uniformly; there is no contact with the
molten steel, and there is no risk of steel leakage; convenient installation
and simple operation are the development trend of electric furnace stirring.
The Swedish company AB has been committed to the development and production of
electromagnetic metallurgical products that improve the quality and output of
molten steel for nearly 70 years, including electric furnace agitators, ladle
furnace agitators, etc. In 1947, the first electric furnace agitator (EAF-EMS)
was put into production at Uddeholms Steel Plant in Sweden, and more than 150
sets have been put into production worldwide. Recently, AB B has developed a
new generation of electric furnace electromagnetic stirrer (brand name
ArcSave?) to meet the stirring requirements of ordinary carbon steel and high
alloy steel electric furnace smelting process. It can help improve the safety
of electric furnace operation, increase output and reduce operation cost. One
set of Arc S ave has been used in the electric furnace smelting of the Ou t o k
ump u Stainless Steel Plant (OS AB) in Sweden, which is a tapping channel for
tapping steel. The technological process of OS AB steelmaking workshop is E A
F→AOD→L F→CC. Electric furnace capacity is 90t, transformer power is 110MV A.
The electric furnace is equipped with 1 set of door spray gun manipulator and 3
gas nozzles. The furnace door manipulator contains 4 spray guns, which are used
to spray O 2, N 2, Fe S i and carbon powder respectively. OS AB mainly produces
special stainless steels with high chromium content. Due to the high melting
point and high density of ferrochrome alloys, unmelted ferrochrome furnace
scales often remain at the bottom of the furnace. The continuously deposited
furnace fouling caused a large change in the tapping volume, high power
consumption and electrode consumption, and reduced the effective volume of the
molten pool, making it difficult to feed the basket. The main purpose of the
installation and use of A r c S a v e at OS AB is to solve the problem of
chromium-ferro alloy melting, while reducing production costs and increasing
output. This article analyzes and summarizes the thermal test results obtained
after the installation of A r cSav e for the 0 SA B electric furnace.
l Stirring mode of molten steel
Figure 1 shows the flow pattern of molten
steel in an electric furnace with a tapping trough equipped with A r c S a v e
agitator at the bottom of the furnace. The stirrer is arranged below the
non-magnetic (austenitic stainless steel) steel plate. The advantage of this arrangement
is that the stirrer has no direct contact with the molten steel, so only a
small amount of system maintenance is required. The low-frequency current
passes through the stirrer coil to generate a traveling wave magnetic field
that penetrates the bottom of the furnace, thereby generating a driving force
in the molten steel [1]. Since the magnetic field can penetrate the entire
height of the molten pool, the molten steel in the entire molten pool will move
in the same direction. When the moving molten steel reaches the furnace wall,
it will flow back to both sides of the furnace wall. When the direction of the
traveling wave magnetic field is opposite, the molten steel will flow in the
reverse direction, and the speed of the molten steel flowing is proportional to
the current. The following figure is the simulation calculation result of the
flow of molten steel in the molten steel slag interface and bottom in a 100 t
stainless steel electric furnace under electromagnetic stirring conditions [2].
From the figure below, it can be seen that ArcS ave produces a full circulation
in the entire molten pool, thus promoting the effective mixing of molten steel
in the entire molten pool. Compared with the gas stirring through the bottom
ventilating brick, a very big advantage of Ar C S a v e is that it can improve
the mixing effect of the entire molten pool, which greatly strengthens the
uniformity of the molten steel temperature and composition and promotes the
slag reaction.
By setting the basic parameters of the
agitator, ArCSaVe can realize fully automated control. Users can also customize
the settings to meet the mixing requirements of the electric furnace for
different smelting stages, such as scrap heating, homogenization, alloy
melting, decarburization, slag removal, and tapping.
ArcS ave has the advantages of low mixing
cost, safe and reliable operation, etc., which is conducive to optimizing the
production conditions of electric furnaces and realizing reasonable control of
logistics.
2 Results and discussion
In the electric furnace smelting process,
the main advantage of molten steel stirring is to accelerate the melting
reaction and homogenization of the molten pool. In order to compare the
improvement effect of Arc S ave on the electric furnace smelting process, two
sets of data were collected, respectively, the three-month production data with
and without ArCSaVe mixing. By comparing these two sets of data, the electric
furnace process after using ArCSaVe The advantages are summarized.
2. 1 Scrap melting and arc stability
The main difference between using and not
using ArcSave is the change in the convective strength of molten steel in the
molten pool. The forced convection generated by electromagnetic stirring can
accelerate the melting of large pieces of scrap steel and ferrochrome, reduce
the probability of scrap collapse, and the layout of the charge is no longer so
strict. C F D simulation results show that the flow rate of molten steel in the
molten pool is about 10 times that of natural convection after using A r c S a
v e [2]. The strong convection in the molten pool helps to evenly distribute
the temperature and accelerate the melting of the scrap. At the same time, it
was also found that by quickly melting large scraps and reducing collapse, Ar C
S a v e can stabilize the arc to a certain extent.
The figure below shows the comparison of
the current fluctuations of one of the electrodes without stirring and using A
r c S a v e. It can be seen that after using A r c S a v e, the current
fluctuation is significantly reduced. The standard deviation of electrode
current without stirring is 9.3, and it is 3.7 after using ArcSave. The
reduction of electrode current fluctuation will increase the effective input
power of the electric furnace, thereby shortening the smelting cycle. The
effect of ArcSave on improving the stability of the electrode arc was also
confirmed in another EBT tapping normal carbon steel electric furnace test [3].
(figure 2)
2.2 Arc heating efficiency and energy saving
According to literature reports, the
temperature gradient of molten steel during the melting period (after the
formation of the molten pool) of molten steel in a traditional AC electric
furnace without stirring is within 50~70°C [4]; but after electromagnetic
stirring, the temperature gradient in the molten pool is only About 25% without
stirring [1]. This means that stirring reduces the overheating of the molten
pool surface during the arc heating process, and the heat of the arc can be
quickly transferred from the arc area to the molten steel [5]. The reduction of
the superheat of the molten pool liquid surface can reduce the heat loss taken
away by the cooling water of the furnace wall and the furnace cover during the
electrification of the electrode, thereby reducing the power consumption. The
OS A B electric furnace does not use stirring furnace wall and furnace cover
cooling! The heat taken away by the water is 81kWh/t, while using ArcSave is
55kWh/t. After using ArCSave, the heat taken by the cooling water is reduced by
26kWh/t, which is roughly equivalent to saving 4% of electricity. At the same
time, stirring can accelerate the melting of scrap steel or ferrochrome alloy,
increase the decarburization rate, thereby shorten the smelting cycle, which
can also reduce the heat loss of the furnace body. The combined effects of
stable electric arc, lower liquid surface superheat, low oxygen activity slag
and lower power consumption make the electrode consumption saving more than 9%.
2.3 Homogenization of the molten pool
and lower tapping temperature
Through the A r c S ave stirring, an
element flow is generated in the molten pool, which makes the entire molten
pool thoroughly mixed, thereby obtaining a very good temperature and
composition uniformity. After turning off the power of the arc, the temperature
was measured at two different locations in the same molten steel with a
temperature measurement interval of 1 ~ 2 min. The result is shown in Figure 4.
The corresponding average temperature difference at the two different locations
is less than 2° C. (Picture 3)
From a process point of view, good bath
uniformity is very important. This means the reliability of molten pool
composition sampling, the accuracy of alloy addition calculation and the
accuracy of temperature measurement. After using A r c S a v e, the compliance
rate of tapping temperature control is almost 100%, which is very important for
reducing the difficulty of subsequent A O D smelting operations. After using A
rC S a v e, the average tapping temperature was reduced by 30 °C, while the
temperature of molten steel reaching the A OD station did not change
significantly.
2.4 Steel slag reaction and C r 203
reduction
Compared with the ladle furnace, the
electric arc furnace has a larger molten pool surface and a shallower molten
pool depth. Therefore, electromagnetic stirring has a more obvious promotion
effect on the slag reaction in the electric furnace than in the ladle furnace.
If stirring is not used, the mass transfer to and from the reaction zone mainly
depends on diffusion. After electromagnetic stirring is used, due to the
frictional force of the steel slag interface, the movement of the molten steel
produced by A r CS a V e can produce a certain stirring effect on the slag, so
that various parts of the steel slag will be stirred to the reaction zone of
the steel slag interface, thereby shortening The diffusion distance in the
reaction interface area is increased, and the speed of the steel slag interface
reaction is accelerated. Convective mass transfer is very beneficial for
desulfurization and the reduction of C r203 in the slag. During the oxidation
period, chromium is oxidized and enters the slag, and it is very important that
Cr 20 3 is reduced so that Cr returns to the molten steel again. By optimizing
the stirring parameters, the C r 203 content in the slag can be effectively
controlled. The optimized combination of Ar C S a V e stirring and oxygen blowing
can reduce the average Cr203 mass fraction in the slag by 3.1%. After the use
of Ar c S ave, the nitrogen consumption, which is mainly used to enhance the
stirring effect, has been reduced by 70%. In fact, after using Arc S ave, there
is no need to inject nitrogen from the furnace door.
2.5 Consumption of FeSi and slagging
agent
Before using ArcSave, the consumption of
ferrosilicon alloy in OSAB electric furnace operation is much higher than the
conventional range. The reason for the high consumption of ferrosilicon alloy
is the large amount of oxygen used to obtain extra energy to help the melting
of ferrochrome alloy. After using Arc S ave, the amount of ferrosilicon alloy
added is significantly reduced, and the consumption is reduced by about 15%. Of
course, the reduction in the consumption of ferrosilicon alloy will increase
the power consumption correspondingly, and the increased power consumption is
to melt the ferrochrome alloy. Reducing the amount of ferrosilicon alloy can
reduce the content of Si 0 2 in the slag, so it can reduce the consumption of
lime by about 9% at the same time, so as to maintain the basicity of the slag
basically unchanged. After using A r c S a v e, a more stable arc can also
reduce the total amount of slag.
2.6 The melting of ferrochrome alloy and
the control of tapping volume
In OS A B electric furnace smelting, too
high Fe C r alloy addition and short tapping cycle (less than 65 min) often
cause furnace bottom fouling problems. Fouling is caused by the deposition of
unmelted Fe C r alloy and large scrap steel on the bottom of the furnace. The
deposition of FeCr alloy on the bottom of the furnace makes it very difficult
to control the tapping volume. At the same time, the gradually rising bottom of
the furnace also reduces the effective volume of the molten pool and increases
the number of charging. As mentioned earlier, one of the main purposes of
installing ArcSave at OS AB steel plant is to solve the problem of Fe C r alloy
melting. Because the Fe C r alloy has a higher melting temperature and a higher
density, it is easy to deposit on the bottom of the furnace, and the bottom of
the furnace is an area with a lower molten steel temperature. Therefore,
without stirring measures, the alloy melts into a large problem. After the
electric furnace is equipped with A r c S a v e, the temperature of the entire
molten pool is uniform. At the same time, the strong convection generated by
stirring accelerates the melting of Fe C r alloys, including large scrap steel.
This means that the uniform temperature of the molten pool and strong
convection together promote the melting of the Fe C r alloy. The test proves
that after using Ar cS ave, the melting rate of F eC r alloy is increased, so
the problem of furnace bottom alloy fouling is eliminated, which makes the
accuracy of tapping volume increased by more than 24%: when stirring is not
used, The tapping accuracy rate is 69%; after using ArcS aV e, the tapping
accuracy rate is increased to 93%.
2.7 Stability of A0D operation
After using ArCSave, the controllability of
the tapping quality and tapping temperature of the electric furnace stabilizes
the initial conditions of the subsequent A0D operation. The correct tapping
quality can reduce the amount of additional alloy added in A O D smelting. This
additional alloying process will increase Fe S i, lime and oxygen consumption
in AOD smelting. The initial tapping temperature lower than the AOD requirement
will also increase Fe S i alloy consumption, because more chemical energy needs
to be obtained from Fe S i to increase the temperature of the AOD molten pool.
The continuous and stable operation of A O D also has great potential for
increasing output and reducing operating costs.
2. 8 Process reliability and safety
Safety and reliability have always been
very important aspects of electric furnace smelting operations. The positive
influence of A r c S a v e discussed above on the electric furnace smelting
process plays an important role in improving the reliability of the electric
furnace process. The complete melting of large scraps and ferrochrome alloys
can promote the rapid and uniform temperature and composition of the molten
pool, which ensures the accuracy of the tapping temperature and quality.
Stirring the molten pool can reduce the collapse of scrap, stabilize the
electrode arc and reduce the risk of electrode fracture. The uniform
performance of the temperature in the entire molten pool ensures smooth
tapping, thereby reducing tapping delays. Eliminating the thermal
stratification in the molten pool can also significantly reduce the tapping
temperature to 30°C. After using A r C S a V e, the hot and cold spots in the molten
pool are also eliminated. -More than years of operating tests have confirmed
that A r c S a v e has no side effects on the refractory layer of the furnace
bottom, but can significantly reduce the corrosion of the refractory material
in the hot spot and slag line area, thereby reducing the consumption of gunning
material.
2. 9 Actual production data
The electromagnetic stirrer can improve the
kinetic conditions of the heat and mass transfer of the molten steel in the
electric furnace smelting process, increase the melting rate of scrap steel and
ferrochrome alloys, obtain a molten pool with uniform temperature, and control
the tapping temperature and tapping quality more accurately and reduce Energy
consumption and electrode consumption optimize the operation of subsequent AOD
stations. Stirring can strengthen the reduction of slag, increase the yield of
chromium and reduce the consumption of ferrosilicon alloy and lime. Shorter
tapping cycles and continuous and stable tapping operations can also increase
the output of electric furnaces. The improvement effect of electromagnetic
stirring on the electric furnace smelting process is shown in the table below.
(Figure 4)
3 conclusion
1) Electromagnetic stirring can uniform the
temperature and composition in the molten pool, and the temperature difference
between different positions is less than 2 °C.
2) After using A r c S ave, the scrap
melting time is shortened by about 4%~5%, the electrode current is more stable,
and the electrode consumption is reduced by 8%~10%.
3) Electromagnetic stirring can reduce
power consumption by about 4%, reduce N 2 consumption by about 70%, and reduce
alloy consumption by about 15%.
4) After using electromagnetic stirring,
the tapping accuracy rate reaches 93%, and the tapping temperature hit rate
reaches 100%.
5) ArcSave can reduce power consumption
time by 4%~5%, and shorter smelting time can increase output by 6%~8%.
6) Compared with bottom-blown air stirring,
the temperature and composition of molten steel are more uniform by
electromagnetic stirring. Because the stirrer does not directly contact the
molten steel, it will not cause leakage accidents, and it is easy to install,
simple to operate, and has a long service life. Ar cS a ve is a very low
maintenance technology that can help the smelting process. It can make the
smelting process safer, faster, and lower production costs.
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