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Metallurgical Characteristics and Effectiveness of Metallic Charges in Electric Arc Furnace.

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Dev. Chem. Eng. Mineral Process. 14(3/4), pp. 353-362, 2006.
Metallurgical Characteristics and
Effectiveness of Metallic Charges
in Electric Arc Furnace
Cheng-Wu Du*, Miao-Yong Zhu, Li-Na Sun' and
Shi-Ze Dong'
School of Materials and Metallurgy, Northeastern University,
PO Box 241, Shenyang 110004, 49 R. China
Department of Metallurgical Engineering, Liaoning Institute
of Science and Technology, Benxi I1 7022, Pr R. China
'
The use of direct reduced iron (DRI), hot metal, cold pig iron, decarburized granular
iron, iron carbide, and complex metal charges to replace scrap metal as the feedstock
to an electric arc furnace not o n b resolves the lack of scrap supply, but is also very
helpful in diluting the residual elements in the scrap, thus improving the quality of the
steel. It has now become possible to produce high-quality steel in an electric arc
furnace. In this paper, the characteristics and effectiveness of various metallic
charges for use in an electric arc furnace (EAF) are discussed.
Introduction
Scrap is a major charge in EAF steelmaking. However, the amounts of impurity
elements in scrap irons increases continuously due to repeated recycling of scrap, and
growth steel coatings on products. This can be detrimental due to the generation of
hot brittleness for a casting slab, decreasing mechanical properties of the steel, and
even cast breakout. Traditional smelting operations only add 10% pig iron into the
furnace as a substitute for scrap. Nowadays, other alternatives such as direct reduced
iron, hot metal, decarburized granular iron, iron carbide, and other complex metal
charges are used as the charge in steelmaking as well as cold pig iron.
Direct Reduced Iron
Direct reduction technology has been developed over 100 years [l]. Direct reduced
iron is a metallic iron product based on the direct reduction process of removing
oxygen from iron oxide. In this process, iron ore or concentrate pellets are added into
* Author for correspondence (ducwducw@163,com).
353
Cheng- Wu Du, Miao- Yong Zhu, Li-Na Sun and Shi-Ze Dong
either a rotary kiln, shaft furnace, fluid-bed or reactive tank in order to reduce ferric
oxide with CO or H2, or by using coal at the softening temperature. The spongy
metallic iron product from iron ore reduction in a rotary kiln or shaft furnace is called
spongy iron. After pelletizing, concentrated fines were reduced to metallic pellets.
The product extruded from spongy iron or metallic pellets at the hot state is called hot
briquetted iron (HBI). HBI has a high density, usually of large size, with good
thermal conductivity, which is similar to scrap. DRI tends to float at the surface of
with top slag, while HBI tends to sink into the bath due to its high density. Therefore,
HBI behaves in a manner similar to pig iron in EAF operations. Since the sulphur,
phosphorus and other tramp element concentrations are very low in DRI, the usage of
DRI in EAF has the function of diluting the concentration of tramp elements and
decreasing their content in gas and inclusions [2-41.
Using DRI for steelmaking can improve steel quality by increasing the yield
strength, ductility, cold-workability, heat-treatability, and welding performance, and
also reducing the strain age. The experimental results of Sidbec-Dosco Co. are given
in Tables 1 and 2 [5]. It can be seen that replacing high-quality scrap with DRI is
more suitable for producing petroleum steel pipe, steel wire rope, and cable conductor,
all of which require strict control of the compositions of nitrogen and tramp elements.
Table 1. Typical residual and nitrogen contents of Sidbec-Dosco products [5].
Composition
cu, %
100% selected scrap
60% DlU, 40% scrap
100% scrau
Property
Temaer rolled
Aged 1 month
Temper rolled
Aged I month
Temper rolled
Aaed I month
Temper rolled
Aped 1 month
353
0.20
0.10
0.03
Ni, %
0.07
0.05
0.02
Charge ppe
100% scrap
24 1
282
35 1
358
34
25
0.21
0.17
cc %
0.08
0.05
0.02
IO0% DRl
203
217
323
325
39
36
0.23
0.2 1
N2>PP"
70
50
25
Metallurgical Characteristics and Effectiveness of Metallic Charges in EA Furnace
As well as the above-mentioned virtues for using DRI, the charge of DRI has
shown the following advantages for the process and its operation [6,7]:
(1) For higher carbon contents that react chemically with residual oxides in the DRI,
the gas generated by the reaction will be helpful for creating foaming slag in the
furnace, which favors increasing the arc operation at higher voltage and
decreasing the electricity consumption. For example, the electricity consumption
can be decreased 16%-20% by adding DRI directly into the furnace at 600°C.
(2) The slag with low density can make DRI penetrate through the slaghetal
interface rapidly, in addition the foaming slag improves heat transfer, stabilizes
the arc, shortens the heating time, and raises the productivity of the electric arc
furnace.
(3) The more carbon monoxide that is produced, the lower the electrode consumption
due to electrode oxidation.
(4) Since arc radiation is absorbed by the foaming slag during the smelting process,
refractory consumption is reduced and more power input is possible into EAF.
( 5 ) Lower electricity consumption can be achieved by the continuous charge, less
power-off time, and heat loss. Hence, the productivity and automatic control
levels are improved.
(6) The noise is lower during the smelting operation.
Hot charging of DRI is an effective means of reducing the cost per metric ton of
liquid steel because of reductions in power and electrode consumption, and hot
charging will increase EAF productivity for an operation that is sized to charge cold
DRI [8]. Although DRI can dilute the impurity content of molten steel and improve
the process and its operation, power consumption will increase since the DRI contains
some iron oxide and silicon dioxide [2, 91. However additional lime increases the
amount of slag, which raises refractory consumption. The high FeO in the slag will
cause easier slag foaming, thus reducing the total recovery ratio of metals [3].
In the past decade, China’s steel industry has paid attention to the use of DRI for
steelmaking. In 1994, the first rotary kiln making DRI was put into production in
Kezuo County of Liaoning Province in China. Two years later, the DRI
manufacturing plant of the Tianjin Pipe Corporation went into operation. In 1997, the
coal-based cold bonded pellet rotary kiln of the Luzhong metallurgical mining
company was built. A one-step method with a rotary hearth is producing DRI at
Miyun Beijing [lo]. At present, there are more than 3 0 plants making DRI in China
and the annual production capacity is approximately 600,000 tons/year, which still
cannot satisfy the demand of the domestic market [l 11.
Hot Metal
Hot metal has two sources, namely blast furnace iron and Corex iron. The charge of
hot metal is one of the important new techniques in modern EAF steelmaking. The
low content of metal residues allows hot metal to become the charge for high quality
steelmaking in EAF. The usage of hot metal not only solves the problem of scrap
shortage, but also shortens the heating time required, decreases power consumption,
dilutes the content of residual and tramp elements in steel, and provides a large
amount of physical heat energy [12-141.
355
Cheng-Wu Du, Miao-Yong Zhu, Li-Na Sun and Shi-Ze Dong
FesC-Iron Carbide
CPI-Cold Pig Iron
'C
8 300
10
20
30
40
50
60
%HM,CPI,DRl,lncharge
Figure 1. The influence of substitution of scrap in EAF charge on the energy
consumption [ I S ] .
Figure 1 shows the influence of the various scrap substitutes on the energy
consumption per ton of steel [15]. It can be seen that the energy consumption
decreases as the charge of hot metal is increased, while the energy rises slightly as
more DRI is used. The main reason is that hot metal contains sensible heat and a high
carbon content. A large amount of heat energy released by oxidation of carbon in hot
metal due to oxygen blowing thus lowers the consumption of electrical energy per ton
of steel. The following factors should be taken into account with hot metal charging in
EAF.
(1) The temperature should be above 1200°C so that the feedstock will melt quickly.
(2) There are moderate Si and Mn contents as higher values will increase the
chemical energy in hot metal, and can result in accelerated heating-up by
oxidating Si and Mn exothermic reactions. However, the amount of slag will
increase and there will be a lower FeO content in the slag, which undesirable for
dephosphorization.
(3) Lower phosphorus (5 0.17%) and sulfur (50.060%) in hot metal, because higher
levels add an extra burden in the smelting process.
(4) An appropriate quantity of hot metal should be added. Large addition will result
in higher contents of carbon and phosphorus after fully smelting, which will
cause expansion over time, more consumption of slagging material, higher iron
loss, and increased power consumption. Smaller addition will result in lower
carbon after filly smelting, which cannot ensure the requirement of gas and
inclusion removal. The best addition of hot metal is 20%-30%.
( 5 ) The addition time of the hot metal should be scheduled to coincide to melting of
the feedstock of about 20%-30%, or in the interval charge time which can
prevent hot metal from splashing. If the time of adding hot metal is too late, its
thermal energy cannot be fully used and it will not utilize the benefit of the
shorter melting stage.
356
Metallurgical Characteristics and Efectiveness of Metallic Charges in EA Furnace
Since October 2002, Xingcheng Special Iron and Steel Co. Ltd. in China began to
adopt the 40%-50% hot metal charging process on the 100 ton DC EBT arc furnace.
The power-on time was decreased from 42 to 36 minutes, the tap-to-tap decreased
from 49 to 44 minutes, and the total electric power consumption was 177 kWh/t.
However, the oxygen consumption increased to 56 m3/t, and the total annual steel
output was 1.1 million tons [161.
Cold Pig Iron
Cold pig iron is also a typical charge to EAF steelmaking. The aim when feeding cold
pig iron is to increase the carbon and other oxidizable elements in the metallic
feedstock, and decrease iron loss. Boiling and slagging caused by adding cold pig iron
into the feedstock ensure trouble-free operation in the smelting process. Oxidizable
elements added by the pig iron increase oxygen consumption, of which Si and Mn
increase the dephosphorizing burden since their oxidized production raising the
amount of slag. The Brazilian MJS Company achieved a good result by adding 35.5%
cold pig iron into the feedstock in a 84 ton UHP,as seen in Table 3 [17].
Table 3 shows that, with the addition of 35.5% pig iron into the metallic charge,
the MJS Company had lowered consumption of power, electrodes and refiactory and
shortened the duration of heating and increased productivity. However, this increases
the consumption of oxygen in the blowing tube and the lime. An increment of 1 m3/t
oxygen consumption is equal to saving 3.6 kWWt electric energy. After adding cold
pig iron and aggravated boiling for ox gen blowing decarbonization, the molten steel
nitrogen content reduced from 90x 10-6? with full scrap to 50x
with 35.5% cold pig
iron plus 64.5% scrap.
Because of very low contents of residual and tramp elements in cold pig iron, the
target level of the diluted residual element can be acquired by adding proper cold pig
iron into the feedstock. There are higher P and S contents in cold pig iron, the P
content in cold pig iron is 0.05-0.08% higher than in scrap. Desulphurization and
dephosphorization will increase energy consumption and also lower productivity. In
addition, a high proportion of pig iron can increase capacity for the diluting scrap,
reduce tramp elements, and lower N2 and HZgases in crude liquid steel.
Table 3. Comparison of a 84 ton Electric Arc Furnace steelmaking index by diferent
charge at MJS Co. f1 71.
357
Cheng- Wu Du, Miao-Yong Zhu, Li-Na Sun and Shi-Ze Dong
Decarburized Granular Iron
Decarburized granular iron is a material for EAF steelmaking, which was obtained by
making granular iron of different graininess by high-pressure water quenching during
tapping o f the blast furnace, it is then charged into a rotary kiln. After heating to a set
temperature, the iron is decarburized by an inflow of a gaseous mixture into the rotary
kiln. Decarburized granular iron is then obtained [18, 191. Table 4 gives the typical
chemical composition of the decarburized granular iron [ 181. The advantages of
utilizing decarburized granular iron include the following:
After smelting in a blast furnace, the gangue content in decarburized granular
iron is 1%-3% lower than in DRI, which can lower the power consumption by
10% in an electric steelmaking process.
The P content in granular iron can be decreased by 10% during hot metal-water
quench, and S content can be reduced by 50% in a blast hrnace and rotary kiln.
Hence, both P and S contents are lower in decarburized granular iron, and other
impurity elements in decarburized granular iron are less than that in DRI.
The Fe content of granular iron is 5%-10% higher than that of DRI, and the
degree of reduction is 3%-5% higher than that of DRI. A small amount of FeO in
the granular iron surface forms foaming slag in the electric steelmaking process.
The C content in granular iron is controlled within 0.2%-3.0%, and industrial
pure iron or other materials for special steels (with carbon less than 0.02%) can
be obtained by using small grain size granular iron (< 0.2mm).
The granular iron made by high temperature oxidization has better anti-oxidation
propeties, which is convenient for hot charging to EAF and lowers power
consumption in EAF. From the discharge calculation 8OO"C, and feeding at
500°C after thermal retardation, EAF power consumption using entirely
decarburized granular iron can be lowered 150 kWh per ton steel.
Bulk density of granular iron is higher than that of DRI, and oxidation resistance
is also stronger than that of DRI, which is advantageous for transportation. There
are simple components and a regular outline in granular iron that can reduce
charging times, and thus lower power consumption and increased productivity.
Simple equipment, less investment and simple operation make production
capacity flexible, and are also suitable for forming a complete production
network with medium and small blast furnaces.
C
Si
S
P
1.5
0.8
<0.04
<0.05
As
< 0.002
Sn
< 0.001
Pb
< 0.001
Sb
< 0.001
Si02
<1
Bi
< 0.001
FeO
<3
Zn
< 0.001
cu
< 0.01
Table 5. Chemical composition (99) of iron carbide [21],
Composition Fe3C Fe3C, SiO2+AI2O3 MFe
2-4
0.5-1
ContentRange 88-94 2-7
2
1
2
Typical Content 92
358
Fe
C
0
89-94
6-6.4
0.4-1.4
90.8
6.2
1.o
Metallurgical Characteristics and Efectiveness of Metallic Charges in EA Furnace
Iron Carbide
In the early 1970s, Sephens [20] conceived the basis of using the iron carbide to
supplement scrap metal for the existing process, and there have been many
subsequent developments. Iron carbide product has now been developed to the
commercial stage by small-scale industry tests. Iron carbide is a chemical compound
consisting of three iron atoms with one carbon atom and has the chemical formula
Fe3C. Iron carbide is a relatively hard and brittle material which resembles black sand,
and has a melting point of 1800OC. Iron carbide is the product fiom preparation of
concentrated powder of iron that has been reduced by natural gas. Other constituents
are gangue and iron oxide. Typical components are shown in Table 5 [21]. The main
advantages of using iron carbide [22-271 are as follows.
(1) Many characteristics, e.g. higher chemical stability, no spontaneous combustion
and insensitive to reoxidation, are beneficial to storage and transportation.
(2) As a metallic burden, the low residues such as sulfur and phosphorus, result in
decreasing concentrations of tramp elements in steel.
(3) Less energy is consumed than that of scrap and DRI in EAF due to the higher
carbon content and more is heat generated.
(4) Iron carbide can be produced directly by the fine powder without blocking, this
ensures quick melting by injection.
(5) It can be used as carburant in the steelmaking process.
( 6 ) Less investment in facilities, lower cost, less environmental pollution.
Since the carbon content of iron carbide reaches 6%, a foaming slag can be
formed rapidly in EAF, even if there is no injection of carbon powder into the bath.
This can prevent impurities in steel from increasing as a result of injecting powdered
carbon. The effect of various injection rates on the generation rates of CO is shown in
Figure 2 [28]. Foaming slag can result in raising thermal efficiency, increasing arc
stability, lowering noise in the smelting process, prolonging the period of EAF
refractory usage, enlarging slag-steel contact area, and quicker refining. In addition,
carbon-oxygen reaction can not only offer thermal power but also form a large
amount of bubbles that improve smelting by good stirring. This can produce uniform
temperatures and components in the bath, raise inclusions and remove gas [29]. Iron
carbide is an ideal material for steelmaking, particularly for producing high-quality
steel and clean steel because it is low in residuals and contains a high chemical energy
capacity. However, iron carbide has not been used as the metal charge to EAF in
China, due to its variable composition, high costs and fewer applications.
Complex Metal Charge
A new high-quality complex metal charge used for EAF has been developed, which is
used to take the place of pig iron steelmaking, spherical agglomeration, certified
burden pallet and scrap. Complex metal charge was produced by putting 15%-25%
stuffing (sintering ore or spherical agglomeration) in molten pig iron and cooling
them to the solid state.
359
Cheng- Wu Du,Miao-Yong Zhu, Li-Na Sun and Shi-Ze Dong
800
700
Equivalent
800
C inj kg/min
500
400
300
200
100
0
0
1000
2000
3000
4000
5000
6000
Iron Carbide Injection Rate kglrnin
Figure 2. Generation of CO vs Fe3C injection rate [28].
There are three main commercial trademarks for the complex metal charge [30]:
0
CK15: includes 15% stuffing, and iron content at least 88% in the complex metal
charge.
0
CK20: includes 20% stuffing, and iron content at least 86% in complex metal
charge.
0
CK25: includes 15% stuffing, and iron content at least 85% in complex metal
charge.
Types CKl5 and CK20 are usually used in the process. For production of 1 ton of
complex metal charge of CK15, then 867.3 kg hot metal and 157.5 kg stuffing were
consumed; for 1 ton of CK20, then 816.3 kg hot metal and 210 kg stuffing were
consumed. Table 6 shows the chemical compositions of pig iron, stuffing, complex
metal charges C K l 5 and CK20.
Complex metal charge has the following advantages when used in the process:
(1) beneficial when smelting high-quality steel;
(2) lower cost than scrap metal;
(3) generates foaming slag at lower temperatures and decreased refractory
consumption;
(4) accelerated residual charge melting, and shortened duration of heating;
( 5 ) higher bulk density than scrap metal.
It has been shown in many applications that each composition of complex metal
charge melts at the same time. The foaming slag formed in smelting can lower the P
and S contents in the metal, shorten the smelting period, and reduce power
consumption. Complex metal charge is a universal metal feedstock that is suitable for
producing high-quality steel, and it is an ideal substitute for scrap metal in
steelmaking. In China, several institutions are performing research related to the
industrial application of this technology.
360
Metallurgical Characteristics and Effectiveness of Metallic Charges in EA Furnace
Table 6. Chemical compositions (%) of pig iron, stufing, complex metal charge
CK15 and CK20 [28].
Mn
Stufing
cK15
CK20
1
I
1
0.1
53.43361.056
88.42589.568
86.36787.891
0
80.41
12.99
12.8817.32
cu
0.01
75.68
P
0.06
I
1
0.1 12-2.
148
SiOz
0
7.649.08
1
0.595
0.085
3.60
0.560
0.08
CaO
0
12.73-1.
27
Impurity
0.02
4.440.64
I
0.079
CK20
10.026
Conclusions
With new processes such as thin slab and near-net shape casting becoming more
widespread, the availability of low residual scrap will continue to decline. Without an
alternative source of clean iron units, it will become increasingly more difficult to
make use of the more abundant and low cost grades of scrap without exceeding
restrictions on residual contents in steel products. Direct reduced iron, hot metal, cold
pig iron, decarburized granular iron, iron carbide and complex metal charge replacing
scrap as electric arc furnace feedstock make it possible to produce steel which
complies with the most stringent quality requirements. Meanwhile these charges offer
additional benefits such as compensating for lack of scrap and enhancing
predictability and uniformity, and hence result in a better quality metallic charge at
lower overall cost. These substitute materials for scrap have become major
contributors to furnace product development by providing reliable and high-quality
iron units to the EAF steelmaking process.
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