U.S. patent application number 13/807442 was filed with the patent office on 2013-04-25 for process for producing molten steel using granular metallic iron.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Shuzo Ito, Itsuo Miyahara, Osamu Tsuge. Invention is credited to Shuzo Ito, Itsuo Miyahara, Osamu Tsuge.
Application Number | 20130098202 13/807442 |
Document ID | / |
Family ID | 45402046 |
Filed Date | 2013-04-25 |
United States Patent
Application |
20130098202 |
Kind Code |
A1 |
Tsuge; Osamu ; et
al. |
April 25, 2013 |
PROCESS FOR PRODUCING MOLTEN STEEL USING GRANULAR METALLIC IRON
Abstract
A process for producing a molten steel (G) is disclosed in which
particulate metallic iron can be more efficiently melted. The
process includes the step of melting, in an electric arc furnace
(2), all charge for iron which comprises: particulate metallic iron
(A) produced by a method including a step in which a feed material
comprising a carbonaceous reducing material and an iron
oxide-containing substance is heated in a rotary hearth furnace (1)
as a reducing/melting furnace and the iron oxide contained in the
feed material is thereby reduced in the solid state to yield
metallic iron and a step in which the resultant metallic iron is
heated to a higher temperature to melt the metallic iron and the
molten iron is aggregated while separating the iron from the slag
(B); and scraps (D) which are another feed material for iron. The
process is characterized in that the content of carbon in the
particulate metallic iron (A) is regulated to 1.0-4.5 mass % and
the carbon in the particulate metallic iron (A) is burned by oxygen
blowing. The process is further characterized in that the
particulate metallic iron (A) is used in an amount of 40-80 mass %
with respect to all charge for iron and that the scraps (D) are
initially charged into the electric arc furnace (2) to obtain
molten iron (F) and then the particulate metallic iron (A) is
continuously charged into the molten iron (F).
Inventors: |
Tsuge; Osamu; (Kobe-shi,
JP) ; Miyahara; Itsuo; (Kobe-shi, JP) ; Ito;
Shuzo; (Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tsuge; Osamu
Miyahara; Itsuo
Ito; Shuzo |
Kobe-shi
Kobe-shi
Kobe-shi |
|
JP
JP
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
45402046 |
Appl. No.: |
13/807442 |
Filed: |
June 27, 2011 |
PCT Filed: |
June 27, 2011 |
PCT NO: |
PCT/JP2011/064717 |
371 Date: |
December 28, 2012 |
Current U.S.
Class: |
75/10.63 |
Current CPC
Class: |
Y02P 10/234 20151101;
Y02P 10/20 20151101; C21C 5/52 20130101; C22B 5/10 20130101; C21C
5/5252 20130101; Y02P 10/214 20151101; C22B 1/245 20130101 |
Class at
Publication: |
75/10.63 |
International
Class: |
C21C 5/52 20060101
C21C005/52 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 2010 |
JP |
2010-146114 |
Claims
1. A process of producing molten steel from a granular metallic
iron, the process comprising: charging an iron raw material into an
electric arc furnace, thereby producing a molten iron, subsequently
continuously charging the granular metallic iron into the molten
iron, and blowing oxygen into the granular metallic iron, thereby
burning carbon in the granular metallic iron, wherein the granular
metallic iron is obtained by a process comprising producing a
metallic iron by heating a raw material of the metallic iron
comprising a solid carbonaceous reductant and an iron
oxide-comprising material in a reducing/melting furnace thereby
reducing iron oxide in the raw material of the metallic iron with
the reductant, heating the metallic iron thereby melting the
metallic iron, and separating the metallic iron from a slag
component while agglomerating the metallic iron, a carbon content
in the granular metallic iron is from 1.0 to 4.5 mass percent, and
a ratio of the granular metallic iron to a total of all iron raw
material including the granular metallic iron is from 40 to 80 mass
percent.
2. The process according to claim 1, wherein a charging speed of
the granular metallic iron per input power of 1 MW is from 40 to
100 kg/min/MW.
3. The process according to claim 1, wherein a position on a
surface of the molten iron, at which the granular metallic iron is
charged, is within an electrode pitch circle.
4. The process according to claim 2, wherein a position on a
surface of the molten iron, at which the granular metallic iron is
charged, is within an electrode pitch circle.
5. The process according to claim 1, wherein an average granular
size of the granular metallic iron is from 1 to 50 mm.
6. The process according to claim 2, wherein an average granular
size of the granular metallic iron is from 1 to 50 mm.
7. The process according to claim 3, wherein an average granular
size of the granular metallic iron is from 1 to 50 mm.
8. The process according to claim 4, wherein an average granular
size of the granular metallic iron is from 1 to 50 mm.
9. The process according to claim 1, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode, during the continuously charging the
granular metallic iron into the molten iron.
10. The process according to claim 2, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode during the continuously charging the
granular metallic iron into the molten iron.
11. The process according to claim 3, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode during the continuously charging the
granular metallic iron into the molten iron.
12. The process according to claim 4, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode during the continuously charging the
granular metallic iron into the molten iron.
13. The process according to claim 5, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode during the continuously charging the
granular metallic iron into the molten iron.
14. The process according to claim 6, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode, during the continuously charging the
granular metallic iron into the molten iron.
15. The process according to claim 7, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode during the continuously charging the
granular metallic iron into the molten iron.
16. The process according to claim 8, further comprising: foaming a
molten slag layer on the molten iron, thereby constantly covering a
lower end of an electrode, during the continuously charging the
granular metallic iron into the molten iron.
17. The process of claim 1, wherein charging the granular metallic
iron into the molten iron comprises charging the granular metallic
iron into the molten iron while a temperature of the granular
metallic iron is from 400 to 700.degree. C., without cooling the
granular metallic iron to room temperature after heating in the
reducing/melting furnace.
18. The process of claim 1, further comprising, prior to charging
the granular metallic iron into the molten iron: producing the
metallic iron by heating the raw material of the metallic iron in a
reducing/melting furnace, thereby reducing iron oxide in the raw
material of the metallic iron with the reductant, heating the
metallic iron, thereby melting the metallic iron, and separating
the metallic iron from a slag component while agglomerating the
metallic iron, thereby obtaining the granular metallic iron.
19. The process of claim 1, wherein the carbon content in the
granular metallic iron is from 1.5 to 3.5 mass percent.
20. The process of claim 5, wherein the average granular size of
the granular metallic iron is from 2 to 25 mm.
Description
DESCRIPTION
[0001] 1. Technical Field
[0002] The present invention relates to a process for producing
molten steel by melting granular metallic iron, which has been
produced in a reducing/melting furnace such as a rotary hearth
furnace, in an electric arc furnace.
[0003] 2. Background Art
[0004] Conventionally, iron raw materials such as scrap, pig iron
(cold pig iron), and reduced iron are batch charged into an
electric arc furnace for steel production using a scrap bucket from
the top of the furnace. After the charged materials have been
melted, a furnace roof is opened and an additional batch of the
iron raw materials is charged so as to be melted. This causes
problems in that, while the furnace roof is opened and the iron raw
material is charged, heat and time are lost and the work
environment is degraded due to many dust particles flying out of
the furnace.
[0005] In order to address the above-described problems, reduced
iron having a comparatively uniform composition and size is
continuously charged (for example, see Patent Literatures 1 to 3).
However, since reduced iron includes gangue or unreduced iron
oxide, there is a problem in that reduced iron requires more
melting energy than scrap or pig iron (cold pig iron) does.
[0006] Regarding pig iron (cold pig iron), the size of pig iron
cannot be reduced due to restrictions in production, and
accordingly, a method in which a large amount of pig iron is
charged and melted using continuous charging has not been
realized.
[0007] Furthermore, in order to improve productivity of the
electric arc furnace for steel production, oxygen addition is
widely performed. Thus, the amount of oxygen used increases and the
amount of carbon source used, which corresponds to the amount of
input oxygen, also increases.
[0008] The carbon source uses a carbon component of pig iron or
cold pig iron, massive coke, coke breeze, or the like.
[0009] However, in order to use pig iron, an own facility for
producing molten pig iron and an own facility used to charge molten
pig iron into the electric arc furnace are required on the upstream
side of the electric arc furnace for steel production.
[0010] In order to use cold pig iron, due to the large size of cold
pig iron as described above, a charging method is limited to batch
charging and longer melting time is required. Thus, there is a
problem in that the amount of cold pig iron to be used is
restricted.
[0011] In order to use massive coke or coke breeze, there are
problems including problems in that the amount of massive coke or
coke breeze to be used may be restricted in accordance with the
total sulfur content or composition of the ash content, massive
coke or coke breeze is caught by slag when massive coke or coke
breeze is charged into the electric arc furnace, and the yield of
added massive coke or coke breeze decreases due to, for example,
exhaustion of massive coke or coke breeze together with exhaust
gas.
[0012] A process for producing high-purity granular metallic iron
has been developed. According to the process, high-purity granular
metallic iron is produced by heating a raw material containing a
carbonaceous reductant and an iron oxide containing material in a
reducing/melting furnace such as a rotary hearth furnace, reducing
iron oxide contained in the raw material using a solid reductant,
and then further heating the produced metallic iron so as to melt
the produced metallic iron while separating the produced metallic
iron from a slag component and agglomerating the produced metallic
iron (for example, see Patent Literatures 4 and 5).
[0013] Compared to reduced iron, the slag component has been
removed from this granular metallic iron in advance and the carbon
content of the granular metallic iron can be increased. Thus, by
continuously charging the granular metallic iron instead of reduced
iron into the electric arc furnace, and in addition, by performing
oxygen blowing, melting energy in the electric arc furnace should
be significantly reduced and molten steel productivity should be
significantly improved.
[0014] However, a technology in which such granular metallic iron
is continuously charged into an electric arc furnace for steel
production so as to be more efficiently melted has not been
established.
CITATION LIST
Patent Literature
[0015] PTL 1: Japanese Unexamined Patent Application Publication
No. 50-64111
[0016] PTL 2: Japanese Unexamined Patent Application Publication
No. 51-65007
[0017] PTL 3: Japanese Unexamined Patent Application Publication
No. 58-141314
[0018] PTL 4: Japanese Unexamined Patent Application Publication
No. 2002-339009
[0019] PTL 5: Japanese Unexamined Patent Application Publication
No. 2003-73722
DISCLOSURE OF INVENTION
Technical Problem
[0020] Accordingly, an object of the present invention is to
provide a process for producing molten steel, the process being a
process in which granular metallic iron can be more efficiently
melted when molten steel is produced by continuously charging the
granular metallic iron having been produced in a reducing/melting
furnace such as a rotary hearth furnace into an electric arc
furnace for steel production so as to melt the granular metallic
iron.
Solution to Problem
[0021] The present invention provides a process for producing
molten steel using granular metallic iron as follows.
[0022] (1) The process includes the step of melting all charged
iron raw materials using an electric arc furnace. All the charged
iron raw materials include the granular metallic iron and another
iron raw material. The granular metallic iron is produced in the
steps of producing metallic iron by heating a raw material of the
metallic iron that includes a carbonaceous reductant and an iron
oxide containing material in a reducing/melting furnace so as to
reduce iron oxide contained in the raw material of the metallic
iron using the solid reductant, and further heating the produced
metallic iron so as to melt the produced metallic iron and
separating the produced metallic iron from a slag component while
agglomerating the produced metallic iron.
[0023] In the process for producing molten steel, a carbon content
in the granular metallic iron is set to 1.0 to 4.5 mass percent and
oxygen is blown so as to burn carbon included in the granular
metallic iron, and
[0024] a usage ratio of the granular metallic iron to all the iron
raw materials to be charged is set to 40 to 80 mass percent and the
other iron raw material is initially charged into the electric arc
furnace so as to produce molten iron, and then the granular
metallic iron is continuously charged into the molten iron.
[0025] (2) The process according to (1), in which a charging speed
of the granular metallic iron per input power of 1 MW is set to 40
to 100 kg/min/MW.
[0026] (3) The process according to (1) or (2), wherein a position
on a surface of the molten iron, the position being a position at
which the granular metallic iron is charged, is set within an
electrode pitch circle.
[0027] (4) The process according to any one of (1) to (3), in which
an average granular size of the granular metallic iron is 1 to 50
mm.
[0028] (5) The process according to any one of (1) to (4), in which
a molten slag layer formed on the molten iron is caused to foam so
as to constantly cover a lower end of an electrode or lower ends of
the electrodes while the granular metallic iron is continuously
charged into the molten iron.
[0029] (6) The process according to any one of (1) to (5), in which
the granular metallic iron having been produced using the
reducing/melting furnace is continuously charged into the molten
iron in the electric arc furnace while the temperature of the
granular metallic iron is maintained at 400 to 700.degree. C.
without being cooled to room temperature.
Advantageous Effects of Invention
[0030] According to the present invention, granular metallic iron
that is produced in a reducing/melting furnace and contains 1.0 to
4.5 mass percent carbon undergoes oxygen blowing so as to burn the
carbon contained in the granular metallic iron. In addition, 40 to
80 mass percent granular metallic iron with respect to all the iron
raw materials to be charged is used and the granular metallic iron
is continuously charged into molten iron produced by initially
charging another iron raw material into an electric arc furnace.
Thus, melting energy can be significantly decreased so as to
increase the energy efficiency of the electric arc furnace and
molten steel productivity can be significantly improved.
BRIEF DESCRIPTION OF DRAWINGS
[0031] FIG. 1 illustrates a general structure of a molten steel
producing facility according to one embodiment of the present
invention showing a flow.
[0032] FIG. 2 is a graph illustrating the relationships between
melting energy and a usage ratio of the granular metallic iron to
all iron raw materials to be charged in an electric arc
furnace.
[0033] FIG. 3 is a graph illustrating the relationships between a
molten steel production rate and the usage ratio of the granular
metallic iron to all the iron raw materials to be charged in an
electric arc furnace.
[0034] FIG. 4 illustrates a general structure of melting text
equipment, part of which is illustrated in a longitudinal sectional
view.
[0035] FIG. 5 is a graph illustrating the relationships between
input power and charging speeds of the granular metallic iron and
reduced iron in the melting test equipment.
DESCRIPTION OF EMBODIMENTS
[0036] An embodiment of the present invention will be described in
detail below with reference to the drawings.
First Embodiment
[0037] FIG. 1 illustrates a general structure of a molten steel
producing facility according to one embodiment of the present
invention. The facility according to the present embodiment is an
example of a case in which a rotary hearth furnace 1 as a
reducing/melting furnace and an electric arc furnace 2 are located
close to each other.
[0038] A granular metallic iron A used in the present invention is
produced, for example, as follows.
[0039] Initially, a carbonaceous reductant such as coal and a raw
material including an iron oxide containing material such as iron
ore are agglomerated into pellets or briquettes. The agglomerated
material is placed on a hearth (not shown), where a carbonaceous
material C is spread, and heated up to, for example, about 1350 to
1400.degree. C. so as to reduce the iron oxide contained in the raw
material in the rotary hearth furnace 1 using a solid reductant.
Then, metallic iron produced is further heated up to about 1400 to
1550.degree. C. to be melted, thereby being separated from a slag
component and agglomerated. After that, the metallic iron produced
is cooled in a chiller of the furnace to a temperature of about
1000 to 1100.degree. C. so as to obtain a mixture of the solidified
granular metallic iron A and a slag B. The mixture together with
bedding carbonaceous material C is discharged from the rotary
hearth furnace 1. After that, the slag B and the bedding
carbonaceous material C are separated and removed using a screen 3
and a magnetic separator 4, thereby obtaining the granular metallic
iron A (see, for example, the aforementioned Patent Literatures 1
to 3, the contents thereof are incorporated herein by
reference).
[0040] A carbon content of the granular metallic iron A is set to
1.0 to 4.5 mass percent. The reason why the lower limit of the
carbon content is set to 1.0 mass percent is because a required
amount of C in accordance with the type of steel to be produced is
ensured so as to improve versatility of the granular metallic iron
A as an iron material. The reason why the upper limit of the carbon
content is set to 4.5 mass percent is because the granular metallic
iron A can be used without load applied through additional
processes such as deoxidation. The carbon content of the granular
metallic iron A is preferably set to a range from 1.5 to 3.5 mass
percent. The carbon content of the granular metallic iron A can be
easily adjusted by changing the amount of the carbonaceous
reductant mixed in the agglomerated material or the atmosphere of
the rotary hearth furnace 1.
[0041] Here, when the granular metallic iron A is melted in the
rotary hearth furnace 1, carbon in the granular metallic iron A
tends to gather near the surface of the granular metallic iron A,
and accordingly, the concentration of carbon is higher as a
position in the solidified granular metallic iron A becomes closer
to the surface. Thus, melting of the granular metallic iron A
charged into the molten iron F in the electric arc furnace 2 easily
begins from part of the granular metallic iron A near the surface,
where the melting point is low due to the high concentration of
carbon. When carbon in the melted molten iron containing highly
concentrated carbon also undergoes oxygen blowing, that is, by
blowing oxygen into the electric arc furnace 2, the carbon is burnt
with the oxygen, and heat caused by burning the carbon causes an
inner part of the granular metallic iron A having a high melting
point due to the low concentration of carbon to be easily
melted.
[0042] The granular metallic iron A together with scrap D as
another iron raw material serves all the iron raw materials to be
charged, and the usage ratio of the granular metallic iron A with
respect to all the iron raw materials to be charged is set to 40 to
80 mass percent.
[0043] First, the scrap D is initially charged (batch charged) into
the electric arc furnace 2, and arc heated by electrodes 7, thereby
producing the molten iron F.
[0044] After that, the arc heating is continued with oxygen blowing
performed (and powder coal is also injected according to need)
while the granular metallic iron A is continuously charged into the
molten iron F and melted. This can improve molten steel G
productivity while the energy efficiency of the electric arc
furnace 2 is improved. Thus, the molten steel G can be more
efficiently produced.
[0045] Here, the usage ratio of the granular metallic iron A with
respect to all the iron raw materials to be charged is set to 40 to
80 mass percent for the following reason.
[0046] That is, with respect to an electric arc furnace used in
actual operation (capacity: 90-ton, capacity of transformer: 74
MVA) as an example, how the usage ratio of granular metallic iron
and the difference in charging method affect melting energy
required to melt all the charged iron raw materials and the rate at
which molten steel is produced are calculated.
[0047] Here, the carbon content of the granular metallic iron is
assumed to be 2.5 mass percent. The temperature of the granular
metallic iron when the granular metallic iron is charged through
"batch charging" or "continuous charging" is assumed to be room
temperature (25.degree. C.), and the temperature of the granular
metallic iron when the granular metallic iron is charged through
"continuous hot charging" is assumed to be 400.degree. C. It is
also assumed that, due to a change from "batch charging" to
"continuous charging" of the granular metallic iron, heat loss in
furnace is decreased by 870 Mcal (here and hereafter, 1
Mcal=4.18605 MJ) per additional charge, power-off time is decreased
by two minutes, and time taken for a boring period is decreased by
two minutes.
[0048] The results of the calculations are illustrated in FIGS. 2
and 3. FIG. 2 illustrates changes in melting energy required for
melting all the charged iron raw materials in accordance with the
usage ratio of the granular metallic iron and the difference in
charging method. FIG. 3 illustrates changes in molten steel
production rate in accordance with the usage ratio of the granular
metallic iron and the difference in charging method.
[0049] In the case where the usage ratio of the granular metallic
iron A is less than 40 mass percent, that is, in the case where the
usage ratio of the scrap D as the other iron raw material exceed 60
mass percent, due to the limitation of capacity of a scrap bucket
(not shown) used for batch charging, the initial charging of the
scrap D needs to be performed twice in a divided amount. Thus, as
illustrated in FIG. 3, even when the granular metallic iron A is
continuously charged, the molten steel production rate is
significantly decreased.
[0050] In the case where the usage ratio of the granular metallic
iron A exceeds 80 mass percent, when the granular metallic iron A
is charged through "continuous hot charging", decarbonization time
becomes longer than melting time, which is determined in accordance
with input power capacity of the electric arc furnace 2. In this
case, the decarbonization time becomes a rate-determining factor
for molten steel productivity. Thus, as illustrated in FIG. 3, the
rate of increase in the molten steel production rate drops.
[0051] In accordance with the above-described results, the usage
ratio of the granular metallic iron A with respect to all the iron
raw material to be charged is set to 40 to 80 mass percent.
[0052] The charging speed of the granular metallic iron A per input
power of 1 MW is preferably set to 40 to 100 kg/min/MW because of
the following reason.
[0053] That is, in order to grasp melting characteristics of the
granular metallic iron in continuous charging, a melting test was
performed on the following melted iron raw materials: a granular
metallic iron having physical and chemical properties as shown in
Table 1 below; a reduced iron as a comparative example.
TABLE-US-00001 TABLE 1 Type of iron Apparent Particle raw material
density diameter Composition (mass %) for melting (g/cm.sup.3) (mm)
T. Fe M. Fe FeO C Particulate 5.5~7.0 2.4~2.5 -- >95.0 --
1.5~3.0 metallic iron Reduced iron 2.8~3.0 2.4~18.0 92.5 85.6 6.3
0.4~0.8
[0054] As generally schematically illustrated in FIG. 4, melting
test equipment includes a 500-kg high-frequency induction furnace
(ratings: 350 kw, 1000 Hz), a raw material feeding device (hopper
capacity: 200 kg, raw material charging speed: 0 to 15 kg/min), a
monitoring camera used for observing a melting behavior, and a data
recorder that records the temperature of molten metal and the raw
material charging speed.
[0055] As for melting conditions, a 250 kg of an initial molten
metal containing 0.2 to 0.3 mass percent carbon, less than 0.03
mass percent silicon, and 0.05 mass percent of manganese was
produced, and the temperature of the initial molten metal was at
1550.degree. C. The raw material charging speed was sequentially
changed while the temperature of the molten metal was maintained at
1550 to 1600.degree. C. The input power is adjusted while a state
in which a continuously charged iron raw material is satisfactorily
melted is checked with the monitoring camera.
[0056] The results of the melting test are listed in Table 2 below
and illustrated in FIG. 5.
TABLE-US-00002 TABLE 2 Max. melting Corrected max. Charging speed
of Input rate melting rate iron raw material power [R] [R']
=[R]/[C] .times. 100 Melting mode (kg/min) (kW) (kg/min/MW)
(kg/min/MW) Maintain 0.0 78 -- -- temperature of initial 0.0 79 --
-- molten metal 0.0 80 -- -- Continuous melting 0.0 80 -- -- of
particulate 4.0 205 32.0 62.4 metallic iron 4.0 225 27.6 53.8 7.0
318 29.4 46.9 7.0 330 28.0 44.7 Continuous melting 1.5 220 10.7 --
of reduced iron 1.8 250 10.6 -- 2.0 250 11.8 --
[0057] As illustrated in these drawing and table, the granular
metallic iron exhibits a maximum melting rate per input power of 1
MW, which is 2.5 to 3.0 times higher than that of the reduced
iron.
[0058] The fact that the granular metallic iron exhibits a maximum
melting rate 2.5 to 3.0 times higher than that of the reduced iron
as observed cannot be explained only by the fact that the amount of
a slag component included in the reduced iron is larger than that
of the granular metallic iron. The cause of higher rate is thought
to be use of high-frequency induction heating as a heat source in
the melting test instead of arc heating.
[0059] That is, granular metallic iron, the apparent density of
which is substantially equal to that of molten iron, is melted in a
state in which the granular metallic iron is floated on the molten
iron. Since the molten iron is sufficiently heated by
high-frequency induction heating, the melting rate of the granular
metallic iron is sufficiently higher. In contrast, the reduced
iron, the apparent density of which is substantially equal to
molten slag, is melted in a state in which the reduced iron is
floated on the molten slag. Unlike arc heating, high-frequency
induction heating cannot sufficiently heat the molten slag. It is
thought that this causes the melting rate of the reduced iron to be
significantly decreased compared to that of the granular metallic
iron.
[0060] Here, the 500-kg high-frequency induction furnace included
in the melting test equipment is small, and accordingly, there is a
significantly large heat loss compared to a 90-ton electric arc
furnace used in actual operation. Thus, the maximum melting rate
per input power of 1 MW of the granular metallic iron obtained in
the melting test is thought to be further increased in the case
where the electric arc furnace used in actual operation is used.
Thus, the melting rate of the granular metallic iron per input
power of 1 MW is estimated for the case where the granular metallic
iron is continuously charged into the 90-ton electric arc furnace
used in actual operation. The estimation is as follows.
[0061] As shown in Table 3, power per unit of production of the
melting test equipment for melting the granular metallic iron is
calculated as follows: 714 kWh/t at charging speed of 4 kg/min, and
584 kWh/t at charging speed of 7 kg/min. There are track records of
the power per unit of production for melting with the
above-described 90-ton electric arc furnace used in actual
operation in the case where the reduced iron is continuously
charged. A power per unit of production for melting the granular
metallic iron, which is estimated on the basis of the track records
of the power per unit of production for the reduced iron with
consideration of the difference in composition between the reduced
iron and the granular metallic iron, is 366 kWh. Thus, the input
power efficiency of the melting test equipment with respect to the
above-described 90-ton electric arc furnace used in actual
operation is, as shown in Table 3, calculated as follows:
366/714=51.3% at the charging speed of 4 kg/min and 366/584=62.7%
at the charging speed of 7 kg/min.
TABLE-US-00003 TABLE 3 90 t electric 500 kg high-frequency
induction furnace arc furnace Input power Power Power efficiency
Iron raw Charging Total amount Total input consumption consumption
[C] = [B]/ material for speed of charge power [A] [B] [A] .times.
100 melting (kg/min) (kg) (kWh) (kWh/t) (kWh) (%) Particulate 4
29.13 20.8 714 366 51.3 metallic 7 72.62 42.4 584 366 62.7 iron
[0062] The maximum melting rate [R] per input power of 1 MW of the
melting test equipment for the granular metallic iron shown in
Table 2 is divided by the above-described input power efficiency
[C]/100 so as to correct the maximum melting rate, thereby
estimating the maximum melting rate per input power of 1 MW of the
90-ton electric arc furnace used in actual operation for the
granular metallic iron (see a "Corrected max. melting rate" column
in Table 2).
[0063] The results of the above-described estimation is listed in
the "Continuous charging" raw in Table 4 below. Table 4 also shows
the results of estimation of the maximum melting rate per input
power of 1 MW for the granular metallic iron, which is obtained by
assuming that the carbon content of the granular metallic iron is
2.5 mass percent and calculating the power per unit of production
of the above-described 90-ton electric arc furnace used in actual
operation in the case where energy is imparted by burning the
carbon component using oxygen blowing and in the case where the
granular metallic iron is charged at a high temperature of
600.degree. C.
TABLE-US-00004 TABLE 4 Power Max. consumption melting rate (KWh/t)
(Kg/min/MW) Continuous charging 366 [base] 44~62 Continuous
charging + C burning 312 51~77 Continuous hot charging + C burning
230 70~98
[0064] From the estimated results shown in the above-described
Table 4, it can be seen that the maximum melting rate per input
power of 1 MW for the granular metallic iron is in a range from 40
to 100 kg/min/MW despite its variation in accordance with the
changing temperature or the carbon content of the granular metallic
iron. Thus, it is recommended that the charging speed of the
granular metallic iron A per input power of 1 MW be set to 40 to
100 kg/min/MW.
[0065] The position at which the granular metallic iron A is
charged on the molten iron F surface is preferably set within an
electrode pitch circle.
[0066] That is, since the apparent density of the related-art
reduced iron is, as described above, substantially equal to that of
the molten slag, the reduced iron having been charged into molten
metal in the electric arc furnace stays in a molten slag layer for
a comparatively long time. This advances melting of the reduced
iron due to arc heating through the molten slag layer. Thus, there
is no specific restriction on the position at which the reduced
iron is charged.
[0067] In contrast, since the apparent density of the granular
metallic iron A according to the present invention is substantially
equal to that of the molten iron F, the granular metallic iron A
having been charged into the molten metal in the electric arc
furnace 2 passes through a molten slag layer E and enters the
molten iron layer F. The granular metallic iron A is melted by arc
heating through the molten slag layer E and the molten iron layer
F. Thus, when the granular metallic iron A is charged at a position
separated from the electrodes 7, sufficient heat may not be
transferred to the granular metallic iron A, and accordingly, the
granular metallic iron A not having been melted may accumulate in
the molten iron layer F. For this reason, it is particularly
recommended that the position at which the granular metallic iron A
is charged on the molten iron F surface be set within the electrode
pitch circle, thereby more directly and efficiently transferring
arc heat to the granular metallic iron A. This prevents the
granular metallic iron A from remaining without being melted, and
accordingly, further improves molten steel G productivity.
[0068] An average granular size of the granular metallic iron A is
preferably set to 1 to 50 mm.
[0069] When the size of the granular metallic iron A is excessively
small, small pieces of the slag component tend to be mixed with the
granular metallic iron A when the granular metallic iron A is
separated and collected after the granular metallic iron A has been
discharged from the rotary hearth furnace 1. This degrades the
purity of iron content and makes the granular metallic iron A
easily dispersed when the granular metallic iron A is charged into
the electric arc furnace 2, thereby decreasing the yield of the
added granular metallic iron A. When the size of the granular
metallic iron A is excessively large, there may be the following
problems: transference of heat into the inside of the agglomerated
material takes long time in production in the rotary hearth furnace
1, and accordingly, productivity is decreased; the inside of a
hopper above a furnace 6 or an exit from the hopper above a furnace
6 become clogged; or the melting rate in the electric arc furnace 2
decreases when the granular metallic iron A is melted. The average
size of the granular metallic iron A is more preferably set to 2 to
25 mm.
[0070] According to the present invention, the average granular
size refers to an average granular size by mass calculated from a
characteristic diameter and mass between meshes after classifying
is performed by screening. For example, suppose that classifying is
performed using sieves having meshes D.sub.1, D.sub.2 . . . ,
D.sub.n, D.sub.n+1 (D.sub.1<D.sub.2< . . .
<D.sub.n<D.sub.n+1). When the mass between the mesh D.sub.k
and D.sub.k+1 is W.sub.k, the average granular size by mass d.sub.m
is defined as follows:
d.sub.m=.SIGMA..sub.k=1,n(W.sub.k.times.d.sub.k)/.SIGMA..sub.k=1,n(Wk)
where d.sub.k is a characteristic diameter between the mesh D.sub.k
and mesh D.sub.k+1 and given by the following expression:
d.sub.k=(D.sub.k+D.sub.k+1)/2.
[0071] When the granular metallic iron A is continuously charged
into the molten iron F in the electric arc furnace 2, the molten
slag layer E formed on the molten iron layer F is preferably caused
to foam so as to performing melting while lower ends of the
electrodes 7 are constantly covered. By doing this, arc heat can be
more efficiently transferred to the molten iron layer F without
radiating ark heat toward an upper space, thereby further
increasing the melting rate of the granular metallic iron A. The
height of the forming of the molten slag layer E is adjustable by,
for example, blowing oxygen into the molten iron layer F and
generating a CO gas through a decarbonizing reaction of carbon in
the molten iron layer F.
[0072] The granular metallic iron A produced in the rotary hearth
furnace 1 is preferably continuously charged into the molten iron F
in the electric arc furnace 2 while the temperature of the granular
metallic iron A is maintained at a high-temperature of 400 to
700.degree. C. without cooling the granular metallic iron A to room
temperature.
[0073] Thus, by effectively utilizing sensible heat of the granular
metallic iron A, the melting energy per unit of production in the
electric arc furnace 2 can be further decreased and molten steel G
productivity (molten steel production rate) can be improved.
[0074] The preferred range of temperature at which the granular
metallic iron A is charged is set to 400 to 700.degree. C. for the
following reason. That is, from the viewpoint of effectively
utilizing sensible heat of the granular metallic iron A, the
granular metallic iron A needs to be heated to a certain
temperature. Thus, the lower limit of the temperature is set to
400.degree. C. In order to separate the granular metallic iron A
from the slag component B and the bedding carbonaceous material C
using magnetic separation, the granular metallic iron A needs to be
magnetized. Thus, the upper limit temperature is set to 700.degree.
C., which is lower than the Curie temperature of iron (770.degree.
C.).
[0075] In order to charge the granular metallic iron A into the
electric arc furnace 2 while the granular metallic iron A is at a
high-temperature, it is sufficient that the granular metallic iron
A is charged as follows: a mixture of the granular metallic iron A,
the slag B, and the bedding carbonaceous material C, the mixture
having been heated to about 1000 to 1100.degree. C. and discharged
from the rotary hearth furnace 1, is slightly cooled for the
purpose of protecting part of the facility including the screen 3,
the magnetic separator 4, and a conveyor 5 provided for the
following process; after that, the granular metallic iron A is
separated and collected by the screen 3 and the magnetic separator
4, which are made to be usable under a high-temperature
environment, and transported by the conveyor 5, which is also made
to be usable under a high-temperature environment, to the hopper
above a furnace 6 of the electric arc furnace 2 and temporarily
stored in the hopper above a furnace 6 such that the temperature of
the granular metallic iron A is from 400 to 700.degree. C. when the
granular metallic iron A is discharged from the hopper above a
furnace 6. A path from a discharge duct, through which the
above-described mixture is discharged from the rotary hearth
furnace 1, to the hopper above a furnace 6 of the electric arc
furnace 2 is desirably maintained in an inert gas atmosphere by
blowing N.sub.2 thereinto in order to prevent the granular metallic
iron A from directly contacting air and being reoxidized.
(Modifications)
[0076] In the above-described embodiment, the rotary hearth furnace
is described as an example of a reducing/melting furnace.
Alternatively, the reducing/melting furnace may use a linear
furnace.
[0077] In the above-described embodiment, the agglomerated
material, which is formed by agglomerating a carbonaceous reductant
and an iron oxide containing material, is described as an example
of a raw material containing a carbonaceous reductant and an iron
oxide containing material. Alternatively, the carbonaceous
reductant and the iron oxide containing material may be used in a
powder state instead of being agglomerated.
[0078] In the above-described embodiment, the conveyor, which is
made to be usable under a high-temperature environment, is
described as an example of a transport device that transports the
granular metallic iron in a high-temperature state. Alternatively,
the granular metallic iron may be contained in a thermally
insulated container, which is moved by transportation truck, crane,
or the like.
[0079] In the above-described embodiment, a case in which the
rotary hearth furnace and the electric arc furnace are located
close to each other is described as an example. However, even in
the case where the rotary hearth furnace and the electric arc
furnace are located at positions separated from each other, the
granular metallic iron can be transported to the electric arc
furnace by cooling the granular metallic iron produced in the
rotary hearth furnace to room temperature and using a generally
used transportation means without performing particular reoxidation
preventing means. This is because the granular metallic iron is
solidified after being melted, and accordingly, more densified
compared to reduced iron.
[0080] In the above-described embodiment, the scrap is described as
an example of the other iron raw material initially charged into
the electric arc furnace. Alternatively, the other iron raw
material initially charged into the electric arc furnace may be
reduced iron or granular metallic iron, or may be combination of
two or more of these materials.
[0081] In the case where the other iron raw material and/or part of
the granular metallic iron are initially charged into the electric
arc furnace, the usage ratio of the granular metallic iron to be
continuously charged into molten iron, which includes this
initially charged granular metallic iron, to all the iron raw
materials to be charged needs to be 40 to 80 mass percent. In other
words, in the case where part or all of the other iron raw material
initially charged into the electric arc furnace is the granular
metallic iron, the usage ratio of the total granular metallic iron
to all the iron raw materials to be charged is the ratio of the
total of the initially charged and continuously charged granular
metallic iron. Thus, there is a possibility of exceeding the usage
ratio of 40 to 80 mass percent, and the total granular metallic
iron to be charged needs be adjusted so that the usage ratio of
this total granular metallic iron falls within the range of 40 to
80 mass percent.
[0082] Although the present application has been described in
detail with reference to a particular form of implementation, it is
clear to those skilled in the art that a variety of changes and
modifications are possible without departing from the spirit and
the scope of the present invention.
[0083] The present application is filed on the basis of a Japanese
Patent Application (Japanese Patent Application No. 2010-146114)
filed on Jun. 28, 2010, the contents of which are incorporated
herein by reference.
INDUSTRIAL APPLICABILITY
[0084] According to the present invention, granular metallic iron
that is produced in a reducing/melting furnace and contains 1.0 to
4.5 mass percent carbon undergoes oxygen blowing so as to burn the
carbon in the granular metallic iron. In addition, 40 to 80 mass
percent granular metallic iron with respect to all iron raw
materials to be charged is used and the granular metallic iron is
continuously charged into molten iron made by initially charging
another iron raw material into an electric arc furnace. Thus,
melting energy can be significantly decreased so as to increase the
energy efficiency of the electric arc furnace and molten steel
productivity can be significantly improved.
REFERENCE SIGNS LIST
[0085] 1 reducing/melting furnace (rotary hearth furnace) [0086] 2
electric arc furnace [0087] 3 screen [0088] 4 magnetic separator
[0089] 5 conveyor [0090] 6 hopper above a furnace [0091] 7
electrode [0092] A granular metallic iron [0093] B slag [0094] C
bedding carbonaceous material [0095] D another iron raw material
(scrap) [0096] E molten slag, molten slag layer [0097] F molten
iron, molten iron layer [0098] G molten steel
* * * * *