Patent Application
20130098202
1. Technical Field
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.
2. Background Art
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.
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.
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.
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.
The carbon source uses a carbon component of pig iron or cold pig iron, massive coke, coke breeze, or the like.
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.
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.
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.
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).
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.
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.
PTL 1: Japanese Unexamined Patent Application Publication No. 50-64111
PTL 2: Japanese Unexamined Patent Application Publication No. 51-65007
PTL 3: Japanese Unexamined Patent Application Publication No. 58-141314
PTL 4: Japanese Unexamined Patent Application Publication No. 2002-339009
PTL 5: Japanese Unexamined Patent Application Publication No. 2003-73722
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.
The present invention provides a process for producing molten steel using granular metallic iron as follows.
(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.
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
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.
(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.
(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.
(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.
(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.
(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° C. without being cooled to room temperature.
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.
An embodiment of the present invention will be described in detail below with reference to the drawings.
A granular metallic iron A used in the present invention is produced, for example, as follows.
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° 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° 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° 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).
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.
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.
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.
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.
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.
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.
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.
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° 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° 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.
The results of the calculations are illustrated in
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
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
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.
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.
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.
As generally schematically illustrated in
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° C. The raw material charging speed was sequentially changed while the temperature of the molten metal was maintained at 1550 to 1600° 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.
The results of the melting test are listed in Table 2 below and illustrated in
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.
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.
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.
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.
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.
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).
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° C.
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.
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.
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.
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.
An average granular size of the granular metallic iron A is preferably set to 1 to 50 mm.
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.
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 D1, D2 . . . , Dn, Dn+1 (D1<D2< . . . <Dn<Dn+1). When the mass between the mesh Dk and Dk+1 is Wk, the average granular size by mass dm is defined as follows:
d
m=Σk=1,n(Wk×dk)/Σk=1,n(Wk)
where dk is a characteristic diameter between the mesh Dk and mesh Dk+1 and given by the following expression:
d
k=(Dk+Dk+1)/2.
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.
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° C. without cooling the granular metallic iron A to room temperature.
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.
The preferred range of temperature at which the granular metallic iron A is charged is set to 400 to 700° 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° 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° C., which is lower than the Curie temperature of iron (770° C.).
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° 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° 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 N2 thereinto in order to prevent the granular metallic iron A from directly contacting air and being reoxidized.
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.
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.
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.
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.
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.
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.
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-146114 | Jun 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/064717 | 6/27/2011 | WO | 00 | 12/28/2012 |
