TW201943856A - Molten steel production method - Google Patents

Abstract

The present invention comprises: a first step for obtaining carbon-containing molten iron by adding a carbon source to molten steel left in an electric furnace as molten seed when a previous ch was tapped; a second step for performing melt-reduction by adding a DRI to the carbon-containing molten iron generated in the first step; a third step for performing desulphurisation processing by adding a deoxidisation material; a fourth step for discharging desulphurisation slag generated by the desulphurisation processing in the third step; a fifth step for performing decarbonation processing by blowing in oxygen; a sixth step for discharging decarbonation slag generated by the decarbonation processing in the fifth step; and a seventh step for tapping steel and leaving a portion as a molten seed for a subsequent ch, after the decarbonation slag has been discharged in the sixth step.

Description

Manufacturing method of molten steel

The present invention relates to a manufacturing method of molten steel that reduces and melts reduced iron (DRI) prepared by pre-reducing iron oxide (iron ore, etc.) in a melting furnace to produce molten steel.

2. Description of the Related Art In the past, new installations of blast furnaces required high costs. Therefore, in countries that produce natural gas, the following processes have been used as the mainstream: pelletization of pellets, etc. through, for example, the Midrex process. Iron oxide such as iron ore is reduced in a shaft furnace to produce reduced iron (DRI) with a metallization rate of 90% or more, and the DRI is melted in an electric furnace to directly produce molten steel.

In addition, a process of reducing iron using carbon materials such as coal instead of natural gas as a reducing agent has also been developed and has been put into practical use. The reduced iron production process includes a method of simultaneously reducing calcined pellets such as iron ore and coal carbon powder by heating in a rotary kiln (SL / RN method), and mixing carbon material with powdered iron oxide and making it lumpy. Method of heating and reduction on a rotary bore to produce reduced iron (RHF method) and the like. Compared with the shaft furnace method, the above method is more difficult to produce DRI with a high metallization rate, and generally, the metallization rate is as high as about 85%. Therefore, when using this DRI, it is necessary to perform the reduction of the remaining iron oxide at the same time when melting the metallic iron in a melting furnace such as an electric furnace.

Patent Document 1 describes a method for producing a DRI having a metallization ratio of 60% or more by the RHF method, and thereafter producing a molten iron having a carbon content of 1.5 to 4.5% by mass in an arc heating melting furnace. After being discharged from the furnace, desulfurization treatment, dephosphorization treatment, and decarburization treatment are performed in other melting furnaces. In this method, in order to reduce the residual iron oxide, a carbon material is added to the melting furnace. However, in this method, molten iron is transferred to another furnace, which causes a large heat loss. In addition, in order to ensure a heat source, a carbon material is further added, and molten iron having a high carbon content is decarburized to produce a molten steel, so that the amount of CO ₂ generated is increased. In addition, Patent Document 2 discloses a technique of supplying a hydrocarbon gas and melting an iron-based raw material. However, this method is based on the premise of using a hydrocarbon gas, so that the cost is high.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Application Laid-Open No. 2001-515138 Patent Literature 2: Japanese Patent Application Laid-Open No. 2016-108575

SUMMARY OF THE INVENTION Problems to be Solved by the Invention In view of the foregoing problems, the present invention aims to provide a method for producing a molten steel. The method for producing the molten steel uses a melting furnace such as an electric furnace for melting and reduction, and particularly has a low metallization rate. In DRI, productivity is high, heat loss is small, and CO ₂ generation is small.

Means for Solving the Problems In the present invention, in order to melt and reduce DRI with a low metallization rate to produce molten steel, a part of the molten steel is left in the furnace and used as the starting molten iron for the next feeding. However, if the starting molten iron is the original molten steel, the melting and reduction of DRI will be delayed, so only the carbon source is supplied to the starting molten iron before the DRI is supplied to increase the C concentration of the starting molten iron. As described later, this C concentration is preferably 0.5% by mass or more and 1.5% by mass or less.

The present invention is as follows.
(1) A method for manufacturing molten steel, which is characterized by having the following steps:
In the first step, a carbon source is added to the molten steel that was left in the electric furnace as the starting molten iron during the previous feeding of steel to obtain a carbon-containing molten iron;
In the second step, DRI is added to the carbon-containing molten iron generated in the foregoing first step to perform melting reduction;
In the third step, a deoxidizing material is added for desulfurization treatment;
The fourth step is to discharge the desulfurization slag generated by the desulfurization treatment of the aforementioned third step;
Step 5 is followed by blowing in oxygen for decarburization treatment;
In the sixth step, the decarburized slag generated by the decarburization treatment in the fifth step is discharged; and in the seventh step, after the aforementioned decarburized slag is discharged in the sixth step, the amount of the starting molten iron for the next feeding is left and Perform tapping.
(2) The method for manufacturing molten steel as described in (1) above, wherein when the furnace diameter of the electric furnace is D (m), the initial amount of molten iron W (t) left in the aforementioned seventh step is set to 0.3 × D ² >W> 1.6 × D ² .
(3) The method for producing a molten steel as described in (1) or (2) above, wherein in the aforementioned first step, a carbon-containing molten iron having a C concentration of 0.5% by mass or more and 1.5% by mass or less is prepared.

According to the present invention can provide a method for producing a molten steel, which, when in an electric furnace and a melting furnace for melting metals in particular reduction of the rate of the DRI, high productivity, low heat loss and a small amount of CO ₂ generated.

Embodiments of the Invention Embodiments of the invention will be described below with reference to the drawings.
FIG. 1 is a diagram for explaining a method for manufacturing a molten steel by melting and reducing DRI with a low metallization rate in a melting furnace such as an electric furnace according to this embodiment.
As shown in FIG. 1, the manufacturing method of this embodiment is established by at least seven steps from the first step to the seventh step.

For the convenience of explanation, the explanation starts from the seventh step. The seventh step is a step of discharging the molten steel, and the molten steel has passed through the decarburization treatment of the fifth step to reduce the C concentration to, for example, less than 0.1% by mass. At this time, the total amount of molten steel will not be discharged, but the molten steel left in the furnace as the starting molten iron amount for the next feeding.

When a DC electric furnace is used as the melting furnace, in the second step described later, a voltage is applied between the upper electrode and the lower electrode provided on the bottom of the furnace to cause an arc to occur, and the heat is used for the melting reduction of DRI. If there is no starting molten iron when the voltage is applied, electricity will flow through the DRI, so the contact resistance between the DRI and the lower electrode of the furnace bottom will increase, and the arc will be unstable at the initial stage of melting, resulting in a longer melting time. In addition, when DRI with a low reduction rate and a large amount of iron oxide is used, electricity does not flow easily, and the melting time is further increased.

On the other hand, if there is an initial molten iron when a voltage is applied, it will be in close contact with the lower electrode of the furnace bottom, so the arc is stable and the melting time can be shortened. Therefore, the important system does not discharge the total amount of molten steel, but leaves a part as the starting molten iron. When the inside diameter of the melting furnace is D (m), the initial amount of molten iron W (t) should satisfy the following formula (1).
0.3 × D ² >W> 1.6 × D ² ... (1)

Here, if the initial amount of molten iron W is 0.3 × D ² or less, as described above, the contact resistance between the DRI and the lower electrode of the furnace bottom tends to increase, and the arc may be unstable. In addition, if the starting amount of molten iron W is 1.6 × D ² or more, the load of the decarburization treatment in the fifth step described later will increase. The values of "0.3" and "1.6" are calculated from the product of the bath depth (m) and the molten iron density (t / m ³ ) in the electric furnace.

Next, the first step will be described. Before adding DRI in the second step described below, in the first step, carbon materials such as coal (fuel coal) and anthracite are added to the furnace to form molten steel that is a starting molten iron into molten iron having a predetermined C concentration. . The carbon material supply method is not particularly limited, but there are the following methods: a method of freely dropping and adding from a hopper provided on the upper part of the furnace; a method of supplying the upper electrode as a hollow electrode and supplying from a hollow portion; using a special spray gun A method of spraying molten steel; a method of directly blowing into the molten steel using an immersion spray gun; and a method of blowing into the molten steel from a bottom blowing port provided for stirring molten soup.

Here, if the C concentration of the starting molten iron is less than 0.1% by mass, the DRI added in the second step cannot be melted when it does not reach the melting point of iron or more. Therefore, in the case of a molten steel that has a C concentration of less than 0.1% by mass of the starting molten iron, a large amount of energy is required for melting. Further, if the operating temperature is equal to or higher than the melting point of iron and the superheating temperature is set to 100 ° C in order to stabilize the operation, a high temperature state of 1650 ° C must be maintained. Therefore, the load on the refractory material is considerable. In addition, in the case of a molten steel whose C concentration of the starting molten iron is less than 0.1% by mass, the iron oxide remaining in the DRI will not be reduced, and slag with a high iron oxide concentration will be generated, which will cause refractory materials. Adverse effects. This is particularly noticeable when using DRI with low metallization rates.

Therefore, in this embodiment, carbon is added in the first step to make the starting molten iron into a C-containing molten soup. Thereby, the metallic iron added with DRI will carburize due to C in the molten soup, lower the melting point, promote the melting speed, and improve productivity. In addition, the working temperature can be reduced in accordance with the C concentration of the starting molten iron, which can reduce the load on the refractory material. In addition, the iron oxide in DRI also reacts with C in the starting molten iron to promote reduction, so the iron oxide concentration in the generated slag will fall to a low level. In addition, denitrification is promoted along with the decarburization reaction in the fifth step, so the nitridation can be reduced. As described above, by making the molten iron containing C from the starting molten iron, productivity can be improved, and the load on the refractory can be reduced.

Here, before entering the second step, the C concentration of the molten iron belonging to the starting molten iron should preferably be 0.5% by mass or more. This is because when the C concentration is less than 0.5% by mass, the rate of carburizing and melting of metallic iron and the reduction rate of iron oxide in DRI are reduced, which leads to deterioration of productivity. On the other hand, when the C concentration of the molten iron becomes too high, the load of the decarburization treatment in the fifth step described later increases, and the amount of CO ₂ generated increases. Therefore, the C concentration of the molten iron, which is the starting molten iron, should preferably be 1.5% by mass or less.

Next, the second step will be described. In the second step, the DRI made by the shaft furnace and RHF is supplied to a melting furnace, and a voltage is applied between the upper electrode and the lower electrode provided at the bottom of the furnace to cause an arc to melt the metallic iron in the DRI, and Reduction of iron oxide remaining in DRI is performed. The supply method of DRI can be, for example, a method in which a block is added to the furnace by being freely dropped from a hopper provided on the upper part, and a powdered method is a method in which the upper electrode is a hollow electrode and is blown in from a hollow portion. The DRI supplied in the second step is, for example, a composition shown in Table 1 below. The main components of the slag component are SiO ₂ and Al ₂ O ₃ , and the others include CaO, MgO, S, P ₂ O _5, and MnO. In addition, when the mass% of the pure iron component in DRI is mass% M.Fe and the mass% of the FeO component in DRI is mass% FeO, the metallization ratio can be metallization ratio = mass% M.Fe / (mass% M.Fe + mass% FeO × 55.75 / 71.85).

[Table 1]

Regarding the C concentration in the molten iron adjusted in the first step, in the second step, carbon materials such as coal and anthracite are added in accordance with the supply rate of DRI. The amount of carbon material input here is based on the sum of the amount required to carburize the iron in the DRI to the C concentration of the molten iron, and the amount required to reduce the iron oxide (FeO, etc.) in the DRI. the amount. The carbon material input in the second step is the same as the carbon material input in the first step, and examples thereof include fuel coal and anthracite. An example of the composition of fuel coal is shown in Table 2 below, and an example of the composition of anthracite is shown in Table 3. In Tables 2 and 3, FC represents Fixed Carbon, and VM represents Volaile Matter. In the first step and the second step, the fuel coal and the anthracite can be used individually or in combination. In addition, other carbon materials can also use carbon sources such as plastic waste and biomass.

[Table 2]

[table 3]

The operating temperature is determined by the C concentration in the molten iron, and the C concentration in the molten iron is related to the difference between the C concentration in the molten iron adjusted in the first step and the second step The sum of the amount of the carbon material input, the amount required to carburize the iron content in the DRI to the C concentration of the molten iron, and the amount required to reduce the iron oxide (FeO, etc.) in the DRI. Fig. 2 is a state diagram of the Fe-C system, showing changes in melting point of iron due to C concentration. In order to stabilize the operation, the superheating temperature must be 100 ° C or higher. For example, when the superheating temperature is 100 ° C, the melting point is 1430 ° C if the molten iron has a C concentration of 1.5% by mass, so the operating temperature is 1530 ° C. . In the second step, a voltage is applied depending on the supply rate of the carbon material and the DRI to maintain the operating temperature determined according to the C concentration in the molten iron.
As described above, the C concentration of the carbon-containing molten iron before the start of the second step is preferably 0.5% by mass or more and 1.5% by mass or less, and is controlled within the range of 0.5% by mass or more and 1.5% by mass or less until the second step. It is even better at the end of the step.

Next, the third step will be described. Although the content varies with the place of origin, both iron ore and coal carbon contain sulfur. Since the iron oxide in DRI cannot be reduced instantaneously, the iron oxide concentration in the slag is high immediately after the completion of the DRI. In a state where the iron oxide concentration in the slag is high, the sulfur distribution between the molten iron (hereinafter sometimes referred to as the molten metal) and the slag is small, and more sulfur exists in the molten metal than in the slag. Since sulfur in the metal liquid is not easily removed in the decarburization treatment in the fifth step described later, if the third step and the fourth step described later are omitted, the molten steel after the decarburization treatment has a high sulfur concentration does not satisfy the low sulfur steel Manufacturing needs. In addition, sulfur-based surface-active components monopolize the adsorption position. Therefore, if the sulfur concentration in the molten metal is high, it will be difficult to remove nitrogen from the molten metal, which will not meet the needs of low nitrogen steel manufacturing. Therefore, it is extremely important to perform a desulfurization treatment after the completion of the second step.

After the second step is completed (DRI supply is completed), in the third step, a deoxidizer such as metal Al or a metal Al content is added to the furnace to reduce iron oxide in the slag and remove oxygen from the molten iron. In this state, the sulfur distribution between the slag and the molten metal becomes more, sulfur moves from the molten metal to the slag, and the sulfur concentration in the molten metal decreases. When a DC electric furnace is used as a melting furnace, the upper electrode is usually a negative electrode and the lower electrode of the furnace bottom is a positive electrode. However, if a voltage is applied when the upper electrode is a positive electrode and the lower electrode is a negative electrode, it can be electrified. Scientifically improve the apparent sulfur distribution, which can promote desulfurization.

Next, the fourth step will be described. If the desulfurization slag formed by the desulfurization in the third step is directly left and subjected to decarburization treatment, the sulfur will be moved from the slag to the metal liquid (resulfurization) again. Therefore, in the fourth step, the desulfurization slag is first removed from the The slag hole is discharged.

Next, the fifth step will be described. In the fifth step, an oxygen lance is inserted into the furnace from the upper part of the furnace, and oxygen is blown into the molten iron, thereby performing a dephosphorization treatment and a decarburization treatment to reduce it to a predetermined phosphorus concentration and carbon concentration. In the decarburization process, oxygen reacts with carbon in the molten iron to generate CO gas. At this time, nitrogen dissolved in the molten iron is incorporated into the CO gas, and nitrogen can be removed from the molten iron.

Next, the sixth step will be described. The sixth step is a step of discharging the decarburized slag produced in the fifth step. During the dephosphorization treatment and decarburization treatment in the fifth step, the phosphorus in the molten iron is transferred to the slag. If the decarburized slag is not discharged to discharge the phosphorus out of the system, the phosphorus will be concentrated and it will be impossible to produce low-P steel. Therefore, it is necessary to discharge the decarburized slag as much as possible.

As described above, according to the first step to the seventh step of this embodiment, it is possible to suppress the heat loss and the amount of CO ₂ generated to produce a molten steel. In particular, by adding a carbon source to the starting molten iron in the first step to make a carbon-containing molten iron, the melting rate and reduction rate of DRI can be increased, and heat loss can be reduced. This makes it possible to suppress the addition of a carbon material to secure a heat source, and as a result, the amount of CO ₂ generated can be suppressed. Table 4 and Table 5 below show the respective molten metal composition and slag composition in each step.

[Table 4]

[table 5]

As shown in Table 4, in the second step in this embodiment, carbon materials such as coal and anthracite are fed in accordance with the DRI feed rate, so that the C concentration in the molten iron is in the range of 0.1 to 1.5% by mass. By suppressing the C concentration, the amount of CO ₂ produced in the decarburization treatment can also be suppressed.

EXAMPLES Next, examples of the present invention will be described. However, the conditions in the examples are an example of a condition adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited by the one example of the condition. As long as the object of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

First, in the previous feeding, the molten steel was tapped from a DC electric furnace with a hollow electrode of 6m in diameter, and 20t of the molten steel was left in the DC electric furnace as the starting molten iron. The C concentration of the molten steel produced in the previous addition was 0.05% by mass. Then, in the first step, a carbon material is added from the hollow electrode, and carbon is added to the starting molten iron while measuring the C concentration with a sub-tube probe having a built-in C sensor capable of measuring the C concentration by thermal analysis. Until the C concentration becomes 1.0% by mass.

Next, in the second step, DRI and a carbon material having a metalization rate of 75% are added at the same time to perform melting reduction. At this time, the C concentration in the molten metal was controlled to be 1.0% by mass, and the working temperature was controlled to be 1570 ° C. The melting reduction time is 30 minutes, the amount of molten soup at the end of adding DRI is 300t, and the amount of slag is 40t.

Next, in the third step, Al ash was added as a deoxidizing agent to perform desulfurization. After the desulfurization, 30 t of slag was discharged from the slag discharge hole of the DC electric furnace in the fourth step. Thereafter, in a fifth step, oxygen is supplied from an oxygen spray gun provided at the upper part of the furnace, and decarburization treatment is performed to produce a molten steel having a C concentration of 0.05% by mass. In the fifth step, the denitrification is promoted along with the decarburization, and the N concentration of the produced molten steel is 30 ppm. Then, in a sixth step, the slag generated by the decarburization treatment is discharged from the slag discharge hole. Thereafter, in the seventh step, 20t of molten steel is left in the furnace as the starting molten iron for the next feeding, and the remaining 280t of molten steel is tapped.

On the other hand, in the case of the two-furnace method of melting and reducing the melt-reduction after decarburization in a single furnace and performing decarburization in other furnaces, it may be caused by milling out of the melting furnace. Reduce the temperature of at least 100 ° C by placing the melt mill in the decarburization furnace. In contrast, in this embodiment, there is no such heat loss, and it is possible to reduce the unit requirement of energy. In addition, since decarburization is performed from a state where the C concentration is 1.0% by mass, compared with the two-furnace method, the amount of decarburization is small, and the amount of CO ₂ generated can be reduced. Specifically, it is as follows.

In this embodiment, since 300t of molten iron having a C concentration of 1.0% by mass is decarburized to 0.05% by mass, CO ₂ as shown below is generated:
300 × (1-0.05) /100/12×22.4=5.3Nm ³ .
On the other hand, in the case of the two-furnace method, the reduction of DRI is carried out at a C concentration of 3.0% by mass, and it is assumed that the melt milling is performed by milling and decarburization is performed in another furnace. The reason for setting the C concentration to 3.0% by mass is that if the concentration is lower than 3.0% by mass, there will be heat loss during the replacement, and the heat generated by the combustion of C that is decarbonized cannot reach the end of the decarbonization treatment. Predetermined temperature. Since 280t molten steel is tapped in this embodiment, 280t smelting and milling can be performed for decarburization in the two furnace mode. Therefore, the amount of CO ₂ produced is
280 × (3-0.05) /100/12×22.4=16.5Nm ³ .
As described above, it can be confirmed that in the case of this embodiment, the amount of CO ₂ generated can be reduced compared to the two-furnace method.

(Comparative example)
First, in the previous feeding, the molten steel was tapped from a DC electric furnace with a hollow electrode of 6m in diameter, and 20t of the molten steel was left in the DC electric furnace as the starting molten iron. The C concentration of the molten steel produced in the previous addition was 0.05% by mass. Next, the first step is omitted, and DRI with a metalization rate of 75% is added in the second step to perform melting reduction. In this case, in addition to the high working temperature of 1640 ° C., a melting and reduction time of 60 minutes is required. Thereafter, desulfurization treatment, decarburization treatment, and the like were performed under the same conditions as in the examples.

As described above, the comparative example takes twice as long as the melting reduction time as compared with the example, and as a result, the productivity is lowered.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a method for producing a molten steel, which, when melted and reduced by a melting furnace such as an electric furnace, particularly DRI having a low metallization rate, has high productivity, low heat loss, and CO ₂ The production volume is small, and the industrial value is high.

FIG. 1 is a diagram for explaining each step of manufacturing molten steel in the embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the C concentration and the melting point of molten iron.

Claims (3)

A method for manufacturing molten steel, which is characterized by having the following steps: In the first step, a carbon source is added to the molten steel that was left in the electric furnace as the starting molten iron during the previous feeding of steel to obtain a carbon-containing molten iron; In the second step, DRI is added to the carbon-containing molten iron generated in the foregoing first step to perform melting reduction; In the third step, a deoxidizing material is added for desulfurization treatment; The fourth step is to discharge the desulfurization slag generated by the desulfurization treatment of the aforementioned third step; Step 5 is followed by blowing in oxygen for decarburization treatment; A sixth step of discharging the decarburization slag generated by the decarburization treatment of the fifth step; and In the seventh step, after the aforementioned decarburization slag is discharged in the sixth step, the amount of the starting molten iron to be fed next time is left and tapping is performed. For example, the method for manufacturing molten steel of claim 1, wherein when the furnace diameter of the electric furnace is D (m), the initial amount of molten iron W (t) left in the aforementioned step 7 is set to 0.3 × D ² >W> 1.6 × D ² . The method for manufacturing a molten steel according to claim 1 or 2, wherein in the aforementioned first step, a carbon-containing molten iron having a C concentration of 0.5 mass% or more and 1.5 mass% or less is prepared.