JP6923075B2 - Method of manufacturing molten steel - Google Patents

Description

The present invention relates to a method for producing molten steel, which produces molten steel by reducing and melting reduced iron (DRI) produced by pre-reducing iron oxide (iron ore, etc.) in a melting furnace.

Conventionally, it costs a lot to construct a new blast furnace, so in countries where natural gas is produced, for example, by the Midrex method, iron oxide such as agglomerated iron ore such as pellets is reduced in a shaft furnace and metal. The mainstream process is to produce reduced iron (DRI) with a conversion rate of 90% or more, melt the DRI in an electric furnace, and directly produce molten steel.

In addition, a reduced iron production process that uses a coal material such as coal as a reducing agent instead of natural gas has also been developed and put into practical use. In this reduced iron production process, a method of heating and reducing calcined pellets such as iron ore together with coal powder with a rotary kiln (SL / RN method), or a method of mixing carbonaceous material and powdered iron oxide to agglomerate and agglomerate them into a rotary hearth. There is a method (RHF method) for producing reduced iron by heating and reducing the above. With these methods, it is difficult to produce a DRI having a high metallization rate as compared with the shaft furnace method, and the metallization rate is generally as high as about 85%. Therefore, when using this DRI, it is necessary to melt the metallic iron in a melting furnace such as an electric furnace and also reduce the remaining iron oxide.

In Patent Document 1, a DRI having a metallized iron content of 60% or more is produced by the RHF method, and then molten iron having a carbon content of 1.5 to 4.5% by mass is produced in an arc heating type melting furnace. A method of performing desulfurization treatment, dephosphorization treatment, and decarburization treatment in another melting furnace after discharging the molten iron to the outside of the furnace is described. In this method, a charcoal material is added to the melting furnace in order to reduce the remaining iron oxide. However, in this method, the heat loss increases by transferring the molten iron to another furnace. Further, in order to secure a heat source, a carbon dioxide material is further added to decarburize molten iron having a high carbon content to produce molten steel, so that the _{amount of CO 2} generated increases. Further, Patent Document 2 discloses a technique for dissolving an iron-based raw material while supplying a hydrocarbon gas. However, this method is costly because it is premised on the use of hydrocarbon gas.

Special Table 2001-515138 Japanese Unexamined Patent Publication No. 2016-108575

In view of the above-mentioned problems, the present invention is a molten steel having _{high productivity, low heat loss, and low CO 2} generation when melting and reducing DRI having a particularly low metallization rate in a melting furnace such as an electric furnace. It is an object of the present invention to provide the manufacturing method of.

In the present invention, in order to dissolve and reduce DRI having a low metallization rate to produce molten steel, a part of the molten steel is left in the furnace and used as a seed bath for the next channel. However, if the seed water remains as molten steel, the dissolution and reduction of DRI is delayed. Therefore, before supplying DRI, only the carbon source is first supplied to the seed water to increase the C concentration of the seed water. As will be described later, the C concentration is preferably 0.5% by mass or more and 1.5% by mass or less.

The present invention is as follows.
(1) The first step of adding a carbon source to the molten steel left in the electric furnace as a seed hot water at the time of steel ejection of the previous channel to obtain carbon-containing molten iron.
The second step of adding DRI to the carbon-containing molten iron produced in the first step to dissolve and reduce it, and the second step.
Next, the third step of adding a deoxidizing material and performing a desulfurization treatment, and
The fourth step of discharging the desulfurized slag produced by the desulfurization treatment of the third step and
Next, the fifth step of blowing oxygen to perform decarburization treatment and
The sixth step of discharging the decarburized slag produced in the decarburization treatment of the fifth step and
In the sixth step, after discharging the decarburized slag, the seventh step, in which the seed hot water of the next channel is left and the steel is discharged,
A method for producing molten steel, which comprises.
(2) When the diameter of the electric furnace is D (m), the amount of seed water W (t) left in the seventh step is 0.3 × D ² <W <1.6 × D ^2. The method for producing molten steel according to (1) above.
(3) Production of the molten steel according to (1) or (2) above, wherein in the 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 obtained. Method.

According to the present invention, when melting and reducing DRI having a particularly low metallization rate in a melting furnace such as an electric furnace, a method for producing molten steel having _{high productivity, low heat loss, and low CO 2 generation amount is provided.} Can be provided.

FIG. 1 is a diagram for explaining each step of manufacturing molten steel in the embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the C concentration and the melting point of molten iron.

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

First, for convenience of explanation, the seventh step will be described. The seventh step is a step of discharging molten steel whose C concentration has been reduced to, for example, less than 0.1% by mass by the decarburization treatment of the fifth step. At this time, instead of discharging the entire amount of molten steel, the amount of molten steel used as the seed bath for the next channel is left in the furnace.

When a DC electric furnace is used as a melting furnace, in the second step described later, a voltage is applied between the upper electrode and the lower electrode installed on the bottom of the furnace to generate an arc, and the heat is used for melting and reducing DRI. do. If there is no seed water when applying voltage, electricity flows through DRI, so the contact resistance between DRI and the lower electrode of the furnace bottom is large, the arc becomes unstable at the initial stage of melting, and the melting time is long. Become. Further, when DRI having a low reduction rate and a high iron oxide content is used, it becomes difficult for electricity to flow and the dissolution time further increases.

On the other hand, if there is a seed bath when applying a voltage, the contact between the lower electrodes of the furnace bottom is close, so that the arc is stable and the melting time can be shortened. For this reason, it is important to leave a part of the water as a seed hot water, rather than producing the entire amount of steel. Further, when the inner diameter of the melting furnace is D (m), the amount of seed water W (t) preferably satisfies the following formula (1).
0.3 x D ² <W <1.6 x D ² ... (1)

Here, if the amount of seed water W is 0.3 × D ² or less, the contact resistance between the DRI and the lower electrode of the furnace bottom tends to increase as described above, and the arc may not be stable. Further, if the amount of seed water W is 1.6 × D ² or more, the load of the decarburization treatment in the fifth step described later increases. The numerical values "0.3" and "1.6" are values calculated from the product of the bath depth (m) and the density of molten iron (t / m ^{3) in the electric furnace.}

Next, the first step will be described. Before adding DRI in the second step, which will be described later, in the first step, coal materials such as coal (general coal) and anthracite are added into the furnace, and molten steel, which is a seed bath, is mixed with molten iron having a predetermined C concentration. do. The method of supplying the charcoal material is not particularly limited, but it is added by free fall from the hopper installed in the upper part of the furnace, the upper electrode is used as a hollow electrode and is supplied from the hollow part, and the molten steel is sprayed using a dedicated lance. There are a method, a method of blowing directly into the molten steel using a dipping lance, a method of blowing into the molten steel from a bottom blowing tuyere installed for stirring the molten metal, and the like.

Here, in the case of molten steel having a C concentration of less than 0.1% by mass in the seed bath, the DRI added in the second step cannot be melted unless it exceeds the melting point of iron. Therefore, if the C concentration of the seed bath remains as molten steel such as less than 0.1% by mass, a large amount of energy is required for melting. Further, the operating temperature is equal to or higher than the melting point of iron, and if the super heat is set to 100 ° C. in order to stabilize the operation, it is necessary to maintain a high temperature state of 1650 ° C. Therefore, the load on the refractory is large. Further, if the C concentration of the seed bath remains as molten steel such as less than 0.1% by mass, the iron oxide content remaining in the DRI is not reduced, and slag having a high iron oxide concentration is generated, which adversely affects the refractory. give. This is particularly noticeable when a low metallization rate DRI is used.

Therefore, in the present embodiment, coal is added in the first step, and the seed water is used as a C-containing molten metal. As a result, the added metallic iron of DRI is carburized by C in the molten metal, the melting point is lowered, the melting rate is promoted, and the productivity is improved. In addition, the operating temperature can be lowered according to the C concentration of the seed hot water, and the load on the refractory is reduced. In addition, iron oxide in DRI also reacts with C in the seed bath to promote reduction, so that the iron oxide concentration in the produced slag is also low. Further, since denitrification is promoted along with the decarburization reaction in the fifth step, it is possible to reduce nitrogen. As described above, by using the C-containing molten metal as the seed water, the productivity can be improved and the load on the refractory can be reduced.

Here, it is preferable that the C concentration of the molten iron, which is the seed hot water, is 0.5% by mass or more before entering the second step. This is because when the C concentration is less than 0.5% by mass, the carburizing dissolution rate of metallic iron and the reduction rate of iron oxide in DRI decrease, and the productivity deteriorates. On the contrary, when the C concentration of molten iron becomes too high, the load of the decarburization treatment increases and the amount of _{CO 2 generated increases in the fifth step described later.} Therefore, the C concentration of molten iron, which is a seed hot water, is preferably 1.5% by mass or less.

Next, the second step will be described. In the second step, the DRI manufactured in the shaft furnace or RHF is supplied to the melting furnace, and a voltage is applied between the upper electrode and the lower electrode installed on the bottom of the furnace to generate an arc, and the metallic iron in the DRI is generated. Is dissolved and the iron oxide remaining in the DRI is reduced. As a method for supplying DRI, for example, a method in which a lumpy one is added into the furnace by free fall from a hopper installed at the upper part, and a powdery one has an upper electrode as a hollow electrode and is blown from the hollow portion can be adopted. The DRI supplied in the second step has, for example, the composition shown in Table 1 below. The slag component is _{mainly composed of SiO 2} , Al ₂ O ₃ , and also contains CaO, MgO, S, P ₂ O ₅ , and MnO. The metallization rate is mass% M.Fe when the mass% of the pure iron component in the DRI is mass% M.Fe and the mass% of the FeO component in the DRI is mass% FeO. It can be calculated by / (mass% M.Fe ＋ mass% FeO × 55.75 / 71.85).

In the second step, coal materials such as coal and anthracite are further added according to the supply rate of DRI with respect to the C concentration in the molten iron adjusted in the first step. The amount of carbonaceous material input here is the sum of the amount required for the iron content in the DRI to carburize to the C concentration of molten iron and the amount required for reducing iron oxide (FeO, etc.) in the DRI. It becomes the base. Examples of the coal material introduced in the second step include steam coal and anthracite coal, as in the case of the coal material introduced in the first step. Table 2 below shows an example of the composition of steaming coal, and Table 3 shows an example of the composition of anthracite coal. FC in Tables 2 and 3 represents fixed carbon, and VM represents a volatile component (Volaile Matter). In the first step and the second step, steaming coal and anthracite may be used alone or in combination. It is also possible to use a carbon source such as waste plastic or biomass as the other carbon material.

The operating temperature is required for the amount of carbonaceous material input in the second step and the iron content in the above-mentioned DRI to carburize to the C concentration of the molten iron with respect to the C concentration in the molten iron adjusted in the first step. It is determined by the C concentration in molten iron, which depends on the difference between the amount and the sum of the amount required to reduce iron oxide (FeO, etc.) in DRI. FIG. 2 is a Fe-C phase diagram showing a change in the melting point of iron depending on the C concentration. It is said that a super heat of 100 ° C or higher is required to stabilize the operation. For example, in order to operate at a super heat of 100 ° C, a molten iron having a C concentration of 1.5% by mass has a melting point of 1430. Since it is ° C, the operating temperature is 1530 ° C. In the second step, a voltage is applied according to the supply rate of the carbonaceous material and the DRI so as to maintain this operating temperature determined by the C concentration in the molten iron.
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 as described above, but at the same time, 0. It is more preferable to control within the range of 5% by mass or more and 1.5% by mass or less.

Next, the third step will be described. Sulfur is contained in iron ore and coal, although the content varies depending on the place of origin. Since iron oxide in DRI is not instantly reduced, the iron oxide concentration in slag is high immediately after the completion of DRI injection. When the iron oxide concentration in the slag is high, the sulfur distribution between the molten iron (hereinafter, may be referred to as metal) and the slag is low, and sulfur is more abundant in the metal than in the slag. Since it is difficult to remove sulfur in the metal in the decarburization treatment of the fifth step described later, if the third step and the fourth step described later are omitted, the sulfur concentration of the molten steel after the completion of the decarburization treatment is high and the low sulfur steel. Does not meet manufacturing needs. Moreover, since sulfur is a surface active component, it monopolizes the adsorption site. Therefore, if the sulfur concentration in the metal is high, it becomes difficult to remove nitrogen from the metal, which does not satisfy the needs for producing low nitrogen steel. Therefore, it is important to perform the desulfurization treatment after the completion of the second step.

After the completion of the second step (end of DRI supply), in the third step, a deoxidizing agent such as metal Al or a metal Al-containing substance is added into the furnace to reduce the iron oxide content in the slag and oxygen in the molten iron. To remove. In this state, the sulfur distribution between the slag and the metal becomes high, the sulfur moves from the metal to the slag, and the sulfur concentration in the metal decreases. When the melting furnace is a DC electric furnace, the upper electrode is usually a negative electrode and the bottom electrode of the furnace bottom is a positive electrode, but when the upper electrode is used as a positive electrode and the lower electrode of the furnace bottom is used as a negative electrode, electrochemically The apparent sulfur distribution can be increased, and desulfurization can be further promoted.

Next, the fourth step will be described. If the desulfurized slag formed by the desulfurization in the third step is left as it is and the decarburization treatment is performed, sulfur is transferred from the slag to the metal (desulfurization) again. Therefore, in the fourth step, the desulfurized slag is discharged from the drain hole. do.

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 onto the molten iron to perform dephosphorization treatment and decarburization treatment to reduce the phosphorus concentration and carbon concentration to a predetermined level. 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 taken into the CO gas and nitrogen is removed from the molten iron. NS.

Next, the sixth step will be described. The sixth step is a step of discharging the decarburized slag produced in the fifth step. In the dephosphorization treatment and decarburization treatment in the fifth step, phosphorus in the molten iron is transferred to slag. Unless decarburized slag is discharged and phosphorus is discharged to the outside of the system, phosphorus will be concentrated and low P steel cannot be produced. Therefore, it is necessary to discharge decarburized slag as much as possible.

As described above, the molten steel can be produced by suppressing _{the heat loss and suppressing the amount of CO 2} generated by the first to seventh steps in the present embodiment. In particular, in the first step, by adding a carbon source to the seed bath to obtain carbon-containing molten iron, the dissolution rate and reduction rate of DRI can be increased and heat loss can be reduced. As a result, the addition of charcoal material for securing a heat source can be suppressed, and as a result, the _{amount of CO 2} generated can also be suppressed. Tables 4 and 5 below show the metal composition and the slag composition in each step, respectively.

As shown in Table 4, in the second step, in the second step, coal materials such as coal and anthracite are further charged according to the supply rate of DRI, and the C concentration in the molten iron is 0.1 to 1.5% by mass. Is in the range of. By suppressing the C concentration, the _{amount of CO 2} generated by the decarburization treatment can also be suppressed.

Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

First, in the front channel, molten steel was ejected from a DC electric furnace having a hollow electrode and having a furnace diameter of 6 m, and 20 tons of molten steel was left in the DC electric furnace as a seed bath. The C concentration of the molten steel produced in the previous channel was 0.05% by mass. Then, in the first step, the carbonaceous material is added from the hollow electrode, and the C concentration of the seed bath is 1.0% by mass while measuring the C concentration with a sublance probe having a built-in C sensor that measures the C concentration by thermal analysis. Charcoal was added until

Subsequently, in the second step, DRI having a metallization rate of 75% was added together with the carbonaceous material to carry out dissolution reduction. At this time, the C concentration in the metal was controlled to remain at 1.0% by mass, and the operating temperature was controlled to be 1570 ° C. The dissolution-reduction time was 30 minutes, and the amount of molten metal was 300 tons and the amount of slag was 40 tons when the addition of DRI was completed.

Next, in the third step, Al ash was added as a deoxidizing agent to perform desulfurization, and after desulfurization, 30 tons of slag was discharged from the drain hole of the DC electric furnace in the fourth step. Then, in the fifth step, acid was sent from an oxygen lance installed in the upper part of the furnace to perform decarburization treatment, and molten steel having a C concentration of 0.05% by mass was produced. In the fifth step, denitrification was promoted along with decarburization, and the N concentration of the produced molten steel was 30 ppm. Then, in the sixth step, the slag produced by the decarburization treatment was discharged from the slag hole. Then, in the seventh step, 20 tons of molten steel was left in the furnace as a seed bath for the next channel, and the remaining 280 tons of molten steel was discharged.

On the other hand, in the case of the two-combustion method, in which the hot metal after thawing and reduction is taken out in one furnace and the decarburization is performed in another furnace, the hot metal from the melting furnace and the hot metal in the decarburization furnace are separated. The charging causes a temperature drop of at least 100 ° C. On the other hand, in this embodiment, there was no heat loss, and the energy intensity could be reduced. Further, since the decarburization was performed from the state where the C concentration was 1.0% by mass, the amount of decarburization was smaller than that of the two-firer method, and the amount of CO ₂ generated could be reduced. Specifically, it is as follows.

In this example, 300 tons of molten iron having a C concentration of 1.0% by mass was decarburized to 0.05% by mass.
300 x (1-0.05) / 100/12 x 22.4 = 5.3 Nm ³
CO ₂ has been generated.
On the other hand, in the case of the two-firer method, when the DRI is reduced, the C concentration is 3.0% by mass, the hot metal is tapped, and the decarburization treatment is performed in another furnace. The reason why the C concentration is set to 3.0% by mass is that if it is lower than 3.0% by mass, there is a heat loss at the time of transfer. This is because it does not reach. In this embodiment, 280 tons of molten steel is discharged, so in the two-firer method, 280 tons of hot metal may be decarburized. Therefore, the amount of _{CO 2 generated is}
280 x (3-0.05) / 100/12 x 22.4 = 16.5 Nm ³
Will be.
As described above, in the case of this example, it was confirmed that the amount of _{CO 2 generated could be reduced as compared with the two-firer method.}

(Comparison example)
First, in the front channel, molten steel was ejected from a DC electric furnace having a hollow electrode and having a furnace diameter of 6 m, and 20 tons of molten steel was left in the DC electric furnace as a seed bath. The C concentration of the molten steel produced in the previous channel was 0.05% by mass. Subsequently, the first step was omitted, and in the second step, DRI having a metallization rate of 75% was added to carry out dissolution reduction. At this time, the operating temperature had to be as high as 1640 ° C., and the dissolution-reduction time was 60 minutes. After that, desulfurization treatment, decarburization treatment and the like were carried out under the same conditions as in the examples.

As described above, in the comparative example, the dissolution-reduction time was twice as long as that in the example, resulting in a decrease in productivity.

According to the present invention, a method for producing molten steel having _{high productivity, low heat loss, and low CO 2} generation when melting and reducing DRI having a particularly low metallization rate in a melting furnace such as an electric furnace can be used. It can be provided and has great industrial value.

Claims (3)

The first step of adding a carbon source to the molten steel left in the electric furnace as a seed hot water when the steel of the previous channel is discharged to obtain carbon-containing molten iron, and
The second step of adding DRI to the carbon-containing molten iron produced in the first step to dissolve and reduce it, and the second step.
Next, the third step of adding a deoxidizing material and performing a desulfurization treatment, and
The fourth step of discharging the desulfurized slag produced by the desulfurization treatment of the third step and
Next, the fifth step of blowing oxygen to perform decarburization treatment and
The sixth step of discharging the decarburized slag produced by the decarburization treatment of the fifth step and
In the sixth step, after discharging the decarburized slag, the seventh step, in which the seed hot water of the next channel is left and the steel is discharged,
A method for producing molten steel, which comprises. When the diameter of the electric furnace is D (m), the amount of seed water W (t) left in the seventh step is 0.3 × D ² <W <1.6 × D ^2. The method for producing molten steel according to claim 1. The method for producing molten steel according to claim 1 or 2, wherein in the 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 obtained.

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