WO2019203278A1

WO2019203278A1 - Molten steel production method

Info

Publication number: WO2019203278A1
Application number: PCT/JP2019/016502
Authority: WO
Inventors: 平田　浩
Original assignee: 日本製鉄株式会社
Priority date: 2018-04-17
Filing date: 2019-04-17
Publication date: 2019-10-24
Also published as: JP6923075B2; CN112004947A; TW201943856A; CN112004947B; TWI698532B; JPWO2019203278A1; KR20200130858A; KR102359738B1

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 method for producing molten steel in which molten iron is produced by reducing and melting reduced iron (DRI) produced by pre-reducing iron oxide (iron ore etc.) in a melting furnace.

Conventionally, since it takes a lot of cost to establish a new blast furnace, in countries where natural gas is produced, iron oxide such as agglomerated iron ore such as pellets is reduced by a shaft furnace, for example, by the Midrex method. The mainstream is a process in which reduced iron (DRI) having a conversion rate of 90% or more is manufactured and the DRI is melted in an electric furnace to directly manufacture molten steel.

In addition, a reduced iron production process using coal and other carbonaceous materials as a reducing agent in place of natural gas has been developed and put into practical use. In this reduced iron production process, calcined pellets of iron ore and the like are heated and reduced together with coal powder in a rotary kiln (SL / RN method), or a mixture of carbonaceous material and powdered iron oxide is agglomerated to form a rotary hearth. There is a method of producing reduced iron by heat reduction (RHF method). In 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 about 85%. Therefore, when this DRI is used, it is necessary to dissolve metallic iron in a melting furnace such as an electric furnace and to reduce the remaining iron oxide content.

In Patent Document 1, a DRI containing 60% or more of metallized iron is produced by the RHF method, and thereafter, molten iron having a carbon content of 1.5 to 4.5% by mass is produced in an arc heating melting furnace, A method is described in which the molten iron is discharged out of the furnace and then desulfurized, dephosphorized and decarburized in another melting furnace. In this method, carbonaceous material is added to the melting furnace in order to reduce the remaining iron oxide content. However, in this method, heat loss is increased by transferring the molten iron to another furnace. Further, when a molten steel is produced by further adding a carbonaceous material to secure a heat source and decarburizing molten iron having a high carbon content, the amount of CO ₂ generated is increased. Furthermore, 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 using hydrocarbon gas.

JP-T-2001-515138 JP 2016-108575 A

In view of the above-mentioned problems, the present invention has high productivity, low heat loss, and low CO ₂ generation amount when melting and reducing DRI having a particularly low metallization rate in a melting furnace such as an electric furnace. It aims at providing the manufacturing method of.

In the present invention, in order to produce a molten steel by dissolving and reducing DRI having a low metallization rate, a part of the molten steel is left in the furnace and used as a seed water for the next channel. However, if the seed hot water remains in the molten steel, the dissolution and reduction of DRI is delayed. Therefore, before supplying DRI, only the carbon source is first supplied to the seed hot water to increase the C concentration of the seed hot 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) a first step of obtaining a carbon-containing molten iron by adding a carbon source to the molten steel left in the electric furnace as seed water at the time of steelmaking in the previous ch;
A second step in which DRI is added to the carbon-containing molten iron produced in the first step to perform dissolution reduction;
Next, a third step of adding a deoxidizer and performing a desulfurization treatment,
A fourth step of discharging the desulfurization slag generated by the desulfurization treatment of the third step;
Next, a fifth step of performing decarburization processing by blowing oxygen,
A sixth step of discharging the decarburized slag generated by the decarburizing process of the fifth step;
After discharging the decarburized slag in the sixth step, the seventh step of leaving the seed ch of the next ch and performing steel output;
The manufacturing method of the molten steel characterized by having.
(2) When the furnace diameter of the electric furnace is D (m), the amount of seed water W (t) remaining in the seventh step is 0.3 × D ² <W <1.6 × D ² The manufacturing method of the molten steel as described in said (1) characterized by these.
(3) In the said 1st process, carbon concentration molten iron with a C density | concentration of 0.5 mass% or more and 1.5 mass% or less is obtained, The manufacture of the molten steel as described in said (1) or (2) characterized by the above-mentioned. 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 with high productivity, low heat loss, and low CO ₂ generation amount is provided. Can be provided.

Drawing 1 is a figure for explaining each process which manufactures 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 view 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 includes at least seven steps from the first step to the seventh step.

First, the seventh step will be described for convenience of explanation. The seventh step is a step of discharging the molten steel whose C concentration has been lowered to, for example, less than 0.1% by mass by the decarburization process of the fifth step. At this time, the entire amount of molten steel is not discharged, but the amount of molten steel used as seed water for the next channel is left in the furnace.

When a DC electric furnace is used as a melting furnace, in the second step to be described later, a voltage is applied between the upper electrode and the lower electrode installed at the bottom of the furnace to generate an arc, and the heat is used for DRI dissolution and reduction. To do. When there is no seed water when applying voltage, electricity flows through the DRI, so the contact resistance between the DRI and the bottom electrode at the bottom of the furnace is large, the arc becomes unstable at the beginning of melting, and the melting time is long. Become. In addition, 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 seed water when applying a voltage, the lower electrode at the bottom of the furnace is in close contact, so that the arc is stable and the melting time can be shortened. For this reason, it is important to leave a part as seed hot water instead of producing the whole amount. Moreover, when the furnace inner diameter in a melting furnace is set to D (m), it is preferable that the seed hot water amount W (t) satisfies the following formula (1).
0.3 × D ² <W <1.6 × D ² (1)

Here, if the seed water amount W is 0.3 × D ² or less, as described above, the contact resistance between the DRI and the bottom electrode of the furnace bottom tends to increase, and the arc may not be stabilized. Moreover, the load of the decarburization process in the 5th process mentioned later will increase that the seed water amount W is 1.6 * D < ² > or more. 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 described later, in the first step, a coal material such as coal (steam coal) or anthracite is added to the furnace, and the molten steel as the seed hot water is molten iron having a predetermined C concentration. To do. There are no particular restrictions on the method of supplying the carbonaceous material, but there is a method of adding free fall from a hopper installed in the upper part of the furnace, a method of supplying the upper electrode from the hollow part as a hollow electrode, and spraying the molten steel using a dedicated lance. There are a method, a method of directly blowing into molten steel using an immersion lance, a method of blowing into molten steel from a bottom blowing tuyer installed for stirring of molten metal, and the like.

Here, in the case of molten steel having a C concentration of the seed hot water of less than 0.1% by mass or the like, the DRI added in the second step cannot be melted unless the melting point of iron is exceeded. Therefore, when the C concentration of the seed hot water remains as molten steel such as less than 0.1% by mass, a large amount of energy is required for melting. Further, the operation temperature is equal to or higher than the melting point of iron, and if the superheat is 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. In addition, when the molten steel with a C concentration of the seed hot water of less than 0.1% by mass remains, the iron oxide remaining in the DRI is not reduced, and high iron oxide concentration slag is generated, which adversely affects the refractory. give. This is particularly noticeable when DRI with a low metalization rate is used.

Therefore, in this embodiment, carburization is performed in the first step, and the seed hot water is made a C-containing molten metal. Thereby, the added metallic iron of DRI is carburized by C in the molten metal, the melting point is lowered, the dissolution rate is accelerated, and the productivity is improved. Also, the operating temperature can be lowered according to the C concentration of the seed hot water, and the load on the refractory is reduced. Moreover, since iron oxide in DRI reacts with C in the seed hot water to promote reduction, the iron oxide concentration in the generated slag is also low. Furthermore, since denitrification is promoted with the decarburization reaction in the fifth step, it is possible to reduce nitrogen. As described above, the productivity can be improved by using the seed hot water as the C-containing molten metal, and the load on the refractory can be reduced.

Here, before entering the second step, the C concentration of the molten iron as the seed hot water is preferably 0.5 mass% or more. This is because when the C concentration is less than 0.5% by mass, the carburizing dissolution rate of metallic iron in DRI and the reduction rate of iron oxide are reduced, and the productivity is deteriorated. On the other hand, if the C concentration of the molten iron becomes too high, the decarburization load increases in the fifth step, which will be described later, and the amount of CO ₂ generated increases. Therefore, it is preferable that the C concentration of the molten iron which is the seed hot water is 1.5% by mass or less.

Next, the second step will be described. In the second step, the DRI manufactured by the shaft furnace or RHF is supplied to the melting furnace, and an arc is generated by applying a voltage between the upper electrode and the lower electrode installed at the bottom of the furnace. And the iron oxide remaining in the DRI is reduced. As a method for supplying DRI, for example, a lump-like material can be added to the furnace by free fall from a hopper installed at the top, and a powdery material can be used in which the upper electrode is a hollow electrode and blown from the hollow portion. 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 and Al ₂ O ₃ , and additionally contains CaO, MgO, S, P ₂ O ₅ and MnO. Further, the metallization rate is such that 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, the metallization rate = mass% M.Fe. /(Mass%M.Fe+mass%FeO×55.75/71.85).

In contrast to the C concentration in the molten iron adjusted in the first step, in the second step, a carbon material such as coal or anthracite is added in accordance with the DRI supply rate. The amount of carbon material introduced here is the sum of the amount necessary for carburizing the iron content in the DRI to the C concentration of the molten iron and the amount necessary for reducing the iron oxide (FeO, etc.) in the DRI. Base. Examples of the carbon material input in the second step include general coal and anthracite as in the case of the carbon material input in the first step. Table 2 below shows examples of the composition of steam coal, and Table 3 shows examples of the composition of anthracite coal. FC in Table 2 and Table 3 represents fixed carbon (Fixed 揮発 Carbon), and VM represents a volatile component (Volaile Matter). In the first step and the second step, steam coal and anthracite may be used alone or in combination. Moreover, it is also possible to use carbon sources such as waste plastic and biomass as other carbon materials.

The operating temperature is necessary for carburizing the amount of carbon material introduced in the second step and the iron content in the DRI 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 the molten iron, which depends on the difference between the amount and the sum required to reduce iron oxide (such as FeO) in the DRI. FIG. 2 is an Fe—C phase diagram showing the change in the melting point of iron with the C concentration. In order to stabilize the operation, it is said that superheat is required to be 100 ° C. or more. For example, in order to operate at superheat 100 ° C., the melting point is 1430 in the case of molten iron having a C concentration of 1.5 mass%. Since it is ° C., the operating temperature is 1530 ° C. In the second step, a voltage is applied according to the supply speed of the carbonaceous material and DRI so as to maintain this operating temperature determined by 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. It is more preferable to control within the range of 5 mass% or more and 1.5 mass% or less.

Next, the third step will be described. Iron ore and coal contain sulfur, although the content varies depending on the production area. Since the iron oxide in the DRI is not reduced instantaneously, the iron oxide concentration in the slag is high immediately after the end of the DRI charging. 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 metal) and the slag is low, and more sulfur is present in the metal than in the slag. In the decarburization process of the fifth step described later, sulfur in the metal is difficult to remove. Therefore, if the third step and the fourth step described later are omitted, the sulfur concentration of the molten steel after the decarburization process is high, and the low-sulfur steel Not satisfying manufacturing needs. Moreover, since sulfur is a surface active component, it occupies the adsorption site. Therefore, if the sulfur concentration in the metal is high, it is difficult to remove nitrogen from the metal, and the need for low-nitrogen steel production is not satisfied. For this reason, it is important to perform the desulfurization treatment after the second step.

After the end of the second step (end of DRI supply), in the third step, a deoxidizer such as metal Al or a metal Al-containing material is added to the furnace to reduce the iron oxide content in the slag, and the oxygen in the molten iron Remove. In this state, the sulfur distribution between the slag and the metal becomes high, the sulfur shifts 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 lower electrode at the bottom of the furnace is a positive electrode. However, if the upper electrode is applied as a positive electrode and the lower electrode at the furnace bottom is applied as a negative electrode, it is electrochemically applied. Apparent sulfur distribution can be increased, and desulfurization can be further promoted.

Next, the fourth step will be described. If the desulfurization slag formed by the desulfurization in the third step is left as it is, desulfurization slag is discharged from the exhaust hole in the fourth step because sulfur is transferred again from the slag to the metal (resulfurization). To do.

Next, the fifth step will be described. In the fifth step, an oxygen lance is inserted into the furnace from the top of the furnace, and oxygen is blown to the molten iron to perform dephosphorization and decarburization, thereby reducing the phosphorus concentration and carbon concentration to a predetermined level. In the decarburization process, oxygen and carbon in the molten iron react 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. The

Next, the sixth step will be described. The sixth step is a step of discharging the decarburized slag generated in the fifth step. In the dephosphorization process and the decarburization process in the fifth step, phosphorus in the molten iron moves to slag. If decarburization slag is discharged and phosphorus is not discharged out of the system, phosphorus is concentrated and low P steel cannot be manufactured. For this reason, decarburization slag needs to be discharged as much as possible.

As described above, the first to seventh steps in the present embodiment can produce molten steel with reduced heat loss and reduced CO ₂ generation. In particular, in the first step, by adding a carbon source to the seed hot water to obtain molten iron containing carbon, it is possible to increase the dissolution rate and reduction rate of DRI and reduce heat loss. Thereby, addition of the carbon material for ensuring a heat source can be suppressed, and as a result, the amount of CO ₂ generation can also be suppressed. Tables 4 and 5 below show the metal composition and slag composition in each step, respectively.

As shown in Table 4, in the present embodiment, in the second step, a carbon material such as coal or anthracite is further added in accordance with the DRI supply rate, and the C concentration in the molten iron is 0.1 to 1.5 mass%. It becomes the range. By suppressing the C concentration, the amount of CO ₂ generated by the decarburization process can also be suppressed.

Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this one condition example. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

First, in the previous channel, molten steel was discharged from a DC electric furnace having a furnace diameter of 6 m having a hollow electrode, and 20 t of molten steel was left as seed water in the DC electric furnace. The C concentration of the molten steel produced in the previous ch was 0.05% by mass. In the first step, carbon 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 sub lance probe incorporating a C sensor for measuring the C concentration by thermal analysis. Carburized until.

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

Next, in the third step, Al ash was added as a deoxidizer to perform desulfurization. After desulfurization, 30 t of slag was discharged from the exhaust hole of the DC electric furnace in the fourth step. Thereafter, in the fifth step, decarburization treatment was carried out by sending oxygen from an oxygen lance installed in the upper part of the furnace to produce molten steel having a C concentration of 0.05% by mass. In the fifth step, denitrification was promoted together with decarburization, and the produced molten steel had an N concentration of 30 ppm. In the sixth step, the slag generated by the decarburization process was discharged from the exhaust hole. Thereafter, in the seventh step, the molten steel 20 t was left in the furnace as the seed ch for the next ch, and the remaining 280 t of molten steel was produced.

On the other hand, in the case of the two-furnace system in which hot metal after smelting and reduction is discharged without decarburization in one furnace and decarburization treatment is performed in another furnace, The charging causes a temperature drop of at least 100 ° C. On the other hand, in this example, there was no heat loss, and the energy intensity could be reduced. Moreover, since the decarburization was performed from a state where the C concentration was 1.0% by mass, the decarburization amount was small as compared with the two-furnace method, and the CO ₂ generation amount could be reduced. Specifically, it is as follows.

In this example, 300 t of molten iron having a C concentration of 1.0% by mass was decarburized to 0.05% by mass,
300 × (1-0.05) /100/12×22.4=5.3 Nm ³
CO ₂ is generated.
On the other hand, in the case of the two-furnace system, when reducing DRI, the C concentration is set to 3.0% by mass, and the molten iron is taken out and decarburized in a separate furnace. The reason why the C concentration is 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. It is because it does not reach. In this embodiment, since 280 t of molten steel is produced, the 280 t hot metal may be decarburized in the 2-furnace system. Therefore, the amount of CO ₂ generated is
280 × (3-0.05) /100/12×22.4=16.5 Nm ³
It becomes.
As described above, in the case of this example, it was confirmed that the amount of generated CO ₂ could be reduced as compared with the two-furnace method.

(Comparative example)
First, in the previous channel, molten steel was discharged from a DC electric furnace having a furnace diameter of 6 m having a hollow electrode, and 20 t of molten steel was left as seed water in the DC electric furnace. The C concentration of the molten steel produced in the previous ch 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 and dissolution reduction was performed. At this time, the operation temperature had to be as high as 1640 ° C., and the dissolution and reduction time took 60 minutes. Thereafter, desulfurization treatment, decarburization treatment, and the like were performed under the same conditions as in the examples.

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

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 with high productivity, low heat loss, and low CO ₂ generation amount is provided. It can be provided and has great industrial value.

Claims

A first step of obtaining a carbon-containing molten iron by adding a carbon source to the molten steel left in the electric furnace as a seed hot water at the time of steelmaking in the previous ch;
A second step in which DRI is added to the carbon-containing molten iron produced in the first step to perform dissolution reduction;
Next, a third step of adding a deoxidizer and performing a desulfurization treatment,
A fourth step of discharging the desulfurization slag generated by the desulfurization treatment of the third step;
Next, a fifth step of performing decarburization processing by blowing oxygen,
A sixth step of discharging the decarburized slag generated by the decarburizing process of the fifth step;
After discharging the decarburized slag in the sixth step, the seventh step of leaving the seed ch of the next ch and performing steel output;
The manufacturing method of the molten steel characterized by having.
When the furnace diameter of the electric furnace is D (m), the amount of seed hot water W (t) left in the seventh step is 0.3 × D 2 <W <1.6 × D 2 The manufacturing method of the molten steel of Claim 1 to do.
3. The method for producing molten steel according to claim 1, wherein in the first step, carbon-containing molten iron having a C concentration of 0.5 mass% to 1.5 mass% is obtained.