JP7667423B2 - Method for producing molten steel and method for removing slag - Google Patents

Description

The present invention particularly relates to a method for producing molten steel and a method for discharging slag, which efficiently discharging slag from a slag discharge hole while leaving the molten iron in the melting furnace.

Traditionally, because of the high costs involved in building new blast furnaces, the mainstream process in countries that produce natural gas is to reduce pelletized iron ore in a shaft furnace to produce reduced iron (DRI) with a reduction rate of 90% or more, and then melt the DRI in an electric furnace to directly produce molten steel. On the other hand, in countries that do not produce natural gas, methods are being developed to produce DRI by gasifying coal and using the gas instead of natural gas, or, in cases where blast furnaces are available, using COG gas discharged from coke ovens, or LDG gas discharged from converters.

Patent Document 1 describes a method for producing molten steel in which oxygen is supplied in combination with an auxiliary heat source such as coke when a cold iron source containing DRI is melted using an arc melting facility. However, since the amount of oxygen supplied is 25 ^Nm3 /t or more, it is understood that the purpose of this method is to burn coke with oxygen and use the reaction heat to melt the scrap, thereby reducing the electricity consumption rate.

Patent Document 2 discloses a method for producing molten steel in which iron ore is reduced in a preliminary reduction furnace to produce semi-reduced iron, the semi-reduced iron produced is sequentially charged into a number of melting reduction furnaces, and the semi-reduced iron is melted and reduced in a high-temperature reducing atmosphere in each melting reduction furnace, after which a decarburization process is performed to produce molten steel. Reduction requires C, as shown in the reaction formula FeO+C=Fe+CO, so carbonaceous material is supplied, but in this method, in addition to the carbonaceous material for reduction, the amount of heat required for reduction is supplied by burning the carbonaceous material with oxygen.

Japanese Patent Application Publication No. 10-292990 Japanese Unexamined Patent Publication No. 59-107013 JP 2020-139180 A JP 2020-139661 A JP 2020-139662 A

In recent years, there has been an increasing need to reduce the concentrations of impurities such as P and S as much as possible, and various methods have been proposed to reduce the concentrations of these components at the molten iron stage.

Both the method described in Patent Document 1 and the method described in Patent Document 2 use the heat of oxidation of carbonaceous material as a heat source. However, because carbonaceous material contains sulfur, the more carbonaceous material is used, the higher the S concentration in the molten steel. It is known that when carbonaceous material is burned with oxygen and the heat generated is used to reduce ore, the S concentration in the molten iron is 0.05 to 0.1 mass%. Since desulfurization during decarburization refining is hardly expected, the S concentration in the steel produced by these methods is high. In recent years, there has been a growing need for low-sulfur steel, and it is not possible to produce products as is.

As a countermeasure, the amount of carbonaceous material used can be reduced by using an electric furnace for the melting furnace and supplying the energy required for reduction with electrical energy, but as long as carbonaceous material is used as a reducing agent, the S concentration in the molten steel will be high, and although it depends on the pre-reduction rate of the DRI, the S concentration is estimated to be 0.04 to 0.05 mass%. Given the growing need for low-sulfur steel in recent years, it still cannot be used as is to make a product.

In addition, in the case of a normal DC electric furnace, the upper electrode is made negative and the bottom electrode is made positive to prevent wear on the upper electrode. On the other hand, it is known that if the upper electrode is made positive and the bottom electrode is made negative, the electrons emitted from the negative electrode electrochemically increase the apparent S distribution ratio, ionizing the S in the molten iron and transferring it to the slag, thereby promoting desulfurization.

However, when the slag is discharged while leaving the molten iron, if the discharge of slag with a high sulfur content is insufficient, the sulfur from the remaining slag will migrate to the molten iron in the next decarburization process, making it impossible to produce low-sulfur steel. Furthermore, if the time taken to discharge the slag is increased, more slag can be discharged, but this also reduces productivity, so there is a limit to the time that can be discharged.

Patent documents 3 to 5 disclose a method of directing slag to a slag discharge port by spraying inert gas from a lance toward the slag surface. However, with these methods, although the slag can be efficiently removed from the area where the inert gas is sprayed and near the slag discharge port, some of the slag is also directed mainly to the opposite side of the slag discharge port, which makes it easier for the slag to remain in a position far from the slag discharge port.

In view of the above problems, the present invention aims to provide a method for producing molten steel and a method for removing slag that reduce impurities from remaining slag.

The gist of the present invention for solving the above problems is as follows.
(1) A method for producing molten steel by melting and reducing reduced iron, which is produced by pre-reducing iron ore, in a melting furnace, comprising the steps of:
energy required for the melting and reduction is supplied by an arc, and the reduced iron is reduced using a carbonaceous material;
When the reduced molten iron is left in the melting furnace and the slag generated in the production of the molten iron is discharged from a slag discharge hole arranged on the side of the melting furnace, a swivellable lance arranged on the upper part of the melting furnace is used to change the blowing angle and position of the lance on the slag surface , the range of the blowing position being changed is from a position directly opposite the slag discharge hole to a range of 30° to 90° on one side along the furnace wall surface, and the slag is discharged by blowing an inert gas onto the slag surface so that the slag moves along the furnace wall surface toward the slag discharge hole,
Thereafter, the inert gas is switched to oxygen gas, and the oxygen gas is sprayed onto the molten iron to perform dephosphorization and decarburization.
(2) A method for producing molten steel as described in (1) above, characterized in that the pressure inside the melting furnace is increased by closing a damper installed in an exhaust gas duct of the melting furnace, thereby discharging the slag.
(3) A method for producing molten steel according to the above (2), characterized in that the pressure inside the melting furnace during slag removal is 1.1 atm or more.
(4) A method for discharging slag from a melting furnace while leaving molten iron remaining therein, comprising the steps of: discharging slag generated during the production of molten iron from a slag discharge hole arranged on the side of the melting furnace; changing the blowing angle and position of the lance toward the slag surface from a rotatable lance installed at the top of the melting furnace ; changing the range of the blowing position from a position directly opposite the slag discharge hole to 30° to 90° on one side along the furnace wall surface, centered on a position directly opposite the slag discharge hole; and blowing inert gas toward the slag surface so that the slag moves toward the slag discharge hole along the furnace wall surface to discharge the slag.
(5) A method for discharging slag according to the above (4), characterized in that the pressure inside the melting furnace is increased by closing a damper installed in an exhaust gas duct of the melting furnace, thereby discharging the slag.
(6) A method for removing slag according to the above (5), characterized in that the pressure inside the melting furnace during removal of slag is 1.1 atm or more.

According to the present invention, it is possible to produce molten steel with fewer impurities migrating from the remaining slag to the molten iron.

FIG. 1 is a diagram for explaining a method for producing molten iron and molten steel in a DC electric furnace according to an embodiment of the present invention. FIG. 1 is a cross-sectional view of a DC electric furnace as seen from above.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram for explaining a method for producing molten iron and molten steel using a DC electric furnace according to this embodiment.
First, reduced iron (DRI), a carbonaceous material such as coal, and quicklime for adjusting basicity are charged into a DC electric furnace 1, and as shown in Fig. 1, an arc is formed from an upper electrode 2 to a lower electrode 3, and the heat generated by this arc reduces and melts the DRI to produce molten pig iron. In a normal DC electric furnace, the upper electrode is made a negative electrode and the lower electrode is made a positive electrode to prevent wear of the upper electrode, but in this embodiment, the upper electrode 2 is made a positive electrode and the lower electrode 3 is made a negative electrode, thereby electrochemically increasing the apparent S distribution ratio and transferring S in the molten iron 4 to the slag 5 to promote desulfurization. That is, S in the molten pig iron 4 is ionized to facilitate the transfer of S to the slag 5.

After this, the upper electrode 2 is raised, and the slag 5 generated by reduction and melting is discharged from the slag discharge hole 6 installed on the side of the DC electric furnace 1 while leaving the molten iron 4 in the DC electric furnace 1. At this time, as shown in Figure 1, inert gas is blown onto the slag surface from the lance 7 installed at the top of the DC electric furnace 1 so that the slag 5 is directed toward the slag discharge hole 6. This lance 7 is structured so that its angle and direction can be freely changed, and by blowing at different angles and directions, the slag 5 is pushed sequentially into the slag discharge hole 6. Figure 2 is a view of the furnace cross section seen from above. When changing the angle and direction of the lance 7, for example, the blowing position is changed from position A on the furnace wall on the center line of the furnace along the furnace wall surface to the opposite position B at a predetermined speed through the opposite side of the slag discharge hole 6, and then the blowing position is changed counterclockwise to position A. This operation is repeated while the slag is being discharged. By changing the blowing position from position A to position B along the furnace wall surface, the slag moves along the furnace wall surface to the slag discharge hole 6, but some of the slag moves past the slag discharge hole 6 and reaches the vicinity of position A. Therefore, by changing the blowing position to position A along the furnace wall surface in a counterclockwise direction after reaching position B, the slag that has reached position A is pushed back into the slag discharge hole 6. By repeating this operation, the slag thickness on the side opposite the slag discharge hole 6 becomes smaller than that on the slag discharge hole 6 side, and the slag can be efficiently discharged. The angle and direction of the lance 7 may be changed in a manner other than this operating procedure.

In the example shown in Figure 2, the spraying position is changed within a range of 180 degrees from position A to position B (90 degrees to the left and right from the position directly opposite the slag discharge hole 6) centered on the position opposite the slag discharge hole 6. The range for changing the spraying position is preferably 30 to 90 degrees on either side from the position directly opposite the slag discharge hole 6. If the spraying range is less than 30 degrees on either side, the effect of pushing the slag into the slag discharge hole 6 will be insufficient, and if the spraying range is more than 90 degrees on one side, the slag will pass through the slag discharge hole 6 and reach the area near the opposite side of the slag discharge hole 6, which will actually reduce the efficiency of slag discharge.

In addition, by increasing the pressure inside the DC electric furnace 1 by closing the damper 9 installed in the exhaust gas duct 8 while spraying inert gas from the lance 7, it becomes possible to discharge the slag even more efficiently. Generally, the pressure inside the furnace is higher than the atmospheric pressure outside the furnace due to the static pressure of the slag, and this pressure difference causes the slag to be discharged from the slag discharge hole. As the slag discharge progresses, the thickness of the slag decreases and the static pressure decreases, so the pressure difference between the inside and outside of the furnace becomes smaller and the discharge speed decreases. Therefore, by increasing the pressure inside the furnace, the pressure difference inside and outside the furnace can be secured and the decrease in the discharge speed can be suppressed.

At this time, it is desirable to set the pressure inside the furnace to 1.1 atm (approximately 1.1 x ¹⁰⁵ Pa) or more. If it is less than 1.1 atm, assuming that the atmospheric pressure is 1.0 x ¹⁰⁵ Pa (1 atm), the effect of increasing the amount of discharged slag is small because the pressure difference between the inside and outside of the furnace is small. It is preferably 1.3 atm or more. In this way, in this embodiment, it is possible to reduce the amount of slag with a high S concentration remaining in the furnace after discharge of slag compared to the conventional method.

Then, the gas supplied from the lance 7 is switched from inert gas to oxygen gas, and oxygen is pumped so that the oxygen jet reaches the surface of the molten pig iron bath, producing molten steel with a C concentration of, for example, 0.05 mass%. Because this dephosphorization and decarburization process is carried out in an oxidizing atmosphere, the molten steel is hardly desulfurized. Once this dephosphorization and decarburization process is completed, the molten steel is discharged from the tapping hole 10 into a container such as a ladle, and, if necessary, undergoes secondary refining before being supplied to a continuous caster or the like.

As described above, in this embodiment, the slag can be efficiently removed, so that the S concentration in the molten steel can be reduced before tapping. Note that, in this embodiment, an example of removing the slag generated after the reduction of DRI has been described, but the above-mentioned slag removal method can be applied to all processes in which only the slag is removed without tilting the furnace and leaving the molten iron. For example, it can also be applied when removing the slag generated by the dephosphorization process in a method of producing molten steel without simultaneously or consecutively performing dephosphorization and decarburization. This can also reduce the amount of P rephosphorization in the molten steel from the remaining slag.

Next, the present invention will be further explained based on an example. However, the conditions in the example are only an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Example 1
First, the pellets were reduced in a shaft furnace to produce DRI with a reduction rate of 90%. Next, 100 tons of this DRI was charged into a 100-ton DC electric furnace, and melted and reduced using 10 tons of coal with the composition shown in Table 1 as a reducing agent to produce molten pig iron at a temperature of 1400°C with [C] = 3.5 mass%. At this time, an arc was generated with the upper electrode as the positive electrode and the lower electrode as the negative electrode, and quicklime was added so that the slag basicity was 1.3. The S concentration in the produced molten pig iron was 0.0033 mass%, and the S concentration in the slag was 0.33 mass%. The amount of slag was 20 tons.

After the melting and reduction of DRI was completed, the lance installed at the top of the DC electric furnace was lowered, and _N2 gas was blown onto the slag surface at 26 ^Nm3 /h/ ^m2 (flow rate per furnace cross-sectional area) so that the slag would move toward the discharge hole, and slag removal was performed for 5 minutes. At this time, the angle and direction of the lance were changed as shown in Figure 2. At this time, as shown in Figure 2, the blowing position was changed 180 degrees (90 degrees on either side from the position directly opposite the slag discharge hole 6) from position A on the furnace wall part on the furnace center line to the opposite position B clockwise at a speed of 10 rpm along the furnace wall surface. After that, the blowing position was changed counterclockwise to position A. As a result of repeating this operation, the amount of slag removed was 15 t.

After that, the gas blown from the lance was changed from _N2 to _O2 to perform dephosphorization and decarburization. At this time, CaO was added to generate slag with a slag basicity of 4.5. Then, when [C] reached 0.05 mass%, oxygen supply was stopped and molten steel was tapped from the tap hole installed at the bottom of the hearth. The S concentration in the molten steel at this time was 0.0040 mass%.

Example 2
First, the pellets were reduced in a shaft furnace to produce DRI with a reduction rate of 90%. Next, 100 tons of this DRI was charged into a 100-ton DC electric furnace, and melted and reduced using 10 tons of coal with the composition shown in Table 1 as a reducing agent, producing molten pig iron at a temperature of 1400°C with [C] = 3.5%. At this time, an arc was generated with the upper electrode as the positive electrode and the lower electrode as the negative electrode, and quicklime was added so that the slag basicity was 1.3. The S concentration in the produced molten pig iron was 0.0034 mass%, and the S concentration in the slag was 0.32 mass%. The amount of slag was 20 tons.

After the melting and reduction of DRI was completed, the lance installed at the top of the DC electric furnace was lowered, and _N2 gas was blown onto the slag surface at 26 ^Nm3 /h/ ^m2 (flow rate per furnace cross-sectional area) so that the slag would move toward the discharge hole, and slag removal was performed for 5 minutes. At this time, the angle and direction of the lance were changed as shown in Figure 2. At this time, as shown in Figure 2, the blowing position was changed 160 degrees (80 degrees on each side from the position directly opposite the slag discharge hole 6) clockwise along the furnace wall surface from position A on the furnace center line to opposite position B at a speed of 10 rpm. Then, the blowing position was changed counterclockwise to position A. This operation was repeated. Furthermore, 2 minutes after the start of slag removal, the damper of the exhaust gas duct was closed, and the furnace pressure was maintained at 1.3 atm (1.3 x ¹⁰⁵ Pa). As a result, the amount of slag discharged after 5 minutes was 17 t.

After that, the gas blown from the lance was changed from _N2 to _O2 to perform dephosphorization and decarburization. At this time, CaO was added to generate slag with a slag basicity of 4.5. Then, when [C] reached 0.05 mass%, oxygen supply was stopped and molten steel was tapped from the tap hole installed at the bottom of the furnace. The S concentration in the molten steel at this time was 0.0035 mass%.

(Comparative Example 1)
First, the pellets were reduced in a shaft furnace to produce DRI with a reduction rate of 90%. Next, 100 tons of this DRI was charged into a 100-ton DC electric furnace, and melted and reduced using 10 tons of coal with the composition shown in Table 1 as a reducing agent, producing molten pig iron at a temperature of 1400°C with [C] = 3.5%. At this time, an arc was generated with the upper electrode as the positive electrode and the lower electrode as the negative electrode, and quicklime was added so that the slag basicity was 1.3. The S concentration in the produced molten pig iron was 0.0033 mass%, and the S concentration in the slag was 0.32 mass%. The amount of slag was 20 tons.

After the dissolution and reduction of DRI was completed, slag was discharged from the slag discharge hole for 5 minutes without gas flowing from the lance. As a result, the amount of slag discharged was 10 tons.

Then, a lance installed at the top of the DC electric furnace was lowered to blow _O2 onto the molten iron to perform dephosphorization and decarburization. At this time, CaO was added to generate slag with a slag basicity of 4.5. Then, when [C] reached 0.05 mass%, oxygen supply was stopped and molten steel was tapped from a tap hole installed at the bottom of the furnace. The S concentration in the molten steel at this time was 0.021 mass%.

Reference Signs List 1 DC electric furnace 2 Upper electrode 3 Lower electrode 4 Molten iron 5 Slag 6 Slag discharge hole 7 Lance 8 Exhaust gas duct 9 Damper 10 Steel tapping hole

Claims (6)

A method for producing molten steel by melting and reducing reduced iron produced by pre-reducing iron ore in a melting furnace, comprising the steps of:
energy required for the melting and reduction is supplied by an arc, and the reduced iron is reduced using a carbonaceous material;
When the reduced molten iron is left in the melting furnace and the slag generated in the production of the molten iron is discharged from a slag discharge hole arranged on the side of the melting furnace, a swivellable lance arranged on the upper part of the melting furnace is used to change the blowing angle and position of the lance on the slag surface , the range of the blowing position being changed is from a position directly opposite the slag discharge hole to a range of 30° to 90° on one side along the furnace wall surface, and the slag is discharged by blowing an inert gas onto the slag surface so that the slag moves along the furnace wall surface toward the slag discharge hole,
a step of: switching the inert gas to oxygen gas after the completion of the slag removal; and spraying the oxygen gas onto the molten iron to perform dephosphorization and decarburization. The method for producing molten steel according to claim 1, characterized in that the pressure in the melting furnace is increased by closing a damper installed in the exhaust gas duct of the melting furnace, thereby discharging the slag. The method for producing molten steel according to claim 2, characterized in that the pressure inside the melting furnace during slag removal is 1.1 atm or more. A method for discharging slag from a melting furnace while leaving molten iron in the furnace, comprising the steps of: discharging slag generated during the production of molten iron from a slag discharge hole arranged on the side of the melting furnace; changing the blowing angle and position of the lance toward the slag surface from a rotatable lance installed at the top of the melting furnace ; changing the range of the blowing position from a position directly opposite the slag discharge hole to 30° to 90° on one side along the furnace wall surface, centered on a position directly opposite the slag discharge hole; and blowing inert gas toward the slag surface so that the slag moves toward the slag discharge hole along the furnace wall surface to discharge the slag. The method for removing slag according to claim 4, characterized in that the pressure inside the melting furnace is increased by closing a damper installed in the exhaust gas duct of the melting furnace to remove the slag. The slag removal method according to claim 5, characterized in that the pressure inside the melting furnace during removal is 1.1 atm or more.

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