Method for refining molten iron in electric furnace

Abstract

Provided is a method for refining molten iron in an electric furnace that has a coal feed lance for top-blowing a carbonaceous material together with a carrier gas, the method being characterized by satisfying formula (1).　(1): 0.04 ≤ LS / LS0 ≤ 1.85, where: LS is a slag dent depth index (mm) calculated as a solution of formula (2); and LS0 is a slug thickness in a static state. (2): d·V0·(cosθ)2 = 0.015·ρ(S / ρg)1 / 2·((ρS / ρm)·LS + H)·(LS)1 / 2, where: v0 is an apparent injection velocity at a nozzle tip of the coal feed lance; θ is a nozzle angle of the coal feed lance; d is an equivalent circle diameter of an inner surface of the nozzle tip of the coal feed lance; H is a vertical distance between the slag surface in a static state and the nozzle tip of the coal feed lance; ρS is a slug density in a static state; ρg is a standard-state carrier gas density; and ρm is a molten iron density.

Description

Electric furnace refining method for molten iron

[0001] The present invention relates to a method for refining molten iron in an electric furnace.

[0002] One of the issues in the production of high-grade steel using an electric furnace is the increase in the N (nitrogen) concentration in the molten iron during processing in the electric furnace. This is due to the increase in the N concentration in the atmosphere that has been mixed into the furnace. ２ Therefore, a technology to reduce the N concentration in electric furnaces is required.

[0003] One method for reducing the N concentration in an electric furnace is the denitrification reaction, which releases the N dissolved in the molten iron into the atmosphere. However, the inside of an electric furnace has a high concentration of N due to the inclusion of air. ２ The atmosphere is a nitrogen-absorbing atmosphere, and when molten iron comes into contact with the atmosphere, a nitrogen absorption reaction occurs. Therefore, in order to cause a denitrification reaction in an electric furnace, one method is to cause a decarburization reaction in the molten iron, release nitrogen in the molten iron into the CO gas bubbles that are generated, and denitrify the CO bubbles as they rise to the surface. Therefore, it is necessary to top-blow carbonaceous material from a coal-feeding lance to supply carbon into the molten iron, and to react the carbon with the oxygen in the molten iron or the iron oxide in the slag to continuously generate CO gas.

[0004] As described in Patent Document 1, techniques for producing low-nitrogen steel using conventional electric arc furnace steelmaking methods have been attempted, such as increasing the carbon concentration in molten iron during melt-down in the arc furnace to actively promote CO boiling associated with the decarburization reaction during refining, or promoting denitrification (denitrification) by gas agitation of the molten steel with Ar gas or the like. Patent Document 1 also discloses a method for producing high-purity steel using a DC arc electric furnace, characterized in that the electrodes of the DC arc electric furnace are hollow, one or more iron-based raw materials containing 50 mass % or more of iron are melted while a hydrocarbon gas is supplied through the hollow portion, the carbon concentration in the molten iron during melt-down is set to be equal to or higher than the average carbon concentration relative to the iron content of the one or more iron-based raw materials, and then oxygen gas is blown onto the molten iron to perform decarburization.

[0005] Patent Document 2 discloses an iron melting furnace having an inlet for iron raw materials and an outlet for molten iron, and equipped with an oxygen burner, in which a nozzle for spraying a recarburizer carried by a carrier gas toward the surface of the molten metal in the melting furnace is provided with the center line of the tip at an angle of 40 degrees or more with respect to the molten metal surface. This is said to achieve a high carbon absorption efficiency of more than 50%.

[0006] JP2016-108575A JP10-8120A

[0007] "Iron Metallurgical Reaction Engineering" by Kiyoshi Segawa, published by Nikkan Kogyo Shimbun on April 30, 1975, p. 90

[0008] To achieve a low nitrogen concentration in molten iron by supplying a carbonaceous material to the molten iron during refining in an electric furnace and reacting the carbon in the carbonaceous material with oxygen in the molten iron or iron oxide in the slag, it is necessary to increase the denitrification reaction rate through this decarburization reaction. To achieve this, it is effective to increase the decarburization reaction rate and increase the reaction interface area between CO bubbles and the molten iron. Increasing the supply rate of the carbonaceous material is effective for increasing the decarburization reaction rate. However, because the decarburization reaction tends to occur mainly in the narrow area where the carbonaceous material is sprayed, the generated CO bubbles coalesce and increase in volume, which increases their buoyancy from the molten iron. This causes the CO bubbles to rise and detach from the molten iron before contributing to the denitrification reaction, which may prevent the denitrification reaction rate from increasing.

[0009] An object of the present invention is to provide an electric furnace refining method for molten iron, which can expand the region where the decarburization reaction occurs in the depth direction of the molten iron and suppress the coalescence of CO bubbles, thereby improving the denitrification reaction rate, without increasing the supply rate of carbonaceous material. In the present invention, the denitrification reaction rate means the amount of reduction in the nitrogen concentration in the molten iron.

[0010] That is, the gist of the present invention is as follows: [1] A method for refining molten iron in an electric furnace, characterized in that in an electric furnace having a coal feeding lance for top-blowing a carbonaceous material together with a carrier gas, the carbonaceous material supply rate is set to 0.10 kg / min or more and 0.78 kg / min or less per ton of molten iron, and the following formula (1) is satisfied: 0.04≦L Ｓ / LＳ０ ≦1.85...(1) L Ｓ is the slag depression depth index (mm) calculated as the solution of the following equation (2), L Ｓ０ : Slag thickness (mm) when not forming (when left stationary), d・v ０ ・(cosθ) ２ = 0.015 (ρ Ｓ / ρ ｇ ) １／２ ・((ρ Ｓ / ρ ｍ )・L Ｓ + H)・(L Ｓ ) １／２ ... (2) d: Equivalent circle diameter (mm) of the inner surface of the nozzle tip of the coal feeding lance, v ０ : apparent injection velocity of carrier gas at the nozzle tip of the coal feeding lance (m / s), θ: angle (rad) made by the nozzle of the coal feeding lance with the vertical direction, H: vertical distance (mm) between the nozzle tip of the coal feeding lance and the surface of the stationary slag, ρ Ｓ : Slag density when left standing (kg / m ３ ), ρ ｇ : Standard carrier gas density (kg / m ３ ), ρ ｍ : Molten iron density (kg / m ３ [2] The electric furnace refining method for molten iron according to [1], characterized in that the following formula (3) is satisfied: 0.60≦L Ｓ / L Ｓ０ ≦1.50 (3) [3] The method for refining molten iron in an electric furnace according to [1] or [2], characterized in that the thickness of the slag when left to stand is 40 mm or more and 270 mm or less. [4] The method for refining molten iron in an electric furnace according to any one of [1] to [3], characterized in that the electric furnace has an oxygen supply lance for top-blowing oxygen. [5] The method for refining molten iron in an electric furnace according to any one of [1] to [4], characterized in that the number of coal supply lances is 2 or more and 4 or less.

[0011] According to the present invention, in an electric furnace, in an operation in which carbonaceous material is top-blown from a coal-feeding lance, the amount of carbonaceous material penetrating into the molten iron is increased, and the amount of denitrification reaction due to decarburization is increased, thereby making it possible to reduce the N concentration of the molten iron.

[0012] 1 is a vertical cross-sectional view of an electric furnace in which an electric furnace refining method for molten iron according to an embodiment of the present invention is carried out.

[0013] FIG. 1 is a vertical cross-sectional view of an electric furnace in which a method for refining molten iron according to an embodiment of the present invention is implemented. The electric furnace 1 is a three-phase AC electric furnace having a furnace bottom 2 at the bottom of the furnace body and a furnace lid 3 at the top of the furnace body. Three graphite electrodes 4 are inserted into the furnace through the lid 3 as upper electrodes to energize the furnace. The electric furnace 1 is not limited to a specific number of upper electrodes or energization method; it can be implemented as a single-phase AC or DC type, or with a number of electrodes appropriate for each type. Scrap or reduced iron is charged into the electric furnace 1 as an iron source. The raw material charging method is also not limited; for example, continuous charging using a horizontal conveyor, batch charging, or continuous charging using a shaft can be used. During operation, the furnace contains molten iron 5, which is the molten iron source, and slag 6, which is generated from auxiliary materials above the molten iron 5. The slag 6 is discharged through a slag discharge port 7 located on the side wall, and molten steel generated from the molten iron is discharged through a tapping port 8 located at the bottom. The electric furnace 1 further has a coal supply lance 9 used for supplying carbonaceous material, and an oxygen supply lance 10 for supplying oxygen gas into the furnace for the purpose of supplying a heat source by oxidation and refining.

[0014] In the present invention, it was discovered that, rather than increasing the supply rate of the carbonaceous material from the top-blowing coal delivery lance, the injection rate of the carbonaceous material carrier gas is increased, the thickness of the slag at the carbonaceous material injection position is reduced, or the surface of the molten iron is exposed from the slag, thereby allowing the carbonaceous material to penetrate into the molten iron. This spreads the area in which the decarburization reaction occurs in the depth direction of the molten iron, prevents the coalescence of CO bubbles, and increases the amount of denitrification reaction.

[0015] Here, we will explain the terms used in the present invention. In this technical field, "top blowing" refers to blowing a predetermined substance downward from above molten iron. "Carbonous material" refers to a substance produced using a carbon source such as coal or coke as a raw material. The particle size and carbon content of the carbonous material can be adjusted by crushing or firing these carbon source raw materials. In the present invention, the sphere-equivalent diameter of the carbonous material is preferably 0.1 to 4.0 mm, and the carbon content is preferably 60% or more. In the present invention, the "carbonous material supply rate" refers to the amount of carbonous material supplied per unit time (1 minute) per ton of molten iron. "Molten iron" refers to a molten iron alloy, and an iron alloy is an alloy containing 50% or more iron atoms by mass. In particular, an iron alloy with an iron atom content of 70% or more is preferred, with an iron atom content of 85% or more being more preferred.

[0016] However, if the injection speed of the carrier gas from the coal feeding lance is too fast, the area of ​​the molten iron exposed from the slag covering the surface of the molten iron will increase, and the N ２ There is a concern that the nitrogen absorption reaction, in which CO2 dissolves in the molten iron, will also be more likely to proceed. In other words, the effectiveness of reducing the nitrogen concentration varies depending on the balance between the denitrification reaction caused by CO bubbles and the nitrogen absorption reaction caused by the carrier gas. Therefore, there are conditions for an appropriate carrier gas injection speed.

[0017] Here, we will find the relationship between the carrier gas blowing conditions and the depth of the depression in the slag. First, Non-Patent Document 1 presents the following formulas (A) and (B) as formula (5.2) for calculating the relationship between the depth of the depression formed at the position where the carrier gas collides with the molten iron when the gas is blown vertically downward. However, in formula (B), ρ in formula (5.2) ｌ (density of hot water) ρ ｍ (density of molten iron) is used. ０ = α (L + h) L １／２ ...(A) α=0.015・(ρ ｍ / ρ ｇ ) １／２ ... (B) d: Equivalent circle diameter (mm) of the inner surface of the nozzle tip of the coal feeding lance, v ０: apparent injection velocity of carrier gas at the nozzle tip of the coal feeding lance (m / s), L: depth of depression in molten iron (mm), h: distance from the nozzle tip of the coal feeding lance to the surface of molten iron (mm), ρ ｍ : Molten iron density (kg / m ３ ), ρ ｇ : carrier gas density (kg / m ３ )

[0018] In equation (A), the vertical flow velocity of the carrier gas is used for the coal delivery lance that blows obliquely into the electric furnace. Specifically, v·cosθ is used, with θ being the angle that the nozzle makes with the vertical direction. Also, h: the distance (mm) from the nozzle tip of the coal delivery lance to the surface of the molten iron is replaced with H: the distance (mm) from the nozzle tip of the coal delivery lance to the surface of the slag when left standing. The distance from the nozzle tip to the bottom of the recess is L / cosθ+H / cosθ. ρ in the equation ｇ is the carrier gas density at standard conditions, and v ０ is found by dividing the carrier gas flow rate (under standard conditions) by the cross-sectional area of ​​the nozzle tip, which is calculated from the equivalent circle diameter of the inner surface of the nozzle tip. This is because the coal feeding lance is water-cooled up to the vicinity of the nozzle tip, and the gas maintains roughly standard conditions up to the nozzle tip. Note that there are cases where the slag is foaming, in which case the thickness of the slag when not foaming (called static slag) is used. From the above, equation (A) can be rewritten as equation (C), and by transforming equation (C) we obtain equation (C)'. d・v ０ ・cosθ=α・(L / cosθ+H / cosθ)・L １／２ ...(C) d・v ０ ・(cosθ) ２ = α (L + H) L １／２ ...(C)' θ: angle (rad) between the nozzle of the coal feeding lance and the vertical direction, H: distance (mm) from the tip of the nozzle of the coal feeding lance to the surface of the slag when left standing

[0019] In addition, while the formula (A) is intended for dents in molten iron, the present invention is intended for dents in slag, so the dent depth index L of the slag is set to L for the dent depth L of the molten iron in the formula. Ｓ It is necessary to calculate the slag depression depth index L Ｓis calculated by multiplying the depth L of the molten iron by a coefficient β as shown in equation (D). The coefficient β is the density ratio between the slag and the molten iron. Ｓ =L・β...(D) L Ｓ : slag depression depth index (mm), β: coefficient (=ρ ｍ / ρ Ｓ ), ρ Ｓ : Slag density when left standing (kg / m ３ )

[0020] By substituting the equations (B) and (D) into the equation (C)′ obtained by rewriting and transforming the equation (A), the equation (i) is obtained. Ｓ and the thickness of the slag when left standing L Ｓ０ The ratio of "L Ｓ / L Ｓ０ The idea was that by setting " in an appropriate range, it would lead to a low N concentration. ０ ・(cosθ) ２ = 0.015 (ρ Ｓ / ρ ｇ ) １／２ ・((ρ Ｓ / ρ ｍ )・L Ｓ + H)・(L Ｓ ) １／２ ...(i)

[0021] Therefore, in a test using an AC electric furnace, the apparent injection velocity of the carrier gas at the nozzle tip of the coal feeding lance v ０ , the circle equivalent diameter d of the inner surface of the nozzle tip of the coal feeding lance, the angle θ that the nozzle of the coal feeding lance makes with the vertical direction, and the vertical distance H between the nozzle tip of the coal feeding lance and the surface of the stationary slag are variously changed to obtain L Ｓ Search for L Ｓ and the thickness of the slag when left standing L Ｓ０ The ratio of "L Ｓ / L Ｓ０ " was calculated. Then, the change in N concentration in the molten iron before and after spraying of the carbonaceous material was evaluated. As a result, it became clear that a high denitrification reaction rate was achieved under conditions that satisfied the following formula (ii), and more preferably under conditions that satisfied the following formula (iii) (see Examples below). 0.04≦L Ｓ / L Ｓ０≦1.85...(ii) 0.60≦L Ｓ / L Ｓ０ ≦1.50...(iii)

[0022] In addition, by setting the slag thickness within an appropriate range, the behavior of reducing the slag thickness and exposing the surface of the molten iron from the slag is stabilized, thereby increasing the effect of reducing the N concentration. If the slag thickness is excessively small, the fluctuation caused by the carrier gas discharged at high speed from the coal feeding lance may create areas where the slag is not locally present other than the collision position of the carrier gas, and a slight nitrogen absorption reaction may occur. From this perspective, the slag thickness when left to stand is preferably 40 mm or more. The slag thickness when left to stand is more preferably 60 mm or more, and even more preferably 80 mm or more. If the slag thickness is excessively large, the amount of carbonaceous material flying on the outer edge of the carrier gas that directly reacts with FeO in the slag may increase, and the amount of CO bubbles that contribute to the denitrification reaction may decrease. From this perspective, the slag thickness when left to stand is preferably 270 mm or less. The type of carrier gas is not particularly limited, but CO ２ and Ar are preferred, CO ２ is more preferred.

[0023] Furthermore, the carbonaceous material supply rate must be a predetermined rate or higher. This is because, if the carbonaceous material supply rate is lower than the predetermined rate, the decarburization reaction rate becomes insufficient, and the denitrification reaction by CO bubbles hardly occurs. From this perspective, the carbonaceous material supply rate is 0.10 kg / min or higher per ton of molten iron. The carbonaceous material supply rate is preferably 0.20 kg / min or higher. On the other hand, as described above, an object of the present invention is to provide an electric furnace refining method for molten iron that can improve the denitrification reaction rate without increasing the carbonaceous material supply rate. If the carbonaceous material supply rate is 0.78 kg / min or lower per ton of molten iron, the carbonaceous material supply rate is not significantly increased compared to general rates, and this object can be achieved. The carbonaceous material supply rate is preferably 0.60 kg / min or lower. Note that the number of coal feed lances may be one or more, but two to four are preferred. When two or more coal feed lances are used, the carbonaceous material supply rate of at least one lance needs only to be within the above range.

[0024] In addition, by blowing oxygen from the oxygen supply lance upward, oxygen is continuously supplied into the molten iron to generate FeO in the slag, which has the effect of stably causing the decarburization reaction. From this viewpoint, it is preferable to blow oxygen from the oxygen supply lance upward. The oxygen supply rate from the oxygen supply lance is 600 Nm ３ / hr or more 6000Nm ３ / hr or less, and ３ / hr or more is more preferable, and 4000 Nm ３ The number of oxygen supply lances is not particularly limited, but it is more preferable that the number is the same as the number of coal supply lances.

[0025] As mentioned above, the slag density ρ Ｓ , slug thickness L Ｓ０ The values ​​used for the above are those when the slag is left to stand. During forming, the density decreases and the slag thickness increases, but the effect of the decrease in density due to forming and the effect of the increase in slag thickness due to forming exactly cancel each other out, so the values ​​when the slag is left to stand can be used.

[0026] As an example of the present invention, a test was carried out using an AC electric furnace with a capacity capable of processing a maximum of 200 tons of molten iron. In the test, 70 tons of molten iron produced by pre-treatment and 1.3 to 3.8 tons of slag were left in the furnace, and in the actual treatment, 110 tons of scrap was supplied to the furnace over a period of 37 to 43 minutes while current was being applied to the electrodes. The horizontal cross-sectional area inside the electric furnace where the molten iron was located was 33 m ２ Furthermore, CaO and SiO are added to form the molten slag. ２ The raw materials used were 60% CaO and 50% SiO2 in terms of the weight ratio of the simple substance. ２ The amount of slag supplied was 20 kg to 200 kg per ton of molten iron. The density of the slag used this time was 3000 kg / m ３ After the scrap melting is completed, the iron atom content of the molten iron is 96 to 99 mass %, and the density of the molten iron is 7000 kg / m ３ is.

[0027] The density of the slag was measured by the maximum bubble pressure method. This method involves increasing the pressure inside the blowpipe immersed in the molten metal, measuring the maximum pressure when bubbles form at the tip, and calculating the density from the pressure difference. The immersion depth of the blowpipe into the molten metal is defined as ΔH, and the maximum difference between the blowpipe tip and the furnace air pressure is defined as P. ｍａｘ Then, the density ρ of the melt is ρ=P ｍａｘ / ΔH.

[0028] After the scrap was completely melted, the carbonaceous material was blown from the coal supply lance upward while the current was still flowing from the electrode, and oxygen was blown from the oxygen supply lance upward. There was one oxygen supply lance. A test was also conducted in which oxygen was not supplied from the oxygen supply lance. The carbonaceous material blown from the coal supply lance upward had a carbon content of 90% by mass or more and a sphere-equivalent diameter of 0.1 mm to 4.0 mm, with 90% or more of the total weight being carbon. CO was used as the carrier gas for the carbonaceous material upward blowing. ２ CO ２ The density of is 1.96 kg / m under standard conditions. ３ The carbonaceous material supply rate was calculated by measuring the weight change per minute of the container containing the carbonaceous material while the carbonaceous material was being top-blown and dividing the result by the weight of the molten iron. On the other hand, the flow rate of oxygen blown from the oxygen supply lance was 600 Nm ３ / hr~6000Nm ３ / hr. The oxygen referred to here is a gas with an oxygen purity of 90% or more by volume. When top blowing of carbonaceous material from the coal feeding lance and top blowing of oxygen from the oxygen feeding lance were used in combination, they were simultaneously blown for 5 minutes. When top blowing of oxygen from the oxygen feeding lance was not performed, top blowing of carbonaceous material from the coal feeding lance was performed for 5 minutes.

[0029] The apparent injection velocity of the carrier gas at the nozzle tip of the coal feeding lance, v ０ , the circle equivalent diameter d of the inner surface of the nozzle tip of the coal feeding lance, the angle θ that the nozzle of the coal feeding lance makes with the vertical direction, and the vertical distance H between the nozzle tip of the coal feeding lance and the surface of the stationary slag are variously changed, and L is obtained as a solution of equation (i). Ｓ Search for L Ｓ and the thickness of the slag when left standing L Ｓ０ The ratio "L Ｓ / L Ｓ０" are shown in Tables 1 and 2.

[0030] The apparent injection velocity of the carrier gas at the nozzle tip is v ０ was calculated by "gas flow rate / cross-sectional area of ​​the nozzle tip." Ｓ０ was calculated from the slag weight, slag density, and the cross-sectional area of ​​the furnace interior where the slag is located. Ｓ０ The thickness of the molten iron was calculated from the weight of the molten iron, the density of the molten iron, and the cross-sectional area of ​​the furnace interior where the molten iron is located.

[0031] Samples of molten iron were taken before and after top blowing of carbonaceous material from the coal delivery lance, and the N concentration was measured. Tables 1 and 2 show the N concentration before top blowing of carbonaceous material, the N concentration after top blowing of carbonaceous material, and the amount of change in N concentration, ΔN. If ΔN was −0.0001 or less, it was determined that the increase in the denitrification reaction rate, which is the object of the present invention, had been achieved.

[0032] L Ｓ / L Ｓ０ The changes in N concentration are shown in Tables 1 and 2.

[0033] In Tables 1 and 2, values ​​outside the range of the present invention are underlined. In Test Nos. 1 to 17, which are comparative examples, the N concentration was higher after coal feeding than before coal feeding, and nitrogen absorption was observed. Test Nos. 1 to 7 were L Ｓ / L Ｓ０ In this case, the carbonaceous material is trapped in the slag, so even if it reaches the surface of the molten iron, it does not penetrate into the molten iron. As a result, the decarburization reaction occurs only on the surface of the molten iron, and the CO bubbles coalesce and rise to the surface before they can contribute to the denitrification reaction. In addition, in Tests Nos. 8 to 11, the L Ｓ / L Ｓ０ is larger than 1.85, the area of ​​the molten iron exposed from the slag by the carrier gas becomes too large, and the atmosphere N ２These are examples where the amount of nitrogen absorption reaction in which carbonaceous material dissolved in molten iron became large. Test Nos. 11 to 14 are examples where the carbonaceous material supply rate was less than 0.10 kg / (t·min), and the lack of carbonaceous material reduced the amount of decarburization reaction, preventing the denitrification reaction from progressing and resulting in nitrogen absorption. Test Nos. 15 and 16 are examples where coal was fed under conditions where oxygen was not fed from the oxygen feeding lance. As will be described later, there are also examples where oxygen was not fed, but in Test No. 15 the carbonaceous material supply rate was insufficient, and therefore Test No. 16 was fed under conditions where oxygen was not fed from the oxygen feeding lance. Ｓ / L Ｓ０ Test No. 17 is an example in which the number of coal feeding lances is two, but both lances have L Ｓ / L Ｓ０ The amount of nitrogen absorption reaction was large because the

[0034] In Test Nos. 21 to 40, which are examples of the present invention, the N concentration was reduced by 0.0005% or more due to denitrification, and in particular, L Ｓ / L Ｓ０ In Test Nos. 25 to 28, where the N concentration was set to a more preferable range, the N concentration was reduced by 0.0016% to 0.0019%, and in Test Nos. 29 to 31, where the slag thickness was set to a preferable range, the N concentration was reduced by 0.0012% to 0.0015%. Ｓ / L Ｓ０ In Test Nos. 32 to 35, in which the N concentration was set to a more preferable range and the slag thickness was also set to a preferable range, the N concentration was reduced by 0.0025% to 0.0027%, and the L Ｓ / L Ｓ０ The denitrification was particularly promoted by setting the slag thickness within an appropriate range. This is because the slag was too thin when it was less than 40 mm, and the injected carrier gas caused the molten iron and slag to oscillate, creating areas where no slag was present. ２This is because the nitrogen absorption reaction proceeds slightly upon contact with the atmosphere, which has a high concentration of nitrogen, and when the slag is thicker than 270 mm, the carbonaceous material flying outside the carrier gas reacts directly with FeO in the slag, reducing the amount of CO bubbles generated in the molten iron. In Tests No. 36 and 37, coal feeding was performed within the range specified in this invention without oxygen feeding from the oxygen feed lance, and the N concentration was reduced by 0.0009%. In Tests No. 38 to 40, there were multiple coal feed lances, and in each test, only one lance had a coal feed rate within the preferred range, but the N concentration was reduced by 0.0005% to 0.0012%.

[0035] In all of the examples of the present invention, the carbonaceous material supply rate was 0.78 kg / min or less per ton of molten iron, and the objective of improving the denitrification reaction rate was achieved without increasing the carbonaceous material supply rate.

[0036]

[0037]

Claims

1. A method for refining molten iron in an electric furnace, characterized in that in an electric furnace having a coal-feeding lance for top-blowing a carbonaceous material together with a carrier gas, the carbonaceous material supply rate is set to 0.10 kg / min or more and 0.78 kg / min or less per ton of molten iron, and the following formula (1) is satisfied: 0.04≦L Ｓ / L Ｓ０ ≦1.85...(1) L Ｓ is the slag depression depth index (mm) calculated as the solution of the following equation (2), L Ｓ０ : Slag thickness (mm) when not forming (when left stationary), d・v ０ ・(cosθ) ２ = 0.015 (ρ Ｓ / ρ ｇ ) １／２ ・((ρ Ｓ / ρ ｍ )・L Ｓ +H)・(L Ｓ ) １／２ ... (2) d: Equivalent circle diameter (mm) of the inner surface of the nozzle tip of the coal feeding lance, v ０ : apparent injection velocity of carrier gas at the nozzle tip of the coal feeding lance (m / s), θ: angle (rad) made by the nozzle of the coal feeding lance with the vertical direction, H: vertical distance (mm) between the nozzle tip of the coal feeding lance and the surface of the stationary slag, ρ Ｓ : Slag density when left standing (kg / m ３ ), ρ ｇ : Standard carrier gas density (kg / m ３ ), ρ ｍ : Molten iron density (kg / m ３ ) 2. The method for refining molten iron in an electric furnace according to claim 1, wherein the following formula (3) is satisfied: 0.60≦L Ｓ / L Ｓ０ ≦1.50 (3) 3. A method for refining molten iron in an electric furnace according to claim 1 or 2, characterized in that the thickness of the slag when left to stand is 40 mm or more and 270 mm or less.

4. A method for refining molten iron in an electric furnace according to any one of claims 1 to 3, characterized in that the electric furnace has an oxygen supply lance for top-blowing oxygen.

5. A method for refining molten iron in an electric furnace according to any one of claims 1 to 4, characterized in that the number of the coal feeding lances is two or more and four or less.

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