Production method of molten steel

Abstract

To disclose a production method of molten steel in which slag can be properly foamed, in which molten steel is produced by using an electric furnace having a lance capable of supplying oxygen gas and a charcoal material into the furnace, dissolving raw materials using an arc as a main heat source while continuously supplying solid reduced iron into the furnace, and subsequently oxidizing and refining molten iron with oxygen gas.SOLUTION: Disclosed is a production method of molten steel comprising, in an electric furnace 1, a first step in which a raw material is melted while solid reduced iron, oxygen gas, and a charcoal material are continuously supplied into the furnace, and a second step in which molten iron is oxidized and refined while the oxygen gas and the charcoal material are continuously supplied into the furnace to produce molten steel. A feed rate of the oxygen gas and a feed rate of the charcoal material into the furnace in the first step are made smaller than the feed rate of the oxygen gas and the feed rate of the charcoal material into the furnace in the second step.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing molten steel in which solid reduced iron is used as a main raw material and the raw material is melted and refined in an electric furnace to obtain molten steel. [Background technology]

[0002] In the blast furnace and converter process in the steel industry, iron ore is used as the iron source, and molten pig iron is produced by reducing the iron ore with coke in a blast furnace, and the molten pig iron is refined in a converter to produce molten steel. Carbon dioxide is emitted during the reduction of iron ore. In contrast, in the electric furnace process, scrap is used as the iron source, and molten steel is obtained by melting the scrap using electricity. From the perspective of reducing the emission of carbon dioxide, a greenhouse gas, attention is being paid to the replacement of the blast furnace and converter process with the electric furnace process. However, from the perspective of replacing the blast furnace and converter process with the electric furnace process, it is necessary to use iron ore as a raw material in the replaced electric furnace process. Therefore, it is expected that solid reduced iron, which is made by directly reducing iron ore, will be used as the iron source supplied to the electric furnace instead of or in addition to scrap. Therefore, in steel production using electric furnaces, it will be necessary to use solid reduced iron as a new raw material and to make the productivity and impurity restrictions equivalent to those of the blast furnace and converter process in the future.

[0003] In the operation of an electric furnace, it is important to control the foaming of slag in the furnace. Foaming is a phenomenon in which the slag in the furnace bubbles due to gases such as carbon monoxide generated in the furnace, and foaming significantly increases the volume of the slag. The increase in slag volume caused by foaming can cover the arc used as a heat source in the electric furnace, which has the effect of improving the efficiency of the energy input to the electric furnace. Therefore, insufficient foaming must be avoided, as it leads to a deterioration in energy efficiency and reduces productivity. On the other hand, excessive foaming leads to an increase in production costs because it leads to the excessive use of oxygen gas and carbonaceous materials to generate the carbon monoxide gas required for foaming. Therefore, the challenge in steel production in electric furnaces is to properly control foaming.

[0004] Patent Document 1 discloses a method for producing molten steel using solid reduced iron obtained from low-grade iron ore as a raw material. The solid reduced iron contains SiO2 and Al2O3 in total at 3.0 mass% or more and carbon at 1.0 mass% or more. The method includes a slag separation step in which the solid reduced iron is heated and melted in an electric furnace without introducing oxygen, separated into molten steel and slag, and the slag is continuously discharged from the electric furnace, and a decarburization step in which the molten steel is decarburized in the electric furnace by blowing the entire amount of oxygen introduced into the electric furnace onto the molten steel after the slag separation step. It is described that by making the carbon content of the solid reduced iron 1.0 mass% or more, FeO and carbon in the solid reduced iron react to generate a sufficient amount of CO gas, and the slag can be sufficiently foamed without introducing oxygen into the electric furnace. Patent Document 2 has a similar purpose. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-080540 [Patent Document 2] JP 2021-102798 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the method for producing molten steel that is the subject of the present invention, the electric furnace process includes a first process for melting raw materials and a second process for oxidizing and refining the molten iron to obtain molten steel. In the first process, solid reduced iron is continuously supplied to the electric furnace, and the solid reduced iron is melted using an arc as the main heat source, and oxygen gas and carbonaceous material are supplied simultaneously, and the heat generated by the combustion of the carbonaceous material is used as an auxiliary source. In the second process, after the first process, oxygen gas is supplied into the furnace together with the carbonaceous material to perform oxidizing and refining, thereby obtaining molten steel with the desired composition. In both the first and second processes, it is necessary to appropriately foam the slag in the electric furnace.

[0007] In the inventions described in Patent Documents 1 and 2, the slag separation step is a step belonging to the first step of the present invention. As described above, Patent Documents 1 and 2 state that the slag can be sufficiently foamed in the slag separation step without introducing oxygen into the electric furnace. However, the inventors have found that the slag cannot be sufficiently foamed if oxygen is not introduced into the electric furnace.

[0008] An object of the present invention is to provide a method for producing molten steel which clarifies the relationship between the oxygen gas supply rate and the carbonaceous material supply rate in the first and second steps and which can appropriately foam slag in both the first and second steps. [Means for solving the problem]

[0009] That is, the gist of the present invention is as follows. [1] A method for producing molten steel from solid reduced iron as a raw material using an electric furnace having a lance capable of supplying oxygen gas and carbonaceous material into the furnace, comprising: a first step of melting the raw materials using an arc as a main heat source while continuously supplying the solid reduced iron, oxygen gas, and carbonaceous material into the furnace; and a second step of oxidizing and refining the molten iron with the oxygen gas while continuously supplying oxygen gas and carbonaceous material into the furnace to obtain molten steel, a supply rate of the oxygen gas and a supply rate of the carbonaceous material into a furnace in the first step are made slower than a supply rate of the oxygen gas and a supply rate of the carbonaceous material into a furnace in the second step, respectively. [2] The apparent density of the solid reduced iron is 3960 kg / m 3 The method for producing molten steel according to [1], characterized in that [3] The method for producing molten steel according to [1] or [2], wherein the solid reduced iron contains 1 to 10 mass % of iron oxide and 0.1 to 2.0 mass % of carbon. [4] The feed rate of the carbonaceous material to the electric furnace is C1 (kg / min) in the first process, C2 (kg / min) in the second process, and the fixed carbon content in the carbonaceous material is N FC(mass%), and the charging rate of solid reduced iron in the first step into the electric furnace is W i (kg / min), the carbon content of the reduced iron is N iC (mass%), C1, C2, N FC , W i , N iC The method for producing molten steel according to any one of [1] to [3], characterized in that the above relationships satisfy the following formulas (1) to (4): C 1C =C2-(N iC / N FC )×W i (1) C 1C -ΔC L ≦C1≦C 1C +ΔC H (2) ΔC H =(0.5 / N FC )×W i (3) ΔC L =(0.2 / N FC )×W i (4) In addition, C 1C -ΔC L If <0, set the left side of equation (2) to 0, and C 1C +ΔC H If <0, set the right side of equation (2) to 0. Effect of the Invention

[0010] It is possible to control the foaming of slag without using excessive amounts of oxygen gas and carbonaceous material. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a cross-sectional view showing an example of an electric furnace. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] In examining foaming control in an electric furnace, the present inventors focused on the introduction routes of carbon and oxygen for generating carbon monoxide, which is the main gas. In electric furnace operation, oxygen gas is usually introduced as the oxygen source, and carbonaceous material such as coal is introduced as the carbon source. On the other hand, solid reduced iron contains unreduced iron oxide and carbon, and therefore it is possible for carbon monoxide gas to be generated by the reaction of these. The present inventors came up with the idea of ​​foaming control by utilizing solid reduced iron as the oxygen source and carbon source.

[0013] An example of operation using solid reduced iron as a raw material in an electric furnace having a lance capable of supplying oxygen gas and carbonaceous material into the furnace is described below. As described above, the process of the electric furnace of the present invention includes a first process of melting raw materials and a second process of oxidizing and refining the molten iron to obtain molten steel. In the first process, the solid reduced iron is melted using an arc as the main heat source while continuously supplying the solid reduced iron, oxygen gas, and carbonaceous material to the electric furnace, and also using the heat generated by the combustion of the carbonaceous material as an auxiliary source by simultaneously supplying oxygen gas and the carbonaceous material. In the second process, after the first process, oxygen gas is supplied into the furnace together with the carbonaceous material to perform oxidizing and refining, thereby obtaining molten steel of the desired composition.

[0014] In both the first and second processes, oxygen and carbon react in the molten metal to generate CO gas, which causes bubbles in the molten slag in the furnace, resulting in slag foaming. The main oxygen sources are the oxygen gas supplied to the furnace and the iron oxide contained in the solid reduced iron. The main carbon sources are the carbonaceous material supplied to the furnace and the carbon content contained in the solid reduced iron.

[0015] First Embodiment In the present invention, attention is paid to the fact that most or all of the solid reduced iron is charged in the first step. In the second step, oxygen gas and carbonaceous material supplied into the furnace serve as oxygen and carbon sources for generating CO gas. First, in the second step, appropriate oxygen gas supply rates and carbonaceous material supply rates for appropriately foaming the slag are determined while adjusting them based on past results. Next, in the first step, it has been found that oxygen gas and carbonaceous material in amounts equivalent to the iron oxide and carbon content contained in the solid reduced iron can be used for foaming, and that slag foaming can be sufficiently ensured while the supply rates of oxygen gas and carbonaceous material in the first step are reduced from those in the second step.

[0016] That is, the first embodiment is characterized in that the supply rate of oxygen gas and the supply rate of carbonaceous material into the furnace in the first step are made slower than the supply rate of oxygen gas and the supply rate of carbonaceous material into the furnace in the second step, which makes it possible to reduce the consumption of oxygen gas and carbonaceous material in the first step and to properly foam the slag as in the second step.

[0017] Second Embodiment Preferred embodiments of the present invention will now be described. In order to utilize the iron oxide and carbon in the solid reduced iron for slag foaming, it is effective to generate carbon monoxide gas when the solid reduced iron sinks as deep as possible into the slag, preferably to the interface between the slag and molten iron. The sinking speed of the solid reduced iron is affected by gravity acting on the solid reduced iron, the buoyant force from the slag, and the drag from the generated gas. For this reason, the depth position at which gas is generated as the solid reduced iron melts is affected by the apparent density of the solid reduced iron. From the results of experiments conducted by the present inventors, it has been found that when foaming is properly maintained, the apparent density of the solid reduced iron should be kept at 3,960 kg / m 3 It was found that the foaming of the slag is promoted by setting the apparent density of the solid reduced iron to 3960 kg / m or more. 3This is presumably because the solid reduced iron quickly settled in the slag and reached the interface between the slag and the molten iron. In addition, the state in which the solid reduced iron remains at the interface between the slag and the molten iron is favorable for promoting foaming, so the apparent density of the solid reduced iron was set to 7000 kg / m 3 It is desirable to have a density of less than 7000 kg / m, but the apparent density of typical solid reduced iron is 7000 kg / m 3 The apparent density is in the range of 3960 to 7000 kg / m 3 In all cases, the solid reduced iron quickly settled and remained at the interface between the slag and the molten iron, and no change in the foaming promotion effect was observed. 3 If the temperature exceeds this range, the solid reduced iron will rapidly settle into the metal, and it can be assumed that a similar foaming promotion effect can be obtained. The apparent density of the solid reduced iron means a density obtained by the so-called Archimedes method, in which the volume of water displaced when the solid reduced iron is immersed in water is taken as the volume of the solid reduced iron, and the mass of the solid reduced iron is divided by the volume. The apparent density of the solid reduced iron varies depending on the amount of bubbles and gangue components in the solid reduced iron. In order to adjust the apparent density of the solid reduced iron to the preferred range of the present invention, the pressure used when the solid reduced iron is compression molded may be adjusted.

[0018] That is, in the second embodiment, the apparent density of the solid reduced iron is set to 3960 kg / m 3 The present invention is characterized in that:

[0019] Third Embodiment The gas exerting a drag on the solid reduced iron is mainly carbon monoxide gas generated by the reaction between oxygen gas and carbonaceous material supplied into the furnace, but the carbon monoxide gas generated by the reaction between the iron oxide and carbon contained in the solid reduced iron and the oxygen gas and carbonaceous material supplied into the furnace, and the carbon dissolved in the molten iron rises due to the difference in specific gravity between the surrounding molten iron and slag, and the gas also exerts a drag on the solid reduced iron. Therefore, if the iron oxide and carbon content in the solid reduced iron are large, this becomes a factor that hinders the settling of the solid reduced iron. On the other hand, if the iron oxide and carbon content in the solid reduced iron are small, the amount of carbon monoxide gas generated by these reactions decreases, and the effect of promoting foaming cannot be enjoyed. From the results of experiments conducted by the present inventors, it was found that the components contained in the solid reduced iron for the purpose of controlling foaming are preferably 1 to 10 mass % of iron oxide and 0.1 to 2.0 mass % of carbon.

[0020] That is, the third embodiment is characterized in that the solid reduced iron contains iron oxide at 1 to 10 mass % and carbon at 0.1 to 2.0 mass %.

[0021] Fourth Embodiment A suitable method for determining the supply rate of the carbonaceous material in the first step will be described. In an electric furnace, most of the carbonaceous material injected from the lance reacts with oxygen to generate carbon monoxide gas, which is used for foaming the slag. Therefore, the supply rate C1 (kg / min) of the carbonaceous material in the first process can be reduced to the same amount of carbon supplied from the solid reduced iron in the first process compared to the supply rate C2 (kg / min) of the carbonaceous material in the second process, and the slag can be sufficiently foamed. Therefore, if the ratio of fixed carbon contained in the carbonaceous material is N FC (mass%), the charging rate of solid reduced iron in the first process is W i (kg / min), the carbon content of the reduced iron is N iC (mass%), the target value C of the carbonaceous material supply rate in the first process 1C is calculated as the following formula (1). Then, ΔC H The supply rate C1 of the carbonaceous material in the first process is set to the target value C1C ΔC H If the carbonaceous material is too high in the first process, the carbon concentration in the molten iron at the start of the second process will be higher than the target, or the proportion of carbonaceous material that does not react in the furnace but scatters and is ultimately discharged from the exhaust system will increase, which is undesirable. L The supply rate C1 of the carbonaceous material in the first process is set to the target value C 1C ΔC L If it is lower than ΔC, the supply of carbonaceous material in the first step will be insufficient, the carbon concentration in the molten iron at the start of the second step will be lower than the target, or it will be difficult to enjoy the effect of promoting foaming, which is undesirable. That is, in the present invention, it is preferable to control C2 within the range of the following formulas (2) to (4). For formulas (3) and (4), ΔC H =(0.3 / N FC )×W i , ΔC L =(0.1 / N FC )×W i It is more preferable to set the following. C 1C =C2-(N iC / N FC )×W i (1) C 1C -ΔC L ≦C1≦C 1C +ΔC H (2) ΔC H =(0.5 / N FC )×W i (3) ΔC L =(0.2 / N FC )×W i (4) In addition, C 1C -ΔC L If <0, set the left side of equation (2) to 0, and C 1C +ΔC H If <0, set the right side of equation (2) to 0.

[0022] That is, in the fourth embodiment, the supply rate of the carbonaceous material to the electric furnace is C1 (kg / min) in the first step, C2 (kg / min) in the second step, and the fixed carbon content contained in the carbonaceous material is N FCis (% by mass), and the charging rate of the solid reduced iron in the first step into the electric furnace is W i (kg / min), the carbon content in the solid reduced iron is N iC (% by mass), when C1, C2, N FC , W i , N iC satisfy the above formulas (1) to (4).

[0023] The fourth embodiment is dependent on the first embodiment. Therefore, regardless of the value on the right side of formula (2), the supply rate C1 of the carbon material in the first step is smaller than the supply rate C2 of the carbon material in the second step. Let δC = 0.9×(N iC / N FC )×W i , and it is more preferable that C1≦C2 - δC.

[0024] When decarburizing and refining the molten iron in the second step, the carbon in the molten iron reacts with the blown oxygen gas to generate CO gas, and this CO gas also contributes to slag formation. Therefore, in calculating the supply rate of the carbon material in the first step, more preferably, a carbon supply rate corresponding to the decarburization rate of the molten iron in the second step can be added to the right side of the above formula (1). Specifically, the value obtained by multiplying the amount of molten iron (kg) by the reduction amount of carbon concentration in the molten iron (% by mass) / 100 in the second step and then dividing by the decarburization required time (min) may be added to the right side of formula (1).

[0025] From the above, it can be seen that the supply rate of the carbon material in the first step in this embodiment is limited to the range of 0≦C1<C2. Therefore, 0≦C 1C <C2(5) If so, when the supply rate C1 of the carbon material in the first step is set to the target value C 1C as described, it can be within the preferable range of the fourth embodiment, which is preferable. Substituting formula (1) into C 1C in formula (5), 0≦C2-(N iC / N FC )×W i <C2(6) is obtained. 0≦C2-(N iC / N FC )×W i ≦C2-δC (7) δC = 0.9 × (N iC / N FC )×W i (8) It is more preferable to set the following. That is, the carbon content of the solid reduced iron, N iC , the charging rate of solid reduced iron W i is adjusted to satisfy formula (6), more preferably formula (7) or formula (8), the supply rate C1 of the carbonaceous material in the first step is set to the target value C 1C When the above is done, the preferred range of the fourth embodiment can be achieved, which is preferable.

[0026] Here, the carbon content of the solid reduced iron is N iC In order to adjust the amount of the iron ore, the composition of the raw materials to be subjected to the reduction step of the iron ore may be adjusted.

[0027] Solid reduced iron charging rate W i The means for adjusting the amount of reduced iron can be, for example, by adjusting the proportion of solid reduced iron in the iron raw material or by adjusting the time required for charging the solid reduced iron in the first step.

[0028] <<Oxygen gas supply rate in the first step>> A suitable method for determining the supply rate of oxygen gas in the first step will be described. The oxygen gas supply rate in the second process is determined based on past performance so as to achieve optimal slag foaming. The total amount of oxygen gas supplied in the second process is a value that includes the amount of oxygen required for foaming, as well as the amount required for oxidation refining, such as decarburization and dephosphorization of molten iron. Since the CO gas generated in the decarburization refining contributes to slag foaming, the oxygen gas used in the decarburization refining also contributes to slag foaming. On the other hand, in the dephosphorization refining, the injected oxygen gas contributes to the formation of FeO in the slag and the oxidation of phosphorus, so it does not contribute to slag foaming.

[0029] When calculating the supply rate of oxygen gas required for forming in the first step based on the supply rate of oxygen gas in the second step, the supply rate of oxygen gas required for forming in the first step can be reduced by the amount of oxygen supplied from iron oxide in the solid reduced iron relative to the supply rate of oxygen gas in the first step in the second step, similar to the case of the carbonaceous material. If it becomes necessary to adjust the iron oxide content in the solid reduced iron, the operating conditions, such as the amount of reducing gas injected in the iron ore reduction process, may be adjusted.

[0030] Furthermore, when calculating the oxygen gas supply rate in the first step based on the oxygen gas supply rate in the second step, it is preferable to subtract the amount required for dephosphorization refining in the second step from the calculated value. Specifically, the total amount of oxygen gas required for generating FeO in the slag is divided by the refining time of the second step according to the increase in the FeO concentration in the slag in the second step to obtain the oxygen gas supply rate for slag oxidation. When calculating the oxygen gas supply rate in the first step, the amount of oxygen supplied from the iron oxide in the solid reduced iron may be subtracted from the oxygen gas supply rate in the second step, and the above-mentioned oxygen gas supply rate for slag oxidation may also be subtracted.

[0031] The iron raw material in the present invention is not limited to solid reduced iron, and scrap or other iron sources can be used in combination. However, if the proportion of solid reduced iron used is small, it becomes difficult to enjoy the promotion of foaming due to the components in the solid reduced iron. Specifically, it is desirable to determine the proportion of solid reduced iron used so that the total mass of carbon contained in the solid reduced iron corresponds to 8% or more of the fixed carbon contained in the total amount of carbonaceous material charged in the first step. EXAMPLES

[0032] The table shows an example of the present invention being implemented in an electric furnace with an inner diameter of 6.5 m, capable of holding 175 t of molten iron, tapping 110 t of steel, and leaving 65 t of seed molten iron in the furnace when tapping, together with a comparative example. As shown in Fig. 1, the electric furnace 1 has a graphite electrode 2 and a slag discharge port 3 with a width of 1000 mm and a height of 995 mm on its side wall, and the insertion of a carbonaceous lance 4 and an oxygen lance 5 of a manipulator 6, which will be described later, or the sampling of molten steel was performed through the slag discharge port 3. Solid reduced iron was charged into the furnace through a raw material charging chute 8.

[0033] The electric furnace 1 has one oxygen lance 5 connected to a movable manipulator 6 as an oxygen supply means, and two lances (hereinafter referred to as furnace wall lances 7) fixed to the inner wall of the electric furnace 1. The oxygen lance 5 of the manipulator 6 can supply up to 4000 Nm 3 / h, and maximum of 3200Nm each from furnace wall lance 7 3 / h of oxygen gas can be supplied into the furnace. In the present invention and comparative examples, oxygen gas with a purity of 99.5% or more was used.

[0034] The electric furnace 1 has one carbonaceous material lance 4 and two furnace wall lances 7 connected to a manipulator 6 as a carbonaceous material supply means, and each lance can supply carbonaceous material into the furnace at a maximum rate of 60 kg / min. The lance for supplying the carbonaceous material is in a separate system from the lance for supplying oxygen, and carbon dioxide is used as the carrier gas for the carbonaceous material. In the present invention and comparative examples, anthracite (i.e., N FC = 81% by mass), the type of carbonaceous material can be changed without departing from the spirit of the present invention, and it is possible to use coal other than anthracite, and carbonaceous materials other than coal, such as graphite, coke, and charcoal. Similarly, the carrier gas can also be changed without departing from the spirit of the present invention, and it is possible to use rare gases such as argon gas and inert gases such as nitrogen.

[0035] In the present invention and comparative examples, solid reduced iron and scrap were used as the iron raw materials. The total amount of the iron raw materials charged corresponds to 110 t of steel tapping. Table 1 shows the proportion (mass%) of the solid reduced iron in the iron raw materials. The rest of the solid reduced iron is scrap. In the first step, scrap was charged into a furnace containing 65 t of seed molten iron, and 6,200 kg of quicklime was charged as a slag-forming material. After that, the solid reduced iron was continuously fed into the furnace from the raw material charging chute 8, while arcing using the graphite electrode 2, supplying oxygen gas using the oxygen lance 5 and furnace wall lance 7, and supplying carbonaceous material using the carbonaceous material lance 4 and furnace wall lance 7, to melt the iron raw materials. In the first step, the time required for supplying the solid reduced iron was constant in all the examples. The charging speed W of the solid reduced iron was adjusted according to the proportion of the solid reduced iron used in each example so that the time required for supplying the solid reduced iron was constant. i was adjusted (Table 1).

[0036] The present invention was implemented in a three-phase AC electric furnace having three graphite electrodes 2, but the slag foaming according to the present invention can also be achieved in single-phase AC and DC electric furnaces, and no limitations are placed on the electrification method of the electric furnace within the scope of the present invention.

[0037] The apparent density of the solid reduced iron was evaluated by the Archimedes method using a part of the solid reduced iron before use. The apparent density, carbon concentration, and iron oxide concentration of the solid reduced iron were varied by using multiple solid reduced iron samples produced under different conditions. The carbon concentration and iron oxide concentration in the solid reduced iron were evaluated by subjecting a portion of the solid reduced iron before use to chemical analysis.

[0038] After the first step, oxygen gas was supplied into the furnace in the second step to perform oxidation refining, and the molten iron was sampled and analyzed to obtain molten steel with the desired composition. After the second step, a tapping hole was opened in the bottom of the furnace, and the molten steel was poured into a ladle (not shown). The desired molten steel compositions in the present invention and comparative examples were 0.04-0.06 mass% carbon and 0.015-0.018 mass% phosphorus. Charcoal supply rate (kg / min), oxygen supply rate (Ncm 3 The carbon feed rate (kcal / h) and the required time (min) for the first and second steps are shown in Table 1. The carbon feed rate column in Table 1 lists the values ​​of the left and right sides of equation (2). When the values ​​of the left and right sides are negative, they are not converted to "0" but are listed as negative values.

[0039] The carbonaceous material reduction effect and the oxygen reduction effect were evaluated as the reduction rate (%) in the first process based on the feed rate in the second process.

[0040] In order to confirm the effect of the slag foaming control according to the present invention, the thickness of the foamed slag was measured and compared in the first process in the present invention and comparative examples using the means described below. The height of the stationary molten iron surface from the bottom of the electric furnace was calculated from the mass of the charged raw materials every minute from the start of reduced iron charging, and the height of the slag surface from the bottom of the electric furnace was determined using a microwave level meter that irradiates microwaves on the slag surface and measures the distance to the slag surface. Furthermore, the slag thickness was calculated by subtracting the height of the stationary molten iron surface from the slag surface height measured at each time point, and the average value over the entire first process is shown in the table.

[0041] The effect of the slag foaming control according to the present invention was evaluated from the viewpoint of the reduction in the amount of carbonaceous material and oxygen used, and the slag thickness. Regarding the slag thickness, the standard arc length of 300 mm in this equipment was marked with an X for thicknesses less than 300 mm, which could not be covered, an O for thicknesses of 300 mm or more, which could be covered, and an ◎ for thicknesses of 450 mm or more, which could be covered particularly well.

[0042] [Table 1]

[0043] Comparative Examples Nos. 1 to 5 in Table 1 are comparative examples. Values ​​outside the range of the present invention are underlined. In Comparative Examples No. 1 to 3, the carbonaceous material supply rate and the oxygen supply rate in the first step were the same as those in the second step. Therefore, no reduction effect was observed for either the carbonaceous material or the oxygen. Furthermore, in the first step, excess CO bubbles were generated in proportion to the carbon and iron oxide supplied from the solid reduced iron, and the slag thickness was rated as ⊚. Comparative Examples Nos. 4 and 5 are comparative examples in which no solid reduced iron was added in the first step.

[0044] In Table 1, examples No. 1 to No. 15 are examples of the present invention. Numerical values ​​that are within the range of the present invention but outside the preferred range are underlined. Note that when the carbonaceous material supply rate C1 in the first step is smaller than the left side of formula (2), the value on the left side of formula (2) is underlined, and when C1 is larger than the right side of formula (2) (the value before being replaced with 0), the value on the right side of formula (2) is underlined. In comparison with Comparative Examples 1 to 3, all of Inventive Examples Nos. 1 to 15 were able to reduce both the carbonaceous material and oxygen in the first step.

[0045] In Inventive Example No. 1, the apparent density of the solid reduced iron was lower than the lower limit of the preferred range of the present invention, and the slag thickness was slightly thin. With regard to the carbon content of the solid reduced iron, Inventive Example No. 6 was outside the upper limit of the suitable range, and Inventive Example No. 8 was outside the lower limit of the suitable range, and in both cases the slag thickness was slightly thin. With regard to the iron oxide content of the solid reduced iron, Inventive Example No. 4 was outside the upper limit of the suitable range, and Inventive Example No. 5 was outside the lower limit of the suitable range, and in both cases the slag thickness was somewhat thin.

[0046] In the present invention, the carbonaceous material supply rate C1 in the first step was larger than the right side of formula (2) (value before replacing with 0) in the case of the invention, and in the case of the invention, the carbonaceous material supply rate C1 in the first step was larger than the right side of formula (2) in the case of the invention, and in all cases, the carbon concentration in the molten iron at the start of the second step was higher than the target, or the proportion of the carbonaceous material discharged from the exhaust system without reacting in the furnace tended to increase. As a result, in the present invention, the carbonaceous material supply rate C1 in the first step was higher than the left side of formula (2), and the slag thickness was somewhat thin. [Explanation of symbols]

[0047] 1 Electric furnace 2. Graphite electrodes 3. Slag outlet 4 Charcoal Lance 5 Oxygen Lance 6 Manipulator 7 Furnace wall lance 8 Raw material input chute 20 Molten Iron 21 Slug

Claims

1. A method for producing molten steel from solid reduced iron as a raw material using an electric furnace having a lance capable of supplying oxygen gas and carbonaceous material into the furnace, the method comprising: a first step of melting the raw materials using an arc as a main heat source while continuously supplying the solid reduced iron, oxygen gas, and carbonaceous material into the furnace; and a second step of oxidizing and refining the molten iron with the oxygen gas while continuously supplying the oxygen gas and carbonaceous material into the furnace to obtain molten steel, a supply rate of the oxygen gas and a supply rate of the carbonaceous material into a furnace in the first step are made slower than a supply rate of the oxygen gas and a supply rate of the carbonaceous material into a furnace in the second step, respectively.

2. The apparent density of the solid reduced iron is 3960 kg / m ３ The method for producing molten steel according to claim 1, characterized in that

3. 3. The method for producing molten steel according to claim 1, wherein the solid reduced iron contains iron oxide in an amount of 1 to 10 mass % and carbon in an amount of 0.1 to 2.0 mass %.

4. The supply rate of the carbonaceous material to the electric furnace is C １ (kg / min), C in the second step ２ (kg / min), the fixed carbon content contained in the carbonaceous material is N ＦＣ (mass%), and the charging rate of the solid reduced iron in the first step into the electric furnace is W ｉ (kg / min), the carbon content of the solid reduced iron is N ｉＣ (mass%), C １ , C ２ , N ＦＣ , W ｉ , N ｉＣ The method for producing molten steel according to claim 1 or 2, characterized in that the relationships satisfy the following formulas (1) to (4): C １Ｃ =C ２ -(N ｉＣ / N ＦＣ )×W ｉ (1) C １Ｃ -ΔC Ｌ ≦C １ ≦C １Ｃ +ΔC Ｈ (2) ΔC Ｈ =(0.5 / N ＦＣ )×W ｉ (3) ΔC Ｌ =(0.2 / N ＦＣ )×W ｉ (4) In addition, C １Ｃ -ΔC Ｌ If <0, set the left side of equation (2) to 0, and C １Ｃ +ΔC Ｈ If <0, the right side of equation (2) is set to 0.

5. The supply rate of the carbonaceous material to the electric furnace is C １ (kg / min), C in the second step ２ (kg / min), the fixed carbon content contained in the carbonaceous material is N ＦＣ (mass%), and the charging rate of the solid reduced iron in the first step into the electric furnace is W ｉ (kg / min), the carbon content of the solid reduced iron is N ｉＣ (mass%), C １ , C ２ , N ＦＣ , W ｉ , N ｉＣ The method for producing molten steel according to claim 3, characterized in that the relationships satisfy the following formulas (1) to (4): C １Ｃ =C ２ -(N ｉＣ / N ＦＣ )×W ｉ (1) C １Ｃ -ΔC Ｌ ≦C １ ≦C １Ｃ +ΔC Ｈ (2) ΔC Ｈ =(0.5 / N ＦＣ )×W ｉ (3) ΔC Ｌ =(0.2 / N ＦＣ )×W ｉ (4) In addition, C １Ｃ -ΔC Ｌ If <0, set the left side of equation (2) to 0, and C １Ｃ +ΔC Ｈ If <0, the right side of equation (2) is set to 0.

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