Method of carburizing molten iron

Abstract

To provide a method for carbonizing molten iron that can realize a high carbonization rate even when an ash content in the carbon material as a carbonizing material is 12 mass% or more.SOLUTION: In a carbonization method in an electric furnace or ladle by adding a solid carbon material (coal material) from above to molten iron while performing gas injection stirring, a carbon material with an ash content ASH of 12 mass% or more is used, and as a stirring power density ε (kW / m3) by gas injection, when a molten iron temperature T (°C) is equal to or lower than a melting point temperature Tm (°C) of the ash in the coal material, the stirring power density ε satisfying the following formula (1) is adopted, and when it is equal to or higher than the melting point temperature Tm (°C), the stirring power density ε satisfying the following formula (2) is adopted. Formula (1): ε>(0.178×ASH-1.696)×{(Tm-T) / (Tm-1400)×0.75+0.25}, formula (2): ε>(0.178×ASH-1.696)×0.25.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for carburizing molten iron for efficiently performing carburization when a solid carbon material is added from above the molten iron in an electric furnace or ladle. [Background technology]

[0002] In electric arc furnaces, cold iron sources such as iron scrap, cold pig iron, and direct reduced iron are melted and refined to produce steel for use in building materials. The main energy source of this arc furnace is the heat of the arc, but auxiliary heat sources such as oxygen gas (for oxidizing and melting iron), gaseous fuel, liquid fuel, and coke powder are used to promote melting and refining and to save on expensive electrical energy. In addition, solid carbon materials are added to the molten iron as a recarburizer, and the carbon in the molten iron is burned with oxygen gas to provide an auxiliary heat source. Recarburizers that have been used include artificial graphite, amorphous graphite, various types of coke, anthracite, wood, and recarburizers made from these raw materials.

[0003] For example, Patent Document 1 discloses a technology for a recarburizer for iron and steel making obtained by calcining amorphous graphite with an ash content of less than 12% by mass, Patent Document 2 discloses a recarburizer technology characterized by adding amorphous graphite, and Patent Document 3 discloses a technology for a recarburizer obtained by dry distilling coconut or oil palm shells as a substitute for coke. Patent Document 4 discloses a technology for adding a carbon source derived from biomass as a recarburizer technology during dephosphorization treatment. In addition, in the smelting reduction method, a large amount of coal is generally charged together with iron ore and oxidizing gas to reduce the iron ore. Auxiliary recarburization may be performed in the ladle to produce high carbon steel.

[0004] When scrap iron is used as the cold iron source in an electric furnace, carbon injection and oxygen enrichment are generally performed, and powdered recarburizers are transported by the blown gas and blown into the molten iron. On the other hand, if the recarburizers can be added by gravity from the top of the furnace, the equipment related to gas transportation can be omitted, and the restrictions on the recarburizers can be relaxed, reducing costs. In addition, when direct reduced iron other than scrap is used as the cold iron source, and low-grade direct reduced iron with a low metallization rate is used, a carbon source for reduction is also required in addition to the carbon source as a heat source, and a large amount of recarburization is required. Furthermore, in order to produce low-N high-grade steel, recarburization is required to perform denitrification during decarburization, and if a large amount of carbon material can be added at high speed by feeding it from above, it is expected that high-grade steel can be produced inexpensively.

[0005] Generally, the price of carbon materials containing a large amount of ash is low, but the ash content in carbon materials is undesirable in many applications, and some literature has reported that the carburization rate is significantly slower when the ash content is high. Here, the carburization rate means the rate at which the carbon concentration in molten iron increases when a carbon source is added to the furnace. For example, Patent Document 1 shows that amorphous graphite with an ash content of less than 12% by mass achieves carburization properties (carburization rate) equivalent to that of artificial graphite, but that the carburization rate is significantly slower when the ash content exceeds that amount, which is thought to be because the components generated from the ash coat the carbonaceous material. In addition, Patent Document 4 shows that the carburization rate decreases with increasing ash content, and discloses a technology related to a biomass-derived carburizer with an ash content of 9% or less. On the other hand, many literatures do not mention the effect of ash when evaluating the carburization rate, and it cannot be said that sufficient knowledge has been accumulated. Furthermore, there are no literature studies that focus on the influence of stirring intensity, molten iron temperature, or ash properties on the relationship between carburization rate and ash concentration.

[0006] In the steelmaking process in an electric furnace, it is common to perform bottom-blowing gas agitation when electricity is applied. The main purpose is to correct the temperature distribution during melting, and the agitation power density is 0.05 kW / ton (approximately 0.4 kW / m 3) is not common. Furthermore, melting cold iron sources such as scrap and direct reduced iron does not require a large amount of carburization, and strengthening the stirring during carburization has not been a major problem. Patent Document 5 discloses that for a bottom-blowing gas stirring method in an arc-type electric furnace, the stirring power density should be 0.1 kW / ton or more and 1 kW / ton or less for stable operation, and it has been found that operation stability drops significantly at 1 kW / ton or more. Furthermore, in a ladle, the upper limit of the stirring strength by gas stirring is about 0.4 kW / ton or less, and Patent Document 6, for example, discloses that the appropriate stirring power density range is 0.18 kW / ton or more and 0.37 kW / ton or less. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 55-38975 [Patent Document 2] Japanese Patent Application Publication No. 1-247527 [Patent Document 3] JP 2009-46726 A [Patent Document 4] JP 2013-72111 A [Patent Document 5] JP 2016-151036 A [Patent Document 6] Patent No. 5803824 [Non-patent literature]

[0008] [Non-Patent Document 1] The Iron and Steel Institute of Japan, 100th and 101st Nishiyama Memorial Technical Lectures, "Recent Trends in Steelmaking Technology Using Stirring", 1984, p.71 Summary of the Invention [Problem to be solved by the invention]

[0009] If cheap carbon materials with a high ash content are used as a recarburizer for molten iron, as mentioned above, the carburization rate may decrease. Furthermore, when the carbon material is added upward by gravity, unlike powder supply by injection or bottom blowing, the contact area between the molten iron and the recarburizer is small, the carburization rate is slow, and the recarburization efficiency is reduced because the material is taken up into the slag or scattered before melting. On the other hand, if the efficiency (recarburization rate) of using carbon materials with a high ash content can be increased beyond the conventional knowledge, it would be preferable because cheap carbon materials can be used with high efficiency. To achieve this, a measure is needed to remove the carbonaceous surface film caused by the ash in the recarburizer and promote carburization.

[0010] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for carburizing molten iron, which can prevent components generated from the ash of a carbonaceous material from coating the carbonaceous material by appropriately controlling the stirring intensity, thereby enabling more efficient use of carbon materials with a high ash content. [Means for solving the problem]

[0011] In order to solve the above problems, the inventors have found that the effect of the ash film on the carbonaceous surface can be neutralized by increasing the stirring intensity more than has been generally done in conventional electric furnaces and ladles. They have also found that this neutralization of the ash film is effective even for coal with a high ash content, for example, for coal with an ASH (ash content) exceeding 12%, which is disclosed in Patent Document 1 as the standard at which a decrease in the carburization rate occurs. They have also found that the stirring intensity required to increase the carburization rate depends on the temperature, and have found a relationship between the melting point temperature and softening temperature, which are determined by the properties of the ash.

[0012] The present invention is based on these findings, and the gist of the present invention is as follows. In a method for carburizing molten iron by adding a solid carbon material (hereinafter also referred to as "carbon material") from above the molten iron while injecting gas and stirring in an electric furnace or ladle, a carbon material having an ash content ASH of 12 mass% or more is used, and for gas injection, the stirring power density ε calculated by the following formula (3) is A method for carburizing molten iron, characterized in that when the molten iron temperature T is equal to or lower than the melting point temperature Tm of the ash in the carbonaceous material, the stirring power density ε satisfies the following formula (1), and when the molten iron temperature T is equal to or higher than the melting point temperature Tm of the ash in the carbonaceous material, the stirring power density ε satisfies the following formula (2). ε>(0.178×ASH-1.696)×{(Tm-T) / (Tm-1400)×0.75+0.25} Equation (1) ε>(0.178×ASH-1.696)×0.25 Formula (2) where ε is the stirring power density (kW / m 3 ), ASH: ash content in the carbonaceous material (mass%), Tm: melting point temperature of the ash in the carbonaceous material (℃). ε=371×Q×(T+273) / V×{ln(1+ρ×g×L / P)+1-(Tn+273) / (T+273)} Equation (3) Where, Q: Total flow rate of bottom blown gas (Nm 3 / s), T: molten iron temperature (℃), V: molten iron volume (m 3 ), ρ: Molten iron density (kg / m 3 ), g: Gravitational acceleration (m / s 2 ), L: floating height of the injected gas (m), P: atmospheric pressure (Pa), Tn: injected gas temperature (℃). Effect of the Invention

[0013] The present invention has made it possible to realize a high carburization rate and extremely efficient carburization when melting carbonaceous materials that contain 12 mass% or more of ash and that would normally have a slow carburization rate by feeding them from above. This has made it possible to further promote the use of low-quality carburizing materials in a low-cost operating method, and has made it possible to significantly reduce the cost of carburizing materials. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a diagram for explaining the carburizing process of the present invention, in which a carbon material is charged from above using an arc-type electric furnace. [Diagram 2] FIG. 1 is a diagram showing the relationship between the stirring power density and the reaction rate constant when carbon materials having a plurality of ash content ratios are used for carburization. [Diagram 3] FIG. 1 is a graph showing the relationship between the stirring power density εa at which the carburization reaction rate increases sharply and the ash concentration in the carbonaceous material. [Figure 4] FIG. 1 is a graph showing the relationship between the stirring power density εa and the molten iron temperature at which the carburization reaction rate increases sharply. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] An embodiment of the present invention will be described with reference to Fig. 1. In an AC electric furnace 1 equipped with a bottom-blowing tuyere 4 shown in Fig. 1, a carbon material is supplied from above molten iron 5 using a lance 3 separate from an electrode 2, and bottom-blowing gas stirring is performed.

[0016] After the carbon material is poured into the molten iron in an electric furnace or ladle, the temperature of the carbon material rises, and the carbonaceous matter dissolves from the surface of the carbon material, while the undissolved ash forms an ash film on the surface of the carbonaceous matter, which is thought to have the effect of preventing the carbonaceous matter from coming into contact with the molten iron. In this case, by increasing the stirring power density by bottom blowing gas stirring, it is expected to have the following effects: washing away the ash film; allowing molten iron to seep through the gaps in the carbonaceous film to dissolve the carbonaceous matter and break down the ash film formed on the surface; and simply rotating the carbonaceous material to expose other uncovered surfaces.

[0017] The properties of the recarburizers used in the experiments of the present invention are summarized in Table 1.

[0018] [Table 1]

[0019] First, the inventors used a small 2 kg melting furnace, controlled the bottom blowing flow rate of bottom blowing gas stirring, added a carburizer while maintaining a specified molten iron temperature, and performed a measurement test of the carburization rate after the addition of the carburizer. After melting electrolytic iron in a small melting furnace, the carbon material shown in Table 1 was dropped from above, bottom blowing gas stirring was performed, and sampling was performed at appropriate time intervals to determine the time change in carbon concentration in the molten iron. The behavior of the carburization rate was assumed to be a first-order reaction driven by the difference between the saturated C concentration and the C concentration in the molten iron, and the reaction rate constant k (cm / s) was calculated assuming that k in the following equation (4) is a constant value (reaction rate constant k). 1400°C was selected as the temperature of the molten iron to be melted, which is approximately equal to the softening temperature Ts of the carburizer used. Here, C S , Ct, and C0 are the C concentrations (mass%) in the molten iron, and C S is the saturated C concentration, Ct is the C concentration at time t (s), and C0 is the C concentration at time t = 0. A is the reaction interface area (cm 2 ), V is the volume of molten iron (cm 3 In the small melting furnace test, A means the surface area of ​​the molten iron in the small melting furnace. (V / A)ln((C S -C0) / (C S -Ct))=k×t Equation (4) For bottom-blowing gas agitation, the agitation power density ε (kW / m 3 ) was calculated, and the reaction rate constant k was measured for each ε by varying ε within the range shown on the horizontal axis of Figure 2. Experiments were conducted using two types of coal (Coal B and Coal C in Table 1). ε=371×Q×(T+273) / V×{ln(1+ρ×g×L / P)+1-(Tn+273) / (T+273)} Equation (3) Where, Q: Total flow rate of bottom blown gas (Nm 3 / s), T: molten iron temperature (℃), V: molten iron volume (m 3 ), ρ: Molten iron density (kg / m 3 ), g: Gravitational acceleration (m / s 2 ), L: floating height of the injected gas (m), P: atmospheric pressure (Pa), Tn: injected gas temperature (℃). In the test of a small melting furnace, L means the depth of the molten iron in the small melting furnace.

[0020] The results are shown in Figure 2. In Figure 2, the vertical axis is the common logarithm of the reaction rate constant k in units of cm / s. Figure 2 shows the relationship between the reaction rate constant k and the stirring power density ε when two types of coal (coal B and coal C) are used as the recarburizer and the molten iron temperature T is set to 1400°C. For both coal B and coal C, when the stirring power density ε is small, log(k) is a small value less than -1.5. However, it was found that at a certain stirring power density that differs for each recarburizer, the reaction rate constant rises abruptly and critically, and log(k) reaches a large value of -1 or more. Therefore, as an index showing the stirring power density at which the reaction rate constant rises critically, an ε value at which log(k) exceeds -1.5 is defined as ε. a It was established as follows.

[0021] The ash content of coal B is 12.1% and that of coal C is 18.09%. From Fig. 2, ε a At higher stirring power densities, coal C with high ASH has k equivalent to that of coal B with low ASH. a was found to be low in low ASH coals and high in high ASH coals.

[0022] Therefore, we used carbon materials with various ash contents ASH [%] to determine the mixing power density ε a The relationship between the graphite material, coal A, coal B, coal C, and a mixed coal of two types of coal B and C shown in Table 1 was used as carbon materials, and the particle size was set to 1.0±0.4 mm by sieving. The molten iron temperature was 1400°C, and the difference with the softening point temperature of each carbon material was within 50°C. The results are shown in Figure 3. When the ash concentration ASH was less than 10%, log(k) was -1.5 or more even at the lowest stirring power density ε tested, and ε a On the other hand, when the ash concentration was about 10% or more, ε a exists and ε a [kW / m 3 ] changes almost linearly with increasing ASH [%], and the equation expressing the relationship is given by the following equation (5A). ε a=(0.178×ASH-1.696) Formula (5A) ε a1400 =(0.178×ASH-1.696) Equation (5)

[0023] In the above experiments, the molten iron temperature was set at 1400°C, which is approximately equal to the softening temperature Ts of the carburized material, and a An experiment was conducted to evaluate ε a Therefore, the ε a ε a1400 Then, the molten iron temperature is changed in various ways, and the ε a Figure 4 shows the effect of molten iron temperature and ε a The melting point temperature Tm of the ash in coal B is 1575°C, and the melting point temperature Tm of the ash in coal C is 1560°C. In both coal B and coal C, the ε a decreases linearly, and ε at the melting point Tm of the ash in each carbon material a is the ε value at a molten iron temperature of 1400℃ a1400 It was also found that the temperature of the molten iron is constant regardless of the temperature above the melting point Tm.

[0024] For both coal B and coal C, when the molten iron temperature T is below the melting point temperature Tm of the ash, ε at a molten iron temperature of 1400°C is ε a1400 , ε at the melting point temperature Tm of the ash is ε a1400 / 4, and since ε varies linearly with temperature, ε a The T dependence of is expressed by the following formula (1A). In addition, when the molten iron temperature is equal to or higher than the melting point temperature Tm of the ash, ε a is ε a1400 It was found that the temperature dependence is given by the fact that the ease with which the ash film is rendered harmless increases as the degree of softening of the ash film increases, and does not change significantly at temperatures above the melting point. ε a =ε a1400×{(Tm-T) / (Tm-1400)×0.75+0.25} =(0.178×ASH-1.696)×{(Tm-T) / (Tm-1400)×0.75+0.25} Formula (1A) ε a =ε a1400 ×0.25 =(0.178×ASH-1.696)×0.25 Formula (2A)

[0025] And the stirring power density ε is ε a Since a high reaction rate constant k is obtained when ε is larger than ε, a That is, it has been found that by determining the stirring power density ε based on the following formula (1) when the molten iron temperature T is equal to or lower than the melting point temperature Tm of the ash, and by determining the stirring power density ε based on the following formula (2) when the molten iron temperature T is equal to or higher than the melting point temperature Tm of the ash, a good carburizing rate can be achieved even if the ash content ASH of the carburizing material is 12% or more. ε>(0.178×ASH-1.696)×{(Tm-T) / (Tm-1400)×0.75+0.25} Equation (1) ε>(0.178×ASH-1.696)×0.25 Formula (2)

[0026] Patent Document 1 discloses a technology for producing recarburizers with an ash content of less than 12%, and it has been found that the reaction rate slows down when the ash content is 12% or more. Although the document does not contain any information on the stirring power density and the melting point of the ash of the carbon material, assuming values ​​for a typical ash composition, the required stirring power density ε a is approximately 0.4kW / m 3 Since such an agitation power density can be obtained under typical agitation conditions, it exceeds the standard of the above formula (1), and therefore it is considered that the necessary reaction rate was obtained at an ash content of less than 12% as in Patent Document 1. In contrast, at an ash content of 12% or more, the necessary agitation power density ε calculated by the above formula (1A) a It is believed that the reaction rate slowed down because the standard value exceeded the experimental standard.

[0027] When bottom-blowing gas stirring is applied to an electric furnace, the normal stirring power density is at most 0.4kW / m 3 The above-mentioned effects of the present invention are realized only by increasing the stirring power density compared to normal gas stirring, and even when coal with a high ash content of 12% or more is used as the carburizing agent, an increase in the carburizing speed to the same level as that of coal with a low ash content of less than 12% is realized.

[0028] In the present invention, the refining vessel for carburization is limited to an electric furnace or a ladle. In contrast, Patent Documents 2 and 4 use converter-type refining equipment under strong stirring conditions, which is different from the subject of the present invention. The reaction rate constant k defined in the above formula (4) is the saturated carbon concentration of molten iron (C S ) and the C concentration at time t (Ct). It does not depend on the C concentration in the molten iron, but it depends on the C concentration when considered in terms of the reaction rate (dC / dt).

[0029] The moisture, ash content (ASH), volatile matter and fixed carbon content in the recarburizer are defined by JIS M 8812, and specifically, are measured by the following methods. Moisture content: The weight loss when 5 g of a sample crushed to a particle size of 250 μm or less is dried at 107 ± 2°C until it reaches a constant weight. Ash content (ASH): Residue when 1 g of sample is heated and incinerated at 815±10℃. Percentage (mass%) relative to 1 g of sample. Volatile content: 1 g of sample is placed in a platinum crucible with a lid and heated at 900±20℃ for 7 minutes while blocking off air; the weight loss is calculated by subtracting water content. Fixed carbon content: Fixed carbon content [mass%] = 100-(moisture content [mass%] + ash content [mass%] + volatile content [mass%]).

[0030] The softening and melting points of ash in recarburized materials are defined by JIS M 8801, and are measured by the following method: A sample is incinerated to prepare minute sample particles, and a triangular test pan sample is prepared with a height of 8 mm, two base lengths of 2.7 mm, one side of 3 mm, and one flank standing upright at a base of 3 mm, and heated continuously in a specified electric furnace under specified conditions. The melting point of the ash is represented by the temperature at which a specific change occurs in the shape of the test pan. Softening point: The temperature at which the top of the test pan begins to melt and round. Melting point: The temperature at which the test pan melts and its height becomes approximately equal to half the apparent width of its base. There are two types of atmospheres, oxidizing and reducing, but the value measured in an atmosphere that corresponds to the gas conditions in the electric furnace or ladle to be applied is used. If the atmosphere in the furnace is unknown, the measured value in a reducing atmosphere is used.

[0031] The particle size of the carbon material used as the recarburizer is desirably 20 mm or less to ensure a sufficient contact area with the molten iron and a high reaction rate. However, when using coal with a high volatile content, for example 10% or more, the volatile content evaporates and breaks into pieces when heated until it comes into contact with the molten iron, so the particle size is not limited to 20 mm or less, and particles up to 100 mm can be used. Also, when adding carbon material from above, small particle sizes will not reach the molten iron and will be discharged outside the furnace with the exhaust gas and lost, so the lower limit of the particle size of the carbon material is desirably 0.2 mm.

[0032] There is no particular upper limit for the ASH (ash content) of the carbon material used as the recarburizer, but from the viewpoint of stable operation, the stirring power density in an electric furnace should be at most 1kW / m 3 The realistic maximum ASH is determined based on the fact that the actual maximum ASH value varies depending on the relationship between the melting point temperature, which depends on the ash composition, and the molten iron temperature. The ash concentration that can achieve the detoxification of the ash film with ε in the range of the present invention is ASH≦16% when the molten iron temperature is equal to the softening point temperature, and ASH≦30% when the molten iron temperature is equal to or higher than the melting point temperature.

[0033] From the viewpoint of operability, the molten iron temperature is preferably 1300°C or higher and 1650°C or lower, but local high temperature areas such as arc spots and hot spots caused by top-blowing oxygen lances are acceptable. In principle, the temperature of the reaction zone should be used as the molten iron temperature in the present invention, but in practice, there are problems with the measurability and uniformity of the temperature distribution, so the overall average molten iron temperature may be used instead.

[0034] From the viewpoint of operability during desulfurization, it is desirable to keep the S concentration in molten iron [S] to a maximum of 0.5% or less.

[0035] The type of bottom blown gas is not important, and gas transportation during supply of the carbon material is not important. There may be a source of cold iron remaining after the carbon material is added, a molten slag layer may be present above the molten metal, solid components may be present in the molten slag layer, there may be multiple carbon material supply lances, the furnace may be a DC electric furnace or not only an AC electric furnace, there may be any number of electrodes, and the gas agitation method may be injection instead of bottom blowing. Also, as long as the carbon material is supplied from above the molten metal surface and gas agitation is present, it does not have to be an electric furnace and may be a ladle. EXAMPLES

[0036] Next, examples carried out to confirm the effects of the present invention will be described. Here, as shown in Figure 1, an actual bottom-blowing electric arc furnace (electric furnace 1) capable of producing 90 tons of molten iron was used, and iron scrap was melted by arc heating from a graphite electrode (electrode 2) while gas was blown in from the bottom-blowing tuyeres 4. After measuring the temperature, carbon material was introduced upward using a lance 3, and temperature and sampling were performed at regular intervals while controlling the stirring intensity. The molten iron temperature and C concentration were measured, and the reaction rate constant k was calculated from equation (4). Arc current was applied under some conditions during carburization. There were six bottom-blowing tuyeres, and the gas flow rate from each tuyeres was adjusted to be equal. The carbon materials used were coal B and coal C shown in Table 1, and amorphous graphite was used as a reference example. The main operating conditions are shown in Table 2. Regarding the evaluation of the reaction rate constant k, in Figure 2, after the ash film is disabled, log(k) is ≧-1.0, and currently, the carburization rate for amorphous graphite and high-quality coal with low ASH is generally log(k): -0.8 or more and less than -0.6, and when it is -0.8 or more, it is judged as "◎". Also, when log(k): is -1.2 or more and less than -0.8, it is judged as "○" because no major operational problems will occur. Also, when log(k): is -1.6 or more and less than -1.2, it is judged as "△", and when log(k): is less than -1.6, it is judged as "×".

[0037] [Table 2]

[0038] In all of Examples 1 to 4 shown in Table 2, the gas stirring conditions satisfied the above formula (1) or (2). In this case, the reaction rate constants all achieved ○ or ◎, and there was no significant hindrance to operational stability regardless of the conditions such as the presence or absence of arc current. However, in Example 3, the stirring intensity was large compared to the other conditions, and the fluctuation of the arc voltage was large, but this did not hinder operation. Reference Example 5 is the result when amorphous graphite with a relatively low ASH (7.41%) was used as the carbon material, which is the condition closest to the current actual operating conditions, but a sufficient reaction rate was obtained under low stirring intensity conditions, which is a target value for high ASH carbon materials.

[0039] In Example 1, the stirring power density ε is ε a In this case, the reaction rate constant was larger than those in Examples 2, 3, and 4, and the evaluation was rated as ⊚, which was equivalent to that of the amorphous graphite in Reference Example 5, and better results were obtained.

[0040] On the other hand, in Comparative Example 6, coal B was used at a stirring intensity similar to that of Reference Example 5, in which amorphous graphite was used. In addition, the stirring intensity was lowered compared to Example 2, and the stirring power density ε was ε a In this case, the operational stability is good, but the stirring strength is not sufficient to render the ash film harmless, and the reaction rate is low, resulting in a rating of x.

[0041] Comparative Example 7 is an example in which the temperature is different from that of Example 2, although the carbon material and stirring strength are the same as those of Example 2. In fact, Comparative Example 7 is an example carried out immediately after Example 2, and since the arc current was OFF in Example 2, the molten iron temperature had dropped when additional carburization was performed in Comparative Example 7, so the arc current was turned ON to gradually increase the temperature and perform additional carburization. At this time, the stirring strength was not changed from that of Example 2, and the carbon material was added to a location other than the arc spot. Under these conditions, the stirring strength was the same as that of Example 2, but the temperature had dropped, so ε a increases, and the stirring power density ε becomes ε a This means that the ash film was not rendered harmless, and the reaction rate was low, resulting in a rating of Δ.

[0042] Comparative Example 8 is a condition in which the type of carbon material added is changed at the same stirring intensity and molten iron temperature as in Example 1. Comparative Example 8 is also a condition in which the stirring intensity is reduced at the same molten iron temperature and carbon material as in Example 3. Under these conditions, the stirring power density ε is ε a The reaction rate was less than 100%, which resulted in a slow reaction rate, and the result was judged as ×.

[0043] Therefore, it was found that by using the stirring intensity standard of the present invention, even a hardly soluble high ASH carbon material can be dissolved and carbonized at a rate equivalent to that of a low ASH carbon material.

[0044] The present invention has been described above with reference to an embodiment, but the present invention is not limited to the configurations described in the above embodiment, and also includes other embodiments and modifications that are possible within the scope of the matters described in the claims. [Explanation of symbols]

[0045] 1 Electric furnace 2 electrodes 3. Lance 4 Bottom-blown tuyere 5. Molten Iron

Claims

[Claim 1] In a method for carburizing molten iron by adding a solid carbon material (hereinafter also referred to as "carbon material") from above the molten iron while injecting and stirring gas in an electric furnace or ladle, a carbon material having an ash content ASH of 12 mass% or more is used, and for gas injection, the stirring power density ε calculated by the following formula (3) is set as follows: A method for carburizing molten iron, characterized in that when the molten iron temperature T is equal to or lower than the melting point temperature Tm of ash in a carbonaceous material, a stirring power density ε that satisfies the following formula (1) is set, and when the molten iron temperature T is equal to or higher than the melting point temperature Tm of ash in a carbonaceous material, a stirring power density ε that satisfies the following formula (2). ε>(0.178×ASH-1.696)×{(Tm-T) / (Tm-1400)×0.75+0.25} Formula (1) ε>(0.178×ASH-1.696)×0.25 Formula (2) where ε is the stirring power density (kW / m ３ ), ASH: ash content in the carbonaceous material (mass%), Tm: melting point temperature of the ash in the carbonaceous material (°C). ε=371×Q×(T+273) / V×{ln(1+ρ×g×L / P)+1−(Tn+273) / (T+273)} Formula (3) Where, Q: total flow rate of bottom blown gas (Nm 3 / s), T: molten iron temperature (°C), V: molten iron volume (m 3 ), ρ: Molten iron density (kg / m 3 ), g: gravitational acceleration (m / s 2 ), L: floating height of the blown gas (m), P: atmospheric pressure (Pa), Tn: blown gas temperature (°C).

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