JP7624142B2 - Method of carburizing molten iron - Google Patents

Description

The present invention relates to a method for carburizing molten iron in an electric furnace or ladle by adding a solid carbon material from above the molten iron to efficiently carburize the iron.

In electric furnaces, cold iron sources such as scrap iron, 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.

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 a ladle to produce high carbon steel.

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. In contrast, if the recarburizers could be added by gravity from above the furnace, the equipment related to gas transportation could be omitted, restrictions on the recarburizers could be relaxed, and costs could be reduced. Furthermore, when using direct reduced iron other than scrap 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 carbon materials can be added in large quantities at high speed by feeding them from above, it is expected that high-grade steel can be produced inexpensively.

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 while amorphous graphite with an ash content of less than 12% by mass achieves the same carburization rate (carburization rate) as artificial graphite, the carburization rate is significantly slower for carburizers with an ash content greater than that, which is thought to be because components generated from the ash coat the carbonaceous material. Patent Document 4 also shows that the higher the ash content, the lower the carburization rate, and discloses a technology related to a biomass-derived carburizer with an ash content of 9% or less. On the other hand, many literature does 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 documents that focus on the effects of stirring intensity, molten iron temperature, or ash properties on the relationship between carburization rate and ash concentration.

In the steelmaking process in an electric furnace, bottom-blowing gas stirring is generally performed during energization, but the main purpose is to correct the temperature distribution during melting, and the stirring power density is generally not more than 0.05 kW/ton (about 0.4 kW/m ³ ). Furthermore, a large amount of carburization is not required when melting cold iron sources such as scrap and direct reduced iron, and strengthening the stirring during carburization has not been a major problem so far. Patent Document 5 discloses that the stirring power density of bottom-blowing gas stirring in an arc-type electric furnace 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 is significantly reduced at 1 kW/ton or more. Furthermore, the upper limit of the stirring strength by gas stirring in a ladle is about 0.4 kW/ton or less, and for example, Patent Document 6 discloses that the appropriate stirring power density range is 0.18 kW/ton or more and 0.37 kW/ton or less.

Japanese Unexamined Patent Publication No. 55-38975 Japanese Patent Application Publication No. 1-247527 JP 2009-46726 A JP 2013-72111 A JP 2016-151036 A Patent No. 5803824

The Iron and Steel Institute of Japan, 100th and 101st Nishiyama Memorial Technical Lectures, "Recent Trends in Steelmaking Technology Using Stirring", 1984, p. 71

When 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 what has been conventionally known, it would be preferable because it would allow the use of cheap carbon materials with high efficiency. To achieve this, a method is needed to remove the carbonaceous surface film caused by the ash in the recarburizer and promote carburization.

The present invention was made in consideration of these circumstances, and its purpose is to provide a method for carburizing molten iron that can prevent components generated from the ash of carbonaceous materials from coating the carbonaceous material by appropriately managing the stirring intensity, thereby enabling more efficient use of carbon materials with high ash content.

In order to solve the above problems, the inventors have discovered that the effects of the ash film on the carbonaceous surface can be neutralized by increasing the stirring intensity beyond what has been commonly done in conventional electric furnaces and ladles. They have also discovered that this neutralization of the ash film is also effective for coal with a high ash content, for example, 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. Furthermore, they have discovered that the stirring intensity required to increase the carburization rate depends on the temperature, and have discovered a relationship between the melting point temperature and softening temperature, which are determined by the properties of the ash.

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 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 is the ash content in the carbonaceous material (mass %), and Tm is the 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 is the total flow rate of the bottom blown gas ( ^Nm3 /s), T is the temperature of the molten iron (°C), V is the volume of the molten iron ( ^m3 ), ρ is the density of the molten iron (kg/ ^m3 ), g is the acceleration of gravity (m/ ^s2 ), L is the floating height of the blown gas (m), P is the atmospheric pressure (Pa), and Tn is the temperature of the blown gas (°C).

The present invention makes it possible to realize a high carburization rate and extremely efficient carburization when melting carbon materials that contain 12% or more by mass of ash and that would normally have a slow carburization rate by feeding them from above. This makes it possible to further promote the use of low-quality carburizing materials in a low-cost operating method, and significantly reduces the cost of carburizing materials.

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. 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. FIG. 1 is a graph showing the relationship between the stirring power density ε _a at which the carburization reaction rate increases sharply and the ash concentration in the carbonaceous material. FIG. 1 is a graph showing the relationship between the stirring power density ε _a and the molten iron temperature at which the carburization reaction rate increases sharply.

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

After carbon material is poured into molten iron contained 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 contact between the carbonaceous matter and the molten iron. At this time, by increasing the stirring power density by bottom-blowing gas stirring, it is expected that the following effects will be achieved: 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.

The properties of the recarburizers used in the experiments of this invention are summarized in Table 1.

First, the inventors used a small melting furnace of 2 kg capacity, controlled the bottom blowing flow rate of bottom blowing gas stirring, added a recarburizer while maintaining a predetermined molten iron temperature, and performed a measurement test of the carburization rate after the addition of the recarburizer. 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 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 molten iron, and the reaction rate constant k (cm/s) was calculated assuming that k in the following formula (4) was a constant value (reaction rate constant k). As the temperature of the molten iron to be melted, 1400°C was selected as a temperature approximately equal to the softening temperature Ts of the recarburizer used. Here, C _S , C t , and C ₀ are all C concentrations (mass%) in molten iron, C _S is the saturated C concentration, C t is the C concentration at time t (s), and C ₀ is the C concentration at time t = 0. In addition, A is the reaction interface area (cm ² ), and V is the molten iron volume (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 -C ₀ )/(C _S -Ct))=k×t Equation (4)
For bottom-blowing gas stirring, the stirring power density ε (kW/m ³ ) was calculated by the following formula (3), and the reaction rate constant k was measured for each ε by varying ε within the range shown on the horizontal axis of Fig. 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)} Formula (3)
where Q is the total flow rate of bottom blown gas ( ^Nm3 /s), T is the temperature of molten iron (°C), V is the volume of molten iron ( ^m3 ), ρ is the density of molten iron (kg/ ^m3 ), g is the acceleration of gravity (m/ ^s2 ), L is the floating height of the blown gas (m), P is the pressure of the atmosphere (Pa), and Tn is the temperature of the blown gas (°C). In the test of the small melting furnace, L means the depth of the molten iron in the small melting furnace.

The results are shown in FIG. 2. In FIG. 2, the vertical axis is the common logarithm of the reaction rate constant k in cm/s. FIG. 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 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, but it was found that the reaction rate constant rises rapidly and critically at a certain stirring power density that differs for each recarburizer, and log(k) becomes a large value of −1 or more. Therefore, ε with log(k) exceeding −1.5 was defined as ε _a as an index indicating the stirring power density at which the reaction rate constant rises critically.

Since the ash content of coal B is 12.1% (ASH) and that of coal C is 18.09%, it can be seen from Fig. 2 that at a stirring power density higher than _εa , the high ASH coal C can obtain k equivalent to that of the low ASH coal B. Furthermore, it was found that _εa is low for low ASH coal and high for high ASH coal.

Therefore, the relationship between k and the agitation power density ε _a , where k changes rapidly, was investigated using carbon materials having various ash concentrations ASH [%]. The carbon materials used were graphite material shown in Table 1, coal A, coal B, coal C, and two types of mixed coals of coal B and C, and the particle size was uniformed 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%, even the lowest agitation power density ε tested had a log(k) of -1.5 or more, and ε _a did not exist. On the other hand, ε _a existed when the ash concentration was about 10% or more, and ε _a [kW/m ³ ] changed almost linearly with the increase in ASH [%], and the equation expressing the relationship was obtained as the following equation (5A).
ε _a = (0.178×ASH-1.696) Formula (5A)
ε _a1400 = (0.178×ASH-1.696) Equation (5)

The above experiment was conducted to evaluate ε _a by setting the molten iron temperature to 1400°C, which is approximately equal to the softening temperature Ts of the recarburizer. Since ε _a is obtained when the molten iron temperature is 1400°C, this ε _a is set to ε _a1400 and is expressed by the above formula (5). Next, the molten iron temperature was changed in various ways, and the effect on ε _a was investigated. Figure 4 shows the relationship between the molten iron temperature and ε _a for coal B and coal C. 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. For both coal B and coal C, ε _a decreased linearly as the molten iron temperature increased, and ε a at the melting point temperature Tm of the ash in each carbon material decreased to 1/4 of ε _a1400 at the molten iron temperature of 1400°C. It was also found that ε _a is constant regardless of temperature when the molten iron temperature is equal to or higher than the melting point temperature Tm.

For both coal B and coal C, when the molten iron temperature T is below the ash melting point temperature Tm, ε is _εa1400 when the molten iron temperature is 1400°C, _εa1400 /4 at the ash melting point temperature Tm, and ε changes linearly with temperature, so the T dependency of _εa is expressed by the following formula (1A). It was also found that when the molten iron temperature is above the ash melting point temperature Tm, _εa is constant at _εa1400 /4, so it can be expressed by the following formula (2A). The ease with which the ash film is rendered harmless increases the greater the degree of softening of the ash film, and does not change significantly at temperatures above the melting point, so it is thought to have this temperature dependency.
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)

And, since a high reaction rate constant k is obtained when the stirring power density ε is larger than ε _a , the effect of the present invention is exerted by making ε>ε _a . That is, it was 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 realized 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} Formula (1)
ε>(0.178×ASH-1.696)×0.25 Formula (2)

Patent Document 1 discloses a technology for producing recarburizer 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 at an ash content of 12% in the above formula (1A) is approximately 0.4 kW/m ^3. Since such a stirring power density can be obtained under general stirring conditions, it is considered that the required reaction rate was obtained at an ash content of less than 12% in Patent Document 1. On the other hand, at an ash concentration of 12% or more, the required stirring power density ε _a calculated by the above formula (1A) exceeded the standard during the experiment, and therefore the reaction rate is considered to have decreased.

When bottom-blowing gas stirring is applied to an electric furnace, the normal stirring power density is at most about 0.4 kW/ ^m3 . The above-mentioned effects of the present invention are only realized 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.

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 are different from the subject of the present invention.
The reaction rate constant k defined in the above formula (4) is the reaction rate constant of a first-order reaction driven by the difference between the saturated C concentration ( _Cs ) of molten iron and the C concentration at time t (Ct), and is independent of the C concentration of molten iron. However, when considered in terms of the reaction rate (dC/dt), it depends on the C concentration.

The moisture, ash (ASH), volatile matter and fixed carbon content in the recarburizer are defined by JIS M 8812, and are specifically measured by the following method.
Moisture content: weight loss when 5 g of a sample pulverized 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° C. Percentage (mass%) relative to 1 g of sample.
Volatile content: 1 g of a sample was placed in a platinum crucible with a lid and heated at 900±20°C for 7 minutes while blocking off air; the weight loss was calculated by subtracting water content.
Fixed carbon content: Fixed carbon content [mass%] = 100 - (moisture content [mass%] + ash content [mass%] + volatile content [mass%]).

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 the 3 mm base, and this is heated continuously under specified conditions in a specified electric furnace, and the temperature at which a specific change occurs in the shape of the test pan represents the melting property of the ash.
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.

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.

Although there is no particular upper limit for the ASH (ash content) of the carbon material used as the recarburizer, from the viewpoint of implementing stable operation, a realistic maximum ASH is determined on the basis that the stirring power density in an electric furnace is at most about 1 kW/ ^m3 . The realistic maximum ASH value varies depending on the relationship between the melting point temperature, which depends on the ash composition, and the molten iron temperature, and the ash concentration at which the ash film can be rendered harmless with ε within the claimed 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.

From the viewpoint of operability, the molten iron temperature is preferably 1300°C or higher and 1650°C or lower, but localized high-temperature areas such as arc spots and fire points 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 this invention, but in practice there are issues with the measurability and uniformity of the temperature distribution, so the overall average molten iron temperature may be used instead.

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

The type of bottom blown gas is not important, and gas transport may or may not be used when the carbon material is supplied. There may be a source of unmelted cold iron when 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, it may be a DC electric furnace or not just an AC electric furnace, there may be any number of electrodes, and the gas agitation method may be injection rather than 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.

Next, examples carried out to confirm the effects of the present invention will be described.
Here, as shown in FIG. 1, an actual arc-type bottom blown electric furnace (electric furnace 1) capable of melting 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 from a bottom blown tuyeres 4. After temperature measurement, carbon material was charged upward using a lance 3, and temperature and sampling were performed at regular intervals while controlling the stirring intensity, and 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 blown tuyeres, and the gas flow rate from each tuyeres was adjusted to be equal. Coal B and coal C shown in Table 1 were used as carbon materials, and amorphous graphite was used as a reference example. Table 2 shows the main operating conditions. Regarding the evaluation of the reaction rate constant k, after the ash film is invalidated in FIG. 2, log(k)≧-1.0, and the carburization rate with amorphous graphite and high-quality coal with low ASH is currently 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):-1.2 or more and less than -0.8, no major operational problems occur, so it is judged as "○". Also, when log(k):-1.6 or more and less than -1.2, it is judged as "△", and when log(k): less than -1.6, it is judged as "x".

In all of Examples 1 to 4 shown in Table 2, the gas stirring conditions satisfied the above formula (1) or (2). In these cases, 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, Example 3 had a higher stirring intensity than the other conditions, and the arc voltage fluctuated significantly, but this did not hinder operation. Reference Example 5 shows the results when a relatively low ASH (7.41%) amorphous graphite was used as the carbon material, which is the closest condition to the current actual operating conditions, but a sufficient reaction rate was obtained under low stirring intensity conditions, and this is the target value for high ASH carbon materials.

In Example 1, the stirring power density ε was set to about 1.8 times ε _a. In this case, the reaction rate constant was larger than those in Examples 2, 3, and 4, and the evaluation was ⊚, which was equivalent to that of the amorphous graphite in Reference Example 5, and a better result was obtained.

On the other hand, in Comparative Example 6, coal B was used at a stirring intensity similar to that of Reference Example 5, where amorphous graphite was used. The stirring intensity was also lower than in Example 2, and the stirring power density ε was less than ε _a . In this case, the operational stability was good, but the stirring intensity was not sufficient to render the ash film harmless, and the reaction rate was low, resulting in a rating of x.

In addition, Comparative Example 7 is a condition in which the temperature is different under the same carbon material and stirring strength as 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 was lowered when additional carburization was performed in Comparative Example 7, and 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 Example 2, and the carbon material was added to a place other than the arc spot. Under these conditions, the stirring strength was the same as Example 2, but ε _a increased due to the decrease in temperature, and the stirring power density ε was less than ε _a , which did not satisfy the conditions of the present invention. As a result, the ash film was not rendered harmless, and the reaction rate was low, resulting in a judgment of △.

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, too, the stirring power density ε is less than _εa , resulting in a low reaction rate, and the result is judged as ×.

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

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

1 Electric furnace 2 Electrode 3 Lance 4 Bottom blowing tuyeres 5 Molten iron

Claims (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 is the ash content in the carbonaceous material (mass %), and Tm is the 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 is the total flow rate of the bottom blown gas ( ^Nm3 /s), T is the temperature of the molten iron (°C), V is the volume of the molten iron ( ^m3 ), ρ is the density of the molten iron (kg/ ^m3 ), g is the acceleration of gravity (m/ ^s2 ), L is the floating height of the blown gas (m), P is the atmospheric pressure (Pa), and Tn is the temperature of the blown gas (°C).

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