JP7575684B2 - Melting method of molten iron using converter type refining furnace - Google Patents

Description

The present invention relates to a method for smelting molten iron using a converter-type refining furnace, in which the molten iron is refined by successively carrying out a molten iron pretreatment process, an intermediate slag removal process, and a decarburization process.

In converter-type refining furnaces, oxygen is supplied to the molten iron to decarbonize it and produce steel through blowing. Prior to this decarbonization process, it is common to perform molten iron pretreatment to remove the silicon and phosphorus from the molten iron, and the process up to decarbonization is called primary refining. Methods for molten iron pretreatment include desiliconization in a torpedo car, which is a vessel for transporting molten iron, and using a converter-type refining furnace exclusively for dephosphorization, but the most widely used method is to perform desiliconization, dephosphorization, and decarbonization in a single converter-type refining furnace.

Patent Document 1 discloses a technology in which, as the first step, scrap iron and molten iron are charged into a top and bottom blowing converter, as the second step (molten iron pretreatment step), dephosphorization is performed by adding flux and blowing acid, and immediately as the third step (intermediate slag removal step), the converter is turned on its side to remove the slag generated in the second step, as the fourth step (decarburization step), decarbonization and dephosphorization are performed to the specified [C] and [P] by adding flux and blowing acid, as the fifth step, steel is tapped while leaving the slag generated in the fourth step, as the sixth step, the furnace is turned upright, and carbonaceous material is added while performing bottom blowing stirring to reduce the amount of FeO in the slag, and the process returns to the first step, and the process is repeated up to the sixth step. This technology is widely used as a primary refining method because it allows hot recycling of slag, allows for large amounts of scrap to be input, and reduces the amount of slag generated.

When smelting molten iron using the above-mentioned method, the slag removal rate of the slag discharged in the intermediate slag removal process (=mass of slag removed/mass of slag in the furnace before removal) greatly affects the refining performance in the decarburization process and subsequent processes. In the dephosphorization reaction, the ratio of P in the molten iron to P in the slag (hereinafter referred to as _P distribution) is determined according to the ratio of CaO to _SiO2 in the slag (CaO/SiO2 mass ratio, hereinafter referred to as basicity C/S). Therefore, in the final dephosphorization in the decarburization process, the higher the slag removal rate in the intermediate slag removal process, the less slag remains in the converter furnace among the slag generated in the hot metal pretreatment process, so that the CaO source added in the subsequent decarburization process can be saved.

Prior to the intermediate slag-removal process, in the hot metal pretreatment process, the C in the hot metal reacts with the FeO in the slag, unavoidably generating CO gas, causing the slag to foam, a phenomenon known as foaming. In the intermediate slag-removal process, the converter-type refining furnace is tilted to discharge this foamed slag outside the furnace, and by making the slag foam as much as possible in the hot metal pretreatment process without interfering with operation, the volume of slag in the converter before intermediate slag-removal can be increased, and the amount of slag to be removed can be increased. However, since it is difficult to directly observe the inside of a converter-type refining furnace during blowing, a technology has been developed to determine the height of the top surface of the slag (hereinafter referred to as the slag surface) using microwaves.

Patent Document 2 discloses a technology for measuring the slag height in a refining furnace using a microwave radar-type slag level meter. This technology makes it possible to grasp the time-varying behavior of the slag surface during blowing in a converter in real time, making it possible to take action such as changing the blowing conditions, such as the oxygen supply rate, according to the slag surface height (hereinafter also referred to as the slag level). By stopping blowing when the slag surface height reaches the top of the converter-type refining furnace, it is possible to increase the amount of slag discharged.

In the hot metal pretreatment process, if the slag level in the converter-type refining furnace reaches a certain level, there is a risk that the slag will overflow from the refining furnace, and the deP treatment cannot be continued any further. For this reason, Patent Document 3 discloses a technique in which "the slag level in the hot metal ladle is measured, and at the same time, the weight of the hot metal in the hot metal ladle is measured, and the level of the hot metal is calculated from the obtained values, and if the difference between the measured value and the calculated value exceeds a predetermined value, a predetermined amount of foaming inhibitor is added to the hot metal ladle to advance the desiliconization reaction of the hot metal." This technique utilizes the measured slag level itself.

On the other hand, when considering increasing the mass of slag removed in the intermediate slag removal process, the volume of the converter-type refining furnace does not change, the volume of molten iron in the furnace remains almost constant, and the critical slag surface height that does not cause operational problems is also determined, so the volume of slag that has risen to the critical slag surface height in the converter before intermediate slag removal is constant. If the tilting angle of the refining furnace during slag removal is also constant, the volume of slag remaining in the converter during intermediate slag removal is also constant, and the volume of the removed slag is also constant. Therefore, in order to increase the mass of removed slag, it is necessary to create foamed slag with the highest possible density as the removed slag.

As a technique for adjusting the density of foamed slag, Patent Document 4 discloses a technique for adjusting the density of slag in a converter-type refining furnace by changing the rate at which CO gas is generated in the furnace during refining. With this technique, the degree of slag foaming can be changed by the rate at which CO gas is generated by the reaction of the gaseous oxygen source and solid oxygen source supplied into the furnace with the carbon in the molten iron, and the slag density tends to decrease as the CO gas generation rate increases.

Japanese Patent Application Publication No. 05-140627 Japanese Patent Application Publication No. 03-281717 Japanese Patent Application Publication No. 08-143925 JP 2015-218338 A

When treating molten iron using the method of Patent Document 1, the initial slag mass and composition in the molten iron pretreatment process can be calculated from the slag mass and composition in the previous channel and the mass and composition of the auxiliary materials added before the molten iron pretreatment process. Since almost all of the Si in the molten iron is oxidized in the molten iron pretreatment process, the slag mass and composition at the end of the molten iron pretreatment process can be calculated from the mass and composition of the auxiliary materials added in the molten iron pretreatment process, and when the mass of the slag that is removed during the process is known, the mass of auxiliary materials containing CaO to be added in the decarburization process can be calculated according to the target P concentration.

The problem at this time is the variation in the mass of slag that is intermediately discharged. When the slag surface height in the furnace at the end of the molten iron pretreatment process is constant and the tilting angle of the converter-type refining furnace during the intermediate slag discharge process is kept constant, the volume of the discharged foamed slag can be estimated as a constant value, as mentioned above. The mass of the discharged slag can be estimated by multiplying the volume of the discharged foamed slag by the apparent density of the discharged foamed slag. However, when the mass of the slag discharged during the intermediate slag discharge process is measured with a weighing machine, it was found that there is variation in the mass of the discharged slag even when slag that has been foamed to the same height as measured by the slag level meter is discharged at the same tilting angle, i.e., even when the same volume of slag is discharged. This variation is large, and if the mass of slag to be discharged is estimated to be less than the actual mass discharged, auxiliary materials will be added in a state where a larger mass of slag than calculated actually remains in the converter-type refining furnace, resulting in a lower slag basicity C/S than the slag basicity C/S set according to the target P concentration, resulting in poor deP. In order to avoid poor deP, the problem was that an excess of CaO source had to be added to take the variation into account so that the basicity C/S was greater than the basicity set according to the target P concentration.

The present invention aims to provide a method for producing molten iron using a converter-type refining furnace, which can produce molten iron without adding an excessive amount of CaO source.

As a result of intensive research by the inventors in order to solve the above-mentioned problems, it was found that the cause of the variation in the mass of slag discharged is the difference in density of the discharged foamed slag, and that even if the height reached by the slag surface in the converter-type refining furnace at the end of the hot metal pretreatment process is the same, the mass of the slag discharged differs depending on the rate of rise of the slag surface height. In other words, the apparent density of the discharged slag was previously treated as a constant value when estimating the mass of slag discharged, but it was found that when the rate of rise of the slag surface height is slow, the bubbles in the foamed slag are small and the apparent density of the foamed slag discharged is large, and conversely, when the rate of rise of the slag surface height is fast, the bubbles in the foamed slag are large and the apparent density of the foamed slag discharged is small. In intermediate slag discharge, the slag in the upper layer of the slag in the converter-type refining furnace is preferentially discharged. When the rate at which the slag surface height rises is slow, compared to when the rate at which the slag surface height rises is fast, it can be assumed that the density of the slag in the upper layer of the slag in the converter-type refining furnace before intermediate slag removal is relatively high.

Based on this knowledge, even if the volume of slag discharged in the intermediate slag discharge process after the hot metal pretreatment process is constant, the density of the discharged slag changes depending on the rate of rise of the slag surface height, so that the apparent density used to estimate the mass of slag discharged can be adjusted according to the rate of rise of the slag surface height, improving the accuracy of estimating the mass of the discharged slag. In addition, it is also possible to determine the relationship between the rate of rise of the slag surface height and the mass of auxiliary materials added in the decarburization process and directly adjust the amount of auxiliary materials added. It would seem that the above-mentioned operations would not be necessary if the mass of the slag discharged was measured directly using a weighing machine; however, since the discharged slag contains iron particles that were scattered into the slag during the blowing process in the hot metal pretreatment process, an error would occur if the mass of the slag measured by the weighing machine was used directly. In addition, the mass of the slag discharged by the weighing machine is determined after the intermediate slag discharge process, which means that it will not be possible to prepare the auxiliary materials to be added in the first half of the decarburization process in time.

The present invention was conceived based on the above-mentioned considerations, and was completed by clarifying the relationship between the rate of rise of the slag surface height and the mass of the added auxiliary material. The specific means are as follows.
[1] A method for producing molten iron, comprising the steps of: a molten iron pretreatment process using a converter-type refining furnace to perform desiliconization and dephosphorization by blowing the slag in a state in which the slag and molten iron coexist; an intermediate slag removal process in which the furnace is tilted to remove the slag from the converter-type refining furnace; and a decarburization process in which CaO-based auxiliary materials are added to the converter-type refining furnace and then blown thereinto to perform decarbonization and finish dephosphorization, characterized in that the slag surface height at the end of the molten iron pretreatment process is measured and the CaO basic unit of the auxiliary materials to be added in the decarburization process is determined in accordance with the rate of change of the slag surface height.
[2] A method for producing molten iron, characterized in that, when producing molten iron by the method for producing molten iron described in the above [1], a CaO basic unit in the auxiliary raw material to be added is determined according to formula (1).
W' _CaO =a・(1/h _foam -1/h' _foam )+W _CaO +E _CaO (1)
W' _CaO : CaO unit content in added auxiliary raw material (kg/ton)
_h'foam : actual change rate of slag surface height (m/s)
a: constant h _foam : rate of change of average slag surface height (m/s)
W _CaO : Thermodynamically necessary CaO unit consumption in auxiliary raw materials (kg/ton) determined according to the target P concentration and temperature under the condition that the rate of change of slag surface height is h _foam
E _CaO : CaO basic unit in auxiliary raw materials added to reduce deP defects and variations (kg/ton)

The present invention not only reduces the amount of auxiliary materials that were added in excess during the decarburization process, but also reduces the rate of dephosphorization failure during the decarburization process.

Processing Flow Relationship between the CaO unit content in added auxiliary materials and [%P] after decarburization treatment

[Definition of terms]
In the present invention, molten iron refers to molten pig iron and molten steel. In the decarburization process, the C content in the molten pig iron is reduced to turn into molten steel, but in the present invention, the molten iron in the furnace is generally referred to as molten iron.

The auxiliary raw materials are mainly refining agents for the purpose of slag formation, which are added to the converter-type refining furnace in the hot metal pretreatment process, the decarburization process, and before and after these processes, and refer to quicklime, limestone, dephosphorization slag, decarburization slag, iron ore, sintered ore, dust, dolomite, calcium ferrite, silica, manganese ore, recycled products containing oxides generated in the ironmaking process including blast furnaces, etc. Note that the components of the auxiliary raw materials are analyzed in advance, so the mass of each component can be calculated according to the amount added. CaO-based auxiliary raw materials refer to auxiliary raw materials with a Ca content of 50% or more calculated as CaO.
The CaO basic unit refers to the amount of CaO per mass of molten iron, which is calculated by multiplying the mass of a CaO-containing auxiliary material by the CaO concentration, and dividing the result by the mass of the molten iron, and is often expressed in units of kg/ton.

[Processing Procedure]
The present invention is assumed to be carried out in succession from the end of the hot metal pretreatment process to the beginning of the decarburization process, which is a series of processes for desiliconization, dephosphorization, and decarbonization, using a converter-type refining furnace, in which a hot metal pretreatment process is carried out by blowing in a state where slag and hot metal coexist, followed by an intermediate slag removal process in which the furnace is tilted to remove the slag from the furnace, and a decarburization process in which CaO-based auxiliary materials are added to the furnace and then blown, to refine molten iron, and to be carried out mainly from the end of the hot metal pretreatment process to the beginning of the decarburization process. The following description will be given with reference to Fig. 1. In Fig. 1, (A) indicates the hot metal charging process, (B) indicates the hot metal pretreatment process, (C) indicates the intermediate slag removal process, (D) indicates the decarburization process, and (E) indicates the tapping process.

When molten iron is produced using one converter-type refining furnace 1, after the decarburization process of the front charge is completed and the molten iron 3 is tapped (see FIG. 1(E)), the furnace body is tilted to discharge the slag (not shown) so that some of the slag remains in the furnace. The volume of the slag remaining in the furnace in the front charge can be calculated according to the tilting angle of the furnace body. Since the slag in the converter-type refining furnace at this time is not formed, it can be converted to the slag mass by multiplying it by the slag density calculated in advance.

To refine the molten iron in the ch, the molten iron is then charged from the pouring ladle 2 into a converter-type refining furnace 1 as shown in Figure 1 (A), and iron sources such as scrap and auxiliary materials for slag production are added. The amount of CaO-based auxiliary materials required in the molten iron pretreatment process shown in Figure 1 (B) can be calculated from information such as the Si in the molten iron, the molten iron temperature, the blast furnace molten iron blending ratio, and the mass of slag carried over from the previous ch. Because almost all of the Si in the molten iron is oxidized by blowing using the oxygen supply lance 5 in the molten iron pretreatment process, the slag basicity C/S at the end of the molten iron pretreatment can be calculated. At the end of the hot metal pretreatment, Si in the hot metal is oxidized, and the reaction between the oxygen blown from the oxygen supply lance 5 and the C in the hot metal as well as between FeO in the slag and the C in the hot metal becomes active, causing foaming of the slag 4 in the furnace. If the foamed slag 4 is blown directly from the top of the furnace, it will hinder operation. If the furnace is tilted after reaching the top of the furnace, the slag removed during tilting may burn auxiliary equipment of the converter. Therefore, when the top surface (slag surface) of the foamed slag 4 reaches a predetermined height, blowing in the hot metal pretreatment is usually stopped and the intermediate slag removal process is started. In the present invention, the measurement result of the slag surface height at the end of the hot metal pretreatment using the slag level gauge 6 is used to determine the completion of the hot metal pretreatment process and to calculate the amount of auxiliary materials to be added in the decarburization process. The rate of change in the slag surface height used to calculate the amount of auxiliary raw materials to be added is calculated by measuring the slag surface height multiple times within about one minute before blowing is stopped, and using a method such as the least squares method.

In the intermediate slag removal process shown in FIG. 1(C), the furnace body of the converter-type refining furnace 1 is tilted to remove slag so that the amount of CaO added in the decarburization process can be reduced and the maximum amount of slag is removed without discharging molten iron. A slag removal ladle 7 is placed on a cart 8, and the slag 4 discharged from the converter-type refining furnace 1 is stored in the slag removal ladle 7.

The volume of the discharged slag is largely determined by the height of the foamed slag at the end of the hot metal pretreatment and the tilting angle of the furnace body, but as mentioned above, the density of the discharged foamed slag varies, so the mass of the discharged slag changes even if the volume is constant. Therefore, the mass of slag remaining in the furnace after intermediate slag discharge, which is necessary to calculate the amount of CaO to be added in the decarburization process, also changes, so the amount of CaO required to obtain the target P concentration changes even if the volume of the discharged slag is the same.

In the conventional method, the CaO consumption rate D _CaO (kg/ton) in the auxiliary material added in the decarburization process shown in FIG. 1(D) can be determined as follows. Since the molten iron after the decarburization process is strongly stirred in the high-temperature environment in the converter-type refining furnace, the P distribution, which is the ratio of the P concentration in the slag to the P concentration in the molten iron, is determined mainly by the temperature and slag composition, especially the basicity C/S, at the end of the decarburization process. The P distribution is one of the indicators including a thermodynamic element, and is greatly influenced by the basicity C/S in the slag. Once the P distribution is determined, the P concentration in the molten iron and the P ₂ O ₅ concentration in the slag can be calculated according to the amount of molten iron and slag. Since the thermodynamically required basicity C/S is determined according to the target P concentration and the target temperature at the end of the decarburization process, the thermodynamically required CaO consumption rate W _CaO (kg/ton) in the auxiliary material is determined according to the slag condition after the intermediate slag removal process. D _CaO is a value obtained by adding W _CaO to the CaO basic unit E _CaO (kg/ton) in the auxiliary raw material added to reduce variation.

In the method of the present invention, the CaO consumption rate W'CaO (kg/ton) in the added auxiliary raw materials can be calculated from the rate of change of the slag surface height _h'foam (m/s) measured at the end of the hot _metal pretreatment process using the following formula (1). As will be described later, it is a feature of the present invention that _ECaO of the present invention can be made smaller than _ECaO of the conventional method.
W' _CaO =a・(1/h _foam -1/h' _foam )+W _CaO +E _CaO (1)

Here, the derivation of formula (1) will be explained. The mass of slag in the molten iron pretreatment process (mass of slag in the furnace before intermediate slag draining) is W ₁ (kg), the basicity C/S of the slag is B ₁ (-), the CaO concentration in the slag is %CaO, the SiO ₂ concentration is %SiO ₂ , the volume of the foamed slag drained in the intermediate slag draining process is V ₁ (m ³ ), the density of the foamed slag drained is ρ ₁ (kg/m ³ ), the mass of slag remaining in the furnace after intermediate slag draining is W ₂ (kg), and the slag basicity C/S in the decarburization process is B ₂ (-). The foamed slag volume _V1 to be discharged can be calculated by estimating the slag volume before slag discharge from the slag surface height in the furnace at the end of the hot metal pretreatment process, and estimating the volume of the slag remaining in the furnace from the slag discharge tilting angle at the time of intermediate slag discharge, so that _V1 can be obtained as the difference between the two volumes. The basicity _B2 should be determined in advance for each type of smelting. The amount of CaO added in the decarburization process is A _CaO (kg).
At this time, equations (2) and (3) hold true.
B ₁ =%CaO/%SiO2 _... (2)
B ₂ = (W ₂ ×%CaO/100+A _CaO )/(W ₂ ×%SiO ₂ /100) ... (3)
Moreover, when considering the mass balance, equation (4) holds true.
W ₂ = W ₁ - V ₁ ×ρ ₁ ...(4)

Here, it has been clarified from experiments that the density of foamed slag discharged ρ ₁ can be expressed as ρ 1 = b1/h _foam when expressed in terms of the reference average slag surface height change rate h foam (m/s) based on the fact that the higher the change rate of the slag surface height, the smaller the density of foamed _slag discharged. Here, b1 is a constant. The reference average slag surface height change rate h _foam can be calculated by averaging several charges using the actual results of the change rate of the _slag surface height under similar refining conditions, i.e., the Si concentration in the hot metal before treatment, the amount of slag carried over from the previous channel, the amount of auxiliary raw materials added in the hot metal preliminary treatment process, and the conditions close to the target basicity C/S.

Furthermore, by transforming equations (2), (3), and (4) and factoring in the amount of CaO A _CaO (kg) added in the decarburization treatment step, equation (5) is obtained.
(Reference: Intermediate steps)
B ₂ = (W ₂ × %CaO/100 + A _CaO ) / (W ₂ × %SiO ₂ /100) (3) W ₂ × %SiO ₂ /100 × B ₂ = W ₂ × %SiO ₂ /100 × B ₁ + A _CaO Transform equation (3) and substitute A _CaO into equation (2) = W ₂ × %SiO ₂ /100 × B ₂ - W ₂ × %SiO ₂ /100 × B ₁
A _CaO = W ₂ ×%SiO ₂ /100 × (B ₂ −B ₁ )
A _CaO = (W ₁ - V ₁ × b1/h _foam ) × %SiO ₂ /100 × (B ₂ - B ₁ ) ... (5)
The value obtained by dividing A _CaO (kg) in formula (5) by the amount of molten iron (tons) is W _CaO (kg/ton) in formula (1) above, which is the CaO basic unit added in the decarburization process under the condition of the reference average slag surface height change rate h _foam (m/s). Also, B ₂ is the set basicity C/S thermodynamically required to obtain the target P concentration.

A _CaO (kg) shown in formula (5) is the amount of CaO required under the condition of the standard average slag surface height change rate h _foam (m/s), but if the slag surface height change rate changes to h' _foam (m/s), ρ ₁ changes according to the change rate, and the mass of slag remaining in the furnace changes, so it is necessary to add an amount of CaO corresponding to the change. At this time, based on formula (4) and "ρ ₁ = b1/h _foam " formula, the amount of CaO A' _CaO (kg) required when the slag surface height change rate is h' _foam (m/s) can be expressed by formula (6).
A' _CaO = (W ₁ - V ₁ × b1/h' _foam ) × %SiO ₂ /100 × (B ₂ - _{B 1} ) ... (6)
(6) Formula - (5) Formula A' _CaO -A _CaO = (1/h _foam -1/h' _foam ) x V ₁ x b1 x %SiO ₂ /100 x (B ₂ - _{B 1} )
A' _CaO = (1/h _foam -1/h' _foam ) x V ₁ x b1 x %SiO ₂ /100 x (B ₂ - _{B 1} ) + A _CaO
Where:
V ₁ × b1 × %SiO ₂ /100 × (B ₂ - B ₁ ) = b2 (8)
Substituting this, _A'CaO = b2 × (1/h _foam - 1/h' _foam ) + _ACaO ... (7)

_A'CaO shown in formula (7) is the amount of CaO required under conditions where the rate of change of slag surface height has changed to _h'foam (m/s), and is divided by the amount of molten iron (tons), and the b2 part of formula (8) above is replaced with a to obtain the first and second terms on the right side of formula (1). The left side of formula (8) is a function with variables _V1 , % _SiO2 , _B2 , and _B1 , but these variables can be treated as approximately constants if the same product is being refined, so a can also be treated as a constant when the same product is being smelted.

E _CaO on the right side of equation (1) is CaO added in addition to the CaO described above so that deP is not insufficient in response to the variations in the deP ability and P concentration of the slag, and can be determined based on the actual results of the slag composition and P concentration at the end of the decarburization process. Generally, a value of up to about 3 kg/ton is sufficient, and it is often set to about 1 to 2 kg/ton. In the present invention, since the rate of change of the slag surface height is taken into consideration, it is possible to set the amount of CaO to a smaller amount than the amount added in excess in the conventional manufacturing method, which is affected by the variations in the density of the foamed slag.

The constant a in formula (1) can be found from the rate of change of the slag surface height, the CaO consumption rate, and the actual value of the P concentration in the molten iron after decarburization. By using formula (1), it is possible to calculate the CaO consumption rate _W'CaO (kg/ton) in the auxiliary materials as soon as _h'foam (m/s) obtained at the stage when the hot metal pretreatment is completed is known, and it is possible to proceed with the cutting out of the auxiliary materials during intermediate slag removal.

The above describes formula (1) for calculating the CaO unit consumption to be added in the decarburization process from the rate of change of the slag surface height, but the method of calculating the CaO unit consumption is not limited to this. The relationship between the rate of change of the slag surface height and the density of the foamed slag may be found, and the CaO unit consumption may be calculated using the density of the foamed slag according to the rate of change of the slag surface height, or the relationship between the rate of change of the slag surface height and the mass of slag discharged may be found, and the CaO unit consumption may be calculated using the amount of slag discharged according to the rate of change of the slag surface height.

[How to check the effect]
The effect of the invention was confirmed by comparing the ultimate P concentration after decarburization treatment when the same steel type was melted and confirmed that when the amount of CaO added was reduced, the ultimate P concentration did not change even if the amount of CaO added was reduced, and that when the amount of CaO added was increased, the ultimate P concentration decreased.

According to the flow shown in Figure 1, the molten iron was desiliconized, dephosphorized, and decarbonized in the same converter-type refining furnace. The amount of processing in the converter-type refining furnace 1 was about 420 tons, including iron sources such as scrap, and auxiliary materials such as quicklime, limestone, and iron ore were added in the first half of the molten iron pretreatment as auxiliary materials for adjusting the basicity C/S and for cooling. The calculated slag amount at the end of the molten iron pretreatment shown in Figure 1 (B) is about 30 tons, and the basicity C/S is about 1.1. During the molten iron pretreatment, the slag surface height in the furnace was measured using a slag level gauge 6 attached to the top of the furnace body while blowing. In the second half of the molten iron pretreatment, slag foaming occurred, and blowing was stopped when the slag surface height reached a predetermined critical height. The invention example and the comparative example each had 8 channels, and in order to match the conditions, the channel in which the P concentration in the sample analysis results taken after the molten iron pretreatment was 0.077% was selected.

In the comparative examples, auxiliary materials were added under all the conditions of the comparative examples to satisfy the CaO consumption unit D _CaO = 21.0 kg/ton required to adjust the basicity C/S during the decarburization treatment to 3.0, which was previously determined assuming a target P concentration of 0.015% at the end of the decarburization treatment, a temperature of 1630°C, and a constant amount of slag discharge. Under these refining conditions, the thermodynamically required CaO consumption unit W _CaO was calculated to be 19.5 kg/ton, and the CaO consumption unit E _CaO in the auxiliary materials added to reduce deP failure and variation was calculated to be 1.5 kg/ton. As a result, D _CaO = W _CaO + E _CaO = 21.0 kg/ton.

On the other hand, in the invention example, the CaO consumption unit _W'CaO required for adjusting to the basicity C/S during decarburization was calculated by substituting the CaO consumption unit WCaO shown on the left, the rate of change of the slag surface height _h'foam obtained from the change in the slag surface height over time for the final 60 seconds of the hot metal pretreatment, and the average rate of change of the slag surface height _hfoam = 0.06 m/s obtained in advance from the change in the slag surface height over time under similar refining conditions into formula (1). It was found that under the standard condition of the average rate of change of the slag surface height _hfoam = 0.06 m/s, the thermodynamically required CaO consumption unit _{WCaO could} be handled at a constant value of 19.5 kg/ton, and the CaO consumption unit _ECaO in the auxiliary material added to reduce deP defects and variation could be handled at a constant value of 0.50 kg/ton, and these values were adopted for calculation. The constant a was determined by calculating the correlation between the amount of CaO added and the rate of change in the slag surface height when the [%P] at the end of the decarburization process was 0.015% based on past data, and was set to 0.15 under these refining conditions.

In the decarburization process shown in Figure 1 (D), prepared auxiliary materials containing CaO were added in the first half of the process and blown, followed by finish deP and deC treatment. After the treatment, molten iron samples were taken and the [%P] concentration in the molten iron was analyzed. Table 1 shows the rate of change in slag surface height, the CaO consumption rate in the added auxiliary materials, and the [%P] concentration in the molten iron after decarburization treatment for the present invention and comparative examples.

The relationship between the CaO consumption unit in the auxiliary raw material and [%P] after decarburization treatment is shown in Figure 2 when _W'CaO , the CaO consumption unit calculated in the present invention, is added, and when D _CaO is added as a comparative example. In the comparative example (black triangles), D _CaO was added at 21.0 kg/ton, and as a result, the [%P] after treatment was 0.013% to 0.015%. (All of them met the target [%P] ≦ 0.015, but some tended to be over-dephosphorized.)

On the other hand, in the invention example (white circle in FIG. 2) in which the CaO consumption unit E _CaO in the auxiliary raw material added to reduce deP failure and variation was reduced to 0.50 kg/ton and the CaO consumption unit W' _CaO calculated by formula (1) was added, the [%P] after treatment was 0.014% to 0.015%. In the invention example, the W' _CaO calculated from formula (1) was less than the D _CaO = 21.0 kg/ton added in the comparative example. Under the condition (invention example 18) in which the rate of change of the slag surface height due to foaming was the largest, the W' _CaO was 20.9 kg/ton, which was close to the comparative example. This is thought to be because the apparent density of the slag discharged in the intermediate slag discharge process was small due to the large rate of change of the slag surface height due to foaming, and the amount of SiO ₂ remaining in the furnace was large. In addition, _W'CaO under the condition (Example 1) in which the rate of change in the slag surface height due to foaming was the smallest was 18.9 kg/ton, but this is thought to be because the rate of change in the slag surface height due to foaming was small, so the apparent density of the slag discharged in the intermediate slag discharge process was large and the amount of _SiO2 remaining in the furnace was small. As described above, it was found that the effect of reducing the CaO consumption rate while maintaining the deP capacity was obtained by optimizing the CaO consumption rate _W'CaO using _equation (1) according to the rate of change in the slag surface height h'foam at the end of the hot metal pretreatment.

In the examples, the constant a is 0.15, the average rate of change of the slag surface height h _foam =0.06 m/s, the CaO consumption unit W _CaO required to adjust the basicity to C/S during decarburization treatment is 19.5 kg/ton, and the CaO consumption unit E _CaO in the auxiliary raw material added to reduce deP failure and variation is reduced from 1.5 kg/ton to 0.5 kg/ton, but these values can be adjusted depending on the refining conditions such as the target P concentration and temperature, and are not limited to the examples described in the examples.

1: Converter-type refining furnace 2: Pouring ladle 3: Molten iron 4: Slag 5: Oxidizing lance 6: Slag level gauge 7: Slag ladle 8: Cart

Claims (2)

A method for producing molten iron, which comprises a hot metal pretreatment process using a converter-type refining furnace to perform desiliconization and dephosphorization by blowing slag and hot metal together, an intermediate slag removal process in which the furnace is tilted to remove the slag from the converter-type refining furnace, and a decarburization process in which CaO-based auxiliary materials are added to the converter-type refining furnace and blown into it to perform decarbonization and final dephosphorization, the method being characterized in that the slag surface height at the end of the hot metal pretreatment process is measured, and the CaO unit value of the auxiliary materials to be added in the decarburization process is determined according to the rate of change in the slag surface height. 2. A method for producing molten iron according to claim 1, characterized in that, in producing molten iron, a basic unit of CaO in the auxiliary raw materials to be added is determined according to formula (1).
W' _CaO =a・(1/h _foam -1/h' _foam )+W _CaO +E _CaO (1)
W' _CaO : CaO unit content in added auxiliary raw material (kg/ton)
_h'foam : actual change rate of slag surface height (m/s)
a: constant h _foam : rate of change of average slag surface height (m/s)
W _CaO : Thermodynamically necessary CaO consumption unit in auxiliary raw material (kg/ton) determined according to the target P concentration and temperature under the condition that the rate of change of slag surface height is h _foam
E _CaO : CaO basic unit in auxiliary raw materials added to reduce deP defects and variations (kg/ton)

Images