Desulfurization method for molten iron

Abstract

To provide a molten iron desulfurization method in which, when, with respect to molten iron charged inside a ladle, a stirring blade is immersed inside the molten iron so as to be stirred, desulfurization treatment is efficiently executed using an inexpensive aluminum-based steel refining secondary material having a low M.Al concentration and a high AlN concentration, based on the decomposition reaction of the AlN to improve desulfurization efficiency.SOLUTION: Provided is a molten iron desulfurization method, in which, with respect to molten iron 4 charged to a ladle 1, an impeller 3 having a stirring blade 2 is immersed inside the molten iron 4 so as to be stirred, using a mechanical stirring desulfurization apparatus in which a desulfurization agent 5 obtained by mixing a steel refining secondary material containing lime and aluminum is added onto a bath face of the molten iron 4 stirred by the rotation of the impeller 3 from an upper part of the ladle 1 to execute desulfurization treatment, where the conditions of a pretreatment temperature of the molten iron 4>1,360°C, the pretreatment temperature of the molten iron 4×0.0003675-0.4575-Ti% in the molten iron 4 before the treatment<0, CaO>0.94 kg / t, mass%CaO / mass%AlN<3.92 and 1.02<mass%CaO / mass%Al2O3 equivalent<1.30 are satisfied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for desulfurizing molten pig iron, in which desulfurization is carried out by adding secondary materials for desulfurization to sulfur-containing molten pig iron in a ladle and using mechanical stirring. [Background technology]

[0002] Generally, molten pig iron tapped from a blast furnace or the like contains a high concentration of sulfur. Therefore, the sulfur-containing molten pig iron is subjected to desulfurization treatment to remove the sulfur from the molten pig iron before charging it into a converter. Methods for desulfurizing molten iron are disclosed in, for example, Patent Documents 1 to 10. Patent Document 1 aims to effectively desulfurize molten pig iron by using aluminum dross, which has a lower metallic aluminum content than conventional steel refining secondary materials, as a method for desulfurizing molten pig iron. Specifically, the method discloses supplying a desulfurization agent to the molten pig iron, the desulfurization agent being a mixture of steel refining secondary materials containing lime and aluminum nitride in a ratio relative to lime that satisfies the following relationship: 0.6<[wt%CaO] / ([wt%AlN]+[Al2O3 equivalent])<45 (Equation (4)).

[0003] Patent Document 2 aims to efficiently reduce sulfur in hot metal by a mechanical stirring desulfurization method in a hot metal desulfurization method. Specifically, when hot metal tapped from a blast furnace is desulfurized by mechanical stirring at the hot metal stage before the decarburization process in a converter, the converted M.Al [kg / t] is set to 1.90-1.34 × 10 -3 It is disclosed that a flux having a composition that satisfies the condition shown in T (Equation 1) is used.

[0004] Patent Document 3 aims to efficiently reduce sulfur in molten pig iron by a mechanical stirring desulfurization method in a molten pig iron desulfurization method. Specifically, when desulfurizing molten pig iron tapped from a blast furnace by mechanical stirring at the molten pig iron stage before the decarburization process in a converter, the converted M.Al [kg / t] is ≧ 2.19-1.49 × 10 -3 T-6.66×10 -2It is disclosed that a flux having a composition that satisfies the condition shown in [Si] (Equation 1) is used.

[0005] Patent Document 4 aims to effectively perform molten iron desulfurization by mechanical stirring in a molten iron desulfurization method. Specifically, it discloses that when molten iron tapped from a blast furnace is desulfurized by mechanical stirring at the molten iron stage before the decarburization process in a converter, a flux having a composition that satisfies the following conditions for molten iron at a temperature of less than 1330°C: converted M.Al [kg / t] ≥ 0.15 kg / t, (CaO + MgO) / (SiO2 + converted Al2O3) ≤ 15, and X / ε = 0.2 to 0.9 is used.

[0006] Patent Document 5 aims to effectively perform molten iron desulfurization by mechanical stirring in a molten iron desulfurization method. Specifically, it discloses that when molten iron tapped from a blast furnace is desulfurized by mechanical stirring at the molten iron stage before the decarburization process in a converter, a flux having a composition that satisfies the following conditions for molten iron at 1330°C or higher: M.Al [kg / t] ≥ 0.05 kg / t, (CaO + MgO) / (SiO2 + Al2O3 equivalent) ≤ 30, and X / ε = 0.2 to 0.9 is used.

[0007] Patent Document 6 discloses a method for desulfurizing molten pig iron using a CaO-based desulfurization agent that does not contain fluorine compounds such as fluorite, which are used as CaO slag-forming promoters, to achieve a desulfurization rate equivalent to that achieved when a CaO-CaF2-based desulfurization agent is used. Specifically, the method discloses a desulfurization agent containing an alumina-metallic Al mixture containing 10-50 mass% metallic Al relative to CaO powder in a range of X mass% to X + 15 mass% calculated by the following equations (2), (3), and (4) depending on the temperature of the molten pig iron before desulfurization. The desulfurization agent is added to the surface of the molten pig iron being stirred by a stirring blade, whereby the molten pig iron is desulfurized. In equation (3), T is the temperature of the molten pig iron before desulfurization (°C).

[0008] Molten iron temperature: 1250°C or less X (mass%) = 20 … (2) Molten iron temperature: Over 1250°C and under 1340°C X (mass%) = 295 - 0.22 × T ... (3) When the molten iron temperature is 1340°C or higher, X (mass%) = 0.2 ... (4) Patent Document 7 aims to further improve the desulfurization efficiency in a method for desulfurizing molten pig iron. Specifically, the method for desulfurizing molten pig iron includes a desulfurization step in which molten pig iron to which a desulfurization agent has been supplied is stirred with a stirring blade, and the concentrations of CaO, Al2O3, and AlN in the desulfurization agent are as follows: It is disclosed that the formulas (1) and (2) are satisfied, and the desulfurization process includes a step S1 of adding a desulfurization agent while rotating the stirring blades with a rotational torque that satisfies the following formula (3), a step S2 of increasing the rotational torque of the stirring blades after the desulfurization agent addition step under conditions that satisfy the following formula (4), and a step S3 of maintaining the rotation of the stirring blades with a rotational torque that satisfies the following formula (5) after the rotational torque increase step.

[0009] 0.055≦AlNwt% / (CaOwt%+Al2O3wt%+AlNwt%)≦0.13 ···(1) 1.0≦CaOwt% / (Al2O3Wt%+AlNwt%)≦2.5 ···(2) 0.3≦τ1≦0.6 (3) 0.4≦Δτ≦1.2 (4) 0.8≦τ2≦1.2 (5) Patent Document 8 aims to stably produce low-sulfur molten pig iron while maintaining a high iron yield in molten pig iron desulfurization by mechanical stirring. Specifically, it discloses that when molten pig iron tapped from a blast furnace is desulfurized by mechanical stirring before being decarburized in a converter 1, the amount of flux added to the molten pig iron satisfies a predetermined relationship, and further, the flux has a composition such that a parameter obtained by dividing the amount of alkali metal oxide in the flux by the amounts of silicon oxide, aluminum oxide, and aluminum nitride in the flux falls within a predetermined numerical range.

[0010] CaO≧3.0kg / t 2≦(CaO+MgO) / (SiO2+ conversion Al2O3+AlN)≦15 However, converted Al2O3 = Al2O3 (kg / t) + 1.89 × M.Al (kg / t) Patent Document 9 aims to improve desulfurization efficiency even when the molten iron temperature is low. Specifically, in a method for desulfurizing molten iron in which the molten iron is desulfurized before the decarburization step, when a desulfurization agent containing CaO, MgO, SiO2, and AlN is added to the molten iron, the CaO consumption rate is set to 3.0 kg / t or more, the converted M.Al is set to 0.020 kg / t or more, and the AlN / (MgO+SiO2) ratio is set to 0.3 to 2.2. It is considered preferable that the AlN / (MgO+SiO2) ratio be set to 0.3 to 1.40.

[0011] Patent Document 10 aims to provide a steel refining auxiliary material that exhibits the effect of desulfurizing molten pig iron while reducing the risk of fire during storage. Specifically, the steel refining auxiliary material is supplied to the molten pig iron together with lime to promote the desulfurization reaction in the molten pig iron, and contains aluminum nitride in an amount of 20 wt% or more by weight percentage (wt%). It discloses that a mixture of lime and the steel refining auxiliary material is used as a desulfurization agent, and the amount of the steel refining auxiliary material in the desulfurization agent is adjusted to, for example, about 5.0 wt% by weight. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-020539 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-218392 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-218391 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-175058 [Patent Document 5] Japanese Patent Application Laid-Open No. 2015-175057 [Patent Document 6] Japanese Patent Application Laid-Open No. 2011-132566 [Patent Document 7] Japanese Patent Application Publication No. 2018-090859 [Patent Document 8] Japanese Patent Application Laid-Open No. 2017-048429 [Patent Document 9] Japanese Patent Application Laid-Open No. 2015-229784 [Patent Document 10] Japanese Patent Application Laid-Open No. 2015-147976 Summary of the Invention [Problem to be solved by the invention]

[0013] In the hot metal desulfurization process using a mechanical agitation desulfurization system, it is common to add a lime-based desulfurization agent (e.g., a secondary material for steel refining containing lime or aluminum) to desulfurize the hot metal, and then remove the slag to prevent resulfurization in the subsequent process. The aluminum-based secondary materials used in steel refining as lime-based desulfurization agents have a high desulfurization capacity as their deoxidizing capacity increases. However, secondary materials with high deoxidizing capacity are expensive due to their high metallic aluminum concentration (hereinafter referred to as M.Al concentration). Therefore, when using secondary materials with high M.Al concentration, cost must be taken into consideration.

[0014] On the other hand, inexpensive Al-based secondary materials for steel refining have a low M.Al concentration but a high content of the impurity AlN. It is known that AlN promotes deoxidation because it generates M.Al through a decomposition reaction at high temperatures. Thus, if a highly efficient desulfurization method can be established using inexpensive aluminum-based secondary steel refining materials with low M.Al concentrations, it will be possible to reduce the cost of desulfurization treatment.

[0015] Furthermore, some inexpensive aluminum-based secondary materials for steel refining are generated when aluminum is melted, and if these by-products could be processed as Al-based secondary materials for steel refining, it would lead to a reduction in industrial waste disposal costs and also reduce the burden on the environment. However, Patent Document 1 does not describe or suggest conditions related to the Ti concentration before treatment, so if the Ti concentration before treatment is low, the desulfurization ability may decrease. Furthermore, although the document describes a case where the desulfurization agent consumption rate is as high as 10 kg / t, it does not describe or suggest a case where the consumption rate is low.

[0016] In Patent Document 2, the addition of MgO reduces the CaO concentration in the desulfurization agent, reducing the activity of CaO. Therefore, a low lime consumption rate may result in reduced desulfurization efficiency. Furthermore, the document specifies the converted M.Al amount according to temperature. However, since the decomposition of AlN hardly progresses unless the temperature is above a certain level, a high AlN content at low temperatures may result in poor desulfurization. Furthermore, the document does not describe or suggest any conditions regarding the Ti concentration before treatment, so a low Ti concentration before treatment may result in reduced desulfurization performance.

[0017] In Patent Document 3, the addition of MgO reduces the CaO concentration in the desulfurization agent, reducing the activity of CaO. Therefore, a low lime consumption rate may result in reduced desulfurization efficiency. Furthermore, the document specifies the converted M.Al amount according to temperature. However, since the decomposition of AlN hardly progresses unless the temperature is above a certain level, a high AlN content at low temperatures may result in poor desulfurization. Furthermore, the document does not describe or suggest any conditions regarding the Ti concentration before treatment, so a low Ti concentration before treatment may result in reduced desulfurization performance.

[0018] In Patent Document 4, the addition of MgO reduces the CaO concentration in the desulfurization agent, which reduces the activity of CaO. Therefore, with a small lime consumption rate, the desulfurization efficiency may decrease. Furthermore, the document does not describe or suggest an appropriate blending method for high temperatures. Furthermore, the document does not describe or suggest conditions related to the Ti concentration before treatment, so that if the Ti concentration before treatment is low, the desulfurization ability may decrease.

[0019] In Patent Document 5, the addition of MgO reduces the CaO concentration in the desulfurization agent, which reduces the activity of CaO. Therefore, with a small lime consumption rate, the desulfurization efficiency may decrease. Furthermore, in this document, the decomposition of AlN does not proceed in the range below 1360°C, which may reduce the desulfurization capacity. Furthermore, this document does not describe or suggest any conditions related to the Ti concentration before treatment, so if the Ti concentration before treatment is low, the desulfurization capacity may decrease.

[0020] Patent Document 6 cannot be applied to equipment that does not project powder. Furthermore, the document does not specify or suggest any temperature or AlN content. Therefore, if a desulfurization agent with a high AlN content is used at low temperatures, the desulfurization rate may be reduced. Furthermore, the document does not specify or suggest any conditions for the Ti concentration before treatment, so if the Ti concentration before treatment is low, the desulfurization capacity may be reduced.

[0021] Patent Document 7 does not specify or suggest any temperature, so the desulfurization rate may decrease at low temperatures. Furthermore, since the upper limit of the AlN value is low in this document, desulfurization may not be possible when inexpensive Al-based secondary steel refining materials are used. Furthermore, since this document does not specify or suggest any conditions regarding the Ti concentration before treatment, desulfurization ability may decrease when the Ti concentration before treatment is low.

[0022] Patent Document 8 does not specify or suggest any temperature, so it is assumed that the desulfurization rate will decrease at low temperatures. Furthermore, since the upper limit of the AlN value in this document is low, desulfurization may not be possible if inexpensive Al-based secondary steel refining materials are used. Furthermore, since the C / A ratio in this document is high, slag formation may not be easily promoted. Furthermore, since this document does not state or suggest any conditions regarding the Ti concentration before treatment, desulfurization ability may decrease if the Ti concentration before treatment is low.

[0023] Patent Document 9 does not specify or suggest any temperature, so if inexpensive Al-based secondary steel refining materials are used, the desulfurization rate may decrease at low temperatures. Furthermore, since the document does not state or suggest any conditions regarding the Ti concentration before treatment, there is a possibility that the desulfurization ability may decrease if the Ti concentration before treatment is low. Although Patent Document 10 contains a large amount of AlN, there is no specification or suggestion regarding the temperature, so the desulfurization rate may decrease at low temperatures. Furthermore, since the C / A is high in this document, it may be difficult to promote slag formation. Furthermore, since this document does not state or suggest any conditions regarding the Ti concentration before treatment, there is a possibility that the desulfurization ability may decrease if the Ti concentration before treatment is low.

[0024] In view of the above problems, an object of the present invention is to provide a method for desulfurizing molten pig iron, which uses an inexpensive aluminum-based secondary material for steel refining that has a low M.Al concentration and a high AlN concentration, and which efficiently performs desulfurization based on the decomposition reaction of AlN when stirring molten pig iron charged in a ladle with a stirring blade immersed in the molten pig iron. This method enables an improvement in desulfurization efficiency. [Means for solving the problem]

[0025] According to the present invention Mechanically agitated desulfurization equipment The desulfurization method for molten iron is as follows: No heating When immersing an impeller having stirring blades in the molten pig iron charged in a ladle to stir the molten pig iron, a powder containing lime and aluminum is deposited on the bath surface of the molten pig iron being stirred by the rotation of the impeller. and does not contain fluorite Desulfurization treatment is carried out by adding a desulfurization agent mixed with secondary materials for steel refining from above the ladle. When , The desulfurization agent is added in a desulfurization agent consumption unit of 1.82 kg / t or more and 2.43 kg / t or less relative to the molten iron, The following conditions (1) to (5) must be satisfied: It is characterized by:

[0026] The temperature of the hot metal before treatment is greater than 1360°C (1) Temperature of the hot metal before treatment × 0.0003675 − 0.4575 − Ti% in the hot metal before treatment < 0 (2) CaO > 0.94 kg / t (3) mass% CaO / mass% AlN < 3.92 ···(4) 1.02 < mass% CaO / mass% Al₂O₃ equivalent < 1.30 ···(5) Preferably, when using the secondary material for steel refining containing aluminum, it is good to satisfy the conditions of formula (6) shown below.

[0027] mass% M.Al < 15% ···(6)

Advantages of the Invention

[0028] According to the present invention, when stirring by immersing the stirring blade into the molten iron charged in the ladle, an inexpensive aluminum-based secondary material for steel refining with a low M.Al concentration and a high AlN concentration is used, and based on the decomposition reaction of AlN, the desulfurization treatment is efficiently performed, making it possible to improve the desulfurization efficiency.

Brief Description of the Drawings

[0029] [Figure 1] It is a diagram schematically showing the configuration of the ladle and the impeller (KR method) in the present embodiment. [Figure 2] It is a diagram schematically showing the rotational diameter and height of the impeller (stirring blade). [Figure 3] It is a diagram regarding formula (1), showing the relationship between the pre-treatment temperature T [°C] of the molten iron and the desulfurization rate η [%] (pre-treatment temperature > 1360 °C). [Figure 4] It is a diagram regarding formula (2), showing the relationship between T × 0.0003675 - 0.4575 - Tii [%] and the desulfurization rate η [%] (T × 0.0003675 - 0.4575 - pre-treatment Ti% < 0). [Figure 5] It is a diagram regarding formula (3), showing the relationship between WCaO and the desulfurization rate η [%] (CaO > 0.94 kg / t). [Figure 6] It is a diagram regarding formula (4), showing the relationship between CCaO / CAlN and the desulfurization rate η [%] (mass% CaO / mass% AlN < 3.92). [Figure 7] It is a diagram related to formula (5), showing the relationship between CCaO / EAl2O3 and the desulfurization rate η [%] (1.02 < mass%CaO / mass%Al2O3 equivalent < 1.30).

Embodiments for Carrying Out the Invention

[0030] Hereinafter, embodiments of the hot metal desulfurization method according to the present invention will be described with reference to the drawings. Note that the embodiments described below are examples that embody the present invention, and do not limit the configuration of the present invention with specific examples. The present invention relates to a technology for a desulfurization method by a mechanical stirring method, in which hot metal 4 is charged into a ladle 1 and an impeller 3 having stirring blades 2 made of refractory is used, and an inexpensive aluminum-based steel refining auxiliary material is mixed. It is an efficient desulfurization method of hot metal 4 using a desulfurizing agent 5.

[0031] In the hot metal 4 desulfurization method according to the present invention, for the hot metal 4 charged into the ladle 1, a refractory 3 (hereinafter referred to as an impeller 3) having a plurality of stirring blades 2 is immersed in the hot metal 4, and lime and a desulfurizing agent 5 containing an aluminum-based steel refining auxiliary material with an M.Al concentration of less than 15 mass% are placed on the bath surface of the hot metal 4 being stirred by the rotation of the impeller 3 and added from above for desulfurization treatment. In the hot metal desulfurization method using a mechanical stirring type desulfurization device, Temperature condition of hot metal 4 before treatment > 1360 °C ··· (1) For the hot metal 4 satisfying formula (1), Ti concentration [mass%] in hot metal 4 before treatment > hot metal pretreatment temperature × 0.0003675 - 0.4575 ··· (2) When formula (2) is satisfied, regarding the blending conditions of the lime-based desulfurizing agent 5, CaO > 0.94 / t ··· (3) mass%CaO / mass%AlN < 3.92 ··· (4) 1.02 < mass%CaO / mass%Al2O3 equivalent < 1.30 ··· (5) The blending of the lime-based desulfurizing agent 5 is carried out so as to satisfy formulas (3) to (5).

[0032] In the present invention, the refining vessel is a ladle 1 (hot metal ladle). The molten iron 4 produced in a blast furnace or the like contains sulfur, and is subjected to desulfurization treatment at the molten iron 4 stage. Before describing the method for desulfurizing molten pig iron 4 of the present invention in detail, an example of the refining process of molten pig iron 4 will be described. Specifically, for example, molten pig iron 4 tapped from a blast furnace is received in a torpedo car and transported to a steelworks, where it is discharged from the torpedo car into a molten pig iron ladle 1 (ladle). In the ladle 1, it is moved to a slag removal position, where blast furnace slag present on the surface of the molten pig iron 4 in the ladle 1 is removed (slag removal). The molten pig iron 4 is then subjected to a desulfurization treatment, where the slag produced by the desulfurization reaction is removed. The desulfurized molten pig iron 4 is transported to the front of the converter and charged into the converter from the ladle 1. The now empty ladle 1 is returned to the discharge position, where the next charge of molten pig iron 4 is discharged from the torpedo car into the ladle 1.

[0033] FIG. 1 is a schematic diagram showing the configuration of a ladle and an impeller (KR method) in this embodiment. The present invention is directed to the KR process (batch processing in which molten pig iron 4 is stirred using a mechanical stirring device). In the KR process, an impeller 3 having a plurality of refractory stirring blades 2 is generally immersed in the molten pig iron 4 and the impeller 3 is rotated to stir the molten pig iron 4. For example, see JP 2003-82409 A and JP 2004-35934 A.

[0034] The method for desulfurizing the molten iron 4 will be described in detail with reference to the drawings. In this embodiment, the desulfurization treatment of the molten iron 4 carried out in such a refining process is carried out in the ladle 1 using a mechanical stirring (KR stirring) device. Specifically, as shown in FIG. 1, the desulfurization process of the molten pig iron 4 is carried out using a mechanical stirring desulfurization device in which a lime-based desulfurization agent 5 is added from above the bath surface to the molten pig iron 4, which has been charged into the ladle 1 and is being stirred by the rotation of an impeller 3, which is a stirring blade 2 made of refractory material. The lime-based desulfurization agent 5 is then introduced into the molten pig iron 4, and the molten pig iron 4 to which the desulfurization agent 5 has been added is forcibly stirred to promote the desulfurization reaction.

[0035] The molten iron 4 produced in a blast furnace or the like contains sulfur, and the sulfur contained in this steel (molten iron 4) is a harmful impurity that generally deteriorates the performance of the steel, so desulfurization is carried out at the molten iron 4 stage. Desulfurization treatment typically involves placing a desulfurization agent 5 on top of the molten iron 4 (adding it from above the bath surface of the molten iron 4) and rotating the impeller 3 to entrain the desulfurization agent 5 into the molten iron 4, thereby desulfurizing the S in the molten iron 4 according to the following chemical reaction formula (formula [a]).

[0036] CaO+ S =CaS+ O [a] Here, the underlined elements in formula [a] are elements present in the molten iron 4. The lime-based desulfurization agent 5 refers to, for example, a combination of quicklime (CaO) required for desulfurization treatment and an aluminum-based secondary material for steel refining (for example, aluminum dross: aluminum refining slag, a mixture of Al2O3 and metallic aluminum (hereinafter referred to as M.Al)) for the purpose of promoting slag formation and deoxidation as shown in formula [b]. A mixture of quicklime and aluminum dross is generally used as the lime-based desulfurization agent 5.

[0037] 2 Al +3 O =Al2O3 [b] The present invention provides a molten iron desulfurization method equivalent to that of conventional agents, using a lime-based desulfurization agent 5 that is mixed with an aluminum-based secondary material for steel refining that has a lower M.Al content than conventional agents and is inexpensive. As described above, in the present invention, when desulfurizing molten pig iron 4 charged into a ladle 1 (refining vessel), an impeller 3 having a plurality of stirring blades 2 made of a refractory material is immersed in the molten pig iron 4, and the molten pig iron 4 is stirred by rotating the impeller 3. A desulfurizing agent 5 containing lime and aluminum-based secondary materials for steel refining is added onto the bath surface of the stirred molten pig iron 4 (added from above the bath surface), thereby carrying out the desulfurization treatment. This is a technology for a method of desulfurizing molten pig iron using a mechanical stirring desulfurization device.

[0038] Now, the formula (1) is as follows. Compared to commonly used aluminum-based secondary materials for steel refining (those with a high M.Al concentration), inexpensive desulfurization agents with a high AlN content steadily promote deoxidation by AlN when the pre-treatment temperature of the molten iron 4 exceeds 1360°C. Therefore, the desulfurization reaction is promoted according to the following chemical reaction formula (formula [c]).

[0039] On the other hand, if the temperature is below 1360°C, the deoxidation capacity will decrease due to insufficient deoxidation. 2AlN+3 O =Al2O3+2 N [c] As a result of extensive research into the above, the temperature of the molten iron 4 before treatment was determined as shown in formula (1). Formula (1) is based on the examples described later. Pre-treatment temperature of hot metal 4 > 1360°C (1) Furthermore, the formula (2) is as follows:

[0040] According to the above formula [c], for AlN to contribute to deoxidation, it is desirable that the N concentration in the molten iron 4 is low. On the other hand, Ti also reacts with N as shown in the following formula [d]. Ti + N =TiN [d] Therefore, when there is a large amount of Ti, Ti and N react with each other, reducing the N concentration in the molten iron 4. On the other hand, when there is a small amount of Ti, the N concentration becomes high, which inhibits the promotion of deoxidation by thermal decomposition of AlN.

[0041] Here, we investigated the Ti concentration and desulfurization results in an actual plant and found that desulfurization is promoted under conditions that satisfy the following: post-treatment temperature of molten iron 4 [°C] × 0.0003675 - 0.4575 < Ti concentration in molten iron 4 after treatment [mass%]. As a result of intensive research into the above, the Ti concentration in the molten iron 4 was determined as shown in formula (2). Note that formula (2) is based on the examples described later.

[0042] Temperature of hot metal 4 before treatment [°C] × 0.0003675 − 0.4575 − Ti concentration in hot metal 4 before treatment [mass%] < 0 (2) Furthermore, the formula (3) is as follows: If the amount of CaO itself is small, sufficient desulfurization cannot be performed. For this reason, the amount of CaO in the desulfurization agent 5 added to the molten iron 4 must be at least 0.94 kg / t per ton of molten iron to be desulfurized.

[0043] As a result of extensive research, we have determined that CaO is defined as shown in formula (3). The details are based on the examples described below. CaO>0.94 kg / t (3) Furthermore, equation (4) is as follows: During the investigation, it was discovered that there is an optimum ratio of AlN to CaO in aluminum-based secondary steel refining materials with a low M.Al content. If the AlN to CaO ratio is 3.92 or higher, the supplied CaO cannot be sufficiently deoxidized, resulting in insufficient desulfurization capacity. Note that mass% refers to the component concentration in the entire lime-based desulfurization agent 5 that is added.

[0044] As a result of extensive research into the above, we have determined that mass% CaO / mass% AlN is defined as shown in formula (4). Note that formula (4) is based on the examples described later. mass%CaO / mass%AlN<3.92 (4) Furthermore, equation (5) is as follows: One way to improve the efficiency of the desulfurization reaction is to dissolve CaO (forming slag) to generate slag and enhance the mass transfer of S within the slag. If the CaO / Al2O3 equivalent is less than 1.1, the formation of CaO slag is not promoted, which may result in a decrease in desulfurization efficiency.

[0045] Al2O3 equivalent: mass%Al2O3+1.89×mass%M.Al+1.24×mass%AlN Note that mass% represents the component concentration in the entire lime-based desulfurizing agent 5 to be input. As a result of intensive research as described above, mass%CaO / mass%Al2O3 equivalent was defined as in formula (5). Note that formula (5) is based on the examples described later. 1.02 < mass%CaO / mass%Al2O3 equivalent < 1.30 ···(5) Furthermore, formula (6) is as follows.

[0046] As described above, the present invention is a desulfurization method aimed at enabling desulfurization even with an aluminum-based auxiliary material for steel refining that has a lower M.Al content and is less expensive than conventional ones. Examples of such aluminum-based auxiliary materials for steel refining with a low M.Al content include, for example, arc furnace ash, which is a by-product in the arc furnace refining of Al. As a result of intensive research as described above, mass%M.Al was defined as in formula (6). Note that formula (6) is based on the examples described later.

[0047] mass%M.Al < 15% ···(6) Table 1 shows the parameter definitions regarding the desulfurization method of hot metal in the present invention.

[0048]

Table 1

[0049] As shown in FIG. 1, in the present embodiment, as the lime-based desulfurizing agent 5, a mixture of lime (CaO) and an aluminum-based auxiliary material for steel refining (Al: deoxidizer, Al2O3, AlN: slagging accelerator) is used. [Examples] Hereinafter, examples implemented according to the desulfurization method of hot metal in the present invention and comparative examples implemented for comparison with the present invention will be described.

[0050] The implementation conditions in this example are as follows. Table 2 shows the implementation conditions in this example. The following is an example of the present invention and is not intended to be limiting.

[0051] [Table 2]

[0052] FIG. 1 is a schematic diagram showing the configuration of a ladle and an impeller (KR method) in this embodiment. Figure 2 shows a schematic diagram of the rotation diameter (thick line) and height of the impeller (agitation blade). Table 3 shows the components of the desulfurizing agent.

[0053] [Table 3]

[0054] Table 4 shows examples carried out in accordance with the method for desulfurizing molten pig iron 4 of the present invention, and comparative examples carried out for comparison with the present invention.

[0055] [Table 4]

[0056] Regarding equation (1), it is as follows: FIG. 3 is a diagram relating to formula (1), showing the relationship between the pre-treatment temperature T [° C.] of the molten iron 4 and the desulfurization rate η [%]. As shown in FIG. 3, above 1360° C., the desulfurization rate η was 50% or more, and good results were obtained (Examples: Nos. 1 to 15 (◯), Comparative Examples: Nos. 16 to 30 (×)).

[0057] Based on the above, it was specified that "temperature before treatment of molten iron 4 > 1360°C". Regarding equation (2), it is as follows: Figure 4 shows the relationship between T and Ti. i The relationship between the desulfurization rate η [%] and the As shown in FIG. 4, when "temperature before treatment × 0.0003675 - 0.4575 - Ti% before treatment" was below 0, the desulfurization rate η was 50% or more, and good results were obtained (Examples: Nos. 1 to 8 (◯), Comparative Examples: Nos. 31 to 34 (×)).

[0058] Based on the above, it was defined as "temperature of molten iron 4 before treatment [°C] × 0.0003675 - 0.4575 - Ti concentration in molten iron 4 before treatment [mass%] < 0". Regarding equation (3), it is as follows: Figure 5 shows the diagram for equation (3), and W CaO and desulfurization rate η [%]. As shown in FIG. 5, when the CaO content exceeded 0.94 kg / t, the desulfurization rate η was 50% or more, and good results were obtained (Examples: Nos. 1 to 8 (◯), Comparative Examples: Nos. 36 and 37 (×)).

[0059] Based on the above, the standard was set as "CaO > 0.94 kg / t." Regarding equation (4), it is as follows: Figure 6 shows the diagram for equation (4), and C CaO / C AlN and desulfurization rate η [%]. As shown in FIG. 6, when "mass% CaO / mass% AlN" was below 3.92, the desulfurization rate η was 50% or more, and good results were obtained (Examples: Nos. 4 to 10 (◯), Comparative Examples: Nos. 34 and 35 (×)).

[0060] Based on the above, the ratio was defined as "mass%CaO / mass%AlN<3.92". Regarding equation (5), it is as follows: Figure 7 shows the diagram for equation (5), and C CaO / C AlN and desulfurization rate η [%]. As shown in FIG. 7, when the "mass% CaO / mass% Al2O3 equivalent" was greater than 1.02 and less than 1.30, the desulfurization rate η was 50% or more, and good results were obtained (Examples: Nos. 1 to 8 (◯), Comparative Examples: Nos. 33-35 (×)).

[0061] From the above, it was defined that "1.02 < mass%CaO / mass%Al2O3 equivalent < 1.30". Now, the present invention aims to perform a desulfurization treatment that exhibits high desulfurization ability using an Al-based secondary material for steel refining (desulfurizing agent 5) with a low M.Al concentration. Regarding the criteria for determining the quality of the present invention, it was as follows. For a desulfurization rate η ≥ 50%, it is the minimum required desulfurization ability in actual operation.

[0062] Also, regarding the S concentration before treatment of the hot metal 4, it was set as a charge of 0.010 mass% to 0.020 mass%. According to the desulfurization method of the hot metal 4 of the present invention, when the stirrer blade 2 is immersed in the hot metal 4 in the ladle 1 and the impeller 3 is rotated to stir, an inexpensive Al-based secondary material for steel refining (desulfurizing agent 5) with a low M.Al concentration and a high AlN concentration is used, and based on the decomposition reaction of AlN, the desulfurization treatment is efficiently performed to improve the desulfurization efficiency.

[0063] Summarizing the above, the desulfurization method of the hot metal of the present invention is a mechanical stirring type desulfurization method using a desulfurization device that, when stirring the hot metal 4 charged in the ladle 1 by immersing an impeller 3 having a plurality of stirrer blades 2 formed of refractory material in the hot metal 4, adds a desulfurizing agent 5 obtained by mixing a secondary material for steel refining containing lime and aluminum from above the ladle 1 to perform the desulfurization treatment, and satisfies the following conditions of formulas (1) to (5).

[0064] Temperature of the hot metal 4 before treatment > 1360°C ···(1) Temperature of the hot metal 4 before treatment × 0.0003675 - 0.4575 - Ti% in the hot metal 4 before treatment < 0 ···(2) CaO > 0.94 kg / t ···(3) mass%CaO / mass%AlN < 3.92 ···(4) 1.02 < mass%CaO / mass%Al2O3 equivalent < 1.30 ···(5) Furthermore, in the method for desulfurizing molten pig iron of the present invention, when using an aluminum-containing secondary material for steel refining (desulfurizing agent 5), it is preferable that the condition of the following formula (6) be satisfied.

[0065] mass% M.Al<15% (6) It should be noted that the embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. In particular, in the embodiments disclosed herein, matters not explicitly stated, such as operating conditions, operating conditions, various parameters, dimensions, weights, volumes of components, etc., do not deviate from the scope of ordinary practice by a person skilled in the art, and values ​​that can be easily assumed by a person ordinary skilled in the art are used. [Explanation of symbols]

[0066] 1 ladle 2 impellers 3 stirring blades 4. Molten iron 5. Lime-based desulfurization agent

Claims

1. When molten pig iron is charged into a ladle that is not being heated and an impeller having stirring blades is immersed in the molten pig iron to stir the molten pig iron, and a desulfurization agent containing lime and aluminum but not fluorite is added from above the ladle onto the bath surface of the molten pig iron being stirred by the rotation of the impeller to perform desulfurization treatment, The desulfurization agent is added in a desulfurization agent consumption unit of 1.82 kg / t or more and 2.43 kg / t or less relative to the molten iron, A method for desulfurizing molten iron using a mechanically stirred desulfurization device, characterized by satisfying the conditions of the following formulas (1) to (5): The temperature of the hot metal before treatment > 1360 ° C. ... (1) Temperature of the molten iron before treatment × 0.0003675 − 0.4575 − Ti% in the molten iron before treatment < 0 ... (2) CaO>0.94kg / t...(3) mass%CaO / mass%AlN<3.92...(4) 1.02<mass%CaO / mass%Al 2 O 3 Equivalent <1.30 ・・・(5)

2. 2. The method for desulfurizing molten iron according to claim 1, wherein when the aluminum-containing secondary steel refining material is used, the condition of the following formula (6) is satisfied: mass%M.Al<15%...(6)

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