Converter operation method

Abstract

We propose a converter operation method that improves the yield of fine-grained auxiliary materials and enables stable operation. [Solution] A method for decarburizing and smelting molten iron in a converter, comprising: a first step of leaving a portion of the decarburized slag produced by decarburizing and smelting after tapping in the previous smelting in the converter; a second step of adding smelting auxiliary material with a particle size of -15 mm to the decarburized slag remaining in the converter; a third step of adding a solidifying agent to the slag in the converter; a fourth step of charging molten iron into the converter; and a fifth step of decarburizing and smelting the molten iron in the converter, the method being carried out in the order of the first to fifth steps.

Description

Technical Field

[0001] The present invention relates to a method for decarburizing and blowing molten iron in a converter, and to an operating method for a converter that leaves decarburization slag in the converter and reuses it. In the following description, the unit of mass "t" represents 10 3 kg. In this specification, "x to y" representing a numerical range represents x or more and y or less, including the boundary values. The particle size - z mm refers to the recovered material passing through a sieve with a mesh size of z mm.

Background Art

[0002] In a converter, by leaving a part of the decarburization slag generated in the previous blowing in the furnace, a thermal margin is created by heat exchange between the slag and the added auxiliary materials or scrap. In addition, this residual slag can be used as an effective lime source for dephosphorization and contains iron oxide (FeOx). Iron oxide reacts with newly added lime (CaO) to form calcium ferrite and lower the melting point, which is also advantageous for slag formation.

[0003] On the other hand, when the slag is in a high-temperature state, that is, in a molten state, and molten iron is charged, FeO in the residual slag reacts violently with C in the charged molten iron. Therefore, there is a risk of a rapid generation of CO gas and the accompanying flame from the furnace mouth, and a large amount of molten iron and slag spouting out to the front side of the furnace due to slag boiling.

[0004] As a countermeasure, to cool the residual slag and reduce its reactivity, fluxing agents such as lime and dolomite and scrap such as magnetic separation chips are added before charging the molten iron to prevent abnormal reactions.

[0005] For example, Patent Document 1 discloses a technique of adding scrap separately before charging molten iron and during blowing. It is stated that the charging amount of scrap can be increased.

[0006] Further, Patent Document 2 discloses a technique of setting the decarburization slag remaining in the converter to 50% by mass or more and specifying the particle size and amount of the solidifying agent for the decarburization slag to rapidly solidify the residual slag. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2013-133484 [Patent Document 2] Japanese Patent Publication No. 2016-037619 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the above-mentioned conventional technology had the following problems. In particular, with regard to fine-grained auxiliary materials, there is a problem that if they are added during smelting, they are sucked into the exhaust gas recovery equipment, reducing the input yield. Therefore, it is desirable to add them before charging the molten iron, but an appropriate method of addition has not been established. For example, if such auxiliary materials are added immediately before charging the molten iron, the flammable materials in the auxiliary materials or the volatile flammable gases in them may burn, causing a large amount of flame and black smoke to erupt, potentially damaging the equipment or causing eruptions. Patent documents 1 and 2 do not mention methods for preventing flames caused by auxiliary materials or scrap added before charging the molten iron.

[0009] This invention has been made in view of the above circumstances, and aims to propose a converter operation method that allows decarburized slag to remain in the converter, enables the selection of the charging order of various raw materials, improves the yield of fine-grained auxiliary raw materials, and achieves stable operation. [Means for solving the problem]

[0010] The converter operation method according to the present invention, which advantageously solves the above problems, is a method of decarburizing and blowing molten iron in a converter, and is characterized by comprising: a first step of leaving a portion of the decarburized slag produced by decarburizing and blowing after tapping of steel in the previous blowing in the converter; a second step of adding blowing auxiliary raw materials with a particle size of -15 mm to the decarburized slag remaining in the converter; a third step of adding a solidifying agent to the slag in the converter; a fourth step of charging molten iron into the converter; and a fifth step of decarburizing and blowing the molten iron in the converter, and being carried out in the order of the first to fifth steps. Here, a particle size of -15 mm refers to the material that passes through a sieve with a mesh size of 15 mm and is collected below the sieve.

[0011] Furthermore, the converter operation method according to the present invention is (a) In the first step, leave at least 25% by mass of the decarburized slag produced in the previous blowing in the converter. (b) In the second step, the amount of magnetically separated converter bricks crushed to a particle size of -15 mm is included as a blowing auxiliary material, and in the third step, the amount of solidifying agent added is 20 parts by mass or more per 100 parts by mass of remaining decarburized slag. These would be more preferable solutions. [Effects of the Invention]

[0012] According to the present invention, fine-grained blowing auxiliary materials can be processed with high yield, and the converter can be operated stably without problems such as flame generation or bumping during molten iron charging. In addition, by utilizing crushed converter bricks, it is possible to reduce the amount of expensive MgO sources and the amount of emissions. [Brief explanation of the drawing]

[0013] [Figure 1] (a) to (e) are schematic diagrams showing a converter operation method according to one embodiment of the present invention in order of process. [Figure 2] (b) to (e) are schematic diagrams showing the conventional converter operation method in order of steps. [Figure 3] This graph shows the relationship between the amount of brick waste used and the MgO concentration Δ(MgO) [mass%] in the slag derived from the brick waste. [Figure 4] This graph shows the relationship between the unit cost of using brick waste and the MgO concentration Δ(MgO) [mass%] in the slag derived from brick waste. [Modes for carrying out the invention]

[0014] The embodiments of the present invention will be described in detail below. The following embodiments are illustrative of equipment and methods for realizing the technical idea of ​​the present invention, and the configuration is not limited to those described below. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0015] <First Embodiment> Figures 1(a) to 1(e) show a schematic diagram of the converter operation method according to the first embodiment of the present invention in order of steps. In this embodiment, first, after tapping steel in the previous smelting, a portion of the decarburized slag 13 produced in the decarburized smelting is left in the converter 1 to form residual slag 14 (first step). Fine-grained auxiliary raw materials 16 are added to the residual slag 14 in the converter 1 (second step, Figure 1(a)). At this time, the particle size of the smelting auxiliary raw materials 16 is set to -15 mm. Next, a solidifying agent 15 is added to the residual slag 14 in the converter 1 (third step, Figure 1(b)). It is preferable to oscillate the converter back and forth once each, so-called furnace shaking 18 (Figure 1(c)). After that, molten iron 10 is charged into the converter 1 from the molten iron ladle 2 (fourth step, Figure 1(d)). Then, oxygen is blown onto the molten iron 10 in the converter 1 from the top blowing lance 3 to perform decarburization blowing (5th step, Figure 1(e)). At this time, decarburized slag 13 is formed. After tapping the molten steel after blowing, some of the remaining decarburized slag is discharged, or the operation is moved on to the next step without discharge. These steps are repeated to carry out hot slag recycling operations.

[0016] Figures 2(b) to (e) are schematic diagrams for explaining a conventional converter operation method disclosed in, for example, Patent Document 2. Conventionally, after tapping in the previous blowing, a part of the decarburization slag 13 generated by decarburization blowing is left in the converter 1 to form a residual slag 14 (first step). A solidifying agent 15 is added to the residual slag 14 in the converter 1 (third step, Fig. 1(b)). It is preferable to perform a so-called furnace rocking 18 in which the converter is rocked back and forth once (Fig. 1(c)). Thereafter, hot metal 10 is charged from the hot metal ladle 2 into the converter 1 (fourth step, Fig. 1(d)). Then, oxygen is blown from the top-blown lance 3 onto the hot metal 10 in the converter 1 to perform decarburization blowing (fifth step, Fig. 1(e)). At this time, coarse-sized auxiliary raw materials 17 can be charged, and a decarburization slag 13 is formed on the hot metal 10 in the converter 1. After tapping the molten steel after blowing, a part of the remaining decarburization slag is discharged, or the operation proceeds to the next operation without discharging. These steps are repeatedly carried out to perform a hot slag recycling operation. The coarse-sized auxiliary raw materials 17 can have, for example, a particle size of -30 mm.

[0017] In the present embodiment, in the second step, highly reactive, fine-sized auxiliary raw materials 16 are added to the residual slag 14 before the addition of the solidifying agent 15, dissolved or dispersed in the slag, so that the charging yield can be improved. Also, no trouble occurs during the charging of hot metal. Conventionally, when adding fine-sized auxiliary raw materials during the decarburization blowing in the fifth step, there have been problems such as being sucked into the waste gas equipment, resulting in a decrease in the addition yield, or a sudden expansion of the volatile content. When added onto the solidified slag after the addition of the solidifying agent in the third step and then immediately charging the hot metal in the fourth step, there is a risk of frame generation or hot metal boiling over.

[0018] (First step) This is a step of leaving a part of the decarburization slag generated by decarburization blowing in the converter after tapping in the previous blowing. It is preferable to leave 25% by mass or more of the decarburization slag generated in the previous blowing in the converter. More preferably, it is 50% by mass or more, and the entire amount may be left.

[0019] (Second step) This is a process of adding fine-grained auxiliary materials 16 to the residual slag 14 in the converter 1. As the fine-grained auxiliary materials, iron sources such as OG dust, MgO sources for protecting the converter refractories, and slag basicity adjusters can be applied. The fine-grained auxiliary materials are sized to -15 mm for the purpose of improving reactivity.

[0020] (Step 3) This is a process of adding a slag solidifying agent to the molten slag 14 in the converter 1. Examples of the slag solidifying agent include cooled and solidified slag, burnt lime, lightly burnt dolomite, raw dolomite magnetic separation chips, etc. It is preferable to add 20 parts by mass or more of the slag solidifying agent based on 100 parts by mass of the slag remaining in the converter 1. Adding a large amount increases the slag amount and hinders the operability of decarbonization blowing. Therefore, it is preferable to determine the upper limit of the addition amount of the solidifying agent so that the total amount of slag during decarbonization blowing falls within a predetermined range based on the ratio of the residual slag left in the converter 1 in the first step.

[0021] (Step 4) This is a process of charging hot metal 10 from the hot metal ladle 2 into the converter 1. The fine-grained auxiliary materials are captured or absorbed in the solidified slag, suppressing the rapid reaction with the hot metal. Therefore, operations can be carried out without troubles such as the generation of flares or the boiling over of hot metal. The hot metal 10 is preferably hot metal that has been desilicated and dephosphorized by pretreatment. Additionally, it is more preferable that desulfurization treatment has been performed.

[0022] (Step 5) This is a step in which a CaO-based solvent is added to the converter 1 as needed, and an oxygen-containing gas is supplied from the top blowing lance 3 as a gaseous oxygen source to decarburize and smelt the molten iron 10. In this fifth step, it is preferable to use only the oxygen-containing gas from the oxygen supply lance 3 as the oxygen source to reduce heat loss. The oxygen-containing gas can be pure oxygen gas or a mixture of oxygen and carbon dioxide or an inert gas, with pure oxygen gas being preferred. It is preferable to blow in bottom-blowing gas from the bottom-blowing tuyere 4 and stir. The bottom-blowing gas can be an oxidizing gas or a non-oxidizing gas. The smelting material may be supplied simultaneously with the oxidizing gas or non-oxidizing gas from the top blowing lance 3 or the bottom-blowing tuyere.

[0023] Subsequently, the upright converter 1 is tilted toward the tapping side, and the molten steel 11 obtained in the fifth step is tapped into a molten steel pan (not shown) through an outlet 6 provided on the side wall of the converter 1. A portion of the decarburized slag 13 generated in the fifth step is tilted toward the opposite side of the tapping side and discharged from the furnace opening, or, without discharge, the process returns to the first step and proceeds to a new decarburization blowing treatment.

[0024] <Second Embodiment> This embodiment investigates how to use inexpensive and environmentally friendly raw materials to achieve a MgO concentration of 6% or more in the decarburized slag 13 in order to suppress the leaching of MgO from the refractory material of the furnace wall of the converter 1. Dolomite, as a source of MgO, is an expensive mineral resource. By crushing converter brick waste containing MgO and adding it to the slag, it is possible to secure MgO in the decarburized slag inexpensively, while minimizing resource waste and reducing environmental impact.

[0025] As shown in Figure 2(e), conventionally, crushed brick waste was added during the decarburization and blowing process in the fifth step, after being adjusted to a particle size of -30 mm. Brick waste of this particle size had poor reactivity, making it difficult to use large quantities of brick waste. In this embodiment, the brick waste is adjusted to a particle size of -15 mm and added as the auxiliary material 16 in the second step, as shown in Figure 1(a), making it possible to use a larger quantity of brick waste than before.

[0026] Figure 3 is a graph showing the relationship between the amount of brick waste used and the MgO concentration Δ(MgO) [mass%] in the slag derived from the brick waste. The amount of brick waste used on the horizontal axis represents the mass ratio [parts by mass] of brick waste to 100 parts by mass of residual slag. Figure 4 is a graph showing the relationship between the unit consumption of brick waste and the MgO concentration Δ(MgO) [mass%] in the slag derived from the brick waste. The unit consumption of brick waste on the horizontal axis represents the mass of brick waste per ton of molten steel. In addition, Δ(MgO) on the vertical axis in Figures 3 and 4 is the value obtained by subtracting the MgO concentration derived from dolomite from the MgO concentration in the slag. Condition A (symbol ○) in Figures 3 and 4 represents the conventional method, while conditions B and C (symbols ◇ and △) in Figure 3 and condition D (symbol ◇) in Figure 4 represent the method according to this embodiment. These parameters include a decarburized slag basicity, i.e., a mass ratio of CaO to SiO2 (C / S) in the range of 3.3 to 3.6, a tapping temperature in the range of 1680 to 1720°C, and a T.Fe content in the decarburized slag in the range of 17.0 to 22.0 mass%. Furthermore, the MgO concentration in the decarburized slag was 7.0 mass% or higher. As shown in Figures 3 and 4, this embodiment was able to stably increase the MgO concentration derived from bricks by 2 to 3 times compared to conventional methods.

[0027] Furthermore, between the second and third steps described above, the furnace may be shaken to allow the brick scraps and residual slag to blend together. [Examples]

[0028] The effects of the present invention were confirmed by the following examples. In this example, the carbon (C) concentration in the molten steel at the end of decarburization and blowing was 0.01 to 0.05 mass%, and the molten steel temperature at the end of blowing was 1600 to 1740°C. The converter used had a capacity of 270 tons, and the amount of molten iron charged was 220 tons. The brick scraps were crushed and then magnetically separated to remove metallic iron.

[0029] In the first step, 1 to 5 tons of slag, representing more than 20% of the decarburized slag produced after the previous blowing, were left behind. In the third step, quicklime and light-calcined dolomite were added as solidifying agents for the slag. These solidifying agents are used as auxiliary raw materials in decarburization blowing. Therefore, the amount of auxiliary raw materials added during blowing was reduced considering the composition and amount of the solidifying agent.

[0030] (Example of Invention 1) In the second step, 1.0 ton of brick waste with a particle size of -15 mm was added, and the solidifying agent in the third step was added without shaking the furnace. Decarburization blowing was performed 21 times, and the flame was investigated and the MgO concentration in the slag was analyzed after each blowing. The results are shown in Table 1.

[0031] (Example of Invention 2) In the second step, 1.5 tons of brick waste with a particle size of -15 mm was added, and after shaking the furnace, the solidifying agent was added in the third step. Forty-five decarburization blowing cycles were performed, and after each blowing cycle, the flame was investigated and the MgO concentration in the slag was analyzed. The results are shown in Table 1.

[0032] (Comparative Example 1) Without performing the second step, 0.5 tons of brick waste with a particle size of -30 mm was added to the impact point of the top-blowing oxygen jet onto the molten metal after the start of the fifth step, blowing. Adding more than 0.5 tons of brick waste resulted in coarse brick waste remaining unmelted, reducing the yield of MgO in the slag. Conversely, adding brick waste with a particle size of -15 mm during blowing caused it to be lifted by the airflow in the converter and drawn into the exhaust gas recovery equipment, similarly reducing the yield of MgO in the slag. 150 decarburization blowing cycles were performed, and the flame was investigated and the MgO concentration in the slag was analyzed after each blowing cycle. The results are shown in Table 1.

[0033] (Comparative Example 2) The second step was omitted, and after adding the solidifying agent in the third step, 0.5 tons of brick waste with a particle size of -15 mm was added, followed by the charging of molten iron in the fourth step. At this time, flames rose up and reached the crane girder or the operator's cab. Several blowing operations were performed under these conditions, but the same flame formation was observed each time. Due to the significant decrease in productivity, including the need for inspections after blowing to ensure the integrity of the equipment and the safety of the workers, it was not possible to continue blowing under these conditions.

[0034] The MgO concentration Δ(MgO) [mass%] in the slag derived from brick waste was calculated by subtracting the MgO concentration of additives, such as dolomite, from the MgO concentration in the slag after the blowing process.

[0035] The height of the frame generated during the molten iron charging process in the fourth stage was investigated and evaluated as follows, and the results are shown in Table 1. Symbol ○: Almost no frame was generated, and operation was possible safely. Symbol △: The frame reached the crane's lifting beam. Symbol ×: The frame reached the crane girder or operator's cab. Symbols ○ and △ are within the operational range. Symbol × means operation should be stopped for safety reasons.

[0036] [Table 1]

[0037] In Invention Example 1, minor flammability occurred in 30% of cases, but this was within a range that did not pose any problems to the integrity or safety of the equipment. Operations were stable. The amount of brick waste used was doubled to 1.0 t per batch compared to Comparative Example 1. The MgO concentration Δ(MgO) in the slag derived from brick waste also reached an average of 4.0 mass%, contributing to a reduction in the amount of dolomite used as a secondary raw material.

[0038] In Invention Example 2, no flame generation was observed. This was presumed to be due to the effect of shaking the furnace after the second process. The amount of brick waste used was increased threefold to 1.5 tons per batch compared to Comparative Example 1. The MgO concentration Δ(MgO) in the slag derived from the brick waste also reached an average of 5.5% by mass, further reducing the amount of dolomite used as a secondary raw material and contributing to cost reduction. [Industrial applicability]

[0039] According to the present invention, by initially adding fine-grained brick waste to the slag remaining in the converter after smelting, it is possible to prevent flame generation during molten iron charging and improve slag hot recycling. This contributes to reducing environmental impact by reducing waste and conserving resources, and is industrially useful. [Explanation of symbols]

[0040] 1 Converter 2. Molten iron pot 3. Upward-blowing lance 4 Bottom-blown tuyere 6 Outlet 10 Molten iron 11 Molten steel 13 Decarburized slag 14 Residual slug 15 (Slag) Solidifying agent 16 (Fine-grained) auxiliary ingredients 17 (Coarse particle size) auxiliary ingredients 18 Hearth Shake

Claims

1. A method of decarburizing and smelting molten iron in a converter, The first step involves leaving a portion of the decarburized slag produced in the decarburized smelting process in the converter after the previous smelting process, A second step involves adding a blowing auxiliary material with a particle size of -15 mm to the decarburized slag remaining in the converter, A third step involves adding a solidifying agent to the slag in the converter, A fourth step involves charging molten iron into the converter, A fifth step involves decarburizing and smelting the molten iron in the converter, A converter operation method that includes and is carried out in the order of the first to fifth steps.

2. The converter operation method according to claim 1, wherein in the first step, 25% by mass or more of the decarburized slag produced in the previous blowing is left in the converter.

3. In the second step, as a secondary raw material for blowing, magnetically separated converter bricks crushed to a particle size of -15 mm are included. The converter operation method according to claim 1, wherein in the third step, the amount of solidifying agent added is 20 parts by mass or more relative to 100 parts by mass of remaining decarburized slag.

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