Steel production method

Abstract

To provide a steel production method capable of increasing an amount of slag to be used in slag recycling while suppressing excessive temperature drop in a furnace, and having excellent stability in operation.SOLUTION: A steel production method using slag discharged after a decarburization treatment includes: a step of mixing converter slag previously produced from slag and having a moisture content of 20% or less by mass and a ratio of grains having a particle diameter of less than 1 mm of 3% or less by mass and hot slag collected from the slag and having a surface temperature of 300°C or higher to produce a mixed slag having a moisture content of 7.0% or less by mass; and a step of carrying out a dephosphorization treatment using the mixed slag.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing steel. [Background technology]

[0002] Molten pig iron produced in a blast furnace or the like is subsequently subjected to dephosphorization and decarburization processes to adjust its chemical composition. In dephosphorization, a flux is added to the molten pig iron and oxygen is blown into it, causing a chemical reaction with the phosphorus in the molten pig iron, which is then discharged into the slag. As a result, phosphorus is removed from the molten pig iron. In decarburization, the flux is added and oxygen is blown into the molten pig iron, causing a reaction between oxygen and carbon, thereby removing carbon from the molten pig iron. The dephosphorization process is generally called hot metal pretreatment, and the decarburization process is generally called primary refining.

[0003] In this hot metal pretreatment process, a method known as slag recycling is known to reduce the amount of flux used in the dephosphorization process. This slag recycling method reduces the amount of flux used in the dephosphorization process by reusing the slag generated after decarburization in the dephosphorization process.

[0004] Slag recycling uses either cold slag, hot slag, or both. Cold slag is slag discharged from a decarburization furnace and cooled by spraying water on it, while hot slag is high-temperature slag that remains in the furnace.

[0005] For example, Patent Document 1 discloses a slag recycling method that specifies the total amount of Fe and the amount of cold slag to be used relative to the total slag in the furnace. Patent Document 2 discloses a method for efficiently separating ferrous and non-ferrous components to produce cold slag. Patent Document 3 discloses a slag recycling method in which hot slag is left in the furnace while cold slag is added. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-88919 [Patent Document 2] Japanese Patent Application Publication No. 6-256047 [Patent Document 3] International Publication No. 2018 / 021019 Summary of the Invention [Problem to be solved by the invention]

[0007] Generally, in slag recycling methods using cold slag, the slag is cooled by spraying water. Therefore, the water content in the cold slag produced by the methods disclosed in Patent Documents 1 and 2 is high, and if an attempt is made to increase the amount of cold slag used, there are problems such as an excessive drop in the temperature inside the furnace and the increased likelihood of abnormal reactions.

[0008] In addition, in the slag recycling method disclosed in Patent Document 3, hot slag and cold slag are reacted in a furnace to reduce the moisture content of the cold slag. This reduces the moisture content of the cold slag. However, time is required to solidify the hot slag in the furnace, which leaves room for improvement in terms of productivity.

[0009] Furthermore, this method leaves hot slag in the furnace, which means that slag recycling cannot be carried out when the hot slag cannot be left in the furnace, for example, during periodic repairs. Therefore, there is a need to develop a slag recycling method that allows stable operation even when the hot slag cannot be left in the furnace due to periodic repairs, etc.

[0010] In view of the above, an object of the present invention is to provide a method for producing steel that can increase the amount of slag used during slag recycling while suppressing excessive temperature drops in the furnace, and that is also excellent in operational stability. [Means for solving the problem]

[0011] The present invention has been made to solve the above problems, and is summarized as the following method for producing steel.

[0012] (1) A method for manufacturing steel using slag discharged after decarburization, a step of mixing converter slag, which is produced in advance from the slag and has a moisture content of 20% by mass or less and a ratio of particles having a particle size of less than 1 mm of 3% by mass or less, with hot slag, which is collected from the slag and has a surface temperature of 300°C or more, to produce mixed slag having a moisture content of 7.0% by mass or less; and performing a dephosphorization treatment using the mixed slag.

[0013] (2) The method for producing steel according to (1) above, wherein in the step of producing the mixed slag, the hot slag is placed on top of the converter slag, and then the two are mixed to produce the mixed slag. [Effects of the Invention]

[0014] According to the present invention, a method for producing steel can be obtained that can increase the amount of slag used in slag recycling while suppressing excessive temperature drops in the furnace, and that is also excellent in operational stability. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have investigated a steel manufacturing method that is excellent in operational stability and that suppresses excessive temperature drops in a furnace, even when a large amount of slag is used during slag recycling, and have obtained the following findings.

[0016] (a) The method disclosed in the above-mentioned Patent Document 3 can reduce the moisture content of the slag, but the hot slag and cold slag are reacted in the same furnace. Therefore, the hot slag must be solidified in the furnace, which leaves room for improvement in terms of operational stability. Therefore, the present inventors came up with the idea of ​​reacting the hot slag and cold slag outside the furnace.

[0017] (b) On the other hand, unlike inside a furnace where dust can be collected by a dust collector, the reaction between hot slag and cold slag outside a furnace generates dust. To prevent this, it is effective to limit the proportion of cold slag with a particle size of less than 1 mm to 3%.

[0018] (c) It is effective to produce mixed slag containing hot slag and cold slag, and it is also effective to keep the moisture content of this mixed slag at 7.0% or less. By keeping the moisture content of the mixed slag at 7.0% or less, excessive temperature drops and abnormal reactions in the furnace can be suppressed even when the mixed slag is used for dephosphorization.

[0019] One embodiment of the present invention has been made based on the above findings, and the method for producing steel of this embodiment is a method for producing steel that utilizes slag discharged after decarburization. Each requirement of the method for producing steel of this embodiment will be described in detail below.

[0020] 1. Production of converter slag In steel production, dephosphorization is usually followed by decarburization. The dephosphorized and decarburized molten steel is then sent to the next process, and at the same time, new, undephosphorized molten pig iron is brought into the facility and subjected to dephosphorization and decarburization again. This process is repeated. In this application, the period from when new molten pig iron is brought in until it is dephosphorized and decarburized is referred to as one charge. In dephosphorization and decarburization facilities, steel is usually produced continuously through multiple charges.

[0021] In the steelmaking method of this embodiment, converter slag is produced in advance from slag. That is, converter slag is produced from slag discharged from the previous charge. Here, slag refers to the slag discharged after decarburization treatment and primarily contains CaO. In addition to CaO, it also contains SiO2, MnO, MgO, PO5, Al2O3, T-Fe (FeO, Fe2O3), etc. The converter slag is what is known as cold slag.

[0022] In the steel manufacturing method of this embodiment, the moisture content of the converter slag is 20% or less by mass. If the moisture content of the converter slag exceeds 20%, the moisture content of the mixed slag described below will not be the desired 13% or less. The moisture content of the converter slag is preferably 18% or less by mass, and more preferably 15% or less. The lower the moisture content of the converter slag, the better, but excessive reduction in the moisture content of the converter slag actually reduces manufacturability. For this reason, the moisture content of the converter slag is preferably 1% or more. The moisture content of the converter slag can be measured using a heat-drying moisture meter. Specifically, the moisture content is measured by collecting approximately 30 g of converter slag as a sample, evaporating the moisture in the heat-drying moisture meter, and determining the mass loss of the sample. Moreover, unlike hot slag described later, converter slag is cooled, and therefore the surface temperature of converter slag is 25°C (room temperature) or lower.

[0023] Furthermore, the proportion of particles with a particle size of less than 1 mm (hereinafter simply referred to as the "proportion of fine particles") in converter slag is 3% or less by mass. This is because if the particles in converter slag are very fine, dust is more likely to be generated. In addition, by increasing the proportion of particles above a certain size, the moisture content of the converter slag also tends to decrease. The smaller the proportion of fine particles, the better.

[0024] Whether the particle size of the converter slag is less than 1 mm can be confirmed by passing it through a sieve with 1 mm openings, as will be described later. The average particle size of the converter slag is not particularly limited. However, from the viewpoint of dust generation, the average particle size is preferably in the range of 13 mm to 50 mm, and more preferably in the range of 13 mm to 25 mm.

[0025] Converter slag satisfying the above-mentioned ranges of moisture content and fine particle ratio can be produced, for example, by the following process. First, molten steel that has been blown and decarburized in a converter is discharged from the furnace. Then, after the molten steel is discharged, some or all of the slag remaining in the furnace is discharged from the furnace.

[0026] The discharged slag is placed in a slag jar, which is then returned to the dirt floor of the slag treatment plant and allowed to cool. Immediately after being returned to the dirt floor, the slag is hot, with a surface temperature of approximately 800 to 1000°C, but as it cools over time, the surface temperature drops. The cooling time is not particularly limited, but is usually in the range of several tens of minutes to several days. This allows the slag to solidify.

[0027] As will be explained later, this slag that is discharged from the furnace and remains at a high temperature is called hot slag. This hot slag is cooled by spraying water on it. In addition to spraying water during cooling, the moisture content can also be adjusted by leaving it in the air.

[0028] The slag that has been sprayed with water and cooled is then crushed and magnetically separated to adjust the size of the slag. Crushing here refers to breaking up the lumps of slag into smaller pieces, and magnetic separation refers to removing the iron components from the slag. The crushing method is not particularly limited, but it is preferable to use a crusher or the like. Through these processes, it is preferable to produce converter slag with a moisture content of 20% by mass or less and a fine particle ratio of 3% by mass or less. After crushing, the slag is sieved to make the particle size uniform. The slag may then be stored for a long period of time to undergo aging, which promotes the hydration reaction.

[0029] 2. Production of hot slag As mentioned above, hot slag refers to slag that is discharged from the furnace after decarburization and maintains its high temperature. Hot slag is unsolidified and is commonly referred to as hot slag. However, hot slag generally refers to slag that remains in the furnace while maintaining its high temperature. In this application, hot slag refers to slag that is discharged from the furnace while maintaining its high temperature. Specifically, hot slag refers to slag that is discharged after decarburization and has a surface temperature of 300°C or higher. While the higher the temperature of hot slag, the better, in normal operation, the surface temperature of hot slag is below 1000°C.

[0030] 3. Mixed slag production The above-described converter slag and hot slag are mixed to produce a mixed slag having a moisture content of 7.0% by mass or less. The moisture content of the mixed slag is preferably 3.0% by mass or less, more preferably 1.0% by mass or less. As long as the moisture content of the mixed slag is 7.0% by mass or less, the mixing ratio (mass ratio) of the converter slag to the hot slag is not particularly limited. For example, even when the amount of hot slag is the smallest, the mixing ratio of converter slag to hot slag is preferably 10:1. However, there is no particular problem if the amount of hot slag is greater than the amount of converter slag. The moisture content of the mixed slag must be lower than that of the converter slag used, and the moisture content of the mixed slag is preferably about 80% when the moisture content of the converter slag is 100%. The moisture content of the mixed slag may be measured in the same manner as in measuring the moisture content of the converter slag.

[0031] Next, the converter slag and the hot slag are mixed. That is, it is preferable to place the above-mentioned hot slag on the converter slag and mix them. In this application, "mixing" includes not only mixing the converter slag and the hot slag by stirring them, but also simply placing the hot slag on the converter slag. Heavy machinery such as a shovel may be used for mixing. Thereafter, if necessary, the mixed converter slag and the hot slag may be air-cooled.

[0032] The components of the mixed slag are not particularly limited, but typically contain CaO as the main component. In addition to CaO, the mixed slag may contain SiO2, MnO, MgO, PO5, Al2O3, T-Fe (FeO, Fe2O3), etc.

[0033] 4. Dephosphorization treatment The mixed slag produced in this manner is used to perform dephosphorization. In addition to the mixed slag produced in advance, a flux typically used in dephosphorization may be used in the dephosphorization process. In this application, the flux refers to a newly added flux, distinct from the mixed slag recycled from slag. A commonly used flux is CaO or the like. The method of adding the mixed slag to the molten iron is not particularly limited. It may be introduced through a chute installed in the furnace and then added through the furnace throat. It may also be added from above the furnace. The addition method may be adjusted as needed. For example, the mixed slag may be added to the furnace or other equipment before the molten iron is added to the equipment, or the mixed slag may be added to the equipment after the molten iron is added to the equipment.

[0034] In the steel manufacturing method of this embodiment, the type of operation in which the dephosphorization and decarburization treatments are performed is not particularly limited. In steel manufacturing, an operation in which the dephosphorization and decarburization treatments are performed in the same converter is called a multi-refining converter (MURC) operation. An operation in which the dephosphorization and decarburization treatments are performed in different converters is called a simple refining process (SRP) operation. Furthermore, the dephosphorization treatment alone may be performed in a transport vessel such as a torpedo car or ladle, or in a specialized vessel. The steel manufacturing method of this embodiment can be applied to the above-mentioned MURC operation, SRP operation, and other operations in which dephosphorization is performed in a transport vessel or the like.

[0035] 5. Reduction of solvent By applying the steel manufacturing method of this embodiment, it is possible to reduce the amount of flux used in the dephosphorization process. Specifically, while the conventional flux used was 15.9 kg per ton of molten steel, by using mixed slag, the amount of newly added flux was reduced to 11.5 kg. Furthermore, while the conventional amount of slag discharged outside the manufacturing process, i.e., outside the system, after decarburization was 55.0 kg per ton of molten steel, by using mixed slag, the amount of slag discharged outside the system could be reduced to 44.6 kg.

[0036] The steel manufacturing method according to the present invention will be explained in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0037] Converter slag was produced from the slag discharged after the decarburization process. Converter slag was produced by spraying water on the slag discharged from the furnace, followed by crushing and magnetic separation. The moisture content of the converter slag was 6.4% by mass, and the proportion of fine particles was 1% by mass or less. Hot slag extracted from the slag with a surface temperature of 500°C or higher was added to the previously produced converter slag and mixed to produce mixed slag. The moisture content of the mixed slag was 0.1% by mass. The resulting mixed slag was added to undephosphorized hot metal in another furnace and dephosphorized. No significant temperature drop or abnormal reactions occurred within the furnace. [Example]

[0038] Converter slag was produced from the slag discharged after the decarburization process. Converter slag was produced by spraying water on the slag discharged from the furnace, followed by crushing and magnetic separation. The moisture content of the converter slag was 20% or less by mass, and the proportion of fine particles was 3% or less by mass. Hot slag extracted from the slag with a surface temperature of 500°C or higher was added to the previously produced converter slag, which was then mixed to produce mixed slag. The moisture content of the mixed slag was 7.9% by mass. The resulting mixed slag was added to undephosphorized hot metal in another furnace for dephosphorization, resulting in an abnormal reaction.

Claims

1. A method for producing steel using slag discharged after decarburization, comprising the steps of: A step of mixing converter slag produced in advance from the slag, the converter slag having a moisture content of 20% by mass or less and a ratio of grains having a particle size of less than 1 mm of 3% by mass or less, with hot slag collected from the slag and having a surface temperature of 300° C. or more, to produce a mixed slag having a moisture content of 7.0% by mass or less; and performing a dephosphorization treatment using the mixed slag.

2. 2. The method for producing steel according to claim 1, wherein in the step of producing the mixed slag, the hot slag is placed on the converter slag, and then the mixed slag is produced by mixing them.