Production method of steel

Abstract

To provide a method for producing steel in which dephosphorization treatment can be applied to a steel grade having a high C content even in the case that free oxygen concentration is low in secondary refining.SOLUTION: A method for producing steel by performing dephosphorization treatment using a vacuum degassing facility includes the steps of: adding a dephosphorization agent to molten steel having a free oxygen concentration of more than 150 ppm and less than 400 ppm and a C content of more than 0.06% by mass%, the dephosphorization agent including calcium ferrite and CaO, CaO contained in the calcium ferrite being 40 to 60% by mass% in the whole CaO; and circulating the molten steel in the vacuum degassing facility at a circulation flow rate of 190 ton / min or more for 4.0 minutes or more after adding the dephosphorization agent to the molten steel.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In the steel manufacturing process, the process of removing the impurity P (phosphorus) is called dephosphorization. Primary refining involves removing carbon and other impurities from pig iron produced in a converter or electric furnace to produce molten steel. Dephosphorization is usually carried out during primary refining. However, for example, when low-grade iron ore is used, the P concentration in the molten pig iron increases, and sufficient dephosphorization may not be possible during primary refining. In such cases, dephosphorization is also carried out in the next process, secondary refining. In secondary refining, degassing and adjustment of chemical composition are usually carried out using RH vacuum degassing equipment.

[0003] For example, Patent Document 1 discloses a method of dephosphorizing steel in secondary refining using vacuum degassing equipment. In this method, CaO and calcium ferrite are used as dephosphorizing agents. Patent Document 2 discloses a method of dephosphorizing steel in secondary refining using vacuum degassing equipment, adjusting the composition and viscosity of the dephosphorizing agent to a predetermined range, and spraying a carrier gas from a top lance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-232157 [Patent Document 2] Japanese Patent Application Publication No. 2019-218580 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the dephosphorization method disclosed in Patent Document 1 cannot be applied to steels with a high C content of more than 0.06%. This is because the oxygen activity (free oxygen concentration) in the molten steel must be increased to a certain level to promote the dephosphorization reaction. The method disclosed in Patent Document 2 can be applied to steels with a high carbon content of 0.7 to 2.0%, but it requires injecting oxygen from a top lance installed in the equipment to increase the free oxygen concentration, leaving room for improvement in manufacturability. For this reason, there is a need for a steel manufacturing method that can apply dephosphorization to steels with a high C content, even when the free oxygen concentration is low, in secondary refining.

[0006] In light of the above, an object of the present invention is to provide a steel manufacturing method that allows dephosphorization treatment to be applied to steel types with a high C content in secondary refining, even when the free oxygen concentration is low. [Means for solving the problem]

[0007] The present invention has been made to solve the above problems, and is summarized as the following method for producing steel.

[0008] (1) A method for manufacturing steel in which dephosphorization is performed using a vacuum degassing facility, adding a dephosphorization agent containing calcium ferrite and CaO, wherein the CaO contained in the calcium ferrite accounts for 40 to 60% by mass of the total CaO, to molten steel having a free oxygen concentration of more than 150 ppm and less than 400 ppm and a C content of more than 0.06% by mass; and refluxing the molten steel in the vacuum degassing equipment at a reflux rate of 190 tons / min or more for 4.0 minutes or more after adding the dephosphorization agent to the molten steel. [Effects of the Invention]

[0009] According to the present invention, a method for producing steel can be obtained that is capable of applying dephosphorization treatment to steel types with high C content even when the free oxygen concentration is low in secondary refining. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing a schematic structure of an RH vacuum degassing facility. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present inventors have investigated a dephosphorization method that can be carried out even when the C content is high, and have obtained the following findings.

[0012] (a) From the viewpoint of equilibrium theory, it is difficult to increase the free oxygen concentration in molten steel to 400 ppm or more in secondary refining for steel types with a C content of more than 0.06%. On the other hand, there is room for improvement in terms of manufacturability when performing oxygen blowing or other methods to increase the free oxygen concentration in molten steel.

[0013] (b) When the free oxygen concentration is low, the dephosphorization reaction does not proceed smoothly. Therefore, it is preferable to set the reflux amount and reflux time in the vacuum degassing equipment within a certain range so that the dephosphorization reaction proceeds at a relatively low free oxygen concentration while maintaining a certain amount of free oxygen concentration at which the dephosphorization reaction proceeds.

[0014] (c) That is, it is effective to use a dephosphorization agent containing calcium ferrite and CaO, in which the CaO contained in the calcium ferrite accounts for 40 to 60% by mass of the total CaO, and to reflux the molten steel for 4.0 minutes or more after adding the dephosphorization agent to the molten steel at a reflux rate of 190 tons / min or more.

[0015] One embodiment of the present invention has been made based on the above findings. The steel manufacturing method of this embodiment is a method of manufacturing steel in which dephosphorization is performed in secondary refining, and in which dephosphorization is performed in vacuum degassing equipment. Each requirement of the steel manufacturing method of this embodiment will be described in detail below.

[0016] 1. Dephosphorization agent In the steel manufacturing method of this embodiment, a dephosphorization treatment is performed using a dephosphorization agent. The dephosphorization agent used contains calcium ferrite and CaO. That is, the dephosphorization agent is a mixture of calcium ferrite and CaO. Calcium ferrite is a compound containing CaO, Fe oxide, etc., and has the property of being easily dissolved in molten steel. For this reason, the dephosphorization agent needs to contain not only the common CaO but also a certain amount of calcium ferrite.

[0017] Therefore, the CaO contained in the calcium ferrite is 40 to 60% by mass of the total CaO. If the CaO contained in the calcium ferrite is less than 40% by mass of the total CaO, the dephosphorization agent is difficult to dissolve in molten steel, and the slag formation reaction slows down. Therefore, the CaO contained in the calcium ferrite is 40% or more by mass, and preferably 42% or more by mass of the total CaO. On the other hand, if the CaO contained in the calcium ferrite is more than 60% by mass of the total CaO, the production cost increases. Therefore, the CaO contained in the calcium ferrite is 60% or less by mass, and preferably 55% or less, and more preferably 50% or less by mass of the total CaO.

[0018] 2. Molten steel The dephosphorization agent is added to molten steel, where the free oxygen concentration in the molten steel is more than 150 ppm and less than 400 ppm. The "free oxygen concentration" refers to oxygen dissolved in the molten steel, i.e., dissolved oxygen.

[0019] If the free oxygen concentration in the molten steel is less than 150 ppm, the dephosphorization reaction does not proceed and dephosphorization cannot be performed. Therefore, the free oxygen concentration in the molten steel is 150 ppm or more, and preferably 200 ppm or more.

[0020] On the other hand, if the free oxygen concentration in the molten steel exceeds 400 ppm, the C content of the produced steel cannot exceed 0.06%. Therefore, the free oxygen concentration in the molten steel is 400 ppm or less, and preferably 350 ppm or less.

[0021] Furthermore, in the steel manufacturing method of this embodiment, the C content of the steel manufactured exceeds 0.06% by mass. Decarburization treatment is performed before secondary refining, and the C content of the molten steel is adjusted. Therefore, the C content of the molten steel exceeds 0.06% by mass. Note that the C content of the molten steel is not particularly limited, but from the viewpoint of manufacturing facilities, etc., the C content is preferably 0.40% or less.

[0022] 3. Reflux Conditions Under the manufacturing conditions of this embodiment, dephosphorization is performed using a vacuum degassing system. FIG. 1 is a schematic diagram showing the structure of an RH vacuum degassing system. As shown in FIG. 1, the RH vacuum degassing system 1 is a device having two immersion pipes 3 and a vacuum vessel 4 above a ladle 2. Initially, molten steel 5 is contained in the ladle 2. The pressure in the vacuum vessel 4 is reduced, and an inert gas 6, such as Ar gas, is blown into one of the immersion pipes 3 (hereinafter also referred to as the "ascending immersion pipe") while the molten steel is pumped into the vacuum vessel 4. Meanwhile, the molten steel is returned to the ladle from the other immersion pipe 3 (hereinafter also referred to as the "descending immersion pipe"), circulating the molten steel throughout the entire system. While circulating the molten steel in this manner, deoxidizers and the like are added to the molten steel to degas it and adjust its chemical composition.

[0023] In this vacuum degassing equipment, the molten steel is refluxed for 4.0 minutes or more after the dephosphorization agent is added to the molten steel, at a reflux rate of 190 ton / min or more. If the time from adding the dephosphorization agent to the molten steel to adding the deoxidizer described below, i.e., the reflux time, is less than 4.0 minutes, the dephosphorization agent will not be sufficiently refluxed, making it difficult for the dephosphorization reaction to proceed. For this reason, the reflux time is 4.0 minutes or more. There are no particular limitations on the upper limit of the reflux time, but it is usually 8.0 minutes, and preferably 7.0 minutes.

[0024] Furthermore, if the reflux rate of the molten steel during the reflux time is less than 190 ton / min, the dephosphorization reaction does not proceed sufficiently at the C content within the above-mentioned range. Therefore, the reflux rate of the molten steel after adding the dephosphorization agent to the molten steel is 190 ton / min or more, and preferably 200 ton / min or more. There is no particular upper limit to the reflux rate of the molten steel after adding the dephosphorization agent to the molten steel, but it is preferably 220 ton / min due to facility limitations.

[0025] The amount of molten steel refluxed can be calculated using the following formula (a). Q=11.4×D 4 / 3 ×G 1 / 3 ×(ln(P1 / P0)) 1 / 3 (a) In the above formula, each symbol is defined as follows:

[0026] Q(t / min): Molten steel return flow rate D(m): Inner diameter of the immersion tube G (Nl / min): Reflux gas flow rate P0 (torr): Pressure inside the vacuum degassing equipment P1 (torr): Pressure at the gas injection point

[0027] The reflux gas flow rate mentioned above refers to the flow rate of gas, such as Ar gas, that is refluxed within the equipment. The pressure within the vacuum degassing equipment refers to the reduced pressure within the equipment. The pressure at the gas injection point refers to the pressure at the point where oxygen gas is injected. The above formula (a) is shown in "Tatsuro Kuwahara et al.: Iron and Steel, 73 (1987), S176."

[0028] The method of adding the dephosphorization agent is not particularly limited. For example, the dephosphorization agent may be added through the alloy injection hole.

[0029] After adding a dephosphorizing agent and refluxing the molten steel, a deoxidizing agent may be added to deoxidize it. The deoxidizing agent is not particularly limited, but Al is usually used as the deoxidizing agent. After adding the deoxidizing agent, alloying elements may be added to adjust the chemical composition, etc.

[0030] 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]

[0031] Molten steel was dephosphorized using RH degassing equipment with a dephosphorization agent having the composition shown in Table 1. Table 1 also shows the time (reflux time) from adding the dephosphorization agent to adding the Al deoxidizer, the amount of molten steel refluxed, etc.

[0032] In Table 1, the free oxygen concentration of the molten steel was measured using an oxygen sensor that utilizes the principle of an oxygen concentration cell. The reflux amount was adjusted by introducing Ar into the RH vacuum degassing equipment through the gas inlet of the immersion tube and adjusting the flow rate.

[0033] The dephosphorization rate is the ratio expressed as (Pi - Pf) / Pi x 100, where Pi is the phosphorus concentration in the molten steel before dephosphorization and Pf is the phosphorus concentration in the molten steel after dephosphorization, and the CaO dephosphorization efficiency is the ratio expressed as (dephosphorization rate) / (total CaO amount in the dephosphorization agent). The phosphorus concentration in the molten steel was measured using an optical emission spectrometer (Shimadzu Corporation) by taking samples from inside the ladle using a dedicated probe before and after the dephosphorization.

[0034] [Table 1]

[0035] In the examples that satisfied the requirements of this embodiment, dephosphorization of molten steel was successfully carried out (Test Nos. 1 to 6), whereas in the examples that did not satisfy this embodiment, the dephosphorization reaction did not proceed sufficiently (Test Nos. 7 to 11).

[0036] In the examples of Test Nos. 7 to 9 (Comparative Examples), the reflux amount was small, and dephosphorization did not proceed sufficiently, resulting in failure to achieve dephosphorization. In the example of Test No. 10 (Comparative Example), the free oxygen concentration was too low, and the dephosphorization reaction did not proceed, resulting in failure to achieve dephosphorization. In the example of Test No. 11 (Comparative Example), the time from the addition of the dephosphorization agent to the introduction of the deoxidizer (Al), i.e., the reflux time, was short, and Al was introduced before the dephosphorization agent had been sufficiently refluxed, resulting in failure to achieve sufficient dephosphorization. [Explanation of symbols]

[0037] 1.RH vacuum degassing equipment 2.Ladle 3.Dip tube 4.Vacuum chamber 5. Molten steel 6. Inert gas

Claims

[Claim 1] A method for producing steel by performing dephosphorization treatment using a vacuum degassing facility, adding a dephosphorization agent containing calcium ferrite and CaO, wherein the CaO contained in the calcium ferrite accounts for 40 to 60% by mass of the total CaO, to molten steel having a free oxygen concentration of more than 150 ppm and less than 400 ppm and a C content of more than 0.06% by mass; and refluxing the molten steel in the vacuum degassing equipment at a reflux rate of 190 tons / min or more for 4.0 minutes or more after adding the dephosphorization agent to the molten steel.

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