Method for producing molten steel and method for removing titanium from molten steel

Abstract

A method for producing molten steel is provided in which alloyed iron containing P and containing Ti as an impurity is added to thereby make it possible to accurately control the P content and reduce a Ti content of molten steel. The method includes a first step in which first P-containing alloyed iron is added to molten steel having an oxygen content of 500-1,200 mass ppm and a second step in which after the first step, Al is added to the molten steel. Preferably, the method includes, after the second step, a third step in which second P-containing alloyed iron is added to the molten steel. The mass proportion of the addition amount of the first P-containing alloyed iron to the total addition amount of the first P-containing alloyed iron and the second P-containing alloyed iron is 30-90%.

Description

Molten steel manufacturing method and detitanization method for molten steel

[0001] The present invention relates to a method for producing molten steel and a method for detitanizing molten steel.

[0002] Molten steel is molten steel obtained by decarburizing pig iron tapped from a blast furnace using a converter or similar furnace. Before being cast, the molten steel is first placed in a ladle where it undergoes secondary refining, such as deoxidation. At the same time, various additive elements are added to the molten steel depending on the steel's intended use. Once the composition of the molten steel has been adjusted in the ladle, it is cast into billets such as slabs, which are easier to roll. These billets are then processed into steel products such as steel plates, shaped steel, and bar steel.

[0003] Among the elements contained in pig iron, P (phosphorus) is known to reduce the strength and toughness of steel. Therefore, dephosphorization is performed in addition to decarburization in a converter. However, it is known that the inclusion of P in electrical steel sheets containing a large amount of silicon improves their electromagnetic properties. For this reason, when producing molten steel for manufacturing electrical steel sheets, a small amount of P is added to the molten steel in the ladle. The addition of P to the molten steel is carried out while accurately controlling the P content in the molten steel. Ferrophosphorus (FeP), a P-containing ferroalloy, is usually used to add P. Ferrophosphorus may contain impurities such as Si, C, S, Mn, and Ti (see Patent Document 1).

[0004] Generally, the timing for adding ferroalloy to molten steel to adjust the composition may be either before or after deoxidation (see Patent Document 2). However, if deoxidation is performed after adding P-containing ferroalloy to molten steel, some of the P added to the molten steel will migrate into the slag generated by the deoxidation. Subsequent removal of the slag containing P may result in a decrease in P yield. Therefore, in order to improve P yield, it is preferable to add P-containing ferroalloy to molten steel after the deoxidation treatment using Al addition is completed and the slag is removed. FIG. 4 is a flowchart showing a method for producing molten steel according to a conventional technique. As illustrated in FIG. 4, in the conventional technique, Al is typically added (S2) to molten steel received in a ladle, and then P-containing ferroalloy is added (S1').

[0005] International Publication No. 2021 / 045212 Japanese Patent Application Laid-Open No. 2020-2411

[0006] Ti is an element that may degrade the electromagnetic properties of electrical steel sheets. Therefore, when producing molten steel for manufacturing electrical steel sheets, it is necessary to remove as much Ti as possible from the molten steel. However, if ferrophosphorus is added to the molten steel after deoxidation to improve the P yield, the opportunity to remove Ti contained as an impurity in the ferrophosphorus is lost. As a result, Ti remains in the molten steel, potentially degrading the electromagnetic properties of the electrical steel sheets. This residual Ti is particularly noticeable when low-purity ferrophosphorus or other ferroalloys with a high Ti content are used.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for producing molten steel and a method for detitanization of molten steel, which are capable of reducing the Ti content in molten steel while accurately controlling the P content by adding a P-containing ferroalloy containing Ti as an impurity.

[0008] As a result of extensive research, the present inventors have found that the above object can be achieved by the following configuration, and have completed the present invention.

[0009] [1] A method for producing molten steel, comprising: a first step of adding a first P-containing ferroalloy to molten steel having an oxygen content of 500 ppm by mass or more and 1200 ppm by mass or less; and a second step of subsequently adding Al to the molten steel.

[0010] [2] The method for producing molten steel according to the above [1], further comprising a third step of adding a second P-containing ferroalloy to the molten steel after the second step, wherein a mass ratio of an added amount of the first P-containing ferroalloy to a total added amount of the first P-containing ferroalloy and the second P-containing ferroalloy is 30% or more and 90% or less.

[0011] [3] The method for producing molten steel according to the above [1] or [2], wherein the time from the end of the first step to the start of the second step is 5 minutes or more and 30 minutes or less.

[0012] [4] The method for producing molten steel according to any one of the above [1] to [3], wherein the first step is carried out by placing the first P-containing ferroalloy in a ladle and receiving the molten steel in the ladle, and the second and subsequent steps are carried out with the molten steel contained in the ladle.

[0013] [5] The method for producing molten steel according to any one of the above [1] to [4], wherein the first P-containing ferroalloy has a maximum diameter of 3 mm or more and 20 mm or less.

[0014] [6] The method for producing molten steel according to any one of the above [1] to [5], wherein at least the second step is carried out while vacuum degassing the molten steel.

[0015] [7] A method for producing molten steel according to any one of the above [1] to [6], wherein the Ti content in the molten steel is set to 1 / 25 or less of the P content by the production method.

[0016] [8] A method for detitanization of molten steel, comprising: a first step of adding a first P-containing ferroalloy to molten steel having an oxygen content of 500 ppm by mass or more and 1200 ppm by mass or less; and a second step of thereafter adding Al to the molten steel, wherein Ti contained as an impurity in the molten steel is removed.

[0017] According to the present invention, even when the composition of molten steel is adjusted using a low-purity P-containing ferroalloy containing Ti as an impurity, Ti can be efficiently removed from the molten steel. When the present invention is applied to the production of molten steel for electrical steel sheets, the electromagnetic properties of the electrical steel sheets can be improved.

[0018] Fig. 1 is a flowchart showing one embodiment of the method for producing molten steel according to the present invention; Fig. 2 is a flowchart showing a preferred embodiment of the method for producing molten steel according to the present invention; Fig. 3 is a flowchart showing a more preferred embodiment of the method for producing molten steel according to the present invention; Fig. 4 is a flowchart showing a method for producing molten steel according to the prior art.

[0019] Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the following description shows examples of preferred embodiments of the present invention, and the embodiments for carrying out the present invention are not limited to these embodiments. Note that all component compositions and quantitative ratios in the present invention are expressed in mass % or mass ppm.

[0020] 1. In one embodiment, the present invention relates to a method for producing molten steel, comprising a first step of adding a first phosphorus-containing ferroalloy to molten steel having an oxygen content of 500 ppm by mass or more and 1200 ppm by mass or less, and a second step of subsequently adding aluminum to the molten steel. The method for producing molten steel according to the present invention relates to secondary refining performed on molten steel after decarburization and dephosphorization. By adjusting the oxygen content in the molten steel to 500 ppm by mass or more, oxidation of Ti contained in the phosphorus-containing ferroalloy is promoted, resulting in the production of titanium oxide. The produced titanium oxide migrates into slag formed by the subsequently added aluminum. By removing the slag from the molten steel, the Ti content in the molten steel can be reduced.

[0021] <Oxygen Content of Molten Steel> In the present invention, the oxygen content of the molten steel in the first step of adding the first P-containing ferroalloy is 500 ppm by mass or more and 1200 ppm by mass or less. If the oxygen content of the molten steel is 500 ppm by mass or more, the oxidation of Ti dissolved in the molten steel and the generation of titanium oxide are promoted. Ti is more easily oxidized than P added to the molten steel by the P-containing ferroalloy, and therefore is oxidized preferentially over P. The oxygen content of the molten steel is preferably 600 ppm by mass or more. On the other hand, if the oxygen content of the molten steel is 1200 ppm by mass or less, excess oxides are generated and remain in the slab, preventing them from becoming the starting point of defects in the steel material. The oxygen content of the molten steel is preferably 900 ppm by mass or less.

[0022] The oxygen content of molten steel can be determined by collecting an analytical sample from the molten steel and measuring the amount of oxygen contained in the collected sample using spark discharge optical emission spectroscopy (quantum back). The contents of other elements contained in the molten steel can also be measured using the same method. When the oxygen content of the molten steel is less than 500 ppm by mass, the oxygen content can be increased by blowing oxygen gas into the molten steel. When the oxygen content of the molten steel is more than 1200 ppm by mass, the oxygen content can be reduced by adding a deoxidizer to the molten steel. For example, a Si-containing ferroalloy can be used as the deoxidizer.

[0023] <First Step> In this embodiment, the first step is carried out, in which a first P-containing ferroalloy is added to molten steel having an oxygen content within the above-mentioned range. As described above, ferrophosphorus (FeP) can be used as the P-containing ferroalloy. The P content in the ferrophosphorus is not particularly limited, but the P content can be, for example, 20 mass% to 30 mass%. Ferrophosphorus may contain impurities such as Si, C, S, Mn, and Ti. As described above, Ti is an element that may degrade the electromagnetic properties of electrical steel sheets. In the present invention, Ti contained in molten steel can be efficiently removed, allowing the use of low-purity ferrophosphorus containing a large amount of Ti. This reduces the raw material cost of P-containing electrical steel sheets.

[0024] In this embodiment, the phosphorus-containing ferroalloy may be added to the molten steel once or twice. Fig. 1 is a flowchart showing one embodiment of a method for producing molten steel according to the present invention. In this embodiment, the first phosphorus-containing ferroalloy is added in the first step (S1), and then the second step of adding Al is carried out, without adding the phosphorus-containing ferroalloy a second time. In this embodiment, the molten steel is first poured into a ladle, and then the first phosphorus-containing ferroalloy is added to the molten steel contained in the ladle (S1). The added first phosphorus-containing ferroalloy dissolves in the molten steel, and the phosphorus component is contained in the molten steel.

[0025] When producing slabs for electrical steel sheets, it is preferable to set the P content of the slab to 40 mass ppm or more and 0.50 mass% or less from the viewpoint of improving electromagnetic properties. The amount of P-containing ferroalloy added can be adjusted so that the P content of the molten steel immediately before casting falls within the above range. The preferred amount of P-containing ferroalloy added to the molten steel can be determined based on the P yield rate obtained from past operational performance and other data.

[0026] When the Ti content of the first P-containing ferroalloy is 0.8 mass% or less, the Ti content does not exceed the Ti content that can be removed by the method for detitanization of molten steel according to this embodiment. Therefore, the Ti content of the P-containing ferroalloy is preferably 0.8 mass% or less. The Ti content of the P-containing ferroalloy is more preferably 0.6 mass% or less, and even more preferably 0.5 mass% or less. The Ti content of the first P-containing ferroalloy is preferably as small as possible, and may be, for example, 0.0 mass%. However, as described above, when low-purity ferrophosphorus containing a large amount of Ti is used as the first P-containing ferroalloy, the raw material cost of the P-containing electrical steel sheet can be reduced. Therefore, from the viewpoint of raw material cost, the Ti content of the P-containing ferroalloy is preferably 0.3 mass% or more, and more preferably 0.4 mass% or more.

[0027] In a preferred embodiment, the time from the end of the first step of adding the first P-containing ferroalloy to the start of the second step of adding Al is set to 5 minutes or more and 30 minutes or less. By setting the time from the end of the first step to the start of the second step to 5 minutes or more, it is possible to ensure the time required for the first P-containing ferroalloy to completely dissolve in the molten steel and for Ti contained as an impurity in the first P-containing ferroalloy to oxidize and form titanium oxide. On the other hand, by setting the time from the end of the first step to the start of the second step to 30 minutes or less, it is possible to shorten the overall time required for producing molten steel.

[0028] In a preferred embodiment, in the method for producing molten steel according to the present invention, the first step is performed by placing a first P-containing ferroalloy in a ladle and receiving molten steel in the ladle, and the second and subsequent steps are performed with the molten steel contained in the ladle. FIG. 2 is a flowchart showing this preferred embodiment. In this embodiment, the first P-containing ferroalloy is placed in the ladle in advance, and then molten steel is received on top of the first P-containing ferroalloy (S1). This allows the first P-containing ferroalloy to be stirred in the molten steel by the flow of molten steel, compared to adding the first P-containing ferroalloy from the surface of the molten steel, resulting in more uniform dispersion of the P component and an improved P yield. While there are no particular limitations on the specific method for placing the first P-containing ferroalloy in the ladle in advance, it is preferable to place the first P-containing ferroalloy in the ladle by using a hopper or a shooter. In this embodiment, the third step (P3) of adding the second P-containing ferroalloy (described later) may or may not be performed.

[0029] In a preferred embodiment, in the method for producing molten steel according to the present invention, the maximum diameter of the first P-containing ferroalloy is 3 mm or more and 20 mm or less. If the maximum diameter of the first P-containing ferroalloy is 3 mm or more, it is possible to prevent the ferroalloy from scattering as dust and reducing the yield of P. On the other hand, if the maximum diameter of the first P-containing ferroalloy is 20 mm or less, it is possible to quickly dissolve the ferroalloy in molten steel and shorten the time required for production.

[0030] <Second Step> In this embodiment, the second step is performed after the first step, in which Al is added to the molten steel. Because Al has a high affinity for oxygen, adding Al to the molten steel can reduce the oxygen content in the molten steel. As described above, in this embodiment, the oxygen content of the molten steel in the first step of adding the first P-containing ferroalloy is adjusted to 500 ppm by mass or more and 1200 ppm by mass or less in order to promote the oxidation of Ti dissolved in the molten steel and the generation of titanium oxide. If molten steel containing a large amount of oxygen is directly solidified and cast, a steel slab containing many bubbles due to the generation of carbon monoxide and iron oxide-based inclusions will be produced. By adding Al to perform deoxidation, a steel slab with fewer bubbles and inclusions can be obtained. For example, a material containing Al as the main component and Si and Cu as impurities can be used for adding Al. Deoxidation by adding Al to molten steel is sometimes referred to as "Al-killed treatment."

[0031] As described above, in the present invention, adding Al to molten steel not only reduces the oxygen content in the molten steel but also removes Ti from the molten steel. While the exact reason why Ti can be removed by the method for producing molten steel according to the present invention is unclear, it is believed that the reason is likely as follows: When Al is added to molten steel, it dissolves and combines with oxygen in the molten steel to form slag mainly composed of aluminum oxide. The formed slag floats and remains on the surface of the molten steel. Meanwhile, Ti contained as an impurity in the first P-containing ferroalloy added to the molten steel before the addition of Al combines with oxygen in the molten steel to form titanium oxide. Titanium oxide has a high affinity with slag. Therefore, after the slag is formed, titanium oxide migrates into the slag and remains there. By removing the slag from the molten steel, the Ti content in the molten steel can be efficiently reduced.

[0032] Since phosphorus pentoxide, an oxide of phosphorus, also has a high affinity with slag, once formed, it also migrates into the slag. As a result, a portion of the phosphorus added to the molten steel by the first phosphorus-containing ferroalloy is incorporated into the slag, resulting in a slight decrease in the P content in the molten steel, as described above. However, since phosphorus has a lower affinity for oxygen than titanium, the amount of phosphorus incorporated into the slag in the form of phosphorus pentoxide is smaller than the amount of titanium incorporated in the form of titanium oxide. Therefore, by predicting the amount of phosphorus lost during the Al-killing process and adjusting the amount of the first phosphorus-containing ferroalloy to be larger, the target P content can be achieved even if the phosphorus-containing ferroalloy is added to the molten steel only once, as shown in FIG. 1 .

[0033] In a preferred embodiment, the basicity of the slag formed by the addition of Al is approximately 3.0. The basicity of the slag is the ratio of the sum of the weights of the basic components divided by the sum of the weights of the acidic components. The weight ratio of calcium oxide to silicon dioxide can be used simply as the basicity. If the basicity of the slag is lower than 3.0, titanium oxide is less likely to migrate into the slag. If the basicity of the slag is higher than 3.0, excessive oxidation of the molten steel is likely to occur. The basicity of the slag can be adjusted by changing the components and amount of the slag-forming agent added to the molten steel. In other words, adding a slag-forming agent to the molten steel for adjusting the basicity of the slag or for other purposes is permitted in the present invention.

[0034] The amount of Al added to the molten steel can be appropriately determined depending on the oxygen content of the molten steel in the first step of adding the first P-containing ferroalloy. If the mass of Al added to the molten steel is too small relative to the mass of the molten steel, the molten steel cannot be sufficiently deoxidized by the Al-killed treatment. Therefore, the mass of Al added to the molten steel is preferably 0.005% or more, more preferably 0.006% or more, of the mass of the molten steel. On the other hand, if the mass of Al added to the molten steel is too large relative to the mass of the molten steel, the amount of Al-based precipitates becomes excessive, resulting in impaired electromagnetic properties. Therefore, the mass of Al added to the molten steel is preferably 0.05% or less, more preferably 0.03% or less, of the mass of the molten steel.

[0035] <Third Step> In a preferred embodiment, the method for producing molten steel according to the present invention includes a third step of adding a second P-containing ferroalloy to the molten steel after the second step of adding Al to the molten steel, and the mass ratio of the amount of the first P-containing ferroalloy to the total amount of the first P-containing ferroalloy and the second P-containing ferroalloy is 30% or more and 90% or less. Fig. 3 is a flowchart showing this more preferred embodiment. In this embodiment, the P-containing ferroalloy is added in two steps (S1 and S3), one before and one after the second step (S2) in which Al is added. The reason for adding the P-containing ferroalloy in two steps is that adding Al in the second step (S2) after adding the first P-containing ferroalloy in the first step (S1) reduces the Ti content in the molten steel and also slightly reduces the P content, and the deficiency is compensated for by adding the second P-containing ferroalloy in the third step (S3).

[0036] When the mass ratio of the first P-containing ferroalloy to the total amount of the first P-containing ferroalloy and the second P-containing ferroalloy is 30% or more and 90% or less, the second P-containing ferroalloy is the remaining 10% or more and 70% or less of the total amount. When the first P-containing ferroalloy is 30% or more of the total amount, Ti contained in 30% or more of the total amount can be combined with oxygen in the molten steel to form titanium oxide, which can be discharged into the slag. The amount of the first P-containing ferroalloy is more preferably 40% or more of the total amount, and even more preferably 50% or more of the total amount. On the other hand, when the first P-containing ferroalloy is 90% or less of the total amount, the P content necessary for improving the electromagnetic properties of the electrical steel sheet can be ensured by adding the second P-containing ferroalloy. The amount of the first P-containing ferroalloy is more preferably 80% or less of the total amount, and even more preferably 70% or less of the total amount.

[0037] Ti contained as an impurity in the second P-containing ferroalloy added to the molten steel in the third step remains in the molten steel without being removed. However, as described above, in this more preferred embodiment, the amount of the second P-containing ferroalloy added is set to 10% or more and 70% or less of the total amount added, so that the amount of Ti ultimately remaining in the molten steel can be reduced to a target value or less.

[0038] The first P-containing ferroalloy added to the molten steel in the first step and the second P-containing ferroalloy added to the molten steel in the second step may have the same or different component compositions. When the first P-containing ferroalloy and the second P-containing ferroalloy have different component compositions, it is preferable that the amount of Ti contained in the second P-containing ferroalloy is smaller than the amount of Ti contained in the first P-containing ferroalloy.

[0039] The time from the end of the second step of adding Al to the start of the third step of adding the second P-containing ferroalloy is preferably 3 minutes or more and 10 minutes or less. By setting the time from the end of the second step to the start of the third step to 3 minutes or more, time for the titanium oxide to migrate into the slag can be ensured. On the other hand, by setting the time from the end of the second step to the start of the third step to 10 minutes or less, the overall time required for producing molten steel can be shortened.

[0040] <Vacuum degassing> In a preferred embodiment, in the method for producing molten steel according to the present invention, at least the second step is performed while vacuum degassing the molten steel. By combining deoxidation by adding Al with vacuum degassing of the molten steel, the oxygen content in the molten steel can be further reduced. Vacuum degassing can simultaneously remove not only oxygen contained in the molten steel but also gas components such as hydrogen, carbon, and nitrogen. The method of vacuum degassing is not particularly limited, and any known method can be used. For example, vacuum degassing can be performed on molten steel received in a ladle using a known method such as RH vacuum degassing or DH vacuum degassing.

[0041] As shown in Fig. 3, the timing of the start of the vacuum degassing treatment (VS) can be set before the first step (S1) of adding the first P-containing ferroalloy. Since it takes about 10 to 15 minutes for the state of the molten steel to stabilize after the molten steel has been poured into the ladle, the timing of starting the vacuum degassing treatment is preferably set 10 to 15 minutes after the completion of the pouring. When the first step (S1) of adding the first P-containing ferroalloy during the vacuum degassing treatment is performed, it is preferably set 5 minutes after the start of the vacuum degassing treatment (VS). The first P-containing ferroalloy and Al can be added using an ferroalloy addition hole or the like provided in the vacuum degassing treatment device.

[0042] As described above, when the first P-containing ferroalloy is placed in the ladle in advance to receive the molten steel, the vacuum degassing treatment (VS) can be started after the molten steel has been received as shown in Fig. 2. In this case, the timing for starting the vacuum degassing treatment (VS) is preferably set to 10 to 15 minutes after the molten steel has been received. As the carbon content in the molten steel decreases about 10 minutes after the start of the vacuum degassing treatment (VS), the oxygen content in the molten steel also decreases. Adding Al (S2) after this is preferable because it improves the yield of Al.

[0043] <Component Composition of Molten Steel> In a preferred embodiment, the production method according to the present invention controls the Ti content in the molten steel to 1 / 25 or less of the P content. By controlling the Ti content to 1 / 25 or less of the P content, it is possible to prevent deterioration of the electromagnetic properties due to the inclusion of Ti in the electrical steel sheet produced from the molten steel. More preferably, the mass proportion of Ti contained in the molten steel immediately before casting is 1 / 30 or less of the mass proportion of P.

[0044] In a preferred embodiment, the molten steel immediately before casting has a composition of C: 0.02 to 0.08% by mass, Si: 2.0 to 4.5% by mass, Mn: 0.01 to 0.5% by mass, P: 40 to 0.50% by mass, acid-soluble Al: 20 to 100 ppm by mass, N: 80 ppm by mass, Ti: less than 50 ppm by mass, the balance being Fe and unavoidable impurities. This composition satisfies the preferred composition range for the composition of electrical steel sheet.

[0045] 2. Method for Detitaniumizing Molten Steel In another embodiment, the present invention is a method for detitaniumizing molten steel, comprising a first step of adding a first P-containing ferroalloy to molten steel having an oxygen content of 500 ppm by mass or more and 1200 ppm by mass or less, and a second step of subsequently adding Al to the molten steel, thereby removing Ti contained as an impurity in the molten steel.

[0046] As described above, according to the present invention, Ti contained in the first P-containing ferroalloy combines with oxygen in the molten steel to form titanium oxide. The titanium oxide migrates into the slag formed by the addition of Al. This allows Ti, which is harmful to the molten steel, to be efficiently removed from the molten steel.

[0047] Shown below are actual values ​​of the amounts of P and Ti contained in molten steel immediately before casting when the molten steel is actually refined by the method for producing molten steel according to the present invention, and the results of a simulation performed based on the actual values.

[0048] Comparative Example: For the purpose of comparing the effects of the present invention with those of the prior art, molten steel of the comparative example was produced according to the flowchart of the prior art molten steel production method shown in Figure 4. First, 200 tons of pig iron produced in a blast furnace was loaded into a pure oxygen top-blown converter and decarburized and dephosphorized. Sample A for analyzing the chemical composition was taken from the molten steel after decarburization and dephosphorization was completed, and the oxygen content was measured by spark discharge optical emission spectroscopy. Next, the molten steel was poured into a ladle, and an RH vacuum degasser was set above the ladle, and vacuum degassing of the molten steel was initiated. 15 minutes after the start of vacuum degassing, 30 kg of Al was added to the molten steel from a hopper installed above the RH vacuum degasser.

[0049] Five minutes after the addition of Al, sample B for analyzing the elemental composition was taken from the molten steel, and the P content was measured. Then, 205 kg of ferrophosphorus containing 25 mass % P was added to the molten steel from a hopper provided above the RH vacuum degassing apparatus. The amount of ferrophosphorus to be added was adjusted based on the P content measured for sample B and past performance values ​​for the increase in P content due to the addition of ferrophosphorus so that the P content in the molten steel immediately before casting would be 50.0 ppm by mass.

[0050] Five minutes after the addition of ferrophosphorus, sample C was taken from the molten steel for composition analysis. Thereafter, the vacuum degassing treatment was completed, and the molten steel was cast using a mold to obtain a steel slab. This series of molten steel refining processes was repeated 50 times. The average values ​​of the 50 measurements for the oxygen content of sample A and the P content and Ti content of sample C are shown in the column for comparative example in Table 1. As shown in Table 1, the average Ti content in the molten steel immediately before casting in the comparative example was 17.1 mass ppm.

[0051] Example 1 Molten steel for Example 1 was produced according to the flowchart of the method for producing molten steel according to the present invention shown in FIG. 3 . First, 200 tons of pig iron produced in a blast furnace was loaded into a pure oxygen top-blown converter and decarburized and dephosphorized. Sample A for analyzing the chemical composition was collected from the molten steel after decarburization and dephosphorization, and the oxygen content was measured by spark discharge optical emission spectroscopy. Next, 100 kg of ferrophosphorus containing 25 mass% P was added to the ladle using a hopper, and the molten steel was poured into the ladle. Next, an RH vacuum degasser was installed above the ladle, and vacuum degassing of the molten steel was initiated. 15 minutes after the start of vacuum degassing, 30 kg of Al was added to the molten steel through a ferroalloy addition hole provided at the top of the RH vacuum degasser.

[0052] Five minutes after the addition of Al, sample B for analyzing the elemental composition was taken from the molten steel, and the P content was measured. Then, 105 kg of ferrophosphorus containing 25 mass % P was added to the molten steel for the second time through the ferroalloy addition hole provided in the upper part of the RH vacuum degassing apparatus. The amount of ferrophosphorus added for the second time to the molten steel was adjusted so that the P content in the molten steel immediately before casting would be 50.0 ppm by mass, based on the P content measured for sample B and the past actual value of the increase in P content due to the second addition of ferrophosphorus.

[0053] Five minutes after the second addition of ferrophosphorus, sample C was taken from the molten steel for composition analysis. The vacuum degassing process was then completed, and the molten steel was cast into a mold to obtain a slab. This series of molten steel production processes was repeated 50 times. The average values ​​of the 50 measurements for the oxygen content of sample A, the proportion of the mass of the initially added ferrophosphorus to the total amount added, and the P and Ti contents of sample C are shown in the column for Example 1 in Table 1. As shown in Table 1, the average Ti content in the molten steel immediately before casting was 14.1 mass ppm. The Ti content in Example 1 was lower than that in the comparative example. This is thought to be because part of the ferrophosphorus to be added to the molten steel was added to the molten steel as the first P-containing ferroalloy before adding Al, and therefore Ti contained in the initially added ferrophosphorus migrated into the slag generated by the addition of Al and was removed from the molten steel.

[0054] Example 2 Based on the actual values ​​of the above Comparative Example and Example 1, the amounts of P and Ti contained in the molten steel immediately before casting were estimated by simulation when the mass of ferrophosphorus initially added was changed from the actual amount of 100 kg to 150 kg, which is 1.5 times the actual amount. Since the total amount of ferrophosphorus added in this simulation was 205 kg, the same as in the Comparative Example and Example 1, the average P content in the molten steel immediately before casting was estimated to be 50.0 ppm by mass, the same as in the Comparative Example and Example 1. Meanwhile, the average Ti content can be estimated by proportional calculation based on the actual values ​​of the Comparative Example and Example 1. The average Ti content in the molten steel immediately before casting estimated by proportional calculation is 12.6 ppm by mass, as shown in the column for Example 2 in Table 1. The reason why the Ti content in the simulation of Example 2 is lower than the actual value of Example 1 is thought to be because the proportion of Ti transferred to the slag generated by the addition of Al increases when the amount of ferrophosphorus previously placed in the ladle as the first P-containing ferroalloy is increased.

[0055] <Invention Example 3> Based on the actual values ​​of the above Comparative Example and Invention Example 1, the estimated amounts of P and Ti contained in molten steel immediately before casting were calculated by simulation when the mass of ferrophosphorus initially added was changed from the actual amount of 100 kg to approximately double that amount, 205 kg. In this manufacturing method, the amount of ferrophosphorus initially added to the ladle was 205 kg, and a second addition of ferrophosphorus was not performed. As described above, the addition of Al after the first addition of ferrophosphorus reduces the Ti content in the molten steel and also slightly reduces the P content. Furthermore, since a second addition of ferrophosphorus is not performed in Invention Example 3, the P deficiency cannot be compensated for. It has been empirically found that in this case, the P content in the molten steel is reduced by approximately 0.5 mass% compared to when a second addition of ferrophosphorus is performed. For this reason, although the total amount of ferrophosphorus added was 205 kg, the same as in the comparative example, invention example 1, and invention example 2, the average value of the P content in the molten steel immediately before casting is estimated to be 49.8 ppm by mass, as shown in the column for invention example 3 in Table 1.

[0056] On the other hand, the average value of the Ti content can be estimated by proportional calculation based on the actual values ​​of the comparative example and invention example 1, as in the case of invention example 2. The average value of the Ti content in the molten steel immediately before casting estimated by proportional calculation is 11.0 mass ppm, as shown in the column for invention example 3 in Table 1. The reason why the Ti content in the simulation of invention example 3 is the lowest in Table 1 is thought to be that when the entire amount of ferrophosphorus is charged into the ladle in advance, the proportion of Ti that migrates into the slag generated by the addition of Al is maximized.

[0057]

[0058] The numerical values ​​of Examples 1 to 3 shown in Table 1 show that by adding the first P-containing ferroalloy to molten steel having an oxygen content of 500 ppm by mass or more and 1200 ppm by mass or less, and then adding Al to the molten steel according to the method for producing molten steel of the present invention, the Ti content in the molten steel can be reduced more than in the comparative example. Also, the numerical values ​​of Examples 1 and 2 shown in Table 1 show that by setting the amount of the first P-containing ferroalloy added to the molten steel to 30% or more and 90% or less of the total amount added, and adding the second P-containing ferroalloy to the molten steel to which Al has been added, according to the method for producing molten steel according to the preferred embodiment of the present invention, the P content can be accurately controlled and the Ti content in the molten steel can be reduced more than in the comparative example.

[0059] S1 Addition of first P-containing ferroalloy (first step) S2 Addition of Al (second step) S3 Addition of second P-containing ferroalloy (third step) VS Start of vacuum degassing VE End of vacuum degassing S1' Addition of P-containing ferroalloy (prior art)

Claims

1. A method for producing molten steel, comprising: a first step of adding a first P-containing ferroalloy to molten steel having an oxygen content of 500 mass ppm or more and 1200 mass ppm or less; and a second step of subsequently adding Al to the molten steel.

2. A method for producing molten steel according to claim 1, further comprising a third step of adding a second P-containing ferroalloy to the molten steel after the second step, wherein the mass ratio of the amount of the first P-containing ferroalloy added to the total amount of the first P-containing ferroalloy and the second P-containing ferroalloy added is 30% or more and 90% or less.

3. A method for producing molten steel according to claim 1 or 2, wherein the time from the end of the first step to the start of the second step is 5 minutes or more and 30 minutes or less.

4. A method for producing molten steel as set forth in any one of claims 1 to 3, wherein the first step is carried out by placing the first P-containing ferroalloy in a ladle and receiving the molten steel in the ladle, and the second and subsequent steps are carried out with the molten steel contained in the ladle.

5. A method for producing molten steel according to any one of claims 1 to 4, wherein the maximum diameter of the first P-containing ferroalloy is 3 mm or more and 20 mm or less.

6. A method for producing molten steel according to any one of claims 1 to 5, wherein at least the second step is carried out while vacuum degassing the molten steel.

7. A method for producing molten steel according to any one of claims 1 to 6, wherein the Ti content in the molten steel is 1 / 25 or less of the P content by the method.

8. A method for detitanization of molten steel, comprising: a first step of adding a first P-containing ferroalloy to molten steel having an oxygen content of 500 ppm by mass or more and 1200 ppm by mass or less; and a second step of subsequently adding Al to the molten steel, wherein Ti contained as an impurity in the molten steel is removed.

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