Method for blowing gas into molten steel in ladle

Abstract

To suppress the clogging of a gas blowing part when nitrogen gas is blown into molten steel in a ladle from the gas blowing part in ladle refining at the time of melting high Mn steel.SOLUTION: In ladle refining, molten steel in a ladle is stirred by blowing nitrogen gas into the molten steel in the ladle from a gas blowing part made of a refractory. The molten steel is adjusted to have the following composition, or a steel having a component specification satisfying the following is a subject. C:0.24% or more and 0.85% or less (% represents mass%, the same shall apply hereinafter), Si:0.12% or more and 0.35% or less, Mn: 6% or more and 16% or less, Al: 0.015% or less, Cr: 1.05% or less, P: 0.0330% or less, S: 0.0300% or less, N: 0.0450% or less, and the balance being Fe and inevitable impurities. The gas blowing part is made of a refractory having a content of Cr2O3 of 0.06% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method of blowing gas into molten steel in a ladle during ladle refining.

Background Art

[0002] Patent Document 1 describes a method for melting austenitic high-Mn stainless steel. In the examples of Patent Document 1, it is described that "···Subsequently, ladle refining was carried out. The raw materials were melted in an electric arc furnace 1 in FIG. 1, and then the molten steel was transferred to a ladle 2 and placed at the bottom of a ladle refining furnace 3. Ar was bottom-blown from a porous brick 6 and slag refining in the ladle was carried out.···" (see

[0016] of Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the examples of Patent Document 1, in ladle refining, Ar gas (argon gas) is blown into the molten steel in the ladle. By blowing Ar gas, the molten steel in the ladle is stirred.

[0005] When melting high-Mn steel, in ladle refining, nitrogen gas may be blown into the molten steel in the ladle from the gas blowing part. From the experiments of the inventors of the present application so far, when melting high-Mn steel, in ladle refining, when nitrogen gas is blown, the gas blowing part may be blocked. When the gas blowing part is blocked, it is necessary to remove the blockage. Therefore, the overall processing time becomes long and the productivity decreases.

[0006] An object of the present invention is to provide a method for suppressing blockage of a gas blowing portion when nitrogen gas is blown into molten steel in a ladle during ladle refining when producing high-Mn steel.

Means for Solving the Problems

[0007] The inventors of the present application studied the cause of blockage of the gas blowing portion when nitrogen gas is blown into the molten steel in the ladle during ladle refining when producing high-Mn steel, and obtained the following findings.

[0008] Nitrogen gas is more likely to dissolve in molten steel than Ar gas. Therefore, when nitrogen gas is blown into the molten steel in the ladle from the gas blowing portion, the linear flow velocity of the nitrogen gas near the gas blowing port is smaller than when Ar gas is blown at the same flow rate, so the molten steel is likely to enter from the gas blowing port into the gas blowing portion. Hereinafter, the entry of molten steel into the gas blowing portion will be expressed as "ingot metal is inserted into the gas blowing portion". In addition, since nitrogen has a large specific heat and an endothermic reaction occurs when it dissolves in molten steel, when nitrogen gas is blown into the molten steel in the ladle, the molten steel near the gas blowing port is cooled and likely to solidify.

[0009] From the above, when nitrogen gas is blown into the molten steel in the ladle, the vicinity of the gas blowing port is likely to be blocked.

[0010] Furthermore, it was found that the following events occur in the high-Mn steel targeted by the present invention.

[0011] As a method of blowing nitrogen gas into the molten steel in the ladle, for example, a method of blowing nitrogen gas from a gas blowing portion formed at the bottom of the ladle, and a method of blowing nitrogen gas using a lance (sometimes referred to as a "lance pipe") can be mentioned. Generally, refractories are used for these gas blowing portions.

[0012] In the high-Mn steel targeted by the present invention, the molten steel in the ladle contains a large amount of Mn (manganese). Due to the Mn (manganese) contained in the molten steel and Al2O3 (aluminum oxide) contained in the refractory of the gas injection part, etc., the reaction shown in formula (1) occurs, and it is considered that MnO·Al2O3 is generated at the interface between the refractory and the molten steel. Note that the underlines represent the components in the molten steel. Al2O3+ Mn + O → MnO·Al2O3···(1) Since Cr2O3 is more easily reduced than other oxides contained in the refractory, when the refractory contains Cr2O3, Cr2O3 becomes an oxygen source, supplies oxygen to the molten steel, and the reaction of formula (1) occurs. Especially when the refractory contains a large amount of Cr2O3, it is considered that the reaction of formula (1) easily proceeds. MnO·Al2O3 generated by the reaction of formula (1) is generated at the interface between the molten steel and the refractory of the gas injection part.

[0013] Figure 1 shows "the case where the wettability is poor" and "the case where the wettability is good" on the surface of the gas injection part. When the "wetting is poor" in the gas injection part, as shown in Figure 1, since the molten steel hardly spreads by wetting on the gas injection part, it is difficult to insert the ingot into the gas injection part. On the other hand, when the "wettability is good" on the surface of the gas injection part, as shown in Figure 1, the molten steel easily spreads by wetting on the gas injection part, and it is easy to insert the ingot.

[0014] When the applicants investigated the Al2O3-Cr2O3-based injection refractory when melting high-Mn steel, it was confirmed that MnO·Al2O3 was generated at the interface between the high-Mn steel and the Al2O3-Cr2O3 refractory. It is assumed that the wetting between the high-Mn steel and MnO·Al2O3 was promoted by MnO·Al2O3 generated at the interface by the reaction between this refractory and the molten steel, and it was considered that it was in a state close to "the case where the wetting is good" in Figure 1, the ingot was inserted into the gas injection part, leading to the blockage of the gas injection part.

[0015] Thus, it is considered that the gas injection part becomes blocked due to the overlap of conditions where the gas injection part, such as the injection of nitrogen gas and the melting of high-Mn steel, is likely to be blocked.

[0016] The inventors of the present application further studied with the aim of suppressing clogging of the gas blowing section even when conditions that make the gas blowing section prone to clogging overlap. Then, focusing on Cr2O3 serving as an O (oxygen) source with the aim of suppressing the reaction of formula (1), the following method was found.

[0017] The method for refining molten steel disclosed in this specification stirs the molten steel in the ladle by blowing nitrogen gas into the molten steel in the ladle from a gas blowing section made of refractory in ladle refining, and the molten steel is adjusted to the following component composition, or is targeted at steel that satisfies the following component specifications. C: 0.24% or more and 0.85% or less (% represents mass%. The same applies hereinafter) Si: 0.12% or more and 0.35% or less Mn: 6% or more and 16% or less Al: 0.015% or less Cr: 1.05% or less P: 0.0330% or less S: 0.0300% or less N: 0.0450% or less The balance consists of Fe and unavoidable impurities. The gas blowing section is made of refractory with a Cr2O3 content of 0.06% or less.

[0018] By using the above refractory for the gas blowing section, the reaction of the above formula (1) is suppressed, so that the formation of MnO·Al2O3, which is assumed to promote wetting between high-Mn steel and MnO·Al2O3, is suppressed, and it is considered that the insertion of the ingot can be suppressed. As a result, clogging of the gas blowing section can be suppressed even under conditions where the gas blowing section is prone to clogging, such as blowing of nitrogen gas and melting of high-Mn steel.

[0019] In the above method, the gas blowing section may be formed in the ladle.

[0020] Also, in the above method, nitrogen gas may be blown into the molten steel in the ladle by a lance, and the lance may have the gas blowing section.

Advantages of the Invention

[0021] When melting high-Mn steel, in the ladle refining, when nitrogen gas is blown into the molten steel in the ladle from the gas injection part, it is possible to suppress the clogging of the gas injection part.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0023] Hereinafter, preferred embodiments of the present invention will be described.

[0024] The method according to this embodiment stirs the molten steel in the ladle by blowing nitrogen gas from the gas injection part made of refractory into the molten steel in the ladle during the melting of high-Mn steel in ladle refining. The gas injection part is made of refractory with a Cr2O3 content of 0.06% or less. Nitrogen gas is blown in during ladle refining for purposes of component adjustment such as, for example, stirring the molten steel in the ladle and adjusting the N (nitrogen) content of the molten steel.

[0025] The high-Mn steel targeted by this embodiment is steel whose molten steel is adjusted to the following component composition or satisfies the following component specifications in ladle refining. C: 0.24% or more and 0.85% or less (% represents mass%. The same applies hereinafter) Si: 0.12% or more and 0.35% or less Mn: More than 6% and less than 16% Al: 0.015% or less Cr: 1.05% or less P: 0.0330% or less S: 0.0300% or less N: 0.0450% or less The balance consists of Fe and inevitable impurities. The high-Mn steel targeted by this embodiment is used, for example, as non-magnetic steel.

[0026] The method of blowing gas into the molten steel in the ladle from the gas injection part is not particularly limited. For example, as shown in FIG. 2, gas may be blown into the molten steel in the ladle from the bottom of the ladle. As shown in FIG. 3, gas may be blown into the molten steel in the ladle using a lance. Hereinafter, an example of the method of blowing gas into the molten steel in the ladle will be described with reference to FIGS. 2 to 5.

[0027] In the method shown in FIG. 2, nitrogen gas is blown into the molten steel 2 in the ladle 1 from the bottom of the ladle 1. The ladle 1 shown in FIG. 2 is mainly made of refractory. A gas injection part 11 is formed at the bottom of the ladle 1. The gas injection part 11 is made of a refractory different from the part of the ladle 1 other than the gas injection part 11. The gas injection part 11 is made of a porous refractory through which gas can pass. When nitrogen gas is blown into the gas injection part 11, the nitrogen gas passes through the gas injection part 11 and is blown into the molten steel 2 in the ladle 1 from the gas injection part 11. The gas injection part 11 is the part that directly contacts the nitrogen gas before it is blown into the molten steel 2. The gas injection part 11 is the part through which the nitrogen gas passes before it is blown into the molten steel 2. The part of the ladle 1 other than the gas injection part 11 does not contact the nitrogen gas before it is blown into the molten steel 2. The nitrogen gas before it is blown into the molten steel 2 does not pass through the part of the ladle 1 other than the gas injection part 11.

[0028] In the method shown in FIG. 3, nitrogen gas is blown into the molten steel 102 in the ladle 101 using the lance 200. The nitrogen gas is blown into the molten steel 102 in the ladle 101 from the lower end of the lance 200.

[0029] Fig. 4 shows an enlarged view of the lower end of the lance 200. One or more gas injection ports 221 are formed at the lower end of the lance 200. Fig. 4 shows, as an example, the case where four gas injection ports 221 are formed in the lance 200.

[0030] Fig. 5 shows a partial cross-sectional view of the lance 200. As shown in Fig. 5, the lance 200 has a core tube 231 and a refractory 232 covering the core tube 231.

[0031] The core tube 231 is, for example, a metal tube. The core tube 231 is made of something other than refractory.

[0032] The refractory 232 covering the core tube 231 has a gas injection part 241 and a covering part 242 covering the core tube 231 and the gas injection part 241. The gas injection part 241 forms a gas flow path from the tip of the core tube 231 to the gas injection port 221. The gas injection part 241 is the part directly in contact with the nitrogen gas before it is blown into the molten steel 2 in the ladle 1. Nitrogen gas passes through the inside of the gas injection part 241. The covering part 242 is not in contact with the nitrogen gas before it is blown into the molten steel 102 in the ladle 101. The refractory of the gas injection part 241 and the refractory of the covering part 242 may be the same or different.

[0033] The nitrogen gas passes through the inside of the core tube 231 and the gas injection part 241 of the lance 200, comes out from the gas injection port 221, and is blown into the molten steel 102 in the ladle 101 shown in Fig. 3.

[0034] As described above, the "gas injection part" (11, 241) is made of refractory and is the part for blowing nitrogen gas into the molten steel (2, 102) in the ladle (1, 101). The "gas injection part" (11, 241) is the part directly in contact with the nitrogen gas before it is blown into the molten steel (2, 102) in the ladle (1, 101).

[0035] The gas injection parts (11, 241) are made of refractories with a Cr2O3 content of 0.06% or less. The refractories of the gas injection parts (11, 241) may be of any type as long as the Cr2O3 content is 0.06% or less. For example, if the Cr2O3 content is 0.06% or less, Al2O3-C refractories, MgO-C refractories, or Al2O3-MgO refractories may be used.

[0036] The refractories other than the gas injection parts (11, 241) may have a Cr2O3 content of 0.06% or less or a Cr2O3 content exceeding 0.06%. The refractories other than the gas injection parts (11, 241) may be of the same type as the gas injection parts (11, 241) or of different types.

[0037] For example, the refractories of the parts other than the gas injection part 11 of the ladle 1 shown in Fig. 2 may have a Cr2O3 content exceeding 0.06%. Similarly, for the refractories of the coating part 242 shown in Fig. 5, they may have a Cr2O3 content of 0.06% or less or a Cr2O3 content exceeding 0.06%.

[0038] Not only nitrogen gas but also gases other than nitrogen gas may be blown from the gas injection parts (11, 241) into the molten steel in the ladle. For example, Ar gas may be blown from the gas injection parts into the molten steel in the ladle. When blowing nitrogen gas and a gas other than nitrogen gas from the gas injection parts into the molten steel in the ladle, the order of the gases to be blown is not limited.

[0039] The specific processes, means, conditions, etc. of ladle refining are not particularly limited, and may be carried out, for example, in accordance with the common methods of those skilled in the art. Ladle refining may be carried out, for example, using a ladle refining apparatus such as LF or a degassing apparatus such as RH.

[0040] Hereinafter, the experiments that led to the above method will be described.

[0041] The molten steel melted in a converter was tapped into a ladle. The tapping temperature was set at 1680 °C or higher. Subsequently, after performing the first ladle refining, degassing treatment, and the second ladle refining in sequence, continuous casting was carried out in the usual manner of those skilled in the art to obtain high-Mn steel.

[0042] The first ladle refining was carried out using an LF (Ladle Furnace). In the first ladle refining, for a while after the start of the treatment, Ar gas was blown into the molten steel in the ladle, and nitrogen gas was blown in from the middle. The components of the molten steel were adjusted by charging alloys into the ladle, etc.

[0043] A lance was used for blowing the gas. In this experiment, a lance with multiple gas injection ports was used. The vicinity of the gas injection ports of the lance is as shown in Fig. 5, and the refractories of the gas injection part are different between Experiment No. 1 - 4 and Experiment No. 5 - 10. In Experiment No. 1 - 4, a lance with a gas injection part made of Al2O3 - Cr2O3 - based refractory was used. In Experiment No. 5 - 10, a lance with a gas injection part made of Al2O3 - C - based refractory was used.

[0044] The lower end of the lance was placed at a height of 0.3 - 0.8 m from the bottom of the ladle. Nitrogen gas with a flow rate of 4.5 - 8.3 L / min per ton of molten steel was blown in to stir the molten steel. In Experiment No. 2, the flow rate of nitrogen gas was changed midway. The experimental conditions are shown in Table 1 described later.

[0045] Other treatments (slag removal, slag formation, etc.) were carried out in the usual manner of those skilled in the art.

[0046] After the first ladle refining, degassing treatment was carried out in the usual manner of those skilled in the art using an RH vacuum degassing device.

[0047] After the degassing treatment, the second ladle refining was carried out. In the second ladle refining, using an LF, the temperature adjustment and fine adjustment of the components of the molten steel were performed. In the second ladle refining, Ar gas with a flow rate of 2.6 - 3.4 L / min per ton of molten steel was blown in to stir the molten steel.

[0048] By the above ladle refining, the molten steel in the ladle was adjusted to have the following component composition ranges. C: 0.24% or more and 0.85% or less Si: 0.12% or more and 0.35% or less Mn: 6% or more and 16% or less Al: 0.015% or less Cr: 1.05% or less P: 0.0330% or less S: 0.0300% or less N: 0.0450% or less The balance consists of Fe and unavoidable impurities. In this experiment, high-Mn steel within the component range satisfying the above was obtained.

[0049] In the first ladle refining, in order to examine whether the portion corresponding to the gas injection part was blocked during the blowing of nitrogen gas, the back pressure of the lance and the gas flow rate were measured. The back pressure of the lance is the pressure at the inlet of the lance during the blowing of nitrogen gas. Table 1 shows the "maximum value of the lance back pressure" during the blowing of nitrogen gas. Also, Table 1 shows the "set value of the gas linear flow velocity" calculated from the set gas flow rate and the "minimum value of the gas linear flow velocity" calculated from the actual value of the minimum gas flow rate. Note that the gas linear flow velocity is the value obtained by dividing the gas flow rate per hole by the cross-sectional area of the blowing hole. When the "maximum value of the lance back pressure" is large, it is considered that the gas injection part is blocked. When the difference between the "set value of the gas linear flow velocity" and the "minimum value of the gas linear flow velocity" is large, it is considered that the gas injection part is blocked. However, a difference of 1 m / s or less is within the range of measurement error, and when the difference is 1 m / s or less, it can be said that there is almost no difference between the "set value of the gas linear flow velocity" and the "minimum value of the gas linear flow velocity". Therefore, when the difference between the "set value of the gas linear flow velocity" and the "minimum value of the gas linear flow velocity" is 1 m / s, it is judged that the gas flow rate has not decreased, that is, the gas injection part is not blocked, and the evaluation is "○". On the other hand, when the difference between the "set value of the gas linear flow velocity" and the "minimum value of the gas linear flow velocity" is greater than 1 m / s, it is judged that the gas flow rate has decreased, that is, the gas injection part is blocked, and the evaluation is "×". Table 1 shows the experimental results.

[0050]

Table 1

[0051] From Table 1, in Experiments No. 1 to 4, the evaluation was "×" and the gas injection part was blocked. In Experiments No. 5 to 10, the evaluation was "〇" and the gas injection part was not blocked.

[0052] In Experiments No. 1 to 4, all the refractories in the gas injection part were Al2O3-Cr2O3 refractories. The Cr2O3 content of this refractory was 3%. On the other hand, in Experiments No. 5 to 10, the refractories in the gas injection part were Al2O3-C refractories. The Cr2O3 content of this refractory was 0.06%. This difference is considered to have affected the presence or absence of blockage of the gas injection part.

[0053] In this experiment, since high-Mn steel is the target, the molten steel contains a lot of Mn (manganese). Due to Mn (manganese) in the molten steel and Al2O3 (aluminum oxide) contained in the refractory, it is assumed that the reaction shown in formula (1) occurs and MnO·Al2O3 that easily blocks the gas injection part is generated. Al2O3+ Mn + O → MnO·Al2O3···(1)

[0054] In Experiments No. 5 to 10, since the Cr2O3 content in the gas injection part of the lance was low, it is considered that the reaction of the above formula (1) was suppressed, so the gas injection part was not blocked. Since the gas injection part was not blocked when the Cr2O3 content of the refractory in the gas injection part was 0.06%, when the Cr2O3 content is less than 0.06%, the reaction of the above formula (1) is more suppressed and the gas injection part is considered not to be blocked.

[0055] From the above, when the Cr2O3 content of the refractory in the gas injection part is 0.06% or less, it is considered that the blockage of the gas injection part is suppressed.

[0056] In the above experiment, a lance was used to blow nitrogen gas. However, as shown in Fig. 2, the same consideration can be applied when blowing nitrogen gas from a part of the gas injection section 11 of the ladle. That is, when blowing nitrogen gas from a part of the gas injection section of the ladle, if the Cr2O3 content of the refractory material in the gas injection section is 0.06% or less, the blockage of the gas injection section is suppressed.

[0057] From the above, the following findings were obtained. When melting high-Mn steel, in ladle refining, the molten steel in the ladle is stirred by blowing nitrogen gas from the gas injection section into the molten steel in the ladle. The molten steel is adjusted to the following component composition or is targeted at steel with a component standard that satisfies the following. C: 0.24% or more and 0.85% or less Si: 0.12% or more and 0.35% or less Mn: 6% or more and 16% or less Al: 0.015% or less Cr: 1.05% or less P: 0.0330% or less S: 0.0300% or less N: 0.0450% or less The balance consists of Fe and inevitable impurities, The gas injection section is made of a refractory material with a Cr2O3 content of 0.06% or less. By the above method, the formation of MnO·Al2O3, which is assumed to easily block the gas injection section, is suppressed. As a result, even when ladle refining is carried out under conditions where the gas injection section is likely to be blocked, such as nitrogen gas blowing and melting of high-Mn steel, the blockage of the gas injection section can be suppressed. Therefore, the operation of removing the blockage can be omitted, and the overall processing time can be prevented from becoming long.

[0058] Also, from the above experiment, as shown in Fig. 6, the relationship between the "maximum value of gas back pressure" and the "presence or absence of blockage of the gas injection section" was obtained. From Fig. 6, it was found that when the "maximum value of gas back pressure" during nitrogen gas blowing is 0.48 MPa or less, the blockage of the gas injection section is suppressed.

[0059] As described above, the embodiments of the present invention have been described with reference to the drawings. However, the specific configuration should be considered not to be limited to these embodiments. The scope of the present invention is defined by the claims rather than the above description, and includes all modifications within the meaning and scope equivalent to the claims.

[0060] In the above experiment, an Al2O3-C refractory was used as the refractory with a Cr2O3 content of 0.06% or less in the gas injection section. However, the refractory with a Cr2O3 content of 0.06% or less is not limited to the Al2O3-C refractory. Other refractories than the Al2O3-C refractory may be used as the refractory with a Cr2O3 content of 0.06% or less.

[0061] In the above embodiment, the methods shown in FIGS. 2 to 5 were exemplified as the method of blowing nitrogen gas into the molten steel in the ladle. However, the method of blowing nitrogen gas into the molten steel in the ladle is not limited to the methods shown in FIGS. 2 to 5. For example, the ladle 1 shown in FIG. 2 has a gas injection section 11 at the bottom. However, the gas injection section 11 may be formed in a part other than the bottom of the ladle 1. For example, the gas injection section 11 may be formed on the side of the ladle 1. Further, the configuration of the lance is not limited to the configurations shown in FIGS. 4 and 5.

Explanation of Reference Numerals

[0062] 1, 101 Ladle 2, 102 Molten Steel 11, 241 Gas Injection Section 200 Lance 221 Gas Injection Port 231 Core Tube 232 Refractory 242 Coating Section

Claims

1. In ladle refining, the molten steel in the ladle is stirred by blowing nitrogen gas into the molten steel in the ladle from a gas blowing part made of refractory, the molten steel is adjusted to the following component composition, or is targeted at steel with a component standard that satisfies the following, C: 0.24% or more and 0.85% or less (% represents mass%). The same applies hereinafter. Si: 0.12% or more and 0.35% or less Mn: 6% or more and 16% or less Al: 0.015% or less Cr: 1.05% or less P: 0.0330% or less S: 0.0300% or less N: 0.0450% or less The balance consists of Fe and inevitable impurities, The gas injection part is made of a refractory material with a Cr 2 O 3 content of 0.06% or less A method for blowing gas into molten steel in a ladle, characterized by the above.

2. The gas blowing part is formed in the ladle A method for blowing gas into molten steel in a ladle according to Claim 1, characterized by the above.

3. Nitrogen gas is blown into the molten steel in the ladle by a lance, The lance has the gas blowing part A method for blowing gas into molten steel in a ladle according to Claim 1, characterized by the above.

Images