Method for producing Al-deoxidized steel

Abstract

To provide an Al-deoxidized steel melting method which can efficiently melt a high cleanliness molten steel without extending a treatment time in an LF step.SOLUTION: Provided is an Al-deoxidized steel melting method in which, in a molten steel which is obtained by executing an LF treatment step after converter tapping and being fed to a casting step, an Al-deoxidized steel in which alloy elements after the LF treatment step probably include C: 0.04 to 0.20 wt%, Si: 0.01 to 0.15 wt%, Mn: 0.15 to 2.13 wt%, Al: 0.024 to 0.035 wt%, and the balance Fe is melted. Regarding an inert gas in a ladle in the LF treatment step, (a sealing gas flow rate Q1+a stirring gas flow rate Q2) / an in-furnace cover volume V≥0.26( / min) is satisfied, the stirring gas flow rate Q2 is controlled to 2.67 (NL / min ton) or less per ton of the monte steel, a cumulative Al input after the second time upon adding Al to the molten steel is controlled to 0.07(kg / ton) or less per ton of the molten steel, and the total Al input in the LF treatment step satisfies a prescribed expression.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for efficiently producing highly clean Al-deoxidized steel in a LF process. [Background technology]

[0002] Steel ingots and steel materials used in manufacturing high-strength parts are required to have high fatigue resistance. Therefore, it is necessary to use high-cleanliness steel (high-cleanliness steel) with very few inclusions that have a significant effect on fatigue properties. Methods for manufacturing high-cleanliness steel involve primary refining in a converter and secondary refining of molten steel tapped from the converter to adjust the composition. Techniques for manufacturing high-cleanliness steel are disclosed in, for example, Patent Documents 1 to 5.

[0003] Patent Document 1 aims to further suppress nitrogen absorption into molten steel while reducing the amount of inert gas and carbon dioxide gas used. Specifically, molten steel containing Al is treated in the following order to produce ultra-low-sulfur, low-nitrogen steel: Step 1: Adding CaO-based flux to molten steel; Step 2: Stirring the molten steel and CaO-based flux by placing a ladle lid over the upper opening of the ladle and injecting stirring gas into the molten steel while preventing air from entering the inside of the ladle lid; and Step 3: Desulfurizing and removing inclusions by injecting stirring gas into the molten steel after stopping the oxygen gas supply. Liquid water is supplied to the inside of the ladle lid during Steps 2 and 3.

[0004] Patent Document 2 aims to sufficiently suppress nitrogen absorption, especially when a gap occurs between the upper edge of the ladle and the ladle lid, reducing the sealing ability. Specifically, the method involves adding CaO-based flux to molten steel containing Al in a ladle under atmospheric pressure, installing a ladle lid covering the ladle opening and equipped with at least one of an oxygen gas top-blowing lance insertion hole, a molten steel stirring lance insertion hole, and an alloy addition hole, injecting stirring gas into the molten steel in the ladle to stir the molten steel and CaO-based flux while preventing atmospheric air from entering the inside of the ladle lid. Furthermore, oxygen gas is blown from above onto the molten steel, and the resulting oxides are mixed with the CaO-based flux to form a cover slag. After this, the oxygen gas supply is stopped, and stirring gas is injected into the molten steel in the ladle under atmospheric pressure to desulfurize and remove inclusions, thereby producing ultra-low-sulfur, low-nitrogen steel. During the top-blowing of oxygen gas, an inert gas or carbon dioxide gas is blown onto the molten steel, entrained around the top-blown oxygen gas.

[0005] Patent Document 3 aims to enable the production of low-Al steel with high productivity even when using a ladle made of a refractory material with a high Al2O3 content used for producing Al-killed steel. Specifically, the method for producing low-Al steel contains, by mass%, C: 0.03-1.2%, Si: 0.03-0.8%, Mn: 0.1-2.5%, P: 0.01% or less, S: 0.150% or less, sol.Al: 0.005% or less, Ti: 0.1% or less, Ca: 0.0020% or less, O: 0.0050% or less, and N: 0.001-0.03%, with the balance being Fe and impurities, in which the ladle is made of a refractory material containing 55% or more by mass of Al2O3, and the area A [m 2 ] and the volume V [m 3 ] ratio A / V is 2.5 [m 2 / m 3 ] or less, and the stirring energy K during stirring of molten steel is 0.3 [MJ / t] or less, or the stirring power density ε L is required to satisfy the requirement of 130 [W / t] or less.

[0006] Patent Document 4 aims to prevent nozzle clogging during continuous casting and surface defects on rolled steel sheets caused by low-melting-point inclusions in a molten steel refining method. Specifically, a simple ladle refining method is used in which molten steel in a ladle is refined while being stirred with Ar gas in an atmospheric pressure atmosphere. Rare earth elements (REM) are added to Al-deoxidized or Al-Si-deoxidized molten steel, and the REM is supplied to the molten steel in an amount ranging from 5 to 20 ppm relative to the mass of the molten steel. The REM is added after the final composition adjustment in the simple ladle refining method and during a time period equal to or shorter than the uniform mixing time. The inclusion composition is set to, by mass, 1 to 25% CaO, 8 to 95% Al2O3, and 3 to 90% REM oxides, thereby increasing the melting point of the formed inclusions and making them harmless.

[0007] Patent Document 5 discloses a method for economically and efficiently detoxifying alumina-based inclusions in steel by ladle refining. Specifically, when molten steel in a ladle is stirred together with slag on the surface of the molten steel, arc heating and gas stirring are used in combination, and the Al concentration in the molten steel is set to 0.005 mass% or more, and the (MgO) concentration in the slag and the molten steel stirring time must satisfy the relationship in formula (1) below.

[0008] y≧-0.11×x+8……(1) In equation (1), y represents the MgO concentration (mass%) in the slag in the ladle at the end of the treatment, and x represents the stirring time (minutes). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2018-016843 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-148737 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-172218 [Patent Document 4] Japanese Patent Application Laid-Open No. 2010-236030 [Patent Document 5] Japanese Patent Application Laid-Open No. 2003-286515 Summary of the Invention [Problem to be solved by the invention]

[0010] In the LF (Ladle Furnace) process, if the slag contains a large amount of lower oxides (T.Fe + MnO), the lower oxides react with the Al in the molten steel during the process from discharging to casting, producing Al2O3. As a result, the molten steel is transferred to the casting process in a state of reduced cleanliness. Therefore, it is necessary to reduce the amount of lower oxides at the stage of discharging the molten steel.

[0011] In order to reduce low-grade oxides in the LF process, it is necessary to (i) suppress atmospheric oxidation and (ii) increase the Al concentration in molten steel, enhance deoxidation, and reduce low-grade oxides.

[0012] Regarding (i), it is necessary to reduce the oxygen partial pressure in the atmosphere inside the ladle and to reduce the plume diameter (contact area between the atmospheric gas and molten steel).

[0013] Regarding (ii), the higher the Al concentration in the molten steel, the more the amount of low-grade oxides can be reduced. However, if the Al concentration in the molten steel is increased too much, it becomes necessary to extend the processing time in the LF process and reduce the Al concentration in the molten steel in order to keep it within the composition specifications (to prevent derailment of the composition).

[0014] Thus, it is necessary to reduce the amount of low-grade oxides and efficiently produce highly clean molten steel without extending the processing time in the LF process.

[0015] Patent Document 1 does not specify the cumulative amount of Al charged from the second time onwards, and therefore cannot reduce lower oxides if initial deoxidation is insufficient. Also, the flow rate of the stirring gas used to stir the molten steel is high, making it impossible to suppress the generation of lower oxides due to atmospheric oxidation.

[0016] Patent Document 2 does not specify the cumulative amount of Al charged from the second time onwards, and if the initial deoxidation is insufficient, it is not possible to reduce the amount of lower oxides.In addition, the flow rate of the stirring gas used to stir the molten steel is high, and it is not possible to suppress the generation of lower oxides due to atmospheric oxidation.

[0017] Patent Document 3 does not specify the flow rate of the seal gas into the furnace roof, and if the flow rate of the seal gas is insufficient, low-grade oxides will be generated by atmospheric oxidation. Note that the present invention is targeted at low-Al steel, and does not add Al alloys according to the product standard composition during steel tapping from a converter. In other words, Patent Document 3 is technically different from the present invention.

[0018] Patent Document 4 does not specify the flow rate of the seal gas into the furnace roof, and if the flow rate is insufficient, low-grade oxides are generated by atmospheric oxidation. Furthermore, it does not specify the amount or number of times of Al addition, and if initial deoxidation is insufficient, it is not possible to reduce low-grade oxides.

[0019] Patent Document 5 is a technology for a cleaning treatment method for aluminum-killed steel, but its purpose is to control the composition of inclusions, and it does not mention whether the treatment time should be extended or whether lower oxides should be cleaned. In other words, the technology of Patent Document 5 is different from that of the present invention.

[0020] In recent years, users have been demanding higher quality due to the trend toward higher added value steel and environmental considerations, etc. Under these circumstances, there is a need to efficiently produce high-purity molten steel without extending the processing time in the LF process.

[0021] In view of the above problems, the present invention aims to extend the processing time of high-cleanliness molten steel in the LF process. The present invention aims to provide a method for producing Al-deoxidized steel that can be produced efficiently without prolonging the process. [Means for solving the problem]

[0022] In order to achieve the above object, the present invention provides the following technical means.

[0023] The method for producing Al-deoxidized steel according to the present invention is The molten iron was poured into a ladle. A method for producing Al-deoxidized steel, the method comprising the steps of: (1) subjecting molten steel to an LF treatment step after the LF treatment step; and (2) sending the molten steel to a casting step as a downstream step; and (3) producing Al-deoxidized steel having alloy elements as shown below after the LF treatment step, C: 0.04 to 0.20 wt% Si: 0.01 to 0.15 wt% Mn: 0.15~2.13wt% Al: 0.024~0.035wt% The balance consists of at least Fe and inevitable impurities, The Al-deoxidized steel to be produced is a high-cleanliness steel intended for steel products having a lower oxide content of 0.54 wt%≦T.Fe+MnO≦1.00 wt%, The LF treatment process has an opening facing upward and a furnace cover that closes the opening facing upward. The volume of the furnace closed by the furnace cover is 20.2 m. 3 For the ladle having a target molten steel weight of 250 tons or less, a seal gas of Ar is supplied into the furnace from the furnace cover, and Ar gas is supplied from the tip of a lance inserted into the furnace from the top to the bottom of the ladle to stir the molten steel in the ladle, and low-grade oxides produced in the furnace are deoxidized by adding Al into the furnace at least twice, once during the first rough alloy adjustment and once during the second or subsequent fine alloy adjustments, The stirring flow rate Q2 of the Ar gas per ton of molten steel is Q2≦2.67NL / (min ton), and the fine-adjustment Al input amount W per ton of molten steel during the fine-adjustment of the alloy is Al-tweak / W Fe W Al-tweak / W Fe For low alloy steel obtained under the conditions of "≦0.07 kg / ton", In the LF processing step The aforementioned Regarding Ar gas in the ladle, (Seal gas flow rate into the furnace roof Q1 + stirring gas flow rate used to stir molten steel Q2) / Volume inside the furnace cover of the ladle (inside the furnace) V≧0.26 ( / min) and The total amount of Al added during the rough alloy adjustment and the fine alloy adjustment in the LF treatment step satisfies formula (1).

[0024]

number

[0025] however, w Al :Al alloy input amount (kg) w Fe : Molten steel amount (ton) N Al :Al alloy pure content (-) t: Target processing time for removal (minutes) [%C] ini : C concentration before treatment (wt%) [%Al] ini : Al concentration before treatment (wt%) [%Al] uplimit : Upper limit of Al concentration (wt%) [Effects of the Invention]

[0026] According to the method for producing Al-deoxidized steel of the present invention, highly clean molten steel can be produced efficiently without extending the processing time in the LF process. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a diagram showing a schematic diagram of the sealing condition inside the furnace cover of the ladle. [Figure 2] This is a diagram showing the relationship between the ratio ((Q1+Q2) / V), which indicates the sealing condition inside the furnace lid, and the amount of lower oxides (T.Fe+MnO). [Figure 3] FIG. 1 is a diagram showing the effect of stirring gas in a ladle. [Figure 4]FIG. 1 is a diagram showing the relationship between the stirring gas flow rate per ton of molten steel (Q2 / WFe) and lower oxides (T.Fe+MnO). [Figure 5] This is a diagram showing the reaction between Al and lower oxides in molten steel in a ladle. [Figure 6] FIG. 1 is a diagram showing the flow of adding Al (showing the case where Al is added three times). [Figure 7] FIG. 1 is a diagram showing the relationship between the amount of fine-tuning Al input per ton of molten steel (WAl-tweak / WFe) and lower oxides (T.Fe+MnO). [Figure 8] FIG. 1 is a diagram showing an image of the transition of the Al concentration in molten steel depending on the amount of Al charged within the range of formula (1). DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, an embodiment of a method for producing Al-deoxidized steel according to the present invention will be described with reference to the drawings. Note that the embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example.

[0029] Steel that requires a high level of cleanliness (high-cleanliness steel) is produced through a steelmaking process consisting of a primary refining process in a converter and a secondary refining process in a ladle refining process (LF process). As shown in Figure 1, in the LF process, the composition is adjusted by gas agitation from a lance 2 installed on the top side of the ladle 1 or a porous refractory material installed at the bottom, and temperature is adjusted by electrode heating.

[0030] Ladle 1 is a container for charging molten steel 3 tapped from a converter. Ladle 1 has an iron container (shell) on the outside, and refractory material is installed on the inside that comes into contact with molten steel 3. In addition, seal gas is blown into the furnace from furnace cover 4 above ladle 1.

[0031] The lance 2 is a rod-shaped refractory material with a gas pipe running through it, and has a discharge hole at its tip for discharging gas into the molten steel 3. In other words, in the LF process, the lance 2 is immersed in the molten steel 3 in the ladle 1, and inert gas (Ar, N2) is blown in through the discharge hole to stir the molten steel 3.

[0032] In the LF (Ladle Furnace) process, if the slag 5 contains a large amount of lower oxides (T.Fe+MnO), the lower oxides react with Al in the molten steel 3 during the process from the discharge of the molten steel 3 to the casting process, producing Al2O3. As a result, the molten steel 3 is transferred to the casting process in a state where its cleanliness has deteriorated. Therefore, it is necessary to reduce the amount of lower oxides at the stage of discharging the molten steel 3.

[0033] In order to reduce the amount of low-grade oxides in the LF process, it is necessary to (i) suppress atmospheric oxidation, and (ii) increase the Al concentration in molten steel 3 to enhance deoxidation and reduce the low-grade oxides.

[0034] Regarding (i), it is necessary to reduce the oxygen partial pressure in the atmosphere inside the ladle 1 and to reduce the plume diameter (contact area between the atmospheric gas and the molten steel 3).

[0035] Regarding (ii), the higher the Al concentration in the molten steel 3, the more the amount of low-grade oxides can be reduced. However, if the Al concentration in the molten steel 3 is increased too much, it becomes necessary to extend the processing time in the LF process and reduce the Al concentration in the molten steel 3 in order to keep the composition within the composition standard (to prevent derailment of the composition).

[0036] As described above, it is required to reduce the amount of low-grade oxides and efficiently produce highly clean molten steel 3 without extending the treatment time in the LF process.

[0037] Therefore, in the present invention, (i) atmospheric oxidation is suppressed by specifying the flow rate of the seal gas and the stirring gas, and (ii) the amount of Al input is specified, making it possible to reduce the amount of low-grade oxides in the slag 5 without extending the processing time in the LF process.

[0038] This embodiment is intended for a case in which the LF treatment process is performed after the converter is tapped, and then the steel proceeds to the downstream casting process (or continuous casting process). This embodiment is also intended for materials sent directly from the LF, and does not go through the RH treatment process. In the present invention, in order to suppress nitrogen pickup during the period from converter tapping to LF delivery, no Al alloy is added during converter tapping according to the product specification composition.

[0039] In this embodiment, the alloy elements in the molten steel 3 after the LF treatment process are set to the following composition ranges at the stage when the molten steel 3 is discharged from the ladle 1 after the LF treatment process. Also, this embodiment targets Al-deoxidized steel.

[0040] C: 0.04 to 0.20 wt% Si: 0.01 to 0.15 wt% Mn: 0.15~2.13wt% Al: 0.024~0.035wt% Balance: Fe and unavoidable impurities First, the inert gas in the ladle 1 in the LF treatment process is to satisfy the following formula.

[0041] (Seal gas flow rate Q1 into the furnace cover 4 + stirring gas flow rate Q2 used to stir the molten steel 3) / Volume V of ladle 1 inside furnace cover 4 ≧ 0.26 ( / min) FIG. 1 shows a schematic diagram of the sealing state inside the furnace cover 4 of the ladle 1.

[0042] As shown in FIG. 1, when the oxygen partial pressure in the atmosphere inside the furnace lid 4 is high, lower oxides (T.Fe+MnO) are produced by atmospheric oxidation as shown in the following formula.

[0043] 1 / 2O2(g)+ Fe , Mn = (FeO, MnO) For this reason, the flow rate of the seal gas using inert gas (Ar, N2) inside the furnace hood 4 is increased to reduce the oxygen partial pressure in the atmosphere inside the furnace hood 4 and suppress atmospheric oxidation. In this embodiment, the reduction of low-grade oxides was evaluated as T.Fe+MnO≦1.00 wt% after the LF treatment process (evaluation (1)). In addition, whether or not the treatment time was extended to prevent derailment was evaluated as evaluation (2).

[0044] FIG. 2 shows the relationship between the ratio ((Q1+Q2) / V) indicating the state of the seal inside the furnace hood 4 and the amount of lower oxides (T.Fe+MnO).

[0045] As shown in FIG. 2, in this embodiment, Nos. 1 to 3 in Table 2, which will be shown later, were compared with Nos. 12 to 15, which satisfied the criteria (1) (T.Fe+MnO≦1.00 wt%), and the ratio of "(seal gas flow rate Q1+stirring gas flow rate Q2) / volume V of ladle 1 inside hood 4" was set to ≧0.26 ( / min).

[0046] The flow rate Q2 of the stirring gas using inert gas (Ar, N2) shall be 2.67 (NL / (min·ton)) or less per ton of molten steel.

[0047] FIG. 3 shows a schematic diagram of the influence of the stirring gas in the ladle 1.

[0048] As shown in Figure 3, if the stirring gas flow rate Q2 using inert gas (Ar, N2) becomes too high, the plume diameter of the molten steel 3 increases, the contact area between the atmospheric gas and the molten steel 3 increases, and atmospheric oxidation becomes more likely to occur. Thus, in order to suppress the generation of lower oxides (T, Fe + MnO) due to atmospheric oxidation, an upper limit is set for the stirring gas flow rate Q2.

[0049] Figure 4 shows the agitation gas flow rate per ton of molten steel (Q2 / W Fe ) and lower oxides (T.Fe+MnO).

[0050] As shown in FIG. 4, in this embodiment, Nos. 4 to 6 in Table 2, which will be shown later, were compared with Nos. 12 to 15, which satisfied evaluation (1) (T.Fe+MnO≦1.00 wt%), and the stirring gas flow rate Q2 was specified to be ≦2.67 (NL / (min ton)) per ton of molten steel.

[0051] When adding Al to the molten steel 3, the cumulative amount of Al added from the second time onwards is set to 0.07 (kg / ton) or less per ton of molten steel.

[0052] FIG. 5 shows a schematic diagram of the reaction between Al and lower oxides in molten steel 3 in ladle 1.

[0053] FIG. 6 shows a flow diagram of Al charging (showing the case where Al is charged three times).

[0054] As shown in Figure 5, in order to reduce the low-grade oxides, it is necessary to increase the Al concentration in the molten steel 3, strengthen deoxidation, and cause the low-grade oxides (T.Fe+MnO) to react with the Al in the molten steel 3 as shown in the following formula (A).

[0055] 3(FeO,MnO)+2 Al →(Al2O3)+3 Fe ,3 Mn (A) Furthermore, in order to promote the reaction of formula (A) and to ensure the reaction time, it is necessary to strengthen the initial deoxidation.

[0056] Therefore, in order to increase the amount of Al charged during the first rough alloy adjustment, an upper limit is set on the cumulative amount of Al charged from the second time (fine alloy adjustment) onwards, as shown in Figure 6. In addition, by increasing the amount of Al charged during the rough alloy adjustment, it is expected that the time required for the generated Al2O3 inclusions to float and separate can be secured.

[0057] Figure 7 shows the amount of finely adjusted Al input per ton of molten steel (W Al-tweak / W Fe ) and lower oxides (T.Fe+MnO).

[0058] As shown in FIG. 7, in this embodiment, Nos. 7 to 10 in Table 2, which will be shown later, were compared with Nos. 12 to 15, which satisfied evaluation (1) (T.Fe+MnO≦1.00 wt%), and the cumulative amount of Al charged from the second time onwards was specified to be ≦0.07 (kg / ton) per ton of molten steel.

[0059] Furthermore, the total amount of Al input in the LF treatment process is set to satisfy formula (1).

[0060]

number

[0061] however, w Al :Al alloy input amount (kg) w Fe : Molten steel amount (ton) N Al :Al alloy pure content (-) t: Target processing time for removal (minutes) [%C] ini : C concentration before treatment (wt%) [%Al] ini : Al concentration before treatment (wt%) [%Al] uplimit : Upper limit of Al concentration (wt%) By specifying the amount of Al to be added so that the left side of equation (1) satisfies "exported Al≧0.024 (wt%)", it is possible to strengthen deoxidation and promote the reaction with lower oxides (T.Fe+MnO) shown in equation (A).

[0062] The right side of equation (1): [%Al] uplimit By setting the value to "-0.0006", it is possible to prevent the need to extend the processing time in order to prevent the Al component from derailing (to keep it within the component specifications). <Derivation of Equation (1)> A linear regression was performed on the Al yield using the "pre-treatment [C] concentration" and "treatment time" to obtain the following formula (B). By modifying this formula (B), the formula for the Al input amount for an arbitrarily set target [Al] concentration in molten steel (formula (C)) was obtained.

[0063] Al yield (%)=84.01[%C] ini -0.06617t+16.01 (B) =(1000×w Fe ×([%Al]-[%Al] ini )) / (N Al ×w Al )

[0064]

number

[0065] FIG. 8 shows an image of the transition of the Al concentration in the molten steel 3 depending on the amount of Al charged within the range of formula (1).

[0066] As shown in Figure 8, when the amount of Al added is determined so as to fall within the range of formula (1), deoxidation is enhanced without extending the treatment time, and the reaction with lower oxides (T.Fe + MnO) is promoted. On the other hand, when more Al is added than the right side of formula (1), the treatment time is extended. Furthermore, when less Al is added than the left side of formula (1), deoxidation is insufficient. [Example] Examples carried out in accordance with the method for producing Al-deoxidized steel of the present invention and comparative examples carried out for comparison with the present invention will be described below.

[0067] The conditions for carrying out this example are as follows.

[0068] Table 1 shows the conditions used in this example.

[0069] [Table 1]

[0070] Table 2 shows examples carried out in accordance with the method for producing Al-deoxidized steel of the present invention, as well as comparative examples carried out for comparison with the present invention.

[0071] [Table 2]

[0072] As shown in Table 2, in Comparative Example 1, (Q1+Q2) / V=0.16( / min)t, which does not satisfy (Q1+Q2) / V≧0.26( / min). Al-tweak / W Fe = 0.19 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton is not met. The left side of formula (1) = 309.8, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 3.80 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). Reasons for this include insufficient sealing inside the furnace cover 2, insufficient initial deoxidation, and insufficient deoxidation during treatment. The overall evaluation was poor due to insufficient cleanliness.

[0073] In Comparative Example 2, (Q1+Q2) / V=0.17( / min)t, which does not satisfy (Q1+Q2) / V≧0.26( / min). Al-tweak / W Fe = 0.14 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton is not met. The left side of formula (1) = 212.6, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 1.45 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). Reasons for this include insufficient sealing inside the furnace cover 2, insufficient deoxidation at the beginning, and insufficient deoxidation during treatment. The overall evaluation was poor due to insufficient cleanliness.

[0074] Comparative Example 3: (Q1+Q2) / V=0.17( / min)t, which does not satisfy (Q1+Q2) / V≧0.26( / min). Al-tweak / W Fe = 0.11 kg / ton, and W Al-tweak / W Fe≦0.07 kg / ton is not met. The left side of formula (1) = 303.4, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 3.52 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). Reasons for this include insufficient sealing inside the furnace cover 2, insufficient initial deoxidation, and insufficient deoxidation during treatment. The overall evaluation was poor due to insufficient cleanliness.

[0075] Comparative Example 4 is Q2 / W Fe =3.03NL / (min·ton), and Q2 / W Fe Does not satisfy ≦2.67NL / (min·ton). Al-tweak / W Fe = 0.17 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton is not satisfied. The right side of formula (1) = 275.6, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 2.25 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). Reasons for this include excessive stirring flow rate and insufficient initial deoxidation. In addition, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to insufficient cleanliness and extended treatment time.

[0076] Comparative Example 5 is Q2 / W Fe =2.70NL / (min·ton), and Q2 / W Fe The right side of equation (1) = 318.6, which is outside the range of equation (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 1.18 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). This is due to an excessive stirring flow rate. Furthermore, because the treatment was extended to prevent derailment, evaluation (2) was also not met. The overall evaluation was poor due to insufficient cleanliness and the extended treatment time.

[0077] Comparative Example 6 is Q2 / W Fe =2.98NL / (min·ton), and Q2 / W FeThe right side of equation (1) = 331.0, which is outside the range of equation (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 1.04 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). This is due to an excessive stirring flow rate. Furthermore, because the treatment was extended to prevent derailment, evaluation (2) was also not met. The overall evaluation was poor due to insufficient cleanliness and the extended treatment time.

[0078] Comparative Example 7 is W Al-tweak / W Fe = 0.24 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton is not satisfied. The right side of formula (1) = 339.1, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 1.60 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). This is due to insufficient initial deoxidation. Furthermore, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to insufficient cleanliness and the extended treatment time.

[0079] Comparative Example 8 is W Al-tweak / W Fe = 0.15 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton is not satisfied. The right side of formula (1) = 346.8, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 1.52 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). This is due to insufficient initial deoxidation. Furthermore, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to insufficient cleanliness and the extended treatment time.

[0080] Comparative Example 9 is W Al-tweak / W Fe = 0.15 kg / ton, and W Al-tweak / W Fe≦0.07 kg / ton is not satisfied. The right side of formula (1) = 427.6, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe+MnO = 2.26 wt%, which does not satisfy evaluation (1): (T.Fe+MnO≦1.00 wt%). This is due to insufficient initial deoxidation. Furthermore, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to insufficient cleanliness and the extended treatment time.

[0081] Comparative Example 10 is W Al-tweak / W Fe = 0.27 kg / ton, and W Al-tweak / W Fe Meets ≦0.07kg / ton The right side of formula (1) = 345.8, which is outside the range of formula (1) and does not satisfy it. After the LF treatment process, T.Fe + MnO = 1.07 wt%, which does not satisfy evaluation (1): (T.Fe + MnO ≦ 1.00 wt%). This is due to insufficient deoxidation in the early stages. Furthermore, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to insufficient cleanliness and the extended treatment time.

[0082] In Comparative Example 11, the left side of formula (1) was 303.9, which is outside the range of formula (1) and does not satisfy it. After the LF treatment step, T.Fe+MnO was 1.55 wt%, which does not satisfy the evaluation (1): (T.Fe+MnO≦1.00 wt%). The reason for this is insufficient deoxidation during treatment. The overall evaluation was poor due to insufficient cleanliness.

[0083] In Comparative Example 12, the right side of formula (1) is 352.2, which is outside the range of formula (1) and does not satisfy it. In addition, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to the extension of the treatment time.

[0084] In Comparative Example 13, the right side of formula (1) was 323.0, which is outside the range of formula (1) and does not satisfy it. In addition, because the treatment was extended to prevent derailment, evaluation (2) was also not satisfied. The overall evaluation was poor due to the extension of the treatment time.

[0085] In Example 14, (Q1+Q2) / V=0.26( / min)t, and (Q1+Q2) / V≧0.26( / min) is satisfied. Fe =2.48NL / (min·ton), and Q2 / W Fe Satisfies ≦2.67NL / (min·ton). W Al-tweak / W Fe = 0.00 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton. The left side of formula (1) = 301.1, and the right side of formula (1) = 391.4, which is within the range of formula (1). After the LF treatment process, T.Fe+MnO = 0.54 wt%, which satisfies evaluation (1): (T.Fe+MnO≦1.00 wt%). In addition, since the treatment was not extended to prevent derailment, evaluation (2) was also met. Overall, good results were obtained.

[0086] In Example 15, (Q1+Q2) / V=0.26( / min)t, and (Q1+Q2) / V≧0.26( / min) is satisfied. Fe =2.67NL / (min·ton), and Q2 / W Fe Satisfies ≦2.67NL / (min·ton). W Al-tweak / W Fe = 0.07 kg / ton, and W Al-tweak / W Fe ≦0.07 kg / ton. The left side of formula (1) = 287.3, and the right side of formula (1) = 293.4, which is within the range of formula (1). After the LF treatment process, T.Fe+MnO = 0.92 wt%, which satisfies evaluation (1): (T.Fe+MnO≦1.00 wt%). In addition, since the treatment was not extended to prevent derailment, evaluation (2) was also met. Overall, good results were obtained.

[0087] The method for producing Al deoxidized steel of the present invention is a method for producing Al deoxidized steel by performing an LF treatment process after steel is tapped from a converter, and then sending molten steel 3 to a casting process, which is a downstream process, so that the alloy elements after the LF treatment process are as follows: C: 0.04 to 0.20 wt% Si: 0.01 to 0.15 wt% Mn: 0.15~2.13wt% Al: 0.024~0.035wt% Balance: Contains at least Fe The inert gas in the ladle 1 during the LF treatment process shall satisfy the following formula: (Seal gas flow rate Q1 into the furnace cover 4 + stirring gas flow rate Q2 used to stir the molten steel 3) / Volume V of ladle 1 inside furnace cover 4 ≧ 0.26 ( / min) The flow rate of the inert gas agitation gas Q2 is set to 2.67 (NL / (min·ton)) or less per ton of molten steel, When adding Al to the molten steel 3, the cumulative amount of Al added from the second time onwards is set to 0.07 (kg / ton) or less per ton of molten steel, Furthermore, the total amount of Al input in the LF treatment process is set to satisfy formula (1).

[0088]

number

[0089] however, w Al :Al alloy input amount (kg) w Fe :Amount of molten steel (ton) N Al :Al alloy pure content (-) t: Target processing time for removal (minutes) [%C] ini : C concentration before treatment (wt%) [%Al] ini : Al concentration before treatment (wt%) [%Al] uplimit : Upper limit of Al concentration (wt%) According to the method for producing Al-deoxidized steel of the present invention, highly clean molten steel 3 can be efficiently produced without extending the treatment time in the LF process.

[0090] It should be noted that the embodiments disclosed herein are illustrative in all respects and should not be considered limiting. In particular, in the embodiments disclosed herein, matters not explicitly stated, such as operating conditions, operating conditions, various parameters, dimensions, weights, volumes of components, etc., do not deviate from the scope of ordinary practice by a person skilled in the art, and values ​​that can be easily assumed by a person skilled in the art are used. [Explanation of symbols]

[0091] 1 ladle 2. Lance 3. Molten Steel 4 Hearth lid 5. Slug

Claims

[Claim 1] A method for producing Al-deoxidized steel, in which molten iron produced in a converter is tapped into a ladle and then subjected to an LF treatment process, and the molten steel is then sent to a downstream casting process, in which the alloy elements after the LF treatment process are as follows: ・C: 0.04 to 0.20 wt% ・Si: 0.01 to 0.15 wt% ・Mn: 0.15 to 2.13 wt% ・Al: 0.024 to 0.035 wt% The balance consists of at least Fe and inevitable impurities, The Al-deoxidized steel to be produced is a high-cleanliness steel intended for steel products having a lower oxide content of 0.54 wt%≦T.Fe+MnO≦1.00 wt%, The LF treatment process has an opening facing upward and a furnace cover that closes the opening facing upward. The volume of the furnace closed by the furnace cover is 20.2 m. 3 For the ladle having a target molten steel weight of 250 tons or less, a seal gas of Ar is supplied into the furnace from the furnace cover, and Ar gas is supplied from the tip of a lance inserted into the furnace from the top to the bottom of the ladle to stir the molten steel in the ladle, and low-grade oxides produced in the furnace are deoxidized by adding Al into the furnace at least twice, once during the first rough alloy adjustment and once during the second or subsequent fine alloy adjustments, The stirring flow rate Q2 of the Ar gas per ton of molten steel is Q2≦2.67 NL / (min ton), and the fine-adjustment Al input amount W per ton of molten steel during the fine-adjustment of the alloy is Al-tweak / W Fe W Al-tweak / W Fe For low alloy steel obtained under the conditions of "≦0.07 kg / ton", Regarding the Ar gas in the ladle in the LF treatment step, (Seal gas flow rate into the furnace roof Q1 + stirring gas flow rate used to stir molten steel Q2) / Volume inside the furnace cover of the ladle (inside the furnace) V≧0.26 ( / min) and The total amount of Al added during the rough alloy adjustment and the fine alloy adjustment in the LF treatment step satisfies formula (1). A method for producing Al-deoxidized steel. [Equation 1] however, W Al : Amount of aluminum alloy added (total amount of aluminum added) (kg) W Fe : Melting volume (tons) N Al Al alloy purity (-) t: Target processing time for removal (minutes) [%C] ini : C concentration before treatment (wt%) [%Al] ini : Al concentration before treatment (wt%) [%Al] uplimit : Upper limit of Al concentration (wt%)

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