High-nitrogen steel refining method

Abstract

To provide a refining method for a high-nitrogen steel capable of making [N]≥150 ppm after an RH treatment by sufficiently increasing the concentration of [N] in molten steel before starting the RH treatment, and performing the method under RH treatment conditions without being affected by denitrification reaction occurring during a reduced pressure treatment in the RH treatment and without adding a nitride alloy to the molten steel.SOLUTION: In a refining method of a high-nitrogen steel, in which a ladle refining treatment and an RH vacuum degassing treatment for carrying out N2 gas circulation flow are carried out on a molten steel 2 discharged from a converter, nitriding is carried out on the molten steel 2 by carrying out stirring with N2 gas in the treatment with the converter and / or in the ladle refining treatment. In the RH vacuum degassing treatment, at least an index value A obtained from "the expression including the value of the molten steel component before the RH vacuum degassing treatment as a variable" becomes 50 or higher.SELECTED DRAWING: Figure 1C

Description

[Technical Field]

[0001] The present invention relates to a technique for adjusting the [N] concentration in molten steel by injecting N2 gas in a method for refining high-nitrogen steel. [Background technology]

[0002] Generally, molten steel undergoes decarburization in a steelmaking furnace such as a converter, and the resulting molten steel is transported to the secondary refining process. In this secondary refining process, processes such as vacuum degassing of the molten steel are carried out. Vacuum degassing (RH treatment) primarily involves adjusting the composition of the molten steel and degassing the molten steel. In addition, RH treatment may also involve adjusting the nitrogen concentration ([N]) in the molten steel.

[0003] In the molten steel processing process, molten steel is subjected to processes such as ladle refining and vacuum degassing. The [N] concentration in molten steel during this process is adjusted based on nitriding (adding nitrogen) using N2 gas bubbling. However, a denitrification reaction occurs during the vacuum degassing process. Therefore, in conventional techniques, achieving a high nitrogen concentration, such as [N] ≥ 150 ppm, is difficult to achieve using only N2 gas reflux. Adjusting the [N] concentration in molten steel to [N] ≥ 150 ppm typically requires the addition of a nitride alloy. For example, Patent Document 1 specifies the degree of vacuum reduction and the reflux gas flow rate (Ar or Ar + N2) during the RH vacuum degassing process, thereby achieving high-precision adjustment of the [N] concentration in molten steel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-224461 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, when steel is produced using the "converter → ladle refining → RH treatment" process, denitrification of molten steel can occur simultaneously during the depressurization process of the RH treatment for the purpose of dehydrogenation. For this reason, it was impossible with conventional technology to adjust the [N] concentration in molten steel to a high nitrogen range of [N] ≥ 150 ppm. To address this issue, it was necessary to supplement the [N] deficiency, which was added using expensive nitride alloys. This increased the cost of the steelmaking process.

[0006] For this reason, Patent Document 1 only adjusts the degree of vacuum and reflux gas conditions, and is limited to a range of [N]<100 ppm. In other words, it is difficult to raise the nitrogen concentration to a high nitrogen range such as [N]≧150 ppm using conventional technology.

[0007] In view of the above problems, the present invention aims to provide a method for refining high-nitrogen steel, which makes it possible to achieve a [N] of 150 ppm or more after RH treatment by sufficiently increasing the [N] concentration in molten steel before the start of RH treatment, and by performing RH treatment under conditions that are not affected by the denitrification reaction that occurs during the reduced pressure treatment in RH treatment and that do not require the addition of a nitride alloy to the molten steel. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides the following technical means.

[0009] The method for refining high-nitrogen steel according to the present invention is a method for refining high-nitrogen steel in which molten steel tapped from a converter is subjected to a ladle refining process and an RH vacuum degassing process in which N2 gas is circulated, and in the process in the converter and / or the ladle refining process, the molten steel is agitated with N2 gas to add nitrogen to the molten steel. and refining the high nitrogen steel so that the [N] after RH treatment is 150 ppm or more. In the RH vacuum degassing treatment, Without adding a nitride alloy to the molten steel, The treatment conditions are characterized in that the value of index A calculated from "Equation (1) including the molten steel composition values ​​before the RH vacuum degassing treatment as variables" is 50 or more.

[0010]

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[0011] However, [N] ini (ppm): [N] concentration in molten steel before RH treatment [N] e,S (ppm): Equilibrium [N] concentration at the surface of the molten steel bath in the vacuum vessel k r,before (m / min / %): Interfacial reaction rate constant of the [N] reaction before S addition k r,after (m / min / %): Interfacial reaction rate constant of [N]ation reaction after S addition t(min): RH treatment time t s (min): Start time of S addition [Effects of the Invention]

[0012] According to the method for refining high-nitrogen steel of the present invention, the [N] concentration in molten steel is sufficiently increased before the start of RH treatment, and the RH treatment is carried out under conditions that are not affected by the denitrification reaction that occurs during the reduced pressure treatment in the RH treatment and that do not require the addition of a nitride alloy to the molten steel, thereby making it possible to achieve [N] ≧ 150 ppm after the RH treatment. [Brief explanation of the drawings]

[0013] [Figure 1A] This figure shows the state of [N] concentration in molten steel in converter, LF treatment, and RH treatment when N2 blowing nitriding was not carried out in converter or LF treatment, as was done with conventional technology, and a nitride alloy was added. [Figure 1B] This figure shows the state of [N] concentration in molten steel in converter, LF treatment, and RH treatment when nitriding by N2 blowing was carried out in converter or LF treatment and a nitride alloy was added, as carried out by conventional technology. [Figure 1C] FIG. 1 shows the state of [N] concentration in molten steel in converter, LF treatment, and RH treatment when nitriding by N2 injection is carried out in converter or LF treatment, as carried out by the method for refining high-nitrogen steel of the present invention, and the RH treatment conditions are set so that the index A is 50 or more. [Figure 2]The relationship between the index A for high [N] in RH treatment and the [N] concentration in molten steel after RH treatment in actual operation is shown (for this example and comparative example). [Figure 3] FIG. 1 is a diagram showing a schematic overview of an RH-type vacuum degassing treatment device. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of a method for refining high-nitrogen 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.

[0015] First, an overview of the RH type vacuum degassing treatment device 1 will be described.

[0016] FIG. 3 shows a schematic overview of an RH vacuum degassing treatment device 1 (a general RH vacuum degassing treatment process).

[0017] As shown in Fig. 3, an RH-type vacuum degassing treatment apparatus 1 has a ladle 3 into which molten steel 2 is charged, and a vacuum vessel 4 which is placed in a vacuum state to degas the molten steel 2. Two immersion pipes 5 which are immersed in the molten steel 2 in the ladle 3 are provided in the lower part (bottom) of the vacuum vessel 4. A gas injection pipe 6 is provided on one side of the immersion pipes 5 for injecting gas into the molten steel 2 flowing toward the vacuum vessel 4. An exhaust port 7 which communicates with the outside and exhausts gas from the vacuum vessel 4 to the outside is provided in the upper part of the vacuum vessel 4.

[0018] To perform vacuum degassing, first, the immersion tube 5 is immersed in the molten steel 2 in the ladle 3. Then, a gas such as argon gas or nitrogen gas is blown into the vacuum vessel 4 from the gas blowing tube 6, and the gas in the vacuum vessel 4 is exhausted to the outside from the exhaust port 7 to maintain a substantial vacuum inside the vacuum vessel 4, and the molten steel 2 is circulated between the vacuum vessel 4 and the ladle 3. At this time, an alloy or the like is supplied to the molten steel 2 to adjust the composition of the molten steel 2. In this manner, the RH vacuum degassing treatment is carried out.

[0019] Next, the method for refining high-nitrogen steel according to the present invention will be described in detail.

[0020] Generally, high-nitrogen steels such as case-hardened steels are subjected to high alloying and temperature adjustment after being tapped from the converter. The high-nitrogen steel after ladle refining is then sent to the ladle refining process. After ladle refining, the high-nitrogen steel undergoes a RH vacuum degassing process (RH treatment) to remove [H], which can cause product defects and delayed fracture. After RH treatment, the high-nitrogen steel is sent to the continuous casting process. In these processing steps, nitriding (adding nitrogen) to the molten steel 2 has traditionally been carried out by adding expensive nitriding alloys. However, using nitriding alloys increases manufacturing costs, so a method that can help reduce costs is needed.

[0021] Therefore, in the present invention, by using N2 as the stirring gas in the bubbling process of the RH treatment and by taking into consideration the denitrification rate in the RH vacuum degassing treatment process, it is possible to produce high-nitrogen steel at low cost without using a nitride alloy.

[0022] In the method for refining high-nitrogen steel of this embodiment, molten steel 2 was treated in the following order: "converter" → "ladle refining" → "vacuum degassing + N2 reflux step." However, "vacuum degassing + N2 reflux step = N2 reflux in RH treatment." The present invention specifies the conditions for nitriding by N2 gas bubbling in the converter or ladle refining, and the conditions for suppressing the rate of denitriding under reduced pressure in the RH treatment.

[0023] In the present invention, N gas bubbling may be performed in either or both of the converter and ladle refining treatment steps, and in this regard, it is possible to inexpensively perform nitriding (adding nitrogen) in a sufficient amount to the molten steel 2 prior to the decompression treatment in the RH treatment.

[0024] In the present invention, an index A relating to the [N] concentration in molten steel has been derived in order to appropriately adjust the [N] concentration in molten steel. That is, the index A is a value for making [N] ≧ 150 ppm, and it has been found that the index A ≧ 50 is preferable. In the present invention, taking into consideration the amount of nitrogen naturally absorbed in the atmosphere into the molten steel 2, the index A is set to ≧ 50 in order to make [N] ≧ 150 ppm.

[0025] Specifically, the value of index A related to high N calculated from the following formula (1), which is composed of the following condition values ​​that contribute to the [N] concentration in molten steel after RH treatment, is set to be 50 or more. In other words, the RH treatment conditions are set so that the value of index A related to the [N] concentration in molten steel, calculated from formula (1) which uses at least the molten steel component values ​​before RH treatment and the RH treatment conditions as parameters, is 50 or more.

[0026]

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[0027] From equation (1), it is possible to reduce the amount of denitrification in the reduced pressure treatment in the RH treatment if the index A is ≥ 50. The concept and derivation of equation (1) will be described below. <Conditional values ​​included in Index A and the concept behind the composition of Index A> The first term of equation (1): initial conditions (initial nitrogen concentration, equilibrium nitrogen concentration) is as follows:

[0028] [N] ini (ppm): [N] concentration in molten steel before RH treatment [N] e,S (ppm): Equilibrium [N] concentration at the surface of molten steel 2 in vacuum vessel 4 [N] ini The higher the value, the higher the N content after RH treatment (higher nitrogen concentration), so it was placed in the numerator of equation (1).

[0029] Also, the equilibrium nitrogen concentration ([N] e,SThe higher the nitrogen content, the higher the nitrogen content after RH treatment. Therefore, this equilibrium nitrogen concentration is mainly determined by the RH reflux conditions (degree of vacuum).

[0030] The second term of equation (1): denitrification rate is as follows:

[0031] ·k r,before (m / min / %): Interfacial reaction rate constant of the [N] reaction before S addition ·k r,after (m / min / %): Interfacial reaction rate constant of [N]ation reaction after S addition t(min): RH treatment time ·t s (min): Start time of S addition However, if S is not added during RH treatment, t s Set =t.

[0032] Basic equation for denitrification rate: -dN / dt=k r × ([N]-[Ne]), the initial driving force ([N] ini -[N] e,S ) was placed in the denominator of equation (1) because it was thought that the smaller the value, the slower the denitrification would be and the higher the nitrogen content would be after treatment.

[0033] In addition, the interfacial reaction rate constant k in the denitrification rate equation r The smaller k is, the slower the denitrification will be and the higher the N content will be after RH treatment. Therefore, k is placed in the denominator in equation (1). r Since the value of changes, the distribution of treatment time before and after the addition of S was multiplied and placed in the denominator of equation (1).

[0034] Each parameter is given a power exponent, and the [N] concentration in molten steel after RH treatment (= [N] fin The exponent of each term was determined so that the coefficient of determination of the approximation line with [N] was close to 1. The larger the exponent A, the greater the fin In this embodiment, the approximate coefficient of determination of the formula (1) that derives the index A is set to 0.94. <Regarding the formula for deriving the condition value in the index A> The method for determining the N equilibrium value at the surface of molten steel 2 in vacuum vessel 4 is shown in equation (2). Note that equation (2) uses the formula described in (Reference: Recommended Values ​​for Steelmaking Reactions (Revised and Enlarged), edited by the 19th Committee on Steelmaking of the Japan Society for the Promotion of Science, Tokyo, (1984), 17.) However, the equilibrium N concentration [N] at the surface of molten steel 2 is e,S For more information, please see P N2 =P v was requested as follows.

[0035]

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[0036] However, P N2 : Partial pressure of N2 gas bubbles (atm) P v : Pressure inside the vacuum layer (atm) ρm: Molten steel density (=7000kg / m 3 ) g: Gravitational acceleration (=9.8m / s 2 ) l: Depth of N2 gas injection point (m) R: Gas constant (=8.314 J / K / mol) T: Molten steel temperature (=1873K) The interfacial reaction rate constant for the N-formation reaction is shown in equation (3). Note that the N-formation reaction is rate-determined by the interfacial reaction rate. Equation (3) is based on the equation described in (Reference: Tsugawa, Mizukami, Ueshima: Iron and Steel 84 (1998), p. 411).

[0037]

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[0038] The activity coefficient of component M is shown below. Note that this coefficient is a scientific general theory (common practice by those skilled in the art).

[0039]

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[0040] <Other conditions> Component values ​​before RH treatment [M] ini The percentages are as follows:

[0041] [C] ini , [Mn] ini , [Si] ini , [Cr] ini , [S] ini , [O] ini C, Mn, Si, Cr, S, O, etc. are the main components that make up the steel grade. Also, the component values ​​before RH treatment [M] ini The actual value before RH treatment was used for O. However, since O was not available as an analytical value, it was assumed to be 0.001%.

[0042] The interaction coefficients are shown in Table 1. For Table 1, the values ​​used are those listed in (Reference: Recommended Equilibrium Values ​​for Steelmaking Reactions (1984), compiled by the 19th Committee on Steelmaking of the Japan Society for the Promotion of Science, or PAC5 (2000), a joint university-commissioned research report by five steel companies).

[0043] [Table 1]

[0044] [Example] Examples carried out in accordance with the method for refining high-nitrogen steel of the present invention, as well as comparative examples carried out for comparison with the present invention and prior art will be described below.

[0045] 1A and 1B show the transition of the [N] concentration in molten steel in each treatment step carried out by the conventional technology.

[0046] As shown in Figure 1A, if there is no nitrogen addition to molten steel 2 by blowing N2 in the converter or LF treatment, adding a nitride alloy in the RH treatment is necessary to achieve [N] ≥ 150 ppm, which is not suitable.

[0047] As shown in Figure 1B, even if nitrogen is added to molten steel 2 by blowing N2 in a converter or LF process, it is denitrified in the RH process, so adding a nitriding alloy is necessary to achieve [N] ≥ 150 ppm, which is not suitable.

[0048] FIG. 1C shows the transition of the [N] concentration in molten steel in each treatment step carried out by the method for refining high-nitrogen steel of the present invention.

[0049] As shown in Figure 1C, a sufficient amount of nitrogen is added to molten steel 2 during the converter-ladle refining process up to the RH treatment. Furthermore, in the RH treatment, by setting the index A, which ensures a high [N], to 50 or more, the amount of denitrification during the reduced pressure treatment is suppressed, and stirring is performed using only N2 gas, making it possible to achieve a [N] of 150 ppm or more. In other words, by setting the RH treatment conditions so that the index A is 50 or greater, it becomes possible to suppress denitrification during the RH treatment.

[0050] Figure 2 shows the relationship between the index A for high [N] in RH treatment and the [N] concentration in molten steel after RH treatment in actual operation (for this example and comparative example). Note that Figure 2 summarizes the data in Table 3, which will be shown later.

[0051] As shown in Figure 2, a lower limit line y for the variation of the data group is created. When an approximation line is drawn between three points estimated from the data group, the approximation line is A = 50, [N] = 150 ppm. In other words, the intersection of [N] = 150 ppm and the lower limit line y is A = 50. From this, it was determined that it would be possible to achieve [N] ≥ 150 ppm by injecting N2 into molten steel 2 in the converter-ladle refining process and by performing RH treatment under conditions that result in an index A ≥ 50.

[0052] Note that for a part of the data group, there are also calculated values obtained by model calculation instead of plotting the actual values. However, the model used in this embodiment was established based on reaction engineering theory for the [N] balance model during N2 gas bubbling in the converter refining process and during the pressure reduction process in the RH process, and the parameters in the formula were adjusted to fit the plot of the actual values. Therefore, the values derived from this model can be said to be as valid as the actual values. The details of this model formula will be described below. <Regarding the [N] calculation model for RH treatment> Regarding the model calculation values in this embodiment, they were calculated by referring to the nitrogen concentration calculation model shown in Japanese Patent No. 5836187 (especially paragraphs

[0025] to

[0057] , mathematical formulas [1] to

[20] , Figure 2, etc.) of the reference.

[0053] Note that the unknown values k m , α S , α Ar , α N2 for "gas type", "vacuum degree", and "presence or absence of oxygen heating" were fitted by the least squares method to match the actual values as shown in Table 2. However, for the volume V V of the molten steel 2 contained in the vacuum chamber 4, it was set as a constant value of 3 m 3 .

[0054] Table 2 shows the parameter values used in the model calculation.

[0055]

Table 2

[0056] Here, Table 3 shows this embodiment implemented according to the refining method of the high nitrogen steel of the present invention, and comparative examples implemented for comparison with the present invention. Note that Charge Nos. 1-1 to 3-1 in Table 3 are the actual values in this experiment. Also, Charge Nos. 4-1 to 4-9 are calculated based on the actual values of No. 3-1 by assuming conditions and calculating [N] fin using a thermodynamic model.

[0057] [Table 3]

[0058] For the comparative examples in Table 3 (charges No. 1-1 and No. 1-2), the adjustment of S in the RH treatment (t s ) timing is slow and the vacuum level (P N2,S =P V ) is also high, so the value of index A is small. That is, since the index A does not satisfy 50, [N] fin The value of [N] is not [N] ≥ 150 ppm.

[0059] For the present example (charges No. 2-1 to No. 2-15) in Table 3, the bubbling process ([N] ini ) a sufficient amount of nitrogen is added to molten steel 2, and the adjustment of S (t s ) timing is too early, or the vacuum level (P N2,S =P V When the RH treatment condition satisfies A≧50, [N] fin The value of [N] is ≥ 150 ppm.

[0060] For this example (charge No. 3-1) in Table 3, the S adjustment (t s ) was not performed, but the bubbling process ([N] ini When the RH treatment condition satisfies A≧50, the amount of nitrogen added is large. fin The value of [N] is ≥ 150 ppm.

[0061] The present examples (Charges No. 4-1 to No. 4-9) in Table 3 are examples in which the [N] concentration was calculated using the N model, assuming molten steel component values ​​and processing conditions for a high Mn composition system based on the results of Charge No. 3-1 described above.

[0062] In addition, in Charges No. 4-1 to No. 4-9, the bubbling process ([N]ini ) is performed to sufficiently nitride molten steel 2. The RH treatment conditions are set to satisfy A≧50. Furthermore, S adjustment (t s ) can be carried out early in the RH treatment, the index A can be further increased, making it possible to achieve [N]≧150 ppm.

[0063] In summary, the method for refining high-nitrogen steel according to the present invention is a method for refining high-nitrogen steel in which molten steel 2 tapped from a converter is subjected to a ladle refining process and an RH vacuum degassing process in which N2 gas is circulated, and in the converter process and / or the ladle refining process, the molten steel 2 is nitrogenated by stirring with N2 gas, and in the RH vacuum degassing process, the processing conditions are such that the value of index A calculated from "Equation (1) including, as variables, the molten steel composition values ​​before the RH vacuum degassing process (preferably immediately before the process)" is 50 or more.

[0064]

number

[0065] However, [N] ini (ppm): [N] concentration in molten steel before RH treatment [N] e,S (ppm): Equilibrium [N] concentration at the surface of molten steel 2 in vacuum vessel 4 k r,before (m / min / %): Interfacial reaction rate constant of the [N] reaction before S addition k r,after (m / min / %): Interfacial reaction rate constant of [N]ation reaction after S addition t(min): RH treatment time t s (min): Start time of S addition As described above, according to the method for refining high-nitrogen steel of the present invention, the [N] concentration in the molten steel is sufficiently increased before the start of the RH treatment, and the RH treatment is performed under RH treatment conditions that are not affected by the denitrification reaction that occurs during the reduced pressure treatment in the RH treatment and that do not add a nitride alloy to the molten steel, i.e., under RH treatment conditions that make the value of equation (1) for deriving index A equal to or greater than 50, thereby making it possible to achieve [N] ≧ 150 ppm after the RH treatment.

[0066] In other words, by using the method for refining high-nitrogen steel of the present invention, it is possible to achieve [N] ≧ 150 ppm using only N gas by performing RH treatment under conditions that satisfy the index A ≧ 50, and high-nitrogen steel can be produced inexpensively without the addition of nitride alloys.

[0067] 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]

[0068] 1 RH type vacuum degassing treatment equipment (RH treatment equipment) 2. Molten steel 3 ladle 4 Vacuum chamber 5 dip tube 6 Gas blowing pipe 7. Exhaust port

Claims

[Claim 1] The molten steel tapped from the converter is subjected to ladle refining and N ２ A method for refining high nitrogen steel by performing RH vacuum degassing treatment and gas reflux, In the converter treatment and / or ladle refining treatment, N ２ The molten steel is nitrogenized by stirring with gas, and the high-nitrogen steel having a [N] of 150 ppm or more after RH treatment is refined. In the RH vacuum degassing treatment, the treatment conditions are such that the value of index A calculated from "Equation (1) including the values ​​of the molten steel components before the RH vacuum degassing treatment as variables" is 50 or more without adding a nitride alloy to the molten steel. A method for refining high nitrogen steel. [Equation 1] Where, [N]ini (ppm): [N] concentration in molten steel before RH treatment [N]e,S (ppm): Equilibrium [N] concentration at the surface of the molten steel bath in the vacuum vessel kr,before(m / min / %): Interfacial reaction rate constant of the [N] reaction before S addition kr,after (m / min / %): Interfacial reaction rate constant of the [N] reaction after adding S t (min): RH treatment time ts (min): Start time of S addition

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