Method for producing high-purity nitrogen-containing steel

Abstract

To provide a method for melting a highly clean nitrogen-containing steel capable of accurately adjusting the nitrogen concentration in a nitrogen-containing steel within a predetermined target range by changing RH treatment conditions in accordance with the nitrogen concentration in the steel during a RH treatment, and improving the cleanliness of the steel.SOLUTION: In melting a nitrogen-containing steel containing 0.03 mass% or more of [C] and having a target nitrogen concentration of 70 ppm to 150 ppm, the relationship 1 between the change of the nitrogen concentration in molten steel during a RH treatment and the treatment condition is obtained. The relation 2 between the difference between the nitrogen concentration in molten steel estimated from the relationship 1 and the nitrogen concentration in molten steel in operation results and the treatment time is obtained. The target nitrogen concentration and its range are set, the nitrogen concentration in molten steel is confirmed, the treatment condition is set using the relationship 1, the relationship between the remaining treatment time tr at the time of treatment and the time tm outside the range of the target nitrogen concentration is confirmed using the relationship 2, the treatment is carried out under the treatment condition. The nitrogen concentration in the molten steel is confirmed in the time Tm outside the range of the target nitrogen concentration when tr≥tm, the treatment is continued when tr<tm, and the treatment is repeated until the nitrogen concentration in molten steel satisfies the target nitrogen concentration.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technique for producing nitrogen-containing steel that requires high cleanliness by controlling the nitrogen concentration to a target level in RH treatment. [Background technology]

[0002] Generally, molten steel decarburized in a steelmaking furnace such as a converter or electric furnace is transported to a secondary refining process, where it is subjected to treatments such as vacuum degassing. RH treatment (vacuum degassing) mainly involves adjusting the composition of the molten steel and degassing it, but in some cases, the nitrogen concentration in the molten steel is also adjusted. Techniques for producing nitrogen-containing steel by controlling the nitrogen concentration in the molten steel during vacuum degassing are disclosed in, for example, Patent Documents 1 to 4.

[0003] Patent Document 1 discloses a method for precisely adjusting the nitrogen concentration while reducing inclusions and promoting dehydrogenation in a vacuum degassing process. thing The purpose is to: Specifically, it is disclosed that when steel having a [C] of 0.03 mass% or more is produced by vacuum degassing, in the first half of the process, the pressure is set to 300 Pa or less, and argon gas alone or a mixed gas of argon gas and nitrogen gas is blown in to reflux the molten steel for 15 minutes or more to adjust the components excluding nitrogen, the flow rate of argon gas is set to 5 L / (min·ton) or more, and the upper limit of the flow rate of the gas blown into the molten steel is set to 20 L / (min·ton) or less, and in the second half of the process, the pressure is maintained at 300 Pa or less, and the molten steel is refluxed with nitrogen gas alone or a mixed gas, and the flow rate of nitrogen gas is determined based on the analytical value of the nitrogen concentration in the molten steel sampled in the first half of the process.

[0004] Patent Document 2 aims to provide a method for adjusting the nitrogen concentration in molten steel in a vacuum degassing apparatus that enables the nitrogen concentration in molten steel to be adjusted more accurately than before. Specifically, it discloses that when adjusting the nitrogen concentration in the molten steel to a target value by blowing nitrogen gas into molten steel held in a vacuum degassing apparatus and simultaneously reducing the pressure inside the apparatus to a nitrogen partial pressure that achieves the equilibrium nitrogen concentration of the molten steel, the nitrogen concentration of the molten steel is quickly measured during the adjustment, the measured value is compared with the expected increase or decrease value at that time, and the pressure inside the apparatus is changed to eliminate the deviation.

[0005] Patent Document 3 describes a bearing material and its manufacturing method that can significantly improve the rolling fatigue life and crushing strength of bearing materials made from ultra-clean steel with an oxygen concentration of 10 ppm by weight in the steel, and aims to improve the life of rolling bearings, etc. Specifically, the document discloses a method for producing a bearing material, which comprises blowing nitrogen gas into molten steel with a C concentration of 0.5 mass% or more and / or an Al concentration of 0.005 mass% or more in a converter, electric furnace, or ladle refining apparatus to raise the nitrogen concentration in the molten steel to 120 ppm or more, then blowing Ar or Ar + nitrogen for 30 minutes or more in an RH-type vacuum degassing apparatus, followed by denitrification and deoxidation treatment, followed by continuous casting of the molten steel, and hot rolling the resulting slab, with a soaking time of 15 hours or less at 1200°C or higher.

[0006] Patent Document 4 discloses a vacuum degassing method for molten steel, which can minimize the amount of deoxidation products at the molten steel refining stage by performing the treatment at an optimal post-deoxidation time, and can prevent interventions caused by the deoxidation products. The aim is to produce clean steel with as little inclusions as possible. Specifically, it discloses that alloy elements are added to molten steel in a vacuum degassing process to adjust the composition, and then the amount of inclusions in the molten steel and the amounts of nitrogen, silicon, and oxides in the slag are measured when producing clean steel by ensuring the time for the inclusions to float up, and the amount of floating primary deoxidation products and the amount of secondary oxidation products produced are quantified, and based on this, the amount of inclusions in the molten steel is calculated as (remaining amount of floating primary deoxidation products) + (secondary oxidation products), and this is compared with the target allowable upper limit amount, and the time for stirring and shaking the molten steel after adding the deoxidizer is determined depending on the difference. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-224461 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-034513 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-272953 [Patent Document 4] Japanese Patent Application Publication No. 4-045220 Summary of the Invention [Problem to be solved by the invention]

[0008] In Patent Document 1, argon gas alone or a mixture of argon gas and nitrogen gas is injected in the first half of the treatment process. However, the present invention is aimed at adjusting the nitrogen concentration in steel to a predetermined range. Therefore, the technique in this document may have difficulty in increasing the [N] to the predetermined [N] range. In Patent Document 2, for example, when [N] is applied to 150 ppm, it is necessary to significantly increase the pressure inside the vacuum vessel. In this case, there is a risk that the amount of molten steel circulating may be significantly reduced, or that the molten steel may not rise into the vacuum vessel and may not be able to circulate. In other words, RH treatment becomes impossible.

[0009] In Patent Document 3, the purpose of nitrogen gas injection is purification, but it only increases the nitrogen concentration in molten steel to 120 ppm or more, and there is no disclosure or suggestion regarding the control of [N], making it very difficult to control [N] (to keep the nitrogen concentration in steel within a predetermined range). Patent Document 4 only determines the treatment time for cleaning after measuring [N], and does not disclose or suggest anything about controlling [N], making it very difficult to control [N].

[0010] That is, when producing nitrogen-containing steel that requires high cleanliness in RH treatment, it is necessary to adjust the nitrogen concentration in the steel and the cleanliness in order to satisfy the hardness, toughness, strength, etc. of the steel material. In view of the above problems, the present invention provides a method for producing a highly clean, nitrogen-containing steel, which can accurately adjust the nitrogen concentration in the steel to within a target predetermined range and improve the cleanliness of the steel material by changing the RH treatment conditions according to the nitrogen concentration in the steel during the RH treatment when producing the highly clean, nitrogen-containing steel. The law The purpose is to provide. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides the following technical means. The method for producing a highly clean nitrogen-containing steel according to the present invention is characterized in that, when producing a nitrogen-containing steel containing 0.03 mass% or more of [C] and having a target nitrogen concentration of 70 ppm to 150 ppm, the steel is produced in an RH vacuum degassing process according to the following steps (a1) to (b7): (a1) The relationship between the change in nitrogen concentration in molten steel during RH treatment and the RH treatment conditions (1) is determined for molten steel in the treatment equipment. (a2) The difference between the nitrogen concentration in the molten steel estimated from the relationship (1) and the nitrogen concentration in the molten steel in actual operation is calculated, and the relationship (2) between the difference and the RH treatment time is determined. (b1) A target nitrogen concentration in molten steel and a range of the target nitrogen concentration are set. (b2) Check the nitrogen concentration in the molten steel at any time during the RH treatment. (b3) The RH treatment conditions are set using the relationship (1) obtained in (a1) above. (b4) Using the relationship (2) obtained in the above (a2), when performing RH treatment based on the RH treatment conditions set in the above (b3), the relationship between the remaining RH treatment time tr and the time tm until the nitrogen concentration in the molten steel deviates from the range of the target nitrogen concentration set in the above (b1) is confirmed, and the treatment is performed under the RH treatment conditions of the above (b3). (b5) When tr ≧ tm, the nitrogen concentration in the molten steel is confirmed at the timing of the time Tm when the nitrogen concentration in the molten steel deviates from the range of the target nitrogen concentration set in the above (b1). (b6) When tr < tm, the treatment is continued under the RH treatment conditions of the above (b3). (b7) The procedures of the above (b3) to (b6) are performed one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in the above (b1). 。

Effect of the Invention

[0012] According to the present invention, when melting a high-purity nitrogen-containing steel, by changing the RH treatment conditions according to the nitrogen concentration in the steel during RH treatment, the nitrogen concentration in the nitrogen-containing steel can be accurately adjusted within a target predetermined range, and the cleanliness of the steel material can be improved.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram schematically showing the outline of an RH type vacuum degassing treatment apparatus. [Figure 2] It is a diagram summarizing the data used for the derivation of the relationship (2). [Figure 3] It is a diagram summarizing the data used for the derivation of the relationship (3). [Figure 4] It is a diagram showing the transition of the predicted nitrogen concentration in the molten steel. [Figure 5] It is a diagram showing the transition of the predicted nitrogen concentration in the molten steel. [Figure 6] It is a diagram showing the transition of the predicted nitrogen concentration in the molten steel. [Figure 7] It is a diagram showing the transition of the predicted nitrogen concentration in the molten steel. [Figure 8] FIG. 4 is a diagram showing the control accuracy of the nitrogen concentration in molten steel according to the present invention. [Figure 9] 1 is a flowchart of a method for producing a highly clean nitrogen-containing steel according to the present invention. [Figure 10] 1 is a flowchart of a method for producing a highly clean nitrogen-containing steel according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a method for producing a highly clean nitrogen-containing steel according to the present invention and a nitrogen-containing steel produced by the method will be described with reference to the drawings. 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. The method for producing highly clean nitrogen-containing steel according to the present invention and the nitrogen-containing steel produced by the method have a high nitrogen concentration in the steel before the start of the RH process, and nitrogen bubbles generated during denitrification in the RH process float and separate inclusions in the molten steel 2, promoting purification. Furthermore, by changing the RH process conditions according to the nitrogen analysis value during the RH process, it is possible to simultaneously control the target nitrogen concentration and the amount of denitrification during purification.

[0015] Specifically, the present invention is a technique that is implemented when producing nitrogen-containing steel that contains 0.03 mass % or more of [C] and has a target nitrogen concentration of 70 ppm to 150 ppm. On the other hand, when steel with [C]<0.03% by mass is produced by RH treatment, CO gas is generated while the molten steel 2 is decarburized during the treatment, and denitrification also occurs. Therefore, the technique for controlling the nitrogen concentration in steel according to the present invention cannot be applied. Therefore, steel with [C]<0.03% by mass is excluded from the scope of the present invention.

[0016] The nitrogen concentration in molten steel must be within a predetermined target range, which is determined in accordance with the hardness, toughness, strength, etc. required for each steel type. If the component concentration after melting deviates from the standard, the steel is discarded. In the RH treatment (vacuum degassing treatment), the components of the molten steel 2 are mainly adjusted and degassed, but in addition to this, the nitrogen concentration in the molten steel 2 may also be adjusted.

[0017] FIG. 1 shows a schematic overview of an RH vacuum degassing treatment device 1 (a general RH vacuum degassing process). As shown in Figure 1, 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 through the gas inlet 6, and the gas in the vacuum vessel 4 is exhausted to the outside through 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 performed. The RH type vacuum degassing treatment apparatus 1 is often used to produce clean steel because it exerts a strong stirring force on the molten steel 2 and is highly effective in agglomerating, floting, and separating inclusions in the molten steel 2.

[0019] In the method for producing a highly clean nitrogen-containing steel of the present invention, the steel is produced in the RH vacuum degassing step according to the following steps (a1) to (b7). (a1) The relationship (1) between the change in nitrogen concentration in molten steel 2 in the treatment device 1 during RH treatment and the RH treatment conditions is obtained.

[0020] However, how to obtain "relationship (1)" for estimating the nitrogen concentration in molten steel will be described in detail in the examples below. The nitrogen concentration ([N]) in molten steel during RH treatment varies depending on the RH treatment conditions (type of blown gas, gas flow rate, degree of vacuum) and the molten steel composition, and the behavior of [N], i.e., the [N] addition rate and [N] removal rate, etc.

[0021] The apparent equilibrium [N] increases as the flow rate of nitrogen gas increases and the pressure in the vacuum chamber 4 increases. On the other hand, the apparent equilibrium [N] concentration decreases as the flow rate of nitrogen gas decreases and the pressure in the vacuum chamber 4 decreases. In addition, the equilibrium [N] concentration, the [N] rate, and the [N] de-rate change depending on the molten steel composition. As described above, there are many factors that affect the behavior of [N], so in order to control [N], it is important to accurately estimate the behavior of [N]. (a2) The difference (deviation) between the nitrogen concentration in molten steel estimated from the relationship (1) obtained in (a1) above and the nitrogen concentration in molten steel in actual operation is calculated, and the relationship (2) between the difference and the RH treatment time is obtained.

[0022] However, how to obtain the "relationship (2)" which indicates the variation in the nitrogen concentration in the molten steel with respect to the RH treatment time will be explained in detail in the examples given later. In actual operation, the behavior of nitrogen varies depending on the processing variations caused by, for example, the degree of vacuum reached in the vacuum chamber 4 of the RH treatment device 1 and the amount of air leaking from the immersion tube, and therefore there may be a discrepancy between the estimated [N] value and the [N] value of the actual operation.

[0023] Therefore, the variation in [N] behavior within the normal operating range of RH treatment must be understood in advance. That is, the time dependency from the start of RH treatment on the difference (deviation) between the estimated [N] value and the actual [N] value must be quantified for each RH treatment condition and each molten steel component. (b1) A target nitrogen concentration in the molten steel 2 and a range of the target nitrogen concentration are set.

[0024] For example, the target nitrogen concentration in the molten steel 2 and the range of the target nitrogen concentration are set based on the [N] standard or the like. (b2) Check the nitrogen concentration in the molten steel at any time during the RH treatment. For example, the molten steel 2 in the ladle 3 is sampled in the usual manner by those skilled in the art. In the early stages of the treatment, the treatment conditions are determined by the addition of alloys, oxygen heating, etc., so sampling is carried out at the timing when these operations are completed and [N] control is started. (b3) The RH treatment conditions are set using the relationship (1) obtained in (a1) above.

[0025] For example, the RH processing conditions are set to the target [N] or to fall within the range of the target [N]. (b4) Using the relationship (2) obtained in (a2) above, the relationship between the remaining RH treatment time tr and the time tm until the nitrogen concentration in the molten steel falls outside the control target nitrogen concentration range set in (b1) when RH treatment is performed based on the RH treatment conditions set in (b3) is confirmed, and treatment is performed under the RH treatment conditions of (b3).

[0026] For example, the timing at which [N] falls outside the target nitrogen concentration range (details will be described later) is determined from the "time transition of the variation in nitrogen concentration in molten steel" obtained from relationships (1) and (2). In order to sample at this timing, the time until the estimated [N] value falls outside the control target [N] range is defined as tm, and the relationship with the remaining processing time tr is confirmed. (b5) When tr≧tm, the nitrogen concentration in the molten steel is checked at the timing of time Tm when the nitrogen concentration in the molten steel falls outside the range of the target nitrogen concentration set in (b1).

[0027] When tr≧tm, that is, when [N] falls outside the range of the target nitrogen concentration, the molten steel 2 is sampled at the timing of the time Tm when [N] falls outside the target range. (b6) On the one hand, when tr < tm, continue the treatment under the RH treatment conditions set in (b3) above. When tr < tm, that is, when [N] is within the target range, continue the RH treatment until the end of the treatment. (b7) Perform the procedures of (b3) to (b6) one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in (b1).

[0028] That is, by repeating (b3) to (b6) and modifying the RH treatment conditions each time if necessary, [N] can be surely controlled within the range of the target nitrogen concentration. Furthermore, when melting a high-purity nitrogen-containing steel containing 0.03% by mass or more of [C] and having a target nitrogen concentration of 70 ppm to 150 ppm, in the RH vacuum degassing process, the following (c1) to (d4) It is advisable to melt by the procedure shown.

[0029] Note that the presence of inclusions in the molten steel 2 causes a decrease in fatigue life, etc., so it is better to have as few inclusions as possible. (c1) Obtain the relationship (3) between the change amount Δ[N] of the nitrogen concentration in the molten steel and the cleanliness of the molten steel 2. However, the method for obtaining the "relationship (3)" indicating the amount of [N] removal and the degree of decrease in inclusions will be described in detail in the examples below.

[0030] Near the surface of the molten steel 2 in the vacuum chamber 4 of the RH treatment apparatus 1, nitrogen gas bubbles are generated during [N] removal. These generated bubbles capture inclusions and promote the floating and separation of inclusions, so the cleanliness of the molten steel 2 is improved. For example, the larger the amount of [N] removal (Δ[N]), the more the number of nitrogen gas bubbles generated increases or the bubble diameter becomes larger, so it becomes easier to capture inclusions, and the purification effect also becomes larger.

[0031] Note that examples of the cleanliness index include the number of inclusions, the T.O concentration, etc., but there is no particular limitation as long as it is an index related to cleanliness. (d1) The RH treatment process is divided into Phase I: a cleaning section and Phase II: an [N] adjustment section. In the present invention, since the RH treatment is carried out while changing the target [N], it is preferable to divide the RH step into two sections and determine the RH treatment conditions for each section.

[0032] First, in the first stage, RH treatment is performed to obtain the amount of [N] removed based on the relationship (3) obtained in (c1), followed by a cleaning treatment. Next, in the second stage, RH treatment is performed to control the post-treatment [N] to the target [N] or within the target [N] range. (d2) Set the processing time for Phase I and Phase II.

[0033] The RH processing time is determined by the preceding processes (such as converters and secondary refining) and the subsequent processes (such as secondary refining and continuous casting). If the RH processing time is extended, the start time of the subsequent processes must be delayed. For example, in the continuous casting process, measures such as slowing down the casting speed are necessary. If measures are not taken to keep up, continuous casting will be interrupted, which could lead to reduced productivity and yield.

[0034] In other words, it is necessary to set the treatment times for Phase I and Phase II so that the RH treatment can be completed at a predetermined time and the respective roles of Phase I and Phase II can be performed. Possible methods for setting the processing time include, for example, setting the processing time from relationship (1) or setting the processing time from past performance, but any setting method may be used. (d3) Using the relationship (3) obtained in (c1) above, the Δ[N] required in the I period is set, and the target nitrogen concentration in the molten steel in the I period (target [N]) and the range of the target nitrogen concentration are set.

[0035] To set the target [N] for Phase I, Δ[N] (amount of de-[N]) is set. To achieve that Δ[N], the target [N] and target [N] range for Phase I are set. Possible methods for setting the target [N] and target [N] range for Period I include, for example, setting them from the [N] in the molten steel analyzed during RH treatment, or setting them from past performance, but any setting method may be used. (d4) In each of the periods I and II, the molten steel 2 is subjected to RH treatment in accordance with the procedures shown in (b1) to (b7).

[0036] In other words, [N] is controlled using the procedures shown in (b1) to (b7) so that the target [N] for each of the first and second periods is achieved. [Example] Examples carried out in accordance with the method for producing a highly clean nitrogen-containing steel of the present invention and comparative examples carried out for comparison with the present invention will be described below.

[0037] The conditions for carrying out this example are as follows. Table 1 shows the conditions for this example.

[0038] [Table 1]

[0039] First, we will explain in detail the derivation of relation (1). In this example, the nitrogen concentration calculation model disclosed in Japanese Patent No. 5836187 was used as the relationship (1). The nitrogen concentration calculation model is described in detail below. Note that the relationship (1) is not limited to the nitrogen concentration calculation model exemplified below. For example, other models or relationships derived from actual values ​​may also be used. <Nitrogen concentration calculation model> The nitrogen concentration in molten steel during RH treatment can be calculated from the material balance between the ladle 3 and the vacuum vessel 4, and is expressed by equation [1], which shows the change in nitrogen concentration in molten steel in the vacuum vessel 4, and equation [2], which shows the change in nitrogen concentration in molten steel in the ladle 3.

[0040]

number

[0041]

number

[0042] where: [N]L: [N] concentration in ladle 3 (mass%) [N]V: [N] concentration in vacuum chamber 4 (mass%) VL: Volume of molten steel in ladle (m 3 ) VV: Volume of molten steel in vacuum chamber (=3m 3 ) t: time (min) RS: De-[N] rate at the bath surface of molten steel 2 (% / min) RAr: Desorption rate of [N] at the interface of refluxing Ar gas bubbles (% / min) RN2: Rate of addition or removal of nitrogen at the interface of refluxing nitrogen gas bubbles (% / min) Rleak: The rate of suction [N] due to air intrusion from the immersion tube 5 (% / min) is.

[0043] The amount of molten steel 2 that is returned is Q (m 3 / min) is calculated using formula [3] described in the reference: Kuwahara et al.: Iron and Steel, 73 (1987) S176.

[0044]

number

[0045] where: Q g : Injection gas flow rate (NL / min) D: Diameter of immersion pipe 5 (m) P0: Pressure at the reflux gas injection point (atm) PV: Pressure inside vacuum chamber 4 (atm) ρF e: Molten steel density (=7ton / m 3 ) is.

[0046] Assuming that RS is a mixed rate-controlling factor between the mass transfer of N on the molten steel 2 side and the chemical reaction of N on the bath surface of molten steel 2, RS can be expressed as the following equation [4].

[0047]

number

[0048] where: AS: Reaction interfacial area at the bath surface of molten steel 2 (m 2 ) [N]i,S: [N] concentration (%) at the bath surface of molten steel [N]e,S: [N] concentration (%) in equilibrium with the atmosphere in vacuum chamber 4 km: Mass transfer coefficient of N in molten steel 2 (m / min) kr:N chemical reaction rate constant (m / (min / %)) By eliminating [N]i,S from equation [4] and rearranging, RS is expressed as equation [5].

[0049]

number

[0050] Similarly, assuming that RAr is controlled by the mass transfer of N on the molten steel 2 side and the chemical reaction of N at the Ar gas bubble interface, RAr can be expressed as the following equation [6].

[0051]

number

[0052] where: AA r: Reaction interfacial area at the Ar bubble surface (m 2 ) [N]e, Ar: Equilibrium [N] concentration (%) at the Ar bubble interface is. RN2 can be expressed in the same way, but the rate of N addition or de-N addition differs because the N2 partial pressure in the bubbles decreases from the N2 injection position toward the bath surface of the molten steel 2 in the vacuum vessel 4. As a result, N addition occurs near the N2 injection position, and de-N addition occurs near the bath surface of the molten steel 2.

[0053] Therefore, the difference between the N addition rate at the injection position and the N removal rate at the bath surface of the molten steel 2 is considered to be the N addition rate or N removal rate at the entire N2 bubble interface (Equation [7]).

[0054]

number

[0055] where: AN2: Reaction interfacial area at the N2 bubble surface (m 2 ) [N]e, O, N2: Equilibrium [N] concentration (%) at the N2 bubble interface at the injection position [N]e, S, N2: [N] concentration (%) at equilibrium at the interface of N2 bubbles on the bath surface of molten steel 2.

[0056] [N]e,S is calculated using equation [8].

[0057]

number

[0058] where: PN2,S: N2 partial pressure (atm) at the bath surface of molten steel 2 f N : Activity coefficient of N T: Molten steel temperature (K) R: gas constant is.

[0059] It should be noted that PN2,S is the same as the degree of vacuum PV inside the vacuum chamber 4, and PN2,S =PV. [N]e,Ar: The equilibrium [N] concentration (%) at the Ar bubble interface is also calculated using equation [9]. However, since the nitrogen partial pressure PN2,Ar inside the Ar bubble is 0, the result is [N]e,Ar = 0.

[0060]

number

[0061] [N]e, O, and N2 are calculated from equation

[10] . Note that P0 and N2 in equation

[10] are calculated from equation

[12] , assuming that the static pressure of the molten steel 2 from the bath surface of the molten steel 2 to the injection position is added to the pressure inside the vacuum vessel 4. [N]e, S, N2 are calculated from equation

[11] . Note that PS, N2 are assumed to be the same as the pressure inside the vacuum chamber 4, and PS, N2 = PV.

[0062]

number

[0063]

number

[0064]

number

[0065] where: l: Distance from the bath surface of molten steel 2 to the injection position (m) g: Gravitational acceleration (=9.8m / sec 2 ) is. In addition, the nitrogen activity coefficient f shown in equations [8] to

[12] N is calculated using equation

[13] .

[0066]

number

[0067] eN j is the interaction coefficient of N with component j, and the value in Table 2 was used (reference: "Japan Society for the Promotion of Science, 19th Committee on Steelmaking, Recommended Equilibrium Values ​​for Steelmaking Reactions, 1984"). [%j] is the concentration of component j (mass%). Table 2 shows the interaction coefficients for N, O, and S components j.

[0068] [Table 2]

[0069] The chemical reaction rate constant (m / (min / %)) of kr:[N] is calculated from formula

[14] in the reference: "Harashima et al.: Iron and Steel, 73 (1987) 1559".

[0070]

number

[0071] f O is the activity coefficient of O, and is calculated from equation

[15] in the same way as equation

[13] . S is the activity coefficient of S, and is calculated from Eq.

[16] in the same way as Eq.

[13] .

[0072]

number

[0073]

number

[0074] eN j ,eS j are the interaction coefficients for the O and S components j, respectively, and the values ​​in Table 2 are used. Used. Additionally, AAr and AN2 are proportional to the 2 / 3 power of QgAr and QgN2, respectively, and are calculated from equations

[17] and

[18] . Note that αAr and αN2 are the respective proportionality constants.

[0075]

number

[0076]

number

[0077] where: QgAr: Ar gas flow rate (Nm 3 / min) QgN2: Injected N2 gas flow rate (Nm 3 / min) is. The suction rate [N] Rl eak (% / min) due to the air intrusion from the immersion tube 5 was calculated using equation

[19] . Regarding Qleak, although the inner diameter and flange structure of the immersion tube 5 are different, it is 0.17 Nm derived in the reference: "Kato et al.: Iron and Steel, 83 (1997) 18". 3 / min was used. Note that MN2 is the molecular weight of N (=28).

[0078]

number

[0079] In the calculation process described above, the unknown values ​​are km, αS, αAr, and αN2. As N moves rapidly in molten steel and is not considered to be mass transfer-limited, km was fixed at a large value of 10,000. αS, αAr, and αN2 were determined using the least squares method so that the calculated value was closest to the measured value of the time change in [N] in molten steel during RH treatment, taking into account the degree of vacuum, whether oxygen heating was used, and the type of gas. The values ​​in Table 3 were used to determine these values.

[0080] Table 3 shows the parameter values ​​used. [Table 3]

[0081] Next, we will explain in detail the derivation of relation (2). Figure 2 shows a graph summarizing the data used to derive relationship (2). The RH treatment was carried out under constant RH treatment conditions, and the nitrogen concentration in the molten steel was analyzed at random timings. The nitrogen concentration in the molten steel at the same timings was also calculated from the relationship (1). As shown in Figure 2, the relationship between the difference (deviation) between the estimated [N] value and the actual operational [N] value in relationship (1) and the RH treatment time was determined.

[0082] Further, when an approximate line was drawn through the origin and at the maximum value for each treatment time so that the slope was at its maximum (in this example, the approximate line was calculated at the maximum values ​​of 11 minutes, 17 minutes, and 19 minutes), relationship (2) was obtained, in which the deviation (variation) increases at 0.93 ppm / min. Regarding the relationship (2), FIG. 2 is an example and is not limiting. Furthermore, we provide a detailed explanation of the derivation of relation (3).

[0083] Figure 3 shows a graph summarizing the data used to derive relationship (3). As shown in Figure 3, the relationship (3) was obtained from the change in nitrogen concentration in molten steel, Δ[N] (ppm), and the rate constant of decrease in the number of inclusions ( / min), where Δ[N] was defined as the difference between the nitrogen concentration in molten steel before treatment and the nitrogen concentration in molten steel after 10 minutes of RH treatment. To count the number of inclusions, a sample of molten steel 2 was taken from inside ladle 3 during RH treatment, and the specimen surface was measured using FE-EPMA. 2 The number of oxide inclusions of 5 μm or more per unit area was evaluated.

[0084] In addition, the inclusion number reduction rate constant k ( / min) is calculated by taking the RH treatment time as t (min) and the number of inclusions before treatment as x0 (pieces / cm 2), and the number of inclusions at the treatment time t (min) is xt (pieces / cm 2 ) and k at RH 10 min was calculated using the following formula. k=-1 / t×ln(xt / x0) In this way, relationship (3) was obtained, where the inclusion number reduction rate constant k increases when Δ[N] is -28 ppm or less.

[0085] Note that, with regard to the relationship (3), FIG. 3 is just an example, and the present invention is not limited to this. In producing a nitrogen-containing steel containing 0.03 mass% or more of [C] and having a target nitrogen concentration of 70 ppm to 150 ppm, the RH vacuum degassing process was carried out in accordance with the following steps (a1) to (b7). In this example, a nitrogen-containing steel containing 0.20 mass% of [C] and having a target [N] of 100 ppm was produced. The steps to be carried out were converter, RH treatment, and continuous casting, and in this example, the RH treatment was carried out. (a1) The relationship (1) between the change in nitrogen concentration in molten steel during RH treatment of the molten steel in the treatment apparatus 1 and the RH treatment conditions is obtained.

[0086] The method for determining the relationship (1) is as described above. (a2) The difference (deviation) between the nitrogen concentration in molten steel estimated from the relationship (1) and the nitrogen concentration in molten steel in actual operation is calculated, and the relationship (2) between this difference and the RH treatment time is obtained. The method for determining the relationship (2) is as described above. (b1) A target nitrogen concentration in molten steel and a range of the target nitrogen concentration are set.

[0087] The target [N] was set at 100 ppm, and the target [N] range was set at the standard range of 80 to 130 ppm. (b2) Check the nitrogen concentration in the molten steel at any time during the RH treatment. The RH treatment was started with a nitrogen gas flow rate of 6 NL / min / ton and a vacuum of 30 Torr. After alloy addition and oxygen heating, the [N] in the molten steel was analyzed 15 minutes into the treatment. The [N] in the molten steel was found to be 126 ppm. (b3) The RH treatment conditions are set using the relationship (1) obtained in (a1) above.

[0088] In this example, using relationship (1), the nitrogen gas flow rate was set to 12 NL / min / ton and the vacuum level to 0.5 Torr so that the [N] transition would fall within the target [N] range. Note that, taking into consideration the possibility of extending the [N] removal time due to trouble, etc., the target [N] was set at 110 ppm, which is higher than the target [N] = 100 ppm. (b4) Using the relationship (2) obtained in (a2) above, the relationship between the remaining RH treatment time tr and the time tm until the nitrogen concentration in the molten steel falls outside the range of the target nitrogen concentration set in (b1) when RH treatment is performed based on the RH treatment conditions set in (b3) is confirmed, and treatment is performed under the RH treatment conditions of (b3).

[0089] FIG. 4 shows the change in predicted nitrogen concentration in molten steel 2. 4, in this example, the remaining processing time tr was 24 min, and the time tm until the target [N] range was exceeded was 19 min according to the relationship (2). The RH processing was performed with a nitrogen gas flow rate of 12 NL / min / ton and a vacuum level of 0.5 Torr. (b5) If tr≧tm, the nitrogen concentration in the molten steel is checked at the timing of time Tm when the nitrogen concentration in the molten steel falls outside the range of the target nitrogen concentration set in (b1).

[0090] As shown in Figure 4, this time, tr (= 24 min) ≥ tm (= 19 min), so [N] in the molten steel was analyzed at Tm = 34 min, a time outside the target [N] range. As a result, [N] in the molten steel was 122 ppm. (b7) The procedures (b3) to (b6) are carried out one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set at (b1).

[0091] In this example, since there is remaining RH treatment time, the second procedure of (b3) to (b6) was carried out. <The second (b3)> Using relation (1), the argon gas flow rate was set at 6 NL / min / ton and the vacuum degree was set at 0.5 Torr so that the [N] transition would fall within the target [N] range.

[0092] Note that when the argon gas flow rate is less than 6 NL / min / ton, there may be poor circulation, and since the nitrogen gas flow rate cannot be decreased, for a target [N] = 100 ppm, the predicted [N] at the end of the treatment was targeted at 105 ppm. <The second (b4)> Fig. 5 shows the transition of the predicted nitrogen concentration in the molten steel 2.

[0093] As shown in Fig. 5, in this example, the remaining treatment time tr was 5 min, and the time tm until going out of the target [N] range was 10 min from relation (2). Note that the RH treatment was carried out with an argon gas flow rate of 6 NL / min / ton and a vacuum degree of 0.5 Torr. <The second (b6)> As shown in Fig. 5, in the second time, since tr (= 5 min) < tm (= 10 min), the treatment was continued under the RH treatment conditions of (b3), and the RH treatment was completed at the treatment end time = 39 min. Note that when [N] in the molten steel was analyzed after the treatment, it was 99 ppm, and the difference from the target [N] = 100 ppm was 1 ppm.

[0094] Furthermore, when producing a highly clean nitrogen-containing steel containing 0.03 mass% or more of [C] and a target nitrogen concentration of 70 ppm to 150 ppm, the RH vacuum degassing process is carried out in accordance with the following steps (c1) to (d4), and in this example, a highly clean nitrogen-containing steel containing 0.40 mass% of [C] and a target [N] of 110 ppm was produced. The process is a converter-LF treatment-RH treatment-continuous casting process, and in this example, the RH treatment was carried out. (c1) The relationship (3) between the change in nitrogen concentration in molten steel Δ[N] and the cleanliness of molten steel 2 is obtained.

[0095] The method for finding the relationship (3) is as described above. (d1) The RH treatment process is divided into Phase I: purification section and Phase II: [N] adjustment section. (d2) Set the processing time for Phase I and Phase II.

[0096] Past performance has confirmed that 10 minutes of nitrogen adjustment time is sufficient to increase the nitrogen from the low nitrogen range to the target nitrogen. In order to improve the cleanliness of the steel, it is preferable to extend the cleaning time as much as possible. By adjusting the start time of continuous casting, the RH treatment time was secured at 41 min, and the I stage was set at 31 min and the II stage at 10 min. (d3) Using the relationship (3) obtained in (c1) above, Δ[N] required in the I period is set, and the target nitrogen concentration in the molten steel in the I period and the range of the target nitrogen concentration are set.

[0097] In this example, Δ[N] was set to −28 ppm using the relationship (3). The target nitrogen concentration and the range of the target nitrogen concentration were set based on the [N] in the molten steel during processing, as described in (b1) and (b2) of the first period below. (d4) In each of the first stage and the second stage, the molten steel 2 is subjected to RH treatment according to the procedures shown in (b1) to (b7).

[0098] For the first stage, it is as follows. <(b1) and (b2) of the first stage> The RH treatment was started with a nitrogen gas flow rate of 12 NL / min / ton and a vacuum degree of 30 Torr. Also, after alloy addition, the [N] in the molten steel was analyzed 12 minutes after the treatment. As a result, the [N] in the molten steel was 136 ppm.

[0099] Also, since the Δ[N] up to the upper limit of the target [N] satisfies -28 ppm or less, the target [N] for the first stage is 80 ppm, and the target [N] range is set to 60 - 100 ppm because the standard width is 40 ppm. <(b3) of the first stage> Using relation (), the nitrogen gas flow rate was set to 12 NL / min / ton and the vacuum degree was set to 0.5 Torr so that the target [N] for the first stage becomes 80 ppm at the end of the first stage. <(b4) of the first stage> Fig. 6 shows the transition of the predicted nitrogen concentration in the molten steel 2.

[0100] As shown in Fig. 6, in the first stage, the remaining treatment time tr was 19 min, and the time tm until it deviated from the target [N] range was 19 min according to relation (2). Note that the RH treatment was carried out with a nitrogen gas flow rate of 12 NL / min / ton and a vacuum degree of 0.5 Torr. <(b5) of the first stage> As shown in Fig. 6, in this first stage, since tr (= 19 min) ≥ tr (= 19 min), the [N] in the molten steel was analyzed at the timing of the time Tm = 31 min when it deviated from the target [N] range. As a result, the [N] in the molten steel was 79 ppm. <(b7) of the first stage> Since there was no remaining treatment time and the actual result was 79 ppm with respect to the target [N] of 80 ppm, the first stage was completed. Note that in the first stage, the procedures of (b3) to (b6) were carried out once.

[0101] ​ For Phase II, it is as follows. <(b1) of Phase II> For the target [N] in Phase II, it was set at 110 ppm. Also, for the target [N] range, it was set at 90 - 130 ppm of the standard range. <(b2) of Phase II> At the end of Phase I, [N] in the molten steel was analyzed. As a result, [N] in the molten steel was 79 ppm. This is described in Phase I (b5). <(b3) of Phase II> Using relationship (1), the nitrogen gas flow rate was set at 6 NL / min / ton and the vacuum degree was set at 30 Torr so that the [N] transition would fall within the target [N] range.

[0102] Note that when the nitrogen gas flow rate is less than 6 NL / min / ton, there may be poor circulation and the nitrogen gas flow rate cannot be decreased. Therefore, for the target [N] = 110 ppm, the predicted [N] at the end of Phase II was targeted at 120 ppm. <(b4) of Phase II> Fig. 7 shows the transition of the predicted nitrogen concentration in the molten steel 2.

[0103] As shown in Fig. 7, in Phase II, the remaining treatment time tr was 10 min, and the time tm until it deviated from the target [N] range was 10 min according to relationship (2). The treatment was carried out with a nitrogen gas flow rate of 6 NL / min / ton and a vacuum degree of 30 Torr. <(b5) of Phase II> In this Phase II, since tr (= 10 min) ≥ tr (= 10 min), [N] in the molten steel was analyzed at the timing of the time Tm = 41 min when it deviated from the target [N] range. As a result, [N] in the molten steel was 109 ppm. <(b7) of Phase II> Since there was no remaining treatment time and the actual result was 109 ppm for the target [N] of 110 ppm, Phase II was completed. That is, the RH treatment was completed. However, in Phase II, the procedures of (b3) - (b6) were carried out once.

[0104] Here, when the number of oxide-based inclusions in the molten steel after RH treatment was evaluated, it was 1.1 pieces / cm 2 It was. The effects of the present invention will now be described. The nitrogen concentration in the molten steel after treatment is as follows. The target control range of the nitrogen concentration in molten steel is predetermined according to the hardness, toughness, strength, etc. required for each steel type, and the nitrogen concentration must be kept within this range.

[0105] FIG. 8 shows the control accuracy of the nitrogen concentration in the molten steel 2 according to the present invention, that is, the improvement effect when the present invention is applied. 8, the present invention reduced the variation σ between the target nitrogen concentration in molten steel and the nitrogen concentration in molten steel after RH treatment from 17.0 ppm to 10.6 ppm, and also improved the probability (derailment rate) that the nitrogen concentration in molten steel deviated from the target range (±25 ppm) of the nitrogen concentration in molten steel from 14.1% to 1.8%.

[0106] The number of inclusions after RH treatment is as follows: Inclusions are the starting points for cracks, fractures, fatigue failures, etc. that occur when steel is processed, so it is best to reduce inclusions as much as possible. For example, in Japanese Patent Laid-Open No. 11-001749, although the target inclusions are different, the number of inclusions is generally 20 / cm 2 It is also disclosed that if the inclusion density exceeds 20 / cm, bending fatigue strength and rolling fatigue strength will decrease. 2 It is disclosed that if the temperature exceeds this range, the wire drawability deteriorates.

[0107] Taking this into consideration, in the present invention, the number of inclusions after RH treatment is set to 0.9 to 12.2 pieces / cm 2 The cleanliness was high enough that the above-mentioned properties were not deteriorated. Finally, the method for producing a highly clean nitrogen-containing steel according to the present invention can be summarized as follows. Fig. 9 shows a flowchart of the method for melting a high-purity nitrogen-containing steel according to the present invention. When melting a nitrogen-containing steel containing [C] of 0.03% by mass or more and having a target nitrogen concentration of 70 ppm to 150 ppm, the method for melting a high-purity nitrogen-containing steel according to the present invention is melted in the RH vacuum degassing process according to the procedures shown in the following (a1) to (b7). (a1) Obtain the relationship (1) between the change in the nitrogen concentration in the molten steel during RH treatment and the RH treatment conditions for the molten steel 2 in the treatment apparatus 1. (a2) Calculate the difference between the nitrogen concentration in the molten steel estimated from the relationship (1) and the nitrogen concentration in the molten steel in the operation results, and obtain the relationship (2) between the difference and the RH treatment time. (b1) Set the target nitrogen concentration in the molten steel 2 and the range of the target nitrogen concentration. (b2) Check the nitrogen concentration in the molten steel at an arbitrary timing during the RH treatment. (b3) Set the RH treatment conditions using the relationship (1) obtained in (a1) above. (b4) Using the relationship (2) obtained in (a2) above, check the relationship between the remaining RH treatment time tr and the time tm until the nitrogen concentration in the molten steel deviates from the range of the target nitrogen concentration set in (b1) when the RH treatment is carried out based on the RH treatment conditions set in (b3), and perform the treatment under the RH treatment conditions of (b3). (b5) When tr ≥ tm, check the nitrogen concentration in the molten steel at the timing of the time Tm when the nitrogen concentration in the molten steel deviates from the range of the target nitrogen concentration set in (b1). (b6) When tr < tm, continue the treatment under the RH treatment conditions of (b3). <00�0708>(b7) Perform the procedures of (b3) to (b6) one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in (b1).

[0108] Fig. 10 shows a flowchart of the method for melting a high-purity nitrogen-containing steel according to the present invention. Furthermore, in the method for producing a highly clean nitrogen-containing steel of the present invention, when producing a highly clean nitrogen-containing steel containing 0.03 mass% or more of [C] and having a target nitrogen concentration of 70 ppm to 150 ppm, the RH vacuum degassing step may further include the steps (c1) to (d4) below in addition to the above. The relationship (3) between the change in nitrogen concentration in molten steel Δ[N] and the cleanliness of molten steel 2 is obtained. (d1) The RH treatment process is divided into Phase I: purification section and Phase II: [N] adjustment section. (d2) Set the processing times for Phase I and Phase II. (d3) Using the relationship (3) obtained in (c1) above, Δ[N] required in the I period is set, and the target nitrogen concentration in the molten steel 2 in the I period and the range of the target nitrogen concentration are set. (d4) In each of the periods I and II, the molten steel is subjected to RH treatment according to the procedures shown in (b1) to (b7).

[0109] The nitrogen-containing steel of the present invention may be produced in accordance with the above-described method for producing a highly clean nitrogen-containing steel. According to the method for producing a highly clean nitrogen-containing steel of the present invention, when producing a highly clean nitrogen-containing steel, the RH treatment conditions are changed depending on the nitrogen concentration in the steel during the RH treatment, whereby the nitrogen concentration in the nitrogen-containing steel can be accurately adjusted to fall within a target predetermined range and the cleanliness of the steel can be improved.

[0110] It should be noted that the embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. 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 ordinary skilled in the art are used. [Explanation of symbols]

[0111] 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] A method for producing a highly clean nitrogen-containing steel, the method comprising the steps of (a1) to (b7) below in a RH vacuum degassing process, when producing a nitrogen-containing steel containing 0.03 mass% or more of [C] and having a target nitrogen concentration of 70 ppm to 150 ppm: (a1) The relationship (1) between the change in nitrogen concentration in molten steel during RH treatment in the treatment equipment and the RH treatment conditions is determined. (a2) The difference between the nitrogen concentration in the molten steel estimated from the relationship (1) and the nitrogen concentration in the molten steel in actual operation is calculated, and the relationship (2) between the difference and the RH treatment time is determined. (b1) A target nitrogen concentration in molten steel and a range of the target nitrogen concentration are set. (b2) The nitrogen concentration in the molten steel is checked at any timing during the RH treatment. (b3) The RH treatment conditions are set using the relationship (1) obtained in (a1) above. (b4) Using the relationship (2) obtained in the above (a2), the relationship between the remaining RH treatment time tr and the time tm until the nitrogen concentration in the molten steel falls outside the range of the target nitrogen concentration set in the above (b1) when the RH treatment is carried out based on the RH treatment conditions set in the above (b3) is confirmed, and treatment is carried out under the RH treatment conditions in the above (b3). (b5) When tr≧tm, the nitrogen concentration in the molten steel is checked at the timing of time Tm when the nitrogen concentration in the molten steel falls outside the range of the target nitrogen concentration set in (b1) above. (b6) If tr<tm, the treatment is continued under the RH treatment conditions of (b3) above. (b7) The steps (b3) to (b6) are carried out one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in (b1).

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