JP2023160452A - Method for melting highly clean nitrogen-containing steel and nitrogen-containing steel produced by the melting method - Google Patents

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

The present invention relates to a technology for melting nitrogen-containing steel that requires cleanliness by controlling the nitrogen concentration to a target level in RH treatment.

Generally, molten steel that has been decarburized in a steelmaking furnace such as a converter or an electric furnace is transported to a secondary refining process, where the molten steel is subjected to treatments such as vacuum degassing. In the RH treatment (vacuum degassing treatment), the composition of molten steel is mainly adjusted and the molten steel is degassed, but in addition, the nitrogen concentration in the molten steel may be adjusted. Techniques for producing nitrogen-containing steel by controlling the nitrogen concentration in molten steel in vacuum degassing are disclosed, for example, in Patent Documents 1 to 4.

Patent Document 1 aims to precisely adjust the concentration of nitrogen while reducing inclusions and promoting dehydrogenation in vacuum degassing treatment.
Specifically, when manufacturing steel with [C] ≥ 0.03 mass% or more by vacuum degassing treatment, the pressure is set to 300 Pa or less in the first half of the treatment, and argon gas alone or a mixture of argon gas and nitrogen gas is used. The molten steel will be refluxed for at least 15 minutes by blowing in argon gas to adjust the components other than nitrogen.The flow rate of argon gas will be set to 5L/(min・ton) or more, and the upper limit of the flow rate of the gas injected into the molten steel will be set to 20L/(min. In the second half of the treatment, the pressure will be maintained at 300Pa or less, and the molten steel will be refluxed with nitrogen gas alone or with a mixture of gases. It is disclosed that the determination is based on an analytical value.

In a method for adjusting the nitrogen concentration in molten steel, Patent Document 2 aims to make it possible to adjust the nitrogen concentration in molten steel in a vacuum degassing device with higher accuracy than before.
Specifically, nitrogen gas is blown into the molten steel held in a vacuum degassing device, and at the same time, the pressure inside the device is reduced to a nitrogen partial pressure that achieves the equilibrium nitrogen concentration of the molten steel, thereby bringing the nitrogen concentration in the molten steel to the target value. During the adjustment, the nitrogen concentration in the molten steel is quickly measured, the measured value is compared with the expected increase or decrease value at that point, and the pressure in the device is adjusted so as to eliminate the deviation. It is disclosed that changes may be made.

Patent Document 3 describes a bearing material and its manufacturing method that can significantly improve the rolling fatigue life and crushing strength of a bearing material using ultra-clean steel with an oxygen concentration of 10 ppm by weight, and is capable of significantly improving rolling bearing fatigue life and crushing strength. The aim is to improve the lifespan of
Specifically, in a converter, electric furnace, or ladle refining device, nitrogen gas is blown into molten steel with a C concentration of 0.5 mass% or more and/or an Al concentration of 0.005 mass% or more to raise the nitrogen concentration in the molten steel to 120 ppm or more. After raising the temperature to 100%, Ar or Ar+Nitrogen is blown into the RH type vacuum degassing equipment for 30 minutes or more, followed by denitrification and deoxidation treatment.Then, the molten steel is continuously cast, and the resulting slab is heated. It is disclosed that a method for manufacturing a bearing material is proposed, which is characterized in that during rolling, the soaking time is 15 hours or less at 1200° C. or higher.

Patent Document 4 discloses that in a vacuum degassing treatment method for molten steel, the amount of deoxidation products can be minimized at the molten steel melting stage by performing the treatment at an optimal post-deoxidation time, and the amount of deoxidation products can be attributed to the deoxidation products. The aim is to produce clean steel with minimal inclusions.
Specifically, alloying elements are added to molten steel during vacuum degassing treatment to adjust the composition, and then by ensuring the floating time of inclusions, the amount of inclusions and nitrogen and silicon content in molten steel can be reduced when manufacturing clean steel. , the amount of oxides in the slag is measured, the floating amount of primary deoxidation products and the amount of secondary oxidation products generated are quantified, and based on this, the amount of inclusions in molten steel (the amount of floating residue of primary deoxidation products) is determined. amount) + (secondary oxidation product), compare this with the target allowable upper limit amount, and determine the molten steel stirring/swinging treatment time after adding the deoxidizer according to the difference. Disclosed.

JP2013-224461A Japanese Patent Application Publication No. 2000-034513 Japanese Patent Application Publication No. 2005-272953 Japanese Unexamined Patent Publication No. 4-045220

By the way, in Patent Document 1, in the first half of the treatment process, argon gas is blown alone or a mixed gas of argon gas and nitrogen gas is blown. However, the present invention is directed to adjusting the nitrogen concentration in steel within a predetermined range. Therefore, with the technique of the same document, it may be difficult to increase [N] to a predetermined [N] range.
In Patent Document 2, for example, when applying [N] to 150 ppm, it is necessary to significantly increase the pressure within the vacuum chamber. In that case, there is a possibility that the amount of molten steel that is returned to the molten steel will be greatly reduced, or that the molten steel will not rise into the vacuum chamber and cannot be recirculated. In other words, RH processing becomes impossible.

In Patent Document 3, the purpose of nitrogen gas blowing is cleaning, 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]. (keeping the nitrogen concentration in steel within a predetermined range) is extremely difficult.
Patent Document 4 only determines the processing time for cleaning after measuring [N], and there is no disclosure or suggestion regarding the control of [N], making it extremely difficult to control [N]. It is.

That is, in the RH treatment, when producing nitrogen-containing steel that requires cleanliness, it is necessary to adjust the nitrogen concentration and cleanliness in the steel in order to satisfy the hardness, toughness, strength, etc. of the steel material.
Therefore, in view of the above-mentioned problems, the present invention aims to improve the nitrogen content of nitrogen-containing steel by changing the RH treatment conditions according to the nitrogen concentration in the steel during RH treatment when producing highly clean nitrogen-containing steel. A method for manufacturing highly clean nitrogen-containing steel that can accurately adjust the concentration within a target predetermined range and improve the cleanliness of steel materials, and a nitrogen-containing steel manufactured by the method. The purpose is to provide

In order to achieve the above object, the following technical measures were taken in the present invention.
The method for producing highly clean nitrogen-containing steel according to the present invention includes RH vacuum degassing when producing nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70 ppm to 150 ppm. The process is characterized in that it is produced in accordance with the steps shown in (a1) to (b7) below.
(a1)
The relationship (1) between the change in the nitrogen concentration in the molten steel during the RH treatment and the RH treatment conditions is determined for the 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 the 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 processing conditions are set using the relationship (1) obtained in (a1) above.
(b4)
Using the relationship (2) obtained in (a2) above, the remaining RH treatment time tr and the nitrogen concentration in molten steel when carrying out the RH treatment based on the RH treatment conditions set in (b3) above are calculated as follows. The relationship with the time tm until the target nitrogen concentration falls outside the range set in (b1) is confirmed, and processing is performed under the RH processing conditions of (b3) above.
(b5)
When tr≧tm, the nitrogen concentration in the molten steel is checked at the time Tm when the nitrogen concentration in the molten steel deviates from the target nitrogen concentration range set in (b1) above.
(b6)
If tr<tm, the process is continued under the RH process condition 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) above.

Preferably, when producing highly clean nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70 ppm to 150 ppm, the following (c1) to It is best to melt it using the procedure shown in (d4).
(c1)
The relationship (3) between the amount of change Δ[N] in the nitrogen concentration in the molten steel and the cleanliness of the molten steel is determined.
(d1)
The RH treatment process is divided into stage I: cleaning section and stage II: [N] adjustment section.
(d2)
Processing times for the I period and the II period are set.
(d3)
Using the relationship (3) obtained in (c1) above, set the necessary Δ[N] in the I period, and set the target nitrogen concentration in the molten steel in the I period and the range of the target nitrogen concentration. do.
(d4)
In each of the I period and the II period, the RH treatment is performed on the molten steel according to the steps shown in (b1) to (b7) above.

The nitrogen-containing steel according to the present invention is characterized in that it is manufactured according to the procedure of the above-described method for producing highly clean nitrogen-containing steel.

According to the present invention, when producing highly clean 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 is reduced to a target predetermined range. The cleanliness of the steel material can be improved.

1 is a diagram schematically showing an outline of an RH type vacuum degassing processing apparatus. FIG. 3 is a diagram summarizing data used to derive relationship (2). FIG. 3 is a diagram summarizing data used for deriving relationship (3). FIG. 3 is a diagram showing the transition of predicted nitrogen concentration in molten steel. FIG. 3 is a diagram showing the transition of predicted nitrogen concentration in molten steel. FIG. 3 is a diagram showing the transition of predicted nitrogen concentration in molten steel. FIG. 3 is a diagram showing the transition of predicted nitrogen concentration in molten steel. FIG. 3 is a diagram showing the control accuracy of nitrogen concentration in molten steel according to the present invention. FIG. 2 is a flowchart of a method for producing highly clean nitrogen-containing steel of the present invention. FIG. 2 is a flowchart of a method for producing highly clean nitrogen-containing steel of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a method for manufacturing highly clean nitrogen-containing steel according to the present invention and a nitrogen-containing steel manufactured by the method will be described with reference to the drawings.
Note that the embodiment described below is an example of embodying the present invention, and the configuration of the present invention is not limited to the 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 include increasing the nitrogen concentration in the steel before the start of the RH process, and denitrifying it in the RH process. By the nitrogen bubbles generated when the molten steel is heated, inclusions in the molten steel 2 are floated and separated, thereby promoting cleaning. Furthermore, by changing the RH processing conditions according to the nitrogen analysis value during the RH processing, it is possible to control both the target nitrogen concentration and the amount of denitrification during cleaning.

Specifically, the present invention is a technology implemented when melting nitrogen-containing steel containing 0.03% by mass or more of [C] and having 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, the molten steel 2 generates CO gas while being decarburized during the treatment, so that it is also denitrified. Therefore, the technique of 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.

By the way, a target nitrogen concentration range for the nitrogen concentration in molten steel is determined in advance according to the hardness, toughness, strength, etc. required for each type of steel, and it is necessary to keep the nitrogen concentration within this target range. In addition, if the component concentration after melting does not meet the specifications, it will be discarded.
Further, in the RH treatment (vacuum degassing treatment), the composition adjustment and degassing treatment of the molten steel 2 are mainly performed, but in addition to this, the nitrogen concentration in the molten steel 2 may be adjusted.

FIG. 1 schematically shows an outline of an RH type vacuum degassing treatment apparatus 1 (general RH vacuum degassing process).
As shown in FIG. 1, the RH type vacuum degassing apparatus 1 includes a ladle 3 into which molten steel 2 is charged, and a vacuum chamber 4 in which the molten steel 2 is degassed in a vacuum state. ing. Two immersion pipes 5 are provided at the lower part (bottom) of the vacuum tank 4 to be immersed in the molten steel 2 in the ladle 3. A gas blowing tube 6 is provided on one side of the immersion tube 5 for blowing gas into the molten steel 2 heading toward the vacuum tank 4. Furthermore, an exhaust port 7 is provided at the top of the vacuum chamber 4 to communicate with the outside and to exhaust the gas in the vacuum chamber 4 to the outside.

In performing the vacuum degassing treatment, 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 gas blowing pipe 6, and the gas inside the vacuum chamber 4 is exhausted to the outside through the exhaust port 7 to keep the inside of the vacuum chamber 4 in a substantially vacuum state. is circulated between the vacuum chamber 4 and the ladle 3. At this time, an alloy or the like is supplied to the molten steel 2 in order to adjust the components of the molten steel 2. In this way, the RH vacuum degassing process is performed. Note that the RH type vacuum degassing treatment apparatus 1 has a large stirring force on the molten steel 2 and has a large agglomeration flotation effect on inclusions in the molten steel 2, so it is often used for melting clean steel.

In the method for producing highly clean nitrogen-containing steel of the present invention, steel is produced by the following steps (a1) to (b7) in the RH vacuum degassing step.
(a1)
The relationship (1) between the change in the nitrogen concentration in the molten steel during the RH treatment and the RH treatment conditions for the molten steel 2 in the processing apparatus 1 is determined.

However, how to obtain the "relationship (1)" for estimating the nitrogen concentration in molten steel will be explained in detail in Examples below.
The nitrogen concentration ([N]) in molten steel during RH treatment depends on the RH processing conditions (type of blown gas, gas flow rate, degree of vacuum) and molten steel composition, and changes in [N] behavior, that is, acceleration [N] rate and de[N] ]The speed etc. will be different.

The higher the flow rate of nitrogen gas and the higher the pressure inside the vacuum chamber 4, the higher the apparent equilibrium [N] becomes. On the other hand, the smaller the flow rate of nitrogen gas and the lower the pressure inside the vacuum chamber 4, the lower the apparent equilibrium [N] concentration becomes. In addition, the equilibrium [N] concentration, the [N] addition rate, the [N] removal rate, etc. change depending on the molten steel composition.
As described above, there are many factors that affect [N] behavior, so in order to control [N], it is important to accurately estimate [N] behavior.
(a2)
Calculate 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 the actual operation, and then calculate the relationship between the difference and the RH treatment time (2 ).

However, how to obtain "Relationship (2)" indicating the variation in the nitrogen concentration in molten steel during the RH treatment time will be explained in detail in Examples below.
In actual operation, nitrogen behavior differs due to variations in processing caused by, for example, the ultimate vacuum in the vacuum chamber 4 of the RH treatment device 1 and the amount of air leakage from the immersion tube. There may be a discrepancy between the [N] value and the [N] value.

Therefore, the variation in [N] behavior within the operating range of normal RH processing should be understood in advance. In other words, the time dependence of the difference (deviation) between the estimated [N] value and the operational performance [N] value after the start of the RH treatment is quantified for each RH treatment condition and for each molten steel composition. put.
(b1)
A target nitrogen concentration in the molten steel 2 and a range of the target nitrogen concentration are set.

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.
(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 a manner common to those skilled in the art. At the initial stage of processing, processing conditions are determined by alloy addition, oxygen heating, etc., so sampling is performed at the timing when these operations are completed and [N] control is started.
(b3)
The RH processing conditions are set using the relationship (1) obtained in (a1) above.

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

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

When tr≧tm, that is, when [N] is outside the range of the target nitrogen concentration, the molten steel 2 is sampled at the timing of time Tm when [N] is outside the range of the target nitrogen concentration.
(b6)
On the other hand, if tr<tm, the process continues under the RH process conditions set in (b3) above.
If tr<tm, that is, [N] is within the target range, the RH process is continued until the process is completed.
(b7)
Perform steps (b3) to (b6) one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in (b1).

That is, by repeating (b3) to (b6) and modifying the RH treatment conditions each time if necessary, [N] can be reliably controlled within the target nitrogen concentration range.
Furthermore, when producing highly clean nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70ppm to 150ppm, the following (c1) to (d4) are applied in the RH vacuum degassing process. ).

Note that the presence of inclusions in the molten steel 2 causes a decrease in fatigue life, so it is better to have as few inclusions as possible.
(c1)
The relationship (3) between the amount of change in nitrogen concentration in molten steel Δ[N] and the cleanliness of molten steel 2 is determined.
However, how to obtain "Relationship (3)" indicating the amount of [N] removed and the degree of reduction of inclusions will be explained in detail in the Examples described later.

Near the surface of the molten steel 2 in the vacuum chamber 4 of the RH treatment device 1, nitrogen gas bubbles are generated during de[N]. The generated bubbles trap inclusions and float the inclusions to promote separation, thereby improving the cleanliness of the molten steel 2.
For example, the larger the amount of [N] removed (Δ[N]), the larger the number of bubbles of nitrogen gas generated and the larger the bubble diameter, which makes it easier to capture inclusions and the greater the cleaning effect. Become.

Indicators of cleanliness include the number of inclusions, TO concentration, etc., but are not particularly limited as long as they are indicators related to cleanliness.
(d1)
The RH treatment process is divided into stage I: cleaning section and stage II: [N] adjustment section.
In the present invention, since the RH process is performed while changing the target [N], it is preferable to divide the RH process into two sections and determine the RH process conditions for each section.

First, in the I period, the RH process is performed so that the amount of [N] removed based on the relationship (3) obtained in (c1) is obtained, and the cleaning process is performed. Next, in the II period, RH processing is performed so that the post-processing [N] can be controlled to the target [N] or within the target [N] range.
(d2)
Set the processing time for period I and period II.

The RH processing time is determined by the front process (converter, secondary refining, etc.) and the post process (secondary refining, continuous casting, etc.). Note that when extending the RH treatment time, it is necessary to delay the start time of the post-process. For example, in a continuous casting process, it is necessary to take measures such as slowing down the casting speed. If the countermeasures are not followed, it may result in continuous breakage (interruption of continuous casting), resulting in a decrease in productivity and yield.

That is, it is necessary to set the processing times for the I period and the II period so that the RH process can be completed at a predetermined time and the roles of the I period and II period can be performed.
Possible methods for setting the processing time include, for example, setting the processing time based on relationship (1), or setting the processing time based on past results, but which setting method should be used? I don't mind.
(d3)
Using the relationship (3) obtained in (c1) above, set the necessary Δ[N] in the I period, and calculate the target nitrogen concentration in the molten steel in the I period (target [N]) and the target nitrogen concentration in the I period. Set the concentration range.

In order to set the target [N] in the I period, Δ[N] (the amount of [N] removed) is set. In order to achieve that Δ[N], set the target [N] and target [N] range for the I period.
Possible methods for setting the target [N] and target [N] range for period I include, for example, setting it from [N] in molten steel analyzed during RH treatment, or setting it from past results. Any setting method may be used.
(d4)
In each of the I stage and II stage, RH treatment is performed on the molten steel 2 according to the procedures shown in (b1) to (b7).

In other words, [N] control is performed according to the procedures shown in (b1) to (b7) so that the target [N] for each of the I period and the II period is achieved.
[Example]
Examples carried out in accordance with the method for producing highly clean nitrogen-containing steel of the present invention and comparative examples carried out for comparison with the present invention will be described below.

The implementation conditions in this example are as follows.
Table 1 shows the implementation conditions in this example.

First, the derivation of relation (1) will be explained in detail.
In this example, the nitrogen concentration calculation model disclosed in Japanese Patent No. 5,836,187 was used as the relationship (1).
The details of the nitrogen concentration calculation model are shown below. Note that the relationship (1) is not limited to the nitrogen concentration calculation model illustrated below. For example, relationships derived from other models or actual values may be used.
<Nitrogen concentration calculation model>
The nitrogen concentration in the molten steel in the RH treatment can be calculated from the mass balance in the ladle 3 to the vacuum chamber 4, and is calculated using the equation [1] that shows the change in the nitrogen concentration in the molten steel in the vacuum chamber 4 and the concentration in the molten steel in the ladle 3. It is expressed by equation [2] which shows the change in nitrogen concentration.

here,
[N] _L : [N] concentration in ladle 3 (mass%)
[N] _V : [N] concentration in vacuum chamber 4 (mass%)
V _L : Volume of molten steel in ladle (m ³ )
V _V : Volume of molten steel in vacuum chamber (=3m ³ )
t: Time (min)
R _S : [N] removal rate (%/min) at the bath surface of molten steel 2
R _Ar : De[N] rate at the reflux Ar gas bubble interface (%/min)
R _N2 : Addition [N] or de[N] rate at the reflux _N2 gas bubble interface (%/min)
R _leak : absorption [N] rate (%/min) due to atmospheric intrusion from submerged tube 5
It is.

Note that the recirculation flow rate Q (m ³ /min) of the molten steel 2 is calculated using the formula [3] described in the reference document: "Kuwabara et al.: Tetsu-to-Hagane, 73 (1987) S176".

here,
Q _g : Blowing gas flow rate (NL/min)
D: Diameter of immersion tube 5 (m)
P ₀ : Pressure at the reflux gas injection position (atm)
P _V : Pressure inside vacuum chamber 4 (atm)
ρ _Fe : Molten steel density (=7ton/m ³ )
It is.

Assuming that R _S is a rate-determining mixture of the mass transfer of N on the molten steel 2 side and the chemical reaction of N on the bath surface of the molten steel 2, R _S is expressed as the following equation [4].

here,
A _S : Reaction interface area on 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 inside the vacuum chamber 4
k _m : Mass transfer coefficient of N in molten steel 2 (m/min)
k _r :N chemical reaction rate constant (m/(min/%))
When [N] _i,S is deleted from the formula [4] and rearranged, R _S is expressed as the formula [5].

Similarly, R _Ar is expressed as the following equation [6], assuming that the mass transfer of N on the molten steel 2 side and the chemical reaction of _N at the Ar gas bubble interface are rate-determining mixtures.

here,
A _Ar : Reaction interfacial area on Ar bubble surface (m ² )
[N] _e,Ar : [N] concentration (%) in equilibrium at the Ar bubble interface
It is.
R _N2 can be expressed in the same way, but the N ₂ partial pressure inside the bubble decreases from the N ₂ injection position toward the bath surface of the molten steel 2 in the vacuum chamber 4. N] have different speeds. Therefore, near the N ₂ injection position, [N] is added, and near the bath surface of the molten steel 2, [N] is removed.

Therefore, the difference between the acceleration [N] rate at the blowing position and the de-[N] rate at the bath surface of molten steel 2 is considered to be the acceleration [N] or de-[N] rate at the entire _N2 bubble interface ([7 ]formula).

here,
A _N2 : Reaction interfacial area on the surface of _N2 bubbles (m ² )
[N] _e,O,N2 : [N] concentration (%) equilibrated at the _N2 bubble interface at the injection position
[N] _e,S,N2 : [N] concentration (%) in equilibrium at the _N2 bubble interface on the bath surface of molten steel 2
It is.

Furthermore, [N] _e,S is obtained from equation [8].

here,
P _N2,S : _N2 partial pressure at the bath surface of molten steel 2 (atm)
f _N :Activity coefficient of N
T: Molten steel temperature (K)
R: Gas constant.

Note that P _N2,S is the same as the degree of vacuum PV in the vacuum chamber 4, and P _N2,S =P _V.
[N] _e,Ar : The [N] concentration (%) that equilibrates at the Ar bubble interface is also found from equation [9], but since the nitrogen partial pressure P _N2,Ar in the Ar bubble is 0, the result is As, [N] _e,Ar =0.

[N] _{e, O, and N2} are obtained from formula [10]. Note that P _0,N2 in the formula [10] is calculated from the formula [12], considering that the static pressure of the molten steel 2 from the bath surface to the injection position is added to the pressure in the vacuum chamber 4.
Moreover, [N] _{e, S, N2} is obtained from equation [11]. Note that P _S,N2 is the same as the pressure inside the vacuum chamber 4, and P _S,N2 =P _V.

here,
l: Distance from the bath surface of molten steel 2 to the injection position (m)
g: Gravitational acceleration (=9.8m/sec ² )
It is.
Further, the nitrogen activity coefficient f _N shown in equations [8] to [12] is determined from equation [13].

e _N ^j is the interaction coefficient for component j of N, and the values in Table 2 (Reference: "Japan Society for the Promotion of Science, 19th Committee on Steelmaking, Recommended Equilibrium Values for Steelmaking Reactions, 1984") were used. . Note that [%j] is the concentration (mass%) of component j.
Table 2 shows the interaction coefficients for component j of N, O, and S.

The chemical reaction rate constant (m/(min/%)) of k _r :[N] is determined from the formula [14] described in the reference document: "Harashima et al.: Tetsu to Hagane, 73 (1987) 1559".

f _O is the activity coefficient of O, and is obtained from equation [15] in the same way as equation [13]. Further, f _S is the activity coefficient of S, and is obtained from equation [16] in the same way as equation [13].

For e _N ^j and e _S ^j , the values in Table 2 were used as interaction coefficients for component j of O and S, respectively.
Furthermore, A _Ar and A _N2 were determined from equations [17] and [18], assuming that they are proportional to the 2/3 power of Q _gAr and Q _gN2 , respectively. Note that α _Ar and α _N2 are respective proportionality constants.

here,
Q _gAr : Ar gas flow rate (Nm ³ /min)
Q _gN2 : Injection _N2 gas flow rate (Nm ³ /min)
It is.
The [N] absorption rate R _leak (%/min) due to air entering from the submerged tube 5 was determined from equation [19]. Regarding Q _leak , although the inner diameter and flange structure of the immersion pipe 5 are different, we used 0.17Nm ³ /min derived in the reference document: "Kato et al.: Tetsu to Hagane, 83 (1997) 18". there was. Note that M _N2 is the molecular weight of N (=28).

In the calculation process described above, the unknown values are _km , α _S , α _Ar , and α _N2 . Considering that N moves quickly in molten steel and is not rate-limiting for mass transfer, _km was fixed at a large value of 10,000. α _S , α _Ar , α _N2 are calculated using the least squares method, and the calculated values are based on the degree of vacuum, presence or absence of oxygen heating, and gas type. I asked him to come closer. In determining these values, the values in Table 3 were used.

Table 3 shows the parameter values used.

Next, the derivation of relation (2) will be explained in detail.
FIG. 2 shows a graph summarizing the data used to derive relationship (2).
The RH treatment was performed under constant RH treatment conditions, and the nitrogen concentration in the molten steel was analyzed at arbitrary timing. Further, the nitrogen concentration in molten steel at the same timing was calculated from relationship (1).
As shown in FIG. 2, the relationship between the difference (deviation) between the estimated [N] value and the actual operation [N] value in relationship (1) and the RH processing time was determined.

Here, an approximate straight line passing through the origin and using the maximum value of each processing time so that the slope becomes the maximum (in this example, an approximate straight line was calculated using the maximum value of 11 minutes, 17 minutes, and 19 minutes). ), relationship (2) was obtained in which the deviation (dispersion) increases at 0.93 ppm/min.
Note that regarding relationship (2), FIG. 2 is an example, and the relationship is not limited to this.
Furthermore, the derivation of relation (3) will be explained in detail.

FIG. 3 shows a graph summarizing the data used to derive relationship (3).
As shown in FIG. 3, relationship (3) was determined from the amount of change Δ[N] (ppm) in the nitrogen concentration in molten steel and the inclusion number reduction rate constant (/min). However, Δ[N] was the difference between the nitrogen concentration in the molten steel before treatment and the nitrogen concentration in the molten steel after 10 minutes of RH treatment.
Regarding the number of inclusions, a sample of molten steel 2 was taken from inside the ladle 3 during the RH treatment, and the number of oxide-based inclusions of 5 μm or more per 1 cm ² was evaluated using FE-EPMA on the microscopic surface of the sample. did.

In addition, regarding the inclusion number reduction rate constant k (/min), the RH treatment time is t (min), the number of inclusions before treatment is x ₀ (pieces/cm ² ), and the treatment time is t (min). The number of inclusions is x _t (pieces/cm ² ),
k at RH 10 minutes was calculated using the following formula.
k=-1/t×ln(x _t /x ₀ )
In this way, relationship (3) was obtained in which the inclusion number reduction rate constant k becomes large when Δ[N]=-28 ppm or less.

Note that regarding relationship (3), FIG. 3 is an example, and the relationship is not limited to this.
When producing nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70ppm to 150ppm, the following steps (a1) to (b7) are performed in the RH vacuum degassing process. In this example, a nitrogen-containing steel containing 0.20% by mass of [C] and a target [N] of 100 ppm was produced. Further, the steps to be carried out are as follows: converter - RH treatment - continuous casting process, and in this example, RH treatment was carried out.
(a1)
The relationship (1) between the change in the nitrogen concentration in the molten steel during the RH treatment and the RH treatment conditions for the molten steel in the processing apparatus 1 is determined.

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

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

In this example, using relationship (1), the nitrogen gas flow rate was set to 12 NL/min/ton and the degree of vacuum was set to 0.5 Torr so that the [N] transition fell within the target [N] range. In addition, considering the extension of the [N] removal time due to troubles, etc., the target was set at 110 ppm, which is higher than the target [N] = 100 ppm.
(b4)
Using the relationship (2) obtained in (a2) above, the remaining RH treatment time tr and the nitrogen concentration in molten steel when RH treatment is performed based on the RH treatment conditions set in (b3) are ( Check the relationship with the time tm until the target nitrogen concentration falls out of the range set in b1), and perform processing under the RH processing conditions of (b3).

FIG. 4 shows the predicted change in nitrogen concentration in the molten steel 2.
As shown in FIG. 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 from relationship (2). Note that the RH treatment was performed at a nitrogen gas flow rate of 12 NL/min/ton and a vacuum degree of 0.5 Torr.
(b5)
If tr≧tm, check the nitrogen concentration in the molten steel at the time Tm when the nitrogen concentration in the molten steel falls outside the range of the target nitrogen concentration set in (b1).

As shown in FIG. 4, this time, since tr(=24min)≧tm(=19min), the [N] in the molten steel was analyzed at the time Tm=34min when it was out of the target [N] range. As a result, [N] in the molten steel was 122 ppm.
(b7)
Perform steps (b3) to (b6) one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in (b1).

In this example, since RH processing time remained, steps (b3) to (b6) were performed for the second time.
<Second (b3)>
Using relationship (1), the argon gas flow rate was set to 6 NL/min/ton and the degree of vacuum was set to 0.5 Torr so that the [N] transition was within the target [N] range.

Note that if the argon gas flow rate is less than 6NL/min/ton, poor circulation may occur, and the nitrogen gas flow rate cannot be lowered. The target was =105ppm.
<Second (b4)>
FIG. 5 shows the predicted change in nitrogen concentration in the molten steel 2.

As shown in FIG. 5, in this example, the remaining processing time tr was 5 min, and the time tm until the target [N] range was exceeded was 10 min from relationship (2). Note that the RH treatment was performed at an argon gas flow rate of 6 NL/min/ton and a degree of vacuum of 0.5 Torr.
<Second (b6)>
As shown in Figure 5, in the second time, tr(=5min)<tm(=10min), so the process was continued under the RH process condition of (b3), and the RH process was completed at a process end time of 39min. . In addition, when [N] in the molten steel was analyzed after the treatment, it was found to be 99 ppm, and the difference from the target [N] = 100 ppm was 1 ppm.

Furthermore, when producing highly clean nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70ppm to 150ppm, the following (c1) to (d4) are applied in the RH vacuum degassing process. ) In this example, a highly clean nitrogen-containing steel containing 0.40% by mass of [C] and a target [N] of 110 ppm was produced. Further, the steps were as follows: converter - LF treatment - RH treatment - continuous casting process, and in this example, RH treatment was performed.
(c1)
The relationship (3) between the amount of change in nitrogen concentration in molten steel Δ[N] and the cleanliness of molten steel 2 is determined.

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

It has been confirmed from past results that if it takes 10 minutes to adjust the nitrogen, it is possible to sufficiently add [N] from the low [N] range to the target [N]. Further, in order to improve the cleanliness of the steel, it is preferable to take as long a time as possible for cleaning.
As a result of adjusting the start time of continuous casting, it was possible to secure an RH treatment time of 41 min. As a result, the I period was set to 31 min, and the II period was set to 10 min.
(d3)
Using the relationship (3) obtained in (c1) above, set the necessary Δ[N] in the I period, and set the target nitrogen concentration in molten steel in the I period and the range of the target nitrogen concentration. .

In this example, Δ[N] was set to -28 ppm using relationship (3).
Note that the target nitrogen concentration and the range of the target nitrogen concentration were set from [N] in the molten steel during treatment, as described in (b1) and (b2) of period I shown below.
(d4)
In each of the I stage and II stage, RH treatment is performed on the molten steel 2 according to the procedures shown in (b1) to (b7).

Regarding stage I, the details are as follows.
<(b1),(b2) of stage I>
The RH treatment was started at a nitrogen gas flow rate of 12 NL/min/ton and a vacuum degree of 30 Torr. In addition, [N] in the molten steel was analyzed 12 minutes after the alloy addition. As a result, [N] in the molten steel was 136 ppm.

In addition, in order to ensure that the Δ[N] to the upper limit of the target [N] satisfies -28 ppm or less, the target [N] of the I period is 80 ppm, and the standard range of the target [N] range is 60 to 40 ppm. It was set at 100ppm.
<I stage (b3)>
Using relationship (1), the nitrogen gas flow rate was set to 12 NL/min/ton and the degree of vacuum was set to 0.5 Torr so that the target [N] of the I period was 80 ppm at the end of the I period.
<I stage (b4)>
FIG. 6 shows the predicted change in nitrogen concentration in the molten steel 2.

As shown in FIG. 6, in the I period, the remaining processing time tr was 19 min, and the time tm until it was out of the target [N] range was 19 min from relationship (2). Note that the RH treatment was performed at a nitrogen gas flow rate of 12 NL/min/ton and a vacuum degree of 0.5 Torr.
<I stage (b5)>
As shown in Figure 6, in this I period, tr(=19min)≧tr(=19min), so the [N] in the molten steel was analyzed at the time Tm=31min when it was out of the target [N] range. . As a result, [N] in the molten steel was 79 ppm.
<I stage (b7)>
Since there was no remaining processing time and the actual result was 79 ppm against the target [N] of 80 ppm, the I period was ended. In addition, in stage I, steps (b3) to (b6) were performed once.

Regarding stage II, the details are as follows.
<Stage II (b1)>
The target [N] for period II was set at 110ppm. Additionally, the target [N] range was set at the standard range of 90 to 130 ppm.
<Stage II (b2)>
[N] in the molten steel was analyzed at the end of the I period. As a result, [N] in the molten steel was 79 ppm. Note that this is stated in Stage I (b5).
<Stage II (b3)>
Using relationship (1), the nitrogen gas flow rate was set to 6NL/min/ton and the degree of vacuum was set to 30 Torr so that the [N] transition was within the target [N] range.

Note that if the nitrogen gas flow rate is less than 6NL/min/ton, poor circulation may occur and the nitrogen gas flow rate cannot be lowered, so the predicted [N ]=120ppm was targeted.
<(b4) of stage II>
FIG. 7 shows the predicted change in nitrogen concentration in the molten steel 2.

As shown in FIG. 7, in the II period, the remaining processing time tr was 10 min, and the time tm until the target [N] range was exceeded was 10 min from relationship (2). The treatment was carried out at a nitrogen gas flow rate of 6NL/min/ton and a vacuum degree of 30Torr.
<Stage II (b5)>
In this II period, since tr(=10min)≧tr(=10min), the [N] in the molten steel was analyzed at the time Tm=41min when it was out of the target [N] range. As a result, [N] in the molten steel was 109 ppm.
<(b7) of stage II>
Since there was no remaining processing time and the actual result was 109 ppm against the target [N] of 110 ppm, period II was ended. That is, the RH process has ended. However, in stage II, steps (b3) to (b6) were performed once.

Here, when the number of oxide inclusions in the molten steel after the RH treatment was evaluated, it was found to be 1.1 inclusions/cm ² .
The effects of the present invention will be described.
The nitrogen concentration in the molten steel after treatment is as follows.
Regarding the nitrogen concentration in molten steel, a target nitrogen concentration control range is determined in advance according to the hardness, toughness, strength, etc. required for each type of steel, and it is necessary to keep the nitrogen concentration within this range.

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.
As shown in FIG. 8, according to the present invention, the variation σ of the difference between the target nitrogen concentration in molten steel and the nitrogen concentration in molten steel after RH treatment was reduced from 17.0 ppm to 10.6 ppm. Additionally, the probability that the nitrogen concentration in molten steel would deviate from the target nitrogen concentration range (±25 ppm) (derailment rate) improved from 14.1% to 1.8%.

The number of inclusions after the RH treatment is as follows.
Since inclusions serve as starting points for cracks, fractures, fatigue failures, etc. that occur when steel materials are processed, it is better to reduce inclusions as much as possible.
For example, Japanese Patent Application Laid-Open No. 11-001749 discloses that although the target inclusions are different, when the number of inclusions exceeds approximately 20 inclusions/cm ² , bending fatigue strength and rolling fatigue strength decrease. For example, Japanese Patent No. 4718359 discloses that wire drawability deteriorates when the number of inclusions exceeds 20 inclusions/cm ² , although the target inclusions are different.

Taking this into consideration, in the present invention, the number of inclusions after the RH treatment was between 0.9 and 12.2 inclusions/cm ² , indicating a high degree of cleanliness without degrading the above-mentioned properties.
Finally, the method for producing highly clean nitrogen-containing steel of the present invention is summarized as follows.
FIG. 9 shows a flowchart of the method for producing highly clean nitrogen-containing steel of the present invention.
The method for producing highly clean nitrogen-containing steel of the present invention includes an RH vacuum degassing process when producing nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70 ppm to 150 ppm. In this step, the solution is prepared using the steps shown in (a1) to (b7) below.
(a1)
The relationship (1) between the change in the nitrogen concentration in the molten steel during the RH treatment and the RH treatment conditions for the molten steel 2 in the processing apparatus 1 is determined.
(a2)
The difference between the nitrogen concentration in molten steel estimated from relationship (1) and the nitrogen concentration in molten steel in the actual operation is calculated, and the relationship (2) between the difference and the RH treatment time is determined.
(b1)
A target nitrogen concentration in the molten steel 2 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 processing conditions are set using the relationship (1) obtained in (a1) above.
(b4)
Using the relationship (2) obtained in (a2) above, the remaining RH treatment time tr and the nitrogen concentration in molten steel when RH treatment is performed based on the RH treatment conditions set in (b3) are calculated as ( Check the relationship with the time tm until the target nitrogen concentration falls out of the range set in b1), and perform processing under the RH processing conditions of (b3).
(b5)
If tr≧tm, check the nitrogen concentration in the molten steel at the time Tm when the nitrogen concentration in the molten steel is out of the range of the target nitrogen concentration set in (b1).
(b6)
If tr<tm, the process is continued under the RH process condition (b3).
(b7)
Perform steps (b3) to (b6) one or more times until the nitrogen concentration in the molten steel satisfies the target nitrogen concentration set in (b1).

FIG. 10 shows a flowchart of the method for producing highly clean nitrogen-containing steel of the present invention.
Furthermore, the method for producing highly clean nitrogen-containing steel of the present invention includes the following steps: In the RH vacuum degassing step, in addition to the above, it is preferable to carry out melt preparation by the following procedures (c1) to (d4). (c1)
The relationship (3) between the amount of change in nitrogen concentration in molten steel Δ[N] and the cleanliness of molten steel 2 is determined.
(d1)
The RH treatment process is divided into stage I: cleaning section and stage II: [N] adjustment section.
(d2)
Set the processing time for period I and period II.
(d3)
Using the relationship (3) obtained in (c1) above, set the necessary Δ[N] in the I period, and set the target nitrogen concentration in molten steel 2 in the I period and the range of the target nitrogen concentration. do.
(d4)
In each of Stage I and Stage II, RH treatment is performed on the molten steel according to the procedures shown in (b1) to (b7).

The nitrogen-containing steel of the present invention is preferably manufactured according to the procedure of the above-described method for producing highly clean nitrogen-containing steel.
According to the method for producing highly clean nitrogen-containing steel of the present invention, when producing highly clean nitrogen-containing steel, by changing the RH treatment conditions according to the nitrogen concentration in the steel during the RH treatment, nitrogen-containing steel It is possible to precisely adjust the nitrogen concentration in the steel within a target predetermined range, and to improve the cleanliness of the steel material.

It should be noted that the embodiments disclosed herein are illustrative in all respects and should not be considered 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 those skilled in the art. Rather, values that can be easily assumed by a person skilled in the art are used.

1 RH type vacuum degassing equipment (RH processing equipment)
2 Molten steel 3 Ladle 4 Vacuum tank 5 Immersion pipe 6 Gas blowing pipe 7 Exhaust port

Claims (3)

When producing nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70ppm to 150ppm, the following steps (a1) to (b7) are performed in the RH vacuum degassing process. A method for producing high-purity nitrogen-containing steel, which is characterized in that it is produced by melting.
(a1)
The relationship (1) between the change in the nitrogen concentration in the molten steel during the RH treatment and the RH treatment conditions is determined for the 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 the 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 processing conditions are set using the relationship (1) obtained in (a1) above.
(b4)
Using the relationship (2) obtained in (a2) above, the remaining RH treatment time tr and the nitrogen concentration in molten steel when carrying out the RH treatment based on the RH treatment conditions set in (b3) above are calculated as follows. The relationship with the time tm until the target nitrogen concentration falls outside the range set in (b1) is confirmed, and processing is performed under the RH processing conditions of (b3) above.
(b5)
When tr≧tm, the nitrogen concentration in the molten steel is checked at the time Tm when the nitrogen concentration in the molten steel deviates from the target nitrogen concentration range set in (b1) above.
(b6)
If tr<tm, the process is continued under the RH process condition 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) above. When producing highly clean nitrogen-containing steel containing 0.03% by mass or more of [C] and a target nitrogen concentration of 70ppm to 150ppm, the following (c1) to (d4) are additionally required in the RH vacuum degassing process. The method for producing highly clean nitrogen-containing steel according to claim 1, characterized in that the steel is produced by the procedure shown in the following.
(c1)
The relationship (3) between the amount of change Δ[N] in the nitrogen concentration in the molten steel and the cleanliness of the molten steel is determined.
(d1)
The RH treatment process is divided into stage I: cleaning section and stage II: [N] adjustment section.
(d2)
Processing times for the I period and the II period are set.
(d3)
Using the relationship (3) obtained in (c1) above, set the necessary Δ[N] in the I period, and set the target nitrogen concentration in the molten steel in the I period and the range of the target nitrogen concentration. do.
(d4)
In each of the I period and the II period, the RH treatment is performed on the molten steel according to the steps shown in (b1) to (b7) above. Nitrogen-containing steel, characterized in that it is manufactured according to the procedure of the method for producing highly clean nitrogen-containing steel according to claim 1 or 2.

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