JP7744308B2 - High-nitrogen steel refining method - Google Patents

Description

The present invention relates to a technique for adjusting the [N] concentration in molten steel by injecting _N2 gas in a method for refining high-nitrogen steel.

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

In the molten steel processing process, molten steel undergoes processes such as ladle refining and vacuum degassing. The [N] concentration in molten steel during this process is controlled by nitriding (adding nitrogen) using _N2 gas bubbling. However, a denitrification reaction occurs during the vacuum degassing process. Therefore, in conventional techniques, achieving a high nitrogen concentration, such as [N] ≥ 150 ppm, is difficult using only _N2 gas reflux. Adjusting the [N] concentration in molten steel to [N] ≥ 150 ppm typically requires the addition of a nitride alloy. For example, Patent Document 1 (Patent Document 1) aims to achieve high-precision control of the [N] concentration in molten steel by specifying the reduced pressure and reflux gas flow rate (Ar or Ar + _N2 ) during the RH vacuum degassing process.

JP 2013-224461 A

Incidentally, when steel is produced using the "converter → ladle refining → RH treatment" process, a denitrification reaction can occur in the molten steel during the reduced pressure RH treatment aimed at dehydrogenation. For this reason, with conventional technology, it was impossible to adjust the [N] concentration in the molten steel to a high nitrogen range of [N] ≧ 150 ppm. To address this, it was necessary to make up for the deficiency in [N], which was added using expensive nitride alloys. This increased the costs associated with the production process.

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

In view of the above problems, the present invention aims to provide a method for refining high-nitrogen steel that can achieve a post-RH treatment [N] level of 150 ppm or greater by sufficiently increasing the [N] concentration in molten steel before the start of RH treatment, and by performing RH treatment under conditions that are not affected by the denitrification reaction that occurs during reduced pressure treatment in RH treatment and that do not require the addition of nitride alloys to the molten steel.

To achieve the above objectives, the present invention employs the following technical measures.

The method for refining high-nitrogen steel according to the present invention comprises carrying out a ladle refining process and an RH vacuum degassing process in which _N2 gas is circulated on molten steel tapped from a converter, wherein in the converter process and/or the ladle refining process, the molten steel is nitrogenized by stirring with _N2 gas, and the high-nitrogen steel having an [N] of 150 ppm or more after the RH process is refined , and the RH vacuum degassing process is characterized in that no nitride alloy is added to the molten steel , and the processing conditions are such that the value of index A calculated from at least "Equation (1) including, as variables, the values of the components of the molten steel before the RH vacuum degassing process" is 50 or more.

where [N] _ini (ppm): [N] concentration in molten steel before RH treatment
[N] _e,S (ppm): Equilibrium [N] concentration at the surface of the molten steel bath in the vacuum vessel
_kr,before (m/min/%): Interfacial reaction rate constant of the [N] reaction before S addition
_kr,after (m/min/%): Interfacial reaction rate constant of the [N] reaction after adding S
t (min): RH treatment time
t _s (min): Start time of S addition

The high-nitrogen steel refining method of the present invention makes it possible to achieve a post-RH treatment [N] level of 150 ppm or greater by sufficiently increasing the [N] concentration in the molten steel before starting the RH treatment and by performing the RH treatment under conditions that are not affected by the denitrification reaction that occurs during the reduced pressure treatment in the RH treatment and that do not require the addition of nitride alloys to the molten steel.

FIG. 1 is a diagram showing the state of the [N] concentration in molten steel in converter, LF treatment, and RH treatment when _N2 blowing nitriding was not performed in converter or LF treatment, as was done in the prior art, and a nitride alloy was added. FIG. 1 is a diagram showing the state of the [N] concentration in molten steel in converter, LF treatment, and RH treatment when nitriding by _N2 blowing was carried out in converter or LF treatment and a nitride alloy was added, as carried out by conventional technology. FIG. 1 is a diagram showing the state of [N] concentration in molten steel in converter, LF treatment, and RH treatment when nitriding by _N2 injection is carried out in converter or LF treatment, which is carried out by the refining method of high-nitrogen steel of the present invention, and the RH treatment conditions are set so that the index A is 50 or more. 1 shows the relationship between the index A for high [N] in RH treatment and the [N] concentration in molten steel after RH treatment in actual operation (Examples and Comparative Examples). 1 is a diagram showing a schematic overview of an RH type vacuum degassing treatment device.

An embodiment of the high-nitrogen steel refining method according to the present invention will be described below with reference to the drawings. Note that the embodiment described below is an example of how the present invention is embodied, and the configuration of the present invention is not limited to this specific example.

First, we will explain the overview of the RH-type vacuum degassing treatment device 1.

Figure 3 shows a schematic overview of an RH vacuum degassing treatment device 1 (a typical RH vacuum degassing treatment process).

As shown in Figure 3, the RH-type vacuum degassing treatment device 1 has a ladle 3 into which molten steel 2 is charged, and a vacuum chamber 4 that is placed in a vacuum state to degas the molten steel 2. Two immersion pipes 5 are provided at the lower part (bottom) of the vacuum chamber 4 and are immersed in the molten steel 2 in the ladle 3. A gas injection pipe 6 is provided on one side of the immersion pipes 5, which inject gas into the molten steel 2 flowing into the vacuum chamber 4. In addition, an exhaust port 7 is provided at the top of the vacuum chamber 4, which communicates with the outside and exhausts the gas inside the vacuum chamber 4 to the outside.

To perform vacuum degassing, first, the immersion tube 5 is immersed in the molten steel 2 in the ladle 3. Then, gas such as argon gas or nitrogen gas is blown in from the gas inlet tube 6, and the gas in the vacuum chamber 4 is exhausted to the outside from the exhaust port 7, creating a near-vacuum state inside the vacuum chamber 4, and the molten steel 2 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 to adjust the composition of the molten steel 2. In this manner, the RH vacuum degassing process is carried out.

Next, we will explain in detail the high-nitrogen steel refining method according to the present invention.

Generally, high-nitrogen steels, such as case-hardened steels, are sent to the ladle refining process after being tapped from a converter for high alloying and temperature adjustment. The high-nitrogen steels after ladle refining undergo a RH vacuum degassing (RH) process to remove [H], which can cause product defects and delayed fracture. The high-nitrogen steels after RH treatment are then sent to the continuous casting process. In this process, nitriding (adding nitrogen) to the molten steel 2 has traditionally been achieved by adding expensive nitriding alloys. Because the use of nitriding alloys increases production costs, a cost-reducing method is needed.

Therefore, in the present invention, by using _N2 as the stirring gas in the bubbling step of the RH treatment and by taking into consideration the denitrification rate in the RH vacuum degassing treatment step, it has become possible to produce high-nitrogen steel at low cost without using a nitride alloy.

In the method for refining high-nitrogen steel of this embodiment, molten steel 2 was processed in the following order: "converter" → "ladle refining" → "vacuum degassing + _N2 reflux step." However, "vacuum degassing + _N2 reflux step = _N2 reflux in RH treatment." The present invention specifies the conditions for nitriding by _N2 gas bubbling in the converter or ladle refining, and the conditions for suppressing the rate of denitriding under reduced pressure in the RH treatment.

In the present invention, _N2 gas bubbling may be performed in either or both of the converter and ladle refining process. Regarding the above, it is possible to inexpensively perform sufficient nitriding (adding nitrogen) to the molten steel 2 before the decompression treatment in the RH treatment.

In the present invention, in order to appropriately adjust the [N] concentration in molten steel, we have derived index A related to the [N] concentration in molten steel. In other words, index A is a value for making [N] ≧ 150 ppm, and we have found that index A ≧ 50 is preferable. In the present invention, taking into account the amount of nitrogen from the atmosphere that is naturally absorbed into molten steel 2, index A ≧ 50 is set to make [N] ≧ 150 ppm.

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

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

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

In addition, the higher the equilibrium nitrogen concentration ([N] _e,S ), the higher the N content after RH treatment, so it was placed in the numerator of equation (1). This equilibrium nitrogen concentration is mainly determined by the RH reflux conditions (degree of vacuum).

The second term of equation (1): denitrification rate:

_kr,before (m/min/%): Interfacial reaction rate constant of the [N] reaction before S addition _kr,after (m/min/%): Interfacial reaction rate constant of the [N] reaction after S addition t (min): RH treatment time _ts (min): Start time of S addition However, if S is not added during RH treatment, set _ts = t.

Regarding the basic equation for denitrification rate: -dN/dt = _kr × ([N]-[Ne]), it was assumed that the smaller the driving force at the beginning of the reaction ([N] _ini - [N] _e,S ), the slower the denitrification will be and the higher the nitrogen content will be after treatment, and this was placed in the denominator of equation (1).

In addition, the smaller the interface reaction rate constant _kr in the denitrification rate equation, the slower the denitrification and the higher the N content after RH treatment, so it was placed in the denominator of equation (1). In addition, since the value of _kr changes depending on the [S] concentration in the molten steel, it was multiplied by the distribution of the treatment time before and after S addition and placed in the denominator of equation (1).

A power exponent was set for each parameter, and the exponent of each term was determined so that the coefficient of determination of the approximation line with the [N] concentration in molten steel after RH treatment (=[N] _fin ) would be close to 1. The larger the exponent A, the higher [N] _fin . In this embodiment, the coefficient of determination of approximation of equation (1) for deriving the exponent A is set to 0.94.
<Regarding the formula for deriving the condition value in index A>
Equation (2) shows how to calculate the N equilibrium value at the surface of molten steel 2 in vacuum vessel 4. For equation (2), the formula used is that described in (Reference: Recommended Values for Steelmaking Reactions (Revised and Enlarged), edited by the 19th Committee on Steelmaking of the Japan Society for the Promotion of Science, Tokyo, (1984), 17.) However, the equilibrium N concentration [N] _e,S at the surface of molten steel 2 was calculated as P _N2 = P _v .

where P _N2 is the partial pressure of _N2 gas bubbles (atm)
P _v : Pressure inside the vacuum layer (atm)
ρm: Molten steel density (=7000kg/m ³ )
g: Gravitational acceleration (=9.8m/s ² )
l: _N2 gas injection point depth (m)
R: Gas constant (=8.314 J/K/mol)
T: Molten steel temperature (=1873K)
The interfacial reaction rate constant for the nitrogen conversion reaction is shown in equation (3). Note that the nitrogen conversion reaction is rate-determining. Equation (3) uses the equation described in (Reference: Tsugawa, Mizukami, Ueshima: Iron and Steel 84 (1998), p. 411).

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

<Other conditions>
The component values [M] _ini (%) before RH treatment are as follows:

・[C] _ini , [Mn] _ini , [Si] _ini , [Cr] _ini , [S] _ini , [O] _ini
C, Mn, Si, Cr, S, O, etc. are the main components that make up the steel. For the component value [M] _ini before RH treatment, the actual value before RH treatment was used. However, since O is not an analytical value, it was assumed to be 0.001%.

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

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

Figures 1A and 1B show the changes in [N] concentration in molten steel during each treatment step performed using conventional technology.

As shown in FIG. 1A, if there is no nitriding of molten steel 2 by blowing N ₂ in a converter or LF treatment, it is necessary to add a nitride alloy in an RH treatment to achieve [N]≧150 ppm, which is not suitable.

As shown in FIG. 1B, even if nitrogen is added to molten steel 2 by blowing N ₂ in a converter or LF treatment, denitrification occurs in the RH treatment, and therefore adding a nitride alloy is required to achieve [N]≧150 ppm, which is not suitable.

Figure 1C shows the change in the [N] concentration in molten steel during each processing step performed using the high-nitrogen steel refining method of the present invention.

As shown in Figure 1C, a sufficient amount of nitrogen is added to molten steel 2 in the converter-ladle refining process up to the RH treatment, and furthermore, in the RH treatment, by setting index A, which ensures high [N], to 50 or more, the amount of denitrification during reduced pressure treatment is suppressed, and stirring is performed using only _N2 gas, making it possible to achieve [N] of 150 ppm or more. In other words, by setting the RH treatment conditions so that index A ≥ 50, it becomes possible to suppress denitrification during the RH treatment.

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

As shown in Figure 2, a lower limit line y for the variation of the data group is created. When an approximation line is drawn between three points estimated from the data group, the approximation line is A = 50, [N] = 150 ppm. In other words, the intersection of [N] = 150 ppm and the lower limit line y is A = 50. From this, it was determined that it would be possible to achieve [N] ≥ 150 ppm by nitriding molten steel 2 by injecting _N2 in the converter-ladle refining process and by performing RH treatment under conditions such that the index A ≥ 50.

Note that for some of the data groups, there are calculated values calculated by model calculation rather than plotting actual values. However, the model used in this example was constructed by formulating a [N] balance model during _N2 gas bubbling in the ladle refining process and during decompression treatment in the RH process based on reaction engineering theory, and fitting the parameters in the formula to the plots of actual values. Therefore, the values derived by this model can be said to be values that are equally valid as actual values. Details of this model formula will be described below.
<About the [N] calculation model for RH treatment>
The model calculation values in this example were calculated with reference to the nitrogen concentration calculation model shown in Japanese Patent No. 5836187 (particularly, paragraphs [0025] to [0057], formulas [1] to [20], FIG. 2, etc.).

The unknown values k _m , α _S , α _Ar , and α _{N 2} in this model were fitted by the least squares method to match the actual values for "type of gas,""degree of vacuum," and "presence or absence of oxygen heating," as shown in Table 2. However, the volume V _V of the molten steel 2 contained in the vacuum vessel 4 was set to a constant value of 3 m ³ .

Table 2 shows the parameter values used in the model calculations.

Table 3 shows examples of the present invention, which were carried out according to the method for refining high-nitrogen steel of the present invention, as well as comparative examples that were carried out for comparison with the present invention. Note that Charges No. 1-1 to No. 3-1 in Table 3 are actual values from this experiment. For Charges No. 4-1 to No. 4-9, conditions were assumed based on the actual value of No. 3-1, and [N] _fin was calculated using a thermodynamic model.

For the comparative examples in Table 3 (charges No. 1-1 and No. 1-2), the timing of the S adjustment ( _ts ) in the RH treatment was late and the degree of vacuum (P _N2,S =P _V ) was high, resulting in a small value of index A. In other words, index A ≥ 50 was not satisfied, and the value of [N] _fin did not satisfy [N] ≥ 150 ppm.

For the present examples (charges No. 2-1 to No. 2-15) in Table 3, a sufficient amount of nitrogen was added to the molten steel 2 in the bubbling process ([N] _ini ) before the RH treatment, and the timing of the S adjustment (t _s ) in the RH treatment was early or the degree of vacuum (P _N2,S =P _V ) was low. When the RH treatment was performed under conditions that satisfied A ≥ 50, the value of [N] _fin became [N] ≥ 150 ppm.

For this example (charge No. 3-1) in Table 3, no S adjustment (t _s ) was performed during RH treatment, but the amount of nitrogen added in the bubbling process ([N] _ini ) prior to RH treatment was large. When RH treatment was performed under conditions that satisfied A≧50, the value of [N] _fin was [N]≧150 ppm.

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

In addition, in Charges No. 4-1 to No. 4-9, sufficient nitriding of the molten steel 2 is performed in the bubbling process ([N] _ini ) before the RH treatment. The RH treatment is performed under conditions that satisfy A≧50. Furthermore, if S adjustment (t _s ) is performed early in the RH treatment, the index A can be further increased, making it possible to achieve [N]≧150 ppm.

In summary, the method for refining high-nitrogen steel according to the present invention is a method for refining high-nitrogen steel in which molten steel 2 tapped from a converter is subjected to a ladle refining process and an RH vacuum degassing process in which _N2 gas is circulated, wherein in the converter process and/or the ladle refining process, _nitriding is performed on the molten steel 2 by stirring with N2 gas, and in the RH vacuum degassing process, the processing conditions are such that the value of index A calculated from "Equation (1) including, as variables, the values of the molten steel components before the RH vacuum degassing process (preferably immediately before the process)" is 50 or more.

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

In other words, by using the high-nitrogen steel refining method of the present invention, RH treatment conditions that satisfy index A ≥ 50 can be performed, making it possible to achieve [N] ≥ 150 ppm using only _N2 gas, and high-nitrogen steel can be produced inexpensively without the need to add nitride alloys.

The embodiments disclosed herein should be considered illustrative in all respects and not restrictive. In particular, matters not explicitly stated in the embodiments disclosed herein, such as operating conditions, parameters, dimensions, weights, and volumes of components, do not deviate from the scope of ordinary skill in the art, and values that would be easily assumed by a person of ordinary skill in the art are used.

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

Claims (1)

A method for refining high-nitrogen steel, comprising the steps of: subjecting molten steel tapped from a converter to a ladle refining process and a RH vacuum degassing process in which _N2 gas is refluxed;
In the converter treatment and/or ladle refining treatment, the molten steel is nitrogenized by stirring with _N2 gas , and the high-nitrogen steel having a [N] of 150 ppm or more after the RH treatment is refined ,
the RH vacuum degassing treatment is performed under treatment conditions in which no nitride alloy is added to the molten steel, and the value of index A calculated from at least "Equation (1) including, as variables, the values of the components of the molten steel before the RH vacuum degassing treatment" is 50 or more.


Where, [N]ini (ppm): [N] concentration in molten steel before RH treatment
[N]e,S (ppm): Equilibrium [N] concentration at the surface of the molten steel bath in the vacuum vessel
kr,before(m/min/%): Interfacial reaction rate constant of the [N] reaction before S addition
kr,after (m/min/%): Interfacial reaction rate constant of the [N] reaction after adding S
t (min): RH treatment time
ts (min): Start time of S addition