JP2009216513A - Oxygen concentration cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oxygen concentration cell that stably measures electromotive force corresponding to the oxygen activity in slag-containing molten steel formed during production of stainless steel. <P>SOLUTION: An oxygen concentration cell 10 includes a reference electrode 12 and a positive electrode 14 made of metallic molybdenum. The distance X (mm) between the reference electrode 12 and the positive electrode 14 satisfies the equation (1) according to the temperature T (°C) of molten steel and the slag mass (kg) per ton of molten steel; X≥0.03×W+2 [where, W represents slag mass contained in one ton of slag-containing molten steel, (10≤W (kg)≤100)]. Moreover, The diameter D (mm) of the positive electrode 14 satisfies the equation (2) : D≥0.01×T-14.0 [where, T represents the temperature of molten steel (1600≤T (°C) ≤1750)]. In the oxygen concentration cell 10, the distance X between both electrodes and the diameter D of the positive electrode 14 are properly adjusted, and thereby suppressing catching of slag in between both electrodes or elution of the positive electrode 14 into molten stainless steel. Thus, it is possible to stably measure accurate electromotive force corresponding to the oxygen activity even in slag-containing hot molten steel. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an oxygen concentration cell for measuring an electromotive force according to the oxygen activity in molten steel containing slag produced during the production of stainless steel.

  Stainless steel is used in a wide range of fields such as automotive parts, building / housing materials, and household appliances, but in recent years it has been used in conventional stainless steels such as flat panel displays (FPD) and fuel cell separators. Application development as difficult electronic parts is expected. Therefore, at present, there is a strong demand for high performance and high quality stainless steel that can be used in various applications.

  As a useful method for changing the properties of stainless steel, there is a method of adding a trace amount of metal element to molten stainless steel to control the metal structure of stainless steel. Al, Si, Mg, Ti and the like are known as additive elements that change the characteristics of stainless steel. For example, Al or Si improves the weldability of stainless steel.

By the way, although oxygen does not melt in solid steel, oxygen is very easily dissolved in molten steel in which steel is melted. Therefore, a large amount of oxygen is present in molten stainless steel. As the oxygen, for example, some exist as ions such as O ²⁻ , and some exist as oxides such as Cr ₂ O ₃ . Since metal elements such as Al, Si, and Mg all have a strong affinity for oxygen, they easily react with oxygen in molten steel to form oxides. As the above reaction occurs, the composition design of stainless steel by Al or Si is hindered, so that it becomes difficult to impart desired characteristics to stainless steel.

  Further, the amount of oxygen in the molten stainless steel greatly affects the electrical and mechanical characteristics and workability of the stainless steel sheet as a molded product. Therefore, in order to improve the function and quality of stainless steel, it is very important to accurately know the oxygen content in the molten stainless steel, that is, the oxygen activity, from the viewpoint of realizing a highly accurate composition design. Become. In fact, in Patent Document 1, high-quality stainless steel is manufactured by measuring the oxygen activity in molten stainless steel and appropriately designing the composition according to the required amount of Al or Si estimated from the measured value. The method of obtaining is described.

An oxygen sensor using the principle of an oxygen concentration cell is mainly used for measuring oxygen activity in molten stainless steel. The oxygen sensor generally has an oxygen concentration cell and a temperature sensor for measuring the molten steel temperature. As this oxygen concentration cell, for example, as disclosed in Patent Document 2, a reference electrode covered with a solid electrolyte having oxygen ion conductivity, and a positive electrode paired with this reference electrode Is typical. The oxygen sensor is based on the electromotive force E (V) detected by the oxygen concentration cell when the oxygen sensor is immersed in molten steel and the molten steel temperature T (° C.) measured by the temperature sensor. The oxygen activity in the molten steel is obtained from the oxygen partial pressure calculated according to the equation.
_{E = (RT / nF) In} (P S / P R)
In this formula,
R is a gas constant,
n is the number of electrons contained in the reaction, i.e. n = 4,
F is a Faraday constant,
P _S is the oxygen partial pressure in the stainless molten steel,
P _R is the oxygen partial pressure in the reference.

The principle of detecting the electromotive force in the molten stainless steel by the oxygen concentration cell is as follows. When the oxygen concentration cell is immersed in the molten stainless steel in which oxygen ions are present, the oxygen ions pass through the solid electrolyte having oxygen ion conductivity and reach the reference electrode covered with the solid electrolyte. As a result, a difference in oxygen partial pressure occurs between the reference electrode and the positive electrode. At this time, 2O ²⁻ → 2O ₂ + 4e ⁻ (oxidation reaction) occurs at the reference electrode, and O ₂ + 4e ⁻ → 2O ²⁻ (reduction reaction) occurs at the positive electrode, so there is a difference in oxygen partial pressure between the two electrodes. A corresponding electromotive force E (V) is generated. Finally, the electromotive force E (V) is read with a voltmeter or the like. From the above, the electromotive force in the molten stainless steel is detected by the oxygen concentration cell.
Japanese Patent No. 3993032 JP 58-014052 A

  As described above, in order to accurately measure the oxygen activity in the molten stainless steel using the oxygen sensor using the oxygen concentration cell, it is necessary to know the exact electromotive force and molten steel temperature in the molten stainless steel. The “accurate electromotive force” measured by an oxygen concentration cell refers to an electromotive force measured as a stable waveform continuously for a certain time after the oxygen concentration cell is immersed in molten steel.

However, in the manufacturing process of stainless steel, a slag phase containing oxides such as CaO, TiO ₂ , Cr ₂ O ₃ , MgO, Al ₂ O ₃ , FeO, and SiO ₂ is formed on the surface of the molten stainless steel. . When the electromotive force in the molten stainless steel containing such a slag phase (also referred to as “slag-containing molten steel”) is measured using a conventional oxygen concentration cell, for example, as shown in FIG. Since it is not stable even after passing through, accurate electromotive force may not be measured. As a cause of this, it is considered that the measurement of the electromotive force generated between the two electrodes becomes difficult due to the slag biting between the reference electrode and the positive electrode.

  In addition, in a conventional oxygen concentration cell measuring high temperature molten stainless steel such as 1600 ° C. or higher, a metal molybdenum positive electrode which is a high melting point material is generally used from the viewpoint of heat resistance. However, when the molten steel temperature is about 1600 to 1750 ° C., there is a problem that the positive electrode made of metal molybdenum is melted into the molten stainless steel during the measurement of the electromotive force and it is difficult to detect an accurate electromotive force.

  Therefore, the present invention provides an oxygen concentration cell that can accurately and stably measure the electromotive force according to the oxygen activity in the molten steel even when the molten steel temperature is 1600 ° C. or higher. The purpose is to provide.

  As a result of intensive studies, the inventors have found that the above problem can be solved by optimizing the distance between the reference electrode and the positive electrode of the oxygen concentration battery and the positive electrode, The present invention has been completed.

That is, the said subject is solved by the oxygen concentration battery of this invention.
[1] An oxygen concentration cell for measuring an electromotive force according to an oxygen activity in a molten steel containing slag produced during the production of stainless steel,
A reference electrode and a metal molybdenum positive electrode;
An oxygen concentration cell in which an interval X between the reference electrode and the positive electrode and a diameter D of the positive electrode satisfy the following expressions (1) and (2).
X ≧ 0.03 × W + 2 (1)
In the formula (1),
X represents a distance [mm] between the reference electrode and the positive electrode,
W represents the mass of slag contained in the slag-containing molten steel 1t (10 ≦ W [kg] ≦ 100).
D ≧ 0.01 × T-14.0 (2)
In the formula (2),
D represents the diameter [mm] of the positive electrode;
T represents the molten steel temperature (1600 ≦ T [° C.] ≦ 1750).
[2] The oxygen concentration cell according to [1], wherein the slag includes 10% by mass or less of Cr ₂ O ₃ and 3 to 30% by mass of MgO per total mass.

  According to the present invention, there is provided an oxygen concentration cell capable of accurately and stably measuring an electromotive force according to an oxygen activity in a molten steel even if the molten steel is a high-temperature slag-containing molten steel having a temperature of 1600 ° C. or higher. be able to.

  Hereinafter, embodiments of the oxygen concentration battery of the present invention will be specifically described with reference to the drawings.

1. Oxygen concentration cell The oxygen concentration cell of the present invention has a reference electrode and a positive electrode made of metallic molybdenum, and the distance X between the reference electrode and the positive electrode and the diameter D of the positive electrode are expressed by the above formulas (1) and (2). ) Is satisfied.

  FIG. 1 is a schematic view of an example of the oxygen concentration cell 10 of the present invention. As shown in FIG. 1, the oxygen concentration cell 10 of the present invention has a structure in which a reference electrode 12 and a cathode 14 are fixed on a support 16 with a predetermined interval X.

The reference electrode 12 is usually an electrode made of a substance having a known oxygen partial pressure. The material of the reference electrode 12 is not particularly limited, but a mixture of Cr and Cr ₂ O ₃ is preferable.

  The reference electrode 12 is covered with a cylindrical solid electrolyte (not shown) whose outer periphery has a bottom. The cylindrical solid electrolyte covering the reference electrode 12 is made of a conductor having oxygen ion conductivity that allows oxygen ions in molten steel to easily pass therethrough. As the solid electrolyte, known ones that are known to be used for oxygen concentration batteries can be used. For example, zirconium oxide (zirconia) or thorium oxide is mainly used, and silicon dioxide, alumina, titanium oxide is used as necessary. Alternatively, a sintered body partially stabilized by dissolving a predetermined amount (several moles) of iron oxide or the like is used. Among these, partially stabilized zirconia is particularly preferable because it has excellent thermal shock resistance and a high response speed to oxygen activity. A filler for holding the reference electrode 12 in the cylindrical solid electrolyte may be filled between the reference electrode 12 and the solid electrolyte.

  The positive electrode 14 has an approximately circular or substantially polygonal cross-sectional shape, and is an electrode that constitutes an oxygen concentration cell in combination with the reference electrode 12. The positive electrode 14 of the present invention is made of metallic molybdenum from the viewpoint of excellent heat resistance even in high-temperature stainless molten steel at 1600 ° C. or higher.

  As an electrode of an oxygen concentration cell in which high-temperature molten steel such as stainless steel is measured for electromotive force, tungsten having the highest melting point and high electric resistance among metal elements is considered useful. However, since tungsten is very expensive, it is economically unsuitable for devices that are mainly disposable, such as oxygen concentration cells. In that respect, metallic molybdenum has an advantage that the melting point is as high as 2600 ° C. or higher and is inexpensive while having a relatively large electric resistance, so that the cost can be suppressed. In addition, since metallic molybdenum is a metal element that improves the mechanical properties of stainless steel, even if a part of the positive electrode is dissolved in the molten steel while using an oxygen concentration cell, the quality of the stainless steel plate is degraded. Less likely.

Further, the distance X between the reference electrode 12 and the positive electrode 14 of the present invention satisfies the following formula (1).
X ≧ 0.03 × W + 2 (1)
In the formula (1),
X represents a distance [mm] between the reference electrode and the positive electrode,
W represents the mass of slag contained per 1 ton of molten steel (10 ≦ W [kg] ≦ 100).

  The “interval X between the reference electrode and the positive electrode” in the present invention refers to the shortest distance connecting arbitrary points on the side surface or one side of the reference electrode 12 and the positive electrode 14. The position where the arbitrary point exists differs depending on the shape of both poles, the proximity state, and the like.

  A method of obtaining the distance X between the reference electrode and the positive electrode will be specifically described with reference to FIG. FIG. 2 is a schematic view of an example of the oxygen concentration cell of the present invention as viewed from above. The symbols in FIG. 2 mean the same members as in FIG. As shown in FIG. 2, in the case of the oxygen concentration battery 10 in which the cross-sectional shape of both electrodes is circular, the shortest distance connecting arbitrary points on the side surfaces of the cylindrical electrodes 12 and 14 is the distance X.

  W in Formula (1) is the slag produced | generated in the middle of manufacture of stainless steel, Comprising: The mass of the slag contained per 1t of stainless steel molten steel used as the object which measures oxygen activity with an oxygen concentration cell is represented. The process of producing stainless steel refers to the period from when the raw material of stainless steel charged in the ladle is melted to form molten stainless steel, which is continuously cast and recovered as CC slab from the ladle. Moreover, in this invention, the slag contained per 1 ton of molten stainless steel at the time of the oxygen activity measurement by an oxygen concentration cell is also called "measurement slag."

  The stage of measuring oxygen activity with an oxygen concentration cell is not particularly limited as long as it is an arbitrary stage during the production, but for the purpose of designing an appropriate composition of stainless steel, immediately before adding a deoxidizer to the finish. It is preferable that This is because the addition amount of the deoxidizer necessary for appropriately designing the composition of the stainless steel can be determined according to the measured value of the oxygen activity of the molten stainless steel. In general, the slag mass W of the slag-containing molten steel immediately before the deoxidizer is added to the finish is in the range of 10 ≦ W [kg] ≦ 100.

  Although the method for obtaining the slag mass W is not particularly limited, it is preferably measured according to one of the following two methods. Specifically, 1) A method for estimating the slag main component from elements ordinarily added to molten stainless steel, and obtaining the total amount of each slag determined stoichiometrically as the slag mass during measurement, 2) CC slab Are obtained from the mass of slag remaining in the ladle after the production. Hereinafter, each method will be described.

Method 1) The method 1) estimates the slag main component produced when producing stainless steel, and the total amount of each main component generated stoichiometrically estimated is the “deemed slag mass”. Furthermore, it is a method of obtaining a value obtained by converting the deemed slag mass per 1 ton of molten steel containing slag as “measurement slag mass W”. The main component of slag is estimated by stoichiometrically estimating the composition of the stainless steel to be manufactured and the type and amount of metal elements added to the molten stainless steel for the purpose of promoting deoxidation and desulfurization. Can do.

The procedure for obtaining the measurement slag mass W by the method 1) will be specifically described. For example, when producing 80t / 1 charge of stainless steel containing 11-30% by mass of Cr in the steel, the purpose is to reduce the amount of carbon in the molten stainless steel compared to the molten stainless steel before measuring the oxygen activity. After adding oxygen (oxygen blowing), addition of CaO for deoxidation and desulfurization (about 600 kg) and addition of Si and Al elements for the purpose of preliminary deoxidation are appropriately performed. Cr in the molten stainless steel is oxidized by oxygen blowing to produce Cr ₂ O ₃ , and Si and Al react with oxygen in the molten stainless steel to produce SiO ₂ and Al ₂ O ₃ . Therefore, the surface of the stainless molten _{steel, CaO, Cr 2 O 3,} SiO 2, Al 2 O 3 slag phase mainly composed of are formed. When the oxygen activity in the molten stainless steel is determined in this state, the amount of slag contained per 1 ton of molten stainless steel is the amount of slag at the time of measurement.

However, it is difficult to actually measure the mass of the slag at the time of measurement mainly containing Cr ₂ O ₃ , SiO ₂ , and Al ₂ O ₃ . However, the oxide amount can be calculated stoichiometrically according to the amount of oxygen sprayed, the composition of the molten stainless steel, and the like. For example, normally, in oxygen blowing, oxygen is sprayed on molten stainless steel so as to reduce the carbon concentration in the molten stainless steel from about 0.3% to 0.01% or less. When the carbon concentration in the molten stainless steel is lowered to about 0.02%, Cr in the molten stainless steel is oxidized, so that Cr ₂ O ₃ is generated. Therefore, from the amount of Cr based on the composition of the molten stainless steel, Cr ₂ O ₃ can be estimated to be about 300 kg stoichiometrically.

On the other hand, for SiO ₂ and Al ₂ O ₃ , the amount of Si and Al added is usually about 1 to 3 kg per 1 ton of molten stainless steel in preliminary deoxidation. Therefore, when manufacturing 80t stainless steel like this example, the production amount of each oxide can be estimated to be about 300 kg stoichiometrically. Therefore, the total of CaO, Cr ₂ O ₃ , SiO ₂ , and Al ₂ O ₃ that are the main components of slag is estimated to be about 1200 kg. Since this approximate value is equivalent to the slag mass only when 80t stainless molten steel is manufactured, the slag mass W at the time of measurement in this case can be determined to be 15kg by converting this per 1t of slag-containing molten steel.

  In addition, the slag mass W at the time of measurement calculated | required by the method of 1) mentioned above fluctuates | varies according to the manufacturing conditions of molten stainless steel like the composition of stainless molten steel, the amount of oxygen spraying, and the addition amount of a deoxidizer, What is necessary is just to calculate suitably like the method. Further, these reactions may be carried out preliminarily on a lab scale, and the amount of slag actually generated may be analyzed to be used as a reference when determining the slag mass W during measurement.

Method 2) The method 2) is a method of estimating the slag at the time of measurement from the slag remaining in the ladle after manufacturing the CC slab obtained after continuous casting (also referred to as residual slag). Specifically, when the mass of only the ladle before refining (before charging the stainless steel component) is a ₀ (kg), and the total mass of residual slag and ladle is a ₁ (kg), from a ₁ a method of obtaining a mere minus a ₀ as measured at the slag mass W. Note that a ₀ and a ₁ may be values actually measured when stainless steel is manufactured on a lab scale.

By the way, when the composition of the slag at the time of measurement and the composition of the residual slag are the same, the slag mass W can be obtained by simply subtracting a ₀ from a ₁ as described above. When a deoxidizer is added to the stainless steel molten steel after obtaining the oxygen activity in the molten steel until the residual slag is obtained, the composition of the slag during measurement and the residual slag may be different. In this case, there is a possibility that the distance X between the reference electrode and the positive electrode in the oxygen concentration cell cannot be properly estimated. However, when the difference in the composition is corrected according to the following procedure, the slag before adding the deoxidizer ( The amount can be accurately determined. Therefore, for example, for slag-containing molten steel containing slag containing Cr ₂ O ₃ and SiO ₂ , oxygen is added to properly grasp the amount of Al added when adding Al as a deoxidizer in the finish deoxidation step. The correction method in the case of measuring the activity will be described.

In this case, the following two reactions occur, and (i) is completed, but (ii) is known to stop during the reaction.
Cr ₂ O ₃ + 2Al → 2Cr + Al ₂ O ₃ ... (I)
3SiO ₂ + 4Al → 3Si + 4Al ₂ O ₃ (ii)
Therefore, first, the amount of SiO ₂ contained in the original slag per unit mass and the amount of SiO ₂ contained in the residual slag per unit mass are measured. The measuring means may be a known wet chemical analysis method as described later. From the difference in SiO _2, the amount of alumina (x) produced by the reaction (ii) is obtained. By subtracting (x) from the amount of alumina (y) contained in the residual slag per unit mass, the amount of alumina produced by the reaction (i) can be determined. Therefore, the amount of Cr ₂ O ₃ present in the original slag per unit mass can be estimated from the amount of alumina.
As mentioned above, the quantity of slag (original slag) which exists in slag containing molten steel before adding Al as a deoxidizer from residual slag is calculated | required correctly.

  In the above example, the reactions (i) and (ii) occur, but the reactions occurring in the molten stainless steel vary depending on the oxides present in the original slag and the type of metal added as a deoxidizer. In that respect, it may be corrected similarly in various cases. Further, if the slag contains an oxide (such as MgO) that is difficult to be reduced by a deoxidizer (such as Al), the amount of the oxide does not change between the residual slag and the original slag, so correction is not necessary. . The mass of such an oxide can be obtained by subtracting the amount of alumina produced by the reactions (i) and (ii) from the mass of the residual slag.

  The oxygen concentration cell with the interval X that satisfies the above formula (1) can stably measure the exact electromotive force in the slag-containing molten steel. The reason is that the slag biting between the two poles usually increases as the amount of slag in the molten stainless steel increases. In this regard, the present inventors have found that there is an optimum coefficient in relation to the slag mass W contained per 1 ton of molten slag-containing steel. That is, the coefficient 0.03 of W in the equation (1) is a value determined to correspond to the slag biting amount that can increase in proportion to the increase in the slag mass.

  Moreover, when the molten steel temperature T of molten stainless steel rises, the slag mass in the molten stainless steel increases, and the amount of slag bite tends to increase in proportion to this. In this regard, the present inventors have found that by using +2 as a correction value in equation (1), it is possible to cope with the amount of slag biting that can be increased in proportion to the increase in the molten steel temperature T.

  Moreover, the oxygen concentration cell of the present invention has an electromotive force in slag-containing molten steel while more significantly suppressing slag biting and positive electrode elution when the molten steel temperature T (° C.) is 1600 ≦ T ≦ 1750. Can be measured accurately. The molten steel temperature T of the present invention refers to the temperature at the time of measuring oxygen activity.

Moreover, the diameter D of the positive electrode 14 made of metallic molybdenum of the present invention satisfies the following formula (2).
D ≧ 0.01 × T-14.0 (2)
In the formula (2),
D represents the diameter [mm] of the positive electrode;
T represents the molten steel temperature (1600 ≦ T [° C.] ≦ 1750).

  As described above, the cross-sectional shape of the positive electrode 14 of the present invention is constituted by a substantially circular shape or a substantially polygonal shape, that is, a combination of straight lines or curved lines. Specifically, a polygon formed by a combination of straight lines, a circle or an ellipse by a curve, and the like can be given. Moreover, the said cross-sectional shape may be comprised by the combination of the straight line and the curve. When the cross-sectional shape of the positive electrode 14 is a polygon, the diameter D is the diameter of the circumscribed circle of the polygon.

  Among these, the cross-sectional shape of the positive electrode 14 of the present invention is preferably circular. The reason for this is that the positive electrode having a circular cross section has a small surface area, so that it is difficult to be influenced by thermal energy even when immersed in molten stainless steel, so that elution of the positive electrode is suppressed. In addition, since the amount of slag attached can be suppressed, there is an advantage that the measurement accuracy of electromotive force is improved.

  The diameter D of the metal molybdenum positive electrode 14 obtained by the above formula (2) is such that the positive electrode melts into the molten steel so that an accurate electromotive force in the slag-containing molten steel can be stably measured using an oxygen concentration cell. It is determined for the reason of suppressing.

  Next, how the equation (2) is obtained will be described. The present inventors investigated how the diameter of the positive electrode made of metallic molybdenum changes when the oxygen concentration cell is immersed in the steel making, and the optimum value of the positive electrode diameter according to the difference in molten steel temperature. Found that there is. Therefore, when a positive electrode made of metallic molybdenum having a melting point of 2600 ° C. is used in a very high temperature stainless molten steel of about 1600 to 1750 ° C., before the positive electrode elutes into the molten stainless steel until it becomes difficult to measure the electromotive force. What is found as a preferable relationship between the molten steel temperature T and the diameter D of the positive electrode so that the electromotive force can be detected quickly is the equation (2).

  As described above, in the oxygen concentration cell 10 of the present invention, since the diameter D of the positive electrode 14 is adjusted within an appropriate range, the elution of the positive electrode into the molten stainless steel is suppressed even when immersed in the hot molten stainless steel. Is done. Therefore, the elution of the positive electrode and the like can be suppressed without using a cover member that covers both electrodes, and the measurement accuracy can be increased. Therefore, there is an advantage that the cost of the oxygen concentration cell 10 can be reduced.

  The length L1 of the reference electrode 12 and the length L2 of the positive electrode 14 are not particularly limited, but are generally in the range of 10 to 20 mm. Any of the above lengths refers to the length of each pole extending from the support 16 with respect to the molten steel to be measured. However, if the length of each pole is longer than necessary, slag is likely to bite between the two poles, and the amount of slag adhering to both poles increases, so care must be taken.

  In the oxygen concentration cell of the present invention, a thermocouple may be juxtaposed along with the reference electrode. An oxygen concentration cell in which a thermocouple is disposed is mainly used as a configuration of an oxygen concentration cell because it can simultaneously measure the temperature of molten stainless steel together with the oxygen activity.

2. Detection method of electromotive force The detection of the electromotive force according to the oxygen activity in the molten stainless steel by the oxygen concentration cell of the present invention may be performed according to the following procedure.

  As shown in FIG. 3, first, the oxygen concentration cell 10 is immersed in a molten stainless steel 31 having a slag phase 30 formed on the surface, and is stopped at a predetermined depth (measurement position). The stopped state of the oxygen concentration cell 10 is maintained for several seconds until the detection of the electromotive force in the molten stainless steel 31 is completed.

The timing for measuring the electromotive force may be at any stage during the production of stainless steel. However, it is preferable to measure the electromotive force immediately before the finish deoxidation step in order to accurately perform composition design for the purpose of improving the quality of stainless steel. Thereby, the addition amount of a deoxidizer etc. required for the finishing deoxidation processes like Al can be estimated correctly. The slag at the time of measurement immediately before the finishing deoxidation step is usually composed mainly of Cr ₂ O ₃ and MgO, and the proportion thereof is 10% by mass or less of Cr ₂ O _{3 and 3} % of MgO per total mass of the slag. 30% by mass. The oxygen concentration cell of the present invention is designed to suppress slag biting between the two electrodes even when immersed in a molten stainless steel in which a slag phase having such a composition exists, so that it is accurate and stable. The electromotive force can be measured.

As a method for measuring the contents of Cr ₂ O ₃ and MgO in the slag phase, wet chemical analysis is preferably used from the viewpoint of realizing high-precision elemental analysis in a short time. The wet chemical analysis may be a known method and is not particularly limited. An example of the basic operation for measuring the content by a wet chemical analysis method is shown below.

First, a sample is prepared using slag to be measured. The sample is prepared by crushing or cutting the slag. Next, the prepared sample is dissolved in an acid (for example, HCl, HF, HNO _3, etc.) to form a solution. The method of making the solution is not particularly limited, and includes alkali melting in addition to the acid. Subsequently, the solution obtained by the solution is subjected to elemental analysis, and the metals Cr and Mg are measured. As the elemental analysis method, a known method such as ICP mass spectrometry or gravimetric method is used. Then, to calculate the amount of stoichiometrically _Cr 2 _{O 3} and MgO from the elemental analysis. The calculated value is the amount of Cr ₂ O ₃ and MgO contained in the slag phase.

  An electromotive force is generated between the reference electrode 12 and the positive electrode 14 constituting the oxygen concentration battery 10 according to the difference in oxygen partial pressure at each electrode. The generated electromotive force is detected by a voltage measuring device (not shown) provided in the oxygen concentration cell 10.

  After confirming that the electromotive force in the stainless steel melt 31 is properly calculated in the voltage measuring instrument, the oxygen concentration cell 10 is pulled out from the slag-containing molten steel. Whether or not the electromotive force has been calculated properly is confirmed by the fact that the waveform change over time due to the electromotive force is kept stable for about 2 to 5 seconds, with the waveform fluctuation being small and stable. Can do. More appropriate electromotive force means that the state in which the waveform is stable is maintained for about 5 seconds.

  When the electromotive force in the molten stainless steel is measured using the oxygen concentration cell 10 of the present invention, as shown in FIG. 4, the electromotive force stabilizes rapidly in about 2 seconds from the start of immersion, and the electromotive force is stabilized for about 5 seconds. It can be seen that the power waveform is stable. As the cause, as described above, the oxygen concentration cell 10 of the present invention has the slag content because the interval X between the reference electrode 12 and the positive electrode 14 is determined according to the molten steel temperature and the amount of slag contained in the molten stainless steel. Even if immersed in molten steel, the slag biting between the two electrodes is suppressed. Moreover, since the diameter D of the positive electrode 14 is determined according to the molten steel temperature of the molten stainless steel, elution into the molten steel is reduced.

  As described above, according to the oxygen concentration cell 10 of the present invention, accurate electromotive force can be stably measured, so that the measurement accuracy can be improved, and consequently the performance of the oxygen sensor can be improved to improve the quality of steelmaking. Can be increased. Further, according to the oxygen concentration cell 10 of the present invention, since the immersion time can be shortened compared to the conventional case, the possibility that the electrode elutes can be reduced, slag adhesion on both electrode surfaces, Slag biting in between is suppressed. Therefore, it is not necessary to increase the diameter and length of the electrode more than necessary, which leads to cost reduction of the apparatus.

3. Application to oxygen sensor The oxygen concentration cell of the present invention can be applied to an oxygen sensor for measuring the oxygen activity in molten stainless steel. The “oxygen sensor” of the present invention is a sensor that can calculate the oxygen activity based on the electromotive force detected by the oxygen concentration cell and the molten steel temperature measured by the temperature sensor, etc., using the principle of the oxygen concentration cell. Say. The form of the oxygen sensor is not particularly limited, and may be an integrated type in which the oxygen concentration battery and the temperature sensor are combined in one housing, or the oxygen concentration battery and the temperature sensor may be independent from each other.

  What is necessary is just to use the well-known thing as a temperature sensor with which an oxygen sensor is equipped as an apparatus which measures the temperature in stainless steel molten steel. For example, a temperature sensor having a thermocouple inside a tube-shaped protective tube made of quartz or the like can be mentioned.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the form of the present invention is not limited to the examples shown here.

1. Production of Stainless Steel In the following Examples and Comparative Examples, stainless steel in which 11 to 30% by mass of Cr was contained in steel was produced as a sample for measuring electromotive force with an oxygen concentration cell described later. First, the manufacturing method of the stainless steel will be described.

1) Deoxidation treatment First, 80 t of molten stainless steel was produced by melting the raw material in an electric furnace, and then the molten stainless steel was roughly decarburized using a converter (decarburization step 50 in FIG. 5). Next, this stainless steel molten steel 16 charge was transferred to the VOD step 40 shown in FIG. 5 and subjected to deoxidation treatment up to the rolling step 52.

In the VOD process 40, first, the decarburized stainless steel melt was subjected to a preliminary deoxidation process 42. In the preliminary deoxidation step 42, after decarburization refining by blowing oxygen gas to the surface of the molten stainless steel until the carbon content becomes 0.01% by mass or less, 6 mass of Cr ₂ O ₃ produced in the molten stainless steel is obtained. In order to reduce it to less than or equal to%, a predetermined amount of Al or Si was added and preliminary deoxidation was performed. Here, the addition amount of Al or Si was appropriately adjusted within a range of 1 to 3 kg per 1 ton of molten stainless steel.

2) Measurement of oxygen activity Next, in the oxygen activity measurement step 43, the oxygen activity in the predeoxidized stainless steel melt was measured by an oxygen sensor having an oxygen concentration cell produced as described later. At this time, the molten steel temperature of the molten stainless steel was measured by a thermocouple disposed in the oxygen concentration cell.

3) Finish deoxidation And, in the finish deoxidation step 44, according to the oxygen activity to be measured, Al is added so that the Al concentration in the molten stainless steel is in the range of 0.0005 to 0.05 mass%. The deoxidation of molten stainless steel was sufficiently advanced.

In the finishing deoxidation step 44, for the amount of Al added to each molten stainless steel, in the charge in which the oxygen activity could be measured, the required amount of Al added was obtained using a calibration curve prepared in advance by the method described later. It was. The amount of Al added is originally determined based on the oxygen activity measured by the oxygen concentration cell, but since Al may be consumed by reduction of Cr ₂ O ₃ present in the slag phase, more accurate Al For the purpose of determining the addition amount of Al, a calibration curve of Al was used. On the other hand, a certain amount of Al was added in the charge whose oxygen activity could not be measured.

4) Continuous casting Finally, in the continuous casting step 45, a stainless steel melt appropriately added with Al was continuously cast to obtain a CC slab. In the continuous casting process, the Al concentration of a sample obtained by sampling molten steel from a tundish was measured. The Al concentration was measured by a wet chemical analysis method. Specifically, a part of the collected sample was dissolved in a mixed acid solution of nitric acid and hydrochloric acid to form a solution, and then the Al in this solution was measured by ICP (inductively coupled plasma) emission spectroscopy, and in the CC slab. The Al concentration of was determined.

2. Oxygen Concentration Battery The oxygen sensor used in each example and comparative example detects oxygen activity based on an electromotive force measured using the principle of an oxygen concentration battery. The oxygen concentration cell used for the oxygen sensor has a reference electrode made of a Cr / Cr ₂ O ₃ mixture and a positive electrode made of metallic molybdenum, and the distance X between both electrodes and the positive electrode diameter D are appropriately adjusted. used. The distance X between the two electrodes and the diameter D of the positive electrode in the oxygen concentration cell used in each example and comparative example are collectively shown in Tables 1 and 2 described later.

3. Evaluation Results In Examples and Comparative Examples, element concentration (mass%) in slag measured during the production of stainless steel, slag mass (kg) per 1 ton of molten stainless steel, molten steel temperature, oxygen activity in molten stainless steel (ppm) ) And the Al concentration (mass%) of the CC slab, the values of Examples 1 to 10 are shown in Table 1, and the values of Comparative Examples 1 to 7 are shown in Table 2. Moreover, the theoretical value of the oxygen concentration cell attached to each table is a value obtained according to the formulas (1) and (2) using the actual measurement values such as the molten steel temperature. Moreover, molten steel temperature and oxygen activity are the values measured in the oxygen activity measurement process.

A method for measuring the Cr ₂ O ₃ and MgO concentrations in the slag will be described. The said measurement made the slag in the stainless steel molten steel in a preliminary deoxidation process a sample, and measured the Cr and Mg amount in the said sample by wet chemical analysis. Specifically, first, the slag in the molten stainless steel collected in the preliminary deoxidation step was dissolved in a mixed acid such as nitric acid, hydrochloric acid, sulfuric acid and the like to form a solution. Next, metals Cr and Mg in the obtained solution were measured by ICP (inductively coupled plasma) emission spectroscopy. Then, to calculate the amount of stoichiometrically Cr 2 _{O 3, MgO} from the measurement results, which was used as a content of the slag phase.

The slag mass (kg) at the time of measuring the oxygen activity was determined by the method 2) in the above-described method for determining the slag mass at the time of measurement. That is, when the mass of only the ladle before refining is a ₀ (kg) and the total mass of the residual slag and ladle obtained after the continuous deoxidation step and the continuous casting step is a ₁ (kg), from a ₁ was determined minus a ₀ as slag weight W during oxygen activity measurement. However, since Al is added to the slag-containing molten steel after measuring the oxygen activity, the composition of the slag during measurement and the composition of the residual slag are different. Therefore, in this example, the difference in the slag composition was corrected based on the reaction change based on the reduction reaction between Cr ₂ O ₃ and SiO ₂ and Al based on the method described above.

  From the results of Examples 1 to 10, it is apparent that the accurate oxygen activity in molten stainless steel can be stably measured by using the oxygen sensor using the oxygen concentration cell of the present invention. Moreover, as a result of adjusting the amount of Al added to the molten stainless steel according to the amount calculated based on this measurement result, the Al concentration of the CC slab is controlled within a desired range (0.005 to 0.05 mass%). We were able to.

  On the other hand, in Comparative Examples 1 to 7, in the oxygen concentration cell constituting the oxygen sensor, the slag was caught between the reference electrode and the positive electrode, or the positive electrode was dissolved in the molten stainless steel. As a result, the oxygen activity could not be measured accurately. Therefore, a certain amount of Al is inevitably added to the molten stainless steel, but Al becomes excessive and insufficient, and as a result, the Al concentration of the CC slab can be controlled in the range of 0.005 to 0.05 mass%. could not. Therefore, as is apparent from the results of the comparative examples, accurate electromotive force is stably measured only when the distance X between the reference electrode and the positive electrode and the diameter D of the positive electrode in the oxygen concentration cell satisfy the predetermined conditions. I found out that I can do it.

The oxygen concentration cell of the present invention has both a reference electrode covered with a cylindrical solid electrolyte having oxygen ion conductivity and a positive electrode made of metallic molybdenum, which is a high melting point material, and the distance between both electrodes and the positive electrode diameter. Are adjusted appropriately, even in molten steel containing slag such as Cr ₂ O ₃ and MgO, slag biting between the reference electrode and the positive electrode, and melting of the positive electrode in the molten stainless steel Suppression is suppressed. Therefore, an accurate electromotive force according to the oxygen activity in molten stainless steel can be measured quickly and stably.

Schematic of an example of the oxygen concentration cell of the present invention Schematic view of an example of the oxygen concentration cell of the present invention viewed from above Schematic when measuring electromotive force according to oxygen activity in molten stainless steel in oxygen concentration cell of the present invention The figure showing the time-dependent change of the electromotive force measured with the oxygen concentration cell of this invention The figure which shows the flow of the VOD process which was implemented with execution example / comparative example The figure showing the time-dependent change of the electromotive force measured with the conventional oxygen concentration cell

Explanation of symbols

10 Oxygen Concentration Battery 12 Reference Electrode 14 Positive Electrode 30 Slag Phase 31 Stainless Steel Molten Steel 40 VOD Process 42 Preliminary Deoxidation Process 43 Oxygen Activity Measurement Process 44 Finish Deoxidation Process 45 Continuous Casting Process 50 Decarburization Process 52 Rolling Process

Claims (2)

An oxygen concentration cell for measuring electromotive force according to oxygen activity in molten steel containing slag produced during the production of stainless steel,
A reference electrode and a metal molybdenum positive electrode;
An oxygen concentration cell in which an interval X between the reference electrode and the positive electrode and a diameter D of the positive electrode satisfy the following expressions (1) and (2).
X ≧ 0.03 × W + 2 (1)
[In the formula (1),
X represents a distance [mm] between the reference electrode and the positive electrode,
W represents the mass of slag contained per 1 t of the slag-containing molten steel (10 ≦ W [kg] ≦ 100)]
D ≧ 0.01 × T-14.0 (2)
[In the above formula (2),
D represents the diameter [mm] of the positive electrode;
T represents molten steel temperature (1600 ≦ T [° C.] ≦ 1750)] The slag, the 10 wt% per total weight less _Cr 2 _{O 3} and comprises 3 to 30 wt% of MgO, oxygen concentration cell according to claim 1.

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