JP4075834B2 - Method for estimating component concentration of molten steel and method for producing ultra-low carbon steel - Google Patents

Description

  The present invention relates to a method for estimating the component concentration of molten steel and a method for producing ultra-low carbon steel. Specifically, the present invention relates to a method for accurately estimating the component concentration of molten steel being subjected to vacuum degassing when producing molten steel using a vacuum degassing apparatus for molten steel, and this method. The present invention relates to a method for manufacturing ultra-low carbon steel at low cost.

  When degassing is performed using a vacuum smelting furnace equipped with a vacuum degassing apparatus (in the following description, the case where this degassing is decarburization is taken as an example), the upper limit value of the carbon concentration as a standard component and It is extremely important for product quality assurance to adjust the components so as not to deviate from the lower limit. Moreover, if the vacuum degassing process can be terminated immediately when a carbon concentration slightly lower than the upper limit value of the carbon concentration is reached, it will lead to shortening of the degassing process time, thereby reducing the processing cost and improving the productivity. It is extremely effective.

  For this purpose, as a matter of course, it is required to accurately estimate the carbon concentration of the molten steel during the vacuum degassing treatment. There have been known methods for estimating the carbon concentration of molten steel during vacuum degassing treatment as listed below.

  In Patent Document 1, molten steel is sampled before or during the vacuum degassing process, and the analytical value of the carbon concentration of this sample is used as a reference. An invention is disclosed in which estimation is performed by calculation using a predetermined decarburization rate constant.

Patent Document 2 discloses an invention for estimating a current carbon concentration based on exhaust gas information without using an analysis value of a carbon concentration of a molten steel sample before or during the vacuum degassing process. Yes.
JP-A-6-256840 JP-A-9-202913

The inventions disclosed in Patent Documents 1 and 2 have problems as described below, and the carbon concentration of molten steel during vacuum degassing cannot be estimated with high accuracy.
In the invention disclosed in Patent Document 1, since the decarburization rate constant is obtained by fitting, it is impossible to detect and correct this when the heat-specific conditions currently processed deviate from the fitting variation. For this reason, in the present invention, there is a high possibility that an error occurs in the estimation of the carbon concentration.

Since the invention disclosed by patent document 2 estimates the carbon concentration of molten steel based on the exhaust gas information measured every moment, there is a possibility that the carbon concentration can be estimated with higher accuracy than the invention disclosed by patent document 1. is there. However, as disclosed in FIG. 1 of Patent Document 2, the gas capable of diluting the CO and CO ₂ gas generated by decarburization inside the vacuum chamber is supplied to the riser side mounted at the lower portion of the vacuum chamber. Only unevenly distributed reflux gas. For this reason, it is difficult to say that the composition of the diluted exhaust gas has a sufficient correlation with the carbon concentration of the molten steel. Therefore, even with the invention disclosed in Patent Document 2, sufficient estimation accuracy cannot be obtained, and an error may occur in the estimation of the carbon concentration.

  The present invention has been made in view of the problems of such conventional techniques, and when performing vacuum degassing using a vacuum refining furnace using a vacuum degassing apparatus, the carbon concentration of molten steel, etc. It aims at providing the method of estimating a component density | concentration with high precision, and the method of manufacturing ultra-low carbon steel using this method.

The present invention is an inert gas stirrer that is blown into the molten steel inside the vacuum tank when degassing the molten steel while immersing the dip tube provided in the lower part of the vacuum tank in the molten steel accommodated in the ladle. An inert gas different from the gas or the inert gas for reflux is blown independently from the inert gas for stirring or the inert gas for reflux , and is contained in the exhaust gas separated from the molten steel inside the vacuum chamber. The molten steel is characterized in that the degassed component is diluted and mixed with an inert gas blown independently, and the concentration of the component contained in the molten steel is estimated by using the analysis value of the concentration of the degassed and mixed degassed component. This is a method for estimating the concentration of components.

In the method for estimating the component concentration of molten steel according to the present invention, it is desirable that the inert gas for mixing is blown from a lance disposed on the canopy or side surface of the vacuum chamber.
In the method for estimating the component concentration of the molten steel according to the present invention, the concentration of the component contained in the molten steel is estimated using the analysis value of the concentration of the degassed component diluted and mixed with the inert gas for mixing. It is desirable.

In the method for estimating the component concentration of the molten steel according to the present invention, it is exemplified that the degassing component is CO gas and / or CO ₂ gas and the component contained in the molten steel is carbon.
In these methods for estimating the component concentration of molten steel according to the present invention, the concentration of carbon contained in the molten steel is obtained by (a) determining the decarburization rate at each time from the concentration of diluted CO gas and / or CO ₂ gas. Calculating the integrated amount of decarburization converted to the concentration using the decarburization rate at each time obtained, and subtracting the integrated amount of decarburization from the analytical value of the carbon concentration before or during the vacuum degassing process, or ( b) By determining the decarburization rate at each time from the concentration of the diluted and mixed CO gas and / or CO ₂ gas, and determining the combination of the decarburization rate constant and the carbon concentration that balances the determined decarburization rate at each time It is desirable to be estimated.

  In these molten steel component concentration estimation methods according to the present invention, the molten steel is mixed with an inert gas for mixing that is blown into the vacuum chamber independently, and an inert gas for stirring or an inert gas for refluxing. Is blown.

Moreover, in the estimation method of the component concentration of the molten steel which concerns on these this invention, the number of installation of a dip tube is one or two.
From another point of view, the present invention is a method for producing an ultra-low carbon steel characterized by including a step of estimating the carbon concentration of molten steel by the above-described method of estimating the component concentration of molten steel according to the present invention.

  According to the present invention, when performing vacuum degassing using a vacuum degassing apparatus, an inert gas for mixing is independently blown into the vacuum chamber, and the molten steel is detached from the molten steel inside the vacuum chamber. Since the degassing components contained in the exhaust gas are diluted and mixed with an inert gas for mixing that is blown independently, the representativeness of the analysis value of the exhaust gas is improved, thereby increasing the concentration of components such as carbon concentration in the molten steel. It is possible to estimate with accuracy. For this reason, according to this invention, it becomes possible to reduce the cost by shortening the vacuum degassing processing time.

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the method for estimating the component concentration of molten steel and the method for producing ultra low carbon steel according to the present invention will be described in detail below with reference to the accompanying drawings. In the following description, the case where degassing is decarburization, the degassing component is CO gas and / or CO ₂ gas, and the vacuum degassing apparatus is an RH vacuum degassing apparatus is taken as an example.
In the present embodiment, the object is vacuum processing of molten steel obtained by decarburizing hot metal transported from a blast furnace in a converter. The hot metal is subjected to various known and common hot metal pretreatments, poured into a converter, decarburized and blown by the converter, and then discharged into a ladle. The molten steel delivered to the ladle is transported to a secondary refining facility and subjected to decarburization processing by an RH vacuum degassing device.

  In this RH vacuum decarburization treatment, a riser pipe and a downfall pipe, which are two immersion pipes provided at the lower part of the vacuum tank, are immersed in molten steel contained in a ladle, the inside of the vacuum tank is exhausted, and the inner surface of the riser pipe Then, an inert gas for reflux is introduced, and the molten steel accommodated in the ladle is lifted up to the inside of the vacuum chamber. The lifted molten steel is reduced in carbon concentration by a decarburization reaction inside the vacuum chamber, and forms a circulating flow returning to the ladle through the downcomer. In this way, the decarburization treatment of the molten steel is performed.

In this RH vacuum degassing apparatus, the flow rate of the inert gas for recirculation, the pressure inside the vacuum chamber, the concentration of CO gas in the generated exhaust gas, CO ₂ gas, etc. are continuously obtained by using a commonly used measuring instrument. Can be measured and recorded automatically or intermittently. The analysis components in the exhaust gas are not limited to CO gas and CO ₂ gas, and other components may be used as analysis values.

Further, in the present embodiment, during this decarburization treatment, a lance for blowing an inert gas for mixing into the inside of the vacuum chamber alone (independently) into the canopy (which may be a side surface) of the vacuum chamber. The CO ₂ gas and / or CO ₂ gas contained in the exhaust gas separated from the molten steel by blowing the inert gas for mixing on the molten steel alone and spraying from the lance to the molten steel. Dilute and mix inside. In addition, naturally, the inert gas for stirring different from this inert gas or the inert gas for reflux are blown in into molten steel.

In the present embodiment, to estimate the concentration of carbon contained in the molten steel with the analysis of CO gas and / or CO ₂ gas diluted mixture. Below, estimation procedure AC of the carbon concentration of molten steel is illustrated.

(Procedure A)
The decarburization rate at each time is obtained from the concentration of the diluted and mixed CO gas and / or CO ₂ gas, and the integrated amount of decarburization converted into the concentration using the decarburization rate at each obtained time is calculated. The accumulated amount of decarburization is subtracted from the analytical value of the carbon concentration before or during the treatment. In particular,
(I) The exhaust gas at time ti during the vacuum degassing process is analyzed to measure the CO fraction CO, i and the CO ₂ fraction CO ₂ , i in the exhaust gas.

(Ii) Measure the exhaust gas flow rate at time ti using an exhaust gas flow meter, and calculate from the tracer gas concentration and the known tracer gas flow rate by introducing the exhaust gas so as to contain a known tracer gas in the exhaust gas. Or by empirically determining the amount of input gas contained in the exhaust gas. The exhaust gas flow rate at time ti thus determined is converted into a volume flow rate Qex, i (Nm ³ / s) converted into a standard state by an appropriate method.

(Iii) The product of the sum of the CO fraction CO, i and the CO ₂ fraction CO ₂ , i in the exhaust gas determined by the above-mentioned item (i) and the exhaust gas flow rate Qex, i determined by the item (ii) (CO , I + CO ₂ , i) · Qex, i is obtained.

(Iv) The decarburization rate dCdt, i (kg / s) per unit time at time ti is obtained from the following equation (1) using the proportionality constant A.
dCdt, i = A · (12 / 22.4) · (CO, i + CO ₂ , i) · Qex, i
... (1)
In the equation (1), since unit conversion is performed by (12 / 22.4), the proportional constant A may be 1.0 in normal use, but the measured value of the exhaust gas thermometer or exhaust gas flow meter. If this error occurs, the proportionality constant A may be finely adjusted.

 (V) The dCdt, i from the time when the concentration becomes Co to the time ti is added to the carbon concentration Co (%) of the molten steel obtained before or during the vacuum degassing treatment, and converted into a concentration. Calculate the amount of decarburization. For example, as shown in the equation (2), the current carbon concentration Ci (%) is calculated by subtracting the integrated amount of decarburization from the analysis value of the carbon concentration before or during the vacuum degassing process.

C, i = Co−ΣdCdt, i × dt, i × 100 / W (2)
In the equation (2), dt, i represents the calculation time increment at time ti, and W represents the molten steel amount (kg). Further, when there is a time lag in the exhaust gas information, it may be calculated by correcting that amount.

  And in order to confirm the effect of this invention regarding this procedure A, the conventional method ((a) method) which inject | pours only the inert gas (Ar gas) for a recirculation | reflux simply from a riser pipe | tube, to this (a) method In addition, the present method of diluting and mixing the exhaust gas by blowing inert gas from the side tuyeres at the bottom of the vacuum chamber (method (b)), or a lance that can be raised and lowered on the canopy located at the top of the vacuum chamber, During the RH decarburization process, three methods of the present invention (method (c)) in which an inert gas for mixing is blown from the gas ejection hole at the tip of the lance toward the substantially central portion of the vacuum chamber to dilute and mix the exhaust gas. The carbon concentration obtained by collecting and analyzing a sample in steel when the carbon concentration of the molten steel reaches 20 ppm (0.002%), and the estimated carbon concentration calculated by the methods (a) to (c) Estimate error (ppm), which is the difference, and calculate standard deviation σ I was determined.

FIG. 1 is an explanatory diagram showing a situation in which the method (a) is performed, FIG. 2 is an explanatory diagram showing a situation in which the method (b) is implemented, and FIG. 3 is a situation in which the method (c) is implemented. It is explanatory drawing which shows.
As shown in FIGS. 1 to 3, the molten steel 4 is degassed while the dip tube 2 provided at the lower part of the vacuum chamber 1 is immersed in the molten steel 4 accommodated in the pan 3. An inert gas (Ar gas) for recirculation is blown from the side wall of one riser pipe 2a of the dip pipe 2, whereby the molten steel 4 is lifted from the ladle 3 to the inside of the vacuum tank 1, and the vacuum tank 1 is degassed. The degassed molten steel 4 forms a circulating flow from the other descending pipe 2 b of the dip pipe 2 to the ladle 3. An evacuation system 5 is connected to the upper side wall of the vacuum chamber 1, and an exhaust gas component is analyzed by an exhaust gas analysis system 6 provided here.

  Further, in FIG. 2, a blowing device (not shown) for blowing an inert gas is attached to the side wall of the vacuum chamber 1 so that the inert gas can be blown into the vacuum chamber 1 independently. It is configured.

  Further, in FIG. 3, a lance 7 for independently blowing an inert gas for mixing downward is arranged at the center of the canopy 1 a of the vacuum chamber 1 so as to be movable up and down. An inert gas for mixing alone is blown into the inside of the vacuum chamber, whereby the degassing component contained in the exhaust gas separated from the molten steel 4 is independently blown inside the vacuum chamber 1 by the inert gas for mixing. Dilute and mix.

As a result, in the method (a), the standard deviation σ of the estimation error was very large as 5.5 ppm, and sufficient estimation accuracy of the carbon concentration could not be obtained. The reason why the standard deviation σ is increased in this way is that, according to the method (a), the inert gas for recirculation blown into the vacuum chamber 1 together with the molten steel 4 is unevenly distributed in the vacuum chamber 1, so that the CO in the exhaust gas A problem arises in the representativeness of the concentration of gas and CO ₂ gas.

  Further, according to the method (b) which is the method of the present embodiment, the standard deviation σ of the estimation error is 4.2 ppm, which is improved from the method (a). This is considered due to the effect of improving the mixing property by introducing an inert gas from the side tuyere of the vacuum chamber 1.

On the other hand, according to the method (c), which is a method of the present preferred embodiment, the standard deviation of the estimation error was significantly reduced to 2.5 ppm. This is because the inert gas for mixing is blown up alone into the molten steel 4 from the lance 7 which is disposed in the central portion of the vacuum chamber 1 so as to be movable up and down and blows the inert gas alone (independently). This is because the representativeness of the concentration of CO gas and CO ₂ gas in the exhaust gas is remarkably improved by spraying. Thus, the superiority of the method (c) was confirmed for the procedure A.

(Procedure B)
Instead of using the analytical value of the carbon concentration before the vacuum degassing treatment as in the above-mentioned procedure A as the procedure B, the sample is collected approximately at a timing of about 80 to 100 ppm of the carbon concentration. The carbon concentration was estimated in the same manner as Procedure A as Co.

  As a result, the standard deviation σ of the estimation error is 5.1 ppm in the method (a) described above, and 3.9 ppm in the method (b) which is the method of the present invention, whereas it is a desirable method of the method of the present invention. In a certain method (c), it was 2.3 ppm, and the superiority of the method (c) was confirmed for procedure B.

(Procedure C)
Obtaining the decarburization rate at each time from the concentration of the diluted and mixed CO gas and / or CO ₂ gas, and obtaining a combination of the decarburization rate constant and the carbon concentration that balances the decarburization rate at each obtained time The exhaust gas at time ti during the vacuum degassing process is analyzed to measure the CO fraction CO, i, CO ₂ fraction CO ₂ , i in the exhaust gas.

(Ii) Measure the exhaust gas flow rate at time ti using the exhaust gas flow meter 6, and introduce the tracer gas with a known flow rate contained in the exhaust gas, and calculate it from the tracer gas concentration and the known tracer gas flow rate. Or by empirically determining the amount of input gas contained in the exhaust gas. The exhaust gas flow rate at time ti determined in this way is converted to a volume flow rate Qex, i (Nm ³ / s) converted to a standard state by an appropriate method.

(Iii) The product of the sum of the CO fraction CO, i and the CO ₂ fraction CO ₂ , i in the exhaust gas determined by the above-mentioned item (i) and the exhaust gas flow rate Qex, i determined by the item (ii) (CO , I + CO ₂ , i) · Qex is obtained.

(Iv) The decarburization rate dCdt, i (kg / s) per unit time at time ti is obtained from the following equation (3) using the proportionality constant A.
dCdt, i = A · (CO, i + CO ₂ , i) · Qex
... (3)
(V) When the carbon concentration during the degassing treatment is C (%) and the equilibrium carbon concentration is Ce (%), the decarburization rate is expressed by the general formula (4). Since the decarburization rate dCdti (kg / s) per unit time at time ti is (−dC / dt), i, it is expressed by equation (6) using the decarburization rate constant Ki at time ti. The dCdti, i from the time when the carbon concentration is reached to the time ti is added to the carbon concentration Co (%) in the molten steel 4 obtained before or during the vacuum degassing treatment, and the current carbon concentration Ci (%) Is calculated using, for example, equations (4) to (6).

dC / dt = −K × (C−Ce) (4)
Ce = Keq / (P × O) (5)
dCdt, i = Ki × (C, i−Ce, i / 100) × W (6)
In equations (4) to (6), Keq represents an equilibrium constant, P represents the pressure inside the vacuum chamber 1, and O represents the O concentration in the steel. In order to simplify the calculation, Ce = 0 may be set as necessary.

 (Vi) Equations (3) and (6) are both decarburization rates per unit time, and since they coincide, Equation (7) or Equation (8) is obtained. Since the right side of the equation (8) is constituted only by the molten steel weight and the exhaust gas information, it is an existing value by measurement. Therefore, if these are set as γ and i, respectively, C combinations C and i in the molten steel 4 can be calculated by obtaining a combination of K, i, C, and i that matches γ and i. For example, C and i can be calculated by equation (9) by estimating K and i by an appropriate means.

A × (CO, i + CO ₂ , i) · Qex, i = K, i × (C, i / 100) × W
(7)
K, i × (C, i−Ce, i) = A × (100 / W) × (CO, i + CO ₂ , i).
Qex, i = γ, i (8)
C, i = Ce, i + γ, i / K, i (9)
(Vii) The method of obtaining the combination of K, i and C, i is not uniform, and various methods can be adopted. For example, Ci can be calculated by setting a function form for substituting operation conditions and estimating K and i using a so-called empirical formula. The coefficient used in the function form may be obtained empirically, for example, by statistical processing.

  For example, an example in which an RH degassing apparatus is used as the degassing apparatus is shown below. The decarburization rate constant K in the RH degassing apparatus can be expressed by the equation (10) using the reflux rate Q of the molten steel 4 and the capacity coefficient ak of the de-C reaction inside the vacuum chamber 1.

K = (Q / W) × ak / (Q + ak) (10)
The circulatory velocity Q is generally known as an empirical formula as a function of the degree of vacuum, the diameter of the dip tube, the flow rate of the circulated gas, etc. The flow rate Qi can be calculated.

  Substituting equation (10) into equation (9) and rearranging results in equation (11) for calculating C and i. If Q, i, ak, and i at time ti are determined by an appropriate method, C, i Can be calculated.

C, i = Ce, i + γ, i × (Q, i + ak, i) / {(Q, i / W) × ak, i} (11)
For example, ak, i can be calculated using the following empirical formula.
ak, i = α · [C], ^iβ · [O], i ^γ · P, i ^δ · Qar, i ^ε
(12)
However, α, β, γ, δ, and ε are constants, [C], i, [O], i are the carbon concentration and oxygen concentration in the molten steel at time ti, and P indicates the vacuum atmosphere pressure.

  Here, the constant can be determined in advance by statistical processing or the like using an actual measurement value. If the equations (11) and (12) are combined and [O], i, Pi, Qar, i are substituted as measured values, [C], i at the time ti can be calculated. Although an example of the exponentiation format is shown as the empirical formula (12), the expression format of the empirical formula is not limited at all as long as the actual measurement value can be expressed.

  In the present embodiment, the concentration of C contained in the molten steel 4 is estimated in this way. At this time, according to the method of the present invention shown in the method (c) described above, the estimation error is remarkably reduced by any of the estimation procedures of the procedures A to C.

  And in this Embodiment, if ultra-low carbon steel is manufactured using the C concentration of the molten steel 4 estimated in this way, the carbon concentration during the vacuum degassing process can be estimated with high accuracy. The vacuum degassing process can be terminated immediately when a carbon concentration slightly lower than the upper limit value of the carbon concentration is reached. For this reason, the degassing processing time can be greatly shortened, and the processing cost can be suppressed and the productivity can be improved.

And the molten steel 4 manufactured by performing a predetermined decarburizing process is then subjected to, for example, continuous casting after further component adjustment by appropriately adding an alloy or the like.
In the above description, the case where the present invention is applied to an RH degassing apparatus having two dip tubes has been described. However, the present invention is applied to a DH degassing apparatus having one dip tube. Since this is also applicable, this point will also be described.

FIG. 4 is an explanatory view schematically showing a situation where the present invention is applied to a DH degassing apparatus.
The degassing treatment of the molten steel 4 is performed while immersing the dip tube 2 provided in the lower part of the vacuum chamber 1 in the molten steel 4 accommodated in the pan 3. An inert gas (Ar gas) for recirculation is blown from the bottom tuyere 3 a of the ladle 3, so that the molten steel 4 is lifted up from the ladle 3 to the inside of the vacuum chamber 1, and is removed inside the vacuum chamber 1. Gas. The degassed molten steel 4 forms a circulating flow that returns from the dip tube 2 to the ladle 3.

  Further, in FIG. 4, a lance 7 for blowing an inert gas for mixing alone downward is arranged at the center of the canopy 1 a of the vacuum chamber 1 so as to be movable up and down. An inert gas for mixing alone is blown into the inside of the vacuum chamber, whereby the degassing component contained in the exhaust gas separated from the molten steel 4 is independently blown inside the vacuum chamber 1 by the inert gas for mixing. Dilute and mix.

  As described above, in the DH degassing apparatus having one dip tube, it is difficult to determine a clear circulation velocity Q of the molten steel 4. For this reason, the decarburization rate constant is quantified in advance as a function of the inert gas flow rate Qar for stirring, the pressure P inside the vacuum chamber 1 and other appropriate variables as shown in the equation (13). Thereby, the carbon concentrations C and i at the time ti can be estimated as shown in the equation (14).

K = f (Qar, P, var1, var2, var3,...)
(13)
C, i = Ce, i + γ, i / K, i (14)
And the carbon concentration of the molten steel 4 was estimated by the method (a)-(c) mentioned above. As a result, the standard deviation σ of the estimation error in method (a) was very large at 5.2 ppm, and a sufficient carbon concentration could not be estimated. The reason why the standard deviation σ is increased in this way is considered to be because there is a problem in the representativeness of the concentration of CO gas and CO ₂ gas in the exhaust gas. Since only the inert gas for recirculation is blown into the inside of the vacuum chamber 1 simply from the bottom of the ladle, the problem of representativeness arises because the inert gas for recirculation is unevenly distributed.

  In the method (b), the standard deviation σ of the estimation error was 4.0 ppm, which was improved as compared with the method (a). This is considered to be caused by the effect of improving the mixing property by blowing the inert gas into the vacuum chamber 1 from the side tuyere of the vacuum chamber 1.

On the other hand, in the method (c), the standard deviation of the estimation error was remarkably improved to 2.4 ppm. This is performed by blowing an inert gas on the molten steel 4 from a lance 7 which is disposed in the central part of the vacuum chamber 1 so as to be movable up and down and independently (independently) for blowing the inert gas for mixing. This is considered to be because the representativeness of the CO gas and CO ₂ gas concentrations in the exhaust gas is remarkably improved.

  In addition, the desirable flow rate of the inert gas for mixing blown into the inside of the vacuum chamber 1 is 0.5 times or more the flow rate of the inert gas for reflux. If it is less than this, the mixing effect is small, and the effect of improving the representativeness of the components of the exhaust gas is small, and more desirably 1.0 times or more. From this point of view, it is not necessary to set the upper limit of the flow rate of the inert gas for mixing, but from the viewpoint of reducing the operating cost and suppressing the load on the vacuum exhaust system, the upper limit of the flow rate of the inert gas for mixing is The upper limit is desirably 10 times, and a more desirable upper limit is 6 times.

  Further, the blowing position of the inert gas for mixing is preferably in the central portion rather than in the vicinity of the wall of the vacuum chamber 1, and desirably in a region within ½ of the diameter inside the vacuum chamber 1. This is because if the inert gas for mixing is blown from the vicinity of the inner wall of the vacuum chamber 1, the effect of improving the representativeness of the concentration of exhaust gas is reduced.

  The blowing height of the inert gas for mixing is preferably between the canopy 1a inside the vacuum chamber 1 and the molten steel 4 surface, but is desirably higher than the intermediate height between the canopy 1a and the molten steel 4 surface. It is desirable that the upper side is also upward. This is because if it is lower than this, the effect of improving the representativeness of the exhaust gas concentration by the inert gas for mixing decreases. More desirably, when the distance between the canopy 1a and the molten steel 4 is X, the distance is higher than the height of X / 3 from the inner canopy.

As described above, according to the method of the present invention, the estimation error can be significantly reduced in any of the estimation procedures A to C.
Thus, according to this embodiment, when performing vacuum degassing using a vacuum smelting furnace using a vacuum degassing apparatus, the component concentration such as C concentration of molten steel 4 is estimated with high accuracy. As a result, ultra-low carbon steel could be reliably produced.

Further, the present invention will be described in detail with reference to examples.
After the hot metal discharged from the blast furnace was transferred to a topped car and transported to a converter plant, one or more hot metal pretreatments such as desiliconization, desulfurization, and dephosphorization were performed, and this hot metal was charged into a 250-ton converter. Then decarburization blowing was performed.

  The molten steel obtained by decarburization blowing is discharged from the converter outlet hole to the ladle, and the ladle containing the molten steel is transferred to the secondary refining equipment (RH vacuum degassing device) for degassing treatment. It was. The carbon concentration after the converter blowing is 0.04%, and the exhaust gas information during the RH vacuum degassing process when the decarburization is performed to approximately 0.002% in the RH degassing apparatus is taken into the calculator. Thus, an estimated value of the carbon concentration of the molten steel was calculated. And the molten steel sample was extract | collected at the time of completion | finish of the decarburization by RH vacuum degassing apparatus, and the standard deviation of the estimation error of the analytical value was investigated. The RH vacuum degassing apparatus was vacuum decarburized under the conditions that the diameter of the dip tube was 0.75 m, the flow rate of the inert gas for reflux was 2000 NL / min, and the ultimate vacuum inside the vacuum chamber was 133 MPa. .

  In this example, when the inert gas for mixing is not blown into the vacuum chamber (Method a), when the inert gas for mixing is blown from the upper side surface of the bath surface of the molten steel accommodated in the vacuum chamber ( The method b) was compared with a case where a lance was provided on the canopy above the vacuum chamber and an inert gas for mixing was blown (method c). As the inert gas for mixing, 4000 NL / min nitrogen gas was used.

  First, Table 1 shows the results of investigating the results of the standard deviation σ of the estimation error of extremely low carbon steel. The results are summarized in Table 2.

  From this result, the carbon concentration was determined with the aim of melting an ultra-low carbon steel having an upper limit of 28 ppm of the C concentration of molten steel without causing desorption. When the standard deviation σ is large, there is a high possibility of component removal, so the target C concentration must be set low, and as a result, the decarburization time becomes longer.

When the effect of shortening the decarburization treatment time was confirmed with the above-mentioned aim, it was shortened by 1.8 minutes in the method b with respect to the method a, but it was possible to shorten it by 4.8 minutes in the method c.
Moreover, as a result of investigating the cost index when the decarburization treatment cost in method a was 1.0, the cost index in method b was 0.89. On the other hand, method c has a large time shortening effect, and the cost index can be greatly reduced to 0.70.

  As described above, the method of the present invention is a method that can greatly reduce the melting cost by shortening the vacuum processing time.

  The same test as in Example 1 was performed, and the carbon concentration of the molten steel was estimated by Procedure C. First, the results in Table 3 were obtained for the standard deviation of the estimation error. Based on this, the effect of shortening the treatment time of ultra-low carbon steel having a carbon concentration of 28 ppm or less was investigated. The results are summarized in Table 4.

  As a result, the method a can be shortened by 1.9 min in the method b and 5.0 min in the method c. As a result, the cost index when method a is set to 1.0 is 0.87 in method b and 0.68 in method c, and the method of the present invention can exhibit time reduction and cost reduction effects. It could be confirmed.

The same experiment as in Example 1 was performed using a DH vacuum degassing apparatus using one large dip tube at the bottom of the vacuum chamber. In this example, the dip tube diameter was 2.0 m, Ar gas was flowed at 1000 NL / min as a stirring gas from the bottom of the ladle, and the ultimate pressure inside the vacuum chamber was 133 Pa. The procedure B similar to Example 1 was employ | adopted as the estimation method of the carbon concentration of molten steel.
First, the results of Table 5 were obtained for the standard deviation of the estimation error. Based on this, the effect of shortening the treatment time of ultra-low carbon steel having a carbon concentration of 28 ppm or less was investigated. The results are summarized in Table 6.

  As a result, the method a can be shortened by 1.9 min in the method b and 5.0 min in the method c. Along with this, the cost index when method a is set to 1.0 is 0.88 for method b and 0.69 for method c, and method c can exhibit significant time reduction and cost reduction effects. Was confirmed.

  As described above, according to the present invention, it can be seen that the melting cost can be significantly reduced by shortening the vacuum processing time.

(A) It is explanatory drawing which shows the condition which enforces a method. (B) It is explanatory drawing which shows the condition which enforces a method. (C) It is explanatory drawing which shows the condition which enforces a method. It is explanatory drawing which shows typically the condition which applied this invention with respect to DH degassing processing apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vacuum chamber 1a Canopy 2 Immersion pipe 2a Rise pipe 2b Downcomer pipe 3 Ladle 4 Molten steel 5 Vacuum exhaust system 6 Exhaust gas analysis system 7 Lance

Claims (9)

An inert gas for stirring that is blown into the molten steel when the degassing of the molten steel is performed while immersing the dip tube provided in the lower part of the vacuum tank in the molten steel contained in the ladle. Or, an inert gas different from the inert gas for recirculation is blown independently from the inert gas for agitation or the inert gas for recirculation, and is contained in the exhaust gas separated from the molten steel inside the vacuum chamber. degassing components were mixed and diluted with an inert gas blown the independently being, characterized by using an analytical value of degassing component concentration which is the diluted mixture, to estimate the concentration of the components contained in the molten steel A method for estimating the component concentration of molten steel.   The said inert gas is an estimation method of the component concentration of the molten steel described in Claim 1 blown from the lance arrange | positioned at the canopy or side surface of the said vacuum chamber. The concentration of the component contained in the molten steel is estimated by using the analysis value of the concentration of the degassed component diluted and mixed with the inert gas. Estimation method. Estimation method of the degassing component CO gas and / or CO ₂ component contained in the molten steel as well as a gas component concentration of the molten steel according to any one of claims 1 is a carbon to claim 3 . The concentration of carbon contained in the molten steel is obtained by calculating the decarburization rate at each time from the concentration of the diluted and mixed CO gas and / or CO ₂ gas, and converted into the concentration using the obtained decarburization rate at each time. The calculated decarburization integrated amount is calculated, and the component concentration of the molten steel according to claim 4 is estimated by subtracting the integrated decarburization amount from the analysis value of the carbon concentration before or during the vacuum degassing process. Estimation method. The concentration of carbon contained in the molten steel is determined by determining the decarburization speed at each time from the concentration of the diluted and mixed CO gas and / or CO ₂ gas, and the decarburization speed balanced with the obtained decarburization speed at each time. The estimation method of the component concentration of the molten steel of Claim 4 estimated by calculating | requiring the combination of a constant and carbon concentration.   The method for estimating the component concentration of molten steel according to any one of claims 1 to 6, wherein an inert gas for stirring or an inert gas for reflux is blown into the molten steel.   The method for estimating the component concentration of molten steel according to any one of claims 1 to 7, wherein the number of said dip tubes is one or two. A method for producing an ultra-low carbon steel, comprising a step of estimating a carbon concentration of molten steel by the method for estimating a component concentration of molten steel according to any one of claims 1 to 8.

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