JP4816513B2 - Molten steel component estimation method - Google Patents

Description

  The present invention relates to a method for estimating molten steel components in a converter-type refining furnace and a method for producing low phosphorus steel using the method.

  In converter blowing, control of molten steel components at the end of blowing, especially control of phosphorus (hereinafter referred to as “P” using element symbols) concentration, is very important for quality control of steel. Conventionally, various methods have been used for monitoring and controlling the P concentration in the molten steel, but there are many restrictions on the blowing conditions, especially when the carbon concentration in the molten steel is high at the end of the blowing. There was no technique for accurately estimating the concentration.

  (1) In Patent Document 1, as a method for estimating the P concentration in molten steel, the oxygen concentration in the molten steel at the end of blowing is measured with an oxygen sensor using a solid electrolyte, and the P concentration is calculated using the measured value. The estimation method is shown. This method measures the oxygen concentration in the molten steel, but this measured value is proportional to the carbon concentration in the molten steel, and is a value that can be sufficiently estimated from the carbon concentration without measuring with an oxygen sensor. is there. In addition, it is known that the influence of oxygen concentration in slag is large as a factor affecting the amount of de-P in blowing. Since this method does not know the oxygen concentration in slag, The estimates were not accurate.

  (2) Patent Document 2 estimates slag components (% FetO) and (% CaO) from molten oxygen concentration measurement results in molten steel as a method for estimating P concentration in molten steel, and estimates P concentration at the end of blowing How to do is shown. However, since the relationship between the oxygen concentration in molten steel and the index of oxygen concentration in slag (% FetO) varies greatly depending on the carbon concentration in molten steel, this method accurately grasps the oxygen concentration in slag over a wide range of carbon concentrations. It is difficult.

  In the above two methods, it is considered that P enters exclusively from hot metal, and no factors other than hot metal are considered. In actual operation, there is an effect of carry-over slag in the previous process, and unless this effect is taken into account, it is impossible to estimate the P concentration in molten steel with high accuracy.

  Furthermore, since the above two methods estimate the phosphorus concentration in the molten steel based on the measurement of the dissolved oxygen concentration in the molten steel, the estimation method and solution of the P concentration in the molten steel at the so-called low carbon concentration blowing end point Is done. This is because the reaction relationship between the molten steel in the converter and its slag during blowing is stable in the low carbon region. Specifically, the oxygen concentration shown in the above two patent documents is 300 ppm or more, and from FIG. 1 of Patent Document 1, it is understood that the oxygen concentration of 300 ppm or more corresponds to a carbon concentration of 0.07% or less. .

  (3) Patent Document 3 discloses a method for estimating the P concentration in molten steel using the oxygen concentration in the slag. In this method, it was found that the amount of oxygen not consumed in the decarburization reaction was proportional to the oxygen concentration in the slag, and the P concentration in the molten steel was estimated from the calculated oxygen concentration in the slag. It is a method to do. However, since this method does not use an oxygen sensor, there may be a large variation in the oxygen concentration in the calculated slag, and it has not been possible to obtain stable P estimation accuracy that is stable over the long term.

(4) Patent Document 4 discloses a method for directly measuring the oxygen concentration in slag as a method for measuring the oxygen concentration in slag. Although it has become possible to grasp the oxygen concentration in the slag by using this method, a method for accurately estimating the P concentration in the molten steel using the measurement results is not shown.
JP 59-136652 A JP-A 61-12811 Japanese Patent Publication No. 6-49890 JP 2000-214127 A

In recent years, with the spread of hot metal dephosphorization, it has become possible to finish blowing in a high carbon concentration region, and there is a need to estimate the P concentration in molten steel for a wide range of molten steel carbon concentrations.
An object of the present invention is to present a method for accurately estimating the P concentration in molten steel using the oxygen concentration in slag with respect to the carbon concentration in molten steel at the end of a wide range of blowing.

The inventors have invented a method for accurately estimating the P concentration in the molten steel even when the carbon concentration in the molten steel at the end of blowing is varied over a wide range.
The present invention is as follows.

(1) Hot metal dephosphorization treatment is performed when decarburization and dephosphorization blowing are performed in a converter to a carbon concentration in molten steel of 1.0% by mass or less and a phosphorus concentration of 0.030% by mass or less. A method of measuring the oxygen concentration in the slag using an oxygen sensor attached to the sub lance, using the molten iron, and estimating the phosphorus concentration in the molten steel at the end of the blowing using the measured value.

(2) Hot metal dephosphorization treatment is performed when decarburization and dephosphorization blowing to a carbon concentration of 1.0% by mass or less and a phosphorus concentration of 0.030% by mass or less in a converter. Using the hot metal performed, the amount of carry-over slag in the previous process of decarburization and dephosphorization was 3 kg or less per ton of hot metal, and the oxygen concentration in the slag was measured with an oxygen sensor attached to the sub lance. To estimate the phosphorus concentration in molten steel at the end of blowing.

(3) The higher the carbon concentration in the molten steel at the end of blowing, the higher the concentration of phosphorus in the molten steel at the end of blowing, and the influence of the oxygen concentration in the slag on the estimated phosphorus concentration in the molten steel. The method for estimating the phosphorus concentration in molten steel at the end of blowing described in (1) or (2) above, wherein the contribution ratio is increased.

(4) A low phosphorus steel that performs decarburization and dephosphorization of hot metal until the carbon concentration in the molten steel is 1.0 mass% or less and the phosphorus concentration is 0.030 mass% or less in a converter. A manufacturing method for estimating phosphorus concentration in molten steel at the end of blowing using the method for estimating phosphorus concentration in molten steel at the end of blowing described in any one of (1) to (1) above A method for producing low-phosphorus steel.

  Here, in the present specification, the “hot metal dephosphorization treatment” means that, prior to decarburization blowing to a carbon concentration of 1.0% by mass or less in a converter, When the concentration of carbon in the hot metal is 3.0% or more, the phosphorus concentration in the hot metal is reduced by 0.5% or more, for example, when the hot metal P concentration is 0.12%, the P concentration is less than 0.07%. Say.

  The “pre-process of decarburization and dephosphorization” is not limited to the hot metal dephosphorization process described above, but for decarburization blown until the carbon concentration in the molten steel becomes 1.0 mass% or less in a converter. It means the refining process of hot metal charged in the converter immediately before charging.

  In the present invention, in order to perform the “molten dephosphorization treatment” prior to “decarburization blowing”, the “pre-process of decarburization and dephosphorization blowing” is often “molten dephosphorization treatment”. If necessary, “hot metal desulfurization treatment” may be performed after “hot metal dephosphorization treatment” and immediately before “decarburization blowing”.

  The “carry-over slag” is slag after the processing of the above “decarburization and dephosphorization pre-process”, and the carbon concentration in the molten steel is reduced to 1.0% by mass or less in the converter. This refers to “mixed slag” that is inserted into the converter together with the hot metal when the hot metal charged into the converter is charged for smelting.

“Contribution of the effect of oxygen concentration in slag on the estimated value of phosphorus concentration in molten steel” means that “index representing oxygen partial pressure in slag” included in the formula for estimating P concentration in molten steel Means the degree of influence on the estimated value of medium P concentration.
As a method of changing the contribution ratio of oxygen partial pressure in slag according to carbon concentration in the P concentration estimation formula in molten steel, the ratio of contained P mass concentration between slag and molten steel according to carbon concentration (hereinafter referred to as “P distribution”). The ratio of the oxygen partial pressure in the slag in the P distribution estimation equation may be varied according to the carbon concentration in the molten steel at the end of blowing. The coefficient of each item in the distribution ratio estimation formula may be calculated as a function of the carbon concentration in the molten steel.

“Low phosphorus steel” refers to a steel material having a phosphorus concentration of 0.030% or less in a product.
The “carbon concentration at the end of a wide range of blowing” means a range in which the carbon concentration in the molten steel in the converter after the oxygen supply stop in the converter is 1.0% by mass or less. The lower limit of this carbon concentration is 0.03%, which is the lower limit of the decarburization capacity of a normal converter, but the more preferable lower limit in the present invention is 0.10%.

  According to the present invention, in the blowing method using a sublance, by using the P distribution ratio estimation formula that takes into account the measurement result of the oxygen sensor using a solid electrolyte, the P concentration in the molten steel at the end of blowing is accurately determined. It is possible to estimate.

  In the present invention, the carbon concentration in the molten steel in the converter at the end of blowing is 1.0% or less and 0.03% or more, and in particular, the carbon concentration is 1.0% or less and 0.10% or more. In a wide range, the phosphorus concentration in the molten steel can be estimated with an estimation accuracy within the target P% ± 0.003% with respect to the target P% in which the phosphorus concentration in the end-point molten steel is 0.03% or less.

The embodiment of the present invention will be specifically described.
In the practice of the present invention, hot metal from which hot metal dephosphorization has been performed in advance is prepared and charged into a converter as a refining furnace. Preferably, the carry-over slag amount at this time is 3 kg or less per ton of hot metal. The hot metal charged in the converter is decarburized and dephosphorized to a carbon concentration of 1.0 mass% or less and a phosphorus concentration of 0.030 mass% or less. The conditions for decarburization and dephosphorization blowing in the converter at this time are not particularly limited as long as target decarburization and dephosphorization are performed. If necessary, bottom blowing of an inert gas may be performed.

  There is no restriction | limiting in particular about the kind of oxygen sensor used in this invention, and a structure, The oxygen concentration in slag should just be measured by immersion in slag. Preferably, an oxygen sensor using zirconia as the solid electrolyte and molybdenum as the reference electrode is used. Specifically, the oxygen sensor disclosed in Patent Document 4 is exemplified.

  Except for measuring the oxygen concentration in the slag with an oxygen sensor, the treatment may be performed under the same conditions as the decarburization / dephosphorization treatment in a normal converter. This oxygen sensor is usually attached to the tip of a sub lance which is one of the facilities installed in the converter, and the oxygen concentration in the slag is measured by immersing the sensor portion in the slag. The timing of measuring the oxygen concentration in the slag with the sublance is the best at estimating the P concentration in the molten steel at the end of the blowing, but the amount of oxygen sent from the sublance measurement to the end of the blowing is even during the blowing. When the amount is small, it is possible to estimate the P concentration in the molten steel at the end of blowing.

  Here, according to the aspect of the present invention, in order to estimate the P concentration in the molten steel at the end of blowing, it is necessary to accurately grasp the de-P amount. As a means for grasping the amount of de-P, there is a method of calculating using the P distribution ratio between the two phases of molten steel and slag at the end of blowing and the P material balance equation.

P distribution ratio formula Lp = P _slag / P _{molten steel} (1)
P substance balance formula:
P _{hot metal} x W _{hot metal} + P _{carryover slag} x W _{carryover slag} = P _slag x W _slag + P _{molten steel} x W _{molten steel} (2)
Formula for calculating _steel output: W _{molten steel} = W _{molten iron} x Y (3)
Lp: P distribution ratio P _slag : P concentration in _molten slag at the end of blowing P _{molten steel} : P concentration in _{molten steel} at the end of blowing P _{hot metal} : P concentration in _{molten iron} at the beginning of blowing W _{hot metal} : charged molten iron weight P _{carryover slag:} carryover slag P concentration W _{carryover slag:} carryover slag weight W _{molten steel:} tapping molten steel weight W _slag: blowing end point when slag weight Y: operation average yield the formula (1), (2), (3) , P _{molten steel} = (P _{molten iron} + P _{carrying slag} x W _{carrying slag} / W _{molten iron} ) / (Lp x W _slag / W _{molten iron} + Y) (4)
It becomes.

  Here, the P concentration in the hot metal at the start of blowing may be determined by using the component analysis value before blowing. The charged hot metal weight can be easily measured from the pot weight before blowing, It can be easily calculated from the past molten iron weight and outgoing steel molten steel weight.

The slag weight at the end of blowing is obtained by the following formula.
W _slag = input raw material weight (slag product such as quicklime) + generated slag weight from hot metal components + carry-over slag weight (5)
The input raw material weight can be easily grasped, and the generated slag weight from the hot metal components can be calculated.

  Here, the carry-over slag weight and the P concentration in the carry-over slag are large variation factors in both equations (4) and (5). However, the carry-over slag weight and the P concentration in the carry-over slag are difficult to measure in advance, and can only be calculated from the material balance using the slag component and the molten steel component after the completion of blowing.

  In the present invention, it has been found that the accuracy of estimating P in molten steel is greatly improved by setting the carry-over slag weight to 3 kg or less per 1 ton of hot metal (hereinafter 3 kg / ton). The most reliable method of reducing the carry-over slag weight to 3 kg / ton or less is to remove the molten iron in the hot metal ladle before charging the converter, but simply reducing the carry-over slag weight using a slag stopper in the previous process. You may let them.

  However, in the present invention, the hot metal dephosphorization treatment is performed in another converter prior to decarburizing and blowing until the carbon concentration in the molten steel reaches 1.0% by mass or less in the converter. In addition, the estimated accuracy of the target P% ± 0.003% for the target P% in the molten steel with a phosphorus concentration in the end-point molten steel of 0.03% or less in the "carbon concentration in the molten steel at the end of extensive blowing" Obtainable.

  In the hot metal dephosphorization treatment by the converter, the hot metal after the dephosphorization treatment is discharged to the transfer pan through the tap hole provided in the upper part of the converter, so that the slag after the dephosphorization process is stably discharged to the transfer pan. Can be reduced. As a result of this investigation, the ratio when the outflow slag amount was 3 kg / t or less with respect to the total number of decarburization / dephosphorization processes in the converter was 90% or more. In addition, for transfer pans with a large amount of outflow slag when the number of treatments is less than 10%, the slag can be easily removed before charging into the converter for decarburization blow-down, so The outflow slag amount can be reduced to 3 kg / t or less.

  For the problem that the carry-over slag weight cannot be grasped in advance, the carry-over slag weight is reduced to 3 kg / ton or less, so the effect of the carry-over slag itself is greatly reduced. The actual average value of normal operation may be used.

  The carry-over slag weight in the range investigated this time was 0.2-3.0 kg / t including the dehulling process from the transfer pan about a part mentioned above. The hot metal conditions and slag conditions after the hot metal preliminary treatment investigated this time are summarized in Tables 1 and 2.

As an equation for calculating the P distribution ratio, the equation of Health is famous.
Health formula Lp = 2.5 × log (% T. Fe) + 0.08 × (% CaO) +
22350 / T-16 (6)
Lp: P distribution ratio = P concentration in slag (mass%) / P concentration in molten steel (mass%)
(% T. Fe): Total iron concentration in slag (% CaO): CaO concentration in slag T: Molten steel absolute temperature Referring to the Health equation, the P distribution ratio is affected by the molten steel temperature and the CaO concentration in the slag. There is a sublance temperature measurement for molten steel. The CaO concentration in the slag can be easily calculated from the amount charged into the converter furnace and the amount of oxide generated during blowing.

  The P distribution ratio is affected by the total iron concentration in the slag. The total iron concentration in slag is the concentration of iron present as iron oxide in slag, and is an index of the degree of slag oxidation. That is, the P distribution ratio is affected by the degree of slag oxidation.

Therefore, this time, the oxygen partial pressure in the slag corresponding to the oxygen concentration in the slag was directly measured, and the P distribution ratio was estimated using the measurement result.
When zirconia is used for the solid electrolyte of the oxygen sensor and molybdenum is used for the reference electrode, the equation for converting the electromotive force into the oxygen partial pressure is as follows.
LogPo ₂ = 8.936 + (20.16 × E-39416) / T (7)
E: Measurement electromotive force T: Absolute temperature FIG. 1 shows the relationship between oxygen partial pressure in slag and P distribution ratio.

  In addition to the oxygen partial pressure in the slag measured this time, a P distribution ratio estimation formula was constructed based on the molten steel temperature and the CaO concentration in the calculated slag, making it possible to calculate the P distribution ratio with high accuracy. At this time, the coefficient used for the P distribution ratio estimation formula needs to be reconstructed by multiple regression analysis, instead of using the Health formula as it is. This is because the contribution rate varies depending on the blowing conditions (top blowing oxygen conditions, bottom blowing gas conditions, etc.).

  Fig. 2 shows the relationship between the absolute value of the difference between the estimated P concentration in molten steel and the P concentration in actual molten steel and the measured value of carried over slag weight by using the carryover slag P concentration and weight, and the P distribution estimation formula. Show.

  In converter blowing, when the carry-over slag weight is 3 kg / ton or less, the difference between the estimated value and the actual value is 0.003% or less, and the effect of carry-over slag amount on the estimated accuracy of P concentration in molten steel It turns out that the influence is great.

  When hot metal dephosphorization treatment is performed in a converter, it is easy to separate the hot metal and slag at the time of tapping, so the amount of dephosphorization slag that flows into the hot metal transfer pan after treatment should be specially taken. It was found that it could be 3 kg / t or less with a probability of 90% or more. The probability of 90% or more can be reliably set to 100% by using a slag stopper such as a so-called dart or by simply removing the slag stopper from the transfer pan with a slag drucker or the like.

Thus, it became possible to calculate the P concentration in molten steel with high accuracy by setting the carry-over slag amount to 3 kg / t or less (see FIG. 2).
When the oxygen concentration in the slag is measured at the sub-lance, the P estimation accuracy in the molten steel is the best at the end of the blowing, but the amount of acid sent from the sub-lance measurement to the end of the blowing is low even during the blowing Can sufficiently estimate the P concentration in the molten steel at the end of blowing.

In order to improve the estimation accuracy of the P concentration in the molten steel at the carbon concentration in the molten steel at the end point of a wide range of blowing, it is effective to change the contribution ratio of the oxygen concentration in the slag according to the carbon concentration in the molten steel.
FIG. 3 shows the relationship between the results of the oxygen partial pressure in molten steel and the oxygen partial pressure in slag measured using an oxygen sensor attached to the sub lance, and the carbon concentration in the molten steel. From the figure, the higher the carbon concentration, the larger the difference between the oxygen partial pressure in the molten steel and the oxygen partial pressure in the slag, and the oxygen partial pressure in the molten steel <the oxygen partial pressure in the slag. As a result, it was discovered that the higher the carbon concentration in the molten steel, the greater the contribution ratio of the oxygen partial pressure in the slag to the P distribution ratio.

  As a method of changing the contribution ratio of oxygen partial pressure in slag according to the carbon concentration in molten steel in the estimation formula of P concentration in molten steel, blow smelting using the results of multiple regression analysis of P distribution ratio for each carbon concentration The coefficient of oxygen partial pressure in the slag of the P distribution estimation equation may be varied according to the carbon concentration at the end point, or the coefficient of each item of the P distribution ratio estimation equation may be calculated as a function of the carbon concentration.

For example, when the equation of Healy is used as it is as an estimation formula for the P concentration in molten steel, the log (% T. Fe) term coefficient 2.5 is increased to 3.0 or the like.
In addition, in order to improve the estimation accuracy of molten steel P concentration in a wide range of molten steel carbon concentrations, the influence of oxygen concentration in molten steel cannot be ignored. A pressure term may be added. Since the oxygen partial pressure in molten steel has a strong correlation with the carbon concentration in molten steel, the carbon concentration in molten steel may be used as an alternative value.

  By using the above method, it became possible to estimate the P concentration in the molten steel with high accuracy for a wide range of carbon concentrations in the molten steel.

The Example which applied the blowing method by this invention is shown.
210 ton of molten steel was blown using a converter having an upper bottom blowing facility as a refining furnace for decarburization and dephosphorization. Oxygen was used from the top blow and inert gas was used from the bottom blow. The sublance was used at the end of blowing or at the end of blowing (85 to 90% of the entire blowing).

The measurement contents of the sublance were the molten steel temperature, the carbon concentration using the solidification temperature, the oxygen partial pressure in the molten steel using the solid electrolyte, and the oxygen partial pressure in the slag.
Table 3 shows the estimated value of the P concentration in molten steel using the operational results and the P distribution ratio estimation formula.

  As operating factors considered in the P distribution ratio estimation formula, oxygen partial pressure in slag, molten steel temperature, calculated CaO concentration in slag, and carbon concentration in molten steel were used. Multiple regression analysis was performed using the above items to construct a P distribution ratio estimation formula, and an estimated value of P concentration in molten steel was calculated using this formula. The carry-over slag weight was 2.0 k / t, which is the operation average value, and the P concentration in the slag was 2%.

First, Examples 1 to 3 are estimation results of the P concentration in molten steel when the amount of carry-over slag ≦ 3 k / t. The estimation errors of P concentration are all ≦ 0.003%.
Examples 4 to 6 are results of changing the contribution ratio of the oxygen concentration in the slag according to the carry-over slag amount ≦ 3 k / t and the carbon concentration. As a method for changing the contribution rate, multiple regression analysis of the P distribution ratio was performed separately according to the carbon concentration, and the regression results were used. In this case, factors other than the oxygen partial pressure in the slag also change the coefficient according to the carbon concentration. As a result, the estimation errors of P concentration are all ≦ 0.003%.

Examples 7 to 9 are results of sampling the sublance during blowing when the amount of carry-over slag ≦ 3 k / t and the contribution ratio of the oxygen concentration in the slag was changed according to the carbon concentration. If the amount of acid sent from the sublance measurement to the end point of blowing is about 1.0 Nm ³ / ton, it is possible to sufficiently estimate the P concentration in the molten steel at the end point of blowing.

Comparative Example 1 is an example in which the oxygen partial pressure in the slag is not considered in the P distribution ratio estimation formula. In this case, since the variation in oxygen partial pressure in the slag cannot be considered, the estimation accuracy of the P concentration in the molten steel deteriorates.
Comparative example 2 is an example of carry-over slag amount> 3 k / t. In this case, since the variation of carry-over slag cannot be taken into account, the estimation accuracy of the P concentration in the molten steel deteriorates.

  Comparative Example 3 is an example in which the contribution ratio of the oxygen concentration in the slag of the P distribution ratio estimation formula is not changed according to the carbon concentration. In this case, since the contribution rate of the oxygen concentration in the slag cannot be taken into account according to the carbon concentration in the molten steel, the estimation accuracy of the P concentration in the molten steel deteriorates.

  Thus, according to the present invention, in the blowing method using a sublance, by using the P distribution ratio estimation formula that takes into account the measurement result of the oxygen sensor using the solid electrolyte, the molten steel at the end of the blowing operation is accurately obtained. It is possible to estimate the medium P concentration.

  These results are summarized in Table 3.

It is a graph which shows the relationship between oxygen partial pressure in slag and P distribution ratio. It is a graph which shows the relationship between carry-over slag weight and P estimation precision. It is a graph which shows the relationship between carbon concentration and the partial pressure of oxygen in molten steel and slag.

Claims (4)

  Hot metal dephosphorization treatment was performed when decarburization and dephosphorization blowing to a carbon concentration of molten steel of 1.0% by mass or less and a phosphorus concentration of 0.030% by mass or less in a converter. The oxygen concentration in the slag is measured with an oxygen sensor attached to the sub lance, and the phosphorus concentration in the molten steel at the end of blowing is estimated using this measured value.   Hot metal dephosphorization treatment was performed when decarburization and dephosphorization blowing to a carbon concentration of molten steel of 1.0% by mass or less and a phosphorus concentration of 0.030% by mass or less in a converter. , The carry-over slag amount in the previous process of decarburization and dephosphorization was set to 3 kg or less per 1 ton of hot metal used, and the oxygen concentration in the slag was measured with an oxygen sensor attached to the sub lance. A method of estimating phosphorus concentration in molten steel at the end of blowing.   The higher the carbon concentration in the molten steel at the end of blowing, the higher the carbon concentration in the molten steel, the more the contribution rate of the influence of the oxygen concentration in the slag on the estimated phosphorus concentration in the molten steel when estimating the phosphorus concentration in the molten steel at the end of blowing. The method for estimating the phosphorus concentration in molten steel at the end of blowing according to claim 1 or 2, wherein the concentration is increased.     In a converter, a low phosphorus steel manufacturing method in which the hot metal is decarburized and dephosphorized until the carbon concentration in the molten steel is 1.0 mass% or less and the phosphorus concentration is 0.030 mass% or less. The method for estimating the phosphorus concentration in the molten steel at the end of blowing is estimated by using the method for estimating the phosphorus concentration in the molten steel at the end of blowing described in any one of claims 1 to 3. A method for producing low phosphorus steel.

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