JP2014037600A - Method for producing clean steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a total oxygen concentration after a RH degassing treatment when producing a clean steel.SOLUTION: A clean steel containing 0.03 to 0.30 mass% of C is produced by using molten steel circulation type vacuum degassing equipment in such a way that molten steel circulation time from the time point of collection of a molten steel sample is controlled to satisfy (1) expression:t-0.5≤t≤t+0.5 and (2) expression:t=-(1/k)×In{(T.[O]-10)/(T.[O]-10)} based on an analysis value of a total oxygen concentration included in the molten steel sample T.[O], while circulating the molten steel at a circulation gas flow rate of 1.0 to 3.0 Nm/min., where t: molten steel circulation time from the time point of sample collection (min.), k:deoxidation rate constant:0.15 (1/min.), T.[O]: a targeted total oxygen concentration after a PH treatment (mass ppm), T.[O]:an analysis value of the total oxygen concentration in the sample (mass ppm).

Description

  The present invention relates to a method for efficiently producing clean steel containing Al and containing 0.03 to 0.30% by mass of C using a molten steel reflux type vacuum degassing facility.

  In recent years, it has been desired to reduce oxide inclusions as much as possible in addition to the reduction of impurity elements such as P and S due to increasing needs for high quality steel materials. Since these oxide inclusions are reduced by molten steel reflux type vacuum degassing treatment (RH degassing treatment), there are many methods for reducing inclusions using molten steel reflux type vacuum degassing equipment (RH degassing equipment). Proposed.

  Patent Document 1 discloses a method of collecting a solidified ingot from molten steel that is being refined in steelmaking operations and quickly analyzing the total oxygen (T. [O]) concentration with high accuracy.

  In Patent Document 2, after decarburizing the molten steel that has not been deoxidized in the converter using RH degassing equipment, deoxidation is performed with Al, and the total oxygen concentration of the molten steel sample immediately after the Al deoxidation is calculated. A method for producing an ultra-low carbon steel having a C concentration of 0.0030% by mass or less, which is analyzed by the method disclosed in Patent Document 1 and optimizes the molten steel reflux time based on the total oxygen concentration, is disclosed.

  In Patent Document 3, the molten steel in an undeoxidized state is circulated into the vacuum chamber by Ar gas for recirculation, and deoxidation is performed by adding Al in the vacuum chamber, and after the recirculation into the vacuum chamber is continued, the treatment is performed. During refining at the RH degassing facility, the treatment time after Al addition is adjusted based on the relationship between the amount of dissolved oxygen in the molten steel immediately before Al addition and the amount of Ar gas for recirculation blown into the rising side dip pipe A method for melting clean steel using an RH degassing facility is disclosed.

  The method disclosed in Patent Document 2 applies the analysis method disclosed in Patent Document 1 to the production of ultra-low carbon steel, and the C concentration in the molten steel is set to 0.0050 using an RH degassing facility. This is a method of determining the RH optimum reflux time after Al deoxidation in the case of performing Al deoxidation after performing vacuum decarburization until the mass% or less. Therefore, various requirements are established so as to be suitable for the production of ultra-low carbon steel that requires decarburization treatment with RH, and management of the total oxygen concentration in the steel is important without requiring decarburization treatment with RH. , And cannot be applied to the production of a general clean steel having a C concentration of 0.03 to 0.30 mass%.

The method disclosed in Patent Document 3 estimates the amount of deoxidation product produced during Al deoxidation with RH, that is, the amount of Al ₂ O ₃ inclusions from the amount of dissolved oxygen. Since the amount is not directly analyzed, it is estimated that the variation in the total oxygen concentration after the RH treatment increases. In RH, a method of spraying an oxidizing gas such as oxygen to molten steel after Al deoxidation for the purpose of heating is often used. However, in this method, the amount of Al ₂ O ₃ generated by heating is not taken into account. Thus, it is impossible to determine the optimum reflux time after the addition of Al and the acid transfer.

JP 2010-261743 A JP 2010-280934 A Japanese Patent Laid-Open No. 11-92821

  One method for evaluating inclusions is the total oxygen (T. [O]) concentration in molten steel. In RH treatment, Al or the like is often added to molten steel for the purpose of increasing heat and an oxidizing gas such as oxygen is sprayed. In such RH treatment, the total oxygen (T. [O]) concentration becomes the highest value immediately after the end of the blowing of the oxidizing gas, and then gradually with the passage of the molten steel reflux time for the purpose of flotation separation of inclusions. Decrease.

1 is a graph showing an example of the relationship between the amount of acid delivered (Nm ³ / t) in RH treatment and the total oxygen (T. [O]; ppm) concentration after acid delivery in RH treatment.

  In the RH treatment for the production of aluminum killed steel, a sample of molten steel after completion of acid feeding was collected and the total oxygen (T. [O]) concentration of the molten steel was investigated offline. In the treatment, the total oxygen (T. [O]) concentration in the molten steel before the start of the RH treatment was about 25 to 30 ppm, whereas immediately after the completion of the acid feed in the treatment of feeding with RH (within 60 seconds). As shown in the graph of FIG. 1, the total oxygen (T. [O]) concentration in the molten steel varies from 30 to 70 ppm, and the total oxygen (T. [O]) concentration increases as the amount of acid delivered increases. It was confirmed that it was not a simple relationship such as becoming higher. Therefore, in the conventional method in which the molten steel recirculation time is kept constant without grasping the total oxygen (T. [O]) concentration after the completion of the acid transfer, an operation loss due to excessive recirculation may occur.

  The object of the present invention is to complete the oxygen delivery, which is considered to have the highest total oxygen (T. [O]) concentration during the RH treatment of molten steel containing 0.03 to 0.30% by mass of C and containing Al. To provide an efficient method for producing clean steel, in which the total oxygen (T. [O]) concentration immediately after analysis can be quickly analyzed and the molten steel reflux time can be optimally adjusted based on the analysis value. .

  By carrying out the present invention, it becomes possible to optimize the molten steel reflux time during the production of clean steel containing 0.03 to 0.30% by mass of C and containing Al, and suppressing excessive reflux time. Rationalization of RH operation can be achieved.

In the production of clean steel containing Al, measures are taken to reduce Al ₂ O ₃ inclusions in the RH degassing treatment, especially when a heat treatment is performed by adding Al to the molten steel and blowing oxygen. For this, measures for reducing Al ₂ O ₃ inclusions after the completion of the oxygen spraying are extremely important.

The Part Al ₂ O ₃ inclusions decrease countermeasure, generally oxygen blowing to ensure molten steel reflux time after completion, inclusions are adopted a method of flotation and flotation effect of Al ₂ O ₃ based inclusions since it depends on the Al ₂ O ₃ based inclusions amount in the molten steel, general trend that it is necessary to increase the Al ₂ O ₃ based inclusions amount greater the molten steel reflux time was known. However, there was no way to know the total oxygen (T. [O]) concentration in the molten steel during the RH treatment before the reflux of the molten steel was completed in order to optimize the reflux time.

  In order to solve this problem, a sample is taken from the molten steel after Al is added and oxygen is blown, and the total oxygen (T. [O]) concentration is analyzed, and before the reflux of the molten steel is finished. It is necessary to determine an appropriate reflux time and perform the reflux at the determined reflux time.

  In response to this need, as a method for analyzing the total oxygen concentration (T. [O]) in molten steel in a short time and with high accuracy, an analysis method as shown below is performed in accordance with the method disclosed in Patent Document 1. Considered and established.

  (A) In a method in which a steel sample is put in a graphite crucible and heated and melted in an inert gas, and the oxygen concentration in the sample is measured from the infrared absorption of one or both of generated carbon monoxide and carbon dioxide. The pre-machined sample is pretreated by removing and cleaning the oxide film on the sample surface by vacuum arc plasma treatment in advance, and then directly put into a graphite crucible without contacting with the atmosphere.

  (B) For a slice piece produced by cutting a steel ingot taken from molten steel, a small piece punched out by a press was used as a sample, pre-baked and pre-treated on a graphite crucible that had been put on standby. throw into.

  (C) Consists of a sample pretreatment device that cleans the surface of the sample by vacuum arc plasma treatment, and an oxygen analyzer that operates in an inert gas by heating-melting-infrared absorption method. At the same time, they are connected by a connecting tube whose inside is replaced with vacuum or inert gas, and the pretreated sample is dropped from the sample pretreatment device through the connecting tube and supplied to the oxygen analyzer.

  By applying this analysis method online, it is possible to grasp the total oxygen (T. [O]) concentration immediately after the completion of the acid delivery until the molten steel reflux is completed, and to determine the optimum reflux time.

  Specifically, it becomes possible to analyze the total oxygen concentration (T. [O] concentration) with the accuracy of the analysis value and the time required for the analysis as follows.

(I) Analytical value accuracy For steel with an oxygen content of 50 ppm or less, the analytical value variation must be within 3 ppm. However, since the value of this variation is determined by three times the standard deviation, it can be said that the standard deviation is within ± 1 ppm.

(Ii) Time required for analysis The time from receipt of a steel ingot sample to sample analysis and pre-cleaning treatment until the oxygen concentration is determined by analysis (hereinafter referred to as analysis required time) is within 4 minutes. about. More preferably within 3 minutes.

Below, this analysis method is demonstrated in detail using drawing.
FIG. 2 is an explanatory view schematically showing an oxygen analyzer in steel for carrying out the analysis method disclosed by the present invention. The symbols in FIG. 2 are collectively shown as follows: 1: Pre-processing device,
2: oxygen analyzer, 3: pre-treatment sample inlet, 4: isolation valve, 5: pre-treated sample inlet, 6: mount, 7: lifter, 8: connecting pipe, 9: pre-treatment sample outlet It is.

  In order to realize a short time and high accuracy analysis required for the present invention, vacuum arc plasma processing is selected as a sample pretreatment method that is quick and highly reproducible among the combined elemental technologies. For example, the adjustment method and apparatus for the metal component analysis sample disclosed in Japanese Patent Application Laid-Open No. 2002-328125 may be applied.

  In FIG. 2, a sample is inserted into the pretreatment apparatus 1 previously maintained in a vacuum through the isolation valves 4 and 4 from the pretreatment sample introduction port 3 with almost no change in the degree of vacuum. Thereafter, the oxide film on the sample surface is removed in a few seconds by vacuum arc plasma treatment in the pretreatment apparatus 1. Here, in order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, and to ensure the analytical accuracy required for the refining operation, it is desirable to perform the arc plasma treatment under the following conditions.

  (1) Degree of vacuum: 10 Pa to 100 Pa (oxygen partial pressure 2 to 20 Torr), more preferably 20 Pa to 50 Pa. The sample surface oxide film removal reaction by the vacuum arc plasma treatment is promoted as the degree of vacuum increases. However, when the degree of vacuum exceeds 100 Pa, the re-oxidation reaction accompanying the rise in the sample temperature becomes remarkable, which is not desirable. On the other hand, when the degree of vacuum is lower than 10 Pa, the oxide film removal reaction itself does not proceed, which is not desirable. Therefore, an optimum degree of vacuum (or oxygen partial pressure) exists. It is further desirable to have a pressure control mechanism that controls the opening and closing of the vacuum exhaust valve and the gas introduction valve so that the degree of vacuum is maintained at a constant value during processing.

  (2) Processing time: 0.2 second or more and 0.6 second or less. More desirably, it is set to 0.2 seconds or more and 0.5 seconds or less.

(3) Number of processing: 1 time.
(4) Arc plasma output current: 20A or more and 40A or less, more preferably 30A.

  The processed sample is finally put into the graphite crucible in the oxygen analyzer 2 through the pretreated sample inlet 5 arranged in the analyzer without being brought into contact with the atmosphere. The pretreated sample inlet 5 of the pretreatment apparatus 1 and the oxygen analyzer 2 is connected by a connecting pipe 8 whose inside is replaced with a vacuum or an inert gas. In view of the difference in specific gravity with air, the inert gas species is gas-replaced reliably in the connecting pipe 8 to prevent reoxidation of the sample after processing, and from an economical viewpoint, Ar is desirable.

  In the apparatus disclosed in Japanese Patent Laid-Open No. 2002-328125, the pretreated sample is discharged and then transferred to a separate oxygen analyzer. However, since rapidity is required in the present invention, as shown in FIG. 2, the pretreatment device 1 and the oxygen analyzer 2 are vertically arranged respectively, and the sample is allowed to fall freely in the connecting pipe 8. Configured to be transported.

  In the apparatus shown in FIG. 2, the oxygen analyzer 2 is arranged at a position close to the floor surface, and cleaning inside the oxygen analyzer 2 hinders the maintenance work of the apparatus such as replacement of the impurity adsorbent in the gas. . Therefore, it is desirable that the entire apparatus incorporated in the gantry 6 is placed on the lifter 7 so that it can be raised and lowered, and in this operation, the entire apparatus is raised to ensure workability. The drive system of the lifter 7 is not particularly limited. However, since the weight of the entire apparatus is considerable, an automatic hydraulic system is desirable from the viewpoint of operability. Further, it is desirable that the movable part of the lifter 7 is covered with a stretchable material and has a structure in consideration of safety so that an operator is not caught.

  Further, in case the connected oxygen analyzer 2 cannot be used due to a failure or when a preprocessed sample waiting for analysis is analyzed by another oxygen analyzer, the connecting pipe between the pretreatment device 1 and the oxygen analyzer 2 It is desirable to provide a pre-processed sample outlet 9 in the middle of 8.

  Among the elemental technologies combined to realize the analysis method according to the present invention, as a method for obtaining an analysis sample more easily and quickly than a steel ingot collected from molten steel, the height produced by cutting the steel ingot collected from molten steel ( It is desirable to use a punched piece as a sample for a slice having a thickness (thickness) larger than 2.5 mm and smaller than 7 mm. Specifically, for example, an analysis sample adjustment method and apparatus disclosed in Japanese Patent Laid-Open No. 10-311782 may be applied. In order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, the ratio S / V of the surface area S to the volume V calculated from the diameter and height of the sample bottom is 1.00 or more and 1.45 or less. It is desirable to secure a shape to become. The reason for this is not fully understood at this time, but it corresponds to the fact that the position suitable for efficient processing is limited in the spatial distribution of the arc plasma depending on the shape of the arc processing part such as the electrode shape. It is guessed.

  Among the elemental technologies combined to realize the analysis method according to the present invention, as a highly accurate analysis method of total oxygen (T. [O]) concentration, the heating principle in an inert gas-infrared absorption method is used as an operating principle. It is desirable to use the oxygen analyzer 2. In this analysis method, a graphite crucible serving as a sample holder and a sample deoxidation reagent (carbon) supply source is used. Prior to analysis, in order to remove oxygen and contamination adsorbed on the surface of the crucible, a so-called “empty baking” process is performed in which only the crucible is preheated at a temperature slightly higher than at the time of analysis.

  When analyzing the total oxygen (T. [O]) concentration in steel with a commercially available oxygen analyzer, the crucible, that is, the sample is usually heated to a temperature of about 1800 ° C. to 2200 ° C. In order to reduce the influence of oxygen, carbon monoxide or carbon dioxide generated from the graphite crucible by fluctuating the analytical value and to achieve the high analytical accuracy required by the present invention, What is necessary is just to heat for 15 second or more at the temperature 100 degreeC or more higher than temperature.

  In a commercially available oxygen analyzer, first, a sample is taken into the analyzer, and while the atmosphere around the sample is replaced with helium gas as a carrier gas, the crucible is exchanged, the electrode is cleaned, and the “blank” process is performed. . Therefore, there is a problem that it takes a relatively long time until the analytical value is determined after the sample is introduced. Therefore, in the present invention, speeding up is realized by putting the sample that has been pre-cleaned into the oxygen analyzer that is ready for analysis, with the replacement of the crucible, the cleaning of the electrodes, and the “blank” process preceded. Can do.

  In addition, when analyzing the total oxygen (T. [O]) concentration, it is necessary to accurately weigh the sample weight in order to convert the detected gas amount into the oxygen concentration in the sample. As a result of evaluating the change in the sample weight before and after the vacuum arc plasma treatment, although there was some variation depending on the shape of the sample and the degree of surface oxidation, the weight loss was about 1 mg at most, so the sample weight was reduced to 0.5 to 1.0 g. On the other hand, it has been found that the error is negligible for practical use. Therefore, when carrying out the present invention, the analytical sample obtained after machining and previously weighed was subjected to vacuum arc plasma treatment and inserted into the oxygen analyzer 2 as it was without contacting with the atmosphere.

  Utilizing this oxygen analysis method in steel, clean steel using 250 to 300 tons of molten steel containing 0.03 to 0.30% by mass of C and Al is used as a target using molten steel reflux type vacuum degassing equipment. As a result of investigating appropriate molten steel reflux treatment conditions by changing the oxygen supply conditions and the molten steel reflux conditions when manufacturing the present invention, the present invention has been completed.

The present invention is listed below.
(1) A method for producing clean steel containing Al in addition to 0.03 to 0.30% by mass of C using a molten steel reflux type vacuum degassing equipment,
After supplying oxygen to molten steel containing 250 to 300 tons containing Al to raise the molten steel temperature, a sample was taken from the molten steel,
While circulating the molten steel at a reflux gas flow rate of 1.0 to 3.0 Nm ³ / min,
Based on the analytical value (T. [O] ₂ ) of the total oxygen concentration of the sample, the molten steel reflux time from the sample collection time is adjusted so as to satisfy the expressions (1) and (2). A method for producing clean steel.

t _C −0.5 ≦ t ≦ t _C +0.5 (1)
t _C = − (1 / k) × ln {(T. [O] ₁ −10) / (T. [O] ₂ −10)}
... (2)
t: Molten steel recirculation time (min) from sample collection
k: Deoxidation rate constant: 0.15 (1 / min)
T. T. et al. [O] ₁ : Target value of total oxygen concentration after RH treatment (mass ppm)
T. T. et al. [O] ₂ : Analytical value of total oxygen concentration of the sample (mass ppm)

(2) The analytical value (T. [O] ₂ ) of the total oxygen concentration of the sample is
A sample collected from the molten steel is machined after solidification to prepare an analytical sample, and the analytical sample is placed in a graphite crucible, heated and melted in an inert gas, and generated either carbon monoxide or carbon dioxide. A method for measuring the oxygen concentration in the sample from one or both infrared absorptions, wherein the vacuum level is set to 10 Pa or more and 100 Pa or less and the arc plasma output current is set to 20 A with respect to the machined sample. Under the condition of 40A or less, the vacuum arc plasma treatment for 0.6 seconds or less is performed only once to perform the pretreatment for removing and cleaning the oxide film on the surface of the sample without contacting with the atmosphere. The method for producing clean steel according to item (1), characterized in that it is obtained by an oxygen analysis method in steel directly put into a graphite crucible.

  According to the present invention, when producing a clean steel containing 0.03 to 0.30% by mass of C and Al using a molten steel recirculation type vacuum degassing facility, oxygen supply conditions and molten steel recirculation between them are produced. Regardless of the conditions, the total oxygen (T. [O]) concentration after the RH degassing process can be controlled to a target value, and efficient according to the required total oxygen (T. [O]) concentration. RH degassing can be performed.

1 is a graph showing an example of the relationship between the amount of acid sent in RH treatment (Nm ³ / t) and the total oxygen concentration (T. [O]; ppm) after acid feed in RH treatment. FIG. 2 is an explanatory view schematically showing an oxygen analyzer in steel for carrying out the analysis method disclosed by the present invention. 3 is a graph showing an example of the relationship between the molten steel reflux time (minutes) after sample collection and the total oxygen (T. [O]) concentration in the molten steel reflux.

  A molten steel having a C concentration of 0.03 to 0.30 mass%, a P concentration of 0.050 mass% or less, and an S concentration of 0.010 mass% or less, which is melted in a converter or the like, is discharged into a ladle. At this time, the molten steel may be deoxidized or non-deoxidized. In the case of deoxidation, the Si concentration is adjusted to 0.02 to 1.0 mass% and the Mn concentration is adjusted to 0.10 to 2.50 mass%.

  The method for producing clean steel according to the present invention includes an RH degassing facility in which a two-leg dip tube is installed in molten steel in a ladle, the inside of the vacuum chamber is evacuated, and an inert gas for reflux is blown from one dip tube. A method for producing clean steel containing Al by supplying oxygen to molten steel containing 0.03 to 0.30% by mass of C and increasing the molten steel temperature and then refluxing the molten steel It is.

  The reason for targeting molten steel containing 0.03% by mass or more of C is that vacuum decarburization treatment with RH is usually required to make C less than 0.03% by mass. This is to clarify that the molten steel that has been subjected to vacuum decarburization treatment is not targeted.

  On the other hand, the reason for restricting the molten steel to contain 0.30% by mass or less of C is that there are many steel types that place importance on management of the total oxygen concentration below this level of C concentration.

In the RH degassing facility, before supplying oxygen to the molten steel, in addition to 0.03 to 0.30% by mass of C, the molten steel components are adjusted to a concentration close to the standard concentration of the product manufactured using the molten steel, Further, the sol. Commensurate with the amount of oxygen supplied reacts to produce Al ₂ O ₃ . Al is added to the molten steel so that the Al concentration is contained.

  In the molten steel before starting the oxygen supply, sol. The Al concentration does not need to be specified, but the sol. The Al concentration needs to be close to the Al standard concentration of a product manufactured by the molten steel being processed.

  Immediately after completing the oxygen supply (within 60 seconds), it is possible to take a sample from molten steel and immediately analyze its total oxygen (T. [O]) concentration, as well as other components at the same time. Therefore, it is possible to finely adjust the concentration of each alloy component in addition to C and Al so as to meet the standard concentration of the product. However, it is natural that it is preferable not to perform such fine adjustment.

  When the total oxygen (T. [O]) concentration of the molten steel is found, the required molten steel reflux time after sampling is calculated by applying the above equation (2), and the calculated value is ± 0.5 minutes. After the lapse of time, the molten steel reflux is stopped to finish the RH treatment.

  In order to make the total oxygen (T. [O]) concentration below the target concentration, the molten steel reflux time after sampling is basically set to the calculated value of equation (2). In consideration of the operational fluctuations related to the slight fluctuation of the molten steel and the operation timing of the molten steel recirculation operation, it is desired to adjust the molten steel recirculation time of the formula (2) ± 0.5 minutes in practice. The RH treatment time for producing a level of clean steel can be optimized.

  Conventionally, in the RH treatment of molten steel containing Al, it has been found that the total oxygen concentration (T. [O]) decreases according to the molten steel reflux time after completion of acid feeding. However, a specific method for accurately knowing the value of the total oxygen concentration (T. [O]) after completion of RH acid feeding before completion of the molten steel reflux has not been known. Therefore, it is difficult to attempt to control the total oxygen (T. [O]) concentration value after completion of RH acid delivery to a required level according to the product, which is less than the required total oxygen (T. [O]) concentration. Has been determined empirically to ensure that the reflux time after completion of the acid delivery is long. Therefore, the molten steel reflux time is often excessive.

  However, since the value of the total oxygen (T. [O]) concentration after completion of RH acid feeding can be accurately and quickly known during the molten steel reflux using the apparatus shown in FIG. 2, the target total oxygen ( The reflux time required for the T. [O]) concentration can be determined, and the excess RH treatment time can be reduced.

  Therefore, first, the inventors focused on establishing a rapid analysis method for the total oxygen (T. [O]) concentration in the molten steel. Then, the analysis method was developed so that it can be used for optimizing the molten steel reflux time during the cleaning treatment of the molten steel containing Al.

  Conventionally, the target of the total oxygen (T. [O]) concentration after the RH treatment may be determined according to the required quality level, but the target value of the total oxygen (T. [O]) concentration after the RH treatment. Specifically, the molten steel reflux time required to reach the temperature was unknown. Therefore, sampling was performed during the molten steel reflux after completion of RH acid feeding, and the relationship between the molten steel reflux time and the total oxygen (T. [O]) concentration was investigated.

  The results are shown graphically in FIG. 3 is a graph showing an example of the relationship between the molten steel reflux time (minutes) after sample collection and the total oxygen (T. [O]) concentration in the molten steel reflux.

The solid line in the graph of FIG. 3 corresponds to the total reflux (T. [O] ₂ ) concentration obtained by analyzing the sample immediately after the completion of acid transfer, and the reflux time after the sample is collected according to equation (2). T. The [O] concentration (T. [O] ₁ ) is calculated and illustrated, and the plot shows the T.O. of the sample collected at that time. [O] Concentration analysis value.

  Even in the case where the acid feeding was not carried out, the sample collected before starting the reflux in RH was used. [O] Concentration analysis was performed, and the calculated values after the start of reflux were indicated by dotted lines, and the analytical values of the samples were plotted.

  From the graph of FIG. 3, it is possible to estimate the transition of the total oxygen (T. [O]) concentration corresponding to the molten steel reflux time of RH using the equation (2). It has also been found that the optimum molten steel reflux time can be predicted by the target value of O]) concentration.

  In order to reflect the above-described knowledge in online operation, the present inventors have used the above-described rapid analysis method for oxygen in steel as a method for analyzing the total oxygen (T. [O]) concentration in a short time and with high accuracy. It was established and used.

  After the steel is removed from the converter, each component such as C and Mn is adjusted, oxygen is further supplied by RH to raise the molten steel temperature, and after about 30 seconds have passed since the oxygen supply was stopped. Sampling was performed. Using the sample, the T.I. Analyzing the [O] concentration, using the equation (2), calculate the molten steel reflux time required to reach the target total oxygen (T. [O]) concentration, and ± 0.5 with respect to the calculated value The molten steel reflux was stopped within a minute to complete the RH degassing process.

  As a comparative example, although sampling of molten steel was performed at the time when about 30 seconds passed from the stop of oxygen supply, it was known in advance that the analysis value was not referred to, and it was not necessary to carry out acid feeding. Since the molten steel sample collected before the start of the RH treatment was used to investigate the example of analyzing the total oxygen (T. [O]) concentration in the same manner as the example of the present invention, the present example was compared with those of the comparative examples. Examples of the invention will be described.

  First, Table 1 summarizes the values obtained by separately analyzing the concentrations of various components in molten steel using the samples for analyzing the total oxygen (T. [O]) concentration in both the inventive examples and the comparative examples. Show.

Furthermore, the target total oxygen (T. [O] ₁ ) concentration and the total oxygen (T. [O] ₂ ) concentration of those samples after the RH treatment in the present invention example and the comparative example, and the equation (2) Table 1 also shows the required reflux time (t _C ) obtained by substituting the concentration, the actual reflux time after sampling, and the actual total oxygen (T. [O]) concentration after RH treatment. During the molten steel recirculation time after sampling, Ar was continuously supplied as a recirculation gas at a flow rate of 1.5 to 2.5 Nm ³ / min. T. in molten steel The [O] concentration was confirmed.

Comparative Examples 1 and 2 and Inventive Examples 1 and 2 having the same target total oxygen (T. [O] ₁ ) concentration after RH treatment of 20 ppm or less are compared, and target total oxygen (T. [O] ₁ ) after RH treatment is compared. [O] ₁ ) When comparing the same Comparative Examples 3 and 4 with Example 3 with a concentration of 25 ppm or less, the Example of the invention can shorten the actual molten steel reflux time by about 4 to 5 minutes, It was confirmed that the target total oxygen (T. [O]) concentration after RH treatment was achieved.

  In Comparative Examples 1 to 4, since the relationship between the molten steel reflux time and the total oxygen (T. [O]) concentration after RH treatment cannot be sufficiently grasped, the management target of the total oxygen (T. [O]) concentration From the need to prevent detachment from the steel, it was found that the molten steel reflux time was longer than necessary, and the RH treatment efficiency and the production cost of clean steel were increased accordingly.

DESCRIPTION OF SYMBOLS 1 Pretreatment apparatus 2 Oxygen analyzer 3 Pretreatment sample inlet 4 Isolation valve 5 Pretreatment specimen inlet 6 Base 7 Lifter 8 Connection pipe 9 Pretreatment specimen intermediate outlet

Claims (2)

A method for producing a clean steel containing Al and containing 0.03 to 0.30% by mass of C using a molten steel reflux type vacuum degassing equipment,
After supplying oxygen to molten steel containing 250 to 300 tons containing Al to raise the molten steel temperature, a sample was taken from the molten steel,
While circulating the molten steel at a reflux gas flow rate of 1.0 to 3.0 Nm ³ / min,
Based on the analytical value (T. [O] ₂ ) of the total oxygen concentration of the sample, the molten steel reflux time from the sample collection time is adjusted so as to satisfy the expressions (1) and (2). A method for producing clean steel.
t _C −0.5 ≦ t ≦ t _C +0.5 (1)
t _C = − (1 / k) × ln {(T. [O] ₁ −10) / (T. [O] ₂ −10)}
... (2)
t: Molten steel recirculation time (min) from sample collection
k: Deoxidation rate constant: 0.15 (1 / min)
T. T. et al. [O] ₁ : Target value of total oxygen concentration after RH treatment (mass ppm)
T. T. et al. [O] ₂ : Analytical value of total oxygen concentration of the sample (mass ppm) The analytical value (T. [O] ₂ ) of the total oxygen concentration of the sample is
A sample collected from the molten steel is machined after solidification to prepare an analytical sample, and the analytical sample is placed in a graphite crucible, heated and melted in an inert gas, and generated either carbon monoxide or carbon dioxide. A method for measuring the oxygen concentration in the sample from one or both infrared absorptions, wherein the vacuum level is set to 10 Pa or more and 100 Pa or less and the arc plasma output current is set to 20 A with respect to the machined sample. Under the condition of 40A or less, the vacuum arc plasma treatment for 0.6 seconds or less is performed only once to perform the pretreatment for removing and cleaning the oxide film on the surface of the sample without contacting with the atmosphere. The method for producing clean steel according to claim 1, wherein the method is obtained by an oxygen analysis method in steel directly put into a graphite crucible.

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