JP2011038125A - Method for smelting clean steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for smelting clean steel, which can stably produce the clean steel containing 13 ppm or less total oxygen, without causing a trouble due to the extension of a time period of vacuum degassing treatment or drop in temperature of the molten steel, when smelting the clean steel containing 13 ppm or less total oxygen. <P>SOLUTION: This smelting method includes: collecting a sample from the molten steel during slag refining treatment; measuring the total oxygen concentration in the sample with a total oxygen rapid analysis method before the end of the slag refining treatment; adjusting the basicity of the slag based on the result of the measurement; and controlling the total oxygen concentration before the vacuum degassing treatment to 18 ppm or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for melting clean steel in which steel is removed by inclusion removal in a vacuum degassing facility after carrying out slag refining in which heating by an arc and stirring by gas or electromagnetic force are performed.

  As a conventional method for melting clean steel, it has been mainstream to remove inclusions by increasing the vacuum degassing time. However, even if the vacuum degassing treatment time is extended, the total oxygen concentration in the steel slab may not be sufficiently reduced, and the variation is large.

In a refining method using vacuum degassing equipment such as clean steel melting, the following method is disclosed as a method for determining a processing time according to the amount of dissolved oxygen in molten steel.
In Patent Document 1, after circulating molten steel into the vacuum chamber by circulating Ar gas, Al is added in the vacuum chamber to deoxidize the undeoxidized molten steel, and after continuing the reflux into the vacuum chamber During refining in the RH degassing apparatus that finishes the treatment, the amount of dissolved oxygen Co in the molten steel immediately before the addition of Al, the amount of Ar gas Q for recirculation blown into the rising side dip pipe, and the treatment time t after the addition of Al , A method for melting clean steel in an RH degassing apparatus characterized by satisfying the expression (1) is disclosed.
t ≧ 20 × (1-0.5 ^{Co / 50} ) × (8 / Q) ^0.5 ... (1)
t: Processing time after addition of Al (min)
Co: dissolved oxygen amount (ppm)
Q: Ring flow rate Ar gas amount (Nl / min / t)

  In the method described in Patent Document 1, the product surface defect index is obtained when the ratio between the actual treatment time after Al addition and the post-Al addition treatment time calculated by the right side of equation (1) is 1 or less. Has been shown to tend to get worse. In this method, since the processing time t after the addition of Al is determined from the result of measuring the dissolved oxygen amount Co, when the dissolved oxygen amount Co is high, it is necessary to extend a predetermined processing time, There was a problem that the molten steel temperature drop trouble occurred due to the time extension.

  Moreover, in patent document 2, the sample used as the determination object extract | collected from the molten steel of the refining process, and the sample extract | collected from the steel of the desired refining state are heated separately in the extraction furnace which is an inert gas atmosphere state, respectively. While gradually raising the temperature, the carbon source in the extraction furnace is reacted with the oxygen contained in each sample, and the reacted oxygen has an oxygen peak and an oxygen peak first appearing in the steel material in the desired refined state. A method for determining the refined state of steel by comparison is disclosed. In this method, the refining state of the steel is determined by comparing the oxygen peaks, but there is no specific method for dealing with the processing content depending on the determined state.

  On the other hand, for the analysis of the total oxygen concentration in steel, the total oxygen concentration analyzer based on the principle of operation of heat melting in an inert gas-infrared absorption method is applied for accurate analysis. However, in order to obtain an analytical sample, a sample is prepared by machining from a steel ingot to a predetermined size, and then chemical polishing, electrolytic polishing, grinder or file is used for the purpose of removing the oxide film on the sample surface. It was necessary to perform pretreatment such as grinding. Such processing and pretreatment are not only complicated, but also require time. As a result, it has been difficult to apply as a total oxygen analysis method for molten steel during refining from the viewpoint of speed.

Japanese Patent Laid-Open No. 11-92821 JP 2001-116710 A JP 2002-328125 A JP-A-10-311782

  The present invention has been made in view of the above-mentioned problems, and the problem is that, in the production of clean steel having a total oxygen concentration of 13 ppm or less, there is no trouble due to the extension of the vacuum degassing treatment time or the temperature drop. It is an object of the present invention to provide a method for producing clean steel, which enables production of clean steel of 13 ppm or less.

  The feature of the present invention provided to solve the above-mentioned problems is that when a clean steel having a total oxygen concentration of 13 ppm or less is melted, a sample is taken from the molten steel during the slag refining process, and before the end of the slag refining process, The total oxygen concentration in the sample is measured by a total oxygen rapid analysis method, the basicity of the slag is adjusted based on the measurement result, and the total oxygen concentration before vacuum degassing is controlled to 18 ppm or less.

The present invention is specifically as follows.
(1) A slag refining method in a clean steel smelting method in which inclusion removal is performed in a vacuum degassing facility after 30 minutes or less of the molten steel reflux time after performing slag refining by heating with an arc and stirring by gas or electromagnetic force. The total oxygen concentration of the molten steel is measured during processing, and the amount of quick lime added per ton of molten steel within the range of formula (2) based on the total oxygen concentration measurement result so that the total oxygen concentration before vacuum degassing is 18 ppm or less A method for producing clean steel characterized in that (kg) is charged and slag refining treatment is performed.

0.18 × total oxygen concentration (ppm) −3.6 ≦ quick lime additional amount (kg / t)
≦ 0.18 × total oxygen concentration (ppm) −1.8 (2)
(2) The above (1) is characterized in that the amount of Al input in the vacuum degassing treatment is suppressed to 0.1 kg or less per ton of molten steel by adjusting the components and temperature during the slag refining treatment. 2. A method for melting clean steel as described in 1.

  (3) As a method of measuring the total oxygen concentration of the molten steel during the slag refining treatment, a steel sample is put in a graphite crucible and heated and melted in an inert gas, and either generated carbon monoxide or carbon dioxide or In this method, the oxygen concentration in the sample is measured from both infrared absorptions. The vacuum arc plasma treatment is performed as a pretreatment for removing and cleaning the oxide film on the sample surface. The ratio of the surface area S to the volume V (S / V) is 1.5 mm to 7 mm in height with respect to the steel ingot collected from the molten steel under the conditions of 35 Pa or less and the arc plasma output current of 15 A or more and 55 A or less. ) Is a sample obtained by machining so as to be 1.05 or more and 1.30 or less, and the arc plasma discharge is applied to the sample a total of 4 times or less, and After applying the total processing time from 0.2 seconds to 1.2 seconds, the sample is heated and cleaned at a temperature higher than the temperature at the time of analysis without contacting with the atmosphere, and then the temperature is set to the temperature to be analyzed. The method for producing clean steel according to (1) or (2) above, wherein the method for analyzing oxygen in steel to be put into a graphite crucible that has been lowered and put on standby is used.

  By controlling the total oxygen concentration before the vacuum degassing treatment to 18 ppm or less according to the present invention, it becomes possible to stably produce clean steel having a concentration of 13 ppm or less without any trouble due to the extension of the vacuum degassing treatment time or the temperature drop. It was.

  In addition to the above, it is possible to stably produce clean steel of 9 ppm or less by suppressing Al input in the vacuum degassing treatment to 0.1 kg / t or less.

It is a graph which shows the relationship between the total oxygen concentration in the molten steel after a slag refining process, and the total oxygen concentration in the steel piece after continuously casting the molten steel. It is a figure which shows the result of having investigated the change of the steel piece total oxygen concentration when the vacuum degassing process time is extended from 20 minutes to 25 minutes. It is a graph which shows the relationship between the total oxygen concentration after slag refining, and the total oxygen concentration before a degassing process. It is a graph which shows the relationship between the total oxygen concentration before slag refining, and the total oxygen concentration 10 minutes after a slag refining start. It is a graph which shows the relationship between the total amount of oxygen 10 minutes after the slag refining start, and the additional input amount (kg) per molten steel ton of quicklime set based on the concentration. It is a graph which shows the relationship between the total oxygen concentration 10 minutes after slag refining start, and the total oxygen concentration after slag refining. It is a graph which shows the relationship between the total oxygen concentration in the molten steel after slag refining, and the total oxygen concentration in the steel slab which passed through continuous casting. It is a figure which shows typically the oxygen analysis equipment in the steel which concerns on this invention.

  The method for melting clean steel according to the present invention will be described below. In the description according to the present invention, “%” or “ppm” indicating the content of a component contained in steel means mass% or mass ppm.

  A feature of the present invention is that when a clean steel having a total oxygen concentration of 13 ppm or less is melted, a sample is taken from the molten steel during the slag refining process, and the total oxygen concentration in the sample is rapidly analyzed before the slag refining process is completed. The measurement is performed by a method, the basicity of the slag is adjusted based on the measurement result, and the total oxygen concentration before the vacuum degassing treatment is controlled to 18 ppm or less.

  In the present invention, clean steel includes C: 0.08 to 1.05%, Si: 0.15 to 0.35%, Mn: 0.30 to 1.10%, P: 0.030% or less, S : 0.025% or less, sol. It is defined as carbon steel or alloy steel having a component range of Al: 0.010 to 0.060% and total oxygen concentration: 0.0013% or less.

For reducing the total oxygen concentration of clean steel, it is effective to extend the vacuum degassing treatment time. However, even if extended, the total oxygen concentration has not been sufficiently reduced.
Therefore, C: 0.08 to 1.05%, Si: 0.15 to 0.35%, Mn: 0.30 to 1.10%, P: 0.00. 030% or less, S: 0.025% or less, sol. About 80t of molten steel of Al: 0.010 to 0.060%, the vacuum degassing treatment time was fixed at 20 minutes, and the total oxygen concentration in the molten steel after slag refining treatment and the steel slab after continuously casting the molten steel The relationship with total oxygen concentration was investigated. In this investigation, the analysis of the total oxygen concentration in the molten steel after the slag refining treatment was performed off-line, and the analysis result was associated with the total oxygen concentration in the steel slab. The result is shown in FIG.

  In FIG. 1, the vacuum degassing treatment time is unified to 20 minutes, and the relationship between the total oxygen concentration in the molten steel and the total oxygen concentration in the steel slab after completion of slag refining in the conventional operation is shown in FIG. The amount of Al to be added to is shown as being layered by more than 0.1 kg / t and less than 0.1 kg / t. This Al amount is basically input as an oxidizing exothermic agent for controlling the molten steel temperature after the vacuum degassing treatment to a predetermined value.

  From this figure, in order to reduce the total oxygen concentration in the steel slab, the total oxygen concentration in the molten steel after the completion of slag refining is reduced, and the amount of Al introduced into the molten steel during the vacuum degassing treatment is 0.1 kg / It has been found that it is effective to reduce it to t or less. In particular, in order to reduce the total oxygen concentration in the steel slab, which is the subject of the present invention, to 13 ppm or less, it can be said that it is effective to eliminate the treatment in which the total oxygen concentration in the molten steel after slag refining exceeds 18 ppm.

  Moreover, since it was found that it is also effective to reduce the amount of Al introduced into the molten steel during vacuum degassing treatment to 0.1 kg / t or less, the vacuum degassing treatment time is basically the same as the investigation conditions in FIG. The effect of extending from 20 minutes to 25 minutes was investigated. The result is shown in FIG.

  From FIG. 1 and FIG. 2, even if the total oxygen concentration in the molten steel after the slag refining is over 18 ppm under the condition that the amount of Al introduced into the molten steel during the vacuum degassing treatment is 0.1 kg / t or less, It can be seen that if it is 22 ppm or less, the total oxygen concentration in the steel slab can be 13 ppm or less, and if the degassing treatment time is extended, it can be handled even if it is higher.

  However, the problem here is that there was no method for quickly knowing the total oxygen concentration in the molten steel after the slag refining, so that the total amount in the molten steel after the slag refining as shown in FIGS. There was no way to effectively cope with variations in oxygen concentration.

  In the past, based on data that had the worst results among the past results, long-term molten steel reflux conditions were set in advance so that the molten steel was cleaned by reflux in RH even under such bad conditions. I was dealing with it. However, such a coping method has a problem because it is wasteful both in terms of heat and processing efficiency.

  Therefore, the present inventors have developed a rapid analysis method for total oxygen in molten steel in parallel, and applied the rapid analysis method to a method for producing clean steel. I decided to plan.

The oxygen rapid analysis method will be described in detail below with reference to the drawings.
FIG. 8 schematically shows an oxygen analyzer in steel for carrying out the analysis method according to the present invention.

  In order to achieve a short time and high accuracy analysis required for the analysis method according to the present invention, vacuum arc plasma treatment was selected as a rapid and reproducible sample pretreatment method among the elemental technologies combined in the present invention. For example, the preparation method and apparatus for a sample for analyzing an in-metal component disclosed in Patent Document 3 may be applied. A sample can be inserted into the sample pretreatment apparatus 1 that has been previously kept in vacuum through the isolation valve 4 through the isolation valve 4 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 this apparatus, since the sample is automatically conveyed, the sample shape is limited to a cylinder or a block (a rectangular parallelepiped). Since the sample is placed on the sample stage and processed, the surface in contact with the sample stage is not processed. Therefore, it is necessary to invert the sample for processing. That is, it is necessary to discharge at least twice for one sample. When the number of discharges increases, the sample is heated for a long time, and once the oxide film is removed, the sample surface is oxidized again. Therefore, in order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, and to ensure the analysis accuracy required for the refining operation, it is necessary to perform an arc plasma treatment under the following conditions.

(a) Degree of vacuum: 5 Pa or more and 35 Pa or less. The sample surface oxide film removal reaction by vacuum arc plasma is promoted as the degree of vacuum increases, but if it exceeds 35 Pa, the reoxidation reaction accompanying the increase in the sample temperature becomes remarkable, which is not preferable. On the other hand, if it is lower than 5 Pa, the oxide film removal reaction itself does not proceed, which is not preferable. There is therefore an optimum degree of vacuum.
It is more preferable to have a pressure control mechanism for controlling 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.

  (b) Arc plasma output current: 15A or more and 55A or less.

(c) Processing time: The total processing time is 0.2 second or more and 1.2 seconds or less for one sample.
(d) Number of treatments: The total number of treatments per sample is 4 or less.

  The treated sample is finally put into the graphite crucible through the pretreated sample inlet 5 arranged in the analyzer 2 without being brought into contact with the atmosphere. The sample pretreatment chamber and the sample inlet of the analyzer are connected by a connecting tube 8 whose inside is replaced with vacuum or an inert gas. As the inert gas species, Ar is preferable from the viewpoint of reliably replacing the gas in the connecting pipe in consideration of the specific gravity difference with air and preventing reoxidation of the sample after processing, and from an economical viewpoint. . In the apparatus configuration disclosed in Patent Document 3, the pretreated sample is dispensed and then transferred to a separate oxygen analyzer. However, since the object of the present invention requires quickness, the sample pretreatment device 1 and the oxygen analyzer 2 are arranged vertically above and below, and freely fall in the connecting tube 8 to transfer the sample. That is, the apparatus configuration as shown in FIG. 8 is adopted.

  In the apparatus configuration of the present invention, the oxygen analyzer 2 is disposed at a position close to the floor surface, and cleaning inside the analyzer 2 hinders maintenance work of the apparatus such as replacement of the impurity adsorbent in the gas. Therefore, the entire apparatus incorporated in the gantry 6 is placed on the lifter 7 so that it can be raised and lowered, and during the operation, the entire apparatus is raised to ensure workability. The driving method of the lifter 7 is not particularly limited. However, since the weight of the entire apparatus is considerable, an automatic hydraulic type is preferable from the viewpoint of operability. Moreover, it is desirable to cover the movable part of the lifter 7 with a stretchable material and to have a structure in consideration of safety so that an operator is not caught.

  Further, the sample pretreatment device 1 and the oxygen analyzer 2 are connected in preparation for the case where 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. An outlet 9 for a pretreated sample is provided in the middle of the tube 8.

  Among the elemental technologies combined in 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 (thickness) produced by cutting the steel ingot collected from molten steel is 1.5 mm or more. A punched cylindrical piece is used as a sample for a slice of 7 mm or less. Specifically, for example, an analysis sample adjustment method and apparatus disclosed in Patent Document 4 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 “1.05 ≦ S / V It is necessary to ensure a shape that satisfies “≦ 1.30”.

  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 in the present invention, an oxygen analyzer based on the operating principle of heating and melting in an inert gas-infrared absorption method was selected as a highly accurate method for analyzing oxygen in steel. 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. The “blank” treatment can reduce the influence of oxygen, carbon monoxide or carbon dioxide generated from the graphite crucible changing the analytical value. When analyzing oxygen in steel with a commercially available oxygen analyzer, a crucible, that is, a sample is usually heated to a temperature of about 1800 ° C. to 2200 ° C. In order to realize the high analysis accuracy required in the present invention, for example, the heating may be performed at a temperature that is 100 ° C. or more higher than the temperature at the time of analysis and for 15 seconds or more.

  In addition, in a commercially available oxygen analyzer, first, a sample is taken into the analyzer, and the atmosphere around the sample is replaced with helium gas as a carrier gas. carry out. Therefore, it takes a relatively long time until the analytical value is determined after the sample is introduced. Replacing the crucible, cleaning the electrodes, and performing the “blank” process in advance, and loading the sample that has been pre-cleaned in a state where the analyzer can analyze, according to the required analysis time. Speeding up can be realized.

  Usually, in the oxygen analysis, in order to convert the detected gas amount into the oxygen concentration in the sample, it is necessary to accurately weigh the sample weight. 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 analysis sample obtained by machining and previously weighed was subjected to vacuum arc plasma treatment and inserted into the oxygen analyzer as it was without being brought into contact with the atmosphere.

  However, even if there is a method of quickly knowing the total oxygen concentration in the molten steel after slag refining, depending on the result, after starting the vacuum degassing treatment, limit the Al input amount or extend the degassing treatment time It is not always appropriate to do. This is because if the degassing time is extended, the molten steel temperature is inevitably lowered, and in order to compensate for it, Al must be introduced to oxidize and the operational constraints are severe. Therefore, basically, the total oxygen concentration in the molten steel after completion of slag refining must be controllable to 18 ppm or less.

Therefore, in the present invention, the total oxygen concentration in the molten steel during the slag refining treatment is quickly measured, and the total oxygen concentration in the slag refining treatment is reduced by changing the slag refining treatment conditions according to the measurement result. In particular, the relationship between the slag refining treatment conditions, the total oxygen concentration in the molten steel after the slag refining treatment, and the total oxygen concentration in the steel slab was investigated. The slag composition at the end of the slag refining in the charge to be investigated is based on conventional experience values, CaO = 50-60 mass%, Al ₂ O ₃ = 33-40 mass%, CaO / Al ₂ O ₃ ≧ 1.5 (FeO + MnO) ≦ 1% by mass, porous bubbling from the bottom of the ladle was set to 1.25 to 1.88 l / min / t, and the treatment was performed for 40 minutes. The total oxygen concentration in the slag refining process is measured by the rapid analysis method by taking a molten steel sample at 10 minutes after the start of processing (= start of porous bubbling) after the slag has hatched. When the total oxygen concentration is high, the basicity is improved by adding quick lime to lower the total oxygen concentration, and the total oxygen concentration after slag refining is controlled to 18 ppm or less. FIG. 3 shows the relationship between the total oxygen concentration after slag refining and before degassing. From this result, it can be seen that there is no change in the total oxygen concentration after slag refining and before degassing.

  In order to reduce the Al input amount during vacuum degassing to 0.1 kg / t or less, the pan temperature was improved by arc heating in slag refining, and the temperature drop in degassing was compensated for heat. Moreover, the [Al] content lost in the vacuum degassing process was estimated, and [Al] was increased in slag refining.

  In the practice of the present invention, first, molten steel is produced using a converter or an electric furnace, and then slag refining is performed in a ladle refining furnace having heating function by arc and stirring function by gas or electromagnetic force. Clean steel with a total oxygen concentration of 13 ppm or less by controlling the total oxygen concentration to 18 ppm or less and continuously casting molten steel from which inclusions have been removed with a molten steel circulation type vacuum degassing facility (RH) within 30 minutes or less. Slabs are manufactured stably.

  Components of molten steel are decarburized and dephosphorized in a converter or electric furnace, and then bubbling is started at the time of steel removal from the converter or electric furnace to the ladle or in the ladle refining furnace (= slag refining is started) 10 minutes In the range, the alloyed iron or Al is added to the molten steel, C: 0.08 to 1.05%, Si: 0.15 to 0.35%, Mn: 0.30 to 1.10%, P: 0.030% or less, S: 0.025% or less, sol. Al: Adjust to 0.010 to 0.060%. Components other than those described here may be included in accordance with product specifications such as Cr, Mo, and N.

For slag components during ladle refining, steel is removed from the converter or electric furnace to the ladle and then stripped, and a suitable amount of solvent such as quick lime or calcium aluminate is put into the pan, and slag refining is completed. The components of the slag at the time are within the range of CaO = 50-60 mass%, Al ₂ O ₃ = 33-40 mass%, CaO / Al ₂ O ₃ ≧ 1.5, (FeO + MnO) ≦ 1 mass%. Adjust so that

  Thus, the fall state of the total oxygen concentration in molten steel at the time of starting slag refining about the molten steel taken out from the converter is shown in FIG. The total oxygen concentration in the molten steel before the start of slag refining varied between 26 and 42 ppm, but at the point of 10 minutes after the start of bubbling, where it was judged that the slag in the ladle was temporarily transformed, The total oxygen concentration of was reduced to a range of 12-25 ppm.

In the practice of the present invention, a sample for analysis is taken from the molten steel at 10 minutes after the start of bubbling, and the sample is quickly analyzed by the oxygen rapid analysis method described above.
Knowing the total oxygen concentration in the sample, taking into account the component status at the time of converter steelmaking, removal before slag refining, and the addition of updated medium material, the range of equation (2) Then add the additional amount of quick lime per ton of molten steel (kg) to the slag. The quicklime used in the present invention has a CaO mass concentration of 92% or more (the balance is mainly CO ₂ ) and a particle size of 5 mm or more and 50 mm or less.
0.18 × total oxygen concentration (ppm) −3.6 ≦ quick lime additional amount (kg / t)
≦ 0.18 × total oxygen concentration (ppm) −1.8 (2)

  Figure 5 shows the relationship between the total oxygen concentration in the sample collected 10 minutes after the start of slag refining, including the additional adjustment of quicklime, and the amount of quicklime added after knowing the concentration, and the resulting slag FIG. 6 shows the relationship with the total oxygen concentration after refining. From these figures, if the amount of quick lime is added as shown in FIG. 5 within the range of the molten steel component / slag component described above, the total oxygen concentration in the molten steel at 12 minutes after the start of slag refining is 12 as shown in FIG. It can be seen that even if variation is observed in the range of ˜25 ppm, the total oxygen concentration in the molten steel after slag refining can be controlled to 18 ppm or less.

In the present invention, the molten steel components and temperature are finely adjusted and the inclusions are removed within 30 minutes of the molten steel reflux time using RH. This fine adjustment of the molten steel component is basically completed by the end of the slag refining, but in the case of a product with a narrow tolerance range of the component standard, or when there is a large decrease in the Al component during slag refining, It is permissible to add a small amount of alloy iron or Al to the molten steel in the RH tank. In addition, the temperature of the molten steel is basically adjusted using the arc heating mechanism of the ladle refining device before the end of slag refining, but it is input at RH using the oxidation heat generated by the use of Al in RH. If the amount of Al to be processed is in the range of 4 kg / t or less, temperature adjustment during the RH treatment is allowed. However, since this oxidation heat generation is accompanied by the generation of Al ₂ O ₃ , it is preferable that the amount of Al used in RH is small. In the present invention, if the amount of Al used in RH is 0.1 kg or less per ton of molten steel, the total oxygen concentration in the steel slab produced through continuous casting can be controlled to 9 ppm or less.

  In the present invention, the relationship between the total oxygen concentration in the molten steel after slag refining and the total oxygen concentration in the steel slab that has undergone continuous casting is as follows: the amount of Al used in RH is more than 0.1 kg / t and less than 4 kg / t. FIG. 7 shows stratification according to the case of 1 kg / t or less. Each data described in FIG. 7 shows that the molten steel reflux time in RH is unified to 20 minutes, and in part, the amount of Al used during RH treatment can be surely suppressed to 0.1 kg / t. The molten steel temperature at the end of slag refining was raised.

  From these data, it can be understood that the total oxygen concentration in the steel slab can be controlled to 13 ppm or less if the total oxygen concentration in the molten steel after slag refining is 18 ppm or less by carrying out the present invention. It can also be seen that if the Al usage is 0.1 kg / t or less, the total oxygen concentration in the steel slab is 9 ppm or less.

After refining by a converter, about 80 tons of molten steel is taken out into a ladle, and alloy iron or Al is added to the molten steel in the steel, C: 0.08 to 1.05%, Si: 0.15 0.35%, Mn: 0.30 to 1.10%, P: 0.030% or less, S: 0.025% or less, sol. Al: Adjusted to 0.010 to 0.060%. After slag refining, lime slag and calcium aluminate are added before slag refining. After slag refining, the slag components are CaO = 50-60 mass%, Al ₂ O ₃ = 33-40 mass%. , CaO / Al ₂ O ₃ ≧ 1.5 and (FeO + MnO) ≦ 1% by mass were charged with quicklime as a slag modifier in the pan.

Thereafter, the ladle was transferred to a ladle refining device having a heating function by arc and a function of stirring molten steel and slag by bottom blowing Ar or N ₂ , and stirring of molten steel and slag was started while heating by arc. A sample is taken from the molten steel at 10 minutes after the start of the stirring, and the total oxygen concentration in the steel is analyzed within 5 minutes by the oxygen analysis method (the analytical value is found within 6 minutes from the time of sampling). ) Immediately after that, quick lime was additionally added according to the formula (2), and the stirring of the molten steel and slag was continued, and then the slag refining was finished.

The flow rate of bottom blown Ar or N ₂ in this slag refining is 1.25 to 1.88 Nl / min / t, and the time of stirring by flowing the bottom blown Ar or N ₂ is 10 minutes, including 10 minutes before sampling. Continued for 30-40 minutes.

As a result of separately analyzing the slag components in the pan at the time when the slag refining was completed, all of them were controlled to the above-described component ranges.
Furthermore, after that, the ladle was transferred to RH, and molten steel circulation was performed for 20 to 30 minutes. At that time, when producing a product having an allowable total oxygen concentration of 13 ppm or less, the amount of Al used in RH was not particularly suppressed, and the amount of Al used as a result was 0 to 4 kg / t. On the other hand, when a product with an allowable total oxygen concentration of 9 ppm or less is targeted, the molten steel temperature is set high by slag refining, and the amount of Al used in RH is suppressed to 0.1 kg / t or less. .

  After finishing the purification and refining of the molten steel in this way, a steel piece for each destination product was manufactured through continuous casting with a cast piece width of 300 to 400 mm and a cast piece thickness of 400 to 500 mm. As a result of collecting a sample from the steel piece and analyzing the total oxygen concentration, it was confirmed that all satisfied a predetermined total oxygen concentration of 13 ppm or less or 9 ppm or less.

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 (3)

In a clean steel melting method in which inclusion removal is performed in a vacuum degassing facility with a molten steel reflux time of 30 minutes or less after performing slag refining with heating by an arc and stirring by gas or electromagnetic force, during the slag refining process Measure the total oxygen concentration of the molten steel, and based on the total oxygen concentration measurement result so that the total oxygen concentration before vacuum degassing treatment is 18 ppm or less, the amount of quick lime added per ton of molten steel within the range of formula (2) (kg) A slag refining process, and a clean steel melting method.
0.18 × total oxygen concentration (ppm) −3.6 ≦ quick lime additional amount (kg / t)
≦ 0.18 × total oxygen concentration (ppm) −1.8 (2)   The cleaning according to claim 1, wherein the amount of Al input in the vacuum degassing process is suppressed to 0.1 kg or less per ton of molten steel by adjusting the components and temperature during the slag refining process. Steel melting method. As a method of measuring the total oxygen concentration of the molten steel during the slag refining treatment,
A method of measuring an oxygen concentration in a sample from an infrared absorption of one or both of carbon monoxide and carbon dioxide generated by placing a steel sample in a graphite crucible and heating and melting in an inert gas,
As a pretreatment for removing and cleaning the oxide film on the surface of the sample, vacuum arc plasma treatment is performed under the conditions of a vacuum degree at the start of arc plasma discharge of 5 Pa to 35 Pa and an arc plasma output current of 15 A to 55 A.
It was obtained by machining a steel ingot collected from molten steel so that the height was 1.5 mm or more and 7 mm or less, and the ratio of surface area S to volume V (S / V) was 1.05 or more and 1.30 or less. Using a small piece as a sample,
After the arc plasma discharge is applied to the sample a total of 4 times or less and a total treatment time of 0.2 seconds or more and 1.2 seconds or less,
Use a method for oxygen analysis in steel that directly heats and cleans the sample at a temperature higher than the temperature at the time of analysis without bringing it into the atmosphere, and then puts it in a graphite crucible that has been lowered to the temperature to be analyzed and placed on standby. The method for melting clean steel according to claim 1 or 2, wherein:

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