JP2007270247A - Method for manufacturing powder for continuous casting, and method for continuously casting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing powder for continuous casting with which in the case of continuously casting a Si-killed steel having <0.015 mass% Al content, the occurrence of a predict signal to the restrictive break-out can be lessened, and a method for continuously casting a steel by using this powder for continuous casting. <P>SOLUTION: The method for manufacturing the powder for continuous casting, is performed, with which in the case of melting and solidifying the raw material for powder, this raw material is cooled and pulverized by injecting the air without using the water. In the continuous casting by using the powder for continuous casting, manufactured with the method, the generation of bubble into the powder film is reduced in the continuous casting of the Si-killed steel having less Al content and the heat-conducting amount from the solidified shell to a mold wall, is increased and as the result, since the frequency of the occurrence of the predict signal to the restrictive break-out can be reduced, the frequency of the occurrence of a non-stationary part in the quality caused by the fluctuation of the casting speed, is reduced and also, the frequency of occurrence of the break-out can be reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing powder for continuous casting added to a mold in continuous casting of steel, and a method for continuously casting steel using the powder for continuous casting.

  In continuous casting of steel, powder for continuous casting is added into the mold. The powder for continuous casting melts on the surface of the molten steel in the mold and forms a lubricating film between the mold wall and the solidified shell. Continuous casting powders are almost all used in slabs and large section blooms. This powder plays a role of preventing oxidation of the molten steel surface in the mold, lubrication between the mold and the slab, capturing the floating inclusions, and keeping the temperature of the molten steel surface in the mold. The powder has many management factors such as its melting rate, viscosity, melting point, and alumina absorption capacity, and the optimum powder differs depending on the steel type, casting speed, slab cross-sectional shape, etc., so the selection is extremely important.

  With regard to the properties of the powder, it has been said that a granular powder that replaces the conventional powder has been developed, which has improved the working environment as well as reducing the vertical cracking and curling in terms of quality. Its adoption has also become common (Non-Patent Document 1). In addition, uniform meltability of powder is important for measures against vertical cracks in slabs, etc. Powder using a premelt base material in which the raw material mixture is previously melted in an electric furnace or cupola and homogenized by rapid cooling glass Have also been developed (Patent Document 1, Non-Patent Document 2).

When producing a premelt base material, it is necessary to melt the raw material of the mold powder base material such as silica, limestone, and fluorite, and then solidify it. As a method of solidification, a method (hydropulverization method) in which high-pressure water is blown, scattered and cooled and granulated is generally used. It is easy to recover the solidified base material, a particle size suitable for the next pulverization step is obtained, and a homogeneous vitrified base material is obtained without generating a compound such as CaO-SiO ₂ system. This is because the water granulation method is optimal because of such advantages.

  When producing granular powder, water is added to the powder raw material to form a slurry, and spray granulation is performed using a spray dryer to produce hollow granules, or water is added to the powder raw material to knead and extrusion granulation. There is a method of producing solid granules by stirring granulation or rolling granulation and then drying (Patent Document 2).

  Steel produced by continuous casting is usually killed steel, and deoxidation is mainly performed by adding 0.015% by mass or more of Al. On the other hand, the steel may be continuously cast using Si deoxidation with an Al content of less than 0.015 mass% and an Si content of 0.05 mass% or more. The main purpose of this is to increase the austenite crystal grain size by reducing the Al content and prevent intergranular fracture at high temperatures. Hereinafter, such steel is sometimes referred to as Si killed steel.

  In the restraint breakout, the solidified shell adheres to the mold wall and breaks in the vicinity of the meniscus. It is to arrive. If a temperature measurement end is installed on the mold wall, the temperature rises unsteadily when passing through the temperature measurement part when the fractured part of the solidified shell moves below the mold, so it is predicted that a breakout will occur can do. If the casting speed is rapidly reduced when the prediction signal is generated, the breakage portion of the solidified shell can be repaired to prevent the occurrence of breakout.

JP 59-27753 A JP-A-9-57407 Third Edition Steel Handbook II Steelmaking and Steelmaking, published by Maruzen Co., Ltd. (October 1979) (12.4-2) 4th Edition Handbook of Steel, July 2002, issued by the Japan Iron and Steel Institute (12.4.2+) 3rd Edition Steel Handbook I Basics, published by Maruzen Co., Ltd. (October 1979) (pages 157-158) J.N.Pontorie et.al., Rev. de Metall. Janvier, (2000), p.35

  In the continuous casting of Si killed steel having an Al content of less than 0.015% by mass, a phenomenon that a predictive signal for restrictive breakout frequently occurred was observed. When the Al content is 0.010% by mass or less, the frequency of occurrence further increases. Although breakout does not occur, it is necessary to rapidly reduce the casting speed each time a prediction signal is issued, so the casting productivity decreases and the solidification speed rapid deceleration part becomes an unsteady part. It can also cause quality degradation. In selecting the powder for continuous casting, a granular powder using a premelt base material is used, and a powder having a low basicity (basicity B is 1.0 to 1.2, giving priority to the inflow of the powder film). However, breakout prediction signals are still frequently generated.

  When the mold temperature in the vicinity of the meniscus was measured, compared with Al killed steel and Al-Si killed steel having an Al content of 0.015% by mass or more, in Si killed steel having an Al content of less than 0.015% by mass, It was found that the mold temperature was low. In the amount of heat removed from the solidified shell to the mold in the vicinity of the meniscus, Si killed steel means that the amount of heat removed is low. In addition, regarding the heat removal amount of the entire mold, it was found that the heat removal amount of Si killed steel was lower than that of other varieties. Despite not using a high basicity powder that slowly cools the heat removal, the heat removal is low, resulting in frequent occurrence of constraining breakout prediction signals.

  The present invention relates to a method for producing a powder for continuous casting and a powder for continuous casting capable of reducing the occurrence of a predictive signal of a constraining breakout when continuously casting Si killed steel having an Al content of less than 0.015% by mass. An object of the present invention is to provide a method for continuous casting of steel using steel.

  As described above, when continuously casting Si killed steel having an Al content of less than 0.015% by mass, the amount of heat removed from the solidified shell to the mold in the vicinity of the meniscus is lower than that of other varieties. This is presumed to be the cause of frequent occurrence of restrictive breakout prediction signals.

  Therefore, the powder film (part 50 mm below the meniscus) attached to the mold after casting was collected, and the cross section of the film was observed. As a result, it was found that when steel with a low Al content was cast, bubbles were generated in the cross section of the film, and the amount of generated bubbles increased as the Al content of the varieties to be cast decreased. In particular, when the Al content in the steel is 0.010% by mass or less, the porosity of the powder film due to air bubbles may exceed 10%. For this reason, in Si killed steel, air bubbles are generated in the powder film being cast and heat insulation is increased. In particular, the amount of heat removed from the solidified shell to the mold wall is reduced near the meniscus, and a constraining breakout prediction signal is generated. It is thought that it is connected to frequent occurrence.

  When the type of gas contained in the bubbles in the powder film was specified, it was found that the hydrogen content was large and the bubble component contained hydrogen gas or water vapor gas. It is considered that bubbles are generated as water vapor in the powder film when the concentration of hydrogen or moisture dissolved in the molten powder film becomes equal to or higher than the saturation solubility.

In basic powder, it exists as OH ⁻ when water vapor is dissolved in the molten powder. When Al is contained in the molten steel in contact with the powder film, OH ^{− in} the powder reacts with Al in the steel, and the hydrogen component moves into the steel, so that the OH ⁻ concentration in the powder decreases. Therefore, there are few bubble generation in a powder film. On the other hand, in the case of Si killed steel with a low Al content, the OH ⁻ concentration in the powder does not decrease and is maintained at a high concentration. Therefore, many bubbles are generated in the powder film.

  If the above hypothesis holds, if the amount of water vapor dissolved in the powder film can be reduced, even when casting Si killed steel with a low Al content, the generation of bubbles in the powder film is reduced. Should be.

  Therefore, in continuous casting of Si killed steel with low Al content, when powder production was performed using a production method with less chance of moisture mixing in the powder production process, the amount of bubbles generated in the powder film decreased, It was found that the amount of heat removed from the solidified shell to the mold wall increased, and as a result, the frequency of breakout prediction signal generation could be reduced.

  Furthermore, if the water vapor saturation solubility of the powder film can be increased, the bubble generation of the powder film should be further reduced. It is known that the solubility of the water vapor in the powder film increases as the basicity increases in a composition having a basicity of 1 or more and as the basicity decreases in a composition having a basicity of less than 1.

  Therefore, in continuous casting of Si killed steel with a low Al content, when casting was performed using powder having a higher basicity than before, the amount of bubbles generated in the Bower film was reduced, and the solidified shell to the mold wall It has been found that the amount of heat extracted from the heat source increases, and as a result, the frequency of breakout prediction signals can be reduced.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) A method for producing a powder for continuous casting for use in continuous casting of steel having an Al content of less than 0.015 mass%, wherein air is used without using water when the powder raw material is melted and solidified. A method for producing powder for continuous casting, characterized by cooling and crushing by spraying.
(2) The method for producing a powder for continuous casting as described in (1) above, wherein the basicity B represented by the following formula (1) is 1.4 or more.
B = T. CaO / SiO ₂ (1)
Here, T.W. CaO represents the CaO content (% by mass) when Ca in the powder is all CaO, and SiO ₂ represents the SiO ₂ content (% by mass) in the powder.
(3) When continuously casting steel having an Al content of less than 0.015% by mass, the continuous casting powder produced by the method described in (1) or (2) above is used. Casting method.

  In the continuous casting using the powder for continuous casting produced by the method of the present invention, the generation of bubbles in the powder film is reduced in the continuous casting of Si-killed steel with low Al content, and the solid shell is removed from the mold wall. Since the amount of heat can be increased, and as a result, the frequency of occurrence of a predictive signal for constraining breakout can be reduced, the frequency of occurrence of quality unsteady parts due to fluctuations in casting speed can be reduced, and the frequency of occurrence of breakout can also be reduced.

  The continuous casting powder of the present invention is intended for continuous casting of steel having an Al content of 0.015% by mass or less. In order to deoxidize the dissolved oxygen remaining in the molten steel by decarburization refining, Si is contained in an amount of 0.05 mass% or more. Therefore, the target steel is referred to as Si killed steel here. The carbon concentration of the steel is not particularly specified, but low carbon steel of about 0.01 to 0.08 mass% is the main target, and medium carbon steel of about 0.09 to 0.40 mass% is also cast. Moreover, although it does not specifically limit about the slab to cast, Slab continuous casting becomes a main object. In the present specification, the Al content in steel means total Al.

  When continuously casting the Si killed steel, the most common powder was used as the powder for continuous casting. In order to obtain a uniform molten slag pool with a constant melt hatching rate, a premelt base material is often used, and granular powder has been used to improve the heat retention of the molten steel surface. In particular, since it is not a variety in which vertical cracking is likely to occur, a powder having a basicity of about 1.0 to 1.2 has been used mainly for improving the inflowability of the powder film.

In the present invention, the basicity B is determined by the following formula (1).
B = T. CaO / SiO ₂ (1)
Here, T.W. CaO represents the CaO content (% by mass) when Ca in the powder is all CaO, and SiO ₂ represents the SiO ₂ content (% by mass) in the powder.

  In continuous casting, a thermocouple is embedded in the mold wall to generate a breakout prediction signal in order to prevent the occurrence of a constraining breakout. As the fractured part of the solidified shell, which causes a constraining breakout, moves below the mold, the temperature rises unsteadily when the fractured part passes through this thermocouple installation, so it is predicted that a breakout will occur can do. If the casting speed is rapidly reduced when the prediction signal is generated, the breakage portion of the solidified shell can be repaired to prevent the occurrence of breakout.

  The occurrence frequency of the breakout prediction signal was compared for each Al content level of steel. In any case, the Si content of the steel is 0.05% by mass or more, and the Al content of 0.015% by mass or more can be called Al-Si killed steel, and the Al content is less than 0.015% by mass. Can be called Si killed steel. FIG. 1 is a diagram in which the horizontal axis represents the Al content and the vertical axis represents the breakout prediction signal generation frequency. As is clear from FIG. 1, as the Al content in the steel decreases, the occurrence frequency of the breakout prediction signal increases, which is particularly remarkable when the Al content is less than 0.015% by mass.

  In a product that frequently generates a breakout prediction signal, a breakout may occur because the countermeasure for preventing the occurrence of the breakout is not in time. Breakout does not occur at all in Al-Si killed steel with an Al content of 0.015 mass% or more, whereas the breakout occurrence rate is about 1.7% for varieties with an Al content of 0.010 mass% or less. It was.

  Therefore, the state of heat transfer between the solidified shell and the mold wall in the mold was investigated for each Al content of steel. The survey was conducted from three viewpoints. First, the cooling water temperature rise allowance of the entire mold long side surface was measured, and the amount of heat removed from the entire mold long side surface was compared. Second, the temperature of thermocouples embedded in a plurality of locations in the casting direction in the mold was measured, and the temperature distribution in the casting direction was compared. Third, paying attention to the secondary dendrite arm spacing in the vicinity of the surface of the slab cross section, the amount of heat removal at a plurality of locations in the casting direction was estimated from this arm spacing. Casting varieties had an Al content of 0.008 mass% (low Al varieties), 0.017 mass% (medium Al varieties), and 0.036 mass% (high Al varieties), and the Si content was low. It is 0.05 mass% or more for Al varieties, and 0.006 to 0.01 mass% for medium Al varieties and high Al varieties. The continuous casting powder used was a granular powder using a premelt base material, and its basicity was about 1.1 to 1.2.

The surface average heat flux of the entire long side surface was determined based on the cooling water temperature rise margin on the long side surface of the mold. The casting speed was about 1.5 m / min. As a result, the surface average heat flux was about 1500 kW / m ² · sec for the medium Al and high Al varieties, while the surface average heat flux was about 1100 kW / m ² · sec for the low Al variety. .

  In the mold wall, thermocouples are embedded at four locations at a pitch of 120 mm in the casting direction. The uppermost thermocouple is 65 mm from the meniscus. The distance from the mold wall surface to the thermocouple tip is about 5 mm. When the thermocouple temperature was measured for the low Al and medium Al varieties, the results shown in FIG. 2 were obtained. The thermocouple temperature in the 2nd to 4th stages in the casting direction does not change much depending on the type, but the temperature of the thermocouple installed near the meniscus is about 160 ° C for the medium Al type, and about 110 ° C for the low Al type. A significantly lower temperature was observed. From this, it is clear that in the low Al varieties, the heat removal from the solidified shell to the mold wall is reduced particularly in the vicinity of the meniscus.

The secondary dendrite arm interval in the vicinity of the surface of the slab cross section was measured, and the cooling rate CR (° C./min) of each part of the slab was calculated from this. The cooling rate CR means an average cooling rate between the liquidus temperature and the solidus temperature. The following formula (2) was used to calculate the cooling rate CR.
_{CR = (λ 2/770)} (-1 / 0.41) (2)
λ ₂ : Secondary dendrite arm spacing (mm)

  The results are shown in FIG. In FIG. 3A, the cooling rate in the vicinity of the meniscus was estimated based on the secondary dendrite arm interval at a depth of 2 mm from the surface. In FIG. 3B, the cooling rate from the middle of the mold to the lower part was estimated based on the secondary dendrite arm spacing at a depth of 15 mm from the surface. As is clear from the figure, the cooling rate calculated from the secondary dendrite arm spacing at a depth of 15 mm from the surface does not show a difference due to the Al content of the steel, but the calculated cooling rate at 2 mm from the surface, that is, near the meniscus. Is affected by the Al content of steel, and it can be seen that the cooling rate of the varieties having an Al content of 0.008% by mass is lower than that of other varieties.

  From the above results, in spite of casting using the same continuous casting powder, in the low Al varieties, the extraction from the solidified shell near the meniscus to the mold wall is different from the medium Al varieties and high Al varieties. It was found that the amount of heat was greatly reduced. Since there was no tendency for the thickness of the powder film existing between the solidified shell and the mold wall to vary by type, the difference in the amount of heat removal is not the difference in the thickness of the powder film. In addition, regarding the crystallization rate of the powder film, since there was no tendency for the crystallization rate to change for each type, the difference in the amount of heat removal is not the difference in the crystallization rate.

  Therefore, a powder film remaining on the wall of the mold after casting was collected and investigated. A powder film was sampled from a position about 50 mm from the meniscus, which has a large effect on the heat removal behavior near the meniscus.

  FIG. 4 shows a cross-sectional photograph of a powder film collected after casting a low Al variety. The black circles seen in the photos are voids. The voids in the film cross section were most severe in the film after casting the low Al type, less in the medium Al type, and hardly observed in the film after casting the high Al type. FIG. 5 is a graph in which the porosity of the powder film is calculated based on the cross-sectional photograph and the relationship between the Al content in the steel and the porosity is plotted. As is clear from FIG. 5, the low Al variety shows a remarkably high porosity. It is clear that the lower the Al content in the steel, the more air bubbles are present in the powder film existing between the solidified shell and the mold wall.

  From the above results, it was estimated that the cause of the decrease in the amount of heat removal in the vicinity of the meniscus in the continuous casting of low Al varieties was caused by the frequent occurrence of bubbles in the powder film and the increased porosity. .

  Next, gas analysis of the gas contained in the bubbles in the powder film was performed, and the spectrum integrated intensity ratio by mass number was obtained. As a result, the most abundant gas was nitrogen or carbon monoxide, but each sample was characterized by containing hydrogen gas at a ratio of 7 to 16%. Since hydrogen gas was detected, it is presumed that the interaction between the molten slag and water vapor is related to the generation of bubbles in the powder film.

According to Non-Patent Document 3, the water vapor solubility of molten slag is proportional to the square root of the water vapor partial pressure under the condition of constant temperature and slag composition. Further, in basic slag having a basicity of 1 or more, it is considered that water vapor is dissolved in the form of OH ^- ions in the slag. Moreover, in the region where the basicity is 1 or more, the water solubility in the molten slag increases as the basicity increases. On the other hand, in acidic slag having a basicity of 1 or less, it is considered that water vapor dissolves in a form that releases oxygen and breaks Si—O—Si bonds in the slag. Further, in a region where the basicity is 1 or less, the lower the basicity, the higher the water vapor solubility in the molten slag.

According to Non-Patent Document 4, at the interface of the molten slag and the molten steel is in contact, and H in the molten steel is transferred into the slag OH ^- reaction and as the ions, in the molten slag OH ^- ions in the molten steel Two types of reactions that react with contained Al and migrate as H in molten steel are considered. From this, OH ^- ions in the powder film tend to increase due to reaction with molten steel when casting low Al type, whereas OH ^- ions in powder film do not increase so much when casting medium and high Al type, or It is estimated that it tends to decrease.

The fact that bubbles in the powder film are not observed in the medium and high Al varieties but is observed in the low Al varieties, the fact that hydrogen is observed in the bubbles, and the facts described in Non-Patent Documents 3 and 4 above. When combined, the following inferences can be assembled. That is, in the medium and high Al varieties, Al in the molten steel reacts with the powder in the molten pool to reduce the OH ⁻ ion concentration in the powder, so no bubbles are generated. since Al is less in powder OH ^- low enough to reduce the ion concentration, OH ^- remaining in the powder ions at high concentrations. As a result, in the low Al variety, the OH ⁻ ion concentration exceeds the dissolution concentration of the powder film and is taken into the powder film as a large amount of bubbles.

  Based on the above inferences, if the amount of water vapor dissolved in the powder film can be reduced, it is possible to prevent the generation of water vapor from the powder film, even if it is a low Al variety, and suppress the generation of bubbles. There is sex.

  By the way, as described above, when manufacturing a premelt base material of premelt powder, a water granulation method is generally used in which a raw material is dissolved and then sprayed with high-pressure water, sprayed, cooled and granulated. Yes. During the water granulation, there is a possibility that a large amount of water vapor is contained in the premelt base material.

  Therefore, in the production of the pre-melt base material, a production method by a method (air crushing method) in which high-pressure air is blown onto the base material of the molten body, is dispersed and cooled, and granulated is applied.

  On top of that, continuous casting is performed using powder produced by the air crushing method (the present invention method) and the water crushing method (conventional method) for Si killed steel having an Al content of 0.004 to 0.010 mass%. The results were compared. Continuous casting using powder produced by a water granulation method was also performed in addition to continuous casting of Al killed steel having an Al content of 0.017% by mass.

  FIG. 6 shows the relationship between the powder manufacturing method / casting type and the powder film porosity, and FIG. 7 shows the relationship between the powder manufacturing method / casting type and the mold thermocouple temperature. In all cases, the horizontal axis represents the basicity of the powder. As for the mold thermocouple temperature in FIG. 7, the difference between the first-stage thermocouple temperature 65 mm from the meniscus and the second-stage thermocouple temperature 120 mm below is displayed. The casting speed was in the range of 1.5 to 1.7 m / min.

  In the case of Si killed steel cast using granular powder produced using a pre-melt base material that has been quenched and crushed by the water granulation method (○ in the figure), many voids were observed in the powder film, In the case of Si killed steel cast using granular powder produced using a premelt base material crushed by the air crushing method (□ in the figure), the porosity in the powder film was low at any powder basicity ( FIG. 6). Therefore, in the former, the mold thermocouple temperature was low particularly at the position corresponding to the molten metal surface, and abnormal slow cooling was achieved, whereas in the latter, the thermocouple temperature at the molten metal surface position was high (FIG. 7).

  As a result, as described later, the frequency of occurrence of a predictive signal for a constraining breakout can be reduced, so that the frequency of occurrence of a quality unsteady part due to fluctuations in casting speed can be reduced and the frequency of occurrence of breakout can also be reduced.

  Furthermore, if the water vapor solubility of the powder film can be increased, the generation of water vapor from the powder film can be prevented even if it is a low Al variety, and the generation of bubbles may be suppressed. And, as described in Non-Patent Document 3, it is known that if the basicity is slag of 1 or more, the higher the basicity is, the more water vapor solubility is increased. By increasing the basicity of the powder, The present inventors have found that there is a possibility that the generation of bubbles in the powder film can be suppressed even in a low Al variety.

  Therefore, as shown in FIGS. 6 and 7, the powder having a low water content produced by the method of the present invention is used, and Si killed steel having an Al content of 0.004 to 0.010% by mass is used as a base. The powder which changed variously the value of the degree in the range of 1.0-2.22 was prepared, and it was evaluated how the slab continuous casting situation in Si killed steel changes with the basicity of the powder.

  In the powder having low basicity, there are many voids in the powder film, and the thermocouple temperature in the meniscus portion is lowered due to the decrease in heat removal associated therewith. On the other hand, when a powder having a high basicity is used, voids in the powder fill are reduced, the thermocouple temperature of the meniscus portion is high, and stable casting similar to Al killed steel can be performed.

  That is, in the continuous casting of steel having an Al content of less than 0.015% by mass, the continuous casting is a powder produced by the method of the present invention, and the basicity B represented by the above formula (1) is 1.4 or more. It was clarified that the use of the powder for powder can suppress the generation of bubbles in the powder film and secure the amount of heat removed from the solidified shell to the mold wall. As a result, as described later, the frequency of occurrence of a predictive signal for a constraining breakout can be reduced, so that the frequency of occurrence of a quality unsteady part due to fluctuations in casting speed can be reduced and the frequency of occurrence of breakout can also be reduced. More preferably, the basicity of the powder is 1.8 or more. On the other hand, if the powder basicity is too high, the powder film coagulation temperature becomes high, and there is a problem that the slab lubricating function by the liquid slag is remarkably impaired. However, if the powder basicity is 2.3 or less, such a problem does not occur. Therefore, it is preferable.

  In addition, the improvement effect becomes more remarkable in the steel having an Al content of 0.010% by mass or less. In addition, the effect is remarkable in slab continuous casting.

  In continuous casting with a vertical bend type slab continuous casting machine, continuous casting is performed using the powder of the present invention and conventional powder as continuous casting powder, and the frequency of occurrence of breakout prediction signals is compared for each Al content level of steel. I tried to. In any case, the Si content of the steel is 0.05% by mass or more. FIG. 8 is a diagram in which the horizontal axis represents the Al content and the vertical axis represents the breakout prediction signal generation frequency.

  The present invention powder (powder 2, 3) is a continuous casting manufactured based on a premelt base material obtained by melting powder raw materials in a cupola furnace in a powder manufacturing process and blowing high pressure air onto slag discharged from the furnace and crushed. For powder. Conventional powder (powder 1) is a powder using a premelt base material obtained by spraying water on slag discharged from a furnace and granulating it. Powders 1 and 2 use base materials of the same composition and have a basicity of 1.2. Powder 3 has a basicity of 1.8.

  As apparent from FIG. 8, in the Si killed steel having an Al content of less than 0.015% by mass, when the powder 1 (●) is used, the frequency of occurrence of breakout prediction signals is extremely high, It can be seen that the frequency of occurrence of the breakout prediction signal is drastically reduced even though the powder 2 (◯) is the same component as the powder 1. In particular, the effect is remarkable in varieties having an Al content of 0.010% by mass or less. Furthermore, the powder 3 (◎) having a higher basicity shows a greater improvement.

  Regarding the breakout occurrence frequency, when the Al content was 0.010% by mass or less and the powder 1 was used, the breakout occurrence frequency was about 1.7%, whereas the powders 2 and 3 were used. In some cases, no breakout occurred at any Al content level.

It is a figure which shows the relationship between Al content in steel, and a breakout prediction signal generation rate. It is a figure which shows the relationship between Al content in steel, and the thermocouple temperature in a casting mold according to casting direction. It is a figure which shows the relationship between Al content in steel, and the cooling rate of a slab, (a) is a value by the secondary dendrite arm space | interval in the depth of 2 mm from the surface, (b) is in the depth of 15 mm from the surface. The value is based on the secondary dendrite arm spacing. It is a figure which shows the cross-sectional photograph of the powder film after low Al kind casting. It is a figure which shows the relationship between Al content in steel, and the porosity in a powder film. It is a figure which shows the relationship between a powder manufacturing method, a casting kind, powder basicity, and a powder film porosity. It is a figure which shows the relationship between powder manufacturing method, casting kind, powder basicity, and mold | type thermocouple temperature. It is a figure which shows the improvement condition of the breakout prediction signal generation rate by this invention according to Al content in steel.

Claims (3)

  A method for producing powder for continuous casting for use in continuous casting of steel having an Al content of less than 0.015% by mass, in which air is blown without using water when the powder raw material is melted and solidified. A method for producing a powder for continuous casting, characterized in that the powder is cooled and crushed by the method. The basicity B shown by following formula (1) is 1.4 or more, The manufacturing method of the powder for continuous casting of Claim 1 characterized by the above-mentioned.
B = T. CaO / SiO ₂ (1)
Here, T.W. CaO represents the CaO content (% by mass) when Ca in the powder is all CaO, and SiO ₂ represents the SiO ₂ content (% by mass) in the powder.   A continuous casting method for steel, characterized by using the powder for continuous casting produced by the method according to claim 1 or 2 when continuously casting a steel having an Al content of less than 0.015 mass%.