JP2017140635A - Manufacturing method of thin cast piece - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a thin cast piece which can suppress nozzle clogging of molten steel by performing property modification of AlOexisting in the molten steel or on the surface of the molten steel to obtain the low melting point even when manufacturing the thin cast piece by using the molten steel T.Al amount of which is 0.005 mass% or more so that long time casting can be stably performed.SOLUTION: In a manufacturing method of a thin cast piece, an immersion nozzle supplying a molten steel gathering part with molten steel comprises: an outer nozzle which includes a bottom face shape along a longer direction of cooling drums with a refractory material for rectification inside thereof; and an inner nozzle which supplies the outer nozzle with the molten steel. Ca vapor is supplied to a closed space above the molten steel between the inner nozzle and the outer nozzle, and atmosphere gas in the closed space is exhausted. Ca vapor partial pressure P(MPa) in the closed space is controlled, when total oxygen amount [T.O] (mass%) is given, to satisfy a relationship: P≥17.077×[T.O]-0.006.SELECTED DRAWING: None

Description

  In the present invention, molten steel is supplied to a molten steel reservoir formed by a pair of cooling drums and a pair of side weirs. The present invention relates to a method for producing a thin-walled slab in which a thin-walled slab is produced using molten steel having an Al content of 0.005% by mass or more.

  As a method for producing a thin metal slab, a cooling drum having a water cooling structure is provided inside, and the molten steel is supplied to a molten steel reservoir formed between a pair of rotating cooling drums, and solidified on the peripheral surface of the cooling drum. A twin-drum continuous casting apparatus that forms and grows shells, joins the solidified shells formed on the outer peripheral surfaces of a pair of cooling drums to each other at a drum kiss point, and produces a thin cast piece of a predetermined thickness by reducing the shell. Is provided. Such a twin drum type continuous casting apparatus is applied to various metals.

In the above-described twin-drum type continuous casting apparatus, the molten steel pool formed between the pair of cooling drums has a shape extending in the longitudinal direction (width direction of the thin cast piece) parallel to the axis of the cooling drum, It is necessary to supply the molten steel so that it is sufficiently distributed in the longitudinal direction of the cooling drum.
For this reason, as shown, for example in patent documents 1-4, the immersion nozzle which has an inner nozzle and the outer nozzle by which the rectification refractory material was arrange | positioned by the bottom face is proposed. Here, the rectifying refractory is structured to add pressure loss to the molten steel flow, and the molten steel supplied from the inner nozzle into the outer nozzle is a longitudinal surface parallel to the axis of the cooling drum on the upper surface of the rectifying refractory. It spreads in the direction (width direction of the thin-walled slab) and is supplied from the discharge port provided in the outer nozzle toward the cooling drum peripheral surface through the passage of the rectifying refractory. That is, in this immersion nozzle, the molten steel is temporarily stored in the outer nozzle, and then the molten steel is supplied to the molten steel pool portion.

When Al deoxidized molten steel is used, the solid phase Al ₂ O ₃ suspended in the molten steel tends to adhere to and accumulate on the inner wall of the passage of the rectifying refractory, and the rectifying refractory tends to block. It was in. Further, an Al ₂ O ₃ coating is formed on the surface of the molten steel stored on the rectifying refractory, and the rectifying refractory may be blocked by the Al ₂ O ₃ coating. For this reason, when casting a thin cast slab using molten steel deoxidized by Al, there is a problem that casting cannot be performed stably for a long time.

Here, in general continuous casting for producing a slab having a thickness of several tens of mm or more, by adding Ca to molten steel, a high melting point Al ₂ O ₃ is converted to a low melting point CaO—Al ₂ O ₃ series. A method of modifying to an oxide has been proposed.
In general, when Ca is added to molten steel, a method of adding Ca wire or a method of adding Ca powder together with flux to a ladle or molten steel in a tundish is performed. However, when Ca solid is added to the molten steel, Ca is rapidly vaporized in the molten steel, and large bubbles rapidly rise, so the Ca addition yield decreases. In addition, the hot water surface shakes violently, resulting in large fluctuations in yield and lack of operational stability.

  As means for improving such drawbacks and stably adding Ca with a good yield, Patent Document 5 and Patent Document 6 include Ca vapor together with a carrier gas in a tundish or molten steel in a mold using an immersion lance. A method of adding to is proposed.

JP-A-62-282853 Japanese Patent Laid-Open No. 07-068357 Japanese Patent Laid-Open No. 08-164450 JP 09-225596 A JP 2005-169404 A Japanese Patent Laying-Open No. 2005-219072

By the way, when it is going to apply the method described in patent document 5 and patent document 6 to a twin drum type continuous casting apparatus, since the molten steel pool part formed between the cooling drums is very narrow, the amount of molten steel is small. Variation of the molten metal surface due to bubbles becomes very large, and it is impossible to keep the molten metal surface level stable. Therefore, it is realistic to supply Ca vapor to the molten steel in the tundish. However, when casting thin-walled slabs, the throughput (casting amount per unit time) is small compared to general continuous casting, so the tundish capacity is small and the bath depth of the molten steel is shallow. For this reason, even if Ca vapor | steam is supplied to the molten steel in a tundish, the bubble of Ca vapor | steam will float up in a short time, the reaction time of molten steel and Ca vapor | steam cannot be ensured, and Ca is yielded in molten steel with a sufficient yield. Cannot be added.
Further, in the methods described in Patent Document 5 and Patent Document 6, since the immersion lance is immersed in the molten steel, heat dissipation proceeds through the non-immersion part, and the metal is attached in the vicinity of the immersion lance outlet, There is a possibility that the immersion lance is blocked and casting cannot be performed stably for a long time.

The present invention has been made in view of the above situation. Even in the case of producing a thin cast slab using molten steel having an Al content of 0.005% by mass or more, Ca can be supplied stably and stably in the molten steel, and the molten steel stored in the nozzle Providing a thin-walled slab manufacturing method that can suppress clogging of molten steel in the nozzle by reforming Al ₂ O ₃ existing on the surface of the molten steel or lowering the melting point, and can stably cast for a long time The purpose is to do.

In order to solve the above problems, a method for producing a thin cast piece according to the present invention supplies molten steel to a molten steel pool formed by a pair of rotating cooling drums and a pair of side weirs, and the peripheral surface of the cooling drum A solidified shell is formed and grown in A thin-walled slab manufacturing method for manufacturing a thin-walled slab using molten steel having an Al amount of 0.005% by mass or more, wherein an immersion nozzle for supplying the molten steel to the molten-steel pool is a longitudinal direction of the cooling drum An outer nozzle having a rectifying refractory inside, and an inner nozzle for supplying molten steel into the outer nozzle, and the molten steel supplied from the inner nozzle is the outer nozzle And supplying Ca vapor to the closed space on the molten steel between the inner nozzle and the outer nozzle and exhausting the atmospheric gas in the closed space, The Ca vapor partial pressure P _Ca (MPa) in the closed space is calculated from the total oxygen amount in the molten steel [T. O] (mass%)
P _Ca ≧ 17.077 × [T. O] −0.006
It is characterized by controlling so as to satisfy the relationship.

According to the method for manufacturing a thin cast slab of this configuration, Ca vapor is supplied to the closed space on the molten steel between the inner nozzle and the outer nozzle, and the atmospheric gas in the closed space is exhausted. Therefore, the Ca vapor partial pressure in the closed space can be stably maintained high. Specifically, the Ca vapor partial pressure P _Ca (MPa) in the closed space is set to the total oxygen amount [T . O] (mass%), P _Ca ≧ 17.077 × [T. O] −0.006 can be controlled. Thus, the contact area and time with the molten steel surface and Ca vapor is ensured, and the molten steel and the Ca vapor is efficiently reacted, in the molten steel stored in the nozzle or the molten steel surface Al ₂ O ₃ having a low melting point It becomes possible to modify to a CaO—Al ₂ O ₃ oxide. Therefore, clogging of the molten steel in the rectifying refractory can be suppressed, and casting can be performed stably for a long time.
Furthermore, since the atmospheric gas in the closed space is exhausted, pressure fluctuations in the closed space can be suppressed, and fluctuations in the molten metal surface can be suppressed. Thereby, operation | movement of the flow volume adjustment mechanism (for example, stopper, sliding nozzle) which controls a hot_water | molten_metal surface level can be stabilized, and casting can be implemented stably.
In addition, since it is not necessary to immerse the Ca vapor supply port in the molten steel, there is no possibility that the steel solidifies and adheres to the Ca vapor supply port and can be stably operated for a long time.

Here, in the manufacturing method of the thin cast piece which concerns on this invention, it is preferable to set it as the structure which supplies the said Ca vapor | steam to the said closed space with the carrier gas which consists of inert gas.
In this case, Ca vapor can be reliably supplied to the closed space by the carrier gas. Further, by adjusting the mixing ratio of the carrier gas and the Ca vapor, the Ca vapor partial pressure in the closed space can be adjusted relatively easily.

As described above, according to the present invention, T.I. Even in the case of producing a thin cast slab using molten steel having an Al content of 0.005% by mass or more, Ca can be supplied stably and stably in the molten steel, and the molten steel stored in the nozzle Providing a thin-walled slab manufacturing method that can suppress clogging of molten steel in the nozzle by reforming Al ₂ O ₃ existing on the surface of the molten steel or lowering the melting point, and can stably cast for a long time can do.

It is explanatory drawing which shows an example of the twin drum type continuous casting apparatus used for the manufacturing method of the thin cast slab which is embodiment of this invention. It is a schematic explanatory drawing of the immersion nozzle used for the manufacturing method of the thin cast slab which is embodiment of this invention. It is a schematic explanatory drawing of the other immersion nozzle used for the manufacturing method of the thin cast slab which is embodiment of this invention. It is a schematic explanatory drawing of the other immersion nozzle used for the manufacturing method of the thin cast slab which is embodiment of this invention. It is a schematic explanatory drawing of the other immersion nozzle used for the manufacturing method of the thin cast slab which is embodiment of this invention. It is a graph which shows the evaluation result of the generation | occurrence | production state of the deposit | attachment to the refractory material for rectification | straightening in a nozzle in an Example. It is a graph which shows the evaluation result of a casting condition in an Example.

Below, the manufacturing method of the thin cast slab which is embodiment of this invention is demonstrated with reference to attached drawing. In addition, this invention is not limited to the following embodiment.
The thin cast slab 1 manufactured in the present embodiment is a T.I. It is supposed to be made of Al deoxidized steel having an Al content of 0.005% by mass or more. In the case of a steel type containing an element that decreases the activity of Ca, for example, Ni or Si, the yield of Ca addition is improved.
Moreover, in this embodiment, the width | variety of the thin cast piece 1 manufactured is set to the range of 500 mm or more and 2000 mm, and the thickness is set to the range of 1 mm or more and 5 mm or less.

Next, a twin-drum type continuous casting apparatus used in the method for manufacturing a thin cast piece according to this embodiment will be described with reference to FIG.
A twin-drum type continuous casting apparatus 10 shown in FIG. 1 includes a pair of cooling drums 11, 11, bender rolls 12, 12 that bend the thin cast piece 1, pinch rolls 13, 13 that support the thin cast piece 1, and a pair The molten steel 3 supplied to the molten steel pool 16 defined by the side weir 15 disposed at the width direction end of the cooling drums 11, 11 and the pair of cooling drums 11, 11 and the side weir 15. A tundish 18 to be held and an immersion nozzle 20 for supplying the molten steel 3 from the tundish 18 to the molten steel pool portion 16 are provided.

  In this twin-drum type continuous casting apparatus 10, the molten steel 3 comes into contact with the rotating cooling drums 11, 11 and is cooled, so that the solidified shells 5, 5 grow on the peripheral surfaces of the cooling drums 11, 11. The solid cast shells 5, 5 formed on the pair of cooling drums 11, 11 are pressed against each other at the drum kiss point, thereby casting the thin cast piece 1 having a predetermined thickness.

  Here, as shown in FIG. 2, the above-described immersion nozzle 20 includes an outer nozzle 21 and an inner nozzle 30 inserted into the outer nozzle 21.

As shown in FIG. 2, the outer nozzle 21 includes a lower region 21a having a bottom shape along a longitudinal direction (width direction of the thin cast piece) parallel to the axis of the cooling drum 11, and above the lower region 21a. And an upper region 21b.
A plurality of discharge ports 22 for the molten steel 3 are provided on the longitudinal side surface of the lower region 21a. In addition, a porous refractory filter 23 is disposed as a rectifying refractory inside the lower region 21a. By disposing the porous refractory filter 23 which is a pressure loss structure, the molten steel 3 supplied from the inner nozzle 30 spreads on the upper surface of the refractory filter 23, and the molten steel 3 is temporarily stored in the outer nozzle 21. Will be.
Here, the length in the longitudinal direction of the lower region 21a of the outer nozzle 21 (the length in the direction along the longitudinal direction parallel to the axis of the cooling drum 11) is 40 to 80% of the width of the thin slab 1 to be cast. It is set within the range.

As shown in FIG. 2, the inner nozzle 30 has a tubular shape and is inserted from above the outer nozzle 21.
The inner nozzle 30 and the outer nozzle 21 define a closed space 40 on the molten steel stored in the outer nozzle 21.

The immersion nozzle 20 includes a Ca vapor supply unit 41 that supplies Ca vapor to the closed space 40 and an exhaust unit 42 that exhausts atmospheric gas in the closed space 40 to the outside.
In the present embodiment, as shown in FIG. 2, the Ca vapor supply unit 41 is arranged in the upper region 21 b of the outer nozzle 21, and blows Ca vapor to the molten steel 3 stored in the outer nozzle 21. Has been placed. The exhaust part 42 is also disposed in the upper region 21 b of the outer nozzle 21 and is disposed above the Ca vapor supply part 41 described above.

  Next, the manufacturing method of the thin cast piece which is this embodiment using the twin drum type continuous casting apparatus 10 mentioned above is demonstrated.

  The molten steel 3 is supplied from the tundish 18 through the immersion nozzle 20 to the molten steel pool 16 formed by the pair of cooling drums 11 and 11 and the side weir 15, and the pair of cooling drums 11 and 11 are moved in the rotation direction R. Each of the cooling drums 11 and 11 is rotated so that the region where the pair of cooling drums 11 and 11 are close to each other is directed in the drawing direction of the thin cast piece 1 (downward in FIG. 1).

  Then, the solidified shell 5 is formed on the peripheral surface of the cooling drum 11. Then, the solidified shell 5 grows on the peripheral surface of the cooling drum 11, and the solidified shells 5, 5 respectively formed on the pair of cooling drums 11, 11 are pressure-bonded at the drum kiss point, so that a thin wall with a predetermined thickness is obtained. The slab 1 is cast.

In this embodiment, the closed space 40 is supplied by supplying Ca vapor from the Ca vapor supply unit 41 to the closed space 40 and exhausting the atmospheric gas in the closed space 40 from the exhaust unit 42 to the outside. The Ca vapor partial pressure P _Ca (MPa) is controlled. Specifically, the total oxygen amount of the molten steel 3 in the tundish 18 [T. O] (mass%) was measured, and this total oxygen amount [T. O] (mass%)
P _Ca ≧ 17.077 × [T. O] −0.006
The Ca vapor partial pressure P _Ca (MPa) is controlled so as to satisfy the following relationship. The upper limit of the Ca vapor partial pressure P _Ca (MPa) is an atmospheric pressure of 0.101 MPa when all atmospheric gases are Ca vapor.

This equation was obtained as follows.
First, Al ₂ O ₃ causing nozzle clogging is modified to a CaO—Al ₂ O ₃ low melting point oxide, and the amount of Ca in molten steel [Ca] (mass%) necessary for preventing clogging is heated. Obtained by mechanical calculation. Here, the amount of Al ₂ O ₃ is [T. O] (mass%) as an index,
[Ca] ≧ a [T. O] + b (a and b are constants) (1)
It can be expressed in the form of [T. It is necessary to increase [Ca] as O] increases, that is, as Al ₂ O ₃ increases. In general, the relationship between [Ca] in molten steel and the Ca vapor partial pressure _PCa is
[Ca] = cP _Ca + d (c and d are constants) (2)
It is shown in the form of Organize both formulas,
P _Ca ≧ e [T. O] + f (e and f are constants) (3)
Is guided. This equation is described in [T. It is shown that the higher the O], that is, the more Al ₂ O ₃ , the higher the Ca vapor partial pressure is required to prevent nozzle clogging. The constants e and f were determined in accordance with the experimental results.

In this embodiment, it is set as the structure which inserts solid Ca into a sealed container and generates Ca vapor | steam by heating with an electric heater from the outside. By constantly measuring the temperature inside the sealed container and controlling the heating temperature, it becomes possible to stably control the amount of Ca vapor generated.
The Ca vapor thus obtained is supplied to the closed space 40 together with a carrier gas made of an inert gas. At this time, the Ca vapor partial pressure in the closed space 40 can be adjusted by adjusting the Ca heating temperature and the carrier gas flow rate. Here, as an inert gas used as a carrier gas, for example, Ar, N ₂ , a mixed gas thereof, or the like can be used.

And by adjusting the Ca vapor partial pressure of the closed space 40, Ca vapor exists on the molten steel 3 stored in the outer nozzle 21, and this Ca vapor reacts with the molten steel 3, Al ₂ O ₃ present in the molten steel ₃ or an Al ₂ O ₃ coating formed on the surface of the molten steel 3 is modified into a low melting point CaO—Al ₂ O ₃ oxide.

In the method for manufacturing the thin cast slab 1 according to the present embodiment configured as described above, the closed space 40 defined by the inner nozzle 30 and the outer nozzle 21 is connected to the Ca via the Ca vapor supply unit 41. Since the steam partial pressure of Ca in the closed space 40 is adjusted by supplying steam and exhausting the atmospheric gas in the closed space 40 from the exhaust portion 42 to the outside, the molten steel 3 stored inside the outer nozzle 21 is adjusted. And Ca vapor contact and react efficiently, and Al ₂ O ₃ present in the molten steel 3 or on the surface of the molten steel 3 can be modified into a CaO—Al ₂ O ₃ oxide having a low melting point. Specifically, in the present embodiment, the Ca vapor partial pressure P _Ca (MPa) in the closed space is set to the total oxygen amount [T. O] (mass%)
P _Ca ≧ 17.077 × [T. O] −0.006
Therefore, the amount of Ca sufficient to modify Al ₂ O ₃ can be secured, and Al ₂ O ₃ present in the molten steel 3 or on the surface of the molten steel 3 can be accurately obtained. Can be modified into a low melting point CaO—Al ₂ O ₃ oxide.
Thereby, clogging of molten steel in the porous refractory filter 23 (rectifying refractory) can be suppressed, and casting can be performed stably for a long time.

Further, since the Ca vapor supply unit 41 is not immersed in the molten steel 3, there is no possibility that the metal is attached to the Ca vapor supply unit 41 and clogs during operation, and the operation can be performed stably for a long time. It becomes.
Furthermore, since the exhaust part 42 which discharges the atmospheric gas of the closed space 40 is arrange | positioned at the outer nozzle 21, the pressure fluctuation in the closed space 40 can be suppressed and a molten metal surface fluctuation | variation can be suppressed. Thereby, operation | movement of the flow volume adjustment mechanism which controls a hot_water | molten_metal surface level can be stabilized, and casting can be implemented stably.

Furthermore, in this embodiment, since Ca vapor is supplied to the closed space 40 by the carrier gas made of an inert gas, the Ca vapor can be reliably supplied to the closed space 40. Moreover, it becomes possible to adjust the Ca vapor partial pressure in the closed space 40 relatively easily by adjusting the mixing ratio of the carrier gas and the Ca vapor.
Specifically, since the heating temperature and carrier gas flow rate when heating solid Ca to generate Ca vapor are adjusted, the Ca vapor partial pressure in the closed space 40 can be accurately controlled. .

Moreover, in this embodiment, since the Ca vapor | steam supply part 41 is arrange | positioned in the upper area | region 21b of the outer nozzle 21, and it is comprised so that Ca vapor | steam may be sprayed with respect to the molten steel 3 stored in the outer nozzle 21. Even in an unsteady part where the Ca vapor partial pressure in the closed space 40 is not sufficiently high in the early stage of casting, the molten steel 3 and the Ca vapor can be reacted, and the Ca content in the molten steel 3 can be quickly increased. It becomes possible to raise, and Al ₂ O ₃ present in the molten steel 3 can be appropriately modified.

As mentioned above, although the manufacturing method of the thin cast slab 1 which is this embodiment which is embodiment of this invention was demonstrated concretely, this invention is not limited to this and does not deviate from the technical idea of the invention. The range can be changed as appropriate.
For example, in this embodiment, as shown in FIG. 1, a twin-drum type continuous casting apparatus in which a bender roll and a pinch roll are arranged has been described as an example. However, there is no limitation on the arrangement of these rolls and the like. The design may be changed.

In the present embodiment, as illustrated in FIG. 2, the Ca vapor supply unit 41 and the exhaust unit 42 are described as being disposed in the outer nozzle 21, but the present invention is not limited thereto.
For example, as shown in FIG. 3, the Ca vapor supply section 41 may be disposed in the outer nozzle 21, and the exhaust section 42 may be disposed in the inner nozzle 30. As shown in FIG. 4, the Ca vapor supply unit 41 may be disposed in the inner nozzle 30 and the exhaust unit 42 may be disposed in the outer nozzle 21. Furthermore, as shown in FIG. 5, a Ca vapor supply unit 41 and an exhaust unit 42 may be disposed in the inner nozzle 30. Since the inside of the inner nozzle 30 is not filled with the molten steel 3 and is formed with a gap, even if the Ca vapor supply part 41 and the exhaust part 42 are disposed in the inner nozzle 30, the closed space 40 has a Ca. It is possible to supply steam or discharge the atmospheric gas in the closed space 40 to the outside.

Moreover, although it demonstrated as what used a porous refractory filter as a refractory for rectification | straightening, it is not limited to this, What is necessary is just a structure which can create a pressure loss and can expand and store molten steel once.
For example, as described in Patent Document 1, a porous refractory filter having a pressure loss of 0.1 kg / mm ² or more may be used.
Moreover, as described in Patent Document 2, a rectifying refractory having a structure having a large number of through-holes having a through-hole diameter of 2 to 30 mm and a hole area ratio of 10 to 30%. There may be.
Furthermore, as described in Patent Document 3, a refractory filter having a hole diameter of 6 to 20 pores per inch having a molten metal passage thickness of 30 mm or more or a through hole having a hole diameter of 3 to 10 mm and a hole area ratio of 10 to 25% is used. It may be a refractory material.
Moreover, as described in Patent Document 4, a rectifying refractory having a structure having a large number of through holes, in which through holes having different hole diameters from the end portion and the central portion, may be provided.

In the following, the results of experiments conducted to confirm the effects of the present invention will be described.
After melting various molten steels having the compositions shown in Tables 1 and 2 in a 60 t melting furnace and adjusting the components, a thin cast slab was produced under the following conditions using the twin drum type continuous casting apparatus shown in FIG. .
Tundish capacity: 5t
Cooling drum diameter: 1200mm
Casting width: 800mm
Casting atmosphere: Ar
Casting thickness: 2.5mm
Casting amount per unit time: 1.1t / min

  Ca was heated to generate Ca vapor, mixed with the carrier gas shown in Tables 3 and 4, and blown into the closed space between the outer nozzle and the inner nozzle. The Ca heating temperature was adjusted, and the carrier gas flow rate was changed in the range of 1 to 20 NL / min (atmospheric pressure, flow rate at room temperature) to adjust the Ca vapor partial pressure in the closed space. Ca vapor | steam and carrier gas were blown in toward the molten steel surface from the supply hole provided in the upper part of the outer nozzle. In addition, an exhaust pipe having a diameter of 5 mm was connected to the upper part of the outer nozzle, the atmosphere gas in the closed space was discharged, and the inside of the closed space was maintained at atmospheric pressure.

Here, a measurement sample was taken from the molten steel in the tundish, and the total oxygen content [T. O] (mass%) was measured. In the present invention example, the Ca vapor partial pressure was controlled so as to satisfy the following formula.
P _Ca ≧ 17.077 × [T. O] −0.006

(Ca vapor partial pressure)
Atmospheric gas in the closed space is collected at the time of casting or under the same gas temperature conditions as at the time of casting (at the time of casting preparation or before and after casting). The same volume of the atmospheric gas containing Ca vapor and the carrier gas only under the condition not containing Ca vapor is sampled and cooled to room temperature, and then the volume ratio of both is measured. Since the Ca vapor gasified at a high temperature is condensed into fine particles at room temperature, the difference in volume between the two can be regarded as a difference in Ca vapor volume. Since the volume of the gas decreases with a decrease in temperature, the volume at room temperature is different from the volume at high temperature, but the Ca vapor partial pressure can be considered to be approximately proportional to the volume fraction in the atmospheric gas. If the volume ratio of the atmospheric gas containing Ca vapor and the carrier gas not containing Ca vapor is known, the Ca vapor partial pressure in the closed space can be obtained.

(Adhesion of inclusions in rectifying refractories in the nozzle after casting)
The rectification refractory after casting is observed from the upper surface to measure the adhesion area ratio of inclusions. When there is no adhesion, “0”, when the adhesion area ratio is ¼ or less, “1”, 2 / 4 or less was evaluated as “2”, 3/4 or less as “3”, and 3/4 or more as “4”.

(Evaluation of casting conditions)
The cast condition of molten steel 60ton is (1) “complete casting (no clogging tendency)” when the entire amount is completely cast without nozzle clogging, and (2) “complete casting (when the total amount is completely cast” (3) When the casting was interrupted due to the occurrence of nozzle clogging, the evaluation was divided into three categories: “interruption”.

  The presence or absence of an obstruction tendency was judged according to the following criteria. The flow rate of the molten steel is adjusted by a sliding nozzle (hereinafter referred to as “SN”) attached to the bottom of the tundish and supplied to the nozzle. The opening of the SN is monitored during casting, assuming that the opening when the SN is fully closed is 0, and the opening when the SN is fully open is 100. Immediately after the start of casting, the opening degree is large in order to raise the molten metal surface between the cooling drums to a predetermined level (initial unsteady part). Thereafter, the opening gradually decreases as the molten metal level rises, and when the molten metal level reaches a predetermined level, the opening of the SN becomes stable. When nozzle clogging does not occur (on average except for fluctuations of several seconds to several tens of seconds), casting proceeds while maintaining the opening degree. And, after pouring the molten steel in the ladle into the tundish at the end of casting, since the molten steel surface in the tundish is lowered and the molten steel static pressure is reduced, in order to supply a predetermined amount of molten steel, Inevitably, the opening of the SN increases (terminal unsteady part).

  Here, if nozzle clogging occurs during a period excluding the initial and final unsteady portions, the SN opening increases in order to keep the molten metal surface between the cooling drums at a predetermined level. If the nozzle is clogged, casting cannot be stopped because the molten steel supply cannot catch up even when the SN is fully opened. Thus, there is a correlation between the nozzle blockage and the SN opening. In this example, the SN opening (initial opening) after the initial unsteady portion was 45 to 55. Then, when the average SN opening increased by 5 or more from the initial opening before the terminal unsteady state was reached, it was determined that “there was a tendency to block”. When the closing tendency occurred near the end of casting, complete casting was possible, and this state was evaluated as “complete casting (with closing tendency)”. When the closing tendency appeared early and could not be completely cast, it was evaluated as “interrupted”. Of course, when there was no closing tendency, the casting was completed and evaluated as “complete casting (no closing tendency)”.

No. 1 to 24 are examples of the present invention that satisfy the conditions of the present invention, and by controlling the Ca vapor partial pressure to a condition that satisfies the above formula, the Al ₂ O _{3 in the} molten steel and the Al ₂ O ₃ coating on the surface of the molten steel Because it has been modified to CaO-Al ₂ O ₃ -based oxide and the inclusion area ratio of the rectifying refractory is suppressed to 2/4 or less, there has been no tendency to block and the entire molten steel has been completely cast. .

On the other hand, no. Reference numerals 31 to 43 are comparative examples outside the conditions of the present invention.
No. 31 and 43, the Ca vapor and carrier gas, for blown using a lance immersed in molten steel tundish, the addition yield of Ca is low, _Al 2 _{O 3} of _Al 2 _{O 3} and the molten steel surface in the molten steel Since the coating was not sufficiently modified to CaO—Al ₂ O ₃ -based oxide, the inclusion adhesion area ratio of the rectifying refractory exceeded 3/4, and the entire molten steel could be cast due to blockage. Without interruption.
No. 32 to 42, since Ca vapor partial pressure does not satisfy the above formula, the content in the molten steel Ca is an _Al 2 _{O 3} coating of _Al 2 _{O 3} and the molten steel surface in the molten steel _CaO-Al 2 _{O 3} based oxide It was insufficient to modify the product. For this reason, the inclusion adhesion area ratio of the rectifying refractory exceeded 2/4, and even when the entire molten steel could be completely cast, a blocking tendency appeared and the operation was unstable. Alternatively, casting was interrupted because the entire molten steel could not be cast due to blockage.

Here, the total amount of oxygen in the molten steel [T. FIG. 6 shows the results of plotting the occurrence of deposits with O] (mass%) as the X axis and Ca vapor partial pressure (MPa) as the Y axis. In addition, No. 1 in which Ca vapor was blown into the tundish. Data of 31 and 43 were excluded. The dotted line in FIG. 6 shows the following formula (1).
P _Ca = 17.077 × [T. O] −0.006 (1)
In the region above the dotted line in Equation (1), it is confirmed that the generation of deposits is suppressed. On the other hand, it is confirmed that many deposits are generated in the region below the dotted line in the formula (1).

Here, the total amount of oxygen in the molten steel [T. FIG. 7 shows the results of plotting the casting state with O] (mass%) as the X axis and Ca vapor partial pressure (MPa) as the Y axis. In addition, No. 1 in which Ca vapor was blown into the tundish. Data of 31 and 43 were excluded. The dotted line in FIG. 7 shows the above equation (1).
In the region above the dotted line in the formula (1), there was no tendency to block and complete casting was possible. On the other hand, in the region below the dotted line of the formula (1), there was a tendency to block even after complete casting, and further, the entire amount could not be cast and the casting was interrupted.

  From the above results, the T.O. It was confirmed that by adjusting the Ca vapor partial pressure in the closed space in accordance with the amount of O, nozzle clogging is suppressed and casting can be performed stably for a long time.

1 Thin-walled slab 3 Molten steel 20 Immersion nozzle 21 Outer nozzle 23 Porous refractory filter (refractory for rectification)
30 Inner nozzle 40 Closed space

Claims (2)

The molten steel is supplied to a molten steel pool formed by a pair of rotating cooling drums and a pair of side weirs, and a solidified shell is formed and grown on the peripheral surface of the cooling drum. A method for producing a thin-walled slab in which a thin-walled slab is produced using molten steel having an Al amount of 0.005 mass% or more,
The immersion nozzle that supplies the molten steel to the molten steel pool portion has a bottom shape along the longitudinal direction of the cooling drum, and includes an outer nozzle having a rectifying refractory inside, and supplies the molten steel into the outer nozzle. An inner nozzle, and the molten steel supplied from the inner nozzle is configured to be stored in the outer nozzle,
Ca vapor is supplied to the closed space on the molten steel between the inner nozzle and the outer nozzle, and the atmospheric gas in the closed space is exhausted to obtain a partial pressure of Ca vapor P _Ca (MPa) in the closed space. Of total oxygen in molten steel [T. O] (mass%)
P _Ca ≧ 17.077 × [T. O] −0.006
A method for manufacturing a thin-walled cast slab characterized by controlling so as to satisfy the following relationship.   The method for producing a thin-walled slab according to claim 1, wherein the Ca vapor is supplied to the closed space by a carrier gas made of an inert gas.

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