JP2020171955A - Continuous steel casting method - Google Patents

Abstract

[Problem] To sufficiently remove non-metallic inclusions without causing troubles such as damage to a long nozzle and leakage of molten steel, and without deteriorating the yield of molten steel, and having a high degree of cleanliness. To provide a continuous casting method for steel capable of producing A long nozzle (20) has a bottom opening hole (24) opening to a bottom part (23) and a side opening hole (25) opening to a side surface. When the opening area of the opening holes 24 is SB (m2), the total opening area of the side opening holes 25 is ST (m2), and the flow velocity in the long nozzle 20 at steady state is v (m/s), (1) Formula: 0.14 ≤ SB / S, Formula (2): 0 < SB / (SB + ST) ≤ 0.55, Formula (3): v2 × (S - SB) ≥ 0.0035, and steady Molten steel is poured from the ladle toward the tundish through the long nozzle 20 so that the molten steel flow rate from the bottom opening 24 during casting does not exceed 3 t/min. [Selection drawing] Fig. 2

Description

The present invention relates to a method for continuously casting steel by injecting molten steel from a ladle toward a tundish, supplying the molten steel from the tundish to a mold for continuous casting, and continuously casting slabs.

For example, ultra-low carbon steel sheets for outer panels for automobiles, low-carbon aluminum killed steel sheets for low-carbon tin steel sheets for cans, hot-rolled steel sheets for steel pipes, high-tensile steel used for structural members for automobiles, etc. A good surface shape is required.
In order to realize these high workability and good surface shape, non-metal inclusions in steel and surface and internal defects containing them are major issues. Therefore, when high workability and good surface shape are required, it is necessary to remove non-metal inclusions from the molten steel to improve the cleanliness.

Conventionally, non-metal inclusions have floated by providing a weir in the tundish to form an ascending flow of molten steel or by using an electromagnetic brake to brake the discharge flow from the immersion nozzle in a continuously cast mold. Although it was removed, it was not sufficiently removed, and non-metal inclusions were brought to the final product, thin steel sheet, and harmful alumina clusters and non-metal inclusions remained in a certain amount.

Therefore, in order to further remove the non-metal inclusions, for example, as shown in Patent Documents 1 to 4, a technique for promoting the floating separation of the non-metal inclusions in the molten steel in the tundish has been proposed.
In Patent Document 1, in order to prevent defects caused by slag and non-metal inclusions, the slag is levitated by forming a recess in the injection flow drop region of the tundish and stirring the slag.

In Patent Document 2, a long nozzle for injecting molten steel into a tundish has a structure having a bottom and a lateral hole opened in the lateral direction is formed, and the molten steel is discharged from the lateral hole to generate an ascending flow. We are proposing a molten steel injection method that effectively floats non-metallic inclusions in the tundish.
Similar to Patent Document 2, Patent Document 3 proposes a long nozzle in which the lower end of the long nozzle is closed and has an upward discharge port on the side surface, the residence time of molten steel in the tundish is lengthened, and a non-metal is used. The floating separation of inclusions is to be performed effectively.
Patent Document 4 has the same concept as Patent Documents 2 and 3, and by proposing a long nozzle having a nozzle having a bottom and one ejection opening formed on the peripheral surface thereof, a tundish. Sufficient time is secured for inclusions in the molten steel to surface inside.

Japanese Unexamined Patent Publication No. 04-344854 JP-A-59-183960 Japanese Unexamined Patent Publication No. 05-200507 Japanese Unexamined Patent Publication No. 07-155911

By the way, as in Patent Document 1, when a recess is formed in the tundish, the amount of molten steel remaining in the tundish increases after the continuous casting is completed, which leads to a decrease in the molten steel yield. As a process, it is not cost effective.
Further, as shown in Patent Documents 2 to 4, when the lower end of the long nozzle is closed, when the injection of molten steel is started from the ladle, a large impact is caused by a large molten steel drop having a large static pressure and a large momentum. There was a risk that force would be applied, the long nozzle would be damaged, and the damaged long nozzle debris would fall into the tundish. Further, the above-mentioned impact force becomes larger than the gripping force for gripping the long nozzle, and a gap is created between the long nozzle and the lower part of the ladle, and there is a possibility that molten steel leaks from the gap. In this way, due to operational problems, it has not been put into practical use.

The present invention has been made in view of the above-mentioned situation, and removes non-metal inclusions without causing troubles such as damage to a long nozzle and leakage of molten steel, and without deteriorating the yield of molten steel. It is an object of the present invention to provide a continuous casting method of steel which can be sufficiently performed and can produce steel slabs having high cleanliness.

As a result of diligent studies by the present inventors in order to solve the above problems, the following findings were obtained.
The present inventors considered that it is important to utilize a long nozzle in order to solve the above problems. That is, a method of utilizing the function of keeping the molten steel in a turbulent flow and stirring state in the long nozzle and promoting the aggregation of fine non-metal inclusions without causing troubles such as damage and leakage of the molten steel. To find out. If the non-metal inclusions are aggregated in the long nozzle to facilitate floating, it is not necessary to form a recess in the tundish, so that the non-metal inclusions can be removed without deteriorating the molten steel yield.
Therefore, by changing the discharge port of the molten steel below the long nozzle and using the turbulent flow and stirring energy in the long nozzle without causing troubles such as damage to the long nozzle and leakage of molten steel, non-metal inclusions Attempts have been made to elucidate the shape of long nozzles, which can promote agglomeration of non-metal inclusions and efficiently levitate and separate to reduce the number density.

When casting low-carbon aluminum killed steel in a continuous casting apparatus, the present inventor changes the flow velocity v (m / s) of molten steel in a long nozzle and the discharge port of molten steel below the long nozzle to create a ladle. We sampled molten steel metal and sampled slabs from tundish and slabs, and analyzed their components, investigated and analyzed non-metal inclusions in detail. The investigation of non-metal inclusions was carried out by the slime extraction method. An aqueous solution of ferrous chloride in a net that collects non-metallic inclusions with a size of 30 μm or more by cutting out a sample from the position of the accumulation zone of inclusions at a depth of about 60 mm from the surface layer of the slab. About 500 g of the non-metal inclusions were electrolyzed and collected as slime. After elutriation (separation and collection of only non-metal inclusions while washing with water) and classification of the collected slime, the particle size distribution and maximum particle size of the non-metal inclusions were determined.

As a result of diligent research, we decided to stop the kinetic energy of the molten steel of the ladle falling in the long nozzle by providing a bottom part with an opening hole at the discharge port of the molten steel below the long nozzle installed at the bottom of the ladle. The sudden change in kinetic energy makes the molten steel inside the long nozzle turbulent and agitated, which can promote the aggregation of non-metal inclusions and the collision force of the molten steel due to the opening hole provided in the bottom surface. It was found that it can be reduced and damage to the long nozzle and leakage of molten steel can be prevented.
Further, if the opening area of the bottom opening hole is made too large, the turbulent flow state of the molten steel in the long nozzle becomes insufficient, and if the opening area of the bottom opening hole is made too small, the molten steel collides with the falling kinetic energy. It was found that it is important to optimize the opening area of the bottom opening hole in order to achieve both, because the long nozzle may be damaged by force or molten steel leakage may occur, causing operational troubles. did.

Further, in order to suppress the phenomenon that the non-metal inclusions directly flow into the mold due to the flow along the bottom in the tundish, a side opening hole is provided on the side surface of the long nozzle, and the non-metal inclusions flow out directly from the bottom opening hole. By limiting the amount of molten steel, it was found that the amount and size of non-metal inclusions brought into the final product, the thin steel sheet, can be cast so as not to exceed the values required for quality.
In this way, non-metal inclusions can be sufficiently removed without causing troubles such as breakage of the long nozzle and leakage of molten steel, and high quality steel slabs can be cast. Continuous steel using the long nozzle. I found a casting method.

The present invention has been made based on the above findings, and in the method for continuous casting of steel of the present invention, molten steel is injected from a ladle into a tundish, and the molten steel is cast from the tundish into a mold for continuous casting. It is a continuous casting method of steel that is supplied and continuously casts slabs. A long nozzle with a bottomed cylinder is arranged at the lower end of the ladle, and the long nozzle is used to the ladle. The molten steel is injected into the tundish, and the long nozzle has a bottom opening hole that opens to the bottom surface and a side opening hole that opens to the side surface, and the long nozzle has the long nozzle. The total area of the bottom surface portion is S (m ² ), the area of the bottom surface opening hole is SB (m ² ), the total opening area of the side surface opening hole is ST (m ² ), and the inside of the long nozzle in a steady state. When the flow velocity is v (m / s), the following equations (1) to (3) are satisfied and the flow velocity is satisfied.
Equation (1): 0.14 ≤ SB / S
Equation (2): 0 <SB / (SB + ST) ≤ 0.55
Equation (3): v ² × (S-SB) ≧ 0.0035
It is characterized in that the molten steel is injected from the ladle toward the tundish so that the amount of molten steel flowing out from the bottom opening hole during steady casting does not exceed 3 t / min.

According to the continuous casting method of steel having this configuration, when the injection of molten steel from the ladle into the tundish is started, the long nozzle is damaged and the molten steel is damaged even if the molten steel with a large static pressure and a large momentum falls. The occurrence of troubles such as leakage can be suppressed, and stable casting can be performed. Further, by having the bottom surface portion, the molten steel in the long nozzle becomes a turbulent flow state, non-metal inclusions can be efficiently aggregated, and convection due to a temperature difference is promoted by the discharge flow from the side opening hole. Floating separation of non-metal inclusions can be promoted, and steel slabs having sufficiently high cleanliness can be produced.

Further, in the continuous steel casting method of the present invention, it is preferable that the molten steel surface of the molten steel in the tundish is covered with one or both of the flux and the inert gas.
In this case, by covering the molten steel surface of the molten steel in the tundish with a flux and an inert gas, the oxidation of the molten steel in the tundish can be suppressed, and the formation of non-metal inclusions can be suppressed. .. Therefore, it is possible to manufacture a steel slab having a sufficiently high cleanliness.

As described above, according to the present invention, it is possible to sufficiently remove non-metal inclusions without causing troubles such as damage to the long nozzle and leakage of molten steel, and without deteriorating the yield of molten steel. It is possible to provide a continuous casting method of steel capable of producing steel slabs having high cleanliness.

It is explanatory drawing which shows an example of the continuous casting apparatus which carries out the continuous casting method of steel which is an embodiment of this invention. It is the schematic explanatory drawing of the long nozzle used in the continuous steel casting method in this embodiment. (A) is a cross-sectional explanatory view along the axis, and (b) is an XX cross-sectional view. It is explanatory drawing explaining the momentum of molten steel at the time of pouring molten steel from a ladle. It is a graph which shows the relationship between the ratio SB / S of the total area S of the bottom surface part of a long nozzle, and the area SB of a bottom opening hole, and the number of molten steel leakage and damage of a long nozzle. It is a graph which shows the relationship between the stirring energy index (v ² × (S-SB)), and the density of the number of non-metal inclusions having a particle size of 30 to 100 μm. It is a graph which shows the relationship between the relational expression SB / (SB + ST) of the area SB of a bottom opening hole and the total opening area ST of a side opening hole, and the maximum diameter of a non-metal inclusion. It is a graph which shows the relationship between the amount of molten steel outflow from a bottom opening hole and the density of the number of non-metal inclusions having a particle size of more than 100 μm.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

In the continuous casting apparatus 10 shown in FIG. 1, molten steel is transferred from a converter by a ladle 11, molten steel 2 is injected into a tundish 13 via a long nozzle 20, and large inclusions are levitated and separated in the tundish 13. After that, molten steel is supplied to the mold 15 via the immersion nozzle 14, a solidified shell is formed in the mold 15, and the obtained slab 1 is continuously cast. Then, the slab 1 produced from the mold 15 is pulled out by a support roll group 17 composed of a plurality of support rolls 18, and further cooled by a secondary cooling means (not shown) to grow a solidified shell and complete. Coagulate.

The continuous steel casting method according to the embodiment of the present invention is characterized by the long nozzle 20 used when injecting molten steel from the ladle 11 into the tundish 13. FIG. 2 shows a long nozzle 20 used in the continuous steel casting method according to the embodiment of the present invention.

As shown in FIG. 2, the long nozzle 20 in the present embodiment has a bottomed tubular shape, specifically, a substantially cylindrical shape, and a flange portion 21 for attaching to a ladle is provided at the upper end thereof. It is formed.
Further, the long nozzle 20 has a molten steel path 22 extending along the axis O, and a bottom surface portion 23 is formed at the lower end of the molten steel path 22. The long nozzle 20 is provided with a bottom opening hole 24 that opens in the bottom surface portion 23 and a side opening hole 25 that opens in the side surface of the molten steel path 22. A plurality of side opening holes 25 are provided at intervals in the circumferential direction, and in the present embodiment, as shown in FIG. 2B, four side opening holes 25 are arranged at equal intervals in the circumferential direction. It is installed.

Here, in the long nozzle 20, the inner area of the bottom surface portion 23 is S (m ² ), the area of the bottom surface opening hole 24 is SB (m ² ), and the total opening area of the side surface opening hole 25 is ST (m ² ). When the flow velocity in the long nozzle at regular times is v (m / s), it is configured to satisfy the following equations (1) to (3). The left side of Eq. (3) is defined as the stirring parameter index.
Equation (1): 0.14 ≤ SB / S
Equation (2): 0 <SB / (SB + ST) ≤ 0.55
Equation (3): v ² × (S-SB) ≧ 0.0035

Further, in the present embodiment, the amount of molten steel injected from the ladle 11 into the tundish 13 is controlled so that the amount of molten steel flowing out from the bottom opening hole 24 during steady casting does not exceed 3 t / min. ing.
The amount of molten steel flowing out from the bottom opening hole 24 is calculated by multiplying the total flow rate of molten steel by the ratio of the area of the bottom opening hole 24 to the area of all the opening holes (SB / (SB + ST)).

Hereinafter, the reasons for defining the above-mentioned equations (1) to (3) and the amount of molten steel flowing out from the bottom opening hole 24 during steady casting will be described in the present embodiment.

In the present embodiment, by providing the bottom surface portion 23 at the lower end of the molten steel flow path 22 of the long nozzle 20, the inside of the long nozzle 20 (inside the molten steel path 22) is turbulently disturbed, and non-metal inclusions are formed. It is agglomerated to promote floating separation of non-metal inclusions.
Further, by providing the bottom opening hole 24 in the bottom surface portion 23, the collision force due to the fall of the molten steel at the time of opening the ladle at the start of casting is reduced, so that the long nozzle 20 is damaged or the long nozzle 20 is misaligned. We are trying to suppress major operational troubles such as molten steel leaks.
Therefore, the experiment was repeated to examine the conditions of the bottom opening hole necessary for avoiding the above-mentioned trouble.

FIG. 3 shows a schematic view for explaining the impact force on the bottom surface of the long nozzle 20 which is installed in the lower part of the ladle 11 and is injected into the tundish 13.
When starting molten steel injection from the ladle 11 into the tundish 13, the slinging nozzle at the bottom of the ladle 11 is opened. Molten steel is free falling inside the long nozzle 20 (molten steel path 22) at an initial velocity v ₀ which is obtained from the molten steel static pressure P by molten steel height H (m) of the ladle 11. The molten steel that has freely fallen in the long nozzle 20 (molten steel path 22) receives the gravitational acceleration g, accelerates, and collides with the bottom surface of the long nozzle 20.

Assuming that the initial velocity of molten steel outflow is v ₀ and the initial flow rate of molten steel outflow is Q ₀ (kg / s),
Q ₀ (kg / s) = long nozzle cross-sectional area x v ₀ x molten steel density If the outlet molten steel outflow velocity v at the lower end of the long nozzle 20 is assumed.
v = v ₀ + g × t, x = v ₀ × t + 0.5 × g × t ²
By setting the length L of the long nozzle and substituting x = L, the fall time t (s) from the upper end to the lower end of the long nozzle 20 can be obtained, and v (m / s) can be obtained. Further, the molten steel outflow flow rate Q (kg / s) at the lower end of the long nozzle 20 can be obtained.

The impact force F of the molten steel flow at the lower end of the long nozzle 20 is calculated by the following formula.
Impact force F (N) = molten steel outflow flow rate Q (kg / s) x molten steel outflow velocity v (m / s)
Since the molten steel height H (m) of the ladle 11 differs depending on the steelmaking factory, the initial velocity v ₀ changes and the collision force F with the bottom surface of the long nozzle 20 also changes.

In the present embodiment, the total area S (m ² ) of the bottom surface portion and the area SB (m ² ) of the bottom surface opening hole are changed in the steelmaking factories in which the amount of molten steel in the ladle 11 differs from about 100 to 400 tons. An actual machine experiment was conducted using a nozzle. At that time, the damage state of the long nozzle 20 and the leakage state of molten steel were confirmed, and FIG. 4 was obtained experimentally. The horizontal axis shows the SB / S of the long nozzle 20 that was used in the experiments at each steelmaking factory, and the number of molten steel leakage troubles at that time and the number of damaged long nozzles 20 are shown on the vertical axis.
From this result, SB / S ≧ 0.14 was obtained under the condition that the molten steel leakage trouble did not occur and the long nozzle 20 was not damaged.

Next, the requirements necessary to sufficiently obtain the agglomeration effect of the non-metal inclusions within the range of SB / S ≧ 0.14 that does not cause molten steel leakage and damage trouble of the long nozzle 20 were examined.
Here, in the present embodiment, "stirring energy index v ² × (S-SB)" is devised. The definition is that the molten steel flow that was shielded at the bottom portion 23 with an opening and was flowing downward is changed to the flow in the opposite direction (upward), and the lost kinetic energy is used as an index. It was considered that the lost amount of the downward kinetic energy acts as stirring energy on the molten steel inside the long nozzle 20. That is, it is considered that a shear force acts on the molten steel flow that is about to flow out from the bottom surface portion 23, and the magnitude of the cohesive energy of the non-metal inclusions is indexed according to the magnitude of the shear force.

The results organized by this index are shown in FIG. It is the relationship between the stirring energy index in the long nozzle 20 and the number density of non-metal inclusions of 100 μm or less in which the aggregation effect works.
As shown in FIG. 5, it was confirmed that the aggregating effect of the non-metal inclusions could be sufficiently obtained by setting the stirring energy index v ² × (S-SB) ≧ 0.0035.

In the present embodiment, as described above, the bottom opening hole 24 is provided in the bottom surface portion 23 in order to obtain the aggregation effect of non-metal inclusions without causing troubles such as damage to the long nozzle 20 and leakage of molten steel. Invented. The presence of the bottom opening hole 24 avoids damage to the long nozzle 20, but as the opening area SB of the bottom opening hole 20 increases, it crawls directly on the bottom of the tundish 13 like a conventional straight type nozzle. The problem of the inflow of relatively large non-metallic inclusions remained due to the molten steel flow flowing into the mold.
This has been solved in the present embodiment by appropriately distributing the molten steel flow through the bottom opening hole 24 provided in the bottom surface portion 23 of the long nozzle 20 and the side opening hole 25 provided in the side surface of the long nozzle 20. .. At this time, the ratio of the opening area SB of the bottom opening hole 24 to the total opening area ST of the side opening hole 25 was determined by the following experiment.

It is considered that when the molten steel flow flowing out from the side opening hole 25 becomes large, an upward flow is generated in the tundish 13, and relatively large non-metal inclusions are easily floated and removed.
Therefore, SB / (SB + ST) is defined as an index showing the distribution ratio of the molten steel flow flowing out from the bottom opening hole 24 and the side opening hole 25, and a long nozzle 20 in which SB / (SB + ST) is changed is used. Continuous casting was performed, and the maximum diameter of non-metal inclusions in the slab 1 was investigated. FIG. 6 shows the relationship of the maximum diameter of non-metal inclusions with respect to SB / (SB + ST).
It was found that by controlling SB / (SB + ST) to 0.55 or less, the non-metal inclusions involved in the slab 1 can be reduced to about 100 μm.

On the other hand, as the flow rate of molten steel flowing out from the bottom opening hole 24 increases, the amount of non-metal inclusions that directly flow into the mold 15 increases. Therefore, the relationship between the flow rate of molten steel flowing out from the bottom opening hole 24 and the number density of non-metal inclusions exceeding 100 μm in the slab 1, which is particularly problematic in products, was investigated. FIG. 7 shows the relationship between the flow rate of molten steel flowing out from the bottom opening hole 24 and the number density of non-metal inclusions exceeding 100 μm. Here, the molten steel outflow flow rate from the bottom opening hole 24 is obtained by multiplying the total molten steel outflow flow rate by the ratio of the area of the bottom opening hole 24 to the area of all the opening holes (SB / (SB + ST)).
It was found that the number density of non-metal inclusions exceeding 100 μm can be reduced by controlling the flow rate of molten steel outflow from the bottom opening hole 24 so as not to exceed 3 t / min.

From the above experimental results, in the present embodiment, the above-mentioned equations (1) to (3) and the amount of molten steel flowing out from the bottom opening hole 24 at the time of steady casting are defined.

Further, in the present embodiment, in order to prevent reoxidation after the molten steel flows out from the long nozzle 20 into the tundish 13, the surface of the molten steel of the tundish 13 is shielded from the atmospheric atmosphere in the tundish 13 to reduce the oxygen concentration. It is preferable to reduce it.
Therefore, it is preferable to cover the molten steel surface in the tundish 13 with one or both of the flux and the inert gas. For high-grade steel, it is desirable to apply both an inert gas seal and a flux seal. For low-grade steel, it is desirable that it can be reliably sealed by either an inert gas seal or a flux seal.

It is desirable to use a flux having a low melting point composition based on the CaO-Al ₂ O _3- SiO ₂ system, which can sufficiently melt at the molten steel temperature and cover the surface of the molten steel.
In this way, in the melting of high-clean steel in which non-metal inclusions are agglutinated and coalesced to strengthen levitation, the non-metal inclusions should be removed while controlling the oxygen concentration on the surface of the molten steel to suppress reoxidation. Therefore, the effect of reducing large alumina clusters can be obtained. The oxygen concentration is preferably at an oxygen partial pressure _P O2 ⁼ 10 -14 (atm) or less.

According to the steel continuous casting method of the present embodiment having the above-described configuration, the long nozzle 20 having a bottom opening hole 24 opening in the bottom surface portion 23 and a side opening hole 25 opening in the side surface is provided. In addition to satisfying the above equations (1) to (3), the ladle 11 is directed toward the tundish 13 so that the amount of molten steel outflow from the bottom opening hole 24 during steady casting does not exceed 3 t / min. Since the molten steel is injected into the tundish 13 from the ladle 11, the long nozzle 20 may drop the molten steel with a large static pressure and a large momentum when the molten steel is injected into the tundish 13. The occurrence of molten steel leakage due to breakage or misalignment can be suppressed, and stable casting can be performed.
Further, by having the bottom surface portion 23, the molten steel in the long nozzle 20 becomes a turbulent flow state, non-metal inclusions can be efficiently aggregated, and convection due to a temperature difference due to the discharge flow from the side opening hole 25. It is possible to promote the floating separation of non-metal inclusions, and it is possible to produce a slab 1 having a sufficiently high cleanliness.

Further, in the present embodiment, when the molten steel surface in the tundish 13 is covered with one or both of the flux and the inert gas, the oxidation of the molten steel in the tundish 13 can be suppressed and the non-metal. It is possible to suppress the formation of inclusions. Therefore, it becomes possible to manufacture a slab having a higher degree of cleanliness.

Although the continuous steel casting method according to the embodiment of the present invention has been specifically described above, the present invention is not limited to this and can be appropriately changed without departing from the technical idea of the invention. is there.

For example, in the present embodiment, it has been described that four side opening holes are provided, but the present invention is not limited to this, and the number of side opening holes is an even number such as 4, 6, 8, ... It is preferable to provide.
When supplying molten steel from the tundish to two strands, by arranging the side opening holes in half (2, 3, 4, ...) in each longitudinal direction of the tundish and in contrast on the left and right sides of both strands. It is possible to obtain a contrasting flow in both strands and avoid a flow directly toward the wall of the tundish or a direct flow directly into the mold.

Further, in the present embodiment, the discharge angle of the side opening hole is not mentioned, but in order to further suppress the flow that crawls on the bottom of the tundish, the discharge angle of the side opening hole is set to a horizontal plane (upward 0 °). ) Is preferable.

The verification test carried out for verifying the effect of the present invention will be described.
Low-carbon aluminum killed steel used for automobiles is degassed in the converter steelmaking and RH secondary refining process, and then taken at the molten steel flow rate shown in Table 1 using a long nozzle under the conditions shown in Table 1. Molten steel was injected into the tundish from the pan and continuous casting was performed. As the long nozzle, four side opening holes were used in the horizontal plane.

For nozzles that had problems such as nozzle breakage or molten steel leakage during casting, casting was stopped at the time of the trouble, and non-metal inclusions were not investigated. For the slabs that could be cast without trouble, the non-metal inclusions were investigated by the slime extraction method described above, and the particle size distribution and maximum particle size of the non-metal inclusions were determined. A slab sample was taken and the density of non-metal inclusions was investigated using the slime method. The test results are shown in Table 2.

No. Examples 1 to 14 are examples of the invention, in which casting can be performed without troubles of nozzle breakage and molten steel leakage, the number density of non-metal inclusions is low, and no coarse inclusions having a diameter of more than 200 μm are observed.
No. Reference examples 15 to 17 are comparative examples in which the SB / S is too small, and the nozzle breakage and molten steel leakage problems occur because the long nozzle bottom opening hole is too small.
No. 18 to 20 are comparative examples in which the stirring energy is too small, and there were non-metal inclusions having a diameter of 30 to 100 μm exceeding 500 pieces / kg.
No. 21 to 23 are comparative examples in which SB / (SB + ST) is too large, and non-metallic inclusions having a diameter of more than 200 μm were present.
No. Reference examples 24 to 26 are comparative examples in which the flow rate of molten steel flowing out from the bottom opening hole exceeds 3.0 t / min, and there are 10 or more large non-metal inclusions having a diameter of more than 100 μm.

1 slab 10 continuous casting device 11 ladle 13 tundish 15 mold 20 long nozzle 22 molten metal path 23 bottom surface 24 bottom opening hole 25 side opening hole

Claims (2)

It is a continuous casting method of steel in which molten steel is injected from a ladle into a tundish, the molten steel is supplied from the tundish to a mold for continuous casting, and slabs are continuously cast.
A bottomed tubular long nozzle is arranged at the lower end of the ladle, and the molten steel is injected from the ladle into the tundish through the long nozzle.
The long nozzle has a bottom opening hole that opens on the bottom surface and a side opening hole that opens on the side surface.
The total area of the bottom surface of the long nozzle is S (m ² ), the area of the bottom surface opening is SB (m ² ), the total opening area of the side surface opening is ST (m ² ), and the long in a steady state. When the flow velocity in the nozzle is v (m / s), the following equations (1) to (3) are satisfied, and the following equations (1) to (3) are satisfied.
Equation (1): 0.14 ≤ SB / S
Equation (2): 0 <SB / (SB + ST) ≤ 0.55
Equation (3): v ² × (S-SB) ≧ 0.0035
A method for continuous casting of steel, which comprises injecting the molten steel from the ladle toward the tundish so that the amount of molten steel flowing out from the bottom opening hole during steady casting does not exceed 3 t / min. The method for continuously casting steel according to claim 1, wherein the molten steel surface of the molten steel in the tundish is covered with one or both of a flux and an inert gas.

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