JP7677438B2 - Submerged nozzle, mold and method for continuous casting of steel - Google Patents

Description

The present invention relates to an immersion nozzle for injecting molten steel into a mold during continuous casting of steel, as well as a mold for a continuous casting machine using the immersion nozzle and a method for continuous casting of steel.

When continuously casting steel, an immersion nozzle is immersed in the molten steel in a mold and the molten steel is poured into it. The flow of molten steel in the mold is split into an upward flow that rises along the inner wall of the mold, and a downward flow that descends along the inner wall of the mold, as the flow discharged from the pair of outlets on the left and right of the immersion nozzle collides with the inner wall on the short side of the mold.

In such cases, especially when the discharge flow velocity is high, uneven flow velocity distribution may occur at the top and bottom of the discharge port. This may disrupt the left and right flow balance for the upward and downward flows, or cause strong discharge flows in certain areas, resulting in large flow fluctuations. Such fluctuations can lead to poor formation of a solidified shell or defects caused by bubbles and inclusions being trapped in the solidified shell.

To solve such problems, it is believed that slowing down the flow of molten steel in the mold and forming a uniform flow will enable continuous casting that prevents defects caused by bubbles and inclusions. In line with this idea, a four-hole submerged nozzle (four-hole nozzle) with two tiers of molten steel outlets in the upper and lower directions has been proposed, for example, in the following patent documents:

Patent Document 1 discloses a nozzle in which the area of the upper outlet is larger than the area of the lower outlet in order to reduce the maximum downward flow velocity as much as possible, and a continuous casting method using the nozzle.

International Publication No. 2010/109887

The technology described in Patent Document 1 succeeded in reducing the downward flow rate, but the nature of gravity tends to bias the flow of molten steel toward the upper and lower outlets, resulting in high pressure at the bottom of the nozzle. This causes stagnation at the bottom of the nozzle, and reactions with inclusions in the molten steel can cause inclusions to adhere to the inner tube of the submerged nozzle or cause melting damage to the inner tube. In addition, because the cross-sectional area of each outlet is relatively small compared to a two-hole nozzle, there is a problem in that adhesion and melting damage can disrupt the flow of molten steel and easily hinder operations.

The present invention has been made to solve the above problems, and aims to provide technology that reduces adhesion of inclusions in the molten steel to the nozzle and nozzle meltdown while appropriately controlling the flow of molten steel in the mold.

In order to solve the above problems, the inventors investigated the pressure distribution within the nozzle to optimize the opening area ratio of the upper and lower outlets of the multi-hole submerged nozzle and the flow rate of each outlet, and arrived at the present invention.

The submerged nozzle of the present invention, which is intended to solve the above problems, is an submerged nozzle that supplies molten steel from a molten steel storage container into the mold of a continuous casting machine that performs continuous casting of steel, and is characterized in that the end of the nozzle body on the side that is immersed in the molten steel in the mold is closed, and the upper and lower parts of the nozzle body that are immersed in the molten steel each have a pair of outlets that are symmetrical about the central axis, and the opening area of the outlet in the lower part is in the range of 1.0 to 1.6 times the opening area of the outlet in the upper part.

The submerged nozzle according to the present invention is
a. The ratio r/R of the inner diameter r from the upper end of the upper hole of the discharge port to the bottom of the lower end of the submerged nozzle to the inner diameter R to the upper end of the upper hole of the discharge port in the flow path inside the submerged nozzle is in the range of 0.6 or more and less than 1.0,
b. The discharge directions of the discharge ports of the upper and lower stages are arranged at an angle θ of 10° or less when viewed from above;
This may be a more preferable solution.

Further, the mold according to the present invention is a mold for a continuous casting machine having any one of the above-mentioned submerged entry nozzles, and is characterized in that an index K, which influences the molten metal level fluctuation and is expressed by the following formula (1), is in the range of 0.09 to 0.14:
The present invention is characterized in that it is configured
K ² =(L ² +W ² /4)/TP ² (1)
Here, L is the distance [m] from the meniscus to the upper end of the upper outlet hole of the submerged nozzle,
W is the short side distance of the mold at the meniscus position [m],
TP is the mass of molten steel passing per unit time [t/min].

A more preferable solution for the mold of the present invention may be to provide an electromagnetic stirring device having a DC coil and an AC coil, which is installed on the outside of the long side of the mold above the discharge outlet of the submerged nozzle and capable of applying a magnetic field in which an AC magnetic field is superimposed on a DC magnetic field, to the molten steel in the mold, and an electromagnetic brake device having a DC coil, which is installed on the outside of the long side of the mold below the discharge outlet of the submerged nozzle and capable of applying a DC magnetic field to the molten steel in the mold.

The method for continuous casting of steel according to the present invention is characterized in that, when continuously casting steel using any of the above-mentioned submerged nozzles, the molten metal level fluctuation index K shown in the above formula (1) is adjusted to be in the range of 0.09 to 0.14.

The method for continuous casting steel according to the present invention comprises the steps of:
c) applying a magnetic field in which a DC magnetic field having a magnetic flux density of 0.1 to 0.8 T and an AC magnetic field having a magnetic flux density of 0.03 to 0.1 T are superimposed on the molten steel in the mold above the discharge port of the submerged nozzle immersed in the molten steel in the mold, and applying a DC magnetic field having a magnetic flux density of 0.1 to 0.8 T to the molten steel in the mold below the discharge port;
d. Ar is introduced from the upper nozzle by adjusting the ratio Q _Ar /TP of the Ar gas flow rate Q _Ar [NL/min] to the molten steel passing mass TP [t/min] to be 2.0 or more and 5.0 or less;
This may be a more preferable solution.

According to the submerged nozzle and continuous casting machine of the present invention, by making the lower outlet area larger than the upper outlet area of the multi-hole submerged nozzle, it is possible to continuously cast steel without forming a high-pressure area due to flow stagnation at the bottom of the nozzle and without forming negative pressure near the outlet. This is expected to prevent adhesion and nozzle melting due to reaction between the nozzle refractory and inclusions, and to reduce the risk of operational disruption. The submerged nozzle of the present invention is suitable for use in a method for continuous casting of steel.

FIG. 1 is a vertical sectional view of a submerged nozzle according to one embodiment of the present invention. 13 is a cross-sectional view of a submerged nozzle according to another embodiment of the present invention, showing the positional relationship between upper and lower discharge ports. 1 is a graph showing the relationship between the maximum pressure inside the submerged nozzle and the ratio of the cross-sectional area of the upper stage outlet to the cross-sectional area of the lower stage outlet. 11 is a graph showing the relationship between the minimum pressure near the outlet and the ratio of the cross-sectional area of the upper outlet to the cross-sectional area of the lower outlet. 1 is a graph showing the relationship between the normalized maximum pressure inside the submerged nozzle and the ratio r/R of the inner diameter of the submerged nozzle. FIG. 13 is an explanatory diagram of the positional relationship of the outlet port of the submerged nozzle that influences the floating of Ar gas bubbles in a mold for a continuous casting machine according to another embodiment of the present invention.

Below, the embodiments of the present invention are specifically described. Note that the drawings are schematic and may differ from the actual ones. Furthermore, the following embodiments are intended to exemplify devices and methods for embodying the technical ideas of the present invention, and are not intended to specify the configurations as described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

[Submerged immersion nozzle]
Fig. 1 is a vertical cross-sectional view showing the tip shape of a multi-hole submerged entry nozzle according to one embodiment of the present invention. During continuous casting of steel, molten steel is poured into the molten steel in a mold by immersing such a submerged entry nozzle. In this embodiment, the nozzle has a total of four discharge ports, two pairs above and below, with a central axis 4 as the axis of symmetry, and is a so-called four-hole submerged entry nozzle.

In this embodiment, the cross-sectional area of the lower outlet 2 is set to be 1.0 to 1.6 times the cross-sectional area of the upper outlet 1. The reason is as follows.

In general, in multi-hole submerged entry nozzles with upper and lower discharge ports, attention is focused on how to obtain the effect of attenuating the discharge flow rate to reduce defects in the cast slab.

However, the inventors found that the flow of molten steel tends to be biased toward the lower stage due to the influence of gravity. As a result, they found that the pressure at the bottom 3 of the inner tube of the submerged entry nozzle increases, making it easier for stagnation to form, and that negative pressure is more likely to form near the discharge port. They found that both of these factors induce a reaction between inclusions in the molten steel and the refractory of the submerged entry nozzle, causing adhesion of the inclusions to the submerged entry nozzle and melting of the nozzle refractory, making stable operation difficult.

First, in an immersion nozzle having an upper stage outlet 1 and a lower stage outlet 2, the opening area of the lower stage outlet 2 is made larger than the opening area of the upper stage outlet 1 to straighten the flow between the upper and lower outlets and reduce the retention area (stagnation area) formed at the bottom 3 of the immersion nozzle. In the retention area, the balance of the outlet areas of the upper and lower stages and the size of the stagnation area change the inner diameter of the nozzle body around the outlet, which are factors that determine the flow of molten steel. In addition, these factors also affect the "continuity" of the molten steel flow field, making it difficult to predict the impact of each factor individually.

Therefore, in order to control the formation of stagnation areas caused by local high pressure or negative pressure areas by balancing the size of the upper and lower outlets, we used numerical calculations to evaluate the effect of the ratio of the outlet areas of the upper and lower stages on stagnation areas.

It was also considered that the stagnation area at the bottom of the nozzle could be controlled by forcibly directing part of the molten steel flow to the upper outlet by colliding it with the refractory material located between the upper and lower outlets, and the effect that the change in the inner diameter of the nozzle body around the outlet has on the stagnation area was investigated by performing numerical calculations.

As shown in FIG. 2, the positions of the upper and lower outlets 1 and 2 are preferably offset by an angle θ (°) in the circumferential direction of the nozzle, up to 10°. The lower outlet 2 is preferably arranged facing the short side of the mold so that the molten steel protruding direction is parallel to the long side of the mold, and the upper outlet 1 is preferably offset in the circumferential direction. This is because, even if alumina or the like adheres to the inside of the nozzle, it is expected that the flow from the upper outlet 1 will collide with the long side of the mold, thereby suppressing the effect of the molten steel flow directly colliding with the molten steel surface. This makes it possible to keep the impact on the molten steel surface level low. On the other hand, if the offset angle is too large, the molten steel flow may be deflected by colliding with the long side of the mold, and the molten steel surface fluctuation may increase due to the upward flow. The angle difference θ is more preferably more than 1° and less than 10°, and even more preferably more than 3° and less than 10°.

<Analysis 1>
First, the immersion nozzle was straight with an inner diameter R of 150 mm, and four-hole immersion nozzles No. 1 to 5 having upper and lower outlets with openings of the shapes shown in Table 1 were subjected to numerical calculation. The analysis was performed using the general-purpose thermal fluid analysis solution STAR-CCM+, and the steady-state total pressure distribution was calculated and evaluated under the conditions that the pressure near the outlet side of the outlet was set to 0 and the maximum flow velocity in the nozzle was 3.0 m/s. In Table 1, "vertical" indicates the vertical direction, and "horizontal" indicates the horizontal direction.

From the analysis results in Table 1, the relationship between the maximum pressure inside the submerged nozzle and the ratio SL/SU of the cross-sectional area of the upper stage outlet 1 to the cross-sectional area of the lower stage outlet 2 is shown in the graph of Figure 3. Here, the cross-sectional area of the upper stage outlet 1 is taken as SU, and the cross-sectional area of the lower stage outlet 2 is taken as SL. As shown in Figure 3, as SL/SU increases, that is, as the cross-sectional area of the lower stage outlet 2 increases relative to the cross-sectional area of the upper stage outlet 1, the maximum pressure tends to decrease, and it is thought that stagnation due to high pressure is eliminated. In particular, when SL/SU was 1.0 or more, a significant pressure reduction effect was obtained.

Figure 4 also shows a graph of the relationship between the minimum pressure near the discharge port and the ratio SL/SU of the cross-sectional area of the upper discharge port 1 to the cross-sectional area of the lower discharge port 2 from the analysis results in Table 1. As shown in Figure 4, as SL/SU increases, the minimum pressure near the discharge port decreases, and negative pressure occurs especially at values exceeding 1.6. Inclusions in the molten steel tend to collect in negative pressure areas, which, like the retention area, is thought to induce a reaction between the inclusions in the molten steel and the refractory of the submerged entry nozzle, causing adhesion of the inclusions to the submerged entry nozzle and melting of the nozzle refractory. Therefore, SL/SU is set to 1.6 or less.

<Analysis 2>
Next, for an immersion nozzle with SL/SU of 1.0, the relationship between the ratio r/R of the inner diameter R up to the top end of the upper hole of the discharge port and the inner diameter r from the top end of the upper hole of the discharge port to the bottom of the lower end of the immersion nozzle, and the maximum pressure in the flow path inside the nozzle was analyzed. The analysis results are shown in Table 2. Here, the maximum pressure inside the nozzle when r/R is 1.0 was normalized to 1.0. The relationship between the normalized maximum pressure and the inner diameter ratio r/R is graphed in Figure 5.

From the results in Table 2 and Figure 5, it can be seen that there is an optimal range for the inner diameter ratio r/R. The normalized maximum pressure is smallest when the inner diameter ratio is around 0.7, and the normalized maximum pressure increases both when the inner diameter ratio is smaller or larger than that. In particular, when r/R was 0.5, the normalized maximum pressure exceeded 1.0. This is thought to be because the proportion of the area where the molten steel flow hits the refractory between the upper and lower discharge ports increased, creating a new high-pressure area and a dangerous area for the formation of a stagnation area. Therefore, it is preferable that the inner diameter ratio r/R is 0.6 or more and less than 1.0. This makes it possible to keep the normalized maximum pressure below 1.0. Preferably, r/R is 0.9 or less.

Furthermore, when actually continuously casting steel, if an immersion nozzle is used, an inert gas such as Ar gas can be mixed into the molten steel through the upper nozzle before casting. This allows the molten steel to be subjected to the buoyancy of the gas bubbles, which can mitigate the formation of a high-pressure area at the bottom 3 of the immersion nozzle.

However, if the amount of inert gas mixed in is excessive, after it leaves the submerged nozzle, the flow will tend to rise to the meniscus in the mold where the steel is cast, causing large fluctuations in the molten metal surface, which will hinder operation. Therefore, there is a range of appropriate amounts of gas to be injected.

[template]
In the continuous casting method using the above-mentioned submerged nozzle, Ar gas or the like is blown into the nozzle to suppress nozzle clogging due to adhesion of alumina or the like to the nozzle. In particular, bubbles flowing out from the upper outlet 1 together with molten steel may cause fluctuations in the molten steel level by floating up. FIG. 6 is an enlarged conceptual cross-sectional view of a mold 20 for a continuous casting machine. In FIG. 6, the trajectory of the floating up of bubbles is shown by an arrow with the symbol Ar. The rising position of the bubbles is related to the diagonal length √(L ² +W ² /4) from the upper outlet 1 to the meniscus position on the short side of the mold and the mass TP of molten steel passing per unit time. According to the study by the inventors, it was found that the molten steel level fluctuations can be significantly suppressed by setting the index K, which is expressed by the following formula (1) and which affects the molten steel level fluctuations, in the range of 0.09 to 0.14. In order to satisfy the relationship of the following formula (1), it is preferable to control the change in the distance between the narrow sides 8 of the mold, the casting speed which affects the mass of passing molten steel, i.e., the strip withdrawal speed, and the immersion depth of the submerged entry nozzle 10. Since the distance between the narrow sides 8 of the mold is fixed at the required width of the strip, it is preferable to adjust the immersion depth of the submerged entry nozzle 10 or the casting speed.
K ² =(L ² +W ² /4)/TP ² (1)
Here, L is the distance [m] from the meniscus 5 to the upper end of the upper outlet hole of the submerged nozzle,
W is the distance between the short sides 8 of the mold at the meniscus 5 position [m],
TP is the mass of molten steel passing per unit time [t/min].

Example 1
The present invention has been configured as described above. The feasibility and effects of the present invention will be further described below with reference to examples.
When casting using this nozzle with a vertical bending continuous casting machine, the nozzle configuration and casting method shown in Table 3 were used. As an index of operational stability in Table 3, an eddy current sensor was installed directly above the molten metal surface at the center of thickness, which is 1/4 of the short side distance W (casting width) from the short side of the mold toward the center of the width. The eddy current sensor was used to measure the change in the molten metal surface level over time. At that time, the magnitude of the molten metal surface level fluctuation in each treatment was indexed, with the magnitude of the molten metal surface level fluctuation in Treatment No. A1 being set as 100. The average value of the first half and the second half of casting was used for evaluation as an index indicating operational stability. Note that both the upper and lower stage discharge ports were opened in a direction facing the short side, and the center of the discharge flow was parallel to the long side of the mold.

From the results in Table 3, all of the inventive examples obtained better results than the comparative examples. When comparing the same Ar gas amount ratio Q _Ar /TP blown from the upper nozzle, process Nos. A2 and A4, in which the inner diameter ratio r/R was set within an appropriate range, obtained better results than process Nos. A1 and A3, in which the inner diameter ratio r/R was 1.0. When comparing the same inner diameter ratio r/R, process Nos. A3 and A4, in which the Ar gas amount ratio Q _Ar /TP blown from the upper nozzle was set within an appropriate range, obtained better results than process Nos. A1 and A2, in which the inner diameter ratio r/R was set within an appropriate range. Among them, process No. A4 showed the lowest average molten metal level variation index and demonstrated high operational stability.

Example 2
Table 4 shows the molten metal level fluctuation index when processing was performed under the conditions of Process No. A1 in Example 1, using a submerged nozzle in which the upper discharge port was inclined at an angle θ in the circumferential direction of the nozzle with respect to the short side of the mold. Process Nos. C2 to C4, in which the angle θ was 3 to 10°, showed improvement in molten metal level fluctuation compared to Process No. A1. Process No. C5 showed a slight increase in molten metal level fluctuation. This is thought to be because the discharge port was inclined too far toward the long side, which increased the influence of the reverse flow that collides with the long side and reaches the molten metal surface.

Example 3
Table 5 shows the molten metal level fluctuation index when processing was performed under the conditions of Process No. A4 in Example 1, using a submerged entry nozzle in which the upper discharge port was inclined by 7° in the circumferential direction of the nozzle with respect to the short side of the mold. In Process No. D1, improvement in molten metal level fluctuation was observed compared to Process No. A4.

Example 4
Table 6 shows the molten metal level fluctuation index when the continuous casting machine of Example 1 was used, and the immersion nozzle with different opening area ratios SL/SU of the upper and lower stage outlets was used, and the K value of the above formula (1) was changed in various ways. The immersion nozzle inner diameter ratio r/R was 1.00, and the Ar gas amount ratio Q _Ar /TP blown from the upper nozzle was 1.50. When the K value is in the range of 0.09 to 0.14, the molten metal level fluctuation is significantly improved. On the other hand, when the K value is too small, the effect of suppressing the molten metal level fluctuation is small due to the influence of the passing molten steel mass being too large, the casting width being too narrow, or the nozzle immersion depth being too shallow. Also, when the K value is too large, the effect of suppressing the molten metal level fluctuation is small due to the influence of the passing molten steel mass being too small, the casting width being too wide, or the nozzle immersion depth being too deep. The inventors believe that maintaining an appropriate deceleration distance of the molten steel flow discharged from the immersion nozzle is effective in suppressing the molten metal level fluctuation.

Example 5
In the mold of the continuous casting machine of Example 1, an electromagnetic stirring device was installed in the upper stage and an electromagnetic brake device was installed in the lower stage as shown in FIG. 6. The electromagnetic stirring device in the upper stage applied an AC magnetic field superimposed on a DC magnetic field. The electromagnetic brake device in the lower stage applied a DC magnetic field. The submerged nozzle inner diameter ratio r/R was 1.00, and the Ar gas amount ratio Q _Ar /TP blown from the upper nozzle was 1.50. Process No. F01 is equivalent to Process No. E11 in Example 4. Various processing conditions and the molten metal level fluctuation index are shown in Table 7. In Process Nos. F02 to F10, the application of a magnetic field reduced the molten metal level fluctuation level index more than when a magnetic field was not applied. On the other hand, in Process No. F02, the DC magnetic field in the lower stage was too weak, the DC magnetic field in the upper stage was too strong, and the AC magnetic field in the upper stage was too weak, which was better than when a magnetic field was not applied, but promoted the molten metal level fluctuation. In addition, in Process No. In the case of F10, the DC magnetic field in the lower stage was too weak, the DC magnetic field in the upper stage was too strong, and the AC magnetic field in the upper stage was too strong, which was better than the case where no magnetic field was applied, but promoted the molten metal level fluctuation. In the case of F11, a magnetic field was applied, but the upper and lower stage discharge port area ratio SL/SU was outside the range of the present invention, so the molten metal level fluctuation index deteriorated. Therefore, it is preferable to apply a DC magnetic field with a magnetic flux density of 0.1 to 0.8 T in the electromagnetic brake device in the lower stage. It is preferable to apply a magnetic field in which an AC magnetic field with a magnetic flux density of 0.03 to 0.1 T is superimposed on a DC magnetic field with a magnetic flux density of 0.1 to 0.8 T in the electromagnetic stirring device in the upper stage. It was found that the molten metal level fluctuation can be further improved by appropriately combining electromagnetic stirring and electromagnetic braking.

In this specification, the unit of volume "L" means ^{10-3 m3} , the unit of mass "t" means metric ton = ¹⁰³ kg, and the symbol "N" for gas volume represents the volume at standard conditions of temperature 0°C and pressure 101325 Pa.

1 Upper discharge port 2 Lower discharge port 3 Bottom 4 Central axis 5 Hot water surface (meniscus)
6 Electromagnetic stirring device 7 Electromagnetic brake device 8 Short side of mold 10 Submerged entry nozzle 20 Mold (for continuous casting machine) R Inner diameter to the top end of the upper hole of the discharge port r Inner diameter from the top end of the upper hole of the discharge port to the bottom of the lower end of the submerged entry nozzle W Distance between the short sides of the mold at the meniscus position L Distance from the meniscus to the top end of the upper hole of the discharge port of the submerged entry nozzle

Claims (8)

An immersion nozzle for supplying molten steel from a molten steel storage container into a mold of a continuous casting machine for performing continuous casting of steel, the immersion nozzle having a nozzle body with an end immersed in the molten steel in the mold that is closed, and a pair of discharge ports each having a central axis as an axis of symmetry at an upper stage and a lower stage of a portion of the nozzle body immersed in the molten steel,
the opening area of the outlet of the lower stage portion is in the range of 1.0 times or more and 1.6 times or less than the opening area of the outlet of the upper stage portion,
The submerged nozzle has an angle θ of more than 0° and less than 10° between the discharge ports of the upper and lower stages when viewed from above. The submerged nozzle according to claim 1, wherein the ratio r/R of the inner diameter r from the upper end of the upper hole of the discharge port to the inner diameter R up to the upper end of the upper hole of the discharge port in the flow path inside the submerged nozzle is in the range of 0.6 or more and less than 1.0. A mold for a continuous casting machine having the submerged nozzle according to claim 1 or 2,
The index K, which affects the molten metal level fluctuation and is shown in the following formula (1), is in the range of 0.09 to 0.14.
It is constructed, a mold.
K ² =(L ² +W ² /4)/TP ² (1)
Here, L is the distance from the meniscus to the upper end of the upper outlet hole of the submerged nozzle [m],
W is the short side distance of the mold at the meniscus position [m],
TP is the mass of molten steel passing per unit time [t/min]. an electromagnetic stirring device having a DC coil and an AC coil, which is installed on the outside of a long side of the mold above the discharge port of the submerged nozzle and can apply a magnetic field in which an AC magnetic field is superimposed on a DC magnetic field to the molten steel in the mold;
an electromagnetic brake device which is installed on the outside of a long side of the mold below the discharge port of the submerged nozzle and has a DC coil capable of applying a DC magnetic field to the molten steel in the mold;
The mold of claim 3 comprising: A method for continuously casting steel, comprising adjusting a molten metal level fluctuation index K represented by the following formula (1) to be in the range of 0.09 to 0.14 when continuously casting steel using the immersion nozzle according to claim 1 or 2.
K ² =(L ² +W ² /4)/TP ² (1)
Here, L is the distance from the meniscus to the top end of the upper hole of the ejection port [m],
W is the mold short side distance at the meniscus position [m],
TP is the mass of molten steel passing per unit time [t/min]
It is. The method for continuous casting of steel according to claim 5, wherein a magnetic field in which a DC magnetic field with a magnetic flux density of 0.1 to 0.8 T is superimposed with an AC magnetic field with a magnetic flux density of 0.03 to 0.1 T is applied to the molten steel in the mold above the discharge port of the submerged nozzle immersed in the molten steel in the mold, and a DC magnetic field with a magnetic flux density of 0.1 to 0.8 T is applied to the molten steel in the mold below the discharge port. 6. The method for continuous casting steel according to claim 5, wherein Ar is introduced from the upper nozzle while adjusting a ratio Q _Ar /TP of an Ar gas flow rate Q _Ar [NL/min] to a passing mass TP of molten steel [t/min] to be 2.0 or more and 5.0 or less. 7. The method for continuous casting steel according to claim 6, wherein Ar is introduced from the upper nozzle while adjusting a ratio Q _Ar /TP of an Ar gas flow rate Q _Ar [NL/min] to a passing mass TP of molten steel [t/min] to be 2.0 or more and 5.0 or less.

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