TWI673122B - Continuous casting method of steel and method for manufacturing thin steel sheet - Google Patents

Abstract

The present invention provides a continuous casting method of steel that can suppress the generation of minute slag-based inclusions and improve the cleanliness of steel. The present invention is a continuous casting method for continuously casting steel that is supplied from a hopper 6 through a trough 1 to a continuous casting mold 14 by supplying molten steel 15 into the continuous casting mold 14. The continuous casting method for steel is characterized by: The side-opened gas flow path supplies nitrogen gas at a flow rate Q (NL / min) to the molten steel stream passing through the long-necked nozzle 5, and the flow rate Q satisfies the following formula (1): 2 ≦ Q / W <200 × H ^{1 / 2} × D ^3/2 ... (1)

Here, Q: nitrogen flow rate (NL / min), W: molten steel flow rate (t / min), H: long neck nozzle immersion depth (m), and D: long neck nozzle inner diameter (m).

Description

Continuous casting method of steel and manufacturing method of thin steel plate

The present invention relates to a continuous casting method for steel, and more particularly, to a continuous casting method using a slag outflow determination technology using a gas blown at a long nozzle and a method using the continuous casting method. In a method for manufacturing a thin steel plate, the long neck nozzle is used to inject molten steel into a continuous casting tank from a ladle.

In the continuous casting of high-cleanness steel, when molten steel is injected from a hopper into a feeding tank, as shown in FIG. 1, via a nozzle 7 on the bottom of the hopper, a sliding nozzle 8 and a long neck nozzle 5) The molten steel is injected into the molten steel in the feeding tank, thereby preventing the air of the molten steel from being oxidized or the slag 19 of the feeding tank being entangled.

Patent Documents 1 and 2 describe techniques for detecting the outflow of a slag in a bucket. In this technology, the slag outflow detection device illustrated in FIG. 2 is used, and an inert gas is flowed in the funnel nozzle or the long-neck nozzle, and the mixing of the slag due to the funnel is detected by the change of the inert gas flow rate or the back pressure. To the change in attraction caused in the molten steel stream. Thereby, the slag outflow is detected as early as possible, thereby reducing the inflow amount of the slag in the hopper, which is the cause of the reoxidation of the molten steel in the feed tank.

Patent Document 3 describes a method for purifying molten steel. In order to reduce alumina inclusions, molten steel in a feed tank is charged with an inclusion absorbent containing CaO / Al ₂ O ₃ of 1.5 or more and a SiO ₂ containing The powder of the melting point reducing agent forms a precipitated slag on the surface of the molten steel, and causes the floating alumina inclusions to be adsorbed in the slag.

Patent Document 4 describes gas blowing conditions, the purpose of which is to make the pressure in the long neck nozzle higher than atmospheric pressure by blowing in an inert gas, (1) to prevent the reoxidation of molten steel caused by the inhalation of air, (2) The pressure of the inert gas in the long-necked nozzle becomes too large, and the molten metal liquid position in the long-necked nozzle is excessively lowered relative to the immersion depth of the long-necked nozzle, thereby preventing a large amount of inert gas from floating (boiling (boiling ( boiling)) to the periphery of the long neck nozzle to cause slag entrainment.

Patent Document 5 discloses a steel sheet for cans which is excellent in strength, workability, and the like in which the nitrogen concentration is increased. Patent Document 6 discloses a method for detecting slag outflow similar to Patent Document 1 and Patent Document 2 for adding nitrogen to molten steel, and describes a method in which a nozzle provided at the bottom of a hopper is more suitable for blowing. The openings of the gas for detecting the outflow of slag are further blown into the openings on the downstream side with nitrogen-containing gas to add nitrogen.

[Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 61-262454

[Patent Document 2] Japanese Patent Laid-Open No. 2-70372

[Patent Document 3] Japanese Patent Laid-Open No. 4-111952

[Patent Document 4] Japanese Patent Laid-Open No. 2-187239

[Patent Document 5] Japanese Patent Laid-Open No. 2007-177315

[Patent Document 6] Japanese Patent Laid-Open No. 4-75767

In the aspect that steel requires high cleanliness, the composition of inclusions that cause defects in thin steel plate products was investigated in detail. As a result, it was found that most of the slag in the feed tank was detected. If the large-scale inclusions of the trough slag system are present in the thin steel plate product, the problem of product defects may become significant in a form different from the case of the inclusions of the alumina cluster system. These inclusions are extended in the rolling direction by rolling. At this time, in the case of inclusions of the alumina cluster system, in a state in which the primary particles of minute alumina are dispersed in the steel with substantially no deformation, the existence range thereof is extended in the rolling direction. On the other hand, in the case of large-scale inclusions of the trough-feeding slag system, the thin and long are expanded while the continuous phase is maintained. Therefore, the steel on the front and back sides is thinly deformed by the inclusions in a layered manner. The separated parts are formed in a line shape. In addition, the deformability of inclusions is significantly inferior to that of steel, so the thickness of the steel on the surface side is significantly reduced. In particular, in a thin steel plate product having a thickness of 0.2 mm or less, such as a material facing a container, it may be broken at the thinned portion of the steel by a process such as bending or deep drawing, and may cause linear defects. Defects are manifested, and it is strongly required that the size of the slag-based inclusions be minimized or the content of the slag-based inclusions be reduced.

The method of Patent Document 3 is effective for reducing alumina-based inclusions, and is also effective for continuous casting of materials for thin steel plate products for containers having a thickness of 0.2 mm or less as described above. However, in the method of Patent Document 3, when the slag outflow detection technology of the gas blown toward the long-necked nozzle of Patent Document 1 and Patent Document 2 is used, there is a concern: Stirring caused by gas bubbles on the steel surface generates slag-based inclusions.

As in Patent Document 4, by controlling the gas blowing conditions toward the long neck nozzle to prevent boiling, the generation of coarse slag-based inclusions can be suppressed to a certain extent. However, it is difficult to sufficiently reduce the fine slag-based inclusions that are a problem in thin steel products for containers having a thickness of 0.2 mm or less as described above.

In the case of manufacturing a product with a high nitrogen concentration as in Patent Document 5, although slag outflow detection is also performed while blowing nitrogen that is easily soluble in molten steel in the manner of Patent Document 6 and adding nitrogen, From the viewpoint of reduction of slag-based inclusions, a sufficient effect cannot be obtained.

That is, in view of the said subject, this invention aims at providing the continuous casting method of steel which can suppress generation | occurrence | production of minute slag-type inclusions, and improves the cleanliness of steel, and the manufacturing method of the thin steel plate using this continuous casting method.

In order to solve the problem, the present inventors and other people conducted diligent research, and as a result, it was found that the reason why the slag in the feeding tank is mixed into the molten steel in the form of fine inclusions is that the slag flowing out of the long-necked nozzle detects The test gas is blown in and the small-diameter gas bubbles float up to the slag / steel steel interface in the feed tank. In addition, it was found that Ar gas which has been conventionally used as an obstacle to reduction of slag-based inclusions. Specifically, Ar gas is not easily dissolved in molten steel, so it floats in the form of bubbles in the molten steel in the feed tank, and when passing through the slag / fused steel interface, minute molten slag droplets are generated. In order to solve this problem, the present inventors and others conducted further research, and found that it is important to use nitrogen gas easily dissolved in molten steel instead of Ar gas in the blowing of gas for slag outflow detection. Flow of molten steel, immersion depth of long neck nozzle and The long-necked nozzle has a relationship in which the nitrogen flow rate is adjusted to a predetermined range. Thereby, during the process from the blowing of the gas until the bubbles reach the surface of the molten steel in the feeding tank, the bubbles are dissolved in the steel, so that the bubbles passing through the slag / fused steel interface in the feeding tank can be reduced or minute. Into. As a result, it was found that generation of fine slag-based inclusions caused by the passage of bubbles through the slag / steel interface in the feed tank can be suppressed.

This invention is completed based on the said knowledge, The summary structure is as follows.

[1] A continuous casting method for steel, which comprises continuously pouring molten steel into a continuous casting mold from a hopper through a feed trough, and the continuous casting method for steel is characterized by immersing the tip of a long neck nozzle in The molten steel supplied from the hopper is injected into the molten steel in the feeding tank through the long-necked nozzle in a state of the molten steel in the feeding tank whose surface is covered with molten slag. The nozzle provided at the bottom of the hopper is communicated; at this time, nitrogen gas with a flow rate Q (NL / min) is supplied from a gas flow path opened on the side of the long neck nozzle to the molten steel stream passing through the long neck nozzle. ; The flow rate Q satisfies the following formula (1): 2 ≦ Q / W <200 × H ^1/2 × D ^3/2 ... (1)

Here, Q: nitrogen flow rate (NL / min), W: molten steel flow rate (t / min), H: long neck nozzle immersion depth (m), and D: long neck nozzle inner diameter (m).

[2] The continuous casting method for steel according to the above [1], wherein the molten slag includes at least a liquid phase slag, the molten slag in a quaternary system of CaO, SiO ₂ , Al ₂ O ₃ and MgO The composition satisfies the following formulas (2) and (3): {(% CaO) + (% MgO)} / (% SiO ₂ ) ≧ 1 ... (2)

25 ≦ (% Al ₂ O ₃ ) ≦ 45 ... (3)

Here, (% CaO), (% SiO ₂ ), (% Al ₂ O ₃ ), and (% MgO) are mass percentages of CaO, SiO ₂ , Al ₂ O _3, and MgO, respectively, and are 100 in total. The value converted by the method.

[3] The continuous casting method for steel according to the above [1] or [2], wherein the upper limit of the target range of the nitrogen content of the steel product manufactured using the molten steel is 50 mass ppm or less, The molten steel is refined so as to have a nitrogen content of 10 mass ppm or more lower than the upper limit value, and then continuously cast.

[4] The continuous casting method for steel according to the above [1] or [2], wherein the lower limit value of the target range of the nitrogen content of the steel product manufactured using the molten steel is 80 mass ppm or more, The molten steel is refined so as to have a nitrogen content higher than the lower limit value, and then continuously cast.

[5] The continuous casting method for steel according to any one of the above [1] to [4], wherein the slag in the funnel is detected by detecting a change in the flow rate and / or back pressure of the nitrogen gas. Outflow towards the feed tank.

[6] A method for manufacturing a thin steel plate, characterized in that a slab of steel manufactured by using the continuous casting method of steel according to any one of [1] to [5] After hot rolling, cold rolling is performed to produce a steel plate for containers or a steel sheet for coiling with a thickness of 0.2 mm or less.

According to the continuous casting method of steel and the manufacturing method of a thin steel plate of the present invention, it is possible to suppress the generation of minute slag-based inclusions and improve the cleanliness of the steel.

1‧‧‧feed trough

2‧‧‧ molten steel outflow hole

3‧‧‧ molten metal butt joint

4‧‧‧ embankment

5‧‧‧long neck nozzle

6‧‧‧ pouring bucket

7‧‧‧ Nozzle on the spout

8‧‧‧Sliding Nozzle

8A, 12A‧‧‧Fixing plate

8B, 12B‧‧‧Sliding plate

9‧‧‧ iron sheet

10‧‧‧ Refractory

11‧‧‧ Nozzle on the feeding trough

12‧‧‧Feeding Sliding Nozzle

13‧‧‧Immersion nozzle

14‧‧‧ continuous casting mold

15‧‧‧ molten steel

16‧‧‧ Cast

17‧‧‧ frozen shell

18‧‧‧ Spoon slag

19‧‧‧Feed trough slag

20‧‧‧Gas piping

21‧‧‧N ₂ piping

22‧‧‧Ar Piping

23, 24‧‧‧ pressure reducing valve

25‧‧‧ Regulating valve

26‧‧‧Pressure gauge

27‧‧‧Flowmeter

28‧‧‧Record Meter

H‧‧‧ Long neck nozzle immersion depth

D‧‧‧Inner diameter of long neck nozzle

FIG. 1 is a schematic cross-sectional view showing a molten steel injection portion of a conventional steel continuous casting apparatus.

FIG. 2 is a schematic diagram showing an outline of a long-necked nozzle and a slag outflow detection device of a continuous casting apparatus used in a continuous casting method of steel according to an embodiment of the present invention.

3 is a graph showing the relationship between the nitrogen flow rate Q and the inclusion density index when the molten steel flow rate W is set as a parameter in Experimental Example 1. FIG.

FIG. 4 is a graph showing the relationship between Q / (W × H ^1/2 × D ^3/2 ) and the inclusion density index when the molten steel flow rate W is set as a parameter in Experimental Example 1. FIG.

5 is a graph showing the relationship between the nitrogen flow rate Q and the inclusion density index when the inner diameter D of the long-necked nozzle is set as a parameter in Experimental Example 2. FIG.

FIG. 6 is a graph showing the relationship between Q / (W × H ^1/2 × D ^3/2 ) and the inclusion density index when the inner diameter D of the long neck nozzle is set as a parameter in Experimental Example 2. FIG.

7 is a graph showing a relationship between a nitrogen flow rate Q and an inclusion density index when a long-neck nozzle immersion depth H is set as a parameter in Experimental Example 3. FIG.

FIG. 8 is a graph showing the relationship between Q / (W × H ^1/2 × D ^3/2 ) and the inclusion density index when the long-neck nozzle immersion depth H is set as a parameter in Experimental Example 3. FIG.

A continuous casting method for steel according to an embodiment of the present invention is to continuously cast molten steel into a continuous casting mold from a hopper through a feed trough. For example, a conventional continuous casting having a molten steel injection portion as shown in FIG. 1 may be used. The device was implemented using the configuration shown in FIG. 2 as a slag outflow detection device.

The continuous casting apparatus shown in FIG. 1 is a two-strand continuous slab continuous casting apparatus. The molten steel outflow holes 2 facing the mold are provided at both ends of the feed tank 1, and the central part of the feed tank 1 is provided. A molten metal abutting portion 3 of molten steel from the hopper 6 is arranged.

In FIG. 1, the outer shell is a metal sheet 9, and a feed tank 1 constructed by the refractory 10 on the inner side of the metal sheet 9 is mounted on a tank car (not shown) and disposed at a predetermined position above the continuous casting mold 14. . Moreover, the hopper 6 which accommodates the molten steel 15 is arrange | positioned in the predetermined position above the feed tank 1. As shown in FIG. An upper nozzle 7 is provided on the bottom of the hopper 6. A hopper slide nozzle 8 including a fixed plate 8A and a slide plate 8B is grounded to the lower surface of the hopper upper nozzle 7 in the form of a molten steel flow control device. Further, a long-necked nozzle 5 for blocking the atmosphere is connected to the lower surface of the hopper slide nozzle 8. The slide plate 8B is connected to a reciprocating actuator (not shown), and moves in a state of being in close contact with the fixed plate 8A by the operation of the reciprocating actuator. By this movement, the opening area of the opening portion of the fixed plate 8A and the opening portion of the slide plate 8B is adjusted, thereby controlling the amount of molten steel injected from the hopper 6 into the feed tank 1. Furthermore, in order to prevent molten steel from being sprayed on the hopper The nozzle 7 is solidified inside the nozzle 7 to block the nozzle 7 on the hopper, and the nozzle 7 is filled with sand filling during the movement of the hopper to prevent intrusion of molten steel. In addition, a nozzle 11 on the feed trough forming the molten steel outflow hole 2 is fitted on the bottom of the feed trough 1 to fit the refractory 10. The slot-feeding sliding nozzle 12 including a fixed plate 12A and a sliding plate 12B is provided in the form of a molten steel flow control device in contact with the lower surface of the top nozzle 11 of the slot. Further, an immersion nozzle 13 whose front end is immersed in the molten steel 15 inside the continuous casting mold 14 is connected to the lower surface of the tank sliding nozzle 12. The slide plate 12B is connected to a reciprocating actuator (not shown), and moves in a state of being in close contact with the fixed plate 12A by the operation of the reciprocating actuator. The opening area of the opening of the fixed plate 12A and the opening of the sliding plate 12B is adjusted by this movement, thereby controlling the amount of molten steel supplied from the feed tank 1 to the continuous casting mold 14.

Regarding the bottom surface of the feed tank 1, the position of the molten metal butt joint 3 located immediately below the long-necked nozzle 5 is the highest, and the position of the molten steel outflow hole 2 located on both sides of the molten metal butt joint 3 is the lowest. The bottom surface of the molten metal liquid butt joint 3 and the bottom surface of the portion of the molten steel outflow hole 2 are horizontal. The bottom surface of the horizontal portion from the horizontal part including the molten metal butt joint 3 to the portion including the molten steel outflow hole 2 is an inclined surface. A bank portion 4 is provided at an end portion of the horizontal portion including the molten metal liquid butt portion 3, respectively.

The molten steel supplied from the hopper 6 is injected through the long-necked nozzle 5 into the molten steel while the tip of the long-necked nozzle 5 is immersed in the molten steel in the feeding tank whose surface is covered with the trough-slag 19 (molten slag). Inside the tank is molten steel. Further, in a state where the molten steel 15 is retained in the feed tank 1, the continuous casting is performed from the feed tank 1 through the molten steel outflow hole 2 Molten steel 15 is supplied in a mold 14. The molten steel 15 supplied to the continuous casting mold 14 contacts the continuous casting mold 14 and is cooled to form a solidified shell 17. The outer shell is a solidified shell 17 and the inner part of the molten steel 15 is an unsolidified molten steel 15. The lower part of the continuous casting mold 14 is continuously pulled out, and it is completely solidified until the center part shortly, and a slab is manufactured. The injection flow of the molten steel 15 from the hopper 6 to the feed tank 1 is blocked from the atmosphere by the long-neck nozzle 5. In addition, the injection flow of the molten steel 15 from the feed tank 1 to the continuous casting mold 14 is blocked from the atmosphere by the immersion nozzle 13. In addition, the surface of the molten steel in the hopper 6 is covered by the slag 18 of the hopper, and the surface of the molten steel in the feeding tank 1 is covered by the slag 19 of the feeding slag.

Here, when the molten steel in the hopper 6 is depleted, the empty hopper is exchanged with another hopper containing the heated molten steel for continuous continuous casting (hereinafter referred to as "continuous continuous casting"). ). When the molten steel in the hopper 6 runs out, the slag 18 flows into the long-necked nozzle 5, so it is necessary to detect the slag 18 as early as possible to prevent the slag 18 from flowing out into the feeding tank 1. This detection method will be described with reference to FIG. 2.

The gas flow path divided by the gas piping 20 is opened on the side of the long-necked nozzle 5, and an inert gas is supplied to the molten steel flow passing through the long-necked nozzle through the gas flow path. The gas piping 20 is formed by joining the piping 21 for N ₂ and the piping 22 for Ar. The types of inert gas in the gas piping 20 can be controlled by controlling the pressure reducing valve 23, the pressure reducing valve 24 provided in the N ₂ piping 21, the Ar piping 22, and the regulating valve 25 provided in the gas piping 20, respectively. And traffic.

In the present embodiment, when molten steel supplied from the hopper 6 is injected into the molten steel in the feed tank through the long-necked nozzle 5, nitrogen gas is supplied from the gas pipe 20 to the molten steel stream passing through the long-necked nozzle. In the stage where only molten steel passes through the long neck nozzle, The attractive force applied by the gas flow path in the gas piping 20 is constant. Therefore, the pressure (back pressure) or flow rate indicated by the pressure gauge 26 or the flow meter 27 provided in the gas piping 20 is constant. On the other hand, when the funnel slag 18 other than the molten steel passes through the long-necked nozzle, the funnel slag 18 is lighter in weight than the molten steel, and thus has a lower attractive force. As a result, the pressure (normally a negative pressure with respect to the atmospheric pressure) indicated by the pressure gauge 26 increases, and the flow rate indicated by the flow meter 27 decreases. By detecting the change in the flow rate and / or the back pressure of the nitrogen gas using the recorder 28, the outflow of the slag 18 into the feed tank 1 is detected.

The present inventors used a two-strand continuous slab casting apparatus shown in FIG. 1 and FIG. 2, and used nitrogen dissolved in molten steel as a blowing gas toward a long neck nozzle for detecting slag outflow of a hopper. , And under the conditions of various molten steel flow W (t / min), long neck nozzle immersion depth H (m) and long neck nozzle inner diameter D (m), the nitrogen flow rate Q (NL / min) will be slightly inclusions. Investigate the impact of the generation of objects. Hereinafter, description will be given in the form of Experimental Examples 1 to 3.

In each experimental example, an inclusion sensor with a leakage magnetic general formula was used to measure a thin steel sheet that was cold rolled to a thickness of 0.2 mm or less, and the number density of micro inclusions with a particle size of about 100 μm or more in the steel was measured. Evaluate. Thus, it was found that the number density of the inclusions can be used to uniformly sort the results of investigations under various conditions by using Q / (W × H ^1/2 × D ^3/2 ) as an index.

Regarding the number density of the inclusions, Ar gas was used as the blowing gas toward the long neck nozzle, and the molten steel flow rate W was set to 7.0 (t / min), and the long neck nozzle immersion depth H was set to 0.5. (m), the inner diameter D of the long neck nozzle is set to 0.175 (m), the Ar gas flow rate Q _Ar at the time of stable casting is set to 20 (NL / min), and the value at this time is used as a reference (ie, 1) The index (inclusion density index) was evaluated under various conditions.

(Experimental example 1)

First, set W (t / min) as a parameter and change it to 5, 7, or 9. Under a certain condition with 0.4 m for H and 0.175 m for D, change Q (within 15 to 100). The change in the density index of inclusions at NL / min) was investigated, and the results are shown in FIG. 3. At any molten steel flow rate W, there is a tendency that the inclusion density index increases as the nitrogen flow rate Q increases, and this tendency becomes significant when W is relatively small.

Therefore, the horizontal axis of the graph was changed to an index that uses W together with Q, and various influences on W were evaluated. The results are shown in FIG. 4. It can be seen that Q / (W × H ^1/2 × D ^{3 / 2} ) It can be used as an index to uniformly organize the survey results under various conditions.

(Experimental example 2)

Next, the long-necked nozzle inner diameter D (m) was set as a parameter and changed to 0.115, 0.145, and 0.175. Under the constant conditions of 0.4 m for H and 7 t / min for W, the range was from 15 to 80. The change in the inclusion density index when Q (NL / min) was changed internally was investigated, and the results are shown in FIG. 5. In any long-neck nozzle inner diameter D, the tendency of the inclusion density index to increase as the nitrogen flow rate Q increases, and this tendency becomes significant when D is relatively small.

Therefore, the horizontal axis of the graph was changed to use the index of D together with Q, and various effects of D were evaluated. The results are shown in FIG. 6. It can be seen that Q / (W × H ^1/2 × D ^{3 / 2} ) It can be used as an index to uniformly organize the survey results under various conditions.

(Experimental example 3)

Next, the long-neck nozzle immersion depth H (m) was set as a parameter and changed to 0.2, 0.3, 0.4, and 0.5. Under certain conditions with D of 0.175 m and W of 7 t / min, the range was 15 to 80. The change of the inclusion density index when Q (NL / min) was changed within the range was investigated, and the results are shown in FIG. 7. At any long-neck nozzle immersion depth H, there is a tendency that the inclusion density index increases as the nitrogen flow rate Q increases, and this tendency becomes significant when H is relatively small.

Therefore, the horizontal axis of the graph was changed to use the index of H together with Q, and various influences on H were evaluated. The results are shown in FIG. 8. It can be seen that Q / (W × H ^1/2 × D ^{3 / 2} ) It can be used as an index to uniformly organize the survey results under various conditions.

As described above, it can be seen that when nitrogen gas is used as the blowing gas toward the long-necked nozzle for detecting the slag outflow of the hopper, the nitrogen flow rate Q (NL / min) and the molten steel flow rate W (t / min ), Long neck nozzle immersion depth H (m) and long neck nozzle inner diameter D (m) under various operating conditions, Q / (W × H ^1/2 × D ^3/2 ) can be set as an index to Unified inclusion density index. In addition, it was confirmed that by setting the index value Q / (W × H ^1/2 × D ^3/2 ) to less than 200, more preferably to less than 150, and using Ar gas as a blowing agent toward the long-necked nozzle Compared with the prior art of injecting gas, the number density of inclusions can be greatly reduced.

Among them, if the nitrogen flow rate Q is excessively reduced with respect to the molten steel flow W, there are cases in which, during stable injection, the blowing pressure (negative pressure) of the slag outflow detection gas also fluctuates and becomes unstable, making it difficult. It was determined that the blowing pressure accompanying the slag outflow increased. Therefore, in order to stabilize the blowing pressure of nitrogen, To determine the slag outflow with good accuracy, Q / W is preferably set to 2 or more, and more preferably 3 or more.

That is, by setting Q / W to a range satisfying the following formula (1), when nitrogen is used as the slag outflow detection gas, the slag outflow can be accurately determined in a short time, and the slag outflow can be suppressed. Formation of fine inclusions caused by slag feeding.

2 ≦ Q / W <200 × H ^1/2 × D ^3/2 ... (1)

Q: nitrogen flow (NL / min)

W: molten steel flow (t / min)

H: long neck nozzle immersion depth (m)

D: Inner diameter of long neck nozzle (m)

Regarding the conditional expression of the formula (1), it has been experimentally confirmed that it can be applied to a wide range of conditions where W is 4t / min to 12t / min, H is 0.2m to 0.8m, and D is 0.1m to 0.25m. And, it can be judged that slag outflow is effective for the suppression of small inclusions.

In addition, in the above-mentioned investigation, as the feed flux for the tank to be added to block the molten steel in the feed tank from the atmosphere, a composition shown in Table 1 was used. It is added according to the amount of sand filling in the nozzle flowing into the feeding tank and the inflow amount of molten steel toward the feeding tank.

[Table 1]

Slot-feeding slag is synthesized by combining the following components: added slot-feeding flux, nozzle sand filling with SiO ₂ as the main component, deoxidation products with Al ₂ O ₃ as the main component and floating from steel, CaO and Al ₂ O ₃ is a main component and contains inflow components of the funnel slag including SiO ₂ , MgO, FeO, etc., and dissolved components of the refractory lining of a feed tank in which a high alumina refractory is generally coated with MgO. Therefore, the composition of the feed slag is changed by the change of the respective mixing ratio or the reaction of the slag and the molten steel. As a result of these results, in the quaternary system of CaO, SiO ₂ , Al ₂ O ₃ and MgO, the composition of the trough slag was {(% CaO) + (% MgO)} / (% SiO ₂ ) ≧ 1 and 25 The range of ≦ (% Al ₂ O ₃ ) ≦ 45 changes during continuous casting. Here, (% CaO), (% SiO ₂ ), (% Al ₂ O ₃ ), and (% MgO) are the mass percentages of CaO, SiO ₂ , Al ₂ O _3, and MgO, respectively, and are the totals of these. It is a value converted to 100. Table 2 shows examples of the composition of the trough slag.

In the slag in the composition range, a large amount of a low-viscosity liquid phase is generated at the molten steel temperature in the feeding tank, and the Al ₂ O ₃ series inclusions floating from the molten steel can be efficiently absorbed, and the Al in the molten steel can be absorbed. Oxidation by slag can suppress the increase of Al ₂ O ₃ based inclusions.

At this time, the steel to be the subject of the present invention is mainly an Al deoxidized steel having an Al content in the steel of 0.005 mass% to 0.06 mass%, but it is not necessarily limited to this. In the case where it is desirable to particularly reduce the inclusions of the slag-based slag system, the present invention is not limited to the Al deoxidized steel.

As mentioned above, slag with high fluidity is liable to generate minute slag-based inclusions due to agitation caused by air bubbles floating in the self-melting steel. Therefore, it is necessary to replace Ar for the gas blowing toward the long-neck nozzle. As the gas, nitrogen that is easily soluble in molten steel is used, and the nitrogen blowing rate is limited to an appropriate nitrogen blowing rate according to the operating conditions.

Among them, when nitrogen gas is used instead of Ar gas as a gas for detecting slag outflow, an increase in nitrogen concentration in steel cannot be avoided. Therefore, in the refining stage, it is necessary to sufficiently reduce the nitrogen concentration in advance, and it is desirable that the nitrogen content is 10 mass ppm or more lower than the upper limit of the target range of the nitrogen content of the steel product manufactured by using molten steel. This molten steel is refined. Therefore, when the upper limit value is 50 mass ppm or less, in the refining step such as a converter, care must be taken to sufficiently reduce the nitrogen content of the molten steel used in continuous casting to determine the auxiliary raw materials or various processes to be used. Gas type, alloy addition method, etc.

Further, in the continuous casting of a steel type having a target nitrogen content of, for example, as high as 80 mass ppm or more, a method of adding nitrogen by blowing a large amount of nitrogen into a long-necked nozzle has previously been used. However, in the method of the present invention, the flow rate of nitrogen gas to be blown into the long-necked nozzle may not necessarily be increased, and a sufficient amount of nitrogen may not be ensured. Therefore, the target nitrogen content In continuous casting of a steel type having a lower limit value of 80 mass ppm or more, it is also necessary to determine a refining method, an auxiliary material, an added alloy, and the like in the refining stage so that the nitrogen content is higher than the lower limit value.

The hot-rolled slabs continuously cast in the above-mentioned manner are cold-rolled to a steel sheet for containers or rolled steel sheets having a thickness of 0.2 mm or less, as compared to the conventional method using Ar gas as a slag outflow detection gas. Compared with the method, the defect rate caused by the linear defects after the pressing process is greatly reduced to 1/2 or less because the slag-based fine inclusions are reduced.

[Example]

Experiments comparing the examples of the present invention with the comparative examples were carried out by continuously casting a low-carbon aluminum deoxidized steel of a container-facing material using a two-strand continuous slab continuous casting apparatus shown in FIGS. 1 and 2.

A 300-ton converter was used to decarburize the molten iron subjected to dephosphorization treatment in the molten iron preliminary treatment, and the obtained molten steel was subjected to steel to a hopper in a non-deoxidized state. After that, aluminum ash was added to the slag in the hopper to reduce and modify the slag, and then, aluminum was added using a RH degassing device to deoxidize the molten steel to perform secondary refining. After secondary refining to obtain molten steel having the composition shown in Table 3, a 260-mm-thick slab cast slab was produced using a two-strand vertical bending slab continuous casting machine as a material for hot rolling. As shown in FIG. 1, molten steel is injected into a feeding tank 1 having a capacity of 50 tons from a nozzle 7 on the bottom of the hopper 6 through a long neck nozzle 5. Furthermore, the upper-feeding-chamber nozzle 11 from the bottom of the feeding tank 1 was injected into the continuous casting mold 14 through the immersion nozzle 13, the slab 16 was pulled out, and continuous casting was performed under each condition shown in Table 4. Automatically control the opening degree of the sliding nozzle 12 of the feeding tank in such a way that the level of molten steel in the continuous casting mold is kept constant, and automatically control the pouring in such a way that the amount of molten steel in the feeding tank is kept constant during stable casting The opening degree of the bucket sliding nozzle 8. The molten steel flow rate W from the hopper is calculated by the sum of the mass of the slab pulled out during the unit time and the amount of increase in the molten steel mass in the feed tank.

In the molten steel injection from a hopper, a device schematically shown in FIG. 2 is used, and a gas for detecting slag outflow is blown from a gas blowing hole provided in an upper portion of the long neck nozzle 5. At this time, based on the measurement gas injection pressure and gas injection speed As a result, it is determined that the slag flowing out of the slag at the end of the molten steel injection from the hopper ends, thereby ending the injection of the molten steel from the hopper and performing hopper exchange. Nitrogen or argon is used as a gas for detecting slag outflow, and after adjusting to a predetermined supply pressure using a pressure reducing valve 23 or a pressure reducing valve 24, it is adjusted to a predetermined blowing rate Q (NL / The opening degree of the control valve 25 is kept constant, and the pressure and pressure of the gas are measured using a pressure gauge 26 and a flow meter 27 provided downstream of the control valve 25 (side of the long-neck nozzle). speed. If at the end of the molten steel injection from the hopper, the slag in the hopper is mixed into the injection flow, the attractive force caused by the falling flow in the long neck nozzle is reduced, the gas injection pressure is increased, and the gas injection speed is increased. reduce. Therefore, at least one of these blowing pressures and blowing speeds is detected, the molten steel injection of the hopper is ended, and the hopper is exchanged to continue the continuous casting.

Under any test conditions, the composition of the tank feeding flux is shown in Table 1, and is added according to the nozzle sand filling and the inflow of molten steel toward the feeding tank. As a result, the composition of the tank slag is CaO, SiO ₂ , In the quaternary system of Al ₂ O ₃ and MgO, continuous casting is performed in a range of ((% CaO) + (% MgO)) / (% SiO ₂ ) ≧ 1 and 25 ≦ (% Al ₂ O ₃ ) ≦ 45. This promotes the absorption of inclusions that float up to the vicinity of the molten steel / slag interface. Regarding the composition of the slag in the feed tank, samples were taken from the feed tank at various time points when the molten steel injection amount from the hopper became 10%, 50%, and 90% of the initial molten steel amount in the hopper. It is obtained by analysis by a fluorescent X-ray analyzer.

For the steel type A with a low target nitrogen concentration (target range of nitrogen content of steel products) and the steel type B with a high target nitrogen concentration, change the gas type and gas blowing speed of the slag outflow detection gas during continuous casting as shown in Table 4. Q (NL / min), long neck spray Each condition of the inside diameter D (m) and the immersion depth H (m) of the nozzle, and the molten steel flow rate W (t / min) from the hopper, was implemented for each invention example and each comparative example.

In Steel Type A with a low target nitrogen concentration, Inventive Example 1 and Comparative Example 2 in which nitrogen was used as a slag outflow detection gas, in order to reduce the nitrogen content of the molten steel, the coke that is usually added is not used as The heat source, as the heat source, uses relatively expensive soil graphite or ferrosilicon. In the steel type B with a high target nitrogen concentration, invented Example 2 of the present invention, in order to sufficiently increase the nitrogen content of the molten steel in the two refining stages in advance, after the RH treatment, manganese nitride was added and blown in Nitrogen, stirring the molten steel to adjust the composition.

After continuous casting machine is used to produce slabs as steel materials for hot rolling, these slabs are hot rolled at hot finishing temperature: 890 ° C and coiling temperature: 650 ° C, pickled and rolled. The cold rolling was performed at a rate of 92% to obtain a plate thickness of 0.14 mm. Then, the temperature was increased to 760 ° C in a continuous annealing furnace, and then continuous annealing was performed to cool to 500 ° C at an average cooling rate of 5 ° C / s. Thereafter, after temper rolling at a rolling rate of 1.5%, the steel sheet thus obtained was passed through to a manufacturing production line for an inclusion sensor having a leakage magnetic general formula capable of detecting inclusion diameters of 100 μm. The index of inclusions is obtained by dividing the number density of inclusions with a diameter of 100 μm or more in steel per unit mass by the value obtained by dividing and indexing the value of Comparative Example 1 using argon as a gas for detecting slag outflow. evaluation of.

Inventive Example 1 and Inventive Example 2 using nitrogen as a gas for detecting slag outflow and satisfying the condition of Q / (W × H ^1/2 × D ^3/2 ) <200, the inclusion index was significantly reduced, and The defect rate due to linear defects of the obtained steel plate product obtained by press-processing into a container for food cans was significantly reduced to 1/2.

In Comparative Example 2 and Comparative Example 3, which did not satisfy the condition of Q / (W × H ^1/2 × D ^3/2 ) <200 even when nitrogen was used as the slag outflow detection gas, no decrease in the inclusion index was confirmed. The defective rate due to linear defects of the product after pressing is also about the same as that of Comparative Example 1 using argon.

Under any of the test conditions, the detection condition of the change in the blowing pressure of the slag outflow detection gas in the late stage of molten steel injection from the hopper was good, and the slag outflow detection was no problem.

[Industrial availability]

According to the continuous casting method of steel and the manufacturing method of a thin steel plate of the present invention, it is possible to suppress the generation of minute slag-based inclusions and improve the cleanliness of the steel.

Claims (6)

A continuous casting method of steel that supplies molten steel into a continuous casting mold through a feed trough to continuously cast. The continuous casting method of the steel is characterized in that the tip of a long neck nozzle is immersed in the surface and melted In the state of molten steel in the feed tank covered by slag, the molten steel supplied from the ladle is injected into the molten steel in the feed tank via the long-necked nozzle, wherein the long-necked nozzle is provided with The nozzle at the bottom of the ladle is in communication; at this time, the gas flow path opened at the side of the long-necked nozzle supplies a flow rate Q (NL / min) of nitrogen to the molten steel flow passing through the long-necked nozzle; The flow rate Q satisfies the following formula (1): 2 ≦ Q / W <200 × H ^1/2 × D ^3/2 ... (1) Here, Q: nitrogen flow rate (NL / min), W: melting Steel flux (t / min), H: Long-neck nozzle immersion depth (m), D: Long-neck nozzle inner diameter (m). The continuous casting method of steel according to item 1 of the patent application scope, wherein the molten slag contains at least a liquid phase slag, the composition of the molten slag in the quaternary system CaO, SiO ₂ , Al ₂ O ₃ and MgO Satisfy the following formula (2) and formula (3): {(% CaO) + (% MgO)} / (% SiO ₂ ) ≧ 1 ... (2) 25 ≦ (% Al ₂ O ₃ ) ≦ 45. .. (3) Here, (% CaO), (% SiO ₂ ), (% Al ₂ O ₃ ) and (% MgO) are the mass percentages of CaO, SiO ₂ , Al ₂ O ₃ and MgO, respectively, and are A value converted so that the total becomes 100. The continuous casting method of steel according to item 1 or 2 of the patent application range, wherein the upper limit value of the target range of nitrogen content of the steel products manufactured using the molten steel is 50 mass ppm or less, so as The molten steel is refined in such a manner that the upper limit value is lower than the nitrogen content of 10 mass ppm or more, and then continuous casting is performed. The continuous casting method of steel according to item 1 or item 2 of the patent application range, wherein the lower limit of the target range of the nitrogen content of the steel products manufactured using the molten steel is 80 mass ppm or more to become high The molten steel is refined at the lower limit of the nitrogen content, and then continuous casting is performed. The continuous casting method of steel according to item 1 or item 2 of the patent application scope, wherein the detection of the change in the flow rate and / or back pressure of the nitrogen gas is used to detect the pouring slag toward the feeding tank Outflow. A method of manufacturing a thin steel plate, characterized by hot rolling a steel slab manufactured by using the continuous casting method of steel according to any one of the patent application items 1 to 5, followed by cold rolling In addition, a steel plate for a container or a steel plate for crimping with a thickness of 0.2 mm or less is manufactured.