JPWO2019044292A1 - Steel continuous casting method and thin steel plate manufacturing method - Google Patents

Abstract

The present invention provides a continuous casting method of steel that can suppress the generation of fine slag inclusions and improve the cleanliness of the steel. The present invention is a continuous casting method of steel in which molten steel 15 is supplied from a ladle 6 through a tundish 1 into a continuous casting mold 14 and continuously cast, and a gas flow path opened on the side of a long nozzle 5 From the above, a continuous casting method of steel characterized in that nitrogen gas having a flow rate Q (NL / min) is supplied to the molten steel flow passing through the long nozzle, and the flow rate Q satisfies the following expression (1). . 2 ≦ Q / W <200 × H1 / 2 × D3 / 2 (1) where Q: nitrogen gas flow rate (NL / min), W: molten steel throughput (t / min), H: long nozzle immersion Depth (m), D: Long nozzle inner diameter (m).

Description

  The present invention relates to a continuous casting method of steel, and in particular, a continuous casting method using a slag outflow judgment technique by gas blowing at a long nozzle for injecting molten steel from a ladle into a continuous casting tundish, and the continuous casting method. It is related with the manufacturing method of the used thin steel plate.

  In continuous casting of high cleanliness steel, when pouring molten steel from the ladle into the tundish, the ladle upper nozzle 7, ladle sliding nozzle 8 installed at the bottom of the ladle as shown in FIG. By injecting molten steel into the molten steel in the tundish via the nozzle 5, air oxidation of the molten steel and entrainment of the tundish slag 19 are prevented.

  Patent Documents 1 and 2 describe techniques for detecting ladle slag outflow. In this technology, using the slag outflow detection device illustrated in FIG. 2, an inert gas is caused to flow through the ladle nozzle or the long nozzle, and the change in suction force due to mixing of the ladle slag into the molten steel flow is inactivated. Detection is based on changes in gas flow and back pressure. Thereby, slag outflow is detected at an early stage, and the inflow amount of ladle slag that causes reoxidation of molten steel in the tundish is reduced.

In Patent Document 3, in order to reduce alumina inclusions, powder containing an inclusion absorbent containing CaO / Al ₂ O ₃ of 1.5 or more and a melting point depressant containing SiO ₂ is introduced into the molten steel in the tundish. In addition, a molten steel purification method is described in which hatched slag is formed on the surface of the molten steel, and the alumina inclusions that have floated are absorbed by the slag.

  In Patent Document 4, an inert gas is blown to make the pressure in the long nozzle higher than the atmospheric pressure, thereby preventing (1) reoxidation of the molten steel due to air suction, and (2) in the long nozzle. A large amount of inert gas floats (boiling) around the long nozzle because the pressure of the inert gas becomes excessive and the level of the molten metal in the long nozzle is lowered too much with respect to the immersion depth of the long nozzle. The gas blowing conditions for the purpose of preventing the occurrence of slag entrainment are described.

  Patent Document 5 discloses a steel plate for cans that is excellent in strength, workability, and the like with increased nitrogen concentration. Patent Document 6 discloses a slag outflow detection method similar to Patent Documents 1 and 2 for adding nitrogen to molten steel, and the nozzle used at the bottom of the ladle is used for gas injection for detecting slag outflow. A method is described in which nitrogen-containing gas is blown from an opening provided downstream of the hole to perform nitriding.

Japanese Patent Laid-Open No. 61-262454 Japanese Patent Laid-Open No. 2-70372 Japanese Patent Laid-Open No. 4-111952 Japanese Patent Laid-Open No. 2-187239 JP 2007-177315 A JP-A-4-75767

  While high cleanliness is required for steel, detailed investigations were made on the components of inclusions that caused defects in thin steel plate products. As a result, it was found that slag in tundish was often detected. When tundish slag-based large inclusions are present in a thin steel plate product, the problem of product defects may become prominent in a form different from that of alumina cluster-based inclusions. These inclusions are elongated in the rolling direction by rolling. At this time, in the case of alumina cluster-based inclusions, the presence range of the fine alumina primary particles is extended in the rolling direction while being dispersed in the steel without much deformation. On the other hand, in the case of large inclusions of tundish slag, they are spread out thin and long while maintaining a continuous phase, so the part where the steel on the front and back sides is separated by inclusions that are thinly deformed in layers is a line. It will be formed in a shape. Moreover, since the deformability of inclusions is significantly inferior to that of steel, the thickness of the steel on the surface side is significantly reduced. In particular, in thin steel plate products with a thickness of 0.2 mm or less, such as container materials, there are cases in which linear wrinkle defects become apparent due to cracking in the thinned part of the steel due to processing such as bending or deep drawing. There is a strong demand for minimizing the size of slag inclusions and reducing the content.

  The method of Patent Document 3 is effective in reducing alumina inclusions, and is also effective for continuous casting of a material for a thin steel plate product for containers having a thickness of 0.2 mm or less as described above. However, in the method of Patent Document 3, when using the slag outflow detection technology by gas blowing into the long nozzle of Patent Documents 1 and 2, due to the stirring by gas bubbles floating on the surface of the molten steel in the tundish, There is a risk that slag inclusions may be generated.

  As in Patent Document 4, by controlling the gas blowing conditions to the long nozzle to prevent boiling, the generation of coarse slag inclusions can be suppressed to some extent. However, it has been difficult to sufficiently reduce even the fine slag inclusions that would be a problem with thin steel plate products for containers having a thickness of 0.2 mm or less as described above.

  When manufacturing a product with a high nitrogen concentration as in Patent Document 5, slag outflow detection is also performed while blowing nitrogen gas that is easily dissolved into molten steel and performing nitriding as in Patent Document 6. However, sufficient effects have not been obtained from the viewpoint of reducing slag inclusions.

  That is, in view of the above problems, the present invention suppresses the generation of minute slag inclusions and improves the cleanliness of the steel, and a thin steel plate using the continuous casting method. An object is to provide a manufacturing method.

  In order to solve the above-mentioned problems, the present inventors diligently studied and the reason why the slag in the tundish is mixed as fine inclusions in the molten steel is that the slag outflow detection gas from the long nozzle is blown into the tank. It was found that gas bubbles with a small diameter float on the slag / molten steel interface in the dish. And it has been found that the Ar gas that has been generally used can hinder the reduction of slag inclusions. Specifically, since Ar gas hardly dissolves in the molten steel, it floats as bubbles in the molten steel in the tundish and generates fine molten slag droplets when passing through the slag / molten steel interface. In order to solve this problem, the present inventors further examined that, in place of Ar gas, nitrogen gas, which is easy to dissolve in molten steel, was used for gas injection for slag outflow detection, and the nitrogen gas flow rate was changed to the molten steel throughput. It has been found that it is important to adjust to a predetermined range in relation to the long nozzle immersion depth and the long nozzle inner diameter. This allows the bubbles to be dissolved in the steel from when the gas is blown until the bubbles reach the surface of the molten steel in the tundish, thereby reducing or miniaturizing the bubbles passing through the slag / molten steel interface in the tundish. it can. As a result, it was found that the generation of fine slag inclusions caused by bubbles passing through the slag / molten steel interface in the tundish can be suppressed.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] A continuous casting method for steel in which molten steel is supplied from a ladle through a tundish into a continuous casting mold and continuously cast.
Molten steel supplied from the ladle through the long nozzle while the tip of the long nozzle communicating with the nozzle provided at the bottom of the ladle is immersed in molten steel in the tundish whose surface is covered with molten slag Is poured into the molten steel in the tundish,
At that time, nitrogen gas at a flow rate Q (NL / min) is supplied to the molten steel flow passing through the inside of the long nozzle from the gas flow path opened on the side of the long nozzle,
The said continuous flow rate Q satisfy | fills the following (1) Formula, The continuous casting method of steel characterized by the above-mentioned.
2 ≦ Q / W <200 × H ^1/2 × D ^3/2 (1)
Here, Q: nitrogen gas flow rate (NL / min), W: molten steel throughput (t / min), H: long nozzle immersion depth (m), D: long nozzle inner diameter (m).

[2] The molten slag contains at least a liquid phase slag, and the composition of the molten slag in a quaternary system of CaO, SiO ₂ , Al ₂ O ₃ and MgO satisfies the following formulas (2) and (3): The continuous casting method for steel according to [1].
{(% 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 the total is 100 It is the value converted so that.

  [3] The upper limit of the target range of the nitrogen content of steel products manufactured using the molten steel is 50 mass ppm or less, and the molten steel is made to have a nitrogen content that is 10 mass ppm or more lower than the upper limit. The steel continuous casting method according to the above [1] or [2], wherein continuous casting is performed after refining.

  [4] Continuous after refining so that the lower limit of the target range of the nitrogen content of steel products manufactured using the molten steel is 80 mass ppm or more, and the molten steel has a nitrogen content higher than the lower limit. The continuous casting method for steel according to the above [1] or [2], wherein casting is performed.

  [5] According to any one of the above [1] to [4], wherein the flow of the nitrogen gas and / or a change in back pressure is detected to detect the outflow of the ladle slag into the tundish. The continuous casting method of the described steel.

  [6] A steel slab manufactured using the steel continuous casting method according to any one of [1] to [5] above is hot-rolled and then cold-rolled to obtain a sheet thickness of 0. A method for producing a thin steel plate, comprising producing a steel plate for containers or a drawing steel plate having a diameter of 2 mm or less.

  According to the steel continuous casting method and the thin steel plate manufacturing method of the present invention, generation of minute slag inclusions can be suppressed, and the cleanliness of the steel can be improved.

It is a schematic sectional drawing which shows the molten steel injection | pouring part of the general continuous casting apparatus of steel. It is a schematic diagram which shows the outline | summary of the long nozzle and the slag outflow detection apparatus of the continuous casting apparatus used in the continuous casting method of steel by one Embodiment of this invention. In Experimental example 1, it is a graph which shows the relationship between nitrogen gas flow volume Q and inclusion density index when molten steel throughput W is used as a parameter. In Experimental example 1, it is a graph which shows the relationship between Q / (W * H1 ^{/ 2} * ^{D3 / 2} ) and inclusion density index when the molten steel throughput W is used as a parameter. In Experimental example 2, it is a graph which shows the relationship between nitrogen gas flow volume Q and inclusion density index when long nozzle inner diameter D is used as a parameter. In Experimental example 2, it is a graph which shows the relationship between Q / (W * H1 ^{/ 2} * ^{D3 / 2} ) and inclusion density index when long nozzle inner diameter D is used as a parameter. In Experimental example 3, it is a graph which shows the relationship between nitrogen gas flow rate Q and inclusion density index when long nozzle immersion depth H is used as a parameter. In Experiment 3, is a graph showing the relationship of when the long nozzle immersion depth H as a ^{parameter, Q / (W × H 1/2} × D 3/2) and the inclusion density index.

  A steel continuous casting method according to an embodiment of the present invention is a method in which molten steel is supplied from a ladle through a tundish into a continuous casting mold and continuously cast, and has a molten steel injection portion shown in FIG. A typical continuous casting apparatus can be used, and the slag outflow detection apparatus can be implemented using the configuration shown in FIG.

  The continuous casting apparatus shown in FIG. 1 is a two-strand slab continuous casting apparatus, in which both ends of the tundish 1 are provided with molten steel outflow holes 2 to the mold, and the tundish 1 has a ladle 6 from the ladle 6. The molten steel hot water contact part 3 is arranged.

  In FIG. 1, a tundish 1 in which an outer shell is an iron skin 9 and an inner side of the iron skin 9 is constructed with a refractory 10 is mounted on a tundish car (not shown) and is predetermined above the continuous casting mold 14. Placed in position. In addition, a ladle 6 containing molten steel 15 is arranged at a predetermined position above the tundish 1. A ladle upper nozzle 7 is installed at the bottom of the ladle 6. A ladle sliding nozzle 8 comprising a fixed plate 8A and a sliding plate 8B is installed as a molten steel flow rate control device in contact with the lower surface of the ladle upper nozzle 7. Further, a long nozzle 5 is connected in contact with the lower surface of the ladle sliding nozzle 8 to block the atmosphere. The sliding plate 8B is connected to a reciprocating actuator (not shown), and moves while being in close contact with the fixed plate 8A by the operation of the reciprocating actuator. The amount of molten steel injected from the ladle 6 to the tundish 1 is controlled by adjusting the opening area of the opening of the fixed plate 8A and the opening of the sliding plate 8B by this movement. In order to prevent the molten steel from solidifying in the ladle nozzle 7 and closing the ladle nozzle 7, the nozzle is packed so that the molten steel does not enter the ladle nozzle 7 during the ladle transfer. Packed with sand. Further, a tundish upper nozzle 11 that forms a molten steel outflow hole 2 is fitted and installed at the bottom of the tundish 1 with the refractory 10. In contact with the lower surface of the tundish upper nozzle 11, a tundish sliding nozzle 12 comprising a fixed plate 12A and a sliding plate 12B is installed as a molten steel flow rate control device. Further, an immersion nozzle 13 is connected in contact with the lower surface of the tundish sliding nozzle 12 and having its tip immersed in molten steel 15 inside the continuous casting mold 14. The sliding plate 12B is connected to a reciprocating actuator (not shown), and moves while being in close contact with the fixed plate 12A by the operation of the reciprocating actuator. The amount of molten steel supplied from the tundish 1 to the mold 14 is controlled by adjusting the opening area of the opening of the fixed plate 12A and the opening of the sliding plate 12B by this movement.

  On the bottom surface of the tundish 1, the portion of the hot water contact portion 3 located immediately below the long nozzle 5 is highest, while the portion of the molten steel outflow hole 2 located on both sides of the hot water contact portion 3 is lowest. The bottom surface of the hot water contact portion 3 and the bottom surface of the molten steel outflow hole 2 are both horizontal, and the bottom surface from the horizontal portion including the hot water contact portion 3 to the horizontal portion including the molten steel outflow hole 2 portion is an inclined surface. A weir 4 is installed at each end of the horizontal part including the hot water contact part 3.

  In the state where the tip of the long nozzle 5 is immersed in the molten steel in the tundish whose surface is covered with the tundish slag 19 (molten slag), the molten steel supplied from the ladle 6 through the long nozzle 5 is contained in the tundish. Inject into molten steel. Further, the molten steel 15 is supplied from the tundish 1 to the continuous casting mold 14 through the molten steel outflow hole 2 while the molten steel 15 stays in the tundish 1. The molten steel 15 supplied to the mold 14 is cooled in contact with the mold 14 to form a solidified shell 17, and a slab 16 having an outer shell as the solidified shell 17 and an inside of the unsolidified molten steel 15 is formed as a continuous casting mold. 14 is continuously pulled out below 14 and finally solidified to the center to produce a slab. The injection flow of the molten steel 15 from the ladle 6 to the tundish 1 is blocked from the atmosphere by the long nozzle 5. In addition, the flow of molten steel 15 from the tundish 1 to the mold 14 is blocked from the atmosphere by the immersion nozzle 13. The surface of the molten steel in the ladle 6 is covered with a ladle slag 18, and the surface of the molten steel in the tundish 1 is covered with a tundish slag 19.

  Here, when the molten steel in the ladle 6 runs out, continuous empty casting (hereinafter referred to as “continuous casting”) is performed by replacing the empty ladle with a ladle containing molten steel of another heat. Done. When the molten steel in the ladle 6 runs out, the ladle slag 18 flows into the long nozzle 5, so it is necessary to detect this early and suppress the ladle slag 18 from flowing into the tundish 1. is there. This detection method will be described with reference to FIG.

A gas flow path defined by the gas pipe 20 is open to the side surface of the long nozzle 5, and an inert gas is supplied to the molten steel flow passing through the long nozzle through the gas flow path. The gas pipe 20 is formed by joining an N ₂ pipe 21 and an Ar pipe 22. By controlling the pressure reducing valves 23 and 24 provided in the pipes 21 and 22 and the control valve 25 provided in the gas pipe 20, the kind and flow rate of the inert gas in the gas pipe 20 can be controlled.

  In this embodiment, when the molten steel supplied from the ladle 6 is poured into the molten steel in the tundish via the long nozzle 5, nitrogen gas is supplied from the gas pipe 20 to the molten steel flow passing through the long nozzle. At the stage where only the molten steel passes through the long nozzle, the suction force applied to the gas flow path in the gas pipe 20 is constant, so the pressure (back) indicated by the pressure gauge 26 and the flow meter 27 provided in the gas pipe 20 is constant. Pressure) and flow rate are constant. On the other hand, when the ladle slag 18 passes through the long nozzle in addition to the molten steel, since the ladle slag 18 is lighter than the molten steel, the suction force is reduced. As a result, the pressure indicated by the pressure gauge 26 (usually a negative pressure relative to the atmospheric pressure) increases, and the flow rate indicated by the flowmeter 27 decreases. By detecting such a change in the flow rate and / or back pressure of the nitrogen gas with the recorder 28, the outflow of the ladle slag 18 into the tundish 1 is detected.

  The present inventors use a 2-strand slab continuous casting apparatus shown in FIGS. 1 and 2 and use nitrogen gas dissolved in molten steel as a blowing gas to a long nozzle for ladle slag outflow detection, Effect of nitrogen gas flow rate Q (NL / min) on the formation of fine inclusions under various molten steel throughput W (t / min), long nozzle immersion depth H (m), and long nozzle inner diameter D (m) Was investigated. Below, it demonstrates as Experimental Examples 1-3.

In each experimental example, a thin steel plate cold-rolled to a plate thickness of 0.2 mm or less was measured with a leakage flux type inclusion sensor, and the number density in the steel of fine inclusions having a particle size of about 100 μm or more was evaluated. Then, the number density of the inclusions was found to be able to unify the survey results under various conditions by using Q / (W × H ^1/2 × D ^3/2 ) as an index.

The number density of the inclusions is Ar gas as the blowing gas to the long nozzle, the molten steel throughput W is 7.0 (t / min), the long nozzle immersion depth H is 0.5 (m), and the long nozzle inner diameter D the 0.175 (m), were evaluated in a variety of conditions as an index based on the value when the Ar gas flow rate Q _Ar in the steady casting and 20 (NL / min) and the reference (i.e. 1) (inclusion density index).

(Experimental example 1)
First, W (t / min) is changed to 5, 7, and 9 as parameters, and H (0.4m) and D (0.175m) are constant and Q (NL / min) is in the range of 15-100. The change in inclusion density index when the change was made was investigated, and the results are shown in FIG. In any molten steel throughput W, the inclusion density index tends to increase as the nitrogen gas flow rate Q increases, and this tendency becomes more pronounced when W is relatively small.

Therefore, as a result of changing the horizontal axis of the graph to an index that uses W together with Q and variously evaluating the influence of W, Q / (W × H ^1/2 × D ^3/2 ) is used as an index as shown in FIG. It was found that the survey results under various conditions can be unified.

(Experimental example 2)
Next, the long nozzle inner diameter D (m) is changed to 0.115, 0.145, 0.175 as parameters, and Q (NL / min) is 15 to 80 under the condition that H is 0.4 m and W is constant at 7 t / min. The change in the inclusion density index when the range was changed was investigated, and the results are shown in FIG. In any long nozzle inner diameter D, the inclusion density index tends to increase as the nitrogen gas flow rate Q increases, and this tendency becomes more pronounced when D is relatively small. .

Therefore, the horizontal axis of the graph is changed to an index that uses D together with Q, and as a result of various evaluations of the influence of D, Q / (W × H ^1/2 × D ^3/2 ) is used as an index as shown in FIG. It was found that the survey results under various conditions can be unified.

(Experimental example 3)
Next, the long nozzle immersion depth H (m) was changed to 0.2, 0.3, 0.4, 0.5 as parameters, D was 0.175 m, and W was constant at 7 t / min, Q (NL / min) The change in inclusion density index when the value was changed in the range of 15 to 80 was investigated, and the results are shown in FIG. At any long nozzle immersion depth H, the inclusion density index tends to increase as the nitrogen gas flow rate Q increases, and this tendency becomes more pronounced when H is relatively small. ing.

Therefore, as a result of changing the horizontal axis of the graph to an index that uses H together with Q and variously evaluating the influence of H, Q / (W × H ^1/2 × D ^3/2 ) is used as an index as shown in FIG. It was found that the survey results under various conditions can be unified.

As described above, when nitrogen gas is used as the blowing gas to the long nozzle for ladle slag outflow detection, nitrogen gas flow rate Q (NL / min), molten steel throughput W (t / min), long nozzle immersion The inclusion density index is unified by using Q / (W × H ^1/2 × D ^3/2 ) as an index under various operating conditions of depth H (m) and long nozzle inner diameter D (m). It was found that can be organized. Then, by setting the index value Q / (W × H ^1/2 × D ^3/2 ) to less than 200, more desirably less than 150, the conventional technique using Ar gas as the blowing gas to the long nozzle is used. It was confirmed that the number density of inclusions can be greatly reduced.

  However, if the nitrogen gas flow rate Q is made too small relative to the molten steel throughput W, the slag outflow detection gas injection pressure (negative pressure) will not be stable even during steady injection, and the slag outflow will occur. It may be difficult to determine an increase in pressure. Therefore, in order to stabilize the nitrogen gas blowing pressure and accurately determine the outflow of slag in a short time, it is preferable to set Q / W to 2 or more, more desirably 3 or more.

That is, by setting Q / W within a range that satisfies the following formula (1), when nitrogen gas is used as the slag outflow detection gas, slag outflow can be accurately determined in a short time. It is possible to suppress the generation of minute inclusions due to the dish slag.
2 ≦ Q / W <200 × H ^1/2 × D ^3/2 (1)
Q: Nitrogen gas flow rate (NL / min)
W: Molten steel throughput (t / min)
H: Long nozzle immersion depth (m)
D: Long nozzle inner diameter (m)
The above conditional expression (1) can be applied over a wide range of conditions where W is 4 to 12 t / min, H is 0.2 to 0.8 m, and D is 0.1 to 0.25 m. It was experimentally confirmed that the suppression was effective.

  In the above investigation, the tundish flux added to the tundish in order to shut off the molten steel in the tundish from the atmosphere, when the composition shown in Table 1 was used, and the nozzle on the ladle was opened. The amount of nozzle-filled sand flowing into the tundish and the amount of molten steel flowing into the tundish were added.

Tundish slag, the added tundish flux, nozzle packed sand SiO ₂ is a major component, _Al 2 _{O 3} is the major component deoxidation product emerged from the steel, the CaO and _Al 2 _{O 3} The inflow of ladle slag containing SiO ₂ , MgO, FeO, etc., which is the main component, and the dissolved content of the tundish lining refractory that is generally coated with MgO coating on high alumina refractories, etc. Are combined. Therefore, the composition of the tundish slag changes depending on the change in the mixing ratio and the reaction between the slag and molten steel. As a result of these, the composition of the tundish slag is {(% CaO) + (% MgO)} / (% SiO ₂ ) ≧ 1 in a quaternary system of CaO, SiO ₂ , Al ₂ O ₃ and MgO, and The range of 25 ≦ (% Al ₂ O ₃ ) ≦ 45 is changed during continuous casting. 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 the sum of these is It is a value converted to be 100. An example of the composition of tundish slag is shown in Table 2.

In the slag having the above composition range, a large amount of low-viscosity liquid phase is generated at the molten steel temperature in the tundish, and Al ₂ O ₃ inclusions floating from the molten steel can be efficiently absorbed, and Al in the molten steel is slag. It can be suppressed that the Al ₂ O ₃ inclusions are increased by being oxidized by the oxygen.

  Under the present circumstances, although the steel made into the object of this invention is mainly Al killed steel whose Al content in steel is 0.005-0.06 mass%, it is not necessarily limited to this. When it is desirable to particularly reduce the tundish slag-based inclusions, the present invention can be applied not only to the Al killed steel described above.

  In the case of slag with high fluidity as described above, minute slag inclusions are likely to be generated by agitation caused by bubbles floating from the molten steel, so that the gas blown into the long nozzle is dissolved in the molten steel instead of Ar gas. While using easy nitrogen gas, it is necessary to restrict | limit to the appropriate nitrogen gas blowing speed according to operation conditions.

  However, when nitrogen gas is used instead of Ar gas as the slag outflow detection gas, an increase in the nitrogen concentration in the steel is inevitable. For this reason, it is necessary to sufficiently reduce the nitrogen concentration in advance in the refining stage, and the nitrogen content is 10 mass ppm or more lower than the upper limit of the target range of the nitrogen content of steel products produced using molten steel. Thus, it is desirable to refine the molten steel. Therefore, when this upper limit is 50 ppm or less, in the refining process such as a converter, the nitrogen content of the molten steel used for continuous casting is sufficiently reduced, and the secondary raw materials and various process gas types to be used are used. It is necessary to determine the alloy addition method.

  Further, in continuous casting of a steel type having a high target nitrogen content of, for example, 80 mass ppm or more, conventionally, a method in which a large amount of nitrogen gas is blown into a long nozzle to perform nitriding has been used. However, in the method of the present invention, the flow rate of nitrogen gas blown into the long nozzle cannot always be increased, and a sufficient amount of nitriding cannot always be ensured. Therefore, even in continuous casting of steel types with a lower limit of the target nitrogen content of 80 ppm by mass or more, the refining method, auxiliary materials, added alloys, etc. are determined so that the nitrogen content is higher than this lower limit at the refining stage. There is a need to.

  In a steel plate for containers or a drawing steel plate produced by hot rolling the slab continuously cast as described above and then cold rolling to a sheet thickness of 0.2 mm or less, Ar gas is used as a slag outflow detection gas. Compared with the conventional method, the percentage of slag-based inclusions was reduced, so that the defect rate due to linear wrinkles after press working could be greatly reduced to 1/2 or less.

  Experiments were performed to compare the present invention example and the comparative example in continuous casting of low-carbon aluminum killed steel as a container material using a two-strand slab continuous casting apparatus shown in FIGS.

  The molten steel obtained by decarburizing the hot metal that had been subjected to dephosphorization treatment in the hot metal pretreatment in a converter having a capacity of 300 tons was received in a ladle in an undeoxidized state. Thereafter, aluminum ash was added onto the slag in the ladle to reduce and reform the slag, and then aluminum was added with an RH degassing device to deoxidize the molten steel, followed by secondary refining. After obtaining the molten steel having the components shown in Table 3 through secondary refining, a steel slab slab having a thickness of 260 mm, which is a material for hot rolling, was produced by a 2-strand vertical bending slab continuous casting machine. As shown in FIG. 1, molten steel was injected from a ladle upper nozzle 7 at the bottom of the ladle 6 through a long nozzle 5 into a tundish 1 having a capacity of 50 tons. Further, the casting was poured into the continuous casting mold 14 from the upper tundish nozzle 11 at the bottom of the tundish 1 through the immersion nozzle 13 and the cast piece 16 was pulled out, and continuous casting was performed under the conditions shown in Table 4. The opening of the tundish sliding nozzle 12 is automatically controlled so as to keep the molten steel level in the continuous casting mold constant, and the ladle sliding nozzle 8 is opened so that the amount of molten steel in the tundish is kept constant during steady casting. The degree was automatically controlled. The molten steel throughput W from the ladle is calculated by the sum of the mass of the slab drawn during the unit time and the increase amount of the molten steel mass in the tundish.

  During the molten steel injection from the ladle, the slag outflow detection gas was injected from the gas injection hole provided in the upper part of the long nozzle 5 using the apparatus schematically shown in FIG. At this time, based on the result of measuring the gas blowing pressure and the gas blowing speed, the ladle slag outflow at the end of the molten steel injection from the ladle was determined, and the injection of the molten steel from the ladle was terminated. The ladle was replaced. Nitrogen gas or argon gas is used as the slag outflow detection gas, adjusted to a predetermined supply pressure with the pressure reducing valve 23 or 24, and then adjusted to a predetermined blowing speed Q (NL / min) during steady casting. The pressure of the gas flow control valve 25 is kept constant, and a pressure gauge 26 and a flow meter 27 provided on the downstream side of the flow rate control valve 25 (on the long nozzle blowing hole side), the gas blowing pressure and blowing speed. Was measured. If ladle slag is mixed into the injection flow at the end of the pouring of molten steel from the ladle, the suction force due to the falling flow in the long nozzle decreases, the gas blowing pressure increases and the gas blowing speed increases. descend. Therefore, at least one of the blowing pressure and the blowing speed was detected, the molten steel injection into the ladle was terminated, the ladle was replaced, and continuous casting was continued.

In any test conditions, the tundish flux has the composition shown in Table 1 and is added according to the amount of nozzle sand and molten steel flowing into the tundish. As a result, the composition of the tundish slag is In the quaternary system of CaO, SiO ₂ , Al ₂ O ₃ and MgO, {(% CaO) + (% MgO)} / (% SiO ₂ ) ≧ 1 and 25 ≦ (% Al ₂ O ₃ ) ≦ 45 Thus, the absorption of inclusions floating near the molten steel / slag interface was promoted. The composition of the tundish slag was obtained by taking a sample from the tundish at each time point when the molten steel injection amount from the ladle became 10%, 50% and 90% of the initial molten steel amount in the ladle. It was determined by analysis with an analyzer.

  For steel type A with low target nitrogen concentration (target range of nitrogen content of steel products) and steel type B with high target nitrogen concentration, gas type and gas injection speed Q (Nl / min for slag outflow detection gas during continuous casting ), The inner diameter D (m) of the long nozzle, the immersion depth H (m), the conditions of the molten steel throughput W (t / min) from the ladle are changed as shown in Table 4, each invention example and each comparative example Carried out.

  In Invention Example 1 and Comparative Example 2 using nitrogen gas as the slag outflow detection gas in steel type A with a low target nitrogen concentration, coke that is normally added as a heat source during converter refining is used to reduce the nitrogen content of molten steel Instead, relatively expensive soil graphite or ferrosilicon was used as the heat source. In Invention Example 2 in which the present invention was carried out in steel type B with a high target nitrogen concentration, in order to sufficiently increase the nitrogen content of the molten steel in the secondary refining stage, manganese nitride was added and nitrogen gas was blown in after RH treatment. Components were adjusted by stirring the molten steel.

  After making steel slabs, which are steel materials for hot rolling, using a continuous casting machine, these steel slabs are hot rolled at a hot finishing temperature of 890 ° C and a winding temperature of 650 ° C, and after pickling, Cold rolling was performed at a rolling rate of 92% to obtain a sheet thickness of 0.14 mm. Next, after the temperature was raised to 760 ° C. in a continuous annealing furnace, continuous annealing was performed at an average cooling rate of 5 ° C./s up to 500 ° C. Then, after temper rolling at a rolling rate of 1.5%, the steel sheet obtained thereby was passed through a production line having a leakage magnetic flux type inclusion sensor capable of detecting inclusion diameters up to 100 μm. (The value obtained by dividing the number density of inclusions with a diameter of 100 μm or more in steel of unit mass by the value in Comparative Example 1 using argon gas as the slag outflow detection gas) was evaluated. .

In the inventive examples 1 and 2 in which nitrogen gas is used as the slag outflow detection gas and the conditions satisfying Q / (W × H ^1/2 × D ^3/2 ) <200, the inclusion index is greatly reduced, The defective rate due to linear wrinkles of the product obtained by pressing the obtained steel plate product into a container for food cans was greatly reduced to 1/2 or less.

Even in the case of using nitrogen gas as the slag outflow detection gas, in Comparative Examples 2 and 3 that did not satisfy the condition of Q / (W × H ^1/2 × D ^3/2 ) <200, a decrease in inclusion index was recognized. In addition, the defect rate due to linear wrinkles of the product after press working was also comparable to that of Comparative Example 1 using argon gas.

  Under any of the test conditions, the state of detection of the change in the injection pressure of the slag outflow detection gas at the end of the molten steel injection from the ladle was good, and there was no problem in detecting the outflow of the ladle slag.

  According to the steel continuous casting method and the thin steel plate manufacturing method of the present invention, generation of minute slag inclusions can be suppressed, and the cleanliness of the steel can be improved.

DESCRIPTION OF SYMBOLS 1 Tundish 2 Molten steel outflow hole 3 Hot water contact part 4 Weir 5 Long nozzle 6 Ladle 7 Ladle upper nozzle 8 Ladle sliding nozzle 9 Tundish iron skin 10 Refractory 11 Tundish upper nozzle 12 Tundish sliding nozzle 13 Immersion nozzle 14 Continuous casting mold 15 Molten steel 16 Cast slab 17 Solidified shell 18 Ladle slag 19 Tundish slag 20 Gas piping 21 N ₂ piping 22 Ar piping 23, 24 Pressure reducing valve 25 Control valve 26 Pressure gauge 27 Flow meter (mass flow meter) )
28 Recorder H Long nozzle immersion depth D Long nozzle inner diameter

Claims (6)

A continuous casting method for steel, in which molten steel is supplied from a ladle through a tundish into a continuous casting mold and continuously cast,
Molten steel supplied from the ladle through the long nozzle while the tip of the long nozzle communicating with the nozzle provided at the bottom of the ladle is immersed in molten steel in the tundish whose surface is covered with molten slag Is poured into the molten steel in the tundish,
At that time, nitrogen gas at a flow rate Q (NL / min) is supplied to the molten steel flow passing through the inside of the long nozzle from the gas flow path opened on the side of the long nozzle,
The said continuous flow rate Q satisfy | fills the following (1) Formula, The continuous casting method of steel characterized by the above-mentioned.
2 ≦ Q / W <200 × H ^1/2 × D ^3/2 (1)
Here, Q: nitrogen gas flow rate (NL / min), W: molten steel throughput (t / min), H: long nozzle immersion depth (m), D: long nozzle inner diameter (m). The molten slag contains at least liquid phase slag, and the composition of the molten slag in a quaternary system of CaO, SiO ₂ , Al ₂ O ₃ and MgO satisfies the following formulas (2) and (3): The continuous casting method of the described steel.
{(% 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 the total is 100 It is the value converted so that.   After refining so that the upper limit of the target range of the nitrogen content of steel products produced using the molten steel is 50 mass ppm or less, and the molten steel has a nitrogen content that is 10 mass ppm or more lower than the upper limit. The continuous casting method of steel according to claim 1 or 2, wherein continuous casting is performed.   The lower limit of the target range of the nitrogen content of the steel product produced using the molten steel is 80 mass ppm or more, and the steel is refined so that the molten steel has a nitrogen content higher than the lower limit, and then continuously cast. The continuous casting method of steel according to claim 1 or 2.   The continuous casting of steel according to any one of claims 1 to 4, wherein a flow of ladle slag is detected in the tundish by detecting a change in flow rate and / or back pressure of the nitrogen gas. Method.   A steel slab produced by using the steel continuous casting method according to any one of claims 1 to 5 is hot-rolled and then cold-rolled to produce a steel plate for containers having a thickness of 0.2 mm or less. Or the manufacturing method of the steel plate for drawing, The manufacturing method of the thin steel plate characterized by the above-mentioned.

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