Immersion nozzle for continuous casting of steel and continuous casting method for steel

Abstract

The purpose of the present invention is to prevent embrittlement of a power line part material due to generation of zirconium carbide inside a powder line part and to prevent the occurrence of resulting sudden trouble such as perforation of the powder line part.　The present invention provides an immersion nozzle for continuous casting, said immersion nozzle being characterized in that: a molten steel flow passage 5 includes an upper flow part 50 to which an inflow port 3 is provided, a lower flow part 52 to which a discharge port 4 is provided, and a boundary part 51 which is located at the boundary between the upper flow part 50 and the lower flow part 52; the equivalent circle diameter DU of the upper flow part 50 is set to be larger than the equivalent circle diameter DL of the lower flow part 52; the boundary part 51 is provided at a position higher than the position of a powder line part 6; and a lining layer 7 which is made of a material differing from a zirconia / graphite material and which has a thickness of not less than 8 mm is provided inside the powder line part 6.

Description

Submerged nozzle for continuous casting of steel and continuous casting method of steel

[0001] The present invention relates to an immersion nozzle for continuous casting of steel and a continuous casting method for steel using the immersion nozzle for continuous casting.

[0002] Submerged entry nozzles used in continuous casting of steel generally have a cylindrical shape. Molten steel in a tundish passes through a molten steel flow passage in the nozzle body and is supplied into the mold from a discharge port provided at the bottom of the submerged entry nozzle. Mold powder is poured on top of the molten steel to prevent reoxidation of the molten steel in the mold. However, mold powder slag is highly susceptible to corrosion by the components of general refractories. For this reason, zirconia-graphite (ZrO ２ Materials such as powder line material (C) are placed on the outer periphery of the submerged entry nozzle in the area where the mold powder slug comes into contact. The area made of such materials is called the powder line section (powder line section material). The inside of the powder line section is made of alumina-silica-graphite, which is used for the nozzle body, or a hard-to-adhere material that prevents alumina clogging of the nozzle inner tube.

[0003] The powder line of the submerged entry nozzle is primarily damaged by mold powder slag, resulting in damage to the outer periphery. The inner part, which does not come into contact with the mold powder slag, was thought to remain unchanged. However, if the submerged entry nozzle is used for a long period of time or the amount of molten steel passing through the inner part (hereinafter referred to as throughput) increases, the structure of the inner part may become embrittled, which can lead to problems.

[0004] For example, Japanese Patent Application Laid-Open No. 2011-224651 (Patent Document 1) reports that the embrittlement of the powder line material of an immersion nozzle during use occurs at a position adjacent to a gas pool (hollow chamber) arranged inside the bricks of the immersion nozzle with a gas blowing function. ２ It has been reported that zirconia-graphite compositions have poor oxidation resistance and are prone to embrittlement when used for long periods of time.

[0005] Japanese Patent Laid-Open Publication No. 2017-080774 (Patent Document 2) reports that zirconium carbide (ZrC) is generated at the boundary between a zirconia-graphite material and an alumina-graphite material, embrittling the structure and selectively damaging the boundary.

[0006] Japanese Patent Application Laid-Open No. 2021-10933 (Patent Document 3) points out that in a calcium zirconate-graphite material that has alumina-resistant adhesion properties and is placed inside a zirconia-graphite material, zirconium carbide is generated around the zirconia particles, which impairs the alumina adhesion suppression effect, particularly during high-speed casting.

[0007] JP 2011-224651 A JP 2017-080774 A JP 2021-10933 A

[0008] Occasionally, problems with the SEN breakage due to holes in the powder line have occurred. No particular abnormalities were found in the powder line material or other materials of the SEN before use, and no notable abnormalities were found in the melted state of the powder line of the SEN after use. However, when the same SEN was observed, zirconium carbide was found to have formed horizontally over a range of several millimeters in thickness, starting from the boundary surface where the powder line meets another material inside the powder line. This zirconium carbide formation was not always uniform in thickness, and the following trends were observed.

[0009] (1) In an immersion nozzle having a structure in which the inner diameter changes, ZrC is likely to be found in a position where the powder line section material is close to the surface of the nozzle inner tube, i.e., in a position where the thickness of another material placed inside the powder line section is thin. (2) When the degree of damage to the material inside the powder line section is uneven due to the influence of wear by molten steel, etc., ZrC is likely to be found on the side of the powder line section where the remaining thickness of the material is thin. (3) The higher the throughput casting conditions, the more likely ZrC is to be found.

[0010] Here, we consider the phenomenon of zirconium carbide formation. Zirconium carbide is thought to be formed by the following reaction occurring in a high-temperature, reducing atmosphere between zirconia particles, which are the main components of the powder line material, and graphite, or between zirconia particles and an amorphous carbon source that is generated when the organic resin added as a binder is reduced and fired: ZrO ２ +3C→ZrC+2CO

[0011] This reaction is thermodynamically reversible, and the following reaction also occurs in some cases: ZrO ２ +3C←ZrC+2CO

[0012] The direction of the reaction is affected by the surrounding atmosphere. Specifically, if the carbon monoxide concentration in the atmosphere inside the powder line material is high, the reaction will be less likely to proceed to the right. The formation of zirconium carbide inside the powder line indicates that the bricks in that area have a low CO atmosphere. When considering where the large amount of carbon monoxide gas (CO gas) generated is moving quickly, it is thought that the gas passes through another material placed inside the powder line and is supplied to the inside of the submerged nozzle.

[0013] The inside of the submerged entry nozzle is in a reduced pressure environment compared to the surrounding area due to the dynamic pressure caused by the downward flow of molten steel. It is believed that if the material placed inside the powder line section is thin or has high breathability, CO gas generated inside the powder line section passes through another material and migrates to the inside of the submerged entry nozzle. It is believed that all of the above-mentioned characteristics (1) to (3) occur under conditions that allow CO gas to easily pass through.

[0014] As shown in the above reaction formula, under conditions where a large amount of ZrC is produced, zirconia (ZrO ２) The carbon source around the grains is consumed in large quantities. When zirconia reacts with graphite in the powder line, the graphite disappears, increasing porosity. Furthermore, when zirconia reacts with amorphous carbon derived from organic resins, the carbon bond that provides strength to the powder line is lost, resulting in a decrease in strength. Therefore, even if the apparent amount of damage to the outer periphery of the powder line is the same as normal, the area where ZrC is not formed and has sufficient strength is narrowed, which is thought to lead to problems such as hole formation and breakage. Furthermore, even if it does not result in problems, similar phenomena are observed relatively frequently, especially in immersion nozzles produced at high throughput. Therefore, it is thought that similar problems could occur in the future if the service life of immersion nozzles is extended.

[0015] The present invention has been made in view of the above-mentioned circumstances, and its object is to suppress the occurrence of sudden problems such as embrittlement of the powder line material due to the formation of zirconium carbide inside the powder line section, which sometimes occurred when the shape of the inside of the nozzle body was changed in conventional immersion nozzles, and the resulting opening of holes in the powder line section.

[0016] The submerged entry nozzle for continuous casting of steel according to the present invention is characterized in that it comprises: a cylindrical nozzle body; a molten steel flow passage provided within the nozzle body so as to extend longitudinally from an inlet for molten steel; a discharge hole having one end communicating with the molten steel flow passage and the other end opening on the outer circumferential surface of the nozzle body; and a powder line section including zirconia-graphite material provided so as to surround a portion of the nozzle body in the circumferential direction, wherein the molten steel flow passage comprises an upper flow section in which the inlet is provided, a lower flow section in which the discharge hole is provided, and a boundary section at the boundary between the upper flow section and the lower flow section, the equivalent circle diameter of the upper flow section being set larger than the equivalent circle diameter of the lower flow section, the boundary section being provided at a position higher than the powder line section, and a lining layer having a thickness of 8 mm or more and made of a material different from the zirconia-graphite material is provided inside the powder line section.

[0017] In the continuous casting submerged entry nozzle according to the present invention, it is preferable that the difference between the equivalent circle diameter of the upper flow portion and the equivalent circle diameter of the lower flow portion is 10 mm or more.

[0018] In the continuous casting submerged entry nozzle according to the present invention, it is preferable that the size of the circle equivalent diameter of the lower flow portion is not less than 50 mm and less than 80 mm.

[0019] In the continuous casting submerged entry nozzle according to the present invention, the air permeability of the lining layer is 1 kgf cm －２ Under the measurement conditions of 0.1 × 10 －１３ m ２ It is preferable that the value is less than 1 / 2.

[0020] In the continuous casting immersion nozzle according to the present invention, it is preferable that the material different from the zirconia-graphite material is at least one selected from the group consisting of alumina, silica, spinel, magnesia, zircon, calcium silicate, calcium zirconate, graphite, carbon black, pitch, silicon carbide, boron carbide, zirconium boride, aluminum, and silicon nitride.

[0021] The method for continuous casting steel according to the present invention is characterized in that it includes flowing molten steel through the above-described submerged entry nozzle for continuous casting according to the present invention.

[0022] In the method for continuous casting steel according to the present invention, it is preferable to circulate the molten steel so that the production rate of steel is 3.5 tons / min or more.

[0023] In the present invention, a lining layer having a thickness of 8 mm or more and made of a material different from the zirconia-graphite material is provided inside the powder line section. Carbon monoxide gas (CO gas) generated by the reaction between zirconia and graphite in the powder line section is likely to remain in the powder line section because the lining layer acts as a barrier. As a result, ZrO ２The reaction +3C → ZrC + 2CO is less likely to proceed, making it difficult for zirconium carbide (ZrC) to be produced. Therefore, according to the present invention, it is possible to suppress the embrittlement of the powder line material due to the production of zirconium carbide inside the powder line, and the resulting sudden problems such as holes in the powder line.

[0024] 1 is a schematic diagram of a vertical cross section of a continuous casting submerged nozzle. FIG.

[0025] (Continuous Casting Immersion Nozzle) An embodiment of a continuous casting Immersion Nozzle 1 (hereinafter referred to as the Immersion Nozzle 1) according to the present invention will be described with reference to the drawings.

[0026] As shown in FIG. 1 , an immersion nozzle 1 according to the present invention comprises a cylindrical nozzle body 2, a molten steel flow passage 5 provided inside the nozzle body 2 so as to extend in the longitudinal direction from an inlet 3 for molten steel, a discharge hole 4 having one end communicating with the molten steel flow passage 5 and the other end opening on the outer peripheral surface of the nozzle body 2, and a powder line section 6 containing zirconia-graphite material provided so as to surround a portion of the nozzle body 2 in the circumferential direction.

[0027] A molten steel flow passage 5 is formed inside the nozzle body 2. The molten steel flow passage 5 includes an upper flow section 50 in which the inlet 3 is provided, a lower flow section 52 in which the discharge hole 4 is provided, and a boundary section 51 at the boundary between the upper flow section 50 and the lower flow section 52.

[0028] The inner diameter (circle-equivalent diameter) DU of the upper flow portion 50 is set larger than the inner diameter (circle-equivalent diameter) DL of the lower flow portion 52. However, the difference between the inner diameter DU of the upper flow portion 50 and the inner diameter DL of the lower flow portion 52 is preferably 10 mm or more. Furthermore, the inner diameter DL of the lower flow portion 52 is preferably 50 mm or more and less than 80 mm. Note that the cross-sectional shapes of the upper flow portion 50 and the lower flow portion 52 are not limited to being perfectly circular, and may be other shapes such as ellipses, and such cases are also included in the term "circle-equivalent diameter."

[0029] Boundary portion 51 is located at a higher position than powder line section 6. In this embodiment, boundary portion 51 is formed in a tapered shape in which the horizontal dimension decreases from the inlet 3 side toward the discharge hole 4 side, but is not limited to this configuration and may be formed in other shapes, such as a stepped shape or a curved surface shape.

[0030] Examples of materials constituting the nozzle body 2 include, but are not limited to, alumina-silica-graphite materials and alumina-graphite materials. The radial thickness A1 of the side wall of the nozzle body 2 corresponding to the upper flow section 50 of the molten steel flow passage 5 is not particularly limited, but is preferably, for example, 20 mm or more and 35 mm or less. The radial thickness A2 of the side wall of the nozzle body 2 corresponding to the lower flow section 52 of the molten steel flow passage 5 is not particularly limited, but is preferably, for example, 25 mm or more and 40 mm or less.

[0031] Powder line portion 6 is provided along the outer peripheral surface of nozzle body 2. Powder line portion 6 contains zirconia and graphite material as its main components. There are no particular limitations on the radial thickness A3 of powder line portion 6, but it is desirable that it be, for example, between 10 mm and 40 mm.

[0032] A lining layer 7 is provided inside the powder line section 6 so as to line the powder line section 6. The lining layer 7 is a layer separate from the powder line section 6 and is made of a material different from the zirconia / graphite material.

[0033] The radial thickness A4 of the lining layer 7 is 8 mm or more, and more preferably 10 mm or more. The upper limit of the radial thickness A4 of the lining layer 7 is preferably 20 mm or less, for example.

[0034] The ventilation characteristics of the lining layer 7 is 1 kgf cm －２ Under the measurement conditions of 0.1 × 10 －１３ m ２It is desirable that the thickness of the lining layer 7 is less than 100 μm. Any material that satisfies the above-mentioned breathability characteristics can be used as the constituent material of the lining layer 7. Examples of such constituent materials include oxide raw materials such as alumina, silica, spinel, magnesia, zircon, calcium silicate, and calcium zirconate, carbon raw materials such as graphite, carbon black, and pitch, and non-oxide additives such as silicon carbide, boron carbide, zirconium boride, aluminum, and silicon nitride. These materials may be used alone or in combination.

[0035] When zirconium carbide is generated inside the powder line, it is important to take care to prevent CO gas generated at the same location from dispersing to other locations. Factors that may cause CO gas to move to other locations include reduced pressure in the nozzle inner tube (molten steel flow path), the thickness and permeability characteristics of other materials placed inside the powder line, and whether or not there is localized concentration of molten steel passing through the nozzle. Various investigations were conducted, and the following findings were obtained.

[0036] By introducing a structure in which the diameter of the nozzle inner tube is reduced by a certain amount or more above the powder line section, the molten steel flow flowing from above is homogenized in the reduced diameter section (lower flow section), thereby mitigating the occurrence of locations where the flow is locally fast in the nozzle inner tube, making it possible to suppress the formation of ZrC at locations in the nozzle inner tube where the flow is locally fast. Regarding the degree of diameter reduction, assuming that the nozzle inner tube has a circular shape, reducing the diameter by about 5 mm on one side can more efficiently homogenize the molten steel flow rate in the nozzle inner tube. If the reduction width is small, the homogenization effect is small, while if it is excessively large, the shape of the nozzle inner tube becomes too large, resulting in dimensional problems.

[0037] The thickness of the material (lining layer) placed inside the powder line section has traditionally been at least about 5 mm, considering that the powder line section should be as thick as possible without considering whether ZrC is generated or not, but problems can arise at this thickness. Therefore, it is desirable that the thickness of the material (lining layer) placed inside the powder line section be at least 8 mm or more, and more preferably 10 mm or more.

[0038] In addition, it is desirable that the material (lining layer) disposed inside the powder line portion has low air permeability. Specifically, it is desirable that the air permeability be low. －２ Under the measurement conditions, the air permeability was 0.1 × 10 －１３ m ２ A material with very low breathability of less than 100% is desirable. Examples of materials for the lining layer include an alumina-silica-graphite material that is applied to the nozzle body and its immersed portion, or a hard-to-adhere material whose main purpose is to prevent alumina from adhering to the nozzle inner tube. When using a hard-to-adhere material, it is more desirable to use a material that has the property of inhibiting breathability by partially generating a liquid phase within its structure.

[0039] The present invention relates to the structure and material arrangement of the inner surface of the nozzle inner tube in the submerged nozzle, and is not restricted by the shape of the discharge hole portion.

[0040] (Method of Manufacturing Immersion Nozzle for Continuous Casting) The immersion nozzle for continuous casting of the above-described embodiment can be manufactured by a known manufacturing method. For example, materials corresponding to the nozzle body, powder line portion, and lining layer are filled and arranged at predetermined positions in a mold, and then hydrostatically molded. The hydrostatically molded body is then fired at a predetermined temperature (e.g., 800°C or higher), thereby manufacturing the immersion nozzle for continuous casting of this embodiment.

[0041] (Method for Continuous Casting Steel) The continuous casting submerged entry nozzle of the above-described embodiment can be applied to ordinary continuous casting equipment equipped with a ladle, a tundish, a continuous casting mold, etc. The continuous casting submerged entry nozzle of this embodiment is attached to the tundish.

[0042] Molten steel, the composition and temperature of which have been adjusted, is first supplied to a ladle, and then to a tundish. The molten steel supplied to the tundish is then supplied to a continuous casting mold via a continuous casting submerged entry nozzle. The molten steel is cooled and solidified in the continuous casting mold, while being gradually withdrawn from below and rolled to form a steel material. By applying the continuous casting submerged entry nozzle of the present invention, it is possible to circulate molten steel at a high throughput, for example, to achieve a production rate of 3.5 tons / min or more.

[0043] Furthermore, if the diameter of the nozzle inner tube in the powder line section (the inner diameter of the lower flow section of the molten steel flow passage) is DL, it is believed that the smaller DL is, the faster the speed of the molten steel passing through that position tends to be. In an extreme example, if DL is too small, the required throughput cannot be ensured. If the cross-sectional area of ​​the nozzle inner tube in the powder line section is S and the throughput is TP, satisfying the relationship S≧650×TP not only ensures casting, but also prevents the flow rate of the molten steel in the nozzle inner tube from increasing more than necessary. Even if the diameter of the nozzle inner tube is increased beyond this relationship, the effect of suppressing ZrC formation remains largely unchanged. Additionally, since there is an upper limit to the outer diameter of the submerged entry nozzle, an excessive increase in S may lead to a reduction in the thickness of the powder line section, potentially causing problems with the number of times the submerged entry nozzle can be used.

[0044] On the other hand, in the case of blooms, billets, or even some slabs, when the production is carried out at a low throughput of less than 3.5 ton / min, the amount of ZrC produced is very small and ZrC production does not pose a problem.

[0045] When performing high-throughput molten steel operations using a conventional nozzle (comparative example) and a nozzle of the present invention (example), we checked whether zirconium carbide (ZrC) was formed inside the powder line (ZrC formation status column). The confirmation method involved cutting the nozzle after use, and if the formed area was visible, it was marked as ++ or + (++ indicates that ZrC formation was particularly widespread and embrittlement was severe). Furthermore, if ZrC formation was not visible but ZrC formation was confirmed by mineral species identification using X-ray diffraction analysis (XRD), it was marked as ±, and if ZrC formation was not confirmed by either method, it was marked as -. Furthermore, if zirconium carbide formation was observed only partially inside the powder line, it was marked as "present" in the local ZrC formation column. The results are shown in Tables 1 and 2 below.

[0046]

[0047] Although the present invention has been described above with reference to the drawings, the present invention is not limited to the configuration of the drawings, and can be implemented in various forms without departing from the spirit of the present invention.

[0048] The submerged entry nozzle for continuous casting of steel according to the present invention is particularly suitable for use in cases where a high flow rate of molten steel passes through the nozzle body.

[0049] 1: Submerged entry nozzle 2: Nozzle body 3: Inlet 4: Discharge hole 5: Molten steel flow path 50: Upper flow section 51: Boundary section 52: Lower flow section 6: Powder line section 7: Lining layer DU: Inner diameter of upper flow section DL: Inner diameter of lower flow section A1: Radial thickness of side wall (upper flow section) of nozzle body A2: Radial thickness of side wall (lower flow section) of nozzle body A3: Radial thickness of powder line section A4: Radial thickness of lining layer

Claims

1. A submerged entry nozzle for continuous casting of steel comprising: a cylindrical nozzle body; a molten steel flow passage provided within the nozzle body so as to extend longitudinally from a molten steel inlet; a discharge port having one end communicating with the molten steel flow passage and the other end opening on the outer circumferential surface of the nozzle body; and a powder line section comprising zirconia-graphite material provided so as to surround part of the nozzle body in the circumferential direction, wherein the molten steel flow passage comprises an upper flow section where the inlet is provided, a lower flow section where the discharge port is provided, and a boundary section at the boundary between the upper flow section and the lower flow section, the equivalent circle diameter of the upper flow section being set larger than the equivalent circle diameter of the lower flow section, the boundary section being provided at a position higher than the powder line section, and a lining layer having a thickness of 8 mm or more and made of a material different from the zirconia-graphite material provided inside the powder line section.

2. The continuous casting submerged nozzle according to claim 1, wherein the difference between the equivalent circle diameter of said upper flow portion and the equivalent circle diameter of said lower flow portion is 10 mm or more.

3. The continuous casting submerged entry nozzle according to claim 2, wherein the equivalent circle diameter of the lower flow portion is 50 mm or more and less than 80 mm.

4. The lining layer has a breathability of 1 kgf cm －２ Under the measurement conditions of 0.1 × 10 －１３ m ２ The continuous casting submerged entry nozzle according to any one of claims 1 to 3, wherein the immersion nozzle has a diameter of less than 1 / 2 mm.

5. The continuous casting submerged entry nozzle according to claim 4, wherein the material different from the zirconia-graphite material is at least one selected from the group consisting of alumina, silica, spinel, magnesia, zircon, calcium silicate, calcium zirconate, graphite, carbon black, pitch, silicon carbide, boron carbide, zirconium boride, aluminum, and silicon nitride.

6. A method for continuous casting of steel, comprising flowing molten steel through the continuous casting submerged entry nozzle according to claim 1.

7. A method for continuous casting of steel according to claim 6, wherein the molten steel is circulated so that the production rate of steel is 3.5 tons / min or more.

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