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

Abstract

To suppress embrittlement of a powder line part material by production of zirconium carbide inside a powder line part, and occurrence of sudden troubles such as hole opening of the powder line part caused by the embrittlement.SOLUTION: A molten steel flow passage 5 includes an upstream flow part 50 provided with an inflow port 3, a downstream flow part 52 provided with a discharge hole 4, and a boundary part 51 in a boundary between the upstream flow part 50 and the downstream flow part 52, wherein a circle equivalent diameter DU of the upstream flow part 50 is set to be larger than a circle equivalent diameter DL of the downstream flow part 52, the boundary part 51 is provided at a position higher than the powder line part 6, and a lining layer 7, which is composed of a material different from a zirconia / graphite material and has thickness of 8 mm or more, is provided inside the power line part 6.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[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. [Background technology]

[0002] Submerged entry nozzles (SENs) used in continuous steel casting are generally cylindrical. Molten steel from the tundish passes through the molten steel flow passage in the nozzle body and is delivered to the mold through a discharge port at the bottom of the nozzle. Mold powder is added above the molten steel to prevent reoxidation of the molten steel in the mold. However, mold powder slag is highly corrosive to the components of common refractories. Therefore, a zirconia-graphite (ZrO2·C) material, which has relatively good corrosion resistance against mold powder slag, is placed on the outer periphery of the SEN. This is called the powder line (powder line material). The inside of the powder line is made of alumina (silica)-graphite, which is used in the main body, or a non-adhesive material that prevents alumina clogging of the inner tube.

[0003] The powder line of the submerged entry nozzle is mainly damaged by mold powder slag, resulting in damage caused by erosion of the outer periphery. The inner part, which does not come into contact with the mold powder, was thought to remain unchanged, but as the lifespan of the submerged entry nozzle increases and the amount of molten steel passing through the inner part (hereinafter referred to as throughput) increases, the structure of the inner part becomes embrittled after use, and problems caused by this embrittlement can occur.

[0004] For example, Japanese Patent Application Laid-Open No. 2011-224651 (Patent Document 1) reports that embrittlement of the powder line material of an immersion nozzle during use occurs at a position in contact with the gas pool surface located inside the brick of the immersion nozzle with a gas blowing function. It has been reported that zirconia-graphite materials with a high ZrO2 composition in particular have reduced oxidation resistance and are prone to embrittlement over long periods of use.

[0005] Japanese Patent Application Laid-Open Publication No. 2017-080774 (Patent Document 2) reports that the formation of zirconium carbide (ZrC) at the boundary between zirconia and graphite and alumina and graphite causes the structure to become embrittled and the material boundary to be selectively damaged.

[0006] Japanese Patent Publication 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 zirconia-graphite, the formation of zirconium carbide around the zirconia particles impairs the alumina adhesion suppression effect, particularly during high-speed casting. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-224651 [Patent Document 2] Japanese Patent Application Publication No. 2017-080774 [Patent Document 3] Patent Publication No. 2021-10933 Summary of the Invention [Problem to be solved by the invention]

[0008] Occasionally, holes have been formed in the powder line, and breakage problems due to these holes have occurred. No particular abnormalities were found in the material of the powder line or other parts before use, and no notable abnormalities were found in the melting condition of the powder line after use. However, when the nozzle was observed, it was found that zirconium carbide had formed in a range of several millimeters in thickness horizontally, starting from the boundary surface where the powder line meets other materials inside. The formation of this zirconium carbide was not always uniform in thickness, and the following trends were observed.

[0009] (1) In an immersion nozzle with a structure in which the inner diameter changes, ZrC is likely to be found in the position where the powder line material is close to the inner tube surface, i.e., in the position where the thickness of other materials placed inside the powder line is thin. (2) When the degree of damage to the material inside the powder line is uneven due to effects such as abrasion by molten steel, ZrC is more likely to be observed on the side where the remaining thickness of the inner material is thinner. (3) The higher the throughput of the casting, the more likely it is that ZrC will be found.

[0010] Here, we consider the phenomenon of zirconium carbide formation. Zirconium carbide is thought to be formed when amorphous carbon sources, which are generated when the main components of the powder line material—zirconia particles and graphite, or the organic resin added as a binder—are reduced and fired, undergo the following reaction under high temperature and reducing atmosphere conditions: ZrO2+3C→ZrC+2CO

[0011] This reaction is thermodynamically reversible, and the following reaction also occurs in some cases: ZrO2+3C←ZrC+2CO

[0012] The direction in which the reaction proceeds 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 interior of the brick in that area is a low CO atmosphere. When considering where the large amount of carbon monoxide gas (CO gas) generated is quickly moving, it is thought that the gas passes through other materials placed inside the powder line and is supplied to the inside of the submerged entry nozzle.

[0013] The inside of the 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. Here, if the material placed inside the powder line is thin or has high breathability, it is thought that CO gas generated inside the powder line passes through the other material and moves to the inside of the nozzle. All of the above characteristics (1) to (3) are thought to occur under conditions that make it easy for CO gas to pass through.

[0014] As the above reaction equation shows, under conditions that result in the formation of large amounts of ZrC, the carbon source surrounding the zirconia (ZrO2) particles is consumed in large quantities. When zirconia reacts with graphite in the powder line, the graphite disappears, increasing porosity. When zirconia reacts with amorphous carbon from the organic resin, the carbon bond that provides strength to the powder line is lost, resulting in a decrease in strength. As a result, even if the amount of damage to the outer periphery of the powder line appears the same as normal, the area where ZrC is not formed and still has sufficient strength becomes smaller, which can lead to problems such as pitting and breakage. Furthermore, even if this does not result in problems, similar phenomena are relatively common, especially in submerged entry nozzles produced at high throughput. It is thought that similar problems could occur in the future as the service life of submerged entry 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. [Means for solving the problem]

[0016] The features of the submerged entry nozzle for continuous casting of steel according to the present invention are: A cylindrical nozzle body; a molten steel flow passage provided inside the nozzle body so as to extend in a longitudinal direction from an inlet for molten steel; a discharge hole having one end communicating with the molten steel flow passage and the other end opening at the outer peripheral surface of the nozzle body; a powder line portion including a zirconia-graphite material and provided so as to surround a portion of the nozzle body in the circumferential direction, 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 a boundary between the upper flow section and the lower flow section, The circle-equivalent diameter of the upper flow portion is set to be larger than the circle-equivalent diameter of the lower flow portion, The boundary portion is provided at a position higher than the powder line portion, The difference is that 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 portion.

[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, 0.1 × 10 -13 m 2 It is preferable that the value is less than 1 / 2.

[0020] In the continuous casting submerged entry 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 ton / min or more. [Effects of the Invention]

[0023] In the present invention, a lining layer of 8 mm or more in thickness made of a material other than zirconia and graphite is provided inside the powder line section. The lining layer acts as a barrier to carbon monoxide gas (CO gas) generated by the reaction between zirconia and graphite in the powder line section, making it more likely to remain within the powder line section. As a result, the reaction ZrO2 + 3C → ZrC + 2CO in the powder line section 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 prevent embrittlement of the powder line section material due to the formation of zirconium carbide inside the powder line section and the resulting sudden problems such as holes in the powder line section. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a schematic diagram of a vertical cross section of a continuous casting submerged nozzle. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0025] (Submerged entry nozzle for continuous casting) An embodiment of a continuous casting submerged entry nozzle 1 (hereinafter referred to as the submerged entry 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 and extending longitudinally 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 desirably 10 mm or more. Furthermore, the inner diameter DL of the lower flow portion 52 is desirably 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 outlet hole 4 side, but is not limited to this configuration and may also 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 breathability of the lining layer 7 is 1 kgf cm -2 Under the measurement conditions, 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. On the other hand, factors that may cause CO gas to move to other locations include reduced pressure in the nozzle inner pipe (molten steel flow path), the thickness and permeability characteristics of other materials placed inside the powder line, and whether or not there is local concentration of molten steel passing through the nozzle. After various investigations, the following findings were obtained.

[0036] By introducing a structure in which the diameter of the inner pipe is reduced by a certain amount above the powder line section, the molten steel flow flowing in from above is homogenized in the reduced diameter section (lower flow section), mitigating the occurrence of locations where the flow is locally fast and making it possible to suppress the formation of ZrC in the inner pipe section where the locally fast flow passes. As for the degree of diameter reduction, assuming the inner pipe shape is circular, reducing the diameter by about 5 mm on one side can more efficiently homogenize the molten steel flow rate in the nozzle inner pipe, but if the reduction width is small, the homogenization effect is small, and if it is made too large, the inner pipe shape becomes too large, causing dimensional problems.

[0037] When considering the material (lining layer) placed inside the powder line section, and excluding the possibility of ZrC generation, and placing the powder line section as thick as possible, the minimum thickness was previously around 5 mm. However, problems can arise with this thickness, and it was found that it is desirable to place other materials at a thickness of at least 8 mm or more, and more preferably 10 mm or more.

[0038] In addition, the material placed inside (lining layer) should have low breathability, i.e., 1 kgf·cm -2 Under the measurement conditions, the air permeability was 0.1×10 -13 m 2 It was found that the following materials with very low breathability are desirable: As for the material of the inner tube, it is possible to use an alumina-silica-graphite material that is used for the nozzle body and its immersion part, or a hard-to-adhere material whose main purpose is to prevent alumina from adhering to the inner tube, but it was found that among hard-to-adhere materials, a material that has the property of inhibiting breathability by partially generating a liquid phase within its structure is also applicable.

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

[0040] (Method of manufacturing a continuous casting submerged entry nozzle) The continuous casting submerged entry nozzle 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 compact is then fired at a predetermined temperature (e.g., 800°C or higher), thereby manufacturing the continuous casting submerged entry nozzle of this embodiment.

[0041] (Continuous Steel Casting Method) 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 the present 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 fed 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 into 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 inner pipe diameter of 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 molten steel will tend to pass through that position. In an extreme example, if DL is too small, the required throughput cannot be achieved. If the cross-sectional area of ​​the inner pipe 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 unnecessary increases in the inner pipe flow velocity. Even if the inner pipe diameter is increased beyond this relationship, the effect of suppressing ZrC formation remains largely unchanged. Furthermore, since there is an upper limit to the outer diameter of the SEN, excessive increases in S lead to a reduction in the thickness of the powder line section, potentially limiting the number of times the SEN can be used.

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

[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 method of confirmation was to cut the nozzle after use, and if the formed area was visible, it was marked as ++, + (of which, if ZrC formation was particularly widespread and embrittlement was significant, it was marked as ++). 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] [Table 1] [Table 2]

[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. [Industrial Applicability]

[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. [Explanation of symbols]

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

Claims

1. A cylindrical nozzle body; a molten steel flow passage provided inside the nozzle body so as to extend in a longitudinal direction from an inlet for molten steel; a discharge hole having one end communicating with the molten steel flow passage and the other end opening at the outer peripheral surface of the nozzle body; a powder line portion including a zirconia-graphite material and provided so as to surround a portion of the nozzle body in the circumferential direction thereof, 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 a boundary between the upper flow section and the lower flow section, The circle-equivalent diameter of the upper flow portion is set to be larger than the circle-equivalent diameter of the lower flow portion, The boundary portion is provided at a position higher than the powder line portion, a lining layer having a thickness of 8 mm or more and made of a material different from the zirconia-graphite material, disposed inside the powder line portion.

2. 2. The continuous casting submerged entry nozzle according to claim 1, wherein a 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.

3. 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. 5. The continuous casting submerged entry nozzle according to claim 4, wherein the material other than 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 continuously casting steel, comprising flowing molten steel through the continuous casting submerged entry nozzle according to claim 1.

7. 7. The 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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