JP2010227958A - Nozzle for continuous casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the breakage in the initial period of passing through molten steel, in a nozzle for continuous casting, such as a long nozzle. <P>SOLUTION: In the nozzle for continuous casting composed of a tubular refractory structure 1 having an inner hole 2 through which a molten metal passes in an axial center and formed as the axial symmetry; in the lengthwise cross section passing through the axial center of the tubular refractory structure 1, the outer periphery at the lower end part of the nozzle has a shape of quadric curve on the basis of arbitrary points of dy in a lower direction from the lower end-edge of the inner hole 2 and dx in the outer peripheral direction, and a stress reducing ratio of the generated stress in the shape of quadric curve is ≥17% on the basis of the generated stress, in case the lengthwise cross section of the outer periphery at the lower end part of the tubular refractory structure 1 is right angle shape. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a continuous casting nozzle comprising a tubular refractory structure having an inner hole through which a molten metal such as molten steel passes in the axial center and formed symmetrically with respect to the axis, and particularly to a technique for preventing damage thereof.

  The nozzle for continuous casting, such as a long nozzle that discharges molten steel from the ladle to the tundish, has an inner hole through which the molten steel passes at the center of the axis, and is composed of an axially symmetric tubular refractory structure. When the molten steel passes through the inner hole, a temperature gradient is generated on the inner hole side and the outer peripheral side. In particular, when the molten steel starts to be discharged and passed, the inner hole side or the outer peripheral side is rapidly heated, and this phenomenon becomes remarkable.

  Such a temperature gradient causes stress distortion inside the refractory regardless of whether the refractory constituting the tubular refractory structure is a single layer or a plurality of layers, and damage such as cracks in the outer peripheral portion. The greater the temperature gradient, the higher the risk.

  As a countermeasure against destruction due to such thermal stress, a refractory can be obtained by adding a large amount of graphite to the refractory constituting the continuous casting nozzle or adding fused silica having a small thermal expansion amount. It is common to prevent thermal stress failure by increasing the thermal conductivity, lowering expansion, and lowering the elastic modulus. However, on the other hand, an increase in graphite or fused silica causes a decrease in oxidation resistance and an increase in reactivity with molten steel components, and thus has a detrimental effect on a decrease in durability due to a decrease in corrosion resistance and wear resistance. So far, refractories made of materials with excellent thermal shock resistance have been used as the basic part of tubular refractory structures. Damage to each part, for example, by installing a material with high wear resistance at the minimum necessary thickness, and installing a material with excellent corrosion resistance on the part that is affected by chemical erosion with slag such as the part immersed in molten steel. The lifetime of the continuous casting nozzle has been extended by arranging appropriate materials according to the shape in a mosaic pattern.

However, in recent years, there has been a growing demand for continuous casting nozzles with high durability, stable casting, and improved cleanliness of molten steel. In response to these demands, the inner hole side of continuous casting nozzles in particular. As for, refractories on the outer peripheral side are being further reduced in carbon and functionality. Recently, materials that do not contain graphite at all, and materials that contain basic components that are excellent in wear resistance, erosion resistance, or adhesion prevention of inclusions such as alumina on the inner surface are applied to the inner hole side. Attempts to do so are also increasing, and there is a tendency to increase the application of immersion nozzles and the like in which a refractory layer containing a CaO component, a MgO component, and a ZrO ₂ component is provided on the inner hole side.

However, the direction of the technology related to the material of the inner hole side refractory layer related to the high durability and stabilization is, as described above, low carbon (high Al ₂ O ₃ component content, etc.), basic It is accompanied by high thermal expansion. Therefore, when these materials are installed on the inner hole side as the reinforcing layer of the outer refractory layer, the outer refractory layer is generated due to a temperature difference in the radial direction of the tubular refractory structure, particularly in the early stage of steel passing. In addition to the thermal stress resulting from the thermal expansion difference, it is subject to a split (radial compressive stress) from the inner hole side refractory layer accompanying the expansion of the inner hole side refractory layer having a large thermal expansibility. As a result, cracks or breaks in the vertical direction or the horizontal direction are likely to occur in the outer peripheral refractory layer.

  On the other hand, for example, Patent Document 1 discloses a long nozzle for continuous casting in which the outer periphery of the tip is tapered. Specifically, it is described that the outer periphery of the tip is made into an R shape (arc cut) or a C shape (straight cut) to suppress thermal and mechanical stress concentration at the tip and adhesion of slag and metal. Yes.

  In Patent Document 1, since the long nozzle is cooled for each charge and immersed in the molten steel in the tundish, the tip of the long nozzle is exposed to severe thermal shock conditions, and the tip is cracked or missing. In view of the fact that slag and metal attached to the tip of the long nozzle shrink during cooling and solidification, tightening the tip of the long nozzle and promoting cracking and chipping of the tip, thermal shock during repeated use It is intended to alleviate the tightening caused by the attached tip.

Japanese Utility Model Publication No. 05-28542

  In Patent Document 1, as described above, the tip of the long nozzle has a tapered shape to reduce thermal shock during repeated use and tightening due to tip deposits, but the tip of the long nozzle is still broken. In particular, it often occurs at the time of the first steel passing in which no slag or metal is attached to the tip.

  The problem to be solved by the present invention is to prevent breakage particularly in the initial stage of steel passing in a continuous casting nozzle such as a long nozzle.

As a result of thermal stress analysis of a continuous casting nozzle (hereinafter also simply referred to as “nozzle”), the present inventor has obtained the following knowledge.
1. The failure of the nozzle in the early stages of steel passing is mainly due to thermal stress. 2. Destruction due to thermal stress is caused by deformation of the nozzle in the direction in which the outer diameter near the lower end of the nozzle expands (direction in which the outer peripheral portion warps upward). 3. The thermal stress largely depends on the shape of the longitudinal section (vertical section passing through the axial center of the tubular refractory structure) on the outer periphery of the lower end of the tubular refractory structure. In the R shape (arc cut) or C shape (straight cut) as shown in Patent Document 1, the effect of relaxing the thermal stress is not sufficient, and the shape along a quadratic curve is the thermal stress. To be effective in further mitigating

The present invention has been completed on the basis of these findings. Specifically, the continuous casting nozzle has a tubular refractory structure having an inner hole through which molten metal passes in the axial center and formed symmetrically. In the longitudinal cross section passing through the axial center of the tubular refractory structure, the outer periphery of the lower end thereof is based on an arbitrary point of dy downward from the edge of the lower end of the inner hole and dx in the outer peripheral direction. The generated stress Ses represented by the equation 2 in the shape of the quadratic curve represented by the equation 1 is a longitudinal section of the outer periphery of the lower end portion of the tubular refractory structure. In the case of a right-angled shape, the condition of Expression 4 is satisfied with respect to the generated stress Sor represented by Expression 3.
y = a (x−dx) ² + dy (1)
Ses = 0.939573-11.497993 * T1 + 2.204375 * a + 0.040648 * rdy + 12.664875 * T1 * T1 + 4.307205 * T1 * a + 0.227584 * T1 * rdy-2.155509 * a * a-1.505593 * a × rdx + 0.0159940 × a × rdy−8.9921956 × rdx × rdx + 0.109840 × rdx × rdy−0.000306851 × rdy × rdy Formula 2
Sor = (0.503748−0.00131236 × T1 × T1 + 0.122377 × T1) Equation 3
Ses <Sor × (1−0.17) ... Formula 4
Where T1 is the thickness of the tubular refractory structure, a is a curve shape parameter, rdx is a value obtained by dividing dx by T1 (dx / T1), and rdy is a value obtained by dividing dy by T1 (dy / T1)

  Hereinafter, the present invention will be described in detail by taking as an example a long nozzle used when pouring molten steel from a ladle into a tundish.

  When the long nozzle starts to be used (at the beginning of steel passing), molten steel of about 1500 to 1600 ° C. passes through its inner hole. For this reason, normally, a long nozzle is preheated and prepared for steel receiving. Although the thermal stress is relieved to some extent by this preheating, the normal preheating temperature is about 800 ° C. to 1200 ° C., and stress is generated inside the long nozzle due to the difference from the molten steel temperature. Furthermore, the stress is further increased by the cooling between the end of preheating and the passing of steel (depending on the individual conditions of operation).

  When passing steel, the long nozzle is rapidly heated from the inner hole surface, and a thermal gradient is generated between the long nozzle and the outer peripheral side, which has a relatively low temperature. Due to this thermal gradient, a difference in volume expansion according to the thermal expansion of the refractory constituting the long nozzle occurs, and the vertical dimension on the inner hole side where thermal expansion is large is the vertical dimension on the outer peripheral side where thermal expansion is small. Bigger than. As a result, in the vicinity of the lower end portion of the long nozzle, a force is generated that causes a deformation that causes the inner hole to expand from the inner hole side to the outer peripheral side, that is, the outer peripheral side warps outwardly to upward. The long nozzle is destroyed by the stress generated by this.

  The present invention mitigates the degree of deformation that causes this breakage and the internal maximum stress by optimizing the shape of the outer periphery near the lower end of the nozzle, thereby preventing the breakage of the nozzle.

  In actual operation, the outer peripheral shape of the lower end of the nozzle, which is an improved shape shown in Patent Document 1, is an R shape (arc cut) as shown in FIG. 5 (a) or an inclination as shown in FIG. 5 (b). Even in the case of a C shape (straight cut) with an angle θ of 30 ° to 60 °, there is a possibility that the nozzle breaks with a probability of about 1 to 5% due to a change in operating conditions, which may be a problem.

  That is, in order to reduce the probability of destruction further than the probability of destruction, it is necessary to make the generated stress in the lower end of the nozzle smaller than the value of the R shape or C shape.

Therefore, the present inventor assigns each parameter in the equation 1 to the orthogonal table, performs stress calculation by FEM for each shape, and sets the response surface with the parameter value at which the calculated value and the predicted value are approximately one correlation coefficient. Approximation was performed to obtain an approximate expression for the thermal stress estimation value (Expression 5).

  Formula 2 is an arrangement of Formula 5 above.

  Further, the present inventor calculated the stress by FEM using the nozzle thickness T1 as a parameter when there is neither an arc cut nor a straight line cut at the outer periphery of the lower end of the nozzle, that is, when the longitudinal cross section of the outer periphery of the lower end of the nozzle is a right angle. The relationship between the nozzle thickness T1 and the stress was approximated by a quadratic equation. This is Equation 3 above. In other words, Sor represented by Equation 3 represents an estimated value of thermal stress generated in the shape before processing the lower end portion of the nozzle.

In addition, the finite element method software “CosmosWorks2007” manufactured by Solid Works Japan Co., Ltd. was used for these thermal stress calculations. The input parameters and conditions are as follows.
Elastic modulus 4.0 Gpa
Poisson's ratio 0.2
Expansion coefficient 3.8 × 10 ⁻⁶ 1 / ° C.
Bulk specific gravity 2.3 g / cm ³
Thermal conductivity: 15W / mK
Specific heat: 837J / kgK
Initial temperature: 800 ° C
Heating surface: Contact temperature 1550 ° C., heat transfer coefficient 1160 W / m ² K

  In the present invention, the degree of stress relaxation is represented by the reduction rate of the stress estimation value based on the Sor. In the present invention, the stress reduction rate of the stress value (Ses) in the outer peripheral shape of the lower end portion that matches the quadratic curve of Equation 1 is the R shape (arc cut), or the inclination angle θ is 30 ° to 30 °. The condition was that it should be higher than the stress reduction rate (hereinafter referred to as “reference reduction rate”) in the case of a 60 ° C shape (straight cut). Since this reference reduction rate is 16.8% at the maximum as will be described later, in the present invention, the stress reduction rate based on the Sor is set to 17% or more. Expression 4 represents this condition.

  Note that a, dx, and dy can be arbitrarily determined in the shape of the quadratic curve that meets the above-described formula 1. These may be determined by any factors such as ease of manufacture and external factors during transportation, for example, prevention of breakage due to mechanical stress due to collision or the like (for example, installing the nozzle lower end during nozzle transportation). For example, the inclination of the edge is reduced to protect the lower end of the edge).

  Further, the generated stress in Equation 2 is affected by the thickness T1 of the nozzle (tubular refractory structure). It was found that this influence can be evaluated as a relative relationship with dx and dy. Therefore, in Equation 2, rdx is a value obtained by dividing the dx by T1 (= dx / T1), and rdy is a value obtained by dividing the dy by T1 (= dy / T1). Here, the thickness (T1) of the nozzle (tubular refractory structure) in Formula 2 refers to the thickness at the upper end of the quadratic curve at the lower end of the nozzle.

  According to the present invention, in a continuous casting nozzle such as a long nozzle, it is possible to more stably prevent breakage particularly in the initial stage of steel passing.

The longitudinal cross-sectional shape of the outer periphery of the lower end part at the time of applying this invention to a long nozzle is shown. The longitudinal direction cross-sectional shape of the outer periphery of the lower end part of a tubular refractory structure corresponding to each calculation example of Table 1 is shown. The longitudinal direction cross-sectional shape of the outer periphery of the lower end part of a tubular refractory structure corresponding to each calculation example of Table 2 is shown. The stress reduction rate by the calculation result of Table 1 and Table 2 is shown. The conventional nozzle lower end part outer periphery shape is shown, (a) shows the example of R shape (arc cut), (b) shows the example of C shape (straight line cut).

  FIG. 1 shows a longitudinal sectional shape of the outer periphery of the lower end when the present invention is applied to a long nozzle.

In the present invention, the longitudinal section of the outer periphery of the lower end of the tubular refractory structure 1 constituting the nozzle is dy downward from the lower edge (point O1) of the inner hole 2 as shown in FIG. The shape of the quadratic curve represented by the above-described formula 1 (y = a (x−dx) ² + dy), with an arbitrary point O2 of dx as a base point.

  Thus, when the longitudinal section of the outer periphery of the lower end of the tubular refractory structure 1 has the shape of a quadratic curve represented by the above equation 1, the generated stress Ses represented by the above equation 2 is calculated by computer simulation. An example is shown in Table 1. Moreover, the longitudinal direction cross-sectional shape of the outer periphery of the lower end part of the tubular refractory structure 1 corresponding to each calculation example of Table 1 is shown in FIG.

  As described above, the present invention is based on the condition that the stress reduction rate based on the Sor is 17% or more. Therefore, in Table 1, the case where the stress reduction rate is less than 17% is used as a comparative example. From Table 1, it can be seen that even if the lower end portion outer peripheral shape is a quadratic curve, the stress reduction rate may be insufficient only by satisfying the condition of the quadratic curve.

  Table 2 shows a calculation example in the case of the conventional technique, that is, the case where the outer peripheral shape of the lower end of the nozzle is an R shape (arc cut) and a C shape (straight cut). Moreover, the longitudinal direction cross-sectional shape of the outer periphery of the lower end part of the tubular refractory structure 1 corresponding to each calculation example of Table 2 is shown in FIG.

  From Table 2, the stress reduction rate is 16.8% at maximum even if the r is changed in the conventional R shape (arc cut) and the inclination angle θ is changed in the C shape (straight cut). I know that there is.

  Since the stress reduction rate is a value as a relative ratio, there is almost no influence on the stress reduction rate of the long nozzle inner diameter in the practical shape of the long nozzle (the inner diameter is about 80 mm to about 300 mm). Incidentally, in the calculation of this example, the inner diameter was set to 180 mm as a representative example.

  In FIG. 4, the stress reduction rate by the calculation result of Table 1 and Table 2 is shown.

  The present invention is not limited to a long nozzle used for pouring molten steel from a ladle into a tundish, but is also composed of a tubular refractory structure, and continuously casting such as an open nozzle used when pouring molten steel from a tundish into a mold. It can be applied to general nozzles.

1 Tubular refractory structure 2 Inner hole

Claims (1)

A continuous casting nozzle comprising a tubular refractory structure having an inner hole through which molten metal passes in the center of the axis and formed symmetrically,
In the longitudinal section passing through the axial center of the tubular refractory structure, the outer periphery of the lower end thereof is expressed by the following equation (1) based on an arbitrary point of dy downward from the edge of the lower end of the inner hole and dx in the outer peripheral direction. Having the shape of the quadratic curve shown,
Further, the generated stress Ses expressed by the equation 2 in the shape of the quadratic curve is the generated stress Sor expressed by the equation 3 when the longitudinal section of the outer periphery of the lower end portion of the tubular refractory structure is a right-angled shape. On the other hand, a nozzle for continuous casting that satisfies the condition of Formula 4.
y = a (x−dx) ² + dy (1)
Ses = 0.939573-11.497993 * T1 + 2.204375 * a + 0.040648 * rdy + 12.664875 * T1 * T1 + 4.307205 * T1 * a + 0.227584 * T1 * rdy-2.155509 * a * a-1.505593 * a × rdx + 0.0159940 × a × rdy−8.9921956 × rdx × rdx + 0.109840 × rdx × rdy−0.000306851 × rdy × rdy Formula 2
Sor = (0.503748−0.00131236 × T1 × T1 + 0.122377 × T1) Equation 3
Ses <Sor × (1−0.17) ... Formula 4
Where T1 is the thickness of the tubular refractory structure, a is a curve shape parameter, rdx is a value obtained by dividing dx by T1 (dx / T1), and rdy is a value obtained by dividing dy by T1 (dy / T1)

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