JP6003850B2 - Manufacturing method of continuous casting mold and continuous casting method of steel - Google Patents

Description

The present invention relates to a method for producing a continuous casting mold capable of continuously casting molten steel while preventing surface cracks due to non-uniform cooling of the solidified shell in the mold, and a method of manufacturing the steel using this mold. The present invention relates to a continuous casting method.

  In continuous casting of steel, the molten steel injected into the mold is cooled by the water-cooled mold, and the molten steel solidifies at the contact surface with the mold to form a solidified layer (called “solidified shell”). While being cooled by the water spray or air-water spray installed on the downstream side of the mold, it is continuously drawn down along with the unsolidified layer inside, and solidified to the center by cooling with the water spray or air-water spray. It is manufactured.

  If the cooling in the mold becomes uneven, the thickness of the solidified shell becomes uneven in the casting direction and the slab width direction. A stress caused by the shrinkage or deformation of the solidified shell acts on the solidified shell, and in the initial stage of solidification, this stress is concentrated on the thin portion of the solidified shell, and the stress causes cracks on the surface of the solidified shell. This crack expands due to subsequent external stresses such as thermal stress, bending stress due to the roll of a continuous casting machine, and straightening stress, resulting in a large surface crack. When the non-uniformity of the solidified shell thickness is large, a vertical crack is generated in the mold, and a breakout in which the molten steel flows out from the vertical crack may occur. Since the cracks present in the slab become surface defects in the subsequent rolling process, it is necessary to care for the surface of the slab and remove the surface cracks at the stage of the slab.

  Inhomogeneous solidification in the mold is likely to occur particularly in steels with a peritectic reaction in which the carbon content is in the range of 0.08 to 0.17% by mass. This is because the solidified shell is deformed due to strain caused by transformation stress due to volumetric shrinkage during transformation from δ iron (ferrite) to γ iron (austenite) due to peritectic reaction, and this deformation causes the site away from the inner wall of the mold. It is considered that surface cracking occurs when the solidification thickness of the solidified shell (the part away from the inner wall surface of the mold is referred to as “depression”) is reduced and the stress is concentrated on this part. In particular, when the slab drawing speed is increased, the average heat flux from the solidified shell to the mold cooling water increases, and the distribution of the heat flux becomes irregular and non-uniform. Tends to increase. Specifically, in a slab continuous casting machine having a slab thickness of 200 mm or more, surface cracks are likely to occur when the slab drawing speed is 1.5 m / min or more.

  Conventionally, in order to prevent surface cracking of the above-mentioned peritectic reaction (referred to as “medium carbon steel”), for example, as proposed in Patent Document 1, a mold powder having a composition that is easy to crystallize is used. Attempts have been made to slowly cool the solidified shell by increasing the thermal resistance of the powder layer. However, only the slow cooling effect by the mold powder does not provide a sufficient improvement in non-uniform solidification, and the occurrence of cracks cannot be prevented with a steel type having a large transformation amount.

  Thus, many methods for slowly cooling the continuous casting mold itself have been proposed. For example, in Patent Document 2, a plurality of grooves are installed along the casting direction on the inner wall surface of a mold, and air cooling is forcibly formed by forcibly cooling the surface of the solidified shell to disperse the surface strain. A method for preventing the vertical cracking of the steel has been proposed. However, in this method, it is necessary to reduce the width and depth of the groove in order to prevent mold powder from entering the groove, and the amount of air gap decreases due to wear of the mold. There is a problem.

  Patent Document 3 proposes a method in which a vertical groove is provided on the inner wall surface of the mold, and mold powder is allowed to flow into the vertical groove so that the mold is cooled slowly. In this method, the inflow of mold powder is inadequate and molten steel penetrates into the recess, or even if the mold powder is filled, the mold powder filled during casting peels off, and the molten steel penetrates into that part. As a result, there is a problem that a restrictive breakout occurs.

  On the other hand, in Patent Document 4 and Patent Document 5, grooves (vertical grooves, lattice grooves) are formed on the inner wall surface of the mold for the purpose of providing a regular heat transfer distribution and reducing the amount of non-uniform solidification. A method of filling metal or ceramics has been proposed. However, in this method, stress due to the thermal strain difference between the low thermal conductivity material and copper acts on the interface between the vertical groove, lattice groove and copper (mold), and at the orthogonal part of the lattice part, and the surface of the mold copper plate is affected. There is a problem that cracks occur.

JP 2005-297001 A Japanese Patent Laid-Open No. 10-193041 Japanese Patent Laid-Open No. 9-276994 Japanese Patent Laid-Open No. 2-6037 JP-A-7-284896

The present invention has been made in view of the above circumstances, and the purpose thereof is to independently form a plurality of parts having lower thermal conductivity than copper on the inner wall surface of a continuous casting mold, As a result, non-uniform cooling of the solidified shell at the initial stage of solidification and δ iron in a medium carbon steel with peritectic reaction without causing constrained breakout of the slab and reduction of mold life due to cracking of the mold surface. It is to provide a method for producing a continuous casting mold capable of preventing surface cracking due to non-uniform thickness of a solidified shell due to transformation from γ iron to γ iron, and also for steel using this continuous casting mold. It is to provide a continuous casting method.

The gist of the present invention for solving the above problems is as follows.
[1] A distance L ₀ or more satisfying the following expression (1) with respect to the slab drawing speed at the time of steady casting from an arbitrary position above the meniscus on the inner wall surface of the water-cooled copper mold. In the inner wall surface up to the lower position, ceramics with a thermal conductivity of 20% or less with respect to the thermal conductivity of copper fills the inside of the circular concave groove or pseudo circular concave groove provided on the inner wall surface. A continuous casting mold comprising a plurality of low thermal conductive ceramic filling portions each having a diameter of 2 to 10 mm or an equivalent circle diameter of 2 to 10 mm.
L ₀ = 2 × Vc × 1000/60 (1)
However, in the formula (1), L ₀ is the distance (mm) from the meniscus, and Vc is the slab drawing speed (m / min) during steady casting.
[2] The filling thickness of the ceramic in the low thermal conductive ceramic filling portion is not more than the depth of the circular concave groove or the pseudo circular concave groove and not less than 0.3 mm, and the diameter of the low thermal conductive ceramic filling portion. Alternatively, the continuous casting mold according to [1] above, wherein the relationship of the following expression (2) is satisfied with respect to the equivalent circle diameter.
0.3 ≦ H ≦ 0.5 × d (2)
In the formula (2), H is the ceramic filling thickness (mm), and d is the diameter (mm) or equivalent circle diameter (mm) of the low thermal conductive ceramic filling portion.
[3] The above-mentioned [1], wherein the interval between the low thermal conductive ceramic filling portions satisfies the relationship of the following formula (3) with respect to the diameter or equivalent circle diameter of the low thermal conductive ceramic filling portions: Or the casting mold for continuous casting described in [2] above.
P ≧ 0.25 × d (3)
However, in the formula (3), P is the interval (mm) between the low thermal conductive ceramic filling portions, and d is the diameter (mm) or the equivalent circle diameter (mm) of the low thermal conductive ceramic filling portions.
[4] Using the continuous casting mold described in any one of [1] to [3] above, the molten steel in the tundish is poured into the continuous casting mold to continuously cast the molten steel. A continuous casting method of steel.
[5] The continuous casting method for steel according to [4], wherein the molten steel is a medium carbon steel having a carbon content of 0.08 to 0.17% by mass.

  According to the present invention, the low thermal conductive ceramic filling portion is disposed in the width direction and the casting direction of the continuous casting mold near the meniscus including the meniscus position. The thermal resistance of the steel plate increases and decreases regularly and periodically, thereby increasing and decreasing the heat flux from the solidified shell in the vicinity of the meniscus, that is, in the initial stage of solidification, to the continuous casting mold, and from δ iron to γ iron. Uneven heat flux distribution due to deformation of the solidified shell caused by transformation stress or thermal stress is made uniform, and the generated stress is dispersed to reduce the amount of individual strain, resulting in cracking. Even if it is easy to use medium carbon steel, generation of cracks on the surface of the solidified shell is prevented.

It is the schematic side view which looked at the long side copper plate which comprises some casting molds concerning this invention from the inner wall surface side. It is an enlarged view of the site | part in which the low thermal conductive ceramic filling part of the casting_mold | template long side copper plate shown in FIG. 1 was formed. It is a figure which shows notionally the thermal resistance in the position of three places of a casting_mold | template long side copper plate according to the installation position of a low thermal conductive ceramic filling part. It is the schematic which shows the example which provided the plating layer for protection of the copper mold surface on the copper mold inner wall surface.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a mold long side copper plate constituting a part of a continuous casting mold according to the present invention, and a mold long side copper plate in which a low thermal conductive ceramic filling portion is formed on the inner wall surface side is viewed from the inner wall surface side. FIG. 2 is a schematic side view, FIG. 2 is an enlarged view of a portion where the low thermal conductive ceramic filling portion of the long-side copper plate shown in FIG. 1 is formed, and FIG. 2 (A) is a side view seen from the inner wall surface side. FIG. 2B is a sectional view taken along line XX ′ in FIG.

  The continuous casting mold shown in FIG. 1 is an example of a continuous casting mold for casting a slab cast, and the continuous casting mold for the slab cast is a pair of long copper plates and a pair of short mold sides. Although it is configured by combining with a copper plate, the mold short side copper plate is also formed with a low thermal conductive ceramic filling part on the inner wall surface side like the mold long side copper plate. Omitted. However, in slab slabs, stress concentration is likely to occur in the solidified shell on the long side surface due to its shape, and surface cracks are likely to occur on the long side surface side. It is not always necessary to install a low thermal conductive ceramic filling portion on the mold short side copper plate of the mold.

  As shown in FIG. 1, the distance L is longer than the meniscus from a position Q (distance Q is an arbitrary value) away from the position of the meniscus (molten steel surface) at the time of steady casting in the long copper plate 1 of the mold. In the range of the inner wall surface of the long-side copper plate 1 to the lower position, ceramics (hereinafter referred to as “low thermal conductive ceramics”) having a thermal conductivity of 20% or less with respect to the thermal conductivity of copper is used as the mold. A plurality of low-thermal-conductivity ceramic filling portions 3 formed by filling the inside of the opening groove provided on the inner wall surface of the long side copper plate 1 are installed. As shown in FIG. 2, the low thermal conductive ceramic filling portion 3 includes a circular concave portion having a diameter (d) of 2 to 10 mm, in which low thermal conductive ceramics are independently processed on the inner wall surface side of the mold long side copper plate 1. The groove 2 is formed by being filled with thermal spraying means, shrink fitting means or the like. Reference numeral 5 in FIG. 2 is a cooling water flow path, and reference numeral 6 is a back plate.

  In FIG. 1 and FIG. 2, the shape of the inner wall surface of the long mold copper plate 1 of the low thermal conductive ceramic filling portion 3 is circular, but it is not necessary to have a circular shape, for example, a so-called “corner” such as an elliptical shape. Any shape may be used as long as the shape is close to a circle and does not include However, even in the case of a shape close to a circle, it is necessary that the equivalent circle diameter determined from the area of the low thermal conductive ceramic filling portion 3 having a shape close to a circle is in the range of 2 to 10 mm.

  By installing the low thermal conductive ceramic filling portion 3 in the width direction and casting direction of the continuous casting mold in the vicinity of the meniscus including the meniscus position, as shown in FIG. 3, the continuous in the mold width direction and the casting direction in the vicinity of the meniscus. The thermal resistance of the casting mold increases and decreases regularly and periodically. As a result, the heat flux from the solidified shell in the vicinity of the meniscus, that is, at the initial stage of solidification, to the continuous casting mold increases and decreases regularly and periodically, and is transformed from δ iron to γ iron (hereinafter referred to as “δ / γ transformation”). Uneven heat flux distribution due to stress and deformation of the solidified shell caused by thermal stress is made uniform, and the generated stress is dispersed to reduce individual strain, resulting in surface cracks on the solidified shell surface. Is prevented. FIG. 3 is a diagram conceptually showing the thermal resistance at three positions of the long-side copper plate 1 according to the installation position of the low thermal conductive ceramic filling portion 3. Thermal resistance is relatively high.

Considering the influence on the initial solidification, the installation position of the low thermal conductive ceramic filling portion 3 is a distance L ₀ or more satisfying the following expression (1) with respect to the slab drawing speed at the scheduled steady casting more than the meniscus It is necessary to reach the lower position.
L ₀ = 2 × Vc × 1000/60 (1)
However, in the formula (1), L ₀ is the distance (mm) from the meniscus, and Vc is the slab drawing speed (m / min) during steady casting.

That is, it is necessary to install the low thermal conductive ceramic filling portion 3 so that the distance L is equal to or greater than the distance L ₀ . Here, the distance L ₀ is related to the time during which the solidified shell of the slab after the start of solidification passes through the range in which the low thermal conductive ceramic filling portion 3 is installed, and solidifies for at least 2 seconds after the start of solidification. Since the immediately following solidified shell needs to stay within the range where the low thermal conductive ceramic filling portion 3 is installed, the formula (1) is derived. Although there is no upper limit of the distance L, it is preferable to set the distance L to 5 times or less of the distance L ₀ from the viewpoint of suppressing the processing cost of the circular groove 2 on the mold surface, the filling processing cost of the low thermal conductive ceramics, and the like. .

  When the solidification shell of the slab after the start of solidification stays in the range where the low thermal conductive ceramic filling portion 3 is installed is less than 2 seconds, the fluctuation of the heat flux by the low thermal conductive ceramic filling portion 3 is periodically changed. Therefore, the effect of preventing the surface crack of the slab becomes insufficient at the time of high-speed casting in which surface cracks are likely to occur or during the casting of medium carbon steel. In order to stably obtain the effect of the periodic fluctuation of the heat flux by the low thermal conductive ceramic filling portion 3, the time required for the solidified shell immediately after solidification to pass through the range where the low thermal conductive ceramic filled portion 3 is installed is 4 seconds or more. It is preferable to ensure.

  On the other hand, the position of the upper end portion of the low thermal conductive ceramic filling portion 3 may be anywhere as long as it is above the meniscus position, and therefore the distance Q may be any value exceeding zero. However, since the meniscus fluctuates in the vertical direction during casting, the low thermal conductive ceramic filling is about 20 mm above the planned meniscus position so that the upper end of the low thermal conductive ceramic filling portion 3 is always located above the meniscus. It is preferable to install the part 3. The meniscus position is generally 60 to 150 mm below the upper end of the mold long side copper plate 1, and the installation range of the low thermal conductive ceramic filling portion 3 may be determined accordingly.

  The shape of the inner wall surface of the mold long side copper plate 1 of the low thermal conductive ceramic filling portion 3 is assumed to be circular or nearly circular. Hereinafter, a shape close to a circle is referred to as a “pseudo circle”. When the shape of the low thermal conductive ceramic filling portion 3 is a pseudo circle, a groove processed on the inner wall surface of the long copper plate 1 for forming the low thermal conductive ceramic filling portion 3 is referred to as a “pseudo circular groove”. The pseudo circle is a shape that does not have a corner, such as an ellipse or a rectangle having a corner or a circle or an ellipse, and may be a shape like a petal pattern.

  When a longitudinal groove or a lattice groove is provided as in Patent Document 4 and Patent Document 5 and this groove is filled with low thermal conductive ceramics, the low heat conductivity is reduced at the boundary surface between the low thermal conductive ceramic and copper and the orthogonal portion of the lattice part. In contrast to the problem that stress due to the thermal strain difference between conductive ceramics and copper concentrates and cracks occur on the surface of the mold copper plate, the shape of the low thermal conductive ceramic filling portion 3 is circular or pseudo-circular as in the present invention. By doing so, since the boundary surface between the low thermal conductive ceramic and copper is curved, an advantage that stress is hardly concentrated on the boundary surface and cracks are hardly generated on the surface of the mold copper plate is exhibited.

  The diameter and equivalent circle diameter of the low thermal conductive ceramic filled portion 3 are required to be 2 mm or more. If the thickness is less than 2 mm, the heat flux in the low thermal conductive ceramic filling portion 3 is not sufficiently lowered, and the above effect cannot be obtained. In addition, the opening area is small, and the low thermal conductive ceramic is rounded by thermal spraying means or shrink fitting means. It is difficult to fill the inside of the concave groove 2 or the pseudo circular concave groove (not shown). On the other hand, when the diameter and equivalent circle diameter of the low thermal conductive ceramic filling portion 3 exceed 10 mm, a large solidification delay occurs due to a decrease in the heat flux in the low thermal conductive ceramic filling portion 3, and the thermal stress on the solidified shell at that position is large. And surface cracks occur in the solidified shell. In order to prevent this, the diameter and the equivalent circle diameter of the low thermal conductive ceramic filling portion 3 need to be 10 mm or less.

In addition, when the shape of the low thermal conductive ceramic filling portion 3 is a pseudo circle, the equivalent circle diameter of the pseudo circle is calculated by the following equation (4).
Equivalent circle diameter = (4 × S / π) ^1/2 (4)
However, in Formula (4), S is an area (mm < ² >) of the low thermal conductive ceramic filling part 3. FIG.

The thermal conductivity of the low thermal conductive ceramic used by filling the circular concave groove and the pseudo circular concave groove is 20% or less (about 76 W / (m) with respect to the thermal conductivity of copper (about 380 W / (m · K)).・ K) or less). When the thermal conductivity is larger than 20% with respect to the thermal conductivity of copper, the effect of the periodic fluctuation of the heat flux by the low thermal conductive ceramic filling portion 3 is insufficient, so that the slab surface cracks are likely to occur during high speed casting. In addition, the effect of preventing slab surface cracking is insufficient when casting medium carbon steel. In particular, alumina (Al ₂ O ₃ , thermal conductivity: about 36 W / (m · K)) that is easily available and has a large difference in thermal conductivity from copper is used as the low thermal conductive ceramic used in the present invention. Silica (SiO ₂ , thermal conductivity: about 10 W / (m · K)) and magnesia (MgO, thermal conductivity: about 60 W / (m · K)) are suitable.

  The filling thickness (H) of the low thermal conductive ceramic filling portion 3 is preferably 0.3 mm or more. When the filling thickness is less than 0.3 mm, the heat flux in the low thermal conductive ceramic filling portion 3 is not sufficiently lowered, and the above effect cannot be obtained.

When the filling thickness of the low thermal conductive ceramic filling portion 3 is larger than 0.5 times the diameter and equivalent circle diameter of the low thermal conductive ceramic filling portion 3, circular grooves and pseudo circular grooves formed by thermal spraying means and shrink fitting means are used. It becomes difficult to fill the low thermal conductive ceramics into the gap, and gaps and cracks occur between the filled low thermal conductive ceramics and the mold copper plate, causing cracks and delamination of the filled low thermal conductive ceramics. It can cause cracks and even constraining breakouts. Therefore, the filling thickness is preferably 0.5 times or less of the diameter of the low thermal conductive ceramic filling portion 3 and the equivalent circle diameter. That is, it is preferable that the filling thickness of the low thermal conductive ceramic filling portion 3 satisfies the following formula (2).
0.3 ≦ H ≦ 0.5 × d (2)
However, in the formula (2), H is the filling thickness (mm) of the low thermal conductive ceramic, and d is the diameter of the circular groove (mm) or the equivalent circle diameter (mm) of the pseudo circular groove. In this case, the filling thickness of the low thermal conductive ceramic is set to be equal to or less than the depth of the circular groove or the pseudo circular groove.

Moreover, it is preferable that the space | interval of the low thermal conductive ceramic filling part is 0.25 times or more of the diameter of the low thermal conductive ceramic filling part 3, and an equivalent circle diameter. That is, it is preferable that the space | interval of the low thermal conductive ceramic filling parts satisfy | fill the relationship of following (3) Formula with respect to the diameter of a low thermal conductive ceramic filling part 3, or a circle equivalent diameter.
P ≧ 0.25 × d (3)
However, in the formula (3), P is the interval (mm) between the low thermal conductive ceramic filling portions, and d is the diameter (mm) or the equivalent circle diameter (mm) of the low thermal conductive ceramic filling portions.

  Here, the space | interval of low heat conductive ceramic filling parts is the shortest distance between the edge parts of the adjacent low heat conductive ceramic filling parts 3, as shown in FIG. When the interval between the low thermal conductive ceramic filled portions is less than “0.25 × d”, the interval is small, and therefore the heat flux and the copper portion (the low thermal conductive ceramic filled portion 3 is formed in the low thermal conductive ceramic filled portion 3). The above effect cannot be obtained because the difference from the heat flux of the non-existing part is small. The upper limit value of the interval between the low thermal conductive ceramic filling portions is not particularly defined. However, since the area ratio of the low thermal conductive ceramic filling portion 3 is reduced when this interval is increased, it is preferably set to “2.0 × d” or less.

  The arrangement of the low thermal conductive ceramic filling portions 3 is preferably a staggered arrangement as shown in FIG. 1, but in the present invention, the arrangement of the low thermal conductive ceramic filling portions 3 is not limited to the staggered arrangement, and any arrangement is possible. It doesn't matter. However, it is preferable that the interval (P) between the low thermal conductive ceramic filling portions is in an arrangement satisfying the above-described conditions.

  When filling a circular groove or pseudo-circular groove with low thermal conductive ceramics by thermal spraying means or shrink fitting means, if the circular groove or pseudo-circular concave groove is deepened by normal spraying means, the groove is completely made of low thermal conductive ceramics. It may be difficult to fill. In that case, the groove portion can be completely filled with the low thermal conductive ceramic by using the shrink fitting means. Further, even when the melting point of the low thermal conductive ceramic is high and it is difficult to melt, the groove portion can be completely filled with the low thermal conductive ceramic by using the shrink fitting means. In order to avoid deterioration of the strength and wear characteristics of the mold copper plate due to temperature rise, it is preferable to perform thermal spraying and shrink fitting within a range of 400 ° C. or less. Since both the thermal spraying means and the shrink-fitting means can increase the temperature of the mold copper plate and the mold copper plate may be thermally deformed, after filling the groove with low thermal conductive ceramics, the surface of the mold copper plate is ground and polished. It is preferable to adjust the dimensions of the copper plate.

  The low thermal conductive ceramic filling portion 3 is basically installed on both the long side mold copper plate and the short side mold copper plate of the continuous casting mold. When the ratio of the long side length of the slab is large, the effect of the present invention can be obtained even if the low thermal conductive ceramic filling portion 3 is installed only on the long piece side.

  Also, as shown in FIG. 4, a plating layer 4 is provided on the inner wall surface of the copper mold on which the low thermal conductive ceramic filling portion 3 is formed in order to prevent wear due to the solidified shell and cracking of the mold surface due to thermal history. It is preferable. The plating layer 4 is sufficient by plating a commonly used nickel-based alloy (Ni-Co alloy, Ni-Cr alloy, etc.), but when the thickness (h) of the plating layer 4 exceeds 2.0 mm. The thickness (h) of the plating layer 4 is preferably 2.0 mm or less because the effect of periodic fluctuations in the heat flux by the low thermal conductive ceramic filling portion 3 becomes insufficient. FIG. 4 is a schematic view showing an example in which a plating layer for protecting the copper mold surface is provided on the inner wall surface of the copper mold.

  As described above, according to the present invention, the low thermal conductive ceramic filling portion 3 is installed in the width direction and the casting direction of the continuous casting mold near the meniscus including the meniscus position. The thermal resistance of the continuous casting mold in the casting direction increases and decreases regularly and periodically, and thereby the heat flux from the solidified shell in the vicinity of the meniscus, that is, in the initial stage of solidification, to the continuous casting mold increases and decreases regularly and periodically. The uneven heat flux distribution due to the deformation of the solidified shell caused by stress due to δ / γ transformation and thermal stress is made uniform, and the generated stress is dispersed to reduce the amount of individual strains. Even in the case of medium-carbon steel that is susceptible to cracking and high-speed casting with a large amount of heat removal, cracking on the solidified shell surface is prevented.

  Moreover, in patent document 4 and patent document 5, since the vertical groove | channel or grating | lattice groove | channel used as a low heat conduction part is continuously installed in the casting direction, the site | part by which solidification shell formation is suppressed is always the same in the slab width direction. And there is a risk of breakout due to the reduced thickness of the solidified shell at that location. On the other hand, in the present invention, since the low heat conductive portions are periodically dispersed and arranged also in the casting direction, the portion where the solidified shell thickness is excessively thin cannot be formed, and the risk of breakout is a normal mold. It is equivalent to using.

  Although the above description has been made with respect to a continuous casting mold for slab slabs, the present invention is not limited to a continuous casting mold for slab slabs, and is continuous for bloom slabs and billet slabs. The present invention can be applied to a casting mold along the above.

  Medium carbon steel (C: 0.08 to 0.17 mass%, Si: 0.10 to 0.30 mass%, Mn: 0.50 to 1.50 mass%, P: 0.010 to 0.030 mass% %, S: 0.002 to 0.010 mass%, Al: 0.02 to 0.04 mass%), a water-cooled copper mold having an inner space size with a long side length of 2450 mm and a short side length of 215 mm When slab slabs were continuously cast using slab, a low thermal conductive ceramic filling part was installed on the inner wall surface of the water-cooled copper mold under various conditions.

The water-cooled copper mold used had a length from the upper end to the lower end (= mold length) of 950 mm, and the meniscus position was set at a position 100 mm below the upper end of the mold. In this water-cooled copper mold, circular grooves are processed on the mold inner wall surface in a range (distance L = 30 mm to 300 mm) from a position 20 mm above the meniscus to a position 30 mm to 300 mm below the meniscus, The circular concave groove is filled with cylindrical alumina (Al ₂ O ₃ , thermal conductivity: 36 W / (m · K)) having a diameter several tens of μm larger than the circular concave groove by shrink fitting. Then, a low thermal conductive ceramic filling portion was formed. All the low thermal conductive ceramic filling parts are the same size and are staggered. The filling depth of the alumina into the circular groove is the same as the depth of the circular groove.

  For comparison, a water-cooled copper mold in which silicon carbide (SiC, thermal conductivity: 200 W / (m · K)) is filled in place of alumina in a circular groove and a low thermal conductive ceramic filling portion are provided. A water-cooled copper mold not installed was also prepared.

In the continuous casting operation, the basicity (mass% CaO / mass% SiO ₂ ) is 1.0 to 2.0, and the viscosity at 1300 ° C. is 0.5 to 2.0 poise (0.05) as the mold powder. A mold powder (˜0.20 Pa · s) was used. The slab drawing speed was 1.5 to 2.5 m / min in the state of steady casting, and the degree of superheated molten steel in the tundish was 25 to 35 ° C. The meniscus position (metal surface position) in the mold was set to 100 mm from the upper end of the mold in a steady casting state, and the meniscus position was controlled so that the meniscus was within the installation range of the low thermal conductive ceramic filling portion.

  After continuous casting was completed, the surface of the long side of the slab was pickled to remove the scale, and the number of occurrences of surface cracks and the length of each slab were measured. The ratio (mm / m) of the total length (mm) of the surface cracks to the length (m) of the slab where the surface cracks were investigated was used as a surface crack index, and the slab surface crack was determined using this surface crack index. The occurrence situation of was evaluated.

Table 1 shows the installation conditions of the low thermal conductive ceramic filling part, the installation conditions of the filling part filled with silicon carbide, and the investigation results of the surface crack index. In the remarks column of Table 1, a test using a water-cooled copper mold within the scope of the present invention is a test using a water-cooled copper mold that does not satisfy the scope of the present invention although it has a low thermal conductive ceramic filling portion. A test using a water-cooled copper mold having a filling portion of silicon carbide and a silicon carbide filling portion is indicated as a comparative example, and a test using a water-cooling mold having no low heat conductive ceramic filling portion is indicated as a conventional example. The distance L _{0 in} Table 1 is a numerical value calculated from the equation (1).

  As shown in Table 1, in tests No. 1 to 6, the diameter (d) and the distance L of the low thermal conductive ceramic filling portion are within the scope of the present invention, and the filling thickness (H) of the low thermal conductive ceramic filling portion. And the interval (P) was within the preferred range of the present invention, no cracks occurred in the mold, and no surface cracks occurred in the slab. In other words, in Test Nos. 1 to 6, the surface crack of the slab can be greatly reduced compared to the conventional steel, such as medium carbon steel, which is prone to surface cracking without causing cracks in the mold. Was confirmed.

  In Test No. 7, the filling thickness (H) of the low thermal conductive ceramic filling portion deviates from the preferred range of the present invention, and in Test No. 8, the interval (P) of the low thermal conductive ceramic filling portion is within the preferred range of the present invention. Although other conditions were within the scope of the present invention and the preferred scope of the present invention, fine surface cracks occurred in the slab, but it was confirmed that the surface cracks can be greatly reduced compared to the conventional case. did it.

Test Nos. 9 to 13 show that the distance L to the distance L ₀ is out of the range of the present invention, test No. 14 is that the thermal conductivity of the filled ceramic is out of the range of the present invention, and test No. 15 is a low heat The diameter (d) of the conductive ceramic filling portion was out of the range of the present invention, and the surface crack of the cast piece could not be prevented.

  In Test Nos. 16 to 19, the low thermal conductive ceramic filling portion was not formed, and cracks occurred on the surface of the slab.

DESCRIPTION OF SYMBOLS 1 Mold long side copper plate 2 Circular groove 3 Low heat conductive ceramic filling part 4 Plating layer 5 Cooling water flow path 6 Back plate

Claims (4)

The inner wall surface of the water-cooled copper mold, which is a distance L ₀ or more below the meniscus from the arbitrary position above the meniscus and satisfying the following equation (1) with respect to the slab drawing speed during steady casting the range of the inner wall surface to the position, is the thermal conductivity of the ceramic to 20% or less with respect to the thermal conductivity of copper Hama charge inside the circular groove or pseudo-circular groove provided in the inner wall surface Each having a plurality of low thermal conductive ceramic filling portions each having a diameter of 2 to 10 mm or an equivalent circle diameter of 2 to 10 mm,
The filling thickness of the ceramic in the low thermal conductive ceramic filling portion is not more than the depth of the circular concave groove or the pseudo circular concave groove and not less than 0.3 mm, and is equivalent to the diameter or circle of the low thermal conductive ceramic filling portion. Satisfying the relationship of the following formula (2) with respect to the diameter,
And, a method for producing a continuous casting mold in which a plating layer is provided on an inner wall surface of a copper mold on which the low thermal conductive ceramic filling portion is formed ,
The method for manufacturing a continuous casting mold , wherein the low thermal conductive ceramic filling portion is formed by filling the ceramic into the circular concave groove or the pseudo circular concave groove by spraying means or shrink fitting means .
L ₀ = 2 × Vc × 1000/60 (1)
0.3 ≦ H ≦ 0.5 × d (2)
However, in the formula (1), L ₀ is a distance (mm) from the meniscus, Vc is a slab drawing speed (m / min) during steady casting, and in the formula (2), H is The ceramic filling thickness (mm) and d are the diameter (mm) or equivalent circle diameter (mm) of the low thermal conductive ceramic filling portion . 2. The continuous state according to claim 1, wherein an interval between the low thermal conductive ceramic filling portions satisfies a relationship of the following expression (3) with respect to a diameter or an equivalent circle diameter of the low thermal conductive ceramic filling portions. A method for producing a casting mold.
P ≧ 0.25 × d (3)
However, in the formula (3), P is the interval (mm) between the low thermal conductive ceramic filling portions, and d is the diameter (mm) or the equivalent circle diameter (mm) of the low thermal conductive ceramic filling portions. Using the continuous casting mold produced by the method for producing a continuous casting mold according to claim 1 or 2, the molten steel in the tundish is poured into the continuous casting mold to continuously cast the molten steel. A method for continuous casting of steel. The continuous casting method of steel according to claim 3 , wherein the molten steel is a medium carbon steel having a carbon content of 0.08 to 0.17 mass%.

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