JP3643460B2 - Continuous casting mold and continuous casting method - Google Patents

Description

【表２】

【００１５】
【発明の効果】
面部にスリット加工を施した鋳型を用いることで、中炭素鋼鋳造時の鋳片表面のへこみを大幅に低減することができる。これにより、電磁撹拌の推力を低減する必要がないため、気泡や介在物の捕捉防止や凝固組織の等軸晶化効果を損なうことなく、高品位の鋳片を高歩留で鋳造することができる。
また、凝固点温度の高いパウダ−を用いる必要がないため、凝固点温度の比較的低いパウダ−を使用でき、その結果、鋳造速度を高速化できる。
【図面の簡単な説明】
【図１】緩冷却度とへこみの深さの関係を示す説明図である。
【図２】スリット加工範囲とへこみの深さの関係を示す説明図である。
【図３】スリット加工条件と鋳片表面形状の関係の模式説明図である。
【図４】スリット長さとへこみ深さの関係を示す説明図である。
【図５】鋳型内表面のスリット加工の模式説明図である。
【図６】スリット巾と鋳片表面形状との関係を示す説明図である。
【符号の説明】[0001]
[Prior art]
When continuous casting of molten metal, particularly steel, is performed at a high drawing speed, there is a problem of cracking of the slab surface due to non-uniform growth of a solidified shell formed in the mold. Various solutions have been proposed so far, but unfortunately these problems are not completely resolved at present.
For example, in the field of slab casting, as in JP-A-6-304709 and JP-A-7-284880, the cause of slab surface cracking is the center of the slab width direction in the solidified shell that is generated in the mold. A method of arranging groove-like slits on the inner wall surface of the mold has been proposed as a means of dispersing strain by slow cooling, assuming that the strain concentrates on the solidified shell.
[0002]
In addition, JP-A-7-80608 and JP-A-7-284896 are similar to the above method, and the conditions for disposing the slits are specified to promote the flow of the mold lubricating flux or have different heat transfer coefficients. A method has been proposed in which materials are arranged in a lattice pattern, and conditions such as the viscosity and melting point of the mold lubricating flux are specified to achieve slow cooling.
[0003]
[Problems to be solved by the invention]
On the other hand, when continuously casting a slab having a relatively small cross-sectional area, an integrally formed water-cooled copper mold is generally used. Moreover, when casting such a small cross section, an electromagnetic stirring coil is installed in the mold to form a horizontal swirling flow to prevent trapping of inclusions / bubbles and equiaxed crystallization of the solidified structure. However, it often appears that the corner portion is swelled and the surface portion is swelled by the stirring flow on the hot water surface. When medium carbon steel is cast in such a molten steel surface level shape, a dent is likely to be formed on the surface of the slab at the swelled portion at the surface portion, and cracking occurs when the dent depth is deep.
[0004]
In general, in the casting of medium carbon steel, a powder having a high freezing point temperature is used in order to suppress the cracking. Increasing the freezing point temperature results in slow cooling, reducing the cracking rate of medium carbon steel. However, when the casting speed is increased, the amount of powder consumption tends to decrease. However, when a powder having a high freezing point temperature is used, this tendency becomes particularly remarkable, and constraining breakout is likely to occur. Therefore, it is necessary to replace the above-described slow cooling function of the powder with some means.
Therefore, an object of the present invention is to present mold conditions that can positively control the heat flux from the mold to the slab surface without depending on the powder properties.
[0005]
[Means for Solving the Problems]
According to the inventor's investigation and analysis, in casting medium carbon steel under such casting conditions, it is extremely important to prevent the vertical cracks from controlling the cooling balance between the face part and the corner part. Turned out to be. That is, it was found that since the solidification start position is higher in the corner portion than in the surface portion, a highly rigid shell is likely to be formed in the corner portion, and a dent is likely to occur in the surface portion having low solidified shell rigidity. The cause of the dent is the tensile strain generated during the δ / γ transformation, and it has been found that the dent can be suppressed by slowly cooling only the surface portion even under such a molten metal level condition.
[0006]
The present invention has been made based on this finding, the gist of which is a mold having at least one corner portion formed by two inner wall surfaces adjacent to each other among the inner wall surfaces constituting the mold. In a continuous casting apparatus for continuously obtaining a slab while supplying molten metal therein, an electromagnetic coil for causing horizontal swirling flow by applying electromagnetic force to the molten metal in the mold is embedded in the mold, slit groove-like is provided on the front surface of the face portion excluding the corner portion of the mold inner wall, further slab width range length of processing the slits (R) and the length of the surface portion of the mold inner wall (W) Is a continuous casting mold that satisfies the following formula (1) .
[0007]
R ≧ 0.7W ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1) Formula
R: Length in the slab width direction of the inner wall surface of the mold for machining the slit (m)
W: Length of the inner wall surface of the mold (m)
More preferably, the width of the slit is 0.3 mm or more and 1 mm or less, and the slit processing range in the mold longitudinal direction is higher than the powder thickness on the upper side and within 150 mm from the molten metal surface on the lower side.
Furthermore, it is more preferable to perform continuous casting using the aforementioned continuous casting mold while applying an electromagnetic force to the molten metal in the mold to cause a horizontal swirling flow.
The “corner portion” referred to here is a corner portion formed by two inner wall surfaces adjacent to each other among the inner wall surfaces constituting the mold, and has a predetermined curvature (curvature in the horizontal section of the mold). ) In the width direction of the slab processed into a curved surface. Further, the “surface portion” means a mold inner wall surface excluding the “corner portion” and a range in the slab width direction having no curvature.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In order to suppress the occurrence of dents by slowly cooling only the surface portion, a plurality of groove-like vertical slits are provided at even intervals in the surface portion, and an air gap is positively formed in the slit portion, and the slab is formed from the mold surface. Reduce the heat flux to the surface. The degree of reduction varies depending on the difference in level of the hot water surface level, but is preferably about -20% to -50% as shown in FIG. By this method, even if solidification delay occurs in the surface portion, the strain generated during the δ / γ transformation can be relieved by slow cooling, and the occurrence of dents on the slab surface can be suppressed.
[0009]
FIG. 2 shows the relationship between the slit machining range and the dent depth. If the machining range is smaller than 70%, that is, slitting is performed only at the center of the surface, the dent at the center of the surface is suppressed, but the dent occurrence position shifts to the vicinity of the corner and cracking occurs at the off-corner part. I found out that For this reason, the range in which the slit is provided is preferably 70% or more of the length of the surface portion excluding the corner portion (FIG. 3), so the following equation (1) is derived.
[0010]
R ≧ 0.7W (1) Formula R: Length in the slab width direction of the mold inner wall surface that processes the slit (m)
W: Length of the inner wall surface of the mold (m)
[0011]
Next, it is appropriate that the slit processing range in the longitudinal direction is more than the powder thickness above the molten metal surface and about 150 mm below the molten metal surface below. The reason is from the relationship between the slit length in FIG. That is, whether or not the dent is generated is almost determined at the upper part of the mold, and it can be understood that the dent generation does not change even if the cooling is performed below 150 mm. On the other hand, if the cooling is slow at the bottom, the thickness of the solidified shell becomes thin, so there is a risk that the solidified shell will be re-dissolved due to the nozzle discharge flow. .
[0012]
FIG. 5 shows a schematic diagram of slit processing. The size of the slit to be processed is preferably as narrow as possible with a width of 0.3 mm or more and 1 mm or less. If the pitch is twice the width and the depth is less than half of the width, the slit conditions should be determined according to the desired degree of slow cooling. Good. The width is set to 1 mm or less when casting a slab having a small slab cross-sectional area, it is difficult to secure a sufficient melt pool thickness of the powder, and even if the powder flow is interrupted, This is because it does not cause a difference. FIG. 6 shows the state of slit transfer onto the surface of the slab under conditions where the slit width is changed. From this, it can be seen that the transfer of the slit is very slight at 0.5 mm.
[0013]
【Example】
A billet slab having a size of 160 mm square was cast at a casting speed of 2.2 m / min. The steel type was 0.1% C steel, and the powder having the composition shown in Table 1 was used. The condition of electromagnetic stirring in the mold was constant within the range of 5 mm to 10 mm in the unevenness of the hot water surface level shape. Table 2 shows the relationship between the slit machining conditions on the mold surface and the occurrence of dents on the surface. From this, it can be understood that the dent generation on the slab surface is greatly reduced by slitting the surface portion and setting the slit processing range to 70 % or more of the length of the surface portion excluding the corner. As described above, this is considered to be due to the fact that the tensile strain generated by the rise at the surface portion, that is, the solidification delay, can be reduced by the effect of slow cooling. On the other hand, when the slow cooling effect is increased below 150 mm from the molten metal surface, the generation of dents is reduced, but conversely, the solidified shell thickness is reduced, and the shell thickness at the center of the surface is reduced. Therefore, it turns out that it is necessary to provide a slow cooling effect at the very upper part of the mold within 150 mm from the molten metal surface.
[0014]
[Table 1]

[Table 2]

[0015]
【The invention's effect】
By using a mold having slits on the surface, it is possible to greatly reduce the dent on the surface of the slab during casting of medium carbon steel. As a result, it is not necessary to reduce the thrust of electromagnetic stirring, so that it is possible to cast high-quality slabs at a high yield without impairing the trapping of bubbles and inclusions and impairing the equiaxed crystallization effect of the solidified structure. it can.
Further, since it is not necessary to use a powder having a high freezing point temperature, a powder having a relatively low freezing point temperature can be used, and as a result, the casting speed can be increased.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the relationship between the degree of slow cooling and the depth of a dent.
FIG. 2 is an explanatory diagram showing a relationship between a slit processing range and a depth of a dent.
FIG. 3 is a schematic explanatory view of a relationship between slit machining conditions and a slab surface shape.
FIG. 4 is an explanatory diagram showing a relationship between a slit length and a dent depth.
FIG. 5 is a schematic explanatory view of slit machining on the inner surface of the mold.
FIG. 6 is an explanatory diagram showing the relationship between the slit width and the slab surface shape.
[Explanation of symbols]

Claims (3)

In a continuous casting apparatus for continuously obtaining a slab while supplying molten metal into a mold having at least one corner portion formed by two inner wall surfaces adjacent to each other among inner wall surfaces constituting the mold, the mold an electromagnetic coil for causing the horizontal swirling fluidized by applying an electromagnetic force to the inner molten metal is embedded in the mold, the slit groove-like is provided on the front surface of the face portion excluding the corner portion of the mold inner wall A continuous casting mold characterized in that the relationship between the length (R) in the slab width direction for further processing the slit and the length (W) of the surface portion of the inner wall surface of the mold satisfies the following expression (1).
R ≧ 0.7W ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1) Formula
R: Length in the slab width direction of the inner wall surface of the mold for machining the slit (m)
W: Length of the inner wall surface of the mold (m)   The continuous width according to claim 1, wherein a width of the slit is 0.3 mm or more and 1 mm or less, and a slit processing range in the mold longitudinal direction is an upper portion of the powder thickness or more and a lower portion thereof is within 150 mm from the molten metal surface. Casting mold. Using a continuous casting mold according to claim 1 or 2, continuous casting method and performing continuous casting while causing horizontal swirling fluidized by applying an electromagnetic force to the template in the molten metal.

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