TW201813740A - Continuous casting method of steel capable of suppressing center segregation occurring in the thickness center of a cast piece - Google Patents

Abstract

The technological object of the present invention is to prevent surface cracking due to uneven cooling of the solidified shell at the early stage of solidification, and to suppress center segregation occurring in the center portion of the thickness of the cast piece. On an inner wall surface of a mold copper plate from a position of at least 20 mm above the meniscus to a position of at least 50 mm and at most 200 mm below the meniscus, a continuous casting mold in the present invention is provided with dissimilar-thermal-conductivity metal-filled parts which are filled with a metal of which the ratio of the thermal conductivity difference respect to the thermal conductivity of the mold copper plate is at least 20%. In addition, the area ratio which is the ratio of the sum of the areas of the dissimilar-thermal-conductivity metal-filled parts to the area of the inner wall surface provided with the dissimilar-thermal-conductivity metal-filled parts is 10% or more and 80% or less. The oscillation mark pitch (F) and the distance (D1) derived from the oscillation frequency (f) and the casting speed (Vc) satisfy the following formula (1): D1 ≤ F=Vc×1000/f, and the distance (D2) satisfies the following formula (2): D2 ≤ 4r; wherein in formula (2), r is a radius (mm) of a circle which takes a gravity center of the dissimilar-thermal-conductivity metal-filled parts as a center and has an area same as that of the dissimilar-thermal-conductivity metal-filled parts.

Description

   Continuous casting method of steel   

The present invention relates to continuous casting technology, and more particularly to a suitable continuous casting method for steel that can improve surface cracking and center segregation of a slab by suppressing uneven solidification of the slab at the initial stage of solidification.

Generally speaking, when manufacturing a steel slab by continuous casting, the molten steel injected into the mold is first brought into contact with the mold to be cooled to form a thin solidified layer (hereinafter, referred to as a "solidified metal shell" ). In this way, while the molten steel is poured into the mold, the solidified metal shell is pulled downward (hereinafter, referred to as "constant casting") to produce a cast piece.

If the cooling caused by the mold is not uniform, the thickness of the solidified metal shell will be uneven. As a result, the surface of the solidified metal shell cannot be smoothed. Especially in the initial stage of solidification, if the thickness of the solidified metal shell grows unevenly, stress concentration will be generated on the surface of the solidified metal shell, and therefore small longitudinal cracks will occur. Such tiny longitudinal cracks remain even after the slab is completely solidified, and become longitudinal cracks on the surface of the slab. If longitudinal cracks occur on the surface of the cast slab, the longitudinal cracks must be removed in advance (hereinafter referred to as "trimming") before the cast slab is sent to a subsequent process (for example, a rolling process).

The mold is vibrated in the casting direction (hereinafter, also referred to as "oscillation"). With the vibration of this mold, the upper end of the solidified metal shell is bent toward the molten steel side, and the molten steel will overflow to the bent and solidified. The gap between the metal shell and the inner wall surface of the mold will thus form a portion (hereinafter, referred to as a "projection piece") protruding toward the molten steel side on the solidified metal shell. The surface of the solidified metal shell is not smooth The gap formed by the curved solid metal shell and the inner wall surface of the mold will become larger, and the protruding piece of the solid metal shell will increase accordingly. If the protruding piece protruding toward the molten steel side is large, the bending In the liquid surface (molten steel soup noodles in the mold), the non-metallic inclusions and bubbles floating in the molten steel will be captured by the protruding piece. The captured non-metallic inclusions and bubbles will become: Causes of surface defects such as surface flaws and bulges are formed in the rolled steel sheet or in the cold rolled steel sheet.

The frequency of such surface cracks such as longitudinal cracks, flaws, and bulges tends to increase with increasing casting speed. At present, the casting speed of a general continuous casting machine for steel slabs is about 1.5 to 2 times higher than that of 10 years ago, so the maintenance work is also increased. In recent years, even in the direct-feed heating (ie, hot charge) and the direct-feed rolling (ie, direct charge) that have been technically established, the maintenance work of slabs has become one of the factors preventing the stabilization of the work. Therefore, it would be extremely economically advantageous to prevent uneven growth of the solidified metal shell thickness and uneven tabs due to uneven cooling in the initial stage of solidification.

In order to prevent uneven cooling in the initial stage of solidification, it is necessary to perform uniform and gentle cooling in the initial stage of solidification so that the thickness of the solidified metal shell grows uniformly and uniformly to prevent the generation of protruding pieces. In this regard, Non-Patent Document 1 describes that in continuous casting of a 280 × 280 mm steel blank, in order to improve the surface properties of the slab, it is effective to form irregularities on the inner surface of the mold. In Patent Document 1, it is described that recesses having a diameter or width of 3 to 80 mm and a depth of 0.1 to 1.0 mm are provided on the inner surface of the mold. In addition, Patent Document 2 describes that a groove having a width of 0.2 to 2 mm and a depth of 6 mm or less is provided on the inner surface of the mold.

These techniques are as follows: the mold powder is poured into the meniscus surface, and a sufficient thickness of the mold powder layer is stably maintained for a long time in the gap between the mold and the solidified metal shell, and is formed in the uneven portion provided on the inner surface of the mold The air layer and the molten mold powder layer use the heat insulation properties of the air layer and the molten mold powder layer to achieve gentle cooling (hereinafter referred to as slow cooling).

However, if these technologies are actually used in continuous casting, various problems will occur. For example, the mold of a continuous casting machine with a changeable width is a mold composed of a long side and a short side. Therefore, when continuous casting is started, if the recess on the inner surface of the mold and the corner of the mold are set, If the positions are the same, the problem of splashing of molten steel at the beginning of casting and splashing into the recesses of the corners will occur.

When the immersion nozzle is replaced or when the casting tank is replaced, the molten steel soup noodle in the mold is lower than the normal casting state. Therefore, the mold powder originally fixed on the inner surface of the mold is likely to peel off. , The phenomenon of shedding, so when the casting is started again, the problem of molten steel or molten steel splashing into the recess of the corners will occur.

This phenomenon of molten steel running into the recess will cause the solidified metal shell to occur: the cause of restrictive casting leakage.

The mechanism of the occurrence of segregation in the center of the slab is considered to be the following reason. With the progress of solidification, segregation components will be concentrated in the solidified structure, that is, between dendritic crystals. When the molten steel with the thickened segregation component solidifies, it will flow out between dendritic crystals due to shrinkage of the slab or expansion of the slab called bulging. The molten steel after the concentrated segregation component that flows out will flow toward the final solidification part, that is, the solidification end point, and solidify while maintaining this state to form a thickened zone of the segregation component. This thickening zone is the central segregation. The effective countermeasures to prevent segregation in the center of the slab are to prevent the movement of the molten steel after the concentration of segregation components existing between dendritic crystals, and to prevent the local accumulation of molten steel after the concentration of segregation components. Some people have proposed technical solutions of several methods using these principles.

One of the methods is to use a roll group to lightly roll the slab. However, if only light rolling is used which is slightly larger than the solidification shrinkage, there is still a limit to the improvement of center segregation. . The technical solution disclosed in Patent Document 3 is to swell the slab at a position where the solid phase rate of the central portion of the slab is 0.1 or less, so that the thickness of the slab in the central portion in the width direction is greater than that in the mold After the thickness of the slab in the short side portion is further increased by 20 to 100 mm, immediately before the solidification end point is reached, at least one pair of rolls is used, and the reduction amount of each pair of rolls is set to 20 mm or more. Rolling in order to achieve a rolling amount equivalent to the amount of swelling.

The technical solution disclosed in Patent Document 4 is to swell the thickness of the slab at the center in the width direction to 10 to the thickness of the slab at the short side during the period before the thickness of the unsolidified portion of the slab reaches 30 mm After the thickness of 50%, at least one pair of rolls is used for rolling to reach a reduction amount equivalent to the bulging amount, until the end of solidification.

In the technical solution disclosed in Patent Document 5, the slab is first bulged to a bulge amount of 3% or more and 25% or less of the slab thickness at the beginning of the bulge, and then the solid phase ratio to the central portion is 0.2 or more and 0.7 or less. A continuous casting method in which steel is rolled at a position of a cast slab to achieve a rolling reduction of a thickness corresponding to a thickness of 30% to 70%.

    [Prior technical literature]         [Patent Literature]    

Patent Document 1: Japanese Patent Application Laid-Open No. 9-94634

Patent Document 2: Japanese Patent Application Laid-Open No. 10-193041

Patent Document 3: Japanese Patent Application Laid-Open No. 7-210382

Patent Document 4: Japanese Patent Application Laid-Open No. 9-206903

Patent Document 5: Japanese Patent Application Laid-Open No. 11-99285

    [Non-patent literature]    

Non-Patent Literature 1: P. Perminov et al, Steel in English, (1968) No. 7.p. 560 ~ 562.

The technical problem of the methods proposed in Patent Documents 3 and 4 is that when the solid phase ratio of the central portion during rolling is not appropriate, or although the solid phase ratio of the central portion during rolling is Appropriate conditions, but if the rolling reduction is not appropriate, there may be problems such as internal segregation at the center of the thickness of the slab and positive segregation near the center. The technical problem of the method proposed in Patent Document 5 is that when the slab is swelled swelled or the swelled amount is too large, internal split defects will occur due to deformation, and if the swelling starts too early, There is a possibility of casting leakage. In addition, depending on the shape of the slab that has been swollen before rolling, sometimes there is a problem that the proper rolling cannot be transmitted to the thickness center of the slab, and the problem of center segregation cannot be improved.

In continuous casting of steel, vertical vibration is applied to a mold, and this vibration is used to prevent a solid metal shell from adhering to the mold. On the surface of the slab that is deformed at the front end portion due to the vibration of the mold, periodic irregularities called "shock marks" are formed. If the unevenness of the shock mark is too large, the contact between the surface of the solidified metal shell and the mold becomes uneven, the cooling amount from the mold becomes uneven, and the unevenness on the inner surface of the solidified metal shell becomes large. If the unevenness of the inner surface of the initial solidified metal shell is too large, the solidified interface in the final solidified part will become uneven, and the effect of light rolling will not be fully obtained, and the center segregation of the slab tends to deteriorate. problem.

In order to solve the above technical problems, the gist of the present invention is as follows.

[1] A continuous casting method of steel, in which molten steel is poured into a continuous casting mold, the aforementioned continuous casting mold is vibrated in the casting direction, and the aforementioned molten steel is pulled out to manufacture the steel of the slab. The continuous casting method, a continuous casting mold, has a plurality of grooves provided on the inner wall surface of the mold copper plate. The mold copper plate is located at a position at least 20 mm above the meniscus in a steady casting state so far. At least 50mm and at most 200mm below the surface; inside the plurality of grooves, a plurality of heterothermally conductive metal filling portions are provided, which are filled: the thermal conductivity difference from the thermal conductivity of the copper plate of the mold The ratio of the values is 20% or more of the metal or metal alloy, and the sum of the areas of all the heterothermally conductive metal filling portions to the area of the inner wall surface provided with the plurality of heterothermally conductive metal filling portions, that is, the area ratio is 10 Above 80% and below 80%; the shock mark distance (F) and distance (D1) derived from the oscillation frequency (f) and casting speed (Vc) are in accordance with the following formula (1) System, and the distance (D2) is consistent with the relationship of the following Equation (2), D1 ≦ F = Vc × 1000 / f. ． ． Equation (1)

D2 ≦ 4r. ． ． Equation (2)

Here, in the formula (1), Vc is the casting speed (m / min), f is the oscillation frequency (cpm), F is the distance between shock marks (mm), and D1 is set from: its center of gravity and a plurality of The center of gravity of one of the different heat-conducting metal filling portions is the other one of the heat-conducting metal filling portions at the same position in the width direction of the mold copper plate, and the other one is adjacent to the other one of the heat-conducting metal filling portions in the casting direction. The distance (mm) from the boundary line between one heterothermal conductive metal filling portion and the mold copper plate to the boundary line between the one different thermal conductive metal filling portion and the mold copper plate, in Equation (2), r is The center of gravity of the heterothermally conductive metal filling part is centered, and is a radius (mm) of a circle having the same area as the area of the aforementioned heterothermally conductive metal filling part, and D2 is set from: the center of gravity of the heterothermally conductive metal filling part is The other heterothermally conductive metal filling portion at the same position in the casting direction is the center of gravity of the other heterothermally conductive metal filling portion adjacent to the aforementioned one of the different heat conductive metal filling portions in the width direction. The distance to a center of gravity of different heat the metal filling portion (mm) conductive.

[2] The continuous casting method for steel according to the above [1], wherein the plurality of heterothermally conductive metal filling portions are provided such that the distance (D1) conforms to a relationship of the following formula (3).

D1 ≦ 2r. ． ． Equation (3).

[3] The continuous casting method for steel according to the above [1] or [2], wherein the shapes of the plurality of grooves are all the same.

[4] A continuous casting method of steel according to any one of the aforementioned [1] to [3], wherein the shape of the plurality of grooves is circular or a pseudo-circular shape without corners.

[5] The continuous casting method for steel according to any one of the above [1] to [4], wherein the plurality of heterothermally conductive metal filling portions are provided in a grid shape.

[6] The continuous casting method for steel according to any one of the above [1] to [4], wherein the plurality of heterothermally conductive metal filling portions are provided in a staggered shape.

[7] The continuous casting method for steel according to any one of the above [1] to [6], wherein the roller opening of a plurality of pairs of slab support rollers provided in the continuous casting machine is directed toward the casting direction On the downstream side of the slab, stepwise increase is made so that the long side surface of the slab with an unsolidified layer inside is relative to the slab thickness at the exit of the mold (thickness between the long side surfaces of the slab) is greater than 0mm And the total bulging amount in the range of 20 mm or less is enlarged, and then the roll opening of the plurality of pairs of slab support rolls is directed to the downstream side in the casting direction, and a lightly rolled area is gradually reduced from the slab. From the point at which the solid phase rate at the center of the thickness is at least 0.2 to the point at which it becomes 0.9, based on the product of the rolling speed (mm / min) and the casting speed (m / min) (mm · m / min ² ) The rolling force corresponding to 0.30 or more and 1.00 or less is applied to the long side surface of the slab, and the rolling force is used to achieve a total reduction amount equal to the total expansion amount or a total reduction amount less than the total expansion amount. The long side surface of the slab is rolled.

Here, the “gravity center of the aforementioned heterothermally conductive metal filling portion” refers to the gravity center of the cross-sectional shape of the heterothermally conductive metal filling portion on the molten steel side plane of the mold copper plate.

According to the present invention, a plurality of heterothermally conductive metal filling portions are provided in the width direction and the casting direction of the continuous casting mold beside the meniscus including the meniscus position, so that the width of the mold beside the meniscus can be made. The thermal resistance of the continuous casting mold in the direction and the casting direction is periodically increased or decreased. In this way, beside the meniscus, in other words, the heat flux transmitted from the solidified metal shell to the continuous casting mold at the initial stage of solidification will increase or decrease periodically. Because of the periodic increase and decrease of this heat flux, the stress and thermal stress caused by the transformation from δ iron to γ iron will decrease, and the deformation of the solidified metal shell due to these stresses will become smaller. Because the deformation of the solidified metal shell becomes smaller, the non-uniform heat flux distribution due to the deformation of the solidified metal shell is also uniformized, and the stress that occurs is also dispersed so that the amount of each deformation is reduced. As a result, the surface of the solidified metal shell can be prevented from cracking.

It can make a part of the heat flow beam increase or decrease at least once between the pitches of the shock marks, and can make the depth of the shock marks shallower, and make the surface of the solidified metal shell uniform. In this way, the inner surface of the solidified metal shell that grows along with the surface also tends to be uniform, so that the solidified interface at the final solidified portion is smoothed, the nodes forming segregation can be reduced, and the internal quality of the steel slab can be improved.

1‧‧‧Steel embryo continuous casting machine

2‧‧‧ casting trough

3‧‧‧ sliding nozzle

4‧‧‧ immersion nozzle

5‧‧‧mould

5a‧‧‧ long copper plate

6‧‧‧ backup roller

7‧‧‧Guide roller

8‧‧‧ pinch roller

9‧‧‧ transport roller

10‧‧‧ Slab Cutting Machine

11‧‧‧ molten steel

12‧‧‧ Cast

12a‧‧‧steel slab

13‧‧‧Solid metal shell

14‧‧‧ unsolidified part

15‧‧‧ End of solidification

16‧‧‧ Forced inflation area

17‧‧‧ light rolling area

18‧‧‧ meniscus position

19‧‧‧Different heat conduction metal filling section

19a‧‧‧ One of the different heat conduction metal filling parts

19b‧‧‧Other hetero-thermal conductive metal filling section

19c‧‧‧Other heterothermal conductive metal filling section

20‧‧‧Different heat conduction metal filling section

20a‧‧‧One of the different heat conduction metal filling sections

20b‧‧‧Other heterothermal conductive metal filling section

20c‧‧‧Other hetero-thermal conductive metal filling parts

[Fig. 1] A schematic side view of a vertical deflection continuous casting machine to which the continuous casting method of steel of the present embodiment can be applied.

[FIG. 2] A diagram showing an example of the opening degree profile of a roll.

[Fig. 3] A schematic side view of a mold long-side copper plate constituting a part of a mold provided in a steel blank continuous casting machine.

[Fig. 4] The thermal impedance of three places of a long-side copper plate of a mold having a heterothermally conductive metal filling portion formed by filling a metal having a lower thermal conductivity than that of a mold copper plate, corresponding to the different thermally conductive metal The position of the filling section is shown conceptually.

5 is a diagram showing an example of the shape of a groove.

Fig. 6 is a partially enlarged view of a region where a heterothermally conductive metal filling portion is provided.

[Fig. 7] Fig. 7 is a diagram showing another example of the arrangement of the heterothermally conductive metal filling portion.

Here is a drawing to illustrate the specific implementation method of the present invention. FIG. 1 is a schematic side view of a vertical deflection continuous casting machine to which the continuous casting method of steel of the present embodiment can be applied.

The steel slab continuous casting machine 1 is provided with a casting mold 5 for injecting molten steel 11 to solidify it to form a shell shape of a casting slab 12 and vibrating the casting slab 12 in the casting direction.

A predetermined position above this mold 5 is provided with a casting tank 2 capable of relaying the molten steel 11 supplied from an unillustrated hopper to the mold 5. Below the mold 5, a plurality of pairs of cast-sheet support rollers composed of a support roller 6, a guide roller 7, and a pinch roller 8 are provided. The pinch roller 8 is a driving roller that can be used to support the cast slab 12 and to pull the cast slab 12 at the same time. The gap between the slab support rollers adjacent in the casting direction is constituted by a secondary cooling zone provided with a water spray nozzle or a spray nozzle (not shown) such as a water mist sprayer. The cooling water (hereinafter, also referred to as "secondary cooling water") sprayed by the mist nozzle, the slab 12 is cooled while being pulled, so the internal non-solidified portion 14 gradually decreases, and the solidified metal shell 13 gradually grows. Casting. A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 is provided at the bottom of the casting tank 2. A dipping nozzle 4 is provided below the sliding nozzle 3.

On the downstream side of the slab support rolls, a plurality of conveyance rolls 9 for conveying the cast slabs 12 are provided. Above the conveyance rolls 9 are provided for cutting from the cast slabs 12. A slab cutter 10 that cuts out a steel slab 12a of a predetermined length. In the casting direction including the solidification end position 15 of the slab 12, a light rolling area 17 composed of a plurality of pairs of guide roller groups is provided. These plurality of pairs of guide roller groups are It is set such that the roller interval of the guide rollers 7 facing each other is gradually narrowed gradually toward the downstream side in the casting direction, in other words, the roller interval has a slope.

This light rolling area 17 can be used for light rolling the slab 12 in the entire area or only in a selected local area. In this embodiment, the light-rolled region 17 is provided so that the solid-phase ratio of the thickness center portion of the slab 12 is at least 0.2 to 0.9 and enters this light-rolled region 17 Within the setting range.

The gradient of the shrinkage amount of the light rolling area 17 is expressed by the reduction of the roll opening per 1 meter in the casting direction, that is, "mm / m". The slab 12 of the light rolling area 17 The rolling speed (mm / min) is obtained by multiplying the reduction gradient (mm / m) and the casting speed (m / min). A mist nozzle for cooling the slab 12 is also arranged between the slab support rolls constituting the light rolling region 17. Although FIG. 1 shows an example in which only the guide roller 7 is disposed in the light rolling region 17, the pinch roller 8 may be disposed in the light rolling region 17. The slab support roll arranged in the light rolling area 17 is also called a "roller".

The opening degree of the guide roller 7 arranged between the lower end of the mold 5 and the liquidus solidification end position of the slab 12 is toward the downstream side of the casting direction, and one guide roller or several guide rollers are used at a time. Sequentially increase the roller opening degree until the expansion amount of the roller opening degree reaches a predetermined value. These guide rollers 7 constitute a forced bulging region 16 that forcibly bulges the long side surface of the slab 12 in which the unsolidified portion 14 still exists. The roll opening of the slab support roller on the downstream side of the forced swell region 16 is reduced to a certain value or a degree corresponding to the amount of shrinkage caused by the decrease in the temperature of the slab 12, and then it is connected to the light Rolled area 17.

FIG. 2 is a diagram showing an example of a profile of a roll opening degree. In the contour shown in FIG. 2, in the forced bulging region 16, the long side of the slab is forcibly swelled by using the static pressure of molten steel to increase the thickness of the central portion of the long side of the slab (area b). On the downstream side after the bulging area 16, the roll opening degree is reduced to a certain value or to a degree corresponding to the amount of shrinkage caused by the decrease in the temperature of the slab 12 (area c). The long side of the slab is rolled (area d).

The areas a and e in FIG. 2 are areas in which the degree of roll opening is reduced to a degree corresponding to the amount of shrinkage caused by the temperature drop of the slab 12. A ′ in FIG. 2 shows that it is not implemented: a method for casting lightly rolled by reducing the roll opening degree to a degree corresponding to the amount of shrinkage caused by the temperature drop of the slab 12 (that is, An example of roll opening in the traditional casting method).

In the forced bulging region 16, the roll opening of the guide roller 7 is sequentially expanded toward the downstream side in the casting direction. In this way, the long side surface other than the short side of the cast piece 12 will receive the unsolidified portion 14. The effect of the static pressure of the molten steel produced is forcibly inflated in accordance with the degree of roll opening of the guide roller 7. The short side of the long side of the slab is held at the short side of the slab after solidification, so the thickness at the time when the forced inflation starts is maintained. In this way, the slab 12 is only forced The bulging is performed in a characteristic manner, and the bulging portion on the long side of the cast piece contacts the guide roller 7.

Fig. 3 is a schematic side view of a mold long-side copper plate constituting a part of a mold provided in a steel blank continuous casting machine. The mold 5 shown in FIG. 3 is an example of a mold for continuous casting for casting a steel slab. The mold 5 is configured by combining a pair of mold long-side copper plates 5a (hereinafter, also referred to as “mold copper plates”) and a pair of mold short-side copper plates. Fig. 3 shows only the long side copper plate 5a of the mold. Similarly to the mold long-side copper plate 5a, the mold short-side copper plate is provided with a different heat conduction metal filling portion 19 on the inner wall surface side, and the description of the mold short-side copper plate is omitted here. However, in the slab 12, if the shape of the steel slab is such that the ratio of the width to the thickness is extremely large, stress concentration is likely to occur in the solidified metal shell 13 on the long side surface of the slab. The sides are prone to surface cracking. Therefore, it is not necessary to provide the hetero-thermal conductive metal filling portion 19 on the short-side copper plate of the mold 5 of the mold 5 for the steel slab.

As shown in FIG. 3, the position is at least 50 mm above the meniscus position 18 at least 20 mm above the meniscus position 18 during the normal casting of the long side copper plate 5a of the mold and at least 50 mm below the meniscus position 18 and A range of the inner wall surface up to a position of R of 200 mm or less is provided in a staggered manner within a range of the length W in the mold width direction: a circular hetero-thermal conductive metal filling portion 19 is filled with respect to: A heterothermally conductive metal or metal alloy (hereinafter referred to as "heterothermally conductive metal") in which the ratio of the thermal conductivity difference of the thermal conductivity of the mold copper plate 5a is 20% or more. The "meniscus" here means "the molten steel soup noodle in the mold".

The heterothermally conductive metal filling portion 19 is formed by filling differently thermally conductive metals with different thermal conductivity from the thermal conductivity of the copper alloy constituting the mold copper plate in the circular grooves that are independently processed on the inner wall surface side of the mold copper plate. .

The technical means for filling the inside of the circular groove with a different thermal conductivity metal having a thermal conductivity different from that of the copper alloy constituting the copper plate of the casting mold is preferably an electroplating treatment or a spraying treatment. Although it is also possible to fill in the heteroconducting metal processed in accordance with the shape of the circular groove into the circular groove, in this way, sometimes the heteroconducting metal and the mold copper plate are filled. There is a gap or crack between them. If there is a gap or crack between the heterothermally conductive metal and the mold copper plate, cracking or peeling of the heterothermally conductive metal will occur, which will reduce the life of the mold, cause slab cracks, and even cause constrained casting leakage. Not appropriate. Therefore, by using a plating process or a spraying process to fill a heterothermally conductive metal, this problem can be prevented in advance.

In this embodiment, a copper alloy used as a mold copper plate may be a copper alloy to which a trace amount of elements such as chromium (Cr) or zirconium (Zr) is added, which is used as a general continuous casting mold. In recent years, in order to uniformize the solidification in the mold or to prevent inclusions in the molten steel from being captured by the solidified metal shell, an electromagnetic stirring device for stirring the molten steel in the mold is generally provided. The attenuation of the magnetic field applied to the molten steel by the electromagnetic coil can also be used: copper alloy with reduced conductivity. In this case, due to the decrease in electrical conductivity, the thermal conductivity also decreases. Therefore, the thermal conductivity of the mold copper plate is only about 1/2 of that of pure copper (thermal conductivity: about 400 W / (m × K)). Generally, the thermal conductivity of a copper alloy used as a mold copper plate is lower than that of pure copper.

FIG. 4 shows the thermal resistances at three positions of the long copper plate of the mold with the heterothermal conductive metal filling portion formed by filling the metal with a lower thermal conductivity than the mold copper plate, and corresponding to the heat resistance of the heterothermal conductive metal filling portion. Diagram showing position conceptually. As shown in FIG. 4, the thermal resistance at the installation position of the heterothermally conductive metal filling portion 19 is relatively increased.

As shown in FIG. 4, when the plurality of heterothermally conductive metal filling portions 19 are provided in the width direction and the casting direction of the continuous casting mold beside the meniscus including the meniscus position 18, as shown in FIG. 4, The distribution of the thermal resistance of the continuous casting mold in the width direction of the mold next to the meniscus and in the casting direction is periodically increased or decreased. In this way, beside the meniscus, in other words, the heat flux transmitted from the solidified metal shell to the mold for continuous casting at the initial stage of solidification can form a cyclic distribution.

If the heterothermally conductive metal filling portion 19 is formed by filling a metal having a higher thermal conductivity than the mold copper plate, the thermal impedance of the position where the heterothermally conductive metal filling portion 19 is installed is different from that of FIG. 4. However, in this case, the thermal resistance of the continuous casting mold in the width direction of the mold beside the meniscus and in the casting direction can be increased or decreased in the same manner as described above. In order to be able to form the above-mentioned periodic distribution of the thermal resistance, it is preferable to form the heterothermally conductive metal filling portions 19 separately from each other.

With the periodic increase and decrease of this heat flux, the stress and thermal stress generated by the phase transition of the solidified metal shell 13 (for example, from δ iron to γ iron) will be reduced, because of these stresses. The deformation of the solidified metal shell 13 becomes smaller accordingly. Because the deformation of the solidified metal shell 13 becomes smaller, the uneven heat flux distribution due to the deformation of the solidified metal shell 13 will tend to be uniform, and the stresses that occur will be dispersed to make the respective deformation amounts smaller. As a result, the problem of surface cracking on the surface of the solidified metal shell can be suppressed.

With the periodic increase and decrease of the heat flux at the initial stage of solidification, the thickness of the solidified metal shell 13 in the mold tends to be uniform (unified) not only in the width direction of the slab but also in the casting direction. . Because the thickness of the solidified metal shell 13 in the mold tends to be uniform, the solidified interface of the solidified metal shell 13 of the slab 12 after being pulled out from the mold 5 is even at the final solidified part of the slab. Both the width direction and the casting direction remain smooth.

However, in order to obtain these effects stably, the periodic increase and decrease of the heat flux caused by the provision of the heterothermally conductive metal filling portion 19 must be adjusted. In other words, if the difference between the periodic increase and decrease of the heat flux is too small, the effect of providing the heterothermal conductive metal filling portion 19 cannot be obtained. On the contrary, if the difference between the periodic increase and decrease of the heat flux is too large, The resulting stress will become too great, resulting in surface cracking due to this stress.

The difference between the increase and decrease of the heat flux caused by the provision of the heterothermally conductive metal filling portion 19 depends on: the difference in thermal conductivity between the mold copper plate and the heterothermally conductive metal; and the total area of all the heterothermally conductive metal filling portions 19 The area ratio is the ratio of the area of the inner wall surface of the mold copper plate in the area having the heterothermally conductive metal filling portion 19.

The mold copper plate used in the continuous casting method of steel according to this embodiment is used: when the thermal conductivity of the heterothermal conductive metal filled in the circular groove is λ _m , the thermal conductivity of the heterothermal conductive metal (λ _m ) is A metal or metal alloy whose ratio ((| λ _c -λ _m | / λ _c ) × 100) of the difference in thermal conductivity (λ _c ) of the copper plate is 20% or more. By using a metal or metal alloy having a ratio of a difference in thermal conductivity (λ _c ) of a copper alloy constituting a mold copper plate of 20% or more, the effect of the periodic fluctuation of the heat flux due to the heterothermally conductive metal filling portion 19 This is sufficient, and even in high-speed casting where slab surface cracking is likely to occur, or medium carbon steel casting, the effect of suppressing the surface cracking of the slab can be sufficiently obtained. The thermal conductivity of the mold copper plate and the thermal conductivity of the different thermally conductive metal are the thermal conductivity at room temperature (about 20 ° C). Generally speaking, the higher the temperature becomes, the smaller the thermal conductivity becomes. If the ratio of the difference in thermal conductivity between different thermal conductive metals and the thermal conductivity of the mold copper plate at normal temperature is 20% or more, Under the conditions of the temperature (approximately 200 to 350 ° C.) at the time of use of the casting mold, the thermal impedance of the portion where the heterothermal conductive metal filling portion 19 is provided and the thermal impedance of the portion where the heterothermal conductive metal filling portion 19 is not provided may be used. There is a difference between them.

The mold copper plate used in the continuous casting method of steel according to the present embodiment is such that the area A (A = (Q + R) × W of the inner wall surface of the mold copper plate within the range where the heterothermally conductive metal filling portion 19 is formed, unit: (ratio mm ²⁾ of which is the area ratio ε (ε = (B / a ) sum of the areas B 19 metal filling portions mm ²⁾ of all of the different heat conductive × 100) falls 10% or more and 80% or less Within the range, a heterothermally conductive metal filling portion 19 is provided. By setting such an area ratio ε to 10% or more, the area occupied by the different heat conduction metal filling portions 19 with different heat fluxes can be ensured, and the difference in heat flux between the different heat conduction metal filling portions 19 and the mold copper plate can be obtained. The effect of suppressing the surface cracking of the slab. On the other hand, if the area ratio ε is higher than 80%, there are too many parts of the heterothermally conductive metal filling portion 19 and the fluctuation period of the heat flux is too long. Therefore, it is difficult to obtain the effect of suppressing the surface cracking of the slab.

Therefore, it is desirable to provide the heterothermally conductive metal filling portion 19 so that the area ratio ε falls within a range of 30% to 60%, and to reduce the area ratio ε to 40% to 50%. It is better to set the hetero-thermal-conductive metal filling portion 19 within a range of 5%.

The heterothermally conductive metal is not particularly limited as long as it is a heterothermally conductive metal having a ratio of a difference in the thermal conductivity (λ _m ) of the filler metal with respect to the thermal conductivity (λ _c ) of the mold copper plate of 20% or more. If you list several metals that can be used as filler metals for reference, for example: pure nickel (Ni, thermal conductivity: 90W / (m × K)), pure chromium (Cr, thermal conductivity: 67W / (m × K )), Pure cobalt (Co, thermal conductivity: 70 W / (m × K)), alloys containing these metals, and the like are applicable. These kinds of pure metals and alloys have lower thermal conductivity than copper alloys, and can be easily filled into circular grooves by electroplating or spraying. Pure copper, which has a higher thermal conductivity than copper alloys, can also be used as the metal filling the circular grooves. For example, if pure copper is used as the filler metal, the thermal impedance of the portion where the heterothermal conductive metal filler portion 19 is provided is smaller than the portion of the mold copper plate.

FIG. 5 is a diagram showing an example of the shape of the groove. In FIGS. 3 and 4, although the shape of the groove is an example of a circle as shown in FIG. 5 (a), the groove may not be circular. For example: the groove can also be an ellipse as shown in Figure 5 (b); it can also be a square or rectangle with rounded corners as shown in Figure 5 (c); or it can be as shown in Figure 5 (d) Donut shape shown. It can also be a triangle as shown in Figure 5 (e); it can also be a trapezoid as shown in Figure 5 (f); it can also be a pentagon as shown in Figure 5 (g); The star-shaped sugar shape shown in Figure 5 (h). In these grooves, a different heat conductive metal filling portion having a shape corresponding to the shape of the groove is provided.

The shape of the groove is preferably a circle as shown in Fig. 5 (a) or a shape without "corners" as shown in Figs. 5 (b) to 5 (d), but also It can be such a shape with "corners" as shown in Figs. 5 (e) to 5 (h). If the shape of the groove is not a "corner" shape, the boundary interface between the heterothermal conductive metal and the mold copper plate is curved, and the stress is not easy to concentrate on the boundary interface, and the surface of the mold copper plate is unlikely to crack.

In this embodiment, among the shapes of these grooves, for example, the non-circular shapes shown in FIGS. 5 (b) to 5 (h) are referred to as "quasi-circular". When the shape of the groove is approximately circular, the groove processed on the inner wall surface of the mold copper plate is referred to as a "quasi-circular groove." The quasi-circular radius is evaluated by the radius of a circle having the same area as the quasi-circular area, that is, the "equivalent circle radius r". The quasi-circular equivalent circle radius r is calculated according to the following formula (4).

Equivalent circle radius r = (S _ma / π) ^1/2 . ． ． Equation (4)

In Equation (4), S _ma is an area (mm ² ) of a pseudo-circular groove.

FIG. 6 is a partially enlarged view of a region where a heterothermally conductive metal filling portion is provided. As shown in FIG. 6, in the mold copper plate of the present embodiment, the circular hetero-thermal conductive metal filling portions 19 are provided in a staggered shape. The term “staggered” as used herein means that the heterothermally conductive metal filling portion 19 is provided so as to be located at positions that are offset by half a pitch from each other.

In Fig. 6, 19a is regarded as one of the heterothermally conductive metal filling portions, and 19b is regarded as the other heterothermally conductive metal filling portion. The center of gravity of the heterothermally conductive metal filling portion 19a and the heterothermally conductive metal filling portion 19b are disposed at the same position in the width direction of the mold copper plate, and are disposed adjacent to each other in the casting direction. The “center of gravity of the heterothermally conductive metal filling portion 19” as referred to herein means the center of gravity of the cross-sectional shape of the heterothermally conductive metal filling portion 19 in the molten steel side plane of the mold copper plate.

If the distance from the boundary line between the heat transfer metal filling portion 19a and the mold copper plate in the casting direction to the boundary line between the heat transfer metal filling portion 19b and the mold copper plate is regarded as D1 (mm), then The distance D1 may be provided on the inner wall surface of the mold copper plate in such a manner that the relationship of the following formula (1) is satisfied.

D1 ≦ F = Vc × 1000 / f. ． ． Equation (1)

In Equation (1), Vc is the casting speed (m / min); f is the oscillation frequency (cpm); and F is the pitch of the shock marks (mm).

In this way, the distance between the heterothermally conductive metal filling portion 19 in the casting direction and the boundary line of the mold copper plate (that is, the distance between the heterothermally conductive metal filling portions 19 in the casting direction) is compared with the shock mark. The method of setting the pitch to a smaller interval allows the heterothermally conductive metal filling portion 19 to be provided on a mold copper plate. In this way, there can be at least one increase and decrease of the heat flux between one pitch of the shock mark, and the protruding pieces generated when the shock mark is formed can be deliberately cooled by using a short pitch. However, the uneven heat flux that causes the deformation of the protruding pieces may be uniformized, so that the amount of deformation of each protruding piece becomes smaller. As a result, the falling of the protruding piece can be reduced, and the depth of the shock mark can be made shallow, and the thickness of the solidified metal shell 13 in the casting direction can be made uniform. Since the thickness of the initial solidified metal shell 13 can be made uniform, the solidified interface at the final solidified portion forming the central segregation also tends to be smooth. As a result, the number of nodes that form segregation is reduced, and the internal quality can be improved. Since the depth of the shock marks becomes shallower, it is also possible to reduce the transverse cracks starting from the shock marks.

The heterothermally conductive metal filling portion 19 is provided on the inner wall surface of the long copper plate 5a of the mold so that the distance D1 can satisfy the relationship of the following formula (3).

D1 ≦ 2r. ． ． Equation (3)

In the formula (3), r is the radius (mm) or the equivalent circle radius (mm) of the heterothermally conductive metal filling portion 19.

The heterothermally conductive metal filling portion 19 is provided on the mold copper plate so that the interval between the heterothermally conductive metal filling portions 19 in the casting direction falls within a range of twice or less the radius of the heterothermally conductive metal filling portion 19 or the equivalent circle radius. . In this way, the difference in the heat flux can be caused without any omission in the casting direction, and the heat flux flowing from the solidified metal shell to the continuous casting mold in the initial stage of solidification can be periodically increased and decreased, and the deformation can be reduced. .

In Fig. 6, 19a is regarded as one of the heterothermally conductive metal filling portions, and 19c is regarded as the other heterothermally conductive metal filling portion. The heat transfer metal filling portion 19a and the heat transfer metal filling portion 19c have their centers of gravity set at the same position with respect to the casting direction, and the two are set adjacent to each other in the width direction of the mold copper plate. Here, if the distance from the center of gravity of the heterothermally conductive metal filling portion 19a to the center of gravity of the differently thermally conductive metal filling portion 19c is regarded as D2 (mm), the distance D2 corresponds to the following equation (2): In other words, the heterothermally conductive metal filling portion 19 is provided on the inner wall surface of the mold long-side copper plate 5a.

D2 ≦ 4r. ． ． Equation (2)

In Equation (2), r is the radius (mm) or the equivalent circle radius (mm) of the heterothermally conductive metal filling portion 19.

The distance from the center of gravity of the heat transfer metal filling portion 19a to the center of gravity of the heat transfer metal filling portion 19c falls within a range of 4 times or less the radius of the heat transfer metal filling portion 19 so that the heat transfer metal is separated from the heat transfer metal. The filling portion 19 is provided on a mold copper plate. In this way, the portion of the heat flux generated by the heterothermal conductive metal filling portion 19 can be increased or decreased at a shorter pitch than the period of the solidification swing space at the front end portion of the solidified metal shell that undergoes uneven solidification. Existence can reduce the deformation of the solidified metal shell 13 at the initial stage of solidification, and the amount of each deformation becomes small, so that cracks on the surface of the solidified metal shell can be reduced.

FIG. 7 is a diagram showing another example of the arrangement of the heterothermally conductive metal filling portion. In FIG. 7, the circular heterothermally conductive metal filling portion 20 is provided on the inner wall surface of the mold copper plate in a grid pattern. The “setting of the heterothermally conductive metal filling portion 20 in a grid shape” as used herein refers to the position where the heterothermally conductive metal filling portion 20 is provided, which is located at a constant width in the casting direction and parallel to the width direction of the mold The group of parallel lines; the width in the mold width direction is constant, and is the position of the intersection of the group of parallel lines parallel to the casting direction.

In Fig. 7, 20a is regarded as one of the different heat conductive metal filling portions, and 20b and 20c are regarded as the other different heat conductive metal filling portions. The center of gravity of the heterothermally conductive metal filling portion 20a and the heterothermally conductive metal filling portion 20b are provided at the same position with respect to the width direction of the mold copper plate and at positions adjacent to each other in the casting direction. The heat transfer metal filling portion 20a and the heat transfer metal filling portion 20c are provided at the same position with respect to the casting direction, and are located adjacent to each other in the width direction of the mold copper plate.

In FIG. 7, the distance D1 is a distance along the casting direction, and must be the distance from the boundary line between the hetero-thermal conductive metal filling portion 20 a and the mold copper plate to the boundary line between the different heat-conductive metal filling portion 20 b and the mold copper plate. The distance D2 is a distance from the center of gravity of the heterothermally conductive metal filling portion 20a to the center of gravity of the differently thermally conductive metal filling portion 20c. In FIG. 7, the heterothermally conductive metal filling portion 20 is provided on the inner wall surface of the mold long-side copper plate 5 a so as to satisfy the relationship of the above-mentioned expressions (1), (2), and (3).

It is also possible to arrange the heterothermally conductive metal filling portion in a grid pattern on the mold copper plate in this way. Even if the heterothermally conductive metal filling portion is provided in a lattice shape, as long as the relationship conforms to the above formula (1), that is, It is possible to reduce the collapse of the protruding piece and to make the depth of the shock mark shallow, and it is possible to obtain the same effect as in the case where the heterothermally conductive metal filling portions are provided in a staggered shape.

Although this embodiment shows an example in which the shapes of the grooves provided in the mold copper plate are all the same circle, it is not limited to this mode. As long as the area ratio is at least within the range of 10% to 80% and the relationship of the formulas (1) and (2) is satisfied, the shapes of the grooves may not be all the same.

If a mold provided with a heterothermally conductive metal filling portion 19 is used, and a slab deliberately swelled the slab more than 0 mm to 20 mm, and the solid phase rate of the central portion of the slab is 0.2 or more and 0.9 or less, and the rolling speed ( mm / min) and casting speed (m / min) product (m · mm / min ² ) of a rolling force equivalent to 0.30 or more and 1.00 or less are performed in accordance with the rolling force of a slab when it is intentionally swollen. The method of light rolling with the same or smaller amount of bulging; if the two are combined, the internal quality of the slab can be further improved.

In this embodiment, the total amount of compulsory inflation (hereinafter referred to as “total bulge”) in the compulsory inflation area 16 is set to the thickness of the slab (thickness between the long sides of the slab) at the exit of the mold. It is a range exceeding 0 mm to 20 mm. According to this embodiment, the initial solidification in the mold is controlled, and even in the final solidified portion of the slab 12, the solidification interface can be kept smooth in the width direction and the casting direction of the slab. The applied rolling force can be uniformly applied to the solidification interface. In this way, even if the total bulging amount exceeds 0 mm to 20 mm, the center segregation can be reduced.

In the light rolling region 17, the slab 12 is rolled at least from the time point when the solid phase ratio of the thickness center of the slab is 0.2 to the time when it becomes 0.9. In the rolling at a period when the solid phase ratio of the central portion has not reached 0.2, the thickness of the unsolidified portion of the slab at the rolling position immediately after the rolling is still very thick. Therefore, with this rolling, As the solidification progresses, central segregation will occur again. When rolling is performed at a time when the solid phase ratio of the center portion exceeds 0.9, the molten steel after the segregation component is concentrated is not easily discharged, and the effect of improving the center segregation is small. Because the thickness of the solidified metal shell 13 of the slab during rolling is still very thick, the rolling force has not yet reached the center of the thickness sufficiently. In addition, if the solid phase ratio of the central portion exceeds 0.9 and the amount of rolling shrinkage is large, as described above, positive segregation occurs near the central portion of the thickness. Therefore, the slabs having a solid phase ratio of the central portion of 0.2 to 0.9 are rolled. Of course, it is also possible to roll the slab 12 in the light rolling area 17 before the solid phase rate at the center of the slab thickness becomes 0.2 and after the solid phase rate at the center of the slab thickness exceeds 0.9.

The solid phase rate at the central part of the thickness of the slab can be calculated by using the two-dimensional heat transfer and solidification calculation. The "solid phase ratio" referred to here means: the solidus ratio above the liquidus temperature of steel is defined as 0, the solidus ratio below the steel is defined as the solidus ratio = 1.0, and the thickness of the slab The position where the solid phase rate of the central part is 1.0 is the solidification end position 15. This solidification end position 15 is equivalent to the slab moving to the downstream side while solidifying, and the solid phase rate of the central part of the slab thickness reaches 1 The position on the most downstream side.

In the present embodiment, the total amount of rolling reduction of the slab 12 in the light rolling area 17 (hereinafter referred to as "total rolling reduction") is set to be the same as or less than the total swelling amount. . By setting the total rolling reduction amount to be equal to or less than the total bulging amount, the solidified portion up to the thickness center portion on the short side of the slab 12 is not subjected to rolling, and the structure can be reduced. The load of the guide roller 7 of the rolling area 17 can reduce the problem of equipment failure such as damage or breakage of the bearing of the guide roller 7.

In the present embodiment, when the light rolling is performed in the light rolling area 17, a rolling force corresponding to a condition that the product of the rolling speed and the casting speed (mm · m / min ² ) is 0.30 or more and 1.00 or less is added. On the long side of the slab. If the rolling is performed with a small rolling reduction of less than 0.30, the thickness of the unsolidified portion of the slab at the rolling position after rolling is still very thick, and the molten steel after the segregation component is concentrated cannot be sufficiently removed from the tree branch. Since the crystals are discharged, the center segregation occurs again after rolling. If the rolling is performed with a rolling reduction exceeding 1.00, the molten steel after the segregation component concentrated between the dendritic crystals is almost completely squeezed and discharged to the upstream side in the casting direction, but it is not solidified. The thickness of the slab is thin. Therefore, it is captured in the solidified shells on both sides of the thickness of the slab located slightly upstream of the casting direction than the rolling position. Positive segregation occurred next to it.

The effect of light rolling to prevent the occurrence of central segregation in the central part of the slab and positive segregation next to the central part is also affected by the solidified structure of the slab. In the case of equiaxed grains, there will be concentrated molten steel that causes semi-microsegregation between the equiaxed grains, and the effect obtained by rolling is very small. Therefore, the solidified structure is preferably not an equiaxed crystal structure but a columnar crystal structure.

In this embodiment, the thickness of the solidified metal shell 11 and the solid phase ratio at the center of the thickness of the slab are determined in advance using methods such as two-dimensional heat transfer and solidification calculation in advance for various casting conditions for continuous casting operations. The rolling zone 14 is a method in which the slab 10 can be rolled at least from the time point at which the solid content of the slab thickness center portion is 0.2 to the time point at 0.9, and the secondary cooling water amount and the secondary cooling can be limited. Adjust one or more of the width and casting speed. "Limiting the width of the secondary cooling" as referred to herein means stopping spraying of cooling water on both ends of the long side surface of the slab. By limiting the width of the secondary cooling, the secondary cooling becomes weak cooling. Generally, the solidification end position 13 can be extended to the downstream side in the casting direction.

As described above, by implementing the continuous casting method of steel according to this embodiment, it is possible to prevent the surface of the slab from being cracked due to uneven cooling of the solidified metal shell at the initial stage of solidification, and to reduce the depth of the shock mark. By making the depth of the shock mark shallow and uniformizing the surface of the initial solidified metal shell 13, the solidification interface of the final solidified portion can be smoothed, and the slab is deliberately swelled and lightly rolled, so that the The rolling force acts equally on the solidification interface, and can reduce the occurrence of center segregation in the thickness center of the slab. In this way, the effect of stably producing high-quality slabs can be achieved.

The above description is about continuous casting of steel slabs, but the continuous casting method of steel of this embodiment is not limited to continuous casting of steel slabs, continuous casting of medium and large slab slabs or small steel slabs, and Applicable.

    [Example 1]    

The water-cooled copper mold with metal arranged on the inner wall surface under various conditions was used, and the total bulging amount in the forced bulging area and the product of the rolling speed and the casting speed in the light rolling area were variously changed to perform medium carbon steel Chemical composition is divided into C: 0.08 ~ 0.17 mass%, Si: 0.10 ~ 0.30 mass%, Mn: 0.50 ~ 1.20 mass%, P: 0.010 ~ 0.030 mass%, S: 0.005 ~ 0.015 mass%, Al: 0.020 ~ 0.040 mass %) Casting, and a test for investigating surface cracking and internal quality (central segregation) was performed on the cast slab.

The product of the rolling speed and the casting speed in the light rolling area is set to 0.28 ~ 0.90mm. m / min ² , in each of the test examples, the slabs were rolled from the time point when the solid phase ratio of the thickness center of the slab thickness was at least 0.2 to 0.9 time point in the light rolling area.

If the slab is subjected to a forced inflation process in the forced inflation area, the total rolling reduction is set to be equal to or less than the total inflation amount. If it is a test example in which the slab is not subjected to a bulging process in the forced swell region, the light-rolling region is also rolled at the solidification end position on the short side of the slab.

The mold used was a mold with an inner surface dimension of 2.1 meters on the long side and 0.26 meter on the short side. The length of the water-cooled copper mold used from the upper end to the lower end (= mold length) is 950 mm. The position of the meniscus (molten steel soup noodles in the mold) at the time of steady casting is set to 100 mm from the upper end of the mold. s position. In order to confirm the effect of the continuous casting method of steel according to the present embodiment, a mold having the following conditions was also separately produced, and a comparative test was performed. The different heat-conducting metal in each mold is a metal having a lower thermal conductivity than the copper plate of the mold. The shape of the heterothermally conductive metal filling portion 19 is a diameter 6mm round shape. The pitch of the shock marks under this casting condition was 13 mm.

Mold 1: The range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold (range length = 220 mm). The heterothermally conductive metal is filled in a staggered pattern as the heterothermally conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of the different thermally conductive metal to that of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 2: The range from the position of 190 mm below the upper end of the mold to the position of 750 mm below the upper end of the mold (range length = 670 mm). The heterothermally conductive metal is filled in a staggered pattern as the heterothermally conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of the different thermally conductive metal to that of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 3: It is a range from 80 mm below the upper end of the mold to 300 mm below the upper end of the mold, and the heterothermal conductive metal is filled in a staggered manner as a heterothermal conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 15 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 4: The range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold is filled with heterothermal conductive metal in a staggered manner as a heterothermal conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 15 mm.

Mold 5: The range from 80mm below the upper end of the mold to 300mm below the upper end of the mold is filled with heterothermally conductive metal in a staggered manner as the heterothermally conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of copper is 15%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 6: The range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold is filled with heterothermal conductive metal in a staggered manner as the heterothermal conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 5%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 7: The range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold is filled with heterothermal conductive metal in a staggered manner as a heterothermal conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 85%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 8: The range from the position 80 mm below the upper end of the mold to the position 300 mm below the upper end of the mold is filled with a heterothermally conductive metal in a grid pattern as a heterothermally conductive metal filling portion. The ratio of the thermal conductivity difference of the thermal conductivity of copper is 20%. The area ratio ε of the heterothermally conductive metal-filled portion was set to 50%. The distance D1 between the different heat conductive metal filling portions 19 in the casting direction was set to 6 mm, and the distance D2 between the centers of gravity of the different heat conductive metal filling portions 19 in the mold width direction was set to 12 mm.

Mold 9: A mold which is not provided with a different heat conduction metal filling portion 19.

In the continuous casting operation, the base powder ((mass% CaO) / (mass% SiO ₂ )) is 1.1; the solidification temperature is 1090 ℃; the viscosity at 1300 ℃ is 0.15Pa. s's mold powder. The so-called "solidification temperature" refers to a temperature at which the viscosity of the molten mold powder rapidly increases during cooling. The position of the meniscus in the mold during steady casting is set at a position 100 mm below the upper end of the mold. The position of the meniscus is controlled during casting so that the meniscus exists within the setting range. The casting speed during steady casting is set at 1.7 ~ 2.2m / min. The slabs used to investigate the surface cracks and internal quality of the slabs were slabs with a casting speed set at 2.0 m / min during steady casting in all test examples.

The superheat degree of molten steel in the casting tank is set at 25 ~ 35 ℃. The temperature management method of the mold is to place a thermocouple from the back of the mold at a depth of 5mm from the mold surface (surface on the molten steel side) below the meniscus of the mold, and measure from the thermocouple. The obtained copper plate temperature measurement value was used to estimate the surface temperature of the mold.

After the continuous casting was completed, the surface of the long side of the slab was pickled to remove scale, and the number of surface cracks was measured. The occurrence of cracks on the surface of the slab is determined by using the casting direction length of the slab to be inspected as the denominator, and using the casting direction length of the slab where the surface crack occurred as the numerator. The evaluation. For the evaluation of the internal quality (central segregation) of the slab, a cross-section sample of the slab was taken. For the range of ± 10 mm at the center of the slab of the mirror-polished surface of the cross-section sample, Mn was measured every 100 μm using EPMA Concentration to evaluate the degree of segregation. Specifically, the ratio (C / C ₀ ) of the Mn concentration (C ₀ ) at the end portion where no segregation is considered to be the average value (C) of the Mn concentration at the center portion ± 10 mm (C / C ₀ ) is defined as the Mn segregation degree. To conduct a comparison.

In addition to these reviews, the non-uniformity σ (mm) of the thickness of the solidified metal shell was also measured according to the conditions of each test level example. The non-uniformity of the thickness of the solidified metal shell is measured by obtaining a Sulfur print from a cross section of a cast piece prepared by melting FeS (iron sulfide) powder into a mold. thickness of. The thickness of the solidified metal shell was measured at a position of 1/4 of the width direction of the mold, from a position of the meniscus to a position 200 mm below, at a distance of 5 mm, and 40 points were measured. The calculation of the non-uniformity σ is based on the following formula (5).

In Equation (5), D is the measured value (mm) of the thickness of the solidified metal shell, and Di is an approximate formula that defines the relationship between the thickness of the solidified metal shell and the solidification time. The distance from the meniscus corresponds to the solidification time, and the calculated thickness of the solidified metal shell (mm). N is the number of measurements, and is 40 in this example.

Table 1 shows the results of investigations on the test conditions of each of the test levels 1 to 14 and the quality of the surface and interior of the slab.

For the examples of the test levels 1, 8, 9, 10, 11, and 13, the conditions for installing the heterothermal conductive filling portion on the mold surface fall within the scope of the present invention. In each case, the surface cracking ratio is significantly improved. The unevenness of the thickness of the solidified metal shell also falls below 0.30, which can make the thickness of the solidified metal shell uniform. However, in the example of the test level 1, since the product of the rolling speed and the casting speed did not fall within the range of 0.30 to 1.00, a slight center segregation was confirmed. As for other examples, it was confirmed that the central segregation also improved.

In the example of the test level 2, the range where the heterothermal conduction filling portion is provided is off-set, and the product of the rolling speed and the casting speed does not fall within the range of 0.30 to 1.00. Therefore, in the example of Test Level 2, a slight surface crack occurred in the slab. Compared with the conventional method, the effect of reducing surface crack was not confirmed. The unevenness of the thickness of the solidified metal shell was also as high as 0.38, and the effect of improving center segregation could not be confirmed.

In the example of the test level 3, the distance D1 in the casting direction is long, and the product of the rolling speed and the casting speed does not fall within the range of 0.30 to 1.00. In the example of Test Level 3, although the problem of cracking on the surface of the slab was improved, the unevenness of the thickness of the solid metal shell was as high as 0.37, and the effect of improving center segregation could not be confirmed.

In the example of the test level 4, the distance D2 in the width direction of the mold is long, and the product of the rolling speed and the casting speed does not fall within a range of 0.30 to 1.00. In the example of the test level 4, the surface cracking of the slab was confirmed, and the effect of improving the surface cracking was not confirmed. The unevenness of the thickness of the solidified metal shell was slightly as high as 0.31, and slight central segregation was confirmed.

In the case of test level 5, the ratio of the difference in the thermal conductivity of the different heat-conducting metal is less than 20%. In the case of test level 6, the area ratio of the filling part of the different heat-conducting metal is less than 10%. The area ratio of the heat-conducting metal filling portion is higher than 80%. Therefore, in the examples of these test levels 5 to 7, the surface cracking of the slab was confirmed, and the effect of improving the surface cracking was not confirmed. The unevenness of the thickness of the solidified metal shell was slightly as high as 0.31 to 0.33, and slight central segregation was confirmed.

In the example of the test level 12, although the product of the rolling speed and the casting speed falls within the range of 0.30 to 1.00, the distance D1 in the casting direction is too long. In the example of test level 12, although the surface cracking and center segregation of the slab were improved, the unevenness of the thickness of the solidified metal shell was as high as 0.37. In the example of the test level 14, since the heterothermally conductive metal filling portion was not provided, it was confirmed that the surface of the slab was cracked. The unevenness of the thickness of the solidified metal shell was slightly as high as 0.32, and it was also confirmed that center segregation occurred.

Claims (7)

    A continuous casting method of steel is a continuous casting method in which molten steel is injected into a continuous casting mold, the aforementioned continuous casting mold is vibrated toward the casting direction, and the aforementioned molten steel is pulled out to make a slab of steel. The continuous casting mold has a plurality of grooves provided on the inner wall surface of the mold copper plate. The mold copper plate is located at a position at least 20 mm above the meniscus in a steady casting state so far, and the meniscus is further below. At least 50mm and at most 200mm; inside the plurality of grooves, a plurality of heterothermally conductive metal filling portions are provided, which are filled: the ratio of the difference in thermal conductivity with respect to the thermal conductivity of the mold copper plate 20% or more of metal or metal alloy, and the sum of the areas of all the heterothermally conductive metal filling parts to the area of the inner wall surface where the plurality of heterothermally conductive metal filling parts are provided, that is, the area ratio is 10% or more and 80 % Or less; The shock mark distance (F) and distance (D1) derived from the oscillation frequency (f) and casting speed (Vc) are in accordance with the following formula (1) And the distance (D2) satisfy the following relation equation (2), D1 ≦ F = Vc × 1000 / f. ． ． Equation (1) D2 ≦ 4r. ． ． Equation (2) Here, in Equation (1), Vc is the casting speed (m / min), f is the oscillation frequency (cpm), F is the distance between shock marks (mm), and D1 is set from: The center of gravity and the center of gravity of one of the plurality of heterothermally conductive metal filling portions are located at the same position in the width direction of the aforementioned mold copper plate, and are in the casting direction with the one of the heterothermally conductive metal filling portions. The distance (mm) from the boundary line between the adjacent another heterothermally conductive metal filling portion and the mold copper plate to the boundary line between the one different thermally conductive metal filling portion and the mold copper plate is shown in Equation (2). , R is the center of gravity of the heterothermally conductive metal filling part as the center, and is the radius (mm) of a circle having the same area as the area of the heterothermally conductive metal filling part, and D2 is set from: its center of gravity is the same as that of the heterothermally conductive metal The center of gravity of the filling portion is another heterothermally conductive metal filling portion at the same position in the casting direction, and is the other heterothermally conductive metal filling portion adjacent to the aforementioned heterothermally conductive metal filling portion in the width direction. The distance (mm) from the center of gravity to the center of gravity of the aforementioned one of the heat-conducting metal filling portions.         The continuous casting method for steel according to claim 1, wherein the plurality of heterothermally conductive metal filling portions are set such that the distance (D1) is in accordance with a relationship of the following formula (3), and D1 ≦ 2r. ． ． Equation (3).         The continuous casting method for steel according to claim 1 or claim 2, wherein the shapes of the plurality of grooves are all the same.         The continuous casting method for steel according to any one of claim 1 to claim 3, wherein the shape of the plurality of grooves is circular or a pseudo-circular shape without corners.         The continuous casting method for steel according to any one of claim 1 to claim 4, wherein the plurality of heterothermally conductive metal filling portions are provided in a lattice shape.         The continuous casting method for steel according to any one of claim 1 to claim 4, wherein the plurality of heterothermally conductive metal filling portions are provided in a staggered shape.     The continuous casting method for steel according to any one of claim 1 to claim 6, wherein the roller opening of a plurality of pairs of slab support rolls provided in the continuous casting machine is directed toward the downstream side in the casting direction, Stepwise increase so that the long side of the slab with an unsolidified layer inside is relative to the thickness of the slab at the exit of the mold (thickness between the long side of the slab) is greater than 0mm and less than 20mm The total bulging amount of the range is expanded. Then, the roll opening of the plurality of pairs of slab support rolls is directed toward the downstream side in the casting direction to gradually reduce the light rolling area from the center of the thickness of the slab. From the point when the solid phase ratio of at least 0.2 is reached to the point when it becomes 0.9, the product of the rolling speed (mm / min) and the casting speed (m / min) (mm · m / min ² ) is equivalent to 0.30 or more Rolling force below 1.00 is applied to the long side of the slab, and the rolling force is used to calculate the total rolling shrinkage amount equal to the total bulging amount or the total rolling shrinkage amount smaller than the total bulging amount. The long side surface of the slab is rolled.