JP6427945B2 - Bloom continuous casting method - Google Patents

Description

The present invention relates to a continuous casting method of the bloom.

  In continuous casting of molten metal, such as steel, molten metal is injected into the opening of a continuous casting mold, the molten metal in contact with the mold wall solidifies to form a solidified shell, and the solidified shell is pulled out as it is, below the mold In the secondary cooling zone, solidification is completed while being supported by the support roll, and it is drawn out as a slab.

Of continuous cast slabs, the slab having a shape with a slab width divided by the slab thickness (aspect ratio) of about 1 to 1.7 and a cross-sectional area of the slab of about 400 to 2000 cm ² is bloom and be called. The steel bloom is used as a material for manufacturing the steel material for the strip.

  Before the solidification is completed in the secondary cooling zone, the molten metal with a concentrated solute between the dendritic trees at the solid-liquid interface is flown in the solid-liquid coexistence phase due to solidification shrinkage and bulging between rolls, and the slab It moves to the center and becomes center segregation. In addition, porosity may occur at the center of the slab due to solidification shrinkage. In the high carbon steel wire rod, C and Mn are concentrated in the central segregation part, so that pro-eutectoid cementite is generated at the austenite grain boundary or micro martensite is generated, which causes breakage during wire drawing. Moreover, the toughness after wire drawing processing also deteriorates.

  It is known that center segregation is improved by increasing the equiaxed crystal ratio, particularly the upper equiaxed crystal ratio, in the cross section of the cast slab after casting. Here, the upper surface equiaxed crystal ratio means the equiaxed crystal ratio on the upper surface side from the center in the height direction of a slab that has been cast through the bent back portion in a continuous casting apparatus having a curved portion and a bent back portion. .

  In bloom continuous casting, it is known that electromagnetic stirring in the mold is effective as a means for increasing the equiaxed crystal ratio of the upper surface of the cast bloom. According to Non-Patent Document 1, when continuous carbon casting of medium carbon steel and high carbon steel is performed, a stirring flow is formed in the mold by electromagnetic stirring in the mold, so that a good upper surface equiaxed axis regardless of the degree of superheated molten steel in the mold. It is disclosed that the crystallinity can be realized. As the coil current for electromagnetic stirring increases, the equiaxed crystal ratio on the upper surface increases. In addition, the upper surface equiaxed crystal ratio is increased and the slab light reduction at the end of solidification is used in combination to reduce the concentration of C in the center.

  It is known that when electromagnetic stirring in the mold is performed, powder for continuous casting is involved and this becomes a defect (Patent Document 1). Patent Document 2 states that when the flow velocity of a horizontal swirl flow is increased in electromagnetic stirring in the mold, surface defects such as powder entrainment, dents on the slab surface, and cracks are induced. When a swirling flow with excessive strength is formed, the meniscus of the molten steel undulates, dents, cracks, and double skin defects occur on the surface of the slab.

  In the mold cross-sectional shape, it is known to provide a corner R at a portion corresponding to the corner portion of the slab. In patent document 3, what has a corner part to which R process was given is disclosed as a continuous casting mold of the square billet of a cylindrical cross section. Patent Document 4 discloses a mold in which a corner R of the mold is larger in the upper part of the mold than in the lower part of the mold.

JP 2004-300502 A JP-A-11-320051 Japanese Patent Laid-Open No. 10-286652 JP-A-8-243688

CAMP-ISIJ Vol.3 (1990) -265

  In bloom continuous casting, by forming a swirl flow in the mold by electromagnetic stirring in the mold, a good top equiaxed crystal ratio can be realized and the center porosity can be reduced. As the flow velocity of the swirl flow is increased, the upper surface equiaxed crystal ratio is increased and the center porosity is improved. In addition, by increasing the equiaxed crystal ratio on the upper surface and using light reduction of the slab at the end of solidification, the concentration of C at the center is reduced, the center segregation of the slab is improved, and the center porosity is reduced. can do. On the other hand, when a swirl flow is formed in the mold by in-mold electromagnetic stirring in bloom continuous casting, a groove as shown in FIG. 10C is formed on the surface of the cast slab. Hereinafter, it is referred to as “depression 23”. The depth of the depression 23 increases as the swirl flow rate by electromagnetic stirring increases. If the depletion becomes too deep, cracks will occur or rolling in the subsequent process will result in problems such as baldness and burrs called rashes, so the slab must be cleaned before rolling in the subsequent process. It is necessary to reduce the difference in level with the surroundings.

An object of the present invention is to provide a continuous casting method capable of reducing the occurrence of depletion of a slab when a swirl flow is formed in a mold by electromagnetic stirring in the mold in bloom continuous casting.

That is, the gist of the present invention is as follows.
(1) A bloom continuous casting method,
With respect to the horizontal cross-sectional shape (hereinafter referred to as “mold cross-sectional shape”) of the opening that accommodates the molten metal of the continuous casting mold, the direction from the inside to the outside of the opening is “outward from the mold”, and the walls that constitute the opening are Called the "mold wall", the surface of the slab that contacts the support roll in the support roll band below the mold during casting is called the "upper and lower surface"
The mold cross-sectional shape of the lower end of the mold has a corner R with a radius of 10 mm or more and a parallel part of 0.5 times or more the slab width at the upper and lower surfaces,
The mold cross-sectional shape of the mold upper meniscus part has a corner R having a radius of 30 mm or more, and the mold wall is curved toward the outside of the mold in the horizontal plane on at least two opposing surfaces, and the mold wall is most out of the mold in the horizontal plane. The distance between the mold walls facing each other in the protruding part increases in the range of 10 mm or more and 60 mm or less compared with the distance between the mold walls at the lower end of the mold (however, in the mold cross-sectional shape, it is directed downward from the meniscus part) Except for the case where the radius of the corner R is increased).
It has an electromagnetic stirrer that can generate a swirl flow in a horizontal plane in the molten metal in the mold, and the average flow velocity at the solid-liquid interface is within the range from the meniscus position in the mold to a depth of 300 mm by the in-mold electromagnetic stirrer. A continuous casting method of bloom, wherein a swirling flow in a horizontal plane of 20 cm / second or more and 40 cm / second or less is formed .

  According to the present invention, the mold cross-sectional shape of the mold upper meniscus portion has a corner R having a radius of 30 mm or more, and the mold wall is curved toward the outside of the mold in the horizontal plane on at least two opposing surfaces, and the mold wall is in the horizontal plane. Since the distance between the opposite mold walls in the portion that protrudes most outward from the mold is increased in the range of 10 mm or more and 60 mm or less compared to the lower end of the mold, the electromagnetic stirrer in the mold allows the distance from the meniscus position in the mold. In the range up to a depth of 300 mm, even if a swirl flow in the horizontal plane having an average flow velocity of 20 cm / second or more at the solid-liquid interface is formed, no depletion occurs in the slab, and the equiaxed crystal ratio on the upper surface can be increased. In addition, the mold cross-sectional shape at the lower end of the mold has a corner R having a radius of 10 mm or more and a parallel part that is 0.5 times or more the slab width at the upper and lower surfaces, so that wrinkles occur during the rolling process. Thus, it is possible to use a conventionally used cylindrical roll as it is, and to ensure stability when stacking and storing slabs.

It is a figure which shows an example of the cross-sectional shape of the casting_mold | template of this invention, (a) is a meniscus part, (b) is a casting mold lower end part. It is a figure which shows an example of the cross-sectional shape of the casting_mold | template in the meniscus part of this invention. It is a figure which shows an example of the cross-sectional shape of the casting_mold | template in the meniscus part of this invention. It is side surface sectional drawing which shows an example of the casting_mold | template of this invention. It is a figure which shows an example of the cross-sectional shape of the casting_mold | template in the meniscus part of this invention. It is a figure which shows the relationship between the flow velocity of the swirl | vortex flow in a casting_mold | template, and a slab upper surface equiaxed crystal ratio. It is a figure which shows the relationship between the rotation speed in a casting_mold | template and a depletion index | exponent in a prior art example. It is a figure which shows the relationship between the corner R in the mold cross-sectional shape of a meniscus part, and the depletion index | exponent of a slab, (a) is a case where the curved part of a _{casting_mold |} template wall does not exist, (b) is a curved part exists and T _Umax -T _{In this} case, _L = 10 mm. It is a figure which shows the relationship between the corner R in the mold cross-sectional shape of a meniscus part, and the depletion index | exponent of a slab, (c) exists in a curved part, and (D) exists in a curved part when _TUmax - _TL = 30mm. In this case, T _Umax −T _L = 60 mm. It is a figure which shows the relationship between the curvature radius of the corner R in the casting_mold | template cross-sectional shape of a casting mold lower end part, and a wrinkle generation | occurrence | production index. It is a slab cross-sectional shape, (a) is a diagram showing the swirl flow in the mold, (b) is a diagram showing the resulting solidified shell development, (c) is a diagram showing the resulting slab depletion. . It is the figure which formed only the corner R in the casting_mold | template cross-sectional shape in a meniscus part.

  The present invention will be described with reference to FIGS.

The present invention is directed to bloom continuous casting of molten metal, particularly steel. As described above, the bloom is a slab having a shape in which the value obtained by dividing the slab width by the slab thickness (aspect ratio) is about 1 to 1.7 and the cross-sectional area of the slab is about 400 to 2000 cm ^2. means.

  The continuous casting mold 1 has an opening 2 for accommodating a molten metal, and the four circumferences of the opening 2 are surrounded by a water-cooled copper mold. Molten metal is poured into the opening of the mold, cooled to the surrounding copper mold to form a solidified shell, and the solidified shell is drawn downward. Here, the horizontal sectional shape of the mold opening is referred to as “mold sectional shape 3”. Further, the direction from the inside to the outside of the opening is referred to as “mold outer side 13”, and the wall constituting the opening is referred to as “mold wall 4”.

After being drawn downward from the mold, the slab shrinks with a decrease in temperature, so the size of the opening 2 at the lower end 12 of the mold takes a slightly larger value than the slab size. Here, the opening width of the lower end 12 of the mold is set to W _{L on} the long side and T _L on the short side (FIG. 1B). In the mold, the solidified shell shrinks as the temperature decreases from the meniscus portion 11 to the mold lower end portion 12, so that the opening width of the meniscus portion 11 is often larger than the opening width of the mold lower end portion 12. Here, the opening width of the meniscus portion 11 is set to W _{U on} the long side and T _U on the short side (FIG. 1A). When the width (thickness) is made different between the meniscus portion 11 and the lower end portion 12 of the mold, the amount of change is usually about 0.3%.

  An electromagnetic coil 8 of an electromagnetic stirring device is disposed outside the continuous casting mold 1 (see FIG. 4), and an alternating current is passed through the electromagnetic coil 8 to form a swirl flow 24 in a horizontal plane in the molten metal in the mold. (See FIG. 10A). The swirl flow rate can be increased as the applied current is increased. FIG. 6 shows the results of investigating the relationship between the swirling flow velocity and the upper surface equiaxed crystal ratio of the cast slab by changing the swirling flow velocity in various ways. Note that the swirl flow velocity refers to the flow velocity in front of the solidification interface in the range from the meniscus position in the mold to a depth of 300 mm. Casting varieties are steels for high carbon wire rods with Al-Si-Mn deoxidation in the carbon concentration range of 0.80 to 0.83%, the bloom size to be cast is 300 to 500 mm, and the degree of superheated molten steel in the mold Is in the range of 23-26 ° C. As the swirling flow velocity, the solidification interface average flow velocity was adopted.

The molten steel flow velocity at the front of the solidified shell is measured by measuring the dendrite tilt angle and using the Okano formula (see “Iron and Steel, vol. 93 (2007) No. 9, pp. 566”) shown in the following formula (1). Calculated. The dendritic tilt angle is measured by cutting a specimen from a cast slab from a cross section in the casting direction and a cross section perpendicular to the casting direction, corroding with acid, imaging the solidified structure at a magnification of 5 times, and measuring the primary dendrite arm of the slab surface layer. This was done by measuring the angle relative to the normal on the slab surface. Further, in order to obtain data in the depth direction from the meniscus, this measurement was performed from the slab surface layer to the lower end position of the core of the electromagnetic stirrer 8.
lnV = (θ + 9.73 × lnf + 33.7) / (1.45 × lnf + 12.5) (1)
Here, V: molten steel flow velocity (cm / second), θ: dendrite tilt angle (degree), f: solidification rate (cm / second).

  As is clear from FIG. 6, the upper surface equiaxed crystal ratio increases as the swirling flow rate increases, and the upper surface equiaxed crystal ratio is 30% or more, desirably 35%. This can be done. In this condition, the lower surface equiaxed crystal ratio is 40% or more.

  On the other hand, when a swirl flow is formed in the mold by electromagnetic stirring in the mold, a depression 23 as shown in FIG. 10C is generated on the surface of the cast slab 21. Here, the depletion index is obtained when the solid flow interface front surface flow velocity is 20 cm / sec in the range from the meniscus position in the mold to a depth of 300 mm by applying electromagnetic stirring with a rectangular mold combined with a normal parallel copper plate. As a value obtained by subtracting the amount of depletion generated when the average flow velocity at the front of the solidification interface in the range from the meniscus position in the mold to a depth of 300 mm is changed by changing the current value of the electromagnetic stirrer, with the generated amount of Define. Here, the amount of depletion was evaluated by measuring the amount of depletion generated on all four surfaces of the bloom slab per unit casting length of bloom, and evaluating it as pieces / m. As shown in FIG. 7, the depletion index increases as the swirl flow rate increases.

  When the swirl flow 24 is formed in the molten metal at the upper part of the mold as shown in FIG. 10A, the horizontal cross-sectional shape of the solidified shell 22 in the vicinity of the lower end of the mold becomes a shape as shown in FIG. This is probably because the solidified shell formation speed of the portion 25 where the swirling flow collides is slow, and the solidified shell thickness is reduced. And it turns out that the depression 23 as shown in FIG.10 (c) is formed in the slab shape after casting in the part where the solidification shell thickness became thin in the mold lower end vicinity.

In the present invention, first, as shown in FIG. 11, depletion can be reduced by forming corners R5 at four corners in the mold cross-sectional shape 3 of the meniscus portion 11 at the upper part of the mold. FIG. 8A shows the depletion index when the radius of curvature of the corner R5 of the mold cross-sectional shape 3 of the meniscus portion 11 as shown in FIG. 11 is variously changed. In addition, in FIG. 8-1 and below-mentioned FIG. 8-2, the description of 20-50 cm / sec described in the upper right of each figure means the turning flow velocity in a casting_mold | template. Figure 8-1 (a) As is apparent from, the larger the curvature radius R _U corner R5, it is possible to reduce the depletion index. By providing the corner R5, it is estimated that the collision flow to the solidified shell due to the swirl flow is reduced. However, only increasing the curvature radius R _U of the corner R, you can not stifle depletion index completely.

  The mold cross-sectional shape has substantially the same shape as the cast slab shape, and the bloom has a rectangular shape. Therefore, the mold cross-sectional shape is also generally rectangular. In the present invention, as will be described later, the mold cross-sectional shape at the lower end of the mold is a rectangle having a corner R in principle (see FIG. 1B).

  On the other hand, according to the present invention, secondly, as shown in FIG. 1 (a), as the mold cross-sectional shape 3 of the meniscus portion 11, the corner R5 is formed as described above, and at the same time, the mold walls are at least on two opposing surfaces. By having the curved portion 6 curved toward the outside of the mold in the horizontal plane, the depletion can be further reduced. In FIG. 1A, the curved portion 6 is formed on the long side surface 15.

An example in which the mold wall is curved toward the outside of the mold will be described with reference to FIG. The bloom size is 300 mm × 500 mm, and T _L = 300 mm and W _L = 500 mm. In the case of a conventional normal mold, the mold size of the meniscus portion is T _U = 301 mm and W _U = 509 mm. FIG. 1A shows a mold cross-sectional shape 3 of the meniscus portion 11, and FIG. 1B shows a mold cross-sectional shape 3 of the mold lower end portion 12. At the lower end portion 12 of the mold, the mold cross-sectional shape 3 has a corner R5 with a radius _RL of 10 mm and is a rectangle of T _L × W _L. In the meniscus 11 relative to it, the four corners and the radius R _U, form a corner R5 of 30mm to about the long side surface 15 of the mold, the radius of curvature R _G is a curved shape of about 820 mm, the mold outer The curved part 6 curved toward is formed. By adopting such a shape, the portion of the mold wall that protrudes most outward in the horizontal plane is the center of the long side, and the distance T _Umax between the mold walls facing each other at the center of the long side is 310 mm. Compared with the distance T _L between the mold walls (T _L = 300 mm), the projection protrudes 10 mm (FIG. 1A).

Using a mold having a mold cross section described in FIG. 1, the projecting amount of the bending portion (T _Umax -T _L) is varied in 10 to 60 mm, the further turning velocity of the radius of curvature R _U and the mold corner R5 Depression indexes when various changes are made are shown in FIGS. 8-1 (b) to 8-2 (d). In the mold cross-sectional shape 3 of the meniscus portion 11, as a result of the long side surface 15 being curved outward from the mold, the depletion index can be completely suppressed in combination with the provision of the corner R5. As is apparent from FIGS. 8-1 (b) to 8-2 (d), the mold wall distance T _Umax is 10 mm compared to the mold lower mold distance T _U due to the curvature of the mold wall in the meniscus portion 11. It is increasing more and if the radius R _U corner R5 in the mold cross section 3 of the meniscus portion 11 is 30mm or more, the turning velocity in the mold is a depression index 0 in the following conditions 40 cm / sec it can. If the swirl flow rate is 40 cm / sec, the equiaxed crystal ratio on the top surface will be 40%, and even if the flow velocity is increased further, the equiaxed crystal ratio will not increase so much and the possibility of entrainment of powder will increase. It is sufficient if the depletion index can be 0 in the range of the flow velocity of 40 cm / second or less. In addition, the improvement effect of a depletion index | exponent increases, so that the protrusion amount of a curved part becomes large (FIGS. 8-1 (b)-FIG. 8-2 (d)).

When the inter-mold distance T _Umax in the meniscus portion 11 is increased as compared with the lower end of the mold, it is necessary to provide a shape that gradually reduces the inter-mold distance from the meniscus portion 11 to the lower end portion 12 of the mold as shown in FIG. The distance between the meniscus portion 11 and the lower end portion 12 of the mold is usually about 900 mm. In order to reduce the distance between the molds while maintaining the solidified shell quality within this distance, the meniscus portion compared to the lower end of the mold is used. It is preferable that the distance between molds (T _Umax −T _L ) is 60 mm or less (one side is 30 mm or less). In addition, when the present invention is realized by remodeling an existing continuous casting apparatus, if the distance increase between the molds is too large, the remodelable range is exceeded, but if the distance increase between the molds is about 60 mm or less, It is possible to fit within the remodelable range.

Therefore, in the present invention, the mold cross-sectional shape 3 of the mold upper meniscus portion 11 has a corner R5 having a radius R _U of 30 mm or more, and the mold wall is curved toward the outside of the mold in a horizontal plane on at least two opposing surfaces. The distance between the mold walls facing each other at the portion of the mold wall that protrudes most outward in the horizontal plane is increased in the range of 10 mm or more and 60 mm or less compared to the lower end of the mold.

  In the mold cross-sectional shape 3 of the meniscus portion, when the cross-sectional area is small, the distance between the vortex sensor for controlling the molten metal surface position of the molten metal in the mold and the mold wall is small, and the sensitivity of the molten metal level sensor is lowered. Surface level control accuracy decreases. In the present invention, since the curved portion 6 is provided, the cross-sectional area does not decrease even when the corner radius of the meniscus portion increases, and this problem can be avoided.

A portion where the mold wall is curved in the horizontal plane toward the outside of the mold is referred to as “curved portion 6” as described above. As a shape for forming the curved portion 6, as shown in FIG. 1 (a), the entire curved portion 6 has a large single curvature radius R _G and is smoothly connected to the corner R5 portion. It is preferable to provide it. Alternatively, as shown in FIG. 2 (a), the radius of curvature R _G1 near the center of the width of the curved portion 6 and the portion connected to the corner R5 can be set to a radius of curvature R _G2 smaller than that. As shown in FIG. 2 (b), the vicinity of the center of the width of the curved portion 6 can be a straight line, and the straight portion and the corner R5 can be smoothly connected with a curve having a large radius of curvature _RG .

  In any of the examples shown in FIGS. 1 and 2, the center of curvature exists on the molten metal side of the mold wall for the portion where the mold wall forms a curved surface. Such a case is referred to as “concave in”. Conversely, the case where the center of curvature is on the anti-melt side of the mold wall is referred to as “convex inward”. In the present invention, it is preferable to be concave or flat in any place on the mold wall. As shown in FIG. 5, there may be a convex portion 18 in a part, but in that case, the mold wall angle and the end of the start portion 19 of the convex portion 18 in the horizontal section It is preferable that the angle difference θ of the mold wall angle of the portion 19 is 5 ° or less.

  As shown in FIGS. 1 and 2, the mold wall forming the curved portion 6 may be at least two surfaces facing each other. In the case of a rectangular mold section having an aspect ratio other than 1, as the two surfaces forming the curved portion, either the long side surface 15 or the short side surface 16 can be selected, but it is preferable to select the long side surface 16. . Moreover, as shown in FIG. 3, it is good also as forming the curved part 6 in all four surfaces of a casting_mold | template wall.

  Next, the mold cross-sectional shape 3 at the mold lower end 12 will be described.

In the conventional normal bloom continuous casting apparatus, the mold cross-sectional shape at the lower end of the mold is rectangular. Therefore, the slab cross-sectional shape after casting is also rectangular. When hot-rolling is performed using a slab having such a shape, the ridgeline portion of the corner is folded to become wrinkles, and there is a problem that care is required in a subsequent process. Therefore, in order to prevent the occurrence of the wrinkles, a test was performed in which the corners R were formed at the four corners in the mold cross-sectional shape 3 of the mold lower end portion 12, thereby giving the corners R at the four corners of the slab cross-sectional shape (see FIG. 9). As a result, as shown in FIG. 1 (b), corners R5 are formed at the four corners as the mold cross-sectional shape 3 of the mold lower end portion 12, and the curvature radius _RL of the corner R portion is set to 10 mm or more. It became clear that it can be prevented. Therefore, in the present invention, the mold cross-sectional shape 3 of the mold lower end 12 is assumed to have a corner R5 having a radius _RL of 10 mm or more.

As described above, in the present invention, the mold wall of the meniscus section 11 of the mold cross-sectional shape 3 is formed with the curved portion 6 curved in the horizontal plane toward the outside of the mold. Therefore, this curved shape can be used as it is in the lower end portion 12 of the mold, or it can be a partially remaining shape. On the other hand, of the slab drawn from the lower end of the mold, the surfaces that become the upper and lower surfaces after casting (hereinafter referred to as “upper and lower surfaces 14”) are all formed of curves, or if there are few parallel straight portions, The shape of the support roll disposed on the surface is not an ordinary cylindrical shape, but needs to have a surface shape that matches the curve of the slab. In addition, if there are few straight portions parallel to the upper and lower surfaces of the cast slab, the stability when the slabs are stacked and stored is impaired. Therefore, in the present invention, the mold cross-sectional shape 3 of the mold lower end portion 12 has the corner R5 having the radius R _L of 10 mm or more as described above, and the upper and lower surfaces 14 are 0.5 times or more the slab width at the portion. It has decided to have the parallel part 7 (refer FIG.1 (b)). This makes it possible to use a conventional cylindrical roll as it is as the support roll shape in the support roll band below the mold, and to ensure stability when stacking and storing the slabs. it can.

However, if the radius R _{L of the} corner R is too large, or if the long side surface of the slab is curved with few top and bottom plane parts, the cross-sectional area of the slab becomes small, so the unit length The per slab mass is reduced and productivity is deteriorated. Or if it is going to maintain productivity, it is necessary to make a casting speed fast, the controllability of a hot_water | molten_metal surface level falls, and quality defects, such as powder entrainment, increase. From the standpoint of preventing wrinkles, the mold cross-sectional shape 3 of the mold lower end 12 is a rectangle having a corner R5 having a radius _RL of 10 mm or more. From the viewpoint of ensuring productivity as described above and ensuring hot water surface controllability. The corner R5 is desirably 50 mm or less.

  The continuous casting apparatus for bloom according to the present invention has a continuous casting mold as described above, and also has an electromagnetic stirring device capable of generating a swirling flow in a horizontal plane in the molten metal in the mold. As shown in FIG. 4, an electromagnetic stirring coil 8 is provided around the mold 1 to generate a swirling flow. Any electromagnetic stirring device may be used as long as it can form a swirl flow in a horizontal plane having an average flow velocity at the solid-liquid interface of 20 cm / second or more in the range from the meniscus position in the mold to a depth of 300 mm. The electromagnetic stirring device has a structure having an electromagnetic coil capable of generating a moving magnetic field on two surfaces of a rectangular mold, a structure having an electromagnetic coil capable of generating a moving magnetic field on four surfaces, and a structure having an arc-shaped electromagnetic coil around the four surfaces Any of these may be sufficient as long as a desired solidification interface front surface flow velocity is ensured by applying electromagnetic force to the molten steel in the mold. What is necessary is just to select the model of the said electromagnetic stirring coil suitably according to the structure of a continuous casting installation.

  Next, a continuous casting method using the bloom continuous casting apparatus of the present invention will be described.

  Using the electromagnetic stirrer included in the continuous casting apparatus of the present invention, a swirl flow in a horizontal plane having an average flow velocity at the solid-liquid interface of 20 cm / second or more is formed in the range from the meniscus position in the mold to a depth of 300 mm. Thereby, for example, when producing a high carbon steel having a C content of 0.4 to 1.2% by mass, the equiaxed crystal ratio of the upper surface of the cast slab is set to 30% or more as shown in FIG. Can do. Thereby, the center segregation and center porosity of a slab can be reduced. Further, the lower surface equiaxed crystal ratio is normally 40% or more, and by increasing the upper surface equiaxed crystal ratio as described above, the upper surface and lower surface equiaxed crystal ratio becomes uniform. Furthermore, the center segregation and center porosity of the slab are further improved by lightly reducing the slab in the support roll band. As a light reduction pattern, the area from the time when the central solid phase ratio of the slab reaches a temperature corresponding to 0.1 to the time when the central solid phase ratio of 0.7, which is the flow limit solid phase ratio, is obtained per unit time. What is necessary is just to make the rolling amount of each roll into the range of 0.5 mm / min-2.5 mm / min by the pattern pulled out continuously in the ratio of 0.5 mm / min or more and less than 2.5 mm / min.

The mold cross-sectional shape 3 of the mold upper meniscus portion 11 of the present invention has a corner R5 having a radius R _U of 30 mm or more, and a curved portion in which the mold wall is curved toward the outside of the mold in a horizontal plane on at least two opposing surfaces. 6, and the distance T _Umax between the mold walls facing each other at the portion of the mold wall that protrudes most outward in the horizontal plane is in the range of 10 mm or more and 60 mm or less compared with the mold distance T _L between the mold lower ends 12. Therefore, even if a swirling flow having a flow rate of 20 cm / second or more is generated in the mold by electromagnetic stirring as described above, it is possible to prevent depletion from occurring in the cast slab.

In the present invention, the mold cross-sectional shape 3 of the mold lower end portion 12 has a corner R5 having a radius R _L of 10 mm or more, and a parallel portion 7 having a width of 0.5 times or more of the slab width at the upper and lower surfaces. Yes. Therefore, a corner R having a radius of 10 mm or more is formed in the cast slab, and generation of wrinkles in the rolling process can be prevented. In addition, since the slab has the above-described parallel portions on the upper and lower surfaces, it is possible to use a conventional cylindrical roll as it is and to improve stability when stacking and storing the slab. Can be secured.

1 Mold 2 Opening 3 Mold Profile 4 Mold Wall 5 Corner R
6 Curved portion 7 Parallel portion 8 Electromagnetic coil 11 Meniscus portion 12 Mold lower end portion 13 Mold outer side 14 Upper and lower surfaces 15 Long side surface 16 Short side surface 17 Recessed portion 19 in mold wall 18 of conventional mold 19 Start portion 20 End portion 21 Slab 22 solidified shell 23 depletion 24 swirl flow 25 part where swirl flow collides

Claims (1)

Bloom continuous casting method,
With respect to the horizontal cross-sectional shape (hereinafter referred to as “mold cross-sectional shape”) of the opening that accommodates the molten metal of the continuous casting mold, the direction from the inside to the outside of the opening is “outward from the mold”, and the walls that constitute the opening are Called the "mold wall", the surface of the slab that contacts the support roll in the support roll band below the mold during casting is called the "upper and lower surface"
The mold cross-sectional shape of the lower end of the mold has a corner R with a radius of 10 mm or more and a parallel part of 0.5 times or more the slab width at the upper and lower surfaces,
The mold cross-sectional shape of the mold upper meniscus part has a corner R having a radius of 30 mm or more, and the mold wall is curved toward the outside of the mold in the horizontal plane on at least two opposing surfaces, and the mold wall is most out of the mold in the horizontal plane. The distance between the mold walls facing each other in the protruding part increases in the range of 10 mm or more and 60 mm or less compared with the distance between the mold walls at the lower end of the mold (however, in the mold cross-sectional shape, it is directed downward from the meniscus part) Except for the case where the radius of the corner R is increased).
It has an electromagnetic stirrer that can generate a swirl flow in a horizontal plane in the molten metal in the mold, and the average flow velocity at the solid-liquid interface is within the range from the meniscus position in the mold to a depth of 300 mm by the in-mold electromagnetic stirrer. A continuous casting method of bloom, wherein a swirling flow in a horizontal plane of 20 cm / second or more and 40 cm / second or less is formed.

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