JP2016168610A - Steel continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel continuous casting method that suppresses surface cracks resultant from an oscillation mark formed in a casting piece when a molten steel such as medium carbon steel liable to generate surface cracks is continuously cast.SOLUTION: A casting piece is produced by drawing out molten steel 11 injected into a continuous casting mold while the casting mold is being oscillated in the casting direction. In the inner wall face of a casting mold long side 1, the casting mold has a plurality of dissimilar metal filling parts 3 formed by filling with metal, that has different thermal conductivity from that of the casting mold, circular recess grooves provided in the inner wall face independently from each other. The filling thickness H(mm) of the dissimilar metal filling part 3 and the diameter d(mm) of the dissimilar metal filling parts 3 satisfy the relation of the following (2) expression. The amplitude S(mm) of oscillation and the frequency f(mm) and the distance PH(mm) between the centers of the dissimilar metal filling parts 3 in the casting direction satisfy the relation of the following (3) expression. 0.5≤H≤d(2)S/2×≤PH≤1000×Vc/f(3)SELECTED DRAWING: Figure 6

Description

  The present invention prevents continuous slab surface cracking due to non-uniform cooling of the solidified shell in a continuous casting mold and also prevents slab surface cracking due to oscillation marks formed on the slab. The present invention relates to a casting method.

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

  If the cooling in the mold becomes non-uniform, the thickness of the solidified shell becomes non-uniform in the casting direction of the slab and in the slab width direction. The solidified shell is subjected to stress resulting from the shrinkage and deformation of the solidified shell. In the initial stage of solidification, this stress is concentrated on the thin portion of the solidified shell, and the stress causes cracks on the surface of the solidified shell. This crack expands due to subsequent external stresses such as thermal stress, bending stress due to the roll of a continuous casting machine, and straightening stress, resulting in a large surface crack. The surface crack becomes a surface defect of the steel product in the subsequent rolling process. Therefore, in order to prevent the occurrence of surface defects in the steel product, it is necessary to remove or break the surface cracks at the slab stage by grinding or grinding the slab surface.

  Inhomogeneous solidification in the mold is likely to occur particularly in a steel having a carbon content of 0.08 to 0.17% by mass (referred to as “medium carbon steel”). In medium carbon steel, a peritectic reaction occurs during solidification. It is believed that the inhomogeneous solidification in the mold is caused by transformation stress due to volume shrinkage during transformation from δ iron (ferrite) to γ iron (austenite) by this peritectic reaction. That is, the solidified shell is deformed by the strain caused by the transformation stress, and the solidified shell is separated from the inner wall surface of the mold by this deformation. The portion separated from the inner wall surface of the mold is cooled by the mold, and the thickness of the solidified shell at the portion away from the inner wall surface of the mold (the portion away from the inner wall surface of the mold is referred to as “depression”) is reduced. It is considered that the stress is concentrated on this portion and the surface cracks are generated by reducing the thickness of the solidified shell.

  In particular, when the slab drawing speed is increased, the average heat flux from the solidified shell to the mold cooling water increases (the solidified shell is rapidly cooled), and the heat flux distribution is irregular and irregular. Since it becomes uniform, the occurrence of slab surface cracks tends to increase. Specifically, in a slab continuous casting machine having a slab thickness of 200 mm or more, surface cracks are likely to occur when the slab drawing speed is 1.5 m / min or more.

  Conventionally, it has been attempted to use a mold powder having a composition that is easily crystallized for the purpose of preventing cracking of the slab surface of a steel type with a peritectic reaction such as the above-mentioned medium carbon steel (see, for example, Patent Document 1). ). When a mold powder having a composition that is easily crystallized is used, the thermal resistance of the mold powder layer increases and the solidified shell is slowly cooled. This is because the stress acting on the solidified shell is lowered by slow cooling, and surface cracks are reduced. However, only the slow cooling effect by the mold powder does not provide sufficient improvement in non-uniform solidification, and it is not possible to prevent the occurrence of cracks in steel types having a large transformation amount.

  Therefore, in order to prevent slab surface cracking of a steel type accompanied by a peritectic reaction, Patent Document 2 discloses that a plurality of parts having lower thermal conductivity than copper are independent on the inner wall surface of a continuous casting mold. The thickness of the solidified shell caused by the transformation from δ iron to γ iron in medium carbon steel with surface cracking and peritectic reaction due to non-uniform cooling of the solidified shell at the initial stage of solidification It is described that surface cracks due to non-uniformity can be effectively prevented.

JP 2005-297001 A International Publication No. 2014/002409

  In continuous casting of steel, molten steel is poured into the mold while applying vertical vibrations to the mold, and mold powder is injected into the molten steel surface in the mold, and the molten slag formed by melting the mold powder is solidified. It flows between the shell and the inner wall of the mold, and the solidified shell is prevented from being baked on the mold by vibration and molten slag. Periodic uneven surfaces called oscillation marks are formed on the surface of the slab obtained by pulling out from the mold the solidified shell whose tip portion will be deformed by vibration (see FIGS. 6B and 6B). See solidified shell 11a in C)).

  When the difference between the top and valley of the concavo-convex surface of the oscillation mark increases, the distance between the inner wall of the mold and the solidified shell locally increases or decreases toward the mold exit, and solidification of the solidified shell in the casting direction occurs. It becomes non-uniform | heterogenous and it becomes easy to generate | occur | produce the vertical crack between the top parts (or trough parts) which an uneven surface adjoins. Also. The valley of the recess has a smaller slab width than the other parts, and stress tends to concentrate on the straightening zone during continuous casting, which is the starting point for transverse cracking of the slab. Cracks are likely to occur. Patent Document 2 does not consider the problem caused by the oscillation mark, and uses the continuous casting mold of Patent Document 2 and appropriately changes the composition of the mold powder as described in Patent Document 1. However, in particular, in the continuous casting of molten steel of medium carbon steel, the actual situation is that a technique for effectively suppressing the surface crack of the slab caused by the aforementioned oscillation mark has not been established.

  The present invention has been made in view of the above circumstances, and the object of the present invention is, in particular, an oscillation formed in a slab when continuously casting a molten steel of a steel type that is susceptible to surface cracking, such as medium carbon steel. It is to provide a continuous casting method of steel that suppresses surface cracks caused by marks.

The gist of the present invention for solving the above problems is as follows.
[1] A continuous casting method for steel in which molten steel is poured into a continuous casting mold and the molten steel is drawn while vibrating the continuous casting mold in a casting direction to produce a slab. The casting mold is from an arbitrary position above the meniscus to a position below the meniscus by a length R (mm) or more below the slab drawing speed Vc (m / min) determined by the following equation (1). In the range of the inner wall surface of the water-cooled copper mold, a metal having a thermal conductivity of 80% or less or 125% or more with respect to the thermal conductivity of the mold is formed into a circular groove or pseudo-circular groove provided on the inner wall surface. A plurality of different metal filling portions each having a diameter of 2 to 20 mm or an equivalent circle diameter of 2 to 20 mm, which are formed by filling the grooves, are independently provided, and the filling thickness H (mm) of the different metal filling portion and the above Equivalent diameter or circle of different metal filling part d (mm) satisfies the relationship of the following equation (2), and the casting amplitude of the vibration casting S of the mold for continuous casting, S (mm) and frequency f (times / min), and the dissimilar metal filling portion. The distance between centers in PH (mm) satisfies the relationship of the following formula (3).
R = 2 × Vc × 1000/60 (1)
0.5 ≦ H ≦ d (2)
S / 2 ≦ PH ≦ 1000 × Vc / f (3)
[2] The interval P (mm) between the dissimilar metal filling portions and the diameter or equivalent circle diameter d of the dissimilar metal filling portions satisfy the relationship of the following expression (4): Steel continuous casting method.
P ≧ 0.2 × d (4)
[3] The continuous casting method of steel according to [1] or [2], wherein the molten steel is a medium carbon steel having a carbon content of 0.08 to 0.17% by mass.

  According to the present invention, a distance between centers of different metal filling portions in a mold is obtained by using a mold in which a plurality of concave grooves formed on the inner wall of the mold is filled with a different metal having a thermal conductivity different from that of the mold. Therefore, by optimizing the vibration conditions, the difference between the top and bottom of the concavo-convex surface of the oscillation mark is reduced, and there is no vertical crack between adjacent tops or horizontal cracks at the bottom of the recess. Pieces can be manufactured.

It is the figure which looked at the mold long side which constitutes a part of mold for continuous casting from the inner wall surface side. It is a figure which shows the site | part of the mold long side in which the dissimilar metal filling part shown in FIG. 1 was formed. It is a figure which shows notionally the change of the thermal resistance in three cross sections of the casting_mold | template long side shown in FIG. It is explanatory drawing which shows the site | part of the casting_mold | template long side in which the dissimilar metal filling part of the form different from FIG. 1 was formed. It is a graph which shows the relationship between heat conduction ratio and slab surface crack length (mm / m). It is a figure which shows the solidification shell formed in the inner wall vicinity of the casting_mold | template long side shown in FIG.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the accompanying drawings. The continuous casting mold is an example of a continuous casting mold for casting a slab cast piece, and is configured by combining a pair of mold long sides and a pair of mold short sides. FIG. 1 is a view of the long side of the mold as viewed from the inner wall surface side. The range of the inner wall surface from the upper position away from the meniscus position in the long casting mold 1 by a distance Q (the distance Q is an arbitrary value) to the lower position away from the meniscus by a distance R is circular. A plurality of concave grooves are provided, and the circular concave grooves are filled with a metal having a thermal conductivity lower or higher than the thermal conductivity of the mold (hereinafter referred to as “foreign metal”). A plurality are formed. “Menicus” means “molten steel surface in mold”.

  Similarly to the long side of the mold, the short metal side not shown is formed with a different metal filling portion on the inner wall surface side, and the description of the short side of the mold will be omitted. However, in a slab slab, stress concentration is likely to occur in the solidified shell on the long side surface due to its shape, and surface cracks are likely to occur on the long side surface side. Therefore, it is necessary to install a dissimilar metal filling part on the long side of the continuous casting mold for slab cast, but it is not always necessary to install a dissimilar metal filling part on the short side of the mold. .

  FIG. 2 is an enlarged view of a portion where the dissimilar metal filling portion of the long side of the mold shown in FIG. 1 is formed, (A) is a view of the portion viewed from the inner wall surface side, and (B) is (A) FIG. The dissimilar metal filling portion 3 is formed on the inner wall surface side of the mold long side 1 independently of the heat of the mold by a plating means, a spraying means or the like inside a circular groove 2 having a diameter d of 2 to 20 mm. A dissimilar metal having a thermal conductivity of 80% or less or 125% or more with respect to the conductivity is filled and formed to have a thickness H. Reference numeral 5 denotes a cooling water flow path, and reference numeral 6 denotes a back plate. The intervals P between all the different metal filling portions 3 do not have to be the same in particular, but the intervals P between all the different metal filling portions 3 are ensured to ensure periodic fluctuations in the thermal resistance described later. It is desirable to be the same. In the following, a case where the thermal conductivity of the dissimilar metal is 80% or less with respect to the thermal conductivity of the mold is described.

  1 and 2, the shape of the inner wall surface of the mold long side 1 of the dissimilar metal filling portion 3 is circular, but it is not necessary to be circular. For example, any shape may be used as long as it has a so-called “corner” -like shape, such as an ellipse, and is close to a circle. However, even in the case of a shape close to a circle, as described later, the equivalent circle diameter determined from the area of the dissimilar metal filling portion 3 having a shape close to a circle is in the range of 2 to 20 mm.

  FIG. 3 is a diagram conceptually showing changes in thermal resistance at three positions on the long side of the mold. By disposing a plurality of different metal filling portions 3 having a thermal resistance larger than that of the mold in the width direction and the casting direction of the continuous casting mold in the vicinity of the meniscus including the position of the meniscus, continuous in the mold width direction and the casting direction in the vicinity of the meniscus. The thermal resistance of the casting mold increases and decreases regularly and periodically. As a result, the heat flux from the solidified shell in the vicinity of the meniscus, that is, in the initial stage of solidification, to the continuous casting mold increases and decreases regularly and periodically. This regular and periodic increase / decrease in the heat flux reduces the stress and thermal stress generated by the transformation from δ iron to γ iron, and reduces the deformation of the solidified shell caused by these stresses. By reducing the deformation of the solidified shell, the non-uniform heat flux distribution resulting from the deformation of the solidified shell is made uniform, and the generated stress is dispersed to reduce the amount of individual strain. As a result, occurrence of surface cracks on the surface of the solidified shell is prevented. In addition, even if the dissimilar metal filling part 3 in which the thermal resistance is lower than the mold long side 1 is installed, the thermal resistance of the continuous casting mold can be regularly increased and decreased.

Considering the influence on the initial solidification, the dissimilar metal filling portion 3 is separated from the meniscus by the distance R calculated from the following equation (1) according to the slab drawing speed Vc (m / min) during steady casting. (Mm) It shall be installed to the position below the meniscus.
R = 2 × Vc × 1000/60 (1)

  That is, the distance R is related to the time required for the slab (solidified shell) after starting solidification to pass through the range in which the dissimilar metal filling portion 3 is installed, and for at least 2 seconds after the start of solidification, It is preferable to stay within the range where the dissimilar metal filling part 3 is installed. In order for the slab to exist in the range where the dissimilar metal filling part 3 is installed for at least 2 seconds after the start of solidification, the dissimilar metal filling part 3 is installed below the meniscus by a distance R or more determined by the equation (1). Need to be.

  By securing the time for the cast slab after the start of solidification to stay in the range where the dissimilar metal filling part 3 is installed for 2 seconds or more, the effect of the periodic fluctuation of the heat flux by the dissimilar metal filling part 3 is sufficient. As a result, the effect of preventing cracks on the slab surface can be obtained even during high-speed casting where a surface crack is likely to occur or during the casting of medium carbon steel. In order to stably obtain the effect of the periodic fluctuation of the heat flux by the dissimilar metal filling part 3, it is more preferable to secure 4 seconds or more as the time for the slab to pass through the range where the dissimilar metal filling part 3 is installed. preferable.

  On the other hand, the position of the upper end portion of the dissimilar metal filling portion 3 may be anywhere as long as it is above the meniscus, and therefore the distance Q may be any value exceeding zero. However, since the meniscus fluctuates in the vertical direction during casting, the upper end portion of the dissimilar metal filling portion 3 is always located above the meniscus to a position about 10 mm above the meniscus, preferably about 20 mm above. The dissimilar metal filling part 3 is preferably installed. In general, the meniscus is positioned 60 to 150 mm below the upper end of the long copper plate 1, and the installation range of the dissimilar metal filling portion 3 may be determined accordingly.

  The shape of the dissimilar metal filling portion 3 is assumed to be circular or nearly circular. Hereinafter, a shape close to a circle is referred to as a “pseudo circle”. When the shape of the dissimilar metal filling portion 3 is a pseudo circle, a groove processed on the inner wall surface of the mold long side 1 to form the dissimilar metal filling portion 3 is referred to as a “pseudo circular groove”. The pseudo circle is a shape that does not have a corner, such as an ellipse or a rectangle with R formed at the corner, and may be a shape like a petal pattern. When a vertical groove or a lattice groove is provided on the surface of the mold copper plate, and this groove is filled with a different metal, the difference between the different metal and copper is caused by a difference in thermal strain at the boundary surface between the different metal and copper and at an orthogonal portion of the lattice portion. There is a problem that stress concentrates and cracks occur on the surface of the mold copper plate. On the other hand, since the boundary surface between the dissimilar metal and copper becomes a curved surface by making the shape of the dissimilar metal filling portion 3 circular or pseudo-circular as in the present invention, stress is concentrated on the boundary surface. The advantage that it is hard to crack and a crack does not generate | occur | produce on the casting_mold | template copper plate surface expresses.

The diameter or equivalent circle diameter of the dissimilar metal filling portion 3 needs to be 2 to 20 mm. By setting it as 2 mm or more, the heat flux in the dissimilar metal filling portion 3 is sufficiently lowered, and the above-described effect can be obtained. Moreover, by setting it as 2 mm or more, it becomes easy to fill a different type metal into the inside of the circular ditch | groove 2 or a pseudo | simulated circular ditch | groove (not shown) by a plating means or a spraying means. On the other hand, by setting the diameter of the dissimilar metal filling portion 3 or the equivalent circle diameter to 20 mm or less, a decrease in heat flux in the dissimilar metal filling portion 3 is suppressed, that is, the solidification delay is suppressed by the dissimilar metal filling portion 3. The stress concentration on the solidified shell at that position is prevented, and the occurrence of surface cracks in the slab can be prevented. That is, when the diameter or equivalent circle diameter exceeds 20 mm, surface cracks occur, so the diameter or equivalent circle diameter of the dissimilar metal filling portion 3 needs to be 20 mm or less. When the shape of the dissimilar metal filling portion 3 is a pseudo circle, the equivalent circle diameter d of the pseudo circle is calculated by the following equation (5).
Equivalent circle diameter d = (4 × C / π) ^1/2 (5)
In the formula (5), C is the area (mm ² ) of the dissimilar metal filling portion 3.

  In FIG. 1, the dissimilar metal filling part 3 having the same shape is installed in the casting direction or the mold width direction. However, in the present invention, it is not always necessary to install the dissimilar metal filling part 3 having the same shape. Moreover, as long as the diameter or equivalent circle diameter of the dissimilar metal filling part 3 is in the range of 2 to 20 mm, the dissimilar metal filling part 3 having a different diameter may be installed in the casting direction or the mold width direction. In this case as well, it is possible to prevent slab surface cracking due to non-uniform cooling of the solidified shell in the mold.

The filling thickness H of the dissimilar metal filling part 3 must be 0.5 mm or more. By setting the filling thickness H to 0.5 mm or more, the heat flux in the dissimilar metal filling portion 3 is sufficiently lowered, and the effect of periodic fluctuations in the heat flux can be obtained. Further, the filling thickness H needs to be equal to or less than the diameter of the dissimilar metal filling portion 3 or the equivalent circle diameter d (mm). Since the filling thickness is equal to or smaller than the diameter of the dissimilar metal filling portion 3 or the equivalent circle diameter, filling of the dissimilar metal into the circular concave groove and the pseudo circular concave groove by the plating means or the spraying means becomes easy, and No gaps or cracks occur between the filled dissimilar metal and the mold copper plate. If a gap or crack occurs between a dissimilar metal and the mold copper plate, the filled dissimilar metal will crack or peel off, leading to a reduction in mold life, cracking of the slab, and even a restrictive breakout. . That is, the filling thickness H of the dissimilar metal filling portion 3 needs to satisfy the following formula (2).
0.5 ≦ H ≦ d (2)

  In this case, the filling thickness H of the dissimilar metal is equal to or less than the depth of the circular concave groove or the pseudo circular concave groove. The upper limit value of the filling thickness H is determined by the diameter d of the circular groove. However, since the above effect is saturated when the filling thickness H exceeds 10.0 mm, the filling thickness H is preferably not more than the diameter d of the circular groove and not more than 10.0 mm.

Moreover, it is preferable that the space | interval P between the different metal filling parts 3 is 0.2 times or more of the diameter of the different metal filling part 3, or the equivalent circle diameter d. That is, it is preferable that the distance P satisfies the relationship of the following expression (4) with respect to the diameter or equivalent circle diameter of the dissimilar metal filling portion 3.
P ≧ 0.2 × d (4)

  The interval P is sufficiently large, and the difference between the heat flux in the dissimilar metal filling part 3 and the heat flux of the inner wall part of the mold (the part where the dissimilar metal filling part 3 is not formed) becomes large, and the above effect can be obtained. The upper limit value of the interval P is not particularly defined, but if the interval is too large, the area ratio of the dissimilar metal filling portion 3 is lowered, so that it is preferably set to “2.0 × d” or less.

  FIG. 4 shows an example in which a plating layer for protecting the copper mold surface is provided on the inner wall surface of the copper mold. It is preferable to provide a plating layer 4 on the inner wall surface of the copper mold on which the dissimilar metal filling portion 3 is formed in order to prevent wear of the solidified shell and cracking of the mold surface due to thermal history. The plating layer 4 is sufficient by plating a commonly used nickel-based alloy such as a nickel-cobalt alloy (Ni-Co alloy). The thickness h of the plating layer 4 is preferably 2.0 mm or less. By setting the thickness h of the plating layer 4 to 2.0 mm or less, the influence of the plating layer 4 on the heat flux can be reduced, and the effect of periodic fluctuation of the heat flux by the dissimilar metal filling portion 3 can be sufficiently obtained. Can be obtained.

  As described in Patent Document 2, if a mold in which a plurality of different metal filling portions are independently formed on the inner wall surface of the mold is used, periodic fluctuations in the heat flux occur, and the slab surface cracks occur. The effect of preventing cracks on the slab surface can be obtained even during high-speed casting, where medium-carbon steel is likely to occur. In order to cause the periodic fluctuation of the heat flux, the thermal conductivity λ of the dissimilar metal needs to be 80% or less or 125% or more with respect to the thermal conductivity λc of the mold (copper). If the thermal conductivity λ is larger than 80% or smaller than 125% with respect to the thermal conductivity λc, the effect of the periodic fluctuation of the heat flux by the dissimilar metal filling portion 3 is insufficient, so that the slab The effect of preventing slab surface cracking is insufficient at the time of high-speed casting in which surface cracks are likely to occur or during casting of medium carbon steel.

  As the dissimilar metal, it is possible to use Ni (thermal conductivity: about 90 W / (m · K)) and Ni alloy (thermal conductivity: about 40 to 90 W / (m · K)) that are easily plated and sprayed. Copper alloy (thermal conductivity: about 100 to 398 W / (m · K)) can be used. Moreover, metals other than Ni alloy and copper alloy can also be used. As the mold, pure copper (having a thermal conductivity of about 398 W / (m · K)) or the above-described copper alloy may be employed. In particular, when performing electromagnetic stirring in the mold, in order not to attenuate the magnetic field strength from the coil into the molten steel, use a mold made of a copper alloy with a few percent of components other than copper added to reduce the conductivity. Therefore, the thermal conductivity of the copper alloy is also lower than that of pure copper. By appropriately selecting the different metal and / or the metal of the mold, the thermal conductivity of the different metal is adjusted to 80% or lower or 125% or higher with respect to the thermal conductivity of the mold (copper).

<Experiment>
In order to investigate the relationship between the thermal conductivity λ of dissimilar metals filled in circular grooves filled in copper plates, the thermal conductivity λc of molds (copper), and the surface cracks of slabs produced by continuous casting, continuous steel An experiment was conducted in which casting was performed multiple times. In the continuous casting of steel in the experiment, a water-cooled copper mold having an inner space size of a long side length of 2.1 m and a short side length of 0.25 m and having a dissimilar metal filling portion 3 formed thereon is used. The length from the upper end to the lower end of the water-cooled copper mold (= mold length) is 900 mm. In the continuous casting of the experimental steel, the meniscus is positioned 80 mm below the upper end of the mold, from 30 mm above the meniscus, and 190 mm from the meniscus. The dissimilar metal filling portion 3 is formed on the inner wall surface of the mold in the range up to the lower position (range length: (Q + R) = 220 mm).

  In each of the continuous castings, a copper alloy is used as the mold copper, a Ni alloy is used as the dissimilar metal, and a continuous casting mold in which the heat conductivity λ of the dissimilar metal and the heat conductivity λc of the copper alloy are appropriately changed is used. Thus, the ratio λ / λc of the thermal conductivity of the dissimilar metal to the copper alloy is changed. In all the continuous castings in the experiment, the diameter d is 2 to 20 mm and satisfies the above-mentioned formulas (1) and (2), but the thermal conductivity of different metals is 80% or less with respect to the copper alloy. There are cases where some are satisfied and cases where it is not.

  In the experiment, slab surface cracks were evaluated. FIG. 5 shows the relationship between the thermal conductivity ratio λ / λc (−) and the slab surface crack length (mm / m) in the experiment. When surface cracks occur in the slab, the surface cracks are often vertical cracks, and the length of the vertical cracks is measured. The vertical crack was confirmed visually by color check, and the slab surface crack was evaluated by the length of the vertical crack relative to the slab length. From the graph of FIG. 5, it can be seen that when the thermal conductivity ratio λ / λc is 0.8 (corresponding to a percentage of 80%) or less, the occurrence of surface cracks on the surface of the solidified shell is suppressed. In addition, if the thermal conductivity ratio λ / λc is 1.25 (125% in percentage) or more, the thermal resistance is periodically changed similarly to the case where the thermal conductivity ratio λ / λc is 0.8 or less. Is produced on the inner wall of the mold, and the effect of suppressing the occurrence of surface cracks in the slab can be expected.

  As a result of earnestly examining the mechanism by which the oscillation mark is formed in order to effectively suppress the surface crack of the slab caused by the oscillation mark in the case of using the above-described continuous casting mold, By adjusting the vibration applied to the casting mold for continuous casting according to the distance between the centers of the metal filling part, the mold inner wall part where the heat flux periodically fluctuates at the top or valley of the uneven surface that becomes the oscillation mark It was thought that this could reduce the difference between the top and valley of the solidified shell portion.

  The formation mechanism of the oscillation mark will be described. A solidified shell 11a formed in the vicinity of the inner wall of the mold long side 1 is shown in FIG. In the continuous casting method of steel, molten steel 11 is poured into a cooling mold, a solidified shell 11a is formed in the vicinity of the inner wall of the mold, and solidified having unsolidified molten steel 11 from the mold while vibrating the mold in the vertical direction. The shell 11a is pulled out to produce a steel slab. In FIG. 6, the solidified shell 11a in the initial state where no vibration is applied to the mold long side 1 is shown in (A), and the solidified shell 11a whose tip is deformed by vibration is shown in (B). Subsequently, (C) shows a solidified shell 11a on which a periodic uneven surface is formed.

  Molten steel 11 is poured into a mold whose outlet is closed by a dummy bar, and as shown in FIG. 6A, the molten steel 11 is solidified near the inner wall of the cooled mold long side 1 to form a solidified shell 11a. Is done. Although illustration is omitted, the inner wall of the mold long side 1 is coated with an anti-seizing agent such as rapeseed oil by spraying to prevent the inner wall of the solidified shell 11a from being seized. Further, by introducing mold powder 21 onto the surface of molten steel 11, mold powder is melted on the surface of molten steel 11 to form a layer of molten slag 24, and then on the layer of molten slag 24. A layer of semi-molten powder 23 that is semi-solid (semi-molten) is formed, and a layer of solid powder 22 is formed on the layer of semi-molten powder 23.

  In the state shown in FIG. 6A, when the solidified shell 11a is drawn from the mold at the slab drawing speed Vc (m / min) and the mold is vibrated, as shown in FIG. A gap is formed between the inner wall of the mold long side 1 and the molten slag 24 flows into the gap. Due to the vibration of the mold, the speed at which the mold moves downward may be larger than the drawing speed Vc of the solidified shell 11a. At that time, the highly viscous portion of the molten slag 24 pushes and bends the tip of the solidified shell 11a.

After the tip is pushed and bent, the solidified shell 11a is pulled out from the mold, but the vibration for moving the mold in the vertical direction is repeated, and the tip is pushed and bent again, as shown in FIG. A periodic uneven surface is formed on the solidified shell 11a, and an oscillation mark is formed on the slab. An interval sd (mm) between adjacent valley portions (or top portions) on the concavo-convex surface is a length (mm) that the solidified shell 11a travels during one period T (minute) of vibration, and a drawing speed Vc (m / Min) and period T [= 1 / frequency f (times / min)] (min), it is expressed by the following equation.
sd = 1000 × Vc / f (6)

When the valley portion or the top portion of the solidified shell 11a moves downward in the interval sd (mm), if it contacts both the dissimilar metal filling portion and the mold copper plate, when the portion of the solidified shell 11a in the valley portion or the top portion is removed, Due to periodic heat flux fluctuations, non-uniform solidification at the site is improved, the difference between the top and valley of the uneven surface is reduced, and eventually surface cracks caused by oscillation marks in the slab Occurrence is suppressed. If the distance PH [mm] between the centers in the casting direction of the dissimilar metal filling portion 3 is equal to or less than the interval sd [mm], the portion of the solidified shell 11a at the valley or top during one period T (minute) of vibration. Reliably contacts both the dissimilar metal filling part and the mold copper plate. Substituting the right side of equation (6) for the right side of the inequality PH ≦ sd, the following equation is derived.
PH ≦ 1000 × Vc / f (7)

When the distance PH is smaller than half of the amplitude S, the solidified shell is always in contact with the low thermal conductive metal part or the mold copper plate part during one oscillation period, so that the heat removal of the convex part between the oscillation marks is eliminated. It is not subject to periodic heat flux fluctuations, and the effect of reducing the oscillation mark depth cannot be obtained. As a result, the solidified shell is cooled in a state where it is difficult to receive periodic fluctuations in the heat flux, and therefore, the effect of reducing the difference between the top and valley of the uneven surface is less likely to occur. Therefore, the distance PH needs to be more than half of the amplitude S, and the following equation is derived.
S / 2 × ≦ PH ≦ 1000 × Vc / f (3)

  The distance PH is more preferably 0.6 times or less of the interval sd. This makes it possible to produce a slab free of surface cracks even in steel types and casting conditions where surface cracks are likely to occur.

  As shown in FIG. 6 (B), the vibration applies a force to the layer made of the mold powder so that the molten slag 14 flows into the gap between the solidified shell 11a and the inner wall of the mold. Depending on the physical properties (such as viscosity) of the mold powder (molten slag) and vibration conditions (such as frequency f), the molten slag may not flow into the gap. Therefore, in actual operation, a mold in which a plurality of dissimilar metal filling portions 3 having a center-to-center distance PH calculated by the equation (3) is formed in accordance with a proven mold powder and vibration conditions, It is desirable to perform continuous casting of steel using the mold. If the distance PH satisfies the expression (3), an effect of reducing the difference between the top part and the trough part of the concavo-convex surface occurs regardless of whether or not the distance P satisfies the expression (4). Even if the vibration is based on a sine wave or a non-sine wave, the effect of reducing the difference between the top and valley of the uneven surface is not changed.

  The slab of hypoperitectic steel with a carbon content of 0.08 to 0.17% by mass has a large amount of transformation shrinkage due to the peritectic reaction during solidification, and stress caused by transformation shrinkage acts on the uneven surface of the oscillation mark. By doing so, the difference between the top part and the trough part of the uneven surface tends to increase. The continuous casting mold in which the dissimilar metal filling portion having the dimensions according to the present invention is formed is vibrated according to the present invention to perform continuous casting of steel, so that the distance between the adjacent top portions (or valley portions) of the uneven surface is less than the distance. At the intervals, the stress is dispersed, and the difference between the top and valley of the uneven surface is reduced. Therefore, the present invention is particularly suitable for reducing the surface cracking of molten steel in the subperitectic region.

  According to the present invention, the unevenness of the oscillation mark can be obtained by optimizing the vibration conditions by using a mold in which a groove formed on the inner wall of the mold is filled with a different metal having a thermal conductivity different from that of the mold. The difference between the top part of a surface and a trough part can be reduced, and the cast piece without the vertical crack between adjacent top parts and the transverse crack in the trough part formed along the width direction of a cast piece can be manufactured.

  As shown in FIG. 1, a water-cooled copper mold in which a plurality of circular dissimilar metal filling portions are formed on an inner wall surface is prepared, and a medium carbon steel (chemical component, C: 0.05 to 0.20 mass%, Si : 0.10 to 0.30 mass%, Mn: 0.50 to 1.20 mass%, P: 0.010 to 0.030 mass%, S: 0.002 to 0.010 mass%, Al: 0 0.020 to 0.050 mass%, the remaining Fe and other inevitable impurities) were cast with the prepared water-cooled copper mold, and a test was conducted to investigate the surface cracks of the cast slab after casting. The water-cooled copper mold has an inner space size with a long side length of 2.1 m and a short side length of 0.22 m.

  The length from the upper end to the lower end of the water-cooled copper mold used (= mold length) was 950 mm, and the position of the meniscus (molten steel surface in the mold) during steady casting was set at a position 100 mm below the upper end of the mold. First, in the range from a position 60 mm below the upper end of the mold to a position 300 mm below the upper end of the mold (Q = 20 mm, R is appropriately changed), a circular groove is processed on the inner wall surface of the mold. Next, using a plating means in the inside of the circular concave groove, nickel alloy (thermal conductivity: 80 (W / (m · K))) is filled as a dissimilar metal to form a dissimilar metal filling portion. did.

The mold material is copper having a thermal conductivity of about 380 (W / (m · K)), the diameter d (mm) of the dissimilar metal filling portion 3, the filling thickness H of the dissimilar metal filling portion 3, and the casting. Continuous casting of steel by using a water-cooled copper mold with different center-to-center distance PH in the direction, and further changing the slab drawing speed Vc (m / min) and the amplitude S and frequency f of vibration applied to the mold Was performed a plurality of times (Nos. 1-33). As the mold powder, one having a basicity (mass% CaO) / (mass% SiO ₂ ) of 1.25 and a viscosity at 1300 ° C. of 1.3 poise was used.

  No. 1-3, the surface crack of the manufactured slab was measured. Surface cracks are confirmed visually by color check, and the lengths of both vertical cracks along the casting direction and transverse cracks along the slab width direction are measured, and the total length (mm) in the casting direction is measured. By dividing by the length (m) of the slab, the surface crack length [mm / m] of the slab was calculated and obtained as a measure of the surface crack. Further, the thickness of the slab was measured at the end and the center in the width direction of the slab, and when the difference between the two was 5 mm or more, it was determined that bulging occurred, and the presence or absence of bulging was evaluated. Furthermore, the difference between the top and valley was measured from the irregularities on the slab surface measured using a laser distance meter.

  Table 1 shows the results of the mold used in each continuous casting, the vibration conditions, the surface cracking index, the occurrence of bulging, and the difference between the top and valley of the oscillation.

  A dissimilar metal filling portion metal having a thermal conductivity of 80% or less with respect to the thermal conductivity of the mold is formed in the range from the meniscus to the position below the meniscus by a length determined by the formula (1) and below, and Using a mold that satisfies the formula (2) and the diameter d of the dissimilar metal filling portion is 2 to 20 mm, No. 1 that satisfies the formula (3) is satisfied. In the case of continuous casting, in the column of “Invention Example / Comparative Example” in Table 1, “Invention Example” is described, and in the case of continuous casting that does not satisfy any of these, “Comparative Example” is described. is there. In the water-cooled copper mold in which the dissimilar metal filling portion 3 filled with the dissimilar metal having a heat conductivity of 80% or less with respect to the heat conductivity of the mold is formed by the above-described experiment, the surface crack of the slab is generated. In the mold used in the example of the present invention, the metal thermal conductivity of the dissimilar metal filling portion 3 is 80% or less with respect to the thermal conductivity of the mold.

  No. For Nos. 1-9, no. In Nos. 2 to 8, the diameter d of the dissimilar metal filling portion is in the range of 2 to 20 mm, and the difference between the top and valley of the uneven surface of the oscillation mark is 0.5 mm or less. On the other hand, no. In 1 and 9, the diameter d is out of the range, and the difference between the top and the valley is 0.6 mm or more.

  No. For Nos. 10 and 11, No. 10 formed in the range from the meniscus to a position below the meniscus by the length R determined by the expression (1). 10, the difference between the top and the valley is 0.45 mm. 11 is 0.59 mm, which is close to 0.6 mm.

  No. For Nos. 12-18, no. 13 to 17, the formula (2) is satisfied, and the difference between the top and the valley is less than 0.5 mm. In 12 and 18, it exceeds 0.6 mm.

  No. For Nos. 19 to 33, no. Nos. 21 to 31 satisfy the formula (3) and the difference between the top and the valley is less than 0.5 mm. 19, 20 and 32, 33 are over 0.5 mm.

  According to the present invention, by using a mold in which a concave groove formed on the inner wall of the mold is filled with a dissimilar metal having a thermal conductivity different from that of the mold, by optimizing the vibration conditions, the uneven surface of the oscillation mark It was found that the difference between the top portion and the trough portion can be reduced, and a slab without vertical cracks between adjacent top portions and transverse cracks at the trough portions formed along the width direction of the slab can be produced.

DESCRIPTION OF SYMBOLS 1 Mold long side 2 Circular groove 3 Dissimilar metal filling part 4 Plating layer 5 Cooling water flow path 6 Back plate 11 Molten steel 11a Solidified shell 21 Mold powder 22 Solid powder 23 Semi-molten powder 24 Molten slag

Claims (3)

Injecting molten steel into a continuous casting mold, pulling out the molten steel while vibrating the continuous casting mold in the casting direction, and producing a slab, a steel continuous casting method,
The continuous casting mold is lower than the meniscus by a length R (mm) or more below the meniscus from the slab drawing speed Vc (m / min) determined by the following equation (1). In the range of the inner wall surface of the water-cooled copper mold up to the position, a metal having a thermal conductivity of 80% or less or 125% or more with respect to the thermal conductivity of the mold, A plurality of dissimilar metal filling portions each having a diameter of 2 to 20 mm or a circle equivalent diameter of 2 to 20 mm, which are formed by filling the pseudo-circular concave grooves, are independently provided, and the filling thickness H (mm of the dissimilar metal filling portion is ) And the diameter or equivalent circle diameter d (mm) of the dissimilar metal filling portion satisfy the relationship of the following formula (2):
The amplitude S (mm) and frequency f (times / minute) of the vibration of the continuous casting mold and the distance PH (mm) between the centers in the casting direction of the dissimilar metal filling portion are expressed by the following equation (3). A continuous casting method of steel characterized by satisfying.
R = 2 × Vc × 1000/60 (1)
0.5 ≦ H ≦ d (2)
S / 2 ≦ PH ≦ 1000 × Vc / f (3) 2. The continuous steel according to claim 1, wherein the gap P (mm) between the different metal filling portions and the diameter or equivalent circle diameter d of the different metal filling portions satisfy the relationship of the following expression (4). Casting method.
P ≧ 0.2 × d (4)   The said molten steel is a medium carbon steel whose carbon content is 0.08-0.17 mass%, The continuous casting method of the steel of Claim 1 or Claim 2 characterized by the above-mentioned.

Images