JP5808313B2 - Slab cooling method at the initial casting stage - Google Patents

Description

  The present invention relates to a method for cooling a slab at the initial stage of continuous casting.

  In casting using a vertical bending type continuous casting machine, the molten steel injected into the mold is cooled in the mold to be in a solidified state only on the surface (primary cooling), and then drawn from the lower part of the mold. Then, the slab is continuously cast by being drawn out in the horizontal direction through the vertical part, the bending part, the arc part and the correction part. After the vertical portion, cooling water is sprayed on the slab (secondary cooling), and solidification is promoted to the inside of the slab. And the method of adjusting the amount of cooling water in the secondary cooling zone after a vertical part is proposed conventionally.

  For example, in patent document 1, the timing which sprays cooling water on a slab bottom part (front-end | tip part) in a vertical part-a correction | amendment part is delayed, and the spray amount to a slab front-end | tip part is decreased. Thereby, it is made for the slab front-end | tip part to bend easily by a bending part or a correction | amendment part.

  Moreover, in patent document 2, the amount of cooling water sprayed on the slab top part (rear end part) which is a position on the opposite side to patent document 1 (bottom part) is prescribed | regulated in the casting completion method. In this method, in addition to the amount of cooling water, the casting speed at the end of casting is also regulated to prevent overcooling, surface cracks, and surface flaws at the top.

  Patent Documents 3 to 5 disclose methods for relatively controlling the amount of cooling water on the bending inner side (IN side) and the amount of cooling water on the outer bending side (OUT side) of the continuous casting machine. For example, in Patent Document 3, the amount of water in the secondary cooling zone is adjusted based on the temperature of the upper and lower surfaces of the slab measured before forging. Thereby, the curvature of the slab which arises at the time of forging is prevented. Moreover, in patent document 4, the quality of a slab is made favorable by strongly cooling the IN side of a slab from the OUT side. Furthermore, in Patent Document 5, the cooling water amount on the IN side and the OUT side is relatively controlled in the vicinity of the correction portion, thereby preventing the slab from being warped in the width direction and improving the quality.

Japanese Patent No. 3528505 Patent No. 4684204 JP-A-6-339761 Japanese Patent Laid-Open No. 55-5115 Japanese Patent Laid-Open No. 52-9629

  Since the slab bottom part (tip part) of Patent Document 1 is commercialized except for the most advanced part, it is desired that the quality is good. However, Patent Document 1 does not specify a specific amount of cooling water, and since the spray amount (cooling water amount) on the bottom portion of the slab is reduced, defects such as internal cracks due to insufficient cooling. Arise. As a result, a good quality product cannot be obtained. Even if the spray amount is reduced, warping due to recuperation (upward warping or downward warping) may occur after passing through the secondary cooling zone.

  Here, in patent document 2, although the amount of cooling water sprayed on a slab top part (rear end part) is prescribed | regulated, a slab top part is a position opposite to a bottom part, and at the time of completion | finish of casting (casting) This is the part that passes through the secondary cooling zone in the last stage. On the other hand, the slab bottom portion of Patent Document 1 is a portion that passes through the secondary cooling zone in the early stage of casting, and casting conditions are different between the initial stage of casting and the end of casting. For this reason, it is not normally performed to apply the amount of cooling water of Patent Document 2 to a slab bottom portion (initial casting) having different casting conditions.

  In addition, Patent Documents 3 to 5 only describe that the amount of cooling water on the IN side and the amount of cooling water on the OUT side are relatively controlled, and the specific water amount is not clearly described. It does not pay attention to the bottom part.

  Therefore, in the methods of Patent Documents 1 to 5, warpage occurs at the bottom of the slab and quality cannot be improved.

  Therefore, an object of the present invention is to provide a cooling method capable of preventing warpage and improving the quality at the bottom of the slab.

According to the cooling method at the initial stage of casting of the present invention, when casting a slab slab using a vertical bend type continuous casting machine, the acceleration acceleration until the target casting speed is reached after the start of casting is 0.10 [m / min. ² ] to 0.70 [m / min. ² ],
About the cooling conditions from the start of casting until the bottom of the slab reaches the end of the secondary cooling zone,
In the area in the range of 1.0 [m] from the bottom to the top of the slab,
(i) The average amount of cooling water sprayed from directly under the mold to the end position of the vertical portion is set to 2.5 [m ³ / m ² · Hr] to 5.7 [m ³ / m ² · Hr],
(ii) The amount of cooling water sprayed from the bending part start position to the correction part end position always satisfies 0.2 [m ³ / m ² · Hr] or more and 2.2 [m ³ / m ² · Hr] or less. West,
(iii) The amount of cooling water inside the bending in the secondary cooling zone is greater than 0.91 times and not more than 1.20 times the amount of cooling water outside the bending,
To satisfy all the conditions.

Here, “the bottom of the slab” is the tip of the slab where the dummy bar is connected at the beginning of casting (the end near the dummy bar), and “the top of the slab” is the top of the slab. It is the end (the slab rear end) opposite to the bottom. The amount of cooling water is the amount [m ³ ] of cooling water injected per unit area [m ² ] on the surface of the slab per unit time [hour], and is also called cooling density.

  Thus, in the present invention, in the secondary cooling zone, the cooling conditions for the region in the range of 1.0 [m] from the bottom part of the slab are defined as “vertical part”, “bending part, arc part and straightening part. And the water amount ratio between the IN side and the OUT side. Thereby, in the area | region of the range of 1.0 [m] from the slab bottom part, it can prevent that a curvature generate | occur | produces and can be made favorable quality.

  Note that, as described above, the method of Patent Document 2 is a casting end method and is performed under casting conditions different from the initial casting. Therefore, the slab bottom part which is the object of the present invention (early casting) and the slab top part which is the object of Patent Document 2 (at the end of casting) are cast under different conditions. Therefore, the cooling conditions are derived from different viewpoints in the present invention and Patent Document 2, and can be said to be different inventions.

  According to the present invention, it is possible to prevent warping from occurring in the slab bottom portion and to achieve good quality.

It is a schematic diagram which shows the structure of a continuous casting machine. (A), (b) is a schematic diagram explaining a casting bottom part, (b) is an enlarged view of the part enclosed by the dotted line of (a). It is a figure explaining the amount of cooling water of the present invention. It is a schematic diagram explaining the upper curvature and lower curvature of a slab. It is a figure which shows the cooling conditions of an Example. It is a figure which shows the cooling conditions of an Example. It is a figure which shows the cooling conditions of a comparative example. It is a figure which shows the cooling conditions of a comparative example. It is a figure which shows the cooling conditions of a comparative example.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

  Here, the continuous casting method and the cooling method of the slab bottom part which are one Embodiment of this invention are demonstrated, referring FIGS. 1-4.

[Continuous casting machine]
As shown in FIG. 1, the continuous casting machine 100 is a vertical bending type continuous casting machine, in which a tundish 1, an immersion nozzle 2 attached to the bottom of the tundish 1, and a lower part of the immersion nozzle 2 are arranged. And a plurality of rolls provided along the casting path Q from directly below the mold 3. The mold 3 is formed with a substantially rectangular opening in plan view so that a slab slab can be cast.

  In the casting path Q, a vertical part 11, a bending part 12, an arc part 13 and a correction part 14 are provided in this order from directly under the mold 3 toward the downstream side. Here, in this embodiment, the side close to the mold 3 along the casting path Q is called an upstream side, and the side far from the mold 3 is called a downstream side. In addition, in FIG. 1, although the structure of the continuous casting machine 100 is shown typically and only several rolls are shown in each path | route part, such as the perpendicular | vertical part 11-the correction | amendment part 14, actually several rolls are arrange | positioned. ing.

  As shown in FIG. 1, the vertical portion 11 extends in the vertical downward direction from directly below the mold 3, and a plurality of foot rolls 21 are arranged. The bent portion 12 is a portion that is bent so that the radius of curvature R gradually decreases, and a plurality of support rolls 22 that bend the slab 7 into an arc shape are arranged. The arc portion 13 is formed in an arc shape having a constant curvature radius R, and a plurality of support rolls 23 are arranged. And the correction | amendment part 14 is the part bent so that the curvature radius R may become large gradually, and the some support roll 24 which corrects a slab is arrange | positioned. Further, a horizontal portion 15 extending in the horizontal direction is provided on the downstream side of the correction portion 14. A plurality of transfer rolls 25 for transferring the slab to the downstream side and a pinch roll 26 for pulling out the slab are disposed in the horizontal portion 15. In addition, the pinch roll 26 may be arrange | positioned at the perpendicular | vertical part 11, the bending part 12, the circular arc part 13, and the correction | amendment part 14. FIG. Moreover, the spray nozzle 5 is arrange | positioned between the rolls adjacent to a casting direction, and the secondary cooling zone is comprised by these spray nozzles 5. FIG.

  Next, a casting method using the continuous casting machine 100 will be described.

  Molten steel 6 accommodated in the tundish 1 is injected into the mold 3 through the immersion nozzle 2 (start of casting). Here, a dummy bar is installed in advance at the bottom of the mold 3. The molten steel 6 in the mold 3 is cooled, and the surface portion (the portion in contact with the mold 3 and the portion in contact with the upper end portion of the dummy bar) is solidified to form a slab in which a solidified shell is formed. Thereafter, when the dummy bar is pulled out downstream, the slab is pulled out of the mold 3 along with the dummy bar, passes through the vertical portion 11 while being supported by the foot roll 21 disposed immediately below the mold 3, and is bent at the bending portion 12. While being supported by the support roll 22, it is bent into an arc shape. Then, after being transferred to the downstream side while being held by the support roll 23 at the arc portion 13, it is corrected by the correction portion 14 so as to face in the horizontal direction by the support roll 24. Then, in the horizontal part 15, the slab slab which was transferred downstream by the transfer roll 25 and solidified to the inside is cast.

Here, after casting starts, the drawing speed of the slab is increased until the casting speed reaches the target speed. In the present invention, the acceleration until the casting speed reaches the target speed is set to 0.10 [m / min. ² ] or more and 0.70 [m / min. ² ] or less. The acceleration is expressed by the following formula.

In addition, when the acceleration after the start of casting is smaller than 0.10 [m / min. ² ], the amount of molten steel supplied into the mold 3 is small, so that heat is not sufficiently supplied to the vicinity of the molten steel surface in the mold 3. . As a result, generation of deckle (skinning) on the surface of the molten steel and constraining breakout due to insufficient hatching of the powder occur. On the other hand, when the acceleration after the start of casting is greater than 0.70 [m / min. ² ], the flow of molten flux in the mold 3 is insufficient to flow between the slab and the mold, resulting in vertical cracks in the slab. As a result, quality is degraded. Therefore, the acceleration until the casting speed reaches the target speed after the start of casting is set to 0.10 [m / min. ² ] or more and 0.70 [m / min. ² ] or less.

In addition, from the start of casting until the bottom of the slab reaches the end of the secondary cooling zone, a region in the range of 1 m from the bottom of the slab to the top (hereinafter referred to as “slab bottom” Is cooled under the following cooling conditions (see “slab bottom B” shown in FIG. 2). Here, in the present invention, as cooling conditions, (i) the average cooling water amount W _av1 of the vertical portion 11, (ii) the cooling water amount W _{2 of} the bending portion 12, the arc portion 13 and the correction portion 14, and (iii) the bending inner side A water amount ratio W _r between the amount of cooling water and the amount of cooling water outside the bend is defined.

  Further, as shown in FIG. 2B, the “bottommost part” of the slab is an end part (end part on the side close to the dummy bar) to which the dummy bar is connected in the initial stage of casting, and the most advanced part of the slab. Indicates. On the other hand, the “topmost part” is an end part opposite to the “most bottom part”, and indicates the end part of the slab. Here, FIG. 2 shows a state where the slab bottom portion B passes through the vertical portion in a state where the dummy bar is connected.

  In addition, in FIG. 2, the dummy bar connection part c which protruded to the dummy bar side (opposite side to a top part) is provided in the slab bottom part (front-most part). In the initial stage of casting, the dummy bar is connected to the dummy bar connecting portion c, so that the cast piece is drawn downstream along with the dummy bar.

(i) Average cooling water amount W _av1 of the vertical portion 11
When the average cooling water amount _Wav1 is small in the vertical portion 11 (from the position immediately below the mold 3 to the end position of the vertical portion 11), the solidified shell is not sufficiently formed due to insufficient cooling. Therefore, the slab bulges, and this causes internal cracks. In addition, as shown in FIG. 2, a dummy bar is connected to the bottom part of the slab at the initial stage of casting. As the material is pulled out, the solidified shell is torn. As a result, the connection between the dummy bar and the slab becomes insufficient, and the slab cannot be pulled out. Such a problem is likely to occur when the average cooling water amount W _av1 is less than 2.5 [m ³ / m ² · Hr].

On the other hand, when the average cooling water amount _Wav1 is too large, the edge portion of the bottom part of the slab becomes too hard due to overcooling and cannot be bent at the bent portion 12 (see FIG. 1). Thereby, since an edge part collides with the support roll 22, a dummy bar cannot be pulled out smoothly. In addition, when the slab bottom part is supercooled by the vertical part 11, the slab bottom part becomes harder when passing through the correction part 14, so that the support roll 24 exerts a large reaction force from the slab bottom part during correction. receive. Thereby, the load to the roll stand that supports the support roll 24 increases, and the equipment is damaged or destroyed. These problems are likely to occur when the average cooling water amount W _av1 exceeds 5.7 [m ³ / m ² · Hr].

Therefore, in the present invention, when the slab bottom part passes through the vertical part 11, the average cooling water amount _Wav1 sprayed on the slab bottom part is 2.5 [m ³ / m ² · Hr] or more and 5.7 [m. ³ / m ² · Hr] or less (see FIG. 3). In FIG. 3, the average cooling water amount W _av1 is illustrated in a diagram illustrating the cooling water amount. However, when the average cooling water amount W _av1 satisfies the above range, the cooling water amount of the vertical portion 11 is 2.5 [m. ³ / m ² · Hr] or may exceed 5.7 [m ³ / m ² · Hr].

(ii) Cooling water amount W _{2 of the} bending part 12, the arc part 13 and the correction part 14
In the bending part 12, the arc part 13, and the correction part 14 (from the bending part 12 start position to the correction part 14 end position), when the cooling water amount W ₂ is less than 0.2 [m ³ / m ² · Hr], cooling is performed. Due to the shortage, the solidified shell is not sufficiently formed. For this reason, a slab bulges and this causes an internal crack. On the other hand, when the cooling water amount W ₂ exceeds 2.2 [m ³ / m ² · Hr] and is too large, the slab bottom portion becomes hard due to overcooling and the strength is increased. The bent bottom of the slab cannot be sufficiently corrected so as to face in the horizontal direction. For this reason, the slab bottom portion cannot be sufficiently bent back, and warpage occurs. The warpage of the bottom of the slab is caused by a transportation trouble (slab slab) by contacting the support roll 24 or the transfer roll 25 in the straightening section 14 or the horizontal section 15 or contacting the roll stand that supports these rolls. For example) and a stand trouble (such as damage to the roll bearing) are caused (see FIG. 4A). Further, if the slab bottom portion is warped, the surface of the bottom portion cannot be uniformly scoured when the slab is subjected to gas welding and scarf (surface treatment) after casting. In addition, when the warpage is large, equipment trouble occurs due to the warped portion coming into contact with the gas nozzle.

Therefore, in the present invention, the slab bottom portion bending portion 12, the cooling water W ₂ sprayed onto the slab bottom portion when passing through the circular arc portion 13 and the correcting section 14, respectively 0.2 [m ^{3 /} m ² · Hr] to 2.2 [m ³ / m ² · Hr].

(iii) Water volume ratio W _r between the amount of cooling water inside the bend and the amount of cooling water outside the bend
In the secondary cooling zone, as shown in FIG. 1, spray nozzles 5 are arranged on the “bending inside (IN side)” and “bending outside (OUT side)” of the continuous casting machine 100. Therefore, cooling water is sprayed on the slab from the IN side and from the OUT side. Therefore, in the present invention, in the secondary cooling zone, “ratio W _r (= W _IN / of total cooling water amount W _IN sprayed from IN side” and “total cooling water amount W _OUT sprayed from OUT side”. W _OUT ) ”is referred to as“ water amount ratio W _r ”. Here, the “secondary cooling zone” is a portion where the spray nozzle 5 immediately below the mold 3 is disposed.

If the "total amount of cooling water W _IN of IN side" is too much for the "total amount of cooling water W _OUT of the OUT side", after the slab bottom part has passed through the secondary cooling zone, the IN side of the slab As the temperature rises (recuperation), the IN surface side expands. As a result, the slab bottom portion warps downward (see FIG. 4B). On the other hand, when the “OUT-side total cooling water amount W _OUT ” is too much than the “IN-side total cooling water amount W _IN ”, the slab bottom portion passes through the secondary cooling zone, and then the slab OUT surface When the temperature on the side increases (recuperation), the OUT surface side expands. As a result, the slab bottom portion warps (see FIG. 4A). Therefore, the water amount ratio W _r (= W _IN / W _OUT ) of the cooling water sprayed on the bottom of the slab in the secondary cooling zone is set to be larger than 0.91 and not larger than 1.20. In addition, in FIG. 4, the state which the curvature generate | occur | produced in the slab bottom part is illustrated with the continuous line, and the state where the curvature does not generate | occur | produce is illustrated with the dashed-dotted line.

Thus, in the present invention, the amount of cooling water sprayed on the slab bottom portion is adjusted to the following range when the slab bottom portion passes through the secondary cooling zone after the start of casting.
・ Vertical section 11
2.5 [m ³ / m ² · Hr] ≦ average cooling water amount W _av1 ≦ 5.7 [m ³ / m ² · Hr]
-Bending part 12, arc part 13, straightening part 14
0.2 [m ³ / m ² · Hr] ≦ cooling water amount W ₂ ≦ 2.2 [m ³ / m ² · Hr]
・ Secondary cooling zone 0.91 <Water ratio W _r (= W _IN / W _OUT ) ≦ 1.20
Further, after the start of casting, the acceleration A is set to the following range until the casting speed reaches the target speed.
0.10 [m / min. ² ] ≦ Acceleration A ≦ 0.70 [m / min. ² ] or less

  When the slab bottom part is cooled under the above-mentioned conditions, the slab bottom part can be sufficiently bent at the bent part, and at the correction part, the arc-shaped slab bottom part is sufficiently oriented in the horizontal direction. Can be corrected. Moreover, since it can prevent that the IN surface side or OUT surface side of a slab bottom part expand | swells by recuperation after passing a secondary cooling zone, generation | occurrence | production of curvature can be prevented. Furthermore, since bulging due to insufficient cooling can be prevented during casting, the occurrence of internal cracks caused by this can be prevented. Thus, when it cools on the said cooling conditions, generation | occurrence | production of a curvature can be prevented in a slab bottom part, and quality can be made favorable.

  Moreover, since the reaction force that the roll receives from the bottom of the slab can be reduced, the load on the roll stand can be reduced. As a result, operational troubles during casting can be prevented. Furthermore, since it is possible to prevent abnormal connection between the slab and the dummy bar at the initial stage of casting, the slab can be pulled out reliably.

  Next, examples and comparative examples of the present invention will be described.

  The presence or absence of internal cracks and the occurrence of warpage when the cooling conditions of the slab bottom portion were changed were examined.

  Table 1 shows the experimental conditions (acceleration acceleration after the start of casting, casting speed in the steady region, and cooling conditions), and experimental results (internal cracks, dummy bar / slab connection abnormality and warpage). 5 to 9 show cooling conditions. In this example, a continuous casting machine having a length of 40.6 [m] and a radius of curvature R of the arc portion of 10.7 [m] was used as the vertical bending type continuous casting machine. And the slab whose slab size after casting is thickness 280 [mm] x width 2100 [mm] was cast.

  Here, the experimental conditions and evaluation results shown in Table 1 will be described.

[Experimental conditions]
<Acceleration of acceleration after casting starts>
After the start of casting, the slab drawing speed is gradually increased, and the acceleration of the slab drawing speed until reaching the target casting speed is defined as “acceleration acceleration”. Here, the “casting speed” is a drawing speed when the dummy bar or slab is drawn by a pinch roll.
<Casting speed in steady range>
After the start of casting, the casting speed is gradually increased until the target casting speed determined according to the molten steel temperature and the molten steel component is reached. After reaching the target casting speed, the slab is pulled out at a constant speed. In this experiment, the period during which the slab is drawn at a constant speed is referred to as a “steady region”. Table 1 shows the casting speed in the “steady region”.
<Cooling conditions>
The cooling conditions of the “slab bottom portion” from the start of casting until the bottom of the slab reaches the end of the secondary cooling zone are shown. In this experiment, all “slab bottom portions” were defined as “regions in the range of 1 m from the bottom of the slab to the top”.
5 to 9 show the amount of cooling water (secondary cooling water amount) sprayed on the “slab bottom portion” in the secondary cooling zone. Here, “W _av1 ” is the average amount of cooling water in the vertical portion, “W ₂ ” is the amount of cooling water in the bent portion, the arc portion, and the straightening portion, and “W _r ” is the amount of cooling water in the secondary cooling zone. it is the ratio (the ratio _{W iN} / _{W OUT} of the total amount of cooling water _{W OUT} of the total amount of cooling water _{W iN} and OUT side of the iN side).
For areas other than the “slab bottom part” (areas exceeding 1 m from the bottom part of the slab toward the top part), conditions different from the cooling conditions shown in Table 1 (normally implemented cooling conditions) ).

[Evaluation results]
<Internal crack presence / absence>
The internal crack is a defect on the metal structure caused by segregation when the slab solidifies from the inside. When a product obtained by rolling a slab or the like is used in a severe environment such as bending or welding, defects caused by internal cracks cause serious troubles such as breakage and vacancy.
In this experiment, the presence or absence of internal cracks was evaluated by conducting a cross-sectional macro test using ammonium persulfate, as in Japanese Patent No. 4397825. Specifically, after casting, a part (sample) of the bottom part of the slab is collected, and after polishing this sample to a predetermined flatness, ammonium persulfate ((NH ₄ ) ₂ S ₂ O ₈ ) is applied to the polished surface. Was applied to corrode (corrosion treatment). Then, the polished surface was visually confirmed using the naked eye or a microscope. Since the progress of corrosion is different between a portion where a defect on the metal structure is present and a portion where the defect is not present, a corrosion spot is generated at a portion where the defect is present. Therefore, when even one corrosion zone has occurred, it is evaluated that an internal crack has occurred, and “Yes” is shown in Table 1. On the other hand, when no corrosion spots have occurred, it is evaluated that no internal crack has occurred, and is indicated as “none”.
<Existence of abnormal connection with dummy bar>
If there is an abnormality in the part where the dummy bar and the bottom part of the slab (particularly the bottom part) are connected (the solidified shell is torn, etc.) and the slab cannot be pulled out, there is an abnormality. As a result of evaluation, “Yes” is shown in Table 1. On the other hand, when the slab can be drawn as usual, it is evaluated that there is no abnormality, and “None” is indicated.
<Swarp of slab (upper warp or lower warp)>
When the slab is warped, as described above, the warped portion comes into contact with the roll or the roll stand, thereby causing a stand trouble (such as damage to the bearing) or a transport trouble (unsuccessful transport of the slab). Further, in the scarf (surface treatment) after casting, when the cast surface is gas-cut by using a gas nozzle, an equipment trouble occurs due to contact between the warped portion and the gas nozzle.
Therefore, when no stand trouble or transport trouble occurred and when no equipment trouble occurred during scarf, it was evaluated that no warpage occurred at the bottom of the slab, and “None” is shown in Table 1. . On the other hand, if any one of the above troubles occurs due to warpage of the slab bottom portion, it is evaluated that the warpage has occurred in the slab bottom portion and is indicated as “present”.
<Comprehensive evaluation>
If all of the internal cracks, abnormal connection with the dummy bar, and warpage of the slab are “No”, the overall evaluation is “○”, and if there is any “Yes”, the overall evaluation is “×”. "

  Next, the results of Examples and Comparative Examples will be described.

  From Table 1, in Examples 1-15 which satisfy | fill this invention, in the slab bottom part, while the internal crack and the connection abnormality with a dummy bar did not arise, curvature did not generate | occur | produce, but the favorable bottom part was obtained. . On the other hand, in Comparative Examples 1 to 7 that do not satisfy the cooling conditions of the present invention, internal cracks and abnormal connections with dummy bars occurred at the bottom of the slab, and warpage also occurred.

Specifically, in Comparative Examples 1 to 3, the average cooling water amount _Wav1 is 2.0 [m ³ / m ² · Hr], 6.4 [m ³ / m ² · Hr], 2.4 [m ³ / ^m is 2 · Hr], was outside the scope of the present invention ^{(2.5 [m 3 / m 2} · Hr] ≦ W av1 ≦ 5.7 [m 3 / m 2 · Hr]). In Comparative Examples 1 and 3, since it was less than the average cooling water amount W _av1 of the present invention, an internal crack or a connection abnormality with a dummy bar occurred due to insufficient cooling. On the other hand, in Comparative Example 2, since the average cooling water amount _Wav1 of the present invention was larger, the edge portion of the slab bottom portion was hardened and increased in strength due to supercooling, and the edge portion collided with the roll when proceeding to the bending portion. The dummy bar could not be pulled out smoothly. In addition, the load on the roll stand was increased, causing a stand trouble.

In Comparative Examples 4 and 5, the bending portion, the cooling water _{W 2} is 0.1 of a circular arc portion and a straightening portion ^{[m 3 / m 2 · Hr} ], 2.5 in ^{[m 3 / m 2 · Hr} ] Yes, it was outside the range of the present invention (0.2 [m ³ / m ² · Hr] <W ₂ ≦ 2.2 [m ³ / m ² · Hr]). In Comparative Example 4, since it was less than the cooling water amount W ₂ of the present invention, internal cracks occurred due to insufficient cooling. On the other hand, in Comparative Example 5, since was greater than the cooling water W ₂ of the present invention, strength is increased harder edge portion of the slab bottom portion by supercooling, that during straightening cast piece unbending becomes insufficient Caused warpage.

In Comparative Examples 6 and 7, the cooling water amount W _{r in the} secondary cooling zone is 1.21 and 0.91, and the range of the present invention (0.91 [m ³ / m ² · Hr] <W ₂ ≦ 1. 20 [m ³ / m ² · Hr]). In these examples, after passing through the secondary cooling zone, warping occurred at the bottom of the slab due to recuperation.

  Thus, in Examples 1-15 which satisfy | fill this invention, while being able to prevent curvature in a slab bottom part, it was able to make quality favorable. Moreover, the connection abnormality of a slab bottom part and a dummy bar did not occur, and the slab and the dummy bar could be pulled out without any problem. On the other hand, in Comparative Examples 1 to 7 that do not satisfy the cooling condition of the present invention, a good bottom portion was not obtained, and trouble occurred.

  As mentioned above, although embodiment of this invention was described based on drawing, it should be thought that a specific structure is not limited to these embodiment. The scope of the present invention is indicated not by the above description but by the scope of claims, and includes all modifications within the meaning and scope equivalent to the scope of claims.

  For example, in the present embodiment and examples, the “slab bottom portion” is set to “a region in a range of 1 m from the bottom of the slab toward the top portion”, and this region is cooled under the cooling conditions of the present invention. , “Casting bottom part” as “Area in the range exceeding 1 m from the bottommost part of the slab to the topmost part” (for example, a region in the range of 2 m from the bottommost part of the slab to the topmost part) This region may be cooled under the cooling conditions of the present invention. Further, portions other than the “slab bottom portion”, for example, a region exceeding 1 m from the bottom of the cast slab toward the top may be cooled under cooling conditions different from those of the present invention.

  Further, the length of the continuous casting machine and the radius of curvature R of the arc portion described in the present embodiment are examples, and these can be changed. Further, the present invention can be applied to casting of various steel types.

  Furthermore, in this embodiment, as shown in FIG. 2, a dummy bar connecting portion protruding to the dummy bar side (opposite the top portion) is provided at the bottom of the slab (the most advanced portion). It is good also as a structure by which the dummy bar connection part is not provided in the bottom part but a dummy bar is directly connected with the slab bottom part.

  INDUSTRIAL APPLICABILITY The present invention can be used for continuous casting of steel because it can prevent warping from occurring at the bottom of the slab and improve the quality.

DESCRIPTION OF SYMBOLS 1 Tundish 2 Immersion nozzle 3 Mold 4 Roll group 5 Spray nozzle 11 Vertical part 12 Bending part 13 Arc part 14 Correction part 15 Horizontal part 100 Continuous casting machine B Bottom part

Claims (1)

When casting a slab slab using a vertical bending type continuous casting machine, the acceleration of acceleration until the target casting speed is reached after the start of casting is 0.10 [m / min. ² ] or more and 0.70 [m / Min. ² ] The following is a range of cooling conditions from the start of casting until the bottom of the slab reaches the end of the secondary cooling zone,
In the area in the range of 1.0 [m] from the bottom to the top of the slab,
(i) The average amount of cooling water sprayed from directly under the mold to the end position of the vertical portion is set to 2.5 [m ³ / m ² · Hr] to 5.7 [m ³ / m ² · Hr],
(ii) The amount of cooling water sprayed from the bending part start position to the correction part end position should be 0.2 [m ³ / m ² · Hr] or more and 2.2 [m ³ / m ² · Hr] or less. ,
(iii) The amount of cooling water inside the bending in the secondary cooling zone is greater than 0.91 times and not more than 1.20 times the amount of cooling water outside the bending,
A slab cooling method in the early stage of casting characterized by satisfying all the conditions of