JP6269543B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a steel continuous casting method for preventing surface cracks typified by transverse cracks in a slab.

  In recent years, from the viewpoint of reducing the manufacturing cost of steel products, the need to improve the straightness rate of slabs manufactured by continuous casting has increased, but this has occurred on the slab surface as one of the factors that hinders the improvement of the straightness rate. There are surface cracks called transverse cracks.

  Recently, the production of low alloy steels containing various alloy elements such as Nb, V, Ni, and Cu has been increasing due to demands on material properties. With the addition of alloy elements, the frequency of occurrence of surface cracks in the slab increases, and the achievement rate continues to be at a standstill in response to demands for reducing manufacturing costs. During secondary cooling of continuous casting, the surface temperature of the slab becomes close to the transformation temperature (about 750 to 850 ° C.) when the metal structure transforms from γ to α, and the hot ductility of the slab decreases. At this time, it is known that surface cracks occur in the slab of low alloy steel due to mechanical stress such as bending or straightening occurring in the slab. Therefore, in order to suppress the occurrence of surface cracks, the surface temperature of the slab is lower than the temperature range where the hot ductility is lowered (hereinafter referred to as “embrittlement temperature range” as appropriate) in the bent part and the straightened part of the slab. The method of making the side or the high temperature side is usually performed.

  However, it is difficult to eliminate surface cracks that occur in the slab simply by avoiding the surface temperature of the slab from the embrittlement temperature range, and there is a technology related to the slab cooling history that focuses on the surface layer structure of the slab. Proposed. In Patent Document 1, primary cooling by the mold is finished with the solidified shell thickness being somewhat thin, secondary cooling is started, and the surface temperature of the entire surface of the slab is within 2 minutes at the longest after leaving the mold. The temperature is once lowered to a range of 600 ° C. or higher and Ar 3 point [° C.] or lower, and then the secondary cooling is performed so that both the slab surface temperature in the bending portion and the straightening portion of the continuous casting machine are 850 ° C. or higher. Has been proposed. According to this invention, in Patent Document 1, reduction in cracking sensitivity of the slab surface structure (unclear structure of γ grain boundaries) and avoidance of the high temperature side of the embrittlement temperature range in the bending part and the straightening part of the vertical bending type continuous casting machine, It is described that the surface cracks of the slab can be effectively eliminated, and as a result, no scarfing / care of the slab and improvement in the straightness rate of the slab are achieved.

JP 9-225607 A

  In patent document 1, it is said that the surface crack of a slab is effectively eliminated, and as a result of conducting an experiment for manufacturing a slab by the method described in patent document 1, the present inventors have confirmed that Although it was confirmed that the occurrence of surface cracks could be prevented to some extent, it was also confirmed that the occurrence of lateral cracks in the slab could not be completely prevented depending on the slab drawing speed.

  The present invention has been made in view of the above circumstances. The object of the present invention is low alloy containing Ni, Cu, V, Nb, etc., which are increasing in surface cracking sensitivity in recent years as well as ordinary steel. Even when steel is continuously cast, it is to provide a continuous casting method of steel that reliably prevents lateral cracking of a slab.

  Usually, in continuous casting of steel, molten steel is poured into the mold while applying vertical vibration to the mold, and mold powder is poured into the molten steel surface in the mold, and the molten slag formed by melting the mold powder Is caused to flow between the solidified shell and the inner wall of the mold to prevent the solidified shell from being seized into the mold by vibration and molten slag. An uneven surface called an oscillation mark is formed on the surface of a slab obtained by pulling out from the mold a solidified shell whose tip is subject to deformation by vibration.

  Regarding the vibration applied to the mold, the amplitude and frequency from the top dead center to the bottom dead center of the mold are generally appropriately changed according to the slab drawing speed. In the case of producing a slab by applying the method described in Patent Document 1, the present inventors particularly examined the reason why the occurrence of transverse cracks in the slab cannot be completely prevented. I guessed it was in the oscillation mark. And even if the present inventors apply the method described in Patent Document 1, depending on the vibration applied to the mold and the cooling method of the slab immediately after coming out of the mold, the casting is caused by the oscillation mark. The depth of the trough formed on the surface of the piece tends to increase, and when the depth increases, it was confirmed that transverse cracks occurred starting from the trough, and the present invention was completed.

The gist of the present invention is as follows.
[1] While injecting molten steel into a mold of a vertical bending type continuous casting machine, the mold is cooled while vibrating the mold in the casting direction to form a solidified shell having a thickness of 9 to 20 mm. A continuous casting method of steel in which a slab is cast by pulling out a shell, wherein the vibration of the mold has an amplitude of 6 mm or less and a frequency of 100 [times / minute] or more, and the slab is removed from an outlet of the mold. Until contact with the first roll, water is sprayed on the slab under the condition that the specific water amount H represented by the following formula (1) is less than 0.4 [l / kg-slab], Next, after cooling the slab until the surface temperature of the slab becomes Ar 3 point [° C.] or less, the slab is reheated, so that at least in the bending portion of the vertical bending die continuous casting machine, The surface temperature of the piece is Ac3 point [° C.] or higher. Continue casting method.
H = Q / (W × T × Vc × ρ) (1)
Here, Q is the amount of water [l / min] sprayed on the surface of the slab formed from the long side of the mold, W is the slab width [m], and T is the slab thickness [m. Vc is the slab drawing speed [m / min], and ρ is the density [kg / m ³ ] of the slab.
[2] Of one cycle of the mold vibration, the time tn when the mold speed Vm [m / min] with the casting direction being positive becomes equal to or higher than the slab drawing speed Vc [m / min] and the speed Vm are The steel continuous casting method according to [1], wherein an index NSR represented by a ratio of the time tn to a total time with the time tp at which the slab drawing speed Vc is less than 30% is 30% or less.

  According to the present invention, it is possible to promote the flattening of the surface of the slab immediately after being drawn out from the mold, and as a result, it becomes possible to shallow the valley of the surface of the slab after the correction portion. By making the appropriate heat history and the trough shallow, transverse cracks that can occur in the slab can be reliably prevented.

It is a figure which shows a vertical bending type continuous casting machine. It is a graph which shows the relationship between specific water quantity H [l / kg-slab] and the depth of a trough [mm]. It is a graph which shows the relationship between the depth [mm] of a trough part, and the number of transverse cracks [piece / m < ² >].

  In the present invention, the molten steel is poured into the mold of the vertical bending type continuous casting machine, the mold is cooled while vibrating the mold, the solidified shell is formed, and the slab formed by pulling out the solidified shell comprises: After leaving the mold and before coming into contact with at least the first roll, the slab is weakly cooled to promote flattening of the slab surface by the roll. A vertical bending type continuous casting machine is shown in FIG. 1, and an example of an embodiment of the present invention will be described with reference to FIG.

  The vertical bending type continuous casting machine 1 includes a mold 5, a tundish 2 installed above the mold 5, a plurality of support rolls 6 arranged below the mold 5, and a plurality of guides. It has a roll 7 and a pinch roll 8 installed between the guide rolls 7.

  Although not shown, a ladle for containing the molten steel 11 is installed above the tundish 2. Molten steel 11 is poured into the tundish 2 from the bottom of the ladle. An immersion nozzle 4 to which a sliding nozzle 3 is attached is installed at the bottom of the tundish 2, and the molten steel 11 is cast through the immersion nozzle 4 in a state where a predetermined amount of molten steel 11 stays in the tundish 2. 5 is injected. A cooling water channel is formed in the mold 5, and the cooling water is passed through the cooling water channel. Thereby, the molten steel 11 is extracted from the inner surface of the mold 5 and solidified to form a solidified shell 13, and an unsolidified layer 14 made of the molten steel 11 is formed inside the solidified shell 13.

  The support roll 6, the guide roll 7, and the pinch roll 8 are composed of one set of upper and lower roll sets suitable for supporting the cast slab 12. While the slab 12 is supported by the guide roll 7, the pinch roll 8 rotates while sandwiching the slab 12, and the slab 12 having the solidified shell 13 and the unsolidified layer 14 inside is pulled out. As shown in FIG. 1, in the vertical bending die continuous casting machine 1, the slab 12 is drawn linearly downward from the mold 5 in the vertical direction, and then bent appropriately below the mold 5, and the bent slab 12 is corrected. A plurality of guide rolls 7 and pinch rolls 8 are arranged so as to be pulled out in the horizontal direction.

  Secondary cooling zones 17a, 17b, 17c, 17d, and 17e divided on the upper surface side and the lower surface side with the slab 2 interposed therebetween are installed in this order along the casting direction from directly below the mold 5. The secondary cooling zone 17 a cools the slab 12 between the mold 5 and the first support roll 6. The starting position of the secondary cooling zone 17d is a bent portion that bends the slab 12 drawn linearly, and the bent slab 12 is pulled out horizontally in the middle of the secondary cooling zone 17e. Thus, the correction part which corrects the slab 12 is provided.

  Although illustration is omitted between the secondary cooling zones 17d and 17e, secondary cooling zones are appropriately provided so that the amount of secondary cooling water can be adjusted independently in each secondary cooling zone. In addition, a spray nozzle such as a water spray nozzle or an air mist spray nozzle is provided between the guide roll 7 and the pinch roll 8. While the slab 12 is supported and conveyed by the guide roll 7, the solidified shell 13 is appropriately cooled and reheated in the secondary cooling zones 17 a to 17 e, the solidification of the unsolidified layer 14 proceeds, and the slab 12 Solidification is complete.

  A plurality of transport rolls 9 are arranged downstream of the guide roll 7, and a gas cutting machine 10 for cutting the slab 12 is installed above the transport roll 9 in synchronization with the drawing speed Vc of the slab 12. Has been. The cut slab 12a is sent to the next process.

  Although not shown, mold powder is poured onto the molten steel 11 poured into the mold 5, and the mold powder is melted by the heat of the molten steel 11, so that molten slag is formed on the molten steel 11. Is generated. In addition, a device for vibrating the mold 5 up and down along the casting direction is attached to the mold 5 (not shown), and the mold 5 is vibrated by the device so that the inner wall of the mold 5 and the solidified shell 13 The molten slag is caused to flow into the gap. Thereby, the solidified shell 13 is prevented from being burned onto the inner wall of the mold 5.

  Due to the vibration of the mold 5, the speed Vm when the mold 5 is directed downward (in the casting direction) may be larger than the slab drawing speed Vc. At that time, the high-viscosity portion of the molten slag generated on the molten metal surface of the molten steel 11 pushes and bends the tip of the solidified shell 13. After the tip portion is pushed and bent, the solidified shell 13 is pulled out from the mold, but during that time, the vibration that moves the mold 5 is repeated, and the tip portion is pushed and bent again. Thereby, a periodic uneven surface is formed on the surface of the solidified shell 13. However, the surface of the solidified shell 13 is pushed by the support roll 6, the guide roll 7, and the pinch roll 8, and the uneven surface is flattened. Therefore, although the surface of the slab 12 (slab 12a) is finally flat to some extent, a trough is formed on the surface of the slab 12a along the width direction. If the trough becomes deeper, it will lead to lateral cracking of the slab 12a.

  In the present invention, the amplitude of vibration applied to the mold 5 is set to 6 mm or less, the frequency is set to 100 [times / min] or more, and the solidified shell 13 having a thickness of 9 to 20 mm is formed by cooling with the mold 5. At least until the slab 12 comes into contact with the first support roll 6, the amount of water sprayed on the slab 12 is suppressed. Thereby, this invention accelerates | stimulates the planarization of the surface of the solidification shell 13 by the support roll 6 just under the casting_mold | template 5, and suppresses the depth of the trough part of the surface of the slab 12a.

  By setting the frequency to 100 [times / minute] or more, the period of vibration (= 1 / frequency) is reduced, and the period [minute / time] is multiplied by the slab drawing speed [m / minute]. The depth of the troughs is suppressed by reducing the interval between the tops or troughs adjacent to the calculated uneven surface and by setting the amplitude to 6 mm or less. On the other hand, if the thickness of the solidified shell 13 is less than 9 mm, the static pressure of the molten steel 11 cannot be withstood and a breakout may occur. On the other hand, if the thickness exceeds 20 mm, the solidified shell 13 is too thick even if the surface temperature of the solidified shell 13 is weakly cooled, and it is difficult to promote the flattening of the surface of the solidified shell 13 by the support roll 6. The metal structure of the slab 12 is improved by setting the surface temperature of the slab 12 to Ar 3 point [° C.] or less, reheating the slab 12 and setting the surface temperature of the slab 12 to Ac 3 point [° C.] or more. It is difficult to obtain an effect.

Until the slab 12 having the solidified shell 13 having a thickness of 9 to 20 mm comes into contact with the support roll 6 from the mold 5, the specific water amount H represented by the following formula (1) is 0.4 [l / Cooling water is sprayed on the slab 12 in the secondary cooling zone 17a under the condition of less than kg-slab], and the surface of the slab 12 is cooled.
H = Q / (W × T × Vc × ρ) (1)

H is the specific water amount [l / kg-slab], Q is the amount of water [l / min] sprayed on the surface of the slab 12 formed from the long side of the mold 5, and W is the slab 12. , T is the thickness [m] of the slab 12, Vc is the drawing speed [m / min] of the slab 12, and ρ is the density of the slab [kg / m] ³ ]. The cooling of the slab 12 in the secondary cooling zone 17a limits the amount of water [l] sprayed per unit mass [kg] of the slab 12, and corresponds to so-called weak cooling. By pressing the surface of the solidified shell 13 in a relatively thin and weakly cooled state with the support roll 6, it is possible to effectively promote the flattening of the uneven surface formed on the slab 12 with a reduced thickness by the roll. Thus, the valley on the surface of the slab 12 becomes shallow. The specific water amount H is desirably 0.01 [l / kg-slab] or more. This is because if the specific water amount H is too small, the solidified shell bulges, causing internal cracks and breakout.

  For the slab 12 that has been weakly cooled after passing through the secondary cooling zone 17a, the slab 12 is then strongly cooled by appropriately adjusting (increasing) the amount of cooling water in the secondary cooling zone 17b. It is desirable that the surface temperature (temperature of the solidified shell 13) be set to Ar3 point [° C.] or less. The surface temperature of the slab 12 becomes Ac3 point [° C.] or higher at least at the start position (bending portion) of the secondary cooling zone 17d by reducing the amount of cooling water sprayed in the secondary cooling zone 17c. It is desirable to reheat the slab 12 as described above. Thereby, the metal structure of the solidified shell 13 is transformed. Note that the slab 12 may be reheated without spraying the cooling water so that the surface temperature of the slab 12 becomes Ac3 point [° C.] or higher.

  By setting the temperature of the solidified shell 13 to Ar 3 point [° C.] or less, the solidified shell 13 has a ferrite-pearlite structure in which the γ grain boundary with low cracking sensitivity is unclear. If the strong cooling in the secondary cooling zone 17b is terminated early and the solidified shell 13 is reheated to the Ac3 point [° C.] or higher in the secondary cooling zone 17c, the temperature of the solidified shell 13 is reduced to the embrittlement region. In addition to the high temperature side, a structure in which the γ grains are refined can be generated. Thereby, even if stress is applied to the bent portion, the crack sensitivity can be reduced.

  In the secondary cooling zone 17b, the surface temperature of the slab 12 (the temperature of the solidified shell 13) is preferably 600 ° C. or higher. This is because if the surface temperature is 600 ° C. or higher, the force required for bending and straightening the slab becomes small. In the secondary cooling zone 17c, it is desirable that the surface temperature of the slab 12 be 1100 ° C. or lower. This is because if the surface temperature is excessive, the solidified shell bulges and the possibility of internal cracks and breakout increases.

  The vibration applied to the mold 5 includes a time tn at which the speed Vm [m / min] of the mold 5 with the casting direction being positive is equal to or higher than the slab drawing speed Vc [m / min] in one cycle of vibration, and the speed The index NSR represented by the ratio of the time tn to the total time of the time tp when Vm is less than the slab drawing speed Vc is preferably 30% or less. The vibration may be a simple vibration represented by a sine wave. However, if the vibration is a non-sine wave and satisfies an index (ratio) NSR of 30% or less, the vibration has an amplitude of 6 mm or less and a vibration frequency of 100. [Times / minute] or more.

  Note that the amplitude and frequency of vibration are preferably determined as appropriate in consideration of the slab drawing speed Vc. When increasing the slab drawing speed Vc, the frequency is increased in proportion to the slab drawing speed Vc in order to keep the index NSR constant. The amplitude is desirably 3 mm or more. This is because, if the amplitude is too small, the index NSR is unlikely to be sufficiently large according to the drawing of the slab even if the frequency is increased. The frequency is preferably 300 times / minute or less. This is because if the frequency is excessive, the vibration acceleration is excessive and the equipment load is excessive.

<Experiment>
A slab 12 to be a thick steel plate was continuously cast a plurality of times by using the vertical bending type continuous casting machine 1 having the configuration shown in FIG. In each continuous casting, the specific water amount H in the secondary cooling zone 17a is appropriately changed as [l / kg-slab], and the relationship between the specific water amount H and the depth of the valley [mm] The relationship between the depth [mm] and the number of transverse cracks [pieces / m ² ] was examined. The mold 5 was configured so that the width W of the slab 12 was 2100 mm and the thickness T was 300 mm.

The composition of the molten steel 11 is such that the C content is 0.08 to 0.14 mass%, the Mn content is 1.60 mass%, the Cu content is 0.10 to 0.15 mass%, and the Ni content is 0.00. The balance is 30 to 0.35% by mass, the balance is Fe and inevitable impurities, and the density ρ of the slab 12 is 7800 [kg / m ³ ]. It can be said that the molten steel 11 contains Cu and Nb, and the slab 12 obtained from the molten steel 11 is highly sensitive to surface cracks. Moreover, Ar3 point was 850 degreeC and Ac3 point became 900 degreeC. These values were obtained by actual measurement with a linear dilatometer. The slab is such that the surface temperature of the slab 12 is not more than Ar3 point [° C.] in the secondary cooling zone 17b and the surface temperature of the slab 12 is not less than Ac3 point [° C] in the secondary cooling zone 17c. The amount of cooling water sprayed on 12 was adjusted. In addition, the thickness of the solidified shell 13 at the outlet of the mold 5 was 9 mm. The amount of cooling water supplied to the mold 5 when forming the solidified shell 13 and the amount of cooling water sprayed on the slab 12 in the secondary cooling zones 17b and 17c are appropriately calculated in advance by heat transfer solidification calculation. I asked for it.

  In the experiment, the slab drawing speed Vc was 0.8 m / min, the amplitude of vibration applied to the mold 5 was 6 mm, and the frequency was 100 [times / min]. The distance from the mold 5 to the support roll 6 with which the slab 12 first comes into contact was 1 m. In each continuous casting of the experiment, the amount of water Q [l / min] sprayed onto the surface of the slab 12 formed from the long side of the mold 5 is appropriately changed, and the specific water amount H represented by the above-described formula (1). Was changed in the range of 0.20 to 0.80 [l / kg-slab].

The surface of the slab 12 is flattened by a pinch roll 8 or the like, but a valley portion due to vibration remains slightly. About the depth [mm] of the trough, the surface of the slab 12a is measured using a laser distance meter, and the depth from the flat surface to the deepest trough is measured using the flat surface of the slab 12a as a reference. The depth of the trough was set to [mm]. For the number of transverse cracks [pieces / m ² ], the slab 12a is pickled, the cracks on the surface of the slab 12a are observed by penetrant flaw inspection, and those having a length of 1 mm or more are identified as transverse cracks. The number of transverse cracks per unit area on the surface of the piece 12a was defined as the number of transverse cracks [pieces / m ² ].

The relationship between the specific water amount H [l / kg-slab] and the depth [mm] of the valley is shown in FIG. 2, and the relationship between the depth [mm] of the valley and the number of transverse cracks [pieces / m ² ]. Is shown in FIG. From the graph of FIG. 2, it can be seen that the depth [mm] of the valley portion is suppressed as the specific water amount H is suppressed, and the surface of the slab 12 is flattened by being weakly cooled in the secondary cooling zone 17a. It can be seen that is effectively promoted. Further, the depth of the trough when the specific water amount H is 0.4 [l / kg-slab] is 0.5 mm at the maximum, and the specific water amount H is 0.4 [l / kg-slab]. In the case of the following, it can be seen that the depth of the valley portion can be 0.5 mm or less. From the graph of FIG. 3, it can be seen that the number of transverse cracks can be set to 0 [piece / m ² ] when the depth of the valley is 0.5 mm or less. Therefore, it was confirmed that the number of transverse cracks [pieces / m ² ] was effectively suppressed by setting the specific water amount H to 0.4 [l / kg-slab] or less.

  According to the present invention, it is possible to promote the flattening of the surface of the slab immediately after being drawn out from the mold, and as a result, it becomes possible to shallow the valley of the surface of the slab after the correction portion. By making the appropriate heat history and the trough shallow, transverse cracks that can occur in the slab can be reliably prevented.

(1) thickness of the solidified shell 13 at the outlet of the mold 5, (2) thickness T of the slab 12, (3) slab drawing speed Vc, (4) vibration conditions such as frequency, and (5) two Whether the surface temperature of the slab 12 in the secondary cooling zone 17b satisfies the Ar3 point [° C] or lower, or whether the surface temperature of the slab 12 in the secondary cooling zone 17c satisfies the Ac3 point [° C] or higher. The slab 12a was continuously cast a plurality of times in the same manner as in the experiment except that the conditions were appropriately changed (Test Nos. 1-32). In each continuous casting, in the same manner as in the experiment, the specific water amount H is appropriately changed as [l / kg-slab], and the relationship between the specific water amount H and the depth [mm] of the valley portion, and the valley portion The relationship between the depth [mm] and the number of transverse cracks [pieces / m ² ] was investigated. Table 1 shows the conditions (1) to (5), the depth [mm] of the valleys, and the number of transverse cracks [pieces / m ² ].

  Test No. In continuous casting of 3 to 11 and 16 to 19, the present invention is satisfied and is an example of the present invention. On the other hand, test no. In the continuous casting of 1 and 2, the thickness of the solidified shell 13 [mm], test No. 12 to 15, the amplitude [mm], test No. 20 to 23, the vibration frequency [times / min], test No. In 24-28 and 32, the specific water amount H does not satisfy the present invention and is a comparative example. In addition, Test No. In 29-31, even after water is sprayed on the slab 12 under the condition that the specific water amount H is less than 0.4 [l / kg-slab], the surface temperature of the slab 12 is set at Ar3 point [° C.]. Then, the slab 12 cannot be reheated so that the surface temperature of the slab 12 becomes Ac3 point [° C.] or higher, which is a comparative example.

Test No. For Nos. 1 and 2, test no. In No. 1, the solidified shell was too thin, breakout occurred, and a slab could not be obtained. Therefore, in Table 1, “−” is described in the depth [mm] of the valleys and the number of transverse cracks [pieces / m ² ]. Test No. In No. 2, the solidified shell was too thick, and the flattening of the surface of the solidified shell 13 by the support roll 6 was not promoted, and the depth value of the valley portion was 0.72 mm. In addition, the slab 12 could be reheated so that the surface temperature of the slab 12 was Ar 3 point [° C.] or lower and then the surface temperature of the slab 12 was Ac 3 point [° C.] or higher. The number of transverse cracks was 7.5 / m ² .

Test No. 3 to 11, in addition to satisfying the present invention, the index NSR was 30% or less, the depth of the valley portion was suppressed to 0.35 mm or less, and the number of transverse cracks was reduced to zero. Test No. 16-19, although the index NSR exceeds 30%, the present invention is satisfied, the depth of the valley is suppressed to less than 0.55 mm, and the number of transverse cracks is 1.0 pieces / m ² or less. did it.

Test No. In 12 to 15, the depth of the valley is greater than 0.6 mm, transverse cracks number is over 5.8 / ^{m 2.} Test No. In 20 to 32, most of the depth of the valley portion exceeds 0.6 mm, and the number of transverse cracks is about 3.0 / m ² , but most is 5.0 / m ^2. Is over.

  According to the present invention, it can be seen that the flattening of the surface of the slab immediately after being drawn out from the mold can be promoted, and as a result, the valley of the surface of the slab after the correction portion can be made shallower. It can also be seen that the appropriate thermal history and the shallow trough can reliably prevent transverse cracks that may occur in the slab.

DESCRIPTION OF SYMBOLS 1 Vertical bending type continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Support roll 7 Guide roll 8 Pinch roll 9 Conveyance roll 10 Gas cutting machine 11 Molten steel 12 Cast slab 12a Cut slab 13 Solidified shell 14 Not yet Solidified layer 17a-17e Secondary cooling zone

Claims (2)

While casting molten steel into the mold of a vertical bending type continuous casting machine, the mold is cooled while vibrating the mold in the casting direction to form a solidified shell having a thickness of 9 to 20 mm, and the solidified shell is pulled out. A continuous casting method of steel for casting slabs,
The vibration of the mold has an amplitude of 6 mm or less and a vibration frequency of 100 [times / minute] or more.
From the outlet of the mold until the slab comes into contact with the first roll, the specific water amount H expressed by the following formula (1) is 0.01 [l / kg-slab] or more and 0.4. Water is sprayed on the slab under the condition of less than [l / kg-slab],
Next, after cooling the slab until the surface temperature of the slab becomes Ar 3 point [° C.] or less, the slab is reheated, so that at least in the bending portion of the vertical bending die continuous casting machine, A method for continuously casting steel, wherein the surface temperature of the piece is set to Ac3 point [° C.] or higher.
H = Q / (W × T × Vc × ρ) (1)
Here, Q is the amount of water [l / min] sprayed on the surface of the slab formed from the long side of the mold,
W is the slab width [m],
T is the slab thickness [m],
Vc is the slab drawing speed [m / min],
ρ is the density [kg / m ³ ] of the slab.   Of one cycle of the mold vibration, the time tn when the mold speed Vm [m / min] with the casting direction being positive becomes equal to or higher than the slab drawing speed Vc [m / min] and the speed Vm are the slab. 2. The continuous casting method for steel according to claim 1, wherein an index NSR represented by a ratio of the time tn to a total time with the time tp at which the drawing speed Vc is less than 30% is 30% or less.

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