JP7031628B2 - Continuous steel casting method - Google Patents

Description

The present invention relates to a continuous steel casting method effective in reducing central segregation of slabs.

In continuous casting of steel, the molten steel injected into the mold discharges solute elements such as P, S, and Mn into the unsolidified molten steel in the process of solidification. These solute elements are concentrated in the remaining molten steel and cause so-called segregation. The degree of this segregation is maximized at the center of the continuously cast slab (hereinafter, also referred to as "slab") that is finally solidified. In addition, molten steel undergoes volume shrinkage of several percent in the process of solidification. This volume shrinkage is a negative pressure in the solid / liquid coexistence region containing a large amount of equiaxed crystals near the end of solidification in the cross section orthogonal to the casting direction of the slab (hereinafter, also referred to as “cross section”). Generates a void in the. As a result, the molten steel in which the solute element is concentrated (hereinafter, also referred to as "concentrated molten steel") passes through a narrow passage in the solid / liquid coexistence region and is sucked into the negative pressure void portion of the slab. A central segregation is formed in the central part.

Central segregation adversely affects product quality. Therefore, various techniques have been proposed and implemented to reduce central segregation. In Patent Document 1, the molten steel in the tundish is injected into a mold at a superheat degree of 50 ° C. or less, and the molten steel in the strand is stirred by applying an electromagnetic force to make the solidified structure at the center of the slab finely equiaxed. It is disclosed to crystallize. Furthermore, when the solid phase ratio at the center of the cross section of the strand is in the range of 10% to 80%, the unsolidified portion is lightly reduced by 5 mm to 50 mm to compensate for solidification shrinkage, thereby suppressing the flow of concentrated molten steel at the end of solidification. It is disclosed to do.

Further, Patent Document 2 discloses that the dendrite is destroyed by giving an impact to the slab from the side and applying energy to the tip of the dendrite at the solidification interface. According to the method of Patent Document 2, by destroying the dendrite, the vicinity of the terminal solidification stage is equiaxially crystallized, and the central segregation is improved.

Japanese Unexamined Patent Publication No. 6-126405 International Publication No. 2009/019969

However, in the technique disclosed in Patent Document 1 in which stirring by electromagnetic force and light reduction are used in combination, the solidified structure in the center of the slab becomes fine equiaxed crystals by stirring the electromagnetic force, so that the center of the slab becomes a fine equiaxed crystal. Flow resistance increases. This reduces the flow and accumulation of concentrated molten steel in the center of the slab. Further, by applying a light reduction at the end of solidification, the solidification shrinkage of the slab is compensated and the flow driving force of the concentrated molten steel is reduced. Further, by suppressing the flow of the concentrated molten steel, a high reduction effect of central segregation can be expected. However, in the technique disclosed in Patent Document 1, electromagnetic force is applied for stirring molten steel, and there is a problem that enormous power consumption and maintenance cost are required.

Further, the technique disclosed in Patent Document 2 is a method for improving segregation by shearing dendrites with mechanical energy and equiaxed crystallization. However, since the striking energy also affects the equipment, there is a problem that the maintenance cost becomes enormous.
Further, it has been reported that in steel having a carbon concentration of 0.4 wt% or more, the central structure hardly becomes equiaxed crystals unless an external force such as the above-mentioned electromagnetic stirring or impact is applied.

Therefore, the present invention solves these problems of the prior art, and can equiaxially crystallize the central structure of the slab by an inexpensive method without introducing new equipment. The purpose is to propose a continuous casting method for steel.

According to one aspect of the present invention, it is a continuous casting method of steel for casting high carbon steel having a carbon concentration of 0.4 wt% or more at a casting position where the central solid phase ratio of the slab is 0.3. Casting is performed under the condition that the temperature gradient G (° C./min ) on the liquid phase side at the solidification interface and the solidification rate V (mm / min) satisfy the relationship of the formula (1), and the central solid phase ratio is more than 0 and 0. At the casting position of less than 3.3, the slab is reduced in the thickness direction with a plurality of first light reduction rolls, and at the casting position where the central solid phase ratio is 0.3 or more and 0.7 or less, a plurality of firsts are used. 2. A method for continuous casting of steel is provided in which the slab is reduced in the thickness direction with a light reduction roll.

According to one aspect of the present invention, there is provided a continuous steel casting method capable of equiaxed crystallization of the central structure of a slab by an inexpensive method without introducing new equipment.

It is sectional drawing which shows the continuous casting machine in one Embodiment of this invention. It is a graph which shows the relationship between the reduction rate and the average segregation grain size in an equiaxed crystal structure.

In the following detailed description, many specific details are described by way of illustration of embodiments of the invention to provide a complete understanding of the invention. However, it is clear that one or more embodiments can be implemented without the description of such particular details. Also, the drawings are schematic representations of well-known structures and equipment for the sake of brevity.

<Continuous casting method of steel>
A steel casting method according to an embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 shows a schematic cross-sectional view showing an example of the continuous casting machine 1 used in the present embodiment. The continuous casting machine 1 is an equipment that solidifies the molten steel 2 and casts a slab 3 having a predetermined cross-sectional shape. As shown in FIG. 1, the continuous casting machine 1 includes a mold 11 and a plurality of rolls including a plurality of first light reduction rolls 12 and a plurality of second light reduction rolls 13.

The mold 11 is a mold for cooling the molten steel 2. The mold 11 is provided so that the cross-sectional shape orthogonal to the casting direction, which is the drawing direction of the slab 3 shown by the broken line in FIG. 1, is a square, depending on the shape of the cross section of the slab to be cast. The molten steel 2 injected into the mold 11 is cooled to form a solidified shell 21 at the interface with the mold 11.
A plurality of rolls are provided side by side in the casting direction on the downstream side of the mold 11 in the casting direction. Further, the plurality of rolls face two rows (inside the curved casting direction) facing the thickness direction of the slab 3 (the direction orthogonal to the casting direction in the cross-sectional view of FIG. 1) so as to sandwich the slab 3. Rows and outer rows) are provided. The number of rolls in each row is the same, and a pair of rolls provided in different rows facing each other in the thickness direction is called a roll set. Hereinafter, the distance between rolls (separation distance) in each roll set is referred to as a roll gap. In the example shown in FIG. 1, only a plurality of first light reduction rolls 12 and a plurality of second light reduction rolls 13, which are rolls on the downstream side in the casting direction, are shown. A plurality of rolls (not shown) will be provided from nearby. The plurality of rolls are for holding the solidified shell 21 and pulling out the slab 3, and are configured so that a part of the rolls on the downstream side in the casting direction is rotationally driven. Further, a plurality of roll sets are provided in each of the plurality of segments provided along the casting direction. The segment can adjust the roll gap by forming a plurality of rolls inside the casting direction of the plurality of roll sets so as to be movable in the thickness direction. In the example shown in FIG. 1, the first light reduction roll 12 and the second light reduction roll 13 are provided in different segments. Then, the slab 3 is formed by adjusting the plurality of first light reduction rolls 12 and the plurality of second light reduction rolls 13 inside the curved casting direction (upper side in FIG. 1) according to the target reduction amount. Be overwhelmed.

The plurality of first light reduction rolls 12 are provided in the segment at the casting position (hereinafter, also referred to as “first section”) where the central solid phase ratio fs is more than 0 and less than 0.3. The casting position is a position in the continuous casting machine 1 with respect to the casting direction in which the slab 3 moves. The plurality of first light reduction rolls 12 are reduced for the purpose of miniaturizing the slab structure, and are in the thickness direction with respect to the slab 3 having a central solid phase ratio fs of more than 0 and less than 0.3. Perform reduction. In the reduction by the plurality of first light reduction rolls 12, the reduction speed is adjusted by adjusting the roll gap and the casting speed of each roll. The reduction speed is a value obtained by multiplying the reduction gradient (mm / m) of the plurality of first light reduction rolls 12 by the casting speed (m / min), and passes through the plurality of first light reduction rolls 12. It indicates the amount of the slab 3 that is pressed down per hour. The reduction gradient is obtained from the relationship between the roll gap of each roll pair of the plurality of first light reduction rolls 12 and the installation position in the casting direction, and is within the range in which the plurality of first light reduction rolls 12 are provided. The amount of decrease in the roll gap with respect to the casting position of is shown. Since the purpose of the reduction in the first section is to crush the dendrite arm, it is desirable to reduce the pressure at least 20% or more of the length in the casting direction of the first section.

The plurality of second light reduction rolls 13 are provided in the segment of the casting position (hereinafter, also referred to as “second section”) in which the central solid phase ratio fs is 0.3 or more and 0.7 or less. The plurality of second light reduction rolls 13 are for the purpose of compensating for the solidification shrinkage of the slab, and are for the slab 3 having a central solid phase ratio fs of 0.3 or more and 0.7 or less. Then, the reduction is performed in the thickness direction. In the reduction by the plurality of second light reduction rolls 113, the reduction speed is adjusted by adjusting the roll gap and the casting speed of each roll, as in the case of the plurality of first light reduction rolls 12. Since the purpose of the reduction in the second section is to prevent the movement of the residual liquid due to solidification shrinkage, it is desirable to always perform a light reduction in the section, and at least 50% of the length in the casting direction of the second section. It is desirable to reduce the pressure in the above. The reduction by the plurality of first light reduction rolls 12 and the plurality of second light reduction rolls 13 is performed at the reduction speed described later.
Further, in the plurality of first light reduction rolls 12 and the plurality of second light reduction rolls, the roll diameter and the roll interval are the bulging amount of the slab 3 between the rolls (the amount of bulging of the slab 3 due to the static iron pressure) and the bulging. It is set so that it does not cause internal cracking due to bulging and does not induce the movement of residual liquid in the center of the slab due to bulging.

The central solid phase ratio fs refers to the solid phase ratio of the central point of the slab 3 in the cross section which is a cross section orthogonal to the casting direction. The central solid phase ratio fs can be calculated from the molten steel temperature at the central point of the molten steel 2. That is, the relational expression between the molten steel temperature and the solid phase ratio can be calculated from the correspondence between the molten steel temperature having a solid phase ratio of 0 and the molten steel temperature having a solid phase ratio of 1.0 and the temperature difference. Then, if the molten steel temperature at the center point of the molten steel 2 can be calculated, the solid phase ratio corresponding to the molten steel temperature can be calculated using this relational expression.

The temperature of the center point of the molten steel 2 can be calculated by using the surface temperature of the solidified shell 21 and the heat transfer calculation formula. The heat transfer calculation formula is described in, for example, The Iron and Steel Institute of Japan, "Heat Transfer Experiment and Calculation Method in Continuous Steel Fragment Heating Furnace", The Iron and Steel Institute of Japan, published on May 10, 1971. Can be used.
The installation positions (or set positions) of the plurality of first light reduction rolls 12 and the plurality of second light reduction rolls 13 in the continuous casting machine 1 are determined as follows. First, continuous casting is performed in advance by the continuous casting machine 1, and a thermocouple is provided in the solidification shell 21. Next, a thermocouple is used to acquire a temperature change in the surface temperature of the solidified shell 21 due to a difference in the position in the casting direction during casting, thereby acquiring a profile of the surface temperature of the solidified shell 21 with respect to the casting direction. Further, using the acquired surface temperature profile of the solidified shell 21 and the heat transfer calculation formula, the temperature profile along the casting direction of the center point of the molten steel 2 is calculated. Then, the profile of the central solid phase ratio fs along the casting direction of the molten steel 2 is calculated by using the profile of the temperature of the center point and the relational expression between the molten steel temperature and the solid phase ratio calculated in advance. From the profile of the central solid phase ratio fs obtained in this way, the plurality of first light reduction rolls 12 and the plurality of second light reduction rolls 13 are installed so as to satisfy the above range of the central solid phase ratio fs. The position is determined.

Further, the continuous casting machine 1 is provided with a cooling facility (not shown) for cooling the slab 3 by spraying water on the surface in a region provided with a plurality of rolls. This cooling facility controls the temperature gradient and the solidification rate of the molten steel 2 inside the unsolidified slab 3 by controlling the injection amount of water and controlling the cooling rate. Further, the cooling equipment controls the cooling of the slab 3 by the temperature gradient and the solidification rate described later.

In the continuous steel casting method according to the present embodiment, the molten steel 2 is continuously cast using the continuous casting machine 1 to cast the bloom-shaped slab 3. The molten steel 2 is a high carbon steel having a carbon concentration of 0.4 wt% or more, which is reported to produce equiaxed crystals without using an external force such as electromagnetic stirring or striking. In the present embodiment, first, the refined molten steel 2 is injected into the mold 11 and the molten steel 2 is cooled to form a solidified shell 21. Then, the solidified shell 21 is further cooled while being pulled out in the casting direction, so that the slab 3 solidified to the inside is cast.

At this time, with respect to the slab 3 in which the unsolidified portion remains inside, in the first section, which is the casting position where the central solid phase ratio fs is more than 0 and less than 0.3, a plurality of first light reduction rolls 12 are used. The slab 3 is reduced (first light reduction step). In the first light reduction step, it is preferable to perform reduction by a plurality of first light reduction rolls 12 or more, and it is more preferable to set the reduction speed to 0.7 mm / min or more and 1.8 mm / min or less. Here, in the range where the central solid phase ratio fs is more than 0 and less than 0.3, the inner tip of the solidification shell 21 is dendrite solidified. In the first light reduction step, the tip of the dendrite inside the solidification shell 21 is destroyed by the physical stress applied by the reduction. Then, the broken dendrite tip becomes a core when the central portion of the slab 3 solidifies, so that a fine equiaxed crystal structure is generated when the central portion of the slab 3 solidifies. If the reduction speed is less than 0.7 mm / min, the dendrite destructive effect may not be sufficiently obtained. On the other hand, when the reduction speed exceeds 1.8 mm / min, cracks may occur inside the slab 3, which is not desirable. The reduction speed is adjusted by adjusting the roll gaps of the plurality of first light reduction rolls 12 according to the determined casting speed (pulling speed of the slab 3 in the casting direction), or by setting the roll gap. On the other hand, it is set by adjusting the casting speed. The reduction speed is set in the same manner for the plurality of second light reduction rolls 13 described later.

After the first light reduction step, in the second section where the central solid phase ratio fs is 0.3 or more and 0.7 or less, the plurality of second light reduction rolls 13 make 0.5 mm / min or more 1 The reduction is performed at a reduction speed of .5 mm / min or less (second light reduction step). In the second light reduction step, it is preferable to apply light reduction by a plurality of second light reduction rolls 13 so as to compensate for solidification shrinkage, and the reduction speed is 0.5 mm / min or more and 1.5 mm / min or less. Is more preferable. In the second light reduction step, the solidification shrinkage is compensated by the reduction, so that the slab 3 having fine central segregated grains is cast. This makes it possible to reduce the central segregation of solute elements such as P, S, and Mn inside the slab 3. When the rolling speed is less than 0.5 mm / min, the coagulation shrinkage cannot be sufficiently compensated, so that the effect of improving the central segregation may not be obtained. On the other hand, when the reduction speed exceeds 1.5 mm / min, reduction is performed more than the solidification shrinkage, so that segregation may worsen.

Further, in the continuous steel casting method according to the present embodiment, the temperature gradient and the solidification rate are controlled, and when the central solid phase ratio fs is 0.3, the temperature gradient and the solidification rate are expressed by the following equation (1). Casting is performed under the conditions that are satisfied. The formula (1) is J.I. D. It is a simplification of Hunt's equation (Material Science and Engineering, 65 (1984) 74). In equation (1), G is the temperature gradient on the liquid phase side at the solidification interface inside the slab 3, and the slab. It is a temperature gradient (° C./min ) in the solidification progress direction at the point where the solid phase ratio inside 3 is 0.99, and V is the moving speed (mm / min) of the gas-liquid interface of the slab 3. By satisfying the equation (1), the dendrites destroyed by the reduction are not redissolved, so that equiaxed crystal nuclei can be efficiently formed. In the control of the temperature gradient and the solidification rate, the influence on the temperature gradient and the solidification rate due to the cooling of the cooling equipment of the continuous casting equipment 1 is calculated in advance by heat transfer calculation, etc. This is done by adjusting the injection area. In the control of the temperature gradient and the solidification rate, the casting rate may be adjusted in addition to the adjustment in the cooling equipment.

<Modification example>
Although the present invention has been described above with reference to specific embodiments, it is not intended to limit the invention by these explanations. By reference to the description of the invention, one of ordinary skill in the art will appreciate the disclosed embodiments as well as other embodiments of the invention including various modifications. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments including these variations described herein alone or in combination.

For example, in the above embodiment, the continuous casting machine 1 shown in FIG. 1 is used, but the present invention is not limited to such an example. As long as the continuous casting machine is a general one that continuously casts slabs such as bloom, the shape of the continuous casting machine, the arrangement of each roll, and the like are not limited to the above-described embodiment.
Further, in the above embodiment, the carbon concentration of the molten steel 2 to be cast is 0.4 wt% or more, but the present invention is not limited to this example. For example, the present invention may be applied to a steel grade having a carbon concentration of less than 0.4 wt% as long as the steel grade has a high concentration of solute elements such as P, S, and Mn and central segregation becomes a problem.

Further, in the above embodiment, the first light reduction roll 12 and the second light reduction roll 13 have the configuration shown in FIG. 1, but the present invention is not limited to such an example. The first light reduction roll 12 and the second light reduction roll 13 do not necessarily have to be provided in different segments, and for example, some rolls may be provided in the same segment. Further, the number of the first light reduction roll 12 and the second light reduction roll 13 may be two or more, and may be two or four or more, respectively. Further, the first light reduction roll 12 and the second light reduction roll 13 may not be provided adjacent to each other, and a roll that does not perform reduction between the first light reduction roll 12 and the second light reduction roll 13 may be provided. It may be provided. Further, when the first light reduction roll 12 and the second light reduction roll 13 are provided adjacent to each other, the first light reduction roll 12 on the most downstream side in the casting direction and the second light reduction roll on the most upstream side in the casting direction are provided. 13 may be the same roll.

<Effect of embodiment>
(1) The continuous steel casting method according to one aspect of the present invention is a continuous steel casting method for casting high carbon steel having a carbon concentration of 0.4 wt% or more, and has a central solid phase ratio of the slab 3. At the casting position where is 0.3, casting is performed under the condition that the temperature gradient G (° C./min ) on the liquid phase side at the solidification interface and the solidification rate V (mm / min) satisfy the relationship of Eq. (1). At the casting position where the central solid phase ratio is more than 0 and less than 0.3, the slab is reduced in the thickness direction by a plurality of first light reduction rolls 12, and the central solid phase ratio becomes 0.3 or more and 0.7 or less. At the casting position, the slab is reduced in the thickness direction by a plurality of second light reduction rolls 13.

According to the configuration of (1) above, the dendrite tip inside the solidification shell 21 is destroyed by the reduction by the first light reduction roll 12. Then, the destroyed dendrite becomes a solidified nucleus at the center of the slab 3, so that a fine equiaxed crystal structure is generated. Further, by controlling the temperature gradient G and the solidification rate V, the dendrites destroyed by the reduction are not redissolved, and equiaxed crystal nuclei can be efficiently formed. Further, the reduction by the second light reduction roll 13 compensates for the solidification shrinkage and suppresses the flow of the concentrated molten steel at the end of solidification. As a result, the central structure of the slab can be equiaxed and crystallized by an inexpensive method without introducing new equipment such as electromagnetic stirring and impact energy.

(2) In the configuration of (1) above, under the reduction by the plurality of first light reduction rolls 12, the reduction speed of the slab 3 is set to 0.7 mm / min or more and 1.8 mm / min or less, and the second light reduction roll. Under the reduction by 13, the reduction speed of the slab 3 is set to 0.5 mm / min or more and 1.5 mm / min or less.
According to the configuration of (2) above, under the reduction by the plurality of first light reduction rolls 12, the dendrite breaking effect can be sufficiently obtained, and the occurrence of internal cracks in the slab 3 can be prevented. Further, under the reduction by the second light reduction roll 13, the solidification shrinkage of the slab 3 can be sufficiently compensated by the reduction, and the reduction of the segregation can be suppressed by not performing the reduction more than the solidification shrinkage.

Next, the examples carried out by the present inventors will be described. In the embodiment, a bloom continuous casting machine having the same configuration as the continuous casting machine 1 shown in FIG. 1, having a machine length of 19.9 m, a bending radius of 15 m, and a cross-sectional size of the slab 3 to be cast is 410 mm × 250 mm. The slab 3 was continuously cast using the slab. The molten steel 2 injected into the mold 11 contained C: 0.7 wt%, Si: 0.2 wt%, and Mn: 0.9 wt% as components. Further, the drawing speed of the slab 3 was set to 0.8 m / min, and the degree of superheat of the molten steel was set to 20 ° C. The molten steel superheat degree is a temperature obtained by subtracting the liquidus temperature from the temperature of the molten steel 2 in the tundish, which is an intermediate container in which the molten steel 2 is housed before being injected into the mold 11.

Further, in the embodiment, the segments provided with the first light reduction roll 12 and the second light reduction roll 13 are installed at the casting positions where the central solid phase ratio fs of the slab 3 is more than 0 and 0.7 or less. Then, the reduction gradient was set so that the reduction speed was 0.3 mm / min to 2.0 mm / min at each casting position with respect to the set casting speed, and continuous casting was performed.
In the examples, in order to confirm the solidified structure in the central part, the slab 3 was investigated by the following method. First, a sample was taken by cutting out the central portion of the slab to a size of 50 mm in thickness, 410 mm in width, and 80 mm in length. Next, a cross section parallel to the casting direction of the collected sample was etched with saturated picric acid to reveal a macrostructure. Furthermore, the type of coagulated tissue was confirmed by visually observing the exposed tissue. In the confirmation of the solidified structure, it was confirmed whether the central structure of the sample was a columnar crystal or an equiaxed crystal, and whether or not there was an internal crack. Then, a macro segregation having a segregated grain size of about 5 mm and a semi-macro segregated grain having a segregated grain size of about 1 mm observed in the central portion of the thickness of the etched slab are photographed. Next, the photograph taken was image-analyzed, the average area of the segregated grains was measured, and the size of the segregated grains was evaluated by calculating the average particle size (average segregated particle size) corresponding to the circle from this average area.
Under each condition of the example, the reduction speed in the range of the central solid phase ratio of 0.3 ≦ fs ≦ 0.7 was set to 0.2 mm / min.

Table 1 shows a plurality of conditions in which the value of G / √V in the equation (1) and the reduction speed in the first light reduction roll 12 are changed in the range of 0 <fs <0.3 in the central solid phase ratio. The results of investigating the central structure of the slabs 3 cast in 1 and the presence or absence of internal cracks are shown. From the results in Table 1, in the range of 0 <fs <0.3, the central structure is equiaxed under the conditions that G / √V is 0.27 or less and the reduction speed is 0.7 mm / min or more. Can be seen. However, if the reduction speed is greater than 1.8 mm / min, internal cracking will occur, so it can be confirmed that the reduction speed must be 0.7 mm / min or more and 1.8 mm / min or less. rice field.

Further, FIG. 2 shows 0.3 ≦ fs ≦ 0.7 when the reduction speed in the range of 0.27 or less and 0 <fs <0.3 is controlled to 1.0 mm / min. The relationship between the rolling speed and the average segregated grain size in the range of is shown.
As shown in FIG. 2, under the condition that G / √V is 0.27, the segregation particle size is small in the range where the reduction speed is 0.5 mm / min or more and 1.5 mm / min or less, and the effect of improving central segregation is obtained. It can be seen that it has been obtained. On the other hand, when the reduction speed was less than 0.5 mm / min, the coagulation shrinkage could not be compensated, so that the segregation improving effect could not be obtained, and it was confirmed that the average segregated particle size became large. Further, it was confirmed that at more than 1.5 mm / min, the average segregated grain size became large due to the deterioration of segregation due to excessive reduction (reverse V segregation). From these results, it was confirmed that the central segregation of solute elements in the slab can be further reduced by the continuous steel casting method according to the above embodiment.

1 Continuous casting machine 11 Mold 12 1st light reduction roll 13 2nd light reduction roll 2 Molten steel 21 Solidified shell 3 Shards

Claims (1)

A continuous steel casting method for casting high carbon steel with a carbon concentration of 0.4 wt% or more.
At the casting position where the central solid phase ratio of the slab is 0.3, the relationship between the temperature gradient G (° C./min ) on the liquidus side at the solidification interface and the solidification rate V (mm / min) is the relationship of Eq. (1). Casting under the conditions that meet the conditions
At the casting position where the central solid phase ratio is more than 0 and less than 0.3, the slab is reduced in the thickness direction with a plurality of first light reduction rolls.
At the casting position where the central solid phase ratio is 0.3 or more and 0.7 or less, the slab is reduced in the thickness direction with a plurality of second light reduction rolls.
Under the reduction by the plurality of first light reduction rolls, the reduction speed of the slab is set to 0.7 mm / min or more and 1.8 mm / min or less.
A method for continuously casting steel in which the rolling speed of the slab is 0.5 mm / min or more and 1.5 mm / min or less under the rolling by the plurality of second light rolling rolls.

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