JP2015226918A - Steel continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel continuous casting method with a low cost of equipment in addition to the capability of manufacturing a sound cast piece by alleviating the center segregation of the cast piece to be cast.SOLUTION: In a steel continuous casting method for casting while supporting two facing sides of a cast piece, the roll pitch is set at 300 mm or less in a first zone with a center solidus rate of 0.1 to 0.5 or 0.6 to guide the cast piece while being supported without draft, and the roll pitch is set at over 300 mm and 500 mm or less in a second zone with a center solidus rate of 0.5 or 0.6 to 1.0 to lightly draft the cast piece by a draft gradient of 0.5 mm/m or more. This process enables a reduction in cost of equipment in addition to the capability of manufacturing a sound cast piece by alleviating the center segregation of the cast piece to be cast.

Description

  The present invention relates to a steel continuous casting method, which relates to a method for producing a sound slab by reducing the center segregation of a cast slab.

  When casting slabs such as slabs and blooms by the continuous casting method, so-called center segregation in which components such as phosphorus and manganese segregate in the center of the slab may occur. Since center segregation may cause a decrease in toughness of steel and hydrogen-induced cracking, it needs to be suppressed as much as possible.

  Central segregation in continuous casting of steel occurs when solute elements concentrated between dendritic trees move to the center due to flow in the solid-liquid coexistence zone at the end of solidification. As a cause of this flow, volume change accompanying solidification shrinkage and bulging between rolls are pointed out.

  In order to suppress the flow accompanying coagulation shrinkage, Patent Document 1 describes a light reduction method at the end of coagulation. Light reduction starts when the solid fraction at the center of the slab (hereinafter also referred to as “central solid fraction”) is 0.1 to 0.3, and light reduction is applied in the interval up to the central solid fraction 0.7. It is described that the center segregation is improved by applying a reduction corresponding to the coagulation shrinkage during this period. The amount of light reduction suitable for coagulation shrinkage is set to 0.7 to 0.8 mm / m. The reason why the upper limit solid phase ratio of the lightly pressed region is 0.7 is that when the rolling is applied in the region exceeding the solid phase rate, overpressure is often caused, which promotes segregation.

  In order to reduce center segregation caused by bulging between rolls, in Patent Document 1, at least two different roll pitches are arranged at a position corresponding to a center solid phase ratio of 0.3-0.9, and a downstream roll is arranged. The pitch is made smaller than the upstream side, and the downstream roll pitch corresponding to the central solid phase ratio of 0.7-0.9 is set to 200-300 mm. This is because central segregation is unlikely to occur even if bulging occurs in the range where the central solid fraction is less than 0.7, and bulging is reduced in the region where the central solid fraction is greater than 0.9.

  Moreover, in order to reduce the center segregation resulting from bulging between rolls, in patent document 2, the area where the local solid-phase rate (corresponding to the center solid-phase rate) of the thickness center part of a slab is 0.30 or more and less than 0.70 If the bulging is suppressed as much as possible, the occurrence of center segregation is almost eliminated. In this section, the slab is guided by a guide roll having a roll pitch of 250 mm or less. On the other hand, when the local solid fraction is less than 0.95, internal cracking is likely to occur due to the reduction, and the slab reduction in this section is avoided.

  Patent Document 3 proposes a continuous casting apparatus that makes the downstream roll pitch 50 mm to 15 m smaller than the upstream roll pitch.

JP 2005-193265 A JP-A-63-72457 JP 61-14057 A

  In continuous casting of steel, various proposals including Patent Documents 1 to 3 have been made in order to reduce the center segregation of the slab, but further improvement in quality is required. There is also a need to reduce continuous casting equipment costs. SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method for steel that can reduce the center segregation of a cast slab and can produce a sound slab and has low equipment costs.

That is, the gist of the present invention is as follows.
(1) A steel continuous casting method in which casting is performed while supporting two opposite sides of a slab extracted from a continuous casting mold with a roll,
The first zone and the second zone are defined in the casting direction using the solid phase rate at the center of the slab thickness (hereinafter referred to as “central solid phase rate”). The transition point between the first zone and the second zone is a position where the central solid fraction is 0.5 or more and less than 0.6, and the second zone has a central solid fraction of 1.0 or later. Ends at
In the first zone, the roll pitch is set to 300 mm or less, and the slab is guided without being pressed down,
In the second zone, the roll pitch is 300 mm to 500 mm or less, and light rolling is performed with a rolling gradient of 0.5 mm / m or more.
(2) The steel according to (1) above, wherein a split roll is used as the roll of the second zone, and the roll diameter of the second zone is equal to or less than the roll diameter of the first zone. Continuous casting method.
(3) The continuous casting of steel as described in (1) above, wherein an integral roll is used as the roll of the second zone, and the roll diameter of the second zone is larger than the roll diameter of the first zone. Method.

  In the continuous casting method of the present invention, in the first zone having a central solid phase ratio of 0.1 to 0.5 to 0.6, the roll pitch is set to 300 mm or less, and the slab is held without being reduced, and guided. In the second zone where the central solid fraction is from 0.5 to 0.6 to 1.0, the roll pitch is set to more than 300 mm and 500 mm or less, and light casting is performed with a rolling gradient of 0.5 mm / m or more, thereby performing casting. While reducing the center segregation of a piece and producing a sound slab, it becomes possible to make an inexpensive installation cost.

It is a schematic sectional drawing which shows the solidification end vicinity during continuous casting. It is a figure which shows the relationship between the center solid phase rate at the time of bulging, the amount of bulging, and Mn segregation degree. It is a figure which shows the relationship between a roll pitch and the amount of bulging. It is a figure which shows the relationship between a light reduction range, a light reduction gradient, and Mn segregation degree. It is sectional drawing of the continuous casting apparatus in the last stage of solidification seen in the cross section perpendicular | vertical to a casting direction, (a) is an example using a division | segmentation roll, (b) is an integrated roll.

  As described above, center segregation in continuous casting of steel is caused by movement of solute elements concentrated between dendritic trees to the center due to flow in the solid-liquid coexistence zone at the end of solidification. As a cause of this flow, volume change accompanying solidification shrinkage and bulging between rolls can be mentioned. Both are phenomena that occur in the slab region where the central part of the slab thickness is in a solid-liquid coexistence state, and the solid phase ratio (central solid phase ratio) of the central part of the slab thickness is not a finite value. It occurs in the region from 1.0 to 1.0. In order to suppress the flow accompanying solidification shrinkage, the slab is lightly reduced. In order to reduce bulging between rolls, it is effective to narrow the roll pitch.

  In order to reduce the center segregation, in which region in the casting direction the measures for improving the center segregation such as light reduction of the slab and narrowing of the roll pitch are performed, in other words, in what region the central solid fraction is. It is important to do it. Various proposals have been made including Patent Documents 1 to 3, but an optimal solution has not yet been found.

  Therefore, using an experimental device that can simulate the behavior in the slab at the end of solidification in continuous casting, bulging that occurs in the region where the central solid fraction has a negative effect on central segregation. An experiment was conducted to clarify the effect of light pressure reduction on the central solid fraction and the effect of light reduction on the reduction of central segregation.

  500 kg of molten steel melted in an induction melting furnace and adjusted to a target component and temperature was poured into an iron mold to cast a steel ingot having a thickness of 200 mm. The temperature was measured and held by a thermocouple inserted in the center of the thickness of the steel ingot, and after reaching a temperature at which a predetermined solid fraction was reached, the solidified shell was pushed and pulled in the thickness direction to simulate bulging. The push-pull amount corresponds to the bulging amount. Further, after reaching a temperature at which a predetermined solid fraction was reached, light pressure was simulated by pushing the solidified shell at a predetermined speed. The experimental rolling speed was converted to the rolling gradient by dividing by the casting speed in the assumed continuous casting. The solidified shell was pushed and pulled with an iron rod attached to a hydraulic cylinder.

  The evaluation was performed using the Mn segregation degree at the center of the slab as an evaluation index of center segregation. The position of 25 mm in height and 45 mm in width at the center of the slab was analyzed by EPMA. The ratio of the average value in the measurement visual field range to the maximum value in the thickness direction of the integrated average value of 45 mm width was defined as the central segregation degree of Mn. The continuous casting method with a Mn center segregation degree of 1.05 or less was evaluated as good, and 1.07 or less was evaluated as substantially good.

  First, it is forcibly required at a specific central solid phase ratio (0.2, 0.5, 0.6, 0.8) under the condition that no flow due to solidification shrinkage occurs in the entire casting area. An experiment was conducted to generate bulging (amount of bulging 0.5 mm, 1.0 mm). The result was evaluated by the Mn segregation degree. The results are shown in FIG. When bulging was generated at the casting position where the central solid phase ratio was 0.2, the degree of segregation of Mn was deteriorated. The greater the amount of bulging, the worse the degree of segregation of Mn. It can be seen that the bulging amount must be suppressed to 0.2 mm or less in order to prevent the deterioration of the degree of Mn segregation due to bulging in the region where the central solid phase ratio is 0.2. On the other hand, if the position at which bulging with a bulging amount of 1.0 mm or less is generated has a central solid phase ratio of 0.6 or more, the Mn segregation degree at the center of the slab is good and the central solid phase ratio is 0.5. It is clear that it is also in the almost good range.

  Regarding the relationship between roll pitch and bulging amount in continuous casting, the bulging amount between rolls at the end of solidification when a 300 mm thick slab was cast at a casting speed of 1.2 m / min with a continuous casting machine with a machine height of 13 m. It was calculated by the method of Matsumiya et al. (Toru Matsumiya, Yasushi Nakamura: Iron and Steel, 67 (1981), S277). The relationship between the roll pitch and the bulging amount is shown in FIG. If the roll pitch is 300 mm or less, the bulging amount can be 0.2 mm or less.

  As described above, for the central solid phase ratio exceeding 0.5, bulging-induced central segregation does not matter even if the narrow roll pitch is not used. It is enough. And in the area | region made into a narrow roll pitch, it is enough if a roll pitch shall be 300 mm or less. As for the lower limit of the central solid phase ratio to be a narrow roll pitch, if the central solid phase ratio is 0.1 and the region of the central solid phase ratio of 0.1 to 0.5 is set to a roll pitch of 300 mm or less, the center segregation can be sufficiently improved. About the point, it decided to confirm in the actual continuous casting described in the below-mentioned Example.

  Second, an experiment was conducted in which light reduction was performed with a predetermined light reduction amount within a specific central solid phase ratio range under the condition that bulging did not occur in the entire casting area. The results are shown in FIG. For light reduction conditions, no light reduction (●), central solid fraction range from 0.4 to 1.0 (◇), 0.6 to 1.0 (◯), 0.8 to 1.0 It was decided to perform light reduction in each range of (△). For conditions (o) for light reduction at a central solid phase ratio of 0.6 to 1.0, five conditions were selected in the range of 0.4 to 1.7 mm / m for the light pressure gradient, and the other two light reduction conditions were selected. Under the rolling condition (◇) (△), the light rolling gradient was 1.0 mm / m. Although the Mn segregation degree was poor under conditions where light reduction was not performed, it is apparent that the Mn segregation degree is improved if the light pressure gradient is 0.5 mm / m or more under the condition (O). Conditions (◇) and (○) showed the same improvement effect at the same light pressure gradient. Therefore, if the light pressure is reduced with a light pressure gradient of 0.5 mm / m or more at least in the range of the condition (◯) (the range of the central solid phase ratio of 0.6 to 1.0), the center caused by the flow caused by the coagulation shrinkage It has been found that segregation can be improved.

  Light reduction is performed at a casting portion where a solid-liquid coexistence region exists on the center side in the width direction of the slab. In the casting portion, the short side of the slab has been solidified over the entire thickness of the slab. Accordingly, if light reduction corresponding to solidification shrinkage is to be performed in the solid-liquid coexistence region, it is necessary to roll the short side where solidification is completed. The casting length for which solidification is completed is not necessarily uniform in the width direction of the slab, solidification is completed quickly in the center of the width, solidification completion is delayed in the vicinity of 1/4 width, and the solidification completion position is connected. In many cases, the wire exhibits a so-called “W shape” in the slab width direction. When lightly reducing such a slab portion, solidification is completed and the entire thickness is a solid phase in a wide range, and a light reduction force necessary for light reduction is a large value.

  In order to perform light reduction against the reduction reaction force of the slab and to secure a predetermined light reduction amount, it is necessary that the deflection of the reduction roll is sufficiently small with respect to the reduction reaction force. The maximum rolling reaction force that falls within the specified deflection when the deflection of the rolling roll against the light rolling reaction force from the slab is called the “stiffness limit”. The rolling reaction force is below the rigidity limit. If it is, it will be referred to as “sufficient rigidity”.

  Focusing on the rigidity of the roll itself, the roll rigidity increases as the roll diameter increases. On the other hand, when the roll diameter is the same, the roll rigidity is higher as the roll length is shorter. Therefore, a split roll method in which a plurality of short rolls are arranged side by side in the width direction is employed. Thereby, even if the roll diameter is small, the deflection of the roll can be reduced.

  FIG. 5 shows a cross-sectional view of the continuous casting apparatus at the end of solidification viewed in a cross section perpendicular to the casting direction. In the segment type roll support system, an upper roll is installed on the upper frame 2, a lower roll is installed on the lower frame 3, and the upper frame 2 and the lower frame 3 are coupled by the frame coupling device 5. The upper and lower frames are steel structures formed of steel plates, and the upper and lower frames will bend when a pressure is applied to push the center of the width of the frame up and down. It can be said that the smaller the amount of deflection of the upper and lower frames at the same pressure, the higher the rigidity of the segment.

  In the split roll method shown in FIG. 5 (a), the roll is supported by the segment frame on the center side in the width direction. Therefore, the reaction force of the slab 10 received by the roll 4a is reduced in the frame (2, 3). A reaction force is applied to the frame not only at the width end but also near the center of the width, which causes the frame to bend. As a result, the roll interval increases near the center of the slab width as viewed from the slab.

  In the integrated roll method shown in FIG. 5B, generally, when the roll 4b receives a rolling reaction force, the roll deflection becomes larger than the frame deflection, and the rigidity limit is often determined by the roll deflection.

  The upper and lower frames are often coupled by a hydraulic mechanism of the frame coupling device 5. In this case, when the rolling reaction force exceeds the hydraulic pressure limit, the upper and lower frames are separated from each other. In such a case, the limit of the upper and lower frame coupling mechanism determines the rigidity limit.

  The segment 1 of the continuous casting apparatus is attached to and detached from the continuous casting apparatus using a segment crane. Since the lifting load capacity of the segment crane is determined, the maximum weight of the segment is determined by the capacity of the segment crane. Therefore, even if it is attempted to increase the rigidity of the segment frame, there is a limit due to the segment weight limitation.

When the rolling gradient G (mm / m) and the roll pitch P _R (m) are determined, the light rolling amount Δh (mm) in roll units is Δh = G × P _R
Determined by The rolling reaction force F _R when performing roll rolling is generally determined by using the roll diameter D and the roll unit rolling amount Δh.
F _R √ √ (D / 2 × Δh)
Have a relationship. When the length of the segment in the casting direction is L, the total rolling reaction force F _S applied to the segment is roughly
F _S = F _R × L / P _R
Are in a relationship. In summary,
F _S = F _R × L / P _R
∝ (√ (D / 2 × Δh)) × L / P _R
= (√ (D / 2 × G × P _R )) × L / P _R
= (√ (D / 2 × G / P _R )) × L
The relationship is obtained. That is, as the segment rolling reaction force F _S roll diameter D is small addition to the smaller roll pitch P _R is too large segment rolling reaction force F _S in the same roll diameter D that is smaller was found possibly .

As described above, the casting direction range in which light reduction should be performed to reduce the center segregation is in the range of a central solid phase ratio of 0.6 to 1.0. Similarly, as described above, when the center solid phase ratio is 0.5 or later, even if the roll pitch is wide and bulging occurs, the center segregation does not occur. Therefore, in the present invention, at least in the range of the central solid phase ratio of 0.6 to 1.0, the rolling reaction force F _S for each segment is reduced by making the roll pitch wider than 300 mm. On the other hand, if the roll pitch exceeds 500 mm, the roll reaction force required for reduction becomes large and it becomes difficult to ensure the reduction amount. Therefore, the upper limit of the roll pitch in this region is set to 500 mm.

In the present invention, as described above, the segment reaction force F accompanying light pressure is reduced by making the roll pitch wider than 300 mm in the range where the central solid phase ratio to be lightly reduced is at least 0.6 to 1.0. _{Since S} is reduced, the required light reduction can be sufficiently performed even in a continuous casting apparatus using a segment having a low rigidity limit.

  In the present invention, as shown in FIG. 1, the first zone and the second zone are defined in the casting direction 15, and the first zone starts from the position where the central solid fraction is 0.1 or less. The turning point of the two zones is defined as a position where the central solid fraction is 0.5 or more and less than 0.6, and the second zone is defined to end at a position where the central solid fraction is 1.0 or later. The first zone necessarily includes the range of the central solid phase ratio of 0.1 to 0.5, and includes the region where the roll pitch should be 300 mm or less. In addition, the second zone necessarily includes the range of the central solid phase ratio of 0.6 to 1.0, and includes a region where light reduction should be performed with a light pressure gradient of 0.5 mm / m or more.

  Since light reduction is not necessary in the first zone, it can be defined as “guide while holding the slab without reducing it”. As a result, since the reduction reaction force is not applied to the segments constituting the first zone, it is possible to use the roll segments that have been used conventionally and that do not have sufficient rigidity to perform light reduction. It becomes possible.

In the second zone, it can be defined that “the roll pitch is more than 300 mm and not more than 500 mm”. As a result, the roll pitch can be widened, and the segment reaction force F _S accompanying light reduction can be reduced. Therefore, the required light pressure can be sufficiently reduced even in a continuous casting apparatus using a segment having a low rigidity limit. When the roll pitch is 500 mm or less, the amount of bulging is less than 1 mm, so that segregation does not deteriorate.

  As a roll used for the second zone, either a split roll or an integrated roll can be selected.

  When a split roll is used in the second zone, the roll length of the split roll is shortened, so the roll deflection due to the light rolling reaction force is small, and the rigidity against the light rolling reaction force is not the roll deflection but the frame deflection. Deflection is the main cause. Therefore, the roll diameter of the split roll can be reduced. As described above, if the other conditions are the same, the smaller the roll diameter, the smaller the light rolling reaction force, which is advantageous because the required rigidity of the segment can be reduced. Therefore, in the present invention, preferably, a split roll is used as the roll of the second zone, and the roll diameter of the second zone is equal to or smaller than the roll diameter of the first zone.

  When an integral roll is used in the second zone, the roll of the integral roll is long, so that the deflection of the roll associated with the light rolling reaction force cannot be ignored. In the present invention, the roll pitch of the second zone is more than 300 mm and 500 mm or less, and the roll pitch is wider than the roll pitch of the first zone (300 mm or less). Therefore, a roll having a diameter larger than that of the first zone can be used as the roll diameter of the second zone, and roll rigidity can be improved by using the roll having a large diameter. Therefore, in the present invention, preferably, an integrated roll is used as the roll of the second zone, and the roll diameter of the second zone is larger than the roll diameter of the first zone.

In the present invention, the central solid phase ratio is calculated by the following equation based on the temperature of the central portion, the liquidus temperature, and the solidus temperature. This formula is not based on a strict metallurgical definition, but this formula is often used for simplicity.
Central solid fraction = (liquidus temperature-central temperature) / (liquidus temperature-solidus temperature)
The liquidus temperature and the solidus temperature are determined from the composition of the molten steel. The temperature at the center is determined by a one-dimensional heat transfer solidification calculation. In the heat transfer / solidification calculation, an enthalpy method or an equivalent specific heat method is known, but any method may be used.

  The present invention was applied using a steel slab vertical bending continuous casting apparatus. The cast slab size has a thickness of 300 mm and a width of 2200 mm. Steels having the components shown in Table 1 were cast. The casting speed during casting was 1.2 m / min. The relationship between the central solid fraction and the casting length (length from the molten steel surface in the mold) is as follows: the central solid fraction is 0.1, the casting length is 18.5 m, the central solid fraction is 0.6, and the casting length is 22.5 m. The center solid phase ratio is 1.0 and the casting length is 24 m.

  The segments were arranged so as to have the central solid fraction at the start and end points of the first zone and the second zone shown in Table 2 (see FIG. 1). The first zone end point and the second zone start point are the same. Table 2 shows the roll pitch and roll diameter of the first zone and the second zone, and the rolling gradient (mm / m) of the second zone. The rolling gradient in the first zone was 0.24 mm / m. The gradient follows the shrinkage of the solidified short side. All rolls used in the first zone were divided rolls. As shown in Table 2, the roll used in the second zone may be a split roll or an integral roll. The roll pitch before entering the first zone was 350 mm, and the roll diameter was 330 mm. The roll pitch after the end of the second zone was 600 mm, and the roll diameter was 450 mm.

In the second zone where light reduction is performed, a light reduction reaction force F (ton) applied to the roll is calculated. The light pressure reaction force at the position where the central solid phase ratio, which is the final solidification position, is 1.0 was defined as the roll reaction force. At the final solidification position, the reaction force generated at the short side shell portion is F ₂ , and the reaction force generated at the portion excluding the short side shell portion is F ₃ . Further, W: slab width (mm), d: slab thickness (mm), Δh: roll reduction amount (mm), and D: roll diameter (mm). The roll reduction amount Δh can be calculated from the reduction gradient and the roll pitch as Δh (mm) = reduction gradient (mm / m) / roll pitch (m).
F = F ₂ + F ₃
F ₂ = k · √ (Δh · D / 2) · d · 1.25 / 10 ³ , k = 15.7,
F ₃ = k ′ · √ (Δh · D / 2) · (Wd · 1.25) / 10 ³ , k ′ = 12.5
The light pressure reaction force F calculated from the above formula is described in “Light pressure reaction force / per roll” in Table 2. Moreover, the value obtained by multiplying the light rolling reaction force F calculated from the above formula by the number of rolls per segment is shown in “light rolling reaction force / per segment” in Table 2.

  When the slab is reduced with a predetermined reduction reaction force, since the roll and the segment are not rigid bodies, the distance between the upper and lower rolls in contact with the slab increases. In the split roll segment, the distance between rolls increases mainly due to the deflection of the segment. In the integral roll segment, the distance between rolls is mainly increased by the deflection of the roll. In addition, since the coupling force for coupling the upper and lower frames is finite, the distance between the rolls is increased by separating the upper and lower frames when the reduction reaction force per segment exceeds the coupling force. Here, the rolling reaction force when the distance between the rolls was separated by 2 mm or more was obtained as rigidity. Table 2 shows the numerical value per segment for segment rolls as segment rigidity. For the integrated roll, the numerical value per roll is shown in Table 2 as roll rigidity.

  About the central segregation degree of Mn, it evaluated by the same evaluation method as the evaluation used with the experimental apparatus of FIG. As for the equipment cost, the target was the first zone and the second zone, and the comparative example was 1 for relative evaluation.

  In the comparative example, the roll pitch of the first zone was out of the range of the present invention, the center segregation occurred due to bulging, and the center segregation degree of Mn as the center segregation evaluation was poor. Further, the roll pitch of the second zone was out of the scope of the present invention, so that the number of rolls per segment was as many as 8, and the equipment cost of the second zone was deteriorated. Light reduction was performed in the first zone. Therefore, the segment of the first zone had rigidity capable of performing light reduction, and the equipment cost was excessive accordingly.

  Examples 1 to 7 were all within the scope of the present invention, and the central segregation degree of Mn was better than that of the comparative example. As for the equipment cost of the first zone, the equipment cost was generally reduced compared to the comparative example because no light reduction was performed in the examples. As for the equipment costs for the second zone, the equipment costs can be generally reduced compared to the comparative example by reducing the cost by reducing the number of rolls per segment and by reducing the cost by not using the split roll for the integrated roll. did it.

In Examples 1 to 7, the start position of the first zone is set to a position having a central solid phase ratio of 0.1, and good center segregation quality is realized. From this, it was confirmed that good results can be obtained by starting the first zone from the position of the central solid phase ratio of 0.1.
In addition, using the light reduction method at the end of solidification described in Patent Document 1, the roll pitch with a central solid phase ratio of 0.3 to 0.7 is 380 mm, the roll pitch with 0.7 to 0.9 is 280 mm, and the roll The segregation ratio obtained when the diameter was 255 mm and light reduction was performed at a central solid phase ratio of 0.3 to 0.7 with a reduction gradient of 0.75 mm / m was 1.07. The roll pitch of the central solid phase ratio 0.3 to 0.5 (corresponding to the first zone of the present invention) exceeds 300 mm, the central solid phase ratio 0.7 to 1.0 (corresponding to the second zone of the present invention) ) Is not performed under light pressure, unlike the present invention, it is considered that the improvement of the segregation ratio was insufficient.

1 Segment 2 upper frame 3 bottom frame 4 rolls 4a divided rolls 4b integrally roll 5 frame coupling device 10 slab 11 15 casting direction solid phase 12 the solid-liquid coexisting layer 13 liquid layer D roll diameter P _R roll pitch

Claims (3)

A steel continuous casting method for casting while supporting two opposite sides of a slab extracted from a continuous casting mold with a roll,
The first zone and the second zone are defined in the casting direction using the solid phase rate at the center of the slab thickness (hereinafter referred to as “central solid phase rate”). The transition point between the first zone and the second zone is a position where the central solid fraction is 0.5 or more and less than 0.6, and the second zone has a central solid fraction of 1.0 or later. Ends at
In the first zone, the roll pitch is set to 300 mm or less, and the slab is guided without being pressed down,
In the second zone, the roll pitch is 300 mm to 500 mm or less, and light rolling is performed with a rolling gradient of 0.5 mm / m or more.   2. The continuous casting method for steel according to claim 1, wherein a split roll is used as a roll of the second zone, and a roll diameter of the second zone is equal to or less than a roll diameter of the first zone. .   2. The steel continuous casting method according to claim 1, wherein an integral roll is used as the roll of the second zone, and the roll diameter of the second zone is set to be larger than the roll diameter of the first zone.

Images