JP2015202510A - continuous casting method of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of steel which applies a rolling reduction force even onto a casting piece in non-steady casting region and reduces center segregation of the casting piece without causing the center segregation due to insufficient rolling reduction amount and producing inner cracking due to excessive rolling reduction amount.SOLUTION: In a continuous casting method for bulging a casting piece 10 in a bulging total amount of 1 to 30 mm and, thereafter, drafting the steel piece 10 of a portion such that at least a solid phase rate on a thickness center part gets to 0.9 on a light rolling-reduction zone 14, the light rolling-reduction zone is divided into rolling-reduction sections of n(n≥2) pieces in a casting direction, the sum (ΣL) of lengths in the casting direction of the respective rolling-reduction sections after division are set so as to satisfy the following relation (1) with respect to a casting direction variation range (β) of solidification completion position 13, the total rolling-reduction amount in the divided respective rolling-reduction sections is set within the range of 0.7 to 1.0 times of the bulging total amount, the rolling-reduction amount is set to be larger or equal as the rolling-reduction section of the downstream side and a rolling-reduction speed on the rolling-reduction section of the lowermost downstream in the casting direction is set within the range of 0.3 to 1.5 mm/min: ΣL≥β+2 (1).

Description

  The present invention relates to a continuous casting method that suppresses component segregation that occurs at the thickness center of a slab by gradually reducing the slab during continuous casting with a reduction amount corresponding to the amount of solidification shrinkage.

  During the solidification process of the slab during continuous casting, solidification shrinkage occurs, and unsolidified molten steel is sucked in order to eliminate the negative pressure generated by this shrinkage, and flows toward the center of thickness of the slab, which is the final solidification part. And accumulated in the center of the thickness of the slab. In this unsolidified molten steel, solute elements such as carbon (C), phosphorus (P), manganese (Mn), and sulfur (S) are concentrated (referred to as “concentrated molten steel”) and accumulated in the center of the slab. As the concentrated molten steel solidifies, central segregation in which solute elements are concentrated more than the surroundings occurs. Factors causing the flow of concentrated molten steel at the end of solidification include bulging between the slab rolls due to static pressure of the molten steel and misalignment of the roll alignment of the slab support roll in addition to the solidification shrinkage described above. Can be mentioned.

  This central segregation deteriorates the quality of steel products, particularly thick steel plates. For example, in a line pipe material for oil transportation or natural gas transportation, hydrogen-induced cracking occurs starting from central segregation due to the action of sour gas. Similar problems also occur in offshore structures, storage tanks, oil tanks, and the like. Moreover, in recent years, the use environment of steel materials is often required to be used in a severe environment such as a lower temperature or a more corrosive environment, and the importance of reducing the center segregation of the slab is increasing.

  Therefore, many countermeasures for reducing or detoxifying the center segregation of the slab have been proposed from the continuous casting process to the rolling process. Among them, as a method for reducing the center segregation, it is known that the end solidification light reduction method of reducing the end solidification slab during continuous casting having an unsolidified layer is effective. In the light reduction method at the end of solidification, a reduction roll is placed near the solidification completion position of the slab, and the slab during continuous casting is gradually reduced by a reduction amount corresponding to the solidification shrinkage amount by this reduction roll. This is a technique for preventing the formation of voids at the center of the piece, thereby suppressing the flow of the concentrated molten steel and suppressing the center segregation of the slab.

  In this end-solid light reduction method, if the amount of reduction is insufficient, the prevention of center segregation and internal defects will be insufficient. On the other hand, if the amount of reduction is too large, reverse V segregation and internal cracks will occur, Deteriorate quality. Therefore, in the end-coagulation light reduction method, it is important to appropriately control the reduction amount within a range in which the amount of coagulation contraction is compensated.

  In the case of applying the light reduction method at the end of solidification, solidification is completed on the short side of the slab, and this portion becomes a rolling resistance, and a predetermined rolling force is hardly applied to the center portion of the slab. Therefore, a technique that does not reduce the short side of the slab has been proposed.

  For example, in Patent Document 1, after the slab is forcibly bulged within a range of 3% to 25% of the thickness of the slab, the solid phase ratio at the center of the slab is 0.2 or more and 0.7 or less. There has been proposed a continuous casting method in which the slab is squeezed by a thickness corresponding to 30% to 70% of the bulging amount. However, in Patent Document 1, the forced bulging amount is excessive, and an internal crack may occur in the slab.

  In Patent Document 2, the gap in the slab thickness direction of the guide roll group arranged in a predetermined range between the position corresponding to the liquid phase crater end of the slab and the position corresponding to the solid phase crater end is widened. Bulging of a total of 5 mm to less than 20 mm is forcibly generated, and then the amount of bulging is performed by at least one pair of rolling rolls with a solid fraction in the center of the slab between 0.1 and 0.8. There has been proposed a continuous casting method in which rolling is performed with a rolling amount of 0.5 to 1.0 times. However, in Patent Document 2, the reduction is performed by a pair of reduction rolls, and the reduction amount per roll position becomes large, and equipment with a large reduction capability is required, and cracking occurs in the slab due to reduction. there's a possibility that. Further, when the solidification completion position changes due to the fluctuation of the slab drawing speed, the solidification completion position may deviate from the reduction zone, and the reduction force may not be applied to the slab.

  In addition, when applying the light reduction method at the end of solidification, proposals have been made for the purpose of applying an appropriate reduction force to the slab at the end of solidification even if the slab drawing speed changes.

  For example, Patent Document 3 uses a continuous casting machine having a light reduction belt composed of a plurality of reduction rolls, and continuously casting a steel slab while rolling down a slab at the end of solidification in the light reduction zone, When the rolling gradient in the light pressure lower zone is set to be smaller toward the downstream side in the casting direction, the light pressure lower belt is used, and at least when the solid phase ratio at the center of the thickness of the slab becomes 0.8 or higher from the time of 0.4 or lower Until now, a continuous casting method has been proposed in which the slab is squeezed at a squeezing speed within a range of 0.9 to 1.3 mm / min. In this technology, the rolling speed is the product of the slab drawing speed and the rolling gradient, so by setting the rolling gradient to be smaller on the downstream side in the casting direction, the slab is almost It is a technology that is reduced at an equivalent reduction speed. However, in Patent Document 3, since the rolling gradient at the upper part of the light pressure lower belt is large, solidification is completed when the slab drawing speed is lowered for some reason and the solidification completion position is located upstream of the light pressure lower belt. The slab is reduced with a large reduction gradient, and the load applied to the slab support roll constituting the light reduction zone increases, which may lead to a reduction in equipment life and equipment damage.

  By the way, in the continuous casting operation, the casting area is maintained at a predetermined value of the slab drawing speed, and when casting is completed or when a plurality of charges are continuously cast (referred to as “continuous casting”). There is an area where casting is performed at a reduced slab pulling speed, such as during ladle replacement. The area where the slab drawing speed is maintained at a predetermined value is called the “steady casting area”, and the area where the slab drawing speed is lower than the steady casting area is called the “unsteady casting area”. . In the unsteady casting zone, the slab drawing speed is changed so as to be lower than that in the steady casting zone, so the solidification completion position in the unsteady casting zone is upstream of the solidification completion position in the steady casting zone. Moving.

  In Patent Documents 1 and 2, the installation range of the rolling roll in the casting direction is short, and the slab at the end of solidification can be rolled down in the steady casting area, but in the unsteady casting area, the slab center is already at the installation position of the rolling roll. There is a high possibility that the solidification will be completed up to the part, and in the unsteady casting region, the slab at the end of solidification may not be reduced. That is, there is a possibility that the center segregation of the slab in the unsteady casting region cannot be suppressed.

  On the other hand, Patent Document 3 is provided with a light rolling belt having a length of 17 m in the casting direction (a range of 15 to 32 m from the molten steel surface position in the mold, see Example 1), and the slab drawing speed is It is said that a slab of 0.85 to 1.6 m / min can be reduced at a predetermined solid phase rate and a reduction speed, but the light reduction zone is too long, for example, when the slab extraction speed is 0.85 m / min. The solidification of the slab is completed at the upper part of the light pressure lower zone (about 17m from the molten steel surface position in the mold), and the reduction in the light pressure lower belt continues thereafter. The load on the single support roll increases, leading to a reduction in equipment life and equipment damage.

JP 2000-288705 A Japanese Patent Laid-Open No. 11-156511 JP 2011-5525 A

  The present invention has been made in view of the above circumstances, and its purpose is to reduce the center segregation of the slab by the light reduction method at the end of solidification, and the load applied to the slab support roll constituting the light reduction zone As a matter of course, the slab in the steady casting area is given a rolling force to the slab in the non-steady casting area, and prevents internal cracking due to excessive forced bulging, It is an object of the present invention to provide a steel continuous casting method capable of reducing the center segregation of a slab without causing center segregation due to insufficient reduction, internal cracking due to excessive reduction, and V segregation.

The gist of the present invention for solving the above problems is as follows.
[1] The roll opening of the plurality of pairs of slab support rolls is gradually increased toward the downstream side in the casting direction to bulge the long side surface of the slab with a total amount of bulging in the range of 1 to 30 mm, and then a plurality of pairs. The length of the slab where the solid phase ratio at the center of the slab thickness is 0.9 at least in the light pressure lower zone where the roll opening of the slab support roll is gradually reduced toward the downstream side in the casting direction. A continuous casting method of steel that reduces the side surface,
The light rolling belt is divided into n (n ≧ 2) rolling sections in the casting direction, and the sum of the casting direction lengths of the respective rolling sections after the division is the casting direction during casting at the solidification completion position. Set the casting direction length of each rolling section to satisfy the following formula (1) for the fluctuation range,
The total reduction amount of the slab, which is the total value of the reduction amount in each divided reduction section, is set within the range of 0.7 to 1.0 times the total bulging amount as shown in the following equation (2). At the same time, the amount of reduction in each reduction section is set to be larger or equal to the downstream reduction section as shown in the following equation (3), and the nth, that is, the most downstream of the casting direction upstream side. A continuous casting method of steel, characterized in that the rolling speed in the rolling section is set within a range of 0.3 to 1.5 mm / min as shown in the following formula (4).
ΣL _i ≧ β + 2 (1)
0.7 × δ _IB ≦ R _total ≦ δ _IB (2)
R _i ≦ R _{i + 1} (3)
0.3 ≦ the reduction speed in the most downstream reduction section (= (R _n / L _n ) × V) ≦ 1.5 (4)
However, in Equations (1), (2), (3), and (4), ΣL _i is the sum (m) of the length in the casting direction of each reduction section, and β is during casting at the solidification completion position. In the casting direction variation range (m), δ _IB is the total amount of bulging of the slab (mm), R _total is the total value of the reduction amount in each reduction section, and the total reduction amount (mm) of the slab, R _i is the reduction amount (mm) in the i-th reduction section from the upstream side in the casting direction, R _{i + 1} is the reduction amount (mm) in the “i + 1” -th reduction section from the upstream side in the casting direction, and R _n is , The amount of reduction (mm) in the n-th or most downstream reduction section from the upstream side in the casting direction, L _n is the length (m) in the casting direction of the n-th or downstream-most reduction section from the upstream side in the casting direction, V is a slab drawing speed (m / min).
[2] The steel according to [1], wherein the total reduction amount of the slab is set so as to satisfy the following formula (5) with respect to the thickness of the slab at the mold outlet: Continuous casting method.
_{R total <D 0/10 ···} (5)
However, in (5), R _total is the total rolling reduction of the slab (mm), D ₀ is the thickness at the mold exit of the slab (mm).
[3] The above [1], wherein the light reduction belt is divided into two sections, ie, a first reduction section upstream of the casting direction and a second reduction section downstream of the casting direction. The steel continuous casting method according to the above [2].

  According to the present invention, since the casting direction length of each reduction section is set so as to satisfy the expression (1), the solid fraction at the center of the slab is 0.9 in the steady casting region and the unsteady casting region. The slab of the part to be in exists in the range of the light reduction zone, and the slab in the steady casting zone is naturally a slab in the unsteady casting zone, even if it is a slab in the non-steady casting zone, Since the slab is squeezed so as to satisfy the formulas (2), (3) and (4), the slab has no internal cracks. Central segregation can be suppressed.

It is a side schematic diagram of an example of a slab continuous casting machine used when carrying out the present invention. It is the schematic which shows one example of the roll segment which forms a light pressure lower belt, and is the schematic seen from the side of the continuous casting machine. It is the schematic which looked at the roll segment shown in FIG. 2 from the direction orthogonal to the slab extraction direction. It is a figure which shows the example of the profile of the roll opening degree of the slab support roll in this invention.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic side view of an example of a slab continuous casting machine used in carrying out the present invention.

  As shown in FIG. 1, a slab continuous casting machine 1 is provided with a mold 5 for injecting and solidifying molten steel 9 to form a rectangular outer shell of a slab 10, and a predetermined position above the mold 5. The tundish 2 for relaying and supplying the molten steel 9 supplied from a ladle (not shown) to the mold 5 is installed. A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is installed at the bottom of the tundish 2, and an immersion nozzle 4 is installed on the lower surface of the sliding nozzle 3. On the other hand, a plurality of pairs of slab support rolls 6 including a support roll, a guide roll, and a pinch roll are arranged below the mold 5. A secondary cooling zone in which a spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is arranged is formed in the gap between the slab support rolls 6 adjacent to each other in the casting direction. The slab 10 is cooled while being drawn out by cooling water (also referred to as “secondary cooling water”) sprayed from the spray nozzle. A plurality of transport rolls 7 for transporting the cast slab 10 are installed on the downstream side of the final slab support roll 6 in the casting direction. A slab cutting machine 8 for cutting a slab 10a having a predetermined length from the slab 10 is disposed.

  On the upstream side in the casting direction of the solidification completion position 13 of the slab 10, an interval between the slab support rolls facing each other with the slab 10 interposed therebetween (this interval is referred to as “roll opening”) is directed downstream in the casting direction. It is composed of a plurality of pairs of slab support rolls that are set so as to be narrowed sequentially, that is, the rolling gradient (the roll opening state set so as to narrow gradually toward the downstream in the casting direction) is set. The light pressure lower belt 14 is installed. In FIG. 1, a lightly reduced belt 14 is installed upstream of the solidification completion position 13 in the casting direction. This is because the slab where the solid phase ratio at the center of the slab thickness is 0.9 is lightly reduced. This is because the slab located at the lower part of the strip 14 and having a solid phase ratio exceeding 0.9 at the center of the slab thickness is immediately moved away from the lightly pressed strip 14. By doing so, the center segregation of the slab 10 is suppressed, the rolling resistance is large, and the rolling force is not applied to the portion where the solid phase ratio at the center of the slab thickness exceeds 0.9. The rolling load of the rolling belt 14 is reduced. However, this shows the ideal state in the steady casting area where the slab drawing speed has reached the target predetermined value. Even in this case, the slab is replaced at the end of continuous casting ladle or at the end of casting. When the drawing speed falls below the target value, the solidification completion position 13 moves upstream in the casting direction and falls within the range of the light pressure lower belt 14. As long as the slab where the solid fraction at the center of the slab thickness is 0.9 is positioned below the light pressure lower belt 14, the solidification completion position 13 is light at the slab drawing speed in the steady casting zone. The light reduction belt 14 may be installed so as to exist below the reduction belt 14.

  The light lower belt 14 is formed by a roll segment in which a plurality of pairs of slab support rolls 6 are integrally arranged. In FIG. 1, the light lower belt 14 is divided into two in the casting direction by two roll segments. Has been. Here, the roll segment 14A on the upstream side in the casting direction forming the light reduction zone 14 is referred to as a first reduction zone, and the roll segment 14B on the downstream side in the casting direction is referred to as a second reduction zone. In FIG. 1, the light pressure lower belt 14 is divided into two in the casting direction by two roll segments 14A and 14B. However, in the present invention, the light pressure lower belt 14 is not limited to only two in the casting direction, and is divided into three. It does not matter as above. That is, the light pressure lower belt 14 may be divided into n (n ≧ 2) or more. However, in any case, at the slab drawing speed in the steady casting zone, the slab where the solid phase ratio at the center of the slab thickness is 0.9 is positioned in the range of the most downstream reduction section. Each rolling section is set according to the scheduled slab drawing speed.

  In this case, the light rolling belt 14 may be divided in roll segment units, or may be divided into two or more rolling sections within the range of one roll segment. However, when dividing within one roll segment, the slab support roll 6 in the roll segment is set to a different roll gradient from the upstream side to the downstream side in advance during the offline roll segment maintenance stage. It is necessary to. In FIG. 1, one roll segment is constituted by three pairs of slab support rolls 6, but the slab support rolls 6 constituting the roll segment are not limited to three pairs, and there are two or more pairs. Any number can be used.

  Usually, the rolling gradient in the light pressure lower belt 14 is indicated by the amount of narrowing of the roll opening per 1 m in the casting direction, that is, “mm / m”. Therefore, the rolling speed of the slab 10 in the light pressure lower belt 14 (mm / m). min) is determined by the product of the rolling gradient (mm / m) and the slab drawing speed (m / min). In the present invention, the light rolling belt 14 is divided into two or more rolling sections in the casting direction, and the rolling speed in each rolling section is obtained by the product of the rolling gradient of the rolling section and the slab drawing speed. . In addition, the amount of reduction in each reduction section is determined by the difference between the roll opening at the inlet and the roll opening at the outlet in the reduction section, in other words, the reduction gradient in the reduction section and the casting in the reduction section. Calculated as the product of the direction length. In addition, the spray nozzle for cooling the slab 10 is arrange | positioned also between each slab support roll which comprises the light pressure lower belt 14. Moreover, the slab support roll 6 arrange | positioned at the light reduction belt 14 is also called a reduction roll.

  2 and 3 are enlarged schematic views of a roll segment 14A as an example of the roll segment forming the light pressure lower belt 14. FIG. FIG. 2 is a schematic view seen from the side of the continuous casting machine, and FIG. 3 is a schematic view seen from a direction orthogonal to the slab drawing direction. The roll segment 14A shown in FIGS. 2 and 3 is a diagram showing an example in which five pairs of slab support rolls 6 are arranged, and one pair of the five pairs of slab support rolls 6 is a pinch roll 6A, and the other These four pairs are examples of the guide roll 6B. The roll segment 14B has the same structure as that shown in FIGS.

  As shown in FIGS. 2 and 3, the roll segment 14 </ b> A includes five pairs of cast slab support rolls (one of five pairs of slab support rolls, one pair is a pinch roll 6 </ b> A, and four pairs are formed via a roll chock 22. A pair of frames 17 and 17 'holding a guide roll 6B), and a total of four tie rods 18 (both upstream side and downstream side) are arranged through the frame 17 and frame 17'. Then, the worm jack 20 installed on the tie rod 18 is driven by the motor 21 so that the distance between the frame 17 and the frame 17 'is adjusted, that is, the reduction gradient in the roll segment 14A is adjusted. It has become. During the casting, the worm jack 20 is self-locked by the molten steel static pressure of the slab 10 having the unsolidified layer 12 and is against the bulging force of the slab 10, under the condition where the slab 10 is not present, that is, The rolling gradient is adjusted under the condition that the load from the slab 10 does not act on the pinch roll 6A and the guide roll 6B installed in the roll segment 14A.

  The tie rod 18 is provided with a disc spring 19 between the frame 17 ′ and the worm jack 20. The disc spring 19 is not configured by a single disc spring, but is configured by stacking a plurality of disc springs (the greater the number of disc springs, the higher the rigidity). The disc spring 19 has a certain thickness without contracting when a load greater than a certain predetermined load does not act on the disc spring 19, but contracts when a certain predetermined load is applied. Then, after a certain predetermined load is exceeded, it is configured to contract in proportion to the load. For example, when the slab 10 has been solidified within the range of the roll segment 14A, an excessive load is applied to the roll segment 14A by reducing the solidified slab 10, and such an excessive load is applied. When the disc spring 19 is loaded, the disc spring 19 is contracted to open the frame 17 ', that is, the roll opening degree is enlarged, and an excessive load is not applied to the roll segment 14A. The lower surface side frame 17 is fixed to the foundation of the continuous casting machine and is configured not to move during casting.

  The guide roll 6B installed on the frame 17 'has the roll chock 22 fixed to the frame 17' and moves in the same manner as the frame 17 ', but the pinch roll 6A arranged on the frame 17' has the roll chock 22 The slab 10 is supported by a cylinder rod 23 of the hydraulic cylinder 16, and the slab 10 is pressed according to the hydraulic pressure of the hydraulic cylinder 16. However, when the pressing force of the pinch roll 6A is smaller than the static iron pressure of the slab 10, the pinch roll 6A is configured to retreat to a position where it has the same gradient as that of the other guide roll 6B. The pinch roll 6A is connected to an electric motor (not shown) and is configured to rotate at a predetermined rotational speed by the electric motor. On the other hand, the guide roll 6B does not drive, but rotates by contacting the moving slab 10.

  On the other hand, the pinch roll 6A installed in the frame 17 is fixed to the frame 17 and configured not to move in the slab thickness direction, like the guide roll 6B installed in the frame 17. However, the pinch roll 6A installed on the frame 17 is also connected to an electric motor (not shown) and is configured to rotate at a predetermined rotational speed.

  The reason why the pinch roll 6A is configured to be able to press the slab 10 regardless of the rolling gradient at the rolling segment 14 is that the slab 10 and a dummy bar (inserted into the continuous casting machine instead of the slab at the start of casting by the pinch roll 6A) This is because the pinch roll 6A needs to have a predetermined pressing force against the slab 10 and the dummy bar. When a predetermined pressing force cannot be secured against the slab 10 or the dummy bar, the slab 10 or the dummy bar is not driven.

  In FIG. 2, a pair of pinch rolls 6 </ b> A are arranged in the rolling-down segment 14, but two or more pairs of pinch rolls 6 </ b> A may be arranged in one rolling-down segment 14. Although not shown, the slab support roll 6 other than the reduction segment 14 also has a roll segment structure.

  In FIG. 1, the slab support roll 6 disposed between the lower end of the mold 5 and the liquidus crater end position of the slab 10 has a predetermined amount of expansion of the roll opening toward the downstream side in the casting direction. The forced bulging band 15 for forcibly bulging the slab 10 having the unsolidified layer 12 therein, in which the roll opening degree is gradually increased for every roll or every several rolls, is formed. The slab support roll 6 on the downstream side of the forced bulging zone 15 is narrowed to such an extent that the roll opening degree corresponds to a constant value or the amount of shrinkage accompanying the temperature drop of the slab 10, and then connected to the roll segment 14 </ b> A.

  In FIG. 4, the example of the profile of the roll opening degree of the slab support roll 6 in this invention is shown. As shown in FIG. 4, the long side surface of the slab is forcibly bulged by the molten steel static pressure in the forced bulging band 15 to increase the thickness of the central part of the long side surface of the slab (region b), and passes through the forced bulging band 15. On the downstream side too much, the roll opening is narrowed to a certain value or the amount of contraction accompanying the temperature drop of the slab 10 (region c), and then the roll segment 14A (first reduction) of the light reduction belt 14 is reached. The profile is such that the long side surface of the slab is crushed in (section) (region d), and then the slab long side surface is crushed in the roll segment 14B (second reduction section) of the light reduction belt 14 (region e). . “A” and “f” in the figure are regions where the roll opening is narrowed to an extent corresponding to the amount of shrinkage associated with the temperature drop of the slab 10. “A ′” in the figure is an example of the roll opening degree of the conventional method in which the roll opening degree is narrowed to an extent corresponding to the amount of shrinkage accompanying the temperature drop of the slab 10.

  In the forced bulging band 15, the roll opening degree of the slab support roll 6 is gradually increased toward the downstream side in the casting direction, so that the long side surface except for the vicinity of the short side of the slab 10 is formed of the molten steel by the unsolidified layer 12. The bulging is forcibly performed according to the roll opening degree of the slab support roll 6 by the pressure. Since the vicinity of the short side of the long side of the slab is held by the short side of the slab after solidification, the thickness at the time when forced bulging is started is maintained. Only the bulging portion of the long side surface of the slab is brought into contact with the slab support roll 6 by the typical bulging.

  In the slab continuous casting machine 1 having this configuration, the molten steel 9 injected from the tundish 2 through the immersion nozzle 4 into the mold 5 is cooled by the mold 5 to form a solidified shell 11. The slab 10 having the solidified shell 11 as an outer shell and having an unsolidified layer 12 inside and having a rectangular cross section is supported by a slab support roll 6 provided below the mold 5 while being below the mold 5. Continuously pulled out. The slab 10 is cooled by the secondary cooling water in the secondary cooling zone while passing through the slab support roll 6 to increase the thickness of the solidified shell 11, and in the forced bulging zone 15, the slab long side surface is short. The thickness of the portion excluding the side end is increased, and the slab long side surface is pulled out while being squeezed down in the light reduction belt 14, and solidification to the inside is completed at the solidification completion position 13. The slab 10 after completion of solidification is cut by the slab cutting machine 8 to become a slab 10a.

  In the present invention, the reason why the forced bulging band 15 is disposed between the lower end of the mold 5 and the liquid phase crater end position of the slab 10 is as follows. That is, on the upstream side in the casting direction from the liquid phase crater end position of the slab 10, the center part of the slab thickness is the unsolidified layer 12 (liquid phase), and the solidified shell 11 of the slab 10 has a temperature of This is because it is high and its deformation resistance is small. When the slab 10 is forcibly bulged, if the bulging is performed at a time when the unsolidified layer 12 present in the slab 10 is small, the concentrated molten steel flows and the center segregation worsens. However, when bulging is performed upstream of the liquidus crater end position of the slab 10 in the casting direction, the molten steel having an initial concentration in which the solute elements are not concentrated is abundant in the slab at this point. And this molten steel flows easily. Even if this molten steel flows, segregation does not occur. Therefore, bulging at this point does not cause central segregation. Further, since the deformation resistance of the solidified shell 11 is small, it can be easily bulged.

  In addition, the liquidus of the slab 10 is a solidification start temperature determined by the chemical component of the slab 10, and can be obtained from, for example, the following equation (6).

TL = 1536- (78 × [% C] + 7.6 × [% Si] + 4.9 × [% Mn] + 34.4 × [% P] + 38 × [% S] + 4.7 × [% Cu] + 3.1 × [ % Ni] + 1.3 × [% Cr] + 3.6 × [% Al]) (6)
However, in the equation (6), TL is the liquidus temperature (° C.), [% C] is the carbon concentration (mass%) of the molten steel, [% Si] is the silicon concentration (mass%) of the molten steel, and [% Mn]. Is the manganese concentration (mass%) of the molten steel, [% P] is the phosphorus concentration (mass%) of the molten steel, [% S] is the sulfur concentration (mass%) of the molten steel, and [% Cu] is the copper concentration (mass%) of the molten steel. ), [% Ni] is the nickel concentration (mass%) of the molten steel, [% Cr] is the chromium concentration (mass%) of the molten steel, and [% Al] is the aluminum concentration (mass%) of the molten steel.

  The liquidus crater end position of the slab 10 can be obtained by comparing the temperature gradient inside the slab obtained by the two-dimensional heat transfer solidification calculation with the liquidus temperature determined by the equation (6).

  The forced bulging band 15 does not require a special mechanism and is formed only by adjusting the roll opening. Therefore, as long as it is within the range from the lower end of the mold 5 to the liquidus crater end position of the slab 10, it is optional. It can be installed in the position.

  In the present invention, in the forced bulging band 15, the total amount of forced bulging needs to be in the range of 1 to 30 mm. If the total amount of bulging is less than 1 mm, the amount that can be reduced in a state where the load is kept low is small in the light reduction belt 14, and the effect of improving the center segregation with respect to the slab 10 becomes insufficient. On the other hand, if the total amount of bulging exceeds 30 mm, internal cracks may be induced in the slab 10 in the forced bulging band 15 due to distortion caused by bulging. Further, in the forced bulging zone 15, the amount of expansion of the roll opening per roll is 1.5 mm or less in order to prevent the occurrence of cracks at the boundary position between the bulging site and the non-bulging site on the long side of the slab. It is preferable that

  In the present invention, the sum of the casting direction lengths of the respective rolling sections of the light rolling belt 14 divided into n satisfies the following formula (1) with respect to the casting direction fluctuation range during casting at the solidification completion position 13. Thus, the casting direction length of each rolling section is set. That is, in the light reduction belt 14 shown in FIG. 1, the length in the casting direction of the roll segment 14A that is the first reduction section on the upstream side in the casting direction and the roll segment 14B that is the second reduction section on the downstream side in the casting direction. The casting direction length of each rolling section is set so that the sum of the length and the casting direction length satisfies the following expression (1).

ΣL _i ≧ β + 2 (1)
However, in equation (1), ΣL _i is the sum (m) of the casting direction lengths of the respective rolling sections of the light rolling belt 14 divided into n pieces, and β is the casting direction fluctuation during casting at the solidification completion position. Range (m). Here, the casting direction fluctuation range β during casting at the solidification completion position 13 is the position of the solidification completion position 13 at the slab drawing speed in the target steady casting region (distance L from the molten steel surface in the mold). _CEst ) and the position of solidification completion position 13 (distance L _CEunst from the molten steel surface in the mold) when the slab drawing speed decreases in the unsteady casting zone at the end of continuous casting ladle replacement or casting Distance (= L _CEst −L _CEunst ).

The sum of the casting direction lengths of the respective rolling sections, that is, the casting direction length of the light rolling belt 14 is 2 m or more than the casting direction fluctuation range β during casting at the solidification completion position 13 as shown in the equation (1). By setting the casting direction length of each reduction section to be longer, the slab where the solid fraction at the center of the slab becomes 0.9 in the steady casting region and the unsteady casting region is the light reduction zone 14 of course, even if the slab in the steady casting region is a slab in the non-steady casting region, it is possible to apply a reduction force by the light reduction belt 14 to the long side surface of the slab at the end of solidification. It becomes possible. Thereby, the center segregation of the slab of an unsteady casting area and the slab of an unsteady casting area is improved. However, excessively increasing the casting direction length of the light pressure lower belt 14 is not preferable from the viewpoint of the load on the roll segment forming the light pressure lower belt 14, and therefore it is preferable to satisfy “β + 4 ≧ ΣL _i ”.

  In the present invention, the total reduction amount of the slab, which is the total value of the reduction amount in each reduction section, is in the range of 0.7 to 1.0 times the total bulging amount as shown in the following equation (2). Set in. That is, in the light reduction belt 14 shown in FIG. 1, the amount of reduction in the roll segment 14A, which is the first reduction section upstream of the casting direction, and the roll segment 14B, which is the second reduction section downstream of the casting direction. The amount of reduction is set so that the sum of the amount of reduction in the range of 0.7 to 1.0 times the total amount of bulging.

0.7 × δ _IB ≦ R _total ≦ δ _IB (2)
However, in the formula (2), δ _IB is the total bulging amount (mm) of the slab, and R _total is the total reduction amount (mm) of the slab, which is the total value of the reduction amount in each reduction section.

  If the total reduction amount is less than 0.7 times the total bulging amount, the total reduction amount is small and the center segregation reduction effect of the slab 10 may not be sufficient, but the total reduction amount is 0.7 times the total bulging amount or more. By doing so, a predetermined amount of rolling force is applied to the long side surface of the slab, and the center segregation of the slab 10 can be suppressed. Further, by setting the total reduction amount to be 1.0 times or less of the total bulging amount, the short side of the slab 10 is not reduced in the light reduction belt 14, and the load load of the roll segment forming the light reduction belt 14 is reduced. It is possible to suppress equipment troubles such as bearing breakage in the roll segment and breakage of the rolling roll.

  Further, in the present invention, the slab reduction amount in each reduction section divided in the casting direction is set to be larger or equal in the downstream reduction section as shown in the following equation (3). That is, in the light reduction belt 14 shown in FIG. 1, the roll which is the second reduction section on the downstream side in the casting direction with respect to the reduction amount in the roll segment 14A which is the first reduction section on the upstream side in the casting direction. The reduction amount in the segment 14B is set to be large or equal.

R _i ≦ R _{i + 1} (3)
However, (3) In the equation, rolling reduction in R _i is the i-th reduction section from the casting direction upstream side (mm), reduction of R i + ₁ from the casting direction upstream side "i + 1" -th pressure section (Mm).

  As described above, in the present invention, the total reduction amount is limited within the range of the bulging total amount. When the reduction amount on the upstream side in the casting direction is increased, the reduction amount on the downstream side in the casting direction is reduced and the slab is reduced. This is because the center segregation of 10 may be deteriorated.

  Here, the amount of reduction of the slab 10 in each reduction section is obtained by multiplying the reduction gradient of the reduction section by the casting direction length of the reduction section. Therefore, as a method of making the amount of reduction in the reduction section downstream in the casting direction larger than the amount of reduction in the upstream reduction section, a method of making the reduction gradient larger than that of the upstream reduction section, Without taking this into account, a method of increasing the length of the reduction section in the casting direction can be employed. In short, the product of the rolling gradient of the rolling section and the casting direction length of the rolling section may be equal to or greater than the value in the upstream rolling section.

  Further, in the present invention, the rolling speed in the nth, that is, the most downstream rolling section from the upstream side in the casting direction is set within a range of 0.3 to 1.5 mm / min as shown in the following equation (4).

0.3 ≦ the reduction speed in the most downstream reduction section (= (R _n / L _n ) × V) ≦ 1.5 (4)
However, in the equation (4), R _n is the amount of reduction (mm) in the n-th or most downstream reduction section from the upstream side in the casting direction, and L _n is the n-th or most downstream side from the upstream in the casting direction. The casting direction length (m) and V of the reduction section are the slab drawing speed (m / min).

  In the present invention, at the slab drawing speed in the steady casting region, the slab of the portion where the solid phase ratio at the center of the slab thickness is 0.9 is located in the range of the most downstream reduction section, and the most downstream. By setting the reduction speed in the reduction section of 0.3 to 1.5 mm / min, a reduction force is applied to the slab 10 in at least the steady casting region at a target reduction speed. Central segregation is suppressed. When the rolling speed is less than 0.3 mm / min, the rolling speed is too small with respect to the amount of solidification shrinkage, and the flow of the concentrated molten steel is insufficient, while the rolling speed exceeds 1.5 mm / min. Then, the reduction speed is too large with respect to the amount of solidification shrinkage, and there is a risk of generating reverse V segregation or internal cracks.

  Further, in the present invention, when the slab drawing speed is in the steady casting region, at least from the time when the solid phase ratio at the center of the slab thickness is 0.3 to 0.9, It is preferable to reduce the piece 10. A slab whose solid fraction at the center of the slab thickness exceeds 0.9 has a large rolling resistance, and a slab whose solid ratio at the center of the slab thickness exceeds 0.9 is within the range of the light reduction zone 14. In this case, there is a possibility that the disc spring 19 contracts and the desired amount of reduction cannot be applied to the slab. Further, the flow of the concentrated molten steel does not occur in the range where the solid phase ratio at the center portion of the slab thickness exceeds 0.9, and the center segregation does not deteriorate even if the reduction is not performed. On the other hand, even if the reduction starts after the solid phase ratio at the center portion of the slab thickness exceeds 0.3 in the light reduction zone 14, the flow of the concentrated molten steel may occur before that. Segregation occurs, and the center segregation reduction effect cannot be obtained sufficiently.

  The solid phase ratio at the center portion of the slab thickness can be obtained by two-dimensional heat transfer solidification calculation as in the case of obtaining the liquidus crater end position. Here, the solid phase ratio is defined as the solid phase ratio = 0 before the start of solidification and the solid phase ratio = 1.0 when the solidification is completed. The position that becomes 0 is the solidification completion position 13 (solidus crater end position), and the liquidus crater end position corresponds to the most downstream position where the solid phase ratio at the center of the slab thickness becomes zero.

  In the present invention, under various casting conditions of continuous casting operation, the thickness of the solidified shell 11 and the solid phase ratio at the center of the slab thickness are obtained in advance using two-dimensional heat transfer solidification calculation, and the slab drawing speed is scheduled. In the steady casting zone, the amount of secondary cooling water, the width of the secondary cooling, the slab so that the slab where the solid phase ratio at the center of the slab is 0.9 is located in the most downstream reduction section Any one or more of the drawing speeds are adjusted. Here, “secondary cooling width cutting” is to stop spraying of cooling water to both ends of the long side surface of the slab. By performing the width cutting of the secondary cooling, the secondary cooling is weakened, and generally, the solidification completion position 13 is extended downstream in the casting direction.

Moreover, in this invention, it is preferable to set the total amount of rolling reduction of the slab 10 so that the following (5) Formula may be satisfied with respect to the thickness in the casting_mold | template exit of the slab 10. FIG. However, in (5), R _total is the total rolling reduction of the slab (mm), D ₀ is the thickness at the mold exit of the slab (mm).

_{R total <D 0/10 ···} (5)
If the total reduction amount increases, there is a risk that an internal crack may occur in the slab 10 during reduction in the light reduction zone 14, but if the total reduction amount is within the range satisfying the formula (5), the slab 10 internal cracks can be suppressed.

  As described above, according to the present invention, since the casting direction length of each reduction section is set so as to satisfy the expression (1), in the steady casting region and the unsteady casting region, the slab center portion is fixed. The slab where the phase ratio is 0.9 exists within the range of the light pressure lower zone, and the slab in the steady casting zone is of course the slab length at the end of solidification even if it is a slab in the unsteady casting zone The slab 10 can be applied to the side surface with the light reduction belt 14 and the slab 10 is squeezed to satisfy the expressions (2), (3), and (4). The center segregation of the slab 10 can be suppressed without causing internal cracks.

  The continuous casting machine shown in FIG. 1 is a vertical bending type continuous casting machine. However, the present invention is not limited to the vertical bending type continuous casting machine. Even if it is a casting machine, this invention can be applied similarly to the above.

As shown in FIG. 1, a slab continuous casting machine in which a forced bulging band is arranged at the upper part and a light pressure lowering band divided into two in the casting direction is arranged therebelow is used. Thickness (D ₀ ), total amount of bulging in forced bulging zone (δ _IB ), length in casting direction (L ₁ , L ₂ ) in _first and second reduction sections, reduction amount (R ₁ , R ₂₎ ), Various reduction speeds ((R ₁ / L ₁ ) × V, (R ₂ / L ₂ ) × V) were performed, and a test for casting low-carbon aluminum killed steel was performed a total of 15 times. The width of the slab was 2100 mm in all tests.

  Table 1 shows the casting conditions, the total amount of bulging, the reduction conditions, and the results of investigation of the center segregation degree, porosity, and internal cracks in the cast slab slabs in each of the casting tests of Conditions 1 to 15. In the remarks column of Table 1, the test under the condition within the scope of the present invention is indicated as “Invention Example”, and the others are indicated as “Comparative Example”.

  Under the casting conditions of each test, in the steady casting area where the slab drawing speed is kept constant by the two-dimensional heat transfer solidification calculation ("Maximum drawing speed" in Table 1) and at the time of ladle replacement or at the end of casting Obtain the solidification completion position at the slab drawing speed in the unsteady casting zone ("Minimum drawing speed" in Table 1), and change the casting direction during casting from the obtained solidification completion position to the solidification completion position. The range β was calculated.

As a result, in conditions 1 to 10 where the slab drawing speed is in the range of 1.00 to 1.60 m / min, the casting direction variation range β is 8 m, and the slab drawing speed is 0.75 to 1.10 m / min. It was found that the casting direction fluctuation range β was 6 m under the conditions 11 to 15 in the range of 5 to 10. Table 1 lists these values. Conditions 1-9 are the sum of the casting direction lengths (L ₁ + L ₂ ) in the first and second reduction sections being 10 m, and conditions 11-15 are the first and second The sum of the lengths in the casting direction (L ₁ + L ₂ ) in the rolling section was 8 m, and both of these satisfied the formula (1). However, in condition 10, the first reduction section is not set, and the expression (1) is not satisfied. In condition 10, the first reduction section is not set, and the description of “second reduction section” in Table 1 is not accurate, but this is because it is displayed in accordance with other conditions.

The method for measuring the center segregation degree of the slab is to analyze the carbon concentration in the thickness direction of the slab at the center of the cross section of the slab, the maximum value is C _max , and the average carbon concentration (the carbon of the sample taken from the molten steel) concentration) and C _0, was defined as the center segregation ratio of C _max / C _0. In this definition, the center segregation decreases as the center segregation degree approaches 1.00. When the center segregation degree is 1.10 or more, adverse effects due to center segregation such as hydrogen-induced cracking become severe. Here, the threshold value of the center segregation degree is set to 1.08, and 1.08 or more is determined to be unacceptable. The porosity and internal crack of the slab are corroded by the macro structure test method (JIS G 0553), and the center of the slab thickness on the corroded surface is observed with a microscope. The presence or absence of was investigated.

  Conditions 1-3, Conditions 6-8, and Conditions 11-13 satisfy all of the expressions (1), (2), (3), and (4) defined in the present invention, and the center segregation. The degree was 1.058 or less, and neither porosity nor internal crack occurred.

  On the other hand, in condition 4, the total reduction amount (= 8 mm) exceeds the bulging total amount (= 6 mm), that is, the equation (2) is not satisfied, the center segregation degree deteriorates, and the internal crack There has occurred. This is thought to be because the total reduction amount of the slab was excessive, the short side surface of the slab was reduced, the load of the roll segment in the light reduction zone increased, and an appropriate reduction force could not be applied to the slab. . Similarly, under condition 14, the total reduction amount (= 12 mm) was excessive with respect to the bulging total amount (= 8 mm), the center segregation degree was high, internal cracks occurred, and reverse V segregation was also confirmed in part.

Condition 5 is that the rolling speed ((R ₂ / L ₂ ) × V) in the _second rolling zone is 2.0 to 3.2 mm / min, and does not satisfy the formula (4). In addition, the rolling speed is excessive, and the formula (5) which is a preferable condition of the present invention is not satisfied, the center segregation degree is high, internal cracks are generated, and reverse V segregation is partially confirmed. .

  Condition 9 is that the amount of reduction in the first reduction section (= 5 mm) is larger than the amount of reduction in the second reduction section (= 4 mm), does not satisfy the expression (3), and internal cracks Although the porosity was good, the effect of reducing the center segregation was not sufficiently obtained.

Condition 10 was that the total amount of reduction relative to the total amount of bulging was within the scope of the present invention, and the reduction speed ((R ₂ / L ₂ ) × V) in the second reduction section was also within the scope of the present invention. The first rolling section is not set, the casting direction length of the light rolling belt is as short as 6 m, the solidification completion position is out of the light rolling belt in the unsteady casting zone, and no rolling amount is given, The central segregation degree was high, and internal cracks and porosity occurred.

Condition 15 is that the rolling speed ((R ₂ / L ₂ ) × V) in the _second rolling zone is 0.25 to 0.28 mm / min, and does not satisfy the formula (4). The rolling speed was too low, the center segregation degree was high, and internal cracks and porosity occurred.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 6A Pinch roll 6B Guide roll 7 Conveying roll 8 Casting piece cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified layer 13 Solidification completion Position 14 Light pressure lower belt 14A Roll segment 14B Roll segment 15 Forced bulging belt 16 Hydraulic cylinder 17 Frame 17 'Frame 18 Tie rod 19 Belleville spring 20 Worm jack 21 Motor 22 Roll chock 23 Cylinder rod

Claims (3)

The roll opening of multiple pairs of slab support rolls is gradually increased toward the downstream side in the casting direction to bulge the long side surface of the slab with a bulging total amount in the range of 1 to 30 mm, and then multiple pairs of slab support are supported. In the light pressure lower zone where the roll opening degree of the roll is gradually reduced toward the downstream side in the casting direction, at least the long side surface of the slab where the solid phase ratio at the center of the slab thickness is 0.9 A method of continuous casting of steel to be rolled down,
The light rolling belt is divided into n (n ≧ 2) rolling sections in the casting direction, and the sum of the casting direction lengths of the respective rolling sections after the division is the casting direction during casting at the solidification completion position. Set the casting direction length of each rolling section to satisfy the following formula (1) for the fluctuation range,
The total reduction amount of the slab, which is the total value of the reduction amount in each divided reduction section, is set within the range of 0.7 to 1.0 times the total bulging amount as shown in the following equation (2). At the same time, the amount of reduction in each reduction section is set to be larger or equal to the downstream reduction section as shown in the following equation (3), and the nth, that is, the most downstream of the casting direction upstream side. A continuous casting method of steel, characterized in that the rolling speed in the rolling section is set within a range of 0.3 to 1.5 mm / min as shown in the following formula (4).
ΣL _i ≧ β + 2 (1)
0.7 × δ _IB ≦ R _total ≦ δ _IB (2)
R _i ≦ R _{i + 1} (3)
0.3 ≦ the reduction speed in the most downstream reduction section (= (R _n / L _n ) × V) ≦ 1.5 (4)
However, in Equations (1), (2), (3), and (4), ΣL _i is the sum (m) of the length in the casting direction of each reduction section, and β is during casting at the solidification completion position. In the casting direction variation range (m), δ _IB is the total amount of bulging of the slab (mm), R _total is the total value of the reduction amount in each reduction section, and the total reduction amount (mm) of the slab, R _i is the reduction amount (mm) in the i-th reduction section from the upstream side in the casting direction, R _{i + 1} is the reduction amount (mm) in the “i + 1” -th reduction section from the upstream side in the casting direction, and R _n is , The amount of reduction (mm) in the n-th or most downstream reduction section from the upstream side in the casting direction, L _n is the length (m) in the casting direction of the n-th or downstream-most reduction section from the upstream side in the casting direction, V is a slab drawing speed (m / min). 2. The continuous casting method for steel according to claim 1, wherein the total reduction amount of the slab is set so as to satisfy the following expression (5) with respect to the thickness of the slab at the mold outlet: .
_{R total <D 0/10 ···} (5)
However, in (5), R _total is the total rolling reduction of the slab (mm), D ₀ is the thickness at the mold exit of the slab (mm).   The light reduction belt is divided into two sections, a first reduction section upstream of the casting direction and a second reduction section downstream of the casting direction. The continuous casting method of the described steel.

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