JP2019155393A - Continuous casting method and continuous casting machine - Google Patents

Abstract

To increase the reduction efficiency of porosities in a slab without increasing the rolling reduction force of rolling reduction rolls.SOLUTION: In a continuous casting method where a slab 20 carried by a continuous casting machine 10 is subjected to rolling reduction by upstream side rolling reduction rolls 42 and downstream side rolling reduction rolls 44 adjacent to each other in the carrying direction of the slab 20, the roll pitch between the respective upstream side rolling reduction roll 42 and the respective downstream side rolling reduction roll 44 adjacent to each other is controlled to 100 to 400 mm, and the slab 20 having a central solid phase ratio of 0.8 or higher is subjected to rolling reduction by the upstream side rolling reduction rolls 42 and the downstream side rolling reduction rolls 44 by 2 mm or higher, respectively.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a continuous casting method and a continuous casting machine.

  There is a continuous casting method in which a slab formed by a mold is reduced by a reduction roll to reduce the porosity in the slab (for example, Patent Documents 1 to 4).

International Publication No. 2015/174395 JP-A-8-164460 JP 2006-022531 A Japanese Patent No. 5929836

  By the way, in order to raise the reduction efficiency of the porosity in a slab, it is possible to enlarge the reduction force (compression force) of the reduction roll with respect to a slab.

  However, in order to increase the reduction force of the reduction roll, a pressing device such as a hydraulic cylinder that presses the reduction roll against the slab may be increased in size.

  In view of the above facts, the present invention aims to increase the reduction efficiency of porosity in a slab without increasing the rolling force of the rolling roll.

  The continuous casting method according to the first aspect is a continuous casting method in which a slab transported by a continuous casting machine is squeezed by a plurality of squeezing rolls adjacent to each other in the transport direction of the slab. The roll pitch is set to 100 mm or more and 400 mm or less, and the slabs having a central solid phase ratio of 0.8 or more are respectively reduced by 2 mm or more by the reduction roll.

  According to the continuous casting method which concerns on a 1st aspect, each slab whose center solid phase rate is 0.8 or more is squeezed 2 mm or more with the rolling roll adjacent to the conveyance direction of a slab. Thereby, the center segregation and internal crack of a slab are suppressed. Therefore, the quality of the slab is improved.

  Moreover, the roll pitch of the rolling rolls adjacent in the conveying direction of the slab is set to 100 mm or more and 400 mm or less.

  Here, when a slab having a high center temperature is reduced by a reduction roll, the compressive stress generated in the slab is as follows. That is, in the slab, a peak of compressive stress (hereinafter referred to as “upstream side peak”) is generated on the upstream side in the slab conveying direction with respect to the rotation center of the rolling roll. Further, a peak of compressive stress (hereinafter referred to as “downstream peak”) is generated in the slab on the downstream side in the conveyance direction of the slab with respect to the rotation center of the rolling roll.

  In view of this, in this aspect, among the adjacent rolling rolls, the peripheral portion of the downstream peak of the compressive stress generated in the slab by the rolling force of the upstream rolling roll and the slab by the rolling force of the downstream rolling roll. The roll pitch of these rolling rolls is set to 100 mm or more and 400 mm or less so that the peripheral portion of the upstream peak of the generated compressive stress overlaps.

  Thereby, in the part which the peripheral part of the downstream peak and the peripheral part of the upstream peak overlapped, compressive stress becomes large. Therefore, in this aspect, the reduction efficiency of the porosity in the slab can be increased without increasing the reduction force of the reduction roll.

  The continuous casting method according to the second aspect is the continuous casting method according to the first aspect, wherein the adjacent rolling rolls roll down the slab having a center temperature of 1350 ° C. or higher.

  According to the continuous casting method which concerns on a 2nd aspect, a reduction roll reduces a slab whose center temperature is 1350 degreeC or more.

  Here, if the center temperature of the slab is low, the slab becomes difficult to be deformed, and the reduction force of the reduction roll required to reduce the porosity in the slab may increase.

  As a countermeasure, in this aspect, a slab having a center temperature of 1350 ° C. or higher is reduced by a reduction roll. Thereby, the reduction force of the reduction roll required in order to reduce the porosity in a slab becomes small. Therefore, it is possible to reduce the size of the pressing device that presses the reduction roll against the slab.

  The continuous casting method according to the third aspect is the continuous casting method according to the second aspect, wherein the continuous casting machine uses the continuous casting machine so that the center temperature of the slab that is reduced by the adjacent reduction rolls is 1350 ° C. or higher. Control at least one of the conveying speed and cooling speed of the slab.

  According to the continuous casting method according to the third aspect, for example, a slab that is reduced by a reduction roll by controlling at least one of the conveyance speed and the cooling rate of the slab according to the thickness of the slab to be continuously cast. Is set to 1350 ° C. or higher. Therefore, even if the thickness of the slab to be continuously cast is changed, the porosity in the slab can be efficiently reduced.

  The continuous casting machine which concerns on a 4th aspect arrange | positions adjacent the conveyance direction of the slab by the conveyance apparatus which conveys a slab, and the said slab by the said conveyance apparatus, and the said slab whose center solid phase rate is 0.8 or more A plurality of reduction rolls that respectively reduce by 2 mm or more, and the roll pitch of the adjacent reduction rolls is set to 100 mm or more and 400 mm or less.

  According to the continuous casting machine which concerns on a 4th aspect, the several reduction roll arrange | positioned adjacent to the conveyance direction of a slab squeezes the slab whose center solid phase rate is 0.8 or more, respectively 2 mm or more. Thereby, the center segregation and internal crack of a slab are suppressed. Therefore, the quality of the slab is improved.

  In addition, the roll pitch of the rolling rolls adjacent in the slab conveying direction is set to 100 mm or more and 400 mm or less.

  Here, when a slab having a high center temperature is reduced by a reduction roll, the compressive stress generated in the slab is as follows. That is, in the slab, a peak of compressive stress (hereinafter referred to as “upstream side peak”) is generated on the upstream side in the slab conveying direction with respect to the rotation center of the rolling roll. Further, a peak of compressive stress (hereinafter referred to as “downstream peak”) is generated in the slab on the downstream side in the conveyance direction of the slab with respect to the rotation center of the rolling roll.

  In view of this, in this aspect, among the adjacent rolling rolls, the peripheral portion of the downstream peak of the compressive stress generated in the slab by the rolling force of the upstream rolling roll and the slab by the rolling force of the downstream rolling roll. The roll pitch of these rolling rolls is set to 100 mm or more and 400 mm or less so that the peripheral portion of the upstream peak of the generated compressive stress overlaps.

  Thereby, in the part which the peripheral part of the downstream peak and the peripheral part of the upstream peak overlapped, compressive stress becomes large. Therefore, in this aspect, the reduction efficiency of the porosity in the slab can be increased without increasing the reduction force of the reduction roll.

  As described above, according to the present invention, the reduction efficiency of the porosity in the slab can be increased without increasing the reduction force of the reduction roll.

FIG. 1 is a side view of a continuous casting machine according to an embodiment as viewed from the width direction of a slab. FIG. 2 is a partially enlarged view of FIG. 1 showing the reduction device. FIG. 3 is a graph showing the relationship between the distance from the upstream reduction roll in the slab conveyance direction and the average compressive stress generated by the slab. FIG. 4 is a graph showing the relationship between the maximum value of the average compressive stress generated in the slab and the roll pitch. FIG. 5 is a graph showing the relationship between the distance from the upstream reduction roll in the slab conveyance direction and the average compressive stress generated in the slab. FIG. 6 is a table showing test conditions and evaluation results of a continuous casting test.

  Hereinafter, a continuous casting machine and a continuous casting method according to an embodiment will be described.

(Continuous casting machine)
First, the configuration of the continuous casting machine will be described.

  FIG. 1 shows a continuous casting machine 10 according to the present embodiment. The continuous casting machine 10 includes a tundish 12, a mold 16, a transfer device 30, and a reduction device 40.

(Tundish)
The tundish 12 is a container that temporarily stores the molten steel W. Molten steel W is poured into the tundish 12 from a ladle (not shown). Further, an immersion nozzle 14 for discharging the molten steel W is provided at the bottom of the tundish 12. A mold 16 is disposed below the tundish 12.

(template)
The mold 16 is, for example, a water-cooled copper mold. The mold 16 cools the molten steel W poured from the immersion nozzle 14 of the tundish 12 and solidifies the surface layer of the molten steel W. Thereby, the slab 20 having a predetermined shape is formed.

  The mold 16 is formed in a cylindrical shape having both ends in the axial direction opened. Moreover, the casting_mold | template 16 is arrange | positioned by making an axial direction into an up-down direction. At the upper end of the mold 16, an injection port 16U is formed. An immersion nozzle 14 for the tundish 12 is inserted into the injection port 16U. Molten steel W is poured into the mold 16 from the immersion nozzle 14.

  The immersion nozzle 14 is provided with an adjustment mechanism such as an adjustment valve for adjusting the discharge amount of the molten steel W. By this adjustment mechanism, the discharge amount of the molten steel W discharged from the immersion nozzle 14 to the injection port 16U is adjusted so that the liquid level of the molten steel W in the mold 16 (hereinafter referred to as “meniscus M”) becomes a predetermined height. The

  The molten steel W poured into the mold 16 is cooled by the mold 16 and gradually solidifies from the surface layer. Thereby, the molten steel W of the surface layer is solidified, and the slab 20 in which the molten steel W remains inside is formed. Moreover, the cross-sectional shape of the casting_mold | template 16 is made into the rectangular shape. Thereby, the cross-sectional shape of the slab 20 is formed in a rectangular shape. Hereinafter, the surface layer portion of the slab 20 where the molten steel W has solidified is referred to as a solidified portion (solidified shell) 20A, and the unsolidified molten steel W remaining inside the slab 20 is referred to as an unsolidified portion 20B.

  A discharge port 16 </ b> L is formed at the lower end of the mold 16. The slab 20 molded by the mold 16 is discharged from the discharge port 16L. In addition, a transfer device 30 is disposed below the mold 16.

(Transport device)
The conveyance device 30 conveys the slab 20 discharged from the mold 16 in a predetermined direction (arrow H direction) while cooling. In the following, the direction of arrow H is defined as the conveyance direction (casting direction) of the conveyance device 30.

  The transport device 30 has a plurality of pairs of support rolls 32. The plurality of pairs of support rolls 32 are arranged on both sides of the slab 20 in the thickness direction (arrow t direction) with an interval in the conveyance direction of the slab 20. Further, both end portions in the axial direction of each support roll 32 are rotatably supported by bearing portions (not shown) on both sides in the width direction of the slab 20. These support rolls 32 form a conveyance path 34 that is gently curved from the outlet 16L of the casting mold 16 toward a reduction device 40 described later, and then extends in a substantially horizontal direction.

  The plurality of pairs of support rolls 32 convey the cast piece 20 in the carrying direction while gripping the cast piece 20 from both sides in the thickness direction. Thereby, the bulging which the slab 20 swells in the thickness direction is suppressed. A part of the plurality of support rolls 32 is a drive roll that is rotationally driven. By this drive roll, the conveyance speed (casting speed) of the slab 20 is adjusted.

  The conveying device 30 has a plurality of coolers (secondary coolers) (not shown) that cool the slab 20. The plurality of coolers have, for example, spray nozzles that inject cooling water. These coolers are arranged at intervals in the conveying direction of the slab 20 and inject cooling water onto the slab 20. Thereby, the slab 20 is cooled and the unsolidified portion 20B of the slab 20 is gradually solidified.

  Moreover, the conveyance speed and the cooling rate of the slab 20 by the conveying apparatus 30 are the center temperature of the slab 20 when the slab 20 is crushed by a pair of upstream-side reduction rolls 42 and a pair of downstream-side reduction rolls 44 described later. Is controlled (adjusted) to be 1350 ° C. or higher, for example.

  The center temperature of the slab 20 here is the temperature at the center of the slab 20 in the thickness direction. Further, the center temperature of the slab 20 when the slab 20 is rolled down by the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44 increases, for example, when the conveyance speed of the slab 20 is increased. When the conveyance speed of the piece 20 is slowed, it decreases. In addition, the center temperature of the slab 20 when the slab 20 is rolled down by the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44 decreases, for example, when the cooling rate of the slab 20 is increased. Increasing the cooling rate of the piece 20 increases.

  Moreover, the conveyance speed of the slab 20 becomes faster, for example, when the rotational speed of the drive roll described above is increased, and becomes slower when the rotational speed of the drive roll is decreased. In addition, the cooling rate of the slab 20 increases, for example, when the amount of cooling water injected from the cooler to the slab 20 is increased, and decreases when the amount of cooling water injected from the cooler to the slab 20 is decreased. Become. Moreover, the cooling rate of the slab 20 becomes faster, for example, when the temperature of the cooling water injected from the cooler to the slab 20 is lowered, and becomes slower when the temperature of the cooling water injected from the cooler to the slab 20 is increased.

  In addition, the conveyance path 34 may be provided with an electromagnetic stirring device that electromagnetically stirs the unsolidified portion 20B of the slab 20.

(Rolling device)
The reduction device 40 is disposed on the downstream side of the conveyance path 34 extending in a substantially horizontal direction. Moreover, the reduction apparatus 40 reduces the porosity etc. in the unsolidified part 20B of the slab 20 by compressing the slab 20 in the thickness direction. The reduction device 40 includes a pair of upstream side reduction rolls 42 and a pair of downstream side reduction rolls 44.

  As shown in FIG. 2, the pair of upstream side reduction rolls 42 conveys the slab 20 in the conveyance direction while gripping the slab 20 from both sides in the thickness direction. Similarly, the pair of downstream side reduction rolls 44 conveys the slab 20 in the conveyance direction while gripping the slab 20 from both sides in the thickness direction. In addition, the pair of downstream side reduction rolls 44 is disposed downstream of the pair of upstream side reduction rolls 42 in the conveying direction of the slab 20. The pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44 form a conveyance path 34 for the slab 20.

  In the present embodiment, the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44 have the same configuration. Therefore, in the following, the pair of upstream-side reduction rolls 42 will be described, and description of the pair of downstream-side reduction rolls 44 will be omitted as appropriate. Further, the upstream-side reduction roll 42 and the downstream-side reduction roll 44 disposed on the upper side of the slab 20 are examples of the reduction rolls adjacent to each other in the conveying direction of the slab 20.

  The pair of upstream-side reduction rolls 42 are disposed on both sides in the thickness direction of the slab 20. Moreover, the pair of upstream-side reduction rolls 42 is formed in a columnar shape. Further, the pair of upstream-side reduction rolls 42 are arranged with the axial direction (longitudinal direction) as the width direction of the slab 20. Both ends in the axial direction of the pair of upstream side reduction rolls 42 are supported by bearings (not shown) on both sides in the width direction of the slab 20. Thereby, the pair of upstream-side reduction rolls 42 can be rotated about the rotation center (rotation axis) C1.

  Further, a bearing portion that supports both ends in the axial direction of the upstream-side reduction roll 42 disposed on the upper side of the slab 20 is a center side (lower side) in the thickness direction of the slab 20 by a pressing device such as a hydraulic cylinder (not shown). Side). Thereby, the slab 20 is rolled down by the upstream-side rolling roll 42.

  Here, the slab 20 is conveyed while being cooled by the plurality of coolers of the conveying device 30 as described above. Thereby, the unsolidified part 20B of the slab 20 is gradually solidified toward the downstream side in the transport direction. In other words, the center solid phase ratio R of the slab 20 becomes higher as the slab 20 moves toward the downstream side in the transport direction.

  The center solid phase ratio R means the thickness of the slab 20 and the ratio (ratio) of the solidified portion at the center in the width direction. For example, when the center solid phase ratio R is 0.8, the ratio of the solidified part at the center of the thickness and width direction of the slab 20 is 80% (80%), and the ratio of the unsolidified part is 20% (20%). It becomes.

  Here, according to the result of the continuous casting test described later, when the slab 20 having a central solid phase ratio R of less than 0.8 is squeezed by the pair of upstream-side squeezing rolls 42, a molten steel flow is generated in the slab 20; Center segregation or internal cracking may increase.

Therefore, the pair of upstream-side reduction rolls 42 is disposed in the conveyance path 34 of the slab 20 at a position where the central solid phase ratio R of the slab 20 is 0.8 or more (0.8 ≦ R ≦ 1). 0.0). Thereby, the center segregation and the internal crack of the slab 20 by the pair of upstream side reduction rolls 42 are suppressed.
Note that the central solid phase ratio R is obtained, for example, by performing solidification analysis on the slab 20. Specifically, the solidus temperature (solid fraction 1.0) from the experimental regression equation called Kawawa's formula (see “Solidification of steel: Japan Society for the Promotion of Science, 19th Committee, 2015, Appendix-13”). And the liquidus temperature (solid phase ratio 0). Then, the solid phase ratio between the solidus temperature and the liquidus temperature was assumed to be linear, and the central solid phase ratio R of the slab was obtained.

  The pair of upstream-side reduction rolls 42 can reduce the slab 20 by 2 mm or more by controlling the pressing force of a pressing device such as the hydraulic cylinder described above. Similarly, the pair of downstream side reduction rolls 44 can reduce the slab 20 by 2 mm or more.

  In addition, from the viewpoint of the maximum output of the pressing device, the reduction amounts S1 and S2 of the pair of upstream side reduction rolls 42 and the pair of upstream side reduction rolls 42 are preferably, for example, 10 mm or less. In addition, the pair of upstream side reduction rolls 42 and the amount of reduction S1 and S2 of the pair of upstream side reduction rolls 42 are a pair of upstream side reduction rolls 42 and a pair of upstream side pressures disposed on the upper side of the slab 20. It means the amount by which the slab 20 is pushed downward by the lower roll 42.

  The roll pitch P between the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44 is set to 100 mm or more and 400 mm or less based on the analysis result described later.

  Note that the roll pitch P here refers to the rotation of the upstream and downstream rolling rolls 42 and 44 when the upstream and downstream rolling rolls 42 and 44 adjacent to each other in the conveying direction of the slab 20 are viewed from above. The distance between the centers C1 and C2. Further, the diameters D1 and D2 of the upstream side reduction roll 42 and the downstream side reduction roll 44 are appropriately set so that the adjacent upstream side reduction roll 42 and downstream side reduction roll 44 do not interfere with each other.

(analysis)
Here, the analysis of the compressive stress generated in the slab 20 by the reduction device 40 will be described.

  In this analysis, the roll pitch P is used as a parameter, and the slab 20 is compressed by the finite element method when the slab 20 is squeezed by the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44, respectively. Asked. The compressive stress to be analyzed was an average value of the compressive stress generated in the thickness direction, the width direction, and the conveyance direction of the slab 20 (hereinafter referred to as “average compressive stress”).

(Analysis model)
The width of the analysis model of the slab 20 (hereinafter simply referred to as “slab 20”) was 2300 mm. Moreover, the thickness T (refer FIG. 2) of the slab 20 was 300 mm. Further, the center solid phase ratio R of the slab 20 was set to 0.8. Further, the temperature distribution of the slab 20 was calculated by a differential method heat transfer calculation program.

  The reduction amounts S1 and S2 of the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44 with respect to the slab 20 were set to 2.5 mm, respectively. Further, the diameters D1 and D2 of the pair of upstream-side rolling rolls 42 and the pair of downstream-side rolling rolls 44 are values obtained by subtracting 20 mm from the roll pitch P (= P−20 mm) when the roll pitch P is less than 500 mm. . On the other hand, when the roll pitch P is 500 mm or more, the diameters D1 and D2 of the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44 are set to 500 mm.

  In this analysis, the average compressive stress generated in the slab 20 when the slab 20 is squeezed only by the pair of upstream-side reduction rolls 42 was also obtained by the finite element method. The reduction amount S1 of the pair of upstream side reduction rolls 42 in this case is the total value of the reduction amounts S1 and S2 when the slab 20 is reduced by both the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44. 5 mm (= 2.5 mm + 2.5 mm). Furthermore, the diameter D1 of the pair of upstream side reduction rolls 42 was 500 mm.

  In the graph shown in FIG. 4 to be described later, for convenience, the roll pitch P when the slab 20 is squeezed by only the pair of upstream-side squeeze rolls 42 is set to 0 (zero).

(Analysis result)
FIG. 3 shows the analysis result of the average compressive stress generated in the slab 20 when the slab 20 is squeezed by the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44.

  Note that the horizontal axis of the graph shown in FIG. 3 is the distance from the rotation center C1 of the upstream-side reduction roll 42 in the conveyance direction of the slab 20. Further, the vertical axis of the graph shown in FIG. 3 is the average compressive stress generated in the slab 20. Further, in the analysis result shown in FIG. 3, the roll pitch P is set to 400 mm.

  As shown in FIG. 3, the average compressive stress generated in the slab 20 has a maximum value Q between the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44.

  FIG. 4 shows the relationship between the maximum value of the average compressive stress generated in the slab 20 and the roll pitch P. Note that the vertical axis (the maximum value of the average compressive stress of the slab) of the graph shown in FIG. 4 is, for example, the maximum value Q in the graph shown in FIG. 3 when the roll pitch P is 400 mm.

  As shown in FIG. 4, the maximum value of the average compressive stress of the slab 20 was larger than the other ranges when the roll pitch P was 100 mm or more and 400 mm or less. Therefore, the roll pitch P is preferably set in a range of 100 mm or more and 400 mm or less.

(Supplement)
In addition, if it supplements about an analysis result, FIG. 5 shows the analysis result of the average compressive stress which generate | occur | produces in the slab 20 when the slab 20 is squeezed only by a pair of upstream side reduction rolls 42.

  As shown in FIG. 5, the slab 20 has an average compressive stress peak (hereinafter, “upstream peak”) on the upstream side in the conveying direction of the slab 20 with respect to the rotation center C1 of the upstream side roll 42. V1 occurs. In the slab 20, an average compressive stress peak (hereinafter referred to as “downstream peak”) V <b> 2 is generated on the downstream side in the conveying direction of the slab 20 with respect to the rotation center C <b> 1 of the upstream side roll 42. . This is considered because the deformation resistance difference between the surface layer portion and the center portion of the slab 20 is increased due to the temperature difference between the surface layer portion and the center portion of the slab 20.

  In a general thick plate rolling process, an average compressive stress peak generated in the thick plate is generated near the rotation center of the rolling roll.

  Although not shown, when the slab 20 is squeezed by the pair of downstream side reduction rolls 44, the slab 20 has an upstream side in the conveyance direction of the slab 20 with respect to the rotation center C2 of the downstream side reduction roll 44. The peak of the average compressive stress (hereinafter referred to as “upstream side peak”) occurs. In the slab 20, a peak of average compressive stress (hereinafter referred to as “downstream peak”) occurs on the downstream side in the conveying direction of the slab 20 with respect to the rotation center C <b> 2 of the downstream side roll 44.

  In this analysis, when the roll pitch P is in the range of 100 mm or more and 400 mm or less, the peripheral portion of the downstream peak V2 of the average compressive stress due to the reduction force of the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44. It is thought that the average compressive stress generated in the slab 20 is increased by overlapping the peripheral portion of the upstream peak of the average compressive stress due to the rolling force.

  Even when the roll pitch P is less than 100 mm, the average compressive stress due to the peripheral portion of the downstream peak V2 of the average compressive stress due to the reduction force of the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44. It is assumed that there is an overlap with the periphery of the upstream peak.

  However, when the roll pitch P is less than 100 mm, the diameters D1 and D2 of the upstream and downstream rolling rolls 42 and 44 are used in order to avoid interference between the adjacent upstream and downstream rolling rolls 42 and 44. Need to be small. In this case, since the upstream side roll 42 and the downstream side roll 44 are likely to bend, it may be difficult to sufficiently reduce the slab 20. Therefore, as described above, the roll pitch P is preferably set in a range of 100 mm or more and 400 mm or less.

(Function)
Next, the operation of this embodiment will be described while explaining the continuous casting method (slab manufacturing method) according to this embodiment.

  In the continuous casting method according to the present embodiment, a slab having a central solid phase ratio R of 0.8 or more by the upstream side reduction roll 42 and the downstream side reduction roll 44 adjacent to each other in the conveying direction (arrow H direction) of the slab 20. 20 is reduced by 2 mm or more. Thereby, the center segregation and internal crack of the slab 20 are suppressed. Therefore, the quality of the slab 20 is improved.

  Further, the roll pitch P between the upstream-side reduction roll 42 and the downstream-side reduction roll 44 adjacent in the conveying direction of the slab 20 is set to 100 mm or more and 400 mm or less.

  Thereby, for example, as shown in FIG. 3, between the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44, the downstream side of the average compressive stress due to the reduction force of the pair of upstream-side reduction rolls 42. The peripheral portion of the peak V2 overlaps with the peripheral portion of the upstream peak of the average compressive stress due to the pair of downstream side rolls 44, and the average compressive stress generated in the slab 20 increases. As a result, the average compressive stress generated in the slab 20 becomes a maximum value Q between the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44.

  Thus, in this embodiment, the average compressive stress which generate | occur | produces in the slab 20 can be enlarged, without enlarging the reduction force of the adjacent upstream side reduction roll 42 and downstream side reduction roll 44. FIG. Therefore, the reduction efficiency of the porosity in the slab 20 can be increased without increasing the reduction force of the upstream side reduction roll 42 and the downstream side reduction roll 44. Furthermore, the enlargement of the pressing device that presses the upstream-side reduction roll 42 and the downstream-side reduction roll 44 against the slab 20 can be suppressed.

  Further, the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44 reduce the slab 20 having a center temperature of 1350 ° C. or higher.

  Here, when the center temperature of the slab 20 is low, the slab 20 is not easily deformed, and a pair of upstream-side reduction rolls 42 and a pair of downstream-side reduction rolls are required to reduce the porosity in the slab 20. There is a possibility that the rolling force of 44 increases.

  As a countermeasure, in the present embodiment, as described above, the slab 20 having a center temperature of 1350 ° C. or higher is squeezed by the pair of upstream squeezing rolls 42 and the pair of downstream squeezing rolls 44, respectively. Thereby, the reduction force of a pair of upstream side reduction rolls 42 and a pair of downstream side reduction rolls 44 required in order to reduce the porosity in the slab 20 can be made small.

  Therefore, it is possible to further suppress an increase in size of the pressing device that presses the upstream-side reduction roll 42 and the downstream-side reduction roll 44 against the slab 20.

  Moreover, even if it is a case where the thickness of the continuous casting slab 20 is changed by controlling at least one of the conveyance speed and the cooling rate of the casting 20 according to the thickness of the continuous casting slab 20. The porosity in the slab 20 can be efficiently reduced.

(Modification)
Next, a modification of the above embodiment will be described.

  In the above embodiment, the diameter D1 of the upstream side roll 42 and the diameter D2 of the downstream side roll 44 are the same. However, the diameter D1 of the upstream side reduction roll 42 and the diameter D2 of the downstream side reduction roll 44 may be different.

  Further, in the above embodiment, the reduction amount S1 of the upstream side reduction roll 42 and the reduction amount S2 of the downstream side reduction roll 44 are the same. However, the reduction amount S1 of the upstream side reduction roll 42 and the reduction amount S2 of the downstream side reduction roll 44 may be different.

(Continuous casting test)
Next, the continuous casting test will be described.

  In this continuous casting test, a plurality of slabs according to Examples 1 to 7 were continuously cast by the continuous casting machine 10 shown in FIG. 1, and the porosity and the like in each slab were evaluated. Moreover, the several slab which concerns on Comparative Examples 1-6 was continuously cast, and the porosity etc. in each slab were evaluated.

(Molten steel)
The composition of the molten steel is mass%, C: 0.05 to 0.15%, Si: 0.1 to 0.4%, Mn: 0.8 to 1.5%, P: 0.02% or less, S: 0.008% or less, with the balance being Fe and impurities.

(template)
Next, a water-cooled copper mold was used as the mold 16. Various dimensions of the mold 16 are shown in Table 1 below.

(Transport device)
Next, the casting speed of the slab by the conveying device 30 was 0.7 m / min. Moreover, the specific water amount of the cooler (secondary cooler) of the conveying device 30 was set to 0.5 to 0.7 L / kg-steel. As a result, the center solid phase ratio R of the slab rolled by the pair of upstream-side rolling rolls 42 and the pair of downstream-side rolling rolls 44 was set to 0.8.

  Further, in Example 4 to be described later, the conveyance speed of the slab by the conveyance device 30 was appropriately adjusted so that the center solid phase ratio R of the slab that was reduced by the reduction device 40 was 0.95. Similarly, in Example 7 to be described later, the conveyance speed of the slab by the conveyance device 30 is appropriately adjusted so that the center solid phase ratio R of the slab that is reduced by the reduction device 40 is 1.0. . In Example 7, the center temperature of the slab that is reduced by the reduction device 40 is 1350 ° C. The center temperature of the slab was estimated from the slab transport speed, cooling rate, and the like. Further, in Comparative Example 4 to be described later, the casting by the conveying device 30 is performed such that the center solid phase ratio R of the slab that is squeezed by the pair of upstream side reduction rolls 42 and the pair of downstream side reduction rolls 44 becomes 0.7. The conveyance speed of the piece was adjusted as appropriate. The center solid phase ratio R of the slab was calculated by two-dimensional solidification analysis of the thickness T and width of the slab.

(Rolling device)
The pair of upstream-side reduction rolls 42 was disposed 20 m downstream from the meniscus M in the mold 16 along the slab conveying direction. Further, a pair of downstream side reduction rolls 44 is disposed downstream of the pair of upstream side reduction rolls 42 in the slab conveying direction. The upstream slab roll 42 and the downstream squeeze roll 44 arranged on the upper side of the slab are pressed by a hydraulic cylinder (not shown), so that the slab is respectively moved by the upstream side roll 42 and the downstream side roll 44. Squeezed. The maximum reduction force (maximum output) of the upstream side reduction roll 42 and the downstream side reduction roll 44 is 400 ton F (3.92 MN).

  As shown in FIG. 6, in the slabs according to Examples 1 to 7 and Comparative Examples 1 to 6, the roll pitch P and the diameters D1 and D2 of the reduction device 40 (upstream side reduction roll 42 and downstream side reduction roll 44). The reduction amounts S1 and S2 and the center solid phase ratio R of the slab are appropriately changed. In Comparative Example 1 and Comparative Example 6, the slab was not squeezed by the downstream side reduction roll 44, and the slab was reduced only by the upstream side reduction roll 42.

(Slab evaluation method)
In the evaluation of the slab, the porosity from the porosity volume V of the slab according to Examples 1 to 7 and Comparative Examples 1 to 6 and the porosity volume V ₀ of the slab (reference slab) that is not reduced by the reduction device 40. An index (= V / V ₀ ) was obtained. And the porosity index set 0.4 or less as the pass ((circle)), and made more than 0.4 the disqualification (x). This is because when the porosity index is 0.4 or less, the porosity in the slab (slab slab) disappears in the rolling process after the continuous casting process.

  Furthermore, the macro sample cut out from the cross section of the slab which concerns on Examples 1-7 and Comparative Examples 1-6 was etched, and the presence or absence of an internal crack and center segregation was confirmed visually. And the case where there was at least one of an internal crack and center segregation was made into a disqualification (x), and the case where there was neither an internal crack and center segregation was made into a pass (o).

  In addition, the calculation method of the porosity volume V of the slab which concerns on Examples 1-7 and Comparative Examples 1-6 is as follows. First, a sample (hereinafter referred to as “center sample”) was cut out at the center position in the thickness direction of the slab. Further, a sample (hereinafter referred to as “¼ thickness sample”) was cut out from the surface of the slab at a position of ¼ of the thickness of the slab. In addition, the dimension of each center sample and 1/4 thickness sample is thickness 7mm, width 100mm, and length 50mm.

Next, according to the solid density and specific gravity measurement method defined in JIS Z8807, the density of each central sample and 1/4 thickness sample was measured. Furthermore, the porosity volume V (cm < ³ > / g) per unit weight was calculated | required from following formula (1). Further, in the same manner as this, porosity volume V ₀ which reference slab that is not pressure by screw down device 40 _(cm 3 / ^g) was also determined.

However,
ρ: density of center sample (g / cm ³ )
ρ ₀ : density of 1/4 thickness sample (g / cm ³ )
It is.

(Evaluation results)
The evaluation result of the slab which concerns on Examples 1-7 and Comparative Examples 1-6 is shown by FIG. Note that the actual measurement values of the amount of reduction S1 and S2 of the reduction device (upstream side reduction roll and downstream side reduction roll) in FIG. 6 were measured by position sensors respectively installed on the upstream side reduction roll 42 and the downstream side reduction roll 44.

(Example)
In Examples 1-7, the evaluation of the porosity index was all acceptable (◯). The reason is considered as follows.

  That is, in Examples 1-7, the amount of reduction of each of the upstream side reduction roll 42 and the downstream side reduction roll 44 is set to 2 mm or more, and the roll pitch P is 100 mm or more and 400 mm or less (100 ≦ P ≦ 400). ) Range. As a result, the peripheral portion of the downstream peak of the compressive stress generated in the slab by the reduction force of the pair of upstream side reduction rolls 42 and the upstream of the compressive stress generated in the slab by the reduction force of the pair of downstream side reduction rolls 44. This is thought to be due to the fact that the compressive stress generated in the slab increased due to the overlap with the peripheral part of the side peak.

  Furthermore, in Examples 1 to 7, internal cracks and center segregation were not confirmed. The reason is considered to be that in Examples 1 to 7, the center solid phase ratio R of the slab when being reduced by the reduction device 40 was set to 0.8 or more (0.8 ≦ R ≦ 1.0). .

  From the above evaluation results, it was confirmed that all the slabs according to Examples 1 to 7 were of high quality.

(Comparative example)
In Comparative Examples 1 and 6, internal cracks and center segregation were not confirmed (◯). However, in Comparative Examples 1 and 6, the evaluation of the porosity index was unacceptable (x). The reason is considered that in Comparative Examples 1 and 6, the slab was squeezed only by the pair of upstream-side reduction rolls 42, so that the compressive stress generated in the slab was smaller than in Examples 1-7.

  In Comparative Example 6, the set value of the reduction amount S1 of the pair of upstream side reduction rolls 42 was 5 mm, but the actual measurement value of the reduction amount S1 was 2.5 mm, which is half of the set value. The reason is considered to be that the maximum reduction force of the pair of upstream side reduction rolls 42 is 400 tonF, so that the reduction force necessary for reducing the slab by 5 mm is insufficient.

  Next, in Comparative Example 2 and Comparative Example 3, internal cracks and center segregation were not confirmed (◯). However, in Comparative Example 2 and Comparative Example 3, the evaluation of the porosity index was rejected (x).

  The reason is that in Comparative Example 2 and Comparative Example 3, the roll pitch P is set to 500 mm or more (500 mm and 700 mm). Therefore, the peripheral part of the downstream peak of the compressive stress generated in the slab by the reduction force of the pair of upstream side reduction rolls 42 and the upstream side of the compression stress generated in the slab by the reduction force of the pair of downstream side reduction rolls 44 It is considered that the peripheral portion of the peak did not overlap and the compressive stress generated in the slab was smaller than in Examples 1-7.

  In Comparative Example 4, the evaluation of the porosity index was a pass (◯). However, in Comparative Example 4, internal cracks or center segregation was confirmed (×). The reason is considered that in Comparative Example 4, the center solid phase ratio R of the slab when being reduced by the reduction device 40 was set to 0.7.

  In Comparative Example 5, no internal crack or center segregation was confirmed (◯). However, in Comparative Example 5, the evaluation of the porosity index was rejected (x). The reason is that in Comparative Example 5, the set value of the reduction amount S1 of the reduction device 40 (the pair of upstream-side reduction rolls 42 and the pair of downstream-side reduction rolls 44) was set to 1 mm. This is thought to be because the compressive stress generated in the film became smaller.

(Summary of evaluation results)
From the above evaluation results, it can be seen that in Examples 1 to 7, high quality slabs with less porosity, center segregation, and internal cracks were obtained.

  Although one embodiment of the present invention has been described above, the present invention is not limited to such an embodiment, and one embodiment and various modifications may be used in combination as appropriate. Needless to say, the present invention can be implemented in various forms without departing from the scope of the invention.

DESCRIPTION OF SYMBOLS 10 Continuous casting machine 20 Slab 30 Conveying apparatus 42 Upstream side reduction roll (reduction roll)
44 Downstream roll (rolling roll)
H Casting direction P Roll pitch S1 Rolling amount of the upstream side roll (rolling amount of the roll)
S2 Reduction amount of the downstream side reduction roll (reduction amount of the reduction roll)

Claims (4)

A continuous casting method in which a slab transported by a continuous casting machine is squeezed by a plurality of squeezing rolls adjacent in the transport direction of the slab,
The roll pitch of the adjacent rolling rolls is set to 100 mm or more and 400 mm or less, and the slabs having a central solid phase ratio of 0.8 or more are respectively rolled down by 2 mm or more by the rolling rolls.
Continuous casting method. The adjacent rolling rolls roll down the slab having a center temperature of 1350 ° C. or higher.
The continuous casting method according to claim 1. Controlling at least one of a conveyance speed and a cooling speed of the slab by the continuous casting machine so that a center temperature of the slab is reduced by 1350 ° C. or more by the adjacent reduction rolls;
The continuous casting method according to claim 2. A conveying device for conveying the slab;
A plurality of reduction rolls arranged adjacent to each other in the conveyance direction of the slab by the conveyance device, and each of the slabs having a central solid phase ratio of 0.8 or more, respectively, by 2 mm or more;
With
The roll pitch of the adjacent reduction rolls is 100 mm or more and 400 mm or less.
Continuous casting machine.