Continuous casting method for steel

Abstract

The continuous casting method for steel can improve the quality of the casting blank cast in the period after the casting is finished. In a continuous casting method for steel in which continuous casting is performed while pressing down a slab (3), pressing down is applied to the continuously cast slab (3), and after completion of casting, control is performed such that the pressing-down speed falls within a prescribed range by changing the pressing-down gradient of a section (14) to which the pressing down is applied.

Description

Technical Field

[0001] The present invention relates to a method for continuous casting of steel. Background Art

[0002] In the continuous casting of steel, in the final process of solidification, unsolidified molten steel (called "unsolidified layer") is attracted and flows in the direction of drawing the billet along with solidification shrinkage. In the unsolidified layer, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated. When the concentrated molten steel flows to the center of the billet and solidifies, so-called center segregation occurs. As the main cause of the flow of concentrated molten steel at the end of solidification, in addition to the above-mentioned solidification shrinkage, the billet bulge between the rolls caused by the static pressure of the molten steel and the mismatch of the roll calibration of the billet support rolls can also be cited.

[0003] This center segregation deteriorates the quality of steel products, especially thick steel plates. For example, in pipelines for oil and natural gas transportation, hydrogen-induced cracks occur from the center segregation as the starting point due to the action of acidic gas. In addition, the same problem also occurs in marine structures, storage tanks, oil tanks, etc. Moreover, in recent years, the use environment of steel materials has been mostly required to be used in harsh environments such as lower temperatures or stronger corrosive environments, and the importance of reducing the center segregation of the ingot has been increasing.

[0004] Therefore, many countermeasures have been proposed to reduce the center segregation or harmlessness of the ingot from the continuous casting process to the rolling process. Among them, the light reduction method at the end of solidification, which is to press down the continuous casting ingot with an unsolidified layer inside the continuous casting machine, is known to be particularly effective in improving the center segregation. Here, the "light reduction method at the end of solidification" refers to a method in which a reduction roll is arranged near the solidification completion position of the ingot, and the ingot in the continuous casting is gradually pressed down by the reduction roll at a reduction speed equivalent to the solidification shrinkage amount. In the light reduction method at the end of solidification, the generation of voids in the center of the ingot and the flow of concentrated molten steel are suppressed by the reduction of the reduction roll, thereby suppressing the center segregation of the ingot.

[0005] In order to effectively prevent the occurrence of center segregation of the cast billet by the soft reduction method in the final solidification stage, it is important to appropriately set the start and end time of the period of applying soft reduction during the final solidification period of the cast billet, as well as the amount of soft reduction applied. Therefore, various setting methods have been proposed for the amount of soft reduction in the soft reduction method in the final solidification stage.

[0006] For example, Patent Document 1 proposes a method for specifying the amount of reduction per unit time of the billet in the interval where light reduction is applied by the billet surface temperature at the start of reduction and the thickness of the unsolidified layer of the billet at the reduction position in a continuous casting method for applying light reduction to the final solidified portion of the continuously cast billet. Patent Document 1 focuses on the thickness of the unsolidified layer of the billet as an indicator for effectively implementing light reduction. This is based on the following insight, that is, according to Patent Document 1, the more the reduction on the downstream side of the casting, that is, the reduction in a state where the thickness of the unsolidified layer of the billet is small, the smaller the proportion of the reduction set by the reduction roll transmitted to the solid-liquid interface of the billet (hereinafter referred to as "reduction efficiency") is.

[0007] In addition, Patent Documents 2 and 3 propose continuous casting methods in which the billet reduction speed is increased on the downstream side in the casting direction as the solid phase ratio of the thickness center portion of the billet increases. In the continuous casting methods of Patent Documents 2 and 3, continuous casting is performed while reducing the area from the time when the solid phase ratio of the thickness center portion of the bloom cast piece is equivalent to a temperature of 0.1 to 0.3 to the time when the solid phase ratio of the bloom cast piece is equivalent to a temperature of the flow limit solid phase ratio.

[0008] Moreover, Patent Document 4 proposes a continuous casting method for setting or adjusting the pressing conditions based on information on the cross-sectional shape of the billet perpendicular to the length direction and information on the shape of the unsolidified portion in the cross-section during continuous casting of steel in which a pressing force is applied to the billet being drawn while continuously casting.

[0009] Furthermore, Patent Document 5 proposes that, when the continuous casting machine is finished casting, the casting speed is not decelerated, stopped, or the bottom of the last end of the ingot is not processed, but the casting is finished while the normal casting speed is maintained unchanged. In addition, Patent Document 5 is characterized in that the crater end of the last end of the ingot is controlled to be located in a predetermined interval, and the vicinity of the crater end is lightly pressed down by a small-diameter roller group arranged in the interval.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 8-132203

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 3-90263

[0014] Patent Document 3: Japanese Patent Application Laid-Open No. 3-90259

[0015] Patent Document 4: Japanese Patent Application Publication No. 2003-71552

[0016] Patent Document 5: Japanese Patent Application Laid-Open No. 7-112255 Summary of the invention

[0017] Problems to be solved by the invention

[0018] Here, in continuous casting, in the period from the end of casting in which molten steel is supplied to the mold to the end of pulling the billet from the machine of the continuous casting machine, the behavior of the billet pulling speed is different from the period in which molten steel is continuously supplied to the mold and continuous casting is performed at a stable casting speed. Therefore, in the case of applying the light reduction method at the end of solidification, if the reduction conditions in the period after the end of casting are made the same as those in the stable period, the quality of the billet may be deteriorated due to center segregation and internal cracks. Here, the period from the end of casting to the end of pulling the billet from the machine of the continuous casting machine is referred to as the period after the end of casting, and the period in which molten steel is continuously supplied to the mold and continuous casting is performed at a stable casting speed is referred to as the stable period.

[0019] However, Patent Documents 1 to 4 do not describe the conditions for light reduction of the ingots remaining in the machine after the casting is completed. Therefore, in the casting methods disclosed in Patent Documents 1 to 4, it is difficult to achieve appropriate reduction conditions in the period after the casting is completed. In addition, according to the casting method disclosed in Patent Document 5, although the quality of the bottom is improved, light reduction is not applied in the same conditions as in the stable period in the period after the casting is completed. Therefore, there is a concern that the quality of the ingots cast in the period after the casting is completed will deteriorate compared to the ingots cast in the stable period.

[0020] Therefore, the present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a continuous casting method of steel capable of improving the quality of a cast slab after the completion of casting.

[0021] Means for solving problems

[0022] (1) According to one embodiment of the present invention, a continuous casting method for steel is provided, which is a continuous casting method for steel in which continuous casting is performed while applying reduction pressure to the ingot, wherein, for the continuously cast ingot, reduction pressure is applied to the ingot, and after the casting is completed, the reduction gradient of the section where the reduction pressure is applied is changed, thereby controlling the reduction speed in a manner that is within a specified range.

[0023] (2) In the continuous casting method of steel of (1) above, after the casting is completed, the method comprises: a deceleration step of decelerating the casting speed of the ingot from the casting speed before the casting is completed, i.e., the first casting speed, to the casting speed required for capping the ingot, i.e., the second casting speed; a capping step of maintaining the casting speed of the ingot at the second casting speed for the time required for capping after the deceleration step; and an acceleration step of increasing the casting speed of the ingot from the second casting speed to the casting speed for re-drawing the ingot, i.e., the third casting speed, after the capping step.

[0024] (3) In the continuous casting method of steel according to (1) or (2), in the deceleration step, the reduction gradient is controlled in a manner satisfying equation (4), in the capping step, the reduction gradient is controlled in a manner satisfying equation (5), and in the acceleration step, the reduction gradient is controlled in a manner satisfying equation (6).

[0025] 0.5<V×Z<3.0…(4)

[0026] 0.5<V×Z<1.5…(5)

[0027] 0.5<V×Z<3.0…(6)

[0028] Wherein, V is the casting speed (m / min), and Z is the reduction gradient (mm / m).

[0029] Effects of the Invention

[0030] According to one aspect of the present invention, it is possible to provide a continuous casting method for steel that can improve the quality of a cast ingot cast after the end of casting. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic side view of a continuous casting machine showing one embodiment of the present invention.

[0032] Figure 2 This is a schematic side view showing an example of a segment constituting a light reduction belt.

[0033] Figure 3 Observed from the casting direction Figure 2 Front view of the section shown.

[0034] Figure 4 This is a graph showing an example of the casting speed after the casting is completed.

[0035] Figure 5 This is a graph showing an example of the reduction gradient and the reduction rate after the casting is completed. DETAILED DESCRIPTION

[0036] In the following detailed description, the embodiments of the present invention are described with reference to the accompanying drawings. In the description of the accompanying drawings, the same or similar parts are marked with the same or similar symbols, and repeated descriptions are omitted. Each drawing is schematic and contains situations that are different from reality. In addition, the embodiments shown below illustrate the devices and methods for concretizing the technical concept of the present invention. The technical concept of the present invention does not specify the materials, structures, configurations, etc. of the constituent parts as follows. The technical concept of the present invention can be variously modified within the technical scope specified by the claims recorded in the claims.

[0037] Figure 1 1 is a schematic diagram of a continuous casting machine 1 to which a continuous casting method according to an embodiment of the present invention is applied. The continuous casting machine 1 is a slab continuous casting machine, and as an example, is a vertical bending type. In addition, the continuous casting machine 1 may also be a curved bending slab continuous casting machine. It should be noted that, in the present embodiment, the length direction of the billet 3, that is, the moving direction of the billet 3 in the continuous casting machine 1 is referred to as the casting direction, and the short side direction (in the cross section) of the rectangle in the cross section of the billet 3 (the cross section orthogonal to the length direction) is referred to as the casting direction. Figure 1 The direction perpendicular to the casting direction in the cross section of the billet 3 is called the thickness direction. Figure 1 The front-to-back direction in the casting (the front-to-back direction in the casting) is referred to as the width direction. Furthermore, the length in the thickness direction of the casting slab 3 is referred to as the thickness, and the length in the width direction is referred to as the width.

[0038] like Figure 1 As shown, the continuous casting machine 1 includes a tundish 10 for injecting molten steel 2 from a molten steel ladle; a copper mold 12 for once cooling the molten steel 2 injected from the tundish 10 via an immersion nozzle 11; and a plurality of pairs of billet support rollers 13 for conveying the semi-solidified billet 3 pulled out from the mold 12.

[0039] The billet support rolls 13 are a general term for support rolls, guide rolls, and drive rolls that are sequentially arranged below the mold 12. Spray nozzles such as water spray nozzles and gas mist spray nozzles (not shown) are arranged in the gaps between the billet support rolls 13 adjacent to each other in the casting direction, and a secondary cooling zone is formed in the range of the billet support rolls 13 from just below the mold to the machine end. The billet 3 is cooled by secondary cooling water sprayed from the spray nozzles of the secondary cooling zone while being pulled.

[0040] In addition, the continuous casting machine 1 has a plurality of sections provided with a plurality of pairs of billet support rollers 13. The section provided in the light reduction zone 15 among the plurality of sections is referred to as a light reduction section 14. The light reduction zone 15 is a region (a group of billet support rollers 13) that presses down the billet 3 at the final stage of solidification in the casting direction of the continuous casting machine 1, and is a region in which the solid phase ratio of the thickness center of the billet 3 is at least 0.2 and less than 1.0. It should be noted that the thickness center of the billet 3 in the present embodiment refers to the center of the thickness direction at the position in the width direction where the solid phase ratio of the thickness center portion in the cross section of the billet 3 is the lowest. Figure 2 and Figure 3 Schematic diagram of the light pressure section 14 is shown. In addition, the number of the sections provided in the light pressure belt 15, that is, the light pressure section 14, may be one or more. In this embodiment, as an example, the case where the number of the light pressure section 14 is one is described. In addition, Figure 1 In FIG. 1 , only the light-pressing section 14 is shown as a section, but sections other than the light-pressing belt 15 are also provided.

[0041] like Figure 2 and Figure 3 As shown in FIG. 1 , the soft-pressing section 14 has six pairs of billet support rollers 13 arranged in the casting direction. It should be noted that one pair of rollers among the six pairs of billet support rollers 13 that apply a pressing force to the billet 3 in the thickness direction while being driven to rotate so as to pull the billet 3 is called a driving roller 140, and the other rollers that rotate under the ferrostatic pressure are called guide rollers 141. The driving rollers 140 can be set at any position in the casting direction of the soft-pressing section 14, and multiple pairs can be set in the soft-pressing section 14.

[0042] The soft reduction section 14 includes an upper frame 142, a lower frame 143, an upstream support 144, and a downstream support 145. The upper frame 142 and the lower frame 143 are disposed opposite to each other in the thickness direction with the cast ingot 3 interposed therebetween, and are connected by the upstream support 144 and the downstream support 145. A plurality of ingot support rolls 13 are rotatably fixed to the upper frame 142 and the lower frame 143 via bearings 146.

[0043] The upstream support 144 and the downstream support 145 are configured to be retractable by hydraulic pressure or the like, and the distance between the upper frame 142 and the lower frame 143 is adjusted by the retractable structure. Thus, the distance between the pair of oppositely disposed casting support rolls 13 in the thickness direction, i.e., the roll interval, is adjusted.

[0044] Next, the continuous casting method of steel of the present embodiment is described. In the present embodiment, continuous casting of steel is performed using a continuous casting machine 1. At this time, the period in which casting is performed while the molten steel 2 is continuously supplied to the mold 12 is referred to as a stable period, and the period after the casting is completed is referred to as a casting completion period. The completion of casting refers to the moment when the supply of the molten steel 2 to the mold 12 is completed. Specifically, after the casting is completed, the moment when the molten steel 2 of the ladle is injected into the tundish 10 and the moment when the molten steel 2 remaining in the tundish 10 is injected into the mold 12 is completed, that is, the moment when the opening of the sliding water gate 16 connected to the immersion water gate 11 finally becomes closed.

[0045] During the stable period, it is preferred that the casting 3 be lightly reduced in the light reduction zone 15. In addition, the roll opening may be expanded in the region on the upstream side of the light reduction zone 15, and the long side surface of the casting 3 may be intentionally bulged by the static pressure of the molten steel. In addition, the intentional bulging is preferably started at a stage where the solid phase ratio of the center portion of the casting 3 is 0, and is ended when the total amount of bulging of the long side surface of the casting 3 is 3 mm or more and 10 mm or less. In addition, in the light reduction of the light reduction zone 15, the casting 3 is preferably reduced at a reduction speed U of 0.3 mm / min or more and 2.0 mm / min or less in the range where the center solid phase ratio is at least 0.2 or more and less than 1.0. When the reduction speed U of the light reduction with the center solid phase ratio within the above range is less than 0.3 mm / min, the possibility of V segregation becomes high. On the other hand, when the reduction speed U of the light reduction with the center solid phase ratio within the above range exceeds 2.0 mm / min, the possibility of reverse V segregation becomes high.

[0046] During the end of casting, if Figure 4 As shown, after the casting is completed, the casting speed V is decelerated, and the low casting speed is maintained for a certain period of time, thereby performing capping to cool and solidify the top of the ingot 3 (the rearmost part of the ingot 3). Capping refers to the following operation: after the casting is completed, when the top part of the ingot 3 pulled out from the mold 12 is not solidified, the molten steel 2 leaks out from the top part, so cooling material is added to the molten steel 2 remaining in the mold 12 to solidify the top part of the ingot 3. When capping, in order to firmly fix the top part, the casting speed is generally decelerated. After capping, the casting speed V (m / min) is increased to draw the ingot 3 in the machine. That is, when the elapsed time from the end of casting is set to t, during the period when the elapsed time t is greater than 0 and less than t1 (time T a ), the casting speed V is decelerated. At this time, the casting speed (pulling speed) V of the billet 3 before the end of casting (immediately before the end of casting) is set as the first casting speed V a(m / min), it changes until the casting speed V of the slab 3 reaches the casting speed (drawing speed) at the capping, i.e., the second casting speed V0 (m / min). The second casting speed V0 is the casting speed required for capping and is appropriately set according to the specifications of the continuous casting machine 1. Next, during the period when the elapsed time t is t1 or more and less than t2 (time T0), the casting speed V is constant at the second casting speed V0 for capping. After that, during the period when the elapsed time t is t2 or more and t3 or less (time T b ), the casting speed V increases. At this time, the casting speed V of the slab 3 changes from the second casting speed V0 to the third casting speed V b (m / min) which is the casting speed during the re-drawing after the casting ends. It should be noted that re-drawing means increasing the drawing speed to draw the slab 3 after the capping ends. Then, when the elapsed time t exceeds t3, during the period before the drawing is completed, the slab 3 is drawn at the third casting speed V b . It should be noted that regarding the period after the casting ends, the period of time T a is also called the deceleration process, the period of time T0 is called the capping process, the period of time T b is called the acceleration process, and the period after time Tb (t > t3) is called the re-drawing process.

[0047] In addition, during the casting end period, as Figure 5 shown, the reduction gradient Z (mm / m) and the reduction speed U (mm / min) change with time. The reduction speed U is the value obtained by multiplying the drawing speed (casting speed) V by the reduction gradient Z (U = V × Z). In this embodiment, control is performed such that the reduction speed U is within a specified range. Specifically, during the casting end period, according to the change in the drawing speed V, the roll opening of the soft reduction section 14, i.e., the reduction gradient Z, is dynamically changed, thereby performing control to make the reduction speed U within the specified range. Furthermore, during the deceleration process, capping process, and acceleration process of time T a , T0, T b , the reduction speed U preferably satisfies the following formulas (1) to (3). It should be noted that the reduction speed U during the stable period when t < 0 before the casting ends is also called the first reduction speed U a , the reduction speed U at the capping when t1 ≤ t < t2 is called the second reduction speed U0, and the reduction speed U during the re-drawing when t < t3 is called the third reduction speed U b . In addition, in this case, the reduction gradient Z satisfies formulas (4) to (6). It should be noted that the reduction gradients Z when t < 0, t1 ≤ t < t2, and t < t3 are also called the first reduction gradient Z a , the second reduction gradient Z0, and the third reduction gradient Z b .

[0048] When 0 ≤ t < t1 (time T a ), 0.5 < U < 3.0 · · · (1)

[0049] When t1 ≤ t < t2 (time T0), 0.5 < U < 1.5 · · · (2)

[0050] When t2 ≤ t ≤ t3 (time T b ), 0.5 < U < 3.0· · · (3)

[0051] When 0 ≤ t < t1 (time T a ), 0.5 < V × Z < 3.0 · · · (4)

[0052] When t1 ≤ t < t2 (time T0), 0.5 < V × Z < 1.5 · · · (5)

[0053] When t2 ≤ t ≤ t3 (time T b ), 0.5 < V × Z < 3.0 · · · (6)

[0054] Figure 5 This represents an example where the reduction gradient Z satisfies equations (4) to (6) at times T a , T0, T b . In Figure 5 , at time T a , the reduction speed U decelerates from the first reduction speed U a and becomes the second reduction speed U0 at the moment when the elapsed time t reaches t1. Then, the second reduction speed U0 is maintained at time T0. Furthermore, at time T b , the reduction speed U accelerates from the second reduction speed U0 and becomes the third reduction speed U b at the moment when the elapsed time t reaches t3. By dynamically changing the reduction gradient Z in this way, the reduction speed U can be dynamically changed to fall within a specified range.

[0055] It should be noted that Figure 4 and Figure 5 the numerical values of the time t on the horizontal axis are examples, and the numerical values of the elapsed times t1, t2, t3 and times T a , T0, T b are appropriately set according to the specifications of the continuous casting machine 1, casting conditions, etc. In addition, the control of the reduction gradient and casting speed in the above-mentioned continuous casting machine 1 is carried out by a control unit (not shown) composed of a computer or the like.

[0056] According to the continuous casting method of steel of the present embodiment, during the end of casting in which the casting speed V is changed for capping, the reduction speed U is kept within a predetermined range by controlling the reduction gradient Z. Thus, the occurrence of center segregation of the ingot due to insufficient reduction and the occurrence of internal cracks of the ingot due to excessive reduction can be prevented, and the requirements for manufacturing steel products of various specifications can be quickly met, bringing about industrially beneficial effects.

[0057] In addition, in the present embodiment, in the deceleration step, the capping step, and the speed-up step, the reduction speed U is controlled as shown in equations (1) to (3), or the reduction gradient Z is controlled as shown in equations (4) to (6). In particular, by making the deceleration step (time T a ) satisfies the formula (1), and when the casting speed V is decelerated, the reduction slope Z increases. Therefore, the solidification shrinkage flow inside the ingot 3 can be blocked, and the absorption of concentrated molten steel can be prevented, so the occurrence of center segregation of the ingot can be reduced. In addition, by making the second reduction speed U0 in the capping process (time T0) greater than 0.5 mm / min and less than 1.5 mm / min, the reduction gradient can be prevented from becoming too large, and internal cracks in the ingot 3 can be prevented. Moreover, by making the speed-up process (time T0) b ) satisfies the formula (3), which can prevent internal cracks in the ingot 3 and increase the speed efficiently, thereby improving production efficiency.

[0058] Furthermore, the casting speed is reduced at the end of casting, so the reduction speed U is reduced. When the reduction speed U is reduced and the flow speed of the molten steel 2 caused by solidification shrinkage exceeds the reduction speed U, the concentrated molten steel is sucked and the segregation of the ingot 3 is deteriorated. However, in this embodiment, by making the reduction speed U within a predetermined range at the end of casting, the suction of the concentrated molten steel can be prevented, and the segregation of the ingot and the loose sand holes can be reduced.

[0059] The present invention has been described above with reference to specific embodiments, but it is not intended to limit the invention by these descriptions. Other embodiments of the present invention will be apparent to those skilled in the art by reference to the description of the present invention, including the disclosed embodiments and various modifications. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments that include these modified examples described in this specification alone or in combination.

[0060] [Example]

[0061] The present invention will be described in more detail below based on examples. Figure 1The continuous casting machine 1 shown in the figure is the same. The continuous casting machine 1 is used to cast low carbon aluminum killed steel. Table 1 shows the casting conditions in the continuous casting method of this embodiment and the investigation results (conditions 1 and 2) of the central segregation degree, the presence of sand holes and the presence of internal cracks of the cast ingot 3. It should be noted that in the embodiment, the test was carried out under the casting conditions in which the pressing speed U was within the range of formula (1) to formula (3). Furthermore, in Table 1, as a comparative example, the casting conditions and investigation results (conditions 3 and 4) in the tests carried out under the conditions in which the pressing speed U was outside the range of formula (1) to formula (3) in each ingot thickness are also shown. In all tests, the thickness of the ingot 3 was 250 mm and the width was 2000 mm. In addition, T during the period is represented. a , T0 and T b They are the deceleration step, capping step and speed-up step in the above-mentioned embodiment.

[0062] [Table 1]

[0063]

[0064] The central segregation degree of the ingot 3 used in the evaluation test was measured by the following method. That is, in a cross section perpendicular to the drawing direction of the ingot, the carbon concentration was analyzed at equal intervals along the thickness direction of the ingot 3. Then, the maximum value in the thickness direction was defined as C max , the carbon concentration of the molten steel 2 extracted from the tundish 10 during casting is analyzed as C0, and C max / C0 is taken as the central segregation degree. Therefore, the closer the central segregation degree is to 1.0, the better the cast piece 3 is with less central segregation. In this embodiment, the difference in the degree of central segregation was determined for the cast pieces 3 with a central segregation degree of 1.10 or more.

[0065] The presence or absence of sand holes and internal cracks in the slab 3 was determined by microscopic observation of the vicinity of the center of the slab thickness in a cross section perpendicular to the drawing direction of the slab 3 .

[0066] In this embodiment, the segregation degree of the ingot 3 was evaluated under the conditions that the pressing speed U was within the range shown in formula (1) to formula (3) and outside the range. From the central segregation degree shown in Table 1, it can be seen that under the condition that the pressing speed U was within the range of formula (1) to formula (3), the central segregation degree was less than 1.10, which was good. In addition, no sand holes or internal cracks were observed in the ingot 3.

[0067] On the other hand, under the conditions performed as a comparative example, when the reduction gradient Z was lower than the range of the above-mentioned embodiment, the central segregation degree exceeded 1.10, and sand holes were also observed inside the ingot 3. When the reduction gradient Z exceeded the range of the above-mentioned embodiment, the reduction rate was excessive, so the central segregation degree exceeded 1.10, and internal cracks in the ingot 3 were also observed.

[0068] Description of Reference Numerals

[0069] 1Continuous Casting Machine

[0070] 10 Tundish

[0071] 11 Immersion nozzle

[0072] 12 Casting

[0073] 13. Casting billet support roller

[0074] 14 Gently press down the section (section)

[0075] 140 driving roller

[0076] 141 Guide roller

[0077] 142 upper frame

[0078] 143 Lower frame

[0079] 144 Upstream side support

[0080] 145 downstream side support

[0081] 146 bearings

[0082] 15 Gently press down the belt

[0083] 16 Sliding nozzle

[0084] 2 Molten Steel

[0085] 3. Casting

Claims

1. A continuous casting method for steel, wherein the continuous casting is performed while applying pressure to the ingot, wherein: The continuous casting process is performed by applying pressure to the casting. After the casting is completed, the reduction gradient of the section where the reduction is applied is changed, thereby controlling the reduction speed so as to be within a predetermined range.

2. The method for continuous casting of steel according to claim 1, wherein: After the casting is completed, the method includes: a deceleration step of decelerating the casting speed of the ingot from a first casting speed, which is a casting speed before the casting is completed, to a second casting speed, which is a casting speed required for capping the ingot; A capping step, after the deceleration step, maintaining the casting speed of the ingot at the second casting speed for a time required for capping; The speed increasing step is to increase the casting speed of the ingot from the second casting speed to a third casting speed which is a casting speed for re-drawing the ingot after the capping step.

3. The method for continuous casting of steel according to claim 2, wherein: In the deceleration step, the pressure reduction gradient is controlled in a manner satisfying equation (4): In the capping step, the pressure reduction gradient is controlled in a manner that satisfies equation (5): In the speed-increasing step, the pressure reduction gradient is controlled in a manner satisfying equation (6): O.5<V×Z<3.O···(4) 0.5<V×Z<1.5·.·(5) 0.5<V×Z<3.0···(6) Wherein, V is the casting speed (m / min), and Z is the reduction gradient (mm / m).

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