Continuous casting method for steel

Abstract

To provide a steel continuous casting method which can improve the quality of a slab to be cast in a period after casting completion.SOLUTION: Provided is a steel continuous casting method of executing continuous casting while applying pressure to a slab 3, in which pressure is applied to the slab 3 to be subjected to continuous casting, and after the casting compression, the pressure gradient of a segment 14 to apply pressure is changed to control a pressure speed within a prescribed range.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] In the final solidification process of continuous casting of steel, solidification shrinkage causes the suction flow of unsolidified molten steel (called the "unsolidified layer") in the direction of drawing the slab. This unsolidified layer contains concentrated solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn). When this concentrated molten steel flows to the center of the slab and solidifies, so-called center segregation occurs. In addition to the above-mentioned solidification shrinkage, factors that cause the concentrated molten steel to flow in the final stage of solidification include slab bulging between rolls due to molten steel static pressure and misalignment of the rolls supporting the slab.

[0003] This center segregation deteriorates the quality of steel products, especially thick steel plates. For example, in line pipe materials for transporting oil or natural gas, hydrogen-induced cracking occurs starting from the center segregation due to the action of sour gas. The same problem also occurs in marine structures, storage tanks, oil tanks, etc. In recent years, moreover, steel materials are often required to be used in harsh environments such as lower temperatures or more corrosive environments, and the importance of reducing center segregation in cast slabs is increasing.

[0004] Therefore, many measures have been proposed to reduce or neutralize the center segregation of the slab from the continuous casting process to the rolling process. Among them, a method of soft reduction at the final stage of solidification, in which a continuously cast slab having an unsolidified layer inside is reduced in a continuous casting machine, is known to be particularly effective in improving center segregation. Here, the "soft reduction method at the final stage of solidification" refers to a method in which a reduction roll is disposed near the solidification completion position of the slab, and the slab during continuous casting is gradually reduced by this reduction roll at a reduction speed equivalent to the amount of solidification shrinkage, thereby suppressing the generation of voids in the center of the slab and the flow of concentrated molten steel, thereby suppressing center segregation of the slab.

[0005] In order to effectively prevent the occurrence of centerline segregation in a slab by this method of soft reduction at the final solidification stage, it is essential to appropriately set the start and end times of the period during which soft reduction is applied during the final solidification period of the slab, and the amount of soft reduction applied, and various methods for setting these have been proposed.

[0006] For example, Patent Document 1 proposes a continuous casting method in which soft reduction is applied to a slab at the final solidification stage of the continuous casting, in which the reduction amount per unit time of the slab in the section where soft reduction is applied is determined by the slab surface temperature at the start of reduction and the thickness of the unsolidified layer of the slab at the reduction position. Patent Document 1 focuses on the thickness of the unsolidified layer of the slab as an index for effectively implementing soft reduction. This is based on the knowledge that, according to Patent Document 1, the more the reduction is performed downstream of casting, i.e., when the thickness of the unsolidified layer of the slab is small, the smaller the rate at which the reduction amount set by the reduction rolls is transmitted to the solid-liquid interface of the slab (hereinafter referred to as "reduction efficiency").

[0007] Furthermore, Patent Documents 2 and 3 propose a continuous casting method in which a bloom slab is continuously cast while being reduced by a plurality of roll pairs in a region from a point where the solid fraction at the center of the thickness of the slab reaches a temperature equivalent to 0.1 to 0.3 to a point where the temperature reaches a temperature equivalent to the flow limit solid fraction, in which the reduction rate of the slab is increased toward the downstream side in the casting direction where the solid fraction at the center of the thickness of the slab becomes larger.

[0008] Furthermore, Patent Document 4 proposes a continuous casting method for steel, in which a rolling force is applied to a slab during drawing, in which rolling conditions are set or adjusted based on information on the cross-sectional shape perpendicular to the longitudinal direction of the slab and information on the shape of the unsolidified portion in the cross-section.

[0009] Furthermore, Patent Document 5 proposes that when casting by a continuous caster is completed, casting is completed while maintaining the normal casting speed, without slowing down or stopping the casting speed or processing the bottom part, which is the rearmost end of the slab. Patent Document 5 also proposes that the crater end at the rearmost end of the slab is controlled to be located in a predetermined section, and the vicinity of the crater end is lightly reduced by a group of small diameter rolls arranged in this section. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 8-132203 [Patent Document 2] Japanese Patent Application Publication No. 3-90263 [Patent Document 3] Japanese Patent Application Publication No. 3-90259 [Patent Document 4] JP 2003-71552 A [Patent Document 5] Japanese Patent Application Publication No. 7-112255 Summary of the Invention [Problem to be solved by the invention]

[0011] In continuous casting, during the period from the end of casting when the supply of molten steel to the mold is completed to the time when the slab is withdrawn from the continuous casting machine (the period after the end of casting), the behavior of the withdrawal speed of the slab is different from that during the period (the steady period) when molten steel is continuously supplied to the mold and continuous casting is performed at a stable casting speed. For this reason, when applying the soft reduction method at the end of solidification, if the reduction conditions during the period after the end of casting are the same as those during the steady period, there is a possibility that the quality of the slab will deteriorate due to center segregation and internal cracks.

[0012] However, Patent Documents 1 to 4 do not describe the conditions for soft reduction of the slab remaining in the machine after casting is completed, and therefore, with the casting methods disclosed in Patent Documents 1 to 4, it is difficult to provide appropriate reduction conditions in the period after casting is completed. In addition, with the casting method disclosed in Patent Document 5, although the quality of the bottom part is improved, soft reduction is not provided under the same conditions as in the steady period in the period after casting is completed. For this reason, there is a concern that the quality of the slab cast in the period after casting is completed may be deteriorated compared to the slab cast in the steady period.

[0013] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a method for continuous casting of steel that can improve the quality of a cast piece that is cast in the period after the end of casting. [Means for solving the problem]

[0014] According to one aspect of the present invention, there is provided a method for continuously casting steel, which comprises applying a reduction to a slab while continuously casting the slab, and controlling, after casting is completed, a reduction in the slab being continuously cast, and changing a reduction gradient of a segment to which the reduction is applied, thereby controlling the reduction rate to be within a predetermined range. Effect of the Invention

[0015] According to one aspect of the present invention, a method for continuous casting of steel can be provided that can improve the quality of a cast piece cast in the period after the end of casting. [Brief description of the drawings]

[0016] [Figure 1] 1 is a side schematic view showing a continuous casting machine according to an embodiment of the present invention. FIG. [Diagram 2] FIG. 2 is a schematic side view showing an example of a segment constituting a soft reduction zone. [Diagram 3] FIG. 3 is a front view of the segment shown in FIG. 2 as viewed from the casting direction. [Figure 4] 1 is a graph showing an example of a casting speed after casting is completed. [Diagram 5] 1 is a graph showing an example of a rolling gradient and a rolling rate after casting is completed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In the following detailed description, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings, identical or similar parts are given identical or similar symbols, and duplicated explanations are omitted. Each drawing is schematic and may differ from the actual one. In addition, the embodiments shown below are examples of devices and methods for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention does not specify the materials, structures, arrangements, etc. of the components as described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims described in the claims.

[0018] FIG. 1 is a schematic diagram showing 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 is a vertical bending type as an example. The continuous casting machine 1 may also be a curved bending type slab continuous casting machine. In this embodiment, the longitudinal direction of the slab 3, which is the moving direction of the slab 3 in the continuous casting machine 1, is referred to as the casting direction, the short direction of the rectangle in the cross section of the slab 3 (cross section perpendicular to the longitudinal direction) is referred to as the thickness direction (direction perpendicular to the casting direction in the cross section of the slab 3 in FIG. 1) is referred to as the thickness direction, and the longitudinal direction of the rectangle in the cross section of the slab 3 (front-rear direction in FIG. 1) is referred to as the width direction. The length in the thickness direction of the slab 3 is referred to as the thickness, and the length in the width direction is referred to as the width.

[0019] As shown in FIG. 1, a continuous casting machine 1 includes a tundish 10 into which molten steel 2 is poured from a molten steel ladle, a copper mold 12 for primarily cooling the molten steel 2 poured from the tundish 10 through an immersion nozzle 11, and several pairs of slab support rolls 13 for transporting a semi-solidified slab 3 pulled out of the mold 12.

[0020] The slab support rolls 13 are a collective term for a support roll, a guide roll, and a drive roll, which are provided in that order below the mold 12. Spray nozzles (not shown), such as water spray nozzles and air mist spray nozzles, are arranged in the gaps between adjacent slab support rolls 13 in the casting direction, and a secondary cooling zone is formed in the area from just below the mold to the slab support rolls 13 at the end of the machine. As the slab 3 is pulled out, it is cooled by secondary cooling water sprayed from the spray nozzles in the secondary cooling zone.

[0021] The continuous casting machine 1 has a plurality of segments each having a plurality of pairs of slab support rolls 13. Among the plurality of segments, the segment provided in the soft reduction zone 15 is referred to as the soft reduction segment 14. The soft reduction zone 15 is a region (a group of slab support rolls 13) in which reduction of the slab 3 at the final stage of solidification is performed in the casting direction of the continuous casting machine 1, and is a region in which the central solid fraction, which is the solid fraction at the thickness center of the slab 3, is at least 0.2 or more and less than 1.0. In this embodiment, the thickness center of the slab 3 means the center in the thickness direction at the width direction position where the solid fraction at the thickness center is the lowest in the cross section of the slab 3. FIGS. 2 and 3 show schematic diagrams of the soft reduction segment 14. The number of the soft reduction segments 14 provided in the soft reduction zone 15 may be one or more. In this embodiment, a case in which the number of the soft reduction segments 14 is one will be described as an example. Although FIG. 1 shows only the soft reduction segment 14 as a segment, other segments than the soft reduction zone 15 may be provided.

[0022] 2 and 3, the soft reduction segment 14 has six pairs of slab support rolls 13 lined up in the casting direction. Of the six pairs of slab support rolls 13, one pair of rolls that pulls out the slab 3 by rotating while applying a pressing force to the slab 3 in the thickness direction is called a drive roll 140, and the other rolls that rotate under ferrostatic pressure are called guide rolls 141. The drive roll 140 may be provided at any position in the casting direction of the soft reduction segment 14, and multiple pairs of rolls may be provided in the soft reduction segment 14.

[0023] The soft reduction segment 14 has an upper frame 142, a lower frame 143, an upstream support pillar 144, and a downstream support pillar 145. The upper frame 142 and the lower frame 143 are disposed opposite to each other in the thickness direction with the strand 3 to be cast sandwiched therebetween, and are connected by the upstream support pillar 144 and the downstream support pillar 145. A plurality of strand support rolls 13 are rotatably fixed to the upper frame 142 and the lower frame 143 via bearings 146.

[0024] The upstream support 144 and the downstream support 145 are configured to be extendable and contractable by hydraulic pressure or the like, and adjust the distance between the upper frame 142 and the lower frame 143 by extending and contracting. This adjusts the roll gap, which is the distance in the thickness direction between a pair of slab support rolls 13 that face each other in the thickness direction.

[0025] Next, a method for continuously casting steel according to this embodiment will be described. In this embodiment, continuous casting of steel is performed using a continuous casting machine 1. In this case, a period during which casting is performed while the supply of molten steel 2 to the mold 12 is continued is referred to as a steady period, and a period after the end of casting is referred to as a casting end period. The period after the end of casting refers to the point in time when the supply of molten steel 2 to the mold 12 is ended, specifically, when the molten steel 2 in the ladle is finished being poured into the tundish 10 and the molten steel 2 remaining in the tundish 10 is further finished being poured into the mold 12, that is, the point in time when the opening of the sliding nozzle 16 connected to the submerged nozzle 11 is finally closed.

[0026] In the steady period, it is preferable that the slab 3 is soft reduced in the soft reduction zone 15. In addition, the roll gap may be widened in an area upstream of the soft reduction zone 15 to intentionally bulge the long side surface of the slab 3 by ferrostatic pressure. This intentional bulging is preferably started when the solid fraction at the center of the slab 3 is 0, and is preferably ended when the total amount of bulging on the long side surface of the slab 3 is 3 mm or more and 10 mm or less. In addition, in the soft reduction in the soft reduction zone 15, it is preferable to reduce the slab 3 at a reduction speed U of 0.3 mm / min or more and 2.0 mm / min or less when the center solid fraction is at least in the range of 0.2 to less than 1.0. If the reduction speed U of the soft reduction in the above range of the center solid fraction is less than 0.3 mm / min, there is a high possibility that V-segregation will occur. On the other hand, if the reduction speed U of the soft reduction in the above range of the center solid fraction is more than 2.0 mm / min, there is a high possibility that inverse V-segregation will occur.

[0027] During the final casting period, as shown in FIG. 4, the casting speed V is slowed down after casting is completed, and the low casting speed is maintained for a certain period of time to cool and solidify the top part of the slab 3 (the rearmost part of the slab 3), which is called head hardening. Head hardening is a process in which, after casting is completed, if the top part of the slab 3 that has been pulled out of the mold 12 is not yet solidified, the molten steel 2 will leak out from the top part, and so cold material is poured toward the molten steel 2 remaining in the mold 12 to solidify the top part of the slab 3. When head hardening is performed, the casting speed is generally slowed down to firmly solidify the top part. After head hardening, the casting speed V (m / min) is increased and the slab 3 is pulled out of the machine. In other words, if the time elapsed from the end of casting is t, the time elapsed is greater than or equal to 0 and less than t. 1 Less than (time T a At this time, the casting speed (drawing speed) V of the slab 3 before the end of casting (just before the end of casting) is reduced to the first casting speed V a (m / min), and the casting speed V of the slab 3 is the casting speed (pulling speed) during head setting, which is the second casting speed V 0 (m / min). The second casting speed V 0is the casting speed required for head hardening, and is set appropriately according to the specifications of the continuous casting machine 1. Next, when the elapsed time t is t 1 more than t 2 Less than (time T 0 ), the casting speed V is the second casting speed V 0 Then, the head is fixed at t. After that, the elapsed time t becomes t 2 more than t 3 During the following time (T b At this time, the casting speed V of the slab 3 is increased to the second casting speed V 0 The third casting speed V is the casting speed at the time of re-drawing after the end of casting. b (m / min). The second withdrawal is performed by increasing the withdrawal speed after the end of the head setting. Then, when the elapsed time t becomes t 3 When the drawing speed exceeds 1000 mm, the slab 3 is cast at the third casting speed V b The period after the casting is completed is called the time T a The period of the deceleration process is T 0 The period is the head hardening process, and the time T b The period after time Tb (t>t 3 ) is also called the re-drawing process.

[0028] During the casting completion period, the reduction gradient Z (mm / m) and the reduction speed U (mm / min) change over time, as shown in Fig. 5. The reduction speed U is the withdrawal speed (casting speed) V multiplied by the reduction gradient Z (U = V x Z). In this embodiment, the reduction speed U is controlled to be within a predetermined range. Specifically, during the casting completion period, the reduction gradient Z, i.e., the roll gap of the soft reduction segment 14, is dynamically changed in response to changes in the withdrawal speed V, thereby controlling the reduction speed U to be within a predetermined range. Furthermore, during the casting completion period, the reduction gradient Z, i.e., the roll gap of the soft reduction segment 14 is dynamically changed in response to changes in the withdrawal speed V, thereby controlling the reduction speed U to be within the predetermined range. a ,T 0 ,T b In the deceleration step, the head hardening step, and the speed increase step, the rolling speed U preferably satisfies the following formulas (1) to (3). The rolling speed U in the steady period t<0 before the end of casting is referred to as the first rolling speed U a , t 1 ≦t <t2 The rolling speed U during the head hardening is the second rolling speed U 0 , t <t 3 The reduction speed U during the second withdrawal is the third reduction speed U b In this case, the rolling reduction gradient Z satisfies the formulas (4) to (6). Note that, t<0, t 1 ≦t <t 2 ,t <t 3 The rolling gradient Z at this time is called the first rolling gradient Z a , 2nd rolling gradient Z 0 and the third reduction gradient Z b It is also called.

[0029] 0≦t <t 1 (Time T a ) , 0.5 <U<3.0 ···(1) t 1 ≦t <t 2 (Time T 0 ) , 0.5 <U<1.5 ···(2) t 2 ≦t≦t 3 (Time T b ) , 0.5 <U<3.0 ···(3) 0≦t <t 1 (Time T a ) , 0.5 <V×Z<3.0 ···(4) t 1 ≦t <t 2 (Time T 0 ) , 0.5 <V×Z<1.5 ···(5) t 2 ≦t≦t 3 (Time T b ) , 0.5 <V×Z<3.0 ···(6)

[0030] In FIG. 5, time T a ,T 0 ,T b In FIG. 5, an example in which the rolling reduction gradient Z satisfies the formulas (4) to (6) is shown. a In the above, the rolling speed U is the first rolling speed U a The speed decelerates from t 1 At this point, the second reduction rate U 0Next, at time T 0 In the second rolling speed U 0 is maintained. Furthermore, for a time T b In the above, the rolling speed U is the second rolling speed U 0 The speed increases from t, and the elapsed time t is t 3 At this point, the third reduction rate U b By dynamically changing the reduction gradient Z in this way, the reduction speed U can be dynamically changed and kept within a predetermined range.

[0031] In addition, the values ​​of time t on the horizontal axis in FIG. 4 and FIG. 5 are merely examples, and the values ​​of elapsed time t 1 ,t 2 ,t 3 and time T a ,T 0 ,T b The numerical values ​​are appropriately set according to the specifications of the continuous casting machine 1, the casting conditions, etc. The reduction gradient and the casting speed in the continuous casting machine 1 are controlled by a control unit (not shown) configured by a computer or the like.

[0032] According to the method for continuous casting of steel according to this embodiment, during the end of casting period when the casting speed V changes due to head hardening, the reduction gradient Z is controlled to keep the reduction speed U within a predetermined range. This makes it possible to prevent the occurrence of central segregation of the slab due to insufficient reduction and the occurrence of internal cracks in the slab due to excessive reduction, and makes it possible to rapidly respond to the demands for manufacturing steel products with diverse specifications, thus providing industrially beneficial effects.

[0033] In this embodiment, in the deceleration process, the head hardening process, and the speed increase process, the rolling speed U is controlled as shown in the formulas (1) to (3), or the rolling gradient Z is controlled as shown in the formulas (4) to (6). a ) satisfies formula (1), the reduction gradient Z increases when the casting speed V is decelerated. This makes it possible to stop the solidification contraction flow inside the slab 3 and prevent the intake of concentrated molten steel, thereby reducing the occurrence of slab center segregation. 0) the second reduction rate U 0 By setting the rolling speed to more than 0.5 mm / min and less than 1.5 mm / min, it is possible to prevent the rolling gradient from becoming excessively large, and it is possible to prevent internal cracks in the slab 3. b When the rolling speed U in the rolling process satisfies formula (3), it is possible to efficiently increase the rolling speed while preventing internal cracks in the slab 3, thereby improving production efficiency.

[0034] Furthermore, at the end of casting, the casting speed decreases, and the reduction speed U also decreases. If the reduction speed U decreases and the flow speed of the molten steel 2 caused by solidification shrinkage exceeds the reduction speed U, concentrated molten steel is sucked in, and segregation of the slab 3 worsens. However, in this embodiment, by setting the reduction speed U within a predetermined range at the end of casting, it is possible to prevent the suction of concentrated molten steel and reduce slab segregation and crevice porosity.

[0035] Although the present invention has been described above with reference to specific embodiments, it is not intended that the invention be limited by these descriptions. By referring to the description of the present invention, other embodiments of the present invention including various modifications in addition to the disclosed embodiments will be apparent to those skilled in the art. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments including these modifications described herein, either alone or in combination. EXAMPLES

[0036] The present invention will be described in more detail below based on examples. The continuous casting machine used in the tests is the same as the continuous casting machine 1 shown in FIG. 1. Using this continuous casting machine 1, low carbon aluminum killed steel was cast. Table 1 shows the casting conditions in the continuous casting method in this example, and the results of investigating the degree of center segregation, the presence or absence of porosity, and the presence or absence of internal cracks in the cast slab 3 (conditions 1 and 2). In the examples, the tests were conducted under casting conditions in which the reduction rate U was within the range of formulas (1) to (3). Furthermore, Table 1 also shows, as comparative examples, the casting conditions and investigation results in tests conducted under conditions in which the reduction rate U was outside the range of formulas (1) to (3) for each slab thickness (conditions 3 and 4). In all tests, the thickness of the slab 3 was 250 mm, and the width was 2000 mm. In addition, the T shown in the period a , T 0 and T b are the deceleration process, the head fixing process, and the acceleration process in the above embodiment, respectively.

[0037] [Table 1]

[0038] The degree of center segregation of the slab 3 used in the evaluation of the test was measured by the following method. That is, the carbon concentration was analyzed at equal intervals along the thickness direction of the slab 3 in a cross section perpendicular to the drawing direction of the slab, and the maximum value in the thickness direction was defined as C max The carbon concentration analyzed in the molten steel 2 sampled from the tundish 10 during casting is defined as C 0 As, C max / C 0 was defined as the center segregation degree. Therefore, the closer the center segregation degree is to 1.0, the better the cast 3 is with less center segregation. In this example, a cast 3 with a center segregation degree of 1.10 or more was determined to have a poor degree of center segregation.

[0039] The presence or absence of porosity and internal cracks in the slab 3 was determined by observing the vicinity of the center of the slab thickness in a cross section perpendicular to the drawing direction of the slab 3 under a microscope. In this example, the segregation degree of the slab 3 was evaluated under conditions where the reduction rate U was within the range of the formulas (1) to (3) and outside the range. As is clear from the centerline segregation degrees shown in Table 1, under conditions where the reduction rate U was within the range of the formulas (1) to (3), the centerline segregation degrees were all less than 1.10, which was good. In addition, no porosity or internal cracks were observed in the slab 3.

[0040] On the other hand, under the conditions of the comparative example where the reduction gradient Z was below the range of the above embodiment, the center segregation degree exceeded 1.10, and porosity was also observed inside the slab 3. Under the conditions where the reduction gradient Z exceeded the range of the above embodiment, the reduction rate was excessive, so the center segregation degree exceeded 1.10, and internal cracks were also observed in the slab 3. [Explanation of symbols]

[0041] 1. Continuous casting machine 10 Tundish 11 Submerged nozzle 12 Mold 13. Cast piece support roll 14 Light reduction segment (segment) 140 Drive Roll 141 Guide Roll 142 Upper Frame 143 Lower Frame 144 Upstream Support 145 Downstream support 146 Bearings 15 Lightly Pressed Zone 16 Sliding Nozzle 2. Molten Steel 3. Casting

Claims

1. A method for continuous casting of steel, comprising the steps of: Applying a rolling reduction to a continuously cast slab; After the casting is completed, the reduction rate is controlled to be within a predetermined range by changing the reduction gradient of the segment to which the reduction is applied; After the casting is completed, a decelerating step of decelerating the casting speed of the slab from a first casting speed, which is a casting speed before the end of casting, to a second casting speed, which is a casting speed required for head hardening of the slab; a head hardening step of maintaining the casting speed of the slab at the second casting speed for a time required for the head hardening step after the deceleration step; an increasing step of increasing the casting speed of the slab from the second casting speed to a third casting speed at which the slab is re-drawn after the head hardening step; A method for continuous casting of steel comprising the steps of:

2. In the deceleration step, the rolling reduction gradient is controlled so as to satisfy formula (4), In the head hardening step, the rolling gradient is controlled so as to satisfy the formula (5), The method for continuous casting steel according to claim 1 , wherein in the speed increasing step, the rolling reduction gradient is controlled so as to satisfy formula (6). 0.5<V×Z<3.0 (4) 0.5<V×Z<1.5 (5) 0.5<V×Z<3.0 (6) Where, V: casting speed (m / min), Z: rolling reduction gradient (mm / m)

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