Continuous casting method for cast slabs

Abstract

Provided is a continuous casting method for a cast slab, the method making it possible to: improve the productivity of a cast slab having unprecedented thickness; and secure surface quality by suppressing cracking of the surface of the cast slab.　The continuous casting method for a cast slab is carried out by a continuous casting machine comprising: a mold 6; a vertical unit 8 disposed on the downstream side of the mold 6 in the movement direction of a cast slab 7 which was drawn out from the mold 6; a bending unit 10 disposed on the downstream side of the vertical unit 8 in the movement direction; and a horizontal unit 11 disposed on the downstream side of the bending unit 10 in the movement direction. The surface temperature of the cast slab 7 on the output side of the vertical unit 8 is 700°C to 1200°C. A first cooling density X calculated from the amount of cooling water at the bending unit 10 is no more than a second cooling density Y calculated from the amount of cooling water at the horizontal unit 11.

Description

Technical Field

[0001] The present invention relates to a method for continuous casting of slabs.

Background Art

[0002] For boiler steel plates, low alloy steel plates for pressure vessels, high strength steel plates for marine structures and industrial machines, etc., there are high-quality extra-thick steel plates with a plate thickness exceeding 100 mm and used as important members. In high-quality extra-thick steel plates, the internal quality may become a problem from the viewpoint of service performance. Therefore, conventionally, large ingots have been produced using the ingot casting method, and high-quality extra-thick steel plates have been produced by rolling or forging this large steel ingot at a sufficient reduction ratio. By this, a manufacturing method for improving the internal quality of high-quality extra-thick steel plates has been adopted.

[0003] On the other hand, since the ingot casting method has low productivity, a method for producing an extra-thick slab with a thick slab thickness using a vertical continuous casting method has been attempted. An example of that method is described in Patent Document 1. In the method described in Patent Document 1, when the slab thickness is 380 mm or more, in order to prevent restrictions on the length of the continuous casting equipment and bulging deformation of the slab, it is disclosed that the casting speed is set to 0.2 m / min or less.

[0004] Furthermore, when producing extremely thick slab castings (hereinafter referred to as "cast slabs") using a vertical bending continuous casting machine, stress is applied to the cast slab at the bending section where the cast slab, which has been drawn from the mold in a direction nearly parallel to the vertical direction, is bent in a direction nearly parallel to the horizontal direction. This increases the risk of corner cracks and lateral cracks occurring in the cast slab. An example of a continuous casting machine that reduces such risks is described in Patent Document 2. The vertical bending continuous casting machine described in Patent Document 2 produces cast slabs without surface defects or internal cracks by setting manufacturing conditions suitable for the chemical composition and thickness of the cast slab to be cast. Specifically, the chemical composition of the cast slab described in Patent Document 2 is C: 0.05~0.55 mass%, Si: 0.10~2.00 mass%, Mn: 0.30~1.90 mass%, P: 0.005~0.070 mass%, S: 0.003~0.120 mass%. The thickness of the cast slab is 280~350 mm. The aforementioned slab is cast using a vertical bending type continuous casting machine with a casting speed set to 0.7 to 1.1 m / min and a specific water content set to 0.15 to 0.40 l / kg·steel. In this case, the vertical length, bending length, secondary cooling length, arc length, straightening length, etc., are set to a predetermined range. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2007-229736 [Patent Document 2] Japanese Patent Publication No. 2009-274116 [Overview of the project] [Problems that the invention aims to solve]

[0006] In the method described in Patent Document 1, the casting speed is limited to 0.2 m / min or less when the thickness of the cast slab is 380 mm or more. In other words, the method described in Patent Document 1 has low productivity when the thickness of the cast slab is 380 mm or more, and there is still room for improvement in this respect.

[0007] In the vertical bending type continuous casting machine described in Patent Document 2, the slab thickness that can achieve both productivity and suppression of surface cracking is 280 to 350 mm. Therefore, when the slab thickness is 350 mm or more, it may not be possible to achieve both productivity and suppression of surface cracking, and there was still room for improvement in this respect.

[0008] The present invention was made to solve the above problems and aims to provide a continuous casting method for cast slabs that can improve the productivity of cast slabs with unprecedented thickness and suppress surface cracking of the cast slabs to ensure surface quality. [Means for solving the problem]

[0009] The means to solve the above problems are as follows: [1] A method for continuous casting of a cast slab using a continuous casting machine comprising a mold, a vertical section located downstream of the mold in the direction of movement of the cast slab withdrawn from the mold, a bent section located downstream of the vertical section in the direction of movement, and a horizontal section located downstream of the bent section in the direction of movement, wherein the surface temperature of the cast slab on the exit side of the vertical section is 700°C or more and 1200°C or less, and the first cooling density X calculated from the amount of cooling water in the bent section is less than or equal to the second cooling density Y calculated from the amount of cooling water in the horizontal section. [2] The continuous casting method for a cast slab according to [1], wherein the vertical section, the bent section, and the horizontal section are each configured to supply cooling water to the cast slab, the first cooling density X is the value obtained by dividing the total amount of cooling water supplied to the cast slab in the bent section by the surface area of ​​the cast slab in the bent section, and the second cooling density Y is the value obtained by dividing the total amount of cooling water supplied to the cast slab in the horizontal section by the surface area of ​​the cast slab in the horizontal section. [3] The continuous casting method for a cast slab according to [1] or [2], wherein the thickness of the cast slab is 350 mm or more and 500 mm or less. [4] The method for continuous casting of a cast slab according to any one of [1] to [3], wherein the withdrawal speed of the cast slab in the vertical section is 0.2 to 1.5 m / min. [Effects of the Invention]

[0010] According to the present invention, it is possible to continuously cast extremely thick slabs with unprecedented thickness, having good internal quality and few or no surface cracks, at unprecedented speeds. Therefore, the productivity of casting unprecedentedly thick slabs can be improved. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows an example of a vertical bending type continuous casting machine to which the continuous casting method of cast slabs according to this embodiment can be applied. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below through embodiments of the present invention (hereinafter referred to as "this embodiment"). The embodiments described below represent preferred examples of the present invention and are not limiting in any way.

[0013] The continuous casting method for slabs according to this embodiment optimizes the amount of cooling in the secondary cooling zone of the continuous casting machine, thereby enabling the continuous casting of unprecedentedly thick slabs with good internal quality and few or no surface cracks, at unprecedented speeds. Figure 1 shows an example of a continuous casting machine to which the continuous casting method for slabs according to this embodiment can be applied. The continuous casting machine 1 shown in Figure 1 is called a vertical bending type, and in this continuous casting machine 1, an immersion nozzle 4 is attached to the bottom of the tundish 2 via a sliding nozzle 3. Molten steel 5 is supplied to the tundish 2 from a ladle (not shown). The sliding nozzle 3 is a device that adjusts the amount of molten steel 5 supplied from the tundish 2 to the mold 6 via the immersion nozzle 4.

[0014] A cooling channel (not shown) is formed in the mold 6, and cooling water is supplied to this cooling channel from a cooling water source (not shown). In the mold 6, a cast slab 7 with a solidified shell on its surface is formed by the cooling water removing heat from the molten steel 5, and the cast slab 7 is pulled out of the mold 6 in a direction approximately parallel to the vertical. Therefore, an unsolidified layer of molten steel 5 that has not yet solidified is formed inside the cast slab 7 directly below the mold. The mold 6 is sometimes referred to as the primary cooling zone. Also, in Figure 1, the unsolidified layer is hatched.

[0015] In this embodiment, the thickness of the cast slab 7 drawn from the mold 6 of the continuous casting machine 1 shown in Figure 1 is set to 350 to 500 mm. The drawing speed of the cast slab 7 from the mold 6 is set to 0.2 to 1.5 m / min. This drawing speed is the drawing speed in a steady state. A steady state means a state in which the casting speed is constant, excluding the start of casting, the end of casting, and when connecting slabs, etc. If the drawing speed of the cast slab 7 is less than 0.2 m / min, the molten steel 5 may solidify in the mold 6, making it impossible to draw the cast slab 7 from the mold 6. If the drawing speed of the cast slab 7 exceeds 1.5 m / min, the solidified shell may not be sufficiently formed, and when the cast slab 7 is drawn from the mold 6, a breakout may occur where the solidified shell ruptures and steel leaks. Therefore, it is preferable to set the drawing speed of the cast slab 7 to 0.2 to 1.5 m / min. It is more preferable to set the withdrawal speed of the cast slab 7 to 0.3 to 1.4 m / min.

[0016] A secondary cooling zone is provided below the mold 6 of the continuous casting machine 1. Specifically, a vertical section 8 is provided directly below the mold 6, extending in a direction approximately parallel to the vertical direction, to support the cast slabs 7 that are continuously drawn from the mold 6. The vertical section 8 is equipped with a plurality of cast slab support rolls 9. As shown in Figure 1, these cast slab support rolls 9 are arranged on both sides of the cast slab 7 in the thickness direction of the slab 7, and are spaced apart in a direction approximately parallel to the vertical direction. The cast slab support rolls 9 support the cast slabs 7 that have been drawn from the mold 6. Therefore, the spacing between the cast slab support rolls 9 in the thickness direction of the slab 7 is set to be approximately the same as the thickness of the cast slab 7 immediately after being drawn from the mold 6. Furthermore, cooling water is supplied to the inside of the cast slab support rolls 9. Note that each cast slab support roll 9 in the vertical section 8, each cast slab support roll 9 in the bending section 10 (described later), and each cast slab support roll 9 in the horizontal section 11 (described later) are configured in approximately the same way. For example, one example of each of these slab support rolls 9 is a roll manufactured by overlay welding 13Cr4Ni onto the surface of a base material made of 21CrMoV511.

[0017] A bend section 10 is provided downstream of the vertical section 8 in the direction of movement of the cast slab 7. The bend section 10 is configured to bend the cast slab 7, which has been drawn from the mold 6 in a direction approximately parallel to the vertical direction, in a direction approximately parallel to the horizontal direction. The bend section 10 comprises multiple segments. As shown in Figure 1, each segment is arranged in an arc shape to smoothly connect the vertical section 8 and the horizontal section 11. Each segment comprises multiple cast slab support rolls 9. These cast slab support rolls 9 are positioned on both sides of the cast slab 7 in the thickness direction of the cast slab 7, and are also spaced apart along the aforementioned arc, that is, in the direction of movement of the cast slab 7. In the thickness direction of the cast slab 7, the spacing between the cast slab support rolls 9 of the bend section 10 is set to be approximately the same as the thickness of the cast slab 7 immediately after being drawn from the mold 6. Note that in Figure 1, for the sake of simplicity, the first segment 10a, the second segment 10b, and the final segment 10c are shown, and the other segments are omitted. The first segment 10a is located furthest upstream in the direction of movement of the cast slab 7 among the multiple segments of the bent section 10. The second segment 10b is located downstream of the first segment 10a. The final segment 10c is located furthest downstream in the direction of movement of the cast slab.

[0018] The first segment 10a is adjacent to the vertical section 8. The first segment 10a is designed to bend the flat-shaped cast slab 7 that extends from the vertical section 8. The final segment 10c is adjacent to the horizontal section 11. The final segment 10c is designed to straighten the curved cast slab 7 back to its original flat shape. In the following description, the first segment 10a will be referred to as the upper straightening band 10a, and the final segment 10c will be referred to as the lower straightening band 10c.

[0019] The horizontal section 11 extends in a direction substantially parallel to the horizontal direction. Also, similar to the bending section 10, the horizontal section 11 includes a plurality of segments. In the example shown in FIG. 1, the horizontal section 11 is composed of two segments. Each segment includes a plurality of slab support rolls 9. Those slab support rolls 9 are arranged on both sides of the slab 7 in the thickness direction of the slab 7 and are spaced apart in a direction substantially parallel to the horizontal direction. The spacing between the slab support rolls 9 of the horizontal section 11 in the thickness direction of the slab 7 is set to be the thickness of the target slab on the outlet side of the continuous casting machine 1.

[0020] Also, in each of the vertical section 8, the bending section 10, and the horizontal section 11, spray nozzles (not shown) are arranged between adjacent slab support rolls 9 in the moving direction of the slab 7. Cooling water is sprayed from those spray nozzles onto the slab 7 to cool the slab 7. Thus, the vertical section 8, the bending section 10, the horizontal section 11, and each spray nozzle provided below the mold 6 correspond to the secondary cooling zone described above. Although details are not shown, the distance between the tip of each spray nozzle and the slab 7 in the thickness direction of the slab 7 is set to be about 90 mm. Also, as an example, the temperature of the cooling water sprayed from each spray nozzle onto the slab 7 may be about 15°C or more and 45°C or less.

[0021] A slab cutting machine 12 for cutting the slab 7 is provided on the downstream side of the horizontal section 11 in the moving direction of the slab 7. In the horizontal section 11, the solidified slab 7 is cut to a predetermined length by the slab cutting machine 12.

[0022] Also, the continuous casting machine 1 shown in FIG. 1 is configured such that the supply amount of cooling water can be controlled independently for each of the vertical section 8, the bending section 10, and the horizontal section 11. Specifically, a cooling water supply source 14 is connected to each spray nozzle of the vertical section 8, each spray nozzle of the bending section 10, and each spray nozzle of the horizontal section 11 via a flow control device 13. The control of the supply amount of cooling water by the flow control device 13 may be performed by a control device (not shown) or by an operator. Also, the flow control device 13 may be, for example, a flow control valve.

[0023] In this embodiment, in order not to cause surface cracking in the slab 7 due to bending or straightening of the slab 7 by the upper straightening band 10a or the lower straightening band 10c, the cooling densities X and Y of the cooling water in the bending portion 10 and the horizontal portion 11 are respectively set. That is, if the surface temperature of the slab 7 in the upper straightening band 10a and the lower straightening band 10c falls within the embrittlement temperature range of the slab 7, there is a possibility that surface cracking of the slab 7 may occur due to bending or straightening of the slab 7. Therefore, the cooling density ratio X / Y of the first cooling density X in the bending portion 10 to the second cooling density Y in the horizontal portion 11 is set to 1.0 or less so that the slab 7 passes through the upper straightening band 10a and the lower straightening band 10c at a temperature exceeding the embrittlement temperature range of the slab 7. That is, the first cooling density X is set to be less than or equal to the second cooling density Y.

[0024] The first cooling density X is a value obtained by dividing the total supply amount of the cooling water supplied to the slab 7 in the bending portion 10 (hereinafter simply referred to as the total amount) by the surface area of the slab 7 in the bending portion 10. The second cooling density Y is a value obtained by dividing the total amount of the cooling water supplied to the slab 7 in the horizontal portion 11 by the surface area of the slab 7 in the horizontal portion 11.

[0025] The surface area of the slab 7 in the bending portion 10 and the surface area of the slab 7 in the horizontal portion 11 can be obtained in design. The total amount of the cooling water in the bending portion 10 means the total amount of the cooling water supplied to the slab 7 in the bending portion 10 when the slab 7 is continuously cast using one charge of the molten steel 5. Similarly, the total amount of the cooling water in the horizontal portion 11 means the total amount of the cooling water supplied to the slab 7 in the horizontal portion 11 when the slab 7 is continuously cast using one charge of the molten steel 5. Also, one charge means the same lot of the molten steel 5. The embrittlement temperature range means the temperature range in which surface cracking is likely to occur due to bending or straightening of the slab 7.

[0026] If the cooling density ratio X / Y exceeds 1.0, the amount of cooling water in the horizontal section 11 and the second cooling density Y calculated from the amount of cooling water in the horizontal section 11 become smaller than the amount of cooling water in the bending section 10 and the first cooling density X calculated from that amount of cooling water. As a result, there may be insufficient cooling of the slab 7 in the horizontal section 11, and the slab 7 may not solidify completely. In addition, when the slab 7 passes through the upper straightening band 10a or lower straightening band 10c of the bending section 10, the surface temperature of the slab 7 may enter the embrittlement temperature range, potentially causing surface cracks in the slab 7. On the other hand, if the cooling density ratio X / Y becomes 0.0, for example, if the first cooling density X in the bending section 10 is 0.0, that is, if the amount of cooling water in the bending section 10 is 0.0, the solidified shell of the slab 7 directly below the mold 6 becomes excessively thin compared to the steady state described above. This can cause large bulging, potentially making it impossible to remove the slab 7 from the mold 6. Therefore, the cooling density ratio X / Y is preferably greater than 0.0 and 1.0 or less, and more preferably greater than 0.0 and 0.9 or less.

[0027] (Effects / Actions) The operation and effects of this embodiment will now be explained. A cast slab 7 with a thickness of 350 to 500 mm is drawn out from the mold 6. The drawing speed of the cast slab 7 is set to 0.2 to 1.5 m / min. Cooling water is sprayed onto the cast slab 7 from a spray nozzle in the vertical section 8. In this way, the surface temperature is lowered to a preset temperature. In this embodiment, the surface temperature of the cast slab 7 on the exit side of the vertical section 8 is preferably 700 to 1200°C. For example, if the surface temperature of the cast slab 7 on the exit side of the vertical section 8 is less than 700°C, the surface temperature of the cast slab 7 may enter the embrittlement temperature range when passing through the upper straightening band 10a or the lower straightening band 10c, potentially causing surface cracking of the cast slab 7. On the other hand, if the surface temperature of the cast slab 7 on the exit side of the vertical section 8 exceeds 1200°C, there is a possibility that drawing defects will occur due to increased bulging.

[0028] Next, in the upper straightening band 10a of the bent section 10, the cast slab 7 is curved, and in this state, the cast slab 7 moves toward the horizontal section 11. Also in the bent section 10, cooling water is sprayed onto the cast slab 7 from each spray nozzle. In this embodiment, the cooling density ratio X / Y between the first cooling density X in the bent section 10 and the second cooling density Y in the horizontal section 11 is set to 1.0 or less, as described above. In other words, the first cooling density X of the cast slab 7 in the bent section 10 is set to be smaller than the second cooling density Y of the cast slab in the horizontal section 11. Therefore, the cooling of the cast slab 7 in the bent section 10 is suppressed more than the cooling in the horizontal section 11. As a result, the surface temperature of the cast slab 7 maintains a temperature above the embrittlement temperature range, and in this state, the cast slab 7 reaches the lower straightening band 10c. Then, in the lower straightening band 10c, the cast slab 7 is straightened from its curved shape back to its original flat shape. Next, in the horizontal section 11, cooling water is sprayed onto the cast slab 7 from each spray nozzle, and the cast slab 7 is completely solidified, even inside.

[0029] Thus, according to this embodiment, the surface temperature of the cast slab 7 in each straightening band 10a and 10c is maintained at a temperature above the embrittlement temperature range. Therefore, according to this embodiment, it is possible to continuously cast an unprecedentedly thick cast slab 7 at an unprecedented speed, with good internal quality and few or no surface cracks. As a result, the productivity of unprecedentedly thick cast slabs 7 can be improved.

[0030] It should be noted that the present invention is not limited to the embodiments described above. For example, the invention may include a control device (not shown) that controls the continuous casting machine 1 shown in Figure 1, and this control device may control the cooling of the cast slab 7 in the secondary cooling zone. In other words, the control device described above controls the amount of cooling water sprayed onto the cast slab 7 from the spray nozzle in the vertical section 8. By doing so, the surface temperature of the cast slab 7 on the exit side of the vertical section 8 is set to 700 to 1200°C. The control device also controls the amount of cooling water sprayed onto the cast slab 7 from the spray nozzle in the bent section 10 and the horizontal section 11. By doing so, the cooling density ratio X / Y is controlled to 1.0 or less. Even with this configuration, the same effects and advantages as in the embodiments described above can be obtained.

[0031] Furthermore, in the embodiment described above, the cast slab 7 was cooled by spraying it with cooling water in the secondary cooling zone. However, instead, dry casting without spraying the cast slab 7 with cooling water may be used. In the case of dry casting, the spray directly below the mold 6 is operated to prevent defects in the withdrawal of the cast slab 7. However, the spray in the bent section 10 is not operated. On the other hand, the spray in the horizontal section 11 is operated. As a result, the cooling density ratio X / Y in the case of dry casting becomes a value very close to 0. [Examples]

[0032] Next, we will describe an example conducted to confirm the effectiveness of the continuous casting method for slabs according to this embodiment. In the example, a continuous casting machine configured in the same manner as in the embodiment described above was used to continuously cast various types of slabs by changing the test conditions. Specifically, molten steel for ordinary steel with a carbon content of 0.07 to 0.50 mass% was used, and slabs with a thickness of 350 to 450 mm were continuously cast at a casting speed of 0.4 to 1.2 m / min. The occurrence of surface cracks in each continuously cast slab was then evaluated. Table 1 summarizes the chemical composition of each steel type used in the example. Table 2 summarizes the steel type, casting speed (i.e., slab withdrawal speed), slab thickness, and secondary cooling conditions of the slabs continuously cast in the example. Note that the water density in Table 2 corresponds to the cooling density in this embodiment.

[0033] [Table 1]

[0034] [Table 2]

[0035] (How to check for surface cracks) The method for checking surface cracks is described below. Approximately 2 mm was machined from each side of a cast slab manufactured under the conditions shown in Table 2. After machining, surface cracks on each side of the cast slab were detected by penetrant testing (sometimes called color checking). The number of surface cracks on each side of the cast slab was counted, and if the total number of surface cracks was 3 or less, the cast slab was evaluated as "good cracking," meaning that surface cracking was suppressed more than before. If the total number of surface cracks exceeded 3, the cast slab was evaluated as "cracks occurring," meaning that surface cracking was approximately the same as before. These evaluation results, along with the manufacturing conditions described above, are summarized in Table 2. Test numbers 1 to 5 in Table 2 are comparative examples where the various manufacturing conditions fall outside the range defined in this invention. Test numbers 6 to 10 are examples where the various manufacturing conditions are within the range defined in this invention.

[0036] (evaluation) As shown in Table 2, the slabs produced under the conditions of comparative examples (test numbers 1-5) were evaluated as having "crack formation." The slabs produced under the conditions of examples (test numbers 6-10) were evaluated as having "good cracking." Therefore, it was confirmed that by implementing the continuous casting method for slabs according to the present invention, it is possible to prevent or suppress the occurrence of surface cracks in slabs when producing slabs with unprecedented thickness at unprecedented speeds. [Explanation of symbols]

[0037] 1. Continuous casting machine 2 Tan Dish 3. Sliding nozzle 4 Immersion nozzle 5 Molten steel 6. Mold 7 cast slabs 8 Vertical section 9. Cast slab support rolls 10 Bending section 10a First segment, upper orthodontic band 10b Second Segment 10c Final segment, lower orthodontic band 11 Horizontal section 12 Slab cutting machine 13 Flow control device 14. Cooling water supply sources

Claims

1. A method for continuous casting of a cast slab using a continuous casting machine comprising a mold, a vertical section positioned downstream of the mold in the direction of movement of the cast slab withdrawn from the mold, a bent section positioned downstream of the vertical section in the direction of movement, and a horizontal section positioned downstream of the bent section in the direction of movement, The surface temperature of the cast slab on the exit side of the vertical portion is 700°C or higher and 1200°C or lower. The first cooling density X calculated from the amount of cooling water in the bent section is less than or equal to the second cooling density Y calculated from the amount of cooling water in the horizontal section. Each of the vertical section, the bent section, and the horizontal section is configured to supply cooling water to the cast slab. The first cooling density X is the value obtained by dividing the total amount of cooling water supplied to the cast slab within the bent portion by the surface area of ​​the cast slab within the bent portion. A method for continuous casting of a slab, wherein the second cooling density Y is the value obtained by dividing the total amount of cooling water supplied to the slab in the horizontal section by the surface area of ​​the slab in the horizontal section.

2. The continuous casting method for a cast slab according to claim 1, wherein the thickness of the cast slab is 350 mm or more and 500 mm or less.

3. The method for continuous casting of a cast slab according to claim 1 or 2, wherein the withdrawal speed of the cast slab in the vertical section is 0.2 to 1.5 m / min.

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