Continuous casting method for cast slab

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

Continuous casting method for slabs

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

[0002] High-quality extra-thick steel plates exceeding 100 mm in thickness are used as important components, such as steel plates for boilers, low-alloy steel plates for pressure vessels, and high-strength steel plates for marine structures and industrial machinery. However, the internal quality of high-quality extra-thick steel plates can be problematic in terms of their performance in use. Therefore, in the past, high-quality extra-thick steel plates were manufactured by producing large ingots using an ingot casting method, and then rolling or forging these large steel ingots with a sufficient reduction ratio. This manufacturing method improved the internal quality of high-quality extra-thick steel plates.

[0003] On the other hand, since the productivity of the ingot casting method is low, attempts have been made to produce thick, extra-thick slabs using a vertical continuous casting method. One example of such a method is described in Patent Document 1. The method described in Patent Document 1 discloses that when the thickness of the slab is 380 mm or more, the casting speed is set to 0.2 m / min or less in order to avoid restrictions on the length of the continuous casting equipment and to prevent bulging deformation of the slab.

[0004] Furthermore, when using a vertical bending continuous caster to produce an extremely thick slab (hereinafter referred to as "bill"), stress is applied to the billet at the bending section where the billet, drawn from the mold in a direction substantially parallel to the vertical direction, is bent in a direction substantially parallel to the horizontal direction. This increases the risk of corner cracks and transverse cracks occurring in the billet. Patent Document 2 describes an example of a continuous casting machine that reduces this risk. The vertical bending continuous casting machine described in Patent Document 2 produces billets free of surface defects and internal cracks by setting production conditions appropriate for the chemical composition and thickness of the billet to be cast. Specifically, the chemical composition of the billet described in Patent Document 2 is C: 0.05 to 0.55 mass%, Si: 0.10 to 2.00 mass%, Mn: 0.30 to 1.90 mass%, P: 0.005 to 0.070 mass%, and S: 0.003 to 0.120 mass%. The billet has a thickness of 280 to 350 mm. The slab is cast by a vertical bending type continuous casting machine with a casting speed set to 0.7 to 1.1 m / min and a specific water amount set to 0.15 to 0.40 L / kg steel, and in this case, the vertical length, bending length, secondary cooling length, arc length, straightening length, etc. are set within predetermined ranges.

[0005] JP 2007-229736 A JP 2009-274116 A

[0006] In the method described in Patent Document 1, when the thickness of the slab is 380 mm or more, the casting speed is limited to 0.2 m / min or less. In other words, in the method described in Patent Document 1, when the thickness of the slab is 380 mm or more, the productivity is low, 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 thickness of the slab that can achieve both productivity and suppression of surface cracks is 280 to 350 mm. Therefore, when the thickness of the slab is 350 mm or more, there is a possibility that both productivity and suppression of surface cracks cannot be achieved, and in this respect, there is still room for improvement.

[0008] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a continuous casting method for slabs that can improve the productivity of thicker slabs than ever before and can suppress surface cracks in the slabs to ensure surface quality.

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

[0010] According to the present invention, it is possible to continuously cast, at a higher speed than ever before, extra-thick slabs that have good internal quality, few or no surface cracks, and are thicker than ever before, thereby improving the productivity of thicker slabs than ever before.

[0011] 1 is a diagram showing an example of a vertical bending type continuous casting machine to which the continuous casting method of a slab according to an embodiment of the present invention can be applied.

[0012] The present invention will be specifically described below through embodiments of the present invention (hereinafter referred to as the present embodiment). The present embodiment described below shows a preferred example of the present invention, and the present invention is not limited to this example.

[0013] The continuous casting method for slabs according to this embodiment is a method for continuously casting slabs of unprecedented thickness with good internal quality and few or no surface cracks at unprecedented high speeds by optimizing the cooling amount in the secondary cooling zone of the continuous casting machine. FIG. 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 FIG. 1 is a so-called vertical bending type, and in this continuous casting machine 1, an immersion nozzle 4 is attached to the bottom of a 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 for adjusting the amount of molten steel 5 supplied from the tundish 2 to a 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 the cooling channel from a cooling water supply source (not shown). In the mold 6, heat is removed from the molten steel 5 by the cooling water, thereby forming a slab 7 with a solidified shell formed on its surface. The slab 7 is then pulled out of the mold 6 in a direction substantially parallel to the vertical direction. Therefore, a wet layer in which the molten steel 5 has not yet solidified is formed inside the slab 7 directly below the mold. The mold 6 is sometimes referred to as a primary cooling zone. In FIG. 1, the wet layer is hatched.

[0015] In this embodiment, the thickness of the slab 7 withdrawn from the mold 6 of the continuous casting machine 1 shown in FIG. 1 is set to 350 to 500 mm. The withdrawal speed of the slab 7 from the mold 6 is set to 0.2 to 1.5 m / min. This withdrawal speed is the withdrawal speed in a steady state. Note that the steady state refers to a state in which the casting speed is constant except at the start and end of casting, when slabs are joined, and so on. If the withdrawal speed of the slab 7 is less than 0.2 m / min, the molten steel 5 may solidify in the mold 6, making it impossible to withdraw the slab 7 from the mold 6. If the withdrawal speed of the slab 7 exceeds 1.5 m / min, the solidified shell may not be sufficiently formed, and when the slab 7 is withdrawn from the mold 6, the solidified shell may break, causing steel leakage, resulting in breakout. Therefore, it is preferable to set the withdrawal speed of the slab 7 to 0.2 to 1.5 m / min. It is more preferable to set the drawing speed of the 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 substantially parallel to the vertical direction and supporting the strand 7 being continuously withdrawn from the mold 6. The vertical section 8 includes a plurality of strand support rolls 9. As shown in FIG. 1 , the strand support rolls 9 are disposed on both sides of the strand 7 in the thickness direction of the strand 7 and spaced apart in a direction substantially parallel to the vertical direction. The strand support rolls 9 support the strand 7 withdrawn from the mold 6. Therefore, the spacing between the strand support rolls 9 in the thickness direction of the strand 7 is set to be substantially the same as the thickness of the strand 7 immediately after withdrawal from the mold 6. Cooling water is supplied to the inside of the strand support rolls 9. The strand support rolls 9 in the vertical section 8, the strand support rolls 9 in the bending section 10 (described later), and the strand support rolls 9 in the horizontal section 11 (described later) are all configured substantially identically. For example, each of the slab support rolls 9 may be a roll manufactured by overlay welding 13Cr4Ni onto the surface of a base material made of 21CrMoV511.

[0017] A bending section 10 is provided downstream of the vertical section 8 in the direction of movement of the slab 7. The bending section 10 is configured to bend the slab 7, which has been withdrawn from the mold 6 in a direction substantially parallel to the vertical direction, in a direction substantially parallel to the horizontal direction. The bending section 10 includes multiple segments. As shown in FIG. 1, each segment is arranged in an arc shape to smoothly connect the vertical section 8 and the horizontal section 11. Each segment includes multiple slab support rolls 9. These 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 along the arc, i.e., in the direction of movement of the slab 7. The spacing between the slab support rolls 9 in the thickness direction of the slab 7 is set to be substantially the same as the thickness of the slab 7 immediately after withdrawal from the mold 6. Note that, for simplicity's sake, FIG. 1 only shows a first segment 10a, a second segment 10b, and a final segment 10c, and does not show the other segments. Of the multiple segments in the bent portion 10, the first segment 10a is located furthest upstream in the direction of movement of the slab 7. 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 slab.

[0018] The first segment 10a is adjacent to the vertical portion 8. The first segment 10a is configured to bend the flat slab 7 emerging from the vertical portion 8. The final segment 10c is adjacent to the horizontal portion 11. The final segment 10c is configured to straighten the curved 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. Like 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 configured by two segments. Each segment includes a plurality of slab support rolls 9. These 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 so that the slab has a target thickness at the outlet side of the continuous casting machine 1.

[0020] In addition, spray nozzles (not shown) are arranged between adjacent strand support rolls 9 in each of the vertical section 8, bending section 10, and horizontal section 11 in the direction of movement of the strand 7. Cooling water is sprayed from these spray nozzles onto the strand 7 to cool it. The vertical section 8, bending section 10, horizontal section 11, and each spray nozzle provided below the mold 6 correspond to the secondary cooling zone described above. Although not shown in detail, the distance between the tip of each spray nozzle and the strand 7 in the thickness direction of the strand 7 is set to about 90 mm. The temperature of the cooling water sprayed from each spray nozzle onto the strand 7 may be, for example, about 15°C or higher and 45°C or lower.

[0021] A slab cutter 12 for cutting the slab 7 is provided downstream of the horizontal section 11 in the direction of movement of the slab 7. In the horizontal section 11, the slab 7 that has completed solidification is cut to a predetermined length by the slab cutter 12.

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

[0023] In this embodiment, the cooling densities X and Y of the cooling water in the bending section 10 and the horizontal section 11 are set so as to prevent surface cracks from occurring in the slab 7 during bending and straightening in the upper straightening band 10a and the lower straightening band 10c. That is, if the surface temperature of the slab 7 in the upper straightening band 10a and the lower straightening band 10c is within the embrittlement temperature range of the slab 7, surface cracks may occur in the slab 7 during bending and straightening. Therefore, the cooling density ratio X / Y between the first cooling density X in the bending section 10 and the second cooling density Y in the horizontal section 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 equal to or less than the second cooling density Y.

[0024] The first cooling density X is a value obtained by dividing the total amount of cooling water supplied to the slab 7 in the bent portion 10 (hereinafter simply referred to as the total amount) by the surface area of ​​the slab 7 in the bent portion 10. The second cooling density Y is a value obtained by dividing the total amount of 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 bent section 10 and the surface area of ​​the slab 7 in the horizontal section 11 can be determined in design. The total amount of cooling water in the bent section 10 refers to the total amount of cooling water supplied to the slab 7 in the bent section 10 when one charge of molten steel 5 is used to continuously cast the slab 7. Similarly, the total amount of cooling water in the horizontal section 11 refers to the total amount of cooling water supplied to the slab 7 in the horizontal section 11 when one charge of molten steel 5 is continuously cast. Furthermore, one charge refers to molten steel 5 from the same lot. The embrittlement temperature range refers to a temperature range in which surface cracks are likely to occur when the slab 7 is bent or straightened.

[0026] When the cooling density ratio X / Y exceeds 1.0, the cooling water volume in the horizontal section 11 and the second cooling density Y calculated from the cooling water volume are smaller than the cooling water volume in the bent section 10 and the first cooling density X calculated from the cooling water volume. This can result in insufficient cooling of the slab 7 in the horizontal section 11, potentially preventing the slab 7 from completely solidifying. Furthermore, the surface temperature of the slab 7 may fall into the embrittlement temperature range when passing through the upper straightening zone 10a or the lower straightening zone 10c of the bent section 10, potentially causing surface cracks in the slab 7. On the other hand, when the cooling density ratio X / Y becomes 0.0, for example, when the first cooling density X in the bent section 10 is 0.0, i.e., when the cooling water volume in the bent 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 result in significant bulging, potentially making it impossible to withdraw the slab 7 from the mold 6. Therefore, the cooling density ratio X / Y is preferably greater than 0.0 and equal to or less than 1.0, and more preferably greater than 0.0 and equal to or less than 0.9.

[0027] (Operations and Effects) The operations and effects of this embodiment will be described. A slab 7 having a thickness of 350 to 500 mm is withdrawn from the mold 6. The withdrawal speed of the slab 7 is set to 0.2 to 1.5 m / min. Cooling water is sprayed onto the slab 7 from a spray nozzle in the vertical section 8. In this manner, the surface temperature is lowered to a predetermined temperature. In this embodiment, the surface temperature of the slab 7 on the outlet side of the vertical section 8 is preferably 700 to 1200°C. For example, if the surface temperature of the slab 7 on the outlet side of the vertical section 8 is less than 700°C, the surface temperature of the slab 7 may fall into the embrittlement temperature range when passing through the upper straightening zone 10a or the lower straightening zone 10c, potentially causing surface cracks in the slab 7. On the other hand, if the surface temperature of the slab 7 on the outlet side of the vertical section 8 exceeds 1200°C, increased bulging may occur, potentially resulting in poor withdrawal.

[0028] Next, the slab 7 is curved in the upper straightening band 10a of the bending section 10 and moves toward the horizontal section 11 in this state. Cooling water is also sprayed onto the slab 7 from each spray nozzle in the bending section 10. In this embodiment, the cooling density ratio X / Y between the first cooling density X in the bending section 10 and the second cooling density Y in the horizontal section 11 is set to 1.0 or less, as described above. That is, the first cooling density X of the slab 7 in the bending section 10 is set to be smaller than the second cooling density Y of the slab 7 in the horizontal section 11. Therefore, cooling of the slab 7 in the bending section 10 is suppressed more than cooling in the horizontal section 11. As a result, the surface temperature of the slab 7 is maintained above the brittle temperature range, and the slab 7 reaches the lower straightening band 10c in this state. The slab 7 is then straightened from its curved shape to its original flat shape in the lower straightening band 10c. Next, cooling water is sprayed onto the slab 7 from the spray nozzles in the horizontal section 11 as well, and the slab 7 is completely solidified to its interior.

[0029] As described above, according to this embodiment, the surface temperature of the slab 7 in each of the straightening bands 10a, 10c is maintained above the brittle temperature range. Therefore, according to this embodiment, it is possible to continuously cast an extremely thick slab 7 having good internal quality and little or no surface cracks at an unprecedented high speed. As a result, it is possible to improve the productivity of the extremely thick slab 7.

[0030] The present invention is not limited to the above-described embodiment. For example, a control device (not shown) may be provided to control the continuous casting machine 1 shown in FIG. 1 , and the cooling of the slab 7 in the secondary cooling zone may be controlled by the control device. That is, the above-described control device controls the amount of cooling water sprayed from the spray nozzles onto the slab 7 in the vertical section 8. By doing so, the surface temperature of the slab 7 on the outlet side of the vertical section 8 is set to 700 to 1200°C. The control device also controls the amount of cooling water sprayed from the spray nozzles onto the slab 7 in the bending 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 functions and effects as those of the above-described embodiment can be obtained.

[0031] In the above-described embodiment, the slab 7 is cooled in the secondary cooling zone by spraying cooling water onto the slab 7. However, dry casting may be used instead, in which cooling water is not sprayed onto the slab 7. In the case of dry casting, spraying is performed immediately below the mold 6 to prevent poor drawing of the slab 7. However, spraying is not performed in the bent portion 10. On the other hand, spraying is performed in the horizontal portion 11. As a result, the cooling density ratio X / Y in the case of dry casting becomes a value that is infinitely close to 0.

[0032] Next, examples conducted to verify the effectiveness of the continuous casting method for slabs according to this embodiment will be described. In these examples, various types of slabs were continuously cast using a continuous casting machine configured similarly to the above-described embodiment, with varying test conditions. Specifically, molten steel for ordinary steel with a carbon content of 0.07 to 0.50 mass % was used, and slabs with thicknesses 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 compositions of the steel types used in the examples. Table 2 summarizes the steel types, casting speeds (i.e., slab withdrawal speeds), slab thicknesses, and secondary cooling conditions of the slabs continuously cast in the examples. The water flow densities in Table 2 correspond to the cooling densities in this embodiment.

[0033]

[0034]

[0035] (Surface crack checking method) The surface crack checking method will be described. Approximately 2 mm was cut off from each of both sides of a cast slab produced under the conditions shown in Table 2. Surface cracks on each cut surface of the cast slab were then detected using a penetrant test (sometimes called a color check). The number of surface cracks on each side of the slab was counted, and if the total number of surface cracks was three or less, the slab was evaluated as "good cracking," meaning that surface cracking was more suppressed than in the past. If the total number of surface cracks was more than three, the slab was evaluated as "crack occurrence," meaning that surface cracking was approximately the same as in the past. These evaluation results, along with the above-mentioned production conditions, are summarized in Table 2. Test numbers 1 to 5 in Table 2 are comparative examples in which various production conditions were outside the ranges specified in the present invention. Test numbers 6 to 10 are examples in which various production conditions were within the ranges specified in the present invention.

[0036] (Evaluation) As shown in Table 2, the cast pieces produced under the conditions of test numbers 1 to 5, which are comparative examples, were evaluated as "cracks occurring." The cast pieces produced under the conditions of test numbers 6 to 10, which are examples, were evaluated as "good cracking." Therefore, it was confirmed that by carrying out the continuous casting method for cast pieces according to the present invention, it is possible to prevent or suppress the occurrence of surface cracks in the cast pieces when producing cast pieces with unprecedented thicknesses at unprecedented high speeds.

[0037] REFERENCE SIGNS LIST 1 continuous casting machine 2 tundish 3 sliding nozzle 4 submerged nozzle 5 molten steel 6 mold 7 strand 8 vertical section 9 strand support roll 10 bending section 10a first segment, upper straightening zone 10b second segment 10c final segment, lower straightening zone 11 horizontal section 12 strand cutting machine 13 flow rate control device 14 cooling water supply source

Claims

1. A method for continuously casting a slab by a continuous casting machine comprising a mold, a vertical part disposed downstream of the mold in the moving direction of the slab withdrawn from the mold, a bending part disposed downstream of the vertical part in the moving direction, and a horizontal part disposed downstream of the bending part in the moving direction, wherein the surface temperature of the slab at the outlet side of the vertical part is 700°C or higher and 1200°C or lower, and a first cooling density X calculated from the amount of cooling water in the bending part is equal to or lower than a second cooling density Y calculated from the amount of cooling water in the horizontal part.

2. Each of the vertical part, the bending part, and the horizontal part is configured to supply cooling water to the slab, the first cooling density X is a value obtained by dividing the total supply amount of the cooling water supplied to the slab in the bending part by the surface area of the slab in the bending part, and the second cooling density Y is a value obtained by dividing the total supply amount of the cooling water supplied to the slab in the horizontal part by the surface area of the slab in the horizontal part. The method for continuously casting a slab according to claim 1.

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

4. The method for continuously casting a slab according to claim 1 or 2, wherein the drawing speed of the slab in the vertical part is 0.2 to 1.5 m / min.

5. The method for continuously casting a slab according to claim 3, wherein the drawing speed of the slab in the vertical part is 0.2 to 1.5 m / min.