Continuous casting method of slab

Abstract

To provide a method for continuous casting of a slab that can avoiding inner cracks by restraining recuperation in secondary cooling.SOLUTION: A method for manufacturing a cast piece 1 composed of steel containing C of 0.08 to 0.29 mass% with a vertical-bend continuous casting machine 10. The casting machine 10 includes, in this order: a casting mold 13; a vertical part 51 connected downstream of the casting mold 13 and extending in a vertical direction: a bent part 52 connected downstream of the vertical part 51 and having a radius of curvature which is gradually smaller toward the downstream side; a curved part 53 connected downstream of the bend part 52 and having a constant radius of curvature; a correction part 54 connected downstream of the curved part 53 and having a radius of curvature which is gradually larger toward the downstream side; and a horizontal part 55 connected downstream of the correction part 54 and extending in a horizontal direction. When the casting piece 1 is extracted at a casting speed of less than 1.1 m / min, cooling is performed so that a temperature rise of the casting piece 1 is less than 15°C on the downstream side of the curved part 53.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] In the continuous casting of steel, the molten steel poured into a water-cooled mold is first cooled, and the surface of the molten steel solidifies in contact with the mold to form a solidified shell. The slab with this solidified shell as the outer shell and the inner part as the non-solidified layer is secondarily cooled by spraying cooling water while being supported by rolls installed on the downstream side of the mold, and is continuously drawn out downstream in the casting direction. In this continuous casting machine, various methods have been proposed to prevent internal cracks from occurring in the slab.

[0003] For example, Patent Document 1 discloses a cooling method directly below the mold when continuously casting round billets from molten steel with a carbon concentration of 0.3 to 1.0% by mass. In this method, when the solidified shell thickness directly below the mold is D (mm) and the reheating amount from directly below the mold until the round billet is first spray-cooled is ΔT (°C), at least one of the secondary cooling water amount and the casting speed directly below the mold is controlled so that ΔT / D is 8.5 °C / mm or less. Thereby, it has become possible to prevent internal cracks in the round billet and breakouts caused by internal cracks.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] According to the invention of Patent Document 1, when continuously casting round billets from molten steel with a carbon concentration of 0.3 to 1.0% by mass, if it is cooled under predetermined conditions directly below the mold, it is said that internal cracks in the obtained round billets can be prevented. However, when casting slabs from molten steel with a carbon concentration of 0.08% by mass or more and 0.29% by mass or less using a vertical bending type continuous casting machine, internal cracks may occur. Here, the vertical bending type continuous casting machine is provided with a vertical portion, a bending portion, a curved portion, a straightening portion, and a horizontal portion in this order directly below the mold. FIG. 4 is a diagram illustrating the occurrence status of internal cracks in a slab obtained by continuous casting from molten steel with a carbon concentration of 0.08% by mass or more and 0.29% by mass or less. As illustrated in FIG. 4, in the slab 1, internal cracks 100 may occur in a region approximately 60 to 70 mm inside from the surface of the short side. The internal cracks 100 are assumed to occur in the region in a solid-liquid coexisting state (solid-liquid coexisting region) of the curved portion (casting length from the molten metal surface in the mold is around 7 to 10 m) in the vertical bending type continuous casting machine from the position where the internal cracks 100 occur.

[0006] In continuous casting, the solid-liquid coexistence region is fragile. Internal cracks occur when tensile deformation occurs in the solidification shell starting from the brittle part of this solid-liquid coexistence region. Fig. 5 is a diagram showing the relationship between the solid fraction and the tensile strength in molten steel with a carbon concentration of 0.18 mass% as an example of molten steel with a carbon concentration of 0.08 mass% or more and 0.29 mass% or less. As shown in Fig. 5, the tensile strength appears around a solid fraction of 0.7, and when the solid fraction exceeds 0.8, the tensile strength exceeds 0.5 MPa and increases rapidly. This tendency is common to molten steel with a carbon concentration of 0.08 mass% or more and 0.29 mass% or less. Therefore, when a tensile stress acts on a slab with a solid fraction exceeding 0.8, the external force acts on the solidification shell, inducing internal cracks. Here, as a means of reducing the tensile stress, it is effective to strengthen secondary cooling or reduce the casting speed Vc. However, when obtaining a slab from molten steel with a carbon concentration of 0.08 mass% or more and 0.29 mass% or less, if expansion due to γ-α transformation occurs with a temperature drop, cracks may occur due to an imbalance in the strain rate caused by the difference in the solidification shell thickness between the long side and the short side of the slab. For this reason, it is not preferable to employ strengthening of secondary cooling that promotes γ-α transformation due to temperature drop as a means of reducing the tensile stress. Therefore, an attempt was made to reduce the casting speed Vc as a means of reducing the tensile stress.

[0007] However, as shown in Fig. 6, it was found that internal cracks occurred instead when the casting speed Vc was reduced. Fig. 6 is a diagram showing the relationship between the casting speed Vc and internal cracks. As a result of intensive research on the reason by the present inventor, the following findings were obtained. That is, when the casting speed Vc is low, while the slab is excessively heat-extracted (cooled) by the roll that it contacts immediately below the mold, the specific water volume in the curved part needs to be restricted in order to prevent the occurrence of γ-α transformation as described above. For this reason, it was found that the amount of reheating of the slab increased in the curved part, and the expansion of the solidification shell due to reheating acted as a tensile stress, resulting in the occurrence of internal cracks.

[0008] Therefore, an example of the object of the present invention is to provide a method for continuous casting of a slab that can suppress reheating in the curved part and avoid internal cracks.

Means for Solving the Problem

[0009] A mold, a vertical part connected to the downstream side of the mold and extending vertically, a bending part connected to the downstream of the vertical part and having a gradually decreasing radius of curvature toward the downstream, a curved part connected to the downstream of the bending part and having a constant radius of curvature, a correcting part connected to the downstream of the curved part and having a gradually increasing radius of curvature toward the downstream, a horizontal part connected to the downstream of the correcting part and extending horizontally, are sequentially provided, and a method for producing a slab made of steel containing 0.08 to 0.29% by mass of C by a vertical bending mold continuous casting machine, When pulling out the slab at a casting speed of 1.1 m / min or less, cooling is performed so that the temperature rise of the slab upstream of the curved part is 15°C or less. A method for continuous casting of slabs.

Advantages of the Invention

[0010] According to the present invention, reheating in the curved part can be suppressed, and internal cracks can be avoided.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiment for Carrying Out the Invention

[0012] Hereinafter, embodiments of the method for manufacturing a slab of the present invention will be described with reference to the drawings.

[0013] FIG. 1 is a schematic view of a continuous casting machine 10 used in the continuous casting method of a slab.

[0014] The continuous casting machine 10 for casting the slab 1 is of a vertical bending type and includes a tundish 11, a submerged nozzle 12, a mold 13, a plurality of rolls 14, a plurality of secondary cooling sprays 15, and a pinch roll 16. Further, the casting path of the continuous casting machine 10 includes a vertical portion 51, a bending portion 52, a curved portion 53, a straightening portion 54, and a horizontal portion 55 in order from the mold on the upstream side in the casting direction toward the downstream pinch roll 16.

[0015] The vertical portion 51 is a portion that extends vertically downward from directly below the mold 13. The bending portion 52 is a portion that gently curves from the vertical portion 51 and reduces the radius of curvature of the casting path, and bends the slab 1 into an arc shape. The curved portion 53 is a portion that is connected to the bending portion 52 and has a constant radius of curvature of the casting path. The straightening portion 54 is provided downstream of the curved portion 53 and is a portion that gradually increases the radius of curvature of the casting path. The horizontal portion 55 is a portion that extends horizontally from the straightening portion 54.

[0016] The molten steel 20 (molten metal) supplied from a ladle (not shown) to the tundish 11 contains 0.08 mass% or more and 0.29 mass% or less of C. Further, when the S content of the molten steel 20 is high, the temperature range in the vicinity of the solid fraction fs = 0.7 to 0.8 described in FIG. 5 expands, and the tensile strength generated in the solidification shell increases. Therefore, the molten steel 20 preferably contains 0.008 mass% or less of S. The lower limit value of the S content is preferably 0.002 mass% from the viewpoints of desulfurization cost and productivity.

[0017] The molten steel 20 in the tundish 11 is poured into the mold 13 from the immersion nozzle 12. The mold 13 is water-cooled, and the outer periphery of the molten steel 20 poured into the mold 13 is cooled (primary cooling) by contacting the mold 13. As a result, a solidification shell 2 is formed on the outer periphery of the molten steel 20.

[0018] With the solidification shell 2 formed on the outside, the molten steel 20 becomes an unsolidified slab 1A directly below the mold 13. The unsolidified slab 1A is supported by a plurality of rolls 14, passes through the vertical portion 51, and is bent in an arc shape while being supported by the rolls 14 at the bending portion 52. Then, after being transferred downstream at the curved portion 53, it is corrected by the rolls 14 in the straightening portion 54 so as to be oriented horizontally. Thereafter, it is transferred downstream by the horizontal portion 55. A secondary cooling spray 15 is arranged between the plurality of rolls 14. The unsolidified slab 1A transferred downstream is cooled (secondary cooling) by the cooling water sprayed from the secondary cooling spray 15. Then, the molten steel 20 is completely solidified into a slab 1 and is pulled out from the continuous casting machine 10 by the pinch roll 16.

[0019] In the process of pulling out this slab 1, the casting speed Vc, which is the speed of pulling out the slab 1, is appropriately set manually or automatically within the range of 0.7 m / min to 1.2 m / min based on the temperature of the molten steel 20 in the tundish 11. However, as described with reference to FIG. 6, when the slab 1 is pulled out at a casting speed Vc of 1.1 m / min or less, internal cracks 100 (see FIG. 4) occur significantly. This is due to the large amount of reheating in the upstream portion of the curved portion 53 as described above.

[0020] Note that the casting speed (m / min) is the horizontal moving speed of the slab 1 on the outlet side of the horizontal portion 55. Also, the upstream portion of the curved portion 53 means a portion that is usually located upstream of the middle position of the curved portion, although it varies depending on the specifications of the apparatus. In particular, temperature control at the position where the solidification shell thickness d on the short side portion side of the slab 1 is about 1 / 4 of the short side width D of the slab 1, that is, the position where d = (1 / 4) × D, is important.

[0021] Therefore, in the present embodiment, in the secondary cooling, when the slab 1 is withdrawn at a casting speed Vc of 1.1 m / min or less, the secondary cooling is performed with a specific water ratio such that the reheating temperature at the upstream portion in the curved portion 53 becomes 15°C or less. The specific water ratio in this case was as shown in Table 1 below. The specific water ratio in Table 1 is the total specific water ratio of a plurality of secondary cooling sprays 15 that inject cooling water at the upstream portion in the curved portion 53, and the water quantity density is the average water quantity density. In Table 1, as comparative examples, the cases where the reheating temperature is set to 55°C or less (Comparative Example 1) and the reheating temperature is set to 35°C or less (Comparative Example 2) are also shown. The temperature of the slab 1 is calculated based on TLL and density due to the composition, heat transfer coefficient, casting speed Vc, molten steel temperature heating degree (T - TLL), nozzle arrangement, and the water quantity of each nozzle.

[0022]

Table 1

[0023] Each specific water ratio shown as an example of the present invention in Table 1 is the specific water ratio for making the reheating temperature at the upstream portion in the curved portion 53 (around a casting length of 7 to 10 m from the molten metal surface in the mold) 15°C or less at the casting speed corresponding to each in the table. Also, these specific water ratios are the specific water ratios for suppressing the internal temperature (temperature in the internal crack generation region) of the slab 1 at the upstream portion in the curved portion 53 from exceeding the γα transformation temperature at the casting speed corresponding to each in the table.

[0024] Figure 2 is a diagram showing the temperature change of the slab when secondary cooling is performed at a casting speed of 0.7 [m / min] among the specific water ratios shown in Table 1. The temperature shown in Figure 2 is the temperature at the position in the internal crack generation region of the slab 1, specifically, the temperature at a position 10 mm inside from the surface of the long side of the slab 1 and 30 mm inside from the surface of the short side of the slab 1. As shown in Figure 2, in the example of the present invention, the reheating temperature at the upstream portion in the curved portion 53 becomes 15°C or less, and the γα transformation temperature is exceeded at the upstream portion of the curved portion 53.

[0025] Figure 3 shows the results of comparing the internal crack generation rates when secondary cooling was performed with water ratio amounts such that the reheating temperature in the upstream portion of the curved portion 53 was 15°C or lower (Example of the present invention), 55°C or lower (Comparative Example 1), and 35°C or lower (Comparative Example 2). Figure 3 is a diagram showing the internal crack generation rates in the example of the present invention and Comparative Examples 1 and 2. In Figure 3, internal cracks were evaluated by visual inspection after cooling. As a result, as shown in Figure 3, it can be seen that internal cracks occurred in Comparative Examples 1 and 2, whereas no internal cracks occurred in the example of the present invention.

[0026] As described above, in the present embodiment, when the slab 1 is withdrawn at a casting speed Vc of 1.1 m / min or less, by performing secondary cooling with a water ratio amount such that the reheating temperature in the upstream portion of the curved portion 53 is 15°C or lower, the occurrence of internal cracks in the slab 1 can be suppressed.

Explanation of reference numerals

[0027] 1 Slab 1A Unsolidified slab 10 Continuous casting machine 11 Tundish 12 Submerged nozzle 13 Mold 14 Roll 15 Secondary cooling spray 16 Pinch roll 20 Molten steel 51 Vertical portion 52 Bending portion 53 Curved portion 54 Straightening portion 55 Horizontal portion

Claims

1. A mold, a vertical part connected to the downstream side of the mold and extending in the vertical direction, a bending part connected to the downstream of the vertical part and having a gradually decreasing radius of curvature toward the downstream, a curved part connected to the downstream of the bending part and having a constant radius of curvature, a correcting part connected to the downstream of the curved part and having a gradually increasing radius of curvature toward the downstream, a horizontal part connected to the downstream of the correcting part and extending in the horizontal direction, which are sequentially provided by a vertical bending mold continuous casting machine, and a method for producing a slab made of steel containing 0.08 to 0.29% by mass of C, comprising: When the slab is drawn out at a casting speed of 1.1 m / min or less, cooling is performed so that the temperature rise of the slab is 15°C or less between directly below the mold and the upstream part of the bending part. A method for continuous casting of slabs.

2. The slab contains 0.002 to 0.008% by mass of S. The method for continuous casting of slabs according to Claim 1.

3. The cooling is performed with the total specific water volume on the upstream side of the bending part being 0.14 L / kg to 0.19 L / kg. The method for continuous casting of slabs according to Claim 1 or Claim 2.

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