Continuous casting method

Abstract

This invention provides a technology to suppress slab defects caused by cooling directly beneath the mold during continuous casting. [Solution] In a continuous casting method for steel, where the secondary cooling water is set to cool at a predetermined flow rate when the secondary cooling water is at a predetermined temperature, the flow rate of the secondary cooling directly below the mold is changed according to the temperature of the secondary cooling water. It is preferable to increase the specific water content of the secondary cooling directly below the mold by 0.5 to 0.9 L / (t-steel·℃) in accordance with the increment in the temperature of the secondary cooling water, and to use medium-carbon steel with a carbon content of 0.08 to 0.16 mass%.

Description

Technical Field

[0001] The present invention relates to a continuous casting method, and particularly to the cooling conditions of a slab by secondary cooling water.

Background Art

[0002] In conventional continuous casting of steel, a method has been carried out to suppress slab corner cracking by controlling the slab temperature from the mold to the correction zone of secondary cooling (see, for example, Patent Document 1). However, cracks may occur at the corner part immediately below the mold due to changes in the cooling water temperature, and countermeasures have been required.

[0003] Therefore, in Patent Document 2, when performing continuous casting of medium carbon steel using a vertical bending type continuous casting facility, cooling water at a water temperature of 32 to 40°C is sprayed onto the slab in the vertical part, bending part, curved part, correction part, and horizontal part where secondary cooling is performed below the mold to cool the slab. A method is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the above prior art has the following problems. In the technology described in the patent document, the determining factor for the water temperature range is to suppress crack generation in the upper correction zone shown by B in FIG. 5. However, regarding slab defects, such as crack generation, in the vertical part immediately below the mold shown by A in FIG. 5, no countermeasures have been taken.

[0006] In continuous casting of steel, secondary cooling water is applied to the area directly below the mold where the cast slab emerges from the mold to lower the surface temperature of the slab, promote solidification, and prevent bulging caused by the static pressure of the molten steel. However, if the temperature of the secondary cooling water is too high, the cooling capacity decreases with the same amount of water as at low temperatures, making it impossible to suppress bulging and potentially leading to internal cracking. On the other hand, if the temperature of the secondary cooling water is too low, the same amount of water may result in excessive cooling, potentially causing surface cracking due to solidification shrinkage.

[0007] This invention was made to solve the above problems and aims to provide a technology that suppresses slab defects caused by cooling directly beneath the mold during continuous casting. [Means for solving the problem]

[0008] The gist of the present invention, which advantageously solves the above problems, is as follows. [1] In a continuous casting method for steel, in which the secondary cooling water is set to cool at a predetermined flow rate when the secondary cooling water is at a predetermined temperature, This is a continuous casting method in which the flow rate of the secondary cooling water is changed according to the temperature of the secondary cooling water. [2] In the above [1], the continuous casting method is characterized in that the specific water content of the secondary cooling directly below the mold is increased by 0.5 to 0.9 L / (t-steel·℃) in accordance with the increase in the temperature of the secondary cooling water. [3] The continuous casting method described in [1] or [2] above, wherein the steel is a medium-carbon steel having a carbon content of 0.08 to 0.16 mass%. [Effects of the Invention]

[0009] According to the continuous casting method of the present invention, by controlling the amount of secondary cooling water according to the temperature of the secondary cooling water, bulging and cast slab defects caused by solidification shrinkage can be suppressed. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram showing a continuous casting apparatus suitable for use in a continuous casting method according to one embodiment of the present invention. [Figure 2] This graph shows the effect of secondary cooling water temperature on the edge hedging rate of the product for secondary cooling pattern b according to the present invention and conventional secondary cooling pattern a. [Figure 3] This graph shows the effect of the secondary cooling pattern on the average value of edge hedging occurrence rate. [Figure 4] This graph shows the effect of the secondary cooling water temperature in the conventional secondary cooling pattern c on the rate of corner cracking in cast slabs. [Figure 5] This is a schematic diagram illustrating the secondary cooling zone of a continuous casting facility. [Modes for carrying out the invention]

[0011] The embodiments of the present invention will be described in detail below. Note that the drawings are schematic and may differ from actual ones. Furthermore, the following embodiments are illustrative examples of equipment and methods for realizing the technical concept of the present invention, and do not limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0012] Figure 1 is a schematic diagram showing a continuous casting apparatus suitable for use in a continuous steel casting method according to one embodiment of the present invention. Reference numeral 1 denotes a ladle for transporting molten steel. Reference numeral 2 denotes a ladle sliding nozzle provided at the bottom of the ladle 1 to control the injection flow rate from the ladle 1. Reference numeral 3 denotes an air seal pipe provided on the ladle sliding nozzle 2 to prevent oxidation of the molten steel injected from the ladle. Reference numeral 4 denotes a tundish that temporarily holds the molten steel injected from the ladle 1 and separates nonmetallic inclusions by flotation. Reference numeral 5 denotes a tundish top nozzle provided at the bottom of the tundish 4 to allow the molten steel to flow out. Reference numeral 6 denotes a sliding nozzle that adjusts the flow rate of molten steel flowing out of the tundish 4. Reference numeral 8 denotes a mold for solidifying the molten steel to form a slab. Reference numeral 7 denotes an immersion nozzle connected to the bottom of the sliding nozzle 6, which immerses its tip in the molten steel inside the mold 8 to inject the molten steel inside the tundish 4. Reference numeral 9 denotes a secondary cooling zone for water cooling the slab. Figure 5 is a schematic diagram illustrating the division of the secondary cooling zone 8 of a vertical bending continuous casting equipment as an example. The secondary cooling zone 9 consists of a vertical zone A, an upper straightening zone B, a curved zone C, a lower straightening zone D, and a horizontal zone E, in order from the position where the cast slab exits the mold.

[0013] In this embodiment, in a continuous casting method for producing slabs from molten steel using the above equipment configuration, the flow rate of the secondary cooling water is changed according to the temperature of the secondary cooling water. In particular, the target area is directly below the mold, that is, from the position where the slab exits the mold (the lower end of the mold) until it advances 500 mm in the casting direction. For example, in a vertical bending continuous casting equipment, this corresponds to the secondary cooling of the vertical band A before the upper straightening band B. The slabs beyond this point have sufficient solidification shell thickness, and it is not necessary to apply the secondary cooling described in this embodiment.

[0014] Directly below the mold, the thickness of the solidified shell of the slab is thin, and due to the static pressure of the molten steel, the slab between the support rolls bulges. So-called bulging needs to be suppressed. If the bulging becomes large, tensile stress is applied inside the solidified shell at the corner part, and there is a risk of internal cracking. To prevent bulging, increasing the cooling capacity of secondary cooling, lowering the surface temperature of the slab, improving the strength of the solidified shell of the slab, and increasing the thickness of the solidified shell are the countermeasures.

[0015] On the other hand, if the slab is strongly cooled directly below the mold, for example, in a steel with a large solidification shrinkage such as medium carbon steel, the corner part becomes supercooled, tensile stress is applied to the corner part of the slab surface, and there is a risk of corner cracking. Corner cracking is likely to occur in medium carbon steel with a large solidification shrinkage. Medium carbon steel has a carbon content of 0.08 to 0.16% by mass.

[0016] When performing secondary cooling on the slab directly below the mold, the water temperature of the secondary cooling water should be in the range of 21 to 34°C, and it is preferable to increase the specific water volume of the secondary cooling directly below the mold by 0.5 to 0.9 L / (t-steel·°C) according to the increase in the water temperature of the secondary cooling water. On the other hand, it is preferable to decrease the specific water volume of the secondary cooling directly below the mold by 0.5 to 0.9 L / (t-steel·°C) according to the decrease in the water temperature of the secondary cooling water. If the range of the specific water volume change per 1°C change in the water temperature of the secondary cooling water is less than 0.5 L / (t-steel·°C), the adjustment of the flow rate with respect to the temperature change is insufficient, and there is a risk of excessive strong cooling at low water temperature and excessive weak cooling at high water temperature. On the other hand, if it exceeds 0.9 L / (t-steel·°C), the flow rate change with respect to the temperature change is too large, and there is a risk of excessive weak cooling at low water temperature and excessive strong cooling at high water temperature.

[0017] When the water temperature of the secondary cooling water is less than the lower limit or exceeds the upper limit of 21 to 34°C, it is difficult to adjust the cooling capacity for the secondary cooling of the slab directly below the mold, so it is preferable to set the water temperature within this range.

[0018] Figures 2 to 3 show the edge heging occurrence rate in cold-rolled products of medium carbon steel in the secondary cooling pattern to which the conventional example (a) and this embodiment (b) are applied. Figure 4 shows the slab corner crack occurrence rate in the secondary cooling pattern to which the conventional example (c) is applied. Edge heging in cold-rolled products is caused by internal cracks generated in the slab corner part due to the bulging of the slab immediately below the mold. In the conventional examples indicated by symbols a and c, a constant specific water volume was used as the secondary cooling water regardless of the water temperature. As is clear from Figure 2, in the conventional example a, when the water temperature is in the range of 21 to 25 °C, the secondary cooling water volume immediately below the mold is sufficient to suppress the bulging of the slab. On the other hand, when the water temperature is 26 °C or higher, the occurrence rate of edge heging increases as the water temperature rises. As the average value of the conventional example a, as shown by symbol a in Figure 3, the edge heging occurrence rate was 7%. Also, as is clear from Figure 4, the conventional example c is sufficient to suppress slab corner cracks due to supercooling at 31 °C or higher. On the contrary, when it is less than 30 °C, the specific water volume of secondary cooling is too much, and slab corner cracks occur frequently due to supercooling of the corner. Here, corner cracks refer to cracks with a length of 1 mm or more occurring.

[0019] In this embodiment indicated by symbol b in Figures 2 and 3, as the water temperature rises in the range of 21 °C to 34 °C, the specific water volume of the secondary cooling zone immediately below the mold was increased by 0.7 L / (t-steel·°C) each time. From the results of Figures 2 to 3, in this temperature range, in the secondary cooling pattern of this embodiment indicated by symbol b, no edge heging was observed. Also, no corner cracks were observed in the slab. Note that the specific water volume of the secondary cooling water in this embodiment b is the same as that of the conventional example a when the water temperature is 21 °C, and the same as that of the conventional example c when the water temperature is 34 °C.

[0020] In the above description, the vertical bending continuous casting equipment was described as an example, but it can also be applied to curved or vertical continuous casting equipment. Also, it can be applied to steels other than medium carbon steel.

Example

[0021] Using the vertical bending continuous casting equipment shown in Figure 1, continuous casting was performed by changing the specific water content of the secondary cooling water for the slab in the secondary cooling zone shown in Figure 5, up to 500 mm in the casting direction from the position where the slab exited the mold, by the specific water content increase shown in Table 1 in relation to the temperature increase of the water temperature. The steel type was medium carbon steel, and steel slabs with a width of 1000 to 1600 mm and a thickness of 235 mm were cast. The steel slabs were hot-rolled and then cold-rolled to produce product steel plates. The water temperature was in the range of 21 to 34°C under all conditions, and the casting speed was in the range of 1.8 to 2.3 m / min.

[0022] The evaluation method for corner cracks in steel slabs was based on the penetrant testing method specified in JIS Z 2343:2017, and the presence or absence of cracks in the corners of the wide surface (length × width) and narrow surface (length × thickness) was evaluated. After applying the developer, surface cracks and defects were checked visually by observing the appearance of the penetrant. In Table 1, the symbol "◎" indicates that no corner cracks were observed, "△" indicates that minor cracks of less than 1 mm were observed, and "×" indicates that cracks of 1 mm or more were observed.

[0023] The edge blemish evaluation of the steel product sheets was as follows: those with an occurrence rate of less than 1% were rated "◎", those with an occurrence rate of 1% or more but less than 3% were rated "△", and those with an occurrence rate of 3% or more were rated "×".

[0024] [Table 1]

[0025] The results in Table 1 show that cracking at the slab corners could be reduced by changing the specific water content directly beneath the mold according to temperature. The rate of edge bends in the product steel plates could be reduced by setting the temperature increment of the specific water content to the range of 0.5 to 0.9 L / (t-steel·℃), and no edge bends were observed in the range of 0.6 to 0.8 L / (t-steel·℃).

[0026] In this specification, the unit of volume "L" is defined as 10 -3 m 3 This means that the unit of mass "t" is a metric ton = 103 The symbol "N" (kg) is used to indicate the volume of a gas, representing the volume at standard conditions: 0°C and 101325 Pa. The numerical range "x~y" means "between x and y," including the boundary value. [Explanation of symbols]

[0027] 1 ladle 2. Pot sliding nozzle 3. Air seal pipe 4 Tan Dish 5. Top nozzle of the tundish 6 Sliding nozzles 7 Immersion nozzle 8. Mold 9. Secondary cooling zone A Vertical band B Upper orthodontic band C Curved Zone D Lower orthodontic band E Horizontal band

Claims

1. In a continuous casting method for steel, where the secondary cooling water is set to cool at a predetermined flow rate when the secondary cooling water is at a predetermined temperature, A continuous casting method in which the flow rate of the secondary cooling water directly below the mold is changed according to the temperature of the secondary cooling water.

2. The continuous casting method according to claim 1, wherein the specific water volume of the secondary cooling directly below the mold is increased by 0.5 to 0.9 L / (t-steel・°C) in accordance with the increment in the temperature of the secondary cooling water.

3. The continuous casting method according to claim 1 or 2, wherein the steel is a medium-carbon steel having a carbon content of 0.08 to 0.16 mass%.

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