Continuous casting equipment and continuous casting method for slabs

Abstract

Provided is cast slab continuous casting equipment capable of inhibiting collision of cooling water jetted from spray nozzles adjacent in the width direction of a cast slab, and of suppressing collapse of a film boiling state.　In the cast slab continuous casting equipment 1 comprising cooling equipment 3 for water-cooling a cast slab S, the cooling equipment 3 has two or more cooling water jetting nozzles 32 disposed in the width direction of the cast slab S, and the two or more cooling water jetting nozzles 32 are disposed such that spray jetting surfaces of the cooling water jetting nozzles 32 adjacent in the width direction of the cast slab S do not overlap.

Description

[Technical Field]

[0001] The present invention relates to a continuous casting facility for slabs and a continuous casting method for slabs. [Background technology]

[0002] In the continuous steel casting process, molten steel is poured into a mold, cooled, and the extracted slab is transported while being cooled at the mold outlet, thereby continuously producing slabs. These slabs are then cut to a predetermined length to produce slabs, blooms, and billets, which are used as rolling materials.

[0003] The slab extracted from the mold contains unsolidified molten steel inside. The unsolidified molten steel is solidified to the center by cooling at the mold outlet, but the characteristics and quality of the slab vary depending on the solidification rate. Therefore, cooling at the mold outlet (hereinafter referred to as secondary cooling) is a process that determines the characteristics and quality of the slab.

[0004] In addition, in recent years, the properties and uniformity required for steel materials have become increasingly strict. Accordingly, the quality requirements for slabs, blooms, and billets have also been increasing year by year, and high-quality casting of slabs across their entire length and width is required. Therefore, it is more important than ever to cool the slab uniformly across its entire width in the continuous casting process, especially in secondary cooling.

[0005] Figure 1 is a graph showing the relationship between the surface temperature of a cast slab and various boiling states during the water-cooling process. As shown in Figure 1, in the initial stage of water-cooling, the slab is cooled in a film boiling state, where a vapor film exists between the slab and the water. In film boiling, the water does not come into direct contact with the slab, resulting in a low heat transfer coefficient (an indicator of cooling capacity) and a gradual decrease in the slab's surface temperature. However, once the slab's surface temperature reaches approximately 700°C, it becomes difficult to maintain a vapor film between the water and the slab, and the slab cools in a transition boiling state, where the water and the slab are in partial contact. When contact occurs, the evaporation of the water in contact with the slab causes the water flow near the slab to intensify, resulting in a rapid increase in the heat transfer coefficient and a rapid decrease in the slab's surface temperature. After this, the slab transitions to a nucleate boiling state, where the slab and water are in steady contact while maintaining a high heat transfer coefficient, and the slab's surface temperature rapidly decreases to the water temperature.

[0006] Generally, the higher the temperature of the object being cooled, the more difficult it is to transition to the nucleate boiling state, and the larger the amount of cooling water required to transition to the nucleate boiling state, which increases production costs. In addition, if a high-temperature object is cooled in the nucleate boiling state, which has a high cooling capacity, large thermal stress is applied to the surface and interior of the object, increasing the risk of defects such as cracks. Therefore, in secondary cooling in the continuous casting process, where the surface temperature of the slab is high, it is common to cool the slab in the film boiling state, which involves a steam film between the slab and the water.

[0007] In view of the above, it is preferable to maintain film boiling over the entire width during secondary cooling in the continuous casting process, because if the vapor film collapses locally or entirely, the slab will transition to nucleate boiling, which has a high cooling capacity, and the temperature deviation between the film boiling and nucleate boiling regions will increase, or the cooling rate will become excessively high, making it impossible to produce a slab of high quality that is uniform over the entire width.

[0008] Possible causes of the collapse of the film boiling state include a local increase in water density, stagnation of cooling water on the upper surface of the slab, and collision of cooling water sprayed from adjacent sprays. Conventionally, attempts have been made to avoid the above causes and maintain the film boiling state in order to cast slabs of uniform quality across the entire width.

[0009] The local increase in water flow density has been addressed by improving the tip of the nozzle that injects the cooling water and flattening the distribution of the cooling water sprayed onto the slab. It is also effective to use a nozzle with a small aspect ratio and a large spray area to prevent localized concentration of cooling water. When the spray injection surface is rectangular, the aspect ratio is the ratio of its short side to its long side. When the spray injection surface is elliptical, the aspect ratio is the ratio of its short axis to its long axis. For square and circular spray injection surfaces, the aspect ratio is 1.

[0010] As a technique for preventing the accumulation of cooling water on a slab, Patent Document 1 discloses a technique in which cooling water is sprayed with the central axis of the spray tilted relative to the central axis of the nozzle, thereby efficiently discharging cooling water that has accumulated between a pair of rolls that support and transport the slab to the outside of the zone and maintaining a film boiling state.As a technique for preventing the collision of cooling water, Patent Document 2 discloses a technique in which cooling water is sprayed with the major axis of the spray spraying surface tilted at an angle θ of 5° or more and less than 45° relative to the width direction of the slab, thereby avoiding the collision of cooling water from adjacent sprays and maintaining a film boiling state. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2018 / 101287 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-255127 Summary of the Invention [Problem to be solved by the invention]

[0012] Patent Document 1 discloses a technology aimed at preventing accumulated water from flowing downstream of vertical and curved bands in continuous casting equipment having vertical and curved bands. However, the technology disclosed in Patent Document 1 is insufficient for removing cooling water accumulated on the slab in horizontal bands, where the surface temperature of the slab drops and film boiling is likely to collapse. Furthermore, even in vertical and curved bands, simply spraying the cooling water upward to prevent it from flowing downstream will result in water stagnation between the roll pairs, increasing temperature deviation in the width direction. Therefore, a technology is needed to smoothly discharge the cooling water sprayed onto the slab outside the slab width, even in vertical and curved bands.

[0013] Even when the technology disclosed in Patent Document 2 is used, if a spray with a low aspect ratio, which is effective in dispersing the water volume distribution and maintaining a film boiling state, is used, the cooling water sprayed from adjacent nozzles may collide.In addition, even when a spray nozzle with a high aspect ratio is used, if the distance between the spray nozzles is short, the cooling water may collide, which promotes the collapse of the film boiling state.

[0014] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide continuous casting equipment and a continuous casting method for slabs that can suppress collision of cooling water sprayed from adjacent spray nozzles in the width direction of the slab and suppress collapse of the film boiling state. [Means for solving the problem]

[0015] The means for solving the above problems are as follows. [1] A continuous casting facility for slabs that is equipped with a cooling facility for water-cooling the slabs, wherein the cooling facility has two or more cooling water injection nozzles arranged in the width direction of the slabs, and the two or more cooling water injection nozzles are arranged so that the spray injection surfaces of adjacent cooling water injection nozzles in the width direction of the slabs do not overlap. [2] The continuous casting equipment for slabs according to [1], wherein the spray injection surface of the cooling water injection nozzle is rectangular or elliptical, and each of the two or more cooling water injection nozzles is arranged within a range that satisfies the following formula (1):

number

number

[10] The method for continuously casting a slab according to [8], wherein the spray injection surface of the cooling water injection nozzle is square, and each of the two or more cooling water injection nozzles is arranged within a range that satisfies the following formula (2): L×sinθ2>t (2) In the above formula (2), L is the installation pitch (m) of the cooling water injection nozzles, θ2 is the angle (°) of one side of the spray injection surface with respect to the width direction, one side of the spray injection surface is the side of the spray injection surface that is closest to the adjacent spray injection surface, and t is the length (m) of the one side.

[11] The method for continuously casting a slab according to [8], wherein the spray injection surface of the cooling water injection nozzle is circular, and each of the two or more cooling water injection nozzles is arranged within a range that satisfies the following formula (3): L>D···(3) In the above formula (3), L is the installation pitch (m) of the cooling water injection nozzles, and D is the diameter (m) of the spray injection surface.

[12] The method for continuously casting slabs according to [9], wherein the aspect ratio of the spray ejection surface is 100 or less.

[13] The continuous casting method according to any one of [8] to

[12] , wherein the cooling rate of the surface layer of the slab in the cooling step is within a range of 0.3°C / sec or more and 100°C / sec or less.

[14] A method for continuous casting of slabs according to any one of [8] to

[13] , wherein the amount of cooling water sprayed from the cooling water spray nozzle and the conveying speed of the slab are controlled to control at least one of the surface cooling rate of the slab and the temperature drop amount of the slab in the cooling process. [Effects of the Invention]

[0016] According to the present invention, collision of cooling water sprayed from adjacent spray nozzles in the width direction of the slab can be suppressed, which suppresses collapse of the film boiling state during secondary cooling of the slab, reduces temperature deviation on the slab surface during cooling, and enables continuous casting of slabs with fewer defects. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a graph showing the relationship between the surface temperature of a slab and each boiling state during the water-cooling process of the slab. [Figure 2] FIG. 2 is a cross-sectional schematic view of a continuous casting facility for slabs according to one embodiment of the present invention. [Figure 3] FIG. 3 is a diagram showing the spray surface of the cooling water sprayed from the cooling water spray nozzle onto the slab S. [Figure 4] FIG. 4 is a schematic diagram showing an example of the configuration of the control device 10. As shown in FIG. [Figure 5] FIG. 5 is a partial cross-sectional schematic view of a continuous casting facility for slabs having a water blowing roll. [Figure 6] FIG. 6 is a partial cross-sectional schematic view of a continuous casting facility for slabs having a purge nozzle. [Figure 7] FIG. 7 is a partial cross-sectional schematic view of a continuous casting facility for slabs having a water blowing roll and a purge nozzle. [Figure 8] FIG. 8 is a cross-sectional schematic view showing another example of continuous casting equipment for slabs according to this embodiment. [Figure 9]FIG. 9 is a cross-sectional schematic view showing another example of continuous casting equipment for slabs according to this embodiment. [Figure 10] FIG. 10 is a cross-sectional schematic view showing another example of continuous casting equipment for slabs according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described below through embodiments of the present invention. The embodiments shown below are examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention is not limited to the following embodiments in terms of the materials, shapes, structures, arrangements, etc. of the components. Furthermore, since the drawings are schematic, it should be noted that the relationships and ratios between thicknesses and planar dimensions may differ from the actual ones, and the relationships and ratios of dimensions may also differ between drawings.

[0019] Fig. 2 is a cross-sectional schematic diagram of a continuous casting facility for slabs according to one embodiment of the present invention. The continuous casting facility for slabs 1 according to one embodiment of the present invention mainly comprises a mold 2 for cooling molten steel poured from a tundish (not shown) to form the outer shell shape of the slab, a cooling facility 3 for cooling the slab extracted from the mold, a thermometer 4 for measuring the temperature of the slab at the outlet of the cooling facility 3, and a control device 10 for controlling the operation of the cooling facility 3. In the following description, the long side surface on the upper right side of the slab in Fig. 2 will be referred to as "side 1," the short side surface on the front side of the page as "side 2," the long side surface on the lower left side as "side 3," and the short side surface on the back side of the page as "side 4."

[0020] [Mold 2] Molten steel produced in a refining facility located separately from the continuous casting facility 1 for slabs is poured into the mold 2. As the poured molten steel is cooled in the mold 2, it solidifies from the contact surface between the molten steel and the mold 2 toward the inner layer, forming an outer shell shape. In the following explanation, the molten steel that has formed the outer shell shape will be referred to as a slab S, including the completely solidified state. The slab S extracted from the mold 2 is cooled in a cooling facility 3 while being supported and transported by slab support rolls 5 installed on the outlet side of the mold 2. A known mold may be used as the mold 2.

[0021] [Cooling equipment 3] The cooling equipment 3 includes a water-cooling device 31 that water-cools the slab S under predetermined cooling conditions. The water-cooling device 31 includes slab support rolls 5 that support and transport the slab S and cooling water injection nozzles 32. The slab support rolls 5 are arranged in pairs on the first and third sides of the slab S at a constant interval in the casting direction. Between adjacent slab support rolls 5 in the casting direction, two or more cooling water injection nozzles 32a on the first side and two or more cooling water injection nozzles 32c on the third side are arranged in pairs with the slab S sandwiched between them at a predetermined pitch in the casting direction. Cooling water W is injected from the cooling water injection nozzles 32 toward the slab S. By feeding the slab S into this cooling equipment 3, the slab S is transported in the casting direction while being secondary-cooled by the cooling water W injected from the cooling water injection nozzles 32. In the following description, a cooling section, consisting of a pair of slab support rolls 5 in the casting direction as one unit, is referred to as a cooling zone, and each cooling section unit is counted as a "zone." In FIG. 2, the cooling zones are depicted as a total of 19 zones, but the number is not limited to this and may be more or less than 19 zones.

[0022] The continuous casting equipment 1 for slabs shown in Fig. 2 is a vertical bending type continuous casting equipment, and is characterized in that the cooling equipment 3 is composed of a vertical band 6 for water-cooling the slab S in a vertical position extracted from the mold 2, a curved band 7 for water-cooling the slab S while bending it with the slab support rolls 5, and a horizontal band 8 for water-cooling the slab S curved by the slab support rolls 5 and placed in a horizontal position. In this embodiment, the description will be given using a vertical bending type continuous casting equipment as an example, but the present invention is not limited to a vertical bending type continuous casting equipment, and can also be applied to a vertical continuous casting equipment having only the vertical band 6 in the cooling equipment 3, a curved continuous casting equipment having only the curved band 7 and the horizontal band 8, and a horizontal continuous casting equipment having only the horizontal band 8.

[0023] The operating parameters of the water cooling device 31 include the amount of cooling water W (cooling water amount) sprayed from the cooling water injection nozzles 32, the amount of compressed air (compressed air amount), and the conveying speed of the slab S. The greater the amount of cooling water, the greater the cooling rate and temperature drop of the slab S. Furthermore, the slower the conveying speed of the slab S, the greater the temperature drop of the slab S. Therefore, by controlling at least one of the conveying speed of the slab S and the amount of cooling water, it is possible to control at least one of the surface cooling rate and temperature drop of the slab S, thereby enabling the casting of a slab S having the desired properties. Furthermore, by adding compressed air to the cooling water W sprayed from the cooling water injection nozzles 32, it is possible to adjust the cooling capacity and the surface distribution of the cooling water W. By adjusting the amount of compressed air, it is possible to maintain optimal conditions for target characteristics, equipment deterioration over time, and even minor changes in nozzle arrangement.

[0024] As an operational parameter of the water cooling device 31, the balance of the amount of cooling water for each cooling zone (for example, increasing the amount of cooling water in the upstream cooling zone and decreasing the amount of cooling water in the downstream cooling zone) may be changed. This allows the cooling rate to be controlled according to the temperature range of the slab S. Furthermore, the number of cooling zones into which cooling water is sprayed may be changed. By changing the number of cooling zones used, the amount of temperature drop of the slab S can be controlled while maintaining the same cooling rate.

[0025] Furthermore, the ratio of the amount of cooling water W sprayed from the cooling water spray nozzles 32a on the first surface side to the amount of cooling water W sprayed from the cooling water spray nozzles 32c on the third surface side may be changed. This makes it possible to control shape defects caused by differences in the amount of temperature drop between the first and third surfaces of the slab S. In addition, the amount of cooling water may be changed depending on the composition of the slab S. This is because the thermal conductivity of the slab S changes depending on the composition, resulting in fluctuations in the cooling state. Furthermore, when changing the amount of cooling water, the amount of compressed air and the conveying speed of the slab S may be changed. The cooling capacity and the surface distribution of the cooling water W can be controlled by changing the amount of compressed air, and the temperature history of the slab S can be controlled by changing the conveying speed of the slab S. This allows for fine-tuning of the cooling conditions to improve the quality of the slab to be cast.

[0026] The operating parameters of the water cooling device 31 may be changed as the casting progresses. In particular, the leading and trailing ends of the slab S are likely to become unsteady parts because they are cooled from their leading and trailing ends. Therefore, fine-tuning the operating parameters of the water cooling device 31 for these parts can contribute to ensuring quality over the entire length and width and improving the yield in the unsteady parts.

[0027] In addition, the operational parameters of the water cooling device 31 may be finely adjusted in accordance with the casting progress in the steady section excluding the leading and trailing ends. Even if the properties of the slab S change in the longitudinal direction in the steady section due to component segregation or other reasons, a slab with uniform properties can be obtained throughout its entire length by finely adjusting the operational parameters of the water cooling device 31.

[0028] The cooling rate can be varied depending on the material, operating conditions, and equipment conditions. If the surface cooling rate exceeds 100°C / sec, the surface often transforms into martensite, causing defects such as cracks on the surface of the slab. Therefore, the surface cooling rate is preferably 100°C / sec or less. If the surface cooling rate is less than 0.3°C / sec, the cooling rate becomes almost the same as natural cooling, resulting in a decrease in production efficiency. In addition, the decrease in cooling rate worsens segregation inside the slab, and the quality of the slab also deteriorates. Therefore, the surface cooling rate is preferably 0.3°C / sec or more.

[0029] As can be seen from the graph shown in Figure 1, film boiling inevitably transitions to nucleate boiling. Therefore, the injection of cooling water W onto the slab S should be stopped when the temperature of the top and bottom surfaces of the slab S reaches 500°C or higher, preferably 600°C or higher. After the injection of cooling water W, if the temperature of the top and bottom surfaces of the slab S again becomes sufficiently high due to internal reheating, cooling water W may be injected again. By increasing the cooling rate through re-injection, production efficiency can be increased and the quality of the slab can be improved.

[0030] It is preferable to use a nozzle that can uniformly spray cooling water at a predetermined flow rate as the cooling water injection nozzle 32. In this embodiment, a spray nozzle is used as the cooling water injection nozzle 32, but it is not limited to a spray nozzle and nozzles such as slit-type nozzles, multi-hole jet nozzles, mist nozzles, and fog nozzles may also be used. Furthermore, the cooling water injection nozzle 32 may be a single-fluid nozzle that sprays only liquid (generally water), or a two-fluid nozzle that sprays a mixed fluid of liquid (generally water) and gas (generally air).

[0031] Although the boiling transition phenomenon does not occur in nozzles that inject only air, they may be arranged in line with or offset from the cooling water injection nozzle 32, and air may be injected from that nozzle to improve drainage and drainage. In addition, it is preferable that the cooling water injection nozzle 32 be a nozzle that can change the amount of cooling water and compressed air in accordance with the target cooling rate.

[0032] When the cooling water W sprayed from the cooling water injection nozzle 32 collides with the slab S, if the cooling water W sprayed from the adjacent cooling water injection nozzle 32 collides with the cooling water W sprayed on the slab S, the horizontal momentum changes to vertical momentum at the collision point, and the vertical downward flow destroys the steam film, causing a local collapse of the film boiling state. Furthermore, in a spray having a rectangular spray injection surface, the horizontal component of the momentum of the cooling water W sprayed in the long axis direction is larger than that of the cooling water W sprayed in the short axis direction. Therefore, it is necessary to avoid collision of the cooling water W sprayed from adjacent cooling water injection nozzles 32 in the long axis direction.

[0033] FIG. 3 is a diagram showing the spray surface of cooling water sprayed from a cooling water injection nozzle on a slab S. FIG. 3(a) is a diagram showing a rectangular spray surface. Consider a cooling water injection nozzle 32 whose spray surface on the slab S is rectangular, with a short side length of t1 (m) and a long side length of t2 (m). Due to geometric relationships, to avoid collisions of the spray surfaces in the long-side direction, the cooling water injection nozzles 32 must be installed so that the spray surfaces of adjacent cooling water injection nozzles 32 in the width direction of the slab S do not overlap. Specifically, the cooling water injection nozzles 32 are installed within a range where the installation pitch of adjacent cooling water injection nozzles 32 in the width direction of the slab S, the angle of the long-side direction with respect to the width direction of the slab, and the length of the short side of the spray surface satisfy the following formula (1). By installing the cooling water injection nozzles 32 within a range that satisfies the following formula (1), collisions of the cooling water W sprayed from adjacent cooling water injection nozzles 32 in the width direction of the slab S in the long-side direction can be suppressed, thereby suppressing the collapse of the film boiling state. The same applies to the case of a cooling water injection nozzle in which the spray injection surface on the slab S is elliptical; simply change the long side direction in the following equation (1) to the long axis direction and the length of the short side to the length of the short axis.

[0034]

number

[0035] When the spray injection surface on the slab S is rectangular or elliptical, the aspect ratio of the injection surface is preferably 100 or less. By using a spray with a small aspect ratio, the spray injection surface becomes wider, which prevents localized concentration of cooling water and allows the film boiling state to be maintained for a longer period of time. The aspect ratio of the spray injection surface is more preferably 50 or less, and even more preferably 30 or less.

[0036] When spraying the same amount of water using nozzles with the same long side length, it is preferable to use a nozzle with a small aspect ratio and a large short axis dimension. A nozzle with a small aspect ratio and a long short side length has a large spray spray injection surface area, which reduces the water flow density and suppresses localized concentration of cooling water. On the other hand, if the aspect ratio is too small, the amount of water flowing in the spray long axis direction decreases, making it easier for cooling water W to stagnate on the slab S. Therefore, the aspect ratio is preferably 2 or more, more preferably 5 or more, and even more preferably 10 or more.

[0037] Furthermore, the angle θ1 in the long side direction is preferably less than 45° so that the cooling water W sprayed onto the slab S can be quickly discharged outside the slab. On the other hand, if the angle θ1 in the long side direction is 45° or more, when the velocity of the cooling water W in the long side direction is decomposed into the slab transport direction and the slab width direction, the velocity component in the transport direction becomes larger than that in the slab width direction, which is undesirable. It is more preferable that the angle θ1 in the long side direction be 30° or less.

[0038] 3(b) is a diagram showing a square spray injection surface. When the spray injection surface on the slab S is square, collision of the cooling water can be suppressed by installing the cooling water injection nozzles 32 within a range in which the installation pitch of adjacent cooling water injection nozzles 32 in the width direction of the slab S, the angle of one side of the spray injection surface with respect to the width direction of the slab, and the length of one side of the spray injection surface satisfy the following formula (2):

[0039] L×sinθ2>t (2) In the above formula (2), L is the installation pitch (m) of the cooling water injection nozzles 32, θ2 is the angle (°) of one side of the spray injection surface with respect to the slab width direction, and t is the length (m) of one side of the spray injection surface. Here, one side of the spray injection surface is the side of the spray injection surface that is closest to the adjacent spray injection surface. That is, in the spray injection surface 20 of FIG. 3(b), the direction of one side is the direction of the side 21 that is closest to the adjacent spray injection surface 22. In addition, in the spray injection surface 22, the direction of one side is the direction of either the side 23 or the side 24 that is closest to the adjacent spray injection surfaces 20 and 25.

[0040] 3(c) is a diagram showing a circular spray injection surface. When the spray injection surface on the slab S is circular, collision of the cooling water can be suppressed by installing the cooling water injection nozzles 32 within a range in which the installation pitch of adjacent cooling water injection nozzles 32 in the width direction of the slab S and the diameter of the spray injection surface satisfy the following formula (3):

[0041] L>D···(3) In the above formula (3), L is the installation pitch (m) of the cooling water injection nozzles 32, and D is the diameter (m) of the spray injection surface.

[0042] The degree of localized concentration of cooling water can also be evaluated by the water flow density, which is defined as flow rate / spray area. Since there is a positive correlation between the water flow density of the sprayed cooling water and the boiling transition temperature, the film boiling state can be maintained for a long time by lowering the water flow density. The localized water flow density of cooling water in the continuous casting equipment 1 for slabs is set to 1000 L / (m 2 × min) or less, and 2 × min), and it is more preferable to keep it below 600 L / (m 2 × min) or less is more preferable.

[0043] [Thermometer 4] The thermometer 4 may be a device that measures the surface temperature of the slab S by scanning the temperature in the width direction of the slab S, or may be a device that is arranged in one or more positions in the width direction of the slab S and measures the surface temperature of the slab S. By using the thermometer 4 to measure the surface temperature of the slab S cooled by the cooling equipment 3, it can be confirmed whether the slab S has been cooled as expected.

[0044] Referring again to Fig. 2, the thermometer 4 is installed on the outlet side of the cooling equipment 3 in Fig. 2, but the thermometer 4 may be installed inside the cooling equipment 3 as long as it is possible to measure the temperature of the steel plate after cooling by the water cooling device 31. In this case, a plurality of thermometers 4 may be installed side by side in the transport direction of the slab S to measure the temperature of the slab S in each cooling zone.

[0045] Furthermore, a thermometer 4 may be installed at the inlet side of the cooling equipment 3 or the inlet side of the mold 2 to measure the initial temperature of the slab S or the pouring temperature of the molten steel. This is because taking into account the temperature measurement results at the inlet side of the cooling equipment 3 improves the accuracy of calculating the cooling rate. One or more thermometers 4 may be installed inside the cooling equipment 3 to measure the temperature of the slab S during water cooling. Taking into account the temperature measurement results inside the cooling equipment 3 improves the accuracy of calculating the cooling rate and also makes it possible to understand the time history.

[0046] Furthermore, by combining the molten steel temperature information and the temperature measurement results of the slab S, heat transfer calculations and heat transfer simulations may be performed to calculate the cooling rate during water cooling and confirm whether the slab S is being cooled as expected. Furthermore, by measuring the in-plane temperature distribution of the slab S during or after water cooling, it may be confirmed whether the slab S is being cooled uniformly. In addition, by measuring the in-plane temperature distribution of the slab S before water cooling, it may be confirmed whether the slab S is being charged into the cooling equipment 3 with a uniform in-plane temperature distribution. Based on these calculation results, the operating parameters of the cooling equipment 3 and the operating conditions of the slab continuous casting equipment 1 may be changed.

[0047] [Control device 10] Next, the control device 10 will be described. FIG. 4 is a schematic diagram showing an example configuration of the control device 10. The control device 10 is an information processing device such as a personal computer. The control device 10 acquires from a host computer 11 information on the molten steel temperature, size information such as the thickness of the slab S, and information on the target range of the cooling amount and the target range of the cooling rate required to obtain the desired material. The control device 10 then calculates the operating conditions of the slab continuous casting equipment 1 to achieve the target cooling amount and the target cooling rate, and determines the operating parameters of each device.

[0048] The control device 10 has a control unit 12 and a memory unit 13. The control unit 12 is, for example, a CPU, and executes a program read from the memory unit 13, causing the control unit 12 to function as a calculation unit 14 and an output unit 15. The memory unit 13 is, for example, an updatable flash memory, a built-in hard disk or a hard disk connected via a data communication terminal, an information recording medium such as a memory card, and a read / write device for the same. The memory unit 13 stores programs for the control unit 12 to execute each function, data used by the programs, and the like.

[0049] The calculation unit 14 performs heat transfer calculations based on the internal model and determines the number of cooling zones to be used, the amount of cooling water, the amount of compressed air, and the conveying speed of the slab S so as to satisfy the target cooling amount and target cooling rate set as the cooling conditions. Command values ​​for the amount of cooling water, the amount of compressed air, and the conveying speed of the slab S determined in this manner are output from the output unit 15 to the water cooling device 31. In the water cooling device 31, commands for the operating pressure and number of cooling water pumps in operation, the operating pressure and number of air compressors in operation, the number of headers provided upstream of the cooling water injection nozzles 32, the opening degree of the flow control valves, and the rotational speed of the slab support rolls 5 are generated based on the command values ​​for the amount of cooling water, the amount of compressed air, and the conveying speed of the slab S, and the operating conditions of the water cooling device 31 are determined.

[0050] Any or all of the number of cooling zones to be used, the amount of cooling water, the amount of compressed air, and the conveying speed of the slab S may be determined in advance using a table based on information such as the composition and size information of the slab S and the target material, and these may be sent as commands to the water cooling device 31. In addition, it is preferable to provide adjustment parameters so that the number of cooling zones to be used, the amount of cooling water, the amount of compressed air, and the conveying speed of the slab S can be changed during operation to respond to changes in the situation during operation.

[0051] [Continuous casting method for slabs] Next, a method for continuously casting a slab using continuous casting equipment 1 for slabs shown in Fig. 2 will be described. First, molten steel produced in a refining facility located separately from the continuous casting equipment 1 for slabs is poured into a mold 2 via a tundish (not shown). The poured molten steel is cooled in the mold 2 and solidifies from the contact surface between the molten steel and the mold 2 toward the inner layer, forming the outer shell shape of the slab S.

[0052] The slab S extracted from the mold 2 is supported by slab support rolls 5 installed on the outlet side of the mold 2 and cooled in the cooling equipment 3 while being transported. In this cooling process, the number of zones to be used, the amount of cooling water, the amount of compressed air, and the transport speed are calculated and set by the control device 10 according to the size of the slab S and the target characteristics of the slab S, but in this embodiment, the slab S is cooled by injecting water and air from all of the zones shown in Fig. 2.

[0053] A predetermined amount of cooling water W and compressed air A are sprayed from 19 pairs of cooling water spray nozzles 32, and the strand support rolls 5 are rotated at a predetermined speed. These parameters are set by the control device 10 so that the target strand characteristics are obtained, and commands are sent to the cooling water spray nozzles 32 and the strand support rolls 5. By cooling the strand S in the cooling equipment 3, it is possible to cast a strand S with the desired characteristics. After this cooling process, the strand S is sent to subsequent processes.

[0054] While the embodiments of the present invention have been described above, the present invention is not limited thereto and various modifications and improvements can be made. Fig. 5 is a partial cross-sectional schematic diagram of a continuous casting facility for a slab having a water-removing roll. As shown in Fig. 5, a water-removing roll 33 may be installed on the outlet side of a water-cooling device 31 of the continuous casting facility 1 for a slab to remove the cooling water W accumulated on the slab S. This prevents the slab S from being locally or entirely cooled by the cooling water W accumulated on the slab S, which would prevent the desired amount of cooling and, further, the desired properties from being locally or entirely obtained.

[0055] To obtain good drainage, the pressing force of the blowing roll 33 against the slab S is preferably 4 tons or more. The pressing force of the blowing roll 33 against the slab S is more preferably 6 tons or more, and even more preferably 8 tons or more. On the other hand, if the pressing force of the blowing roll 33 against the slab S is too large, the blowing roll 33 will bend due to elastic deformation, creating a gap between the slab S and the blowing roll 33 and deteriorating drainage. For this reason, the pressing force of the blowing roll 33 against the slab S is preferably 20 tons or less.

[0056] The mechanism for applying a pressing force by the water-removing roll 33 may be a spring type mechanism or a mechanism capable of applying a constant pressing force, such as air pressure or hydraulic pressure. In order to adjust the deflection of the water-removing roll 33, a mechanism capable of maintaining a constant pressing force is preferred, and it is further preferred to use a mechanism capable of changing the pressing force in the longitudinal direction of the slab S.

[0057] The slab support rolls 5 may also serve as draining rolls. In this case, the pressing force of the slab support rolls 5 is not limited to the above range. This is because a higher quality slab may be obtained by rolling down the slab S with the slab support rolls 5.

[0058] FIG. 6 is a partial cross-sectional schematic diagram of a continuous casting facility for slabs having a purge nozzle. As shown in FIG. 6, a purge nozzle 34 may be provided instead of the water-removing roll 33, and a water-removing purge 35 may be sprayed to remove the cooling water W remaining on the slab S. The water-removing purge 35 may be a liquid, a gas, or a mixture thereof. However, if a liquid is used as the water-removing purge 35, the sprayed portion may be cooled, which may increase the in-plane temperature deviation of the slab S. For this reason, it is preferable to use a gas as the water-removing purge 35. From the viewpoint of production costs, it is more preferable to use air as the water-removing purge 35.

[0059] 7 is a partial cross-sectional schematic diagram of a continuous casting facility for slabs having a blower roll and a purge nozzle. As shown in FIG. 7, a blower roll 33 and a purge nozzle 34 may be used together. Alternatively, the blower roll 33 and / or the purge nozzle 34 may be disposed on the inlet side of the water-cooling device 31 to cut off the cooling water W leaking from the water-cooling device 31. This prevents a decrease in the temperature of the slab S at the time of delivery to the cooling device 3 and prevents the cooling water W from flowing into other devices (e.g., the mold 2) located upstream of or around the cooling device 3.

[0060] Furthermore, not only at the inlet and outlet sides of the water cooling device 31, but also at the inlet and outlet sides of each cooling zone, one or both of the water blowing rolls 33 and the purge nozzles 34 may be arranged to separate the cooling zones. When different amounts of water are sprayed in each cooling zone, separating the zones into zones with different amounts of cooling water makes it possible to grasp the temperature history of the steel sheet.

[0061] Furthermore, it is not necessary for all of the cooling water injection nozzles 32 of the cooling equipment 3 to satisfy the above formula (1), (2), or (3); it is sufficient that two or more cooling water injection nozzles 32 adjacent to each other in the slab width direction satisfy the above formula (1), (2), or (3). This makes it possible to suppress collision of cooling water and to suppress collapse of the film boiling state more effectively than in a continuous casting equipment for slabs that does not have any cooling water injection nozzles 32 that satisfy the above formula (1), (2), or (3).

[0062] FIG. 8 is a cross-sectional schematic diagram showing another example of a continuous casting system for slabs according to the present embodiment. As shown in FIG. 8 , in a continuous casting system for slabs 40, cooling water injection nozzles 36 capable of injecting cooling water at a rate sufficient to bring the boiling state to a nucleate boiling state are installed on both the inlet and outlet sides of the water cooling device 31, and the slab S is cooled using these nozzles according to the target characteristics of the slab S. In this manner, cooling water injection nozzles 36 for injecting cooling water at a rate sufficient to bring the boiling state to a nucleate boiling state may be installed on both the inlet and outlet sides of the water cooling device 31. While FIG. 8 illustrates an example in which cooling water injection nozzles 36 are installed on both the inlet and outlet sides of the water cooling device 31, this is not a limitation, and the cooling water injection nozzles 36 may be installed on either the inlet or outlet side. Furthermore, although FIG. 8 illustrates an example in which cooling water injection nozzles 36 are installed in three zones on the inlet side and three zones on the outlet side of the water cooling device 31, this is not a limitation, and the number of cooling zones in which the cooling water injection nozzles 36 are installed may be more than three.

[0063] FIG. 9 is a cross-sectional schematic diagram showing another example of continuous casting equipment for slabs according to this embodiment. As shown in FIG. 9 , in continuous casting equipment for slabs 50, a cooling water injection nozzle 32 and a cooling water injection nozzle 36 capable of injecting a quantity of cooling water sufficient to bring the boiling state to a nucleate boiling state are arranged in the same cooling zone. In this manner, the cooling water injection nozzle 32 and the cooling water injection nozzle 36 may be arranged in the same cooling zone. By combining the cooling by the cooling water injection nozzle 32 and the cooling by the cooling water injection nozzle 36, cooling with a more diverse temperature history can be achieved. While FIG. 9 shows an example in which the cooling water injection nozzle 32 and the cooling water injection nozzle 36 are arranged in the same cooling zone in two zones, the arrangement is not limited to this, and other zones may be used.

[0064] FIG. 10 is a cross-sectional schematic diagram showing another example of a continuous casting system for slabs according to this embodiment. As shown in FIG. 10, in a continuous casting system for slabs 60, cooling water injection nozzles 37 are arranged at some of the cooling water injection nozzles in the water cooling device 31, injecting only the amount of water required to bring the boiling state to a nucleate boiling state. In this manner, the cooling water injection nozzles 37 may be arranged at some of the cooling water injection nozzles in the water cooling device 31. This is because, if the portion other than the cooling water injection nozzles 37 can be considered the water cooling device 31 and the collision of cooling water can be suppressed in this portion, the collapse of the film boiling state can be suppressed. The cooling water injection nozzles 36 capable of injecting the amount of water required to bring the boiling state to a nucleate boiling state may be a single-fluid nozzle that injects only air, a single-fluid nozzle that injects only water, or a dual-fluid nozzle that injects a mixture of water and air.

[0065] The slab S extracted from the mold 2 is generally conveyed while being cooled and reduced by the slab support rolls 5. This is because reducing the slab S reduces internal segregation and improves the quality of the slab. Therefore, the continuous casting method for slabs according to this embodiment may be combined with a known slab reduction technique, thereby further improving the quality of the slabs produced. In this case, it is preferable to couple the operating parameters of the water cooling device 31 with the operating parameters of the known slab reduction technique.

[0066] Furthermore, in the continuous casting equipment 1 for slabs according to this embodiment, an example has been shown in which the slab support rolls 5 are arranged on the first and third surface sides of the slab S, but this is not limiting, and the slab support rolls 5 may also be arranged on the second and fourth surface sides. By supporting and pressing down the second and fourth surface sides with rolls, it is possible to suppress expansion in the width direction that occurs under pressure from the slab support rolls 5 on the first and third surface sides. [Example]

[0067] Next, an example will be described in which a slab S continuously cast using the continuous casting equipment 1 for slabs shown in Figure 2 was cooled in a cooling equipment 3 to produce a slab for rolling. In the continuous casting equipment 1 for slabs, the cooling equipment 3 is located downstream of the mold 2, and inside it are arranged 19 pairs of cooling water injection nozzles 32 that constitute a water cooling device 31, and 20 pairs of slab support rolls 5. The cooling water injection nozzles 32 have a structure in which rectangular spray nozzles, square spray nozzles, and circular spray nozzles can be interchangeably attached, and these nozzles were changed depending on the casting conditions.

[0068] A thermometer 4 was installed 5 m downstream from the outlet of the cooling equipment 3, and the temperature distribution of the surface layer in the width direction of the slab S after passing through the cooling equipment 3 was measured. The temperature deviation value within the slab S was evaluated by subtracting the maximum value from the minimum value of the width direction temperature distribution measured by the thermometer 4. A temperature deviation value of less than 50°C was deemed acceptable. Furthermore, the cast slab S was cut into slabs, and microcracks on the slab surface were investigated in a downstream inspection. The number of segregated grains at the center of the slab thickness was also investigated. Furthermore, the slab was inspected for defects after hot and cold rolling in a downstream process. Furthermore, a heat transfer simulation was performed based on the temperature measurement results from the thermometer 4, and the surface cooling rate of the slab S was calculated. The casting conditions and evaluation results for the slab S in this example are shown in Table 1 below. In Table 1, the angle is θ1 for a rectangle and θ2 for a square. The length is t1 for a rectangle, t for a square, and D for a circle.

[0069] [Table 1]

[0070] The cast slab evaluation results for Examples 1-6 all passed. High-quality slabs were obtained uniformly across the entire width and length. No defects were found in the cold-rolled steel strip, making them suitable for shipping. In Example 4, no defects were found in the cold-rolled steel strip, and the temperature deviation was less than 50°C. However, the temperature deviation was larger than that of Example 2. This is thought to be because the increased spray angle θ1 increased the velocity component of the spray water in the direction of travel. This prevented the cooling water sprayed onto the slab from being quickly discharged outside the slab, resulting in localized overcooling. Furthermore, even under the same conditions, the temperature deviation in the width direction was smaller when using a rectangular spray nozzle than when using a circular or square spray nozzle. This result is thought to be due to the fact that the rectangular spray nozzle allowed the cooling water W, which flows faster in the longitudinal direction, to be quickly discharged outside the width of the slab S.

[0071] When a rectangular spray nozzle was used, the temperature deviation was smallest when the spray nozzle had an aspect ratio of 30 for the spray injection surface. A small aspect ratio reduces the drainage effect of the cooling water W, while a large aspect ratio causes the cooling water W to concentrate locally, increasing the temperature deviation. These results confirmed that there is an optimal value for the aspect ratio of the spray injection surface.

[0072] Inventive Examples 7-9 were cast using rectangular, square, and circular spray nozzles with a high water flow rate. In Inventive Example 7-9, the temperature deviation was less than 50°C, passing the test, but several small cracks occurred on the slab surface. These small cracks are thought to be due to an excessive cooling rate, which caused the surface layer of the slab S to transform into martensite. However, because the temperature deviation in Inventive Example 7-9 was acceptable, no defects were found in the steel strip after cold rolling, making it a shippable product.

[0073] Inventive Example 10-12 is a casting example in which a rectangular, square, or circular spray nozzle was used and the water flow density was reduced. In Inventive Example 10-12, the temperature deviation was less than 50°C, which was acceptable, but the number of segregated grains at the center of the slab thickness increased. This increase in the number of segregated grains at the center of the slab thickness is thought to be due to an excessively low cooling rate, which reduced the temperature gradient inside the slab S. However, because the temperature deviation in Inventive Example 10-12 was acceptable, no defects were found in the steel strip after cold rolling, making it a shippable product.

[0074] Comparative Example 1 is a casting example in which a flat spray nozzle was used, and the spray angle was reduced so that the above formula (1) was no longer satisfied. In Comparative Example 1, the temperature deviation was 90°C, making it impossible to produce a slab that was uniform across the entire width, and defects occurred in the steel strip after rolling. For this reason, the steel strip produced from this slab could not be shipped. This is thought to be because the cooling water W sprayed from adjacent nozzles interfered with each other, causing the film boiling state to collapse in that area, thereby improving the cooling capacity.

[0075] Comparative Example 2 is a casting example in which a rectangular spray nozzle was used, and the spray angle was increased so that the above formula (1) was no longer satisfied. In Comparative Example 2, the temperature deviation was 218°C, making it impossible to produce a slab that was uniform across the entire width, and defects occurred in the steel strip after rolling. For this reason, the steel strip produced from this slab could not be shipped. This is thought to be because the cooling water W above the slab S could not be discharged outside the width of the slab S, and the cooling water W remained above the slab S, causing a local transition from film boiling to nucleate boiling.

[0076] Comparative Examples 3 and 4 are casting examples in which square or circular spray nozzles were used, and the nozzle installation pitch was shortened, so that the above formula (2) or (3) was no longer satisfied. In Comparative Examples 3 and 4, the temperature deviation was 86°C and 92°C, respectively, making it impossible to produce a slab that was uniform across the entire width, and defects occurred in the steel strip after cold rolling. For this reason, the steel strip produced from this slab could not be shipped. This is thought to be because the cooling water W sprayed from adjacent spray nozzles interfered with each other, and the cooling capacity was improved only in that area. [Explanation of symbols]

[0077] 1. Continuous casting equipment for slabs 2. Mold 3 Cooling equipment 4 thermometer 5. Casting support roll 6 Vertical Strips 7 Curved Band 8 Horizontal band 10 Control device 11 Host Computer 12 Control Unit 13 Storage section 14 Arithmetic section 15 Output section 20 spray surface 21 sides 22 spray surface 23 sides 24 sides 25 spray surface 31 Water cooling system 32 Cooling water injection nozzle 32a Cooling water injection nozzle 32c Cooling water injection nozzle 33 Draining Roll 34 Purge nozzle 35 Drain purge 36 Cooling water injection nozzle 37 Cooling water injection nozzle 40 Continuous casting equipment for slabs 50 Continuous casting equipment for slabs 60 Continuous casting equipment for slabs S slab W Cooling water

Claims

1. A continuous casting facility for cast slabs, comprising a cooling facility for water-cooling the slabs, the cooling equipment has two or more cooling water injection nozzles arranged in the width direction of the slab, The spray injection surface of the cooling water injection nozzle is rectangular or elliptical, the two or more cooling water injection nozzles are arranged so that spray injection surfaces of adjacent cooling water injection nozzles in the width direction of the slab do not overlap; Each of the two or more cooling water injection nozzles is arranged within a range that satisfies the following formula (1), The aspect ratio of the spray ejection surface is 10 or more and 50 or less. [Equation 1] In the above formula (1), L is the installation pitch (m) of the cooling water injection nozzles, and θ 1 is is the angle (°) of the long side or major axis direction of the spray ejection surface with respect to the width direction, and t 1 is the length (m) of the short side or minor axis of the spray ejection surface.

2. A continuous casting method for a cast slab, comprising a cooling step of water-cooling the cast slab in a cooling facility, the cooling equipment has two or more cooling water injection nozzles arranged in a width direction of the slab, and in the cooling step, cooling water is injected from the two or more cooling water injection nozzles to cool the slab; The spray injection surface of the cooling water injection nozzle is rectangular or elliptical, the two or more cooling water injection nozzles are arranged so that spray injection surfaces of adjacent cooling water injection nozzles in the width direction of the slab do not overlap; Each of the two or more cooling water injection nozzles is arranged within a range that satisfies the following formula (1), The aspect ratio of the spray ejection surface is 10 or more and 50 or less. [Equation 2] In the above formula (1), L is the installation pitch (m) of the cooling water injection nozzles, θ 1 is the angle (°) of the long side or major axis direction of the spray injection surface relative to the width direction, and t 1 is the length (m) of the short side or minor axis of the spray injection surface.

3. 3. The method for continuously casting a slab according to claim 2, wherein the cooling rate of the surface layer of the slab in the cooling step is within a range of 0.3°C / sec to 100°C / sec.

4. 4. The method for continuously casting a slab according to claim 2, wherein at least one of a surface cooling rate of the slab and a temperature drop amount of the slab in the cooling step is controlled by controlling the amount of cooling water sprayed from the cooling water spray nozzles and a transport speed of the slab.

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