A secondary cooling method for improving the surface quality of an ultra-thick slab

Abstract

The application discloses a secondary cooling method for improving the surface quality of an ultra-thick slab, and produces a continuous casting slab with a thickness of 400 mm or above. The method adopts an optimized cooling density of the casting slab, and the cooling density gradually decreases from top to bottom according to the solidification characteristics of the casting slab, so that the surface temperature of the casting slab uniformly decreases in the length direction, and the periodic rise of the surface temperature of the casting slab and the repeated phase change of the solidified shell are avoided, thereby preventing the subcutaneous cracks of the casting slab. Meanwhile, the cooling water amount of the edge part and the center area of the casting slab is controlled in an optimal proportion range, so that the temperature uniformity of the casting slab in the width direction is ensured, and the undercooling problem of the edge corner part of the casting slab caused by the fast heat transfer is improved. The temperature of the corner part of the casting slab is prevented from being too low to generate transverse cracks under the action of a straightening force. Meanwhile, the optimized cooling water and compressed air pressure values and pressure ratio values of each area are set, so that the atomization effect of the cooling water is ensured, the water accumulation problem of the casting slab surface is improved, and the surface cooling uniformity of the casting slab is improved.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and more specifically to a secondary cooling method for improving the surface quality of ultra-thick slabs. Background Technology

[0002] Continuous casting is the process by which molten steel gradually cools into a solid billet as heat is transferred outwards. This process is continuous and ongoing, hence the name continuous casting. After entering the crystallizer, the molten steel is cooled by the water-cooled copper walls, gradually forming a billet with a fixed cross-sectional shape and a certain shell thickness. At this point, the billet, still containing molten steel at its core, is continuously pulled out from the bottom of the crystallizer and enters the secondary cooling zone. Secondary cooling involves spraying water or atomized water at a certain pressure directly onto the surface of the high-temperature billet, allowing the heat inside the billet to be continuously transferred out until the billet completely solidifies.

[0003] Heat transfer in the secondary cooling zone is a crucial part of the continuous casting process. An inadequate cooling regime can lead to defects such as surface cracks, internal cracks, bulging deformation, and rhomboid deformation in the billet. The formation of center segregation and center cracks in the billet is also closely related to the secondary cooling system.

[0004] In current slab continuous casting machines, the secondary cooling zone encompasses the entire slab casting area along the length of the casting process, including the foot rolls, bending section, arc section, straightening section, and horizontal section after the slab exits the crystallizer. The width of the secondary cooling zone is equal to the width of the casting equipment, exceeding the width of the cast slab to ensure cooling of the slab with the maximum cross-section. The secondary cooling zone is typically divided into several cooling loops for control, ensuring the cooling requirements of slabs of different steel grades.

[0005] Uneven cooling along the length of the secondary cooling zone causes a periodic rise in the surface temperature of the billet, leading to repeated phase transformations in the solidified shell and contributing to subcutaneous cracks. An improper secondary cooling process, such as overcooling the billet and placing it in the brittle temperature range during straightening, makes it prone to transverse cracks at the troughs of oscillation marks under straightening forces. Localized intense cooling can also create surface tension, resulting in surface cracks.

[0006] Extra-thick plates are widely used in various fields such as cross-sea bridges, military industry, shipbuilding, nuclear power, chemical industry, large machinery, hydropower stations, and marine engineering. The use of large-section continuous casting billets to replace traditional ingot casting for the production of extra-thick steel plates has become a development trend. It has many advantages such as short process flow, high steel yield, and high steel plate yield, which can greatly improve production efficiency and reduce production costs.

[0007] As the thickness of the billet increases, controlling the uniformity of secondary cooling becomes more difficult. Traditional secondary cooling processes for slabs are no longer suitable for the surface quality requirements of extra-thick slabs with a thickness of 400mm or more. Therefore, developing a secondary cooling method suitable for extra-thick slabs has become extremely important. Summary of the Invention

[0008] In view of this, the present invention provides a secondary cooling method for improving the surface quality of ultra-thick slabs. It employs an optimized cooling water density for the slab, gradually decreasing from top to bottom according to the solidification characteristics of the slab, ensuring a uniform temperature drop across the slab's surface along its length. Simultaneously, the cooling water volume at the slab's edges and center is controlled within an optimal ratio range to ensure temperature uniformity across the slab's width, mitigating the overcooling problem caused by rapid heat transfer at the slab's corners. Furthermore, optimized cooling water and compressed air pressure values ​​and their ratios are set for each zone to ensure effective cooling water atomization while addressing surface water accumulation and improving the cooling uniformity of ultra-thick slabs (thicker than 400 mm), resulting in ultra-thick slabs with excellent surface quality.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A secondary cooling method for improving the surface quality of ultra-thick slabs, wherein the secondary cooling zone comprises a bending section, an arc section, a straightening section, and a horizontal section arranged sequentially.

[0011] The cooling water density of the curved section is 40 L / min·m 2 Gradually decrease to 20 L / min·m 2 ;

[0012] The cooling water density of the arc-shaped section is 10 L / min·m 2 Gradually decrease to 4 L / min·m 2 ;

[0013] The ratio of cooling water volume between the inner and outer arc sides of the arc segment is 1:1.2 to 1:1.5, and gradually increases.

[0014] The ratio of cooling water volume at the center to the edge of the curved section to the front section of the arc section is 3:1. The cooling water at the edge of the rear section of the arc section is turned off, and air cooling is used.

[0015] The pressure ratio of cooling water to compressed air in the curved section is 1.4, and the pressure ratio of cooling water to compressed air in the arc section is 1.5.

[0016] Preferably, the curved section is divided into three zones: zone 2, zone 3, and zone 4, wherein the cooling water density in zone 2 is 40 L / min·m³. 2 The cooling water density in the three zones is 30 L / min·m³. 2 The cooling water density in the four zones is 20 L / min·m³. 2 .

[0017] Furthermore, the total length of the curved section is 4.23m, and the lengths of zones 2, 3, and 4 are 0.54m, 1.55m, and 2.14m, respectively.

[0018] Preferably, the arc segment is divided into 5, 6, 7, 8, 9, 10 and 11 zones in sequence, wherein zones 5, 6, 7 and 8 are the front section of the arc segment, and zones 9, 10 and 11 are the rear section of the arc segment.

[0019] Preferably, the cooling water pressure of the curved section is 0.35 MPa, and the cooling water pressure of the arc section is 0.30 MPa; the compressed air pressure of the curved section is 0.25 MPa, and the compressed air pressure of the arc section is 0.20 MPa.

[0020] Preferably, the slab drawing speed is 0.45 m / min.

[0021] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a secondary cooling method for improving the surface quality of ultra-thick slabs, which has the following beneficial effects:

[0022] This invention, within the existing equipment and process layout, controls the cooling water density in each zone of the secondary cooling area of ​​the billet, maintains appropriate water ratios between the inner and outer arcs and between the center and edges of each cooling zone, and controls the cooling water pressure, compressed air pressure, and a reasonable cooling water to compressed air pressure ratio in each zone. This ensures a uniform decrease in the surface temperature of the billet along its solidification length, preventing periodic temperature rises and repeated phase transformations in the solidified billet shell that could lead to subcutaneous cracks. Furthermore, the invention achieves a uniform surface temperature distribution along the width of the billet, mitigating the problem of undercooling at the corners and preventing transverse cracks caused by excessively low corner temperatures under straightening forces. This invention is simple, reliable, easy to implement, and suitable for widespread use. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the secondary cooling zone of the continuous casting machine according to the present invention;

[0025] Figure 2 This is a diagram showing the original process for producing the cast billet in Example 1;

[0026] Figure 3 Transverse cracks on the edge of the cast billet were produced using the original process in Example 1.

[0027] Figure 4 The diagram shows the production of the billet using the new process after adjustments to Example 1. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] As attached Figure 1 As shown, the secondary cooling zone of the continuous casting machine includes a bending section 1, an arc section 2, a straightening section 3, and a horizontal section 4 arranged sequentially. The bending section 1 is divided into zones 2, 3, and 4, with a total length of 4.23m. Zones 2, 3, and 4 have lengths of 0.54m, 1.55m, and 2.14m, respectively. The arc section 2 is divided into zones 5, 6, 7, 8, 9, 10, and 11, respectively. The straightening section 3 is zone 12, and the horizontal section 4 is divided into zones 13 and 14.

[0030] The secondary cooling method for improving the surface quality of ultra-thick slabs in this invention involves producing a 450mm thick continuously cast slab at a constant casting speed of 0.45m / min. The optimal cooling density of the slab cooling water is used, and the cooling density gradually decreases from top to bottom according to the solidification characteristics of the slab, ensuring that the surface temperature of the slab decreases uniformly along its length, thereby obtaining an ultra-thick slab with good surface quality.

[0031] The cooling water density in Zone 2 is 40 L / min·m³. 2 Cooling water density in Zone 3 is 30 L / min·m³ 2 Cooling water density in Zone 4 is 20 L / min·m³ 2 The temperature decreases sequentially from top to bottom. By controlling the density of the cooling water, the surface temperature of the billet decreases uniformly.

[0032] The density of cooling water in the arc section is 10 L / min·m 2 Gradually decrease to 4 L / min·m 2 Furthermore, the water accumulation effect on the inner arc of the billet becomes increasingly pronounced as the curvature of the billet changes. Therefore, while reducing the amount of cooling water in the inner arc, the proportion of water in the outer arc is appropriately increased, and the ratio of water in the inner and outer arcs of the billet's curved section is controlled between 1:1.2 and 1:1.5, and gradually increased.

[0033] The ratio of water volume at the center and edge of the inner and outer arcs in zones 2 to 8 is controlled at 3:1. Cooling water is turned off at the inner and outer arc edges of the billet in zones 9 to 11, and air cooling is used. Zones 12 to 14 are for indirect secondary cooling of the billet. A small amount of mist cooling water is sprayed onto the surface of the fan-shaped section roller to cool the rollers. The heat on the surface of the billet is carried away through the contact between the roller and the billet.

[0034] The cooling water pressure in the bending section is controlled at 0.35 MPa, and the compressed air pressure in the bending section is controlled at 0.28 MPa. The cooling water pressure in the arc-shaped section is controlled at 0.30 MPa, and the compressed air pressure in the arc-shaped section is controlled at 0.25 MPa. This achieves the best secondary cooling effect for the billet and ensures uniform cooling of the billet surface.

[0035] The following is a description using specific embodiments.

[0036] Example 1

[0037] The production process involves casting billets with a cross-section of 450mm × 2270mm, using low-alloy steel Q345R, with a casting speed of 0.45m / min and a superheat of 10–20℃ in the tundish. The original cooling water density, inner and outer arc cooling water ratio, and center to edge cooling water ratio for each zone are shown in Table 1. The adjusted cooling water density, inner and outer arc cooling water ratio, and center to edge cooling water ratio for each zone are shown in Table 2. The original cooling water and compressed air pressures for each zone are shown in Table 3. The adjusted cooling water and compressed air pressures for each zone are shown in Table 4.

[0038] Table 1. Cooling density, water ratio between inner and outer arcs, and water ratio between center and edge of each zone.

[0039]

[0040] Table 2 shows the adjusted cooling density, water ratio between the inner and outer arcs, and water ratio between the center and the edge of each zone.

[0041]

[0042] Table 3 Original Cooling Water and Compressed Air Pressures for Each Zone

[0043] Secondary cooling zone Zones 2-4 5-11 districts Cooling water pressure (MPa) 0.45 0.45 Compressed air pressure (MPa) 0.35 0.35 Water-air pressure ratio 1.29 1.29

[0044] Table 4 shows the adjusted cooling water and compressed air pressures for each zone.

[0045] Secondary cooling zone Zones 2-4 5-11 districts Cooling water pressure (MPa) 0.35 0.3 Compressed air pressure (MPa) 0.25 0.2 Water-air pressure ratio 1.4 1.5

[0046] Example 2

[0047] The production process involves casting billets with a cross-section of 450mm × 2070mm, using low-alloy steel Q355D, with a casting speed of 0.45m / min and a superheat of 12–20℃ in the tundish. The original cooling water density, inner and outer arc cooling water ratio, and center to edge cooling water ratio for each zone are shown in Table 5. The adjusted cooling water density, inner and outer arc cooling water ratio, and center to edge cooling water ratio for each zone are shown in Table 6. The original cooling water and compressed air pressures for each zone are shown in Table 7. The adjusted cooling water and compressed air pressures for each zone are shown in Table 8.

[0048] Table 5 shows the original cooling density, water ratio between the inner and outer arcs, and water ratio between the center and the edge of each zone.

[0049]

[0050]

[0051] Table 6 shows the adjusted cooling density, inner and outer arc water ratio, and center and edge water ratio for each zone.

[0052]

[0053] Table 7 Original Cooling Water and Compressed Air Pressures for Each Zone

[0054] Secondary cooling zone Zones 2-4 5-11 districts Cooling water pressure (MPa) 0.45 0.45 Compressed air pressure (MPa) 0.35 0.35 Water-air pressure ratio 1.29 1.29

[0055] Table 8. Adjusted Cooling Water and Compressed Air Pressures for Each Zone

[0056] Secondary cooling zone Zones 2-4 5-11 districts Cooling water pressure (MPa) 0.35 0.3 Compressed air pressure (MPa) 0.25 0.2 Water-air pressure ratio 1.4 1.5

[0057] Example 1: The original process for producing cast billets is shown in the image. Figure 2-3 The cast billet products produced by the modified process of this invention are shown below. Figure 4 , Figure 2-4 This indicates that the casting produced using the original process exhibits extremely uneven surface cooling, with severe overcooling at the edges and corners. After exiting the fan-shaped section, a significant temperature difference is visually apparent (see...). Figure 2 Meanwhile, numerous surface quality issues were observed in the cast billet, with clearly visible transverse cracks at the edges after flame grinding (see...). Figure 3 With the new process, the overall surface temperature of the cast billet increased and cooling became more uniform, with a significant increase in corner temperature. Figure 4 No transverse cracks occurred on the surface of the cast billet after grinding.

[0058] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0059] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A secondary cooling method for improving the surface quality of ultra-thick slabs, wherein the secondary cooling zone comprises a bending section, an arc-shaped section, a straightening section, and a horizontal section arranged sequentially, characterized in that, The cooling water density of the curved section is 40 L / min·m 2 Gradually decrease to 20 L / min·m 2 ; The cooling water density of the arc-shaped section is 10 L / min·m 2 Gradually decrease to 4 L / min·m 2 ; The ratio of cooling water volume between the inner and outer arc sides of the arc segment is 1:1.2 to 1:1.5, and gradually increases. The ratio of cooling water volume at the center to the edge of the curved section to the front section of the arc section is 3:

1. The cooling water at the edge of the rear section of the arc section is turned off, and air cooling is used. The pressure ratio of cooling water to compressed air in the curved section is 1.4, and the pressure ratio of cooling water to compressed air in the arc section is 1.

5.

2. The secondary cooling method for improving the surface quality of ultra-thick slabs according to claim 1, characterized in that, The curved section is divided into zones 2, 3, and 4 in sequence, wherein the cooling water density in zone 2 is 40 L / min·m³. 2 The cooling water density in the three zones is 30 L / min·m³. 2 The cooling water density in the four zones is 20 L / min·m³. 2 .

3. The secondary cooling method for improving the surface quality of ultra-thick slabs according to claim 1, characterized in that, The arc-shaped segment is divided into zones 5, 6, 7, 8, 9, 10, and 11 in sequence. Zones 5, 6, 7, and 8 are the front section of the arc-shaped segment, while zones 9, 10, and 11 are the rear section.

4. The secondary cooling method for improving the surface quality of ultra-thick slabs according to claim 1, characterized in that, The cooling water pressure in the curved section is 0.35 MPa, and the cooling water pressure in the arc section is 0.30 MPa; the compressed air pressure in the curved section is 0.25 MPa, and the compressed air pressure in the arc section is 0.20 MPa.

5. A secondary cooling method for improving the surface quality of ultra-thick slabs according to any one of claims 1 to 4, characterized in that, The slab drawing speed is 0.45 m / min.

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