Method for rapid cooling of a super-thick plate to obtain a uniform structure in the thickness direction

Abstract

A method for rapidly cooling a super-thick plate to obtain uniform microstructure in the thickness direction, the temperature of the plate after rolling being 700-980 DEG C; for a plate with thickness of 80-100 mm, first implement a large water flow on the surface of the plate to reduce the surface temperature to 350-500 DEG C within 12 seconds, then implement a small water flow mode on the surface of the plate to maintain the surface temperature at 350-500 DEG C for more than 20 seconds; then increase the water flow on the surface to make the surface cooling rate greater than 2 DEG C / s until the surface temperature is below 200 DEG C; for a plate with thickness of 100-120 mm, when implementing a large water flow on the surface of the plate, reduce the surface temperature to 350-450 DEG C within 12 seconds, then implement a small water flow mode on the surface to maintain the surface temperature at 350-450 DEG C for more than 20 seconds; then continue to increase the water flow on the surface to make the surface cooling rate greater than 2 DEG C / s until the surface temperature is below 200 DEG C. Under the above process, bainite phase transformation microstructure is obtained in the core and the surface of the plate.

Description

Technical Field

[0001] This invention relates to the field of thick plate manufacturing technology, and in particular to a rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate. Background Technology

[0002] Controlled rolling and controlled cooling technology has been widely used in thick plate products. However, with the increasing thickness of thick plates, new requirements have been placed on the water cooling process after rolling. For steel plates with a thickness of less than 60mm, conventional water cooling processes can generally achieve rapid cooling rates on both the surface and core of the steel plate simultaneously, thereby realizing the phase transformation structure of high-strength phases. Figure 1 As shown, the cooling rate of water cooling after rolling of a 30mm thick steel plate can achieve bainitic structure in both the surface and core of the steel plate, so the properties obtained in the surface and core are also relatively similar.

[0003] As the thickness increases to 80mm and above, the goal is to achieve a faster cooling rate simultaneously on both the surface and core of the steel plate by continuously adjusting the cooling water flow rate using conventional water cooling processes. For example... Figure 2 As shown, due to the thickness effect of the steel plate, when the surface achieves a faster cooling rate, the cooling rate of the steel plate core increases more slowly. The steel plate obtains a martensitic structure with particularly high hardness on the surface, while the steel plate core has a bainitic structure. This results in an excessively high hardness distribution deviation in the thickness direction of the steel plate.

[0004] Chinese Patent Publication No. CN105268750A discloses a rapid head and tail shielding device, shielding system, and shielding method for a medium-thick plate cooling system. The shielding device, installed below the cooling manifold, includes a water bucket, two parallel sliding shafts, and a cylinder. Both ends of the water bucket are connected to the two sliding shafts via connectors. The cylinder's push rod is connected to the connectors, causing the water bucket to slide on the sliding shafts. At least one end of the water bucket's bottom has an overflow port. The cylinder of the shielding device is controlled by a PLC controller, which shields or does not shield the cooling manifold at the head and / or tail of the steel plate as it passes under the cooling manifold. This invention can compensate for the over-cooling problem at the head and tail of the steel plate during the cooling process due to the rapid temperature drop at the head and tail, thereby improving the uniformity of cooling for medium-thick plates. The response time is rapid, completing the complete interception of cooling water within 0.2 seconds, significantly improving the quality of the steel.

[0005] Chinese Patent Publication No. CN106363029A discloses a method for accelerated cooling of extra-thick plates based on inclined jet flow. This method employs a three-stage accelerated cooling system with jet cooling zones consisting of individual units ("forward jet + reverse jet") or combined units ("forward jet + forward jet + reverse jet"), and achieves accelerated cooling of extra-thick plates through the matching and setting of cooling rates, surface cooling temperatures, reheating times, and corresponding water cooling parameters at different temperature ranges. This invention integrates the cooling path with the cooling of extra-thick plates by controlling different temperature ranges, cooling rates, and phase transformation structures in a unified manner, significantly reducing the temperature gradient along the thickness direction of the extra-thick plate and providing a technological window for the development of new extra-thick steel grades with diverse performance requirements.

[0006] Chinese Patent Publication No. CN113976646A discloses an ultra-rapid cooling method for hot-rolled extra-thick plates, solving the problems of uneven cooling within the thickness range when relying solely on cooling water for ultra-rapid cooling, and the long production line and slow cooling rate of air mist cooling methods, which fail to meet the requirements of ultra-rapid cooling. The ultra-rapid cooling method of this invention includes the following steps: moving the hot-rolled extra-thick plate to a cooling water area for cooling; moving the hot-rolled extra-thick plate from the cooling water area to an air mist area for air mist cooling; reheating the air-mist-cooled hot-rolled extra-thick plate; repeating steps 1 to 2 at least once to complete the ultra-rapid cooling of the hot-rolled extra-thick plate. The ultra-rapid cooling method of this invention can be used for the ultra-rapid cooling of hot-rolled extra-thick plates. Summary of the Invention

[0007] The purpose of this invention is to provide a rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate, which solves the problem of excessive hardness distribution deviation in the thickness direction of the steel plate and can obtain a more uniform phase transformation microstructure in the thickness direction of the thick steel plate.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: A rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate, wherein the rolled steel plate is in the austenitic temperature range of 700~980℃; For steel plates with a thickness of 80~100mm, a large water flow is first applied to the surface of the steel plate to rapidly reduce the surface temperature to below 500℃ but above 350℃, creating a temperature difference between the surface and the core. Then, the surface is switched to a low water flow mode to maintain the surface temperature between 350~500℃ for more than 20 seconds, allowing the surface to completely pass through the bainitic transformation region. Subsequently, the water flow is increased to make the surface cooling rate greater than 2℃ / s until the surface temperature is below 200℃, at which point both the core and surface of the steel plate enter the bainitic transformation region. For steel plates with a thickness of 100~120mm, a large water flow rate is applied to rapidly reduce the surface temperature of the steel plate to below 450℃, but not below 350℃. The surface temperature of the steel plate is maintained between 350~450℃ for more than 20 seconds, allowing the surface of the steel plate to completely pass through the bainitic phase transformation region. The water flow rate is then increased further, so that the cooling rate of the steel plate surface is greater than 2℃ / s until the surface temperature of the steel plate is below 200℃, at which point the core and surface of the steel plate acquire a bainitic phase transformation structure. Through the above process, bainitic structure is formed on both the surface and core of the steel plate, achieving uniform properties on both the surface and the core.

[0009] Preferably, for steel plates with a thickness of 80~100mm, after the surface temperature of the steel plate is below 200℃, the heat dissipation from the core of the steel plate to the surface is further accelerated, and the cooling rate of the core is controlled to be greater than 2℃ / s, so that the core of the steel plate enters the bainitic phase transformation region.

[0010] Preferably, for steel plates with a thickness of 100~120mm, after the surface temperature of the steel plate is below 200℃, the heat dissipation from the core of the steel plate to the surface is further accelerated, and the cooling rate of the core is controlled to be greater than 2℃ / s, so that the core of the steel plate enters the bainitic phase transformation region.

[0011] Preferably, for steel plates with a thickness of 80~100mm, a large water flow rate is first applied to the surface of the steel plate, with a water flow rate density greater than 370L / m³. 2 The surface temperature of the steel plate is rapidly reduced to below 500℃ but above 350℃ to create a temperature difference between the surface and the core. Then, the surface enters a low-flow mode with a water density of 180~360 L / m³. 2 For a period of time, maintain the surface temperature of the steel plate at 350~500℃ for at least 20 seconds; then continue to increase the water flow rate to a density greater than 400L / m³. 2 min, so that the surface cooling rate is greater than 2℃ / s, until the surface temperature of the steel plate is below 200℃; further accelerate the heat dissipation from the core of the steel plate to the surface of the steel plate, and control the cooling rate of the core of the steel plate to be greater than 2℃ / s, so that the core of the steel plate also enters the bainitic phase transformation region.

[0012] Preferably, for steel plates with a thickness of 100~120mm, a larger water flow rate and a water flow density greater than 400L / m³ are applied to the steel plate. 2 For min, rapidly reduce the surface temperature of the steel plate to below 450℃, but not below 350℃, and then the surface enters a low-flow mode with a water density of 180~360L / m³. 2 Maintain the steel plate surface temperature at 350~450℃ for at least 20 seconds; then continue to increase the water flow rate to a density greater than 400L / m³. 2min, so that the cooling rate of the steel plate surface is greater than 2℃ / s until the surface temperature of the steel plate is below 200℃; further accelerate the heat dissipation from the core of the steel plate to the surface of the steel plate, and control the cooling rate of the core of the steel plate to be greater than 2℃ / s, so that the core of the steel plate enters the bainitic phase transformation region.

[0013] The technical solution of this invention differs from the previous DQ or rapid cooling process for extra-thick plates in that: In traditional cooling processes, the thickness of thick plate products is usually below 60mm. In order to obtain a faster cooling rate for the core of the steel plate, a large volume of water is used at the beginning of the cooling process, so that the surface of the steel plate enters the low temperature zone in the shortest possible time, and the core of the steel plate enters the low temperature zone as soon as possible. To cooperate with this cooling process, almost all mainstream thick plate production lines arrange DQ+ACC in the cooling equipment after rolling. That is, the DQ with a large volume of water and strong cooling is first, followed by the subsequent ACC cooling of the steel plate. Its cooling rate is also slower than the cooling rate that the steel plate can obtain in the previous DQ process.

[0014] As the thickness of plate products continues to increase, for extra-thick plates greater than 80mm, the traditional cooling process, limited by the plate thickness and the heat transfer rate along the thickness direction, actually has the opposite effect of being too hasty. The cooling of the steel core lags behind the surface, and the core experiences a significantly different cooling rate than the surface. In other words, under this extremely fast surface cooling rate, the surface often completes the cooling process before the steel core even begins to cool. This results in the surface and core of the extra-thick plate experiencing two completely opposite cooling processes, leading to significantly different cooling structures on the surface and in the core. The rapid entry of the steel plate surface into the low-temperature zone initially brings two drawbacks: 1) the steel plate surface acquires high-strength, low-toughness martensite; 2) due to the steel plate thickness and thermal conductivity, the steel core has not yet entered the high-strength phase transformation zone.

[0015] Based on the above two analyses, it is impossible to obtain a uniform microstructure in the thickness direction of extra-thick plates using traditional cooling methods.

[0016] This invention optimizes the cooling process by designing a segmented water flow rate for the cooling process and combining it with the continuous cooling phase transformation CCT diagram of the product. It uses the finite element method to simulate the cooling process after rolling of thick plates, calculates the cooling rate of the thick plate at the surface, one-quarter of the thickness, and one-half of the thickness, and optimizes the cooling rate of each thickness layer of the thick plate to form the required cooling rate at each thickness layer, thereby obtaining the target phase transformation structure and achieving a more uniform performance in the thickness direction.

[0017] The cooling rate range is determined based on the product's continuous cooling phase transition (CCT) curve, and the target cooling rate range for obtaining the desired phase change microstructure is defined, i.e., the maximum cooling rate C. max Minimum cooling rate Cmin Two values. For example... Figure 3 The figure shows a typical continuous cooling phase transformation (CCT) curve for steel materials. For high-strength steel, a cooling rate exceeding 30℃ / s, and below 350℃, will cause the steel plate surface to enter the martensitic transformation (high hardness) region. Cooling rates below 2℃ / s will lead to the ferrite (F) and pearlite (P) phase transformation (low strength, low hardness) regions, making it impossible to obtain the high-strength bainitic phase. It is generally desirable to obtain bainitic (B) regions both inside and on the surface of the steel plate, achieving a balance of strength and hardness. The bainitic phase transformation cooling temperature range is 2~30℃ / s, which means the CCT curve can be determined. max =30℃ / s, C min =2℃ / s.

[0018] In the technical solution of this invention, the rolled steel plate is in the austenitic temperature range of 700~980℃. For extra-thick plates with a thickness of 80~120mm, the cooling process adopts controlled variable water flow density control. That is, during the initial cooling, a large amount of water is given to the surface, so that the surface temperature drops rapidly to below 500℃ but above 350℃. Then, an innovative small amount of water is given to maintain this temperature range for more than 20 seconds. This can achieve two beneficial effects: 1) avoid the formation of a high-strength, low-toughness martensite phase on the surface of the steel plate, and give sufficient time for the surface of the steel plate to obtain a bainitic structure; 2) give the core of the steel plate sufficient time to dissipate heat.

[0019] During the subsequent increase in water volume, two beneficial effects are maintained: 1) the bainitic transformation of the steel plate surface has been completed, and martensite will not be acquired. 2) The core of the steel plate is given faster cooling conditions, allowing it to enter the bainitic transformation region.

[0020] Through the above process, bainitic structure is formed on both the surface and core of the steel plate, achieving uniform properties on both the surface and the core.

[0021] Preferably, the present invention further specifies the specific flow rates at each stage for steel plates with a thickness of 80-100mm and 100-120mm, such as: For steel plates with a thickness of 80-100mm, a large water flow rate with a water flow density greater than 370L / m³ is first applied to the surface of the steel plate. 2 The steel plate surface temperature is reduced to below 500℃ but above 350℃ within 12 seconds, creating a temperature difference between the steel plate surface and core. Then, the surface enters a low-flow mode with a water flow density of 180~360 L / m³. 2 For a period of time, maintain the surface temperature of the steel plate at 350~500℃ for at least 20 seconds; then continue to increase the water flow rate to a density greater than 400L / m³. 2min, so that the surface cooling rate is greater than 2℃ / s, until the surface temperature of the steel plate is below 200℃, further accelerating the heat dissipation from the core of the steel plate to the surface of the steel plate. The cooling rate of the core of the steel plate is controlled to be greater than 2℃ / s, so that the core of the steel plate also enters the bainitic phase transformation region. For steel plates with a thickness of 100~120mm, a large water flow rate is applied to the steel plate, with a water flow density greater than 400L / m³. 2 The steel plate surface temperature is reduced to below 450℃ within 12 seconds, but not below 350℃. Then, the surface enters a low-flow mode with a water density of 180~360 L / m³. 2 Maintain the steel plate surface temperature at 350~450℃ for at least 20 seconds, then continue to increase the water flow rate to a density greater than 400L / m³. 2 The cooling rate of the steel plate surface is increased to greater than 2℃ / s until the surface temperature of the steel plate is below 200℃, and the core and surface of the steel plate also obtain bainitic phase transformation structure.

[0022] Compared with conventional rapid cooling processes for steel plates less than 60mm thick, the rapid cooling process described in this invention is more suitable for cooling steel plates with a thickness of 80~120mm. Its main advantages are: Steel plates with a thickness of less than 60 mm have a higher heat transfer efficiency between the core and surface than steel plates with a thickness of 80-120 mm. When using conventional cooling processes, a more consistent cooling rate can be achieved between the surface and core of steel plates with a thickness of less than 60 mm, resulting in a more similar phase transformation structure and a more uniform structure in the thickness direction.

[0023] When conventional cooling processes are used for steel plates with a thickness of 80~120mm, the heat transfer efficiency is slow due to the thickness of the steel plate. According to the traditional process design concept, the water volume is increased to obtain a faster cooling rate in the core of the steel plate. However, this operation can only make the surface of the steel plate obtain a martensitic structure, while the cooling efficiency of the core is not significantly improved.

[0024] Using the present invention, the water volume is initially adjusted on the surface of the steel plate, and the surface quickly enters the bainitic transformation region. At this stage, the temperature of the core of the steel plate has not changed significantly. After the bainitic transformation is completed on the surface of the steel plate, the water volume is increased to lower the temperature to below 200°C. At this time, the temperature difference between the surface and the core of the steel plate increases, and the core begins to enter the bainitic transformation region until the entire steel plate completes the transformation, thereby obtaining bainitic structure on both the surface and the core of the steel plate.

[0025] The cooling method described in this invention can obtain a relatively uniform phase transformation structure in the thickness direction of thick steel plates. Attached Figure Description

[0026] Figure 1This is a schematic diagram illustrating the cooling rate of a 30mm thick steel plate after rolling and water cooling in the steel plate surface and core, based on existing technology. Figure 2 This is a schematic diagram illustrating the cooling rate of water cooling after rolling of steel plates with a thickness of 80mm or more in the steel plate surface and core, as shown in existing technologies. Figure 3 This is a typical CCT curve of continuous cooling phase transformation for steel materials. Figure 4 This is a cooling curve diagram of Embodiment 1 of the present invention; Figure 5 This is a superimposed graph of the cooling curve and CCT curve of Embodiment 1 of the present invention; Figure 6 This is a cooling curve diagram of Embodiment 2 of the present invention; Figure 7 This is a superimposed graph of the cooling curve and CCT curve of Embodiment 2 of the present invention; Figure 8 This is a cooling curve diagram of Embodiment 3 of the present invention; Figure 9 This is a superimposed graph of the cooling curve and CCT curve of Embodiment 3 of the present invention; Figure 10 The cooling curve is a comparative example of the present invention; Figure 11 This is a superimposed graph of the cooling curve and CCT curve of Embodiment 4 of the present invention. Detailed Implementation

[0027] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0028] Example 1 A type of ordinary carbon steel with a thickness of 80mm.

[0029] First, apply a large water flow rate of 370 L / m to the surface of the steel plate. 2 The minimum water flow rate density reduces the steel plate surface temperature to 480℃ in 5 seconds, creating a temperature difference between the steel plate surface and the core. Then, the steel plate surface enters a low-flow mode with a water flow rate density of 180 L / m³. 2 Over 22 seconds, the surface temperature of the steel plate decreased from 480°C to 350°C. Subsequently, the water flow rate was increased further on the steel plate surface, with a water flow density of 400 L / m³. 2 The surface is rapidly cooled to below 200℃ at a rate of 3.5℃ / s, further accelerating the heat dissipation from the core of the steel plate to the surface. The cooling rate of the core is controlled at 7.5℃ / s.

[0030] By performing segmented water flow rate cooling process calculations on a certain 80mm thick carbon steel plate, the cooling curve was obtained as follows: Figure 4 As shown.

[0031] See Figure 5 After coordinate transformation and overlay with the product's CCT diagram, the beneficial effects of this cooling process can be observed. The steel plate surface temperature rapidly reaches 480℃ within 5 seconds, widening the temperature difference between the steel plate surface and the core. Then, the water volume is reduced, and after 22 seconds, the steel plate surface temperature passes through the bainitic transformation region, completing the bainitic transformation. Subsequently, the cooling water volume is increased further on the steel plate surface, causing the surface temperature to drop further as quickly as possible. The temperature of the steel plate core also begins to pass through the ferrite region and enter the bainitic transformation region. Because the core cooling lags behind the surface cooling by 60 seconds, the core cooling temperature curve can be shifted forward by 60 seconds. The actual calculated cooling rate is 7.5℃ / s. This cooling method ensures that both the steel plate surface and core complete the bainitic transformation, thereby obtaining a microstructure with uniform surface and core properties.

[0032] Example 2 For a 100mm thick ordinary carbon steel plate, a large water flow rate of 450L / m is first applied to the surface of the steel plate. 2 Rapid cooling at a minimum water flow rate density reduced the steel plate surface temperature to 441°C within 10 seconds, creating a temperature difference between the steel plate surface and the core. The water flow rate then decreased on the steel plate surface, entering a low-flow mode to maintain a density of 270 L / m³. 2 Over 20 seconds, the temperature on the steel plate surface decreased from 441℃ to 350℃. Subsequently, the water flow rate was increased further to maintain a water flow density of 450 L / m³. 2 The steel plate surface is rapidly cooled to below 200℃ at a rate of 3℃ / s, further accelerating the heat dissipation from the steel plate core to the steel plate surface. The core cooling rate is controlled at 5.5℃ / s.

[0033] By performing segmented water flow rate cooling process calculations on a 100mm thick carbon steel plate, the cooling curve was obtained as follows: Figure 6 As shown.

[0034] See Figure 7 After coordinate transformation and overlay with the product's CCT diagram, the beneficial effects of this cooling process can be observed. The steel plate surface is first rapidly cooled, with the temperature dropping to 441℃ after 10 seconds, widening the temperature difference between the steel plate surface and the core. Then, the water flow is reduced, and after 20 seconds, the steel plate surface temperature decreases from 441℃ to 350℃, indicating that the steel plate surface has passed through the bainitic transformation region. Subsequently, the cooling water flow is increased, and the steel plate surface is cooled to below 200℃. The temperature of the steel plate core also begins to pass through the ferrite region and enter the bainitic transformation region. Because the core cooling lags behind the surface by 80 seconds, the core cooling temperature curve can be shifted forward by 80 seconds. The actual calculated cooling rate is 5.5℃ / s. This cooling method ensures that both the steel plate surface and core complete the bainitic transformation, thereby obtaining a microstructure with uniform surface and core properties.

[0035] Example 3 For a 120mm thick carbon steel plate, a large water flow rate of 500L / m is first applied to the surface of the plate. 2 Rapid cooling with a minimum water flow rate lowers the steel plate surface temperature to 440°C within 10 seconds, creating a temperature difference between the steel plate surface and the core. Then, the water flow rate is reduced to maintain a density of 360 L / m³ on the steel plate surface. 2 After 20 seconds, the surface temperature of the steel plate decreased from 440℃ to 350℃. Then, the water flow rate was increased to maintain a water flow density of 400L / m³. 2 The steel plate surface is rapidly cooled to below 200℃ at a rate of 2.5℃ / s, further accelerating the heat dissipation from the steel plate core to the steel plate surface. The core cooling rate is controlled at 4.2℃ / s.

[0036] By performing segmented water flow rate cooling process calculations on a 120mm thick carbon steel plate, the cooling curve was obtained as follows: Figure 8 As shown.

[0037] See Figure 9 After coordinate transformation and overlay with the product's CCT diagram, the beneficial effects of this cooling process can be observed. The steel plate surface is first rapidly cooled to 440℃, creating a temperature difference between the surface and the core. Then, the water flow is reduced, and the surface temperature decreases from 440℃ to 350℃ over 20 seconds, indicating that the surface has passed through the bainitic transformation region. Subsequently, the cooling water flow is increased, and the surface temperature is cooled to below 200℃. At this point, the core temperature also begins to pass through the ferrite region and enter the bainitic transformation region. Because the core cooling lags behind the surface cooling by 110 seconds, the core cooling temperature curve can be shifted forward by 110 seconds. The actual calculated cooling rate is 4.2℃ / s. This cooling method ensures that both the surface and core of the steel plate complete the bainitic transformation, resulting in a microstructure with uniform surface and core properties.

[0038] Example 4 For a 100mm thick steel plate, using both traditional ultra-fast cooling technology and the cooling technology of this invention, the cooling curves of different layers of the steel plate surface and thickness are shown below. Figure 10 , Figure 11 As shown.

[0039] See Figure 10 Under traditional ultra-fast cooling process conditions, a relatively large water volume of 700L / m³ is used. 2 The cooling process continued until the end without any water volume adjustment. The surface of the steel plate quickly entered the martensitic transformation region, while the steel plate was in the bainitic transformation region at half and a quarter of its thickness. This caused excessive deviations in the surface and core microstructure properties.

[0040] See Figure 11 Through the cooling process conditions of this invention, a large water flow rate of 400 L / m is first applied to the surface of the steel plate. 2 Rapid cooling is achieved using a minimum water flow rate, bringing the steel plate surface to 500°C within 12 seconds. This creates a temperature difference between the steel plate surface and the core. The water flow rate is then reduced to 220 L / m³. 2 With a minimum water flow rate, the steel plate surface temperature decreases from 500℃ to 350℃ over 20 seconds, ensuring the steel plate surface passes through the bainitic phase transformation region. Then, the water flow rate over the steel plate surface is increased to 400 L / m². 2 With a minimum water flow density, the surface of the steel plate is rapidly cooled to below 200℃ at a cooling rate of 2.2℃ / s, while the core is cooled to the bainitic phase transformation range at a rate of 4.8℃ / s, thereby obtaining bainitic structure in both the surface and core of the steel plate, i.e., obtaining a uniform structure in the thickness direction of the steel plate.

Claims

1. A rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate, characterized in that, The rolled steel plate is in the austenitic temperature range of 700~980℃; For steel plates with a thickness of 80mm to 100mm, a large water flow rate is first applied to the surface of the steel plate to reduce the surface temperature to below 500℃ but above 350℃ within 12 seconds, creating a temperature difference between the surface and the core. Then, the surface of the steel plate is switched to a small water flow rate mode, maintaining the surface temperature between 350 and 500℃ for more than 20 seconds, allowing the surface to completely pass through the bainitic transformation region. Subsequently, the water flow rate is increased further to make the surface cooling rate greater than 2℃ / s until the surface temperature of the steel plate is below 200℃, at which point the core and surface of the steel plate acquire a bainitic transformation structure. For steel plates with a thickness of 100 mm and a thickness of 120 mm, a large water flow rate is first applied to the surface of the steel plate to reduce the surface temperature to below 450°C but above 350°C within 12 seconds, creating a temperature difference between the surface and the core. Then, the surface of the steel plate is switched to a small water flow rate mode to maintain the surface temperature between 350 and 450°C for more than 20 seconds, allowing the surface of the steel plate to completely pass through the bainitic phase transformation region. The water flow rate is then increased further, so that the surface cooling rate of the steel plate is greater than 2°C / s, until the surface temperature of the steel plate is below 200°C, at which point the core and surface of the steel plate acquire a bainitic phase transformation structure. Through the above process, bainitic structure is formed on both the surface and core of the steel plate, achieving uniform properties on both the surface and the core.

2. The rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate as described in claim 1, characterized in that, For steel plates with a thickness of 80mm to 100mm, after the surface temperature of the steel plate is below 200℃, further accelerate the heat dissipation from the core of the steel plate to the surface. The cooling rate of the core is controlled to be greater than 2℃ / s, so that the core of the steel plate enters the bainitic phase transformation region.

3. The rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate as described in claim 1, characterized in that, For steel plates with a thickness of 100 < 120 mm, after the surface temperature of the steel plate is below 200 ℃, further accelerate the heat dissipation from the core of the steel plate to the surface of the steel plate. The cooling rate of the core is controlled to be greater than 2 ℃ / s, so that the core of the steel plate enters the bainitic phase transformation region.

4. The rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate as described in claim 1, characterized in that, For steel plates with a thickness of 80mm to 100mm, a large water flow rate with a water flow density greater than 370L / m³ is first applied to the surface of the steel plate. 2 The surface temperature of the steel plate is reduced to 350-500℃ within 12 seconds, creating a temperature difference between the surface and the core. Then, the surface enters a low-flow mode with a water density of 180-360 L / m³. 2 For a period of time, maintain the surface temperature of the steel plate at 350~500℃ for at least 20 seconds; then continue to increase the water flow rate to a density greater than 400L / m³. 2 The surface cooling rate is increased to greater than 2℃ / s until the surface temperature of the steel plate is below 200℃, which further accelerates the heat dissipation from the core of the steel plate to the surface. The cooling rate of the steel plate core is controlled to be greater than 2℃ / s, so that the core of the steel plate also enters the bainitic phase transformation region.

5. The rapid cooling method for obtaining a uniform microstructure in the thickness direction of an extra-thick plate as described in claim 1, characterized in that, For steel plates with a thickness of 100 mm to 120 mm, a larger water flow rate and a water flow density greater than 400 L / m³ are applied to the steel plate. 2 The surface temperature of the steel plate is reduced to 350~450℃ within 12 seconds; then the surface enters a low-flow mode with a water flow rate density of 180~360 L / m³. 2 Maintain the steel plate surface temperature at 350~450℃ for at least 20 seconds, then continue to increase the water flow rate to a density greater than 400L / m³. 2 min, so that the cooling rate of the steel plate surface is greater than 2℃ / s, until the surface temperature of the steel plate is below 200℃; further accelerate the heat dissipation from the core of the steel plate to the surface of the steel plate, and control the cooling rate of the core to be greater than 2℃ / s, so that the core of the steel plate enters the bainitic phase transformation region.

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