Rapid cooling method for obtaining uniform structure in thickness direction of extra-thick plate

Abstract

The invention discloses a rapid cooling method for obtaining a uniform structure in the thickness direction of an extra-thick plate. The temperature of a rolled steel plate is 700-980 DEG C; for a steel plate with the thickness of 80-100 mm, a large water flow is applied to the surface of the steel plate, the surface temperature of the steel plate is reduced to 350-500 DEG C within 12 seconds, then a small water flow mode is started to be applied to the surface of the steel plate, and the surface temperature of the steel plate is kept at 350-500 DEG C for 20 seconds or above; increasing the water quantity on the surface to enable the surface cooling speed to be greater than 2 DEG C / s until the surface temperature of the steel plate is below 200 DEG C; for a steel plate with the thickness of 100-120 mm, when large water flow is applied to the surface of the steel plate, the surface temperature of the steel plate is reduced to 350-450 DEG C within 12 seconds, then a small water flow mode is applied to the surface, and the surface temperature of the steel plate is kept at 350-450 DEG C for 20 seconds or above; and continuously increasing the water quantity to enable the surface cooling speed to be greater than 2 DEG C / s until the surface temperature of the steel plate is below 200 DEG C. According to the process, bainite phase change structures are obtained on the core part and the surface of the steel plate.

Description

Technical Field

[0001] The invention relates to the technical field of thick plate manufacturing, and in particular to a rapid cooling method for obtaining a uniform structure in a thickness direction of an extra-thick plate. Background Art

[0002] From 1890 to World War II, Germany first focused on the relationship between the hot working conditions, material and microstructure of steel. By the 1960s, the United States qualitatively explained the dynamic changes of austenite recrystallization after hot rolling. Since the 1970s, Japan has begun specific research on controlled water cooling after rolling to obtain controlled cooling effects. Subsequently, TMCP technology replaced heating and quenching heat treatment, and was widely used in direct quenching technology after hot rolling, especially for ensuring high strength and good low-temperature toughness of thick plates.

[0003] This idea of ​​adding alloy elements combined with controlled rolling and controlled cooling has been widely used in thick plate products. As the thickness of thick plate products continues to increase, new requirements have been placed on the water cooling process of thick plate products after rolling. For steel plates with a thickness of less than 60mm, conventional water cooling processes can generally achieve faster cooling rates on both the surface and the core of the steel plate, thereby achieving a phase change structure of a high-strength phase. Figure 1 As shown, the cooling rate formed on the surface and core of the 30mm thick steel plate after water cooling after rolling can obtain bainite structure, so the performance obtained on the surface and the core is also relatively close.

[0004] As the thickness increases to 80 mm and above, the cooling water flow rate is continuously adjusted through conventional water cooling process, hoping to obtain faster cooling speed on both the surface and the core of the steel plate. Figure 2 As shown, due to the thickness effect of the steel plate, when a faster cooling rate is obtained on the surface, the cooling rate of the core of the steel plate increases slowly, and the steel plate obtains a martensite structure with particularly high hardness on the surface, while the core of the steel plate is a bainite structure, so that the hardness distribution deviation of the steel plate in the thickness direction is too high.

[0005] The Chinese patent publication number CN105268750A discloses a quick shielding device, shielding system and shielding method for the head and tail of a medium and thick plate cooling system. The shielding device is installed under the cooling manifold, and includes: a water bucket, two parallel sliding shafts and a cylinder; the two ends of the water bucket are respectively connected to the two sliding shafts through a connecting piece; the push rod of the cylinder is connected to the connecting piece to drive the water bucket to slide on the sliding shaft; an overflow port is set at least at one end of the bottom of the water bucket; the cylinder of the shielding device is controlled by a PLC controller, and when the steel plate passes under the cooling manifold, the cooling manifold at the head and / or tail of the steel plate is shielded or not. The invention can compensate for the problem of overcooling of the head and tail of the steel plate due to the rapid temperature drop of the head and tail of the steel plate during the cooling process, thereby improving the uniformity of cooling of the medium and thick plate; the response time is fast, and the cooling water can be completely intercepted within 0.2 seconds, which can significantly improve the quality of steel.

[0006] Chinese patent publication number CN106363029A discloses a method for accelerating cooling of extra-thick plates based on inclined jet flow. The method adopts "forward jet + reverse jet" or "forward jet + forward jet + reverse jet" as a single unit and a three-stage accelerated cooling of the jet cooling area of ​​the combined unit. The accelerated cooling of the extra-thick plate is achieved through the cooling rate control of different temperature sections, surface cooling temperature control, return temperature time control and matching setting of corresponding water cooling parameters. The invention will organically combine the cooling path with the cooling of the extra-thick plate through the integrated control of different temperature sections, cooling rate and phase change structure of the extra-thick plate, greatly reduce the temperature gradient in the thickness direction of the extra-thick plate, and provide a process window for the development of new steel grades for extra-thick plates with diversified performance requirements.

[0007] Chinese Patent Publication No. CN113976646A discloses a method for ultra-fast cooling of hot-rolled extra-thick plates, which solves the problem in the prior art that ultra-fast cooling that relies solely on cooling water results in uneven cooling within the thickness range, and that the production line for mist cooling is long and the cooling rate is slow, which leads to the inability to meet the requirements of ultra-fast cooling. The ultra-fast cooling method of the invention comprises the following steps: moving the hot-rolled extra-thick plate to a cooling water area for cooling with cooling water; moving the hot-rolled extra-thick plate from the cooling water area to an aerosol area for aerosol cooling; returning the temperature of the hot-rolled extra-thick plate after aerosol cooling; repeating steps 1 to 2 at least once to complete the ultra-fast cooling of the hot-rolled extra-thick plate. The ultra-fast cooling method of the present invention can be used for ultra-fast cooling of hot-rolled extra-thick plates. Summary of the invention

[0008] The purpose of the present invention is to provide a rapid cooling method for obtaining uniform structure in the thickness direction of an extra-thick plate, so as to solve the problem of excessive deviation in hardness distribution of the steel plate in the thickness direction and obtain a relatively uniform phase change structure in the thickness direction of the thick steel plate.

[0009] To achieve the above object, the technical solution of the present invention is:

[0010] A rapid cooling method for obtaining uniform structure in the thickness direction of an extra-thick plate, wherein the steel plate after rolling is in the austenite temperature range of 700 to 980°C;

[0011] For steel plates with a thickness of 80-100 mm, a large water flow rate is first applied to the surface of the steel plate to quickly reduce the surface temperature of the steel plate to below 500°C but above 350°C, so that a temperature difference is formed between the surface and the core of the steel plate. Then the surface begins to enter a small water flow mode to maintain the surface temperature of the steel plate between 350 and 500°C for more than 20 seconds. The surface of the steel plate completely passes through the bainite phase transformation region. Subsequently, the surface water flow is continued to increase to make the cooling rate of the steel plate surface greater than 2°C / s until the surface temperature of the steel plate is below 200°C, and the core and surface of the steel plate also enter the bainite phase transformation region.

[0012] For steel plates with a thickness of 100-120mm, a large water flow rate is applied to the steel plates to rapidly reduce the surface temperature of the steel plates to below 450℃, but not below 350℃. The surface temperature of the steel plates is maintained between 350℃ and 450℃ for more than 20 seconds until the surface of the steel plates completely passes through the bainite phase transformation region. The water flow is then increased to make the cooling rate of the steel plate surface greater than 2℃ / s until the surface temperature of the steel plate is below 200℃, and the core and surface of the steel plate obtain bainite phase transformation structure.

[0013] Through the above process, bainite structure is formed on the surface and core of the steel plate, achieving uniform performance on the surface and core.

[0014] Preferably, for a steel plate with a thickness of 80 to 100 mm, after the surface temperature of the steel plate is below 200°C, the heat dissipation from the core of the steel plate to the surface of the steel plate is further accelerated, and the cooling rate of the core is controlled to be greater than 2°C / s, so that the core of the steel plate enters the bainite phase transformation region.

[0015] Preferably, for a steel plate with a thickness of 100 to 120 mm, after the surface temperature of the steel plate is below 200°C, the heat dissipation from the core of the steel plate to the surface of the steel plate is further accelerated, and the cooling rate of the core is controlled to be greater than 2°C / s, so that the core of the steel plate enters the bainite phase transformation region.

[0016] Preferably, for a steel plate with a thickness of 80 to 100 mm, a large water flow rate is first applied to the surface of the steel plate, and the water flow rate density is greater than 370 L / m 2 min, the surface temperature of the steel plate is rapidly reduced to below 500℃ but above 350℃, so that a temperature difference is formed between the surface and the core of the steel plate, and then the surface begins to enter a small water flow mode, with a water flow density of 180~360L / m 2 min, so that the surface temperature of the steel plate is maintained at 350-500℃ for more than 20 seconds; then continue to increase the amount of water on the surface, and the water flow density is greater than 400L / m 2min, making the surface cooling rate 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 bainite phase transformation region.

[0017] Preferably, for a steel plate with a thickness of 100 to 120 mm, a relatively large water flow rate is applied to the steel plate, and the water flow rate density is greater than 400 L / m 2 min, quickly reduce the surface temperature of the steel plate to below 450℃, but not below 350℃, and then the surface begins to enter the small water flow mode, with a water flow density of 180~360L / m 2 min, maintain the steel plate surface temperature at 350-450℃ for more than 20 seconds; then continue to increase the water volume, the water flow density is greater than 400L / m 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 of the steel plate to be greater than 2℃ / s, so that the core of the steel plate enters the bainite phase transformation region.

[0018] The technical solution of the present invention is different from the previous DQ or rapid cooling process of extra thick plates in that:

[0019] In the traditional cooling process, the thickness of the thick plate product itself is usually below 60mm. In order to obtain a faster cooling speed of the core of the steel plate, a large amount of water is used at the beginning of the cooling of the steel plate, so that the surface of the steel plate enters the low temperature zone in the shortest time, so that the core of the steel plate enters the low temperature zone as soon as possible. In order to cooperate with this cooling process system, almost all mainstream thick plate production lines have arranged DQ+ACC in the cooling equipment after rolling, that is, the DQ with large amount of water and strong cooling is in the front, and then the steel plate is given subsequent ACC cooling, and its cooling rate is also slower than the cooling rate that the steel plate can obtain in the DQ of the previous process.

[0020] With the further increase in the thickness of thick plate products, for extra-thick plates with a thickness greater than 80mm, the traditional cooling process system has the effect of haste makes waste due to the thickness of the steel plate and the heat transfer speed in the thickness direction. The cooling of the core of the steel plate lags behind the surface, and the cooling rate of the core is very different from that of the surface. In other words, under this ultra-fast surface cooling rate, the surface has completed the cooling process system before the core of the steel plate begins to cool. This results in the surface and core of the extra-thick plate obtaining two completely divergent cooling process systems, thereby obtaining cooling structures with greatly different performance on the surface and core of the extra-thick plate, that is, the rapid entry of the steel plate surface into the low temperature zone at the beginning brings two disadvantages: 1) The steel plate surface obtains high-strength and low-toughness martensite; 2) Due to the thickness and thermal conductivity of the steel plate, the core of the steel plate has not yet entered the high-strength structure phase transformation zone.

[0021] Based on the above two aspects of analysis, it is impossible to obtain uniform structure in the thickness direction of extra thick plates through traditional cooling schemes.

[0022] The present invention optimizes the cooling process by reasonably designing the segmented water flow rate of the cooling process in combination with the continuous cooling phase change CCT diagram of the product, and adopts the finite element method to simulate the cooling process of the thick plate after rolling, and calculates the cooling rates of the thick plate at the surface, one quarter of the thickness, and one half of the thickness. By optimizing the cooling rate of each thickness level of the thick plate, the required cooling rate is formed at each thickness level, and then the target phase change structure is obtained, thereby obtaining a relatively uniform performance in the thickness direction.

[0023] Determine the cooling rate range according to the product continuous cooling phase transformation CCT curve, and define the target cooling rate range for the product to obtain the required phase transformation structure, that is, the maximum cooling rate C max , minimum cooling rate C min Two values. Figure 3 The figure shows a typical CCT curve of continuous cooling phase transformation of steel materials. For high-strength steel, if the cooling rate exceeds 30℃ / s and the temperature is close to 350℃ or below, the surface of the steel plate will enter the martensitic phase transformation (high hardness) region. If the cooling rate is less than 2℃ / s, the steel plate will enter the ferrite (F) and pearlite (P) phase transformation (low strength and low hardness) region, and the bainite high-strength phase cannot be obtained. It is usually hoped that the bainite structure (B) region can be obtained both inside and on the surface of the steel plate, with good strength and hardness properties. The cooling temperature range of the bainite phase transformation is 2-30℃ / s, which means that the CCT can be determined. max =30℃ / s, C min =2℃ / s.

[0024] In the technical solution of the present invention, the rolled steel plate is in the austenite temperature range of 700-980°C. For the extra thick plate with a thickness of 80-120 mm, 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 is quickly cooled to below 500°C but above 350°C, and then a small amount of water is innovatively given to maintain the temperature range for more than 20s, which can achieve two beneficial effects: 1) avoid the formation of high-strength and low-toughness martensite phase on the surface of the steel plate, and give sufficient time for the surface of the steel plate to obtain bainite structure; 2) give the core of the steel plate sufficient time to dissipate heat.

[0025] In the subsequent process of increasing the water volume, two beneficial effects are maintained: 1) the steel plate surface has completed the bainite phase transformation and will not obtain martensite. 2) The core of the steel plate is given a faster cooling condition and enters the bainite phase transformation region.

[0026] Through the above process, bainite structure is formed on the surface and core of the steel plate, achieving uniform performance on the surface and core.

[0027] As a preference, the present invention further sets specific flow rates at each stage for steel plates with a thickness of 80 to 100 mm and steel plates with a thickness of 100 to 120 mm, such as:

[0028] For steel plates with a thickness of 80 to 100 mm, a large water flow rate is first applied to the surface of the steel plate, and the water flow density is greater than 370 L / m 2 min, the surface temperature of the steel plate is reduced to below 500℃ but above 350℃ within 12 seconds, so that a temperature difference is formed between the surface and the core of the steel plate, and then the surface begins to enter a small water flow mode, with a water flow density of 180~360L / m 2 min, so that the surface temperature of the steel plate is maintained at 350-500℃ for more than 20 seconds; then continue to increase the amount of water on the surface, and the water flow density is greater than 400L / m 2 min, making the surface cooling rate 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, and controlling 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 bainite phase transformation region;

[0029] For steel plates with a thickness of 100 to 120 mm, a larger water flow rate is applied to the steel plate, and the water flow density is greater than 400 L / m 2 min, reduce the surface temperature of the steel plate to below 450℃ within 12 seconds, but not below 350℃, and then the surface begins to enter the small water flow mode, with a water flow density of 180~360L / m 2 min, maintain the steel plate surface temperature at 350-450℃ for more than 20 seconds, and then continue to increase the water volume, the water flow density is greater than 400L / m 2 min, making the cooling rate of the steel plate surface 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 bainite phase transformation structure.

[0030] Compared with the conventional rapid cooling process for steel plates with a thickness of less than 60 mm, the rapid cooling process of the present invention is more suitable for cooling steel plates with a thickness of 80 to 120 mm. Its main advantages are:

[0031] Steel plates with a thickness of less than 60 mm are thinner, and the heat transfer efficiency between the core and the surface is higher than that of steel plates with a thickness of 80 to 120 mm. When using conventional cooling technology, a relatively consistent cooling rate can be obtained on the surface and core of steel plates with a thickness of less than 60 mm, thereby obtaining a relatively close phase change structure, and then obtaining a relatively uniform structure in the thickness direction.

[0032] When the conventional cooling process is used for steel plates with a thickness of 80 to 120 mm, due to the thick thickness of the steel plates and the slow heat transfer efficiency, according to the traditional process design idea, the amount of water will be increased to obtain a faster cooling rate in the core of the steel plate. In fact, this operation can only make the surface of the steel plate obtain martensitic structure, while the cooling rate efficiency of the core is not significantly improved.

[0033] By adopting the scheme of the present invention, the amount of water is initially adjusted on the surface of the steel plate, and the surface quickly enters the bainite phase transformation region. At this stage, the temperature of the core of the steel plate has not changed significantly. After the bainite phase transformation of the steel plate surface is completed, the amount of water is increased to cool down to below 200°C. At this time, the temperature difference between the surface and the core of the steel plate is enlarged, and the core begins to enter the bainite phase transformation region until the entire steel plate completes the phase transformation, thereby obtaining bainite structure on the surface and core of the steel plate.

[0034] By means of the cooling method of the present invention, a relatively uniform phase change structure can be obtained in the thickness direction of the thick steel plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the cooling rate formed on the surface and core of a 30mm thick steel plate by water cooling after rolling in the prior art;

[0036] Figure 2 It is a schematic diagram of the cooling rate formed on the surface and core of a steel plate with a thickness of 80 mm or more by water cooling after rolling in the prior art;

[0037] Figure 3 This is a typical CCT curve of continuous cooling phase transformation of steel materials;

[0038] Figure 4 This is a cooling curve diagram of Example 1 of the present invention;

[0039] Figure 5 It is a superposition diagram of the cooling curve and the CCT curve of Example 1 of the present invention;

[0040] Figure 6 This is a cooling curve diagram of Example 2 of the present invention;

[0041] Figure 7 It is a superposition diagram of the cooling curve and the CCT curve of Example 2 of the present invention;

[0042] Figure 8 This is a cooling curve diagram of Example 3 of the present invention;

[0043] Fig. 9 It is a superposition diagram of the cooling curve and the CCT curve of Example 3 of the present invention;

[0044] Fig.10 It is a cooling curve diagram of the comparative example of the present invention;

[0045] Fig.11 It is a superposition diagram of the cooling curve and the CCT curve of Example 4 of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be further described below in conjunction with the embodiments and drawings.

[0047] Example 1

[0048] Steel plate thickness 80mm ordinary carbon steel.

[0049] First, a large water flow rate of 370L / m is applied to the steel plate surface. 2 The water flow density of min reduces the surface temperature of the steel plate to 480℃ in 5 seconds, forming a temperature difference between the surface and the core of the steel plate. Then the surface of the steel plate begins to enter the small water flow mode, with a water flow density of 180L / m 2 min, after 22 seconds, the surface temperature of the steel plate decreases from 480℃ to 350℃, and then the amount of water on the surface of the steel plate continues to increase, and the water flow density is 400L / m 2 min, so that the surface is quickly cooled to below 200℃ at 3.5℃ / s, further accelerating the heat dissipation from the core of the steel plate to the surface of the steel plate, and the cooling rate of the core is controlled at 7.5℃ / s.

[0050] The cooling curve is obtained by calculating the cooling process of a common carbon steel thick plate product with a thickness of 80 mm by setting the segmented water flow rate. Figure 4 shown.

[0051] See also Figure 5 , after coordinate conversion and superposition with the product CCT diagram, the beneficial effect of the cooling process can be seen. The surface temperature of the steel plate quickly reaches 480℃ in 5 seconds, widening the temperature difference between the surface and the core of the steel plate, and then reducing the amount of water. After 22 seconds, the surface temperature of the steel plate passes through the bainite phase transformation area and completes the bainite phase transformation; then continue to increase the cooling water on the surface of the steel plate, the surface temperature of the steel plate drops further as soon as possible, and the temperature of the core of the steel plate also begins to pass through the ferrite area and enter the bainite phase transformation area. Because the cooling of the core lags behind the surface by 60 seconds, the cooling temperature curve of the core can be moved forward by 60 seconds. The actual calculated cooling rate is 7.5℃ / s. Cooling in this way ensures that the surface and core of the steel plate have completed the bainite phase transformation, thereby obtaining a uniform structure of the surface and core performance.

[0052] Example 2

[0053] For a common carbon steel plate with a thickness of 100 mm, a large water flow rate of 450 L / m is first applied to the surface of the steel plate. 2The water flow density of min is quickly cooled, and the surface temperature of the steel plate drops to 441℃ after 10 seconds, forming a temperature difference between the surface and the core of the steel plate. Then the surface of the steel plate begins to reduce the water volume and enters the small water volume mode, maintaining the water flow density of 270L / m 2 min, after 20 seconds, the temperature of the steel plate surface decreases from 441℃ to 350℃, and then the amount of water on the steel plate surface continues to increase, maintaining the water flow density at 450L / m 2 min, the surface of the steel plate is rapidly cooled to below 200°C at 3°C / s, and the heat dissipation from the core of the steel plate to the surface of the steel plate is further accelerated. The cooling rate of the core is controlled at 5.5°C / s.

[0054] By calculating the cooling process of a common carbon steel thick plate product with a thickness of 100 mm by setting the segmented water flow rate, the cooling curve is obtained as follows: Figure 6 shown.

[0055] See also Figure 7 After coordinate transformation and superposition with the product CCT diagram, the beneficial effect of the cooling process can be seen. The surface of the steel plate is cooled quickly first, and the temperature drops to 441°C after 10 seconds, widening the temperature difference between the surface and the core of the steel plate. Then the amount of water is reduced. After 20 seconds, the surface temperature of the steel plate decreases from 441°C to 350°C, and the surface of the steel plate passes through the bainite phase transformation area. Then the cooling water is increased, and the surface of the steel plate is cooled to below 200°C. The temperature of the core of the steel plate also begins to pass through the ferrite area and enter the bainite phase transformation area. Because the cooling of the core lags behind the surface by 80 seconds, the cooling temperature curve of the core can be moved forward by 80 seconds. The actual calculated cooling rate is 5.5°C / s. Cooling in this way ensures that the surface and core of the steel plate have completed the bainite phase transformation, thereby obtaining a uniform structure of the surface and core performance.

[0056] Example 3

[0057] For a common carbon steel plate with a thickness of 120 mm, a large water flow rate of 500 L / m is first applied to the surface of the steel plate. 2 min water flow rate for rapid cooling, the surface temperature of the steel plate drops to 440℃ after 10 seconds, so that a temperature difference is formed between the surface and the core of the steel plate, and then the water volume on the surface of the steel plate is reduced to maintain the water flow density of 360L / m 2 min, after 20 seconds, the surface temperature of the steel plate decreases from 440℃ to 350℃, and then the water volume is increased to maintain the water flow density at 400L / m 2 min, the surface of the steel plate is rapidly cooled to below 200°C at 2.5°C / s, and the heat dissipation from the core of the steel plate to the surface of the steel plate is further accelerated. The cooling rate of the core is controlled at 4.2°C / s.

[0058] The cooling curve is obtained by calculating the cooling process of a common carbon steel thick plate product with a thickness of 120 mm by setting the segmented water flow rate. Figure 8shown.

[0059] See also Fig. 9 , after coordinate transformation and superposition with the product CCT diagram, the beneficial effect of the cooling process can be seen. The surface of the steel plate is first quickly cooled to 440℃, so that the temperature difference between the surface of the steel plate and the core is widened, and then the water volume is reduced. After 20 seconds, the temperature of the steel plate surface decreases from 440℃ to 350℃, and the surface of the steel plate passes through the bainite phase transformation area. Then, the cooling water is continued to increase, and the surface of the steel plate is cooled to below 200℃. The temperature of the core of the steel plate also begins to pass through the ferrite area and enter the bainite phase transformation area. Because the cooling of the core lags behind the surface by 110 seconds, the cooling temperature curve of the core can be moved forward by 110 seconds. The actual calculated cooling rate is 4.2℃ / s. Cooling in this way ensures that the surface and core of the steel plate have completed the bainite phase transformation, thereby obtaining a uniform structure of the surface and core performance.

[0060] Example 4

[0061] For a 100 mm thick steel plate, the conventional ultra-fast cooling process and the cooling process of the present invention are used. The cooling curves of the steel plate surface and different thickness layers are as follows: Fig.10 , Fig.11 shown.

[0062] See also Fig.10 Under the traditional ultra-fast cooling process, a large water volume of 700L / m 2 min until the cooling is completed, and there is no water adjustment in the middle. The surface of the steel plate quickly enters the martensitic phase transformation zone, while the half and one quarter of the thickness of the steel plate are in the bainitic phase transformation zone, which will cause a large deviation in the microstructure and performance of the surface and core.

[0063] See also Fig.11 Through the cooling process conditions of the present invention, a large water flow rate of 400L / m 2 min water for rapid cooling, the steel plate surface reaches 500℃ after 12 seconds, so that the temperature difference between the steel plate surface and the core is formed, and then the water flow rate is reduced to 220L / m 2 min water volume, the temperature of the steel plate surface drops from 500℃ to 350℃ after 20 seconds to ensure that the steel plate surface passes through the bainite phase transformation area, and then increase the water flow rate on the steel plate surface to 400L / m 2 min water flow density, the surface of the steel plate is rapidly cooled to below 200℃ at a cooling rate of 2.2℃ / s, and the core is cooled to the bainite phase transformation range at a rate of 4.8℃ / s, so that bainite structure is obtained on both the surface and the core of the steel plate, that is, a uniform structure is obtained in the thickness direction of the steel plate.

Claims

1. A rapid cooling method for obtaining uniform structure in the thickness direction of a particularly thick plate, characterized in that: The steel plate after rolling is in the austenite temperature range of 700-980°C; For steel plates with a thickness of 80 to 100 mm, a large water flow rate is first applied to the surface of the steel plate to reduce the surface temperature of the steel plate to below 500°C but above 350°C within 12 seconds, so that a temperature difference is formed between the surface and the core of the steel plate. Then the surface of the steel plate begins to enter a small water flow mode, so that the surface temperature of the steel plate is maintained between 350 and 500°C for more than 20 seconds, and the surface of the steel plate completely passes through the bainite phase transformation region. Subsequently, the water flow on the surface of the steel plate is continued to increase, so that the surface cooling rate is greater than 2°C / s, until the surface temperature of the steel plate is below 200°C, and the core and surface of the steel plate obtain bainite phase transformation structure; For steel plates with a thickness of 100-120 mm, a large water flow rate is first applied to the surface of the steel plate to reduce the surface temperature of the steel plate to below 450°C but above 350°C within 12 seconds, so that a temperature difference is formed between the surface and the core of the steel plate. Then the surface of the steel plate begins to enter a small water flow mode, maintaining the surface temperature of the steel plate between 350 and 450°C for more than 20 seconds, and the surface of the steel plate completely passes through the bainite phase transformation region; then the water flow is continued to be increased, so that the cooling rate of the steel plate surface is greater than 2°C / s, until the surface temperature of the steel plate is below 200°C, and the core and surface of the steel plate obtain a bainite phase transformation structure; Through the above process, bainite structure is formed on the surface and core of the steel plate, achieving uniform performance on the surface and core.

2. The rapid cooling method for obtaining uniform structure in the thickness direction of a particularly thick plate as claimed in claim 1, characterized in that: For steel plates with a thickness of 80 to 100 mm, after the surface temperature of the steel plate is below 200°C, the heat dissipation from the core of the steel plate to the surface of the steel plate is further accelerated, and the cooling rate of the core is controlled to be greater than 2°C / s, so that the core of the steel plate enters the bainite phase transformation region.

3. The rapid cooling method for obtaining uniform structure in the thickness direction of a particularly thick plate as claimed in claim 1, characterized in that: For steel plates with a thickness of 100 to 120 mm, after the surface temperature of the steel plate is below 200°C, the heat dissipation from the core of the steel plate to the surface of the steel plate is further accelerated, and the cooling rate of the core is controlled to be greater than 2°C / s, so that the core of the steel plate enters the bainite phase transformation region.

4. The rapid cooling method for obtaining uniform structure in the thickness direction of a particularly thick plate as claimed in claim 1, characterized in that: For steel plates with a thickness of 80 to 100 mm, a large water flow rate is first applied to the surface of the steel plate, and the water flow density is greater than 370 L / m 2 min, the surface temperature of the steel plate is reduced to 350-500℃ within 12 seconds, so that the temperature difference between the surface and the core of the steel plate is formed, and then the surface begins to enter the small water flow mode, and the water flow density is 180-360L / m 2 min, so that the surface temperature of the steel plate is maintained at 350-500℃ for more than 20 seconds; then continue to increase the amount of water on the surface, and the water flow density is greater than 400L / m 2 min, making the surface cooling rate 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, and controlling 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 bainite phase transformation region.

5. The rapid cooling method for obtaining uniform structure in the thickness direction of a particularly thick plate as claimed in claim 1, characterized in that: For steel plates with a thickness of 100 to 120 mm, a larger water flow rate is applied to the steel plate, and the water flow density is greater than 400 L / m 2 min, reducing the surface temperature of the steel plate to 350-450°C within 12 seconds; then the surface begins to enter a small water flow mode, with a water flow density of 180-360L / m 2 min, maintain the steel plate surface temperature at 350-450℃ for more than 20 seconds, and then continue to increase the water volume, the water flow density is greater than 400L / m 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 bainite phase transformation region.

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