A heat treatment method of a nuclear power steel P460NH with high temperature resistance and high strength and toughness

Abstract

The application provides a heat treatment method of a nuclear power steel P460NH with high-temperature resistance and high strength and toughness, which comprises quenching, stage control cooling and tempering, and the steel plate has good hardenability, high-temperature resistance and toughness through high-pressure quenching in stages, the production efficiency is improved, the problem of uneven temperature caused by multiple quenching for improving the strength is avoided, and the production cost is reduced to a certain extent. The unevenness of the steel plate produced according to the process can reach 2mm / 1m. The red temperature of the steel plate is controlled through the stage quenching and stage control cooling process, a certain amount of bainite structure is ensured in the steel plate under a large cooling speed, the strength of the steel plate is ensured, a small amount of proeutectoid ferrite makes the steel plate have certain low-temperature toughness when bearing load, the yield strength is greater than or equal to 520MPa, the tensile strength in the 200 DEG C and room temperature tensile test is greater than or equal to 570MPa, the elongation is greater than or equal to 25-30%, the 0 DEG C impact energy is 160J-200J, the plate shape, high-temperature tensile property and excellent impact toughness fully meet the use requirements of the nuclear power steel.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of medium-thickness steel plate heat treatment, and particularly relates to a heat treatment method of a nuclear power steel P460NH with high-temperature-resistant and high-toughness characteristics. BACKGROUND

[0002] With the gradual commissioning of nuclear island unit equipment, the quality requirements of nuclear power steels are higher and higher. The nuclear power steels need to work in high-temperature and high-pressure environments for a long time, and the yield strength is required to be greater than 430 Mpa, and the room-temperature tensile strength and 200 DEG C high-temperature tensile strength are required to be greater than 570 Mpa, the elongation is required to be greater than 20%, the hardness value is required to be in the range of 128 HB-192 HB, and the 0 DEG C impact performance is required to be greater than 56 J. There is a serious contradiction between high toughness and high strength in the performance requirements.

[0003] In order to solve the problem among high-temperature-resistant strength, low hardness and high toughness of the steel plate, the following measures are generally used to improve: 1. improving the purity of molten steel in the smelting process, reducing the influence of harmful elements such as P and S on the cold and hot brittleness of the steel plate, but the high-temperature-resistant performance of the steel plate is not obviously improved; 2. adding a certain amount of Ni, Nb and Ti alloy elements for toughening, strengthening and high-temperature resistance, but the scheme has certain limitations due to the limitation of carbon equivalent and low hardness; 3. continuously refining grains and enhancing the tensile capacity of the steel plate when bearing load by combining multiple quenching and tempering heat treatment processes, but the nuclear power steel is strictly controlled in the heat treatment process, and multiple heat treatment is prohibited, and the process is relatively complicated and not suitable for large-scale industrial production.

[0004] Therefore, the application is provided. SUMMARY

[0005] The application aims to provide a heat treatment method of a nuclear power steel P460NH with high-temperature-resistant and high-toughness characteristics, which simplifies the heat treatment process, reduces the production cost and realizes batch production on the basis of meeting the high-temperature-resistant and high-toughness requirements of the nuclear power steel.

[0006] To achieve the above-mentioned purpose, the application adopts the following technical scheme: a heat treatment method of a nuclear power steel P460NH with high-temperature-resistant and high-toughness characteristics, the steel plate is subjected to heat treatment after smelting, rolling and stack cooling in a quenching line, and the specific heat treatment process is as follows:

[0007] The heat treatment of the steel includes quenching, staged controlled cooling and tempering. The quenching process requires the steel plate to be heated to 900-930℃ in a roller quenching machine, the holding time is 1.9-2.3 min / mm, after the holding time, the steel plate enters a high-pressure zone and a low-pressure zone for quenching in sequence, the roller speed is maintained at 3-5 m / min, the quenching water temperature is controlled at 15-20℃, and the water quantity is adjusted, wherein the ratio of the water quantity in the high-pressure zone to the water quantity in the high-pressure zone is 1.1-1.3, the ratio of the water quantity in the low-pressure zone to the water quantity in the low-pressure zone is 1.3-1.5, and the high-pressure zone is divided into three sections, which are controlled as follows:

[0008] a. The first section of the high-pressure zone: the water pressure is 0.8±0.05 Mpa, the controlled water quantity is 400-450 m 3 / h, and the spray angle of the cooling nozzle is 20-30° to ensure that the cooling speed is 12-15℃ / S. It should be noted that the spray angle of the nozzle in the first section of the high-pressure zone is adjusted to maximize the absorption of the heat of the steel plate surface by the water, so that the upper and lower surfaces of the steel plate maintain the same cooling speed, the flatness of the plate shape is maintained, and buckling is avoided. At the same time, the distribution of the transverse water flow is ensured to prevent water accumulation on the surface of the steel plate.

[0009] b. The second section of the high-pressure zone: the water pressure is 0.8±0.05 Mpa, the controlled water quantity is 100-150 m 3 / h to ensure that the cooling speed is 8-10℃ / S, and the surface temperature of the steel plate is as low as possible to generate a strengthening phase of bainite structure.

[0010] c. The third section of the high-pressure zone: the water pressure is 0.8±0.05 Mpa. In order to better solve the limitation of heat transfer of thick plates, ensure the consistency of the core temperature and the surface temperature of the steel plate, and avoid the generation of a large amount of proeutectoid ferrite which affects the overall tensile properties, the controlled water quantity is 230-280 m 3 / h to ensure that the cooling speed is 7-8℃ / S. After the high-pressure zone water cooling is completed, the steel plate is controlled to have a red temperature of 500-550℃.

[0011] d. The low-pressure zone: the water pressure is 0.4±0.05 Mpa, the controlled water quantity is 300-350 m 3 / h to ensure that the cooling speed is 5-8℃ / S, and the steel plate is quenched to below 50℃. It should be noted that the low-pressure zone quenching further removes the heat conducted from the core of the steel plate to the surface to prevent reheat, the uniform moderate water temperature continuously exchanges heat with the steel plate to ensure a large cooling speed, retains undissolved ferrite and a small amount of pearlite to have certain toughness, and obtains certain impact toughness.

[0012] The quenched steel plate is sent to a tempering furnace for tempering, the holding temperature is 650-670℃, the holding time is 3.5-4.0 min / mm, and then air cooling is performed to room temperature.

[0013] The beneficial effects of the present application include:

[0014] The present application uses segmented control of water pressure and water volume to control the cooling capacity of the steel plate. First, the steel plate obtains high hardenability to obtain supercooled austenite, and then a small amount of proeutectoid ferrite and a large amount of bainite structure are obtained by controlling the cooling rate. The flatness of the plate is maintained by reasonable angle distribution of the cooling nozzle and cooling in the low pressure area, avoiding the temperature inhomogeneity problem existing in multiple quenching to improve strength, improving production efficiency and reducing production cost to a certain extent. The red temperature of the steel plate is controlled by the segmented quenching and staged controlled cooling process. Under a large cooling rate, a certain amount of bainite structure is generated to ensure the strength of the steel plate, and a small amount of proeutectoid ferrite makes the steel plate have certain low-temperature toughness when bearing load. The flatness of the steel plate produced according to the process is also greatly improved, and the unevenness can reach 2mm / 1m.

[0015] The high-temperature-resistant and high-strength and toughness nuclear power steel P460NH thick plate obtained according to the present application has a yield strength of ≥520MPa, a tensile strength of ≥570MPa in 200℃ and room temperature tensile test, and an elongation of ≥25-30%, and a 0℃ impact energy of 160J-200J. The flatness, high-temperature tensile properties and excellent impact toughness fully meet the use requirements of nuclear power steel. BRIEF DESCRIPTION OF DRAWINGS

[0016] The technical features of the present application will be further described below in combination with the drawings and examples.

[0017] Figure 1 The metallographic structure schematic diagram of the steel plate obtained in Example 1 of the present application

[0018] Figure 2 The metallographic structure schematic diagram of the steel plate obtained in Comparative Example 2

[0019] Figure 3 The metallographic structure schematic diagram of the steel plate obtained in Comparative Example 3 DETAILED DESCRIPTION

[0020] The present application will be further described below in combination with the specific embodiments. It should be noted that the following embodiments or technical features can be combined to form new embodiments without conflict.

[0021] The quenching section of the roller quenching machine is briefly introduced as follows: the whole quenching section is divided into a high-pressure zone and a low-pressure zone. The high-pressure zone is 5 m long and has a water pressure of 0.8 MPa, and is divided into three sections, namely, a high-pressure zone section 1, a high-pressure zone section 2 and a high-pressure zone section 3. The high-pressure zone section 1 is 1.2 m long, and is provided with three groups of angle-adjustable slit cooling nozzles, including upper slit cooling nozzles and lower slit cooling nozzles. In the present scheme, the spray angle of the slit cooling nozzles is set to be between 20-30°. The high-pressure zone section 2 is 1.8 m long, and is provided with four groups of double super-fast cooling nozzles. The high-pressure zone section 3 is 2 m long, and is provided with six groups of fast cooling nozzles symmetrically arranged above and below. The low-pressure zone is 17145 mm long and has a water pressure of 0.4 MPa, and is also divided into three sections, namely, a low-pressure zone section 1, a low-pressure zone section 2 and a low-pressure zone section 3. Each section is provided with 15 groups of nozzles symmetrically arranged above and below, and the interval between adjacent two groups of nozzles is 381 mm.

[0022] Example 1

[0023] A preparation method of a 45 mm thick nuclear power steel P460NH with high temperature resistance and high strength and toughness is as follows:

[0024] (1) Smelting: the molten iron after KR desulfurization and high-quality scrap steel are sequentially smelted in a converter-LF furnace-VD vacuum furnace, and a high-alkali slag former is added for 30 min of large slag amount slagging, and 60 Kg of calcium wire is added to improve the purity of the molten steel. The spectral smelting composition is as follows (unit: wt%): C (0.161), Si (0.20), Mn (1.55), P≤0.015, S≤0.003, Cu (0.18), Nb (0.02), V (0.044), Ti (0.007), Alt (0.030), [H]≤1.0ppm, [0]≤15ppm.

[0025] (2) Pouring: the molten steel obtained in step (1) is poured into an ingot using a water-cooled mold with a thickness of 530 mm, a width of 2000 mm and a weight of 24.5 t.

[0026] (3) Ingot heating: the ingot obtained in step (2) is brushed with high-temperature paint and then full-plate cleaning is performed, and the ingot is heated to 1270℃ in a soaking pit at a heating time of 10 min / cm, and the holding time is 10 h.

[0027] (4) Rolling breakdown: after the ingot heated in step (3) is high-pressure descaled, rolling is started, the breakdown temperature is 1050℃, the pass reduction is 40 mm, after the ingot thickness is below 350 mm, the pass reduction is adjusted to 30 mm, and the intermediate billet thickness is 300 mm.

[0028] (5) Intermediate billet heating: the intermediate billet obtained in step (4) is stacked in a slow cooling pit for 24 h, then enters a regenerative heating furnace for 5 h of holding, and the holding temperature is 1270℃.

[0029] (6) Controlled rolling: the intermediate blank after heating in step (5) is high-pressure water descaled, the first rolling temperature is 1020°C, and the rolling is carried out to 130mm using a 40mm pass reduction, then the steel is air-cooled, the second rolling temperature is 820°C, and the rolling is carried out to the finished thickness and enters the ACC fast cooling, the re-red temperature is 580°C. After straightening, the steel plate is sent to the slow cooling pit for slow cooling, and the slow cooling time is 48h.

[0030] (7) Quenching: the steel plate after slow cooling in step (6) is continuously heated in stages in a roller quenching machine, the heating temperature is 920°C, and the heating time is 2.2min / mm; after heating, the high-pressure zone water pipe is opened for quenching, the roller speed is kept at 4m / min, the water temperature is 20°C, the lower water and upper water ratio is 1.1, 3 groups of high-pressure zone one are fully opened, the water flow is 450m 3 / h, and the control cooling speed is 14°C / s; 3 groups of double super-fast cooling nozzles in high-pressure zone two are opened, the water flow is 150m 3 / h, and the control cooling speed is 10°C / s; 4 groups of fast cooling nozzles in high-pressure zone three are opened for cooling, the water flow is 250m 3 / h, and the control cooling speed is 7°C / s, after the high-pressure section, the re-red temperature is 510°C. The lower water and upper water ratio in the low-pressure zone is 1.4, the water flow is 300m 3 / h, 13 groups of nozzles are opened for cooling in each section, and the control cooling speed is 7°C / s, and the final temperature of the steel plate is 25°C.

[0031] (8) Tempering: the steel plate in step (7) is tempered in a tempering furnace, the tempering temperature is 670°C, the holding time is 3.6min / mm, and then air-cooled to room temperature, thereby obtaining a 45mm thick high-temperature-resistant and high-toughness nuclear power steel P460NH plate.

[0032] The performance of the steel plate is shown in Table 1

[0033] Example 2

[0034] A method for preparing a 60mm thick high-temperature-resistant and high-toughness nuclear power steel P460NH is as follows:

[0035] (1) Smelting: the molten iron after KR desulfurization and high-quality scrap are sequentially smelted in a converter-LF furnace-VD vacuum furnace, 25min of large slag amount slagging is carried out by adding high-basicity slagging agent, and 60Kg of calcium wire is used to improve the purity of the molten steel. The spectral smelting composition is as follows (unit: wt%): C(0.165), Si(0.25), Mn(1.62), P≤0.013, S≤0.003, Cu(0.17), Nb(0.037), V(0.055), Ti(0.002), Alt(0.030), [H]≤1.0ppm, [0]≤15ppm.

[0036] (2) Pouring: The molten steel obtained in step (1) is poured into an ingot using a water-cooled mold with a thickness of 580 mm, a width of 2000 mm, and a weight of 27.2 t.

[0037] (3) Ingot heating: The ingot obtained in step (2) is brushed with high-temperature paint and then full-plate cleaning is performed. The ingot is heated to 1270°C in a soaking pit at a heating time of 10 min / cm, and the holding time is 11 h.

[0038] (4) Rolling breakdown: After the ingot heated in step (3) is high-pressure descaled, rolling is started, the breakdown rolling temperature is 1050°C, the pass reduction is 40 mm, and after the ingot thickness is below 350 mm, the pass reduction is adjusted to 30 mm, and the intermediate billet thickness is 320 mm.

[0039] (5) Intermediate billet heating: After the intermediate billet obtained in step (4) is stacked in a slow cooling pit for 24 h, it is put into a regenerative heating furnace for 5 h of holding at a holding temperature of 1270°C.

[0040] (6) Controlled rolling: The intermediate billet heated in step (5) is high-pressure water descaled, the first breakdown rolling temperature is 1020°C, and after rolling to 130 mm using a pass reduction of 40 mm, the steel is allowed to stand, the second breakdown rolling temperature is 820°C, and the steel is rolled to the finished thickness and enters the ACC fast cooling, the re-red temperature is 600°C. After straightening, it is sent to the slow cooling pit for stacking and cooling for 48 h.

[0041] (7) Quenching: The steel plate after stacking and cooling in step (6) is continuously heated in stages in a roller quenching machine, the heating temperature is 910°C, and the heating time is 2.3 min / mm; after heating, the high-pressure zone water pipes are opened for quenching, the roller speed is maintained at 3 m / min, the water temperature is 18°C, the lower water to upper water ratio is 1.2, the high-pressure zone first segment is fully open, the water flow is 420 m 3 / h, the controlled cooling rate is 12°C / s; the high-pressure zone second segment opens 3 groups of double super-fast cooling nozzles, the water flow is 150 m 3 / h, the controlled cooling rate is 8°C / s; the high-pressure zone third segment opens 5 groups of fast cooling nozzles for cooling, the water flow is 280 m 3 / h, the controlled cooling rate is 7°C / s, and after the high-pressure segment, the re-red temperature is 500°C. The lower water to upper water ratio in the low-pressure zone is 1.4, the water flow is 350 m 3 / h, each segment opens 13 groups of nozzles for cooling, the controlled cooling rate is 6°C / s, and the final temperature of the steel plate is 20°C.

[0042] (8) Tempering: The steel plate of step (7) is tempered in a tempering furnace at a tempering temperature of 660°C and a holding time of 4 min / mm, and then air-cooled to room temperature to obtain a 60 mm thick plate of high-temperature resistant, high-toughness nuclear power steel P460NH.

[0043] The properties of the steel plate are shown in Table 1

[0044] Comparative Example 1

[0045] Comparative Example 1 differs from Example 1 in that in step (7), the gap cooling nozzles in the high-pressure zone first section are closed, the re-red temperature is measured to be 580℃ after the high-pressure zone, then the low-pressure zone 15 groups of nozzles are opened for cooling, the roller speed is 3m / min, and the final temperature of the steel plate is 22℃. The rest is the same as Example 1.

[0046] Comparative Example 2

[0047] Comparative Example 2 differs from Example 1 in that in step (7), 2 groups of double cooling nozzles are opened in the high-pressure zone second section, and 2 groups of fast cooling nozzles are opened in the high-pressure zone third section for cooling, and the re-red temperature is measured to be 650℃ after the high-pressure zone. The rest is the same as Example 1.

[0048] Comparative Example 3

[0049] Comparative Example 3 differs from Example 2 in that in step (7), 4 groups of cooling nozzles are opened in the high-pressure zone second section, and 15 groups of nozzles are opened in the low-pressure zone, and the re-red temperature is measured to be 420℃ after the high-pressure zone. The rest is the same as Example 2.

[0050] Comparative Example 4

[0051] Comparative Example 4 differs from Example 1 in that in step (7), 8 groups of nozzles are opened in the low-pressure zone for cooling, and the final temperature of the steel plate is 45℃. The rest is the same as Example 1.

[0052] The yield strength, tensile strength, 200℃ high-temperature tensile, 0℃ impact, hardness and flatness of the nuclear power steel P460NH thick plate with high temperature resistance and high strength and toughness prepared by Examples 1 to 2 and Comparative Examples 1 to 4 are tested. The test standards are consistent, and the specific test performance data are shown in Table 1.

[0053] Table 1

[0054]

[0055] As can be seen from Table 1, the yield strength of the nuclear power steel P460NH thick plate material with high temperature resistance and high strength and toughness obtained by Examples 1 to 2 is ≥520MPa, the room temperature tensile strength is ≥600MPa, the 200℃ high-temperature tensile strength is ≥570MPa, the average impact energy is greater than 175J, the Brinell hardness is below the required critical value of 180HB, the plate shape is flat, and the flatness is 2mm<1m. It can be seen that the product obtained by the present application has high tensile strength, excellent high temperature resistance and high strength and toughness performance characteristics.

[0056] Comparative Example 1 and Example 1 (such asFigure 1 ) In comparison, the cold water spray in the high pressure zone 1 section is turned off, the roll speed is appropriately reduced, and the time of the steel plate in the high pressure quenching zone is increased, and it can be found that the room temperature and high temperature tensile properties are slightly lower than those of Example 1. Turning off the high pressure zone 1 section spray reduces the water entry temperature of the steel plate indirectly, and the cooling rate is relatively reduced compared to Example 1, and the cooling rate is lower than the critical transformation temperature of bainite, and mainly granular bainite and granular structure are formed. Compared with Comparative Example 2, the cooling rate of the steel plate of Comparative Example 1 is improved compared with Comparative Example 2 when the cooling rate is greater than 5°C / S. At low temperature, the austenite will transform into massive ferrite, and the massive ferrite and C-rich austenite will form M / A islands, so the tensile strength at this time will be improved compared with Comparative Example 2.

[0057] Comparative Example 2 (as shown in Table 2) Figure 2 ) Compared with Example 1, both groups of tests are subjected to high pressure zone 3 cooling quenching, but the cooling rate of Comparative Example 2 is slower, the ferrite in the structure gradually increases, and the formed bainite structure is relatively less, so the tensile strength will gradually decrease, but the residual austenite structure in the structure will be transformed into fine-grained pearlite under sufficient high-temperature tempering, so the low-temperature impact performance at this time will be improved to a certain extent. But its hardness will rise to a certain extent, and finally it does not meet the use requirements.

[0058] Comparative Example 3 (as shown in Table 2) Figure 3 ) Compared with Example 1, its plate thickness is 60mm, so 4 groups of high pressure water are opened in the high pressure zone 2 section to enhance the quenching capacity, so that the 1 / 2 thickness can also be quenched. The low pressure section is fully opened, and the roll speed is relatively slow, resulting in poor overall plate shape. The high pressure zone implementation method is the same as Example 1, and the cooling rate is larger than that of Example 1, which is basically 15°C / S. The greater the cooling rate, the lower the transformation temperature of bainite, and the diffusion of C atoms is insufficient, so a large amount of M / A island structure is formed, the grain size is reduced, the morphology is gradually transformed from granular bainite to lath bainite, and the strength is gradually improved, but the impact performance is overall low.

[0059] Comparative Example 4 compared with Example 1, the amount of low pressure zone spray is reduced, the temperature cannot be conducted from the core of the steel plate to the surface, and tempering occurs, so the tensile strength of the steel plate is low.

[0060] In summary, the application provides a heat treatment method of high-temperature-resistant and high-toughness steel P460NH for nuclear power, which is suitable for industrial production. The cooling speed of the steel plate is controlled by controlling high and low pressure water cooling nozzles in stages, so that the steel plate has a certain tensile strength under high temperature load, while ensuring good hardness and impact toughness, and meeting the use environment of nuclear power under high temperature and high pressure. By reasonably adjusting the cooling ratio of high and low pressure, the re-red temperature in different stages is controlled to ensure effective transformation of lath bainite. Under very high cooling speed, the phase change temperature point is reduced, and the diffusion of carbon atoms is also insufficient, resulting in an increase in the number of M / A islands in the structure and a decrease in the size of the M / A islands. Under the condition of very high cooling speed, the phase change driving force gradually increases, and the dislocation density increases. The dispersed distribution of bainite ensures a high tensile strength. In the interior of the thick plate, the quenching machine cannot fully quench, so a certain amount of ferrite is formed to increase the toughness of the steel plate. At the same time, through accurate cooling in the low pressure area, the temperature conduction from the inside to the surface of the steel plate is effectively avoided, so that the temperature difference between the core and the surface is reduced, and the good flatness of the plate is maintained. The high-temperature-resistant and high-toughness steel P460NH thick plate prepared by the application has fine grains, uniform bainite structure distribution, good interface bonding, high-temperature-resistant tensile properties and excellent impact toughness.

[0061] The above embodiments are only preferred embodiments of the application, and cannot be used to limit the protection scope of the application. Any non-essential changes and replacements made by those skilled in the art based on the application belong to the protection scope of the application.

Claims

1. A heat treatment method for nuclear power steel P460NH with high temperature resistance, high strength and toughness, characterized in that, The heat treatment of the steel includes staged controlled cooling quenching and tempering. The staged controlled cooling quenching process requires the steel plate to be heated to 900℃-930℃ in a roller quenching mill, with a holding time of 1.9-2.3 min / mm. After holding, it sequentially enters the high-pressure zone and low-pressure zone for quenching, maintaining a roller speed of 3-5 m / min, controlling the quenching water temperature at 15-20℃, and adjusting the water volume. Specifically, the ratio of water flow to top water flow in the high-pressure zone is 1.1-1.3, and the ratio in the low-pressure zone is 1.3-1.

5. The high-pressure zone is divided into three sections, and the specific control is as follows: a. High-pressure zone section: water pressure 0.8±0.05Mpa, water flow controlled at 400-450m³. 3 / h, the spray angle of the cooling nozzle is 20-30° to ensure a cooling rate of 12-15℃ / S; b. High-pressure zone, section two: water pressure 0.8±0.05Mpa, water flow controlled at 100-150m³. 3 / h, ensuring a cooling rate of 8-10℃ / S; c. High-pressure zone, three sections: water pressure 0.8±0.05Mpa, water flow controlled at 230-280m³. 3 / h, ensuring a cooling rate of 7-8℃ / S, water cooling in the high-pressure zone ends, and the steel plate reddening temperature is controlled at 500-550℃; d. Low-pressure zone: water pressure 0.4±0.05Mpa, water flow controlled at 300-350m³. 3 / h, ensuring a cooling rate of 5-8℃ / S, and quenching the steel plate to below 50℃; The quenched steel plate is sent to a tempering furnace for tempering at a holding temperature of 650℃-670℃ for 3.5-4.0 min / mm, and then air-cooled to room temperature.

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