A production method and simulated hot forming process of thick steel plates for high-temperature and high-pressure vessel heads

Abstract

A production method and simulated hot forming process for thick steel plates for high-temperature and high-pressure vessel heads. The chemical composition of the steel is as follows by weight: C=0.11%-0.13%, Si=0.05%-0.12%, Mn=1.40%-1.60%, P≤0.010%, S≤0.002%, Sn≤0.010%, Cr=0.20%-0.40%, Mo=0.20%-0.40%, Ni=0.60%-1.0%, Al=0.05%-0.08%, Nb=0.010%-0.030%, B=0.0010%-0.0020%, N=0.0040%-0.0060%, with the remainder being Fe and unavoidable impurity elements. The thickness of the steel plates is 75-145 mm, and key production process steps include smelting, heating, controlled rolling and controlled cooling, normalizing, and tempering. The key steps in the simulated hot forming process for steel plates include hot forming and simulated post-weld heat treatment. The thick steel plates for high-temperature and high-pressure vessel heads described in this invention can be directly subjected to simulated post-weld heat treatment after hot forming without requiring heat treatment to restore properties, while still meeting the mechanical property requirements of the steel plates prior to hot forming.

Description

Technical Field

[0001] The present invention belongs to the technical field of low alloy steel production, and relates to a production method and a simulated hot forming process of thick steel plates for high temperature and high pressure vessel heads. Background Art

[0002] Steel plates for high-temperature and high-pressure vessel heads are widely used in reactors, heat exchangers, nuclear reactor pressure vessels, boiler drums, and other high-temperature and high-pressure vessels. As equipment becomes larger, the strength and wall thickness of head plates continue to increase, leading to increasingly stringent requirements for their thermoformability. While increasing the thermoforming temperature improves formability, it also leads to coarsening of the re-austenitized grains and a significant decrease in the mechanical properties of the steel. Therefore, heat treatment is generally required to restore the properties of the head plates after thermoforming. Chinese patent CN108315539A discloses a method for producing extra-wide, extra-thick steel plates for vessel heads with excellent hot formability, utilizing a normalizing, water cooling, and tempering process to restore performance. Chinese patent CN108754340A proposes Q345R steel plates for pressure vessel heads and a production method thereof, subjecting the plates to two normalizing and tempering heat treatments to restore performance. Chinese patent CN112176156A discloses a production method and simulated hot forming method for SA387Gr22CL2 steel plates for pressure equipment heads. To ensure final performance, a quenching and tempering heat treatment process is performed after hot forming. Undoubtedly, this performance-restoring heat treatment not only places higher demands on the equipment capabilities of the head manufacturer, requiring supporting cooling equipment, but also increases the number of manufacturing steps and prolongs the product production cycle, increasing costs and impacting delivery. Therefore, developing thick steel plates for head heads that do not require a performance-restoring heat treatment after hot forming is an important path to improving the competitiveness of high-temperature and high-pressure vessel manufacturers. Summary of the Invention

[0003] One of the objects of the present invention is to provide a method for producing thick steel plates for high-temperature and high-pressure vessel heads, wherein the steel plates have a thickness of 75 to 145 mm and are designed with low Si, low P, low S, and low Sn, with the addition of Mn, Cr, Mo, and Ni alloying elements, and controlled Al, Nb, B, and N trace elements. In the delivery state of normalizing and tempering, the steel plates have a yield strength ≥390 MPa, a tensile strength of 570 to 720 MPa, an elongation ≥18%, a 0°C Charpy V-type impact energy ≥47 J, and a yield strength ≥330 MPa at 360°C. Another object of the present invention is to provide a simulated hot forming process for the thick steel plates for high-temperature and high-pressure vessel heads, wherein no recovery performance heat treatment is performed after hot forming, and simulated post-weld heat treatment is directly performed, thereby meeting the mechanical property requirements of the steel plates before hot forming.

[0004] The technical solution of the present invention is:

[0005] A method for producing thick steel plates for high-temperature and high-pressure vessel heads, wherein the steel plates have a thickness of 75 to 145 mm, and the chemical composition of the steel is as follows by weight: C = 0.11% to 0.13%, Si = 0.05% to 0.12%, Mn = 1.40% to 1.60%, P ≤ 0.010%, S ≤ 0.002%, Sn ≤ 0.010%, Cr = 0.20% to 0.40%, Mo = 0.20% to 0.40%, Ni = 0.60% to 1.0%, Al = 0.05% to 0.08%, Nb = 0.010% to 0.030%, B = 0.0010% to 0.0020%, N = 0.0040% to 0.0060%, with the remainder being Fe and unavoidable impurity elements. The key process steps include:

[0006] (1) Smelting: After LF refining, the molten steel is treated with RH vacuum. After breaking the vacuum, 400~600m pure calcium wire is fed. After feeding the wire, the soft argon blowing time is maintained for ≥10min; then full protection casting is carried out, the superheat is controlled at 10~20℃, and weak cooling is used for secondary cooling; the total compression ratio of the cross-sectional size of the billet is ≥3; after the continuous casting billet stack is cooled to room temperature, the surface quality inspection and flame cleaning are carried out;

[0007] (2) Heating: The soaking temperature is 1150±10℃ and the soaking time is 60~80min;

[0008] (3) Controlled rolling and controlled cooling: adopt two-stage controlled rolling, with the rough rolling reduction ratio ≥2 and the rolling temperature in the range of 950~1000℃, and at least 3 passes of the rough rolling stage with a reduction ratio ≥15%; then the intermediate billet is water-cooled, and the finishing rolling start temperature is controlled to be ≤880℃, and the final rolling temperature is 820~840℃; after rolling, it is rapidly cooled to 680~720℃ at a cooling rate of ≥2℃ / s, and then stacked and slowly cooled to room temperature;

[0009] (4) Normalizing: heating temperature is 960±10℃, heating time is (1.8~2.0)×plate thickness mm×min / mm, holding time is 40~60min;

[0010] (5) Tempering: heating temperature is 680±10℃, holding time is 50~80min.

[0011] A simulated hot forming process for thick steel plates used in high-temperature and high-pressure vessel heads. The key process steps include:

[0012] (1) Thermoforming: heating temperature is 960±10℃, holding time is (1.2~1.6) plate thickness mm×min / mm, and then air cooling to room temperature;

[0013] (2) Simulated post-weld heat treatment: heating temperature is 570~600℃, holding time is 9~11h, and the heating and cooling rate above 300℃ is ≤55℃ / h.

[0014] Principle of the invention:

[0015] The present invention adopts a high-temperature normalizing + tempering process. In order to ensure sufficient toughness, the appropriate amount of C content is controlled, Mn, Cr, and Mo are added to improve the strength, and Ni is added to improve the toughness. At the same time, Cr and Mo have the effect of improving the high-temperature tensile strength, and Mo has the effect of inhibiting the embrittlement of the steel plate during tempering and simulated post-weld heat treatment; the use of low Si, low P, low S, and low Sn is beneficial to improving the toughness of the steel plate. At the same time, the lower P+Sn content is also to avoid the embrittlement of the grain boundaries during long-term tempering and simulated post-weld heat treatment; the present invention adopts a higher Al content and adds Nb and B to generate second-phase particles such as BN, AlN, NbC, and NbN. Not only does it improve the strength through precipitation strengthening, but these precipitated particles also play an important role in refining the original austenite grains during the controlled rolling process, making the structure fine and uniform during the controlled cooling process, and inhibiting the growth of austenite grains during the normalizing process, so even if a much higher content is used, A c3 The normalizing temperature can also obtain good strength and toughness; however, the present invention does not use too much N content. On the one hand, too much N content is likely to cause cracks on the surface of the continuous casting billet. On the other hand, too high N content affects the formability and causes hazards such as aging brittleness.

[0016] The present invention adopts LF refining + RH vacuum treatment for molten steel. Compared with VD vacuum treatment, RH vacuum treatment can retain more N content. Since it is necessary to control a certain N content in the molten steel, there is no need to worry about the problem of a small amount of nitrogen absorption by the molten steel. In order to ensure the castability of high-Al molten steel, RH vacuum breaking is selected and then heavy calcium treatment is carried out. At the same time, after calcium treatment, the soft blowing time is guaranteed to allow the inclusions to fully float up. The continuous casting process of the present invention controls the overheating, secondary cooling and cross-sectional dimensions of the ingot, which is a comprehensive consideration of the control of the center segregation, looseness and surface quality of the continuous casting ingot. The heating and controlled rolling and controlled cooling process parameters adopted by the present invention are to obtain a fine and uniform initial structure with relatively few defects in the structure and the energy stored therein, so as to prevent abnormal growth of grains during subsequent high-temperature normalizing. At the same time, it is required that the rough rolling stage with a rolling temperature within the range of 950~1000℃ has at least 3 passes with a reduction rate ≥15%. This is of great significance for promoting the full precipitation of secondary phases such as BN, AlN, NbC, and NbN and making them finely dispersed. The present invention adopts a temperature far higher than A c3 The high temperature normalizing is carried out at a temperature of 1000℃ because the final performance of the steel plate depends on the hot forming process of the head factory. Therefore, it is more practical to deliver the steel plate in a heat treatment state similar to the hot forming process of the head.

[0017] The simulated hot forming process proposed in the present invention does not require heat treatment to restore performance, which reduces the manufacturing cost of the head and improves production efficiency. It is a green process. The thick steel plate for high-temperature and high-pressure vessel heads described in the present invention is a green product. This is the most prominent advantage of the present invention compared with the existing technology. In addition, the beneficial effects of the present invention also include: (1) The thick steel plate for high-temperature and high-pressure vessel heads produced by the present invention and after simulated hot forming have good strength and toughness: yield strength ≥390MPa, tensile strength of 570~720MPa, elongation ≥18%, 0℃ Charpy V-type impact energy ≥47J, and yield strength of 360℃ high-temperature tensile ≥330MPa. Moreover, the performance of the two is similar and relatively stable, so the quality assurance certificate of the steel plant can only test the mechanical properties of the delivered steel plate and provide it to the customer, thereby saving sampling and performance testing after the simulated hot forming process, which not only improves the steel plate yield rate, but also speeds up the production cycle and delivery; (2) The present invention adopts the normalizing + tempering process for delivery, and the steel plate has low residual stress, high flatness and good surface quality, which provides a good basis for the subsequent head size and quality assurance when the steel plate is hot formed; (3) The production method of the present invention can serve as a reference for the development of carbon steel, low alloy steel, CrMo steel, hydrogen steel, low temperature steel and other steels for head use. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the metallographic structure diagram of the steel plate at 1 / 4 thickness of Example 1 of the present invention.

[0019] Figure 2 This is the metallographic structure diagram at 1 / 4 thickness of the steel plate sample after the simulated hot forming process in Example 1 of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be further described below with reference to the embodiments and accompanying drawings. Example 1

[0021] A method for producing thick steel plates for high-temperature and high-pressure vessel heads, wherein the steel plates have a thickness of 80 mm and a chemical composition by weight of C=0.11%, Si=0.10%, Mn=1.47%, P=0.008%, S=0.002%, Sn=0.004%, Cr=0.30%, Mo=0.28%, Ni=0.70%, Al=0.065%, Nb=0.015%, B=0.0012%, and N=0.0048%, with the remainder being Fe and unavoidable impurity elements. The key process steps include:

[0022] (1) Smelting: After LF refining, the molten steel is treated with RH vacuum. After breaking the vacuum, 500m of pure calcium wire is fed. After feeding the wire, soft argon blowing time is maintained for 12 minutes. Then full-process protection pouring is carried out, the superheat is controlled at 15~17℃, and weak cooling is used for secondary cooling. The cross-sectional size of the ingot is 300mm×2270mm, and the total compression ratio is 3.75. After the continuous casting ingot stack is cooled to room temperature, the surface quality inspection and flame cleaning are carried out.

[0023] (2) Heating: soaking temperature is 1150℃, soaking time is 65min;

[0024] (3) Controlled rolling and controlled cooling: Two-stage controlled rolling is adopted, with a rough rolling reduction ratio of 2.5, three rough rolling passes with a rolling temperature in the range of 950-1000°C, and reduction rates of 15.6%, 16.8%, and 18.5% for each pass; then the intermediate billet is water-cooled, and the finishing rolling start temperature is controlled at 880°C and the final rolling temperature is 825°C; after rolling, the billet is rapidly cooled to 685-700°C at a cooling rate of about 4.0°C / s, and then slowly cooled to room temperature by stacking;

[0025] (4) Normalizing: heating temperature is 960℃, heating time is 150min, holding time is 55min;

[0026] (5) Tempering: heating temperature is 680℃ and holding time is 75min.

[0027] A simulated hot forming process for thick steel plates used in high-temperature and high-pressure vessel heads. The key process steps include:

[0028] (1) Thermoforming: heating temperature is 960℃, holding time is 120min, and then air cooling to room temperature;

[0029] (2) Simulated post-weld heat treatment: heating temperature is 580℃, holding time is 9.5h, heating rate above 300℃ is about 50℃ / h, cooling rate above 300℃ is about 50℃ / h.

[0030] Figure 1 This is the metallographic structure diagram of the steel plate at 1 / 4 thickness in the delivery state of Example 1 of the present invention. Figure 2The microstructure at 1 / 4 thickness of a steel plate sample taken from Example 1 of the present invention after a simulated hot forming process is shown. The microstructure is primarily composed of ferrite, pearlite, and a small amount of bainite. However, precipitates at grain boundaries are more pronounced after the simulated hot forming process. Mechanical property testing of a transverse sample (at 1 / 4 thickness) of the as-delivered steel plate from Example 1 revealed yield strength of 553 MPa, tensile strength of 669 MPa, elongation of 20%, Charpy V-type impact energy at 0°C of 175 J, and yield strength at 360°C tensile stress of 428 MPa. Simultaneously, a sample was taken from an adjacent location of the as-delivered steel plate from Example 1 and subjected to a simulated hot forming process. Mechanical property testing of the transverse sample (at 1 / 4 thickness) also revealed yield strength of 550 MPa, tensile strength of 662 MPa, elongation of 19.5%, Charpy V-type impact energy at 0°C of 181 J, and yield strength at 360°C tensile stress of 432 MPa. This demonstrates that the performance differences between the two steel plates are not significant. Example 2

[0031] A method for producing thick steel plates for high-temperature and high-pressure vessel heads, wherein the steel plates have a thickness of 120 mm and a weight percentage composition of the steel of C=0.13%, Si=0.08%, Mn=1.53%, P=0.007%, S=0.001%, Sn=0.004%, Cr=0.35%, Mo=0.33%, Ni=0.89%, Al=0.073%, Nb=0.025%, B=0.0018%, and N=0.0055%, with the remainder being Fe and unavoidable impurity elements. The key process steps include:

[0032] (1) Smelting: After LF refining, the molten steel is treated with RH vacuum. After breaking the vacuum, 500m of pure calcium wire is fed. After feeding the wire, soft argon blowing time is maintained for 15 minutes. Then full-process protection pouring is carried out, the superheat is controlled at 12~15℃, and weak cooling is used for secondary cooling. The cross-sectional size of the ingot is 450mm×2070mm, and the total compression ratio is 3.75. After the continuous casting ingot stack is cooled to room temperature, the surface quality inspection and flame cleaning are carried out.

[0033] (2) Heating: soaking temperature is 1150℃, soaking time is 75min;

[0034] (3) Controlled rolling and controlled cooling: Two-stage controlled rolling is adopted, with a rough rolling reduction ratio of 2.5, three rough rolling passes with a rolling temperature in the range of 950-1000°C, and reduction rates of 15.2%, 16.5%, and 18.0% for each pass; then the intermediate billet is water-cooled, and the finishing rolling start temperature is controlled at 870°C and the final rolling temperature is 836°C; after rolling, it is rapidly cooled to 695-715°C at a cooling rate of about 2.8°C / s, and then stacked and slowly cooled to room temperature;

[0035] (4) Normalizing: heating temperature is 960℃, heating time is 220min, and holding time is 45min;

[0036] (5) Tempering: heating temperature is 680℃ and holding time is 60min.

[0037] A simulated hot forming process for thick steel plates used in high-temperature and high-pressure vessel heads. The key process steps include:

[0038] (1) Thermoforming: heating temperature is 960℃, holding time is 180min, and then air cooling to room temperature;

[0039] (2) Simulated post-weld heat treatment: heating temperature is 590℃, holding time is 10.5h, heating rate above 300℃ is about 42℃ / h, cooling rate above 300℃ is about 45℃ / h.

[0040] Mechanical property testing of a transverse specimen (at 1 / 4 thickness) of the as-delivered steel plate from Example 2 revealed yield strength of 520 MPa, tensile strength of 652 MPa, elongation of 18.5%, Charpy V-type impact energy at 0°C of 128 J, and yield strength at 360°C of 418 MPa. Simultaneously, a sample was taken from an adjacent location of the as-delivered steel plate from Example 2 to simulate a hot forming process. Mechanical property testing of the transverse specimen (at 1 / 4 thickness) also revealed yield strength of 522 MPa, tensile strength of 649 MPa, elongation of 19%, Charpy V-type impact energy at 0°C of 131 J, and yield strength at 360°C of 415 MPa. This demonstrates that the performance differences between the two are not significant.

Claims

1. A method for producing thick steel plates for high-temperature and high-pressure vessel heads, characterized by: The thickness of the steel plate is 75~145mm, and the chemical composition of the steel is as follows by weight: C=0.11%~0.13%, Si=0.05%~0.12%, Mn=1.40%~1.60%, P≤0.010%, S≤0.002%, Sn≤0.010%, Cr=0.20%~0.40%, Mo=0.20%~0.40%, Ni=0.60%~1.0%, Al=0.05%~0.08%, Nb=0.010%~0.030%, B=0.0010%~0.0020%, N=0.0040%~0.0060%, and the balance is Fe and unavoidable impurity elements; The key process steps include: (1) Smelting: After LF refining, the molten steel is treated with RH vacuum. After breaking the vacuum, 400~600m pure calcium wire is fed. After feeding the wire, the soft argon blowing time is maintained for ≥10min; then full protection casting is carried out, the superheat is controlled at 10~20℃, and weak cooling is used for secondary cooling; the total compression ratio of the cross-sectional size of the billet is ≥3; after the continuous casting billet stack is cooled to room temperature, the surface quality inspection and flame cleaning are carried out; (2) Heating: The soaking temperature is 1150±10℃ and the soaking time is 60~80min; (3) Controlled rolling and controlled cooling: adopt two-stage controlled rolling, with the rough rolling reduction ratio ≥2 and the rolling temperature in the range of 950~1000℃, and at least 3 passes of the rough rolling stage with a reduction ratio ≥15%; then the intermediate billet is water-cooled, and the finishing rolling start temperature is controlled to be ≤880℃, and the final rolling temperature is 820~840℃; after rolling, it is rapidly cooled to 680~720℃ at a cooling rate of ≥2℃ / s, and then stacked and slowly cooled to room temperature; (4) Normalizing: heating temperature is 960±10℃, heating time is (1.8~2.0)×plate thickness mm×min / mm, holding time is 40~60min; (5) Tempering: heating temperature is 680±10℃, holding time is 50~80min.

2. The method for producing a thick steel plate for a high-temperature and high-pressure vessel head according to claim 1, characterized in that: The key process steps include: (1) Thermoforming: heating temperature is 960±10℃, holding time is (1.2~1.6) plate thickness mm×min / mm, and then air cooling to room temperature; (2) Simulated post-weld heat treatment: heating temperature is 570~600℃, holding time is 9~11h, and the heating and cooling rate above 300℃ is ≤55℃ / h.

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