Method for producing a nickel-containing cryogenic pressure vessel steel, nickel-containing cryogenic pressure vessel steel and steel product

Abstract

The application discloses a method for preparing a nickel-containing low-temperature pressure vessel steel, the nickel-containing low-temperature pressure vessel steel and a steel product, and the method comprises the following steps: performing primary quenching treatment on a steel plate after hot rolling to obtain a primary quenching plate; performing secondary quenching treatment on the primary quenching plate to obtain a secondary quenching plate; and performing tempering on the secondary quenching plate to obtain the nickel-containing low-temperature pressure vessel steel. After the primary quenching treatment, the steel plate is continuously heated to a two-phase zone temperature and is kept for a certain time, and then the secondary quenching treatment is performed; and the holding time is controlled according to the thickness of the steel plate, respectively. In the tempering process, reversed austenite generated can effectively absorb stable elements such as C, Ni and Mn, purifies the matrix martensite structure, and improves the toughness of the martensite structure; meanwhile, due to the fact that the reversed austenite absorbs a large amount of stable elements, the stability of the reversed austenite is improved, and the low-temperature toughness of the steel plate in a low-temperature environment of-196 DEG C is greatly improved.

Description

Technical Field

[0001] This application belongs to the field of steelmaking technology, specifically relating to a method for preparing nickel-containing low-temperature pressure vessel steel, the nickel-containing low-temperature pressure vessel steel, and steel products. Background Technology

[0002] Cryogenic pressure vessel steel is generally designed for containers that store cryogenic temperatures and can withstand certain pressures. In existing technologies, cryogenic pressure vessel steel is typically 9Ni steel, which exhibits good low-temperature toughness and strength at low temperatures and is widely used in cryogenic engineering fields, such as in the construction of large structural storage tanks like liquefied natural gas storage tanks and liquid nitrogen storage tanks.

[0003] Among related technologies, the 9Ni heat treatment process mainly includes three offline heat treatment processes: quenching + tempering (QT), normalizing + normalizing + tempering (NNT), and quenching + sub-temperature quenching + tempering (QLT). However, the resulting steel plates still cannot fully meet the application requirements in terms of strength and low-temperature toughness, and urgently need improvement. Summary of the Invention

[0004] In view of this, this application provides a method for preparing nickel-containing low-temperature pressure vessel steel, nickel-containing low-temperature pressure vessel steel and steel products, and aims to provide a method for preparing nickel-containing low-temperature pressure vessel steel with high strength, high toughness and high elongation.

[0005] In a first aspect, embodiments of this application provide a method for preparing nickel-containing low-temperature pressure vessel steel, comprising:

[0006] The hot-rolled steel plate undergoes a single quenching treatment to obtain a single-quenched plate. The cooling rate of the single quenching treatment is 40-50℃ / s, which can be selected as 45℃ / s. The single quenching treatment includes quenching heating to the austenitizing temperature of the steel plate and quenching holding treatment. The thickness of the hot-rolled steel plate is denoted as h, and the quenching holding time is denoted as t1. The relationship between the thickness of the steel plate and the quenching holding time satisfies: t1=1.4×h+a, where the unit of t1 is minutes; the unit of h is mm; and a is 14-16.

[0007] A secondary quenching treatment is performed on a first-quenched plate to obtain a second-quenched plate. The cooling rate of the first quenching treatment is 40-50℃ / s, which can be selected as 45℃ / s. The secondary quenching treatment includes a secondary quenching heating treatment to the two-phase temperature of the steel plate and a secondary quenching holding treatment. The time of the secondary quenching holding treatment is denoted as t2. The relationship between the thickness of the steel plate and the time of the secondary quenching holding treatment is: t2=1.4×h+b, where the unit of t2 is minutes; the unit of h is mm; and b is 14-16.

[0008] The secondary quenched plate is tempered to obtain nickel-containing low-temperature pressure vessel steel.

[0009] According to an embodiment of one aspect of this application, the method satisfies at least one of the following conditions:

[0010] The hot-rolled steel plate contains, by weight percentage: 8.5%-9.5% Ni;

[0011] The thickness of the steel plate is 5mm to 25mm;

[0012] The austenitizing temperature of the steel plate is 815-825℃;

[0013] The two-phase temperature of the steel plate is 665-675℃.

[0014] According to one embodiment of this application, the cooling rate of the first quenching process is equal to the cooling rate of the second quenching process.

[0015] According to one embodiment of this application, the time for the first quenching and holding treatment is equal to the time for the second quenching and holding treatment.

[0016] According to one embodiment of this application, tempering includes heating to 575-585°C and performing tempering heat preservation treatment.

[0017] According to one aspect of the embodiment of this application, the tempering and heat preservation time is denoted as t3, and the relationship between the thickness of the steel plate and the tempering and heat preservation time satisfies: t3=1.4×h+c, where the unit of t3 is minutes; the unit of h is mm, and c is 34-36.

[0018] According to one embodiment of this application, the secondary quenched plate is cooled by air cooling after tempering to obtain nickel-containing low-temperature pressure vessel steel.

[0019] According to one embodiment of this application, before subjecting a hot-rolled steel plate to quenching heat treatment and quenching heat holding treatment to obtain a primary quenching heating plate, the following steps are included:

[0020] The billet is hot-rolled and air-cooled to obtain a hot-rolled plate; the hot rolling includes rough rolling and finish rolling, the exit temperature of the rough rolling is 1160-1180℃, and the exit temperature of the finish rolling is 810-850℃.

[0021] According to one embodiment of this application, the process between hot rolling and air cooling further includes: winding, cooling, and unwinding.

[0022] Secondly, embodiments of this application provide a nickel-containing cryogenic pressure vessel steel, which is prepared by the method described in the first aspect.

[0023] Thirdly, embodiments of this application provide a steel product, which is made by processing nickel-containing low-temperature pressure vessel steel obtained by the method of the first aspect or nickel-containing low-temperature pressure vessel steel of the second aspect.

[0024] Compared with the prior art, this application has at least the following beneficial effects:

[0025] The method for preparing nickel-containing low-temperature pressure vessel steel provided in this application involves a first quenching treatment followed by heating the steel plate to the two-phase region temperature and holding it thereafter, and then performing a second quenching treatment. The holding time is controlled according to the thickness of the steel plate, which greatly reduces the quenching stress during the quenching process and reduces the tendency of the slab to deform during quenching. In addition, the steel plate produces a small amount of acicular ferrite at the two-phase region temperature, which is also beneficial to improving the low-temperature toughness. During the tempering process, the reversed austenite produced can effectively absorb stabilizing elements such as C, Ni, and Mn, purify the matrix martensite structure, and improve the toughness of the martensite structure. At the same time, since the reversed austenite absorbs a large number of stabilizing elements, the stability of the reversed austenite is improved, which greatly improves the low-temperature toughness of the steel plate at a low temperature of -196℃. The heat treatment method of this application determines the optimal heat treatment temperature and time mechanism, which is beneficial to reduce energy consumption, improve the strength, toughness, and elongation of the steel plate, and also improve the production efficiency of the steel plate. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0027] Figure 1 A flowchart of the steel plate production and processing technology according to an embodiment of this application is shown;

[0028] Figure 2 A temperature-time diagram illustrating a method for preparing nickel-containing low-temperature pressure vessel steel according to an embodiment of this application is shown.

[0029] Figure 3 Microscopic images of the nickel-containing cryogenic pressure vessel steel of Embodiment 1 of this application are shown;

[0030] Figure 4 Microscopic images of nickel-containing cryogenic pressure vessel steel after cryogenic impact testing according to Embodiment 1 of this application are shown;

[0031] Figure 5 The X-ray diffraction pattern of nickel-containing cryogenic pressure vessel steel after a cryogenic impact test according to Embodiment 1 of this application is shown. Detailed Implementation

[0032] To make the purpose, technical solution, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the embodiments described in this specification are merely for explaining this application and are not intended to limit it.

[0033] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, every point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value or with other lower or upper limits to form a range not explicitly stated.

[0034] In the description of this application, it should be noted that, unless otherwise stated, "above" and "below" include the stated number, and "multiple" in "one or more" means two or more.

[0035] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0036] Cryogenic pressure vessel steel is generally designed for containers that store cryogenic temperatures and can withstand certain pressures. In existing technologies, cryogenic pressure vessel steel is typically 9% Ni steel, but other steel grades are also possible. 9% Ni steel, with a nickel content of approximately 8.5%-9.5%, is called 9Ni steel or 06Ni9 steel. It exhibits good low-temperature toughness and strength and is widely used in cryogenic engineering. The production of 9% Ni steel employs a steelmaking-refining-continuous casting process. LF white slag refining and desulfurization, RH vacuum treatment to control inclusion content, and high steel purity all play crucial roles in improving the steel's low-temperature toughness. Since the presence of impurities such as phosphorus (P) and sulfur (S) deteriorates the steel's low-temperature toughness, the content of these impurities must be strictly controlled at low levels.

[0037] With technological advancements, cryogenic pressure vessel steel is increasingly failing to meet the requirements of end users. Mechanical properties, particularly cryogenic impact toughness, are now subject to more stringent requirements than before.

[0038] In one related technology, 9Ni steel is hot-rolled without heat treatment to obtain the final product. The metallographic structure of this steel plate is mainly tempered martensite and reverse-transformed austenite, resulting in low yield strength and a small yield-to-tensile ratio, which cannot meet the requirements of relevant applications. In another related technology, the steel plate is laminar cooled to below 200°C after hot rolling, generating significant residual stress. This is unsuitable for controlling the shape of thin and wide steel plates. While the yield strength of the resulting steel plate meets the requirements, it sacrifices the high toughness of 9Ni steel. The actual impact energy of the steel plate at -196°C is only 120J, which is relatively low and cannot meet the relevant performance requirements.

[0039] Generally, the production process of 9Ni steel includes: hot metal pretreatment desulfurization → hot metal slag removal → 210t converter simultaneous double dephosphorization → converter decarburization → slag-blocking tapping → LF ladle deep desulfurization and inclusion removal → RH vacuum removal of harmful gases → slab continuous casting → slow cooling → billet surface grinding and coating → heating → rolling → quenching → tempering. Based on the inventor's research, the method in this application employs heating, double quenching, and tempering processes, optimizing the heating temperature, quenching temperature, and time mechanism to achieve heat treatment of the steel plate.

[0040] Based on this, the inventors conducted extensive research to provide a method for heat-treating 9Ni steel or steel grades with similar properties. The method of this application optimizes the heat treatment process in steelmaking, determines the optimal heat treatment temperature and time mechanism, and provides a method for preparing nickel-containing low-temperature pressure vessel steel that reduces energy consumption and improves production efficiency, resulting in steel plates with advantages such as high strength, high toughness, and high elongation.

[0041] Method for preparing nickel-containing low-temperature pressure vessel steel

[0042] In a first aspect, embodiments of this application provide a method for preparing nickel-containing low-temperature pressure vessel steel, comprising:

[0043] The hot-rolled steel plate undergoes a single quenching treatment to obtain a single-quenched plate. The cooling rate of the single quenching treatment is 40-50℃ / s, which can be selected as 45℃ / s. The single quenching treatment includes quenching heating to the austenitizing temperature of the steel plate and quenching holding treatment. The thickness of the hot-rolled steel plate is denoted as h, and the quenching holding time is denoted as t1. The relationship between the thickness of the steel plate and the time of the single quenching holding treatment satisfies: t1=1.4×h+a, where the unit of t1 is minutes; the unit of h is mm; and a is 14-16.

[0044] A secondary quenching treatment is performed on a first-quenched plate to obtain a second-quenched plate. The cooling rate of the secondary quenching treatment is 40-50℃ / s, and 45℃ / s can be selected. The secondary quenching treatment includes quenching heating to the two-phase temperature of the steel plate and quenching holding treatment. The time of the secondary quenching holding treatment is denoted as t2. The relationship between the thickness of the steel plate and the quenching holding treatment time satisfies: t2=1.4×h+b, where the unit of t2 is minutes; the unit of h is mm; and b is 14-16.

[0045] The secondary quenched plate is tempered to obtain nickel-containing low-temperature pressure vessel steel.

[0046] Through research, the inventors discovered a linear relationship between the thickness of the steel plate and the holding time after heating, optimizing the temperature and holding time during heat treatment. After the first quenching, the steel plate is further heated to the two-phase region temperature and held there before undergoing a second quenching. The relationship between the thickness of the steel plate and the holding time of the first and second quenching processes is controlled, i.e., t1 = 1.4 × h + a, t2 = 1.4 × h + b. This significantly reduces the quenching stress during the two quenching processes, reduces the tendency of the slab to deform during quenching, and causes the steel plate to produce a small amount of acicular ferrite at the two-phase region temperature, improving low-temperature toughness. This effectively saves a large amount of energy consumption required during heat treatment, achieves a good match between the strength and toughness of the steel plate, and greatly improves the production efficiency of steel.

[0047] According to the embodiments of this application, primary quenching can be understood as performing a first quenching on the hot-rolled steel plate, and secondary quenching can be understood as performing a second quenching after the first quenching. This application employs a two-quenching process. The steel plate undergoes primary quenching heating to the austenitizing temperature, followed by quenching holding and another primary quenching, resulting in a large amount of martensite, effectively improving the strength of the steel plate. The steel plate undergoes secondary quenching heating to the two-phase region temperature, which allows the steel plate to have both γ and α phases, followed by a second quenching holding. During the two-phase region holding, alloying elements diffuse, reversing the austenite enrichment and stabilizing alloying elements, while simultaneously purifying the martensite matrix, effectively improving the low-temperature toughness of the steel plate.

[0048] According to the embodiments of this application, the above-mentioned primary quenching heat treatment, primary quenching holding treatment, primary quenching treatment, secondary quenching heat treatment, secondary quenching holding treatment, secondary quenching treatment, and tempering treatment constitute a complete heat treatment process. On the one hand, austenite is fully formed during the primary quenching heat treatment. On the other hand, the two quenching processes greatly reduce the quenching stress during the quenching process, reduce the tendency of the slab to deform during the quenching process, and the steel plate produces a small amount of acicular ferrite at the two-phase temperature, which is also beneficial to improving low-temperature toughness. During the tempering process, the reversed austenite produced can effectively absorb stabilizing elements such as C, Ni, and Mn, purify the matrix martensite structure, and improve the toughness of the martensite structure. At the same time, since the reversed austenite absorbs a large number of stabilizing elements, the stability of the reversed austenite is improved, which greatly improves the low-temperature toughness of the steel plate in the -196℃ low-temperature environment. The combined effect of the above heat treatments gives the steel plate high strength, high toughness, and high elongation.

[0049] According to the embodiments of this application, the two-phase region temperature is a temperature range in which two phases γ and α can be generated. The α phase is a low-temperature stable phase, and the γ phase is a strengthening phase. During the two-phase region temperature process, the nucleation points of reversed austenite increase significantly. At the same time, during the quenching process after heat preservation, the newly generated martensite α' phase is formed. Due to the adaptive increase of phase transformation and the orientation difference with the surrounding martensite, the lath martensite structure is refined, thereby greatly improving the low-temperature toughness and elongation of the steel plate.

[0050] Furthermore, the resulting steel plate still exhibits good low-temperature toughness and low-temperature impact rate in subsequent low-temperature environments.

[0051] In some embodiments, the quenching medium used in the primary quenching treatment and the secondary quenching treatment can be water, or an aqueous solution with cooling properties, mineral oil, such as an aqueous solution containing organic matter, quenching oil, etc.

[0052] In some embodiments, t1 = 1.4 × h + 15.6. For example, for a 16 mm thick steel plate, the optimal quenching holding time t1 is 38 minutes and the tempering holding time is 58 minutes.

[0053] In some embodiments, t2 = 1.4 × h + 15.6. For example, for a 16 mm thick steel plate, preferably, the single quenching holding time t1 is 38 minutes.

[0054] In some embodiments, according to one aspect of this application, the method satisfies at least one of the following conditions:

[0055] The hot-rolled steel plate contains, by weight percentage: 8.5%-9.5% Ni;

[0056] The thickness of the steel plate is 5mm to 25mm;

[0057] The austenitizing temperature of the steel plate is 815-825℃;

[0058] The two-phase temperature of the steel plate is 665-675℃.

[0059] In some embodiments, the hot-rolled steel sheet comprises, by mass percentage: C ≤ 0.1%; Si, 0.1%–0.35%; Mn, 0.3%–0.8%; P, 0.0038%–0.015%; S, 0.001%–0.003%; Al, 0.015%–0.045%; Ni, 8.5%–9.5%; N, ≤ 0.012%; with the balance being Fe and unavoidable inclusions.

[0060] In some embodiments, the tempered plate is cooled to room temperature by cold bed stacking or water cooling to obtain nickel-containing low-temperature pressure vessel steel. The specific cooling method is not limited and can be adjusted according to actual conditions.

[0061] In some embodiments, the cooling rate of the primary quenching process is equal to the cooling rate of the secondary quenching process. Martensite and newly formed fine-grained martensite structures are obtained during the primary and secondary quenching processes, respectively, effectively improving the strength and toughness of the steel plate.

[0062] In some embodiments, the time for the first quenching and holding treatment is equal to the time for the second quenching and holding treatment. The first quenching and holding treatment refines the primary austenite grains, while the second quenching and holding treatment enhances the stability of the reversed austenite by enriching it with a large number of stabilizing alloying elements, thus effectively improving the low-temperature toughness of the steel plate.

[0063] In some embodiments, tempering includes heating to 575-585°C and performing a tempering holding process.

[0064] According to the embodiments of this application, the tempering process of heating to 575-585℃ can enable the reversed austenite generated in the steel plate to further absorb stabilizing elements such as C, Ni, and Mn, further purify the matrix martensite structure, improve the toughness of the martensite structure, and thus improve the stability of the reversed austenite.

[0065] In some embodiments, the tempering and heat preservation time is denoted as t3, and the relationship between the thickness of the steel plate and the tempering and heat preservation time satisfies: t3=1.4×h+c, where t3 is in minutes, h is in mm, and c is 34-36.

[0066] Through research, the inventors discovered a linear relationship between the thickness of the steel plate and the holding time during tempering, thus optimizing the holding time during heat treatment. Controlling the relationship between the thickness of the steel plate and the tempering holding time, i.e., t3 = 1.4 × h + c, has a positive effect on the microstructure and stress relief during tempering, thereby improving the mechanical properties of nickel-containing low-temperature pressure vessel steel.

[0067] In some embodiments, t3 = 1.4 × h + 35.6. For example, for a 16 mm thick steel plate, the tempering and heat preservation time, i.e., the tempering and heat preservation time, is 58 minutes.

[0068] In some embodiments, the secondary quenched plate is air-cooled and then tempered to obtain nickel-containing low-temperature pressure vessel steel. Using air cooling not only saves costs but also helps reduce thermal stress during cooling, improves the quality of the steel plate, and stabilizes the microstructure of the nickel-containing low-temperature pressure vessel steel.

[0069] In some embodiments, the process of subjecting the hot-rolled steel plate to a first quenching heating treatment and a first quenching holding treatment to obtain a first quenching heating plate includes:

[0070] The billet is hot-rolled and air-cooled to obtain a hot-rolled plate; the hot rolling includes rough rolling and finish rolling, the exit temperature of the rough rolling is 1160-1180℃, and the exit temperature of the finish rolling is 810-850℃.

[0071] According to the embodiments of this application, rough rolling can control the thickness of the billet, and then fine rolling can obtain a steel plate of a predetermined thickness, such as a 16mm thick steel plate after fine rolling.

[0072] According to the embodiments of this application, air cooling or laminar flow cooling can be used for cooling. By controlling the temperature during hot rolling, the coarse grains in the casting state are broken up, cracks are significantly healed, the as-cast structure is transformed into a deformable structure, and the processing performance of the steel plate is improved.

[0073] In some embodiments, such as Figure 1As shown, the slab is heated in heating furnace 1 for homogenization, then rolled to the specified thickness by roughing mill 2, roughing mill 3, and finishing mill 4. The hot-rolled steel plate is then straightened by straightening machine 5 to obtain the hot-rolled steel plate. After leveling, the hot-rolled steel plate is directly sent to the heat treatment unit. It can be held in heating furnace 6 for 38 minutes, then enter quenching unit 7, where it can be rapidly cooled to room temperature at a cooling rate of approximately 45℃ / s. Alternatively, the steel plate can be heated to the two-phase region temperature of 670±5℃, held in holding unit 8 for 38 minutes, then entered quenching unit 9, and cooled to room temperature at a cooling rate of approximately 45℃ / s. After completing the two-phase region treatment, the steel plate enters tempering furnace 10 for tempering. The tempering temperature can be controlled at 580±5℃, and the tempering holding time is approximately 58 minutes, followed by air cooling to room temperature.

[0074] In some embodiments, the process between hot rolling and air cooling also includes winding, cooling, and unwinding.

[0075] In some embodiments, the steel plate is rolled at a temperature of 580-600°C, and then cooled by means of air cooling or other methods. It can be uncoiled at a temperature of 580-600°C and then air cooled to room temperature.

[0076] Secondly, embodiments of this application provide a nickel-containing cryogenic pressure vessel steel, which is prepared by the method described in the first aspect.

[0077] In some embodiments, the metallographic structure of nickel-containing low-temperature pressure vessel steel is determined according to GB / T 13298-2015 "Methods for Testing the Microstructure of Metals". The metallographic structure includes, by volume fraction, 82%-89% tempered lamellar martensite, 2%-5% acicular ferrite and 9%-13% reverse-transformed austenite.

[0078] In some embodiments, the mechanical properties of nickel-containing low-temperature pressure vessel steel are determined according to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Test method at room temperature", including: tensile strength (transverse) of 680-730 MPa, yield strength of 620-680 MPa, elongation of 38%-50%, and impact energy at -196℃ of 215-240 J.

[0079] Thirdly, embodiments of this application provide a steel product, which is made by processing nickel-containing low-temperature pressure vessel steel obtained by the method of the first aspect or nickel-containing low-temperature pressure vessel steel of the second aspect.

[0080] According to the embodiments of this application, steel products can be liquefied natural gas storage tanks, LNG cryogenic transport ships, liquid nitrogen storage tanks, etc.

[0081] Example

[0082] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0083] Examples 1-3

[0084] Preparation of hot-rolled plate: The continuously cast slab is heated, straightened and then slowly cooled to obtain a cast slab; the chemical composition of the cast slab is shown in Table 1 as a mass percentage, and the process parameters of heating, straightening and slow cooling are shown in Table 2.

[0085] Using the above-mentioned billet as raw material, it is heated, hot-rolled, and coiled to obtain a hot-rolled plate; the specific parameters are shown in Table 3. The metallographic structure of the hot-rolled plate is quenched martensite + bainite + retained austenite.

[0086] Table 1. Chemical composition (wt%) of 9Ni steel in Examples 1-3

[0087]

[0088] Table 2 Continuous casting process parameters.

[0089]

[0090] Table 3 Rolling process parameters

[0091]

[0092] Examples 4-6

[0093] Method for preparing nickel-containing low-temperature pressure vessel steel:

[0094] The hot-rolled plate obtained in Example 1 is subjected to a single quenching treatment to obtain a single-quenched plate. The quenching speed is 40-50℃ / s, and can be selected as 45℃ / s. The single quenching treatment includes a single quenching heating treatment to the austenitizing temperature of the steel plate and a single quenching holding treatment. The thickness of the hot-rolled steel plate is denoted as h, and the time of the single quenching holding treatment is denoted as t1. The relationship between the thickness of the steel plate and the time of the single quenching holding treatment satisfies: t1=1.4×h+a, where the unit of t1 is minutes; the unit of h is mm, and a is 14-16.

[0095] A secondary quenching treatment is performed on a first-quenched plate to obtain a second-quenched plate. The secondary quenching rate is 40-50℃ / s, and 45℃ / s is optional. The secondary quenching treatment includes heating the steel plate to the two-phase temperature and holding it at that temperature. The holding time is denoted as t2. The relationship between the thickness of the steel plate and the holding time is: t2 = 1.4 × h + b, where t2 is in minutes, h is in mm, and b is 14-16.

[0096] The secondary quenched plate was tempered to obtain nickel-containing low-temperature pressure vessel steel. The specific process parameters for the primary quenching treatment, the secondary quenching treatment, and the tempering are shown in Table 4.

[0097] Examples 7-9

[0098] The difference between this embodiment and Embodiment 4 is that the hot-rolled plate obtained in Embodiment 2 is processed. The specific process parameters for the first quenching treatment, the second quenching treatment, and the tempering are shown in Table 4.

[0099] Examples 10-12

[0100] The difference between this embodiment and Embodiment 4 is that the hot-rolled plate obtained in Embodiment 3 is processed. The specific process parameters for the first quenching treatment, the second quenching treatment, and the tempering are shown in Table 4.

[0101] Comparative Examples 1-3

[0102] The difference between the comparative example and Example 4 in this application is that the hot-rolled plate obtained in Example 1 is treated with a quenching and tempering (QT) process. The specific process parameters for quenching and tempering are shown in Table 5.

[0103] Comparative Examples 4-6

[0104] The difference between the comparative example and Example 7 in this application is that the hot-rolled plate obtained in Example 2 is treated with a quenching and tempering (QT) process. The specific process parameters for quenching and tempering are shown in Table 5.

[0105] Comparative Examples 7-9

[0106] The difference between the comparative example and Example 10 in this application is that the hot-rolled plate obtained in Example 3 is treated with a quenching and tempering (QT) process, and the specific process parameters for quenching and tempering are shown in Table 5.

[0107] Comparative Examples 10-12

[0108] The difference between this comparative example and Example 4 is that the hot-rolled plate obtained in Example 3 is processed. The specific process parameters for the first quenching treatment, the second quenching treatment, and the tempering are shown in Table 4.

[0109] Table 4 Heat treatment process parameters for Examples 4-12 and Comparative Examples 10-12

[0110]

[0111] Table 5. Heat treatment process parameters for comparative examples 1-9.

[0112]

[0113]

[0114] Test section

[0115] 1) Metallographic structure: The nickel-containing low-temperature pressure vessel steels prepared in Examples 4-12 and Comparative Examples 1-12 were determined according to GB / T13298-2015 "Metallic Microstructure Test Method".

[0116] 2) Tensile strength, yield strength, elongation and -196℃ low temperature impact energy: The properties of the nickel-containing low temperature pressure vessel steels prepared in Examples 4-12 and Comparative Examples 1-12 were tested according to GB / T228.1-2010 "Metallic materials - Tensile testing - Part 1: Room temperature test method".

[0117] The test results of the mechanical properties of the nickel-containing low-temperature pressure vessel steels prepared in Examples 4-12 and Comparative Examples 1-12 are shown in Table 6.

[0118] Table 6. Test results of mechanical properties of nickel-containing low-temperature pressure vessel steel

[0119]

[0120]

[0121] In the table above, M represents tempered lath martensite, B represents bainite, F represents ferrite, mainly acicular ferrite, and A represents reverse-transformed austenite. As can be seen from the mechanical property results of nickel-containing low-temperature pressure vessel steel in Table 6 above, after using the heat treatment method described in this invention, the tensile strength and yield strength of 9Ni steel do not change significantly from the comparative example, but its impact energy at -196℃ reaches approximately 220J. Furthermore, the production method for nickel-containing low-temperature pressure vessel steel provided in this application has advantages such as low process control difficulty, strong operability, and simple method.

[0122] 3) The nickel-containing cryogenic pressure vessel steel of Example 1 was observed using a FEI Nova Nano SEM400 field emission scanning electron microscope, and the results were as follows: Figure 3 The images shown are microscopic observations of the examples after the low-temperature shock test, yielding the following results: Figure 4 The image shown Figure 3 , 4 The microstructure images show that the steel plate has a lath-like tempered martensite microstructure. As can be seen from the images, after the steel plate has undergone two-phase region treatment, the martensite laths are even finer. Observation under high magnification microstructure reveals a large number of dotted and linear bright background areas. Extensive research has confirmed that the dotted and linear structure between the martensite laths is reverse-transformed austenite. Due to the presence of a large amount of reverse-transformed austenite, the low-temperature toughness of the steel plate can be improved.

[0123] 4) The steel plates of the examples and comparative examples after the low-temperature impact test were subjected to NMR analysis using a Rigaku D / max 2500PC XRD diffractometer and NMR spectra, and the XRD patterns were obtained as follows: Figure 5 As shown. Figure 5 The XRD patterns of nickel-containing low-temperature pressure vessel steel samples from Example 1 and Comparative Example 1 after low-temperature impact are shown. 1# is the heat-treated plate from Comparative Example 1 and 2# is the heat-treated plate from Example 1. It can be seen that the characteristic peak value of reversed austenite in sample 2# is significantly higher than that in sample 1#. Based on the average intensity of the (220)γ and (311)γ peaks and the intensity of the (211)α peak, according to the austenite content calculation formula... Calculate. Where V α and V γ These represent the volume percentages of martensite and austenite, respectively. α (211) α The cumulative intensity, I γ (220) γ and (311) γ The average cumulative intensity.

[0124] After low-temperature impact testing, the volume percentages of reversed austenite γ' in samples #1 and #2 were found to be 0.88% and 6.6%, respectively. This low-temperature impact test simulates the actual use environment of heat-treated steel, such as its preparation into low-temperature pressure storage containers, for example, for storing low-temperature liquid nitrogen.

[0125] The reasons for the above phenomena are as follows: After being held in liquid nitrogen at -196℃, most of the reverse-transformed austenite in sample #1 underwent martensitic transformation. In contrast, during the two-phase region holding treatment of sample #2, alloying elements underwent redistribution, with elements such as C and Si becoming sufficiently enriched in the γ' phase. Simultaneously, Ni and a small amount of Mn also enriched in the γ' phase. These austenite-stabilizing elements, enriched in the γ' phase, meant that only a small portion of the reverse-transformed austenite underwent martensitic transformation during liquid nitrogen holding. These stabilized reverse-transformed austenite effectively hindered crack propagation at grain boundaries. This microstructure further explains why sample #2 exhibited superior low-temperature toughness compared to sample #1 in the comparative example. Furthermore, the absorption of stabilizing elements from the matrix by the reverse-transformed austenite γ' in sample #2 weakened the solid solution strengthening effect of the matrix, consistent with the decrease in yield strength in sample #2.

[0126] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing nickel-containing cryogenic pressure vessel steel, comprising: A hot-rolled steel plate is subjected to a primary quenching treatment to obtain a primary quenched plate. The hot-rolled steel plate contains 8.5%-9.5% Ni by mass percentage. The cooling rate of the primary quenching treatment is 40-50℃ / s. The primary quenching treatment includes quenching heating to the austenitizing temperature of the steel plate and quenching holding treatment. The thickness of the hot-rolled steel plate is denoted as h, and the quenching holding time is denoted as t1. The relationship between the thickness of the steel plate and the quenching holding time satisfies: t1 = 1.4 × h + a, where t1 is in minutes, h is in mm, and a is 14-16. The first-quenched plate is subjected to a second quenching treatment to obtain a second-quenched plate, wherein the cooling rate of the second quenching treatment is 40-50℃ / s; the second quenching treatment includes a second quenching heating treatment to the two-phase region temperature of the steel plate and a second quenching holding treatment, the time of the second quenching holding treatment is denoted as t2; the relationship between the thickness of the steel plate and the time of the second quenching holding treatment satisfies: t2=1.4×h+b, where the unit of t2 is minutes; the unit of h is mm; and b is 14-16; wherein the two-phase region temperature of the steel plate is 665-675℃; The secondary quenched plate is tempered to obtain nickel-containing low-temperature pressure vessel steel. The tempering includes heating to 575-585℃ and tempering holding treatment. The tempering holding treatment time is denoted as t3. The relationship between the thickness of the steel plate and the tempering holding treatment time satisfies: t3 = 1.4 × h + c, where t3 is in minutes, h is in mm, and c is 34-36.

2. The method according to claim 1, characterized in that, The method satisfies at least one of the following conditions: The thickness of the steel plate is 5mm to 25mm; The austenitizing temperature of the steel plate is 815-825℃.

3. The method according to claim 1, characterized in that, The cooling rate of the first quenching treatment is 45℃ / s; and / or, the cooling rate of the second quenching treatment is 45℃ / s.

4. The method according to claim 1, characterized in that, The cooling rate of the first quenching process is equal to the cooling rate of the second quenching process.

5. The method according to claim 1, characterized in that, The secondary quenched plate is then cooled by air cooling after tempering to obtain nickel-containing low-temperature pressure vessel steel.

6. The method according to any one of claims 1-5, characterized in that, The process of quenching and holding the hot-rolled steel plate to obtain the quenched heated plate includes: The billet is hot-rolled and air-cooled to obtain a hot-rolled plate; wherein the hot rolling includes rough rolling and finish rolling, the exit temperature of the rough rolling is 1160-1180 ℃, and the exit temperature of the finish rolling is 810-850 ℃.

7. The method according to claim 6, characterized in that, Between the hot rolling and the air cooling process, there are also: winding, cooling, and unwinding.

8. A nickel-containing cryogenic pressure vessel steel, prepared by the method described in any one of claims 1-7.

9. A steel product, which is made by processing the nickel-containing low-temperature pressure vessel steel or the nickel-containing low-temperature pressure vessel steel of claim 8, by processing the method described in any one of claims 1-7.

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