A 9Ni steel and a method for producing the same

Abstract

The application discloses a kind of 9Ni steel and preparation method thereof, it is related to steel preparation technical field.The 9Ni steel provided by the application adopts C-Si-Mn-Ni alloy component system, including C 0.035%~0.05%, Si 0.5%~0.8%, Mn 0.4%~1.4%, Ni 8%~10%, P≤0.006%, S≤0.002%, the balance is Fe.LF+VD control smelting process is used, high-quality billet is obtained;Then after billet heating, two-stage steel plate rolling process, directly using continuous cooling and warm straightening process is prepared to obtain 9Ni steel.The application directly quenches by reasonable component design combined with deformation heat treatment process technology, keeps the dislocation in material after plastic processing, after the steel plate is tempered and the like process, obtains the steel plate with good matching of strength and toughness and good flatness, simplifies production process, optimizes production process, shortens construction period, reduces energy consumption.

Description

Technical Field

[0001] This application relates to the field of steel plate preparation technology, and in particular to a 9Ni steel and its preparation method. Background Technology

[0002] Currently, 9Ni steel is one of the key materials used in the construction of LNG storage and transportation equipment worldwide. At present, the production of this steel requires multiple heat treatment processes to ensure a good balance of strength and toughness. The main processes used include double quenching and tempering, double normalizing and tempering, and single quenching and tempering. However, all of these processes have drawbacks such as high resource consumption in heat treatment, long production processes, high carbon emissions, and long delivery cycles, affecting the production efficiency of this steel. Summary of the Invention

[0003] This application provides a 9Ni steel and its preparation method to solve the technical problems of long preparation cycle and low production efficiency of existing 9Ni steel.

[0004] In a first aspect, this application provides a 9Ni steel, wherein the chemical composition of the 9Ni steel plate, calculated by weight, includes 0.035% to 0.05% C, 0.5% to 0.8% Si, 0.4% to 1.4% Mn, 8% to 10% Ni, P ≤ 0.006%, S ≤ 0.002%, with the balance being Fe and unavoidable impurities.

[0005] Optionally, the microstructure of the 9Ni steel plate includes tempered martensite and rotated austenite.

[0006] Optionally, the tempered martensitic laths are distributed with a predetermined amount of the spun austenite, the predetermined amount being 3%-7%.

[0007] Secondly, the present invention provides a method for preparing 9Ni steel, comprising the following steps:

[0008] Molten steel is prepared, and the smelting process is controlled by LF+VD to obtain a steel billet containing the aforementioned chemical composition;

[0009] The steel billet is heated using a step-by-step heating method;

[0010] The heated steel plate is rolled using a two-stage rolling process to obtain a rolled steel plate;

[0011] The rolled steel plate is directly quenched using a continuous cooling method to obtain a quenched steel plate;

[0012] The quenched steel plate is then subjected to warm straightening.

[0013] The straightened steel plate is then tempered to obtain a 9Ni steel plate.

[0014] Optionally, the steel billet has a sulfur content of less than 0.002% and a phosphorus content of less than 0.006%. The inclusions of types A, B, and C are required to be ≤1.5 grade, wherein type A inclusions are sulfides, type B inclusions are alumina, and type C inclusions are silicates.

[0015] Optionally, the parameters of the step heating method are as follows: at 200℃ to 700℃, the heating rate is 4℃ / min to 6℃ / min; at 700℃ to 900℃, the heating rate is 6℃ / min to 10℃ / min; and at temperatures > 900℃, the heating rate is 2℃ / min, until the temperature is heated to 1130-1150℃.

[0016] Optionally, the two-stage rolling process includes a roughing stage and a finishing stage. The starting temperature of the roughing stage is 1000℃~1050℃, and the finishing temperature is 970℃~1000℃. During the finishing stage, the starting temperature is 900℃~980℃, and the finishing temperature is 800℃~890℃.

[0017] Optionally, during the quenching process of the steel plate, the continuous cooling rate is 20℃ / s to 30℃ / s, and the final cooling temperature is ≤100℃.

[0018] Optionally, the temperature of the thermal straightening is less than 100°C.

[0019] Optionally, the tempering temperature during the tempering process is 550℃~600℃; the heating time is 40min~60min; and the holding time is 30min~50min.

[0020] The technical solution provided by this invention has the following advantages compared with the prior art:

[0021] This invention provides a 9Ni steel and its preparation method. By combining reasonable composition design with deformation heat treatment technology, the steel plate is directly quenched while retaining the dislocations in the material after plastic processing. After tempering and other processes, a steel plate with good strength and toughness matching and good flatness is obtained. This simplifies the production process, optimizes the production technology, shortens the preparation cycle, reduces energy consumption, and also reduces costs. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of a method for preparing 9Ni steel provided in this application.

[0025] Figure 2 This is a schematic diagram of the microstructure of the 9Ni steel provided in Example 1.

[0026] Figure 3 This is a schematic diagram of the microstructure of 9Ni steel provided in the comparative example. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0029] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0030] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0031] In a first aspect, this application provides a 9Ni steel, wherein the chemical composition of the 9Ni steel plate, calculated by weight, includes 0.035% to 0.05% C, 0.5% to 0.8% Si, 0.4% to 1.4% Mn, 8% to 10% Ni, P ≤ 0.006%, S ≤ 0.002%, with the balance being Fe and unavoidable impurities.

[0032] In some embodiments, the microstructure of the 9Ni steel plate comprises tempered martensite and rotated austenite.

[0033] In some embodiments, a predetermined amount of rotatable austenite is distributed at the tempered martensitic lath boundaries, the predetermined amount being 3%-7%, and optionally 3%, 4%, 5%, 6%, or 7%.

[0034] Secondly, the present invention provides a method for preparing 9Ni steel, comprising the following steps:

[0035] Molten steel is prepared, and the smelting process is controlled by LF+VD to obtain a steel billet containing the aforementioned chemical composition;

[0036] The steel billet is heated using a step-by-step heating method;

[0037] The heated steel plate is rolled using a two-stage rolling process to obtain a rolled steel plate;

[0038] The rolled steel plate is directly quenched using a continuous cooling method to obtain a quenched steel plate;

[0039] The quenched steel plate is not subjected to hot straightening process, but is directly conveyed to a warm straightening machine for high-intensity warm straightening;

[0040] The straightened steel plate is then tempered to obtain a 9Ni steel plate.

[0041] In some embodiments, the steel billet has a sulfur content controlled below 0.002% and a phosphorus content controlled below 0.006%. Inclusions of types A, B, and C are required to be ≤1.5 grade, wherein type A inclusions are sulfides, type B inclusions are alumina, and type C inclusions are silicates.

[0042] The steel billet adopts a C-Si-Mn-Ni alloy composition system, and the designed proportions of each alloy are: C 0.035%~0.05%, Si 0.5%~0.8%, Mn 0.4%~1.4%, Ni 8%~10%, P≤0.006%, S≤0.002%;

[0043] The composition system design in this invention adopts a low carbon content design, and the reasonable alloy matching ensures that the steel plate base material has excellent low-temperature toughness and strength after the final heat treatment.

[0044] Adding an appropriate amount of Mn has deoxidizing and desulfurizing effects during steelmaking and prevents hot brittleness. This invention uses 0.4%-1.4% Mn alloy, for example, the Mn content can be selected as 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, which can improve the strength and hardness of steel.

[0045] The 9Ni steel provided in this invention has a carbon content between 0.035% and 0.05%, for example, values ​​of 0.035%, 0.040%, 0.045%, and 0.05%. Increasing the Mn element in combination with the C element can also improve hardenability, improve the situation of large hardness differences in the thickness direction of the steel plate, and improve the uniformity of the steel plate's performance.

[0046] Ni is an austenite-expanding element. Ni-containing low-temperature steel forms reverse-transformed austenite during heat treatment, which is a key reason for the improved low-temperature toughness of Ni-containing low-temperature steel. The rotatable austenite inhibits crack propagation, a phenomenon known as crack tip passivation. When a crack tip encounters a rotatable austenite region, it will deflect and bifurcate, increasing energy consumption and absorption, thus hindering crack tip propagation. Therefore, the Ni content in this invention is 8%–10%, for example, 8.0%, 8.5%, 9.0%, 9.5%, and 10.0%.

[0047] 9Ni steel has a carbon equivalent of over 0.7% and lower thermal conductivity precipitation compared to ordinary carbon steel. To prevent cracking during heating and ensure uniform heat conduction in the slab, a slow heating rate is used before the austenitizing temperature of the billet. Therefore, in some embodiments, the step-by-step heating method during the billet heating process specifically includes: a heating rate of 4℃ / min to 6℃ / min when the temperature is between 200℃ and 700℃, which can be selected as 4℃ / min, 5℃ / min, or 6℃ / min; a heating rate of 6℃ / min to 10℃ / min when the temperature is between 700℃ and 900℃, which can be selected as 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min; due to the decrease in thermal conductivity in the furnace at high temperatures, considering the load on the furnace, the heating rate is 2℃ / min when the temperature is above 900℃, until the temperature reaches 1130-1150℃.

[0048] In some embodiments, to fully utilize the grain-refining effect of plastic deformation, the steel plate adopts a two-stage rolling process, which includes a roughing rolling stage and a finishing rolling stage. The roughing rolling stage adopts a large reduction process, with an initial rolling temperature of 1000℃~1050℃ and a finishing rolling temperature of 970℃~1000℃, to ensure sufficient recrystallization of the material. During the finishing rolling process, the grains are further refined through plastic deformation, with an initial rolling temperature of 900℃~980℃ and a finishing rolling temperature between 800℃~890℃.

[0049] In some embodiments, to preserve dislocations formed during rolling and improve the strength of the quenched steel plate, continuous cooling is employed after rolling, with direct quenching at a rate greater than the critical cooling rate for 9Ni steel. During the quenching process, the continuous cooling rate is 20°C / s to 30°C / s, for example, 20°C / s, 22°C / s, 24°C / s, 25°C / s, 26°C / s, 28°C / s, or 30°C / s, and the cooling termination temperature is ≤100°C, such as 60°C, 70°C, 80°C, 85°C, 90°C, 95°C, or 100°C.

[0050] Compared with conventional hot rolling process, the temperature of warm straightening process is lower, and the steel plate has stronger deformation resistance. It can better avoid defects such as pitting on the steel plate surface caused by foreign objects being pressed in. Therefore, in some embodiments, the temperature of warm straightening is 0℃-300℃, and 0℃, 50℃, 100℃, 150℃, 200℃, 250℃, and 300℃ can be selected.

[0051] After online quenching, Ni steel forms a structure mainly composed of lath martensite. After tempering, tempered martensite is obtained. A certain amount of reversed austenite is distributed at the lath boundaries of the tempered martensite. This structure has a significant effect on low-temperature toughness. If the tempering temperature is too high, the stability of the reversed austenite will decrease; if the tempering temperature is too low, the uneven distribution of the reversed austenite will be aggravated. Therefore, in some embodiments, the tempering temperature during the tempering process is 550℃~600℃, for example, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃.

[0052] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0053] Example 1

[0054] This embodiment provides a 9Ni steel, the preparation method of which is as follows: Figure 1 As shown, the specific steps include the following:

[0055] 1) Steelmaking process

[0056] The alloy specific gravity design for smelting steel billets is as follows:

[0057] The steel billet adopts a C-Si-Mn-Ni alloy composition system, and the designed proportions of each alloy are: C 0.035%, Si 0.5%, Mn 1.4%, Ni 9.5%, S 0.00085%, P 0.003%.

[0058] The smelting process adopts LF+VD control, with sulfur content controlled at 0.00085% and phosphorus content controlled at 0.0015%. The quality control requirements for continuous casting billets are A, B, and C inclusions of grade 1.5, 1.0, and 1.0, respectively.

[0059] 2) Steel billet heating process

[0060] The steel billet is cold-fed and fed into the furnace at a temperature of 100℃. When the billet temperature is ≤700℃, the heating rate is controlled at 4℃ / min. When the billet temperature is between 700-900℃, the heating rate is controlled at 6℃ / min. When the billet temperature is >900℃, the heating rate is controlled at 2℃ / min until it reaches 1150℃. The resulting billet thickness is 18mm.

[0061] 3) Steel plate rolling process

[0062] A two-stage rolling process is adopted for 18mm thick steel plates. The initial rolling temperature of roughing is 1000℃, the initial rolling temperature of finishing is 900℃, and the final rolling temperature is 800℃. After the steel plates are rolled, they are straightened by a pre-straightening machine and then quenched in online quenching equipment.

[0063] 5) Direct quenching process for steel plates

[0064] The cooling rate of the rolled 18mm thick steel plate reaches 25℃ / second, and the cooling temperature is terminated at 70℃, forming a microstructure mainly composed of martensite inside the steel body.

[0065] 6) Steel plate straightening

[0066] The quenched steel plate is not subjected to a hot straightening process. Instead, it is directly transported to a warm straightening machine via a cooling bed for one straightening cycle. The temperature of the warm straightening is 50℃.

[0067] 7) Tempering of steel plates

[0068] The straightened steel plate was tempered at a temperature of 550℃ for 43 minutes and a holding time of 30 minutes. The resulting 9Ni steel plate was mainly composed of tempered martensite with rotated austenite distributed at the martensite lath boundaries.

[0069] Example 2

[0070] This embodiment provides a 9Ni steel, specifically including the following steps:

[0071] 1) Steelmaking process

[0072] The alloy specific gravity design for smelting steel billets is as follows:

[0073] The steel billet adopts a C-Si-Mn-Ni alloy composition system, and the designed proportions of each alloy are: C 0.05%, Si 0.5%, Mn 0.4%, Ni 8%, S 0.0015%, P 0.005%.

[0074] The smelting process adopts LF+VD control, with sulfur content controlled at 0.0015% and phosphorus content controlled at 0.005%. The quality control requirements for continuous casting billets are A, B, and C inclusions of grade 1.0, 1.5, and 1.0, respectively.

[0075] 2) Steel billet heating process

[0076] The steel billet is cold-fed and fed into the furnace at a temperature of 150℃. When the billet temperature is ≤700℃, the heating rate is controlled at 6℃ / min. When the billet temperature is between 700-900℃, the heating rate is controlled at 10℃ / min. When the billet temperature is >900℃, the heating rate is controlled at 2℃ / min until it reaches 1150℃. The resulting billet thickness is 18mm.

[0077] 3) Steel plate rolling process

[0078] A two-stage rolling process is adopted for 18mm thick steel plates. The initial rolling temperature of roughing is 1050℃, the initial rolling temperature of finishing is 970℃, and the final rolling temperature is 890℃. After the steel plates are rolled, they are straightened by a pre-straightening machine and then quenched in online quenching equipment.

[0079] 5) Direct quenching process for steel plates

[0080] The cooling rate of the rolled 18mm thick steel plate reaches 20℃ / second, and the cooling temperature is 90℃, forming a microstructure mainly composed of martensite inside the steel body.

[0081] 6) Steel plate straightening

[0082] The quenched steel plate is not subjected to a hot straightening process. Instead, it is directly transported to a warm straightening machine via a cooling bed for one straightening cycle. The temperature of the warm straightening is 70℃.

[0083] 7) Tempering of steel plates

[0084] The straightened steel plate was tempered at a temperature of 580℃ for 40 minutes and held for 30 minutes. The resulting 9Ni steel plate was mainly composed of tempered martensite and rotated austenite.

[0085] Example 3

[0086] This embodiment provides a 9Ni steel, specifically including the following steps:

[0087] 1) Steelmaking process

[0088] The alloy specific gravity design for smelting steel billets is as follows:

[0089] The steel billet adopts a C-Si-Mn-Ni alloy composition system, and the designed proportions of each alloy are: C 0.045%, Si 0.5%, Mn 0.8%, Ni 9%, S 0.001%, P 0.004%.

[0090] The smelting process adopts LF+VD control, with sulfur content controlled at 0.001% and phosphorus content controlled at 0.004%. The quality control requirements for continuous casting billets are A, B, and C inclusions of grade 1.5, 1.0, and 1.0, respectively.

[0091] 2) Steel billet heating process

[0092] The steel billet is cold-fed and fed into the furnace at a temperature of 130℃. When the billet temperature is ≤700℃, the heating rate is controlled at 5℃ / min. When the temperature is 700-900℃, the heating rate is controlled at 8℃ / min. When the billet temperature is >900℃, the heating rate is controlled at 2℃ / min until it reaches 1150℃. The resulting billet thickness is 30mm.

[0093] 3) Steel plate rolling process

[0094] A two-stage rolling process is adopted for 30mm thick steel plates. The initial rolling temperature of roughing is 1020℃, the initial rolling temperature of finishing is 950℃, and the final rolling temperature is 830℃. After the steel plates are rolled, they are straightened by a pre-straightening machine and then quenched in online quenching equipment.

[0095] 5) Direct quenching process for steel plates

[0096] The cooling rate of the rolled 30mm thick steel plate reaches 15℃ / second, and the cooling temperature is terminated at 100℃, forming a microstructure mainly composed of martensite inside the steel body.

[0097] 6) Steel plate straightening

[0098] The quenched steel plate is not subjected to a hot straightening process. Instead, it is directly transported to a warm straightening machine via a cooling bed for one straightening cycle. The temperature of the warm straightening is 90℃.

[0099] 7) Tempering of steel plates

[0100] The straightened steel plate was tempered at a temperature of 600℃ for 60 minutes and a holding time of 50 minutes. The resulting 9Ni steel plate was mainly composed of tempered martensite with rotated austenite distributed at the martensite lath boundaries.

[0101] Comparative Example

[0102] This comparative example provides a 9Ni steel, which specifically includes the following steps:

[0103] 1) Steelmaking process

[0104] The alloy specific gravity design for smelting steel billets is as follows:

[0105] The steel billet adopts a C-Si-Mn-Ni alloy composition system, and the designed proportions of each alloy are: C 0.035%, Si 0.5%, Mn 1.4%, Ni 9.5%, S 0.00085%, P 0.003%.

[0106] The smelting process adopts LF+VD control, with sulfur content controlled at 0.00085% and phosphorus content controlled at 0.003%. The quality control requirements for continuous casting billets are A, B, and C inclusions of grade 1.5, 1.0, and 1.0, respectively.

[0107] 2) Steel billet heating process

[0108] The steel billet is cold-fed. The billet temperature is 100℃ when it enters the furnace. When the billet temperature is ≤700℃, the heating rate is controlled at 4℃ / min. When the billet temperature is 700-900℃, the heating rate is controlled at 6℃ / min. When the billet temperature is >900℃, the heating rate is controlled at 2℃ / min until it is heated to 1150℃. The resulting billet thickness is 18mm.

[0109] 3) Steel plate rolling process

[0110] The steel plate adopts a two-stage rolling process. The thickness of the steel plate is 18mm. The initial rolling temperature of the rough rolling is 1000℃, the initial rolling temperature of the finish rolling is 900℃, and the final rolling temperature is 800℃. After the steel plate is rolled, it is cooled to 600℃ at 5℃ / second for hot straightening, and then cooled to room temperature in the air.

[0111] 5) Steel plate quenching

[0112] The rolled 18mm thick steel plate is heated to 870℃ and cooled in a quenching machine at a cooling rate of 25℃ / second, forming a microstructure mainly composed of martensite inside the steel body.

[0113] 6) Tempering of steel plates

[0114] The quenched 18mm thick steel plate was heated to 560℃ for 45 minutes and held for 30 minutes. The resulting 9Ni steel plate had a microstructure mainly composed of tempered martensite with rotated austenite distributed at the martensite lath boundaries.

[0115] The performance of the 9Ni steel plates obtained in Examples 1-3 and Comparative Example 1 was tested, and the data results are shown in Table 1 below:

[0116] Table 1. Performance data and production cycle of 9Ni steel obtained from Examples 1-3 and comparative examples.

[0117]

[0118]

[0119] As can be seen from the data in the table above, the 9Ni steel plates provided in Examples 1-3 and the comparative example all meet the product performance requirements. In particular, the 9Ni steel plates provided in Examples 1-3 eliminate the offline quenching process during production, shortening the steel plate manufacturing process and requiring only about 45 days to produce the finished product. Furthermore, the yield strength of the obtained product reaches over 680 MPa, the tensile strength reaches over 740 MPa, and the average impact energy at -196℃ reaches at least 128 J, demonstrating excellent overall performance and meeting relevant standards. However, the 9Ni steel plate provided in the comparative example uses a process flow from existing technology. Although the product performance is not significantly different from that of the products in the examples, it requires a 60-day production cycle, reducing production efficiency by 30%. Therefore, the preparation method of this invention greatly improves production efficiency.

[0120] In addition, performance tests were performed on the 9Ni steel plates obtained in Example 1 and the comparative example, and their microstructure images were compared, such as... Figure 2 and Figure 3 As shown, Figure 2 The microstructure of the 9Ni steel plate obtained in Example 1 is shown below. Figure 3 To illustrate the microstructure of the 9Ni steel plate obtained in the comparative example, a comparison in the figures reveals that... Figure 1 The amount of rotated austenite (black dots) distributed at the martensite lath boundaries is significantly greater than that in the middle. Figure 2 The amount of austenite in the 9Ni steel further illustrates that the strength and toughness of the 9Ni steel provided in this invention are improved.

[0121] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing 9Ni steel, characterized in that, Includes the following steps: Molten steel is prepared, and the smelting process is controlled by LF+VD to obtain steel billets containing chemical components; The steel billet is heated using a step-by-step heating method; A two-stage rolling process is used to roll the heated steel billet to obtain rolled steel plate; The rolled steel plate is directly quenched using a continuous cooling method to obtain a quenched steel plate; The quenched steel plate is then subjected to warm straightening. The straightened steel plate is then tempered to obtain a 9Ni steel plate; The chemical composition includes C 0.035%~0.05%, Si 0.5%~0.8%, Mn 0.4%~1.4%, Ni 8%~10%, P ≤0.006%, S ≤0.002%, with the balance being Fe and unavoidable impurities; The parameters of the step heating method are as follows: the heating rate is 4℃ / min to 6℃ / min in the range of 200℃ to 700℃; the heating rate is 6℃ / min to 10℃ / min in the range of 700℃ to 900℃; and the heating rate is 2℃ / min when the temperature is >900℃, until the temperature is heated to 1130-1150℃. The two-stage rolling process includes a roughing stage and a finishing stage. The starting temperature of the roughing stage is 1000℃~1050℃, and the finishing temperature is 970℃~1000℃. During the finishing stage, the starting temperature is 900℃~980℃, and the finishing temperature is 800℃~890℃. During the quenching process of the steel plate, the continuous cooling rate is 20℃ / s~30℃ / s, and the termination cooling temperature is ≤100℃. The temperature for thermal straightening is less than 100℃; The microstructure of the 9Ni steel plate includes tempered martensite and rotten austenite, with 3%-7% of the rotten austenite distributed at the tempered martensite lath boundaries; The 9Ni steel has a yield strength of 680 MPa or higher, a tensile strength of 740 MPa or higher, and an average impact energy of at least 128 J at -196℃.

2. The method for preparing 9Ni steel according to claim 1, characterized in that, The sulfur content of the steel billet is less than 0.002%, and the phosphorus content of the steel billet is less than 0.006%.

3. The method for preparing 9Ni steel according to claim 1, characterized in that, The tempering temperature during the tempering process is 550℃~600℃.

Images