A 9Ni steel plate and a method for manufacturing the same

Abstract

The application provides a 9Ni steel plate and a preparation method thereof, and relates to the technical field of steel rolling.The chemical components of the 9Ni steel plate include C 0.04-0.06%, Si 0.15-0.35%, Mn 0.50-0.80%, Ni 8.5-9.5%, P≤0.004%, S≤0.002%, Als 0.015-0.045%, and the balance of Fe and impurities, calculated by percentage by weight.The preparation method provided by the application is a hot mechanical rolling method, which is combined with a corresponding cooling process, can ensure appropriate initial grain size, adopts a QT heat treatment process, can control the microstructure in the quenching and tempering process, can obtain appropriate strength and good super-low temperature impact toughness, and can control the yield ratio of the steel plate to be below 0.90.The 9Ni steel produced according to the application does not need to add micro-alloy elements such as Nb, V and Ti, and the manufacturing cost of the steel plate is reduced.Meanwhile, the heat treatment process is simple and easy to operate, and is suitable for batch production.

Description

Technical Field

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

[0002] Against the backdrop of building a global ecological community, the clean energy strategy is being accelerated, and natural gas, as the cleanest fossil energy source, is increasingly demonstrating its important position. Liquefied Natural Gas (LNG), with its advantages of easy storage and transportation, and good safety, accounts for more than half of the natural gas trade. 9Ni steel, with its high strength, excellent -196℃ ultra-low temperature impact toughness, and good weldability, is widely used in all aspects of LNG storage, transportation, and use.

[0003] To achieve suitable strength and good cryogenic toughness, 9Ni steel, after thermomechanical rolling, must undergo heat treatments such as double normalizing + tempering, quenching + tempering, or quenching + two-phase quenching + tempering. As the inner liner material primarily used in large land-based LNG storage tanks, marine LNG fuel tanks, and LNG carrier cargo maintenance systems, 9Ni steel comes into direct contact with flammable and explosive LNG at -162℃, operating in harsh environments. To ensure a sufficient safety factor, designers have specific requirements for the material's yield strength ratio. However, the yield strength ratio of heat-treated 9Ni steel plates is typically quite high, making it challenging to meet design requirements for low yield strength ratios.

[0004] In the prior art, Chinese patent CN110229998 discloses a thin-gauge 9Ni steel plate with a low yield strength ratio, produced using a hot-rolled coil process, with a plate thickness as low as 5mm, combined with an offline quenching-offline normalizing-high-temperature tempering heat treatment process. However, this method is only applicable to 5mm thin-gauge 9Ni steel plates and cannot fully cover the actual range of supplied steel plate thicknesses. Furthermore, the heat treatment process is complex and has low production efficiency, impacting heat treatment capacity and limiting its suitability for mass production.

[0005] Chinese patent CN114836692 discloses a high-compression-ratio, low-yield-strength-ratio marine high-nickel steel plate and its manufacturing method. It provides a 10-25mm thick 9Ni steel plate for LNG ships with large compression ratio and low yield-strength-ratio, and its manufacturing method. This method adds a two-phase zone secondary quenching process between the normal quenching and tempering heat treatment to reduce alloying and impurity elements in the ferrite matrix, thereby reducing the solid solution strengthening effect and lowering the yield-strength-ratio. However, this method cannot completely cover the actual thickness range of the supplied steel plates. Furthermore, the addition of a two-phase zone secondary quenching process will have a certain impact on the manufacturing cost and production efficiency of the steel plate.

[0006] Chinese patent CN110541110A discloses a high-strength, low-yield-strength-ratio 9Ni steel plate for marine LNG storage tanks and its manufacturing method. By increasing the Nb and Cr content in the steel and employing a QLT heat treatment process, a 9Ni steel with good strength and toughness and a low yield-strength-ratio is obtained. This method achieves the goal of increasing the strength and reducing the yield-strength-ratio of 9Ni steel through the addition of alloying elements and an additional intermediate two-phase quenching process. However, this method also inevitably has a certain impact on the manufacturing cost and production efficiency of the steel plate.

[0007] In summary, in order to solve the above-mentioned technical problems, it is necessary to find a new 9Ni steel and its preparation method. Summary of the Invention

[0008] This application provides a 9Ni steel and its preparation method to solve the technical problems of high alloy cost and complex heat treatment process in the existing 9Ni steel production process.

[0009] In a first aspect, this application provides a 9Ni steel plate, the chemical composition of which, by weight percentage, comprises C 0.04%–0.06%, Si 0.15%–0.35%, Mn 0.50%–0.80%, Ni 8.5%–9.5%, P ≤0.004%, S ≤0.002%, Als 0.015%–0.045%, with the balance being Fe and unavoidable impurity elements.

[0010] Optionally, the 9Ni steel meets the following properties: yield strength ≥ 590 MPa, tensile strength ≥ 690 MPa, yield-to-tensile ratio ≤ 0.90, elongation ≥ 19%, and impact energy at -196℃ ≥ 100 J.

[0011] Optionally, the microstructure of the 9Ni steel plate is a mixed microstructure with tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite.

[0012] Optionally, the thickness of the 9Ni steel plate is 5mm to 50mm.

[0013] Secondly, the present invention provides a method for preparing 9Ni steel plate, comprising the following steps:

[0014] Prepare molten steel and refine it into steel billets;

[0015] The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0016] Heat the steel billet to 1030℃~1200℃ and hold for 1h~2h;

[0017] After descaling, the heated steel billet is rolled into a steel plate. The thickness of the steel plate is determined to be greater than 10 mm. If it is, rough rolling and finish rolling are used, followed by a cooling process. If the thickness is 10 mm or less, it is directly rolled and then air-cooled.

[0018] The rolled steel plate was heat-treated by quenching and tempering process to finally obtain 9Ni steel plate.

[0019] Optionally, in the billet smelting step, the chemical composition of the molten steel, calculated by weight percentage, includes: C 0.04%–0.06%, Si 0.15%–0.35%, Mn 0.50%–0.80%, Ni 8.5%–9.5%, P ≤0.004%, S ≤0.002%, Al 0.015%–0.045%, with the balance being Fe and unavoidable impurity elements.

[0020] Optionally, the billet smelting steps specifically include: converter → LF furnace refining → VD vacuum treatment → slab continuous casting process to produce billets.

[0021] Optionally, the thickness of the steel billet is 200mm to 400mm.

[0022] Optionally, during rough rolling, the initial rolling temperature is 1000℃~1050℃, and the final rolling temperature is 970℃~1000℃; during finish rolling, the initial rolling temperature is 900℃~980℃, and the final rolling temperature is 800℃~860℃.

[0023] Optionally, the cooling process adopts water cooling, with a final cooling temperature of 550℃~650℃ and a cooling rate of 5℃ / S~10℃ / S.

[0024] Optionally, the heat treatment process specifically includes: heating the steel plate to A c3 After holding at 80℃~120℃ for 20min~50min, quench the steel plate, then heat it to A. c1 The following conditions apply: Heat at 10℃~60℃ for 20min~60min followed by air cooling and tempering; A c1 A is the temperature at which the pearlite transforms into austenite upon heating. c3 This is the final temperature at which all proeutectoid ferrite transforms into austenite during heating.

[0025] The technical solution provided by this invention has the following advantages compared with the prior art:

[0026] This invention provides a 9Ni steel plate and its preparation method. Besides Ni, Mn, and Al, no additional microalloying elements such as Nb, V, and Ti are required in this steel plate, reducing manufacturing costs. For steel plates of different thicknesses, direct rolling and two-stage controlled rolling processes are employed, combined with appropriate water cooling processes, to achieve precise control, ensuring the steel plate's microstructure and initial grain size. This creates favorable conditions for the steel plate to achieve excellent comprehensive mechanical properties. While ensuring the steel plate's strength and -196℃ ultra-low temperature impact toughness, the yield strength ratio is effectively reduced to below 0.90, meeting downstream user design requirements. After rolling, the steel plate undergoes quenching and tempering heat treatment, a simple and easy process suitable for mass production. Attached Figure Description

[0027] 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.

[0028] 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.

[0029] Figure 1 A flowchart of a method for preparing 9Ni steel plate provided in this application.

[0030] Figure 2 This is a microstructure of a 9Ni steel plate with a low yield strength ratio provided in Embodiment 1 of the present invention.

[0031] Figure 3 This is a microstructure diagram of a 9Ni steel plate with a low yield strength ratio provided in Embodiment 2 of the present invention.

[0032] Figure 4 This is a microstructure of a 9Ni steel plate with a low yield strength ratio provided in Embodiment 3 of the present invention. Detailed Implementation

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] In a first aspect, this application provides a 9Ni steel plate, the chemical composition of which, by weight percentage, comprises C 0.04%–0.06%, Si 0.15%–0.35%, Mn 0.50%–0.80%, Ni 8.5%–9.5%, P ≤0.004%, S ≤0.002%, Als 0.015%–0.045%, with the balance being Fe and unavoidable impurity elements.

[0038] The functions and limitations of the aforementioned main alloying elements are explained in detail below:

[0039] Carbon (C) expands the austenite phase region and has a strong solid solution strengthening effect. However, excessively high C content results in poor low-temperature toughness and weldability. Considering all factors, this steel grade requires excellent strength and toughness; therefore, the carbon content is controlled within the range of 0.04% to 0.06%, for example, 0.04%, 0.05%, or 0.06%.

[0040] Si does not form carbides with C and exists in steel in a solid solution manner. By interacting with the stress field of movable dislocations, it hinders dislocation movement and improves the strength of the steel plate. However, a high Si content is detrimental to the weldability of the steel. In this invention, the Si content is controlled at 0.15% to 0.35%, for example, 0.15%, 0.20%, 0.25%, 0.30%, and 0.35%.

[0041] Mn strengthens the microstructure through solid solution strengthening, and an appropriate Mn content can also significantly improve the stability of austenite. Increasing the Mn content can increase the hardenability of steel, thus enabling the steel to obtain martensite or even full martensite over a wide range of cooling rates. However, excessive Mn content can easily lead to segregation, which is detrimental to the low-temperature toughness of the steel plate. Therefore, the steel should have excellent comprehensive mechanical properties. In this invention, the Mn content is controlled at 0.50% to 0.80%, for example, it can be selected as 0.50%, 0.60%, 0.70%, or 0.80%.

[0042] Ni is an element that improves the hardenability of steel plates and can significantly enhance low-temperature toughness at -196℃. It also has a positive effect on impact toughness and the ductile-brittle transition temperature. However, when the Ni content is too high, iron oxide scale easily forms on the surface of the slab, which is difficult to remove and affects the surface quality of the steel plate. In addition, Ni is also a precious metal, and excessive content will increase costs. Therefore, this invention controls the Ni content to 8.50% to 9.50%, for example, 8.50%, 9.0%, or 9.50%.

[0043] Polymer (P) has a strong solid solution strengthening effect in steel. When added as an alloying element to low-alloy structural steel, it can improve its strength and atmospheric corrosion resistance. However, the biggest drawback of P is its severe segregation, which increases temper brittleness and significantly increases the plasticity and toughness of steel, making it prone to brittle fracture during cold working, a phenomenon known as "cold brittleness." P also has an adverse effect on weldability. As a harmful element, P should be strictly controlled. In this invention, the P content is controlled to ≤0.004%, for example, it can be 0.002%, 0.003%, or 0.004%.

[0044] Sulfur (S) segregates severely in steel, deteriorating its quality and reducing its plasticity at high temperatures, making it a harmful element. It exists as FeS, a form with a low melting point. The melting point of FeS alone is only 1190℃, while its eutectic temperature in steel, where it forms a eutectic with iron, is even lower, only 988℃. When steel solidifies, iron sulfide precipitates at the primary grain boundaries. When steel is rolled at 1100–1200℃, the FeS at the grain boundaries melts, significantly weakening the bonding force between grains and leading to hot brittleness. Therefore, sulfur content must be strictly controlled; in this invention, the S content is controlled to ≤0.002%.

[0045] Al increases the driving force for phase transformation. In steel, Al interacts with N to form fine and dispersed AlN precipitates, which can inhibit grain growth, thereby refining the grains and improving the toughness of the steel at low temperatures. In this invention, the Al content is controlled at 0.015% to 0.045%, for example, 0.015%, 0.020%, 0.025%, 0.030%, and 0.035%.

[0046] In some embodiments, the 9Ni steel meets the following properties: yield strength ≥ 590 MPa, tensile strength ≥ 690 MPa, yield-to-tensile ratio ≤ 0.90, elongation ≥ 19%, and impact energy at -196℃ ≥ 100 J.

[0047] In some embodiments, the microstructure of the 9Ni steel plate is a mixture of tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite.

[0048] In some embodiments, the thickness of the 9Ni steel plate is 5mm to 50mm, and can be 5mm, 10mm, 15mm, 20mm, 30mm, 40mm, or 50mm.

[0049] Secondly, the present invention provides a method for preparing 9Ni steel plate, comprising the following steps:

[0050] Prepare molten steel and refine it into steel billets;

[0051] The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0052] Heat the steel billet to 1030℃~1200℃ and hold for 1~2 hours;

[0053] After descaling, the heated steel billet is rolled into a steel plate. The thickness of the steel plate is determined to be greater than 10 mm. If it is, rough rolling and finish rolling are used, followed by a cooling process. If the thickness is 10 mm or less, it is directly rolled and then air-cooled.

[0054] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process to finally obtain 9Ni steel plate.

[0055] In some embodiments, during the billet smelting step, the chemical composition of the molten steel, calculated by weight percentage, includes: C 0.04%–0.06%, Si 0.15%–0.35%, Mn 0.50%–0.80%, Ni 8.5%–9.5%, P ≤0.004%, S ≤0.002%, Al 0.015%–0.045%, with the balance being Fe and unavoidable impurity elements.

[0056] In some embodiments, the billet smelting steps specifically include: converter → LF furnace refining → VD vacuum treatment → slab continuous casting process to produce billets.

[0057] In some embodiments, the thickness of the steel billet is 200mm to 400mm, and can be 200mm, 250mm, 300mm, 350mm, or 400mm.

[0058] In some implementations, mechanical grinding is used to clean and smooth the surface of the steel billet, followed by an anti-oxidation coating treatment.

[0059] In some embodiments, a walking beam furnace is used to heat the billet during the billet heating process;

[0060] In some embodiments, during rough rolling, the initial rolling temperature is 1000℃~1050℃ and the final rolling temperature is 970℃~1000℃; during finish rolling, the initial rolling temperature is 900℃~980℃ and the final rolling temperature is 800℃~860℃.

[0061] In some embodiments, the cooling process employs water cooling, with a final cooling temperature of 550°C to 650°C and a cooling rate of 5°C / s to 10°C / s.

[0062] In some embodiments, the heat treatment process specifically includes: heating the steel plate to A c3 After holding at 80℃~120℃ for 20min~50min, quench the steel plate, then heat it to A. c1 The following conditions apply: Heat at 10℃~60℃ for 20min~60min followed by air cooling and tempering; A c1 A is the temperature at which the pearlite transforms into austenite upon heating. c3 This is the final temperature at which all proeutectoid ferrite transforms into austenite during heating.

[0063] 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.

[0064] Example 1

[0065] This embodiment provides a 9Ni steel plate with the following chemical composition by weight percentage: C 0.050%, Si 0.26%, Mn 0.71%, Ni 9.12%, Al 0.038%, P 0.0021%, S 0.0010%, with the balance being Fe and impurities. The preparation method is as follows:

[0066] Steel billet smelting: Prepare molten steel with the above chemical composition ratio and smelt it into 200mm thick steel billets;

[0067] Steel billet surface treatment: The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0068] Billet heating: Heat the billet to 1200℃ and hold for 1 hour;

[0069] Rolling and cooling: After the heated steel billet comes out of the furnace, it undergoes a descaling process, is directly rolled into a 7mm thick steel plate, and then air-cooled.

[0070] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process. The steel plate is heated to 820℃, held for 20 minutes and water quenched, then heated to 590℃, held for 20 minutes and air-cooled and tempered to obtain 9Ni steel plate.

[0071] Its microstructure is as follows Figure 2 As shown, it can be clearly seen that its microstructure is a mixture of tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite. The austenite content was tested to be 5.32%.

[0072] Example 2

[0073] This embodiment provides a 9Ni steel plate with the following chemical composition by weight percentage: C 0.044%, Si 0.23%, Mn 0.66%, Ni 9.13%, P 0.0019%, S 0.00085%, Al 0.043%, with the balance being Fe and impurities. The preparation method is as follows:

[0074] Steel billet smelting: Prepare molten steel and smelt it into 200mm thick steel billets;

[0075] Steel billet surface treatment: The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0076] Billet heating: Heat the steel billet to 1180℃ and hold for 1 hour;

[0077] Rolling and Cooling: After the steel billet is descaled after exiting the furnace, it is rolled by rough rolling and finish rolling respectively, followed by a cooling process to obtain a 30mm thick steel plate;

[0078] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process. The steel plate is heated to 830℃, held for 40 minutes and water quenched, then heated to 580℃, held for 40 minutes and air-cooled and tempered to obtain 9Ni steel plate.

[0079] Its microstructure is as follows Figure 3 As shown, it can be clearly seen that its microstructure is a mixture of tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite. The austenite content in the microstructure is 4.96% as tested.

[0080] Example 3

[0081] This embodiment provides a 9Ni steel plate with the following microstructure: Figure 4 As shown, its chemical composition by weight percentage is: C 0.044%, Si 0.24%, Mn 0.67%, Ni 9.14%, P 0.0017%, S 0.00098%, Al 0.036%, with the balance being Fe and impurities. The preparation method is as follows:

[0082] Steel billet smelting: Prepare molten steel and smelt it into 300mm thick steel billets;

[0083] Steel billet surface treatment: The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0084] Billet heating: Heat the steel billet to 1160℃ and hold for 1.5 hours;

[0085] Rolling and cooling: After the steel billet exits the furnace, it undergoes a descaling process, and is then rolled using rough rolling and finish rolling methods before cooling to finally obtain a 50mm thick steel plate;

[0086] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process. The steel plate is heated to 810℃, held for 50 minutes and water quenched, then heated to 570℃, held for 50 minutes and air-cooled and tempered to obtain 9Ni steel plate.

[0087] Its microstructure is as follows Figure 4As shown, it can be clearly seen that its microstructure is a mixture of tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite. The austenite content in the microstructure is 4.96% as tested.

[0088] Comparative Example 1

[0089] This comparative example provides a 9Ni steel plate with the following chemical composition by weight percentage: C 0.05%, Si 0.25%, Mn 0.72%, Ni 9.19%, P 0.0051%, S 0.0020%, Al 0.030%, with the balance being Fe and impurities. The preparation method is as follows:

[0090] Steel billet smelting: The molten steel with the above composition is smelted into 200mm thick steel billets;

[0091] Steel billet surface treatment: The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0092] Billet heating: Heat the billet to 1250℃ and hold for 1 hour;

[0093] Rolling and Cooling: After the heated steel billet is taken out of the furnace, it is descaled and then directly rolled to obtain a 10mm thick steel plate, followed by a cooling process;

[0094] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process. The steel plate is heated to 810℃, held for 30 minutes and water quenched, then heated to 600℃, held for 20 minutes and air cooled and tempered.

[0095] Comparative Example 2

[0096] This comparative example provides a 9Ni steel plate with the following chemical composition by weight percentage: C 0.04%, Si 0.27%, Mn 0.70%, Ni 9.06%, P 0.0019%, S 0.0019%, Al 0.019%, with the balance being Fe and impurities. The preparation method is as follows:

[0097] Steel billet smelting: Prepare molten steel and smelt it into 200mm thick steel billets;

[0098] Steel billet surface treatment: The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0099] Billet heating: Heat the billet to 1200℃ and hold for 1 hour;

[0100] Rolling and Cooling: After the steel billet is descaled after exiting the furnace, it is rolled by rough rolling and finish rolling respectively, and then cooled to obtain a 20mm thick steel plate.

[0101] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process. The steel plate is heated to 790℃, held for 30 minutes and water quenched, then heated to 580℃, held for 40 minutes and air cooled and tempered.

[0102] Comparative Example 3

[0103] This comparative example provides a 9Ni steel plate with the following chemical composition by weight percentage: C 0.038%, Si 0.29%, Mn 0.77%, Ni 9.12%, P 0.0050%, S 0.0018%, Al 0.034%, with the balance being Fe and impurities. The preparation method is as follows:

[0104] Steel billet smelting: Prepare molten steel and smelt it into 200mm thick steel billets;

[0105] Steel billet surface treatment: The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied.

[0106] Billet heating: Heat the billet to 1150℃ and hold for 1 hour;

[0107] Rolling and Cooling: After the steel billet is descaled after exiting the furnace, it is rolled by rough rolling and finish rolling respectively, followed by a cooling process to obtain a 40mm thick steel plate;

[0108] Heat treatment: The rolled steel plate is heat treated by quenching and tempering process. The steel plate is heated to 810℃, held for 40 minutes and water quenched, then heated to 560℃, held for 40 minutes and air-cooled for tempering.

[0109] Experimental results

[0110] The 9Ni steel plates obtained in Examples 1-3 and Comparative Examples 1-3 were compared. First, the compositions of each example and comparative example are shown in Table 1:

[0111] Table 1. Components (wt%) of the embodiments and comparative examples of the present invention.

[0112] C Si Mn Ni Als P S Example 1 0.050 0.26 0.71 9.12 0.038 0.0021 0.0010 Example 2 0.044 0.23 0.66 9.13 0.043 0.0019 0.00085 Example 3 0.044 0.24 0.67 9.14 0.036 0.0017 0.00098 Comparative Example 1 0.050 0.25 0.72 9.19 0.030 0.0051 0.0020 Comparative Example 2 0.040 0.27 0.75 9.06 0.019 0.0033 0.0019 Comparative Example 3 0.038 0.29 0.77 9.12 0.034 0.0050 0.0018

[0113] The heat treatment process data used in each embodiment and comparative example are shown in Table 2:

[0114] Table 2 Heat treatment processes of embodiments and comparative examples of the present invention

[0115]

[0116] The steel plates obtained in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing. The testing methods were as follows: the tensile properties of the steel plates were determined according to GB / T228 "Metallic Materials - Tensile Testing at Room Temperature", and the impact properties at 1 / 4 of the thickness of the steel plates were determined according to GB / T229-2007 "Metallic Materials - Charpy Impact Test". The test results are shown in Table 3.

[0117] Table 3 Mechanical properties of embodiments and comparative examples of the present invention

[0118]

[0119] As shown in Tables 1-3, Examples 1-3 of this invention, through a composition design with low C content and strict control of P and S impurity elements, and by employing quenching and tempering treatment, obtain a mixed microstructure of tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite, resulting in a 9Ni steel plate with a low yield strength ratio. The different C, P, and S content in Comparative Examples 1-3, as well as the failure to set the quenching and tempering temperatures according to the requirements specified in this invention, resulted in product performance far inferior to that of Examples 1-3.

[0120] The low yield strength ratio 9Ni steel plate prepared under the composition design and process conditions of this invention has a yield strength ≥618MPa, tensile strength ≥702MPa, yield strength ratio ≤0.90, elongation after fracture ≥23.5%, impact energy of 5*10*55mm specimen at -196℃ ≥93J, and impact energy of 10*10*55mm specimen at -196℃ ≥200J, with good mechanical properties. By comparing the data of the proportions and examples, it can be seen that when the quenching temperature, tempering temperature or chemical composition is not within the range provided in the examples of this application, a higher yield strength ratio will occur.

[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 plate, characterized in that, Includes the following steps: Prepare molten steel and refine it into steel billets; The surface of the steel billet is ground to make it smooth, and then an anti-oxidation coating is applied. The treated steel billet is heated to 1030~1200℃ and held at that temperature for 1h~2h to obtain the heated steel billet. After descaling, the heated steel billet is rolled into a steel plate. The thickness of the steel plate is determined to be greater than 10 mm. If it is, rough rolling and finish rolling are used, followed by a cooling process. If the thickness is 10 mm or less, it is directly rolled and then air-cooled. The rolled steel plate was heat-treated by quenching and tempering process to finally obtain 9Ni steel plate; The cooling process uses water cooling, with a final cooling temperature of 550℃~650℃ and a cooling rate of 5℃ / S~10℃ / S. The heat treatment process specifically includes: heating the steel plate to A c3 After holding at 80℃~120℃ for 20min~50min, quench the steel plate, then heat it to A. c1 The following temperatures are set at 10℃ to 60℃, held for 20 to 60 minutes, and then air-cooled for tempering; among which, A c1 A is the temperature at which the pearlite transforms into austenite upon heating. c3 This is the final temperature at which all preeutectoid ferrite transforms into austenite during heating; The chemical composition of the 9Ni steel plate, calculated by weight percentage, includes C 0.04%~0.06%, Si 0.15%~0.35%, Mn 0.50%~0.80%, Ni 8.5%~9.5%, P ≤0.004%, S ≤0.002%, Als 0.015%~0.045%, with the balance being Fe and unavoidable impurity elements. The 9Ni steel meets the following properties: yield strength ≥590MPa, tensile strength ≥690MPa, yield-to-tensile ratio ≤0.90, elongation ≥19%, and impact energy at -196℃ ≥100J. The microstructure of the 9Ni steel plate is a mixed microstructure consisting of tempered martensite as the matrix, plus ferrite and not less than 2.0% austenite. The thickness of the 9Ni steel plate is 5mm~50mm. During rough rolling, the initial rolling temperature is 1000℃~1050℃, and the final rolling temperature is 970℃~1000℃; during finish rolling, the initial rolling temperature is 900℃~980℃, and the final rolling temperature is 800℃~860℃.

2. The method for preparing 9Ni steel plate according to claim 1, characterized in that, The specific steps of billet smelting include: converter → LF furnace refining → VD vacuum treatment → slab continuous casting process to produce thick steel billets.

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