Method for producing 5.5 ni steel for use in low-temperature container rated for -196°c services

Abstract

Disclosed in the present invention is a method for producing 5.5 Ni steel for use in low-temperature container rated for -196°C services, the 5.5 Ni steel comprising the following components in percentage by mass: 0.03-0.06% of C, 0.15-0.20% of Si, 0.9-1.2% of Mn, less than or equal to 0.002% of S, less than or equal to 0.005% of P, 5.3-5.7% of Ni, 0.3-0.7% of Cr, 0.12-0.22% of Mo, 0.010-0.030% of Alt, and less than or equal to 0.0020% of O, the balance being Fe and impurities; and the method comprising sequentially carrying out converter smelting, LF+RH refining, and continuous casting into a slab having a thickness of 150 mm. The method has the advantages that a controlled rolling and controlled cooling and offline heat treatment process is used to obtain a structure in which tempered martensite is used as a matrix and a small amount of austenite is added, and steel for use in a low-temperature container rated for -196°C services and having a thickness in the range of 5-30 mm is produced.

Description

A method for producing 5.5Ni steel for cryogenic containers that can operate at -196℃. Technical Field

[0001] This invention belongs to the field of steel production technology, and in particular relates to a method for producing 5.5Ni steel for cryogenic containers that can operate at -196℃. Background Technology

[0002] With the development of my country's LNG industry chain and the increasing size of cryogenic storage tanks in the chemical sector, the demand for high-strength cryogenic container steel is growing daily. 9Ni steel is generally used for constructing large LNG storage tanks or large ethane and ethylene storage tanks. However, with advancements in smelting and material microstructure control technologies, the performance margin of 9Ni steel is now significantly excessive. Furthermore, Ni is a precious metal, keeping the cost of 9Ni steel high. To support the high-quality, low-cost development of large cryogenic storage tanks, it is necessary to develop more economical materials to replace traditional 9Ni steel while ensuring material performance, thereby enhancing the competitiveness of the industry chain.

[0003] To reduce the cost of 9Ni steel alloys, nickel-saving alloy design is a trend in low-temperature materials research and development. Internationally, nickel-saving 7Ni steel has been successfully developed and applied, and it is gradually replacing 9Ni steel in China, but the cost reduction is limited. This invention further reduces the amount of precious metal Ni used while ensuring material performance, but its performance still reaches the level of traditional 9Ni steel.

[0004] Currently, the following Chinese patents relate to 5.5Ni steel, including those concerning alloy design and manufacturing processes:

[0005] Chinese patent CN116770170A discloses a 5.5Ni steel plate for ultra-low temperature environments and its manufacturing method, producing 5.5Ni steel plates with thicknesses ranging from 5 to 30 mm while ensuring mechanical and technological properties. Based on this example, a 12mm plate can meet the toughness requirements at -196℃, but a 25mm plate can only meet the toughness requirements at -100℃, and cannot be used under -196℃ conditions. The heat treatment employs a QLT process, requiring two quenchings: one above the A3 line and one in the two-phase region, resulting in high heat treatment costs.

[0006] Chinese patent CN115341140A discloses a steel for cryogenic pressure vessels and its production method. The steel has a wide composition range, with a Ni content of 5.2%–6.8%. According to standards, the alloy element range covers three steel grades: 5.5Ni, 6Ni, and 7Ni. The Ni content required to meet the -196℃ impact performance requirement is high, resulting in high alloy costs. This patent uses two-phase quenching at around 750℃. However, in industrial production, the normalizing or quenching temperatures for most steel grades are above 800℃. Cooling the furnace to such a low temperature for quenching leads to long empty furnace cooling times during heat treatment, severely impacting heat treatment output. Two-phase quenching is highly sensitive to water immersion temperature. If the water immersion temperature and cooling rate are not properly matched, the ratio of α to γ ​​phases cannot be precisely controlled within a reasonable range, leading to significant performance fluctuations. Furthermore, phase transformation occurs simultaneously during water immersion, and the complex conditions of thermal stress and phase transformation stress make plate shape difficult to control.

[0007] Summary of the Invention

[0008] The purpose of this invention is to solve the problems of high cost, low efficiency, poor plate shape, and inability to meet the low temperature service requirements of existing 5.5Ni steel heat treatment. It provides a production method for 5.5Ni steel for cryogenic containers that can meet the service requirements of -196℃, which can reduce alloy costs, meet the service requirements of -196℃, facilitate production organization and plate shape control, and improve processing efficiency and output.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A method for producing 5.5Ni steel for cryogenic vessels that meet the requirements of -196℃ includes the following composition design, by mass percentage: C 0.03~0.06%, Si 0.15~0.20%, Mn 0.9~1.2%, S≤0.002%, P≤0.005%, Ni 5.3~5.7%, Cr 0.3~0.7%, Mo 0.12~0.22%, Alt 0.010-0.030%, O≤0.0020%, with the balance being Fe and impurities.

[0011] To further achieve the objectives of this invention, a method for producing 5.5Ni steel for cryogenic vessels operating at -196°C using the above-mentioned composition is also provided. The specific steps are as follows:

[0012] (1) Steelmaking: using converter smelting and LF+RH refining;

[0013] (2) Continuous casting: The continuous casting is carried out using a secondary cooling water weak cooling and low casting speed scheme. The thickness of the continuous casting billet is 150mm, and the billet is subjected to stacking cooling treatment.

[0014] (3) Slab heating: In order to control the original austenite grain size, surface quality and plate shape before hot rolling, the temperature uniformity of the entire slab is ≤10℃;

[0015] (4) Hot rolling: Steel plates with a thickness ≥15mm are rolled in two stages with controlled rolling. After hot rolling, laminar cooling is performed to a certain temperature before being removed from the line for stacking cooling.

[0016] (5) Heat treatment: Offline tempering heat treatment process is adopted.

[0017] Furthermore, the heat treatment ultimately yields a microstructure with tempered martensite as the matrix and a small amount of austenite.

[0018] Furthermore, the steel plate has a yield strength ≥630MPa, a tensile strength of 690~780MPa, an elongation ≥20%, a standard sample KV2 ≥120J at -196℃, and a plate shape of 5~30mm that is ≤6mm / m.

[0019] Furthermore, in step (1), the vacuum degree is controlled to be ≤0.3 torr and the vacuum treatment time is ≥30 min during the RH treatment process.

[0020] Furthermore, in step (2), the superheat of the molten steel during continuous casting is controlled at 25-40°C.

[0021] Furthermore, in step (3), the furnace exit temperature of slabs rolled to a size of 15mm or larger is 1130-1160℃, and the furnace exit temperature of slabs rolled to a size of less than 15mm is 1180-1220℃.

[0022] Furthermore, in step (4), the hot rolling of steel plates with a thickness ≥15mm adopts a two-stage controlled rolling process, the specific steps of which are as follows:

[0023] S41: Rough rolling is performed in the austenite recrystallization zone, with a rough rolling start temperature of 1050-1100℃ and a total reduction of 30-60%.

[0024] S42: Finish rolling is performed in the non-recrystallized austenite region, with an initial rolling temperature below 880℃ and a final rolling temperature of 800~830℃.

[0025] Furthermore, in step (4), after hot rolling, laminar cooling is performed, and the reddening temperature is controlled between 600-650℃. After laminar cooling to a certain temperature, the product is removed from the production line and stacked for cooling.

[0026] Furthermore, in step (5), the hot-rolled plate undergoes offline tempering heat treatment, using a quenching and tempering process above Ac3 temperature. The steel plate is heated to 800-840℃ and held for 20-50 minutes, then water-quenched on a roller quenching machine; it is then heated to 610-640℃ and tempered for 50-120 minutes, and then air-cooled after being taken out of the furnace.

[0027] The design principle of the components of this invention is as follows:

[0028] Carbon (C): A strengthening element and austenite stabilizing element in steel. Reversing austenite enrichment with C significantly lowers the Ms point and improves its stability. However, excessive C content leads to an increase in the ductile-brittle transition temperature, which is detrimental to low-temperature toughness in the high-temperature zone (HAZ). In design, the C content should be as low as possible while ensuring strength; this patent specifies a range of 0.03% to 0.06%.

[0029] Ni can form α and γ phase solid solutions with Fe, and can be infinitely dissolved in the γ phase. It can expand the γ phase region and is an austenite forming and stabilizing element. It can make screw dislocations less prone to decomposition, ensuring the occurrence of cross slip and improving the plastic deformation properties of materials. It is very important for stabilizing reversed austenite. Ni-rich reversed austenite with other austenite stabilizing elements is stable at extremely low temperatures and can absorb part of the strain energy during deformation. It can also transform into the α′ phase through deformation-induced phase transformation, which is one of the toughening mechanisms. It is beneficial for improving hardenability and increasing strength through solid solution strengthening. However, Ni is a precious metal element and mainly relies on imports. Its usage should be reduced while ensuring performance. The scope of this patent is 5.3% to 5.7%.

[0030] Mn is an austenite stabilizing element; its enrichment in austenite helps reverse austenite stability. It is also a matrix strengthening element, capable of improving strength through solid solution strengthening and precipitation strengthening; and it significantly improves the hardenability of materials. Excessive Mn content can easily lead to segregation, which is detrimental to performance stability. Considering all factors, the Mn content in this patent ranges from 0.90% to 1.2%.

[0031] Si is a deoxidizing element; it can inhibit the segregation of P and Mn at grain boundaries; however, excessive Si content is not conducive to the precipitation of cementite in the heat-affected zone (HAZ) during the welding cooling process, thus affecting the low-temperature toughness of the welded joint. This patent controls the Si content to be between 0.15% and 0.20%.

[0032] S and P: S readily forms MnS precipitates with Mn, reducing low-temperature toughness. P tends to segregate at grain boundaries, reducing the grain boundary's resistance to crack propagation and worsening low-temperature toughness. Therefore, S should be controlled to the minimum extent possible, taking into account material properties, weldability, and production efficiency.

[0033] Mo can improve hardenability, thereby increasing strength; improve the tempering stability of steel; when coexisting with chromium or manganese, it can reduce or suppress temper brittleness caused by other elements. However, if the Mo content is too high, it will not be conducive to welding performance. The Mo content range of this invention is 0.12% to 0.22%.

[0034] Cr: A Cr content of 0.2% or higher can improve the hardenability of steel, which is beneficial for the formation of a full martensitic structure during quenching. This results in a stable microstructure after heat treatment and improves low-temperature toughness. However, a Cr content exceeding 0.70% will cause larger sparks during welding, affecting weld quality. Therefore, the Cr content range of this invention is 0.40% to 0.60%.

[0035] O: The purity of steel is a prerequisite for ensuring its toughness. Excessive O content will lead to the formation of too many oxide inclusions in the steel. O, N and Al easily form high-melting-point precipitates Al2O3 and AlN. Moreover, the precipitates are relatively large, reaching several micrometers in diameter. Stress concentration can easily occur near the precipitates, becoming crack initiation sites and seriously affecting the low-temperature toughness of the matrix. Therefore, this design controls the O content to ≤0.0020%.

[0036] Compared with the prior art, the advantages of the technical solution of the present invention are as follows:

[0037] (1) The alloy range in the composition designed in this invention is narrow, which is beneficial to controlling the stability of the microstructure and properties;

[0038] (2) This invention reduces the content of two precious metal elements, Mo and Ni, thereby reducing the cost of the alloy;

[0039] (3) The 5.5Ni steel of the present invention can meet the service conditions of -196℃ and has excellent low-temperature toughness;

[0040] (4) The present invention uses quenching at a temperature above A3 line, which is beneficial for production organization and control of plate shape. Attached Figure Description

[0041] Figure 1 is a metallographic photograph of the tempered microstructure at 1 / 4 thickness of a 5mm steel plate etched with 4% nitric acid alcohol solution in this invention.

[0042] Figure 2 is a metallographic photograph of the tempered microstructure at 1 / 4 thickness of a 25mm steel plate etched with 4% nitric acid alcohol solution in this invention.

[0043] Figure 3 shows a photograph of the steel plate structure of the present invention taken under a scanning electron microscope. Detailed Implementation

[0044] Example

[0045] To make the present invention clearer, the following description, in conjunction with the accompanying drawings, further illustrates a method for producing 5.5Ni steel for cryogenic containers that can operate at -196℃. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0046] In this embodiment, the composition of 5.5Ni steel can be designed as follows, see Table 1.

[0047] Table 1 Chemical composition of steel plates produced in Examples 1-4

[0048] The steel plates with the different chemical compositions mentioned above are produced according to the following methods, with the specific steps as follows:

[0049] (1) Steelmaking: using converter smelting and LF+RH refining;

[0050] (2) Continuous casting: The continuous casting is carried out using a secondary cooling water weak cooling and low casting speed scheme. The thickness of the continuous casting billet is 150mm, and the billet is subjected to stacking cooling treatment.

[0051] (3) Slab heating: In order to control the original austenite grain size, surface quality and plate shape before hot rolling, the temperature uniformity of the entire slab is ≤10℃;

[0052] (4) Hot rolling: Steel plates with a thickness ≥15mm are rolled in two stages with controlled rolling. After hot rolling, laminar cooling is performed to a certain temperature before being removed from the line for stacking cooling.

[0053] (5) Heat treatment: Offline tempering heat treatment process is adopted.

[0054] In this embodiment, an LF furnace is used for deoxidation, desulfurization, and precise alloying, with continuous casting superheat control. To reduce the impact of inclusion gaseous elements such as hydrogen on low-temperature toughness, RH vacuum degassing is employed, maintaining a vacuum level of ≤0.3 torr for ≥30 minutes, and controlling the hydrogen content to below 2 ppm. 150 mm thick billets are produced by continuous casting, with gas-protected casting throughout the entire process, controlling the molten steel superheat within the range of 25–40°C to ensure billet quality.

[0055] To control the shape of thin-gauge rolled plates, a high-temperature tapping process is used for plates with a thickness <15mm, while a low-temperature tapping process is used for plates with a thickness ≥15mm. To refine the rolled microstructure, a two-stage controlled rolling process is used for plates with a thickness ≥15mm.

[0056] Specifically, for the different chemical compositions in Table 1, the manufacturing process parameters for heating, rolling, and heat treatment of the steels in Examples 1 to 4 are shown in Table 2.

[0057] Table 2 Manufacturing process parameters for steel heating, rolling, and heat treatment in Examples 1-4 of this invention

[0058] After adopting the production method of the present invention, the final mechanical properties of the steel plates in Examples 1 to 4 are shown in Table 3 below. Figure 1 shows the tempered microstructure at 1 / 4 of the thickness of the 5mm thick steel plate after etching with a 4% nitric acid alcohol solution, and Figure 2 shows the tempered microstructure at 1 / 4 of the thickness of the 25mm thick steel plate after etching with a 4% nitric acid alcohol solution.

[0059] Table 3 shows the mechanical properties of steel plates obtained through production processes in Examples 1-4.

[0060] In this invention, all steel plates undergo offline heat treatment to obtain a mixed microstructure consisting mainly of tempered martensite and a small amount of austenite. This ensures that the material possesses high strength as well as excellent low-temperature toughness, fully meeting the service requirements at -196℃. The shape of the finished plates within the thickness range of 5–30 mm is controlled to ≤6 mm / m in any direction.

[0061] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A method for producing 5.5Ni steel for cryogenic vessels operating at -196℃, comprising the following composition design, characterized in that: The composition by mass percentage is as follows: C 0.03-0.06%, Si 0.15-0.20%, Mn 0.9-1.2%, S≤0.002%, P≤0.005%, Ni 5.3-5.7%, Cr 0.3-0.7%, Mo 0.12-0.22%, Alt 0.010-0.030%, O≤0.0020%, with the balance being Fe and impurities.

2. The method for producing 5.5Ni steel for cryogenic containers operating at -196℃ according to claim 1, characterized in that: (1) Steelmaking: using converter smelting and LF+RH refining; (2) Continuous casting: The continuous casting is carried out using a secondary cooling water weak cooling and low casting speed scheme. The thickness of the continuous casting billet is 150mm, and the billet is subjected to stacking cooling treatment. (3) Slab heating: In order to control the original austenite grain size, surface quality and plate shape before hot rolling, the temperature uniformity of the entire slab is ≤10℃; (4) Hot rolling: Steel plates with a thickness ≥15mm are rolled in two stages with controlled rolling. After hot rolling, laminar cooling is performed to a certain temperature before being removed from the line for stacking cooling. (5) Heat treatment: Offline tempering heat treatment process is adopted.

3. The method for producing 5.5Ni steel for cryogenic containers that meet the requirements of -196℃ service temperature according to claim 2, characterized in that: The heat treatment ultimately yields a microstructure with tempered martensite as the matrix and a small amount of austenite.

4. The method for producing 5.5Ni steel for cryogenic containers that meet the requirement of operating at -196℃ according to claim 2, characterized in that: The steel plate has a yield strength ≥630MPa, tensile strength 690~780MPa, elongation ≥20%, standard sample KV2 ≥120J at -196℃, and the plate shape of the 5~30mm steel plate is ≤6mm / m.

5. The method for producing 5.5Ni steel for cryogenic containers operating at -196℃ according to claim 2, characterized in that: In step (1), the vacuum degree is controlled to be ≤0.3 torr and the vacuum treatment time is ≥30 min during the RH treatment process.

6. The method for producing 5.5Ni steel for cryogenic containers operating at -196℃ according to claim 2, characterized in that: In step (2), the superheat of the molten steel during continuous casting is controlled at 25-40°C.

7. The method for producing 5.5Ni steel for cryogenic containers operating at -196℃ according to claim 2, characterized in that: In step (3), the furnace exit temperature of slabs rolled to a size of 15mm or larger is 1130-1160℃, and the furnace exit temperature of slabs rolled to a size of less than 15mm is 1180-1220℃.

8. The method for producing 5.5Ni steel for cryogenic containers operating at -196℃ according to claim 2, characterized in that: In step (4), the hot rolling of steel plates with a thickness ≥15mm adopts a two-stage controlled rolling process, and the specific steps are as follows: S41: Rough rolling is performed in the austenite recrystallization zone, with a rough rolling start temperature of 1050-1100℃ and a total reduction of 30-60%. S42: Finish rolling is performed in the non-recrystallized austenite region, with an initial rolling temperature below 880℃ and a final rolling temperature of 800~830℃.

9. The method for producing 5.5Ni steel for cryogenic containers operating at -196℃ according to claim 2, characterized in that: In step (4), after hot rolling, laminar cooling is performed, and the reddening temperature is controlled between 600-650℃. After laminar cooling to a certain temperature, the product is removed from the production line and stacked for cooling.

10. The method for producing 5.5Ni steel for cryogenic containers serving at -196℃ according to claim 2, characterized in that: In step (5), the hot-rolled plate undergoes offline tempering heat treatment, using a quenching and tempering process above Ac3 temperature. The steel plate is heated to 800-840℃ and held for 20-50 minutes, then water-quenched on a roller quenching machine; then heated to 610-640℃ and tempered for 50-120 minutes, and finally air-cooled after being taken out of the furnace.

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