Seamless gas cylinders made of low-carbon steel, their preparation methods and applications

Abstract

This invention provides a seamless low-carbon steel for gas cylinders, its preparation method, and its application. The preparation method includes: Step S1, preparing raw materials according to the composition of the seamless low-carbon steel for gas cylinders; Step S2, smelting the raw materials to obtain a first steel billet; heating the first steel billet to obtain a second steel billet; Step S3, hot rolling the second steel billet at least 8 passes under the conditions of an initial rolling temperature of 1120℃~1180℃ and a final rolling temperature of 880℃~980℃ to obtain a hot-rolled steel billet; Step S4, cooling the hot-rolled steel billet to obtain a third steel billet; Step S5, heat-treating the third steel billet to obtain the seamless low-carbon steel for gas cylinders. This invention carefully designs the alloy element composition of the seamless low-carbon steel for gas cylinders, and coordinates it with the preparation process, which not only optimizes the comprehensive mechanical properties of the obtained low-carbon steel, but more importantly, enables it to exhibit excellent impact toughness under low-temperature conditions.

Description

Technical Field

[0001] This invention relates to the field of low-carbon steel, and more specifically, to a seamless low-carbon steel for gas cylinders, its preparation method, and its application. Background Technology

[0002] In the context of the current global industrial and energy transformation, seamless steel gas cylinders, as key pressure-bearing equipment in fields such as industrial gases, medical devices, aerospace, and deep-sea exploration, play a crucial role in driving technological innovation in related industries, ensuring personnel safety, and promoting energy efficiency. As a filling tool, reducing the weight of gas cylinders and increasing the gas capacity of a single cylinder are essential to minimizing losses during transportation and handling, thus conserving resources. Therefore, to achieve lightweight design and ensure safe use, the steel used for gas cylinders must possess both high strength and high toughness.

[0003] Currently, seamless gas cylinders mainly use Cr-Mo series alloy steels, such as 30CrMo and 35CrMo. After quenching and tempering (quenching + tempering), their tensile strength is less than 1000MPa, and their working pressure is limited to around 20MPa. However, with the increasing demand for high-pressure gas cylinders, especially in applications where the working pressure exceeds 35MPa and the tensile strength requirement reaches over 1100MPa, the strength and toughness of traditional Cr-Mo steel can no longer meet the higher requirements. Among the steel strengthening methods, precipitation strengthening, grain boundary strengthening, and phase transformation strengthening techniques have been widely studied. Among these, controlling the addition of alloying elements to refine grain size, increase dislocation density, and stabilize the retained austenite structure can effectively improve the overall performance of the material. For example, the addition of microalloying elements such as Nb and V can promote the precipitation of fine carbides, producing a strong pinning effect on the material, significantly increasing strength without significantly impairing toughness. The addition of Ni can lower the ductile-brittle transition temperature and improve low-temperature toughness. As exemplified by CN102409242A, the mechanical properties of seamless gas cylinder steel are improved by adding Ti and Nb microalloying elements. CN115074603A, on the other hand, enhances the steel's strength by adding small amounts of Mo and Ni elements, along with specific preparation methods. Similarly, the strength improvement of most traditional CrMo steels mainly relies on various carbides formed during high-temperature tempering, while the improvement in toughness depends primarily on grain refinement. The strength improvement is limited, generally less than 1300 MPa, and the addition of alloying elements significantly increases production costs.

[0004] Meanwhile, in terms of heat treatment processes, although traditional quenching and tempering can improve the overall performance of materials, its application in the large-scale production of seamless gas cylinder steel is limited due to its complex processes, high energy consumption, and stringent equipment requirements during quenching. New-generation heat treatment technologies, such as quenching and partitioning (Q&P) processes, have become effective means to improve the strength and toughness of seamless gas cylinder steel. The core of the Q&P process lies in controlling the tempering temperature and time after quenching to allow carbon elements to diffuse from supersaturated martensite to retained austenite, forming stable and dispersed retained austenite, thereby improving plasticity and toughness while maintaining high strength. IN202111045218(A) uses low-carbon alloy composition, quenching, and non-isothermal partitioning processes to develop a hot-rolled steel with a multiphase microstructure containing martensite, bainite, retained austenite, and carbides, achieving certain improvements in its various mechanical properties. CN 105441814 A describes a steel product with superior mechanical properties obtained by using a segmented cooling process to obtain a three-phase microstructure containing a certain volume fraction of proeutectoid ferrite, martensite, and retained austenite.

[0005] In summary, stabilizing a small amount of residual austenite at room temperature through the Q&P heat treatment process can effectively improve ductility and toughness. However, current research on Q&P steel focuses more on the strength-ductility product, with less attention paid to toughness, especially low-temperature transverse impact toughness. Furthermore, for seamless gas cylinder steel, the special manufacturing process prevents the use of controlled rolling and cooling techniques during hot rolling, posing a challenge to the application of Q&P steel. Moreover, while improving the strength-ductility product, Q&P steel often neglects to enhance low-temperature transverse impact toughness, especially for gas cylinders operating in extreme low-temperature environments, where the material's low-temperature toughness is a crucial factor determining the cylinder's safety and reliability.

[0006] Therefore, how to design the steel composition and preparation process in a coordinated manner to develop a seamless low-carbon steel for gas cylinders that combines high strength with high toughness at low temperatures has become a research hotspot in the field of materials science and engineering. Summary of the Invention

[0007] The main objective of this invention is to provide a seamless low-carbon steel for gas cylinders, its preparation method and application, so as to solve the problem that gas cylinder steel in the prior art is difficult to simultaneously possess high strength and high low-temperature impact toughness.

[0008] To achieve the above objectives, a first aspect of the present invention provides a method for preparing low-carbon steel for seamless gas cylinders, the method comprising: step S1, preparing raw materials according to the composition of low-carbon steel for seamless gas cylinders; the composition of the low-carbon steel for seamless gas cylinders, by weight percentage, comprises: 0.2wt%~0.3wt% C, 1.0wt%~1.8wt% Si, 2.5wt%~4.0wt% Mn, ... The raw material contains 0.32wt%~0.50wt% Mo, 0.5wt%~1.0wt% Ni, 0.1wt%~0.3wt% V, and 0.02wt%~0.05wt% Nb, with the balance being Fe and unavoidable impurity elements. Step S2 involves smelting the raw material to obtain a first steel billet; the first steel billet is then heat-treated to obtain a second steel billet. Step S3 involves hot-rolling the second billet at least eight times under conditions of an initial rolling temperature of 1120℃~1180℃ and a final rolling temperature of 880℃~980℃ to obtain a hot-rolled steel billet. Step S4 involves cooling the hot-rolled steel billet to obtain a third steel billet. Step S5 involves heat-treating the third steel billet to obtain seamless low-carbon steel for gas cylinders.

[0009] Furthermore, in step S2, the heat treatment temperature is 1200±50℃, and the heat treatment time is 2h~2.5h.

[0010] Furthermore, in step S3, the initial rolling temperature is 1140℃~1160℃, and the final rolling temperature is 900℃~960℃.

[0011] Furthermore, in step S3, the reduction amount of each hot rolling is ≤30%.

[0012] Furthermore, in step S4, the metallographic structure of the third steel billet includes lath martensite phase and retained austenite phase, and the area of ​​the retained austenite phase in the metallographic structure has a volume fraction of 11.5% to 12.5%.

[0013] Furthermore, in step S5, the heat treatment holding temperature is 200℃~350℃, and the holding time is 0.5h~2.0h.

[0014] A second aspect of the present invention provides a low-carbon steel for seamless gas cylinders, which is prepared by the above-described method for preparing low-carbon steel for seamless gas cylinders.

[0015] Furthermore, the metallographic structure of the low-carbon steel for seamless gas cylinders includes a lath martensite matrix phase and an austenite second phase, and the area of ​​the austenite second phase accounts for 10% to 15% of the volume fraction of the metallographic structure.

[0016] Furthermore, the yield strength of the low-carbon steel used in seamless gas cylinders is 1300 MPa to 1400 MPa; and / or, the tensile strength of the low-carbon steel used in seamless gas cylinders is 1500 MPa to 1600 MPa; and / or, the elongation after fracture of the low-carbon steel used in seamless gas cylinders is 8.0% to 14.0%; and / or, at 25±2℃, the transverse impact toughness of the low-carbon steel used in seamless gas cylinders is 40 J·cm. -2 ~70J·cm -2 ; and / or, at -50±2℃, the transverse impact toughness of low carbon steel for seamless gas cylinders is 35 J·cm. -2 ~55J·cm -2 .

[0017] A third aspect of the present invention provides the application of the aforementioned low-carbon steel for seamless gas cylinders as a metallic material for seamless gas cylinders in the energy, industrial, medical, and aerospace fields.

[0018] By applying the technical solution of this invention, the alloy element composition of low-carbon steel for seamless gas cylinders is carefully designed and coordinated with the manufacturing process. Specifically, controlling the low-carbon content ensures that the resulting low-carbon steel retains an appropriate amount of retained austenite, while the addition of various elements in specific amounts further refines the microstructure. The combination of multi-pass hot rolling and tempering heat treatment not only optimizes the comprehensive mechanical properties of the resulting low-carbon steel, but more importantly, it enables it to exhibit excellent impact toughness at low temperatures, ultimately achieving a comprehensive improvement in the low-temperature toughness and strength of the low-carbon steel for seamless gas cylinders. Attached Figure Description

[0019] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0020] Figure 1 This is a time-temperature process diagram of the preparation method used in Example 1 of the present invention;

[0021] Figure 2 This is an engineering stress-strain curve of the low-carbon steel for seamless gas cylinders obtained in Example 1 of the present invention;

[0022] Figure 3 The metallographic characterization results of the seamless gas cylinder low-carbon steel obtained in Example 1 of this invention are shown below. Figure 3 (a) is a metallographic photograph obtained by an optical microscope (OM). Figure 3 (b) is the metallographic structure distribution obtained by electron backscatter diffraction (EBSD). Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0024] As described in the background section, existing gas cylinder steels suffer from the problem of simultaneously possessing high strength and high low-temperature impact toughness. To address this technical problem, a first aspect of the present invention provides a method for preparing seamless low-carbon steel for gas cylinders. This method includes: step S1, preparing raw materials according to the composition of seamless low-carbon steel for gas cylinders; the composition of the seamless low-carbon steel for gas cylinders, by weight percentage, includes: 0.2wt%~0.3wt% C, 1.0wt%~1.8wt% Si, 2.5wt%~4.0wt% Mn, 0.32wt%~0.50wt% Mo, 0.5wt%~1.0wt% Ni, and 0.1wt%~0.3wt%... The raw material contains wt% V and 0.02wt%~0.05wt% Nb; the balance is Fe and unavoidable impurity elements; Step S2: Smelting the raw material to obtain a first steel billet; heating the first steel billet to obtain a second steel billet; Step S3: hot rolling the second billet at least 8 times under the conditions of an initial rolling temperature of 1120℃~1180℃ and a final rolling temperature of 880℃~980℃ to obtain a hot-rolled steel billet; Step S4: cooling the hot-rolled steel billet to obtain a third steel billet; Step S5: heat-treating the third steel billet to obtain seamless low-carbon steel for gas cylinders.

[0025] This invention achieves a comprehensive improvement in the low-temperature toughness and strength of low-carbon steel for seamless gas cylinders through careful design of alloy element composition and synergistic coordination of the manufacturing process. Regarding the aforementioned alloy element composition, based on traditional Cr-Mo gas cylinder steel, the C content is reduced to minimize toughness loss caused by excessive C content, while the Mn content is increased to lower the Ms point, allowing for a stable retained austenite structure at room temperature, thereby improving the material's ductility and toughness. Small amounts of Nb and V microalloying elements are added, significantly enhancing the steel's strength through the precipitation of nano-carbide. An appropriate amount of Ni is added, effectively improving the low-temperature impact toughness of the seamless gas cylinder steel. More specifically:

[0026] C: In the seamless low-carbon steel for gas cylinders prepared in this invention, carbon is the main element ensuring strength. If the carbon content is too low, the strength requirements cannot be met, but excessive carbon easily forms a brittle phase, which impairs the toughness of the steel. Based on this, the carbon content is preferably controlled at 0.2wt%~0.3wt% in this invention.

[0027] Si: In the seamless low-carbon steel for gas cylinders prepared in this invention, silicon plays a role in solid solution strengthening, improving the strength of the steel. Furthermore, silicon can effectively inhibit the formation of cementite, allowing carbon to distribute from martensite to untransformed austenite during low-temperature tempering, thereby enhancing the stability of austenite. Based on this, the Si content is preferably controlled between 1.0 wt% and 1.8 wt% in this invention.

[0028] Mn: In the seamless low-carbon steel for gas cylinders prepared in this invention, manganese is the most effective element for improving strength and toughness, and is also one of the important alloying elements used in this invention. Manganese is a typical austenite stabilizing element, which can significantly delay the transformation of pearlite and bainite, reduce the critical cooling rate for martensite formation, and thus significantly improve the hardenability of the steel. In addition, manganese combines with sulfur in steel to form MnS to prevent hot brittleness, so the smelting process needs to ensure a low S content. However, while a high Mn content delays the pearlite transformation, it also delays ferrite precipitation and causes segregation in the center of the steel. Based on this, the Mn content is preferably controlled between 2.5 wt% and 4.0 wt% in this invention.

[0029] Mo: In the seamless low-carbon steel for gas cylinders prepared in this invention, molybdenum can dissolve into ferrite to play a role in solid solution strengthening. Furthermore, molybdenum is a strong carbide-forming element, precipitating carbides during tempering, refining the grain size, and simultaneously hindering the growth of Nb, V, and other carbides, refining the precipitated phase, and promoting tempering stability. Based on this, the Mo content is preferably controlled at 0.32wt%~0.50wt% in this invention.

[0030] Ni: In the seamless low-carbon steel for gas cylinders prepared in this invention, nickel can strengthen ferrite, lower the ductile-brittle transition temperature, and effectively reduce the resistance to dislocation movement, thereby enhancing the strength of the steel while improving its low-temperature toughness. Therefore, the Ni content is preferably controlled at 0.5wt%~1.0wt% in this invention.

[0031] V: In the seamless low-carbon steel for gas cylinders prepared in this invention, vanadium is a strong carbide-forming element. During tempering, it precipitates to form carbonitrides, playing a precipitation strengthening role. Furthermore, VC particles have a high dissolution temperature, effectively preventing grain boundary movement and refining grains well; however, excessive V can impair low-temperature impact toughness. Therefore, this invention preferably controls the V content to be between 0.1wt% and 0.3wt%.

[0032] Nb: In the seamless low-carbon steel for gas cylinders prepared in this invention, niobium precipitates during hot rolling to form carbonitrides. Fine Nb(C, N) particles pin the austenite grain boundaries, preventing grain boundary movement and inhibiting dynamic recrystallization of the steel. If dynamic recrystallization occurs during deformation, the resulting grain boundary migration can enclose the microcracks formed at the original grain boundaries within new grains, preventing crack aggregation, growth, and extension, thus improving the steel's ductility. Therefore, this invention preferably controls the Nb content to be between 0.02wt% and 0.05wt%.

[0033] Furthermore, there are synergistic effects among the elemental components within the aforementioned content range. Firstly, there is the C-Si synergistic effect: the combination of low carbon content and Si prevents the formation of brittle phases, while Si inhibits carbide precipitation and maintains the stability of the retained austenite, which is crucial for improving low-temperature toughness. Secondly, there is the Mn-Ni coupling: the synergy between Mn and Ni significantly reduces the martensitic transformation temperature, and the addition of Ni lowers the ductile-brittle transformation temperature; together, they improve the material's toughness at low temperatures. Thirdly, there is the Mo-V-Nb tri-element effect: the synergistic effect of Mo, V, and Nb not only refines the grains and inhibits grain boundary movement, but also further enhances the material's low-temperature toughness by precipitating fine carbides and carbonitrides. In other words, the elements in the composition of the seamless low-carbon steel for gas cylinders provided by this invention do not act alone, but rather work synergistically in pairs or among multiple elements to form a complete alloy composition, comprehensively optimizing the various properties of the low-carbon alloy steel.

[0034] Based on the precise control of the content of the aforementioned elements, this invention optimizes the preparation method accordingly. In actual production, vacuum smelting is employed, effectively reducing the hydrogen and nitrogen content in the steel, minimizing hydrogen and nitrogen embrittlement effects, and improving low-temperature toughness. Following this, the resulting first steel billet undergoes a heat treatment to fully dissolve and homogenize the alloying elements, laying the foundation for a stable microstructure during subsequent hot rolling and heat treatment. In step S3, the initial rolling temperature is 1120℃~1180℃, and the final rolling temperature is 880℃~980℃, with at least eight hot rolling passes. This process not only promotes microstructure refinement but also, through final rolling temperature control, retains a certain amount of austenite phase, providing favorable conditions for the stable distribution of retained austenite during subsequent low-temperature heat treatment, thereby effectively improving the material's low-temperature toughness. During the subsequent heat treatment, i.e., the low-temperature tempering distribution process, carbon diffuses from martensite to retained austenite. The resulting stable retained austenite can absorb the energy of crack propagation at low temperatures, significantly improving the material's low-temperature fracture toughness.

[0035] Furthermore, it is worth mentioning that the above-mentioned preparation method involves direct low-temperature tempering and fractionation after hot rolling, while traditional seamless gas cylinder steel requires a quenching and tempering (quenching + tempering) heat treatment process. Therefore, compared with existing processes, the above-mentioned preparation method of this invention, by combining alloy composition and process route, can obtain low-carbon steel with superior comprehensive performance, thereby simplifying the heat treatment process route, reducing energy consumption from processing, saving resources, and particularly conforming to the development concept of a resource-saving society.

[0036] In the above preparation method, for the heat treatment in step S2, in order to make the alloying elements more fully dissolved and distributed, so as to improve the uniformity of the microstructure of the obtained first steel billet in the subsequent processing, and ultimately improve the various properties of the obtained low carbon steel for seamless steel cylinders, the preferred heat treatment temperature is 1200±50℃ and the heat treatment time is 2h~2.5h.

[0037] In the multi-pass hot rolling process of step S3, the initial rolling temperature is further preferably 1140℃~1160℃, and the final rolling temperature is 900℃~960℃. The initial rolling temperature of 1140℃~1160℃ can better ensure the full formation of austenite, while the narrower final rolling temperature range of 900℃~960℃ ensures that the microstructure transformation of the billet during the cooling process is more controllable, thereby more significantly promoting the formation of stable retained austenite and more effectively enhancing the low-temperature toughness of the final low-carbon steel.

[0038] Building upon this, in step S3, it is preferable that the reduction in each hot rolling pass is ≤30% to more effectively control the deformation during hot rolling, reducing internal cracks or structural defects caused by excessive deformation. This also further promotes grain refinement and improves the plasticity and toughness of the resulting low-carbon steel, especially its impact toughness at low temperatures. Extensive experiments have shown that, in several typical implementations, 8 hot rolling passes are preferred. This allows for more complete plastic deformation while significantly simplifying the process, reducing energy consumption, and without damaging the material's internal structure. It also more efficiently refines the microstructure of the billet, thereby more effectively improving the overall plasticity and toughness of the material, especially its transverse impact toughness at low temperatures. Furthermore, in practical applications, to better control the degree of deformation and the cooling rate of the billet during hot rolling, obtain a more uniform metallographic structure, and achieve superior toughness and plasticity, the thickness of the first billet is preferably 80±5mm, and the thickness of the second billet is preferably 6mm~7.5mm.

[0039] Based on the optimization of the above process parameters, in step S4, the metallographic structure of the obtained third steel billet preferably includes lath martensite phase and retained austenite phase, and the volume fraction of the area of ​​the retained austenite phase in the metallographic structure is 11.5%~12.5%. By optimizing the volume fraction of retained austenite in the metallographic structure of the intermediate product, the third steel billet, as described above, the billet can be more responsive in subsequent heat treatment (low-temperature tempering and partitioning), thereby significantly promoting the effective diffusion of carbon and the stable retention of retained austenite. Therefore, the final low-carbon steel can maintain high strength while significantly improving low-temperature toughness, better meeting the performance requirements of seamless gas cylinder steel in extreme environments.

[0040] In several typical implementations, in step S5, the heat treatment holding temperature is 200℃~350℃, and the holding time is 0.5h~2.0h. During this low-temperature tempering stage of 200℃~350℃, carbon can diffuse more efficiently from supersaturated martensite into the retained austenite, forming a more stable and dispersed retained austenite structure. The holding time of 0.5h~2.0h further promotes this diffusion process, thereby further enhancing the stability of the retained austenite phase, allowing it to function as a toughening phase at low temperatures, ultimately significantly improving the low-temperature toughness of the resulting low-carbon steel. Furthermore, air cooling is preferred for the heat treatment, promoting more stable preservation of the retained austenite phase and reducing the increase in martensite supersaturation that may be caused by rapid cooling (such as water cooling), thus reducing the risk of embrittlement. It also improves the surface integrity of the resulting seamless steel cylinder low-carbon steel, reduces crack formation, and optimizes surface quality and subsequent safety in use.

[0041] A second aspect of the present invention provides a low-carbon steel for seamless gas cylinders, which is prepared by the aforementioned method for preparing low-carbon steel for seamless gas cylinders. Through the synergistic effect of the above-described manufacturing process and composition design, a low-carbon steel for seamless gas cylinders that possesses both high strength and excellent low-temperature toughness is prepared, thus effectively meeting the requirements of seamless gas cylinders for lightweight design and high safety.

[0042] Furthermore, the microstructure of the obtained seamless low-carbon steel for gas cylinders includes a lath martensite matrix phase and an austenite second phase, and the area of ​​the austenite second phase in the microstructure has a volume fraction of 10% to 15%, preferably 11.5% to 12.0%. Within the above-mentioned preferred and more preferred volume fraction ranges, the residual austenite phase in the obtained low-carbon steel microstructure can be more uniformly distributed, which can further reduce crack initiation and propagation, and more significantly improve the fracture toughness of the obtained low-carbon steel at low temperatures.

[0043] In several preferred embodiments, the yield strength of the low-carbon steel used for seamless gas cylinders is 1300 MPa to 1400 MPa; and / or, the tensile strength of the low-carbon steel used for seamless gas cylinders is 1500 MPa to 1600 MPa; and / or, the elongation after fracture of the low-carbon steel used for seamless gas cylinders is 8.0% to 14.0%; and / or, at 25±2℃, the transverse impact toughness of the low-carbon steel used for seamless gas cylinders is 40 J·cm. -2 ~70J·cm -2 ; and / or, at -50±2℃, the transverse impact toughness of low carbon steel for seamless gas cylinders is 35 J·cm. -2 ~55J·cm -2 In other words, the seamless low-carbon steel for gas cylinders prepared by the above method has superior mechanical properties, including yield strength, tensile strength, and elongation after fracture. It also exhibits superior transverse impact toughness at room temperature, especially at low temperatures. When preparing various metal products, it can provide them with more significant safety and reliability.

[0044] A third aspect of this invention provides the application of the aforementioned low-carbon steel for seamless gas cylinders as a metallic material in the energy, industrial, medical, and aerospace fields. The low-carbon steel obtained by this invention possesses high strength, high fracture toughness, a low ductile-brittle transition temperature, and good ductility even at low temperatures. Therefore, it can significantly improve the reliability and safety of seamless gas cylinder products, thereby meeting the application requirements of multiple fields such as energy, industry, medical, and aerospace.

[0045] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0046] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0047] Example 1

[0048] A method for preparing low-carbon steel for seamless gas cylinders:

[0049] (1) The raw materials are prepared according to the composition of the low carbon steel for seamless gas cylinders to be prepared as follows: 0.25wt% C, 1.5wt% Si, 3.0wt% Mn, 0.4wt% Mo, 0.8wt% Ni, 0.13wt% V, 0.04wt% Nb, with the balance being Fe and unavoidable impurity elements (the composition is the same as in Table 1).

[0050] (2) The chemical composition as described above is smelted to obtain a target steel billet of 80×80 mm, namely the first steel billet; the obtained steel billet sample is sent into a heating furnace and heated to the complete austenitizing temperature of 1200 ℃, and held for 2 h to obtain the second steel billet;

[0051] (3) The steel billet sample after exiting the furnace was hot rolled in 8 passes. The initial rolling was at 1160 ℃. After 8 passes of hot rolling, the steel billet was rolled from 80 mm to 7 mm. The single pass reduction was 30%-30%-30%-25%-25%-25%-20%-20% respectively. The final rolling temperature was 950 ℃.

[0052] (4) The sample in step (3) was air-cooled to room temperature to obtain the third steel billet, whose metallographic structure included lath martensite phase and retained austenite phase;

[0053] (5) Place the sample from step (4) in an electric arc furnace at a temperature of 200 °C, temper and keep it at the temperature for 1 h, and air cool it to room temperature to obtain seamless low carbon steel for gas cylinders.

[0054] Furthermore, the time-temperature process diagram for the above preparation method is shown below. Figure 1 .

[0055] Example 2

[0056] A method for preparing low-carbon steel for seamless gas cylinders:

[0057] (1) The raw materials are prepared according to the composition of the low carbon steel for seamless gas cylinders to be prepared as follows: 0.27wt% C, 1.2wt% Si, 3.3wt% Mn, 0.32wt% Mo, 1.0wt% Ni, 0.15wt% V, 0.03wt% Nb, with the balance being Fe and unavoidable impurity elements (the composition is the same as in Table 1).

[0058] (2) The chemical composition as described above is smelted to obtain a target steel billet of 80×80 mm, namely the first steel billet; the obtained steel billet sample is sent into a heating furnace and heated to the complete austenitizing temperature of 1200 ℃, and held for 2 h to obtain the second steel billet;

[0059] (3) The billet sample after exiting the furnace was hot rolled in 8 passes. The rolling was started at 1170 ℃ and after 8 passes of hot rolling, the billet was rolled from 80 mm to 6.5 mm. The reduction per pass was the same as in Example 1, and the final rolling temperature was 960 ℃.

[0060] (4) The sample in step (3) was air-cooled to room temperature to obtain the third steel billet, whose metallographic structure included lath martensite phase and retained austenite phase;

[0061] (5) Place the sample from step (4) in an electric arc furnace at a temperature of 300 °C, temper and keep it at the temperature for 2 h, and air cool it to room temperature to obtain seamless low carbon steel for gas cylinders.

[0062] Example 3

[0063] A method for preparing low-carbon steel for seamless gas cylinders:

[0064] (1) The raw materials are prepared according to the composition of the low carbon steel for seamless gas cylinders to be prepared as follows: 0.23wt% C, 1.0wt% Si, 3.8wt% Mn, 0.43wt% Mo, 0.65wt% Ni, 0.12wt% V, 0.02wt% Nb, with the balance being Fe and unavoidable impurity elements (the composition is the same as in Table 1).

[0065] (2) The chemical composition as described above is smelted to obtain a target steel billet of 80×80 mm, namely the first steel billet; the obtained steel billet sample is sent into a heating furnace and heated to the complete austenitizing temperature of 1200 ℃, and held for 2 h to obtain the second steel billet;

[0066] (3) The steel billet sample after exiting the furnace was hot rolled in 8 passes. The rolling was started at 1160 ℃ and after 8 passes of hot rolling, the steel billet was rolled from 80 mm to 6.5 mm. The single pass reduction was the same as in Example 1, and the final rolling temperature was 950 ℃.

[0067] (4) The sample in step (3) was air-cooled to room temperature to obtain the third steel billet, whose metallographic structure included lath martensite phase and retained austenite phase;

[0068] (5) Place the sample from step (4) in an electric arc furnace at a temperature of 350 °C, temper and hold for 0.5 h, and air cool to room temperature to obtain seamless low carbon steel for gas cylinders.

[0069] Example 4

[0070] A method for preparing low-carbon steel for seamless gas cylinders:

[0071] The only difference between this embodiment and embodiment 1 is that in step (2), the heat preservation temperature is changed to 1100℃ and the heat preservation time is changed to 5h.

[0072] Example 5

[0073] A method for preparing low-carbon steel for seamless gas cylinders:

[0074] The only difference between this embodiment and embodiment 1 is that in step (3), the initial rolling temperature is changed to 1120°C and the final rolling temperature is changed to 880°C.

[0075] Example 6

[0076] A method for preparing low-carbon steel for seamless gas cylinders:

[0077] The only difference between this embodiment and embodiment 1 is that in step (3), the initial rolling temperature is changed to 1180°C and the final rolling temperature is changed to 980°C.

[0078] Example 7

[0079] A method for preparing low-carbon steel for seamless gas cylinders:

[0080] The only difference between this embodiment and embodiment 1 is that in step (3), the thickness of the steel billet before hot rolling is changed to 100mm, and 8 passes of hot rolling are performed to obtain a hot-rolled steel billet with a thickness of 5mm, wherein the single pass reduction is 35%-35%-35%-30%-30%-30%-30%-30%.

[0081] Example 8

[0082] A method for preparing low-carbon steel for seamless gas cylinders:

[0083] The only difference between this embodiment and embodiment 1 is that in step (5), the heat preservation temperature is changed to 150°C and the heat preservation time is changed to 3h.

[0084] Example 9

[0085] A method for preparing low-carbon steel for seamless gas cylinders:

[0086] The only difference between this embodiment and embodiment 1 is that in step (5), the heat preservation temperature is changed to 400℃ and the heat preservation time is changed to 20min.

[0087] Comparative Example 1 and Comparative Example 2

[0088] The only difference between Comparative Examples 1 and 2 and Example 1 is the element content, as detailed in Table 1.

[0089] Comparative Example 3

[0090] A method for preparing seamless steel for gas cylinders:

[0091] The only difference between this comparative example and Example 1 is that in step (3), the initial rolling temperature is changed to 1100°C and the final rolling temperature is changed to 800°C.

[0092] Comparative Example 4

[0093] A method for preparing seamless steel for gas cylinders:

[0094] The only difference between this comparative example and Example 1 is that in step (3), the initial rolling temperature is changed to 1200°C and the final rolling temperature is changed to 1000°C.

[0095] Comparative Example 5

[0096] A method for preparing seamless steel for gas cylinders:

[0097] The only difference between this comparative example and Example 1 is that in step (3), the number of hot rolling passes is changed to 5, and the billet is rolled from 80 mm to 7.5 mm, wherein the single-pass reduction is 40%-40%-40%-35%-35%-35%.

[0098] Test methods

[0099] Yield strength: Tested in accordance with GB / T 221.1-2021.

[0100] Tensile strength: Tested in accordance with GB / T 221.1-2021.

[0101] Elongation after fracture: Tested in accordance with GB / T 221.1-2021.

[0102] Transverse impact toughness: Tested according to GB / T 229-2020, and the transverse impact toughness of steel samples at room temperature (25±2℃) and -50℃ were obtained respectively.

[0103] Metallographic structure and phase distribution diagram: Tests were conducted according to GB / T 38720-2020 and GB / T 341720-2017, respectively, and the volume percentage of retained austenite phase in the metallographic structure of the third steel billet in each embodiment and comparative example was obtained, as well as the volume percentage of retained austenite phase in the metallographic structure of the final seamless low carbon steel for gas cylinders.

[0104] The above tests were performed on each embodiment and comparative example, and the results are shown in Table 2. The engineering stress-strain curve of Embodiment 1 obtained from the tests is shown in Table 2. Figure 2 Metallographic photographs of the seamless gas cylinder low-carbon steel obtained in Example 1 are shown below. Figure 3 (a) Metallographic structure distribution diagram is shown in Figure 1. Figure 3 (b).

[0105] Table 1

[0106]

[0107] Table 2

[0108]

[0109] As can be seen from the above description, compared with the comparative examples, the embodiments of the present invention achieve the preparation of seamless low-carbon steel for steel cylinders with superior comprehensive performance. The resulting low-carbon steel possesses superior mechanical properties, including yield strength, tensile strength, and elongation after fracture. It also exhibits superior transverse impact toughness at room temperature, especially at low temperatures, providing more significant safety and reliability when preparing various metal products.

[0110] As can be seen from Comparative Examples 1 and 2, the failure to synergistically control and adjust the elements in low-carbon steel will lead to a significant decrease in the strength and plasticity of Comparative Example 1; although Comparative Example 2 exhibits high strength, it has poor plasticity and poor impact toughness at both room temperature and low temperature.

[0111] As can be seen from Comparative Examples 3 to 5, in step S3, the initial rolling temperature is strictly controlled at 1120℃~1180℃ and the final rolling temperature is 880℃~980℃. The hot rolling process of at least 8 passes not only promotes the refinement of the microstructure, but also retains a certain amount of austenite phase through the control of the final rolling temperature. This provides favorable conditions for the stable distribution of residual austenite in the subsequent low-temperature heat treatment, thereby effectively improving the low-temperature toughness of the material.

[0112] In each embodiment, comparing Example 4 with Example 1, it can be seen that the preferred heating treatment in step (2) can make the alloying elements more fully dissolved and distributed, so as to improve the uniformity of the structure of the first steel billet in the subsequent processing, and ultimately improve the various properties of the obtained seamless steel cylinder low carbon steel, especially the low temperature impact toughness.

[0113] Comparing Examples 5 and 6 with Example 1, it can be seen that by further optimizing the initial rolling temperature and final rolling temperature in step (3), the full formation of austenite can be better guaranteed. At the same time, the microstructure transformation of the billet during the cooling process is more controllable, which significantly promotes the formation of stable retained austenite, optimizes the area of ​​the retained austenite phase in the microstructure, and ultimately enhances the low-temperature toughness of the final low-carbon steel more effectively.

[0114] Comparing Example 7 with Example 1, it can be seen that by further optimizing the rolling passes in step (3) and the amount of reduction per pass, more complete plastic deformation can be achieved under the conditions of significantly simplifying the process, reducing energy consumption, and not damaging the internal structure of the material. The microstructure of the billet at this time is also refined more efficiently, thereby more effectively improving the overall plasticity and toughness of the material, especially the transverse impact toughness at low temperature.

[0115] Comparing Examples 8 and 9 with Example 1, it can be seen that by optimizing the heat treatment temperature and time in step (5), the C element can diffuse more efficiently from the supersaturated martensite into the retained austenite, forming a more stable and dispersed retained austenite structure, thereby further promoting and improving the stability of the retained austenite phase, so that it can still play a role as a toughening phase at low temperatures, and ultimately significantly improving the low-temperature toughness of the obtained low-carbon steel.

[0116] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0117] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing low-carbon steel for seamless gas cylinders, characterized in that, The preparation method includes: Step S1: Prepare raw materials according to the composition of the seamless gas cylinder low-carbon steel; by weight percentage, the composition of the seamless gas cylinder low-carbon steel includes: 0.2wt%~0.3wt% C, 1.0wt%~1.8wt% Si, 2.5wt%~4.0wt% Mn, 0.32wt%~0.50wt% Mo, 0.5wt%~1.0wt% Ni, 0.1wt%~0.3wt% V, 0.02wt%~0.05wt% Nb; the balance is Fe and unavoidable impurity elements; Step S2: Smelt the raw material to obtain a first steel billet; heat the first steel billet to obtain a second steel billet; Step S3: Under the conditions of an initial rolling temperature of 1120℃~1180℃ and a final rolling temperature of 880℃~980℃, the second steel billet is hot rolled at least 8 times to obtain a hot-rolled steel billet. Step S4: The hot-rolled steel billet is air-cooled to obtain a third steel billet; Step S5: The third steel billet is heat-treated to obtain the seamless low-carbon steel for gas cylinders; the heat treatment temperature is 200℃~350℃ and the heat treatment time is 0.5h~2.0h; the cooling method of the heat treatment is air cooling.

2. The method for preparing seamless gas cylinder low-carbon steel according to claim 1, characterized in that, In step S2, the heat treatment temperature is 1200±50℃ and the heat treatment time is 2h~2.5h.

3. The method for preparing seamless gas cylinder low-carbon steel according to claim 1, characterized in that, In step S3, the initial rolling temperature is 1140℃~1160℃, and the final rolling temperature is 900℃~960℃.

4. The method for preparing low-carbon steel for seamless gas cylinders according to any one of claims 1 to 3, characterized in that, In step S3, the reduction amount of each hot rolling is ≤30%.

5. The method for preparing low-carbon steel for seamless gas cylinders according to any one of claims 1 to 3, characterized in that, In step S4, the metallographic structure of the third steel billet includes lath martensite phase and retained austenite phase, and the area of ​​the retained austenite phase in the metallographic structure has a volume fraction of 11.5% to 12.5%.

6. A type of low-carbon steel for seamless gas cylinders, characterized in that, The seamless gas cylinder low-carbon steel is prepared by the method for preparing seamless gas cylinder low-carbon steel according to any one of claims 1 to 5.

7. The low-carbon steel for seamless gas cylinders according to claim 6, characterized in that, The metallographic structure of the seamless gas cylinder low carbon steel includes a lath martensite matrix phase and an austenite second phase, and the area of ​​the austenite second phase in the metallographic structure has a volume fraction of 10% to 15%.

8. The low-carbon steel for seamless gas cylinders according to claim 6 or 7, characterized in that, The seamless gas cylinder uses low-carbon steel with a yield strength of 1300 MPa to 1400 MPa; and / or, The seamless gas cylinder uses low-carbon steel with a tensile strength of 1500MPa~1600MPa; and / or, The seamless gas cylinder uses low-carbon steel with an elongation after fracture of 8.0% to 14.0%; and / or, At 25±2℃, the transverse impact toughness of the low-carbon steel used for the seamless gas cylinder is 40 J·cm. -2 ~70J·cm -2 ; and / or, At -50±2℃, the transverse impact toughness of the low-carbon steel used for the seamless gas cylinder is 35 J·cm. -2 ~55J·cm -2 .

9. The application of any one of the low-carbon steel for seamless gas cylinders according to claims 6 to 8 as a metallic material for seamless gas cylinders in the energy, industrial, medical and aerospace fields.

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