Low-carbon steel for seamless gas cylinder as well as preparation method and application of low-carbon steel

Abstract

The invention provides low-carbon steel for a seamless gas cylinder as well as a preparation method and application of the low-carbon steel. The preparation method comprises the steps that S1, raw materials are prepared according to components of the low-carbon steel for the seamless gas cylinder; s2, the raw materials are smelted, and a first steel billet is obtained; heating the first steel billet to obtain a second steel billet; s3, under the conditions that the initial rolling temperature ranges from 1120 DEG C to 1180 DEG C, and the final rolling temperature ranges from 880 DEG C to 980 DEG C, at least eight passes of hot rolling are conducted on the second steel billet, and a hot-rolled steel billet is obtained; s4, cooling the hot-rolled steel billet to obtain a third steel billet; and S5, the third steel billet is subjected to heat treatment, and the low-carbon steel for the seamless gas cylinder is obtained. Alloy element components of the low-carbon steel for the seamless gas cylinder are meticulously designed, meanwhile, a preparation technology is cooperatively matched, the comprehensive mechanical property of the obtained low-carbon steel is optimized, and more importantly, the low-carbon steel shows excellent impact toughness under the low-temperature condition.

Description

Technical Field

[0001] The present invention relates to the field of low-carbon steel, and in particular to low-carbon steel for seamless gas cylinders, a preparation method thereof, and applications thereof. Background Art

[0002] Against the backdrop of global industrial and energy transformation, seamless steel gas cylinders, as critical pressure-bearing equipment in fields such as industrial gases, medical equipment, aerospace, and deep-sea exploration, are crucial for driving technological innovation in related industries, ensuring personnel safety, and promoting efficient energy utilization. As a filling tool, reducing the weight of gas cylinders and increasing the gas capacity of individual cylinders can reduce losses during transportation and handling and conserve resources. Therefore, to achieve lightweight design and ensure safe use, cylinder steel must possess both high strength and high toughness.

[0003] Currently, seamless gas cylinder steel primarily utilizes Cr-Mo alloy steels, such as 30CrMo and 35CrMo. After quenching and tempering (quenching and tempering), their tensile strength is less than 1000 MPa, and their operating pressure is limited to approximately 20 MPa. However, with the growing demand for high-pressure gas cylinders, particularly in applications where operating pressures exceed 35 MPa and tensile strength requirements reach over 1100 MPa, the strength and toughness of traditional Cr-Mo steels are no longer sufficient. Among steel strengthening methods, precipitation strengthening, grain boundary strengthening, and phase transformation strengthening have been extensively researched. Controlled alloying element additions can effectively improve the overall material properties by refining the grain size, increasing dislocation density, and stabilizing the retained austenite structure. For example, the addition of microalloying elements such as Nb and V promotes the precipitation of fine carbides, exerting a strong pinning effect on the material, significantly increasing strength without significantly compromising toughness. The addition of Ni can lower the ductile-brittle transition temperature and enhance low-temperature toughness. For example, CN102409242A improves the mechanical properties of seamless gas cylinder steel by adding Ti and Nb microalloying elements. CN115074603A improves steel strength by adding small amounts of Mo and Ni in conjunction with a preparation method. Similarly, the strength improvement of most traditional CrMo steels relies primarily on the formation of various carbides during high-temperature tempering, while the improvement in toughness relies 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, while traditional quenching and tempering can improve the overall performance of the material, its application in the large-scale production of seamless gas cylinder steel is limited by its complex process, high energy consumption, and stringent equipment requirements during the quenching process. New-generation heat treatment technologies, such as the Quenching and Partitioning (Q&P) process, have become effective means of improving 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 the carbon element to diffuse from the supersaturated martensite into the retained austenite, forming a stable and dispersed retained austenite, thereby improving plasticity and toughness while maintaining high strength. IN202111045218 (A) uses a low-carbon alloy composition and quenching and non-isothermal partitioning processes to develop a hot-rolled steel with a multiphase microstructure consisting of martensite, bainite, retained austenite, and carbides, achieving improvements in various mechanical properties. CN 105441814 A adopts a segmented cooling process to obtain a three-phase structure of proeutectoid ferrite, martensite, and retained austenite containing a certain volume fraction, and also obtains a steel product with relatively superior mechanical properties.

[0005] In summary, the Q&P heat treatment process can effectively improve plasticity and toughness by stabilizing a small amount of retained austenite to room temperature. However, at present, the research on the strength-ductility product of Q&P steel is more focused, and there is less research on toughness, especially low-temperature transverse impact toughness. However, for seamless gas cylinder steel, due to its special manufacturing process, controlled rolling and controlled cooling technology cannot be used during the hot rolling process, which poses a challenge to the application of Q&P steel. In addition, while Q&P steel improves its strength-ductility product, it often ignores the improvement of low-temperature transverse impact toughness. This is especially true for gas cylinders that need to operate in extremely low-temperature environments. The low-temperature toughness of the material is an important factor in determining the safety and reliability of the gas cylinder.

[0006] Based on this, how to coordinately design the steel composition and preparation process to develop a low-carbon steel for seamless gas cylinders that has both high strength and high toughness at low temperatures has become a research hotspot in the current field of materials science and engineering. Summary of the Invention

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

[0008] In order to achieve the above object, the first aspect of the present application provides a preparation method of low carbon steel for seamless gas cylinder, which comprises the following steps: S1, preparing raw materials according to the composition of the low carbon steel for seamless gas cylinder; according to the weight percentage, the composition of the low carbon steel for seamless gas cylinder comprises 0.2wt%-0.3wt% of C, 1.0wt%-1.8wt% of Si, 2.5wt%-4.0wt% of Mn, 0.32wt%-0.50wt% of Mo, 0.5wt%-1.0wt% of Ni, 0.1wt%-0.3wt% of V, and 0.02wt%-0.05wt% of Nb; the balance is Fe and inevitable impurity elements; S2, smelting the raw materials to obtain a first steel billet; heating the first steel billet to obtain a second steel billet; S3, hot-rolling the second steel billet for at least 8 passes under the condition that the open rolling temperature is 1120°C-1180°C and the final rolling temperature is 880°C-980°C to obtain a hot-rolled steel billet; S4, cooling the hot-rolled steel billet to obtain a third steel billet; S5, heat-treating the third steel billet to obtain the low carbon steel for seamless gas cylinder.

[0009] Further, in the step S2, the holding temperature of the heating treatment is 1200±50°C, and the holding time is 2h-2.5h.

[0010] Further, in the step S3, the open rolling temperature is 1140°C-1160°C, and the final rolling temperature is 900°C-960°C.

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

[0012] Further, in the step S4, the metallographic structure of the third steel billet comprises lath martensite phase and residual austenite phase, and the volume fraction of the area of the residual austenite phase in the metallographic structure is 11.5%-12.5%.

[0013] Further, in the step S5, the holding temperature of the heat treatment is 200°C-350°C, and the holding time is 0.5h-2.0h.

[0014] The second aspect of the present application provides a low carbon steel for seamless gas cylinder, which is prepared by the above preparation method of low carbon steel for seamless gas cylinder.

[0015] Further, the metallographic structure of the low carbon steel for seamless gas cylinder comprises lath martensite matrix phase and austenite second phase, and the volume fraction of the area of the austenite second phase in the metallographic structure is 10%-15%.

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

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

[0018] Applying the technical solutions of the present invention, the alloying element composition of low-carbon steel for seamless gas cylinders is carefully designed, while the manufacturing process is coordinated. Controlling the carbon content ensures that the resulting low-carbon steel retains an appropriate amount of retained austenite, while the addition of multiple elements at specific concentrations further refines the microstructure. The combination of multiple hot rolling passes and tempering heat treatment not only optimizes the overall mechanical properties of the resulting low-carbon steel but, more importantly, exhibits excellent impact toughness at low temperatures, ultimately achieving a comprehensive improvement in both low-temperature toughness and strength for the low-carbon steel used in seamless gas cylinders. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying 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 structure characterization results of the low carbon steel for seamless gas cylinders obtained in Example 1 of the present invention are shown in FIG. Figure 3 (a) is a metallographic photograph obtained by optical microscope (OM). Figure 3 (b) is the metallographic structure distribution map obtained by electron backscatter diffraction (EBSD). DETAILED DESCRIPTION

[0023] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0024] As described in the background art, the existing gas cylinder steel has the problem of being difficult to have both high strength and high low-temperature impact toughness. In order to solve the above technical problems, the first aspect of the present invention provides a method for preparing low-carbon steel for seamless gas cylinders, the preparation method comprising: step S1, preparing raw materials according to the composition of low-carbon steel for seamless gas cylinders; in terms of weight percentage, the composition of low-carbon steel for seamless gas cylinders 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%~0.3 wt% V, 0.02wt%~0.05wt% Nb; the balance is Fe and inevitable impurity elements; step S2, smelting the raw materials to obtain a first steel billet; heat-treating the first steel billet to obtain a second steel billet; step S3, hot-rolling the second steel billet for at least 8 passes under the conditions of a starting rolling temperature of 1120℃~1180℃ and a finishing 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 low-carbon steel for seamless gas cylinders.

[0025] The present invention achieves a comprehensive improvement in the low-temperature toughness and strength of low-carbon steel for seamless gas cylinders through the careful design of alloying element components and the coordinated coordination of the preparation process. With respect to the above-mentioned alloying elements, on the basis of traditional Cr-Mo gas cylinder steel, the C content is reduced to reduce the toughness loss caused by excessive C content, and the Mn content is increased to lower the Ms point so that a stable residual austenite structure can be obtained at room temperature, thereby improving the plasticity and toughness of the material; a small amount of Nb and V microalloying elements is added to significantly improve the strength of the steel by precipitating nanocarbides; and an appropriate amount of Ni is added to effectively improve the low-temperature impact toughness of the steel for seamless gas cylinders. More specifically:

[0026] C: In the low-carbon steel used in seamless gas cylinders produced by this invention, carbon is the primary element that ensures strength. Too low a carbon content will not meet the required strength, while excessive carbon can easily form brittle phases, impairing the steel's toughness. Therefore, the present invention preferably controls the carbon content to between 0.2% and 0.3% by weight.

[0027] Si: In the low-carbon steel for seamless gas cylinders prepared in this invention, silicon acts as a solid solution strengthener, increasing the steel's strength. Furthermore, silicon effectively prevents the formation of cementite, allowing carbon to partition from martensite into untransformed austenite during low-temperature tempering, thereby improving austenite stability. Therefore, the Si content in this invention is preferably controlled within a range of 1.0wt% to 1.8wt%.

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

[0029] Mo: In the low-carbon steel for seamless gas cylinders prepared in this invention, molybdenum dissolves into ferrite, providing solid solution strengthening. Furthermore, molybdenum is a strong carbide-forming element, precipitating carbides during tempering, refining the grain size. It also hinders the growth of carbides such as Nb and V, refines the precipitated phase, and improves tempering stability. Therefore, the Mo content in this invention is preferably controlled between 0.32wt% and 0.50wt%.

[0030] Nickel: In the low-carbon steel for seamless gas cylinders prepared in this invention, nickel strengthens ferrite and lowers the ductile-brittle transition temperature. It also effectively reduces the resistance to dislocation movement, enhancing the steel's strength while also improving its low-temperature toughness. Therefore, the Ni content is preferably controlled within a range of 0.5wt% to 1.0wt%.

[0031] V: In the low-carbon steel for seamless gas cylinders produced in this invention, vanadium is a strong carbide-forming element. It precipitates during tempering to form carbonitrides, contributing to precipitation strengthening. Furthermore, vanadium particles have a high dissolution temperature, effectively preventing grain boundary migration and effectively refining grains. However, excessive V content can impair low-temperature impact toughness. Therefore, the present invention preferably controls the V content to 0.1wt% to 0.3wt%.

[0032] Nb: In the low-carbon steel prepared by the application for seamless gas cylinders, niobium forms carbonitride during hot rolling, and fine Nb(C, N) particles pin the austenite grain boundaries, prevent grain boundary migration, and inhibit the dynamic recrystallization of the steel. If dynamic recrystallization occurs during deformation, the grain boundary migration triggered can enclose the microcracks formed at the original grain boundaries in new grains, prevent the aggregation, growth and extension of the cracks, and improve the ductility of the steel. Therefore, the Nb content is preferably controlled at 0.02wt%-0.05wt%.

[0033] Further, the element components in the above content range also have a synergistic effect. First, C-Si synergistic effect: the combination of low carbon content and Si can prevent the formation of brittle phases, and Si prevents the precipitation of carbides, thereby maintaining the stability of residual austenite, which is crucial for improving low-temperature toughness. Mn-Ni coupling: the synergy of Mn and Ni can significantly reduce the martensite transformation temperature, and the addition of Ni reduces the ductile-brittle transition temperature. Under the combined action of the two, the toughness of the material at low temperature is improved. Mo-V-Nb three-element effect: under the synergistic action of Mo, V and Nb, not only the grain is refined and the grain boundary movement is inhibited, but also the low-temperature toughness of the material is further enhanced by the precipitation of fine carbides and carbonitrides. That is to say, the elements in the above low-carbon steel composition for seamless gas cylinders provided by the application do not act alone, but synergistically between two or more elements, forming a complete alloy overall formula, which comprehensively optimizes the performance of the low-carbon alloy steel.

[0034] On the basis of fine control of the content of each element, the preparation method is also optimized. In actual production and preparation process, smelting is vacuum smelting, which effectively reduces the content of hydrogen and nitrogen in the steel, reduces the hydrogen embrittlement and nitrogen embrittlement effect, and improves the low-temperature toughness. On this basis, the obtained first steel billet is subjected to a heat treatment, so that the alloy elements are fully dissolved and homogenized, laying a foundation for the formation of stable microstructure in the subsequent hot rolling and heat treatment process. In step S3, the opening rolling temperature is 1120-1180 DEG C, the final rolling temperature is 880-980 DEG C, and the hot rolling process of at least 8 passes not only promotes the refinement of the microstructure, but also controls the final rolling temperature to retain a certain amount of austenite phase, which provides favorable conditions for the stable partitioning of residual austenite in the subsequent low-temperature heat treatment, thereby effectively improving the low-temperature toughness of the material. During the subsequent heat treatment, i.e. during the low-temperature tempering partitioning process, the C element diffuses from the martensite to the residual austenite, and the stable residual austenite formed can absorb the energy of crack propagation at low temperature, significantly improving the low-temperature fracture toughness of the material.

[0035] In addition, it is worth mentioning that the preparation method directly performs low-temperature tempering and distribution after hot rolling, while the traditional seamless cylinder steel needs to be quenched and tempered. Therefore, compared with the existing process, the preparation method can obtain low-carbon steel with excellent comprehensive performance by matching the alloy composition and process route, thereby simplifying the heat treatment process route, reducing energy consumption caused by processing, saving resources, and particularly meeting the development concept of a resource-saving society.

[0036] In the preparation method, for the heating treatment in step S2, in order to make the alloy elements more fully dissolved and distributed, so as to improve the uniformity of the microstructure of the obtained first billet in the subsequent treatment process, and finally improve the performance of the obtained low-carbon steel for seamless cylinder, the holding temperature is preferably 1200±50℃, and the holding time is 2h~2.5h.

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

[0038] On this basis, in step S3, the reduction of each hot rolling is preferably ≤30%, so as to more effectively control the deformation amount in the hot rolling process, reduce internal cracks or organizational defects caused by excessive deformation, and further promote grain refinement and improve the plasticity and toughness of the obtained low-carbon steel, especially the impact toughness at low temperature. Through a large number of experiments, in several typical embodiments, the number of hot rolling passes is preferably 8, so as to more significantly simplify the process, reduce energy consumption, and not damage the internal structure of the material, while achieving more sufficient plastic deformation and more efficiently refining the microstructure of the billet at this time, thereby more effectively improving the overall plasticity and toughness of the material, especially the transverse impact toughness at low temperature. In practical applications, in order to better control the deformation degree in the hot rolling process and the cooling speed of the billet, obtain more uniform metallographic structure, and obtain superior toughness and plasticity, the thickness of the first billet is preferably 80±5mm, and the thickness of the second billet is 6mm~7.5mm.

[0039] Based on the optimization of the process condition parameters, in step S4, the microstructure of the obtained third billet preferably includes lath martensite phase and residual austenite phase, and the volume fraction of the area of the residual austenite phase in the microstructure is 11.5% to 12.5%. By optimizing the volume fraction of the residual austenite in the microstructure of the third intermediate product billet as above, the steel billet can respond more sensitively in the subsequent heat treatment (low-temperature tempering and partitioning), thereby more significantly promoting the effective diffusion of C element and the stable retention of residual austenite, so that the final obtained low-carbon steel can significantly improve the low-temperature toughness while maintaining high strength, and better meet the performance requirements of seamless gas cylinder steel in extreme environments.

[0040] In several typical embodiments, in step S5, the holding temperature of the heat treatment is 200°C to 350°C, and the holding time is 0.5h to 2.0h. In this low-temperature tempering stage of 200°C to 350°C, the C element can more efficiently diffuse from the supersaturated martensite to the residual austenite to form a more stable and dispersed residual austenite structure; and the holding time of 0.5h to 2.0h promotes the diffusion process to be more fully carried out, thereby further promoting the stability of the residual austenite phase, so that it can still play a role as a toughness phase at low temperature, and finally more significantly improves the low-temperature toughness of the obtained low-carbon steel. In addition, the cooling mode of the heat treatment is preferably air cooling, thereby promoting the more stable preservation of the residual austenite phase, reducing the increase in the degree of supersaturation of the martensite phase that may be caused by rapid cooling (such as water cooling), thereby reducing the risk of embrittlement; at the same time, it also improves the integrity of the surface of the obtained low-carbon steel for seamless gas cylinders, reduces the generation of cracks, and optimizes the surface quality and subsequent use safety.

[0041] The second aspect of the present application provides a low-carbon steel for seamless gas cylinders, which is prepared by the above-mentioned method for preparing a low-carbon steel for seamless gas cylinders. Through the synergistic effect of the above-mentioned manufacturing process and component design, a low-carbon steel for seamless gas cylinders with both high strength and excellent low-temperature toughness is prepared, so as to better meet the lightweight and high safety requirements of seamless gas cylinders.

[0042] Further, the microstructure of the prepared low-carbon steel for seamless gas cylinders includes lath martensite matrix phase and austenite second phase, and the volume fraction of the area of the austenite second phase in the microstructure is 10% to 15%, preferably 11.5% to 12.0%. Within the above-mentioned preferred and more preferred volume fraction range, the residual austenite phase in the microstructure of the obtained low-carbon steel can be more uniformly distributed, so as to further reduce the initiation and propagation of cracks and more significantly improve the fracture toughness of the obtained low-carbon steel at low temperature.

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

[0044] The third aspect of the present application provides an application of the above-mentioned low carbon steel for seamless gas cylinder as a metal material for seamless gas cylinder in the fields of energy, industry, medicine and aerospace. The low carbon steel obtained by the present application has high strength, high fracture toughness, low ductile-brittle transition temperature, and good ductility at low temperature, so it can significantly improve the reliability and safety of the obtained seamless gas cylinder product as a metal material for seamless gas cylinder, and thus can well meet the application requirements in the fields of energy, industry, medicine and aerospace, etc.

[0045] The present application will be further described in detail below in combination with specific examples, which should not be understood as limiting the scope of the present application.

[0046] Unless otherwise defined, all the professional terms used herein have the same meaning as generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific examples and are not intended to limit the protection scope of the present application.

[0047] Example 1

[0048] A preparation method of a low carbon steel for seamless gas cylinder:

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

[0050] (2) smelt the chemical components as above to obtain a target billet of 80x80 mm, i.e. a first billet; the obtained billet sample is sent into a heating furnace and heated to a complete austenitization temperature of 1200 ℃, and held for 2 h to obtain a second billet;

[0051] (3) the billet sample after tapping is subjected to 8-pass hot rolling, with opening rolling at 1160 ℃, and after 8-pass hot rolling, the billet is rolled from 80 mm to 7 mm, wherein the single-pass reduction is 30%-30%-30%-25%-25%-25%-20%-20% respectively, and the finish rolling temperature is 950 ℃;

[0052] (4) the sample in step (3) is air-cooled to room temperature to obtain a third billet, which has a microstructure including lath martensite phase and residual austenite phase;

[0053] (5) the sample in step (4) is placed in an electric arc furnace at a temperature of 200 ℃, tempered and held for 1 h, and air-cooled to room temperature to obtain a low-carbon steel for seamless gas cylinders.

[0054] In addition, the time-temperature process chart of the preparation method is shown in Figure 1 .

[0055] Example 2

[0056] A preparation method of a 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% of C, 1.2wt% of Si, 3.3wt% of Mn, 0.32wt% of Mo, 1.0wt% of Ni, 0.15wt% of V, 0.03wt% of Nb, and the balance being Fe and inevitable impurity elements (the composition is shown in Table 1);

[0058] (2) smelt the chemical components as above to obtain a target billet of 80x80 mm, i.e. a first billet; the obtained billet sample is sent into a heating furnace and heated to a complete austenitization temperature of 1200 ℃, and held for 2 h to obtain a second billet;

[0059] (3) the billet sample after tapping is subjected to 8-pass hot rolling, with opening rolling at 1170 ℃, and after 8-pass hot rolling, the billet is rolled from 80 mm to 6.5 mm, wherein the single-pass reduction is the same as in Example 1, and the finish rolling temperature is 960 ℃;

[0060] (4) the sample in step (3) is air-cooled to room temperature to obtain a third billet, which has a microstructure including lath martensite phase and residual austenite phase;

[0061] (5) The sample in step (4) is placed in an electric arc furnace at a temperature of 300 ℃, tempered for 2 h, and air-cooled to room temperature to obtain a low-carbon steel for seamless gas cylinders.

[0062] Example 3

[0063] A method for preparing a 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% of C, 1.0wt% of Si, 3.8wt% of Mn, 0.43wt% of Mo, 0.65wt% of Ni, 0.12wt% of V, 0.02wt% of Nb, and the balance of Fe and unavoidable impurity elements (see Table 1 for composition);

[0065] (2) The chemical composition as described above is smelted to obtain a target billet of 80x80 mm, i.e., a first billet; the obtained billet sample is sent to a heating furnace and heated to a complete austenitization temperature of 1200 ℃, and held for 2 h to obtain a second billet;

[0066] (3) The billet sample after being discharged is subjected to 8-pass hot rolling, with opening rolling at 1160 ℃, and after 8-pass hot rolling, the billet is rolled from 80 mm to 6.5 mm, wherein the single-pass reduction is the same as in Example 1, and the finish rolling temperature is 950 ℃;

[0067] (4) The sample in step (3) is air-cooled to room temperature to obtain a third billet, and the metallographic structure thereof includes lath martensite phase and residual austenite phase;

[0068] (5) The sample in step (4) is placed in an electric arc furnace at a temperature of 350 ℃, tempered for 0.5 h, and air-cooled to room temperature to obtain a low-carbon steel for seamless gas cylinders.

[0069] Example 4

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

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

[0072] Example 5

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

[0074] The difference between this example and Example 1 is only that in step (3), the opening rolling temperature is changed to 1120 ℃, and the finish rolling temperature is changed to 880 ℃.

[0075] Example 6

[0076] A method for producing a low carbon steel for a seamless gas cylinder

[0077] This example differs from Example 1 only in that in step (3), the starting rolling temperature is changed to 1180°C and the finishing rolling temperature is changed to 980°C.

[0078] Example 7

[0079] A method for producing a low carbon steel for a seamless gas cylinder

[0080] This example differs from Example 1 only in that in step (3), the billet thickness before hot rolling is changed to 100 mm and 8 passes of hot rolling are performed to obtain a hot rolled billet having a thickness of 5 mm, wherein the single pass reduction is 35%-35%-35%-30%-30%-30%-30%-30%, respectively.

[0081] Example 8

[0082] A method for producing a low carbon steel for a seamless gas cylinder

[0083] This example differs from Example 1 only in that in step (5), the holding temperature is changed to 150°C and the holding time is changed to 3 h.

[0084] Example 9

[0085] A method for producing a low carbon steel for a seamless gas cylinder

[0086] This example differs from Example 1 only in that in step (5), the holding temperature is changed to 400°C and the holding time is changed to 20 min.

[0087] Comparative Example 1 and Comparative Example 2

[0088] Comparative Example 1 and Comparative Example 2 differ from Example 1 only in the element contents, as shown in Table 1.

[0089] Comparative Example 3

[0090] A method for producing a low carbon steel for a seamless gas cylinder

[0091] This comparative example differs from Example 1 only in that in step (3), the starting rolling temperature is changed to 1100°C and the finishing rolling temperature is changed to 800°C.

[0092] Comparative Example 4

[0093] A method for producing a low carbon steel for a seamless gas cylinder

[0094] This comparative example differs from Example 1 only in that in step (3), the starting rolling temperature is changed to 1200°C and the finishing rolling temperature is changed to 1000°C.

[0095] Comparative Example 5

[0096] A method for preparing steel for seamless 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 steel billet is rolled from 80 mm to 7.5 mm, wherein the reduction in a single pass is 40%-40%-40%-35%-35%-35% respectively.

[0098] Test Method

[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: The test was carried out in accordance with 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 in accordance with GB / T 38720-2020 and GB / T 341720-2017, respectively, and the volume fraction of the retained austenite phase in the metallographic structure of the third steel billet in each embodiment and comparative example, as well as the volume fraction of the retained austenite phase in the metallographic structure of the resulting seamless low-carbon steel for gas cylinders, were obtained.

[0104] The above test was carried out on each embodiment and the comparative example and the comparative example, and the results are shown in Table 2. Also, the engineering stress-strain curve of Example 1 obtained by the test is shown in Figure 2 ; Metallographic photographs of the seamless gas cylinder obtained in Example 1 with low carbon steel are shown in Figure 3 (a), metallographic structure distribution diagram is shown in Figure 3 (b).

[0105] Table 1

[0106]

[0107] Table 2

[0108]

[0109] From the above description, it can be seen that, compared with the respective comparative examples, the above-mentioned embodiments of the present application achieve the preparation of seamless steel bottle low carbon steel with superior comprehensive performance. The obtained low carbon steel has superior mechanical properties, including yield strength, tensile strength, and elongation after fracture, and also exhibits superior transverse impact toughness at room temperature, especially at low temperature, which can provide more significant safety and reliability for the preparation of various metal products.

[0110] As can be seen from Comparative Examples 1 and 2, if the elements in the low carbon steel are not synergistically controlled and adjusted, the strength and plasticity of Comparative Example 1 will decrease significantly; Comparative Example 2 exhibits high strength, but poor plasticity, and also poor impact toughness at room temperature and low temperature.

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

[0112] In the embodiments, as can be seen by comparing Example 4 with Example 1, the preferred heating treatment in step (2) can make the alloying elements more fully dissolved and distributed, so as to improve the organization uniformity of the obtained first steel billet in the subsequent treatment process, and finally improve the properties of the obtained seamless steel bottle low carbon steel, especially the low temperature impact toughness.

[0113] As can be seen by comparing Example 5 and Example 6 with Example 1, by further optimizing the rolling temperature and finishing temperature in step (3), the formation of austenite can be better ensured, and at the same time, the microstructure transformation of the steel billet during cooling can be more controllable, thereby more significantly promoting the formation of stable residual austenite, optimizing the volume fraction of the residual austenite phase in the metallographic structure, and finally more effectively enhancing the low temperature toughness of the obtained low carbon steel.

[0114] As can be seen by comparing Example 7 with Example 1, by further optimizing the rolling passes and the reduction amount of each pass in step (3), more sufficient plastic deformation can be achieved under the conditions of more significantly simplifying the process, reducing energy consumption, and not damaging the internal structure of the material, and the microstructure of the steel billet at this time can also be more efficiently refined, 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 be more efficiently diffused from the supersaturated martensite into the retained austenite, forming a more stable and dispersed retained austenite structure, thereby further promoting the stability of the retained austenite phase, so that it can still function as a toughness 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," and the like 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 precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, so that the embodiments of the application described herein can, for example, be implemented in an order other than that described herein.

[0117] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be 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 comprises: Step S1, preparing raw materials according to the composition of the low-carbon steel for seamless gas cylinders; the composition of the low-carbon steel for seamless gas cylinders includes, by weight percentage, 0.2wt% to 0.3wt% of C, 1.0wt% to 1.8wt% of Si, 2.5wt% to 4.0wt% of Mn, 0.32wt% to 0.50wt% of Mo, 0.5wt% to 1.0wt% of Ni, 0.1wt% to 0.3wt% of V, and 0.02wt% to 0.05wt% of 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 steel billet for at least 8 passes at a starting rolling temperature of 1120° C. to 1180° C. and a finishing rolling temperature of 880° C. to 980° C. 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 low-carbon steel for the seamless gas cylinder.

2. The method for preparing low carbon steel for seamless gas cylinders according to claim 1, characterized in that: In step S2, the holding temperature of the heating treatment is 1200±50° C., and the holding time is 2 h to 2.5 h.

3. The method for preparing low carbon steel for seamless gas cylinders according to claim 1, characterized in that: In step S3, the starting rolling temperature is 1140°C to 1160°C, and the finishing rolling temperature is 900°C to 960°C.

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 the step S3, the reduction 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 a lath martensite phase and a retained austenite phase, and the volume fraction of the retained austenite phase in the metallographic structure is 11.5% to 12.5%.

6. 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 S5, the heat treatment is carried out at a holding temperature of 200° C. to 350° C., and a holding time of 0.5 h to 2.0 h.

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

8. The low carbon steel for seamless gas cylinders according to claim 7, characterized in that: 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 volume fraction of the austenite second phase in the metallographic structure is 10% to 15%.

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

10. Use of the low carbon steel for seamless gas cylinders according to any one of claims 7 to 9 as a metal material for seamless gas cylinders in the energy, industrial, medical and aerospace fields.

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