Pure ferritic structure Ti-Mo hydrogen transmission pipeline steel and preparation process thereof

Abstract

The invention discloses Ti-Mo hydrogen transmission pipeline steel with a pure ferritic structure. The Ti-Mo hydrogen transmission pipeline steel comprises the following chemical components in percentage by mass: 0.025 to 0.05 percent of C, 1.4 to 1.7 percent of Mn, 0.04 to 0.15 percent of Si, 0.05 to 0.30 percent of Ti, 0.05 to 0.30 percent of Mo, less than or equal to 0.02 percent of P, 0.001 to 0.01 percent of N and the balance of Fe and inevitable impurities. The invention further discloses a preparation process. The invention aims to provide pure ferritic structure Ti-Mo hydrogen transmission pipeline steel and a preparation process thereof, on the basis of traditional Nb and Ti system microalloy hydrogen transmission pipeline steel, nano particle precipitation type high-strength steel with Ti and Mo as main alloy elements is prepared from components by referring to the application of Mo element in heat-resistant steel, and (Ti, Mo) is introduced, so that the high-strength steel is prepared. Mo) C nano particle second phase is used as a strengthening phase, and is also used as a hydrogen trap.

Description

Technical Field

[0001] This invention belongs to the technical field of steel for hydrogen pipelines, and particularly relates to a Ti-Mo steel for hydrogen pipelines with a pure ferritic structure and its preparation process. Background Technology

[0002] Currently, with the gradual implementation of national low-carbon and environmental protection policies, the application of hydrogen energy is becoming increasingly widespread. Hydrogen energy is an abundant, green, and low-carbon secondary energy source, an important component of the future national energy system, a crucial carrier for achieving green and low-carbon development at the energy end, and a key development direction for strategic emerging industries. Developing the hydrogen energy industry is of great significance for building a clean, low-carbon, safe, and efficient energy system and achieving the goal of carbon peaking and carbon neutrality. The "Medium- and Long-Term Plan for the Development of the Hydrogen Energy Industry (2021-2035)," jointly formulated by the National Development and Reform Commission and the National Energy Administration, places innovation at the core of industrial development, focusing on the construction of the entire chain of hydrogen energy production, storage, transportation, and application to promote the deep integration and development of the industrial chain and innovation chain.

[0003] The high hydrogen content in steel used for hydrogen transportation and storage pipelines significantly increases the risk of hydrogen-induced cracking in the base material. The higher the grade of the base material, the higher the risk. Therefore, currently used hydrogen pipeline steels are generally Nb-Ti composition system steels of grades L245M, L360M, and L415M. The hydrogen compatibility of hydrogen pipeline steels produced by different steel mills varies considerably. The microstructure of this type of steel is typically ferrite + pearlite. To improve the hydrogen compatibility of pipeline steel, researchers have conducted extensive studies on composition optimization and microstructure control through controlled rolling and cooling.

[0004] For example, Qu Jinbo et al. of Shagang Group (inventor nationality: China; publication number CN120290989A; publication date: 2025.07.11) prepared a hydrogen pipeline steel containing quasi-polygonal ferrite and acicular ferrite by adding a certain amount of Mg and Al to the traditional Nb-Ti microalloyed pipeline steel composition system and combining the design of the casting method and rolling process.

[0005] Gao Qing et al. of Hunan Valin Steel (inventor nationality: China; publication number CN202510715628; publication date: 2025.05.30) have developed a marine hydrogen pipeline steel with a certain density hydrogen trap by adding a certain amount of Mo to the traditional Nb-Ti microalloyed pipeline steel composition system and making comprehensive use of the grain refinement and precipitation strengthening effects of Nb, V, Ti and Mo microalloying elements.

[0006] Peng Shiyao et al. of China Oil & Gas Pipeline Network Corporation (inventors' nationality: China; publication number CN202510069925; publication date: 2025.05.13) have prepared a high-niobium X60 grade pipeline steel for hydrogen transportation by increasing the Nb content based on the traditional Nb-Ti microalloyed pipeline steel composition system.

[0007] In the implementation of this patent, the hydrogen compatibility of commonly used pipeline steel grades was first evaluated, and the correspondence between microstructure and hydrogen compatibility was analyzed. The results showed that a microstructure with a high proportion of equiaxed grains, similar to pure ferrite, often exhibits excellent hydrogen compatibility. This is because this type of microstructure has relatively fewer phase interfaces compared to the ferrite-pearlite dual-phase microstructure. These phase interfaces are often regions where hydrogen accumulates. When pipeline steel is used in a hydrogen environment, cracks generally initiate and propagate from these regions, leading to a decrease in the hydrogen compatibility of the base material. Therefore, this patent focuses on research aimed at preparing a pure ferrite microstructure with equiaxed grains. Based on the traditional pipeline steel composition, a series of Ti-Mo composition system pipeline steels were prepared by adding appropriately designed amounts of Ti, Mo, and C elements, combined with controlled rolling and controlled cooling processes.

[0008] Mo is a commonly used microalloying element in high-temperature alloys. When applied to microalloyed steel, it reduces the activity of other elements at high temperatures, preventing the rapid growth of the second phase. Adding a certain amount of Ti and Mo to microalloyed steel allows TiC nanoparticles to precipitate in the austenite region during the high-temperature rolling process, reducing the austenite grain size. In the low-temperature phase transformation region and the subsequent coiling and heat preservation region, a certain amount of (Ti,Mo)C nanoparticles can precipitate either dispersedly or interphase. These nanoparticles have small diameters and certain thermal stability, ensuring that Ti-Mo pipeline steel meets the strength requirements of relevant national standards. When the Ti and Mo content reaches a certain level, a pure ferrite structure can be achieved, which in turn provides good toughness to the Ti-Mo pipeline steel. Simultaneously, to achieve a pure ferrite structure, the carbon content in Ti-Mo pipeline steel needs to be controlled below 0.05%, as low carbon content helps improve the weldability of Ti-Mo pipeline steel.

[0009] However, there are few reports on research into pipeline steel with a pure ferritic structure prepared by Ti-Mo microalloying combined with controlled rolling and cooling processes, relying on interphase precipitation and dispersed (Ti-Mo)C nanoparticles as hydrogen traps during rolling to prepare hydrogen pipeline steel with excellent hydrogen compatibility. The application of Mo in pipeline steel is limited to adding a small amount of Mo to traditional high-strength Nb-Ti pipeline steel, improving the performance of traditional pipeline steel through the grain refinement and solid solution strengthening effects of Nb, V, Ti, and Mo. Summary of the Invention

[0010] The purpose of this invention is to provide a Ti-Mo hydrogen pipeline steel with a pure ferritic structure and its preparation process. Based on traditional Nb and Ti-based microalloyed hydrogen pipeline steels, this invention focuses on compositional analysis, drawing on the application of Mo in heat-resistant steels to prepare a nanoparticle precipitation-type high-strength steel with Ti and Mo as the main alloying elements. A (Ti,Mo)C nanoparticle second phase is introduced as a strengthening phase and also acts as a hydrogen trap. This steel primarily relies on the precipitation strengthening effect of (Ti,Mo)C nanoparticles to provide high strength. Its hydrogen compatibility is improved by the deep hydrogen trapping effect of the (Ti,Mo)C nanoparticle second phase.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0012] This invention discloses a Ti-Mo hydrogen pipeline steel with a pure ferritic structure, the chemical composition of which by mass percentage is: C 0.025-0.05%, Mn 1.4-1.7%, Si 0.04-0.15%, Ti 0.05-0.30%, Mo 0.05-0.30%, P≤0.02%, N 0.001-0.01%, with the remainder being Fe and unavoidable impurities.

[0013] Furthermore, its chemical composition by mass percentage is as follows: C 0.025-0.05%, Mn 1.5-1.6%, Si 0.09%, Ti 0.05-0.30%, Mo 0.05-0.30%, P 0.007%, N 0.005%, with the remainder being Fe and unavoidable impurities.

[0014] Furthermore, its chemical composition by mass percentage is as follows: C 0.03%, Mn 1.55%, Si 0.09%, Ti 0.15%, Mo 0.25%, P 0.007%, N 0.005%, with the remainder being Fe and unavoidable impurities.

[0015] Further steps include: heating the billet that meets the composition requirements in a heating furnace to 1250±50℃ and holding it for 1-3 hours, then rolling it at 1100±50℃, rolling it into a strip after 6-8 passes, controlling the final rolling temperature at 890±10℃, water cooling it to 600±10℃, holding it for 1-3 hours, and then cooling it to room temperature in the furnace.

[0016] Further, specifically: after heating in the furnace to 1250℃, hold for 2 hours, start rolling at 1100℃, and after 6-8 rolling passes, roll into a strip. The final rolling temperature is controlled at 890℃, water-cooled to 600℃, and held for 2 hours before being cooled to room temperature in the furnace.

[0017] Furthermore, Ti-Mo hydrogen pipeline steel with a large number of (Ti,Mo)C nanoparticles dispersed in the matrix was prepared.

[0018] Further, it is rolled into a strip with a thickness of 8 mm.

[0019] Further, specifically: after heating in the furnace to 1250℃, hold for 1.5 hours, start rolling at 1100℃, and after 6-8 rolling passes, roll into a strip. The final rolling temperature is controlled at 890℃, water-cooled to 600℃, and held for 1 hour before being cooled to room temperature in the furnace.

[0020] Furthermore, the cooling rate of the water cooling system is 30-50℃ / s.

[0021] This application achieves a fully ferritic microstructure for hydrogen pipelines by controlling the weight percentage of alloying elements added to the steel and designing a controlled rolling and cooling process. The bonding ability of Ti and Mo with C is greater than that of Fe with C. The precipitation of (Ti,Mo)C nanoparticles consumes C in the matrix, avoiding the formation of cementite, thus producing a pure ferritic microstructure for hydrogen pipelines, providing excellent plasticity to meet application requirements. Due to the characteristics of a pure ferritic microstructure, the alloying elements appear in the form of nanoparticles, therefore there is no significant elemental segregation in the matrix. Furthermore, banded defects in the pipeline steel will be greatly reduced during the rolling process.

[0022] The grade of steel used in hydrogen pipelines is specified based on the minimum yield strength. When preparing steel for hydrogen pipelines with different strength grades, the content of C, Ti, and Mo elements can be adjusted, and controlled rolling and cooling processes can be used in conjunction with the rolling process.

[0023] The carbides precipitated after the addition of Ti can enhance the strength of the matrix. Simultaneously, due to the low solid solubility of Ti in the matrix at high temperatures, it exhibits the characteristic of high-temperature precipitation. The Ti carbide nanoparticles precipitated in the austenite stage can effectively refine the austenite grains, resulting in finer ferrite grains after the phase transformation. Ti is a representative microalloying element that combines grain refining and nanoscale second-phase precipitation strengthening effects.

[0024] The role of Mo in the matrix is ​​mainly to effectively reduce the diffusion rate of C and alloying elements in ferrite, thereby inhibiting the growth of nanoparticle carbides and fully leveraging the precipitation strengthening effect of nanoparticle precipitates. The amounts of Ti and Mo added to the experimental steel were controlled at the commonly used levels of alloying elements in microalloyed steels.

[0025] To achieve both good strength and elongation, the target microstructure of this steel was designed to be pure ferrite. The composition design primarily considered the addition of C and the alloying elements Ti and Mo. Excessive C content can lead to cementite formation during the rolling phase transformation process. Since the austenitic-ferrite transformation is a diffusion-dependent phase transformation, achieving a fully ferrite microstructure requires more alloying elements to form carbides with C. However, the high rolling speed during rolling leaves insufficient diffusion time, resulting in localized cementite formation and impacting the elongation of the experimental steel. Conversely, insufficient C content results in a low increase in solid solution strengthening strength, making it difficult for the experimental steel to achieve a high yield strength. Therefore, the carbon content was controlled between 0.025% and 0.05%.

[0026] Simultaneously, by relying on controlled rolling and cooling technologies during the rolling process, such as controlling the final rolling temperature, the cooling rate, and simulating the coiling temperature, it is possible to control the diameter of the second phase of (Ti,Mo)C nanoparticles, control the grain size of the steel matrix involved in the invention, and ultimately achieve the regulation of the strength of the steel used in hydrogen transportation pipelines involved in the invention.

[0027] The (Ti,Mo)C nanoparticle second phase is identified as a deep hydrogen trap. In the steel obtained by this invention, the dispersed (Ti,Mo)C nanoparticles grow at a slower rate in the high-temperature environment of the weld heat-affected zone than the grain growth rate of pipeline steel with traditional Nb and Ti composition systems. Therefore, the number density will be greater, the ability to capture hydrogen will be stronger, and hydrogen atoms can be evenly distributed.

[0028] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0029] This invention optimizes the composition of hydrogen pipeline steel by adding the microalloying element Mo to replace Nb, thus preparing a hydrogen pipeline steel with interphase precipitation and dispersed precipitation of (Ti,Mo)C as the reinforcing phase. In addition, this steel has a single ferrite structure with small element segregation. During the rolling process, it can effectively reduce the banded structure caused by pearlite and ferrite in the traditional Nb-Ti composition system of hydrogen pipeline steel, thereby improving hydrogen compatibility. Attached Figure Description

[0030] The present invention will be further described below with reference to the accompanying drawings.

[0031] Figure 1 for Figure 1 Rolling process of steel for Ti-Mo hydrogen pipelines;

[0032] Figure 2 Microstructure of steel for Ti-Mo hydrogen pipelines;

[0033] Figure 3 Nanoparticles are the second phase in Ti-Mo hydrogen pipeline steel. Detailed Implementation

[0034] Example 1

[0035] A Ti-Mo hydrogen pipeline steel with a pure ferritic structure is designed based on the traditional Nb-Ti microalloyed composition system for hydrogen storage pipeline steel, replacing Nb with an appropriate amount of Mo. The specific composition is shown in Table 2.

[0036] Table 2. Composition of a Ti-Mo hydrogen pipeline steel with a pure ferritic structure.

[0037]

[0038] After steelmaking according to the designed composition, the steel is processed into a billet. The billet is then forged into a forging mill to the specifications that can be rolled. In this invention, it can be forged into an intermediate billet with a length * width * thickness of 500mm * 100mm * 40mm. The billet is heated to 1250℃ in a furnace and held for 1.5 hours, then rolled at 1100℃. After 6-8 passes, it is rolled into a strip with a thickness of 8mm. The final rolling temperature is controlled at 890℃, water-cooled to 600℃, and held for 1 hour before being cooled to room temperature in the furnace (simulated coiling). This process produces Ti-Mo hydrogen pipeline steel with a large number of dispersed (Ti,Mo)C nanoparticles in the matrix. The specific process flow is as follows... Figure 1 As shown.

[0039] The microstructure of the hydrogen pipeline steel obtained by this process and the (Ti,Mo)C nanoparticles therein are as follows: Figure 2 and Figure 3As shown, the microstructure of this steel is pure ferrite, in which (Ti,Mo)C nanoparticles are mainly interphase precipitation.

[0040] In Example 1, the steel has a yield strength of 629 MPa, a tensile strength of 741 MPa, and an elongation of 21%. This invention considers second-phase nanoparticles as important hydrogen traps in hydrogen storage materials, enabling uniform hydrogen distribution within the pipeline steel and preventing aggregation, thereby improving hydrogen compatibility during the service life of the hydrogen pipeline steel. The strengthening effect of this type of steel mainly relies on nanoparticles. The diameter of the nanoparticles and the spacing between their phases determine the strength index. The diameter of the nanoparticles and the spacing between their phases can be adjusted by controlling the Ti and Mo content and the rolling process, thus achieving different strength levels of pure ferritic Ti-Mo hydrogen pipeline steel.

[0041] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A Ti-Mo hydrogen pipeline steel with a pure ferritic structure, characterized in that, Its chemical composition by mass percentage is as follows: C 0.025-0.05%, Mn 1.4-1.7%, Si 0.04-0.15%, Ti 0.05-0.30%, Mo 0.05-0.30%, P≤0.02%, N 0.001-0.01%, with the remainder being Fe and unavoidable impurities.

2. The Ti-Mo hydrogen pipeline steel with a pure ferritic structure according to claim 1, characterized in that, Its chemical composition by mass percentage is: C 0.025-0.05%, Mn 1.5-1.6%, Si 0.09%, Ti 0.05-0.30%, Mo 0.05-0.30%, P 0.007%, N 0.005%, with the remainder being Fe and unavoidable impurities.

3. The Ti-Mo hydrogen pipeline steel with a pure ferritic structure according to claim 1 or 2, characterized in that, Its chemical composition by mass percentage is: C 0.03%, Mn 1.55%, Si 0.09%, Ti 0.15%, Mo 0.25%, P 0.007%, N 0.005%, with the remainder being Fe and unavoidable impurities.

4. The preparation process of Ti-Mo hydrogen pipeline steel with pure ferritic structure according to any one of claims 1-3, characterized in that, include: The billet that meets the composition requirements is heated to 1250±50℃ in a heating furnace and held for 1-3 hours. It is then rolled at 1100±50℃ and rolled into a strip after 6-8 passes. The final rolling temperature is controlled at 890±10℃, and then water-cooled to 600±10℃. After holding for 1-3 hours, it is cooled to room temperature in the furnace.

5. The preparation process of Ti-Mo hydrogen pipeline steel with pure ferritic structure according to claim 4, characterized in that, Specifically: After heating the furnace to 1250℃, hold for 2 hours, start rolling at 1100℃, and after 6-8 rolling passes, roll into a strip. The final rolling temperature is controlled at 890℃, water-cooled to 600℃, and held for 2 hours before being cooled to room temperature in the furnace.

6. The Ti-Mo hydrogen pipeline steel with a pure ferritic structure according to claim 4, characterized in that, Ti-Mo hydrogen pipeline steel with a large number of dispersed (Ti,Mo)C nanoparticles in the matrix was prepared.

7. The Ti-Mo hydrogen pipeline steel with a pure ferritic structure according to claim 1, characterized in that, Rolled into a sheet or strip with a thickness of 8 mm.

8. The Ti-Mo hydrogen pipeline steel with a pure ferritic structure according to claim 4, characterized in that, Specifically: after heating the furnace to 1250℃, hold it at that temperature for 1.5 hours, start rolling at 1100℃, and after 6-8 rolling passes, roll it into a strip. The final rolling temperature is controlled at 890℃, water-cooled to 600℃, and held at that temperature for 1 hour before cooling to room temperature in the furnace.

9. The Ti-Mo hydrogen pipeline steel with a pure ferritic structure according to claim 5 or 8, characterized in that, The cooling rate of the water cooling system is 30-50℃ / s.

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