A low-silicon high-pressure header steel and a manufacturing method thereof

Abstract

The application provides a low-silicon high-pressure manifold steel and a manufacturing method thereof, and the steel comprises the following chemical components in percentage by weight: C 0.13-0.17%, Si ≤0.10%, Mn 0.50-0.70%, Cr 0.60-0.80%, Mo 0.30-0.50%, Ni 0.80-1.00%, Al 0.030-0.050%, Nb 0.030-0.060%, V 0.10-0.50%, P ≤0.010%, S ≤0.005%, T.O ≤15ppm, 60 ppm ≤[N] ≤120ppm, and the rest is Fe and inevitable impurity elements. By reducing the silicon content to ≤0.10% and precisely controlling the easily-oxidized elements such as Cr and Mn, and simultaneously inhibiting internal oxidation by Mo, Nb and V, the depth of the oxidation layer after carburizing is ≤35 μm, which can significantly improve the internal oxidation phenomenon in the carburizing process of the high-pressure manifold; in combination with the optimized electric furnace smelting and continuous casting process, the high purity and uniformity of the steel are ensured, and the final product has high strength, high toughness and excellent contact fatigue performance, thereby effectively improving the service life and reliability of the high-pressure manifold.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas transportation technology, and relates to a manifold steel and its manufacturing process. Background Technology

[0002] High-pressure manifolds are important equipment in oil and gas production enhancement operations. The quality of high-pressure manifolds directly determines the effectiveness and economy of production enhancement operations. As the depth of oil and gas extraction increases, the pressure of high-pressure manifolds rises from 70MPa to 140MPa. The heat treatment process of manifolds has also changed from traditional quenching and tempering to carburizing. Carburizing technology is the main process technology for hardening the inner wall surface of manifolds.

[0003] During the carburizing process of manifolds, in addition to the oxidation of the surface carburized layer, trace amounts of oxygen penetrate into the steel interior, reacting with alloying elements to form oxides. These elements have a much higher oxidation tendency than iron, thus reacting with the more reactive elements in the alloy to form particulate oxide precipitates, known as internal oxidation. During operation, manifolds are subjected to high-speed flow of fracturing fluid, and these internal oxide precipitates become fatigue sources, ultimately leading to stress concentration in certain areas and fatigue cracks. With increasing oil and gas extraction depths, higher demands are placed on the pressure-bearing capacity of high-pressure manifolds, and the phenomenon of internal oxidation in the carburized layer is receiving increasing attention and urgently needs improvement. Summary of the Invention

[0004] To address the urgent need to improve the oxidation phenomenon within the carburized layer of high-pressure manifolds as described in the background art, this invention provides a low-silicon high-pressure manifold steel and its manufacturing method, suitable for high-pressure manifolds used in high-performance shale gas transportation.

[0005] This invention provides a low-silicon high-pressure manifold steel, comprising the following chemical composition by weight percentage: C 0.13-0.17%, Si ≤0.10%, Mn 0.50-0.70%, Cr 0.60-0.80%, Mo 0.30-0.50%, Ni 0.80-1.00%, Al 0.030-0.050%, Nb 0.030-0.060%, V 0.10-0.50%; P≤0.010%, S≤0.005%, TO≤15ppm, 60ppm≤[N]≤120ppm, with the remainder being Fe and unavoidable impurity elements.

[0006] The chemical composition and weight percentage of this low-silicon high-pressure manifold steel are analyzed as follows: C: As the most basic and efficient strengthening element in steel, C plays a prominent role in improving hardenability and also has a cost advantage. In order to give high-pressure manifold steel sufficient strength and hardenability, stabilize microalloying elements and prevent carburizing oxidation, a certain C content needs to be ensured. However, excessive carbon content will weaken the toughness of steel and impair fatigue performance. Therefore, the carbon content range is determined to be 0.13-0.17%. Si: Si is a strong oxidizing element. Although it can enhance the activity of C, it is also very easy to induce internal oxidation. Because the oxides formed are far from the surface, they are difficult to remove in subsequent processing of high-pressure manifolds, which seriously affects the fatigue performance of the material. In order to eliminate the risk of internal oxidation, the Si content is controlled at ≤0.10%. Mn: Mn can broaden the austenite phase region, stabilize the austenite structure, and effectively improve the hardenability of steel, so the Mn content needs to be ≥0.50%; however, excessive Mn will reduce the plasticity of steel, worsen the toughness during rolling, and also aggravate internal oxidation; in summary, the Mn content should be controlled in the range of 0.50-0.70%. Cr: Cr can significantly enhance the hardenability and strength of steel. The fine carbides formed by combining with carbon can improve the material's strength and fatigue performance. Therefore, the Cr content should be ≥0.60%. However, Cr is easily oxidized, and excessively high content will worsen the oxide layer depth. Therefore, the Cr content should be ≤0.80%, and the Cr content should be controlled between 0.60-0.80%. Mo: Mo can significantly improve the hardenability of steel and effectively prevent temper brittleness and overheating. In this invention, the reasonable combination of Mo and Cr can significantly enhance hardenability and tempering resistance, and also refine the grain size. However, if the Mo content is too low, it will not be effective, while if it is too high, it will affect the hot plasticity of the steel, increase the risk of reheat cracking, and increase costs. Mo is a non-oxidizing element and can effectively inhibit internal oxidation during carburizing. Therefore, the Mo content is controlled at 0.30-0.50%. Ni: Ni can significantly improve the core toughness of steel, lower the ductile-brittle transition temperature, enhance low-temperature impact performance and fatigue strength, and also improve stacking fault energy and torsional resistance; however, due to the high cost of Ni and the fact that excessive content can reduce machinability after hot working, the Ni content is controlled at 0.80-1.00%. Al: Al is a highly efficient deoxidizer that can form refined AlN grains; its effect is poor when the Al content is below 0.030%, and it is prone to producing coarse inclusions when it is above 0.050%; by optimizing the timing of Al addition during steelmaking, its content can be stabilized at 0.030-0.050%. Nb: As a strong carbide, nitride, and carbonitride forming element, Nb can effectively refine austenite grains through pinning and dragging effects, and has important applications in steel. Nb can refine grains and enhance strength and toughness by solid solution or by combining with C and N atoms in the steel to form fine Nb(C,N) atoms; however, excessive Nb will reduce hardenability. Therefore, the Nb content should be controlled between 0.030-0.060%. V: Carbonitrides formed by the combination of V with C and N can pin grain boundaries and refine grains. In addition, the combined addition of V and Nb can promote the precipitation of fine Nb, thereby improving the strength and toughness of steel. Therefore, the V content should be ≥0.1%. However, excessive V can cause corner cracks in continuously cast billets, affecting the yield. Therefore, the V content should be ≤0.5%, and its content range was finally determined to be 0.10-0.50%. P and S: P is an element with a strong tendency to segregate, which increases the cold brittleness of steel, reduces plasticity, and is detrimental to the uniformity of product structure and properties. Therefore, P is controlled to be ≤0.010%. Although trace amounts of S can improve machinability, in order to ensure high purity and low internal oxidation, this invention strictly controls S to be ≤0.005%, sacrificing some machinability in exchange for fatigue performance. TO: TO refers to total oxygen content, which is the main cause of inclusions and internal oxidation in steel. Strictly controlling the oxygen content in steel is the key to ensuring the performance of steel used in high-pressure manifolds. Therefore, TO ≤ 15 ppm. [N]: [N] refers to nitrogen content. It can form compounds with Nb, Al, etc., which can refine grains. A reasonable Al / [N] ratio has a significant effect on grain refinement. However, excessive [N] can cause continuous casting defects. Therefore, the [N] content is controlled at 60-120ppm.

[0007] Furthermore, the chemical composition ratio satisfies: G T =10×G(Cr)+13×G(Mn)+15×G(Si)+10×G(TO)-4×G(Mo)-12×G(Nb)-10×G(V), 8.0≤G T ≤13.0, Wherein, G(Cr) is the weight percentage of Cr; G(Mn) is the weight percentage of Mn; G(Si) is the weight percentage of Si; G(TO) is the ppm value of TO divided by 100; G(Mo) is the weight percentage of Mo; G(Nb) is the weight percentage of Nb; and G(V) is the weight percentage of V.

[0008] This invention provides a low-silicon high-pressure manifold steel to avoid internal oxidation during carburizing, thereby reducing carburizing deformation and improving the fatigue life and quality of the high-pressure manifold. Cr, Mn, Si, and TO in the steel are all easily oxidized elements, which are detrimental to oxide layer control during carburizing. Based on the affinity of alloying elements for oxygen from strongest to weakest, Si, Mn, and Cr, their contribution coefficients to oxide layer depth, GT, are positive, with coefficients of 15, 13, and 10 for Si, Mn, and Cr, respectively. Mo, V, and Nb, on the other hand, are not easily oxidized and are beneficial to improving the strength and toughness of the manifold steel; therefore, their contribution coefficients to oxide layer depth, GT, are negative, with coefficients of 4, 12, and 10 for Mo, Nb, and V, respectively. To meet the requirement of low carburizing oxidation in the manifold steel, the GT value should be ≤13.0; however, to ensure good mechanical properties and production stability, the GT value should be ≥8.0. In summary, to achieve the best alloying effect, an oxide layer depth contribution coefficient, G, is introduced. T (8.0≤G) T ≤13.0), this formula quantifies the combined effects of easily oxidizable elements (Si, Mn, Cr, TO) and less easily oxidizable but beneficial elements (Mo, Nb, V) on carburizing oxidation behavior.

[0009] The manufacturing method of the low-silicon high-pressure manifold steel described above includes electric arc furnace smelting, LF furnace refining, RH vacuum degassing, billet continuous casting, rolling, and material heat treatment. During the electric arc furnace smelting process, the strong deoxidation capacity of the electric furnace is utilized to add Cr and Mn alloys during the electric furnace smelting stage and adjust them to the target values. During the LF furnace refining process, Al material is added and the Al element is adjusted to the target value; During the continuous casting of the billet, the superheat of the tundish is controlled at 20-50°C above the liquidus line. During the rolling process, the billet is fed into a walking beam furnace for rolling and heating. The heating temperature of the round steel is ≥1230℃, the initial rolling temperature is ≥1180℃, and the final rolling temperature is 700-950℃. After rolling, it is cooled by a heat preservation cover or by entering a slow cooling pit.

[0010] In the manufacturing method of this invention, Al material is added during the refining process in the LF furnace. This ensures the aluminum content and prevents excessive Al inclusions in the steel, thus avoiding nozzle accumulation. During the continuous casting of the billet, the superheat of the tundish is controlled at 20-50°C above the liquidus line. Light compression (reduction of 0.8-1.2 mm / m) can effectively reduce the central carbon segregation index, while heavy compression (2.5-3.0 mm / m) may further eliminate shrinkage cavities. However, excessive compression should be avoided to prevent internal cracks.

[0011] Preferably, during the RH vacuum degassing process, the vacuum degree is ≥30Pa and the vacuum degassing time is ≥20min. Through a series of manufacturing processes, such as enhanced deoxidation in an electric arc furnace, precise control of Al content in LF refining, and deep RH vacuum degassing (vacuum degree ≥30Pa, time ≥20min), the total oxygen content TO is controlled to ≤15ppm, which greatly reduces oxide inclusions and lowers the inducing factors of internal oxidation.

[0012] Preferably, during the rolling process, the residual oxygen content of the steel billet in the heating furnace is ≤3%. Controlling the residual oxygen content in the heating furnace (≤3%) and employing a high-temperature rolling and slow cooling process during rolling avoids excessive oxidation of the steel surface and the generation of internal cracks, ensuring the uniformity and density of the microstructure.

[0013] Preferably, during the heat treatment of the material, carburizing is performed at a temperature of 950℃, followed by oil quenching at 840-880℃, cooling to room temperature, and then isothermal quenching at 280-300℃. After air cooling to room temperature, low-temperature tempering is performed at 180-200℃, followed by air cooling. This optimized carburizing + quenching + low-temperature tempering heat treatment process achieves high surface hardness (≥620HV), effectively resisting the erosion and wear of fracturing fluid. Simultaneously, the strong and tough core structure provides stable support for the hardened layer, preventing plastic collapse or spalling under alternating contact stress.

[0014] More preferably, the heat treatment of the material specifically includes the following steps: Step 1, Multi-stage carburizing treatment: The process involves sequentially performing the first stage of low-temperature precarburizing, the second stage of high-temperature main carburizing, and the third stage of diffusion homogenization. The first stage of low-temperature precarburizing is carried out at a temperature of 820-850℃, a carbon potential of 0.80-0.95%, and a time of 2-4 hours, forming a dense carbide barrier layer of 5-15μm thickness on the surface of the low-silicon high-pressure manifold steel. This carbide barrier layer is composed of granular Cr7C3, VC and NbC carbides. The second stage of high-temperature main carburizing is carried out at a temperature of 930-950℃, a carbon potential of 1.00-1.15%, and a time of 6-10 hours. The temperature for the third stage of carbon diffusion and homogenization is 900-920℃, the carbon potential is 0.75-0.85%, and the time is 2-3 hours. Step 2, oil quenching: Heat the multi-stage carburized manifold to 840-880℃ for austenitization, hold for 30-60 minutes, and then oil quench to room temperature; Step 3, cryogenic treatment: After oil quenching and cooling to room temperature, the manifold is cooled to -80 to -140℃ at a rate of 1-3℃ / min and held for 1-3 hours to reduce the residual austenite content to <3%. Then, it is naturally warmed back to room temperature at a rate of ≤5℃ / min. Step 4, Salt bath magnetic field-assisted isothermal quenching: The manifold, which has been cryogenically treated and then warmed to room temperature, is placed in a mixed salt bath of NaNO2 and KNO3 at a temperature of 280-300℃ for isothermal quenching. At the same time, a DC steady magnetic field of 0.2-0.5T is applied along the manifold axis during the isothermal quenching process. After holding at this temperature for 30-60 minutes, it is taken out and air-cooled to room temperature. Step 5, Low-temperature tempering: Hold the isothermal quenched manifold at 180-200℃ for 2-4 hours, then air cool to room temperature.

[0015] More preferably, it also includes step six, surface strengthening treatment, which is performed after low-temperature tempering: laser shock strengthening treatment or ultrasonic surface rolling treatment on the carburized surface of the manifold after tempering, or a combination treatment of laser shock strengthening treatment and ultrasonic surface rolling treatment in sequence.

[0016] Preferably, the parameters of the laser shock peening treatment are: laser wavelength 1064nm, pulse energy 5-12J, pulse width 10-20ns, spot diameter 2-4mm, overlap rate 50-70%, number of impacts 2-3, constraint layer is a flowing water film, and absorption layer is aluminum foil.

[0017] Preferably, the parameters for the ultrasonic surface rolling treatment are: frequency 20-27kHz, static pressure 80-150N, feed speed 50-100mm / min, and rolling times 3-5 times.

[0018] The low-silicon high-pressure manifold steel prepared by this invention has the following properties: tensile strength 930-1080 MPa, yield strength 750-820 MPa, elongation after fracture ≥15%, reduction of area ≥40%, impact energy at -20℃ ≥60 J, oxide layer depth after carburizing ≤35 μm, surface hardness after carburizing ≥620 HV, and contact fatigue performance meets fatigue life L. 10 ≥1.0×10 5 Median fatigue life L 50 ≥5.0×10 5 .

[0019] Compared with the prior art, the present invention has the following advantages: (1) Significantly inhibits oxidation within the carburized layer: By strictly controlling the content of the strong oxidizing element Si to ≤0.10%, and in conjunction with controlling the content of easily oxidized elements Mn and Cr, the number of alloying elements that react with oxygen during the carburizing process is reduced from the source; through the precise ratio of the components, the performance and the requirements for resistance to internal oxidation are effectively balanced, ensuring that the final oxide layer depth after carburizing is ≤35μm. (2) High purity and excellent fatigue performance: The reasonable ratio of C, Cr and Mo ensures that the steel has sufficient hardenability and strength; at the same time, the addition of Ni element significantly improves the toughness of the steel core and the low temperature impact performance (impact energy ≥60J at -20℃); the use of Nb and V to form fine carbonitrides pins the grain boundaries and effectively refines the austenite grains, thereby improving the strength (tensile strength 930-1080MPa, yield strength 750-820MPa) while maintaining good plasticity (elongation after fracture ≥15%, reduction of area ≥40%). (3) Refined microstructure and uniformity: Optimized manufacturing processes achieve high surface hardness (≥620HV), effectively resisting the erosion and wear of fracturing fluid; simultaneously, the strong and tough core structure provides stable support for the hardened layer, preventing plastic collapse or spalling under alternating contact stress; furthermore, thanks to the clean steel (low P, S, TO), refined grains, and effectively suppressed internal oxidation, fatigue crack initiation is reduced, resulting in a significant improvement in the contact fatigue life of the steel under 140MPa contact stress. 10 ≥1.0×10 5 L 50 ≥5.0×10 5 ; (4) Excellent surface hardness and wear resistance: Through a series of manufacturing processes such as electric arc furnace enhanced deoxidation, LF refining to precisely control Al content, and RH vacuum deep degassing, the total oxygen content TO is controlled at ≤15ppm, which greatly reduces oxide inclusions and reduces the internal oxidation induction; low superheat (20-50℃ above the liquidus line) continuous casting combined with appropriate light pressure reduces center segregation; high temperature rolling and slow cooling process are used during rolling to avoid excessive oxidation of the steel surface and the generation of internal cracks, ensuring the uniformity and density of the structure.

[0020] In summary, this invention significantly improves the internal oxidation phenomenon during the carburizing process of high-pressure manifolds by reducing the silicon content to ≤0.10%, precisely controlling easily oxidizable elements such as Cr and Mn, and using Mo, Nb, and V to suppress internal oxidation, resulting in an oxide layer depth of ≤35μm after carburizing. Combined with optimized electric furnace smelting and continuous casting processes, it ensures high purity and uniformity of the steel structure. The final product possesses high strength, high toughness, and excellent contact fatigue performance, effectively improving the life and reliability of high-pressure manifolds. Attached Figure Description

[0021] Figure 1 The image shows a metallographic image of the carburized layer in Example 1.

[0022] Figure 2 The image shows the metallographic structure of the carburized layer in Comparative Example 2. Detailed Implementation

[0023] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0024] A low-silicon high-pressure manifold steel comprises the following chemical composition by weight percentage: C 0.13-0.17%, Si ≤0.10%, Mn 0.50-0.70%, Cr 0.60-0.80%, Mo 0.30-0.50%, Ni 0.80-1.00%, Al 0.030-0.050%, Nb 0.030-0.060%, V 0.10-0.50%; P≤0.010%, S≤0.005%, TO≤15ppm, 60 ppm≤[N]≤120ppm, with the remainder being Fe and unavoidable impurity elements.

[0025] The chemical composition ratio satisfies: G T =10×G(Cr)+13×G(Mn)+15×G(Si)+10×G(TO)-4×G(Mo)-12×G(Nb)-10×G(V), 8.0≤G T ≤13.0, Wherein, G(Cr) is the weight percentage of Cr; G(Mn) is the weight percentage of Mn; G(Si) is the weight percentage of Si; G(TO) is the ppm value of TO divided by 100; G(Mo) is the weight percentage of Mo; G(Nb) is the weight percentage of Nb; and G(V) is the weight percentage of V.

[0026] The manufacturing method of the low-silicon high-pressure manifold steel described above includes electric arc furnace smelting, LF furnace refining, RH vacuum degassing, billet continuous casting, rolling, and material heat treatment. During the electric arc furnace smelting process, the strong deoxidation capacity of the electric furnace is utilized to add Cr and Mn alloys during the electric furnace smelting stage and adjust them to the target values. During the LF furnace refining process, Al material is added and the Al element is adjusted to the target value; this can ensure the aluminum content and prevent excessive Al inclusions in the steel, thus avoiding nozzle buildup. During the RH vacuum degassing process, the vacuum degree is ≥30Pa and the vacuum degassing time is ≥20min.

[0027] During the continuous casting of billets, the superheat of the tundish should be controlled 20-50°C above the liquidus line; light reduction (reduction of 0.8-1.2 mm / m) can effectively reduce the central carbon segregation index, while heavy reduction (2.5-3.0 mm / m) may further eliminate shrinkage cavities, but excessive compression should be avoided to prevent internal cracks. During the rolling process, the billet is fed into a walking beam furnace for heating. The heating temperature of the round steel is ≥1230℃, the initial rolling temperature is ≥1180℃, and the final rolling temperature is 700-950℃. After rolling, it is cooled by a heat preservation cover or a slow cooling pit. The residual oxygen content of the billet in the heating furnace is ≤3%. During the heat treatment of the material, carburizing is performed at a carburizing temperature of 950℃. After carburizing, oil quenching is performed at 840-880℃. After cooling to room temperature, isothermal quenching is performed at a temperature of 280-300℃. Then, after air cooling to room temperature, low-temperature tempering is performed at a temperature of 180-200℃, followed by air cooling.

[0028] Preferably, the heat treatment process for the material includes the following steps: Step 1, Multi-stage carburizing treatment: The multi-stage carburizing treatment includes a first stage of low-temperature pre-carburizing, a second stage of high-temperature main carburizing, and a third stage of diffusion homogenization. The first stage of low-temperature precarburizing involves a temperature of 820-850℃, a carbon potential of 0.80-0.95%, and a time of 2-4 hours. This process forms a dense carbide barrier layer of 5-15 μm thickness on the surface of the low-silicon high-pressure manifold steel. This carbide barrier layer is composed of granular Cr7C3, VC, and NbC carbides, covering the grain boundaries and surface areas. It blocks the channels for oxygen to diffuse inward along the grain boundaries and fixes the easily oxidized elements Cr, V, and Nb in a stable carbide form, preventing them from undergoing selective oxidation reactions with the infiltrated oxygen. The second stage of high-temperature main carburizing is carried out at a temperature of 930-950℃, a carbon potential of 1.00-1.15%, and a time of 6-10 hours. Under the oxygen barrier protection of the carbide barrier layer, carbon is deeply diffused to achieve the target carburized layer depth. The third stage of carbon diffusion and homogenization is carried out at a temperature of 900-920℃, a carbon potential of 0.75-0.85%, and a time of 2-3 hours, so as to smooth out the carbon concentration gradient. Step 2, oil quenching: Heat the multi-stage carburized manifold to 840-880℃ for austenitization, hold for 30-60 minutes, and then oil quench to room temperature; Step 3, cryogenic treatment: After oil quenching and cooling to room temperature, the manifold is cooled to -80 to -140℃ at a rate of 1-3℃ / min and held for 1-3 hours to transform the retained austenite into martensite and reduce the retained austenite content to <3%. Then, it is naturally warmed back to room temperature at a rate of ≤5℃ / min. Step 4, Salt bath magnetic field-assisted isothermal quenching: The manifold, which has been cryogenically treated and then warmed to room temperature, is placed in a salt bath at a temperature of 280-300℃ for isothermal quenching. The salt bath is composed of a mixture of NaNO2 and KNO3. At the same time, a DC steady magnetic field of 0.2-0.5T is applied along the manifold axis during the isothermal quenching process. After holding at this temperature for 30-60 minutes, the manifold is removed and air-cooled to room temperature. Step 5, Low-temperature tempering: Hold the isothermal quenched manifold at 180-200℃ for 2-4 hours, then air cool to room temperature; Step 6, Surface strengthening treatment: The carburized surface of the manifold after tempering is subjected to laser shock strengthening treatment or ultrasonic surface rolling treatment, or a combination of laser shock strengthening treatment and ultrasonic surface rolling treatment in sequence.

[0029] The parameters of the laser shock peening treatment are as follows: laser wavelength 1064nm, pulse energy 5-12J, pulse width 10-20ns, spot diameter 2-4mm, overlap rate 50-70%, number of shocks 2-3, constraint layer is a flowing water film, and absorption layer is aluminum foil.

[0030] The parameters for the ultrasonic surface rolling treatment are: frequency 20-27kHz, static pressure 80-150N, feed speed 50-100mm / min, and rolling times 3-5 times.

[0031] After the surface strengthening treatment, the residual compressive stress on the carburized surface of the manifold is ≥-400MPa, and the depth of influence of the residual compressive stress is ≥0.5mm.

[0032] The improved heat treatment process consists of a unique combination of six steps, with synergistic effects between each step: (1) Synergy of step one → step four: the carbide barrier layer formed by multi-stage carburizing blocks oxygen → reduces the depth of internal oxidation → isothermal quenching to obtain a purer bainitic structure → magnetic field-assisted refinement of bainitic laths → more uniform structure + fewer internal oxidation defects → higher fatigue life. (2) Synergy between step three and step four: cryogenic treatment reduces the amount of retained austenite to <3% → the initial microstructure is more uniform during isothermal quenching (mainly martensite rather than martensite + a large amount of retained austenite) → the bainite transformation is more complete and more uniform under magnetic field assistance → the residual stress distribution is better. (3) Synergy of step four to step six: Salt bath + magnetic field isothermal quenching to obtain uniform and refined bainitic structure → reasonable hardness gradient after tempering → LSP / USR introduces deep residual compressive stress on the surface of high-quality carburized layer → compressive stress and high hardness carburized layer work together → dual suppression of fatigue crack initiation and propagation. (4) The full-chain synergy of step one → step three → step six: pre-carburizing to block oxygen → cryogenic elimination of residual austenite → surface strengthening to introduce compressive stress. The three elements comprehensively suppress fatigue failure from different dimensions (chemical barrier → structural stability → mechanical protection), forming a unique "oxygen barrier-structural stability-compressive stress" triple protection system for low-silicon high-pressure manifold steel.

[0033] The technical solutions and effects of this application will be further illustrated by the following embodiments and comparative examples.

[0034] The chemical compositions of the examples and comparative examples are shown in Table 1. Examples 1-3 are low-silicon high-pressure manifold steels produced using the specific compositions and smelting processes of this invention. Comparative Example 1 uses the compositions of this invention but does not employ the specific smelting and rolling processes, which results in the TO content not being controlled to the target requirements. Comparative Example 2 is 15CrNiMo produced according to GB / T 3077 standards and using conventional smelting and rolling processes. Other smelting and rolling processes are the same for the examples and comparative examples.

[0035] Table 1 Chemical composition of the embodiments and comparative examples of the present invention (unit: TO, [N] are ppm, others are wt%) Table 2 shows the mechanical properties, contact fatigue properties (contact stress 140 MPa), and oxide layer depth test results of the materials in the examples and comparative examples after tempering heat treatment. The heat treatment regime for mechanical properties was: 860℃×1h (oil cooling) + 200℃×2h (air cooling).

[0036] Table 2 Comparison of Mechanical and Fatigue Properties between Examples and Comparative Examples Figure 1 The image shown is a metallographic image of the carburized layer in Example 1, illustrating the metallographic structure of the carburized layer of low-silicon high-pressure manifold steel prepared using the composition and process described in this invention. Figure 1 The carburized layer is uniform and dense, with no obvious internal oxide layer. The oxide layer depth is controlled at approximately 26 μm (see Table 2), which meets the target of ≤35 μm set in this invention. There are no obvious oxide particles or oxidation defects distributed along grain boundaries in the microstructure, indicating that the control of Si content (0.05%) and total oxygen content (TO) (10 ppm), combined with reasonable G... TThe value (10.2) effectively suppressed internal oxidation; this microstructure helps improve the contact fatigue life of the material (see Table 2, L). 10 Reaching 5.8×10 5 This indicates that the present invention has a significant effect in suppressing oxidation within the carburized layer.

[0037] Figure 2 The image shown is a metallographic image of the carburized layer in Comparative Example 2, illustrating the microstructure of the carburized layer made from conventional 15CrNiMo steel (Comparative Example 2), with a Si content of 0.21%, TO content of 20 ppm, and G content of 10 ppm. T The value is 20.8, which is outside the scope of this invention. Figure 2 A distinct oxide layer is visible, reaching a depth of 61 μm, far exceeding the control target of this invention (≤35 μm). Numerous oxide particles distributed along grain boundaries are present in the microstructure, indicating severe internal oxidation during carburizing. These oxides act as fatigue crack initiation sites, leading to a significant reduction in contact fatigue life (see Table 2, L). 10 Only 0.4×10 5 ); Figure 2 This, in turn, verifies that the invention achieves its effect by controlling the Si content, TO content, and G content. T When the value is within a reasonable range, it plays a key role in inhibiting internal oxidation and improving fatigue life.

[0038] As can be seen from the above, the present invention has the following technical effects: I. Effective Suppression of Internal Oxidation in Carburizing: This invention reduces the formation of internal oxides at the source by strictly controlling the silicon (Si) content to ≤0.10%, and by controlling the total oxygen content (TO ≤ 15ppm) and the content of other easily oxidized elements (such as Mn and Cr); it also introduces and calculates the oxide layer depth contribution coefficient G. T (8.0 ≤ GT ≤ 13.0) Through the synergistic effect of alloying elements, the composition design was quantified and optimized to ensure that the tendency of internal oxidation was minimized while maintaining performance; Effect verification: The oxide layer depth of all embodiments was ≤35μm, while the oxide layer depth of comparative examples 1-3 was significantly increased (46-72μm) due to excessive Si and TO content or excessive GT value, which proved the effectiveness of the composition design of the present invention in suppressing internal oxidation.

[0039] II. Achieving Excellent Strength and Toughness: A reasonable ratio of elements such as C, Cr, Mo, and Ni ensures sufficient hardenability and strength in the steel. Simultaneously, Ni enhances core toughness, and Nb and V carbonitrides refine grain size, ensuring good plasticity and toughness even at high strength. Verification of Results: The tensile strength (973-1012 MPa) and yield strength (788-816 MPa) of Examples 1-3 all meet the requirements, and the elongation after fracture (≥21%), reduction of area (≥50%), and impact energy at -20℃ (≥75 J) all demonstrate excellent performance. In contrast, Comparative Example 2, due to excessively high Mn and Cr content, exhibits a sharp decrease in plasticity (reduction of area 24%) and toughness (impact energy at -20℃ 28 J). III. Significantly Improved Contact Fatigue Performance: The reduction of internal oxides directly eliminates potential fatigue crack initiation sites. Combined with the material's excellent strength and toughness and refined grain structure, this contributes to the long service life of the high-pressure manifold under high-pressure fracturing fluid erosion. Performance Verification: Under a contact stress of 140 MPa, the fatigue life L of all embodiments... 10 All far exceed 1.0×10 5 The requirement for this (Example 1: 5.8 × 10) 5 Example 2: 6.4 × 10 5 Example 3: 8.2 × 10 5 In contrast, in comparative examples 1-3, due to severe internal oxidation (deep oxide layer) and / or insufficient toughness, the fatigue life L... 10 All are below the required value (0.4 × 10). 5 -0.9×10 5 This invention has a significant effect on improving fatigue life; IV. Stable and controllable manufacturing process, ensuring material purity and uniformity: Optimized smelting and continuous casting processes (such as electric arc furnace pre-alloying, LF refining and aluminum control, RH vacuum degassing, and control of continuous casting superheat) effectively reduce the oxygen content and inclusions in the steel, alleviate central segregation, and lay the foundation for obtaining high-performance materials; controlling the residual oxygen content (≤3%) during the rolling process and adopting slow cooling measures help prevent surface oxidation and internal stress concentration; specific heat treatment methods (carburizing followed by oil quenching + isothermal quenching + low-temperature tempering) help obtain ideal microstructure and further optimize the performance of the carburized layer and the toughness of the core.

[0040] In summary, by Figure 1 , Figure 2As shown in Tables 1 and 2, this invention proposes a low-silicon high-pressure manifold steel and its manufacturing method to solve the problems of easy surface oxidation and low contact fatigue life during the carburizing process of the inner wall of the high-pressure manifold. Based on 15CrNiMo steel, this invention does not add Si and reduces the content of Mn and Cr, while adding appropriate amounts of Nb and V elements to refine the material's microstructure and improve its mechanical properties. Furthermore, by adjusting the order of alloy addition during the steelmaking process, the contact time between easily oxidized elements Cr and Mn and oxygen is reduced to achieve ultra-low oxygen content control.

[0041] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings and specific examples. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. A low-silicon high-pressure manifold steel, characterized in that, The chemical composition includes the following weight percentages: C 0.13-0.17%, Si ≤0.10%, Mn 0.50-0.70%, Cr 0.60-0.80%, Mo 0.30-0.50%, Ni 0.80-1.00%, Al 0.030-0.050%, Nb 0.030-0.060%, V 0.10-0.50%; P≤0.010%, S≤0.005%, TO≤15ppm, 60ppm≤[N]≤120ppm, with the remainder being Fe and unavoidable impurity elements.

2. The low-silicon high-pressure manifold steel according to claim 1, characterized in that: The chemical composition ratio satisfies: G T =10×G(Cr)+13×G(Mn)+15×G(Si)+10×G(TO)-4×G(Mo)-12×G(Nb)-10×G(V), 8.0≤G T ≤13.0， Wherein, G(Cr) is the weight percentage of Cr; G(Mn) is the weight percentage of Mn; G(Si) is the weight percentage of Si; G(TO) is the ppm value of TO divided by 100; G(Mo) is the weight percentage of Mo; G(Nb) is the weight percentage of Nb; and G(V) is the weight percentage of V.

3. A method for manufacturing low-silicon high-pressure manifold steel according to claim 1 or 2, characterized in that: This includes electric arc furnace smelting, LF furnace refining, RH vacuum degassing, billet continuous casting, rolling, and material heat treatment; During the electric arc furnace smelting process, the strong deoxidation capacity of the electric furnace is utilized to add Cr and Mn alloys during the electric furnace smelting stage and adjust them to the target values. During the LF furnace refining process, Al material is added and the Al element is adjusted to the target value; During the continuous casting of the billet, the superheat of the tundish is controlled at 20-50°C above the liquidus line. During the rolling process, the billet is fed into a walking beam furnace for rolling and heating. The heating temperature of the round steel is ≥1230℃, the initial rolling temperature is ≥1180℃, and the final rolling temperature is 700-950℃. After rolling, it is cooled by a heat preservation cover or by entering a slow cooling pit.

4. The method for manufacturing a low-silicon high-pressure manifold steel according to claim 3, characterized in that: During the RH vacuum degassing process, the vacuum degree is ≥30Pa and the vacuum degassing time is ≥20min.

5. The method for manufacturing a low-silicon high-pressure manifold steel according to claim 3, characterized in that: During the rolling process, the residual oxygen content of the steel billet in the heating furnace is ≤3%.

6. The method for manufacturing a low-silicon high-pressure manifold steel according to claim 3, characterized in that: The material undergoes heat treatment by carburizing at a temperature of 950℃, followed by oil quenching at 840-880℃, cooling to room temperature, and then isothermal quenching at a temperature of 280-300℃. After air cooling to room temperature, it is then tempered at a low temperature of 180-200℃ and air cooled.

7. The method for manufacturing a low-silicon high-pressure manifold steel according to claim 6, characterized in that: The heat treatment of the material specifically includes the following steps: Step 1, Multi-stage carburizing treatment: The process involves sequentially performing the first stage of low-temperature precarburizing, the second stage of high-temperature main carburizing, and the third stage of diffusion homogenization. The first stage of low-temperature precarburizing is carried out at a temperature of 820-850℃, a carbon potential of 0.80-0.95%, and a time of 2-4 hours, forming a dense carbide barrier layer of 5-15μm thickness on the surface of the low-silicon high-pressure manifold steel. This carbide barrier layer is composed of granular Cr7C3, VC and NbC carbides. The second stage of high-temperature main carburizing is carried out at a temperature of 930-950℃, a carbon potential of 1.00-1.15%, and a time of 6-10 hours. The temperature for the third stage of carbon diffusion and homogenization is 900-920℃, the carbon potential is 0.75-0.85%, and the time is 2-3 hours. Step 2, oil quenching: Heat the multi-stage carburized manifold to 840-880℃ for austenitization, hold for 30-60 minutes, and then oil quench to room temperature; Step 3, cryogenic treatment: After oil quenching and cooling to room temperature, the manifold is cooled to -80 to -140℃ at a rate of 1-3℃ / min and held for 1-3 hours to reduce the residual austenite content to <3%. Then, it is naturally warmed back to room temperature at a rate of ≤5℃ / min. Step 4, Salt bath magnetic field-assisted isothermal quenching: The manifold, which has been cryogenically treated and then warmed to room temperature, is placed in a mixed salt bath of NaNO2 and KNO3 at a temperature of 280-300℃ for isothermal quenching. At the same time, a DC steady magnetic field of 0.2-0.5T is applied along the manifold axis during the isothermal quenching process. After holding at this temperature for 30-60 minutes, it is taken out and air-cooled to room temperature. Step 5, Low-temperature tempering: Hold the isothermal quenched manifold at 180-200℃ for 2-4 hours, then air cool to room temperature.

8. The method for manufacturing a low-silicon high-pressure manifold steel according to claim 7, characterized in that: It also includes step six, surface strengthening treatment, which is located after low-temperature tempering: laser shock strengthening treatment or ultrasonic surface rolling treatment is performed on the carburized surface of the manifold after tempering, or a combination of laser shock strengthening treatment and ultrasonic surface rolling treatment is performed in sequence.

9. A method for manufacturing a low-silicon high-pressure manifold steel according to claim 8, characterized in that: The parameters of the laser shock peening treatment are as follows: laser wavelength 1064nm, pulse energy 5-12J, pulse width 10-20ns, spot diameter 2-4mm, overlap rate 50-70%, number of shocks 2-3, constraint layer is a flowing water film, and absorption layer is aluminum foil.

10. A method for manufacturing a low-silicon high-pressure manifold steel according to claim 8, characterized in that: The parameters for the ultrasonic surface rolling treatment are: frequency 20-27kHz, static pressure 80-150N, feed speed 50-100mm / min, and rolling times 3-5 times.

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