High-strength precision hydraulic cold-rolled steel pipe

Abstract

This invention provides a high-strength precision hydraulic cold-rolled steel pipe, relating to the field of precision machining of metal pipes and manufacturing of hydraulic pipe fittings. The process includes selecting a seamless hydraulic steel pipe blank, performing surface pretreatment on the blank, and then cold-rolling it with a mandrel to achieve a single-stage diameter and wall reduction, obtaining a cold-rolled steel pipe semi-finished product. The inner diameter of the cold-rolled steel pipe semi-finished product is controlled to be larger than the target inner diameter of the finished product, and a micro-cold-rolled sealing allowance is retained on the inner wall side of the semi-finished product for subsequent defect sealing. This high-strength precision hydraulic cold-rolled steel pipe can improve the bonding stability between the sealing area and the inner wall matrix of the steel pipe, reduce the risk of the active sealing medium falling off under high-pressure hydraulic scouring or alternating pressure, and inhibit the reopening and expansion of shallow micro-defects with open inner walls under pulsating hydraulic loads, thereby improving the service reliability and service life of the high-strength precision hydraulic cold-rolled steel pipe.

Description

Technical Field

[0001] This invention relates to the field of precision machining of metal pipes and manufacturing of hydraulic pipe fittings, specifically a high-strength precision hydraulic cold-rolled steel pipe. Background Technology

[0002] High-strength precision hydraulic steel pipes are essential components in hydraulic transmission systems, widely used in engineering machinery, hydraulic cylinders, automotive hydraulic lines, mining equipment, and high-pressure fluid transport systems. Existing precision hydraulic steel pipes are typically manufactured from hot-rolled billets through processes such as cold rolling, cold drawing, heat treatment, pickling, cleaning, straightening, and diameter adjustment to improve dimensional accuracy, wall thickness uniformity, surface quality, and mechanical properties. To further improve the internal wall condition, existing technologies often employ methods such as internal wall grinding, honing, rolling, shot peening, ultrasonic finishing, or stress-relieving heat treatment to reduce internal wall roughness, minimize the impact of local defects, or improve surface stress conditions.

[0003] However, under high pressure and pulsating hydraulic conditions, scratches, pits, and shallow microcracks remaining on the inner wall of steel pipes can still become the initiation points of fatigue cracks. Existing methods of inner wall grinding, finishing, or surface strengthening mostly focus on improving surface roughness or passivating defect morphology, making it difficult to simultaneously ensure stable sealing of defect areas, maintain inner diameter accuracy, and ensure the quality of the non-defective inner wall reference surface. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a high-strength precision hydraulic cold-rolled steel pipe. Through a synergistic process of micro-cold rolling sealing allowance reservation, selective defect filling, sealing secondary micro-cold rolling, and low-temperature diffusion stabilization, the problem of easy reopening and expansion of shallow micro-defects with open inner walls under pulsating hydraulic loads is solved.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a high-strength precision hydraulic cold-rolled steel pipe, comprising:

[0006] S1. Select a seamless steel pipe blank for hydraulic applications. After surface pretreatment, the seamless steel pipe blank is subjected to a one-time diameter and wall reduction forming process using a mandrel-type cold rolling method to obtain a cold-rolled steel pipe semi-finished product. The inner diameter of the cold-rolled steel pipe semi-finished product is controlled to be larger than the target inner diameter of the finished product, and a micro-cold rolling sealing allowance is reserved on the inner wall side of the cold-rolled steel pipe semi-finished product for subsequent defect sealing.

[0007] S2. The inner wall of the cold-rolled steel pipe semi-finished product is cleaned, activated, and defect identified. Scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. Subsequently, an active sealing medium with diffusion bonding ability with the steel pipe inner wall matrix of the cold-rolled steel pipe semi-finished product is applied to the inner wall of the cold-rolled steel pipe semi-finished product, so that the active sealing medium enters the sealable defect and removes the excess active sealing medium located on the non-defect inner wall reference surface of the cold-rolled steel pipe semi-finished product, so that the active sealing medium remains in the sealable defect in a discontinuous state, and the non-defect inner wall reference surface does not form a continuous sealing layer.

[0008] S3. The cold-rolled steel pipe semi-finished product after step S2 is subjected to a sealing secondary micro-cold rolling. The reduction in inner diameter of the sealing secondary micro-cold rolling is not greater than the micro-cold rolling sealing allowance. Through the sealing secondary micro-cold rolling, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes plastic flow in opposite directions, which covers, compacts and locks the active sealing medium retained in the sealable defect inside the sealable defect, forming a filling and compaction zone and a plastic sealing layer covering the filling and compaction zone.

[0009] S4. Under conditions lower than the recrystallization temperature of the inner wall matrix of the steel pipe, the steel pipe treated in step S3 is subjected to low-temperature diffusion stabilization treatment, so that the active sealing medium in the filling and compaction zone covered by the plastic sealing layer forms a metallurgical bonding transition zone with the defect wall of the sealable defect and the plastic sealing layer, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0010] S5. The steel pipe processed in step S4 is subjected to low deformation calibrating and straightening to obtain a high-strength precision hydraulic cold-rolled steel pipe. The shallow micro-defects with open openings on the inner wall of the high-strength precision hydraulic cold-rolled steel pipe are sealed by the filling and compaction zone, the plastic sealing layer, the metallurgical bonding transition zone and the residual compressive stress sealing zone to suppress the reopening and expansion of the shallow micro-defects with open openings on the inner wall under pulsating hydraulic load.

[0011] Preferably, the inner wall substrate of the steel pipe is the original metal material on the inner wall side of the cold-rolled steel pipe semi-finished product, and the inner wall substrate of the steel pipe does not include the active sealing medium; the micro cold rolling sealing allowance is the difference between the inner diameter of the cold-rolled steel pipe semi-finished product and the target inner diameter of the finished product, and the micro cold rolling sealing allowance is 0.02mm-0.15mm.

[0012] Preferably, the sealable defect is a shallow inner wall defect that meets preset sealing conditions after defect identification. The preset sealing conditions include: the opening depth of the sealable defect is not greater than 0.08 mm, and the opening width of the sealable defect is not greater than 0.20 mm.

[0013] Preferably, the cleaning includes degreasing and cleaning the inner wall of the cold-rolled steel pipe semi-finished product and removing free particles; the activation includes removing the oxide film on the defective wall surface of the sealable defect, and drying the inner wall of the cold-rolled steel pipe semi-finished product after activation.

[0014] Preferably, the active encapsulating medium is a paste-like or suspension-like medium formed by iron-based, nickel-based, or iron-nickel-based metal powder and a diffusion activation component; the particle size of the metal powder is 0.5 μm-15 μm, and the diffusion activation component includes boron-containing components, silicon-containing components, or boron-silicon-containing components.

[0015] Preferably, after the active sealing medium enters the sealable defect, excess active sealing medium located on the reference surface of the non-defect inner wall is removed by negative pressure suction, inner wall scraping or directional airflow purging, so that the thickness of the active sealing medium remaining on the reference surface of the non-defect inner wall is no more than 2μm.

[0016] Preferably, the sealing secondary micro cold rolling is carried out by mandrel cold rolling, the inner diameter reduction of the sealing secondary micro cold rolling is 50%-100% of the micro cold rolling sealing allowance, and the single-pass wall reduction rate of the sealing secondary micro cold rolling is less than the single-pass wall reduction rate of the primary diameter and wall reduction forming.

[0017] Preferably, the sealing secondary micro-cold rolling causes the inner wall matrix of the steel pipe on both sides of the sealable defect opening to undergo opposing plastic flow along the opening width direction of the sealable defect, and radially compacts the active sealing medium in the sealable defect; the thickness of the plastic sealing layer is 0.01mm-0.10mm.

[0018] Preferably, the temperature of the low-temperature diffusion stabilization treatment is 220℃-520℃, the holding time is 20min-180min, and the temperature of the low-temperature diffusion stabilization treatment is at least 50℃ lower than the recrystallization temperature of the inner wall matrix of the steel pipe; the low-temperature diffusion stabilization treatment is carried out under a protective atmosphere or vacuum conditions.

[0019] Preferably, the inner diameter correction amount of the low deformation calibrator is less than 30% of the inner diameter reduction amount of the sealing secondary micro cold rolling; in the obtained high-strength precision hydraulic cold-rolled steel pipe, the active sealing medium is only distributed in the sealing compaction zone and the metallurgical bonding transition zone, and the non-defect inner wall reference surface does not have a continuous sealing layer.

[0020] This invention provides a high-strength, precision hydraulic cold-rolled steel pipe. It possesses the following beneficial effects:

[0021] This high-strength precision hydraulic cold-rolled steel pipe, by reserving a micro-cold rolling sealing allowance in the first cold rolling forming stage, allows the active sealing medium to selectively enter the sealable defects in the inner wall of the steel pipe in subsequent processes. Then, a sealing secondary micro-cold rolling is used to cause the steel pipe inner wall matrix on both sides of the defect opening to undergo opposite plastic flow, thereby covering, compacting and locking the active sealing medium inside the defect. Compared with simple grinding, honing, rolling or surface coating, this invention does not simply remove or cover the inner wall defects, but locally seals the shallow defects with openings in the inner wall without forming a continuous sealing layer. This is beneficial to maintaining the surface condition of the non-defect inner wall reference surface and the accuracy of the inner diameter of the steel pipe, and reducing dimensional deviations and inner wall quality fluctuations caused by overall coating, excessive grinding or conventional strengthening treatment.

[0022] Simultaneously, this invention employs low-temperature diffusion stabilization treatment to create a metallurgical bonding transition zone between the active sealing medium in the compacted filling zone and the defect wall surface of the sealable defects, as well as the plastic sealing layer. Furthermore, a residual compressive stress sealing zone is formed near the surface of the inner wall adjacent to the plastic sealing layer. This improves the bonding stability between the filling area and the inner wall matrix of the steel pipe, reduces the risk of the active sealing medium detaching under high-pressure hydraulic scouring or alternating pressure, and inhibits the reopening and expansion of shallow micro-defects with open openings on the inner wall under pulsating hydraulic loads. Ultimately, this enhances the service reliability and lifespan of the high-strength precision hydraulic cold-rolled steel pipe. Attached Figure Description

[0023] Figure 1 This is a flowchart illustrating the overall process of the preparation method of the present invention.

[0024] Figure 2 This is a flowchart of the inner wall defect identification and sealable defect screening process of the present invention;

[0025] Figure 3 This is a flowchart illustrating the selective filling process of the active filling medium of the present invention.

[0026] Figure 4 This is a flowchart of the sealing secondary micro-cold rolling and low-temperature diffusion stabilization treatment process of the present invention. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The basic process flow of this invention is as follows: a first cold rolling with a reserved micro-cold rolling sealing allowance, followed by cleaning, activation and defect identification of the inner wall, so that the active sealing medium selectively enters the sealable defect and removes the excess medium on the reference surface of the non-defect inner wall, and then the sealing secondary micro-cold rolling causes the steel pipe inner wall matrix on both sides of the sealable defect opening to undergo opposite plastic flow, and finally the finished product is obtained by low temperature diffusion stabilization, low deformation calibrating and straightening.

[0029] The high strength in the high-strength precision hydraulic cold-rolled steel pipe of this invention refers to the steel pipe's suitability for pressure-bearing fittings in hydraulic systems. Seamless steel pipe blanks for hydraulic applications can be made of 27SiMn, 20CrMo, 35CrMo, or other steels suitable for hydraulic pressure-bearing components. Through a single-stage diameter and wall reduction forming process, a sealing-type secondary micro-cold rolling process, and a low-temperature diffusion stabilization treatment, the resulting steel pipe maintains the strength and dimensional stability required for hydraulic pressure-bearing fittings. In this invention, the filling and compaction zone refers to the area where the active filling medium is compacted and retained within the sealable defect under the sealing-type secondary micro-cold rolling action; the plastic sealing layer refers to the area near the surface of the inner wall of the steel pipe on both sides of the sealable defect opening, after plastic flow in opposite directions, covering the filling and compaction zone; the metallurgical bonding transition zone refers to the transition area formed by the diffusion bonding of the active filling medium with the defect wall and the plastic sealing layer after low-temperature diffusion stabilization treatment; and the residual compressive stress sealing zone refers to the area near the surface of the inner wall adjacent to the plastic sealing layer where residual compressive stress is formed and used to inhibit the reopening of the defect.

[0030] The preferred allowance for micro-cold rolling sealing is 0.02mm-0.15mm, which provides plastic flow space for the sealing secondary micro-cold rolling. If the allowance is too small, it may lead to insufficient sealing at the opening of sealable defects; if the allowance is too large, it may increase the difficulty of controlling the accuracy of the inner diameter of the finished product.

[0031] The sealable defects are preferably shallow inner wall defects that meet preset sealing conditions after defect identification. The preset sealing conditions may include an opening depth of no more than 0.08 mm and an opening width of no more than 0.20 mm. For defects that do not meet the preset sealing conditions, they can be removed, ground down, or downgraded before applying the active sealing medium.

[0032] The preferred particle size of the metal powder in the active sealing medium is 0.5μm-15μm. The particle size can be selected according to the opening size of the defect that can be sealed. Smaller particle size powder can be used for shallow microcracks with narrower openings, while larger particle size powder can be used for pits or scratches with wider openings.

[0033] The thickness of the residual active sealing medium on the non-defective inner wall reference surface is preferably no more than 2 μm to avoid forming a continuous sealing layer and affecting the inner diameter accuracy, inner wall roughness or hydraulic cleanliness.

[0034] The reduction in inner diameter during the sealing secondary micro-cold rolling should not exceed the micro-cold rolling sealing allowance, preferably 50%-100% of the micro-cold rolling sealing allowance, to balance defect sealing and finished product inner diameter control. The thickness of the plastic sealing layer is preferably 0.01mm-0.10mm.

[0035] The low-temperature diffusion stabilization treatment is preferably carried out at 220℃-520℃, with a holding time of 20min-180min, and the treatment temperature is at least 50℃ lower than the recrystallization temperature of the inner wall matrix of the steel pipe, so as to maintain the cold-rolled strengthening state while promoting metallurgical bonding.

[0036] The reduction rate, wall reduction rate, feed rate, and mandrel size for one-time diameter reduction and wall reduction forming can be determined based on the billet specifications, material strength, target finished product size, and equipment capacity, but it should be ensured that the inner diameter of the cold-rolled steel pipe semi-finished product is greater than the target inner diameter of the finished product.

[0037] The cleaning, activation, and drying conditions can be determined based on the degree of contamination of the inner wall of the steel pipe, the state of the oxide film, and the type of active sealing medium, in order to meet the need for the active sealing medium to enter the sealable defects and form a diffusion bond with the matrix of the inner wall of the steel pipe.

[0038] The active sealing medium can be a paste or a suspension. Paste media are suitable for scratches or pits with wider openings, while suspension media are suitable for shallow microcracks with narrower openings.

[0039] Excess active sealing medium can be removed by negative pressure suction, inner wall scraping, or directional airflow purging. The specific method depends on the length of the steel pipe, the inner diameter, and the viscosity of the medium.

[0040] The inner diameter correction amount for low deformation correction is preferably less than 30% of the inner diameter reduction amount of the sealing secondary micro cold rolling, so as to avoid damaging the formed filling and compaction zone, plastic sealing layer and residual compressive stress sealing zone.

[0041] Surface pretreatment, degreasing and cleaning, drying, straightening, cutting and routine inspection can be carried out using conventional methods in this field. The parameters can be adjusted according to the steel pipe material, specifications, equipment capacity and target dimensional accuracy, as long as they do not affect the selective entry of the active sealing medium into sealable defects, the formation of a plastic sealing layer by secondary micro-cold rolling, and the formation of a metallurgical bonding transition zone by low-temperature diffusion stabilization.

[0042] Example 1

[0043] like Figure 1-4As shown, this embodiment of the invention provides a high-strength precision hydraulic cold-rolled steel pipe, comprising: S1. Selecting a seamless steel pipe blank for hydraulic applications, performing surface pretreatment on the seamless steel pipe blank for hydraulic applications, and then performing a one-time diameter reduction and wall reduction forming using a mandrel-type cold rolling method to obtain a cold-rolled steel pipe semi-finished product; wherein, the inner diameter of the cold-rolled steel pipe semi-finished product is controlled to be greater than the target inner diameter of the finished product, and a micro-cold rolling sealing allowance is reserved on the inner wall side of the cold-rolled steel pipe semi-finished product for subsequent defect sealing.

[0044] Specifically, 27SiMn seamless steel pipe blanks for hydraulic applications are selected and subjected to degreasing, pickling, cleaning, and drying. Then, a mandrel-driven cold rolling mill is used for a single-stage diameter and wall reduction forming process. After the first cold rolling, a cold-rolled steel pipe semi-finished product is obtained. The inner diameter of the semi-finished product is 45.08 mm, while the target inner diameter of the finished product is 45.00 mm. Therefore, a micro-cold rolling sealing allowance of 0.08 mm is retained on the inner wall side of the semi-finished product. The inner wall substrate of the steel pipe is the original metal material on the inner wall side of the semi-finished cold-rolled steel pipe, excluding any subsequently applied active sealing medium.

[0045] S2. The inner wall of the cold-rolled steel pipe semi-finished product is cleaned, activated, and defect identified. Scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. Subsequently, an active sealing medium with diffusion bonding ability with the steel pipe inner wall matrix of the cold-rolled steel pipe semi-finished product is applied to the inner wall of the cold-rolled steel pipe semi-finished product, so that the active sealing medium enters into the sealable defects. Excess active sealing medium located on the non-defective inner wall reference surface of the cold-rolled steel pipe semi-finished product is removed, so that the active sealing medium remains in the sealable defects in a discontinuous state, and no continuous sealing layer is formed on the non-defective inner wall reference surface.

[0046] Specifically, the inner wall of the cold-rolled steel pipe semi-finished product is degreased and cleaned to remove free particles. A weak acid activation method is used to remove the oxide film on the defective wall surface of sealable defects. After activation, the inner wall is dried. Subsequently, an industrial endoscope is used to identify defects on the inner wall of the steel pipe. Defects that meet the preset sealing conditions among scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. In this embodiment, the opening depth of the sealable defect is no greater than 0.05 mm, and the opening width is no greater than 0.12 mm.

[0047] Specifically, the active sealing medium is a suspension formed by iron-nickel-based metal powder and boron-silicon diffusion-activated components, with the metal powder having a particle size of 3μm-8μm. The active sealing medium is introduced into the cold-rolled steel pipe semi-finished product, allowing it to enter the sealable defect. Subsequently, excess active sealing medium located on the non-defective inner wall reference surface is removed by negative pressure suction and directional airflow purging, causing the active sealing medium to remain in a discontinuous state within the sealable defect. In this embodiment, the thickness of the active sealing medium remaining on the non-defective inner wall reference surface is no greater than 2μm.

[0048] S3. The cold-rolled steel pipe semi-finished product after step S2 is subjected to sealing secondary micro-cold rolling. The reduction in inner diameter of the sealing secondary micro-cold rolling is not greater than the micro-cold rolling sealing allowance. Through sealing secondary micro-cold rolling, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes plastic flow in opposite directions. The active sealing medium retained in the sealable defect is covered, compacted and locked inside the sealable defect, forming a filling and compaction zone and a plastic sealing layer covering the filling and compaction zone.

[0049] Specifically, the sealing secondary micro-cold rolling is carried out using a mandrel cold rolling method, controlling the inner diameter reduction of the sealing secondary micro-cold rolling to be 0.06 mm. This inner diameter reduction is less than the 0.08 mm micro-cold rolling sealing allowance and is 75% of the micro-cold rolling sealing allowance. At the same time, the single-pass wall reduction rate of the sealing secondary micro-cold rolling is less than the single-pass wall reduction rate of the single-pass diameter reduction and wall reduction forming.

[0050] Specifically, during the sealing secondary micro-cold rolling process, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes opposing plastic flow along the opening width direction of the sealable defect, and radially compacts the active sealing medium located within the sealable defect, so that the active sealing medium is covered, compacted, and locked inside the sealable defect. In this embodiment, the thickness of the formed plastic sealing layer is 0.04mm-0.06mm.

[0051] S4. Under conditions lower than the recrystallization temperature of the inner wall matrix of the steel pipe, the steel pipe treated in step S3 is subjected to low-temperature diffusion stabilization treatment, so that the active sealing medium in the filling and compaction zone covered by the plastic sealing layer forms a metallurgical bonding transition zone with the defect wall surface of the sealable defect and the plastic sealing layer, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0052] Specifically, the steel pipe, after undergoing a sealing secondary micro-cold rolling process, is placed in a protective atmosphere furnace for low-temperature diffusion stabilization treatment. The protective atmosphere is argon, the treatment temperature is 360°C, and the holding time is 90 minutes. In this embodiment, the treatment temperature is at least 50°C lower than the recrystallization temperature of the inner wall matrix of the steel pipe. After the low-temperature diffusion stabilization treatment, the active sealing medium in the filling and compaction zone forms a metallurgical bonding transition zone with the defect wall surface of the sealable defect and the plastic sealing layer, respectively, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0053] S5. The steel pipe processed in step S4 is calibrated and straightened with low deformation to obtain a high-strength precision hydraulic cold-rolled steel pipe. The shallow micro-defects with open openings on the inner wall of the high-strength precision hydraulic cold-rolled steel pipe are sealed by the filling and compaction zone, the plastic sealing layer, the metallurgical bonding transition zone and the residual compressive stress sealing zone to suppress the reopening and expansion of shallow micro-defects with open openings on the inner wall under pulsating hydraulic load.

[0054] Specifically, the steel pipes after low-temperature diffusion stabilization treatment undergo low-deformation calibrating and straightening. The inner diameter correction amount for low-deformation calibrating is less than 0.018 mm, which is less than 30% of the inner diameter reduction amount during the sealing secondary micro-cold rolling. After calibrating and straightening, a high-strength precision hydraulic cold-rolled steel pipe with an inner diameter of 45.00 mm is obtained.

[0055] Cross-sectional observation revealed that the sealable defect area of ​​the high-strength precision hydraulic cold-rolled steel pipe forms a filling and compaction zone, a plastic sealing layer, and a metallurgical bonding transition zone. The inner wall near the surface adjacent to the plastic sealing layer forms a residual compressive stress sealing zone. The active sealing medium is only distributed in the filling and compaction zone and the metallurgical bonding transition zone, and the non-defect inner wall reference surface does not have a continuous sealing layer.

[0056] Example 2

[0057] This embodiment illustrates that, even with a small cold-rolled sealing allowance and small shallow defects on the inner wall, the present invention can still achieve selective defect filling, plastic sealing, and maintenance of the finished product's inner diameter accuracy.

[0058] S1. Select a seamless steel pipe blank for hydraulic applications. After surface pretreatment, perform a one-time diameter and wall reduction forming using a mandrel-driven cold rolling process to obtain a cold-rolled steel pipe semi-finished product. The inner diameter of the cold-rolled steel pipe semi-finished product is controlled to be larger than the target inner diameter of the finished product, and a micro-cold rolling sealing allowance is reserved on the inner wall side of the cold-rolled steel pipe semi-finished product for subsequent defect sealing.

[0059] Specifically, 20CrMo seamless steel pipe blanks for hydraulic applications are selected and subjected to degreasing, pickling, cleaning, and drying. Then, a mandrel-driven cold rolling mill is used for a single-stage diameter and wall reduction forming process. After the first cold rolling, a cold-rolled steel pipe semi-finished product is obtained. The inner diameter of the semi-finished product is 32.03 mm, while the target inner diameter of the finished product is 32.00 mm. Therefore, a micro-cold rolling sealing allowance of 0.03 mm is retained on the inner wall side of the semi-finished product. The inner wall substrate of the steel pipe is the original metal material on the inner wall side of the semi-finished cold-rolled steel pipe, excluding any subsequently applied active sealing medium.

[0060] S2. The inner wall of the cold-rolled steel pipe semi-finished product is cleaned, activated, and defect identified. Scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. Subsequently, an active sealing medium with diffusion bonding ability with the steel pipe inner wall matrix of the cold-rolled steel pipe semi-finished product is applied to the inner wall of the cold-rolled steel pipe semi-finished product, so that the active sealing medium enters into the sealable defects. Excess active sealing medium located on the non-defective inner wall reference surface of the cold-rolled steel pipe semi-finished product is removed, so that the active sealing medium remains in the sealable defects in a discontinuous state, and no continuous sealing layer is formed on the non-defective inner wall reference surface.

[0061] Specifically, the inner wall of the cold-rolled steel pipe semi-finished product is degreased and cleaned to remove free particles. The defective wall surface with sealable defects is activated to remove the oxide film. After activation, the inner wall is dried. Subsequently, industrial endoscopy and inner wall surface contour detection are used for defect identification. Defects that meet preset sealing conditions and open into small scratches and shallow microcracks on the inner wall surface are identified as sealable defects. In this embodiment, the opening depth of the sealable defect is no greater than 0.03 mm, and the opening width is no greater than 0.08 mm.

[0062] Specifically, the active sealing medium is a suspension formed by iron-based metal powder and boron-containing diffusion-activating components, with the metal powder having a particle size of 0.5μm-5μm. The active sealing medium is introduced into the cold-rolled steel pipe semi-finished product, allowing it to enter the sealable defect. Subsequently, excess active sealing medium located on the non-defective inner wall reference surface is removed by negative pressure suction and directional airflow purging, causing the active sealing medium to remain in a discontinuous state within the sealable defect. In this embodiment, the thickness of the active sealing medium remaining on the non-defective inner wall reference surface is no greater than 1.5μm.

[0063] S3. The cold-rolled steel pipe semi-finished product after step S2 is subjected to sealing secondary micro-cold rolling. The reduction in inner diameter of the sealing secondary micro-cold rolling is not greater than the micro-cold rolling sealing allowance. Through sealing secondary micro-cold rolling, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes plastic flow in opposite directions. The active sealing medium retained in the sealable defect is covered, compacted and locked inside the sealable defect, forming a filling and compaction zone and a plastic sealing layer covering the filling and compaction zone.

[0064] Specifically, the sealing secondary micro-cold rolling is carried out using a mandrel cold rolling method, controlling the inner diameter reduction of the sealing secondary micro-cold rolling to be 0.024 mm. This inner diameter reduction is less than the 0.03 mm micro-cold rolling sealing allowance and is 80% of the micro-cold rolling sealing allowance. At the same time, the single-pass wall reduction rate of the sealing secondary micro-cold rolling is less than the single-pass wall reduction rate of the single-pass diameter reduction and wall reduction forming.

[0065] Specifically, during the sealing secondary micro-cold rolling process, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes opposing plastic flow along the opening width direction of the sealable defect, and radially compacts the active sealing medium located within the sealable defect, so that the active sealing medium is covered, compacted, and locked inside the sealable defect. In this embodiment, the thickness of the formed plastic sealing layer is 0.015mm-0.03mm.

[0066] S4. Under conditions lower than the recrystallization temperature of the inner wall matrix of the steel pipe, the steel pipe treated in step S3 is subjected to low-temperature diffusion stabilization treatment, so that the active sealing medium in the filling and compaction zone covered by the plastic sealing layer forms a metallurgical bonding transition zone with the defect wall surface of the sealable defect and the plastic sealing layer, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0067] Specifically, the steel pipe after the sealing secondary micro-cold rolling is placed in a protective atmosphere furnace for low-temperature diffusion stabilization treatment. The protective atmosphere is nitrogen, the treatment temperature is 280°C, and the holding time is 120 minutes. In this embodiment, the treatment temperature is at least 50°C lower than the recrystallization temperature of the inner wall matrix of the steel pipe. After the low-temperature diffusion stabilization treatment, the active sealing medium in the filling and compaction zone forms a metallurgical bonding transition zone with the defect wall surface of the sealable defect and the plastic sealing layer, respectively, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0068] S5. The steel pipe processed in step S4 is calibrated and straightened with low deformation to obtain a high-strength precision hydraulic cold-rolled steel pipe. The shallow micro-defects with open openings on the inner wall of the high-strength precision hydraulic cold-rolled steel pipe are sealed by the filling and compaction zone, the plastic sealing layer, the metallurgical bonding transition zone and the residual compressive stress sealing zone to suppress the reopening and expansion of shallow micro-defects with open openings on the inner wall under pulsating hydraulic load.

[0069] Specifically, the steel pipes after low-temperature diffusion stabilization treatment undergo low-deformation calibrating and straightening. The inner diameter correction amount for low-deformation calibrating is less than 0.007 mm, which is less than 30% of the inner diameter reduction amount during sealing secondary micro-cold rolling. After calibrating and straightening, a high-strength precision hydraulic cold-rolled steel pipe with an inner diameter of 32.00 mm is obtained.

[0070] Cross-sectional observation revealed that the sealable defect area of ​​the high-strength precision hydraulic cold-rolled steel pipe forms a filling and compaction zone, a plastic sealing layer, and a metallurgical bonding transition zone. The inner wall near the surface adjacent to the plastic sealing layer forms a residual compressive stress sealing zone. The active sealing medium is only distributed in the filling and compaction zone and the metallurgical bonding transition zone, and the non-defect inner wall reference surface does not have a continuous sealing layer.

[0071] Example 3

[0072] This embodiment illustrates that, even with a large micro-cold rolling sealing allowance and large shallow defects on the inner wall, the present invention can still achieve selective defect filling, plastic sealing, metallurgical bonding, and maintenance of the finished product's inner diameter accuracy.

[0073] S1. Select a seamless steel pipe blank for hydraulic applications. After surface pretreatment, perform a one-time diameter and wall reduction forming using a mandrel-driven cold rolling process to obtain a cold-rolled steel pipe semi-finished product. The inner diameter of the cold-rolled steel pipe semi-finished product is controlled to be larger than the target inner diameter of the finished product, and a micro-cold rolling sealing allowance is reserved on the inner wall side of the cold-rolled steel pipe semi-finished product for subsequent defect sealing.

[0074] Specifically, 35CrMo seamless steel pipe blanks for hydraulic applications are selected and subjected to degreasing, pickling, cleaning, and drying. Then, a mandrel-driven cold rolling mill is used for a single-stage diameter and wall reduction forming process. The resulting cold-rolled steel pipe semi-finished product has an inner diameter of 60.14 mm, while the target inner diameter of the finished product is 60.00 mm. This leaves a 0.14 mm micro-cold-rolling sealing allowance on the inner wall side of the semi-finished product. The inner wall substrate is the original metal material on the inner wall side of the semi-finished cold-rolled steel pipe, excluding any subsequently applied active sealing medium.

[0075] S2. The inner wall of the cold-rolled steel pipe semi-finished product is cleaned, activated, and defect identified. Scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. Subsequently, an active sealing medium with diffusion bonding ability with the steel pipe inner wall matrix of the cold-rolled steel pipe semi-finished product is applied to the inner wall of the cold-rolled steel pipe semi-finished product, so that the active sealing medium enters into the sealable defects. Excess active sealing medium located on the non-defective inner wall reference surface of the cold-rolled steel pipe semi-finished product is removed, so that the active sealing medium remains in the sealable defects in a discontinuous state, and no continuous sealing layer is formed on the non-defective inner wall reference surface.

[0076] Specifically, the inner wall of the cold-rolled steel pipe semi-finished product is degreased and cleaned to remove free particles. The defective wall surface with sealable defects is activated to remove the oxide film. After activation, the inner wall is dried. Subsequently, industrial endoscopy and inner wall surface contour detection are used for defect identification. Defects that meet preset sealing conditions among scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. In this embodiment, the opening depth of the sealable defect is no greater than 0.08 mm, and the opening width is no greater than 0.20 mm.

[0077] Specifically, the active sealing medium is a paste-like medium formed by nickel-based metal powder and silicon-containing diffusion-activating components, with the metal powder having a particle size of 8μm-15μm. The active sealing medium is introduced into the cold-rolled steel pipe semi-finished product, allowing it to enter the sealable defect. Subsequently, excess active sealing medium located on the non-defective inner wall reference surface is removed by inner wall scraping and negative pressure suction, causing the active sealing medium to remain in a discontinuous state within the sealable defect. In this embodiment, the thickness of the active sealing medium remaining on the non-defective inner wall reference surface is no greater than 2μm.

[0078] S3. The cold-rolled steel pipe semi-finished product after step S2 is subjected to sealing secondary micro-cold rolling. The reduction in inner diameter of the sealing secondary micro-cold rolling is not greater than the micro-cold rolling sealing allowance. Through sealing secondary micro-cold rolling, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes plastic flow in opposite directions. The active sealing medium retained in the sealable defect is covered, compacted and locked inside the sealable defect, forming a filling and compaction zone and a plastic sealing layer covering the filling and compaction zone.

[0079] Specifically, the sealing secondary micro-cold rolling is carried out using a mandrel cold rolling method, controlling the inner diameter reduction of the sealing secondary micro-cold rolling to be 0.13 mm. This inner diameter reduction is less than the 0.14 mm micro-cold rolling sealing allowance and is 92.9% of the micro-cold rolling sealing allowance. At the same time, the single-pass wall reduction rate of the sealing secondary micro-cold rolling is less than the single-pass wall reduction rate of the single-pass diameter reduction and wall reduction forming.

[0080] Specifically, during the sealing secondary micro-cold rolling process, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes opposing plastic flow along the opening width direction of the sealable defect, and radially compacts the active sealing medium located within the sealable defect, so that the active sealing medium is covered, compacted, and locked inside the sealable defect. In this embodiment, the thickness of the formed plastic sealing layer is 0.06mm-0.10mm.

[0081] S4. Under conditions lower than the recrystallization temperature of the inner wall matrix of the steel pipe, the steel pipe treated in step S3 is subjected to low-temperature diffusion stabilization treatment, so that the active sealing medium in the filling and compaction zone covered by the plastic sealing layer forms a metallurgical bonding transition zone with the defect wall surface of the sealable defect and the plastic sealing layer, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0082] Specifically, the steel pipe, after undergoing a sealing secondary micro-cold rolling process, is placed in a protective atmosphere furnace for low-temperature diffusion stabilization treatment. The protective atmosphere is argon, the treatment temperature is 480°C, and the holding time is 60 minutes. In this embodiment, the treatment temperature is at least 50°C lower than the recrystallization temperature of the inner wall matrix of the steel pipe. After the low-temperature diffusion stabilization treatment, the active sealing medium in the filling and compaction zone forms a metallurgical bonding transition zone with the defect wall surface of the sealable defect and the plastic sealing layer, respectively, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0083] S5. The steel pipe processed in step S4 is calibrated and straightened with low deformation to obtain a high-strength precision hydraulic cold-rolled steel pipe. The shallow micro-defects with open openings on the inner wall of the high-strength precision hydraulic cold-rolled steel pipe are sealed by the filling and compaction zone, the plastic sealing layer, the metallurgical bonding transition zone and the residual compressive stress sealing zone to suppress the reopening and expansion of shallow micro-defects with open openings on the inner wall under pulsating hydraulic load.

[0084] Specifically, the steel pipes after low-temperature diffusion stabilization treatment undergo low-deformation calibrating and straightening. The inner diameter correction amount for low-deformation calibrating is less than 0.039 mm, which is less than 30% of the inner diameter reduction amount during the sealing secondary micro-cold rolling. After calibrating and straightening, a high-strength precision hydraulic cold-rolled steel pipe with an inner diameter of 60.00 mm is obtained.

[0085] Cross-sectional observation revealed that the sealable defect area of ​​the high-strength precision hydraulic cold-rolled steel pipe forms a filling and compaction zone, a plastic sealing layer, and a metallurgical bonding transition zone. The inner wall near the surface adjacent to the plastic sealing layer forms a residual compressive stress sealing zone. The active sealing medium is only distributed in the filling and compaction zone and the metallurgical bonding transition zone, and the non-defect inner wall reference surface does not have a continuous sealing layer.

[0086] Example 4

[0087] This embodiment has the same basic process flow as Embodiment 1. The difference is that the active filling medium is a suspension medium formed by iron-based metal powder and boron-containing diffusion activation components. The particle size of the iron-based metal powder is 0.5μm-6μm.

[0088] Specifically, after the inner wall of the cold-rolled steel pipe semi-finished product is cleaned, activated and defect identified, the iron-based active sealing medium is introduced into the inner wall of the steel pipe so that it enters the sealable defect; then, the excess active sealing medium on the non-defect inner wall reference surface is removed by negative pressure suction and directional airflow purging, so that the iron-based active sealing medium remains in the sealable defect in a discontinuous state, and no continuous sealing layer is formed on the non-defect inner wall reference surface.

[0089] Following this, a sealing secondary micro-cold rolling process is performed. The inner diameter reduction during this sealing secondary micro-cold rolling is 70%-90% of the sealing allowance. This causes the inner wall matrix of the steel pipe on both sides of the sealable defect opening to undergo opposing plastic flow, encapsulating and compacting the iron-based active sealing medium. A subsequent low-temperature diffusion stabilization treatment then forms a metallurgical bonding transition zone between the iron-based active sealing medium, the defect wall surface, and the plastic sealing layer.

[0090] Example 5

[0091] This embodiment has the same basic process flow as Embodiment 1. The difference is that the active filling medium is a paste-like medium formed by nickel-based metal powder and silicon-containing diffusion activation components. The particle size of the nickel-based metal powder is 6μm-15μm.

[0092] Specifically, after cleaning, activating and identifying defects on the inner wall of the cold-rolled steel pipe semi-finished product, a nickel-based active sealing medium is applied to the inner wall of the steel pipe, allowing it to enter into the sealable defects such as scratches or pits with relatively wide openings. Subsequently, excess active sealing medium on the non-defective inner wall reference surface is removed by inner wall scraping and negative pressure suction, so that the thickness of the active sealing medium remaining on the non-defective inner wall reference surface is no more than 2μm.

[0093] Following this, a sealing secondary micro-cold rolling process is performed, reducing the inner diameter by 80%-100% of the sealing allowance of the micro-cold rolling. This allows the nickel-based active sealing medium within the sealable defect to be encapsulated, compacted, and locked inside the defect. Subsequently, a low-temperature diffusion stabilization treatment is carried out, forming a metallurgical bonding transition zone between the nickel-based active sealing medium, the defect wall, and the plastic sealing layer. A residual compressive stress sealing zone is also formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer.

[0094] Example 6

[0095] The basic process flow of this embodiment is the same as that of Embodiment 1. The difference is that the active filling medium is a suspension medium or paste medium formed by iron-nickel-based metal powder and boron-silicon diffusion activation component. The particle size of the iron-nickel-based metal powder is 3μm-10μm.

[0096] Specifically, the iron-nickel-based active sealant is introduced into the inner wall of the cold-rolled steel pipe semi-finished product, allowing it to enter into sealable defects such as scratches, pits, or shallow microcracks; then, excess active sealant on the reference surface of the non-defect inner wall is removed by negative pressure suction, inner wall scraping, or directional airflow purging, so that the iron-nickel-based active sealant remains in the sealable defects in a discontinuous state.

[0097] Following this, a sealing secondary micro-cold rolling process is performed. The inner diameter reduction during this secondary micro-cold rolling is 60%-90% of the sealing allowance of the micro-cold rolling process. This causes the matrix of the inner wall of the steel pipe on both sides of the sealable defect opening to undergo opposing plastic flow, encapsulating, compacting, and locking the iron-nickel-based active sealing medium inside the sealable defect. Subsequently, a low-temperature diffusion stabilization treatment is carried out under a protective atmosphere or vacuum, allowing the iron-nickel-based active sealing medium to form a metallurgical bonding transition zone with the defect wall and the plastic sealing layer.

[0098] Example 7

[0099] This embodiment has the same basic process flow as Embodiment 1, the difference being that the low-temperature diffusion stabilization treatment uses process conditions of lower temperature and longer holding time.

[0100] Specifically, after completing the cold rolling process with retained allowance, selective filling of micro-defects on the inner wall, and sealing secondary micro-cold rolling, the steel pipe is placed in a protective atmosphere furnace for low-temperature diffusion stabilization treatment. The protective atmosphere is nitrogen, the treatment temperature is 240℃, the holding time is 120 min, and the treatment temperature is at least 50℃ lower than the recrystallization temperature of the inner wall matrix of the steel pipe.

[0101] After low-temperature diffusion stabilization treatment, a metallurgical bonding transition zone is formed between the active sealing medium in the filling and compaction zone, the defect wall of the sealable defect, and the plastic sealing layer. A residual compressive stress sealing zone is formed near the surface of the inner wall adjacent to the plastic sealing layer. This process is suitable for high-strength precision hydraulic cold-rolled steel pipes that require high maintenance of the cold-rolled strengthening state and have small sealable defect sizes.

[0102] Example 8

[0103] This embodiment has the same basic process flow as Embodiment 1, the difference being that the low-temperature diffusion stabilization treatment uses medium temperature and medium holding time process conditions.

[0104] Specifically, after the sealing secondary micro-cold rolling is completed, the steel pipe is placed in an argon protective atmosphere for low-temperature diffusion stabilization treatment. The treatment temperature is 360℃, the holding time is 90 min, and the treatment temperature is at least 50℃ lower than the recrystallization temperature of the matrix on the inner wall of the steel pipe.

[0105] After processing, a metallurgical bonding transition zone is formed between the active sealing medium, the defect wall of the sealable defect, and the plastic sealing layer. Simultaneously, a residual compressive stress sealing zone is formed near the surface of the inner wall adjacent to the plastic sealing layer. This process balances metallurgical bonding stability with the preservation of the cold-rolled structure, making it suitable for high-strength, precision hydraulic cold-rolled steel pipes of general specifications.

[0106] Example 9

[0107] This embodiment has the same basic process flow as Embodiment 1, the difference being that the low-temperature diffusion stabilization treatment uses process conditions of higher temperature and shorter holding time.

[0108] Specifically, after the sealing secondary micro-cold rolling, the steel pipe is placed in a vacuum furnace or an argon-protected atmosphere furnace for low-temperature diffusion stabilization treatment. The treatment temperature is 500℃, the holding time is 30 minutes, and the treatment temperature is at least 50℃ lower than the recrystallization temperature of the matrix on the inner wall of the steel pipe.

[0109] After processing, the active sealing medium in the compacted zone can form a metallurgical bonding transition zone with the defect wall and the plastic sealing layer, while maintaining the residual compressive stress sealing effect in the vicinity of the plastic sealing layer. This process is suitable for high-strength precision hydraulic cold-rolled steel pipes with relatively large sealable defects that require improved diffusion bonding efficiency.

[0110] Comparative Example 1

[0111] This comparative embodiment is based on the process conditions of Embodiment 1. The difference is that after the inner wall cleaning, activation and defect identification of the cold-rolled steel pipe semi-finished product are completed, no active sealing medium is applied to the sealable defects. Instead, the sealing secondary micro cold rolling, low temperature diffusion stabilization treatment, low deformation diameter correction and straightening are directly carried out.

[0112] Cross-sectional observation revealed that although the open area of ​​the sealable defect underwent some plastic deformation due to the secondary micro-cold rolling for sealing, and the defect tip was somewhat blunted, a filling and compaction zone was not formed inside the defect. Even after subsequent low-temperature diffusion stabilization treatment, a metallurgical bonding transition zone involving the active filling medium was not formed, and some open, shallow defects still had unfilled areas inside. This indicates that relying solely on secondary micro-cold rolling is insufficient to achieve sufficient sealing inside open, shallow defects, and the defects may reopen under pulsating hydraulic loads due to internal voids or lack of bonding interfaces. This invention improves the integrity of the sealing inside the defect by selectively introducing an active filling medium into the sealable defect before secondary micro-cold rolling, providing a compactable and lockable filling object for subsequent plastic flow.

[0113] Comparative Example 2

[0114] This comparative embodiment is based on the process conditions of Embodiment 1. The difference is that after the inner wall cleaning, activation and defect identification of the cold-rolled steel pipe semi-finished product are completed, an active sealing medium is applied to the sealable defect and the excess active sealing medium on the reference surface of the inner wall of the non-defect is removed. However, no sealing secondary micro cold rolling is performed. Instead, low temperature diffusion stabilization treatment, low deformation calibrating and straightening are performed directly.

[0115] Cross-sectional observation revealed that while the active sealing medium could penetrate part of the sealable defect, the active sealing medium was not fully encapsulated, compacted, and locked inside the defect because the matrix of the inner wall of the steel pipe on both sides of the defect opening did not undergo opposing plastic flow. Consequently, a stable plastic sealing layer did not form at the defect opening. This result indicates that simple sealing and low-temperature diffusion stabilization are insufficient to guarantee the mechanical locking state of the sealing medium inside the defect. The sealed area is at risk of loosening, peeling, or re-exposing the defect opening under high-pressure hydraulic scouring or alternating pressure. This invention utilizes a sealing-oriented secondary micro-cold rolling process to induce opposing plastic flow in the matrix of the inner wall of the steel pipe on both sides of the defect opening, encapsulating, compacting, and locking the active sealing medium inside the defect. This allows the sealed and compacted area and the plastic sealing layer to jointly perform the sealing function, improving the stability of the sealing structure.

[0116] Comparative Example 3

[0117] This comparative embodiment is based on the process conditions of Embodiment 1. The difference is that after applying the active sealing medium to the inner wall of the cold-rolled steel pipe semi-finished product, negative pressure suction, inner wall scraping or directional airflow purging are not performed, or only simple liquid drainage is performed, so that the active sealing medium forms a continuous or nearly continuous residual layer on the non-defect inner wall reference surface; then sealing secondary micro cold rolling, low temperature diffusion stabilization treatment, low deformation calibrating and straightening are performed.

[0118] Testing revealed a continuous or near-continuous layer of residual sealing medium on the non-defective inner wall reference surface. This residual layer affects the inner diameter accuracy and inner wall surface condition of the steel pipe. Compared to Example 1, the surface uniformity of the non-defective inner wall reference surface is poor, and there is a risk of residual medium peeling off or forming a secondary contamination source during subsequent use of the hydraulic system. This result indicates that if the active sealing medium does not remain primarily in a discontinuous state within the sealable defect, but instead forms a continuous layer on the non-defective inner wall reference surface, it weakens the requirements for inner diameter accuracy, inner wall cleanliness, and surface consistency of precision hydraulic cold-rolled steel pipes. This invention removes excess active sealing medium from the non-defective inner wall reference surface, allowing the active sealing medium to selectively remain within the sealable defect. This achieves local sealing of the defect area while avoiding the adverse effects of a continuous sealing layer on the inner wall reference surface of the steel pipe.

[0119] As can be seen from the above comparative examples, the active sealing medium, the sealing secondary micro-cold rolling, and the removal of excess medium from the non-defective inner wall reference surface are not simply parallel conventional steps, but rather work together to form a continuous defect sealing mechanism. The active sealing medium is used to enter the interior of the open-type shallow defect, providing a sealing basis for subsequent compaction and metallurgical bonding; the sealing secondary micro-cold rolling is used to cause the inner wall matrix of the steel pipe on both sides of the defect opening to undergo opposing plastic flow, covering, compacting, and locking the active sealing medium inside the defect; the removal of excess active sealing medium from the non-defective inner wall reference surface is used to avoid the formation of a continuous sealing layer, thereby maintaining the accuracy of the steel pipe's inner diameter and the quality of the inner wall reference surface.

[0120] Compared to solutions that do not apply an active sealing medium, do not undergo sealing secondary micro-cold rolling, or do not sufficiently remove excess active sealing medium, this invention can form a composite sealing structure in the defect area, consisting of a filling and compaction zone, a plastic sealing layer, a metallurgical bonding transition zone, and a residual compressive stress sealing zone. This structure not only improves the sealing integrity and interfacial bonding stability of shallow defects with open inner walls, but also reduces the adverse effects of continuous coating or excessive filling on the reference surface of the non-defective inner wall, thereby better meeting the requirements of high-strength precision hydraulic cold-rolled steel pipes for inner diameter accuracy, inner wall cleanliness, and service reliability under pulsating hydraulic loads.

[0121] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high-strength precision hydraulic cold-rolled steel pipe, characterized in that, include: S1. Select a seamless steel pipe blank for hydraulic applications. After surface pretreatment, the seamless steel pipe blank is subjected to a one-time diameter and wall reduction forming process using a mandrel-type cold rolling method to obtain a cold-rolled steel pipe semi-finished product. The inner diameter of the cold-rolled steel pipe semi-finished product is controlled to be larger than the target inner diameter of the finished product, and a micro-cold rolling sealing allowance is reserved on the inner wall side of the cold-rolled steel pipe semi-finished product for subsequent defect sealing. S2. The inner wall of the cold-rolled steel pipe semi-finished product is cleaned, activated, and defect identified. Scratches, pits, and shallow microcracks opening on the inner wall surface are identified as sealable defects. Subsequently, an active sealing medium with diffusion bonding ability with the steel pipe inner wall matrix of the cold-rolled steel pipe semi-finished product is applied to the inner wall of the cold-rolled steel pipe semi-finished product, so that the active sealing medium enters the sealable defect and removes the excess active sealing medium located on the non-defect inner wall reference surface of the cold-rolled steel pipe semi-finished product, so that the active sealing medium remains in the sealable defect in a discontinuous state, and the non-defect inner wall reference surface does not form a continuous sealing layer. S3. The cold-rolled steel pipe semi-finished product after step S2 is subjected to a sealing secondary micro-cold rolling. The reduction in inner diameter of the sealing secondary micro-cold rolling is not greater than the micro-cold rolling sealing allowance. Through the sealing secondary micro-cold rolling, the inner wall matrix of the steel pipe on both sides of the sealable defect opening undergoes plastic flow in opposite directions, which covers, compacts and locks the active sealing medium retained in the sealable defect inside the sealable defect, forming a filling and compaction zone and a plastic sealing layer covering the filling and compaction zone. S4. Under conditions lower than the recrystallization temperature of the inner wall matrix of the steel pipe, the steel pipe treated in step S3 is subjected to low-temperature diffusion stabilization treatment, so that the active sealing medium in the filling and compaction zone covered by the plastic sealing layer forms a metallurgical bonding transition zone with the defect wall of the sealable defect and the plastic sealing layer, and a residual compressive stress sealing zone is formed in the near-surface layer of the inner wall adjacent to the plastic sealing layer. S5. The steel pipe processed in step S4 is subjected to low deformation calibrating and straightening to obtain a high-strength precision hydraulic cold-rolled steel pipe. The shallow micro-defects with open openings on the inner wall of the high-strength precision hydraulic cold-rolled steel pipe are sealed by the filling and compaction zone, the plastic sealing layer, the metallurgical bonding transition zone and the residual compressive stress sealing zone to suppress the reopening and expansion of the shallow micro-defects with open openings on the inner wall under pulsating hydraulic load.

2. The high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The inner wall substrate of the steel pipe is the original metal material on the inner wall side of the cold-rolled steel pipe semi-finished product, and the inner wall substrate of the steel pipe does not include the active sealing medium; the micro cold rolling sealing allowance is the difference between the inner diameter of the cold-rolled steel pipe semi-finished product and the target inner diameter of the finished product, and the micro cold rolling sealing allowance is 0.02mm-0.15mm.

3. The high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The sealable defect is a shallow inner wall defect that meets preset sealing conditions after defect identification. The preset sealing conditions include: the opening depth of the sealable defect is not greater than 0.08 mm, and the opening width of the sealable defect is not greater than 0.20 mm.

4. The high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The cleaning includes degreasing and cleaning the inner wall of the cold-rolled steel pipe semi-finished product and removing free particles; the activation includes removing the oxide film on the defective wall surface of the sealable defect, and drying the inner wall of the cold-rolled steel pipe semi-finished product after activation.

5. A high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The active encapsulating medium is a paste-like or suspension-like medium formed by iron-based, nickel-based, or iron-nickel-based metal powder and a diffusion-activating component; the particle size of the metal powder is 0.5μm-15μm, and the diffusion-activating component includes boron-containing components, silicon-containing components, or boron-silicon-containing components.

6. A high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: After the active sealing medium enters the sealable defect, excess active sealing medium located on the reference surface of the non-defect inner wall is removed by negative pressure suction, inner wall scraping or directional airflow purging, so that the thickness of the active sealing medium remaining on the reference surface of the non-defect inner wall is no more than 2μm.

7. A high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The sealing secondary micro cold rolling is carried out by mandrel cold rolling. The inner diameter reduction of the sealing secondary micro cold rolling is 50%-100% of the sealing allowance of the micro cold rolling, and the single-pass wall reduction rate of the sealing secondary micro cold rolling is less than the single-pass wall reduction rate of the primary diameter and wall reduction forming.

8. A high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The sealing secondary micro-cold rolling causes the inner wall matrix of the steel pipe on both sides of the sealable defect opening to undergo opposing plastic flow along the opening width direction of the sealable defect, and radially compacts the active sealing medium in the sealable defect; the thickness of the plastic sealing layer is 0.01mm-0.10mm.

9. A high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The low-temperature diffusion stabilization treatment is carried out at a temperature of 220℃-520℃ and a holding time of 20min-180min. The temperature of the low-temperature diffusion stabilization treatment is at least 50℃ lower than the recrystallization temperature of the inner wall matrix of the steel pipe. The low-temperature diffusion stabilization treatment is carried out under a protective atmosphere or vacuum conditions.

10. A high-strength precision hydraulic cold-rolled steel pipe according to claim 1, characterized in that: The inner diameter correction amount of the low deformation calibrator is less than 30% of the inner diameter reduction amount of the sealing secondary micro cold rolling; in the obtained high-strength precision hydraulic cold-rolled steel pipe, the active sealing medium is only distributed in the sealing compaction zone and the metallurgical bonding transition zone, and the non-defect inner wall reference surface does not have a continuous sealing layer.

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