Preparation method of steel skeleton plastic composite pipe and steel skeleton plastic composite pipe

Abstract

The application discloses a preparation method of a steel skeleton plastic composite pipe and the steel skeleton plastic composite pipe and belongs to the technical field of pipes. The method is characterized in that: low-carbon steel wires are subjected to electrochemical etching to form a nano-scale porous structure, and are woven into a gradient porosity steel wire mesh with porosity gradually decreasing in the thickness direction of the pipe wall; the steel wire mesh is subjected to immersion in a molten mixed liquid containing maleic anhydride grafted high-density polyethylene and an initiator under ultrasonic vibration, so as to form a chemical bonding interface layer on the surface of the steel wire; and the steel wire mesh is sent into a co-extrusion die to fill high-density polyethylene melt under high pressure and is once composite formed to obtain a pipe blank, and the pipe blank is subjected to step temperature control cooling and heat treatment to obtain a finished product. The steel wire surface of the composite pipe is connected with the plastic matrix through a covalent chemical bond, and the steel wire end face at the pipe end is completely closed by the continuous plastic matrix. The application realizes synergistic effects in the aspects of interface bonding strength, corrosion resistance and thermal cycle stability.

Description

Technical Field

[0001] This invention relates to the field of pipeline technology, and in particular to a method for preparing a steel-reinforced plastic composite pipe and the steel-reinforced plastic composite pipe itself. Background Technology

[0002] Steel-reinforced plastic composite pipe is a type of composite pipe that uses welded steel wire mesh or spiral steel wire mesh as a reinforcing skeleton and thermoplastic plastic as the matrix material. It is widely used in municipal water supply and drainage, oilfield gathering and transportation, chemical corrosive media transportation, and marine engineering. This type of pipe combines the advantages of both metal and plastic pipes through a composite mechanism where the steel skeleton provides strength support and the plastic matrix provides corrosion resistance.

[0003] However, existing steel-reinforced plastic composite pipes have revealed several technical problems that affect the long-term operational reliability of pipelines in engineering applications.

[0004] Firstly, insufficient bonding strength at the steel-plastic interface is the most critical technical problem currently facing steel-reinforced plastic composite pipes. In existing technologies, the bond between the steel wire and the plastic matrix primarily relies on the physical adhesion of the hot-melt adhesive layer. Due to the smooth surface of the steel wire and the inherently difficult-to-bond nature of polyethylene, the adhesive strength between the hot-melt adhesive layer and the steel wire is limited, typically below 3.5 MPa. In actual operating conditions, the circumferential and axial stresses generated by the internal pressure of the pipe are transmitted to the steel wire reinforcement layer through the plastic matrix. The weak bond at the steel-plastic interface leads to low stress transmission efficiency, resulting in interface debonding. Statistical data shows that steel-plastic interface failure accounts for 53% of leakage accidents in steel-reinforced plastic composite pipes, and the problem of interface debonding is particularly prominent under operating pressures exceeding 1.6 MPa.

[0005] Secondly, exposed steel wires at the pipe ends and electrochemical corrosion severely impact the service life of pipelines. At the cut ends of steel-reinforced plastic composite pipes, the end faces of the steel wire skeleton are directly exposed to the transported medium or the external environment. When the pipeline transports fluids containing corrosive media such as H2S, CO2, and Cl⁻ (e.g., produced water from oil fields), the corrosive media penetrates along the gaps between the steel wire and plastic interface, triggering electrochemical corrosion of the steel wires. Corrosion reduces the cross-sectional area of ​​the steel wires, decreasing their load-bearing capacity, ultimately leading to pressure failure and even pipe bursting. Studies show that under sulfur-containing gas field water transport conditions, the oxidation induction time of steel-reinforced plastic composite pipes drops to 1.4 to 17 minutes, far below the standard requirement of over 20 minutes, indicating that material aging and media penetration have seriously affected the safety of pipeline operation.

[0006] Third, the difference in thermal expansion coefficients between the steel wire and the plastic matrix leads to the accumulation of residual stress at the interface under temperature cycling conditions. The linear expansion coefficient of low-carbon steel wire is approximately 1.2 × 10⁻⁻⁻⁶. 5 / ℃, while the linear expansion coefficient of high-density polyethylene is approximately 2.0×10⁻ 4 The temperature difference is approximately 16 times. When pipelines transport high-temperature media or experience diurnal temperature variations, the steel wire and plastic matrix undergo varying degrees of thermal expansion or contraction under these temperature changes, resulting in significant residual stress at the interface. As the number of temperature cycles increases, this residual stress accumulates, accelerating fatigue debonding at the steel-plastic interface. Studies show that in applications with significant temperature fluctuations, the adhesive strength of traditional hot melt adhesive layers can decrease by 30% to 40% after 1500 thermal cycles.

[0007] Fourth, while existing methods for manufacturing steel-reinforced plastic composite pipes alleviate the aforementioned problems to some extent, they still have significant shortcomings. Some existing technologies employ a multi-stage molding process: first, extruding the inner plastic layer; then applying hot melt adhesive; then winding steel wire; then applying hot melt adhesive again; and finally, extruding the outer plastic layer. This results in distinct physical interfaces between the pipe layers, making them prone to delamination under temperature changes or long-term pressure conditions. Other existing technologies, although employing simultaneous steel wire mesh welding and plastic extrusion, still maintain a mechanical coating relationship between the steel wire and plastic. During long-term service, temperature stress and internal pressure ring stress can still cause gaps at the metal-plastic interface. When the pipe ends are poorly sealed, interlayer water seepage can occur, leading to corrosion of the steel wire reinforcement layer.

[0008] In summary, there are still clear technical requirements in the field of steel-reinforced plastic composite pipe manufacturing technology for substantially improving the interfacial bonding strength between the steel wire reinforced skeleton and the plastic matrix, eliminating the causes of electrochemical corrosion on the end face of the steel wire at the pipe end, and effectively suppressing the residual stress at the interface under temperature cycling conditions. There is an urgent need to provide a method for manufacturing steel-reinforced plastic composite pipe that can achieve breakthrough improvements in the above aspects simultaneously. Summary of the Invention

[0009] To achieve the above objectives, the present invention provides a method for preparing a steel-reinforced plastic composite pipe, comprising the following steps:

[0010] Step 1: An electrolyte solution prepared by mixing 8% to 12% oxalic acid solution and 2% to 5% hydrofluoric acid solution at a volume ratio of 3:1 is used to perform electrochemical etching treatment with low carbon steel wire as the anode, so that a nanoscale porous structure is formed on the surface of the low carbon steel wire, and a pretreated steel wire is obtained.

[0011] Step 2: Weave the pretreated steel wire into a wire mesh, and make the wire mesh spacing decrease gradually from the outer layer to the inner layer along the radial direction of the pipe, forming a gradient porosity wire mesh with porosity decreasing gradually from the outer layer to the inner layer.

[0012] Step 3: The gradient porosity steel wire mesh is continuously impregnated under ultrasonic vibration conditions through a molten mixture containing maleic anhydride-grafted high-density polyethylene and dicumyl peroxide initiator at a temperature of 170°C to 190°C. This allows the maleic anhydride groups to undergo an in-situ grafting polymerization reaction with the hydroxyl groups on the nanoscale porous structure of the pretreated steel wire surface, forming a chemically bonded interface layer on the surface of the pretreated steel wire, thus obtaining a gradient porosity steel wire mesh after interface grafting treatment.

[0013] Step 4: The gradient porosity steel wire mesh after the interface grafting treatment is fed into the co-extrusion die. High-density polyethylene melt is filled into the gradient porosity steel wire mesh under an extrusion pressure of 10 MPa to 18 MPa. At the same time, an inner plastic layer is extruded on the inner side of the gradient porosity steel wire mesh and an outer plastic layer is extruded on the outer side. The tube blank is obtained by one composite molding.

[0014] Step 5: The tube blank is subjected to stepped temperature control cooling in a first cooling section of 50℃ to 60℃, a second cooling section of 25℃ to 35℃, and a third cooling section of 10℃ to 15℃, and then heat-treated at 100℃ to 120℃ for 4 to 8 hours, and naturally cooled to obtain a steel-reinforced plastic composite pipe.

[0015] Preferably, the diameter of the low-carbon steel wire in step 1 is 2.5 mm to 4.0 mm; the current density of the electrochemical etching treatment is 0.3 A / cm² to 0.8 A / cm², the electrolyte temperature is 25°C to 45°C, and the treatment time is 60 seconds to 180 seconds; after the electrochemical etching treatment is completed, the low-carbon steel wire is taken out of the electrolyte, rinsed with deionized water 3 to 5 times, and dried at a temperature of 80°C to 100°C for 30 minutes to 60 minutes to obtain the pretreated steel wire.

[0016] Preferably, in step 2, the braiding area is sequentially divided into a first braiding section, a second braiding section, and a third braiding section along the pipe axis. The third braiding section corresponds to the inner wall region of the finished steel-reinforced plastic composite pipe, the first braiding section corresponds to the outer wall region of the finished steel-reinforced plastic composite pipe, and the second braiding section is located between the first and third braiding sections. The wire mesh spacing of the first braiding section is set to 3.0 mm to 4.0 mm, the wire mesh spacing of the second braiding section is set to 2.0 mm to 3.0 mm, and the wire mesh spacing of the third braiding section is set to 1.0 mm to 2.0 mm. During the braiding process, the warp tension is 80 Newtons to 120 Newtons, and the weft tension is 60 Newtons to 100 Newtons. The porosity of the gradient porosity wire mesh decreases gradually from the first braiding section to the third braiding section, and the porosity of the third braiding section is 30% to 60% of the porosity of the first braiding section.

[0017] Preferably, the preparation method of the melt mixture containing maleic anhydride-grafted high-density polyethylene and dicumyl peroxide initiator in step 3 is as follows: 100 parts by weight of maleic anhydride-grafted high-density polyethylene and 0.3 to 0.8 parts by weight of dicumyl peroxide initiator are mixed in a high-speed mixer at a speed of 800 to 1200 rpm for 10 to 15 minutes to obtain a mixture. The mixture is then fed into a twin-screw extruder, and the mixture is extruded at a screw speed of 60 to 100 rpm. The mixture is melt-extruded at 0 rpm and a temperature of 170°C to 190°C, and directly introduced into an ultrasonic-assisted impregnation tank to form the melt mixture. During impregnation, the ultrasonic frequency of the ultrasonic generator is 20 kHz to 40 kHz, the ultrasonic power density is 0.5 W / cm² to 1.5 W / cm², the traction speed of the gradient porosity steel wire mesh is 0.3 m / min to 0.8 m / min, and the impregnation time is 2 minutes to 5 minutes. The thickness of the chemically bonded interface layer formed is 5 micrometers to 20 micrometers.

[0018] Preferably, in step 4, the melt index of the high-density polyethylene melt being filled is 0.2 g / 10 min to 0.5 g / 10 min, and the melt index is measured according to GB / T3682.1-2018 standard under conditions of 190°C and 2.16 kg load; the thickness of the inner plastic layer is 2.0 mm to 4.0 mm, and the thickness of the outer plastic layer is 2.5 mm to 5.0 mm; the die temperature of the co-extrusion die is 190°C to 200°C, and the traction speed is 0.5 m / min to 1.5 m / min.

[0019] Preferably, in step 5, the cooling time in the first cooling section is 30 to 60 seconds, the cooling time in the second cooling section is 60 to 120 seconds, and the cooling time in the third cooling section is 30 to 60 seconds, with water being the cooling medium in all three cooling sections.

[0020] Preferably, after obtaining the pretreated steel wire in step 1 and before step 2, the method further includes: scanning the surface of the pretreated steel wire using an atomic force microscope to collect the average pore size parameters and average pore depth parameters of the nanoscale porous structure on the surface of the pretreated steel wire; in step 3, adjusting the ultrasonic power density and impregnation time according to the collected average pore size parameters and average pore depth parameters so that the thickness of the chemical bonding interface layer is in the range of 5 micrometers to 20 micrometers.

[0021] The steel-reinforced plastic composite pipe includes an inner plastic layer, a steel reinforcement layer, and an outer plastic layer. The steel reinforcement layer is a gradient porosity steel wire mesh. There are chemically bonded interface layers between the surface of the low-carbon steel wire constituting the gradient porosity steel wire mesh and the high-density polyethylene matrix of the inner plastic layer, and between the surface of the low-carbon steel wire and the high-density polyethylene matrix of the outer plastic layer. The high-density polyethylene matrix of the inner plastic layer, the chemically bonded interface layer, and the high-density polyethylene matrix of the outer plastic layer are continuously and seamlessly connected throughout the entire area of ​​the steel reinforcement layer to completely seal the end faces of the low-carbon steel wires.

[0022] Preferably, the gradient porosity wire mesh is composed of a first braided section, a second braided section, and a third braided section arranged radially from the outside to the inside of the pipe. The wire mesh spacing of the first braided section is 3.0 mm to 4.0 mm, the wire mesh spacing of the second braided section is 2.0 mm to 3.0 mm, and the wire mesh spacing of the third braided section is 1.0 mm to 2.0 mm. The porosity of the third braided section is 30% to 60% of the porosity of the first braided section. The thickness of the chemically bonded interface layer is 5 μm to 20 μm. The thickness of the inner plastic layer is 2.0 mm to 4.0 mm, and the thickness of the outer plastic layer is 2.5 mm to 5.0 mm.

[0023] Preferably, at the pipe cutting end, the end faces of all the low-carbon steel wires in the steel skeleton reinforcement layer are completely sealed and covered by a continuous high-density polyethylene matrix through which the inner plastic layer and the outer plastic layer are connected.

[0024] The beneficial effects of this invention are:

[0025] 1. By combining the synergistic effect of electrochemical etching of steel wire surface to construct a nanoscale porous structure and in-situ grafting polymerization reaction at the interface, a covalent chemical bond interface layer with a thickness of 5μm to 20μm is formed between the low carbon steel wire surface and the high density polyethylene matrix. The steel-plastic interface bonding strength reaches more than 6.0MPa, which is more than 60% higher than the traditional hot melt adhesive physical bonding. This fundamentally solves the problem of interface debonding and pipeline failure caused by steel skeleton plastic composite pipe under high pressure conditions.

[0026] 2. By linking the design of the gradient porosity steel wire mesh with the high-pressure co-extrusion process, the plastic melt is driven by the extrusion pressure to fully fill the pores of all woven sections of the steel wire mesh, achieving a pore filling rate of over 99%. With the cooperation of the chemically bonded interface layer, all steel wire end faces at the pipe cutting ends are completely sealed and covered by a continuous high-density polyethylene matrix, blocking the penetration channels of corrosive media along the interface. After the corrosion test, the percentage of the remaining cross-sectional area of ​​the steel wire exceeds 99%.

[0027] 3. The chemically bonded interface layer adapts to the differential thermal expansion between the steel wire and the high-density polyethylene matrix through molecular chain segment conformation adjustment. Combined with the residual stress release effect of stepped temperature control cooling and heat treatment, the interface bonding strength retention rate of the pipe reaches more than 90% after 1000 thermal cycles. This effectively solves the fatigue debonding problem caused by the accumulation of residual stress at the interface under temperature cycling conditions, and significantly improves the long-term reliability of the pipeline. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a flowchart of the steps of the method of the present invention. Detailed Implementation

[0030] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. It should also be noted that, to make the embodiments more comprehensive, the following embodiments are the best and preferred embodiments, and those skilled in the art can use other alternative methods to implement some well-known technologies; moreover, the accompanying drawings are only for more specific description of the embodiments and are not intended to specifically limit the present invention.

[0031] Please see Figure 1 This invention provides a method for preparing a steel-reinforced plastic composite pipe and the pipe itself. The application scenario for this embodiment is an oilfield produced water transportation pipeline, transporting media containing H2S, CO2, and Cl⁻, with an operating pressure of 2.0 MPa, an operating temperature of 45℃ to 65℃, an ambient day-night temperature difference of up to 25℃, and a designed service life of 20 years. This application scenario directly corresponds to the described oilfield gathering and transportation of corrosive media, and places strict requirements on the steel-plastic interface bonding strength, electrochemical corrosion resistance, and thermal cycling stability.

[0032] Example

[0033] This embodiment is performed according to the following steps in sequence, and the step numbers correspond to the preparation method flow of the present invention:

[0034] Step 1: Nanostructuring pretreatment of steel wire surface;

[0035] Low-carbon steel wire with a diameter of 3.2 mm was selected. The grade of the low-carbon steel wire was Q195, with a carbon content of 0.08%, a manganese content of 0.35%, a silicon content of 0.20%, a sulfur content not exceeding 0.030%, a phosphorus content not exceeding 0.030%, and the remainder being iron and unavoidable impurities.

[0036] First, the surface of the low-carbon steel wire is degreased: the low-carbon steel wire is immersed in an 8% sodium hydroxide solution at 70℃ for 20 minutes. The purpose of degreasing is to remove residual lubricating grease and rust-preventive oil from the surface of the low-carbon steel wire during the drawing process, so that the subsequent pickling treatment can act evenly on the metal surface. After the degreasing treatment is completed, the low-carbon steel wire is removed and the alkaline solution on the surface is drained.

[0037] Next, the degreased low-carbon steel wire undergoes pickling: the wire is immersed in a 12% hydrochloric acid solution for 8 minutes. The purpose of pickling is to remove the rolling scale and rust from the surface of the wire, exposing the fresh metal surface. After pickling, the wire is removed.

[0038] Then, the pickled low-carbon steel wire is washed with water: the low-carbon steel wire is repeatedly rinsed with deionized water 4 times to remove the residual acid and pickling reaction products on the surface. After rinsing, the low-carbon steel wire is placed in an oven and dried at 90°C.

[0039] The washed and dried low-carbon steel wire is fed into an electrochemical etching tank as the anode. The electrolyte in the electrochemical etching tank is prepared by mixing 10% oxalic acid solution and 3% hydrofluoric acid solution in a volume ratio of 3:1. The role of the oxalic acid solution is to form a soluble complex on the surface of the low-carbon steel wire through the complexation of oxalate ions and iron ions, achieving uniform anodic dissolution. The role of the hydrofluoric acid solution is to locally pit and break down the passivation film formed on the surface of the low-carbon steel wire due to anodizing through fluoride ions, promoting the formation of a nanoscale porous structure. A lead plate is used as the cathode in the electrochemical etching tank. The purity of the lead plate is not less than 99.9%, and the ratio of the lead plate area to the surface area of ​​the low-carbon steel wire is controlled at 2:1. During the electrolysis process, a DC regulated power supply is used to apply the current, with the current density set at 0.5 A / cm². The electrolyte temperature is controlled at 35°C by a constant temperature circulating water bath, and the etching time is 120 s. After the electrochemical etching process is completed, the low-carbon steel wire is taken out of the electrochemical etching tank, and the surface of the low-carbon steel wire is immediately rinsed 5 times with deionized water. Then it is dried in an oven at 100°C for 45 minutes to obtain the pretreated steel wire.

[0040] The pretreated steel wire obtained after this step exhibits a nanoscale porous structure on its surface. The formation mechanism of this nanoscale porous structure is as follows: In an oxalic acid-hydrofluoric acid mixed electrolyte, the iron on the surface of the low-carbon steel wire undergoes electrochemical dissolution under anodic polarization. Oxalate ions react with the dissolved iron ions to form a soluble ferrous oxalate complex that continuously detaches from the metal surface. Meanwhile, fluoride ions in the hydrofluoric acid, under the influence of the current, precisely penetrate the surface oxide layer. The synergistic effect of these two processes results in a uniformly distributed nanoscale pit and channel structure on the surface of the low-carbon steel wire. The pore size of the nanoscale porous structure ranges from 150 nm to 450 nm, and the pore depth ranges from 80 nm to 250 nm.

[0041] Step 2: Weaving of gradient porosity wire mesh;

[0042] The pretreated steel wire obtained in step 1 is braided using a braiding machine in a warp and weft braiding manner.

[0043] The braided area is divided into three sections along the pipe axis: a first braided section, a second braided section, and a third braided section. The first braided section corresponds to the outer wall region of the finished steel-reinforced plastic composite pipe, meaning it is located on the outermost side of the pipe's radial direction. The third braided section corresponds to the inner wall region of the finished steel-reinforced plastic composite pipe, meaning it is located on the innermost side of the pipe's radial direction. The second braided section is located between the first and third braided sections, corresponding to the middle region of the finished steel-reinforced plastic composite pipe.

[0044] In the first weaving section, the wire mesh spacing in both the warp and weft directions is set to 3.5 mm. In the second weaving section, the wire mesh spacing in both the warp and weft directions is set to 2.5 mm. In the third weaving section, the wire mesh spacing in both the warp and weft directions is set to 1.5 mm.

[0045] During the weaving process, the warp feeding mechanism's tension is set to 100N, and the weft feeding mechanism's tension is set to 80N. The feeding tension is monitored in real-time by a tension sensor, and the tension deviation is controlled within ±5N. After weaving, the measured porosity of the first weaving section is 48%, the second weaving section is 32%, and the third weaving section is 18%. The porosity of the third weaving section is 37.5% of that of the first weaving section, falling within the range of 30% to 60%, which meets the requirement of a gradual decrease. Here, porosity is defined as the percentage of void volume in the wire mesh relative to the total volume of the weaving section.

[0046] The decreasing porosity gradient serves several purposes: In the subsequent co-extrusion process (step 4), the third braided section has the smallest mesh spacing, resulting in the greatest local flow resistance encountered by the molten plastic in this section. Under high extrusion pressures of 10 MPa to 18 MPa, the melt is fully compressed and filled into the fine mesh of the third braided section. The first braided section has the largest mesh spacing, minimizing local flow resistance and allowing the melt to quickly fill and expand outwards, forming a continuous transition with the outer plastic layer. The second braided section acts as a transitional connector. By setting a porosity gradient across the three braided sections, the molten plastic is guided to form a directional filling channel that gradually accelerates from the inner to the outer layer along the pipe wall thickness direction. This ensures that all pores in the braided sections are fully filled by the molten plastic, eliminating the microscopic void defects caused by poor melt flow at the wire intersections of traditional uniform wire mesh.

[0047] Step 3: In-situ interfacial polymerization reaction;

[0048] The gradient porosity wire mesh woven in step 2 is then fed into an ultrasonic-assisted impregnation tank.

[0049] An ultrasonically assisted impregnation tank contains a molten mixture of maleic anhydride-grafted high-density polyethylene (HDPE) and dicumyl peroxide initiator at a temperature of 180°C. The molten mixture is prepared as follows: 100 parts by weight of maleic anhydride-grafted HDPE with a maleic anhydride grafting rate of 1.8% are accurately weighed; 0.5 parts by weight of dicumyl peroxide initiator with a purity of ≥99.0% are accurately weighed. Both are then placed in a high-speed mixer and mixed at 1000 rpm for 12 minutes to obtain a homogeneous mixture. The mixture is fed into a twin-screw extruder, with the screw speed set at 80 r / min. The temperatures of each section of the extruder, from the feeding section to the extrusion section, are set to 170℃, 175℃, 180℃, 185℃, and 190℃ respectively. The mixture is melt-extruded in the twin-screw extruder and directly introduced into an ultrasonic-assisted impregnation tank to form a molten mixture. The twin-screw extruder has a length-to-diameter ratio (L / D) of 40:1 and a screw diameter of 35 mm.

[0050] In an ultrasonic-assisted impregnation tank, an ultrasonic generator applies ultrasonic vibration to the molten mixture. The ultrasonic frequency is set to 28 kHz, and the ultrasonic power density is set to 1.0 W / cm². Ultrasonic transducers are installed at the bottom and side walls of the ultrasonic-assisted impregnation tank to ensure uniform distribution of ultrasonic energy in the molten mixture. Gradient porosity steel wire mesh is continuously passed through the ultrasonic-assisted impregnation tank at a traction speed of 0.5 m / min, and the impregnation time is 3.5 min.

[0051] Under ultrasonic cavitation and thermal initiation conditions, dicumyl peroxide decomposes at temperatures between 170°C and 190°C to generate free radicals. These free radicals initiate an in-situ grafting polymerization reaction between the maleic anhydride groups on the maleic anhydride-grafted high-density polyethylene molecular chains and the hydroxyl groups on the nanoscale porous structure of the pretreated steel wire surface from step 1. The hydroxyl groups on the nanoscale porous structure originate from the iron hydroxyl chemisorption layer formed on the surface of the low-carbon steel wire after electrochemical etching due to water molecule adsorption and surface oxidation. Ultrasonic vibration plays two roles in this reaction process: firstly, it generates a localized high-temperature and high-pressure micro-region (local temperature can reach 5000K, and local pressure can reach 5×10⁻⁶ K). 7 (1) The ultrasonic vibration can instantly destroy the gas layer and pollutant layer adsorbed on the surface of low carbon steel wire, so that the reaction sites of maleic anhydride groups and hydroxyl groups are fully exposed; (2) The micro-jet and acoustic flow effect generated by ultrasonic vibration can form forced convection in the molten mixture inside the pores of the nanoscale porous structure, overcome the mass transfer resistance caused by the scale effect inside the pores, and allow the maleic anhydride grafted high-density polyethylene molecular chains to fully penetrate into the depths of the nanoscale pores and effectively graft bond with the hydroxyl groups on the pore walls.

[0052] After the reaction is complete and a chemically bonded interface layer is formed on the surface of the pretreated steel wire, a gradient porosity steel wire mesh with interface grafting treatment is obtained. The measured thickness of the chemically bonded interface layer is 12 μm, which meets the requirement of a thickness ranging from 5 μm to 20 μm. The formation of the chemically bonded interface layer indicates that the relationship between the steel wire surface and the plastic matrix is ​​no longer a physical adhesion relationship as in traditional technologies, but a chemically bonded relationship connected by covalent chemical bonds.

[0053] The graft polymerization chemical reaction of the chemically bonded interface layer is as follows: cumene peroxide decomposes under heating conditions to generate cumeneoxy radicals.

[0054] (CH3)2C(C6H5)-OOC(C6H5)(CH3)2—isopropylphenoxy free radical abstracts tertiary hydrogen atoms from maleic anhydride-grafted high-density polyethylene molecular chains, generating macromolecular free radicals:

[0055]

[0056] Macromolecular free radicals undergo a grafting reaction with hydroxyl groups on the nanoscale porous structure of low-carbon steel wire, forming a covalently bonded chemically bonded interface layer:

[0057]

[0058] In the reaction formula, PE-g-MAH represents maleic anhydride grafted onto high-density polyethylene molecular chains, Fe (surface) represents iron atoms on the surface of low-carbon steel wire, and —O—Fe represents covalent chemical bonds formed through oxygen bridges.

[0059] Step 4: Multilayer co-extrusion composite

[0060] The gradient porosity wire mesh obtained from step 3 after interface grafting is continuously fed into the co-extrusion composite mold.

[0061] The co-extrusion die has three extruders: a main extruder, an inner extruder, and an outer extruder. All three extruders are single-screw extruders with a screw length-to-diameter ratio (L / D) of 33:1 and a screw diameter of 65 mm. The main extruder, inner extruder, and outer extruder are connected to different flow channel inlets of the co-extrusion die.

[0062] The main extruder injects high-density polyethylene (HDPE) melt at 195°C into the reinforcing layer filling area of ​​the co-extrusion composite die through the main extruder channel, with an extrusion pressure of 15 MPa. Driven by the 15 MPa extrusion pressure, the HDPE melt fills all the pores in the first, second, and third braided sections of the gradient porosity wire mesh along the pipe wall thickness direction from the outer layer to the inner layer. The HDPE resin used in the main extruder has a melt index of 0.35 g / 10 min, measured according to GB / T3682.1-2018 standard under conditions of 190°C and 2.16 kg load. The HDPE resin has a density of 0.951 g / cm³, a tensile yield stress of 24.5 MPa, and a nominal strain at break of 680%.

[0063] The inner extruder injects high-density polyethylene melt at 195℃ into the inner channel of the co-extrusion composite die, forming an inner plastic layer with a thickness of 3.0 mm inside the gradient porosity steel wire mesh. The extrusion pressure of the inner extruder is 8 MPa.

[0064] The outer extruder injects high-density polyethylene melt at 195℃ into the outer channel of the co-extrusion composite die, forming an outer plastic layer with a thickness of 3.5mm on the outside of the gradient porosity steel wire mesh. The extrusion pressure of the outer extruder is 8MPa.

[0065] The inner plastic layer, the gradient porosity steel wire mesh reinforcement layer filled with high-density polyethylene melt, and the outer plastic layer are composited in a single step within the compression and shaping sections of the co-extrusion die. The co-extrusion die has a die diameter of 219 mm and a die temperature of 195 °C. The traction speed is 1.0 m / min. The compression ratio of the internal flow channel of the co-extrusion die is 2.5:1, and the shaping section length is 80 mm. A vacuum sizing sleeve with a vacuum degree of -0.03 MPa is installed at the die orifice of the co-extrusion die to precisely control the outer diameter of the pipe.

[0066] In the high-pressure co-extrusion process of step 4, the gradient porosity steel wire mesh constructed in step 2 plays a crucial role in guiding differentiated filling. The third braided section has the smallest wire mesh spacing, and the melt exhibits the greatest abrupt change in flow channel cross-section in this section. Under an extrusion pressure of 15 MPa, it generates a high local shear rate. The apparent viscosity of the high-density polyethylene melt decreases due to the shear thinning effect, which facilitates the melt's full penetration into the fine mesh of the third braided section. The first braided section has the largest wire mesh spacing and the lowest melt flow resistance, allowing the high-density polyethylene melt to quickly pass through the first braided section and uniformly coat the wire surface. The pore filling status of the three braided sections was verified by microscopic observation of the cross-section of the extruded pipe. The pore filling rates of the first, second, and third braided sections were 99.5%, 99.3%, and 99.1%, respectively.

[0067] Step 5: Stepped temperature controlled cooling and shaping, and heat treatment;

[0068] The tube blank obtained in step 4 is fed into a stepped temperature-controlled cooling device. The stepped temperature-controlled cooling device is divided into a first cooling section, a second cooling section, and a third cooling section along the direction of tube blank travel. Each of the three cooling sections is equipped with an independent circulating cooling water spray system and a temperature control unit.

[0069] The cooling medium in the first cooling section is water, with a temperature set at 55℃ and a cooling time of 45 seconds. The purpose of the first cooling section is to allow the surface temperature of the high-temperature tube blank, which has just been extruded from the die at 195℃, to decrease gradually, avoiding a large temperature gradient between the outer and inner layers of the tube caused by sudden cooling, thereby preventing compressive thermal stress on the inner layer due to the outer layer shrinking first.

[0070] The cooling medium in the second cooling section is water, with a temperature set at 30°C and a cooling time of 90 seconds. The purpose of this second cooling section is to further reduce the overall temperature of the pipe to near the crystallization temperature range of high-density polyethylene (HDPE), within which the HDPE matrix begins to crystallize and solidify. The second cooling section uses a lower cooling rate to ensure the HDPE crystallization process proceeds sufficiently, avoiding insufficient crystallinity and uneven grain size caused by rapid cooling, which would affect the mechanical properties of the pipe.

[0071] The cooling medium in the third cooling section is water, with a temperature set at 12°C and a cooling time of 45 seconds. The function of the third cooling section is to perform final cooling and shaping of the pipe after the high-density polyethylene matrix has completed its main crystallization process, bringing the pipe to near room temperature and fixing its final shape and size.

[0072] After the tube blank undergoes stepped cooling and shaping through the first, second, and third cooling sections, the shaped tube is sent to a heat treatment unit. The heat treatment unit is a hot air circulating oven, with the temperature uniformity controlled within ±2℃. The heat treatment temperature is set at 110℃, and the heat treatment time is 6 hours.

[0073] The mechanism and purpose of the heat treatment process are as follows: During the co-extrusion process in step 4, when the high-density polyethylene melt passes through the die under extrusion pressure, the polyethylene molecular chains align along the extrusion direction. After cooling and solidification, the orientation of the molecular chains is partially frozen within the pipe, forming residual orientation stress. Simultaneously, during the transition from the molten state to the crystalline state, high-density polyethylene also generates crystalline stress due to the density difference between the crystalline and amorphous phases. Heating the pipe to 110°C, which is above the crystallization temperature of high-density polyethylene but far below the melting temperature, allows the polyethylene molecular chains sufficient mobility, relaxes the frozen orientation segments, and releases the residual orientation stress. Simultaneously, at 110°C, the maleic anhydride-grafted high-density polyethylene molecular chains within the chemically bonded interface layer formed in step 3 undergo further thermal motion adjustment, promoting the continued grafting and solidification reaction between the unreacted maleic anhydride groups in the chemically bonded interface layer and the residual hydroxyl groups on the surface of the low-carbon steel wire, thereby improving the crosslinking density and uniformity of the chemically bonded interface layer.

[0074] After heat treatment, the pipe is naturally cooled to room temperature in the heat treatment apparatus at a rate of 0.5℃ / min. After natural cooling, the finished steel-reinforced plastic composite pipe is obtained.

[0075] The steel-reinforced plastic composite pipe prepared through steps 1 to 5 above has the following structural characteristics: The steel-reinforced plastic composite pipe consists of an inner plastic layer, a steel reinforcement layer, and an outer plastic layer. The inner plastic layer is a high-density polyethylene matrix with a thickness of 3.0 mm. The outer plastic layer is a high-density polyethylene matrix with a thickness of 3.5 mm. The steel reinforcement layer is a gradient porosity steel wire mesh woven from low-carbon steel wires with a nanoscale porous structure on its surface. A 12 μm thick chemically bonded interface layer exists between the surface of the low-carbon steel wire constituting the gradient porosity steel wire mesh and the high-density polyethylene matrix of the inner plastic layer, and between the surface of the low-carbon steel wire and the high-density polyethylene matrix of the outer plastic layer. This chemically bonded interface layer is composed of a covalent bond structure formed by the in-situ graft polymerization reaction of maleic anhydride-grafted high-density polyethylene with hydroxyl groups on the nanoscale porous structure of the low-carbon steel wire surface. The high-density polyethylene matrix of the inner plastic layer, the chemically bonded interface layer, and the high-density polyethylene matrix of the outer plastic layer are a continuous whole, and all three are seamlessly connected throughout the entire area of ​​the steel skeleton reinforcement layer, including the pipe cutting end. At the pipe cutting end, the end faces of all low-carbon steel wires in the steel skeleton reinforcement layer are completely sealed and covered by the continuous high-density polyethylene matrix that runs through the inner and outer plastic layers, and there is no situation where the end faces of low-carbon steel wires are exposed on the surface of the cutting end.

[0076] Comparative example;

[0077] To verify the technical effect of this embodiment, Comparative Example 1 and Comparative Example 2 are set up for comparison.

[0078] The preparation method of Comparative Example 1 is as follows: Low-carbon steel wire with a diameter of 3.2 mm, the same as in this example, is selected. No electrochemical etching nanostructure pretreatment is performed; only conventional degreasing, pickling, and water washing are applied to the surface of the low-carbon steel wire. Then, a 0.3 mm thick ethylene-vinyl acetate copolymer hot melt adhesive layer is coated onto the surface of the steel wire using a hot melt adhesive coating process. The hot melt adhesive-coated low-carbon steel wire is woven into a uniform mesh steel wire mesh with a uniform mesh spacing of 2.5 mm. The uniform mesh steel wire mesh is fed into the same co-extrusion die as in this example. High-density polyethylene melt is filled into the uniform mesh steel wire mesh under an extrusion pressure of 8 MPa. Simultaneously, an inner plastic layer with a thickness of 3.0 mm is extruded on the inner side of the steel wire mesh, and an outer plastic layer with a thickness of 3.5 mm is extruded on the outer side. After extrusion, the billet is cooled in water at 25°C using a conventional single-stage cooling method for 180 s. After cooling, the steel-reinforced plastic composite pipe is obtained by heat treatment at 110℃ for 6 hours and natural cooling.

[0079] The preparation method of Comparative Example 2 is as follows: except for omitting the interfacial in-situ polymerization reaction in step 3, the remaining steps are the same as in this embodiment. Specifically, after performing nanostructural pretreatment on the surface of the steel wire, a gradient porosity steel wire mesh is woven, and then the gradient porosity steel wire mesh is directly fed into a co-extrusion die for co-extrusion composite, without the ultrasonic-assisted impregnation and in-situ graft polymerization reaction steps, and the remaining process parameters are consistent with those in this embodiment.

[0080] The following performance tests were conducted on the steel-reinforced plastic composite pipes prepared in this embodiment, Comparative Example 1, and Comparative Example 2: The interlayer bond strength test of the steel-plastic interface was performed according to the interlayer bond strength test method in CJ / T189-2007 standard. A 25mm wide annular sample was cut from the pipe and subjected to a peel test at a speed of 10mm / min on a universal testing machine. The maximum force during interface peeling was recorded, and the interlayer bond strength value was obtained by dividing it by the sample width. The corrosion resistance test employed an accelerated corrosion test, directly exposing the cut end of the pipe to a corrosive medium simulating oilfield produced water. The simulated oilfield produced water consisted of: NaCl concentration 50000mg / L, Na2SO4 concentration 2000mg / L, NaHCO3 concentration 500mg / L, with H2S gas introduced to saturation. The pH was adjusted to 5.0 with acetic acid. The test temperature was 60℃, and the corrosion test duration was 720h. After the test, the corrosion condition of the steel wire at the cut end of the pipe was visually inspected, and the remaining cross-sectional area of ​​the steel wire was measured. The thermal cycling stability test involves subjecting the pipe sample to 1000 thermal cycles between 20℃ and 80℃. Each thermal cycle includes heating to 80℃ and holding for 30 minutes, followed by cooling to 20℃ and holding for 30 minutes. After the thermal cycling test, the steel-plastic interface bond strength is tested, and the retention rate of the interface bond strength after thermal cycling is calculated.

[0081] The results of various performance tests are shown in Table 1:

[0082] Table 1 Comparison of Performance Test Results of Steel-Reinforced Plastic Composite Pipes

[0083] Test Project This embodiment Comparative Example 1 Comparative Example 2 Steel-Plastic Interface Bond Strength (Initial Value) / MPa 6.5 3.8 5.2 Percentage of remaining cross-sectional area of ​​steel wire after corrosion test / % 99.6 78.3 92.1 Interfacial bonding strength after 1000 thermal cycles / MPa 6.1 1.9 4.0 Interfacial bonding strength retention rate after thermal cycling / % 93.8 50.0 76.9 Corrosion of steel wire at pipe cutting ends No visible corrosion The end face of the steel wire was severely corroded. Slight rust on the end face of the steel wire

[0084] As can be seen from the data in the table above, the steel-plastic interfacial bonding strength of this embodiment reaches 6.5 MPa, which is significantly higher than 3.8 MPa of Comparative Example 1 and 5.2 MPa of Comparative Example 2. The advantage of this embodiment comes from the synergistic effect of three technical features: the nano-structural pretreatment of the steel wire surface in step 1 increases the actual surface area of ​​the steel wire, providing a grafting reaction site density far exceeding that of the untreated surface for the in-situ interfacial polymerization reaction in step 3; the in-situ interfacial polymerization reaction in step 3 establishes covalent chemical bonds between the steel wire surface and the high-density polyethylene matrix, and its bonding strength far exceeds the physical bonding strength of the hot melt adhesive in Comparative Example 1; the gradient porosity steel wire mesh in step 2 achieves full melt filling in step 4, eliminating microscopic voids at the interface, and allowing the role of the chemically bonded interfacial layer to be fully utilized.

[0085] Regarding corrosion resistance, in this embodiment, the percentage of the remaining cross-sectional area of ​​the steel wire after the corrosion test was 99.6%, indicating that the steel wire was basically uncorroded, while in Comparative Example 1 it was 78.3% and in Comparative Example 2 it was 92.1%. The fundamental reason for the superior corrosion resistance of this embodiment is that the chemically bonded interface layer eliminates the physical gap between the steel wire surface and the high-density polyethylene matrix. The gradient porosity steel wire mesh, combined with high-pressure co-extrusion, achieves complete sealing and coating of the entire steel wire surface. At the pipe cutting end, all steel wire end faces are completely sealed by the continuous high-density polyethylene matrix, preventing corrosive media from contacting the steel wire surface. Comparative Example 1 uses a uniform mesh steel wire mesh and is only physically bonded by hot melt adhesive. Numerous microscopic gaps exist at the interface, directly exposing the steel wire end faces at the pipe cutting end. Corrosive media penetrates along the interface gaps and directly corrodes the steel wire. Although Comparative Example 2 adopted a gradient porosity steel wire mesh and nanostructured steel wire surface, it lacked a chemically bonded interface layer. A weak physical interface still existed between the low-carbon steel wire and the high-density polyethylene matrix, and the corrosive medium could still slowly penetrate along the physical interface, resulting in slight corrosion.

[0086] Regarding thermal cycling stability, this embodiment maintained a high interfacial bonding strength retention rate of 93.8% after 1000 thermal cycles, compared to only 50.0% in Comparative Example 1 and 76.9% in Comparative Example 2. The superior resistance to thermal cycling degradation in this embodiment stems from the synergistic effect of the molecular chain conformation adjustment mechanism of the chemically bonded interface layer and the step-controlled temperature cooling heat treatment. In the chemically bonded interface layer, the maleic anhydride-grafted high-density polyethylene molecular chain segments can adapt to the differential thermal expansion between the steel wire and the high-density polyethylene matrix through conformational adjustments during temperature changes. This dissipates the strain caused by thermal expansion mismatch through molecular chain segment movement, preventing stress concentration accumulation at the interface. Furthermore, the covalent bond connection in the chemically bonded interface layer itself exhibits a uniform stress distribution characteristic, making stress concentration points less likely and effectively suppressing thermal fatigue debonding. In Comparative Example 1, the hot melt adhesive layer experienced a significant decrease in bonding strength after 1000 thermal cycles due to repeated cycles of thermal softening and cold hardening. In Comparative Example 2, due to the lack of a chemically bonded interface layer, the bond between steel and plastic is still mainly physical, consisting of mechanical interlocking and van der Waals forces. Under temperature cycling conditions, differential thermal expansion directly affects the physical interface, leading to a decrease in bond strength.

[0087] In this embodiment, the synergistic effect among the various technical features is manifested in the following ways: the nanostructured pretreatment of the steel wire surface provides a basis for grafting reaction sites for the in-situ polymerization reaction at the interface. Without the high-density hydroxyl sites provided by the nano-porous structure, simply grafting maleic anhydride onto the smooth steel wire surface would result in a significantly reduced grafting density, making it impossible to form a chemically bonded interface layer of effective thickness. The gradient porosity steel wire mesh design allows for more complete melt filling under high extrusion pressure conditions, eliminating microscopic voids at the interface caused by insufficient filling, and ensuring that the chemically bonded interface layer can form a continuous and defect-free closed layer on the entire steel wire surface. The conformational adjustment capability of the molecular chain segments of the chemically bonded interface layer and the stress release effect of the stepped temperature-controlled cooling heat treatment work together to achieve excellent resistance to thermal cycling degradation. The above technical features form a close linkage, enabling this embodiment to achieve significant improvements in interface bonding strength, corrosion resistance, and thermal cycling stability simultaneously, producing a synergistic effect that cannot be achieved by implementing a single technical feature independently.

[0088] The melt flow index measurement method in this embodiment is based on GB / T3682.1-2018 standard, using a melt flow indexer at 190℃ and a load of 2.16kg. The density measurement of high-density polyethylene is based on GB / T1033.1-2008 standard, using the immersion method at 23℃. The tensile yield stress and nominal strain at break are measured according to GB / T1040.2-2006 standard, using a type 1A dumbbell-shaped specimen at a test speed of 50mm / min. The interlaminar bond strength measurement is based on CJ / T189-2007 standard. The temperature control accuracy for the thermal cycling test is ±1℃, and the heating / cooling rate is 5℃ / min. All measurement data are the arithmetic mean of five parallel measurements, and the relative standard deviation of the measurement results is controlled within 5%.

[0089] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0090] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method of producing a steel skeleton plastic composite pipe, characterized by, Includes the following steps: Step 1: An electrolyte solution prepared by mixing 8% to 12% oxalic acid solution and 2% to 5% hydrofluoric acid solution at a volume ratio of 3:1 is used to perform electrochemical etching treatment with low carbon steel wire as the anode, so that a nanoscale porous structure is formed on the surface of the low carbon steel wire, and a pretreated steel wire is obtained. Step 2: Weave the pretreated steel wire into a wire mesh, and make the wire mesh spacing decrease gradually from the outer layer to the inner layer along the radial direction of the pipe, forming a gradient porosity wire mesh with porosity decreasing gradually from the outer layer to the inner layer. Step 3: The gradient porosity steel wire mesh is continuously impregnated under ultrasonic vibration conditions through a molten mixture containing maleic anhydride-grafted high-density polyethylene and dicumyl peroxide initiator at a temperature of 170°C to 190°C. This allows the maleic anhydride groups to undergo an in-situ grafting polymerization reaction with the hydroxyl groups on the nanoscale porous structure of the pretreated steel wire surface, forming a chemically bonded interface layer on the surface of the pretreated steel wire, thus obtaining a gradient porosity steel wire mesh after interface grafting treatment. Step 4: The gradient porosity steel wire mesh after the interface grafting treatment is fed into the co-extrusion die. High-density polyethylene melt is filled into the gradient porosity steel wire mesh under an extrusion pressure of 10 MPa to 18 MPa. At the same time, an inner plastic layer is extruded on the inner side of the gradient porosity steel wire mesh and an outer plastic layer is extruded on the outer side. The tube blank is obtained by one composite molding. Step 5: The tube blank is subjected to stepped temperature control cooling in a first cooling section of 50℃ to 60℃, a second cooling section of 25℃ to 35℃, and a third cooling section of 10℃ to 15℃, and then heat-treated at 100℃ to 120℃ for 4 to 8 hours, and naturally cooled to obtain a steel-reinforced plastic composite pipe.

2. The method for preparing the steel-reinforced plastic composite pipe according to claim 1, characterized in that, The low-carbon steel wire in step 1 has a diameter of 2.5 mm to 4.0 mm; the current density of the electrochemical etching process is 0.3 A / cm² to 0.8 A / cm², the electrolyte temperature is 25°C to 45°C, and the processing time is 60 seconds to 180 seconds; after the electrochemical etching process is completed, the low-carbon steel wire is removed from the electrolyte, rinsed with deionized water 3 to 5 times, and dried at a temperature of 80°C to 100°C for 30 minutes to 60 minutes to obtain the pretreated steel wire.

3. The method for preparing the steel-reinforced plastic composite pipe according to claim 1, characterized in that, In step 2, the braiding area is sequentially divided into a first braiding section, a second braiding section, and a third braiding section along the pipe axis. The third braiding section corresponds to the inner wall region of the finished steel-reinforced plastic composite pipe, the first braiding section corresponds to the outer wall region of the finished steel-reinforced plastic composite pipe, and the second braiding section is located between the first and third braiding sections. The wire mesh spacing of the first braiding section is set to 3.0 mm to 4.0 mm, the wire mesh spacing of the second braiding section is set to 2.0 mm to 3.0 mm, and the wire mesh spacing of the third braiding section is set to 1.0 mm to 2.0 mm. During the braiding process, the warp tension is 80 Newtons to 120 Newtons, and the weft tension is 60 Newtons to 100 Newtons. The porosity of the gradient porosity wire mesh decreases gradually from the first braiding section to the third braiding section, and the porosity of the third braiding section is 30% to 60% of the porosity of the first braiding section.

4. The method for preparing the steel-reinforced plastic composite pipe according to claim 1, characterized in that, The preparation method of the melt mixture containing maleic anhydride-grafted high-density polyethylene and dicumyl peroxide initiator in step 3 is as follows: 100 parts by weight of maleic anhydride-grafted high-density polyethylene and 0.3 to 0.8 parts by weight of dicumyl peroxide initiator are mixed in a high-speed mixer at a speed of 800 to 1200 rpm for 10 to 15 minutes to obtain a mixture. The mixture is then fed into a twin-screw extruder at a screw speed of 60 to 100 rpm. The mixture is melt-extruded at a temperature of 170°C to 190°C and directly introduced into an ultrasonic-assisted impregnation tank to form the molten mixture. During impregnation, the ultrasonic frequency of the ultrasonic generator is 20 kHz to 40 kHz, the ultrasonic power density is 0.5 W / cm² to 1.5 W / cm², the traction speed of the gradient porosity steel wire mesh is 0.3 m / min to 0.8 m / min, and the impregnation time is 2 minutes to 5 minutes. The thickness of the chemically bonded interface layer formed is 5 micrometers to 20 micrometers.

5. The method for preparing the steel-reinforced plastic composite pipe according to claim 1, characterized in that, In step 4, the melt index of the filled high-density polyethylene melt is 0.2 g / 10 min to 0.5 g / 10 min, and the melt index is measured under conditions of 190°C and 2.16 kg load; the thickness of the inner plastic layer is 2.0 mm to 4.0 mm, and the thickness of the outer plastic layer is 2.5 mm to 5.0 mm; the die temperature of the co-extrusion die is 190°C to 200°C, and the traction speed is 0.5 m / min to 1.5 m / min.

6. The method for preparing the steel-reinforced plastic composite pipe according to claim 1, characterized in that, In step 5, the cooling time in the first cooling section is 30 to 60 seconds, the cooling time in the second cooling section is 60 to 120 seconds, and the cooling time in the third cooling section is 30 to 60 seconds. The cooling medium in all three cooling sections is water.

7. The method for preparing the steel-reinforced plastic composite pipe according to any one of claims 1 to 6, characterized in that, After obtaining the pretreated steel wire in step 1 and before step 2, the method further includes: scanning the surface of the pretreated steel wire using an atomic force microscope to collect the average pore size parameters and average pore depth parameters of the nanoscale porous structure on the surface of the pretreated steel wire; in step 3, adjusting the ultrasonic power density and impregnation time according to the collected average pore size parameters and average pore depth parameters so that the thickness of the chemical bonding interface layer is in the range of 5 micrometers to 20 micrometers.

8. A steel-reinforced plastic composite pipe, prepared by the method according to any one of claims 1 to 7, characterized in that, It includes an inner plastic layer, a steel skeleton reinforcement layer, and an outer plastic layer. The steel skeleton reinforcement layer is a gradient porosity steel wire mesh. There are chemically bonded interface layers between the surface of the low-carbon steel wire constituting the gradient porosity steel wire mesh and the high-density polyethylene matrix of the inner plastic layer, and between the surface of the low-carbon steel wire and the high-density polyethylene matrix of the outer plastic layer. The high-density polyethylene matrix of the inner plastic layer, the chemically bonded interface layer, and the high-density polyethylene matrix of the outer plastic layer are continuously and seamlessly connected throughout the entire area of ​​the steel skeleton reinforcement layer to completely seal the end face of the low-carbon steel wire.

9. The steel-reinforced plastic composite pipe according to claim 8, characterized in that, The gradient porosity wire mesh is composed of a first braided section, a second braided section, and a third braided section arranged radially from the outside to the inside of the pipe. The wire mesh spacing of the first braided section is 3.0 mm to 4.0 mm, the wire mesh spacing of the second braided section is 2.0 mm to 3.0 mm, and the wire mesh spacing of the third braided section is 1.0 mm to 2.0 mm. The porosity of the third braided section is 30% to 60% of the porosity of the first braided section. The thickness of the chemically bonded interface layer is 5 μm to 20 μm. The thickness of the inner plastic layer is 2.0 mm to 4.0 mm, and the thickness of the outer plastic layer is 2.5 mm to 5.0 mm.

10. The steel-reinforced plastic composite pipe according to claim 8, characterized in that, At the pipe cutting end, the end faces of all the low-carbon steel wires in the steel skeleton reinforcement layer are completely sealed and covered by a continuous high-density polyethylene matrix through which the inner plastic layer and the outer plastic layer are connected.

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