A forming method for small-diameter bimetallic metallurgical composite welded pipe

Abstract

This invention provides a forming method for small-diameter bimetallic composite welded pipes, belonging to the field of steel pipe welding and forming technology. The method involves using a bimetallic composite steel strip, composed of a carbon steel base layer and a corrosion-resistant metal layer, as raw material. The strip is continuously bent into a C-shape and further closed into an O-shaped pipe blank using several roll forming assemblies with upper and lower pressure rollers. The corrosion-resistant metal layer forms the inner surface of the pipe blank after forming. During the forming process, differentiated pressing forces are applied to the upper and lower pressure rollers based on the pre-measured yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer. This guides the coordinated deformation of the two metal layers and reduces interlayer shear stress. The forming method for small-diameter bimetallic composite welded pipes provided by this invention applies different pressing forces to the upper and lower pressure rollers, guiding the two metal layers to achieve coordinated deformation during bending and reducing the shear stress at the interlayer interface.

Description

Technical Field

[0001] This invention belongs to the field of steel pipe welding and forming technology, and more specifically, it relates to a forming method for small-diameter bimetallic composite welded pipes. Background Technology

[0002] Bimetallic composite pipes are made of carbon steel base material and stainless steel or corrosion-resistant alloy cladding through metallurgical bonding. They combine the excellent mechanical properties of carbon steel with the superior corrosion resistance of the cladding material, making them promising for applications in pipeline transportation of corrosive media in fields such as oil, natural gas, chemical engineering, and marine engineering. Small-diameter bimetallic composite pipes are particularly suitable for transporting dispersed, short-distance corrosive media in oil and gas field gathering and transportation networks.

[0003] For the forming of straight seam welded pipes, continuous roll bending is a highly efficient production method. Its basic principle is to pass the steel strip sequentially through multiple sets of roll forming assemblies consisting of upper and lower pressure rollers, gradually bending the flat strip into a C-shape, and then further closing it into an O-shaped pipe blank, thus providing a well-formed pipe body for subsequent welding processes. This process has advantages such as high production efficiency and a high degree of automation.

[0004] However, applying continuous roll forming technology to bimetallic composite steel strips presents unique technical challenges. The inconsistent deformation resistance of the inner and outer metal layers of the composite steel strip leads to uncoordinated deformation during bending, resulting in significant shear stress at the interlayer interface. When this shear stress exceeds the metallurgical bonding capacity, it can cause a decrease in interfacial bond strength, or even delamination failure, severely impacting the quality and reliability of the finished pipe. Summary of the Invention

[0005] The purpose of this invention is to provide a forming method for small-diameter bimetallic composite welded pipes, which aims to solve the problem that the existing continuous roll bending forming process for bimetallic composite steel strips suffers from the difference in yield strength between the inner and outer metals, resulting in inconsistent interlayer deformation, shear stress, and even delamination.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: to provide a method for forming a small-diameter bimetallic metallurgical composite welded pipe, comprising: The bimetallic composite steel strip is continuously rolled and formed by several roll forming assemblies, so that the bimetallic composite steel strip is gradually bent into a C shape and further closed into an O shape to form a tube blank. The roll forming assembly includes an upper pressure roller and a lower pressure roller. The bimetallic composite steel strip includes a carbon steel base and a corrosion-resistant metal layer covering one side of the carbon steel base. The corrosion-resistant metal layer forms the inner surface of the tube blank after forming. During the continuous roll bending process, differentiated pressing forces are applied to the upper and lower pressure rollers based on the measured yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer. The pressing force applied to the carbon steel base layer is greater than the pressing force applied to the corrosion-resistant metal layer, so as to guide the carbon steel base layer and the corrosion-resistant metal layer of the bimetallic composite steel strip to deform in a coordinated manner, maintain a synchronous deformation state, and reduce the interlayer shear stress of the bimetallic composite steel strip.

[0007] In one possible implementation, the continuous roll bending forming adopts an ERW / COE integrated forming process.

[0008] In one possible implementation, measured yield strength data of the carbon steel base layer and the corrosion-resistant metal layer are obtained, and the pressing amount of the upper pressure roller and the lower pressure roller is automatically adjusted by the control system based on the measured yield strength data to achieve the differentiated pressing force.

[0009] In one possible implementation, at the location where the bimetallic composite steel strip closes into an O-shape to form the tube blank, the roundness data and joint gap data of the tube blank are collected in real time, and the pressing force of the upper pressure roller and the lower pressure roller is compensated and adjusted by the control system based on the roundness data and the joint gap data.

[0010] One possible implementation also includes the following steps: The seam of the tube blank is pre-welded to form a pre-welded steel pipe; The pre-welded steel pipe is cut to a predetermined length to obtain a fixed-length pipe section; The carbon steel base layer of the specified length pipe section is welded internally and externally; After the carbon steel base layer is welded, the weld is inspected for defects. Once it is confirmed that there are no defects, the corrosion-resistant metal layer of the fixed-length pipe section is welded to obtain the target welded pipe.

[0011] In one possible implementation, when welding the corrosion-resistant metal layer of the fixed-length pipe section, the welding material and welding process are selected according to the material of the corrosion-resistant metal layer.

[0012] In one possible implementation, the bimetallic composite steel strip is sequentially leveled, trimmed, sheared, and butt-welded before the continuous roll bending process.

[0013] In one possible implementation, the trimming includes machining a V-shaped, U-shaped, or X-shaped bevel on the edge of the bimetallic composite steel strip.

[0014] In one possible implementation, after obtaining the target welded pipe, the target welded pipe is sequentially subjected to sizing and straightening, non-destructive testing of the weld, hydrostatic testing, and pipe end finishing.

[0015] In one possible implementation, during the non-destructive testing of the weld, the testing data is recorded and analyzed synchronously in real time. When a defect is detected, the control system automatically positions the defect by spraying a marker.

[0016] The beneficial effects of the forming method for small-diameter bimetallic composite welded pipe provided by this invention are as follows: Compared with the prior art, the forming method of this invention for small-diameter bimetallic composite welded pipe obtains the measured yield strength data of the carbon steel base layer and the corrosion-resistant metal layer, and applies different pressing forces to the upper and lower pressure rollers according to the yield strength difference between the two. This allows the metal layer with higher yield strength to bear a larger pressing force, while the metal layer with lower yield strength bears a smaller pressing force, or the force can be adjusted in the opposite direction according to deformation requirements, thereby guiding the two metal layers to achieve coordinated deformation during bending deformation. This effectively reduces the shear stress at the interlayer interface, prevents delamination defects, and ensures the forming quality of the pipe blank. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, 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 some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating a method for forming a small-diameter bimetallic composite welded pipe according to an embodiment of the present invention; Figure 2 Metallographic images of the tube blank interface obtained in an embodiment of the present invention; Figure 3 Here is a metallographic photograph of the tube blank interface obtained in the comparative example of this invention; In the figure, mark 1 represents the corrosion-resistant metal layer, mark 2 represents the carbon steel base layer, and mark 3 represents the weld. Detailed Implementation

[0019] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0020] Before describing the specific embodiments of the present invention in detail, several specific terms used herein will be explained to help understand the meaning of subsequent process steps.

[0021] Bimetallic composite steel strip refers to a steel strip composed of two metal layers of different materials bonded together through a metallurgical bonding method. In this article, the bimetallic composite steel strip includes a carbon steel base layer and a corrosion-resistant metal layer covering one side of the carbon steel base layer. The carbon steel base layer typically uses pipeline steel (such as X65, X70, etc.), primarily bearing the mechanical load-bearing function of the pipeline; the corrosion-resistant metal layer typically uses stainless steel (such as 316L, 304L, etc.) or nickel-based alloys (such as 825 alloy, 625 alloy, etc.), primarily providing corrosion protection. The "metallurgical bonding" between the two refers to the atomic bonding between the two metal layers at the interface through processes such as explosive welding and hot rolling composite bonding, rather than simple mechanical bonding or lining. The interfacial bonding strength of the metallurgical bond is typically not less than 80% of the lower limit of the base material strength, far exceeding the interfacial bonding strength of mechanically bonded pipes. In the finished pipe, the corrosion-resistant metal layer constitutes the inner surface of the pipe body, directly contacting the transported medium; the carbon steel base layer constitutes the outer surface of the pipe body, bearing external loads and the mechanical stress of the pipeline system.

[0022] ERW / COE integrated forming is a forming method that combines "continuous roll forming of electric resistance welding pipe" with the "continuous operation concept of coiled tubing." In the field of ERW (electric resistance welding) pipe manufacturing, continuous roll forming refers to the gradual and continuous bending of the steel strip from a flat state into a tubular shape using multiple sets of rollers arranged sequentially along the steel strip's travel direction. In the field of COE (coiled tubing) manufacturing, the continuous operation concept means that the entire production process, from uncoiling the steel strip to the finished tubing, is completed uninterruptedly on a single production line. This article refers to the integration of the above forming method and operation concept as "ERW / COE integrated forming," which is essentially a highly efficient continuous roll forming process capable of completing all forming processes from steel strip to tubing blank on a single production line.

[0023] Roll forming assembly: refers to the roller group set in pairs on the production line. Depending on the forming stage, the roller forming assembly is divided into front roller forming assembly and rear roller forming assembly.

[0024] The front-end roll forming assembly is located in the roughing stage of the production line. It consists of a pair of rollers, an upper roller and a lower roller, with the upper roller positioned above the steel strip and the lower roller below, forming a gap between them for the steel strip to pass through. During continuous roll bending, the steel strip passes through multiple sets of front-end roll forming assemblies sequentially. The upper and lower rollers of each set apply different pressure forces to bend and deform the steel strip, gradually bending the flat steel strip into a C-shape. In application, the pressure reduction of the upper and lower rollers in each set of front-end roll forming assemblies can be independently adjusted, providing the equipment basis for differentiated pressure force control in this invention.

[0025] The rear roller pressing assembly is located in the finishing section of the production line, behind the front roller pressing assembly. The rear roller pressing assembly consists of a pair of rollers, a left pressure roller and a right pressure roller, positioned on the left and right sides of the steel strip respectively, forming a roller gap between them for the steel strip to pass through. During continuous roll bending, the C-shaped tube blank formed by the front roller pressing assembly passes through multiple sets of rear roller pressing assemblies, further closing the C-shaped tube blank into an O-shaped tube blank.

[0026] Pressure Reduction Force and Pressure Reduction Amount: Pressure reduction force refers to the pressure applied to the surface of the steel strip by the upper or lower pressure roller, measured in Newtons (N) or kilonewtons (kN). Pressure reduction amount refers to the vertical displacement of the pressure roller relative to the surface of the steel strip, measured in millimeters (mm). There is a corresponding relationship between pressure reduction amount and pressure reduction force: the greater the pressure reduction amount, the greater the squeezing of the steel strip by the pressure roller, and the greater the pressure reduction force generated. In actual production, the control system can indirectly control the pressure reduction force by adjusting the pressure reduction amount, or it can directly monitor and adjust the pressure reduction force through a force sensor. In this article, "differentiated pressure reduction force" refers to the unequal pressure reduction force applied by the upper and lower pressure rollers, with the specific difference determined based on the measured yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer.

[0027] In actual production, when using efficient and continuous roll forming processes to process bimetallic metallurgical composite steel strips, a problem arises that is not prominent in traditional single-metal tubes or when using discontinuous forming processes such as JCO: delamination easily occurs between the base layer and the cladding layer of the tube blank. In-depth analysis revealed that the root cause lies in the significant difference in yield strength between the carbon steel base layer and the corrosion-resistant metal layer. Under the same bending deformation, the deformation resistance of the two metal layers is inconsistent; the higher-strength material resists deformation, while the lower-strength material is prone to plastic flow. This uncoordinated deformation generates significant shear stress at the interlayer interface. Further research found that once this shear stress exceeds the metallurgical bond strength between the two materials, delamination occurs.

[0028] Based on this mechanism, conventional roll forming methods that only focus on equal pressure control of the macroscopic geometry of the billet cannot solve the problem of interlayer coordinated deformation caused by differences in material properties. However, if the measured yield strength difference between the two metal layers can be obtained in advance, and based on this, the deformation forces acting on the two metal layers (i.e., the pressure of the upper and lower pressure rollers) can be differentiated during the bending process, so that the material with greater deformation resistance bears greater pressure and the material with less deformation resistance bears less pressure. This can guide the two metal layers to change from "each doing its own thing" to "synchronous coordination" during the deformation process, thereby reducing the generation of interlayer shear stress from the source and achieving the effect of preventing delamination.

[0029] Please see Figure 1 The following describes a method for forming a small-diameter bimetallic composite welded pipe according to the present invention. The method for forming the small-diameter bimetallic composite welded pipe specifically includes the following steps: S1: Steel strip preparation S11. Uncoiling. The bimetallic composite steel coil is uncoiled using an uncoiler to supply strip for the continuous production line. This bimetallic composite steel strip is composed of a carbon steel base layer and a corrosion-resistant metal layer through a metallurgical bonding method. The carbon steel base layer is pipeline steel X65, primarily bearing the pressure-bearing function of the pipeline; the corrosion-resistant metal layer is 316L stainless steel coated on one side of the carbon steel base layer, used to contact the transported medium and provide corrosion protection. After forming, the corrosion-resistant metal layer forms the inner surface of the pipe blank, and the carbon steel base layer forms the outer surface. The metallurgical bonding interface between the two is formed through an explosive welding process, and the interface bonding strength is not less than 80% of the lower limit of the base material strength.

[0030] S12. Leveling. A multi-roll straightener is used to level the unfolded bimetallic composite steel strip. The bimetallic composite steel strip in its coiled state exhibits curling stress and shape defects. Directly introducing it into the forming process would lead to increased roundness deviation in the tube blank and uneven joint gaps. The multi-roll straightener applies repeated bending stress to the bimetallic composite steel strip using staggered leveling rollers, eliminating residual stress through elasto-plastic deformation and obtaining a flat strip. The straightness of the leveled bimetallic composite steel strip should be controlled to not exceed 2mm per meter of length.

[0031] S13. Trimming. High-precision disc shears or milling machines are used to trim the edges of the leveled bimetallic composite steel strip. During rolling and transportation, the edges of the bimetallic composite steel strip may have defects such as burrs, oxide scale, and micro-cracks, which can affect the subsequent welding quality. The purpose of trimming is to ensure that the strip edges are straight, smooth, and burr-free, and to precisely control the strip width within the tolerances required by the process.

[0032] S14. Beveling. After trimming, V-shaped, U-shaped, or X-shaped beveling is made on one or both sides of the bimetallic composite steel strip. The purpose of the beveling is to provide a reasonable molten pool space and fusion surface shape for subsequent welding processes, allowing the weld metal to fully fill the joint gap and ensuring the weld penetration and fusion quality. The beveling type can be selected according to the total thickness of the steel strip, the thickness ratio of the base layer to the cladding layer, and the welding process requirements; V-shaped, U-shaped, or X-shaped beveling is acceptable. By pre-beveling, the welding heat input can be reduced, welding deformation can be decreased, and the fusion effect of the weld can be optimized.

[0033] S15. Shearing and Butt Welding. A shearing and butt welding machine is used to connect the ends of two consecutive rolls of bimetallic composite steel strip to form a continuous strip. The shearing and butt welding machine first shears the tail of the preceding roll and the head of the following roll to remove irregular ends, then aligns the two ends and welds them. The butt weld needs to be ground online to remove excess weld height, ensuring a smooth transition on the strip surface and preventing weld protrusions from damaging the roller surface or causing strip deviation when passing through the rolling assembly. To buffer the conflict between the brief pause during the connection of the preceding and following rolls and continuous production, a looper device is installed in the production line to store a certain length of strip, ensuring the continuous and stable operation of subsequent continuous roll bending forming processes.

[0034] S16. Raw Material Inspection. Before forming, the leveled bimetallic composite steel strip can be subjected to online ultrasonic testing to screen for defects such as delamination, bubbles, and lack of bonding within the steel strip. Only steel strips that pass the inspection can proceed to subsequent processes.

[0035] S2: Continuous roll forming S21. Acquisition of Yield Strength Data. Before continuous roll bending begins, samples are taken from the same batch of bimetallic composite steel strips to be processed, and tensile tests are conducted according to national standards to determine the yield strength of the carbon steel base layer and the corrosion-resistant metal layer respectively. The measured yield strength data is then input into the production line control system as the basis for setting differentiated pressing forces.

[0036] S22, ERW / COE integrated forming. Continuous roll bending is achieved using the ERW / COE integrated forming process. This process combines the continuous roll bending technology of resistance welded pipes with the concept of continuous operation, enabling the bimetallic metallurgical composite steel strip to be gradually formed from a flat state into an O-shaped tube blank on a continuous production line through multiple sets of roll forming assemblies arranged sequentially along the steel strip's travel direction.

[0037] S23. Setting and Controlling Differentiated Reduction Force. In this step, the control system automatically calculates the differentiated reduction amount between the upper and lower pressure rollers in each roller pressing assembly based on the pre-input measured yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer, according to a preset algorithm model.

[0038] In production, the deformation resistance of the carbon steel base layer is typically greater than that of the corrosion-resistant metal layer. During bending forming, if the upper and lower pressure rollers apply the same downward force, the corrosion-resistant metal layer will yield and exhibit greater plastic flow before the carbon steel base layer, while the deformation of the carbon steel base layer will lag behind. This difference in deformation response generates shear stress at the interface between the two metal layers. When this shear stress exceeds the shear strength of the metallurgical interface, it triggers interface separation, i.e., delamination.

[0039] To address the aforementioned issues, this invention employs a control system to apply a larger pressing force to the roller acting on the carbon steel base layer side and a relatively smaller pressing force to the roller acting on the corrosion-resistant metal layer side. The magnitude of the difference in pressing force is positively correlated with the yield strength difference, and the specific proportionality coefficient can be determined using empirical formulas or finite element simulations based on factors such as the total thickness of the steel strip, the thickness ratio of the base layer to the coating layer, and the number of forming passes. In this way, the two metal layers can maintain a relatively synchronized deformation state during bending, and the interlayer shear stress is effectively controlled.

[0040] It should be noted that in actual production control, there is a definite physical relationship between the pressing force F and the pressing amount Δh of the pressure roller. According to the principles of material mechanics and elastic-plastic deformation, the larger the pressing amount Δh, the more severe the local deformation applied by the pressure roller to the steel strip surface, and the larger the resulting pressing force F. That is, the pressing force F is a monotonically increasing function of the pressing amount Δh. Therefore, in the technical solution of this invention, the control system indirectly achieves differentiated and precise adjustment of the pressing force acting on the carbon steel base layer and the corrosion-resistant metal layer by differentially setting and controlling the pressing amounts of the upper and lower pressure rollers. Specifically, the control system outputs a larger pressing amount command to the pressure roller acting on the carbon steel base layer side through a servo drive mechanism, resulting in a larger displacement; and outputs a relatively smaller pressing amount command to the pressure roller acting on the corrosion-resistant metal layer side, resulting in a smaller displacement. In this way, the difference in material properties, such as the yield strength difference, is transformed into an executable and quantifiable pressing amount control parameter, thereby ensuring the engineering feasibility of the differentiated pressing force control strategy.

[0041] S24. Rough Forming (C-shaped Bending). The bimetallic composite steel strip first enters the rough forming section. Multiple sets of roller pressing assemblies in the rough forming section progressively bend the flat strip into an open C-shaped tube blank according to a preset roll gap profile. The bending amount of each roller pressing assembly is precisely designed to ensure uniform distribution of bending deformation in the steel strip, avoiding localized stress concentration caused by excessive deformation in a single pass. During the rough forming process, the upper and lower pressure rollers of each roller pressing assembly are controlled according to differentiated pressure settings to ensure that the carbon steel base layer and the corrosion-resistant metal layer maintain coordinated deformation throughout the entire bending process from flat to C-shape.

[0042] S25, Finishing (O-shaped closing forming). The C-shaped billet after rough forming enters the finishing section. The roller assembly in the finishing section further bends the C-shaped billet, gradually closing the edges to form an O-shaped billet. At this critical station, a laser diameter gauge and a gap detector are installed to collect the roundness data and gap data of the billet in real time. In this application, the roundness target tolerance is set to ±0.5mm, and the gap target value is set to below 0.2mm. When the control system detects that the roundness or gap deviates from the target range, it automatically compensates and adjusts the pressing force of the upper and lower pressure rollers, forming a "feedforward + feedback" closed-loop control circuit. Among them, the feedforward control sets the reference pressing force based on the yield strength difference, and the feedback control performs fine-tuning compensation based on geometric dimensional deviations.

[0043] By using a combination of feedforward and feedback control, the problem of interlayer deformation coordination is solved from the perspective of material properties, while the dimensional quality of the tube blank is guaranteed from the perspective of geometric accuracy, thus achieving dual optimization.

[0044] S26. Weld orientation setting. During the forming process, the weld orientation of the tube blank can be flexibly set to downward or upward according to the layout and process requirements of subsequent welding stations, both of which are within the protection scope of this invention.

[0045] S3: Pre-welding and cutting S31. Pre-welding. The seam of the closed O-shaped tube blank is pre-welded to form a pre-welded steel pipe. The purpose of pre-welding is to initially fix the butt joint edges, enabling the tube blank to form an integral structure capable of withstanding subsequent welding heat input and mechanical handling, providing a stable base for the subsequent formal welding. Pre-welding can be performed using any of gas shielded welding, TIG welding, or submerged arc welding. The welding parameters are determined based on the total wall thickness and material of the bimetallic composite steel strip.

[0046] S32. Fixed-length cutting. After pre-welding, a servo-driven flying saw is used to cut the pre-welded steel pipe to a predetermined length, obtaining fixed-length pipe sections. During implementation, the flying saw moves synchronously with the pipe body, completing the cutting during the movement to ensure cutting accuracy and cut quality. After cutting, the cut is ground or chamfered online to remove burrs, ensuring smooth end faces and accurate dimensions.

[0047] S4: Layered welding The cut pipe sections are transported to the welding station and welded in layers, first the carbon steel base layer and then the corrosion-resistant metal layer.

[0048] S41. Carbon steel base welding. First, the carbon steel base of the fixed-length pipe section undergoes internal and external welding. The welding method for the carbon steel base can be selected from at least one of submerged arc welding, gas shielded welding, or laser welding, depending on its material, wall thickness, and production efficiency requirements. The order of internal and external welding can be flexibly set according to the production line layout or pipe specifications, and there are no fixed requirements.

[0049] S42. Defect Inspection of Carbon Steel Base Welds. After the carbon steel base welding is completed, before welding the corrosion-resistant metal layer, defect inspection of the carbon steel base welds is performed. X-ray inspection can be used in this step. The purpose of defect inspection in this step is to ensure that any defects such as porosity, cracks, or lack of fusion in the carbon steel base welds are detected and addressed at this stage. If the corrosion-resistant metal layer is welded without inspection, the base defects will be covered by the coating, making them difficult to detect in subsequent routine inspections and posing a safety hazard to the finished pipe. Only after confirming that the welds are defect-free can the next process proceed.

[0050] S43. Welding of the Corrosion-Resistant Metal Layer. After confirming the carbon steel base weld is defect-free, the corrosion-resistant metal layer is welded. When welding the corrosion-resistant metal layer, matching welding materials and welding processes must be selected based on the material of the corrosion-resistant metal layer to ensure that the corrosion resistance and chemical composition of the weld are consistent with the cladding body. For example, when the corrosion-resistant metal layer is 316L stainless steel, TIG welding can be used, and ER316L welding wire, which is compatible with 316L stainless steel, can be selected as the welding material. After welding, the target welded pipe is obtained.

[0051] S5: Post-weld finishing and inspection S51. Sizing and Straightening. The target welded pipe undergoes online sizing and multi-roll straightening. During welding and cooling, the welded pipe may experience minor deformations such as bending and ellipticization due to thermal stress and phase transformation stress. The sizing process precisely calibrates the outer diameter of the welded pipe using sizing roller dies, while the straightening process corrects the straightness of the welded pipe using a multi-roll straightener, ensuring that the dimensional accuracy and straightness of the finished pipe meet standard requirements.

[0052] S52. Non-destructive testing of welds. An ultrasonic testing system is used to perform full-coverage inspection of the welds and heat-affected zones of the welded pipe. Ultrasonic probes are arranged around the pipe wall to perform a comprehensive circumferential and axial scan of the welds. Testing data is recorded and analyzed in real time. When the system detects a defect echo, it automatically marks the defect location with a marker, facilitating subsequent defect verification and processing. Only after passing the testing can the pipe proceed to the hydrostatic testing process.

[0053] S53. Hydrostatic Test. A hydrostatic test is conducted on welded pipes that have passed non-destructive testing. High-pressure water is injected into the pipe, and the pressure resistance and sealing performance of the pipe body are tested according to the test pressure and holding time specified in the standard. A welded pipe that passes the hydrostatic test indicates that its overall strength and airtightness meet the usage requirements.

[0054] S54. Pipe End Finishing. For welded pipes that have passed the hydrostatic test, the pipe ends are machined and trimmed. The pipe end is a critical part of the on-site welding connection; its dimensional accuracy and end face quality directly affect the quality of the on-site butt weld. Machined ends ensure the pipe end face is flat and perpendicular to the pipe axis, while trimming removes burrs and flash from the inner and outer walls of the pipe end.

[0055] S55. Appearance and Dimension Re-inspection. The finished pipes that have undergone end finishing are subjected to a final re-inspection of appearance quality and geometric dimensions. After confirming that all indicators meet the delivery standard requirements, they are coated with oil for rust prevention and packaged for storage.

[0056] Example This embodiment takes a small-diameter bimetallic metallurgical composite straight seam welded pipe with an outer diameter of 168mm and a total wall thickness of 5.5mm (carbon steel base layer X65, wall thickness 4.0mm; corrosion-resistant metal layer 316L, wall thickness 1.5mm) as an example. The specific steps are as follows: (I) Steel strip preparation The bimetallic composite steel coil is uncoiled using an uncoiler. The steel strip is formed by explosive welding of a carbon steel base layer (X65, 4.0 mm thick) and a corrosion-resistant metal layer (316L stainless steel, 1.5 mm thick). The corrosion-resistant metal layer forms the inner surface of the finished pipe that comes into contact with the transported medium.

[0057] A multi-roller straightener is used to level the steel strip, eliminating curling stress. After leveling, the straightness of the steel strip is controlled within 2mm per meter. A milling machine is used to trim the edges of both sides of the steel strip, with the width tolerance controlled within ±0.3mm. After trimming and before shearing and welding, V-shaped bevels are made on both sides of the steel strip, with a bevel angle of 30° and a blunt edge of 1.0mm. The front and rear rolled steel strips are connected by a shearing and welding machine. The butt weld is smoothly transitioned after online grinding. A looper device stores a certain length of steel strip to ensure continuous subsequent feeding.

[0058] (II) Continuous roll forming Before production, samples were taken from the same batch of steel strips, and the yield strength of the carbon steel base layer and the corrosion-resistant metal layer were measured according to GB / T 228.1. The measured values ​​were 485 MPa and 260 MPa, respectively, with a yield strength difference of 225 MPa. This set of data was then input into the control system.

[0059] This embodiment employs an ERW / COE integrated molding process. The control system automatically calculates the differentiated reduction amount based on the yield strength difference: a larger reduction force is applied to the roller acting on the carbon steel base layer side, and a smaller reduction force is applied to the roller acting on the corrosion-resistant metal layer side, in order to guide the coordinated deformation of the two metal layers.

[0060] The steel strip sequentially enters the roughing and finishing sections, where it is gradually bent into a C-shape and further closed into an O-shaped tube blank. At the O-shaped closing station, a laser diameter gauge and a gap detector collect roundness and gap data in real time, with target values ​​of roundness tolerance ±0.5mm and gap ≤0.2mm, respectively. When the measured values ​​deviate from the targets, the control system automatically compensates and adjusts the pressing force of the pressure rollers.

[0061] (III) Pre-welding and cutting The closed tube blank seam is pre-welded with gas shielded welding to form a pre-welded steel pipe. After pre-welding, a servo-driven flying saw is used for fixed-length cutting, with a cutting length of 12 meters. The cut is then ground.

[0062] (iv) Layered welding The cut-length pipe section is delivered to the welding station. First, submerged arc welding is performed on the carbon steel base layer (X65, 4.0mm), with the welding sequence being internal welding first, then external welding. After the carbon steel base layer welding is completed, X-ray inspection is performed to confirm that the weld is defect-free. Then, the corrosion-resistant metal layer (316L stainless steel) is welded using TIG welding with ER316L welding wire. The target welded pipe is obtained after welding.

[0063] (V) Post-weld finishing and inspection The target welded pipes undergo the following sequential processes: sizing and straightening (outer diameter tolerance controlled within ±1mm, straightness ≤1.5mm per meter), ultrasonic testing (UT probe for full coverage scanning of the weld and heat-affected zone, with automatic defect marking and location), hydrostatic testing, pipe end trimming, and visual dimensional re-inspection. After passing inspection, the pipes are coated with rust-preventive oil and packaged for storage.

[0064] In this embodiment, for the combination of a carbon steel base layer X65 (measured yield strength 485MPa) and a corrosion-resistant metal layer 316L (measured yield strength 260MPa), based on finite element simulation and on-site debugging, in the rough forming section (taking the first pass as an example), the control system is set to increase the reduction of the lower pressure roller acting on the carbon steel base layer by 0.8mm compared to the reduction of the upper pressure roller acting on the corrosion-resistant metal layer. Under this setting, the measured lower pressure force is approximately 1.4 times the upper pressure force, which can effectively achieve coordinated deformation of the two metal layers. The difference in reduction between each pass is gradient-distributed according to the forming roll pattern. The final billet undergoes ultrasonic flaw detection, and no abnormal echoes are found, indicating good interface bonding and a yield rate exceeding 99.5%. Comparative Example This comparative example uses the same bimetallic metallurgical composite steel strip as the example (carbon steel base layer X65, wall thickness 4.0mm; corrosion-resistant metal layer 316L, wall thickness 1.5mm), and performs continuous roll bending on the same ERW / COE integrated forming equipment. The production process parameters are consistent with those of the example. The only difference is that in this comparative example, the upper and lower pressure rollers are formed with equal pressing force, that is, the differential pressing force control is not implemented according to the yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer.

[0065] The other steps in the comparative example—steel strip preparation (leveling, trimming, beveling, shearing and butt welding), pre-welding and cutting, layer welding, post-weld finishing and inspection—are the same as in the example.

[0066] Effect verification Ultrasonic testing and metallographic examination were performed on the tube blanks after molding in the examples and comparative examples, respectively.

[0067] The ultrasonic testing data are detailed in Table 1.

[0068] Table 1 (Comparison of ultrasonic testing data between the examples and comparative examples) As shown in Table 1, the ultrasonic testing of the tube blanks in the embodiment showed uniform echo signals at the interface between the carbon steel base layer and the corrosion-resistant metal layer, with no abnormal echoes, indicating a complete interface bond. In contrast, ultrasonic testing of the tube blanks in the comparative example revealed multiple abnormal interface echoes in the bend areas on both sides of the tube blanks with significant bending deformation.

[0069] Please see Figure 2 and Figure 3 ,in, Figure 2 Metallographic images of the tube blank interface obtained in an embodiment of the present invention; Figure 3 Here is a metallographic photograph of the tube blank interface obtained in the comparative example of this invention; Figure 2 and Figure 3 All images were taken from views of both sides of the weld seam of the welded pipe. Figure 2 and Figure 3 In the diagram, mark 1 represents the corrosion-resistant metal layer, mark 2 represents the carbon steel base layer, and mark 3 represents the weld. Figure 2 In this embodiment of the invention, the bonding interface between the carbon steel base layer and the corrosion-resistant metal layer of the tube blank is good, and no delamination or cracking defects were found. Figure 3 In the comparative tube blank of this invention, there is a cracking defect at the interface between the carbon steel base layer and the corrosion-resistant metal layer.

[0070] The above comparative results demonstrate that the present invention, by applying differentiated pressing forces to the upper and lower pressure rollers based on the measured yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer during continuous roll bending, can effectively guide the coordinated deformation of the two metal layers, significantly reduce interlayer shear stress, and prevent delamination defects, thereby ensuring the forming quality of small-diameter bimetallic composite welded pipes. The comparative example, which did not employ differentiated pressing force control, exhibited interface delamination under the same forming conditions, further verifying the necessity and effectiveness of the present invention.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for forming a small-diameter bimetallic metallurgical composite welded pipe, characterized in that, include: The bimetallic composite steel strip is continuously rolled and formed by several roll forming assemblies, so that the bimetallic composite steel strip is gradually bent into a C shape and further closed into an O shape to form a tube blank. The roll forming assembly includes an upper pressure roller and a lower pressure roller. The bimetallic composite steel strip includes a carbon steel base and a corrosion-resistant metal layer covering one side of the carbon steel base. The corrosion-resistant metal layer forms the inner surface of the tube blank after forming. During the continuous roll bending process, differentiated pressing forces are applied to the upper and lower pressure rollers based on the measured yield strength difference between the carbon steel base layer and the corrosion-resistant metal layer. The pressing force applied to the carbon steel base layer is greater than the pressing force applied to the corrosion-resistant metal layer, so as to guide the carbon steel base layer and the corrosion-resistant metal layer of the bimetallic composite steel strip to deform in a coordinated manner, maintain a synchronous deformation state, and reduce the interlayer shear stress of the bimetallic composite steel strip.

2. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 1, characterized in that, The continuous roll bending forming adopts the ERW / COE integrated forming process.

3. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 1, characterized in that, The measured yield strength data of the carbon steel base layer and the corrosion-resistant metal layer are obtained, and the pressing amount of the upper pressure roller and the lower pressure roller is automatically adjusted by the control system according to the measured yield strength data to achieve the differentiated pressing force.

4. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 3, characterized in that, At the location where the bimetallic composite steel strip closes into an O-shape to form the tube blank, the roundness data and joint gap data of the tube blank are collected in real time. Based on the roundness data and joint gap data, the pressing force of the upper pressure roller and the lower pressure roller is compensated and adjusted by the control system.

5. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 1, characterized in that, It also includes the following steps: The seam of the tube blank is pre-welded to form a pre-welded steel pipe; The pre-welded steel pipe is cut to a predetermined length to obtain a fixed-length pipe section; The carbon steel base layer of the specified length pipe section is welded internally and externally; After the carbon steel base layer is welded, the weld is inspected for defects. Once it is confirmed that there are no defects, the corrosion-resistant metal layer of the fixed-length pipe section is welded to obtain the target welded pipe.

6. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 5, characterized in that, When welding the corrosion-resistant metal layer of the fixed-length pipe section, the welding materials and welding process are selected according to the material of the corrosion-resistant metal layer.

7. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 1, characterized in that, Before the continuous roll bending process, the bimetallic composite steel strip is sequentially leveled, trimmed, sheared, and butt-welded.

8. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 7, characterized in that, The trimming includes processing V-shaped, U-shaped, or X-shaped bevels on the edge of the bimetallic composite steel strip.

9. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 5, characterized in that, After obtaining the target welded pipe, the target welded pipe is subjected to sizing and straightening, non-destructive testing of the weld, hydrostatic testing, and pipe end finishing in sequence.

10. The forming method of a small-diameter bimetallic composite welded pipe as described in claim 9, characterized in that, During the non-destructive testing of the weld, the testing data is recorded and analyzed in real time. When a defect is detected, the control system automatically positions the defect by spraying a marker.

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