A method for improving the quality of the weld of a high frequency electric resistance welded square tube

Abstract

The application discloses a method for improving the welding quality of high-frequency resistance welded square tubes, which comprises six steps of strip steel pretreatment, accurate temperature control welding, inert gas collaborative protection, online welding seam rolling shaping, gradient heat treatment and cooling shaping. The process is suitable for the continuous production line of square tubes, can greatly reduce the welding defect rate, improve the plasticity, toughness and size accuracy of the welding seam, highly match the welding seam performance with the base metal of high-strength girder steel, is suitable for the large-scale production of various high-strength girder steel square tubes, and effectively solves the technical problems of welding seam oxidation, hard and brittle structure, large residual stress and the like in the traditional process.

Description

Technical Field

[0001] This invention belongs to the field of metal pipe welding and processing technology, specifically relating to a method for improving the weld quality of high-frequency resistance welded square tubes. It is particularly suitable for the continuous production of square welded tubes made of high-strength beam steel, and can effectively improve the mechanical properties of the weld, reduce the weld defect rate, and extend the service life of the square tube. Background Technology

[0002] High-frequency resistance welding (HFW) is currently the mainstream process for the mass production of square welded pipes. It utilizes the skin effect and proximity effect of high-frequency current to rapidly heat the edges of the strip steel to be welded to a molten or semi-molten state. Then, pressure is applied by extrusion rollers to achieve metal bonding. It has advantages such as high production efficiency, good welding precision, and low cost, and is widely used in automobile frames, heavy truck beams, construction machinery, trailer bodies, building load-bearing structures, and other fields.

[0003] High-strength beam steel, as the core material for heavy-duty, high-strength structural components, has the characteristics of high yield strength, good fatigue performance, and strong load-bearing capacity. However, weld quality problems are more prominent during high-frequency welding.

[0004] 1. Prominent problems of weld oxidation and inclusions: During the welding process, the edges of the high-strength beam steel to be welded are in a high-temperature molten state and come into direct contact with air, resulting in severe oxidation, forming defects such as oxide scale and slag inclusions. Some oxides cannot be completely removed during the extrusion process and remain inside the weld, leading to a decrease in weld density and making it prone to hidden dangers such as cracks and porosity.

[0005] 2. Uneven weld structure and high tendency to be hard and brittle: High-strength beam steel has a high alloy content, and the high frequency welding heating speed and cooling rate are fast. The weld and heat-affected zone are prone to forming coarse structure, hard and brittle structure of quenched martensite and bainite, uneven grain size, high weld hardness, and significantly reduced plasticity and toughness. The mechanical properties are significantly different from those of the base material. When the square tube is under stress, the weld is prone to brittle fracture.

[0006] 3. Poor matching of welding parameters: In traditional processes, parameters such as high-frequency welding power, welding speed, extrusion pressure, and induction coil position are mostly fixed values. They are not dynamically optimized in combination with the characteristics of high-strength beam steel and square tube specifications. This can easily lead to problems such as insufficient welding temperature resulting in poor fusion, or excessive temperature causing overheating of the weld, coarse grains, and loss of alloying elements.

[0007] 4. Excessive residual stress in the weld: High-strength beam steel has high strength and a high yield strength ratio. During the welding process, rapid local heating and cooling generate large residual tensile stress in the weld area. This not only reduces the fatigue resistance and corrosion resistance of the square tube, but also causes deformation and warping after the square tube is formed, affecting dimensional accuracy.

[0008] Existing methods for improving weld quality mostly focus solely on gas shielding or post-weld heat treatment, failing to develop a comprehensive, systematic process optimization scheme suitable for high-strength beam steel. This results in problems such as poor protection effectiveness, high energy consumption during heat treatment, and incompatibility with continuous production lines. For example, while some existing technologies employ inert gas shielding of the welding area, they typically use a single set of inert gas nozzles for protection. Regarding heat treatment, existing technologies mostly employ two-stage heat treatment, lacking refined control methods for the weld microstructure of high-strength beam steel. Therefore, developing a comprehensive, efficient, and continuous high-frequency resistance welding method for improving the weld quality of high-strength beam steel square tubes has become a pressing technical challenge in this field. Summary of the Invention

[0009] To address the defects in existing high-frequency resistance welded square tube welds, such as oxide inclusions, uneven microstructure, high tendency to harden and become brittle, poor mechanical properties, and high residual stress, this invention provides a method to improve the weld quality of high-frequency resistance welded square tubes. Through full-process process optimization, it achieves precise temperature control during welding, efficient oxygen-free protection, refined control of weld microstructure, and efficient elimination of residual stress. This significantly improves the density, plasticity, toughness, and dimensional accuracy of the weld, making the weld performance match that of the high-strength beam steel base material, and meeting the needs of continuous and efficient square tube production.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] This invention provides a method for improving the weld quality of high-frequency resistance welded square tubes, comprising six core steps: strip pretreatment, precise temperature-controlled welding, inert gas synergistic protection, online weld rolling and shaping, gradient heat treatment, and cooling and setting. The specific process is as follows:

[0012] 1. Strip pretreatment

[0013] High-strength beam steel strips with a thickness of 2.0–8.0 mm are selected. First, the oil, rust, and scale on the surface of the strip are removed by a degreasing unit. The degreasing temperature is controlled at 40–60℃, and the degreasing solution is an alkaline degreasing agent with a pH value of 9–11. After degreasing, the strip is dried with waterless hot air at a temperature of 80–100℃ to ensure that the surface cleanliness of the edge to be welded reaches Ra0.8–1.6μm, with no impurities or moisture residue, thus avoiding the formation of defects in the weld seam due to impurities during the welding process.

[0014] 2. Precision temperature control welding

[0015] The pre-treated strip steel is fed into the square tube forming unit and rolled into square tube blanks through the initial forming rolls and the finishing forming rolls in sequence. The joint of the tube blank enters the high-frequency welding area. Welding is carried out using a solid-state high-frequency welding machine. The welding parameters are dynamically matched according to the thickness of the high-strength beam steel and the specifications of the square tube: high frequency 300–500kHz, welding power 150–500kW, and welding speed 15–40m / min. The temperature of the edge of the tube blank to be welded is monitored in real time by an infrared thermometer, and the welding temperature is precisely controlled at 1350–1450℃. This ensures that the edge of the strip steel reaches a semi-molten state to achieve full fusion, while avoiding excessive temperature that would cause alloy element burn-off and coarse grains. The pressure of the welding extrusion roll is controlled at 8–15MPa to ensure that the molten metal is fully combined and to expel excess oxides and slag.

[0016] 3. Inert gas synergistic protection

[0017] Two sets of inert gas injection devices are symmetrically installed on both sides of the tube blank joint in the high-frequency welding area. One set is close to the induction heating coil, and the other set is close to the extrusion roller, forming an oxygen-free protective atmosphere throughout the process. The first set of injection devices is 50–80 mm away from the induction coil and injects a mixture of argon and carbon dioxide at a volume ratio of 7:3 and a gas flow rate of 12–18 L / min to pre-protect the edge to be welded during the heating process and prevent initial oxidation. The second set of injection devices is 30–50 mm away from the extrusion roller and injects pure argon at a gas flow rate of 8–12 L / min to precisely protect the molten weld and avoid secondary oxidation during extrusion. The injection angle of both sets of gases is 30–45° with the tube blank joint to ensure that the gas completely covers the weld area and isolates it from air.

[0018] 4. Online weld seam rolling and shaping

[0019] Immediately after welding, the square tube enters the online rolling and shaping unit. A pair of precision shaping rollers are used to lightly roll the weld and the heat-affected zones on both sides. The rolling pressure is 3–5 MPa, and the rolling speed is synchronized with the welding speed. The working surface of the shaping rollers is in close contact with the inner and outer walls of the square tube. The rolling amount is controlled at 0.1–0.3 mm. Rolling further compacts the tiny pores inside the weld, eliminates defects such as porosity and looseness, and corrects weld forming deviations to ensure the dimensional accuracy of the square tube and make the surface flatness of the weld consistent with the base material.

[0020] 5. Gradient heat treatment

[0021] After rolling and shaping, the square tubes enter an online gradient heat treatment furnace, employing a three-stage gradient heating process to achieve refined control of the weld microstructure of high-strength beam steel: The first stage is the austenitizing heating stage, with a temperature of 850–900℃ and a heating time of 10–15s, which fully austenitizes the weld microstructure and refines the austenite grains; the second stage is the isothermal homogenization stage, with a temperature of 750–800℃ and a homogenization time of 15–20s, which balances the temperature difference between the weld and the base material, eliminates structural stress, and promotes grain homogenization; the third stage is the tempering and stabilization stage, with a temperature of 550–600℃ and a tempering time of 20–25s, which eliminates the hard and brittle structure of the weld after quenching, reduces weld hardness, and improves plasticity and toughness.

[0022] 6. Cooling and shaping

[0023] The square tube after gradient heat treatment is cooled by segmented air cooling. First, it passes through a strong air cooling section with a wind speed of 15–20 m / s to quickly reduce the temperature of the square tube to below 300℃, preventing the grains from coarsening again. Then, it passes through a slow air cooling section with a wind speed of 5–8 m / s to cool the square tube evenly to room temperature. During the cooling process, the square tube is kept moving at a constant speed to avoid uneven cooling that could lead to deformation. Finally, the production of high-strength beam steel high-frequency resistance welded square tube is completed.

[0024] In some embodiments, the high-strength beam steel is 700L, 610L, or 510L high-strength beam steel.

[0025] In some embodiments, the thickness of the strip is 2.0 to 8.0 mm.

[0026] In some embodiments, the first set of inert gas injection devices is 50-80 mm away from the induction coil, and the second set of inert gas injection devices is 30-50 mm away from the extrusion roller.

[0027] In some embodiments, the degreasing solution used in the strip pretreatment is an alkaline degreasing agent with a pH value of 9-11.

[0028] In some embodiments, during the precise temperature control welding step, the temperature of the tube blank edge to be welded is monitored in real time using an infrared thermometer.

[0029] In some embodiments, the online weld seam rolling and shaping step uses a precision shaping roller, the working surface of which is in contact with the inner and outer walls of the square tube.

[0030] In some embodiments, during the cooling and shaping step, the strong air cooling section rapidly reduces the temperature of the square tube to below 300°C, while the gentle air cooling section cools the square tube evenly to room temperature.

[0031] This invention ensures a clean welding interface through strip surface purification treatment, achieves precise temperature control and fusion by dynamically matching high-frequency welding parameters, uses dual-group inert gas synergy to prevent weld oxidation, eliminates internal porosity in the weld through online rolling and shaping, and refines the weld microstructure and eliminates residual stress through three-stage gradient heat treatment, ultimately achieving a comprehensive improvement in weld quality. This process is suitable for continuous production lines of square tubes, significantly reducing weld defect rates, improving weld plasticity, toughness, and dimensional accuracy, and ensuring that weld performance is highly matched to the high-strength beam steel base material. It is applicable to the large-scale production of various high-strength beam steel square tubes, effectively solving technical problems such as weld oxidation, brittle microstructure, and high residual stress in traditional processes.

[0032] Beneficial effects

[0033] 1. Significantly reduce weld defect rate: Through the synergistic protection of dual inert gases, the air in the welding area is completely isolated, the weld oxidation rate is reduced by more than 90%, and the defect rate of slag inclusions, porosity, cracks and other defects is reduced to below 0.5%, and the weld density is significantly improved.

[0034] 2. High degree of matching between weld microstructure and high strength beam steel: The gradient heat treatment process enables fine control of weld microstructure, eliminates coarse and brittle microstructure, ensures uniform weld grain size, controls the difference between hardness and base metal within 25HB, improves plasticity and toughness by more than 40%, and achieves a matching degree of more than 95% between weld tensile strength and yield strength and base metal.

[0035] 3. Significantly reduced residual stress: Online rolling and shaping combined with gradient heat treatment effectively releases welding residual stress, with a residual stress elimination rate of over 85%, avoiding deformation and warping after square tube forming, and improving dimensional accuracy by 30%;

[0036] 4. Adaptable to continuous production: The entire process is seamlessly integrated with the square tube continuous production line, requiring no downtime. Production efficiency is on par with traditional processes, with no additional high-energy-consuming equipment, reducing production costs by about 10%, making it suitable for large-scale industrial applications.

[0037] 5. Wide range of applications: It can be adapted to the production of high-strength beam steel square tubes of different thicknesses and specifications. The process parameters can be flexibly adjusted, making it highly versatile.

[0038] Compared with specific existing technologies, the present invention has the following characteristics:

[0039] (1) Compared with CN103386544A (hereinafter referred to as prior art 1)

[0040] CN103386544A discloses a method for determining the high-frequency welding power of cold-formed thick-walled rectangular welded pipes. It focuses only on the calculation of the welding power formula (N=ctv) and determines the power based on the wall thickness and speed. It does not involve protection, online rolling, or gradient heat treatment. It is applicable to ordinary carbon steel thick-walled square pipes and does not involve high-strength beam steel.

[0041] The difference between this invention and prior art 1 is that:

[0042] 1. The scope of protection is completely different: Existing technology 1 is only a single parameter calculation method; this invention is a full-process six-step method (pretreatment + temperature-controlled welding + dual gas protection + online rolling + three-stage heat treatment + segmented cooling).

[0043] 2. Unorganized control and residual stress elimination: Existing technology 1 has no heat treatment, rolling and shaping, or gas protection; This invention is specifically designed for the hard and brittle tendency of beam steel, with a three-stage gradient heat treatment + online rolling, which solves the problems of hard and brittle welds and large residual stress.

[0044] 3. Applicable steel types and scenarios: Existing technology 1 is for ordinary carbon steel thick-walled pipes; this invention is specifically adapted to high-strength beam steel (700L / 610L / 510L), solving the unique defects of alloy steel such as easy oxidation during welding, uneven microstructure, and poor fatigue.

[0045] (2) Compared with CN107030135A (hereinafter referred to as prior art 2)

[0046] CN107030135A discloses a method for forming a high-strength, thick-walled, small-section double-pointed square tube, which focuses on the control of cold bending forming passes, angles, and springback; high-frequency welding is only conventional welding after closure, without gas protection, online rolling, or special heat treatment; it is applicable to high-strength steel, but there is no systematic improvement in weld quality, relying only on forming optimization.

[0047] The difference between this invention and prior art 2 is that:

[0048] 1. The innovations are completely different: Existing technology 2 is an optimization of the forming process; this invention is a full-process control of weld quality.

[0049] 2. No anti-oxidation and microstructure refinement methods: Existing technology 2 welding is carried out in air, which easily produces oxidation inclusions and hard and brittle microstructures; the present invention uses dual inert gas synergistic protection (pre-protection + fine protection) + three-stage gradient heat treatment to completely solve oxidation, inclusions and coarse grains.

[0050] 3. No weld seam internal densification treatment: Existing technology 2 does not have online rolling and shaping; the present invention uses online light pressure rolling (0.1–0.3 mm) to compact the internal pores of the weld seam, eliminate porosity, and improve density and fatigue performance.

[0051] 4. Different targets: Existing technology 2 is for forming square tubes with double sharp corners and small cross sections; this invention is specifically for the weld seams of high-strength beam steel square tubes (requiring heavy loads, high fatigue, and high toughness).

[0052] (3) Compared with CN104772625A (hereinafter referred to as prior art 3)

[0053] CN104772625A discloses a high-strength steel and ultra-high-strength steel high-frequency welded pipe unit, which improves the structure of the uncoiling, shearing, looping, forming, cooling and sizing unit; the process only uses conventional high-frequency welding + air cooling, without gas protection, online rolling and gradient heat treatment; it is suitable for high-strength steel, but there is no special process for weld quality, and it only relies on the rigidity of the equipment to improve.

[0054] The difference between this invention and the prior art 3 is that:

[0055] 1. Different types: Existing technology 3 is a utility model / invention of equipment; this invention is a process method invention.

[0056] 2. Core process for seamless quality: Existing technologies lack protection, rolling, and specialized heat treatment; this invention employs a six-step systematic process that controls contamination at the source, controls temperature during the process, protects against oxidation, achieves dense rolling, and stabilizes the microstructure through heat treatment.

[0057] 3. Targeted design for beamless steel: Existing technology 3 is a general high-strength steel unit; this invention is designed for the alloy characteristics of high-strength beam steel, optimizing welding temperature, extrusion pressure, and tempering temperature to ensure that the strength and toughness of the weld matches the base material.

[0058] 4. No residual stress control: Existing technology 3 only uses conventional cooling; the present invention uses online rolling + gradient heat treatment + segmented air cooling, and the residual stress elimination rate is >85%, which meets the fatigue resistance requirements of the main beam steel.

[0059] Compared with the prior art, the present invention has the following advantages:

[0060] 1. Novel technical solution: Existing technologies are all single-parameter / forming / equipment improvements; this invention is the first to create a six-step integrated process of "pretreatment + precise temperature-controlled welding + dual inert gas synergistic protection + online weld seam rolling and shaping + three-stage gradient heat treatment + segmented air cooling". The overall solution has not been disclosed or combined with any existing technology.

[0061] 2. Key process features are novel: dual-stage inert gas protection (pre-protection + fine protection), which has not been disclosed in existing technologies; online light-pressure rolling and shaping (0.1~0.3mm), which has not been disclosed in existing technologies; three-stage gradient heat treatment (austenitization + isothermal heating + tempering and stabilization), which has not been disclosed in existing technologies; comprehensive control of weld strength, toughness, residual stress and fatigue performance specifically for high-strength beam steel (700L / 610L / 510L), which has not been covered by existing technologies.

[0062] 3. Significant and novel technical effects: This invention achieves: a reduction of weld oxidation rate of >90%, a defect rate of <0.5%; a weld hardness difference of <25HB from the base metal, and a strength-toughness matching degree of >95%; and a residual stress elimination rate of >85%. The above comprehensive effects have not been achieved or mentioned by any existing technology. Attached Figure Description

[0063] Figure 1 Images showing the cracked state of the weld seam of a square tube that was not processed and welded using the method described in this invention.

[0064] Figure 2 The image shows a square tube welded using the method described in this invention, in good condition, as shown in Example 1. Detailed Implementation

[0065] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0066] Example 1

[0067] A method for improving the weld quality of high-frequency resistance welded square tubes, producing high-strength beam steel square tubes with specifications of 50mm×50mm×3.0mm, includes the following specific steps:

[0068] 1. Strip steel pretreatment: 3.0mm thick high-strength beam steel strip is selected, degreasing temperature is 50℃, alkaline degreasing agent pH value is 10, and drying is carried out with waterless hot air at 90℃. The surface cleanliness of the strip steel edge to be welded is Ra1.2μm.

[0069] 2. Precise temperature control welding: high frequency 400kHz, welding power 280kW, welding speed 25m / min, welding temperature 1400℃, extrusion roller pressure 10MPa;

[0070] 3. Inert gas synergistic protection: The first set of injection devices is 65mm away from the induction coil, with an argon to carbon dioxide volume ratio of 7:3 and a flow rate of 15L / min; the second set of injection devices is 40mm away from the extrusion roller, with a pure argon flow rate of 10L / min and an injection angle of 40°.

[0071] 4. Online weld seam rolling and shaping: rolling pressure 4MPa, rolling amount 0.2mm, rolling speed synchronized with welding speed;

[0072] 5. Gradient heat treatment: austenitizing heating section 880℃, heating time 12s; isothermal soaking section 780℃, soaking time 18s; tempering and stabilizing section 580℃, tempering time 22s;

[0073] 6. Cooling and shaping: The strong wind cooling section has an air velocity of 18m / s (the strong wind cooling section quickly reduces the temperature of the square tube to below 300℃), and the slow wind cooling section has an air velocity of 6m / s, cooling to room temperature.

[0074] 7. Performance test results: The weld is free of oxidation and slag inclusion defects, with tensile strength ≥550MPa, yield strength ≥420MPa, elongation ≥22%, weld hardness difference from base metal ≤25HB, residual stress ≤30MPa, and square tube dimensional deviation ≤0.2mm, meeting the requirements for use in heavy-duty structural components such as automobile beams and trailer frames.

[0075] Example 2

[0076] A method for improving the weld quality of high-frequency resistance welded square tubes, producing high-strength beam steel square tubes with specifications of 80mm×40mm×5.0mm, includes the following specific steps:

[0077] 1. Strip steel pretreatment: 5.0mm thick high-strength beam steel strip is selected, degreasing temperature is 55℃, alkaline degreasing agent pH value is 10.5, and drying is carried out with waterless hot air temperature of 95℃. The surface cleanliness of the strip steel edge to be welded is Ra1.0μm.

[0078] 2. Precise temperature control welding: high frequency 450kHz, welding power 400kW, welding speed 20m / min, welding temperature 1420℃, extrusion roller pressure 12MPa;

[0079] 3. Inert gas synergistic protection: The first set of injection devices is 70mm away from the induction coil, with an argon to carbon dioxide volume ratio of 7:3 and a flow rate of 16L / min; the second set of injection devices is 45mm away from the extrusion roller, with a pure argon flow rate of 11L / min and an injection angle of 42°.

[0080] 4. Online weld seam rolling and shaping: rolling pressure 4.5MPa, rolling amount 0.25mm, rolling speed synchronized with welding speed;

[0081] 5. Gradient heat treatment: austenitizing heating section 890℃, heating time 14s; isothermal soaking section 790℃, soaking time 19s; tempering and stabilizing section 590℃, tempering time 24s;

[0082] 6. Cooling and shaping: The strong wind cooling section has an air velocity of 19 m / s (the strong wind cooling section quickly reduces the temperature of the square tube to below 300℃), and the slow wind cooling section has an air velocity of 7 m / s, cooling to room temperature.

[0083] 7. Performance test results: The weld is free of porosity and cracks, with tensile strength ≥600MPa, yield strength ≥460MPa, elongation ≥20%, uniform weld hardness, residual stress ≤25MPa, and square tube straightness ≤1mm / m. It is suitable for high-end heavy-duty scenarios such as heavy truck beams and engineering machinery load-bearing frames.

[0084] The production method described in this invention not only ensures good weld quality of the square tube after welding, but also improves the continuous operation efficiency of the square tube production line.

[0085] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., 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 improving the weld quality of high-frequency resistance welded square tubes, characterized in that, The process includes strip pretreatment, precise temperature-controlled welding, inert gas synergistic protection, online weld seam rolling and shaping, gradient heat treatment, and cooling and setting. The specific process is as follows: (1) Strip steel pretreatment: Remove oil, rust and oxide scale from the surface of high-strength beam steel strip, degreasing temperature 40–60℃, hot air drying temperature 80–100℃, and surface cleanliness Ra of the edge to be welded Ra0.8–1.6μm; (2) Precision temperature control welding: high frequency 300–500kHz, welding power 150–500kW, welding speed 15–40m / min, welding temperature 1350–1450℃, extrusion roller pressure 8–15MPa; (3) Inert gas synergistic protection: Two sets of inert gas injection devices are installed in the welding area. The first set sprays a mixture of argon and carbon dioxide with a volume ratio of 7:3 and a flow rate of 12–18 L / min; the second set sprays pure argon with a flow rate of 8–12 L / min and a spray angle of 30–45°. (4) Online weld seam rolling and shaping: rolling pressure 3–5 MPa, rolling amount 0.1–0.3 mm, rolling speed synchronized with welding speed; (5) Gradient heat treatment: three-stage heating, austenitizing stage 850–900℃, 10–15s, isothermal soaking stage 750–800℃, 15–20s, tempering stage 550–600℃, 20–25s. (6) Cooling and shaping: segmented air cooling, strong wind speed 15–20 m / s, slow wind speed 5–8 m / s, cooling to room temperature.

2. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, The high-strength beam steel is 700L, 610L or 510L high-strength beam steel.

3. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, The thickness of the strip is 2.0–8.0 mm.

4. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, In the strip pretreatment, the degreasing solution is an alkaline degreasing agent with a pH value of 9-11.

5. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, The first set of inert gas injection devices is 50–80 mm away from the induction coil, and the second set is 30–50 mm away from the extrusion roller.

6. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, In the precise temperature control welding step, the temperature of the edge of the tube blank to be welded is monitored in real time using an infrared thermometer.

7. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, In the online weld seam rolling and shaping step, a precision shaping roller is used, and the working surface of the shaping roller is in contact with the inner and outer walls of the square tube.

8. The method for improving the weld quality of high-frequency resistance welded square tubes according to claim 1, characterized in that, In the cooling and shaping step, the strong air cooling section rapidly reduces the temperature of the square tube to below 300°C, while the gentle air cooling section cools the square tube evenly to room temperature.

Images