Production Process, Classification, and Applications of Q235 Straight-Seam Welded Steel Pipe

Q235 straight-seam welded steel pipe is made from Q235 steel strip or plate, formed by coiling and then welding. Because it utilizes high-frequency welding technology in its production process, it is also known as high-frequency welded steel pipe. As a representative product of carbon structural steel welded pipe, Q235 straight-seam welded steel pipe has become one of the preferred pipe materials in industries such as construction, machinery, and transportation due to its excellent mechanical properties, affordable price, and wide range of applications. As a key branch of welded steel pipe, Q235 straight-seam welded steel pipe occupies a key position in low-pressure fluid transportation and structural support due to its unique longitudinal weld structure and stable quality.

First, Material Properties and Standard Specifications of Q235 Straight-Seam Welded Steel Pipe

Q235 steel is a common carbon structural steel with a yield strength of 235 MPa and excellent ductility and weldability. According to the national standard GB/T 3091-2015 "Welded Steel Pipes for Low-Pressure Fluid Transportation," Q235 straight seam welded steel pipes must meet the following technical requirements:

1. Chemical Composition: Carbon content ≤ 0.22%, manganese content ≤ 1.4%, and impurity levels such as sulfur and phosphorus must be strictly controlled to ensure weld quality and corrosion resistance.

2. Mechanical Properties: Tensile strength 375-500 MPa, elongation ≥ 20%, and the ability to withstand certain impact and bending deformation.

3. Process Standards: Longitudinal welded steel pipes utilize the JCOE (pre-bending, forming, welding, and expansion) process. Welds must undergo ultrasonic or X-ray testing to ensure the absence of defects such as incomplete fusion and porosity.

Second, Key Production Process and Technical Points for Q235 Longitudinal Welded Steel Pipes

The production process for Q235 straight seam welded steel pipes primarily includes uncoiling, leveling, shearing, forming, welding, cooling, sizing, and cutting. High-frequency welding (HFW) is the core process:

- High-frequency current welding: Utilizing the skin effect and proximity effect of high-frequency current, the edges of the steel strip are instantly heated to a molten state, achieving a metallurgical bond under the action of squeeze rollers. This technology is highly efficient and energy-efficient, and the weld strength can reach over 90% of the parent metal.

- Weld seam treatment: After welding, internal and external burrs are removed, and in-line heat treatment is performed to eliminate residual stress and improve weld toughness. Some high-end products also undergo hydrostatic testing or eddy current testing to ensure sealing.

Third, Classification and Specifications of Q235 Straight-Seam Welded Steel Pipe

Based on application and processing methods, Q235 straight-seam welded steel pipe can be divided into the following categories:

1. By Weld Seam Form:

- Straight-Seam Welded Steel Pipe: The weld seam is parallel to the pipe axis. It is suitable for pipes with a diameter of ≤406mm. Common specifications range from Φ20 to Φ426mm, with a wall thickness of 1.0 to 20mm. - Spiral welded steel pipe: Its weld seam is spiral, suitable for large-diameter (Φ219-Φ3000mm) pipelines, but its compressive strength is slightly lower than that of straight-seam steel pipe.

2. By surface treatment:

- Galvanized welded steel pipe: The hot-dip galvanizing layer improves corrosion resistance and is widely used in outdoor guardrails, scaffolding, etc.

- Black pipe (non-galvanized): Lower cost, mostly used for structural support or after painting.

Fourth, Application Scenarios and Advantages of Q235 Straight Seam Welded Steel Pipe

1. Construction: As scaffolding and steel structure support columns, Q235 straight seam welded steel pipe, with its high strength and lightweight characteristics, can reduce construction load costs. For example, some auxiliary structures of the Guangzhou Tower use this type of steel pipe.

2. Fluid transportation: Used in low-pressure pipelines such as tap water and gas, its sealing and pressure resistance fully meet the requirements of the GB/T 3091 standard. 3. Machinery Manufacturing: Q235 straight seam welded steel pipe is often used for components such as agricultural machinery frames and automotive drive shafts. During processing, it can be adapted to different needs through secondary forming processes such as cold bending and punching.

Fifth, Market Status and Development Trends of Q235 Straight Seam Welded Steel Pipe

By 2025, with continued increases in domestic infrastructure investment, the annual demand for Q235 straight seam welded steel pipe is expected to exceed 30 million tons. Market competition exhibits the following characteristics:

- Green Production: Steel mills are gradually promoting high-frequency welding waste heat recovery technology, reducing energy consumption by over 30%.

- High-end Transformation: Some companies are developing Q235B-grade welded steel pipe (with trace alloying elements) for use in low-temperature (-20°C) oil and gas pipelines.

- Intelligent Inspection: AI visual recognition systems are being introduced into weld quality inspection, increasing the defect detection rate to 99.5%.

Sixth, Purchasing and Usage Recommendations

1. Quality Identification: Check the steel pipe surface for a smooth, crack-free surface and smooth welds. Require suppliers to provide third-party inspection reports (such as SGS certification).

2. Corrosion Prevention: For outdoor use, hot-dip galvanizing or epoxy coating is recommended. Regular inspection for rust is required in humid environments.

3. Cutting Precautions: Avoid flame cutting. Abrasive saws or plasma cutting are recommended to prevent thermal effects in the weld area, which may degrade performance.

Q235 straight seam welded steel pipe, as a foundational industrial material, is technologically mature and offers cost-effective advantages that will be difficult to replace in the short term. In the future, with the integration of new materials and processes, its application will further expand into emerging fields such as new energy equipment and smart warehousing.