Characteristics and Production of Q235B Longitudinal Welded Pipe

Q235B longitudinally welded pipe is widely used in construction, machinery manufacturing, steel structures, and other fields. Its performance characteristics, production process, and market applications have always been a focus of attention in the industrial sector. The following will provide a detailed analysis from five perspectives: material properties, production process, application scenarios, market status, and purchasing considerations.

First, Analysis of Q235B Material Characteristics

Q235B is a carbon structural steel, a common grade in China's GB/T700 standard. Its chemical composition is carbon ≤ 0.20%, silicon ≤ 0.35%, and manganese ≤ 0.35%-0.80%. Phosphorus and sulfur contents are each controlled below 0.045%. The letter "B" denotes the quality grade, distinguishing it from Grade A (no impact resistance) and Grade C (higher low-temperature toughness). Q235B has an impact energy requirement of ≥ 27J at a room temperature of 20°C, making it more suitable for environments subject to dynamic loads. In terms of mechanical properties, Q235B has a yield strength of ≥235 MPa, a tensile strength of 375-500 MPa, and an elongation of ≥26%. This moderate strength, combined with good plasticity, makes it an ideal base material for welded pipe manufacturing. Notably, Q235B also offers excellent weldability, with a carbon equivalent (Ceq) typically kept below 0.3%. This eliminates the need for complex preheating during welding, significantly reducing processing costs.

Second, the production process for straight seam welded pipes.

Long seam welded pipe (LSAW) is primarily produced using high-frequency electric resistance welding (ERW) or submerged arc welding (SAW). Taking a supplier's processing flow as an example, a typical production line includes the following steps:

1. Uncoiling and Leveling: Unrolling and leveling the Q235B hot-rolled coil to eliminate curling stress.

2. Edge Milling: Using a double-edge milling machine, the strip edges are precisely machined to ensure a smooth weld surface.

3. Forming and Welding: Forming into a tubular shape through multiple roll bending passes, using high-frequency induction welding (100-400kHz) to achieve instant fusion.

4. In-Line Heat Treatment: The weld undergoes medium-frequency induction annealing to eliminate residual weld stress.

5. Sizing and Straightening: A hydraulic straightening machine ensures straightness deviation ≤1mm/m.

6. Hydrostatic Testing: The standard requires a holding pressure of 6MPa for ≥5 seconds.

Compared to spiral-welded pipe, straight seam welded pipe features higher dimensional accuracy (diameter tolerance ±0.5mm) and lower ovality (≤0.6%D), making it particularly suitable for applications requiring precision assembly. Reports indicate that advanced production lines using JCOE forming technology can achieve wall thickness uniformity within ±5%.

Third, Application Scenarios and Project Case Studies of Q235B Straight Seam Welded Pipe

1. Building Structure: A commercial complex project uses Φ219×6mm welded pipe as a curtain wall support system, with a single pipe bearing up to 8 tons. Its advantage lies in its convenient joint connection, which can be directly welded to the steel plate, eliminating the need for flanges.

2. Fluid Transportation: In low-pressure (≤1.6MPa) water and gas pipelines, DN150 welded pipe replaces traditional cast iron pipe, reducing weight by 40% and improving corrosion resistance. It is important to note that the operating temperature should be maintained between -20°C and 300°C.

3. Machinery Manufacturing: An engineering machinery company uses 80×40mm rectangular welded pipe to manufacture forklift masts. A cold-bending strengthening process increases the local yield strength to 345MPa.

Specific application cases demonstrate that galvanized Q235B welded pipe can achieve a service life of up to 25 years in transmission towers in coastal areas. When the zinc coating has a zinc coating adhesion of ≥610g/m², it exhibits no red rust after more than 1000 hours of salt spray testing.

Fourth, Market Status and Price Trends of Q235B Longitudinal Welded Pipe

The latest quote shows that the wholesale price of Φ48×3.5mm Q235B longitudinally welded pipe in April 2025 is 3,850-4,200 yuan/ton, a 1.2% increase from the previous month.

1. Regional Difference: Inventory turnover in East China has been shortened to 12 days, while delivery times in North China have been extended to 20 days due to environmental protection production restrictions.

2. Specification Premium: The price gap between thin-walled (δ≤3mm) and thicker-walled products has widened to 300 yuan/ton.

3. Alternative Competition: The market share of Q355B welded pipe increased by 5% year-on-year, but Q235B still maintains a 75% cost advantage in conventional applications.

Notably, one supplier has launched a "Customized Intelligent Cutting" service that allows 6-meter standard pipes to be cut on demand, increasing material utilization from 85% to 98%, making it particularly suitable for small projects.

Fifth, Key Points for Selecting and Quality Controlling Q235B Longitudinal Welded Pipe

1. Qualification Documents: Suppliers are required to provide a CMA-certified mechanical properties test report, with particular attention to the yield strength, which should not be less than 245 MPa.

2. Appearance Inspection: The weld reinforcement should be ≤2.5 mm and free of undercut defects. When spot-checking with an eddy current flaw detector, the defect signal should not exceed 50% of the artificial defects observed in standard sample pipes.

3. Usage Recommendations:

- For pressure-bearing use, the recommended operating pressure is ≤1.6 MPa.

- For low-temperature environments (<-20°C), consider switching to Q235D grade.

- For cutting, plasma cutting is recommended to avoid localized hardening caused by carbon arc gouging.

A failure analysis case study revealed that the improper use of Q235B welded pipe in the production of pressure vessel supports (designed for a stress of 310 MPa) resulted in premature fracture. This highlights the importance of strictly adhering to the applicable scope of the GB/T 13793-2016 standard.

As steel structures evolve toward prefabricated construction, Q235B straight seam welded pipe is being upgraded toward higher precision and lighter weight. Some leading companies have begun offering value-added services such as pre-piercing and end threading, revitalizing this traditional material in the market.