Introduction: The Core Role of Carbon Steel Pipe in Construction
In the construction of modern large buildings, bridges, marine platforms, and wind turbine foundations, carbon steel pipe serves as a critical structural member for piling, load-bearing columns, and trusses. These structural components demand strict adherence to material yield strength, geometric dimensions, and weldability. In the Chinese and international markets, Q235 and Q345 are two of the most commonly specified structural steel grades. The choice between them directly impacts project cost, safety, and structural efficiency. This guide will analyze the performance differences and application scenarios for these two grades, assisting engineers and procurement staff in selecting the most suitable structural steel pipe for large construction projects.
Part I: Decoding Structural Steel Grades: The Fundamental Difference Between Q235 and Q345
Q235 and Q345 belong to the Chinese GB standard system for carbon or low-alloy high-strength steel, with their nomenclature based on minimum yield strength.
1.1 Yield Strength: The Key to Structural Efficiency
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Q235 (Common Carbon Structural Steel):
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Yield Strength: Minimum yield strength is approximately 235 MPa.
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Characteristics: Excellent plasticity, toughness, and weldability, making it the most widely used and economical structural steel.
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Application: Typically used for small-to-medium structures, non-primary load-bearing trusses, or large-diameter piling that does not require extremely high strength.
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Q345 (Low-Alloy High-Strength Structural Steel):
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Yield Strength: Minimum yield strength is approximately 345 MPa (varies by thickness).
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Characteristics: Strength is increased by adding trace alloying elements (such as Mn, Si, Nb, V, Ti). Under the same load, using Q345 allows for a reduction in wall thickness and structural weight, thereby saving material and reducing foundation loads.
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Application: Primarily used for large bridges, heavy load-bearing columns in high-rise buildings, long-span structures, or projects requiring seismic performance.
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1.2 Weldability and Low-Temperature Toughness
Weldability and low-temperature performance are crucial secondary considerations when choosing between Q235 and Q345:
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Q235: Has a low Carbon Equivalent (CE) and excellent weldability, simplifying field welding control.
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Q345: Due to alloying elements, its CE is slightly higher. Field welding demands stricter control, often requiring preheating and tighter welding procedures to prevent cold cracking in the weld area.
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Low-Temperature Toughness: Q345 can be supplied in different impact temperature grades (e.g., Q345B, Q345C), making it suitable for structures in cold climates.
Part II: Structural Pipe Manufacturing Methods and Grade Correlation
The selection of a structural steel pipe grade must be integrated with the manufacturing process (SSAW or ERW) to ensure project success.
2.1 SSAW (Spiral Welded Pipe): Large Piling and Deep Foundations
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Feature: SSAW pipe can be economically manufactured in super large diameters (e.g., over 60 inches) with thick walls.
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Grade Application:
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Large Diameter Piling: Q235 is the most common grade for pile pipe, as it meets the basic strength needed for axial load-bearing capacity at the lowest cost.
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Wind Turbine Towers / Wharf Piling: Q345 is often chosen to reduce wall thickness or increase the bearing safety margin when higher load capacity and longer lifespan are required.
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To explore the cost advantages and applications of SSAW in large diameter projects, please refer to: 【 Large Diameter Spiral Welded Pipe (SSAW): Length, Diameter, and Cost Advantages】
2.2 ERW (Electric Resistance Welded Pipe): Trusses and Smaller Structures
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Feature: ERW pipe offers extremely high geometric accuracy and good surface quality, suitable for small to medium diameters.
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Grade Application: Q235 or Q345 ERW pipe is frequently used for steel trusses, support structures, or frames requiring precise connections in buildings, where its high accuracy facilitates rapid on-site assembly and welding.
Part III: Procurement Decisions and Standard Comparison
When procuring Q235 or Q345 structural steel pipe, attention must be paid to the governing standards and how they differ from line pipe standards like API 5L.
3.1 Structural Pipe Standards vs. Line Pipe Standards
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Structural Pipe (GB/T 3091, GB/T 13793): Focuses on yield strength, dimensional accuracy, and weldability.
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Line Pipe (API 5L): Focuses on pressure capacity, brittle fracture toughness, and fluid containment (e.g., Charpy impact testing required for PSL2).
While Q235}/Q345 focus on structural integrity, high-pressure pipeline projects must strictly adhere to the API 5L standard. To understand the difference between PSL1 and PSL2, please consult: 【API 5L Line Pipe Procurement Guide: Selecting Between PSL1 and PSL2】
3.2 International Standard Cross-References
Q235 and Q345 have common international equivalents:
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Q235 Equivalent: ASTM A36 or A53 Grade B (Yield Strength approx 240 MPa).
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Q345 Equivalent: ASTM A572 Grade 50 or EN S355 (Yield Strength approx 355 MPa).
For more on the correspondence between GB grades like Q235 and JIS grades, please refer to: 【Decoding Chinese & Japanese Standards: Equivalents and Procurement of JIS G3444 STKX400】
Conclusion
The Q235 and Q345 grades are foundational to the selection of structural steel pipe in large construction projects. Q235 offers economy, excellent weldability, and sufficient strength for general piling and structures. Q345, with its higher yield strength, enables structural weight reduction and increased load-bearing efficiency, making it the ideal choice for high-rise buildings and heavy-duty bridges. The correct selection strategy involves a precise trade-off between the strength and cost of the two grades, based on the project's specific load requirements, welding environment, and budget constraints.
To formulate a comprehensive welded steel pipe procurement strategy, please return to our authoritative guide for a full comparative analysis of all pipe type selection frameworks: 【【Definitive Guide】SSAW, ERW, and LSAW Welded Steel Pipe Full Analysis: Manufacturing Processes, API 5L Standards, and Application Selection Strategy】
