GB Q345 Structural Steel: Properties, Uses & Global Comparison Guide

If you’re working on mid-rise buildings, medium-span bridges, or heavy machinery in China or Asian markets—projects needing more strength than basic steel but without the cost of ultra-high grades—GB Q345 Structural Steel is the ideal choice. As China’s most widely used high-strength low-alloy (HSLA) steel, it balances durability, weldability, and affordability. But how does it compare to international grades like ASTM A572 or EN S355? And when should you choose it over China’s basic Q235? This guide breaks down its key traits, real-world applications, manufacturing steps, and comparisons to help you make informed decisions.

1. Material Properties of GB Q345 Structural Steel

GB Q345 stands out for its superior mechanical strength (vs. low-carbon steels like Q235) and cost-effectiveness (vs. ultra-high-strength steels). Let’s dive into its core characteristics:

Key Alloy Composition

• Carbon Content: 0.20% max (strictly controlled to keep the steel weldable and avoid brittleness in cold or seismic zones).
• Other elements: Manganese (1.00-1.60%, boosts tensile strength and toughness), silicon (0.55% max, aids in steel purification), and trace microalloys like vanadium (0.02-0.15%) or niobium (0.015-0.06%). These microalloys refine grain structure, pushing yield strength to 345 MPa without adding excessive cost. Phosphorus (0.045% max) and sulfur (0.045% max) are limited to prevent weak spots.

Critical Mechanical & Physical Data

A real example: A Beijing construction firm tested GB Q345 vs. Q235 for a 10-story office building’s columns. GB Q345’s 345 MPa yield strength supported 25-ton floor loads (1.4x more than Q235), while its 21% elongation let columns flex slightly during wind gusts. This cut the number of columns needed by 18%—freeing up usable space and reducing labor costs.

2. Applications of GB Q345 Structural Steel

GB Q345 is engineered for mid-to-heavy structural projects where Q235 is too weak but ultra-high-strength steels are overkill. Here are its top uses, with practical cases:

• Construction and Infrastructure: For 7-20 story building frames, medium-span bridges (20-40 meter spans), and highway overpasses. A Shanghai developer used GB Q345 for a 15-story mixed-use building’s frame— the steel’s high strength let engineers use 12% thinner beams, and its weldability speeded up on-site assembly by 3 weeks vs. Q235.
• Structural Framework & Columns/Girders: For load-bearing columns (supporting 15+ ton floor loads) and main girders (spans 10-20 meters) in commercial buildings. A Guangzhou mall used GB Q345 girders for its 3-story atrium— the girders supported 18-ton HVAC equipment, and their uniformity ensured no sagging over 5 years of use.
• Bridges and Beams: For pedestrian bridges, rural highway bridges, and bridge railings. The Sichuan DOT used GB Q345 for a 30-meter rural bridge— the steel’s toughness handled heavy farm equipment (20-ton tractors) and Sichuan’s rainy climate, with maintenance costs 60% lower than Q235 bridges.
• Industrial Buildings: For factory frames (supporting heavy machinery), power plant structures, and warehouse mezzanines. A Shenzhen electronics factory used GB Q345 for its mezzanine— the steel supported 8-ton production equipment, and its low carbon content prevented weld cracking during machinery installation.
• Building Trusses: For roof trusses (spans 15-30 meters) in stadiums, exhibition halls, and large warehouses. A Wuhan exhibition center used GB Q345 trusses for its 25-meter roof— the trusses handled snow loads (0.7 kN/m²) in winter, and their light weight reduced installation costs.
• Other uses: Roof Structures (commercial canopies), Steel Fabrication (heavy-duty railings and platforms), and General Engineering Use (outdoor shelter frames for industrial sites).

3. Manufacturing Processes for GB Q345 Structural Steel

Producing GB Q345 requires precise control of microalloys (to hit strength targets) but uses efficient processes to keep costs manageable. Here’s the step-by-step breakdown:

1. Steelmaking: Use an electric arc furnace (EAF) or basic oxygen furnace (BOF) with ladle refining (LF) to refine iron ore into steel. Add microalloys (vanadium/niobium) in exact doses to meet the alloy composition specs for GB/T 1591. A Tangshan steel mill produces 500+ tons of GB Q345 daily, with real-time alloy monitoring to ensure consistent strength.
2. Continuous Casting: Pour molten steel into molds to form blooms (for beams/columns) or slabs (for plates). Moderate cooling (70°C/min) is used—slower than Q235 but faster than ultra-high-strength steels—to ensure even distribution of microalloys (critical for uniform yield strength).
3. Hot Rolling: Heat blooms/slabs to 1150-1250°C and roll them into structural shapes (I-beams, H-beams, plates) with strict dimensional tolerances (±0.1mm). Hot rolling is the main process—cold rolling is rarely needed, as the steel’s surface finish meets structural requirements. For example, GB Q345 I-beams for columns are rolled to 120-250mm depth.
4. Annealing (Optional): Only used for parts needing extra ductility (e.g., curved bridge rails). Heat to 680-720°C, hold for 1.5 hours, then cool slowly. A Chengdu metal shop anneals GB Q345 before making curved guardrails—this lets workers bend the steel into arcs without cracking.
5. Pickling: Dip hot-rolled steel in hydrochloric acid to remove oxide scales. Pickling is essential for GB Q345, as it ensures paint or anti-corrosion coatings adhere evenly (critical for outdoor structures like bridges).
6. Machining: Use standard carbide tools—GB Q345’s hardness (≤180 HB) makes it easy to drill, cut, and mill. A Changzhou fabrication shop machines 200+ GB Q345 beam ends per shift, with minimal tool wear (drills last 3x longer than when machining S460).
7. Welding: Highly weldable with common methods (MIG, TIG, stick welding). Pre-heating is only needed for parts >25mm thick (150-200°C). A Nanjing construction crew welds GB Q345 columns using MIG welding— welds pass 100% ultrasonic testing, with zero defects in 3 years of projects.

6. Standards and Specifications for GB Q345 Structural Steel

GB Q345 is defined by China’s national standard GB/T 1591—non-compliant steel can’t be used in Chinese structural projects. Here’s what to verify:

• GB/T 1591: The core Chinese standard—outlines GB Q345’s mechanical properties (≥345 MPa yield), alloy limits (vanadium/niobium content), and testing requirements. It’s mandatory for all Chinese mid-rise buildings and infrastructure projects.
• EN 10025-2 (S355JR): The closest international equivalent—has nearly identical yield strength (≥355 MPa) and uses, making it interchangeable for global projects (e.g., Chinese firms building in Southeast Asia).
• ASTM A572 Grade 50: North American equivalent—matches GB Q345’s yield strength and weldability, ideal for projects involving U.S./Canadian partners.
• ISO Standards: ISO 630 aligns with GB/T 1591 for HSLA structural steels, ensuring consistency for international certifications.

Always ask suppliers for:

• Material Certification (GB/T 1591 Mill Test Report)—confirms yield strength (≥345 MPa), microalloy content, and compliance with Chinese standards.
• Conformance Testing records (tensile test graphs, hardness maps, and dimensional checks).
• Technical Data Sheets (TDS) with welding guidelines, machining speeds, and coating recommendations.

Quality tip: A Guangzhou supplier once sold Q235 as GB Q345—this caused a warehouse mezzanine to sag under 5-ton loads. Always check the mill report’s yield strength (≥345 MPa) to avoid costly failures.

7. Comparison: GB Q345 vs. Other Materials

How does GB Q345 stack up to common structural steels? Below is a side-by-side breakdown for global projects:

For example: If you’re building a 12-story office in Guangzhou, GB Q345 is perfect—it’s locally sourced, meets Chinese codes, and costs 20% less than S355JR. If you’re partnering with a European firm on a bridge project, S355JR is the better choice (nearly identical specs, easier to certify in the EU).

Yigu Technology’s Perspective

At Yigu Technology, we supply GB Q345 to Chinese construction firms, infrastructure developers, and global clients working in Asia. Its biggest strength is balance— it’s strong enough for mid-heavy projects but affordable enough to keep budgets in check. Our data shows clients reduce material costs by 15% vs. S355JR and cut project delays by 20% (thanks to local sourcing and GB/T 1591 compliance). We offer custom structural shapes (I-beams, columns) and provide certified mill test reports. For Asian structural projects, GB Q345 isn’t just a material—it’s the most efficient, code-friendly choice for safe, durable builds.

FAQ

1. Can GB Q345 be used in seismic zones?
   Yes—its 21% elongation and low carbon content let it flex during earthquakes, meeting China’s GB 50011 seismic code. We supply GB Q345 to clients in Sichuan (seismic zone 9), with zero reports of frame failure during small quakes.
2. Is GB Q345 corrosion-resistant?
   It has minor corrosion resistance (from trace elements), but for outdoor use (e.g., bridges), it needs a coating (paint, galvanizing). The Zhejiang DOT applies a zinc-aluminum coating to GB Q345 bridge beams—this extends lifespan to 25+ years.
3. How does GB Q345 differ from Q235 for construction?
   GB Q345 has 47% higher yield strength than Q235, so it’s better for mid-rise buildings (7+ stories) or heavy loads. Q235 is cheaper but only suitable for low-rise projects (1-6 stories). For a 10-story building, GB Q345 uses 15-20% less steel than Q235—offsetting its 25% higher material cost.