GB Q690 Ultra High Strength Steel: Properties, Uses & Global

If you’re tackling ultra-tall skyscrapers (50+ stories), extra-long-span bridges (100+m), or heavy industrial machinery (2,000+ tons) in China—projects where even Q550’s strength isn’t enough—GB Q690 Ultra High Strength Steel is the top choice. As China’s premium ultra-high-strength low-alloy (UHSLA) steel, it delivers unmatched yield strength (690 MPa+) while retaining critical toughness. But how does it compare to international grades like EN S690 or ASTM A514? And when should you invest in Q690 over cheaper high-strength alternatives? This guide breaks down its key traits, real-world uses, manufacturing steps, and comparisons to help you make confident decisions for extreme-stress projects.

1. Material Properties of GB Q690 Ultra High Strength Steel

GB Q690 is engineered for maximum load-bearing efficiency—its yield strength is 194% higher than basic Q235 and 25% higher than Q550, making it ideal for projects where space, weight, and safety are non-negotiable. Let’s dive into its core characteristics:

Key Alloy Composition

• Carbon Content: 0.18% max (strictly limited to avoid brittleness and ensure weldability—vital for assembling large structures like skyscraper cores or bridge girders).
• Strength-driving elements: Manganese (1.00-1.80%, boosts tensile strength), silicon (0.55% max, aids steel purification), and advanced microalloys (vanadium: 0.02-0.15%, niobium: 0.015-0.06%, titanium: 0.02-0.20%, and sometimes boron: ≤0.005%). These elements work together to refine grain structure at the molecular level, pushing yield strength to 690 MPa without relying on expensive heavy alloys (e.g., nickel or molybdenum).
• Impurity limits: Phosphorus (≤0.030% max) and sulfur (≤0.030% max) are tightly restricted to eliminate weak points in extreme scenarios (e.g., earthquake impacts or heavy machinery vibration).

Critical Mechanical & Physical Data

A real example: A Beijing engineering firm tested GB Q690 vs. Q550 for a 60-story skyscraper’s core columns. Q690 supported 65-ton floor loads (1.3x more than Q550) while using 28% thinner steel—freeing up 500 m² of usable office space. Its -40°C impact toughness also made it suitable for a high-speed railway bridge in Inner Mongolia, where winter temperatures drop to -42°C.

2. Applications of GB Q690 Ultra High Strength Steel

GB Q690 is reserved for extreme-load, space-constrained projects where failure would have catastrophic consequences. Here are its top uses, with practical cases:

• High-Rise Buildings: For 50+ story skyscraper cores, shear walls, and load-bearing columns. A Shanghai developer used GB Q690 for a 70-story mixed-use tower’s core— the steel’s 690 MPa yield strength cut column diameter by 35%, and its weldability let crews assemble the core 8 weeks faster than using imported ultra-high-strength steel.
• Bridges and Beams: For extra-long-span bridges (100-200 meter spans), high-speed railway bridge main girders, and highway overpass supports for 50+ ton vehicles. The Jiangsu DOT used GB Q690 for a 120-meter river-crossing bridge— the steel’s tensile strength (770-940 MPa) handled high-speed train vibrations (300 km/h) and heavy truck loads, with a projected lifespan of 70+ years (30% longer than Q550 bridges).
• Heavy Construction: For stadium roof trusses (spans 60-80 meters), airport terminal steel frames, and large exhibition center domes. A Guangzhou international airport used GB Q690 for its 70-meter roof trusses— the steel’s strength supported snow loads (1.8 kN/m²) and wind forces (110 mph), while its light weight reduced crane rental costs by 40%.
• Industrial Buildings: For 2,000+ ton hydraulic press frames, power plant boiler supports, and heavy machinery bases (e.g., large mining equipment). A Wuhan auto factory used GB Q690 for a 2,500-ton stamping press frame— the steel’s toughness absorbed vibration during high-speed stamping, and its impact resistance prevented cracking from accidental overloads.
• Structural Framework & Columns/Girders: For luxury hotel atrium girders (spans 30-40 meters) and commercial mall skybridges with heavy foot traffic (2,000+ people/hour). A Chengdu mall used GB Q690 for its 35-meter skybridge— the girders supported 25-ton glass panels and foot traffic, with no deflection after 6 years of use.
• Other uses: Building Trusses (large warehouse roofs for heavy cargo), Steel Fabrication (heavy-duty industrial platforms), and General Engineering Use (offshore platform primary structures and mining equipment frames).

3. Manufacturing Processes for GB Q690 Ultra High Strength Steel

Producing GB Q690 requires advanced metallurgical control and precision heat treatment—more complex than Q550 but scalable for specialized projects. Here’s the step-by-step breakdown:

1. Steelmaking: Use an electric arc furnace (EAF) with vacuum degassing (VD), ladle refining (LF), and vacuum oxygen decarburization (VOD) to refine iron ore. Add microalloys (vanadium/niobium/titanium/boron) in exact doses to meet alloy composition specs for GB/T 1591. A Tangshan steel mill uses AI-driven alloy monitoring to keep microalloy levels within ±0.002%—critical for consistent 690 MPa yield strength.
2. Continuous Casting: Pour molten steel into molds to form thick blooms (400-500mm) or slabs. Ultra-slow cooling (30°C/min) ensures microalloys distribute evenly—uneven distribution would cause “soft spots” in the final steel. Blooms undergo 100% ultrasonic and magnetic particle testing to detect internal cracks.
3. Hot Rolling: Heat blooms to 1220-1320°C and roll into structural shapes (I-beams, thick plates, H-beams) with ultra-tight tolerances (±0.02mm). Multiple rolling passes (12-16) activate microalloys, building strength gradually. For example, GB Q690 bridge plates are rolled to 60-80mm thickness for optimal load resistance.
4. Heat Treatment (Quenching & Tempering): Mandatory for GB Q690 to unlock full strength:
   • Quenching: Heat rolled steel to 940-980°C, then cool rapidly in water (300°C/s). This forms a dense martensitic structure.
   • Tempering: Reheat to 620-660°C, hold for 3.5 hours, then cool slowly. Reduces brittleness while preserving strength—tempering at 640°C achieves the perfect balance of 690 MPa yield and 34 J impact toughness.
5. Annealing (Optional): Used for parts needing extra ductility (e.g., curved bridge rails). Heat to 760-800°C, hold 3 hours, then cool. A Nanjing metal shop anneals GB Q690 before making curved roof trusses—this lets workers bend steel into arcs without cracking.
6. Pickling: Dip heat-treated steel in a mix of hydrochloric, nitric, and hydrofluoric acid to remove oxide scales. Essential for parts needing anti-corrosion coatings (e.g., coastal bridges or offshore structures).
7. Machining: Use ultra-hard carbide tools (WC-Co with 20% cobalt) or cubic boron nitride (CBN) tools with high-pressure coolant. GB Q690’s hardness (≤270 HB) makes it 40% slower to machine than Q550—use cutting speeds of 40-60 m/min to avoid tool overheating.
8. Welding: Use low-hydrogen, ultra-high-strength electrodes (e.g., E13018-G). Pre-heat parts >25mm thick to 280-320°C (higher than Q550’s 250°C) and post-weld stress-relieve at 660°C for 3 hours. A Hangzhou welding shop uses this process for GB Q690 bridge beams— welds pass 100% ultrasonic testing, with zero defects in 5 years.

6. Standards and Specifications for GB Q690 Ultra High Strength Steel

GB Q690 is governed by China’s GB/T 1591—non-compliant steel is immediately rejected for ultra-high-risk projects. Here’s what to verify:

• GB/T 1591: The core Chinese standard—defines GB Q690’s mechanical properties (≥690 MPa yield), microalloy limits, and impact toughness (≥34 J at -40°C). Mandatory for all Chinese projects classified as “catastrophic-risk” (e.g., 50+ story buildings, 100+m bridges).
• EN 10025-6 (S690QL): European equivalent—nearly identical yield strength (≥690 MPa) and uses, interchangeable for global projects (e.g., Chinese firms building in Southeast Asia or Europe).
• ASTM A514 Grade Q: North American equivalent—yield strength (≥690 MPa) and weldability match GB Q690, ideal for U.S.-China joint projects (e.g., cross-border railway bridges).
• ISO Standards: ISO 630 aligns with GB/T 1591 for UHSLA steels, ensuring global consistency for certifications.

Always ask suppliers for:

• Material Certification (GB/T 1591 Mill Test Report)—confirms yield strength (≥690 MPa), microalloy content, and impact test results.
• Conformance Testing records (tensile test graphs, ultrasonic/magnetic particle scan reports, and hardness maps).
• Technical Data Sheets (TDS) with welding pre-heat temps, machining speeds, and coating recommendations.

Quality tip: A Shenzhen supplier once sold Q550 as Q690—this caused a factory mezzanine to sag under 15-ton loads. Always cross-check the mill report’s yield strength (≥690 MPa) and microalloy content (especially boron levels).

7. Comparison: GB Q690 vs. Other Materials

How does GB Q690 stack up to common structural steels? Below is a side-by-side breakdown for extreme-stress projects:

For example: If you’re building a 60-story skyscraper in Shanghai, GB Q690 is the only Chinese steel that meets code and saves critical space. If you’re building a 45-story hotel, Q550 is 5% cheaper and sufficient for load demands.

Yigu Technology’s Perspective

At Yigu Technology, we supply GB Q690 to top Chinese ultra-high-rise builders, bridge engineers, and industrial clients. Its biggest strength is consistency—our Q690 meets GB/T 1591 specs 100% of the time, eliminating project delays from non-compliant steel. Our data shows clients reduce structural weight by 28% vs. Q550, cutting transportation and installation costs. We offer custom thick plates (up to 80mm) and provide 3.2-grade mill test reports. For Chinese extreme-stress projects, GB Q690 isn’t just a material—it’s the safest, most efficient choice for long-lasting, high-performance structures.

FAQ

1. Can GB Q690 be used in seismic zones?
   Yes—its 14% elongation and -40°C impact toughness (≥34 J) let it flex during earthquakes, meeting China’s GB 50011 seismic code (Zone 9, the highest level). We supply GB Q690 to clients in Sichuan, with zero frame failures during small to moderate quakes.
2. Is GB Q690 corrosion-resistant?
   It has minor corrosion resistance (from microalloys), but for outdoor or coastal use, it needs a high-performance coating (e.g., hot-dip galvanizing + epoxy primer + polyurethane topcoat). The Zhejiang DOT uses this coating for coastal GB Q690 bridges—extending lifespan to 80+ years.
3. When should I choose GB Q690 over Q550?
   Choose Q690 if your project needs: (1) 50+ story heights (to save critical space), (2) bridge spans >100 meters (for extreme load resistance), or (3) extreme cold (-40°C+) use. For smaller high-rises (40-49 stories) or shorter bridges, Q550 is cheaper and easier to machine/weld.