Q690C is a high-strength low-alloy (HSLA) structural steel plate/material specified under the Chinese National Standard GB/T 1591. The designation breaks down as follows:
Q: "Qu" (屈), meaning Yield Strength.
690: Minimum Yield Strength in MPa (690 MPa).
C: Quality Class, specifically indicating the required impact toughness (Charpy V-notch) at 0°C.
Here are its key properties:
Chemical Composition:
Higher Alloy Content: Compared to lower grades like Q460D, Q690C typically contains higher levels of strengthening elements like Manganese (Mn), Chromium (Cr), Molybdenum (Mo), Nickel (Ni), Vanadium (V), Niobium (Nb), and Titanium (Ti). Carbon (C) is still controlled but might be slightly higher than Q460D.
Typical Max Limits (refer to specific mill certs for exacts):
Carbon (C): ≤ 0.20%
Silicon (Si): ≤ 0.80%
Manganese (Mn): ≤ 2.00% (often higher than Q460D)
Phosphorus (P): ≤ 0.025% (Very low for weldability & toughness)
Sulfur (S): ≤ 0.015% (Very low for weldability & toughness)
Alloying Elements: Significant additions of Cr, Ni, Mo, V, Nb, Ti, B (Boron) are common. Specific limits vary by producer and thickness.
Carbon Equivalent (CEV/Pcm): Significantly higher than Q460D due to the increased alloy content required for the higher strength. This is a critical factor affecting weldability and requiring strict procedures.
Mechanical Properties:
Yield Strength (ReH): ≥ 690 MPa (Minimum guaranteed at room temp). This is substantially higher than Q460D (460 MPa).
Tensile Strength (Rm): Typically between 770 MPa to 940 MPa.
Elongation (A5): Minimum values specified (e.g., ≥ 14% for plates t ≤ 16mm). Lower elongation than Q460D is typical due to the much higher strength.
Impact Toughness (KV2 @ 0°C): This is the defining property for grade "C". Minimum average absorbed energy per specimen at 0°C is specified (e.g., ≥ 34J or 47J, depending on thickness/sub-grade). Good toughness, but the test temperature requirement is less stringent than Q460D (-20°C).
Delivery Conditions: Achieving the high strength of Q690C requires specific processing:
Quenching and Tempering (Q+T): This is the most common and reliable method for Q690C, especially for thicker plates. Provides excellent strength, toughness, and through-thickness properties.
Thermo-Mechanically Controlled Process (TMCP): Can be used, often combined with accelerated cooling (TMCP+ACC) or direct quenching (TMCP+DQ). Requires precise control. May not achieve the same level of toughness or through-thickness consistency as Q+T for very thick plates.
Normalized (N) or Normalized Rolling (NR): Generally insufficient to achieve the required 690 MPa yield strength consistently for Q690C.
Specific Characteristics:
Very High Strength: Primary characteristic, offering a significant strength-to-weight advantage over lower grades like Q355 or Q460.
Good Toughness: Adequate impact toughness guaranteed at 0°C ("C" grade).
Weldability: More challenging than lower-strength steels like Q460D due to the high CEV/Pcm. Requires strictly controlled procedures:
Mandatory preheating (often higher temperatures).
Strict control of interpass temperatures.
Use of low-hydrogen or ultra-low hydrogen electrodes/wires.
Often requires Post-Weld Heat Treatment (PWHT) to relieve stresses and restore HAZ toughness, especially for thick sections or critical joints.
Limited Thickness Availability: Maximum available thickness is generally less than for lower grades (e.g., often max 50-100mm vs. 150mm+ for Q460D), primarily due to quenching limitations.
Higher Cost: Significantly more expensive than Q460D due to alloy content and complex processing (especially Q+T).
Typical Applications:
High-Pressure Equipment: Pressure vessels, penstocks, pipelines (where high strength reduces wall thickness).
Mining & Heavy Machinery: Excavator booms, arms, buckets, crane structures requiring extreme strength and wear resistance.
Lifting Equipment: Crane booms, heavy-duty hooks, spreader beams.
Transportation: Chassis frames for heavy trucks, trailers, specialized vehicles.
Offshore Structures: Jacket legs, flanges, critical nodes requiring high strength.
Advanced Structural Applications: Bridges needing very long spans or minimal weight, specialized building components.
Armor Plate (Military Applications).
Key Distinction vs. Other Grades:
Compared to Q690A/B: Q690C offers superior guaranteed toughness at 0°C.
Compared to Q690D/E: Q690C has a less stringent toughness requirement (tested at 0°C vs. -20°C or -40°C).
Compared to Q460D: Q690C offers ~50% higher yield strength (690MPa vs 460MPa) but is more challenging to weld, has lower elongation, potentially lower toughness (depending on grade comparison), higher cost, and limited max thickness.
Similar grades internationally include S690QL (EN 10025-6 - Quenched & Tempered), A514 Gr. S (ASTM A514 - Quenched & Tempered), and HT690 (JIS G3128 - High Strength Steel Plates).
In summary: Q690C is characterized by its very high yield strength (≥690 MPa), good toughness at 0°C, and typically requires delivery in Quenched & Tempered (Q+T) condition. Its high carbon equivalent makes welding complex and demanding strict procedures. It's used in applications where maximizing strength-to-weight ratio is critical, justifying the increased material and fabrication costs. Always consult the specific GB/T 1591 revision and mill certificates for exact properties and approved welding procedures.
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