First, Q690D Material Characteristics
Q690D is a high-strength, low-alloy steel. The "Q" in its designation stands for yield strength, "690" indicates a minimum yield strength of 690 MPa, and "D" represents the quality grade (-20°C impact resistance). This material achieves a balance of high strength and good toughness through the addition of microalloying elements such as niobium, vanadium, and titanium, combined with a controlled rolling and controlled cooling process (TMCP) or quenching and tempering (QT) heat treatment. Typical mechanical properties are as follows:
- Yield strength: ≥690 MPa
- Tensile strength: 770-940 MPa
- Elongation: ≥14%
- Low-temperature impact toughness: Impact energy ≥34 J at -20°C
In addition, the carbon equivalent (Ceq) of Q690D is typically kept below 0.47. Low-hydrogen welding consumables and a preheating process (preheat temperature approximately 100-150°C) are required during welding to avoid the risk of cold cracking.

Second, the production process of straight seam welded pipes.
Q690D straight seam steel pipes are primarily manufactured using high-frequency electric resistance welding (ERW) or submerged arc welding (SAW). The specific process includes:
1. Steel plate pretreatment: The Q690D steel plate is flattened and milled to ensure edge flatness.
2. Forming: The steel plate is bent into a tube using the JCOE (progressive bending) or UOE (compression forming) processes.
3. Welding:
- ERW process: This process uses high-frequency current to heat the steel plate edges until they are molten, then welds them under pressure. This process is highly efficient and cost-effective, suitable for small and medium-diameter pipes (Φ20-Φ660mm).
- SAW process: This process is suitable for thick-walled, large-diameter pipes (Φ406-Φ1420mm). It utilizes multi-wire submerged arc welding, resulting in deep weld penetration and stable weld formation.
4. Weld seam treatment: Weld seam quality is inspected online using ultrasonic or X-ray technology, and heat treatment is performed to eliminate residual stress if necessary. 5. Sizing and Finishing: Cold expansion improves dimensional accuracy, while chamfered ends facilitate on-site docking.

Third, Core Applications of Q690D Straight Seam Steel Pipe
1. Construction Machinery: Used in load-bearing components such as crane booms and pump truck outriggers, reducing deadweight while increasing load capacity.
2. Mine Support: As columns and top beams in hydraulic supports, they must withstand high pressure and impact loads. The high strength of Q690D significantly extends its service life.
3. Bridge Construction: Suitable for chords and supporting structures in long-span bridges. For example, the steel tube trusses of the Hutong Yangtze River Bridge utilize this same high-strength steel.
4. Shipbuilding and Offshore Engineering: Used in ship frames and offshore platform jackets. Seawater corrosion resistance is further enhanced through coating or alloying.

Fourth, Market Status and Development Trends of Q690D Straight Seam Steel Pipe
According to industry data, the market size of high-strength welded pipes in my country will exceed 20 billion yuan in 2023, of which Q690D and above grades will account for approximately 30%. As the "dual carbon" goals advance, the demand for lightweight design is driving high-strength steel to replace traditional materials. For example, new energy wind turbine towers are beginning to trial the use of Q690D straight seam steel pipe to reduce tower weight.
However, technical bottlenecks remain:
- Domestic Substitution: Some high-end Q690D steel plates rely on imports. Companies such as Baowu Steel Group and Anshan Iron and Steel Group are accelerating the development of low-cost controlled rolling processes.
- Welding Process Optimization: New technologies such as narrow-gap welding and laser-arc hybrid welding can further improve efficiency and reduce softening in the heat-affected zone.

Fifth, Recommendations for the Selection and Use of Q690D Straight Seam Steel Pipe
1. Certification Standards: Prefer pipes that comply with GB/T 1591-2018 or EN 10025-6 standards, and require suppliers to provide third-party inspection reports.
2. Processing Precautions:
- Avoid high temperatures during cutting that may cause edge hardening. Plasma or water jet cutting is recommended.
- Dehydrogenation treatment is required after welding to prevent delayed cracking. 
3. Anti-corrosion measures: Hot-dip galvanizing or epoxy powder spraying is recommended for outdoor use. In corrosive environments, "Q690D+weather-resistant layer" composite pipes can be used.