As a representative product of high-strength, low-alloy structural steel, Q690MD square steel tubes have been widely used in recent years in fields such as construction, bridges, and machinery manufacturing. With its excellent mechanical properties, weldability, and weather resistance, this steel has become an indispensable material in modern engineering structures.

First, Material Properties and Standard Specifications of Q690MD Square Steel Tubes
Q690MD is a high-strength, low-alloy structural steel. Its nomenclature complies with the GB/T 1591-2018 "Low-Alloy High-Strength Structural Steel" standard. "Q" stands for yield strength, "690" indicates a minimum yield strength of 690 MPa, "M" indicates the delivery condition is thermomechanically rolled (TMCP), and "D" indicates the steel meets -20°C low-temperature impact toughness requirements. This steel achieves an optimal balance of strength and toughness through the addition of microalloying elements such as niobium, vanadium, and titanium, combined with controlled rolling and cooling processes.

Second, the production process and technical difficulties of Q690MD square steel pipes.
The production of Q690MD square and rectangular steel pipes involves multiple processes, including steelmaking, rolling, forming, and welding, each requiring precise control. During the steelmaking stage, converter or electric furnace smelting, combined with LF refining and RH vacuum degassing, is employed. Sulfur and phosphorus contents are strictly controlled (≤ 0.015% and 0.025%, respectively), and microalloying elements are precisely added. Regarding forming, square and rectangular steel pipes are primarily produced using high-frequency electric resistance welding (ERW) or submerged arc welding (SAW). The ERW process offers high production efficiency (speeds of up to 20-40 m/min) and a narrow weld heat-affected zone, but requires strict control of forming pressure and welding temperature. The SAW process is suitable for thick-walled steel pipes, offering consistent weld quality but lower efficiency. For structural parts with higher requirements, a cold-bending process followed by an overall annealing step is also employed.

Third, Q690MD Square Steel Tube Applications and Project Case Studies.
Due to its high strength, lightweight, and durability, Q690MD square and rectangular steel tubes have been widely used in a variety of fields:
1. Building Steel Structures: Used in key locations such as core support and long-span spatial structure joints in super-high-rise buildings. For example, the tree-like support structure of the Beijing Daxing Airport terminal building utilizes Q690MD rectangular tubes, reducing weight by 15% while also improving seismic performance.
2. Bridge Engineering: Suitable for arch ribs, pylons, and other load-bearing components of long-span bridges. The arched pylons of the Shanghai-Suzhou-Jiangyin Yangtze River Bridge utilize extensive Q690MD square steel tubes, achieving a world record main span of 1,092 meters.

Fourth, Market Status and Supply Chain Analysis of Q690MD Square Steel Tubes.
According to industry data, the market size of Q690MD-grade high-strength steel square tubes in my country is expected to reach approximately 850,000 tons in 2024, with an annual growth rate of approximately 12%. Market prices typically fluctuate between 6,500 and 8,500 yuan per ton, representing a 20%-30% premium over standard Q355 grades. Notably, with the advancement of the "dual carbon" strategy, green manufacturing of Q690MD products has become a new trend. Many companies have developed environmentally friendly processes such as biomass heating and waste acid recovery. A leading company has reduced energy consumption per ton of pipe to 0.38 tons of standard coal, and its wastewater reuse rate has reached over 95%.

Fifth, Technical Development Trends and Challenges of Q690MD Square Steel Pipes.
1. Performance Upgrade: Developing products with higher strength than Q890D or even Q960D grades while maintaining excellent toughness. Using the TMCP+tempering process, a pilot product has achieved a yield strength of 890 MPa and an impact energy of 54 J at -40°C.
2. Intelligent Manufacturing: Applying machine vision for automated weld inspection and employing digital twin technology to optimize the forming process. A demonstration plant has reduced its product defect rate from 1.2% to 0.3% using an AI control system. 
3. Composite Applications: Develop composite square tube structures, such as steel-aluminum and steel-carbon fiber. Tests have shown that the bending stiffness of carbon fiber-reinforced Q690MD square steel tubes can be increased by over 40%.
4. Green Transformation: Promote low-carbon production technologies such as hydrogen heating and electric arc furnace short-process technology. A pilot project using hydrogen heating reduced CO2 emissions by 62%.

Sixth: Recommendations for Selection and Usage Considerations for Q690MD Square Steel Tubes.
1. Specification Matching: Select an appropriate aspect ratio (usually 1-1.5 is appropriate) and wall thickness (generally 6-40mm) based on the load characteristics. When the aspect ratio exceeds 2:1, local stability issues should be considered.
2. Connection Design: Bolted end-plate connections are preferred. Preheat to 120-150°C and control the interpass temperature during welding. One project demonstrated that proper preheating can increase the impact energy of the weld zone by 35%. 
3. Anti-corrosion Treatment: For marine environments, a dual protection system of hot-dip galvanizing and fluorocarbon spraying is recommended. For inland areas, an epoxy zinc-rich primer system can be used.

With the transformation and upgrading of my country's manufacturing industry and the advancement of "new infrastructure," the application breadth and depth of Q690MD square and rectangular steel pipes will continue to expand. Its penetration rate in prefabricated buildings is expected to increase from the current 18% to 30% by 2026, and its use in construction machinery is expected to exceed 400,000 tons.