Q345D Seamless Square Steel Pipe Processing Technology

First, Material Properties and Chemical Composition of Q345D Seamless Square Steel Pipe
Q345D seamless square steel pipe is made from low-alloy, high-strength structural steel Q345D. Its chemical composition strictly adheres to national standards:
Carbon: 0.12%-0.18% to ensure basic strength;
Silicon: 0.20%-0.50% to enhance oxidation resistance;
Manganese: 1.2%-1.7% to enhance toughness and weldability;
Phosphorus and Sulfur: ≤0.025% (≤0.030% for some standards) to reduce brittleness;
Alloying Elements: Niobium ≤0.07%, Vanadium ≤0.15%, Aluminum ≥0.015%. Microalloying refines grain size and improves strength and low-temperature toughness. Low-Temperature Toughness: The "D" grade for Q345D indicates that it maintains excellent impact resistance (longitudinal impact value ≥40J, transverse impact value ≥27J) at -20°C, making it suitable for cold regions or marine engineering.

Second, Mechanical Properties and Processing Performance of Q345D Seamless Square Steel Pipe
Mechanical Properties of Q345D Seamless Square Steel Pipe:
Yield Strength: ≥345 MPa for wall thickness ≤16 mm, ≥335 MPa for wall thicknesses 16 mm to 40 mm, and ≥325 MPa for wall thicknesses >40 mm;
Tensile Strength: 490 to 670 MPa;
Elongation: ≥21% in longitudinal direction, ≥19% in transverse direction;
Hardness: Brinell hardness (HB) ≤217, Rockwell hardness (HRC) ≤15. Processing Performance of Q345D Seamless Square Steel Pipe:
Weldability: Low sulfur and phosphorus content (≤0.025%) requires the use of low-hydrogen welding consumables, along with preheating (100-150°C) and heat treatment to prevent cold cracking.
Cold Bending: No cracking after 180° cold bending, meeting the processing requirements of complex geometries.
Fatigue Resistance: Optimized grain size (grade 8 and above) improves fatigue life under cyclic loading.

Third, Q345D Seamless Square Steel Pipe Production Process and Quality Control
Q345D Seamless Square Steel Pipe Production Process:
Hot Rolling: Suitable for large-size square steel pipes (side length ≥ 100mm), high-temperature rolling at 1150°C with a compression ratio of 6:1 to ensure grain refinement;
Cold Drawing Finishing: Suitable for small-size square steel pipes (side length < 100mm), a three-pass cold drawing process with a surface roughness Ra ≤ 3.2μm and a wall thickness tolerance of ±0.2mm;
Heat Treatment: Normalizing (880-940°C) + tempering to eliminate internal stress and improve toughness.
Q345D Seamless Square Steel Pipe Quality Control:
Three-Level Inspection System: Raw Material Spectral Testing; Online Ultrasonic Testing; Finished Product Mechanical Testing.
Standard Certification: Complies with GB/T 8162, GB/T 6728-2017, ASTM A500, or EN10210 standards. Some manufacturers are ISO 3834 welding system certified.

Fourth, Specifications and Weight Calculation for Q345D Seamless Square Steel Tubes
Common Specifications:
Small Diameter: 30 × 40 × 4 mm (suitable for lightweight structures);
Medium Diameter: 150 × 150 × 10 mm (suitable for building frames);
Large Diameter: 300 × 300 × 20 mm (suitable for bridge beams).
Weight Calculation Formula:
Square Tube: Weight (kg/m) = 4 × Wall Thickness × (Side Length - Wall Thickness) × 0.00785
Rectangular Tube: Same as above, substituting the length and width for the dimensions. Example: The weight per meter of a 200 × 200 × 16 mm square tube is 4 × 16 × (200 - 16) × 0.00785 = 92.6 kg.

Fifth, Q345D Seamless Square Steel Tube Applications and Advantages
Building Structures: High strength (yield strength ≥ 345 MPa) and excellent seismic resistance, suitable for high-rise buildings, stadiums, and other applications;
Bridge Engineering: High flexural and torsional strength, lightweight (30% lighter than solid steel), enabling increased bridge spans;
Machinery: Excellent weldability and cold-bending properties, suitable for dynamically loaded components such as crane booms and vehicle frames;
Energy Transmission: High-pressure resistance (pressure rating up to PN160) and corrosion resistance (surface galvanized or spray-coated), suitable for oil and natural gas pipelines.