What Is Q355 Steel Pipe
Q355 steel pipe is a Chinese low-alloy high-strength structural tubing grade that succeeds the earlier Q345 designation, produced as hot rolled or hot expanded seamless pipe. Under GB/T 1591-2018 the series includes three common quality classes, Q355B, Q355C, and Q355D, differentiated by Charpy impact test temperature. The Q indicates yield point and 355 the minimum yield strength in megapascals for thin sections.
Chemical Composition
For Q355B the limits are carbon 0.24 percent, silicon 0.55 percent, manganese 1.60 percent, phosphorus 0.035 percent, sulfur 0.035 percent, chromium 0.30 percent, nickel 0.30 percent, copper 0.40 percent, and nitrogen 0.012 percent. Q355C holds carbon to 0.20 percent with phosphorus and sulfur at 0.030 percent, and Q355D reduces phosphorus and sulfur further to 0.025 percent for better low-temperature toughness.
Mechanical Properties
The minimum yield strength is 355 MPa for wall thicknesses up to 16 mm, decreasing gradually with thickness to about 275 MPa for diameters above 400 mm. The tensile strength is 470 to 630 MPa for thicknesses up to 100 mm and 450 to 600 MPa above 100 mm, with elongation of 17 to 22 percent depending on sampling direction. Impact energies are 34 J longitudinal and 27 J transverse at plus 20 degrees C for Q355B, 0 degrees C for Q355C, and minus 20 degrees C for Q355D.
Production and Inspection
The pipe is produced by hot rolling or hot expanding from round billet, then straightened on a multi-roller straightener and inspected by eddy current testing for surface defects before finishing. Pipes are oiled or varnished per customer requirement, end-capped, and marked before packing. Random lengths, fixed lengths, or custom lengths up to 12 m are available.
Applications
Q355 seamless pipe is used in petroleum storage facilities, gas transmission pipelines, petrochemical equipment, shipbuilding structures, boiler and pressure vessel manufacturing, and structural frames subject to substantial mechanical loads. Its forming performance and compressive strength also suit medium to high pressure gas vessels below 500 liters capacity.
Size Range and Selection
Common outer diameters range from 34 mm to 610 mm with wall thicknesses from 3.5 mm to 50 mm. Select the quality class by the minimum ambient design temperature, and confirm the required yield and tensile values against the pipe dimensions. Contact our team with your pipe sizes, grades, and quantities for a quotation and delivery program.
Frequently Asked Questions
Q: What does Q355 mean?
A: Q stands for yield point and 355 the minimum yield strength in megapascals.
Q: What are the quality classes of Q355?
A: Q355B, Q355C, and Q355D, with impact testing at plus 20, 0, and minus 20 degrees C.
Q: What sizes are available?
A: Outer diameters from 34 mm to 610 mm with wall thicknesses from 3.5 mm to 50 mm.
Q: How is Q355 seamless pipe produced?
A: By hot rolling or hot expanding, followed by straightening and eddy current inspection.
Q: What replaced Q345?
A: Q355 per GB/T 1591-2018 is the successor grade.
Q: What applications suit Q355 pipe?
A: Gas pipelines, petrochemical equipment, shipbuilding, boilers, and structural frames.
Contact us for Q355 hot rolled seamless steel pipe stock, custom lengths, and export packaging options.