.Unsintered seamless steel tubes for fluid conveyance (GB/T14976-1994) are hot-rolled (extruded, expanded) and cold-drawn (rolled) seamless steel tubes made of stainless steel for conveying fluids.

17. Special-shaped seamless steel tubes refer to seamless steel tubes with cross-sectional shapes other than round tubes. They can be further divided into equal-thickness special-shaped seamless steel tubes (code: D), uneven-thickness special-shaped seamless steel tubes (code: BD), and variable-diameter special-shaped seamless steel tubes (code: BJ) based on different cross-sectional shape dimensions. These tubes are widely used in various structural components, tools, and mechanical parts. Compared to round tubes, special-shaped tubes generally have larger inertia moments and cross-sectional moduli, offering greater bending and torsional resistance, which can significantly reduce structural weight and save on steel.