GB/T 6728-2002, GB/T 6725-2002, GB 5310-2008 (Seamless Steel Tubes for High-Pressure Boilers), GB 3087-2008 (Seamless Steel Tubes for Low and Medium-Pressure Boilers), etc. Non-standard square and rectangular hollow sections are a type of hollow profile, also known as square and rectangular cold-formed hollow steel bars. Their cross-sectional shapes are equal-sided squares and rectangles. Hot-rolled thick-walled seamless steel tubes are primarily used in the diesel injection systems of the automotive industry; they are also widely used in the pipeline transportation of oil and natural gas in hydraulic support engineering and in the field of steel structure engineering, etc.