The square steel pipe is a term for square pipes which have the equal side length and rectangular pipes which have the unequal side length. The square steel pipe is made by the strip steel with the rolling process. Generally, the strip steel will be unpacked, flattened, curled and welded to be a round pipe, be rolled into a square pipe, and will be cut into the needed length.

The classification of square steel pipes according to materials
According to materials, the square steel pipe can be divided into the common carbon steel square pipe and the low alloy square steel pipe.

Applications
Square steel pipes are mainly used in fields including the mechanical industry, the chemical industry, shipbuilding, oil casings and pipelines for transporting water, gas, sewage, etc. 

The classification of square steel pipes according to production processes 
According to production processes, square steel pipes can be classified into the hot rolled  seamless square pipe, the cold drawn seamless square pipe, the extruded seamless square pipe and the welded square pipe.

Welded square steel pipes can be further divided into the arc welding square pipe, the ERW square pipe (high frequency, low frequency), the gas welding square pipe and the furnace welding square pipe according to the process; the longitudinal welded square pipe and the spiral welding square pipe according to the welding seam.

The classification of square steel pipes according to surface treatments
According to production processes, square steel pipes can be classified into the hot galvanized square steel pipe, the electrogalvanizing square steel pipe, the oiled square steel pipe and the acid pickling square steel pipe.

Standards and applications
1. The standard number of the galvanized welded square steel pipes for transporting low pressure fluid is GB/T3091-2008. The typical materials of galvanized welded square steel pipes are Q235A, Q235B, Q345B, etc. Galvanized welded square steel pipes are mainly used for transporting water, coal gas, air, oil, hot water or steam, and they are limited to the transportation of low-pressure fluid.

2. The standard number of the mining welded square steel pipe is GB/T14291-2000. 

3. The standard number of the large-diameter welded square steel pipes for transporting low pressure fluid is GB/T14980-1994, and the typical materials of these pipes are Q235 Grade A and Grade B. The large-diameter welded square steel pipes are mainly used to transport the low pressure fluid such as water, sewage, coal gas and air. 

4. The standard number of the stainless welded square pipes for transporting fluid is GB/T12771-1991, and the typical materials of these pipes are 0Cr13, 0Cr19Ni9, 00Cr19Ni11, 00Cr17, 0Cr18Ni11Nb, 0017Cr17Ni14Mo, etc. The stainless welded square pipes are mainly used for transporting low pressure corrosive media.

5. The standard number of the spiral welded square pipe for transporting low pressure fluid is SY/T5037-2008, and the typical materials of these pipes are Q235B, Q345B, etc. The spiral welded square steel pipe are mainly applied to low pressure pipelines for transporting water, oil, natural gas, etc.

6. The standard number of the square pipe for transporting flammable fluid and non-flammable fluid is B/T9711.1（GB/T9711.2）-1997, and the typical materials of these pipes are from L245 to L415. The requirements on these pipes are usually high.