主要特点：承受压力大，支撑时间久，易安装不易变形等特点。
Main features: high pressure, long support time, easy to install not easy to deformation and other characteristics.
主要用途：主要用于矿井巷道、矿井巷道二次支护、以及穿山隧道的支撑支护等用途。
Main uses: mainly used in mine roadway, roadway support, and two times of the tunnel supporting etc..
作为制造巷道可缩性金属支架的主要型钢，在国内外广泛采用的都是U型钢。
U steel is the main type of steel which can be used as the reducible metal support in the roadway.
但由于对U型钢本身的性能和要求认识不一致，因而各国U型钢的断面形状、几何参数以及材质等不尽相同。
However, because of the differences in the performance and requirements of the U type steel itself, the shape, geometric parameters and material of the U steel are not the same.
我国生产的U型钢型号主要有18U、25U、29U、36U四种。其中前两种系60年代产品，属腰定位； 后两种是80年代产品，属耳定位。U18由于承载能力很低，现很少生产。
There are four types of U - type steel produced in China: 18U, 25U, 29U and 36U. The first two kinds of products in 60s are located in the waist, and the latter two are in 80s, which are located in the ear. U18 is rarely produced because of its low bearing capacity.