Low alloy steelis carbon content Wc ≤ 0.20% carbon structural steel basis, adding a small amount of alloy elements developed, is higher than the strength of carbon structural steel such steel in addition to contain a certain amount of manganese silicon or basic elements, also contain other suits our country resource condition elements. ( V ) such as vanadium, niobium, titanium ( Nb ) ( Ti ), aluminum ( Al ), molybdenum ( Mo ), nitrogen ( N ), and rare earth ( RE ) and other trace elements. According to the chemical composition and performance requirements, its brand by Q295A, B, Q345A, B, C, D, E, Q390A, B, C, D, E, Q420A, B, C, D, E, Q460C, D, E and other steel grade, its meaning with the carbon structure steel. V, Nb, Ti, Al and other trace elements in grain refinement, such steel in addition to A, B steel, C, D, E grade steel should at least contain one; in order to improve the performance of steel, A, B grade steel also can join one of the. In addition, this kind of steel Cr, Ni, Cu residual element content not greater than 0.30%. Q345A, B, C, D, E is representative of such steel grades, where A, B grade steel usually called 16Mn; C class steel need to add more than one trace element, its mechanical properties increased 1 low temperature impact properties of. This kind of steel and carbon structural steel ratio. With high intensity, good comprehensive performance, long life, wide application range, comparative economic advantages. The steel rolled into sheet, bar, seamless steel pipe and so on, are widely used in bridges, ships, boilers, vehicles and important in building structure.