方管在使用狀態下以鐵素體組織為主的叫合金方管。含鉻量在11%~30%，具有體心立方晶體結構。這類鋼一般不含鎳，有時還含有少量的Mo、Ti、Nb等到元素，這類鋼具導熱系數大，膨脹系數小、抗氧化性好、抗應力腐蝕優良等特點，多用于制造耐大氣、水蒸氣、水及氧化性酸腐蝕的零部件。這類合金方管存在塑性差、焊后塑性和耐蝕性明顯降低等缺點，因而限制了它的應用。爐外精煉技術（AOD或VOD）的應用可使碳、氮等間隙元素大大降低，因此使這類鋼獲得廣泛應用。

The square tube is called alloy square tube, which is mainly composed of ferrite in service state. The chromium content ranges from 11% to 30%. It has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel tool has the characteristics of high thermal conductivity, small expansion coefficient, good oxidation resistance and excellent stress corrosion resistance. It is mostly used to manufacture parts resistant to atmospheric, steam, water and oxidative acid corrosion. This kind of alloy square pipe has some shortcomings, such as poor plasticity, obvious decrease of post-weld plasticity and corrosion resistance, which limits its application. The application of out-of-furnace refining technology (AOD or VOD) can greatly reduce the gap elements such as carbon and nitrogen, thus making this kind of steel widely used.