Abstract: Phosphide precipitates in weathering steels with Ti addition produced by CSP processing were studied by using TEM and X-ray energy dispersive spectroscopy (EDX).The specimens were hot strips and a continuous casting slab.The slab was reheated and compressed by 20% at 900 ℃,followed by isothermal holding for 30 min.The results show that phosphides M_xP in nanometer scale exist in the weathering steels.The value x is 2 to 3,and metallic elements M are Fe,Ti and Cr or Ni.The crystalline structure of the phosphides is hexagonal,the lattice parameters a=0.609 nm and c= 0.351 nm.In the hot strips,the size of most phosphide particles is smaller than 20 nm.But in the slab,the phosphides are with varied dimension and shape.The larger phosphide particles are rod-like,about 50 nm in width and 300 nm in length,but the others are smaller than 50 nm and in square shape.The phosphides may precipitate during the processing of the last pass of hot rolling and cooling until 400 to 500 ℃.Phosphide precipitation can result in strengthening the steels,but the phosphor content in solid solution of the steels decreases.