Abstract: Five heats of industrial trials were performed to investigate the cleanliness of high level welding wire steel produced with hot metal after desulfurization and dephosphorization pretreatment．The variation of T.O contents and the transformation route and mechanism of inclusions in the steel were studied by system sampling during steelmaking processes．The results showed that the average oxygen mass fraction was 610.2×10-6 at the converter end，484×10-6 at RH start，220×10-6 before adding aluminum alloy，4.6×10-6 at RH end，and the overall control was better. The inclusions were mainly MnO-SiO2-Al2O3 at RH start，which shapes were mostly spherical，and the sizes of some inclusions were more than 20 μm. The inclusions were mainly Al2O3 inclusions and a little MgO-SiO2 inclusions after RH treatment to slabs.

Key words: welding wire steel, cleanliness, inclusions, dephosphorization, dephosphorization