Abstract: The 440 MPa HSLA steel was welded with gas metal arc welding (GMAW),and the microstructure of weld metals and the morphology of inclusions were analyzed with OM, TEM,SEM,and EBSD.The effects of shielding gas on the microstructure, impact toughness of the weld metals and the size, quantity,compositions of inclusions were investigated.The results show that impact toughness of weld metal is bad with 100% CO₂ as shielding gas,and weld metals with good impact toughness can be obtained with 80% Ar+20% CO₂ and 90% Ar+10% CO₂ (volume fraction) as shielding gas.The microstructure of weld metal of 100% CO₂ GMAW is made up of ferrite and bainite,and that of both 20% CO₂ GMAW and 10% CO₂ GMAW are predominantly acicular ferrite with small areas of ferrite side plates.With the increasing of the proportion of CO₂ in shielding gas,the amount and dimension of inclusions increase,and the compositions of its main ingredients change.For 440 MPa HSLA steel,welding with proper gas composition can produce welded joints with good properties.