The manual arc welding and CO

2

shielding gas arc welding of 3. 8 mm DP590 steel sheet (/% ： 0. 07C

0. 45Si

1.61Mn) are carried out respectively by Φ3 mm E4303 carbon steel welding rod (/% : ≤0. 12C

≤0. 25Si

0. 30 〜0. 60Mn) and Φ 1 mm H10MnSi welding wire (/% : 0. 14C

0. 65 ~0. 95Si

0. 80 ~ 1. 10Mn) with different welding parameters

and the structure and microhardness of welding zone are observed and analyzed by ZEISS optical microscope and LEICA microhardness meter. Results show that in welding metal zone the structure of steel by manual welding is proeutectoid ferrite and pearlite distributed along columnar crystal and that by CO

2

shielding gas welding is acicular ferrite and minor bainite； in coarse grain r

egion the structure of steel by manual welding is bainite and proeutectoid ferrite

and that by CO

2

shielding gas welding is lath martensite and bainite

of which the grain size is larger than that by manual welding ；with CO

2

shielding gas welding as using larger heat input the Widmannstatten structure in welding metal and coarse grain region is eliminated; the maximum of microhardness is occurred at coarse grain region and the width of heat affected zone of welding steel by manual arc welding is less than that by CO

2

shielding gas welding.