The manual welding on 7 mm plate of ultra-fine-grained steel SS400 (/% : 0.18C

0.04Si

0.43Mn

0.022Al

0.002Ca) has been carried out by using J442 (/% : ≤0.12C

0.3-0.6Mn)

J506 (/% :≤0.12C

1.6Mn) and J507 (/% : ≤0.12C

≤1.25Mn) welding rod with heat input 6-14 kJ/cm

and the structure and microhardness of welded joint are observed and measured by optical microscope and microhardness tester. Results show that with increasing alloy element content in welding rod

the amount of acicular ferrite in welded zone increases； as heat input ≤10 kJ/cm

the change of size of coarse grain in heat affected zone is minor

and as heat input is larger than 10 kJ/cm

the size of coarse grain in heat affected zone increases obviously ； the microhardness of welded joint with different welding process is all higher than the microhardness of base metal

there is no soften zone in welded joint.