Abstract: The effects of unbalance factor B controlled by nitride forming elements Al, Ti etc on the impact toghness of weld metal by CO₂ arcwelding with flux cored wire were studied.The experimental results showed that as theBincreases, the impact toghness increases at first and than deacreases.Because of the fluxes oxidability, the optimal value ofBraised as compared by CO₂ arc welding with solid wire.As the increases from the first to optimal value, the main microstructure of the weld metal changed from granular bainite and ferrite side-plate to acicular ferrite, so the impact toghness was improved.But in case of theBover the optimal value very much, the toghness deacreased because the surplus nitride forming elements resided in the parent of ferrite.