Alloying elements, which do not have a significant effect on microstructure, can have an effect on toughness:

• Ni additions decrease the Ductile-Brittle Transition Temperature (DBTT). Small additions can be made to an alloy to increase toughness to meet customer specifications. Unfortunately Ni is expensive so there is a cost penalty.
• Nb, V, Ti additions tend to increase the DBTT - they increase the strength of the alloy by precipitate strengthening but decrease the toughness.
• Si, Mn additions tend to increase the DBTT - they strengthen the alloy by solid solution strengthening but decrease the toughness.