For Micro-Alloyed Carbon Steels (vanadium, niobium, and titanium alloyed ferritic steels), the vanadium content should not exceed 0.05%, the niobium 0.05%, the titanium 0.02%, and their combined content shall not exceed 0.10%. When these elements are used for alloying, the soluble aluminum to nitrogen ratio should not be less than 2:1, with the nitrogen less than 0.015%.
Just some related information about sour service materials, cast iron cannot be used for sour service. However, for certain specific applications (non-pressure containment), the use of ductile forms of cast iron (e.g. ASTM A395) may be suitable and will require case-by-case study prior to use.