In this paper, we have investigated the structure, the phase composition and the distribution of alloying elements in the structure and the creep strength of 12% chromium martensite-ferritic steels with a nitrogen content within the grade composition of 0.017 and increased to 0.06% (mass.). It is found that increasing the nitrogen content leads to a decrease in the amount of ferrite in the steel structure, to an increase in the density of dislocations, as well as to the precipitation of fine carbonitrides of niobium, located on the borders. It was found that increasing the amount of nitrogen in a steel of grade 07Cr12NiMoVB increases its long-term strength. The conditional parametric diagrams for the steel of grade 07Cr12NiMoVB with different nitrogen content using the values of long-term strength at temperatures of 550 and 600°C were constructed. A significant increase was discovered in the value of long-term strength with increasing content of nitrogen in the steel of grade 07Cr12NiMoVB.