Abstract
	Development of low alloy steels started on steels with low Mo and/or Cr contents about 0.3 wt %. Formerly top CrMo steels were steels on 2,25Cr 1Mo and 9Cr 1Mo base. 
	Vanadium bearing steel 0,5Cr 0,5Mo 0,3V was utilized in power stations from later. The improvement of creep resistance was attained due to precipitation strengthening by vanadium carbide V4C3. The precipitation strengthening depends on the interparticle spacing (IPS) ? which depends on number of V4C3 particles per unit volume NV and mean diameter of particles d.

	? = ? (NV · d)-? - d ??

	When the interparticle spacing decreases, the  proof stress at room temperature and the creep rupture strength (CRS) at high temperatures increases , but creep rate decreases.In some steel grades two secondary phases contribute to precipitation strengthening  e.g.Mo2C and Cr7C3 in the steel 2.25Cr-1Mo, similarly in the steel 9Cr 1Mo 0.3V 0.003N there are M23C6 and VN. In these cases CRS is controlled by means of effective IPS - ?eff, that takes into account IPS of both phases 

    1     =      1     +   1     
   ?eff           ?e          ?s

where ?e means interparticle spacing of large particles (M23C6 – in chromium steels, Cr7C3 in low alloy steels. ?s means IPS of small particles VN in chromium steels containg V and available nitrogen or IPS of Mo2C in low alloy vanadium free steels. Available nitrogen is this part of nitrogen in steel which is not bound on Al; Ti and/or Nb.
	It was found that creep resistance of ferritic steels increases only this part of Mo in the steel which remains in solid solution. Therefore it is no reason to increase Mo content in the steel over solubility limit. In low alloy steels it is about 0.5% Mo and in modified chromium  steels about 1% Mo. In the low alloy CrMoV steel containing 0.12% C and 0.3% V, the best creep rupture strength  is attained by the Mo content about 0.5% .
	The most significant improvement of 9 to 12% Cr steels has been attained by nitrogen addition. It was found that creep rupture strength of known 9 to 12% Cr steels depends on the same available nitrogen content.
	With respect of performed analysis the improvement of creep resistance of ferritic steels can be achieved by increasing precipitation strengthening using nanoparticles in the structure (V4C3 or VCN in low alloy steels or VN in chromium steels – 9 to 12 mass % Cr).
The efforts to increase creep resistance by means of increasing Mo and/or W in the steel have been unsuccessful in low alloy steels as well as in chromium steels. Improvement of CRS creep rupture strength can be attained only by increasing precipitation strengthening by nanoparticles of V4C3 VCN or VN.