Precipitation of MC phase and precipitation strengthening in hot rolled Nb–Mo and Nb–Ti steels

Hot rolled Nb–Mo steel of yield strength 600 MPa and Nb–Ti steel of yield strength 525 MPa with polygonal and acicular ferrite microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (<10 nm in size) of Nb–Mo steel was 58.4%, and higher than that of Nb–Ti steel with 30.0%. Therefore, the results of strengthening mechanisms analysis showed the higher strength of Nb–Mo steel than that of Nb–Ti steel is attributed to its relatively more prominent precipitation strengthening effect. The yield strength increments from precipitation hardening of Nb–Mo steel attained 182.7 MPa and higher than that of Nb–Ti steel.