Extensive investigations of metallurgical roles played by Nb microalloying in advanced products of seamless steel tube have been carried out.The results show that with Nb microalloyed,the recrystallized austenite grain(RAG)and final ferrite grain of tubular steel are evidently refined even experiencing a piercing and a continuous rolling at very high temperature,and a certain quantity of(Nb,V)(C,N)and(Ti,Nb,V)(C,N)particles form on air cooling.Moreover,for quenching(Q)& tempering(T)treated tubular steels,the nanoscale particles of(Nb,V)(C,N)further precipitate on heating stage of Q at 900-1000oC,leading to a significant refinement of prior austenite grain(PAG)and final martensitic or bainitic packet/block structures,and during subsequent T at 600-700oC,producing an improved resistance to softening.The microalloying with 0.02-0.04wt%Nb in Q&T martensitic OCTGs of 155ksi or higher grades,have produced an adequate combination of ultrahigh strength and good toughness in casings for ultra-deep well and high-collapse services,perforation gun tube and drill pipe,etc.The SSCC resistance of Q&T martensitic OCTGs including C110 casings and SS105 drill pipe has been also evidently improved due to the 0.02-0.04wt%Nb microalloyed.With 0.03-0.05wt%Nb microalloyed and carbon,Ceq and Pcm reduced,an excellent strength/toughness/weldability combination has been achieved in Q&T bainitic line pipe and structural tube of X60-X100.In addition,the N80_1 class OCTGs with 0.24wt%C-0.02wt%Nb in hot rolling delivery has an improved and qualified mechanical property.