Nb is often considered to be a powerful alloying element for controlling the recrystallization process in microalloyed high strength steels.However,Nb can be presented either as solute in solution,where it is thought to exhibit a strong solute drag effect,or as NbC precipitates,which are thought to be effective at pinning grain boundaries.Therefore,it is very important to quantitatively measure Nb in solution or in NbC precipitates.A quantitative analysis method of Nb in solution and in precipitates was proposed.The test procedure involved chemical dissolution,filtration and inductively coupled plasma atomic emission spectroscopic(ICP-AES) analysis.The amount of Nb in solution in Nb-microallyed steels under different treatment conditions was evaluated.The results show that the niobium and carbon contents in steels have a great effect on niobium dissolution kinetics.The solute Nb is more effective to retard dynamic recrystallization,while the NbC precipitates are more effective to inhibit static recrystallization.The results may help to comprehend effect of Nb in steels,and provide some guides in the design of new high strength Nb-bearing steels.
XIAO Fu-renCAO Ya-binQIAO Gui-yingZHANG Xiao-bingLIAO Bo
The accurate prediction of soluble Nb content during reheating is helpful for the design of chemical composition and reheating parameters for Nb-containing steels.The dissolution behavior of Nb in high Nb microalloyed steels was investigated.The results show that Nb does not entirely dissolve for high Nb microalloyed steels containing Ti after holding at 1 300℃ for 3h.The soluble Nb content increases with the decrease of C content and/or the increase of Nb content in steels.Moreover,an expression has been established to describe the amount evolution of soluble Nb in high Nb microalloyed steels during reheating and the validity of this expression has also been verified by experiment.
The mechanical properties of heat affected zone(HAZ)of two commercial high-Nb X80grade pipeline steels with different alloy elements were investigated using thermal simulation performed on a Gleeble-3500thermal simulator.The results showed that the high-Nb steels have excellent weldability.Embrittlement regions appear in coarse grain heat affected zone(CGHAZ)and intercritically heat affected zone(ICHAZ);Softening region appears in fine-grain heat affected zone(FGHAZ),and the strength here was even lower than 555 MPa as required in the standard.Meanwhile,with the increase of heat input,the strength and the toughness of HAZ of steel with high Nb,C and lower alloy decrease notably.Therefore,take into account the welding procedure during manufacture of weld pipe,suitable amount of alloy elements,such as Cr,Ni,Cu,Mo and so on,is necessary for high Nb X80heavythick steel plate.
