Research On Solution/Precipitation Of NB And Their Effects On Microstructure Evolution Of High NB Microalloyed Teels

High-Nb X80pipeline steels have being applied in many pipeline projects. In orderto increase the effectiveness of Nb in Nb-bearing microalloyed steel, it is necessary toquantitatively research the dissolution and precipitation regular of Nb and their effects onmicrostrctures and properties. In this paper, six high-Nb microalloyed steels are taken asresearch objects, the dissolution regular of Nb during heating process and the effects ofdissolution and precipitation of Nb on the hot deformation, static softening and phasetransformation behaviors were investigated quantitatively to elucidate the mechanism ofNb in high-Nb microalloyed steels.A method of determining the content of soluble and undissolved Nb in steel has beenpresented. By dissolving, filtering and Inductively Coupled Plasma Atomic EmissionSpectroscopic (ICP-AES) analysis, the content of Nb in filtrate (soluble Nb) and residue(undissolved Nb) was determined. The dissolution regular of Nb in tested steels duringheating process was investigated with this method. The result shows that the soluble Nbcontent in the traceTi treated high-Nb steels increases with the heating temperature. Yet,the Nb in tested steels still can not completely dissolve after the holding of3h at1300°C.The soluble Nb content increases with the decrease of C content and/or the increase of Nbcontent in steel. On this basis, a model was built to describe the relation among the solubleNb content, heating temperature and the content of C and Nb in steel.The effects of soluble Nb, undissolved precipitates and dynamic precipitation of Nbduring deformation on hot deformation behavior were researched by Gleeble-3500thermo-mechanical simulator and the method of determining soluble Nb content. Theresults show that the undissolved precipitates have no obvious effect on the deformationactivation energy and peak strain, but increase the flow stress during deformation. Thedynamic precipitation of Nb during deformation can not significantly affect the peakstress and strain because the precipitates are less. Soluble Nb can strongly delay thedynamic recrystallization. For every1wt%increase in soluble Nb, the deformationactivation energy increases1599.9±1.37.7kJ/mol.By the stress relaxation way and back extrapolation method, the strain induced precipitation and static softening behaviors during the holding process after deformationwere investigated; the effects of undissolved Nb and soluble Nb on the strain inducedprecipitation of Nb were researched quantitatively and the influences of soluble Nb andprecipitation on static softening were also analyzed. During the holding process afterdeformation, the undissolved precipitates can also act as heterogenous nucleation sitesfor the precipitates induced by strain. However, this precipitation has no obvious effecton the stress relaxation curves. In contrast, the supersaturation of Nb in austenite isreduced due to the heterogeneous nucleation, which delays the precipitation start timeshown in the stress relaxation curves. Moreover, the incubation time of strain-inducedprecipitation is linearly related to the content of supersaturated Nb. Increasing the contentof supersaturated Nb can shift the nose of the precipitation C-curve to shorter time andhigher temperature.Both soluble Nb and strain induced precipitates can restrain the recover and staticrecrystallization and then decrease the softening ratio. Yet, the precipitates induced bystrain have more strong retarding effect on the recover and static recrystallization than thesolid solution Nb atoms. The softening ratio is linear with the dissolved Nb content beforethe strain-induced precipitation and with the precipitable Nb content after thestrain-induced precipitation. However, the retarding effect decreases with the prolongingof holding time and the coarsening of precipitate.The continuous cooling transformation (CCT) curves were constructed and the phasetransformation behanviors of tested steels were researched by Gleeble-3500thermo-mechanical simulator and metallographic observation. The lower soluble Nbcontent, hot deformation and holding treatment after deformation can increase phasetransformation temperature, promote the transformation of polygonal ferrite and move theCCT curves upper left. The increase of soluble Nb content promotes the low temperaturemicrostructure transformation and enlarges the acicular ferrite area in CCT curves.Deformation not only increases the phase transformation temperature, but also refinesferrite grains. This effect is more obvious with the increase of deformation pass. Therecovery and recrystallization occur during holding process, which decreases thenucleation rate and cause the coarsening of microstructure. The hardness of phase transformation microstructure increases with the cooling rate.Moreover, both deformation and the holding process after deformation promote straininduced precipitation of NbC and decrease the content of Nb and C in solid solution,which weakens the solid solution strengthening of Nb and C and decreases the hardness ofpolygonal ferrite.