The Research Of Strength Toughening Mechanism Of Added Nano-oxides High Grade Pipeline Steel

Pipeline Steel, as the main carried tool to transport petroleum and natural gas, needto be high heat input welding. However, the worse microstructure and lowed propertiesrestrict the performance of these excellent steel. We add nanometer oxides into X80pipeline steel to improve its microstructure and property. Therefore, Gleeble-2000welding thermal simulation machine, OLYMPU-CK40M metalloscope, S-4800scanning electron microscope, NCINI750impact test machine and so on were adoptedin this experiment to study the inclusions, microstructure, impact toughness and strengthtoughening mechanism, the main results are as follows:1. The nano-oxides’s addition decrease the inclusion’s size and change its contentin nano-oxides added X80pipeline steel after rolling and HAZ. The0.02MgO%wt and0.02CaO%wt’s addition decrease the inclusion’s diameter to1~2μm and make Mg andCa elements enter into its core, leading the lack-Mn region to appear and acceleratingacicular ferrite’s nucleation around it.2. The nano-oxides’s addition can refine the grains in nano-oxides added X80pipeline steel after rolling and HAZ. The average grain size is the smallest in0.02MgO%wt and0.02CaO%samples, respectively62.9μm and81.3μm in rolled ones，and respectively121.7μm and137.6μm in HAZ.3. The nano-oxides’s addition can improve impact toughness in HAZ. In contrastto raw steel, the impact toughness of0.02MgO%wt samples is improved from121J to190J. Then, the crack mechanism is converted from cleavage fracture to dimple fracture,improving toughness.4. The HAZ’s samples are quenched at different temperatures in the coolingprocess of the welding thermal circle, the observed result shows that: when adding0.02MgO%wt, the acicular ferrite begin to nucleate around the inclusions at about620 ℃，and the acicular ferrite apparently stretch and become thick at620℃~580℃; whenadding0.02CaO%wt, the acicular ferrite begin to nucleate around the inclusion at about600℃, and the acicular ferrite apparently stretch in the length’s direction and becomethick at600~580℃.5. After adding nano-MgO and nano-CaO, the composite inclusions can induceacicular ferrite to nuclear in HAZ, refining microstructure and segmenting grains, andhave the function to fine crystal strengthening. The nano-oxides’s addition not onlychange the M/A component’s shape from strips to particles, dispersing uniformly andrestraining crack propagation, but also have the function to dispersion strengthening,improving its impact toughness.