Effect Of Ti-containing Oxides On The Properties Of Pressure Vessel Steel

With the increasing tendency of large-scale in mechanical engineering structure, the requirement for strength and toughness of steel made for mechanical engineering structure is much higher, and the steel become thicker. In the mean time, high heat input welding is widely used in order to enhance efficiency and reduce steel structure construction cost, however, the high heat input welding technology may lead to the grain coarsening and a sharp decline in toughness of heat-affected-zone(HAZ) for traditional low-alloy steels. So it is a great challenge for large thickness structure steel to be used for heat input welding. While steels processed by Oxide Metallurgy technology(an technology) can meet the demand of high heat input welding by introducing fine inclusions into steel to induce intragranular-acicular-ferrite (IAF) formation, so the Oxide Metallurgy technology is attracting more attention and recognition.The microstructure and properties of HAZ in pressure vessel steel produced by oxide metallurgy technology were studied in this paper to clarify the mechanism of microstructure refinement in HAZ and the contribution of IAF to toughness of HAZ. The characteristics such as the type and crystal relationship of inclusions that induce IAF nucleation were analyzed. The appropriate size of inclusions that promote IAF nucleation was calculated. In addition the corrosion resistance of steel processed by oxide metallurgy route was also investigated. Based on these studies, the following conclusions can be summerized:(1) In the pressure vessel steel by oxide metallurgy, there were not only nano-scaled TiN, TiOx particles that could refine austenite grain, but also effective inclusions in size of 1～3μm that could induce formation of IAF so that the prior austenite grains were divided into smaller regions and the microstructure of HAZ was refined. That was the underlying reason for that the effective grain size of the HAZ of steel by oxide metallurgy technology was small. The small effective grain size of the HAZ is beneficial for improving the toughness of HAZ.(2) The low temperature toughness of HAZ was improved by IAF. The observation and analysis of the cross-section of impact fracture indicated that IAFs suppressed crack initiation and impeded crack propogation. Crack initiation energy (CIE) was enhanced by the refined prior austenite grain and superior deformability of IAFs. Furthermore, EBSD analysis of secondary cracks indicated that IAF impeded crack propagation and increased crack propagation energy by forming high angle grain boundaries with surrounding bainite and its own superior deformability.(3) The TiOx/MnS complex inclusion which induced the formation of IAF in HAZ in steel processed by oxide metallurgy was determined to be Ti2O3/MnS inclusion by EBSD. It was found that MnS precipitated on Ti2O3 at specific habit plane and direction, MnS had a specific orientation relationship ({0001}Ti2O3//{111} MnS),<10-10> Ti2O3//<110>MnS) with respect to Ti2O3.(4) IAF nucleated on MnS of the Ti2O3/MnS complex inclusion had no specific orientation relationship with MnS. IAF keeps K-S relationship with prior austenite, moreover, the IAFs formed on the same inclusion tend to be in one bain group. Moreover, IAF and the surrounding bainite belonged to different bain groups, so that there was increase in high angle boundaries (>45°), which was beneficial for the toughness of HAZ.(5) It is calculated that the size of Mn-depleted zone will decrease along with the increase in size of TiOx/MnS inclusion, and the critical nucleation size of IAF will decrease along with the increasing the size of inclusion, therefore, the over sized inclusion (>3 μm) may not promote IAF nucleation due to its small Mn-depleted zone.(6) The corrosion resistance of steel processed by oxide metallurgy technology and steel produced by traditional method was comparatively analyzed by salt-fog test. It is found that the corrosion resistance of steel by oxide metallurgy technology is not worse than the contrast steel, the higher oxygen content in steel processed by oxide metallurgy technology could enhance the self-corrosion potential of the steel. The corrosion resistance is not deteriorated by the oxide inclusion.