Effect of Nb on Austenite Formation and Decomposition in an X80 Linepipe Steel
Advanced high strength steels for pipeline applications, e.g. X80 grades, have complex microstructures and are frequently microalloyed with Nb. In the hot rolled product it is sought to have Nb precipitated as Nb(CN). However, when processing these steels Nb may be in solution and critically affects the microstructure evolution, e.g. austenite decomposition on the run-out table of a hot mill. Further, microstructure changes in the heat affected zone (HAZ) during girth welding of these linepipe steels may occur with Nb precipitated or in solution. In the HAZ, depending on welding procedures, the material undergoes a number of austenite formation and decomposition cycles and the amount of Nb in solution varies along these stages.In selected positions of the HAZ, thermal cycles peak at the intercritical region and the partial formation of austenite and subsequent decomposition constitutes additional complexity. Developing reliable process models for run-out table cooling and the HAZ hinges on an accurate tracking of microstructure evolution, which is strongly influenced by the amount of Nb in solution.The present study provides more insight into the effect of Nb on austenite formation and decomposition. Firstly, a novel experimental methodology is presented to measure quantitatively the effect of Nb on transformation temperatures pertinent to austenite decomposition, notably ferrite. A model for ferrite formation that accounts for solute drag of Nb is proposed to describe the experimental observations. Secondly,an experimental study will be presented to quantify the effect of Nb in and out of solution on austenite formation in the intercritical region. It is found that the morphology of intercritical austenite, as well as the kinetics of its formation is strongly affected by the starting microstructure and the state of Nb.
Fateh Fazeli Matthias Militzer
The Centre for Metallurgical Process Engineering,The University of British Columbia Vancouver,BC,Canada V6T 1Z 4
国际会议
The 6th International Conference on High Strength Low Alloy Steels(第六届高强度低合金钢国际会议HSLA Steels2011)
北京
英文
246-249
2011-05-31（万方平台首次上网日期，不代表论文的发表时间）