A fundamental method to quantify phase change in microalloyed steels

Abstract

Microalloyed steels are a class of High Strength Low Alloy (HSLA) steels that contain a low concentration of carbon and up to 0.1% by weight of microalloying elements such as Ti, Nb, V and B. Microalloyed steels are known for their excellent combination of strength, toughness and weldability. The trio of properties makes these steels a material of choice for the pipeline industry. In addition to the above mentioned properties, pipelines can benefit from large wall thickness. This allows microalloyed steel pipes to be used at higher operating pressures that result in better productivity for the pipeline. However, manufacturing heavy sections of high strength microalloyed steels is hindered by inhomogeneous cooling cycles through the thickness of the plate. This results in non-homogeneous mechanical properties that stem from differences in phase transformations which depend on the cooling conditions. The issue with non-homogenous phase transformations is more pronounced for higher strength grades of microalloyed steel since the need for higher strength dictates that transformations occur at lower temperatures which necessitates employing higher cooling rates on the runout table. These higher cooling rates result in an even more complex cooling cycle that includes a period of heating in the near surface layers that could induce auto tempering. To understand the effect of the complex cooling cycle on the kinetics of phase transformations, an analysis tool is designed that can calculate the amount of phase transformation and product type using dilatometry data. This computer code is named the Master Fitter program and it executes an algorithm called Unit Cell Dilation. The Unit Cell Dilation technique is capable of calculating the amount of phase transformation considering the compositional changes to the product and austenite phases during phase transformation. It is shown that the results generated by the Master Fitter program agree with phase content measurements using quantitative metallography.