A qualitative study of multiphase steel microstructure developed in four different steel alloys

Abstract

In this work, quench and partitioning (Q&P) heat treatment was performed on four different steel alloys in order to obtain multiphase steels with a microstructure of ferrite, martensite, bainite, and retained austenite. The four alloys are SS1672, 38MnVS6, R350HT and SS2244. They were selected to study the effect of increasing the Mn/Si ratio and the Cr effect on the microstructure. Multiphase steels with retained austenite of different fractions and morphologies in a ferritic-bainitic matrix were obtained depending on the Cr and Si amounts. The detailed characterisation of the microstructural evolution of four heat-treated Q&P steel samples by optical microscopy (OM) was done only to provide a qualitative understanding. Despite these limitations, the OM micrographs were sufficient to confirm and identify the formed phases, especially the bainite phase. This study shows that a high Si levels are not strictly required to suppress pearlite formation during the quenching and partitioning (Q&P) treatment of medium and eutectoid steels, due to the influence of manganese. A Mn/Si ratio greater than 2 is found to be essential for effectively preventing pearlite formation. Additionally, a lower martensite starting temperature (Ms) significantly refines the microstructural features, particularly bainite and retained austenite.

Keywords

• quench and partitioning
• bainite
• martensite
• retained austenite