Formation of martensite austenite constituent in continuously cooled Nb-bearing low carbon steels

Fe-0.15%C-1.5%Mn-0.2%Si (Nb-free alloy) and Fe-0.15%C-1.5%Mn-0.2%Si-0.03%Nb (Nb-added alloy) were continuously cooled to room temperature at constant cooling rates in the range from 0.1 to 20K/s. At lower cooling rates, such as 0.1K/s, the Nb addition retards the ferrite transformation, resulting in a decrease in the transformation temperature and an increase in the volume fraction of bainite. The fraction of martensite-austenite constituent (MA) increases by the Nb addition and the largest fraction of MA, about 0.5 %, is observed in the Nb-added specimen cooled at 5K/s. In the specimens cooled at 5K/s, relatively coarse bainite without cementite precipitation is formed near the austenite (γ) grain boundary in both alloys. Most of MA is localized between such relatively coarse bainitic ferrite (BF). On the other hand, MA is hardly observed in the bainite formed with cementite precipitation in γ grain. Based on microstructure observation of the continuously cooled specimens down to intermediate temperatures followed by quenching, it is concluded that small-sized untransformed γ near γ grain boundary partly remains as MA whereas relatively larger untransformed γ in the γ grain decompose into bainite with cementite precipitation.