Effect of finish cooling temperature on microstructure and mechanical properties of high-strength bainitic steels containing Cr, Mo, and B

Six low-carbon high-strength bainitic steels containing Cr, Mo, and B were fabricated by controlling finish cooling temperature, and the effect of bainitic microstructure on tensile and Charpy impact properties were investigated. All the specimens were composed primarily of bainitic ferrite, together with small amounts of granular bainite, acicular ferrite, martensite-austenite constituent. These bainitic microstructures were more critically affected by the finish cooling temperature than by the alloying elements. The H-series specimens with a high finish cooling temperature had larger amount of acicular ferrite and smaller amount of granular bainite and bainitic ferrite, compared to the L-series specimens with the low finish cooling temperature at the same chemical composition. The L-series specimens exhibited higher strength and yield ratio, and lower uniform and total elongations than the H-series specimens because the volume fraction of BF was higher in the L-series specimens than in the H-series specimens. On the other hand, the energy transition temperature decreased with increasing the volume fraction of AF having fine effective grain size, while it increased with an increase in the volume fraction of GB having coarse effective grain size. Thus, the energy transition temperature of the H-series specimens with the high finish cooling temperature were slightly lower than that of the L-series specimens with the low finish cooling temperature because the H-series specimens had a larger amount of AF than the L-series specimens.