Effects of intermediate cooling and tempering process on martensite-austenite constituents in niobium-bearing low carbon steel

Heat treatment processes were simulated with a thermo-analyse simulator for niobium-bearing low carbon steel (0.028%C-0.25%Si-1.82%Mn-0.085%Nb). In the processes, the samples were cooled down to a final cooling temperature of 350 to 550℃ after being normalized at 950℃, then reheated at different ratios, and tempered at different temperatures for different holding time periods. The morphology, the size and distribution of martensite-austenite (MA) constituents obtained under different tempering conditions were investigated by optical microscopy, scanning electron microscopy (SEM) and image analysis. The results show that the enhancement of reheating ratio and the final cooling temperature in the range of bainite transition lead to an increase of MA volume fraction, and the maximum volume fraction of MA constituents is up to 7.9%. Tempering at high holding temperature for long holding time can result in an occurrence of the volume fraction peak of MA constituents. The average grain size of MA constituents is 0.77 to 1.48 μm after tempering. MA grains grow up and tend to be in polygonal shape when the final cooling temperature, reheating ratio and holding temperature increase and the holding time prolongs. The volume fraction and average grain size of MA constituents are mainly affected by the quantity of untransformed austenite after intermediate cooling, the evolution of carbon diffusion from ferrite to retained austenite during tempering and the stability of retained austenite after tempering.