The effects of Sn on austenite decomposition in low carbon low alloy steels

The effects of Sn additions on austenite decomposition, using the same initial prior austenite grain size (PAGS) distribution, under continuous cooling and isothermal holding conditions have been investigated. Sn levels from 0.06 to 0.21 wt%, in a C-Mn base steel, have been used, representing potential Sn contents in high scrap content steelmaking. The initial austenite to ferrite transformation (5% transformation) is delayed to slightly lower temperatures and longer times with Sn additions during continuous cooling at rates of 0.1 - 50 °C/s and isothermal transformation at 700 °C respectively. This is consistent with delayed nucleation at prior austenite grain boundaries (PAGBs) due to Sn segregation increasing the critical barrier for nucleation. Increasing Sn content slowed ferrite development along PAGBs for isothermal transformation, although no clear Sn effect on ferrite growth could be discerned for continuous cooling. The ferrite grain size was slightly refined for the same cooling rate with Sn additions. Interestingly, pearlite formation at low cooling rates occurred at slightly higher temperatures with the Sn additions. Bainite formation at a higher cooling rate of 50 °C/s is retarded by Sn additions, which is suggested to be due to suppression of bainitic cementite formation. The martensite start temperature (M S ) is independent of Sn content. Overall, this study demonstrates the effects, and identifies potential benefits, from residual Sn additions in (i) refining ferrite grain sizes and (ii) more significantly, delaying bainite formation in low carbon low alloy steels.