Effect of Sn on Microstructure Evolution of a HSLA Steel

The recycling of scrap could cause continuous enrichment of Sn in steels. To reveal the influences of Sn on the mechanical properties and microstructure evolution of high-strength low-alloy (HSLA) steels, HSLA steels with varying Sn contents were designed and a series of continuous cooling tests were conducted. The results show that the effect of Sn on microhardness is mainly affected by the cooling rate. Under cooling rates lower than 10 °C/s, the addition of Sn increases the microhardness due to the solid solution strengthening effect of Sn. The ferrite grain size decreases with the increase of Sn content at a cooling rate of 0.1 °C/s because of the possible solute drag effect of Sn, while no refining effect was found for cooling rates between 0.5 °C/s and 10 °C/s. Under cooling rates higher than 10 °C/s, Sn reduces the ferrite and bainite transformation start temperatures and increases the bainite and martensite phase fractions. The higher hard phase fraction with increasing Sn content results in a significant increase in microhardness, and the contribution of solid solution strengthening plays a supplementary role under this condition. As one of the major residual elements in scrap, the strengthening and hardenability increasing effects of Sn should not be overlooked and may even be properly utilized in alloy design.