Effects of cooling rate and isothermal holding on the characteristics of MnS particles in high-carbon heavy rail steels

The characteristics of MnS particles were intensively investigated at three different cooling rate of 80.4 K · s −1 (water cooling), 3.8 K · s −1 (air cooling) and 1.8 K · s −1 (furnace cooling) as well as the different isothermal holding temperature and time in laboratory experiments. The three-dimensional (3D) morphology of MnS particles was extracted from steel samples using non-aqueous solution electrolysis. The results showed that the 3D morphology of MnS changed from a nearly spherical into rod-like and the area fraction and average diameter of MnS increased with decreasing cooling rate. During isothermal holding process, the morphology of MnS changed little at 1473 K (1200 °C), but their shape profiles varied from a nearly spherical and spindle-like to irregular at higher holding temperature 1673 K (1400 °C) when the holding time exceeded 60 min. Moreover, the number density and area fraction of MnS decreased with increasing holding time at 1573 K (1300 °C) and 1673 K (1400 °C), respectively. Especially at 1573 K (1300 °C), the 1 ∼ 3 µm MnS inclusions were dissolved and lead to decreasing of number density, but that > 3 µm one occurs growth and resulted in increasing of average diameter. The calculation results show that the starting temperature of precipitation of MnS was about 1627 K (1354 °C) and effect of cooling rate on the segregation of Mn and S is insignificant. Considering the segregation of solutes, MnS formation and growth takes place in the solid/liquid interface of steel when the solid fraction is close to 0.9567 during solidification. It has been found that the increase of cooling rate gives rise to the decreased of MnS diameter because the growth time of MnS is short. Furthermore, thermodynamic calculations of MnS solid solubility product were carried out to reveal the high holding temperature and long holding time favors the dissolution of MnS particles. It is necessary to decrease the sulfur content by less than 16 ppm in order to assure that the larger MnS which formed during solidification redissolves in the steel matrix, rather than relying on increasing the heating temperature which is above 1649 K (1376 °C). Subsequent, the MnS will precipitate again in a finely dispersive state during rolling process, and it can hinder annealing grain growth and finally make for the improvement of the toughness property of the steel.