Effect of heat treatment conditions on shape control of large-sized elongated MnS inclusions in sulfurized free-cutting steels

Abstract

Effect of heat treatment conditions on shape evolution of large-sized elongated MnS inclusions in resulfurized free-cutting rolled steel has been investigated using confocal scanning laser microscope and Si 2Mo resistance furnace. The results show that the heating rate, soaking temperature and soaking time impose significant effects upon shape profiles of elongated MnS. The split of slender MnS was oberved in continuous heating with heating rate in the range of 0.5-2 K/s. In addition, split degree of MnS indicated a negative relation with the rise of heating rate. As a result, separation of elongated MnS was not observed at the heating rate of 6 K/s. During soaking experiments, there was no remarkable shape change for MnS at temperature lower than 1 073 K. While the elongated MnS splited up and spheroidized obviously at 1 473 K. Correspondingly, number density of inclusions increased while mean length reduced as the soaking time increased from 1 h to 4 h at 1 473 K. Significant shape change from slender to spindlelike or spherical was identified only when the soaking time exceeds 3 h or 4 h. Based on the Gibbs Thompson relation and the obtained experimental results, mechanism of shape evolution of MnS inclusions was discussed and morphology evolution of MnS was divided into three major steps: (1) first, the shrinkage occurred in the longitudinal direction at the beginning of heating process, (2) expansion and contraction in radial direction followed after the shrinkage which caused the split of slender MnS; (3) eventually, the spherical particles emerged from the split parts. © 2011 ISIJ.