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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 Si2Mo 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.KEY WORDS: large-sized elongated MnS inclusions; in-situ observation; resulfurized free-cutting steel.

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The stringer shaped B type non‐metallic inclusions in linepipe steel plates deteriorates the properties of low temperature toughness, hydrogen induced crack, etc. At present the formation mechanism and controlling method of B type inclusions in mass production are rarely reported. In this paper, the characteristics, origin, and controlling method of stringer shaped B type non‐metallic inclusions were studied for linepipe steel. The stringer shaped B type non‐metallic inclusions in API X80 linepipe steel plates, produced through “BOF‐LF‐RH‐Ca treatment” steelmaking route, were mainly of CaO–Al2O3 system with lower melting temperatures, which formed because the presence of: (i) small sized liquid inclusions of CaO–Al2O3 system in liquid steel after the secondary refining and Ca treatment. These small inclusions could aggregate to larger ones of 10–20 μm in continuous casting and be deformed into stringer shaped inclusions in steel plates during rolling. (ii) The large sized low melting temperature CaO–Al2O3 inclusions, in Ca treatment, their surface layers could be modified into high melting temperature CaO, CaS, or CaO–CaS system, but the centers were still of CaO–Al2O3 system. During rolling, these inclusions could also be elongated to stringer typed ones because of their soft CaO–Al2O3 centers. A new strategy for control of B type inclusions in linepipe steel plates was adopted. The key of the control was shifted from removing low melting temperature inclusions of CaO–Al2O3 system after Ca treatment to removing as much as possible inclusions especially large sized inclusions before the Ca treatment. With the new strategy, amount of inclusions after RH refining was remarkably decreased and the efficiency of Ca treatment significantly improved. The inclusions found in steel plates were all of high melting temperature CaO–CaS system and the severities of B‐type inclusions have been lowered from “≤2.0” to “0”(ASTM E45 standard).