In Situ Observation on the Heterogeneous Nucleation of MnS in Heavy Rail Steels with Varied Aluminum Content

Abstract

Heavy rail steels are widely used in railway transportation owing to their high strength and great toughness properties nowadays. [1][2][3] The number, size, composition, morphology, and spatial distribution of nonmetallic inclusions in steel have a crucial impact on the cleanliness and performance of the steel product. [4][5][6][7][8] Particularly, large and long MnS inclusions precipitated during heat treatment after rolling of the steel and have a detrimental effect on the strength and corrosion resistance of heavy rail steels. [9][10][11][12][13] Many studies were reported to control large-sized MnS inclusions according to the characteristic and source of MnS in the steel, such as slag refining, [14,15] calcium/magnesium addition, [16][17][18][19] zirconium/cerium addition, [20][21][22] heat treatment process, [23,24] and heterogeneous nucleation improvement. [25][26][27] Oikawa et al. [28,29] studied the effect of Ti addition on the formation and distribution of MnS inclusions in Fe-0.1%C-1%Mn-0.02%S steels during solidification. The reason for the significant size reduction of MnS inclusions was the (Ti, Mn)O composite oxides generated at solid/liquid interface of the steel with Ti addition, which acted as heterogeneous nucleation cores for the formation of MnS. Zhang et al. [30] investigated the characteristics of MnS particles at three different cooling rates of 80.4 K s À1 (water cooling), 3.8 K s À1 (air cooling), and 1.8 K s À1 (furnace cooling). With the decreasing cooling rate, the 3D morphology of MnS changed from nearly spherical into rod like and the area fraction and average diameter of MnS increased. Li et al. [25] studied the mechanism of MnS precipitation on Al 2 O 3 -SiO 2 inclusions during the solidification of nonoriented silicon steel. MnS can precipitate on micrometer-sized oxides and its precipitation behavior was governed by the phase structure of the oxides. Nowadays, promoting the heterogeneous nucleation of MnS inclusions was acknowledged as an effective method to reduce flaw detection defects of steel heavy rails.The precipitation of MnS inclusions mostly occurred in steel during cooling and during heating processes. [31,32] It was reported that MnS inclusions near the grain boundary of the steel were usually large and long and well deformed during steel rolling while MnS that precipitated around oxide inclusions were nearly spherical and difficultly deformed during steel rolling.

Precipitation Characteristic of Sulfide in Medium‐Carbon Sulfur‐Containing Steel

“…[6] Thus, regarding the important role of MnS inclusions in steel, lots of investigations have been performed, especially, the mechanism of morphology evolution by optimizing cooling rates. [15][16][17][18][19][20][21][22][23] At present, according to the classic work by Sims et al, the MnS in an as-cast steel can be typically classified as follows: globular MnS (Type I); fine rodlike MnS (Type II); and angular MnS (Type III). [15,16] However, the mechanisms of morphology evolution by various cooling rates still remain controversial.…”

Morphological Differences of MnS Inclusions in Medium‐Carbon Steel with Different Manganese and Sulfur Contents

“…[19][20][21][22] Al is considered as the element to increase the interfacial energy between the steel and MnS, allowing MnS to nucleate at higher subcooling, which would promote the divorce eutectic transformation of sulfide to form type III MnS from type II. [23,24] However, Dahl et al [12] and Baker et al [13][14][15] concluded that when the carbon and silicon content in the steel increases, it facilitates the formation of type III sulfide; if no carbon and silicon are present, high Al content will not form type III sulfide. Fredriksson et al [6] also suggested that MnS changes from type I and type II to type III when the carbon or aluminum content is sufficiently high.…”