Effect of Magnesium Treatment and Cooling Method on Manganese Sulfide in U75V Heavy‐Rail Steel Deoxidized by Si–Mn

Abstract

The effect of magnesium (Mg) on the morphology of sulfides in silicon–manganese deoxidized low‐sulfur and low‐aluminum steels is investigated by adding different amounts of nickel–magnesium (Ni–Mg) alloy to a heavy‐rail steel U75V melt and cooling the melt from 1600 °C to room temperature under different conditions. Herein, the effects of Mg content and cooling rate on MnS inclusions are investigated. It is indicated that with an increase in the Mg content in molten steel from 0 to 64 ppm, the main composition of inclusions changes following the path of CaO–SiO2–Al2O3 → MgO–Al2O3–MnS → MgO–MnS–MgS. Mg significantly inhibits the precipitation of long‐strip MnS and can decrease the average size and aspect ratio of the inclusions. Under furnace cooling, when Mg is 41 ppm, the minimum aspect ratio of the inclusions is 1.61. The higher the cooling rate, the smaller the inclusion size; however, compared to water and furnace coolings, with cooling rates of 126.3 and 0.12 °C s−1, respectively, the inclusion aspect ratio of the air‐cooling conditions with a cooling rate of 67.7 °C s−1 is low. Finally, thermal simulations verify the effect of Mg treatment on the suppression of MnS deformation during 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.…”