Behaviour of Hydrogen During the Manufacture of Rail Steels

JSW Steel Ltd. supplies hydrogen-sensitive rail steels blooms for the manufacture of steel rails for Euro-rail application through AFERPI, Lucchini. The hydrogen distribution in the production of two high-carbon near-eutectoid rail steel grades VAR89S (typically, 0.70%C–0.3%Si–1.1%Mn–0.015%S) and VAR101 (typically, 0.80%C–0.42%Si–1.01%Mn–0.65%Cr–0.005%S) has been examined from the steel making stage to the as-cast blooms. In the steel grade VAR89S, the hydrogen distribution has been examined in the hot-rolled steel rails at different locations. The steel making involves primary steel making using energy optimizing furnace, ladle furnace followed by vacuum degassing. The hydrogen content before secondary steel making varies between 3 and 5.5 ppm H, which decreases to less than 1.5 ppm H after vacuum degassing. The steel picks up hydrogen, when transferred to tundish to a value, less than 2.5 ppm, associated with moisture in spray mass and tundish compound. On solidification in continuous casting, the hydrogen level decreases to less than 1.6 ppm H, which is associated with lowering of hydrogen solubility associated phase transformation of molten steel to solid. When the bloom is converted to steel rails, the hydrogen level further decreases due to the increased surface area-to-volume ratio. The final hydrogen levels in steel rails are < 1.6 ppm H, which is safe enough against defects such as shatter crack or Tache Ovales in steel rails.