To manufacture a high-strength rail which is suitable to suppress the damage by internal fatigue by utilizing cooling buffer action of a surface layer which is generated by executing γ-granulating work on the head surface at the time of rolling rails and applying stable rapid cooling to the rail and controlling hardness distribution in the vicinity of the peak hardness in the inside after cooling.
In the rolling of a bloom for rails, finish rolling stage is divided into two stages, after the first-half finish rolling, time between passes of ≥10sec is imparted. One pass or plural passes of the second-half finish rolling is executed at 800-950°C at the draft of ≤10% per pass, rapid cooling at a surface cooling rate of ≥6°C/sec is started after 0.1-10sec, restaining or cooling is executed at 0-3°C/sec for 1-40sec when the surface temp. becomes the range of 450-650°C and accelerating cooling is again executed at the cooling rate of ≥1°C/sec down to the surface temp. of 400°C. This method is not a high-cost method such as salt bath and the hardness distribution in the vicinity of the peak hardness in the head part is freely set without producing different structure such as martensite.
UCHINO KOICHI
HIRAGAMI DAISUKE
