METHOD FOR MANUFACTURING SURFACE FINE GRAIN STEEL

摘要

PURPOSE: A method for manufacturing surface fine grain steel is provided to manufacture steel products for reinforcing concrete runway, that is, steel products for structural, and steel products requiring a high yield ratio by increasing the yield strength of the surface fine grain steel with the effect of surface fine grain. CONSTITUTION: The method comprises the processes of rough rolling the heated steel product after heating a steel product containing 0.10 to 0.50 wt.% of C, 0.30 to 1.50 wt.% of Mn, 0.10 to 0.50 wt.% of Si, 0.010 to 0.050 wt.% of Al, 0.002 to 0.020 wt.% of N, 0.01 to 0.10 wt.% of S and a balance of Fe and other inevitable impurities to a temperature ranging from 900 to 1250 deg.C; controlling a surface temperature of the rough rolled steel product to a temperature of 520 to 560 deg.C with a water zone I when passing through the water zone after the rough rolling; forming a surface hardening layer by cooling the surface temperature of the middle rolled steel product to 340 to 410 deg.C with a water zone II after middle rolling the resulting steel product in a temperature ranging from 843 to 887 deg.C with the restoration of the internal recuperative heat; and finally rolling the resulting steel product in a temperature ranging from 730 to 800 deg.C with the internal recuperative heat and the processing heat.