METHOD FOR MANUFACTURING LOW CARBON ALLOY STEELS FOR COLD FORGING THAT CAN BE SHORTENING THE SPHERODIZING ANNEALING TIME

摘要

The present invention provides a method for manufacturing steel for cold forging capable of shortening spherodizing annealing time, performed for assuring a cold forging property by controlling a micro-structure via controlled rolling and controlled cooling technology, which comprises the steps of: maintaining total in-furnace time of 80 minutes or more at temperatures of 950-1140C, desirably maintaining 30-60 minutes in a crack temperature range and total in-furnace time of 80-120 minutes and performing a rolling process for the steel containing 0.17-0.23 wt% of C, 0.10-0.35 wt% of Si, 0.55-0.95 wt% of Mn, 0.020 wt% or less of P (not including 0), 0.010-0.030 wt% of S, 0.25 wt% or less of Ni (not including 0), 0.85-1.45 wt% of Cr, 0.02-0.65 wt% of Mo, 0.010-0.030 wt% of Al, 0.015-0.035 wt% of Nb, 0.0010-0.0030 wt% of B, 0.0150 wt% or less of N (not including 0) and the remainder consisting of Fe and inevitable impurities; lowering finishing rolling temperature to 1000C or less, desirably 850-950C by using a water cooling facility and performing a rolling process; and cooling the rolled steel at 600-700C. The present invention provides the method for manufacturing the steel for cold forging capable of shortening the spherodizing annealing time by forming a low temperature transformation structure on a surface and a micro-structure in the inside via the controlled rolling and the controlled cooling technology.