A Method of Manufacturing the Fine-Grained Ferrite High-Strength Steel

摘要

PURPOSE: A method of manufacturing fine-grained ferrite high strength steel (tensile strength 600 MPa) is provided. CONSTITUTION: The method includes the steps of repeating thermal treatment of a steel slab comprising C 0.05 to 0.2 wt.%, Si 0.01 to 1.0 wt.%, Mn 0.05 to 2.0 wt.%, Al 0.005 to 0.1 wt.%, 0.01 to 0.1 wt.% of at least one element selected from Nb, Ti and V, N 0.003 to 0.015 wt.%, 0.03 wt.% or less of P, 0.03 wt.% or less of S, a balance of Fe and incidental impurities within the temperature range of Ac3 to Ac3+100 deg.C, wherein heat treatment is conducted in such a manner that heating and cooling is repeatedly performed less than three times at a rate of greater than 5 deg.C/min, so that austenite grain size thereof is refined less than 20 micrometers; rolling the steel slab over the non-recrystallization temperature range of Ar3 to Ar+50 deg.C at a reduction ratio of greater than 15 % per pass, wherein total cumulative reduction ratio is greater than 30 %; accelerated-cooling the rolled steel sheet at a cooling rate of 5C/sec to 20C/sec in order to transform microstructure of the steel into dual phase of ferrite+pearlite or ferrite+bainite, wherein ferrite grain size is 2 to 7 micrometers.