Manufacturing of high-strength ultrafine grained steels with high toughness using Strain Induced Dynamic Transformations

摘要

PURPOSE: A method for manufacturing high strength high toughness steel formed of composite structure of ultra-fine ferrite having an average grain size of 5 μm or less and pearlite having 40% or less of volume fraction by optimizing austenite grain size and hot working conditions is provided. CONSTITUTION: The method for manufacturing ultra-fine grained steel using strain induced dynamic transformation comprises a step of preparing a steel slab comprising 0.3 wt.% or less of C, 1.5 wt.% or less of Si, 2.0 wt.% or less of Mn, 0.01 to 0.08 wt.% of Nb and a balance of Fe and inevitable impurities; a step of controlling an average grain size of austenite structure of the steel slab to 50 μm or less before finally rolling the steel slab; a step of hot rolling the steel slab having austenite structure in multistage in such a way that the total reduction ratio is maintained to 60% or more as maintaining a reduction ratio per one pass of the steel slab to 30% or less in the temperature range of Ar3 to Ar3+100 deg.C; and a step of slowly cooling the hot rolled steel sheet so that fraction of pearlite in the steel structure becomes 40% or less, wherein the hot rolled steel sheet is slowly cooled in a cooling rate of 10 deg.C/sec or less.