| 摘要 |
PURPOSE: A low carbon steel sheet for structure purposes of 400 MPa and a method for manufacturing the same are provided which has superior toughness at the heat-affected zone by stably forming a fine Acicular ferrite structure at the heat-affected zone. CONSTITUTION: The low carbon steel sheet having superior toughness at heat-affected zone comprises 0.03 to 0.15 wt.% of C, 0.2 wt.% or less of Si, 0.6 to 2.0 wt.% of Mn, 0.25 to 0.7 wt.% of Al, 0.002 to 0.02 wt.% of N and a balance of Fe and inevitable impurities, wherein a prior Austenitic grain size is 150 microns or less in the structure of heat-affected zone that is heated to a temperature of 1200 deg.C or more, and the structure in the prior Austenitic grain consists of fine Acicular ferrite having a lath width of 3 microns or less. The method for manufacturing the low carbon steel sheet having superior toughness at heat-affected zone comprises the process of finishing hot rolling the reheated slab to a temperature ranging from 800 to 950 deg.C after reheating a slab comprising 0.03 to 0.15 wt.% of C, 0.2 wt.% or less of Si, 0.6 to 2.0 wt.% of Mn, 0.25 to 0.7 wt.% of Al, 0.002 to 0.02 wt.% of N and a balance of Fe and inevitable impurities to the temperature range of 1150 to 1250 deg.C.
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