Normalizing heat-treated steel sheet having good low-temperature impact toughness and method for manufacturing same

Abstract

A normalizing heat-treated steel sheet having good low-temperature impact toughness and a method for manufacturing the same is provided. The normalizing heat treated steel sheet of the present invention contains, by weight %, C: 0.04 to 0.1%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Sol. Al: 0.015 to 0.04%, Nb: 0.003 to 0.03%, Ti: 0.005 to 0.02%, Cu: 0.35% or less, Ni: 0.05 to 0.8%, N: 0.002 to 0.008%, P: 0.01% or less (excluding 0%), S: 0.003% or less, and a balance of Fe and unavoidable impurities and has a steel microstructure composed of 70 to 90 area % of polygonal ferrite having a grain size of 20 μm or less, and 10 to 30 area % of spheroidized pearlite.

1 . A normalizing heat-treated steel sheet having good low-temperature impact toughness, comprising:
 by weight %, C: 0.04 to 0.1%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Sol. Al: 0.015 to 0.04%, Nb: 0.003 to 0.03%, Ti: 0.005 to 0.02%, Cu: 0.35% or less, Ni: 0.05 to 0.8%, N: 0.002 to 0.008%, P: 0.01% or less (excluding 0%), S: 0.003% or less, and a balance of Fe and unavoidable impurities and has a steel microstructure comprising 70 to 90 area % of polygonal ferrite having a grain size of 20 μm or less, and 10 to 30 area % of spheroidized pearlite.   
     
     
         2 . The normalizing heat-treated steel sheet having good low-temperature impact toughness of  claim 1 , wherein the heat-treated steel sheet has yield strength of 350 MPa or more, and an impact absorption energy value of 150 J or more at −60° C. 
     
     
         3 . A method for manufacturing a normalizing heat-treated steel sheet having good low-temperature impact toughness, comprising processes of:
 reheating a steel slab including, by weight %, C: 0.04 to 0.1%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Sol. Al: 0.015 to 0.04%, Nb: 0.003 to 0.03%, Ti: 0.005 to 0.02%, Cu: 0.35% or less, Ni: 0.05 to 0.8%, N: 0.002 to 0.008%, P: 0.01% or less (excluding 0%), S: 0.003% or less, and a balance of Fe and unavoidable impurities at a temperature of 1020 to 1150° C.;   manufacturing a hot-rolled steel sheet by finish hot rolling the reheated steel slab in a non-recrystallization temperature region at a Ar3 temperature or higher;   air-cooling or water-cooling the hot-rolled steel sheet; and   normalizing in which the cooled hot-rolled steel sheet is heated to a temperature range of 850 to 960° C., and then maintained for [1.3 t+(10 to 30)] minutes, where t is a value measured in mm of a thickness of the hot-rolled steel sheet.   
     
     
         4 . The method for manufacturing a normalizing heat-treated steel sheet having good low-temperature impact toughness of  claim 3 , wherein the finish rolling temperature is in a range of 760 to 810° C. 
     
     
         5 . The method for manufacturing a normalizing heat-treated steel sheet having good low-temperature impact toughness of  claim 3 , wherein, when the hot-rolled steel sheet is water-cooled, it is cooled at a cooling rate of 2 to 30° C./s to a temperature range of 500 to 300° C. 
     
     
         6 . The method for manufacturing a normalizing heat-treated steel sheet having good low-temperature impact toughness of  claim 3 , wherein the normalizing steel sheet has a steel microstructure comprising 70 to 90 area % of polygonal ferrite having a grain size of 20 μm or less, and 10 to 30 area % of spheroidized pearlite.