High-strength steel having superior brittle crack arrestability, and production method therefor

Abstract

Provided are high-strength steel having superior brittle crack arrestability and a production method therefor. The high-strength steel comprises 0.05-0.1 wt % of C, 0.9-1.5 wt % of Mn, 0.8-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, 0.005-0.1 wt % of Ti, 0.1-0.6 wt % of Cu, 0.1-0.4 wt % of Si, at most 100 ppm of P, and at most 40 ppm of S with the remainder being Fe and other inevitable impurities, and has microstructures including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite. The high-strength steel has high yield strength and superior brittle crack arrestability.

1 . A high-strength steel having excellent brittle crack arrestability, comprising:
 0.05 wt % to 0.1 wt % of carbon (C), 0.9 wt % to 1.5 wt % of manganese (Mn), 0.8 wt % to 1.5 wt % of nickel (Ni), 0.005 wt % to 0.1 wt % of niobium (Nb), 0.005 wt % to 0.1 wt % of titanium (Ti), 0.1 wt % to 0.6 wt % of copper (Cu), 0.1 wt % to 0.4 wt % of silicon (Si), 100 ppm or less of phosphorous (P), 40 ppm or less of sulfur (S), and the remainder being iron (Fe) and other inevitably contained impurities, the high-strength steel having a microstructure including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite, and having a thickness of 50 mm or more.   
     
     
         2 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein the contents of Cu and Ni are set such that a Cu/Ni weight ratio is 0.6 or less. 
     
     
         3 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein the ferrite is acicular ferrite or polygonal ferrite, and the bainite is granular bainite. 
     
     
         4 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein when the microstructure of the steel is a complex-phase structure including the pearlite, a fraction of pearlite is 20% or less. 
     
     
         5 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein in the high-strength steel, a grain size having a high-angle boundary of 15 degrees or more measured, in an electron backscattered diffraction (EBSD) method, in a central portion of a steel thickness, is 30 μm or less. 
     
     
         6 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein in the high-strength steel, yield strength is 390 MPa or more. 
     
     
         7 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein an area ratio of a (100) plane forming an angle within 15 degrees with respect to a plane perpendicular to a rolling direction in a region of the high-strength steel in a range of 20% of an overall steel thickness based on a position equal to ½ of the steel thickness is 40% or less. 
     
     
         8 . The high-strength steel having excellent brittle crack arrestability of  claim 1 , wherein a steel thickness is 80 mm to 100 mm. 
     
     
         9 . A method of manufacturing a high-strength steel having excellent brittle crack arrestability, the method comprising:
 reheating a slab to a temperature of 950° C. to 1100° C., the slab including 0.05 wt % to 0.1 wt % of carbon (C), 0.9 wt % to 1.5 wt % of manganese (Mn), 0.8 wt % to 1.5 wt % of nickel (Ni), 0.005 wt % to 0.1 wt % of niobium (Nb), 0.005 wt % to 0.1 wt % of titanium (Ti), 0.1 wt % to 0.6 wt % of copper (Cu), 0.1 wt % to 0.4 wt % of silicon (Si), 100 ppm or less of phosphorous (P), 40 ppm or less of sulfur (S), and the remainder being iron (Fe) and other inevitably contained impurities, and then rough rolling the slab at a temperature of 1100° C. to 900° C.;   obtaining a steel sheet having a thickness of 50 mm or more by finish rolling a rough-rolled bar at a temperature of 850° C. to Ar 3 ; and   cooling the steel sheet to a temperature of 700° C. or less,   wherein a temperature difference between a central portion and a surface of the slab or bar before the rough rolling is 70° C. or more when the rough rolling is performed.   
     
     
         10 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein the contents of Cu and Ni are set such that a Cu/Ni weight ratio is 0.6 or less. 
     
     
         11 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein a temperature difference between a central portion in a thickness direction of the slab or bar and an outer surface of the slab or bar is 100° C. to 300° C. 
     
     
         12 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein a temperature difference between a central portion in a thickness direction and an outer surface of the slab or bar is a difference between a temperature of a surface of the slab or bar measured immediately before the rough rolling, and a temperature of a central portion calculated in consideration of a cooling condition and a thickness of the slab or bar immediately before the rough rolling. 
     
     
         13 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein the rough rolling is performed in two or more passes, and
 a temperature difference between a central portion in the thickness direction of the slab or bar and an outer surface of the slab or bar is a temperature difference in which a temperature difference therebetween for each pass of the rough rolling is measured and a total average value is calculated.   
     
     
         14 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein a reduction ratio per pass, with respect to three final passes when the rough rolling is performed, is 5% or more, and a total cumulative reduction ratio is 40% or more. 
     
     
         15 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein a crystal grain size of a central portion of the bar before the finish rolling after the rough rolling is 200 μm or less. 
     
     
         16 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein a reduction ratio during the finish rolling is set such that a ratio of a slab thickness (mm)/a steel sheet thickness (mm) after the finish rolling is 3.5 or above. 
     
     
         17 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein the cooling of the steel sheet is performed at a cooling rate of a central portion of the steel sheet of 2° C./s or more. 
     
     
         18 . The method of manufacturing a high-strength steel having excellent brittle crack arrestability of  claim 9 , wherein the cooling of the steel sheet is performed at an average cooling rate from 3° C./s to 300° C./s.