Structural ultra-thick steel having excellent resistance to brittle crack propagation, and production method therefor

Abstract

Provided are structural ultra-thick steel having excellent resistance to brittle crack propagation and a production method therefor. The structural ultra-thick steel comprises 0.02-0.1 wt % of C, 0.8-2.5 wt % of Mn, 0.05-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, and 0.005-0.1 wt % of Ti 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 ultra-thick structural steel has excellent resistance to brittle crack propagation, excellent yield strength and an excellent impact transition temperature in the center.

1 . A structural ultra-thick steel having excellent resistance to brittle crack propagation, the structural ultra-thick steel comprising:
 0.02-0.1 wt % of C, 0.8-2.5 wt % of Mn, 0.05-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, 0.005-0.1 wt % of Ti, and the remainder of Fe and other inevitable impurities, the structural ultra-thick steel having 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.   
     
     
         2 . The structural ultra-thick steel of  claim 1 , wherein the ferrite is acicular ferrite or polygonal ferrite, and the bainite is granular bainite. 
     
     
         3 . The structural ultra-thick steel of  claim 1 , having a grain size of 15 μm or less, the grain size having a high-angle grain boundary of 15° or higher measured in an ESBD manner in a central portion in a plate thickness direction. 
     
     
         4 . The structural ultra-thick steel of  claim 1 , having a yield strength of 350 MPa or more, and an impact transition temperature of −60° C. or lower in of a central portion thereof. 
     
     
         5 . The structural ultra-thick steel of  claim 1 , having a thickness of 10-100 mm. 
     
     
         6 . A method of producing a structural ultra-thick steel having excellent resistance to brittle crack propagation, the method comprising:
 reheating a slab or a bar including 0.02-0.1 wt % of C, 0.8-2.5 wt % of Mn, 0.05-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, 0.005-0.1 wt % of Ti, and the remainder of Fe and other inevitable impurities to 950-1,100° C. and then rough rolling the reheated slab or bar at 900-1,100° C.;   obtaining a steel sheet by finish rolling the rough rolled slab or bar at an Ar3 transformation point or higher; and   cooling the steel sheet to 700° C. or lower,   wherein a temperature difference between a central portion of the slab or the bar in a thickness direction thereof and an external surface of the slab or the bar before rolling at the time of rough rolling is 100° C. or higher.   
     
     
         7 . The method of  claim 6 , wherein the temperature difference between the central portion of the slab or the bar in the thickness direction and the external surface of the slab or the bar is from 100-300° C. 
     
     
         8 . The method of  claim 6 , wherein the temperature difference between the central portion of the slab or the bar in the thickness direction and the external surface of the slab or the bar is a difference between a surface temperature of the slab or the bar measured immediately before rough rolling and a temperature of the central portion calculated by considering cooling conditions and a thickness of the slab or the bar immediately before rough rolling. 
     
     
         9 . The method of  claim 6 , wherein the rough rolling is performed in two passes or more, and the temperature difference between the central portion of the slab or the bar in the thickness direction and the external surface of the slab or the bar is a temperature difference obtained by measuring temperature differences in the respective passes during the rough rolling and calculating a total average value. 
     
     
         10 . The method of  claim 6 , wherein the temperature difference between the central portion of the slab or the bar in the thickness direction and the external surface of the slab or the bar is obtained by cooling the slab or the bar with a cooling device. 
     
     
         11 . The method of  claim 10 , wherein a cooling medium of the cooling device is at least one of water, air, a liquid coolant, and a vapor coolant. 
     
     
         12 . The method of  claim 6 , wherein a total cumulative reduction ratio at the time of rough rolling is 40% or higher. 
     
     
         13 . The method of  claim 6 , wherein the cooling the steel sheet is performed at a central portion cooling rate of 2° C./s or more. 
     
     
         14 . The method of  claim 6 , wherein the cooling the steel sheet is performed at an average cooling rate of 3-300° C./s.