Ultrathick steel materials for flange having excellent strength and low temperature impact toughness, and manufacturing method therefor

Abstract

Provided is an ultrathick steel material for a flange having excellent strength and low temperature impact toughness, and a manufacturing method therefor. The steel material of the present disclosure comprises, in weight %, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.005-0.07%, V: 0.001-0.3%, Ti: 0.001-0.05%, Cr: 0.01-0.3%, Mo: 0.01-0.12%, Cu: 0.01-0.6%, Ni: 0.05-4.0%, Ca: 0.0005-0.004%, and the balance being Fe and other unavoidable impurities, wherein the steel material has a microstructure having a prior austenite crystallite grain size of 35 μm or less and including 90 area % or more of at least one of bainite and martensite, and the remainder of ferrite or pearlite.

1 . An ultrathick steel material for a flange, comprising:
 in weight %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.005 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.05%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 4.0%, Ca: 0.0005 to 0.004%, and the balance being Fe and other unavoidable impurities,   the ultrathick steel material has a microstructure having a grain size of prior austenite to be 35 μm or less and comprising 90 area % or more of at least one of bainite and martensite, and the remainder of ferrite or pearlite, the low temperature transformation phase has a packet size of 15 μm or less based on a high angle grain boundary of 15° or more,   the number of strain-induced NbC precipitates of 5 to 50 nm is 10 or more, and the number of coarse precipitates of 100 nm or more is 5 or less, per 1 μm 2 , and   a porosity of the central portion of the steel material, which is an area of ⅜t to ⅝t in a thickness direction from the surface, is 0.05 mm 3 /g or less.   
     
     
         2 . The ultrathick steel material of  claim 1 , wherein the steel material further includes Zr: 0.001 to 0.15%. 
     
     
         3 . The ultrathick steel material of  claim 1 , wherein the steel material has a thickness of 200 to 500 mm. 
     
     
         4 . The ultrathick steel material of  claim 1 , wherein the steel material has a tensile strength of 590 to 820 MPa, a yield strength of 440 MPa or more, and a Charpy impact test absorption energy value of 50 J or more at −50° C. 
     
     
         5 . The ultrathick steel material of  claim 1 , wherein a maximum surface crack depth of the steel material is 0.1 mm or less (including 0). 
     
     
         6 . A manufacturing method for an ultrathick steel material for a flange, comprising:
 preparing a slab comprising, in weight %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.005 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.05%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 4.0%, Ca: 0.0005 to 0.004%, and the balance being Fe and other unavoidable impurities, and then heating the slab to a temperature within a range of 1100 to 1300° C.;   performing primary upsetting on the heated slab at a forging ratio of 1.3 to 2.4 and then bloom forging on the heated slab at a forging ratio of 1.5 to 2.0;   reheating the bloom-forged material to a temperature within a range of 1100 to 1300° C.;   performing secondary upsetting on the reheated bloom-forged material at a forging ratio of 1.3 to 2.3 and then round forging on the reheated bloom-forged material at a forging ratio of 1.65 to 2.25;   performing tertiary upsetting on the round-forged material at a forging ratio of 2.0 to 2.8 so that a cumulative reduction amount is 10% or more at a temperature of recrystallization temperature or lower defined by the following Equation 1;   performing hole processing on the material subjected to the tertiary upsetting, reheating the hole processed material to a temperature within a range of 1100 to 1300° C., and then performing ring forging on the reheated material at a forging ratio of 1.0 to 1.6; and   heating the ring-forged material to a temperature within a range of 820 to 930° C. that is a temperature measured based on the central portion thereof, maintaining the heated ring-forged material for 5 to 600 minutes, air cooling the heated ring-forged material to room temperature, and then raising and maintaining the temperature to 550 to 700° C.   
       
         
           
             
               
                 
                   
                     
                       
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         7 . The manufacturing method of  claim 6 , wherein the slab is manufactured using one of a continuous casting process, a semi-continuous casting process, and an ingot casting process. 
     
     
         8 . The manufacturing method of  claim 6 , wherein a size of a forged surface punched during the primary upsetting is 1000 to 1200 mm×1800 to 2000 mm when an initial size is 700 mm×1800 mm. 
     
     
         9 . The manufacturing method of  claim 6 , wherein, for the bloom forging, the size of the forged surface upon the completion of forging is 1450 to 1850 mm×2100 to 2500 mm when an initial size is 1000 to 1200 mm×1800 to 2000 mm. 
     
     
         10 . The manufacturing method of  claim 6 , wherein, when the secondary upsetting and round forging end, a size of the product is 1450 to 1850Ø×1300 to 1700 mm. 
     
     
         11 . The manufacturing method of  claim 6 , wherein, when the tertiary upsetting ends, a size of the product is 2300 to 2800Ø×400 to 800 mm. 
     
     
         12 . The manufacturing method of  claim 6 , wherein a maximum thickness of the flange made of the steel material is 200 to 500 mm, an inner diameter is 4000 to 7000 mm, and an outer diameter is 5000 to 8000 mm.