Steel material having excellent sulfide stress corrosion cracking resistance and method of manufacturing same

Abstract

The present disclosure relates to a thick steel material that can be appropriately used as a line pipe, a sour-resistant material and, more particularly, to a high-strength steel material having excellent sulfide stress corrosion cracking resistance and excellent resistance against propagation of sulfide stress corrosion cracking, and a method of manufacturing the steel material.

1 . A steel material comprising, by weight %, carbon (C): 0.02˜0.06%, silicon (Si): 0.1˜0.5%, manganese (Mn): 0.8˜1.8%, chrome (Cr): less than 0.05%, phosphorous (P): 0.03% or less, sulfur (S) 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005˜0.08%, titanium (Ti): 0.005˜0.05%, calcium (Ca): 0.0005˜0.005%; one or more of nickel (Ni): 0.05˜0.3%, molybdenum (Mo): 0.02˜0.2%, and vanadium (V): 0.005˜0.1%, and a balance of Fe and unavoidable impurities,
 wherein the Ca and the S satisfy the following Equation 1, 
 the steel material has a microstructure of a surface portion composed of ferrite or a complex structure of ferrite and pearlite, and 
 a microstructure of the center portion is composed of acicular ferrite,
   0.5≤ Ca/S≤ 5.0  [Equation 1]
 
 
 where each element represents the content of each element by weight %. 
 
     
     
         2 . The steel material of  claim 1 , wherein Vickers hardness in the surface portion is 200 Hv or less. 
     
     
         3 . The steel material of  claim 1 , wherein the steel material has yield strength of 450 MPa or more. 
     
     
         4 . A method of manufacturing a steel material, the method comprising: heating a steel slab, which includes, by weight %, carbon (C): 0.02˜0.06%, silicon (Si): 0.1˜0.5%, manganese (Mn) 0.8˜1.8%, chrome (Cr): less than 0.05%, phosphorous (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al) 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb) 0.005˜0.08%, titanium (Ti): 0.005˜0.05%, calcium (Ca): 0.0005˜0.005%; one or more of nickel (Ni): 0.05˜0.3%, molybdenum (Mo): 0.02˜0.2%, and vanadium (V): 0.005˜0.1%, and a balance of Fe and unavoidable impurities and in which the Ca and the S satisfy the following Equation 1, at a temperature range of 1100˜1300° C. for 2 hours or more;
 obtaining a hot-rolled plate by hot-rolling the heated steel slab; and 
 cooling the hot-rolled plate after hot rolling, 
 wherein the cooling includes primary cooling, air cooling, and secondary cooling, and 
 the primary cooling is performed at a cooling rate of 5˜40° C./s such that a temperature of a surface portion of the hot-rolled plate becomes Ar1−50° C.˜Ar3−50° C. and the secondary cooling is performed at a cooling rate of 50˜500° C./s such that the temperature of the surface portion of the hot-rolled plate becomes 300˜600° C.
   0.5≤ Ca/S≤ 5.0  [Equation 1]
 
 
 where each element represents the content of each element by weight %. 
 
     
     
         5 . The method of  claim 4 , wherein the hot rolling is performed at an accumulated reduction ratio of 50% or more in a temperature range of Ar3+80° C.˜Ar3+200° C. 
     
     
         6 . The method of  claim 4 , further comprising maintaining for 30 second or more before cooling after the hot rolling. 
     
     
         7 . The method of  claim 4 , wherein the primary cooling is started when the temperature of the surface of the hot-rolled plate is Ar3−20° C.˜Ar3+50° C. 
     
     
         8 . The method of  claim 4 , wherein a temperature of a center portion of the hot-rolled plate is Ar3−30° C.˜Ar3+30° C. after the primary cooling is finished. 
     
     
         9 . The method of  claim 4 , wherein the temperature of the surface portion of the hot-rolled plate is Ar3−10° C.˜Ar3−50° C. after the air cooling is finished.