Steel plate for linepipe having ultra-high strength and excellent low temperature toughness and manufacturing method of the same

Abstract

A steel plate for linepipes having ultra-high strength and excellent low temperature toughness, and a method for manufacturing the same are disclosed. The steel plate has a strength of 930 MPa or more and excellent toughness even with much smaller amounts of alloying elements than that of conventional steel plates, and a method for manufacturing the same. The steel includes by weight %: 0.03˜0.10% C; 0˜0.6% Si; 1.6-2.1% Mn; 0˜1.0% Cu; 0-1.0% Ni; 0.02-0.06% Nb; 0-0.1% V; 0.1-0.5% Mo; 0-1.0% Cr; 0.005-0.03% Ti; 0.01-0.06% Al; 0.0005-0.0025% B; 0.001-0.006% N; 0-0.006% Ca; 0.02% or less P; 0.005% or less S; and the balance Fe and unavoidable impurities. The microstructure includes at least about 75 area percent of a mixture of bainitic ferrite and acicular ferrite.

1 - 8 . (canceled) 
   
   
       9 . A steel plate having ultra-high strength and excellent low temperature toughness, comprising, by weight %: 0.03˜0.10% C; 0˜0.6% Si; 1.6˜2.1% Mn; 0˜1.0% Cu; 0˜1.0% Ni; 0.02˜0.06% Nb; 0˜0.1% V; 0˜10.5% Mo; 0˜1.0% Cr; 0.005˜0.03% Ti; 0.01˜0.06% Al; 0.0005˜0.0025% B; 0.001˜0.006% N; 0˜0.006% Ca; 0.02% or less P; 0.005% or less S; and the balance of Fe and unavoidable impurities, wherein the microstructure comprises at least about 75 area percent of a mixture of bainitic ferrite and acicular ferrite. 
   
   
       10 . The steel plate according to  claim 9 , wherein the steel plate comprises 0.015 wt % or less Mo. 
   
   
       11 . The steel plate according to  claim 9 , wherein the microstructure of the steel plate includes 5% or less granular bainite in terms of area fraction. 
   
   
       12 . The steel plate according to  claim 9 , wherein the through thickness dimension of austenite grains should less than 15 μm. 
   
   
       13 . A steel plate having ultra-high strength and excellent low temperature toughness, comprising, by weight %: 0.03˜0.10% C; 0˜0.6% Si; 1.6˜2.1% Mn; 0˜1.0% Cu; 0˜1.0% Ni; 0.20˜0.06% Nb; 0˜0.1 V; 0.1˜0.5% Mo; 0˜1.0 Cr; 0.005˜0.03% Ti; 0.01˜0.06% Al; 0.0005˜0.0025% B; 0.001˜0.006% N; 0˜0.006% Ca; 0.02% or less P; 0.005% or less S; and the balance of Fe and unavoidable impurities, wherein the microstructure comprises at least about 75 area percent of a mixture of bainitic ferrite and acicular ferrite, yield strength is 930 MPa or more, and Charpy impact absorbed energy at −40° C. is 230 Joules or more. 
   
   
       14 . The steel plate according to  claim 13 , wherein the steel plate comprises 0.015 wt % or less Mo. 
   
   
       15 . The steel plate according to  claim 13 , wherein the microstructure of the steel plate includes 5% or less granular bainite in terms of area fraction. 
   
   
       16 . The steel plate according to  claim 13 , wherein the through thickness dimension of austenite grains should less than 15 μm. 
   
   
       17 . A method of manufacturing a steel plate having ultra-high strength and excellent low temperature toughness, the method comprising:
 reheating a steel slab to 1,050˜1,150° C., the slab comprising, by weight %, 0.03˜0.10% C, 0˜0.6% Si, 1.6˜2.1% Mn, 0˜1.0% Cu, 0˜1.0% Ni, 0.02˜0.06% Nb, 0˜0.1 V, 0.1˜0.5% Mo, 0˜1.0 Cr, 0.005˜0.03% Ti, 0.01˜0.06% Al, 0.0005˜0.0025% B, 0.001˜0.006% N, 0˜0.006% Ca, 0.02% or less P, 0.005% or less S, and the balance of Fe and unavoidable impurities;   rolling the reheated steel slab by one or more hot rolling passes in an austenite recrystallization temperature region or more with a reduction ratio of 20˜80%;   rolling the rolled steel slab by one or more hot rolling passes in the range from the austenite recrystallization temperature or less to Ar 3  temperature or more with a reduction ratio of 40˜80% to provide a steel plate;   cooling the rolled steel plate at a cooling rate of 20˜50° C./sec; and   stopping the cooling of the steel plate at a temperature of 200˜400° C.   
   
   
       18 . The method according to  claim 17 , wherein the steel slab comprises 0.015 wt % or less Mo. 
   
   
       19 . The method according to  claim 17 , further comprising: air cooling or room temperature-cooling the steel plate after the cooling finish step. 
   
   
       20 . The method according to  claim 18 , further comprising: air cooling or room temperature-cooling the steel plate after the cooling finish step.