Steel plate and method for manufacturing same (as amended)

Abstract

A steel plate includes a predetermined chemical composition, in which B is controlled to be less than 0.0003%, and the balance is Fe and incidental impurities. The steel plate also includes precipitates containing Ti, Nb, and Mo and having a mean particle size of 20 nm or less, the relationship [Nb]/([Ti]+[Nb]+[Mo])≧0.3 being satisfied, where [Ti] is the Ti content, [Nb] is the Nb content, and [Mo] is the Mo content, thereby providing a thick, high tensile strength steel plate that is suitable for use in steel structures such as marine structures, ships, pressure vessels, and penstocks, has a yield stress (YS) of 460 MPa or greater, and has excellent low-temperature toughness of the heat-affected zone in a multilayer weld (CTOD property) and excellent strength and toughness after Post Weld Heat Treatment (PWHT property).

1 . A steel plate, comprising:
 a chemical composition including, by mass %,   C: 0.020% to 0.090%,   Si: 0.01% to 0.35%,   Mn: 1.40% to 2.00%,   P: 0.008% or less,   S: 0.0035% or less,   Al: 0.010% to 0.060%,   Ni: 0.40% to 2.00%,   Mo: 0.05% to 0.50%,   Nb: 0.005% to 0.040%,   Ti: 0.005% to 0.025%,   N: 0.0020% to 0.0050%,   Ca: 0.0005% to 0.0050%, and   0: 0.0035% or less,   Ceq specified by formula (1) below being in a range of 0.420% to 0.520%, formulas (2), (3), and (4) below being satisfied, B being controlled to be less than 0.0003%, and a balance being Fe and incidental impurities; and   precipitates including Ti, Nb, and Mo and having a mean particle size of 20 nm or less, wherein [Nb]/([Ti]+[Nb]+[Mo]) 0.3, where [Ti] is the Ti content, [Nb] is the Nb content, and [Mo] is the Mo content:
   Ceq=[C]+[Mn]/6 +([Cu]+[Ni])/15 +([Cr]+[Mo]+[V])/5   (1)
 
   1.5 5≦[Ti]/[N]≦4.0   (2)
 
   0<{[Ca]−(0.18+130×[Ca])×[O]}/1.25/[S]<1.5   (3)
 
   5.5[C] (4/3) +15[P]+0.90[Mn]+0.12[Ni]+7.9[Nb] (1/2) +0.53[Mo]≦3.70   (4)
 
   where [M] is the content of element M by mass %.   
     
     
         2 . The steel plate of  claim 1 , wherein the chemical composition further includes, by mass %, at least one selected from the group consisting of Cu: less than 0.7%, Cr: 0.1% to 1.0%, and V: 0.005% to 0.05%, Mg: 0.0002% to 0.0050%, and a REM: 0.0010% to 0.0200%. 
     
     
         3 . (canceled) 
     
     
         4 . A method for manufacturing a steel plate, the method comprising:
 heating steel having the chemical composition of  claim 1  to 950° C. to 1150° C.;   subsequently subjecting the steel to hot rolling at a cumulative rolling reduction of 30% or higher in a temperature range of 900° C. or higher and a cumulative rolling reduction of 30% to 70% in a temperature range of less than 900° C.; and   subsequently cooling the steel at least to 500° C. with a cooling rate of 1.0° C./s or higher.   
     
     
         5 . The method of  claim 4 , further comprising tempering at 450° C. to 650° C. after the cooling. 
     
     
         6 . A method for manufacturing a steel plate, the method comprising:
 heating steel having the chemical composition of any one of  claims 2  to 950° C. to 1150° C.;   subsequently subjecting the steel to hot rolling at a cumulative rolling reduction of 30% or higher in a temperature range of 900° C. or higher and a cumulative rolling reduction of 30% to 70% in a temperature range of less than 900° C.; and   subsequently cooling the steel at least to 500° C. with a cooling rate of 1.0° C./s or higher.   
     
     
         7 . The method of  claim 6 , further comprising tempering at 450° C. to 650° C. after the cooling.