Steel

Abstract

Steel according to one embodiment of the present invention has predetermined chemical components, wherein in a region where a distance r from the center of a cross-section perpendicular to the length direction satisfies 0.7R≤r≤0.9R, structures include ferrite and bainite, the average fraction of the ferrite is in the range of 40 to 70% in terms of area ratio, the total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and the balance includes bainite; and the standard deviation of a ferrite fraction in the region is 4% or less.

1 . Steel comprising in % by mass:
 C: 0.17 to 0.21%,   Si: 0.40 to 0.60%,   Mn: 0.25 to 0.50%,   Cr: 1.35 to 1.55%,   Mo: 0.20 to 0.40%,   S: 0.010 to 0.05%,   N: 0.005 to 0.020%,   Al: 0.001% to 0.100%,   Nb: 0.001 to 0.030%   Ni: 0 to 3.0%,   Cu: 0 to 1.0%,   Co: 0 to 3.0%,   W: 0 to 1.0%,   V: 0 to 0.3%,   Ti: 0 to 0.3%,   B: 0 to 0.005%   O: 0.005% or less,   P: 0.03% or less,   Pb: 0 to 0.5%,   Bi: 0 to 0.5%,   Ca: 0 to 0.01%,   Mg: 0 to 0.01%,   Zr: 0 to 0.05%,   Te: 0 to 0.1%,   rare earth element: 0 to 0.005%, and   a balance including Fe and impurities, wherein   in a region where a distance r from the center of a cross-section perpendicular to a length direction satisfies the following expression, structures comprise ferrite and bainite, an average fraction of the ferrite is in a range of 40 to 70% in terms of area ratio, a total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and a balance includes bainite; and   a standard deviation of a ferrite fraction in the region is 4% or less:
   0.7R≤r≤0.9R
 
   where R represents a circle equivalent radius of the steel.   
     
     
         2 . The steel according to  claim 1 , which comprises in % by mass one or two or more of:
 Ni: 0.01 to 3.0%,   Cu: 0.01 to 1.0%,   Co: 0.01 to 3.0%,   W: 0.01 to 1.0%,   V: 0.01 to 0.3%,   Ti: 0.001 to 0.3%, and   B: 0.0001 to 0.005%.   
     
     
         3 . The steel according to  claim 1 , which comprises in % by mass one or two or more of:
 Pb: 0.01 to 0.5%,   Bi: 0.0001 to 0.5%,   Ca: 0.0001 to 0.01%,   Mg: 0.0001 to 0.01%,   Zr: 0.0001 to 0.05%,   Te: 0.0001 to 0.1%, and   Rare earth element: 0.0001 to 0.005%.   
     
     
         4 . The steel according to  claim 2 , which comprises in % by mass one or two or more of:
 Pb: 0.01 to 0.5%,   Bi: 0.0001 to 0.5%,   Ca: 0.0001 to 0.01%,   Mg: 0.0001 to 0.01%,   Zr: 0.0001 to 0.05%,   Te: 0.0001 to 0.1%, and   Rare earth element: 0.0001 to 0.005%.