Hot-rolled steel sheet for high strength linepipe having tensile strength of 540 mpa or more

Abstract

A hot-rolled steel sheet for a high strength linepipe having a tensile strength of 540 MPa or more and a method for making the same. The steel sheet having a chemical composition containing, by mass %, C: 0.02% or more and 0.06% or less, Si: 0.05% or more and 0.25% or less, Mn: 0.60% or more and 1.10% or less, P: 0.008% or less, S: 0.0010% or less, Nb: 0.020% or more and 0.060% or less, Ti: 0.001% or more and 0.020% or less, Al: 0.01% or more and 0.08% or less, Ca: 0.0005% or more and 0.0050% or less, one or more selected from among Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, and V: 0.10% or less, and the balance being Fe and inevitable impurities, in which the relationships 0.60≦CP≦0.90 and CM≦3.05 are satisfied.

1 . A hot-rolled steel sheet for a high strength linepipe, the steel sheet having a tensile strength of 540 MPa or more and having a chemical composition comprising:
 C: 0.02% or more and 0.06% or less, by mass %;   Si: 0.05% or more and 0.25% or less, by mass %;   Mn: 0.60% or more and 1.10% or less, by mass %;   P: 0.008% or less, S: 0.0010% or less, by mass %;   Nb: 0.020% or more and 0.060% or less, by mass %;   Ti: 0.001% or more and 0.020% or less, by mass %;   Al: 0.01% or more and 0.08% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %;   one or more selected from the group consisting of Cu: 0.50% or less, by mass %, Ni: 0.50% or less, by mass %, Cr: 0.50% or less, by mass %, Mo: 0.50% or less, by mass %, and V: 0.10% or less, by mass %; and   remaining Fe and unavoidable impurities as a balance,   wherein the relational expressions (1) and (2) below are satisfied:
   0.60≦CP≦0.90   (1),
 
   where CP is calculated from CP=4.46×C+2.37×Mn/6+(1.18×Cr+1.95×Mo+1.74×V)/5+(1.74×Cu+1.70×Ni)/15, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %; and
   CM≦3.05   (2),
 
   where CM is calculated from CM=2.37×Mn+2.34×Mo+0.59×Cr+0.17×Ni, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %.   
     
     
         2 . The hot-rolled steel sheet for a high strength linepipe according to  claim 1 , wherein the steel sheet has a metallographic structure including a bainitic-ferrite microstructure in a center segregation part, the bainitic-ferrite microstructure being present in the center segregation part in the range of 95% or more in terms of area fraction, and
 the average grain diameter of the bainitic-ferrite microstructure is in the range of 8.0 μm or less.   
     
     
         3 . The hot-rolled steel sheet for a high strength linepipe according to  claim 2 , wherein the steel sheet has a yield strength in the range of 493 MPa to 548 MPa. 
     
     
         4 . The hot-rolled steel sheet for a high strength linepipe according to  claim 2 , wherein the steel sheet has a tensile strength in the range of 565 MPa to 637 MPa. 
     
     
         5 . The hot-rolled steel sheet for a high strength linepipe according to  claim 2 , wherein the steel sheet has a fracture transition temperature in the range of −80° C. or less. 
     
     
         6 . A method for manufacturing a hot-rolled steel sheet for a high strength linepipe, the method comprising:
 heating a steel slab having the chemical composition according to  claim 1  at a temperature in the range of 1100° C. to 1300° C.;   rough rolling on the steel slab;   then finish rolling the rough-rolled steel slab to generate a finish-rolled steel sheet under conditions that a cumulative rolling reduction ratio is in the range of 20% or more in a temperature in the range of 930° C. or less;   cooling the finish-rolled steel sheet to a temperature in the range of 380° C. to 600° C. at an average cooling rate in the range of 10° C./s to 100° C./s in terms of a temperature of a central part of the finish-rolled steel sheet in a thickness direction; and   coiling the cooled finish-rolled steel sheet into a coil shape,   wherein the cooled finish-rolled steel sheet has a tensile strength of 540 MPa or more.