Hot-rolled steel sheet for high strength linepipe

Abstract

A hot-rolled steel sheet for a high strength linepipe having a metallic structure composed of bainitic-ferrite 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: 0.010% or more and 0.060% or less, Ti: 0.001% or more and 0.020% or less, Mo: 0.05% or less, Cr: 0.05% or more and 0.50% or less, Al: 0.01% or more and 0.08% or less, Ca: 0.0005% or more and 0.0050% or less, O: 0.005% or less, one or more selected from among Cu: 0.50% or less, Ni: 0.50% or less, and V: 0.10% or less, and the balance being Fe and inevitable impurities, in which the ratio of the hardness of a center segregation part to the hardness of a non-segregation part is less than 1.20 and the relationship SP≦1.90 is satisfied.

1 . A hot-rolled steel sheet for a high strength linepipe, the steel sheet having a metallic structure composed of bainitic-ferrite 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, by mass %;   S: 0.0010% or less, by mass %;   Nb: 0.010% or more and 0.060% or less, by mass %;   Ti: 0.001% or more and 0.020% or less, by mass %;   Mo: 0.05% or less, by mass %;   Cr: 0.05% or more and 0.50% 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 %;   O: 0.005% 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 %, and V: 0.10% or less, by mass %; and   remaining Fe and unavoidable impurities as a balance,   wherein expression (1) below is satisfied:
     SP≦ 1.90  (1),
 
   where SP is calculated from SP=Mn+Mo+11.3×C+0.29×(Cu+Ni)+0.60×Cr+0.88×V, and where atomic symbols in the equation respectively represent the contents of the corresponding chemical elements by mass %,   and a ratio of the hardness of a center segregation part to the hardness of a non-segregation part is less than 1.20.   
     
     
         2 . The hot-rolled steel sheet for a high strength linepipe according to  claim 1 , wherein the steel sheet has a yield strength in the range of 435 MPa to 534 MPa. 
     
     
         3 . The hot-rolled steel sheet for a high strength linepipe according to  claim 1 , wherein the steel sheet has a tensile strength in the range of 508 MPa to 616 MPa. 
     
     
         4 . The hot-rolled steel sheet for a high strength linepipe according to  claim 1 , wherein the steel sheet has a fracture transition temperature in the range of −70° C. or less. 
     
     
         5 . 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 on 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 5° 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 steel sheet has a metallic structure composed of bainitic-ferrite.