Seamless steel pipe and method for producing same

1. A hot-rolled seamless steel pipe having a chemical composition consisting of, by mass %,

C: 0.10 to 0.17%,

Si: 0.05 to 1.0%,

Mn: 0.05 to 1.2%,

P: 0.025% or less,

S: 0.005% or less,

Cu: 0.20% or less,

N: 0.007% or less,

Ni: 0.20 to 0.50%,

Cr: 0.30% or more and less than 0.50%,

Mo: 0.30 to 0.50%,

Nb: 0.01 to 0.05%,

Al: 0.001 to 0.10%,

B: 0.0005 to 0.0020%,

Ti: 0.003 to 0.050%,

V: 0.01 to 0.20%,

a total of any one or more elements among Ca, Mg and REM: 0 to 0.025%, and the balance: Fe and impurities;

wherein:

a value of Pcm that is represented by Formula [A] below is 0.30 or less,

a steel micro-structure comprises, in area %, tempered martensite: 90% or more,

a tensile strength is 980 MPa or more,

a Charpy impact value at −40° C. using a 2 mm V-notch test specimen is 75 J/cm 2 or more,

Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B  [A]

where, each symbol of an element in Formula [A] represents a content (mass %) of a corresponding element in the steel, with a value of a symbol being zero if the corresponding element is not contained, and

wherein the hot-rolled seamless steel pipe is produced by hot rolling the hot-rolled seamless steel pipe, cooling the hot-rolled seamless steel pipe, subjecting the hot-rolled seamless steel pipe to a quenching process, and subjecting the hot-rolled seamless steel pipe to a tempering process.

2. A method for producing a hot-rolled seamless steel pipe having a chemical composition consisting of, by mass %,

C: 0.10 to 0.17%,

Si: 0.05 to 1.0%,

Mn: 0.05 to 1.2%,

P: 0.025% or less,

S: 0.005% or less,

Cu: 0.20% or less,

N: 0.007% or less,

Ni: 0.20 to 0.50%,

Cr: 0.30% or more and less than 0.50%,

Mo: 0.30 to 0.50%,

Nb: 0.01 to 0.05%,

Al: 0.001 to 0.10%,

B: 0.0005 to 0.0020%,

Ti: 0.003 to 0.050%,

V: 0.01 to 0.20%,

a total of any one or more elements among Ca, Mg and REM: 0 to 0.025%, and the balance: Fe and impurities;

wherein:

a value of Pcm that is represented by Formula [A] below is 0.30 or less,

a steel micro-structure comprises, in area %, tempered martensite: 90% or more,

a tensile strength is 980 MPa or more,

a Charpy impact value at −40° C. using a 2 mm V-notch test specimen is 75 J/cm 2 or more,

Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B  [A]

where, each symbol of an element in Formula [A] represents a content (mass %) of a corresponding element in the steel, with a value of a symbol being zero if the corresponding element is not contained,

the method comprising performing processes [i] to [iv] hereunder in sequence using a cast piece having a chemical composition described in claim 1 :

[i]: a hot rolling process of producing a material pipe by heating the cast piece to 1200 to 1300° C., and thereafter subjecting the cast piece to working with a reduction of area in a range of 40 to 99%;

[ii]: a cooling process of cooling the material pipe to a temperature that is less than an Ac 1 point;

[iii]: a quenching process of heating the cooled material pipe to a temperature in a range from an Ac 3 point to 950° C., and thereafter rapidly cooling the material pipe; and

[iv]: a tempering process of heating the quenched material pipe to a temperature in a range from 500 to 600° C., and thereafter cooling to room temperature.

3. The hot-rolled seamless steel pipe according to claim 1 , wherein the tempering process includes heating the hot-rolled seamless steel pipe, after quenching, to a temperature in a range from 500 to 600° C., and thereafter cooling to room temperature.

4. The hot-rolled seamless steel pipe according to claim 1 , wherein a wall thickness of the hot-rolled seamless steel pipe is 10 to 45 mm.