Oil country tubular goods with dual phase structure and producing method thereof

1. A steel pipe with a dual phase structure, the steel pipe comprising, as a chemical composition, by mass %,

C: 0.07% to 0.15%,

Si: 0.1% to 0.5%,

Mn: 0.8% to 1.9%,

Nb: 0.020% to 0.10%,

P: limited to 0.05% or less,

S: limited to 0.01% or less,

Al: limited to 0.1% or less, and

a balance consisting of iron and unavoidable impurities,

wherein a carbon equivalent Ceq defined by an (Equation 1) is 0.25 to 0.40 and [Nb]×[C]≧0.002 is satisfied when a [X] is an amount of an element X in mass %, and

the steel pipe with the dual phase structure comprising, as a metallographic structure, a ferrite and a secondary hard phase,

wherein, by area % the ferrite is 80% to 98% and the secondary hard phase is 2% to 20%,

wherein the secondary hard phase consists of a martensite, a residual austenite, or a mixture thereof, and

wherein an average grain size of the ferrite is 1 μm to less than 8 μm and an average grain size of the secondary hard phase is 0.1 μm to 2 μm,

Ceq=[C]+[Mn]/6  (Equation 1)

where the [X] expresses the amount of the element X in mass %.

2. The steel pipe with the dual phase structure according to claim 1 , further comprising, as the chemical composition, by mass %, at least one of

V: 0.0001% to 0.02%,

Ti: 0.005% to 0.03%,

Ca: 0.001% to 0.010%, and

N: 0.001% to 0.01%,

wherein [V]/[Nb]≦1/3 is satisfied when the [X] is the amount of the element X [in mass %] and the carbon equivalent Ceq is defined as an (Equation 2) on behalf of the (Equation 1),

Ceq=[C]+[Mn]/6+[V]/5  (Equation 2)

where the [X] expresses the amount of the element X in mass %.

3. The steel pipe with the dual phase structure according to claim 2 ,

wherein the Nb content of the chemical composition is 0.040% to 0.10% and [Nb]×[C]≧0.003 is satisfied when the [X] is the amount of the element X in mass %.

4. The steel pipe with the dual phase structure according to claim 2 ,

wherein a plate thickness of the steel pipe with the dual phase structure is 5 mm to 15 mm.

5. The steel pipe with the dual phase structure according to claim 2 ,

wherein the chemical composition, by mass %, consists of

C: 0.07% to 0.15%;

Si: 0.1% to 0.5%;

Mn: 0.8% to 1.9%;

Nb: 0.020% to 0.10%;

P: limited to 0.05% or less;

S: limited to 0.01% or less;

Al: limited to 0.1% or less;

at least one selected from the group consisting of

V: 0.0001% to 0.02%,

Ti: 0.005% to 0.03%,

Ca: 0.001% to 0.010%, and

N: 0.001% to 0.01%; and

the balance consisting of iron and unavoidable impurities.

6. The steel pipe with the dual phase structure according to claim 2 , wherein the chemical composition is free of Ni, Cu, Cr and Mo.

7. The steel pipe with the dual phase structure according to claim 2 ,

wherein, by area %, the ferrite is 80% to 98% and the secondary hard phase is 2% to 7%.

8. The steel pipe with the dual phase structure according to claim 2 ,

wherein the metallographic structure, by area %, consists of the ferrite and the secondary hard phase.

9. The steel pipe with the dual phase structure according to claim 2 ,

wherein in the metallographic structure, by area %, a total amount of portions except for the ferrite and the secondary hard phase are limited to 2% or less.

10. The steel pipe with the dual phase structure according to claim 1 ,

wherein the Nb content of the chemical composition is 0.040% to 0.10% and [Nb]×[C]≧0.003 is satisfied when the [X] is the amount of the element X in mass %.

11. The steel pipe with the dual phase structure according to claim 1 ,

wherein a plate thickness of the steel pipe with the dual phase structure is 5 mm to 15 mm.

12. The steel pipe with the dual phase structure according to claim 1 ,

wherein the chemical composition, by mass %, consists of

C: 0.07% to 0.15%,

Si: 0.1% to 0.5%,

Mn: 0.8% to 1.9%,

Nb: 0.020% to 0.10%,

P: limited to 0.05% or less,

S: limited to 0.01% or less,

Al: limited to 0.1% or less, and

the balance consisting of iron and unavoidable impurities.

13. The steel pipe with the dual phase structure according to claim 1 , wherein the chemical composition is free of Ni, Cu, Cr and Mo.

14. The steel pipe with the dual phase structure according to claim 1 ,

wherein, by area %, the ferrite is 80% to 98% and the secondary hard phase is 2% to 7%.

15. The steel pipe with the dual phase structure according to claim 1 ,

wherein the metallographic structure, by area %, consists of the ferrite and the secondary hard phase.

16. The steel pipe with the dual phase structure according to claim 1 ,

wherein in the metallographic structure, by area %, a total amount of portions except for the ferrite and the secondary hard phase are limited to 2% or less.

17. A producing method of a steel pipe with a dual phase structure, the method comprising,

a hot rolling process of making a hot rolled steel plate under a condition in which an average grain size of a ferrite is controlled to be 1 μm to less than 10 μm by using a steel material which includes, as a chemical composition, by mass %,

C: 0.07% to 0.15%,

Si: 0.1% to 0.5%,

Mn: 0.8% to 1.9%,

Nb: 0.020% to 0.10%,

P: limited to 0.05% or less,

S: limited to 0.01% or less,

Al: limited to 0.1% or less, and

a balance consisting of iron and unavoidable impurities,

wherein a carbon equivalent Ceq defined by an (Equation 3) is 0.25 to 0.40 and [Nb]×[C]≧0.002 is satisfied when a [X] is an amount of an element X in mass %,

a pipe-making process of making a steel pipe of the hot rolled steel plate,

a heating and quenching process of heating the steel pipe to a temperature of more than Ac 1 at which an austenite transformation starts to less than Ac 3 at which the austenite transformation finishes and of quenching,

Ceq=[C]+[Mn]/6  (Equation 3)

where the [X] expresses the amount of the element X in mass %, and

the steel pipe with the dual phase structure comprising, as a metallographic structure, a ferrite and a a secondary hard phase,

wherein, by area %, the ferrite is 80% to 98% and the secondary hard phase is 2 to 20%, and

wherein the secondary hard phase consists of a martensite, a residual austenite, or a mixture thereof.

18. The producing method of the steel pipe with the dual phase structure according to claim 17 , wherein the steel material further includes, as the chemical composition, by mass %, at least one of

V: 0.0001% to 0.02%,

Ti: 0.005% to 0.03%,

Ca: 0.001% to 0.010%, and

N: 0.001% to 0.01%,

wherein [V]/[Nb]≦1/3 is satisfied when the [X] is the amount of the element X [in mass %] and the carbon equivalent Ceq is defined as an (Equation 4) on behalf of the (Equation 3),

Ceq=[C]+[Mn]/6+[V]/5  (Equation 4)

where the [X] expresses the amount of the element X in mass %.

19. The producing method of the steel pipe with the dual phase structure according to claim 18 ,

wherein the chemical composition, by mass %, consists of

C: 0.07% to 0.15%;

Si: 0.1% to 0.5%;

Mn: 0.8% to 1.9%;

Nb: 0.020% to 0.10%;

P: limited to 0.05% or less;

S: limited to 0.01% or less;

Al: limited to 0.1% or less;

at least one selected from the group consisting of

V: 0.0001% to 0.02%,

Ti: 0.005% to 0.03%,

Ca: 0.001% to 0.010%, and

N: 0.001% to 0.01%; and

the balance consisting of iron and unavoidable impurities.

20. The producing method of the steel pipe with the dual phase structure according to claim 17 ,

wherein the chemical composition, by mass %, consists of

C: 0.07% to 0.15%,

Si: 0.1% to 0.5%,

Mn: 0.8% to 1.9%,

Nb: 0.020% to 0.10%,

P: limited to 0.05% or less,

S: limited to 0.01% or less,

Al: limited to 0.1% or less, and

the balance consisting of iron and unavoidable impurities.