High-strength steel excellent in low temperature toughness and toughness at weld heat-affected zone, mehtod for producing the same, and method for producing high-strength steel pipe

• US 20040031544A1
• Filed: 05/23/2003
• Published: 02/19/2004
• Est. Priority Date: 05/27/2002
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation
• Inventors: Hara, Takuya, Asahi, Hitoshi
• Granted Patent: US 7,601,231 B2
• Time in Patent Office: Days
• US Class Current: 148/521
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/10 during manufacturing of tubular bodies
• CPC Class Codes: C21D 8/1261 following hot rolling
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/42 with copper
• CPC Class Codes: C22C 38/44 with molybdenum or tungsten
• CPC Class Codes: C22C 38/46 with vanadium
• CPC Class Codes: C22C 38/58 with more than 1.5% by weight of manganese
• CPC Class Codes: Y10S 148/909 Tube

Abstract

A high-strength steel pipe excellent in weldability on site and a method for producing the steel pipe by improving the reliability of the low temperature toughness of a steel are provided. For example, the steel pipe includes elements to enhance hardenability for furthering high-strengthening and also improving toughness at a weld heat affected zone subjected to double or more layer welding. In the method, the steel is made to consist of a structure composed of bainite and/or martensite by containing prescribed amounts of C, Si, Mn, P, S, Ni, Mo, Nb, Ti, Al and N, and, as occasion demands, one or more of B, V, Cu, Cr, Ca, REM, and Mg, and regulating C, Si, Mn, Cr, Ni, Cu, V and Mo. Such elements enhancing hardenability, by a specific relational expression. The diameter of prior austenite grains may be regulated in a prescribed range. The method may include heating a casting to a temperature not lower than the Ac₃ point, hot rolling such casting, and thereafter cooling the resulting hot-rolled steel plate at a prescribed cooling rate.

Claims

1. A high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

at least one portion which contains, in mass;

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.2 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%,

wherein a balance of the at least one portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 1.9 and 3.5, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+Mo−

0.5.

2. The steel according to claim 1, wherein an average diameter of a prior austenite grains in the steel is not larger than 10 μ

m.

3. A high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

at least one portion which contains, in mass;

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein a balance of the at least one portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 2.5 and 4.0, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo.

4. The steel according to claim 3, wherein an average diameter of a prior austenite grains in the steel is not larger than 10 μ

m.

5. A high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

at least one portion which contains, in mass, one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%,

Ca;

0.0001 to 0.01%,

REM;

0.0001 to 0.02%, and

Mg;

0.0001 to 0.006%.

6. The steel according to claim 5, wherein an average diameter of a prior austenite grains in the steel is not larger than 10 μ

m.

7. A high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

at least one portion which contains, in mass;

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.001%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%, and

Cr;

0.01 to 1.0%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the steel being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo.

8. A high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

at least one portion which contains, in mass;

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%, and

Ca;

0.0001 to 0.01%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the steel being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo.

9. A method for producing a steel plate made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass, one or more of:

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.2 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%,

wherein a balance of the at least one portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 1.9 and 3.5, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+Mo−

0.5, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet; and

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.

10. The method according to claim 9, further comprising the steps of:

d) cold-forming the cooled resulting steel plate into a pipe; and

e) after step (d), applying a seam welding to an abutted portion thereof of the pipe.

11. A method for producing a steel plate made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass, one or more of:

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein a balance of the at least one portion consists of Fe and unavoidable impurities,

wherein a P value of the steel is provided in a range of 2.5 and 4.0, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet; and

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.

12. The method according to claim 11, further comprising the steps of:

d) cold-forming the cooled resulting steel plate into a pipe; and

e) after step (d), applying a seam welding to an abutted portion thereof of the pipe.

13. A method for producing a steel plate made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass:

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%,

Ca;

0.0001 to 0.01%,

REM;

0.0001 to 0.02%, and

Mg;

0.0001 to 0.006%, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet; and

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.

14. The method according to claim 13, further comprising the steps of:

d) cold-forming the cooled resulting steel plate into a pipe; and

e) after step (d), applying a seam welding to an abutted portion thereof of the pipe.

15. A method for producing a steel plate made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass:

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.001%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%, and

Cr;

0.01 to 1.0%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the steel being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, the method comprising the steps of;

a) reheating the. casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet; and

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.

16. The method according to claim 15, further comprising the steps of:

d) cold-forming the cooled resulting steel plate into a pipe; and

e) after step (d), applying a seam welding to an abutted portion thereof of the pipe.

17. A method for producing a steel plate made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass:

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%, and

Ca;

0.0001 to 0.01%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the steel being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet; and

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.

18. The method according to claim 17, further comprising the steps of:

d) cold-forming the cooled resulting steel plate into a pipe; and

e) after step (d), applying a seam welding to an abutted portion thereof of the pipe.

19. A high-strength steel pipe having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

a seam-welded portion; and

a base steel portion containing, in mass;

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.2 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%,

wherein a balance of the base steel portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 1.9 and 3.5, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+Mo−

0.5.

20. The pipe according to claim 19, wherein an average diameter of a prior austenite grains in the pipe is not larger than 10 μ

m.

21. A high-strength steel pipe having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

a seam-welded portion; and

a base steel portion containing, in mass;

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein a balance of the base steel portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 2.5 and 4.0, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo.

22. The pipe according to claim 21, wherein an average diameter of a prior austenite grains in the pipe is not larger than 10 μ

m.

23. A high-strength steel pipe having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

a seam-welded portion; and

a base steel portion containing, in mass, one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%,

Ca;

0.0001 to 0.01%,

REM;

0.0001 to 0.02%, and

Mg;

0.0001 to 0.006%.

24. The pipe according to claim 23, wherein an average diameter of a prior austenite grains in the pipe is not larger than 10 μ

m.

25. A high-strength steel pipe having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

a seam-welded portion; and

a base steel portion containing, in mass;

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.001%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the base steel portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%, and

Cr;

0.01 to 1.0%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the pipe being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo.

26. A high-strength steel pipe having a particular low temperature toughness and a particular weld heat-affected zone toughness, comprising:

a seam-welded portion; and

a base steel portion containing, in mass;

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%, and

Ca;

0.0001 to 0.01%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the pipe being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo

27. A method for producing a steel pipe made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass, one or more of:

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.2 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%,

wherein a balance of the at least one portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 1.9 and 3.5, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+Mo−

0.5,

the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet;

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.;

d) cold-forming the cooled resulting steel plate into the pipe;

e) after step (d), applying a submerged arc welding to an abutted portion thereof of the pipe from outer and inner sides thereof; and

f) after step (e), subjecting the pipe to a pipe expansion.

28. The method according to claim 27, further comprising the step of, prior to step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

29. The method according to claim 27, further comprising the step of, after step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

30. A method for producing a steel pipe made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass, one or more of:

C;

0.02 to 0.10%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein a balance of the at least one portion consists of Fe and unavoidable impurities, wherein a P value of the steel is provided in a range of 2.5 and 4.0, wherein a microstructure of the steel being mainly composed of martensite and bainite, and wherein the P value is defined by;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet;

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.;

d) cold-forming the cooled resulting steel plate into the pipe;

e) after step (d), applying a submerged arc welding to an abutted portion thereof of the pipe from outer and inner sides thereof; and

f) after step (e), subjecting the pipe to a pipe expansion.

31. The method according to claim 30, further comprising the step of, prior to step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

32. The method according to claim 31, further comprising the step of, after step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

33. A method for producing a steel pipe made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass:

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%,

Ca;

0.0001 to 0.01%,

REM;

0.0001 to 0.02%, and

Mg;

0.0001 to 0.006%, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet;

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.;

d) cold-forming the cooled resulting steel plate into the pipe;

e) after step (d), applying a submerged arc welding to an abutted portion thereof of the pipe from outer and inner sides thereof, and

f) after step (e), subjecting the pipe to a pipe expansion.

34. The method according to claim 33, further comprising the step of, prior to step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

35. The method according to claim 33, further comprising the step of, after step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

36. A method for producing a steel pipe made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass:

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.001%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%, and

Cr;

0.01 to 1.0%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the steel being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet;

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.;

d) cold-forming the cooled resulting steel plate into the pipe;

e) after step (d), applying a submerged arc welding to an abutted portion thereof of the pipe from outer and inner sides thereof, and

f) after step (e), subjecting the pipe to a pipe expansion.

37. The method according to claim 36, further comprising the step of, prior to step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

38. The method according to claim 36, further comprising the step of, after step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

39. A method for producing a steel pipe made of from a casting containing at least one portion of a high-strength steel having a particular low temperature toughness and a particular weld heat-affected zone toughness, wherein the at least one portion contains, in mass:

C;

0.02 to less than 0.05%,

Si;

not more than 0.6%,

Mn;

1.5 to 2.5%,

P;

not more than 0.015%,

S;

not more than 0.003%,

Ni;

0.01 to 2.0%,

Mo;

0.1 to 0.6%,

Nb;

less than 0.010%,

Ti;

not more than 0.030%,

B;

0.0003 to 0.0030%,

Al;

not more than 0.070%, and

N;

not more than 0.0060%, so as to satisfy the expression Ti−

3.4N≧

0,

wherein the at least one portion further includes one or more of;

V;

0.001 to 0.10%,

Cu;

0.01 to 1.0%,

Cr;

0.01 to 1.0%, and

Ca;

0.0001 to 0.01%,

with the balance consisting of Fe and unavoidable impurities,

wherein a microstructure of the steel is composed of martensite and bainite; and

an average diameter of a prior austenite grains in the steel being not larger than 10 μ

m, and wherein a P value of the steel is in the range from 2.5 to 4.0, and defined as follows;

P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, the method comprising the steps of;

a) reheating the casting to a temperature of not lower than the Ac₃ point;

b) hot rolling the casting to produce a resulting steel sheet;

c) after step (b), cooling the resulting steel sheet at a cooling rate of not lower than 1°

C./sec. to a temperature of not higher than 550°

C.;

d) cold-forming the cooled resulting steel plate into the pipe;

e) after step (d), applying a submerged arc welding to an abutted portion thereof of the pipe from outer and inner sides thereof; and

f) after step (e), subjecting the pipe to a pipe expansion.

40. The method according to claim 39, further comprising the step of, prior to step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.

41. The method according to claim 39, further comprising the step of, after step (f), heating the seam-welded portion of the pipe to a temperature between 300°

C. and 500°

C.