Steel pipe for high strength line pipe superior in strain aging resistance and steel plate for high strength line pipe and methods of production of the same
The present invention provides steel plate for high strength line pipe suppressing the rise in yield strength in the longitudinal direction of expanded steel pipe due to the heating at the time of coating to prevent corrosion and superior in strain aging resistance and steel pipe for the material for the same, that is, high strength steel pipe for line pipe superior in strain aging resistance characterized in that a base material having a composition of chemical elements containing, by mass %, Mo: over 0% to less than 0.15% and Mn: 1.7 to 2.5%, satisfying Mo/Mn: over 0 to 0.08, containing C, Si, P, S, Al, Ti, N, and B, furthermore containing one or more of Ni, Cu, and Cr, having a balance of iron and unavoidable impurities, having a P value of 2.5 to 4.0 in range, and having a metallurgical structure comprised of bainite and martensite: P value=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2Mo. Furthermore, it may contain one or more of Nb, V, Ca, REM, and Mg.
1 . High strength steel pipe for line pipe superior in strain aging resistance characterized in that the base material has a composition of chemical elements containing, by mass %,
C: over 0.03% to 0.07,
Si: 0.6% or less,
Mn: 1.7 to 2.5%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.1% or less,
Mo: over 0% to less than 0.15%,
Ti: 0.005 to 0.03%,
N: 0.001 to 0.006%, and
B: 0.0006 to 0.0025%,
furthermore containing one or more of
Ni: 1.5% or less,
Cu: 1.0% or less, and
Cr: 1.0% or less,
having a balance of iron and unavoidable impurities,
satisfying
Mo/Mn: over 0 to 0.08
having a P value expressed by the following (formula 1) of 2.5 to 4.0 in range, having a metallurgical structure comprised of bainite or bainite and martensite complex structure, and having a circumferential direction tensile strength TS Cpp [MPa] of 900 to 1100 MPa:
P-value=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2Mo (formula 1)
where, C, Si, Mn, Cr, Ni, Cu, and Mo are the contents of the elements [mass %].
2 . High strength steel pipe for line pipe superior in strain aging resistance as set forth in claim 1 , characterized in that the base material has a composition of chemical elements containing, by mass %, one or more of:
Nb: 0.1% or less,
V: 0.1% or less,
Ca: 0.01% or less,
REM: 0.02% or less, and
Mg: 0.006% or less.
3 . High strength steel pipe for line pipe superior in strain aging resistance as set forth in claim 1 , characterized in that the base material has contents of Ti and N satisfying:
Ti−3.4N>0.
4 . Steel plate for a material for high strength steel pipe for line pipe superior in strain aging resistance, containing, by mass %,
C: over 0.03% to 0.07%,
Si: 0.6% or less,
Mn: 1.7 to 2.5%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.1% or less,
Mo: over 0% to less than 0.15%,
Ti: 0.005 to 0.03%,
N: 0.001 to 0.006%, and
B: 0.0006 to 0.0025%,
furthermore containing one or more of:
Ni: 1.5% or less,
Cu: 1.0% or less, and
Cr: 1.0% or less
having a balance of iron and unavoidable impurities,
Mo/Mn: over 0 to 0.08
having a P value expressed by the following (formula 1) of 2.5 to 4.0 in range, having a metallurgical structure comprised of bainite or bainite and martensite complex structure, and having a circumferential direction tensile strength TScpp [MPa] of 880 to 1080 MPa:
P-value=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2Mo (formula 1)
where, C, Si, Mn, Cr, Ni, Cu, and Mo are the contents of the elements [mass %].
5 . Steel plate for a material for high strength steel pipe for line pipe superior in strain aging resistance as set forth in claim 4 , said high strength steel plate for line pipe superior in strain aging resistance as set forth in claim 4 characterized by containing, by mass %, one or more of:
Nb: 0.1% or less,
V: 0.1% or less,
Ca: 0.01% or less,
REM: 0.02% or less, and
Mg: 0.006% or less.
6 . Steel plate for a material for high strength steel pipe for line pipe superior in strain aging resistance as set forth in claim 4 , said high strength steel plate for line pipe superior in strain aging resistance characterized in that contents of Ti and N satisfy
Ti−3.4N>0.
7 . A method of production of high strength steel plate for line pipe superior in strain aging resistance as set forth in claim 4 , said method of production of high strength steel plate for line pipe superior in strain aging resistance characterized by heating a slab obtained by melting and casting steel comprised of chemical elements as set forth in claim 4 to 1000 to 1250° C., then rough rolling in a recrystallization temperature region of over 900° C., then rolling in the non-recrystallization region at 700 to 900° C. with a cumulative reduction amount of 75% or more, then acceleratedly cooling by a cooling rate at a center part of plate thickness of 1 to 30° C./s until a temperature of 500° C. or less.
8 . A method of production of high strength steel plate for line pipe superior in strain aging resistance, characterized by shaping steel plate for high strength line pipe produced by the method as set forth in claim 7 into a pipe so that a rolling direction of the steel plate and a longitudinal direction of the steel pipe match, welding the seam parts, then expanding the pipe.
9 . A method of production of high strength steel plate for line pipe superior in strain aging resistance as set forth in claim 8 , characterized by shaping the pipe by a UO process and welding the seam parts from inner and outer surfaces by submerged arc welding.
10 . A method of production of high strength steel plate for line pipe superior in strain aging resistance as set forth in claim 8 , characterized in that a pipe expansion rate is 0.7 to 2%.