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.