To improve the strength and toughness at extremely low temps. of a bent pipe by prescribing the elemental contents in the base metal and weld metal part.
This steel pipe has a low C-low-Si-Nb-trace Ti base metal and metallc components contg. low Chigh Mn-Cr-Nb-trace Ti and in which the balance among the contents of Al, N, O and Ti are taken into consideration. Namely, in the base metal, particularly, the contents of Si and Al are extremely reduced to suppress the growth of MA even in the case of the formation of upper bainite, and also, they are finely dispersed to improve its low temp. toughness. On the other hand, as for the weld metal part, particularly, the simultaneous addition of Mn and Cr is extremely effective for the improvement of its low temp. toughness by the suppression of coarse intergranular ferrite and for the increase of low temp. hardness in the T-cross part. Moreover, by optimizing each component so as to regulate the value of P expressed by P=(1.5(0-0.89 Al+3.4N)-Ti to -0.010 to 0.010%, its low temp. toughness improves.
OGATA YOSHINORI
TAKAMOTO TERUHISA
