The main objective of the present invention is to provide a high-strength martensitic stainless steel pipe which has resistance to carbon dioxide gas corrosion and is composed of an inexpensive component system, which can ensure high strength andsatisfactory toughness, without adding expensive Ni and Mo in large quantities, as in the "super 13Cr", and also have excellent hot workability.
It is another objective of the present invention to provide a method for stably and definitely producing a high-strength martensitic stainless steel pipe, composed of an inexpensive component system, which is excellent in toughness and also inhot workability by reducing the influence of a straightening treatment by a straightener.
The techniques proposed in Patent Documents 1 and 2 require reductions in the P content to less than 0.008% by mass and also to 0.008% by mass or less, respectively. However, in the current refining technique, an increase in the frequency ofdephosphorization is necessary in order to stably and definitely reduce the P content in the 13Cr-type martensitic stainless steel to 0.008% by mass or less in an industrial mass production scale, and this leads to a significant increase in cost. Evenif the frequency of dephosphorization is increased, it is difficult to definitely control the P content up to 0.008% by mass or less. Therefore, development of an inexpensive martensitic stainless steel pipe, which never requires the reduction in Pinvolving an increase in cost and the addition of large quantities of expensive Ni and Mo, has been requested.
In order to satisfy such a request, the present inventors variously examined the effects of chemical compositions of martensitic stainless steel pipe, particularly a 13Cr-type martensitic stainless steel pipe on hot workability, toughness,tempering temperature, and the straightening treatment by a straightener. As the result, the following findings (a) to (c) were obtained.
(a) The hot workability and the toughness of a martensitic stainless steel pipe can be enhanced by controlling the chemical compositions, particularly, the contents of C, Mn, N and Al.
(b) Particularly, among the above-mentioned elements, by reducing the content of Al to a specified range, the quantity of carbides precipitated in grain boundary, especially the M23C.sub.6 type carbide, is extremely minimized, and thetoughness is greatly improved.
(c) Since Nb, Mo and V can raise the tempering temperature by an addition of trace amounts thereof, a high temperature exceeding 510 degrees C. can be ensured even in case of performing a straightening treatment by a straightener successively totempering treatment, and so the influence of working by the straightener can be suppressed.