A martensitic stainless steel pipe having a heat-affected zone with high resistance to intergranular stress corrosion cracking is provided. In particular, the martensitic stainless steel pipe contains less than 0.0100% of C; less than 0.0100% of N; 10% to 14% of Cr; and 3% to 8% of Ni on a mass basis.
Alternatively, the martensitic stainless steel pipe may further contain Si, Mn, P, S, and Al within an appropriate content range. The martensitic stainless steel pipe may further contain one or more selected from the group consisting of 4% or less of Cu, 4% or less of Co, 4% or less of Mo, and 4% or less of W and one or more selected from the group consisting of 0.15% or less of Ti, 0.10% or less of Nb, 0.10% or less of V, 0.10% or less of Zr, 0.20% or less of Hf, and 0.20% or less of Ta on a mass basis.
In recent years, in order to cope with a high increase in the price of crude oil and in order to guard against the depletion of oil resources that may occur in near future, the following wells have been extensively developed worldwide: deep oil wells that have not attracted much attention and sour gas wells which are highly corrosive and of which the development has therefore been abandoned once. Steel pipes used for such oil wells and gas wells must have high corrosion resistance.
In environments containing a large amount of, for example, carbon dioxide, inhibitors have been used to prevent corrosion. However, the use of such inhibitors causes an increase in cost and the inhibitors cannot provide sufficient advantages under high temperature conditions in some cases. Therefore, steel pipes with high corrosion resistance have been recently used without using the inhibitors.
The API standards specify that 12%-Cr martensitic stainless steel with a reduced C content be suitable for line pipes. In recent years, martensitic stainless steel pipes have been used for pipelines for natural gas containing CO2. There is a problem in that such martensitic stainless steel pipes must be preheated or subjected to post-welding heat treatment when they are girth-welded. Furthermore, there is a problem in that welded portions thereof are inferior in toughness.
In order to cope with such problems, for example, Japanese Unexamined Patent Application Publication No. 9-316611 discloses martensitic stainless steel having a C content of 0.02% or less, an N content of 0.07% or less, an appropriate Cr content, an appropriate Ni content, and an appropriate Mo content. The Cr content, the Ni content, and the Mo content are adjusted in relation to the C content or the C content and the N content and the Ni content and the Mo content are adjusted in relation to the C content and the N content. A martensitic stainless steel pipe manufactured using the steel disclosed in this document is superior in CO2 corrosion resistance, resistance to stress corrosion cracking, weldability, and high-temperature strength and the toughness of a welded section of the pipe is high.
The following new problem has recently arisen: a problem in that cracking occurs in heat-affected zones (hereinafter referred to as HAZs) of martensitic stainless steel pipes which is subject to girth-welding in environments containing CO2.
The martensitic stainless steel pipe is useful in manufacturing a welded structure by welding. Examples of the welded structure include oil or natural gas production facilities such as pipelines manufactured by girth-welding line pipes, chemical plant pipes such as risers and manifolds, and bridges. The welded structure specified herein may be manufactured by welding the martensitic stainless steel pipes of the present invention, welding the martensitic stainless steel pipe of the present invention to another type of steel pipe, or welding the martensitic stainless steel pipe to a member made of another material.