This invention relates to steel pipes for use in crude oil wells or natural gas wells. In particular, the invention relates to a high strength stainless steel having superior corrosion resistance, which is suitably used in an oil well and gas well in a very severe corrosion environment containing carbon dioxide (CO2), chloride ions (Cl-), and the like. The "high strength stainless steel pipe" indicates a stainless steel pipe having a yield strength of 654 MPa (95 ksi) or more.
In recent years, in response to steep rise in crude oil price and to depletion of petroleum oil resources anticipated in the near future, deeper oil fields, which have not be taken into consideration in the past, very corrosive sour gas fields, the development of which was abandoned once in the past, and the like have been aggressively developed on a worldwide basis. The oil fields and gas fields as described above are generally located in very deep places, and in addition, these oil and gas fields are in a very severe corrosive environment in which the temperature is high and CO2, CF, and the like are present. Hence, as an oil-well steel pipe used for mining oil and gas fields as described above, a steel pipe having high strength and also having superior corrosion resistance is required.
In addition, in recent years, development of oil wells in cold regions has been increasingly carried out. Hence, besides high strength, superior low-temperature toughness has also been required for the steel pipe in many cases.
In response to the requirements described above, for example, in Japanese Unexamined Applications 8-120345, 9-268349 and 10-1755 and Japanese Patents 28-14528 and 32-51648, improved martensite stainless steel or a steel pipe thereof have been proposed which are obtained by improving the corrosion resistance of 13% Cr martensite stainless steel or a steel pipe thereof.
According to the situations described above, a high strength 13% Cr martensite stainless steel pipe for use in oil wells has been strongly desired, which is primarily formed of inexpensive 13% Cr martensite stainless steel having excellent hot workability and which has a high yield strength of more than 654 MPa (95 ksi), superior CO2 corrosion resistance, and a high toughness.
Heretofore, in oil wells and gas wells in an environment containing CO2, Cl-, and the like, 13% Cr martensite stainless steel pipes, which have superior CO2 corrosion resistance, have been generally used as an oil-well steel pipe. However, there has been a problem in that a general martensite stainless steel cannot withstand the use in an environment in which a large amount of Cl- is present and the temperature is high, such as more than 100�� C. Hence, in a well in which steel pipes and the like are required to have corrosion resistance, a dual phase stainless steel pipe has been used. However, since the dual phase stainless steel pipe contains a large amount of alloy elements, hot workability thereof is not superior, and hence a specific hot working can only be used for forming the dual phase stainless steel pipe, thereby causing the increase in cost. In addition, when the yield strength of a conventional 13% Cr martensite stainless steel pipe is more than 654 MPa, the toughness thereof is seriously degraded, and hence there has been a problem in that the 13% Cr martensite stainless steel pipe may not be used.
A technique described in Japanese Patent 32-51648 relates to a precipitation hardened martensite stainless steel having superior strength and toughness, the stainless steel having a steel composition in which 10.0% to 17% or Cr is contained, 0.08% or less of C, 0.015% or less of N, 6.0% to 10.0% of Ni, and 0.5% to 2.0% of Cu are contained, and 0.5% to 3.0% of Mo is further contained, and having a texture in which, owing to a cold working of 35% or more and annealing, the average crystal particle diameter is set to 25 ��m or less and the number of precipitates, which are precipitated in a matrix and which have a particle diameter of 5��10-2 ��m or more, is reduced to 6��106/mm2 or less. Since a texture is formed containing fine crystal particles and having a small amount of precipitates, precipitation hardened martensite stainless steel, which has a high strength and causes no decrease in toughness, can be provided.
However, there has been a problem in that improved 13% Cr martensite stainless steel pipe manufactured by the techniques discussed above cannot stably exhibit desired corrosion resistance in a severe corrosive environment in which CO2, Cl-, and the like are present and the temperature is high, such as more than 180�� C