Fe-Cr-Ni ALLOY MATERIAL

The present disclosure relates to an alloy material, and more particularly relates to a Fe—Cr—Ni alloy material.

In oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as “oil wells”), alloy materials for oil wells which are typified by oil country tubular goods are used. Many oil wells are sour environments that contain hydrogen sulfide, which is corrosive. As used in the present description, the term “sour environment” means an acidified environment containing hydrogen sulfide. In some cases, sour environments also contain carbon dioxide, and not just hydrogen sulfide. Materials used in such sour environments are required to have excellent corrosion resistance.

Examples of materials which are required to have excellent corrosion resistance include 18-8 stainless steel materials such as SUS304H, SUS316H, SUS321H, and SUS347H, and Fe—Cr—Ni alloy materials represented by Alloy 800H, which is defined as NCF800H by the JIS Standard. Fe—Cr—Ni alloy materials have excellent corrosion resistance in comparison to 18-8 stainless steel. Fe—Cr—Ni alloy materials are also more excellent in economic efficiency in comparison to a Ni-base alloy material represented by Alloy 617. Therefore, Fe—Cr—Ni alloy materials may in some cases be used as alloy materials for oil wells for use in a sour environment.

Japanese Patent Application Publication No. 2-217445 (Patent Literature 1) and International Application Publication No. WO2015/072458 (Patent Literature 2) each proposes an alloy material for oil wells that has excellent corrosion resistance.

Patent Literature 1 discloses an alloy material which is a Fe—Cr—Ni alloy that consists essentially of Ni: 27 to 32%, Cr: 24 to 28%, Cu: 1.25 to 3.0%, Mo: 1.0 to 3.0%, Si: 1.5 to 2.75%, and Mn: 1.0 to 2.0%, and the following elements whose amounts are controlled as follows: N: 0.015% or less, B: 0.10% or less, C: 0.10% or less, Al: 0.30% or less, P: 0.03% or less, and S: 0.02% or less, with the balance being Fe and impurities. It is disclosed in Patent Literature 1 that this alloy material has high strength, galling resistance, and corrosion resistance under stress.

Patent Literature 2 discloses an alloy material that is an Ni—Cr alloy material having a chemical composition consisting of, by mass %, Si: 0.01 to 0.5%, Mn: 0.01 to less than 1.0%, Cu: 0.01 to less than 1.0%, Ni: 48 to less than 55%, Cr: 22 to 28%, Mo: 5.6 to less than 7.0%, N: 0.04 to 0.16%, sol. Al: 0.03 to 0.20%, REM: 0.01 to 0.074%, W: 0 to less than 8.0%, Co: 0 to 2.0%, one or more of Ca and Mg: 0.0003 to 0.01% in total, and one or more of Ti, Nb, Zr, and V: 0 to 0.5% in total, with the balance being Fe and impurities, and in which C, P, S, and O in the impurities are as follows: C: 0.03% or less, P: 0.03% or less, S: 0.001% or less, and O: 0.01% or less, and a dislocation density p satisfies the formula (7.0×10≤ρ≤2.7×10−2.67×10×[REM (%)]). It is disclosed in Patent Literature 2 that this alloy material is excellent in hot workability and toughness, and is also excellent in corrosion resistance (stress corrosion cracking resistance in environments in which the temperature is a high temperature of more than 200° C. and which contain hydrogen sulfide), and has a yield strength (0.2% proof stress) of 965 MPa or more.

Patent Literature 1: Japanese Patent Application Publication No. 2-217445

Patent Literature 2: International Application Publication No. WO2015/072458

In recent years, oil wells are being made deeper, and consequently there is a need to increase the strength of alloy materials for oil wells. In other words, in regard to Fe—Cr—Ni alloy materials for which use as alloy materials for oil wells is assumed, there has been a need for the Fe—Cr—Ni alloy materials to have high strength.

Furthermore, recent oil wells include an increased number of inclined wells in addition to vertical wells that are drilled straight down vertically. An inclined well is formed by drilling in such a way that the extending direction of the well is bent from vertically downward to the horizontal direction. By including a portion that extends horizontally (horizontal well), an inclined well can cover a wide range of strata in which a production fluid such as crude oil or gas is buried, and can thus increase the efficiency of producing a production fluid.

On the other hand, when used for such kinds of inclined wells, the alloy material may be loaded with a compressive force. For such cases, it is preferable that not only the tensile yield strength, but also the compressive yield strength of the alloy material is high. That is, it is preferable that a Fe—Cr—Ni alloy material which is expected to be used in an inclined well not only has high strength, but also has a reduced strength anisotropy of the alloy material. However, in the aforementioned Patent Literatures 1 and 2, as the strength of the relevant Fe—Cr—Ni alloy material, only the tensile yield strength is investigated. That is, in the aforementioned Patent Literatures 1 and 2, the strength anisotropy of the alloy material has not been investigated.

An objective of the present disclosure is to provide a Fe—Cr—Ni alloy material that has high strength and in which strength anisotropy has been reduced.

A Fe—Cr—Ni alloy material according to the present disclosure consists of, by mass %,

The Fe—Cr—Ni alloy material according to the present disclosure has high strength, and strength anisotropy of the Fe—Cr—Ni alloy material is reduced.

First, as a Fe—Cr—Ni alloy material having high strength, the present inventors focused their attention on a Fe—Cr—Ni alloy material having a tensile yield strength of 110 ksi (758 MPa) or more. Next, the present inventors conducted studies from the viewpoint of the chemical composition with regard to the strength anisotropy of a Fe—Cr—Ni alloy material having a tensile yield strength of 758 MPa or more.

As a result, the present inventors considered that if a Fe—Cr—Ni alloy material consists of, by mass %, C: 0.030% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 2.00%, P: 0.030% or less, S: 0.0050% or less, Ni: 29.0 to 36.5%, Cr: 23.0 to 27.5%, Mo: 2.00 to 6.00%, Al: 0.01 to 0.30%, rare earth metal: 0.016 to 0.100%, N: 0.220 to 0.500%, O: 0.010% or less, W: 0 to 6.0%, Cu: 0 to 2.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, V: 0 to 0.50%, Ti: 0 to 0.50%, Nb: 0 to 0.50%, and Co: 0 to 2.00%, with the balance being Fe and impurities, there is a possibility that the Fe—Cr—Ni alloy material will have a tensile yield strength of 758 MPa or more, and also that strength anisotropy can be reduced.

On the other hand, even in the case of Fe—Cr—Ni alloy materials that had the chemical composition described above, when the Fe—Cr—Ni alloy materials had a tensile yield strength of 758 MPa or more, the strength anisotropy increased in some cases. Therefore, the present inventors conducted detailed studies with regard to reducing the strength anisotropy of an alloy material having the chemical composition described above and having a tensile yield strength of 758 MPa or more.

In an alloy material having the above chemical composition, because the content of Ni is high, the stacking fault energy is liable to become large. When the stacking fault energy is large, the degree of work hardening in response to applied strain decreases. That is, if the stacking fault energy can be made small, it will be easier for work hardening to occur in response to strain. As a result, there is a possibility that the alloy material will be less susceptible to the influence of anisotropy of strain applied in the production process, and thus the strength anisotropy of the alloy material can be reduced.

Therefore, the present inventors focused their attention on the stacking fault energy of a Fe—Cr—Ni alloy material having the chemical composition described above and a tensile yield strength of 758 MPa or more, and conducted detailed studies regarding a technique for reducing the strength anisotropy of the alloy material. As a result of the detailed studies of the present inventors, it was revealed that in a Fe—Cr—Ni alloy material having the chemical composition described above, when the chemical composition also satisfies the following Formula (1), on the condition that the other requirements of the present embodiment are satisfied, the Fe—Cr—Ni alloy material has a tensile yield strength of 758 MPa or more and, in addition, strength anisotropy can be reduced.

Let I be defined as I=3×Ni−2×Cr−150×N. I is an index of the stacking fault energy of an alloy material having the above chemical composition. Further, in the present description, a ratio (compressive YS/tensile YS) of the compressive yield strength (compressive YS) to the tensile yield strength (tensile YS) is also referred to as an “anisotropy index AI”. Hereunder, the relation between I that is an index of the stacking fault energy of an alloy material and the anisotropy index AI of the alloy material is described specifically using the drawings.is a view illustrating the relation between a value of I and the anisotropy index AI in the present examples.was created using the value of I and the anisotropy index AI in, among examples to be described later, those examples in which the composition and the like other than I satisfied the conditions of the present embodiment.

Referring to, in an alloy material having the chemical composition described above and a tensile yield strength of 758 MPa or more, when I is less than 15.0, the anisotropy index AI is increased to 0.800 or more. On the other hand, when I is 15.0 or more, the anisotropy index AI decreases to less than 0.800. Therefore, in the Fe—Cr—Ni alloy material according to the present embodiment, the chemical composition described above is satisfied, and in addition, I is made less than 15.0. As a result, in the Fe—Cr—Ni alloy material according to the present embodiment, on the condition that the other requirements of the present embodiment are satisfied, the strength anisotropy can be reduced.

Firstly, a Fe—Cr—Ni alloy material having the chemical composition described above has the microstructure consisting of austenite. In the present description, the phrase “the microstructure consisting of austenite” means that the amount of any phase other than austenite is negligibly small. Therefore, the present inventors focused their attention on the austenite grains of a Fe—Cr—Ni alloy material having the above chemical composition including Formula (1) and having a tensile yield strength of 758 MPa or more, and conducted detailed studies regarding a technique for reducing the strength anisotropy of the alloy material.

As a result of the detailed studies conducted by the present inventors, it was revealed that in a Fe—Cr—Ni alloy material having the above chemical composition including Formula (1) and having a tensile yield strength of 758 MPa or more, the standard deviation σ of the grain size numbers in the microstructure influences the strength anisotropy of the alloy material. This point will be specifically described using the drawings.is a view illustrating the relation between a value of the standard deviation σ of the grain size numbers and the anisotropy index AI in the present examples.was created using the value of the standard deviation σ of the grain size numbers and the anisotropy index AI in, among examples to be described later, those examples in which the composition and the like other than the standard deviation σ of the grain size numbers satisfied the conditions of the present embodiment.

Referring to, in a Fe—Cr—Ni alloy material having the above chemical composition including Formula (1) and having a tensile yield strength of 758 MPa or more, when the standard deviation σ of the grain size numbers is 0.80 or less, the anisotropy index AI is increased to 0.800 or more. On the other hand, when the standard deviation σ of the grain size numbers is more than 0.80, the anisotropy index AI decreases to less than 0.800. Therefore, in the Fe—Cr—Ni alloy material according to the present embodiment, the chemical composition described above is satisfied, I is less than 15.0, the tensile yield strength is 758 MPa or more, and in addition, the standard deviation σ of the grain size numbers is made 0.80 or less. As a result, in the Fe—Cr—Ni alloy material according to the present embodiment, the strength anisotropy can be reduced.

The reason why the strength anisotropy of an alloy material can be reduced by making the standard deviation σ of the grain size numbers 0.80 or less has not been clarified in detail. However, the fact that the strength anisotropy can be reduced by satisfying the chemical composition described above, making I less than 15.0, having a tensile yield strength of 758 MPa or more, and in addition, making the standard deviation σ of the grain size numbers 0.80 or less has been proven by examples which are described later.

The gist of the Fe—Cr—Ni alloy material according to the present embodiment, which has been completed based on the findings described above, is as follows.

[1]

The Fe—Cr—Ni alloy material according to [1], containing one or more elements selected from a group consisting of:

The Fe—Cr—Ni alloy material according to [1] or [2], wherein:

Note that, the shape of the Fe—Cr—Ni alloy material according to the present embodiment is not particularly limited. The shape of the Fe—Cr—Ni alloy material according to the present embodiment may be a plate shape, may be a bar shape having a circular cross section, or may be a pipe shape. In other words, the Fe—Cr—Ni alloy material according to the present embodiment may be an alloy plate, may be an alloy bar having a circular cross section, or may be an alloy pipe. Note that, the term “alloy pipe” may refer to a seamless alloy pipe or may refer to a welded alloy pipe. In a case where the alloy material is an alloy pipe for oil wells, the alloy material is preferably a seamless alloy pipe.

Hereunder, the Fe—Cr—Ni alloy material according to the present embodiment is described in detail. The symbol “%” in relation to an element means mass percent unless otherwise stated.

The chemical composition of the Fe—Cr—Ni alloy material according to the present embodiment contains the following elements.

Carbon (C) is an impurity which is unavoidably contained. That is, a lower limit of the content of C is more than 0%. If the content of C is too high, even if the contents of other elements are within the range of the present embodiment, Cr carbides will form at grain boundaries. The Cr carbides will increase cracking susceptibility at grain boundaries. As a result, corrosion resistance of the alloy material will decrease. Therefore, the content of C is to be 0.030% or less. A preferable upper limit of the content of C is 0.028%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.015%. The content of C is preferably as low as possible. However, extremely reducing the content of C will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of C is 0.001%, and more preferably is 0.003%.

Silicon (Si) deoxidizes the alloy. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is too high, hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Si is to be 0.01 to 1.00%. A preferable lower limit of the content of Si is 0.05%, more preferably is 0.10%, and further preferably is 0.20%. A preferable upper limit of the content of Si is 0.80%, more preferably is 0.60%, and further preferably is 0.50%.

Manganese (Mn) deoxidizes and desulfurizes the alloy. If the content of Mn is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is too high, hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Mn is to be 0.01 to 2.00%. A preferable lower limit of the content of Mn is 0.10%, more preferably is 0.20%, and further preferably is 0.30%. A preferable upper limit of the content of Mn is 1.80%, more preferably is 1.50%, further preferably is 1.20%, further preferably is 1.00%, and further preferably is 0.80%.

Phosphorus (P) is an impurity which is unavoidably contained. That is, a lower limit of the content of P is more than 0%. P segregates to grain boundaries. Therefore, if the content of P is too high, hot workability and corrosion resistance of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment. Accordingly, the content of P is to be 0.030% or less. A preferable upper limit of the content of P is 0.025%, and more preferably is 0.020%. The content of P is preferably as low as possible. However, extremely reducing the content of P will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.

Sulfur(S) is an impurity which is unavoidably contained. That is, a lower limit of the content of S is more than 0%. S segregates to grain boundaries. Therefore, if the content of S is too high, hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of S is to be 0.0050% or less. A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0020%. The content of S is preferably as low as possible. However, extremely reducing the content of S will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.

Nickel (Ni) is an austenite forming element, and stabilizes the austenite in the alloy material. If the content of Ni is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, the amount of dissolved N will decrease, and in some cases strength of the alloy material will decrease. In such a case, in addition, the production cost will significantly increase. Therefore, the content of Ni is to be 29.0 to 36.5%. A preferable lower limit of the content of Ni is 29.5%, and more preferably is 30.0%. A preferable upper limit of the content of Ni is 36.0%, more preferably is 35.5%, and further preferably is 35.0%.

Chromium (Cr) increases corrosion resistance of the alloy material. Cr also increases the amount of dissolved N, thereby increasing strength of the alloy material. If the content of Cr is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Cr is too high, hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment. In such a case, in addition, intermetallic compounds typified by the σ phase will be easily formed, and corrosion resistance of the alloy material will decrease. Therefore, the content of Cr is to be 23.0 to 27.5%. A preferable lower limit of the content of Cr is 23.5%, more preferably is 24.0%, and further preferably is 24.5%. A preferable upper limit of the content of Cr is 27.0%, and more preferably is 26.5%.

Molybdenum (Mo) contributes to stabilization of a corrosion protection film, thereby increasing corrosion resistance of the alloy material. Mo also increases strength of the alloy material by solid-solution strengthening. If the content of Mo is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment. In such a case, in addition, the production cost will significantly increase. Therefore, the content of Mo is to be 2.00 to 6.00%. A preferable lower limit of the content of Mo is 2.20%, more preferably is 2.40%, and further preferably is 2.50%. A preferable upper limit of the content of Mo is 5.50%, more preferably is 5.00%, further preferably is 4.50%, and further preferably is 4.00%.

Aluminum (Al) deoxidizes the alloy. Al also forms oxides to immobilize oxygen, and thereby increases hot workability of the alloy material. In addition, Al enhances an impact resistance property and corrosion resistance of the alloy material. If the content of Al is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, Al oxides will excessively form and hot workability of the alloy material will, on the contrary, decrease. Therefore, the content of Al is to be 0.01 to 0.30%. A preferable lower limit of the content of Al is 0.02%, more preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of Al is 0.25%, and more preferably is 0.20%. Note that, in the present description, the term “content of Al” means the content of “acid-soluble Al”, that is, the content of sol. Al.

Rare earth metal (REM) fixes S in the alloy as a sulfide to make it harmless, thereby increasing hot workability of the alloy material. REM also increases the corrosion resistance of the alloy material. If the content of REM is too low, aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will be formed in the alloy material, and hot workability of the alloy material will, on the contrary, decrease. Therefore, the content of REM is to be 0.016 to 0.100%. A preferable lower limit of the content of REM is 0.018%, and more preferably is 0.020%. A preferable upper limit of the content of REM is 0.080%, more preferably is 0.060%, and further preferably is 0.050%.

Note that, in the present description the term “REM” means one or more elements selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description, the term “content of REM” means the total content of these elements.

Nitrogen (N) increases strength of the alloy material by solid-solution strengthening. If the content of N is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of N is too high, even if the contents of other elements are within the range of the present embodiment, a large amount of Cr nitrides will be formed, and corrosion resistance of the alloy material will decrease. Therefore, the content of N is to be 0.220 to 0.500%. A preferable lower limit of the content of N is 0.225%, more preferably is 0.230%, further preferably is 0.235%, and further preferably is 0.240%. A preferable upper limit of the content of N is 0.480%, more preferably is 0.450%, and further preferably is 0.400%.

Oxygen (O) is an impurity which is unavoidably contained. That is, a lower limit of the content of O is more than 0%. O combines with REM to form oxides. Therefore, if the content of O is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will be formed in the alloy material, and hot workability of the alloy material will decrease. In such a case, in addition, corrosion resistance of the alloy material will decrease. Therefore, the content of O is to be 0.010% or less. A preferable upper limit of the content of O is 0.008%, and more preferably is 0.005%. The content of O is preferably as low as possible. However, extremely reducing the content of O will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of O is 0.0001%, more preferably is 0.001%, and further preferably is 0.002%.

The balance of the chemical composition of the Fe—Cr—Ni alloy material according to the present embodiment is Fe and impurities. Here, the term “impurities” means substances which are mixed in from ore and scrap as the raw material or from the production environment or the like when industrially producing the Fe—Cr—Ni alloy material, and which are permitted within a range that does not have a noticeable adverse effect on the operational advantages of the Fe—Cr—Ni alloy material according to the present embodiment.

The chemical composition of the Fe—Cr—Ni alloy material according to the present embodiment may further contain one or more elements selected from the group consisting of W and Cu. Each of these elements increases corrosion resistance of the alloy material.