Duplex Stainless Steel Material

The present disclosure relates to a duplex stainless steel material.

Oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as “oil wells”) may be in a corrosive environment containing a corrosive gas. Here, the term “corrosive gas” means carbon dioxide gas and/or hydrogen sulfide gas. That is, steel materials for use in oil wells are required to have excellent corrosion resistance in a corrosive environment.

To date, as a method for enhancing the corrosion resistance of a steel material, there has been a known method that increases the content of chromium (Cr) and forms a passive film mainly composed of Cr oxides on the surface of the steel material. Therefore, a duplex stainless steel material in which the content of Cr has been made high may in some cases be used in an environment where excellent corrosion resistance is required. It is known that, in particular, a duplex stainless steel material exhibits excellent corrosion resistance in seawater.

In recent years, deep wells below sea level are being actively developed. On the other hand, steel materials used for deep wells below sea level are required to not only have excellent corrosion resistance, but also to have high strength and excellent low-temperature toughness. Therefore, there is a need for duplex stainless steel materials that have high strength and excellent low-temperature toughness.

Japanese Patent Application Publication No. 10-60597 (Patent Literature 1), International Application Publication No. WO2012/111536 (Patent Literature 2), and Japanese Patent Application Publication No. 2016-3377 (Patent Literature 3) each propose a technique for increasing the strength and low-temperature toughness of a duplex stainless steel material.

Patent Literature 1 discloses a duplex stainless steel material which contains ferrite in an amount of 60 to 90% in area fraction, in which a Ni balance value (=Ni+0.5Mn+30 (C+N)−1.1 (Cr+1.5Si+Mo+0.5Nb)+8.2) is −15 to −10, and which satisfies the formula (content of Al×content of N≤0.0023×Ni balance value+0.357). It is described in Patent Literature 1 that this duplex stainless steel material has high strength and excellent toughness.

Patent Literature 2 discloses a duplex stainless steel material that has a chemical composition consisting of, by mass %, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 8.00% or less, P: 0.040% or less, S: 0.0100% or less, Cu: more than 2.00 to 4.00% or less, Ni: 4.00 to 8.00%, Cr: 20.0 to 30.0%, Mo: 0.50 to less than 2.00%, N: 0.100 to 0.350%, and Al: 0.040% or less, with the balance being Fe and impurities, and that has a microstructure in which a ferrite ratio is 30 to 70%, and the hardness of ferrite is 300 Hvor more. It is described in Patent Literature 2 that this duplex stainless steel material has high strength and high toughness.

Patent Literature 3 discloses a duplex stainless steel material that is a duplex stainless steel tube which has a chemical composition consisting of, by mass %, C: 0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0%, P: 0.040% or less, S: 0.010% or less, Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to 4.0%, N: 0.1 to 0.35%, O: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, and B: 0.0005 to 0.02%, with the balance being Fe and impurities, and has a microstructure composed of a duplex microstructure of a ferrite phase and an austenite phase in which there is no precipitation of sigma phase, and in which a proportion occupied by the ferrite phase in the steel microstructure is 50% or less in area fraction, and the number of oxides having a particle size of 30 μm or more present in a visual field of 300 mmis 15 or less. It is described in Patent Literature 3 that this duplex stainless steel material is excellent in strength, pitting resistance, and low-temperature toughness.

Patent Literature 1: Japanese Patent Application Publication No. 10-60597

Patent Literature 2: International Application Publication No. WO2012/111536

Patent Literature 3: Japanese Patent Application Publication No. 2016-3377

As described above, the aforementioned Patent Literatures 1 to 3 disclose duplex stainless steel materials that have high strength and excellent low-temperature toughness. However, a duplex stainless steel material that has high strength and excellent low-temperature toughness may also be obtained by a technique other than the techniques disclosed in the aforementioned Patent Literatures 1 to 3.

An objective of the present disclosure is to provide a duplex stainless steel material that has high strength and excellent low-temperature toughness.

A duplex stainless steel material according to the present disclosure consists of, by mass %,

The duplex stainless steel material according to the present disclosure has high strength and excellent low-temperature toughness.

First, the present inventors conducted studies regarding obtaining a duplex stainless steel material having a yield strength of 80 ksi (552 MPa) or more as a high strength. That is, the present inventors conducted investigations and studies regarding a method for obtaining a duplex stainless steel material that achieves both a yield strength of 80 ksi or more and excellent low-temperature toughness. As a result, the present inventors obtained the following findings.

First, the present inventors conducted studies from the viewpoint of the chemical composition with respect to a duplex stainless steel material that achieves both a yield strength of 80 ksi or more and excellent low-temperature toughness. As a result, the present inventors considered that if a duplex stainless steel material has a chemical composition consisting of, by mass %, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.50 to 7.00%, P: 0.040% or less, S: 0.0200% or less, Al: 0.100% or less, Ni: 4.20 to 9.00%, Cr: 20.00 to 30.00%, Mo: 0.50 to 2.00%, Cu: 0.50 to 3.00%, N: 0.150 to 0.350%, V: 0.01 to 1.50%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, W: 0 to 0.200%, Co: 0 to 0.500%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, B: 0 to 0.020%, and rare earth metal: 0 to 0.200%, with the balance being Fe and impurities, there is a possibility that a yield strength of 80 ksi or more and excellent low-temperature toughness can be obtained.

Here, the microstructure of a duplex stainless steel material having the chemical composition described above is composed of ferrite and austenite. Specifically, the microstructure of a duplex stainless steel material having the chemical composition described above is composed of, in volume ratio, ferrite in an amount of 35 to 55% with the balance being austenite. Note that, in the present description, the phrase “composed of ferrite and austenite” means that the amount of any phase other than ferrite and austenite is negligibly small.

Next, with respect to a duplex stainless steel material having the chemical composition described above in which the volume ratio of ferrite is 35 to 55%, the present inventors conducted detailed studies regarding a method for obtaining a yield strength of 80 ksi or more and excellent low-temperature toughness. Here, in a duplex stainless steel material having the chemical composition described above, ferrite is harder than austenite. Therefore, especially in a low-temperature environment, minute cracks generated in the duplex stainless steel material tend to easily propagate through the ferrite.

Therefore, with respect to a duplex stainless steel material having the chemical composition described above in which the volume ratio of ferrite is 35 to 55%, the present inventors conducted studies regarding causing fine austenite to disperse in the ferrite so as to increase the low-temperature toughness of the ferrite. If austenite is finely dispersed in ferrite, there is a possibility that the low-temperature toughness of the ferrite can be selectively increased. In such case, there is a possibility that the low-temperature toughness of a duplex stainless steel material having the chemical composition described above can be increased.

On the other hand, as the result of detailed studies conducted by the present inventors it was revealed that in a duplex stainless steel material having the chemical composition described above, there is a possibility that the yield strength will be reduced by causing fine austenite to disperse in ferrite. In other words, in a duplex stainless steel material having the chemical composition described above, there is a possibility that if the austenite is merely refined, even if the low-temperature toughness is increased, a yield strength of 80 ksi or more will not be obtained.

Therefore, the present inventors considered causing ferrite having a volume ratio of 35 to 55%, coarse austenite, and fine austenite to be intermixed in the microstructure of a duplex stainless steel material. In this case, there is a possibility that not only will the low-temperature toughness of the duplex stainless steel material be increased by the fine austenite dispersed in the ferrite, but also that the yield strength can be maintained by the coarse austenite.

Specifically the present inventors classified austenite in the microstructure of a duplex stainless steel material into coarse austenite grains with a minor axis of 20 μm or more, and fine austenite grains which is the remaining austenite other than the coarse austenite grains. More specifically, in the austenite of the microstructure of a duplex stainless steel material, the present inventors defined an austenite grain with a minor axis of 20 μm or more as “primary austenite”, and defined the balance of the austenite as “secondary austenite”. This point will be described more specifically using the drawings.

is a schematic diagram illustrating the appearance of microstructure during microstructure observation at a cross section at a central portion of the wall thickness of a duplex stainless steel seamless pipe that is one example of the duplex stainless steel material according to the present embodiment, the cross section being perpendicular to the pipe axis direction of the duplex stainless steel seamless pipe. The vertical direction of an observation visual field regionincorresponds to the pipe radial direction of the duplex stainless steel seamless pipe. The horizontal direction of the observation visual field regionincorresponds to the pipe circumferential direction of the duplex stainless steel seamless pipe. That is, the observation visual field regionincorresponds to a plane perpendicular to the pipe axis direction. Note that, the observation visual field regioninhas a length of 200 μm in the vertical direction and a length of 200 μm in the horizontal direction.

Referring to, regions shown in black represent ferrite, and regions shown in white represent austenite. Among the austenite, an austenite grain with a minor axis of 20 μm or more is defined as primary austenite, and an austenite grain with a minor axis of less than 20 μm is defined as secondary austenite. Note that, in the observation visual field region, the ferrite, the primary austenite, and the secondary austenitecan be identified by a method described later.

Next, the present inventors used method described later to evaluate the yield strength and the low-temperature toughness of duplex stainless steel materials which had the chemical composition described above and in which the volume ratio of ferrite was 35 to 55%. As a result, it was revealed that in a duplex stainless steel material which has the chemical composition described above and in which the volume ratio of ferrite is 35 to 55%, if the volume ratio of primary austenite is 40 to 55% and, in addition, the volume ratio of secondary austenite is 5 to 20%, both a yield strength of 80 ksi or more and excellent low-temperature toughness can be achieved. This point will now be described specifically using the drawings.

is a view illustrating the relation between a volume ratio (%) of secondary austenite and a lowest temperature (° C.) at which an absorbed energy is 30 J/cmor more in duplex stainless steel materials which satisfy the chemical composition described above among examples that are described later.was created using a volume ratio of secondary austenite (%) and a lowest temperature (° C.), that is an index of low-temperature toughness, at which an absorbed energy was 30 J/cmor more in, among examples described later, duplex stainless steel materials which satisfied the chemical composition described above and which had a microstructure including ferrite in an amount of 35 to 55% in volume ratio, and primary austenite in an amount of 40 to 55% in volume ratio.

Note that, the volume ratio of secondary austenite and the lowest temperature at which an absorbed energy is 30 J/cmor more were determined using methods described later. Further, a white circle (◯) inmeans a steel material whose yield strength was 552 MPa or more. A black circle (●) inmeans a steel material whose yield strength was less than 552 MPa.

Referring to, it can be confirmed that in a duplex stainless steel material having the chemical composition and microstructure described above, when the volume ratio of secondary austenite is 5% or more, the lowest temperature at which an absorbed energy is 30 J/cmor more is −20° C. or less, and thus excellent low-temperature toughness is exhibited. Referring further to, it can be confirmed that in a duplex stainless steel material having the chemical composition and microstructure described above, when the volume ratio of secondary austenite is more than 20%, although excellent low-temperature toughness is exhibited, a yield strength of 552 MPa or more is not obtained. That is, referring to, it can be confirmed that in a duplex stainless steel material having the chemical composition and microstructure described above, if the volume ratio of secondary austenite is 5 to 20%, both a yield strength of 552 MPa or more and excellent low-temperature toughness can be achieved.

Therefore, the duplex stainless steel material according to the present embodiment has the chemical composition described above, and is composed of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5 to 20%. As a result, the duplex stainless steel material according to the present embodiment has a yield strength of 80 ksi (552 MPa) or more and has excellent low-temperature toughness.

The gist of the duplex stainless steel material according to the present embodiment, which has been completed based on the above findings, is as follows.

[1]

A duplex stainless steel material consisting of, by mass %,

[2]

The duplex stainless steel material according to [1], containing one or more elements selected from a group consisting of:

Note that, the shape of the duplex stainless steel material according to the present embodiment is not particularly limited. The duplex stainless steel material according to the present embodiment may be a steel pipe, may be a round steel bar (a solid material), or may be a steel plate. Note that, the term “round steel bar” means a steel bar in which a cross section perpendicular to the axial direction is a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.

Hereunder, the duplex stainless steel material according to the present embodiment is described in detail.

The chemical composition of the duplex stainless steel material according to the present embodiment contains the following elements. The symbol “%” relating to an element means “mass %” unless otherwise noted.

Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C forms Cr carbides at grain boundaries and increases corrosion susceptibility at the grain boundaries. Therefore, if the content of C is too high, corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of C is to be 0.030% or less. A preferable upper limit of the content of C is 0.028%, and more preferably is 0.025%. 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%, more preferably is 0.003%, and further preferably is 0.005%.

Silicon (Si) deoxidizes the steel. 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, the low-temperature toughness of the steel 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 0.20 to 1.00%. A preferable lower limit of the content of Si is 0.25%, and more preferably is 0.30%. A preferable upper limit of the content of Si is 0.80%, more preferably is 0.70%, and further preferably is 0.60%.

Manganese (Mn) deoxidizes the steel, and desulfurizes the steel. Mn also increases hot workability of the steel material. If the content of Mn 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, Mn segregates to grain boundaries together with impurities such as P and S. Therefore, if the content of Mn is too high, corrosion resistance of the steel 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 0.50 to 7.00%. A preferable lower limit of the content of Mn is 0.75%, and more preferably is 1.00%. A preferable upper limit of the content of Mn is 6.50%, and more preferably is 6.20%.

Phosphorus (P) is unavoidably contained. That is, the 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, the low-temperature toughness and corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of P is to be 0.040% or less. A preferable upper limit of the content of P is 0.035%, and more preferably is 0.030%. 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%, and more preferably is 0.003%.

Sulfur(S) is unavoidably contained. That is, the 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, the low-temperature toughness and corrosion resistance of the steel 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 0.0200% or less. A preferable upper limit of the content of S is 0.0180%, and more preferably is 0.0160%. 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 0.0005%, and more preferably is 0.0010%.

Aluminum (Al) is unavoidably contained. That is, the lower limit of the content of Al is more than 0%. Al deoxidizes the steel. On the other hand, if the content of Al is too high, coarse oxide-based inclusions will form and the low-temperature toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Al is to be 0.100% or less. A preferable lower limit of the content of Al is 0.001%, more preferably is 0.005%, and further preferably is 0.010%. A preferable upper limit of the content of Al is 0.090%, and more preferably is 0.085%. Note that, as used in the present description, the term “content of Al” means the content of “acid-soluble Al,” that is, the content of sol. Al.

Nickel (Ni) stabilizes the austenitic microstructure of the steel material. That is, Ni is an element necessary for obtaining a stable duplex microstructure composed of ferrite and austenite. Ni also enhances corrosion resistance of the steel material. If the content of Ni 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 Ni is too high, even if the contents of other elements are within the range of the present embodiment, the volume ratio of austenite will be too high and the yield strength of the steel material will decrease. Therefore, the content of Ni is to be 4.20 to 9.00%. A preferable lower limit of the content of Ni is 4.25%, more preferably is 4.30%, further preferably is 4.35%, further preferably is 4.40%, and further preferably is 4.50%. A preferable upper limit of the content of Ni is 8.75%, more preferably is 8.50%, further preferably is 8.25%, further preferably is 8.00%, and further preferably is 7.75%.

Chromium (Cr) forms a passive film as an oxide on the surface of the steel material and thereby enhances corrosion resistance of the steel material. Cr also increases the volume ratio of the ferritic microstructure of the steel material. By obtaining a sufficient ferritic microstructure, corrosion resistance of the steel material is stabilized. 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 steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is to be 20.00 to 30.00%. A preferable lower limit of the content of Cr is 20.50%, more preferably is 21.00%, further preferably is 21.50%, and further preferably is 22.00%. A preferable upper limit of the content of Cr is 29.50%, more preferably is 29.00%, and further preferably is 28.00%.

Molybdenum (Mo) enhances corrosion resistance of the steel material. Mo also dissolves in the steel and increases the yield strength of the steel material. In addition, Mo forms fine carbides in the steel and increases the yield strength of the steel material. 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 steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Mo is to be 0.50 to 2.00%. A preferable lower limit of the content of Mo is 0.55%, more preferably is 0.60%, and further preferably is 0.70%. A preferable upper limit of the content of Mo is less than 2.00%, more preferably is 1.85%, and further preferably is 1.50%.

Copper (Cu) increases the yield strength of the steel material. If the content of Cu 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 Cu is too high, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cu is to be 0.50 to 3.00%. A preferable lower limit of the content of Cu is 0.60%, more preferably is 0.80%, further preferably is 0.90%, further preferably is 1.00%, and further preferably is 1.50%. A preferable upper limit of the content of Cu is 2.90%, more preferably is 2.75%, and further preferably is 2.50%.

Nitrogen (N) stabilizes the austenitic microstructure of the steel material. That is, N is an element necessary for obtaining a stable duplex microstructure composed of ferrite and austenite. N also enhances corrosion resistance of the steel material. If the content of N 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 N is too high, the low-temperature toughness and hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of N is to be 0.150 to 0.350%. A preferable lower limit of the content of N is 0.170%, more preferably is 0.180%, and further preferably is 0.190%. A preferable upper limit of the content of N is 0.340%, and more preferably is 0.330%.

Vanadium (V) increases the yield strength of the steel material. If the content of V 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 V is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high, and the low-temperature toughness and hot workability of the steel material will decrease. Therefore, the content of V is to be 0.01 to 1.50%. A preferable lower limit of the content of V is 0.02%, more preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.10%. A preferable upper limit of the content of V is 1.20%, and more preferably is 1.00%.