Stainless steel pipe and oil well pipe

Abstract

ABSTRACT     An object of the present invention is to provide a stainless steel pipe having high strength and excellent hydrogen embrittlement resistance.  The expression "having excellent hydrogen embrittlement resistance" means that, in a tensile test under hydrogen charging, the elongation after fracture is 9.0% or more, and that, in the tensile test, when the horizontal axis represents the true strain, the vertical axis represents the true stress, and a true strain at a maximum load is denoted by εmax, a rate of change d2σ / dε2 of a work hardening rate dσ / dε with respect to ε is -200,000 MPa or more in a true strain range of 0.8εmax or more and εmax or less.  σ denotes the true stress (MPa), and ε denotes the true strain (no units).  The term "high strength" refers to a yield strength of 450 MPa or more.     The stainless steel pipe has a specific microstructure, in which an average KAM value which is an average of a KAM (Kernel Average Misorientation) value of the steel microstructure is 2.0° or more and 4.0° or less, and a standard deviation of a distribution of the KAM value is 1.5° or less.

Description

DESCRIPTIONTitle of Invention: STAINLESS STEEL PIPE AND OIL WELL PIPETechnical Field

[0001] The present invention relates to a stainless steel pipe suitable for use in an oil well pipe and an oil well pipe.Background Art

[0002] In recent years, from the viewpoint of the surging price of crude oil and depletion of petroleum resources which is expected in the near future, there has been active development of, for example, deep oil fields, and oil fields and gas fields in severe corrosive environments that contain carbon dioxide gas, chloride ions, and hydrogen sulfide, which have not received much attention.

[0003] In hydrogen sulfide environments, hydrogen embrittlement caused by corrosion is a problem; therefore, there has been a growing need for oil well pipes with hydrogen embrittlement resistance.

[0004] Patent Literature 1 proposes a martensitic stainless steel seamless pipe whose corrosion resistance is improved by controlling the amounts of elements added so that the amount of retained austenite, the repassivation potential, and the pitting potential are within suitable ranges.

[0005] Patent Literature 2 proposes a dual phase stainless steel pipe whose corrosion resistance is improved by controlling the distribution of austenite and ferrite.Citation ListPatent Literature

[0006] PTL 1: Japanese Patent No. 6315159PTL 2: Japanese Unexamined Patent Application Publication No. 2022-111733Summary of InventionTechnical Problem

[0007] However, in the steel pipes described in Patent Literatures 1 and 2, sufficient hydrogen embrittlement resistance may not be achieved, and there is room for improvement in hydrogen embrittlement resistance.

[0008] The present invention has been made in view of the circumstances described above, and an object of the present invention is to provide a stainless steel pipe having high strength and excellent hydrogen embrittlement resistance.

[0009] The expression "having excellent hydrogen embrittlement resistance" as used herein means that, in a tensile test under hydrogen charging described later, the elongation after fracture is 9.0% or more, and that, in the tensile test, when the horizontal axis represents the true strain, the vertical axis represents the true stress, and a true strain at a maximum load is denoted by εmax, a rate of change d2σ / dε2 of a work hardening rate dσ / dε with respect to ε is -200,000 MPa or more in a true strain range of 0.8εmax or more and εmax or less.Here, σ denotes the true stress (MPa), and ε denotes the true strain (no units).

[0010] The term "high strength" as used herein refers to a yield strength of 450 MPa or more.Solution to Problem

[0011] The inventors of the present invention have conducted extensive studies and consequently found that when a region having a high dislocation density is locally present in the steel microstructure, hydrogen embrittlement resistance deteriorates.

[0012] It has also been found that the above region tends to be formed near an interface between ferrite and martensite or an interface between ferrite and austenite, that is, an interface between two phases having a large difference in strength.

[0013] The present invention has been completed on the basis of the above findings, and the summary of the present invention is as follows.[1] A stainless steel pipe, wherein:in a steel microstructure at a wall-thickness center of the stainless steel pipe,a total volume fraction of ferrite and martensite is 30% or more and 99% or less, a volume fraction of a σ phase is 0% or more and 3% or less, and the balance is austenite, an average KAM value which is an average of a KAM (Kernel Average Misorientation) value of the steel microstructure is 2.0° or more and 4.0° or less, anda standard deviation of a distribution of the KAM value is 1.5° or less.[2] The stainless steel pipe according to [1], having a chemical composition containing,in mass%:C: 0.060% or less,Si: 1.00% or less,Mn: 6.00% or less,P: 0.050% or less,S: 0.0300% or less,Al: 0.005% or more and 0.100% or less,N: 0.400% or less,Cr: 11.00% or more and 30.00% or less,Mo: 5.00% or less, andNi: 15.00% or less, andfurther optionally containing one or two or more selected fromCu: 4.00% or less,V: 0.300% or less,Nb: 0.300% or less,Ti: 0.300% or less,B: 0.0050% or less,W: 3.00% or less,Ca: 0.0050% or less,Co: 0.500% or less,Sn: 0.100% or less,Mg: 0.020% or less,Zr: 0.020% or less,REM: 0.020% or less,Ta: 0.10% or less, andSb: 0.100% or less,with the balance being Fe and incidental impurities.[3] The stainless steel pipe according to [1] or [2],wherein the stainless steel pipe is a seamless steel pipe.[4] An oil well pipe including the stainless steel pipe according to any one of [1] to [3] above.Advantageous Effects of Invention

[0014] The present invention can provide a stainless steel pipe and an oil well pipe that have high strength and excellent hydrogen embrittlement resistance.Brief Description of Drawing

[0015] [Fig. 1] Fig. 1 is a schematic view of a curve in which the true strain ε and the true stress σ are plotted on the horizontal axis and the vertical axis, respectively, when a tensile test under cathodic hydrogen charging was performed.Description of Embodiments

[0016] A stainless steel pipe according to the present invention will be described below.

[0017] At a wall-thickness center of the stainless steel pipe according to the present invention, a total volume fraction of ferrite and martensite is 30% or more and 99% or less, a volume fraction of a σ phase is 0% or more and 3% or less, the balance is austenite, an average KAM value which is an average of a KAM (Kernel Average Misorientation) value is 2.0° or more and 4.0° or less, and a standard deviation of a distribution of the KAM value is 1.5° or less.

[0018] Hereinafter, the "volume fraction" is also referred to as "fraction".

[0019] Steel microstructure at wall-thickness center: A total volume fraction of ferrite and martensite is 30% or more and 99% or less, a volume fraction of a σ phase is 0% or more and 3% or less, and the balance is austeniteFerrite is a soft microstructure but can improve the strength and / or corrosion resistance of stainless steel by refinement through hot working or heat treatment and by dislocation strengthening through cold working. Martensite is a hard microstructure and contributes to an increase in the strength of stainless steel. Austenite is a soft microstructure at room temperature but is a microstructure having high corrosion resistance.

[0020] If the total volume fraction of ferrite and martensite is less than 30%, the yield strength decreases. Accordingly, the total volume fraction of ferrite and martensite is preferably 30% or more. The total volume fraction of ferrite and martensite is more preferably 35% or more, still more preferably 37% or more, and most preferably 40% or more. If the total volume fraction of ferrite and martensite exceeds 99%, the amount of austenite decreases, and corrosion resistance deteriorates. Accordingly, the total volume fraction of ferrite and martensite is preferably 99% or less. The total volume fraction of ferrite and martensite is more preferably 95% or less, still more preferably 93% or less, and most preferably 90% or less.

[0021] The σ phase is an intermetallic compound that deteriorates corrosion resistance and toughness of stainless steel and decreases the elongation after fracture and d2σ / dε2 under hydrogen charging; therefore, the volume fraction of the σ phase is preferably small. Specifically, the volume fraction of the σ phase is 0% or more and 3% or less. The volume fraction of the σ phase is 3% or less, preferably 2% or less, more preferably 1% or less, and still more preferably 0%.

[0022] The volume fractions of austenite and the σ phase are determined by a SEM / EBSD method. The measurement surface is a cross section parallel to both the wall thickness direction of the pipe and the pipe axis direction. The measurement region (one field of view) is 400 µm × 400 µm, the measurement step size is 0.1 µm, and measured values in five fields of view are averaged. On the basis of the obtained EBSD data, a phase distribution map is obtained using crystal orientation analysis software OIM Analysis (trademark). The area fractions of austenite and the σ phase obtained in this manner are taken as the volume fractions of austenite and the σ phase, respectively.

[0023] Note that a region having a high dislocation density is locally formed at an interface between two phases having a large difference in strength, resulting in a deterioration of hydrogen embrittlement resistance; therefore, the interface is preferably decreased as much as possible. Thus, the microstructure is more preferably a single-phase microstructure. Furthermore, by controlling the temperature history during cooling after heat treatment in the production of the stainless steel pipe, the formation of the above-described region is suppressed, and hydrogen embrittlement resistance is improved.

[0024] The volume fractions of ferrite and martensite are determined by observation with an optical microscope. The above measurement surface is subjected to electrolytic etching using an aqueous solution of potassium hydroxide. Ten fields of view were observed with an optical microscope at an observation magnification of 400 times, each of the phases was distinguished as described below, and the volume fraction of each phase was determined as described below as an average value of the ten fields of view. In this case, ferrite, austenite, and the σ phase are microstructures with a bright contrast, and martensite is a microstructures with a dark contrast. A photograph of these microstructures is taken with the optical microscope, and the total area fraction of ferrite, austenite, and the σ phase and the area fraction of martensite are each determined from the obtained image. Furthermore, the area fractions of austenite and the σ phase determined by the SEM / EBSD method are subtracted from the total area fraction of ferrite, austenite, and the σ phase to determine the area fraction of ferrite. The area fractions of ferrite and martensite determined as described above are taken as the volume fractions of ferrite and martensite, respectively.

[0025] Average KAM value: 2.0° or more and 4.0° or lessThe KAM (Kernel Average Misorientation) value represents a local misorientation. The higher the KAM value, the higher the dislocation density at the measurement point tends to be and the higher the hardness tends to be. If the average KAM value is less than 2.0°, the amount of soft ferrite or austenite having a low dislocation density is large, a stress concentrates at an interface with a surrounding hard phase, which acts as a starting point of cracking, and the elongation after fracture and d2σ / dε2 under hydrogen charging decrease. Accordingly, the average KAM value is 2.0° or more. The average KAM value is preferably 2.1° or more, more preferably 2.2° or more, still more preferably 2.3° or more, and most preferably 2.4° or more. On the other hand, if the average KAM value exceeds 4.0°, the amount of hard martensite having a high dislocation density or a work-hardened microstructure is large, and the elongation after fracture and d2σ / dε2 under hydrogen charging decrease. Accordingly, the average KAM value is 4.0° or less. The average KAM value is preferably 3.9° or less, more preferably 3.8° or less, still more preferably 3.7° or less, and most preferably 3.6° or less.

[0026] Standard deviation of KAM value distribution: 1.5° or lessIf a variation in the KAM value is large, soft portions and hard portions coexist, and the difference in hardness between the portions increases; consequently, a stress concentrates at an interface between them, which acts as a starting point of cracking, and the elongation after fracture under hydrogen charging decreases. Accordingly, the standard deviation of a KAM value distribution is 1.5° or less. The standard deviation of the KAM value distribution is preferably 1.4° or less, more preferably 1.3° or less, still more preferably 1.2° or less, and most preferably 1.1° or less. The smaller the standard deviation of the KAM value distribution, the better. However, since an excessive reduction results in an increase in the production cost and the production load, the standard deviation of the KAM value distribution is preferably 0.4° or more, more preferably 0.5° or more, and still more preferably 0.6° or more.

[0027] The average KAM value and the standard deviation of the KAM value distribution are measured by a SEM / EBSD method. The measurement region (one field of view) is 400 µm × 400 µm, the measurement step size is 0.1 µm, and measured values in five fields of view are averaged. On the basis of the obtained EBSD data, a distribution image of the KAM value (KAM map) is obtained using crystal orientation analysis software OIM Analysis (trademark). Herein, the KAM value is determined by the following method. In each measurement point (regular hexagonal pixel), a misorientation between pixels is determined using up to three neighboring pixels (total 37 pixels) around the measurement point serving as the center. The average value of the determined misorientation is taken as the KAM value of the pixel at the center. This operation is performed for all pixels in the field of view to obtain a KAM map. From the obtained distribution of the KAM value, the average KAM value and the standard deviation of the KAM value distribution are determined by formula (1) and formula (2), respectively.

[0028] [Math. 1]Here, n is the number of measurement points in the field of view.

[0029] [Math. 2]Here, n is the number of measurement points in the field of view.

[0030] Preferably, the chemical composition of the stainless steel pipe according to the present invention contains, in mass%,C: 0.060% or less, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, and Ni: 15.00% or less and further optionally contains one or two or more selected from Cu: 4.00% or less, V: 0.300% or less, Nb: 0.300% or less, Ti: 0.300% or less, B: 0.0050% or less, W: 3.00% or less, Ca: 0.0050% or less, Co: 0.500% or less, Sn: 0.100% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, Ta: 0.10% or less, and Sb: 0.100% or less, with the balance being Fe and incidental impurities.

[0031] In the present Description, "%" in the steel composition is "mass%" unless otherwise specified.

[0032] C: 0.060% or lessC is an element that increases the strength of steel through solid solution strengthening. Moreover, C is an element that lowers the transformation start temperature to refine crystal grains, thereby increasing the strength of steel. In addition, C is an element that stabilizes austenite and increases the austenite fraction and the martensite fraction. To provide such effects, the C content is preferably 0.002% or more. The C content is more preferably 0.003% or more, still more preferably 0.004% or more, and most preferably 0.005% or more. However, if C is contained in an excessive amount, Cr carbide is formed at crystal grain boundaries, corrosion sensitivity at grain boundaries increases, and hydrogen embrittlement resistance deteriorates. Accordingly, the C content is preferably 0.060% or less. The C content is more preferably 0.050% or less, still more preferably 0.040% or less, and most preferably 0.035% or less.

[0033] Si: 1.00% or lessSi is an element that acts as a deoxidizing agent and is an element that stabilizes ferrite and increases the ferrite fraction. To provide such effects, the Si content is desirably 0.01% or more. The Si content is more preferably 0.05% or more, still more preferably 0.07% or more, and most preferably 0.10% or more. However, if the Si content exceeds 1.00%, hot workability of an intermediate product (such as a billet), which is produced in the middle of production of the product, deteriorates. Accordingly, the Si content is preferably 1.00% or less. The Si content is more preferably 0.90% or less, still more preferably 0.80% or less, and most preferably 0.70% or less.

[0034] Mn: 6.00% or lessMn is an element that increases the strength of steel through solid solution strengthening. Moreover, Mn is an element that lowers the transformation start temperature to refine crystal grains, thereby increasing the strength of steel. In addition, Mn is an element that stabilizes austenite and increases the austenite fraction and the martensite fraction. To provide such effects, the Mn content is preferably 0.05% or more. The Mn content is more preferably 0.10% or more, still more preferably 0.15% or more, and most preferably 0.20% or more. However, if Mn is contained in an excessive amount, the steel has excessively high strength, and hydrogen embrittlement resistance deteriorates. Accordingly, the Mn content is preferably 6.00% or less. The Mn content is more preferably 5.00% or less, still more preferably 3.00% or less, and most preferably 2.50% or less.

[0035] P: 0.050% or lessP segregates at grain boundaries and deteriorates toughness. Thus, it is preferable that the amount of P, which is an incidental impurity, be reduced as much as possible. The P content is preferably 0.050% or less. The P content is more preferably 0.040% or less, still more preferably 0.030% or less, and most preferably 0.025% or less. The lower limit of the P content is not particularly specified but is preferably 0.001% or more because an excessive reduction in the P content results in an increase in smelting cost. The P content is more preferably 0.002% or more, and still more preferably 0.003% or more.

[0036] S: 0.0300% or lessS is an element that significantly deteriorates hot workability in the pipe production process, and it is desirable that the S content be as low as possible. However, if the S content is reduced to 0.0300% or less, a pipe can be produced by using an ordinary process. Accordingly, the S content is preferably 0.0300% or less. The S content is more preferably 0.0100% or less, still more preferably 0.0050% or less, and most preferably 0.0030% or less. The lower limit of S is not particularly specified but is preferably 0.0001% or more because an excessive reduction in the S content results in an increase in smelting cost. The S content is more preferably 0.0002% or more, and still more preferably 0.0003% or more.

[0037] Al: 0.005% or more and 0.100% or lessAl is an element that acts as a strong deoxidizing agent. Moreover, Al is an element that stabilizes ferrite and increases the ferrite fraction. To provide such effects, the Al content is preferably 0.005% or more. The Al content is more preferably 0.010% or more, still more preferably 0.015% or more, and most preferably 0.018% or more. However, an excessively high Al content deteriorates weldability, increases the amount of alumina-based inclusions, and deteriorates surface properties. Accordingly, the Al content is preferably 0.100% or less. The Al content is more preferably 0.080% or less, still more preferably 0.070% or less, and most preferably 0.065% or less.

[0038] N: 0.400% or lessN has an effect of improving hydrogen embrittlement resistance. Moreover, N is an element that stabilizes austenite and increases the austenite fraction and the martensite fraction. To provide such effects, the N content is preferably 0.002% or more. The N content is more preferably 0.005% or more, still more preferably 0.010% or more, and most preferably 0.020% or more. On the other hand, if the N content exceeds 0.400%, hot workability deteriorates. In addition, nitrides are formed at grain boundaries, and hydrogen embrittlement resistance deteriorates. Accordingly, the N content is preferably 0.400% or less. The N content is more preferably 0.380% or less, still more preferably 0.360% or less, and most preferably 0.340% or less.

[0039] Cr: 11.00% or more and 30.00% or lessCr has an effect of forming a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. Moreover, Cr is an element that stabilizes ferrite and increases the ferrite fraction. If the Cr content is less than 11.00%, corrosion resistance is insufficient, and target hydrogen embrittlement resistance in the present invention cannot be ensured. Accordingly, the Cr content is 11.00% or more. The Cr content is preferably 11.20% or more, more preferably 11.50% or more, still more preferably 11.70% or more, and most preferably 11.90% or more. On the other hand, if the Cr content exceeds 30.00%, the ferrite fraction excessively increases, and the strength decreases. Accordingly, the Cr content is 30.00% or less. The Cr content is preferably 28.00% or less, more preferably 26.00% or less, still more preferably 24.00% or less, and most preferably 20.00% or less.

[0040] Mo: 5.00% or lessMo has an effect of stabilizing a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. Moreover, Mo is an element that stabilizes ferrite and increases the ferrite fraction. To provide such effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, still more preferably 0.10% or more, and most preferably 0.15% or more. On the other hand, if the Mo content exceeds 5.00%, the ferrite fraction excessively increases, and the strength decreases. Accordingly, the Mo content is preferably 5.00% or less. The Mo content is more preferably 4.50% or less, still more preferably 4.30% or less, and most preferably 4.00% or less.

[0041] Ni: 15.00% or lessNi has an effect of stabilizing a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. Moreover, Ni is an element that stabilizes austenite and increases the austenite fraction and the martensite fraction. To provide such effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 1.00% or more, still more preferably 3.00% or more, and most preferably 3.50% or more. On the other hand, if the Ni content exceeds 15.00%, the austenite fraction increases, and the strength decreases. Accordingly, the Ni content is preferably 15.00% or less. The Ni content is more preferably 10.00% or less, still more preferably 9.00% or less, and most preferably 8.50% or less.In addition to the above components, one or two or more selected from Cu, V, Nb, Ti, B, W, Ca, Co, Sn, Mg, Zr, REM, Ta, and Sb can be arbitrarily selected.

[0042] Cu: 4.00% or lessCu has an effect of stabilizing a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. Moreover, Cu is an element that stabilizes austenite and increases the austenite fraction and the martensite fraction. To provide such effects, the Cu content is desirably 0.01% or more. The Cu content is more preferably 0.05% or more, still more preferably 0.10% or more, most preferably 0.20% or more, and even still more preferably 0.30% or more. On the other hand, if the Cu content exceeds 4.00%, CuS precipitates at grain boundaries, resulting in deterioration of hot workability. Accordingly, in the case where Cu is contained, the Cu content is 4.00% or less. The Cu content is preferably 3.50% or less, more preferably 3.00% or less, still more preferably 2.80% or less, and most preferably 2.50% or less.

[0043] V: 0.300% or less, Nb: 0.300% or less, and Ti: 0.300% or lessV, Nb, and Ti are elements that form fine carbides and nitrides in steel and thereby contribute to improvement in the strength of the steel. Moreover, V, Nb, and Ti also have an effect of improving hydrogen embrittlement resistance by trapping hydrogen atoms when hydrogen generated by corrosion enters the steel. In addition, V and Ti are elements that stabilize ferrite and increase the ferrite fraction. To provide the above effects, it is preferable to contain V: 0.002% or more, Nb: 0.002% or more, and Ti: 0.002% or more. The contents are more preferably V: 0.005% or more, Nb: 0.005% or more, and Ti: 0.005% or more, still more preferably V: 0.010% or more, Nb: 0.010% or more, and Ti: 0.010% or more, and most preferably V: 0.015% or more, Nb: 0.015% or more, and Ti: 0.015% or more. However, if these elements are contained in excessive amounts, the steel has excessively high strength, and hydrogen embrittlement resistance deteriorates. The toughness also deteriorates. Accordingly, in the case where V, Nb, and Ti are contained, V: 0.300% or less, Nb: 0.300% or less, and Ti: 0.300% or less, preferably V: 0.200% or less, Nb: 0.200% or less, and Ti: 0.200% or less, more preferably V: 0.150% or less, Nb: 0.150% or less, and Ti: 0.150% or less, still more preferably V: 0.140% or less, Nb: 0.140% or less, and Ti: 0.140% or less, and most preferably V: 0.120% or less, Nb: 0.120% or less, and Ti: 0.120% or less.

[0044] B: 0.0050% or lessB is an element that lowers the transformation start temperature and thereby contributes to refinement of the microstructure to increase the strength of steel. Moreover, B has an effect of suppressing segregation of S at grain boundaries to improve hot workability. To provide the above effects, the B content is preferably 0.0002% or more. The B content is more preferably 0.0005% or more, still more preferably 0.0008% or more, and most preferably 0.0010% or more. However, if B is contained in an excessive amount, a nitride is formed at grain boundaries, and hydrogen embrittlement resistance deteriorates. Accordingly, in the case where B is contained, the B content is 0.0050% or less. The B content is preferably 0.0045% or less, preferably 0.0040% or less, more preferably 0.0035% or less, still more preferably 0.0030% or less, and most preferably 0.0025% or less.

[0045] W: 3.00% or lessW is an element that contributes to improvement in the strength of steel through solid solution strengthening and can stabilize a protective coating on the surface of the steel to improve hydrogen embrittlement resistance. Moreover, W is an element that stabilizes ferrite and increases the ferrite fraction. To provide the above effects, the W content is preferably 0.02% or more. The W content is more preferably 0.05% or more, still more preferably 0.10% or more, and most preferably 0.15% or more. However, if W is contained in an excessive amount, the toughness deteriorates due to formation of an intermetallic compound. Accordingly, in the case where W is contained, the W content is 3.00% or less. The W content is preferably 2.50% or less, more preferably 2.40% or less, still more preferably 2.20% or less, and most preferably 2.00% or less.

[0046] Ca: 0.0050% or lessCa is an element that spheroidizes sulfides such as MnS and can thereby improve hydrogen embrittlement resistance. To provide the above effect, the Ca content is preferably 0.0005% or more. The Ca content is more preferably 0.0008% or more, still more preferably 0.0010% or more, and most preferably 0.0012% or more. However, if Ca is contained in an excessive amount, Ca oxide clusters are formed in steel, and the toughness deteriorates. Accordingly, in the case where Ca is contained, the Ca content is 0.0050% or less. The Ca content is preferably 0.0040% or less, more preferably 0.0035% or less, still more preferably 0.0030% or less, and most preferably 0.0025% or less.

[0047] Co: 0.500% or lessCo has an effect of stabilizing a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. Moreover, Co is an element that stabilizes austenite and increases the austenite fraction and the martensite fraction. To provide the above effects, the Co content is preferably 0.002% or more. The Co content is more preferably 0.005% or more, still more preferably 0.010% or more, and most preferably 0.015% or more. However, if the content exceeds 0.500%, the obtained effects are saturated, and the production cost only increases. Accordingly, in the case where Co is contained, the Co content is 0.500% or less. The Co content is preferably 0.450% or less, more preferably 0.400% or less, still more preferably 0.0300% or less, and most preferably 0.200% or less.

[0048] Sn: 0.100% or lessSn has an effect of stabilizing a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. To provide the above effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.002% or more, and still more preferably 0.005% or more. However, if the content exceeds 0.100%, hot workability deteriorates. Accordingly, in the case where Sn is contained, the Sn content is 0.100% or less. The Sn content is preferably 0.070% or less, more preferably 0.050% or less, still more preferably 0.040% or less, and most preferably 0.030% or less.

[0049] Mg: 0.020% or lessMg has an effect of improving hydrogen embrittlement resistance by shape control of inclusions. To provide the above effect, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.002% or more, and still more preferably 0.005% or more. However, if the content exceeds 0.020%, hot workability deteriorates. Accordingly, in the case where Mg is contained, the Mg content is 0.020% or less. The Mg content is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.009% or less, and most preferably 0.008% or less.

[0050] Zr: 0.020% or lessZr is an element that forms a fine carbide and a fine nitride in steel and thereby contributes to improvement in the strength of the steel. To provide the above effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.002% or more and still more preferably 0.005% or more. However, if Zr is contained in an excessive amount, the steel has excessively high strength, and hydrogen embrittlement resistance deteriorates. The toughness also deteriorates. Accordingly, in the case where Zr is contained, the Zr content is 0.020% or less. The Zr content is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.009% or less, and most preferably 0.008% or less.

[0051] REM: 0.020% or lessREM has an effect of improving hydrogen embrittlement resistance by shape control of inclusions. To provide the above effect, the REM content is preferably 0.001% or more. The REM content is more preferably 0.002% or more, still more preferably 0.005% or more, and most preferably 0.006% or more. However, if the content exceeds 0.020%, hot workability deteriorates. Accordingly, in the case where REM is contained, the REM content is 0.020% or less. The REM content is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.009% or less, and most preferably 0.008% or less.Herein, REM is a generic name for a total of 17 elements of Sc, Y, and lanthanoid elements. The REM content refers to the total amount of these elements contained.

[0052] Ta: 0.10% or lessTa is an element that forms a fine carbide and a fine nitride in steel and thereby contributes to improvement in the strength of the steel. To provide the above effect, the Ta content is preferably 0.01% or more. The Ta content is more preferably 0.02% or more, still more preferably 0.03% or more, and most preferably 0.04% or more. However, if Ta is contained in an excessive amount, the steel has excessively high strength, and hydrogen embrittlement resistance deteriorates. The toughness also deteriorates. Accordingly, in the case where Ta is contained, the Ta content is 0.10% or less. The Ta content is preferably 0.08% or less, more preferably 0.07% or less, still more preferably 0.06% or less, and most preferably 0.05% or less.

[0053] Sb: 0.100% or lessSb has an effect of stabilizing a protective coating on the surface of steel to inhibit hydrogen entry into the steel and improving hydrogen embrittlement resistance. To provide the above effect, the Sb content is preferably 0.001% or more. The content is more preferably 0.002% or more, still more preferably 0.005% or more, and most preferably 0.008% or more. However, if the content exceeds 0.100%, hot workability deteriorates. Accordingly, in the case where Sb is contained, the Sb content is 0.100% or less. The content is preferably 0.070% or less, more preferably 0.050% or less, still more preferably 0.040% or less, and most preferably 0.030% or less.

[0054] The balance is Fe and incidental impurities. Examples of incidental impurities in the balance include As, Bi, Pb, Zn, O, Te, Hf, Ge, Sr, and Cs. However, As, Te, Hf, Ge, Sr, and Cs can each be contained in an amount of 0.10% or less, and Bi, Pb, Zn, and O can each be contained in an amount of 0.005% or less as long as the effects of the present invention are not impaired.

[0055] According to the stainless steel pipe according to the present invention, in the case where a tensile test under hydrogen charging is performed, the elongation after fracture is 9.0% or more, and when a true strain at a maximum load is denoted by εmax, a rate of change d2σ / dε2 of a work hardening rate dσ / dε with respect to ε is -200,000 MPa or more in a true strain range of 0.8εmax or more and εmax or less. Here, σ denotes a true stress (MPa), and ε denotes a true strain (no units).

[0056] Fig. 1 is a schematic view of a curve in which the true strain ε and the true stress σ are plotted on the horizontal axis and the vertical axis, respectively, when a tensile test under cathodic hydrogen charging is performed. The true strain range of 0.8εmax or more and εmax or less corresponds to a stage which is generally referred as uniform elongation and in which the mode changes from elastic deformation (elastic region) to plastic deformation (plastic region). The range of 0.8εmax or more and εmax or less is focused and the range is limited to the above because the work hardening behavior is stable. Regarding the range, the above-described range is basically focused, but the range may be 0.9εmax or more.

[0057] A stainless steel pipe that achieves the above characteristics has excellent hydrogen embrittlement resistance.

[0058] As for the method of hydrogen charging, for example, in a solution at room temperature (21°C to 27°C) in which 3.0 g of ammonium thiocyanate is added per 1 L of a 3.0 mass% aqueous solution of sodium chloride, cathodic hydrogen charging is performed at a current density of 0.05 mA / cm2 for 24 hours. Subsequently, a tensile test is conducted while cathodic hydrogen charging is continuously performed under the conditions described above.

[0059] The method of hydrogen charging is not limited to the above method as long as hydrogen charging is performed under conditions such that the hydrogen content in the steel at the time of break in the tensile test is 1.5 mass ppm or more. Hydrogen charging is more preferably performed under conditions such that the hydrogen content is 1.6 mass ppm or more and still more preferably performed under conditions such that the hydrogen content is 1.7 mass ppm or more. Although the upper limit is not particularly limited, hydrogen charging is preferably performed under conditions such that the hydrogen content is 10.0 mass ppm or less. The hydrogen content in the steel is measured by, for example, a thermal desorption method in which the heating rate is 100°C / s, heating is performed to 600°C, and a gas chromatograph is used as an analyzer.

[0060] The strain rate in the tensile test is, for example, 1.7 × 10-5 s-1. The strain rate in the tensile test is not particularly limited but is preferably 1.0 × 10-5 s-1 or more, more preferably 1.5 × 10-5 s-1 or more, still more preferably 1.8 × 10-5 s-1 or more, and most preferably 1.9 × 10-5 s-1 or more. The strain rate is preferably 9.0 × 10-5 s-1 or less, and more preferably 6.0 × 10-5 s-1 or less.

[0061] For the tensile test, a round-bar test piece including a parallel portion with a diameter of 3.8 mm and a length of 15 mm is used. The test piece is taken from a central portion in the wall-thickness direction such that the tensile direction is parallel to the pipe axis direction.

[0062] Elongation after fracture: 9.0% or moreIf the elongation after fracture in the above-described tensile test under hydrogen charging is small, ductility in hydrogen environments is insufficient. Accordingly, in the present invention, the elongation after fracture is 9.0% or more. The elongation after fracture is preferably 9.5% or more, more preferably 10.0% or more, still more preferably 10.2% or more, and most preferably 10.4% or more. The larger the elongation after fracture, the better. However, since an excessive increase results in an increase in the production cost and the production load, the elongation after fracture is preferably 30.0% or less. The elongation after fracture is more preferably 28.0% or less, still more preferably 25.0% or less, and most preferably 24.0% or less.The elongation after fracture (%) is determined as ((total length in state where test pieces after test are abutted against each other) - (total length of test piece before test)) / (total length of test piece before test) × 100.

[0063] In true strain range of 0.8εmax or more and εmax or less, d2σ / dε2: -200,000 MPa or moreWhile work hardening progresses in the plastic region, the work hardening rate dσ / dε decreases with an increase in ε.That is, d2σ / dε2 becomes a negative value. The smaller d2σ / dε2, the more rapidly dσ / dε decreases; therefore, necking occurs in an early stage, the maximum load decreases, and ductility deteriorates. In particular, it is important to control d2σ / dε2 in a true strain range of 0.8εmax or more and εmax or less. Accordingly, in the present invention, in the true strain range of 0.8εmax or more and εmax or less, d2σ / dε2 in the above-described tensile test under hydrogen charging is -200,000 MPa or more. d2σ / dε2 is preferably -180,000 MPa or more, more preferably -160,000 MPa or more, still more preferably -150,000 MPa or more, and most preferably -140,000 MPa or more. A larger d2σ / dε2 is more preferable. However, since an excessive increase results in an increase in the production cost and the production load, d2σ / dε2 is preferably less than 0 MPa. d2σ / dε2 is more preferably -10,000 MPa or less, and still more preferably -15,000 MPa or less.

[0064] dσ / dε and d2σ / dε2 are determined from time history data of the load and the cross-head displacement in the tensile test. First, σ and ε at each time are determined from the load and the cross-head displacement. Subsequently, in order to reduce the fluctuation of σ, at each time, σ is averaged in a range corresponding to ε = 0.001. Specifically, for example, in the case where the strain rate is 1.7 × 10-5 s-1, σ at certain time t(s) is determined as an average value of σ from time (t - 30)(s) to time (t + 30)(s). This is performed for all times. Subsequently, dσ / dε is determined at each time. Specifically, dσ / dε at certain time t(s) is determined as an average rate of change from time (t - 60)(s) to time t(s). This is performed for all times. Furthermore, d2σ / dε2 is determined at each time. Specifically, (d / dε)(dσ / dε) at certain time t(s) is determined as an average rate of change from time (t - 60)(s) to time t(s). This is performed for all times.

[0065] Note that, in the present invention, since the tensile test is conducted in a solution, it is difficult to directly measure the displacement of the parallel portion with a tensometer or the like. Therefore, ε is determined from the cross-head displacement. In this case, the value of ε includes an error due to elastic deformation of the testing machine; however, the error in the present invention is negligibly small because dσ / dε and d2σ / dε2 are calculated from the amount of change in ε.

[0066] In addition, the stainless steel pipe according to the present invention has a yield strength of 450 MPa or more to withstand the internal pressure and its own weight. The yield strength is preferably 550 MPa or more, more preferably 580 MPa or more, and still more preferably 600 MPa or more. On the other hand, as the yield strength increases, corrosion resistance deteriorates. Accordingly, the yield strength is preferably 1,100 MPa or less. The yield strength is more preferably 1,000 MPa or less, still more preferably 990 MPa or less, and most preferably 980 MPa or less.

[0067] The yield strength is measured by a tensile test at room temperature (10°C to 38°C) in air. A test piece is taken in accordance with ASTM E8 / E8M (2021). Specifically, a round-bar test piece is taken from a central portion in the wall-thickness direction such that the tensile direction is parallel to the pipe axis direction. Regarding the size of the round-bar test piece, for example, the diameter of the parallel portion is 8.9 mm, and the gauge length is 35.6 mm. If a round-bar test piece cannot be taken from the steel pipe, an arc-shaped test piece is taken. Regarding the size of the arc-shaped test piece, for example, the thickness is the full wall thickness, the width is 25.4 mm, and the gauge length is 50.8 mm. A tensile test is performed in accordance with ASTM E8 / E8M (2021) using the taken test piece for tensile test. An obtained 0.2% offset proof stress (MPa) is taken as the yield strength (MPa).

[0068] The stainless steel pipe described above is preferably a seamless steel pipe.

[0069] Furthermore, an oil well pipe using the above stainless steel pipe is preferred.

[0070] Next, a method for producing a stainless steel pipe according to an embodiment of the present invention will be described.

[0071] The stainless steel pipe according to the present invention is produced by, for example, subjecting a steel material having the chemical composition described above to heating and hot working to provide a cylindrical shape, and subjecting the resulting steel pipe to cooling and then heat treatment.

[0072] In the description of the following production method, the expression "°C" related to a temperature represents a surface temperature unless otherwise specified. The surface temperature can be measured with a radiation thermometer or the like. The temperature at a wall-thickness center can be determined by calculating the temperature distribution in the wall-thickness direction using heat transfer analysis and correcting the result with the surface temperature.

[0073] In the present invention, the smelting method for the steel material is not particularly limited, and any known smelting method such as a method using a converter, an electric arc furnace, or a vacuum melting furnace is suitably used. The casting method is also not particularly limited. A known casting method such as a continuous casting method is used to produce a steel material with desired dimensions. Instead of the continuous casting method, an ingot casting-blooming method may be used without any problem. The molten steel may be subjected to secondary refining such as ladle refining. Preferably, the steel material is cast into a round-bar shape such as a billet, and this is used as a steel pipe material. The cast steel may be further subjected to hot rolling, and the cast steel having desired dimensions and shape may be used as a steel pipe material.

[0074] Subsequently, the obtained steel pipe material is heated and subjected to hot working to provide a stainless steel pipe having a predetermined shape. The hot pipe making process is preferably a Mannesmann-plug mill process or a Mannesmann-mandrel mill process. A stainless steel pipe may be produced by hot extrusion by a pressing method. In the hot pipe making process, it is only necessary that a stainless steel pipe having a predetermined shape can be produced, and conditions for the process are not particularly specified.

[0075] In the heating process, the heating temperature of the steel pipe material (for example, a billet) is preferably a temperature in a range of 1,100°C to 1,350°C. If the heating temperature is lower than 1,100°C, hot workability of the billet deteriorates, and flaws are more likely to be generated during pipe making. In addition, the load on the equipment becomes excessive. Accordingly, the heating temperature is preferably 1,100°C or higher. The heating temperature is more preferably 1,150°C or higher, still more preferably 1,160°C or higher, and most preferably 1,170°C or higher. On the other hand, if the heating temperature is a high temperature of higher than 1,350°C, crystal grains coarsen, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases. Accordingly, the heating temperature in the heating process is preferably 1,350°C or lower. The heating temperature is more preferably 1,300°C or lower, still more preferably 1,290°C or lower, and most preferably 1,280°C or lower.

[0076] In the hot pipe making process, a reduction (%) in area of a cross section perpendicular to the pipe axis direction is preferably 25% or more. Herein, the reduction (%) in area is determined by ((cross-sectional area of steel pipe material) - (cross-sectional area after hot pipe making)) / (cross-sectional area of steel pipe material) × 100. If the reduction in area is low, crystal grains coarsen, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases. The reduction in area is more preferably 30% or more, still more preferably 35% or more, and most preferably 40% or more. On the other hand, if the reduction in area is excessively high, the obtained effects are saturated, and the load on the facility becomes excessive. Accordingly, the reduction in area is preferably 75% or less. The reduction in area is more preferably 70% or less, still more preferably 65% or less, and most preferably 60% or less.

[0077] After the hot pipe making process, the obtained stainless steel pipe is subjected to cooling treatment. In the cooling treatment, at the wall-thickness center, an average cooling rate in the range of 650°C to 300°C is preferably 10°C / s or more, and a minimum cooling rate is preferably 3°C / s or more. The minimum cooling rate is determined by dividing the time in the range of 650°C to 300°C into sections of 3 s each, calculating the average cooling rate in each section, and taking the minimum value of the average cooling rates. The average cooling rate in each section can be determined by dividing a temperature obtained by subtracting a cooling stop temperature from a cooling start temperature in a predetermined section by the time required for cooling in the predetermined section. The average cooling rate in the range of 650°C to 300°C can be determined by (650 - 300) / (the time required for cooling in the range of 650°C to 300°C) (°C / s).At a low average cooling rate and a low minimum cooling rate, crystal grains coarsen, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases. Moreover, the σ phase, which is a brittle phase, is generated, and the elongation after fracture and d2σ / dε2 under hydrogen charging decrease. Accordingly, the average cooling rate in the range of 650°C to 300°C is preferably 10°C / s or more. The average cooling rate is more preferably 15°C / s or more, still more preferably 18°C / s or more, and most preferably 20°C / s or more. On the other hand, if the average cooling rate exceeds 100°C / s, the load on the cooling apparatus becomes excessive; therefore, the average cooling rate is preferably 100°C / s or less. The average cooling rate is more preferably 70°C / s or less, still more preferably 60°C / s or less, and most preferably 50°C / s or less. The minimum cooling rate is preferably 3°C / s or more. The minimum cooling rate is more preferably 4°C / s or more, still more preferably 5°C / s or more, and most preferably 6°C / s or more. On the other hand, if the minimum cooling rate exceeds 15°C / s, the load on the cooling apparatus becomes excessive; therefore, the minimum cooling rate is preferably 15°C / s or less. The minimum cooling rate is more preferably 12°C / s or less, still more preferably 11°C / s or less, and most preferably 10°C / s or less. The cooling method is preferably water cooling in order to ensure the necessary cooling rate.

[0078] Subsequently, in the present invention, the seamless steel pipe is preferably subjected to heat treatment involving quenching treatment and tempering treatment. However, in the case where the Cr content is 20.00% or more, the tempering treatment can be omitted because austenite is stable at room temperature, and the martensite fraction is low.

[0079] In the quenching treatment, the stainless steel pipe is reheated to a temperature (heating temperature) in the range of 850°C to 1,150°C and preferably held for 300 s or more. Subsequently, in the range of 650°C to 300°C, the average cooling rate is set to 10°C / s or more, and the minimum cooling rate is set to 3°C / s or more at the wall-thickness center. The minimum cooling rate is determined by dividing the time in the range of 650°C to 300°C into sections of 3 s each, calculating the average cooling rate in each section, and taking the minimum value of the average cooling rates. The average cooling rate in each section can be determined by dividing a temperature obtained by subtracting a cooling stop temperature from a cooling start temperature in a predetermined section by the time required for cooling in the predetermined section. The average cooling rate in the range of 650°C to 300°C can be determined by (650 - 300) / (the time required for cooling in the range of 650°C to 300°C) (°C / s).

[0080] At a low heating temperature of the quenching treatment, the austenite fraction decreases, and the ferrite fraction increases; therefore, the average KAM value decreases. The yield strength also decreases. In addition, the σ phase may remain without melting in some cases; therefore, the elongation after fracture and d2σ / dε2 under hydrogen charging decrease. Accordingly, the heating temperature of the quenching treatment is preferably 850°C or higher. The heating temperature is more preferably 880°C or higher, still more preferably 900°C or higher, and most preferably 920°C or higher. At a high heating temperature of the quenching treatment, crystal grains coarsen, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases. Accordingly, the heating temperature of the quenching treatment is preferably 1,150°C or lower. The heating temperature is more preferably 1,130°C or lower, still more preferably 1,100°C or lower, and most preferably 1,080°C or lower.

[0081] At a low average cooling rate and a low minimum cooling rate, crystal grains coarsen, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases.Moreover, the σ phase is generated, and the elongation after fracture and d2σ / dε2 under hydrogen charging decrease. Accordingly, the average cooling rate in the range of 650°C to 300°C is preferably 10°C / s or more. The average cooling rate is more preferably 15°C / s or more, still more preferably 18°C / s or more, and most preferably 20°C / s or more. If the average cooling rate exceeds 100°C / s, the load on the cooling apparatus becomes excessive; therefore, the average cooling rate is preferably 100°C / s or less. The average cooling rate is more preferably 70°C / s or less, still more preferably 60°C / s or less, and most preferably 50°C / s or less. The minimum cooling rate is preferably 3°C / s or more. The minimum cooling rate is more preferably 4°C / s or more, still more preferably 5°C / s or more, and most preferably 6°C / s or more. If the minimum cooling rate exceeds 15°C / s, the load on the cooling apparatus becomes excessive; therefore, the minimum cooling rate is preferably 15°C / s or less. The minimum cooling rate is more preferably 12°C / s or less, still more preferably 11°C / s or less, and most preferably 10°C / s or less. The cooling method is preferably water cooling in order to ensure the necessary cooling rate.

[0082] Subsequently, the stainless steel pipe that has been subjected to the quenching treatment is preferably subjected to tempering treatment.

[0083] In the tempering treatment, the stainless steel pipe is heated to 500°C or higher and 750°C or lower, and preferably held for 30 minutes or more, and then cooled preferably at a cooling rate faster than or equal to natural cooling preferably to room temperature. At a low heating temperature of the tempering treatment, the recovery of dislocation does not proceed sufficiently, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases. Accordingly, the heating temperature of the tempering treatment is preferably 500°C or higher. The heating temperature is more preferably 550°C or higher, still more preferably 560°C or higher, and most preferably 570°C or higher. On the other hand, at a high heating temperature of the tempering treatment, crystal grains coarsen, the variation in the KAM value in crystal grains increases, and the standard deviation of the KAM value distribution increases. Moreover, the σ phase is generated, and the elongation after fracture and d2σ / dε2 under hydrogen charging decrease. Accordingly, the heating temperature of the tempering treatment is preferably 750°C or lower. The heating temperature is more preferably 720°C or lower, still more preferably 700°C or lower, and most preferably 680°C or lower.

[0084] In the present invention, subsequent to the tempering treatment, cold working may be optionally performed to correct defects in the steel pipe shape and adjust the yield strength through work hardening. The method of cold working may be, for example, cold drawing. If the reduction (%) in area of a cross section perpendicular to the pipe axis direction in the cold working exceeds 40%, work hardening increases, and thus the average KAM value increases. Accordingly, the reduction (%) in area in the cold working is preferably 40% or less. The reduction (%) in area is more preferably 35% or less, still more preferably 30% or less, and most preferably 25% or less. Although the lower limit is not particularly limited, the reduction (%) in area in the cold working is preferably 5% or more. Herein, the reduction (%) in area in the cold working is determined by ((cross-sectional area before working) - (cross-sectional area after working)) / (cross-sectional area before working) × 100.EXAMPLES

[0085] Hereinafter, the present invention will be further described in detail with reference to Examples. The present invention is not limited to the following Examples.

[0086] Molten steel having the chemical composition shown in Table 1 was obtained by smelting to produce a billet (steel pipe material).The obtained billet was heated, subjected to hot working, and cooled under the conditions shown in Table 2 to obtain a stainless steel pipe having the outer diameter (mm) and the wall thickness (mm) shown in Table 2.

[0087] Test pieces were taken from the obtained stainless steel pipe, and measurement of the KAM value, a tensile test, and a tensile test under cathodic hydrogen charging described below were performed. Note that the microstructure evaluation is conducted on the basis of the contents described in Description of Embodiments. 

[0088] [Measurement of KAM value]The average KAM value and the standard deviation of the KAM value distribution were measured by the SEM / EBSD method. The acceleration voltage was 15 kV. The measurement region (one field of view) was 400 µm × 400 µm, the measurement step size was 0.1 µm, and measured values in five fields of view were averaged. On the basis of the obtained EBSD data, a distribution image of the KAM value (KAM map) was obtained using crystal orientation analysis software OIM Analysis (trademark). Herein, the KAM value was determined by the following method. In each measurement point (regular hexagonal pixel), a misorientation between pixels was determined using up to three neighboring pixels (total 37 pixels) around the measurement point serving as the center. The average value of the determined misorientation was taken as the KAM value of the pixel at the center. This operation was performed for all pixels in the field of view to obtain a KAM map. From the obtained distribution of the KAM value, the average KAM value and the standard deviation of the KAM value distribution were determined by formula (1) and formula (2), respectively.

[0089] [Math. 3]Here, n is the number of measurement points in the field of view.

[0090] [Math. 4]Here, n is the number of measurement points in the field of view.

[0091] [Tensile test]A test piece was taken in accordance with ASTM E8 / E8M (2021). Specifically, an arc-shaped test piece was taken such that the tensile direction was parallel to the pipe axis direction. Regarding the size of the arc-shaped test piece, the thickness was the full wall thickness, the width was 25.4 mm, and the gauge length was 50.8 mm. A tensile test was performed in accordance with ASTM E8 / E8M (2021) using the taken test piece for tensile test. The obtained 0.2% offset proof stress (MPa) was taken as the yield strength (MPa).

[0092] [Tensile test under cathodic hydrogen charging]A round-bar test piece was taken from a central portion in the wall-thickness direction such that the tensile direction was parallel to the pipe axis direction. The diameter of a parallel portion was 3.8 mm, and the length of the parallel portion was 15 mm. In a solution at room temperature in which 3.0 g of ammonium thiocyanate was added per 1 L of a 3.0 mass% aqueous solution of sodium chloride, cathodic hydrogen charging was performed for the test piece at a current density of 0.05 mA / cm2 for 24 hours. Subsequently, a tensile test was conducted while cathodic hydrogen charging was continuously performed under the conditions described above. The strain rate in the tensile test was 2.0 × 10-5 s-1.The elongation after fracture (%) was determined as ((total length in state where test pieces after test are abutted against each other) - (total length of test piece before test)) / (total length of test piece before test) × 100.

[0093] dσ / dε and d2σ / dε2 were determined from time history data of the load and the cross-head displacement in the tensile test. First, σ and ε at each time were determined from the load and the cross-head displacement. Subsequently, in order to reduce the fluctuation of σ, at each time, σ was averaged in a range corresponding to ε = 0.001. Specifically, σ at certain time t(s) was determined as an average value of σ from time (t - 30)(s) to time (t + 30)(s). This was performed for all times. Subsequently, dσ / dε was determined at each time. Specifically, dσ / dε at certain time t(s) was determined as an average rate of change from time (t - 60)(s) to time t(s). This was performed for all times. Furthermore, d2σ / dε2 was determined at each time. Specifically, (d / dε)(dσ / dε) at certain time t(s) was determined as an average rate of change from time (t - 60)(s) to time t(s). This was performed for all times.

[0094] The obtained results are shown in Table 3.

[0095] In Table 3, stainless steel pipes of Nos. 1, 3, 5, 8, 10, 12, 15, 16, 17, 20, 26, and 27 are Inventive Examples, and stainless steel pipes of Nos. 2, 4, 6, 7, 9, 11, 13, 14, 18, 19, and 21 to 25 are Comparative Examples.

[0096] In the steel microstructure at the wall-thickness center of each of the stainless steel pipes of Inventive Examples, the total volume fraction of ferrite and martensite was 30% or more and 99% or less, the volume fraction of the σ phase was 0% or more and 3% or less, the balance was austenite, the average KAM value was 2.0° or more and 4.0° or less, and the standard deviation of the KAM value distribution was 1.5° or less. In addition, the yield strength was 450 MPa or more, the elongation after fracture was 9.0% or more, and the rate of change d2σ / dε2 of the work hardening rate dσ / dε with respect to ε was -200,000 MPa or more in the true strain range of 0.8εmax or more and εmax or less.

[0097] In contrast, the stainless steel pipe of No. 2 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 4 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 6 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 7 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 9 of Comparative Example had a low average KAM value, and consequently, the elongation after fracture and d2σ / dε2 required in the present invention were not achieved. The yield strength also tended to be low.The stainless steel pipe of No. 11 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 13 of Comparative Example had a large standard deviation of the KAM value distribution and a high volume fraction of the σ phase, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 14 of Comparative Example had a high average KAM value, and consequently, the elongation after fracture and d2σ / dε2 required in the present invention were not achieved.The stainless steel pipe of No. 18 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 19 of Comparative Example had a low average KAM value, and consequently, the elongation after fracture and d2σ / dε2 required in the present invention were not achieved.The stainless steel pipe of No. 21 of Comparative Example had a large standard deviation of the KAM value distribution, and consequently, the elongation after fracture required in the present invention was not achieved.The stainless steel pipe of No. 22 of Comparative Example had a high average KAM value, and consequently, the elongation after fracture and d2σ / dε2 required in the present invention were not achieved.The stainless steel pipe of No. 23 of Comparative Example had a high total volume fraction of ferrite and martensite, and consequently, the elongation after fracture and d2σ / dε2 required in the present invention were not achieved.The stainless steel pipe of No. 24 of Comparative Example had a low total volume fraction of ferrite and martensite, and consequently, the yield strength tended to be low.The stainless steel pipe of No. 25 of Comparative Example had a high volume fraction of the σ phase, and consequently, d2σ / dε2 required in the present invention was not achieved.

[0098]   [Table 1]NoChemical composition (mass%)CSiMnPSAlNCrMoNiCuVNbTiBWCaCoSnMgZrREMTaSb10.006 0.250.070.035 0.0085 0.012 0.015 11.541.895.61--------------20.006 0.250.070.035 0.0085 0.012 0.015 11.541.895.61--------------30.056 0.421.230.004 0.0007 0.012 0.044 17.803.153.770.04--0.261--0.0032-0.042----0.07240.056 0.421.230.004 0.0007 0.012 0.044 17.803.153.770.04--0.261--0.0032-0.042----0.07250.023 0.152.220.006 0.0077 0.034 0.367 25.694.706.781.660.125-----0.263---0.0100.06-60.023 0.152.220.006 0.0077 0.034 0.367 25.694.706.781.660.125-----0.263---0.0100.06-70.023 0.152.220.006 0.0077 0.034 0.367 25.694.706.781.660.125-----0.263---0.0100.06-80.040 0.240.410.006 0.0104 0.045 0.033 14.672.015.75-0.0400.231---0.0016--0.008----90.040 0.240.410.006 0.0104 0.045 0.033 14.672.015.75-0.0400.231---0.0016--0.008----100.019 0.935.790.014 0.0036 0.029 0.043 13.180.033.131.37-0.1100.129-2.73--------110.019 0.935.790.014 0.0036 0.029 0.043 13.180.033.131.37-0.1100.129-2.73--------120.015 0.310.180.018 0.0108 0.041 0.169 22.423.346.800.520.017--0.0022---------130.015 0.310.180.018 0.0108 0.041 0.169 22.423.346.800.520.017--0.0022---------140.015 0.310.180.018 0.0108 0.041 0.169 22.423.346.800.520.017--0.0022---------150.011 0.122.350.030 0.0054 0.010 0.029 15.113.284.220.630.057----0.0023-------160.023 0.330.340.008 0.0086 0.037 0.017 16.652.425.150.89-0.024-------0.011---170.043 0.133.100.027 0.0098 0.027 0.065 17.253.033.610.080.0350.0150.0320.00140.090.00100.0060.0230.0070.0110.0140.040.009180.043 0.133.100.027 0.0098 0.027 0.065 17.253.033.610.080.0350.0150.0320.00140.090.00100.0060.0230.0070.0110.0140.040.009190.043 0.133.100.027 0.0098 0.027 0.065 17.253.033.610.080.0350.0150.0320.00140.090.00100.0060.0230.0070.0110.0140.040.009200.026 0.341.420.011 0.0072 0.045 0.016 12.971.734.60--------------210.026 0.341.420.011 0.0072 0.045 0.016 12.971.734.60--------------220.026 0.341.420.011 0.0072 0.045 0.016 12.971.734.60--------------230.022 0.191.930.009 0.0012 0.040 0.051 16.415.062.612.420.0170.0110.040--0.0015-------240.016 0.266.210.007 0.0024 0.038 0.296 24.953.136.150.330.076---0.12--------250.034 0.352.420.010 0.0008 0.044 0.256 25.244.056.211.480.1030.0120.021--0.0031-------260.034 0.352.420.010 0.0008 0.044 0.256 25.244.056.211.480.1030.0120.021--0.0031-------270.034 0.352.420.010 0.0008 0.044 0.256 25.244.056.211.480.1030.0120.021--0.0031-------· The chemical composition other than the above is the balance being Fe and incidental impurities.

[0099]  [Table 2]NoHot pipe makingQuenchingTemperingCold workingStainless steel pipeHeating temperature(°C)Reduction in area(%)Average cooling rate in range of 650°C to 300°C(°C / s)Minimum cooling rate in range of 650°C to 300°C(°C / s)Heating temperature(°C)Average cooling rate in range of 650°C to 300°C(°C / s)Minimum cooling rate in range of 650°C to 300°C(°C / s)Heating temperature(°C)Reduction in area(%)Outer diameter(mm)Wall thickness(mm)111504532895035553061141321200223413950528520516827313203219131020221171028716413203284102062471028716512803541121120239-3451961280356221120587-3451971280352291170409-34519812006964993021962011762159120069691282049773011762151013406122109305111560913927111310614489303867609139271213203433111110405-028436131250346171110401-0284361412703459141110696-4128436151340704513960707630034616118056278930257570740640171160351812890418560304069181160352528904010570304069191160351413830401154015406920123027456940555610288712211240272689406147602887122212402742119503846604287122311904435129404375901040612241230302491100516-487925120051208106084-3251826120051971060279-32518271200512671040127-0518· "-" means "not performed".· In cold working, a reduction in area of 0% means that cold working was not performed.

[0100] [Table 3]NoSteel microstructure at wall-thickness center of stainless steel pipeMechanical properties of stainless steel pipeRemarksVolume fraction of F(%)Volume fraction of M(%)Total volume fraction of F and M(%)Volume fraction of σ phase(%)Volume fraction of A(%)Average KAM value(°)Standard deviation of KAM value distribution(°)Yield strength(MPa)Elongation after fracture under cathodic hydrogen charging(%)d2σ / dε2 under cathodic hydrogen charging(MPa)169298023.80.691415.8-164880Inventive Example299099013.21.99298.5-136004Comparative Example34441850152.91.370219.7-152762Inventive Example4454893072.51.86018.8-186559Comparative Example5360360642.61.088320.5-39763Inventive Example6510515442.21.87477.2-112499Comparative Example7670672312.91.68448.6-43689Comparative Example81370830173.31.178422.4-150240Inventive Example9276592081.91.44327.6-254104Comparative Example1069298023.40.886718.6-111664Inventive Example1189098023.61.78787.5-197882Comparative Example12590591402.40.973915.4-166744Inventive Example13320324642.91.76758.1-130512Comparative Example14400402584.21.08206.9-272598Comparative Example153356890113.10.873416.2-4409Inventive Example163647830173.41.382121.0-87546Inventive Example173848860143.51.29949.2-195635Inventive Example184048880123.31.99488.4-165239Comparative Example19673198021.81.17527.5-243861Comparative Example2079097033.91.49819.1-196399Inventive Example21118798023.51.79648.1-101758Comparative Example2249599014.31.29968.1-267090Comparative Example235644100003.71.18617.4-235548Comparative Example24110112872.10.841719.1-153495Comparative Example25670674293.41.391511.5-222593Comparative Example26540543433.21.290612.8-192367Inventive Example27380381612.30.948324.8-6778Inventive Example· The underlined values indicate that the values are outside the scope of the present invention or outside the target range.· F indicates ferrite, M indicates martensite, and A indicates austenite.  

Claims

 

1. A stainless steel pipe, wherein: in a steel microstructure at a wall-thickness center of the stainless steel pipe, a total volume fraction of ferrite and martensite is 30% or more and 99% or less, a volume fraction of a σ phase is 0% or more and 3% or less, and the balance is austenite, an average KAM value which is an average of a KAM (Kernel Average Misorientation) value of the steel microstructure is 2.0° or more and 4.0° or less, and a standard deviation of a distribution of the KAM value is 1.5° or less.  

2. The stainless steel pipe according to Claim 1, comprising a chemical composition containing, in mass%: C: 0.060% or less, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, and Ni: 15.00% or less, and further optionally containing one or two or more selected from Cu: 4.00% or less, V: 0.300% or less, Nb: 0.300% or less, Ti: 0.300% or less, B: 0.0050% or less, W: 3.00% or less, Ca: 0.0050% or less, Co: 0.500% or less, Sn: 0.100% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, Ta: 0.10% or less, and Sb: 0.100% or less, with the balance being Fe and incidental impurities.  

3. The stainless steel pipe according to Claim 1 or 2, wherein the stainless steel pipe is a seamless steel pipe.  

4. An oil well pipe comprising the stainless steel pipe according to any one of Claims 1 to 3.

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