High-nitrogen stainless-steel pipe with high strength high ductility, and excellent corrosion and heat resistance

Abstract

Nitrogen (N) absorption and diffusion treatments are performed for the inner and/or outer surfaces of austenite stainless steel pipe materials in N gas atmosphere at temperatures near 1,100 C. to obtain nitrided stainless steel pipe materials having 0.251.7% (mass) of solid solution nitrogen (N) including a gradient structure formed within the pipe wall in which the concentration of solid solution N continuously decreases gradually from the surface. The solid solution N present in the gradient structure promotes short range ordering (SRO) of substitutional alloying elements leading to homogenization of distribution of alloying elements in the austenite phase, generating an extremely high proof strength (yield strength) about 3 times as high as that of conventional austenite stainless steel pipe materials and enhancing characteristic of anti-hydrogen gas embrittlement (anti-HGE) so as to be suitable for use in a high pressure hydrogen tank utilized in hydrogen cell vehicle (FCV) and a liquid hydrogen tank.

Claims

1. A high nitrogen stainless steel pipe material with ductility, and a corrosion and heat resistance, comprising: an austenite steel having a chemical composition comprising Cr, Ni, Mo, C, and Fe, containing 0.25 to 1.7% (by mass) of solid solution nitrogen including a nitrogen concentration gradient structure throughout, and including an outside surface and/or an inside surface of a pipe in which a concentration of solid solution nitrogen continuously decreases gradually from the outside and/or inside surface, wherein said austenite steel including said gradient structure comprises a part that is the outside and/or inside surface and has a high concentration solid solution of nitrogen and a part in which ductility gradually increases toward around a center of a cross-section of the pipe as the nitrogen concentration decreases, and an extended dislocation generated in an austenite phase by the solid solution nitrogen present in said gradient structure is stabilized to enhance yield strength level of the austenite phase and ductility thereof so that the solid solution nitrogen contained in said gradient structure also still more enhances characteristic of anti-hydrogen gas embrittlement, that can be seen in high nickel austenite stainless steel, as a strong austenite stabilizer element of nitrogen itself.

2. The high nitrogen stainless steel pipe material with ductility, and a corrosion and heat resistance according to claim 1, wherein said pipe material comprises a kind of steel selected from the group consisting of austenitic stainless steel, ferritic stainless steel, and ferrite-austenite stainless steel.

3. A high nitrogen stainless steel pipe material with ductility, and a corrosion and heat resistance, comprising: the pipe according to claim 1, the pipe comprising a plurality of pipe members wherein the pipe members are disposed one over another and metallurgically bonded to obtain interfaces between nitrided surfaces of the pipe members and surfaces not undergoing nitriding of the pipe members leading to a united high nitrogen austenite steel pipe material.

4. A product formed of the high nitrogen austenitic stainless steel pipe material having high yield strength, leading to weight reducing and the ductility, and enhancing anti-hydrogen gas embrittlement through stabilization of the austenite phase ascribed to solid solution nitrogen present in said nitrogen-concentration gradient structure, according to claim 1, wherein said product is a high-pressure hydrogen gas container and a liquid hydrogen container which are for fuel cell vehicles or a stainless steel pipe or a hollow material.

5. A high nitrogen stainless steel pipe material with ductility, and a corrosion and heat resistance, comprising: an austenite steel having a chemical composition comprising Cr, Ni, Mo, C, and Fe (iron), and containing 0.25 to 1.7% (by mass) of solid solution nitrogen including a nitrogen-concentration gradient structure throughout including a surface layer, within a pipe wall in which a concentration of solid solution nitrogen continuously decreases gradually from a surface, wherein said high nitrogen stainless steel has austenite grains of an order of a few tens of m refined through a eutectoid transformation of an austenite or refined through a recrystallization of a work-hardened austenite, and wherein a synergistic effect of the solid solution nitrogen present in said gradient structure combined with crystal grains refined leads to austenite stainless steel pipe materials having a high yield strength and ductility originated from the refined and enhanced crystal grains included in said gradient structure, and extended dislocation containing stacking faults is stabilized to enhance yield strength as well as heat resistance due to the extended dislocation generated in an austenite phase.

6. The high nitrogen stainless steel pipe material with ductility, and a corrosion and a heat resistance according to claim 5, wherein said pipe material comprises a kind of steel selected from the group consisting of austenitic stainless steel, ferritic stainless steel, and ferrite-austenite stainless steel.

7. A high nitrogen stainless steel pipe material with ductility, and a corrosion and heat resistance, comprising: the pipe consisting of all austenite stainless steel pipe according to claim 5, the pipe comprising a plurality of pipe members, wherein the pipe members are disposed one over another and metallurgically bonded to obtain interfaces between nitrided surfaces of the pipe members and surfaces not undergoing nitriding of the pipe members leading to a united high nitrogen austenite stainless steel pipe material having a predetermined thickness, and having a high yield strength and ductility, with a multi-gradient structure.

8. A product formed of the high nitrogen austenitic stainless steel pipe material having high yield strength leading to weight reducing and ductility, and enhancing anti-hydrogen gas embrittlement through stabilization of the austenite phase ascribed to solid solution nitrogen present in said nitrogen-concentration gradient structure, according to claim 5, wherein said product is a high-pressure hydrogen gas container and a liquid hydrogen container which are for fuel cell vehicles or a stainless steel pipe or a hollow material.

Description

BRIEF EXPLANATION OF THE DRAWINGS

(1) FIG. 1 is illustrative of Vickers hardness (Hv) of cross-section of austenite stainless steel pipe of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) having undergone the treatments, as used in one specific example of the invention.

(2) .circle-solid. (sample b): 30-hr holding in N.sub.2 gas at 1075 C. (N absorption and diffusion into solid phase)

(3) (sample c): 30-hr holding in N.sub.2 gas at 1075 C.+24-hr holding in argon gas at 1075 C. (Annealing)

(4) (sample a): Commercial material.

(5) FIG. 2 is illustrative of temperature dependence of tensile strength (.sub.B) of austenite stainless steel pipe samples a and b of Fe-18Cr-12Ni-2.5Mo-0.06C (mass %), as used in one specific example of the invention.

(6) (sample b): 30-hr holding in N.sub.2 gas at 1075 C.+24-hr holding in argon gas at 1075 C.+grain refining

(7) (sample a): Commercial material.

BEST MODE FOR CARRYING OUT THE INVENTION

(8) Some embodiments of the invention are now explained. In one embodiment of the invention, nitrogen absorption and diffusion processing is applied to an austenite stainless steel pipe. In this processing, the austenite stainless steel pipe, surface-electropolished, (outside dia 26 mm, 3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) is treated in nitrogen gas at a temperature of 1070 to 1100 C., which is not higher than the critical temperature for crystal grain enlargement of the steel material, for 30 hr to cause nitrogen (N) to be absorbed into the surface of the steel pipe material and diffused into the solid phase, followed by annealing in argon gas at a temperature of 1070 to 1100 C. for 24 hr.

(9) The resulting stainless steel pipe material has a gradient structure which comprises a part that is close to the steel surface part and has been highly strengthened by the formation of a high concentration solid solution of N and a part in which ductility gradually increases toward around the center of the cross-section of the steel as the N concentration decreases, and the enlargement of crystal grain is minimized during the processing, thereby obtaining a high nitrogen austenite stainless steel pipe material with high strength and ductility.

(10) Furthermore, if grain refining (i.e., crystal grain size reducing) treatment utilizing, for instance, eutectoid transformation of the austenite is applied to the austenite stainless steel pipe material having undergone both the nitrogen absorption-diffusion and the annealing processing, it is then possible to manufacture much more improved steel pipe material, because there is obtained an extremely fine austenite with crystal grains of the order of 10 to 30 m leading to a marked elongation of the pipe material.

(11) Synergistic effects of the solid solution strengthening originally coming from the gradient structure formed by nitrogen and the enhanced crystal grain refinement are combined with the ductility (elongation) inherent in the austenite phase to make it easy to manufacture a highly strong and ductile austenite stainless steel pipe material.

(12) Moreover, the resulting steel pipe material is more strengthened by plastic working such as drawing, rolling, extrusion or the like.

(13) It is thus possible to achieve effective manufacture of high-nitrogen-concentration austenite stainless steel pipe materials having the gradient structure as mentioned above.

(14) In yet another embodiment of the invention, adhesion processing is applied to a plurality of austenite stainless steel pipes with the same chemical composition but different dimensions having undergone said treatments of nitrogen absorption-diffusion, annealing, and grain refining as described above using a hot drawing, hot rolling or other methods at a temperature of 1050 to 1100 C. in H.sub.2 gas atmosphere.

(15) The resultant united austenite stainless steel pipe or hollow material has dimensions, e.g., diameter and wall thickness according to the use or strength level required and has repetitions of the gradient structure, as mentioned above, within the pipe wall, existence of such gradient structures in the united stainless steel material, making it easy to adjust pertinently the mechanical properties thereof.

(16) It is also thus possible to achieve more effective manufacture of the united austenite stainless steel pipe or hollow material with particularly large dimensions and high strength level that cannot be manufactured by using single stainless steel pipe material alone.

EXAMPLES

(17) Examples of the invention are now explained with reference to the accompanying drawings.

Example 1

(18) Set out in Table 1 are the mean crystal grain diameter D, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of stainless steel pipe samples (26 mm outside dia150 mm long3 mm thick) of (a) Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) and (b) Fe-20Cr-8Ni-0.03C (mass %) obtained by applying nitrogen (N) absorption and solid diffusion treatment to their surface-electropolished samples in 0.1 MPa N.sub.2 gas at both the temperatures of 1075 C. and 1200 C. for 30 hr, followed by annealing in argon gas at the same temperatures as those of N absorption and diffusion treatment for 24 hr.

(19) TABLE-US-00001 TABLE 1 Mean crysral grain diameter D, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of stainless steal pipe samples (26 mm outside dia 150 mm long 3 mm thick) of (a) Fe18Cr12Ni2.5Mo0.02C (mass %) and (b) Fe20Cr8Ni0.03C (mass %) obtained by applying nitrogen (N) absorption and solid diffusion treatment to their surface-elec- tropolished samples in 0.1 MPa N.sub.2 gas at both the temperatures of 1075 C. and 1200 C. for 30 hr, followed by annealing in argon gas at the same temperatures as those of N absorption and diffusion treatment for 24. D .sub.0.2 .sub.B Temperature C. m MPa MPa % a 1075 115 510 850 50.0 1200 562 593 872 5.7 (Commercial material) 100 265 549 68.0 b 1075 107 525 890 45.0 1200 493 624 908 4.6 (Commercial material) 95 285 598 66.4 Value of D: microscopically determined. Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

(20) From a comparison of the results obtained at 1075 C. of both the sample (a) and the sample (b) with those obtained at 1200 C., it has been found that according to the invention although the values of offset yield strength .sub.0.2 and tensile strength .sub.B of the samples processed at 1075 C. and 1200 C. are remarkably increased, the values of elongation of these samples processed at 1200 C. are extremely decreased.

Example 2

(21) Set out in Table 2 are the mean crystal grain diameter D, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of stainless steel pipe sample (26 mm outside dia150 mm long3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) obtained by applying nitrogen absorption and solid diffusion treatment to the surface-electropolished sample in 0.1 MPa NH.sub.3 gas at a temperature of 520 C. for 60 h, followed by annealing in argon gas at a temperature of 1075 C. for 24 h.

(22) TABLE-US-00002 TABLE 2 Mean crystal grain diameter D, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of stainless steel pipe sample (26 mm outside dia 150 mm long 3 mm thick) of Fe18Cr12Ni2.5Mo0.02C (mass %) obtained by applying nitrogen absorption and solid diffusion treatment to the surface- electropolished sample in 0.1 MPa NH.sub.3 gas at a temperature of 520 C. for 60 h, followed by annealing in argon gas at a temperature of 1075 C. for 24 h. D .sub.0.2 .sub.B m MPa MPa % N absorption .Math. solid diffusion: 107 520 865 40.3 520 C. 60 hr in NH.sub.3 gas annealing: 1075 C. 24 hr in argon gas (Commercial material) 100 265 549 68.0 Value of D: microscopically determined. Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

Example 3

(23) The stainless steel pipe sample surface-electropolished (26 mm outside dia150 mm long3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %), was processed by 30-hr holding in N.sub.2 gas at 1075 C. [N absorption-Diffusion] (sample a), followed by 24-hr annealing in argon gas [Annealing] (sample b) and 20%* cold drawing [Drawing] (sample d)
*[(D.sub.0D.sub.x)/D.sub.0]100=20(%)

(24) where D.sub.0 and D.sub.x are outside diameter of pipe before and after cold drawing, respectively.

(25) The offset yield strength .sub.0.2, tensile strength .sub.B, elongation , and mean crystal grain diameter D microscopically determined are shown in table 3.

(26) TABLE-US-00003 TABLE 3 The offset yield strength .sub.0.2, tensile strength .sub.B, elongation , and mean crystal grain diameter D of said sample a, sample b, and sample d described in Example 3 of Fe18Cr12Ni2.5Mo0.02C (mass %) pipe material. a b c d .sub.0.2 MPa 502 510 265 769 .sub.B MPa 685 850 549 1016 % 28.3 50.0 68.0 42.0 c: commercial material test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

Example 4

(27) FIG. 1 is illustrative of Vickers hardness (Hv) of cross-section of austenite stainless pipe (outside dia 26 mm150 mm long3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) having undergone the treatments. .circle-solid. (sample b): 30-hr holding in N.sub.2 gas at 1075 C. (N absorption and diffusion into solid phase) (sample c): 30-hr holding in N.sub.2 gas at 1075 C.+24-hr holding in argon gas at 1075 C. (Annealing) (sample a): Commercial material.
Hv value/N (mass %) of the surface part of sample (b) and (c) were about 550/0.9 and 400/0.7, respectively.

(28) From FIG. 1, it has been seen that although Vickers hardness Hv of the surface part of the N absorption processed sample (b) is markedly increased due to the formation of solid solution of N compared to the center part remaining a little increase in Hv, in annealing processed sample (c), diffusion of N is promoted so that there appeared a considerable N solid solution hardening even in the center part, the gradient of Hv vs D curve becoming gentle.

(29) It has been here noted that within the pipe wall in the sample, e.g. C, is formed a gradient structure which comprise a part that is close to the surface part of the pipe and has been highly strengthened by the formation of a high-concentration solid solution of N and a part in which ductility gradually increases toward around the center of the cross-section of the pipe as the N concentration decreases.

Example 5

(30) The stainless steel pipe sample surface-electropolished (outside dia 26 mm150 mm long3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) was processed by 30-hr holding in N.sub.2 gas at 1075 C. [N absorption and Diffusion] and 24-hr annealing in argon gas at 1075 C. [Annealing](sample a) followed by crystal grain refining (1) (sample b) and crystal grain refining (2) (sample c) described below.

(31) The offset yield strength .sub.0.2, tensile strength .sub.B, elongation , and mean crystal grain diameter D are shown in Table 4. The values of D were microscopically determined.

(32) TABLE-US-00004 TABLE 4 Mean crystal grain diameter D, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of said samples a, b, and c described in explanation of Example 5 of Fe18Cr12Ni2.5Mo0.02C (mass %) pipe material. D .sub.0.2 .sub.B Sample m MPa MPa % a 115 510 850 50.0 b 30 716 1004 65.0 c 34 692 973 59.7 Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.
Crystal Grain Refining (Treatment) (1)

(33) N absorptionDiffusion and Annealing processed said sample (a) was grain refined by the following processes [1] to [7]:

(34) [1] heating to 1200 C. (austenitizing)

(35) [2] 5 to 6-minutes holding at 1200 C.

(36) [3] air cooling (decomposition of austenite () into fine ferrite () and nitride (Cr.sub.2N))

(37) [4] reheating to 1200 C. (formation fine of austenite ())

(38) [5] 3 to 4-minutes holding at 1200 C.

(39) [6] rapid cooling (water quenching) to room temperature

(40) [7] fine austenite sample (b)

(41) More fine austenite sample (b) is obtained by making use of 2 repetitions of a series of the above procedures [1] to [7]

(42) i.e., crystal grain refining treatment (1) is based on the eutectoid transformation of austenite as shown in the following equation

(43) ##STR00001##
Crystal Grain Refining (Treatment) (2)
N absorption-Diffusion and Annealing processed said sample (a) was grain-refined by the following procedures [1] to [4]:

(44) [1] 50% drawing* near 200 to 250 C., i.e., below recrystallization temperature, (work hardening)

(45) [2] heating to 1200 C. (formation of fine austenite) and 3 to 4-minutes holding at the same temperature

(46) [3] rapid cooling (water cooling)

(47) [4] fine austenite sample (c)
*[(D.sub.0D.sub.x)/D.sub.0]100=50(%)

(48) where D.sub.0 and D.sub.x are outside diameter of pipe before and after drawing, respectively.

(49) The stainless steel pipe sample surface-electropolished (26 mm outside dia150 mm long3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) was processed by 30-hr holding in N.sub.2 gas at 1130 C., which is in a range of temperatures exceeding the critical temperature for crystal grain enlargement, and 24-hr annealing in argon gas at said temperature (sample a), followed by grain refining utilizing the grain refining treatment (1) stated in the example 5 (sample b).

(50) The crystal grain diameter D microscopically determined, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation are shown in Table 5.

(51) TABLE-US-00005 TABLE 5 Mean crystal grain diameter D, offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of said samples a and b described in the present Example 6 of Fe18Cr12Ni2.5Mo0.02C (mass %) pipe material. D .sub.0.2 .sub.B sample m MPa MPa % a 250 569 860 15.0 b 65 755 942 45.5 Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

Example 7

(52) The ferritic stainless steel pipe samples surface-electropolished (outside dia 26 mm20 mm long4.0 mm thick) of (a) Fe-18.0Cr-0.07C (mass %) pipe and (b) Fe-18.0Cr-0.06C-0.07Al (mass %) pipe whose cuter surface was shielded from ambient atmosphere by welding both edges of the pipe covered with 0.2 mm thick nickel pipe, were processed by 30-hr holding in 0.65 MPa N.sub.2 gas at 1075 C., followed by cooling in 0.65 MPa N.sub.2 gas and 24-hr annealing in 0.1 MPa argon gas.

(53) The mean crystal grain diameter D and the resulting structure of the outer surface part and the inner surface part are described in Table 6.

(54) TABLE-US-00006 TABLE 6 Mean crystal grain diameter D and structure of outer surface part and inner surface part of ferritic stainless steel pipe samples of the dimension of outside diameter 26 mm, length 20 mm, and thickness 4.0 mm of said pipe (a) Fe18.0Cr0.07C (mass %) and (b) Fe18.0Cr0.06C0.07Al (mass %) processed by 30-hr holding in 0.65 MPa N.sub.2 gas at 1075 C., followed by cooling in 0.65 MPa N.sub.2 gas and 24-hr annealing in 0.1 MPa argon gas at 1075 C. D m Al inner surface part* Outer surface part* sample mass % <structure> <structure> a 0.001 453 550 <austenite> <austenite> b 0.070 112 133 <austenite> <ferrite> *Distance from surface: 0.20 mm
The mean crystal grain diameter of the patent samples of (a) and (b) without the treatment of N absorption-diffusion and Annealing were 100 m and 90 m, respectively.

Example 8

(55) FIG. 2 is illustrative of temperature dependence of tensile strength (.sub.B) of austenite stainless steel pipe samples (outside dia 26 mm150 mm long3 mm thick) (a) and (b) of Fe-18Cr-12Ni-2.5Mo-0.06C (mass %). (sample b): 30-hr holding in N.sub.2 gas at 1075 C.+24-hr holding in argon gas at 1075 C.+grain refining (sample a): Commercial material.

(56) From Example 8, FIG. 2, it has been found that the tensile strength .sub.B of the N absorption-diffusion and grain refining processed sample (b) is strikingly increased to compared to that of sample (a) in a wide range of temperature due to the synergistic effects of N solid solution strengthening and reduction of crystal grain.

(57) It has also been found that according to the invention in the sample (b), nitrogen (N) contained in the gradient structure, whose N concentration range is wide, decreases stacking fault energy (SFE) of the austenite phase thereof, so that the extended dislocation containing stacking fault (SF) is stabilized to higher temperature side and the softening temperature rises about 100 C.

Example 9

(58) The austenite stainless steel pipe samples surface electropolished (22 mm outside dia130 mm long3 mm thick) of

(59) (a) Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) (SUS316L), (b) Fe-20Cr-8Ni-0.03C (mass %) (SUS304L), and (c) Fe-25Cr-20Ni-0.06C (mass %) (SUS310S) were processed by 30-hr holding in N.sub.2 gas at 1075 C. [N absorption and Diffusion], followed by 24-hr annealing in argon gas at 1075 C.
[Annealing], then grain refining using grain refining treatment (1) described in the example 5, and finally 20%* drawing
*[(D.sub.0D.sub.x)/D.sub.0]100=20(%)
where D.sub.0=outside diameter of pipe before drawing

(60) D.sub.x=outside diameter of pipe after drawing

(61) The offset yield strength .sub.0.2, tensile strength .sub.e, and elongation of the samples (a), (b), and (c) are shown in Table 7. Here the samples (A), (B), and (C) are the parent materials (commercial materials) of the samples (a), (b), and (c), respectively.

(62) TABLE-US-00007 TABLE 7 Offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of said samples (a) and (A) of Fe18Cr12Ni2.5Mo0.02C (mass %), said samples (b) and (B) of Fe20Cr8Ni0.03C (mass %), and said samples (c) and (C) of Fe25Cr20Ni0.06C (mass %) described in explanation of Example 9. .sub.0.2 .sub.B sample MPa MPa % a 794 1150 50.0 A 265 549 68.0 b 850 1206 46.2 B 285 598 66.4 c 948 1300 37.0 C 312 656 43.0 Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

(63) From Example 9, Table 7, it has been noted that according to the invention, the values of offset yield strength .sub.0.2 and tensile strength .sub.B of the austenite stainless steel pipe samples, having undergone the treatments of absorption-diffusion, annealing, grain refining, and slight plastic working as mentioned in said explanation, are around 3 times and 2 times as high as those of commercial materials, respectively, in addition to the values of elongation compared favorably with those of commercial ones.

Example 10

(64) Inner surface of pipe (sample (a), outer surface of pipe (sample (b)), and both surface of pipe (sample (c)) of stainless steel of Fe-18Cr-12Ni-3.5Mo-0.02C (mass %) were processed by 30-hr holding in N.sub.2 gas atmosphere at 1075 C., followed by 24-hr annealing in argon gas atmosphere at 1075 C., then crystal grain refining using grain refining treatment (2) described in Example 5, and finally 20%* cold drawing as shown in Example 9. Here atmosphere of outer surface side of sample (a) and inner surface side (i.e. in pipe) of example (b) were filled with argon gas.

(65) The offset yield strength .sub.0.2, tensile strength .sub.B, and elongation are shown in Table 8.

(66) TABLE-US-00008 TABLE 8 Offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of said sample a, b, and, c described in explanation of Example 10. .sub.0.2 .sub.B sample MPa MPa % a 669 1004 49.0 b 690 1020 48.0 c 775 1095 46.3 Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

Example 11

(67) Set out in Table 9 are the offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of the stainless steel pipe sample (), (.sup.ND), () and () manufactured from a plurality of nitrogen absorption-diffusion and annealing treated stainless steel pipes (Fe-18Cr-12Ni-3.5Mo-0.02C, mass %) using the following process (as stated in <7> above).

(68) The processes of manufacture of said samples (), (.sup.ND), () and () are as follows.

(69) Sample ():

(70) pipe sample A (37.sup.OD250.sup.L3t, mm, i.e., 37 mm in outside dia by 250 mm long by 3 mm thick), pipe sample B (47.sup.OD250.sup.L3.sup.t, mm), and pipe sample C (57.sup.OD250.sup.L3.sup.t, mm) were treated by 30-hr holding in N.sub.2 gas at 1075 C. and 24-hr annealing in argon gas at 1075 C., followed by practicing 2 repetitions of grain-refining treatment (1) described in Example 5 and disposing a plurality of said A, B, and C one over another, then adhesion-processing through 15% hot drawing in H.sub.2 gas at 1075 C. to unite said plurality of the pipe A, B, and C, and finally 20% cold drawing to result in higher strength level.

(71) [M2] The manufacturing process is summarized as follows.

(72) Manufacturing process:

(73) $N\mathrm{Absorption}\text{-}\mathrm{Diffusion}.fwdarw..fwdarw.\mathrm{Annealing}.fwdarw.\mathrm{Grain}\mathrm{refining}.fwdarw..fwdarw.\left\{\begin{array}{c}\mathrm{Disposing}\mathrm{one}\mathrm{over}\mathrm{another}-\\ \mathrm{Adhesion}\mathrm{processing}\\ \left(\mathrm{Hot}\mathrm{drawing}\right)\end{array}\right\}$$\mathrm{Cold}\mathrm{drawing}\mathrm{.Math.}\mathrm{pipe}\mathrm{sample}\left(\right).fwdarw.\left\{\begin{array}{c}\mathrm{Without}\mathrm{cold}\\ \mathrm{drawing}\end{array}\right\}\mathrm{.Math.}\mathrm{pipe}\mathrm{sample}\left({}^{\mathrm{ND}}\right)$
Sample ():

(74) Pipe sample A (37.sup.OD250.sup.L3.sup.t, mm), whose inner surface was shielded from ambient atmosphere by welding both edges of the pipe covered with 0.2 mm thick Ni pipe, pipe sample B (47.sup.OD250.sup.L3.sup.t, mm), and pipe sample C (57.sup.OD250.sup.L3.sup.t, mm), surface-electropolished were treated by 30-hr holding in N.sub.2 gas at 1075 C. and 24-hr annealing in argon gas at 1075 C., followed by disposing a plurality of said samples A, B, and C one over another, then adhesion-processing through 15% hot drawing in H.sub.2 gas at 1075 C. to unite the plurality of said pipes A, B, and C, and finally 20% cold drawing to result in higher strength level.

(75) [M3]

(76) Manufacturing Process:

(77) $N\mathrm{Absorption}\text{-}\mathrm{Diffusion}\text{.Math.}\left(\begin{array}{c}\mathrm{all}\mathrm{the}\mathrm{surfaces}\mathrm{of}\mathrm{samples}A,B,\mathrm{and}C,\\ \mathrm{except}\mathrm{for}\mathrm{inner}\mathrm{surface}\mathrm{of}\mathrm{sample}A)\end{array}\right).fwdarw..fwdarw.\mathrm{Annealing}.fwdarw.\left\{\begin{array}{c}\mathrm{Disposing}\mathrm{one}\mathrm{over}\mathrm{another}-\\ \mathrm{Adhesion}\mathrm{processing}\\ \left(\mathrm{Hot}\mathrm{drawing}\right)\end{array}\right\}.fwdarw..fwdarw.\mathrm{Cold}\mathrm{drawing}\mathrm{.Math.}\mathrm{pipe}\mathrm{sample}\left(\right)$
Sample ():

(78) Each of 2 austenite stainless steel pipes, surfaced-polished, with 0.7 mm thickness as adhesion materials (having the name quality with SUS316L stainless steel pipe) without N absorption-Diffusion and Annealing treatment was sandwiched between pipe samples A and B, and between pipe samples B and C, respectively.

(79) Thus obtained adhesion material-sandwiched steel pipe were treated as in the case of the sample (a)

(80) [M4]

(81) Manufacturing Process:

(82) $Na\mathrm{bsorption}\text{-}\mathrm{Diffusion}.fwdarw.\mathrm{Annealing}.fwdarw.\mathrm{Grain}\mathrm{refining}.fwdarw.\text{.Math.}\left\{\begin{array}{c}\mathrm{Adhesion}\mathrm{material}\mathrm{sandwiching}\\ \left(\mathrm{Between}A/B\mathrm{and}B/C\right)\end{array}\right\}.fwdarw.\left\{\begin{array}{c}\mathrm{Disposing}\mathrm{one}\mathrm{over}\mathrm{another}-\\ \mathrm{Adhesion}\mathrm{processing}\\ \left(\mathrm{Hot}\mathrm{drawing}\right)\end{array}\right\}.fwdarw.\mathrm{Cold}\mathrm{drawing}\mathrm{.Math.}\mathrm{pipe}\mathrm{sample}\left(\right)$

(83) TABLE-US-00009 TABLE 9 Offset yield strength .sub.0.2, tensile strength .sub.B, and elongation of said sample , .sup.ND , and described in the present Example 11 of Fe18Cr12Ni3.5Mo0.02C (mass %) pipe material sample .sup.ND .sub.0.2 MPa 787 720 725 760 .sub.B MPa 1110 995 945 1077 % 52.4 62.0 43.0 55.0 Test pieces: as cut from tublar material/gauge length 50 mm/gripped ends inserted with metal plugs.

(84) From Example 11, Table 9, it has been found that according to the invention, the values of offset yield strength .sub.0.2 of each of the samples , .sup.ND, , and , which are basically formed from N absorption-diffusion and annealing processed steel pipe materials, of Fe-18Cr-12Ni-3.5Mo-0.02C (mass %) are about 3 times as high as that of the commercial pipe material in addition to the values of elongation of these samples compared favorably with that of the commercial one.

(85) Furthermore, it has been noted that the elongation of the sample .sup.ND without final cold drawing processing is as good as the commercial pipe material in spite of the high strength similar to those of other samples , and .

(86) Mechanics of materials teaches that when fluid like gas of pressure (P) is filled in cylindrical vessel with the inside dia (D) and the wall thickness (t), the maximum stress acting along the circumference direction ((e) so as to tear up the vessel is given by the following equation, in the case where the value of t is within 10% lower than that of D.
.sub.=(PD)/2t[M5].
According to eq [M5], since the parameters P, D, and t are kept in equilibrium with one another, when the values of P and D are constant, the value of thickness of the cylindrical vessel t decreases in inverse proportion to the value of .sub.e, i.e., the strength level of the vessel itself; the required thickness t of the vessel decreases to around compared to that of the vessel manufactured from conventional pipe material.

POSSIBLE APPLICATIONS OF THE INVENTION TO THE INDUSTRY

(87) The high-nitrogen austenite stainless steel pipe and hollow materials formed therefrom obtained herein are now explained with reference to what purposes they are used for.

(88) High-Nitrogen Austenite Stainless Steel Pipe

(89) High-nitrogen austenite steel pipe materials have common properties as stated below. They have high strength and ductility, and show excellent pitting corrosion resistance, crevice corrosion resistance and non-magnetism as well. Furthermore, they do not undergo sharp softening from the temperature of near 200 to 300 C. on temperature rises, which is usually experienced with steel materials of the martensite or ferrite type, and they are less susceptible to low-temperature brittleness at a temperature at or lower than room temperature.

(90) Another important feature of noteworthiness is that one exemplary high-nitrogen austenite stainless steel pipe material of SUS 316L type of the invention which has undergone the nitrogen (N) absorption-diffusion and the grain refining processing has an offset yield strength about three times as high as that of the SUS 316L stainless steel pipe material, in addition to the value of elongation compared favorably with those of commercial materials.

(91) According to the invention as the N absorption-diffusion processing is applied to austenite stainless steel pipe material, there is obtained a gradient structure whose N concentration range is wide. The N contained in the gradient structure decreases stacking fault energy (SFE) of the austenite phase in the pipe, so that the extended dislocation containing stacking fault is stabilized to enhance the strength level as well as heat resistance thereof.

(92) Furthermore, as adhesion processing is applied to a plurality of austenite stainless steel pipes which have undergone the N absorption-diffusion and the grain refining processing, a united austenite stainless steel pipe with dimensions according to the use or strength level required can easily be produced, which cannot be realized to produce by using single such austenite pipe alone.

(93) In addition, in high nitrogen austenite stainless steel of the invention, characteristic of anti-HGE (anti-hydrogen gas embrittlement) that can be seen in high nickel austenite stainless steel such as SUS 316L or SUS 310S austenite stainless steel unlike many other metallic materials is still more enhanced by the solid solution nitrogen, contained in said gradient structure, that promotes short range ordering (SRO) leading to homogenization of distribution of alloying elements in the austenite phase through increase in concentration of free electron providing more metallic character of interatomic bonds.

(94) Thus, the high-nitrogen austenite stainless steel pipe materials of the invention, because of having such features as mentioned above, can suitably find a wide spectrum of applications inclusive of high strength and ductility materials as follows:

(95) Compressed hydrogen gas storage tank materials for fuel cell vehicle (FCV), Pipe arrangement materials for steam power generation, petrochemistry etc. and Sea related machinery and tools materials.