Super austenitic stainless steel pipe and preparation method thereof

Abstract

The present invention discloses a super austenitic stainless steel pipe and a preparation method thereof. The chemical composition of the super austenitic stainless steel pipe, measured by weight percentage, is as follows: C: 0.03-0.05%; Cr: 20-21%; Mn: 1.2-1.4%; Nb: 0.6-0.7%; Ni: 10-11%; Cu: 2-2.5%; Mo: 0.2-0.3%; V: 0.1-0.2%; Ti: 0.1-0.2%; Co: 0.15-0.2%; N: 0.07-0.09%; B: 0.002-0.008%; Al: ≤0.01%; Si: ≤0.3%; P: ≤0.03%; the balance being Fe and unavoidable impurity elements. The present invention can improve the intergranular corrosion resistance of the super austenitic stainless steel pipe.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and in particular to a super austenitic stainless steel pipe and a preparation method thereof. Background Art

[0002] Supercritical and ultra-supercritical thermal power units offer significant energy savings and environmental benefits, and are a future trend in thermal power generation. Because the steel used in these units is exposed to high temperatures, high pressures, and high humidity for extended periods, ultra-supercritical boiler steels place stringent performance requirements on the unit. These steels must possess not only excellent high-temperature mechanical properties, such as high-temperature endurance strength, creep resistance, and thermal fatigue performance, but also good high-temperature corrosion resistance.

[0003] The steels used earlier in the industry for ultra-supercritical boiler units are austenitic stainless steel, ferrite and martensitic steel. Because austenitic stainless steel is non-magnetic and has high toughness and plasticity, as well as good corrosion resistance, it has attracted widespread attention from researchers. At present, the austenitic stainless steel pipes produced by existing common technologies have the problem of insufficient resistance to intergranular corrosion after sensitization, resulting in a short service life of the steel pipes in the operating environment of ultra-supercritical boiler units, which seriously affects the use of the boiler. Summary of the Invention

[0004] The main purpose of the present invention is to provide a super austenitic stainless steel pipe and a preparation method thereof, so as to solve the problem that the austenitic stainless steel pipe prepared by the prior art has insufficient intergranular corrosion resistance after sensitization.

[0005] According to one aspect of the present invention, a super austenitic stainless steel pipe is provided. The chemical composition of the super austenitic stainless steel pipe is as follows, by weight percentage: C: 0.03-0.05%; Cr: 20-21%; Mn: 1.2-1.4%, Nb: 0.6-0.7%; Ni: 10-11%; Cu: 2-2.5%; Mo: 0.2-0.3%; V: 0.1-0.2%; Ti: 0.1-0.2%; Co: 0.15-0.2%; N: 0.07-0.09%; B: 0.002-0.008%; Al: ≤0.01%; Si: ≤0.3%; P: ≤0.03%; the balance being Fe and unavoidable impurity elements.

[0006] According to one embodiment of the present invention, at a sensitization temperature of 675°C, the content of σ phase in the super austenitic stainless steel tube is 0-0.5%, M 23 The content of C6 carbide phase is 1.0-1.2%.

[0007] According to one embodiment of the present invention, within the temperature range of 600-1500°C, the maximum content of σ phase in the super austenitic stainless steel pipe is 2.5-2.9%, M 23 The maximum content of C6 carbide phase is 1.21-1.24%.

[0008] According to another aspect of the present invention, a method for preparing a super austenitic stainless steel pipe according to any of the above embodiments is provided, comprising: sequentially extruding, quenching, cold rolling in a first pass, heating and softening, cold rolling in a second pass, and solution treatment on a tube blank to obtain the super austenitic stainless steel pipe.

[0009] According to one embodiment of the present invention, during cold rolling, a feed amount of 2 to 3 mm and a rolling speed of 50 to 80 times / min are first selected, and then the feed amount and rolling speed are gradually increased.

[0010] The technical solution of the present invention can improve the intergranular corrosion resistance of super austenitic stainless steel pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0012] Figure 1 A phase diagram of a super austenitic stainless steel pipe according to an embodiment of the present invention is shown;

[0013] Figure 2 Another phase diagram of a super austenitic stainless steel pipe according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0014] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0015] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. Subsequent embodiments will not explain this one by one.

[0016] The present invention provides a super austenitic stainless steel pipe. The chemical composition of the super austenitic stainless steel pipe is as follows, by weight percentage: C: 0.03-0.05%; Cr: 20-21%; Mn: 1.2-1.4%, Nb: 0.6-0.7%; Ni: 10-11%; Cu: 2-2.5%; Mo: 0.2-0.3%; V: 0.1-0.2%; Ti: 0.1-0.2%; Co: 0.15-0.2%; N: 0.07-0.09%; B: 0.002-0.008%; Al: ≤0.01%; Si: ≤0.3%; P: ≤0.03%; the balance being Fe and unavoidable impurity elements.

[0017] The inventors of this application realized that the chemical composition of steel determines the properties of steel. According to the "chromium-poor theory", the main reason affecting the corrosion performance of austenitic stainless steel is the combination of carbon and chromium at the grain boundaries to form Cr 23 C6 is precipitated. Since the radius of chromium atoms is larger than that of carbon atoms, the diffusion rate inside the grains is slower and there is no time to diffuse to the grain boundaries. Therefore, while forming chromium carbides, a chromium-poor zone is generated at the grain boundaries, leading to the occurrence of intergranular corrosion. In addition, a σ phase containing Fe-Cr-Mo intermetallic phase is produced in almost all high-alloy stainless steels. It has a great influence on corrosion resistance and mechanical properties. Super austenitic stainless steel has a high Cr and Mo content, so the precipitation rate of the σ phase is generally faster, and the mesh volume is also relatively large. In addition, the dissolution temperature of the σ phase in austenitic stainless steel is relatively high, making it difficult for the σ phase to fully dissolve in the austenite during the solid solution process. Since the σ phase is rich in Cr and Mo elements, it will cause Cr-poor Mo zones to form in the surrounding area, which will eventually reduce the corrosion resistance of the alloy. Therefore, Cr can be avoided by controlling the composition and process. 23 The key to preventing intergranular corrosion of stainless steel is the large-scale precipitation of C6 carbides and σ phase to form a Cr-poor zone.

[0018] The present invention reduces Cr in steel by designing a reasonable chemical composition range. 23 The precipitation of C6 phase and σ phase. Through thermodynamic calculation, it is calculated that Figure 1 and Figure 2 The results of the property diagram shown are referenced Figure 1 The M23C6 curve (indicates M 23 C6 carbide phase content change) and Figure 2 The SIGMA curve (indicating the change in σ phase content) is obtained from the calculation of the chemical composition range according to the present invention. At 675°C, i.e., the sensitization temperature, the σ phase content in the super austenitic stainless steel is 0-0.5%, M 23 The content of C6 carbide phase is 1.0~1.2%; in the calculation temperature range of 600℃~1500℃, the maximum content of σ phase is 2.5~2.9%, M23 The maximum content of the C6 carbide phase is 1.21-1.24%, which is at a relatively low level, thereby achieving the purpose of improving the intergranular corrosion resistance of the alloy while also having good plasticity and toughness.

[0019] The present invention also provides a method for preparing a super austenitic stainless steel pipe according to any of the above embodiments, comprising: sequentially extruding, quenching, first-pass cold rolling, heating and softening, second-pass cold rolling, and solution treatment on a tube blank to obtain the super austenitic stainless steel pipe.

[0020] In some embodiments, the production process of the austenitic stainless steel pipe of the present invention is as follows: tube blank → heating → horizontal extruder extrusion (obtaining extruded rough tube) → quenching (heat treatment) → straightening → pickling → grinding → lubrication → first pass cold rolling (obtaining semi-finished steel pipe) → high temperature softening treatment → pickling → grinding → shot blasting → pickling → lubrication → second pass cold rolling → pickling → heat treatment (finished product solution treatment) → pickling → flaw detection → sampling and testing → finishing inspection → packaging and warehousing.

[0021] Among them, the surface of the tube blank should be of good quality and free of cracks visible to the naked eye. After pickling and grinding, all visible defects on the inner and outer surfaces of the extruded raw tube must be removed before it can flow into the subsequent cold rolling process.

[0022] After high-temperature softening and pickling, the semi-finished steel pipes are inspected and ground, and defects visible to the naked eye are removed with a fine grinding wheel. After the semi-finished products are ground, they must be shot blasted as a whole, then pickled and cleaned, and then cold rolled.

[0023] The cold rolling process selects matching tooling according to the rolling specifications. When rolling the finished product, based on the material characteristics, a small feed rate (2-3mm) and low speed rolling of 50-80 times / min are first selected. After the rolling is normal, the feed rate and rolling speed are gradually increased to ensure the surface quality and dimensional tolerance accuracy of the finished pipe. The dimensional tolerance control of the finished product rolling needs to take into account the allowance for subsequent grinding and polishing of the finished product. During the rolling process, the surface of the plug is checked every 3-5 rolls to prevent the plug hair from scratching the inner wall. At the same time, rolling hair, rolling folds and bamboo joint marks are strictly prohibited.

[0024] In summary, to address the thermal fatigue and intergranular corrosion issues of austenitic stainless steel pipes during use, the present invention addresses the problems of thermal fatigue and intergranular corrosion in austenitic stainless steel pipes. By adding a certain amount of alloying or non-metallic elements to 18-8 stainless steel, adjusting the upper and lower limits of the elements already present, and selecting an appropriate pipe preparation process, a super austenitic stainless steel pipe with relatively excellent high-temperature mechanical properties and intergranular corrosion resistance is obtained. The present invention provides a super austenitic stainless steel pipe for supercritical and ultra-supercritical boiler units. This steel pipe has excellent high-temperature mechanical properties and intergranular corrosion resistance, and has good overall performance. It can better meet the operating environment requirements of ultra-supercritical boiler units and can provide long-term stable service at 500°C to 800°C.

[0025] The following describes the specific embodiments.

[0026] Example 1

[0027] This embodiment provides a super austenitic stainless steel tube for supercritical and ultra-supercritical units, wherein the chemical composition thereof is as follows, by weight percentage: C 0.033%, Cr 20.22%, Mn 1.23%, Nb 0.61%, Ni 10.08%, Cu 2.25%, Mo 0.23%, V 0.11%, Ti 0.13%, Co 0.15%, N 0.072%, B 0.003%, Al 0.005%, Si 0.12%, P 0.008%, and the balance is Fe and unavoidable impurity elements.

[0028] According to the thermodynamic calculation of the above chemical composition, at 675℃, i.e. the sensitization temperature, the σ phase content in the stainless steel of this composition is 0.12%, M 23 The content of C6 carbide phase is 1.04%, the maximum content of σ phase in the calculation temperature range is 2.54%, M 23 The maximum content of C6 carbide phase is 1.21%, which is relatively low.

[0029] This embodiment also provides a production process for the above-mentioned austenitic stainless steel pipe. The specific process route is: tube blank → heating → extrusion in a horizontal extruder (obtaining an extruded rough tube) → quenching (heat treatment) → straightening → pickling → grinding → lubrication → first pass cold rolling (obtaining a semi-finished steel pipe) → high temperature softening treatment → pickling → grinding → shot blasting → pickling → lubrication → second pass cold rolling → pickling → heat treatment (solution treatment of the finished product) → pickling → flaw detection → sampling and testing → finishing inspection → packaging and warehousing.

[0030] Among them, the surface of the tube blank should be of good quality and free of cracks visible to the naked eye. After pickling and grinding, all visible defects on the inner and outer surfaces of the extruded raw tube must be removed before it can flow into the subsequent cold rolling process.

[0031] After high-temperature softening and pickling, the semi-finished steel pipes are inspected and ground, and defects visible to the naked eye are removed with a fine grinding wheel. After the semi-finished products are ground, they are shot blasted as a whole, pickled, cleaned, and then cold rolled.

[0032] The cold rolling process selects matching tooling according to the rolling specifications. When rolling the finished product, based on the characteristics of the material itself, first use a smaller feed of 2mm and a low speed rolling of 52 times / min. After the rolling is normal, gradually increase the feed and rolling speed.

[0033] The tolerance control of the finished product rolling size takes into account the allowance for subsequent grinding and polishing of the finished product. During the rolling process, the surface of the plug is checked every three pieces to prevent the plug hair from scratching the inner wall. At the same time, rolling hair, rolling folds and bamboo joint marks are strictly prohibited.

[0034] The finished pipe was sensitized at 675℃ for 1 hour and then tested for intergranular corrosion performance, and no intergranular corrosion occurred.

[0035] Example 2

[0036] This embodiment provides a super austenitic stainless steel pipe for supercritical and ultra-supercritical units, wherein the chemical composition thereof is, by weight percentage, C 0.045%, Cr 20.89%, Mn 1.34%, Nb 0.67%, Ni 10.84%, Cu 2.42%, Mo 0.27%, V 0.15%, Ti 0.18%, Co 0.19%, N 0.083%, B 0.003%, Al≤0.004%, Si 0.23%, P 0.006%, and the balance is Fe and unavoidable impurity elements.

[0037] According to the thermodynamic calculation of the above chemical composition, at 675℃, i.e. the sensitization temperature, the σ phase content in the stainless steel of this composition is 0.35%, M 23 The content of C6 carbide phase is 1.16%, the maximum content of σ phase in the calculation temperature range is 2.88%, M 23 The maximum content of C6 carbide phase is 1.24%, which is relatively low.

[0038] This embodiment also provides a production process for the above-mentioned austenitic stainless steel pipe. The specific process route is: tube blank → heating → extrusion in a horizontal extruder (obtaining an extruded rough tube) → quenching (heat treatment) → straightening → pickling → grinding → lubrication → first pass cold rolling (obtaining a semi-finished steel pipe) → high temperature softening treatment → pickling → grinding → shot blasting → pickling → lubrication → second pass cold rolling → pickling → heat treatment (solution treatment of the finished product) → pickling → flaw detection → sampling and testing → finishing inspection → packaging and warehousing.

[0039] Among them, the surface of the tube blank should be of good quality and free of cracks visible to the naked eye. After pickling and grinding, all visible defects on the inner and outer surfaces of the extruded raw tube must be removed before it can flow into the subsequent cold rolling process.

[0040] After high-temperature softening and pickling, the semi-finished steel pipes are inspected and ground, and defects visible to the naked eye are removed with a fine grinding wheel. After the semi-finished products are ground, they must be shot blasted as a whole, then pickled and cleaned, and then cold rolled.

[0041] The cold rolling process selects matching tooling according to the rolling specifications. When rolling the finished product, based on the characteristics of the material itself, first use a smaller feed of 3mm and a low speed rolling of 75 times / min. After the rolling is normal, gradually increase the feed and rolling speed.

[0042] The tolerance control of the finished product rolling size must take into account the allowance left for subsequent grinding and polishing of the finished product. During the rolling process, the surface of the plug should be checked every 5 pieces to prevent the plug hair from scratching the inner wall. At the same time, rolling hair, rolling folds and bamboo joint marks are strictly prohibited.

[0043] The finished pipe was sensitized at 675℃ for 1 hour and then tested for intergranular corrosion performance, and no intergranular corrosion occurred.

[0044] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples. Within the spirit of the embodiments of the present invention, the technical features of the above embodiments or different embodiments may be combined, and there are many other variations of the different aspects of the embodiments of the present invention described above, which are not provided in detail for the sake of clarity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included within the scope of protection of the embodiments of the present invention.

Claims

1. A super austenitic stainless steel pipe, characterized in that: The chemical composition of the super austenitic stainless steel pipe is as follows by weight percentage: C: 0.03-0.05%; Cr: 20-21%; Mn: 1.2-1.4%, Nb: 0.6-0.7%; Ni: 10-11%; Cu: 2.25-2.5%; Mo: 0.2~0.3%; V：0.1～0.2%； Ti: 0.13-0.2%; Co: 0.15~0.2%; N：0.07～0.09%； B: 0.002~0.008%; Al: ≤0.005%; Si: ≤0.3%; P: ≤0.008%; the balance is Fe and unavoidable impurity elements; At a sensitization temperature of 675°C, the content of σ phase in the super austenitic stainless steel tube is 0-0.5%, M 23 The content of C6 carbide phase is 1.0-1.2%.

2. The super austenitic stainless steel pipe according to claim 1, characterized in that: The chemical composition of the super austenitic stainless steel pipe is: C: 0.033%; Cr: 20.22%; Mn: 1.23%, Nb: 0.61%; Ni: 10.08%; Cu: 2.25%; Mo: 0.23%; V: 0.11%; Ti: 0.13%; Co: 0.15%; N: 0.072%; B: 0.003%; Al: 0.005%; Si: 0.12%; P: 0.008%; the balance is Fe and unavoidable impurity elements.

3. The super austenitic stainless steel pipe according to claim 2, characterized in that: At a sensitization temperature of 675°C, the content of σ phase in the super austenitic stainless steel tube is 0.12%, M 23 The content of C6 carbide phase is 1.04%.

4. The super austenitic stainless steel pipe according to claim 1, characterized in that: The chemical composition of the super austenitic stainless steel pipe is: C: 0.045%; Cr: 20.89%; Mn: 1.34%, Nb: 0.67%; Ni: 10.84%; Cu: 2.42%; Mo: 0.27%; V: 0.15%; Ti: 0.18%; Co: 0.19%; N: 0.083%; B: 0.003%; Al: ≤0.004%; Si: 0.23%; P: 0.006%; the balance is Fe and unavoidable impurity elements.

5. The super austenitic stainless steel pipe according to claim 4, characterized in that: At a sensitization temperature of 675°C, the content of σ phase in the super austenitic stainless steel tube is 0.35%, M 23 The content of C6 carbide phase is 1.16%.

6. A method for preparing a super austenitic stainless steel pipe according to any one of claims 1 to 5, characterized in that: include: The tube blank is sequentially subjected to extrusion, quenching, first-pass cold rolling, heating and softening, second-pass cold rolling and solution treatment to obtain the super austenitic stainless steel tube.

7. The method according to claim 6, characterized in that When performing the second cold rolling, first select a feed amount of 2 to 3 mm and a rolling speed of 50 to 80 times / min, and then gradually increase the feed amount and rolling speed.

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