An oxidation-resistant and heat-resistant steel pipe and its manufacturing method

Abstract

This invention discloses an oxidation-resistant and heat-resistant steel pipe containing Fe and unavoidable impurity elements, and further containing the following chemical elements in the following mass percentages: C: 0.09–0.2%, Si: 0.2–1.2%, Mn: 0.2–1.0%, Cr: 12–16%, V: 0.05–0.2%, W: 0.5%–1.5%, Co: 1.8%–2.3%, Nb: 0.12–0.3%, Cu: 0.2%–0.8%, B: 0.001–0.008%, Al: 0.005–0.05%, N: 0.01–0.08%. Accordingly, the present invention also discloses a method for manufacturing the above-mentioned anti-oxidation and heat-resistant steel pipe, which includes the following steps: (1) electric furnace steelmaking and continuous casting of square billets; (2) rolling the billet into a round billet and then annealing it; (3) heating the round billet and then homogenizing it; (4) hot piercing; (5) rolling the steel pipe; (6) reheating in a reheating furnace and then using a tension reducing mill to reduce the diameter and wall thickness of the billet, and then naturally cooling it; (7) heat treatment: normalizing + tempering.

Description

Technical Field

[0001] This invention relates to a steel pipe and its manufacturing method, and more particularly to a heat-resistant steel pipe and its manufacturing method. Background Technology

[0002] In recent years, as the world has placed increasingly higher demands on environmental protection and energy conservation, the parameter requirements for boiler equipment in thermal power generation systems have also become more stringent.

[0003] As is well known, the power generation efficiency of thermal power generating units can be significantly improved by increasing the steam temperature and steam pressure of the boilers. Furthermore, the higher the operating parameters of the unit, the higher the thermal efficiency, the lower the coal consumption, the less greenhouse gas emissions, and the greater the conservation of resources and environmental protection.

[0004] However, it is important to note that to ensure the stable operation of boiler equipment, while increasing the steam temperature and pressure in the boiler, the steel pipes used must also maintain structural stability under high-temperature, long-term operating conditions. Simultaneously, these steel pipes must possess excellent resistance to high-temperature steam oxidation and corrosion to prevent oxidation by the high-temperature steam in the boiler, thus preventing severe oxidation during service and the resulting scale buildup and blockage of the pipes.

[0005] Therefore, in order to meet existing needs, the inventors hope to obtain a heat-resistant steel pipe with good resistance to high-temperature creep and high-temperature oxidation corrosion.

[0006] While existing technologies have developed heat-resistant steel pipes with resistance to high-temperature creep and high-temperature oxidation corrosion, these heat-resistant steel pipes still have some unavoidable defects in their design.

[0007] For example, Chinese patent document CN103695806A, published on April 2, 2014, entitled "A Novel Austenitic Heat-Resistant Steel," describes an austenitic heat-resistant steel with high-temperature resistance to steam corrosion and good high-temperature strength. Its chemical composition (mass percentage) is: C: 0.02–0.10%, Si: 0.05–1.00%, Mn: 0.4–2.0%, Cr: 20–28%, Ni: 30–39%, Nb: 0.9– The alloy composition is as follows: 2.0%, Ti: 1.6–2.8%, Al: 0.9–2.0%, Cu: 0.05–3.50%, Co: 0.1–3.0%, V: 0.08–0.80%, Zr: 0.01–0.30%, Ce: 0.003–0.200%, B: 0.001–0.010%, with the remainder being Fe and impurities. W: 1.5–3.0% and Mg: 0.001–0.010% can be added to the above formula. This technical solution primarily achieves high high-temperature creep resistance and oxidation resistance by adding elements such as Cr and Co to form an austenitic structure. However, this chemical composition design results in a high alloy content and high alloy cost. Furthermore, in practical applications, it presents the problem of weakened weld joints between austenitic and ferritic steels. Summary of the Invention

[0008] One objective of this invention is to provide a novel oxidation-resistant heat-resistant steel pipe, which is a ferritic martensitic heat-resistant steel pipe. It can be effectively applied to 630℃ high-temperature ultra-supercritical boilers and exhibits excellent resistance to high-temperature creep and high-temperature oxidation corrosion under high-temperature and high-pressure operating conditions. Furthermore, this oxidation-resistant heat-resistant steel pipe has good processing and welding properties, and thus possesses promising prospects and application value.

[0009] To achieve the above objectives, the present invention provides an oxidation-resistant and heat-resistant steel pipe containing Fe and unavoidable impurity elements, and further containing the following chemical elements in the following mass percentages:

[0010] C: 0.09~0.2%, Si: 0.2~1.2%, Mn: 0.2~1.0%, Cr: 12~16%, V: 0.05~0.2%, W: 0.5%~1.5%, Co: 1.8 %~2.3%, Nb: 0.12~0.3%, Cu: 0.2%~0.8%, B: 0.001~0.008%, Al: 0.005~0.05%, N: 0.01~0.08%.

[0011] Furthermore, in the antioxidant and heat-resistant steel pipe described in this invention, the mass percentage content of each chemical element is as follows:

[0012] C: 0.09–0.2%, Si: 0.2–1.2%, Mn: 0.2–1.0%, Cr: 12–16%, V: 0.05–0.2%, W: 0.5%–1.5%, Co: 1.8%–2.3%, Nb: 0.12–0.3%, Cu: 0.2%–0.8%, B: 0.001–0.008%, Al: 0.005–0.05%, N: 0.01–0.08%; the balance is Fe and unavoidable impurity elements.

[0013] The design principles of each chemical element in the anti-oxidation and heat-resistant steel pipe of the present invention are as follows:

[0014] C: In the anti-oxidation and heat-resistant steel pipe described in this invention, C is one of the essential elements for improving the strength of steel. At the same time, C can also form carbides with Cr, Nb, and W at high temperatures, and improve the thermal strength of the material through dispersion strengthening. Increasing the C content in the steel can improve M... 23 The precipitation of C6, MX, etc., enhances the dispersion strengthening effect. However, the C content in the steel should not be too high. When the C content in the steel is too high, it will lead to excessive precipitates and affect the weldability of the steel. Therefore, considering the influence of C on the performance of steel pipes, the mass percentage of C in the anti-oxidation and heat-resistant steel pipe of this invention is controlled between 0.09% and 0.2%.

[0015] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be preferably controlled between 0.09 and 1.5%.

[0016] Si: In the anti-oxidation and heat-resistant steel pipe described in this invention, Si is one of the main deoxidizing agents in smelting. Si can combine with elements such as Mo and Cr, thereby improving the material's resistance to high-temperature oxidation corrosion. However, it should be noted that the Si content in the steel should not be too high. Excessive Si content will adversely affect the impact toughness of the material, and during long-term service at high temperatures, Si will promote the precipitation of brittle phases, which is detrimental to the stability of creep resistance. In addition, Si is also a ferrite-forming element, tending to increase the formation of δ-ferrite at high temperatures, which is detrimental to the stability of the steel's creep resistance. Therefore, in order to maximize the beneficial effects of Si, the mass percentage of Si in the anti-oxidation and heat-resistant steel pipe described in this invention is controlled between 0.2% and 1.2%.

[0017] Mn: In the anti-oxidation and heat-resistant steel pipe described in this invention, Mn is an austenite-forming element. Adding an appropriate amount of Mn can improve the material strength while inhibiting the formation of high-temperature ferrite. Simultaneously, Mn can stabilize P and S elements to prevent the formation of low-melting-point sulfides and improve the hot working performance of the material. Therefore, if the mass percentage of Mn in the steel is too low, P and S cannot be effectively stabilized, failing to achieve the desired effect. Furthermore, during high-temperature oxidation, Mn has a relatively high high-temperature diffusion coefficient and is easily oxidized. When the Mn content in the steel is too high, it will adversely affect the material's resistance to steam oxidation corrosion. Therefore, considering the influence of Mn content on steel properties, the Mn content must be strictly controlled. In this invention, the mass percentage of Mn is controlled between 0.2% and 1.0%. This ensures that the steel pipe obtains suitable mechanical strength without compromising its resistance to steam oxidation corrosion.

[0018] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be preferably controlled between 0.4% and 0.7%.

[0019] Cr: In the anti-oxidation heat-resistant steel pipe described in this invention, the dispersed precipitation of carbides formed by Cr and C elements has a strengthening effect during high-temperature service. Furthermore, Cr is the most important alloying element for resisting high-temperature oxidation corrosion. When the mass percentage of Cr in the steel reaches a certain level, continuous Cr2O3, (CrFe)3O4, and (CrNi)3O4 oxide films can be formed on the surface of the heat-resistant steel. These oxide films endow the heat-resistant steel with good resistance to high-temperature steam oxidation corrosion. However, it should be noted that when the mass percentage of Cr is too low, the desired solid solution strengthening and precipitation strengthening effects will not be achieved, and the material surface will not be able to form a continuous Cr2O3 or (CrFe)3O4 oxide film, which is detrimental to the material's resistance to high-temperature corrosion. Moreover, the consumption of Cr during long-term high-temperature oxidation must also be considered to ensure that there is always a sufficient Cr content at the oxide matrix interface to repair the degradation and peeling of the protective oxide film. Furthermore, the Cr content in steel should not be too high. Cr is a ferrite-forming element, and an excessively high Cr content will increase the amount of high-temperature δ-ferrite in the steel, which will adversely affect the high-temperature performance of the material. Based on this, in the oxidation-resistant and heat-resistant steel pipe described in this invention, the mass percentage of Cr is controlled between 12% and 16%.

[0020] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Cr element can be further preferably controlled between 13.1% and 16%.

[0021] V: In the anti-oxidation and heat-resistant steel pipe described in this invention, element V can form fine nitrides and / or carbonitrides in the steel, and the addition of V can improve the high-temperature creep rupture strength of the material. Therefore, to ensure the beneficial effects of element V, its addition amount should not be too low; however, it should be noted that the V content in the steel should also not be too high. Adding excessive V may lead to coarse carbonitrides and coarse grains of surface oxides, reducing the high-temperature creep strength of the material. Based on this, in this invention, the mass percentage content of element V is controlled between 0.05% and 0.2%.

[0022] W: In the anti-oxidation and heat-resistant steel pipe described in this invention, W alloy has a strong solid solution strengthening effect, which can significantly improve the hot strength of the steel. Furthermore, W is also a major component of the Lavas phase, and the addition of W has a significant effect on improving the hot strength of the steel. However, it should be noted that W should not be added in excess, as excessive W content will cause a decrease in weldability and machinability. Therefore, considering the influence of W content on material properties, to maximize the beneficial effects of W, the mass percentage of W element in this invention is controlled between 0.5% and 1.5%.

[0023] Co: In the oxidation-resistant and heat-resistant steel pipe described in this invention, Co has a strong solid solution strengthening effect, which can improve the tensile strength and high-temperature creep strength of austenitic steel. Furthermore, Co is also an austenite-forming element, which can effectively inhibit the increase of high-temperature δ-ferrite, thereby improving the high-temperature creep strength of the steel. However, the alloying cost of Co is relatively high. Considering overall cost factors, in this invention, the mass percentage of Co is controlled between 1.8% and 2.3%.

[0024] Nb: In the oxidation-resistant and heat-resistant steel pipe described in this invention, Nb is a stabilizing element for C and N, and it is also a major component of the MX precipitate phase. It can form Nb carbonitrides in the steel, thus achieving precipitation strengthening. However, it should be noted that the Nb content in the steel should not be too high. Excessive Nb content leads to coarse carbonitrides, resulting in decreased thermal strength, and also reduces the material's machinability and oxidation resistance. Therefore, in the oxidation-resistant and heat-resistant steel pipe described in this invention, the mass percentage of Nb is controlled between 0.12% and 0.3%.

[0025] Cu: In the anti-oxidation and heat-resistant steel pipe described in this invention, Cu is also an austenite-forming element. At high temperatures, Cu can form a Cu-rich phase and improve thermal strength. However, it should be noted that the addition of Cu will reduce the hot workability of the steel. Therefore, Cu should not be added in excess. In this invention, the mass percentage of Cu is controlled between 0.2% and 0.8%.

[0026] B: In the anti-oxidation and heat-resistant steel pipe described in this invention, element B can strengthen the grain boundaries and simultaneously inhibit carbide growth. However, it should be noted that the content of element B should not be too high, as excessive B content can also lead to a decrease in the processing performance of the steel. Therefore, in this invention, the mass percentage content of element B is controlled between 0.001% and 0.008%.

[0027] Al: In the anti-oxidation and heat-resistant steel pipe described in this invention, Al element has a significant effect on improving the steel's resistance to high-temperature steam oxidation corrosion. However, it should be noted that Al element easily combines with N element in steel to form AlN, which is detrimental to the material's high-temperature creep performance. Therefore, in this invention, the mass percentage content of Al element is controlled between 0.005% and 0.05%.

[0028] N: In the oxidation-resistant and heat-resistant steel pipe described in this invention, nitrogen (N) is a strong austenite-forming element, which can improve the thermal strength of the material. Furthermore, N and Nb can form M(C,N) type carbonitrides, which can also suppress the precipitation of the Z phase during high-temperature service. Therefore, in this invention, the mass percentage of N is controlled between 0.01% and 0.08%.

[0029] In the composition design described above, the inventors primarily employ the addition of appropriate proportions of Cr and W to enhance the steel's thermal strength and resistance to high-temperature oxidation. This is achieved by adding a suitable proportion of Cr to prevent the rapid growth of the oxide layer during high-temperature service. Simultaneously, the inventors also incorporated elements such as Co and Cu to improve the steel's thermal strength while inhibiting the formation of high-temperature δ-ferrite, thus reducing the weakening effect of Cr content on long-term creep resistance. This ensures that the steel pipe achieves high oxidation resistance while maintaining creep stability and high creep strength at 630°C for extended periods.

[0030] In addition, the inventors also designed and added Nb, V and N elements in the above chemical composition design. These elements can ensure that the steel precipitates fine strengthening phases during high-temperature service and does not produce a large growth rate during long-term service, thus ensuring the thermal strength of the steel.

[0031] Furthermore, in the anti-oxidation and heat-resistant steel pipe of the present invention, the content of each impurity element among the unavoidable impurity elements satisfies the following: P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.003%, Pb+Sn+As+Sb≤0.01%.

[0032] In the above technical solution, P, S, O, H, Pb, Sn, As and Sb are all impurity elements in the anti-oxidation and heat-resistant steel pipe of the present invention. Under the condition that the technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in the anti-oxidation and heat-resistant steel pipe should be reduced as much as possible.

[0033] Furthermore, in the oxidation-resistant and heat-resistant steel pipe of the present invention, each chemical element satisfies at least one of the following formulas:

[0034] 5Co + 10Cu ≥ Cr;

[0035] N / Al≥2;

[0036] Substitute the values ​​of each chemical element in the formula into the values ​​before the percentage sign of the chemical element's mass.

[0037] In the above-described technical solution of this invention, while controlling the mass percentage of a single chemical element in the steel, this invention can also control the Co, Cu, and Cr elements in the oxidation-resistant and heat-resistant steel pipe to satisfy the limiting relationship 5Co + 10Cu ≥ Cr. This is because Co and Cu are austenite-forming elements, and Cr is a ferrite-forming element. While improving thermal strength, another major role of Co and Cu is to suppress the formation of high-temperature δ-ferrite caused by the addition of Cr. The formation of high-temperature δ-ferrite has a significant reducing effect on the high-temperature creep resistance of steel. Therefore, by calculating the thermodynamic phase diagram, satisfying this relationship can effectively suppress the high-temperature δ-ferrite structure in the steel, thereby ensuring the thermal strength of the steel.

[0038] Accordingly, this invention, while controlling the mass percentage of a single chemical element in the steel, can also control the N and Al elements in the oxidation-resistant and heat-resistant steel pipe to meet the constraint relationship of N / Al≥2. This is because N is the main element that forms dispersed strengthening particles in steel during high-temperature service. Since Al has a strong bonding performance with N, if N / Al cannot meet the condition of ≥2, N cannot effectively combine with alloying elements such as Nb, V, and W to form dispersed strengthening particles, ensuring the thermal strength of the steel under high-temperature conditions and strengthening grain boundaries during long-term high-temperature service, inhibiting the aging of the microstructure, and thus possessing high high-temperature creep strength.

[0039] Furthermore, in the oxidation-resistant and heat-resistant steel pipe of the present invention, the mass percentage content of each chemical element also satisfies at least one of the following conditions:

[0040] C: 0.09–1.5%;

[0041] Mn: 0.4–0.7%.

[0042] Furthermore, in the anti-oxidation and heat-resistant steel pipe of the present invention, its microstructure matrix at room temperature is tempered martensite and high-temperature ferrite with a proportion not exceeding 2%.

[0043] Furthermore, in the oxidation-resistant and heat-resistant steel pipe described in this invention, its microstructure at room temperature has a diffusely precipitated MX phase, M... 23 C6, Laves and Cu-rich phases.

[0044] Furthermore, in the antioxidant and heat-resistant steel pipe described in this invention, its high-temperature microstructure after creep at 630°C for 10,000 hours is M. 23 C6 phase, MX phase, Laves phase, Cu-rich phase and Z phase, among which M 23 The size of the C6 and Laves phases is no greater than 300 nm, the size of the Cu-rich phase is no greater than 60 nm, the size of the MX phase is no greater than 20 nm, and the proportion of the z phase is <1%.

[0045] Furthermore, in the oxidation-resistant and heat-resistant steel pipe described in this invention, its room temperature mechanical properties satisfy: yield strength 500MPa≤Rp0.2≤750MPa, tensile strength 650MPa≤Rm≤880MPa, and elongation A 50 ≥15%, impact energy ≥120J.

[0046] Furthermore, in the anti-oxidation and heat-resistant steel pipe described in this invention, its high-temperature mechanical properties at 630℃ satisfy the following: yield strength Rp0.2 ≥ 300 MPa, tensile strength Rm ≥ 360 MPa, and elongation A 50 ≥30%; extrapolated endurance strength at 630℃ for 100,000 hours ≥70MPa, creep rate at 630℃ and 100MPa ≤8×10 -6 At a speed of mm / h, the weight gain after 10,000 hours of oxidation corrosion at 630℃ is no higher than 20 mg / cm³. 2 .

[0047] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned anti-oxidation and heat-resistant steel pipe of the present invention. The anti-oxidation and heat-resistant steel pipe obtained by the manufacturing method has good resistance to high-temperature creep, excellent resistance to high-temperature oxidation corrosion, and good processing and welding performance.

[0048] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned oxidation-resistant and heat-resistant steel pipe, comprising the following steps:

[0049] (1) Electric furnace steelmaking, continuous casting of square billets;

[0050] (2) The tube blank is rolled into a round blank, and then annealed;

[0051] (3) Heat the round tube blank until it is homogenized;

[0052] (4) Thermal perforation;

[0053] (5) Steel pipe rolling;

[0054] (6) The tube blank is heated in a reheating furnace, and then the tube blank is reduced in diameter and wall thickness by a tension reducing machine, and then cooled naturally.

[0055] (7) Heat treatment: normalizing + tempering.

[0056] Furthermore, in the manufacturing method described in this invention, in step (2), the compression ratio of the tube blank rolling is controlled to be ≥8, the tube blank annealing temperature is 750-900℃, and the annealing time is 180-300min.

[0057] In the above-mentioned technical solution of the present invention, the inventors have optimized the rolling process of the tube blank. In some preferred embodiments, the compression ratio of the tube blank rolling can be controlled to be ≥8, so as to ensure that the tube does not produce surface defects in the subsequent production process.

[0058] Furthermore, in the manufacturing method described in this invention, in step (3), the heating temperature of the round tube blank is 1160-1220℃, and the heat soaking time is 40-100min.

[0059] Furthermore, in the manufacturing method described in this invention, in step (6), the temperature of the reheat furnace is 800-1000°C, and the tensile deformation is controlled to be 20-60%.

[0060] Furthermore, in the manufacturing method described in this invention, in step (7), the normalizing temperature is 950-1150℃ and the holding time is 30-60min; the tempering temperature is 600-850℃ and the holding time is 90-180min.

[0061] Compared with the prior art, the anti-oxidation and heat-resistant steel pipe and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0062] In this invention, the inventors employed a novel chemical element composition design, primarily by adding appropriate proportions of Cr and W to enhance the steel's thermal strength and resistance to high-temperature oxidation. The addition of Cr prevents the excessively rapid growth of the oxide layer during high-temperature service. Simultaneously, the inventors also incorporated elements such as Co and Cu to improve thermal strength while inhibiting the formation of high-temperature δ-ferrite, reducing the weakening effect of Cr content on long-term creep resistance. This ensures that the steel pipe achieves high oxidation resistance while also possessing structural stability and high creep strength during long-term creep at 630℃.

[0063] In addition, the inventors designed and added Nb, V and N elements, which can ensure that the steel precipitates fine strengthening phases during high-temperature service and does not produce a large growth rate during long-term service, thus ensuring the thermal strength of the steel.

[0064] Accordingly, this invention, while ensuring the steel's oxidation resistance and high-temperature creep strength, further optimizes the pipe-making process to guarantee the steel pipe's room-temperature microstructure and mechanical properties. In the pipe-making process designed in this invention, the rolling ratio during billet rolling is controlled to prevent surface defects in subsequent steel production processes; the heating time in the furnace is controlled to ensure the steel pipe's workability and prevent internal surface defects; and final normalizing and tempering heat treatments ensure that the steel pipe has a tempered martensitic microstructure and strengthening precipitates at room temperature, while simultaneously ensuring that the steel pipe does not have excessively high strength at room temperature, but possesses high creep strength during long-term high-temperature service.

[0065] The room temperature mechanical properties of the oxidation-resistant and heat-resistant steel pipe obtained by adopting the design scheme of this invention are: yield strength 500MPa≤Rp0.2≤750MPa, tensile strength 650MPa≤Rm≤880MPa, and elongation A 50 ≥15%, room temperature impact energy ≥120J; the high-temperature mechanical properties of this oxidation-resistant and heat-resistant steel pipe at 630℃ meet the following requirements: yield strength Rp0.2≥300MPa, tensile strength Rm≥360MPa, elongation A 50 ≥30%; extrapolated endurance strength at 630℃ for 100,000 hours ≥70MPa, creep rate at 630℃ and 100MPa ≤8×10 -6 At a speed of mm / h, the weight gain after 10,000 hours of oxidation corrosion at 630℃ is no higher than 20 mg / cm³. 2 . Attached Figure Description

[0066] Figure 1 The image shows the metallographic structure of the oxidation-resistant and heat-resistant steel pipe of Example 1 under a 500x electron microscope at room temperature.

[0067] Figure 2 The image shows the tissue of the antioxidant and heat-resistant steel pipe of Example 2 at 2000x magnification under a scanning electron microscope when it is at room temperature.

[0068] Figure 3 The image shows the metallographic structure of the oxidation-resistant and heat-resistant steel pipe of Example 3 after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100MPa under a 500x electron microscope.

[0069] Figure 4 The image shows the tissue of the antioxidant heat-resistant steel pipe of Example 4 after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100 MPa, magnified by a scanning electron microscope at 2000x.

[0070] Figure 5 The image shows the transmission microstructure of the oxidation-resistant and heat-resistant steel pipe of Example 5 after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100 MPa under a transmission microscope at 5000x magnification. Detailed Implementation

[0071] The following will further explain and illustrate the antioxidant and heat-resistant steel pipe and its manufacturing method according to the present invention with reference to specific embodiments and accompanying drawings. However, such explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0072] Examples 1-10 and Comparative Examples 1-5

[0073] The oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5 of the present invention were all prepared using the following steps:

[0074] (1) Smelting and casting are carried out according to the chemical element mass percentages shown in Table 1-1 and Table 1-2: During smelting, electric furnace steelmaking is used, and after VD and VOD, continuous casting of square billets with side length > 400mm is obtained by vertical continuous casting machine.

[0075] (2) Roll the tube blank into a round blank and then anneal it: During rolling, control the compression ratio of the tube blank to be ≥8. During annealing, control the annealing temperature of the tube blank to be 750-900℃ and the annealing time to be 180-300min.

[0076] (3) Heat the round tube blank and then homogenize it: control the heating temperature of the round tube blank to 1160-1220℃ and control the homogenization and heat preservation time to 40-100min.

[0077] (4) Use a vertical conical piercing machine for hot piercing.

[0078] (5) The steel pipe is rolled to the finished product using a fully floating mandrel rolling mill.

[0079] (6) Heating in a reheating furnace, controlling the temperature of the reheating furnace to 800-1000℃, then using a tension reducing machine to reduce the diameter and wall thickness of the tube blank, and controlling the tension reduction deformation to 20-60%, and then cooling naturally.

[0080] (7) Heat treatment: normalizing + tempering, control the normalizing temperature at 950-1150℃ and the holding time at 30-60min; control the tempering temperature at 600-850℃ and the holding time at 90-180min.

[0081] It should be noted that the chemical composition design and related processes of the anti-oxidation and heat-resistant steel pipes in Examples 1-10 all meet the design specifications of this invention. Although the comparative steel pipes in Comparative Examples 1-5 are also prepared using the above steps (1)-(7), their chemical composition design contains parameters that do not meet the design requirements of this invention.

[0082] Tables 1-1 and 1-2 list the mass percentage of each chemical element in the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5.

[0083] Table 1-1. (wt%, balance Fe and other unavoidable impurities other than P, S, H, O, Pb, Sn, As and Sb)

[0084]

[0085] Table 1-2.

[0086]

[0087]

[0088] Note: In the table above, in the formulas “5Co+10Cu-Cr” and “N / Al”, the elements in the formula should be substituted with the values ​​before the percentage sign of the mass percentage of that element.

[0089] Table 2 lists the specific process parameters of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5.

[0090] Table 2.

[0091]

[0092]

[0093] Samples were taken from the finished anti-oxidation and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5 obtained through the above process steps, and the mechanical properties of the steel pipes of Examples 1-10 and Comparative Examples 1-5 of this application were tested.

[0094] In this invention, the inventors controlled the steel pipe samples of Examples 1-10 and Comparative Examples 1-5 to be tested under two different temperature environments: room temperature and high temperature (630°C), so as to obtain the mechanical properties of these steel pipe samples at room temperature and high temperature (630°C), and the obtained mechanical property test results are listed in Table 3.

[0095] Tensile test: A room temperature tensile test was conducted at room temperature according to the requirements of GB / T228, and a high temperature tensile test was conducted at 630℃ according to the requirements of GB / T4338, in order to test the yield strength, tensile strength and elongation of the steel plates of each embodiment and comparative example.

[0096] Whether the mechanical properties are tested at room temperature or at high temperature (630℃), the yield strength, tensile strength and elongation are all measured by the above tensile test.

[0097] Impact test: Impact tests were conducted at room temperature according to the requirements of GB / T229 to test the impact energy of the steel plates of each embodiment and comparative example at room temperature.

[0098] Table 3 lists the mechanical property test results of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5.

[0099] Table 3.

[0100]

[0101]

[0102] Accordingly, after completing the above tests on yield strength, tensile strength and elongation, the inventors further tested the mechanical properties of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5 under high temperature conditions, in order to obtain the creep rate of each steel pipe at 630℃ and 100MPa, the extrapolated endurance strength at 630℃ for 100,000 hours, and the weight gain at 630℃ for 10,000 hours of oxidation corrosion, and the test results are listed in Table 4 below.

[0103] The relevant testing methods are as follows:

[0104] Under the condition of 630℃, according to the GB / T2039 standard, φ5mm cylindrical standard specimens were taken from the longitudinal direction of the steel pipe and subjected to creep rupture test on the RD2-3 type creep rupture tester to obtain the creep rate of each embodiment and comparative example steel pipe at 630℃ and 100MPa, and the extrapolated rupture strength at 630℃ for 100,000 hours.

[0105] Accordingly, a high-temperature oxidation test was also conducted at 630℃ in accordance with the standard GB / T13303 to measure the weight gain of the steel pipes in each embodiment and comparative example after 10,000 hours of oxidation corrosion at 630℃.

[0106] Table 4 lists the creep rate, extrapolated endurance strength at 630℃ for the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5 at 630℃ and 100MPa, and the weight gain at 630℃ for 10,000 hours of oxidation corrosion.

[0107] Table 4.

[0108]

[0109] As shown in Tables 3 and 4, compared with the comparative steel pipes of Comparative Examples 1-5, the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 of this case have superior resistance to high-temperature creep and high-temperature oxidation corrosion.

[0110] As shown in Table 3 above, in this invention, the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 have a yield strength Rp0.2 of 590-720 MPa at room temperature, a tensile strength Rm of 680-830 MPa, and an elongation A. 50 Between 16-19%, its room temperature impact energy is between 125-145 J. At a high temperature of 630℃, the yield strength Rp0.2 of the oxidation-resistant and heat-resistant steel pipes in Examples 1-10 is between 302-320 MPa, its tensile strength Rm is between 365-378 MPa, and its elongation A... 50 Between 30.5% and 36%.

[0111] Furthermore, as shown in Table 4, the creep rate of the oxidation-resistant and heat-resistant steel pipes in Examples 1-10 at 630℃ and 100MPa is 6.8-7.5×10⁻⁶. -6 Between mm / h, its extrapolated creep strength at 630℃ for 100,000 hours is between 71-78 MPa, and its weight gain after 10,000 hours of oxidation corrosion at 630℃ is between 14-19 mg / cm³. 2 between.

[0112] In summary, it can be seen that the anti-oxidation and heat-resistant steel pipes of Examples 1-10 obtained by adopting this technical solution of the present invention not only have good resistance to high-temperature creep, but also have excellent resistance to high-temperature oxidation corrosion, and have good processing performance.

[0113] Accordingly, after completing the above-mentioned tests on mechanical properties, the inventors further observed the microstructure of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5.

[0114] When observing the microstructure, the inventors also divided into two observation groups:

[0115] (a) The samples in one observation group were the finished anti-oxidation and heat-resistant steel pipes of Examples 1-10 and the control steel pipes of Comparative Examples 1-5 at room temperature. The observation was of the microstructure matrix of the example and comparative example steel pipes at room temperature. The observation results are listed in Table 5 below.

[0116] Table 5 lists the microstructure matrix observation results of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-5 at room temperature.

[0117] Table 5.

[0118]

[0119]

[0120] As shown in Table 5 above, in this invention, at room temperature, the microstructure of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 is: tempered martensite and high-temperature ferrite with a phase ratio of less than 2%.

[0121] Furthermore, the inventors observed that, at room temperature, the microstructure of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 also exhibited a dispersed MX phase. 23 C6, Laves and Cu-rich phases.

[0122] (b) The samples in the other observation group were the steel pipes of the examples and comparative examples that had been creeped at 630°C for 10,000 hours. The observations were made on the high-temperature microstructure of the steel pipes of the examples and comparative examples that had been creeped at 630°C for 10,000 hours. The relevant observation results are listed in Table 6 below.

[0123] Table 6.

[0124]

[0125]

[0126] As can be seen from Table 6, in this invention, the high-temperature microstructure of the oxidation-resistant and heat-resistant steel pipes of Examples 1-10 prepared using the design scheme of this invention is as follows after creep at 630℃ for 10,000 hours: M 23 C6 phase, MX phase, Laves phase, Cu-rich phase, and Z phase. Among them, M 23 The size of the C6 phase is between 220-280 nm; the size of the MX phase is between 10-20 nm; the size of the Laves phase is between 220-280 nm; and the size of the Cu-rich phase is between 30-58 nm. Furthermore, in the high-temperature microstructures of Examples 1-10, the proportion of the z-phase is between 0.2% and 0.9%.

[0127] Figure 1 The image shows the metallographic structure of the oxidation-resistant and heat-resistant steel pipe of Example 1 under a 500x electron microscope at room temperature.

[0128] Figure 2 The image shows the tissue of the antioxidant and heat-resistant steel pipe of Example 2 at 2000x magnification under a scanning electron microscope when it is at room temperature.

[0129] like Figure 1 and Figure 2 As shown, in this embodiment, the microstructure of the oxidation-resistant and heat-resistant steel pipes in Examples 1 and 2 is tempered martensite and dispersed precipitates of M. 23 C6 phase, MX phase, Laves phase, Cu-rich phase, and Z phase.

[0130] Figure 3 The image shows the metallographic structure of the oxidation-resistant and heat-resistant steel pipe of Example 3 after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100MPa under a 500x electron microscope.

[0131] Figure 4 The image shows the tissue of the antioxidant heat-resistant steel pipe of Example 4 after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100 MPa, magnified by a scanning electron microscope at 2000x.

[0132] like Figure 3 and Figure 4 As shown, in this embodiment, the microstructure of the oxidation-resistant and heat-resistant steel pipes in Examples 3 and 4 is tempered martensite and dispersed precipitates of M. 23 C6 phase, MX phase, Laves phase, Cu-rich phase, and Z phase.

[0133] Figure 5 The image shows the transmission microstructure of the oxidation-resistant and heat-resistant steel pipe of Example 5 after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100 MPa under a transmission microscope at 5000x magnification.

[0134] like Figure 5 As shown, in this embodiment, after creeping for 10,000 hours at a temperature of 630°C and a pressure of 100 MPa, M in Example 5... 23 The size of the C6 and Laves phases is no greater than 300 nm, the size of the Cu-rich phase is no greater than 60 nm, the size of the MX phase is no greater than 20 nm, and the proportion of the z phase is <1%.

[0135] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0136] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. An oxidation-resistant and heat-resistant steel pipe, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.09–0.2%, Si: 0.2–1.2%, Mn: 0.2–1.0%, Cr: 12–16%, V: 0.05–0.2%, W: 0.5%–1.5%, Co: 1.8%–2.3%, Nb: 0.12–0.3%, Cu: 0.2%–0.8%, B: 0.001–0.008%, Al: 0.005–0.05%, N: 0.01–0.08%; the balance is Fe and unavoidable impurity elements. Among them, each chemical element also satisfies the following formulas: 5Co + 10Cu ≥ Cr; N / Al ≥ 2; where each chemical element is represented by the value before the mass percentage sign.

2. The oxidation-resistant and heat-resistant steel pipe as described in claim 1, characterized in that, Among the unavoidable impurity elements, the contents of each impurity element satisfy the following: P≤0.02%, S≤0.01%, H≤0.0002%, O≤0.003%, and Pb+Sn+As+Sb≤0.01%.

3. The oxidation-resistant and heat-resistant steel pipe as described in claim 1, characterized in that, The mass percentage content of each chemical element also meets the following requirements: Mn: 0.4-0.7%.

4. The oxidation-resistant and heat-resistant steel pipe as described in claim 1, characterized in that, Its microstructure at room temperature consists of tempered martensite and high-temperature ferrite in proportion not exceeding 2%.

5. The oxidation-resistant and heat-resistant steel pipe as described in claim 4, characterized in that, Its microstructure at room temperature exhibits a diffusely precipitated MX phase, M 23 C6, Laves and Cu-rich phases.

6. The oxidation-resistant and heat-resistant steel pipe as described in claim 1, characterized in that, Its high-temperature microstructure after creep at 630℃ for 10,000 hours is M. 23 C6 phase, MX phase, Laves phase, Cu-rich phase and Z phase, among which M 23 The size of the C6 and Laves phases is no greater than 300 nm, the size of the Cu-rich phase is no greater than 60 nm, the size of the MX phase is no greater than 20 nm, and the proportion of the z phase is <1%.

7. The oxidation-resistant and heat-resistant steel pipe as described in claim 1, characterized in that, Its room temperature mechanical properties meet the following requirements: yield strength 500MPa≤Rp0.2≤750MPa, tensile strength 650MPa≤Rm≤880MPa, elongation A 50 ≥15%, impact energy ≥120J.

8. The oxidation-resistant and heat-resistant steel pipe as described in claim 1, characterized in that, Its high-temperature mechanical properties at 630℃ meet the following requirements: yield strength Rp0.2 ≥ 300 MPa, tensile strength Rm ≥ 360 MPa, and elongation A 50 ≥30%; extrapolated endurance strength at 630℃ for 100,000 hours ≥70MPa, creep rate at 630℃ and 100MPa ≤8×10 -6 At a speed of mm / h, the weight gain after 10,000 hours of oxidation corrosion at 630℃ is no higher than 20 mg / cm³. 2 .

9. The method for manufacturing the oxidation-resistant and heat-resistant steel pipe according to any one of claims 1-8, characterized in that, It includes the following steps: (1) Electric furnace steelmaking, continuous casting of square billets; (2) The tube blank is rolled into a round blank, and then annealed; (3) Heat the round tube blank until it is homogenized; (4) Thermal perforation; (5) Steel pipe rolling; (6) The tube blank is heated in a reheating furnace, and then the tube blank is reduced in diameter and wall thickness by a tension reducing machine, and then cooled naturally. (7) Heat treatment: normalizing + tempering.

10. The manufacturing method as described in claim 9, characterized in that, In step (2), the compression ratio of the tube blank rolling is controlled to be ≥8, the tube blank annealing temperature is 750-900℃, and the annealing time is 180-300min.

11. The manufacturing method as described in claim 9, characterized in that, In step (3), the heating temperature of the round tube blank is 1160-1220℃, and the heat soaking time is 40-100min.

12. The manufacturing method as described in claim 9, characterized in that, In step (6), the temperature of the reheat furnace is 800-1000℃, and the tensile deformation is controlled at 20-60%.

13. The manufacturing method as described in claim 9, characterized in that, In step (7), the normalizing temperature is 950-1150℃ and the holding time is 30-60min; the tempering temperature is 600-850℃ and the holding time is 90-180min.

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