Austenitic stainless steel pipe containing antioxidant infiltrated layer and preparation method of austenitic stainless steel pipe

Abstract

The invention relates to an austenitic stainless steel pipe containing an antioxidant infiltrated layer and a preparation method thereof, and belongs to the technical field of austenitic stainless steel. The stainless steel pipe comprises a pipe body and an anti-oxidation permeation layer; the pipe body comprises the following chemical components in percentage by weight: 0.07 to 0.08 percent of C, 0.20 to 0.25 percent of Si, 0.7 to 0.9 percent of Mn, 17.5 to 18.5 percent of Cr, 8.0 to 9.5 percent of Ni, 0.20 to 0.60 percent of Mo, 0.35 to 0.45 percent of Nb, 3.0 to 3.5 percent of Cu, 0.08 to 0.10 percent of N, less than or equal to 0.008 percent of Al, 0.002 to 0.004 percent of B, 0.02 to 0.08 percent of Y and the balance of Fe; the thickness of the anti-oxidation permeation layer is larger than or equal to 3 microns, and the phase of the anti-oxidation permeation layer is a FeAl phase. And a layer of Al2O3 film is formed in the use process, so that the pipe body is prevented from being further oxidized, and effective protection is formed.

Description

Technical Field

[0001] The present invention relates to the technical field of austenitic stainless steel, and particularly to an austenitic stainless steel pipe with an antioxidant infiltration layer and a preparation method thereof. Background Art

[0002] Austenitic heat-resistant stainless steel (such as S30432 steel) has good high-temperature creep strength, tissue stability and economy, and has become the main candidate material for reheaters and superheaters of 600-700 °C ultra-supercritical power plants. However, S30432 steel has weak steam oxidation resistance and intergranular corrosion resistance. During service, oxide scales are likely to appear on the inner wall, and intergranular corrosion cracking occurs on the outer wall due to flue gas corrosion. Especially under the background of "deep peak shaving" in thermal power generation, frequent periodic temperature changes are more likely to cause large-area peeling of the oxide scales on the inner wall, and the accumulation at the elbow leads to pipe explosion, causing serious economic losses.

[0003] Therefore, S30432 steel usually requires a fine-grained structure to improve the corrosion resistance of the outer wall, and at the same time, shot peening treatment is carried out on the inner wall to further improve the steam oxidation resistance. However, on the one hand, the shot peening layer will gradually fade with service time, and the antioxidant effect will decrease until it disappears; on the other hand, the fine-grained structure is not conducive to improving the creep strength of the heat-resistant steel, and there are potential safety hazards.

[0004] Therefore, there is an urgent need for an austenitic stainless steel pipe with better overall performance and higher steam oxidation resistance. Summary of the Invention

[0005] In view of the above analysis, the present invention aims to provide an austenitic stainless steel pipe with an antioxidant infiltration layer and a preparation method thereof, so as to solve the problems of low strength and poor steam oxidation resistance of the existing austenitic stainless steel pipes.

[0006] On the one hand, the present invention provides an austenitic stainless steel pipe with an antioxidant infiltration layer, and the stainless steel pipe includes a pipe body and an antioxidant infiltration layer coated on the inner wall of the pipe body;

[0007] The chemical composition of the pipe body, by mass percentage, includes: C: 0.07-0.08%, Si: 0.20-0.25%, Mn: 0.7-0.9%, Cr: 17.5-18.5%, Ni: 8.0-9.5%, Mo: 0.20-0.60%, Nb: 0.35-0.45%, Cu: 3.0-3.5%, N: 0.08-0.10%, Al≤0.008%, B: 0.002-0.004%, Y: 0.02-0.08%, and the balance is Fe;

[0008] The thickness of the antioxidant infiltration layer is ≥3 μm, and its phase is FeAl phase.

[0009] Furthermore, the chemical composition of the pipe body, by mass percentage, includes: C: 0.078 - 0.08%, Si: 0.22 - 0.25%, Mn: 0.8 - 0.9%, Cr: 17.5 - 18.2%, Ni: 8.5 - 9.5%, Mo: 0.45 - 0.60%, Nb: 0.35 - 0.42%, Cu: 3.2 - 3.5%, N: 0.092 - 0.10%, Al ≤ 0.008%, B: 0.002 - 0.004%, Y: 0.05 - 0.08%, and the balance is Fe.

[0010] Furthermore, the grain size of the pipe body is 6.0 - 7.0 grades.

[0011] On the other hand, the present invention provides a method for preparing an austenitic stainless steel pipe with an antioxidant infiltration layer, including the following steps:

[0012] S1: Steel billet preparation, producing steel billets by continuous casting, and using periodic electromagnetic stirring at the solidification end;

[0013] S2: Rolling and blooming, heating the steel billets by a sectional heating method, and rolling and blooming the steel billets heated and discharged from the furnace into steel bars;

[0014] S3: Rotary piercing, heating the obtained steel bars in a heating furnace and then performing rotary piercing, and directly putting the pierced rough pipe into the heating furnace for solution heat treatment;

[0015] S4: Cold rolling the solution-treated rough pipe to obtain a steel pipe;

[0016] Among them, the single-pass deformation is not less than 40%, and the deformation matching parameter Q value is 1.0 - 1.5;

[0017] S5: Heat treatment, performing solution treatment on the cold-rolled steel pipe to obtain an austenitic stainless steel pipe;

[0018] S6: Finally, adding an infiltration agent to the obtained austenitic stainless steel pipe, heating and holding for heat preservation, and after cooling, coating an antioxidant infiltration layer on the inner wall of the obtained austenitic stainless steel pipe.

[0019] Furthermore, in step S1, the cross-sectional size of the steel billet is 280×280 mm - 350×350 mm.

[0020] Furthermore, in step S2, the heating process includes a preheating section, a heating section, and a soaking section; the temperature of the preheating section is 500 - 800 °C, and after preheating, the steel billets are heated to the heating section at a rate not higher than 120 °C / h; the temperature of the heating section is 1200 - 1240 °C, after holding for 12 - 24 h, it is cooled with the furnace to the soaking section; the temperature of the soaking section is 1170 - 1190 °C.

[0021] Further, in step S3, the heating temperature during the cross-rolling pipe piercing process is 1150 - 1180 °C;

[0022] The solution temperature is 1180 - 1200 °C, and the solution time is 8 - 13 min.

[0023] Even further, in step S5, the solution temperature during heat treatment is 1160 - 1180 °C, and the solution time is 8 - 10 min.

[0024] Even further, in step S6, the heating temperature is 750 - 800 °C, and the heat preservation time is 2 - 4 h; then it is cooled in the furnace at a rate not exceeding 120 °C / h to below 400 °C.

[0025] Further, by mass percentage, the components of the penetrant are: FeAl powder: 70 - 75%; NH4Cl: 3 - 7%; Y2O3: 1.5%; Al2O3 powder: the balance;

[0026] Both the FeAl powder and the Al2O3 powder are 50 - 300 mesh.

[0027] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0028] 1. In the present invention, the austenitic stainless steel pipe includes a pipe body and an antioxidant penetration layer coated on its inner wall. Through the mutual cooperation of elements, the contents of elements such as Nb, N, and Al are optimized. At the same time, the precipitation tendency of large-sized Nb(C,N) can be reduced. Meanwhile, the Y element is added, which can improve the strength of the austenitic stainless steel pipe. The tensile strength at room temperature is not lower than 660 MPa, and the yield strength is not lower than 400 MPa; a part of the antioxidant penetration layer penetrates into the pipe body, which can improve the bonding force between the inner wall of the pipe body and the antioxidant penetration layer and prevent the problem of peeling during use; another part forms a micron-level FeAl phase penetration layer on the inner wall of the pipe body, with a thickness of more than 3 μm; during use, an Al2O3 film is quickly formed to prevent further oxidation of the pipe body, forming an effective protection for the entire life cycle of the steel pipe;

[0029] 2. The preparation method of the austenitic stainless steel pipe in the present invention mainly includes steps such as billet preparation, rolling and blooming, cross-rolling pipe piercing, cold rolling, and heat treatment. The obtained steel pipe has no cracks, the precipitation phases such as Nb(C,N) are evenly distributed, the grain size is between 6.0 and 7.0 grades and there is no mixed grain, and the overall strength is relatively high;

[0030] 3. In the present invention, surface modification is carried out by means of an aluminizing process on the inner wall. The aluminizing agent is FeAl powder, NH4Cl, Y2O3 and Al2O3 powder, and their proportions are controlled. After aluminizing, a micron-level FeAl phase aluminized layer is formed on the inner wall of the pipe body. This layer not only has a thermal expansion performance close to that of the matrix and is not easily detached, but also rapidly forms a layer of Al2O3 film during the oxidation process to prevent further oxidation of the matrix.

[0031] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combined solutions. Other features and advantages of the present invention will be described in the subsequent specification. Moreover, some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained from the content specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings are only used for the purpose of showing specific embodiments and are not considered to be a limitation on the present invention. Throughout the drawings, the same reference signs represent the same components.

[0033] Figure 1 It is the metallographic photograph of the pipe body in Embodiment 1;

[0034] Figure 2 It is the morphology of the oxide film in Embodiment 1;

[0035] Figure 3 It is the metallographic photograph of the pipe body in Comparative Example 3;

[0036] Reference signs in the drawings: 1. Pipe body; 2. Oxide layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] The following will specifically describe the preferred embodiments of the present invention in conjunction with the drawings. Among them, the drawings form a part of the present invention and are used together with the embodiments of the present invention to explain the principle of the present invention, rather than to limit the scope of the present invention.

[0038] Existing austenitic stainless steels have weak resistance to steam oxidation and intergranular corrosion. During service, scale is likely to appear on the inner wall, and intergranular corrosion cracking occurs on the outer wall due to flue gas corrosion. Especially under the background of "deep peak shaving" in thermal power generation, frequent periodic temperature changes are more likely to cause large-area peeling of the scale on the inner wall, and the scale accumulates at the elbow to trigger tube rupture, resulting in serious economic losses.

[0039] After shot peening treatment on the inner wall, the steam oxidation resistance can be further improved. However, the shot peening layer will gradually fade with the service time, and the antioxidant effect will decrease until it disappears.

[0040] Therefore, the present invention provides an austenitic stainless steel pipe with an antioxidant aluminized layer, and the stainless steel pipe includes a pipe body and an antioxidant aluminized layer coated on the inner wall of the pipe body;

[0041] The chemical composition of the pipe body, by mass percentage, includes: C: 0.07 - 0.08%, Si: 0.20 - 0.25%, Mn: 0.7 - 0.9%, Cr: 17.5 - 18.5%, Ni: 8.0 - 9.5%, Mo: 0.20 - 0.60%, Nb: 0.35 - 0.45%, Cu: 3.0 - 3.5%, N: 0.08 - 0.10%, Al ≤ 0.008%, B: 0.002 - 0.004%; Y: 0.02 - 0.08%; the balance is Fe.

[0042] The thickness of the antioxidant infiltration layer is ≥ 3 μm, and its phase is the FeAl phase.

[0043] Compared with the prior art, the austenitic stainless steel pipe provided by the present invention includes a pipe body and an antioxidant infiltration layer coated on its inner wall. Through the mutual cooperation between elements, the contents of elements such as Nb, N, and Al are optimized. At the same time, the precipitation tendency of large-sized Nb(C,N) can be reduced. At the same time, the Y element is added, which can improve the strength of the austenitic stainless steel pipe. The tensile strength at room temperature is not less than 660 MPa, and the yield strength is not less than 400 MPa. A part of the antioxidant infiltration layer penetrates into the pipe body, which can improve the bonding force between the inner wall of the pipe body and the antioxidant infiltration layer and prevent the problem of peeling during use. Another part forms a micron-scale FeAl phase infiltration layer on the inner wall of the pipe body, with a thickness of more than 3 μm. During use, an Al2O3 film is quickly formed to prevent the further oxidation of the pipe body, forming an effective protection for the entire life cycle of the steel pipe.

[0044] The functions of each element are as follows:

[0045] Carbon (C): Stabilizes and strengthens austenite; however, a high C content will increase carbides and reduce corrosion resistance; therefore, the content is controlled at 0.07 - 0.08%.

[0046] Silicon (Si): Si can generate a dense SiO2 oxide film on the surface and inside of the stainless steel matrix, significantly improving the oxidation resistance of austenitic stainless steel. However, Si is a ferrite-forming element, and excessive addition may result in ferrite, reducing the plasticity and toughness of the stainless steel. Therefore, the content is controlled at 0.2 - 0.25%.

[0047] Manganese (Mn): Manganese is an austenite stabilizing element and can significantly improve the strength of austenitic steel. Its synergistic effect with Ni can significantly increase the solubility of N in austenitic steel, but adding Mn does not significantly improve the corrosion resistance of austenitic steel. Therefore, the content is controlled at 0.7 - 0.9%.

[0048] Chromium (Cr): It can significantly improve the corrosion resistance of austenitic steel. However, Cr is a ferrite stabilizing element and can also form compounds with C and N in the steel, deteriorating the toughness and plasticity of stainless steel. Therefore, the content is controlled at 17.5 - 18.5%.

[0049] Nickel (Ni): A typical austenite stabilizing element, it can improve the mechanical properties and hot working properties of steel. However, adding too much Ni will reduce the solubility of N, leading to the precipitation of nitrides in the steel. Therefore, the content is controlled at 8.0 - 9.5%.

[0050] Molybdenum (Mo): It can significantly improve the high-temperature mechanical properties of austenitic stainless steel and reduce the corrosion rate of stainless steel in reducing corrosive media. However, Mo is a ferrite-forming element and can cause the precipitation of intermetallic compounds such as σ-phase, χ-phase, and Laves phase. These precipitates will deteriorate the corrosion resistance and plasticity of austenitic stainless steel. Therefore, the content is controlled at 0.2 - 0.6%.

[0051] Niobium (Nb): Adding Nb to austenitic steel can reduce the combination of N and C with Cr, ensuring the stable existence of Cr in the matrix of stainless steel. And Nb-containing compounds can refine the grains of stainless steel and improve the mechanical properties. Therefore, the content is controlled at 0.35 - 0.45%.

[0052] Copper (Cu): It can improve the corrosion resistance of stainless steel in reducing corrosive media and form nano copper-rich phases to improve the creep strength. However, its solubility in stainless steel is low. Therefore, the content is controlled at 3.0 - 3.5%.

[0053] Nitrogen (N): An austenite-forming and stabilizing element, it can significantly expand the proportion and region of the austenite phase, and significantly improve the strength, creep properties, fatigue resistance, and work hardening ability, etc. without changing the strength and toughness of stainless steel. However, too high a content will form large-sized nitride precipitation phases, reducing the plasticity and toughness of the steel. Therefore, the content is controlled at 0.08 - 0.1%.

[0054] Boron (B): It can improve the hardenability of austenitic steel, strengthen the grain boundaries, improve the welding performance and corrosion resistance. However, too high a content will reduce the toughness of stainless steel and affect the processing performance. Therefore, the content is controlled at 0.002 - 0.004%

[0055] Aluminum (Al): Since a high Al content in heat-resistant steel is likely to form Al2O3 inclusions and further generate large-sized composite inclusions as the nucleation substrate of Nb(C,N), seriously affecting the matrix properties, the Al content is limited to not more than 0.008%.

[0056] Yttrium (Y): It can improve the purity of heat-resistant steel and inhibit the coarsening of the microstructure and precipitation phases during long-term service through solid solution strengthening, thereby improving the mechanical properties. At the same time, during the steam oxidation corrosion process of S30432 steel after aluminizing, Y can promote the formation of surface α-aluminum oxide and form Y-rich particles near the oxide film, further improving the compactness and stability of the aluminized layer structure.

[0057] Specifically, the chemical composition of the pipe body, by mass percentage, includes: C: 0.078 - 0.08%, Si: 0.22 - 0.25%, Mn: 0.8 - 0.9%, Cr: 17.5 - 18.2%, Ni: 8.5 - 9.5%, Mo: 0.45 - 0.60%, Nb: 0.35 - 0.42%, Cu: 3.2 - 3.5%, N: 0.092 - 0.10%, Al ≤ 0.008%, B: 0.002 - 0.004%; Y: 0.05 - 0.08%.

[0058] Specifically, the grain size of the pipe body is 6.0 - 7.0 grades.

[0059] It should be noted that for heat-resistant steel, an increase in grain size helps to eliminate weak grain boundaries during the long-term creep process, thereby improving the long-term strength. Therefore, by optimizing the composition and combining internal wall aluminizing to improve the corrosion resistance, and controlling the grain size at 6.0 - 7.0 grades, it helps to further improve the long-term strength of austenitic stainless steel pipes or pipe bodies.

[0060] The present invention also provides a method for preparing an austenitic stainless steel pipe with an antioxidant aluminized layer, including the following steps:

[0061] S1: Steel billet preparation, using continuous casting to produce steel billets, and applying periodic electromagnetic stirring at the solidification end;

[0062] S2: Rolling and blooming, heating the steel billets using a segmented heating method, and rolling and blooming the steel billets heated and discharged from the furnace into steel bars;

[0063] S3: Rotary piercing, heating the obtained steel bars in a heating furnace and then performing rotary piercing, and directly putting the pierced rough pipe into the heating furnace for solution heat treatment;

[0064] S4: Cold rolling the solution-treated rough pipe to obtain steel pipes;

[0065] wherein the single-pass deformation is not less than 40%, and the deformation matching parameter Q value is 1.0 - 1.5;

[0066] S5: Heat treatment, performing solution treatment on the cold-rolled steel pipes to obtain austenitic stainless steel pipes;

[0067] S6: Finally, a penetrant is added to the obtained austenitic stainless steel pipe, heated and kept warm, and after cooling, an anti-oxidation penetrant layer is coated on the inner wall of the obtained austenitic stainless steel pipe.

[0068] Compared with the prior art, the preparation method of the austenitic stainless steel pipe in the present invention mainly includes the steps of billet preparation, billet rolling, oblique rolling through the pipe, cold rolling and heat treatment. The obtained steel pipe has no cracks, the precipitation phases such as Nb (C, N) are evenly distributed, the grain size is 6.0 to 7.0, no mixed crystals will appear, and the overall strength is high.

[0069] In the present invention, the inner wall aluminizing process is adopted for surface modification, the aluminizing agents are FeAl powder, NH4Cl, Y2O3 and Al2O3 powder, and their proportions are controlled. After aluminizing, a micron-scale FeAl phase infiltration layer is formed on the inner wall of the tube body, which not only has a thermal expansion performance close to that of the matrix and is not easy to fall off, but also quickly forms a layer of Al2O3 film during the oxidation process to prevent further oxidation of the matrix.

[0070] Specifically, in step S1, the size of the steel billet is 280×280 mm to 350×350 mm.

[0071] It should be noted that the continuous casting production process adopted in the present invention has the advantages of low cost and high yield rate. At the same time, under the element ratio, core segregation and large-sized Nb (C, N) will not appear during the continuous casting process.

[0072] The cross-sectional size of the continuous casting billet of the present invention is 280×280~350×350mm. On the one hand, the continuous casting billet size is too small, the deformation ratio is too low in the subsequent rolling process, and the deformation of the core is insufficient, which cannot play the role of dense organization and improving the segregation of precipitated phases; on the other hand, when the cross-sectional size exceeds 350×350mm, the solidification quality of the core is seriously deteriorated, which is not conducive to subsequent forming. Therefore, this scheme requires that the cross-sectional size of the continuous casting square billet is between 280×280~350×350mm.

[0073] Specifically, the ingot segregation of the continuous casting billet is at level 0-1.0, and the residual shrinkage cavity is at level 0-1.0.

[0074] It should be noted that the factors affecting the degree of segregation in the core of the continuous casting billet are complex. In addition to the cross-sectional dimensions, it is also related to the V-type segregation at the end of solidification. In the segregation channel, chain Nb (C, N) is easily formed, which seriously affects the processing performance. Therefore, it is necessary to use periodic electromagnetic stirring at the end of solidification to ensure the quality of the inner wall of the seamless pipe, further improve the continuity of the infiltration layer, and then ensure that the ingot segregation of the continuous casting billet is 0-1.0 level and the residual shrinkage cavity is 0-1.0 level.

[0075] Specifically, in step S2, the heating process includes a preheating section, a heating section, and a temperature equalizing section; the temperature of the preheating section is 500 - 800 °C, and after preheating, the steel billet is heated to the heating section at a rate not higher than 120 °C / h; the temperature of the heating section is 1200 - 1240 °C, and after holding for 12 - 24 h, it is cooled with the furnace to the temperature equalizing section; the temperature of the temperature equalizing section is 1170 - 1190 °C.

[0076] It should be noted that in the present invention, the steel billet needs to be preheated at 500 - 800 °C first, which can ensure the uniform temperature of the whole steel billet and ensure that the core temperature and surface temperature of the steel billet are the same when in the heating section. The temperature of the preheating section can be 500 °C, 520 °C, 530 °C, 550 °C, 570 °C, 590 °C, 600 °C, 610 °C, 630 °C, 660 °C, 670 °C, 680 °C, 700 °C, 725 °C, 740 °C, 760 °C, 780 °C or 800 °C.

[0077] After preheating, the steel billet is heated to the heating section at a slower heating rate (such as a rate not higher than 120 °C / h, preferably 60 - 120 °C / h) to avoid excessive surface temperature and grain coarsening. Subsequently, it is held at 1200 - 1240 °C for 12 - 24 h to promote the re - solution of primary Nb(C,N) during the solidification process, and then cooled to 1170 - 1190 °C for rolling to obtain a steel with uniform grain structure and good hot workability.

[0078] In the present invention, the temperature of the heating section can be 1200 °C, 1205 °C, 1210 °C, 1218 °C, 1220 °C, 1225 °C, 1230 °C, 1235 °C or 1240 °C. The holding time can be 12 h, 15 h, 18 h, 20 h, 22 h or 24 h.

[0079] In the present invention, the temperature of the temperature equalizing section can be 1170 °C, 1172 °C, 1175 °C, 1180 °C, 1185 °C or 1190 °C.

[0080] Specifically, in step S3, the heating temperature during the cross - rolling piercing process is 1150 - 1180 °C; the solution temperature is 1180 - 1200 °C, and the solution time is 8 - 13 min.

[0081] It should be noted that in the present invention, after cross - rolling piercing, solution treatment is carried out to dissolve the precipitated phases and promote the dispersed precipitation during the subsequent cooling process. After adopting the cross - rolling piercing and cold rolling processes, the precipitated phases are broken and deformed, which is more conducive to re - solution.

[0082] In the present invention, the heating temperature during the oblique rolling and pipe-piercing process may be 1150° C., 1155° C., 1160° C., 1165° C., 1170° C., 1175° C. or 1180° C. The solution temperature may be 1180° C., 1185° C., 1190° C., 1195° C. or 1200° C.

[0083] Specifically, in step S4, the deformation of a single cold rolling pass is not less than 40%, and the deformation matching parameter Q value is 1.0-1.5.

[0084] It should be noted that the steel billet is solid-solutionized after oblique rolling and pipe-piercing, which plays a role in further homogenizing the structure and promoting the dispersion of the precipitated phase. After that, it is cold-rolled for multiple times to obtain a seamless steel pipe, and the cold-rolling deformation is required to be not less than 40% (preferably 40%-60%) to ensure that the matrix stores sufficient deformation energy and recrystallization is more sufficient. At the same time, the Q value (ratio of inner diameter reduction rate and wall thickness reduction rate) is controlled to 1.0-1.5, reducing the incidence of inner wall cracks and improving the yield rate.

[0085] Specifically, in step S5, the solution temperature during heat treatment is 1160-1180° C., and the solution time is 8-10 min.

[0086] It should be noted that the original grains and precipitated phases are broken and deformed during the cold rolling process, and direct heat treatment and solid solution treatment after cold rolling is more conducive to the dissolution of the precipitated phases. At the same time, the solid solution temperature during the heat treatment process can adjust the final grain size and ensure the mechanical properties.

[0087] In the present invention, the solution temperature during heat treatment is 1160°C, 1165°C, 1170°C, 1175°C or 1180°C.

[0088] Specifically, in step S6, the heating temperature is 750-800°C, and the holding time is 2-4h; then the furnace is cooled to below 400°C at a rate not exceeding 120°C / h.

[0089] The composition of the penetrant is as follows by mass percentage: FeAl powder: 70-75%; NH4Cl: 3-7%; Y2O3: 1.5%; Al2O3 powder: the balance;

[0090] The FeAl powder and Al2O3 powder are both 50-300 mesh.

[0091] In the present invention, the steps of preparing the anti-oxidation diffusion layer on the inner wall of the pipe body are as follows:

[0092] First, one end of the seamless tube (i.e., the tube body or the austenitic stainless steel tube obtained after heat treatment) is sealed using an iron sheet by welding. Then, the well-mixed infiltrant is poured into the unsealed end. Subsequently, the seamless tube is continuously tapped or vibrated to ensure that the infiltrant is loaded continuously without cavities. After filling, the unsealed end of the seamless tube is welded and sealed using an iron sheet, and ventilation holes are left on the iron sheet.

[0093] Then, the steel tube filled with the infiltrant is placed vertically (with the ventilation holes facing upward) into a heating furnace and heated at 750 - 800 °C for 2 - 4 h to complete aluminizing of the inner wall. Finally, it is cooled in the furnace at a rate not exceeding 120 °C / h to below 400 °C, the iron sheet is removed, and the infiltrant is taken out from the tube body to complete the preparation of the antioxidant infiltration layer.

[0094] It should be noted that the main reactions during the aluminizing process are as follows:

[0095]

[0096] 2FeAl + 6HCl → AlCl3 + FeCl3 + 3H2 (2)

[0097] AlCl3 + FeCl3 → FeAl + 3Cl2 (3)

[0098] The aluminizing process provided by the present invention can achieve effective aluminizing at 700 - 750 °C. Meanwhile, this temperature can perform age hardening on the tube body. Keeping it at this temperature range for 2 - 4 h helps the precipitation of nano - Cu - rich phases in the austenitic stainless steel tube, providing sufficient strength for the steel tube.

[0099] It should be noted that in the present invention, FeAl powder in the infiltrant serves as the main Al source, and a stable FeAl infiltration layer can be formed on the inner wall of the tube through equations (1) - (3); NH4Cl serves as an accelerating agent to promote the decomposition of FeAl powder to form halide gases, which are then deposited on the inner wall of the tube to form an infiltration layer; Y2O3 serves as an activator, which does not participate in the reaction itself but plays a role in accelerating the formation of the infiltration layer. Al2O3 is a stabilizer to control the reaction rate.

[0100] It should be noted that under heating conditions, NH4Cl decomposes into ammonia and hydrogen chloride gases. Among them, hydrogen chloride gas reacts with FeAl powder to generate gaseous AlCl3 and FeCl3. At high temperatures, gaseous AlCl3 and FeCl3 re - form solid FeAl (i.e., FeAl undergoes a process of solid → gas → solid). It is worth noting that this process occurs on the inner wall surface, forming an antioxidant infiltration layer on the inner wall surface and improving the antioxidant ability.

[0101] The austenitic stainless steel tube with an antioxidant infiltration layer obtained by the present invention is mainly used for super-supercritical power station superheaters and reheater heating surface tubes at 600-700°C. During use, the antioxidant infiltration layer will not fall off, the service life is longer, and it is safer.

[0102] To describe the present invention more clearly, it is further illustrated by the following examples and comparative examples.

[0103] Example 1

[0104] The preparation method of the austenitic stainless steel tube with an antioxidant infiltration layer includes the following steps:

[0105] S1: Steel billet preparation. The steel billet is produced by continuous casting, and periodic electromagnetic stirring is used at the solidification end.

[0106] By mass percentage, the chemical composition of the steel billet is C: 0.078%, Si: 0.22%, Mn: 0.8%, Cr: 18.2%, Ni: 8.5%, Mo: 0.45%, Nb: 0.42%, Cu: 3.2%, N: 0.092%, Al: 0.003%, B: 0.0025%; Y: 0.05%.

[0107] At the solidification end, periodic electromagnetic stirring is used, stirring clockwise for 15 s → pausing for 5 s → stirring counterclockwise for 15 s. The cross-sectional size of the obtained steel billet is 320×320 mm, and the ingot segregation is 0.5 level, and the residual shrinkage cavity is 0.5 level.

[0108] S2: Rolling and blooming. The steel billet is heated by a segmented heating method, and the steel billet is heated and rolled into a steel bar after being taken out of the furnace.

[0109] After the preheating section is kept at 600°C for 6 h, it is heated to the heating section at 1220°C at a rate of 90°C / h, kept at 1220°C for 24 h, then cooled with the furnace to 1180°C, kept at 1180°C for 2 h, and then taken out of the furnace and rolled into Steel bar;

[0110] S3: Rotary piercing. The obtained steel bar is heated to 1160°C in a heating furnace and then subjected to rotary piercing. The obtained rough tube is immediately water-cooled and quenched, and then placed in a heating furnace and solution-treated at 1190°C for 8 min; the size of the rough tube is Φ116×20 mm.

[0111] S4: The solution-treated rough tube is cold-rolled twice. The deformation amount of the first cold rolling is 47%, and the deformation matching parameter Q value is 1.5. The size of the first cold-rolled tube is Φ72×16 mm; the deformation amount of the second cold rolling is 54%, and the deformation matching parameter Q value is 1.1. The size of the manufactured steel tube is Φ54×11 mm.

[0112] S5: Heat treatment. The cold-rolled steel pipe is solutionized at 1180 °C for 10 min to obtain an austenitic stainless steel pipe.

[0113] S6: Finally, an infiltration agent is added to the obtained austenitic stainless steel pipe, heated at 760 °C and held for 4 h, then cooled in the furnace at a rate of 80 °C / h to below 400 °C, and an antioxidant infiltration layer with a thickness of 5 μm is coated on the inner wall of the obtained austenitic stainless steel pipe.

[0114] The composition of the infiltration agent is as follows: FeAl powder: 70%; NH4Cl: 4%; Y2O3: 1.5%; Al2O3 powder: 24.5%. Both the FeAl powder and the Al2O3 powder are powders with a mesh size of 50 - 300.

[0115] Example 2

[0116] The preparation process of Example 2 is generally the same as that of Example 1. The difference is that in Example 2, the chemical composition of the steel billet is C: 0.074%, Si: 0.22%, Mn: 0.8%, Cr: 17.8%, Ni: 8.4%, Mo: 0.38%, Nb: 0.43%, Cu: 3.2%, N: 0.088%, Al: 0.004%, B: 0.0022%; Y: 0.02%.

[0117] Example 3

[0118] The preparation process of Example 3 is generally the same as that of Example 1. The difference is that in Example 3, in step S1, the cross-sectional size of the steel billet is 280×280 mm.

[0119] In step S2, the temperature of the preheating section is 800 °C. After preheating, the steel billet is heated to the heating section at a rate of 120 °C / h. The temperature of the heating section is 1200 °C. After holding for 12 h, it is cooled in the furnace to the temperature equalizing section. The temperature of the temperature equalizing section is 1170 °C.

[0120] Example 4

[0121] The preparation process of Example 4 is generally the same as that of Example 1. The difference is that in Example 4, in step S4, the deformation amount of the first cold rolling is 44%, the deformation matching parameter Q value is 1.1, and the size of the first cold-rolled pipe is Φ85×15 mm. The deformation amount of the second cold rolling is 58%, the deformation matching parameter Q value is 1.1, and the size of the obtained steel pipe is Φ55×10 mm.

[0122] Example 5

[0123] Example 5 is generally the same as Example 1 in the preparation process. The difference is that in Example 5, the components of the infiltrant are: FeAl powder: 75%; NH4Cl: 6%; Y2O3: 1.5%; Al2O3 powder: 17.5%, where both the FeAl powder and the Al2O3 powder are powders with a mesh size of 50 - 300.

[0124] Comparative Example 1

[0125] Comparative Example 1 is generally the same as Example 1 in the preparation process. The difference is that the steel billet in Comparative Example 1 does not contain Y.

[0126] Comparative Example 2

[0127] Comparative Example 2 is generally the same as Example 1 in the preparation process. The difference is that in Comparative Example 2, C: 0.13%, Mn: 0.3%, Mo: 0.3%.

[0128] Comparative Example 3

[0129] Comparative Example 3 is generally the same as Example 1 in the preparation process. The difference is that the cold rolling process in Comparative Example 3 is as follows: The deformation amount of the first cold rolling is 39%, and the Q value is 3.3. The deformation amount of the second cold rolling is 54%, and the Q value is 1.1.

[0130] After solution treatment, the grain size is 6.5 - 8.5 grades, and obvious mixed grains exist, as Figure 3 shown.

[0131] Comparative Example 4

[0132] Comparative Example 4 is generally the same as Example 1 in the preparation process. The difference is that in step S2 of Comparative Example 4, after preheating at 600°C for 6 hours, it is heated to the soaking section at 1180°C at a rate of 90°C / h, and after soaking for 6 hours, it is taken out of the furnace and rolled into steel bars.

[0133] Comparative Example 5

[0134] Comparative Example 5 is generally the same as Example 1 in the preparation process. The difference is that in step S6 of Comparative Example 5, the heating temperature of high-temperature aluminizing is 1000°C, and then it is cooled to room temperature in the furnace.

[0135] Comparative Example 6

[0136] Comparative Example 6 is generally the same as Example 1 in the preparation process. The difference is that in Comparative Example 6, the infiltrant is: Al powder: 50%, NH4Cl: 4%, Y2O3: 1.5%, Al2O3: 44.5%.

[0137] After detection, the main phases in the infiltration layer of Comparative Example 6 are Fe2Al5 and FeAl3, and there is a large hardness difference between it and the matrix. Cracks are extremely likely to occur during the frequent temperature changes under the peak shaving working conditions, affecting the performance.

[0138] Performance detection

[0139] The above-mentioned Examples 1-4 and Comparative Examples 1-6 were subjected to performance detection, mainly including yield strength, tensile strength, creep strength, grain size, intergranular corrosion detection, and the oxide film thickness after 1000 h of steam oxidation at 700 °C. The detection results are shown in Table 1.

[0140] Table 1 Performance detection results

[0141]

[0142] Combining Examples 1-4 and Comparative Examples 1-6 and combining Table 1 and Figures 1-3 , it can be seen that by using the preparation method provided in the embodiments of the present invention, the obtained austenitic stainless steel pipe includes a pipe body and an antioxidant infiltration layer coated on the inner wall of the pipe body. At room temperature, the tensile strength is 679-690 MPa, the yield strength is 470-477 MPa, the grain size is 6.5-7.0 grades, the creep strength extrapolated for 100,000 hours at 650 °C is 168-175 MPa, and the oxide film thickness after 1000 h of steam oxidation at 700 °C (the smaller the value, the less severe the oxidation).

[0143] Combining Example 1 and referring to Figure 2 , a part of the antioxidant infiltration layer penetrates into the pipe body, which can improve the bonding force between the inner wall of the pipe body and the antioxidant infiltration layer and prevent the problem of peeling during use; the other part forms a micron-level FeAl phase infiltration layer on the inner wall of the pipe body, with a thickness of more than 3 μm; during use, an Al2O3 film (the brighter the color, the higher the element content) is quickly formed to prevent the further oxidation of the pipe body and form an effective protection for the entire life cycle of the steel pipe.

[0144] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. An austenitic stainless steel pipe containing an oxidation-resistant layer, characterized in that: The stainless steel pipe comprises a pipe body and an anti-oxidation permeation layer coated on the inner wall of the pipe body; The chemical composition of the tube body, by mass percentage, includes: C: 0.07-0.08%, Si: 0.20-0.25%, Mn: 0.7-0.9%, Cr: 17.5-18.5%, Ni: 8.0-9.5%, Mo: 0.20-0.60%, Nb: 0.35-0.45%, Cu: 3.0-3.5%, N: 0.08-0.10%, Al≤0.008%, B: 0.002-0.004%, Y: 0.02-0.08%, and the balance is Fe; The thickness of the anti-oxidation diffusion layer is ≥3 μm, and its phase is FeAl phase.

2. The austenitic stainless steel pipe with an oxidation resistant layer according to claim 1, characterized in that: The chemical composition of the tube body, by mass percentage, includes: C: 0.078-0.08%, Si: 0.22-0.25%, Mn: 0.8-0.9%, Cr: 17.5-18.2%, Ni: 8.5-9.5%, Mo: 0.45-0.60%, Nb: 0.35-0.42%, Cu: 3.2-3.5%, N: 0.092-0.10%, Al≤0.008%, B: 0.002-0.004%, Y: 0.05-0.08%, and the balance is Fe.

3. The austenitic stainless steel pipe with an oxidation resistant layer according to claim 1, characterized in that: The grain size of the tube body is 6.0-7.

0.

4. A method for preparing an austenitic stainless steel pipe containing an oxidation resistant layer as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: S1: Billet preparation, using continuous casting to produce billets, and periodic electromagnetic stirring at the end of solidification; S2: rolling and blanking, heating the steel billet by a segmented heating method, and the steel billet is heated and rolled out of the furnace to form a steel bar; S3: Oblique rolling and tube penetration, the obtained steel bar is heated in a heating furnace and then oblique rolling and tube penetration is performed, and the rough tube after tube penetration is directly placed in a heating furnace for heating and solid solution; S4: cold rolling the rough pipe after solid solution to obtain a steel pipe; The single-pass deformation is not less than 40%, and the deformation matching parameter Q value is 1.0 to 1.5; S5: heat treatment, subjecting the cold-rolled steel pipe to solid solution treatment to obtain an austenitic stainless steel pipe; S6: Finally, a penetrant is added to the obtained austenitic stainless steel pipe, heated and kept warm, and after cooling, an anti-oxidation penetrant layer is coated on the inner wall of the obtained austenitic stainless steel pipe.

5. The method for preparing an austenitic stainless steel pipe containing an oxidation-resistant diffusion layer according to claim 4, characterized in that: In step S1, the cross-sectional dimensions of the steel billet are 280×280 mm to 350×350 mm.

6. The method for preparing an austenitic stainless steel pipe containing an oxidation resistant diffusion layer according to claim 4, characterized in that: In step S2, the heating process includes a preheating section, a heating section and a temperature-averaging section; the temperature of the preheating section is 500-800°C, and the billet is heated to the heating section at a rate not higher than 120°C / h after preheating; the temperature of the heating section is 1200-1240°C, and after being kept warm for 12-24 hours, it is cooled to the temperature-averaging section along with the furnace; the temperature of the temperature-averaging section is 1170-1190°C.

7. The method for preparing an austenitic stainless steel pipe containing an oxidation-resistant diffusion layer according to claim 4, characterized in that: In step S3, the heating temperature during the oblique rolling and tube threading process is 1150-1180°C; The solution temperature is 1180-1200°C, and the solution time is 8-13 minutes.

8. The method for preparing an austenitic stainless steel pipe containing an oxidation-resistant diffusion layer according to claim 4, characterized in that: In step S5, the solution temperature during heat treatment is 1160-1180°C, and the solution time is 8-10 minutes.

9. The method for preparing an austenitic stainless steel pipe containing an oxidation-resistant diffusion layer according to claim 4, characterized in that: In step S6, the heating temperature is 750-800°C, and the holding time is 2-4h; then the furnace is cooled to below 400°C at a rate not exceeding 120°C / h.

10. The method for preparing an austenitic stainless steel pipe containing an oxidation resistant layer according to claim 4, characterized in that: The composition of the penetrant is as follows by mass percentage: FeAl powder: 70-75%; NH4Cl: 3-7%; Y2O3: 1.5%; Al2O3 powder: the balance; The FeAl powder and Al2O3 powder are both 50-300 mesh.

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