A ferrite-free ultra-pure urea-grade austenitic stainless steel plate and preparation method thereof

Abstract

The present invention provides an ultra-pure urea-grade austenitic stainless steel plate without ferrite, wherein the chemical composition and weight percentage thereof are C: 0.01-0.03%, Si≤1.00%, Mn: 1.00-2.00%, P≤0.020%, S≤0.010%, Ni: 9.00-11.00%, Cr: 25.00-30.00%, Mo: 3.00-4.00%, and the remainder is Fe and unavoidable impurities, and the preparation method thereof comprises smelting-continuous casting-forging-homogenization-compounding-rolling-heat treatment. The stainless steel plate provided by the present invention takes into account good corrosion resistance and high-purity single structure without ferrite, and has excellent room temperature and high temperature performance and low Ni content.

Description

Technical Field

[0001] The invention relates to the technical field of urea-grade stainless steel, in particular to an ultra-pure urea-grade austenitic stainless steel plate without ferrite and a preparation method thereof. Background Art

[0002] As we all know, urea-grade stainless steel is a kind of austenitic stainless steel specially used in the urea production process. In the process of urea production, the intermediate product ammonium carbamate has strong corrosion resistance to stainless steel. General stainless steel cannot meet its corrosion resistance requirements, so a large amount of Cr and Mo need to be added to the steel. However, these elements are ferrite-forming elements in steel, and a certain amount of ferrite is inevitably generated during the continuous casting process. The appearance of these ferrites will have an adverse effect on the performance of austenitic stainless steel, such as increasing the tendency of hot working of steel plates to crack, resulting in the deterioration of the corrosion resistance of steel plates; in addition, when the steel plates are heated at room temperature for a long time, ferrite will form a brittle phase, affecting the toughness of the steel plates. Although the ferrite content of the steel plate will be reduced after heat treatment, the blocky ferrite distributed at the grain boundary in the center of the steel plate cannot be completely eliminated by heat treatment or reprocessing. In addition, although the large amount of austenite-forming element Ni can inhibit the formation of ferrite to a certain extent, due to its high price, the production cost of the steel plate is high, and it is also easy to form thermal fatigue cracks.

[0003] For thick-gauge urea-grade stainless steel, in order to ensure the compression ratio and internal quality, ingots or electroslag billets are generally used for production. However, the cooling speed of the core of the extra-thick billet is extremely slow during the cooling process, and there is a certain degree of looseness and segregation, resulting in a large amount of ferrite generated on the grain boundary there, and its content is much higher than that of the ferrite in the continuous casting billet. Therefore, the thicker the steel plate, the more difficult it is to eliminate the ferrite in the center.

[0004] The invention patent "A Urea-grade Stainless Steel Pipe and Its Production Process" (grant number CN103243279B) discloses a production process for a urea-grade austenitic stainless steel pipe, which has a Ni content of 19.6-19.8% and a ferrite content of less than 0.6%. However, the patent is not applicable to thick gauge steel plates and does not completely eliminate the ferrite content, while the Ni content is relatively high.

[0005] In order to achieve urea-grade austenitic stainless steel with good corrosion resistance and high-purity single structure without ferrite, as well as excellent room temperature and high temperature performance and low Ni content, it is urgent to develop a new type of ultra-pure urea-grade austenitic stainless steel plate without ferrite and its manufacturing method. Summary of the invention

[0006] In view of the above technical problems, a ferrite-free ultra-pure urea-grade austenitic stainless steel plate and a preparation method thereof are provided.

[0007] The technical means adopted by the present invention are as follows:

[0008] An ultra-pure urea-grade austenitic stainless steel plate without ferrite, the chemical composition and weight percentage of the stainless steel plate are as follows:

[0009] C: 0.01-0.03%, Si≤1.00%, Mn: 1.00-2.00%, P≤0.020%, S≤0.010%, Ni: 9.00-11.00%, Cr: 25.00-30.00%, Mo: 3.00-4.00%, and the rest are Fe and unavoidable impurities.

[0010] The effects of the above chemical elements are analyzed as follows:

[0011] C is an important element of austenitic stainless steel. It can expand the austenite zone and inhibit the formation of ferrite in the steel. In addition, the presence of C in the form of solid solution or carbide precipitation in the steel can effectively improve the room temperature and high temperature strength of the steel plate. However, too high C content not only increases the hardness of the heat affected zone of welding and cracking after reheating after welding, but also is not conducive to the intergranular corrosion resistance of the steel plate. Therefore, the C content is limited to 0.01-0.03%.

[0012] Si is the main deoxidizing element of austenitic stainless steel, which can effectively reduce the oxygen content in steel and improve the purity of steel. However, too high Si content will promote the formation of brittle σ phase or the precipitation of silicon-rich G phase at the grain boundary. Therefore, the Si content is ≤1.00%.

[0013] Mn is an important element of austenitic stainless steel. Mn can expand the austenite zone and improve the strength of steel, but too high Mn content will affect the welding performance of the steel plate. Therefore, the Mn content is limited to 1.00-2.00%.

[0014] Ni is an important element of austenitic stainless steel. Ni can expand the austenite zone and inhibit the formation of ferrite in the steel. At the same time, it is used in combination with Cr to ensure that the steel plate has good anti-oxidation and corrosion performance. Due to its high price, the Ni content is limited to 9.00-11.00%.

[0015] Cr is an important element for oxidation resistance and corrosion resistance in austenitic stainless steel. In order to ensure that the steel plate of the present invention has excellent corrosion resistance, the grain boundary is prevented from 23 The precipitation of C6 carbides causes chromium-poor areas to appear near the grain boundaries, and excessive addition of C6 carbides will lead to carbide coarsening, which in turn causes a decrease in the high-temperature strength and toughness of the steel plate. Therefore, the Cr content is limited to 25.00-30.00%.

[0016] Mo is an important element for improving high temperature strength and corrosion resistance in austenitic stainless steel. In addition, it has a strong affinity for P. It reduces the amount of P segregated at the grain boundary, which is beneficial to reduce the sensitivity of welding reheating cracking. Therefore, the Mo content is limited to 3.00-4.00%.

[0017] P and S are harmful elements of austenitic stainless steel. Theoretically, the lower the content, the better. However, excessive reduction will lead to a significant increase in manufacturing costs. Therefore, the P content is limited to ≤0.020% and the S content is limited to ≤0.010%.

[0018] The present invention also discloses a method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate. For products with a finished steel plate thickness of ≤20 mm, a smelting-continuous casting-forging-homogenization-rolling-heat treatment method is used. For products with a finished steel plate thickness of >20 mm and ≤100 mm, a smelting-continuous casting-forging-homogenization-compounding-rolling-heat treatment method is used. The specific method is as follows:

[0019] (1) Smelting and continuous casting process: Electric furnace smelting, VOD vacuum treatment and other processes are used for smelting. The continuous casting process is adopted, and the key to the process is to control the casting temperature. The casting temperature of the molten steel in the tundish is 1555-1575℃. Low temperature casting is better to refine the original cast structure. In order to control the central segregation and looseness of the continuous casting billet, electromagnetic stirring or continuous casting billet light reduction process is adopted, and the light reduction rate is controlled at 7-11%.

[0020] (2) Forging process: heat the continuous casting billet to 1220-1250℃, keep it warm for 5-10h, and control the deformation of each forging process to 10-20mm. First forge the length direction, then forge the width direction, and finally forge the thickness direction. The three-directional deformation is cumulatively performed for 5-10 times until the forging process is completed. This process is the key process to eliminate ferrite. Its main purpose is to deform the block ferrite in the center of the billet thickness into long strip ferrite through forging.

[0021] (3) Homogenization process: The forged billet is sent to the heating furnace and first heated to 650-700℃, and kept warm for 2-4h, then heated to 1240-1260℃, and kept warm for 80-100h. After the end of the heat preservation, in order to prevent water cooling after being taken out of the furnace, the cooling rate is 50-100℃ / min. This process is the key process for eliminating ferrite. Its main purpose is to promote the precipitation of a large amount of carbides at the grain boundary through the first stage of heat preservation, and cut the long strip ferrite into several small pieces of ferrite. The second stage of heat preservation is to completely eliminate the above small pieces of ferrite through high-temperature diffusion.

[0022] (4) Composite welding process: After forging, the blank is ground and its surface roughness is controlled at 4-6μm, and then packaged in a vacuum electron beam welder. The welding voltage is 150-260KV, the welding current is 40-200mA, the welding speed is 40-350mm / min, and the packaged blank is chamfered by 20-40°. The number of packaged blanks is determined by the thickness of the final product. When the thickness of the finished product is greater than 20 and ≤70mm, the number of packaged blanks is two; when the thickness of the finished product is greater than 70 and ≤100mm, the number of packaged blanks is three.

[0023] (5) Rolling process: billet heating temperature 1200-1230℃, first stage rolling temperature ≥1150℃, single pass reduction rate controlled at 10-15%, first stage rolling termination temperature ≥1000℃. For encapsulated composite billets, the first two passes single pass reduction rate ≥20%, and the rolling temperature at this time ≥1120℃. The intermediate billet thickness is determined according to the finished product thickness, generally 1.5-2 times the thickness of the finished steel plate. Do not use mill cooling water to remove phosphorus during the entire rolling process to prevent recrystallization from stopping due to too fast temperature drop of the billet surface; the second stage rolling temperature is 920℃-960℃, the final rolling temperature is controlled at 860-900℃, and the single pass reduction rate is 5-8%; the steel plate is directly laminar cooled after rolling, and the final cooling temperature is controlled at 650-700℃, the surface temperature of the steel plate.

[0024] (6) Heat treatment process: The steel plate adopts a two-stage heat treatment process. The main temperature of the first stage heat treatment is 1000-1050℃, and the net insulation time is 10-60mm. After the insulation is completed, the temperature is quickly raised to the second stage. The heat treatment temperature is 1100-1120℃, and the net insulation time is 10min. After leaving the furnace, it is water-cooled to room temperature.

[0025] The new nitrogen-containing urea-grade austenitic stainless steel with a width of 2000-5000 mm and a thickness of 10-100 mm manufactured by the above composition and process has a yield strength of ≥210 MPa, a tensile strength of ≥510 MPa, and an elongation of ≥50% at room temperature; a yield strength of ≥180 MPa and a tensile strength of ≥480 MPa at 200°C; a ferrite content of 0 and a grain size of 3-6; after sensitization treatment at 650°C×(1-4)h, it meets the requirements of method E of GB / T4334-2018 "Corrosion of Metals and Alloys - Test Method for Intergranular Corrosion of Stainless Steels", and at the same time, according to the requirements of method C of ASTMA262 "Standard Method for Detecting Intergranular Corrosion Sensitivity of Austenitic Stainless Steels", the average corrosion rate within five corrosion cycles is ≤3.0μm / 48h.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. In terms of corrosion resistance, the present invention greatly improves the corrosion resistance of the steel plate by adding a large amount of Cr and Mo elements, ensuring that the steel plate still meets the E method requirements of GB / T4334-2018 "Corrosion of Metals and Alloys - Test Method for Intergranular Corrosion of Stainless Steel" after sensitization treatment at 650°C and insulation for 1-4h, and at the same time meets the C method requirements of ASTMA262 "Standard Method for Detecting Intergranular Corrosion Susceptibility of Austenitic Stainless Steel" to meet the average corrosion rate within five corrosion cycles ≤3.0μm / 48h.

[0028] 2. In terms of manufacturing process, the present invention adopts forging + homogenization + (composite welding) process and cooperates with staged rolling and multi-stage heat treatment, which can not only completely eliminate the ferrite in the finished steel plate with a thickness of 10-100mm, but also ensure that the steel plate has a uniform grain size and the grain size reaches level 3-6.

[0029] Based on the above reasons, the present invention can be widely promoted in the fields of austenitic stainless steel plates and the like. DETAILED DESCRIPTION

[0030] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] The present invention provides an ultra-pure urea-grade austenitic stainless steel plate without ferrite, the chemical composition and weight percentage of the stainless steel plate are as follows:

[0032] C: 0.01-0.03%, Si≤1.00%, Mn: 1.00-2.00%, P≤0.020%, S≤0.010%, Ni: 9.00-11.00%, Cr: 25.00-30.00%, Mo: 3.00-4.00%, and the rest are Fe and unavoidable impurities.

[0033] For finished steel plate products with a thickness of ≤20mm, the method of smelting-continuous casting-forging-homogenization-rolling-heat treatment is adopted for preparation. For finished steel plate products with a thickness of >20mm and ≤100mm, the method of smelting-continuous casting-forging-homogenization-compounding-rolling-heat treatment is adopted for preparation. The specific method is as follows:

[0034] (1) Smelting and continuous casting process: Electric furnace smelting, VOD vacuum treatment and other processes are used for smelting. The continuous casting process is adopted, and the key to the process is to control the casting temperature. The casting temperature of the molten steel in the tundish is 1555-1575℃. Low temperature casting is better to refine the original cast structure. In order to control the central segregation and looseness of the continuous casting billet, electromagnetic stirring or continuous casting billet light reduction process is adopted, and the light reduction rate is controlled at 7-11%.

[0035] (2) Forging process: heat the continuous casting billet to 1220-1250℃, keep it warm for 5-10 hours, and control the deformation of each forging process to 10-20mm. First forge the length direction, then forge the width direction, and finally forge the thickness direction. The three-directional deformation is cumulatively performed for 5-10 times until the forging process is completed. This process is the key process to eliminate ferrite. Its main purpose is to deform the block ferrite in the center of the billet thickness into long strip ferrite through forging.

[0036] (3) Homogenization process: The forged billet is sent to the heating furnace and first heated to 650-700℃, and kept warm for 2-4h, then heated to 1240-1260℃, and kept warm for 80-100h. After the end of the heat preservation, in order to prevent water cooling after being taken out of the furnace, the cooling rate is 50-100℃ / min. This process is the key process for eliminating ferrite. Its main purpose is to promote the precipitation of a large amount of carbides at the grain boundary through the first stage of heat preservation, and cut the long strip ferrite into several small pieces of ferrite. The second stage of heat preservation is to completely eliminate the above small pieces of ferrite through high-temperature diffusion.

[0037] (4) Composite welding process: After forging, the blank is ground and its surface roughness is controlled at 4-6μm, and then packaged in a vacuum electron beam welder. The welding voltage is 150-260KV, the welding current is 40-200mA, the welding speed is 40-350mm / min, and the packaged blank is chamfered by 20-40°. The number of packaged blanks is determined by the thickness of the final product. When the thickness of the finished product is greater than 20 and ≤70mm, the number of packaged blanks is two; when the thickness of the finished product is greater than 70 and ≤100mm, the number of packaged blanks is three.

[0038] (5) Rolling process: billet heating temperature 1200-1230℃, first stage rolling temperature ≥1150℃, single pass reduction rate controlled at 10-15%, first stage rolling termination temperature ≥1000℃. For encapsulated composite billets, the first two passes single pass reduction rate ≥20%, and the rolling temperature at this time ≥1120℃. The intermediate billet thickness is determined according to the finished product thickness, generally 1.5-2 times the thickness of the finished steel plate. Do not use mill cooling water to remove phosphorus during the entire rolling process to prevent recrystallization from stopping due to too fast temperature drop of the billet surface; the second stage rolling temperature is 920℃-960℃, the final rolling temperature is controlled at 860-900℃, and the single pass reduction rate is 5-8%; the steel plate is directly laminar cooled after rolling, and the final cooling temperature is controlled at 650-700℃, the surface temperature of the steel plate.

[0039] (6) Heat treatment process: The steel plate adopts a two-stage heat treatment process. The main temperature of the first stage heat treatment is 1000-1050℃, and the net insulation time is 10-60mm. After the insulation is completed, the temperature is quickly raised to the second stage. The heat treatment temperature is 1100-1120℃, and the net insulation time is 10min. After leaving the furnace, it is water-cooled to room temperature.

[0040] The present specific implementation mode is described in detail below with 7 examples, and the chemical compositions of the 7 examples are shown in Table 1;

[0041] Table 1 Chemical composition of examples (wt%)

[0042] Example C Si Mn P S Ni Cr Mo 1 0.013 0.15 1.16 0.010 0.005 9.13 25.1 3.13 2 0.017 0.26 1.25 0.013 0.006 9.23 25.4 3.28 3 0.019 0.31 1.43 0.017 0.005 9.78 26.8 3.47 4 0.022 0.58 1.54 0.011 0.002 10.15 27.7 3.55 5 0.025 0.67 1.69 0.014 0.008 10.56 28.3 3.61 6 0.027 0.89 1.80 0.017 0.004 10.87 29.2 3.73 7 0.029 0.92 1.96 0.010 0.002 10.98 29.9 3.99

[0043] The smelting, continuous casting and forging processes of the 7 embodiments are shown in Table 2;

[0044] Table 2 Smelting and forging process of the embodiment

[0045]

[0046] The homogenization processes of the 7 embodiments are shown in Table 3;

[0047] Table 3 Homogenization process of the embodiment

[0048] Example First stage temperature ℃ Phase 1 time h First stage temperature ℃ Phase 1 time h Cooling rate℃ / min 1 660 2 1251 87 68 2 650 2 1249 93 73 3 680 3 1255 89 85 4 675 4 1258 99 93 5 690 2 1247 85 67 6 685 4 1250 97 56 7 670 3 1259 98 64

[0049] The composite welding processes of Examples 3 to 7 are shown in Table 4;

[0050] Table 4 Example composite process

[0051] Example Roughness μm Voltage KV Current mA Speed ​​mm / min Chamfer° Number of continuous casting billets 3 4.6 150 40 40 34 2 4 5.1 180 80 110 27 2 5 5.9 200 120 230 39 2 6 4.7 220 160 300 21 3 7 5.5 260 200 350 40 3

[0052] The rolling processes of the seven embodiments are shown in Table 5;

[0053] Table 5 Rolling process of the embodiment

[0054]

[0055] The heat treatment processes of the 7 embodiments are shown in Table 6;

[0056] Table 6 Heat treatment process of the embodiment

[0057]

[0058] The mechanical properties of a ferrite-free ultra-pure urea-grade austenitic stainless steel plate prepared according to the above-mentioned composition ratio and preparation parameters are shown in Table 7;

[0059] Table 7 Mechanical properties of examples

[0060]

[0061]

[0062] The corrosion resistance of a ferrite-free ultra-pure urea-grade austenitic stainless steel plate prepared according to the above-mentioned composition ratio and preparation parameters is shown in Table 8;

[0063] Table 8 Corrosion performance of examples

[0064] Example Thickness mm Sensitization temperature℃ Sensitization time h Corrosion results Average corrosion rate μm / 48h 1 10 650 1 qualified 2.94 2 20 650 2 qualified 2.71 3 30 650 3 qualified 2.68 4 50 650 3 qualified 2.61 5 70 650 4 qualified 2.74 6 80 650 2 qualified 2.88 7 100 650 4 qualified 2.92

[0065] It can be seen from Tables 6 and 7 that the ferrite-free ultra-pure urea-grade austenitic stainless steel plate provided by the present invention has both good corrosion resistance and ferrite-free high-purity single structure, and has excellent room temperature and high temperature performance and low Ni content.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate, characterized in that: The chemical composition and weight percentage of the stainless steel plate are as follows: C: 0.01-0.03%, 0.31≤Si≤1.00%, Mn: 1.00-2.00%, P≤0.020%, S≤0.010%, Ni: 9.00-10.87%, Cr: 25.00-30.00%, Mo: 3.13-4.00%, the rest is Fe and unavoidable impurities; The preparation method thereof comprises the following steps: Smelting and continuous casting: Smelting is performed according to the above composition, followed by continuous casting; During the continuous casting process, the tundish molten steel casting temperature is 1555-1575℃ to refine the original cast structure; the continuous casting billet light reduction process is adopted, in which the light reduction rate is controlled at 7-11% to control the center segregation and looseness of the continuous casting billet; forging: The continuous casting billet is heated to 1220-1250℃ and kept warm for 5-10h. The deformation amount of each forging pass is controlled to be 10-20mm. During forging, the length direction is forged first, then the width direction is forged, and finally the thickness direction is forged. The deformation in the three directions is cumulatively performed for 5-10 passes until the forging process is completed. The block ferrite in the center of the billet thickness is deformed into a long strip of ferrite by forging; Homogenization: The forged billet is sent to a heating furnace and first heated to 650-700°C for 2-4h of first-stage net heat preservation, then heated to 1240-1260°C for 80-100h of second-stage net heat preservation, and then water-cooled at a cooling rate of 50-100°C / min. The first-stage net heat preservation promotes the precipitation of a large amount of carbides at the grain boundaries, and the long strip ferrite is cut into several small pieces of ferrite. The second-stage net heat preservation completely eliminates the small pieces of ferrite through high-temperature diffusion. Rolling: The billet heating temperature is 1200-1230℃, the first-stage rolling temperature is ≥1120℃, and the first-stage rolling termination temperature is ≥1000℃; the second-stage rolling temperature is 920℃-960℃, the final rolling temperature is controlled at 860-900℃, and the single-pass reduction rate is 5-8%; the steel plate is directly laminar-cooled after rolling, and the final cooling temperature is controlled at the steel plate surface temperature of 650-700℃; During the rolling process, the mill cooling water is not used for dephosphorization to prevent the recrystallization from stopping due to the rapid temperature drop of the billet surface; After heat treatment, the finished product of the stainless steel plate is obtained; A two-stage heat treatment process is used during the heat treatment. The first stage heat treatment is 1000-1050℃, with a net insulation time of 10-60mm. After the insulation is completed, the temperature is raised to the second stage. The temperature of the second stage heat treatment is 1100-1120℃, with a net insulation time of 10min. After being taken out of the furnace, it is water-cooled to room temperature.

2. The method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate according to claim 1, characterized in that: The finished product thickness of the stainless steel plate is 10-20 mm; and during the rolling process, the first-stage rolling temperature is ≥1150° C., and the single-pass reduction rate of the first stage is controlled at 10-15%.

3. The method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate according to claim 1, characterized in that: The finished thickness of the stainless steel plate is greater than 20 mm and less than or equal to 100 mm. Before the rolling, a composite welding step is also performed, in which two or three homogenized billets are packaged by composite welding. The packaged billets are rolled, and during the rolling process, the reduction rate of the first two passes in one stage is greater than or equal to 20%.

4. The method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate according to claim 3, characterized in that: During the composite welding process, the homogenized blank is ground and its surface roughness is controlled at 4-6μm, and then packaged in a vacuum electron beam welder with a welding voltage of 150-260KV, a welding current of 40-200mA, a welding speed of 40-350mm / min, and a chamfer of 20-40° for the packaged blank.

5. The method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate according to claim 4, characterized in that: During the composite welding process, when the finished thickness of the stainless steel plate is greater than 20 mm and less than or equal to 70 mm, two blanks are packaged; when the finished thickness of the stainless steel plate is greater than 70 mm and less than or equal to 100 mm, three blanks are packaged.

6. The method for preparing a ferrite-free ultra-pure urea-grade austenitic stainless steel plate according to claim 2 or 3, characterized in that: During the rolling process, the thickness of the intermediate billet is 1.5-2 times the thickness of the finished stainless steel plate.

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