A low-temperature ethylene spherical tank steel plate with a yield strength of 690mpa and a manufacturing method thereof

Abstract

The application provides a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa and a manufacturing method thereof, the steel plate comprises the following components in percentage by weight: C: 0.04%-0.10%, Si: 0.10%-0.30%, Mn: 1.00%-2.00%, P: ≤0.010%, S: ≤0.006%, Cr: 1.00%-1.50%, Mo: 0.20%-0.50%, Ni: 0.50%-1.00%, Nb: 0.10%-0.50%, Al: 0.0025%-0.0045%, and the balance of Fe and inevitable impurities; the manufacturing method comprises smelting, continuous casting, heating, rolling and heat treatment; the steel plate produced by the application has a thickness specification of 10-50 mm, a yield strength R eL ≥690 MPa, a tensile strength R m : 800-920 MPa, an elongation A ≥18%, and a KV2 of-50 ℃ ≥100 J; after simulating post-welding heat treatment, the mechanical properties of the steel plate are a yield strength R eL ≥680 MPa, a tensile strength R m : 780-900 MPa, an elongation A ≥18%, and a KV2 of-50 ℃ ≥90 J.

Description

Technical Field

[0001] This invention belongs to the field of steel materials, and particularly relates to a steel plate for low-temperature ethylene spherical tanks with a yield strength of 690 MPa and its manufacturing method. Background Technology

[0002] With the continuous development of my country's ethylene industry, the domestic demand for ethylene spherical tanks is also increasing. Steel plates used for constructing -50℃ cryogenic ethylene spherical tanks typically employ 15MnNiNbDR, 07MnNiMoDR, and 09MnNiDR. However, as China's demand for petrochemical energy continues to grow, more spherical tanks are needed annually for raw material storage or production facilities. Ethylene spherical tanks are characterized by high pressure, low temperature, and the flammability and explosiveness of the media. Simultaneously, to continuously improve the economic efficiency of petrochemical plants, spherical storage tanks for ambient temperature ethylene storage are gradually developing towards larger sizes and higher parameters. Furthermore, based on the continuous advancements in material manufacturing, welding, equipment manufacturing, and installation technologies, China's domestic production capacity for large spherical tanks needs further improvement. The development of steel plates for ethylene spherical tanks towards high strength, large size, and lightweight design not only demands higher strength and toughness but also excellent comprehensive performance. Existing domestic steel plates for ethylene spherical tanks cannot meet the high strength requirements; therefore, researching a steel plate for cryogenic ethylene spherical tanks with a yield strength of 690MPa has become an urgent task for China's steel industry.

[0003] Steel plates for ethylene spherical tanks are typically produced through a quenching and tempering heat treatment process, resulting in good low-temperature impact toughness (-50℃), high strength, and excellent comprehensive mechanical properties. Subsequent testing, including simulated post-weld heat treatment to simulate the welding and hot forming processes of the vessels, demonstrates that the steel plates still meet the required mechanical properties. Therefore, there is an urgent need to develop a low-temperature ethylene spherical tank steel plate with a yield strength of 690 MPa and its manufacturing method to meet the needs of the steel industry.

[0004] The patent document "Heat Treatment Method for Thin-gauge Quenched and Tempered 07MnNiMoDR ​​Steel Plate" (CN111041163A) adopts a quenching and tempering heat treatment process, selecting a quenching temperature lower than the conventional quenching temperature, and adding a suitable tempering process to obtain a mixed structure of tempered bainite and ferrite with a certain proportion. The grains are fine, with a grain size of 8.0-10.0 grade. The resulting steel plate has moderate strength and excellent toughness, and the transverse impact energy at -50℃ reaches more than 180J, which meets the requirements for steel plates for low-temperature spherical tanks. However, its yield strength is difficult to reach 690MPa, which cannot meet the requirements.

[0005] The patent document "A 07MnNiMoDR ​​Steel for Low Temperature Pressure Vessels and Its Texture Control Method" (CN114507811A) adopts a low C, Mo, Ni alloy and Nb, V, Ti micro-alloying design, and through a reasonable "sub-temperature quenching + tempering" process, it achieves a high degree of randomization of texture, relatively diffuse grain orientation and relatively balanced content of each typical texture in the 07MnNiMoDR ​​steel for low temperature pressure vessels, thereby significantly improving the toughness of the 07MnNiMoDR ​​steel for low temperature pressure vessels. However, the quenching temperature is low, the microstructure transformation is insufficient, mixed crystals are prone to occur, and the strength does not meet the 690MPa requirement.

[0006] The main chemical composition of the steel for spherical tanks with low surface hardness and its production method (CN117026093A) is C 0.05-0.07%, Si 0.15-0.30%, Mn 1.40-1.60%, P≤0.008%, S≤0.003%, Alt 0.020-0.050%, Nb 0.020-0.030%, V 0.040-0.060%, Ni 0.30-0.50%, Mo 0.20-0.30%, Sn≤0.008%, with a yield strength of 550-570MPa. The designed composition cannot meet the requirement of a yield strength of 690MPa.

[0007] Therefore, in view of the above situation, it is urgent to develop a new type of high-strength steel by adjusting the manufacturing method and designing new components, and to develop a steel plate for low-temperature ethylene spherical tanks with a yield strength of 690MPa and its manufacturing method. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a steel plate for low-temperature ethylene spherical tanks with a single tempered sorbite microstructure, which eliminates the damage of ferrite to the strength of the steel plate, so that the steel plate has excellent strength and toughness, and a yield strength of 690 MPa with sufficient strength and excellent low-temperature toughness, as well as a method for manufacturing the same.

[0009] The objective of this invention is achieved as follows:

[0010] A steel plate for low-temperature ethylene spherical tanks with a yield strength of 690 MPa, the steel plate having the following composition by weight percentage: C: 0.04%–0.10%, Si: 0.10%–0.30%, Mn: 1.00%–2.00%, P: ≤0.010%, S: ≤0.006%, Cr: 1.00%–1.50%, Mo: 0.20%–0.50%, Ni: 0.50%–1.00%, Nb: 0.10%–0.50%, Al: 0.0025%–0.0045%, with the balance being Fe and unavoidable impurities.

[0011] The microstructure of the steel plate is a single tempered sorbite, with carbide sizes of 2-4 μm.

[0012] The steel plate has a thickness of 10–50 mm, ReL ≥ 690 MPa, Rm: 800–920 MPa, A ≥ 18%, and KV2 ≥ 100 J at -50℃. The simulated mechanical properties of the steel plate after post-weld heat treatment are: yield strength ReL ≥ 680 MPa, tensile strength Rm: 780–900 MPa, elongation A ≥ 18%, and KV2 ≥ 90 J at -50℃.

[0013] The rationale for the design of the components in this invention is as follows:

[0014] In steel, carbon (C) forms various carbides or solid solutions with alloying elements to strengthen the steel. It is the element that directly improves the strength of steel plates. However, excessively high C content leads to poor toughness, plasticity, and weldability. Therefore, to ensure that steel plates have a good balance of low-temperature impact toughness, high strength, and weldability during use, and because increased C content can easily lead to an increase in bainite and cementite, affecting toughness, the C content is limited to 0.04%–0.10%.

[0015] Silicon (Si) is an element that increases carbon activity in steel. In high-concentration regions, it reduces the diffusion flux of carbon to carbides, inhibiting carbide coarsening. Furthermore, Si can dissolve in ferrite to improve steel plate strength and hardenability, and within a certain range, it can lower the ductile-brittle transition temperature and improve the plasticity of steel. Simultaneously, Si improves the fluidity of molten steel, which is beneficial for casting performance. Therefore, the Si content is limited to 0.10%–0.30%.

[0016] Mn plays a role in solid solution strengthening in steel. It can dissolve in large quantities in the Fe matrix to increase the strength of steel plates, improve toughness, hot workability, and sulfide distribution, and prevent hot cracking. Furthermore, with increasing Mn content, it can improve the stability of austenite in the steel plate during heating, lower the critical austenite transformation temperature during cooling, and significantly improve the hardenability and tempering stability of the steel during quenching and tempering heat treatment. However, excessively high Mn content can easily lead to segregation in the steel plate, which is detrimental to plasticity and toughness. Therefore, Mn content is limited to 1.00%–2.00%.

[0017] Phosphorus (P) is a harmful element in steel, significantly impairing low-temperature impact toughness and being highly prone to segregation. This invention controls P within a low range, thus keeping the P content below 0.010%.

[0018] Sulfur (S) is also a harmful element in steel, readily forming MnS within the steel. MnS can easily become a crack initiation point during subsequent processing and significantly impacts the toughness of the steel. Therefore, S content should be controlled below 0.006%.

[0019] Cr, as the main element in this invention, is a weak carbide-forming element. Adding chromium reduces the dissolution rate of carbides, improves the toughness and hardenability of the steel plate, and simultaneously promotes the formation of bainite, a product of low-temperature phase transformation. During tempering heat treatment, the generated Cr7C3 or Cr... 23 C6 carbides precipitate, which enhances the strength and toughness of the steel plate through precipitation strengthening, and the hardenability of Cr ensures uniform surface and core microstructure. For the offline quenched and tempered steel plate of this invention, a certain amount of Cr is required to guarantee the hardenability of the steel plate; therefore, the Cr content is controlled between 1.00% and 1.50%.

[0020] Mo, the main element in this invention, improves hardenability in steel. During controlled rolling and cooling, it inhibits the formation of grain boundary ferrite and promotes bainite transformation. Appropriate amounts of Mo provide nucleation sites for acicular ferrite during welding, improving weldability. Through solid solution strengthening, it dissolves in austenite and ferrite to enhance the strength and toughness of steel, while also suppressing temper brittleness. Mo is relatively expensive, and adding too much increases cost; therefore, the Mo content is between 0.20% and 0.50%.

[0021] Ni, as the main element in this invention, is a key element for stabilizing austenite. It can exist in austenite and ferrite in a solid solution with Fe, thereby improving the strength of steel and refining the grain size. Simultaneously, Ni can improve the ultra-low temperature toughness of offline tempered steel plates. Therefore, Ni simultaneously improves the strength, elongation, and low-temperature toughness of tempered steel plates. However, due to the high cost of Ni, it also makes it difficult to remove the iron oxide scale from the steel plate, further increasing costs. Therefore, the Ni content is limited to between 0.50% and 1.00%.

[0022] Nitrogen (Nb), as the main element in this invention, can suppress austenite recrystallization during steel rolling, control rolling of the non-recrystallized zone to increase the dislocation density within the austenite, promote grain refinement, improve the strength and toughness of quenched and tempered steel plates, reduce overheating sensitivity and temper brittleness, improve weldability, and also produce solid solution strengthening. It causes a large amount of Nb (C,N) to precipitate at grain boundaries and dislocations. Furthermore, high-Nb content steel plates experience slow cooling rates in the heat-affected zone during high-heat welding, making it easier for Nb segregated at grain boundaries to precipitate, creating carbon-depleted zones and promoting the nucleation and growth of ferrite at austenite grain boundaries at high temperatures. Therefore, the Nb content is limited to 0.10%–0.50%.

[0023] Al acts as a primary deoxidizer in steel, refining grain size and improving toughness. However, excessive Al content can lead to the formation of larger oxides, reducing the low-temperature impact toughness of the steel plate. Therefore, to ensure more complete deoxidation and nitrogen fixation, the Al content is limited to 0.0025%–0.0045%.

[0024] The second technical solution of the present invention is to provide a method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa, including smelting, continuous casting, heating, rolling and heat treatment;

[0025] (1) Smelting: Iron pretreatment is adopted, with iron temperature of 1330~1430℃; during refining treatment, LF time is 20~40min, RH time is 15~30min, and vacuum degree is ≤5.0mbar.

[0026] (2) Continuous casting: The molten steel casting temperature in the tundish is 1350~1410℃, electromagnetic stirring is used, and the preferred electromagnetic stirring frequency is 3~5Hz; the billet speed is 0.70~0.90m / min, the secondary cooling water is 5600~6100L / min, the superheat is 25~35℃, and the straightening temperature is 970~1000℃.

[0027] (3) Heating: The heating of the continuously cast billet is divided into a preheating section, a heating section and a soaking section; the temperature range of the heating section is 1170~1200℃; the temperature range of the soaking section is 1150~1190℃; the total time in the furnace is 120~150min. By heating the billet in three stages, the core temperature of the billet reaches 1150~1190℃, ensuring that the billet structure is completely austenitic.

[0028] (4) Rolling: After the billet exits the furnace, a two-stage controlled rolling and cooling technology is adopted. In the roughing stage, the initial rolling temperature is 1120-1160℃, the roll speed is 0.6-0.8m / s, and the single-pass reduction rate is 12%-20%; in the finishing stage, the initial rolling temperature is 830-880℃, the final rolling temperature is 770-820℃, the roll speed is 0.5-0.6m / s, and the single-pass reduction rate is 10%-15%; in the cooling stage, the initial cooling temperature is 740-790℃, and the final cooling temperature is 480-530℃; after the steel plate is rolled and cooled, it is straightened at a speed controlled at 0.8-1.8m / s to eliminate residual stress generated inside the steel plate due to cooling and ensure good plate shape; finally, the steel plates are stacked and slowly cooled at a temperature of 400-430℃ for 900-1000min. Through two-stage rolling and controlled cooling, the steel plate grains are refined and the microstructure is completely transformed into bainitic microstructure.

[0029] (5) Heat treatment: After slow cooling in stacks, the steel plates undergo quenching and tempering heat treatment. The quenching temperature is 910–930℃, and the holding time is 20–60 min. The tempering temperature is 620–640℃, and the holding time is 60–200 min. After being taken out of the furnace, the plates are air-cooled. Through quenching and tempering heat treatment, the steel plate has a uniform microstructure, with carbides precipitating and being evenly distributed in the microstructure. The microstructure is tempered sorbite, and the mechanical properties of the steel plate are improved after heat treatment.

[0030] Simulated post-weld heat treatment process: Samples of finished steel plates are taken and subjected to simulated post-weld heat treatment process to test the mechanical properties of the steel plates. The simulated welding heat treatment temperature is 550-650℃ and the holding time is 120-480min.

[0031] The beneficial effects of this invention are as follows:

[0032] 1. The composition of this invention is based on traditional carbon steel. By adding sufficient amounts of Nb, Cr, Mo, and Ni elements, and by adjusting the content ratio of Cr and Mo elements, the hardenability and tempering resistance of the steel plate are further improved, ensuring that the steel plate has sufficient strength and excellent low-temperature toughness.

[0033] 2. The manufacturing method of this invention is reasonable. By adopting controlled rolling and cooling combined with tempering heat treatment, it is ensured that the finished product has a single tempered sorbite structure with carbide size between 2 and 4 μm and uniform distribution. This eliminates the damage of ferrite to the strength of the steel plate, giving the steel plate excellent strength and toughness.

[0034] 3. The steel plate of this invention has a thickness of 10-50 mm and a yield strength R. eL ≥690MPa, tensile strength R m 800~920MPa, elongation A≥18%, KV2≥100J at -50℃; simulated mechanical properties of steel plate after post-weld heat treatment: yield strength R eL ≥680MPa, tensile strength R m 780~900MPa, elongation A≥18%, KV2≥90J at -50℃. Detailed Implementation

[0035] The present invention will be further illustrated below through examples.

[0036] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, heating, rolling and heat treatment.

[0037] (1) Heating: The heating of the continuously cast billet is divided into a preheating section, a heating section and a soaking section; the temperature range of the heating section is 1170~1200℃; the temperature range of the soaking section is 1150~1190℃; the total time in the furnace is 120~150min;

[0038] (2) Rolling: After the billet exits the furnace, a two-stage controlled rolling and cooling technology is adopted; the initial rolling temperature of the roughing stage is 1120-1160℃, the roll speed is 0.6-0.8m / s, and the single-pass reduction rate is 12%-20%; the initial rolling temperature of the finishing stage is 830-880℃, the final rolling temperature is 770-820℃, the roll speed is 0.5-0.6m / s, and the single-pass reduction rate is 10%-15%; the initial cooling temperature of the cooling stage is 740-790℃, and the final cooling temperature is 480-530℃; then the steel plates are stacked and slowly cooled at a temperature of 400-430℃ for 900-1000min.

[0039] (3) Heat treatment: After stacking and slow cooling, the steel plates are subjected to quenching and tempering heat treatment, wherein the quenching temperature is 910~930℃ and the holding time is 20~60min; the tempering temperature is 620~640℃ and the holding time is 60~200min.

[0040] Further smelting: hot metal pretreatment is adopted, with hot metal temperature of 1330~1430℃; during refining treatment, LF time is 20~40min, RH time is 15-30min, and vacuum degree is ≤5.0mbar.

[0041] Further; continuous casting: the tundish steel pouring temperature is 1350~1410℃, electromagnetic stirring technology is used, the billet pulling speed is 0.70~0.90m / min, the secondary cooling water is 5600~6100L / min, the superheat is 25~35℃, and the straightening temperature is 970~1000℃. Preferably, the electromagnetic stirring frequency is 3~5Hz.

[0042] Furthermore, after rolling and cooling but before slow cooling, the steel plate is straightened at a speed controlled between 0.8 and 1.8 m / s.

[0043] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for smelting and casting of the steel in this embodiment of the invention are shown in Table 2. The main process parameters for heating and rolling of the steel in this embodiment of the invention are shown in Table 3. The process parameters for cooling, slow cooling, and heat treatment of the steel in this embodiment of the invention are shown in Table 4. The properties of the steel in this embodiment of the invention are shown in Table 5.

[0044] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0045] Example C Si Mn P S Nb Mo Ni Cr Al 1 0.04 0.13 1.18 0.001 0.001 0.14 0.22 0.56 1.07 0.0026 2 0.05 0.12 1.04 0.001 0.001 0.12 0.20 0.55 1.06 0.0026 3 0.08 0.22 1.79 0.006 0.004 0.36 0.43 0.84 1.38 0.0034 4 0.05 0.13 1.26 0.001 0.001 0.14 0.21 0.58 1.08 0.0026 5 0.06 0.16 1.39 0.003 0.002 0.18 0.26 0.68 1.21 0.0028 6 0.09 0.24 1.89 0.008 0.004 0.36 0.45 0.86 1.41 0.0034 7 0.07 0.19 1.63 0.005 0.003 0.23 0.35 0.78 1.35 0.0029 8 0.06 0.16 1.40 0.005 0.003 0.19 0.26 0.68 1.22 0.0029 9 0.06 0.14 1.35 0.002 0.001 0.16 0.26 0.66 1.13 0.0027 10 0.07 0.17 1.58 0.005 0.003 0.22 0.29 0.74 1.31 0.0029 11 0.05 0.13 1.02 0.004 0.001 0.11 0.21 0.51 1.02 0.0026 12 0.10 0.27 1.96 0.009 0.005 0.41 0.48 0.98 1.47 0.0040 13 0.08 0.21 1.77 0.006 0.004 0.30 0.36 0.82 1.38 0.0033 14 0.09 0.26 1.93 0.008 0.004 0.39 0.46 0.87 1.43 0.0040 15 0.10 0.30 1.98 0.01 0.005 0.49 0.50 0.99 1.5 0.0040

[0046] Table 2 Main process parameters for steel smelting and casting in the embodiments of the present invention.

[0047]

[0048] Table 3 Main process parameters for steel heating and rolling in the embodiments of the present invention.

[0049]

[0050] Table 4. Process parameters for steel cooling, slow cooling, and heat treatment in embodiments of the present invention.

[0051]

[0052] Table 5 Properties of the steel in the embodiments of the present invention

[0053]

[0054] Table 6 Mechanical properties of steel after post-weld heat treatment in embodiments of the present invention

[0055]

[0056] Note: The heat treatment temperature for mold welding is 550-650℃, and the holding time is 120-480min.

[0057] As can be seen from the above, the steel plates produced using this invention have a thickness of 10-50mm and a yield strength R. eL ≥690MPa, tensile strength R m 800~920MPa, elongation A≥18%, KV2≥100J at -50℃; simulated mechanical properties of steel plate after post-weld heat treatment: yield strength R eL ≥680MPa, tensile strength R m 780~900MPa, elongation A≥18%, KV2≥90J at -50℃.

[0058] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa, comprising smelting, continuous casting, heating, rolling, and heat treatment; characterized in that: (1) Heating: The heating of the continuously cast billet is divided into a preheating section, a heating section and a soaking section; the temperature range of the heating section is 1170~1200℃; the temperature range of the soaking section is 1150~1190℃; the total time in the furnace is 120~150min; (2) Rolling: After the billet exits the furnace, a two-stage controlled rolling and cooling technology is adopted; the initial rolling temperature of the roughing stage is 1120-1160℃, the roll speed is 0.6-0.8m / s, and the single-pass reduction rate is 12%-20%; the initial rolling temperature of the finishing stage is 830-880℃, the final rolling temperature is 770-820℃, the roll speed is 0.5-0.6m / s, and the single-pass reduction rate is 10%-15%; the initial cooling temperature of the cooling stage is 740-790℃, and the final cooling temperature is 480-530℃; then the steel plates are stacked and slowly cooled at a temperature of 400-430℃ for 900-1000min. (3) Heat treatment: After stacking and slow cooling, the steel plates are subjected to quenching and tempering heat treatment, wherein the quenching temperature is 910~930℃ and the holding time is 20~60min; the tempering temperature is 620~640℃ and the holding time is 60~200min. The steel plate comprises the following components by weight percentage: C: 0.04%–0.10%, Si: 0.10%–0.30%, Mn: 1.00%–2.00%, P: ≤0.010%, S: ≤0.006%, Cr: 1.00%–1.50%, Mo: 0.20%–0.50%, Ni: 0.50%–1.00%, Nb: 0.10%–0.50%, Al: 0.0025%–0.0045%, with the balance being Fe and unavoidable impurities.

2. The method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa according to claim 1, characterized in that, The microstructure of the steel plate is a single tempered sorbite, with carbide sizes ranging from 2 to 4 μm.

3. The method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa according to claim 1, characterized in that, The steel plate has a thickness of 10-50mm, ReL≥690MPa, Rm:800-920MPa, A≥18%, and KV2≥100J at -50℃.

4. The method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa according to claim 1, characterized in that: Smelting: Hot metal pretreatment is adopted, with hot metal temperature of 1330~1430℃; during refining treatment, LF time is 20~40min, RH time is 15~30min, and vacuum degree is ≤5.0mbar.

5. The method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa according to claim 1, characterized in that: Continuous casting: The tundish steel pouring temperature is 1350~1410℃, electromagnetic stirring technology is used, the billet pulling speed is 0.70~0.90m / min, the secondary cooling water is 5600~6100L / min, the superheat is 25~35℃, and the straightening temperature is 970~1000℃.

6. The method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa according to claim 5, characterized in that: The electromagnetic stirring frequency is 3-5 Hz.

7. The method for manufacturing a steel plate for a low-temperature ethylene spherical tank with a yield strength of 690 MPa according to claim 1, characterized in that: After rolling and cooling but before slow cooling, the steel plate is straightened at a speed controlled between 0.8 and 1.8 m / s.

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