Steel sheet for propylene spherical tank with yield strength of 690mpa and method for manufacturing the same

Abstract

The application provides a steel plate for a propylene spherical tank with a yield strength of 690 MPa and a manufacturing method thereof, the steel plate has the following components in percentage by weight: C: 0.10%-0.15%, Si: 0.10%-0.30%, Mn: 1.00%-2.00%, P: <=0.010%, S: <=0.005%, Mo: 0.50%-0.90%, B: 0.0007%-0.0014%, V: 0.10%-0.50%, Al: 0.0025%-0.0045%, N: 0.02%-0.06%, and the balance of Fe and inevitable impurities, by adopting two-stage rolling + controlled cooling, cooperating with quenching and tempering heat treatment + slow cooling after tempering, the finished product structure is ensured to be single tempering sorbite, the carbide size in the structure is between 1-3 mu m, and the distribution is uniform, by eliminating the damage of ferrite to the strength of the steel plate, the steel plate has a good strength and toughness ratio, and after simulated post-welding heat treatment, the steel plate still has excellent strength and toughness.

Description

Technical Field

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

[0002] Propylene is a fundamental raw material for the three major synthetic materials: plastics, synthetic rubber, and synthetic fibers. The demand for propylene is increasing year by year with the continuous and rapid development of my country's industry. Simultaneously, to continuously improve the economic efficiency of petrochemical plants, spherical storage tanks for storing propylene at room temperature are gradually developing towards larger sizes and higher parameters. Since propylene belongs to the Group A liquefied hydrocarbon category, it is a colorless gas with a hydrocarbon odor at room temperature, is flammable and explosive, insoluble in water, and has a high saturated vapor pressure. Propylene spherical tanks are used for storing gaseous propylene at room temperature; therefore, the steel plates need to possess excellent comprehensive mechanical properties such as high strength, good low-temperature impact toughness (-20℃), and good weldability. Existing domestic steel plates for propylene spherical tanks cannot meet the high-parameter requirements. Therefore, this study researches a steel plate for propylene spherical tanks with a yield strength of 690 MPa to meet the development needs of China's steel industry.

[0003] Steel plates used in propylene spherical tanks are generally subjected to normalizing or quenching and tempering heat treatment processes to give them high strength and toughness and good comprehensive mechanical properties. Subsequent testing of the steel plates after simulated post-weld heat treatment to simulate the welding and hot forming process of the container shows that the various mechanical properties of the steel plates still meet the requirements.

[0004] For example, the container steel plate disclosed in the patent document "A 40-60mm thick easy-to-weld core with excellent low-temperature toughness and its manufacturing method" (CN110184531A) has the following main chemical composition: C 0.04-0.09%, Si≤0.40%, Mn 1.00-1.60%, P≤0.008%, S≤0.002%, Mo 0.12-0.30%, Ni 0.20-0.60%, and satisfies Pcm≤0.20% and 10≤w(Nb+V) / w(Ti)≤30. Its shortcoming is that its yield strength is lower than the designed yield strength of 690MPa.

[0005] The patent document "Steel Plate for Large Thickness Ultra-Low Temperature High Toughness Spherical Tank and its Rolling Method" (CN112795839A) discloses a steel plate for spherical tanks with the following chemical composition: C: 0.06-0.09%, Si≤0.55%, Mn 1.30-1.60%, P≤0.012%, S≤0.005%, Ni 0.40-0.70%, Nb 0.020-0.030%, Alt 0.020-0.050%, with the balance being Fe and other elements. Its shortcomings are that the strength of the designed large-thickness steel plate is lower than the designed yield strength of 690MPa, and the mechanical properties of the thick steel plate after welding heat treatment are difficult to guarantee, affecting subsequent processing.

[0006] The patent document "A Steel for Spherical Tanks with Low Surface Hardness and Its Production Method" (CN117026093A) discloses a steel for spherical tanks with the following main chemical composition: 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%, and Sn≤0.008%. Its shortcoming is that its yield strength is within the range of 550-570 MPa, and the designed composition cannot meet the requirement of a yield strength of 690 MPa.

[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 propylene 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 propylene spherical tanks with a single tempered sorbite microstructure, which eliminates the damage of ferrite to the strength of the steel plate, has a good strength-toughness ratio, and still has excellent strength and toughness after simulated welding heat treatment, with a yield strength of 690 MPa, and a method for manufacturing the same.

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

[0010] A steel plate for propylene spherical tanks with a yield strength of 690 MPa, the steel plate having the following composition by weight percentage: C: 0.10%–0.15%, Si: 0.10%–0.30%, Mn: 1.00%–2.00%, P: ≤0.010%, S: ≤0.005%, Mo: 0.50%–0.90%, B: 0.0007%–0.0014%, V: 0.10%–0.50%, Al: 0.0025%–0.0045%, N: 0.02%–0.06%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, in the steel plate, V / N: 7.5~9.

[0012] Furthermore, the microstructure of the steel plate is tempered sorbite, and the carbide size in the microstructure is 1-3 μm.

[0013] Furthermore, the steel plate thickness is 10–50 mm, ReL ≥ 690 MPa, Rm: 800–920 MPa, A ≥ 18%, and KV2 ≥ 100 J at -20℃; the simulated mechanical properties of the steel plate after post-weld heat treatment include yield strength R. eL ≥680MPa, tensile strength Rm 780~900MPa, elongation A≥18%, KV2≥100J at -20℃.

[0014] The rationale for the design of the components in this invention is as follows:

[0015] Based on traditional C-Mn low-alloy steel, the hardenability and tempering stability of the steel plate are improved by adding a certain amount of V. V mainly exists in the form of V(C,N) in steel. The increase of precipitates can inhibit grain boundary movement and grain growth. In addition, V can also undergo compound precipitation with inclusions, which is beneficial to deformation-induced ferrite phase transformation. At the same time, the precipitation strengthening and dispersion strengthening effects of V element ensure that the steel plate has sufficient strength and toughness. In addition, by adding B and Mo elements, the diffusion of C and carbide growth in the microstructure are inhibited, which improves the strength of the steel plate while increasing its hardenability and tempering stability.

[0016] C: In steel, C forms various carbides or solid solutions with alloying elements to strengthen the steel. Among them, V has a very strong affinity with C atoms, and the V-containing carbides formed in the steel play a role in improving the strength of the steel. Moreover, V and C can form stable and refractory carbides during the smelting process, so that the steel can maintain a fine-grained structure at high temperatures and greatly reduce the overheating sensitivity of the steel. Therefore, the C content is limited to 0.10% to 0.15%.

[0017] Si: Si dissolves in ferrite in steel to improve the strength and hardenability of the steel plate, and within a certain range, it can lower the ductile-brittle transition temperature and improve the plasticity of the steel. Therefore, the Si content is limited to 0.10% to 0.30%.

[0018] Mn: Mn in steel enhances strength and hardenability through solid solution strengthening. Increased Mn content improves the stability of austenite and tempered structures, and reduces the critical cooling rate. However, excessive Mn content leads to grain coarsening at high temperatures, increasing Ceq (coarse grain size), affecting weldability, and easily causing segregation in steel plates, which is detrimental to plasticity and toughness. Therefore, Mn content is limited to 1.00%–2.00%.

[0019] Phosphorus (P) is a harmful element in steel, significantly impacting low-temperature impact toughness and being highly prone to segregation. Therefore, its content should be kept low during steelmaking. The P content should be controlled below 0.010%.

[0020] Sulfur (S) is also a harmful element in steel. It easily forms MnS in steel, which can become a source of crack formation during subsequent processing and has a significant impact on the toughness of steel. Therefore, the S content should be controlled below 0.005%.

[0021] Mo: As the main element in this invention, Mo improves the strength and toughness of steel, reduces the critical cooling rate of steel to improve the hardenability of steel plates, shrinks the austenite phase region and promotes bainite transformation during two-stage rolling, and generates Mo-containing carbides that inhibit ferrite formation. When coexisting with Mn, Mo can also reduce temper brittleness. After quenching and tempering heat treatment, Mo can improve low-temperature impact toughness. Therefore, the Mo content is between 0.50% and 0.90%.

[0022] V: As the main element in this invention, V can dissolve into austenite at high temperatures, improving the strength and hardenability of steel through solid solution strengthening. V exists in steel in the form of VC, VN, and V(C,N) composite compounds. Increased V(C,N) precipitation can also inhibit grain boundary movement and grain growth, improving the toughness and tempering stability of the steel. Furthermore, V can undergo composite precipitation with inclusions, facilitating deformation-induced ferrite phase transformation. Adjusting the V to N ratio serves two purposes: firstly, V can play a denitrification role during smelting; secondly, appropriate N can promote V precipitation, generating vanadium nitride to achieve good precipitation strengthening. If the V content is too high, not only will the cost increase, but the precipitate size will also increase. Therefore, V is limited to 0.10%–0.50%, and the V:N ratio is 7.5–9.

[0023] B: During the two-stage rolling process, the segregation of dissolved boron at the original austenite grain boundaries occupies the nucleation sites of ferrite on the original austenite grain boundaries, inhibiting ferrite formation; B can promote the transformation of bainite structure, thereby improving the hardenability of the steel plate; and B can stabilize the M on the original austenite grain boundaries. 23 C6 carbides inhibit grain boundary slip and grain growth, enhancing grain boundary strengthening and thus ensuring the hot workability of the steel sheet. Therefore, B is limited to 0.0007%–0.0014%.

[0024] Al: Al acts as a major deoxidizer in steel, fixing nitrogen in the molten steel, refining grains, and improving the toughness of steel. It can also reduce the formation of nitrogen (BN) in steel and increase the content of dissolved boron (B) in the steel. However, excessive Al content can lead to the formation of large-sized oxides, affecting the toughness of the steel plate. Therefore, the Al content is limited to 0.0025%-0.0045%.

[0025] Nitrogen (N): In steel, nitrogen mainly precipitates as VN or V(CN) during the austenite-ferrite transformation, inhibiting austenite grain growth and improving the steel's strength and low-temperature toughness. Therefore, the nitrogen content is limited to 0.02–0.06%.

[0026] The second technical solution of the present invention is to provide a method for manufacturing a steel plate for a propylene spherical tank with a yield strength of 690 MPa, including smelting, continuous casting, heating, rolling and heat treatment;

[0027] (1) Smelting: Iron pretreatment is adopted, with iron temperature of 1350~1450℃; during refining treatment, LF time is 30-50min, RH time is 30-50min, and vacuum degree is ≤5.0mbar.

[0028] (2) Continuous casting: The tundish steel pouring temperature is 1380-1400℃, preferably using a light reduction technique with a reduction of 3-6mm; the billet pulling speed is 0.75-0.95m / min, the secondary cooling water is 5500-6000L / min, the superheat is 20-35℃, and the straightening temperature is 950-1000℃. The light reduction technique involves slightly reducing the billet at the point where it is about to solidify to reduce center segregation.

[0029] (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 1180~1210℃; the temperature range of the soaking section is 1160~1190℃; the total time in the furnace is 120~180min. By heating the billet in three stages, the core temperature of the billet reaches 1160~1190℃, ensuring that the billet structure is completely austenitic.

[0030] (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 1140-1170℃, the roll speed is 0.6-0.8m / s, and the single-pass reduction rate is 10%-18%; in the finishing stage, the initial rolling temperature is 850-900℃, the final rolling temperature is 800-850℃, the roll speed is 0.5-0.6m / s, and the single-pass reduction rate is 8%-10%; in the cooling stage, the initial cooling temperature is 770-820℃, and the final cooling temperature is 500-550℃; after cooling, the steel plates are stacked and slowly cooled at a temperature of 400-450℃ for 960-1140min. Through two-stage rolling, using high reduction and slow roll speed rolling, the deformation storage energy in the microstructure is increased, the time for dynamic recrystallization is prolonged, and the grains are refined; by controlling the cooling, the ferrite transformation is suppressed, and the bainite transformation is promoted to be complete.

[0031] (5) Heat treatment: After slow cooling in stacks, the steel plates undergo quenching and tempering heat treatment. The quenching temperature is 930–950℃, and the holding time is 15–75 min. The tempering temperature is 610–630℃, and the holding time is 30–200 min. After being removed from the furnace, the plates are slowly cooled to 500–540℃ and then air-cooled to room temperature. The slow cooling rate is 0.4–0.6℃ / s to ensure that the steel plate microstructure is fully transformed into tempered sorbite. Through quenching and tempering heat treatment followed by slow cooling after tempering, the steel plate microstructure is uniform and is tempered sorbite. The mechanical properties of the steel plate can be improved after heat treatment.

[0032] The beneficial effects of this invention are as follows:

[0033] 1. The composition design of this invention is simple. Based on traditional C-Mn low alloy steel, a certain amount of V is added to improve the hardenability and tempering stability of the steel plate. V mainly exists in the form of V(C,N) in steel. The increase of precipitates can inhibit grain boundary movement and grain growth. In addition, V can also undergo compound precipitation with inclusions, which is beneficial to deformation-induced ferrite phase transformation. At the same time, the precipitation strengthening and dispersion strengthening effects of V element ensure that the steel plate has sufficient strength and toughness. In addition, by adding B and Mo elements, the formation of ferrite is inhibited, the diffusion of C in the microstructure and the growth of carbides are inhibited, thereby increasing the hardenability and tempering stability of the steel plate.

[0034] 2. The manufacturing method of this invention is reasonable. By adopting two-stage rolling and controlled cooling, combined with tempering heat treatment and slow cooling after tempering, the finished product structure is ensured to be a single tempered sorbite with carbide size between 1-3μm and uniform distribution. By eliminating the damage of ferrite to the strength of the steel plate, the steel plate has a good strength-toughness ratio. Even after simulated welding heat treatment, it still has excellent strength and toughness.

[0035] 3. The steel plate of this invention has a thickness of 10-50mm and a tensile strength R. eL ≥690MPa, yield strength R m 800~920MPa, elongation A≥18%, KV2≥100J at -20℃; simulated mechanical yield strength R of steel plate after post-weld heat treatment. eL ≥680MPa, tensile strength R m 780~900MPa, elongation A≥18%, KV2≥100J at -20℃. Detailed Implementation

[0036] The present invention will be further illustrated below through examples.

[0037] 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.

[0038] (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 1180~1210℃; the temperature range of the soaking section is 1160~1190℃; the total time in the furnace is 120~180min;

[0039] (2) 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 1140-1170℃, the roll speed is 0.6-0.8m / s, and the single-pass reduction rate is 10%-18%; in the finishing stage, the initial rolling temperature is 850-900℃, the final rolling temperature is 800-850℃, the roll speed is 0.5-0.6m / s, and the single-pass reduction rate is 8%-10%; in the cooling stage, the initial cooling temperature is 770-820℃, and the final cooling temperature is 500-550℃; after cooling, the steel plates are stacked and slowly cooled at a temperature of 400-450℃ for a time of 960-1140min.

[0040] (3) Heat treatment: After stacking and slow cooling, the steel plates are subjected to quenching and tempering heat treatment, wherein the quenching temperature is 930~950℃ and the holding time is 15~75min; the tempering temperature is 610~630℃ and the holding time is 30-200min. After being taken out of the furnace, the plates are slow cooled to 500~540℃ and then air cooled to room temperature. The slow cooling rate is 0.4~0.6℃ / s.

[0041] Further; Smelting: Iron pretreatment is adopted, with iron temperature of 1350~1450℃; during refining treatment, LF time is 30-50min, RH time is 30-50min, and vacuum degree is ≤5.0mbar.

[0042] Further; Continuous casting: tundish steel pouring temperature 1380~1400℃, billet pulling speed 0.75~0.95m / min, secondary cooling water 5500~6000L / min, superheat 20~35℃, straightening temperature 950~1000℃.

[0043] Furthermore, a light reduction technique is adopted during the continuous casting process, with the reduction amount controlled at 3-6 mm.

[0044] 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 main process parameters for cooling and heat treatment of the steel in this embodiment of the invention are shown in Table 4. The mechanical properties of the steel in this embodiment of the invention are shown in Table 5.

[0045] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0046] Example C Si Mn P S Al V Mo B N V / N 1 0.10 0.10 1.00 0.005 0.001 0.0025 0.10 0.50 0.0007 0.0149 6.7 2 0.12 0.18 1.47 0.003 0.002 0.0034 0.30 0.67 0.0009 0.037 8.1 3 0.11 0.14 1.27 0.002 0.002 0.0029 0.25 0.59 0.0008 0.0316 7.9 4 0.15 0.30 1.98 0.010 0.005 0.0045 0.50 0.86 0.0014 0.0562 8.9 5 0.11 0.15 1.39 0.003 0.002 0.0031 0.29 0.61 0.0009 0.0363 8.0 6 0.12 0.21 1.56 0.004 0.003 0.0034 0.35 0.68 0.0011 0.0427 8.2 7 0.13 0.21 1.57 0.004 0.003 0.0036 0.38 0.70 0.0011 0.0463 8.2 8 0.14 0.24 1.91 0.009 0.004 0.0045 0.48 0.82 0.0014 0.0539 8.9 9 0.13 0.21 1.64 0.005 0.003 0.0039 0.41 0.74 0.0011 0.0488 8.4 10 0.11 0.12 1.12 0.002 0.002 0.0026 0.21 0.57 0.0008 0.0273 7.7 11 0.13 0.22 1.75 0.006 0.004 0.004 0.44 0.76 0.0011 0.0524 8.4 12 0.10 0.10 1.09 0.001 0.001 0.0025 0.20 0.55 0.0007 0.0267 7.5 13 0.13 0.22 1.77 0.007 0.004 0.0042 0.46 0.76 0.0012 0.0535 8.6 14 0.14 0.22 1.82 0.008 0.004 0.0043 0.48 0.81 0.0013 0.0552 8.7 15 0.15 0.30 2.00 0.010 0.005 0.0045 0.50 0.90 0.0014 0.0556 9.0

[0047] Table 2 Main process parameters for steel smelting and casting in the embodiments of the present invention.

[0048]

[0049] Table 3 Main process parameters for steel heating and rolling in the embodiments of the present invention.

[0050]

[0051]

[0052] Table 4 Main process parameters for steel cooling and heat treatment in embodiments of the present invention

[0053]

[0054] Table 5 Mechanical properties of steel in embodiments of the present invention

[0055]

[0056] Table 6 Mechanical properties of steel after post-weld heat treatment in embodiments of the present invention

[0057]

[0058] Note: The heat treatment temperature for mold welding is 550-650℃, and the holding time is 120-480min.

[0059] The microstructure of the steel plate produced using this invention is tempered sorbite. The thickness of the steel plate produced using this invention is 10-50 mm, and the yield strength R... eL ≥690MPa, tensile strength R m 800~920MPa, elongation A≥18%, KV2≥100J at -20℃; simulated tensile strength R of steel plate after post-weld heat treatment. eL ≥680MPa, yield strength R m 780~900MPa, elongation A≥18%, KV2≥100J at -20℃.

[0060] 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 steel plate for propylene spherical tanks with a yield strength of 690 MPa, characterized in that, The composition of the steel plate by weight percentage is as follows: C: 0.10%–0.15%, Si: 0.10%–0.30%, Mn: 1.00%–2.00%, P: ≤0.010%, S: ≤0.005%, Mo: 0.50%–0.90%, B: 0.0007%–0.0014%, V: 0.10%–0.50%, Al: 0.0025%–0.0045%, N: 0.02%–0.06%, with the balance being Fe and unavoidable impurities; the microstructure of the steel plate is tempered sorbite, with carbide sizes of 1–3 μm.

2. The steel plate for propylene spherical tanks with a yield strength of 690 MPa according to claim 1, characterized in that, In the steel plate, V / N: 7.5~9.

3. The steel plate for a propylene spherical tank with a yield strength of 690 MPa according to claim 1, characterized in that, The steel plate has a thickness of 10–50 mm and a tensile strength R. eL ≥690MPa, yield strength R m 800~920MPa, elongation A≥18%, KV2≥100J at -20℃.

4. A method for manufacturing a propylene spherical tank steel plate with a yield strength of 690 MPa as described in any one of claims 1-3, 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 1180~1210℃; the temperature range of the soaking section is 1160~1190℃; the total time in the furnace is 120~180min; (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 1140-1170℃, the roll speed is 0.6-0.8m / s, and the single-pass reduction rate is 10%-18%; the initial rolling temperature of the finishing stage is 850-900℃, the final rolling temperature is 800-850℃, the roll speed is 0.5-0.6m / s, and the single-pass reduction rate is 8%-10%; the initial cooling temperature of the cooling stage is 770-820℃, and the final cooling temperature is 500-550℃; after cooling, the steel plates are stacked and slowly cooled at a temperature of 400-450℃ for a time of 960-1140min. (3) Heat treatment: After stacking and slow cooling, the steel plates are subjected to quenching and tempering heat treatment, wherein the quenching temperature is 930~950℃ and the holding time is 15~75min; the tempering temperature is 610~630℃ and the holding time is 30~200min. After being taken out of the furnace, the plates are slow cooled to 500~540℃ and then air cooled to room temperature. The slow cooling rate is 0.4~0.6℃ / s.

5. The method for manufacturing a steel plate for a propylene spherical tank with a yield strength of 690 MPa according to claim 4, characterized in that: Smelting: Hot metal pretreatment is adopted, with hot metal temperature of 1350~1450℃; during refining treatment, LF time is 30~50min, RH time is 30~50min, and vacuum degree is ≤5.0mbar.

6. The method for manufacturing a steel plate for a propylene spherical tank with a yield strength of 690 MPa according to claim 4, characterized in that: Continuous casting: The tundish steel pouring temperature is 1380~1400℃, electromagnetic stirring or light pressure technology is used, the billet pulling speed is 0.75~0.95m / min, the secondary cooling water is 5500~6000L / min, the superheat is 20~35℃, and the straightening temperature is 950~1000℃.

7. The method for manufacturing a steel plate for a propylene spherical tank with a yield strength of 690 MPa according to claim 6, characterized in that: The reduction amount of the light reduction technology is controlled at 3-6 mm.

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