Steel plate for high-strength storage tank with low ductile-brittle transition temperature and preparation method of steel plate

Abstract

The invention relates to the technical field of steel plate production, in particular to a low-ductile-brittle-transition-temperature high-strength steel plate for a storage tank and a preparation method of the steel plate. The steel comprises the following chemical components in percentage by mass: 0.18-0.21% of C; si: 0.31%-0.42%; mn: 1.12% to 1.28%; p < = 0.012%; s is less than or equal to 0.01%; 0.01%-0.03% of Cr, 0.062%-0.074% of Nb, and the ratio of Nb to Cr is 2.5-4; ce: 0.0036% to 0.0072%; and the balance of Fe and inevitable impurities. By optimizing the smelting process, the purity of molten steel is improved, and original structure grains are controlled; the internal structure of the steel plate is optimized through two-stage controlled rolling, and the plate shape is improved; and through temperature control normalizing heat treatment, the microstructure of the material is adjusted while strength and toughness matching is improved, and the mechanical property and low-temperature service performance of the steel plate are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of steel plate production, and particularly relates to a high-strength steel plate for low-temperature storage tanks with a low ductile-brittle transition temperature and a preparation method thereof. Background Art

[0002] With the rapid development of industries such as chemical engineering and petroleum, the steel used for container storage tanks is gradually developing towards the directions of large-scale, complex structure, high parameters, and adapting to complex and harsh service environments. Due to the increasing demand for liquefied energy such as liquefied petroleum gas and liquefied natural gas, the manufacturing volume of steel for low-temperature storage tanks has gradually increased, but at the same time, higher requirements are also put forward for the steel. For example, it needs to serve in complex environments such as low temperature and high pressure and hydrogen-containing media for a long time, and requires the steel plate to have good strength-ductility matching and a low ductile-brittle transition temperature.

[0003] The Chinese patent application with the publication number CN114875304A discloses "a quenched and tempered high-strength steel plate for SA537MCL2 pressure vessels and its production method". The steel plate is composed of the following components by weight percentage: C 0.105 - 0.130%, Mn 1.25 - 1.34%, Si 0.20 - 0.30%, S ≤ 0.005%, P ≤ 0.010%, Nb 0.01 - 0.02%, V 0.020 - 0.025%, Ti ≤ 0.005%, Ni 0.40 - 0.50%, Mo 0.05 - 0.07%, Alt 0.025 - 0.040%, Cu 0.15 - 0.20%, and the rest is Fe and unavoidable impurities. Quenching and tempering treatment is adopted. Since the steel plate contains alloying elements such as Ni with high cost and high addition amounts, the production cost of the steel plate is relatively high; because multi-stage heat treatment is required and the average time consumption is ≥ 110 min, the production cost is further increased. In addition, although the impact toughness of the steel plate at -68°C has been studied, the impact toughness at lower temperatures below -68°C has not been involved. Therefore, its method is not applicable to the large-scale production of steel for low-temperature storage tanks.

[0004] The Chinese invention patent with the authorization announcement number CN114395733B discloses "A production method of 07MnNiVDR steel for low-cost low-temperature high-strength containers". The steel plate is composed of the following components by weight percentage: C: 0.06 - 0.08%, Si: 0.15 - 0.25%, Mn: 1.40 - 1.50%, P ≤ 0.015%, S ≤ 0.005%, Als: 0.015 - 0.040%, Ni: 0.3 - 0.5%, Cr: 0.2 - 0.3%, Mo: 0.10 - 0.25%, V: 0.02 - 0.04%, and the rest are Fe and residual elements. The high-content elements Cr and Ni in its composition lead to an increase in the production cost of the steel plate. In addition, the production process parameters and mechanical properties of steel plates with a thickness less than 15 mm are not involved in its embodiments. And it only studies the impact toughness at -40°C, but does not involve the low-temperature impact toughness below -40°C. Therefore, this method is not applicable to the large-scale production of full-thickness low-temperature storage tank steel.

[0005] The Chinese patent application with the application publication number CN112080684A discloses "A thick plate for high-strength containers with excellent core toughness and its manufacturing method". The chemical composition and mass percentage of the steel are as follows: C: 0.08% - 0.12%, Si: 0.10% - 0.30%, Mn: 1.10% - 1.50%, Ni: 0.50% - 0.80%, Mo: 0.08% - 0.15%, V: 0.03% - 0.06%, P ≤ 0.006%, S ≤ 0.002%, and the balance is Fe and inevitable impurities. Its composition only targets steel plates with a thickness specification of 50 - 80 mm, and does not involve steel plates with a thickness less than 50 mm and greater than 80 mm. And the high-content element Ni in its composition leads to an increase in the production cost of the steel plate. In its manufacturing method, the total time in the furnace during slab heating ≥ 240 min, and the heat treatment adopts a quenching + tempering heat treatment process, and the total heat treatment time ≥ 70 min. The process of these two stages takes a long time and occupies a large amount of resources. Therefore, its method is not applicable to the large-scale production of full-thickness low-temperature storage tank steel.

[0006] Compared with the above-mentioned published literature, the present invention is a high-strength storage tank steel plate with a low ductile-brittle transition temperature and its preparation method, which relates to a storage tank steel plate with a plate thickness of 8 - 28 mm and good comprehensive performance, with a low production cost and is suitable for large-scale production applications. Summary of the Invention

[0007] The present invention provides a high-strength steel plate for storage tanks with a low ductile-brittle transition temperature and a preparation method thereof. By optimizing the smelting process, adopting the molten iron inoculation treatment technology in combination with the whole-process protected casting, the purity of molten steel and the purity of continuous casting billets are improved, the influence of elements such as P, S, and O is reduced, and the grain size of the original structure is controlled; a three-stage slab heating process is adopted to shorten the processing aging and improve the production efficiency; through two-stage controlled rolling, the internal structure of the steel plate is further optimized, and the plate shape is improved; through the temperature-controlled normalizing heat treatment, while improving the strength-ductility matching, the microstructural organization of the material is adjusted, and the mechanical properties and low-temperature service performance of the steel plate are improved.

[0008] In order to achieve the above object, the present invention is realized by adopting the following technical solutions: A high-strength steel plate for storage tanks with a low ductile-brittle transition temperature, the chemical composition in the steel is by mass percentage: C: 0.18% - 0.21%; Si: 0.31% - 0.42%; Mn: 1.12% - 1.28%; P ≤ 0.012%; S ≤ 0.01%; Cr: 0.01% - 0.03%; Nb: 0.062% - 0.074%, and Nb / Cr = 2.5 - 4; Ce: 0.0036% - 0.0072%; the balance is Fe and unavoidable impurities.

[0009] The metallographic structure of the finished steel plate is ferrite + troostite + spherical bainite. Among them, by volume ratio, the ratio of troostite to ferrite is 1.6 - 3.0, the ratio of ferrite to spherical bainite is 1.2 - 1.4, and the size of spherical bainite is not greater than 100 nm; the grain size of the finished steel plate is 8 - 9 grades.

[0010] The performance of the finished steel plate is as follows: at room temperature, 690 MPa ≤ tensile strength ≤ 740 MPa, 520 MPa ≤ yield strength ≤ 590 MPa, elongation ≥ 25%, hardness HV10 ≥ 240; under the condition of -80 °C, 700 MPa ≤ tensile strength ≤ 740 MPa, 490 MPa ≤ yield strength ≤ 530 MPa, elongation ≥ 25%, transverse impact energy KV 2 average value ≥ 160 J; ductile-brittle transition temperature ≤ -80 °C.

[0011] A preparation method of a high-strength steel plate for storage tanks with a low ductile-brittle transition temperature, the production process includes: smelting, continuous casting, slab heating, two-stage controlled rolling, online cold straightening and temperature-controlled normalizing heat treatment; the specific control is as follows: 1) Smelting: The smelting process includes hot metal pretreatment, converter smelting, LF refining, and RH vacuum degassing. During converter smelting, an inoculant with a magnesium content of 5% - 6.2% by mass percentage is added. The decarburization oxygen-blowing time is controlled within 3 - 5 minutes, the dephosphorization oxygen-blowing time is controlled within 8 - 10 minutes, and the mass percentage of phosphorus in the molten steel is controlled below 0.012%. During LF refining, deep desulfurization treatment is carried out. The desulfurization oxygen-blowing time is controlled within 9 - 12 minutes, and the mass percentage of sulfur in the molten steel is controlled below 0.01%. The starting temperature of RH vacuum degassing is 1618 - 1640 °C, the oxygen-blowing amount is controlled within 3.35 - 3.65 m³ / t steel·min, and the net circulation time is controlled within 8 - 18 minutes. The calming time before casting is 5 - 8 minutes. 2) Continuous casting: The pouring temperature of the molten steel is 1540 - 1566 °C, the superheat is controlled within 9 - 12 °C, and the casting speed is 1.0 - 1.6 m / min. The soft reduction process for continuous casting billets is adopted, and the reduction rate is controlled within 6% - 8%. 3) Slab heating: The slab is heated in three stages and then taken out of the furnace. The temperature range of the first heating stage is 985 - 1011 °C, the temperature range of the second heating stage is 1200 - 1225 °C, and the temperature range of the soaking stage is 1085 - 1120 °C. The total time of the slab in the furnace is controlled within 1.8 - 3.2 h. 4) Two-stage controlled rolling: The starting rolling temperature for the first-stage recrystallization zone rolling is 1130 - 1155 °C, and the final rolling temperature is 1020 - 1050 °C. The controlled rolling process with a "large reduction rate - small reduction rate" cycle is adopted, and the rolling speed is 3.0 - 4.4 m / s. The starting rolling temperature for the second-stage non-recrystallization zone rolling is 900 - 915 °C, and the final rolling temperature is 826 - 860 °C. The controlled rolling process with a decreasing single-pass reduction rate from large to small is adopted. 5) Online cold straightening: The starting temperature of cold straightening is 170 - 190 °C, and the reduction rate is 0.6% - 1.4%. 6) Temperature-controlled normalizing heat treatment: In the first stage, it is heated to 880 - 935 °C under temperature control, and the holding time is 15 - 28 minutes. In the second stage, it is cooled to 625 - 640 °C under temperature control, and the cooling rate is controlled within 45 - 56 °C / s. In the third stage, it is cooled to room temperature with the furnace.

[0012] During the smelting process, scrap steel and hot metal are used as raw materials. The size of the charged scrap steel is controlled between 80 - 110 mm, and the mass percentage of hot metal is controlled above 75%.

[0013] After the continuous casting process, the cast billets are taken off the production line for stacking and slow cooling, and the stacking and slow cooling time is 24 - 36 h.

[0014] During the rolling in the first-stage recrystallization zone, a large reduction ratio refers to a reduction ratio of 11% - 15%, and a small reduction ratio refers to a reduction ratio of 6% - 9%. During the rolling in the second-stage non-recrystallization zone, the single-pass reduction ratio is controlled within 5% - 12%.

[0015] After the rolling in the first-stage recrystallization zone is completed, the soaking time of the steel plate is controlled within 50 - 65 s.

[0016] The thickness of the finished steel plate is 8 - 28 mm.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) On the basis of strengthening elements such as C, Si, and Mn, appropriate alloying elements such as Cr, Nb, and Ce are added, and at the same time, the contents of harmful elements P and S are strictly controlled. Combining with the optimized production process, a uniformly refined "ferrite + troostite + spherical bainite" structure is obtained, in which the ratio of troostite to ferrite is 1.6 - 3.0 by volume ratio, the ratio of ferrite to spherical bainite is 1.2 - 1.4, and the size of spherical bainite is not greater than 100 nm. The grain size of the finished steel plate is 8 - 9 grades, and the second-phase Cr / Nb carbide particles with a size in the range of 40 - 60 nm are uniformly and dispersedly distributed, as well as Ce(O / S / N) with a size not greater than 30 nm, ensuring the strength, plasticity, low-temperature toughness, and service performance of the steel plate.

[0018] (2) The mechanical properties of the steel plate for storage tanks obtained through the unique production process are as follows: at room temperature, 690 MPa ≤ tensile strength ≤ 740 MPa, 520 MPa ≤ yield strength ≤ 590 MPa, elongation ≥ 25%, hardness HV10 ≥ 240; under the condition of -80 °C, 700 MPa ≤ tensile strength ≤ 740 MPa, 490 MPa ≤ yield strength ≤ 530 MPa, elongation ≥ 25%, and the average value of the transverse impact energy KV 2 is ≥ 160 J; the ductile-brittle transition temperature ≤ -80 °C, that is, the steel plate has good strength-ductility matching and low-temperature service performance.

[0019] (3) According to the "Evaluation Method for Resistance to Hydrogen-Induced Cracking of Pipeline Steels and Pressure Vessel Steels" of GB / T8650-2006 and NACE-TM0284, a hydrogen-induced crack (HIC) experiment is carried out. After 96 h of testing of the steel plate in Solution A and Solution B, the crack sensitivity CSR (%), crack length rate CLR (%), and crack width rate CTR (%) are all 0, indicating that the steel plate has excellent resistance to hydrogen-induced cracking; according to the test of GB / T17897-2016 "Corrosion of Metals and Alloys - Method of Ferric Chloride Pitting Corrosion Test for Stainless Steels", the corrosion rates of the steel plate in Solution A and Solution B are both not greater than 0.004 g / m 2 ·h; it shows that the steel plate has excellent corrosion resistance.

[0020] (4)The test was carried out according to GB / T 3960-2016 "Test Method for Sliding Friction and Wear of Plastics". The results showed that the volume wear of the steel plate was not more than 0.0004 cm 3 , indicating that the steel plate has good wear resistance. Specific implementation mode

[0021] For the high-strength storage tank steel plate with low ductile-brittle transition temperature described in the present invention, the chemical components in the steel are calculated by mass percentage as follows: C: 0.18% - 0.21%; Si: 0.31% - 0.42%; Mn: 1.12% - 1.28%; P≤0.012%; S≤0.01%; Cr: 0.01% - 0.03%; Nb: 0.062% - 0.074%, and Nb / Cr = 2.5 - 4; Ce: 0.0036% - 0.0072%; the balance is Fe and unavoidable impurities.

[0022] The reasons for the selection of the content of each chemical element C, Si, Mn, P, S, Cr, Nb, and Ce in the steel plate and the mechanism of action are as follows: C is a basic element of steel and plays an important role in both the strength and toughness of the steel plate. When the C content in the steel plate is low, it is difficult to ensure the strength, hardness, and hardenability of the steel plate, and the wear resistance of the steel plate decreases, directly affecting the service performance of the material. However, too high a C content will affect the machining performance of the steel, and supersaturated carbides will also have an adverse effect on the low-temperature toughness of the steel plate, increasing the ductile-brittle transition temperature range of the steel plate. Therefore, the C content range in the present invention is set to 0.18% - 0.21%.

[0023] Si is a common element in steel, has a high binding property with O, and can play a role in deoxidation. Adding an appropriate content of Si in the steel can make the steel have better wear resistance and tensile strength. However, when the Si content in the steel plate is too high, it is easy to cause an increase in non-metallic inclusions in the steel plate, which has a negative impact on the low-temperature toughness of the steel plate. Therefore, the Si content range in the present invention is set to 0.31% - 0.42%.

[0024] Mn mainly plays a role in solid solution strengthening in steel, but its role in strengthening ferrite or austenite is less than that of carbon, phosphorus, and silicon. While increasing the strength of the steel, it has no impact on ductility. In addition, the price of Mn is relatively cheap. However, Mn is easy to combine with S and segregate at the grain boundaries, which has an adverse effect on the hydrogen-induced cracking resistance of the steel plate. Therefore, the Mn content range in the present invention is set to 1.12% - 1.28%.

[0025] S and P are harmful elements in steel. To ensure the purity, plasticity, and toughness of the steel quality, their contents must be strictly controlled. Therefore, in the present invention, P≤0.012% and S≤0.01% are specified.

[0026] Cr is a strong carbide-forming element, and its affinity with C is higher than that of Fe. When a certain amount of Cr is added to steel, fine chromium carbide particles will be formed, which can still exist stably in high temperature areas, play the role of pinning dislocation movement, increasing grain boundary area, reducing austenite grain size, etc., and can also prevent grain growth, ensuring good strength and toughness matching of the steel plate; adding a certain amount of Cr element to steel is beneficial to improving the corrosion resistance and oxidation resistance of the steel plate; Cr element can also increase the hardness of the steel plate, thereby ensuring that the steel plate has good wear resistance. When an excessive amount of Cr element is added to steel, large-sized carbides are easily generated, which has a negative impact on the toughness and plasticity of the steel plate, and is also not conducive to the hydrogen-induced cracking resistance of the steel plate; therefore, the present invention sets the Cr content range to 0.01% to 0.03%.

[0027] Nb is a strong carbon / nitriding element, which forms precipitation strengthening phases such as Nb (C, N) in steel to improve the strength of the steel plate. The Nb element has a strong effect on increasing the area of ​​grain boundaries and subgrain boundaries for nucleation and inhibiting grain growth. The presence of the Nb element mainly plays a role in refining the grain structure and ensuring the strength and toughness matching and low-temperature toughness of the steel plate. However, when the content is too high, the brittleness of the steel plate will increase. Therefore, the present invention sets the Nb content range to 0.062% to 0.074%. The fine and dispersed Cr-C particles precipitated and stably exist in the high temperature section to ensure the strength of the steel plate, and the Nb (C, N) particles precipitated at low temperature to ensure the toughness of the steel plate and the lower tough-brittle transition temperature. The two cooperate with each other to ensure good strength and toughness matching and good service performance of the steel plate; therefore, the present invention sets the content ratio of Nb to Cr in the range of 2.5 to 4.

[0028] Ce element has strong chemical activity and can combine with impurity elements such as sulfur, oxygen, and nitrogen in steel to form stable rare earth inclusions, thereby reducing the harmful effects of these impurity elements and improving the purity of steel; rare earth elements can also promote the precipitation of harmful elements such as phosphorus and sulfur in steel in the form of rare earth inclusions, thereby reducing the adverse effects of these elements on the performance of steel. The precipitation strengthening effect reduces the toughness-brittle transition temperature of the steel plate and improves its toughness. However, when excessive Ce is added to the steel, pointed non-spherical large-sized inclusions will be generated, which will become the source of fracture cracks and the starting point of corrosion, and have an adverse effect on the steel plate's resistance to hydrogen-induced cracking and corrosion resistance. At the same time, taking into account the production and manufacturing costs, the present invention sets the Ce content range to 0.0036% to 0.0072%.

[0029] The production process of a high-low temperature impact toughness high-strength steel plate described in the present invention includes: molten iron pretreatment - refining outside the furnace - vacuum degassing - slab continuous casting - stacking slow cooling - casting cleaning - slab three-stage heating - two-stage controlled rolling - online cold straightening - temperature controlled normalizing heat treatment, etc. The produced high-strength storage tank steel plate with a thickness of 8 to 28 mm has uniform structure, good strength and toughness matching, and excellent low toughness-brittle transition temperature, corrosion resistance and wear resistance. The specific description is as follows: 1. Smelting process: The molten steel smelting is carried out in a converter. High-quality scrap steel and hot metal are used as raw materials. The size of the charged materials is controlled between 80 - 110 mm, and the amount of hot metal is controlled above 75% (mass percentage) to ensure the purity of the steel quality, shorten the process time, and reduce the difficulty of subsequent processes. During the smelting process, an inoculant with a magnesium content of 5% - 6.2% (mass percentage) is added to ensure the uniformity of the original structure, refine the as-cast grains, and shorten the smelting time. The process parameters of converter dephosphorization and decarburization are strictly controlled. The decarburization oxygen blowing time is controlled within 3 - 5 min; to effectively reduce the content of harmful element P, the dephosphorization oxygen blowing time is controlled within 8 - 10 min, and the phosphorus content in the molten steel is controlled below 0.012% (mass percentage) at the end of converter smelting; further, deep desulfurization treatment is carried out using an LF refining furnace, and the desulfurization oxygen blowing time is controlled within 9 - 12 min. The sulfur content is controlled below 0.01% (mass percentage) at the end of LF refining; degassing is completed in an RH furnace. The starting temperature is controlled within 1618 - 1640 °C, the oxygen blowing amount is controlled within 3.35 - 3.65 m³ / t steel·min, the net circulation time is controlled within 8 - 18 min, and the calming time before casting is 5 - 8 min.

[0030] By optimizing the smelting process parameters, the purpose of reducing the oxidation of molten steel, controlling the inclusion content in the steel, reducing internal defects in the steel plate, and purifying the steel quality is achieved.

[0031] 2. Continuous casting process: After breaking the vacuum, slab continuous casting machine is used for casting. The casting temperature is mainly controlled. The molten steel pouring temperature in the tundish is 1540 - 1566 °C, and the superheat is controlled within 9 - 12 °C. The casting speed during pouring is 1.0 - 1.6 m / min. The soft reduction process for continuous casting billets is adopted, and the reduction rate is controlled within 6% - 8%. After the casting billets are taken off the production line, they are stacked and slowly cooled, and the stacking and slow cooling time is 24 - 36 h.

[0032] By controlling the pouring temperature, the original as-cast structure is refined; adopting the soft reduction process for continuous casting billets is beneficial to optimizing the internal quality of continuous casting billets and reducing defects such as segregation and cavities.

[0033] 3. Slab heating process: The continuous casting slabs are sent into the heating furnace for heating and taken out after three-stage heating. The temperature range of the first heating section is 985 - 1011 °C, the temperature range of the second heating section is 1200 - 1225 °C, and the temperature range of the soaking section is 1085 - 1120 °C. The total time of the slab in the furnace is controlled within 1.8 - 3.2 h.

[0034] Through the three-stage heating method, the uniformity of the internal structure of the steel billet is further improved, the original size of the precipitated phase particles is controlled, the internal stress of the steel plate is fully released, and at the same time, the temperature inside and outside the steel billet is ensured to be uniform, which is beneficial for further processing.

[0035] 4. Rolling process: The two-stage controlled rolling method is adopted for rolling. In the first stage of rolling in the recrystallization zone, the starting rolling temperature is 1130 - 1155°C, and the finishing rolling temperature is 1020 - 1050°C to fully refine the original austenite structure. The controlled rolling process of "large reduction ratio - small reduction ratio" cyclic rolling is adopted, and the rolling speed is 3.0 - 4.4 m / s. The large reduction ratio refers to a reduction ratio of 11% - 15%, and the small reduction ratio refers to a reduction ratio of 6% - 9%. The purpose is to reduce the deformation resistance of the steel plate, ensure full recrystallization of grains, refine the internal structure of the steel plate, and improve the tissue uniformity at the same time.

[0036] After the rolling in the first-stage recrystallization zone is completed, the holding time of the steel plate is controlled within 50 - 65 s.

[0037] In the second stage of rolling in the non-recrystallization zone, the starting rolling temperature is 900 - 915°C, and the finishing rolling temperature is 826 - 860°C. The controlled rolling process with a single-pass reduction ratio decreasing from large to small is adopted, and the reduction ratio is controlled within the range of 5 - 12%. With the increase of the grain boundary area, the ferrite nucleation rate increases during the subsequent phase transformation process, fully refining the internal structure of the steel plate, and the austenite grains are further flattened and elongated. The internal stress of the steel plate is released, which is beneficial to optimizing the shape control of the steel plate. The rolled steel plate is air-cooled.

[0038] 5. Online cold straightening process: The fine and uniform grain structure helps to optimize the strength and toughness matching of the steel plate. The fine grain strengthening effect increases the hardness of the steel plate, thereby further achieving the purpose of improving the service performance such as the wear resistance of the steel plate. The online cold straightening method is used to further refine the structure of the steel plate. At the same time, small-angle grain boundaries are introduced during the straightening process to improve the corrosion resistance of the steel plate. The starting temperature of the cold straightening of the steel plate is controlled at 170 - 190°C, and the reduction ratio is 0.6% - 1.4%.

[0039] 6. Temperature-controlled normalizing heat treatment process: Since elements such as C, Si, Mn, Cr, Nb, and Ce are added to the steel, the steel plate can obtain an "ferrite + troostite + spherical bainite" structure with excellent strength and toughness after rolling. However, the grain size distribution of the steel plate is uneven, and there are concentration of tissue stress and thermal stress, which is prone to produce delayed cracks during flame cutting. Therefore, heat treatment should be used in time for softening and stress elimination. To further control the internal structure of the steel plate and ensure high production efficiency at the same time, the present invention adopts temperature-controlled normalizing heat treatment to ensure that the strength of the steel plate is not lost, while enabling the steel plate to have appropriate plasticity and toughness, low-temperature impact toughness, corrosion resistance, and good processing performance. In the first stage of the temperature-controlled normalizing heat treatment, it is heated to 880 - 935°C in a temperature-controlled manner and held for 15 - 28 min; in the second stage, it is cooled to 625 - 640°C in a temperature-controlled manner, and the cooling rate is controlled at 45 - 56°C / s. In the third stage, it is cooled to room temperature with the furnace.

[0040] The thickness of the finished steel plate is 8 - 28 mm.

[0041] The metallographic structure of the finished steel plate is "ferrite + troostite + spherical bainite" structure. Among them, by volume ratio, the ratio of troostite to ferrite is 1.6 - 3.0, the ratio of ferrite to spherical bainite is 1.2 - 1.4, and the size of spherical bainite is not more than 100 nm; the grain size of the finished steel plate is 8 - 9 grades.

[0042] The properties of the finished steel plate are as follows: at room temperature, 690 MPa ≤ tensile strength ≤ 740 MPa, 520 MPa ≤ yield strength ≤ 590 MPa, elongation ≥ 25%, hardness HV10 ≥ 240; under the condition of -80 °C, 700 MPa ≤ tensile strength ≤ 740 MPa, 490 MPa ≤ yield strength ≤ 530 MPa, elongation ≥ 25%, and the transverse impact energy KV 2 average value ≥ 160 J; the ductile-brittle transition temperature ≤ -80 °C.

[0043] The purpose of the present invention is to obtain a steel plate with a thickness specification of 8 - 28 mm, which has excellent strength, low-temperature toughness, service performance and flatness, by means of a brand-new chemical composition design and a production manufacturing process of "molten iron inoculation treatment smelting process + three-stage high-efficiency slab heating process + optimized two-stage controlled rolling process + on-line pre-treatment (cold straightening) process + temperature-controlled normalizing heat treatment process", meeting the manufacturing and application requirements of high-performance steel plates for storage tanks.

[0044] In order to more intuitively reflect the present invention, the embodiments of the present invention will be further described in combination with examples. The following examples are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the technical scope disclosed by the present invention, including simple changes or equivalent replacements, are within the protection scope of the present invention.

[0045] Example:

[0046] Table 1 shows the chemical compositions of the steel in each example and comparative example, Table 2 shows the smelting-continuous casting and heating process parameters of the steel in each example and comparative example, Table 3 shows the slab rolling and cold straightening process parameters in each example and comparative example, Table 4 shows the heat treatment process parameters of the steel plate in each example and comparative example, Table 5 shows the mechanical properties of the finished steel plate in each example and comparative example, Table 6 shows the evaluation test results of the grain size and second-phase particles of the finished steel plate in each example and comparative example, and Table 7 shows the service performance test results of the finished steel plate in each example and comparative example - corrosion resistance (hydrogen-induced cracking test, pitting corrosion test) test, friction and wear test results.

[0047] Table 1: Chemical Compositions of Steel (wt, %)

[0048] Table 2: Smelting - continuous casting and heating process parameters

[0049] Table 3: Slab rolling and cold straightening process parameters

[0050] Table 4: Heat treatment process parameters of steel plates

[0051] Table 5: Mechanical properties of finished steel plates

[0052] Table 6: Evaluation test results of the grain size of the microstructure and second - phase particles of finished steel plates

[0053] Table 7: Service performance test results of finished steel plates

[0054] As described above, it is only a preferred specific embodiment of the present invention. However, the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered within the protection scope of the present invention.

Claims

1. A low-ductile-brittle transition temperature high-strength steel plate for storage tanks, characterized in that: The chemical composition of the steel by mass percentage is C: 0.18% ~ 0.21%; Si: 0.31% ~ 0.42%; Mn: 1.12% ~ 1.28%; P≤0.012%; S≤0.01%; Cr: 0.01% ~ 0.03%; Nb: 0.062% ~ 0.074%, and Nb / Cr=2.5 ~ 4; Ce: 0.0036% ~ 0.0072%; the balance is Fe and unavoidable impurities.

2. The low-ductile-brittle transition temperature high-strength steel plate for storage tanks according to claim 1, characterized in that: The metallographic structure of the finished steel plate is ferrite + troostite + spherical bainite, wherein, by volume ratio, the ratio of troostite to ferrite is 1.6-3.0, the ratio of ferrite to spherical bainite is 1.2-1.4, and the size of spherical bainite is not greater than 100nm; the grain size of the finished steel plate is grade 8-9.

3. The low-ductile-brittle transition temperature high-strength steel plate for storage tanks according to claim 1, characterized in that: The properties of the finished steel plate are: at room temperature, 690MPa≤ tensile strength≤740MPa, 520MPa≤ yield strength≤590MPa, elongation≥25%, hardness HV10≥240; at -80℃, 700MPa≤ tensile strength≤740MPa, 490MPa≤ yield strength≤530MPa, elongation≥25%, transverse impact strength KV 2 Average value ≥160J; ductile-brittle transition temperature ≤-80℃.

4. A method for preparing a low-ductile-brittle transition temperature high-strength storage tank steel plate as claimed in any one of claims 1 to 3, characterized in that: The production process includes: smelting, continuous casting, slab heating, two-stage controlled rolling, online cold straightening and temperature-controlled normalizing heat treatment; the specific control process is as follows: 1) Smelting: The smelting process includes molten iron pretreatment, converter smelting, LF refining and RH vacuum degassing. During the converter smelting process, an inoculant containing 5% to 6.2% magnesium by mass is added, the decarburization oxygen blowing time is controlled at 3 to 5 minutes, the dephosphorization oxygen blowing time is controlled at 8 to 10 minutes, and the mass percentage of phosphorus in the molten steel is controlled below 0.012%. During LF refining, deep desulfurization treatment is carried out, the desulfurization oxygen blowing time is controlled at 9 to 12 minutes, and the mass percentage of sulfur in the molten steel is controlled below 0.01%. The starting temperature of RH vacuum degassing is 1618 to 1640°C, the oxygen blowing amount is controlled at 3.35 to 3.65 m³ / t steel·min, and the net cycle time is controlled at 8 to 18 minutes. The calming time before pouring is 5 to 8 minutes. 2) Continuous casting: The casting temperature of molten steel is 1540~1566℃, the superheat is controlled at 9~12℃, and the casting rate is 1.0~1.6m / min. The continuous casting billet light reduction process is adopted, and the reduction rate is controlled at 6%~8%. 3) Slab heating: The slab is heated in three stages before leaving the furnace. The temperature range of the first heating stage is 985-1011°C, the temperature range of the second heating stage is 1200-1225°C, and the temperature range of the soaking stage is 1085-1120°C. The total time of the slab in the furnace is controlled at 1.8-3.2h. 4) Two-stage controlled rolling: The first stage of rolling in the recrystallization zone has a starting rolling temperature of 1130-1155°C and a final rolling temperature of 1020-1050°C. The controlled rolling process of "large reduction rate-small reduction rate" cycle is adopted, and the rolling speed is 3.0-4.4m / s. The second stage of rolling in the non-recrystallization zone has a starting rolling temperature of 900-915°C and a final rolling temperature of 826-860°C. The controlled rolling process of decreasing the reduction rate from large to small in a single pass is adopted. 5) Online cold straightening: The starting temperature of cold straightening is 170-190°C, and the reduction rate is 0.6%-1.4%; 6) Temperature controlled normalizing heat treatment: The first stage is temperature controlled heating to 880-935°C, and the insulation time is 15-28 minutes; the second stage is temperature controlled cooling to 625-640°C, and the cooling rate is controlled at 45-56°C / s; the third stage is cooling to room temperature with the furnace.

5. The method for preparing a low-ductile-brittle transition temperature high-strength storage tank steel plate according to claim 4, characterized in that: During the smelting process, scrap steel and molten iron are used as raw materials. The size of the scrap steel loaded into the furnace is controlled between 80 and 110 mm, and the mass percentage of the molten iron is controlled at more than 75%.

6. The method for preparing a low-ductile-brittle transition temperature high-strength storage tank steel plate according to claim 4, characterized in that: After the continuous casting process is completed, the ingots are stacked and slowly cooled down, and the stacking and slow cooling time is 24 to 36 hours.

7. The method for preparing a low-ductile-brittle transition temperature high-strength storage tank steel plate according to claim 4, characterized in that: During the first stage of rolling in the recrystallization zone, the maximum reduction rate refers to a reduction rate of 11% to 15%, and the minimum reduction rate refers to a reduction rate of 6% to 9%; during the second stage of rolling in the non-recrystallization zone, the single-pass reduction rate is controlled at 5% to 12%.

8. The method for preparing a low-ductile-brittle transition temperature high-strength storage tank steel plate according to claim 4, characterized in that: After the first stage of recrystallization zone rolling is completed, the steel plate waiting time is controlled at 50 to 65 seconds.

9. The method for preparing a low-ductile-brittle transition temperature high-strength storage tank steel plate according to claim 4, characterized in that: The thickness of the finished steel plate is 8 to 28 mm.

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