A high-strength steel plate for storage tanks with low ductile-brittle transition temperature and its preparation method

Abstract

The present invention relates to the technical field of steel plate production, and particularly to a high-strength storage tank steel plate with a low ductile-brittle transition temperature and a preparation method thereof. The chemical components in the steel are 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 inevitable impurities. By optimizing the smelting process, the purity of the molten steel is improved and the grain of the original structure is controlled; by two-stage controlled rolling, the internal structure of the steel plate is optimized and the plate shape is improved; by temperature-controlled normalizing heat treatment, the strength-ductility matching is improved while the microscopic structure of the material is adjusted, and the mechanical properties 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 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 adaptation to complex and harsh service environments. Due to the increasing demand for liquefied energy such as liquefied petroleum gas and liquefied natural gas, the production volume of steel for low-temperature storage tanks has also gradually increased, but at the same time, higher requirements are put forward for the steel. For example, it is required to serve in complex environments such as low temperature and high pressure and hydrogen-containing media for a long time, and the steel plate is required to have good strength-ductility matching and a low ductile-brittle transition temperature.

[0003] Chinese patent application with publication number CN114875304A discloses "a quenched and tempered high-strength steel plate for SA537MCL2 pressure vessels and a production method thereof". 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 inevitable impurities. Quenching and tempering treatment is adopted. Since the steel plate contains expensive alloy elements such as Ni and the addition amount is relatively high, the production cost of the steel plate is relatively high; because multi-stage heat treatment is required and the average time-consuming 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, this 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 and 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 components 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 suitable for 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 suitable for 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, has 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 low-temperature service in 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 full protection 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 controlled-temperature 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] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A high-strength steel plate for low-temperature service in storage tanks with a low ductile-brittle transition temperature, 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.

[0010] 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 more than 100 nm; the grain size of the finished steel plate is 8 - 9 grades.

[0011] The performance of the finished steel plate is: at room temperature, 690 MPa ≤ tensile strength ≤ 740 MPa, 520 MPa ≤ yield strength ≤ 590 MPa, elongation ≥ 25%, hardness HV10 ≥ 240; at -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.

[0012] A preparation method of a high-strength steel plate for low-temperature service in 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 controlled-temperature normalizing heat treatment; the specific control is as follows:

[0013] 1) Smelting:

[0014] Smelting includes the processes of 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 min, the dephosphorization oxygen blowing time is controlled within 8 - 10 min, 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 min, 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 min; the calming time before casting is 5 - 8 min;

[0015] 2) Continuous casting:

[0016] The pouring temperature of the molten steel is 1540 - 1566 °C, the superheat is controlled within 9 - 12 °C, and 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%;

[0017] 3) Slab heating:

[0018] The slab is taken out of the furnace after three-stage heating. 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;

[0019] 4) Two-stage controlled rolling:

[0020] The starting rolling temperature for the first-stage recrystallization zone rolling is 1130 - 1155 °C, and the finishing rolling temperature is 1020 - 1050 °C; a 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 finishing rolling temperature is 826 - 860 °C; a controlled rolling process with a single-pass reduction rate decreasing from large to small is adopted;

[0021] 5) Online cold straightening:

[0022] The starting temperature of cold straightening is 170 - 190 °C, and the reduction rate is 0.6% - 1.4%;

[0023] 6) Temperature-controlled normalizing heat treatment:

[0024] In the first stage, it is heated to 880 - 935 °C under temperature control, and the holding time is 15 - 28 min; 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.

[0025] 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 and 110 mm, and the mass percentage of hot metal is controlled above 75%.

[0026] After the continuous casting process is completed, the cast slabs are taken offline for stacking and slow cooling, and the stacking and slow cooling time is 24 to 36 h.

[0027] During the rolling in the first-stage recrystallization zone, the large reduction ratio refers to a reduction ratio of 11% to 15%, and the small reduction ratio refers to a reduction ratio of 6% to 9%. During the rolling in the second-stage non-recrystallization zone, the single-pass reduction ratio is controlled at 5% to 12%.

[0028] After the rolling in the first-stage recrystallization zone is completed, the holding time of the steel plate is controlled at 50 to 65 s.

[0029] The thickness of the finished steel plate is 8 to 28 mm.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) On the basis of strengthening elements such as C, Si, and Mn, appropriate amounts of 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. Combined 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 to 3.0 by volume ratio, the ratio of ferrite to spherical bainite is 1.2 to 1.4, and the size of spherical bainite is not greater than 100 nm. The grain size of the finished steel plate is 8 to 9 grades, the second-phase Cr / Nb carbide particles with a size in the range of 40 to 60 nm are uniformly and dispersedly distributed, and Ce(O / S / N) with a size not greater than 30 nm ensure the strength, plasticity, low-temperature toughness, and service performance of the steel plate.

[0032] (2) The mechanical properties of the steel plate for storage tanks obtained through a 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; at -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.

[0033] (3)According to the "Evaluation Method for Resistance to Hydrogen-Induced Cracking of Pipeline Steel and Pressure Vessel Steel" in GB / T8650-2006 and NACE-TM0284, the hydrogen-induced cracking (HIC) experiment was carried out. After 96 hours of testing the steel plate in Solution A and Solution B, the crack sensitivity CSR (%), crack length rate CLR (%), and crack width rate CTR (%) were all 0, indicating that the steel plate has excellent resistance to hydrogen-induced cracking. According to the test method of GB / T17897-2016 "Corrosion of Metals and Alloys - Pitting Corrosion Test Method for Stainless Steels in Ferric Chloride Solution", the corrosion rates of the steel plate in Solution A and Solution B were both not greater than 0.004g / m 2 ·h; it shows that the steel plate has excellent corrosion resistance.

[0034] (4)According to the test method of GB / T3960-2016 "Plastics - Determination of Sliding Friction and Wear Properties", the test results show that the volume wear of the steel plate is not greater than 0.0004cm 3 ,indicating that the steel plate has good wear resistance. Specific Embodiments

[0035] For the high-strength steel plate for low-temperature toughness storage tanks of 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 inevitable impurities.

[0036] The reasons for selecting the content of each chemical element of C, Si, Mn, P, S, Cr, Nb, and Ce in the steel plate and the action mechanism are as follows:

[0037] C is a basic element of steel and plays an important role in 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%.

[0038] Si is a common element in steel. It has a high binding property with O and can play a role in deoxidation. Adding an appropriate amount of Si to 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%.

[0039] Manganese (Mn) primarily contributes to solid solution strengthening in steel, but its effect on strengthening ferrite or austenite is less pronounced than that of carbon, phosphorus, and silicon. While increasing steel strength, it has no effect on ductility. Furthermore, Mn is relatively inexpensive. However, Mn readily combines with sulfur and segregates at grain boundaries, adversely affecting the steel's resistance to hydrogen-induced cracking. Therefore, the present invention sets the Mn content range to 1.12% to 1.28%.

[0040] S and P are harmful elements in steel. To ensure the purity and plasticity of the steel, their contents must be strictly controlled. Therefore, the present invention limits P to 0.012% and S to 0.01%.

[0041] Cr is a strong carbide-forming element with a higher affinity for carbon than Fe. Adding a certain amount of Cr to steel forms fine chromium carbide particles that remain stable even in high-temperature zones. These particles pin dislocations, increase grain boundary area, and reduce austenite grain size, while also preventing grain growth and ensuring a good balance of strength and toughness between the steel plate. Adding a certain amount of Cr to steel improves the corrosion and oxidation resistance of the steel plate. Cr also increases the hardness of the steel plate, thereby ensuring good wear resistance. However, adding excessive Cr to steel easily forms large carbides, negatively impacting the steel plate's toughness and ductility, as well as its resistance to hydrogen-induced cracking. Therefore, the present invention sets the Cr content range to 0.01% to 0.03%.

[0042] Nb is a strong carbonitriding element, forming precipitation-strengthening phases such as Nb (C, N) in steel, thereby increasing the strength of the steel plate. Nb has a strong effect on increasing the area of grain boundaries and subgrain boundaries for nucleation and inhibiting grain growth. The presence of Nb primarily refines the microstructure grains, ensuring the steel plate's strength-toughness match and low-temperature toughness. However, excessively high Nb content increases the steel plate's brittleness, so the present invention sets the Nb content range to 0.062% to 0.074%. Fine, dispersed Cr-C particles that precipitate and stably exist in the high-temperature range ensure the steel plate's strength, while Nb (C, N) particles precipitated at low temperatures ensure the steel plate's toughness and a low ductile-brittle transition temperature. The two work together to ensure the steel plate's good strength-toughness match and excellent service performance. Therefore, the present invention sets the Nb:Cr content ratio range to 2.5 to 4.

[0043] 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, thus reducing the adverse effects of these elements on the properties of steel. The precipitation strengthening effect reduces the ductile-brittle transition temperature of the steel plate and improves its toughness. However, when excessive Ce is added to the steel, acicular non-spherical large-size inclusions will be generated, which become the source of fracture cracks and the starting point of corrosion, having an adverse impact on the hydrogen-induced crack resistance and corrosion resistance of the steel plate. At the same time, considering the production and manufacturing costs comprehensively, the content range of Ce in this invention is set at 0.0036% - 0.0072%.

[0044] The production process flow of the high and low temperature impact toughness high-strength steel plate described in this invention includes: hot metal pretreatment - secondary refining - vacuum degassing - slab continuous casting - stacking slow cooling - billet cleaning - three-stage slab heating - two-stage controlled rolling - on-line cold straightening - temperature-controlled normalizing heat treatment, etc. The high-strength steel plate for storage tanks with a thickness specification of 8 - 28 mm produced has a uniform structure, good strength-ductility matching, as well as excellent low ductile-brittle transition temperature, corrosion resistance, and wear resistance. The specific description is as follows:

[0045] 1. Smelting process: The steelmaking is carried out in a converter, using high-quality scrap steel and hot metal 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 dephosphorization and decarburization smelting process parameters in the converter 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; the 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.

[0046] By optimizing the smelting process parameters, the purpose of reducing the oxidation of molten steel, controlling the inclusion content in the steel, reducing the internal defects of the steel plate, and purifying the steel quality is achieved.

[0047] 2. Continuous casting process: After breaking the vacuum, slab continuous casting machine is used for casting. The pouring temperature is mainly controlled. The molten steel pouring temperature in the tundish is 1540 - 1566 °C, the superheat is controlled at 9 - 12 °C, and the drawing speed during pouring is 1.0 - 1.6 m / min. The soft reduction process for continuous casting billets is adopted, with the reduction rate controlled at 6% - 8%. After the billets are taken off the production line, they are stacked and slowly cooled, and the stacking and slow cooling time is 24 - 36 h.

[0048] 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.

[0049] 3. Slab heating process: The continuous casting slabs are sent into the heating furnace for heating and taken out of the furnace after three-stage heating. 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 - 3.2 h.

[0050] Through the three-stage heating method, the uniformity of the internal structure of the steel billet is further improved, the original size of the precipitation 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 to further processing.

[0051] 4. Rolling process:

[0052] Two-stage controlled rolling method is adopted for rolling. The starting rolling temperature in the first-stage recrystallization zone rolling is 1130 - 1155 °C, and the finishing rolling temperature is 1020 - 1050 °C, which fully refines the original austenite structure; the controlled rolling process of "large reduction rate - small reduction rate" cyclic rolling is adopted, and the rolling speed is 3.0 - 4.4 m / s. The large reduction rate means the reduction rate is 11% - 15%, and the small reduction rate means the reduction rate is 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 structure uniformity at the same time.

[0053] After the first-stage recrystallization zone rolling is completed, the waiting temperature time of the steel plate is controlled at 50 - 65 s.

[0054] The starting rolling temperature in the second-stage non-recrystallization zone rolling is 900 - 915 °C, and the finishing rolling temperature is 826 - 860 °C. The controlled rolling process with the single-pass reduction rate decreasing from large to small is adopted, and the reduction rate 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, which fully refines 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.

[0055] 5. Online cold straightening process:

[0056] 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 steel plate structure. 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 rate is 0.6%-1.4%.

[0057] 6. Temperature-controlled normalizing heat treatment process:

[0058] Due to the addition of elements such as C, Si, Mn, Cr, Nb, and Ce in the steel, the steel plate can obtain an excellent "ferrite + troostite + spherical bainite" structure in terms of 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 delayed cracking during flame cutting. Therefore, heat treatment should be promptly used 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, which ensures that the strength of the steel plate is not lost, and at the same time enables 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 under temperature control and held for 15-28 minutes; in the second stage, it is cooled to 625-640°C under temperature control, and the cooling rate is controlled at 45-56°C / s. In the third stage, it is cooled to room temperature with the furnace.

[0059] The thickness of the finished steel plate is 8-28 mm.

[0060] 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 the spherical bainite is not greater than 100 nm; the grain size of the finished steel plate is 8-9 grades.

[0061] 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; at -80°C, 700 MPa ≤ tensile strength ≤ 740 MPa, 490 MPa ≤ yield strength ≤ 530 MPa, elongation ≥ 25%, transverse impact energy KV 2 The average value ≥ 160 J; the ductile-brittle transition temperature ≤ -80°C.

[0062] The object of the present invention is to obtain steel plates with a thickness specification of 8 - 28 mm, which have excellent strength, low-temperature toughness, service performance and flatness, by means of a production process that combines a new chemical composition design with an "iron melt inoculation treatment and 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", so as to meet the manufacturing and application requirements of high-performance steel plates for storage tanks.

[0063] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the 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 obtained obviously 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.

[0064] Example:

[0065] 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 plates in each example and comparative example. Table 5 shows the mechanical properties of the finished steel plates in each example and comparative example. Table 6 shows the evaluation test results of the grain size and second-phase particles of the microstructure of the finished steel plates in each example and comparative example. Table 7 shows the service performance test results of the finished steel plates in each example and comparative example - corrosion resistance (hydrogen-induced cracking test, pitting corrosion test) test, friction and wear test results.

[0066] Table 1: Chemical Composition of Steel (wt, %)

[0067]

[0068] Table 2: Smelting-Continuous Casting and Heating Process Parameters

[0069]

[0070] Table 3: Slab Rolling and Cold Straightening Process Parameters

[0071]

[0072] Table 4: Heat Treatment Process Parameters of Steel Plates

[0073]

[0074] Table 5: Mechanical Properties of Finished Steel Plates

[0075]

[0076] Table 6: Test results of the grain size of the microstructure and the second-phase particles of the finished steel plate

[0077]

[0078] Table 7: Test results of the service performance of the finished steel plate

[0079]

[0080] As mentioned above, it is only the preferred specific implementation mode of the present invention, but 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, shall be covered by the protection scope of the present invention.

Claims

1. A high-strength steel plate for storage tanks with a low ductile-brittle transition temperature, characterized in that The chemical components in the steel are 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; the properties of the finished steel plate are: at room temperature, 690 MPa ≤ tensile strength ≤ 740 MPa, 520 MPa ≤ yield strength ≤ 590 MPa, elongation ≥ 25%, hardness HV10 ≥ 240; at -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 ≥ 160 J; the ductile-brittle transition temperature ≤ -80 °C.

2. The high-strength storage tank steel plate with low ductile-brittle transition temperature according to claim 1, wherein 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 more than 100 nm; the grain size of the finished steel plate is 8 - 9 grades.

3. A preparation method of a high-strength storage tank steel plate with a low ductile-brittle transition temperature as described in claim 1 or 2, 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 is as follows: 1) Smelting: Smelting includes hot metal pretreatment, converter smelting, LF refining and RH vacuum degassing processes; 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 at 3 - 5 min, the dephosphorization oxygen blowing time is controlled at 8 - 10 min, 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 - 12 min, 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 at 3.35 - 3.65 m³ / t steel·min, and the net circulation time is controlled at 8 - 18 min; the calming time before pouring is 5 - 8 min; 2) Continuous casting: The molten steel casting temperature is 1540 - 1566 °C, the superheat is controlled at 9 - 12 °C, and the casting speed during pouring is 1.0 - 1.6 m / min; the continuous casting slab soft reduction process is adopted, and the reduction rate is controlled at 6% - 8%; 3) Slab heating: The slab is taken out of the furnace 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 in-furnace time of the slab is controlled at 1.8 - 3.2 h; 4) Two-stage controlled rolling: The starting rolling temperature of the first-stage recrystallization zone rolling is 1130 - 1155 °C, and the final rolling temperature is 1020 - 1050 °C; a 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 of the second-stage non-recrystallization zone rolling is 900 - 915 °C, and the final rolling temperature is 826 - 860 °C; a controlled rolling process with a single-pass reduction rate decreasing 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 temperature-controlled and heated to 880 - 935 °C, and the holding time is 15 - 28 min; in the second stage, it is temperature-controlled and cooled to 625 - 640 °C, and the cooling rate is controlled at 45 - 56 °C / s. In the third stage, it is cooled to room temperature with the furnace.

4. The preparation method of a high-strength storage tank steel plate with a low ductile-brittle transition temperature according to claim 3, characterized in that, During smelting, 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%.

5. The preparation method of a high-strength storage tank steel plate with a low ductile-brittle transition temperature according to claim 3, characterized in that, After the continuous casting process, the cast slab is taken off the production line for stacking and slow cooling, and the stacking and slow cooling time is 24 - 36 h.

6. The preparation method of a high-strength storage tank steel plate with low ductile-brittle transition temperature according to claim 3, characterized in that, When 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%; when rolling in the second-stage non-recrystallization zone, the single-pass reduction ratio is controlled within 5% - 12%.

7. The preparation method of a high-strength storage tank steel plate with a low ductile-brittle transition temperature according to claim 3, characterized in that, After the rolling in the first-stage recrystallization zone is completed, the soaking time of the steel plate is controlled within 50 - 65 s.

8. The preparation method of a high-strength storage tank steel plate with low ductile-brittle transition temperature according to claim 3, characterized in that, The thickness of the finished steel plate is 8 - 28 mm.

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