A container steel plate for a crude oil storage tank having an excellent high strength and high toughness ratio and a method for manufacturing the same

Abstract

The application discloses a container steel plate for crude oil storage tanks with excellent high-strength and high-toughness and a preparation method thereof, and is characterized by a novel chemical composition design, a smelting process, a three-stage efficient slab heating system, a four-stage TMCP controlled rolling process, a two-stage controlled cooling process, a plate shape and structure comprehensive control process technology and a controlled tempering heat treatment mode. The prepared steel plate has a uniform structure, good strength and toughness matching, low ductile-brittle transition temperature, excellent corrosion resistance and wear resistance, and is suitable for the production and manufacturing of large-scale efficient low-temperature storage tank steel.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, and more specifically, to a container steel plate for crude oil storage tanks with excellent yield strength ratio and high strength and toughness, and a method for preparing the same. Background Technology

[0002] With the growth of my country's oil consumption and the need for strategic reserve construction, large crude oil storage tanks are developing towards larger capacity and higher pressure. Previously, the reliance on imported high-strength steel plates constrained energy security and engineering efficiency. Domestic development can reduce costs, ensure supply, and has profound significance in promoting technological upgrading in the steel and petrochemical industries.

[0003] However, traditional steels used in storage tanks have significant shortcomings in key performance indicators, especially in terms of strength-toughness matching, adaptability to low-temperature / ultra-low-temperature service environments, and tolerance to complex environments: high-strength steels generally face the dilemma of insufficient toughness reserves, which can easily lead to catastrophic brittle fracture accidents under extreme working conditions.

[0004] Patent CN114657458A discloses a thick, high-strength, high-heat-input weldable crude oil storage tank steel plate and its preparation method. The steel plate is composed of the following components by weight percentage: C: 0.07%–0.13%, Si: 0.15%–0.35%, Mn: 1.45%–1.60%, Nb≤0.030%, V: 0.030%–0.050%, Ti: 0.010%–0.025%, Mo≤0.15%, Als: 0.020%–0.060%, P≤0.015%, S≤0.005%, with Pcm≤0.22% and PSR≤-0.50%, and the balance being Fe and other unavoidable impurity elements. Steel plates produced by sequential quenching and tempering treatment with the above-mentioned composition have low strength, and the impact toughness in low-temperature environments below -20℃ is not addressed. Therefore, they are not suitable for the production of high-strength and high-toughness steel for large-scale cryogenic storage tanks. Patent CN116356199A discloses a high-strength, corrosion-resistant crude oil storage tank steel plate for high heat input welding and its manufacturing method. The steel plate is composed of the following components by weight percentage: C: 0.07~0.12%, Si: 0.25~0.45%, Mn: 1.40~2.0%, P≤0.012%, S≤0.005%, Cu: 0.1~0.4%, Ni: 0.1~0.5%, Mo: 0.1 The composition is as follows: 0~0.30%, V: 0.01~0.05%, Ti: 0.005~0.035%, B: 0.0005~0.0035%, Sn: 0.01~0.06%, La: 0.001~0.03%, Ca: 0.0002~0.005%, Zr: 0.001~0.02%, O≤0.0030%, N: 0.0045~0.0065%, with the remainder being Fe and unavoidable impurities. Patent CN116121651A also discloses a high-strength, corrosion-resistant crude oil storage tank steel plate and its manufacturing method for high heat input welding. However, the aforementioned patent technology has not studied the service conditions of the steel plate at temperatures below -20℃ or its resistance to hydrogen embrittlement, therefore it is not suitable for large-scale production of high-strength, high-toughness cryogenic storage tank steel with excellent yield strength ratio. Summary of the Invention

[0005] This invention aims to address the limitations of existing storage tank steels in terms of strength-toughness matching, adaptability to ultra-low temperature service environments, and tolerance to complex environments. These steels are prone to brittle fracture under extreme operating conditions and struggle to simultaneously meet the comprehensive requirements of 19-45mm thickness steel plates for high strength, high and low temperature toughness, excellent service performance, and plate shape. This invention provides a high-strength, high-toughness container steel plate for crude oil storage tanks with an excellent yield strength ratio and a method for its preparation. Through a novel chemical composition design and optimized smelting process, the purity of molten steel and cast billets is improved, reducing the influence of elements such as P, S, and O, and controlling the original microstructure grain size. A three-stage high-efficiency slab heating process is employed to improve slab heating efficiency and ensure and steadily improve slab quality. Four-stage TMCP controlled rolling combined with two-stage controlled cooling, along with comprehensive control technology for steel plate shape and microstructure, further optimizes the internal microstructure of the steel plate, improves plate shape, and ensures production efficiency. Controlled tempering heat treatment improves strength-toughness matching while adjusting the material's microstructure, improving the mechanical properties or low-temperature service performance of the steel plate. The steel plate produced by this invention has a uniform structure, good strength and toughness matching, low ductile-brittle transition temperature, excellent corrosion resistance and wear resistance, and is suitable for the production and manufacturing of steel for large-scale, high-efficiency cryogenic storage tanks.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A high-strength and high-toughness crude oil storage tank steel plate with excellent yield strength ratio comprises the following components by weight percentage: C: 0.08%~0.10%, Si: 0.19%~0.21%, Mn: 1.12%~1.28%, P≤0.015%, S≤0.005%, N: 0.007%~0.01%, Ni: 0.22%~0.29%, Cr: 0.21%~0.28%, Nb: 0.017~0.034%, with the balance being Fe and unavoidable impurities.

[0008] The present invention also discloses a method for preparing a container steel plate for crude oil storage tanks with excellent yield strength ratio and high strength and toughness as described above, comprising the following steps: smelting, continuous casting, three-stage high-efficiency slab heating, four-stage rolling, two-stage controlled cooling process and heat treatment. In the three-stage high-efficiency slab heating process, the continuously cast slab is heated in three stages before being removed from the furnace. The temperature range of the first stage continuous heating section is 942~965℃, and the heating rate is controlled at 4.6~5.4℃ / min. The temperature range of the second stage high-temperature soaking section is 1185~1220℃, the heating rate is 4.1~4.6℃ / min, and the holding time is 18~36min. The temperature range of the third stage low-temperature soaking section is 1041~1062℃. The total net holding time of the continuously cast slab is controlled at 96~145min. In the four-stage rolling and two-stage controlled cooling process, the first stage involves high-temperature section initial microstructure control rolling, with an initial rolling temperature of 1142~1161℃ and a final rolling temperature of 1034~1059℃. A combination of large and small reductions is used, sequentially employing a large reduction rolling, small reduction rolling, large reduction rolling, and small reduction cyclic rolling control process. The rolling speed is 3.4~3.9 m / s, with a large reduction range of 9%~12% and a small reduction range of 4%~7%. The second stage involves high-temperature section microstructure optimization control rolling, with an initial rolling temperature of 1008~1031℃ and a final rolling temperature of 982~996℃. Rapid small deformation rolling is performed in the recrystallization zone, using a small reduction rolling method, with the small reduction controlled at 3%~5%. The rolling speed is 5.8~6.2 m / s; the third stage is finished product microstructure controlled rolling, with an initial rolling temperature of 941~954℃ and a final rolling temperature of 814~843℃. The single-pass reduction adopts a combination of small and large reduction, with a cyclic rolling control process of small reduction rolling followed by large reduction rolling. The large reduction range is 7%~10%, and the small reduction range is 3%~5%; the fourth stage is the steel plate service performance optimization rolling stage, with an initial rolling temperature of 786~822℃ and a reduction of 2%~4%; the rolled steel plate adopts a two-stage controlled cooling method, with the first stage starting cooling temperature at 738~769℃ and a cooling rate of 33~38℃ / s; the second stage starting cooling temperature at 592~628℃ and a cooling rate of 12~16℃ / s; The heat treatment is carried out by tempering; the tempering temperature is 562~589℃, the heating rate is 3.9~4.4℃ / min, the holding time is 42~56min, and the heat treatment is followed by air cooling to room temperature.

[0009] Implementing the embodiments of the present invention will have the following beneficial effects: (1) This invention, through a novel chemical composition design, adopts a smelting process + a three-stage high-efficiency slab heating system + an optimized design of a four-stage TMCP controlled rolling process + a two-stage controlled cooling process + a comprehensive control process technology for plate shape and microstructure + a controlled tempering heat treatment method. The steel plate for storage tanks obtained through this unique production process exhibits the following mechanical properties: at room temperature, the tensile strength is ≥817MPa, the yield strength is ≥798MPa, the yield ratio is in the range of 0.87~0.91, A≥22%, and Vickers hardness ≤235HV10; at -60℃, the tensile strength is ≥815MPa, the yield strength is ≥800MPa, the yield ratio is in the range of 0.87~0.92, A≥20%, and the average transverse impact energy KV2 is ≥240J; at -196℃, the average transverse impact energy KV2 is ≥200J, which means good strength and toughness matching and low-temperature service performance.

[0010] (2) Based on the strengthening elements of C, Si and Mn, by adding appropriate amounts of alloying elements such as Ni, Cr and Nb, while strictly controlling the content of harmful elements P and S, and combining with the optimization of production process, a uniform and refined troostite + spherical bainite structure is obtained. The grain size grade is 8~9, and the sum of the grades of A, B, C and D inclusions is ≤1.0. The volume percentage of troostite and bainite is (5~8):(2~5). The size of spherical bainite is 32~46nm. The second phase Cr / Nb (C / N) particles with a size in the range of 22~35nm are uniformly dispersed, which ensures the strength, plasticity, low temperature toughness and service performance of the steel plate.

[0011] (3) It has good corrosion resistance (resistance to hydrogen-induced cracking and pitting corrosion). According to the hydrogen-induced cracking (HIC) test GB / T 8650-2006 and NACE TM0284 "Evaluation Method for Hydrogen-Induced Cracking Resistance of Pipeline Steel and Pressure Vessel Steel", after 96 hours of testing in solution A and solution B, the crack sensitivity CSR (%), crack length ratio CLR (%), and crack width ratio CTR (%) of the steel plate are all 0, indicating excellent resistance to hydrogen-induced cracking. According to GB / T 17897-2016 "Corrosion of Metals and Alloys - Test Method for Pitting Corrosion of Stainless Steel with Ferric Chloride", the corrosion rate of the steel plate in solution A and solution B is not greater than 0.0046 g / m. 2 The results indicate that the steel plate has excellent corrosion resistance.

[0012] (4) The test was conducted according to GB / T 3960-2016 "Test Method for Sliding Friction and Wear of Plastics". The results showed that the volumetric wear of the steel plate was no greater than 0.00035 cm. 3 Steel plates have good wear resistance.

[0013] (5) The steel plates with a thickness of 19~45mm prepared by the present invention meet the comprehensive requirements for high strength, high and low temperature toughness, excellent service performance and plate shape. They are low in cost and suitable for the production and manufacturing of steel for large-scale and high-efficiency cryogenic storage tanks. Detailed Implementation

[0014] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0015] This invention discloses a container steel plate for crude oil storage tanks with excellent yield strength ratio and high strength and toughness, comprising the following components by weight percentage: C: 0.08%~0.10%, Si: 0.19%~0.21%, Mn: 1.12%~1.28%, P≤0.015%, S≤0.005%, N: 0.007%~0.01%, Ni: 0.22%~0.29%, Cr: 0.21%~0.28%, Nb: 0.017~0.034%, with the balance being Fe and unavoidable impurities.

[0016] In one specific embodiment, the weight percentage ratio of Ni and N in the container steel plate for crude oil storage tanks conforms to the following: 2.7≤Ni / 9N≤4.2.

[0017] In one specific embodiment, the weight percentage ratio of the chemical composition of C, Nb, and Cr in the container steel plate for crude oil storage tanks conforms to the following: C≤Nb / Cr.

[0018] The specific reasons for limiting the amounts of each chemical element in steel plates are detailed below: Carbon (C) is a fundamental element in steel, playing a crucial role in both the strength and toughness of steel plates. This is especially true for steel plates used in storage tanks, which require high low-temperature performance and a good balance between strength and toughness. To ensure the required strength grade, the C content in the steel plate must be controlled to prevent it from becoming too low, thus guaranteeing the material's service performance. On the other hand, to control the negative impact of supersaturated carbides on the low-temperature toughness and machinability of the steel plate, and to ensure a low ductile-brittle transition temperature range and uniform microstructure, the C content in the steel should not be too high. Therefore, this invention sets the C content range to 0.08%~0.10%.

[0019] Si is a common deoxidizer in steel. Adding an appropriate amount of Si to steel can improve its wear resistance and tensile strength. However, excessive Si content in steel plates can easily lead to segregation in the core structure, increasing non-metallic inclusions and negatively impacting the strength-toughness ratio (yield-to-tensile ratio deviating from the expected range), low-temperature toughness, and the uniformity of microstructure properties in the thickness direction. Therefore, this invention sets the Si content range to 0.19%~0.21%.

[0020] Mn mainly plays a role in solid solution strengthening and precipitation strengthening in steel, increasing the strength of steel plates without affecting ductility. In addition, it is relatively inexpensive. However, Mn readily combines with sulfur to form a hard second phase and segregates at grain boundaries, affecting the steel plate's resistance to hydrogen-induced cracking. Therefore, this invention sets the Mn content range to 1.12%~1.28%.

[0021] S and P are harmful elements in steel. To ensure the purity and toughness of steel, they must be strictly controlled. Therefore, this invention limits P to ≤ 0.015% and S to ≤ 0.005%.

[0022] In steel, chromium (Cr) exists as an alloying element. In steel, Cr forms fine chromium carbide particles, reducing the austenite grain size while inhibiting grain growth, ensuring good strength properties of the steel plate without sacrificing its toughness. Cr also forms a continuous solid solution with iron, shrinking the austenite phase region and contributing to the formation of carbides (such as Cr...). 23 C6) strengthens the steel matrix, especially when synergistically combined with other alloying elements (such as Nb). On the other hand, adding a certain amount of Cr to steel is beneficial for improving the steel plate's corrosion resistance and oxidation resistance. Cr can increase the hardness of the steel plate, thus ensuring good wear resistance. However, adding excessive Cr to steel easily generates large-sized carbides, negatively impacting the steel plate's toughness and plasticity, and also hindering its resistance to hydrogen-induced cracking. Therefore, this invention sets the Cr content range to 0.21%~0.28%.

[0023] Nitrogen (Nb) primarily functions as a grain refiner, precipitation strengthener, and dislocation strengthener. As a strong carbide / nitride element, Nb carbides and nitrides can recrystallize during steel heat treatment, forming fine precipitates. These precipitates hinder dislocation movement, thus increasing the steel's strength. Furthermore, the uniform distribution of Nb carbides and nitrides in the steel improves its plasticity and toughness, as well as its fatigue and impact resistance. This ensures a balance between strength and toughness, and adequate low-temperature toughness in the steel plate. However, excessively high Nb content increases the steel plate's brittleness; therefore, the recommended Nb content is 0.017% to 0.034%.

[0024] The synergistic effect of C, Cr, and Nb significantly enhances the overall performance of steel: C, as the main strengthening element, improves the hardness and hardenability of steel by forming carbides; Cr not only participates in the formation of strengthening carbides but also generates a dense Cr2O3 passivation film on the steel surface, greatly enhancing corrosion resistance, while its property of slowing austenite decomposition improves hardenability; Nb preferentially combines with C to form stable NbC carbides, which both inhibits Cr depletion near grain boundaries to maintain corrosion resistance, refines austenite grains and hinders grain growth at high temperatures, strengthening the strength and toughness of the steel, and also reduces softening in the weld heat-affected zone and prevents joint embrittlement. The combined effect of these three elements creates a complementary synergistic effect in strengthening, corrosion resistance, heat treatment stability, and weldability, enabling the steel to exhibit superior overall performance under high loads, harsh corrosive environments, and during welding. Therefore, this patent controls the C / Cr ratio to be within the range of C ≤ Nb / Cr based on elemental composition.

[0025] Ni primarily functions as a solid solution strengthener and a grain refiner in steel. Existing in steel as a solid solution, it significantly improves the steel's tensile strength through this strengthening mechanism. Ni refines the grain size, improves the steel's toughness, effectively lowers the ductile-brittle transition temperature, and ensures good low-temperature toughness. As a key alloying element for austenite formation and stabilization, Ni expands the austenite region of iron, preventing martensitic transformation at low temperatures and thus avoiding brittle fracture. By lowering the critical transformation temperature, Ni ensures good hardenability, making it easier to achieve a uniform microstructure and excellent mechanical properties during heat treatment. Furthermore, adding a certain amount of Ni improves the steel's resistance to corrosion from acids, alkalis, and the atmosphere. Therefore, the recommended Ni content is controlled at 0.22%~0.29%.

[0026] Ni (ni) plays a similar role to Ni in steel, primarily functioning as a solid solution strengthener and a precipitation strengthener. It expands the austenite phase region and stabilizes the austenite structure, thus maintaining the low-temperature toughness of the steel while reducing Ni content. Adding Ni increases strength, reduces dependence on nickel, and lowers costs. On one hand, Ni dissolves in ferrite or austenite to form interstitial solid solutions, increasing steel strength through lattice distortion. Nitrogen also forms fine nitrides (such as NbN and CrN) with elements like Cr / Nb, which precipitate during tempering or aging, significantly improving yield strength. Simultaneously, Ni reduces grain growth tendency, refines ferrite grains, and improves toughness. Therefore, Ni content should be controlled within the range of 0.007% to 0.01%.

[0027] Synergistic Effects of Ni and N: In crude oil storage tank container steel, nitrogen (N) and nickel (Ni) significantly improve steel performance through synergistic effects: N enhances strength through solid solution strengthening and precipitation hardening (such as forming NbN and CrN), while refining grains; Ni stabilizes the austenitic structure, lowers the ductile-brittle transition temperature, ensures low-temperature toughness, and improves the cooling behavior of the weld heat-affected zone. This synergy achieves a balance between strength, toughness, corrosion resistance, and weldability. However, due to the high cost of Ni, its addition amount needs to be optimized; simultaneously, the N content must be strictly controlled to avoid embrittlement, thus achieving a balance between performance improvement and cost control. Therefore, the Ni / 9N ratio is controlled at 2.7~4.2.

[0028] In one specific embodiment, the crack sensitivity index PCM of the container steel plate for crude oil storage tanks is ≤0.2%; and the carbon equivalent CE is ≤0.40%.

[0029] Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B.

[0030] CE (%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0031] In one specific embodiment, the thickness of the steel plate is 19~45mm.

[0032] In one specific embodiment, the steel plate for the crude oil storage tank has a tensile strength ≥817MPa, a yield strength ≥798MPa, a yield-to-tensile ratio of 0.87~0.91, and an A ≥22% at room temperature; its Vickers hardness is ≤235HV10; at -60℃, its tensile strength ≥815MPa, its yield strength ≥800MPa, its yield-to-tensile ratio of 0.87~0.92, and an A ≥20%; and its average transverse impact energy KV2 is ≥240J; at -196℃, its average transverse impact energy KV2 is ≥200J.

[0033] In one specific embodiment, the steel plate for the crude oil storage tank has a tensile strength of 817~950MPa and a yield strength of 798~853MPa at room temperature; and a tensile strength of 815~956MPa and a yield strength of 800~864MPa at -60℃.

[0034] In one specific embodiment, the metallographic structure of the container steel plate for crude oil storage tank is troostite and bainite. The bainite includes spherical bainite with a size of 32~46nm and a grain size grade of 8~9. The sum of the grades of inclusions of type A, type B, type C and type D is ≤1.0. The volume percentage of troostite and bainite is (5~8):(2~5).

[0035] In one specific embodiment, the matrix of the container steel plate for crude oil storage tanks contains dispersed Cr (C / N) and Nb (C / N) particles with a size of 22~35nm.

[0036] In one specific embodiment, according to the hydrogen-induced cracking test in GB / T 8650-2006 and NACE-TM0284 "Evaluation Method for Hydrogen-Induced Cracking Resistance of Pipeline Steel and Pressure Vessel Steel", after 96 hours of testing, the crack sensitivity CSR (%), crack length ratio CLR (%), and crack width ratio CTR (%) of the steel plate were all 0; according to GB / T 17897-2016 "Corrosion of Metals and Alloys - Test Method for Ferric Chloride Pitting Corrosion of Stainless Steel", the corrosion rate of the steel plate was ≤0.0046 g / m. 2 •h; According to GB / T 3960-2016 "Test Method for Sliding Friction and Wear of Plastics", the volumetric wear of the steel plate is ≤0.0035cm. 3 .

[0037] The present invention also discloses a method for preparing a container steel plate for crude oil storage tanks with excellent yield strength ratio and high strength and toughness as described above, comprising the following steps: smelting, continuous casting, three-stage high-efficiency slab heating, four-stage rolling, two-stage controlled cooling process and heat treatment.

[0038] (1) In the smelting process, molten iron and scrap steel are added to the converter for smelting treatment; the size of the charge is controlled to be 76~98mm, and the amount of molten iron added accounts for ≥76% of the total charge mass to ensure the purity of the steel, shorten the process time, and reduce the difficulty of subsequent processes; the parameters for dephosphorization and decarburization in the converter are strictly controlled, and the decarburization oxygen blowing time is controlled to be 311~422s; in order to effectively reduce the content of harmful element P, the dephosphorization oxygen blowing is controlled to be 492~566s, and the P in the molten steel is controlled to be ≤0.015%; deep desulfurization is carried out using the LF refining furnace, and the desulfurization oxygen blowing is controlled to be 532~682s, and the S is controlled to be ≤0.005%; degassing is completed in the RH furnace, the starting temperature is controlled to be 1639~1673℃, and the oxygen blowing amount is 15~29m³. 3 The net circulation time is 511~879s, and the pre-casting settling time is 294~552s. By optimizing the smelting process parameters, the oxidation of molten steel is reduced, the content of inclusions in the steel is controlled, internal defects are reduced, and the purity of the steel is improved.

[0039] In one specific embodiment, a spheroidizing inoculant with a magnesium content of 4% to 5% by weight is added during the smelting process to enhance the fluidity of the molten iron, ensure the uniformity of the original microstructure composition, refine the grain size of the as-cast original microstructure, and improve smelting production efficiency.

[0040] (2) In continuous casting, after vacuum breaking, a slab continuous casting machine is used for casting treatment. The key is to control the casting temperature. The casting temperature of molten steel in the tundish is 1562~1589℃, the superheat is set to 8~12℃, and the billet pulling speed during casting is 16.4~22.8mm / s. Through high-temperature continuous casting, the excellent original as-cast structure of the steel plate is guaranteed. In order to optimize the internal quality of the continuously cast billet and reduce defects such as segregation and voids, the continuous casting billet light reduction process and electromagnetic stirring process are adopted. The billet is put into the stack for slow cooling after leaving the line, and the stacking slow cooling time is 24~36h.

[0041] In one specific embodiment, the light reduction process controls the reduction rate to be 2% to 4%.

[0042] In one specific embodiment, the electromagnetic stirring process controls the magnetic field frequency to be 4~12Hz and the current intensity to be 344~421A.

[0043] (3) In the three-stage high-efficiency slab heating, the continuous casting slab is heated in three stages before being taken out of the furnace; the temperature range of the first stage continuous heating section is 942~965℃, and the heating rate is controlled at 4.6~5.4℃ / min to ensure uniform heating of the slab and reduce the internal stress of the steel plate; the temperature range of the second stage high-temperature homogenization section is 1185~1220℃, the heating rate is 4.1~4.6℃ / min, and the holding time is 18~36min; the temperature range of the third stage low-temperature homogenization section is 1041~1062℃; the total net holding time of the continuous casting slab is controlled at 96~145min; through the three-stage heating method, the uniformity of the internal structure of the steel slab is further improved, the original size of the precipitated phase particles is controlled, the internal stress of the steel plate is fully released, and the uniform temperature inside and outside the steel slab is ensured to facilitate reprocessing.

[0044] (4) In the four-stage rolling and two-stage controlled cooling process, the first stage high-temperature section is used for original microstructure control rolling, with an initial rolling temperature of 1142~1161℃ and a rolling end temperature of 1034~1059℃; the original austenite microstructure is fully refined, and rolling is carried out by a combination of large reduction and small reduction, in the following order: large reduction rolling, small reduction rolling, large reduction rolling and small reduction cyclic rolling control process, with a rolling speed of 3.4~3.9m / s and a large reduction range of 9%~12%. The initial reduction ranges from 4% to 7%, reducing the deformation resistance of the steel plate, ensuring sufficient recrystallization of grains, refining the internal structure of the steel plate, and improving the uniformity of the structure. The second stage involves high-temperature section microstructure optimization and controlled rolling, with an initial rolling temperature of 1008~1031℃ and a final rolling temperature of 982~996℃. Rapid, low-deformation rolling is performed in the recrystallization zone using a low reduction method, with a reduction controlled at 3%~5% and a rolling speed of 5.8~6.2 m / s. This further optimizes the microstructure and reduces the steel's deformation resistance. The third stage is controlled rolling for finished product microstructure, with an initial rolling temperature of 941~954℃ and a final rolling temperature of 814~843℃. The single-pass reduction is achieved by combining small and large reductions, using a reciprocating rolling process with small and large reductions in sequence. The large reduction ranges from 7% to 10%, and the small reduction ranges from 3% to 5%, increasing grain boundary area and ferrite nucleation rate, fully refining the internal microstructure of the steel plate, and further compressing austenite grains. The rolling process begins with flattening and elongation; the fourth stage is the steel plate service performance optimization rolling stage, with an initial rolling temperature of 786~822℃ and a reduction of 2%~4%. This optimizes the microstructure and increases the proportion of small-angle grain boundaries in the steel plate, ensuring good corrosion resistance and wear resistance. After rolling, the steel plate undergoes a two-stage controlled cooling process. The first stage initial cooling temperature is controlled at 738~769℃, with a cooling rate of 33~38℃ / s; the second stage initial cooling temperature is controlled at 592~628℃, with a cooling rate of 12~16℃ / s. This releases internal stress in the steel plate and optimizes the plate shape control.

[0045] (5) During heat treatment, due to the addition of elements such as C, Si, Mn, N, Ni, Cr, and Nb to the steel, the steel plate can obtain a troostite + spheroidal bainite structure with excellent strength and toughness after rolling. However, the grain size distribution of the steel plate is uneven, and there is a concentration of structural stress and thermal stress, which can easily cause delayed cracks during flame cutting. Therefore, heat treatment should be used in time to soften and relieve stress. In order to further control the internal structure of the steel plate and at the same time ensure high production efficiency, tempering heat treatment is adopted to ensure that the strength of the steel plate is not lost, while giving the steel plate suitable plasticity and toughness, low-temperature impact toughness, corrosion resistance and good processing performance. The tempering heat treatment temperature is 562~589℃, the heating rate is 3.9~4.4℃ / min, the holding time is 42~56min, and the heat treatment is followed by air cooling to room temperature.

[0046] The following are specific embodiments. The method for preparing the high-strength and high-toughness crude oil storage tank steel plate of this embodiment includes the following steps: smelting, continuous casting, three-stage high-efficiency slab heating, four-stage rolling, two-stage controlled cooling process and heat treatment.

[0047] (1) In the smelting process, molten iron and scrap steel are added to the converter for smelting treatment; the size of the charge is controlled; the parameters for dephosphorization and decarburization in the converter are strictly controlled; and the deep desulfurization treatment is carried out using the LF refining furnace; degassing is completed in the RH furnace.

[0048] (2) In continuous casting, after vacuum breaking, a slab continuous casting machine is used for casting treatment. The key is to control the casting temperature. Through high-temperature continuous pouring, the excellent original as-cast structure of the steel plate is guaranteed. In order to optimize the internal quality of the continuous casting billet and reduce defects such as segregation and voids, a continuous casting billet light reduction process and an electromagnetic stirring process are adopted; the billet is put into the stack for slow cooling after it leaves the line.

[0049] (3) In the three-stage high-efficiency slab heating, the continuous casting slab is heated in three stages and then taken out of the furnace; the temperature and heating rate of the first stage continuous heating section are controlled; the temperature, heating rate and holding time of the second stage high-temperature homogenization section are controlled; and the temperature range of the third stage low-temperature homogenization section and the total net holding time of the continuous casting slab are controlled.

[0050] (4) In the four-stage rolling and two-stage controlled cooling process, the first stage is the original microstructure control rolling in the high-temperature section, which adopts a combination of large reduction and small reduction for rolling, and the control process is large reduction rolling, small reduction rolling, large reduction rolling and small reduction cyclic rolling in sequence; the second stage is the microstructure optimization control rolling in the high-temperature section, and the recrystallization zone is rolled with rapid small deformation; the third stage is the finished product microstructure control rolling, and the single-pass reduction adopts a combination of small reduction and large reduction for rolling, and the control process is small reduction rolling and large reduction reciprocating cyclic rolling in sequence; the fourth stage is the steel plate service performance optimization rolling stage, and the rolled steel plate adopts a two-stage controlled cooling method.

[0051] (5) In heat treatment, tempering heat treatment is adopted.

[0052] Table 1 shows the chemical composition of the embodiments of the present invention; Table 2 shows the steel smelting-continuous casting and heating process parameters of the embodiments; Table 3 shows the steel slab rolling process parameters of the embodiments; Table 4 shows the steel cooling and heat treatment process parameters of the embodiments; Table 5 shows the mechanical properties of the steel of the embodiments; Table 6 shows the test results of microstructure grain size and second phase particles; Table 7 shows the test results of service performance of the embodiments - corrosion resistance (hydrogen-induced cracking test, pitting test) test, and friction and wear test results.

[0053] Table 1 Chemical composition of the examples (wt, %)

[0054] Table 2 Smelting-continuous casting and heating process parameters for the examples

[0055] Table 3 shows the steel rolling process in the examples.

[0056] Table 4. Steel Cooling and Heat Treatment Processes in Examples

[0057] Table 5 Final Mechanical Properties of Examples

[0058] Table 6 shows the results of the microstructure grain size and second-phase particle evaluation tests.

[0059] Table 7 shows the service performance test results of the examples.

[0060] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A type of steel plate for crude oil storage tanks with excellent yield strength ratio and high strength and toughness, characterized in that, Includes the following components by weight percentage: C: 0.08%~0.10%, Si: 0.19%~0.21%, Mn: 1.12%~1.28%, P≤0.015%, S≤0.005%, N: 0.007%~0.01%, Ni: 0.22%~0.29%, Cr: 0.21%~0.28%, Nb: 0.017~0.034%, with the balance being Fe and unavoidable impurities.

2. The high strength and toughness crude oil storage tank steel plate with excellent yield strength ratio as described in claim 1, characterized in that, The weight percentage ratio of Ni and N in the steel plate of the crude oil storage tank conforms to the following: 2.7≤Ni / 9N≤4.2; The weight percentage ratio of C, Nb, and Cr in the steel plate of the crude oil storage tank conforms to the following: C≤Nb / Cr; The crack sensitivity index (PCM) of the steel plate used for the crude oil storage tank is ≤0.2%; the carbon equivalent (CE) is ≤0.40%. Wherein, Pcm (%) = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B; CE (%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

3. The high strength and toughness crude oil storage tank steel plate with excellent yield strength ratio as described in claim 1, characterized in that, The thickness of the steel plate is 19~45mm; The steel plates used for crude oil storage tanks have the following properties at room temperature: tensile strength ≥817MPa, yield strength ≥798MPa, yield ratio 0.87~0.91, A≥22%; Vickers hardness ≤235HV10; at -60℃, tensile strength ≥815MPa, yield strength ≥800MPa, yield ratio 0.87~0.92, A≥20%, average transverse impact energy KV2 ≥240J; at -196℃, average transverse impact energy KV2 ≥200J.

4. The high strength and toughness crude oil storage tank steel plate with excellent yield strength ratio as described in claim 1, characterized in that, The metallographic structure of the steel plate for the crude oil storage tank is troostite and bainite. The bainite includes spherical bainite with a size of 32~46nm and a grain size grade of 8~9. The sum of the grades of inclusions of type A, type B, type C and type D is ≤1.

0. The volume percentage of troostite and bainite is (5~8):(2~5). The matrix of the container steel plate for the crude oil storage tank contains dispersed Cr (C / N) and Nb (C / N) particles with a size of 22~35nm.

5. The high strength and toughness crude oil storage tank steel plate with excellent yield strength ratio as described in claim 1, characterized in that, According to the hydrogen-induced cracking test in GB / T 8650-2006 and NACE-TM0284 "Evaluation Method for Hydrogen-Induced Cracking Resistance of Pipeline Steel and Pressure Vessel Steel", after 96 hours of testing, the crack susceptibility (CSR) (%), crack length ratio (CLR) (%), and crack width ratio (CTR) of the steel plate were all 0. According to GB / T 17897-2016 "Corrosion of Metals and Alloys - Test Method for Ferric Chloride Pitting Corrosion of Stainless Steel", the corrosion rate of the steel plate was ≤0.0046 g / m³. 2 •h; According to GB / T 3960-2016 "Test Method for Sliding Friction and Wear of Plastics", the volumetric wear of the steel plate is ≤0.0035cm. 3 .

6. A method for preparing a crude oil storage tank container steel plate with excellent yield strength ratio and high strength and toughness as described in any one of claims 1-5, characterized in that, Includes the following steps: Smelting, continuous casting, three-stage high-efficiency slab heating, four-stage rolling, two-stage controlled cooling process and heat treatment; In the three-stage high-efficiency slab heating process, the continuously cast slab is heated in three stages before being removed from the furnace. The temperature range of the first stage continuous heating section is 942~965℃, and the heating rate is controlled at 4.6~5.4℃ / min. The temperature range of the second stage high-temperature soaking section is 1185~1220℃, the heating rate is 4.1~4.6℃ / min, and the holding time is 18~36min. The temperature range of the third stage low-temperature soaking section is 1041~1062℃. The total net holding time of the continuously cast slab is controlled at 96~145min. In the four-stage rolling and two-stage controlled cooling process, the first stage involves high-temperature section initial microstructure control rolling, with an initial rolling temperature of 1142~1161℃ and a final rolling temperature of 1034~1059℃. A combination of large and small reductions is used, sequentially employing a large reduction rolling, small reduction rolling, large reduction rolling, and small reduction cyclic rolling control process. The rolling speed is 3.4~3.9 m / s, with a large reduction range of 9%~12% and a small reduction range of 4%~7%. The second stage involves high-temperature section microstructure optimization control rolling, with an initial rolling temperature of 1008~1031℃ and a final rolling temperature of 982~996℃. Rapid small deformation rolling is performed in the recrystallization zone, using a small reduction rolling method, with the small reduction controlled at 3%~5%. The rolling speed is 5.8~6.2 m / s; the third stage is finished product microstructure controlled rolling, with an initial rolling temperature of 941~954℃ and a final rolling temperature of 814~843℃. The single-pass reduction adopts a combination of small and large reduction, with a cyclic rolling control process of small reduction rolling followed by large reduction rolling. The large reduction range is 7%~10%, and the small reduction range is 3%~5%; the fourth stage is the steel plate service performance optimization rolling stage, with an initial rolling temperature of 786~822℃ and a reduction of 2%~4%; the rolled steel plate adopts a two-stage controlled cooling method, with the first stage starting cooling temperature at 738~769℃ and a cooling rate of 33~38℃ / s; the second stage starting cooling temperature at 592~628℃ and a cooling rate of 12~16℃ / s; The heat treatment is carried out by tempering; the tempering temperature is 562~589℃, the heating rate is 3.9~4.4℃ / min, the holding time is 42~56min, and the heat treatment is followed by air cooling to room temperature.

7. The method for preparing the high-strength and high-toughness crude oil storage tank steel plate according to claim 6, characterized in that, In the smelting process, molten iron and scrap steel are added to a converter for smelting treatment; the charge size is controlled at 76~98mm, and the amount of molten iron added accounts for ≥76% of the total charge mass; the decarburization oxygen blowing time is controlled at 311~422s; the dephosphorization oxygen blowing time is controlled at 492~566s, and the P in the molten steel is controlled at ≤0.015%; deep desulfurization treatment is carried out in an LF refining furnace, with the desulfurization oxygen blowing time controlled at 532~682s, and the S ≤0.005%; degassing is completed in an RH furnace, with the initial temperature controlled at 1639~1673℃ and the oxygen blowing rate at 15~29m³. 3 The net circulation time is 511~879s, and the settling time before pouring is 294~552s.

8. The method for preparing the high-strength and high-toughness crude oil storage tank steel plate according to claim 6, characterized in that, In the continuous casting process, after vacuum breaking, casting treatment is carried out. The tundish steel pouring temperature is 1562~1589℃, the superheat is set to 8~12℃, and the billet pulling speed during pouring is 16.4~22.8mm / s. The continuous casting billet light reduction process and electromagnetic stirring process are adopted. The billet is put into the stack for slow cooling after leaving the line, and the stacking slow cooling time is 24~36h.

9. The method for preparing the high-strength and high-toughness crude oil storage tank steel plate according to claim 8, characterized in that, The light reduction process controls the reduction rate to be 2% to 4%.

10. The method for preparing the high-strength and high-toughness crude oil storage tank steel plate according to claim 8, characterized in that, The electromagnetic stirring process controls the magnetic field frequency to be 4~12Hz and the current intensity to be 344~421A.

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