A high-strength thick steel plate for low-temperature service storage tanks with a 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 thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature and a preparation method thereof. The chemical components in the steel are as follows: C: 0.22% - 0.26%, Si: 0.13% - 0.144%, Mn: 0.73% - 0.92%, P≤0.015%, S≤0.01%, Cr: 0.03% - 0.049%, V: 0.01% - 0.019%, Nd: 0.00016% - 0.00047%, and the balance is Fe and impurities. Microalloying is used to ensure the excellent internal structure and service performance of the steel plate; the purity of the molten steel is improved by optimizing the smelting process; the internal structure of the steel plate is optimized and the plate shape is improved by adopting a four-stage slab heating process and a three-stage controlled rolling process; the simulated post-weld heat treatment process is used to improve the strength-ductility matching and adjust the microstructure of the material, ensuring the low-temperature service performance of the steel plate in the full thickness direction.

Description

Technical Field

[0001] The present invention relates to the technical field of steel plate production, and particularly relates to a thick steel plate for high-strength 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 also gradually increased. Since the steel for low-temperature storage tanks needs to serve in complex environments such as low temperature, high pressure, and hydrogen-containing media for a long time, higher requirements are put forward for its performance. At the same time, with the large-scale development of low-temperature storage tanks, the requirement for the thickness of the steel plate is further increased, and it is required that the thick steel plate has 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 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 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 suitable for 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 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 15mm 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 publication number CN112080684A discloses "a thick plate for high-strength containers with excellent core toughness and a 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 unavoidable impurities. Its composition only targets steel plates with a thickness specification of 50 - 80mm and does not cover steel plates with a thickness greater than 80mm. And due to the high-content element Ni, the production cost of the steel plate is increased. In its manufacturing method, the total time in the furnace during slab heating ≥240min; the impact resistance only involves KV2 at 70°C, and the yield strength ≥420MPa, the tensile strength ≥560MPa, which does not belong to the high-strength grade steel plate. Therefore, this method is not applicable to the production of large-scale full-thickness low-temperature storage tank steel.

[0006] Compared with the above-mentioned publicly disclosed patents, the present invention is a thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature and its preparation method, which relates to a production method of storage tank steel plates with a plate thickness of 54 - 120mm and good comprehensive performance, has a low cost, and is suitable for large-scale production. Summary of the Invention

[0007] The present invention provides a thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature and a preparation method thereof. Through microalloying, the excellent internal structure and service performance of the steel plate are fundamentally guaranteed; by optimizing the smelting process, adopting an optimized hot metal treatment process 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 of the original structure is controlled; a four-stage slab heating process is adopted to improve production efficiency; through a three-stage controlled rolling process, the internal structure of the steel plate is further optimized and the plate shape is improved; through a reasonable cooling process, the service performance of the steel plate is further ensured; through a simulated post-weld heat treatment process, while improving the strength-toughness matching, the microscopic structure of the material is adjusted, the mechanical properties of the steel plate are improved, and the low-temperature service performance in the full thickness direction of the steel plate is ensured.

[0008] In order to achieve the above object, the present invention is realized by adopting the following technical solutions:

[0009] A thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature, the chemical composition of the steel by mass percentage is: C: 0.22% - 0.26%, Si: 0.13% - 0.144%, Mn: 0.73% - 0.92%, P ≤ 0.015%, S ≤ 0.01%, Cr: 0.03% - 0.049%, V: 0.01% - 0.019%, Nd: 0.00016% - 0.00047%, and the balance is Fe and inevitable impurities.

[0010] The thickness of the finished steel plate is 54 - 120 mm.

[0011] The metallographic structure of the finished steel plate is ferrite + sorbite + granular pearlite, and by volume ratio, ferrite: sorbite: granular pearlite = 3 - 5: 1 - 3: 1 - 2; the distance between the lamellar structures of sorbite is not greater than 100 nm, and the size of granular pearlite is 60 - 90 nm; the grain size of the finished steel plate is 7 - 9 grades.

[0012] The performance of the finished steel plate is as follows:

[0013] At room temperature, at 1 / 2 of the steel plate: 675 MPa ≤ tensile strength ≤ 720 MPa, 506 MPa ≤ yield strength ≤ 560 MPa, elongation ≥ 28%; at 1 / 4 of the steel plate: 660 MPa ≤ tensile strength ≤ 715 MPa, 500 MPa ≤ yield strength ≤ 540 MPa, elongation ≥ 28%;

[0014] At -80°C, the average value of the transverse impact energy KV2 ≥ 180 J; at the 1 / 2 position of the steel plate: 680 MPa ≤ tensile strength ≤ 735 MPa, 485 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 31%; at the 1 / 4 position of the steel plate: 670 MPa ≤ tensile strength ≤ 725 MPa, 480 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 32%; the ductile-brittle transition temperature of the steel plate ≤ -80°C, and the nil-ductility transition temperature ≤ -83°C;

[0015] At 400°C, the tensile strength ≥ 278 MPa.

[0016] A preparation method for a thick steel plate with low ductile-brittle transition temperature and high strength for storage tanks. The production process includes smelting, continuous casting, heating, rolling, cooling, and heat treatment; specifically as follows:

[0017] a. Smelting:

[0018] Smelting includes hot metal pretreatment, converter smelting, LF refining, and RH vacuum degassing processes; during converter smelting, an inoculant with a magnesium content of 3.9% - 4.8% by mass percentage is added, the decarburization oxygen blowing time is controlled within 220 - 385 s; the dephosphorization oxygen blowing time is controlled within 320 - 415 s, and the mass percentage of phosphorus in the molten steel is controlled within 0.01%; during LF refining, deep desulfurization treatment is carried out, the desulfurization oxygen blowing time is controlled within 405 - 465 s, and the mass percentage of sulfur in the molten steel is controlled below 0.015%; the starting temperature of RH vacuum degassing is 1632 - 1654°C, the oxygen blowing amount is controlled within 3.16 - 3.34 m³ / t steel·min, the net circulation time is controlled within 600 - 725 s, and the pre-casting calming time is 260 - 320 s;

[0019] b. Continuous casting:

[0020] The molten steel casting temperature is 1560 - 1576°C, the superheat is controlled within 8 - 11°C, and the casting withdrawal rate is 1.7 - 2.4 mm / s; the continuous casting slab soft reduction process is adopted, and the reduction rate is controlled within 3% - 5%;

[0021] c. Heating:

[0022] The slab is heated and then discharged after going through four stages. The temperature range of the preheating section is 980 - 1004°C, the temperature range of the low-temperature soaking section is 1034 - 1058°C, the temperature range of the high-temperature soaking section is 1127 - 1149°C, and the temperature range of the high-temperature homogenization section is 1230 - 1254°C; the slab heating rate is controlled within 20 - 28°C / min, and the total time in the furnace is 2.5 - 3.4 h;

[0023] d. Rolling:

[0024] The rolling process adopts the three-stage controlled rolling method; in the first stage, the original austenite structure is refined by rolling, the starting rolling temperature is 1148 - 1172 °C, the finishing rolling temperature is 1065 - 1082 °C, the "decreasing reduction ratio" rolling control process is adopted, and the total reduction ratio is controlled within the range of 42% - 56%; in the second stage, the dual-phase zone rolling is carried out, the starting rolling temperature is 974 - 985 °C, the finishing rolling temperature is 852 - 870 °C, the reciprocating rolling control process with "large - large - small - small" reduction ratio is adopted, where the large reduction ratio refers to a reduction ratio of 8% - 10%, and the small reduction ratio refers to a reduction ratio of 4% - 7%; in the third stage, the performance strengthening rolling is carried out, the starting rolling temperature is 784 - 815 °C, and the single-pass reduction ratio is 2% - 4%.

[0025] e. Cooling:

[0026] The cooling is divided into two stages; the first stage is the water cooling stage, the starting temperature is 756 - 774 °C, and the cooling rate is controlled at 37 - 59 °C / s; the second stage is the ultra-fast cooling stage, the starting temperature is 584 - 635 °C, and the cooling rate is controlled at 145 - 165 °C / s.

[0027] f. Heat treatment:

[0028] The simulated post-weld heat treatment process is adopted, the heating temperature is 645 - 675 °C, the heating rate is controlled at 0.9 - 1.2 min / mm, the holding time is 1.0 - 2.2 min / mm, and finally it is air-cooled to room temperature.

[0029] During the smelting process, scrap steel and hot metal are used as raw materials. The size of the charged scrap steel is controlled between 72 - 80 mm, and the mass percentage of hot metal is controlled above 70%.

[0030] After the continuous casting process is completed, the cast billet is taken offline for stacking and slow cooling. The cooling rate is 12 - 16 °C / h, and the stacking slow cooling time is 36 - 48 h.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] (1)On the basis of strengthening elements such as C, Si, and Mn, appropriate amounts of alloying elements such as Cr, V, and Nd are added, while strictly controlling the contents of harmful elements P and S. Combining with the optimized production process, a uniformly refined "ferrite + sorbite + granular pearlite" structure is obtained. And by volume ratio, ferrite: sorbite: granular pearlite = (3 - 5):(1 - 3):(1 - 2), the grain size is 7 - 9 grades, the distance between the lamellar structures of sorbite is not more than 100 nm, and the size of granular pearlite is between 60 - 90 nm; the second-phase Cr / V carbide particles with a size ≤ 50 nm are uniformly and dispersedly distributed, ensuring the strength, plasticity, and low-temperature toughness of the steel plate. The spherical Nd(O / S / N) with a size not more than 10 nm strengthens the mechanical properties of the steel plate while ensuring that the steel plate has good corrosion resistance and wear resistance.

[0033] (2)The steel plate for storage tanks obtained through a unique production process shows the following mechanical properties:

[0034] At room temperature, at the 1 / 2 position of the steel plate: 675 MPa ≤ tensile strength ≤ 720 MPa, 506 MPa ≤ yield strength ≤ 560 MPa, elongation ≥ 28%; at the 1 / 4 position of the steel plate: 660 MPa ≤ tensile strength ≤ 715 MPa, 500 MPa ≤ yield strength ≤ 540 MPa, elongation ≥ 28%;

[0035] At -80 °C, the average value of transverse impact energy KV2 ≥ 180 J; at the 1 / 2 position of the steel plate: 680 MPa ≤ tensile strength ≤ 735 MPa, 485 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 31%; at the 1 / 4 position of the steel plate: 670 MPa ≤ tensile strength ≤ 725 MPa, 480 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 32%; the ductile-brittle transition temperature of the steel plate ≤ -80 °C, and the nil-ductility transition temperature ≤ -83 °C;

[0036] At 400 °C, the tensile strength ≥ 278 MPa.

[0037] That is, the steel plate for storage tanks has good strength-ductility matching and high-temperature and low-temperature service performance.

[0038] (3)According to the "Evaluation Method for Resistance to Hydrogen-Induced Cracking of Pipeline Steels and Pressure Vessel Steels" in GB / T8650-2006 and NACE-TM0284, a hydrogen-induced cracking (HIC) experiment is carried out. After 96 h of testing the steel plate in Solution A and Solution B, the crack sensitivity CSR (%), crack length ratio CLR (%), and crack width ratio CTR (%) are all 0, indicating excellent hydrogen-induced crack resistance of the steel plate; according to the test method in GB / T17897-2016 "Corrosion of Metals and Alloys - Method of Pitting Corrosion Testing for Stainless Steels in Ferric Chloride Solution", when the steel plate is tested in Solution A and Solution B, the corrosion rate is not more than 0.0032 g / m 2·h; indicating that the steel plate has excellent corrosion resistance.

[0039] (4) Test was carried out according to GB / T3960-2016 "Test Method for Sliding Friction and Wear of Plastics". The result shows that the volume wear amount of the steel plate is not more than 0.00018 cm 3 , and the steel plate has good wear resistance. Specific embodiments

[0040] For the thick steel plate for high-strength storage tanks 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: 0.22% - 0.26%, Si: 0.13% - 0.144%, Mn: 0.73% - 0.92%, P ≤ 0.015%, S ≤ 0.01%, Cr: 0.03% - 0.049%, V: 0.01% - 0.019%, Nd: 0.00016% - 0.00047%, and the balance is Fe and inevitable impurities. The thickness of the finished steel plate is 54 - 120 mm.

[0041] The selection reasons and action mechanisms of the contents of chemical elements such as C, Si, Mn, P, S, Cr, V, and Nd in the steel plate are as follows:

[0042] 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 present invention sets the C content range as 0.22% - 0.26%.

[0043] Si is a common element in steel. Adding an appropriate amount of Si to the steel can make the steel have better wear resistance, elastic limit, yield strength, and yield ratio. 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, having a negative impact on the low-temperature toughness of the steel plate. Therefore, the present invention sets the Si content range as 0.13% - 0.144%.

[0044] Mn is often used as a deoxidizer and desulfurizer in steel. At the same time, Mn is dissolved in ferrite and austenite, which can expand the austenite region, increase the critical temperature, and make the quenched and tempered structure of the steel uniform and refined, playing a role of solid solution strengthening. While increasing the strength and hardness of the steel, it has little impact on plasticity. In addition, the price of Mn is relatively cheap. However, Mn is easy to combine with S and segregate at the grain boundaries, having an adverse effect on the hydrogen-induced cracking resistance of the steel plate. Therefore, the present invention sets the Mn content range as 0.73% - 0.92%.

[0045] 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 0.015% and S to 0.01%.

[0046] Cr is a strong carbide-forming element. Cr combines with carbon in steel to form fine chromium carbide particles, which remain stable even in high-temperature zones. These stable second-phase particles pin dislocations, further inhibiting dislocation movement, increasing grain boundary area, and reducing austenite grain size while preventing grain growth, ensuring a good balance of strength and toughness in the steel. Cr also has a high affinity for oxygen, and the oxides it forms with oxygen help improve the corrosion and oxidation resistance of the steel. Cr also increases the hardness of the steel, thereby ensuring good wear resistance. However, excessive Cr addition to steel can negatively impact its toughness, ductility, and resistance to hydrogen-induced cracking. Therefore, the present invention sets the Cr content range to 0.03% to 0.049%.

[0047] V is a strong carbonitriding element, forming stable carbides in steel. This increases the area of grain boundaries and subgrain boundaries for nucleation, significantly refining the microstructure and ensuring a balanced strength-toughness balance and toughness. VN microalloying in steel acts as a precipitation strengthening agent, promoting intragranular ferrite nucleation, effectively refining the lamellar structure, and reducing the lamellar spacing, which is key to the present invention's goal of maintaining the troostite lamellar spacing below 100 nm. Appropriate heat treatment processes allow vanadium carbides to disperse and precipitate throughout the steel, thereby enhancing its strength and toughness, weldability, and intergranular corrosion resistance. However, excessive V content can lead to aggregation and growth of V-containing carbides within the steel, increasing its brittleness. Therefore, the present invention sets the V content within a range of 0.01% to 0.019%, controlling the particle size of the secondary Cr / V carbide phase to ≤50 nm.

[0048] The Nd element has a purification effect of deoxidizing and desulfurizing, a modification effect of controlling inclusions, and a strong microalloying effect. When a certain amount of Nd element is added to steel, on the one hand, it can react with oxygen and sulfur in manganese sulfide, alumina, and aluminosilicate inclusions formed in the steel to form non-metallic compounds with relatively high melting points. These compounds have high melting points, are easy to float on the surface and be removed with the steel slag, thereby reducing the content of inclusion elements in the steel and realizing the purification of the molten steel. On the other hand, Nd(O / S / N) particles with small size and smooth edges have the effect of strengthening the grain boundary, can reduce the segregation of harmful elements at the grain boundary, and hinder the propagation of intergranular cracks, thus improving the plasticity and high-temperature and low-temperature properties of the steel plate. Therefore, in the present invention, the size of spherical Nd(O / S / N) is controlled not to be greater than 10 nm. However, when an excessive amount of Nd 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. In addition, considering the production and manufacturing cost comprehensively, the Nd content range in the present invention is set at 0.00016% - 0.00047%.

[0049] The production process flow of the high-strength thick steel plate for low-temperature toughness and brittle transition temperature storage tanks described in the present invention includes: hot metal pretreatment - secondary refining - vacuum degassing - slab continuous casting - stacking slow cooling - billet cleaning - four-stage heating of the slab - three-stage controlled rolling - two-stage cooling - simulated post-weld heat treatment, etc. The high-performance storage tank steel plate with a thickness specification of 54 - 120 mm produced has a uniform structure, good strength and toughness matching, a low ductile-brittle transition temperature, good high-temperature tensile properties, excellent corrosion resistance and wear resistance. Specifically as follows:

[0050] 1. Smelting process:

[0051] The molten steel smelting 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 72 and 80 mm, and the amount of hot metal is controlled above 79% (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 3.9% - 4.8% (mass percentage) is added to ensure the purity and uniformity of the original structure, refine the as-cast grains, and shorten the smelting time. The process parameters of converter dephosphorization and decarburization smelting are strictly controlled. The decarburization oxygen blowing time is controlled between 220 and 385 s; to effectively reduce the content of harmful element P, the dephosphorization oxygen blowing time is controlled between 320 and 415 s, and the phosphorus content in the molten steel is controlled below 0.01% (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 between 405 and 465 s, and the sulfur content is controlled below 0.015% (mass percentage) at the end of LF refining; degassing is completed in an RH furnace, the starting temperature is controlled between 1632 and 1654 °C, the oxygen blowing amount is controlled between 3.16 and 3.34 m³ / t steel·min, the net circulation time is 600 - 725 s, and the calming time before casting is 260 - 320 s.

[0052] By optimizing the smelting process parameters, the purposes 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 are achieved.

[0053] 2. Continuous casting process:

[0054] After breaking the vacuum, slab continuous casting machine is used for casting. High-temperature casting is adopted to make impurities float on the surface of the molten steel, thereby ensuring the internal quality of the original casting blank. The casting temperature is controlled between 1560 and 1576 °C, the superheat is controlled between 8 and 11 °C, and the drawing speed during casting is 1.7 - 2.4 mm / s.

[0055] The original as-cast structure is refined by controlling the casting temperature. In order to optimize the internal quality of the continuous casting billet and reduce defects such as segregation and cavities, the soft reduction process of the continuous casting billet is adopted, and the reduction rate is controlled between 3% and 5%. The casting billet is taken offline for stacking and slow cooling, the cooling rate is 12 - 16 °C / h, and the stacking slow cooling time is 36 - 48 h.

[0056] 3. Heating process:

[0057] The continuous casting slab is sent to the heating furnace for heating, and the slab is taken out of the furnace after four-stage heating. The temperature range of the preheating section is 980 - 1004 °C, the temperature range of the low-temperature soaking section is 1034 - 1058 °C, the temperature range of the high-temperature soaking section is 1127 - 1149 °C, and the temperature range of the high-temperature homogenization section is 1230 - 1254 °C. The heating rate of the slab is controlled between 20 and 28 °C / min, and the total time in the furnace is controlled between 2.5 and 3.4 h.

[0058] By means of four-stage heating, further improve the uniformity of the internal structure of the steel billet, control the original size of the precipitated phase particles, fully release the internal stress of the steel plate, and at the same time ensure the uniform temperature inside and outside the steel billet, which is beneficial to reprocessing.

[0059] 4. Rolling process:

[0060] The rolling adopts the three-stage controlled rolling method. In the first stage, the original austenite structure is refined by rolling. The starting rolling temperature is 1148 - 1172 °C, and the finishing rolling temperature is 1065 - 1082 °C. The rolling control process of "decreasing reduction ratio" is adopted, and the total reduction ratio is controlled within the range of 42% - 56%. By rolling in the high-temperature austenite region, the deformation resistance of the steel plate is reduced, the original austenite structure is fully refined, full recrystallization of grains is ensured, and while the internal structure of the steel plate is refined, the structural uniformity is improved.

[0061] In the second stage, two-phase region rolling is carried out. The starting rolling temperature is 974 - 985 °C, and the finishing rolling temperature is 852 - 870 °C. The reciprocating rolling control process of "large - large - small - small" reduction ratio is adopted, where the large reduction ratio is 8% - 10% and the small reduction ratio is 4% - 7%. 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.

[0062] In the third stage, performance strengthening rolling is carried out. The starting rolling temperature is 784 - 815 °C, and the reduction ratio per pass is controlled at 2% - 4%. Further improve the uniformity of the steel plate structure, introduce more small-angle grain boundaries on the steel plate surface, further ensure the service performance such as wear resistance and corrosion resistance of the steel plate, release the internal stress of the steel plate, optimize the steel plate shape, and the rolled steel plate is air-cooled after rolling.

[0063] 5. Cooling process:

[0064] The cooling is divided into two stages; the first stage is the water cooling stage, the starting temperature is controlled at 756 - 774 °C, and the cooling rate is controlled at 37 - 59 °C / s. By controlling the cooling rate and temperature, the grain growth can be prevented or the premature precipitation of carbides to form network carbides can be avoided, the dislocations caused by deformation can be fixed, and the supercooling degree of phase transformation can be increased, thereby optimizing the organizational structure of the steel. The second stage is the ultra-fast cooling stage, the starting temperature is controlled at 584 - 635 °C, and the cooling rate is controlled at 145 - 165 °C / s. By high-rate cooling, the austenite hardening state is maintained, nucleation is promoted, and the structure is refined; strain-induced precipitation is inhibited, more micro-alloying elements are retained, the precipitation strengthening effect is improved, and the mechanical properties of the steel plate are enhanced.

[0065] 6. Heat treatment process:

[0066] Due to the addition of elements such as C, Si, Mn, Cr, V, and Nd in the steel, the steel plate can obtain an excellent "ferrite + sorbite + granular pearlite" structure in terms of strength and toughness after rolling. However, the grain size distribution of the steel plate is uneven, there are concentration of structure stress and thermal stress, and delayed cracks are likely to occur during flame cutting. Therefore, heat treatment should be carried out in a timely manner 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 a simulated post-weld heat treatment method to ensure that the strength of the steel plate is not lost, and at the same time, the steel plate has appropriate plasticity and toughness, low-temperature impact toughness, corrosion resistance, and good processing performance. The heating temperature of the simulated post-weld heat treatment is 645 - 675 °C, the heating rate is controlled at 0.9 - 1.2 min / mm, the holding time is 1.0 - 2.2 min / mm, and finally it is air-cooled to room temperature.

[0067] The metallographic structure of the finished steel plate is ferrite + sorbite + granular pearlite, and by volume ratio, ferrite: sorbite: granular pearlite = 3 - 5: 1 - 3: 1 - 2; the distance between the lamellar structures of sorbite is not greater than 100 nm, and the size of granular pearlite is 60 - 90 nm; the grain size of the finished steel plate is 7 - 9 grades.

[0068] The properties of the finished steel plate are as follows:

[0069] At room temperature, at the 1 / 2 position of the steel plate: 675 MPa ≤ tensile strength ≤ 720 MPa, 506 MPa ≤ yield strength ≤ 560 MPa, elongation ≥ 28%; at the 1 / 4 position of the steel plate: 660 MPa ≤ tensile strength ≤ 715 MPa, 500 MPa ≤ yield strength ≤ 540 MPa, elongation ≥ 28%;

[0070] At -80 °C, the average value of the transverse impact energy KV2 ≥ 180 J; at the 1 / 2 position of the steel plate: 680 MPa ≤ tensile strength ≤ 735 MPa, 485 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 31%; at the 1 / 4 position of the steel plate: 670 MPa ≤ tensile strength ≤ 725 MPa, 480 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 32%; the ductile-brittle transition temperature of the steel plate ≤ -80 °C, and the nil-ductility transition temperature ≤ -83 °C;

[0071] At 400 °C, the tensile strength ≥ 278 MPa.

[0072] The purpose of the present invention is to obtain a steel plate with a thickness specification of 54 - 120 mm that is excellent in strength, low-temperature toughness, high-temperature tensile and service performance, and flatness through a production and manufacturing process of "molten iron optimization treatment process + four-stage high-efficiency slab heating process + three-stage controlled rolling process + simulated post-weld heat treatment process" with a new chemical composition design, meeting the manufacturing and application requirements of high-performance steel plates for storage tanks.

[0073] To more intuitively illustrate the present invention, the implementation manners of the present invention will be further described in conjunction with embodiments. The following embodiments are only preferred specific implementation manners 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.

[0074] Embodiment:

[0075] Table 1 shows the chemical compositions of the steel in each embodiment and comparative example. Table 2 shows the steelmaking, continuous casting, and heating process parameters in each embodiment and comparative example. Table 3 shows the slab rolling process parameters in each embodiment and comparative example. Table 4 shows the steel plate cooling and heat treatment process parameters in each embodiment and comparative example. Table 5 shows the mechanical properties of the finished steel plates in each embodiment and comparative example. Table 6 shows the high-temperature tensile mechanical property test results of the finished steel plates in each embodiment and comparative example. Table 7 shows the evaluation test results of the grain size of the microstructure and the second-phase particles of the finished steel plates in each embodiment and comparative example. Table 8 shows the service performance test results of the steel plates in each embodiment and comparative example, including corrosion resistance (hydrogen-induced cracking test, pitting corrosion test) test and friction and wear test results.

[0076] Table 1: Chemical Compositions of Steel (wt, %)

[0077]

[0078] Table 2: Steelmaking - Continuous Casting and Heating Process Parameters

[0079]

[0080] Table 3: Slab Rolling Process Parameters

[0081]

[0082] Table 4: Steel Plate Cooling and Heat Treatment Process Parameters

[0083]

[0084] Table 5: Mechanical Properties of Finished Steel Plates

[0085]

[0086] Table 6: Test Results of High-Temperature Tensile Properties (Yield Strength) of Finished Steel Plates

[0087]

[0088] Table 7: Evaluation Test Results of Grain Size of Microstructure and Second-Phase Particles of Finished Steel Plates

[0089]

[0090] Table 8: Experimental Results of Service Performance of Finished Steel Plates

[0091]

[0092] As described above, it is only a preferred specific implementation manner 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 preparation method of a high-strength thick steel plate for low-temperature ductile-brittle transition temperature storage tanks, characterized in that, The chemical components in the steel are by mass percentage: C: 0.22% - 0.26%, Si: 0.13% - 0.144%, Mn: 0.73% - 0.92%, P ≤ 0.015%, S ≤ 0.01%, Cr: 0.03% - 0.049%, V: 0.01% - 0.019%, Nd: 0.00016% - 0.00047%, and the balance is Fe and unavoidable impurities; the production process includes smelting, continuous casting, heating, rolling, cooling and heat treatment; specifically 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 3.9% - 4.8% by mass percentage is added, the decarburization oxygen blowing time is controlled within 220 - 385 s; the dephosphorization oxygen blowing time is controlled within 320 - 415 s, and the mass percentage of phosphorus in the molten steel is controlled within 0.01%; during LF refining, deep desulfurization treatment is carried out, the desulfurization oxygen blowing time is controlled within 405 - 465 s, and the mass percentage of sulfur in the molten steel is controlled below 0.015%; the starting temperature of RH vacuum degassing is 1632 - 1654 °C, the oxygen blowing amount is controlled within 3.16 - 3.34 m³ / t steel·min, the net circulation time is controlled within 600 - 725 s, and the calming time before casting is 260 - 320 s; 2) Continuous casting: The pouring temperature of the molten steel is 1560 - 1576 °C, the superheat is controlled within 8 - 11 °C, and the casting speed during pouring is 1.7 - 2.4 mm / s; the soft reduction process of continuous casting billets is adopted, and the reduction rate is controlled within 3% - 5%; 3) Heating: The slab is discharged from the furnace after four-stage heating. The temperature range of the preheating section is 980 - 1004 °C, the temperature range of the low-temperature soaking section is 1034 - 1058 °C, the temperature range of the high-temperature soaking section is 1127 - 1149 °C, and the temperature range of the high-temperature homogenization section is 1230 - 1254 °C; the heating rate of the slab is controlled within 20 - 28 °C / min, and the total time in the furnace is 2.5 - 3.4 h; 4) Rolling: Rolling adopts the three-stage controlled rolling method; in the first stage, the original austenite structure is refined by rolling, the starting rolling temperature is 1148 - 1172 °C, the finishing rolling temperature is 1065 - 1082 °C, the "decreasing reduction rate" rolling control process is adopted, and the total reduction rate is controlled within the range of 42% - 56%; in the second stage, two-phase zone rolling is carried out, the starting rolling temperature is 974 - 985 °C, the finishing rolling temperature is 852 - 870 °C, the reciprocating rolling control process of "large - large - small - small" reduction rate is adopted, where the large reduction rate refers to a reduction rate of 8% - 10%, and the small reduction rate refers to a reduction rate of 4% - 7%; in the third stage, property strengthening rolling is carried out, the starting rolling temperature is 784 - 815 °C, and the single-pass reduction rate is 2% - 4%; 5) Cooling: Cooling is divided into two stages; the first stage is the water cooling stage, the starting temperature is 756 - 774 °C, and the cooling rate is controlled within 37 - 59 °C / s; the second stage is the ultra-fast cooling stage, the starting temperature is 584 - 635 °C, and the cooling rate is controlled within 145 - 165 °C / s; 6) Heat treatment: The simulated post-weld heat treatment process is adopted, with the heating temperature being 645 - 675°C, the heating rate controlled at 0.9 - 1.2 min / mm, the holding time being 1.0 - 2.2 min / mm, and finally air-cooled to room temperature.

2. The preparation method of a thick steel plate for a high-strength storage tank with a low ductile-brittle transition temperature according to claim 1, characterized in that, The thickness of the finished steel plate is 54 - 120 mm.

3. The preparation method of a thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature according to claim 1, characterized in that, The metallographic structure of the finished steel plate is ferrite + sorbite + granular pearlite. And by volume ratio, ferrite:sorbite:granular pearlite = 3 - 5:1 - 3:1 - 2; the distance between the sorbite lamellar structures is not greater than 100 nm, and the size of the granular pearlite is 60 - 90 nm; the grain size of the finished steel plate is 7 - 9 grades.

4. The preparation method of a thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature according to claim 1, characterized in that, The properties of the finished steel plate are as follows: At room temperature, at 1 / 2 of the steel plate: 675 MPa ≤ tensile strength ≤ 720 MPa, 506 MPa ≤ yield strength ≤ 560 MPa, elongation ≥ 28%; at 1 / 4 of the steel plate: 660 MPa ≤ tensile strength ≤ 715 MPa, 500 MPa ≤ yield strength ≤ 540 MPa, elongation ≥ 28%. At -80°C, the average value of the transverse impact energy KV2 ≥ 180 J; at 1 / 2 of the steel plate: 680 MPa ≤ tensile strength ≤ 735 MPa, 485 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 31%; at 1 / 4 of the steel plate: 670 MPa ≤ tensile strength ≤ 725 MPa, 480 MPa ≤ yield strength ≤ 520 MPa, elongation ≥ 32%; the ductile-brittle transition temperature of the steel plate ≤ -80°C, and the nil-ductility transition temperature ≤ -83°C. At 400°C, the tensile strength ≥ 278 MPa.

5. The preparation method of a thick steel plate for a high-strength storage tank with a low ductile-brittle transition temperature according to claim 1, characterized in that, During the smelting process, scrap steel and hot metal are used as raw materials. The size of the charged scrap steel is controlled between 72 - 80 mm, and the mass percentage of hot metal is controlled above 70%.

6. The preparation method of a thick steel plate for a high-strength storage tank with a low ductile-brittle transition temperature according to claim 1, characterized in that, After the continuous casting process, the cast billet is taken off the line for stacking slow cooling. The cooling rate is 12 - 16°C / h, and the stacking slow cooling time is 36 - 48 h.

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