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

Abstract

The invention relates to the technical field of steel plate production, in particular to a low-ductile-brittle-transition-temperature high-strength thick steel plate for a storage tank and a preparation method of the thick steel plate. The steel comprises the following chemical components in percentage by weight: 0.22%-0.26% of C, 0.13%-0.144% of Si, 0.73%-0.92% of Mn, less than or equal to 0.015% of P, less than or equal to 0.01% of S, 0.03%-0.049% of Cr, 0.01%-0.019% of V, 0.00016%-0.00047% of Nd and the balance of Fe and impurities. The excellent performance of the internal structure and the service performance of the steel plate is guaranteed through microalloying; the purity of the molten steel is improved by optimizing a smelting process; a four-stage plate blank heating process and a three-stage controlled rolling process are adopted to optimize the internal structure of the steel plate and improve the plate shape; the microstructure of the material is adjusted while the obdurability matching is improved by simulating the postweld heat treatment process, and the low-temperature service performance of the steel plate in the full-thickness direction is guaranteed.

Description

Technical Field

[0001] 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. 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 production 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] 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 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 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, this 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 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 steel for full-thickness low-temperature storage tanks.

[0005] The Chinese patent application with the application publication number CN112080684A discloses "A thick plate for high-strength containers with excellent core toughness and 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 cover steel plates with a thickness greater than 80 mm. And due to the high-content element Ni, the production cost of the steel plate increases. In its manufacturing method, the total time in the furnace during slab heating ≥ 240 min; the impact resistance only involves KV at 70°C 2 , and the yield strength ≥ 420 MPa, the tensile strength ≥ 560 MPa, which does not belong to the high-strength grade steel plate. Therefore, this method is not suitable for the production of steel for large-scale full-thickness low-temperature storage tanks.

[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 steel plates for storage tanks with a plate thickness of 54 - 120 mm and good comprehensive performance, with low cost and 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 whole-process protected casting, the purity of molten steel and the purity of continuous casting billets are improved, the influence of elements such as P, S, and O is reduced, and the grain size of the original structure is controlled; a 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 guaranteed; by simulating the post-weld heat treatment process, while improving the strength-ductility matching, the microscopic structure of the material is adjusted, the mechanical properties of the steel plate are improved, and the low-temperature service performance of the steel plate in the full thickness direction is ensured.

[0008] To achieve the above object, the present invention is implemented by the following technical solutions: 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 unavoidable impurities.

[0009] The thickness of the finished steel plate is 54 - 120 mm.

[0010] 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.

[0011] The performance of the finished steel plate is as follows: 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%; At -80 °C, the transverse impact energy KV 2 The average value ≥ 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; At 400 °C, the tensile strength ≥ 278 MPa.

[0012] A preparation method of a thick steel plate for high-strength storage tanks with a low ductile-brittle transition temperature. The production process includes smelting, continuous casting, heating, rolling, cooling and heat treatment; specifically as follows: a. 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 pre-casting calming time is 260 - 320 s; b. Continuous casting: The molten steel casting temperature is 1560 - 1576 °C, the superheat is controlled within 8 - 11 °C, and the casting speed 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%; c. Heating: The slab is discharged from the furnace after four-stage heating. The preheating section temperature range is 980 - 1004 °C, the low-temperature soaking section temperature range is 1034 - 1058 °C, the high-temperature soaking section temperature range is 1127 - 1149 °C, and the high-temperature homogenization section temperature range is 1230 - 1254 °C; the slab heating rate is controlled within 20 - 28 °C / min, and the total in-furnace time is 2.5 - 3.4 h; d. Rolling: Rolling adopts a 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, and the "rolling reduction rate decreasing" rolling control process is adopted, and the total rolling 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, and the reciprocating rolling control process of "large - large - small - small" rolling reduction rate is adopted, where the large rolling reduction rate refers to a rolling reduction rate of 8% - 10%, and the small rolling reduction rate refers to a rolling reduction rate of 4% - 7%; in the third stage, performance strengthening rolling is carried out, the starting rolling temperature is 784 - 815 °C, and the single-pass rolling reduction rate is 2% - 4%; e. Cooling: The cooling is divided into two stages; the first stage is the water cooling stage, with the starting temperature being 756 - 774 °C and the cooling rate controlled at 37 - 59 °C / s; the second stage is the ultra-fast cooling stage, with the starting temperature being 584 - 635 °C and the cooling rate controlled at 145 - 165 °C / s; f. Heat treatment: Adopt the simulated post-weld heat treatment process, 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.

[0013] 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 content of hot metal is controlled above 70%.

[0014] After the continuous casting process ends, the billet is taken offline for stacking slow cooling, with the cooling rate being 12 - 16 °C / h and the stacking slow cooling time being 36 - 48 h.

[0015] Compared with the prior art, the beneficial effects of the present invention are: (1) On the basis of strengthening elements such as C, Si, and Mn, appropriate alloying elements such as Cr, V, and Nd are added, and at the same time, the contents of harmful elements P and S are strictly controlled. Combining with the optimized production process, a uniformly refined "ferrite + 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 sorbite lamellar structures is not greater than 100 nm, and the size of the granular pearlite is between 60 - 90 nm; the second-phase Cr / V carbide particles with a size ≤ 50 nm are uniformly dispersed, ensuring the strength, plasticity, and low-temperature toughness of the steel plate. The spherical Nd(O / S / N) with a size not greater than 10 nm strengthens the mechanical properties of the steel plate while ensuring that the steel plate has good corrosion resistance and wear resistance.

[0016] (2) The mechanical properties of the steel plate for storage tanks obtained through the unique production process are as follows: 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%; At -80 °C, the transverse impact energy KV 2The mean value ≥ 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, nil-ductility transition temperature ≤ -83 °C; At 400 °C, the tensile strength ≥ 278 MPa.

[0017] That is, the steel plate for storage tanks has good strength-ductility matching and service performance at high and low temperatures.

[0018] (3)According to the "Evaluation Method for Resistance to Hydrogen-Induced Cracking of Pipeline Steels and Pressure Vessel Steels" of GB / T8650-2006 and NACE-TM0284, the hydrogen-induced cracking (HIC) test was carried out. After 96 h 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 - Stainless Steels - Ferric Chloride Pitting Corrosion Test Method", the corrosion rates of the steel plate in Solution A and Solution B were both not greater than 0.0032 g / m 2 ·h; indicating that the steel plate has excellent corrosion resistance.

[0019] (4)According to the test of GB / T3960-2016 "Plastics - Methods of Test for Sliding Friction and Wear Properties", the test results showed that the volume wear of the steel plate was not greater than 0.00018 cm 3 , indicating that the steel plate has good wear resistance. Specific embodiments

[0020] 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 unavoidable impurities. The thickness of the finished steel plate is 54 - 120 mm.

[0021] 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: C is a basic element of steel and plays an important role in both the strength and toughness of steel plates. When the C content in the steel plate is relatively low, it is difficult to ensure the strength, hardness, and hardenability of the steel plate. The wear resistance of the steel plate decreases, directly affecting the service performance of the material. However, an excessive 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 this invention is set to 0.22% - 0.26%.

[0022] Si is a common element in steel. Adding an appropriate amount of Si to steel can endow the steel with 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 Si content range in this invention is set to 0.13% - 0.144%.

[0023] 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 in solid solution strengthening. While increasing the strength and hardness of the steel, it has less impact on plasticity. In addition, the price of Mn is relatively cheap. However, Mn is prone to combine with S and segregate at grain boundaries, having an adverse effect on the hydrogen-induced cracking resistance of the steel plate. Therefore, the Mn content range in this invention is set to 0.73% - 0.92%.

[0024] S and P are harmful elements in steel. To ensure the purity, plasticity, and toughness of the steel, their contents must be strictly controlled. Therefore, in this invention, P ≤ 0.015% and S ≤ 0.01% are specified.

[0025] Cr is a strong carbide-forming element. The fine chromium carbide particles formed by Cr combining with C in steel can still stably exist in the high-temperature zone. The stable second-phase particles pin dislocations, further preventing dislocation movement, increasing the grain boundary area, reducing the austenite grain size, and at the same time preventing grain growth, ensuring good strength-toughness matching of the steel plate; on the other hand, Cr also has a high affinity with O, and the oxides formed with O are beneficial to improving the corrosion resistance and oxidation resistance of the steel plate; the Cr element can increase the hardness of the steel plate, thus ensuring that the steel plate has good wear resistance. However, when an excessive amount of Cr element is added to the steel, it will have a negative impact on the ductility and plasticity and hydrogen-induced cracking resistance of the steel plate. Therefore, the Cr content range in this invention is set at 0.03% - 0.049%.

[0026] V is a strong carbon / nitriding element, which forms stable carbides in steel, increases the area of ​​grain boundaries and subgrain boundaries for nucleation, and strongly refines the grain structure to ensure the strength and toughness matching and toughness of the steel plate. VN microalloying plays a precipitation strengthening role in steel. At the same time, by promoting the nucleation of intracrystalline ferrite, it effectively refines the lamellar structure and reduces the distance of the lamellar structure, which is the key to controlling the distance of the troostite lamellar layer to be no more than 100nm in the present invention. With appropriate heat treatment process, vanadium carbides are dispersed, precipitated and distributed in steel, thereby improving the strength and toughness of steel, and at the same time, the welding performance and intergranular corrosion resistance of steel can be improved. However, when the V content is too high, the V-containing carbides in the steel plate will aggregate and grow, which will increase the brittleness of the steel plate. Therefore, the present invention sets the V content range to 0.01% to 0.019%, and controls the particle size of the second phase Cr / V carbide to ≤50nm.

[0027] Nd element has the purification effect of deoxidation and desulfurization, the 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, aluminum oxide and aluminosilicate inclusions generated in the steel to generate non-metallic compounds with a higher melting point. These compounds have a higher melting point and are easy to float on the surface and be removed with the slag, thereby reducing the content of inclusion elements in the steel and achieving the purification of the molten steel. On the other hand, the Nd (O / S / N) particles with small size and smooth edges have the effect of strengthening the grain boundary, and can reduce the segregation of harmful elements at the grain boundary, hinder the expansion of intergranular cracks, and thus improve the plasticity and high temperature and low temperature properties of the steel plate. Therefore, the present invention controls the size of spherical Nd (O / S / N) to be no more than 10nm. However, when excessive Nd is added to the steel, pointed non-spherical large-sized inclusions will be generated, which will become the source of fracture cracks and the starting point of corrosion, and have an adverse effect on the hydrogen-induced crack resistance and corrosion resistance of the steel plate. In addition, taking the production cost into comprehensive consideration, the present invention sets the Nd content range to 0.00016% to 0.00047%.

[0028] The production process of a high-strength thick steel plate for storage tanks with low toughness-brittleness transition temperature described in the present invention includes: molten iron pretreatment - refining outside the furnace - vacuum degassing - slab continuous casting - stacking slow cooling - casting cleaning - slab four-stage heating - three-stage controlled rolling - two-stage cooling - simulated post-weld heat treatment, etc. The high-performance steel plate for storage tanks with a thickness of 54 to 120 mm produced has uniform structure, good strength-toughness matching, low toughness-brittleness transition temperature, good high-temperature tensile properties, and excellent corrosion resistance and wear resistance. The details are as follows: 1. Smelting process: 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 hot metal amount 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 pouring is 260 - 320 s.

[0029] 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.

[0030] 2. Continuous casting process: 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 rate during pouring is 1.7 - 2.4 mm / s.

[0031] 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 off the production line for stacking and slow cooling, the cooling rate is 12 - 16 °C / h, and the stacking slow cooling time is 36 - 48 h.

[0032] 3. Heating process: The continuous casting slab is sent into 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.

[0033] 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 uniform temperature inside and outside the steel billet, which is beneficial to reprocessing.

[0034] 4. Rolling process: 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 structure uniformity is improved.

[0035] 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.

[0036] In the third stage, performance strengthening rolling is carried out. The starting rolling temperature is 784 - 815 °C, and the single-pass reduction ratio 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.

[0037] 5. Cooling process: 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 or premature precipitation of carbides to form network carbides can be prevented, 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 microalloying elements are retained, the precipitation strengthening effect is improved, and the mechanical properties of the steel plate are enhanced.

[0038] 6. Heat treatment process: 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.

[0039] 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.

[0040] The properties of the finished steel plate are as follows: 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%; At -80 °C, the transverse impact energy KV 2 The average value ≥ 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; At 400 °C, the tensile strength ≥ 278 MPa.

[0041] The purpose of the present invention is to obtain steel plates with thickness specifications of 54 - 120 mm that are excellent in strength, low-temperature toughness, high-temperature tensile and service performance, and flatness through a brand-new chemical composition design and 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", meeting the manufacturing and application requirements of high-performance steel plates for storage tanks.

[0042] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with examples. The following examples are only 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.

[0043] Example:

[0044] 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 process parameters in each example and comparative example. Table 4 shows the cooling and heat treatment process parameters of the steel plate in each example and comparative example. Table 5 shows the mechanical properties of the finished steel plate in each example and comparative example. Table 6 shows the high-temperature tensile mechanical property test results of the finished steel plate in each example 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 plate in each example and comparative example. Table 8 shows the service performance test results of the steel plate in each example and comparative example, including corrosion resistance (hydrogen-induced cracking test, pitting corrosion test) test and friction and wear test results.

[0045] Table 1: Chemical Compositions of Steel (wt, %)

[0046] Table 2: Smelting - Continuous Casting and Heating Process Parameters of Steel

[0047] Table 3: Slab Rolling Process Parameters

[0048] Table 4: Cooling and Heat Treatment Process Parameters of Steel Plate

[0049] Table 5: Mechanical Properties of Finished Steel Plate

[0050] Table 6: Test Results of High-Temperature Tensile Properties (Yield Strength) of Finished Steel Plate

[0051] Table 7: Evaluation Test Results of Grain Size of Microstructure and Second-Phase Particles of Finished Steel Plate

[0052] Table 8: Service Performance Test Results of Finished Steel Plate

[0053] The above are only the preferred specific embodiments 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 should be covered within the protection scope of the present invention.

Claims

1. A thick steel plate for storage tanks with low toughness-brittle transition temperature and high strength, characterized in that: 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 unavoidable impurities.

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

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

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

5. A method for preparing a thick steel plate with low toughness-brittle transition temperature and high strength for a storage tank as claimed in any one of claims 1 to 4, characterized in that: The production process includes smelting, continuous casting, heating, rolling, cooling and heat treatment; the details are as follows: a. Smelting: The smelting process includes molten iron pretreatment, converter smelting, LF refining and RH vacuum degassing. During the converter smelting process, an inoculant containing 3.9% to 4.8% magnesium by mass is added, and the decarburization oxygen blowing time is controlled at 220 to 385 seconds. The dephosphorization oxygen blowing time is controlled at 320 to 415 seconds, 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 at 405 to 465 seconds, and the mass percentage of sulfur in the molten steel is controlled below 0.015%. The starting temperature of RH vacuum degassing is 1632 to 1654°C, the oxygen blowing amount is controlled at 3.16 to 3.34 m³ / t steel·min, the net cycle time is controlled at 600 to 725 seconds, and the calming time before pouring is 260 to 320 seconds. b. Continuous casting: The casting temperature of molten steel is 1560-1576℃, the superheat is controlled at 8-11℃, and the casting rate is 1.7-2.4mm / s. The continuous casting billet light reduction process is adopted, and the reduction rate is controlled at 3%-5%. c. Heating: 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 slab heating rate is controlled at 20-28°C / min, and the total time in the furnace is 2.5-3.4h. d. Rolling: The rolling adopts a three-stage controlled rolling method; the first stage is the original austenite structure refinement rolling, the start rolling temperature is 1148-1172℃, the final rolling temperature is 1065-1082℃, the "reduction rate decreasing" rolling control process is adopted, and the total reduction rate is controlled within the range of 42%-56%; the second stage is the dual-phase region rolling, the start rolling temperature is 974-985℃, the final rolling temperature is 852-870℃, and the reduction rate is "large-large-small-small" reciprocating rolling control process is adopted, where the large reduction rate refers to the reduction rate of 8%-10%, and the small reduction rate refers to the reduction rate of 4%-7%; the third stage is the strengthening performance rolling, the start rolling temperature is 784-815℃, and the single-pass reduction rate is 2%-4%; e. Cooling: The cooling process is divided into two stages. The first stage is water cooling, with a starting temperature of 756-774°C and a cooling rate of 37-59°C / s. The second stage is ultra-fast cooling, with a starting temperature of 584-635°C and a cooling rate of 145-165°C / s. f. Heat treatment: A simulated post-weld heat treatment process was adopted, with a heating temperature of 645-675°C, a heating rate controlled at 0.9-1.2 min / mm, a holding time of 1.0-2.2 min / mm, and finally air cooling to room temperature.

6. The method for preparing a thick steel plate with low toughness-brittle transition temperature and high strength for storage tanks according to claim 5, characterized in that: During the smelting process, scrap steel and molten iron are used as raw materials. The size of the scrap steel charged into the furnace is controlled between 72 and 80 mm, and the mass percentage of the molten iron is controlled at more than 70%.

7. The method for preparing a thick steel plate with low toughness-brittle transition temperature and high strength for storage tanks according to claim 5, characterized in that: After the continuous casting process is completed, the ingots are stacked and slowly cooled down at a cooling rate of 12 to 16 °C / h, and the stacking slow cooling time is 36 to 48 hours.

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