LPG marine low yield ratio high strength low temperature steel plate and manufacturing method

Abstract

The application provides a kind of LPG ship low yield ratio high strength low temperature steel plate and manufacturing method, the component of the steel plate is as follows according to percentage by weight: C:0.060%~0.140%, Si:0.25%~0.60%, Mn:1.20%~1.60%, P≤0.007%, S≤0.004%, V:0.050%~0.150%, Nb:0.050%~0.100%, N:0.0150%~0.0190%, Cu:0.45%~0.80%, Ni:0.65%~0.95%, Ti:0.010%~0.030%, Als:0.025%~0.045%, Zr:0.080%~0.150%, the balance is Fe and inevitable impurities.Manufacturing method, including smelting, continuous casting, reheating, slab cooling, rolling, on-line accelerated cooling, stack slow cooling;The steel plate produced by the application has yield strength ≥540MPa, tensile strength ≥685MPa, elongation after fracture ≥24.0%, yield ratio ≤0.83, impact energy at-80℃ ≥260J, CTOD at-40℃ ≥1.30mm or more, and yield ratio of steel plate at-40℃ ≤0.80.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a low yield strength ratio high strength low temperature steel plate for LPG marine applications and its manufacturing method. Background Technology

[0002] LPG has become an indispensable clean fuel. Currently, LPG transportation is mainly carried out by sea. To ensure greater carrying capacity, LPG carriers are gradually becoming larger. The increase in ship size places higher demands on the safety and stability of the ships during service. Therefore, not only are the design and construction processes of ships more stringent, but the performance of the materials required for ship construction is also subject to higher requirements. As a key material in the construction of LPG ships, low-temperature steel needs to possess key properties such as high strength and toughness, low yield strength ratio, and high fracture toughness.

[0003] Patent document titled "A Low Yield-to-Strength Ratio Steel Plate for Ship Storage Tanks and Its Manufacturing Method," application number 202210781891.3, discloses a low yield-to-strength ratio steel plate for ship storage tanks. Its chemical composition, by weight percentage, is: C 0.08%~0.12%, Mn 1.00%~1.50%, Si 0.10%~0.50%, P≤0.015%, S≤0.005%, Nb≤0.029%, Ti≤0.02%, Cr 0.20%~0.50%, Mo 0.20%~0.50%, Ni 0.10%~0.29%, Alt 0.020%~0.060%, B The steel plate contains 0.001%~0.002% Fe, with the remainder being Fe and unavoidable impurities. Its yield strength is ≥690MPa, tensile strength is ≥780MPa, low-temperature impact energy at -40℃ is ≥100J, and yield strength ratio is ≤0.90. However, its manufacturing process involves online quenching + offline critical zone quenching + tempering, making the steel plate manufacturing process complex.

[0004] Patent document entitled "A 500MPa Grade LPG Fuel Tank Steel and Its Manufacturing Method", with application number 202210951264.0, discloses a 500MPa grade LPG fuel tank steel with the following chemical composition: C 0.06%~0.09%, Si 0.20%~0.40%, Mn 1.20%~1.60%, P≤0.012%, S≤0.002%, Als 0.040%~0.060%, N≤0.0040%, Nb 0.005%~0.030%, Ti 0.005%~0.020%, Cr≤0.15%, Ni≤0.20%, Cu≤0.15%, Cu+Cr+Ni≤0.30%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of over 400 MPa and a tensile strength of over 500 MPa, but it has the disadvantage of low strength, which cannot meet the construction requirements of large-scale LPG ships.

[0005] Patent document entitled "A 540MPa grade LPG fuel tank steel and its manufacturing method", application number 202010951101.2, discloses a 540MPa grade LPG fuel tank steel with the following chemical composition: C 0.07%~0.10%, Si 0.20%~0.40%, Mn 1.4%~1.6%, P≤0.012%, S≤0.002%, Als 0.040%~0.060%, N≤0.0040%, Nb 0.005%~0.040%, Ti 0.005%~0.020%, Cr≤0.20%, Ni≤0.25%, Cu≤0.20%, Cu+Cr+Ni≤0.50%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of over 450 MPa and a tensile strength of over 570 MPa, but its maximum thickness is only 50 mm, which also cannot meet the construction requirements for larger LPG ships.

[0006] Patent document entitled "540MPa Grade VL4-4MOD Marine Steel Plate and Manufacturing Method" with application number 202110392192.5 discloses a 540MPa grade VL4-4MOD marine steel plate with the following chemical composition: C 0.04%~0.08%, Si 0.05%~0.14%, Mn 1.00%~1.30%, S≤0.002%, P≤0.008%, Als 0.015%~0.045%, N 0.003%~0.015%, Nb 0.02%~0.04%, Cu 0.16%~0.35%, Ni 0.10%~0.30%, Mo 0.20%~0.40%, Ti 0.008%~0.014%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of ≥360MPa and a tensile strength of ≥540MPa, which is too low to meet the construction requirements for larger LPG ships.

[0007] In summary, the main problems currently existing in the production of LPG marine steel plates are as follows.

[0008] 1) The complex process increases the production cost of steel plates.

[0009] 2) The steel plate contains expensive alloying elements in its chemical composition, resulting in high manufacturing costs.

[0010] 3) The steel plates have low strength, toughness, and thickness specifications, which cannot meet the construction requirements of larger LPG vessels. Summary of the Invention

[0011] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a high-strength, low-yield-strength, low-yield-strength low-temperature steel plate for LPG marine use and a method for manufacturing it.

[0012] The objective of this invention is achieved as follows:

[0013] A low yield strength ratio, high strength, low temperature steel plate for LPG marine applications, comprising the following components by weight percentage: C: 0.060%~0.140%, Si: 0.25%~0.60%, Mn: 1.20%~1.60%, P≤0.007%, S≤0.004%, V: 0.050%~0.150%, Nb: 0.050%~0.100%, N: 0.0150%~0.0190%, Cu: 0.45%~0.80%, Ni: 0.65%~0.95%, Ti: 0.010%~0.030%, Als: 0.025%~0.045%, Zr: 0.080%~0.150%, with the balance being Fe and unavoidable impurities.

[0014] The steel plate has an Al / Zr ratio of 0.24 to 0.41.

[0015] The microstructure of the steel plate is ferrite + bainite; wherein, by volume percentage, it is as follows: ferrite 10%~15%, bainite 85%~90%; average grain size ≤18.0μm; the precipitates are composed of Ti(C,N), Nb(C,N) and V(C,N), and the average size of the precipitates is ≤40nm, and they are diffusely distributed.

[0016] The steel plate has a yield strength ≥540MPa, tensile strength 680~760MPa, elongation after fracture ≥24.0%, yield strength ratio ≤0.83, impact absorption energy at -80℃ ≥260J, CTOD at -40℃ ≥1.30mm, and yield strength ratio of the steel plate at -40℃ ≤0.80.

[0017] The rationale for the design of the components in this invention is as follows:

[0018] C: An important strengthening element in steel. In this invention, it controls the microstructure of the steel plate, optimizes the precipitation of microalloying elements, and regulates the formation of hard phases such as cementite and pearlite, thereby reducing the yield strength ratio of the steel. However, when the content is too high, the cementite content increases, its size increases, and its distribution becomes more dense, leading to cementite aggregation at grain boundaries. The aggregation of large-sized cementite at grain boundaries causes stress concentration, significantly reducing the plasticity and toughness of the steel and adversely affecting its weldability. Considering factors such as cost and performance, this invention controls the range of C to be 0.060%~0.140%.

[0019] Si plays a deoxidizing role in steelmaking, which can improve the strength and hardness of steel. However, if the content is too high, it will cause the ductile-brittle transition temperature of steel to rise, reduce the low-temperature toughness of steel plates, and increase the yield strength ratio of steel. Taking into account factors such as cost and performance, the Si content in this invention is controlled within the range of 0.25% to 0.60%.

[0020] Mn: An element that expands the austenite phase region, forming a substitutional solid solution in steel. It can dissolve extensively in the Fe matrix, increasing the strength of steel through solid solution strengthening. Its contribution to yield strength is less than that to tensile strength, thus reducing the yield-to-tensile ratio and positively impacting grain refinement. It also delays the transformation between ferrite and pearlite in steel, significantly increases hardenability, lowers the ductile-brittle transition temperature, and alters impact toughness. However, excessively high Mn content can easily lead to segregation in steel, negatively affecting both plasticity and toughness. Considering all factors, this invention controls the Mn content to be in the range of 1.20% to 1.60%.

[0021] V: A key strengthening and toughening element in this invention, it has a strong affinity for nitrogen and carbon, and is a potent carbide and nitride forming element. In steel, it improves the strength of the steel through grain refinement, precipitation strengthening, and solid solution strengthening. By combining with N, another strengthening and toughening element in this invention, it promotes the formation of ferrite in the steel, thereby reducing the yield strength ratio. As the V content in steel increases, the ductile-brittle transition temperature of the steel decreases. However, if the content is too high, the ductile-brittle transition temperature actually increases. Taking all factors into consideration, this invention controls the V content within the range of 0.050% to 0.150%.

[0022] Nitrogen (Nb): It can form dispersed, strong carbides (NbC), which strongly inhibit grain growth, thus refining the grain size of steel. Adding Nb to steel can effectively refine the grain size and improve its strength and toughness. However, excessive addition can form the δ phase or other brittle phases in the steel. Considering all factors, this invention controls the Nb content to be in the range of 0.050% to 0.100%.

[0023] Nitrogen (N): A key strengthening and toughening element in this invention. The addition of nitrogen promotes the formation of large amounts of vanadium (CN) in the steel, promotes the formation of ferrite, reduces the yield strength ratio, refines the grain size, and improves the plasticity and toughness of the steel. Nitrogen-containing steel not only eliminates the cost increases caused by degassing and refining during steelmaking, but also allows for the fuller utilization of microalloying elements, saving on the amount of alloying elements used, significantly reducing production costs, and thus improving its overall mechanical properties. Considering both performance and cost, this invention controls the range of nitrogen (N) to be 0.0150%~0.0190%.

[0024] Cu can improve the stability of austenite in steel, increase the hardenability of steel, and enable steel to obtain bainitic structure over a wide range of cooling rates. When added in appropriate amounts, it improves the strength, plasticity, and low-temperature toughness of steel. During low-temperature treatment, it can produce ε-Cu precipitation, thereby increasing the strength of the steel. However, if the content is too high, the hot brittleness of the steel deteriorates, making it prone to hot cracking. In this invention, the Cu content is controlled within the range of 0.45% to 0.80%.

[0025] Ni has no adverse effect on the hardening and toughness of the weld heat-affected zone of steel, and can improve the plasticity, low-temperature toughness and low-temperature strength of steel. It is beneficial to reduce the yield strength ratio of steel plates in low-temperature environments. In addition, the addition of Ni can also reduce the tendency of hot cracking when the Cu content is high. Taking into account factors such as cost and performance, the range of Ni in this invention is controlled to be 0.65%~0.95%.

[0026] Ti (Ti) exhibits strong precipitation strengthening, increasing the strength of steel and preventing austenite recrystallization. Simultaneously, it refines grain size, improving the yield strength of steel. Ti carbides and nitrides have high dissolution temperatures, inhibiting austenite grain growth during heating, thus refining the grain size of the steel plate and improving its strength and toughness. However, excessively high content leads to extremely strong precipitation strengthening, deteriorating the toughness of the steel. Considering all factors, this invention controls the Ti content within the range of 0.010% to 0.030%.

[0027] Al: A strong deoxidizer in steel. Adding a small amount can generate highly fine, ultra-microscopic oxides, which has a beneficial effect on improving the purity of steel. The purpose of adding Al in this invention is to ensure sufficient deoxidation of the molten steel, creating conditions for the subsequent addition of Zr, and preventing the formation of large amounts of ZrO2 after Zr addition. The Al content in this invention is controlled within the range of 0.025% to 0.045%.

[0028] Zr: A strong carbide-forming element, its addition in small amounts has degassing, purification, and grain refinement effects, which is beneficial for improving the low-temperature performance of steel. To better exert its grain refinement effect, it should be added in combination with Al, controlling the Al / Zr ratio to be 0.24~0.41. Considering factors such as cost and performance, the Zr range in this invention is controlled to be 0.080%~0.150%.

[0029] The second technical solution of this invention provides a method for manufacturing LPG marine low yield strength ratio high strength low temperature steel plate, including smelting, continuous casting, reheating, billet cooling, rolling, online accelerated cooling, and stacking slow cooling.

[0030] Smelting: Smelting steel according to the above composition, including converter smelting, refining, and RH treatment;

[0031] a) Adjust the content of elements such as C, Si, Mn, P, and S during converter smelting to bring them within the range of this invention, and add other alloying components as required for smelting;

[0032] b) Refine the molten steel and adjust the content of other alloying elements to the range of this invention;

[0033] c) The refined molten steel is subjected to RH treatment for 40-60 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 580-640 Pa to ensure that the final N content of the steel is 0.0150%-0.0190% and to control the [H] in the steel to ≤2.0 ppm and [O] to ≤10 ppm.

[0034] Continuous casting: The molten steel obtained after smelting is continuously cast to obtain the required billet. The tundish uses a low superheat, 10~20℃, with full protective pouring. The casting speed is controlled at 0.80~1.20m / min, and the secondary cooling water ratio is 0.15~0.35m³ / min. 3 / t, electromagnetic stirring is used during continuous casting with a stirring current of 450~550A, and a light reduction process is used at the end of continuous casting with a reduction of 6.0~11.0mm;

[0035] To control the grain size of the continuously cast billet, the billet is stacked and slowly cooled to promote the precipitation of Nb(C,N) and V(C,N) in the billet, thereby inhibiting the growth of the grains in the billet. The stacking start temperature is 700~750℃ and the slow cooling time is ≥36h.

[0036] Reheating: Heat the billet obtained in step (3) to 1100℃~1220℃, hold for 0.5~1.5h, and the total time in the furnace is ≤5.5h;

[0037] Slab cooling: High-pressure water is used to descale the slab after it exits the furnace, so that a temperature gradient is formed between the surface and the core of the slab. After descaling, the slab is directly rolled.

[0038] Rolling: The cast billet is rolled into hot-rolled steel plate in two stages. The first stage uses a high-speed, high-reduction method. The billet is reheated before exiting the furnace and descaled using high-pressure water. Preferably, the high-pressure descaling water pressure is 1.00~1.30MPa, creating a temperature gradient between the surface and core of the billet, with a temperature difference >70℃. After descaling, the billet is directly rolled, with the roll speed controlled at 1.5~2.5m / s, the first pass reduction controlled at >50mm, and the final rolling temperature above 1050℃. The thickness of the billet after heating is (2.0~2.5)t, where t is the final product thickness. The second stage starts rolling at 900~950℃ and ends rolling at 850~900℃.

[0039] Online accelerated cooling: In order to maintain fine grains after rolling and prevent grain growth, the rolled steel plate is accelerated cooled with an initial cooling temperature of 720~810℃, a cooling rate of 10.0~20.0℃ / s, and a reddening temperature of 500~600℃.

[0040] Stacking and slow cooling: In order to release the internal stress formed during the rolling and cooling process of steel plates and to further form fine VC precipitates, the cooled steel plates are stacked and slow cooled. The stacking temperature is ≥450℃ and the stacking time is ≥20h.

[0041] This invention employs a low-carbon chemical composition system to improve the low-temperature toughness of steel. The yield strength ratio of the steel is controlled by adding alloying elements such as Mn, VN, Ni, and Zr. The smelting process uses low superheat to reduce center segregation in the billet, and electromagnetic stirring and light pressure are incorporated during continuous casting to further control billet segregation. The rolling process employs a two-stage controlled rolling process. The first stage uses high-speed, high-pressure rolling to better transfer deformation to the core and break up the core grains. The second stage uses rolling in the non-recrystallized zone, significantly increasing the austenite-ferrite phase transformation storage energy, thereby increasing the number of ferrite nuclei and refining the grain size of the steel plate. After rolling, the steel plate undergoes accelerated cooling + slow stacking cooling to control the internal stress and precipitate distribution of the final steel plate.

[0042] The metallographic structure of the steel plate is 10%~15% ferrite + 85%~90% bainite, with an average grain size ≤18.0μm. The ratio of soft phase (ferrite) to hard phase (bainite) in this invention ensures that the steel plate has a low yield strength ratio while achieving high strength and toughness. The precipitated phases are composed of Ti(C,N), Nb(C,N) and V(C,N), with an average size ≤40nm and a dispersed distribution. The resulting steel plate has excellent comprehensive mechanical properties: yield strength ≥540MPa, tensile strength ≥685MPa, elongation after fracture ≥24.0%, yield strength ratio ≤0.83, impact absorption energy at -80℃ ≥260J, CTOD at -40℃ ≥1.30mm, and yield strength ratio of the steel plate at -40℃ ≤0.80. Detailed Implementation

[0043] The present invention will be further illustrated below through examples.

[0044] According to the component ratio of the technical solution, the embodiments of the present invention involve smelting, continuous casting, reheating, billet cooling, rolling, online accelerated cooling, and stacking slow cooling. Specifically:

[0045] Reheating: The billet obtained from continuous casting is heated to 1100℃~1220℃ and held for 0.5~1.5h, with a total furnace time ≤5.5h;

[0046] Slab cooling: High-pressure water is used to descale the slab after it exits the furnace, so that a temperature gradient is formed between the surface and the core of the slab. After descaling, the slab is directly rolled.

[0047] Rolling: The cast billet is rolled into hot-rolled steel plate in two stages.

[0048] The first stage is carried out using a high-speed, high-reduction method, with the roll speed controlled at 1.5~2.5m / s, the first-pass reduction controlled at >50mm, the final rolling temperature above 1050℃, and the thickness of the billet to be heated to (2.0~2.5)t, where t is the final product thickness; the second stage has an initial rolling temperature of 900~950℃ and a final rolling temperature of 850~900℃.

[0049] Online accelerated cooling: The rolled steel plate is accelerated cooled at an initial cooling temperature of 720~810℃, a cooling rate of 10.0~20.0℃ / s, and a reddening temperature of 500~600℃.

[0050] Stacking and slow cooling: The cooled steel plates are stacked and slow cooled, with a stacking temperature ≥450℃ and a stacking time ≥20h.

[0051] Furthermore, after continuous casting and before reheating, the continuously cast billet is stacked and slowly cooled, with the initial stacking temperature at 700~750℃ and the slow cooling time at ≥36h.

[0052] Further; smelting: including converter smelting, refining, and RH treatment;

[0053] The refined molten steel is subjected to RH treatment for 40-60 minutes, with nitrogen blowing throughout the RH treatment process at a pressure of 580-640 Pa, to ensure that the final N content of the steel is 0.0150%-0.0190%, and to control the [H] content in the steel to ≤2.0 ppm and [O] content to ≤10 ppm.

[0054] Further; Continuous casting: The molten steel obtained after smelting is continuously cast to obtain the required billet. The tundish uses a low superheat, 10~20℃, with full-process protective casting. The billet pulling speed is controlled at 0.80~1.20m / min, and the secondary cooling water ratio is 0.15~0.35m³. 3 / t, electromagnetic stirring is used during continuous casting with a stirring current of 450~550A, and a light reduction process is used at the end of continuous casting with a reduction of 6.0~11.0mm.

[0055] Furthermore, after continuous casting and before reheating, the continuously cast billets are stacked and slowly cooled, with the initial stacking temperature at 700~750℃ and the slow cooling time at least 36 hours.

[0056] Further; High-pressure water descaling process: descaling water pressure 1.0~1.3MPa, temperature difference between billet surface and core >70℃.

[0057] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for smelting the steel in this embodiment of the invention are shown in Table 2. The main process parameters for heating the steel in this embodiment of the invention are shown in Table 3. The main process parameters for rolling the steel in this embodiment of the invention are shown in Table 4. The main process parameters for cooling the steel in this embodiment of the invention are shown in Table 5. The microstructure and properties of the steel in this embodiment of the invention are shown in Tables 6-8.

[0058] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0059] serial number C Si Mn V Nb N Cu Ni P S Ti Al Zr Al / Zr 1 0.074 0.56 1.47 0.058 0.098 0.0154 0.46 0.66 0.006 0.003 0.019 0.027 0.109 0.25 2 0.137 0.33 1.24 0.143 0.053 0.0189 0.74 0.72 0.003 0.002 0.022 0.042 0.097 0.43 3 0.076 0.26 1.38 0.087 0.081 0.0173 0.71 0.94 0.002 0.004 0.012 0.034 0.084 0.40 4 0.092 0.42 1.32 0.112 0.068 0.0176 0.66 0.92 0.005 0.002 0.016 0.036 0.146 0.25 5 0.106 0.39 1.59 0.127 0.062 0.0188 0.63 0.87 0.004 0.003 0.018 0.031 0.089 0.35 6 0.068 0.51 1.42 0.074 0.089 0.0169 0.54 0.74 0.002 0.001 0.027 0.039 0.143 0.27 7 0.082 0.46 1.49 0.094 0.077 0.0158 0.78 0.89 0.001 0.003 0.029 0.044 0.128 0.34 8 0.089 0.38 1.34 0.106 0.074 0.0164 0.52 0.77 0.006 0.004 0.014 0.029 0.134 0.22 9 0.117 0.58 1.26 0.136 0.057 0.0183 0.58 0.81 0.005 0.002 0.013 0.038 0.094 0.40 10 0.122 0.36 1.53 0.066 0.092 0.0184 0.49 0.69 0.002 0.001 0.024 0.033 0.116 0.28

[0060] Table 2 Main process parameters for steel smelting in the embodiments of the present invention

[0061] serial number RH processing time / min Nitrogen blowing pressure / Pa Superheat / °C Casting speed / m / min <![CDATA[Secondary cooling water specific flow rate / m 3 / t]]> Stirring current / A Light pressure reduction / mm Stacking temperature / ℃ Stacking time / h 1 49 593 13 0.84 0.18 456 9.6 726 38 2 54 614 16 1.17 0.22 542 10.2 708 42 3 59 637 18 0.92 0.27 483 10.6 704 39 4 43 584 12 1.06 0.31 516 6.3 719 44 5 52 603 11 1.12 0.19 527 9.4 732 41 6 48 618 19 1.01 0.28 508 8.7 741 47 7 44 626 14 0.98 0.23 533 6.6 747 44 8 57 621 17 0.86 0.21 479 7.3 738 43 9 56 632 16 0.89 0.26 468 7.8 712 37 10 41 597 18 1.09 0.33 492 8.4 722 46

[0062] Table 3 Main process parameters for steel heating in embodiments of the present invention

[0063] serial number Heating temperature / ℃ Insulation time / h Total time in the furnace / h Descaling water pressure Temperature difference of billet after descaling / ℃ 1 1188 0.9 4.7 1.03 76 2 1179 0.8 4.8 1.27 82 3 1109 1.1 4.9 1.16 88 4 1136 1.3 5.3 1.09 73 5 1207 1.1 5.1 1.24 77 6 1211 0.6 4.6 1.12 71 7 1193 0.7 4.7 1.21 74 8 1128 0.9 4.8 1.07 84 9 1146 1.2 5.4 1.01 86 10 1157 1.3 5.2 1.14 81

[0064] Table 4. Main process parameters for steel rolling in the embodiments of the present invention

[0065]

[0066] Table 5. Main process parameters for steel cooling in embodiments of the present invention.

[0067] serial number Cooling temperature / ℃ Cooling rate / ℃ / s Redness temperature / ℃ Stacking temperature / ℃ Stacking time / h 1 729 10.4 507 463 24 2 804 13.8 593 484 26 3 767 15.7 516 476 31 4 753 12.6 527 488 32 5 744 11.4 532 477 27 6 772 14.7 556 474 23 7 784 18.4 543 453 29 8 739 19.1 561 459 24 9 792 16.2 578 496 33 10 761 17.1 584 481 28

[0068] The microstructure and mechanical properties of the steel in the embodiments of the present invention were tested, and the results are shown in Tables 6-8.

[0069] Table 6. Steel microstructure characteristics of embodiments of the present invention

[0070]

[0071] Table 7. Steel properties of embodiments of the present invention

[0072]

[0073] Table 8 Low-temperature tensile properties of steels according to embodiments of the present invention

[0074]

[0075] The steel plate produced using this invention has a microstructure of 10%~15% ferrite + 85%~90% bainite, with an average grain size ≤18.0μm. The ratio of soft phase (ferrite) to hard phase (bainite) in this invention ensures that the steel plate has high strength and toughness while maintaining a low yield strength ratio. The precipitated phases are composed of Ti(C,N), Nb(C,N) and V(C,N), with an average size ≤40nm and a dispersed distribution. The resulting steel plate has excellent comprehensive mechanical properties: yield strength ≥540MPa, tensile strength ≥685MPa, elongation after fracture ≥24.0%, yield strength ratio ≤0.83, impact absorption energy at -80℃ ≥260J, CTOD at -40℃ ≥1.30mm, and yield strength ratio at -40℃ ≤0.80.

[0076] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A LPG marine low yield ratio high strength cryogenic steel plate, characterized by, The steel plate has the following components in percentage by weight: C: 0.060%-0.140%, Si: 0.25%-0.60%, Mn: 1.20%-1.60%, P≤0.007%, S≤0.004%, V: 0.050%-0.150%, Nb: 0.050%-0.100%, N: 0.0150%-0.0190%, Cu: 0.45%-0.80%, Ni: 0.65%-0.95%, Ti: 0.010%-0.030%, Als: 0.025%-0.045%, Zr: 0.080%-0.150%, the balance being Fe and inevitable impurities, the steel plate having a yield strength≥540 MPa, a tensile strength of 680-760 MPa, an elongation after fracture≥24.0%, a yield ratio≤0.83, an impact energy at-80 ℃≥260 J, a CTOD at-40 ℃≥1.30 mm, and a yield ratio of the steel plate at-40 ℃≤0.

80.

2. The LPG ship low yield ratio high strength cryogenic steel plate according to claim 1, characterized in that, In the steel plate, Al / Zr=0.24-0.

41.

3. The LPG ship low yield ratio high strength cryogenic steel plate according to claim 1, characterized in that, The steel plate has a microstructure of ferrite+ bainite, wherein the ferrite is 10%-15% and the bainite is 85%-90% by volume percentage, the average grain size is≤18.0 μm, and the precipitated phase is composed of Ti(C, N), Nb(C, N) and V(C, N) and has an average size≤40 nm.

4. A method of manufacturing the LPG carrier low yield ratio high strength cryogenic steel plate according to any one of claims 1 to 3, characterized by: The manufacturing method comprises smelting, continuous casting, reheating, slab cooling, rolling, on-line accelerated cooling, and stack slow cooling. The reheating is performed by heating the continuous casting slab to 1100 ℃-1220 ℃, maintaining for 0.5-1.5 h, and keeping the total furnace time≤5.5 h. The slab cooling is performed by using high-pressure water to remove the scale after the slab is discharged from the furnace, so as to form a temperature gradient between the surface and the core of the slab, and the slab is directly rolled after the scale is removed. The rolling is performed by two-stage rolling of the slab into a hot-rolled steel plate, The first stage is performed by using high speed and large reduction, the roller speed is controlled to be 1.5-2.5 m / s, the reduction of the first pass is controlled to be >50 mm, the finish rolling temperature is≥1050 ℃, the warm slab thickness is (2.0-2.5) t, and t is the thickness of the final product; the second stage is performed at a starting rolling temperature of 900-950 ℃ and a finish rolling temperature of 850-900 ℃. The on-line accelerated cooling is performed by accelerated cooling of the rolled steel plate at a starting cooling temperature of 720-810 ℃ and a cooling speed of 10.0-20.0 ℃ / s, and the re-red temperature is 500-600 ℃. The stack slow cooling is performed by stack slow cooling of the cooled steel plate at a stack temperature≥450 ℃ and a stack time≥20 h.

5. The method of producing an LPG carrier use low yield ratio high strength low temperature steel plate according to claim 4, characterized in that: The continuous casting slab is stack slow cooled before the reheating, the stack starting temperature is 700-750 ℃, and the slow cooling time is≥36 h.

6. The manufacturing method of the LPG marine low-yield-ratio high-strength low-temperature steel plate according to claim 4, characterized in that: The smelting comprises converter smelting, LF refining and RH treatment. LF refining molten steel is treated by RH, RH treatment time is 40-60 min, nitrogen is blown in the whole process during RH treatment, pressure is 580-640 Pa, final N content of the steel is ensured to be 0.0150%-0.0190%, [H] in the steel is controlled to be ≤2.0 ppm, and [O] is controlled to be ≤10 ppm.

7. The method of manufacturing the LPG ship low yield ratio high strength low temperature steel plate according to claim 4, characterized in that: Continuous casting: the molten steel obtained after smelting is used to produce the required casting blank, the tundish adopts low superheat, the superheat is 10-20 °C, the whole process is protected pouring, the control casting speed is 0.80-1.20 m / min, the secondary cooling water ratio is 0.15-0.35 m 3 / t, electromagnetic stirring is used in the continuous casting process, the stirring current is 450-550 A, and the light press-down process is used at the end of the continuous casting, the press-down amount is 6.0-11.0 mm.

8. The method of manufacturing the LPG ship low yield ratio high strength low temperature steel plate according to claim 4, characterized in that: The continuous casting billet is stacked and slowly cooled after continuous casting and before reheating, the stacking starting temperature is 700-750 DEG C, and the slow cooling time is ≥36 h.

9. The method of producing an LPG-ship use low yield ratio high strength cryogenic steel plate according to claim 4, characterized by: High-pressure water descaling process: descaling water pressure is 1.0-1.3 MPa, and a temperature difference between the surface and the core of the billet is > 70 DEG C.

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