Low-yield-ratio high-strength low-temperature steel plate for LPG (Liquefied Petroleum Gas) ship and manufacturing method

Abstract

The invention provides a low-yield-ratio high-strength low-temperature steel plate for an LPG (liquefied petroleum gas) ship and a manufacturing method. The steel plate comprises the following components in percentage by weight: 0.060%-0.140% of C, 0.25%-0.60% of Si, 1.20%-1.60% of Mn, less than or equal to 0.007% of P, less than or equal to 0.004% of S, 0.050%-0.150% of V, 0.050%-0.100% of Nb, 0.0150%-0.0190% of N, 0.45%-0.80% of Cu, 0.65%-0.95% of Ni, 0.010%-0.030% of Ti, 0.025%-0.045% of Als, 0.080%-0.150% of Zr and the balance of Fe and inevitable impurities. The manufacturing method comprises the steps of smelting, continuous casting, reheating, casting blank cooling, rolling, online accelerated cooling and stacking slow cooling. According to the steel plate produced through the method, the yield strength is larger than or equal to 540 MPa, the tensile strength is larger than or equal to 685 MPa, the percentage elongation after fracture is larger than or equal to 24.0%, the yield ratio is smaller than or equal to 0.83, the impact absorption energy at the temperature of-80 DEG C is larger than or equal to 260 J, CTOD at the temperature of-40 DEG C is larger than or equal to 1.30 mm, and the yield ratio of the steel plate at the temperature of-40 DEG C is smaller than or equal to 0.80.

Description

Technical Field

[0001] The invention belongs to the field of metal materials, and in particular relates to a low-yield ratio high-strength low-temperature steel plate for LPG ships and a manufacturing method thereof. Background Art

[0002] With the increasing attention and reliance on clean energy at home and abroad, the demand for LPG energy is increasing, and LPG has become one of the indispensable clean fuels. At present, the transportation of LPG is mainly completed by sea. In order to ensure a larger carrying capacity, LPG transport ships are gradually developing towards large-scale. The large-scale ships put forward higher requirements on the safety and stability of ship service. Therefore, not only the design and construction process of the ship are more stringent, but also the performance of the materials required for ship construction is put forward. As the main material for the construction of LPG ships, low-temperature steel needs to have key properties such as high strength and toughness, low yield ratio, and high fracture toughness.

[0003] The patent document entitled “A low yield ratio steel plate for ship storage tanks and its manufacturing method” with application number 202210781891.3 discloses a low yield ratio steel plate for ship storage tanks, whose 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%, Mo0.20%~0.50%, Ni 0.10%~0.29%, Alt0.020%~0.060%, B 0.001%~0.002%, the rest is Fe and unavoidable impurities, the steel plate has a yield strength ≥690MPa, a tensile strength ≥780MPa, a -40℃ low temperature impact energy ≥100J, and a yield strength ratio ≤0.90. However, its process includes online quenching + offline critical zone quenching + tempering process, and the steel plate process is complicated.

[0004] The patent document with the invention name “A 500MPa grade LPG fuel tank steel and its manufacturing method” and application number 202210951264.0 discloses a 500MPa grade LPG fuel tank steel, whose chemical composition is: C 0.06%~0.09%, Si 0.20%~0.40%, Mn 1.20%~1.60%, P≤0.012%, S≤0.002%, Als0.040%~0.060%, N≤0.0040%, Nb 0.005%~0.030%, Ti0.005%~0.020%, Cr≤0.15%, Ni≤0.20%, Cu≤0.15%, Cu+Cr+Ni≤0.30%, and the balance is Fe and unavoidable impurities. The steel plate has a yield strength of more than 400 MPa and a tensile strength of more than 500 MPa, but has a disadvantage of being relatively low in strength and cannot meet the requirements for the construction of large-scale LPG ships.

[0005] The patent document with the invention name “A 540MPa grade LPG fuel tank steel and its manufacturing method” and application number 202010951101.2 discloses a 540MPa grade LPG fuel tank steel, whose chemical composition is: 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%, and the balance is Fe and unavoidable impurities. The yield strength of the steel plate is over 450 MPa and the tensile strength is over 570 MPa, but its maximum thickness is only 50 mm, which also cannot meet the construction requirements of large-scale LPG ships.

[0006] The patent document with the invention name “540MPa grade VL4-4MOD ship steel plate and manufacturing method” and application number 202110392192.5 discloses a 540MPa grade VL4-4MOD ship steel plate, whose chemical composition is: 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%, and the balance is Fe and unavoidable impurities. The yield strength of the steel plate is ≥360MPa, and the tensile strength is ≥540MPa. Its strength is relatively low and cannot meet the construction requirements of large-scale LPG ships.

[0007] In summary, the current production of LPG marine steel plates mainly has the following problems.

[0008] 1) The process is complicated, resulting in an increase in the production cost of the steel plate.

[0009] 2) The chemical composition of the steel plate contains precious alloy elements, and the manufacturing cost is relatively high.

[0010] 3) The strength, toughness and thickness specifications of steel plates are relatively low and cannot meet the construction requirements of large-scale LPG ships. Summary of the invention

[0011] The object of the present invention is to overcome the above problems and shortcomings and to provide a low yield ratio high strength low temperature steel plate for LPG ships with high toughness and low yield ratio and a manufacturing method thereof.

[0012] The object of the invention is achieved in this way:

[0013] The invention discloses a low yield ratio high strength low temperature steel plate for LPG ships. The components of the steel plate are as follows 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%, and the balance is Fe and unavoidable impurities.

[0014] In the steel plate, Al / Zr=0.24-0.41.

[0015] The microstructure of the steel plate is ferrite + bainite; wherein, by volume percentage, the ferrite is 10% to 15%, the bainite is 85% to 90%; the average grain size is ≤18.0 μm; the precipitated phase is composed of Ti(C, N), Nb(C, N) and V(C, N), and the average size of the precipitated phase is ≤40 nm and is dispersed.

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

[0017] The reasons for the composition design of the present invention are as follows:

[0018] C: An important strengthening element in steel. In the technical solution of the present invention, it controls the composition of the steel plate, optimizes the precipitation of microalloying elements in the steel, and can regulate the formation of hard phase structures such as cementite and pearlite in the steel, thereby reducing the yield strength ratio of the steel. However, when the content is too high, the cementite content in the steel increases, the size increases, and the distribution becomes more dense, and cementite will aggregate at the grain boundary. The aggregation of large-sized cementite at the grain boundary will cause stress concentration, thereby significantly reducing the plasticity and toughness of the steel, and having an adverse effect on its welding performance. Taking into account factors such as cost and performance, the present invention controls C in the range of 0.060% to 0.140%.

[0019] Si: It plays a role of deoxidation in the steelmaking process and can improve the strength and hardness of steel. However, when the content is too high, it will cause the ductile-brittle transition temperature of steel to rise, reduce the low-temperature toughness of the steel plate, and increase the yield strength ratio of steel. Taking into account factors such as cost and performance, the present invention controls the range of Si to be 0.25% to 0.60%.

[0020] Mn: an element that expands the austenite phase region, forms a substitutional solid solution in steel, can be dissolved in a large amount in the Fe matrix, improves the strength of steel through solid solution strengthening, and contributes less to the yield strength than the tensile strength, so it can reduce the yield strength ratio of steel and has a positive effect on grain refinement; delays the transformation of ferrite and pearlite in steel, greatly increases the hardenability of steel, reduces the toughness-brittle transition temperature of steel, and changes the impact toughness, but if the Mn content is too high, it is easy to form segregation in the steel, which has an adverse effect on the plasticity and toughness of the steel. Taking all factors into consideration, the range of Mn controlled by the present invention is 1.20% to 1.60%.

[0021] V: An important toughening element of the present invention, which has a strong affinity with nitrogen and carbon and is a strong carbide and nitride forming element. The strength of steel is improved by fine grain strengthening, precipitation strengthening and solid solution strengthening. By combining with another toughening element N of the present invention, the formation of ferrite in steel is promoted, thereby reducing the yield strength ratio of steel. As the V content in steel increases, the ductile-brittle transition temperature of the steel decreases. However, when the content is too high, the ductile-brittle transition temperature increases instead. Taking all factors into consideration, the present invention controls V in the range of 0.050% to 0.150%.

[0022] Nb: It can form dispersed and strong carbides NbC, which has a strong inhibitory effect on the growth of grains, so it can refine the grain size of steel. Adding Nb to steel can effectively refine the grain size of steel and improve the strength and toughness of steel. However, excessive addition will form δ phase or other brittle phases in steel. Taking comprehensive considerations, the present invention controls the range of Nb to 0.050% to 0.100%.

[0023] N: An important strengthening and toughening element of the present invention. The addition of N element is conducive to promoting the formation of a large amount of V (CN) in steel, promoting the formation of ferrite in steel, reducing the yield strength ratio of steel, and refining its grains, improving the plasticity and toughness of steel. Nitrogen-containing steel not only eliminates the cost increase caused by degassing and refining and denitrification in the steelmaking process, but also the nitrogen addition in steel can give full play to the role of micro-alloying elements, save the amount of alloying elements, greatly reduce production costs, and thus improve its comprehensive mechanical properties. Taking into account the performance and cost, the present invention controls the range of N to be 0.0150% to 0.0190%.

[0024] Cu: It can improve the stability of austenite in steel, increase the hardenability of steel, and make steel obtain bainite structure at a wide cooling rate. When added in appropriate amount, it can improve the strength, plasticity and low-temperature toughness of steel. During low-temperature treatment, ε-Cu can be precipitated, thereby improving the strength of steel. However, when the content is too high, the hot brittleness of steel deteriorates and hot cracks are easily generated. The present invention controls the range of Cu to be 0.45% to 0.80%.

[0025] Ni: has no adverse effect on the hardenability and toughness of the welding heat affected zone of steel, and can improve the plasticity, low-temperature toughness and low-temperature strength of steel, which is beneficial to reducing the yield strength ratio of the steel plate in a low-temperature environment. In addition, the addition of Ni can also reduce the tendency of thermal cracking when the Cu content is high. Considering factors such as cost and performance, the present invention controls the range of Ni to be 0.65% to 0.95%.

[0026] Ti: It can produce a strong precipitation strengthening effect, improve the strength of steel, and prevent austenite recrystallization; at the same time, it can produce a grain refinement effect and improve the yield strength of steel. The carbon and nitride of Ti have a high dissolution temperature, which will prevent the growth of austenite grains when heated, thereby refining the grain size of the steel plate and improving its strength and toughness. However, when the content is too high, it will show a very strong precipitation strengthening effect and deteriorate the toughness of the steel. Taking all factors into consideration, the present invention controls the range of Ti to be 0.010% to 0.030%.

[0027] Al: A strong deoxidizer in steel. A small amount of Al can generate highly fine and ultra-micro oxides, which has a beneficial effect on improving the purity of steel. The purpose of adding Al in the present invention is to create conditions for the subsequent addition of Zr on the basis of ensuring that the molten steel is fully deoxidized, and to prevent the formation of a large amount of ZrO2 after the addition of Zr. The range of Als controlled in the present invention is 0.025% to 0.045%.

[0028] Zr: A strong carbide-forming element. A small amount of Zr has the functions of degassing, purification and grain refinement, which is beneficial to improving the low-temperature performance of steel. In order to better play its role in grain refinement, it needs to be used in combination with Al when added, and Al / Zr is controlled to be 0.24-0.41. Considering factors such as cost and performance, the present invention controls Zr in the range of 0.080%-0.150%.

[0029] The second technical solution of the present invention is to provide a method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships, including smelting, continuous casting, reheating, billet cooling, rolling, online accelerated cooling, stacking slow cooling,

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

[0031] a) adjusting the contents of elements such as C, Si, Mn, P, S, etc. during converter smelting to be within the scope of the present invention, and adding other alloy components for smelting as required;

[0032] b) refining the molten steel and adjusting the contents of other alloying elements within the range of the present invention;

[0033] c) subjecting the refined molten steel to RH treatment for 40 to 60 minutes, blowing nitrogen throughout the RH treatment at a pressure of 580 to 640 Pa, ensuring that the final nitrogen content of the steel is 0.0150% to 0.0190%, and controlling [H] ≤ 2.0 ppm and [O] ≤ 10 ppm in the steel;

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

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

[0036] Reheating: heating the ingot obtained in step (3) to 1100° C. to 1220° C., keeping the temperature for 0.5 to 1.5 h, and the total time in the furnace is ≤ 5.5 h;

[0037] Ingot cooling: The ingot is descaled by high-pressure water after it leaves the furnace, so that a temperature gradient is formed between the surface and the core of the ingot. After descaling, the ingot can be rolled directly;

[0038] Rolling: The ingot is rolled into hot-rolled steel plate in two stages. The first stage is carried out by high speed and large reduction. The ingot is reheated and descaled by high-pressure water. Preferably, the pressure of high-pressure descaling water is 1.00-1.30 MPa, so that a temperature gradient is formed between the surface and the core of the ingot, and the temperature difference is greater than 70°C. After descaling, the ingot is directly rolled, the roller speed is controlled at 1.5-2.5 m / s, the first pass reduction is controlled to be greater than 50 mm, the final rolling temperature is above 1050°C, and the thickness of the warm ingot is (2.0-2.5) t, where t is the thickness of the final product. The second stage rolling temperature is 900-950°C, and the final rolling temperature is 850-900°C. ;

[0039] Online accelerated cooling: In order to maintain fine grains after rolling and prevent grain growth, the steel plate is cooled at an accelerated rate of 720-810°C, 10.0-20.0°C / s, and 500-600°C.

[0040] Stacking slow cooling: In order to release the internal stress formed during the rolling and cooling process of the steel plate and further form fine VC precipitation phase, the cooled steel plate is stacked and slowly cooled, with a stacking temperature of ≥450°C and a stacking time of ≥20h;

[0041] The beneficial effects of the present invention are:

[0042] (1) The present invention adopts a low-carbon chemical composition system to improve the low-temperature toughness of steel, controls the yield strength ratio of steel by adding alloy elements such as Mn, VN, Ni, and Zr, adopts low superheat in the smelting process to reduce the center segregation of the ingot, and uses electromagnetic stirring and light pressure reduction in the continuous casting process to further control the segregation of the ingot; the rolling process adopts two-stage controlled rolling, and the first stage rolling adopts a high-speed and high-pressure process to better transfer the deformation to the core and break the grains in the core; the second stage adopts non-recrystallization zone rolling to significantly increase the austenite-ferrite phase transformation storage energy, thereby increasing the number of ferrite nuclei and further refining the grain size of the steel plate; the steel plate after rolling adopts an accelerated cooling + stacking slow cooling process to control the internal stress and precipitation phase distribution of the final steel plate.

[0043] (2) The metallographic structure of the steel plate is 10% to 15% ferrite + 85% to 90% bainite, and the average grain size is ≤18.0 μm. The ratio of the soft phase (ferrite) to the hard phase (bainite) of the present invention can ensure that the steel plate has a low yield strength ratio while obtaining high strength and toughness. The precipitated phase is composed of Ti (C, N), Nb (C, N) and V (C, N), with an average size of ≤40 nm and a dispersed distribution. The final steel plate has excellent comprehensive mechanical properties, yield strength ≥540 MPa, tensile strength ≥685 MPa, elongation after fracture ≥24.0%, yield strength ratio ≤0.83, -80°C impact absorption energy ≥260 J, -40°C CTOD ≥1.30 mm or more, and the yield strength ratio of the -40°C steel plate ≤0.80. DETAILED DESCRIPTION

[0044] The present invention will be further described below by way of examples.

[0045] The embodiment of the present invention performs smelting, continuous casting, reheating, cooling of ingots, rolling, online accelerated cooling, and stacking slow cooling according to the component ratio of the technical solution. Specifically:

[0046] Reheating: Heat the continuous casting ingot to 1100℃~1220℃, keep warm for 0.5~1.5h, and the total furnace time is ≤5.5h;

[0047] Ingot cooling: The ingot is descaled by high-pressure water after it leaves the furnace, so that a temperature gradient is formed between the surface and the core of the ingot. After descaling, the ingot can be rolled directly;

[0048] Rolling: The ingot is rolled into hot-rolled steel plate in two stages.

[0049] The first stage is carried out by high-speed and large-reduction method, with the roller speed controlled at 1.5-2.5 m / s, the first-pass reduction amount controlled to be greater than 50 mm, the final rolling temperature above 1050°C, the thickness of the warm billet to be heated to (2.0-2.5) t, t being the thickness of the final product; the second stage has a starting rolling temperature of 900-950°C and a final rolling temperature of 850-900°C;

[0050] Online accelerated cooling: The steel plate is cooled quickly after rolling, with a cooling temperature of 720-810°C, a cooling rate of 10.0-20.0°C / s, and a red-return temperature of 500-600°C;

[0051] Stacking and slow cooling: The cooled steel plates are stacked and slowly cooled, with the stacking temperature ≥450℃ and the stacking time ≥20h.

[0052] Furthermore, after the continuous casting and before reheating, the continuously cast billets are stacked and slowly cooled, the stacking starting temperature is 700-750°C, and the slow cooling time is ≥36h.

[0053] Further; smelting: including converter smelting, refining, RH treatment;

[0054] The refined molten steel is subjected to RH treatment for 40 to 60 minutes. Nitrogen is blown throughout the RH treatment at a pressure of 580 to 640 Pa to ensure that the final N content of the steel is 0.0150% to 0.0190%, and [H] in the steel is controlled to be ≤2.0ppm and [O] ≤10ppm.

[0055] Further; continuous casting: the molten steel obtained after smelting is continuously cast to obtain the required ingot, the tundish adopts low superheat, the superheat is 10-20℃, the whole process is protected, the billet drawing speed is controlled to be 0.80-1.20m / min, the water volume of the secondary cooling water is 0.15-0.35m 3 / t, electromagnetic stirring is used during continuous casting, with a stirring current of 450-550A, and a light reduction process is adopted at the end of continuous casting, with a reduction of 6.0-11.0mm.

[0056] Furthermore, after continuous casting and before reheating, the continuous casting billets are stacked and slowly cooled, the stacking starting temperature is 700-750°C, and the slow cooling time is ≥36h.

[0057] Further; high-pressure water descaling process: descaling water pressure 1.0 ~ 1.3MPa, the temperature difference between the surface and the core of the ingot is > 70℃.

[0058] The composition of the steel of the embodiment of the present invention is shown in Table 1. The main process parameters for smelting the steel of the embodiment of the present invention are shown in Table 2. The main process parameters for heating the steel of the embodiment of the present invention are shown in Table 3. The main process parameters for rolling the steel of the embodiment of the present invention are shown in Table 4. The main process parameters for cooling the steel of the embodiment of the present invention are shown in Table 5. The microstructure and properties of the steel of the embodiment of the present invention are shown in Tables 6 to 8.

[0059] Table 1 Composition of steel according to the present invention (wt%)

[0060]

[0061] Table 2 Main process parameters for steel smelting in the embodiment of the present invention

[0062]

[0063] Table 3 Main process parameters for heating steel in the embodiment of the present invention

[0064]

[0065] Table 4 Main process parameters of steel rolling in the embodiment of the present invention

[0066]

[0067] Table 5 Main process parameters for cooling steel in the embodiment of the present invention

[0068] serial number Cooling temperature / ℃ Cooling speed / ℃ / s Red return 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

[0069] The microstructure and mechanical properties of the steel according to the present invention were tested, and the results are shown in Tables 6 to 8.

[0070] Table 6 Steel structure characteristics of the embodiment of the present invention

[0071]

[0072] Table 7 Steel properties of the present invention

[0073]

[0074] Table 8 Low temperature tensile properties of steel according to the present invention

[0075]

[0076] The metallographic structure of the steel plate produced by the present invention is 10%-15% ferrite + 85%-90% bainite structure, and the average grain size is ≤18.0μm. The ratio of the soft phase (ferrite) to the hard phase (bainite) of the present invention can ensure that the steel plate has a low yield strength ratio while obtaining high strength and toughness. The precipitated phase is composed of Ti (C, N), Nb (C, N) and V (C, N), with an average size of ≤40nm and dispersed distribution. The finally obtained steel plate has excellent comprehensive mechanical properties, yield strength ≥540MPa, tensile strength ≥685MPa, elongation after fracture ≥24.0%, yield strength ratio ≤0.83, -80℃ impact absorption energy ≥260J, -40℃ CTOD ≥1.30mm or more, and the yield strength ratio of the -40℃ steel plate ≤0.80.

[0077] In order to describe the present invention, the present invention is appropriately and fully illustrated by the examples in the above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A low yield ratio high strength low temperature steel plate for LPG ships and a manufacturing method thereof, characterized in that: The components of the steel plate are as follows 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%, and the balance is Fe and unavoidable impurities.

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

41.

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

4. The low yield ratio high strength low temperature steel plate for LPG ships according to claim 1, characterized in that: The steel plate has a yield strength of ≥540MPa, a tensile strength of 680-760MPa, an elongation after fracture of ≥24.0%, a yield strength ratio of ≤0.83, an impact absorption energy of -80°C of ≥260J, a CTOD of -40°C of ≥1.30mm, and a yield strength ratio of the steel plate at -40°C of ≤0.

80.

5. A method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships according to any one of claims 1 to 4, characterized in that: Including smelting, continuous casting, reheating, billet cooling, rolling, online accelerated cooling, stacking slow cooling; Reheating: Heat the continuous casting ingot to 1100℃~1220℃, keep warm for 0.5~1.5h, and the total furnace time is ≤5.5h; Ingot cooling: The ingot is descaled by high-pressure water after it leaves the furnace, so that a temperature gradient is formed between the surface and the core of the ingot. After descaling, the ingot can be rolled directly; Rolling: The ingot is rolled into hot-rolled steel plate in two stages. The first stage is carried out by high-speed and large-reduction method, with the roller speed controlled at 1.5-2.5 m / s, the first-pass reduction amount controlled to be greater than 50 mm, the final rolling temperature above 1050°C, the thickness of the warm billet to be heated to (2.0-2.5) t, t being the thickness of the final product; the second stage has a starting rolling temperature of 900-950°C and a final rolling temperature of 850-900°C; Online accelerated cooling: The steel plate is cooled quickly after rolling, with a cooling temperature of 720-810°C, a cooling rate of 10.0-20.0°C / s, and a red-return temperature of 500-600°C; Stacking and slow cooling: The cooled steel plates are stacked and slowly cooled, with the stacking temperature ≥450℃ and the stacking time ≥20h.

6. The method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships according to claim 5, characterized in that: After the continuous casting and before reheating, the continuously cast billets are stacked and slowly cooled, the stacking start temperature is 700-750° C., and the slow cooling time is ≥36 hours.

7. The method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships according to claim 5, characterized in that: Smelting: including converter smelting, LF refining, and RH treatment; The molten steel after LF refining is subjected to RH treatment for 40 to 60 minutes. Nitrogen is blown throughout the RH treatment at a pressure of 580 to 640 Pa to ensure that the final N content of the steel is 0.0150% to 0.0190%, and [H] in the steel is controlled to be ≤2.0ppm and [O] ≤10ppm.

8. The method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships according to claim 5, characterized in that: Continuous casting: The molten steel obtained after smelting is continuously cast to obtain the required ingots. The tundish adopts low superheat, superheat of 10-20℃, full protection casting, controlled billet drawing speed of 0.80-1.20m / min, secondary cooling water ratio of 0.15-0.35m 3 / t, electromagnetic stirring is used during continuous casting, with a stirring current of 450-550A, and a light reduction process is adopted at the end of continuous casting, with a reduction of 6.0-11.0mm.

9. The method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships according to claim 5, characterized in that: After continuous casting and before reheating, the continuous casting billets are stacked and slowly cooled. The stacking starting temperature is 700-750°C and the slow cooling time is ≥36h.

10. The method for manufacturing a low yield ratio high strength low temperature steel plate for LPG ships according to claim 5, characterized in that: High-pressure water descaling process: The descaling water pressure is 1.0-1.3MPa, and the temperature difference between the surface and the core of the ingot is >70℃.

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