An economic cryogenic toughness steel plate for LPG carrier and a method for manufacturing the same

Abstract

This invention provides an LPG marine economic low-temperature toughness steel plate and its manufacturing method, relating to the field of heavy plate production technology. Its chemical composition is as follows: C: 0.05~0.08%, Si: 0.15~0.30%, Mn: 1.50~1.60%, B: 0.0006~0.0010%, Alt: 0.02~0.05%, Nb: 0.020~0.040%, Ti: 0.010~0.015%, Ni: 0.20~0.40%, P≤0.010%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities. The production method includes continuous casting billet smelting, continuous casting billet heating, rolling, and cooling. The steel plate adopts a two-stage rolling process, with a finishing rolling start temperature of 760~790℃ and a finishing rolling finish temperature of 760±20℃. After rolling, ACC cooling is used, with a reddening temperature of 580~620℃ and an average cooling rate of 5~8℃ / s. The steel plate produced by this invention has a microstructure of 90%~95% ferrite + 5%~10% pearlite, a grain size of not less than 9.5, a yield strength ≥400MPa, a tensile strength ≥550MPa, an elongation after fracture ≥22%, an impact energy of -95℃ ≥350J, an impact energy of -105℃ ≥200J, and a thickness of 10~60mm. It can replace 1.5Ni and 2.25Ni at low cost for the construction of fuel tanks, liquid cargo tanks, and related components with service temperatures not lower than -105℃.

Description

Technical Field

[0001] This invention relates to the field of LPG marine heavy steel plate production technology, specifically to an economical low-temperature toughness steel plate for LPG marine applications and its manufacturing method. Background Technology

[0002] In recent years, with increasingly stringent global shipping decarbonization regulations and the development of the world energy market, the demand for LPG fuel carriers and LPG storage and transportation vessels has surged and is trending towards larger sizes. Larger vessels mean increased strength and thickness of steel plates. According to the specifications of major classification societies such as CCS, DNV, LR, and BV, the impact test temperature of low-temperature toughness steel decreases with increasing thickness, which places higher demands on the toughness of thicker steel plates. For example, when the design temperature of the LPG tank is not lower than -65℃, the low-temperature steels used are 1.5Ni and 2.25Ni steels. When the thickness increases to 60mm, the steel plate is typically required to meet impact toughness requirements down to -105℃. C-Mn steel has a cost advantage over Ni-based steels, and when the service temperature is not lower than -105℃, it can replace nickel-based steels by optimizing its composition and processing.

[0003] CN101497961A discloses a low-temperature toughness 1.5Ni steel and its manufacturing method. The steel plate is produced using a controlled rolling + quenching and tempering process, achieving a yield strength of over 400 MPa, a tensile strength of over 500 MPa, and low-temperature impact toughness meeting the -100℃ requirement. However, this method suffers from high alloy costs due to the addition of 1.2~1.6% Ni, and the steel plate undergoes quenching and tempering heat treatment without employing a cost-saving composition control or TMCP process, resulting in high manufacturing costs and complex processes.

[0004] CN117305720A discloses a cost-effective 360MPa grade carbon-manganese cryogenic steel with ultra-low temperature toughness and its manufacturing method, which adopts a two-stage controlled rolling + relaxation + ultra-rapid cooling process. However, this design results in steel plate strength that is relatively low, with impact toughness only guaranteed up to -60℃, and a maximum thickness of only 40mm. This makes it unsuitable for the increasing size requirements of LPG vessels and cannot replace 1.5Ni and 2.25Ni steels for service environments not lower than -105℃.

[0005] CN119956212A discloses a low yield strength ratio high-strength low-temperature steel plate for LPG marine applications and its manufacturing method. The steel plate has a 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 a yield strength ratio at -40℃ ≤0.80. This design uses a high Ni content of 0.65%-0.95% and adds 0.080%-0.15% Zr, resulting in high alloy cost. Furthermore, the impact toughness is only guaranteed up to -80℃, making it unsuitable as a substitute for 1.5Ni and 2.25Ni steels for service environments not lower than -105℃.

[0006] The above preliminary analysis shows that low-temperature steel plates generally have the following problems: (1) high content of precious alloys, complex process flow and high production cost; (2) low thickness, low-cost low-temperature tough steel cannot guarantee the performance requirements of the service environment of -105℃; (3) no patent has proposed to replace 1.5Ni and 2.25Ni with C-Mn steel composition process optimization for service environment of not less than -105℃. Summary of the Invention

[0007] The purpose of this invention is to provide an economical low-temperature toughness steel plate for LPG marine applications and its manufacturing method. The steel plate manufactured by this invention can replace 1.5Ni and 2.25Ni for the construction of fuel tanks, liquid cargo tanks and related components with service temperatures not lower than -105℃.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A type of LPG marine economical low-temperature toughness steel plate, the steel plate comprising the following components by mass percentage: C: 0.05~0.08%, Si: 0.15~0.30%, Mn: 1.50~1.60%, B: 0.0006~0.0010%, Alt: 0.02~0.05%, Nb: 0.020~0.040%, Ti: 0.010~0.015%, Ni: 0.20~0.40%, P≤0.010%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities.

[0009] Furthermore, the steel plate comprises the following components by mass percentage: C: 0.05~0.08%, Si: 0.20~0.25%, Mn: 1.50~1.55%, B: 0.0008~0.0010%, Alt: 0.035~0.045%, Nb: 0.025~0.040%, Ti: 0.010~0.013%, Ni: 0.28~0.40%, P≤0.008%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities.

[0010] This invention also provides a method for manufacturing LPG marine economic low-temperature toughness steel plates, including continuous casting billet smelting, continuous casting billet heating, steel plate rolling, and steel plate cooling, wherein... 1) Continuous casting billet smelting: Molten iron is desulfurized and slag removed by KR, and after converter smelting, it is refined by LF+RH. Low silicon ferrotitanium is added in the RH process and then ferroboron is added. It is put into use under light pressure during continuous casting, and constant casting speed is used for production. The casting speed is 0.45~0.50m / min, the superheat is 15~30℃, and the center segregation is controlled within C1.0. The 320~370mm thick continuous casting billet is stacked and cooled slowly. 2) Heating of continuous casting billet: The continuous casting billet is cold-charged into the furnace and heated in sections using a walking beam furnace. The temperature of the first heating section is 800~1040℃, the temperature of the second heating section is 1060~1140℃, the temperature of the third heating section is 1150~1180℃, and the temperature of the soaking section is 1150~1160℃. The heating time of the continuous casting billet in the furnace is 352~407 min. 3) Steel plate rolling: The rolling process is a two-stage process of roughing and finishing. The initial rolling temperature of the roughing rolling is not lower than 1100℃, and the reduction of the roughing rolling is not less than 30mm in at least 3 passes. After the roughing rolling is completed, the intermediate billet is cooled by water cooling device to the starting rolling temperature of finishing rolling. The starting rolling thickness of finishing rolling is ≥3.3h (h is the target steel plate thickness), the starting rolling temperature of finishing rolling is 760~790℃, and the finishing rolling temperature is 760±20℃. 4) Steel plate cooling: After finishing rolling, the steel plate is cooled by ACC with an average cooling rate of 5~8℃ / s and a reddening temperature of 580~620℃.

[0011] Furthermore: the thickness of the steel plate is 10~60mm.

[0012] Furthermore, the metallographic structure of the steel plate is 90-95% ferrite and 5-10% pearlite, with a grain size ≥9.5 and fine, dispersed pearlite distribution.

[0013] Furthermore: the steel plate has a yield strength ≥400MPa, tensile strength ≥550MPa, elongation after fracture ≥22%, impact energy at -95℃ ≥350J, and impact energy at -105℃ ≥200J.

[0014] The working principle of this invention is as follows: (1) This invention effectively fixes free N through Al+Ti microalloying, thereby ensuring that trace amounts of B are dissolved in the steel. 0.0006~0.0010% of the B element segregates at the grain boundaries to increase the stability of austenite, delays the ferrite + pearlite phase transformation to occur at a lower temperature, and increases the phase transformation undercooling, which is beneficial to refine the microstructure (too high B content inhibits the ferrite + pearlite phase transformation, and too low B content has no obvious effect on delaying the ferrite + pearlite phase transformation). (2) The present invention adopts a two-stage rolling process at a relatively low temperature of 760~790℃ to obtain the optimal recrystallized austenite flattening state, providing a large number of nucleation sites for subsequent phase transformation, thereby refining the grains; Under the influence of the above two factors, a microstructure of 90%~95% ferrite + 5%~10% pearlite is obtained, with a grain size of not less than 9.5 grade, thereby ensuring an impact energy of ≥300J at -95℃ and ≥250J at -105℃.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The 10-60mm low-temperature tough steel produced according to the manufacturing method of the present invention has a metallographic structure of 90-95% ferrite + 5-10% pearlite, a grain size of not less than 9.5, a yield strength ≥400MPa, a tensile strength ≥550MPa, an elongation after fracture ≥22%, an impact energy of -95℃ ≥350J, and an impact energy of -105℃ ≥200J. It can replace 1.5Ni and 2.25Ni at low cost for the construction of fuel tanks, liquid cargo tanks and related components with service temperatures not lower than -105℃.

[0016] 2. The low Nb content in the composition design of this invention reduces the alloy cost of steel plates and broadens their applications. Attached Figure Description

[0017] Figure 1 This is a metallographic diagram of the steel plate prepared in Example 2 of the present invention. Detailed Implementation

[0018] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

[0019] According to the chemical element composition, mass percentage, and production method requirements of this invention, four embodiments were set up, namely Embodiment 1, Embodiment 2, Embodiment 3, and Embodiment 4. To verify the effects of the content of each chemical component and its mass percentage, as well as the effects of the billet heating temperature, finishing rolling temperature, and tempering temperature after finishing rolling on the performance parameters, three comparative examples were prepared, namely Comparative Example 1, Comparative Example 2, and Comparative Example 3, in which seven batches of steel plates were smelted and rolled. Specifically, the mass percentage content of the chemical components in Comparative Example 1 is outside the scope of this invention, while the process parameters of the preparation process are within the scope of this invention; the mass percentage content of the chemical components in Comparative Example 2 is within the scope of this invention, while the process parameters of the preparation process are outside the scope of this invention; and both the mass percentage content of the chemical components and the process parameters of the preparation process in Comparative Example 3 are outside the scope of this invention. The weight percentages of the chemical element composition of the four embodiments and the three comparative examples are shown in Table 1, where the balance is Fe and unavoidable impurities.

[0020] Table 1. Chemical composition (wt%) of steel plates from embodiments and comparative examples of the present invention.

[0021] A method for manufacturing an LPG marine economical low-temperature toughness steel plate includes continuous casting billet smelting, continuous casting billet heating, steel plate rolling, and steel plate cooling, wherein... 1) Continuous casting billet smelting: Molten iron is desulfurized and slag removed by KR, and after converter smelting, it is refined by LF+RH. Low silicon ferrotitanium is added in the RH process and then ferroboron is added. It is put into use under light pressure during continuous casting, and constant casting speed is used for production. The casting speed is 0.45~0.50m / min, the superheat is 15~30℃, and the center segregation is controlled within C1.0. The 320~370mm thick continuous casting billet is stacked and cooled slowly. 2) Heating of continuous casting billet: The continuous casting billet is cold-charged into the furnace and heated in sections using a walking beam furnace. The temperature of the first heating section is 800~1040℃, the temperature of the second heating section is 1060~1140℃, the temperature of the third heating section is 1150~1180℃, and the temperature of the soaking section is 1150~1160℃. The heating time of the continuous casting billet in the furnace is 352~407 min. 3) Steel plate rolling: The rolling process is a two-stage process of roughing and finishing. The initial rolling temperature of the roughing rolling is not lower than 1100℃, and the reduction of the roughing rolling is not less than 30mm in at least 3 passes. After the roughing rolling is completed, the intermediate billet is cooled by water cooling device to the starting rolling temperature of finishing rolling. The starting rolling thickness of finishing rolling is ≥3.3h (h is the target steel plate thickness), the starting rolling temperature of finishing rolling is 760~790℃, and the finishing rolling temperature is 760±20℃. 4) Steel plate cooling: After finishing rolling, the steel plate is cooled by ACC with an average cooling rate of 5~8℃ / s and a reddening temperature of 580~620℃.

[0022] The production process control parameters of the steel plates in the embodiments and comparative examples of this invention are shown in Table 2.

[0023] Table 2. Production process control parameters for steel plates in the embodiments and comparative examples of the present invention.

[0024] The mechanical properties of the steel plates in the embodiments and comparative examples of this invention are shown in Table 3.

[0025] Table 3 Mechanical properties of steel plates produced in the embodiments and comparative examples of the present invention

[0026] As can be seen from Tables 1, 2, and 3, the steel plates produced by the chemical composition, mass percentage, and rolling temperature controlled during the production process of Examples 1-4 of this invention all exhibit impact energy exceeding 300J at -95℃ and exceeding 200J at -105℃, demonstrating excellent low-temperature toughness. In contrast, the steel plates of Comparative Examples 1, 2, and 3 show extremely low impact energy at -95℃ and -105℃, indicating a plateau in the ductile-brittle transition process.

[0027] The steel plate prepared in Example 2 of this invention has a yield strength of 456 MPa, a tensile strength of 566 MPa, an elongation after fracture of 24.5%, an impact energy of over 370 J at -95℃, and an impact energy of over 275 J at -105℃. It has excellent comprehensive mechanical properties and can effectively avoid brittle fracture when manufacturing low-temperature structural components, ensuring safe operation. It is the best embodiment.

[0028] Figure 1 The metallographic morphology of the preferred embodiment 2 has the typical characteristics of the low-temperature steel structure in this invention. It can be seen that the carbon manganese low-temperature steel structure is composed of ferrite and pearlite, with ferrite as the main matrix, ferrite content greater than 90%, and grain size of 9.5 grade. Therefore, this structure has good low-temperature impact toughness. In addition, the structure is fine and uniform, and the pearlite is dispersed, ensuring the stability of performance.

Claims

1. An economical low-temperature toughness steel plate for LPG marine applications, characterized in that, The steel plate comprises the following components by mass percentage: C: 0.05~0.08%, Si: 0.15~0.30%, Mn: 1.50~1.60%, B: 0.0006~0.0010%, Alt: 0.02~0.05%, Nb: 0.020~0.040%, Ti: 0.010~0.015%, Ni: 0.20~0.40%, P≤0.010%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities.

2. The steel plate according to claim 1, characterized in that, The steel plate comprises the following components by mass percentage: C: 0.05~0.08%, Si: 0.20~0.25%, Mn: 1.50~1.55%, B: 0.0008~0.0010%, Alt: 0.035~0.045%, Nb: 0.025~0.040%, Ti: 0.010~0.013%, Ni: 0.28~0.40%, P≤0.008%, S≤0.003%, N≤0.005%, with the remainder being Fe and unavoidable impurities.

3. A method for manufacturing an economical low-temperature toughness steel plate for LPG marine applications, characterized in that, This includes continuous casting billet smelting, continuous casting billet heating, steel plate rolling, and steel plate cooling, among which... 1) Continuous casting billet smelting: Molten iron is desulfurized and slag removed by KR, and after converter smelting, it is refined by LF+RH. Low silicon ferrotitanium is added in the RH process and then ferroboron is added. It is put into use under light pressure during continuous casting, and constant casting speed is used for production. The casting speed is 0.45~0.50m / min, the superheat is 15~30℃, and the center segregation is controlled within C1.

0. The 320~370mm thick continuous casting billet is stacked and cooled slowly. 2) Heating of continuously cast billets: The continuously cast billets are cold-charged into the furnace and heated in sections using a walking beam furnace; 3) Steel plate rolling: The rolling process is a two-stage process of roughing and finishing. The initial rolling temperature of the roughing rolling is not lower than 1100℃, and the reduction of the roughing rolling is not less than 30mm in at least 3 passes. After the roughing rolling is completed, the intermediate billet is cooled by water cooling device to the starting rolling temperature of finishing rolling. The starting rolling thickness of finishing rolling is ≥3.3h (h is the target steel plate thickness), the starting rolling temperature of finishing rolling is 760~790℃, and the finishing rolling temperature is 760±20℃. 4) Steel plate cooling: After finishing rolling, the steel plate is cooled by ACC with an average cooling rate of 5~8℃ / s and a reddening temperature of 580~620℃.

4. The manufacturing method according to claim 3, characterized in that, In section 2), the temperature of the first heating section is 800~1040℃, the temperature of the second heating section is 1060~1140℃, the temperature of the third heating section is 1150~1180℃, the temperature of the soaking section is 1150~1160℃, and the heating time of the continuous casting billet in the furnace is 352~407min.

5. The manufacturing method according to claim 3, characterized in that, The thickness of the steel plate is 10~60mm.

6. The manufacturing method according to claim 3, characterized in that, The metallographic structure of the steel plate is 90-95% ferrite and 5-10% pearlite, with a grain size ≥9.5 and fine, dispersed pearlite distribution.

7. The manufacturing method according to claim 3, characterized in that, The steel plate has a yield strength ≥400MPa, tensile strength ≥550MPa, elongation after fracture ≥22%, impact energy at -95℃ ≥350J, and impact energy at -105℃ ≥200J.

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