Low-nickel low-temperature steel and manufacturing method thereof

Abstract

The invention relates to the technical field of low alloy steel manufacturing, in particular to low-nickel low-temperature steel suitable for manufacturing liquefied propylene or liquefied propane storage tanks and a manufacturing method of the low-nickel low-temperature steel. The steel plate comprises the following chemical components in percentage by weight: 0.10%-0.20% of C, 0.20%-0.60% of Si, 1.10%-1.80% of Mn, 0.30%-0.80% of Ni, 0.03%-0.07% of Nb, less than or equal to 0.005% of S, less than or equal to 0.008% of P and the balance of Fe and inevitable impurities. The manufacturing method of the steel plate comprises the following steps: 1) converter smelting + LF refining + VD refining; (2) continuous casting; (3) slowly cooling the plate blank; (4) heating the plate blank; (5) controlled rolling; and (6) normalizing and tempering heat treatment is conducted, specifically, on the basis that C, Si, Mn and Ni are added, only a small amount of Nb is added, high-purity smelting, controlled rolling and the normalizing and tempering heat treatment process are combined, and the low-temperature steel capable of being used in the environment with the temperature not lower than-60 DEG C is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy steel manufacturing, and particularly relates to a low nickel cryogenic steel suitable for manufacturing liquefied propylene or liquefied propane storage tanks and a manufacturing method thereof. Background Art

[0002] Propane is commonly used as a fuel for engines, a raw material for propylene, or a solvent in the refining industry. The domestic demand exceeded 30 million tons in 2023; while propylene is an important chemical raw material, 70% of which is used in the production of polypropylene, and the domestic demand exceeded 50 million tons in 2023. Compared with oil-based and coal-based products, propane and propylene, which have obvious environmental protection advantages and cost advantages, have good development prospects. PDH (propane dehydrogenation) projects also entered the peak production period after 2022 and will be in the continuous development stage in the next few years. According to statistics, more than 30 million tons of PDH plants are in the proposed, under construction, or planned construction stage.

[0003] The boiling point of propane is -42°C, and the boiling point of propylene is -47°C. A large amount of cryogenic steel is required for the storage and transportation of propane and propylene. Currently, low nickel steel that can serve in a low temperature environment of -60°C is commonly used in liquefied propane and liquefied propylene projects.

[0004] For working conditions below -40°C, most foreign countries use 0.5Ni cryogenic steel. For example, the French standard NFA36-208 (1982) first included the -60°C grade 0.5Ni cryogenic steel 10N2 (divided into two grades according to the yield point of 285 MPa and 355 MPa). Subsequently, the German standard DIN 17280-1985 included two -60°C grade 0.5Ni cryogenic steels, 11MnNi5-3 and 13MnNi6-3. The 0.5Ni cryogenic steels included in the international standard ISO9328-3 published at the end of 1987 are also the two grades of 11MnNi5-3 and 13MnNi6-3.

[0005] The Chinese patent application with the application number 201911045689.9 discloses "A 0.5Ni low-temperature steel and its manufacturing method". Its chemical composition by weight percentage is as follows: C: 0.05% - 0.15%, Si: 0.20% - 0.50%, Mn: 1.0% - 1.60%, Ni: 0.30% - 0.70%, Al: 0.020% - 0.035%, B: 0.001% - 0.005, S: ≤0.005%, P: ≤0.008%, and the balance is Fe and impurities. The manufacturing method includes: 1) converter + LF + vacuum refining; 2) continuous casting; 3) slab slow cooling; 4) rolling; 5) normalizing + tempering heat treatment. By strictly controlling the continuous casting and rolling processes, a high-quality initial steel plate structure is obtained. The addition of element B and the normalizing + tempering heat treatment reduce the minimum service temperature of the 0.5Ni steel from -60°C to -80°C.

[0006] The Chinese patent application with the application number 201911047191.6 discloses "A low-nickel steel for low-temperature storage tanks and its manufacturing method". Its chemical composition by weight percentage is as follows: C: 0.05 - 0.15%, Si: 0.20 - 0.60%, Mn: 1.0 - 1.70%, Ni: 0.30 - 1.0%, Cr: 0.20 - 0.70%, Mo: 0.20 - 0.70%, Nb: 0.01 - 0.05%, Ti: 0.01 - 0.05%, S: ≤0.005%, P: ≤0.008%, and the balance is Fe and impurities. Converter + LF + VD refining is adopted to ensure precise control of the steel composition and the gas content in the steel; full protection casting during the continuous casting process and slab slow cooling ensure the internal quality of the cast slab; controlled rolling ensures the refinement of the initial structure; the heat treatment adopts quenching + tempering process to obtain a uniform and fine tempered sorbite structure, fully ensuring the strength and low-temperature toughness of the steel plate. Finally, a steel for LPG liquid tanks that can be used in an environment with a temperature not lower than -80°C is obtained.

[0007] The Chinese patent application with the application number 201610503624.4 discloses "A normalized -50°C low-temperature steel and its manufacturing method". The steel plate composition by weight percentage is as follows: C: 0.09% - 0.15%, Si: 0.16% - 0.50%, Mn: 0.60% - 1.18%, Ni: 0.10% - 0.50%, Mo: 0.01% - 0.09%, Cr: 0.15% - 0.30%, V: 0.06% - 0.10%, Ti: 0.015% - 0.030%, S: ≤0.005%, P: ≤0.008%, and the balance is Fe and unavoidable impurities. The manufacturing method includes: converter smelting, refining, continuous casting, slab slow cooling, cleaning, rolling, heat treatment. The low-temperature steel produced has a yield strength ≥350 MPa, a tensile strength of 500 - 630 MPa, and an elongation rate ≥25%. Summary of the Invention

[0008] The present invention provides a low-nickel low-temperature steel and a manufacturing method thereof. Based on the addition of C, Si, Mn, and Ni in the chemical composition design, only a small amount of Nb is added. Combining high-purity smelting, controlled rolling, and normalizing + tempering heat treatment processes, a low-temperature steel that can be used in an environment with a temperature not lower than -60°C is obtained.

[0009] To achieve the above object, the present invention is implemented by the following technical solutions:

[0010] For a low-nickel low-temperature steel, the chemical composition of the steel plate is by weight percentage: C: 0.10% - 0.20%, Si: 0.20% - 0.60%, Mn: 1.10% - 1.80%, Ni: 0.30% - 0.80%, Nb: 0.03% - 0.07, S ≤ 0.005%, P ≤ 0.008%, and the balance is Fe and unavoidable impurities.

[0011] The metallographic structure of the finished steel plate is ferrite + pearlite structure, and by volume percentage, the proportion of ferrite is 50% - 80%, and the proportion of pearlite is 20% - 50%.

[0012] The yield strength of the finished steel plate ≥ 360 MPa, the tensile strength ≥ 500 MPa, the elongation ≥ 30%, and the impact energy KV at -60°C 2 ≥ 150 J, and the lateral expansion ≥ 1.0 mm.

[0013] A manufacturing method of a low-nickel low-temperature steel includes the following steps:

[0014] (1) Converter smelting + LF refining + VD refining: The LF furnace makes a reducing slag for desulfurization, reduces inclusions, and adjusts the composition; then the molten steel is degassed in the VD vacuum furnace, and the pressure holding time of the VD vacuum furnace is 15 - 20 min; ensure [H] ≤ 2 ppm and [O] ≤ 20 ppm;

[0015] (2) Continuous casting: Protect the whole process of pouring, and control the superheat temperature at 15 - 20°C; control the casting speed < 1 m / min;

[0016] (3) Slab slow cooling: The continuous casting slab enters the slow cooling pit for slow cooling, and the slow cooling time ≥ 48 h;

[0017] (4) Slab heating: The heating temperature is 1200 - 1250°C, and the holding time is 4 - 6 h;

[0018] (5) Controlled rolling: Adopt two-stage controlled rolling; the starting rolling temperature in the first stage ≥ 1050°C, and the single-pass reduction rate ≥ 15%; the starting rolling temperature in the second stage is 850 - 890°C, and the finishing rolling temperature is 800 ± 20°C;

[0019] (6) Normalizing + tempering heat treatment: The steel plate at room temperature is fed into a heating furnace, normalized at 880 - 940 °C for 2 - 4 min / mm, tempered at 580 - 640 °C for 4 - 6 min / mm, and finally air-cooled to room temperature.

[0020] The thickness of the finished steel plate is 6 - 50 mm.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) For the low-nickel cryogenic steel manufactured by the method of the present invention, the yield strength ≥ 360 MPa, the tensile strength ≥ 500 MPa, the elongation ≥ 30%, the impact energy KV at -60 °C 2 ≥ 150 J, and the lateral expansion ≥ 1.0 mm;

[0023] (2) Improve the quality of the continuous casting billet through high-purity smelting, thereby ensuring that the steel plate has good toughness;

[0024] (3) By adding Nb element, refine the grain and improve the strength of the steel;

[0025] (4) Achieve precise control of the rolling process through two-stage controlled rolling, and give full play to the microalloying effect of Nb;

[0026] (5) Through normalizing + tempering heat treatment, obtain a "ferrite + pearlite" structure, refine the grains, improve the strength and toughness of the steel, and ensure the stability of the structure.

[0027] (6) The manufactured low-nickel cryogenic steel can be applied in an environment with a temperature not lower than -60 °C, and is suitable for the construction of liquefied propane and liquefied propylene storage tanks. Description of the Drawings

[0028] Figure 1 is a metallographic structure photo (corroded by 4% nitric acid alcohol solution) of the steel plate manufactured in Example 1 magnified 500 times.

[0029] Figure 2 is a metallographic structure photo (corroded by 4% nitric acid alcohol solution) of the steel plate manufactured in Example 3 magnified 500 times. Detailed Embodiments

[0030] The chemical composition of the steel plate according to the present invention is by weight percentage: C: 0.10% - 0.20%, Si: 0.20% - 0.60%, Mn: 1.10% - 1.80%, Ni: 0.30% - 0.80%, Nb: 0.03% - 0.07, S ≤ 0.005%, P ≤ 0.008%, and the balance is Fe and unavoidable impurities.

[0031] The reasons for the composition design of the low-nickel cryogenic steel are as follows:

[0032] (1) Carbon: Carbon is the most effective chemical element for increasing the strength of steel. However, carbon also significantly reduces the toughness of steel and damages its weldability. After comprehensive consideration, the carbon content in this invention is controlled within 0.10% - 0.20%, which is suitable for low-nickel cryogenic steel.

[0033] (2) Silicon: Silicon can increase the strength of steel, but it is not beneficial to the toughness of steel. In this invention, its content is controlled within 0.20% - 0.60%.

[0034] (3) Manganese: Manganese can increase the strength and toughness of steel. However, when the manganese content is too high, it will promote grain growth and cause temper brittleness. In this invention, the manganese content is controlled within 1.10% - 1.80%.

[0035] (4) Nickel: Nickel can increase the strength of steel and endow the steel with excellent low-temperature toughness. Nickel is one of the elements that infinitely expand the austenite region. Therefore, after quenching and tempering treatment, high-nickel steel can obtain a fully refined tempered sorbite structure, making the strength and toughness of the steel well-matched. However, nickel is a scarce resource and expensive. In this invention, the nickel content is controlled within 0.30% - 0.80%.

[0036] (5) Niobium: Niobium can refine grains, reduce the overheating sensitivity and temper brittleness of steel, increase the strength of steel, and improve its weldability. In this invention, the niobium content is controlled within 0.03% - 0.07%.

[0037] (6) Sulfur: Sulfur easily forms FeS and MnS inclusions in steel, which will then cause hot brittleness and significantly reduce the toughness of steel. Therefore, the sulfur content in steel should be minimized. Considering cost issues, the sulfur content in this invention is controlled below 0.005%.

[0038] (7) Phosphorus: Phosphorus often segregates at grain boundaries in steel, destroying the continuity of the matrix, significantly reducing the toughness of steel, deteriorating the weldability, and being prone to cold brittleness. Therefore, the phosphorus content in steel should be minimized. Considering cost issues, the phosphorus content in this invention is controlled below 0.008%.

[0039] The manufacturing method of a low-nickel cryogenic steel described in this invention specifically includes the following steps:

[0040] (1) Converter smelting + LF refining + VD refining: The LF furnace makes a reducing slag for desulfurization, reduces inclusions, and adjusts the composition. Then the molten steel is degassed in a VD vacuum furnace, and the pressure-holding time of the VD furnace is 15 - 20 min. Finally, the H and O contents are measured to ensure [H] ≤ 2 ppm and [O] ≤ 20 ppm.

[0041] (2) Continuous casting: Protect the whole process of casting, control the superheat within 15 - 20 °C, and control the casting speed < 1 m / min; reduce secondary oxidation during continuous casting, reduce the inclusion content in steel, and improve the purity of steel.

[0042] (3) Slab slow cooling: The continuous casting slab is sent to the slow cooling pit for slow cooling to allow the gas to fully diffuse and escape, minimizing the gas content in the slab. The slow cooling time is ≥ 48 hours.

[0043] (4) Slab heating: The heating temperature is 1200 - 1250 °C, and the holding time is 4 - 6 hours to ensure that Nb is completely dissolved into the matrix and diffuses evenly.

[0044] (5) Controlled rolling: Two-stage controlled rolling is adopted. In the first stage, the rolling start temperature is ≥ 1050 °C, and the reduction per pass is maximized as much as possible, with the reduction rate per pass ≥ 15%. In the second stage, the rolling start temperature is 850 - 890 °C, and the finishing temperature is 800 ± 20 °C.

[0045] For rolling in the recrystallization zone above 1050 °C in the first stage, the reduction per pass should be maximized as much as possible. The purpose is to transform the original coarse austenite structure into a fine and uniform austenite structure through static recrystallization, giving full play to the strengthening effect of controlled rolling.

[0046] For rolling in the non-recrystallization zone at 850 - 890 °C, through the flattening of austenite and the increase in the number of intragranular deformation bands after rolling deformation, the effective grain boundary area for γ-α phase transformation nucleation is increased, and the phase transformation nucleation rate is increased, thus obtaining a refined structure. This is the core and key of Nb microalloying technology.

[0047] Lowering the finishing temperature can promote the precipitation of Nb(C, N) in austenite and prevent the growth of austenite grains. At the same time, due to controlled rolling in the non-recrystallization zone, the number of deformation bands in austenite grains can be increased, enabling ferrite to nucleate not only at grain boundaries but also on intragranular deformation bands. As the finishing temperature decreases, the ferrite grain size becomes finer. Therefore, the finishing temperature should be lowered as much as possible according to the mill's capacity. After comprehensive consideration, the finishing temperature in this invention is controlled at 800 ± 20 °C.

[0048] (6) Normalizing + tempering heat treatment: The room-temperature steel plate is sent into the heating furnace, normalized at 880 - 940 °C for 2 - 4 min / mm, tempered at 580 - 640 °C for 4 - 6 min / mm, and finally air-cooled to room temperature.

[0049] The purpose of normalizing is to obtain a "ferrite + pearlite" structure, refine the grains, and improve strength and toughness; the purpose of tempering is to improve the tissue stability and eliminate internal stress.

[0050] The low-nickel cryogenic steel of the present invention adopts a smelting process of "converter smelting + LF refining + VD refining" to ensure precise control of the steel grade composition and the gas content in the steel; the continuous casting process adopts full protection casting and slab slow cooling to ensure the internal quality of the cast slab; the refinement of the initial structure is ensured by controlling the rolling process; the "normalizing + tempering" heat treatment process is adopted to obtain a uniform and fine "ferrite + pearlite" structure, ultimately ensuring the strength and low-temperature toughness of the steel plate.

[0051] The metallographic structure of the finished steel plate is a ferrite + pearlite structure, and by volume percentage, the proportion of ferrite is 50% - 80%, and the proportion of pearlite is 20% - 50%.

[0052] The yield strength of the finished steel plate is ≥360 MPa, the tensile strength is ≥500 MPa, the elongation is ≥30%, and the impact energy KV at -60 °C 2 is ≥150 J, and the lateral expansion is ≥1.0 mm.

[0053] The thickness of the finished steel plate is 6 - 50 mm.

[0054] To more intuitively reflect the present invention, the implementation manners of the present invention will be further described in combination with embodiments. The following embodiments are only preferred specific implementation manners of the present invention, but the protection scope of the present invention is not limited thereto. Any technical solution that can be obviously obtained by those skilled in the art within the technical scope disclosed by the present invention, including simple changes or equivalent replacements, is within the protection scope of the present invention.

[0055]

Embodiment

[0056] The main chemical components in the steel of each embodiment are shown in Table 1; the smelting and continuous casting process parameters of each embodiment are shown in Table 2; the heating and rolling process parameters of each embodiment are shown in Table 3; the heat treatment process parameters of each embodiment are shown in Table 4; the transverse mechanical properties of the finished steel plates of each embodiment are shown in Table 5.

[0057] Table 1 Main chemical components in steel wt%

[0058] Example C Si Mn P S Ni Nb 1 0.13 0.52 1.65 0.005 0.002 0.60 0.07 2 0.10 0.40 1.62 0.005 0.002 0.80 0.05 3 0.20 0.20 1.10 0.005 0.002 0.30 0.03 4 0.15 0.35 1.35 0.005 0.002 0.50 0.05 5 0.12 0.60 1.80 0.005 0.002 0.45 0.06 6 0.16 0.43 1.45 0.005 0.002 0.72 0.03

[0059] Table 2 Steel smelting and continuous casting process parameters

[0060] Example Pouring superheat, °C Slab thickness, mm Slab casting speed, m / min Final thickness, mm 1 18 300 0.8 9 2 15 300 0.8 6 3 16 250 0.7 50 4 16 250 0.7 12 5 18 300 0.8 40 6 15 250 0.8 25

[0061] Table 3 Slab heating and rolling process parameters

[0062]

[0063] Table 4 Heat treatment process parameters

[0064] Example Normalizing temperature (°C) / holding time (mm / min) Tempering temperature (°C) / holding time (mm / min) 1 890 / 3 600 / 5 2 940 / 2 580 / 4 3 930 / 3 620 / 5 4 880 / 4 595 / 5 5 925 / 2 640 / 6 6 900 / 2 630 / 4

[0065] Table 5 Transverse Mechanical Properties of Finished Steel Plates

[0066]

[0067] The metallographic structure photograph of the steel plate manufactured in Example 1 (etched with 4% nitric acid alcohol solution) magnified 500 times is as Figure 1 shown. The metallographic structure photograph of the steel plate manufactured in Example 3 (etched with 4% nitric acid alcohol solution) magnified 500 times is as Figure 2 shown. The microscopic structure of the steel plate is ferrite + pearlite. As the carbon content in the steel increases, the amount of pearlite significantly increases and the grain size decreases.

[0068] As mentioned above, 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 low-nickel low-temperature steel, characterized in that: The chemical composition of the steel plate is C: 0.10% to 0.20%, Si: 0.20% to 0.60%, Mn: 1.10% to 1.80%, Ni: 0.30% to 0.80%, Nb: 0.03% to 0.07, S≤0.005%, P≤0.008%, and the balance is Fe and unavoidable impurities.

2. A low-nickel low-temperature steel according to claim 1, characterized in that: The metallographic structure of the finished steel plate is ferrite + pearlite structure, and in terms of volume percentage, ferrite accounts for 50% to 80%, and pearlite accounts for 20% to 50%.

3. The low-nickel and low-temperature steel according to claim 1, characterized in that: The yield strength of the finished steel plate is ≥360MPa, the tensile strength is ≥500MPa, the elongation is ≥30%, the impact energy KV2 at -60℃ is ≥150J, and the lateral expansion is ≥1.0mm.

4. A method for manufacturing low-nickel and low-temperature steel according to any one of claims 1 to 3, characterized in that: The steps include: (1) Converter smelting + LF refining + VD refining: Desulfurization of LF furnace-made reducing slag to reduce inclusions and adjust the composition; then the molten steel is degassed in a VD vacuum furnace, and the pressure holding time of the VD vacuum furnace is 15 to 20 minutes; ensure that [H] ≤ 2ppm, [O] ≤ 20ppm; (2) Continuous casting: Protect pouring throughout the process, control superheat at 15-20°C; control casting speed <1m / min; (3) Slow cooling of slab: The continuous casting slab is slowly cooled in the slow cooling pit, and the slow cooling time is ≥48h; (4) Slab heating: heating temperature is 1200-1250°C, holding time is 4-6h; (5) Controlled rolling: two-stage controlled rolling is adopted; the first-stage starting rolling temperature is ≥1050℃, and the single-pass reduction rate is ≥15%; the second-stage starting rolling temperature is 850~890℃, and the final rolling temperature is 800±20℃; (6) Normalizing + tempering heat treatment: The room temperature steel plate enters the heating furnace, is kept at 880-940℃ for 2-4min / mm for normalizing, is kept at 580-640℃ for 4-6min / mm for tempering, and finally air-cooled to room temperature.

5. The method for manufacturing low-nickel and low-temperature steel according to claim 4, characterized in that: The thickness of the finished steel plate is 6 to 50 mm.

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