A low-temperature high-strength container steel with a tensile strength of ≥ 600 mpa and a production method thereof

Abstract

The application discloses a low-temperature high-strength container steel with tensile strength greater than or equal to 600 MPa, which comprises C, Si, Mn, P, S, Ni, Mo, Alt, Cr, V, Fe and residual elements; the steel plate obtained through controlled rolling and controlled cooling and normalizing and fast cooling has a main structure of 60-75% of bainite, 25-35% of ferrite and a small amount of pearlite, the yield strength of the heat-treated delivery state steel plate is 533-620 MPa, the tensile strength is 640-700 MPa, the elongation is greater than or equal to 19%, and the impact energy at-20 DEG C is 200-310 J; after maximum die welding at 580-620 DEG C for 10-12 h, the yield strength is 530-600 MPa, the tensile strength is 630-680 MPa, the elongation is greater than or equal to 20%, and the impact energy at-20 DEG C is 170-250 J. The scheme has low alloy cost and short production process, the obtained steel plate has stable performance, and after maximum die welding, the tensile strength and impact energy of the steel plate still meet the use requirements of low-temperature container steel.

Description

Technical Field

[0001] This invention applies to the field of medium and heavy plate production, specifically relating to a low-temperature high-strength container steel with a tensile strength ≥600MPa and its production method. Background Technology

[0002] Low-temperature high-strength vessel steel plates with a tensile strength ≥600MPa play an indispensable role in many important industries due to their excellent physical and chemical properties. Especially in the petroleum, chemical, power, and energy sectors, their high strength, high toughness, and good weldability make them the preferred material for manufacturing pressure vessels, boilers, storage tanks, and pipelines. However, after simulated post-weld heat treatment, both tensile strength and impact energy decrease to some extent. The decrease in tensile strength is typically between 30-50MPa, while the decrease in impact energy can reach 30-60%. More seriously, the instability of impact values ​​increases, even resulting in single-digit impact values. These changes undoubtedly have an adverse impact on the performance of the steel plates and bring greater challenges to equipment manufacturing and quality control.

[0003] Patent publication number CN110088339A discloses a "pressure vessel steel plate with excellent PWHT resistance and its manufacturing method," with the following composition by weight percentage: C: 0.10%–0.20%, Si: 0.15%–0.40%, Mn: 1.15%–1.50%, Mo: 0.45%–0.60%, Cu: 0.03%–0.30%, P≤0.025%, S≤0.025%, sol.Al: 0.005%–0.06%, Cr: 0.03%. The steel contains approximately 0.30% Nb, 0.002%–0.025% Zr, with the remainder being Fe and unavoidable impurities. The hot-rolled steel sheet is normalized at 820–950°C, followed by tempering at 550–680°C after cooling. After post-weld heat treatment at 600–660°C (630°C in all examples) for a maximum of 60 hours, its tensile strength is 550 MPa or higher, and its Charpy impact energy at -10°C is 100 J or higher. However, this steel contains a relatively high amount of the precious element Mo (0.45%–0.60%), and the additional tempering process in addition to normalizing results in higher production costs.

[0004] Patent CN108431272A discloses "a low-temperature pressure vessel steel plate with excellent resistance to PWHT and its manufacturing method." The steel plate maintains a tensile strength of ≥600 MPa or higher even after PWHT treatment at 580–640°C (630°C in the examples), and a Charpy impact energy of ≥200 J at -110°C. However, this steel contains a large amount of the precious element Ni (3.0%–4.0%) and a small amount of the precious element W (0.03%–0.25%). Furthermore, the hot-rolled steel plate undergoes a quenching followed by high-temperature tempering heat treatment, resulting in significantly higher production costs. Summary of the Invention

[0005] To address the aforementioned technical deficiencies, the present invention aims to provide a low-temperature high-strength container steel with a tensile strength ≥600MPa. This steel plate alloy has low cost, short production process, stable performance, and a tensile strength that can reach ≥600MPa.

[0006] Another objective of this invention is to provide a method for producing low-temperature high-strength container steel with a tensile strength ≥600MPa.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a low-temperature high-strength container steel with a tensile strength ≥600MPa, the steel plate thickness being 36mm~100mm, and employing the following chemical composition by mass percentage: C: 0.06~0.09, Si: 0.15~0.25, Mn: 1.35~1.48, P≤0.008, S≤0.003, Ni: 0.30~0.40, Mo: 0.16~0.30, Alt: 0.018~0.030, Cr: 0.15~0.20, V: 0.025~0.040, with the remainder being Fe and residual elements.

[0008] Carbon: As the carbon content increases, tensile strength increases, but impact energy decreases. To ensure impact energy, the carbon content needs to be appropriately reduced; therefore, the carbon content is limited to 0.06–0.09 wt%.

[0009] Silicon: In subsequent heat treatment processes, there is normalizing and rapid cooling, which requires increasing the Si content to increase the steel's ability to obtain martensite. However, high Si content can cause cracks in the steel plate after rolling. Therefore, the Si content is limited to 0.15% to 0.25 wt%.

[0010] Manganese: Increasing the Mn content can increase the number of Mn-substituted atoms in the matrix, thereby improving the strength of the steel plate. However, excessively high Mn content makes the banded structure more pronounced, reducing the low-temperature toughness of the steel plate. To ensure strength while improving impact toughness, Mn content needs to be set at the lower end of the range, limiting it to 1.35–1.48 wt%.

[0011] Nickel: Ni is an important component to ensure low-temperature impact toughness at the 1 / 2 thickness position. Increasing the Ni content will improve the stability of austenite and enhance the inhibition of ferrite and pearlite phase transformations. However, this steel does not require impact toughness at very low temperatures, and excessive Ni will increase the cost of the alloy. Therefore, the Ni content is limited to 0.30 to 0.40 wt%.

[0012] Molybdenum: Adding a small amount of Mo can promote the low-temperature bainitic phase transformation, resulting in more lath bainite in the final microstructure. Adding Mo can also promote the bainitic phase transformation kinetics under low-temperature conditions and effectively shorten the phase transformation time. Therefore, the Mo content is limited to 0.16-0.30 wt%.

[0013] Vanadium (V) can refine the microstructure and grain size of steel, thereby improving its strength and toughness. When vanadium dissolves into a solid solution at high temperatures, it increases the hardenability of the steel. It also improves the resistance to die welding of steel plates used in low-temperature, high-strength containers; therefore, the V content is limited to 0.25-0.40 wt%.

[0014] The production methods for low-temperature high-strength container steel with a tensile strength ≥600MPa include continuous casting, billet heating, controlled rolling and controlled cooling, and normalizing and rapid cooling, as detailed below:

[0015] ① Continuous casting: Use billets with a thickness of ≥350mm, and the low magnification of the billets reaches Class C 1.5 or above;

[0016] ② Billet heating: The billet temperature when loaded into the furnace is 250-600℃, the first heating temperature in the heating furnace is 860-1000℃, the second heating temperature is 1130-1150℃, the soaking zone heating temperature is 1220-1240℃, and the total heating time is 15-20 min / cm. This steel is a crack-sensitive steel, and excessive heating temperature will cause hot cracks.

[0017] ③ Controlled rolling and cooling: Two-stage rolling is adopted. The roughing rolling temperature is 1000-1020℃, the reduction per pass is controlled at 25-35mm, the deformation rate per pass is >14%, and the deformation coefficient for more than 3 passes is controlled at >0.55. The cumulative reduction in the first stage is 150-180mm. The finishing rolling temperature is 800-830℃, the deformation per pass is 25-30mm, and the deformation in the last pass is controlled at 1-3mm to level the steel plate. The final rolling temperature is 750-790℃. After rolling, the steel plate enters ACC cooling, the water ratio between the upper and lower layers is controlled at 1:1.2-1.4, and the reheating temperature is 630-650℃.

[0018] It should be noted that the recrystallization temperature of this steel is 845℃. The entire rough rolling process is carried out in the range above the austenite recrystallization temperature, promoting dynamic recrystallization, refining austenite grains, and forming recrystallized austenite. During the rough rolling and the subsequent drying process, the upper surface is in contact with air, and the lower surface is in contact with the roller table, resulting in a temperature difference in the thickness direction of the slab. The upper surface cools more slowly and does not undergo proeutectoid ferrite transformation, but compared with the core, its cooling rate is relatively faster. Therefore, the deformed austenite grains do not have time to fully recrystallize, retaining some coarse austenite grains. These coarse deformed austenite grains are retained in the subsequent finish rolling process. To avoid the formation of coarse bainite structure during the ACC water cooling process, which would result in uneven grain size distribution on the upper surface, the water-to-water ratio is optimized to be 1:1.2 to 1.4.

[0019] ④ Normalizing and rapid cooling: Normalizing temperature 865~885℃, holding time 1.5~2.0mm / min. After holding, the steel plate enters the quenching machine for rapid cooling, controlling the water temperature of the steel plate to 840~860℃, the cooling rate to 65~75℃ / S, and the red-hot temperature to 635~645℃. Then, it is air-cooled to room temperature.

[0020] This alloying method boasts low cost and a short production process. The steel plates obtained using this method have a microstructure primarily consisting of 60-75% bainite, 25-35% ferrite, and a small amount of pearlite. The heat-treated steel plates have a yield strength of 533-620 MPa, a tensile strength of 640-700 MPa, an elongation ≥19%, and an impact energy of 200-310 J at -20℃. After maximum die welding at 580-620℃ for 10-12 hours, the yield strength becomes 530-600 MPa, the tensile strength 630-680 MPa, the elongation ≥20%, and the impact energy at -20℃ 170-250 J. Even after maximum die welding, the tensile strength and impact energy of the steel plates still meet the requirements for cryogenic container steel. Attached Figure Description

[0021] The present invention will now be described in further detail with reference to the accompanying drawings.

[0022] Figure 1 This is a 500X metallographic image of the 60mm thick steel plate of the present invention at 1 / 4 of its thickness.

[0023] Figure 2 This is a 500X metallographic image of the 60mm thick steel plate of the present invention at half its thickness. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0025] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0026] Several types of low-temperature high-strength container steels with thicknesses ranging from 36mm to 100mm and tensile strengths ≥600MPa were produced using the chemical compositions (unit: wt%) shown in Table 1 below:

[0027] Table 1: Chemical composition (Wt, %) of steel for low-temperature high-strength containers with thicknesses ranging from 36mm to 100mm.

[0028] Example Thickness (mm) C Si Mn P S Als Ni Mo Cr V 1 36 0.08 0.25 1.40 0.007 0.003 0.025 0.35 0.25 0.22 0.035 2 60 0.09 0.25 1.40 0.007 0.003 0.025 0.35 0.25 0.24 0.035 3 100 0.09 0.25 1.40 0.007 0.003 0.025 0.35 0.25 0.25 0.035

[0029] The key processes in the steel production include converter smelting, LF refining, VD vacuum refining, continuous casting, billet heating, controlled rolling and cooling, and heat treatment, as detailed below:

[0030] (1) Converter smelting: P ≤ 0.007% and C ≤ 0.10% of the steel is tapped; slag must be blocked when tapping steel from the converter, and the slag layer thickness must be ≤ 65 mm; argon must be blown throughout the tapping process.

[0031] (2) LF refining: During the refining process, the white residue is kept for ≥20 min to fully desulfurize and control the finished product S≤0.003wt%; the finished product C content is controlled within the range of 0.08~0.10wt%.

[0032] (3) VD vacuum refining: The holding time is controlled at 15-25 min, and the hydrogen content is required to be ≤1.6ppm; the VD leaving station temperature ensures that the superheat of the molten steel in the ladle is 15-25℃.

[0033] (4) Continuous casting: Ensure that the billet thickness is at least 350mm, the superheat of the casting is controlled at 15-25℃, and the casting speed is 0.85-0.88m / min. Low magnification must be at or above Class C 1.0-1.5 level.

[0034] (5) Billet heating: The billet temperature in the furnace is 250-600℃, the heating temperature of the first heating furnace is 860-1000℃, the heating temperature of the second heating furnace is 1130-1150℃, the heating temperature of the soaking zone is 1220-1240℃, and the total heating time is 15-20 min / cm.

[0035] (6) Controlled rolling and cooling: Two-stage rolling is adopted. The roughing rolling temperature is 1000-1020℃, the reduction per pass is controlled at 25-35mm, the deformation rate per pass is >14%, and the deformation coefficient of more than 3 passes is controlled at >0.55. The cumulative reduction in the first stage is 150-180mm. The finishing rolling temperature is 800-830℃, the deformation per pass is 25-30mm, and the deformation of the last pass is controlled at 1-3mm to level the steel plate. The final rolling temperature is 750-790℃. After rolling, the steel plate enters ACC cooling, the water ratio between the upper and lower layers is controlled at 1:1.2-1.4, and the reheating temperature is 630-650℃.

[0036] (7) Normalizing and rapid cooling: Normalizing temperature 865~885℃, holding time 1.5~2.0mm / min, after holding, the steel plate enters the quenching machine for rapid cooling, controlling the water temperature of the steel plate to 840~860℃, the cooling rate to 65~75℃ / S, the red temperature to 635~645℃, and then air cooling to room temperature.

[0037] The steel plates obtained from the example were subjected to performance testing. The maximum mold welding holding temperature was 580–620℃, with a holding time of 10–12 hours; the minimum mold welding holding temperature was 580–620℃, with a holding time of 2–3 hours; the furnace loading and unloading temperature was ≤400℃; the heating and cooling rate was 50–80℃ / h; and air cooling was performed after unloading. Performance indicators are shown in Table 2.

[0038] Table 2: Mechanical properties of steel plates with tensile strength ≥600MPa for low-temperature high-strength containers, thickness 36mm~100mm

[0039]

[0040]

[0041] This trial production involved 10 batches each of 36mm, 60mm, and 100mm thick steel plates for low-temperature high-strength containers with a tensile strength ≥600MPa. Through reasonable chemical composition design and production process control, the microstructure of the steel plates was 60-75% bainite + 25-35% ferrite + a small amount of pearlite. The heat-treated steel plates had a yield strength of 533-620MPa, a tensile strength of 640-700MPa, an elongation ≥19%, and an impact energy of 200-310J at -20℃. After maximum die welding at 580-620℃ for 10-12 hours, the yield strength remained at 530-600MPa, the tensile strength at 630-680MPa, the elongation ≥20%, and the impact energy at -20℃ at 170-250J. Even after maximum die welding, the tensile strength and impact energy of the steel plates still met the requirements for low-temperature container steel.

Claims

1. A method for producing low-temperature high-strength container steel with a tensile strength ≥600MPa, characterized in that, The steel has a thickness of 36mm to 100mm and contains the following chemical composition by mass percentage: C: 0.06 to 0.09, Si: 0.15 to 0.25, Mn: 1.35 to 1.48, P≤0.008, S≤0.003, Ni: 0.30 to 0.40, Mo: 0.16 to 0.30, Alt: 0.018 to 0.030, Cr: 0.15 to 0.20, V: 0.025 to 0.040, with the remainder being Fe and residual elements; The main microstructure of the steel is 60-75% bainite, 25-35% ferrite, and a small amount of pearlite. The yield strength of the heat-treated steel plate is 533-620 MPa, the tensile strength is 640-700 MPa, the elongation is ≥19%, and the impact energy at -20℃ is 200-310 J. After maximum die welding at 580-620℃ for 10-12 hours, its yield strength is 530-600 MPa, the tensile strength is 630-680 MPa, the elongation is ≥20%, and the impact energy at -20℃ is 170-250 J. The production methods for the aforementioned low-temperature high-strength container steel with a tensile strength ≥600MPa include continuous casting, billet heating, controlled rolling and controlled cooling, and normalizing and rapid cooling, as detailed below: ① Continuous casting: Use billets with a thickness of ≥350mm, and the low magnification of the billets reaches Class C 1.5 or above; ② Billet heating: The billet temperature in the furnace is 250-600℃, the first heating temperature in the heating furnace is 860-1000℃, the second heating temperature is 1130-1150℃, the soaking zone heating temperature is 1220-1240℃, and the total heating time is 15-20 min / cm. ③ Controlled rolling and cooling: Two-stage rolling is adopted. The roughing rolling temperature is 1000-1020℃, the reduction per pass is controlled at 25-35mm, the deformation rate per pass is >14%, and the deformation coefficient for more than 3 passes is controlled at >0.

55. The cumulative reduction in the first stage is 150-180mm. The finishing rolling temperature is 800-830℃, the deformation per pass is 25-30mm, and the deformation in the last pass is controlled at 1-3mm to level the steel plate. The final rolling temperature is 750-790℃. After rolling, the steel plate enters ACC cooling, the water ratio is controlled at 1:1.2-1.4, and the reheating temperature is 630-650℃. ④ Normalizing and rapid cooling: Normalizing temperature 865~885℃, holding time 1.5~2.0mm / min. After holding, the steel plate enters the quenching machine for rapid cooling, controlling the water temperature of the steel plate to 840~860℃, the cooling rate to 65~75℃ / S, and the red-hot temperature to 635~645℃. Then, it is air-cooled to room temperature.

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