Steel plate with lamellar tearing resistance, low hardness and high toughness for quenched and tempered pressure vessel and production method of steel plate

Abstract

The invention discloses a lamellar-tear-resistant, low-hardness and high-toughness steel plate for a quenched and tempered pressure vessel and a production method of the steel plate. The steel plate comprises the following chemical components: C, Si, Mn, Cu, Ni, Mo, Nb, V, Ti, P, S, N, O, As, Ca and the balance of Fe and impurities. The production method comprises a strict smelting process, a rolling process and a heat treatment process, through component design and production process design, the thickness of the prepared finished steel plate is 36-60 mm, the yield strength is larger than or equal to 510 MPa, the tensile strength is 610-730 MPa, the ductility is larger than or equal to 18.0%, the-50 DEG C low-temperature impact KV2 at the 1 / 4 and 1 / 2 plate thickness position is larger than or equal to 160 J, the lamellar tearing resistance in the thickness direction is larger than or equal to 50%, the Vickers hardness HV10 of the surface of the steel plate is smaller than or equal to 220, and the steel plate is suitable for being used for manufacturing the steel plate. And the NDT drop hammer detection temperature is less than or equal to-50 DEG C. The steel has good lamellar tearing resistance, processability and toughness, can be used for manufacturing large spherical tanks and hydropower station pressure steel pipes, can be produced on a large scale, and has a wide market prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy steel manufacturing, and in particular to a steel plate for a quenched and tempered pressure vessel that is resistant to lamellar tearing and has low hardness and high toughness, and a production method thereof. Background Art

[0002] In recent years, with the continuous development of society, the demand for the energy industry has gradually increased, and the safety requirements for storage and transportation have become increasingly strict. In terms of storage, large spherical tanks, liquefied gas tanks, and fixed storage tanks are mainly used, and the main material used is pressure vessel steel. Depending on the storage medium, storage pressure, and storage environment, the materials used are divided into medium- and high-temperature pressure vessels, low-temperature pressure vessels, and quenched and tempered pressure vessels. The demand for quenched and tempered pressure vessels is increasing year by year due to their high strength and high toughness. At the same time, with the improvement of design requirements, steel plates have been put forward with excellent lamellar tear resistance and low hardness in processing while meeting mechanical properties.

[0003] Prior to the present invention, Chinese invention patent publication number CN 117626122 A disclosed a low-hardness quenched and tempered steel plate for pressure vessels and its production method. Its chemical composition and weight percentages are: C: 0.04-0.05%, Mn+Cr: 1.40-1.50%, Si: 0.20-0.30%, Ni: 0.60-0.80%, Nb+V: 0.050-0.070%, Mo: 0.20-0.30%, with the remainder being Fe and unavoidable impurities. This invention's steel plate has a hardness of ≤220 HBW, an impact energy of only ≥100 J at 1 / 4 and 1 / 2 of the plate thickness at -50°C, and its lamellar tearing resistance is unknown. However, this invention requires a holding period of 1240°C for 4-5 hours during the heating process of the ingot, significantly reducing production efficiency, increasing production costs, and making it inconvenient for mass production. Therefore, the development of higher-quality steel plate for pressure vessels and its production method has become an urgent issue in the industry. Summary of the Invention

[0004] The purpose of the present invention is to address the problem that the performance of existing quenched and tempered steel for pressure vessels is poor and cannot meet the current market performance requirements for pressure vessel steel. The present invention provides a quenched and tempered steel plate for pressure vessels that is resistant to lamellar tearing and has low hardness and high toughness, and a production method thereof. The steel produced by the method of the present invention has the characteristics of resistance to lamellar tearing, high strength, high plasticity, high toughness and low hardness, and can be used to manufacture large spherical tanks and hydropower station pressure pipes.

[0005] The present invention provides a tempered steel plate for a pressure vessel that is resistant to lamellar tearing and has low hardness and high toughness. The steel plate comprises the following components in weight percentage: C: 0.05-0.07%, Si: 0.15-0.35%, Mn: 1.40-1.60%, Cu: 0.10-0.20%, Ni: 0.40-0.60%, Mo: 0.20-0.50%, Nb: 0.020-0.050%, V: 0.020-0.050%, Ti: 0.010-0.030%, P≤0.007%, S≤0.002%, N≤35ppm, O≤20ppm, As≤0.006%, Ca / S≥1.2, and the rest are Fe and unavoidable impurities.

[0006] The finished steel plate has a thickness of 36 to 60 mm.

[0007] The steel plate has a yield strength of ≥510 MPa, a tensile strength of 610-730 MPa, an elongation of ≥18.0%, a -50°C low-temperature impact KV2 of ≥160J at 1 / 4 and 1 / 2 plate thickness, a lamellar tearing resistance of ≥50% in the thickness direction, a Vickers hardness HV10 of ≤220 on the steel plate surface, and an NDT drop hammer test temperature of ≤-50°C.

[0008] The present invention provides a method for producing a quenched and tempered steel plate for a pressure vessel that is resistant to lamellar tearing and has low hardness and high toughness, comprising the following steps: (1) Incoming molten iron: P content ≤ 0.100%, S content ≤ 0.030%, temperature ≥ 1350℃, no slag treatment required.

[0009] (2) Converter smelting: Control the smelting endpoint C≤0.04%, slag basicity 3.5~5.5, nickel pig iron and copper plate are added with scrap steel, nickel plate, ferroniobium, ferromanganese, ferrosilicon manganese and ferrovanadium are added when the tapping volume reaches 1 / 3, and slag plate is added when the tapping volume reaches 5 / 6. Spot blowing is strictly prohibited during the tapping process to avoid nitrogen addition; (3) Refining outside the furnace: argon blowing, refining time ≥30min, white slag basicity controlled at 5.5-6.5, fine-tuning the alloy; (4) Vacuum treatment: vacuum degree ≤ 67 Pa, ultimate vacuum holding time ≥ 10 min, feeding 300-500 m of Ca-Fe wire, soft blowing time ≥ 10 min; (5) Continuous casting: superheat 10-25℃, casting speed 0.80-0.95m / min, electromagnetic stirring is used in zones 2 and 3 of the continuous casting machine, and soft reduction is used in zones 8 and 9. The slow cooling time of the casting is not less than 48h; (6) Billet heating: heating rate ≥ 12 min / cm, soaking section heating temperature 1260 ± 30 °C, soaking section holding time ≥ 5 min / mm, furnace discharge temperature 1230 ~ 1280 °C, high-pressure water descaling is performed once or twice according to the surface quality of the billet after discharge; (7) Rolling and cooling: In stage I, the starting rolling temperature of rough rolling is ≥1170℃, and the rolling is performed in 2 to 4 passes. The reduction per pass is ≥20mm, and the cumulative reduction is ≥100mm. In stage II, the intermediate billet waiting temperature thickness is 2H+10mm, the starting rolling temperature is ≤890℃, and the final rolling temperature is 820-880℃. In stage II, the first and second to last passes are machine body descaling, and the last pass is a leveling pass. The steel throwing speed is ≥4m / s. After rolling, the steel plate is cooled by ACC, and the red-return temperature is controlled at 600-700℃. The H is the thickness of the finished steel plate in mm. (8) Quenching: The steel plates after rolling and cooling are quenched in a vacuum radiation heat treatment furnace. The quenching temperature is 900-940°C, and the holding time after reaching the temperature is ≥20 minutes. Then the temperature is lowered to 860-880°C, and the temperature is held for 10-20 minutes after reaching the temperature. After the steel plates are taken out of the furnace, they are water-cooled to room temperature. (9) Tempering: Tempering is carried out in a vacuum radiation heat treatment furnace at a temperature of 600-670°C for a furnace time of H+(70-90) min. The steel plate is air-cooled after being taken out of the furnace. The H is the thickness of the finished steel plate in mm.

[0010] The reasons for limiting the amounts of the chemical components and production process parameters of the present invention are described in detail below: 1. Influence of alloying elements on steel properties Carbon increases strength through solid solution strengthening but compromises low-temperature toughness. Carbon expands the austenite phase and lowers the phase transition temperature. Excessive carbon content can easily form Fe3C, increasing the proportion of pearlite in the steel and impairing weldability. Taking all performance factors into consideration, the present invention limits the carbon content to 0.05-0.07%.

[0011] Adding silicon as a reducing agent and deoxidizer during the steelmaking process can significantly improve the elastic limit, yield point and tensile strength of steel. However, too high Si content will reduce the welding performance of steel, so the Si content is limited to 0.15-0.35%.

[0012] Mn has a strong affinity with carbon and is an effective element for expanding the austenite phase, refining grains, and ensuring overall performance, thereby improving the hardenability of steel. However, Mn is also an element that easily segregates. When the Mn and C contents in the segregation zone reach a certain ratio, a martensite phase will form during steel production and welding. This phase exhibits very high hardness, significantly affecting the welding performance of equipment. Therefore, to ensure the low-temperature toughness of steel, the Mn content is limited to 1.40-1.60%.

[0013] Cu is an element with strong solid solution strengthening, which can improve hardenability. However, too high a content will damage the low-temperature toughness, so the Cu content is limited to 0.10-0.20%.

[0014] Ni can significantly improve the low-temperature toughness of steel, so the Ni content is limited to 0.40-0.60%.

[0015] Mo can reduce the overheating tendency of steel, improve strength and thermal stability, and reduce temper brittleness, so the Mo content is limited to 0.20-0.50%.

[0016] Niobium (Nb) significantly enhances the dynamic recrystallization of austenite and effectively refines the matrix grains. This grain refinement simultaneously improves the strength and low-temperature toughness of the steel plate. However, excessive Nb content increases the size of the second-phase particles, impairing weldability. Therefore, in the steel of this invention, a certain amount of Nb is added, within the range of 0.020% to 0.050%.

[0017] V is a carbide-forming element that effectively increases the strength of steel plates, second only to Nb and Ti in its effectiveness in steel. Adding V to steel forms V (CN), which increases the melting point, hardness, and wear resistance of cementite. V also improves tempering stability and undergoes dispersion strengthening at moderate temperatures, contributing to the core strength of thick steel plates. Therefore, the V content is limited to 0.020% to 0.050%.

[0018] Ti can precipitate fine TiN particles at higher temperatures to pin austenite grains, thereby limiting the growth of austenite grains and refining the austenite grain size. Therefore, the Ti content is limited to 0.010-0.030%.

[0019] P and S are harmful elements in steel. They increase the cold brittleness of steel, deteriorate welding performance, easily form defects such as segregation and inclusions, hinder grain boundary movement and grain growth. S is also prone to form strip-shaped MnS inclusions. Therefore, the lower the content in steel, the better. Considering production costs, P in steel is limited to 0.007% and S ≤ 0.002%.

[0020] As gaseous elements in steel, nitrogen and oxygen react with alloying elements to form nitrides, which are non-metallic inclusions. O has a negative impact on low-temperature toughness. Excessive oxygen in steel forms particles or inclusions with elements such as Nb and Ti, hindering grain growth, further impairing the toughness of the weld heat-affected zone, and reducing steel cleanliness. Therefore, the nitrogen and oxygen content in steel should be minimized. Considering production costs, the limit for nitrogen in steel is ≤35ppm, and the limit for oxygen in steel is ≤20ppm.

[0021] As is a harmful element in steel. It increases the cold brittleness of steel, deteriorates welding performance, and easily forms defects such as segregation and inclusions. Therefore, the lower the content in steel, the better. Therefore, As is limited to 0.006%.

[0022] 2. Reasons for setting production process parameters (1) Steelmaking process By controlling the phosphorus and sulfur content of incoming hot metal, the P and S content is reduced, reducing slag removal processes, lowering costs, and accelerating production. The C content of the tapped steel is controlled in a 130t top- and bottom-blown converter. Adding a certain percentage of nickel pig iron reduces alloy costs. The corresponding alloy is added when the tapping volume reaches 1 / 3 to effectively dissolve the alloy in the molten steel. Slag baffles are used to prevent excessive slag layers, and spot blowing is strictly prohibited to prevent oxygen enrichment. The refining time outside the furnace is controlled to ≥ 30 minutes to maximize desulfurization and deoxidation, preventing the deterioration of impact toughness due to high S and O contents. The vacuum process utilizes a cyclic degassing process, controlling the vacuum level and ensuring the maximum vacuum treatment time, effectively reducing impurity and gas content in the steel. Similar to the Ca-Fe line, the type of inclusions in the molten steel is modified, and soft blowing is performed to fully float the inclusions, ensuring high steel purity and low-temperature toughness. The superheat is controlled at 10-25°C and the casting speed at 0.80-0.95 m / min. The use of electromagnetic stirring, dynamic soft reduction and slow cooling of the billet is to reduce the looseness and segregation in the center of the billet and improve the internal quality.

[0023] (2) Steel rolling process The present invention controls the billet discharge temperature within a specific range to ensure full austenitization and homogenization of the billet, mitigate internal defects such as center segregation, and control the heating rate and holding time to achieve a certain depth of decarburization on the billet surface, thereby reducing the impact of martensite formed during subsequent quenching on the surface hardness of the steel plate. Controlling the start temperature, number of rolling passes, and reduction in stage I rough rolling promotes proper fragmentation of austenite grains. Controlling the start temperature, warming thickness, and final rolling temperature in stage II prevents the formation of mixed crystals and achieves an appropriate grain size.

[0024] (3) Heat treatment process The steel plate produced by this invention features high strength, lamellar tear resistance, low hardness, and high toughness. Therefore, a quenching and tempering heat treatment process is designed. During the quenching process, the steel plate is heated to the quenching temperature and then held at that temperature to ensure full austenitization of the core. The subsequent cooling phase reduces the surface temperature of the steel plate during quenching, utilizing the decarburized layer to prevent the formation of large amounts of martensite on the surface. Tempering is performed to reduce segregation in the steel plate and improve grain size uniformity.

[0025] Compared with the prior art, the technical solution provided by the present invention has the following advantages: The steel plates produced by the present invention were sampled and tested for room temperature tensile properties, -50℃ impact toughness, Z-direction properties, surface hardness, and NDT drop hammer. The test results show that the products produced by the composition and process of the present invention have good strength and toughness matching, and their Z-direction properties reach more than 53%, indicating good lamellar tear resistance; their surface hardness HV(10) is controlled within 220, and their elongation is ≥18%, indicating good processing performance; their -50℃ impact absorption energy at 1 / 4 and 1 / 2 of the plate thickness is both above 160J, and the NDT drop hammer temperature is ≤-50℃, indicating good toughness. They can be used to manufacture large spherical tanks and hydropower station penstocks, and can be mass-produced, with broad market prospects. DETAILED DESCRIPTION

[0026] In order to better explain the technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any form.

[0027] Table 1 below lists the chemical composition values ​​(wt%) of the steel plates of various embodiments of the present invention and comparative examples; Table 2 below is a list of the main process parameter values ​​for the smelting process of the steel plates of various embodiments of the present invention and comparative examples; Table 3 below lists the main process parameter values ​​for the rolling and heat treatment processes of the steel plates of various embodiments of the present invention and comparative examples; Table 4 below lists the main mechanical properties test results of the steel grades of various embodiments of the present invention and comparative examples.

[0028] A method for producing a lamellar tear-resistant, low-hardness, and high-toughness quenched and tempered steel plate for a pressure vessel according to various embodiments of the present invention comprises the following steps: (1) Incoming molten iron: P content ≤ 0.100%, S content ≤ 0.030%, temperature ≥ 1350℃, no slag treatment required.

[0029] (2) Converter smelting: Control the smelting endpoint C≤0.04%, slag basicity 3.5~5.5, nickel pig iron and copper plate are added with scrap steel, nickel plate, ferroniobium, ferromanganese, ferrosilicon manganese and ferrovanadium are added when the tapping volume reaches 1 / 3, and slag plate is added when the tapping volume reaches 5 / 6. Spot blowing is strictly prohibited during the tapping process to avoid nitrogen addition; (3) Refining outside the furnace: argon blowing, refining time ≥30min, white slag basicity controlled at 5.5-6.5, fine-tuning the alloy; (4) Vacuum treatment: vacuum degree ≤ 67 Pa, ultimate vacuum holding time ≥ 10 min, feeding 300-500 m of Ca-Fe wire, soft blowing time ≥ 10 min; (5) Continuous casting: superheat 10-25℃, casting speed 0.80-0.95m / min, electromagnetic stirring is used in zones 2 and 3 of the continuous casting machine, and soft reduction is used in zones 8 and 9. The slow cooling time of the casting is not less than 48h; (6) Billet heating: heating rate ≥ 12 min / cm, soaking section heating temperature 1260 ± 30 °C, soaking section holding time ≥ 5 min / mm, furnace discharge temperature 1230 ~ 1280 °C, high-pressure water descaling is performed once or twice according to the surface quality of the billet after discharge; (7) Rolling and cooling: In stage I, the starting rolling temperature of rough rolling is ≥1170℃, and the rolling is performed in 2 to 4 passes. The reduction per pass is ≥20mm, and the cumulative reduction is ≥100mm. In stage II, the intermediate billet waiting temperature thickness is 2H+10mm, the starting rolling temperature is ≤890℃, and the final rolling temperature is 820-880℃. In stage II, the first and second to last passes are machine body descaling, and the last pass is a leveling pass. The steel throwing speed is ≥4m / s. After rolling, the steel plate is cooled by ACC, and the red-return temperature is controlled at 600-700℃. The H is the thickness of the finished steel plate in mm. (8) Quenching: The steel plates after rolling and cooling are quenched in a vacuum radiation heat treatment furnace. The quenching temperature is 900-940°C, and the holding time after reaching the temperature is ≥20 minutes. Then the temperature is lowered to 860-880°C, and the temperature is held for 10-20 minutes after reaching the temperature. After the steel plates are taken out of the furnace, they are water-cooled to room temperature. (9) Tempering: Tempering is carried out in a vacuum radiation heat treatment furnace at a temperature of 600-670°C for a furnace time of H+(70-90) min. The steel plate is air-cooled after being taken out of the furnace. The H is the thickness of the finished steel plate in mm.

[0030] Table 1 Chemical composition values ​​of steel plates of various embodiments of the present invention and comparative examples (wt%)

[0031] Table 2 List of main process parameter values ​​of the smelting process of the steel plates of various embodiments of the present invention and comparative examples Table 3 List of main process parameter values ​​for rolling and heat treatment of steel plates in various embodiments of the present invention and comparative examples

[0032] Table 4 Main mechanical properties test results of steel grades in various embodiments of the present invention and comparative examples

[0033] As can be seen from Table 4 above, the steel plates of the embodiments of the present invention were sampled and tested for room temperature tensile properties, -50°C impact toughness, Z-axis properties, surface hardness and NDT drop hammer.

[0034] The test results show that the product produced according to the composition and process of the present invention has a good match between strength and toughness. Its Z-direction performance reaches more than 50%, indicating good resistance to lamellar tearing. Its surface hardness HV(10) is controlled within 220, and its elongation is ≥18%, indicating good processing performance. Its -50℃ impact absorption energy at 1 / 4 and 1 / 2 of the plate thickness is both above 160J, and the NDT drop hammer temperature is ≤-50℃, indicating good toughness. It can be used to manufacture large spherical tanks and hydropower station penstocks, and can be produced in large quantities. In contrast, the steel plate produced by the composition design and production process of Comparative Example 1 has various properties far inferior to the steel plate of the embodiment of the present invention.

[0035] The above embodiments are merely specific examples given by the present invention to explain the present invention and do not limit the present invention in any form. Any non-substantial changes made by anyone based on the above content and form that do not deviate from the scope of protection of the claims of the present invention should be deemed to fall within the scope of protection of the claims of the present invention.

Claims

1. A quenched and tempered steel plate for pressure vessels that is resistant to lamellar tearing and has low hardness and high toughness, characterized in that The steel plate contains the following components in weight percentage: C: 0.05-0.07%, Si: 0.15-0.35%, Mn: 1.40-1.60%, Cu: 0.10-0.20%, Ni: 0.40-0.60%, Mo: 0.20-0.50%, Nb: 0.020-0.050%, V: 0.020-0.050%, Ti: 0.010-0.030%, P≤0.007%, S≤0.002%, N≤35ppm, O≤20ppm, As≤0.006%, Ca / S≥1.2, and the rest are Fe and unavoidable impurities.

2. The lamellar tear resistant, low hardness, high toughness quenched and tempered steel plate for pressure vessels according to claim 1, characterized in that: The finished steel plate has a thickness of 36 to 60 mm.

3. The lamellar tear resistant, low hardness, high toughness quenched and tempered steel plate for pressure vessels according to claim 1 or 2, characterized in that: The steel plate has a yield strength of ≥510 MPa, a tensile strength of 610-730 MPa, an elongation of ≥18.0%, a -50°C low-temperature impact KV2 of ≥160J at 1 / 4 and 1 / 2 plate thickness, a lamellar tearing resistance of ≥50% in the thickness direction, a Vickers hardness HV10 of ≤220 on the steel plate surface, and an NDT drop hammer test temperature of ≤-50°C.

4. A method for producing a lamellar tear resistant, low hardness, high toughness quenched and tempered steel plate for pressure vessels according to claim 1, 2 or 3, characterized in that The steps include: (1) Incoming molten iron: P content ≤ 0.100%, S content ≤ 0.030%, temperature ≥ 1350℃, no slag treatment required; (2) Converter smelting: Control the smelting endpoint C≤0.04%, slag basicity 3.5~5.5, nickel pig iron and copper plate are added with scrap steel, nickel plate, ferroniobium, ferromanganese, ferrosilicon manganese and ferrovanadium are added when the tapping volume reaches 1 / 3, and slag plate is added when the tapping volume reaches 5 / 6. Spot blowing is strictly prohibited during the tapping process to avoid nitrogen addition; (3) Refining outside the furnace: argon blowing, refining time ≥30min, white slag basicity controlled at 5.5-6.5, fine-tuning the alloy; (4) Vacuum treatment: vacuum degree ≤ 67 Pa, ultimate vacuum holding time ≥ 10 min, feeding 300-500 m of Ca-Fe wire, soft blowing time ≥ 10 min; (5) Continuous casting: superheat 10-25℃, casting speed 0.80-0.95m / min, electromagnetic stirring is used in zones 2 and 3 of the continuous casting machine, and soft reduction is used in zones 8 and 9. The slow cooling time of the casting is not less than 48h; (6) Billet heating: heating rate ≥ 12 min / cm, soaking section heating temperature 1260 ± 30 °C, soaking section holding time ≥ 5 min / mm, furnace discharge temperature 1230 ~ 1280 °C, high-pressure water descaling is performed once or twice according to the surface quality of the billet after discharge; (7) Rolling and cooling: In stage I, the starting rolling temperature of rough rolling is ≥1170℃, and the rolling is performed in 2 to 4 passes. The reduction per pass is ≥20mm, and the cumulative reduction is ≥100mm. In stage II, the intermediate billet waiting temperature thickness is 2H+10mm, the starting rolling temperature is ≤890℃, and the final rolling temperature is 820-880℃. In stage II, the first and second to last passes are machine body descaling, and the last pass is a leveling pass. The steel throwing speed is ≥4m / s. After rolling, the steel plate is cooled by ACC, and the red-return temperature is controlled at 600-700℃. The H is the thickness of the finished steel plate in mm. (8) Quenching: The steel plates after rolling and cooling are quenched in a vacuum radiation heat treatment furnace. The quenching temperature is 900-940°C, and the holding time after reaching the temperature is ≥20 minutes. Then the temperature is lowered to 860-880°C, and the temperature is held for 10-20 minutes after reaching the temperature. After the steel plates are taken out of the furnace, they are water-cooled to room temperature. (9) Tempering: Tempering is carried out in a vacuum radiation heat treatment furnace at a temperature of 600-670°C for a furnace time of H+(70-90) min. The steel plate is air-cooled after being taken out of the furnace. The H is the thickness of the finished steel plate in mm.

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