Low hardness quenched and tempered steel plate for pressure vessels and method for producing the same

Abstract

The application discloses a low-hardness tempered steel plate for a pressure vessel and a production method thereof, and the chemical composition and mass percentage of the steel plate are as follows: 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%, and the rest is Fe and inevitable impurities; and the production method comprises the following steps: smelting, casting blank heating, rolling, on-line quenching and tempering heat treatment processes. The tempered steel plate provided by the application has a thickness of 30-50 mm, excellent conventional comprehensive performance, a hardness of less than or equal to 220 HBW, meets market demands, stable product quality and adaptability to mass production.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and in particular relates to a low-hardness quenched and tempered steel plate for pressure vessels and its production method. Background Technology

[0002] Quenching and tempering heat treatment, as an efficient method to improve the comprehensive performance of steel materials, not only enables the material to obtain high strength but also excellent plasticity and toughness. Therefore, quenched and tempered steel plates are widely used in the manufacture of key equipment in industries such as petroleum, chemical, marine structures, power plants, and boilers. In recent years, with the rapid development of industries such as petrochemicals and energy, the demand for quenched and tempered steel for pressure vessels has surged, and the design and technical requirements have become increasingly stringent. For example, cryogenic pressure vessels such as spherical tanks, oil and gas tanks, and liquefied gas tanks have stringent service requirements, needing to withstand high pressure and low temperature environments, and requiring high strength, high and low temperature toughness, and high plasticity. Secondly, the material processing and manufacturing process requires processes such as pressing and deformation, and hardness refers to the material surface's ability to resist the indentation of hard objects, so low hardness is also required to facilitate equipment processing and manufacturing.

[0003] Generally, materials with low strength also have low hardness, while materials with high strength also have high hardness. However, tempered steel plates typically have a matrix structure consisting of hard bainite or martensite, resulting in a relatively high level of strength and hardness. For steel plates with a yield strength of 490-550 MPa, the surface Brinell hardness is usually 250-280 HBW. Currently, there is virtually no production of tempered steel plates with a hardness below 220 HBW in the market, which cannot meet the growing market demand. This invention innovates the steel plate composition and the smelting, rolling, and heat treatment processes to provide a low-hardness tempered steel plate for pressure vessels and its production method. Summary of the Invention

[0004] The purpose of this invention is to provide a low-hardness quenched and tempered steel plate for pressure vessels and its production method. The steel plate provided has a thickness of 30-50 mm and its performance meets the high technical requirements for quenched and tempered steel plates for pressure vessels.

[0005] To achieve the above objectives, the technical solution provided by the present invention is as follows: A low-hardness quenched and tempered steel plate for pressure vessels has the following chemical composition and mass percentage: 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.

[0006] The steel plate described in this invention has a thickness of 30-50 mm.

[0007] The production method of the low-hardness quenched and tempered steel plate for pressure vessels according to the present invention includes smelting, billet heating, rolling, online quenching, and tempering heat treatment processes.

[0008] The smelting process described in this invention is as follows: a smelting process of converter primary refining + LF refining + VD vacuum degassing is adopted, and the steel produced by primary refining has a carbon content of ≤0.03%, achieving over-oxidation steel production; aluminum wire deep deoxidation is adopted, with a usage of 8-10 m / t, and is added in three stages: 6-7 m / t during LF refining, 1-2 m / t before VD vacuuming, and 1-2 m / t after VD vacuuming to ensure good deoxidation; soft blowing for 10-15 min.

[0009] The billet heating process of the present invention involves heating the billet to 1270-1280°C in the heating section for 1-1.5 hours, and then holding it at 1230-1240°C for 4-5 hours in the holding section. By using the process of rapid heating and extended holding time, the billet is fully heated and the microstructure does not coarsen.

[0010] The rolling process described in this invention adopts a two-stage controlled rolling process. The deformation in the roughing stage is 60-65%, and the reduction in the final pass is 30-35 mm at a temperature of 950℃-955℃. The starting rolling temperature of the finishing rolling is ≤850℃, and the finishing rolling temperature is ≤800℃.

[0011] In the online quenching process described in this invention, after the steel plate is rolled, it is first pre-straightened by a pre-straightening machine and straightened 1 to 3 times. When the surface temperature drops to 730 to 750°C, it is immediately put into a water cooling device and rapidly cooled to 280 to 300°C at a rate of 5 to 7°C / s. After exiting the water cooling device, it is air-cooled to room temperature.

[0012] The tempering heat treatment process described in this invention involves holding at a temperature of 650–670°C for 60–80 minutes, followed by air cooling after removal from the furnace.

[0013] The principles and beneficial effects of this invention are as follows: It uses an ultra-low carbon composition. Carbon (C) is one of the most important elements in steel, determining its strength and hardness. Low carbon content is a necessary condition for ensuring the low hardness of the steel plate.

[0014] Mn and Cr: As substitution elements, they play a role in solid solution strengthening; at the same time, Mn easily combines with O and S, and can also play an effective role in deoxidation and desulfurization.

[0015] Ni: can significantly improve the toughness of the steel matrix and is an essential element for ensuring the toughness of steel plates under high strength.

[0016] Mo: It plays a role in improving the hot strength of steel plates, so that the strength of steel plates does not decrease significantly after high-temperature tempering.

[0017] Nb and V are strong carbide-forming elements. They are used to prevent coarse microstructure during high-temperature heating of the billet and to refine the grain structure during controlled rolling.

[0018] Ultra-low carbon, well-deoxidized molten steel was obtained by using an oxidation process followed by batch-wise, quantitative aluminum feeding. Large-scale trials have demonstrated that proper batch-wise aluminum feeding can reduce the size and quantity of inclusions.

[0019] Rapid heating is used when heating the billet, which is fast and has a high nucleation rate, resulting in a finer original austenite structure. Low temperature and long-term heat preservation are used in the soaking zone to prevent austenite from growing and coarsening.

[0020] The process employs controlled rolling combined with online quenching. Rough rolling primarily involves large reduction rolling in the recrystallization zone to break up the austenite structure. Especially in the final pass, a large reduction is used at the critical temperature of the recrystallization zone, allowing the rolling force to be better transmitted to the core and homogenizing the microstructure along the thickness direction. Finish rolling utilizes low-temperature rolling, resulting in a steel plate with more deformation substructures.

[0021] After rolling, the steel sheet is pre-straightened. This serves two purposes: firstly, to improve the flatness of the sheet shape; and secondly, and most importantly, to delay its immersion in water. During air cooling, the equiaxed ferrite transformation occurs first, especially at the surface where the temperature drops faster, resulting in the highest ferrite content. This is crucial for achieving a low surface hardness in the steel sheet. When the steel sheet cools to a temperature slightly above AC1, it is then rapidly immersed in water for further cooling, forming a hard bainite structure. The final steel sheet exhibits a granular bainite + equiaxed ferrite structure with a grain size of 9-10. The microstructure is uniform along the thickness direction, and the grain size difference between the surface and the core is ≤1.

[0022] The steel plate obtained by the method of this invention has excellent comprehensive properties, with a tensile strength ReL ≥ 500 MPa and a tensile strength Rm of 610~730 MPa at room temperature; an impact energy ≥ 100 J at 1 / 4 and 1 / 2 of the plate thickness at -50℃; and a Brinell hardness HBW ≤ 220. It is suitable for mass production and has strong market competitiveness. Attached Figure Description

[0023] Figure 1 Metallographic structure of the steel plate in Example 1 (500X); Figure 2 Metallographic structure of steel plate in Example 2 (500X); Figure 3 Metallographic structure of steel plate in Example 3 (500X); Figure 4 Metallographic structure of steel plate in Example 4 (500X); Figure 5 Metallographic structure of steel plate in Example 5 (500X); Figure 6 Metallographic structure of steel plate in Example 6 (500X). Detailed Implementation

[0024] The technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0025] The chemical composition and mass percentage of the steel plates for low-hardness quenched and tempered pressure vessels in each embodiment are shown in Table 1.

[0026] Table 1 Chemical composition and mass percentage of each embodiment

[0027] In Table 1, the balance of chemical composition is Fe and unavoidable impurities.

[0028] The production methods for low-hardness quenched and tempered steel plates for pressure vessels in each embodiment include smelting, billet heating, rolling, online quenching, and tempering heat treatment processes, specifically: Smelting process: The smelting process adopts converter primary smelting + LF refining + VD vacuum degassing. The steel tapped from the primary smelting has a carbon content ≤ 0.03%, achieving over-oxidation. Deep deoxidation is performed using aluminum wire, with a dosage of 8-10 m³ / t, added in three stages: 6-7 m³ / t during LF refining, 1-2 m³ / t before VD vacuuming, and 1-2 m³ / t after VD vacuuming to ensure good deoxidation. Soft blowing lasts 10-15 minutes.

[0029] Heating process for the billet: In the heating section, the temperature is raised to 1270-1280℃ in 1-1.5 hours, and then held at 1230-1240℃ in the holding section for 4-5 hours.

[0030] Rolling process: Two-stage controlled rolling is adopted. The deformation in the roughing stage is 60-65%, and the reduction in the final pass is 30-35mm at a temperature of 950℃-955℃. The starting rolling temperature of the finishing rolling is ≤850℃, and the final rolling temperature is ≤800℃.

[0031] Online quenching process: After rolling, the steel plate is first pre-straightened in an advanced pre-straightening machine, and straightened back and forth 1 to 3 times. When the surface temperature drops to 730 to 750°C, it is immediately put into a water cooling device and rapidly cooled to 280 to 300°C at a rate of 5 to 7°C / s. After exiting the water cooling device, it is air-cooled to room temperature.

[0032] Tempering heat treatment process: holding temperature 650~670℃, holding time 60~80min, then air cooling after removal from the furnace.

[0033] The specific parameters for the smelting and billet heating processes in each embodiment are shown in Table 2, and the specific parameters for the steel plate rolling process, online quenching process, and tempering heat treatment process are shown in Table 3.

[0034] Table 2 Parameters of smelting and billet heating processes in each embodiment

[0035] Table 3 Heat treatment process parameters in each embodiment

[0036] The product standard for low-hardness quenched and tempered steel plates for pressure vessels refers to GB / T19189-2011. The metallographic structures of the steel plates obtained in each embodiment are shown below. Figures 1-6 As can be seen, it has a granular bainite + equiaxed ferrite structure with uniform structure in the thickness direction; the test results of grain size and mechanical properties are shown in Table 4.

[0037] Table 4. Test results of mechanical properties of steel plates obtained in each embodiment.

[0038] As can be seen from Table 4, the low-hardness tempered steel plate for pressure vessels provided by the present invention has stable quality and is suitable for mass production.

Claims

1. A low-hardness quenched and tempered steel plate for pressure vessels, characterized in that, The chemical composition and mass percentage of the steel plate are as follows: 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; The steel plate is 30-50 mm thick; The microstructure of the steel plate is granular bainite + equiaxed ferrite with a grain size of 9-10; the microstructure is uniform in the thickness direction and the grain size difference between the surface and the core of the steel plate is ≤1. The mechanical properties of the steel plate meet the following requirements: tensile strength ReL ≥ 500 MPa at room temperature, Rm: 610~730 MPa; impact energy at 1 / 4 and 1 / 2 of the plate thickness at -50℃ ≥ 100 J; Brinell hardness HBW ≤ 220. The steel plate production method includes smelting, billet heating, rolling, online quenching, and tempering heat treatment processes; In the online quenching process, after the steel plate is rolled, it is first pre-straightened by a pre-straightening machine and straightened 1 to 3 times. When the surface temperature drops to 730 to 750°C, it is immediately put into a water cooling device and rapidly cooled to 280 to 300°C at a rate of 5 to 7°C / s. The plate is then air-cooled to room temperature after exiting the water. The tempering heat treatment process involves holding at a temperature of 650–670°C for 60–80 minutes, followed by air cooling after removal from the furnace.

2. The method for producing low-hardness quenched and tempered steel plates for pressure vessels according to claim 1, characterized in that, The production method includes smelting, billet heating, rolling, online quenching, and tempering heat treatment processes; In the online quenching process, after the steel plate is rolled, it is first pre-straightened by a pre-straightening machine and straightened 1 to 3 times. When the surface temperature drops to 730 to 750°C, it is immediately put into a water cooling device and rapidly cooled to 280 to 300°C at a rate of 5 to 7°C / s. The plate is then air-cooled to room temperature after exiting the water. The tempering heat treatment process involves holding at a temperature of 650–670°C for 60–80 minutes, followed by air cooling after removal from the furnace.

3. The method for producing low-hardness quenched and tempered steel plates for pressure vessels according to claim 2, characterized in that, The smelting process adopts a smelting process of converter primary smelting + LF refining + VD vacuum degassing; the steel produced from the primary smelting has C≤0.03%, achieving over-oxidation; aluminum wire deep deoxidation is used, with a usage of 8-10 m / t, added in three stages: 6-7 m / t during LF refining, 1-2 m / t before VD vacuuming, and 1-2 m / t after VD vacuuming; soft blowing for 10-15 min.

4. The method for producing low-hardness quenched and tempered steel plates for pressure vessels according to claim 2, characterized in that, The billet heating process involves raising the temperature to 1270-1280°C in the heating section for 1-1.5 hours, and then holding it at 1230-1240°C for 4-5 hours in the holding section.

5. The method for producing low-hardness quenched and tempered steel plates for pressure vessels according to claim 2, characterized in that, The rolling process adopts a two-stage controlled rolling process. The deformation in the roughing stage is 60-65%, and the reduction in the final pass is 30-35 mm at a temperature of 950℃-955℃. The starting rolling temperature of the finishing rolling is ≤850℃, and the finishing rolling temperature is ≤800℃.

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