A pressure vessel steel plate for a small, normally heated crude oil storage tank and its manufacturing method.

Abstract

This invention relates to the field of steel technology, and more particularly to a pressure vessel steel plate for a normalized small crude oil storage tank and its manufacturing method. The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.145%–0.175%, Si: 0.15%–0.30%, Mn: 0.80%–0.90%, P≤0.015%, S≤0.003%, Nb: 0.01%–0.02%, Ti: 0.010%–0.025%, V: 0.015%–0.025%, Alt: 0.020%–0.050%, with the remainder being Fe and unavoidable impurities. The present invention adopts the process of niobium, vanadium and titanium microalloying + continuous casting billet smelting + controlled rolling and controlled cooling + normalizing treatment, so that the product has good strength and toughness matching and a flat plate shape, and the internal quality of the continuous casting billet and the purity of the steel are greatly improved. The pressure vessel steel plate product of the present invention has good process performance and mechanical properties, and is widely used in crude oil storage tank projects.

Description

Technical Field

[0001] This invention relates to the field of steel technology, and more particularly to a pressure vessel steel plate for a normalized small crude oil storage tank and its manufacturing method. Background Technology

[0002] Small crude oil storage tanks are crucial petrochemical storage and transportation equipment, and their safety and reliability directly affect the stable operation of the entire production chain. Pressure vessel steel plates, as a key material for these tanks, are of paramount importance in terms of quality. However, in existing industrial production processes, the manufacture of pressure vessel steel plates for normalized small crude oil storage tanks faces a series of pressing technical challenges that need to be addressed.

[0003] (1) Due to factors such as insufficient purity of raw materials and improper control of inclusions during smelting and continuous casting, defects such as cracks and inclusions frequently appear in the finished steel plates during flaw detection, which seriously reduces the pass rate and reliability of the products.

[0004] (2) Due to uneven internal structure, unreasonable distribution of inclusions or micro-defects, these weak points are prone to fracture or delamination during the stretching process, which seriously affects the performance and safety of the steel plate.

[0005] (3) Poor board shape not only affects the appearance quality of the product, but also reduces its processing performance and subsequent use effect, and increases production cost and scrap rate.

[0006] Therefore, there is an urgent need for a new technology that can effectively solve these problems in order to improve product quality and production efficiency. Summary of the Invention

[0007] The purpose of this invention is to overcome the above-mentioned problems and deficiencies, and to provide a pressure vessel steel plate for a normalized small crude oil storage tank and its manufacturing method.

[0008] This invention employs a process of niobium-vanadium-titanium microalloying + continuous casting billet smelting + controlled rolling and cooling + normalizing treatment, which greatly improves the internal quality and steel purity of the continuous casting billet, resulting in products with good strength and toughness matching and a flat plate shape.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] This invention provides a pressure vessel steel plate for a normalized small crude oil storage tank, the chemical composition of which, by weight percentage, is as follows:

[0011] C: 0.145%–0.175%, Si: 0.15%–0.30%, Mn: 0.80%–0.90%, P≤0.015%, S≤0.003%, Nb: 0.01%–0.02%, Ti: 0.010%–0.025%, V: 0.015%–0.025%, Alt: 0.020%–0.050%, with the remainder being Fe and unavoidable impurities.

[0012] The reasons for using the above-mentioned components are as follows:

[0013] (1) C: C is the main component element of steel. The strength of steel mainly depends on the C content. Excessive C content will lead to poor toughness, plasticity and weldability of steel; low C content will lead to lower strength and performance after simulated stress relief treatment. In order to ensure that the steel plate has a good balance of low-temperature impact toughness, strength and weldability during use, the C content in the steel of this invention is controlled at 0.145% to 0.175%.

[0014] (2) Si: Si is a common solid solution strengthening alloying element in steel. It is essential for the strength, toughness, hardenability and even the deoxidation of steel. However, a high content will also lead to a decrease in the toughness of steel. Therefore, the Si content in the steel of this invention is controlled at 0.15% to 0.30%.

[0015] (3) Mn: Mn can strengthen pearlite in steel through solid solution strengthening. C-Mn strengthening is also the main way to improve the strength of low carbon steel. However, if the Mn content is too high, it will increase the production cost. Mn is easy to combine with S to form MnS, which will reduce the material's resistance to hydrogen-induced cracking. At the same time, the Mn content will reduce the activity of carbon. Therefore, the Mn content in the steel of this invention is controlled at 0.80% to 0.90%.

[0016] (4) P: Phosphorus is a harmful element in steel, increasing its cold brittleness, worsening its weldability, reducing its plasticity, and worsening its cold bending performance. Furthermore, P is particularly sensitive to radiation embrittlement. Therefore, the lower the P content in steel, the better; this invention requires it to be below 0.015%.

[0017] (5) S: Sulfur is a harmful element under normal circumstances. S usually forms brittle sulfides with alloying elements in steel, causing hot brittleness and reducing the ductility and toughness of the steel. At the same time, S also tends to accelerate radiation embrittlement. Therefore, the S content in the steel of this invention is controlled below 0.003%.

[0018] (6) V: V is a microalloying element. V microalloying in steel can form fine second phase particles, which can play the role of pinning grain boundaries and precipitation strengthening. It can effectively refine grains and greatly improve the comprehensive mechanical properties of steel such as strength, toughness, ductility and thermal fatigue resistance. The V content in the steel of this invention is controlled at 0.015% to 0.025%.

[0019] (7) Nb: As a strong carbide-forming element, Nb forms a highly dispersed NbC phase with good high-temperature stability in steel, playing a precipitation strengthening role. Through multi-stage rolling, it can effectively refine the grains and improve the reduction in toughness caused by precipitation strengthening, thereby enabling the steel plate to obtain comprehensive properties of high strength and high toughness. In addition, in Nb-Mo composite steel, Mo can also agglomerate at the NbC matrix interface, preventing the coarsening of NbC particles, thereby greatly improving the high-temperature strength of the steel. In this invention, the Nb content is controlled at 0.01% to 0.02%.

[0020] (8) Ti: Adding an appropriate amount of Ti can form a large number of dispersed fine TiN or Ti2O3 particles, which can serve as heterogeneous nucleation sites for acicular pearlite during solidification, thereby refining the microstructure. Ti also has a deoxidizing effect, ensuring that B is not oxidized or nitrided. B can lower the transformation temperature from austenite to pearlite, promoting the formation of acicular pearlite within the grains and refining the grains. However, when w(Ti)≥0.09%, the content of acicular pearlite will decrease, causing the low-temperature toughness of the steel plate to deteriorate. Therefore, the Ti content in the steel of this invention is controlled at 0.010% to 0.025%.

[0021] (9) Alt: Adding a small amount of Al to steel can effectively refine the austenite grains, thereby refining the ferrite grains and microstructure, and improving the impact toughness of the steel. However, Al has the disadvantage of affecting the hot working performance, weldability, and machinability of the steel. Therefore, the Alt content in the steel of this invention is controlled at 0.020% to 0.050%.

[0022] In the above technical solution, the finished steel plate has a tensile strength of 520-570 MPa, a yield strength of 400-450 MPa, an elongation after fracture of 19-24%, and an impact energy of 210-310 J at -40℃.

[0023] In the above technical solution, the thickness of the finished steel plate is further 6-26 mm.

[0024] Another aspect of the present invention provides a method for manufacturing the above-mentioned steel plate, comprising the following steps:

[0025] (1) Continuous casting billet smelting:

[0026] The RH inlet temperature is 1590–1620℃, and the RH oxygen blowing rate is 65–85 m³ / h. 3 / h, circulating oxygen blowing time 30-40min, effectively prevents secondary oxidation of molten steel, reduces inclusions, protects the casting from ladle to tundish, and the thickness of the continuously cast billet is 200-250mm;

[0027] (2) Controlled rolling and controlled cooling:

[0028] After heating, the continuously cast billet is rough rolled at a temperature of 1100–1130℃ and a final rolling temperature of 1030–1060℃. After rough rolling, the intermediate billet is air-cooled by reciprocating oscillation on the roller table until it reaches a temperature of 855–885℃, and then subjected to continuous multi-pass finishing rolling at a final rolling temperature of 815–845℃. After finishing rolling, the steel is shot at high speed at a speed of 3–4 m / s to reduce the temperature drop of the steel plate. Then, it is pre-straightened and then subjected to ACC laminar flow cooling at an initial cooling temperature of 750–800℃ to allow the austenite grains to retain more stored energy. Water cooling is used at a cooling rate of 40–60℃ / s, and the steel plate reddening temperature is 100–250℃. At the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar flow cooling to ensure that the temperature difference between the head and tail of the steel plate is less than 20℃ and the flatness is ≤8mm / m.

[0029] (3) Normalizing heat treatment:

[0030] The temperature is 880-910℃, and the net heat preservation time is 20-120 minutes. After the heat preservation time is up, air cool to room temperature.

[0031] (4) Straightening:

[0032] The flatness of the finished steel plate after straightening is ≤5mm / m.

[0033] In the above technical solution, further, in step (1), during converter smelting, the size of the furnace charge is 50-100mm, the hot charging temperature of the furnace charge is 1150-1200℃, and the net heat preservation time is 3-6min.

[0034] In the above technical solution, further, in step (1),

[0035] In the above technical solution, further, in step (1), the casting temperature is 1530~1550℃, and the casting speed is (0.9~1.0) m·min. -1 After smelting, the continuously cast billet is placed in a slow cooling pit for slow cooling at a temperature of 300-400℃ and a holding time of 12-24 hours.

[0036] In the above technical solution, further, in step (1), the specific method for protective casting from ladle to tundish is as follows: a. A covering agent is added to the tundish to reduce heat loss of molten steel, isolate air, reduce secondary oxidation of molten steel, and absorb inclusions floating in molten steel. SiO2 in tundish slag is the most direct oxygen source. To prevent the oxygen content in molten steel from increasing, the SiO2 content in the covering agent should be reduced as much as possible. Therefore, the covering agent used is composed of the following components by weight percentage: CaO 40-50%, SiO2 5%-10%, Al2O3 20%-30%, MgO 5%–10%, with the remainder being unavoidable impurities; b. A hood-type protective casting method is adopted, with an argon protective hood sealing the molten steel and tundish to keep the molten steel in a protective atmosphere at a pressure of 0.3–0.6 MPa; c. The protective casting from the tundish to the crystallizer uses an immersion nozzle with protective slag, inserting the long nozzle 90–100 mm into the molten steel in the tundish to isolate the molten steel from the air and prevent secondary oxidation caused by exposed molten steel splashing in the impact zone of the tundish.

[0037] In the above technical solution, further, in step (1), in order to keep the slag system in the liquid phase region, the mold protective slag is composed of the following components by weight percentage: CaO 30%~45%, Al2O3 25%~40%, MnO 0~10%, SiO2 5%~20%, with the balance being unavoidable impurities; the thickness of the slag film is 0.1~1.5mm, to ensure that the inclusions are fully floated and removed, and to prevent the generation of linear defects in the billet.

[0038] In the above technical solution, further, in step (2), the heating adopts a four-stage heating method: the temperature of heating stage I is 600-650℃, the temperature of heating stage II is 1000-1050℃, the temperature of heating stage III is 1210-1250℃, the temperature of the heat soaking stage is 1190-1230℃, the heat soaking stage time is 30-40min, and the total heating time is H*(1-1.3)min·mm. -1 H represents the thickness of the continuously cast billet, in mm; appropriate heating regime effectively controls the original austenite grain size and ensures sufficient solid solution of alloying elements, thus guaranteeing good final performance of the product.

[0039] In the above technical solution, further, in step (2), rolling is carried out in a double-stand rolling mill. In the roughing stage, longitudinal-transverse rolling or full longitudinal rolling is adopted. The thickness of the intermediate billet is 2 to 3 times the thickness of the finished steel plate. The reduction rate of each pass is 15% to 25%, and the reduction rate of each pass increases with each pass to make it greater than the critical deformation rate, so as to avoid the formation of mixed crystals. The austenite grains are repeatedly deformed and recrystallized. In the finishing rolling, full longitudinal rolling is adopted to the final finished product thickness. The reduction rate of each pass is 5% to 15%, and the reduction rate of each pass decreases with each pass. During the rolling process, the austenite is fully flattened and elongated along the rolling direction. The reduction rate of each pass decreases with each pass, which is conducive to plate shape control. After rolling in the non-recrystallized austenite zone, a large number of deformation bands and dislocations are formed in the deformed austenite grains, increasing the position of phase deformation nuclei and the driving force of phase transformation.

[0040] In the above technical solution, further, in step (2), during ACC laminar flow cooling, the head and tail of the steel plate are shielded according to the outlet plate type of the steel plate, and the shielding flow rate is 5% to 10%.

[0041] The principle of shielding control is: the temperature of the head and tail of the steel plate is close to that of the body; reduce the number of cooling manifold groups that use shielding control and shorten the flow adjustment time; through the use of ACC head and tail shielding function, the temperature uniformity of the steel plate is good and the plate shape is straight, which meets the requirements of the plate shape of the steel plate entering the furnace.

[0042] In the above technical solution, further, in step (2), during the pre-straightening process, the roller gap setting parameters of the pre-straightening machine system are adjusted according to the thickness of the finished steel plate: the tilt correction range is 0 to +1.5 mm, the roller gap correction range is ~1.5 to +0.5 mm, and the load reduction range is +0.5 to +1.0 mm, so that the steel plate after pre-straightening is straight and can smoothly enter the ACC area, avoiding a series of problems such as uneven temperature and plate warping caused by local water accumulation, asymmetrical cooling, and uneven cooling during the water cooling process.

[0043] The beneficial effects of this invention are as follows:

[0044] 1. By adopting niobium-vanadium-titanium microalloying, the expected transformation of the microstructure is controlled, laying the foundation for the strength and toughness of the steel plate.

[0045] 2. By optimizing the protective casting, slag ratio, and RH circulation process, the inclusion content in the steel is reduced, effectively preventing secondary oxidation of the molten steel, improving the purity of the steel, and improving the low-magnification quality of the continuously cast billet. This lays the foundation for improving the flaw detection pass rate and reducing the tensile delamination rate. The center segregation (grade) should be ≤B1.0, the center porosity should be ≤1.0, and the non-metallic inclusions such as A (sulfides), B (alumina), C (silicates), D (spherical oxides), and DS (single-particle spherical inclusions) should not exceed grade 1.0.

[0046] 3. By gradually increasing the reduction rate in the roughing stage, the repeated deformation and recrystallization of austenite grains are promoted, making the grain size more uniform and thus improving the overall performance of the material. The reduction rate in the finishing stage is gradually reduced, which reduces stress concentration and uneven deformation during the rolling process, making it easier to control the flatness and dimensional accuracy of the plate. Therefore, a product with a fully uniform and refined microstructure and a good flatness is obtained, with a flatness ≤5mm / m after cold straightening.

[0047] 4. The microstructure of bainite + ferrite + pearlite is obtained through normalizing heat treatment. The second phase is dispersed and the structure is uniform. The product has good strength and toughness. The tensile strength of the finished steel plate is 520-570MPa, the yield strength is 400-450MPa, the elongation after fracture is 19-24%, and the impact energy at -40℃ is 210-310J. Attached Figure Description

[0048] Figure 1 The metallographic structure of the steel plate in Example 1 is shown. Detailed Implementation

[0049] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0050] Examples 1-6

[0051] The chemical composition of the steel plates in Examples 1-6 of this invention is shown in Table 1.

[0052] Table 1. Chemical composition (wt%) of the steel plates in Examples 1-6 of this invention.

[0053]

[0054]

[0055] Table 2 Main process parameters for continuous casting billet smelting in Examples 1-6 of the present invention

[0056]

[0057] Table 3 Chemical composition and slag film thickness parameters of the crystallizer protective slag in Examples 1-6 of the present invention

[0058]

[0059] Table 4 Casting process parameters for Examples 1-6 of the present invention

[0060]

[0061]

[0062] Table 5. Low-magnification evaluation grades and gaseous element content of continuously cast billets in Examples 1-6 of the present invention.

[0063]

[0064] Table 6 Heating process parameters for continuous casting billets in Examples 1-6 of the present invention

[0065]

[0066]

[0067] Table 7 Steel plate rolling process parameters of Examples 1-6 of the present invention

[0068]

[0069] Table 8. Reduction rates of roughing and finishing passes for steel plates in Examples 1-6 of the present invention.

[0070]

[0071] Table 9. Steel plate pre-straightening process parameters in Examples 1-6 of the present invention.

[0072]

[0073]

[0074] Table 10 ACC laminar flow cooling process parameters for steel plates in Examples 1-6 of the present invention

[0075]

[0076] Table 11. Normalizing heat treatment process parameters for steel plates in Examples 1-6 of the present invention.

[0077]

[0078] Table 12 Comprehensive mechanical properties of steel plates in Examples 1-6 of the present invention

[0079]

[0080]

[0081] Table 13 Non-metallic inclusions in steel plates of Examples 1-6 of the present invention

[0082]

[0083] like Figure 1 As shown, the metallographic structure of the steel plate consists of bainite, ferrite and pearlite.

[0084] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A pressure vessel steel plate for a normalized small crude oil storage tank, characterized in that, The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.145%~0.175%, Si: 0.15%~0.30%, Mn: 0.80%~0.90%, P≤0.015%, S≤0.003%, Nb: 0.01%~0.02%, Ti: 0.010%~0.025%, V: 0.015%~0.025%, Alt: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities; The method for manufacturing the steel plate includes the following steps: (1) Continuous casting billet smelting: The RH inlet temperature is 1590~1620℃, and the RH oxygen blowing rate is 65~85m³. 3 / h, circulating oxygen blowing time 30~40min, protective casting from ladle to tundish, continuous casting billet thickness 200~250mm; (2) Controlled rolling and controlled cooling: After heating, the continuously cast billet is rough rolled at a temperature of 1100~1130℃ and a final rolling temperature of 1030~1060℃. After rough rolling, the intermediate billet is air-cooled by reciprocating oscillation on the roller table until it reaches a temperature of 855~885℃, and then subjected to continuous multi-pass finishing rolling at a final rolling temperature of 815~845℃. After finishing rolling, the steel is shot at high speed at a speed of 3~4m / s, followed by pre-straightening. After pre-straightening, it is subjected to ACC laminar flow cooling at a starting temperature of 750~800℃ and water cooling at a cooling rate of 40~60℃ / s. The steel plate reddening temperature is 100~250℃. At the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar flow cooling to ensure that the reddening temperature difference between the head and tail of the steel plate is less than 20℃ and the flatness is ≤8mm / m. (3) Normalizing heat treatment: The temperature is 880~910℃, and the net heat preservation time is 20~120 minutes. After the heat preservation time is reached, air cool to room temperature. (4) Straightening: The flatness of the straightened finished steel plate is ≤5mm / m; In step (2), the roughing stage adopts longitudinal-transverse rolling or full longitudinal rolling, the intermediate billet thickness is 2 to 3 times the finished steel plate thickness, the reduction rate of each pass is 15% to 25%, and the reduction rate of each pass increases with each pass; the finishing rolling adopts full longitudinal rolling to the final finished product thickness, the reduction rate of each pass is 5% to 15%, and the reduction rate of each pass decreases with each pass.

2. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, The finished steel plate has a tensile strength of 520~570MPa, a yield strength of 400~450MPa, an elongation after fracture of 19~24%, and an impact energy of 210~310J at -40℃.

3. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, The thickness of the finished steel plate is 6~26mm.

4. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, In step (1), during converter smelting, the furnace charge size is 50~100mm, the furnace charge hot charging temperature is 1150~1200℃, and the net heat preservation time is 3~6min.

5. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, Casting temperature 1530~1550℃, casting speed (0.9~1.0) m·min -1 After smelting, the continuously cast billet is placed in a slow cooling pit for slow cooling at a temperature of 300~400℃ and a holding time of 12~24h.

6. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, In step (1), the specific method for protective casting from the ladle to the tundish is as follows: a. A covering agent is added to the intermediate tundish. The covering agent consists of the following components by weight percentage: CaO 40%~50%, SiO2 5%~10%, Al2O3 20%~30%, MgO 5%~10%, with the balance being unavoidable impurities. b. A hood-type protective pouring method is adopted, and an argon gas protective hood is used to seal the molten steel and the tundish. The argon gas pressure is 0.3~0.6MPa. c. The protective casting from the tundish to the crystallizer adopts the submerged nozzle and protective slag casting method, inserting the long nozzle into the molten steel in the tundish for 90~100mm.

7. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, In step (1), the mold protective slag is composed of the following components by weight percentage: CaO 30%~45%, Al2O3 25%~40%, MnO 0~10%, SiO2 5%~20%, with the balance being unavoidable impurities; the thickness of the slag film is 0.1~1.5mm.

8. The pressure vessel steel plate for a normalized small crude oil storage tank according to claim 1, characterized in that, According to the manufacturing method of claim 4, the characteristic is that, in step (2), the heating adopts a four-stage heating method, with the heating stage I temperature at 600-650℃, the heating stage II temperature at 1000-1050℃, the heating stage III temperature at 1210-1250℃, the soaking stage temperature at 1190-1230℃, the soaking stage time at 30-40 min, and the total heating time being H×(1~1.3) min·mm. -1 H represents the thickness of the continuously cast billet, in mm.

Images