Normalized pressure vessel steel plate for small crude oil storage tank and manufacturing method of normalized pressure vessel steel plate

Abstract

The invention relates to the technical field of steel, in particular to a normalized pressure vessel steel plate for a small crude oil storage tank and a manufacturing method of the normalized pressure vessel steel plate. The steel plate comprises the following chemical components in percentage by weight: 0.145 percent to 0.175 percent of C, 0.15 percent to 0.30 percent of Si, 0.80 percent to 0.90 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.003 percent of S, 0.01 percent to 0.02 percent of Nb, 0.010 percent to 0.025 percent of Ti, 0.015 percent to 0.025 percent of V, 0.020 percent to 0.050 percent of Alt and the balance of Fe and inevitable impurities. The process means of niobium-vanadium-titanium microalloying, continuous casting billet smelting, controlled rolling and controlled cooling and normalizing treatment is adopted, so that the product has good obdurability and is matched with a flat and straight plate shape, the internal quality and the steel purity of a continuous casting billet are greatly improved, and the pressure vessel steel plate product has good process performance and mechanical performance and is suitable for large-scale production. The device is widely applied to crude oil storage tank engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of steel, and in particular to a pressure vessel steel plate for a normalized small crude oil storage tank and a manufacturing method thereof. Background Art

[0002] As an important petrochemical storage and transportation equipment, the safety and reliability of small crude oil storage tanks are directly related to the stable operation of the entire production chain. As a key material for storage tanks, the quality of pressure vessel steel plates is even more important. However, in the existing industrial production process, the manufacture of pressure vessel steel plates for small crude oil storage tanks in normalized state faces a series of technical problems that need to be solved urgently.

[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 occur in finished steel plates during flaw detection, which seriously reduces the product qualification rate and reliability.

[0004] (2) Due to the uneven internal structure of the steel plate, unreasonable distribution of inclusions or the existence of microscopic defects, these weak points are prone to fracture or delamination during the stretching process, seriously affecting 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, increasing production costs 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 object of the present invention is to overcome the above problems and shortcomings and to provide a normalized pressure vessel steel plate for a small crude oil storage tank and a method for manufacturing the same.

[0008] The present invention adopts the process of niobium vanadium titanium micro-alloying + continuous casting billet smelting + controlled rolling and controlled cooling + normalizing treatment, and the internal quality of the continuous casting billet and the purity of the steel are greatly improved, so that the product has good strength and toughness matching and a flat plate shape.

[0009] In order to achieve the above object, the technical solution of the present invention is as follows:

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

[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%, and the rest is Fe and unavoidable impurities.

[0012] The reasons for adopting the above components are as follows:

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

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

[0015] (3) Mn: The Mn element can strengthen pearlite in steel by 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, the production cost will be increased. The Mn element is easy to combine with the S element to form MnS, which reduces the material's resistance to hydrogen-induced cracking. At the same time, too high a Mn content will reduce the activity of the carbon element. Therefore, the Mn content in the steel of the present invention is controlled at 0.80% to 0.90%.

[0016] (4) P: Phosphorus is a harmful element in steel. It increases the cold brittleness of steel, deteriorates welding performance, reduces plasticity, and deteriorates cold bending performance. In addition, P is particularly sensitive to radiation embrittlement. Therefore, the lower the P content in steel, the better. The present invention requires that the P content be less than 0.015%.

[0017] (5) S: S is usually a harmful element. S is usually easy to form brittle sulfides with alloy elements in steel, making the steel hot brittle 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 the present invention is controlled to be less than 0.003%.

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

[0019] (7) Nb: As a strong carbide-forming element, Nb forms a NbC phase with large dispersion and good high-temperature stability in steel, which plays a role in precipitation strengthening. Through multi-stage rolling, it can effectively refine the grains and improve the reduction of toughness caused by precipitation strengthening, so that the steel plate can obtain the comprehensive performance of high strength and high toughness. In addition, in the steel with Nb-Mo composite addition, Mo can also be segregated on the NbC matrix interface, preventing the coarsening of NbC particles, thereby greatly improving the high-temperature strength of the steel. The Nb content of the present invention 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 the heterogeneous nucleation core of needle-shaped pearlite during the solidification of the structure, thereby refining the structure. Ti also has a deoxidizing effect, ensuring that B is not oxidized and nitrided. B can reduce the transformation temperature from austenite to pearlite phase, promote the formation of needle-shaped pearlite in the grains, and play a role in refining the grains. However, when w(Ti) ≥ 0.09%, the needle-shaped pearlite content will be reduced, and the low-temperature toughness of the steel plate will deteriorate. Therefore, the Ti content in the steel of the present invention is controlled to be 0.010% to 0.025%.

[0021] (9) Alt: Adding a small amount of Al to steel can effectively refine austenite grains, thereby refining ferrite grains and structure, and improving the impact toughness of steel. However, the disadvantage of Al is that it affects the hot working performance, welding performance and cutting performance of steel. Therefore, the Alt content in the steel of the present invention is controlled at 0.020% to 0.050%.

[0022] In the above technical solution, further, 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°C.

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

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

[0025] (1) Continuous casting smelting:

[0026] RH inlet temperature is 1590~1620℃, RH oxygen blowing volume is 65~85m 3 / h, circulating oxygen blowing time 30 ~ 40min, effectively prevent secondary oxidation of molten steel to reduce inclusions, protection pouring from ladle to tundish, continuous casting billet thickness 200 ~ 250mm;

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

[0028] After heating the continuous casting billet, rough rolling is performed to open the billet. The rough rolling start temperature is 1100-1130°C, and the rough rolling final rolling temperature is 1030-1060°C. After rough rolling, the intermediate billet roller is reciprocated and air-cooled to wait for the temperature to reach 855-885°C for continuous multi-pass finishing rolling. The finishing rolling final rolling temperature is 815-845°C. After finishing rolling, the steel is thrown at a speed of 3-4m / s to reduce the temperature drop of the steel plate. Then, pre-straightening is performed. After pre-straightening, ACC laminar cooling is performed. The start cooling temperature is 750-800°C, so that the austenite grains can retain more storage energy. The cooling rate is 40-60°C / s water cooling, and the steel plate red-return temperature is 100-250°C. At the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar cooling to make the red-return temperature difference between the head and tail of the steel plate less than 20°C, and the unevenness is ≤8mm / m.

[0029] (3) Normalizing heat treatment:

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

[0031] (4) Straightening:

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

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

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

[0035] In the above technical solution, further, in step (1), the pouring temperature is 1530-1550°C, the pulling speed is (0.9-1.0) m·min -1 After smelting, the continuous casting billet is put into the slow cooling pit for slow cooling. The slow cooling temperature is 300-400℃ and the holding time is 12-24h.

[0036] In the above technical scheme, further, in step (1), the specific method of protective pouring from the ladle to the tundish is as follows: a. Adding a covering agent to the tundish is used to reduce the heat loss of the molten steel, isolate the air, reduce the secondary oxidation of the molten steel, and absorb the inclusions floating from the molten steel. SiO2 in the tundish slag is the most direct oxygen source. In order to prevent the increase of oxygen content in the molten steel, 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 in weight percentage: CaO 40-50%, SiO2 5%-10%, Al2O3 20%-30%, MgO 5% to 10%, the remainder is inevitable impurities; b. Use a hood-type protective pouring method, and use an argon protective hood to seal between the molten steel and the tundish to keep the molten steel in a protective atmosphere. The argon pressure is 0.3 to 0.6 MPa; c. The protective pouring from the tundish to the crystallizer adopts an immersed nozzle plus protective slag pouring, and the long nozzle is inserted into the molten steel in the tundish for 90 to 100 mm, which separates the molten steel from the air and avoids the secondary oxidation caused by the exposed splashing of the molten steel in the impact zone of the tundish.

[0037] In the above technical solution, further, in step (1), in order to make the slag system in the liquid phase, the crystallizer protection slag is composed of the following components in weight percentage: CaO 30% to 45%, Al2O3 25% to 40%, MnO 0 to 10%, SiO2 5% to 20%, and the balance is inevitable impurities; the thickness of the slag film is 0.1 to 1.5 mm, which ensures that the inclusions are fully floated and removed to prevent the generation of linear defects in the ingot.

[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°C, the temperature of heating stage II is 1000-1050°C, the temperature of heating stage III is 1210-1250°C, the temperature of the soaking stage is 1190-1230°C, the soaking stage time is 30-40min, and the total heating time is H*(1-1.3)min·mm -1 , H is the continuous casting billet thickness, mm; the appropriate heating system effectively controls the original austenite grain size and ensures sufficient solid solution of alloy elements, ensuring good final performance of the product.

[0039] In the above technical scheme, further, in step (2), rolling is carried out on a double-stand rolling mill, and longitudinal-transverse rolling or full longitudinal rolling is adopted in the rough rolling stage, 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 from pass to pass, so that it is greater than the critical deformation rate, so as to avoid the generation of mixed crystals, repeated deformation and repeated recrystallization of austenite grains; 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 from pass to pass. During the rolling process, the austenite is fully flattened and elongated along the rolling direction. The reduction of the reduction rate from pass to pass is beneficial to the plate shape control. After rolling in the austenite non-recrystallization zone, a large number of deformation bands and dislocations are formed in the deformed austenite grains, which increases the phase deformation nucleus position and the phase transformation driving force.

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

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

[0042] In the above technical solution, further, in step (2), during the pre-straightening process, according to the thickness of the finished steel plate, the roller gap setting parameters of the pre-straightening machine system are adjusted: the tilt correction range is 0 to +1.5mm, the roller gap correction range is ~1.5 to +0.5mm and the load reduction range is +0.5 to +1.0mm, 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 warping of the steel plate caused by local water accumulation, asymmetric cooling and uneven cooling during the water cooling process.

[0043] The beneficial effects of the present invention are:

[0044] 1. Use niobium, vanadium and titanium microalloying to control the expected transformation of the organizational structure and lay the foundation for the strength and toughness of the steel plate.

[0045] 2. By optimizing the protective pouring, slag system ratio and RH circulation process, the inclusion content in the steel can be reduced, the secondary oxidation of the molten steel can be effectively prevented, the purity of the steel can be improved, and the low-power quality of the continuous casting billet can be improved, laying the foundation for improving the flaw detection pass rate and reducing the tensile delamination rate. The center segregation (grade) is ≤B1.0, the center looseness is ≤1.0, and the non-metallic inclusions such as Class A (sulfide), Class B (alumina), Class C (silicate), Class D (spherical oxide) and Class DS (single-particle spherical) shall not be greater than Class 1.0.

[0046] 3. By controlling the rough rolling stage, the reduction rate is gradually increased. By increasing the reduction rate, the repeated deformation and recrystallization of austenite grains are promoted, and the grain size is made more uniform, thereby improving the overall performance of the material. The reduction rate of the finishing pass is gradually reduced to reduce the 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 fully uniform and refined structure and good plate straightness is obtained, and the unevenness after cold straightening is ≤5mm / m.

[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 strength and toughness of the product are well matched. 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is the metallographic structure of the steel plate in Example 1. DETAILED DESCRIPTION

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

[0050] Examples 1-6

[0051] The chemical compositions of the steel plates of Examples 1-6 of the present invention are shown in Table 1.

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

[0053]

[0054]

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

[0056]

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

[0058]

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

[0060]

[0061]

[0062] Table 5 Macroscopic evaluation grade and gas element content of continuous casting slabs of Examples 1-6 of the present invention

[0063]

[0064] Table 6 Continuous casting slab heating process parameters of 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 Rough rolling and finishing rolling reduction ratios of steel plates in Examples 1-6 of the present invention

[0070]

[0071] Table 9 Pre-straightening process parameters of steel plates in Examples 1-6 of the present invention

[0072]

[0073]

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

[0075]

[0076] Table 11 Normalizing heat treatment process parameters of 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 is composed of bainite, ferrite and pearlite.

[0084] In order to describe the present invention, the present invention is appropriately and fully illustrated by the examples in the above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A normalized small crude oil storage tank pressure vessel steel plate, characterized in that: The chemical composition of the steel plate is as follows by weight percentage: 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%, and the rest is Fe and unavoidable impurities.

2. The normalized small crude oil storage tank pressure vessel steel plate according to claim 1, characterized in that: 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°C.

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

4. A method for manufacturing a pressure vessel steel plate for a normalized small crude oil storage tank according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) Continuous casting smelting: RH inlet temperature is 1590~1620℃, RH oxygen blowing volume is 65~85m 3 / h, circulating oxygen blowing time 30-40min, protection pouring from ladle to tundish, continuous casting billet thickness 200-250mm; (2) Controlled rolling and controlled cooling: After heating the continuous casting billet, rough rolling is carried out to open the billet. The rough rolling start temperature is 1100-1130℃, the rough rolling final rolling temperature is 1030-1060℃, and after rough rolling, the intermediate billet roller is reciprocated and air-cooled to wait for the temperature to reach 855-885℃ for continuous multi-pass finishing rolling. The finishing rolling final rolling temperature is 815-845℃. After finishing rolling, the steel is cast at a speed of 3-4m / s, and then pre-straightening is carried out. After pre-straightening, ACC laminar cooling is carried out. The start cooling temperature is 750-800℃, and the cooling rate is 40-60℃ / s. Water cooling, the steel plate red-return temperature is 100-250℃. At the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar cooling, so that the red-return temperature difference between the head and tail of the steel plate is less than 20℃, and the unevenness is ≤8mm / m; (3) Normalizing heat treatment: The temperature is 880-910℃, and the net heat preservation is 20-120min. After the heat preservation time is up, air cool to room temperature; (4) Straightening: The unevenness of the finished steel plate after straightening is ≤5mm / m.

5. The manufacturing method according to claim 4, characterized in that: In step (1), during converter smelting, the charge block size is 50-100 mm, the charge hot charging temperature is 1150-1200° C., and the net insulation time is 3-6 minutes.

6. The manufacturing method according to claim 4, characterized in that: Casting temperature 1530~1550℃, pulling speed (0.9~1.0)m·min -1 After smelting, the continuous casting billet is put into the slow cooling pit for slow cooling. The slow cooling temperature is 300-400℃ and the holding time is 12-24h.

7. The manufacturing method according to claim 4, characterized in that: In step (1), the specific method of protective pouring from the ladle to the tundish is: a. Add covering agent to the tundish, the covering agent is composed of the following components in weight percentage: CaO 40%-50%, SiO25%-10%, Al2O3 20%-30%, MgO 5%-10%, and the balance is unavoidable impurities; b. Use hood-type protection for pouring, and use an argon protective cover to seal the molten steel and the tundish. The argon pressure is 0.3-0.6MPa; c. The protective pouring from the tundish to the crystallizer adopts an immersed nozzle plus protective slag pouring, and the long nozzle is inserted into the molten steel in the tundish for 90 to 100 mm.

8. The manufacturing method according to claim 4, characterized in that: In step (1), the mold protection slag is composed of the following components in weight percentage: CaO 30% to 45%, Al2O3 25% to 40%, MnO 0 to 10%, SiO2 5% to 20%, and the remainder is inevitable impurities; the thickness of the slag film is 0.1 to 1.5 mm.

9. The manufacturing method according to claim 4, characterized in that: The manufacturing method according to claim 4 is characterized in that in step (2), the heating adopts a four-stage heating method, the heating stage I temperature is 600-650°C, the heating stage II temperature is 1000-1050°C, the heating stage III temperature is 1210-1250°C, the soaking stage temperature is 1190-1230°C, the soaking stage time is 30-40min, and the total heating time is H*(1-1.3)min·mm -1 , H is the thickness of the continuous casting billet, mm.

10. The manufacturing method according to claim 4, characterized in that: In step (2), longitudinal-transverse rolling or full longitudinal rolling is adopted in the rough rolling stage, 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; 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.

Images