Low-temperature HRB500DW steel bar for large-size liquefied natural gas storage tank and preparation method of low-temperature HRB500DW steel bar

Abstract

The invention discloses a low-temperature HRB500DW reinforcing steel bar for a large-size liquefied natural gas storage tank and a preparation method of the low-temperature HRB500DW reinforcing steel bar, and relates to the technical field of steel production, and the low-temperature HRB500DW reinforcing steel bar comprises the following chemical components in percentage by mass: 0.06%-0.10% of C, 0.20%-0.35% of Si, 1.50%-1.60% of Mn, less than 0.010% of P, less than 0.008% of S, 1.00%-1.10% of Ni, 0.020%-0.040% of Al, 0.05%-0.09% of V, 0.0060%-0.0100% of N, less than or equal to 0.0002% of H and the balance of Fe and inevitable impurities. The low-temperature thread HRB500DW for liquefied natural gas with the specification of phi 32 mm is developed, the unnotched elongation Agt under the maximum low-temperature stretching force reaches 2.0%-4.0%, and the one-time success rate of low-temperature stretching is increased to 95% or above.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a low-temperature HRB500DW steel bar for a large-size liquefied natural gas storage tank and a preparation method thereof. Background Art

[0002] Low-temperature steel bar HRB500DW is used for the construction of prestressed concrete outer tanks of LNG liquefied natural gas storage tanks. It not only requires the steel bar to have high room temperature mechanical properties (lower yield strength 500-650MPa) and plasticity (elongation under maximum force Agt ≥ 5.0%), but also requires high elongation (notched Agt ≥ 1.0%, no-notch Agt ≥ 3.0%), yield strength ≥ 575MPa and crack sensitivity indicator NSR (notched tensile strength / no-notch lower yield strength) ≥ 1.0 in the ultra-low temperature harsh environment below -165℃, to avoid brittle fracture of steel bars under low temperature conditions, to ensure safety protection and personnel evacuation time in the event of a leakage accident, and is a safety barrier for LNG storage tanks, known as the "iron guard of LNG liquefied natural gas storage tanks". Therefore, low-temperature steel bars are mainly required to have high strength and good toughness and notch sensitivity at low temperatures. There are many factors that affect its low-temperature performance, mainly including steel purity, microalloying process, controlled rolling, controlled cooling, rolled material structure, processing and inspection of low-temperature tensile samples, etc., so the comprehensive technical difficulty is relatively high. In particular, the larger the diameter of the steel bar, the higher the requirements, such as requiring better uniformity of the entire cross-section of the steel bar, and requiring smaller coaxiality deviation between the end of the low-temperature tensile steel bar and the external screw. As a result, only a few steel mills in the world can produce this steel and conduct low-temperature tensile inspections, and the maximum specification is Φ28mm. However, with the increasing application of liquefied natural gas and the increasing demand, the capacity design of LNG storage tanks is also increasing, and the storage tanks need to withstand greater loads. Therefore, the production of Φ32mm specifications has become an urgent need for many LNG storage tank design units. It is particularly important to study how to meet the limit specifications when stretched at low temperatures below -165℃, with good toughness and notch sensitivity, as well as processing inspection. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the disadvantages of the prior art and provide a low-temperature HRB500DW steel bar for a large-size liquefied natural gas storage tank and a preparation method thereof.

[0004] In order to solve the above technical problems, the technical solution of the present invention is as follows: A low-temperature HRB500DW steel bar for a large-size liquefied natural gas storage tank, the chemical composition and mass percentage of which are: C: 0.06%-0.10%, Si: 0.20%-0.35%, Mn: 1.50%-1.60%, P < 0.010%, S: < 0.008%, Ni: 1.00%-1.10%, Al: 0.020%-0.040%, V: 0.05%-0.09%, N: 0.0060%-0.0100%, H≤0.0002%, and the balance is Fe and unavoidable impurities.

[0005] As a preferred solution of the low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks described in the present invention, its chemical composition and mass percentage are: C: 0.08%, Si: 0.26%, Mn: 1.55%, P < 0.010%, S: < 0.008%, Ni: 1.05%, Al: 0.030%, V: 0.07%, N: 0.0080%, H ≤ 0.0002%, and the balance is Fe and unavoidable impurities.

[0006] The present invention also provides a method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks, which comprises the following steps in sequence: electric furnace smelting, LF refining, VD refining, continuous casting, slow cooling or hot delivery of continuous casting billets, continuous casting billet testing, heating, rolling, and steel bar inspection.

[0007] As a preferred solution of the method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks of the present invention, the refining time of the LF refining step is greater than or equal to 30 minutes.

[0008] As a preferred solution of the method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks of the present invention, the vacuum degree of the VD refining process is less than or equal to ≤1 mbar, and the holding time is ≥10 min.

[0009] As a preferred solution of the method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks of the present invention, the superheat degree of the tundish in the continuous casting process is 10-30°C.

[0010] As a preferred solution of the method for preparing the low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks of the present invention, the heating temperature in the heating process is 1150-1200°C.

[0011] As a preferred embodiment of the method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to the present invention, the starting rolling temperature in the rolling process is 1045-1095°C, and after rolling, graded controlled cooling is performed using graded controlled cooling equipment, the temperature of the cooling bed on the steel bars after rolling is 550-600°C, and the rolling speed of the finished product is 6.0-7.0 m / s.

[0012] As a preferred solution of the method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks of the present invention, the hierarchical cooling control equipment includes a first cooling water pipe group, a second cooling water pipe group and a third cooling water pipe group, the first cooling water pipe group includes a group of one-stage water pipes and a group of two-stage water pipes, the second cooling water pipe group includes two groups of one-stage water pipes and two groups of two-stage water pipes, and the third cooling water pipe group includes three groups of one-stage water pipes and three groups of two-stage water pipes; The one group of one-section water pipes, the one group of two-section water pipes, the two groups of one-section water pipes, the two groups of two-section water pipes, the three groups of one-section water pipes and the three groups of two-section water pipes are arranged in sequence and their axes coincide. The lengths of the one group of one-section water pipes, the one group of two-section water pipes, the two groups of one-section water pipes, the two groups of two-section water pipes, the three groups of one-section water pipes and the three groups of two-section water pipes are all 1.4m. The spacing between the two sections of water pipes in any cooling water pipe group is 0.2m. The spacing between the one group of one-section water pipes and the three groups of two-section water pipes is 5.4m.

[0013] As a preferred embodiment of the method for preparing low-temperature HRB500DW steel bars for large-sized liquefied natural gas storage tanks according to the present invention, the method for preparing low-temperature HRB500DW steel bars for large-sized liquefied natural gas storage tanks according to claim 3 is characterized in that the steel bar inspection includes: Inspect the steel bar surface and weight deviation; Performing normal temperature stretching, low temperature stretching, normal temperature bending and reverse bending in sequence; Among them, in the low-temperature stretching process, after the end of the low-temperature steel bar is connected to the threaded connector by friction welding, the coaxiality deviation between the two is less than or equal to 0.8 mm.

[0014] The beneficial effects of the present invention are: The present invention develops the extreme specification φ32mm low-temperature thread HRB500DW for liquefied natural gas by optimizing alloy design, post-rolling controlled cooling process, and low-temperature tensile specimen processing process, providing feasibility for designing larger storage tanks for users such as Sinopec, PetroChina, and CNOOC. At the same time, the successful experience of developing the φ32mm specification from the entire process of component design, production process, and processing inspection is applied to other specifications smaller than φ32mm, so that the quality indicators of these specifications are significantly improved, such as the most critical indicator, the elongation Agt under the maximum force of low-temperature tensile without notch is increased from the normal 1.5%~3.5% to 2.0~4.0%, the no-notch is increased from 3.5~6.0% to 4.5~7.0%, and the low-temperature tensile one-time success rate is increased from an average of 75% to more than 95%, greatly reducing the inspection cycle and cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0016] Figure 1 Schematic diagram of the effect of VN and V reinforcement on performance; Figure 2 It is a schematic diagram of the steel part in the iron-carbon phase diagram; Figure 3 A schematic diagram of a graded cooling control device in a method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks provided by the present invention; Figure 4 This is the metallographic diagram of the original component production at 1 / 4 diameter of φ32mm specification; Figure 5 Schematic diagram of CCT curve designed for the original molybdenum-containing composition; Figure 6 Schematic diagram of CCT curve after optimizing composition design for vanadium replacing molybdenum; Figure 7 A schematic flow chart of a method for preparing low-temperature HRB500DW steel bars for large-sized liquefied natural gas storage tanks provided by the present invention; Figure 8 A schematic diagram of the temperature of the cooling bed on which any steel billet is rolled after the process of the present invention is collected by the system; Fig. 9 A schematic diagram of the temperature of the cooling bed on which any steel billet is rolled when the traditional process is used; Fig.10 It is a low-magnification comparison diagram of steel bars produced by the traditional process and the process of the present invention; Fig.11This is a schematic diagram showing the high magnification metallographic structure comparison at 1 / 4 diameter of a steel bar produced by a conventional process and the process of the present invention; Fig.12 This is a schematic diagram of the analysis of precipitates using ultra-high magnification transmission electron microscopy; Fig.13 This is a schematic diagram of the φ32 specification unnotched steel bars produced by the process of the present invention after low-temperature stretching. DETAILED DESCRIPTION

[0017] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific implementation modes and in combination with the accompanying drawings.

[0018] The present application provides a low-temperature HRB500DW steel bar for a large-size liquefied natural gas storage tank, whose chemical composition and mass percentage are: C: 0.06%~0.10%, Si: 0.20%~0.35%, Mn: 1.50%~1.60%, P<0.010%, S: <0.008%, Ni: 1.00%~1.10%, Al: 0.020%~0.040%, V: 0.05%~0.09%, N: 0.0060%~0.0100%, H≤0.0002%, and the balance is Fe and unavoidable impurities.

[0019] Among them, the determination principle of the above chemical composition and mass percentage is as follows: In view of the fact that the current molybdenum-containing composition design easily leads to the formation of supercooled structure B (bainite) in the steel bar, the study uses vanadium to replace molybdenum, and combines the steel composition to calculate the formula for room temperature tensile properties, as follows: Tensile strength Rm:

[0020] Lower yield strength Rel: Elongation A (%)

[0021] Where t q It is the quenching temperature, which specifically refers to the temperature of the water cooling bed after the steel bar is finally rolled, which is generally 570~600℃; Ac3 is the final temperature at which the first eutectoid ferrite is completely dissolved into austenite when the steel is heated. The general calculation formula based on the composition is as follows: Ac3=908-223.7C+438.5P+30.49Si+37.92V-34.4Mn-23Ni-200(C-0.54+0.06.Ni). At the same time, considering the combination of V and N, the influence on the room temperature performance of Φ16~Φ32mm steel bars is shown below Figure 1 .

[0022] Through the above three formulas and Figure 1 , the regulations for determining the main elements of this steel, such as C, Mn, Si, V, Al, Ni, N, H, etc. are as follows: C: The main element for high strength and hardenability, the C content must be above 0.06%. The higher the carbon content, the higher the strength of the steel, but the lower the plasticity, so the upper limit of the C content is no more than 0.10%, and the target value is 0.08%.

[0023] Si: It can improve the elastic limit of steel, but affects the cold working performance and has little effect on hardenability. It also affects the plastic elongation and cross-sectional shrinkage of steel. Therefore, its content is controlled above 0.20%, but not more than 0.35%, and the target value is 0.26%.

[0024] Mn: can improve the strength of steel, weaken and eliminate the adverse effects of sulfur, and improve the hardenability of steel. However, when the manganese content is high, there is a more obvious temper brittleness phenomenon and the effect of promoting grain growth. These disadvantages can be overcome by adding grain refinement elements such as molybdenum, vanadium, aluminum, titanium, etc. Since vanadium and aluminum are added in the present invention, more manganese can be added to maximize its advantages, so the content is controlled at 1.50-1.60%, and the target value is 1.55%.

[0025] P: It increases the cold brittleness of steel and is a harmful residual element. It forms micro segregation when the molten steel solidifies, increasing the delayed fracture sensitivity of the steel. Therefore, the content is controlled below 0.010%, and the goal is to keep it as low as possible.

[0026] S: Increases the hot brittleness of steel and deteriorates hot working properties; forms MnS inclusions (Type A inclusions) in the molten steel, deteriorating the cold working properties and delayed fracture properties of the steel, so its content is controlled below 0.008%, with the goal of being as low as possible.

[0027] Ni: It can significantly improve the low-temperature toughness of steel and is the most important element of this steel. However, since it is a precious metal, in order to control costs, it is more appropriate to control it at 1.00-1.10%, and the target value is 1.05%.

[0028] Al: It can improve the hardenability of steel and refine the grains without damaging the toughness of the steel. However, too much Al will affect the hot working performance and welding performance of the steel. Therefore, it should be controlled at 0.020-0.040%, with a target value of 0.030%.

[0029] V: can refine grains, improve the strength and toughness of steel, combined with Figure 1 The blue VN line in the middle does not accelerate cooling, and requires the tensile yield strength of the steel bar at room temperature to be between 500 and 650 MPa, so it should be controlled at 0.05-0.09%, with a target value of 0.07%.

[0030] N: It can form fine nitrides with Al and V in steel to refine the grains. Increasing the nitrogen content can increase the precipitation rate of vanadium. However, nitrogen can cause strain aging in steel and affect the performance of the steel bars. At the same time, if the nitrogen content is too high, the strength, strength-to-yield ratio and other properties of the steel are also very unstable. Therefore, it is controlled at 0.0060-0.0100%, with a target value of 0.0080%. The target value of V / N is 0.07 / 0.0080=8.75.

[0031] H: Hydrogen dissolved in steel can cause defects such as hydrogen embrittlement and white spots in the steel, and easily lead to low-temperature tensile brittle fracture, so it is controlled at ≤0.0002%.

[0032] The present application also provides a method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks, which comprises the following steps in sequence: electric furnace smelting, LF refining, VD refining, continuous casting, slow cooling or hot delivery of continuous casting billets, continuous casting billet testing, heating, rolling, and steel bar inspection, see Figure 7 .

[0033] Specifically, in the electric furnace smelting process, the electric furnace smelting steel is produced with the focus on controlling the carbon and phosphorus content, using an eccentric furnace bottom to produce steel, and proportioning slag testing to ensure that there is no slag inclusion in the molten steel.

[0034] LF furnace process: refining time ≥30min, using special refining slag and aluminum deoxidation to ensure the effective removal of inclusions in the molten steel, while adding corresponding alloys to ensure that the main elements are within the required range.

[0035] VD refining process: vacuum degree ≤ 1 mbar, holding time ≥ 10 min, effectively remove oxygen, nitrogen and hydrogen in the steel, feed high silicon nitride cored wire after vacuum to ensure that the nitrogen content is controlled within the required range.

[0036] Continuous casting process: The superheat of the tundish is controlled at 10-30℃, the pouring and casting speed are protected throughout the whole process, and the end electromagnetic stirring is used normally to ensure the good low-multiple quality of the continuous casting billet.

[0037] Slow cooling or hot delivery process of continuous casting billets: The continuous casting billets are first put into the pit for slow cooling. If the conditions for slow cooling in the pit are not met, they are piled up in the factory to avoid wind or hot delivered in an insulated car to ensure the slow release of hydrogen content and internal stress in the steel. Because the steel contains a high amount of Ni and Mn is also higher than general alloy structural steel, the internal stress of the billet is relatively large. At the same time, the hydrogen content is further released, which is beneficial to improving the low-temperature tensile properties.

[0038] Continuous casting slab inspection process: inspect the surface, curvature and internal quality of the slab (take a low-magnification sample for each stream) to ensure that the slab is qualified before rolling.

[0039] Heating process: The key to the billet heating process is to determine the billet heating temperature range, which is generally based on Figure 2(the steel part in the iron-carbon phase diagram) to preliminarily determine the maximum and minimum values ​​of the allowable heating temperature of the steel. Due to the different compositions and structures of various steels, their heating temperature ranges are also different. According to the changes in the structure of the steel during the heating process, the most suitable temperature range for carbon steel is the single-phase austenite region ( Figure 2 The heating temperature range of hypoeutectoid steel (carbon content <0.77%) is between 30~50℃ above Ac3 in the iron-carbon phase diagram and 100~150℃ below the solidus line HJE, generally between 800~1350℃; the maximum heating temperature of hypereutectoid steel should be 50~100℃ lower than the JE line.

[0040] Since the carbon content of this steel is far less than 0.77%, it belongs to hypoeutectoid steel, that is, the maximum temperature cannot exceed 1350℃. However, the heating temperature of the steel billet generally exceeds 1200℃, and the austenite grains grow rapidly, forming coarse grains, which seriously deteriorates the product performance. Although vanadium can inhibit the growth of austenite grains, manganese and phosphorus will promote the growth of austenite grains, so the maximum temperature cannot exceed 1200℃. However, it is necessary to ensure that vanadium has a high solid solution in the steel. The more solid solution, the more conducive it is to control the precipitation of V (C, N) in the steel bar and improve the product performance. The higher the billet temperature, the more conducive it is to the solid solution of vanadium, so the heating temperature must not be lower than 1150℃. Therefore, the suitable heating temperature range of low-temperature steel bar HRB500DW is 1150~1200℃.

[0041] Rolling process: rolling start temperature 1045 ~ 1095 ℃, steel bar upper cooling bed temperature 560 ~ 590 ℃, of which the same billet rolled steel bar upper cooling bed temperature ≤ 10 ℃, if it does not meet the requirements, by fine-tuning the roller speed of each section from the rolling to the upper cooling bed to ensure that the curvature is ≤ 2.5‰.

[0042] The optimization research of controlled cooling process after rolling is determined as follows: a) Realize graded cooling There are three groups of cooling water pipes for controlled cooling after rolling, each group is 3 meters long. In order to ensure relatively uniform cooling of the steel bars, only the 1st and 3rd sections can be cooled. The 2nd section cannot be cooled as a temperature recovery section. However, in this way, there is only one section for temperature recovery, and the total length is 3.4m, which is relatively short and the effect is slightly worse. It is easy to cause poor temperature uniformity of the steel bar, especially for the development of large specifications of φ32. Because the larger the specification, the greater the temperature difference between the core and the surface of the steel bar, it is necessary to try to increase the temperature recovery length or number of the steel bar.

[0043] In this embodiment, the original controlled cooling water pipe is transformed from one section of 3 meters long in each group to two sections of 1.5 meters each. The simplified schematic diagram after the transformation is shown in FIG. Figure 3, the total length remains unchanged, that is, 9.8 meters. After optimization and transformation, one group of one-stage water pipes, one group of two-stage water pipes and three groups of two-stage water pipes were opened, so that there were two temperature recovery sections: first, a short temperature recovery section with a length of 20cm was added between the first and second stages of the first cooling water pipe group; second, the length between the second stage of the first cooling water pipe group and the second stage of the third cooling water pipe group was 5.4 meters, and the φ32 specification final rolling speed was 6.0m / s, so the temperature recovery time of this section could reach nearly 1s, and the temperature recovery effect was significantly improved, realizing a simple graded controlled cooling process, so that the steel bar was cooled more evenly and the purpose of uniform organization was achieved.

[0044] b) Research and find the optimal cooling intensity When φ32 was first developed, the original molybdenum-containing and vanadium-free components were used, which easily led to the low-temperature tensile properties of -165℃ not meeting the requirements. Metallographic analysis showed that the steel bar had an overcooled structure of upper bainite (B 上 ), typical appearance see Figure 4 In the red box, this structure makes it easy for ferrite (F) strips to fracture brittlely, which easily leads to the failure of the plasticity index Agt tested at low temperature to meet the requirements. This is caused by excessive cooling after rolling, but if the cooling intensity is reduced to avoid the formation of this structure, the overall overcooling structure of the steel bar will be reduced, making the room-temperature yield strength of the steel bar unable to meet the required lower limit of 500MPa. The feasibility of replacing molybdenum with vanadium under similar cooling intensity is studied. First, the target values ​​of the two components are input into the classic thermodynamic calculation software Thermo-Calc, and their CCT curves (continuous cooling transformation curve of supercooled austenite) are calculated. The resulting curve is shown in Figure 5 and Figure 6 .

[0045] Figure 5 and Figure 6 Explanation of the key part labels in the figure: 1) The rightmost "Ferrite (1%)" is the ferrite transformation of 1%, "Pearlite (1%)" is the pearlite transformation of 1%, "Bainite (1%)" is the bainite transformation of 1%, "Austenite (1%)" is the austenite transformation of 1%, "Martensite start" is the martensite start transformation, "Martensite 50%" is the martensite transformation of 50%, and "Martensite 90%" is the martensite transformation of 90%. 2) The ordinate represents "temperature" and the abscissa represents "time", where the inverse "C" curves corresponding to "10", "100", "1000", "10000", and "100000" indicate that the cooling rates of the steel bars are "100℃ / s", "10℃ / s", "1℃ / s", "0.1℃ / s", and "0.01℃ / s" respectively.

[0046] Depend on Figure 5The "blue square" in the red box indicates the minimum cooling rate at which bainite transformation will not occur. The corresponding red arrow is about 1 / 3 between the cooling rates "0.1℃ / s" and "0.01℃ / s", and the cooling rate is about 0.03℃ / s, that is, as long as the cooling rate of the internal temperature of the steel bar exceeds 0.03℃ / s, bainite will be produced. This is actually inevitable, especially the lower the temperature and the larger the specification, the greater the impact. Figure 6 It is inferred that the minimum cooling rate at which bainite transformation will not occur is approximately between the cooling rates of "1℃ / s" and "10℃ / s" (red arrow), that is, 4.5℃ / s. In other words, as long as the cooling rate is less than 4.5℃ / s, bainite will not be produced inside the steel bar. Figure 6 The green square in the red circle corresponds to a cooling rate of 1°C / s, that is, pearlite transformation begins when the cooling rate is lower than this value. Figure 6 It is judged that the cooling rate must be less than 0.03℃ / s before pearlite transformation begins.

[0047] From the above analysis, it is known that the use of vanadium is more conducive to obtaining pearlite P and avoiding the appearance of bainite B. The characteristics of pearlite are both strong and tough, which is exactly the ideal structure pursued by this steel, especially for large specifications of φ32mm. Figure 7 Based on theoretical calculations, it was initially found that the temperature of the water pipe after rolling is 950~1000℃, the temperature of the cooling bed after rolling is 550~600℃, and the rolling speed of the finished product is 6.0~7.0m / s.

[0048] Steel bar inspection process: According to the requirements of the metallurgical standard YB / T 4641-2018 "Low-temperature steel bars for liquefied natural gas storage tanks", the steel bar surface, weight deviation, normal and low-temperature stretching, normal temperature bending and reverse bending are inspected. Among them, the most critical and core is the sample processed before the low-temperature stretching process, that is, when the steel bar end and the threaded connector are friction welded, the optimized specification is used to detect whether the two are aligned, and the specified top forging pressure, welding flow, pressure and time are used.

[0049] Among them, the research on reducing the coaxiality deviation between the end of the low-temperature tensile specimen steel bar and the threaded connection head is as follows: When the low-temperature steel bar and the threaded connector are friction welded into a specimen, the coaxiality deviation between the two is large (usually ≥1.5mm). During low-temperature stretching, the specimen is not only subjected to axial tension, but also to a transverse bending moment (equivalent to bending), and the closer it is to the weld. Therefore, during low-temperature stretching, the specimen basically breaks near the weld, generally within 20mm. The greater the coaxiality deviation, the greater the bending moment. In this way, the Agt tested is basically lower than the value that the steel bar itself should have. In severe cases, it breaks directly at the weld, resulting in low-temperature tensile strength and Agt failures, and re-inspection is required. For the developed extreme specification φ32mm, the tensile force reaches more than 650KN, which is more than 150KN larger than the current maximum φ28 specification in normal production. That is, under the same coaxiality between the low-temperature steel bar and the threaded connector, the greater the transverse bending moment (equivalent to bending) the steel bar is subjected to during low-temperature stretching, the easier it is for the fracture position to be close to the weld, and even brittle fracture at the weld, resulting in low-temperature stretching failure.

[0050] Through research, it is found that the control of coaxiality deviation mainly depends on the steel bar itself and the processing specifications, as follows: 1) Ensure that the curvature of the steel bar meets the national standard GB / T 1499.2-2018 of ≤4‰, and further reduce the curvature to reduce the difficulty of controlling the centering of the end and the threaded connector. Because the steel bar surface has transverse ribs and total ribs, the control difficulty during centering is significantly higher than that of round steel. The measures to control the curvature of steel bars are: a) Ensure that the heating temperature of the billet wire is ≤50℃, that is, within 1150~1200℃, and the target is controlled at 1175±15℃, to create conditions for the uniformity of the steel bar rolling process and cooling after rolling; b) Optimize the post-rolling controlled cooling process, that is, the above-mentioned Article 4.a; c) In actual production, explore the speed adjustment in the 4-section roller after the steel bar is rolled and cooled to the cooling bed, to ensure that a 12m long 165mm2 billet with a total rolling length of about 400m (before the cooling bed, cut into 6 pieces) of φ32mm The temperature fluctuation is ≤10℃. In this way, the steel bar is cooled evenly and the curvature can generally be controlled below 2.5‰, which is better.

[0051] 2) Detection of the main processes and quantification of parameters during friction welding between the end of the low-temperature steel bar and the threaded connector, such as detecting whether the two remain aligned, specifying the upsetting pressure, welding flow, pressure and time, etc.

[0052] 3) Through the above research, after the low-temperature steel bar end is friction welded with the threaded connector, the coaxiality deviation between the two is reduced from ≥1.5mm to ≤0.8mm.

[0053] The following is described by specific examples: Example: Rolling furnace 1 produces low-temperature thread HRB500DW with a specification of φ32mm. The main production process parameters of steelmaking and rolling, as well as performance inspection are as follows: 1) After the steel is smelted in an electric furnace, the main controlled elements C and P are 0.040% and 0.0045%, respectively. The slag test shows that there is no large slag inclusion in the molten steel, and the purity of the molten steel is good.

[0054] 2) LF furnace refining for 31 minutes. During the smelting process, the corresponding alloys were added to the main elements according to the target values, and refined lime was added to control the content of S and P. The results showed that C was 0.069%, Si was 0.25%, Mn was 1.54%, V was 0.068%, Ni was 1.04%, Al was 0.025%, S was 0.0049%, and P was 0.0052%.

[0055] 3) VD vacuum furnace refining, vacuum degree ≤ 1 mbar, holding time 14min. For main elements that do not meet the target value, the composition is further adjusted by feeding corresponding wires, such as carbon wire and aluminum wire. After fine-tuning the composition, C is 0.078%, Si is 0.25%, Mn is 1.54%, V is 0.068%, Ni is 1.04%, Al is 0.031%, S is 0.0036%, P is 0.0048%, N is 0.0078%, and H is 0.00017%.

[0056] 4) CCM continuous casting of 150mm*150mm square billets, through on-site scheduling to coordinate the upstream and downstream processes, control the production rhythm, ensure the whole furnace molten steel billet drawing speed 2.0 ~ 2.1m / min basic constant drawing speed control, molten steel superheat of 21 ~ 23 ℃, small temperature fluctuation, crystallizer electromagnetic stirring normal start, these aspects of control are conducive to the stability and consistency of the quality of the whole furnace castings, take a low magnification sample for each stream (a total of 5 streams).

[0057] 5) After the billet is slowly cooled in the pit for 24 hours, the surface and curvature are inspected. The above item 4) is required to be inspected at a low magnification. The results are in line with the national standards and the company's internal strict requirements.

[0058] 6) The bar mill rolls φ32mm HRB500DW. The hot section temperature of the billet is 1153~1194℃, the rolling temperature is 1052~1091℃, and the upper cooling bed temperature is 569~586℃. The upper cooling bed temperature of the same billet rolling steel bar is ≤6℃. Figure 8 In the yellow box, the temperature is controlled at 573-579℃, which is very beneficial to reducing the bending of the steel bars. The bending of the steel bars in the spot check is basically controlled at 2.1-2.4‰. However, the temperature fluctuation of the cooling bed on the same billet rolled steel bars produced by the original process is as high as 25℃. Fig. 9 In the red box, the temperature is controlled at 570-595°C, which results in the bending degree of the steel bars being basically controlled at around 4‰ as required by the national standard.

[0059] Figure 8The schematic diagram of the temperature of the cooling bed on any steel billet after the process of the present invention is collected by the system. Fig. 9 This is a schematic diagram of the temperature of the cooling bed on which any steel billet is rolled when using the traditional process.

[0060] See also Fig.10 The comparison of the steel bars produced by the traditional process and the process of the present invention is as follows: Fig.10 .a It can be seen that the thickness of the quench layer (white circle) at the edge of the steel bar in the red circle pointed by the two red arrows is obviously shorter than that at other parts, indicating that the cooling uniformity of the steel bar is slightly poor; Fig.10 .b The thickness of the white quenching layer on the edge of the entire specimen is relatively uniform, indicating that the cooling uniformity of the steel bar is good.

[0061] Fig.11 This is a schematic diagram of the high-magnification metallographic structure comparison of the 1 / 4 diameter of the steel bar produced by the traditional process and the process of the present invention. Fig.11 .a Known: The original process of producing steel bars is prone to produce upper bainite (B 上 ), in the red box in the figure, this structure makes it easy for ferrite (F) to break easily; there is also a needle-shaped F (in the red circle), this shape of structure will also deteriorate the plasticity of the steel bar. The steel bar produced by the present invention does not have B 上 And needle-shaped F, which is basically ferrite (F) and pearlite (P) in the form of clusters. In this way, when the steel bar is stretched by external force, the deformation in all directions is uniform, and of course the plasticity index will be very good.

[0062] Fig.12 This is a schematic diagram of the analysis of precipitates by ultra-high magnification transmission electron microscopy. By comparison, it is found that in this process, vanadium is used to replace molybdenum, and after finding the best ratio of vanadium and nitrogen, more V (C, N) precipitates are produced. The white dots in the figure give full play to the precipitation strengthening effect of vanadium, which is very beneficial to the high strength and toughness requirements of this steel.

[0063] The steel bars are tested for surface quality, room temperature stretching and bending, and low temperature stretching. The key indicator is the processing of the sample before low temperature stretching. According to the requirements of the present invention, it is tested whether the two are kept aligned. After the top forging pressure, welding flow, pressure and time are completed, after stretching in a low temperature box at -165~-170℃, the fracture of the steel bar without a notch has obvious necking, indicating that the steel bar has good plasticity. The distance between the fracture and the nearest welding end is more than 50cm. Fig.13 .

[0064] Table 1 shows the comparison of the normal temperature and low temperature tensile test results of the original process and the process of the present invention for the production of the limit specification φ32mm.

[0065] Table 1 It can be seen from Table 1 that the various indicators of room temperature and low temperature stretching of the limit specification φ32mm produced by the process of the present invention are better than those of the original process, and fully meet the requirements of standard YB / T 4641-2018. However, the original process is lower than the maximum force total extension without notch, the maximum force total extension with notch and the notch sensitivity index, which do not meet the requirements of standard YB / T 4641-2018.

[0066] In addition to the above embodiments, the present invention may also have other implementation modes; any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope required by the present invention.

Claims

1. A low-temperature HRB500DW steel bar for large-size liquefied natural gas storage tanks, characterized by: Its chemical composition and mass percentage are: C: 0.06%~0.10%, Si: 0.20%~0.35%, Mn: 1.50%~1.60%, P<0.010%, S: <0.008%, Ni: 1.00%~1.10%, Al: 0.020%~0.040%, V: 0.05%~0.09%, N: 0.0060%~0.0100%, H≤0.0002%, and the balance is Fe and unavoidable impurities.

2. The low-temperature HRB500DW steel bar for large-size liquefied natural gas storage tanks according to claim 1 is characterized in that: Its chemical composition and mass percentage are: C: 0.08%, Si: 0.26%, Mn: 1.55%, P < 0.010%, S: < 0.008%, Ni: 1.05%, Al: 0.030%, V: 0.07%, N: 0.0080%, H ≤ 0.0002%, and the balance is Fe and unavoidable impurities.

3. A method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 1 or 2, characterized in that: The process includes the following steps: electric furnace smelting, LF refining, VD refining, continuous casting, slow cooling or hot delivery of continuous casting billets, continuous casting billet testing, heating, rolling, and steel bar testing.

4. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 3, characterized in that: The refining time of the LF refining step is greater than or equal to 30 minutes.

5. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 3, characterized in that: The vacuum degree of the VD refining process is less than or equal to ≤1 mbar, and the holding time is ≥10 min.

6. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 3, characterized in that: The superheat degree of the tundish in the continuous casting process is 10-30°C.

7. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 3, characterized in that: The heating temperature in the heating process is 1150-1200°C.

8. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 3, characterized in that: In the rolling process, the starting rolling temperature is 1045-1095° C., and after rolling, graded controlled cooling equipment is used for graded controlled cooling. The temperature of the cooling bed on the steel bar after rolling is 550-600° C., and the rolling speed of the finished product is 6.0-7.0 m / s.

9. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 8, characterized in that: The hierarchical cooling control equipment comprises a first cooling water pipe group, a second cooling water pipe group and a third cooling water pipe group, wherein the first cooling water pipe group comprises a group of one-stage water pipes and a group of two-stage water pipes, the second cooling water pipe group comprises two groups of one-stage water pipes and two groups of two-stage water pipes, and the third cooling water pipe group comprises three groups of one-stage water pipes and three groups of two-stage water pipes; The one group of one-section water pipes, the one group of two-section water pipes, the two groups of one-section water pipes, the two groups of two-section water pipes, the three groups of one-section water pipes and the three groups of two-section water pipes are arranged in sequence and their axes coincide. The lengths of the one group of one-section water pipes, the one group of two-section water pipes, the two groups of one-section water pipes, the two groups of two-section water pipes, the three groups of one-section water pipes and the three groups of two-section water pipes are all 1.4m. The spacing between the two sections of water pipes in any cooling water pipe group is 0.2m. The spacing between the one group of one-section water pipes and the three groups of two-section water pipes is 5.4m.

10. The method for preparing low-temperature HRB500DW steel bars for large-size liquefied natural gas storage tanks according to claim 3, characterized in that: The steel bar inspection includes: Inspect the steel bar surface and weight deviation; Performing normal temperature stretching, low temperature stretching, normal temperature bending and reverse bending in sequence; Among them, in the low-temperature stretching process, after the end of the low-temperature steel bar is connected to the threaded connector by friction welding, the coaxiality deviation between the two is less than or equal to 0.8 mm.

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