Low-temperature high-strength steel bar and method for manufacturing the same

Abstract

The application discloses a low-temperature high-strength steel bar and a preparation method thereof. The preparation method of the low-temperature high-strength steel bar comprises the following steps: selecting low-phosphorus molten iron with a phosphorus content of less than or equal to 0.14% to carry out smelting; carrying out converter smelting, controlling the final phosphorus content to be less than or equal to 0.009%, controlling the carbon content to be less than or equal to 0.05%, controlling the aluminum content to be 0.020%-0.035%, and controlling the nickel content to be 1%-1.15%; carrying out LF furnace refining, and controlling the sulfur content to be less than or equal to 0.01%; carrying out VD furnace refining, and removing hydrogen, oxygen and other gases; carrying out continuous casting to obtain a steel billet; and carrying out steel rolling to obtain the steel bar. According to the application, the cold brittleness of the steel bar is reduced as much as possible by means of low carbon, low phosphorus and low hydrogen, the cold brittleness tendency is reduced by means of appropriate aluminum, the low-temperature toughness of the steel bar is improved by means of high nickel, meanwhile, the low-temperature resistance of the steel bar is improved by means of a complex alloy system, the strength of the steel bar is further improved according to each alloy element, finally, the surface of the steel bar is formed with a tempered sorbite, and the core is formed with a complex structure of bainite + pearlite, so that the mechanical properties and the durability of the steel bar in a low-temperature environment of-165 DEG C are met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel smelting and processing, in particular to a low-temperature high-strength steel bar and a preparation method thereof. BACKGROUND

[0002] Strong liquefied natural gas (LNG) construction drives huge demand for low-temperature steel bars. With the rising status of natural gas energy, the country is also speeding up the development of large-scale liquefied natural gas storage tank technology and actively striving for the country's natural gas development road. Therefore, the low-temperature high-strength steel bar for LNG storage tank has a better development prospect in national construction.

[0003] Steel bars and concrete are important structural materials in liquefied natural gas storage tank construction. However, the low-temperature resistance of ordinary low-temperature steel bars on the market cannot meet the demand, so it is urgent to develop low-temperature high-strength steel bars suitable for LNG storage tanks.

[0004] Low-temperature steel bars serve in low-temperature environments. To prevent cold brittleness, the steel grade is strictly controlled for P. At present, the steel grade with P≤0.015% is strictly controlled for P, and the steel grade with P≤0.010% is difficult to control. Improper control can easily cause a large amount of waste products during production. Therefore, the present application aims to develop a low-P steel control process technology by improving and optimizing the process, controlling P throughout the smelting process, to meet the requirements of stable steel and batch production. SUMMARY

[0005] Therefore, it is necessary to propose a low-temperature high-strength steel bar and a preparation method thereof in view of the above technical problems.

[0006] In one aspect of the present application, a preparation method of a low-temperature high-strength steel bar is proposed, which comprises the following steps:

[0007] Low-phosphorus molten iron with a phosphorus content of ≤0.14% is selected for smelting;

[0008] Converter smelting, the low-phosphorus molten iron is smelted in a converter, the target phosphorus content of the molten iron is controlled to be ≤0.006% at the time of tapping, the converter is tapped by using a slide plate to block slag, the target phosphorus reversion is controlled to be ≤0.002% before and after the tapping, low-phosphorus iron alloy is selected for alloying, the alloy phosphorus reversion is controlled to be ≤0.001%, the final phosphorus content is controlled to be ≤0.009%, the carbon content is controlled to be ≤0.05%, the aluminum content is controlled to be 0.020%-0.035%, and the nickel content is controlled to be 1%-1.15%, to obtain low-phosphorus molten iron smelted by a converter;

[0009] LF furnace refining, the low-phosphorus molten iron smelted by the converter is placed in an LF furnace for refining, the sulfur content is controlled to be ≤0.01%, to obtain molten iron refined by the LF furnace;

[0010] VD furnace refining, placing the LF furnace refined liquid iron into a VD furnace for refining to remove hydrogen, oxygen and other gases to obtain VD furnace refined liquid iron;

[0011] continuous casting, placing the VD furnace refined liquid iron into a continuous casting machine for continuous casting to obtain a billet;

[0012] rolling, rolling the billet to obtain a steel bar.

[0013] In some embodiments, the converter smelting adopts high-ladle-slagging, large amount of slag, and double-slag smelting.

[0014] In some embodiments, the low-phosphorus iron alloy is metal manganese + ferrosilicon + nickel + vanadium-nitrogen alloy.

[0015] In some embodiments, in the VD furnace refining, the pressure is controlled to be greater than or equal to 67 Pa, the holding time is greater than or equal to 10 min, and the soft blowing time is greater than or equal to 15 min.

[0016] In some embodiments, the whole process of the continuous casting adopts protective casting, and the long nozzle and submerged entry nozzle are argon sealed and protected by the stopper flow control.

[0017] In some embodiments, the superheat degree of the continuous casting is controlled as a whole to be 40℃-52℃, the specific water amount is controlled to be 0.20L / kg-0.26L / kg, and the average pulling speed is controlled to be 1.8m / min-2.2m / min.

[0018] In some embodiments, the continuous casting adopts end electromagnetic stirring technology, and the temperature of the straightening and correcting is controlled to be 945℃-1030℃.

[0019] In some embodiments, in the continuous casting, the temperature of the billet when it is discharged is controlled to be 400℃, a layer of 450℃ ordinary HRB400 billet is laid in the lower layer of the slow cooling pit, and then the 400℃ billet is laid flat on the ordinary HRB400 billet, covered, and kept for 36h.

[0020] In some embodiments, the rolling adopts long-time high-temperature heating-high-temperature rolling-strong water cooling process, which includes the following steps: heating the billet in a heating furnace, keeping the heating section at 1020℃-1050℃ for 2.5h, controlling the opening rolling temperature to be 1000℃-1050℃, the finishing rolling temperature to be 900℃-990℃, and the temperature on the cooling bed to be 500℃-590℃.

[0021] In the second aspect of the present application, a low-temperature high-strength steel bar is also provided, which is prepared by any one of the above preparation methods of the low-temperature high-strength steel bar.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] By low carbon, low phosphorus, low hydrogen, the cold brittleness of the reinforcing bar is reduced as much as possible, the cold brittleness tendency is reduced by appropriate aluminum, the low temperature toughness of the reinforcing bar is improved by high nickel, at the same time, the low temperature resistance of the reinforcing bar is improved by a complex alloy system, and the strength of the reinforcing bar is further improved according to each alloy element, finally, the surface of the reinforcing bar is formed into a tempered sorbite, the core is a complex phase structure of bainite + pearlite by refining, continuous casting and rolling, the mechanical properties and the durability of the reinforcing bar in the low temperature environment of-165 DEG C are met. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The preparation method flow chart in the embodiment of the application. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0026] In the description of the application, it should be noted that the terms "upper", "lower", "inner", "outer", "front end", "rear end", "two ends", "one end", "the other end" and the like indicate the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore, it cannot be understood as a limitation on the application. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0027] In the description of the application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "provided with", "connection" and the like should be broadly understood, for example, "connection" can be fixed connection, can also be detachable connection, or integral connection; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0028] As described in the background, the strong liquefied natural gas (LNG) construction drives the huge demand for low-temperature reinforcing steel, with the rising status of natural gas energy, the state is also speeding up the research and development of large-scale liquefied natural gas storage tank technology, and actively striving for the development of the country's natural gas development road. The "14th Five-Year" plan proposes to develop high-quality, high-strength, and long-life steel products, so the low-temperature high-strength steel bar for LNG storage tank has a better development prospect in national construction. Reinforcing steel and concrete are important structural materials in the construction of liquefied natural gas storage tanks. However, the low-temperature performance of ordinary low-temperature steel bars on the market cannot meet the demand, so it is urgent to develop low-temperature high-strength steel bars suitable for LNG storage tanks. The service temperature of low-temperature steel is relatively low, in order to prevent cold brittleness, the steel grade is strictly controlled for P, and the steel grade with P≤0.015% is currently strictly controlled for P, and the steel grade with P≤0.010% is difficult to control, and improper control can easily cause a large amount of waste during production. Therefore, the present application aims to develop a low-P steel control process technology by improving and optimizing the process, controlling P throughout the smelting process, and meeting the requirements of stable steel and batch.

[0029] To improve the above problems, in one aspect of the present application, a preparation method of low-temperature high-strength steel bar is proposed, which mainly includes the following steps:

[0030] S1, low-phosphorus molten iron with a phosphorus content of ≤0.14% is selected for smelting.

[0031] In order to reduce the cold brittleness of the steel bar, it is crucial to select low-phosphorus molten iron as the raw material for smelting. In some embodiments, low-phosphorus molten iron with a phosphorus content of 0.12% can be selected.

[0032] S2, converter smelting

[0033] The low-phosphorus molten iron is smelted in a converter. In some embodiments, a high-ladle-replacement-blowing, large-slag-amount, and double-slag-smelting method can be adopted to control the target phosphorus content of the molten steel to be ≤0.006%, which helps to reduce the impurity content in the steel and improve the purity of the steel; further, in some embodiments, the converter tapping adopts a slide plate slag stopping, and the front and rear double stopping tapping is performed to control the target phosphorus reversion to be ≤0.002%; further, in some embodiments, low-phosphorus ferroalloy is selected for alloying, specifically metal manganese + aluminum + silicon iron + nickel + vanadium-nitrogen alloy, the alloy phosphorus reversion is controlled to be ≤0.001%, the final phosphorus content is controlled to be ≤0.009%, the carbon content is controlled to be ≤0.05%, the aluminum content is controlled to be 0.020%-0.035%, and the nickel content is controlled to be 1%-1.15%, to obtain low-phosphorus molten iron for converter smelting.

[0034] S3, LF furnace refining

[0035] The low phosphorus molten iron obtained by converter smelting is placed in an LF furnace for refining. The LF furnace refining can further reduce the sulfur content in the steel and improve the purity of the steel. In some embodiments, the sulfur content can be controlled to be ≤0.01%.

[0036] S4, VD furnace refining

[0037] The molten iron obtained by LF furnace refining is placed in a VD furnace for refining. The main purpose of VD furnace refining is to remove hydrogen, oxygen and other gases in the molten iron, and to improve the purity of the steel. In some embodiments, the pressure in the furnace can be controlled to be ≥67 Pa, the holding time is ≥10 min, and the soft blowing time is ≥15 min.

[0038] S5, continuous casting

[0039] The molten iron obtained by VD furnace refining is placed in a continuous casting machine for continuous casting to obtain a steel billet. In the embodiments, protective casting is used throughout the continuous casting process to prevent secondary oxidation of the molten steel. A carbon-free covering agent and carbonized rice husk are used in the tundish, and the existing low-carbon steel crystallizer is used for continuous casting. The long nozzle and submerged nozzle are argon sealed, and the flow is controlled by the stopper. At the same time, the superheat degree of continuous casting is controlled in the range of 40-52°C, the specific water amount is controlled in the range of 0.20-0.26 L / kg, and the average casting speed is controlled in the range of 1.8-2.2 m / min, which fully guarantees the surface quality of the continuous casting billet. End electromagnetic stirring technology can be used to improve the internal organization of the steel billet, remove inclusions, and improve segregation. The temperature of the straightening and correction is controlled in the range of 945-1030°C, which meets the requirements of the good plastic zone, reduces the generation of cracks in the straightening stage, and verified by production, the continuous casting billet has no off-square and rhombus, the pickling surface has no cracks, and the macro test piece has no cracks of all kinds. Further, in some embodiments, to prevent the occurrence of micro-cracks caused by rapid temperature drop after the steel billet is taken offline, the steel billet is required to enter the slow cooling pit for heat preservation treatment. Specifically, the temperature of the steel billet when taken offline is controlled at 400°C, a layer of 450°C ordinary HRB400 steel billet is laid in the lower layer of the slow cooling pit, and then the 400°C steel billet is laid flat on the ordinary HRB400 steel billet, covered, and heat preserved for 36 h.

[0040] S6, rolling

[0041] The billet is rolled to obtain the steel bar. In the embodiment, a long-time high-temperature heating-high-temperature rolling-strong water penetration process can be adopted. The specific steps include: heating the billet in a heating furnace, the heating section temperature is 1020-1050℃, the holding time is 2.5 hours, then the open rolling temperature is controlled to be 1000-1050℃, the entry finish rolling temperature is 900-990℃, and the temperature on the cooling bed is 500-590℃. According to the full-flow continuous temperature control rolling technology, according to the actual production equipment parameters of the rough, medium and finish rolling units of the steel bar rolling production line, the recrystallization, non-recrystallization and deformation induced ferrite mechanisms and post-rolling controlled cooling are comprehensively utilized. The purpose of controlling the grain growth and uniformizing the structure is achieved. The prestressed steel bar has obvious same requirements in surface size, mechanical properties, process performance and other aspects as the conventional GB1499.2 steel bar, and the quality requirement is very high. Compared with HRB400, the anchor rod force steel bar is a large-diameter, high-strength and high-precision straight steel bar with discontinuous external threads on the entire surface, which has the characteristics of simple connection and anchoring, strong adhesion, safety and reliability and the like.

[0042] Further, in some embodiments, the following technical route can also be adopted:

[0043] Converter→LF refining furnace→continuous casting machine→VD furnace→billet slow cooling→shot blasting flaw detection→billet inspection→rod three, rod five heating furnace heating→rough and medium rolling unit rolling→pre-water cooling section to achieve controlled cooling→finish rolling unit controlled rolling→post-rolling water cooling section controlled cooling→cut-to-length shearing→cold bed natural cooling→cold shearing to size→short length rejection→counting, finishing→bundling, weighing→hoisting, warehousing. The above technical route adopts the low-temperature high-strength steel bar controlled rolling and controlled cooling technology, so that the steel bar surface forms tempered sorbite and the core is a complex phase structure of bainite+pearlite, thereby improving the strength and toughness.

[0044] According to the above steps, the following is a detailed description of some specific embodiments:

[0045] Embodiment 1: Low-phosphorus molten iron with a phosphorus content of 0.12% is selected for smelting. High-ladle-replenishment blowing, large slag amount and double-slag smelting are adopted in the converter smelting process. The sulfur content is controlled to be 0.008% in the LF furnace refining. The VD furnace refining control pressure is 70Pa, the holding time is 12min, and the soft blowing time is 20min. The continuous casting process adopts protective casting, opens the long nozzle, and the immersion nozzle is argon sealed. The stopper controls the flow. The overheat degree of continuous casting is controlled at 45℃, the specific water amount is controlled at 0.22L / kg, and the average drawing speed is controlled at 2.0m / min. The end electromagnetic stirring technology is adopted, and the temperature control of straightening and correction is 980℃. The steel rolling adopts a long-time high-temperature heating-high-temperature rolling-strong water penetration process, the heating section temperature is 1030℃, the holding time is 2.5 hours, the open rolling temperature is 1020℃, the entry finish rolling temperature is 920℃, and the temperature on the cooling bed is 550℃.

[0046] Example 2: Low phosphorus molten iron with a phosphorus content of 0.14% is selected for smelting. High pull compensation blowing, large slag amount, and double slag smelting are adopted in the converter smelting process. The sulfur content is controlled to be 0.009% in the LF furnace refining. The pressure is controlled to be 75 Pa in the VD furnace refining, the holding time is 15 min, and the soft blowing time is 25 min. The continuous casting process adopts protective casting, and the long nozzle, submerged entry nozzle argon sealing protection is opened. The stopper controls the flow. The overheat degree is controlled to be 52 ℃ in the continuous casting, the specific water amount is controlled to be 0.26 L / kg, and the average pulling speed is controlled to be 2.2 m / min. The end electromagnetic stirring technology is adopted, and the temperature is controlled to be 1030 ℃ in the pulling and straightening. The rolling adopts long time high temperature heating-high temperature rolling-strong water cooling process. The heating section temperature is 1050 ℃, the holding time is 2.5 hours, the opening rolling temperature is 1000 ℃, the entering finishing rolling temperature is 990 ℃, and the upper cooling bed temperature is 590 ℃.

[0047] The principle of the preparation method proposed in the application is as follows:

[0048] With the increase of carbon content, the welding performance of the steel material becomes poor, the cold brittleness and aging sensitivity increase, and the atmospheric corrosion resistance decreases.

[0049] Hydrogen dissolved in steel can cause hydrogen embrittlement, white spots and other defects of the steel. Like oxygen and nitrogen, the solubility of hydrogen in solid steel is extremely small, and it dissolves into the molten steel at high temperature, accumulates in the structure and forms high-pressure fine pores when it cannot escape during cooling, which sharply reduces the plasticity, toughness and fatigue strength of the steel, and can cause cracks and brittle fracture in severe cases.

[0050] With the increase of nitrogen content, the strength of the steel material can be significantly improved, and the plasticity, especially the toughness, is also significantly reduced, but the cold brittleness is intensified.

[0051] Phosphorus makes the steel material significantly brittle at low temperature.

[0052] Adding high content of nickel improves the fatigue performance of the steel, reduces the sensitivity of the steel to notches, improves the strength of the steel without reducing its plasticity, improves the low temperature toughness of the steel, and itself has certain corrosion resistance, good corrosion resistance to some reducing acids, and improves the service life of concrete buildings.

[0053] Nickel improves the strength and hardness of the steel through solid solution strengthening. It can keep the concentration of eutectoid carbon at a low level, thereby avoiding the brittleness of the steel caused by high carbon content in the process of improving the strength of ferrite, and also improving the hardenability by reducing the critical transformation temperature of the steel, so that the steel can form high hardness and high strength phases such as martensite phase at a slower cooling speed, thereby improving its hardness.

[0054] The main role of aluminum in steel is to refine the grain size and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel, reducing the cold brittleness and aging tendency.

[0055] Compared with the prior art, the low-temperature high-strength steel bar preparation method has the following beneficial effects:

[0056] Reducing the cold brittleness of the steel bar: by designing low carbon, low phosphorus, and low hydrogen, the impurity content in the steel is reduced, and the cold brittleness of the steel bar is reduced.

[0057] Improving the low-temperature toughness of the steel bar: by adding an appropriate amount of aluminum and nickel, the toughness and ductility of the steel bar in a low-temperature environment can be improved.

[0058] Improving the low-temperature performance of the steel bar: through complex alloy systems and refining, continuous casting, and rolling processes, the purity and internal structure of the steel bar are improved, and the low-temperature performance of the steel bar is enhanced.

[0059] Achieving the mechanical properties and durability of the steel bar in a -165°C low-temperature environment: by controlling the content of each alloying element and fine process control, the surface of the steel bar forms tempered sorbite, and the core forms a complex structure of bainite + pearlite, thereby meeting the mechanical property requirements in a -165°C low-temperature environment and ensuring the durability of the steel bar.

[0060] In summary, the low-temperature high-strength steel bar and its preparation method proposed in the present application improve the performance of the steel bar in a low-temperature environment by optimizing the material formula and process parameters, and have important practical application value.

[0061] In the second aspect of the present application, a low-temperature high-strength steel bar is also proposed, which is prepared by the above preparation method.

[0062] According to the preparation method of the above embodiment 1, the composition system of the low-temperature high-strength alloy obtained is as follows:

[0063]

[0064] Finally, it should be noted that the above is only a preferred embodiment of the present application and does not limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing low-temperature high-strength steel bars, characterized in that... This includes the following steps: Low-phosphorus molten iron with a phosphorus content of ≤0.14% was selected for smelting; In the converter smelting process, the low-phosphorus molten iron is smelted in a converter, and the phosphorus content of the target tapped steel is controlled to be ≤0.006%. The converter tapping adopts a sliding plate slag blocking method and implements front and rear double tapping to control the target phosphorus recovery amount to be ≤0.002%. Low-phosphorus ferroalloys are selected for alloying, and the phosphorus recovery amount of the alloy is controlled to be ≤0.001%. The final phosphorus content is controlled to be ≤0.009%, the carbon content is controlled to be ≤0.05%, the aluminum content is controlled to be 0.020%-0.035%, and the nickel content is controlled to be 1%-1.15%, thus obtaining low-phosphorus molten iron smelted in a converter. LF furnace refining involves placing the low-phosphorus molten iron smelted in the converter into an LF furnace for refining, controlling the sulfur content to ≤0.01%, to obtain LF furnace refined molten iron. VD furnace refining involves placing the LF furnace refined molten iron into a VD furnace for refining to remove hydrogen and oxygen gases, resulting in VD furnace refined molten iron. Continuous casting involves placing the refined molten iron from the VD furnace into a continuous casting machine for continuous casting to obtain steel billets. Steel rolling: Rolling the steel billet to obtain reinforcing bars; The converter smelting adopts high-pulling and supplemental blowing, large slag volume, and double-slag smelting; The low-phosphorus iron alloy is a metallic manganese + aluminum + ferrosilicon + nickel + vanadium nitrogen alloy. In the VD furnace refining process, the pressure is controlled at ≥67Pa, the holding time is ≥10min, and the soft blowing time is ≥15min. The continuous casting process employs protective casting, with open long nozzles, immersion nozzles argon sealing, and stopper rods for flow control. The continuous casting superheat is controlled at 40℃-52℃, the specific water content is controlled at 0.20 L / kg-0.26 L / kg, and the average casting speed is controlled at 1.8 m / min-2.2 m / min; The continuous casting adopts end electromagnetic stirring technology, and the tensioning temperature is controlled at 945℃-1030℃. In the continuous casting process, the temperature of the billet is controlled at 400℃ when it comes off the line. A layer of ordinary HRB400 billet at 450℃ is laid in the lower layer of the slow cooling pit. Then, the 400℃ billet is laid flat on the ordinary HRB400 billet, covered, and kept warm for 36 hours. The steel rolling process adopts a long-term high-temperature heating-high-temperature rolling-strong water piercing process, including the following steps: heating the steel billet in a heating furnace, maintaining the heating section at 1020℃-1050℃ for 2.5 hours, controlling the initial rolling temperature at 1000℃-1050℃, the finishing rolling temperature at 900℃-990℃, and the cooling bed temperature at 500℃-590℃.

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