Prestressed steel bar with memory recovery function

Abstract

The present application relates to the field of steel production, especially to a prestressed steel bar with memory recovery function, which takes an iron-based shape memory alloy as a raw material, and its chemical composition is as follows according to weight percentage: Mn (manganese): 10-20%, Si (silicon): 1-10%, Cr (chromium): 5-15%, Ni (nickel): 0-8%, V (vanadium): 0.2-2.0%, N (nitrogen): 0.1-0.5%, C (carbon): ≤0.03%, and the rest is Fe (iron), which has excellent recovery stress, low excitation temperature, and can obtain considerable recovery stress without complicated thermal mechanical training, and is reasonable in cost, suitable for industrial production and large-scale use.

Description

Technical Field

[0001] This invention relates to the field of steel bar production technology, and in particular to a prestressed steel bar with memory recovery function. Background Technology

[0002] Prestressed steel bars are used to strengthen concrete structures, commonly found in bridges, high-rise buildings, and underground structures. During use, concrete structures may develop tiny cracks that can gradually widen, reducing structural strength and durability. Furthermore, earthquakes can cause severe damage to building structures, threatening human lives. Currently, these problems can be addressed by using steel bars made of shape memory alloys (SMA). SMA steel bars possess a shape memory effect; when the structure is subjected to external loads, SMA steel bars can automatically contract and close cracks, achieving self-healing. They can also absorb and disperse seismic energy, reducing vibration amplitude and improving structural resilience. However, current Ni-based and Cu-based shape memory alloys are expensive and difficult to industrialize and use on a large scale. Prestressed steel bars made from ordinary iron-based shape memory alloys suffer from insufficient recovery stress and high excitation temperatures. Steel bars made from these alloys require thermomechanical training to improve the alloy's shape recovery rate, but the thermomechanical training process is complex, costly, and difficult to train alloys with complex shapes, hindering engineering applications. Summary of the Invention

[0003] (a) Technical problems to be solved

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a prestressed steel bar with shape memory recovery function, which solves the problems existing in the prior art. By rationally distributing the components and using the rolling process, the recovery stress of the iron-based shape memory alloy is significantly improved, and the excitation temperature is reduced. This allows the iron-based shape memory alloy to obtain considerable recovery stress without the need for cumbersome thermomechanical training. Furthermore, the cost is reasonable, making it suitable for industrial production and large-scale use.

[0005] (II) Technical Solution

[0006] To achieve the above objectives, the present invention provides the following technical solution: a prestressed steel bar with memory recovery function, the raw material of which is an iron-based shape memory alloy, and its chemical composition, expressed as a percentage by weight, is as follows: Mn (manganese): 12-20%, Si (silicon): 2-9%, Cr (chromium): 6-14%, Ni (nickel): 0-8%, V (vanadium): 0.2-2.0%, N (nitrogen): 0.1-0.5%, C (carbon): ≤0.03%, wherein 5.5N≥V≥4.5N, and the remainder is Fe (iron), wherein the vanadium (V) content in the chemical composition is at least 4.5 times and does not exceed 5.5 times the nitrogen (N) content, and its chemical purity index meets eight requirements. The total inclusions are ≤6.5 grade; its physical properties meet the following requirements: yield strength ≥500MPa, tensile strength ≥800MPa, elongation after fracture ≥20%, and recovery stress (200℃) ≥280MPa; the preparation method includes the following steps: S1, electric furnace smelting: melting the raw materials into molten steel in an electric furnace and reducing the content of harmful elements in the molten steel; S2, AOD furnace smelting: oxygen blowing to decarburize the molten steel, adding nitrogen, and adjusting the alloy composition; S3, LF furnace refining: purifying the molten steel, removing inclusions and fine-tuning the composition; S4, die casting: using a mold to directly and continuously cast the liquid molten steel into billets; S5, rolling: processing and rolling the billets or profiles into reinforcing bars through a series of rollers.

[0007] Preferably, in step S1, the raw materials and auxiliary materials are added in 3-4 batches during the electric furnace steelmaking process, and the tapping temperature is 1600℃.

[0008] Preferably, in step S4, the baking temperature of the ingot mold during the casting process is >50°C before pouring; before pouring, the ingot mold is filled with argon gas, and argon gas is used to protect the pouring process, with the pouring temperature controlled within 1500-1510°C.

[0009] Preferably, step S5, the rolling process, includes: heating the steel ingot to 1200℃-1250℃, holding it in the furnace for ≥4 hours, and rolling it into a square billet after it is taken out of the furnace; heating it again to 1000℃-1100℃, holding it for ≥1.5 hours, and hot rolling, with isothermal rolling in the intermediate process.

[0010] Preferably, the isothermal rolling temperature is controlled at 850–900℃.

[0011] (III) Beneficial Effects

[0012] The wire rod produced by this method has the following physical properties: yield strength ≥ 500 MPa, tensile strength ≥ 800 MPa, elongation after fracture ≥ 20%, and recovery stress (200℃) ≥ 280 MPa. By rationally combining the contents of elements such as Mn, Si, Cr, Ni, V, and N, the volume fraction of ε-martensite generated during the pre-deformation process is increased, thereby further improving the recovery stress and reducing the excitation temperature to a certain extent. In addition, during the rolling process, reasonable heating temperature and TMCP process ensure that the iron-based shape memory alloy has a yield strength ≥ 500 MPa, tensile strength ≥ 800 MPa, and A ≥ 20%, which improves the recovery performance to a certain extent. Detailed Implementation

[0013] This invention provides a technical solution: a prestressed steel bar with memory recovery function, the preparation method of which includes the following steps:

[0014] S1. Electric Furnace Steelmaking: First, a suitable electric furnace is selected for melting raw materials. An electric arc furnace is used, which provides high temperatures and high melting capacity. Raw material and auxiliary material addition: Raw materials and auxiliary materials are added to the electric furnace in 3-4 batches. This batch addition process helps to mix the raw materials and auxiliary materials more evenly, ensuring a uniform composition. Throughout the electric furnace steelmaking process, special attention is paid to controlling the sulfur (S) and phosphorus (P) content. Sulfur and phosphorus are common impurities that adversely affect steel quality; therefore, their content must be strictly controlled to ensure it does not exceed the upper limit specified in the process. The electric arc in the electric furnace provides sufficient heat to melt the raw materials and auxiliary materials into liquid steel. The temperature of the molten steel is maintained at around 1600℃. Once the molten steel reaches the required temperature and composition, it can be discharged through the electric furnace's tapping system for use in subsequent smelting and processing steps.

[0015] S2, AOD furnace smelting: In the initial stage, the temperature of the molten steel is measured and samples are taken for chemical analysis. This helps determine the initial composition and temperature of the molten steel. The entire smelting process is divided into the following stages:

[0016] Pretreatment stage: Start temperature: 1500℃-1600℃, end temperature: 1600℃-1620℃. In this stage, the temperature of the molten steel is increased, and lime and fluorite are added to make slag. The role of the slag is to control the reaction and absorb some unwanted components, and to make preliminary chemical adjustments. The temperature setting in this stage helps to prepare the molten steel for subsequent treatment.

[0017] Oxygen blowing and nitrogen enrichment stage: The initial temperature is set at 1630℃-1650℃. First, oxygen is introduced at a rate of 10L / min-30L / min. The purpose of this step is to oxidize some of the carbon in the steel through oxygen reaction, gasifying the carbon into gases such as carbon monoxide (CO), thereby reducing the carbon content. Subsequently, the temperature is gradually increased to 1650℃-1700℃, and nitrogen is injected at a rate of 40L / min-50L / min. The purpose of this step is to increase the nitrogen content in the steel, thereby improving the alloy's properties, especially the properties of shape memory alloys. Nitrogen enrichment can enhance the alloy's corrosion resistance, improve the shape memory effect, and adjust its martensitic phase transformation temperature and stability.

[0018] Pre-reduction stage: Starting temperature: 1700℃-1750℃, ending temperature: 1750℃-1780℃. During this stage, a reducing agent is added to reduce elements such as Cr, Mn, and Si, which are oxidized during oxygen blowing and nitrogen enrichment, from the steel slag. Increasing the temperature helps accelerate the reduction reaction.

[0019] Reduction stage: starting temperature: 1700℃-1750℃, ending temperature: 1600℃-1700℃. During this stage, reducing agent is added to ensure that the oxygen content in the molten steel is minimized. This helps to eliminate the reaction between oxygen and other elements, making the steel purer.

[0020] The use of stepped temperature control in this process serves the following purposes:

[0021] 1. Adjustment of alloying element distribution: The diffusion rate of alloying elements varies at different temperatures. Stepped temperature control helps to adjust the distribution of alloying elements in the crystal structure to achieve a uniform distribution and improve the performance of the alloy.

[0022] 2. Reduce stress: By gradually increasing or decreasing the temperature, the temperature gradient of the metal during the cooling or heating process can be reduced, thereby reducing internal stress, lowering the risk of deformation and cracking, and improving the stability of the material.

[0023] S3, LF Furnace Refining: First, the molten steel from the AOD furnace enters the LF furnace. At the beginning of the LF furnace process, samples are taken and the temperature is measured to determine the composition and temperature of the molten steel. The LF furnace is then electrically heated to raise the molten steel to the required temperature for at least 20 minutes. Impurities in the molten steel are removed by adding lime and fluorite in small batches. Based on the steel composition measured by the samples, specific alloying elements are added in appropriate amounts to fine-tune the alloy composition. The tapping temperature is 1520-1530℃. The purpose of this step is to improve the quality and purity of the molten steel, remove impurities, and fine-tune the alloy composition to ensure that the prepared prestressed steel bars have the required performance and quality. This meticulous control and purification process helps ensure that the prestressed steel bars have excellent performance in subsequent processes.

[0024] S4. Ingot Casting: In the preparation stage, first clean the inner wall of the ingot mold of slag, rust, cold steel, and debris to ensure a smooth inner wall. Preheat the ingot mold to a temperature greater than 50°C before pouring. This is to avoid problems caused by temperature differences. If there is a large temperature difference between the ingot mold and the molten steel, it will lead to uneven cooling and solidification, which can cause porosity, cracks, or other defects. Preheating the ingot mold also helps reduce thermal stress on the material. Before pouring, the ingot mold is filled with argon gas for argon protection during pouring. This is to prevent the molten steel from contacting oxygen in the air and avoiding oxidation. Argon protection can also reduce gas inclusions in the molten steel and reduce the formation of bubbles. The pouring temperature is controlled at 1500-1510°C. Once the mold is ready, the preheated molten steel is poured from the ladle into the mold. In the mold, the molten steel will cool and solidify rapidly to form an ingot. Once the billet has fully solidified, the mold can be opened to remove the ingot.

[0025] S5. Rolling: The steel ingots made from S4 are ground and heated to 1200℃-1250℃ to ensure sufficient plasticity during processing, making them easy to deform and form into the required shape. The ingots are held in the furnace for ≥4 hours and then rolled into square billets. A second heating to 1000℃-1100℃ and holding for ≥1.5 hours improves the temperature uniformity of the material, ensuring successful subsequent rolling. Hot rolling is then performed, where the square billets are rolled through a series of rollers and rolling tools to further refine them. The prestressed steel bars are manufactured to the required shape and size. In the intermediate stage of rolling, isothermal rolling is required to further control the temperature and structure of the steel. The isothermal temperature is controlled between 850 and 900 degrees Celsius, and the rolling time is controlled between 15 and 60 minutes. The purpose is to optimize the microstructure of the material and ensure that it has the required properties. After rolling, the prestressed steel bars are transferred to a cooling bed for cooling for 1 to 4 hours. The purpose of this process is to solidify the structure of the material and ensure that it has the required mechanical properties.

[0026] The final prestressed steel bar with self-recovery function has the following chemical composition: Mn: 12-20%, Si: 2-9%, Cr: 6-14%, Ni: 0-8%, V: 0.2-2.0%, N: 0.1-0.5%, C: ≤0.03%, with the remainder being Fe, of which 5.5N≥V≥4.5N.

[0027] It's important to clarify that this ratio means the nitrogen (N) content is at least 4.5 times and no more than 5.5 times the vanadium (V) content. This is to ensure the stability of the shape memory effect. The properties of shape memory alloys depend on the martensitic phase transformation, where the nitrogen and vanadium contents can affect the temperature and stability of the transformation. By ensuring a ratio of 5.5N ≥ V ≥ 4.5N, a controllable martensitic phase transformation can be achieved, which helps to realize the desired shape memory effect. By adjusting the N and V ratio, the recovery ability of the alloy can be adjusted to meet the needs of specific engineering projects. The ratio of (5.5N ≥ V ≥ 4.5N) plays an important role in iron-based shape memory alloys, enabling the realization of the desired shape memory effect and self-recovery function, and improving the stability and corrosion resistance of the material. This helps to ensure that prestressed steel bars perform their required functions in prestressed concrete structures.

[0028] The following will describe the technical solution of the present invention clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] Example 1: A prestressed steel bar with memory recovery function, comprising the following chemical composition by weight: Mn (manganese): 12%, Si (silicon): 2%, Cr (chromium): 6%, Ni (nickel): 1%, V (vanadium): 0.45%, N (nitrogen): 0.1%, C (carbon): 0.01%, with the remainder being Fe (iron);

[0030] The above-mentioned processing method for prestressed steel bars includes the following steps:

[0031] S1. Electric furnace molten steel: Raw materials and auxiliary materials are gradually added to the electric furnace in 3-4 batches to melt into molten steel, and the temperature of the molten steel is controlled at about 1600℃.

[0032] S2, AOD furnace smelting: The whole process is divided into pretreatment stage, oxygen blowing and nitrogen enrichment stage, pre-reduction stage and reduction stage;

[0033] Pretreatment stage: starting temperature 1550℃, lime and fluorite are added to make slag, ending temperature 1600℃;

[0034] Oxygen blowing and nitrogen enrichment stage: The initial temperature is 1630℃, oxygen is introduced at a rate of 10L / min to reduce the carbon content, and the temperature is gradually increased to 1650℃, and nitrogen is injected at a rate of 40L / min.

[0035] Pre-reduction stage: Starting temperature: 1700℃, adding reducing agent to reduce the Cr, Mn, Si and other elements oxidized during oxygen blowing and nitrogen enrichment from the steel slag; Ending temperature: 1750℃.

[0036] Reduction stage: Starting temperature: 1700℃. During this stage, reducing agent is added to ensure that the oxygen content in the molten steel is minimized. Ending temperature: 1600℃.

[0037] S3 and LF furnace refining: purifying molten steel, removing inclusions and fine-tuning the composition, with a tapping temperature of 1520℃;

[0038] S4. Ingot casting: Liquid steel is directly and continuously cast into billets using molds, with the initial pouring temperature controlled at 1500℃.

[0039] S5. Rolling: First, heat the ground steel ingot to 1230℃ and heat it in the furnace for 4 hours. Then, roll it into a square billet. Heat it again to 1050℃ and heat it in the furnace for 1.5 hours. Then, heat it into a steel bar. In the intermediate process, perform isothermal rolling at 850℃ for 30 minutes. After rolling, air cool it on a cooling bed for 1 hour.

[0040] Example 2: A prestressed steel bar with memory recovery function, comprising the following chemical composition by weight: Mn (manganese): 14%, Si (silicon): 4%, Cr (chromium): 8%, Ni (nickel): 3%, V (vanadium): 0.9%, N (nitrogen): 0.2%, C (carbon): 0.01%, with the remainder being Fe (iron).

[0041] The above-mentioned processing method for prestressed steel bars includes the following steps:

[0042] S1. Electric furnace molten steel: Raw materials and auxiliary materials are gradually added to the electric furnace in 3-4 batches to melt into molten steel, and the temperature of the molten steel is controlled at about 1600℃.

[0043] S2, AOD furnace smelting: The whole process is divided into pretreatment stage, oxygen blowing and nitrogen enrichment stage, pre-reduction stage and reduction stage;

[0044] Pretreatment stage: starting temperature 1550℃, lime and fluorite are added to make slag, ending temperature 1600℃;

[0045] Oxygen blowing and nitrogen enrichment stage: The initial temperature is 1630℃, oxygen is introduced at a rate of 20L / min to reduce the carbon content, and the temperature is gradually increased to 1650℃, and nitrogen is injected at a rate of 40L / min.

[0046] Pre-reduction stage: Starting temperature: 1730℃, adding reducing agent to reduce the Cr, Mn, Si and other elements oxidized during oxygen blowing and nitrogen enrichment from the steel slag; Ending temperature: 1760℃.

[0047] Reduction stage: Starting temperature: 1730℃. During this stage, reducing agent is added to ensure that the oxygen content in the molten steel is minimized. Ending temperature: 1620℃.

[0048] S3 and LF furnace refining: purifying molten steel, removing inclusions and fine-tuning the composition, with a tapping temperature of 1520℃;

[0049] S4. Ingot casting: Liquid steel is directly and continuously cast into billets using molds, with the initial pouring temperature controlled at 1510℃.

[0050] S5. Rolling: First, heat the ground steel ingot to 1200℃ and heat it in the furnace for 4 hours. Then, roll it into a square billet. Heat it again to 1000℃ and heat it in the furnace for 1.5 hours. Then, heat it into a steel bar. In the intermediate process, perform isothermal rolling at 850℃ for 15 minutes. After rolling, air cool it on a cooling bed for 2 hours.

[0051] Example 3: A prestressed steel bar with memory recovery function, comprising the following chemical composition by weight: Mn (manganese): 16%, Si (silicon): 6%, Cr (chromium): 10%, Ni (nickel): 5%, V (vanadium): 1.35%, N (nitrogen): 0.3%, C (carbon): 0.01%, with the remainder being Fe (iron).

[0052] The above-mentioned processing method for prestressed steel bars includes the following steps:

[0053] S1. Electric furnace molten steel: Raw materials and auxiliary materials are gradually added to the electric furnace in 3-4 batches to melt into molten steel, and the temperature of the molten steel is controlled at about 1600℃.

[0054] S2, AOD furnace smelting: The whole process is divided into pretreatment stage, oxygen blowing and nitrogen enrichment stage, pre-reduction stage and reduction stage;

[0055] Pretreatment stage: starting temperature 1550℃, lime and fluorite are added to make slag, ending temperature 1600℃;

[0056] Oxygen blowing and nitrogen enrichment stage: The initial temperature is 1630℃, oxygen is introduced at a rate of 20L / min to reduce the carbon content, and the temperature is gradually increased to 1650℃, and nitrogen is injected at a rate of 40L / min.

[0057] Pre-reduction stage: Starting temperature: 1730℃, adding reducing agent to reduce the Cr, Mn, Si and other elements oxidized during oxygen blowing and nitrogen enrichment from the steel slag; Ending temperature: 1760℃.

[0058] Reduction stage: Starting temperature: 1730℃. During this stage, reducing agent is added to ensure that the oxygen content in the molten steel is minimized. Ending temperature: 1620℃.

[0059] S3 and LF furnace refining: purifying molten steel, removing inclusions and fine-tuning the composition, with a tapping temperature of 1520℃;

[0060] S4. Ingot casting: Liquid steel is directly and continuously cast into billets using molds, with the initial pouring temperature controlled at 1510℃.

[0061] S5. Rolling: First, heat the ground steel ingot to 1200℃ and heat it in the furnace for 4 hours. Then, roll it into a square billet. Heat it again to 1000℃ and heat it in the furnace for 1.5 hours. Then, heat it into a steel bar. In the intermediate process, perform isothermal rolling at 850℃ for 15 minutes. After rolling, air cool it on a cooling bed for 2 hours.

[0062] Example 4: A prestressed steel bar with memory recovery function, comprising the following chemical composition by weight: Mn (manganese): 18%, Si (silicon): 8%, Cr (chromium): 12%, Ni (nickel): 6%, V (vanadium): 1.8%, N (nitrogen): 0.4%, C (carbon): 0.02%, with the remainder being Fe (iron).

[0063] The above-mentioned processing method for prestressed steel bars includes the following steps:

[0064] S1. Electric furnace molten steel: Raw materials and auxiliary materials are gradually added to the electric furnace in 3-4 batches to melt into molten steel, and the temperature of the molten steel is controlled at about 1600℃.

[0065] S2, AOD furnace smelting: The whole process is divided into pretreatment stage, oxygen blowing and nitrogen enrichment stage, pre-reduction stage and reduction stage;

[0066] Pretreatment stage: starting temperature 1600℃, lime and fluorite are added to make slag, ending temperature 1620℃;

[0067] Oxygen blowing and nitrogen enrichment stage: The initial temperature is 1650℃, oxygen is introduced at a rate of 20L / min to reduce the carbon content, and the temperature is gradually increased to 1700℃, and nitrogen is injected at a rate of 40L / min.

[0068] Pre-reduction stage: Starting temperature: 1750℃, adding reducing agent to reduce the Cr, Mn, Si and other elements oxidized during oxygen blowing and nitrogen enrichment from the steel slag; Ending temperature: 1780℃.

[0069] Reduction stage: Starting temperature: 1750℃. During this stage, reducing agent is added to ensure that the oxygen content in the molten steel is minimized. Ending temperature: 1700℃.

[0070] S3 and LF furnace refining: purifying molten steel, removing inclusions and fine-tuning the composition, with a tapping temperature of 1520℃;

[0071] S4. Ingot casting: Liquid steel is directly and continuously cast into billets using molds, with the initial pouring temperature controlled at 1510℃.

[0072] S5. Rolling: First, heat the ground steel ingot to 1200℃ and heat it in the furnace for 4 hours. Then, roll it into a square billet. Heat it again to 1000℃ and heat it in the furnace for 1.5 hours. Then, heat it into a steel bar. In the intermediate process, perform isothermal rolling at 850℃ for 15 minutes. After rolling, air cool it on a cooling bed for 2 hours.

[0073] Example 5: A prestressed steel bar with memory recovery function, comprising the following chemical composition by weight: Mn (manganese): 20%, Si (silicon): 9%, Cr (chromium): 14%, Ni (nickel): 8%, V (vanadium): 2.25%, N (nitrogen): 0.5%, C (carbon): 0.03%, with the remainder being Fe (iron).

[0074] The above-mentioned processing method for prestressed steel bars includes the following steps:

[0075] S1. Electric furnace molten steel: Raw materials and auxiliary materials are gradually added to the electric furnace in 3-4 batches to melt into molten steel, and the temperature of the molten steel is controlled at about 1600℃.

[0076] S2, AOD furnace smelting: The whole process is divided into pretreatment stage, oxygen blowing and nitrogen enrichment stage, pre-reduction stage and reduction stage;

[0077] Pretreatment stage: starting temperature 1600℃, lime and fluorite are added to make slag, ending temperature 1620℃;

[0078] Oxygen blowing and nitrogen enrichment stage: The initial temperature is 1650℃, oxygen is introduced at a rate of 20L / min to reduce the carbon content, and the temperature is gradually increased to 1700℃, and nitrogen is injected at a rate of 40L / min.

[0079] Pre-reduction stage: Starting temperature: 1750℃, adding reducing agent to reduce the Cr, Mn, Si and other elements oxidized during oxygen blowing and nitrogen enrichment from the steel slag; Ending temperature: 1780℃.

[0080] Reduction stage: Starting temperature: 1750℃. During this stage, reducing agent is added to ensure that the oxygen content in the molten steel is minimized. Ending temperature: 1700℃.

[0081] S3 and LF furnace refining: purifying molten steel, removing inclusions and fine-tuning the composition, with a tapping temperature of 1520℃;

[0082] S4. Ingot casting: Liquid steel is directly and continuously cast into billets using molds, with the initial pouring temperature controlled at 1510℃.

[0083] S5. Rolling: First, heat the ground steel ingot to 1200℃ and heat it in the furnace for 4 hours. Then, roll it into a square billet. Heat it again to 1000℃ and heat it in the furnace for 1.5 hours. Then, heat it into a steel bar. In the intermediate process, perform isothermal rolling at 850℃ for 15 minutes. After rolling, air cool it on a cooling bed for 2 hours.

[0084] Example 6: The rest is the same as Example 2, except that in the smelting step, the chemical composition content of V (vanadium) added in Example 6 is 1.0% (5 times the N content).

[0085] Example 7: The rest is the same as Example 2, except that in the smelting step, the chemical composition content of V (vanadium) added in Example 7 is 1.1% (5.5 times the N content).

[0086] Example 8: The rest is the same as Example 3, except that in the smelting step, the chemical composition content of V (vanadium) added in Example 8 is 1.5% (5 times the N content).

[0087] Example 9: The rest is the same as Example 3, except that in the smelting step, the chemical composition content of V (vanadium) added in Example 9 is 1.65% (5.5 times the N content).

[0088] Example 10: The rest is the same as Example 4, except that in the smelting step, the chemical composition content of V (vanadium) added in Example 10 is 2.0% (5 times the N content).

[0089] Example 11: The rest is the same as Example 4, except that in the smelting step, the chemical composition content of V (vanadium) added in Example 11 is 2.2% (5.5 times the N content).

[0090] Comparative Example 1: Prestressed steel bars made of commercially available common iron-based shape memory alloys

[0091] The prestressed steel bars prepared in the above embodiments and comparative examples were subjected to the following tests, and the results are shown in Table 1.

[0092]

[0093] Table 1

[0094] As can be seen from the data in Table 1, the prestressed steel bar with memory recovery function processed by the present invention has significant advantages in yield strength, tensile strength, elongation and recovery stress compared with the prestressed steel bar of the prior art. In particular, the advantages of the prestressed steel bars processed in Examples 2 and 3 are more obvious. Therefore, the chemical composition ratio and processing technology of the prestressed steel bars in Examples 2 and 3 are the preferred solutions.

[0095] The principle of this type of prestressed steel bar with self-recovery function is as follows: the shape memory alloy in the prestressed steel bar is in a pre-stretched state, that is, stress is applied before the original shape, causing it to elongate. When external force or strain is applied to the prestressed steel bar, the alloy begins to deform. At the excitation temperature, the reverse phase transformation of martensite triggers the recovery effect. Under heating conditions, the steel bar partially shrinks back, and considerable recovery stress is generated during the shrinkage process.

[0096] Through numerous experiments, the applicant broke with convention and significantly increased the volume fraction of ε-martensite generated during the pre-deformation process by using a combination of chemical elements: Mn: 12-20%, Si: 2-9%, Cr: 6-14%, Ni: 0-8%, V: 0.2-2.0%, N: 0.1-0.5%, and C: ≤0.03%. This further increased the recovery stress and, to some extent, reduced the excitation temperature.

[0097] Through numerous experiments, the applicant has discovered that the prestressed steel bars with memory recovery function produced through these steps have the best effect. The physical properties of the steel bars produced by this invention are as follows: yield strength ≥ 500 MPa, tensile strength ≥ 800 MPa, elongation after fracture ≥ 20%, and recovery stress (200℃) ≥ 280 MPa. The prestressed steel bars produced by this invention can be used to reduce the permanent deformation of structures under earthquakes, temperature changes, or other external stresses, which is of great significance for maintaining the safety and stability of structures.

[0098] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A prestressed steel bar with memory recovery function, characterized in that, Its raw material is an iron-based shape memory alloy, and its chemical composition, expressed as a percentage by weight, is as follows: Mn (manganese): 12-20%, Si (silicon): 2-9%, Cr (chromium): 6-14%, Ni (nickel): 1-8%, V (vanadium): 0.2-2.0%, N (nitrogen): 0.1-0.5%, C (carbon): ≤0.03%, with the remainder being Fe (iron). The vanadium (V) content in the chemical composition is at least 4.5 times, but not more than 5.5 times, the nitrogen (N) content. Its preparation method includes the following steps: S1. Electric furnace steelmaking: This involves melting raw materials into molten steel using an electric furnace, thereby reducing the content of harmful elements in the molten steel. S2, AOD furnace smelting: oxygen blowing to decarburize molten steel, nitrogen addition, and adjustment of alloy composition; S3 and LF furnace refining: purifying molten steel, removing inclusions and fine-tuning the composition; S4. Die casting: Using molds to directly and continuously cast molten steel into billets; S5. Rolling: The process of rolling billets or profiles into steel bars by passing them through a series of rollers; Step S2, the AOD furnace smelting process, employs a stepped temperature control method, including: Pretreatment stage: Start temperature: 1500℃-1600℃, End temperature: 1600℃-1620℃; Oxygen blowing and nitrogen enrichment stage: starting temperature: 1630℃-1650℃, ending temperature: 1650℃-1700℃; Pre-reduction stage: Start temperature: 1700℃-1750℃, End temperature: 1750℃-1780℃; Reduction stage: Start temperature: 1700℃-1750℃, End temperature: 1600℃-1700℃; Step S5, the rolling process, includes: heating the steel ingot to 1200℃-1250℃, holding it in the furnace for ≥4 hours, and rolling it into a square billet after exiting the furnace; reheating it to 1000℃-1100℃, holding it for ≥1.5 hours, and hot rolling, with isothermal rolling in between; the isothermal rolling temperature is controlled at 850-900℃, the rolling time is controlled at 15-60 minutes, and after rolling, cooling is performed for 1-4 hours.

2. A prestressed steel bar with memory recovery function according to claim 1, characterized in that, In step S1, during the electric furnace steelmaking process, raw materials and auxiliary materials are added in 3-4 batches, and the tapping temperature is 1600℃.

3. A prestressed steel bar with memory recovery function according to claim 1, characterized in that, Step S2AOD furnace smelting process includes: Pretreatment stage: Lime and fluorite are added to form slag and the chemical composition is initially adjusted; Oxygen blowing and nitrogen addition stage: Oxygen is blown in at a rate of 10L / min-30L / min and nitrogen is blown in at a rate of 40L / min-50L / min to control the carbon content and ensure that Cr can fully enter the molten steel. Pre-reduction stage: A reducing agent is added to initially reduce the oxidized Cr, Mn, and Si elements from the steel slag; Reduction stage: Continue to add reducing agent to ensure that the oxygen content in the molten steel is minimized.

4. A prestressed steel bar with memory recovery function according to claim 1, characterized in that, During the refining process in step S3LF furnace, the electric heating time is ≥20min, and the tapping temperature is 1520-1530℃.

5. A prestressed steel bar with memory recovery function according to claim 1, characterized in that, In step S4, the baking temperature of the ingot mold during the casting process is >50℃ before pouring; before pouring, the ingot mold is filled with argon gas, and argon gas is used to protect the pouring process. The pouring temperature is controlled within 1500-1510℃.

6. A prestressed steel bar with memory recovery function according to any one of claims 1-5, characterized in that, Its physical properties meet the following requirements: yield strength ≥ 500 MPa, tensile strength ≥ 800 MPa, elongation after fracture ≥ 20%, and recovery stress ≥ 280 MPa.

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