A 500mpa grade low-temperature-resistant high-manganese non-magnetic steel bar and a production method thereof

Abstract

The application provides a 500MPa-grade low-temperature-resistant high-manganese non-magnetic steel bar and a production method, and the composition of the steel bar is as follows: C 0.25-0.45%, Si 0.15-0.35%, Mn 20-30%, P≤0.025%, S≤0.025%, V 0.05-0.15%, Nb 0.01-0.03%, Al 2.0-4.0%, N 0.01-0.03%, and the rest is Fe and inevitable impurity elements. 4.6≤L=1.5×[C]+0.20×[Mn]+10.0×[N]≤6.5. Compared with the prior art, the steel bar is smelted according to the above composition, the production method is adopted, the product has normal temperature mechanical properties meeting the requirements, good toughness and plasticity and good notch sensitivity in a low-temperature environment of-165 DEG C. Moreover, the production cost of the steel bar is low, the production process is feasible, and the steel bar has a good market prospect.

Description

Technical Field

[0001] This invention belongs to the field of non-magnetic steel bar production, specifically relating to a 500MPa grade low-temperature resistant high-manganese non-magnetic steel bar and its production method. Background Technology

[0002] Non-magnetic steel has a wide range of applications, spanning numerous fields including power, rail transportation, machinery, defense, and construction. In the power industry, generator retaining rings experience high stress, necessitating the use of non-magnetic steel to reduce leakage magnetic flux at the rotor ends. Rail transportation systems such as the guidance mechanisms of maglev trains, superconducting power generation, transmission, and energy storage equipment, as well as large-scale nuclear fusion devices, also require significant amounts of non-magnetic steel to minimize the impact of magnetic fields on materials. In the defense industry, non-magnetic steel is used to manufacture the hulls of minesweepers, non-magnetic engines, and structural materials for stealth submarines. In the construction industry, non-magnetic steel is used in magnetic shielding structures to protect against magnetic fields. Furthermore, non-magnetic bearings, oil drill collars, and non-magnetic molds also require non-magnetic steel materials.

[0003] Therefore, the development of low-cost, high-performance non-magnetic steel materials is of great significance to my country's civilian and national defense construction.

[0004] Patent CN 117512308 A, published on February 6, 2024, discloses a non-magnetic steel bar and its preparation method. The chemical composition is set to include: C, Si, Mn, P, S, Cr, V, O, N, and Fe. Among them, by mass fraction, the content of C is 0.55-0.65%, the content of Si is 0.45-0.55%, the content of Mn is 18.0-18.5%, the content of P is ≤0.010%, the content of S is ≤0.010%, the content of Cr is 3.0-3.5%, the content of V is 1.70-1.73%, the content of O is 0-0.0010%, and the content of N is 0.0140-0.0150%. The production method includes: heating a billet with a set chemical composition; rolling the heated billet and controlling the rolling process parameters to obtain hot-rolled steel; cooling the hot-rolled steel and controlling the final temperature of the cooling to obtain non-magnetic steel bars.

[0005] However, in the aforementioned existing technologies, the added C and V content is too high, which can easily lead to brittle and hard steel bars, affecting low-temperature performance. Therefore, it is essential to provide a non-magnetic steel bar with good strength and low-temperature performance. Summary of the Invention

[0006] The purpose of this invention is to provide a 500MPa grade low-temperature resistant high-manganese non-magnetic steel bar and its production method. By adding Mn, Al, and N, and combining steelmaking and rolling processes, an austenitic structure is formed at room temperature, meeting the requirement of magnetic permeability ≤1.05. By adding Nb and V, the precipitation strengthening effect is utilized to improve the mechanical properties of the steel bar, and the room temperature performance reaches R eL ≥500MPa, R m ≥630MPa, A≥30%, and all exhibit good low-temperature resistance. The unnotched sample R at -165℃... eL ≥575MPa, A gt ≥3%, notched specimen A gt ≥1%, Notch Sensitivity Index (NSR) ≥1.0 (NSR = R of notched specimen) m / R of unnotched specimens eL This invention can meet the technical requirements for the mechanical properties of non-magnetic steel bars in frigid winter regions. The manufacturing process of this invention is not complex, making it suitable for large-scale industrial production and widespread application.

[0007] The specific technical solution of this invention is as follows:

[0008] A 500MPa grade low-temperature resistant high-manganese non-magnetic steel for reinforcing bars comprises the following components by weight percentage:

[0009] C 0.25-0.45%, Si 0.15-0.35%, Mn 20-30%, P≤0.025%, S≤0.025%, V0.05-0.15%, Nb 0.01-0.03%, Al 2.0-4.0%, N 0.01-0.03%, with the remainder being Fe and unavoidable impurity elements.

[0010] The composition of the steel used for the 500MPa grade low-temperature resistant high-manganese non-magnetic steel reinforcement also satisfies: 4.6 ≤ L = 1.5 ×

[0011] [C]+0.20×[Mn]+10.0×[N]≤6.5. This improves the strength and non-magnetic properties of the steel.

[0012] In the formula, each symbol represents the content of its corresponding chemical component × 100%.

[0013] The room temperature microstructure of the 500MPa grade low-temperature resistant high-manganese non-magnetic steel is austenitic.

[0014] The magnetic permeability of the 500MPa grade low-temperature resistant high-manganese non-magnetic steel used for reinforcing bars is ≤1.05; its room temperature performance R eL ≥500MPa, R m ≥630MPa, A≥30%; unnotched sample R at -165℃ low temperature environment eL ≥575MPa, Agt ≥3%; Notched specimen A gt ≥1%; Notch Sensitivity Index (NSR) ≥1.0 (NSR = R of notched specimen) m / R of unnotched specimens eL ).

[0015] Preferred, R eL ≥530MPa, R m ≥690MPa, A≥42%; unnotched sample R at -165℃ low temperature environment eL ≥575MPa, A gt ≥5%; Notched specimen A gt ≥3%; Gap Sensitivity Index (NSR) ≥1.3.

[0016] This invention provides a method for producing 500MPa grade low-temperature resistant high-manganese non-magnetic steel bars, comprising the following process flow:

[0017] Smelting → LF furnace refining → RH vacuum treatment → continuous casting → rolling.

[0018] The smelting process includes an electric furnace or converter. First, scrap steel and ferromanganese alloy are added, followed by molten iron, with the molten iron comprising 30-50% and the scrap steel and ferromanganese alloy comprising 70-50%. Electrodes are turned on, and the oxygen lance is started to heat the steel. The endpoint requirements are: P ≤ 0.010%, S ≤ 0.010%, and temperature 1560-1610℃. The steel is tapped while slag is blocked. When about 1 / 4 of the molten steel has been tapped, aluminum ferroalloy and slag are added. When about 3 / 4 of the molten steel has been tapped, aluminum cakes are added. After tapping, an appropriate amount of aluminum granules is evenly sprinkled onto the slag surface according to the amount of slag discharged.

[0019] The LF furnace refining process involves bottom-blowing argon into the ladle throughout the entire process, with the argon flow rate controlled to prevent molten steel from splashing out of the ladle. Pre-melted refining slag (0.83–1.65 kg / t) and lime (6.67–8.33 kg / t) are added, with TFe+MnO in the slag ≤ 1.0%. Based on the composition analysis results before entering the LF furnace, alloys are added before, during, and after the LF furnace refining process to adjust the content of Si, Mn, V, and Al elements. Al is added via aluminum wire feeding, while other alloys are added in block form through the charging system. The temperature exiting the LF furnace is 1565–1585℃.

[0020] The RH vacuum degassing process involves the following steps: The temperature of the molten steel entering the RH vacuum furnace is 1525-1545℃; during the initial vacuum phase, the vacuum holding time is ≥12 minutes; based on the composition analysis results from the initial vacuum phase, composition adjustments are made during the middle vacuum phase, ensuring a vacuum holding time of ≥8 minutes after adjustment. The temperature exiting the RH vacuum furnace is 1485-1505℃.

[0021] The continuous casting process involves casting 150 square billets using full-process protective casting. The initial casting temperature is 1450-1470℃, the cooling water flow rate of the crystallizer is 3000 liters / minute, and the secondary cooling water ratio is 0.8-1.2 liters / kg to ensure the surface quality of the square billets.

[0022] The rolling process is carried out using a bar mill: the heating temperature is controlled at 1150-1250℃, the tapping temperature is controlled at 980-1080℃, and after finishing rolling, a three-stage water cooling process is adopted, with the temperature dropping by 150-250℃, and the temperature of the upper cooling bed is controlled at 800-900℃.

[0023] The role of alloying elements in this invention is as follows:

[0024] Carbon (C): A favorable element for forming single-phase austenite structures, it also has excellent solid solution strengthening properties, effectively improving the strength of steel. However, with increasing C content, although a single austenite structure can be obtained under water quenching or air cooling, excessively high C content reduces the plasticity of the steel and increases the tendency for austenite grain growth, thereby leading to increased magnetic permeability. In this invention, the C content is controlled at 0.25-0.45%.

[0025] Si (Si) primarily functions as a deoxidizer in steel. Due to its much smaller atomic radius compared to austenite, Si exhibits significant solid solution strengthening. However, as a non-carbide-forming element, Si reduces the solubility of carbon (C) in austenite, leading to carbide precipitation and negatively impacting the stability of the austenitic structure. In this invention, the Si content is controlled at 0.15-0.35%.

[0026] Mn is a strong austenite-forming element that expands the austenite region, stabilizes the austenite structure, and improves toughness and plasticity at low temperatures. Most of the Mn in steel can dissolve in austenite to form a substitutional solid solution. Furthermore, as the Mn content increases, the temperature of the austenite-to-martensite transformation further decreases, increasing the stability of austenite. In this invention, the Mn content is controlled at 20-30%.

[0027] V: A strong carbonitride-forming element, the finely dispersed VC, VN, and V(CN) precipitates it forms inhibit austenite grain growth. Simultaneously, these finely dispersed VC, VN, and V(CN) precipitates promote austenite nucleation and improve steel strength through coherent distortion and dispersion strengthening. In this invention, the V content is controlled at 0.05-0.15%.

[0028] Nb is a strong carbonitride-forming element. The fine, dispersed NbC, NbN, and Nb(CN) precipitates it forms inhibit austenite grain growth, while simultaneously promoting austenite nucleation. Furthermore, these fine, dispersed precipitates enhance steel strength through coherent distortion and dispersion strengthening. In this invention, the Nb content is controlled at 0.01-0.03%.

[0029] Al (Al) increases the stacking fault energy of austenite and strongly inhibits martensitic transformation, stabilizing the austenitic microstructure. It is one of the key elements in austenitic steel. However, the upper limit of Al content depends on whether high-temperature δ-ferrite appears, while the lower limit depends on whether the transformation from low-temperature austenite to martensite can be avoided. In this invention, the Al content is controlled between 2.0% and 4.0%.

[0030] Nitrogen (N) is a strong austenite-forming element and also an interstitial solid solution element. It forms VN and V(CN) compounds with V, significantly increasing the strength of steel. In this invention, the N content is controlled at 0.01-0.03%.

[0031] P and S are harmful impurity elements that are detrimental to ensuring the strength, ductility, and low magnetic properties of steel. In this invention, the P and S content is controlled to ≤0.025%.

[0032] The key to this invention is to achieve non-magnetic properties by using a high C-high Mn composition system and adding a certain amount of Al to expand the austenite phase region, thereby obtaining a single-phase austenite structure that significantly reduces the magnetic permeability of steel and improves its toughness and plasticity at low temperatures. At the same time, V-Nb composite microalloying is used to enhance the mechanical properties of steel bars by utilizing its precipitation strengthening effect.

[0033] The non-magnetic and mechanical properties of the reinforcing steel of this invention are achieved by utilizing the combined effect of the above-mentioned elements, rather than the individual effect of each element.

[0034] Compared with existing technologies, this invention, by smelting according to the above-mentioned composition and employing electric furnace or converter smelting, LF furnace refining, RH vacuum degassing, 150 square billet continuous casting, and bar rolling, can produce non-magnetic steel bars with excellent properties. Their room temperature mechanical properties reach: yield strength R... eL ≥500MPa, tensile strength R m ≥630MPa, elongation after fracture A≥30%; Mechanical properties at -165℃: unnotched specimen R eL ≥575MPa, A gt ≥3%, notched specimen A gt The magnetic permeability is ≥1%; the notch sensitivity index (NSR) is ≥1.0; and the relative permeability is ≤1.05, as evaluated using the measurement method for the permeability of weak magnetic materials in GJB 937—90. In summary, the mechanical properties of the product at room temperature meet the requirements for HRB500E steel bars in GB 1499.2, and it exhibits good toughness, ductility, and notch sensitivity at -165℃, meeting the needs of non-magnetic construction in severely cold winter regions of my country. Furthermore, the steel bars of this invention have low production costs and feasible production processes, indicating a promising market prospect. Attached Figure Description

[0035] Figure 1This is the room temperature microstructure of a typical 500MPa low-temperature resistant high-manganese non-magnetic steel bar (magnification 100). Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Example 1-Example 2

[0038] A 500MPa grade low-temperature resistant high-manganese non-magnetic steel for reinforcing bars comprises the following components by weight percentage:

[0039] As shown in Table 1, the balance not shown in Table 1 is for Fe and unavoidable impurity elements.

[0040] Comparative Example

[0041] A high-manganese non-magnetic steel for reinforcing bars comprises the following composition by mass percentage as shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurity elements.

[0042] Table 1. Smelting chemical composition (%) of the examples and comparative examples

[0043] Case C Si Mn P S V Nb Al N L value Example 1 0.43 0.32 20.2 0.011 0.009 0.135 0.015 3.5 0.025 4.94 Example 2 0.40 0.33 21.3 0.012 0.007 0.124 0.016 2.9 0.022 5.08 Comparative Example 0.25 0.30 20 0.013 0.01 0.12 0.012 3.5 0.012 4.50

[0044] The production method of the 500MPa grade low-temperature resistant high-manganese non-magnetic steel bar described in Example 1 includes the following steps:

[0045] (1) Electric furnace smelting (120 tons): First add scrap steel + ferromanganese alloy, then add molten iron, with molten iron accounting for 40% and scrap steel + ferromanganese alloy accounting for 60%. Turn on the electrodes and start the oxygen lance to heat up. The endpoint is 0.010% P, 0.009% S, and the temperature is 1595℃. Tap the steel while blocking the slag. When about 1 / 4 of the molten steel has been tapped, add aluminum iron and slag. When about 3 / 4 of the molten steel has been tapped, add aluminum cake. After tapping, according to the amount of slag, evenly sprinkle an appropriate amount of aluminum particles onto the steel slag surface.

[0046] (2) LF furnace refining: Argon is blown from the bottom of the ladle throughout the process, with the argon flow rate determined by preventing molten steel from splashing out of the ladle; 155 kg of pre-melted refining slag and 910 kg of lime are added, with the slag containing 1.0% TFe + MnO. Based on the composition analysis results before entering the LF furnace, alloys are added before, during, and after the LF furnace refining process to adjust the content of Si, Mn, V, and Al elements. Al is added via aluminum wire feeding, while other alloys are added in block form through the charging system. The temperature exiting the LF furnace is 1580℃.

[0047] (3) RH vacuum degassing: The temperature of the molten steel entering the RH vacuum furnace is 1530℃. The vacuum holding time in the early stage is 13 minutes. After the composition is adjusted in the middle stage, the vacuum holding time is 8.5 minutes. The temperature of the RH vacuum furnace exit is 1495℃.

[0048] (4) Billet continuous casting: initial casting temperature 1460℃, crystallizer cooling water flow rate 3000 liters / minute, secondary cooling water ratio 1.0 liters / kg.

[0049] (5) Bar rolling mill: heating temperature, tapping temperature control and upper cooling bed temperature are shown in Table 2.

[0050] The production method of the 500MPa grade low-temperature resistant high-manganese non-magnetic steel bar described in Example 2 includes the following steps:

[0051] (1) Electric furnace smelting (120 tons): First add scrap steel + ferromanganese alloy, then add molten iron, with molten iron accounting for 31% and scrap steel + ferromanganese alloy accounting for 69%. Turn on the electrodes and start the oxygen lance to heat to the endpoint: 0.008% P, 0.006% S, temperature 1560℃. Slag blocking and tapping. When about 1 / 4 of the molten steel has been tapped, add aluminum iron and slag. When about 3 / 4 of the molten steel has been tapped, add aluminum cake. After tapping, according to the amount of slag, evenly sprinkle an appropriate amount of aluminum particles onto the steel slag surface.

[0052] (2) LF furnace refining: Argon is blown from the bottom of the ladle throughout the process, with the argon flow rate determined by preventing molten steel from splashing out of the ladle; 120 kg of pre-melted refining slag and 815 kg of lime are added. The slag contains 0.81% TFe + MnO. Based on the composition analysis results before entering the LF furnace, alloys are added before, during, and after the LF furnace refining process to adjust the content of Si, Mn, V, and Al elements. Al is added by feeding aluminum wire, while other alloys are added in block form through the charging system. The temperature exiting the LF furnace is 1568℃.

[0053] (3) RH vacuum degassing: The temperature of the molten steel entering the RH vacuum furnace is 1525℃; the vacuum holding time in the early stage is 13.5 minutes. Based on the composition analysis results in the early stage of vacuum, the vacuum holding time is adjusted to 10 minutes after the composition in the middle stage of vacuum, and the temperature of the RH vacuum furnace exit is 1490℃.

[0054] (4) Continuous casting of 150 square billets: initial casting temperature 1450℃, cooling water flow rate of the crystallizer 3000 liters / minute, secondary cooling water flow rate 0.8 liters / kg.

[0055] (5) Bar rolling mill: heating temperature, tapping temperature control and upper cooling bed temperature are shown in Table 2.

[0056] The production method of the high-manganese non-magnetic steel bar described in the comparative example includes the following steps:

[0057] (1) Electric furnace smelting: First add scrap steel + ferromanganese alloy, then add molten iron. The molten iron accounts for 45%, and the scrap steel + ferromanganese alloy accounts for 55%. Turn on the electrodes and start the oxygen lance to heat up. The endpoint requirement is: 0.012％P 0.010% S, temperature 1605℃. Slag blocking and tapping: When about 1 / 4 of the molten steel has been tapped, aluminum iron and slag are added; when about 3 / 4 of the molten steel has been tapped, aluminum cakes are added; after tapping, according to the amount of slag discharged, an appropriate amount of aluminum granules are evenly sprinkled onto the steel-slag surface.

[0058] (2) LF furnace refining: Argon is blown from the bottom of the ladle throughout the process, and the argon flow rate is set so that the molten steel does not splash out of the ladle; pre-melted refining slag is added. 220kg 1000 kg of lime and 0.7% TFe + MnO in the slag were added. Based on the composition analysis results before entering the LF furnace, alloys were added before, during and after refining in the LF furnace to adjust the content of Si, Mn, V and Al elements. Al was added by feeding aluminum wire, and other alloys were added in block form through the charging system. The temperature at the LF furnace outlet was 1585℃.

[0059] (3) RH vacuum degassing: The temperature of the molten steel entering the RH vacuum furnace from the ladle 1550℃ The initial vacuum holding time is 15 minutes. Based on the compositional analysis results of the initial vacuum period, the composition is adjusted during the middle vacuum period to ensure a vacuum holding time of 12.5 minutes. The temperature exiting the RH vacuum furnace is... 1510℃ .

[0060] (4) Continuous casting of billets: the initial casting temperature is 1470℃, the flow rate of cooling water in the crystallizer is 3000 liters / minute, and the secondary cooling water ratio is 1.15 liters / kg to ensure the surface quality of the billets.

[0061] (5) Bar rolling mill: heating temperature, tapping temperature control and upper cooling bed temperature are shown in Table 2.

[0062] Table 2 Rolling parameters for each embodiment and comparative example

[0063] Case Heating temperature (°C) Steel tapping temperature (°C) Temperature of the upper cooling bed (°C) Example 1 1180 1020 840 Example 2 1220 1060 860 Comparative Example 1185 1021 860

[0064] The mechanical properties, metallographic structure, and relative magnetic permeability of the embodiments and comparative examples of this invention at room temperature and -165℃ are shown in Table 4. Wherein: R eL R is the yield strength. m NSR is the tensile strength; A is the elongation after fracture at a gauge length of 5d (d is the nominal diameter of the steel bar); NSR = R of the notched specimen. m / R of unnotched specimens eL .

[0065] Table 3. Mechanical properties at room temperature and low temperature of the examples and comparative examples.

[0066]

[0067] The data underlined above do not meet the requirements of this invention.

[0068] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A type of 500MPa grade low-temperature resistant high-manganese non-magnetic steel for reinforcing bars, characterized in that, The 500MPa grade low-temperature resistant high-manganese non-magnetic steel bar comprises the following components by mass percentage: C 0.25-0.45%, Si 0.15-0.35%, Mn 20-30%, P≤0.025%, S≤0.025%, V 0.05-0.15%, Nb0.01-0.03%, Al 2.0-4.0%, N 0.01-0.03%, with the remainder being Fe and unavoidable impurity elements; The composition of the steel used for the 500MPa grade low-temperature resistant high-manganese non-magnetic steel bars also satisfies: 4.6≤L=1.5×[C]+0.20×[Mn]+10.0×[N] ≤6.5; The 500MPa grade low-temperature resistant high-manganese non-magnetic steel used for reinforcing bars has a magnetic permeability ≤1.05; its room temperature performance R eL ≥500MPa, R m ≥630MPa, A≥30%; unnotched sample R at -165℃ low temperature environment eL ≥575MPa, A gt ≥3%; Notched specimen A gt ≥1%; Gap Sensitivity Index (NSR) ≥1.

0.

2. The 500MPa grade low-temperature resistant high-manganese non-magnetic steel for reinforcing bars according to claim 1, characterized in that, The room temperature microstructure of the 500MPa grade low-temperature resistant high-manganese non-magnetic steel bar is austenitic.

3. A method for producing 500MPa grade low-temperature resistant high-manganese non-magnetic steel bars as described in claim 1 or 2, characterized in that, The production method includes the following process flow: smelting → LF furnace refining → RH vacuum treatment → continuous casting → rolling.

4. The production method according to claim 3, characterized in that, The smelting process includes an electric furnace or converter, where scrap steel and ferromanganese alloy are added first, followed by molten iron, with the molten iron accounting for 30-50% and the scrap steel and ferromanganese alloy accounting for 70-50%. The endpoint requirements are: P≤0.010%, S≤0.010%, and temperature 1560-1610℃.

5. The production method according to claim 3, characterized in that, The LF furnace refining process involves adding 0.83-1.65 kg / t of pre-melted refining slag and 6.67-8.33 kg / t of lime, with TFe+MnO ≤ 1.0% in the slag, and the LF furnace exit temperature being 1565-1585℃.

6. The production method according to claim 3, characterized in that, The RH vacuum degassing process involves the following steps: the temperature of the molten steel entering the RH vacuum furnace is 1525-1545℃; in the early stage of vacuum, the vacuum holding time is ≥12 minutes; in the middle stage of vacuum, the composition is adjusted, and after adjustment, a vacuum holding time of ≥8 minutes is guaranteed; the temperature exiting the RH vacuum furnace is 1485-1505℃.

7. The production method according to claim 3, characterized in that, The continuous casting process involves casting 150 square billets, employing full-process protective casting, with an initial casting temperature of 1450-1470℃, a crystallizer cooling water flow rate of 3000 liters / minute, and a secondary cooling water ratio of 0.8-1.2 liters / kg.

8. The production method according to claim 3, characterized in that, During the rolling process, the heating temperature is controlled at 1150-1250℃, the tapping temperature is controlled at 980-1080℃, and after finishing rolling, a three-stage water cooling process is adopted, with the temperature dropping by 150-250℃. The temperature of the upper cooling bed is controlled at 800-900℃.

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