A 20Mn2 rare earth optimized steel synchronous nitriding preparation method for DAT level electric hoist chain

Abstract

This invention provides a method for simultaneous nitriding of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains, belonging to the field of electric hoist chain manufacturing technology. The method includes the following steps: placing a cast billet body into a sleeve, filling the gap between the cast billet body and the sleeve with iron nitride powder, then evacuating and sealing the inside of the sleeve to form a combined cast billet; subjecting the combined cast billet to sequential heat treatment and hot rolling to obtain wire rod; then performing a tempering treatment, which involves sequential quenching and tempering; during the quenching and tempering processes, ammonia-containing gas is introduced; the tempered wire rod is shaped into chain blanks, and post-processed to obtain the finished chain. By vacuum sealing the iron nitride in the sleeve and achieving simultaneous surface nitriding through hot rolling and heat treatment, a separate subsequent nitriding process is eliminated, ensuring that the chain strength reaches 90Kg level, surface hardness ≥HRC50, and significantly improving core toughness and elongation.

Description

Technical Field

[0001] This invention relates to the field of electric hoist chain manufacturing technology, and in particular to a method for simultaneous nitriding of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains. Background Technology

[0002] As a core load-bearing component of lifting equipment, the performance of electric hoist chains directly determines the safety and reliability of lifting operations. DAT-grade electric hoist chains, as high-end products, explicitly require a chain strength of 90kg or higher, a surface hardness ≥ HRC50, and a core with good toughness, elongation, and hardness matching characteristics to suit heavy-duty, high-frequency applications. Currently, the industry commonly uses 20Mn2 steel to manufacture electric hoist chains. To meet DAT-grade standards, the composition of 20Mn2 steel needs to be optimized, and after tempering treatment, a separate surface nitriding process is implemented to achieve the dual technical requirements of high surface hardness and high core toughness.

[0003] However, traditional surface nitriding processes and 20Mn2 steel preparation processes have many insurmountable technical defects, which seriously affect the production efficiency, production cost, and product quality of the chains, as follows: 1. Cumbersome process and low production efficiency: Traditional 20Mn2 steel mostly adopts electric arc furnace smelting process, which has a long production cycle and low efficiency. It also requires smelting, billet casting, hot rolling, heat treatment and surface nitriding to be completed in sequence. The additional independent process further prolongs the production cycle and cannot meet the needs of large-scale production. 2. High energy consumption and high production costs: Nitriding alone requires dedicated nitriding equipment, and the nitriding process needs to maintain specific temperature and atmosphere conditions. Its energy consumption accounts for 20% to 25% of the total production cost of the chain, and the equipment maintenance cost is also high; the energy consumption of traditional electric arc furnace smelting is significantly higher than that of converter process. 3. Unstable nitriding quality: During the nitriding process alone, the ammonia decomposition rate must be strictly controlled within the range of 30% to 40%. If the decomposition rate is too high, it will reduce the nitriding efficiency and result in insufficient nitrided layer thickness; if the decomposition rate is too low, it will cause uneven distribution of the nitrided layer, resulting in quality problems such as surface hardness fluctuations and local denitrification. In addition, oxidation of the chain surface is prone to occur during the nitriding process, which affects the bonding force between the nitrided layer and the substrate, and thus leads to peeling and detachment of the nitrided layer. 4. High risk of chain deformation: Nitriding alone requires secondary heating of the tempered chain, which can easily generate residual stress, leading to defects such as chain deformation and bending. A straightening process is required afterward, which further increases production costs and process complexity. 5. Significant environmental pressure: Nitriding alone can cause ammonia leakage and waste gas emissions, requiring corresponding environmental protection equipment; traditional electric arc furnace smelting produces a large amount of pollutants, which does not conform to the concept of green production. Summary of the Invention

[0004] To address the problems of low efficiency, high energy consumption, cumbersome separate nitriding process, and unstable quality in traditional 20Mn2 steel preparation processes (electric arc furnace), this invention aims to provide a method for simultaneous nitriding of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains. This invention employs a converter-refining-continuous casting process to prepare 20Mn2 rare earth optimized steel billets, then vacuum-seales iron nitride powder within the billet, utilizing the temperatures of hot rolling and heat treatment to achieve simultaneous surface nitriding. This eliminates the need for a separate subsequent nitriding process, ensuring a chain strength of 90Kg, surface hardness ≥HRC50, and significantly improved core toughness and elongation, fully meeting the usage standards for DAT-grade electric hoist chains.

[0005] To achieve the above objectives, the present invention provides the following technical solution: One of the technical solutions of this invention is a method for simultaneous nitriding of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains, comprising the following steps: (1) The billet body is placed into the sleeve, and iron nitride powder is filled in the gap between the billet body and the sleeve. Then the inside of the sleeve is evacuated and sealed to form a combined billet. (2) The combined casting billet is subjected to heat treatment and hot rolling treatment in sequence to obtain wire rod; (3) The wire rod is subjected to quenching and tempering treatment, which is to perform quenching and tempering treatment in sequence; during the quenching and tempering treatment, ammonia-containing gas is introduced; (4) The heat-treated wire rod is formed into chain blanks and then post-processed to obtain finished chains.

[0006] The second technical solution of the present invention is an electric hoist chain prepared by the above preparation method.

[0007] The present invention discloses the following technical effects: 1. By vacuum sealing iron nitride powder in a cast billet, the surface nitriding is achieved simultaneously using the temperatures of hot rolling and quenching and tempering, completely eliminating the separate surface nitriding process in traditional processes and shortening the production cycle.

[0008] 2. Stable nitriding quality and excellent surface properties: By controlling the ammonia decomposition rate to 30%~40% during the quenching and tempering process, combined with a vacuum environment and Ar inert gas protection, problems such as uneven nitriding layer and surface oxidation are effectively avoided. The nitriding layer thickness is uniform (0.15~0.25mm), and the surface hardness reaches HRC52~56, far exceeding the DAT grade standard requirements (≥HRC50). Moreover, the nitriding layer is tightly bonded to the substrate, with no peeling or detachment, and the wear resistance and corrosion resistance are significantly improved.

[0009] 3. Low risk of chain deformation and high product qualification rate: Simultaneous nitriding utilizes the existing heating processes of hot rolling and tempering, eliminating the need for secondary separate heating of the chain, effectively reducing residual stress generation, reducing chain deformation rate by more than 80%, eliminating the need for additional straightening processes, and resulting in a high product qualification rate.

[0010] 4. The entire process uses Ar inert gas protection to reduce ammonia leakage and exhaust emissions, making it environmentally friendly and pollution-free; Detailed Implementation

[0011] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0012] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0013] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0014] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0015] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0016] Existing converter steelmaking technologies for 20Mn2 steel often fail to employ precise rare-earth-titanium-boron composite microalloying schemes, or the cored wires used have issues such as poor sealing performance and low density, resulting in insufficient element yield. Consequently, the steel's strength and toughness cannot meet the performance requirements of DAT-grade chains, and the process is not combined with simultaneous nitriding, requiring separate nitriding treatment.

[0017] Furthermore, the existing composition design of 20Mn2 steel suffers from insufficient matching between toughness and strength. Even with the addition of rare earth, titanium, and boron elements, it is difficult to meet the core performance requirements of DAT-grade electric hoist chains without precise control of the element addition amount and yield through reasonable processes. Therefore, this invention provides a method for preparing 20Mn2 rare earth optimized steel that achieves precise microalloying using dedicated cored wire and eliminates the need for a separate nitriding process.

[0018] The first aspect of this invention provides a method for simultaneous nitriding of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains, characterized by comprising the following steps: (1) The billet body is placed into the sleeve, and iron nitride powder is filled in the gap between the billet body and the sleeve. Then the inside of the sleeve is evacuated and sealed to form a combined billet. (2) The combined casting billet is subjected to heat treatment and hot rolling treatment in sequence to obtain wire rod; (3) The wire rod is subjected to quenching and tempering treatment, which is to perform quenching and tempering treatment in sequence; during the quenching and tempering treatment, ammonia-containing gas is introduced; (4) The heat-treated wire rod is formed into chain blanks and then post-processed to obtain finished chains.

[0019] In a preferred embodiment of the present invention, the composition of the billet body, by mass percentage, is as follows: C 0.18%~0.23%, Mn 1.40%~1.80%, Si 0.20%~0.40%, P≤0.030%, S≤0.020%, La-Ce mixed rare earth 0.015%~0.025%, Ti 0.015%~0.02%, B 0.002%~0.004%, N≤0.008%, with the remainder being Fe and unavoidable impurities; the mass ratio of La to Ce in the La-Ce mixed rare earth is 3:7~4:6.

[0020] Ingredient optimization explanation: (1) Strictly control the C content within the range of 0.18%~0.23% to avoid excessive C content leading to a decrease in core toughness, and at the same time prevent excessively low C content from affecting chain strength, so as to ensure that the core hardness and toughness are well matched after tempering; (2) The Mn content is controlled at 1.40%~1.80% to improve the hardenability and strength of the steel, laying the foundation for the chain to reach the 90Kg strength standard; (3) La-Ce mixed rare earth (La:Ce=3:7~4:6), Ti and B elements are introduced through refining wire feeding. Rare earth can spheroidize harmful inclusions (MnS, Al2O3) in steel, refine grains, significantly improve core impact toughness and elongation, and improve the bonding force between the nitrided layer and the matrix. Ti can combine with free N in steel to form TiN, inhibit Fe4N precipitation, avoid aging embrittlement, and at the same time play a role in refining grains. B can strengthen grain boundaries, improve the hardenability of steel, and work synergistically with rare earth to achieve a match between strength and toughness. (4) Strictly control the N content to ≤0.008% to avoid excessive N forming harmful nitrides with rare earth and Ti, which would affect the core toughness and nitriding effect.

[0021] In a preferred embodiment of the present invention, the method for preparing the billet body includes the following steps: (S1) The raw materials are prepared according to the composition of the billet body, and then melted to obtain molten steel; (S2) The molten steel is refined. At the end of the refining process, a rare earth-titanium-boron composite vacuum-sealed high-density cored wire is fed into the molten steel to obtain refined molten steel. (S3) Under a protective atmosphere, the refined molten steel is continuously cast to obtain the billet body.

[0022] More preferably, after smelting, the tapping temperature is controlled at 1660~1680℃. 。

[0023] More preferably, the refining temperature is 1550~1600℃, and the holding time is 30~40min.

[0024] More preferably, the preparation method of rare earth-titanium-boron composite vacuum-sealed high-density cored wire is as follows: (1) Weigh out lanthanum cerium rare earth alloy powder, titanium powder and boron powder for batching, and then mix them evenly under a protective atmosphere to obtain a mixed powder; (2) After pretreatment of the low carbon steel cylinder, the mixed powder is filled in under a protective atmosphere, and then pre-sealed to obtain a composite blank; (3) The composite blank is reduced in hydrogen and then sealed and welded under vacuum conditions to obtain the vacuum-sealed composite blank. (4) Heating the vacuum-sealed composite blank and then hot drawing it to obtain the cored wire; In step (1), by mass percentage, the mixed powder contains 50%-70% lanthanum-cerium rare earth alloy powder, 20%-35% titanium powder, and 5%-15% boron powder; the mass ratio of La to Ce in the lanthanum-cerium rare earth alloy powder is 3:7-4:6, and the total rare earth content is ≥99%; the mixed powder contains ≤0.03% oxygen, ≤0.05% carbon, ≤0.2% silicon, and ≤0.5% iron.

[0025] In step (2), the filling rate of the mixed powder is 75%-85%. In step (2), the preliminary sealing is performed by inserting low-carbon steel plugs into both ends of the low-carbon steel cylinder and pre-sealing them by argon arc spot welding.

[0026] In step (3), the temperature of the reduction reaction is 350-550℃ and the time is 2-4h; after the reduction reaction is completed, the furnace is cooled to ≤50℃. In step (4), the composite blank after vacuum sealing is heated to 600-800℃; the hot drawing is performed in 4-8 passes, with a deformation of 15%-25% per pass. The drawing process is carried out under a protective atmosphere, and vacuum annealing at 450-550℃ for 1 hour is performed every 2-3 passes until the finished product diameter is reached. 8-13mm, density ≥95%.

[0027] In this invention, the rare earth-titanium-boron composite vacuum-sealed high-density cored wire achieves high yields (≥75%) of rare earth, titanium, and boron elements through composite addition, with composition fluctuations ≤±5ppm. The three elements work synergistically: rare earth can spheroidize harmful inclusions and refine grains; titanium can fix nitrogen and prevent aging; and boron can improve hardenability. Combined with composition optimization, the chain core performance achieves: core hardness 28~32HRC, elongation ≥22%, and impact toughness (A... KV With a strength of ≥52J, reaching the 90Kg standard, it fully meets the load-bearing requirements of DAT-grade electric hoist chains; and it is compatible with the industrial continuous production of converter-refining-continuous casting process, requiring no secondary processing and can be directly used in the refining wire feeding process.

[0028] More preferably, the feeding speed of the rare earth-titanium-boron composite vacuum-sealed high-density cored wire is 2~4m / min, and the addition amount is 0.8~1.2kg / t steel.

[0029] In the final stage of refining, a wire feeder is used to feed the above-mentioned rare earth-titanium-boron composite vacuum-sealed high-density cored wire into the molten steel to ensure that the cored wire is completely melted, so that rare earth, titanium and boron elements are uniformly diffused and precise microalloying is achieved. During the wire feeding process, Ar inert gas is continuously introduced for protection to avoid element oxidation and ensure that the element recovery rate is ≥75%. During the refining process, the N content of the molten steel is strictly controlled to be ≤0.008% to ensure the subsequent nitriding effect and core toughness.

[0030] More preferably, the continuous casting temperature is controlled at 1500~1550℃, and the casting speed is 1.2~1.8m / min. After continuous casting, a billet body is obtained; the process also includes surface grinding and rust removal of the billet body to remove surface oxide scale, inclusions and burrs, ensuring a clean and smooth surface, laying the foundation for subsequent encapsulation and nitrogen atom diffusion.

[0031] In a preferred embodiment of the present invention, the sheath is a seamless low-carbon steel cylinder with a length consistent with the billet body, and vacuum ports are reserved at both ends of the sheath.

[0032] In this invention, the seamless low-carbon steel cylinder is made of cold-rolled low-carbon steel, preferably SPCC grade low-carbon steel (C≤0.05%). The sheath adopts a seamless structure, which can avoid vacuum seal failure caused by lap joints. The low-carbon steel material can ensure the compatibility between the sheath and the billet body, and at the same time, it can be partially fused with the billet body during hot rolling without affecting the final performance of the chain.

[0033] In a preferred embodiment of the present invention, before placing the billet body into the sleeve, a pretreatment step of the sleeve is further included; the pretreatment consists of: sequentially performing alkaline washing to remove oil, water washing, acid pickling to remove rust, water washing, and drying. This pretreatment removes oil, scale, and impurities from the inner wall of the sleeve, preventing contamination of the iron nitride powder and the billet body.

[0034] In a preferred embodiment of the present invention, the iron nitride powder is γ'-Fe4N powder; the purity of the γ'-Fe4N powder is ≥99%, and the particle size is 100~200 mesh; the filling rate of the iron nitride powder is 80%~90%. During the powder filling process, a vibration compaction process is adopted to ensure that the iron nitride powder is densely packed. γ'-Fe4N powder has a high nitrogen content and stable nitrogen release, and can slowly release nitrogen atoms at hot rolling and heat treatment temperatures, achieving simultaneous nitriding of the billet surface, while avoiding the pollution problems caused by traditional ammonia nitriding.

[0035] In a preferred embodiment of the present invention, evacuating and sealing the inside of the casing specifically involves: evacuating the inside of the casing to a vacuum level of 1×10⁻⁶. -2 ~1×10 -3Pa, during the vacuuming process, heat to 200~300℃ and keep warm for 1~2 hours to completely remove residual air and moisture from the casing. After vacuuming, vacuum seal the vacuum interface and both ends of the casing to ensure that the casing is in a vacuum state, prevent air from entering and causing oxidation of iron nitride powder, and provide a stable environment for nitrogen atom diffusion.

[0036] In a preferred embodiment of the present invention, the heat treatment is: holding at 1100~1150℃ for 2~3 hours; the hot rolling process is: performing multiple hot rolling passes on the heat-treated composite billet, with the final rolling temperature controlled at 880~920℃, and rolling it into wire rod.

[0037] Ar inert gas (purity ≥99.99%) is introduced throughout the heat treatment process to prevent oxidation of the cladding surface and ensure uniform heating. The heating temperature avoids excessively high temperatures that could cause the cladding to melt, while also preventing excessively low temperatures that could affect nitrogen atom diffusion.

[0038] During hot rolling, the γ'-Fe4N powder inside the bladder slowly decomposes under high temperature, releasing nitrogen atoms. The nitrogen atoms diffuse rapidly to the surface of the billet body in a vacuum environment, forming a preliminary nitrided layer (thickness 0.08~0.12mm). At the same time, the pressure during hot rolling can promote the diffusion of nitrogen atoms and improve the bonding force between the nitrided layer and the matrix.

[0039] After hot rolling, the wire rod is cooled to room temperature under Ar inert gas protection to avoid surface oxidation during cooling and to ensure the stability of the initial nitriding layer. After cooling, the sheath has partially fused with the wire rod surface during hot rolling. The remaining sheath can be naturally detached in subsequent processes or removed by simple grinding, without the need for a complex peeling process.

[0040] In a preferred embodiment of the present invention, the quenching treatment is as follows: the wire rod is heated to 860~880°C in a mixed atmosphere of inert gas and ammonia, held at that temperature for 2~3 hours, and then oil-cooled to room temperature; during this period, the ammonia decomposition rate is controlled to be 30%~40%.

[0041] During the quenching process, nitrogen atoms produced by the decomposition of ammonia and nitrogen atoms released by the incompletely decomposed γ'-Fe4N powder in the cladding work together to further diffuse to the surface of the wire rod, thickening the nitriding layer to a thickness of 0.15~0.25mm. Controlling the ammonia decomposition rate at 30%~40% can avoid excessively high decomposition rates that lead to rapid release of nitrogen atoms and a decrease in nitriding efficiency, while also preventing excessively low decomposition rates that result in an uneven nitriding layer.

[0042] In a preferred embodiment of the present invention, the tempering treatment is as follows: the quenched wire rod is heated to 420~450°C in a mixed atmosphere of inert gas and ammonia, held at that temperature for 3~4 hours, and then air-cooled to room temperature; during this period, the ammonia decomposition rate is controlled to be maintained at 30%~40%.

[0043] During the tempering process, the ammonia decomposition rate is maintained at 30%~40%, and nitrogen atoms further diffuse and homogenize, making the hardness of the nitrided layer tend to stabilize. At the same time, the tempering treatment is completed, so that the core of the wire rod forms a uniform tempered sorbite structure, ensuring that the core performance meets the standards.

[0044] After the quenching and tempering process is completed, the thickness and hardness of the nitrided layer on the wire rod surface are tested to ensure that the nitrided layer thickness is 0.15~0.25mm and the surface hardness is ≥HRC50. Unqualified products need to be quenched and tempered again.

[0045] In a preferred embodiment of the present invention, the heat-treated wire rod is formed into a chain blank, specifically by: cold heading, bending, and welding the heat-treated wire rod to form an electric hoist chain blank; due to the excellent toughness and elongation of the core, defects such as cracking and deformation of the chain can be effectively avoided during the cold heading process; then the chain blank is surface polished to remove residual casing debris and oxide scale, ensuring a smooth chain surface; subsequently, rust prevention treatment is performed, and after packaging, the finished DAT-grade electric hoist chain is obtained; the entire preparation process does not require a separate nitriding process, effectively simplifying the production process.

[0046] A second aspect of the present invention provides an electric hoist chain prepared by the above-described preparation method.

[0047] The electric hoist chain provided by this invention is made of advanced steel material. Electric hoist chains are typical wire products manufactured from wire (coiled wire) through processes such as drawing and chain braiding. The electric hoist chain obtained using the process of this invention achieves a strength of 90 kg, a surface hardness ≥ HRC50, and significantly improved core toughness and elongation.

[0048] Unless otherwise specified, the technical solutions described in this invention are all conventional solutions in the field, and the reagents or raw materials used are all purchased from commercial channels or are publicly available unless otherwise specified.

[0049] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0050] Example 1 1. Composition (mass percentage) of 20Mn2 rare earth optimized steel (cast billet body): C 0.20%, Mn 1.60%, Si 0.30%, P 0.025%, S 0.015%, La-Ce mixed rare earth (La:Ce=3.5:6.5) 0.018%, Ti 0.018%, B 0.003%, N 0.006%, the remainder being Fe and unavoidable impurities; The method for preparing the billet body includes the following steps: (S1) The billet is batched according to its composition, then melted. After melting, the tapping temperature is controlled at 1665℃ to obtain molten steel. (S2) The molten steel is refined (the refining temperature is 1580℃ and the holding time is 35min). At the end of the refining process, a rare earth-titanium-boron composite vacuum-sealed high-density cored wire is fed into the molten steel at a feeding speed of 3m / min and an addition amount of 1.0kg / t steel. Argon gas is used for protection during the feeding process to obtain the refined molten steel. The preparation method of rare earth-titanium-boron composite vacuum-sealed high-density cored wire is as follows: 1.1. Core powder composition (mass percentage): 60% lanthanum-cerium rare earth alloy powder (La:Ce=3.5:6.5, RE=99.2%), 30% titanium powder (Ti=99.1%), 10% boron powder (B=97.5%); Core powder impurity control: O=0.022%, C=0.04%, Si=0.15%, Fe=0.4%; Particle size control is 200-325 mesh.

[0051] 1.2. Sheath Parameters: Seamless low-carbon steel cylinder (08Al material, C=0.04%), initial diameter 25mm, wall thickness 1.0mm; the low carbon steel cylinder is sequentially subjected to alkaline washing to remove oil, acid washing to remove rust, water washing, drying at 110℃, and Ar purging for 5min.

[0052] 1.3. Powder filling: The filling rate is 80%. During the filling process, the powder is vibrated and compacted. The vibration parameters are 55Hz and 4min. After filling, the powder is pre-sealed. The pre-sealing is done by argon arc spot welding with a weld spacing of 5mm.

[0053] 1.4. Hydrogen reduction process: The core powder was kept at 450℃ for 3 hours in a pure H2 atmosphere (purity 99.999%); after reduction, the oxygen content of the core powder was reduced to 0.012%.

[0054] 1.5. Vacuum sealing: Start the vacuum pump and evacuate to a vacuum level of 1×10⁻⁶. -3 Pa, while heating the temperature inside the vacuum sealing furnace to 250℃ and holding it for 1 hour; then keeping the vacuum level constant, electron beam welding was used with a current of 65mA and a welding speed of 8mm / s; helium mass spectrometry leak detection showed no leakage.

[0055] 1.6. Hot Drawing: The vacuum-sealed composite billet is fed into an induction heating furnace and heated to 700℃. It undergoes six passes of hot drawing, with deformation amounts of 25%, 22%, 20%, 18%, 16%, and 15% respectively per pass. Vacuum annealing (500℃, 1 hour) is performed every two passes. The finished product diameter... 10mm, density 96.5%.

[0056] (S3) Under a protective atmosphere, the refined molten steel is continuously cast at a temperature controlled at 1520℃ and a casting speed of 1.5 m / min. A 100mm×1000mm billet body is produced. After the billet cools, the surface is polished and rust is removed to remove the oxide scale, inclusions and burrs, ensuring a clean and flat surface to obtain the billet body.

[0057] 2. The cladding is a seamless SPCC low-carbon steel cylinder with a wall thickness of 1.2mm, an inner diameter of 101mm, and a length of 1000mm. The cast billet body is placed inside the cladding, and the gap between the cast billet body and the cladding is filled with γ'-Fe4N (purity 99.2%, particle size 150 mesh) at a filling rate of 85%. Subsequently, the inside of the cladding is evacuated to a vacuum degree of 5×10⁻⁶. -3 Pa was sealed using electron beam welding (65mA current, 8mm / s welding speed), and leak detection by helium mass spectrometry confirmed no leakage, forming a composite casting billet; 3. The combined cast billet is subjected to heat treatment and hot rolling in sequence to obtain wire rod; The parameters for heat treatment are: temperature 1120℃, holding time 2.5h, and Ar inert gas protection; the parameters for hot pressing treatment are: 8 passes of hot rolling, final rolling temperature 900℃, and rolling to... 11mm wire rod, cooled to room temperature under Ar inert gas protection; 4. The wire rod is subjected to quenching and tempering treatment, which involves quenching and tempering in sequence; during the quenching and tempering process, ammonia-containing gas is introduced. The quenching temperature was 870℃, held for 2.5 hours, and then oil-cooled to room temperature; the tempering temperature was 430℃, held for 3.5 hours, and then air-cooled to room temperature; Ar inert gas protection was used throughout the process, and the ammonia decomposition rate was controlled at 35%. 5. The heat-treated wire rod is cold-forged, bent, and welded to form an electric hoist chain, which is then polished and rust-proofed to obtain the finished chain. 6. Finished product performance testing: (1) Surface properties: The nitrided layer is 0.20 mm thick, the surface hardness is HRC54, the nitrided layer is evenly distributed, and it is tightly bonded to the substrate without peeling. (2) Core properties: Core hardness 30HRC, elongation 23%, impact toughness (A KV 55J; (3) Elemental yields: La-Ce rare earth 78%, Ti 82%, B 85%; (4) Overall performance: The strength reaches the 90Kg standard, with no defects such as deformation or cracking. It has excellent wear resistance and corrosion resistance, and fully meets the usage standards of DAT grade electric hoist chains. (5) Production efficiency and cost: The production cycle is shortened by 52% compared with the traditional electric arc furnace + separate nitriding process, energy consumption is reduced by 23%, equipment maintenance costs are reduced by 42%, and the product qualification rate is 98.5%.

[0058] Example 2 1. Composition (mass percentage) of 20Mn2 rare earth optimized steel (cast billet body): C 0.18%, Mn 1.40%, Si 0.25%, P 0.020%, S 0.010%, La-Ce mixed rare earth (La:Ce=3:7) 0.025%, Ti 0.020%, B 0.004%, N 0.005%, the remainder being Fe and unavoidable impurities; The method for preparing the billet body includes the following steps: (S1) The billet is batched according to its composition, then melted. After melting, the tapping temperature is controlled at 1672℃ to obtain molten steel. (S2) The molten steel is refined (the refining temperature is 1550℃ and the holding time is 30min). At the end of the refining process, a rare earth-titanium-boron composite vacuum-sealed high-density cored wire is fed into the molten steel at a feeding speed of 2.5m / min and an addition amount of 1.2kg / t steel. Argon gas is used for protection during the feeding process to obtain the refined molten steel. The preparation method of rare earth-titanium-boron composite vacuum-sealed high-density cored wire is as follows: 1.1. Core powder composition (mass percentage): 55% lanthanum-cerium rare earth alloy powder (La:Ce=3:7, RE=99.0%), 25% titanium powder (Ti=99.0%), 20% boron powder (B=97.2%); Core powder impurity control: O=0.028%, C=0.045%, Si=0.18%, Fe=0.45%; Particle size control is 100-200 mesh.

[0059] 1.2. Sheathing parameters: Seamless low-carbon steel cylinder (SPCC material, C=0.05%), initial diameter 28mm, wall thickness 1.2mm; the low carbon steel cylinder is sequentially subjected to alkaline washing to remove oil, acid washing to remove rust, water washing, drying at 120℃, and Ar purging for 6 minutes.

[0060] 1.3. Powder filling: The filling rate is 78%. During the filling process, the powder is vibrated and compacted. The vibration parameters are 60Hz and 3.5min. After filling, the powder is pre-sealed. The pre-sealing is done by argon arc spot welding with a weld spacing of 4mm.

[0061] 1.4. Hydrogen reduction process: The core powder is kept at 500℃ for 3.5h in a pure H2 atmosphere (purity 99.999%); after reduction, the oxygen content of the core powder is reduced to 0.015%.

[0062] 1.5. Vacuum sealing: Start the vacuum pump and evacuate to a vacuum level of 1×10⁻⁶. -3 Pa, while the temperature inside the vacuum sealing furnace was heated to 280℃ and held for 1.5h; then, the vacuum level was kept constant and laser welding was used with a power of 1.5kW and a welding speed of 12mm / s; helium mass spectrometry leak detection showed no leakage.

[0063] 1.6. Hot Drawing: The vacuum-sealed composite billet is fed into an induction heating furnace and heated to 750℃. It undergoes seven passes of hot drawing, with deformation amounts of 25%, 22%, 20%, 18%, 16%, 15%, and 15% respectively per pass. Vacuum annealing (520℃, 1h) is performed every three passes. The finished product diameter... 11mm, density 95.8%.

[0064] (S3) Under a protective atmosphere, the refined molten steel is continuously cast at a temperature controlled at 1520℃ and a casting speed of 1.5 m / min. The billet body is 80mm×800mm; after the billet cools, the surface is polished and rust removed to remove the oxide scale, inclusions and burrs, ensuring a clean and flat surface to obtain the billet body.

[0065] 2. The cladding is a seamless SPCC low-carbon steel cylinder with a wall thickness of 1.0 mm, an inner diameter of 80.8 mm, and a length of 800 mm. The cast billet body is placed inside the cladding, and the gap between the cast billet body and the cladding is filled with γ'-Fe4N (purity 99.0%, particle size 120 mesh) at a filling rate of 80%. Subsequently, the inside of the cladding is evacuated to a vacuum degree of 1×10⁻⁶. -3 Pa was sealed using electron beam welding (current 55mA, welding speed 6mm / s), and leak detection by helium mass spectrometry confirmed no leakage, forming a composite casting billet; 3. The combined cast billet is subjected to heat treatment and hot rolling in sequence to obtain wire rod; The parameters for heat treatment are: temperature 1100℃, holding time 2 hours, and Ar inert gas protection; the parameters for hot pressing treatment are: 6 passes of hot rolling, final rolling temperature 880℃, and rolling to... 10mm wire rod, cooled to room temperature under Ar inert gas protection; 4. The wire rod is subjected to quenching and tempering treatment, which involves quenching and tempering in sequence; during the quenching and tempering process, ammonia-containing gas is introduced. The quenching temperature was 860℃, held for 2 hours, and then oil-cooled to room temperature; the tempering temperature was 420℃, held for 3 hours, and then air-cooled to room temperature; Ar inert gas protection was used throughout the process, and the ammonia decomposition rate was controlled at 30%. 5. The heat-treated wire rod is cold-forged, bent, and welded to form an electric hoist chain, which is then polished and rust-proofed to obtain the finished chain. 6. Finished product performance testing: (1) Surface properties: The nitrided layer is 0.18 mm thick, the surface hardness is HRC52, and the nitrided layer is uniformly distributed; (2) Core properties: Core hardness 28HRC, elongation 24%, impact toughness (A KV 58J; (3) Elemental yields: La-Ce rare earth 76%, Ti 80%, B 83%; (4) Overall performance: The strength reaches the 90Kg standard, the toughness is excellent, it is suitable for high frequency and heavy load scenarios, and meets the DAT standard; the production cycle is shortened by 50% compared with the traditional process, and the energy consumption is reduced by 20%.

[0066] Example 3 1. Composition (mass percentage) of 20Mn2 rare earth optimized steel (billet body): C 0.23%, Mn 1.80%, Si 0.40%, P 0.030%, S 0.020%, La-Ce mixed rare earth (La:Ce=4:6) 0.015%, Ti 0.015%, B 0.002%, N 0.008%, the remainder being Fe and unavoidable impurities; The method for preparing the billet body includes the following steps: (S1) The billet is batched according to its composition, then melted. After melting, the tapping temperature is controlled at 1350℃ to obtain molten steel. (S2) The molten steel is refined (the refining temperature is 1600℃ and the holding time is 40min). At the end of the refining process, a rare earth-titanium-boron composite vacuum-sealed high-density cored wire is fed into the molten steel at a feeding speed of 3.5m / min and an addition amount of 0.8kg / t steel. Argon gas is used for protection during the feeding process to obtain the refined molten steel. The preparation method of rare earth-titanium-boron composite vacuum-sealed high-density cored wire is as follows: 1.1. Core powder composition (mass percentage): 70% lanthanum-cerium rare earth alloy powder (La:Ce=4:6, RE=99.1%), 20% titanium powder (Ti=99.0%), 10% boron powder (B=97.0%); Core powder impurity control: O=0.025%, C=0.05%, Si=0.2%, Fe=0.5%; Particle size control is 100-325 mesh.

[0067] 1.2. Sheathing parameters: Seamless low-carbon steel cylinder (SPCC material, C=0.05%), initial diameter 20mm, wall thickness 0.8mm; the low carbon steel cylinder is sequentially subjected to alkaline washing to remove oil, acid washing to remove rust, water washing, drying at 100℃, and Ar purging for 4 minutes.

[0068] 1.3. Powder filling: The filling rate is 85%. During the filling process, the powder is vibrated and compacted. The vibration parameters are 50Hz and 5min. After filling, the powder is pre-sealed. The pre-sealing is done by argon arc spot welding with a weld spacing of 6mm.

[0069] 4. Hydrogen reduction process: The core powder is kept at 380℃ for 2.5 hours in a pure H2 atmosphere (99.999% purity); after reduction, the oxygen content of the core powder is reduced to 0.01%.

[0070] 1.5. Vacuum sealing: Start the vacuum pump and evacuate to a vacuum level of 1×10⁻⁶. -2 Pa, while heating the temperature inside the vacuum sealing furnace to 220℃ and holding it for 1 hour; then keeping the vacuum level constant, electron beam welding was used with a current of 55mA and a welding speed of 6mm / s; helium mass spectrometry leak detection showed no leakage.

[0071] 1.6. Hot Drawing: The vacuum-sealed composite billet is fed into an induction heating furnace and heated to 650℃. It undergoes four passes of hot drawing, with deformation amounts of 25%, 22%, 20%, and 18% respectively per pass. Vacuum annealing (480℃, 1h) is performed every two passes. The finished product diameter... 8mm, density 95.2%.

[0072] (S3) Under a protective atmosphere, the refined molten steel is continuously cast at a temperature controlled at 1550℃ and a casting speed of 1.8 m / min. The billet body is 120mm×1200mm; after the billet cools, the surface is polished and rust removed to remove the oxide scale, inclusions and burrs, ensuring a clean and flat surface to obtain the billet body.

[0073] 2. The cladding is a seamless SPCC low-carbon steel cylinder with a wall thickness of 1.5mm, an inner diameter of 121.0mm, and a length of 1200mm. The cast billet body is placed inside the cladding, and the gap between the cast billet body and the cladding is filled with γ'-Fe4N (purity 99.0%, particle size 180 mesh) at a filling rate of 90%. Subsequently, the inside of the cladding is evacuated to a vacuum degree of 1×10⁻⁶. -8 Pa was sealed using electron beam welding (75mA current, 9mm / s welding speed), and leak detection by helium mass spectrometry confirmed no leakage, forming a composite casting billet; 3. The combined cast billet is subjected to heat treatment and hot rolling in sequence to obtain wire rod; The parameters for heat treatment are: temperature 1150℃, holding time 3 hours, and Ar inert gas protection; the parameters for hot pressing treatment are: 7 passes of hot rolling, final rolling temperature 920℃, and rolling to... 13mm wire rod, cooled to room temperature under Ar inert gas protection; 4. The wire rod is subjected to quenching and tempering treatment, which involves quenching and tempering in sequence; during the quenching and tempering process, ammonia-containing gas is introduced. The quenching temperature was 880℃, held for 3 hours, and then oil-cooled to room temperature; the tempering temperature was 450℃, held for 4 hours, and then air-cooled to room temperature; Ar inert gas protection was used throughout the process, and the ammonia decomposition rate was controlled at 40%. 5. The heat-treated wire rod is cold-forged, bent, and welded to form an electric hoist chain, which is then polished and rust-proofed to obtain the finished chain. 6. Finished product performance testing: (1) Surface properties: The nitrided layer is 0.25 mm thick, the surface hardness is HRC56, and the wear resistance is excellent; (2) Core properties: Core hardness 32HRC, elongation 22%, impact toughness (A KV 52J; (3) Elemental yields: La-Ce rare earth 75%, Ti 79%, B 82%; (4) Overall performance: The strength reaches the 90Kg standard, the surface hardness is high, it is suitable for scenarios with high wear resistance requirements, and meets the DAT standard; the production cycle is shortened by 55% compared with the traditional process, and the energy consumption is reduced by 25%.

[0074] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for the preparation of 20Mn2 rare earth optimized steel for DAT grade electric hoist chain by simultaneous nitriding, characterized in that, Includes the following steps: (1) Place the billet body into the sleeve, fill the gap between the billet body and the sleeve with iron nitride powder, then evacuate and seal the inside of the sleeve to form a composite billet; (2) The combined casting billet is subjected to heat treatment and hot rolling treatment in sequence to obtain wire rod; (3) The wire rod is subjected to quenching and tempering treatment, which is to perform quenching and tempering treatment in sequence; during the quenching and tempering treatment, ammonia-containing gas is introduced; (4) The heat-treated wire rod is formed into chain blanks and then post-processed to obtain finished chains.

2. The method for preparing 20Mn2 rare earth optimized steel for DAT grade electric hoist chain by synchronous nitriding according to claim 1, characterized in that, The composition of the billet body, by mass percentage, is as follows: C 0.18%~0.23%, Mn 1.40%~1.80%, Si 0.20%~0.40%, P≤0.030%, S≤0.020%, La-Ce mixed rare earth 0.015%~0.025%, Ti 0.015%~0.02%, B 0.002%~0.004%, N≤0.008%, with the remainder being Fe and unavoidable impurities; the mass ratio of La to Ce in the La-Ce mixed rare earth is 3:7~4:

6.

3. The method for preparing 20Mn2 rare earth optimized steel for DAT grade electric hoist chain by synchronous nitriding according to claim 1, characterized in that, The sheath is a seamless low-carbon steel cylinder with a length consistent with the billet body, and vacuum ports are reserved at both ends of the sheath.

4. The method for preparing 20Mn2 rare earth optimized steel for DAT grade electric hoist chain by synchronous nitriding according to claim 1, characterized in that, The iron nitride powder is γ'-Fe4N powder; the purity of the γ'-Fe4N powder is ≥99%, and the particle size is 100~200 mesh; the filling rate of the iron nitride powder is 80%~90%.

5. The method for simultaneous nitriding preparation of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains according to claim 1, characterized in that, The heat treatment is: holding at 1100~1150℃ for 2~3 hours; the hot rolling process is: hot rolling the heat-treated composite billet in multiple passes, with the final rolling temperature controlled at 880~920℃, to roll it into wire rod.

6. The method for simultaneous nitriding preparation of 20Mn2 rare earth optimized steel for DAT-grade electric hoist chains according to claim 1, characterized in that, The quenching process is as follows: the wire rod is heated to 860~880℃ in a mixed atmosphere of inert gas and ammonia, held at that temperature for 2~3 hours, and then oil-cooled to room temperature; during this period, the ammonia decomposition rate is controlled to be 30%~40%.

7. The method for preparing 20Mn2 rare earth optimized steel for DAT grade electric hoist chain by synchronous nitriding according to claim 1, characterized in that, The tempering process is as follows: the quenched wire rod is heated to 420~450℃ in a mixed atmosphere of inert gas and ammonia, held for 3~4 hours, and then air-cooled to room temperature; during this period, the ammonia decomposition rate is controlled to be maintained at 30%~40%.

8. The electric hoist chain prepared by the preparation method according to any one of claims 1 to 7.