Method for improving fatigue performance of 304 austenitic stainless steel root canal file

Abstract

The invention provides a method for improving the fatigue performance of a 304 austenitic stainless steel root canal file, which comprises the following steps: carrying out high-temperature solution treatment and heat preservation on a 304 austenitic stainless steel bar, and carrying out multi-pass cold deformation and heat treatment co-treatment on the treated stainless steel bar to implement drawing reducing treatment to obtain the 304 austenitic stainless steel root canal file. Directionally inducing the Sigma 3 crystal boundary to extend along the {111} crystal face to obtain a wire material; the wire is subjected to plastic shape righting and straightening, high-temperature annealing is conducted after straightening, grain boundary reconstruction is achieved, and the treated wire is obtained and serves as a raw material of the stainless steel root canal file; and grinding the treated wire material to obtain a blank for preparing the root canal file, and then applying periodic torsion to the blank to obtain the root canal file with the core part being the penetrating low-energy grain boundary network. By the adoption of the technical scheme, the proportion of the low sigma CSL grain boundary of the material is increased, the bending fatigue life of the root canal file is prolonged by about 60%, cost is low, and efficiency is high.

Description

Technical Field

[0001] The present invention relates to the technical field of material processing, and in particular to a method for improving the fatigue performance of a 304 austenitic stainless steel root canal file. Background Art

[0002] In the field of oral root canal treatment, the mechanical properties and fatigue life of root canal instruments are crucial to clinical treatment effects and medical safety. Nickel-titanium alloy instruments have excellent flexibility in the root canal shaping stage and occupy a mainstream application position, but traditional stainless steel instruments still play an irreplaceable role in operations such as root canal exploration and calcification channel dredging, relying on the precise tactile feedback and controllable cutting brought by rigidity. Among them, the cost-effective 304 austenitic stainless steel K-type / H-type files are widely used preparatory tools in domestic clinical practice. However, stainless steel instruments often have structural failure problems in clinical applications. About 60% of intraoperative instrument separations occur at the bend of the root canal, and the fracture mechanisms include torsional fracture and bending fatigue fracture. The geometric parameters of the instrument and the root canal curvature radius will affect the stress distribution, but the internal defects of the material are the key to restricting the fracture resistance.

[0003] Scanning electron microscopy analysis of fracture interfaces reveals that stainless steel instrument failure often manifests as a mixed intergranular and transgranular fracture pattern characterized by micropore aggregation and dimple formation. Defects such as brittle inclusions (e.g., Al₂O₃, SiO₂) and Cr / Mo segregation within the material not only disrupt matrix continuity but also create stress concentration points at grain boundaries, accelerating crack propagation dynamics. Studies have shown that the fragility of the grain boundary structure in austenitic stainless steel is a key factor contributing to its increased fracture susceptibility. On the one hand, the high proportion of high-angle random grain boundaries (HAGBs) (>70%) significantly reduces the interfacial energy of the grain boundaries due to their high interfacial strength. On the other hand, the precipitation of second-phase particles, such as σ phase and carbides, at the grain boundaries triggers localized embrittlement, forming preferential nucleation sites for microcracks. Under torsional loading, the synergistic effect of the high-angle random grain boundaries and second-phase particles promotes rapid crack propagation along the grain boundaries. During bending fatigue, dislocations accumulate at the grain boundaries, inducing stress concentrations that ultimately lead to fracture.

[0004] In response to the low-cycle fatigue strength and grain boundary-dominated failure problems of 304 austenitic stainless steel, recent studies have used additive manufacturing, surface nanomaterials and other process optimization methods. However, they can only form gradient nanostructures (thickness < 200 μm) on the surface, and cannot control the core structure and grain boundary defects of the material. The overall fatigue life is limited, and because it is difficult to reconstruct the full-section grain boundary network, cracks can still extend along the original large-angle grain boundaries. Summary of the Invention

[0005] In response to the above technical problems, the present invention discloses a method for improving the fatigue performance of 304 austenitic stainless steel root canal files. Without increasing costs and production processes, grain boundary engineering technology is integrated into the forming process of the stainless steel root canal files. The deformation amount is controlled during the forming process, and the content of special grain boundaries is increased through repeated recrystallization processes, thereby increasing the stability of the grain boundaries and improving the fatigue performance of the stainless steel root canal files.

[0006] To this end, the technical solution adopted in the present invention is:

[0007] A method for improving the fatigue performance of a 304 austenitic stainless steel root canal file comprises the following steps:

[0008] Step S1: subjecting a 304 austenitic stainless steel bar to a high-temperature solution treatment, keeping the temperature, and water-cooling to obtain a stainless steel bar with a completely austenitic structure.

[0009] Step S2, performing a coordinated process of multiple cold deformation and heat treatment on the stainless steel bar obtained in step S1 to perform a drawing and diameter reduction process to obtain a wire;

[0010] The coordinated treatment of cold deformation and heat treatment in each pass includes cold deformation and drawing, followed by high-temperature short-time annealing to directionally induce the expansion of the ∑3 grain boundary along the {111} crystal plane; the strain of a single cold deformation pass is no more than 8%, the annealing temperature is higher than the recrystallization critical temperature of 304 austenitic stainless steel, each annealing time is no more than 20 minutes, and water cooling is used;

[0011] In this step, a staged small strain accumulation method is adopted, and high-temperature short-time annealing is performed after each cold deformation. The annealing temperature is higher than the recrystallization critical value and the annealing time is sufficient to eliminate the work hardening, directionally inducing the ∑3 grain boundary to expand along the {111} crystal plane.

[0012] In step S3, the wire obtained in step S2 is plastically straightened and then subjected to high-temperature annealing to restructure the grain boundaries and form a penetrating low-energy grain boundary network. The treated wire is then used as raw material for stainless steel root canal files. During plastic straightening, the single-roller penetration is ≤0.1 mm. The high-temperature annealing is performed at a temperature above the critical recrystallization temperature of 304 austenitic stainless steel for no more than 15 minutes, with water cooling to eliminate residual stress and promote grain boundary migration. This straightening process, through the combination of low-stress bending deformation and annealing, optimizes both the wire's geometric accuracy and grain boundary network.

[0013] Step S4: Grinding the treated wire to obtain a blank for preparing an endodontic file. Periodic torsion is then applied to the blank to produce an endodontic file with a core composed of a penetrating low-energy grain boundary network, significantly improving the fatigue resistance of the stainless steel endodontic file. In this step, periodic torsion is applied to the precision-ground and ground wire to control the surface grain boundary distribution through thermomechanical coupling. Surface material is removed through micro-grinding, and annealing conditions ensure the restoration of grain boundary migration capacity, forming a continuous low-energy grain boundary pathway, thereby simultaneously improving surface quality and grain boundary stability.

[0014] The above-mentioned technical solution integrates multiple plastic strain passes during straightening and grinding processes in endodontic file manufacturing with a step annealing process, achieving full-cross-section grain boundary engineering and constructing a high-∑CSL grain boundary network throughout the cross-section. This approach improves material fatigue resistance by optimizing process parameters without modifying existing production line equipment, providing a solution for upgrading the reliability of stainless steel endodontic files. This method is applicable to the manufacture of endodontic files of varying specifications, eliminating the need for specialized equipment or complex composition adjustments. The fatigue life of these files is significantly improved compared to traditional processes, with the proportion of low-∑CSL grain boundaries on the surface and core of these stainless steel endodontic files exceeding 65%.

[0015] As a further improvement of the present invention, in step S2, the strain of the single-pass cold deformation is 4%-8%, ensuring uniform distribution of deformation energy storage and providing sufficient driving force for ∑3 grain boundary nucleation in the subsequent annealing process.

[0016] As a further improvement of the present invention, in step S2, the drawing speed is ≤2 m / min to ensure that the wire is evenly stressed and to avoid the initiation of micro cracks caused by local stress concentration.

[0017] As a further improvement of the present invention, in step S2, the annealing temperature is 1000-1100°C, and each annealing time is 10-20 minutes. After annealing, water cooling is performed to fix the recrystallized structure. After drawing, a short annealing is performed to utilize residual strain energy to drive the ∑3 grain boundary to expand along the {111} plane.

[0018] As a further improvement of the present invention, step S3 utilizes a multi-roller continuous straightening process to plastically straighten the wire obtained in step S2. Furthermore, in this multi-roller continuous straightening process, the single-roller penetration is 0.05-0.1 mm, and the cumulative true strain is 4%-8%. Multi-roller progressive straightening is employed, and the single-roller penetration and cumulative true strain are controlled to avoid microstructural inhomogeneities caused by localized shear bands.

[0019] As a further improvement of the present invention, in step S3, the high-temperature annealing temperature is 1050-1100°C for 5-15 minutes. After annealing, water cooling is performed to fix the recrystallized structure. Annealing after straightening using this process utilizes the thermal activation effect to eliminate residual stress, restore grain boundary migration ability, and form penetrating low-energy grain boundary pathways.

[0020] As a further improvement of the present invention, in the grinding process of step S4, the single grinding depth is ≤10μm, the cumulative true strain is 4% to 8%, and the grinding speed is ≤30m / s, so as to avoid fatigue performance degradation caused by excessive grain refinement of the surface layer.

[0021] As a further improvement of the present invention, in the periodic torsion in step S4, the single torsion angle is 5°-15° and the strain is 4%-8%. This technical solution adopts a more matched strain range and annealing parameters for the periodic torsion, promoting dynamic recrystallization and increasing the ∑3 grain boundary ratio.

[0022] As a further improvement of the present invention, after the periodic torsion is completed in step S4, annealing treatment is performed; the annealing treatment is to keep the temperature at 1000-1100° C. for no more than 15 minutes, and then water-cool to fix the recrystallized structure.

[0023] As a further improvement of the present invention, in step S1, the diameter of the 304 austenitic stainless steel bar is φ0.5-φ1.5 mm.

[0024] As a further improvement of the present invention, in step S1, the temperature of the high-temperature solution treatment is 1050-1150°C, the treatment time is greater than 60 minutes, and then the solution is cooled to room temperature to inhibit the precipitation of the σ phase and restructure the structure into a completely austenitic state.

[0025] The present invention also discloses a 304 austenitic stainless steel root canal file, which is prepared by using any of the above methods for improving the fatigue performance of a 304 austenitic stainless steel root canal file; the proportion of low ∑CSL grain boundaries on the surface and core of the 304 austenitic stainless steel root canal file is not less than 65%.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The technical solution of the present invention is based on grain boundary engineering technology. In the multi-pass drawing, straightening, grinding and pin forming process, by controlling the deformation amount and annealing parameters of a single pass, the wire cross section is induced to form a high ratio of ∑3 nThe (n=1,2,3) low-energy grain boundary network achieves grain boundary network reconstruction through process-accumulated strain, combined with short-term annealing to promote recrystallization and twin expansion. This technical solution utilizes grain boundary engineering technology to achieve strain path design that aligns plastic rheology with the direction of ∑3 grain boundary ratio growth. This increases the proportion of low-∑CSL grain boundaries in the cross-section of 304 austenitic stainless steel root canal files to over 65%, effectively improving the fatigue performance of 304 austenitic stainless steel root canal files and increasing their bending fatigue life by approximately 60%. This process is low-cost and highly efficient, and while achieving key performance improvements, it is fully compatible with existing production systems and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the preparation process of a 304 austenitic stainless steel wire root canal file according to an embodiment of the present invention.

[0029] Figure 2 Orientation difference distribution maps and different types of grain boundary reconstruction maps obtained by backscattered electron diffraction (EBSD) for the samples of Comparative Examples 1 and 2 of the present invention (Non-GBE-1, Non-GBE-2); wherein, (a) and (b) are the orientation difference distribution maps and different types of grain boundary reconstruction maps of the Non-GBE-1 sample, and (c) and (d) are the orientation difference distribution maps and different types of grain boundary reconstruction maps of the Non-GBE-2 sample.

[0030] Figure 3 Orientation difference distribution maps and different types of grain boundary reconstruction maps obtained by backscattered electron diffraction (EBSD) of samples (GBE-1, GBE-2) of Examples 1 and 2 of the present invention; among them, (a) and (b) are orientation difference distribution maps and different types of grain boundary reconstruction maps of the GBE-1 sample, and (c) and (d) are orientation difference distribution maps and different types of grain boundary reconstruction maps of the GBE-2 sample.

[0031] Figure 4 Statistical distribution diagram of grain boundary data of samples of Example 1 and Example 2 (GBE-1, GBE-2) of the present invention, and samples of Comparative Example 1 and Comparative Example 2 (Non-GBE-1, Non-GBE-2).

[0032] Figure 5 The figures are the comparison results of fatigue life test of 304 austenitic stainless steel of the embodiment of the present invention and the comparative example. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present invention are described in further detail below.

[0034] Example 1

[0035] A method for improving the fatigue performance of a 304 austenitic stainless steel root canal file comprises the following steps:

[0036] Step S1, original silk tissue regulation:

[0037] Commercial 304 austenitic stainless steel bars (ASTM A276) with a diameter of 1.0 mm were solution treated in an SX2-12-12A resistance furnace at 1100°C for 90 minutes to ensure complete dissolution of carbides and the formation of a single austenite phase. The steel was then water-quenched to room temperature (cooling rate >200°C / s) to inhibit σ phase precipitation and achieve an initial austenitic structure with a uniform average grain size.

[0038] Step S2, multi-pass strain-annealing coordinated control:

[0039] The solution-treated rods were drawn to reduce diameter, with a single-pass strain controlled at 6% and a drawing speed of 1.5 m / min. After drawing, they were briefly annealed at 1050°C for 15 minutes, followed by water cooling. The above process was repeated, with a single-pass strain controlled at 4%, reducing the wire diameter to 0.9 mm. The wire was then annealed at 1050°C for 15 minutes and water-cooled to eliminate work hardening and promote grain recrystallization.

[0040] Step S3: Multi-roll progressive straightening is performed, with a single roller pressing depth of 0.08 mm and strain controlled at 6%. After straightening, annealing is performed at 1080°C for 10 minutes, followed by water cooling.

[0041] In step S4, surface material is removed by micro-grinding, with a single grinding depth of 8 μm and a cumulative true strain of 6%. The grinding speed is 25 m / s. The ground wire is fixed to a CNC rotary swaging machine, and a servo motor drives the mandrel to apply a periodic torsional torque, with a single torsion angle of 10° and a strain range of 6%. After the torsional process is completed, a short annealing treatment is performed at 1050°C for 10 minutes, followed by water cooling. This sample is labeled GBE-1.

[0042] The flow chart of the sample of the above steps S2 to S4 is as follows Figure 1 shown.

[0043] The obtained samples were characterized, mainly including:

[0044] The microstructure of the stainless steel root canal file after forming was characterized. The root canal file was cut slowly using a precision wire cutting machine, and its cross section was cut and polished to a mirror surface step by step using silicon carbide sandpaper (800#→5000#). It was then electropolished under constant temperature control, and the electropolishing fluid was HCLO. 3:The sample was polished with a CH3COOH ratio of 10:90 (volume ratio) at a voltage of 30 V for 3 minutes to remove residual surface strain. Deionized water ultrasonic oscillation and anhydrous ethanol cleaning were then used to ensure that the sample surface was free of residual contamination. EBSD data were collected using a field emission scanning electron microscope (FESEM). The EBSD test area was selected at the center of the cross section, i.e., the core of the material. The electric field acceleration voltage was 20 kV, the beam spot size was 5.5, and the step size was 0.5 μm.

[0045] The Palumbo-Aust criterion is used to classify grain boundary characteristics. <111> Orientation error, the grain boundary with the axis angle tolerance value Δθ = 6° is defined as ∑3CSL grain boundary (grain boundary is generally marked in red), that is, twin boundary; <110> Orientation error, the grain boundary with the axis angle tolerance value Δθ=2.4° is defined as ∑9CSL grain boundary (grain boundary is generally marked in green), and the grain boundary with 31.6° / <110> and 35.4 / <210> Grain boundaries with an orientation misalignment tolerance of Δθ = 0.9° are defined as ∑27CSL grain boundaries (grain boundaries are generally marked in purple); all other grain boundaries are collectively referred to as "other grain boundaries." This criterion outperforms the traditional Brandon criterion (Δθ = 15° / √∑) by setting a stricter angle tolerance threshold. This effectively suppresses the interference of orientation noise on grain boundary type discrimination, significantly improving the recognition accuracy of low-angle grain boundaries and special grain boundary structures.

[0046] Comparative Example 1

[0047] On the basis of Example 1, a sample obtained according to the above steps but without adopting grain boundary engineering (i.e., there is no high-temperature short-time annealing after each single drawing pass in step S2, and the first strain is 9% and the second strain is 3%; there is no high-temperature annealing after straightening in step S3, and there is no short-time annealing treatment in step S4) is used as a comparison sample, namely Non-GBE-1.

[0048] The Non-GBE-1 sample was tested by EBSD, and the distribution of grain boundary characteristics in the core area is as follows Figure 2 (a) Figure 2 (b) shows the data analysis results. Figure 4 As shown in the figure, the proportion of low-∑CSL grain boundaries (∑≤29) is only 45.7%, of which ∑3 grain boundaries account for 35.4%, and ∑9 and ∑27 grain boundaries together account for 10.3%. Within this grain boundary network, high-angle random grain boundaries (HAGBs) dominate (>50%), indicating significant grain boundary structural fragility in the material core, making them a preferred path for crack propagation.

[0049] In contrast, the core region of the GBE-1 sample of Example 1 showed significant optimization, such as Figure 3 (a) Figure 3(b) shows the data analysis results. Figure 4 As shown in the figure, it can be seen that the proportion of low ∑CSL grain boundaries increased to 70.5%, of which ∑3 grain boundaries accounted for 64.5% (including ∑3 n Multiple twins), ∑9 grain boundaries and ∑27 grain boundaries account for 6%. This result shows that the systematic integration of multi-pass processing strain and step annealing successfully induced the reconstruction of the grain boundary network in the entire cross section, forming a ∑3 n A penetrating structure dominated by low-energy grain boundaries.

[0050] Fatigue performance test of samples:

[0051] The test was carried out using a rotary bending fatigue testing machine. A machine-used root canal file with an operating length of 25 mm was selected and placed in a constant temperature and humidity chamber (36°C ± 1°C) for 5 minutes to ensure that the sample temperature was consistent with the oral environment.

[0052] The test parameters were set as follows: continuous rotation mode, torque of 410 mN·m, and rotation speed of 300 rpm. The fatigue life of the root canal file was recorded until the sample broke or the preset number of cycles was reached.

[0053] The fatigue test was used to evaluate the cyclic deformation resistance of the samples. Figure 5 As shown in the figure, the Non-GBE-1 sample without GBE treatment fractured after 152 cycles. In contrast, the GBE-1 sample of Example 1, which underwent GBE, showed significantly improved fatigue performance, with a cycle life of 246 cycles, a 61.8% increase over the Non-GBE-1. This improvement is attributed to the high ∑CSL grain boundary network (∑3 n Such grain boundaries effectively disperse stress, avoid stress concentration, and synergistically delay crack initiation and propagation rates.

[0054] Example 2

[0055] A method for improving the fatigue performance of a 304 austenitic stainless steel root canal file comprises the following steps:

[0056] Step S1, original silk tissue regulation:

[0057] Untreated 304 austenitic stainless steel bars with a diameter of φ1.2 mm were selected as raw materials. The 304 austenitic stainless steel bars were solution treated at 1120°C in a resistance furnace with a model number of SX2-12-12A. The holding time was 80 min and the bars were water quenched to room temperature so that their structure was completely transformed into austenite.

[0058] Step S2, stainless steel root canal file shaping and control:

[0059] The solution-treated rods were drawn to reduce diameter, with a single-pass strain controlled at 8% and a drawing speed of 1.5 m / min. After drawing, they were briefly annealed at 1050°C for 15 minutes, followed by water cooling. The above process was repeated, with a single-pass strain controlled at 6%, reducing the wire diameter to φ1.0 mm. The wires were then annealed at 1050°C for 15 minutes and water-cooled to eliminate work hardening and promote grain recrystallization.

[0060] Step S3: Multi-roll progressive straightening is performed, with a single roller press depth of 0.09 mm and strain controlled at 5%. After straightening, annealing is performed at 1080°C for 10 minutes, followed by water cooling.

[0061] In step S4, surface material is removed by micro-grinding, with a single grinding depth of 9 μm and a cumulative true strain of 5%. The grinding speed is 25 m / s. The ground wire is fixed to a CNC rotary swaging machine, and a servo motor drives the mandrel to apply a periodic torsional torque, with a single torsion angle of 12° and a strain range of 5%. After the torsional process is completed, a short annealing treatment is performed at 1050°C for 10 minutes, followed by water cooling. This sample is labeled GBE-2.

[0062] Comparative Example 2

[0063] On the basis of Example 2, a sample obtained according to the above steps but without adopting grain boundary engineering (i.e., there is no high-temperature short-time annealing after each single drawing pass in step S2, and the first strain is 3%, and the second strain is 9%; there is no high-temperature annealing after straightening in step S3, and there is no short-time annealing treatment in step S4) is used as a comparison sample, namely Non-GBE-2.

[0064] The Non-GBE-2 sample was tested by EBSD, and the distribution of grain boundary characteristics in the core area is as follows: Figure 2 (c) Figure 2 (d) shows the data analysis results. Figure 4 As shown, the proportion of low ∑CSL grain boundaries (∑≤29) is only 47.1%, of which ∑3 grain boundaries account for 37.3%, and ∑9 and ∑27 grain boundaries account for a total of 9.8%.

[0065] In contrast, the core region of the GBE-2 sample treated in Example 2 showed significant optimization, such as Figure 3 (c) Figure 3 (d) shows the data analysis results. Figure 4 As shown in the figure, the proportion of low ∑CSL grain boundaries increased to 69.2%, of which ∑3 grain boundaries accounted for 63.3%, and ∑9 grain boundaries and ∑27 grain boundaries accounted for 5.9%.

[0066] The fatigue test was used to evaluate the cyclic deformation resistance of the samples. Figure 5 As shown, the comparative example Non-GBE-2 sample which did not adopt grain boundary engineering treatment broke after 158 cycles, while the fatigue life of the sample GBE-2 of Example 2 was increased to 253 cycles, an increase of 60.1%.

[0067] In summary, the technical solution of this embodiment is used to perform solution treatment on 304 stainless steel bars. Subsequently, strain is accumulated through processes such as drawing, straightening, and grinding. Combined with short-term annealing, recrystallization and twin expansion are promoted to achieve grain boundary network reconstruction, increase the proportion of low-∑CSL grain boundaries in the material, and increase the bending fatigue life of root canal files by approximately 60%, providing an efficient solution for upgrading the reliability of clinical instruments.

[0068] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all of these should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for improving the fatigue performance of a 304 austenitic stainless steel root canal file, characterized in that: The steps include: Step S1, subjecting a 304 austenitic stainless steel bar to a high-temperature solution treatment, keeping the temperature, and water-cooling to obtain a stainless steel bar having a completely austenitic structure; Step S2, performing a coordinated process of multiple cold deformation and heat treatment on the stainless steel bar obtained in step S1 to implement drawing and diameter reduction to obtain a wire; The coordinated treatment of cold deformation and heat treatment in each pass includes cold deformation and drawing, followed by high-temperature short-time annealing to directionally induce the expansion of the ∑3 grain boundary along the {111} crystal plane; the strain of a single cold deformation pass is no more than 8%, the annealing temperature is higher than the recrystallization critical temperature of 304 austenitic stainless steel, each annealing time is no more than 20 minutes, and water cooling is used; Step S3, performing plastic straightening on the wire obtained in step S2, and then performing high-temperature annealing and water cooling after straightening to achieve grain boundary reconstruction and form a penetrating low-energy grain boundary network to obtain a processed wire as a raw material for a stainless steel root canal file; wherein, during the plastic straightening, the single roller indentation is ≤0.1 mm, and the high-temperature annealing temperature is higher than the recrystallization critical temperature of 304 austenitic stainless steel, and the time is no more than 15 minutes; Step S4: grinding the treated wire to obtain a blank for preparing an endodontic file, and then applying periodic torsion to the blank to obtain an endodontic file with a core having a penetrating low-energy grain boundary network.

2. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 1, characterized in that: In step S2, the strain of the single-pass cold deformation is 4%-8%; and the drawing speed is ≤2m / min.

3. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 2, characterized in that: In step S2, the annealing temperature is 1000-1100° C., and each annealing time is 10-20 minutes.

4. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 1, characterized in that: In step S3, the wire obtained in step S2 is plastically straightened by a multi-roller continuous straightening process, with a single roller pressing amount of 0.05-0.1 mm and a cumulative true strain of 4% to 8%.

5. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 4, characterized in that: In step S3, the high temperature annealing temperature is 1050-1100° C. and the time is 5-15 minutes.

6. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 1, characterized in that: In the grinding process of step S4 , the single grinding depth is ≤10 μm, the accumulated true strain is 4% to 8%, and the grinding speed is ≤30 m / s.

7. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 1, characterized in that: In the periodic torsion in step S4, the single torsion angle is 5°-15° and the strain is 4%-8%.

8. The method for improving fatigue performance of a 304 austenitic stainless steel root canal file according to claim 7, characterized in that: After the periodic twisting is completed in step S4, annealing treatment is performed; the annealing treatment is to keep the temperature at 1000-1100° C. for no more than 15 minutes, and then water-cooling.

9. The method for improving the fatigue performance of a 304 austenitic stainless steel root canal file according to any one of claims 1 to 8, characterized in that: In step S1, the diameter of the 304 austenitic stainless steel bar is φ0.5-φ1.5 mm; the temperature of the high-temperature solution treatment is 1050-1150° C., the treatment time is greater than 60 minutes, and then water cooling is performed.

10. A 304 austenitic stainless steel root canal file, characterized in that: The 304 austenitic stainless steel root canal file is prepared by the method for improving fatigue performance of the 304 austenitic stainless steel root canal file according to any one of claims 1 to 9; the proportion of low ΣCSL grain boundaries on the surface and core of the 304 austenitic stainless steel root canal file is not less than 65%.

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