Composite heat treatment method for improving toughness of high-carbon steel fine wire

Abstract

The application discloses a composite heat treatment method for improving the toughness of high-carbon steel fine wire, and belongs to the technical field of metal material heat treatment. The method comprises the following steps: performing surface purification treatment on high-carbon steel fine wire to obtain pretreated wire material; performing rapid austenitizing treatment on the pretreated wire material under a protective atmosphere to obtain austenitized wire material; performing step cooling treatment on the austenitized wire material, and synchronously applying axial tensile deformation and pulse current auxiliary treatment in the cooling process to obtain composite strengthened wire material; performing two-temperature-zone partition isothermal phase change treatment on the composite strengthened wire material to obtain phase change strengthened wire material; performing cryogenic stabilization treatment on the phase change strengthened wire material to obtain stabilized wire material; performing low-temperature relaxation tempering treatment on the stabilized wire material to obtain tempered wire material; and performing surface residual compressive stress strengthening treatment on the tempered wire material to obtain high-carbon steel fine wire with improved toughness.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology for metallic materials, and more specifically, to a composite heat treatment method for improving the strength and toughness of high-carbon steel fine wires. Background Technology

[0002] High-carbon steel fine wires (typically ranging from 0.05 mm to 1.0 mm in diameter) are widely used in precision springs, medical guide wires, cable core wires, braided meshes, precision elastic elements, and other fields with stringent requirements for strength, toughness, and fatigue life. To meet these requirements, high-carbon steel fine wires typically undergo heat treatment processes such as austenitization, cooling phase transformation, and tempering before leaving the factory to obtain comprehensive mechanical properties that combine high strength with good ductility and toughness.

[0003] However, existing heat treatment processes for high-carbon steel fine wires generally have the following shortcomings: First, conventional austenitization often uses box furnaces or continuous furnaces for overall heating, resulting in a slow heating rate, easy grain growth, and difficulty in obtaining a fine and uniform austenitic structure. Second, conventional quenching and cooling are mostly single-rate continuous cooling or single isothermal treatment. During the cooling process, the temperature gradient between the inside and the surface of the wire is not precisely controlled, leading to uneven microstructure and difficulty in accurately controlling the ratio of martensite to bainite. Consequently, it is difficult to balance strength and toughness, i.e., when strength is increased, toughness decreases, and when toughness is increased, strength is insufficient. Third, the amount of residual austenite in conventional processes is unstable, and the wire is prone to dimensional changes and performance degradation during subsequent use or storage, affecting reliability. Fourth, conventional processes do not adequately control the residual stress state on the surface of the wire, resulting in limited fatigue resistance and bending resistance of the fine wires, making it difficult to meet the requirements of high-end precision applications for long service life and high reliability of fine wires.

[0004] Therefore, how to simultaneously improve the strength and toughness of high-carbon steel by coordinating and controlling multiple processes such as austenitization, cooling phase transformation, cryogenic treatment, tempering and surface strengthening while ensuring production efficiency is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To overcome a series of shortcomings in the existing technology, the purpose of this application is to provide a composite heat treatment method for improving the strength and toughness of high-carbon steel fine wires, comprising: Surface cleaning treatment is performed on high-carbon steel fine wires to obtain pretreated wire; Under a protective atmosphere, the pretreated wire is subjected to rapid austenitization treatment to obtain austenitized wire. Austenitic wire was subjected to step cooling treatment, and axial tensile deformation and pulsed current assisted treatment were applied simultaneously during the cooling process to obtain composite reinforced wire. The composite reinforced filament is subjected to a dual-temperature zone isothermal phase change treatment to obtain a phase change reinforced filament. The phase change reinforced filament is subjected to cryogenic stabilization treatment to obtain stabilized filament; The stabilized filaments were subjected to low-temperature relaxation tempering treatment to obtain tempered filaments; Tempered wire is subjected to surface residual compressive stress strengthening treatment to obtain high carbon steel fine wires with improved strength and toughness.

[0006] In some embodiments, the method for surface cleaning treatment of high-carbon steel fine wires is as follows: High-carbon steel fine wires are subjected to alkaline degreasing and acidic rust removal treatments in sequence to remove oil and oxide layers from the surface of the wires. The temperature of the alkaline degreasing tank is controlled at 55°C to 70°C and the treatment time is 20 to 60 seconds. High-carbon steel fine wires, after degreasing and rust removal, are subjected to ultrasonic-assisted cleaning to remove residual impurities. The ultrasonic frequency is controlled between 28kHz and 40kHz, and the ultrasonic power density is not less than 0.5W / cm². 2 ; After ultrasonic-assisted cleaning, the high-carbon steel fine wires are rinsed in multiple stages with deionized water and then dried using hot air drying or infrared drying to obtain pretreated wire with a surface moisture content of no more than 0.1%.

[0007] In some embodiments, a method for rapidly austenitizing pretreated wire is as follows: The pre-treated filament is continuously fed into a medium-frequency induction heating device and passes sequentially through at least three independently controlled induction heating coils arranged along the filament's running direction. The pre-treated wire is rapidly heated in segments by induction heating coils at a heating rate of 80℃ / second to 150℃ / second to the austenitizing temperature range of 880℃ to 930℃, and the pre-treated wire is held in the austenitizing temperature range for 3 to 8 seconds to obtain austenitized wire. During the rapid heating and austenitization holding process, the temperature of the wire is collected in real time by an infrared temperature measuring device set at the outlet of the medium frequency induction heating device, and the output power of each section of the induction heating coil is dynamically adjusted according to the collected temperature data, so that the austenitization temperature of the wire is controlled within the range of ±5℃ of the set temperature.

[0008] In some embodiments, the method for performing austenitic wire step cooling treatment is as follows: The austenitized wire is placed in a molten salt bath or fluidized bed cooling medium and rapidly cooled to a temperature range of 550°C to 650°C at a cooling rate of 40°C / second to 70°C / second, while controlling the dwell time to not exceed 1 second. The filament material, after the first stage of rapid cooling, is continuously cooled to reduce its temperature to the range of 450°C to 500°C. The filament material in the temperature range of 450℃ to 500℃ is subjected to a second stage of cooling, and the temperature is further reduced to the temperature range of 380℃ to 420℃ at a cooling rate of 10℃ / second to 25℃ / second. The wire is subjected to a temperature uniformization treatment for 2 to 5 seconds within a temperature range of 380℃ to 420℃ to make the temperature of the wire cross section uniform. After the filament has undergone uniform temperature treatment, it is transferred to the first isothermal zone of the dual-temperature zone isothermal phase change treatment.

[0009] In some embodiments, the method of simultaneously applying axial tensile deformation and pulsed current-assisted processing is as follows: In the second cooling stage of the stepped cooling process, the filament in the temperature range of 450°C to 500°C is cooled, and axial tensile deformation and pulse current assisted treatment are applied simultaneously as it cools down to the temperature range of 380°C to 420°C. An axial tensile load is applied to the filament by active tension rollers set at both ends of the filament's running direction, causing the filament to undergo axial tensile deformation. The tensile strain is controlled to be 1.5% to 6%, and the tensile strain rate is controlled to be 0.01 / s to 0.1 / s. While applying axial tensile deformation, a pulsed direct current is applied to the wire through electrodes surrounding the wire's outer periphery, wherein the current density is controlled to be 15 A / mm². 2 Up to 40A / mm 2 The pulse frequency is 500Hz to 2000Hz, the duty cycle is 30% to 60%, and the duration of a single pulse is 100μs to 500μs. The axial tensile deformation process and the pulsed current assisted processing process are kept synchronized during the second cooling stage to obtain the wire material after force-electric synergistic treatment.

[0010] In some embodiments, the dual-temperature zone isothermal phase transition treatment method is as follows: The composite reinforced wire was subjected to the first stage of isothermal phase transformation treatment in the first isothermal zone, wherein the isothermal temperature was controlled at 350℃ to 400℃ and the holding time was 15 seconds to 40 seconds, so that some of the supercooled austenite transformed into upward bainite. The composite reinforced filament, after completing the first stage of isothermal phase change treatment, is transported to the second isothermal zone via a continuous transition conveyor section, wherein the length of the continuous transition conveyor section is controlled to not exceed 0.5 meters. The composite reinforced wire is subjected to a second-stage isothermal phase transformation treatment in the second isothermal zone, wherein the isothermal temperature is controlled at 230°C to 280°C and the holding time is 30 to 70 seconds, so that the remaining supercooled austenite transforms into downward bainite, fine martensite and retained austenite. A phase change-strengthened filament was obtained by completing a dual-temperature zone isothermal phase change treatment.

[0011] In some embodiments, the phase transformation reinforced wire includes upper bainite, lower bainite, fine martensite, and retained austenite, wherein the volume fraction of the upper bainite is 10% to 20%, the volume fraction of the lower bainite is 35% to 50%, the volume fraction of the fine martensite is 15% to 25%, and the volume fraction of the retained austenite is 15% to 30%.

[0012] In some embodiments, the cryogenic stabilization treatment method is as follows: The phase change reinforced filament is subjected to cryogenic cooling treatment, and the filament is cooled to a cryogenic temperature range of -120°C to -180°C at a cooling rate of 5°C / second to 10°C / second. The filament is subjected to isothermal insulation treatment in the cryogenic temperature range for 20 to 45 minutes. After the cryogenic insulation treatment is completed, the filament is subjected to a heating recovery treatment, and the temperature is raised to room temperature at a heating rate of no more than 3℃ / second. During the heating process, when the temperature of the filament reaches the temperature range of -40℃ to 0℃, the filament is subjected to an intermediate isothermal residence treatment for 10 minutes to 20 minutes. The temperature is further increased to room temperature to obtain stabilized filament material after cryogenic stabilization treatment.

[0013] In some embodiments, the low-temperature relaxation tempering treatment method is as follows: The stabilized filaments after cryogenic stabilization are placed in a tempering heating device; The stabilized filament is heated by hot air circulation heating or infrared radiation heating at a heating rate of 3℃ / second to 8℃ / second to a temperature range of 150℃ to 220℃. The stabilized filament is subjected to heat preservation treatment within a temperature range of 150℃ to 220℃, and the heat preservation time is controlled to be 40 minutes to 90 minutes to obtain tempered filament.

[0014] In some embodiments, the surface residual compressive stress strengthening treatment method is as follows: The tempered wire material, after undergoing low-temperature relaxation and tempering treatment, is conveyed to the shot peening strengthening device. The tempered wire is subjected to rough shot strengthening treatment. High-hardness cast steel shot or ceramic shot with an average diameter of 0.1 mm to 0.3 mm is used as the shot peening medium. The shot peening pressure is controlled at 0.3 MPa to 0.6 MPa, the shot peening coverage is not less than 150%, and the shot peening direction is deflected by 10 degrees to 20 degrees relative to the normal direction of the wire surface. After the rough shot blasting strengthening treatment, the wire is subjected to fine shot blasting strengthening treatment. A shot blasting medium with a particle size smaller than that in the rough shot blasting stage is used, and the blasting pressure is reduced. Based on the continuous effect of coarse spraying and fine spraying strengthening treatment, a gradient residual compressive stress structure with a gradually decreasing radial direction is formed from the surface layer to the core of the wire. After completing the surface residual compressive stress strengthening treatment, a reinforced wire with a surface residual compressive stress peak of not less than 800 MPa and a residual compressive stress layer depth of 8% to 15% of the wire radius is obtained.

[0015] Compared with the prior art, this application has the following beneficial effects: 1) Segmented rapid austenitization is achieved through multi-segment induction heating coils, with high heating rate and short holding time, which can obtain fine and uniform austenite grains, providing a good microstructure basis for subsequent phase transformation and avoiding the grain coarsening problem that is easily caused by conventional overall heating process. 2) During the stepped cooling process, axial tensile deformation and pulsed current assisted treatment are applied simultaneously to refine the substructure of the undercooled austenite by utilizing the synergistic effect of the stress field and electric field, thereby promoting the refinement of subsequent phase transformation products and improving the axial strength of the wire. 3) A dual-temperature isothermal phase transformation treatment is adopted. Through two stages of isothermal treatment at different temperatures and with different holding times, a multi-phase synergistic structure of upper bainite, lower bainite, fine martensite and appropriate amount of retained austenite is formed inside the wire, so as to achieve a good match between strength and toughness. 4) By using cryogenic stabilization treatment combined with intermediate isothermal dwell, the fluctuation of residual austenite content is reduced while avoiding internal stress concentration and microcracks generated during the cryogenic process, thereby improving the long-term stability of the wire structure and dimensions. 5) By using low-temperature relaxation tempering and surface gradient residual compressive stress strengthening treatment, while maintaining the core strength of the filament, a higher residual compressive stress is introduced into the surface layer, which significantly improves the fatigue resistance and bending resistance of the filament and extends its service life. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process flow for a composite heat treatment method to improve the strength and toughness of high-carbon steel fine wires provided by the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the scope of protection of the invention. Furthermore, two comparative examples are provided to illustrate the technical effects of the composite heat treatment method of this invention on strength and toughness compared to conventional heat treatment processes. For example... Figure 1 As shown, the composite heat treatment method of the present invention sequentially includes six main steps: surface cleaning treatment, rapid austenitization treatment, stepped cooling and force-electric synergistic treatment, dual-temperature zone isothermal phase transformation treatment, cryogenic stabilization treatment, low-temperature tempering and surface residual compressive stress strengthening treatment. The following embodiments and comparative examples are all based on... Figure 1 The process sequence shown is implemented in sequence, with only the process parameters differing.

[0018] Example 1 This embodiment uses 0.20 mm diameter 70 steel (carbon content approximately 0.70 wt%) fine wire as raw material and processes it using the composite heat treatment method described in this invention. The specific steps are as follows: (1) Surface cleaning treatment: The wire was subjected to alkaline degreasing (tank temperature 60℃, treatment time 40 seconds) and acidic rust removal treatment in sequence. After degreasing and rust removal, it was heated to 32kHz and power density 0.6W / cm². 2 The filaments were subjected to ultrasonic-assisted cleaning under certain conditions, followed by multi-stage rinsing with deionized water and hot air drying to obtain a pretreated filament with a surface moisture content of approximately 0.05%. (2) Rapid austenitization treatment: Under the protection of a nitrogen-hydrogen mixed atmosphere (volume ratio 90:10, dew point -45℃, oxygen content in the furnace about 30ppm, flow rate 0.8 standard cubic meters per hour), the pretreated wire is continuously fed into a three-stage medium frequency induction heating device and heated to 900℃ at a heating rate of 100℃ / second, and held for 5 seconds to obtain austenitized wire; (3) Step cooling and force-electric synergistic treatment: The austenitized wire was placed in a molten salt bath and rapidly cooled to 600°C at a rate of 55°C / s and held for 0.5 seconds. Then it was continuously cooled to 480°C and then cooled to 400°C at a rate of 18°C / s. At this temperature, a 3-second isothermal treatment was performed. In the second cooling stage (from 480°C to 400°C), axial tensile deformation (tensile strain 3%, strain rate 0.05 / s) and pulsed current (current density 25A / mm) were applied simultaneously. 2 (with a pulse frequency of 1000Hz, a duty cycle of 45%, and a single pulse duration of 300μs), a composite reinforced filament was obtained. (4) Dual-temperature isothermal phase change treatment: the temperature of the first isothermal zone is 375℃ and the holding time is 25 seconds; after a 0.3-meter transition section, the temperature of the second isothermal zone is 255℃ and the holding time is 50 seconds, to obtain phase change reinforced filament. (5) Cryogenic stabilization treatment: Cool to -150℃ at a rate of 7℃ / second and hold for 30 minutes; then heat to room temperature at a rate of 2℃ / second and hold at an intermediate isothermal temperature for 15 minutes in the range of -20℃ to 0℃ to obtain stabilized filament. (6) Low-temperature relaxation tempering treatment: Heat to 185°C at a rate of 5°C / second and hold for 60 minutes to obtain tempered wire; (7) Surface residual compressive stress strengthening treatment: rough blasting strengthening is carried out with cast steel shot with an average diameter of 0.2 mm, a blasting pressure of 0.45 MPa, a shot coverage of 180%, and a deflection angle of 15 degrees. Then, fine blasting strengthening is carried out with a finer particle size shot blasting medium and a reduced blasting pressure. Finally, high carbon steel fine wire with improved strength and toughness is obtained, which is numbered S-1.

[0019] Example 2 This embodiment uses 0.35 mm diameter 82B steel (carbon content approximately 0.82 wt%) fine wire as raw material and employs the composite heat treatment method described in this invention. The process parameters are selected from the lower end of the range for each step. The specific steps are as follows: (1) Surface cleaning treatment: alkaline degreasing bath temperature 55℃, treatment time 25 seconds; ultrasonic frequency 28kHz, power density 0.5W / cm²; after drying, pretreated filament is obtained; (2) Rapid austenitization treatment: Using a high-purity nitrogen atmosphere (purity 99.995%, dew point -42℃, oxygen content in the furnace about 25ppm, flow rate 0.5 standard cubic meters per hour), heat to 885℃ at a heating rate of 85℃ / second and hold for 7 seconds; (3) Stepped cooling and force-electric synergistic treatment: Rapidly cool to 630℃ at a rate of 42℃ / s and hold for 0.8 seconds; continuously cool to 495℃, then cool to 415℃ at a rate of 12℃ / s and homogenize for 4 seconds; in the second cooling stage, axial tensile deformation (strain 1.8%, strain rate 0.02 / s) and pulsed current (current density 17A / mm) are applied simultaneously. 2 (Pulse frequency 600Hz, duty cycle 32%, single pulse duration 150μs). (4) Dual-temperature isothermal phase transition treatment: the first isothermal zone is 355℃, held for 18 seconds; the second isothermal zone is 235℃, held for 35 seconds; (5) Cryogenic stabilization treatment: Cool to -125℃ at a rate of 5.5℃ / second and hold for 22 minutes; heat to room temperature at a rate of 1.5℃ / second and hold isothermally in the range of -35℃ to -5℃ for 12 minutes; (6) Low-temperature relaxation tempering treatment: Heat to 155°C at a rate of 3.5°C / second and hold for 45 minutes; (7) Surface residual compressive stress strengthening treatment: coarse spraying pressure 0.32 MPa, coverage 155%, deflection angle 11 degrees, fine spraying stage reduce pressure and use finer shot peening medium, finally obtain high carbon steel fine wire with improved strength and toughness numbered S-2.

[0020] Example 3 This embodiment uses 0.50 mm diameter 65Mn steel fine wire as raw material and adopts the composite heat treatment method described in this invention. The process parameters are selected from the higher end of the range for each step. The specific steps are as follows: (1) Surface cleaning treatment: alkaline degreasing bath temperature 68℃, treatment time 55 seconds; ultrasonic frequency 38kHz, power density 0.9W / cm³ 2 ; (2) Rapid austenitization treatment: using a nitrogen-hydrogen mixed atmosphere (volume ratio 86:14), heating to 925℃ at a heating rate of 140℃ / second, and holding for 7.5 seconds; (3) Stepped cooling and force-electric synergistic treatment: rapidly cool to 555℃ at a rate of 65℃ / s and hold for 0.9 seconds; continuously cool to 455℃, then cool to 385℃ at a rate of 23℃ / s and homogenize for 4.5 seconds; in the second cooling stage, simultaneously apply axial tensile deformation (strain 5.5%, strain rate 0.09 / s) and pulse current (current density 37A / mm) 2 (Pulse frequency 1800Hz, duty cycle 58%, single pulse duration 460μs). (4) Two-temperature zone isothermal phase transition treatment: the first isothermal zone is 395℃, held for 38 seconds; the second isothermal zone is 275℃, held for 65 seconds; (5) Cryogenic stabilization treatment: Cool to -175℃ at a rate of 9.5℃ / second and hold for 42 minutes; heat to room temperature at a rate of 2.8℃ / second and hold isothermally in the range of -5℃ to 0℃ for 19 minutes; (6) Low-temperature relaxation tempering treatment: Heat to 215°C at a rate of 7.5°C / second and hold for 85 minutes; (7) Surface residual compressive stress strengthening treatment: coarse spraying pressure 0.58 MPa, coverage 195%, deflection angle 19 degrees, fine spraying stage further refines the shot peening medium and reduces pressure, finally obtaining high carbon steel fine wire with improved strength and toughness numbered S-3.

[0021] Example 4 This embodiment uses 0.10 mm diameter T9A steel fine wire as raw material and adopts the composite heat treatment method described in this invention. The process parameters are selected from the intermediate values ​​within the range of each step. The specific steps are as follows: (1) Surface cleaning treatment: alkaline degreasing bath temperature 62℃, treatment time 30 seconds; ultrasonic frequency 34kHz, power density 0.7W / cm³2 ; (2) Rapid austenitization treatment: using a high-purity nitrogen atmosphere (purity above 99.99%), heating to 910℃ at a heating rate of 115℃ / second, and holding for 4 seconds; (3) Stepped cooling and force-electric synergistic treatment: rapidly cool to 615℃ at a rate of 48℃ / s and hold for 0.6 seconds; continuously cool to 470℃, then cool to 395℃ at a rate of 15℃ / s and homogenize for 3 seconds; in the second cooling stage, simultaneously apply axial tensile deformation (strain 4%, strain rate 0.06 / s) and pulsed current (current density 30A / mm) 2 (Pulse frequency 1400Hz, duty cycle 50%, single pulse duration 350μs). (4) Dual-temperature isothermal phase transition treatment: the first isothermal zone is 380℃, held for 30 seconds; the second isothermal zone is 260℃, held for 55 seconds; (5) Cryogenic stabilization treatment: Cool to -160℃ at a rate of 8℃ / second and hold for 35 minutes; heat to room temperature at a rate of 2.5℃ / second and hold isothermally in the range of -30℃ to -10℃ for 17 minutes; (6) Low-temperature relaxation tempering treatment: Heat to 200°C at a rate of 6°C / second and hold for 70 minutes; (7) Surface residual compressive stress strengthening treatment: coarse spraying pressure 0.5 MPa, coverage 170%, deflection angle 16 degrees, fine spraying stage reduce pressure and use finer shot peening medium, finally obtain high carbon steel fine wire with improved strength and toughness numbered S-4.

[0022] Comparative Example 1 This comparative example uses 70 steel fine wire of the same diameter and batch as in Example 1 as raw material, but is treated with conventional heat treatment process. Specifically, the pretreated wire is placed in a box-type resistance furnace and heated to 900°C at a conventional heating rate (about 10°C / second) and held for 60 seconds for conventional austenitization. Then, it is directly quenched in oil and continuously cooled to room temperature (without stepped cooling, force-electric synergistic treatment, or dual-temperature isothermal phase transformation). Finally, it is placed in a 180°C box tempering furnace and held for 60 minutes for conventional tempering. No cryogenic stabilization treatment or surface residual compressive stress strengthening treatment is performed. The resulting comparative wire is numbered D-1.

[0023] Comparative Example 2 This comparative example uses 82B steel fine wire of the same diameter and from the same batch as in Example 2 as the raw material. The process steps include rapid austenitization treatment (parameters same as in Example 2) and single isothermal treatment (isothermal temperature 300℃, holding time 60 seconds, without dual-temperature zoned isothermal phase transformation). No axial tensile deformation or pulsed current assisted treatment is applied during the cooling process, and no cryogenic stabilization treatment is performed. The tempering and surface strengthening treatment parameters are the same as in Example 2. The resulting comparative wire material is numbered D-2, which is used to examine the contribution of the stepped cooling composite force-electric synergistic treatment, dual-temperature zoned isothermal phase transformation, and cryogenic stabilization treatment in this invention to the strength and toughness of the wire material.

[0024] Performance testing and results analysis Mechanical and fatigue properties were tested on the wires obtained in Examples 1 to 4 (S-1 to S-4) and the wires obtained in Comparative Examples 1 and 2 (D-1 and D-2). The test items included tensile strength, reduction of area, number of bidirectional torsion cycles (characterizing ductility, toughness and fracture resistance), and rotational bending fatigue life (median fatigue life measured under stress ratio R=-1). The test results are shown in the table below.

[0025]

[0026] As shown in the table above, the wires (S-1 to S-4) prepared by the composite heat treatment method described in this invention are significantly superior to the wires obtained by Comparative Example 1 (D-1) in terms of tensile strength, reduction of area, number of bidirectional torsion cycles, and fatigue life. The reduction of area is increased by more than 35%, the number of bidirectional torsion cycles is increased by more than 87% to 110%, and the fatigue life is increased by more than 1.4 times. This indicates that the present invention, through multiphase synergistic microstructure regulation and surface gradient residual compressive stress strengthening, can significantly improve the plasticity, toughness, and fatigue resistance of the wires while maintaining high strength.

[0027] Comparative Example 2 (D-2), due to retaining rapid austenitization and single-temperature isothermal treatment, showed a slight improvement in tensile strength compared to Comparative Example 1. However, because it did not employ stepped cooling combined force-electric synergistic treatment and dual-temperature zoned isothermal phase transformation treatment, the multiphase ratio in the microstructure did not reach the range described in this invention. Furthermore, it did not undergo cryogenic stabilization treatment, resulting in significantly lower reduction of area, bidirectional torsion cycles, and fatigue life compared to the wires obtained in Examples 1 to 4. Nevertheless, due to the same surface residual compressive stress strengthening treatment, its surface residual compressive stress was similar to that of the examples. The above results demonstrate that the stepped cooling combined force-electric synergistic treatment, dual-temperature zoned isothermal phase transformation treatment, and cryogenic stabilization treatment in this invention make significant contributions to the synergistic improvement of the wire's strength and toughness, and each process is mutually complementary and indispensable.

[0028] Furthermore, comparing Examples 1 to 4, it can be seen that within the parameter range defined by the present invention, when the process parameters are selected at a higher value (such as in Example 3), the tensile strength and fatigue life of the filament are further improved, while the reduction of area decreases slightly; when the process parameters are selected at a lower value (such as in Example 2), the reduction of area of ​​the filament is relatively high and the strength is slightly low. This indicates that those skilled in the art can make reasonable selections and adjustments within the parameter range defined by the present invention according to actual usage requirements to obtain the best match between strength and toughness.

[0029] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite heat treatment method for improving the strength and toughness of high-carbon steel fine wires, characterized in that, include: Surface cleaning treatment is performed on high-carbon steel fine wires to obtain pretreated wire; Under a protective atmosphere, the pretreated wire is subjected to rapid austenitization treatment to obtain austenitized wire. Austenitic wire was subjected to step cooling treatment, and axial tensile deformation and pulsed current assisted treatment were applied simultaneously during the cooling process to obtain composite reinforced wire. The composite reinforced filament is subjected to a dual-temperature zone isothermal phase change treatment to obtain a phase change reinforced filament. The phase change reinforced filament is subjected to cryogenic stabilization treatment to obtain stabilized filament; The stabilized filaments were subjected to low-temperature relaxation tempering treatment to obtain tempered filaments; Tempered wire is subjected to surface residual compressive stress strengthening treatment to obtain high carbon steel fine wires with improved strength and toughness.

2. The composite heat treatment method according to claim 1, characterized in that, The method for surface cleaning treatment of high carbon steel fine wires is as follows: High-carbon steel fine wires are subjected to alkaline degreasing and acidic rust removal treatments in sequence to remove oil and oxide layers from the surface of the wires. The temperature of the alkaline degreasing tank is controlled at 55°C to 70°C and the treatment time is 20 to 60 seconds. High-carbon steel fine wires, after degreasing and rust removal, are subjected to ultrasonic-assisted cleaning to remove residual impurities. The ultrasonic frequency is controlled between 28kHz and 40kHz, and the ultrasonic power density is not less than 0.5W / cm². 2 ; After ultrasonic-assisted cleaning, the high-carbon steel fine wires are rinsed in multiple stages with deionized water and then dried using hot air drying or infrared drying to obtain pretreated wire with a surface moisture content of no more than 0.1%.

3. The composite heat treatment method according to claim 1, characterized in that, The method for rapid austenitization of pretreated wire is as follows: The pre-treated filament is continuously fed into a medium-frequency induction heating device and passes sequentially through at least three independently controlled induction heating coils arranged along the filament's running direction. The pre-treated wire is rapidly heated in segments by induction heating coils at a heating rate of 80℃ / second to 150℃ / second to the austenitizing temperature range of 880℃ to 930℃, and the pre-treated wire is held in the austenitizing temperature range for 3 to 8 seconds to obtain austenitized wire. During the rapid heating and austenitization holding process, the temperature of the wire is collected in real time by an infrared temperature measuring device set at the outlet of the medium frequency induction heating device, and the output power of each section of the induction heating coil is dynamically adjusted according to the collected temperature data, so that the austenitization temperature of the wire is controlled within the range of ±5℃ of the set temperature.

4. The composite heat treatment method according to claim 1, characterized in that, The method for step cooling treatment of austenitic wire is as follows: The austenitized wire is placed in a molten salt bath or fluidized bed cooling medium and rapidly cooled to a temperature range of 550°C to 650°C at a cooling rate of 40°C / second to 70°C / second, while controlling the dwell time to not exceed 1 second. The filament material, after the first stage of rapid cooling, is continuously cooled to reduce its temperature to the range of 450°C to 500°C. The filament material in the temperature range of 450℃ to 500℃ is subjected to a second stage of cooling, and the temperature is further reduced to the temperature range of 380℃ to 420℃ at a cooling rate of 10℃ / second to 25℃ / second. The wire is subjected to a temperature uniformization treatment for 2 to 5 seconds within a temperature range of 380℃ to 420℃ to make the temperature of the wire cross section uniform. After the filament has undergone uniform temperature treatment, it is transferred to the first isothermal zone of the dual-temperature zone isothermal phase change treatment.

5. The composite heat treatment method according to claim 4, characterized in that, The method of simultaneously applying axial tensile deformation and pulsed current assisted treatment is as follows: In the second cooling stage of the stepped cooling process, the filament in the temperature range of 450°C to 500°C is cooled, and axial tensile deformation and pulse current assisted treatment are applied simultaneously as it cools down to the temperature range of 380°C to 420°C. An axial tensile load is applied to the filament by active tension rollers set at both ends of the filament's running direction, causing the filament to undergo axial tensile deformation. The tensile strain is controlled to be 1.5% to 6%, and the tensile strain rate is controlled to be 0.01 / s to 0.1 / s. While applying axial tensile deformation, a pulsed direct current is applied to the wire through electrodes surrounding the wire's outer periphery, wherein the current density is controlled to be 15 A / mm². 2 Up to 40A / mm 2 The pulse frequency is 500Hz to 2000Hz, the duty cycle is 30% to 60%, and the duration of a single pulse is 100μs to 500μs. The axial tensile deformation process and the pulsed current assisted processing process are kept synchronized during the second cooling stage to obtain the wire material after force-electric synergistic treatment.

6. The composite heat treatment method according to claim 1, characterized in that, The dual-temperature zone isothermal phase transition treatment method is as follows: The composite reinforced wire was subjected to the first stage of isothermal phase transformation treatment in the first isothermal zone, wherein the isothermal temperature was controlled at 350℃ to 400℃ and the holding time was 15 seconds to 40 seconds, so that some of the supercooled austenite transformed into upward bainite. The composite reinforced filament, after completing the first stage of isothermal phase change treatment, is transported to the second isothermal zone via a continuous transition conveyor section, wherein the length of the continuous transition conveyor section is controlled to not exceed 0.5 meters. The composite reinforced wire is subjected to a second-stage isothermal phase transformation treatment in the second isothermal zone, wherein the isothermal temperature is controlled at 230°C to 280°C and the holding time is 30 to 70 seconds, so that the remaining supercooled austenite transforms into downward bainite, fine martensite and retained austenite. A phase change-strengthened filament was obtained by completing a dual-temperature zone isothermal phase change treatment.

7. The composite heat treatment method according to claim 1, characterized in that, The phase transformation reinforced wire comprises upper bainite, lower bainite, fine martensite, and retained austenite, wherein the volume fraction of upper bainite is 10% to 20%, the volume fraction of lower bainite is 35% to 50%, the volume fraction of fine martensite is 15% to 25%, and the volume fraction of retained austenite is 15% to 30%.

8. The composite heat treatment method according to claim 1, characterized in that, The cryogenic stabilization treatment method is as follows: The phase change reinforced filament is subjected to cryogenic cooling treatment, and the filament is cooled to a cryogenic temperature range of -120°C to -180°C at a cooling rate of 5°C / second to 10°C / second. The filament is subjected to isothermal insulation treatment in the cryogenic temperature range for 20 to 45 minutes. After the cryogenic insulation treatment is completed, the filament is subjected to a heating recovery treatment, and the temperature is raised to room temperature at a heating rate of no more than 3℃ / second. During the heating process, when the temperature of the filament reaches the temperature range of -40℃ to 0℃, the filament is subjected to an intermediate isothermal residence treatment for 10 minutes to 20 minutes. The temperature is further increased to room temperature to obtain stabilized filament material after cryogenic stabilization treatment.

9. The composite heat treatment method according to claim 1, characterized in that, The low-temperature relaxation tempering treatment method is as follows: The stabilized filaments after cryogenic stabilization are placed in a tempering heating device; The stabilized filament is heated by hot air circulation heating or infrared radiation heating at a heating rate of 3℃ / second to 8℃ / second to a temperature range of 150℃ to 220℃. The stabilized filament is subjected to heat preservation treatment within a temperature range of 150℃ to 220℃, and the heat preservation time is controlled to be 40 minutes to 90 minutes to obtain tempered filament.

10. The composite heat treatment method according to claim 1, characterized in that, The surface residual compressive stress strengthening treatment method is as follows: The tempered wire material, after undergoing low-temperature relaxation and tempering treatment, is conveyed to the shot peening strengthening device. The tempered wire is subjected to rough shot strengthening treatment. High-hardness cast steel shot or ceramic shot with an average diameter of 0.1 mm to 0.3 mm is used as the shot peening medium. The shot peening pressure is controlled at 0.3 MPa to 0.6 MPa, the shot peening coverage is not less than 150%, and the shot peening direction is deflected by 10 degrees to 20 degrees relative to the normal direction of the wire surface. After the rough shot blasting strengthening treatment, the wire is subjected to fine shot blasting strengthening treatment. A shot blasting medium with a particle size smaller than that in the rough shot blasting stage is used, and the blasting pressure is reduced. Based on the continuous effect of coarse spraying and fine spraying strengthening treatment, a gradient residual compressive stress structure with a gradually decreasing radial direction is formed from the surface layer to the core of the wire. After completing the surface residual compressive stress strengthening treatment, a reinforced wire with a surface residual compressive stress peak of not less than 800 MPa and a residual compressive stress layer depth of 8% to 15% of the wire radius is obtained.

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