A heat treatment process to prevent steel wire from breaking

Abstract

The present invention discloses a heat treatment process to avoid the self-breaking of steel wire, and belongs to the technical field of heat treatment of metal products. The process includes the following steps: steel wire unfolding, austenitization, surface high temperature treatment, quenching cooling, tempering, surface softening and cooling treatment, and steel wire winding. The present invention mainly uses the skin effect of high-frequency induction heating to selectively heat the surface of the steel wire, and uses the current density change of the induced eddy current at the defect to form a small molten pool locally, change the morphology of surface defects, reduce the sensitivity of the finished steel wire to surface defects, avoid the self-breaking of the finished steel wire during the subsequent storage process, improve the qualified rate of the product, and reduce the quality cost; at the same time, the present invention eliminates the need for flaw detection and grinding treatment, and can realize the continuous production of steel wires of different specifications and different tensile strengths. It also has the advantages of simple process, good stability, easy implementation, simple operation, high qualified rate, cost saving, and no pollution.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of metal products, and more specifically relates to a heat treatment process for preventing a steel wire from breaking by itself. Background Art

[0002] Springs made of quenched and tempered steel wire are widely used in the automotive, electrical, and machinery industries as important energy storage and motion components. As the stress on the spring increases, the tensile strength of the quenched and tempered steel wire is required to be higher and higher. In the production process of high-strength quenched and tempered steel wire, induction heating is generally used for heat treatment. The advantages are rapid heating, high efficiency, low energy consumption, and no lead pollution. In addition, it can be combined with water or water medium to achieve rapid cooling, which can refine the grains and improve the strength and plasticity of the steel wire. It is particularly suitable for the production of coarse-gauge high-strength steel wire. However, in the actual production process of high-strength steel wire, especially high-strength steel wire with a tensile strength requirement of more than 1900MPa, the finished steel wire often breaks by itself. The self-breaking of the steel wire not only causes a waste of materials, but also poses a great safety hazard. Relevant theoretical analysis shows that the spontaneous breakage of high-strength steel wire originates from defects such as microcracks on the surface. Since the steel wire is coiled during storage, microcracks produce stress corrosion under stress and external hydrogen-containing environment, which causes the steel wire to break instantly after expansion. Especially in winter, the notch sensitivity of the steel wire increases sharply with the decrease in temperature, and the phenomenon of spontaneous breakage of the steel wire occurs more frequently. In order to reduce the occurrence of spontaneous breakage of steel wire, the traditional practice is that manufacturers often add flaw detection and grinding to the drawing process before heat treatment. The disadvantages of the flaw detection and grinding method are: on the one hand, the production efficiency is low, and on the other hand, defects less than 50㎛ and transverse microcracks cannot be detected, so the spontaneous breakage of steel wire cannot be completely avoided by flaw detection and grinding. Summary of the invention

[0003] The purpose of the present invention is to provide a heat treatment process to prevent the self-breaking of the above-mentioned quenched and tempered spring steel wire in the storage process. The present invention mainly uses the skin effect of high-frequency induction heating to selectively heat the surface of the steel wire, and uses the change of current density of the induced eddy current at the defect to form a small molten pool locally, change the morphology of surface defects, reduce the sensitivity of the finished steel wire to surface defects, and avoid the self-breaking of the finished steel wire during the subsequent storage process, thereby improving the qualified rate of the product and reducing the quality cost; at the same time, the present invention eliminates the need for flaw detection and grinding, and can realize the continuous production of steel wires of different specifications and different tensile strengths. It also has the advantages of simple process, good stability, easy implementation, simple operation, high qualified rate, cost saving, and no pollution.

[0004] The purpose of the present invention can be achieved through the following technical measures:

[0005] A heat treatment process for preventing a steel wire from breaking spontaneously according to the present invention comprises the following steps:

[0006] A. Wire unwinding: Driven by the front traction machine, the wire rod wound on the pay-off drum after rounding and drawing is gradually unwound;

[0007] B. Austenitization: At the same time, the steel wire moves forward and passes through coil I and coil II in the induction heating coil in sequence. That is, the steel wire is first preheated by coil I with a power frequency of 5 kHz to 10 kHz, and then the steel wire (1) is heated by coil II with a power frequency of 40 kHz to 50 kHz for 20 seconds to 50 seconds and is heated to 850°C to 900°C to obtain an austenitized steel wire.

[0008] C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through a high-frequency heating power coil with a power frequency higher than 400KHz for surface high temperature heating, and the surface temperature of the steel wire is heated to 1080℃~1230℃; the steel wire after surface high temperature treatment immediately enters the insulation pipe and moves forward while being heated;

[0009] D. Quenching and cooling: After being removed from the insulation tube, the steel wire is moved forward and immersed in the cold water medium of the quenching and cooling tank for quenching until it cools to room temperature;

[0010] E. Tempering: The steel wire cooled to room temperature continues to move forward and passes through the tempering heating coil with a power frequency of 5KHz to 10KHz. The steel wire is heated to 400℃ to 500℃, and then enters the tempering insulation pipe for 27S to 56S for tempering treatment;

[0011] F. Surface softening and cooling treatment: After tempering and heat preservation, the steel wire moves forward and passes through the softening heating coil with a power frequency higher than 300KHz for surface softening treatment, and the surface temperature of the steel wire is heated to 480℃~540℃; then the steel wire moves forward and enters the softening cooling tank for cooling treatment to obtain the finished steel wire;

[0012] G. Steel wire winding: The finished steel wire obtained in step F is wound onto a take-up drum under the traction of the rear traction machine.

[0013] In the step C of the present invention, the heating depth of the steel wire surface is less than 1 mm and the heating time is less than 2 s.

[0014] In step F of the present invention, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is less than 2 s, and then the steel wire is immediately cooled to room temperature.

[0015] The finished steel wire of the present invention has a tensile strength of more than 1800 MPa, a cross-sectional shrinkage rate of 44.7% to 52.4%, a Vickers hardness of 502 to 544 HV at 0.5 mm from the surface, a Vickers hardness of 527 to 596 HV at 1.0 mm from the surface, a Vickers hardness of 558 to 637 HV at 1.5 mm from the surface, and a Vickers hardness of 554 to 643 HV at 3.0 mm from the surface.

[0016] The design principle of the present invention is as follows:

[0017] The present invention is a heat treatment process designed to prevent the self-breaking of thick-sized high-strength quenched-tempered steel wires. The process of the present invention adopts an expanded online induction heating-quenching-tempering heat treatment method for the steel wire rods after rounding and drawing. The skin effect of high-frequency induction heating is mainly used to selectively heat the surface of the steel wire, and the current density change of the induced eddy current at the defect is used to form a small molten pool locally, change the morphology of surface defects, reduce the sensitivity of the finished steel wire to surface defects, and avoid the self-breaking of the finished steel wire during the subsequent storage process, thereby improving the qualified rate of the product and reducing the quality cost. At the same time, the process of the present invention omits the flaw detection and grinding process, and can realize the continuous production of steel wires of different specifications and different tensile strengths. It also has the advantages of simple process, good stability, easy implementation, simple operation, high qualified rate, cost saving, and no pollution.

[0018] More specifically, the steel wire rod after rounding and drawing in step A provides good shape, size and surface state for subsequent induction heating. Steps B to G are all completed continuously on an induction heat treatment line, and the steel wire is in an unfolded state, ensuring the stable performance of the finished steel wire. In step B, medium and low frequency induction power supplies are used for heating in two stages, which is beneficial to the uniformity of austenitization of the steel wire. Step C uses an ultra-high frequency heating power supply, and utilizes the skin effect of induction heating to heat only within 1mm of the surface of the steel wire, and the heating time is short. The subsequent steel wire is subjected to heat conduction, and the surface temperature and core temperature can be unified, avoiding the coarse grain phenomenon on the surface of the steel wire caused by high heating temperature and long heating time; at the same time, due to the eddy current effect of the induced current, if there are defects or microcracks on the surface of the steel wire, its current density increases sharply at the tip of the defect, and a small molten pool is instantly formed at the tip of the defect, and the tip of the defect on the surface of the steel wire tends to be smooth, reducing the sensitivity of the finished steel wire to such defects, thereby avoiding stress concentration at the tip of the defect during the storage of the finished steel wire and self-breaking. Step D is the quenching and cooling of the steel wire. The steel wire, which has been kept warm for a certain period of time, enters the cooling medium and is quickly cooled to room temperature to obtain a uniform and fine martensite structure. Step E is the steel wire tempering process. The steel wire quenched into a martensite structure is transformed into a tempered troostite structure by tempering heating and heat preservation. The selected power supply frequency is 5KHz to 10KHz, which is conducive to the consistency of the temperature inside and outside the steel wire. After the heat preservation treatment, the cross-sectional structure of the steel wire is uniform; according to the different tensile strength requirements of the finished steel wire, the tempering temperature is set between 400℃ and 500℃. In step F, an ultra-high frequency heating power source is used to heat only the surface of the steel wire within a depth of 1.5 mm. This is because the higher the tempering temperature of the steel wire, the lower the tensile strength and hardness, the lower the defect sensitivity of the finished steel wire, and the less likely it is to break spontaneously during storage. By increasing the tempering temperature of the steel wire surface, the hardness of the steel wire surface is reduced, thereby reducing the sensitivity to surface defects of the steel wire and avoiding spontaneous breaking of the steel wire. The heating time in step F is less than 2 seconds and the cooling is performed immediately to avoid the heat on the steel wire surface from diffusing to the core of the steel wire, resulting in a significant decrease in the tensile strength of the overall steel wire. It is obvious that different softening treatment temperatures are further selected according to different tensile strength requirements. The softening heating temperature is 40 to 80°C higher than the tempering heating temperature in step E. This can not only avoid the self-breakage of the finished steel wire, but also reduce the tensile strength loss of the steel wire in the surface softening process; the surface hardness of the finished steel wire after softening has a certain gradient, and the Vickers hardness of the steel wire at 0.5mm after softening is about 60 to 100HV different from the Vickers hardness of the steel wire at a position greater than 1.5mm, which can effectively avoid the overall tensile strength of the steel wire being significantly reduced due to softening.

[0019] The beneficial technical effects of the present invention are as follows:

[0020] The present invention mainly uses the skin effect of high-frequency induction heating to selectively heat the surface of the steel wire, and uses the current density change of the induced eddy current at the defect to form a small molten pool locally, change the surface defect morphology, reduce the sensitivity of the finished steel wire to surface defects, avoid the finished steel wire from breaking itself during subsequent storage, improve the qualified rate of the product, and reduce the quality cost; at the same time, the present invention omits the flaw detection and grinding treatment, can realize the continuous production of steel wires of different specifications and different tensile strengths, and also has the advantages of simple process, good stability, easy realization, simple operation, high qualified rate, cost saving, and no pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a simplified process flow diagram of the present invention.

[0022] Parts numbers in the figure: 1. Steel wire, 2. Pay-off drum, 3. Front traction machine, 4. Induction heating coil, 4-1. Coil I, 4-2. Coil II, 5. High-frequency heating power coil, 6. Insulation pipe, 7. Quenching cooling tank, 8. Tempering heating coil, 9. Tempering insulation pipe, 10. Softening heating coil, 11. Softening cooling tank, 12. Rear traction machine, 13. Take-up drum. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with the accompanying drawings and embodiments:

[0024] Embodiment 1

[0025] A heat treatment process for preventing the steel wire from breaking spontaneously in the first embodiment includes the following steps:

[0026] A. Steel wire unwinding: driven by the front traction machine 3, the steel wire rod made of 55SiCr, which is drawn to Φ12.6mm by the wire drawing machine and the drawing die and then wound on the pay-off drum 2, is gradually unwound;

[0027] B. Austenitization: At the same time, the steel wire 1 moves forward and passes through the coil I4-1 and the coil II4-2 in the induction heating coil 4 in sequence. That is, the steel wire 1 first passes through the coil I4-1 with a power frequency of 5KHz for preheating, and then passes through the coil II4-2 with a power frequency of 40KHz for heating for 30S and is heated to 850°C to obtain an austenitized steel wire.

[0028] C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through the high-frequency heating power coil 5 with a power frequency of 450KHz for surface high temperature heating, and the surface temperature of the steel wire is heated to 1080°C; the steel wire 1 after the surface high temperature treatment immediately enters the insulation pipe 6 while being kept warm and moving forward;

[0029] D. Quenching and cooling: After being removed from the insulation tube 6, the steel wire 1 is moved forward and immersed in the cold water medium of the quenching and cooling tank 7 for quenching until it is cooled to room temperature;

[0030] E. Tempering: The steel wire 1 cooled to room temperature continues to move forward, passes through the tempering heating coil 8 with a power frequency of 10KHz, and is heated to 500°C. Then, it enters the tempering insulation pipe 9 for 40S for tempering treatment;

[0031] F. Surface softening and cooling treatment: After tempering and heat preservation, the steel wire 1 moves forward and enters the softening heating coil 10 with a power frequency of 350KHz for surface softening treatment, and the surface temperature of the steel wire is heated to 540°C; then the steel wire 1 moves forward and enters the softening cooling tank 11 for cooling treatment to obtain the finished steel wire;

[0032] G. Steel wire winding: The finished steel wire obtained in step F is wound onto the take-up drum 13 under the traction of the rear traction machine 12 for winding.

[0033] In the step C of the present invention, the heating depth of the steel wire surface is less than 1 mm and the heating time is 1.8 s.

[0034] In step F of the present invention, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is 1.8 s, and then the steel wire is immediately cooled to room temperature.

[0035] The finished steel wire of the present invention has a tensile strength of 1850 MPa, a cross-sectional shrinkage rate of 52.4%, a Vickers hardness of 502 HV at 0.5 mm from the surface, a Vickers hardness of 527 HV ​​at 1.0 mm from the surface, a Vickers hardness of 558 HV at 1.5 mm from the surface, and a Vickers hardness of 554 HV at 3.0 mm from the surface.

[0036] The finished steel wire obtained in Example 1 was inspected after a seven-day storage test: no self-breaking phenomenon occurred.

[0037] Embodiment 2

[0038] A heat treatment process for preventing the steel wire from breaking spontaneously in the second embodiment comprises the following steps:

[0039] A. Steel wire unwinding: driven by the front traction machine 3, the steel wire rod made of 55SiCr, which is rounded to Φ11.8mm by the wire drawing machine and the drawing die and then wound on the pay-off drum 2, is gradually unwound;

[0040] B. Austenitization: At the same time, the steel wire 1 moves forward and passes through the coil I4-1 and the coil II4-2 in the induction heating coil 4 in sequence. That is, the steel wire 1 first passes through the coil I4-1 with a power frequency of 5KHz for preheating, and then passes through the coil II4-2 with a power frequency of 50KHz for heating for 20S and is heated to 900°C to obtain an austenitized steel wire.

[0041] C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through the high-frequency heating power coil 5 with a power frequency of 480KHz for surface high temperature heating, and the surface temperature of the steel wire is heated to 1150°C; the steel wire 1 after the surface high temperature treatment immediately enters the insulation pipe 6 while being kept warm and moving forward;

[0042] D. Quenching and cooling: After being removed from the insulation tube 6, the steel wire 1 is moved forward and immersed in the cold water medium of the quenching and cooling tank 7 for quenching until it is cooled to room temperature;

[0043] E. Tempering: The steel wire 1 cooled to room temperature continues to move forward, passes through the tempering heating coil 8 with a power frequency of 10KHz, and is heated to 400°C. Then, it enters the tempering insulation pipe 9 for 27S for tempering treatment;

[0044] F. Surface softening and cooling treatment: After tempering and heat preservation, the steel wire 1 moves forward and enters the softening heating coil 10 with a power frequency of 380KHz for surface softening treatment, and the surface temperature of the steel wire is heated to 480°C; then the steel wire 1 moves forward and enters the softening cooling tank 11 for cooling treatment to obtain the finished steel wire;

[0045] G. Steel wire winding: The finished steel wire obtained in step F is wound onto the take-up drum 13 under the traction of the rear traction machine 12 for winding.

[0046] In the step C of the present invention, the heating depth of the steel wire surface is less than 1 mm and the heating time is 1.2 s.

[0047] In step F of the present invention, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is 1.2 s, and then the steel wire is immediately cooled to room temperature.

[0048] The finished steel wire of the present invention has a tensile strength of 2172 MPa, a cross-sectional shrinkage rate of 46.8%, a Vickers hardness of 541 HV at 0.5 mm from the surface, a Vickers hardness of 596 HV at 1.0 mm from the surface, a Vickers hardness of 637 HV at 1.5 mm from the surface, and a Vickers hardness of 643 HV at 3.0 mm from the surface.

[0049] The finished steel wire obtained in Example 2 was inspected after a seven-day storage test: no self-breaking phenomenon occurred.

[0050] Embodiment 3

[0051] A heat treatment process for preventing the steel wire from breaking spontaneously in the third embodiment comprises the following steps:

[0052] A. Steel wire unwinding: driven by the front traction machine 3, the steel wire rod made of 55SiCr, which is drawn to Φ13.6mm by the wire drawing machine and the drawing die and then wound on the pay-off drum 2, is gradually unwound;

[0053] B. Austenitization: At the same time, the steel wire 1 moves forward and passes through the coil I4-1 and the coil II4-2 in the induction heating coil 4 in sequence. That is, the steel wire 1) first passes through the coil I4-1) with a power frequency of 5KHz for preheating, and then the steel wire 1) passes through the coil II4-2) with a power frequency of 45KHz for heating for 30S and is heated to 880°C to obtain an austenitized steel wire;

[0054] C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through the high-frequency heating power coil 5 with a power frequency of 450KHz for surface high temperature heating, and the surface temperature of the steel wire is heated to 1230°C; the steel wire 1 after the surface high temperature treatment immediately enters the insulation pipe 6 while being kept warm and moving forward;

[0055] D. Quenching and cooling: After being removed from the insulation tube 6, the steel wire 1 is moved forward and immersed in the cold water medium of the quenching and cooling tank 7 for quenching until it is cooled to room temperature;

[0056] E. Tempering: The steel wire 1 cooled to room temperature continues to move forward, passes through the tempering heating coil 8 with a power frequency of 8KHz, and is heated to 450°C. Then, it enters the tempering insulation pipe 9 for 40S for tempering treatment;

[0057] F. Surface softening and cooling treatment: After tempering and heat preservation, the steel wire 1 moves forward and enters the softening heating coil 10 with a power frequency of 350KHz for surface softening treatment, and the surface temperature of the steel wire is heated to 500°C; then the steel wire 1 moves forward and enters the softening cooling tank 11 for cooling treatment to obtain the finished steel wire;

[0058] G. Steel wire winding: The finished steel wire obtained in step F is wound onto the take-up drum 13 under the traction of the rear traction machine 12 for winding.

[0059] In the step C of the present invention, the heating depth of the steel wire surface is less than 1 mm and the heating time is 1.8 s.

[0060] In step F of the present invention, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is 1.8 s, and then the steel wire is immediately cooled to room temperature.

[0061] The finished steel wire of the present invention has a tensile strength of 2021 MPa, a cross-sectional shrinkage rate of 48.2%, a Vickers hardness of 539 HV at 0.5 mm from the surface, a Vickers hardness of 533 HV at 1.0 mm from the surface, a Vickers hardness of 599 HV at 1.5 mm from the surface, and a Vickers hardness of 605 HV at 3.0 mm from the surface.

[0062] The finished steel wire obtained in Example 3 was inspected after a seven-day storage test: no self-breaking phenomenon occurred.

[0063] Embodiment 4

[0064] A heat treatment process for preventing the steel wire from breaking spontaneously in the fourth embodiment comprises the following steps:

[0065] A. Steel wire unwinding: driven by the front traction machine 3, the steel wire rod made of 55SiCr, which is drawn to Φ14.5mm by the wire drawing machine and the drawing die and then wound on the pay-off drum 2, is gradually unwound;

[0066] B. Austenitization: At the same time, the steel wire 1 moves forward and passes through the coil I4-1 and the coil II4-2 in the induction heating coil 4 in turn. That is, the steel wire 1 first passes through the coil I4-1 with a power frequency of 10KHz for preheating, and then passes through the coil II4-2 with a power frequency of 50KHz for heating for 45S to 880°C to obtain an austenitized steel wire.

[0067] C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through the high-frequency heating power supply coil 5 with a power supply frequency of 500KHz for surface high temperature heating, and the surface temperature of the steel wire is heated to 1200°C; the steel wire 1 after the surface high temperature treatment immediately enters the insulation pipe 6 and moves forward while being heated;

[0068] D. Quenching and cooling: After being removed from the insulation tube 6, the steel wire 1 is moved forward and immersed in the cold water medium of the quenching and cooling tank 7 for quenching until it is cooled to room temperature;

[0069] E. Tempering: The steel wire 1 cooled to room temperature continues to move forward, passes through the tempering heating coil 8 with a power frequency of 6KHz, and is heated to 440°C. Then, it enters the tempering insulation pipe 9 for 50S of insulation for tempering treatment;

[0070] F. Surface softening and cooling treatment: The steel wire 1 after tempering and heat preservation moves forward and enters the softening heating coil 10 with a power frequency of 400KHz for surface softening treatment, and the surface temperature of the steel wire is heated to 500°C; then the steel wire 1 moves forward and enters the softening cooling tank 11 for cooling treatment to obtain the finished steel wire;

[0071] G. Steel wire winding: The finished steel wire obtained in step F is wound onto the take-up drum 13 under the traction of the rear traction machine 12 for winding.

[0072] In the step C of the present invention, the heating depth of the steel wire surface is less than 1 mm and the heating time is 1.7 s.

[0073] In step F of the present invention, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is 1.7 s, and then the steel wire is immediately cooled to room temperature.

[0074] The finished steel wire of the present invention has a tensile strength of 2039 MPa, a cross-sectional shrinkage rate of 45.1%, a Vickers hardness of 544 HV at 0.5 mm from the surface, a Vickers hardness of 562 HV at 1.0 mm from the surface, a Vickers hardness of 611 HV at 1.5 mm from the surface, and a Vickers hardness of 612 HV at 3.0 mm from the surface.

[0075] The finished steel wire obtained in Example 4 was inspected after a seven-day storage test: no self-breaking phenomenon occurred.

[0076] Embodiment 5

[0077] A heat treatment process for preventing the steel wire from breaking spontaneously in the fifth embodiment comprises the following steps:

[0078] A. Steel wire unwinding: driven by the front traction machine 3, the steel wire rod made of 55SiCr, which is drawn to Φ15.3mm by the wire drawing machine and the drawing die and then wound on the pay-off drum 2, is gradually unwound;

[0079] B. Austenitization: At the same time, the steel wire 1 moves forward and passes through the coil I4-1 and the coil II4-2 in the induction heating coil 4 in turn. That is, the steel wire 1 first passes through the coil I4-1 with a power frequency of 8KHz for preheating, and then passes through the coil II4-2 with a power frequency of 50KHz for heating for 50S to 860°C to obtain an austenitized steel wire.

[0080] C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through the high-frequency heating power supply coil 5 with a power supply frequency of 500KHz for surface high temperature heating, and the surface temperature of the steel wire is heated to 1180°C; the steel wire 1 after the surface high temperature treatment immediately enters the insulation pipe 6 while being kept warm and moving forward;

[0081] D. Quenching and cooling: After being removed from the insulation tube 6, the steel wire 1 is moved forward and immersed in the cold water medium of the quenching and cooling tank 7 for quenching until it is cooled to room temperature;

[0082] E. Tempering: The steel wire 1 cooled to room temperature continues to move forward, passes through the tempering heating coil 8 with a power frequency of 5KHz, and is heated to 450°C. Then, it enters the tempering insulation pipe 9 for 56S for tempering treatment;

[0083] F. Surface softening and cooling treatment: After tempering and heat preservation, the steel wire 1 moves forward and enters the softening heating coil 10 with a power frequency of 400KHz for surface softening treatment, and the surface temperature of the steel wire is heated to 520°C; then the steel wire 1 moves forward and enters the softening cooling tank 11 for cooling treatment to obtain the finished steel wire;

[0084] G. Steel wire winding: The finished steel wire obtained in step F is wound onto the take-up drum 13 under the traction of the rear traction machine 12 for winding.

[0085] In the step C of the present invention, the heating depth of the steel wire surface is less than 1 mm and the heating time is 1.9 s.

[0086] In step F of the present invention, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is 1.9 s, and then the wire is immediately cooled to room temperature.

[0087] The tensile strength of the finished steel wire of the present invention is 2016MPa, and the cross-sectional shrinkage is 44.7%.

[0088] The Vickers hardness at 0.5mm from the surface is 525HV, the Vickers hardness at 1.0mm from the surface is 543HV, the Vickers hardness at 1.5mm from the surface is 594HV, and the Vickers hardness at 3.0mm from the surface is 602HV.

[0089] The finished steel wire obtained in Example 5 was inspected after a seven-day storage test: no self-breaking phenomenon occurred.

Claims

1. A heat treatment process for preventing a steel wire from breaking by itself, characterized in that: The process comprises the following steps: A. Steel wire unwinding: The steel wire coil wound on the pay-off drum (2) is gradually unwound by the front traction machine (3); B. Austenitization: At the same time, the steel wire (1) moves forward and passes through coil I (4-1) and coil II (4-2) of the induction heating coil (4) in sequence. That is, the steel wire (1) first passes through coil I (4-1) with a power frequency of 5 kHz to 10 kHz to be preheated, and then passes through coil II (4-2) with a power frequency of 40 kHz to 50 kHz to be heated for 20 seconds to 50 seconds to be heated to 850° C. to obtain an austenitized steel wire; C. Surface high temperature treatment: The austenitized steel wire continues to move forward and passes through a high-frequency heating power coil (5) with a power frequency higher than 400 kHz to perform surface high temperature heating, and the surface temperature of the steel wire is heated to 1080°C to 1230°C; the steel wire (1) after the surface high temperature treatment then enters the insulation pipe (6) and moves forward while being heat-insulated; D. Quenching and cooling: after being removed from the insulation tube (6), the steel wire (1) is moved forward and immersed in the cold water medium of the quenching and cooling tank (7) for quenching until it is cooled to room temperature; E. Tempering: The steel wire (1) cooled to room temperature continues to move forward and passes through a tempering heating coil (8) with a power frequency of 5 kHz to 10 kHz. The steel wire is heated to 400° C. to 500° C. and then enters a tempering insulation pipe (9) to be kept warm for 27 seconds to 56 seconds for tempering treatment. F. Surface softening and cooling treatment: the steel wire (1) after tempering and heat preservation is moved forward and passed through a softening heating coil (10) with a power frequency higher than 300 kHz for surface softening treatment, and the surface temperature of the steel wire is heated to 480° C. to 540° C.; then the steel wire (1) is moved forward and entered into a softening cooling tank (11) for cooling treatment, thereby obtaining a finished steel wire; G. Steel wire winding: The finished steel wire obtained in step F is wound onto a take-up drum (13) for winding under the traction of a rear traction machine (12).

2. A heat treatment process for preventing the steel wire from breaking by itself according to claim 1, characterized in that: In step C, the heating depth of the steel wire surface is less than 1 mm and the heating time is less than 2 s.

3. A heat treatment process for preventing the steel wire from breaking itself according to claim 1, characterized in that: In step F, the heating depth of the steel wire surface is less than 1.5 mm, the heating time is less than 2 seconds, and then the steel wire is immediately cooled to room temperature.

4. A heat treatment process for preventing the steel wire from breaking itself according to claim 1, characterized in that: The finished steel wire has a tensile strength of more than 1800 MPa, a cross-sectional shrinkage rate of 44.7% to 52.4%, a Vickers hardness of 502 to 544 HV at 0.5 mm from the surface, a Vickers hardness of 527 to 596 HV at 1.0 mm from the surface, a Vickers hardness of 558 to 637 HV at 1.5 mm from the surface, and a Vickers hardness of 554 to 643 HV at 3.0 mm from the surface.

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