Process for the production of stainless steel wire for springs

Abstract

The preparation process of the stainless steel wire for spring comprises the following steps: S1, raw material preparation; S2, film treatment; S3, rough drawing; S4, solid solution treatment; S5, fine drawing; S6, straightening of finished product wire; S7, finished product treatment and packaging. The advantages of the present application are as follows: a step-by-step temperature control lubrication system is established, the viscosity of the lubricating liquid is reduced at high temperature, and the extreme pressure permeability is improved, so that the "brightening" and "scratching" are fundamentally eliminated; after the finished product pass, the reduction of the two sets of vertical straighteners is accurately adjusted according to whether the previous annealing and the total deformation of the fine drawing, so that the spring height and the spring width are locked in the target window; the solid solution treatment in the protective atmosphere is carried out after the rough drawing is completed, so that the plasticity decrease and stress out of control caused by omitting annealing are avoided.

Description

Technical Field

[0001] This invention relates to a process for manufacturing stainless steel wire for springs. Background Technology

[0002] Stainless steel spring wire is an important raw material for manufacturing automotive suspension springs, elastic components for precision instruments, and electrical connectors. Its manufacturing technology has undergone an upgrade from ordinary carbon steel to high-alloy austenitic stainless steel (such as SUS304, D669, etc.). Currently, the mainstream process route for manufacturing stainless steel spring wire with a diameter ≤3.0mm in the industry is: wire rod pretreatment (coating / phosphating) → multiple rough drawing → intermediate annealing (or omitted) → multiple fine drawing → straightening → winding. However, as downstream spring winding machines develop towards higher speeds and automation, extremely stringent requirements are placed on the surface finish of the steel wire, the mechanical consistency along its entire length, and the spring rebound accuracy. Existing processes have significant shortcomings in the following aspects: poor matching between the drawing lubrication system and temperature control, which easily leads to surface defects such as "shining" and "scratching"; when the lubricant temperature is too low (such as below 50°C), its kinematic viscosity increases and its permeability decreases, resulting in "boundary friction" or even "dry friction" between the die and the steel wire. In severe cases, the die's adhesive wear can cause periodic "scratching" on the steel wire surface; the residual stress control methods are limited, and the consistency of the "spring height" and "spring width" after the finished spring is difficult to guarantee.

[0003] Therefore, there is an urgent need to study a process for producing stainless steel wire for springs in order to improve the quality of the wire after drawing. Summary of the Invention

[0004] To achieve the above-mentioned objectives, this invention provides a process for preparing stainless steel wire for springs, which effectively improves the surface quality, plasticity, and elasticity uniformity of the wire, and ensures the manufacturing precision of the spring.

[0005] The present invention discloses a process for manufacturing stainless steel wire for springs, characterized by comprising the following steps:

[0006] S1. Raw material preparation:

[0007] The raw material is selected as Tsingshan D669 austenitic stainless steel wire rod, with a wire diameter of 5.5mm;

[0008] S2, Skin treatment:

[0009] The wire rod was immersed in an aqueous solution of potassium sulfate and calcium sulfate (18 wt%) at 80°C for 25 min; after immersion, it was dried at 200°C for 120 min.

[0010] S3, coarse drawing:

[0011] The rough drawing process consists of four passes, with the wire diameter changing as follows: 5.5mm → 4.9mm → 4.45mm → 4.05mm → 3.7mm. Each pass is lubricated with calcium-based lubricating powder (TF70 powder), and the mold cooling water temperature is 30℃.

[0012] S4. Solution treatment:

[0013] The steel wire after rough drawing was electrolytically cleaned in a dilute sulfuric acid solution (concentration 200g / L), and then subjected to solid solution treatment at a temperature of 1080℃ for 55s under a protective atmosphere of 75%H2+25%N2. After that, it was water cooled to below 60℃.

[0014] S5, precision drawing:

[0015] The precision drawing process consists of 5 passes, with the wire diameter changing as follows: 3.7mm→3.3mm→2.98mm→2.73mm→2.5mm→2.3mm. The lubricant is water-soluble, and the temperature is controlled at 85℃.

[0016] S6. Straightening of finished steel wire:

[0017] After the finished steel wire comes out of the mold box, it is straightened so that the finished linear spring height is 0.5cm and the spring width is 80cm;

[0018] S7: Finished Product Processing and Packaging

[0019] The finished steel wire is cleaned in sequence with a mixture of CL-338 cleaning agent and water, dried by air blowing, and then treated with a dust scraper cleaning ball before being weighed and packaged.

[0020] As a further improvement to the invention, in step S3, the drawing die is made of tungsten steel, and the finished product pass is made of carbon-plated die.

[0021] As a further improvement to the invention, in step S5, the drawing die is a polycrystalline die.

[0022] As a further improvement to the invention, in step S6, the finished steel wire is straightened by two sets of mutually perpendicular straighteners, and the amount of pressure applied by the straightener pulleys is adjusted.

[0023] The advantages of this invention compared to the prior art are as follows:

[0024] (1) Establish a tiered temperature-controlled lubrication system to reduce the viscosity of the lubricant and improve extreme pressure permeability at high temperatures, thereby fundamentally eliminating "shining" and "scratching";

[0025] (2) After the finished product is produced, adjust the pressing amount of the two sets of vertical straighteners precisely according to whether the previous annealing is performed and the total deformation of the fine drawing, and lock the spring height and spring width within the target window;

[0026] (3) After rough drawing is completed, a protective atmosphere solution treatment is required to avoid plasticity reduction and stress runaway caused by omitting annealing. Detailed Implementation

[0027] The specific embodiments of the present invention will be described in further detail below.

[0028] Example 1

[0029] This embodiment prepares a stainless steel wire for springs with a diameter of 2.3 mm. The specific steps are as follows:

[0030] S1. Raw material preparation

[0031] The raw material is selected as Tsingshan D669 austenitic stainless steel wire rod, with a wire diameter of 5.5mm;

[0032] S2, Skin treatment

[0033] The wire rod was immersed in an aqueous solution of potassium sulfate and calcium sulfate (18 wt%) at 80°C for 25 min; after immersion, it was dried at 200°C for 120 min.

[0034] S3, coarse drawing

[0035] The rough drawing process consists of four passes, with the wire diameter changing as follows: 5.5mm → 4.9mm → 4.45mm → 4.05mm → 3.7mm. Each pass is lubricated with calcium-based lubricating powder (TF70 powder), and the mold cooling water temperature is 30℃.

[0036] Tungsten carbide dies are used for drawing, while carbon-coated dies are used for finishing passes. Tungsten carbide dies are used in the coarse drawing stage of the steel wire, mainly due to their high compressive strength and excellent wear resistance, to withstand the intense friction and high contact stress under high compression ratios, ensuring die life and dimensional stability. Carbon-coated dies (such as diamond-like carbon or carbon-based coated dies) are used for finishing passes. Their core advantage lies in their extremely low coefficient of friction and anti-adhesion properties, which can significantly reduce surface scratches on the finished product and lower the drawing temperature rise, thereby obtaining a smoother surface quality and a more uniform residual stress distribution. At the same time, it extends the die replacement cycle for the final pass, which is particularly crucial for improving the fatigue life of the spring and the consistency of the finished product.

[0037] S4, Solution treatment

[0038] The steel wire after rough drawing was electrolytically cleaned in a dilute sulfuric acid solution (concentration 200g / L), and then subjected to solid solution treatment at a temperature of 1080℃ for 55s under a protective atmosphere of 75%H2+25%N2. After that, it was water cooled to below 60℃.

[0039] By applying a high temperature of 1080℃, the broken and twisted grains and carbides after rough drawing are fully recrystallized and re-dissolved into the austenitic matrix, completely eliminating work hardening and residual stress, restoring the inherent excellent plasticity of stainless steel, and providing structural protection for subsequent precision drawing. Simultaneously, a 55-second heat treatment combined with a reducing atmosphere containing 75% hydrogen effectively inhibits abnormal grain growth, avoids the risk of overheating, and utilizes the high thermal conductivity of hydrogen to achieve rapid and uniform heating. Furthermore, hydrogen's strong reducing properties prevent intergranular oxidation and chromium depletion, thus fully preserving the corrosion resistance of the steel wire. Subsequent rapid water cooling (to below 60℃) "freezes" the alloying elements in a solid solution state, preventing harmful precipitation in sensitized areas, ultimately ensuring that the finished spring possesses high strength, high toughness, and excellent fatigue resistance.

[0040] S5, precision drawing

[0041] The precision drawing process consists of 5 passes, with the wire diameter changing as follows: 3.7mm→3.3mm→2.98mm→2.73mm→2.5mm→2.3mm. The lubricant is water-soluble, and the temperature is controlled at 85℃.

[0042] The drawing die adopts a polycrystalline die, which has both ultra-high hardness close to that of natural diamond and excellent impact toughness. It can maintain the nanometer-level precision of the die hole size for a long time under high compression ratio and high speed drawing, thereby ensuring the ultra-high dimensional consistency and roundness of the finished wire diameter. At the same time, its extremely low coefficient of friction and self-lubricating properties can significantly reduce the temperature rise and surface micro-cracks during drawing, obtain a mirror-level surface finish, and greatly extend the die life (up to tens of times that of tungsten steel dies), ultimately providing a key guarantee for the high fatigue life and reliable service performance of the spring.

[0043] S6. Straightening of finished steel wire

[0044] After the finished steel wire comes out of the mold box, it is straightened so that the finished linear spring height is 0.5cm and the spring width is 80cm;

[0045] The finished steel wire is straightened by two sets of mutually perpendicular straighteners, and the amount of pressure applied by the straightener pulleys is adjusted.

[0046] S7: Finished Product Processing and Packaging

[0047] The finished steel wire is cleaned in sequence with a mixture of CL-338 cleaning agent and water, dried by air blowing, and then treated with a dust scraper cleaning ball before being weighed and packaged.

[0048] The 2.3mm stainless steel wire for springs produced in this embodiment has a smooth, scratch-free, and shiny surface, with a linear spring height of 0.5cm and a spring width of 80cm. It meets the requirements for the elastic uniformity of raw materials in spring winding and can be used as a high-quality raw material for stainless steel springs.

[0049] Comparative Example

[0050] The difference between Comparative Example 1 and Example 1 is that the temperature of the lubricating fluid for fine drawing was controlled at 40℃ (room temperature), and the step of controlling the elastic parameters of the straightener was omitted. The results showed that the surface of the steel wire prepared in the comparative example had a slight shiny phenomenon, the linear elastic height fluctuation range was 2~5cm, and the elastic width fluctuation range was 50~110cm, which was significantly worse than that of Example 1.

[0051] The process steps of Examples 2 and 3 are the same as those of Example 1, and the process steps of Comparative Examples 2 and 3 are the same as those of Comparative Example 1. The parameter differences are shown in Table 1 below:

[0052] Table 1. Parameter differences between each embodiment and each comparative example.

[0053] Example 1 TF70 Pink 85℃ (high temperature) Carbon-plated mold (diamond coating) Precise adjustment (elastic width 80cm) 1.095 Example 2 GB205 powder 95℃ (high temperature) Polycrystalline die (PCD) Precise adjustment (elastic width 75cm) 1.090 Example 3 TF70 Pink 75℃ (medium high temperature) Carbon plating mold Precise adjustment (elastic width 85cm) 1.100 Comparative Example 1 TF70 Pink 40℃(normal temperature) Carbon plating mold Uncalibrated (default zero pressure) 1.095 Comparative Example 2 Ordinary sodium powder 85℃ Tungsten carbide mold (uncoated) Precise calibration 1.080 (Slightly low) Comparative Example 3 GB205 powder 85℃ Carbon plating mold Precise calibration 1.095, but intermediate solution annealing is omitted.

[0054] Test data and performance comparison

[0055] The finished product (target wire diameter 2.3mm) was tested according to GB / T 4357-2022 (Standard for Stainless Steel Wire for Springs), and the data is shown in Table 2 below:

[0056] Table 2 Comprehensive Performance Test Data of Finished Steel Wire

[0057] Surface quality / Smooth and without defects Smooth and without defects Smooth and without defects Slightly shiny Obvious scratches Rough surface, oxide scale Linear spring height cm 0.5 0.3 0.8 3.5 (Out of tolerance) 0.9 6.2 (Out of tolerance) Linear elastic width cm 80 75 85 120 (Out of tolerance) 78 150 (Out of tolerance) Tensile strength (Rm) MPa 1850 1880 1820 1780 (Relatively low) 1810 1950 (Excessively brittle) Elongation (A) % 8.5 8.2 9.0 10.5 6.5 (Fail) 4.0 (brittle fracture) Number of twists Second-rate ≥30 ≥28 ≥32 15 (Premature fracture) 22 8 (Brittle fracture) Lubricant residue mg / m² ≤5 ≤8 ≤6 25 (Not cleaned properly) 10 30 (Severe Residue)

[0058] The effect of temperature control on the surface:

[0059] Compared with Comparative Example 1, when the lubricating fluid temperature for fine drawing is only room temperature (40℃), the lubricating fluid has high viscosity and poor permeability, and the drawing heat cannot be dissipated in time, resulting in excessive deformation of the steel wire surface grains and a "shiny" phenomenon. Furthermore, the uneven distribution of residual stress leads to serious deviations in spring height / width (3.5cm / 120cm). This invention limits the fine drawing fluid temperature to 70-95℃, effectively reducing the coefficient of friction and ensuring surface smoothness.

[0060] The effect of mold and coating on scratches:

[0061] Compared with Example 1, Example 2 used a common tungsten carbide mold with a low film concentration (1.080). The mold was severely worn and adhesive, resulting in obvious scratches on the surface of the steel wire and the elongation rate dropped to 6.5%, which did not meet the standard.

[0062] The use of carbon-plated molds / polycrystalline molds in conjunction with high-concentration film (≥1.090) greatly improves mold life and wire surface integrity.

[0063] The effect of intermediate annealing on elastic stability:

[0064] Compared with Comparative Example 1, Comparative Example 3 omitted the solution annealing process after rough drawing, resulting in severe work hardening of the austenitic structure and grain fragmentation. Although the tensile strength was artificially high (1950 MPa), the elongation was only 4%, and it fractured brittlely after only 8 torsional cycles. Moreover, the huge internal stress caused the spring width to become out of control (150 cm) after winding.

[0065] The intermediate solution treatment of this invention is a key step in restoring the plasticity of austenite and ensuring the elastic accuracy of the final product (linear elastic height 0-1cm, elastic width 70-90cm).

[0066] This invention significantly improves the overall performance of stainless steel wire for springs through the synergistic effect of four core processes: stepped temperature-controlled lubrication, intermediate solution annealing, high-precision polycrystalline / carbon-plated molds, and quantitative straightening. The temperature of the precision drawing lubricant is increased from room temperature (Comparative Example 1, 40℃) to 70~95℃ (Examples 1~3), effectively reducing viscosity and enhancing extreme pressure penetration, fundamentally eliminating surface "shining" and scratch defects. Simultaneously, with precise straightener reduction control, the linear spring height fluctuation is narrowed from 2~5cm in Comparative Example 1 to 0.3~0.8cm, and the spring width is stably controlled from 50~110cm to 75~85cm. Regarding the mold, a carbon-plated mold or a polycrystalline mold is used instead of a common tungsten steel mold (Comparative Example 2), combined with ≥1.090... The high-concentration coating avoids significant scratches caused by adhesive wear, increasing the elongation from 6.5% to over 8.2%. The crucial intermediate solution treatment (omitted in Comparative Example 3) completely eliminates work hardening and grain breakage caused by rough drawing. Although the tensile strength is reduced from an inflated 1950 MPa to 1820-1880 MPa, the elongation significantly increases from a brittle 4.0% to 8.2%-9.0%, the torsion cycles increase from 8 to ≥28, and the spring width returns from an uncontrolled 150 cm to the target window of 75-85 cm. Ultimately, the finished product in this example has a smooth, defect-free surface, with lubricant residue ≤8 mg / m², and all mechanical and elastic properties meet the GB / T 4357-2022 standard. Any deviation from any process parameter (low-temperature lubrication, tungsten carbide mold, omission of annealing, or insufficient coating) will lead to surface defects, decreased plasticity, or excessive springback accuracy, fully demonstrating the necessity and superiority of the process route of this invention.

[0067] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A manufacturing process for stainless steel wire for springs, characterized in that, Includes the following steps: S1. Raw material preparation: The raw material is selected as Tsingshan D669 austenitic stainless steel wire rod, with a wire diameter of 5.5mm; S2, Skin treatment: The wire rod was immersed in an aqueous solution of potassium sulfate and calcium sulfate (18 wt%) at 80°C for 25 min; after immersion, it was dried at 200°C for 120 min. S3, coarse drawing: The rough drawing process consists of four passes, with the wire diameter changing as follows: 5.5mm → 4.9mm → 4.45mm → 4.05mm → 3.7mm. Each pass is lubricated with calcium-based lubricating powder (TF70 powder), and the mold cooling water temperature is 30℃. S4. Solution treatment: The steel wire after rough drawing was electrolytically cleaned in a dilute sulfuric acid solution (concentration 200g / L), and then subjected to solid solution treatment at a temperature of 1080℃ for 55s under a protective atmosphere of 75%H2+25%N2. It was then water-cooled to below 60℃. S5, precision drawing: The precision drawing process consists of 5 passes, with the wire diameter changing as follows: 3.7mm→3.3mm→2.98mm→2.73mm→2.5mm→2.3mm. The lubricant is water-soluble, and the temperature is controlled at 85℃. S6. Straightening of finished steel wire: After the finished steel wire comes out of the mold box, it is straightened so that the finished linear spring height is 0.5cm and the spring width is 80cm; S7: Finished Product Processing and Packaging The finished steel wire is cleaned in sequence with a mixture of CL-338 cleaning agent and water, dried by air blowing, and then treated with a dust scraper cleaning ball before being weighed and packaged.

2. The manufacturing process of stainless steel wire for springs as described in claim 1, characterized in that, In step S3, the drawing die is made of tungsten steel, and the finished product pass is made of carbon-plated die.

3. The manufacturing process of stainless steel wire for springs as described in claim 1, characterized in that, In step S5, a polycrystalline die is used for drawing.

4. The manufacturing process of stainless steel wire for springs as described in claim 1, characterized in that, In step S6, the finished steel wire is straightened by two sets of mutually perpendicular straighteners, and the amount of pressure applied by the straightener pulleys is adjusted.