Preparation process of high-strength cold-drawn stainless steel wire

Abstract

The application provides a preparation process of high-strength cold-drawn stainless steel wire, which adopts a duplex preparation process of vacuum induction melting and electroslag remelting to melt metal, and prepares a wire rod after forging and hot rolling of an ingot; the wire rod is cold-rough-drawn by controlling the strain rate of the material; the deformation amount and the area reduction of the material are strictly controlled during fine drawing, so that the martensite transformation amount in the material reaches a saturation state; finally, the steel wire is tempered at low temperature to cause precipitation strengthening and phase transformation strengthening of the steel wire, and the residual austenite is further transformed into martensite during tempering and cooling, so that the strength of the steel wire after cold drawing is further improved. The stainless steel wire prepared by the process can make the martensite transformation amount in the steel wire reach about 98%, and the tensile strength of the steel wire reach about 2280MPa on average, so that the prepared high-strength steel wire can meet the use demand of high-strength steel wire in the existing application environment.

Description

Technical Field

[0001] This invention relates to the field of special steel wire preparation technology, and in particular to a preparation process for high-strength cold-drawn stainless steel wire. Background Technology

[0002] In current industrial manufacturing, the concept of lightweighting has been fully integrated into various fields due to environmental protection and energy conservation considerations. Lightweight manufacturing mainly includes lightweight structural design and lightweight material research. In practical applications, lightweight structural design is widely used to achieve weight reduction, which means reducing the weight of components, including reducing component wall thickness or shrinking component size. This requires significantly increasing the strength of materials while ensuring the overall structural strength. Therefore, in mechanical fields, the research and development of high-strength stainless steel materials, such as stainless steel which has extremely wide applications, has received increasing attention. Stainless steel wire is one of the most widely used stainless steel materials in many fields, including mechanical seals, automotive tire cords, biomedicine, chemical energy, and robot transmission systems.

[0003] For most applications of stainless steel wire, high strength and hardness are required, while plasticity and toughness are not particularly demanding. Currently, the stainless steel materials produced are often austenitic stainless steels. Austenitic stainless steel is an unsteady or metastable steel. This type of steel has excellent corrosion resistance, plasticity, and weldability, but its main drawback is its low strength, making it unsuitable for producing high-strength stainless steel wires. Furthermore, it generally cannot be strengthened through phase transformation; cold working is one of the main methods for strengthening steel wires. During cold working, the softer austenitic parent phase can transform into a harder martensitic phase, known as strain-induced martensite. Martensitic phase transformation has a significant impact on the mechanical properties and work hardening behavior of austenitic stainless steel. Due to the formation of stacking faults, twins, ε-martensite, and α′-martensite during cold working, austenitic stainless steel exhibits a very high work hardening capacity. Theoretically, the more transformed martensite obtained during cold working, the higher the hardness of the material. However, traditional cold working processes generally reach saturation at around 85% martensite content, and in rare cases, they can only reach a maximum of close to 90% martensite content before reaching complete saturation, making it impossible to further increase the amount of martensite transformation in the material. Therefore, in the preparation of high-strength or ultra-high-strength stainless steel wire, how to further increase the amount of martensite transformation in the material becomes a key factor in the preparation process. Summary of the Invention

[0004] Based on the application requirements for high-strength stainless steel wire in the existing technology and the problem that the existing cold working process cannot significantly increase the martensite transformation amount and thus improve the strength of the steel wire, the purpose of this application is to provide a preparation process for high-strength cold-drawn stainless steel wire. By strictly controlling the metal smelting, cold working and heat treatment processes, the martensite transformation amount in stainless steel can be significantly increased, and high-strength stainless steel wire can be prepared.

[0005] To achieve the objectives of this invention, a process for preparing high-strength cold-drawn stainless steel wire is provided, comprising the following steps: Step 1: Prepare steel ingots by smelting metal using a dual process of vacuum induction melting and electroslag remelting. The elemental and weight percentage content of the stainless steel is as follows: C: 0.08%, Si: 1.0%, Mn: 2.0%, P: 0.045%, S: 0.03%, Cr: 17%, Ni: 13%, Mo: 2.7%, Ti: 0.45%, with the balance being Fe. During vacuum induction melting, the vacuum degree is ≤1.2 Pa during the metal melting period, and the melting time is 1.5–2 h. During the refining period, the vacuum degree is maintained at <1.0 Pa, and the melting temperature is 1550℃–1630℃. Step 2, Forging: Before forging, the remelted ingot prepared in Step 1 is heated to a temperature of 1150℃–1250℃; then it undergoes two forging processes, with the total deformation of the ingot exceeding 85%, and the deformation of the last forging process controlled within 10%–15%, with the final forging temperature controlled at 850℃–900℃. Step 3, Hot rolling and wire rod preparation: The billet after forging in Step 2 is heated to 980℃–1020℃. The deformation of the first rolling is controlled at about 25%, the deformation of the second rolling is about 45%, and the deformation of the third rolling is above 65%. The final rolling temperature is controlled within the range of 800℃–850℃. Then, the material rolled in the third rolling process is drawn into wire rod to obtain a wire rod with a diameter of 4mm.

[0006] Step 4: Rough drawing of wire rod: A 4mm diameter wire rod is drawn multiple times at room temperature using a drawing machine to produce a 1mm diameter stainless steel wire; the strain rate of the material is controlled at 6×10⁻⁶ during drawing. -4 / S, and the error is controlled within ±30%; Step 5, Solution treatment and fine drawing of steel wire: The 1mm diameter steel wire prepared in Step 4 is solution treated and kept at 1030℃ for 30 minutes. After water quenching, it is cooled to room temperature. Then, the solution-treated steel wire is cold-drawn to 0.3mm in multiple passes at room temperature. The final reduction rate of the steel wire reaches about 90%.

[0007] Step 6: Temper the steel wire prepared in Step 5 at a low temperature: the tempering temperature is 250 ℃-350 ℃, the tempering holding time is not less than 2 hours, and then air cool to room temperature.

[0008] Preferably, in step one, during vacuum induction melting, the vacuum degree is 1.0 Pa and the melting time is 2 hours during the metal melting period; the vacuum degree is 0.8 Pa and the melting temperature is 1590℃ during the refining period.

[0009] More preferably, in step two, the remelted ingot is heated to 1200°C before forging; after two forging processes, the total deformation of the ingot is 90%, the deformation of the last forging process is controlled at 10%, and the final forging temperature is 890°C.

[0010] More preferably, in step six, the low-temperature tempering temperature is 300°C, the tempering holding time is 2.5 hours, and then the temperature is air-cooled to room temperature.

[0011] The high-strength cold-drawn stainless steel wire preparation process of this invention employs a dual-process manufacturing method combining vacuum induction melting and electroslag remelting, which allows for precise control of the strengthening element content in the steel. During the cold drawing and diameter reduction of the steel wire, the strain rate of the material is controlled by adjusting the drawing speed to avoid the influence of strain heat on the transformation of austenite to martensite. During fine drawing, the deformation amount and reduction rate of the material are strictly controlled to ensure that the martensite transformation amount in the material reaches a saturated state. Finally, low-temperature tempering causes precipitation strengthening and phase transformation strengthening of the steel wire. During the tempering cooling process, the retained austenite further transforms into martensite, thereby significantly improving the strength of the cold-drawn steel wire. The stainless steel wire prepared by the process of this invention can achieve a martensite transformation amount of approximately 98% and an average tensile strength of approximately 2280 MPa. The resulting high-strength steel wire can meet the requirements for high-strength steel wire in current application environments. Attached Figure Description

[0012] Figure 1 The diagram shown is a schematic flow chart of the preparation process of high-strength cold-drawn stainless steel wire according to an embodiment of the present invention. Detailed Implementation

[0013] The preparation process and technical effects of the high-strength cold-drawn stainless steel wire of the present invention are described in detail below with reference to the accompanying drawings.

[0014] This application takes austenitic stainless steel wire with the standard grade 06Cr17Ni12Mo2Ti and code S31668 as defined in the national standard GB / T4240-2019 (Stainless Steel Wire). This material has excellent corrosion resistance and is commonly used in seawater and acidic environments. The steel wire products made from this material can be used in, for example, the fixed structures of flexible photovoltaic systems on the sea surface. This invention aims to prepare high-strength stainless steel wire from this austenitic stainless steel by controlling the metal smelting, cold working, and heat treatment processes. The specific preparation process is as follows: Step 1: Steel ingots are prepared by metal smelting through a dual process of vacuum induction melting and electroslag remelting; specifically, the element and weight percentage content of stainless steel in one embodiment of the present invention is as follows: C: 0.08%, Si: 1.0%, Mn: 2.0%, P: 0.045%, S: 0.03%, Cr: 17%, Ni: 13%, Mo: 2.7%, Ti: 0.45%, with the balance being Fe.

[0015] First, the metal is melted in a high-vacuum environment in a vacuum induction furnace. This allows for control of the O, N, and H content in the induction ingot and reduces impurity elements in the alloy. Furthermore, refining and stirring in the induction furnace yields a consumable electrode with uniform composition. The consumable electrode obtained in the induction furnace is then remelted in an electroslag remelting furnace. This process effectively desulfurizes, degass ...

[0016] During vacuum induction melting, the electromagnetic stirring of the molten metal prevents the segregation of metal elements, resulting in a more uniform composition of the steel ingot. However, removing sulfur (S) and phosphorus (P) is relatively difficult in vacuum induction melting. Therefore, a two-step melting process is combined with electroslag remelting. This produces ingots with a uniform microstructure, smooth surface, and good machinability. It also allows for precise control of the content of strengthening elements such as Ti and Mo in the steel, while effectively removing impurities such as S, P, and O. In this embodiment, during vacuum induction melting, the vacuum degree is ≤1.2 Pa during the metal melting period, and the melting time is 1.5–2 h; during the refining period, the vacuum degree is maintained at <1.0 Pa, and the melting temperature is 1550℃–1630℃. Step Two: Forging; Specifically, before forging, the remelted ingot prepared in Step One must be heated to a temperature of 1150℃–1250℃. Excessive temperature will generate ferrite within the microstructure, reducing the steel's plasticity and making it prone to fracture during forging. If the temperature is too low, the carbides will not be fully dissolved. In this case, in the initial stage of carbide dissolution, the carbon content near the carbides is high, while the surrounding area has a low carbon content. This will lead to an increase in retained austenite during subsequent cold working, reducing the martensite conversion rate and affecting the material's strength.

[0017] The steel ingot is forged using an electro-hydraulic hammer and then subjected to two forging processes. The total deformation of the steel ingot exceeds 85%, and the deformation in the final forging process is controlled within 10%–15%. The final forging temperature is controlled at 850℃–900℃.

[0018] During large deformation forging, the grains are fully broken down. This large deformation forging not only helps eliminate defects in the as-cast structure but also significantly improves the high-temperature ductility of the steel, providing a good material basis for subsequent hot rolling processes. During forging, the internal structure of the billet is optimized, the grains are refined, and the dislocation density increases, all of which contribute to improving the mechanical and machinability of the final product.

[0019] Step 3: Hot rolling and wire rod preparation; specifically, the billet after forging in Step 2 is heated to 980℃-1020℃, the deformation of the first rolling is controlled at about 25%, the deformation of the second rolling is about 45%, and the deformation of the third rolling is above 65%; the final rolling temperature is controlled within the range of 800℃-850℃; then the material rolled in three rolling processes is drawn into wire rod to obtain a wire rod with a diameter of 4mm.

[0020] Step Four: Rough Drawing of Wire Rod; Specifically, a 4mm diameter wire rod is drawn multiple times using a drawing machine to obtain a 1mm diameter stainless steel wire. In this step, the drawing speed of the wire rod needs to be strictly controlled. Since austenitic stainless steel is an unstable alloy, according to the test results of the trial production process, plastic deformation at different strain rates at a certain temperature directly affects the strain-induced α′-martensite transformation rate and the amount of transformation, thus exhibiting different mechanical behaviors. Performance indicators such as yield strength, tensile strength, uniform elongation, and strain hardening rate will change significantly with the strain rate. Excessive drawing speed leads to a faster strain rate in the material, and an excessively fast strain rate will generate a deformation heat effect, thus tending to stabilize the austenite and affecting the amount of martensite transformation. According to the product trial production test results, when the strain rate of the material approaches 3×10⁻⁶, the strain rate is less than ideal. -2 When the temperature reaches 6 × 10⁻⁶, it will significantly affect the transformation of martensite. Therefore, in this application, when rough drawing the wire rod, firstly, a cold drawing process must be used, that is, rough drawing must be carried out at room temperature, and stretching at the annealing temperature is strictly prohibited; secondly, the strain rate of the material needs to be strictly controlled; in this way, the thermal effect on the transformation of martensite can be avoided. In this embodiment, based on the actual trial production results, the strain rate of the material is controlled at 6 × 10⁻⁶ during rough drawing. -4 When the value of / S is within ±30%, the transformation of martensite can achieve relatively excellent results.

[0021] Step 5: Solution Treatment and Reduction Drawing of Steel Wire. The 1mm diameter steel wire prepared in Step 4 is solution treated by holding it at 1030℃ for 30 minutes, followed by water quenching and cooling to room temperature. The solution-treated wire is then cold-drawn to 0.3mm in multiple passes at room temperature. Solution treatment ensures uniform stainless steel composition, moderate hardness, and improved plasticity, facilitating subsequent precise reduction drawing. During room temperature drawing, the strain rate must be controlled, gradually reducing the wire diameter to 0.85mm, 0.7mm, 0.55mm, and 0.3mm through multiple draw passes, achieving a final reduction in surface area of ​​approximately 90%. According to the test results of the samples, when the final reduction ratio of the steel wire reaches about 90%, the content of α′ martensite with high hardness can reach about 85.6%, the content of ε martensite with slightly lower hardness can reach about 6.7%, and the remainder is retained austenite. The average tensile strength of the steel wire is about 2090 MPa. When the reduction ratio continues to increase, the content of martensite and the average tensile strength of the steel wire increase slightly but do not change significantly, indicating that the transformation of martensite has basically reached saturation. When the reduction ratio is too low, the transformation of martensite cannot be maximized.

[0022] Step Six: Perform low-temperature tempering on the steel wire prepared in Step Five. Low-temperature tempering can further improve the strength of stainless steel wire. When the tempering temperature is 250℃, the tensile strength and yield strength of the steel wire begin to show a significant increasing trend. When the tempering temperature is increased to 300℃, the tensile strength of the steel wire can increase from 2090MPa to about 2280MPa. When the tempering temperature continues to rise, the strength of the steel wire will increase slightly, but the increase is not particularly significant. When the tempering temperature is increased to 350℃, the strength of the steel wire begins to show a decreasing trend. The tempering holding time should not be less than 2 hours, followed by air cooling to room temperature.

[0023] Low-temperature tempering of steel wire has three main effects: First, because the tempering temperature is below the recrystallization temperature, large-angle grain boundary migration does not occur, and the shape and size of the grains remain unchanged. Tempering can significantly reduce the internal stress caused by cold drawing deformation. Second, when the tempering temperature reaches 250℃, carbides precipitate in the material. Trace amounts of nano-sized carbides precipitate from the retained austenite, playing a role in dispersion strengthening, thus improving the hardness and strength of the material. Third, since carbon is the element that stabilizes austenite, the chemical composition of the retained austenite changes after carbide precipitation, becoming more unstable. This leads to the further transformation of the retained austenite into martensite during tempering cooling, significantly improving the strength and elastic limit of the material. Therefore, through low-temperature tempering, precipitation strengthening and phase transformation strengthening can occur in steel wire. During tempering cooling, the retained austenite further transforms into martensite, thereby significantly improving the strength of the cold-drawn steel wire. According to the test results, the martensite transformation rate in the steel wire after low-temperature tempering can reach about 98%, and the tensile strength can reach up to about 2280 MPa. This proves that by controlling the material strain rate, reduction ratio, and low-temperature tempering during cold drawing, the austenite-to-martensite transformation rate in the steel wire can be significantly increased, thereby significantly improving the strength of the steel wire.

[0024] The high-strength cold-drawn stainless steel wire preparation process of this invention employs a dual-process preparation method of vacuum induction melting + electroslag remelting in the metal smelting stage. This method can precisely control the content of strengthening elements such as Ti and Mo, effectively remove impurity elements such as S, P, and O, and effectively control the loss and segregation of strengthening elements in the steel ingot. Following two-stage forging, controlling the deformation and final forging temperature of the steel ingot helps eliminate defects in the as-cast structure and significantly improves the high-temperature ductility of the steel, providing a good material basis for subsequent hot rolling processes. During the cold drawing and diameter reduction of the steel wire, the process is controlled... The strain rate of the material is controlled by the drawing speed to avoid the influence of strain heat on the transformation of austenite to martensite. Before fine drawing, solution treatment is performed to make the stainless steel composition uniform, with moderate hardness, and to improve its plasticity, so as to facilitate subsequent precise diameter reduction cold drawing. During fine drawing, the deformation amount and reduction rate of the material are strictly controlled to ensure that the amount of martensite transformation in the material reaches a saturated state. Finally, low-temperature tempering is used to cause precipitation strengthening and phase transformation strengthening of the steel wire. During the tempering cooling process, the retained austenite is further transformed into martensite, thereby significantly improving the strength of the cold-drawn steel wire. The stainless steel wire prepared by the process of this invention can achieve an average tensile strength of about 2280 MPa, and the resulting high-strength steel wire can meet the requirements of high-strength steel wire in current application environments.

[0025] The present invention has been described by the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. It must be noted that the disclosed embodiments do not limit the scope of the present invention. Conversely, any modifications and refinements made without departing from the spirit and scope of the present invention are within the scope of patent protection of the present invention.

Claims

1. A process for preparing high-strength cold-drawn stainless steel wire, characterized in that, The preparation process includes the following steps: Step 1: Prepare steel ingots by smelting metal using a dual process of vacuum induction melting and electroslag remelting. The elemental and weight percentage content of the stainless steel is as follows: C: 0.08%, Si: 1.0%, Mn: 2.0%, P: 0.045%, S: 0.03%, Cr: 17%, Ni: 13%, Mo: 2.7%, Ti: 0.45%, with the balance being Fe. During vacuum induction melting, the vacuum degree is ≤1.2 Pa during the metal melting period, and the melting time is 1.5–2 h. During the refining period, the vacuum degree is maintained at <1.0 Pa, and the melting temperature is 1550℃–1630℃. Step 2, Forging: Before forging, the remelted ingot prepared in Step 1 is heated to a temperature of 1150℃–1250℃; then it undergoes two forging processes, with the total deformation of the ingot exceeding 85%, and the deformation of the last forging process controlled within 10%–15%, with the final forging temperature controlled at 850℃–900℃. Step 3, Hot Rolling and Wire Rod Preparation: The billet forged in Step 2 is heated to 980℃–1020℃. The deformation of the first rolling is controlled at about 25%, the deformation of the second rolling is about 45%, and the deformation of the third rolling is above 65%. The final rolling temperature is controlled within the range of 800℃–850℃. Then, the material rolled in the third rolling process is drawn into wire rod to obtain a wire rod with a diameter of 4mm. Step 4: Rough drawing of wire rod: A 4mm diameter wire rod is drawn multiple times at room temperature using a drawing machine to produce a 1mm diameter stainless steel wire; the strain rate of the material is controlled at 6×10⁻⁶ during drawing. -4 / S, and the error is controlled within ±30%; Step 5, Solution treatment and fine drawing of steel wire: The 1mm diameter steel wire prepared in Step 4 is solution treated and kept at 1030℃ for 30 minutes. After water quenching, it is cooled to room temperature. Then, the solution-treated steel wire is cold-drawn to 0.3mm in multiple passes at room temperature. The final reduction rate of the steel wire reaches about 90%. Step 6: Temper the steel wire prepared in Step 5 at a low temperature: the tempering temperature is 250 ℃-350 ℃, the tempering holding time is not less than 2 hours, and then air cool to room temperature.

2. The preparation process of high-strength cold-drawn stainless steel wire as described in claim 1, characterized in that, In step one, during vacuum induction melting, the vacuum degree is 1.0 Pa and the melting time is 2 h during the metal melting period; the vacuum degree is 0.8 Pa and the melting temperature is 1590℃ during the refining period.

3. The preparation process of high-strength cold-drawn stainless steel wire as described in claim 1, characterized in that, In step two, the remelted ingot is heated to 1200℃ before forging; after two forging processes, the total deformation of the ingot is 90%, and the deformation in the last forging process is controlled at 10%, with a final forging temperature of 890℃.

4. The preparation process of high-strength cold-drawn stainless steel wire as described in claim 1, characterized in that, In step six, the low-temperature tempering temperature is 300℃, the tempering holding time is 2.5 hours, and then it is air-cooled to room temperature.

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