A method for producing a high-strength fastener wire for cold heading with large deformation

Abstract

The application relates to a manufacturing method of high-strength fastener steel wire for large deformation cold upsetting, and relates to the technical field of steel wire production. The element composition of the wire rod for steel wire production is as follows in percentage by weight: C: 0.33-0.44%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P: <=0.020%, S: <=0.020%, Cr: 0.90-1.20%, Mo: 0.45-0.75%, V: 0.10-0.35%, and the rest is Fe and inevitable impurities. The manufacturing process is as follows: configuring raw materials according to chemical composition, sequentially performing KR molten iron pretreatment-converter-LF refining-vacuum degassing-casting-breaking-wire controlled rolling-wire controlled cooling-deep processing annealing-deep processing wire drawing-steel wire. Through large compression ratio rolling, low temperature final rolling and rapid cooling after rolling, the organization level of spheroidizing annealing is improved, and the annealing hardness is reduced; meanwhile, the spheroidizing annealing cycle is shortened through rapid heating, cooling and increasing supercooling degree. The steel wire produced through the process can meet the demand of downstream customers for large deformation cold upsetting processing, reduce the production cost, and save energy and protect the environment.

Description

Technical Field

[0001] This invention pertains to a method for producing steel wire, specifically a method for producing fastener steel wire for large deformation cold heading. Background Technology

[0002] CrMoV alloy steel can be used to manufacture high-strength fasteners of grade 12.9 and above. Due to the large deformation during subsequent cold heading, hot-rolled wire rods cannot be directly cold-headed. They need to be modified before processing to reduce hardness and improve cold heading performance. Since CrMoV alloy steel hot-rolled wire rods cannot be directly drawn into wires, the current modification process for obtaining excellent mechanical properties and cold heading microstructure is: spheroidizing annealing → pickling and coating → wire drawing → spheroidizing annealing → pickling and phosphating → wire drawing. The existing modification process increases internal defects such as dislocations through the wire drawing step. These defects are more prone to melting during annealing, increasing the spheroidization rate. Simultaneously, wire drawing accumulates a large amount of stored energy within the steel, which provides energy for the aggregation and growth of carbides.

[0003] However, spheroidizing annealing in the wire rod refining process is a high-energy-consuming and high-cost processing step, and pickling generates wastewater and acid mist, which are harmful to the environment and human health. How to reduce these high-energy-consuming and polluting processes to achieve steel wire production and make the spheroidized structure and mechanical properties meet customer requirements is a common problem faced by the industry, and also the key to improving product competitiveness.

[0004] Furthermore, the annealing cycle in current industrial production is relatively long. For example, the spheroidizing annealing disclosed in patent document CN109487052A involves multiple heating and cooling cycles, resulting in a long heat treatment period. Annealing schemes disclosed in publications CN114196806A and CN112779404A all employ furnace cooling or slow cooling after the heat treatment period, further extending the spheroidizing annealing cycle. Therefore, shortening the annealing cycle is one of the problems that this invention aims to solve. Summary of the Invention

[0005] The purpose of this invention is to address the problems of long processing steps and long annealing cycles in the current wire rod modification process, and to provide a manufacturing method for high-strength fastener steel wire for large deformation cold heading that can meet the needs of downstream customers. By adopting controlled rolling and controlled cooling technology, combined with improvements to the spheroidizing annealing process, the subsequent pickling, annealing, and wire drawing processes are reduced, and the annealing cycle is shortened. The steel wire produced fully meets the performance requirements of downstream customers for large deformation cold heading, reduces production costs, and is energy-saving and environmentally friendly.

[0006] The preferred elemental composition of the high-strength fastener steel wire for large deformation cold heading in this application is, by weight percentage: C: 0.33-0.44%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P: ≤0.020%, S: ≤0.020%, Cr: 0.90-1.20%, Mo: 0.45-0.75%, V: 0.10-0.35%, with the remainder being Fe and unavoidable impurities.

[0007] The manufacturing method of fastener steel wire includes the following: Step 1, Smelting and Casting: Based on the elemental composition of the wire rod, the molten steel is designed and smelted into steel billets. The smelting process is divided into multiple steps, such as KR molten iron pretreatment, primary furnace smelting, refining and die casting or continuous casting, to produce steel billets with accurate chemical composition, good uniformity and high purity.

[0008] Step 2, Billeting: The steel billet is heated at high temperature and rolled to obtain an intermediate billet. After peeling and grinding, the intermediate billet removes surface defects and oxide layers, improves surface quality, and avoids severe decarburization during subsequent heating.

[0009] Step 3, Controlled Rolling of Wire Rods: The intermediate billet is heated in sections in the heating furnace. Preferably, the preheating section temperature is controlled to ≤720℃, the high-temperature section temperature is controlled to 980℃~1120℃, the holding time in the high-temperature section is not less than 60 minutes, and the residual oxygen content in the furnace is ≤2.0%. After holding, the intermediate billet is descaled to remove the surface oxide layer before rolling. As one of the control schemes: Roughing stage: initial rolling temperature is 850~950℃, total compression ratio ≥60%; secondary descaling is performed before intermediate rolling; Intermediate and finishing rolling stage: intermediate and finishing rolling temperature is 820~900℃, total compression ratio ≥90%; the temperature entering the reducing mill is 780~820℃, and the compression ratio of the last two passes is ≥30%.

[0010] Step 4, Wire Cooling Control: After sizing, the wire rod is spun out at a temperature of 790-810℃. After spun out, the wire rod coil is rapidly cooled by air cooling or water bath cooling. The cooling rate of the wire rod at this stage is ensured to be ≥2℃ / s. If air cooling is used, the roller speed of the wire rod coil is controlled to be 0.20m / s-0.35m / s, and the heat preservation cover is fully opened.

[0011] Step 5, Deep Processing Annealing: Place the wire rods, after pickling or shot blasting to remove surface iron oxide, into an annealing furnace and fill with protective gas. Control the oxygen content in the furnace to below 20 ppm and the dew point to below -40℃ to prevent increased decarburization. Perform the first heating at a rate of ≥50℃ / h to 700-720℃, and hold at 700-720℃ for 2-5 hours. After the first holding, perform the second heating at a rate of ≤20℃ / h to 760-780℃, and perform the third annealing at 760-780℃. The process involves two heat treatments, each lasting 2-5 hours. After the second heat treatment, a first cooling process is performed at a rate of ≥50℃ / h to 650-680℃. A third heat treatment is then performed at 650-680℃ for 2-3 hours. After the third heat treatment, a third heating process is performed at a rate of ≤20℃ / h to 700-720℃. A fourth heat treatment is then performed at 700-720℃ for 6-10 hours. After the fourth heat treatment, a second cooling process is performed at a rate of ≥50℃ / h to below 400℃ before the product is removed from the furnace.

[0012] Step 6, Deep processing wire drawing: The wire rod is finely drawn to reduce the surface area to the target specification of steel wire. It is preferable to perform pickling and phosphating saponification coating before drawing to meet the lubrication requirements of subsequent large deformation cold heading. The fine drawing reduction rate is controlled between 3% and 15%.

[0013] The high-strength fastener steel wire produced by the above manufacturing method has a specification of Φ5.2~Φ24.5mm.

[0014] Preferably, in step one, high-quality blast furnace iron, scrap steel, and raw and auxiliary materials are selected for smelting to reduce the content of harmful elements in the molten steel. The molten steel smelting involves KR iron pretreatment, converter primary smelting, LF refining, and vacuum degassing. KR iron pretreatment and converter slag formation achieve carbon and phosphorus removal, and slag discharge should be avoided during tapping. One of the purposes of vacuum treatment is to control the hydrogen content below 0.5 ppm, effectively reducing the harm of hydrogen embrittlement of high-strength fasteners. The molten steel is cast into steel billets using continuous casting or ingot casting processes. If continuous casting is used, the superheat during continuous casting is 20-35°C. The continuous casting process uses light reduction and electromagnetic stirring to improve billet segregation. The entire continuous casting process uses protective casting to isolate the billet from air and prevent secondary oxidation.

[0015] Preferably, in step two, the continuously cast billet is heated at a high temperature in a heating furnace, at a temperature of 1220–1320°C, to ensure that the alloy components in the steel are fully dissolved, and then the billet is cut into a cross-section of 150*150mm. 2 ~200*200mm 2 The intermediate billet is then removed from the production line and allowed to cool slowly after the billet is opened.

[0016] The high-strength fastener steel wire obtained by the above manufacturing method has a metallographic structure of spheroidal pearlite with a spheroidization rate of ≥95%, an actual grain size of 8 to 12, a tensile strength of ≤600MPa, a reduction of area of ​​≥65%, and a hardness of ≤90HRB, which can meet the needs of downstream customers for large deformation cold heading.

[0017] Compared with the prior art, the advantages of the present invention are as follows: (1) When smelting steel, it is preferred to use a converter to produce steel with low S and P content. LF refining + vacuum degassing helps to produce steel with uniform chemical composition and purity. Two-fire forming is adopted. During the billet opening process, the alloy composition is fully dissolved and diffused. At the same time, the surface of the intermediate billet is peeled off to ensure the surface quality and decarburization of the wire rod.

[0018] (2) The roughing and intermediate finishing rolling of wire rods adopts a large compression ratio, which forms a large number of dislocations and deformation bands inside the steel, and at the same time accumulates a large amount of stored energy, which increases the transformation temperature of austenite to ferrite, preparing for the subsequent reduction sizing rolling. When the temperature of the wire rod entering the reduction sizing mill is controlled at 780-820℃, combined with the compression ratio of ≥30% in the last two rolling passes, deformation-induced phase transformation occurs, resulting in the transformation of austenite to ferrite, precipitating fine proeutectoid ferrite, and refining the grain size. For wire rods with a diameter of φ30mm or more, a rapid cooling process (air cooling or water bath cooling) is adopted after wire drawing. The austenite that has not been transformed during the rolling process undergoes martensitic transformation, so that the dislocations and stored energy generated during the rolling process are retained as much as possible. Meanwhile, the refined grains increase the grain boundary area of ​​the steel. The stored energy of grain boundaries, dislocations and other defects in hot-rolled wire rod, as well as rolling deformation and phase transformation, can replace the wire drawing process before spheroidizing annealing in traditional processes, significantly accelerating the transfer of carbon atoms during annealing and accelerating the spheroidizing process of wire rod.

[0019] (3) The annealing process shortens the annealing time by adopting rapid heating and cooling below Ac1. At the same time, the rapid cooling and large temperature difference after the two-phase region is held at a high temperature increases the supercooling, which is beneficial to shorten the phase transformation time of the two-phase region to ferrite. Then, the temperature is slowly raised to a temperature lower than AC1 and held for a long time, which causes spontaneous spheroidization and carbide growth. The size and number of carbides are reduced, the hardness and tensile strength are reduced, and the surface shrinkage is increased.

[0020] (4) By improving the controlled rolling and cooling and annealing process, one spheroidizing annealing is used instead of two spheroidizing annealing processes. The actual grain size of the steel wire obtained is grade 8 to 12. At the same time, the spheroidization rate is ≥95%, the tensile strength is ≤600MPa, the reduction of area is ≥65%, and the hardness is ≤90HRB. The structure and properties are comparable to those of two-spheroidized and two-drawn steel wires, but save one pickling, one wire drawing and one annealing process. This can meet the needs of downstream customers for large deformation cold heading, reduce production costs, and save energy and protect the environment. Attached Figure Description

[0021] Figure 1This is a schematic diagram of the spheroidizing annealing process for the wire rod of the present invention; Figure 2 This is a schematic diagram of a typical metallographic spheroidized structure of the steel wire of the present invention, magnified 1000 times. Figure 3 A flowchart illustrating the modification process of traditional hot-rolled wire rod; Figure 4 This is a flowchart illustrating the modified process of the hot-rolled wire rod of this invention. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0023] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention in any way. Example 1

[0024] The high-strength fastener steel wire for large deformation cold heading involved in this embodiment has a specification of Φ5.2mm and contains the following components and their mass percentages: C: 0.39%, Si: 0.19%, Mn: 0.81%, P: ≤0.010%, S: ≤0.002%, Cr: 1.08%, Mo: 0.61%, V: 0.26%, with the remainder being Fe and unavoidable impurities.

[0025] The smelting raw materials are prepared according to the chemical composition described above and proceeded sequentially as follows: KR hot metal pretreatment → BOF converter smelting → LF refining → RH vacuum degassing → continuous casting (the continuous casting billet size is 390*510mm). 2 → Heating of continuously cast billet → Billet preparation (intermediate billet size 200*200mm) 2 → Surface treatment of intermediate billet → Heating of wire rod → High-pressure water descaling → Controlled rolling → Controlled cooling → Annealing → Wire drawing → Finished product.

[0026] During continuous casting, the pouring temperature is 25℃. Light reduction and electromagnetic stirring are used to improve billet segregation. Protective pouring is employed throughout the continuous casting process to isolate the billet from air and prevent secondary oxidation. The billet is heated at 1250℃ in a furnace to ensure complete dissolution of the alloy components in the steel. After billet preparation, it is slowly cooled after being removed from the production line.

[0027] Controlled rolling of wire rod: The intermediate billet is heated in sections in the heating furnace. The temperature of the preheating section is controlled at 650℃, and the temperature of the high-temperature section is controlled at 980℃~1120℃. The holding time in the high-temperature section is 80 minutes, and the residual oxygen content in the furnace is 1.2%. After the holding time, the intermediate billet is descaled to remove the surface oxide layer, and then rolled. Rough rolling stage: the initial rolling temperature is 910℃, and the total compression ratio is 68%. Secondary descaling is performed before intermediate rolling. Intermediate and finish rolling stage: the intermediate and finish rolling temperature is 845℃, and the total compression ratio is 99.6%. The temperature of the sizing mill is 794℃, and the compression ratio of the last two passes is ≥43%. Wire cooling control: After sizing, the wire rod is spun out at a temperature of 801℃. The wire rod coil is cooled by a water bath after spun out. The roller speed of the wire rod coil is set to 0.20m / s. The heat preservation cover is fully opened. The cooling rate of the wire rod at this stage is ensured to be 8℃ / s.

[0028] Deep processing annealing: After pickling or shot blasting to remove surface iron oxide, the wire rod is placed in an annealing furnace and filled with protective gas. The oxygen content in the furnace is controlled below 30 ppm, and the dew point is controlled below -45℃ to prevent increased decarburization. The first heating is carried out at a rate of 80℃ / h to 720℃, and then held at 720℃ for 3 hours. After the first holding, the second heating is carried out at a rate of 10℃ / h to 770℃, and then held at 770℃ for 3 hours. After the second holding, the first cooling is carried out at a rate of 80℃ / h to 660℃, and then held at 660℃ for 2 hours. After the third holding, the third heating is carried out at a rate of 10℃ / h to 715℃, and then held at 715℃ for 9 hours. After the fourth holding, the second cooling is carried out at a rate of 100℃ / h to below 400℃ before being removed from the furnace.

[0029] Deep processing and wire drawing: Before wire drawing, acid pickling and phosphating are performed to form a coating, and the surface area reduction rate is controlled at 10.6% during fine drawing.

[0030] The high-strength fastener steel wire produced by the above manufacturing method has a high spheroidization rate, low tensile strength, high reduction of area and low hardness, which meets the needs of downstream customers for large deformation cold heading. Its performance is shown in Table 1. Example 2

[0031] The high-strength fastener steel wire for large deformation cold heading involved in this embodiment has a specification of Φ13.7mm and contains the following components and their mass percentages: C: 0.41%, Si: 0.22%, Mn: 0.82%, P: ≤0.011%, S: ≤0.002%, Cr: 1.06%, Mo: 0.63%, V: 0.27%, with the remainder being Fe and unavoidable impurities.

[0032] The smelting raw materials are prepared according to the chemical composition described above and proceeded sequentially as follows: KR hot metal pretreatment → BOF converter smelting → LF refining → RH vacuum degassing → continuous casting (the continuous casting billet size is 390*510mm). 2 → Heating of continuously cast billet → Billet preparation (intermediate billet size 150*150mm) 2 → Surface treatment of intermediate billet → Heating of wire rod → High-pressure water descaling → Controlled rolling → Controlled cooling → Annealing → Wire drawing → Finished product.

[0033] During continuous casting, the pouring temperature is 24℃. Light reduction and electromagnetic stirring are used to improve billet segregation. Protective pouring is employed throughout the continuous casting process to isolate the billet from air and prevent secondary oxidation. The billet is heated at a high temperature of 1260℃ in a furnace to ensure complete dissolution of the alloy components in the steel. After billet preparation, it is slowly cooled after being removed from the production line.

[0034] Controlled rolling of wire rod: The intermediate billet is heated in sections in the heating furnace. The temperature of the preheating section is controlled at 680℃, and the temperature of the high-temperature section is controlled at 980℃~1120℃. The holding time in the high-temperature section is 95 minutes, and the residual oxygen content in the furnace is 1%. After the holding time, the intermediate billet is descaled to remove the surface oxide layer, and then rolled. Rough rolling stage: the initial rolling temperature is 895℃, and the total compression ratio is 68%. Secondary descaling is performed before intermediate rolling. Intermediate and finish rolling stage: the intermediate and finish rolling temperature is 860℃, and the total compression ratio is 98%. The temperature of the sizing mill is 801℃, and the compression ratio of the last two passes is 36%. Wire cooling control: After sizing, the wire rod is spun out at a temperature of 801℃. After spun out, the wire rod coil is cooled by blowing air. The roller speed of the wire rod coil is set to 0.30m / s. The heat insulation cover is fully opened. The cooling rate of the wire rod at this stage is 6℃ / s.

[0035] Deep processing annealing: After pickling or shot blasting to remove surface iron oxide, the wire rod is placed in an annealing furnace and filled with protective gas. The oxygen content in the furnace is controlled below 30 ppm, and the dew point is controlled below -45℃ to prevent increased decarburization. The first heating is carried out at a rate of 70℃ / h to 720℃, and then held at 720℃ for 4 hours. After the first holding, the second heating is carried out at a rate of 8℃ / h to 775℃, and then held at 775℃ for 3 hours. After the second holding, the first cooling is carried out at a rate of 90℃ / h to 650℃, and then held at 650℃ for 3 hours. After the third holding, the third heating is carried out at a rate of 10℃ / h to 720℃, and then held at 720℃ for 10 hours. After the fourth holding, the second cooling is carried out at a rate of 100℃ / h to below 400℃ before being removed from the furnace.

[0036] Deep processing and wire drawing: Before wire drawing, acid pickling and phosphating are performed to form a coating, and the surface area reduction rate is controlled at 4.2% during fine drawing.

[0037] The high-strength fastener steel wire produced by the above manufacturing method has a high spheroidization rate, low tensile strength, high reduction of area and low hardness, which meets the needs of downstream customers for large deformation cold heading. Its performance is shown in Table 1.

[0038] Table 1. Microstructure and properties of steel wire produced in the two examples.

Claims

1. A method for producing high-strength fastener steel wire for large deformation cold forging, characterized in that: Includes the following steps, Step 1, Smelting and Casting: Molten steel is smelted according to the elemental composition of the wire rod, and then cast into steel billets. The elemental composition of the wire rod, by weight percentage, is: C: 0.33-0.44%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P: ≤0.020%, S: ≤0.020%, Cr: 0.90-1.20%, Mo: 0.45-0.75%, V: 0.10-0.35%, with the remainder being Fe and unavoidable impurities. Step 2, Billeting: After the steel billet is heated at high temperature, it is rolled to obtain an intermediate billet with a compressed cross section. The intermediate billet is peeled and / or ground to remove surface defects and improve surface quality. Step 3, wire rod controlled rolling: After the intermediate billet is heated and held in sections in the heating furnace, it is descaled to remove the surface oxide layer after exiting the furnace, and then rolled into wire rod in the following order: rough rolling, intermediate rolling, intermediate finishing rolling and finishing rolling on the sizing mill. During the rolling process, a second descaling is performed to remove the surface oxide layer. Step 4, wire cooling control: After the sizing is reduced, the wire rod is spun out. After the wire rod is spun out, the coil is cooled by air or water bath. The insulation cover is fully opened, and the cooling rate of the wire rod in this stage is controlled to be ≥2℃ / s. Step 5, Deep Processing Annealing: Place the wire rods, after pickling or shot blasting to remove surface iron oxide, into an annealing furnace and fill it with protective gas. Control the oxygen content in the furnace to below 20 ppm and the dew point to below -40℃ to inhibit decarburization. Perform the first heating at a rate of ≥50℃ / h to 700-720℃. After reaching the temperature, perform the first holding for 2-5 hours. After the first holding, perform the second heating at a rate of ≤20℃ / h to 760-780℃. After reaching the temperature, perform the second holding. The second holding time is 2-5 hours. After the second holding time, the first cooling is carried out at a rate of ≥50℃ / h to 650-680℃. After reaching the temperature, the third holding time is carried out for 2-3 hours. After the third holding time, the third heating is carried out at a rate of ≤20℃ / h to 700-720℃. After reaching the temperature, the fourth holding time is carried out for 6-10 hours. After the fourth holding time, the second cooling is carried out at a rate of ≥50℃ / h to below 400℃. The oven is then removed from the furnace. Step 6, Deep processing wire drawing: Finely draw the wire rod to reduce its surface area to the target wire specifications.

2. The method according to claim 1, characterized in that: In step three, the segmented heating process for the intermediate billet is as follows: preheating section temperature ≤720℃, high-temperature section temperature: 980℃~1120℃, high-temperature section holding time not less than 60min, residual oxygen content in the furnace ≤2.0%; roughing stage: initial rolling temperature 850~950℃, total compression ratio ≥60%; secondary descaling is performed before intermediate rolling; intermediate and finishing rolling stage: intermediate and finishing rolling temperature 820~900℃, total compression ratio ≥90%; temperature entering the reducing and sizing mill 780~820℃, compression ratio ≥30% for the last two passes.

3. The method according to claim 1, characterized in that: In step four, the wire rod spinning temperature is 790–810℃, and the roller conveyor speed of the wire rod coil is set to 0.20 m / s–0.35 m / s.

4. The method according to claim 1, characterized in that: In step six, an acid-washed and phosphated coating is applied before drawing, and the surface area reduction rate is controlled between 3% and 15% during fine drawing.

5. The method according to claim 1, characterized in that: In step one, high-quality blast furnace iron, scrap steel, and raw and auxiliary materials are selected for smelting. The steel smelting involves KR iron pretreatment, converter primary refining, LF refining, and vacuum degassing. KR iron pretreatment and converter primary refining slag formation achieve decarburization and dephosphorization. Vacuum degassing controls the hydrogen content in the molten steel to be below 0.5 ppm. The molten steel is cast into steel billets using continuous casting or ingot casting processes. If continuous casting is used, the pouring superheat during continuous casting is 20-35℃. The continuous casting process uses light reduction and electromagnetic stirring processes to improve billet segregation. The entire continuous casting process uses protective pouring to isolate the billet from air.

6. The method according to claim 1, characterized in that: The steel billet is heated at a high temperature of 1220–1320℃ in a heating furnace to ensure that the alloy components in the steel are fully dissolved, and then cut into a cross-section of 150×150mm. 2 ~200×200mm 2 The intermediate billet is slowly cooled after it leaves the production line.

7. The method according to claim 1, characterized in that: The method is used to produce fastener wires with a diameter of φ5.2 to φ24.5 mm.

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