Corrosion fatigue-resistant spring steel wire, and wire rod and production method therefor

Abstract

Disclosed in the present invention are a corrosion fatigue-resistant spring steel wire, and a wire rod and a production method therefor. The wire rod and the steel wire each comprise the following chemical components in percentage by mass: C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P: 0.02% or less, S: 0.02% or less, and the balance of Fe and inevitable impurities.

Description

Corrosion-resistant fatigue spring steel wire, wire rod and production method thereof

[0001] This application is based on the Chinese patent application with application number CN202311085007.3 and application date of August 28, 2023, and claims the priority of the Chinese patent application. The entire content of the above patent application is hereby introduced into this application as a reference. Technical Field

[0002] The present invention belongs to the technical field of steel production, and relates to a corrosion-resistant fatigue spring steel wire, a wire rod for the corrosion-resistant fatigue spring steel wire, and a production method of the wire rod for the corrosion-resistant fatigue spring steel wire. Background Art

[0003] As safety-relevant load-bearing components, springs are widely used in automobiles, machinery, railways, and other fields. They often bear high-frequency alternating loads during their service. With technological advancements and improved manufacturing standards, new energy vehicles are gaining popularity, and their sales are steadily increasing within the automotive industry. However, the heavy weight of new energy vehicles places stringent demands on the spring wire used in suspension springs. Not only does it require high strength to increase the vehicle's load capacity, but it also requires lightweighting to reduce energy consumption during driving and improve endurance.

[0004] However, current spring steels have poor corrosion resistance. When used in the humid environment of northern China, where snow is thick in winter, salting is often used to clear roads. This creates a large amount of chloride ions on the roads, which can easily cause corrosion pits on the surface of the spring steel, leading to stress concentration and reduced fatigue strength. Furthermore, hydrogen ingress during the corrosion process can lead to hydrogen-induced delayed cracking, causing components to break suddenly at loads far below their service life, severely impacting their service life and posing a significant risk to the safe operation of automobiles and railways. Therefore, it is necessary to develop spring steel wire that is both high-strength, lightweight, and corrosion-resistant to meet the application requirements of new energy vehicles in northern China.

[0005] Summary of the Invention

[0006] The present invention aims to provide a wire rod for corrosion-resistant fatigue spring steel wire and a production method thereof, and also relates to a corrosion-resistant fatigue spring steel wire.

[0007] To achieve the above-mentioned object of the invention, one embodiment of the present invention provides a method for producing a wire rod for corrosion-resistant fatigue spring steel wire, wherein the chemical composition of the wire rod comprises, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the remainder is Fe and unavoidable impurities;

[0008] The production method includes the following steps: pre-desulfurization of molten iron, converter smelting, LF refining, RH refining, continuous casting, cogging, flaw detection and grinding, high-speed wire rolling, and controlled cooling; wherein,

[0009] In the converter smelting process, a smelting raw material consisting of high-quality scrap steel, ferromolybdenum and pre-desulfurized molten iron is fed into a converter for smelting, wherein the weight ratio of the pre-desulfurized molten iron to the high-quality scrap steel is 7:1 to 8:1, and the temperature of the molten iron when fed into the converter is 1350 to 1450°C;

[0010] In the continuous casting process, the mold liquid level is protected by high-Al steel special protective slag for casting, and the chemical composition of the high-Al steel special protective slag includes, by mass percentage, CaO+Al2O3=50-60%, and CaO / Al2O3=1.4-1.5, Na2O:15-20%, MgO:5-12%, K2O:3-6%, Li2O:2-5%, BaO:1-2%, SiO2:0.8-1.1%, and CaF2:4-9%;

[0011] In the high-speed wire rolling process, the intermediate billet is heated in a heating furnace and then descaled with high-pressure water, and then the high-speed wire is continuously rolled into a wire rod. The heating process includes a heating stage, a heating stage, and a soaking stage. The heating temperature of the heating stage is 780-820°C, and the heating time is ≤30 minutes; the heating temperature of the heating stage is 900-950°C, and the heating time is ≤30 minutes; the heating temperature of the soaking stage is 1030-1070°C, and the heating time is ≤60 minutes. The high-speed wire continuous rolling process includes rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and wire laying in sequence. The starting rolling temperature of rough rolling is 890-910°C, the starting rolling temperature of finishing rolling is 860-890°C, the final rolling temperature of finishing rolling is ≤960°C, and the wire laying temperature is 860-880°C.

[0012] In the controlled cooling process, the wire rod is controlled and cooled by a Stelmore air cooling line. When the temperature of the edge of the wire rod is 810-860°C, the 1#-5# fans on the Stelmore cooling line are turned on, and the air volume of the fan is controlled to be 65-75%, the roller speed is 0.50-0.65m / s, and the cooling rate is 4.5-5.5°C / s; when the temperature of the edge of the wire rod is 690-810°C, the 6#-8# fans on the Stelmore cooling line are turned on, and the air volume of the fan is controlled to be 50-60%, the roller speed is 0.2-0.3m / s, and the cooling rate is 1.5-2°C / s.

[0013] As a further improvement of one embodiment of the present invention, the chemical composition of the wire rod includes, in mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities; wherein, [Si] / [Al]=1.5-3.5.

[0014] As a further improvement of one embodiment of the present invention, in the molten iron pre-desulfurization process, the blast furnace molten iron is desulfurized in a KR desulfurization device until S in the molten iron is ≤ 0.002%, the desulfurization slag rate is ≥ 97%, and the molten iron temperature after desulfurization is ≥ 1350°C.

[0015] As a further improvement of one embodiment of the present invention, in the converter smelting process, the converter smelting adopts top and bottom combined blowing, constant pressure variable gun, and double slag operation. The C in the molten steel at the end of the converter smelting is ≥0.15%, P ≤0.01%, and the tapping temperature is ≥1650°C; there is no slag chip in the ladle before tapping, and the cold slag steel at the bottom of the ladle is less than 0.5 tons. Before tapping, ferromanganese, ferrosilicon, ferrochrome, lime, and 1 / 3 of aluminum blocks are laid on the bottom of the ladle.

[0016] As a further improvement of one embodiment of the present invention, in the converter smelting process, the bottom blowing of the ladle is turned on before the converter taps the steel, and the pressure of the bottom blowing argon is controlled to be 0.5-0.6 MPa, and the bright ring diameter is 250-400 mm. After 3 / 4 of the steel is tapped, the pressure of the bottom blowing argon of the ladle is adjusted to 0.4-0.5 MPa, and the bright ring diameter is controlled to be 200-350 mm.

[0017] As a further improvement of one embodiment of the present invention, in the LF refining process, during the waiting period for smelting, the flow rate of argon gas blown from the bottom of the ladle is 40 to 80 NL / min; when adding the refining mold slag and alloy, the flow rate of argon gas blown from the bottom of the ladle is 200 to 600 NL / min; during heating after adding the alloy, the flow rate of argon gas blown from the bottom of the ladle during heating is 200 to 400 NL / min;

[0018] The chemical components of the refining protective slag include, by mass percentage, CaO: 45-50%, Al2O3: 30-40%, with CaO / Al2O3=1.7-1.8, MgO: 4-8%, and SiO2: 4-7%.

[0019] As a further improvement of one embodiment of the present invention, in the RH refining process, after the molten steel circulates, the remaining 2 / 3 of the aluminum block is first added for alloying. After the alloying is completed, vacuum treatment is started, the vacuum degree of the vacuum treatment is ≤67Pa, and the vacuum treatment time is ≥20min, so that O ≤18ppm and H ≤1.5ppm in the molten steel; after the vacuum treatment is completed, argon is blown from the bottom of the ladle for soft stirring, and calcium silicon core wire is added for inclusion modification treatment, the soft stirring time is ≥20min, and the feeding speed of the calcium silicon core wire is 120-150m / min.

[0020] As a further improvement of one embodiment of the present invention, in the continuous casting process, a large ladle with a long nozzle, an immersed nozzle, a tundish covered with a pre-melted hollow particle covering agent and carbonized rice husks is used for continuous casting, and an argon seal is used for full protection pouring. The depth of the immersed nozzle is 200 to 300 mm, and the back pressure of the argon seal is ≥0.05 Bar. The carbonized rice husk is covered on the outer layer of the pre-melted hollow particle covering agent, and the weight ratio of the pre-melted hollow particle covering agent to the carbonized rice husk is 1:3.

[0021] To achieve the above-mentioned purpose of the invention, one embodiment of the present invention also provides a wire rod for corrosion-resistant fatigue spring steel wire, the chemical composition of the wire rod comprising, by mass percentage: C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities.

[0022] As a further improvement of one embodiment of the present invention, the wire rod for corrosion-resistant fatigue spring steel wire is prepared by the production method for wire rod for corrosion-resistant fatigue spring steel wire as described above.

[0023] To achieve the above-mentioned purpose of the invention, one embodiment of the present invention further provides a corrosion-resistant fatigue spring steel wire, the chemical composition of the steel wire comprising, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the remainder being Fe and unavoidable impurities.

[0024] As a further improvement of one embodiment of the present invention, the corrosion-resistant fatigue spring steel wire is prepared by sequentially performing pickling, drawing, and heat treatment steps using the above-mentioned corrosion-resistant fatigue spring steel wire wire rod as a base material.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) In the chemical composition design scheme of the present invention, by adding a certain amount of Al, not only the density of the wire rod and spring steel wire can be reduced, making them lightweight, but also the corrosion resistance can be improved. Combined with the addition and content control of other elements, especially Nb, it can prevent the austenite grains from coarsening during the heating process, play the role of fine grain strengthening and improving the elasticity reduction resistance, and solve the problems of grain coarsening, many inclusions and difficulty in improving strength caused by the addition of Al. Further combined with the control of Si, the elasticity reduction resistance and corrosion resistance of the spring steel wire can be improved. Cr, as an important corrosion-resistant element, can form a passivation film on the surface of the spring steel wire, effectively prevent oxidation, and improve the corrosion resistance of the matrix. Further combined with Mo element, it can improve the pitting resistance. Mn, as a solid solution strengthening element, can improve the hardenability of the spring steel wire. Mn has a strong binding force with S and O, which can not only deoxidize, but also form MnS compounds, reducing the probability of S combining with Fe to form FeS, thereby reducing or eliminating the hot brittleness effect of S. Overall, this chemical composition design scheme can improve the strength, toughness, corrosion resistance, and resistance to elasticity loss of wire rods and spring steel wires, making the spring steel wire lightweight. It is also suitable for the cold and humid northern environment and has excellent corrosion fatigue resistance even in the road environment of salt spreading and snow melting in winter, which is beneficial to extending the service life of the spring steel wire and improving its safety in use.

[0027] (2) On the basis of the chemical composition design scheme of the present invention, combined with the control of the production process, not only the cleanliness and surface quality of the wire rod are improved, the structure of the wire rod is refined, and the wire rod finally prepared has high strength and high corrosion fatigue resistance, wherein the wire rod is a two-phase structure of pearlite and ferrite, and the volume fraction of pearlite is ≥95%, the tensile strength is 1000-1200MPa, the cross-sectional shrinkage is ≥45%, and the elongation after fracture is ≥14%. It can not only effectively reduce the wire breakage rate when it is further drawn to prepare spring steel wire, but also lay the foundation for further drawing to prepare spring steel wire with high strength and high corrosion fatigue resistance, which is beneficial to prolonging the service life of the spring steel wire and improving its safety in use.

[0028] (3) The spring steel wire prepared by further drawing on the basis of the aforementioned wire rod has both high strength and excellent plasticity, with a tensile strength of >2000 MPa and a cross-sectional shrinkage rate of ≥40%, which can meet the high requirements of the heavy-load automobile suspension system for the strength and toughness of the spring steel wire in service. In addition, the bending fatigue life of the obtained spring steel wire exceeds 260,000 cycles, and the corrosion fatigue life in the salt spray test exceeds 18,000 cycles. It can be used in the cold and humid northern environment and has excellent corrosion fatigue resistance even in the road environment of salting and melting snow in winter. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below in conjunction with specific implementation methods, but the scope of protection required is not limited to the description.

[0030] This embodiment provides a method for producing a wire rod for corrosion-resistant fatigue spring steel wire, and a wire rod for corrosion-resistant fatigue spring steel wire prepared by the production method.

[0031] Specifically, the chemical composition of the wire rod for corrosion-resistant fatigue spring steel wire includes, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities.

[0032] One embodiment of the present invention further provides a corrosion-resistant fatigue spring steel wire, which is prepared using the above-mentioned wire rod for corrosion-resistant fatigue spring steel wire as a base material and has the same chemical composition as the wire rod for corrosion-resistant fatigue spring steel wire.

[0033] In the chemical composition design of the present invention, by adding a certain amount of Al, the density of the wire rod and spring steel wire can be reduced, making them lightweight and improving the corrosion resistance. In combination with the addition and content control of other elements, especially Nb, it can prevent the coarsening of austenite grains during heating, exert the effect of fine grain strengthening, and improve the resistance to elastic degradation, thus solving the problems of grain coarsening, high inclusions, and difficulty in increasing strength caused by the addition of Al. Further combined with the control of Si, the resistance to elastic degradation and corrosion resistance of the spring steel wire can be improved. Cr, as an important corrosion-resistant element, can form a passivation film on the surface of the spring steel wire, effectively preventing oxidation and improving the corrosion resistance of the matrix. Further combination with Mo can improve the pitting corrosion resistance. Mn, as a solid solution strengthening element, can improve the hardenability of the spring steel wire. Mn has a strong binding force with S and O, not only deoxidizing but also forming MnS compounds, reducing the probability of S combining with Fe to form FeS, thereby reducing or eliminating the hot brittleness of S. Overall, this chemical composition design scheme can improve the strength, toughness, corrosion resistance, and resistance to elasticity loss of wire rods and spring steel wires, making the spring steel wire lightweight. It is also suitable for the cold and humid northern environment and has excellent corrosion fatigue resistance even in the road environment of salt spreading and snow melting in winter, which is beneficial to extending the service life of the spring steel wire and improving its safety in use.

[0034] Preferably, the chemical composition of the wire rod for corrosion-resistant fatigue spring steel wire includes, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.01%, S≤0.02%, and the rest is Fe and unavoidable impurities; wherein, [Si] / [Al]=1.5-3.5.

[0035] Here, [Si] represents the mass percentage of Si, and [Al] represents the mass percentage of Al.

[0036] By limiting the [Si] / [Al] ratio, the final spring steel wire can have both corrosion resistance and elasticity reduction resistance, so that the spring steel wire has excellent anti-relaxation performance and excellent elastic deformation recovery ability.

[0037] The following is a detailed introduction to each process in the production method of the wire rod for corrosion-resistant fatigue spring steel wire.

[0038] (1) Hot metal pre-desulfurization process

[0039] The blast furnace molten iron is desulfurized in the KR desulfurization device until the S in the molten iron is ≤ 0.002%, the desulfurization slag rate is ≥ 97%, and the temperature of the molten iron after desulfurization is ≥ 1350°C.

[0040] (2) Converter smelting process

[0041] The smelting raw materials, consisting of high-quality scrap steel, ferromolybdenum, and pre-desulfurized hot metal, are fed into a converter for smelting. The weight ratio of pre-desulfurized hot metal to high-quality scrap steel is 7:1 to 8:1, and the hot metal temperature upon entry into the converter is 1350-1450°C. Adding ferromolybdenum before the start of converter smelting avoids excessive alloy addition mid-stream, which would cause a significant temperature drop in the molten steel and compromise steel purity and inclusion removal.

[0042] The high-quality scrap steel here refers to scrap steel with S≤0.012% and P≤0.015%. The use of high-quality scrap steel can effectively control the residual element content in molten steel, reduce the amount of resulfurization in molten steel, effectively reduce the slag-metal reaction in the refining process, reduce the generation of large-sized inclusions in molten steel, and improve the cleanliness of molten steel.

[0043] Preferably, the converter smelting adopts top and bottom combined blowing, constant pressure changing gun, and double slag operation, and the C in the molten steel at the end of the converter smelting is ≥ 0.15%, P ≤ 0.01%, and the tapping temperature is ≥ 1650° C. The so-called double slag operation means that the slag is poured twice.

[0044] Ideally, before tapping, ensure that the ladle is free of dross and debris, and that the amount of cold slag steel at the bottom of the ladle is less than 0.5 tons to ensure smooth bottom blowing. Ferromanganese, ferrosilicon, ferrochrome, lime, and one-third of aluminum blocks are placed at the bottom of the ladle before tapping to facilitate deoxidation and alloying during the converter tapping process. Only one-third of the aluminum blocks are placed here, with the remaining two-thirds added during the RH refining process.

[0045] Preferably, before tapping the converter, the bottom blowing of argon in the ladle is turned on, and the pressure of the bottom blowing argon is controlled to 0.5-0.6 MPa so that the molten steel surface can be turned over and the diameter of the bright ring is 250-400 mm. After 3 / 4 of the steel is tapped, the pressure of the bottom blowing argon in the ladle is adjusted to 0.4-0.5 MPa, and the diameter of the bright ring is controlled to 200-350 mm.

[0046] (3) LF refining process

[0047] Molten steel from the converter is fed into the LF furnace for smelting. Upon arrival, the ladle is connected to the bottom argon blowing system and mold slag is added. Alloys are then added to adjust the chemical composition and control inclusions. Electrical heating is applied to restore the temperature of the molten steel, which has dropped after the alloy addition. Temperature measurement, sampling, diffusion deoxidation, and heating are all performed continuously during this process. Steel is then tapped after the molten steel composition, slag composition, and temperature all meet specified standards. The alloys include metallic manganese, ferrosilicon, and ferrochrome.

[0048] Preferably, while waiting for smelting, the ladle bottom argon blowing flow rate is 40-80 NL / min to allow the slag surface to fluctuate. When adding refined mold slag and alloy, the ladle bottom argon blowing flow rate is 200-600 NL / min. Furthermore, during heating after adding the alloy, the ladle bottom argon blowing flow rate is 200-400 NL / min. Specifically, the argon blowing process can be performed according to the automatic argon blowing model.

[0049] Preferably, aluminum particles and calcium carbide are used to deoxidize the slag surface to reduce the oxygen content in the molten steel, and lime is added to adjust the slag in time until the slag turns white, and the white slag time is controlled to be ≥15 minutes.

[0050] Among them, the chemical components of the refined protective slag include, in mass percentage, CaO: 45-50%, Al2O3: 30-40%, and CaO / Al2O3 = 1.7-1.8, MgO: 4-8%, SiO2: 4-7%. The refined protective slag has good fluidity and the ability to adsorb non-metallic inclusions, which can effectively control the type and content of inclusions, improve the purity of molten steel, and thus improve the fatigue resistance of the finally prepared wire rod and spring steel wire.

[0051] (4) RH refining process

[0052] The molten steel is fed into an RH vacuum furnace for vacuum refining to further remove inclusions. Specifically, after the molten steel circulates, the remaining 2 / 3 of the aluminum blocks are added for alloying. After alloying is complete, vacuum treatment begins. The vacuum level during vacuum treatment is ≤67 Pa and the treatment time is ≥20 minutes. This further reduces the oxygen content in the molten steel to ≤18ppm for O and ≤1.5ppm for H. After the vacuum treatment, argon is blown from the bottom of the ladle for soft stirring. Calcium silicon core wire is added for inclusion modification. The soft stirring time is ≥20 minutes, and the feed speed of the calcium silicon core wire is 120-150m / min to promote the floating of non-metallic inclusions and improve the purity of the molten steel.

[0053] The chemical composition of the calcium silicon core wire is as follows: SiO2: 48-55%, CaO: 40-50%, Al2O3: 3-5%, MgO: 1-2% by mass. The tapping temperature is 1575-1585°C.

[0054] (5) Continuous casting process

[0055] A continuous casting machine is used to cast the molten steel produced in the RH refining process into a continuous casting billet, and a high-Al steel special protective slag is used to protect the casting at the liquid level of the crystallizer. The chemical composition of the high-Al steel special protective slag includes, in terms of mass percentage, CaO+Al2O3=50~60%, and CaO / Al2O3=1.4~1.5, Na2O:15~20%, MgO:5~12%, K2O:3~6%, Li2O:2~5%, BaO:1~2%, SiO2:0.8~1.1%, and CaF2:4~9%.

[0056] Preferably, continuous casting adopts a large ladle with a long water nozzle, an immersed water nozzle, a tundish covered with a pre-melted hollow particle covering agent and carbonized rice husks, and an argon seal for full protection pouring. The depth of the immersed water nozzle is 200-300 mm, and the back pressure of the argon seal is ≥0.05 Bar. Specifically, the granular pre-melted hollow particle covering agent is covered on the tundish, and then the carbonized rice husk is covered on the outer layer of the pre-melted hollow particle covering agent, and the weight ratio of the pre-melted hollow particle covering agent to the carbonized rice husk is 1:3.

[0057] Preferably, the chemical composition of the pre-melted hollow particle covering agent includes, by mass percentage, CaO: 30-35%, Al2O3: 10-15%, SiO2: 40-45%, Fe2O3: 2-4%, and MgO: 1-10%.

[0058] Preferably, the superheat of the tundish is controlled at 38-45°C, the nitrogen addition amount during the continuous casting process is ≤0.0002%, the liquid level fluctuation of the crystallizer is <2mm, the surface temperature of the continuous casting billet in the straightening section is ≥920°C, and after the continuous casting is completed, the continuous casting billet is sent to the insulation pit for slow cooling to below 200°C and then taken out of the pit, and the slow cooling time is ≥36h.

[0059] Preferably, the cross-sectional size of the continuous casting billet is 300 mm×390 mm.

[0060] (6) Blanking process

[0061] The continuous casting slabs are heated in a slab furnace and then rolled into intermediate slabs with a cross-section of 150mm x 150mm. The heating temperature is 1160-1190°C, the furnace time is 170-190 minutes, and the initial rolling temperature is 1040-1070°C. After slab opening, the intermediate slabs are inspected and tested using full slab grinding and fluorescence testing to ensure that there are no surface quality issues.

[0062] (7) Flaw detection and grinding process

[0063] The surface of the intermediate billet obtained from the billet opening process is fully ground, and the surface quality of the intermediate billet is detected by fluorescence. Grinding is performed according to the detection results until the surface of the intermediate billet is free of surface defects and decarburization layer, so as to facilitate the implementation of surface quality control in the subsequent rolling process.

[0064] (8) High-speed wire rolling process

[0065] The intermediate billet is heated in a heating furnace and then descaled with high-pressure water. The intermediate billet is then subjected to high-speed continuous rolling to be rolled into a wire rod with a diameter of 5 to 17 mm.

[0066] The heating process includes a first heating stage, a second heating stage, and a soaking stage. The first heating stage is heated to 780-820°C for 30 minutes or less; the second heating stage is heated to 900-950°C for 30 minutes or less; and the soaking stage is heated to 1030-1070°C for 60 minutes or less. By controlling the heating temperature and time in different stages, the formation of a difficult-to-remove oxide scale on the wire rod surface is avoided, which reduces the difficulty of descaling, improves descaling efficiency, and enhances the effectiveness of descaling, thereby preventing any impact on the fatigue life of the final spring steel wire.

[0067] The high-speed continuous rolling process includes rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and wire drawing in sequence. The starting rolling temperature of rough rolling is 890-910℃, the starting rolling temperature of finishing rolling is 860-890℃, the final rolling temperature of finishing rolling is ≤960℃, and the wire drawing temperature is 860-880℃.

[0068] Preferably, the air-fuel ratio of the heating furnace is ≤0.55, and the descaling water pressure is ≥15 MPa.

[0069] (9) Control cooling process

[0070] The wire rod is cooled in a controlled manner using a Stelmore air cooling line. When the temperature of the wire rod edge is 810-860°C, fans 1#-5# on the Stelmore cooling line are turned on, and the air volume of the fans is controlled to be 65-75%, the roller speed is 0.50-0.65m / s, and the cooling rate is 4.5-5.5°C / s. When the temperature of the wire rod edge is 690-810°C, fans 6#-8# on the Stelmore cooling line are turned on, and the air volume of the fans is controlled to be 50-60%, the roller speed is 0.2-0.3m / s, and the cooling rate is 1.5-2°C / s. In other words, the cooling process controls the number and position of the fans, the roller speed, and the cooling rate according to the different temperatures of the wire rod edge.

[0071] In summary, based on the chemical component design of the present invention, combined with the control of the aforementioned production process, not only the cleanliness and surface quality of the wire rod are improved, the structure of the wire rod is refined, and the finally prepared wire rod has high strength and high corrosion fatigue resistance, wherein the wire rod is a two-phase structure of pearlite and ferrite, and the volume fraction of pearlite is ≥95%, the tensile strength is 1000-1200 MPa, the cross-sectional shrinkage rate is ≥45%, and the elongation after fracture is ≥14%. It can not only effectively reduce the wire breakage rate when it is further drawn to prepare spring steel wire, but also lay the foundation for further drawing and preparing spring steel wire with high strength and high corrosion fatigue resistance, which is beneficial to extending the service life of the spring steel wire and improving its safety in use.

[0072] Specifically, the wire rod is further drawn into spring steel wire through sequential pickling, drawing, and heat treatment processes.

[0073] (10) Pickling process

[0074] Use 20% hydrochloric acid aqueous solution to pickle the wire rod at 20-30℃ for 15-25min, then rinse with clean water and put it into phosphating solution for phosphating treatment. The unit area film weight of the phosphating film is 10g / m 2 After phosphating treatment, the wire rod is washed with water, saponified with soapy water and then naturally dried.

[0075] (11) Drawing process

[0076] The pickled wire rod is cold drawn into spring steel wire with a diameter of 3.5 to 15 mm using dies of different diameters, and the surface reduction rate during drawing is greater than 10%.

[0077] (12) Heat treatment process

[0078] The spring steel wire obtained in the drawing process is quenched and tempered successively on a production line equipped with online induction heating. The quenching temperature is Ar3+20°C to Ar3+50°C, where Ar3 is the austenitizing temperature of the wire rod. In this embodiment, the quenching temperature is 915-945°C and the tempering temperature is 390-400°C.

[0079] Based on the aforementioned wire rod, a heat treatment process can be used to form fine grains in the spring steel wire, thus combining high strength and excellent plasticity, with a tensile strength of >2000MPa and a cross-sectional shrinkage rate of ≥40%. This can meet the high requirements of heavy-load automobile suspension systems for the strength and toughness of the spring steel wire in service. In addition, the resulting spring steel wire has a bending fatigue life of more than 260,000 cycles and a corrosion fatigue life of more than 18,000 cycles in a salt spray test. It can be used in the cold and humid northern environment and has excellent corrosion fatigue resistance even in the winter road environment where salt is spread to melt snow.

[0080] The following 13 examples further illustrate the specific embodiments of the present invention. Of course, these 13 examples are only a portion of the numerous variations of this embodiment, and are not exhaustive. Other examples based on the aforementioned embodiments do not depart from the technical spirit of the present invention.

[0081] Specifically, the 13 embodiments all provide a wire rod for corrosion-resistant fatigue spring steel wire and a spring steel wire, the chemical composition of which is shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0082] Table 1

[0083] The production method of the wire rod in each embodiment adopts the following steps including molten iron pre-desulfurization, converter smelting, LF refining, RH refining, continuous casting, blanking, flaw detection and grinding, high-wire rolling, and controlled cooling. The specific operation of each step is described above and will not be repeated here.

[0084] The wire rods of Examples 1 to 13 were sampled and subjected to tissue testing and mechanical property testing according to the same testing method. The specific testing methods and test results are as follows:

[0085] (1) In terms of structure, 10 samples were taken from the head, middle and tail of the wire rod, totaling 30 samples, and metallographic samples were prepared. After mechanical polishing and nitric acid etching, the samples were placed under a metallographic microscope for structural observation. The pearlite lamellar spacing of each sample was measured at a magnification of 10,000 times. The average pearlite lamellar spacing of the 30 samples was calculated as the pearlite lamellar spacing of the wire rod of each embodiment. The structure of the wire rods of Examples 1 to 13 was measured to be a two-phase structure of pearlite and ferrite, wherein the percentage of pearlite and the pearlite lamellar spacing are shown in Table 2.

[0086] (2) In terms of mechanical properties, referring to the standard test methods and definitions for mechanical properties of steel products in ASTM A370, a tensile testing machine was used to test the mechanical properties of the wire rods. The tensile strength, cross-sectional shrinkage, and elongation after fracture of the wire rods of Examples 1 to 13 were measured, as shown in Table 2.

[0087] Table 2

[0088] The wire rods of Examples 1 to 13 are further drawn to prepare spring steel wires by the aforementioned sequential pickling, drawing, and heat treatment steps. The specific operations of each step are described above and will not be repeated here.

[0089] The obtained spring steel wire was subjected to mechanical property testing, fatigue life testing, and corrosion fatigue life testing. The specific test methods and test results are as follows:

[0090] (1) In terms of mechanical properties, referring to the standard test methods and definitions for mechanical properties of steel products in ASTM A370, a tensile testing machine was used to test the mechanical properties of the spring steel wire. The tensile strength and cross-sectional reduction rate of the spring steel wires of Examples 1 to 13 were measured, as shown in Table 3.

[0091] (2) In terms of fatigue life and corrosion fatigue life, referring to KS B ISO 1143, a rotating bending fatigue test was performed on a spring steel wire specimen with a diameter of 4 mm, and the fatigue life of the spring steel wires of Examples 1 to 13 was measured as shown in Table 3; referring to KSD 9502 ISO 3768 / 7263, a salt spray test was performed to measure the corrosion fatigue life of the spring steel wires of Examples 1 to 13 in a salt spray environment as shown in Table 3.

[0092] Table 3

[0093] In summary, the wire rods of Examples 1 to 13 produced according to this embodiment have a two-phase structure of pearlite and ferrite, with a volume fraction of pearlite ≥95%, a tensile strength of 1000 to 1200 MPa, a cross-sectional reduction rate ≥45%, and an elongation after fracture ≥14%. The spring steel wires of Examples 1 to 13 produced according to this embodiment have a tensile strength >2000 MPa, a cross-sectional reduction rate ≥40%, and excellent toughness, which can meet the high requirements of high-load automobile suspension systems for the strength and toughness of spring steel wires in service. In addition, the bending fatigue life of the obtained spring steel wire exceeds 260,000 cycles, and the corrosion fatigue life in the salt spray test exceeds 18,000 cycles. It can be used in the cold and humid northern environment and has excellent corrosion fatigue resistance even in the road environment of salting and melting snow in winter.

[0094] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention within the technical scope disclosed by the present invention, and they should be covered by the scope of protection of the present invention.

Claims

1. A method for producing a wire rod, characterized in that: The chemical composition of the wire rod includes, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities; The production method comprises the following steps: pre-desulfurization of molten iron, converter smelting, LF refining, RH refining, continuous casting, blanking, flaw detection and grinding, high-speed wire rolling, and controlled cooling; wherein: In the converter smelting process, a smelting raw material consisting of high-quality scrap steel, ferromolybdenum and pre-desulfurized molten iron is fed into a converter for smelting, wherein the weight ratio of the pre-desulfurized molten iron to the high-quality scrap steel is 7:1-8:1, and the temperature of the molten iron when fed into the converter is 1350-1450°C; In the continuous casting process, the liquid level of the crystallizer is protected by high-Al steel protective slag for casting, and the chemical composition of the high-Al steel protective slag includes, by mass percentage, CaO+Al2O3=50-60%, and CaO / Al2O3=1.4-1.5, Na2O: 15-20%, MgO: 5-12%, K2O: 3-6%, Li2O: 2-5%, BaO: 1-2%, SiO2: 0.8-1.1%, and CaF2: 4-9%; In the high-speed wire rolling process, the intermediate billet is heated in a heating furnace and then descaled by high-pressure water, and then the high-speed wire is continuously rolled into a wire rod. The heating process includes a heating stage, a heating stage and a soaking stage. The heating temperature of the heating stage is 780-820°C, and the heating time is ≤30min; the heating temperature of the heating stage is 900-950°C, and the heating time is ≤30min; the heating temperature of the soaking stage is 1030-1070°C, and the heating time is ≤60min; the high-speed wire continuous rolling process includes rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and wire laying in sequence. The starting rolling temperature of the rough rolling is 890-910°C, the starting rolling temperature of the finishing rolling is 860-890°C, the final rolling temperature of the finishing rolling is ≤960°C, and the wire laying temperature is 860-880°C. In the controlled cooling process, the Stelmore air cooling line is used to control the cooling of the wire rod. When the temperature of the edge of the wire rod is 810-860°C, the 1#-5# fans on the Stelmore cooling line are turned on, and the air volume of the fan is controlled to be 65-75%, the roller speed is 0.50-0.65m / s, and the cooling rate is 4.5-5.5°C / s; when the temperature of the edge of the wire rod is 690-810°C, the 6#-8# fans on the Stelmore cooling line are turned on, and the air volume of the fan is controlled to be 50-60%, the roller speed is 0.2-0.3m / s, and the cooling rate is 1.5-2°C / s.

2. The method for producing wire rod according to claim 1, characterized in that: The chemical composition of the wire rod includes, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities; wherein, [Si] / [Al]=1.5-3.

5.

3. The method for producing wire rod according to claim 1, characterized in that: In the molten iron pre-desulfurization process, the blast furnace molten iron is desulfurized in a KR desulfurization device until the S in the molten iron is ≤ 0.002%, the desulfurization slag removal rate is ≥ 97%, and the temperature of the molten iron after desulfurization is ≥ 1350°C.

4. The method for producing wire rod according to claim 1, characterized in that: In the converter smelting process, the converter smelting adopts top and bottom composite blowing, constant pressure changing gun, and double slag operation. The C in the molten steel at the end of the converter smelting is ≥0.15%, P ≤0.01%, and the tapping temperature is ≥1650°C; there is no slag chips in the ladle before tapping, and the cold slag steel at the bottom of the ladle is less than 0.5 tons. Ferromanganese, ferrosilicon, ferrochrome, lime, and 1 / 3 of aluminum blocks are laid on the bottom of the ladle before tapping.

5. The method for producing wire rod according to claim 1, characterized in that: In the converter smelting process, before the converter taps steel, the bottom blowing of the ladle is started to blow argon, and the pressure of the bottom blowing argon is controlled to be 0.5-0.6MPa, and the bright ring diameter is 250-400mm. After 3 / 4 of the steel is tapped, the pressure of the bottom blowing argon is adjusted to 0.4-0.5MPa, and the bright ring diameter is controlled to be 200-350mm.

6. The method for producing wire rod according to claim 1, characterized in that: In the LF refining process, during the waiting period for smelting, the flow rate of argon gas blown from the bottom of the ladle is 40-80NL / min; when adding refining protection slag and alloy, the flow rate of argon gas blown from the bottom of the ladle is 200-600NL / min; during heating after adding the alloy, the flow rate of argon gas blown from the bottom of the ladle is 200-400NL / min; The chemical components of the refining protection slag include, by mass percentage, CaO: 45-50%, Al2O3: 30-40%, and CaO / Al2O3=1.7-1.8, MgO: 4-8%, and SiO2: 4-7%.

7. The method for producing wire rod according to claim 4, characterized in that: In the RH refining process, after the molten steel is circulated, the remaining 2 / 3 of the aluminum block is first added for alloying, and after the alloying is completed, vacuum treatment is started, the vacuum degree of the vacuum treatment is ≤67Pa, the vacuum treatment time is ≥20min, and O in the molten steel is made ≤18ppm and H ≤1.5ppm; after the vacuum treatment is completed, argon is blown from the bottom of the ladle for soft stirring, and calcium silicon core wire is added for inclusion modification treatment, the soft stirring time is ≥20min, and the feeding speed of the calcium silicon core wire is 120-150m / min.

8. The method for producing wire rod according to claim 1, characterized in that: In the continuous casting process, a large ladle long nozzle, an immersion nozzle, and a tundish are used for continuous casting to cover the pre-melted hollow particle covering agent and the carbonized rice husk, and an argon seal is used for full protection pouring. The depth of the immersion nozzle is 200-300 mm, and the back pressure of the argon seal is ≥0.05 Bar. The carbonized rice husk is covered on the outer layer of the pre-melted hollow particle covering agent, and the weight ratio of the pre-melted hollow particle covering agent to the carbonized rice husk is 1:

3.

9. A wire rod, characterized in that: The chemical composition of the wire rod includes, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities.

10. A steel wire, characterized in that: The chemical composition of the steel wire includes, by mass percentage, C: 0.2-0.7%, Si: 0.8-1.6%, Mn: 0.5-1.7%, Al: 0.35-0.85%, Nb: 0.1-0.5%, Cr: 0.5-1.0%, Mo: 0.1-0.4%, P≤0.02%, S≤0.02%, and the rest is Fe and unavoidable impurities.

Images