Component gradient high-strength steel wire and preparation method thereof

Abstract

The invention belongs to the field of steel product manufacturing, and particularly relates to a component gradient high-strength steel wire and a preparation method thereof. According to the machining and service characteristics of the high-strength steel wire, the carbon element of the carbon steel or the low-alloy steel wire is in gradient distribution on the radial section in a carburizing or / and decarburizing mode, and the requirements of the high-strength steel wire for microstructures and mechanical properties are met. According to the method, phase change products (pearlite / bainite / martensite) and mechanical properties in the radial direction after hot working are adjusted by utilizing the carbon content difference of the cross section of the wire rod or the pre-drawn steel wire, the work hardening rate and residual stress distribution of the drawn steel wire are optimized, and the processability and extreme service potential of the high-strength steel wire are better exerted; and the development requirement of special steel wires towards customization is met. Compared with a homogeneous component high-strength steel wire method, the method provided by the invention not only meets the harsh requirements of the steel wire on certain specific service performance such as high strength, wear resistance, torsion / bending resistance and fatigue resistance, but also can effectively reduce the drawing damage of the steel wire, and greatly improves the machinable and extreme service performance of the steel wire.

Description

TECHNICAL FIELD

[0001] The present application relates to the preparation of high-strength steel wire for special fields, in particular a composition gradient high-strength steel wire and a preparation method thereof, and belongs to the field of steel product manufacturing. BACKGROUND

[0002] High-strength steel wire has a wide range of application requirements in automobiles, bridges, and special cables. Currently, high-strength steel wire mainly uses carbon steel or low-alloy steel as raw materials, and through hot rolling and heat treatment, etc., a relatively uniform structure of pearlite, bainite and martensite is obtained, and then single or multi-pass drawing deformation is performed to meet the requirements of high-strength steel wire for high strength and size specifications, etc. With the demand for higher strength levels and extremely harsh service environment steel wire, the high-strength steel wire with uniform composition and structure has bottlenecks in research and development. For example, taking high-carbon pearlite steel wire as an example, in order to meet the demand for higher strength, the drawing deformation is further increased, and the large deformation drawing strengthening leads to excessive strain (damage) in the local position (subsurface) of the steel wire cross section, greatly sacrificing the plasticity and toughness of the high-strength steel wire, resulting in a sharp decline in the steel wire's resistance to torsion, bending and impact performance, and is also not conducive to the steel wire's heat resistance and corrosion resistance. If the carbon or nitrogen content in the steel is further increased, such as the document with Chinese patent publication number CN102936688A, which relates to a bridge cable wire with a tensile strength of ≥2000MPa and a production method, the carbon content is as high as 0.95-1.2wt.%, and the nitrogen content is as high as 0.01-0.03wt.%. Although high carbon and nitrogen content increases the strength of the steel wire, it is difficult to produce, and if the production is not properly controlled, it often leads to a large fluctuation in the torsion performance, which is not conducive to the control of high-quality high-strength steel wire. For martensitic high-strength steel wire, although the dependence on drawing deformation strengthening is weakened, it relies more on solid solution strengthening and phase transformation strengthening to improve the strength of the steel wire, but continuously increasing the carbon content or other alloying elements in the steel not only increases the cost of the alloy, but also brings difficulties to the smelting and cold and hot working of the martensitic steel, and the intrinsic brittleness and hydrogen embrittlement sensitivity of high-alloy martensitic steel limit its wide application.

[0003] Currently, the composition and structure of high-strength steel wire are relatively uniform and single in the cross-sectional direction, but during cold drawing and subsequent service, there is a big difference in the strain or load borne by the steel wire in the cross-sectional direction, so the processing and comprehensive service performance of high-strength steel wire cannot be fully utilized. In order to solve the above problems, some technical personnel have proposed a composite steel wire preparation method in recent years. For example, the invention patent with Chinese patent application publication number CN114015946A relates to a high-strength corrosion-resistant stainless steel coated steel wire for bridge cables and a preparation method thereof, which uses a stainless steel corrosion-resistant outer layer (austenitic steel), a high-strength steel core (high-carbon steel), and a metallurgical transition layer to solve the demand for high strength and corrosion resistance of high-strength steel wire for bridge cables, but the preparation and processing of the composite steel wire are relatively complex, and there may be problems of metallurgical bonding of dissimilar materials. SUMMARY

[0004] The present application aims to provide a kind of component gradient high-strength steel wire and its preparation method, for the application demand of high-strength steel wire to high toughness plasticity, wear resistance and torsional properties, on the basis of existing carbon steel or low alloy steel wire preparation, by carburizing and / or decarburization treatment to high-strength steel wire rod or pre-drawing steel wire, then control the microstructure evolution of hot working process, optimize the work hardening rate and residual stress distribution of steel wire, realize that the final drawing steel wire has better strength, plasticity, wear resistance, heat resistance and drawing performance.

[0005] The technical scheme adopted by the present application is as follows:

[0006] A kind of component gradient high-strength steel wire preparation method, comprising the following steps:

[0007] (1) selected carbon steel or low alloy steel material is hot-rolled into wire rod or further pre-drawing into steel wire;The carbon content of the carbon steel or low alloy steel material is: mass fraction is 0.2-1.2%, preferably 0.4-1.0%;

[0008] (2) the hot-rolled wire rod or pre-drawing steel wire is treated on the surface, then is placed in controllable atmosphere furnace and is treated by carburizing or decarburization, or is treated by carburizing after decarburization, or is treated by decarburization after carburizing;

[0009] (3) the hot-rolled wire rod or pre-drawing steel wire treated in step (2) is quenched;

[0010] (4) the hot-rolled wire rod or pre-drawing steel wire after quenching is drawn, and high-strength steel wire of required size specification and performance is obtained.

[0011] Further, in step (1), the low alloy steel material contains, by weight percentage: 0.20%≤C≤1.2%, 0.15%≤Si≤0.85%, 0.1%≤Mn≤1.5%, Cr≤3.0%, Mo≤0.8%, Ni≤1.2%, the sum of alloying elements (including C, Si, Mn, Cr, Mo, Ni) ≤5.0%, and the rest is Fe and unavoidable impurities, and the impurity element is one or both of P or S.

[0012] Further, in step (1), the diameter of the hot-rolled wire rod is φ6-18mm, and the diameter of the pre-drawing steel wire is φ3-9mm, and the radial carbon segregation degree of the wire rod or pre-drawing steel wire is within ±0.05wt.%.

[0013] Further, in step (2), the surface treatment is pickling and phosphating treatment.

[0014] Further, in step (2), the atmosphere in the controllable atmosphere furnace is a mixture of carbon-containing component or carbon-containing component and inert atmosphere gas, the carbon-containing component is a mixture of CO2 and CO or methane, and the inert atmosphere gas is one or more of nitrogen, argon or helium, and the proportion of the mixture is automatically adjusted to control the carbon potential in the furnace (the carbon potential can be controlled at 0.2-1.2%); the carburizing or decarburizing treatment, or the decarburizing treatment after carburizing treatment, or the carburizing treatment after decarburizing treatment, is carried out at a temperature of 800-1150°C for 4-24 hours; and the effective thickness of the carburized or decarburized layer is 1 / 6-1 / 2 of the diameter of the hot-rolled wire rod or pre-drawing steel wire.

[0015] Further, the thickness of the decarburized layer after carburizing treatment needs to be less than the thickness of the carburized layer; and the thickness of the carburized layer after decarburizing treatment needs to be less than the thickness of the decarburized layer.

[0016] Further, in step (3), the quenching is reasonably selected according to the carbon content distribution of the cross section of the hot-rolled wire rod or pre-drawing steel wire and the corresponding hardenability characteristics; if isothermal quenching is carried out in a lead bath, the temperature of the lead bath is 480-600°C, and the time in the lead bath is 0.5-15 minutes; if the quenching medium is water, oil or gas (argon, nitrogen or air), the quenching cooling rate is 2-100°C / s.

[0017] Further, in step (4), in order to obtain a multi-layer composite structure, the wire rod or pre-drawing steel wire can also be subjected to multiple carburizing or decarburizing treatments, or decarburizing treatment after carburizing treatment, or carburizing treatment after decarburizing treatment during subsequent drawing; during drawing of the hot-rolled wire rod or pre-drawing steel wire, steps (2) and (3) can be repeated between 1-4 drawing passes, and then the next drawing pass is carried out, until the high-strength steel wire with the required size specification and performance is obtained.

[0018] Further, in step (4), during drawing of the hot-rolled wire rod or pre-drawing steel wire after quenching, the deformation amount of each pass is controlled in the range of 3-20%, the number of drawing passes is 6-12, and the drawing speed is 5-300 m / min.

[0019] A component gradient high-strength steel wire prepared by the method described above.

[0020] When the raw material is high-carbon steel in carbon steel, after decarburization treatment and quenching, pearlite structure with gradient increase of cementite content (8-15%) or gradient decrease of lamellar spacing (100-600 nm) from edge to core along the radial direction of the cross section can be obtained; when the raw material is low-alloy medium-carbon steel in low-alloy steel, after decarburization treatment and quenching, gradient distribution of lower bainite-low-carbon martensite-medium-carbon martensite structure from edge to core along the radial direction of the cross section can be obtained; when the raw material is low-alloy medium-carbon steel in low-alloy steel, after decarburization treatment and quenching, multi-gradient distribution of martensite structure with different carbon contents from edge to core along the radial direction of the cross section can be obtained.

[0021] The main physical metallurgical principles and design ideas of the present application are completely different from the prior art, and are introduced as follows:

[0022] Currently, most of the bridge cable and special rigging steels are drawn from the hypereutectoid steel, and different strength grades of steel wires correspond to different grades (mainly different carbon contents) of hot-rolled wire rods, such as 82B high-carbon steel (C content 0.82wt.%) corresponding to a strength of 1770 MPa; 87Mn high-carbon steel (C content 0.87wt.%) corresponding to a strength of 1860 MPa; 97Si high-carbon steel (C content 0.97wt.%) corresponding to a steel wire strength of 2000 MPa. With the increase of the strength of the steel wire, the carbon content of the wire rod steel is continuously increased, which increases the difficulty of controlling composition segregation, network carbide, etc., and the process window of smelting, rolling and drawing is increasingly narrow. At the same time, increasing the overall carbon content of the steel wire increases the strength and brittleness of the steel wire, significantly reducing the toughness and safety of the steel wire. In some special fields, high-strength martensitic steel wires are applied, and with the increase of the strength, the overall increase of the alloying elements and carbon content of the steel wire, the torsion and bending performance of the steel wire decreases sharply, and micro-cracks easily appear on the surface of the steel wire during the drawing process, and then the phenomenon of broken wire occurs. Therefore, the traditional uniform composition steel wire is difficult to meet the performance requirements of special high-strength steel wire and has limited development prospects.

[0023] In order to solve the problems of preparation and interface combination of dissimilar material composite steel wire, the present application mainly considers that the strain and load state of the steel wire are different along the radial direction during drawing process and service, that is, the strength and plasticity requirements are different at different positions, that the carbon element of the wire rod or pre-drawing steel wire is continuously distributed along the radial direction by carburizing and / or decarburizing, that the composition gradient is constructed in the cross-section direction of the steel wire, that the microstructure of the steel wire cross-section is gradiently distributed by subsequent hot working, that the strain damage is reduced during subsequent drawing process, that the microstructure characteristics of the wire rod or pre-drawing steel wire are optimized after suitable heat treatment, that the work hardening rate and residual stress distribution of the drawing steel wire are adjusted, and that the drawing performance and service performance of the steel wire are finally improved. It can be seen that the main physical and metallurgical principles and design ideas of the high-strength steel wire of the present application are completely different from those of the prior art, the method has relatively low cost and high operability.

[0024] The present application has the following advantages and benefits:

[0025] 1. The present application can prepare high-strength steel wires meeting different service performance requirements according to the specific performance requirements of the high-strength steel wire, and realize customized development of high-strength steel wires according to customer requirements. According to the processing and service characteristics of the high-strength steel wire, the carbon element of the carbon steel or low-alloy steel wire is gradiently distributed in the radial cross-section by carburizing and / or decarburizing, so as to meet the microstructure and mechanical property requirements of the high-strength steel wire.

[0026] 2. The present application adjusts the overall work hardening rate of the high-strength steel wire by changing the carbon content and microstructure distribution of the high-strength steel wire in the radial direction, so that the high-strength steel wire can reach the critical stress of drawing deformation damage under larger plastic strain conditions, and the residual stress distribution on the cross-section of the drawing steel wire can be optimized, thereby improving the drawing performance and service performance of the high-strength steel wire.

[0027] 3. Since the steel wire is an axisymmetric workpiece with a relatively small diameter, the carburizing and / or decarburizing time is short, the quenching cooling parameters are easy to adjust, and the heat treatment structure control is relatively simple. Therefore, in actual production, heat treatment equipment can be appropriately added to the existing production line, or a continuous production line can be developed, which is beneficial to mass production of the high-strength steel wire.

[0028] The method utilizes the difference in carbon content of the wire rod or pre-drawing steel wire cross-section, adjusts the phase transformation products (pearlite / bainite / martensite) and mechanical properties in the radial direction after hot working, optimizes the work hardening rate and residual stress distribution of the drawing steel wire, better utilizes the processing performance and extreme service potential of the high-strength steel wire, and meets the development needs of customized special steel wire. Compared with the method of high-strength steel wire with homogeneous composition, the present application not only meets the harsh requirements of the steel wire for high strength, wear resistance, torsional resistance / bending resistance and fatigue resistance, but also effectively reduces the drawing damage of the steel wire and greatly improves the processability and extreme service performance of the steel wire. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 SEM microstructure of Example 1 compositionally gradient high strength steel wire at different locations of radial cross section, wherein (a) is surface layer location, (b) is 1 / 2 location from surface layer to core, (c) is core location;

[0030] Figure 2 Distribution diagram of microhardness of Example 2 compositionally gradient high strength steel wire at radial cross section, wherein abscissa is distance from point to center of circle (mm) in diameter direction of radial cross section, origin is center of cross section, left ordinate is carbon content (wt.%), right ordinate is hardness (HV);

[0031] Figure 3 SEM microstructure of Example 3 compositionally gradient high strength steel wire at different locations of radial cross section, wherein (a) is surface layer location, (b) is subsurface layer location, (c) is core location. DETAILED DESCRIPTION

[0032] In the specific implementation process, the preparation method of the compositionally gradient high strength steel wire is as follows:

[0033] A carbon steel or low alloy steel is selected, and no obvious harmful second phase is precipitated after cold and hot working in a wide carbon content range. The microstructure and hardness change significantly with the change of carbon content and present a certain rule. The steel has a certain drawing performance. In the wide carbon content change range of 0.2-1.2 wt.%, the second phase of carbide or proeutectoid ferrite deteriorating the performance of the steel wire is more easily controlled.

[0034] The selected carbon steel or low alloy steel ingot or bar is hot-rolled into a wire rod with a required diameter of φ6-18 mm or further pre-drawn into a steel wire with a diameter of φ3-9 mm. The sampling of the wire rod or pre-drawn steel wire at different locations of the radial cross section should be controlled within ±0.05 wt.%.

[0035] The hot-rolled wire rod or pre-drawn steel wire is subjected to surface treatment, and then is placed in a controllable atmosphere furnace for carburizing or / and decarburizing treatment. The controllable atmosphere for carburizing and / or decarburizing can be CO2 / CO or methane, or a certain proportion of inert other gases such as nitrogen, argon or helium, etc. According to the change of carbon potential in the furnace, the mixing of other proportions can be automatically adjusted to carry out decarburizing and carburizing treatment. In order to ensure that the decarburized wire rod or pre-drawn steel wire is not oxidized, P CO2 / P CO should be less than 0.37, P CO2 is CO2 partial pressure, P COThe carbon monoxide partial pressure; carburizing and / or decarburizing temperature is carried out at 800-1150°C, ensuring that the steel wire does not oxidize and abnormal grain growth phenomenon does not occur at the carburizing and / or decarburizing temperature, and the time of each step of carburizing and decarburizing is determined according to the required gradient distribution requirement of the rod / bar or pre-drawing steel wire, and is generally controlled at 4h-24h; the effective thickness of the carburized or decarburized layer (the distance from the surface to the point of the specified carbon content or hardness level) is 1 / 6-1 / 2 of the diameter of the rod / bar or pre-drawing steel wire, and is generally controlled at 0.5-4.5mm.

[0036] The carburized or / and decarburized hot-rolled rod / bar or pre-drawing steel wire is quenched, and the quenching cooling rate / medium and quenching (isothermal transformation) temperature are reasonably selected to make the radial section at different positions obtain different characteristics of pearlite, bainite and / or martensite and the like; if the steel wire section is prepared to have a gradient distribution of pearlite structure with different cementite contents or lamella spacings, lead bath quenching is adopted, the lead bath temperature is controlled at 480-600°C, and the lead bath time is controlled at 0.5-15min; if the steel wire section is prepared to have a gradient composite of different types of martensite or bainite structure, the quenching cooling rate is ≥ the critical quenching cooling rate of bainite obtained at the lowest carbon position, and it is ensured that the high carbon area does not appear quenching cracks, and is generally controlled at 2-50°C / s.

[0037] When the quenched carburized or / and decarburized hot-rolled rod / bar or pre-drawing steel wire is drawn, the deformation amount of each drawing pass should be reasonably distributed according to the gradient distribution characteristics of the steel wire structure, and the deformation amount of each pass should be controlled in the range of 3-20%, and the cold drawing speed is 5-300m / min, and the high-strength steel wire of the required size specification is finally drawn.

[0038] Hereinafter, the present application is further illustrated in detail through examples.

[0039] Example 1

[0040] In this example, a 97Si (C: 0.97wt.%, Si: 0.35wt.%, Mn: 0.35wt.%, Cr: 0.25wt.%, P: 0.008wt.%, S: 0.002wt.%) high carbon steel rod with a diameter of φ100mm is used as raw material, and a wire rod with a diameter of φ8.5mm is obtained by controlled rolling and controlled cooling process. According to YB / 4413 rating, the carbon center segregation is 0 level, the proportion of sorbitization reaches 100%, and the network cementite is all within 1.0 level. The hot-rolled wire rod is pickled to remove the rust and oil on the surface of the wire rod. The specific process is to immerse the hot-rolled wire rod in hydrochloric acid with a mass concentration of 18% for 1h, then wash with water, and then perform surface phosphating treatment. The specific process is to immerse the hot-rolled wire rod after water washing in zinc phosphate with a concentration of 15g / L for 5min. The wire rod after surface treatment is placed in a controllable atmosphere furnace at 950°C for complete austenitization and decarburization treatment. By controlling the CO2 / CO mixed gas ratio and introducing a certain amount of nitrogen, the carbon potential in the furnace is controlled at 0.7wt.% by using a flowmeter for self-feedback, and the decarburization time is controlled at 12h. Decarburization occurs in the circumferential direction about 2.2mm (decarburization effective thickness) away from the edge of the wire rod. Finally, the carbon content at a distance of 2mm from the edge is controlled at about 0.8wt.%, and then the sample is immediately taken out for isothermal lead bath quenching. The isothermal temperature is controlled in the range of 530-550°C, and the isothermal temperature is 540°C. The isothermal time is 15min. The wire rod obtains pearlite structure with different cementite content / lamellar spacing in the radial direction, as shown in (a)-(c). Figure 1 (a)- Figure 1 (c). The cementite content is less and the lamellar spacing is larger at the surface position (1mm inward from the edge) (carbon content is 0.73wt.%, cementite volume fraction is 11.2%, and lamellar spacing is 180nm), followed by the surface at a distance of 1 / 2 from the center (a ring with a width of 1mm) (carbon content is 0.85wt.%, cementite volume fraction is 12.8%, and lamellar spacing is 164nm), and the cementite content is the most and the lamellar spacing is the smallest at the center position (a circle with a diameter of 1mm) (carbon content is 0.97wt.%, cementite volume fraction is 14.6%, and lamellar spacing is 138nm). Finally, the wire rod forms a pearlite structure with increasing cementite content from the edge to the center in the radial section.

[0041] The wire rod after the above heat treatment is drawn for 10 passes, and the deformation amount of each pass should be controlled in the range of 8-12%, which is 10% here. The cold drawing speed is 200m / min. Finally, the steel wire is drawn to a diameter of φ2.6mm. The mechanical property test results of the finally drawn steel wire show that the diameter of the steel wire is φ

[0042] 2.6mm of the component gradient high carbon pearlitic steel wire tensile strength reaches 2230MPa, torsion performance ≥25 turns, bending performance ≥15 times, detection standard respectively according to GBT-228.1-2021, GB / T 239.1-2023 and GB / T 238-2013 (same below).

[0043] The homogeneous 97Si high carbon pearlitic steel wire with a diameter of 2.6mm is prepared by using the traditional method, and the preparation process is the same as that of the above-mentioned embodiment 1, the only difference is that after surface treatment, it does not go through the decarburization process, but only goes through the heat treatment, that is, it is kept at 950℃ for 12h under vacuum conditions, and the mechanical properties of the gradient steel wire of the application are greatly improved, see Table 1 for details.

[0044] Table 1 Performance comparison of high carbon pearlitic steel wire under different preparation processes

[0045]

[0046] Example 2

[0047] In this embodiment, first, a medium-low alloy medium carbon steel bar with a diameter of φ300mm is prepared, and the chemical composition (mass fraction, %) is: C: 0.45, Cr: 1.5, Mo: 0.3, Si: 0.25, Mn: 0.65, P: 0.013, S: 0.024, and Fe balance. The carbon segregation degree of the sample taken at different positions of the radial section of the bar is controlled at ±0.02wt.%; the low alloy steel bar with a diameter of φ300mm is hot rolled into a wire rod with a diameter of φ12mm as required, and is further pre-drawn into a steel wire with a diameter of φ6mm; then it is pickled to remove the surface rust and oil stains of the wire rod or pre-drawn steel wire, and then it is phosphated, the pickling and phosphating processes are the same as those of embodiment 1.

[0048] The surface treated pre-drawing steel wire is heated to 1050°C austenitizing zone in a controllable atmosphere furnace for decarburization treatment, i.e. CO2 / CO mixed gas is introduced during the austenitizing holding stage, and a certain amount of nitrogen is introduced, the carbon potential in the furnace is controlled at 0.15wt.% by using a flow meter for self feedback, and the sample is immediately quenched by high pressure nitrogen after holding for 12 hours, and the cooling speed is 30°C / s. The effective thickness of the decarburized layer is about 2.5mm, due to the difference in carbon content and quenching cooling speed in the radial direction of the steel wire, medium carbon martensite structure is formed in the core (a circle with a diameter of 0.5mm), lower bainite or low carbon martensite is formed in the subsurface (a circular ring with a width of 0.5mm from the surface inward), and lower bainite is formed in the surface layer (a region with a width of 0.5mm from the edge inward), and finally a lower bainite-low carbon martensite-medium carbon martensite structure is formed in the radial section from the edge to the core. The surface heat treated pre-drawing steel wire is drawn for 8 passes, the deformation amount of each pass should be controlled in the range of 6-10%, which is 8% here, and the cold drawing speed is 50m / min, and finally the steel wire can be drawn to a diameter of φ2mm, and the radial composition distribution is the same as that of the steel wire before drawing.

[0049] When the low alloy composition gradient steel wire is drawn, no micro cracks or wire breakage phenomenon is found on the surface. The mechanical property test results show that the tensile strength of the diameter φ2mm composition gradient low alloy medium carbon martensite steel wire reaches 2050MPa, and the bending performance is ≥10 times. It can be seen that, by the process of the present application, the surface layer and subsurface of the low alloy medium carbon martensite steel wire form lower bainite or lath martensite structure with good toughness, and the core forms medium carbon martensite structure with higher strength, which improves the work hardening rate of the martensite steel and is beneficial to drawing, and greatly improves the strength and bending performance of the steel wire.

[0050] As shown in Figure 2 As shown in the cross-sectional microhardness distribution of the high strength steel wire of the composition gradient martensite / bainite steel of Example 2, the hardness at different positions in the radial section of the steel wire is positively correlated with the carbon content distribution, the higher the carbon content, the higher the hardness; since the surface layer of the pre-drawing steel wire is decarburized, the carbon content gradually increases from the surface layer to the core, resulting in a gradual increase in the hardness of the cross-section of the drawn steel wire from the surface layer to the core, which meets the requirement of 'tough surface and hard core' of the designed steel wire.

[0051] Example 3

[0052] In this example, a diameter φ400mm medium carbon steel bar is first prepared, and the chemical composition (mass fraction, %) is: C: 0.50, Si: 0.25, Mn: 0.65, P: 0.012, S: 0.017, Fe balance. The carbon segregation degree of the sample taken at different positions in the radial section of the bar is controlled at ±0.03wt.%; the φ

[0053] 400mm medium carbon steel rod is hot-rolled into a diameter of φ9mm wire rod according to the specification requirements, and then is further pre-drawn into a diameter of φ6mm steel wire; then the wire rod or the pre-drawn steel wire is pickled by using hydrochloric acid to remove the surface rust and oil stains, and then is subjected to surface phosphating treatment.

[0054] The surface-treated pre-drawn steel wire is heated to an austenitizing zone of 1000℃ under an argon protective atmosphere for decarburization / cementation treatment, that is, CO2 / CO mixed gas is introduced during the austenitizing holding stage, and a certain amount of nitrogen is introduced, and the carbon potential in the furnace is controlled at 0.15wt.% and 0.7wt.% respectively by using a flowmeter for self-feedback to perform decarburization and cementation treatment, the decarburization and cementation time is 4 hours and 8 hours respectively, and then the sample is immediately taken out for oil quenching to room temperature, the cooling speed is 30℃ / s, the carbon and hardness distribution of the pre-drawn steel wire cross section is measured, it can be seen that the effective thickness of the cementation / decarburization layer is about 1.5mm, about 1 / 4 of the diameter of the pre-drawn steel wire, the carbon content of the steel wire at the subsurface position (a ring with a width of 0.5mm from the surface) is as low as 0.25wt.% (the torsional deformation strain in this area is larger), a low-carbon martensite structure is formed; the carbon content of the core (a circle with a diameter of 1mm) and the surface (0.5mm from the edge) is 0.5wt.% and 0.7wt.% respectively, a medium and high carbon martensite structure is formed (the carbon content of high-carbon martensite is 0.6-1.0%, the carbon content of medium-carbon martensite is 0.3-0.6%, and the carbon content of low-carbon martensite is 0.15-0.3%), finally a high-carbon martensite-low-carbon martensite structure which is gradiently distributed from the edge to the subsurface and a low-carbon martensite-medium carbon martensite structure which is gradiently distributed from the subsurface to the core are formed in the radial cross section, and the specific microstructure characteristics are shown in Figure 3 The pre-drawn steel wire after surface heat treatment is subjected to 8 passes of drawing, the deformation amount of each pass should be controlled in the range of 6-10%, here it is 8%, and the cold drawing speed is 20m / min, finally the steel wire can be drawn to a diameter of φ3mm.

[0055] Since the subsurface structure of the steel wire cross section is low-carbon martensite, it has high deformation capacity and anti-drawing damage capacity. When the composite martensite high-strength steel wire is drawn multiple times, no micro-cracks or wire breakage phenomenon is found on the surface. The mechanical property test results show that the tensile strength of the composition gradient medium carbon martensite steel wire with a diameter of φ3mm reaches 2200MPa, the torsional performance is ≥28 turns, and the bending performance is ≥19 times. It can be seen that, by the process of the present application, the low-carbon martensite structure with good toughness is formed in the subsurface of the medium carbon martensite steel wire, and the medium and high carbon martensite structure with high strength is formed in the core and the surface, and the residual stress distribution of the steel wire cross section is optimized, which improves the work hardening rate of the martensite steel and is beneficial to drawing, at the same time, the strength, torsional performance and stability of the steel wire are greatly improved.

[0056] The implementation result shows that the preparation method utilizes the section carbon content difference of the wire rod or pre-drawing steel wire, adjusts the phase change product (pearlite / bainite / martensite) and mechanical property in the radial direction after hot working, optimizes the work hardening rate and residual stress distribution of the drawing steel wire, better plays the processing performance and extreme service potential of the high-strength steel wire, and meets the development demand of the special steel wire to the customization direction. Compared with the high-strength steel wire method with homogeneous composition, the application can meet the harsh requirements of the steel wire on high strength, wear resistance, torsion / bending resistance and fatigue and some other specific service performances, effectively reduce the drawing damage of the steel wire, and greatly improve the processability and extreme service performance of the steel wire. Especially for the high-strength steel wire processing and service characteristics, the carbon element of the carbon steel or low-alloy steel wire is distributed in a gradient in the radial section through carburizing and / or decarburizing, so as to meet the microstructure and mechanical property demand of the high-strength steel wire.

Claims

1. A method for preparing high-strength steel wire with composition gradient, characterized in that, The method includes the following steps: (1) The selected carbon steel or low alloy steel material is hot-rolled into wire rod or further pre-drawn into steel wire; the carbon content of the carbon steel or low alloy steel material is: 0.2-1.2% by mass, preferably 0.4-1.0%; (2) The hot-rolled wire rod or pre-drawn steel wire is surface treated and then placed in a controlled atmosphere furnace for carburizing or decarburizing treatment, or carburizing treatment followed by decarburizing treatment, or decarburizing treatment followed by carburizing treatment. (3) Quench the hot-rolled wire rod or pre-drawn steel wire processed in step (2); (4) The quenched hot-rolled wire rod or pre-drawn steel wire is drawn to obtain high-strength steel wire with the required size and performance.

2. The method for preparing high-strength steel wire according to claim 1, characterized in that, The low-alloy steel material described in step (1) has the following elemental composition by weight percentage: 0.20%≤C≤1.2%, 0.15%≤Si≤0.85%, 0.1%≤Mn≤1.5%, Cr≤3.0%, Mo≤0.8%, Ni≤1.2%, and the sum of alloying elements (including C, Si, Mn, Cr, Mo, Ni) ≤5.0%, with the remainder being Fe and unavoidable impurities.

3. The method for preparing high-strength steel wire according to claim 1, characterized in that, The diameter of the hot-rolled wire rod mentioned in step (1) is φ6~18mm, the diameter of the pre-drawn steel wire is φ3~9mm, and the radial carbon segregation degree of the wire rod or pre-drawn steel wire is within ±0.05wt.%.

4. The method for preparing high-strength steel wire according to claim 1, characterized in that, The surface treatment described in step (2) is pickling and phosphating.

5. The method for preparing high-strength steel wire according to claim 1, characterized in that, In step (2), the atmosphere in the controllable atmosphere furnace is a carbon-containing gas or a mixture of a carbon-containing gas and an inert atmosphere gas. The carbon-containing gas is a mixture of CO2 and CO or methane. The inert atmosphere gas is one or more of nitrogen, argon or helium. The carbon potential in the furnace is controlled by automatically adjusting the ratio of the mixed gas (the carbon potential can be controlled between 0.2% and 1.2%). The carburizing or decarburizing treatment, or carburizing treatment followed by decarburizing treatment, or decarburizing treatment followed by carburizing treatment, is carried out at a temperature of 800-1150℃ for a time of 4-24 hours. The effective thickness of the carburized or decarburized layer is 1 / 6 to 1 / 2 of the diameter of the wire rod or pre-drawn steel wire.

6. The method for preparing high-strength steel wire according to claim 1, characterized in that, The quenching described in step (3) should be carried out by selecting a quenching method according to the carbon content distribution of the hot-rolled wire rod or pre-drawn steel wire section and its corresponding hardenability characteristics. If isothermal quenching is carried out by lead bath, the lead bath temperature is 480-600℃ and the lead bath time is 0.5-15min. If the quenching medium is water, oil or gas (argon, nitrogen or air), the quenching cooling rate is 2-100℃ / s.

7. The method for preparing high-strength steel wire according to claim 1, characterized in that, In step (4), in order to obtain a multi-layered multiphase structure, the wire rod or pre-drawn steel wire can also undergo multiple carburizing or decarburizing treatments in the subsequent drawing stage, or carburizing treatment followed by decarburizing treatment, or decarburizing treatment followed by carburizing treatment. When drawing hot-rolled wire rod or pre-drawn steel wire, steps (2) and (3) can be repeated between 1 to 4 drawing passes before the next drawing pass is performed until the high-strength steel wire with the required size and performance is obtained.

8. The method for preparing high-strength steel wire according to claim 1, characterized in that, In step (4), when drawing the quenched hot-rolled wire rod or pre-drawn steel wire, the deformation amount of each pass is controlled within the range of 3% to 20%, the number of drawing passes is 6 to 12, and the drawing speed is 5 to 300 m / min.

9. A composition-gradient high-strength steel wire prepared by the method for preparing high-strength steel wire according to any one of claims 1-8.

10. The high-strength steel wire according to claim 9, characterized in that: When the raw material is high-carbon steel, after decarburization and quenching, a pearlitic structure with a gradient increase in cementite content (8-15%) or a decrease in interlamellar spacing (100-600 nm) along the radial direction of the cross section from the edge to the core can be obtained. When the raw material is low-alloy medium-carbon steel, after decarburization and quenching, a lower bainite-low-carbon martensite-medium-carbon martensite structure with a gradient distribution along the radial direction of the cross section from the edge to the core can be obtained. When the raw material is low-alloy medium-carbon steel, after decarburization followed by carburizing or carburizing followed by decarburization and quenching, a martensite structure with different carbon contents with multiple gradient distributions along the radial direction of the cross section from the edge to the core can be obtained.

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