High plasticity carbon tool steel wire rod and method of manufacturing the same

Abstract

The present application relates to a kind of high plasticity carbon tool steel wire rod and its manufacturing method, the chemical composition of wire rod and mass percentage include: C:0.51%~0.56%, Si:0.15%~0.25%, Mn:0.45%~0.55%, P≤0.015%, S≤0.015%, the rest is Fe and inevitable impurities, wire rod after spinning is treated by on-line molten salt isostatic toughening, control from high temperature austenite state into sorbite phase region phase transition, isothermal tempering in isothermal interval, promote sorbite sheet layer fuse, toughening stress relief treatment, finally by roller slow cooling, be made into wire rod including microstructure of tempering sorbite, ferrite and fuse sorbite, can improve the plasticity of wire rod, reach tensile strength is 903~952MPa, reduction of area is 53%~58%, reduce material cost and the risk of wire breakage or cracking in tool cold working process, improve tool quality.

Description

Technical Field

[0001] The present invention belongs to the technical field related to hot-rolled wire rods, and in particular relates to a high-plasticity carbon tool steel wire rod and a manufacturing method thereof. Background Art

[0002] Common tools such as wrenches, pliers, screwdrivers, tape measures, and hammers are processed into finished products through cold forming and heat treatment of hot-rolled wire rod. As quality requirements for tool steel products, such as durability, high strength, and wear resistance, increase, the comprehensive performance requirements for hot-rolled wire rod for tool steel are also gradually increasing. To improve product durability, hot-rolled wire rod for tool steel is often processed by adding hardenability elements or increasing the content of hardenability elements. However, this also increases the difficulty of hot-rolled wire rod production and material costs. Therefore, it is necessary to develop a high-plasticity carbon tool steel wire rod to reduce material costs, improve cold working performance, and expand the application scenarios of carbon tool steel.

[0003] Existing tool steel wire rods are generally produced using Stelmor air-cooled wire controlled cooling. To improve the plasticity of tool steel wire rods, alloying elements are added to the wire rods, and low-temperature rolling and Stelmor air-cooled wire rods are used to produce sorbite wire rods. For example, patent CN119640155A discloses a low-cost, high-plasticity sorbite tool alloy steel wire rod and its manufacturing method. This utilizes a C-Si-Mn-Cr composition design, combined with low-temperature rolling, air cooling, and heat-insulating cooling to obtain a hot-rolled structure dominated by sorbite, achieving an area reduction of 50% to 55%. The main factors limiting the development of high-plasticity carbon tool steel wire rods include:

[0004] On the one hand, hardenability elements such as Mn and Cr can reduce the sorbite transformation temperature, so that austenite can obtain refined lamellar structure at a lower temperature on the air cooling line. However, it also increases the material cost. Affected by the segregation of hardenability elements, it is difficult to control abnormal structures such as martensite in hot-rolled wire rods for tool steel, which leads to wire breakage or cracking during the cold working of tools. Reducing the alloy content or using carbon components will significantly reduce the stability of supercooled austenite, widen the pearlite transformation temperature range, increase the critical cooling rate, and be limited by the maximum cooling capacity of the Stelmor air cooling line. Due to the influence of the instability of the control and temperature control, pearlite transformation is likely to occur in the early stage of air cooling, or because of the large difference in cooling rates at different positions of the wire rod, the cooling rate of the core is lower than the critical value, resulting in the formation of ferrite and pearlite in the core. During cooling, the pearlite transformation rate is fast and the lamellar layer is easy to coarsen. The interface bonding force between ferrite and cementite in the pearlite with coarse lamellar structure is weak, which leads to low strength and hardness of the wire rod and decreased toughness, or the phenomenon of hard surface and soft core, aggravating the fluctuation of toughness. When subjected to force, stress concentration is likely to occur at the interface, leading to cracks, reducing the quality and durability of the tool.

[0005] On the other hand, the wire rod needs to be controlled rolled and cooled at low temperature to refine the microstructure grains in order to suppress the coarse pearlite structure on the air-cooling line, but it also leads to a large load demand and wear on the rolling line, affecting the rolling efficiency. After the carbon wire rod increases the rolling spinning temperature, the driving force of the sorbite phase transformation will be further reduced, and it is limited by the minimum cooling capacity of the Stelmor air-cooling line. The wire rod undergoes a short time of phase transformation incubation during the continuous cooling process, which can easily lead to a low sorbitization rate. Due to the influence of the non-uniform cooling and temperature gradient between the surface and core of the wire rod during the air-cooling process, the wire rod will produce thermal stress and microstructure stress, and the wire rod is in a low temperature state after the phase transformation. The thermal stress and microstructure stress will be retained in the wire rod in the form of residual stress, resulting in deterioration of the plasticity of the wire rod. When cold working is carried out, the residual stress and the processing stress are superimposed, which can easily lead to local plastic exhaustion or cracks on the surface, resulting in wire breakage and cracking problems. Summary of the Invention

[0006] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a high-plasticity carbon tool steel wire rod and a manufacturing method thereof, which can improve the plasticity of the wire rod, reduce material costs and the risk of wire breakage or cracking during tool cold working, and improve tool quality.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] A method for manufacturing a high-plasticity carbon tool steel wire rod, in a preferred technical solution, comprises:

[0009] The steel billet is rolled and spun into a wire rod at a spinning temperature of ≥845°C, and then subjected to an online molten salt strong isothermal toughening treatment. The wire rod is controlled to enter the sorbite phase region from the high-temperature austenite state at a cooling rate of ≥33°C / s to form a structure mainly composed of sorbite. The wire rod is controlled to be isothermal tempered in the isothermal range to promote the melting of the sorbite lamellar layer and toughening and stress relief treatment. Finally, it is slowly cooled by a roller to form a wire rod with a microstructure including tempered sorbite, ferrite and fused sorbite. The chemical composition and mass percentage of the wire rod include: C: 0.51%~0.56%, Si: 0.15%~0.25%, Mn: 0.45%~0.55%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities.

[0010] The chemical composition and mass percentage of the above-mentioned wire rod are designed based on the following:

[0011] (1) Carbon: As an effective carbide strengthening element and austenite forming element, C is relatively cheap. It can improve the strength of wire rod by dissolving in ferrite to form interstitial solid solution, and improve the hardness of wire rod by carbide strengthening. As the carbon content increases, it can expand the austenite stable zone, reduce the starting temperature of sorbite formation, and make sorbite transformation easier to complete within a narrow temperature range. Combined with molten salt treatment, it promotes the refinement of sorbite lamellae, provides sufficient carbon source for lamella melting, and provides strength and toughness. However, too high carbon content will increase the decarburization tendency, prolong the incubation period of sorbite phase transformation, and reduce the plasticity and toughness of steel. Therefore, in order to meet the strength and wear resistance requirements of tool steel and control material costs, and at the same time facilitate tempering regulation toughening and stress relief treatment, improve the plasticity and cold working properties of wire rod, the carbon content is appropriately reduced, and the mass percentage of C is controlled to be 0.51%~0.56%.

[0012] (2) Silicon: Si is a deoxidizing element that can improve the purity of steel and inhibit grain coarsening during online molten salt treatment to inhibit pearlite transformation, refine the spacing between troostite lamellae and hinder the aggregation and growth of carbides during tempering, so that the wire rod maintains appropriate hardness and strength. A small amount of silicon is dissolved in ferrite, which can improve the matrix strength through the solid solution strengthening effect. However, too high a silicon content will lead to an increase in inclusions, inhibit the precipitation of cementite, delay the phase transformation and incubation transformation speed, reduce the plasticity and impact toughness of the wire rod, and affect the cold working performance. Therefore, in order to adapt to the regulation of the organization by online molten salt, take into account the cold working performance of the wire rod, and facilitate the improvement of plasticity, a low silicon content is adopted, and the mass percentage of Si is controlled to be 0.15%~0.25%.

[0013] (3) Manganese: Mn is a hardenability element that can expand the austenite zone and reduce the equilibrium temperature of the transformation from austenite to sorbite, which is conducive to the formation of sorbite at a lower temperature. In combination with molten salt treatment, the nucleation rate of sorbite is improved. Because the atomic diffusion is slow at low temperatures, it is easier to obtain a fine lamellar structure. The refinement of sorbite lamellar can increase the grain boundary area, hinder the dislocation movement, and improve the strength of the wire rod. However, when the Mn content is too high, it will promote the growth of austenite grains. When heated at high temperature, it is easy to cause grain coarsening and increase the central segregation of the steel billet, resulting in uneven phase transformation structure or prolonging the incubation period of sorbite phase transformation, affecting the impact toughness of the wire rod and deteriorating the fatigue resistance of the tool. Therefore, in order to facilitate the control of the microstructure phase transformation by molten salt treatment and reduce the difficulty of improving plasticity, the Mn content is appropriately reduced. The mass percentage of Mn is controlled to be 0.45%~0.55%.

[0014] (4) Phosphorus and sulfur: P and S are impurity elements. The lower the better. Therefore, P is controlled to be ≤ 0.015% and S is controlled to be ≤ 0.015%.

[0015] The above-mentioned wire rod is designed with a carbon component and the content is relatively low. There is no need to add alloying elements such as Cr and Ni, which can reduce material costs. At the same time, the sorbite transformation temperature is appropriately reduced by optimizing the ratio of C, Si, and Mn, which is favorable for inhibiting the precipitation of coarsened pearlite, promoting the refinement of sorbite lamellae, avoiding too long incubation time of sorbite phase transformation, and increasing the melting sites with the diffusion power and fine initial lamellae, so as to facilitate the improvement of plasticity. On this basis, the wire rod adopts a higher spinning temperature, that is, the quenching temperature, so that the wire rod is in a uniform high-temperature austenite state, avoiding pearlite transformation before spinning, and preparing for the formation of sorbite structure with fine lamellar spacing. After spinning, the wire rod is directly subjected to online molten salt strong isothermal toughening treatment without air cooling:

[0016] 1. Compared with the Stelmor air-cooled line, the maximum cooling capacity and cooling control capacity are limited, resulting in coarse pearlite structure and difficulty in sufficient phase transformation. After the wire rod is treated with molten salt, on the one hand, the high heat exchange capacity of the molten salt can be used to promote the rapid cooling of the wire rod. The wire rod quickly passes through the pearlite phase region from the high-temperature austenite state to the sorbite phase region, increasing the phase transformation supercooling, which can inhibit the coarse pearlite and promote the reduction of the spacing between the sorbite lamellae and the rapid transformation. Compared with the alloy steel wire rod, it makes up for the adverse effects of not adding hardenability alloy components such as Cr, which can reduce the restrictions on the wire drawing and rolling temperature and avoid excessive strength loss. On the other hand, when the wire rod passes through the molten salt, the molten salt can cover the Uniform heat exchange is carried out on the surface of the wire rod, and there is no temperature difference problem between the winded side and the winded side, or between the overlap and non-overlap. The temperature difference from the surface to the core of the wire rod can be reduced, thereby adapting to the phase transformation range limitation of carbon steel, promoting the uniform precipitation of fine lamellar sorbite, avoiding too slow cooling of the core to form pearlite, avoiding local overcooling to form bainite or martensite abnormal structure, and improving the uniformity of the structure. At the same time, as the processing time increases, the incubation time of the wire rod in the sorbite phase region can be extended, promoting the formation of austenite structure dominated by sorbite with fine lamellar spacing, increasing the sorbite content, overcoming the difficulty of controlling sufficient phase transformation of the structure in the air-cooling line, and reducing the fluctuation of mechanical properties and the risk of abnormal structure.

[0017] 2. Compared with the Stelmor air-cooled line, the minimum cooling capacity and cooling control capacity are limited, resulting in insufficient microstructure plasticity and large residual stress. On the one hand, the wire rod can quickly and evenly reach the sorbite phase temperature when passing through the molten salt. By reducing the temperature gradient from the surface to the core of the wire rod, the synchronous phase transformation of the austenite in the surface and core of the wire rod is promoted, which can reduce the thermal stress and microstructure stress caused by the temperature gradient and the difference in phase transformation progress; on the other hand, the molten salt can control the wire rod to undergo strong isothermal treatment in the high-temperature isothermal temperature range instead of continuous cooling, and can stably maintain the temperature of the sorbite lamellar melting, providing a continuous thermodynamic driving force for melting, so that the wire rod can obtain sufficient energy in a uniform thermal environment, and the sorbite lamellar layers in each region are uniformly melted, further releasing the microstructure stress, reducing the slip resistance of the lamellar interface, performing toughening and stress relief treatment, and effectively improving the overall plasticity. Afterwards, slow cooling is carried out on the roller to prevent the wire rod from increasing stress due to excessive cooling rate, promote further toughening of the wire rod microstructure, and achieve strong-plastic matching of the wire rod.

[0018] Before the rolling, selecting appropriate heating furnace soaking temperature and furnace time can promote uniform diffusion of components, reduce defects such as segregation of the steel billet, improve the plasticity of the steel billet for rolling, and avoid grain coarsening caused by excessive temperature or excessive furnace time. In the preferred technical solution, before the rolling, the heating furnace soaking temperature is controlled to be 1080~1140℃, and the furnace time is 120~200min.

[0019] Due to the high spinning temperature, the online molten salt can control the refinement and rapid nucleation of the sorbite lamellae, which can reduce the restrictions on rolling. A higher rolling temperature is selected to improve the high-temperature plasticity of the steel billet and reduce the deformation resistance, so as to increase the rolling speed and efficiency, reduce the load demand and wear of the rolling line, control the final rolling temperature and the final rolling reduction to inhibit work hardening, avoid excessive final rolling reduction to increase deformation resistance and stress concentration, promote dynamic recrystallization and grain refinement in the final rolling process, and improve the dimensional accuracy of the wire rod. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1010~1060℃, the final rolling temperature is 850~895℃, and the final rolling reduction is 10%~16%.

[0020] During the spinning, the spinning temperature can be further controlled to inhibit excessive growth of austenite grains and match the stability of the spinning process and the shape of the wire rod. In a preferred technical solution, during the spinning, the spinning temperature is controlled to be 845-890°C.

[0021] In the preferred technical solution, the online molten salt strong isothermal toughening treatment is divided into a front-stage molten salt treatment and a rear-stage molten salt treatment. The molten salt circulation volume of the front-stage molten salt treatment is greater than that of the rear-stage molten salt treatment. The use of a larger molten salt circulation volume in the front-stage molten salt treatment can promote the rapid cooling of the wire rod to the sorbite phase region, reduce the temperature gradient of various parts of the wire rod, inhibit the coarsening of pearlite, promote the uniform and rapid transformation of various parts of the wire rod to the sorbite structure with fine lamellar spacing, reduce the initial stress, and appropriately reduce the molten salt circulation volume in the rear-stage molten salt treatment, which can reduce production energy consumption and equipment operation load on the basis of controlling the molten salt temperature.

[0022] The molten salt temperature of the front-stage molten salt treatment is in the sorbite phase region. The lower the molten salt temperature and the longer the treatment time, the more favorable it is to improve the supercooling degree of the wire rod, promote the rapid and uniform transformation of austenite into sorbite structure with fine lamellar spacing, inhibit the early precipitation of ferrite or the coarsening of carbides, and provide the wire rod with appropriate strength and good toughness; but if the molten salt temperature is too low and the treatment time is too long, the temperature is lower than the efficient range of sorbite phase transformation, which will affect the transformation rate of austenite to sorbite and increase the phase transformation stress. Due to the higher molten salt circulation volume, the production energy consumption will increase; on the contrary, the higher the molten salt temperature, the lower the temperature difference from the edge to the core of the wire rod, which is favorable to reduce the tissue stress. As the treatment time increases, the molten salt temperature increases, and the temperature difference from the edge to the core of the wire rod decreases, which is favorable to reduce the tissue stress. Shortening is beneficial to reducing production energy consumption, but the molten salt temperature is too high, the treatment time is too short, the supercooling degree is reduced, the phase transformation products are less and the interlamellar spacing is coarser in a short time, and there is more retained austenite, which is not conducive to inhibiting coarse pearlite structure, affecting the subsequent toughness improvement, and causing the strength and organizational uniformity of the wire rod to decrease. Therefore, the molten salt temperature and treatment time of the front-stage molten salt treatment can be controlled to promote the wire rod to be rapidly cooled from the high-temperature austenite state to the sorbite phase region, taking into account the rapid transformation of austenite, organizational refinement and reduction of production energy consumption requirements. In the preferred technical solution, the molten salt temperature of the front-stage molten salt treatment is 555~590℃, and the treatment time is 60~110s.

[0023] Since the spinning temperature is quite different from the molten salt temperature of the front-stage molten salt treatment, selecting a larger molten salt circulation rate can increase the heat exchange efficiency between the molten salt and the wire rod, reduce the molten salt temperature rise, promote the uniform transformation of the wire rod structure, and avoid excessive energy consumption due to excessive molten salt circulation rate. In the preferred technical solution, the molten salt circulation rate of the front-stage molten salt treatment is 360~560t / h, and the molten salt temperature rise is ≤10℃.

[0024] The latter molten salt treatment is in the high-temperature isothermal temperature range. The higher the molten salt temperature and the longer the treatment time, the more conducive it is to improving the atomic diffusion capacity, promoting further homogenization and melting of the sorbite lamellae, eliminating the residual deformation and phase change internal stress and thermal stress of rolling and the former molten salt treatment, and releasing the core stress accumulated over time, thereby significantly improving the plasticity and cold working performance of the wire rod, and avoiding cracking due to stress concentration in subsequent cold working; however, if the molten salt temperature is too high and the treatment time is too long, the sorbite lamellae will melt too much, the cementite lamellae will coarsen and soften excessively, and the wire rod strength and production energy consumption will be excessively lost; on the contrary, the lower the molten salt temperature and the shorter the treatment time, the lower the atomic diffusion rate. The decrease is beneficial to inhibit the coarsening of cementite lamellae, reduce production energy consumption and strength loss, but the molten salt temperature is too low and the treatment time is too short, the melting of sorbite lamellae is reduced, the elimination of core stress is slow, and the toughness is insufficiently improved, which will increase the risk of wire breakage or cracking due to stress concentration in subsequent processing. Therefore, the molten salt temperature and treatment time of the latter molten salt treatment can be controlled, and the wire rod can be controlled to perform high-temperature isothermal annealing of the organization in the high-temperature isothermal range to promote the melting of sorbite organization lamellae, toughen and relieve stress, and improve the strength-plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the latter molten salt treatment is 575~585℃, and the treatment time is 200~350s.

[0025] Since the difference in molten salt temperature between the front-stage molten salt treatment and the rear-stage molten salt treatment is small, appropriately reducing the molten salt circulation rate can control the molten salt temperature rise, take into account the uniformity of the wire rod structure, and effectively reduce production energy consumption. In the preferred technical solution, the molten salt circulation rate of the rear-stage molten salt treatment is 240~340t / h.

[0026] Since the wire rod has undergone sufficient phase transformation after the online molten salt strong isothermal toughening treatment, the residual austenite can be avoided and the brittle structure can be continued to be formed. However, in order to prevent the wire rod from physical shrinkage and increased stress due to excessively fast cooling rate during the cooling process, slow roller cooling can be adopted. The high temperature state of the wire rod after leaving the molten salt is used to promote further toughening of the wire rod structure and improve the softening effect. Further controlling the cooling rate of the wire rod can promote the rapid offline of the wire rod and improve production efficiency. In the preferred technical solution, the slow roller cooling controls the wire rod to slowly cool to below 310°C at a cooling rate of 0.3~0.6°C / s.

[0027] A high-plasticity carbon tool steel wire rod is manufactured by any one of the above-mentioned methods for manufacturing high-plasticity carbon tool steel wire rods.

[0028] The above-mentioned tool steel wire rod is designed with relatively low C, Si, and Mn carbon components, and does not add alloy components such as Cr, Ni, V, and Mo, which can effectively reduce material costs. At the same time, the wire rod microstructure includes a mixed structure mainly composed of tempered bainite, partially melted bainite, and a small amount of ferrite structure. The bainite structure has finer interlamellar spacing than the pearlite structure, has higher strength and toughness, and can reduce the ferrite structure content in the structure, taking into account the appropriate strength of the wire rod and the wear resistance of the tool steel. At the same time, through the release of tissue stress, the bainite lamellar melts and transforms into tempered bainite, an intermediate transition state of melting bainite and carbide to spheroidized structure, which reduces the splitting of the matrix, makes the structure have both certain strength and better plasticity, makes the material more prone to plastic deformation when subjected to stress, and is not easily brittlely fractured by local stress concentration. It can be directly used for cold processing of tool steel to improve the yield rate.

[0029] The higher the volume percentage of the tempered bainite and the finer the lamellar spacing, the more conducive it is to maintaining the strength of the wire rod. The higher the proportion of the fused bainite and ferrite, the more conducive it is to regulating the plasticity and processing properties. In the preferred technical solution, the volume percentage of the tempered bainite is 58%~73%, the lamellar spacing is 105~135nm, the volume percentage of the fused bainite is 20%~30%; and the volume percentage of the ferrite is 7%~12%.

[0030] In the preferred technical solution, the diameter of the wire rod is 5.5~9mm, the tensile strength is 903~952MPa, the cross-sectional shrinkage rate is 53%~58%, the mechanical property same-circle difference is ≤25MPa, and the wire rod diameter specification is medium and fine, which can adapt to the deformation needs of downstream users in cold drawing, cold heading and other processes, reduce processing passes and improve material utilization. The wire rod has appropriate tensile strength and can meet the tool force strength requirements. The wire rod has a high cross-sectional shrinkage rate and small fluctuations in mechanical properties. It can withstand greater deformation during cold drawing, which can reduce the risk of wire breakage or cracking in cold processing of downstream users, buffer impact loads or stress concentration, and is conducive to improving the processing yield rate and consistency of finished product performance.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) In view of the current situation that carbon steel wire rod production is difficult and has insufficient plasticity due to the control of air temperature and cooling rate of Stelmor air cooling line, the present invention controls the wire rod after spinning to quickly enter the sorbite phase region from the high-temperature austenite state by combining the design of carbon chemical composition with the online molten salt strong isothermal toughening technology, which can reduce the restriction on rolling, inhibit coarse pearlite, and form a structure dominated by sorbite with fine lamellar spacing, thereby compensating for the adverse effects caused by not adding hardenability elements such as Cr. Then, the structure is subjected to high-temperature isothermal tempering in the high-temperature isothermal range to promote the melting of the sorbite structure lamellar layer and toughen and relieve stress, which can improve the strength and plasticity matching of the wire rod. Finally, slow cooling by roller promotes further toughening of the wire rod structure, thereby realizing the manufacture of high-plasticity carbon tool steel wire rod, which has good industrial adaptability.

[0033] (2) Aiming at the current situation that the addition of hardenability elements to alloy tool steel wire rods makes it difficult to control abnormal structures and leads to insufficient plasticity, which makes it easy for wires to break or crack during tool cold working, seriously reducing the quality of tools, the wire rods of the present invention are designed with relatively low carbon components such as C, Si, and Mn, and do not add alloy components such as Cr, Ni, V, and Mo, which can effectively reduce material costs. At the same time, the wire rod microstructure includes tempered bainite, ferrite, and fused bainite, which can improve the strength-plasticity matching of the wire rods, achieving a tensile strength of 903~952MPa and a cross-sectional shrinkage rate of 53%~58%. It is used in application fields such as the manufacture of high-plasticity tool steel, which is beneficial to reducing the risk of wire breakage or cracking during tool cold working, improving tool quality and yield rate, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0035] Figure 1 is a metallographic structure diagram of Example 1 of the present invention;

[0036] Figure 2 is a metallographic structure diagram of Example 2 of the present invention;

[0037] Figure 3 This is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0038] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention. They are to propose the best way to implement the present invention, are intended to be used to explain the present invention, and are sufficient to enable those skilled in the art to practice the present invention, but should not be understood as limiting the scope of the present invention, which is limited only by the appended claims; the wire rods obtained in the following embodiments and comparative examples are subjected to organizational and performance testing, including: tensile testing using "GB-T228.1-2021 Metallic Materials Tensile Testing Part 1: Room Temperature Test Method" to obtain tensile strength and cross-sectional shrinkage; organizational testing is performed in accordance with the metal microstructure detection method of GB / T13298 standard; mechanical property same-circle difference test method: take 2 circles of wire rod 5m away from the end of the coil, and divide each circle of wire rod into 8 sections with the overlap area position as the base point, and take 1 tensile specimen on each section. The extreme difference in strength of the tensile specimens after tensile testing is the mechanical property same-circle difference. Example 1:

[0039] A preferred embodiment of the method for manufacturing high-plasticity carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.55%, Si: 0.19%, Mn: 0.45%, P: 0.015%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt strong isothermal toughening treatment → roller slow cooling → coiling, specifically:

[0040] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote component homogenization, and control the heating furnace according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 6.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1090°C, the furnace time is 150min, the initial rolling temperature is 1030°C, the final rolling temperature is 880°C, and the final rolling reduction is 14.5%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, preparing for the formation of a sorbite structure with fine lamellar spacing; specifically, the wire-spinning temperature is controlled to be 860°C.

[0041] The online molten salt strong isothermal toughening treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 34°C / s, and quickly enters the sorbite phase region from the high-temperature austenite state, suppresses coarse pearlite, and forms a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to be isothermal tempered in the isothermal range, promoting the melting of the sorbite lamellar layer, toughening and stress relief treatment, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 577°C, the treatment time is 75s, the molten salt circulation volume is 445t / h, and the molten salt temperature rise is ≤10°C; the molten salt temperature of the rear-stage molten salt treatment is 582°C, the treatment time is 310s, and the molten salt circulation volume is 275t / h.

[0042] The roller slow cooling process adopts the method of closing the heat preservation cover, and the wire rod conveyed by the conveyor roller through the second salt bath tank is slowly cooled through the heat preservation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to an increase in stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to be slowly cooled to 305°C at a cooling rate of 0.5°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0043] Comparative Example 1:

[0044] A method for manufacturing a wire rod, which differs from Example 1 in that: the manufacturing method is manufactured according to the process flow of rolling → spinning → Stelmor air cooling line → coiling, specifically: in the rolling process, the heating furnace soaking temperature is controlled to be 1050°C, the furnace time is 200min, the initial rolling temperature is 980°C, the final rolling temperature is 820°C, the spinning temperature is controlled to be 805°C, the Stelmor air cooling line uses 85% of the air volume of 1~4# fans to control the wire rod to cool to 710°C at a speed of 8.9°C / s, 5~14# fans are opened at 40%, the wire rod is controlled to cool to 295°C at a speed of 3.7°C / s, and the total air volume of the fan is 200,000 m 3 / h, collected by a collecting drum to obtain a finished wire rod, the volume percentage of pearlite in the wire rod microstructure is 70%, the interlamellar spacing is 236nm, the volume percentage of ferrite is 30%, the tensile strength is 665MPa, the cross-sectional shrinkage rate is 40%, and the mechanical property difference in the same circle is 58MPa.

[0045] Comparative Example 2:

[0046] A method for manufacturing a wire rod, which differs from Example 1 in that: the soaking temperature of the heating furnace is controlled to 1050°C, the time in the furnace is 200 minutes, the initial rolling temperature is 980°C, the final rolling temperature is 840°C, the spinning temperature is controlled to be 830°C, the wire rod is treated with molten salt in the front section and cooled at a cooling rate of 32°C / s to obtain a finished wire rod. Example 2:

[0047] A preferred embodiment of the method for manufacturing high-plasticity carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.51%, Si: 0.25%, Mn: 0.45%, P: 0.011%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt strong isothermal toughening treatment → roller slow cooling → coiling, specifically:

[0048] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote component homogenization, and the heating furnace is controlled according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1120°C, the time in the furnace is 175min, the initial rolling temperature is 1045°C, the final rolling temperature is 890°C, and the final rolling reduction is 12%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, in preparation for promoting the formation of a sorbite structure with fine lamellar spacing; specifically, the wire-spinning temperature is controlled to be 875°C.

[0049] The online molten salt strong isothermal toughening treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 35°C / s, and quickly enters the sorbite phase region from the high-temperature austenite state, suppresses coarse pearlite, and forms a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to be isothermal tempered in the isothermal range, promoting the melting of the sorbite lamellar layer, toughening and stress relief treatment, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 567°C, the treatment time is 95s, the molten salt circulation volume is 510t / h, and the molten salt temperature rise is ≤10°C; the molten salt temperature of the rear-stage molten salt treatment is 578°C, the treatment time is 260s, and the molten salt circulation volume is 300t / h.

[0050] The roller slow cooling process adopts the method of closing the heat preservation cover, and the wire rod conveyed by the conveyor roller through the second salt bath tank is slowly cooled through the heat preservation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to an increase in stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to be slowly cooled to 308°C at a cooling rate of 0.4°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0051] Comparative Example 3:

[0052] A method for manufacturing a wire rod, which differs from Example 2 in that: the wire rod undergoes a front-stage molten salt treatment and is cooled at a cooling rate of 33°C / s, the molten salt temperature of the front-stage molten salt treatment is 600°C, and the treatment time is 45s to obtain a finished wire rod.

[0053] Comparative Example 4:

[0054] A method for manufacturing wire rod, which differs from Example 2 in that: the wire rod is subjected to a front-stage molten salt treatment and cooled at a cooling rate of 37°C / s, the molten salt temperature of the front-stage molten salt treatment is 525°C, and the treatment time is 125s to obtain a finished wire rod. Example 3:

[0055] A preferred embodiment of the method for manufacturing high-plasticity carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.51%, Si: 0.15%, Mn: 0.49%, P: 0.015%, S: 0.011%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt strong isothermal toughening treatment → roller slow cooling → coiling, specifically:

[0056] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote component homogenization, and control the heating furnace according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 5.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1080°C, the furnace time is 200min, the initial rolling temperature is 1010°C, the final rolling temperature is 850°C, and the final rolling reduction is 16%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, in preparation for promoting the formation of a sorbite structure with fine lamellar spacing; specifically, the wire-spinning temperature is controlled to be 845°C.

[0057] The online molten salt strong isothermal toughening treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 33°C / s, and quickly enters the sorbite phase region from the high-temperature austenite state, suppresses coarse pearlite, and forms a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to be isothermal tempered in the isothermal range, promoting the melting of the sorbite lamellar layer, toughening and stress relief treatment, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 586°C, the treatment time is 60s, the molten salt circulation volume is 360t / h, and the molten salt temperature rise is ≤10°C; the molten salt temperature of the rear-stage molten salt treatment is 585°C, the treatment time is 350s, and the molten salt circulation volume is 240t / h.

[0058] The roller slow cooling process adopts the method of closing the heat preservation cover, and the wire rod conveyed by the conveyor roller through the second salt bath tank is slowly cooled through the heat preservation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to an increase in stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 302°C at a cooling rate of 0.6°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 3 shown.

[0059] Comparative Example 5:

[0060] A method for manufacturing a wire rod, which differs from the manufacturing method of Example 3 in that the molten salt temperature of the latter molten salt treatment is 595° C., the treatment time is 400 s, and the finished wire rod is obtained.

[0061] Comparative Example 6:

[0062] A method for manufacturing a wire rod, which differs from the manufacturing method of Example 3 in that the molten salt temperature of the latter molten salt treatment is 560° C., the treatment time is 150 s, and the finished wire rod is obtained. Example 4:

[0063] A preferred embodiment of the method for manufacturing high-plasticity carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.56%, Si: 0.22%, Mn: 0.55%, P: 0.013%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt strong isothermal toughening treatment → roller slow cooling → coiling, specifically:

[0064] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote component homogenization, and the heating furnace is controlled according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 9mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1140°C, the time in the furnace is 120min, the initial rolling temperature is 1060°C, the final rolling temperature is 890°C, and the final rolling reduction is 10%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, in preparation for promoting the formation of a sorbite structure with fine lamellar spacing; specifically, the wire-spinning temperature is controlled to be 890°C.

[0065] The online molten salt strong isothermal toughening treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 36°C / s, and quickly enters the sorbite phase region from the high-temperature austenite state, suppresses coarse pearlite, and forms a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to be isothermal tempered in the isothermal range, promoting the melting of the sorbite lamellar layer, toughening and stress relief treatment, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 555°C, the treatment time is 110s, the molten salt circulation volume is 560t / h, and the molten salt temperature rise is ≤10°C; the molten salt temperature of the rear-stage molten salt treatment is 575°C, the treatment time is 200s, and the molten salt circulation volume is 340t / h.

[0066] The roller slow cooling process adopts the method of closing the insulation cover, and the wire rod transported by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to increased stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 309°C at a cooling rate of 0.3°C / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the finished wire rod is obtained after packaging and storage.

[0067] Comparative Example 7:

[0068] A method for manufacturing a wire rod, which differs from that of Example 4 in that: the manufacturing method is manufactured according to a process flow of rolling → spinning → online molten salt strong isothermal toughening treatment → air cooling → coiling; specifically: the air cooling process uses a wire rod transported by a conveyor roller through a second salt bath tank to naturally cool in the air, and the wire rod is cooled to 300°C at a cooling rate of 1.3°C / s to obtain the wire rod.

[0069] The structure and performance of the wire rods obtained in Examples 1 to 4 and Comparative Examples 2 to 7 were tested, and the comparative results are shown in Table 1 below:

[0070] Table 1. Comparison of microstructure and properties of different wire rod compositions and manufacturing methods

[0071]

[0072] From the comparison results of Example 1 and Comparative Example 1, it can be seen that under the carbon steel composition, the pearlite transformation speed is fast and the lamellae are easy to coarsen, weakening the strengthening effect due to the uncontrollable cooling rate during the air-cooled line cooling phase transformation process. The thermal stress and tissue stress of the wire rod will be retained in the wire rod in the form of residual stress, resulting in deterioration of the plasticity of the wire rod, the phenomenon of hard surface and soft core, and aggravated fluctuations in toughness. The present invention controls the wire rod after spinning to quickly enter the sorbite phase region from the high-temperature austenite state through the design of carbon chemical composition combined with the online molten salt strong isothermal toughening technology, which can inhibit coarse pearlite, perform high-temperature isothermal tempering on the tissue in the high-temperature isothermal range, promote the melting of the sorbite tissue lamellae, toughen and relieve stress, and improve the strength-plasticity matching of the wire rod. It can be seen from the results of Examples 1 to 4 that the wire rod can reach a tensile strength of 903~952MPa and a cross-sectional reduction rate of 53%~58%. It is used in application fields such as manufacturing high-plasticity tool steel, which is beneficial to reducing the risk of wire breakage or cracking during tool cold working.

[0073] From the comparison results of Example 1 and Comparative Example 2, it can be seen that the online molten salt strong isothermal toughening technology can reduce the restrictions on rolling, adopt a higher spinning temperature, that is, the quenching temperature, to prepare for the formation of a bainite structure with fine lamellar spacing, so that a higher rolling temperature can be selected to improve the rolling speed and efficiency.

[0074] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the higher the molten salt temperature of the front-stage molten salt treatment, the lower the temperature difference from the edge to the core of the wire rod can be, which is beneficial to reducing the tissue stress. As the treatment time is shortened, it is beneficial to reduce production energy consumption. However, if the molten salt temperature is too high and the treatment time is too short, the supercooling degree is reduced, the phase change products are less in a short time and the interlamellar spacing is coarser, resulting in a decrease in the strength and tissue uniformity of the wire rod.

[0075] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the lower the molten salt temperature and the longer the treatment time of the front-stage molten salt treatment, the more conducive it is to improving the supercooling degree of the wire rod, promoting the rapid and uniform transformation of austenite into sorbite structure with fine lamellar spacing, and providing the wire rod with appropriate strength and good toughness. However, if the molten salt temperature is too low and the treatment time is too long, it will affect the transformation rate of austenite to sorbite, increase the phase transformation stress and production energy consumption.

[0076] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the higher the molten salt temperature and the longer the treatment time of the later molten salt treatment, the more conducive it is to promote further homogenization and melting of the sorbite lamellae, increase the melting of sorbite, release tissue stress, and significantly improve the plasticity and cold working performance of the wire rod. However, if the molten salt temperature is too high and the treatment time is too long, the sorbite lamellae will melt too much and soften excessively, resulting in excessive loss of wire rod strength and production energy consumption.

[0077] From the comparison results of Example 3 and Comparative Example 6, it can be seen that the lower the molten salt temperature and the shorter the treatment time of the later molten salt treatment, the more conducive it is to reducing production energy consumption and strength loss. However, if the molten salt temperature is too low and the treatment time is too short, the melting of the troostite lamellae will be reduced, the elimination of the core stress will be slow, and the toughness will not be improved enough, which will increase the risk of wire breakage or cracking due to stress concentration in subsequent processing.

[0078] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow roller cooling can prevent the stress of the wire rod from increasing due to excessively fast cooling rate during the cooling process, and utilize the high temperature state of the wire rod after leaving the molten salt to promote further toughening of the wire rod structure and improve the softening effect.

[0079] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing a high plasticity carbon tool steel wire rod, characterized in that: The manufacturing method includes: The steel billet is rolled and spun into a wire rod at a spinning temperature of ≥845°C, and then subjected to an online molten salt strong isothermal toughening treatment. The wire rod is controlled to enter the sorbite phase region from the high-temperature austenite state at a cooling rate of ≥33°C / s to form a structure mainly composed of sorbite. The wire rod is controlled to be isothermal tempered in the isothermal range to promote the melting of the sorbite lamellar layer and toughening and stress relief treatment. Finally, it is slowly cooled by a roller to form a wire rod with a microstructure including tempered sorbite, ferrite and fused sorbite. The chemical composition and mass percentage of the wire rod include: C: 0.51%~0.56%, Si: 0.15%~0.25%, Mn: 0.45%~0.55%, P≤0.015% , S≤0.015%, and the rest are Fe and unavoidable impurities; the online molten salt strong isothermal toughening treatment is divided into a front-stage molten salt treatment and a rear-stage molten salt treatment, the molten salt temperature of the front-stage molten salt treatment is 555~590℃, and the treatment time is 60~110s; the molten salt temperature of the rear-stage molten salt treatment is 575~585℃, and the treatment time is 200~350s, and the roller slow cooling controls the wire rod to be slowly cooled to below 310℃ at a cooling rate of 0.3~0.6℃ / s; the volume percentage of the tempered bainite is 58%~73%, the volume percentage of the fused bainite is 20%~30%; the volume percentage of the ferrite is 7%~12%.

2. The method for manufacturing high plasticity carbon tool steel wire rod according to claim 1, characterized in that: Before the rolling, the soaking temperature of the heating furnace is controlled to be 1080-1140° C., and the soaking time in the furnace is 120-200 minutes.

3. The method for manufacturing high plasticity carbon tool steel wire rod according to claim 1, characterized in that: During the rolling, the initial rolling temperature is controlled to be 1010-1060° C., the final rolling temperature is controlled to be 850-895° C., and the final rolling reduction is controlled to be 10%-16%; during the wire drawing, the wire drawing temperature is controlled to be 845-890° C.

4. The method for manufacturing high plasticity carbon tool steel wire rod according to claim 1, characterized in that: The molten salt circulation amount of the front-stage molten salt treatment is greater than the molten salt circulation amount of the back-stage molten salt treatment.

5. The method for manufacturing high plasticity carbon tool steel wire rod according to claim 1, characterized in that: The molten salt circulation rate of the front-stage molten salt treatment is 360~560t / h, and the molten salt temperature rise is ≤10°C; the molten salt circulation rate of the back-stage molten salt treatment is 240~340t / h.

6. A high plasticity carbon tool steel wire rod, characterized in that: The wire rod is manufactured by the method for manufacturing high-plasticity carbon tool steel wire rod according to any one of claims 1 to 5.

7. The high plasticity carbon tool steel wire rod according to claim 6, characterized in that: The interlamellar spacing of the tempered troostite is 105-135 nm.

8. The high plasticity carbon tool steel wire rod according to claim 6, characterized in that: The diameter of the wire rod is 5.5-9 mm, the tensile strength is 903-952 MPa, the cross-sectional shrinkage is 53%-58%, and the mechanical property difference within the same circle is ≤25 MPa.

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