CN118639120B - High-carbon cold heading steel wire rod for 14.9-grade non-quenched and tempered bolt and manufacturing method thereof - Google Patents

Abstract

The invention relates to a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt and a manufacturing method thereof, wherein a high-carbon component of high C-Si-Mn is adopted to control rolling and spinning to form the wire rod, then online molten salt phase transformation regulation and control are carried out, the wire rod is firstly subjected to first-stage molten salt and cooled to a bainite and sorbite phase region at a cooling speed of more than or equal to 44 ℃/s to carry out tissue phase transformation, partial austenite is promoted to be transformed into bainite and sorbite, then is heated to a pearlite region to isothermal temperature through a last-stage molten salt, the transformation of non-transformed austenite into pearlite is promoted, tempering and softening are carried out at the same time, and finally, a roller way is used for slowly cooling, so that the cold heading steel wire rod with a microstructure comprising most tempered pearlite, a small amount of tempered sorbite, a small amount of tempered bainite and fused pearlite is manufactured, the tensile strength is 1305-1345 MPa, and the section shrinkage is 44% -49%, and the method is used for efficiently green manufacturing the 14.9-grade bolt under a non-quenched and tempered process, and the material cost is effectively reduced, and the yield is improved.

Description

High-carbon cold heading steel wire rod for 14.9-grade non-quenched and tempered bolt and manufacturing method thereof

Technical Field

The invention belongs to the technical field of cold heading steel wire rods, and particularly relates to a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt and a manufacturing method thereof.

Background

The bolt connection is used as an important connection mode, is widely applied in the fields of machinery, automobiles, buildings and the like, and under the development trend of light weight of structural steel, the strength grade of the steel is continuously improved, and the service stress condition of the bolt serving as a connecting member is also continuously improved, so that the strength grade of the bolt is required to be continuously improved, and the application in the increasingly developed high-stress industrial field is met. The bolts use cold heading steel wire rods as parent materials, plays a vital role in improving the strength grade of the bolts, so the strength grade of the cold heading steel wire rods is required to be continuously improved, but in the prior art, the researches on the production of the bolts with the grade of 8.8-12.9 and the cold heading steel wire rods thereof are more, and the bolts with the higher performance grade of 13.9 and above, such as 14.9 bolts with the tensile strength of more than 1400MPa, and the development difficulty of high-speed wire cold heading steel are higher, the strength grade of a component system is ensured mainly by improving the content of alloy elements with hardenability on the basis of low and medium carbon and adding a plurality of microalloy elements, but the material cost is obviously increased, the abnormal tissue precipitation risk of the deterioration of drawing performance such as martensite and bainite of the component system is aggravated due to the influence of the cooling speed of the stell air cooling line after hot rolling wire rods, the high hardenability of the component system is easy to be broken, the rolling coil is difficult to be coiled, the yield is lower, the development of round steel forms is adopted for developing round steel, for mass production, for the round heading steel, for example, the patent CN110106443A is a round heading steel production method is used for producing the round steel, and the round heading steel 42, the round steel is difficult to be developed into the round steel with low cost, and the production line is difficult to be 14, the production is difficult to be in production with low in production process grade, and has high production quality and high quality, and has high quality and high quality, and quality is difficult to be easily produced.

Although, for example, patent CN118186298a discloses a high-plasticity delayed fracture-resistant ultra-high strength fastener steel wire rod and a manufacturing method thereof, which adopts the design of medium C-Si-Mn-Cr-Mo-V-Al-Ni components, and combines the hot rolled metallographic structure which is mainly made of bainite and contains a small amount of martensite and ferrite by air cooling and heat preservation on a stelmor air cooling line after spinning, and can obtain the lower bainite structure through molten salt isothermal heat treatment to achieve the mechanical property of the 14.9 grade fastener, on one hand, the higher Cr content and the addition of noble valence components such as Mo, V, ni and the like also lead to higher cost of steel materials, and the aim of reducing cost is to save alloy elements or reduce alloy content and lead to obvious loss of strength and hardenability of the wire rod, and the bainite and martensite are used as hard brittle phase structures with higher distortion and dislocation density, and are difficult to be softened in the continuous cooling process of heat preservation treatment on the stelmor air cooling line, so that the wire rod is broken in the coiling, downstream user transportation and unreeling processes of steel factories, leading to material waste and increased manufacturing cost; on the other hand, although salt bath treatment is employed, it is an off-line salt bath, and downstream users need to unpack the coil, unreel, heat to austenitizing temperature, and perform long-time molten salt treatment, resulting in increased process, energy consumption, and cost, which is disadvantageous for efficient production of fasteners.

The existing 12.9 grade and below grade cold heading steel wire rods also need to be subjected to multiple spheroidizing annealing, drawing, cold heading and tempering processes to manufacture the bolts, the cold deformation performance is improved by spheroidizing annealing, the comprehensive mechanical properties are further improved by tempering after drawing and cold heading face strengthening so as to achieve the final performance grade, and for the 14.9 grade cold heading steel wire rods, the non-quenched and tempered type cold heading steel wire rods for the bolts, which can dispense with spheroidizing annealing and tempering processes, are further required to be developed to simplify the manufacture of the bolts, reduce emission and energy consumption and realize efficient green manufacture, but the higher strength grade corresponding to the cold heading steel wire rods also brings challenges to the improvement of the plasticity of the wire rods.

The high-carbon steel uses carbon element to replace other alloy elements, has the advantage of lower material cost, but the existing high-carbon steel wire rod is mainly concentrated on realizing the improvement of strength grades such as bridge ropes, cords and the like and obtaining proper drawing performance by improving sorbite content in a structure, and is difficult to be applied to cold heading steel wire rods, and the reasons of the high-carbon steel wire rod include: 1. the stelmor air-cooling line adopts heat preservation or delayed cooling to reduce abnormal tissue risk and promote sorbite inoculation, but the time of a precipitation temperature interval of secondary cementite is longer, high carbon elements can cause the wire rod to form a netlike carbide deteriorated phase in the air-cooling line cooling process, the tissue continuity can be damaged, particularly the brittleness of the material is increased, annealing or tempering is difficult to eliminate, and the wire rod is easy to crack in the drawing and cold heading processes under the non-tempering process; 2. because the supercooling degree is small, the time of the wire rod passing through the sorbite phase region is short, the wire rod after the wire rod is inoculated by sorbite is in a low-temperature state under the condition of continuous cooling in heat preservation, so that the obtained sorbite tissue has large lamellar spacing, high tissue stress and high dislocation density, the wire rod has insufficient final plasticity, poor cold deformation capacity and easy drawing hardening, the drawing cold heading requirement cannot be met, the strength of the wire rod can be obviously deteriorated due to the improvement of the ferrite content in the tissue, and the performance grade of a 14.9-grade bolt without tempering is not good; 3. the increase of the carbon content makes the sheet thicker, the increase of the alloy element makes the diffusion of carbon slower, and although the quenching property of the wire rod can be used for properly increasing the cooling speed and utilizing the phase transformation to form a certain amount of bainite, the bainite as a low-temperature structure can also obviously increase the brittleness because the wire rod is reduced to a low-temperature state in the continuous cooling phase transformation process and cannot be softened, the wire rod with high strength and high plasticity cannot be directly obtained, and the yield loss and the cracking risk of a non-tempering process are caused.

Disclosure of Invention

The invention aims to solve at least one of the technical problems to a certain extent, and provides a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt and a manufacturing method thereof, which can effectively reduce material cost by utilizing high-carbon components, realize wire rod tissue regulation and control, high strength and high plasticity matching, are used for efficiently manufacturing a 14.9-grade bolt in a green mode under a non-quenched and tempered process, effectively reduce cracking risk and improve yield.

The technical scheme adopted for solving the technical problems is as follows:

the high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolt comprises the following chemical components in percentage by mass: c:0.79% -0.83%, si:0.15% -0.25%, mn: 0.58-0.72%, P less than or equal to 0.015%, S less than or equal to 0.015%, and the balance of Fe and unavoidable impurities; the microstructure comprises 50-60% by volume of tempered pearlite, 5-10% by volume of tempered bainite, 25-35% by volume of tempered sorbite and the balance of mixed structure formed by fused pearlite.

The design basis of the chemical components and the mass percentage of the cold heading steel wire rod comprises:

(1) Carbon: the C element is used as a main additive element, is cheaper than other alloy elements, is used for ensuring the matrix strength, high hardenability and wear resistance of steel through solid solution strengthening, so that the performance grade of a 14.9-grade bolt can be achieved through drawing cold heading after the annealing and tempering processes are omitted, but the decarburization risk of the steel is increased, cementite is in net distribution risk and ageing sensitivity along a grain boundary, the C element content is too high, a cementite sheet layer is thickened, carbon diffusion is slowed down, and the on-line fused salt phase-change regulated tissue fusing softening is unfavorable, so that the cold deformation performance and strength characteristics of a cold heading steel wire rod are conveniently regulated and controlled, and the mass percentage of C is controlled to be 0.79% -0.83%.

(2) Silicon: si element is a deoxidizer in the steelmaking process, can be dissolved in austenite to improve the hardness and strength of steel, can inhibit online molten salt phase transformation to regulate and control grain coarsening of first-stage molten salt treatment, is beneficial to nucleation of bainite and sorbite, but excessive silicon can increase nonmetallic inclusion and decarburization risks, improves the cold work hardening degree of steel to reduce the cold plastic deformation and toughness of the steel, and therefore, the mass percentage of Si is controlled to be 0.15% -0.25%.

(3) Manganese: mn element is often added into steel as deoxidizer and desulfurizing agent in smelting process, can be infinitely dissolved with Fe, can improve strength grade, hardenability and wear resistance of steel after being dissolved in austenite, increases supercooling degree when austenite is cooled, is favorable for rapidly converting wire rods into bainite and sorbite with finer lamellar spacing through first-stage molten salt, but too high Mn content can aggravate segregation in billet solidification process, lead to uniformity deterioration of steel, increase grain coarsening tendency in material heating process, influence activity of C and increase tempering softening difficulty, and therefore Mn mass percent is controlled to be 0.58% -0.72%.

(4) Phosphorus, sulfur: the lower the P element and the S element are, the better, so that the P is less than or equal to 0.015 percent and the S is less than or equal to 0.015 percent.

The cold heading steel wire rod adopts the design of high carbon components of high C-Si-Mn, and uses low-price C as a main component to replace Cr, mo, ti, V, nb, B, ni and other alloy components, so that the material cost can be effectively reduced, the matrix strength and high hardenability are ensured, the cold heading steel wire rod structure comprises most tempered pearlite, a small amount of tempered sorbite, a small amount of tempered bainite and fused pearlite, the lamellar spacing of the pearlite is larger than that of the sorbite, the plasticity is better than that of the sorbite, the strength is higher than that of the ferrite, the tempered pearlite maintains the characteristics of moderate pearlite strength and plasticity, after the pearlite is fused and transformed into a spheroidized structure to form the fused pearlite and the tempered pearlite, the structure stress and dislocation density are further reduced, the plastic property is improved, the lamellar spacing of the sorbite is smaller than that of the pearlite, the strength is higher than that of the pearlite is better, the drawing property is better, the pearlite phase transformation forms the sorbite, the pearlite density is further reduced, the matrix strength is effectively reduced, the hardening is further reduced, the bainite phase transformation is retained, and the soft bainite phase transformation is performed after the bainite phase transformation is performed, and the high-dislocation density is further reduced, and the characteristics of the bainite transformation is further remarkably performed after the bainite transformation is performed.

Therefore, compared with the existing hot rolled medium and low carbon alloy cold heading steel wire rod structure mainly containing bainite, the cold heading steel wire rod can provide higher matrix strength through matching of high carbon components with tempered bainite and tempered sorbite, make up for the loss of strength caused by saving alloy elements and reducing alloy content, can effectively and remarkably improve wire rod plasticity through regulating and controlling the tempered state structure of pearlite, bainite and sorbite, make up for the adverse effect of the high carbon components and bainite hard and brittle phase structure on cold deformation performance, and compared with the existing high carbon steel wire rod with high sorbite conversion rate, the cold heading steel wire rod also can effectively improve the structure stress and the sorbite dislocation density through properly adding tempered pearlite and fused pearlite, can effectively reduce drawing hardening and cold heading cracking, further realize the high strength and high plasticity matching of the high carbon components, and is suitable for manufacturing 14.9-level non-quenched and tempered bolts.

In a preferred embodiment, the lamellar spacing of the tempered pearlite is 155-185 nm, the lamellar spacing of the tempered sorbite is 75-105 nm, the decrease of lamellar spacing of the tempered sorbite is accompanied by an increase in tempered bainite ratio, the increase of lamellar spacing of the tempered pearlite is accompanied by an increase in fused pearlite ratio, the decrease of wire rod strength and the increase of plasticity.

In the preferred embodiment, the mesh carbide grade of the cold heading steel wire rod is 0 grade, so that the formation of the mesh carbide can be effectively inhibited, the cracking risk caused by the mesh carbide is avoided, and the economic value of carbon element is improved.

In a preferred embodiment, the diameter of the cold heading steel wire rod is 5.5-12.0 mm, the tensile strength is 1305-1345 MPa, the reduction of area is 44% -49%, the cold heading steel wire rod has smaller diameter specification than the cold heading round steel for the existing 14.9-grade bolt, the manufacturing difficulty of the small-specification bolt is facilitated to be simplified and reduced, compared with the cold heading steel wire rod for the existing low-grade or quenched and tempered fastener, the cold heading steel wire rod has obviously improved tensile strength and reduction of area, compared with the existing bainitic wire rod, the coil collecting, transportation and downstream unreeling brittle fracture can be effectively avoided, the raw material waste is reduced, the yield is improved, the wire rod tensile strength and the forming manufacturing can be further improved through drawing and cold heading after the annealing process is omitted, the wire rod can reach the 14.9-grade performance grade through simple ageing treatment under the annealing process, the wire rod has higher plasticity, the wire rod cannot crack in the cold working process of the annealing and quenching and tempering process is easy to be improved, the whole wire rod cannot be broken, and the manufacturing cost is reduced.

The manufacturing method of the high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolt comprises the following steps: according to the method, the wire rod is produced by controlling and rolling the chemical components of the 14.9-grade non-quenched and tempered steel wire rod, the wire rod is subjected to online molten salt phase change regulation and control after being spun into the wire rod according to the spinning temperature of more than or equal to 935 ℃, the wire rod is subjected to first-stage molten salt, the wire rod is cooled to a bainite and sorbite phase region at a cooling speed of more than or equal to 44 ℃/s to carry out tissue phase change, the wire rod is promoted to be converted into bainite and sorbite, the wire rod is heated to a pearlite phase region to isothermal through a last-stage molten salt, the conversion of non-converted austenite into pearlite is promoted, the tempering softening of the bainite, the sorbite and the pearlite is promoted, and finally, the wire rod is cooled by a roller way, so that the cold heading steel wire rod with a microstructure comprising 50-60% tempered pearlite, 5-10% tempered bainite and 25-35% tempered pearlite by volume percent and the balance of a mixed structure formed by fusing the pearlite is produced.

Compared with the prior art that the method is limited to reduce the risk of abnormal tissue precipitation or to prolong the phase transformation inoculation of sorbite but the online time is not suitable to be too long, the wire laying temperature of the high-carbon steel wire rod cannot be too high, the manufacturing method adopts the wire laying to directly perform online molten salt phase transformation regulation and control, can quickly cool and regulate the tissue, therefore, the low-temperature wire laying limit can be broken, and a higher wire laying temperature is selected, so that on one hand, the method is used for avoiding the formation of secondary cementite precipitation in the wire laying stage and inhibiting the netlike carbide, and on the other hand, the method provides favorable conditions for the subsequent formation of higher supercooling degree through the first-stage molten salt and the promotion of the phase change of sorbite with finer lamellar spacing.

When wire rods subjected to wire spinning are subjected to online molten salt phase change regulation and control, firstly, the wire rods pass through first-stage molten salt, compared with the highest cooling speed of a Steyr air cooling line, the wire rods can only reach about 10 ℃/s, the high heat exchange capacity of the molten salt can be utilized to enable the wire rods to be extremely rapidly cooled, on one hand, the wire rods can pass through a secondary network carbon precipitation zone at an ultra-rapid cooling speed, the formation of netlike carbides is inhibited, the economic value of carbon elements is effectively improved, on the other hand, larger supercooling degree can be formed, the precipitation of proeutectoid ferrite is inhibited, the wire rods rapidly enter a bainite and sorbite phase zone to carry out tissue phase change, and the transformation from part of high-temperature austenite to small-thickness sorbite of bainite and sheet layer space is promoted through lower isothermal temperature and short isothermal phase change, so that the tempered bainite and the content of sorbite are controlled, meanwhile, compared with a strong air cooling environment, the wire rods can be coated with the molten salt, the problem of temperature difference between a windward surface and a back surface is avoided, the occurrence of martensite abnormal structure of the wire rods can be avoided, and the mechanical property fluctuation of the wire rods is reduced.

The wire rod passing through the first molten salt can quickly reach a pearlite phase transition temperature zone after passing through the first molten salt, the transition from unconverted high-temperature austenite in the wire rod to pearlite tissue is promoted, meanwhile, compared with the existing continuous cooling phase transition control process for preserving heat and slowly cooling, the temperature of the wire rod can be increased by utilizing the tail end molten salt after tissue phase transition, the temperature of the wire rod is consistent with that of the molten salt, the time of the wire rod in the high-temperature zone is prolonged, bainite, sorbite and pearlite tissue can be promoted to be tempered and softened after phase transition, the dislocation density of a matrix and the carbide morphology can be continuously reduced, the strength and plasticity of the matrix are regulated, the occurrence of martensite abnormal tissue in the subsequent cooling process of the wire rod is avoided, the tempered bainite, sorbite and pearlite tissue can be promoted to be softened in one step after being cooled by utilizing the high-temperature state after the wire rod passes through the molten salt, the final tissue regulation and the strong plasticity can be matched with the offline salt bath process, the problem that the temperature of the wire rod in the online salt bath can be directly regulated and controlled by the molten salt can be solved, and the problem that the temperature of the wire rod in the prior art is difficult to be greatly influenced by the heat and the brittle wire rod cooling efficiency and the heat treatment rate is avoided.

In a preferred embodiment, during the controlled rolling, the temperature of a heating furnace is controlled to 1050-1100 ℃ before rolling, the furnace time is controlled to be 2-4 hours, the oxygen content in the furnace is less than or equal to 1.4%, the lower heating furnace temperature and the shorter furnace time are selected, the components are promoted to be uniformly diffused, the low furnace oxygen content is combined, the high-temperature oxidation is restrained, and the decarburization degree of the high-carbon steel billet is controlled.

Because the high wire laying temperature is adopted, the limitation of low-temperature wire laying on low-temperature rolling can be broken, the high rolling temperature is selected to reduce the abrasion to a rolling line and improve the rolling speed, in a preferred embodiment, during the controlled rolling, the initial rolling temperature is controlled to 1030-1075 ℃, the initial rolling reduction is controlled to 20% -28%, the high initial rolling temperature is selected to promote component homogenization, and austenite grains are refined through repeated deformation and recrystallization.

In a preferred embodiment, during the controlled rolling, the finishing temperature is controlled to be more than or equal to 950 ℃, the finishing rolling reduction is 32% -42%, the higher finishing rolling temperature is selected, the carbide in the high-carbon steel is dissolved, the austenite stability is improved, the larger finishing rolling reduction is selected, the growth tendency of austenite grains is reduced, and favorable conditions are created for refining the structure and controlling the net-shaped carbide.

In a preferred embodiment, the molten salt temperature of the first molten salt is 420-450 ℃, the treatment time is 5-15 s, the lower the molten salt temperature of the first molten salt is, the longer the treatment time is, the more austenite is converted into bainite, the finer the inter-lamellar spacing of sorbite is, the tempered bainite is increased in proportion, the wire rod strength is increased, the plasticity is reduced, but the lower the molten salt temperature of the first molten salt is, the tempered bainite is too high in proportion or the sorbite structure cannot be obtained, the longer the treatment time is, the more the bainite is converted into sorbite, and the plasticity of the wire rod is unfavorable; on the contrary, the higher the molten salt temperature is, the shorter the time is, the tempered bainite proportion is reduced, the spacing between the sorbite sheets is increased, the strength of the wire rod is reduced, the plasticity is increased, but the molten salt temperature is too high and the time is too short, the bainite transformation is unfavorable, even the sorbite tissue with thinner sheets cannot be obtained, the strength of the matrix is obviously lost, and the excessive degradation of the strength of the wire rod is caused.

In a preferred embodiment, the molten salt circulation amount of the first-stage molten salt is 350-550 t/h, the molten salt temperature rise is less than or equal to 10 ℃, the molten salt temperature rise can be controlled by the molten salt circulation amount to maintain the treatment precision, unnecessary energy consumption increase caused by overlarge molten salt circulation amount is avoided, and abnormal tissue and regulation difference caused by overlarge molten salt circulation amount is avoided.

In a preferred embodiment, the molten salt temperature of the final molten salt is 460-520 ℃, the treatment time is 200-400 s, the lower the molten salt temperature of the final molten salt is, the finer the formed pearlite lamellar spacing is, the fusing difficulty can be reduced, but the lower the molten salt temperature is, the higher the sorbite duty ratio is caused by supercooling degree, the plasticity of the wire rod can be influenced, otherwise, the thicker the formed pearlite lamellar spacing is, the more thermal power can be provided for tempering and softening of bainite, sorbite and pearlite tissues, the strength of the wire rod is reduced, the plasticity is increased, but the too high molten salt temperature easily causes the lamellar coarseness or ferrite tissue to be formed, and the excessive tempering and softening are unfavorable for the strength of the wire rod; the longer the treatment time of the final molten salt is, the more obvious the tempering softening effect is for reducing the dislocation density of the matrix and adjusting the form of carbide to spheroidize transformation, the strength of the wire rod is reduced, the plasticity is increased, but the longer the treatment time is, the larger strength loss can be caused by excessive softening, meanwhile, the adverse effect on efficient production is caused, the shorter the treatment time is, the worse the tempering softening effect is, the plasticity of the wire rod is reduced, even the short treatment time affects the full inoculation of pearlite to increase the risk of abnormal tissue precipitation, and the adverse effect on tissue regulation is caused, so the molten salt temperature and the treatment time of the final molten salt can be further controlled, the transformation of unconverted high-temperature austenite to pearlite is promoted, and meanwhile, the strength and the plasticity of the matrix are regulated and controlled by tempering softening.

In a preferred embodiment, the molten salt circulation amount of the final molten salt is 350-550 t/h, the temperature rise of the molten salt is less than or equal to 5 ℃, and the control of the higher molten salt circulation amount can promote the uniform transformation and tempering softening of the structure, and avoid the unnecessary energy consumption increase caused by the overhigh molten salt circulation amount.

In a preferred embodiment, the roller way slow cooling adopts a closed heat preservation cover, the wire rod is controlled to be cooled to below 300 ℃ at a cooling speed of 0.35-0.80 ℃/s, a heat preservation channel structure is formed by closing the heat preservation cover, the wire rod conveyed by the roller way to pass through the final fused salt is continuously and slowly cooled in the heat preservation cover, hot gas generated in the online fused salt phase change regulation and control process can be input into the heat preservation cover to control the cooling speed of the wire rod if necessary, the time of the wire rod at high temperature can be prolonged by using the slow cooling speed, further softening of the wire rod tissue is promoted, and then coil collection is carried out.

Compared with the prior art, the invention has the beneficial effects that:

(1) Aiming at the current situation that the existing cold heading steel wire rod improves the content of alloy elements with hardenability and adds a plurality of micro-alloy elements to improve the system strength, but brings about high material cost, aggravated risk of abnormal structure precipitation and difficulty in plasticity, the cold heading steel wire rod adopts a high-carbon component design with high C-Si-Mn, can save Cr, mo, ti, V, nb, B, ni alloy components, effectively reduces the material cost, has a microstructure comprising most tempered pearlite, a small part of tempered sorbite, a small amount of tempered bainite and fused pearlite, does not comprise a martensitic structure, can provide matrix strength by high-carbon solid solution strengthening and tempering bainite and tempering sorbite tissues, compensates for strength loss brought by alloy components after the alloy components are saved, can also effectively reduce tissue stress and dislocation density by combining regulation and control on the tempered state tissues of pearlite, bainite and sorbite, remarkably improves the plasticity of the wire rod, compensates for adverse effects of high-carbon components and bainite hard brittle phase tissues on cold deformation performance, reduces drawing hardening and cold heading cracking, realizes high strength and high plasticity matching, has a tensile strength of 1305-1345 MPa, has a section rate of 44% -49%, can realize shrinkage and low-down-stream rolling reduction of the cost of the bolt, has good manufacturing effect, and is suitable for the manufacturing of the bolt, has a good market-quality and has a good manufacturing prospect of the bolt, and a green-quality is suitable for the problem of being manufactured.

(2) Aiming at the current situation that the cold heading steel is developed in a round steel form or the application requirement of a 14.9-grade non-quenched and tempered bolt is difficult to meet due to the influence of the cooling speed of a Steyr air cooling line on high-speed wire cold heading steel, the manufacturing method provided by the invention can select a higher wire laying temperature through the combination of a high-carbon component design and an online molten salt phase change regulation technology, is combined with the subsequent rapid cooling of online molten salt, passes through a secondary network carbon precipitation interval at an ultra-fast speed through a first-stage molten salt, inhibits the formation of netty carbides, effectively improves the economic value of carbon elements, promotes the transformation of part of high-temperature austenite to bainite and sorbite tissues, further effectively controls the contents of matrix tempered bainite and tempered sorbite, ensures the strength and plasticity of the matrix, promotes the transformation of unconverted high-temperature austenite to pearlite through a last-stage molten salt, simultaneously, continuously reduces the density of the matrix and regulates the form of carbides, and the strength and plasticity of the matrix, and finally promotes the further softening of a wire rod tissue through the slow cooling, and has better adaptability to the industrial coil structure, and can simplify the steps of the offline salt process, and avoid the dislocation coil breakage and high-phase transition structure.

Drawings

The foregoing and/or additional aspects and advantages of the invention will become apparent and may be better understood from the following description of embodiments taken in conjunction with the accompanying drawings in which:

FIG. 1 is a metallographic structure diagram of example 1 of the present invention;

FIG. 2 is a metallographic structure diagram of example 2 of the present invention;

FIG. 3 is a metallographic structure of example 3 of the present invention.

Detailed Description

The embodiments described below are exemplary only and are not intended to limit the description of the features and characteristics of the invention, in order to set forth the best mode of carrying out the invention, intended to explain the invention and to enable those skilled in the art to practice the invention without any limit the scope of the invention.

Example 1:

The invention relates to a preferred embodiment of a manufacturing method of a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt, which comprises the following chemical components in percentage by mass: c:0.82%, si:0.15%, mn:0.63%, P:0.013%, S:0.012%, the balance being Fe and unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, online molten salt phase change regulation and control, roller way slow cooling and coil collecting, and specifically comprises the following steps of:

The controlled rolling is used for heating a steel billet with the specification of 180mm multiplied by 180mm through a heating furnace to obtain a high-temperature steel billet with rolling plasticity, selecting a lower heating furnace temperature and a shorter in-furnace time, controlling the decarburization degree of the high-carbon steel billet, rolling the steel billet discharged from the heating furnace into a wire rod with the diameter of 8mm through a rolling line, reducing the abrasion to the rolling line by using a higher rolling temperature, improving the rolling speed, and selecting a larger rolling reduction to create favorable conditions for refining tissues and controlling net carbide, and is characterized in that: controlling the temperature of the heating furnace to 1070 ℃, the oxygen content in the furnace to be less than or equal to 1.3 percent in the furnace for 3 hours, the initial rolling temperature to 1045 ℃, the initial rolling reduction to 26 percent, the final rolling temperature to 970 ℃ and the final rolling reduction to 38 percent; the wire rod that the wire rod process of laying wire is used for going out the pass line is the wire rod through the wire rod mechanism of laying wire, and the wire rod spreads on the roll table and carries along the roll table, selects for use higher wire temperature, does benefit to control wire rod secondary net carbon and separates out and form great supercooling degree, promotes bainite and sorbite nucleation, and is specific: the laying temperature was controlled to 945 ℃.

The online molten salt phase change regulation and control adopts two salt baths with built-in molten salt, wire rods after spinning are conveyed through a first salt bath of the salt, namely first-stage molten salt, are quickly cooled to the molten salt temperature, lower isothermal temperature is selected, the wire rods are cooled at a cooling speed of 44 ℃/s, the wire rods quickly pass through a secondary network carbon precipitation zone, transformation of part of high-temperature austenite into bainite and sorbite tissues is promoted, so that the strength and plasticity of a matrix are ensured, the wire rods passing through the first-stage molten salt are conveyed through a second salt bath, namely last-stage molten salt, are heated to the pearlite zone to be isothermal, transformation of unconverted high-temperature austenite into pearlite is promoted, tempering softening of bainite, sorbite and pearlite is promoted, and the following steps are realized: the molten salt temperature of the first-stage molten salt is 442 ℃, the treatment time is 8s, the circulating amount of the molten salt is 400t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃; the molten salt temperature of the final molten salt is 500 ℃, the treatment time is 350s, the circulating amount of the molten salt is 480t/h, and the temperature rise of the molten salt is less than or equal to 5 ℃.

The roller way slow cooling procedure adopts to close the heat preservation cover to form a heat preservation channel structure, the wire rods coming out of the second salt bath groove are conveyed by the heat preservation roller way to enter the heat preservation cover, the wire rods are subjected to slow cooling treatment, and further softening of wire rod tissues is promoted, and the roller way slow cooling procedure is specific: controlling the wire rod to be cooled to 286 ℃ at a cooling speed of 0.65 ℃/s, and then collecting and coiling; the coil collecting procedure is used for collecting and coiling the wire rods into coils through the coil collecting drum, and obtaining cold heading steel wire rod finished products after packaging and warehousing, and a metallographic structure diagram of the cold heading steel wire rod finished products is shown in figure 1.

Comparative example 1:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 1 in that the manufacturing method is manufactured according to the process flow of controlled rolling, low-temperature wire laying and stelmor heat preservation slow cooling, and specifically comprises the following steps: controlling the temperature of a heating furnace to 990 ℃, controlling the furnace time to 5 hours, controlling the initial rolling temperature to 950 ℃, controlling the final rolling temperature to 840 ℃ and controlling the spinning temperature to 820 ℃, wherein the stelmor heat preservation slow cooling adopts the steps of closing all fans and heat preservation covers, conveying the wire rods by using a roller way, finishing the phase change of the wire rods in the covers at a cooling speed of 1.2 ℃/s, and obtaining the finished wire rod products after the wire rods are taken off line.

Comparative example 2:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 1 in that the manufacturing method is manufactured according to the process flow of controlled rolling, low-temperature wire laying, air cooling, stelmor heat preservation and slow cooling, and specifically comprises the following steps: the temperature of the heating furnace is controlled to be 1000 ℃, the furnace time is 4 hours, the initial rolling temperature is 990 ℃, the final rolling temperature is 885 ℃, the spinning temperature is 860 ℃, the air cooling adopts the steps of starting a heat insulation cover, closing a fan, conveying the wire rod by a roller way, cooling the wire rod to be 685 ℃ at a cooling speed of 2.2 ℃/s, the Steyr heat insulation slow cooling adopts the steps of closing the fan and the heat insulation cover, conveying the wire rod by the roller way, finishing phase change of the wire rod in the cover at a cooling speed of 0.9 ℃/s, and obtaining a finished wire rod product after the wire rod is taken off line.

Comparative example 3:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 1 in that the manufacturing method is manufactured according to the process flow of controlled rolling, spinning, stelmor quick cooling and heat preservation slow cooling, and specifically comprises the following steps: the temperature of a heating furnace is controlled to be 1035 ℃, the furnace time is 4 hours, the initial rolling temperature is 1015 ℃, the final rolling temperature is 935 ℃, the spinning temperature is 910 ℃, a1 # fan to 7# fan is adopted for carrying out the stelmor rapid cooling and the heat preservation slow cooling according to 45 percent, the wire rod is cooled to 690 ℃ at the cooling speed of 5.8 ℃/s, then the rear fan and the heat preservation cover are closed, the wire rod is conveyed by a roller way, the phase change of the wire rod is completed in the cover at the cooling speed of 0.6 ℃/s, and the finished wire rod product is obtained after the wire rod is taken off line.

The wire rods obtained in example 1 and comparative examples 1 to 3 were subjected to structure and performance detection: tensile testing was performed using the metal material tensile test section 1 of GB-T228.1-2021: room temperature test method, to obtain tensile strength and reduction of area, and to perform tissue detection according to the metal microstructure detection method of GB/T13298 standard, the comparison results obtained are shown in table 1 below:

TABLE 1 comparative results of the tissue properties of wire rods from different manufacturing methods

As can be seen from the comparison result of the embodiment 1 and the comparative example 1, compared with the design of the high-carbon component with high C-Si-Mn, the sorbite+ferrite structure obtained by the low-temperature wire laying and heat preservation slow cooling process has higher strength but forms a netty carbide deterioration phase in the process, which can destroy the continuity of the structure, the obtained sorbite structure has larger lamellar spacing, higher structural stress and higher dislocation density, so that the final plasticity of the wire rod is insufficient, the cold deformation capability is poor and the wire rod is easy to draw and harden.

As can be seen from the comparison result of the embodiment 1 and the comparative example 2, compared with the method of adopting air cooling and heat preservation cooling after slightly increasing the spinning temperature, the method has the advantages that the improvement on net carbon is limited, meanwhile, bainite is used as a hard and brittle phase structure with higher distortion and dislocation density, softening is difficult to obtain in the continuous cooling process of heat preservation treatment on a Steyr air cooling line, material waste is easy to cause, the yield is reduced, the tempered bainite of the invention maintains the characteristic that the strength of the bainite is higher than that of soft phase structures such as pearlite and ferrite, the dislocation density is further reduced after the bainite is transformed into the tempered bainite through tempering, and the plasticity is obviously improved.

As can be seen from the comparison result of the embodiment 1 and the comparative example 3, compared with the method adopting air cooling to rapidly cool and heat-preserving cool after further increasing the spinning temperature, the method has the advantages that the improvement on the net carbon is limited, the risks of precipitation of abnormal structures deteriorating the drawing performance such as martensite, bainite and the like are aggravated, the formed materials are easy to brittle fracture and difficult to collect, the abnormal structure of the martensite is avoided, the fluctuation of the mechanical property of the wire rod is reduced, and the adverse effect of high-carbon components and the hard and brittle phase structure of the bainite on the cold deformation performance is compensated.

Example 2:

The invention relates to a preferred embodiment of a manufacturing method of a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt, which comprises the following chemical components in percentage by mass: c:0.83%, si:0.23%, mn:0.67%, P:0.013%, S:0.014%, the balance of Fe and unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, online molten salt phase change regulation and control, roller way slow cooling and coil collecting, and specifically comprises the following steps of:

The controlled rolling is used for heating a steel billet with the specification of 180mm multiplied by 180mm through a heating furnace to obtain a high-temperature steel billet with rolling plasticity, selecting a lower heating furnace temperature and a shorter in-furnace time, controlling the decarburization degree of the high-carbon steel billet, rolling the steel billet discharged from the heating furnace into a wire rod with the diameter of 10mm through a rolling line, reducing the abrasion to the rolling line by using a higher rolling temperature, improving the rolling speed, and selecting a larger rolling reduction to create favorable conditions for refining tissues and controlling net carbide, and is characterized in that: the temperature of the heating furnace is controlled to 1085 ℃, the oxygen content in the furnace is less than or equal to 1.2% in 3.5 hours, the initial rolling temperature is 1065 ℃, the initial rolling reduction is 22%, the final rolling temperature is 980 ℃, and the final rolling reduction is 36%; the wire rod that the wire rod process of laying wire is used for going out the pass line is the wire rod through the wire rod mechanism of laying wire, and the wire rod spreads on the roll table and carries along the roll table, selects for use higher wire temperature, does benefit to control wire rod secondary net carbon and separates out and form great supercooling degree, promotes bainite and sorbite nucleation, and is specific: the laying temperature was controlled to 950 ℃.

The online molten salt phase change regulation and control adopts two salt baths with built-in molten salt, wire rods after spinning are conveyed through a first salt bath of the salt, namely first-stage molten salt, are quickly cooled to the molten salt temperature, lower isothermal temperature is selected, the wire rods are cooled at a cooling speed of 44 ℃/s, the wire rods quickly pass through a secondary network carbon precipitation zone, transformation of part of high-temperature austenite into bainite and sorbite tissues is promoted, so that the strength and plasticity of a matrix are ensured, the wire rods passing through the first-stage molten salt are conveyed through a second salt bath, namely last-stage molten salt, are heated to the pearlite zone to be isothermal, transformation of unconverted high-temperature austenite into pearlite is promoted, tempering softening of bainite, sorbite and pearlite is promoted, and the following steps are realized: the molten salt temperature of the first-stage molten salt is 450 ℃, the treatment time is 5s, the circulating amount of the molten salt is 350t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃; the molten salt temperature of the final molten salt is 472 ℃, the treatment time is 400s, the circulating amount of the molten salt is 350t/h, and the temperature rise of the molten salt is less than or equal to 5 ℃.

The roller way slow cooling procedure adopts to close the heat preservation cover to form a heat preservation channel structure, the wire rods coming out of the second salt bath groove are conveyed by the heat preservation roller way to enter the heat preservation cover, the wire rods are subjected to slow cooling treatment, and further softening of wire rod tissues is promoted, and the roller way slow cooling procedure is specific: controlling the wire rod to be cooled to 284 ℃ at a cooling speed of 0.5 ℃/s, and then collecting and coiling; the coil collecting procedure is used for collecting and coiling the wire rods into coils through the coil collecting drum, and obtaining cold heading steel wire rod finished products after packaging and warehousing, and a metallographic structure diagram of the cold heading steel wire rod finished products is shown in figure 2.

Comparative example 4:

A manufacturing method of wire rods is different from that of the embodiment 2 in that the temperature of a heating furnace is controlled to 1040 ℃, the initial rolling temperature is 1020 ℃, the final rolling temperature is 940 ℃, the wire-throwing temperature is 920 ℃, the wire rods after wire throwing are conveyed through a first section of molten salt through a roller way, the wire rods are cooled at a cooling speed of 39 ℃/s, and wire rod finished products are obtained after wire drawing.

Comparative example 5:

the manufacturing method of the wire rod is different from the manufacturing method of the embodiment 2 in that the online molten salt phase change regulation adopts a salt bath tank with molten salt, the wire rod after spinning is conveyed through the salt bath tank by a roller way, the wire rod is cooled at a cooling speed of 42 ℃/s, the temperature of the molten salt is 465 ℃, the treatment time is 405s, and a finished wire rod product is obtained after the wire rod is taken off line.

Comparative example 6:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 2 in that the wire rod after spinning is conveyed through a first section of molten salt through a roller way, so that the wire rod is cooled at a cooling speed of 48 ℃/s, the molten salt temperature of the first section of molten salt is 410 ℃, the treatment time is 20s, the circulating amount of the molten salt is 500t/h, and a finished wire rod product is obtained after the wire rod is taken off line.

Example 3:

the invention relates to a preferred embodiment of a manufacturing method of a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt, which comprises the following chemical components in percentage by mass: c:0.81%, si:0.2%, mn:0.58%, P:0.015%, S:0.014%, the balance of Fe and unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, online molten salt phase change regulation and control, roller way slow cooling and coil collecting, and specifically comprises the following steps of:

The controlled rolling is used for heating a steel billet with the specification of 180mm multiplied by 180mm through a heating furnace to obtain a high-temperature steel billet with rolling plasticity, selecting a lower heating furnace temperature and a shorter in-furnace time, controlling the decarburization degree of the high-carbon steel billet, rolling the steel billet discharged from the heating furnace into a wire rod with the diameter of 5.5mm through a rolling line, reducing the abrasion to the rolling line by using a higher rolling temperature, improving the rolling speed, and selecting a larger rolling reduction to create favorable conditions for refining tissues and controlling net carbide, and is characterized in that: controlling the temperature of the heating furnace to 1050 ℃, the oxygen content in the furnace to be less than or equal to 1.4% in 2 hours, the initial rolling temperature to 1030 ℃, the initial rolling reduction to 28%, the final rolling temperature to 950 ℃ and the final rolling reduction to 42%; the wire rod that the wire rod process of laying wire is used for going out the pass line is the wire rod through the wire rod mechanism of laying wire, and the wire rod spreads on the roll table and carries along the roll table, selects for use higher wire temperature, does benefit to control wire rod secondary net carbon and separates out and form great supercooling degree, promotes bainite and sorbite nucleation, and is specific: the laying temperature was controlled to 935 ℃.

The online molten salt phase change regulation and control adopts two salt baths with built-in molten salt, wire rods after spinning are conveyed through a first salt bath of the salt, namely first-stage molten salt, are quickly cooled to the molten salt temperature, lower isothermal temperature is selected, the wire rods are cooled at a cooling speed of 45 ℃/s, the wire rods quickly pass through a secondary network carbon precipitation zone, transformation of part of high-temperature austenite into bainite and sorbite tissues is promoted, so that the strength and plasticity of a matrix are ensured, the wire rods passing through the first-stage molten salt are conveyed through a second salt bath, namely last-stage molten salt, are heated to the pearlite zone to be isothermal, transformation of unconverted high-temperature austenite into pearlite is promoted, tempering softening of bainite, sorbite and pearlite is promoted, and the following steps are realized: the molten salt temperature of the first-stage molten salt is 420 ℃, the treatment time is 15s, the molten salt circulation amount is 550t/h, and the molten salt temperature rise is less than or equal to 10 ℃; the molten salt temperature of the final molten salt is 520 ℃, the treatment time is 270s, the circulating amount of the molten salt is 550t/h, and the temperature rise of the molten salt is less than or equal to 5 ℃.

The roller way slow cooling procedure adopts to close the heat preservation cover to form a heat preservation channel structure, the wire rods coming out of the second salt bath groove are conveyed by the heat preservation roller way to enter the heat preservation cover, the wire rods are subjected to slow cooling treatment, and further softening of wire rod tissues is promoted, and the roller way slow cooling procedure is specific: controlling the wire rod to be cooled to 295 ℃ at a cooling speed of 0.8 ℃/s, and then collecting and coiling; the coil collecting procedure is used for collecting and coiling the wire rods into coils through the coil collecting drum, and obtaining cold heading steel wire rod finished products after packaging and warehousing, and a metallographic structure diagram of the cold heading steel wire rod finished products is shown in figure 3.

Comparative example 7:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 3 in that the molten salt temperature of the final molten salt is 455 ℃, and the finished wire rod product is obtained after the wire rod is taken off line.

Comparative example 8:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 3 in that the molten salt temperature of the final molten salt is 535 ℃, the treatment time is 420s, the circulating amount of the molten salt is 600t/h, and the finished wire rod product is obtained after the wire rod is taken off line.

Comparative example 9:

The manufacturing method of the wire rod is different from the manufacturing method of the embodiment 3 in that the processing time of the final molten salt is 165s, and the finished wire rod product is obtained after off-line.

Example 4:

The invention relates to a preferred embodiment of a manufacturing method of a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt, which comprises the following chemical components in percentage by mass: c:0.79%, si:0.25%, mn:0.72%, P:0.014%, S:0.015% of Fe and the balance of unavoidable impurities; the manufacturing method comprises the following steps of rolling control, wire laying, online molten salt phase change regulation and control, roller way slow cooling and coil collecting, and specifically comprises the following steps of:

The controlled rolling is used for heating a steel billet with the specification of 180mm multiplied by 180mm through a heating furnace to obtain a high-temperature steel billet with rolling plasticity, selecting a lower heating furnace temperature and a shorter in-furnace time, controlling the decarburization degree of the high-carbon steel billet, rolling the steel billet discharged from the heating furnace into a wire rod with the diameter of 12mm through a rolling line, reducing the abrasion to the rolling line by using a higher rolling temperature, improving the rolling speed, and selecting a larger rolling reduction to create favorable conditions for refining tissues and controlling net carbide, and is characterized in that: controlling the temperature of the heating furnace to 1100 ℃, the oxygen content in the furnace to be less than or equal to 1.4% in 4 hours, the initial rolling temperature to be 1075 ℃, the initial rolling reduction to be 20%, the final rolling temperature to be 990 ℃ and the final rolling reduction to be 32%; the wire rod that the wire rod process of laying wire is used for going out the pass line is the wire rod through the wire rod mechanism of laying wire, and the wire rod spreads on the roll table and carries along the roll table, selects for use higher wire temperature, does benefit to control wire rod secondary net carbon and separates out and form great supercooling degree, promotes bainite and sorbite nucleation, and is specific: the laying temperature was controlled to 965 ℃.

The online molten salt phase change regulation and control adopts two salt baths with built-in molten salt, wire rods after spinning are conveyed through a first salt bath of the salt, namely first-stage molten salt, are quickly cooled to the molten salt temperature, lower isothermal temperature is selected, the wire rods are cooled at a cooling speed of 47 ℃/s, the wire rods quickly pass through a secondary network carbon precipitation zone, transformation of part of high-temperature austenite into bainite and sorbite tissues is promoted, so that the strength and plasticity of a matrix are ensured, the wire rods passing through the first-stage molten salt are conveyed through a second salt bath, namely last-stage molten salt, are heated to the pearlite zone to be isothermal, transformation of unconverted high-temperature austenite into pearlite is promoted, tempering softening of bainite, sorbite and pearlite is promoted, and the following steps are realized: the molten salt temperature of the first-stage molten salt is 430 ℃, the treatment time is 12s, the circulating amount of the molten salt is 500t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃; the molten salt temperature of the final molten salt is 460 ℃, the treatment time is 200s, the circulating amount of the molten salt is 380t/h, and the temperature rise of the molten salt is less than or equal to 5 ℃.

The roller way slow cooling procedure adopts to close the heat preservation cover to form a heat preservation channel structure, the wire rods coming out of the second salt bath groove are conveyed by the heat preservation roller way to enter the heat preservation cover, hot gas generated in the online molten salt phase change regulation and control process is input into the heat preservation cover, the wire rods are subjected to slow cooling treatment, and further softening of wire rod tissues is promoted, and the method is characterized in that: controlling the wire rod to be cooled to 295 ℃ at a cooling speed of 0.35 ℃/s, and then collecting and coiling; the coil collecting procedure is used for collecting and winding the coil rod into coils through the coil collecting drum, and obtaining a cold heading steel coil rod finished product after packaging and warehousing.

The wire rods obtained in examples 2 to 4 and comparative examples 4 to 10 were subjected to structure and performance detection: tensile testing was performed using the metal material tensile test section 1 of GB-T228.1-2021: room temperature test method, to obtain tensile strength and reduction of area, and to perform tissue detection according to the metal microstructure detection method of GB/T13298 standard, the comparison results obtained are shown in table 2 below:

TABLE 2 comparison of the composition of the steel wire rods with the properties of the wire rod structure of the manufacturing method

As can be seen from the results of examples 1-4, the cold heading steel wire rod disclosed by the invention adopts a high-carbon component design with high C-Si-Mn, can save Cr, mo, ti, V, nb, B, ni and other alloy components, effectively reduces the material cost, has a microstructure comprising most tempered pearlite, a small part tempered sorbite, a small amount of tempered bainite and fused pearlite and does not comprise martensite, realizes high-strength and high-plasticity matching, achieves the tensile strength of 1305-1345 MPa and the reduction of area of 44% -49%, and can be suitable for manufacturing 14.9-grade bolts in a high-efficiency green manner under a non-tempering process, and reduces the carbon emission and the bolt cost.

As can be seen from the comparison result of the embodiment 2 and the comparative example 4, the wire rod is extremely rapidly cooled by selecting a higher spinning temperature and utilizing the high heat exchange capability of the molten salt, thereby effectively avoiding the inhibition of the netlike carbide and providing favorable conditions for promoting the phase change of the sorbite with finer lamellar spacing; as can be seen from the comparison result of the embodiment 2 and the comparative example 5, the use of one salt bath treatment is equivalent to the improvement of the molten salt temperature of the first-stage molten salt, the higher the molten salt temperature is, the shorter the time is, the tempered bainite proportion is reduced, the spacing between sorbite sheets is increased, the strength of the wire rod is reduced, the plasticity is increased, but the higher the molten salt temperature is, the shorter the time is, the transformation to bainite is unfavorable, and the strength of the matrix is obviously lost; as can be seen from the comparison result between the example 2 and the comparative example 6, the lower the molten salt temperature of the first molten salt and the longer the treatment time, the more austenite is transformed into bainite, the finer the inter-lamellar spacing of sorbite is, the tempered bainite is increased in proportion, the strength of the wire rod is increased, and the plasticity is lowered, but the lower the molten salt temperature of the first molten salt and the longer the time are, the tempered bainite is excessively high in proportion, which is disadvantageous to the plasticity of the wire rod.

As can be seen from the comparison result of the example 3 and the comparative example 7, the lower the molten salt temperature of the final molten salt is, the finer the interlayer spacing of the formed pearlite is, but the lower the molten salt temperature is, the higher the sorbite ratio is caused by supercooling degree, and the plasticity of the wire rod is affected; as can be seen from the comparison result of the example 3 and the comparative example 8, the higher the molten salt temperature is, the thicker the interlayer spacing of the formed pearlite is, and more thermal power can be provided for tempering and softening bainite, sorbite and pearlite tissues, so that the strength of the wire rod is reduced, the plasticity is increased, but the too high molten salt temperature easily causes the excessive coarseness of the lamellar, excessive tempering and softening, and the strength of the wire rod is not good; as can be seen from the comparison of example 3 and comparative example 9, the shorter the treatment time, the worse the tempering softening effect, and the lower the wire rod plasticity; as can be seen from the comparison of the results of example 4 and comparative example 10, after the roller table is slowly cooled, the tempered bainite, sorbite and pearlite structure can be promoted to be softened in one step by using the high temperature state of the wire rod after the wire rod is molten salt.

The above list of detailed descriptions is only specific to practical embodiments of the present invention, and they are not intended to limit the scope of the present invention, and all equivalent embodiments or modifications that do not depart from the spirit of the present invention should be included in the scope of the present invention.

Claims (8)

1. The high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolt is characterized by comprising the following chemical components in percentage by mass: c:0.79% -0.83%, si:0.15% -0.25%, mn: 0.58-0.72%, P less than or equal to 0.015%, S less than or equal to 0.015%, and the balance of Fe and unavoidable impurities; the microstructure comprises 50-60% by volume of tempered pearlite, 5-10% by volume of tempered bainite, 25-35% by volume of tempered sorbite and the balance of mixed structure formed by fused pearlite, and the manufacturing method comprises the following steps:

And (3) rolling the wire rod according to the chemical components of the high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolt, carrying out online molten salt phase change regulation and control after the wire rod is wire rod according to the wire-laying temperature of more than or equal to 935 ℃, so that the wire rod is firstly subjected to first-stage molten salt, the wire rod is cooled to a bainite and sorbite phase region at a cooling speed of more than or equal to 44 ℃/s to carry out tissue phase change, promoting transformation of part of austenite to bainite and sorbite, heating the wire rod to a pearlite phase region to isothermal temperature through a last-stage molten salt, promoting transformation of non-transformed austenite to pearlite, simultaneously promoting tempering and softening of bainite, sorbite and pearlite, and finally carrying out slow cooling through a roller way, so as to prepare the cold heading steel wire rod of a mixed structure consisting of tempered pearlite, tempered bainite, tempered sorbite and fused pearlite, wherein the molten salt temperature of the first-stage molten salt is 420-450 ℃, the molten salt temperature of the last-stage molten salt is 5-15 s, the molten salt temperature of the last-stage molten salt is 460-460 ℃ and the treatment time is 200-520 s, the slow cooling of the roller way adopts a closing cover, and the slow cooling speed of controlling the wire rod to be cooled to below 300 ℃ at a cooling speed of 0.35-0.80 ℃.

2. The high carbon cold heading steel wire rod for 14.9 grade non-quenched and tempered bolts according to claim 1, wherein the lamellar spacing of the tempered pearlite is 155-185 nm, the lamellar spacing of the tempered sorbite is 75-105 nm, and the net carbide grade of the cold heading steel wire rod is 0 grade.

3. The high carbon cold heading steel wire rod for 14.9-grade non-quenched and tempered bolts according to claim 1, wherein the diameter of the cold heading steel wire rod is 5.5-12.0 mm, the tensile strength is 1305-1345 mpa, and the area reduction is 44% -49%.

4. The manufacturing method of the high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolt is characterized by comprising the following steps of: the method comprises the steps of according to claim 1, carrying out chemical composition controlled rolling on a 14.9-grade non-quenched and tempered steel wire rod for producing wires, carrying out online molten salt phase change regulation and control after the wires are spun into wire rods according to the spinning temperature of more than or equal to 935 ℃, enabling the wire rods to firstly pass through first-stage molten salt, cooling the wire rods to a bainite and sorbite phase region at a cooling speed of more than or equal to 44 ℃/s for carrying out tissue phase change, promoting transformation of part of austenite into bainite and sorbite, heating the wire rods to isothermal in a pearlite phase region through last-stage molten salt, promoting transformation of non-transformed austenite into pearlite, simultaneously promoting tempering and softening of bainite, sorbite and pearlite, and finally carrying out roller way slow cooling, wherein the microstructure comprises 50% -60% tempered pearlite, 5% -10% tempered bainite, 25% -35% tempered sorbite by volume, and the balance of cold-headed steel wire rod with a mixed structure formed by fusing the pearlite; the molten salt temperature of the first-stage molten salt is 420-450 ℃, the treatment time is 5-15 s, the molten salt temperature of the last-stage molten salt is 460-520 ℃, the treatment time is 200-400 s, the roller way slow cooling adopts a closed heat preservation cover, and the wire rod is controlled to be cooled to below 300 ℃ at a cooling speed of 0.35-0.80 ℃/s.

5. The method for manufacturing a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt according to claim 4, wherein the temperature of a heating furnace is controlled to be 1050-1100 ℃ before rolling, the time of the heating furnace is 2-4 h, and the oxygen content in the furnace is less than or equal to 1.4%.

6. The method for manufacturing a high-carbon cold heading steel wire rod for a 14.9-grade non-quenched and tempered bolt according to claim 4, wherein during the controlled rolling, the initial rolling temperature is controlled to 1030-1075 ℃, the initial rolling reduction is controlled to 20% -28%, the final rolling temperature is controlled to be more than or equal to 950 ℃ and the final rolling reduction is controlled to 32% -42%.

7. The method for manufacturing the high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolts, which is characterized in that the molten salt circulation amount of the first-stage molten salt is 350-550 t/h, and the temperature rise of the molten salt is less than or equal to 10 ℃.

8. The method for manufacturing the high-carbon cold heading steel wire rod for the 14.9-grade non-quenched and tempered bolts, which is characterized in that the molten salt circulation amount of the final molten salt is 350-550 t/h, and the temperature rise of the molten salt is less than or equal to 5 ℃.