CN117926136B - 1550 MPa-grade hot-rolled wire rod free of Cr element at low cost and production method thereof - Google Patents

Abstract

The invention relates to a low-cost 1550 MPa-grade hot rolled wire rod without Cr element and a production method thereof, wherein the design of C-Si-Mn-V and without Cr component is adopted, the wire rod after high-line rolling and spinning is subjected to online molten salt cooling treatment, the molten salt temperature is 550-580 ℃, the isothermal time is 200-350 s, the wire rod cooling speed is more than or equal to 35 ℃/s, the hot rolled wire rod which takes sorbite tissues as a main part and contains/does not contain a small amount of ferrite and contains/does not contain residual austenite to form a mixed structure is obtained, the spacing between sorbite tissue sheets is 75-90 nm, the mesh carbide can reach 0 grade, the excellent hydrogen embrittlement resistance and strong plasticity matching are realized, the tensile strength is more than or equal to 1570MPa, the section shrinkage rate is more than or equal to 43%, the development of the hot rolled wire rod to the ultrahigh strengthening, the light weight and the low cost direction is realized, and the hot rolled wire rod is used for manufacturing the steel for ultrahigh-strength bridge cables and other application fields.

Description

1550 MPa-grade hot-rolled wire rod free of Cr element at low cost and production method thereof

Technical Field

The invention belongs to the technical field of alloy materials, and particularly relates to a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost and a production method thereof.

Background

At present, hot rolled wire rods at 1400MPa level and below are entry materials of various steel plants, wire rod productivity is far beyond market demands, product competition caused by low threshold is changed into price competition, and in the field of hot rolled wire rods for part of bridge cables, 10% of the hot rolled wire rods for the bridge cables can be reduced when the strength level is improved by 100MPa, so that hot rolled wire rod products with higher strength level and lower cost are developed, and the hot rolled wire rod has important significance for improving the competitiveness of the steel plants and realizing the light weight and low cost targets of end users.

The main causes of insufficient strength grade, insufficient plasticity and higher cost of the existing hot rolled wire rod include:

On the one hand, microalloying is a wire rod strengthening method widely applied, high-carbon steel utilizes the precipitation strengthening and solid solution strengthening of microalloying elements such as V, al, cr, si, ti and the like to achieve the purposes of refining a pearlite lamellar structure and improving strength and plasticity, for example, an ultrahigh-strength vanadium-titanium composite microalloying high-carbon steel wire rod disclosed in a patent CN102352469B and a preparation method thereof adopt a C-Mn-Si-Cr-V-Ti component and a Steyr air cooling line control cooling process design, the product performance tensile strength Rm is 1330-1410 MPa, the area shrinkage Z is more than or equal to 32%, and therefore, the further improvement of the strength of the hot-rolled wire rod can be prevented in an alloy content reduction mode aiming at cost reduction.

On the other hand, the hot rolled wire rods of 1400MPa grade and below at home and abroad usually adopt a Steyr air cooling line to carry out final structure and performance regulation and control, but because of the limitation of cooling capacity of the Steyr air cooling line, the cooling speed of the wire rods is 3-12 ℃/s, for the hot rolled wire rods for high-carbon large-specification bridge cables, the carbon content of the steel components is more than or equal to 0.77%, the diameter of the hot rolled wire rods is more than or equal to 12mm, the risk of precipitation of network carbide is increased by high carbon content, the cooling speed of air cooling is slower and longer, the high Wen Pantiao can pass through a network carbide precipitation sensitive zone of about 700-800 ℃ in ten seconds to twenty seconds, so that the high-carbon content steel is easy to precipitate harmful network cementite along the grain boundary, the strength of the steel is degraded, the plastic toughness of the material is greatly reduced, meanwhile, the uneven air distribution of a fan is also easy to cause abnormal local structure and large performance fluctuation, the space between sheets is difficult to be controlled by improper cooling, the strengthening effect of microalloy carbide is played, and the strength level is difficult to improve.

Although the cooling strength of water bath cooling is higher than that of an air cooling line, for example, in the method for producing the wire rod and the steel strand for the stress corrosion resistant ultra-high strength steel strand disclosed in the patent CN114369760B, the C-Mn-Si-Cr-V-B component and the on-line water bath process design are adopted, so that the tensile strength of the hot rolled wire rod is 1360-1450 MPa and the reduction of area is 20-25%, a large number of bubbles are formed on the surface of the wire rod after water heating when the high-temperature wire rod after rolling and spinning passes through the water bath, the heat exchange is affected, the cooling speed is at the lowest possible to be up to 100 ℃/s, the highest possible to be very uneven, the same-circle mechanical property of the wire rod is poor, the yield is low, and the strength grade and the plasticity of the hot rolled wire rod are still insufficient; and although the prior art adopts salt bath heat treatment to improve the mechanical property and uniformity of the mechanical property of the wire rod in the same circle, for example, the high-carbon steel wire rod disclosed in patent CN113088798A and the production method thereof adopt C-Mn-Si-Cr-Nb/V/Ti components, molten steel smelting, continuous casting, cogging, high-line rolling, temperature control cooling and salt bath heat treatment processes, the tensile strength can reach 1662MPa, but the reduction of area is less than or equal to 33 percent, the plasticity is insufficient, and meanwhile, the processes adopt reheating and salt bath isothermal processes after the stelmor air cooling line temperature control cooling, and the process belongs to offline salt bath, and has more processes and higher cost.

Disclosure of Invention

The invention aims to solve at least one of the technical problems to a certain extent, and provides a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost and a production method thereof, which are used for improving the current situations of insufficient strength grade and insufficient plasticity of the existing hot-rolled wire rod with low cost, and realizing the development of the hot-rolled wire rod towards the directions of ultrahigh reinforcement, light weight and low cost so as to meet the use demands of markets.

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

A production method of a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost comprises the following chemical components in percentage by mass: c:0.85% -1.05%, si:0.70 to 1.15 percent of Mn:0.40% -0.80%, al:0.20 to 0.40 percent, P is less than or equal to 0.020 percent, S is less than or equal to 0.008 percent, and V: 0.030-0.070 percent, and the balance of Fe and unavoidable impurities; the production method comprises the following steps: the wire rod subjected to high-speed wire rolling and wire spinning is subjected to online molten salt cooling treatment, the molten salt temperature of the online molten salt cooling treatment is 550-580 ℃, the isothermal time is 200-350 s, and the wire rod cooling speed is more than or equal to 35 ℃/s.

The design basis of the chemical components and the mass percentages of the hot rolled wire rod comprises:

(1) Carbon: the element C is used for forming enough carbide strengthening phase and microstructure mainly comprising sorbite in the controlled cooling process of the wire rod after high-temperature rolling, so that the strength of the wire rod is greatly improved, the strength grade of steel is ensured, but the control difficulty of component segregation in the alloy solidification process is aggravated due to the excessive carbon content, and the toughness and uniformity of the material are deteriorated, so that the mass percentage of C is controlled to be 0.85-1.05%.

(2) Silicon: si is often added into steel as a deoxidizer in the smelting process, and simultaneously plays a solid solution strengthening role in the steel, so that the ferrite forming range can be enlarged, the cementite formation and coarsening are inhibited, the loss of the strength of the steel wire is reduced, but the material is embrittled due to the excessively high silicon content, and therefore, the mass percentage of Si is controlled to be 0.70-1.15%.

(3) Manganese: mn can enlarge an austenite phase region, obviously improves the hardenability of the steel by improving the thermodynamic stability of austenite, and obtains stable residual austenite and reverse transformation austenite, meanwhile, mn element strongly delays pearlite and bainite transformation, which is beneficial to regulating and controlling the matrix structure content, in addition, mn element can also improve the strength of a wire rod by solid solution strengthening effect so as to ensure the strength grade of the steel, but the excessive manganese content easily increases the overheat sensitivity of the steel, promotes the segregation of the residual element and increases the control difficulty of a controlled cooling structure, so that the mass percentage of Mn is controlled to be 0.40-0.80%.

(4) Aluminum: al has similar effect to Si element, and the most effective deoxidizing element in the steelmaking process plays a solid solution strengthening effect in steel, can enlarge the ferrite formation range, can inhibit cementite formation and coarsening, and is beneficial to improving the plasticity of the wire rod, but the high temperature strength and toughness of the steel are reduced due to the excessively high Al content, so that the mass percentage of the Al is controlled to be 0.20-0.40%.

(5) Alum: the V microalloy element mainly has the effects of forming VN refined grains in a high-temperature rolling region, forming alloy carbide VC in a medium-temperature isothermal region by diffusion precipitation, enhancing a matrix, and simultaneously forming a deep hydrogen trap to reduce the hydrogen-induced crack resistance sensitivity of the wire rod, so that the product has excellent hydrogen embrittlement resistance, but the cost of the steel grade is increased due to the excessively high V content, so that the mass percentage of V is controlled to be 0.030-0.070%.

(6) Phosphorus, sulfur: the P can cause grain embrittlement, so that the plasticity and toughness of the steel are obviously reduced, and the S can increase harmful impurities of steel types to cause hot shortness, so that the mass percent of the P is less than or equal to 0.020%, and the mass percent of the S is less than or equal to 0.008%.

The Cr element can strengthen the matrix, but can simultaneously lead to the improvement of abnormal tissue risks such as wire rod martensite and the like and the reduction of plasticity, on the basis of adopting the design of C-Si-Mn-V and containing no Cr component, the production method of the hot rolled wire rod adopts high-line rolling and wire spinning to directly carry out line molten salt cooling treatment, and the temperature, isothermal time and cooling speed are controlled, so that the wire rod can quickly pass through a harmful reticular carbide precipitation temperature interval in steel at a higher cooling speed by quickly reducing the heat exchange between the wire rod and the molten salt to the molten salt temperature, and the reticular carbide precipitation temperature interval is only 1-2 seconds, thereby effectively avoiding the risk of reticular carbide precipitation caused by high carbon content and avoiding the degradation of the reticular carbide on the steel strength and particularly plasticity.

Meanwhile, the isothermal temperature of the steel grade is increased due to the fact that Cr is not contained, the molten salt temperature is increased, the isothermal time is prolonged, dislocation density of a wire rod tissue is reduced, tissue and mechanical uniformity are improved, and therefore the plasticity of a hot rolled wire rod can be improved; the temperature of the molten salt is reduced, the isothermal time is shortened, the dislocation density of the wire rod tissue is increased, the interaction between the dislocations is enhanced, the resistance to plastic deformation is formed, and the strength of the hot rolled wire rod is improved; however, the temperature of molten salt is too high, the isothermal time is too short, the residual austenite is increased, the strength of the wire rod is influenced, the ferrite content is increased due to the too low temperature of the molten salt and the too long isothermal time, and the plasticity of the wire rod is influenced.

Preferably, the production method comprises the steps of high-line rolling by using a continuous casting billet, wherein the continuous casting billet is obtained by molten steel through a continuous casting process, the C segregation index of the continuous casting billet is less than or equal to 1.05, the superheat degree of a continuous casting tundish in the continuous casting process is controlled to be 15-35 ℃, the fluctuation of the molten steel surface of a crystallizer is controlled to be within +/-5 mm, the continuous casting billet can be cast at a stable speed, the electromagnetic stirring current at the head end is 250-450A, and the frequency is 1-5 Hz; the tail end electromagnetic stirring current is 400-600A, the frequency is 8-11 Hz, the isometric crystal interval in the center of the continuous casting billet is increased by reducing the superheat degree, and the head end and tail end electromagnetic stirring technology is used, so that impurities in molten steel fully float up to obtain the continuous casting billet with good internal quality, the influence of high carbon content on the center segregation of the continuous casting billet is further controlled, the problem that after-rolling segregation causes uneven structure or uneven carbide distribution due to cooling after-rolling is avoided, the performance of a wire rod is influenced, and the mechanical property level difference is increased.

Preferably, the scale on the surface of the continuous casting billet is removed by high-pressure descaling water before blooming, the pressure of the high-pressure descaling water is more than or equal to 18MPa, the continuous casting billet is oxidized to form the scale by tapping the red steel surface of a heating furnace in the process of blooming, and the scale on the surface of the red steel is removed as once as possible by the high-pressure water before blooming, so that the influence of the scale pressed into the billet on the surface quality and performance is avoided.

Preferably, the finish rolling temperature of the high-line rolling is lower than 880 ℃, the finish rolling reduction is 6-10%, and because online molten salt cooling treatment can rapidly pass through a network carbide precipitation temperature interval, compared with the prior stelmor wire cooling, the high finish rolling temperature is required to reduce the risk of network carbide precipitation.

Preferably, the cooling speed of the wire rod subjected to online molten salt cooling treatment is less than or equal to 55 ℃/s, so that the phenomenon that the plasticity of the wire rod is influenced due to the fact that the ferrite content of the wire rod is increased and the dislocation density is increased due to the fact that the cooling speed is too high and the isothermal time is too short can be avoided.

Preferably, the molten salt circulation speed of the online molten salt cooling treatment is 100-200 t/h, the molten salt temperature precision is controlled within +/-4 ℃, the temperature rise of the molten salt caused by heat exchange with a high-temperature coil can be reduced by increasing the molten salt circulation speed, the molten salt temperature precision is controlled, the coil is further kept in a proper temperature interval and a sufficient isothermal time, the sorbite content is further improved, and carbide micro-alloy carbide is controlled to be stably dispersed and precipitated to achieve a strengthening effect.

A low-cost 1550 MPa-grade hot-rolled wire rod free of Cr element, which is obtained by the production method of any one of the above low-cost 1550 MPa-grade hot-rolled wire rod free of Cr element.

Preferably, the hot rolled wire rod structure is a mixed structure composed of sorbite structure with the volume percentage being more than or equal to 95%, ferrite or residual austenite, the sorbite rate is high, the wire rod is suitable for drawing, the sheet spacing of the sorbite structure is 75-90 nm, the thinner sheet spacing has higher plasticity, and the larger area reduction rate is reflected.

Preferably, the net carbide of the hot rolled wire rod is < 0.5 grade, and can reach 0 grade.

Preferably, the diameter of the hot rolled wire rod is 12.5-15.0 mm, the tensile strength is more than or equal to 1570MPa, the area shrinkage is more than or equal to 43%, and the same-circle mechanical fluctuation is less than or equal to 22MPa.

According to the application of 1550 MPa-grade hot-rolled wire rods without Cr element at low cost, the ultra-high-strength bridge cable comprises the step of manufacturing the ultra-high-strength bridge cable.

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

(1) According to the invention, a low-cost component design which does not contain Cr and contains V and is added with proper Mn and Si alloy elements is adopted, an online ultra-fast molten salt cooling process design after rolling is combined, harmful net carbide precipitation is effectively avoided at a larger cooling speed by controlling the temperature of molten salt, and the molten salt temperature and isothermal time are controlled to quickly enter a sorbite phase region at a larger supercooling degree, so that tiny uniform sorbite tissues and dispersed microalloy carbide are obtained, the strength is enhanced, the plasticity is greatly improved, and the austenitic structure before refining phase change by low Wen Zhongga deformation energy storage can be further combined with proper low-temperature rolling and online molten salt cooling process design, so that good tissue preparation is provided for subsequent wire rod tissue transformation, the carbide microalloy carbide is controlled to stably disperse and precipitate to achieve a strengthening effect, and the same-circle mechanical fluctuation is less than or equal to 22MPa.

(2) The invention successfully develops a low-cost 1550 MPa-grade hot-rolled wire rod without Cr element, the microstructure type is a mixed structure which is mainly composed of sorbite tissues, contains/does not contain a very small amount of ferrite and contains/does not contain residual austenite, the spacing between layers of the sorbite tissues is 75-90 nm, meanwhile, the product has excellent hydrogen embrittlement resistance and strong plasticity matching, can achieve the tensile strength of more than or equal to 1570MPa, the reduction of area of more than or equal to 43 percent, can be used for manufacturing steel for ultra-high-strength bridge cables and other application fields to reduce the cost, solves the current situations of insufficient strength grade and plastic deficiency of the existing hot-rolled wire rod, and can not realize the development of the hot-rolled wire rod to the ultra-high strengthening, light-weight and low-cost directions so as to meet the market use requirements, and has good market application prospects.

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 a hot rolled wire rod according to example 1 of the present invention;

FIG. 2 is a metallographic structure diagram of a hot rolled wire rod of comparative example 1 of the present invention;

FIG. 3 is a metallographic structure diagram of a hot rolled wire rod according to example 2 of the present invention;

FIG. 4 is a metallographic structure diagram of a hot rolled wire rod according to comparative example 2 of the present invention;

FIG. 5 is a metallographic structure diagram of a hot rolled wire rod according to example 3 of the present invention;

FIG. 6 is a metallographic structure diagram of a hot rolled wire rod of comparative example 3 of the present invention;

FIG. 7 is a metallographic structure diagram of a hot rolled wire rod according to example 4 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 illustrate it and to enable those skilled in the art to practice it, without any limitation to its scope, which is defined solely by the claims appended hereto.

Example 1:

The invention relates to a production method of a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost, which comprises the following chemical components in percentage by mass: c:0.85%, si:1.15%, mn:0.75%, al:0.36%, P:0.018%, S:0.008%, V:0.065%, the balance being Fe and unavoidable impurities; the production method comprises the following steps of continuous casting, heating furnace, high-line rolling, wire laying and online molten salt cooling treatment, and specifically comprises the following steps of:

The continuous casting process is used for preparing high-temperature molten steel obtained through converter smelting, LF refining and VD vacuum degassing into a continuous casting blank through a continuous casting machine, wherein the superheat degree of a continuous casting tundish is controlled to be 15-20 ℃, the fluctuation of the molten steel surface of a crystallizer is controlled to be within +/-5 mm, the electromagnetic stirring current at the head end is 300A, and the frequency is 3Hz; the electromagnetic stirring current at the tail end is 500A, the frequency is 9Hz, the C segregation index of the continuous casting billet is 1.01, and the specification is 160mm multiplied by 160mm square billets.

The heating furnace procedure is used for heating the continuous casting billet through a heating furnace to reach the high Wen Gonggang of rolling plasticity; the high-line rolling procedure is used for rolling high Wen Gonggang into a wire rod through hot rolling of a rolling line, oxidizing a continuous casting billet by a red steel surface discharged from a heating furnace to form an oxide skin, removing the oxide skin on the surface of the continuous casting billet by using high-pressure descaling water before initial rolling, wherein the high-pressure descaling water pressure is 20MPa, the finishing rolling temperature is 875 ℃, and the finishing rolling reduction is 6.5%, so that the grain size of the wire rod is less than or equal to 15 mu m, and refining the grain size by using low Wen Zhongga deformation energy storage, thereby providing good tissue preparation for the subsequent wire rod tissue transformation; the wire rod of rolling line off-line is used for passing through the wire rod machine of laying head to be wire rod, and the wire rod specification is 13mm in diameter with the wire temperature of laying head being 892 ℃.

The online molten salt cooling treatment adopts a salt bath tank with molten salt arranged therein, when the wire rods after spinning pass through the salt bath tank, heat exchange is carried out between the wire rods and the molten salt to quickly reduce the temperature of the molten salt to 554 ℃, the circulating speed of the molten salt is 150t/h, the precision of the molten salt temperature is controlled to be within +/-4 ℃, the isothermal time is 243s, the cooling speed of the wire rods is 52 ℃/s, the material is enabled to obtain a temperature interval for quickly passing through harmful reticular carbide precipitation in steel at a higher cooling speed, micro-alloy carbides which are distributed in tiny and uniform sorbite tissue phase transformation and dispersion are quickly entered into a sorbite phase area at a lower isothermal temperature, the wire rods discharged from the salt bath tank are cleaned, air-cooled to 425 ℃ along the rails for collecting and rolling, and then the wire rods are transported along the rails for finishing and packaging and warehousing, so as to obtain coiled products of hot rolled wire rods, and the metallographic structures of the hot rolled wire rods are shown in figure 1.

Comparative example 1:

A method of producing a hot rolled wire rod, which differs from example 1 in that: the production method comprises the steps of continuous casting, heating furnace, high-line rolling, wire spinning and stelmor air cooling, wherein the finish rolling temperature of the high-line rolling is below 930 ℃, the finish rolling reduction is 5%, the wire spinning temperature is 950 ℃, the stelmor air cooling is that the wire rods subjected to wire spinning pass through stelmor air cooling lines, the stelmor line heat preservation covers are all opened, the roller speed is 1.2m/s, the first fans of 1-13 are all opened by 100%, the total air quantity of each fan is 26 ten thousand m ³/h, the average cooling speed of the wire rods is 10.5 ℃/s, and the hot rolled wire rod is obtained, and the metallographic structure of the hot rolled wire rod is shown in figure 2.

Example 2:

the invention relates to a production method of a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost, which comprises the following chemical components in percentage by mass: c:0.96%, si:0.7%, mn:0.8%, al:0.22%, P:0.018%, S:0.007%, V:0.046% and the balance of Fe and unavoidable impurities; the production method comprises the following steps of continuous casting, heating furnace, high-line rolling, wire laying and online molten salt cooling treatment, and specifically comprises the following steps of:

the continuous casting process is used for preparing high-temperature molten steel obtained through converter smelting, LF refining and VD vacuum degassing into a continuous casting blank through a continuous casting machine, wherein the superheat degree of a continuous casting tundish is controlled to be 20-25 ℃, the fluctuation of the molten steel surface of a crystallizer is controlled to be within +/-5 mm, the head end electromagnetic stirring current is 350A, and the frequency is 2Hz; the electromagnetic stirring current at the tail end is 400A, the frequency is 10Hz, the C segregation index of the continuous casting billet is 0.98, and the specification is 160mm multiplied by 160mm square billets.

The heating furnace procedure is used for heating the continuous casting billet through a heating furnace to reach the high Wen Gonggang of rolling plasticity; the high-line rolling procedure is used for rolling high Wen Gonggang into wires through hot rolling of a rolling line, oxidizing a continuous casting billet by a red steel surface discharged from a heating furnace to form oxide skin, removing the oxide skin on the surface of the continuous casting billet by using high-pressure descaling water before initial rolling, wherein the high-pressure descaling water pressure is 21MPa, the finishing rolling temperature is 870 ℃, and the finishing rolling reduction is 8%, so that the grain size of the wires is less than or equal to 16 mu m, refining the grain size by using low Wen Zhongga deformation energy storage, and providing good tissue preparation for the subsequent wire rod tissue transformation; the wire rod of rolling line off-line is used for passing through the wire rod machine of spinning to be wire rod, and the wire rod specification is 13mm in diameter with the wire rod temperature of 889 ℃.

The online molten salt cooling treatment adopts a salt bath tank with molten salt arranged therein, when the wire rods after spinning pass through the salt bath tank, heat exchange is carried out between the wire rods and the molten salt to quickly reduce the temperature of the molten salt to 565 ℃, the circulating speed of the molten salt is 170t/h, the precision of the molten salt temperature is controlled to be within +/-4 ℃, the isothermal time is 225s, the cooling speed of the wire rods is 50 ℃/s, the material can quickly pass through a harmful reticular carbide precipitation temperature interval in steel at a higher cooling speed, micro-alloy carbides which are distributed in tiny and uniform sorbite structure phase transformation and dispersion are quickly entered into a sorbite phase area at a lower isothermal temperature, the wire rods discharged from the salt bath tank are cleaned, air-cooled to 430 ℃ along a track, and then transported, finished, packaged and put in storage along the line to obtain coiled products of hot rolled wire rods, and the metallographic structures of the hot rolled wire rods are shown in figure 3.

Comparative example 2:

a method of producing a hot rolled wire rod, which differs from example 2 in that: and when the wire rod subjected to spinning passes through a salt bath, heat exchange with molten salt is rapidly reduced to the molten salt temperature, the molten salt temperature is 531 ℃, the isothermal time is 369s, the wire rod cooling speed is 38 ℃/s, and the hot-rolled wire rod is obtained, and the metallographic structure of the hot-rolled wire rod is shown in figure 4.

Example 3:

the invention relates to a production method of a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost, which comprises the following chemical components in percentage by mass: c:1.05%, si:0.86%, mn:0.5%, al:0.4%, P:0.02%, S:0.008%, V:0.03% of Fe and the balance of unavoidable impurities; the production method comprises the following steps of continuous casting, heating furnace, high-line rolling, wire laying and online molten salt cooling treatment, and specifically comprises the following steps of:

The continuous casting process is used for preparing high-temperature molten steel obtained through converter smelting, LF refining and VD vacuum degassing into a continuous casting blank through a continuous casting machine, wherein the superheat degree of a continuous casting tundish is controlled to be 20-30 ℃, the fluctuation of the molten steel surface of a crystallizer is controlled to be within +/-5 mm, the head end electromagnetic stirring current is 400A, and the frequency is 4Hz; the electromagnetic stirring current at the tail end is 400A, the frequency is 8Hz, the C segregation index of the continuous casting billet is 1.02, and the specification is 160mm multiplied by 160mm square billets.

The heating furnace procedure is used for heating the continuous casting billet through a heating furnace to reach the high Wen Gonggang of rolling plasticity; the high-line rolling procedure is used for rolling high Wen Gonggang into wires through hot rolling of a rolling line, oxidizing a continuous casting billet by a red steel surface discharged from a heating furnace to form oxide skin, removing the oxide skin on the surface of the continuous casting billet by using high-pressure descaling water before initial rolling, wherein the high-pressure descaling water pressure is 21MPa, the finishing rolling temperature is 872 ℃, and the finishing rolling reduction is 9%, so that the grain size of the wires is less than or equal to 18 mu m, refining the grain size by using low Wen Zhongga deformation energy storage, and providing good tissue preparation for the subsequent wire rod tissue transformation; the wire rod of rolling line off-line is used for passing through the wire rod machine of spinning to become wire rod, and the wire rod specification is 15mm in diameter with the wire rod temperature of spinning being 890 ℃.

The online molten salt cooling treatment adopts a salt bath tank with molten salt arranged therein, when the wire rods after spinning pass through the salt bath tank, heat exchange is carried out between the wire rods and the molten salt to quickly reduce the temperature of the molten salt, the temperature of the molten salt is 558 ℃, the circulating speed of the molten salt is 180t/h, the precision of the molten salt temperature is controlled within +/-4 ℃, the isothermal time is 341s, the cooling speed of the wire rods is 52 ℃/s, the material can obtain a temperature interval for quickly passing through harmful reticular carbide precipitation in steel at a higher cooling speed, micro-alloy carbides which are distributed in tiny and uniform sorbite structure phase transformation and dispersion are quickly entered into a sorbite phase area at a lower isothermal temperature, the wire rods discharged from the salt bath tank are cleaned, air-cooled to 440 ℃ along a track, and then transported, finished, packaged and put in storage along the track to obtain coiled products of hot rolled wire rods, and the metallographic structures of the hot rolled wire rods are shown in figure 5.

Comparative example 3:

A method of producing a hot rolled wire rod, which differs from example 3 in that: and when passing through a salt bath, the wire rod subjected to spinning is quickly cooled to the molten salt temperature by heat exchange with the molten salt, the molten salt temperature is 595 ℃, the isothermal time is 175s, the wire rod cooling speed is 40 ℃/s, and the hot-rolled wire rod is obtained, and the metallographic structure of the hot-rolled wire rod is shown in figure 6.

Example 4:

the invention relates to a production method of a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost, which comprises the following chemical components in percentage by mass: c:1.05%, si:0.86%, mn:0.5%, al:0.4%, P:0.016%, S:0.006%, V:0.055% of Fe and the balance of unavoidable impurities; the production method comprises the following steps of continuous casting, heating furnace, high-line rolling, wire laying and online molten salt cooling treatment, and specifically comprises the following steps of:

The continuous casting process is used for preparing high-temperature molten steel obtained through converter smelting, LF refining and VD vacuum degassing into a continuous casting blank through a continuous casting machine, wherein the superheat degree of a continuous casting tundish is controlled to be 15-25 ℃, the fluctuation of the molten steel surface of a crystallizer is controlled to be within +/-5 mm, the head end electromagnetic stirring current is 450A, and the frequency is 2Hz; the electromagnetic stirring current at the tail end is 600A, the frequency is 11Hz, the C segregation index of the continuous casting billet is 1.01, and the specification is 160mm multiplied by 160mm square billets.

The heating furnace procedure is used for heating the continuous casting billet through a heating furnace to reach the high Wen Gonggang of rolling plasticity; the high-line rolling procedure is used for rolling high Wen Gonggang into wires through hot rolling of a rolling line, oxidizing a continuous casting billet by a red steel surface discharged from a heating furnace to form oxide skin, removing the oxide skin on the surface of the continuous casting billet by using high-pressure descaling water before initial rolling, wherein the high-pressure descaling water pressure is 22MPa, the finishing rolling temperature is 874 ℃, and the finishing rolling reduction is 7%, so that the grain size of the wires is less than or equal to 17 mu m, refining the grain size by using low Wen Zhongga deformation energy storage, and providing good tissue preparation for the subsequent wire rod tissue transformation; the wire rod of rolling line off-line is used for passing through the wire rod machine of spinning to be wire rod, and the wire rod specification is 15mm in diameter with the wire rod temperature of 887 ℃.

The online molten salt cooling treatment adopts a salt bath tank with molten salt arranged therein, when the wire rods after spinning pass through the salt bath tank, heat exchange is carried out between the wire rods and the molten salt to quickly reduce the temperature of the molten salt, the temperature of the molten salt is 575 ℃, the circulating speed of the molten salt is 120t/h, the precision of the molten salt temperature is controlled to be within +/-4 ℃, the isothermal time is 288s, the cooling speed of the wire rods is 39 ℃/s, the material is enabled to obtain a temperature interval for quickly passing through harmful reticular carbide precipitation in steel at a higher cooling speed, micro-alloy carbides which are distributed in tiny and uniform sorbite tissue phase change and dispersion are quickly entered into a sorbite phase area at a lower isothermal temperature, the wire rods discharged from the salt bath tank are cleaned, air-cooled to 435 ℃ coil collection is carried along a track, the coil is transported along the track to be finished and packaged and put in storage, and the coil finished product of the hot rolled wire rods is obtained, and the metallographic structure of the hot rolled wire rods is shown in a figure 7.

Comparative example 4:

A method of producing a hot rolled wire rod, which differs from example 4 in that: the superheat degree of a continuous casting tundish in the continuous casting process is controlled to be 35-40 ℃, fluctuation of the molten steel surface of a crystallizer is controlled within +/-8 mm, the electromagnetic stirring current at the tail end is 300A, the frequency is 5Hz, and the C segregation index of a continuous casting blank is 1.18, so that a hot-rolled wire rod is obtained.

Comparative example 5:

a method of producing a hot rolled wire rod, which differs from example 4 in that: the final rolling temperature of the high-line rolling process is 910 ℃, the final rolling reduction is 3%, the grain size of the wire rod is less than or equal to 17 mu m, and the wire-spinning temperature is 932 ℃, so that the hot-rolled wire rod is obtained.

The hot rolled wire rods obtained in the above examples and comparative examples were subjected to structure and property detection: the method for testing the same-circle mechanical fluctuation range comprises the following steps: taking 2 coils from the end part of the coil, taking the position of a lap joint area as a base point, equally dividing each coil into 8 sections, respectively taking 1 tensile sample on each section, wherein the extremely poor strength of the tensile sample after tensile test is the same coil difference of the coil, and the tensile test adopts the section 1 of the tensile test of GB-T228.1-2021 metal materials: testing the room temperature test method to obtain tensile strength and area reduction rate; the microstructure detection is carried out according to a metal microstructure detection method of GB/T13298 standard, and the obtained comparison results are shown in the following table 1:

TABLE 1 comparison of the composition of the hot rolled wire rods with the properties of the wire rod structure of the production process

As can be seen from the comparison results of examples 1-4 and the prior art, aiming at the current situations of insufficient strength grade and insufficient plasticity of the existing hot rolled wire rod, the development of the hot rolled wire rod towards the directions of ultrahigh reinforcement, light weight and low cost can not be realized so as to meet the market use requirement, and the invention adopts the low-cost component design which does not contain Cr and contains V and is added with proper Mn and Si alloy elements, can be further combined with the online ultra-fast molten salt cooling process design after rolling, and obviously improves the strength loss of reducing the alloy content aiming at reducing the cost; compared with the on-line water bath cooling treatment, the strength grade and the plasticity of the hot rolled wire rod can be improved, and the fluctuation of the mechanical properties of the same circle is smaller; compared with off-line salt bath heat treatment, the method can achieve plastic promotion, process reduction and energy consumption cost reduction.

As can be seen from the comparison results of examples 1-4 and comparative example 1 and the comparison of the accompanying drawings 1 and 2, the invention can effectively avoid precipitation of harmful reticular carbide at a higher cooling rate, and can quickly enter a sorbite phase region at a higher supercooling degree by controlling the temperature and isothermal time of molten salt, so that tiny uniform sorbite tissues and dispersed microalloy carbide are obtained, the strength is enhanced and the plasticity is greatly improved compared with the conventional controlled rolling and cooling, the microstructure is a mixed tissue mainly comprising sorbite tissues, containing/not containing a very small amount of ferrite and containing/not containing residual austenite, the lamellar spacing of the sorbite tissues is 75-90 nm, meanwhile, the product has excellent hydrogen embrittlement resistance and strong plasticity matching, the coil rod with the diameter of 12.5-15.0 mm can reach the current situation that the tensile strength is more than or equal to 1570MPa, the section shrinkage rate is more than or equal to 43%, the invention can be used for manufacturing steel for ultra-high-strength bridge cables and other application fields, the strength grade of the existing hot rolled coil rod is insufficient, the plasticity is insufficient, the market demand is satisfied, the market demand is low, and the market demand is good, and the market is light.

According to the invention, when the temperature of the molten salt is increased and the isothermal time is prolonged, the dislocation density of the wire rod tissue is reduced, the tissue and mechanical uniformity is increased, and the plasticity of the hot rolled wire rod can be increased; the temperature of the molten salt is reduced, the isothermal time is shortened, the dislocation density of the wire rod tissue is increased, the interaction between the dislocations is enhanced, the resistance to plastic deformation is formed, and the strength of the hot rolled wire rod is improved; as can be seen from the comparison result of the example 2 and the comparative example 2, the excessively low molten salt temperature and excessively long isothermal time adopted by the invention can cause the increase of ferrite content and affect the plasticity of the wire rod; as can be seen from the comparison result of the embodiment 3 and the comparative example 3, the too high temperature and the too short isothermal time of the molten salt adopted by the invention can cause the increase of residual austenite and affect the strength of the wire rod, so that the composition and dislocation density of the wire rod can be prepared by controlling the temperature and the isothermal time of the molten salt, fine and uniform sorbite structure and dispersed microalloy carbide can be obtained, and the microalloy strengthening effect can be fully exerted.

As can be seen from the comparison result of the embodiment 4 and the comparative embodiment 4, the invention can avoid the uneven structure or uneven carbide distribution formed by cooling after rolling caused by segregation after rolling by further controlling the influence of high carbon content on the center segregation of the continuous casting blank, thereby reducing the level difference of mechanical properties; as can be seen from the comparison result of the embodiment 4 and the comparative example 5, the invention further adopts low-temperature finish rolling and large rolling reduction, and the austenite structure before refining phase transformation is effectively utilized by the low Wen Zhongga deformation energy storage through the chemical composition free of Cr+low-temperature rolling and on-line molten salt cooling technology, and the subsequent wire rod structure transformation provides good structure preparation, so that the strength of steel is further improved.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limited thereto; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.

Claims (9)

1. A production method of a 1550 MPa-grade hot-rolled wire rod free of Cr element with low cost is characterized by comprising the following chemical components in percentage by mass: c:0.85% -1.05%, si:0.70 to 1.15 percent of Mn:0.40% -0.80%, al:0.20 to 0.40 percent, P is less than or equal to 0.020 percent, S is less than or equal to 0.008 percent, and V: 0.030-0.070 percent, and the balance of Fe and unavoidable impurities; the production method comprises the following steps: performing online molten salt cooling treatment on the wire rod subjected to high-speed wire rolling and wire spinning, wherein the molten salt temperature of the online molten salt cooling treatment is 554-580 ℃, the isothermal time is 200-350 s, and the wire rod cooling speed is more than or equal to 35 ℃/s; the reduction of area of the hot rolled wire rod is more than or equal to 43%, the same-circle mechanical fluctuation is less than or equal to 22MPa, the hot rolled wire rod tissue is a sorbite tissue with the volume percentage of more than or equal to 95%, a mixed tissue consisting of ferrite and residual austenite is contained or not, and the lamellar spacing of the sorbite tissue is 75-90 nm.

2. The production method of the low-cost 1550 MPa-grade hot-rolled wire rod free of Cr elements, which is characterized by comprising the steps of high-line rolling by using a continuous casting billet, wherein the continuous casting billet is obtained by molten steel through a continuous casting process, the C segregation index of the continuous casting billet is less than or equal to 1.05, the superheat degree of a continuous casting tundish in the continuous casting process is controlled to be 15-35 ℃, the fluctuation of the molten steel surface of a crystallizer is controlled to be within +/-5 mm, the head-end electromagnetic stirring current is 250-450A, and the frequency is 1-5 Hz; the electromagnetic stirring current at the tail end is 400-600A, and the frequency is 8-11 Hz.

3. The method for producing a low-cost 1550 MPa-grade hot-rolled wire rod free of Cr element according to claim 1, wherein the scale on the surface of the continuous casting billet is removed by high-pressure descaling water before the blooming of the high-line rolling, and the pressure of the high-pressure descaling water is not less than 18MPa.

4. The production method of the 1550 MPa-grade hot rolled wire rod free of Cr elements, which is characterized in that the final rolling temperature of high-line rolling is below 880 ℃, the final rolling reduction is 6-10%, and the grain size of the wire rod is less than or equal to 20 mu m.

5. The production method of the 1550 MPa-grade hot rolled wire rod free of Cr elements, which is characterized by 1, wherein the molten salt circulation speed of the online molten salt cooling treatment is 100-200 t/h, and the molten salt temperature precision is controlled within +/-4 ℃.

6. A low-cost 1550 MPa-grade hot-rolled wire rod free of Cr element, characterized in that the hot-rolled wire rod is obtained by the production method of 1550 MPa-grade hot-rolled wire rod free of Cr element according to any one of claims 1 to 5.

7. The low cost 1550MPa grade hot rolled wire rod free of Cr element as claimed in claim 6 wherein the net-like carbide of the hot rolled wire rod is < 0.5 grade.

8. The low-cost 1550 MPa-grade hot-rolled wire rod free of Cr element as claimed in claim 6, wherein the diameter of the hot-rolled wire rod is 12.5-15.0 mm, and the tensile strength is more than or equal to 1570MPa.

9. The use of a low cost, non-Cr element containing 1550MPa grade hot rolled wire rod as claimed in claim 6 comprising for the manufacture of bridge cables.