CN117845137A - Mn-Si-V-Ti-Nb-Cr multi-element alloy hot-rolled wire rod and preparation method thereof - Google Patents

Abstract

The invention relates to an Mn-Si-V-Ti-Nb-Cr multi-element alloying hot rolled wire rod and a preparation method thereof, belongs to the technical field of wire rods for prestressed steel, and solves the problems of low tensile strength, low area shrinkage and insufficient strong plasticity matching and stress corrosion resistance of a prestressed steel strand prepared subsequently in the prior art. The alloy components of the hot rolled wire rod are as follows by mass percent: c:0.75 to 0.90 percent, mn:0.60% -0.90%, si:0.10 to 0.60 percent, cr: less than or equal to 0.35 percent, V:0.04 to 0.10 percent of Ti:0.010% -0.025%, nb: less than or equal to 0.03 percent, al: less than or equal to 0.06 percent, P: less than or equal to 0.02 percent, S: less than or equal to 0.005 percent, N: less than or equal to 0.004 percent, and the balance of Fe and unavoidable impurities. The tensile strength of the hot rolled wire rod disclosed by the invention reaches 1150-1350 MPa, the reduction of area is more than or equal to 35%, and the preparation process is simple and suitable for large-scale manufacturing.

Description

Mn-Si-V-Ti-Nb-Cr multi-element alloy hot-rolled wire rod and preparation method thereof

Technical Field

The invention relates to the technical field of wire rods for prestressed steel, and in particular to a Mn-Si-V-Ti-Nb-Cr multi-element alloyed hot-rolled wire rod and a preparation method thereof.

Background technique

Prestressed steel strands are widely used in transportation, construction, water conservancy, hydropower, energy and other fields related to the national economy and people's livelihood. At present, 1860MPa grade prestressed steel strands are the most widely used. In recent years, corresponding standards such as GB/T5224-2023 "Steel Strands for Prestressed Concrete", JG/T 369-2012 "Retarded Bonding Prestressed Steel Strands", JG/T 161-2016 "Unbonded Prestressed Steel Strands" have added 1960MPa grade higher strength prestressed steel strands. JGJ 369-2016 "Design Code for Prestressed Concrete Structures" clearly points out that 1960MPa grade tensile strength steel strands can be used as post-tensioning prestressed reinforcement. In addition to the development direction of ultra-high strength and lightweight, high-strength prestressed steel wires of 1860MPa and above have high ductility, and the total elongation at maximum force is significantly higher than the 3.5% specified in GB/T 5224-2023, so that they can have higher tension redundancy during construction and achieve higher load-bearing and lightweight effects. Therefore, high strengthening and high plasticization of prestressed steel strands have also become an important development direction.

Prestressed steel wire rod (mostly hot-rolled wire rod) is the main raw material/process product for the preparation of prestressed steel strands. The mechanical properties and cross-sectional shrinkage of the wire rod are closely related to the performance of the final steel strand. At present, ordinary 82B hot-rolled wire rods are generally used to prepare 1860MPa prestressed steel strands, but ordinary 82B hot-rolled wire rods have shortcomings such as low tensile strength (1130-1250MPa) and low cross-sectional shrinkage (30%-37%), resulting in the subsequent preparation of prestressed steel strands. The strength and ductility (total elongation at maximum force) of the prestressed steel strands above 1860MPa are generally insufficient. The prestressed steel strands above 1860MPa are prepared by 82B hot-rolled wire rods with larger diameters, that is, high strengthening is achieved by large deformation drawing, but the ductility and stress corrosion resistance of the prepared prestressed steel strands are insufficient. The strength and plasticity of ordinary 82B wire rods can be improved by offline salt bath heat treatment, but the complexity and cost of the preparation process of prestressed steel strands above 1860MPa are increased, and the stress corrosion resistance is still insufficient.

Summary of the invention

In view of the above analysis, the embodiments of the present invention aim to provide a Mn-Si-V-Ti-Nb-Cr multi-alloyed hot-rolled wire rod and a preparation method thereof, so as to solve the problems of insufficient tensile strength and low cross-sectional shrinkage rate of the existing prestressed hot-rolled wire rod, thereby improving the strength, ductility and stress corrosion resistance of the subsequently prepared prestressed steel strand.

The invention discloses a Mn-Si-V-Ti-Nb-Cr multi-element alloyed hot-rolled wire rod. The alloy components of the hot-rolled wire rod are as follows by mass percentage: C: 0.75%-0.90%, Mn: 0.60%-0.90%, Si: 0.10%-0.60%, Cr: ≤0.35%, V: 0.04%-0.10%, Ti: 0.010%-0.025%, Nb: ≤0.03%, Al: ≤0.06%, P: ≤0.02%, S: ≤0.005%, N: ≤0.004%, and the balance is Fe and unavoidable impurities.

Preferably, the alloy composition of the hot-rolled wire rod is calculated by mass percentage as follows: C: 0.80%-0.90%, Mn: 0.60%-0.90%, Si: 0.10%-0.60%, Cr: 0.10%-0.35%, V: 0.04%-0.10%, Ti: 0.010%-0.025%, Nb: ≤0.03%, Al: ≤0.06%, P: ≤0.02%, S: ≤0.005%, N: ≤0.004%, and the balance is Fe and unavoidable impurities.

Specifically, the microalloy ratio of V, Ti and Nb in the hot-rolled wire rod satisfies (V: 0.04% to 0.06%, Ti: 0.010% to 0.025%, Nb: 0.01% to 0.03%) or (V: 0.07% to 0.10%, Ti: 0.010% to 0.025%, Nb: <0.01%).

Specifically, the hot-rolled wire rod has a diameter of 12.5 to 14 mm, and a microstructure of complete lamellar pearlite without network cementite.

Specifically, the hot-rolled wire rod has a tensile strength of 1150 MPa to 1350 MPa, and a cross-sectional shrinkage of ≥35%.

The present invention also discloses a method for preparing the hot-rolled wire rod, which specifically comprises the following steps:

S1: Prepare the materials according to the alloy composition, and obtain the ingot through smelting and casting;

S2: homogenizing the ingot, that is, heating it to the austenite homogenization temperature and keeping it warm;

S3: Descaling the ingot after heat preservation and then continuously rolling it until the target diameter is reached to obtain a semi-finished wire rod;

S4: Curling the semi-finished wire rod, air-cooling, and obtaining the finished hot-rolled wire rod.

Specifically, the austenite homogenization temperature in step S2 is 1180-1250° C., and the holding time is 0.5-3 h.

Preferably, when the Nb content in the alloy is between 0.01% and 0.03%, the austenite homogenization temperature is between 1200 and 1250°C.

Specifically, in step S4, the air cooling rate from the hot rolling finish to 500°C is 1.5°C/s to 3.5°C/s.

The present invention also discloses a prestressed steel strand, which is made from the hot-rolled wire rod or the hot-rolled wire rod prepared by the preparation method, and is pickled, cold drawn, stranded and subjected to stabilization heat treatment.

Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

1. The hot-rolled wire rod disclosed in the present invention has high tensile strength and high cross-sectional shrinkage. In the wire rod of the present invention, on the basis of the main alloying of Mn-Si, Cr is added as an alloying element, which can refine the pearlite lamellar spacing and improve the mechanical strength of the alloy, which is particularly beneficial for the manufacture of high-strength prestressed steel/steel strands; assisted by the V-Ti(-Nb) strong carbide-forming element microalloy design, the austenite grains are refined by graded precipitation of MX-type second phase particles, thereby refining the pearlite cluster size and increasing the diversity of lamellar orientation, improving the cross-sectional shrinkage and improving the drawing performance; wherein Nb is an optional microalloying element, which is added according to the V content in the alloy. The strong carbide-forming elements of V, Ti, and Nb have the following graded precipitation characteristics. Ti fixes N and is almost completely precipitated as TiN or Ti(C,N) to refine the homogenized austenite grains before hot rolling, and Ti(C,N) may contain a small amount of Nb or V. Nb is partially precipitated during the hot rolling process, and the NbC particles and the solid solution Nb work together to refine the austenite grains, increase the austenite deformation energy storage, increase the driving force of the pearlite phase transformation, and refine the pearlite clusters and interlamellar spacing; the incompletely precipitated Nb continues to precipitate during the cooling process, further improving the strength. V precipitates in small quantities during the hot rolling process, but precipitates during the air cooling process after hot rolling. On the one hand, VC is precipitated alone, or (Nb, V) C is precipitated together with Nb to play a precipitation strengthening role, and on the other hand, it participates in the precipitation of cementite and refines the interlamellar spacing of pearlite; the V and Nb dissolved after air cooling will continue to precipitate in small quantities during the stabilization treatment after drawing and stranding. The precipitation of Ti and Nb accounts for 95% to 100% of the total addition, and the precipitation of V accounts for 50% to 80% of the total addition. The above graded precipitated MX-type second phase particles work together to resist stress corrosion (i.e., improve the tensile strength). The hot-rolled wire rod disclosed by the invention has a tensile strength of 1150MPa to 1350MPa and a cross-sectional shrinkage rate of ≥35%.

2. The prestressed steel strand disclosed in the present invention has good mechanical properties. In view of the fact that the hot-rolled wire rod disclosed in the present invention has excellent tensile properties and cross-sectional shrinkage, the subsequently prepared steel strand has excellent mechanical properties. On the basis of the microalloying of strong carbide-forming elements of V, Ti, and Nb (i.e., on the basis of coiling characteristics), the present invention adopts a higher heat treatment temperature and a suitable heat treatment line speed in the stabilization heat treatment process section in the preparation of steel strands, which cooperates with the precipitation of V, Ti, and Nb nano MX second phases to achieve high strength and high plasticity of the steel strands. The tensile strength of the prestressed steel strands is 1860 to 2060 MPa, and the maximum force total elongation is ≥5.5%.

3. The prestressed steel strand disclosed in the present invention has good stress corrosion resistance. The tensile strength of the steel strand prepared by the hot-rolled wire rod of the present invention after pickling, drawing, stranding, and stabilization heat treatment can reach 1860-2060MPa, and the stress corrosion resistance meets the following requirements: according to the provisions of GB/T 21839, solution A (analytical pure ammonium thiocyanate aqueous solution) is carried out, and the stress corrosion test is carried out with a stress of 80% of the actual maximum force, and the test group is not less than 5 groups, and the minimum stress corrosion fracture time of the prestressed steel is ≥2h, and the median is ≥5h.

4. The wire rod and steel strand preparation process disclosed in the present invention is relatively mature and suitable for large-scale production. The various raw materials and production equipment required by the present invention are common materials/equipment on the market, the process flow is relatively simple, the process conditions are easy to achieve, the process is relatively mature, and it is suitable for large-scale production and large-scale promotion.

In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are only for the purpose of illustrating specific embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

FIG1 is a flow chart of a method for preparing a hot rolled coil;

Figure 2 is a microstructure photograph of the hot-rolled wire rod of Example 5.

Detailed ways

The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

The invention discloses a Mn-Si-V-Ti-Nb-Cr multi-element alloyed hot-rolled wire rod. The alloy components of the hot-rolled wire rod are as follows by mass percentage: C: 0.75%-0.90%, Mn: 0.60%-0.90%, Si: 0.10%-0.60%, Cr: ≤0.35%, V: 0.04%-0.10%, Ti: 0.010%-0.025%, Nb: ≤0.03%, Al: ≤0.06%, P: ≤0.02%, S: ≤0.005%, N: ≤0.004%, and the balance is Fe and unavoidable impurities.

The basis for selecting the effects/synergies and contents of each component is as follows:

C: An interstitial solid solution strengthening element in steel that effectively improves the tensile strength of wire rods and a second phase strengthening element such as cementite and MX phases, but too high a C content impairs plasticity. In order to achieve a tensile strength of 1150-1350 MPa for the wire rods and good plasticity, with a cross-sectional shrinkage of ≥35%, the C content is not less than 0.75% but not more than 0.90%. After alloying with Mn, Si, and Cr, it becomes hypereutectoid steel, and through proper air cooling after hot rolling, complete pearlite can be obtained without network cementite. The C content of the steel of the present invention is between 0.75% and 0.90%.

Mn: an element that stabilizes austenite, reduces the eutectoid transformation temperature and eutectoid carbon content, and has little effect on the spacing between the pearlite lamellae of the equilibrium phase transformation. The present invention mainly utilizes its effect of stabilizing austenite to reduce the temperature of the continuous cooling pearlite phase transformation, thereby refining the pearlite lamellae spacing. Mn is the main alloying element, and the Mn content is not less than 0.60% and not more than 0.90%. In addition, Mn also has a deoxidizing effect. The present invention simultaneously utilizes the deoxidizing effect of Mn and the effect of stabilizing austenite. In summary, the Mn content of the steel of the present invention is between 0.6% and 0.90%.

Si: A strong substitutional solid solution strengthening element in steel, with excellent anti-relaxation performance, increases the eutectoid transformation temperature, reduces the eutectoid carbon content, and refines the interlamellar spacing of pearlite formed below the eutectoid transformation temperature. The higher the content, the more significant the strengthening and refinement effects, but higher Si additions will cause greater damage to plasticity. Therefore, the Si content is controlled to be lower, not exceeding 0.60%. In addition, Si can also play a deoxidation role, and ultra-low Si control will increase costs. In summary, the Si content of the steel of the present invention is 0.10% to 0.60%.

Cr: a medium-strong carbide-forming element, one of the elements that make up cementite in alloys, increases the eutectoid phase transition temperature, reduces the eutectoid carbon content, and significantly refines the interlamellar spacing of pearlite formed below the eutectoid phase transition temperature. Adding 0.1% Cr has a significant refining effect. The higher the Cr content, the finer the pearlite interlamellar spacing and the higher the strength. Since Cr is also an alloying element that significantly improves hardenability, the present invention mainly adopts a process strategy of Mn main alloying and hot rolling air cooling to obtain pearlite structure, so that the strength level of the steel strand reaches 2060MPa level. Therefore, the upper limit of the Cr content in the present invention is controlled to 0.35%. The prestressed steel strand with a tensile strength of 1860MPa in the present invention does not need to be deliberately added with Cr, and the content is ≤0.10%.

V: A strong carbonitride-forming element. Its solid solubility in high carbon steel is much larger than that of Ti and Nb. At a lower phase transition temperature, it can combine with C to form a relatively fine MX-type VC, which plays a significant role in resisting stress corrosion. Its mechanism of action mainly includes two aspects: on the one hand, the atomic vacancies in the VC lattice provide hydrogen traps, and on the other hand, VC forms a semi-coherent interface with the iron matrix, providing hydrogen traps. In addition, V can also enter the cementite to replace part of the Fe atoms, that is, participate in the pearlite phase transformation, and play a role in refining the pearlite lamella spacing. The V content in the steel of the present invention is controlled to be 0.04% to 0.10%. In the high-temperature homogenization stage before hot rolling, V may participate in the precipitation of Ti(C,N) in a very small amount. Due to the high final rolling temperature of hot rolling, a small amount of V is precipitated in the hot rolling stage. During the air cooling process after hot rolling, VC is precipitated alone, or (Nb, V) C is precipitated together with Nb to strengthen the precipitation. On the other hand, it participates in the precipitation of cementite and refines the spacing between pearlite lamellae. The remaining V is in a solid solution state. After drawing, due to the dissolution of C atoms in cementite, a small amount of MX-type second phase particles continue to be precipitated during the stabilization process. The precipitation of V accounts for 50% to 80% of the total addition. Too low V content is not conducive to the stress corrosion resistance of prestressed steel strands of 1860MPa and above. Adding a higher V content (such as more than 0.10%) increases the alloy cost.

Ti: A strong carbonitride-forming element, mainly forming TiN or TiC. The solid solubility of TiN in steel is very small, and the solid solubility of TiC in steel is also small. The steel of the present invention uses TiN to fix N, but in order to avoid the precipitation of TiN in the liquid, the N content is controlled not to exceed 0.004%, and at the same time, a lower Ti addition amount is controlled. The atomic ratio of TiN is 3.42, and the lower limit of Ti addition is about 3.42 times the N content, which is set to 0.01%, and the upper limit does not exceed 0.025%. After fixing N, the trace amount of Ti also precipitates TiC during the hot rolling and air cooling process. After drawing, due to the dissolution of C atoms in cementite, the Ti that is not fully precipitated during hot rolling and cooling continues to precipitate a small amount of MX-type second phase particles during the stabilization treatment. The precipitation of Ti in the steel of the present invention accounts for 95% to 100% of the total addition. The semi-coherent interface formed by TiC and the iron matrix can become a hydrogen trap, which plays a role in resisting stress corrosion.

Nb: A strong carbonitride-forming element. The solid solubility of NbN formed by Nb and N is one order of magnitude greater than that of TiN. Therefore, Ti in the steel of the present invention fixes N at high temperature, while Nb mainly combines with C to precipitate NbC. Since the C content in the steel of the present invention is relatively high, the solid solubility of NbC is more than one order of magnitude smaller than that of VC. Therefore, the Nb content of the steel of the present invention is controlled to be 0.01% to 0.03%. NbC is partially precipitated during the hot rolling process, and the NbC particles and solid solution Nb work together to refine the austenite grains, increase the austenite deformation energy storage, increase the driving force of the pearlite phase transformation, and refine the pearlite blocks and interlamellar spacing; the incompletely precipitated Nb continues to precipitate during the cooling process, further improving the strength; after drawing, due to the re-dissolution of C atoms in the cementite, the Nb that is not fully precipitated during hot rolling cooling can continue to precipitate a small amount of MX-type second phase particles during the stabilization treatment. The proportion of Nb precipitation in the total addition is 95% to 100%, and the atomic vacancies in the NbC lattice and the semi-coherent interface between NbC and the iron matrix act as hydrogen traps to resist stress corrosion.

Al: a strong deoxidizing element. The steel of the present invention uses Al as an optional deoxidizing element with a controlled content of ≤0.06%. Too high an Al content easily leads to coarse size of some inclusions.

P: reduces the toughness and plasticity of steel. The P content of the steel of the present invention is controlled to be ≤0.02%.

S: reduces the plasticity and toughness of steel. The S content of the steel of the present invention is controlled to be ≤0.005%.

N: An effective interstitial solid solution strengthening element and second phase strengthening element in steel. Free N atoms lead to increased strength of wire rod and decreased plastic toughness, and are prone to cause aging and blue brittleness, so the total N content and free N content should be as low as possible. Ti is used to fix N to form TiN. In order to avoid the formation of coarse and large amounts of TiN, which in turn deteriorates the drawing performance of the wire rod, the total N content needs to be controlled as low as possible. Combined with the technical level of the test or production equipment, the N content of the steel of the present invention does not exceed 0.004%.

Preferably, the alloy composition of the hot-rolled wire rod is, by mass percentage, C: 0.75% to 0.85%, Mn: 0.60% to 0.90%, Si: 0.10% to 0.60%, Cr: <0.10%, V: 0.04% to 0.10%, Ti: 0.010% to 0.025%, Nb: ≤0.03%, Al: ≤0.06%, P: ≤0.02%, S: ≤0.005%, N: ≤0.004%, and the remainder is Fe and unavoidable impurities; the hot-rolled wire rod of this composition can be used to prepare prestressed steel strand with a tensile strength of 1860 MPa.

Preferably, the alloy composition of the hot-rolled wire rod is, by mass percentage, C: 0.75% to 0.85%, Mn: 0.60% to 0.90%, Si: 0.10% to 0.60%, Cr: 0.10% to 0.35%, V: 0.04% to 0.10%, Ti: 0.010% to 0.025%, Nb: ≤0.03%, Al: ≤0.06%, P: ≤0.02%, S: ≤0.005%, N: ≤0.004%, and the remainder is Fe and unavoidable impurities; the hot-rolled wire rod of this composition can be used to prepare prestressed steel strand with a tensile strength of 1960 MPa.

Preferably, the alloy composition of the hot-rolled wire rod is, by mass percentage, C: 0.80% to 0.90%, Mn: 0.60% to 0.90%, Si: 0.10% to 0.60%, Cr: 0.10% to 0.35%, V: 0.04% to 0.10%, Ti: 0.010% to 0.025%, Nb: ≤0.03%, Al: ≤0.06%, P: ≤0.02%, S: ≤0.005%, N: ≤0.004%, and the remainder is Fe and unavoidable impurities; the hot-rolled wire rod of this composition can be used to prepare prestressed steel strand with a tensile strength of 2060 MPa.

Furthermore, the V, Ti, and Nb microalloying ratios in the hot-rolled wire rods satisfy (V: 0.04% to 0.06%, Ti: 0.010% to 0.025%, Nb: 0.01% to 0.03%) or (V: 0.07% to 0.10%, Ti: 0.010% to 0.025%, Nb: <0.01%). The above ratios can better exert the strengthening effect of Nb microalloying during the hot rolling process and match with V microalloying, which helps to improve the strength and plasticity of the wire rods/strands.

Specifically, the hot-rolled wire rod has a diameter of 12.5 to 14 mm, and a microstructure of complete lamellar pearlite without network cementite.

The common diameter of the wire rod used to prepare the prestressed steel strand is 12.5-13 mm. Since the hot-rolled wire rod of the present invention obtains a refined lamellar pearlite structure with diversified lamellar orientation through the proprietary microalloying design and process control, the plasticity of the hot-rolled wire rod with a diameter of 14 mm is also very excellent. It can be used to prepare prestressed steel wire and steel strand with the same diameter as the 12.5-13 mm wire rod, and can obtain excellent mechanical properties.

Lamellar pearlite with refined cluster size and interlamellar spacing and diversified lamellar orientation has high strength, good deformation ability and excellent drawing performance. However, if some network cementite is formed, the proeutectoid cementite at the austenite grain boundary is coarse and brittle, which not only damages the strength but also deteriorates the drawing performance.

Specifically, the hot-rolled wire rod has a tensile strength of 1150MPa to 1350MPa and a cross-sectional shrinkage of ≥35%. The hot-rolled wire rod has excellent tensile strength and high cross-sectional shrinkage, which is conducive to the preparation of prestressed steel/steel strands with high strength and high toughness.

The present invention also discloses a method for preparing the hot-rolled wire rod, which specifically comprises the following steps:

S1: Prepare the materials according to the alloy composition, and obtain the ingot through smelting and casting;

S2: homogenizing the ingot, that is, heating it to the austenite homogenization temperature and keeping it warm;

S3: Descaling the ingot after heat preservation and then continuously rolling it until the target diameter is reached to obtain a semi-finished wire rod;

S4: Curling the semi-finished wire rod, air-cooling it, and obtaining the finished hot-rolled wire rod.

Specifically, in step S1, excess raw materials of various alloy elements are weighed according to the alloy composition and added to a high-temperature converter, and then cast into ingots through converter smelting, LF refining, RH or VD degassing, electromagnetic stirring, and continuous casting. By controlling the relevant parameters of converter smelting, LF refining, RH or VD degassing, and real-time sampling and analysis of the molten steel in the furnace, when the components of each element reach a preset value/range, the molten steel is poured out for electromagnetic stirring and continuous casting. The above-mentioned process and parameter control are relatively mature alloy preparation processes in the prior art, and the parameters can be adjusted during implementation according to practical experience and specific alloy composition.

Specifically, the austenite homogenization temperature described in step S2 is 1180-1250°C, and the holding time is 0.5-3h. When the homogenization temperature is too high, the TiN part in the ingot will be completely dissolved, resulting in coarsening of the austenite grains and increasing energy costs; too low homogenization will lead to insufficient solid solution of strong carbonitride-forming elements such as Nb and V in the steel, thereby affecting the performance of subsequent finished products. If the homogenization holding time is too long, the austenite grains will coarsen, increasing energy costs and being detrimental to production efficiency; if the homogenization holding time is too short, the uniformity of the thickness and temperature of the ingot is difficult to guarantee. Therefore, the holding time is controlled to be 0.5-3h.

Preferably, when the Nb content in the alloy is between 0.01% and 0.03%, the austenite homogenization temperature is between 1200 and 1250° C., causing more than 80% of the Nb to be in a solid solution state.

For example, the descaling operation in step S3 can be performed by high pressure water descaling technology or other conventional descaling processes. The descaling operation is to remove the iron oxide scale to prevent it from being pressed into the alloy/wire rod surface to produce defects, thereby improving the surface quality of the product.

Specifically, the heating temperature before hot rolling in step S3 is 1180-1250° C. (i.e., the austenite homogenization temperature in step S2, and hot rolling is directly performed after heat preservation), and the hot rolling final rolling temperature is 890-950° C. If the final rolling temperature is too high, the hot-rolled austenite grains are relatively coarse, and the effect of Nb microalloying on refining the hot-rolled austenite grains is not significant; if the final rolling temperature is too low, more V will be precipitated and the rolling force will be significantly increased.

Specifically, the air cooling rate from the hot rolling to 500°C in step S4 is 1.5°C/s to 3.5°C/s. When the air cooling rate is ≥1.5°C/s, the formation of pro-eutectoid cementite (network cementite) can be avoided; when the air cooling rate is ≥3.5°C, a large amount of bainite structure will be formed, and even part of the super-hard martensite structure will be formed in the subsequent cooling process, which is not conducive to drawing, so excessive air cooling rate should be avoided.

The present invention also discloses a prestressed steel strand, which is made from the hot-rolled wire rod or the hot-rolled wire rod prepared by the preparation method, and is pickled, cold drawn, stranded and subjected to stabilization heat treatment.

Exemplarily, pickling can be performed by the following operations: unbundling and loosening the wire rods and immersing them in a pickling solution. The pickling solution can be 10% to 15% hydrochloric acid. Pickling is performed at room temperature for about 30 minutes. After pickling, the wire rods are lifted to a suspended bracket above the pickling tank and shaken in a small range to allow the pickling solution brought out by the wire rods to flow back into the tank.

For example, cold drawing is to select a wire drawing die/die hole with a suitable aperture at room temperature, and allow the wire rod to pass through the die hole under the action of strong external force, so that the wire rod undergoes plastic deformation, thereby obtaining a steel wire of corresponding size.

Exemplarily, twisting is to wrap multiple strands of steel wires together once to form a steel strand, which can be done using a mechanical stranding machine. The number of steel wires in the steel strand of the present invention is not less than 3.

Specifically, the diameter of the drawn single wire in the cold drawing process is 5mm, the stabilization heat treatment temperature after stranding is 400-460℃, and the line speed is 1.2-2.0m/s. The stabilization heat treatment process matches the full precipitation of V, Ti, and Nb microalloys, and the temperature matches the line speed. When the temperature is high, the line speed is fast, and when the temperature is low, the line speed is slow, which is conducive to the precipitation of V, Ti, and Nb nano MX second phase particles to improve strength and plasticity.

Specifically, the tensile strength of the prestressed steel strand is 1860-2060MPa, the total elongation at maximum force is ≥5.5%, and the stress corrosion resistance meets the following requirements: Solution A (analytical pure ammonium thiocyanate aqueous solution) is carried out in accordance with the provisions of GB/T 21839, and the stress corrosion test is carried out by loading a stress of 80% of the actual maximum force, with no less than 5 test groups, and the minimum stress corrosion fracture time of the prestressed steel is ≥2h, and the median is ≥5h. Solution A (analytical pure ammonium thiocyanate aqueous solution) is an ammonium thiocyanate solution prepared by dissolving 200g NH ₄ SCN in 800mL distilled water or demineralized water. Ammonium thiocyanate is analytical pure, with an NH ₄ SCN content of at least 98.5%, Cl ^- <0.005%, ^S2- ＜0.001%.

The advantages of precise control of the composition and process parameters of the wire rod/strand of the present invention will be demonstrated below with specific examples and comparative examples. The chemical composition of the steel of Examples 1 to 8 and Comparative Examples 1 to 5 is shown in Table 1, the specific rolling and cooling process parameters are shown in Table 2, and the mechanical properties are shown in Table 3.

Examples 1 to 6 were smelted in a converter, LF refining, RH degassing, electromagnetic stirring, and continuous casting into 160 mm (thick) × 160 mm (wide) square billets, cut into 12000 mm long billets, rolled and heated to 1180-1250° C., respectively kept warm for 0.5-3 h, and hot rolled into wire rods with a diameter of 12.5-14 mm after being taken out of the furnace, with a final rolling temperature of 890-950° C. The air cooling rate before cooling to 500° C. after hot rolling was 1.5-3.5° C./s.

Examples 7 to 8 are smelted in a converter, and are subjected to LF refining, VD degassing, electromagnetic stirring, and continuous casting to form 160 mm (thick) × 160 mm (wide) square billets, which are cut into 12000 mm long billets, rolled and heated to 1200 to 1250°C, and kept warm for 1 to 2 hours respectively. After being taken out of the furnace, they are hot rolled into wire rods with a diameter of 13 mm, and the final rolling temperature is 900 to 930°C. After hot rolling, the air cooling rate before cooling to 500°C is 1.8 to 2.7°C/s.

The preparation methods of Comparative Examples 1 to 5 are substantially the same as those of Example 1, and the specific differences in formulations or process parameters are detailed in Tables 1 and 2.

Table 1 Chemical composition of Examples and Comparative Examples wt.%

serial number C Mn Si Cr V Ti Nb Al P S N Example 1 0.75 0.69 0.36 0.06 0.05 0.013 0.026 0.023 0.012 0.0044 0.0038 Example 2 0.83 0.68 0.22 0.02 0.08 0.016 0.004 0.015 0.011 0.0028 0.0029 Example 3 0.78 0.87 0.42 0.11 0.09 0.022 0.005 0.013 0.014 0.0034 0.0037 Example 4 0.80 0.76 0.46 0.18 0.04 0.018 0.028 0.011 0.013 0.0029 0.0036 Example 5 0.81 0.88 0.58 0.12 0.06 0.017 0.012 0.008 0.012 0.0020 0.0035 Example 6 0.86 0.77 0.46 0.34 0.08 0.011 0.003 0.016 0.014 0.0041 0.0031 Example 7 0.84 0.62 0.37 0.27 0.05 0.023 0.018 0.007 0.013 0.0048 0.0023 Example 8 0.90 0.79 0.45 0.26 0.10 0.025 0.004 0.015 0.011 0.0037 0.0036 Comparative Example 1 0.81 0.74 0.48 0.19 0.004 0.007 0.004 0.022 0.013 0.0048 0.0044 Comparative Example 2 0.83 0.84 0.22 0.03 0.02 0.004 0.003 0.005 0.012 0.0043 0.0038 Comparative Example 3 0.85 0.63 0.28 0.16 0.006 0.005 0.005 0.013 0.014 0.0036 0.0033 Comparative Example 4 0.80 0.77 0.12 0.12 0.03 0.006 0.004 0.011 0.014 0.0038 0.0037 Comparative Example 5 0.80 0.65 0.11 0.04 0.003 0.003 0.003 0.007 0.013 0.0048 0.0040

Table 2 Specific hot rolling cooling process parameters of the embodiments and comparative examples

Table 3 Diameter specifications and mechanical properties of embodiments and comparative examples

serial number Diameter/mm Tensile strength MPa rate of reduction in area/% Example 1 14 1170 41.0 Example 2 13 1220 39.0 Example 3 13 1295 39.5 Example 4 12.5 1310 38.5 Example 5 13 1355 38.5 Example 6 13 1380 37.0 Example 7 14 1270 39.0 Example 8 12.5 1385 35.5 Comparative Example 1 13 1210 35.0 Comparative Example 2 13 1225 34.0 Comparative Example 3 14 1190 34.0 Comparative Example 4 12.5 1230 33.0 Comparative Example 5 13 1135 36.5

It can be seen from Tables 2 and 3 that the tensile strength of the hot-rolled wire rods of Examples 1 to 8 is 1150 MPa to 1350 MPa (e.g., 1170 to 1385 MPa), the cross-sectional shrinkage is ≥35% (e.g., 35.5% to 41.0%), and the diameter specification is 12.5 to 14 mm.

The tensile strength of the hot-rolled wire rods with a diameter of 12.5 to 14 mm in Comparative Examples 1 to 5 is 1130 to 1230 MPa, and the cross-sectional shrinkage is 33.0% to 36.5%. Compared with Examples 1, 2, 3 and 7 with similar strength, the plasticity of Comparative Examples 1 to 5 is relatively low.

In general, the comprehensive performance of Examples 1 to 8 is significantly better than that of the comparative example, and they have higher tensile strength and higher cross-sectional shrinkage.

Table 4 Stabilized heat treatment process and performance of prestressed steel corresponding to wire rod in Example

As shown in Table 4, the high-strength wire rods of Examples 1 to 8 are pickled and drawn into 5mm diameter 4-grade steel wire (5.05-5.25mm), and after stranding (7-wire steel strand), the tensile strength of the prestressed steel strand reaches 1860-2060MPa, and the stress corrosion resistance meets the following requirements: according to the provisions of GB/T 21839, solution A (analytical pure ammonium thiocyanate aqueous solution) is carried out, and the stress of 80% of the actual maximum force is loaded to carry out stress corrosion test, and the test group is not less than 5 groups, and the minimum stress corrosion fracture time of prestressed steel is ≥2h, and the median is ≥5h. The high-strength wire rods of Comparative Examples 1 to 5 are pickled and drawn into 5mm diameter (5.05-5.25mm) steel wire, and after stranding, the stabilization heat treatment is carried out, and the tensile strength of the prestressed steel strand is 1860MPa, and the minimum stress corrosion fracture time is ≥2h, but the median is only 2.8-4.1h, which does not meet the requirement of ≥5h.

The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims (10)

1. A Mn-Si-V-Ti-Nb-Cr multi-element alloyed hot rolled wire rod is characterized in that: the alloy components of the hot rolled wire rod are as follows by mass percent: c:0.75 to 0.90 percent, mn:0.60% -0.90%, si:0.10 to 0.60 percent, cr: less than or equal to 0.35 percent, V:0.04 to 0.10 percent of Ti:0.010% -0.025%, nb: less than or equal to 0.03 percent, al: less than or equal to 0.06 percent, P: less than or equal to 0.02 percent, S: less than or equal to 0.005 percent, N: less than or equal to 0.004 percent, and the balance of Fe and unavoidable impurities.

2. The hot rolled wire rod as claimed in claim 1, wherein: the alloy components of the hot rolled wire rod are as follows by mass percent: c:0.80 to 0.90 percent, mn:0.60% -0.90%, si:0.10 to 0.60 percent, cr:0.10 to 0.35 percent, V:0.04 to 0.10 percent of Ti:0.010% -0.025%, nb: less than or equal to 0.03 percent, al: less than or equal to 0.06 percent, P: less than or equal to 0.02 percent, S: less than or equal to 0.005 percent, N: less than or equal to 0.004 percent, and the balance of Fe and unavoidable impurities.

3. The hot rolled wire rod as claimed in claim 1, wherein: the ratio of the V, ti and Nb microalloy in the hot rolled wire rod is as follows: 0.04 to 0.06 percent of Ti:0.010% -0.025%, nb:0.01% -0.03%, or V:0.07 to 0.10 percent of Ti:0.010% -0.025%, nb: less than 0.01%.

4. The hot rolled wire rod as claimed in claim 1, wherein: the diameter of the hot rolled wire rod is 12.5-14 mm, the microstructure is complete lamellar pearlite, and no network cementite exists.

5. The hot rolled wire rod as claimed in claim 1, wherein: the tensile strength of the hot rolled wire rod is 1150-1350 MPa, and the area shrinkage rate is more than or equal to 35%.

6. A method for producing a hot rolled wire rod according to any one of claims 1 to 5, comprising the steps of:

s1: batching according to alloy composition, and smelting and casting to obtain a casting blank;

s2: homogenizing the casting blank, namely heating to the austenite homogenizing temperature and preserving heat;

s3: continuously rolling the heat-preserving casting blank after descaling until reaching the target diameter to obtain a semi-finished product wire rod;

s4: and (3) curling and air-cooling the semi-finished wire rod to obtain the finished hot rolled wire rod.

7. The method of manufacturing according to claim 6, wherein: the austenite homogenizing temperature in the step S2 is 1180-1250 ℃, and the heat preservation time is 0.5-3 h.

8. The method of manufacturing according to claim 6, wherein: when the Nb content in the alloy is 0.01% -0.03%, the austenite homogenizing temperature is 1200-1250 ℃.

9. The method of manufacturing according to claim 6, wherein: and S4, the air cooling speed after hot rolling and before finishing rolling to 500 ℃ is 1.5 ℃/S-3.5 ℃/S.

10. The utility model provides a prestressing force steel strand wires which characterized in that: the steel strand is manufactured by acid washing, cold drawing, stranding and stabilizing heat treatment of the hot rolled wire rod according to any one of claims 1 to 5 or the hot rolled wire rod manufactured by the manufacturing method according to any one of claims 6 to 9.