CN117965862A

CN117965862A - Pearlite-bainite ultra-high strength wire rod and preparation method and application thereof

Info

Publication number: CN117965862A
Application number: CN202410048706.9A
Authority: CN
Inventors: 李昭东; 曹燕光; 高博; 张超; 陈颖; 王慧敏; 左越; 杨忠民; 雍岐龙
Original assignee: Zhonglian Advanced Steel Technology Co ltd; Central Iron and Steel Research Institute
Current assignee: Zhonglian Advanced Steel Technology Co ltd; Central Iron and Steel Research Institute
Priority date: 2024-01-12
Filing date: 2024-01-12
Publication date: 2024-05-03

Abstract

The invention relates to a pearlite-bainite ultra-high strength wire rod, a preparation method and application thereof, belonging to the technical field of prestressed steel wire rods, wherein a volume fraction of 10% -50% of bainitic structure is introduced through cooling phase transition control to form a pearlite-bainite composite structure, so that the tensile strength and the strong plasticity matching degree of the wire rod/prestressed steel are improved. The invention discloses a preparation method of a pearlite-bainite ultra-high strength wire rod, which comprises the following steps: s1: smelting and casting; s2: homogenizing; s3: descaling and rolling to obtain a semi-finished wire rod; s4: curling and post-processing the semi-finished wire rod to obtain a finished product of the ultra-high strength wire rod; wherein, step S4 includes "isothermal salt bath short-time heat treatment". The wire rod prepared by the method has the tensile strength reaching 1400-1680 MPa, the reduction of area is more than or equal to 40 percent, the drawing performance is excellent, the strength loss of hot galvanizing or stabilizing treatment is small, the preparation process is simple, and the wire rod is suitable for preparing the downstream products such as the ultra-high strength prestressed steel wires, ropes and the like.

Description

Pearlite-bainite ultra-high strength wire rod and preparation method and application thereof

Technical Field

The invention relates to the technical field of prestressed steel wire rods, in particular to a pearlite-bainite ultra-high strength wire rod, a preparation method and application thereof.

Background

The prestressed steel comprises prestressed steel strands, cables, steel wire ropes and the like which are widely applied to the fields related to national life such as buildings, bridges, engineering machinery, water conservancy and hydropower, energy and ocean engineering. The requirements of ultra-high bearing, light weight and easy construction are put forward for prestressed steel strands, cables and steel wire ropes, and the strength of the prestressed steel is upgraded to be necessarily selected.

The strength upgrade of prestressed steel is usually aimed by either increasing the basic strength of the wire rod or by an unconventional means of increasing the initial wire rod diameter, i.e. increasing the amount of drawing deformation. The traditional wire rod for the prestressed steel adopts the technical thought of a full pearlite structure or a pearlite structure with the volume fraction of more than 95%, the higher the strength is, the lower the plasticity is, or the larger the drawing deformation is, the more difficult the drawing is.

In conclusion, the existing wire rod/prestressed steel taking the pearlite structure as an absolute dominant structure type has the problems of poor strong plastic matching, difficult drawing and uncontrollable strength loss in the post-treatment process (cold drawing and the like).

Disclosure of Invention

In view of the above analysis, the embodiments of the present invention aim to provide a pearlite-bainite ultra-high strength wire rod, and a preparation method and application thereof, so as to solve at least one of the problems of poor strong plastic matching, difficult drawing, and uncontrollable strength loss in post-treatment process (cold drawing, etc.) of the existing wire rod/prestressed steel.

The invention discloses a preparation method of a pearlite-bainite ultra-high strength wire rod, which comprises the following steps:

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: curling and post-processing the semi-finished wire rod to obtain a finished product of the ultra-high strength wire rod;

The post-treatment mode in step S4 is one of "online air cooling+isothermal salt bath short-time heat treatment+air cooling" or "primary air cooling+offline heat treatment+secondary air cooling+isothermal salt bath short-time heat treatment+air cooling".

Specifically, the ultra-high strength wire rod alloy comprises ：C：0.90％～1.10％,Si：0.70％～1.20％,Mn：0.10％～1.00％,V：0.03％～0.15％,Ti：0.010％～0.025％,Cr：≤0.80％,Mo：≤0.35％,Nb：≤0.03％,Al：≤0.04％,P：≤0.02％,S：≤0.005％,N：≤0.004％, parts by mass of Fe and unavoidable impurities as the balance; and Si, mn, cr, mo alloy components in percentage by mass satisfy the following conditions: si+Mn+Cr+Mo is 1.2-2.4 and Cr+Mo is 1.0.

Specifically, the austenite homogenization temperature in the step S2 is 1180-1250 ℃, and the heat preservation time is 0.5-3 h.

Specifically, the rolling in the step S3 is hot rolling, the heating temperature before hot rolling is 1180-1250 ℃, and the hot rolling finishing temperature is 890-950 ℃.

Specifically, the cooling speed of the online air cooling or the primary air cooling or the secondary air cooling is 1.5 ℃/s-3.5 ℃/s.

Specifically, the isothermal temperature of the isothermal salt bath short-time heat treatment is 450-550 ℃, and the isothermal time is controlled to be 10-200 s.

The invention also discloses a pearlite-bainite ultra-high strength wire rod, which is prepared by the preparation method.

Specifically, the diameter of the ultra-high strength wire rod is less than or equal to 15mm, the microstructure is lamellar pearlite and bainite, no network cementite is uniformly distributed in the diameter range, wherein the volume fraction of lamellar pearlite is 50% -90%, and the volume fraction of bainite is 10% -50%.

Specifically, the tensile strength of the ultra-high strength wire rod reaches 1400-1680 MPa, and the area shrinkage is more than or equal to 40%.

The invention also discloses pearlite-bainite ultra-high strength prestressed steel, which is prepared from the ultra-high strength wire rod.

Compared with the prior art, the invention has at least one of the following beneficial effects:

1. The ultra-high strength wire rod disclosed by the invention has high tensile strength, higher area reduction rate than wire rods with the same strength level and more than 95% of pearlite volume fraction, the wire rod tensile strength reaches 1400-1680 MPa level, the area reduction rate is more than or equal to 40%, and the wire rod is easy to draw.

In the wire rod, the thought of taking Mn or Mn-Si as a main alloy design is changed, the Si-Mn-V-Ti multi-element alloy design is adopted, and Cr, mo and Nb can be added in a proper amount in a matching way; the technical thought of obtaining the full pearlite structure or pearlite with the volume fraction of more than 95% for the traditional wire rod for the prestressed steel is changed, the volume fraction of 10% -50% of bainite structure is introduced by combining isothermal salt bath short-time (within 200 s) heat treatment and air cooling control, the bainite structure consists of a bainitic ferrite soft phase with high movable dislocation density and a martensite-austenite hard phase with high hardness, the pearlite-bainite composite structure is finally obtained, the strong plasticity matching is excellent, and the ultrahigh strength targets of the easy-drawing and prestressed steel are realized.

It is worth emphasizing that the isothermal salt bath short-time heat treatment (within 200 s) time in the post-treatment process should be strictly controlled and matched with the heat treatment temperature, and when the heat treatment temperature is lower, the heat treatment time can be properly prolonged, but the heat treatment time is inadvisable to be too long, otherwise, more than 95% of pearlite tissues can be formed, and the plasticity of the wire rod/prestressed steel is reduced.

2. The ultra-high strength wire rod disclosed by the invention has small strength loss after being pulled out. V, ti, cr, mo, nb in the wire rod alloy composition is used as a strong carbide forming element, and by inhibiting the recovery of a drawing deformation structure of the prestressed steel or precipitating MX type second phase reinforcement, the high-temperature stability of the structure is improved, and the strength loss of the prestressed steel wire caused by hot galvanizing and stabilizing treatment after the wire rod is drawn is reduced.

3. The stress corrosion resistance of the downstream prestressed steel product is improved through the pre-design of the wire rod alloy composition and the preparation process. The V, ti and Nb strong carbide forming elements have the following grading precipitation characteristics and are synergistically reinforced with the medium-strength carbide forming element Mo. Ti fixes N and is almost completely precipitated as TiN or Ti (C, N) refined austenite grains homogenized before hot rolling, and Ti (C, N) may contain a small amount of Nb or V. Nb is partially separated out in the hot rolling process, and NbC particles and solid-solution Nb act together to refine austenite grains, so that austenite deformation energy storage is improved, pearlite transformation driving force is improved, and meanwhile, pearlite clusters and lamellar spacing are refined; the incompletely precipitated Nb is continuously precipitated in the cooling process, so that the strength is further improved; nb precipitates almost completely during austenitizing reheating in an off-line isothermal heat treatment, refining austenite grains. V precipitates in a small amount in the hot rolling process, but precipitates VC alone or precipitates (Nb, V) C together with Nb when austenitizing and reheating in off-line isothermal heat treatment; in the air cooling or isothermal salt bath heat treatment process after hot rolling, V is separated out of VC alone or together with Nb (Nb, V) C to play a role in precipitation strengthening, cementite separation is participated in on the other hand, the interlayer spacing of pearlite sheets is thinned, and the rest V is in a solid solution state. After pulling, a small amount of MX-type second phase particles are continuously separated out in the galvanization or stabilization treatment process due to the dissolution of C atoms in cementite. Because the steel wire rod has relatively high area shrinkage and relatively good stress corrosion resistance, the V addition amount can be properly reduced compared with the full pearlite wire rod with the same strength level, and the lower limit is 0.03 percent. The precipitation of Ti and Nb accounts for 95-100% of the total addition, and the precipitation of V accounts for 50-80% of the total addition. Mo participates in precipitation of MX type second phase particles such as TiN, ti (C, N), nbC, (Nb, V) C and VC, so that the total precipitation amount of the MX type second phase is increased, and coarsening of the MX type second phase particles is prevented. The above-classified MX-type second phase particles collectively act as stress corrosion resistance. The diameter of the prestressed steel subjected to cold drawing and stabilizing heat treatment (not hot galvanizing) is less than or equal to 7mm, the tensile strength is 2260-2460 MPa, and the stress corrosion resistance meets the following requirements: and carrying out a solution A (analyzing pure ammonium thiocyanate aqueous solution) according to the specification of GB/T21839, carrying out a stress corrosion test by loading stress with 80% of the actual maximum force, wherein the test group is not less than 5 groups, the minimum value of the stress corrosion breaking time of the prestressed steel is not less than 2 hours, and the median value is not less than 5 hours.

4. The raw materials are easy to obtain, the preparation process is relatively simple, and the method is suitable for large-scale production. The preparation method of the ultra-high strength plate wire/prestressed steel disclosed by the invention has the advantages that various raw materials and production equipment required by the preparation method of the ultra-high strength plate wire/prestressed steel are common materials/equipment on the market, the process flow is relatively simple, the process condition is easy to realize, the process flow is mature, and the preparation method is suitable for large-scale production and large-scale popularization.

In the invention, the technical schemes can be mutually combined to realize more preferable combination schemes. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and drawings.

Drawings

The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, like reference numerals being used to refer to like parts throughout the several views.

FIG. 1 is a flow chart of ultra-high strength wire rod preparation;

fig. 2 is a photograph of a microstructure of the ultra-high strength wire rod of example 1.

Detailed Description

The following detailed description of preferred embodiments of the application is made in connection with the accompanying drawings, which form a part hereof, and together with the description of the embodiments of the application, are used to explain the principles of the application and are not intended to limit the scope of the application.

The invention discloses a pearlite-bainite ultra-high strength wire rod, which comprises ：C：0.90％～1.10％,Si：0.70％～1.20％,Mn：0.10％～1.00％,V：0.03％～0.15％,Ti：0.010％～0.025％,Cr：≤0.80％,Mo：≤0.35％,Nb：≤0.03％,Al：≤0.04％,P：≤0.02％,S：≤0.005％,N：≤0.004％, parts by mass of Fe and unavoidable impurities as the balance.

The following is a specific description of the action and the selection of the amounts of the components contained in the invention:

C: and the steel can effectively improve the tensile strength of the wire rod, and the second phase strengthening elements such as cementite and MX are added in the steel. In order to ensure that the tensile strength of the wire rod reaches more than 1400MPa, the C content is not less than 0.90 percent, and the wire rod is hypereutectoid steel, but pearlite-bainite structure is obtained through proper air cooling, isothermal salt bath short-time heat treatment and air cooling after heating austenitizing by hot rolling or off-line, and no network cementite exists; however, the higher the C content is, the more difficult the control of C segregation and network cementite is increased, and too high C content can also lead to the reduction of wire rod plasticity and influence drawing. In the invention, the upper limit of the C content can be up to 1.10 percent by controlling the components and the process doubly. The C content of the steel is 0.90-1.10%.

Si: the steel has excellent relaxation resistance by replacing solid solution strengthening elements stronger than Mn, cr and Mo, improves the eutectoid phase transition temperature, reduces the eutectoid carbon content, refines the lamellar spacing of pearlite formed by being lower than the eutectoid phase transition temperature, has more remarkable strengthening and refining effects, but has larger plastic damage when Si is excessively high, such as more than 1.2 percent, and aggravates decarburization in the off-line heat treatment process. Si is also an alloy element for improving the hardenability of steel, but the effect of deferring pearlite transformation is lower than Mn, cr and Mo, and under the addition of Si exceeding 0.70 percent, the steel wire rod can realize lamellar pearlite structure with the volume fraction of 50-90 percent by heat treatment in isothermal salt bath short time (within 200 seconds). In addition, si also plays a role in deoxidization. By combining the above, the Si content of the steel of the invention is 0.70% -1.20%.

Mn: the austenite element is stabilized, the eutectoid phase transition temperature and the eutectoid carbon content are reduced, and the influence on the equilibrium phase transition pearlite lamellar spacing is small. Therefore, the Mn content is not higher than 1.00%. In addition, the Mn is more than or equal to 0.10 percent and plays a role in deoxidization. The invention utilizes the deoxidization effect of Mn and the effect of stabilizing austenite. By combining the above, the Mn content of the steel of the invention is 0.1% -1.00%.

V: the strong carbonitride forms elements, the solid solution quantity of the elements in high-carbon steel is much larger than that of Ti and Nb, and the elements can be combined with C to form finer MX-type VC at a lower phase transition temperature, so that the elements play a remarkable role in resisting stress corrosion, and the action mechanism mainly comprises two aspects: on one hand, atoms in the VC lattice are in a shortage position to provide a hydrogen trap, and on the other hand, VC and an iron matrix form a semi-coherent interface to provide the hydrogen trap. In addition, V can also enter cementite to replace part of Fe atoms, namely participate in pearlite transformation, and play a role in refining the lamellar spacing of pearlite. Therefore, the steel wire rod has relatively high area shrinkage, relatively good stress corrosion resistance, and the V addition amount can be properly reduced compared with the full pearlite wire rod with the same strength level, and the lower limit is 0.03%. In the high-temperature homogenization stage before hot rolling, V may be involved in Ti (C, N) precipitation in a very small amount. Because the hot rolling finishing temperature is higher, V is precipitated in a small amount in the hot rolling stage, and VC is mainly precipitated singly or (Nb, V) C is precipitated together with Nb during austenitizing and reheating of offline isothermal heat treatment; in the air cooling or isothermal salt bath heat treatment process after hot rolling, V is separated out of VC alone or together with Nb (Nb, V) C to play a role in precipitation strengthening, cementite separation is participated in on the other hand, the interlayer spacing of pearlite sheets is thinned, and the rest V is in a solid solution state. After pulling, the C atoms in the cementite are dissolved back, and a small amount of MX-type second phase particles are continuously separated out from V in the galvanization or stabilization treatment process. An excessively low V content is detrimental to the stress corrosion resistance of ultra-high strength pre-stressed steels of the order of 2160MPa and above (i.e. downstream products of the present wire rod), and when a higher V content (e.g. exceeding 0.15%) is added, V will precipitate at a higher temperature, although the total amount of precipitation will increase, the precipitated particles will also increase, which is detrimental to significantly improving the stress corrosion resistance, while also increasing the alloy cost. The V precipitation in the steel accounts for 50-80% of the total addition, wherein the hot rolling process can precipitate 50-65%, and the isothermal salt bath heat treatment process can precipitate 65-80%.

Ti: the strong carbonitride forming element mainly forms TiN or TiC. The solid solution amount of TiN in the steel is small, and the solid solution amount of TiC in the steel is also small. The steel of the invention utilizes TiN to fix N, but in order to avoid the precipitation of TiN in liquid, the lower Ti addition amount is controlled on the basis of controlling the N content to be not more than 0.004%, the atomic ratio of TiN is 3.42, the lower limit of Ti addition is about 3.42 times of the N content and is set to be 0.01%, and the upper limit is not more than 0.025%. The trace Ti after N fixation also precipitates TiC in a lower off-line austenitizing reheating process or an air cooling process or an isothermal salt bath heat treatment process. After pulling, the C atoms in cementite are dissolved back, and Ti atoms which are not fully separated out by hot rolling can be continuously separated out a small amount of MX type second phase particles in the stabilizing treatment process. The precipitation of Ti in the steel accounts for 95-100% of the total addition. The semi-coherent interface formed by TiC and the iron matrix can be a hydrogen trap to play a role in resisting stress corrosion.

Cr: the medium-strength carbide forming element and one of the alloy cementite constituent elements can improve the eutectoid phase transition temperature, reduce the eutectoid carbon content, obviously refine the lamellar spacing of pearlite formed below the eutectoid phase transition temperature, and the addition of 0.1% Cr has obvious refining effect. Therefore, cr in the steel is used as an optional additive element and is added in a matching way with Si, mn, mo and other elements. Too high Cr content also significantly delays pearlite transformation, which is unfavorable for the high-carbon wire rods to obtain lamellar pearlite structure with volume fraction of 50% -90% under the isothermal salt bath heat treatment condition of short time (less than or equal to 200 s). When the content of Si, mn and Mo alloy in the steel is close to the lower limit, the highest adding amount of Cr is not more than 0.80 percent. By combining the above, the Cr content of the steel is less than or equal to 0.80 percent.

Mo: the medium-strength carbonitride forming element, one of the alloying cementite and MX carbonitride forming elements, improves the eutectoid phase transition temperature, reduces the eutectoid carbon content, obviously refines the lamellar spacing of pearlite formed below the eutectoid phase transition temperature, has a stronger effect than Cr, and has obvious effect of refining the lamellar spacing of the pearlite when Mo is more than or equal to 0.05 percent. However, mo has a remarkable effect of improving hardenability and postpones pearlite transformation. When the Si, mn and Cr alloy content of the steel is close to the lower limit, mo exceeding 0.35% can obviously prolong the transformation time of the wire rod pearlite, which is unfavorable for obtaining the lamellar pearlite structure with the volume fraction of 50-90% by the short-time (within 200 s) heat treatment of the steel in isothermal salt bath. In addition, mo inhibits the recovery of the drawing deformation structure of the prestressed steel or participates in the strengthening of the precipitated MX type second phase, improves the high-temperature stability of the structure, and reduces the strength loss of the prestressed steel wire caused by hot galvanizing and stabilizing treatment after wire rod drawing. By combining the above, mo of the steel is taken as an optional additive element, and is added in a matching way with elements such as Si, mn, cr and the like, and the content of Mo is less than or equal to 0.35 percent.

Nb: the strong carbonitride forming element, nb combines with N to form NbN with a solid solubility that is an order of magnitude greater than TiN, so Ti in the steel of the present invention fixes N at a high temperature stage, while Nb mainly combines with C to precipitate NbC. Nb is used as an optional additive element in the steel, the content of C is higher, and the solid solubility of NbC is more than 1 order of magnitude less than that of VC, so that the Nb content of the steel is controlled to be not more than 0.03 percent. NbC is completely precipitated during the hot rolling process or the off-line austenitizing reheating process. Nb is partially separated out in the hot rolling process, and NbC particles and solid-solution Nb act together to refine austenite grains, so that austenite deformation energy storage is improved, pearlite transformation driving force is improved, and meanwhile, pearlite block mass and lamellar spacing are refined; the incompletely precipitated Nb is continuously precipitated in the cooling process, so that the strength is further improved; nb is almost completely precipitated during austenitizing reheating in off-line isothermal heat treatment, refining austenite grains, and further refining the pearlite block size. After pulling, a small amount of MX-type second phase particles can be continuously precipitated in the stabilizing treatment process of insufficient precipitation of hot rolling due to dissolution back of C atoms in cementite. The precipitation of Nb accounts for 95-100% of the total addition, and the stress corrosion resistance is achieved by taking the atomic defect positions in the NbC lattice and the semi-coherent interface of NbC and the iron matrix as hydrogen traps.

Al: the steel of the invention takes the strong deoxidizing element as an optional deoxidizing element, the control content is less than or equal to 0.04 percent, and the excessive Al content easily causes the coarse size of partial inclusion.

P: the toughness and plasticity of the steel are reduced, and the P content of the steel is controlled to be less than or equal to 0.02 percent.

S: the toughness of the steel is reduced, and the S content of the steel is controlled to be less than or equal to 0.005%.

N: an effective interstitial solid solution strengthening element and a second phase strengthening element in the steel. The free N atoms cause the strength of the wire rod to be increased, the toughness to be reduced, and phenomena such as aging and blue embrittlement to be easily caused, so that the total N content and the free N content are as low as possible. Ti is adopted to fix N, so that TiN is formed. In order to avoid the formation of coarse, large amounts of TiN and thus the deterioration of wire rod drawing processability, the total N content needs to be controlled as low as possible. In connection with the experimental or production equipment technical level, the N content of the steel according to the invention does not exceed 0.004%.

Specifically, si, mn, cr, mo alloy components in the ultra-high strength wire rod satisfy the following mass percentages: si+Mn+Cr+Mo is 1.2-2.4 and Cr+Mo is 1.0. The alloy composition ratio can further effectively refine the pearlite colony spacing of the ultra-high strength wire rod. Si, mn, cr, mo, especially Cr and Mo, cannot be excessively high, otherwise, on one hand, 50-90% of lamellar pearlite with volume fraction cannot be obtained by isothermal salt bath short-time (within 200 s) heat treatment, and on the other hand, the hardenability is obviously improved when the composite addition content is high, so that the bainite forming temperature is low, even a massive martensitic structure is formed, the strength is excessively high, and the plasticity (area reduction rate) is obviously reduced.

Preferably, in order to exert the strengthening effect of Nb microalloying in the hot rolling process and match with V microalloying, pearlite-bainite structure, the ultra-high strength wire rod manufactured by adopting an on-line air cooling, isothermal salt bath short-time heat treatment and air cooling mode is adopted, and the ratio of the V, nb and Ti microalloys is as follows: v:0.03 to 0.10 percent of Ti:0.01% -0.025%, nb:0.01 to 0.03 percent.

Preferably, in order to exert the effect of refining austenite grains by Nb microalloying in the off-line heat treatment process and thereby refining the size of pearlite block, and match and cooperate with the V microalloying refining effect, the ultra-high strength wire rod manufactured by adopting a mode of primary air cooling, off-line isothermal heat treatment, secondary air cooling, isothermal salt bath short-time heat treatment and air cooling is adopted, and the ratio of the V, nb and Ti microalloys follows the following two conditions: (1) V:0.03 to 0.10 percent of Ti:0.01% -0.025%, nb:0.01% -0.03%; (2) V:0.11 to 0.15 percent of Ti:0.01% -0.025%, nb: less than 0.01%.

Specifically, the diameter of the ultra-high strength wire rod is less than or equal to 15mm, the microstructure is lamellar pearlite and bainite, no network cementite is uniformly distributed in the diameter range, wherein the volume fraction of lamellar pearlite is 50% -90%, and the volume fraction of bainite is 10% -50%. The bainite structure consists of a soft bainitic ferrite phase with high movable dislocation density and a martensite-austenite hard phase with high hardness, the pearlite-bainite composite structure is finally obtained, the strong plastic matching is excellent, and the ultrahigh strength target of the easy-to-draw and prestressed steel is realized; however, the volume fraction of bainite is not too large, which leads to a decrease in the strength of the wire rod/prestressed steel.

The diameter specification of the wire rod used for preparing the prestressed steel wire, wire and rope is generally not more than 15mm, and the diameter specification is usually 10-15 mm. The diameter of the wire rod for preparing the prestress steel strand is commonly used to be 12.5-13 mm, and the wire rod can be expanded to 14mm when the strength and the plasticity are excellent; the diameter of the wire rod for preparing the prestressed steel wire is commonly used to be 10-13 mm; the diameter of the wire rod for preparing the prestressed wire rope is usually 12.5-15 mm.

The lamellar pearly-luster structure with refined mass size and lamellar spacing, diversified lamellar orientation has high strength, good deformability and excellent drawing performance. However, if some network cementite is formed, the proeutectoid cementite at these austenite grain boundaries is coarser in size, more brittle, not only is the strength impaired, but also the drawing performance is deteriorated.

The steel of the invention obtains a pearlite-bainite composite structure through chemical components and cooling phase transformation control, the bainite structure is formed in an isothermal salt bath short-time heat treatment process or in an air cooling higher temperature section after the isothermal salt bath short-time heat treatment process, and consists of a bainitic ferrite soft phase with high movable dislocation density and a martensite-austenite hard phase with high hardness, and the steel has excellent strength-plasticity (reduction of area) matching and excellent drawing performance.

Specifically, the tensile strength of the ultra-high strength wire rod reaches 1400-1680 MPa, and the area shrinkage is more than or equal to 40%. The ultra-high strength wire rod has excellent tensile strength-plasticity (area reduction rate) matching, and is beneficial to preparing the downstream products such as prestressed steel/steel stranded wires with high strength and high toughness.

The invention also discloses a preparation method of the ultra-high strength wire rod, which comprises the following steps:

s4: and curling and post-processing the semi-finished wire rod to obtain the finished product of the ultra-high strength wire rod.

Specifically, in the step S1, a casting blank can be obtained by adopting vacuum induction smelting, die casting and forging modes.

In step S1, the excessive raw materials of each alloy element are weighed according to the alloy composition and added into a high-temperature converter, and the mixture is smelted by the converter, refined by LF, degassed by RH or VD, stirred electromagnetically and continuous cast into a casting blank. The method comprises the steps of controlling relevant parameters of converter smelting, LF refining, RH or VD degassing, carrying out real-time sampling analysis on molten steel in the converter, pouring out the molten steel when each element component reaches a preset value/range, and carrying out magnetic stirring and continuous casting. The process and the parameters are controlled to be a mature alloy preparation process in the prior art, and the parameters can be adjusted in the implementation process according to practical experience and specific alloy composition.

Specifically, the austenite homogenization temperature in the step S2 is 1180-1250 ℃, and the heat preservation time is 0.5-3 h. When the homogenization temperature is too high, the TiN part in the casting blank can be completely dissolved in a solid state, so that austenite grains are coarsened, and the energy cost is increased; too low homogenization may result in insufficient solid solution of strong carbonitride forming elements such as Nb, V, etc. in the steel, thereby affecting the subsequent finished product properties. The homogenization heat preservation time is too long, austenite grains coarsen, the energy cost is increased, and the production efficiency is not facilitated; the homogenization heat preservation is too short, and the thickness and temperature uniformity of the casting blank are difficult to ensure. Therefore, the heat preservation time is controlled to be 0.5-3 h.

Preferably, when the Nb content in the alloy is 0.01% -0.03%, the austenite homogenization temperature is 1200-1250 ℃, so that more than 80% of Nb is in a solid solution state.

Illustratively, the descaling operation in step S3 may employ a high-pressure water descaling technique or other conventional descaling process. The descaling operation has the function of removing the iron scales so as to prevent the iron scales from being pressed into the surface of the alloy/wire rod to generate defects, thereby improving the surface quality of the product.

Specifically, the heating temperature before hot rolling in the step S3 is 1180-1250 ℃, and the hot rolling finishing temperature is 890-950 ℃. The final rolling temperature is too high, the hot-rolled austenite grains are coarser, and the effect of refining the hot-rolled austenite grains by Nb microalloying is not obvious; too low a finish rolling temperature will result in more V precipitation and a significant increase in rolling force. The air cooling speed of cooling to 450 ℃ after hot rolling is more than or equal to 1.5 ℃/s, and the aim is to avoid forming a network cementite by hot rolling the wire rod, thereby avoiding the adverse effect of the network cementite on the final performance and the adverse effect on the uncoiling performance before offline salt bath.

Specifically, the post-treatment mode in step S4 is one of "online air cooling+isothermal salt bath short-time heat treatment+air cooling" or "primary air cooling+offline isothermal heat treatment+secondary air cooling+isothermal salt bath short-time heat treatment+air cooling".

Specifically, the cooling speed of the online air cooling or the primary air cooling or the secondary air cooling is 1.5 ℃/s-3.5 ℃/s. The air cooling speed is more than or equal to 1.5 ℃/s, and the formation of proeutectoid cementite (network cementite) can be avoided; when the air cooling speed is more than or equal to 3.5 ℃, a large amount of bainite tissues can be formed, and even part of superhard martensite tissues are formed in the subsequent cooling process, so that the drawing is not facilitated, and the excessive air cooling speed is avoided. The formation of bainite should be strictly limited in the subsequent isothermal salt bath short-time heat treatment and air cooling stages, so that the tissue ratio is convenient to control and is not excessively high.

Specifically, when the post-treatment mode is 'on-line air cooling + isothermal salt bath short-time heat treatment + air cooling', the isothermal salt bath heat treatment temperature is controlled to be 450-550 ℃, and the isothermal time is controlled to be 10-200 s. According to the preset chemical components, the pearlite structure with the volume fraction of 50-90% can be obtained by selecting proper temperature and time range. For the chemical composition of the invention, the higher the temperature, the greater the lamellar spacing of the pearlite is, the invention aims at ultra-high strength wire rods, so the temperature does not exceed 550 ℃, and the upper temperature does not exceed the fastest isothermal transformation temperature of the pearlite. On the other hand, the temperature is not lower than 450 ℃, as the lower limit temperature. The temperature is lower than the fastest pearlite isothermal transformation temperature, so the lower the temperature is, the longer the pearlite isothermal transformation time is, the pearlite isothermal transformation time of the steel is less than or equal to 200s at the lower limit temperature of 450 ℃, the pearlite isothermal transformation time at the upper limit temperature of 550 ℃ is less than or equal to 10 s, and the volume fraction of the pearlite can reach more than 50 percent by combining with the subsequent pearlite transformation in the air cooling section. It is worth emphasizing that the post-treatment mode is adopted without cooling the wire rod to room temperature after finish rolling, and the wire rod is directly cooled to the preset isothermal salt bath short-time heat treatment temperature in an online manner.

Specifically, when the post-treatment mode is primary air cooling, off-line isothermal heat treatment, secondary air cooling, isothermal salt bath heat treatment and air cooling, the off-line austenitizing temperature is controlled to 900-980 ℃, the heat preservation time is controlled to 10-60 s, the air cooling speed before entering the salt bath is more than or equal to 1.5 ℃/s, the formation of proeutectoid cementite (network cementite) is avoided, the isothermal salt bath heat treatment temperature is controlled to 450-550 ℃, and the isothermal time is controlled to 10-200 s. It is emphasized that before adopting off-line isothermal heat treatment, the wire rod after final rolling needs to be air-cooled to room temperature and then reheated.

The offline austenitizing temperature control principle is as follows: the lower limit is 30-100 ℃ above the equilibrium transformation temperature of the complete austenitization of the steel of the invention, so that complete austenitization can occur at the temperature during the offline heat treatment of the wire rod; the upper limit must not be too high, on the one hand, based on controlling the austenite grains to be fine and, on the other hand, on controlling the energy consumption. The off-line heat preservation time is matched with the off-line austenitizing temperature, the higher the temperature is, the shorter the time is, the austenitizing heat preservation is carried out for 10s at 980 ℃, the austenitizing heat preservation is carried out for 1min at 900 ℃, the fineness of austenite grains and the energy consumption can be controlled at a high temperature in a short time, and meanwhile, the austenitic phase transformation is sufficiently carried out and the components are homogenized.

The invention also discloses pearlite-bainite ultra-high strength pre-stressed steel, which is prepared from the ultra-high strength wire rod.

Specifically, the ultra-high strength wire rod is subjected to cold drawing and stabilizing heat treatment to prepare the high strength prestressed steel, and the specific operation and parameters are as follows:

Cold drawing: acid washing is carried out before cold drawing, the acid washing time is 15-25 min, a surface oxide layer is cleaned, and the surface quality of the wire rod before drawing is improved; because the wire rod has high strength, in order to establish better drawing lubrication conditions, the wire rod after pickling is subjected to phosphating treatment for 5-10 min, so that a compact phosphating film is formed on the surface of the wire rod; drawing times after phosphating are 7-15 times, and drawing speed is 2-4 m/s.

Stabilizing heat treatment: the cold drawn monofilament has a diameter of 5-7 mm, and the steel has high heat stability, high ductility, post-stranding stabilizing heat treatment temperature of 420-520 deg.c and linear speed of 1.2-2.0 m/s. The stabilization heat treatment process is fully separated out and matched with V, ti, nb, mo microalloy, the temperature is matched with the linear velocity, the linear velocity is high when the temperature is high, the linear velocity is low when the temperature is low, and the separation of V, ti, nb, mo nanometer MX second phase particles is facilitated, and the strong plasticity is improved.

Specifically, the diameter of the prestressed steel is less than or equal to 7mm, the tensile strength is 2260-2460 MPa, and the stress corrosion resistance meets the following requirements: and carrying out a solution A (analyzing pure ammonium thiocyanate aqueous solution) according to the specification of GB/T21839, carrying out a stress corrosion test by loading stress with 80% of the actual maximum force, wherein the test group is not less than 5 groups, the minimum value of the stress corrosion breaking time of the prestressed steel is not less than 2 hours, and the median value is not less than 5 hours. Solution a (analytically pure aqueous ammonium thiocyanate) was: 200g of NH ₄ SCN was dissolved in 800mL of distilled or demineralised water to prepare an ammonium thiocyanate solution. Ammonium thiocyanate is analytically pure, wherein the content of NH ₄ SCN is at least 98.5%, cl ^- is less than 0.005%,S^2-<0.001％。

Specifically, the ultra-high strength wire rod is subjected to cold drawing, hot galvanizing and stabilizing heat treatment to prepare the high strength prestressed steel, and specific operations and parameters are as follows:

Hot galvanizing: the diameter of the cold drawn monofilament is 3-7 mm, and the temperature of hot galvanizing (zinc aluminum, zinc aluminum magnesium) is 420-500 ℃ and the time is 20-40 s.

Stabilizing heat treatment: the hot galvanizing time is short, the heating processes experienced by the surface and the interior of the cold drawn steel wire are inconsistent, so that the original relatively uniform residual stress can become unevenly distributed, and macroscopic appearance is that the torsion performance of the steel wire is sharply reduced. Because the steel of the invention has good thermal stability, and simultaneously, in order to obtain higher ductility and torsion performance, the post-stranding stabilization heat treatment temperature is 420-480 ℃ and the linear velocity is 0.5-2.0 m/s. The stabilization heat treatment process is fully separated out and matched with V, ti, nb, mo microalloy, the temperature is matched with the linear velocity, the linear velocity is high when the temperature is high, the linear velocity is low when the temperature is low, and the separation of V, ti, nb, mo nanometer MX second phase particles is facilitated, and the strong plasticity is improved.

The diameter of the prestressed steel subjected to cold drawing, hot galvanizing and stabilizing heat treatment of the ultra-high strength wire rod is less than or equal to 7mm, the tensile strength is 2100-2200 MPa, and the torsion performance reaches: the torsion is more than or equal to 20 times.

The advantages of the precise control of the composition and process parameters of the steel according to the invention will be demonstrated in the following in the specific examples and comparative examples. The chemical composition of the steels of examples 1 to 8 and comparative example 1 are shown in Table 1, the specific rolling and cooling process parameters are shown in Table 2, the heat treatment process parameters are shown in Table 3, and the mechanical properties are shown in Table 4.

Comparative examples 1 and 3, which selected conventional 82B hot rolled wire rods as example components, processes and properties, comparative examples 2, 4 and 5 have higher carbon content with little or no microalloy elements added.

The specific steps are as follows:

Examples 1 to 2 were vacuum induction melted, die cast, and forged into a 160mm thick 160mm wide billet, 160mm thick 160mm wide 1000mm long 1 block was cut, welded to a conventional 82B160mm thick 160mm wide 11000mm long billet, heated to 1200-1210 c, heat-preserved for 1-1.5 hours, hot rolled to 13-15 mm wire rods after tapping, a finish rolling temperature of 896-905 c, and air cooling rate of 1.6-2.4 c/s after hot rolling to 450 c. The temperature of the offline austenitizing is 900 ℃, the temperature is kept for 60 seconds, the air cooling speed of the offline salt bath heat treatment is 2.0-2.5 ℃/s after austenitizing, the offline isothermal salt bath temperature is 475-500 ℃, the isothermal time is 100-150 seconds, and then the air cooling is carried out.

Examples 3-4 were smelted in a converter, LF refined, RH degassed, electromagnetic stirred, continuous cast to 160mm (thickness). Times.160 mm (width) square billets, cut to 12000mm billets, rolled to 1180-1250 ℃, kept at temperature for 2h, hot rolled to wire rods with diameters of 10-15 mm after tapping, and final rolling temperature of 892-912 ℃. The air cooling speed is 1.8-2.5 ℃/s before cooling to 450 ℃ after hot rolling, the on-line isothermal salt bath short-time heat treatment is adopted, the temperature of the salt bath is 525-550 ℃, the isothermal time is 80-100 s, and then air cooling is carried out.

Examples 5 to 6 were smelted in a converter, subjected to LF refining, RH degassing, electromagnetic stirring, continuous casting to give square billets 160mm (thick) ×160mm (wide), cutting into billets 12000mm long, rolling and heating to 1220-1250 ℃, respectively maintaining the temperature for 0.5-1.5 h, hot rolling to give wire rods 13-14 mm in diameter after discharging, and final rolling at 924-948 ℃. Cooling to the air cooling speed of 2.2-3.2 ℃/s before 450 ℃ after hot rolling, off-line austenitizing temperature of 930-950 ℃, preserving heat for 20-30 s, cooling to the air cooling speed of 2.2-2.6 ℃/s after austenitizing, off-line isothermal salt bath temperature of 450-550 ℃ and isothermal time of 30-200 s, and then air cooling.

Examples 7 to 8 were smelted by a converter, LF refined, VD degassed, electromagnetic stirred, continuous cast into 160mm thick by 160mm wide billets, cut into 12000mm long billets, rolled and heated to 1200-1210 ℃, kept for 2-3 hours, hot rolled into 13mm diameter wire rods after tapping, hot rolled to a finish rolling temperature of 900-910 ℃, cooled to an air cooling speed of 2.6-3.4 ℃/s before 450 ℃, heat treated by an off-line salt bath, heat preserved for 10 s-15 s at an off-line austenitizing temperature of 980 ℃, cooled to an air cooling speed of 3.0-3.5 ℃/s after austenitizing, and cooled to an off-line salt bath heat treatment at an off-line isothermal salt bath temperature of 525-550 ℃, isothermal time of 10-50 s, and then air cooled.

Table 1 chemical composition wt% of examples and comparative examples

Table 2 specific hot rolling cooling process parameters of examples and comparative examples

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Table 3 example on-line or off-line salt bath heat treatment process parameters

Table 4 diameter specification, tissue type and mechanical properties of the example and comparative wire rods

Numbering device	Diameter/mm	Tissue type (volume fraction)	Tensile strength/MPa	Area reduction/%
					Example 1	13	68% Pearlite+32% bainite	1600	44.0
Example 2	15	70% Pearlite+30% bainite	1690	42.0
					Example 3	15	87% Pearlite+13% bainite	1445	45.5
Example 4	10	80% Pearlite+20% bainite	1550	42.0
					Example 5	13	50% Pearlite+50% bainite	1605	41.5
Example 6	14	60% Pearlite+40% bainite	1590	42.0
					Example 7	13	55% Pearlite +45% bainite	1520	45.0
Example 8	13	58% Pearlite +42% bainite	1560	44.5
					Comparative example 1	13	100% Pearlite	1210	35.0
Comparative example 2	15	100% Pearlite	1410	32.0
					Comparative example 3	14	100% Pearlite	1190	34.0
Comparative example 4	15	100% Pearlite	1380	31.0
					Comparative example 5	14	100% Pearlite	1495	26.0

As can be seen from tables 2, 3 and 4, examples 1, 2 and 5 to 8 are "primary air cooling+offline isothermal heat treatment+secondary air cooling+isothermal salt bath heat treatment+air cooling" wire rods, which satisfy tensile strength of 1500 to 1680MPa (for example, 1520 to 1690 MPa), reduction of area of not less than 40% (for example, 41.5 to 45.0%), diameter specification of 13 to 15mm, microstructure of pearlite+bainite, wherein volume fraction of pearlite is 50 to 70%, volume fraction of bainite is 30 to 50%.

Comparative examples 2 and 5 are "primary air cooling + offline isothermal heat treatment + secondary air cooling + isothermal salt bath heat treatment + air cooling" wire rods, the wire rod tensile strength of diameter 14-15 mm is 1400-1480 MPa level, but the area shrinkage is only 26.0% -32.0%, because isothermal salt bath heat treatment time is longer, the microstructure is 100% pearlite.

Examples 3 and 4 are "on-line air cooling+isothermal salt bath short-time heat treatment+air cooling" wire rods, which satisfy the tensile strength of 1400-1500 MPa (for example, 1445-1550 MPa), the reduction of area of not less than 40% (for example, 42.0% -45.5%), the diameter specification of 10-15 mm, and the microstructure of pearlite+bainite, wherein the volume fraction of pearlite is 80% -87%, and the volume fraction of bainite is 10% -13%.

And comparative examples 1 and 3 are also common 82B hot rolled wire rods, the wire rod tensile strength level of 13-14 mm is only 1200MPa, and the area reduction rate is only 34.0% -35.0%. Comparative example 4 is an "on-line air cooling + isothermal salt bath short-time heat treatment + air cooling" wire rod, the wire rod with the diameter of 15mm has the tensile strength of 1380MPa and the area reduction rate of 31.0%. The microstructures of comparative examples 1,3 and 4 were all 100% pearlite.

Table 5 example wire rod corresponding prestressed steel hot galvanizing and stabilizing heat treatment process and performance

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The stabilizing heat treatment temperature is 420-520 ℃ and the linear velocity is 0.5-2.0 m/s

Because the steel is matched and added with the strong carbide element and the heat-strength element, the steel can obtain higher basic strength before cold drawing, and can also adopt higher temperature and longer time for hot galvanizing and stabilizing heat treatment after cold drawing, thereby improving plasticity and residual stress distribution and having smaller strength loss. The temperature and time of the cold drawing post treatment of the comparative example are relatively low, the strength loss is still large, and the strong plastic matching and the service performance are far inferior to those of the invention example.

As shown in Table 5, the prestressed steel with the diameter of 5-7 mm obtained by cold drawing and stabilizing heat treatment of the ultra-high strength wire rods of examples 1,2 and 5-7 has the tensile strength of 2260-2460 MPa, and the stress corrosion resistance meets the following requirements: and carrying out a solution A (analyzing pure ammonium thiocyanate aqueous solution) according to the specification of GB/T21839, carrying out a stress corrosion test by loading stress with 80% of the actual maximum force, wherein the test group is not less than 5 groups, the minimum value of the stress corrosion breaking time of the prestressed steel is not less than 2 hours, and the median value is not less than 5 hours. The high-strength wire rods of comparative examples 1, 3 and 4 were cold drawn, stabilized heat treated, prestressed steels having diameters of 5 to 7mm, and the minimum stress corrosion fracture time was 2.3 to 2.6 hours, and the median value was only 2.8 to 3.0 hours.

The prestressed steel with the diameter of 3-7 mm is prepared by carrying out cold drawing, hot galvanizing and stabilizing heat treatment on the ultrahigh-strength wire rods of examples 3, 4 and 8, wherein the tensile strength reaches 2100-2200 MPa, and the torsion performance reaches: the torsion is more than or equal to 20 times. The high-strength wire rods of comparative examples 2 and 5 were cold drawn, hot dip galvanized, and stabilized heat treated to 5mm diameter prestressed steels having tensile strengths of 2060 to 2160MPa, but the number of pass of torsion was only 8 to 10.

The present invention is not limited to the above-mentioned embodiments, and any changes or substitutions that can be easily understood by those skilled in the art within the technical scope of the present invention are intended to be included in the scope of the present invention.

Claims

1. The preparation method of the pearlite-bainite ultra-high strength wire rod is characterized by comprising the following steps of:

2. The method of manufacturing according to claim 1, characterized in that: the ultra-high strength wire rod alloy comprises ：C：0.90％～1.10％,Si：0.70％～1.20％,Mn：0.10％～1.00％,V：0.03％～0.15％,Ti：0.010％～0.025％,Cr：≤0.80％,Mo：≤0.35％,Nb：≤0.03％,Al：≤0.04％,P：≤0.02％,S：≤0.005％,N：≤0.004％, parts by mass of Fe and unavoidable impurities as the rest; and Si, mn, cr, mo alloy components in percentage by mass satisfy the following conditions: si+Mn+Cr+Mo is 1.2-2.4 and Cr+Mo is 1.0.

3. The method of manufacturing according to claim 1, characterized in that: the austenite homogenizing temperature in the step S2 is 1180-1250 ℃, and the heat preservation time is 0.5-3 h.

4. The method of manufacturing according to claim 1, characterized in that: the rolling in the step S3 is hot rolling, the heating temperature before hot rolling is 1180-1250 ℃, and the hot rolling finishing temperature is 890-950 ℃.

5. The method of manufacturing according to claim 1, characterized in that: the cooling speed of the on-line air cooling or the primary air cooling or the secondary air cooling is 1.5 ℃/s to 3.5 ℃/s.

6. The method of manufacturing according to claim 1, characterized in that: isothermal temperature of the isothermal salt bath short-time heat treatment is 450-550 ℃, and isothermal time is controlled to be 10-200 s.

7. The pearlite-bainite ultra-high strength wire rod is characterized in that: the ultra-high strength wire rod is produced by the production method of any one of claims 1 to 6.

8. The ultra-high strength wire rod of claim 7, wherein: the diameter of the ultra-high strength wire rod is less than or equal to 15mm, the microstructure is lamellar pearlite and bainite, the non-network cementite is uniformly distributed in the diameter range, wherein the volume fraction of lamellar pearlite is 50-90%, and the volume fraction of bainite is 10-50%.

9. The ultra-high strength wire rod of claim 7, wherein: the tensile strength of the ultra-high strength wire rod reaches 1400-1680 MPa, and the area shrinkage rate is more than or equal to 40%.

10. A pearlite-bainite ultra-high strength prestressed steel characterized by: the pre-stressed steel is made of an ultra-high strength wire rod according to any one of claims 7 to 9.