A steel strand with excellent fatigue resistance and its production method

Abstract

The present invention relates to a steel strand with excellent fatigue resistance and a production method thereof. By making full use of existing equipment conditions, optimizing the ratio of alloy elements, and combining the following production methods, including heating in a heating furnace, continuous rolling on multiple stands, cooling by flow control in different water tanks after finish rolling, Stelmor phase transformation cooling, coiling to form wire rods, drawing the wire rods and twisting the steel wires to form steel strands, and stabilizing and coiling the steel strands, the plasticity of the wire rods is improved under the existing production level, and the residual stress state of the steel wire is improved to increase the fatigue stress amplitude of the steel strand, thereby meeting the high stress amplitude fatigue requirement of 2000MPa-grade steel strands.

Description

Technical Field

[0001] The invention relates to a steel strand with excellent fatigue resistance and a production method thereof, belonging to the technical field of metallurgy. Background Art

[0002] Prestressed steel strands feature high tensile strength, low relaxation, and excellent fatigue resistance, making them widely used in the construction of roads, bridges, airports, hydropower dams, and other projects. With the rapid development of infrastructure in my country, the annual demand for prestressed steel strands has continued to increase by more than 10%, and strength requirements have gradually increased from 1860 MPa to 2000 MPa and above. Currently, the 2000 MPa level is the most widely used. Furthermore, as requirements for the load-bearing capacity and safety of prestressed steel strands increase, so too do the demands for their fatigue performance. In some structural systems, such as prestressed cables, the load on the strands varies widely, placing higher demands on the stress amplitude during fatigue testing. Currently, the fatigue stress amplitude requirement for prestressed cables has risen to over 300 MPa. Wire rods produced using conventional compositions and processes cannot meet the high-strength and high-fatigue performance requirements of steel strands.

[0003] In the prior art, application publication number CN 1778981A discloses a microalloying method for producing wire rod for steel strands. By adding a certain amount of Cr and V to 82B, the diameter of the pearlite pellets is reduced, the phase transformation point is lowered, and the pearlite interlamellar spacing suitable for drawing is obtained, thereby increasing the tensile strength of the wire rod and improving the drawing performance of the wire rod. The 13mm wire rod produced by this method can be used to draw 2000MPa steel strands. However, this application uses Cr combined with V microalloying to increase the strength of the wire rod. The V content added to the composition is above 0.065%, resulting in high alloy costs. The process window is also narrow, and the Cr content needs to be adjusted according to monthly changes to adapt to the cooling rate in different months, making the operation complicated. In addition, the wire rod produced by this method can be used to draw 2000MPa steel strands, but it cannot meet the fatigue requirements under high stress amplitudes.

[0004] Therefore, in order to solve the above problems, it is urgent to provide a steel strand that can meet the high stress amplitude fatigue requirements of 2000MPa grade steel strand. Summary of the Invention

[0005] The present invention provides a steel strand with excellent fatigue resistance and a production method thereof. By making full use of existing equipment conditions, optimizing the alloy element ratio and adjusting the additional controlled rolling and controlled cooling process, the plasticity of the wire rod is improved under the existing production level, thereby increasing the tolerance to surface defects and meeting the high stress amplitude fatigue requirements of 2000MPa grade steel strand.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A steel strand with excellent fatigue resistance. The wire rod used to make the steel strand comprises the following chemical elements by mass fraction: C: 0.80%-0.85%, Si: 0.40%-0.55%, Mn: 0.65%-0.75%, Cr: 0.16%-0.22%, V: 0.01-0.04%, Cr+V≤0.25%, Al: 0.01-0.03%, P≤0.015%, S≤0.015%, and the remainder is iron and unavoidable impurities.

[0008] The method for producing a steel strand with excellent fatigue resistance specifically comprises the following steps:

[0009] Step S1: Heating in a heating furnace. The preheating section temperature of the heating furnace is set according to the surface temperature of the billet when it enters the furnace. The preheating section temperature range is 910-1000°C. At the same time, the heating section temperature is set to 1070-1110°C, the soaking section temperature is set to 1120-1160°C, the heating time is set to 60-70 minutes, the air-fuel ratio is ≤0.65, and the starting rolling temperature is 1020-1040°C.

[0010] Step S2: multi-stand continuous rolling, performing preliminary rolling on the heated raw materials through rough rolling equipment, and then performing finishing rolling on the basis of rough rolling, with the inlet temperature set at 920-950°C and the outlet temperature set at ≤1000°C during finishing rolling;

[0011] Step S3: After finishing rolling, cooling is performed by controlling the flow of different water tanks, and the spinning temperature is set to 890-900°C;

[0012] Step S4: Stelmore phase change cooling: 10 fans are set on the Stelmore air cooling line and numbered from 1 to 10. Fans numbered 1 to 6 are turned on 100%, fans numbered 7 to 8 are turned on 50%, and fans numbered 9 to 10 are turned on 100%.

[0013] Step S5: After Stelmor cooling, the wire rod is coiled to form a wire rod;

[0014] Step S6: Selecting wire rods that meet quality requirements for drawing and twisting the steel wires to form stranded wires;

[0015] Step S7: Stabilizing the steel strands and coiling them, setting the stabilization temperature to 450-460°C and the treatment time to 80-100 seconds;

[0016] Furthermore, in step S1, when the surface temperature of the billet is less than 300°C when entering the furnace, the temperature of the preheating section of the heating furnace is set to 960-1000°C; when the surface temperature of the billet is greater than or equal to 300°C when entering the furnace, the temperature of the preheating section of the heating furnace is set to 910-950°C;

[0017] Furthermore, in step S2, during the finish rolling, the maximum temperature of the entire finish rolling process is maintained at ≤ 1000°C;

[0018] Furthermore, in step S3, after finishing rolling, cooling is performed by flow control of five water tanks, which are numbered from 1 to 5, and only the water tanks numbered 1, 3, and 5 are opened, and the water tanks numbered 2 and 4 are closed. The flow rate of the water tank numbered 1 is set to 1500-1800 L / min, the flow rate of the water tank numbered 3 is set to 1100-1300 L / min, and the flow rate of the water tank numbered 5 is set to 550-650 L / min.

[0019] Furthermore, in step S4, the air volume of the fan configured on the Stelmor air cooling line is 280,000 m 3 ; During the operation of the fan, the phase change temperature is maintained at 580-620℃;

[0020] Furthermore, in step S6, the wire rod is drawn 9 times to form a steel wire with a diameter of 5.06-5.20 mm;

[0021] Furthermore, the wire rod produced by steps S1-S5 of the production method has a tensile strength of ≥1250 MPa, an area reduction rate of ≥40%, a sorbite ratio of ≥90%, a grain size finer than grade 8.5, no complete decarburization, and a semi-decarburized layer depth of ≤0.1 mm;

[0022] Furthermore, the steel strand produced by steps S6-S7 of the production method did not break after 2 million tests when a cyclic load of 360 MPa was applied in an axial fatigue test.

[0023] Through the above technical solution, compared with the existing technology, the present invention has the following beneficial effects:

[0024] 1. The steel strand with excellent fatigue resistance provided by the present invention fully utilizes the influence of various elements on the strength and plasticity of steel. Through the associated design of alloy elements, a wire rod with excellent microstructure and mechanical properties is obtained. The production cost is low, which can improve market competitiveness.

[0025] 2. The production method of the steel strand with excellent fatigue resistance provided by the present invention, under the premise of the chemical composition design scheme of the wire rod used to produce the steel strand, uses the existing equipment level to design the controlled rolling and controlled cooling process, thereby ensuring the strengthening effect of the alloy elements and the acquisition of excellent structure. In the process of wire rod production and steel strand stabilization treatment, the release of internal stress of the material is fully considered in the process design. The obtained steel strand is subjected to a 360MPa stress amplitude fatigue performance test and can meet the requirement of 2 million times without breaking. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below with reference to the accompanying drawings and examples.

[0027] Figure 1 The present invention provides a flow chart of a method for producing a steel strand with excellent fatigue resistance. DETAILED DESCRIPTION

[0028] The present invention will now be described in further detail with reference to the accompanying drawings. In the description of this application, the specific dimensions used in the embodiments are only for illustrating the technical solutions and do not limit the scope of protection of the present invention.

[0029] As described in the background technology, the wire rods produced in the prior art can be used to draw 2000MPa steel strands, but cannot meet the fatigue requirements under high stress amplitudes in fatigue tests. Therefore, it is necessary to produce a steel strand that can meet the drawing requirements of 2000MPa steel strands and meet the relevant fatigue requirements tests.

[0030] To achieve the above requirements, it is necessary to start with the fatigue test itself. Here we will specifically explain what fatigue testing for steel strands is. The fatigue test for steel strands is tensile fatigue. Before the fatigue test, a certain tension needs to be applied to both ends of the steel strand, and then the fatigue test is carried out at a given stress amplitude. If the surface of the steel strand has a high axial tensile stress, then during the fatigue test, after the axial tensile stress is applied to it, the tensile stress on the surface of the steel wire will be superimposed. If the tensile stress on the surface of the steel strand exceeds its yield strength, failure will occur.

[0031] Therefore, the fatigue stress amplitude of the steel strand can be increased by improving the residual stress distribution of the steel wire and reducing the surface residual stress of the steel wire. Obviously, there are two factors that affect the fatigue resistance of the steel wire, namely residual stress and redistribution and diffusion of interstitial atoms.

[0032] The biggest innovation of this application is that it makes full use of existing equipment conditions. By optimizing the ratio of alloy elements and combining the adjustment of the entire steel strand production process, the plasticity of the wire rod is improved under the existing production level, which not only meets the drawing conditions but also meets the fatigue resistance requirements.

[0033] First of all, regarding the design of each chemical element, the wire rod used to make steel strand includes the following chemical elements in mass fraction: C: 0.80%-0.85%, Si: 0.40%-0.55%, Mn: 0.65%-0.75%, Cr: 0.16%-0.22%, V: 0.01-0.04%, Cr+V≤0.25%, Al: 0.01-0.03%, P≤0.015%, S≤0.015%, and the rest are iron and unavoidable impurities.

[0034] The above-mentioned chemical element components are set according to their respective characteristics. For example, C is the most economical strengthening element in steel materials. Increasing the C content can increase the initial strength of the wire rod; however, an increase in the C content will make the toughness of the wire rod poor and make segregation difficult to control. Therefore, in this application, the C content is controlled to 0.80-0.85%.

[0035] Mn can improve the hardenability of steel, thereby increasing the strength and hardness of steel. At the same time, Mn can also combine with the harmful element S to reduce the hot brittleness of steel. However, a high Mn content will lead to core segregation and grain coarsening. Therefore, in this application, the Mn content is controlled to 0.65-0.75%.

[0036] Cr can effectively improve the hardenability of steel and increase the work hardening rate during cold drawing of steel wire; however, too high a Cr content will reduce the toughness of the steel, and Cr has a large tendency to segregate, which can easily lead to the formation of core martensite. Therefore, in this application, the Cr content is controlled to 0.16-0.22%.

[0037] V can form fine compounds such as vanadium carbide or carbonitride with elements such as C and N in steel, and exists in cementite to play a strengthening role, which can make up for the insufficient strength caused by Stelmor controlled cooling; during the cold drawing process, V strengthens the cementite and reduces the deformation coordination of pearlite, which will aggravate the fragmentation of cementite and affect the mechanical properties of steel wire. Therefore, as a micro-alloying element, the content of V should be controlled at 0.01-0.04%.

[0038] It should also be noted that both Cr and V can significantly reduce the bainite transformation temperature, causing the bainite and pearlite regions to transform and separate, increasing the tendency of bainite formation; therefore, the total amount of Cr and V should be controlled below 0.25%.

[0039] Al is a deoxidizing element that removes free oxygen from steel. It can also refine grains and improve the plasticity of steel. In order to achieve better deoxidation and toughening effects, in this application, the aluminum content is controlled to 0.01-0.03%.

[0040] As for P and S, both are harmful elements and their contents should be as low as possible. Therefore, in this application, the contents of P and S are controlled below 0.015%.

[0041] When designing chemical elements, one of the biggest highlights is the Si element. Si, as a commonly used deoxidizer, can be dissolved in ferrite to play a role in solid solution strengthening, which can improve the strength and hardenability of wire rod. The segregation tendency of Si is much lower than that of Mn and Cr, and its cost is low. Therefore, by adding Si, the amount of Mn and Cr added can be reduced. However, as the Si content increases, the area reduction rate of steel decreases significantly; therefore, in this application, the Si element content is controlled at 0.40%-0.55%.

[0042] Secondly, the present application also provides a method for producing the steel strand with excellent fatigue resistance. Figure 1 As shown, the specific steps include:

[0043] Step S1: Heating in the heating furnace. The preheating section temperature of the heating furnace is set according to the surface temperature of the billet when entering the furnace. The temperature range of the preheating section is 910-1000°C. Specifically, when the surface temperature of the billet is less than 300°C when entering the furnace, the preheating section temperature of the heating furnace is set to 960-1000°C; when the surface temperature of the billet is ≥300°C when entering the furnace, the preheating section temperature of the heating furnace is set to 910-950°C.

[0044] At the same time, set the heating section temperature to 1070-1110℃, the soaking section temperature to 1120-1160℃, the heating time to 60-70min, the air-fuel ratio ≤0.65, and the rolling start temperature to 1020-1040℃.

[0045] The above-mentioned settings of the various parameters within the heating furnace can, on the one hand, prevent complete decarburization of the wire rod and ensure that the depth of the semi-decarburized layer is ≤0.1mm; on the other hand, they can control grain growth, which helps to more evenly distribute interstitial atoms in the billet, thereby reducing residual stresses caused by uneven microstructure during phase transformation and eliminating residual stresses generated during solidification. In particular, the setting of the preheating section temperature is determined by the surface temperature of the billet before entering the furnace (the temperature is determined by hot charging or cold charging). If the surface temperature of the billet before entering the furnace is 300°C or above, reducing the preheating section temperature range by 50°C relative to room temperature can reduce the billet surface heating rate, increase the core-surface temperature difference, increase the rate of residual heat transfer from the core, reduce the billet's residence time in the high-temperature section, and avoid core grain coarsening.

[0046] Step S2: Multi-stand continuous rolling, the heated raw materials are preliminarily rolled by rough rolling equipment, and finishing rolling is performed on the basis of rough rolling. During finishing rolling, the inlet temperature is set to 920-950℃, and the outlet temperature is ≤1000℃. Finer austenite grains can be obtained by low-temperature rolling, thereby improving the plasticity of the wire rod.

[0047] Step S3: After finishing rolling, cooling is performed by controlling the flow rates of different water tanks. The coordination of the opening and closing of the water tanks and the water flow rate prevents a sharp drop in the surface temperature of the wire rod while also facilitating a temperature recovery and uniformity between the core and surface temperatures. Specifically, cooling is performed by controlling the flow rates of five water tanks, numbered 1-5. Only water tanks 1, 3, and 5 are open, while water tanks 2 and 4 are closed. The flow rate of water tank 1 is set to 1500-1800 L / min, that of water tank 3 to 1100-1300 L / min, and that of water tank 5 to 550-650 L / min.

[0048] At the same time, the spinning temperature is set to 890-900℃. After spinning, the cooling rates at the core and surface of the wire rod are relatively close, the cooling is more uniform, and the microstructure differences between the core and surface are small. The reason for setting the spinning temperature range is that a higher spinning temperature can increase the austenite undercooling, reduce the phase transformation temperature, and increase the cooling rate before the phase transformation.

[0049] In this step, the opening and closing of the water volume in the water cooling box is controlled under the premise of ensuring the spinning temperature, and rapid cooling and temperature recovery are carried out after rolling to make the temperature of the entire cross section of the wire rod as uniform as possible, thereby reducing the residual stress inside the wire rod before phase transformation, so as to reduce the stress inside the wire rod after rolling; at the same time, it promotes the reduction of the wire rod grain size and the diameter of the sorbite pellets, so that the wire rod obtained after air cooling has higher plasticity.

[0050] Step S4: Stelmore phase change cooling, the air volume of the fan configured on the Stelmore air cooling line is 280,000 m 3 Therefore, 10 fans are set up and numbered from 1 to 10. The fans numbered 1 to 6 are turned on 100%, and the cooling rate before phase transformation can reach 14-15℃ / s. Increasing the cooling rate before phase transformation can make the wire rod quickly cool to below the formation temperature of network cementite, avoiding the formation of network cementite; fans numbered 7-8 are turned on 50%, and fans numbered 9-10 are turned on 100% to avoid the formation of bainite structure due to too low temperature in the early stage of phase transformation and the high temperature return due to the release of latent heat of phase transformation in the late stage of phase transformation, so that the phase transformation temperature is maintained at 580-620℃, which can increase the proportion of troostite to more than 90%.

[0051] Step S5: After Stelmor cooling, the wire rod is coiled to form a wire rod. The wire rod produced through steps S1-S5 now has a tensile strength of 1250 MPa or higher, an area reduction of 40% or higher, a sorbite fraction of 90% or higher, a grain size finer than grade 8.5, no complete decarburization, and a semi-decarburized layer depth of 0.1 mm or less. All of these wire rod specifications fully meet existing technical requirements for producing 15.2 mm diameter steel strand with a strength of 2000 MPa.

[0052] Next is the production process of steel strands, step S6: select wire rods that meet the quality requirements and draw them 9 times to form steel wires with a diameter of 5.06-5.20 mm, and then twist the steel wires to form steel strands.

[0053] Step S7: Stabilize the steel strands and coil them, then pack them. During the stabilization process, the temperature is set at 450-460°C and the treatment time is 80-100 seconds.

[0054] Step S7 is a more prominent innovation in the steel strand production process. It needs to be explained here in conjunction with the setting of the Si element content in the chemical composition design. In order to improve the strength of conventional products, the stabilization treatment temperature will be lower, but the lower stabilization temperature is not conducive to the release of steel wire stress. This application reasonably increases the Si element content in the steel strand (the Si element content is controlled at 0.40%-0.55%), reduces the speed of cementite spheroidization during the stabilization treatment of the steel strand, and reduces the strength loss, so that the heating temperature of the steel strand can be increased to 450-460°C during the stabilization treatment. Increasing the heating temperature will promote the thermal motion of atoms, accelerate the diffusion of interstitial atoms, and make the residual stress more fully released, thereby improving the fatigue resistance of the ultra-high strength steel strand.

[0055] Furthermore, tests have shown that when stabilizing at 450-460°C, the axial residual stress on the wire surface initially increases and then gradually decreases with increasing heating time. However, excessive heating time can reduce dislocation density and deteriorate relaxation properties. Therefore, a stabilization treatment time of 80-100 seconds can maintain a low residual stress on the wire surface, allowing sufficient time for atoms to diffuse and redistribute, while ensuring stable relaxation properties.

[0056] Finally, the steel strand produced by steps S6-S7 of the production method did not break after 2 million tests when a cyclic load of 360 MPa was applied in an axial fatigue test, thereby greatly improving the service life.

[0057] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as such, will not be interpreted in an idealized or overly formal sense.

[0058] The meaning of "and / or" in this application means that both situations where each exists alone or both exist at the same time are included.

[0059] The term “connection” as used in this application may mean a direct connection between components or an indirect connection between components via other components.

[0060] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A method for producing a steel strand with excellent fatigue resistance, characterized in that: The specific steps include: Step S1: Heating in a heating furnace. The preheating section temperature of the heating furnace is set according to the surface temperature of the billet when it enters the furnace. The preheating section temperature range is 910-1000°C. At the same time, the heating section temperature is set to 1070-1110°C, the soaking section temperature is set to 1120-1160°C, the heating time is set to 60-70 minutes, the air-fuel ratio is ≤0.65, and the starting rolling temperature is 1020-1040°C. Step S2: Multi-stand continuous rolling, performing preliminary rolling on the heated raw materials through rough rolling equipment, and then performing finishing rolling on the basis of rough rolling. During finishing rolling, the inlet temperature is set at 920-950°C, the outlet temperature is ≤1000°C, and the maximum temperature of the entire finishing rolling process is maintained at ≤1000°C; Step S3: After finishing rolling, cooling is performed by flow control of five water tanks, which are numbered 1-5. Only water tanks 1, 3, and 5 are opened, and water tanks 2 and 4 are closed. The flow rate of water tank 1 is set to 1500-1800 L / min, the flow rate of water tank 3 is set to 1100-1300 L / min, and the flow rate of water tank 5 is set to 550-650 L / min. At the same time, the spinning temperature is set to 890-900°C. Step S4: Stelmore phase change cooling. Ten fans are set on the Stelmore air cooling line and numbered from 1 to 10. Fans numbered 1 to 6 are turned on 100%, fans numbered 7 to 8 are turned on 50%, and fans numbered 9 to 10 are turned on 100%. The air volume of the fans configured on the Stelmore air cooling line is 280,000 m 3 ; During the operation of the fan, the phase change temperature is maintained at 580-620℃; Step S5: After Stelmor cooling, the steel strand is coiled to form a wire rod; the wire rod used to make the steel strand comprises the following chemical elements by mass fraction: C: 0.80%-0.85%, Si: 0.40%-0.55%, Mn: 0.65%-0.75%, Cr: 0.16%-0.22%, V: 0.01-0.04%, Cr+V≤0.25%, Al: 0.01-0.03%, P≤0.015%, S≤0.015%, and the remainder is iron and unavoidable impurities; Step S6: Selecting wire rods that meet quality requirements for drawing and twisting the steel wires to form stranded wires; Step S7: Stabilizing the steel strands and coiling them, setting the stabilization temperature to 450-460°C and the treatment time to 80-100 seconds; The steel strand produced by steps S6-S7 of the production method did not break after 2 million tests when a cyclic load of 360 MPa was applied in an axial fatigue test.

2. The method for producing a steel strand with excellent fatigue resistance according to claim 1, characterized in that: In step S1, when the surface temperature of the blank is less than 300°C when entering the furnace, the temperature of the preheating section of the heating furnace is set to 960-1000°C; when the surface temperature of the blank is greater than or equal to 300°C when entering the furnace, the temperature of the preheating section of the heating furnace is set to 910-950°C.

3. The method for producing a steel strand with excellent fatigue resistance according to claim 1, characterized in that: In step S6, the wire rod is drawn 9 times to form a steel wire with a diameter of 5.06-5.20 mm.

4. The method for producing a steel strand with excellent fatigue resistance according to claim 1, characterized in that: The wire rod produced by steps S1-S5 of the production method has a tensile strength of ≥1250 MPa, an area reduction rate of ≥40%, a sorbite ratio of ≥90%, a grain size finer than grade 8.5, no complete decarburization, and a semi-decarburized layer depth of ≤0.1 mm.

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