Preparation method of steel strand with fatigue stress amplitude above 300 MPa

Abstract

The present invention discloses a method for preparing a steel strand with a fatigue stress amplitude of more than 300 MPa. The method comprises the following steps. The chemical composition of the wire rod used is as follows in mass percentage: C 0.80 - 0.85%, Si 0.8 - 1.2%, Mn 0.65 - 0.85%, Cr + 2V > 0.30%, and the rest is Fe and inevitable inclusions. Pickling and phosphating are carried out with this wire rod as the bus bar. The bus bar is subjected to 9 - 11 passes of dry drawing, the reduction ratio of the last pass is 5 - 10%, and the reduction ratio of the remaining passes is ≤18% to obtain an intermediate product wire. The intermediate product wire is twisted into a prestressed steel strand of 1 center wire + 6 side wires. The twisted steel strand is subjected to a stabilization treatment, and the temperature of the stabilization treatment is 415 - 440°C and the heat preservation time is 10 - 20 s. Thus, the obtained finished steel strand can meet the requirement of a fatigue stress amplitude of more than 300 MPa.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel material preparation, and relates to a method for preparing a steel strand with a fatigue stress amplitude of more than 300 MPa. Background Art

[0002] Prestressed steel strands have the characteristics of high tensile strength, low relaxation value, and good fatigue resistance, and are widely used in engineering construction such as roads, bridges, airports, and water conservancy dams. With the rapid development of infrastructure construction, the annual demand for prestressed steel strands increases greatly, and the development and application of prestressed steel strands will further promote the industrial upgrading in the field of engineering construction.

[0003] In recent years, the requirements for the bearing capacity and safety of prestressed steel strands have been continuously improved, and the requirements for their fatigue performance have also been gradually increased. In some structural systems such as prestressed cables, the load of the steel strand has the characteristic of large variation amplitude, which puts forward higher requirements for the stress amplitude during its fatigue detection. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a steel strand, in which the residual stress in the produced steel strand is low and can meet the performance requirements of a fatigue stress amplitude of more than 300 MPa.

[0005] To achieve the above-mentioned invention purpose, an embodiment of the present invention provides a method for preparing a steel strand with a fatigue stress amplitude of more than 300 MPa, which includes the following steps.

[0006] The chemical composition of the used wire rod is, by mass percentage: C 0.80 - 0.85%, Si 0.8 - 1.2%, Mn 0.65 - 0.85%, Cr + 2V > 0.30%, and the rest is Fe and inevitable inclusions; pickling and phosphating are carried out with this wire rod as the bus bar.

[0007] The phosphated bus bar is subjected to 9 - 11 passes of dry drawing to obtain an intermediate product wire; among them, the reduction ratio of the last pass is 5 - 10%, and the reduction ratio of the remaining passes is ≤18%.

[0008] The obtained intermediate product wire is twisted into a prestressed steel strand of 1 center wire + 6 side wires.

[0009] The twisted steel strand is subjected to stabilization treatment to obtain a finished product of a steel strand with a fatigue stress amplitude of more than 300 MPa; among them, the temperature of the stabilization treatment is 415 - 440°C, and the holding time of the stabilization treatment is 10 - 20 s.

[0010] Preferably, the diameter of the used wire rod is 12.5 - 13 mm.

[0011] Preferably, in the step of "performing 9 to 11 passes of dry drawing on the phosphated busbar to obtain an intermediate wire rod", the obtained intermediate wire rod includes edge wires with a diameter of 5.0 mm and a center wire with a diameter of 5.3 mm.

[0012] Preferably, in the step of "performing 9 to 11 passes of dry drawing on the phosphated busbar to obtain an intermediate wire rod",

[0013] The reduction ratio of the first pass is 12 to 15%;

[0014] The reduction ratio of the second pass is higher than that of the other passes, and the reduction ratio is ≤ 18%;

[0015] The reduction ratios of the passes after the third pass decrease one by one;

[0016] The reduction ratio of the last pass is 5 to 10%.

[0017] Preferably, in the step of "performing 9 to 11 passes of dry drawing on the phosphated busbar to obtain an intermediate wire rod", the working cone angle of the wire drawing die used for dry drawing is controlled at 8 to 10°.

[0018] Preferably, in the step of "performing 9 to 11 passes of dry drawing on the phosphated busbar to obtain an intermediate wire rod", the drawing speed is not higher than 4 m / s.

[0019] Preferably, in the step of "twisting the obtained intermediate wire rod into a prestressed steel strand of 1 center wire + 6 edge wires", the diameter of the twisted prestressed steel strand is 15.24 mm.

[0020] Preferably, in the step of "performing a stabilization treatment on the twisted steel strand to obtain a steel strand finished product with a fatigue stress amplitude of more than 300 MPa", the tensile stress during the stabilization treatment is not greater than 0.5σ b , where σ b is the nominal tensile strength of the steel strand finished product.

[0021] Furthermore, in the step of "performing a stabilization treatment on the twisted steel strand to obtain a steel strand finished product with a fatigue stress amplitude of more than 300 MPa", the fatigue life of the obtained steel strand finished product is not less than 2 million times when the stress amplitude is more than 300 MPa.

[0022] Furthermore, in the step of "performing a stabilization treatment on the twisted steel strand to obtain a steel strand finished product with a fatigue stress amplitude of more than 300 MPa", the tensile strength of the obtained steel strand finished product is 1860 MPa or more.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: Based on the design of chemical components, combined with comprehensive improvements in dry drawing technology and stabilization treatment technology, on the one hand, it can significantly reduce the axial residual tensile stress on the surface of the steel wire, improve the fatigue limit of the steel wire; on the other hand, based on the composition design, it can inhibit the strength loss of the steel wire during the stabilization process, and based on the technical improvements in the reduction ratio and stabilization treatment, it can avoid strength loss, ensuring that the steel wire has excellent tensile strength. On the third hand, it can improve the consistency of the core-surface deformation of the steel wire, avoid the defects of uneven core-surface, and finally obtain a steel strand with excellent comprehensive performance, making the finished steel strand have high strength and fatigue resistance not less than 2 million times when the stress amplitude ≥ 300 MPa. Detailed implementation mode

[0024] The technical solution of the present invention will be further introduced below in combination with specific implementation modes.

[0025] In an embodiment of the present invention, a method for preparing a steel strand is provided, which can reduce the residual stress in the steel strand and increase the fatigue stress amplitude of the steel strand to more than 300 MPa. That is, the residual stress in the finished steel strand prepared by this preparation method is low, and it can meet the performance requirements of a fatigue stress amplitude of more than 300 MPa.

[0026] Specifically, the bus bar used in this preparation method is a wire rod that meets the following chemical components. The chemical components of the wire rod are in mass percentage: C 0.80 - 0.85%, Si 0.8 - 1.2%, Mn 0.65 - 0.85%, Cr + 2V > 0.30%, and the rest are Fe and inevitable inclusions.

[0027] The basic functions of the above chemical elements are briefly introduced here.

[0028] C is the most basic strengthening element in steel. For every 0.01% increase in the C content, the strength of the wire rod increases by about 10 MPa. However, excessive C will promote the precipitation of proeutectoid cementite at the center segregation, and in severe cases, it will form a network of cementite, reducing the plasticity of the wire rod and causing wire breakage during drawing. In one embodiment, the C content range is limited to 0.80 - 0.85%, preferably 0.82% - 0.84%.

[0029] Si is a ferrite strengthening element, which can improve the strength of ferrite through solid solution strengthening. Si is also an important deoxidizer, which helps to reduce the oxygen content in steel and reduce inclusions. In addition, the enrichment of Si at the ferrite / cementite interface helps to improve the thermal stability of cementite during the stabilization treatment, thereby reducing strength loss. However, too much Si will cause decarburization and reduce the plasticity of the wire rod. In one embodiment, the Si content range is 0.8 - 1.2%, preferably 0.9% - 1.0%.

[0030] Mn is mainly used in steel to increase the strength of the steel. At the same time, it can increase the stability of austenite, lower the phase transformation temperature. Meanwhile, Mn can change the composition of sulfides and reduce the harmful effects of S. However, too high Mn content is likely to cause segregation, which has an adverse effect on microstructure control. Therefore, the Mn content should not be too high. In one embodiment, the range of the Mn content is limited to 0.65 - 0.85%, preferably 0.65% - 0.70%.

[0031] Both Cr and V are carbide - forming elements. On the one hand, appropriately adding Cr and V can improve the hardenability of the wire rod and increase the strength of the wire rod under the same cooling conditions. On the other hand, it can increase the thermal stability of the cementite lamellae and inhibit the spheroidization of the steel wire during the heat treatment process. In one embodiment, the content range of Cr + 2V is > 0.30%, preferably the upper limit of the content is 0.5%, and more preferably the range is 0.35% - 0.45%.

[0032] The preparation method uses the above - mentioned wire rod as the bus bar and prepares the finished steel strand through a process route of pickling, phosphating, dry drawing, twisting, and stabilization treatment in sequence. The specific introduction is as follows.

[0033] Step 1), pickle and phosphate the above - mentioned wire rod. The specific pickling process and phosphating process can be implemented using existing known feasible technologies and will not be elaborated.

[0034] Step 2), perform 9 - 11 passes of dry drawing on the phosphatized bus bar to obtain the intermediate - product steel wire.

[0035] Among them, the reduction ratio of the last pass is 5 - 10%, and the reduction ratio of the remaining passes is ≤ 18%. Preferably, the reduction ratio of the first pass is 12 - 15%; the reduction ratio of the second pass is higher than that of the remaining passes and the reduction ratio is ≤ 18%; the reduction ratios of the passes after the third pass decrease one by one; the reduction ratio of the last pass is 5 - 10%. In this way, on the basis of dry drawing, by controlling the reduction ratios of each pass, on the one hand, the tensile strength of the steel wire can be ensured, on the other hand, the axial residual tensile stress on the surface of the steel wire can be greatly reduced (for example, the axial residual tensile stress on the surface is reduced to below 500 MPa), thereby avoiding the negative impact of the residual tensile stress on the fatigue limit and further greatly improving the fatigue performance. On the third hand, the consistency of the core - surface deformation of the steel wire can be ensured, avoiding the defect of uneven core - surface.

[0036] Preferably, the working cone angle of the wire - drawing die used for dry drawing is controlled at 8 - 10°.

[0037] Preferably, the drawing speed is not higher than 4 m / s.

[0038] Preferably, the diameter of the wire rod used is 12.5 - 13 mm, and the intermediate product wire obtained by drawing includes edge wires with a diameter of 5.0 mm and a center wire with a diameter of 5.3 mm.

[0039] Step 3), twist the obtained intermediate product wire into a prestressed steel strand of 1 center wire + 6 edge wires.

[0040] Among them, the specific twisting process can be implemented by using existing known feasible technologies, which will not be elaborated.

[0041] Preferably, the twisted prestressed steel strand is a steel strand with a diameter of 15.24 mm and including 1 center wire with a diameter of 5.3 mm and 6 edge wires with a diameter of 5.0 mm.

[0042] Step 4), perform a stabilization treatment on the twisted steel strand to obtain a finished steel strand product.

[0043] Among them, the temperature of the stabilization treatment is 415 - 440 °C, and the heat preservation time of the stabilization treatment is 10 - 20 s. In this way, on the one hand, the stress state on the surface of the wire can be optimized, thereby reducing the tensile stress on the surface of the wire, improving the fatigue limit and fatigue performance. On the other hand, the spheroidization of the wire structure can be avoided, thus ensuring the excellent tensile strength of the wire.

[0044] Preferably, the tensile stress during the stabilization treatment is not greater than 0.5σ b , where σ b is the nominal tensile strength of the finished steel strand product. The nominal tensile strength here refers to the standard tensile strength of the finished steel strand (after stabilization treatment), such as levels like 1770 MPa, 1860 MPa, etc. In fact, the actual tensile strength of the obtained steel strand is higher than the nominal tensile strength. Here, when controlling the tensile stress during the stabilization treatment, it can be implemented according to the requirements of the nominal tensile strength of the finished steel strand product to be produced (for example, planning to produce a finished steel strand product of 1860 MPa grade).

[0045] Based on the above method, the fatigue stress amplitude of the obtained finished steel strand product is above 300 MPa, that is, when meeting the stress amplitude test requirements of 300 MPa and above, for example, the fatigue life reaches more than 2 million times when the stress amplitude is above 300 MPa. And, the residual stress of this finished steel strand product is low, and the tensile strength is above the 1860 MPa level, specifically above 1890 MPa.

[0046] Compared with the prior art, the beneficial effects of this embodiment are as follows: Based on the design of chemical components, combined with comprehensive improvements in dry drawing technology and stabilization treatment technology, on the one hand, it can significantly reduce the axial residual tensile stress on the surface of the steel wire and improve the fatigue limit of the steel wire. On the other hand, based on the component design, it can inhibit the strength loss of the steel wire during the stabilization process, and based on the technical improvements in the compression ratio and stabilization treatment, it can avoid strength loss, ensuring that the steel wire has excellent tensile strength. On the further hand, it can improve the consistency of the core-surface deformation of the steel wire and avoid the defect of uneven core-surface, and finally obtain a steel strand with excellent comprehensive performance, making the finished steel strand have high strength and fatigue resistance of not less than 2 million times when the stress amplitude ≥ 300 MPa.

[0047] The detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

[0048] The following provides three embodiments of the present invention to further illustrate the technical solutions of the present invention. Of course, these embodiments are only a part of the numerous variant embodiments included in the present invention, rather than all of them.

[0049] Example 1

[0050] Select wire rods with a diameter of 12.5 mm as the base material to produce 1860 MPa grade (i.e., σ b is 1860 MPa) prestressed steel strands. The chemical composition of the wire rods is as follows by mass percentage: C 0.82%, Si 0.90%, Mn 0.65%, Cr 0.25%, V 0.06%, and the rest is Fe and unavoidable inclusions.

[0051] Using the above wire rods as the busbars, pickling and phosphating are carried out.

[0052] Then, the phosphated wire rods are subjected to 10 - 11 passes of dry drawing to obtain intermediate steel wires (including side wires with a diameter of 5.0 mm and a center wire with a diameter of 5.3 mm). Among them, the compression ratio of each pass is shown in Table 1; the working cone angle of the wire drawing dies for the first three passes is 10°, and the working cone angle of the subsequent wire drawing dies is 8°; the drawing speed is 5 m / s.

[0053] Table 1 Compression ratio of each pass

[0054]

[0055] The obtained intermediate steel wires are twisted into a 1×7 (1 center wire with a diameter of 5.3 mm and 6 side wires with a diameter of 5.0 mm) prestressed steel strand with a diameter of 15.24 mm.

[0056] The twisted steel strand is subjected to stabilization treatment at a temperature of 420 °C, a heat preservation time of 15 s, and a tensile stress of 0.46σ b , and the finished steel strand is obtained.

[0057] The mechanical properties of the obtained finished steel strand and the axial residual tensile stress on the surface of the single wire are shown in Table 2.

[0058] Table 2 Mechanical properties of the steel strand and surface residual stress

[0059] Specification Tensile strength Total elongation at maximum force Number of fatigue cycles Axial residual tensile stress on the surface of single wire 15.24mm 1926MPa 7.5％ > 2 million times 410MPa

[0060] The fatigue performance test is carried out on the obtained finished steel strand. The fatigue stress amplitude is 300 MPa, and the upper limit of the fatigue pulsating load is 0.45F ma , F ma is the actual maximum force of the steel strand, and no fracture occurs after 2 million times of pulsating load.

[0061] Example 2

[0062] Wire rods with a diameter of 13 mm are selected as the base material to produce 1860 MPa grade (i.e., σ b is 1860 MPa) prestressed steel strands. The chemical composition of the wire rods is as follows by mass percentage: C 0.83%, Si 0.94%, Mn 0.70%, Cr 0.30%, V 0.03%, and the rest is Fe and unavoidable inclusions.

[0063] Using the above wire rods as the bus bars, pickling and phosphating are carried out.

[0064] Then, the phosphated wire rods are subjected to 10 - 11 passes of dry drawing to obtain intermediate product steel wires (including side wires with a diameter of 5.0 mm and a center wire with a diameter of 5.3 mm). Among them, the reduction ratio of each pass is shown in Table 3; the working cone angle of the wire drawing dies for the first three passes is 10°, and the working cone angle of the subsequent wire drawing dies is 8°; the drawing speed is 6 m / s.

[0065] Table 3 Reduction ratio of each pass

[0066]

[0067] The obtained intermediate product steel wires are twisted into a 1×7 (1 center wire with a diameter of 5.3 mm and 6 side wires with a diameter of 5.0 mm) prestressed steel strand with a diameter of 15.24 mm.

[0068] The twisted steel strand is subjected to stabilization treatment at a temperature of 435 °C, a heat preservation time of 15 s, and a tensile stress of 0.43σ b , and the finished steel strand is obtained.

[0069] The mechanical properties of the obtained finished steel strand and the axial residual tensile stress on the surface of the single wire are shown in Table 4.

[0070] Table 4 Mechanical Properties and Surface Residual Stress of Steel Strand

[0071] Specification Tensile strength Total elongation at maximum force Number of fatigue cycles Axial residual tensile stress on the surface of single wire 15.24mm 1902MPa 8.0％ > 2 million times 380MPa

[0072] For the fatigue performance test of the obtained finished steel strand, the fatigue stress amplitude is 380 MPa, and the upper limit of the fatigue pulsating load is 0.45F ma , F ma is the actual maximum force of the steel strand, and no fracture occurs after 2 million times of pulsating load.

[0073] Example 3

[0074] Wire rods with a diameter of 13 mm are selected as the base material to produce 1860 MPa grade (i.e., σ b is 1860 MPa) prestressed steel strands. The chemical composition of the wire rods is as follows by mass percentage: C 0.83%, Si 1.0%, Mn 0.68%, Cr 0.33%, V 0.02%, and the rest is Fe and inevitable inclusions.

[0075] Using the above wire rods as the bus bars, pickling and phosphating are carried out.

[0076] Then, the phosphated wire rods are subjected to 10 - 11 passes of dry drawing to obtain intermediate product wires (including 5.0 mm side wires and 5.3 mm center wires). Among them, the reduction ratio of each pass is shown in Table 5; the working cone angle of the wire drawing dies for the first three passes is 10°, and the working cone angle of the subsequent wire drawing dies is 8°; the drawing speed is 6 m / s.

[0077] Table 5 Reduction Ratio of Each Pass

[0078]

[0079] The obtained intermediate product wires are twisted into a 1×7 (1 center wire with a diameter of 5.3 mm and 6 side wires with a diameter of 5.0 mm) prestressed steel strand with a diameter of 15.24 mm.

[0080] The twisted steel strand is subjected to stabilization treatment. The temperature of the stabilization treatment is 430 °C, the heat preservation time is 20 s, and the tensile stress is 0.45σ b , to obtain the finished steel strand.

[0081] The mechanical properties of the obtained finished steel strand and the axial residual tensile stress on the surface of the single wire are shown in Table 6.

[0082] Table 6 Mechanical Properties and Surface Residual Stress of Steel Strand

[0083] Specification Tensile strength Total elongation at maximum force Number of fatigue cycles Axial residual tensile stress on the surface of single wire 15.24mm 1912MPa 8.2％ > 2 million times 392MPa

[0084] The fatigue performance test of the obtained finished steel strand was carried out with a fatigue stress amplitude of 350 MPa and the upper limit of the fatigue pulsating load being 0.45F ma , F ma being the actual maximum force of the steel strand, and no fracture occurred after 2 million times of pulsating load.

Claims

1. A preparation method of a steel strand with a fatigue stress amplitude of more than 300 MPa, characterized in that, including the following steps, The chemical composition of the wire rod used is by mass percentage: C 0.80 - 0.85%, Si ≥ 0.8 and <1.0%, Mn 0.65 - 0.85%, containing Cr and V and Cr + 2V > 0.30% and ≤ 0.5%, the rest being Fe and inevitable inclusions; pickling and phosphating are carried out with this wire rod as the busbar; The phosphated busbar is subjected to 9 - 11 passes of dry drawing to obtain an intermediate product wire; among them, the reduction ratio of the first pass is 12 - 15%, the reduction ratio of the last pass is 5 - 10%, the reduction ratios of the remaining passes are ≤ 18%, the reduction ratio of the second pass is higher than that of the remaining passes, and the reduction ratios of the passes after the third pass decrease one by one; The obtained intermediate product wire is twisted into a prestressed steel strand of 1 center wire + 6 side wires; The twisted steel strand is subjected to stabilization treatment to obtain a finished steel strand with a fatigue stress amplitude of more than 300 MPa; the temperature of the stabilization treatment is 415 - 440 °C, the heat preservation time of the stabilization treatment is 10 - 20 s, and the tensile stress during the stabilization treatment is 0.43σ b ~0.5σ b , where σ b is the nominal tensile strength of the finished steel strand.

2. The preparation method of the steel strand with a fatigue stress amplitude of more than 300 MPa according to claim 1, characterized in that, The diameter of the wire rod used is 12.5 - 13 mm; In the step "The phosphated busbar is subjected to 9 - 11 passes of dry drawing to obtain an intermediate product wire", the obtained intermediate product wire includes side wires of 5.0 mm and a center wire of 5.3 mm.

3. The preparation method of the steel strand with a fatigue stress amplitude of more than 300 MPa according to claim 1, characterized in that, The chemical composition of the wire rod is by mass percentage: C 0.82 - 0.84%, Si 0.9 - 1.0%, Mn 0.65 - 0.70%, Cr + 2V is 0.35 - 0.45%, the rest being Fe and inevitable inclusions.

4. The preparation method of the steel strand with a fatigue stress amplitude of more than 300 MPa according to claim 1, characterized in that, In the step "The phosphated busbar is subjected to 9 - 11 passes of dry drawing to obtain an intermediate product wire", the working cone angle of the wire drawing die used for dry drawing is controlled at 8 - 10°.

5. The preparation method of the steel strand with a fatigue stress amplitude of more than 300 MPa according to claim 1, characterized in that, In the step "The obtained intermediate product wire is twisted into a prestressed steel strand of 1 center wire + 6 side wires", the diameter of the twisted prestressed steel strand is 15.24 mm.

6. The preparation method of the steel strand with a fatigue stress amplitude of more than 300 MPa according to claim 1, characterized in that In the step of "performing stabilization treatment on the twisted steel strand to obtain a finished steel strand with a fatigue stress amplitude of more than 300 MPa", the tensile stress during the stabilization treatment is 0.43σ b ~0.46σ b .

7. The method for preparing a steel strand with a fatigue stress amplitude of 300 MPa or more according to claim 1, wherein In the step "The twisted steel strand is subjected to stabilization treatment to obtain a finished steel strand with a fatigue stress amplitude of more than 300 MPa", the obtained finished steel strand has a fatigue life of not less than 2 million times when the stress amplitude is more than 300 MPa.

8. The preparation method of the steel strand with a fatigue stress amplitude of more than 300 MPa according to claim 1, characterized in that, In the step "The twisted steel strand is subjected to stabilization treatment to obtain a finished steel strand with a fatigue stress amplitude of more than 300 MPa", the tensile strength of the obtained finished steel strand is 1860 MPa or more.

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