A hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand and its production method

Abstract

This invention relates to the technical field of equipment for manufacturing metal wire, specifically to a hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand and its production method. The method includes the following steps: smelting, continuous casting, billet grinding, rolling, and post-rolling residual heat isothermal treatment. The rolling process employs a one-fire forming process, heating the continuous casting billet to 1000-1200℃ and holding it at that temperature for 2-3 hours. The rolling temperature is not lower than the Armm temperature of the steel, and the wire drawing temperature is Armm + (30-50)℃. In the post-rolling residual heat isothermal treatment, after wire drawing, the wire rod is laid out in loose coils and passed through a constant-temperature salt bath. The wire rod must be completely immersed in the salt bath, with the salt bath temperature set to the Ms point temperature of the steel ±20℃, and the isothermal time 5-15 minutes. The microstructure of the wire rod of this invention consists of lower bainite, a small amount of low-carbon martensite, and retained austenite, with volume fractions of 75%-90%, 0-10%, and 5%-20%, respectively.

Description

Technical Field

[0001] This invention relates to the field of metal wire manufacturing technology, specifically to a hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand and its production method. Background Technology

[0002] my country consumes a huge amount of prestressed steel strands, with an annual national consumption of approximately 5 million tons, currently mainly consisting of steel strands with a strength ≤1860MPa. In recent years, with the continuous development of prestressed concrete structure construction, there has been a greater demand for improving the overall technical performance of steel strands. Among these improvements, increasing the strength of steel strand materials has become an important development direction and a key factor in optimizing the design of prestressed concrete structures. Increasing the strength of prestressed steel strands to 2000MPa-2400MPa is a crucial technical direction, effectively enabling structural optimization and improving the technical and economic efficiency of the structure.

[0003] Currently, steel strand manufacturing generally uses high-carbon pearlitic wire rod, with a processing technology of hot-rolled wire rod - 9-pass drawing - twisting - stabilization - finished product. The strength of steel strand comes from the initial strength of the wire rod + cold work hardening - strength loss during stabilization treatment. Measures to improve tensile strength mainly involve increasing the carbon content of the wire rod to increase the original tensile strength, and increasing the diameter of the original wire rod to increase deformation and enhance cold work strengthening. However, these measures all lead to a decrease in the plasticity of the steel strand. Plasticity, as a crucial indicator for ensuring structural safety, is even more important than strength requirements. The plasticity index of existing conventional 1860MPa grade steel strand is generally around 5% elongation after fracture. Developing ultra-high strength steel strands above 2000MPa using conventional production processes makes it difficult to exceed this value in terms of plasticity.

[0004] From a manufacturing process perspective, developing ultra-high strength steel strands with strengths exceeding 2000MPa using traditional processes requires modifications to existing conventional strength steel strand production lines due to the increased strength and diameter of the wire rods. These modifications might include increasing the number of drawing passes, redesigning drawing dies, and even adding intermediate heat treatment. However, the increased wire rod strength and drawing passes further increase the likelihood of surface damage and susceptibility to cracking in the steel wire, negatively impacting the strand's plasticity. Furthermore, traditional steel strand wire rods typically have a fine pearlitic microstructure, with cementite acting as the strength phase and ferrite as the plastic phase. Increasing strength requires increasing the carbon content to raise the proportion of cementite, inevitably leading to a reduction in ferrite and consequently, lower plasticity. Therefore, entirely new chemical compositions and microstructure designs are necessary to address the technical challenges of achieving high strength and high ductility in prestressed steel wires / strands. Summary of the Invention

[0005] The purpose of this invention is to provide a hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand and a production method thereof, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand, having the following chemical composition by weight: C: 0.55-0.70 wt.%; Si: 1.0-2.5 wt.%; Mn: 0.30-1.50 wt.%; Cr: 0-0.80 wt.%; P: ≤0.020 wt.%; S: ≤0.010 wt.%; the remainder being Fe and unavoidable impurities; the microstructure of the wire rod is lower bainite, a small amount of low-carbon martensite, and retained austenite, with volume fractions of 75%-90%, 0-10%, and 5%-20%, respectively.

[0007] This invention also provides a method for producing hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand. The method sequentially includes the steps of smelting, continuous casting, billet grinding, rolling, and post-rolling residual heat isothermal treatment. The rolling process employs a one-fire forming process, heating the continuous casting billet to 1000-1200℃ and holding it at that temperature for 2-3 hours. The rolling temperature is not lower than the Arcm temperature of the steel, and the wire drawing temperature is Arcm + (30-50)℃, where the Arcm temperature refers to the temperature at which carbides begin to precipitate in the steel. The post-rolling residual heat isothermal treatment involves the wire rod being laid out in loose coils after wire drawing. A constant-temperature salt bath is used, requiring the entire wire rod to be immersed in the salt bath. The salt bath temperature is set to ±20℃ of the steel's Ms point temperature, and the time is 5-15 minutes. The Ms point temperature of the steel refers to the temperature at which the martensite begins to transform. When a temperature lower than the Ms point temperature is selected, the isothermal time should not be less than 5 minutes to allow the carbon and other chemical elements in the martensite to be fully distributed to the untransformed austenite. When a temperature higher than the Ms point temperature is selected, the isothermal time is determined by referring to the TTT curve of the steel grade, and the time selected from the curve should ensure that at least 70% of the bainite transformation is completed. The salt bath temperature fluctuation during heat treatment should not exceed ±2℃.

[0008] Preferably, the smelting and continuous casting process is as follows: blast furnace hot metal → KR desulfurization → top and bottom combined blowing converter steelmaking → LF refining → RH refining → continuous casting.

[0009] Preferably, the billet repair includes shot blasting and flaw detection, which are performed according to the surface condition of the continuously cast billet. The surface of the continuously cast billet after repair should not have burrs or sharp edges.

[0010] Preferably, the continuously cast billet undergoes high-pressure water descaling after exiting the heating furnace during the rolling process to ensure the removal of iron oxide scale.

[0011] Preferably, the Ms point temperature is tested by a thermal expansion meter, predicted using JMat / Pro software, or estimated using the formula Ms(°C)=520-320C-50Mn-30Cr-20(Ni+Mo)-5(Cu+Si), where the element symbols represent the percentage values ​​of the corresponding elements.

[0012] Compared with the prior art, the beneficial effects of the present invention are:

[0013] The microstructure of the wire rod of this invention consists of lower bainite, a small amount of low-carbon martensite, and retained austenite, with volume fractions of 75%-90%, 0-10%, and 5%-20%, respectively; tensile strength is 1800-1950 MPa, reduction of area is over 40%, and elongation after fracture is 6-12%. When using the wire rod of this invention to manufacture prestressed steel wire / strand, only surface treatment of the wire rod is required, followed by 1-2 draw passes or no drawing, and then processing such as rib rolling / twisting using existing production lines. Finally, prestressed products with strengths of 1860, 1960, and 2000 MPa or higher are obtained as needed, with an elongation after fracture of 6.5-10%. Attached Figure Description

[0014] Figure 1 This is a microstructure diagram of the wire rod in Embodiment 1 of the present invention;

[0015] Figure 2 This is a microstructure diagram of the wire rod in Embodiment 2 of the present invention;

[0016] Figure 3 This is a microstructure diagram of the wire rod in Embodiment 3 of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the present invention clear and complete, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only some, not all, embodiments of the present invention, and are merely illustrative of the embodiments of the present invention. They are not intended to limit the embodiments of the present invention. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1

[0019] Please see Figure 1 A hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand has the following chemical composition by weight: C: 0.61 wt.%; Si: 1.82 wt.%; Mn: 0.81 wt.%; Cr: 0.055 wt.%; P: 0.015 wt.%; S: 0.005 wt.%; the remainder being Fe and unavoidable impurities.

[0020] This embodiment also provides a production method for the above-mentioned high-strength, high-ductility prestressed steel wire / strand hot-rolled wire rod. The method includes, in sequence, blast furnace hot metal → KR desulfurization → top and bottom blowing converter steelmaking → LF refining → RH refining → continuous casting, to obtain a rectangular billet of 180*240mm. After KR desulfurization, the sulfur content of the blast furnace hot metal entering the converter is ≤0.005%. The converter adopts a double-slag blowing method, which can reduce the content of impurity elements in the molten steel. The final P content in the converter is ≤0.015%, and the S content is ≤0.010%. The LF furnace uses aluminum deoxidation and a high-basicity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25min, controlling the O content in the steel to ≤15ppm and the N content to ≤55ppm. The continuous casting process is protected during casting to avoid secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billet. During continuous casting, the superheat of molten steel is controlled at 16-25℃, and electromagnetic stirring, automatic water distribution, and light pressure are used to control the compositional segregation of the continuously cast billet, with a segregation index of 1.10.

[0021] After shot blasting, flaw detection, and grinding, the continuously cast billet is rolled to prevent surface defects from affecting the surface quality of the wire rod. A one-fire rolling process is used. The continuously cast billet is heated to 1080-1120℃ and held for 150 minutes. After exiting the furnace, it undergoes high-pressure water descaling to ensure the removal of iron oxide scale. It is then rolled into wire rod with a diameter of 5.5 mm. In this embodiment, the Arcm temperature of the steel is 870℃. The rolling process temperature and wire drawing temperature are controlled at 900-920℃. After wire drawing, the wire rod is laid out in loose coils and undergoes isothermal transformation in a constant-temperature salt bath. The wire rod must be completely immersed in the salt bath. In this embodiment, the Ms point temperature of the steel is 305℃, which is obtained through thermal expansion testing. The salt bath temperature is 300℃, and the isothermal time is 7 minutes. The isothermal time in this embodiment is determined by referring to the TTT curve of this steel grade, selecting a time from the curve to ensure at least 70% completion of the bainite transformation.

[0022] The microstructure of the wire rod produced in this embodiment is shown in the figure. Figure 1 The wire rod is composed of 85% lower bainite, 11% retained austenite, and a small amount of martensite. The wire rod has a diameter of 5.5mm, a tensile strength of 1945 MPa, a reduction of area of ​​46%, and an elongation after fracture of 11%. After de-oxidation, two-pass drawing, wire twisting, and stabilization treatment, the wire rod is made into steel strand with a diameter of 15.2mm, a tensile strength of 2055 MPa, an elongation after fracture of 9%, and a fatigue life of 2.5 million cycles.

[0023] Example 2

[0024] Please see Figure 2A hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand has the following chemical composition by weight: C: 0.55 wt.%; Si: 2.5 wt.%; Mn: 0.3 wt.%; Cr: 0.08 wt.%; P: 0.020 wt.%; S: 0.01 wt.%; the remainder being Fe and unavoidable impurities.

[0025] This embodiment also provides a production method for the above-mentioned high-strength, high-ductility prestressed steel wire / strand hot-rolled wire rod. The method includes, in sequence, blast furnace hot metal → KR desulfurization → top and bottom blowing converter steelmaking → LF refining → RH refining → continuous casting, to obtain a rectangular billet of 180*240mm. After KR desulfurization, the sulfur content of the blast furnace hot metal entering the converter is ≤0.005%. The converter adopts a double-slag blowing method, which can reduce the content of impurity elements in the molten steel. The final P content in the converter is ≤0.020%, and the S content is ≤0.010%. The LF furnace uses aluminum deoxidation and a high-basicity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25min, controlling the O content in the steel to ≤15ppm and the N content to ≤40ppm. The continuous casting process is protected during casting to avoid secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billet. During continuous casting, the superheat of molten steel is controlled at 16-25℃, and electromagnetic stirring, automatic water distribution, and light pressure are used to control the compositional segregation of the continuously cast billet, with a segregation index of 1.10.

[0026] After shot blasting, flaw detection, and grinding, the continuously cast billet is rolled to prevent surface defects from affecting the surface quality of the wire rod. A one-fire rolling process is used. The continuously cast billet is heated to 1000-1080℃ and held for 120 minutes. After exiting the furnace, it undergoes high-pressure water descaling to ensure the removal of iron oxide scale. It is then rolled into wire rod with a diameter of 5.5mm. In this embodiment, the Arcm temperature of the steel is 840℃, and the rolling and wire-drawing temperatures are controlled at 870-890℃. After wire drawing, the wire rod is isothermally transformed in a constant-temperature salt bath in a loose, flat-lay manner. The entire wire rod must be immersed in the salt bath. In this embodiment, the Ms point temperature of the steel is 282℃, which was predicted using JMat / Pro software. The salt bath temperature is 275℃, and the isothermal time is 13 minutes.

[0027] The microstructure of the wire rod produced in this embodiment is shown in the figure. Figure 2 The wire rod is composed of 80% lower bainite, 10% retained austenite, and 10% martensite. The wire rod has a diameter of 6.0 mm, a tensile strength of 1865 MPa, a reduction of area of ​​50%, and an elongation after fracture of 12%. After de-oxidation, two-pass drawing, wire twisting, and stabilization treatment, the wire rod is made into steel strand with a diameter of 15.2 mm, a tensile strength of 2015 MPa, an elongation after fracture of 11%, and a fatigue life of 2.5 million cycles.

[0028] Example 3

[0029] Please see Figure 3 A hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand has the following chemical composition by weight: C: 0.7 wt.%; Si: 1.0 wt.%; Mn: 1.5 wt.%; P: 0.015 wt.%; S: 0.005 wt.%; the remainder being Fe and unavoidable impurities.

[0030] This embodiment also provides a production method for the above-mentioned hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand. The method includes, in sequence, blast furnace hot metal → KR desulfurization → top and bottom blowing converter steelmaking → LF refining → RH refining → continuous casting, to obtain a rectangular billet of 180*240mm. After KR desulfurization, the sulfur content of the blast furnace hot metal entering the converter is ≤0.005%. The converter adopts a double-slag blowing method, which can reduce the content of impurity elements in the molten steel. The final P content in the converter is ≤0.015%, and the sulfur content is ≤0.010%. The LF furnace adopts aluminum deoxidation and a high-basicity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25min, controlling the O content in the steel to ≤15ppm and the N content to ≤40ppm. The continuous casting process is protected during casting to avoid secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billet. During continuous casting, the superheat of molten steel is controlled at 16-25℃, and electromagnetic stirring, automatic water distribution, and light pressure are used to control the compositional segregation of the continuously cast billet, with a segregation index of 1.10.

[0031] After shot blasting, flaw detection, and grinding, the continuously cast billet is rolled to prevent surface defects from affecting the surface quality of the wire rod. A one-fire rolling process is used. The continuously cast billet is heated to 1120-1200℃ and held for 180 minutes. After exiting the furnace, it undergoes high-pressure water descaling to ensure the removal of iron oxide scale. It is then rolled into wire rod with a diameter of 5.5mm. In this embodiment, the Arcm temperature of the steel is 830℃, and the rolling and wire-drawing temperatures are controlled at 860-880℃. After wire drawing, the wire rod is laid out in loose coils and undergoes isothermal transformation in a constant-temperature salt bath. The entire wire rod must be immersed in the salt bath. In this embodiment, the Ms point temperature of the steel is 216℃, the salt bath temperature is 236℃, and the isothermal time is 5 minutes. During heat treatment, the salt bath temperature fluctuation is no greater than ±2℃. In this embodiment, the isothermal time is estimated using the formula Ms(°C)=520-320C-50Mn-30Cr-20(Ni+Mo)-5(Cu+Si), where the element symbols represent the percentage values ​​of the corresponding elements.

[0032] The microstructure of the wire rod produced in this embodiment is shown in the figure. Figure 3The wire rod is composed of 90% lower bainite, 6% retained austenite, and 4% martensite. The wire rod has a diameter of 5.0 mm, a tensile strength of 1950 MPa, a reduction of area of ​​43%, and an elongation after fracture of 10%. After de-oxidation, one-pass drawing, wire twisting, and stabilization treatment, the wire rod is made into steel strand with a diameter of 15.2 mm, a tensile strength of 2025 MPa, an elongation after fracture of 9%, and a fatigue life of 2.5 million cycles.

[0033] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for producing hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand, characterized in that, The hot-rolled wire rod for high-strength and high-ductility prestressed steel wire / strand has the following chemical composition by weight percentage: C: 0.55-0.70 wt.%; Si: 1.0-2.5 wt.%; Mn: 0.30-1.50 wt.%; Cr: 0-0.80 wt.%; P: ≤0.020 wt.%. S: ≤0.010wt.%; the remainder is Fe and unavoidable impurities. The microstructure of the wire rod is lower bainite, a small amount of low-carbon martensite, and retained austenite, with volume fractions of 75%-90%, 0-10%, and 5%-20%, respectively. The method includes the following steps in sequence: smelting, continuous casting, billet grinding, rolling, and post-rolling heat treatment. The rolling process adopts a one-fire forming process, heating the continuous casting billet to 1000-1200℃ and holding it for 2-3 hours. The rolling process temperature is not lower than the Arcm temperature of the steel, and the wire drawing temperature is Arcm + (30-50)℃. After rolling, the wire rod is subjected to isothermal heat treatment with residual heat. After the wire rod is unrolled, it is laid out in a loose coil and passed through a constant temperature salt bath. The wire rod must be completely immersed in the salt bath. The salt bath temperature is set to ±20℃ of the steel's Ms point temperature. The isothermal time is 5-15 minutes. When a temperature lower than the Ms point temperature is selected, the isothermal time should not be less than 5 minutes to ensure that chemical elements such as carbon in the martensite are fully distributed to the untransformed austenite. When a temperature higher than the Ms point temperature is selected, the isothermal time is determined by referring to the TTT curve of the steel grade. The time selected from the curve should ensure that at least 70% of the bainite transformation is completed. The salt bath temperature fluctuation during the heat treatment should not exceed ±2℃.

2. The method for producing hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand according to claim 1, characterized in that: The smelting and continuous casting process is as follows: blast furnace hot metal → KR desulfurization → top and bottom combined blowing converter steelmaking → LF refining → RH refining → continuous casting.

3. The method for producing hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand according to claim 1, characterized in that: The billet grinding includes shot blasting and flaw detection, which are carried out according to the surface condition of the continuous casting billet. The surface of the continuously casting billet after grinding must not have burrs or sharp edges.

4. The method for producing hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand according to claim 1, characterized in that: After exiting the heating furnace during the rolling process, the continuously cast billet undergoes high-pressure water descaling to ensure the removal of iron oxide scale.

5. The method for producing hot-rolled wire rod for high-strength, high-ductility prestressed steel wire / strand according to claim 1, characterized in that: The Ms point temperature is tested by thermal expansion test, predicted by JMat / Pro software, or estimated by the formula Ms (°C) = 520 - 320C - 50Mn - 30Cr - 20(Ni + Mo) - 5(Cu + Si), where the element symbols represent the weight percentage values ​​of the corresponding elements.

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