A heat-treatable 1670MPa grade hot-rolled prestressed steel bar and its production method

Abstract

This invention proposes a heat-treatment-free 1670MPa grade hot-rolled prestressed steel bar and its production method. The production method includes steel smelting, continuous casting, high-speed wire rod rolling, online salt bath quenching and aging, and uncoiling and shearing to length. By optimizing the wire rod composition and production process, this invention directly rolls the final steel bar to its desired diameter and utilizes the residual heat after rolling for direct online heat treatment to achieve optimal performance. Users can eliminate the need for drawing and tempering heat treatment, aligning with the current industry trend towards green and low-carbon development, and offering significant economic and social benefits.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and particularly relates to a heat-free 1670MPa grade hot-rolled prestressed steel bar and its production method. Background Technology

[0002] Steel bars for prestressed concrete (PC steel bars) belong to the intermediate strength grade of prestressed concrete. Due to their high strength and toughness, low relaxation, strong bond with concrete, good weldability, forgeability, and material savings (e.g., φ11mm PC steel bars can replace φ20mm hot-rolled steel bars), they are widely used in high-strength prestressed concrete centrifugal pipe piles, utility poles, viaduct piers, railway sleepers, and other prestressed components, and have a very broad market. The main raw material for PC steel bars is non-torsion controlled-cooling hot-rolled wire rod, with main steel grades including 30MnSi and 35Si2Mn. After cold drawing and scoring, these wire rods require tempering heat treatment to obtain a tempered sorbite structure, thus achieving a tensile strength of 1420-1570MPa. Although this product and its production process can save materials, it requires secondary heat treatment, resulting in high energy consumption and low production efficiency. Summary of the Invention

[0003] This invention addresses the technical problem of high energy consumption in the production of PC steel bars, which requires secondary heat treatment. It proposes a heat treatment-free 1670MPa grade hot-rolled prestressed steel bar and its production method, which can eliminate the two major steps of drawing and tempering heat treatment for users.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A heat-treatment-free 1670MPa grade hot-rolled prestressed steel bar has the following chemical composition and weight percentage: C 0.32-0.38%, Si 1.20-2.20%, Mn 1.50-2.20%, Cr 0.30-0.80%, Ni≤0.30%, S≤0.010%, P≤0.020%, N≤0.0040%, O≤0.0020%, with the remainder being Fe and other unavoidable impurities.

[0006] In the chemical composition of this invention:

[0007] C is the most important strengthening element in steel. To ensure the high tensile strength and good weldability of prestressed steel bars, the present invention limits the C content to 0.32-0.38%.

[0008] Si is a strengthening and deoxidizing element in steel, which helps to improve the steel's resistance to relaxation. In addition, it has the function of inhibiting carbide formation and promoting the formation of retained austenite. In this invention, the Si content is limited to 1.20-2.20%.

[0009] Mn is a strengthening element in steel, which can improve the strength and hardenability of steel and ensure a good work hardening rate of wire rod. In this invention, the Mn content is limited to 1.50-2.20%.

[0010] Cr can refine pearlite lamellars and improve the stability of carbides; however, excessive Cr content increases the difficulty of controlling segregation. This invention limits the Cr content to 0.30-0.80%.

[0011] Ni is a strengthening element in steel and can improve the toughness of steel and enhance the high-temperature performance of materials. To control costs, the amount added is limited to less than 0.30% in this invention.

[0012] S tends to segregate at grain boundaries, which can embrittle the grain boundaries and reduce the strength and plasticity of steel. In addition, S can react with Mn to form MnS, thereby reducing the solid solution strengthening effect of Mn. In this invention, the S content is limited to ≤0.010%.

[0013] P is an impurity element in steel. It tends to segregate at grain boundaries, which can cause grain boundary embrittlement and thus reduce the strength and plasticity of steel. In this invention, the P content is limited to ≤0.020%.

[0014] Nitrogen (N) can reduce the plasticity of steel, increase the risk of delayed fracture of wire rod, and high N content can coarsen AlN and TiN, which is not conducive to fine grain strengthening. In addition, free nitrogen atoms undergo dynamic aging in steel wire, which reduces the plasticity of steel wire. In this invention, the nitrogen content is limited to ≤0.0040%.

[0015] O combines with alloying elements in steel to form non-metallic inclusions, affecting the strength and plasticity of wire rods and the fatigue and corrosion resistance of steel wires. Therefore, O should be limited to ≤0.0020%.

[0016] As a preferred embodiment, its chemical composition and weight percentage are as follows: C 0.33-0.36%, Si 1.45-2.10%, Mn 1.55-2.15%, Cr 0.35-0.75%, Ni≤0.20%, S≤0.008%, P≤0.012%, N≤0.0036%, O≤0.0008%, with the remainder being Fe and other unavoidable impurities.

[0017] Preferably, the diameter of the steel bar is 5.5-16 mm.

[0018] This invention also proposes a production method for 1670MPa grade hot-rolled prestressed steel bars that do not require heat treatment. The method includes steel smelting, continuous casting, high-speed wire rod rolling, online salt bath quenching and aging, and uncoiling and shearing to length. In the online salt bath quenching and online aging steps, the wire rod is drawn at a temperature of 880-920℃ to ensure the wire has a fully austenitic structure with fine grain size, thus promoting phase transformation and improving final plasticity. Then, the wire rod is directly immersed in a salt bath for isothermal quenching at a temperature of Ms~420℃ for 60-200s, allowing the wire rod to begin phase transformation in the lower bainite region with a transformation amount exceeding 50%. The wire rod after online salt bath quenching is then hot-coiled and subjected to online aging in an annealing furnace at a temperature of 300-400℃ for ≥60min, allowing the bainite to continue its phase transformation completely and eliminating internal stress.

[0019] Preferably, in the continuous casting step, the superheat of the molten steel is 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water content is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total reduction under light pressure is 15.0±0.2mm. The combination of low superheat and casting speed control with electromagnetic stirring and light reduction ensures good segregation in the continuously cast billet. Specific parameters can be set according to the equipment characteristics and billet size. The continuous casting yields a rectangular billet with a cross-sectional dimension of not less than 240mm×180mm.

[0020] Preferably, in the rolling process, the initial rolling temperature is 1050-1100℃, the temperature entering the pre-finishing mill is 910±20℃, the temperature entering the finishing mill is 910±20℃, and the temperature entering the reducing and sizing mill is 910±20℃, resulting in a smooth or ribbed wire rod with a diameter of 5.5-16mm. The rolling temperature is determined according to the characteristics of the production line to ensure that the billet is rolled in a fully austenitic state, guaranteeing a fine and uniform austenite grain size.

[0021] As a preferred option, in the online salt bath quenching and aging steps, the salt bath temperature is 350~405℃ and the time is 60-120s.

[0022] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0023] (1) This invention optimizes the design and production process of wire rod composition, directly rolls to obtain the final diameter of the steel bar, and uses the residual heat after rolling to directly perform online heat treatment to obtain the best performance. Users can eliminate the two major steps of drawing and tempering heat treatment, which is in line with the current green and low-carbon development direction of the industry and has good economic and social benefits.

[0024] (2) The present invention improves the tensile strength level of prestressed steel bars to 1670MPa and the elongation is greater than 15%, with excellent strength and plasticity combination, and can be used for special needs such as coal mine support and large prestressed concrete buildings. Attached Figure Description

[0025] Figure 1 A scanning electron microscope image of the microstructure of the wire rod of Example 1 is shown;

[0026] Figure 2 Scanning electron microscope images of the microstructure of the wire rod from Example 2 are shown;

[0027] Figure 3 A scanning electron microscope image of the microstructure of the wire rod of Example 3 is shown;

[0028] Figure 4 A scanning electron microscope image of the microstructure of the wire rod in Comparative Example 1 is shown. Detailed Implementation

[0029] To better understand the present invention, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. Example 1

[0030] A method for producing 1670MPa grade hot-rolled prestressed steel bars that do not require heat treatment includes the following steps:

[0031] (1) Steel smelting steps

[0032] The molten steel was smelted through converter smelting and LF furnace refining steps. The chemical composition of the molten steel at the end of the smelting process, expressed as a percentage by mass, is shown in Table 1. The chemical composition of the continuously cast billet obtained from the continuous casting step, as well as the chemical composition of the prestressed steel bar finally prepared, are consistent with the chemical composition of the molten steel at the end of the smelting process, and are also shown in Table 1.

[0033] In the converter smelting step, molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, oxygen blowing and decarburization are carried out. When tapping the steel, alloys are added to the ladle for deoxidation and alloying. In the refining step, the molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment and temperature control. The inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0034] (2) Continuous casting steps

[0035] In the continuous casting process, the superheat of the molten steel is controlled at 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water content is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total reduction under light pressure is 15.0±0.2mm. The continuous casting process yields a rectangular billet with a cross-sectional dimension of not less than 240mm×180mm to ensure sufficient compression ratio after rolling into wire rod to guarantee the homogeneity of the wire rod.

[0036] (3) High wire rod rolling process

[0037] In the rolling process, the initial rolling temperature is controlled at 1050-1100℃ by adjusting the heating temperature and time, the water tank flow rate is adjusted to make the pre-finishing mill temperature 910±10℃, the finishing mill temperature 900±10℃, and the sizing mill temperature 900±10℃, so as to obtain a smooth wire rod with a diameter of 5.5mm.

[0038] (4) Online salt bath quenching and aging steps

[0039] In the online salt bath quenching and aging process, the wire drawing temperature is 900±10℃. After wire drawing, the wire is directly immersed in a salt bath for isothermal quenching at 350±2℃ for 120s. The isothermally quenched wire is then hot-rolled and placed in an insulated corridor for online aging at 350±10℃ for 70min.

[0040] Unwinding, cutting, and length setting steps

[0041] After the wire rod comes off the production line, it is uncoiled, straightened, and cut to length according to the order requirements to obtain prestressed steel bars. Example 2

[0042] A method for producing 1670MPa grade hot-rolled prestressed steel bars that do not require heat treatment includes the following steps:

[0043] (1) Steel smelting steps

[0044] The molten steel was smelted through converter smelting and LF furnace refining steps. The chemical composition of the molten steel at the end of the smelting process, expressed as a percentage by mass, is shown in Table 1. The chemical composition of the continuously cast billet obtained from the continuous casting step, as well as the chemical composition of the prestressed steel bar finally prepared, are consistent with the chemical composition of the molten steel at the end of the smelting process, and are also shown in Table 1.

[0045] In the converter smelting step, molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, oxygen blowing and decarburization are carried out. When tapping the steel, alloys are added to the ladle for deoxidation and alloying. In the refining step, the molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment and temperature control. The inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0046] (2) Continuous casting steps

[0047] In the continuous casting process, the superheat of the molten steel is controlled at 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water content is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total reduction under light pressure is 15.0±0.2mm. The continuous casting process yields a rectangular billet with a cross-sectional dimension of not less than 240mm×180mm to ensure sufficient compression ratio after rolling into wire rod to guarantee the homogeneity of the wire rod.

[0048] (3) High wire rod rolling process

[0049] In the rolling process, the initial rolling temperature is set to 1050-1100℃ by adjusting the heating temperature and time, the water tank flow rate is adjusted to set the pre-finishing mill temperature to 910±10℃, the finishing mill temperature to 900±10℃, and the sizing mill temperature to 900±10℃, thus producing a smooth wire rod with a diameter of 15mm.

[0050] (4) Online salt bath quenching and aging steps

[0051] In the online salt bath quenching and aging process, the wire drawing temperature is 900±10℃. After wire drawing, the wire is directly immersed in a salt bath for isothermal quenching at 400±5℃ for 60 seconds. The wire is then hot-coiled after isothermal quenching and placed in an insulated corridor for online aging at 380±10℃ for 60 minutes.

[0052] (5) Unwinding, cutting and length setting steps

[0053] After the wire rod comes off the production line, it is uncoiled, straightened, and cut to length according to the order requirements to obtain prestressed steel bars. Example 3

[0054] A method for producing 1670MPa grade hot-rolled prestressed steel bars that do not require heat treatment includes the following steps:

[0055] (1) Steel smelting steps

[0056] The molten steel was smelted through a series of converter smelting and LF furnace refining steps. The chemical composition of the molten steel at the end of the smelting process is shown in Table 1 as a mass percentage. In addition, the chemical composition of the continuously cast billet obtained from the continuous casting step and the chemical composition of the prestressed steel bar finally prepared are consistent with the chemical composition of the molten steel at the end of the smelting process, as shown in Table 1.

[0057] In the converter smelting step, molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, oxygen blowing and decarburization are carried out. When tapping the steel, alloys are added to the ladle for deoxidation and alloying. In the refining step, the molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment and temperature control. The inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0058] (2) Continuous casting steps

[0059] In the continuous casting process, the superheat of the molten steel is controlled at 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water content is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total reduction under light pressure is 15.0±0.2mm. The continuous casting process yields a rectangular billet with a cross-sectional dimension of not less than 240mm×180mm to ensure sufficient compression ratio after rolling into wire rod to guarantee the homogeneity of the wire rod.

[0060] (3) High wire rod rolling process

[0061] During the rolling process, the initial rolling temperature is controlled at 1050-1100℃ by adjusting the heating temperature and time. The water tank flow rate is adjusted to make the temperature at the pre-finishing mill 910±10℃, the temperature at the finishing mill 900±10℃, and the temperature at the sizing mill 900±10℃. The rolled product is a ribbed wire rod with a diameter of 16mm, and the shape and size of the ribs conform to the requirements of GB1499.

[0062] (4) Online salt bath quenching and aging steps

[0063] In the online salt bath quenching and aging process, the wire drawing temperature is 900±10℃. After wire drawing, the wire is directly immersed in a salt bath for isothermal quenching at 370±5℃ for 100s. The wire rod is then hot-coiled after isothermal quenching in the salt bath and then aging online in an insulated corridor at 360±10℃ for 90min.

[0064] (5) Unwinding, cutting and length setting steps

[0065] After the wire rod comes off the production line, it is uncoiled, straightened, and cut to length according to the order requirements to obtain prestressed steel bars.

[0066] Comparative Example

[0067] The comparative example used a standard 35Si2Mn composition and employed a conventional smelting-rolling-Stelmo cooling-aging-drawing-scribing-tempering heat treatment process to produce a 15mm diameter steel bar. The specific steps are as follows:

[0068] (1) Steel smelting steps

[0069] The molten steel is smelted through a series of steps including converter smelting and LF furnace refining. The chemical composition of the molten steel at the end of the smelting process is shown in Table 1 as a percentage by mass.

[0070] In the converter smelting step, molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, oxygen blowing and decarburization are carried out. When tapping the steel, alloys are added to the ladle for deoxidation and alloying. In the refining step, the molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment and temperature control. The inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0071] (2) Continuous casting steps

[0072] In the continuous casting process, the superheat of the molten steel is controlled at 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water content is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total reduction under light pressure is 15.0±0.2mm. The continuous casting process yields a rectangular billet with a cross-sectional dimension of not less than 240mm×180mm to ensure sufficient compression ratio after rolling into wire rod to guarantee the homogeneity of the wire rod.

[0073] (3) High wire rod rolling process

[0074] During the rolling process, the initial rolling temperature is adjusted to 1050-1100℃ by adjusting the heating temperature and time, the water tank flow rate is adjusted to make the pre-finishing mill temperature 920±10℃, the finishing mill temperature 920±10℃, and the sizing mill temperature 920±10℃, resulting in a smooth wire rod with a diameter of 16mm.

[0075] (4) Stellmore cooling and aging steps

[0076] In the Stellmore cooling process, the spinning temperature is 910±10℃. After spinning, cooling is achieved using fans and a thermal insulation cover for slow cooling. Specifically, fans for frames 1-8 are operated at 100%, and fans for frames 9-12 are operated at 50%. Subsequent frames are then covered with a thermal insulation cover for slow cooling. The aging process involves placing the product at room temperature for 15 days before use.

[0077] (5) Pulling-scribing-tempering steps

[0078] After aging, the wire rod is drawn to 15mm in two passes. The scored wire is induction heated to 920℃, cooled in water, and then induction heated to 550-600℃. Finally, it is cut to length to obtain the final product.

[0079] Table 1. Chemical composition of molten steel at the smelting endpoint in Examples 1-3 and Comparative Examples.

[0080]

[0081] The steel bars prepared in Examples 1-3 and the comparative example were sampled and subjected to metallographic structure and mechanical property testing according to the same testing methods. The specific testing methods and results are as follows:

[0082] (1) Metallographic analysis: Samples with a length of 10 cm were taken from one end of the steel rod to prepare metallographic samples. After mechanical polishing and etching with nitric acid and alcohol, the samples were observed under a metallographic microscope. The metallographic images of the steel rod samples prepared in Examples 1-3 and the comparative examples are shown below. Figures 1 to 4 As shown in the figure. Figure 1-3 All are quasi-bainitic structures, also known as carbide-free bainite, composed of fine lath-shaped bainitic ferrite and a thin film of retained austenite. The chemical composition system designed in this invention contains a large amount of Si, which can inhibit carbide formation. This allows the wire to cool rapidly from austenite to the lower bainitic transformation temperature after being drawn, forming a large amount of bainitic ferrite but preventing carbide precipitation. As a result, the untransformed austenite forms carbon-enriched deposits and stabilizes at room temperature. Figure 4 The traditional process shown produces a typical tempered sorbite structure.

[0083] (2) Mechanical property testing: In accordance with the test methods and definitions of GB / T228 standard, the mechanical properties of the steel bar samples were tested using a tensile testing machine. The tensile strength and reduction of area of ​​the steel bars prepared in Examples 1-3 and the comparative examples are shown in Table 2. As can be seen from Table 2, the tensile strength of the prestressed steel bars prepared in Examples 1-3 is increased to 1670 MPa, and the elongation is greater than 15%, with excellent strength and plasticity. It can be seen that under the combined effect of alloying elements and microstructure, the present invention significantly improves the tensile strength and plasticity of the steel bars compared with the traditional process. In particular, the fine lath bainitic ferrite in the microstructure ensures that the steel bars have extremely high tensile strength, while the residual austenite and alloying elements such as Ni ensure good plasticity.

[0084] Table 2 Performance parameters of steel bars prepared in Examples 1-3 and comparative examples

[0085]

[0086] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A non-heat treated 1670 MPa grade hot-rolled prestressed steel bar, characterized in that, The chemical composition and weight percentage are: C 0.32-0.38%, Si 1.20-2.20%, Mn 1.50-2.20%, Cr 0.30-0.80%, Ni≤0.30%, S≤0.010%, P≤0.020%, N≤0.0040%, O≤0.0020%, and the rest is Fe and other inevitable impurities; The steel rod is produced by the following method: a molten steel smelting step-a continuous casting step-a high-speed wire rolling step-an online salt bath quenching and aging step-a decoiling and shearing sizing step, wherein in the online salt bath quenching and aging step, the wire rod laying temperature is 880-920℃, and then the wire rod is directly immersed in a salt bath for isothermal quenching, the salt bath temperature is Ms~420℃, and the time is 60-200s; the wire rod after online salt bath quenching is hot coiled, and then enters an annealing furnace for online aging, the aging temperature is 300-400℃, and the time is≥60min.

2. The non-heat treated 1670 MPa grade hot rolled prestress steel bar according to claim 1, characterized in that, The chemical composition and weight percentage are: C 0.33-0.36%, Si 1.45-2.10%, Mn 1.55-2.15%, Cr 0.35-0.75%, Ni≤0.20%, S≤0.008%, P≤0.012%, N≤0.0036%, O≤0.0008%, and the rest is Fe and other inevitable impurities.

3. The non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1, wherein: The diameter size of the steel rod is 5.5-16mm.

4. The non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1, wherein: In the continuous casting step, the superheat of the molten steel is 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water consumption is 0.22±0.01L / kg, the terminal stirring current is 450±25A, the terminal stirring frequency is 8±0.5Hz, the total amount of light reduction is 15.0±0.2mm, and the rectangular billet with a cross-sectional size not less than 240mm×180mm is obtained by continuous casting.

5. The non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1, wherein: In the rolling step, the open rolling temperature is 1050-1100℃, the temperature entering the pre-finishing rolling mill is 910±20℃, the temperature entering the finishing rolling mill is 910±20℃, the temperature entering the reducing mill is 910±20℃, and the bright or surface-ribbed wire rod with a diameter size of 5.5-16mm is obtained by rolling.

6. The non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1, wherein: In the online salt bath quenching and aging step, the salt bath temperature is 350~405℃, and the time is 60-120s.

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