Heat-treatment-free 1670MPa-grade hot-rolled prestressed steel bar and production method thereof

Abstract

The invention provides a heat-treatment-free 1670MPa-grade hot-rolled prestressed steel bar and a production method thereof. The production method comprises a molten steel smelting step, a continuous casting step, a high-speed wire rolling step, an online salt bath quenching and aging step and an uncoiling, shearing and sizing step. According to the method, the final diameter size of the steel bar is obtained through direct rolling by optimizing the component design and the production process of the wire rod, the optimal performance is obtained by directly carrying out online heat treatment through waste heat after rolling, the two steps of drawing machining and quenching and tempering heat treatment can be omitted for a user, and the method conforms to the green and low-carbon development direction of the current industry; and the method has good economic and social benefits.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a heat treatment-free 1670MPa grade hot-rolled prestressed steel bar and a production method thereof. Background Art

[0002] Steel bar for prestressed concrete (PC bar) belongs to the intermediate strength grade within the prestressing strength range. Due to its high strength and toughness, low relaxation, strong bond with concrete, good weldability, forgeability, and material savings (for example, φ11mm PC bar can replace φ20mm hot-rolled rebar), it has been widely used in prestressed components such as high-strength centrifugal pipe piles, electric poles, viaduct piers, and railway sleepers, enjoying a broad market. The primary raw material for PC bar is non-torsion-controlled cold-rolled and hot-rolled wire rod, with grades such as 30MnSi and 35Si2Mn being the main steel grades. After cold drawing and notching, this wire rod undergoes a quenching and tempering heat treatment to achieve a tempered bainite structure, resulting in a tensile strength of 1420-1570 MPa. While this product and production process can save material, the need for secondary heat treatment results in high energy consumption and low production efficiency. Summary of the Invention

[0003] In response to the technical problem in the prior art that the production of PC steel bars requires secondary heat treatment and consumes high energy, the present invention proposes a heat-treatment-free 1670MPa grade hot-rolled prestressed steel bar and a production method thereof, which can relieve users of the two major steps of drawing and tempering heat treatment.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is: A heat treatment-free 1670MPa grade hot-rolled prestressed steel bar, the chemical composition and weight percentage of which 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 remainder is Fe and other inevitable impurities.

[0005] In the chemical composition of the present invention: C is the most important strengthening element in steel. In order to ensure high tensile strength and good welding performance of prestressed steel bars, the present invention limits the C content to 0.32-0.38%.

[0006] Si is a strengthening and deoxidizing element in steel, which helps to improve the steel's anti-relaxation performance. It also has the effect of inhibiting the formation of carbides and promoting the formation of retained austenite. In the present invention, the Si content is limited to 1.20-2.20%.

[0007] Mn is a strengthening element in steel, which can improve the strength and hardenability of steel and ensure a good work hardening rate of the wire rod. In the present invention, the Mn content is limited to 1.50-2.20%.

[0008] Cr can refine pearlite lamellae and improve the stability of carbides; however, excessive Cr content makes segregation control more difficult. The present invention limits the Cr content to 0.30-0.80%.

[0009] Ni is a strengthening element in steel and can improve the toughness of steel and enhance the high-temperature performance of the material. To control costs, the present invention limits the addition amount to within 0.30%.

[0010] S is easily segregated at the grain boundaries and makes the grain boundaries embrittled, thereby reducing the strength and plasticity of the steel. In addition, S will react with Mn to form MnS, thereby reducing the solid solution strengthening effect of Mn. In the present invention, the S content is limited to ≤0.010%.

[0011] P is an impurity element in steel, which easily segregates at the grain boundaries and makes the grain boundaries embrittled, thereby reducing the strength and plasticity of the steel. In the present invention, the P content is limited to ≤0.020%.

[0012] Nitrogen can cause the plasticity of steel to deteriorate, increasing the risk of delayed fracture of wire rod. A high nitrogen content can coarsen AlN and TiN, which is not conducive to fine grain strengthening. In addition, free nitrogen atoms undergo dynamic aging in the steel wire, reducing the plasticity of the steel wire. In the present invention, the nitrogen content is limited to ≤0.0040%.

[0013] O combines with alloying elements in steel to form non-metallic inclusions, affecting the strength and plasticity of the wire rod and the fatigue and corrosion resistance of the steel wire. It is necessary to limit O to ≤ 0.0020%.

[0014] Preferably, its 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 are Fe and other inevitable impurities.

[0015] Preferably, the diameter of the steel rod is 5.5-16 mm.

[0016] The present invention also proposes a method for producing heat-treatment-free 1670MPa-grade hot-rolled prestressed steel bars, comprising a molten steel smelting step, a continuous casting step, a high-speed wire rolling step, an online salt bath quenching and aging step, and an uncoiling, shearing, and sizing step. During the online salt bath quenching and aging steps, the wire rod is extruded at a temperature of 880-920°C to ensure a fully austenitic structure with a fine grain size, thereby promoting phase transformation and improving ultimate plasticity. The wire rod is then directly immersed in a salt bath for isothermal quenching at a temperature of Ms~420°C for 60-200 seconds, causing the wire rod to initiate phase transformation in the lower bainite region and achieve a transformation amount exceeding 50%. The wire rod, after online salt bath quenching, is then hot-coiled and placed in an annealing furnace for online aging at a temperature of 300-400°C for 60 minutes or longer. The online aging process allows the bainite phase transformation to continue completely and eliminates internal stresses.

[0017] Preferably, during the continuous casting step, the molten steel superheat is 20-25°C, the mold stirring current is 270±25A, the mold stirring frequency is 3±0.5Hz, the casting speed during continuous casting is 1.2±0.05m / min, the continuous casting 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 soft reduction is 15.0±0.2mm. Low superheat and casting speed control combined with electromagnetic stirring and soft reduction ensure good segregation of the continuously cast billet. Specific parameters can be set according to equipment characteristics and billet size. The continuous casting produces a rectangular billet with a cross-sectional size of not less than 240mm×180mm.

[0018] Preferably, in the rolling step, the starting rolling temperature is 1050-1100°C, the temperature entering the pre-finishing mill is 910±20°C, the temperature entering the finishing mill is 910±20°C, and the temperature entering the reducing and sizing mill is 910±20°C. The rolled wire rod is smooth or ribbed, and the wire rod diameter is 5.5-16 mm. The temperature during the rolling process is determined according to the characteristics of the production line, so that the ingot is rolled in a fully austenitic state, ensuring a small and uniform austenite grain size.

[0019] Preferably, in the online salt bath quenching and aging steps, the salt bath temperature is 350-405° C., and the time is 60-120 seconds.

[0020] Compared with the prior art, the advantages and positive effects of the present invention are: (1) The present invention optimizes the design of wire rod composition and the production process, directly rolling to obtain the final diameter of the steel bar, and using the residual heat after rolling to directly perform heat treatment online to obtain the best performance. Users can avoid 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.

[0021] (2) The present invention increases the tensile strength of prestressed steel bars to 1670 MPa, and the elongation is greater than 15%, with excellent strength and plasticity, and can be used for special needs such as coal mine support and large prestressed concrete buildings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A scanning electron microscope image showing the microstructure of the wire rod of Example 1; Figure 2 A scanning electron microscope image showing the microstructure of the wire rod of Example 2; Figure 3 A scanning electron microscope image showing the microstructure of the wire rod of Example 3; Figure 4 The scanning electron microscope image of the microstructure of the wire rod of Comparative Example 1 is shown. DETAILED DESCRIPTION

[0023] In order to better understand the present invention, the following is a detailed description with reference to the accompanying drawings and embodiments. Example 1

[0024] A method for producing a heat treatment-free 1670 MPa grade hot-rolled prestressed steel bar comprises the following steps: (1) Molten steel smelting steps The molten steel was smelted through the converter smelting and LF furnace refining steps. The chemical composition of the molten steel at the end of the smelting step is shown in Table 1 in terms of mass percentage. The chemical composition of the continuous casting billet obtained in the continuous casting step and the chemical composition of the prestressed steel rod finally produced were consistent with the chemical composition of the molten steel at the end of the molten steel smelting step, as shown in Table 1.

[0025] Among them, in the converter smelting step, the molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, and oxygen blowing decarburization are carried out. When tapping, alloy is 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, and the inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0026] (2) Continuous casting steps During the continuous casting step, the superheat of the molten steel is controlled at 20-25°C, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed during continuous casting is 1.2±0.05m / min, the continuous casting 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 soft reduction is 15.0±0.2mm. The continuous casting step produces a rectangular billet with a cross-sectional size of no less than 240mm×180mm to ensure a sufficient compression ratio after rolling into wire rod to ensure the homogeneity of the wire rod.

[0027] (3) High-speed wire rolling steps During the rolling step, the starting rolling temperature is controlled at 1050-1100°C by adjusting the heating temperature and time, the water tank flow is adjusted to make the pre-finishing rolling temperature 910±10°C, the finishing rolling mill temperature 900±10°C, and the reducing and sizing unit temperature 900±10°C, and a smooth wire rod with a diameter of 5.5 mm is obtained by rolling.

[0028] (4) Online salt bath quenching and aging steps During the online salt bath quenching and aging steps, the wire rod is laid at a temperature of 900±10°C. After laying, the wire rod is directly immersed in a salt bath for isothermal quenching at a temperature of 350±2°C for 120 seconds. The wire rod is then hot-coiled and placed in a heat preservation corridor for online aging at a temperature of 350±10°C for 70 minutes.

[0029] Uncoiling and cutting to length steps 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

[0030] A method for producing a heat treatment-free 1670 MPa grade hot-rolled prestressed steel bar comprises the following steps: (1) Molten steel smelting steps The molten steel was smelted through the converter smelting and LF furnace refining steps. The chemical composition of the molten steel at the end of the smelting step is shown in Table 1 in terms of mass percentage. The chemical composition of the continuous casting billet obtained in the continuous casting step and the chemical composition of the prestressed steel rod finally prepared were consistent with the chemical composition of the molten steel at the end of the molten steel smelting step, as shown in Table 1.

[0031] Among them, in the converter smelting step, the molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, and oxygen blowing decarburization are carried out. When tapping, alloy is 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, and the inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0032] (2) Continuous casting steps During the continuous casting step, the superheat of the molten steel is controlled at 20-25°C, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed during continuous casting is 1.2±0.05m / min, the continuous casting 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 soft reduction is 15.0±0.2mm. The continuous casting step produces a rectangular billet with a cross-sectional size of no less than 240mm×180mm to ensure a sufficient compression ratio after rolling into wire rod to ensure the homogeneity of the wire rod.

[0033] (3) High-speed wire rolling steps During the rolling step, the starting rolling temperature is adjusted to 1050-1100°C by adjusting the heating temperature and time, the water tank flow is adjusted to make the pre-finishing rolling temperature 910±10°C, the finishing rolling mill temperature 900±10°C, and the reducing and sizing unit temperature 900±10°C, and a smooth wire rod with a diameter of 15 mm is obtained by rolling.

[0034] (4) Online salt bath quenching and aging steps During the online salt bath quenching and aging steps, the wire rod is laid at a temperature of 900±10°C. After laying, the wire rod is directly immersed in a salt bath for isothermal quenching at a temperature of 400±5°C for 60 seconds. After salt bath austempering, the wire rod is hot-coiled and then placed in a heat-insulating corridor for online aging at a temperature of 380±10°C for 60 minutes.

[0035] (5) Uncoiling and cutting to length steps 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

[0036] A method for producing a heat treatment-free 1670 MPa grade hot-rolled prestressed steel bar comprises the following steps: (1) Molten steel smelting steps Molten steel was smelted through the sequential steps of 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 in terms of mass percentage. In addition, the chemical composition of the continuous casting billet obtained in the continuous casting step and the chemical composition of the prestressed steel rods finally produced were consistent with the chemical composition of the molten steel at the end of the molten steel smelting step, as shown in Table 1.

[0037] Among them, in the converter smelting step, the molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, and oxygen blowing decarburization are carried out. When tapping, alloy is 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, and the inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0038] (2) Continuous casting steps During the continuous casting step, the superheat of the molten steel is controlled at 20-25°C, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed during continuous casting is 1.2±0.05m / min, the continuous casting 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 soft reduction is 15.0±0.2mm. The continuous casting step produces a rectangular billet with a cross-sectional size of no less than 240mm×180mm to ensure a sufficient compression ratio after rolling into wire rod to ensure the homogeneity of the wire rod.

[0039] (3) High-speed wire rolling steps During the rolling step, the starting rolling temperature is controlled at 1050-1100°C by adjusting the heating temperature and time, the water tank flow is adjusted to make the pre-finishing rolling temperature 910±10°C, the finishing rolling mill temperature 900±10°C, and the reducing and sizing unit temperature 900±10°C. The steel wire rod is rolled into a surface ribbed wire rod with a diameter of 16 mm, and the shape and size of the ribs comply with the provisions of GB1499.

[0040] (4) Online salt bath quenching and aging steps During the online salt bath quenching and aging steps, the wire rods are laid at a temperature of 900±10°C. After laying, they are directly immersed in a salt bath for isothermal quenching at a temperature of 370±5°C for 100 seconds. After salt bath austempering, the wire rods are hot-coiled and then placed in a heat-insulating corridor for online aging at a temperature of 360±10°C for 90 minutes.

[0041] (5) Uncoiling and cutting to length steps 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.

[0042] Comparative Example The comparative example uses conventional 35Si2Mn composition and adopts the conventional smelting-rolling-Stelmor cooling process-aging-drawing-notching-quenching and tempering heat treatment process to produce a steel bar with a diameter of 15 mm. The specific steps are as follows: (1) Molten steel smelting steps The molten steel was smelted through the steps of converter smelting and LF furnace refining in sequence. The chemical composition of the molten steel at the end of smelting is shown in Table 1 in terms of mass percentage.

[0043] Among them, in the converter smelting step, the molten iron is sent into the converter and mixed with scrap steel to form molten steel, and desiliconization, dephosphorization, and oxygen blowing decarburization are carried out. When tapping, alloy is 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, and the inclusions in the molten steel are controlled by soft stirring. The steel is tapped after the temperature and chemical composition meet the standards.

[0044] (2) Continuous casting steps During the continuous casting step, the superheat of the molten steel is controlled at 20-25°C, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed during continuous casting is 1.2±0.05m / min, the continuous casting 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 soft reduction is 15.0±0.2mm. The continuous casting step produces a rectangular billet with a cross-sectional size of no less than 240mm×180mm to ensure a sufficient compression ratio after rolling into wire rod to ensure the homogeneity of the wire rod.

[0045] (3) High-speed wire rolling steps During the rolling step, the heating temperature and time are adjusted to make the starting rolling temperature 1050-1100℃, the water tank flow is adjusted to make the pre-finishing rolling temperature 920±10℃, the finishing rolling mill temperature 920±10℃, and the reducing and sizing unit temperature 920±10℃, and the wire rod is rolled into a smooth wire rod with a diameter of 16mm.

[0046] (4) Stelmor cooling and aging steps During the Stelmore cooling step, the spinning temperature is 910±10°C. After spinning, fans are used for cooling and a heat shield is used for slow cooling. Specifically, fans 1-8 are operated at 100% and fans 9-12 at 50%. Afterwards, a heat shield is used for slow cooling. The aging step is to place the yarn at room temperature for 15 days before use.

[0047] (5) Drawing-score-tempering steps The aged wire rod is drawn to 15mm in two passes. The scored wire is induction heated to 920℃, water cooled, and then induction heated to 550-600℃. Finally, it is cut to a fixed length to obtain the final product.

[0048] Table 1 Chemical composition of molten steel at the end of smelting of Examples 1-3 and Comparative Examples

[0049] The steel bars prepared in Examples 1-3 and the comparative example were sampled and subjected to metallographic structure testing and mechanical property testing according to the same testing method. The specific testing methods and test results are as follows: (1) Metallographic structure detection: 10 cm long samples were taken from one end of the steel rod to prepare metallographic samples. After mechanical polishing and nitric acid etching, the samples were placed under a metallographic microscope for structure observation. The metallographic structure pictures of the steel rod samples prepared in Examples 1-3 and the comparative example are shown in FIG. Figures 1 to 4 As shown in the figure, Figure 1-3Both structures are meta-bainite, also known as carbide-free bainite, consisting of fine lamellar bainitic ferrite and a film of retained austenite. The chemical composition system designed by the present invention contains a large amount of Si, which inhibits carbide formation. This allows the wire to rapidly cool from austenite to the lower bainite transformation temperature after spinning, forming a large amount of bainitic ferrite without precipitating carbides. This allows the untransformed austenite to become carbon-enriched and stabilize at room temperature. Figure 4 The traditional process shown is a typical tempered martensite structure.

[0050] (2) Mechanical property testing: With reference to the test method and definition of GB / T228 standard, the mechanical property test of the steel bar samples was carried out using a tensile testing machine, and the tensile strength and cross-sectional shrinkage of the steel bars prepared in Examples 1-3 and the comparative example were measured as shown in Table 2. As can be seen from Table 2, the tensile strength level of the prestressed steel bars prepared in Examples 1-3 was improved to 1670 MPa, and the elongation was greater than 15%, and the strength and plasticity were excellent. It can be seen that the present invention has greatly improved the tensile strength and plasticity of the steel bar compared with the traditional process under the combined effect of alloying elements and microstructure. In particular, the fine lamellar bainitic ferrite in the microstructure ensures that the steel bar has extremely high tensile strength. At the same time, good plasticity is guaranteed under the effect of retained austenite and alloying elements such as Ni.

[0051] Table 2 Performance parameters of steel bars prepared in Examples 1-3 and Comparative Examples

[0052] The above description is merely a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any person skilled in the art may utilize the technical contents disclosed above to change or modify them into equivalent embodiments with equivalent changes for application in other fields. However, any simple modification, equivalent change, and modification of the above embodiments made in accordance with the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A heat treatment-free 1670MPa grade hot-rolled prestressed steel bar, characterized in that: Its 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 are Fe and other inevitable impurities.

2. The heat treatment-free 1670 MPa grade hot-rolled prestressed steel bar according to claim 1, characterized in that: Its 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 are Fe and other inevitable impurities.

3. The heat treatment-free 1670 MPa grade hot-rolled prestressed steel bar according to claim 1, characterized in that: The diameter of the steel rod is 5.5-16 mm.

4. A method for producing heat-treatment-free 1670MPa grade hot-rolled prestressed steel bars, characterized in that: The method comprises a molten steel smelting step, a continuous casting step, a high-speed wire rolling step, an online salt bath quenching and aging step, and an uncoiling, shearing and sizing step. In the online salt bath quenching and online aging steps, the wire rod is laid at a temperature of 880-920° C., and then is directly immersed in a salt bath for isothermal quenching, the salt bath temperature is Ms~420° C., and the time is 60-200 seconds. The wire rod after the online salt bath quenching is hot-coiled and then placed in an annealing furnace for online aging, the aging temperature is 300-400° C., and the time is ≥60 minutes.

5. The method for producing heat treatment-free 1670 MPa grade hot-rolled prestressed steel bars according to claim 1, characterized in that: In the continuous casting step, the superheat of the molten steel is 20~25°C, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the pulling speed during continuous casting is 1.2±0.05m / min, the continuous casting water content is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, the total reduction under light pressure is 15.0±0.2mm, and the continuous casting obtains a rectangular billet with a cross-sectional size of not less than 240mm×180mm.

6. The method for producing heat treatment-free 1670 MPa grade hot-rolled prestressed steel bars according to claim 1, characterized in that: In the rolling step, the starting rolling temperature is 1050-1100°C, the temperature entering the pre-finishing mill is 910±20°C, the temperature entering the finishing mill is 910±20°C, and the temperature entering the reducing and sizing mill is 910±20°C. The wire rod with a smooth surface or a ribbed surface is obtained by rolling, and the wire rod diameter is 5.5-16mm.

7. The method for producing heat treatment-free 1670 MPa grade hot-rolled prestressed steel bars according to claim 1, characterized in that: In the online salt bath quenching and aging steps, the salt bath temperature is 350~405℃ and the time is 60-120s.

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