A hot-rolled wire rod of grade 14.9 quasi-bainitic non-quenched and tempered steel and its manufacturing method

Abstract

This invention discloses a hot-rolled wire rod of grade 14.9 quasi-bainitic non-quenched and tempered steel and its manufacturing method, belonging to the field of metal wire technology. The wire rod composition is: C 0.28-0.32%, Si 1.20-1.50%, Mn 2.00-2.20%, Cr 0.90-1.20%, Mo 0.10-0.25%, Al 0.020-0.035%, P≤0.015%, S≤0.010%, with the balance being Fe. The wire rod has a quasi-bainitic microstructure, a tensile strength ≥1400MPa, and a reduction of area ≥60%. The method involves smelting, continuous casting, rolling, and isothermal quenching with residual heat after wire drawing, with high silicon content to suppress carbide precipitation. This invention solves the problems of strength-plasticity mismatch and low plasticity in existing grade 14.9 steel, and the process is low-carbon and low-cost, making it suitable for high-strength bolts, wind power fasteners, etc.

Description

Technical Field

[0001] This invention belongs to the field of metal wire technology, and particularly relates to a hot-rolled wire rod of grade 14.9 quasi-bainitic non-quenched and tempered steel and its manufacturing method. Background Technology

[0002] High-strength non-quenched and tempered steel, by achieving carbon reduction at the source through a streamlined process without quenching and tempering, and by reducing carbon at the end through high strength and lightweight properties, is a landmark material for green and low-carbon transformation. It has irreplaceable strategic significance for enhancing the resilience of the industrial chain and building a manufacturing powerhouse. Currently, research on 14.9 grade non-quenched and tempered steel is still in the exploratory stage.

[0003] Existing technologies related to grade 14.9 non-quenched and tempered steel have significant shortcomings: CN118639120A: Discloses a 14.9 grade non-quenched and tempered high-carbon cold heading steel wire rod for bolts, with a composition of C: 0.79%~0.83%, Si: 0.15%~0.25%, and Mn: 0.58%~0.72%. After online molten salt quenching and tempering, the microstructure is a mixture of tempered pearlite, sorbite, and bainite. The tensile strength is 1305-1345MPa, the reduction of area is only 44%-49%, and the plasticity is insufficient.

[0004] CN118653095A: Discloses a 14.9 grade non-quenched and tempered cold heading steel wire rod, containing C: 0.80%~0.84%, Cr, Nb, V, Mo and other alloys, with the microstructure mainly composed of tempered sorbite (>88%), strength of 1350-1400MPa, but with a shrinkage of only 42-48%, and poor strength-plasticity matching.

[0005] CN121228119A: Discloses a 14.9 grade steel with delayed fracture resistance and non-quenched and tempered, with low carbon design (C: 0.33%~0.55%), low Si (Si≤0.10%), and microstructure of lath bainite. However, if the Si content is too low, brittle cementite is easily precipitated, which will impair toughness.

[0006] In summary, existing technologies generally suffer from problems such as poor strength-plasticity matching, low plasticity (section reduction rate <50%), high alloy cost, and complex processes, which cannot meet the comprehensive requirements of 14.9 grade high-strength steel for high strength, high plasticity, low cost, and green manufacturing. Summary of the Invention

[0007] To achieve the above objectives, the present invention provides the following technical solution: A 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod has the following chemical composition by weight percentage: C: 0.28-0.32 wt.%; Si: 1.20-1.50 wt.%; Mn: 2.00-2.20 wt.%; Cr: 0.90-1.20 wt.%; Mo: 0.10-0.25 wt.%; Al: 0.020-0.035%; P: ≤0.015 wt.%; S: ≤0.010 wt.%; with the remainder being Fe and unavoidable impurities.

[0008] In the component design of this invention: C: To obtain a quasi-bainitic microstructure, a low-carbon composition design is adopted in this invention. A certain amount of carbon is beneficial to improving the stability of untransformed austenite, so that after the wire rod is salt bath treated and cooled to room temperature, a certain volume fraction of residual austenite can be retained, thereby obtaining high strength and high plasticity. The carbon content in this invention ranges from 0.28% to 0.32%.

[0009] Si: A non-carbide-forming element that effectively prevents the precipitation of brittle cementite during bainite transformation. The Si content in this invention ranges from 1.20% to 1.50%.

[0010] Mn: On the one hand, the addition of Mn can effectively delay the high-temperature ferrite phase transformation; on the other hand, similar to the addition of C mentioned above, Mn can increase the stability of untransformed austenite, so that a certain volume fraction of residual austenite can be retained after the wire rod is cooled to room temperature after salt bath treatment. The Mn content in this invention ranges from 2.00 to 2.20 wt%.

[0011] Cr and Mo: To further suppress the high-temperature ferrite phase transformation, a certain amount of Cr and Mo is added in this invention. Considering that Mo is an expensive element, the content ranges of Cr and Mo in this invention are 0.90~1.20% and 0.10~0.25%, respectively.

[0012] S and P: reduce the plasticity of high-strength steel. In this invention, the content of S and P is controlled to be S≤0.010% and P≤0.015%, respectively.

[0013] Furthermore, the microstructure of the hot-rolled wire rod is mainly quasi-bainite, with a volume fraction of ≤6% martensite allowed.

[0014] Furthermore, the wire rod has a tensile strength of not less than 1400 MPa, a reduction of area of ​​≥60%, and an elongation after fracture of ≥12%.

[0015] This invention also provides a method for manufacturing 14.9 grade quasi-bainitic non-quenched and tempered steel hot-rolled wire rod, the method comprising the steps of steel smelting, continuous casting, high-speed wire rolling, wire drawing, and isothermal quenching with residual heat from rolling, and the steps are as follows: 1) Steelmaking: Through KR desulfurization of molten iron, converter smelting, LF refining and RH refining, molten steel that meets the composition requirements is obtained; 2) Continuous casting: casting with low superheat, reasonable control of casting speed, and control of segregation in continuous casting by means of electromagnetic stirring in the crystallizer, electromagnetic stirring at the end and light reduction. 3) High-speed wire rod rolling and wire drawing: This includes the continuous casting billet heating, rough rolling, finish rolling and wire drawing processes. The continuous casting billet heating temperature is 1060-1160℃ and the time is not less than 120min. The rough rolling start temperature is 1000-1050℃, the finish rolling inlet temperature is 860-920℃, and the wire drawing temperature is 830-900℃. 4) Isothermal quenching with residual heat from rolling: After the wire rod is wired, it is directly immersed in a constant temperature salt bath at 350-400℃ in the austenitic state using the residual heat after rolling, and the isothermal time is not less than 1080s.

[0016] When wire rod is isothermally quenched in the lower bainitic transformation temperature range, the carbide precipitation process is delayed due to the large amount of Si element in the steel during the austenite-bainite transformation. Carbon is enriched and retained in the untransformed austenite, thus obtaining a quasi-bainitic structure composed of bainitic ferrite and retained austenite.

[0017] Preferably, the isothermal quenching of the rolling residual heat can also be carried out by using a salt bath or aqueous solution spray to rapidly cool the wire rod to a temperature 20-50°C above the Ms point, and then put it into a constant temperature heating furnace for an isothermal time of not less than 1800s.

[0018] Preferably, the cross-sectional dimensions of the continuously cast billet are not less than 180mm × 240mm to ensure a sufficient compression ratio after rolling into wire rod.

[0019] Compared with the prior art, the beneficial effects of the present invention are: A composition system of high Si (1.20-1.50%), low C (0.28-0.32%), and low Mo (0.10-0.25%) was established. High Si inhibits cementite and stabilizes residual austenite, low carbon reduces brittleness, and low Mo controls costs. There are no other precious elements, resulting in low cost. Through reasonable composition matching, wire rod mass production can be achieved on conventional hot rolling and controlled cooling equipment, taking into account both mechanical properties and the needs of large-scale production.

[0020] A carbide-free quasi-bainitic microstructure (bainitic ferrite plus retained austenite) is obtained, avoiding brittle phases. The retained austenite plasticizes the microstructure, achieving a super-strong plasticity match with tensile strength ≥1400MPa and area shrinkage ≥60%, breaking through the plasticity bottleneck of existing technologies.

[0021] The residual heat after rolling isothermal quenching achieves high-strength plasticization of raw materials, reducing the need for secondary heat treatment by downstream users and meeting the industry's demand for green and low-carbon development. Attached Figure Description

[0022] Figure 1 Example 1: Microstructure of the wire rod (quasi-bainite, 10 μm); Figure 2 Example 2: Microstructure of the wire rod (quasi-bainite, 10 μm); Figure 3 Example 3: Microstructure of the wire rod (quasi-bainite, 10 μm); Figure 4 Example 4: Microstructure of the wire rod (quasi-bainite, 10 μm). Detailed Implementation

[0023] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below through examples.

[0024] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0025] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments. Example 1

[0026] A 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod has the following chemical composition by weight percentage: C: 0.3 wt.%; Si: 1.25 wt.%; Mn: 2.20 wt.%; Cr: 0.90 wt.%; Mo: 0.10 wt.%; Al: 0.015%; P: 0.009 wt.%; S: 0.007 wt.%; with the remainder being Fe and unavoidable impurities.

[0027] The manufacturing method includes the following steps: (1) Steel smelting After desulfurization in KR, smelting in a converter, refining in LF and RH, molten steel that meets the composition requirements is obtained.

[0028] 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 LF 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.

[0029] (2) Continuous casting steps In the continuous casting process, low superheat casting is employed, and measures such as electromagnetic stirring in the crystallizer, electromagnetic stirring at the end of solidification, and gentle reduction are used to control the segregation of the continuously cast billet. Specific parameters can be adjusted according to the actual conditions of the equipment. The dimensions of the continuously cast billet are 180mm × 240mm.

[0030] (3) High-speed wire rolling and wire drawing The continuous casting billet is heated at 1060-1160℃ for 140 minutes. The roughing rolling temperature is 1000-1050℃, the finishing rolling inlet temperature is 870-900℃, and the wire drawing temperature is 830-850℃. The resulting smooth wire rod has a diameter of 20mm.

[0031] (4) Isothermal quenching with residual heat from rolling After the wire rod is spun into wire, it is directly immersed in a constant temperature salt bath at 350-370℃ for isothermal quenching via roller conveyor. The isothermal time is 1080s. After the isothermal time is completed, the final product is obtained. Example 2

[0032] A 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod has the following chemical composition by weight percentage: C: 0.31 wt.%; Si: 1.35 wt.%; Mn: 2.10 wt.%; Cr: 1.0 wt.%; Mo: 0.12 wt.%; Al: 0.016%; P: 0.011 wt.%; S: 0.008 wt.%; with the remainder being Fe and unavoidable impurities.

[0033] The manufacturing method includes the following steps: (1) Steel smelting After desulfurization in KR, smelting in a converter, refining in LF and RH, molten steel that meets the composition requirements is obtained.

[0034] 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 LF 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.

[0035] (2) Continuous casting steps In the continuous casting process, low superheat casting is employed, and measures such as electromagnetic stirring in the crystallizer, electromagnetic stirring at the end of solidification, and gentle reduction are used to control the segregation of the continuously cast billet. Specific parameters can be adjusted according to the actual conditions of the equipment. The dimensions of the continuously cast billet are 180mm × 240mm.

[0036] (3) High-speed wire rolling and wire drawing The continuous casting billet is heated at 1060-1160℃ for 140 minutes. The roughing rolling temperature is 1000-1050℃, the finishing rolling inlet temperature is 870-900℃, and the wire drawing temperature is 830-850℃. The resulting smooth wire rod has a diameter of 20mm.

[0037] (4) Isothermal quenching with residual heat from rolling After the wire rod is spun, it passes through the salt bath / aqueous solution spraying section at a roller speed of not less than 1 m / s, and is cooled to 360-380℃ at a cooling rate of 20℃ / s. Then it enters a constant temperature furnace at 370±5℃ and isothermally treated for 1800s before naturally cooling to obtain the final product. Example 3

[0038] A 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod has the following chemical composition by weight percentage: C: 0.28 wt.%; Si: 1.5 wt.%; Mn: 2.0 wt.%; Cr: 1.20 wt.%; Mo: 0.18 wt.%; Al: 0.02%; P: 0.012 wt.%; S: 0.007 wt.%; with the remainder being Fe and unavoidable impurities.

[0039] The manufacturing method includes the following steps: (1) Steel smelting After desulfurization in KR, smelting in a converter, refining in LF and RH, molten steel that meets the composition requirements is obtained.

[0040] 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 LF 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.

[0041] (2) Continuous casting steps In the continuous casting process, low superheat casting is employed, and measures such as electromagnetic stirring in the crystallizer, electromagnetic stirring at the end of solidification, and gentle reduction are used to control the segregation of the continuously cast billet. Specific parameters can be adjusted according to the actual conditions of the equipment. The dimensions of the continuously cast billet are 180mm × 240mm.

[0042] (3) High-speed wire rolling and wire drawing The continuous casting billet is heated at 1060-1160℃ for 140 minutes. The roughing rolling temperature is 1000-1050℃, the finishing rolling inlet temperature is 870-900℃, and the wire drawing temperature is 830-850℃. The resulting smooth wire rod has a diameter of 8mm.

[0043] (4) Isothermal quenching with residual heat from rolling After the wire rod is spun into wire, it is directly immersed in a constant temperature salt bath at 380-400℃ for isothermal quenching via roller conveyor. The isothermal time is 1200s. After the isothermal time is completed, the final product is obtained. Example 4

[0044] A 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod has the following chemical composition by weight percentage: C: 0.32 wt.%; Si: 1.45 wt.%; Mn: 2.05 wt.%; Cr: 1.11 wt.%; Mo: 0.25 wt.%; Al: 0.013%; P: 0.012 wt.%; S: 0.007 wt.%; with the remainder being Fe and unavoidable impurities.

[0045] The manufacturing method includes the following steps: (1) Steel smelting After desulfurization in KR, smelting in a converter, refining in LF and RH, molten steel that meets the composition requirements is obtained.

[0046] 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 LF 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.

[0047] (2) Continuous casting steps In the continuous casting process, low superheat casting is employed, and measures such as electromagnetic stirring in the crystallizer, electromagnetic stirring at the end of solidification, and gentle reduction are used to control the segregation of the continuously cast billet. Specific parameters can be adjusted according to the actual conditions of the equipment. The dimensions of the continuously cast billet are 180mm × 240mm.

[0048] (3) High-speed wire rolling and wire drawing The continuous casting billet is heated at 1060-1160℃ for 140 minutes. The roughing rolling temperature is 1000-1050℃, the finishing rolling inlet temperature is 870-900℃, and the wire drawing temperature is 830-850℃. A smooth wire rod with a diameter of 16mm is obtained by rolling.

[0049] (4) Isothermal quenching with residual heat from rolling After the wire rod is spun into wire, it is directly immersed in a constant temperature salt bath at 370-390℃ for isothermal quenching via roller conveyor. The isothermal time is 1300s. After the isothermal time is completed, the final product is obtained.

[0050] The wire rods prepared in Examples 1-4 were sampled and subjected to metallographic and mechanical property tests using the same testing methods. The specific testing methods and results are as follows: (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 scanning electron microscope. The metallographic images of the steel rod samples prepared in Examples 1-4 are shown below. Figures 1 to 4 As shown in the figure. Figure 1-4 All are quasi-bainitic structures, also known as carbide-free bainite, composed of bainitic ferrite and a thin film / block 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 winding, 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.

[0051] (2) Mechanical property testing: In accordance with the test methods and definitions of GB / T228 standard, the mechanical properties of the wire rod samples were tested using a tensile testing machine. The tensile strength and reduction of area of ​​the wire rods prepared in Examples 1-4 are shown in Table 1. As can be seen from Table 1, the tensile strength of the wire rods prepared in Examples 1-4 is increased to above 1400 MPa, and the reduction of area is ≥60%, and the elongation after fracture is greater than 12%. The combination of strength and plasticity is excellent. 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 wire rod compared with the traditional process. In particular, the fine lath-shaped bainitic ferrite in the microstructure ensures that the wire rod has extremely high tensile strength, while the residual austenite ensures good plasticity.

[0052] Table 1 Performance parameters of the wire rods prepared in Examples 1-4

[0053] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A type of 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod, characterized in that, Its chemical composition by weight percentage is as follows: C: 0.28-0.32 wt.%; Si: 1.20-1.50 wt.%; Mn: 2.00-2.20 wt.%; Cr: 0.90-1.20 wt.%; Mo: 0.10-0.25 wt.%; Al: 0.020-0.035%; P: ≤0.015 wt.%; S: ≤0.010wt.%; the remainder is Fe and unavoidable impurities.

2. The 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod according to claim 1, characterized in that, The microstructure of the hot-rolled wire rod is mainly quasi-bainite, with a volume fraction of martensite ≤6%.

3. The 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod according to claim 1, characterized in that, The wire rod has a tensile strength of not less than 1400 MPa, a reduction of area of ​​≥60%, and an elongation after fracture of ≥12%.

4. A method for manufacturing 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod, the method comprising the steps of steel smelting, continuous casting, high-speed wire rolling, wire drawing, and isothermal quenching with residual heat from rolling, characterized in that, The steps are as follows: 1) Steelmaking: Through KR desulfurization of molten iron, converter smelting, LF refining and RH refining, molten steel that meets the composition requirements is obtained; 2) Continuous casting: casting with low superheat, reasonable control of casting speed, and control of segregation in continuous casting by means of electromagnetic stirring in the crystallizer, electromagnetic stirring at the end and light reduction. 3) High-speed wire rod rolling and wire drawing: This includes the continuous casting billet heating, rough rolling, finish rolling and wire drawing processes. The continuous casting billet heating temperature is 1060-1160℃ and the time is not less than 120min. The rough rolling start temperature is 1000-1050℃, the finish rolling inlet temperature is 860-920℃, and the wire drawing temperature is 830-900℃. 4) Isothermal quenching with residual heat from rolling: After the wire rod is wired, it is directly immersed in a constant temperature salt bath at 350-400℃ in the austenitic state using the residual heat after rolling, and the isothermal time is not less than 1080s.

5. The method for manufacturing 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod according to claim 4, characterized in that, The isothermal quenching of the rolling residual heat can also be carried out by using a salt bath or aqueous solution spray to rapidly cool the wire rod to a temperature 20-50°C above Ms point, and then put it into a constant temperature heating furnace, with an isothermal time of not less than 1800s.

6. The method for manufacturing 14.9 grade quasi-bainitic non-quenched and tempered hot-rolled wire rod according to claim 4, characterized in that, The cross-sectional dimensions of the continuously cast billet are not less than 180mm × 240mm to ensure a sufficient compression ratio after rolling into wire rod.

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