Hot-rolled air-cooled bainite steel and method of manufacturing and use thereof

Abstract

This invention discloses a hot-rolled air-cooled bainitic steel, its preparation method, and its applications. The chemical composition of this hot-rolled air-cooled bainitic steel, by mass percentage, includes: C 0.18%–0.28%, Mn 1.6%–2.4%, Si 0.8%–1.5%, Cr 0.2%–0.6%, Mo 0.1%–0.2%, V 0.02%–0.08%, Ti 0–0.015%, Al(t) 0.01%–0.02%, with the remainder being Fe and residual trace impurities. The hot-rolled air-cooled bainitic steel preparation process provided by this invention is simple. It uses natural air cooling followed by low-temperature tempering of hot-rolled round steel to form carbide-free bainite, retained austenite, and martensite, avoiding processes such as quenching, quenching followed by medium-high temperature tempering, and salt bath treatment. This can save heat treatment costs and reduce the emission of harmful gases. In addition, this hot-rolled air-cooled bainitic steel has excellent mechanical properties and can be used to manufacture large-size bolts.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, and in particular to a hot-rolled air-cooled bainitic round steel, its preparation method, and its application. Background Technology

[0002] In recent years, with economic and social development, the demand for ultra-high strength and toughness bolts has increased in fields such as rail transit, wind power, and bridges, especially for ultra-large bolt steel. Currently, bolts of grade 10.9 or higher are mainly obtained through quenching and tempering, i.e., using medium carbon steel, which is quenched and then tempered at medium to high temperatures. For example, grade 10.9 bolts are generally made from 42CrMo steel through quenching at 880℃ and tempering at 550℃ to form tempered sorbite. However, 42CrMo steel is difficult to meet the manufacturing requirements of grade 12.9 or higher bolts. In addition, 42CrMo steel has poor hardenability, with a maximum hardenable diameter of only 42mm, making it unsuitable for manufacturing larger bolts. Against this backdrop, the development of bolt steel suitable for higher strength levels, larger sizes, and superior performance has become a hot topic of concern both domestically and internationally.

[0003] Baoshan Iron & Steel Co., Ltd.'s patent application for "A High-Homogeneous and High-Hardenability Steel for Wind Turbine Bolts, Bars and Their Manufacturing Methods" (202111121073.2) discloses a high-homogeneous and hardenable steel suitable for wind turbine bolts with a diameter of 90mm or less. It adopts a medium-carbon, high-Cr composition design, supplemented with precious elements such as Mo and Ni. However, this patent requires "water quenching + medium-high temperature tempering at 520~640℃". The heat treatment process used in the patent application "High-strength bolt steel for wind power and high-strength bolts for wind power" (202211653246.X) by Aipu Parts Manufacturing (Suzhou) Co., Ltd. is as follows: "S5. Low-temperature aging treatment of bolt blanks from S4: heating temperature 200-300℃, holding for 6-8 hours to remove bolt processing stress and reduce heat treatment deformation; S6. Austenitizing treatment of the aged bolts: sending them to a salt bath quenching equipment, heating the bolts to 920℃~940℃ and holding for 2-3 hours for austenitization treatment." During the process, carbon monoxide, hydrogen, and nitrogen are introduced to protect the bolts from oxidation and decarburization during heating; S7, after S6, the bolts are transferred to a quenching salt bath for isothermal quenching. The isothermal quenching is a two-stage quenching method to ensure that the bainite content is greater than 99.8%; S8, after quenching, the bolts are air-cooled to room temperature and then sent to an ultrasonic cleaner for cleaning and air drying; S9, the cleaned bolts are subjected to hard chrome plating to obtain high-strength bolts with higher surface hardness, good wear resistance, and good corrosion resistance. The overall heat treatment process of this patent is too complicated, requiring salt bath isothermal quenching, which is not environmentally friendly. The production method disclosed in the patent application filed by Maanshan Iron & Steel Co., Ltd. for "A high-strength and tough non-quenched and tempered steel for wind turbine bolts and its production method" (202211055609.X) includes "after the wire is spun, the Stellmore cooling line uses strong wind to rapidly cool it to 450-500°C at a cooling rate of ≥30°C / s, then adds an insulation cover to naturally cool it to 400-440°C, coils it up, hooks it, and then air-cools it to room temperature." This patent requires strong wind to rapidly cool it at a cooling rate of ≥30°C / s and is only applicable to wires with specifications of 6.5-25mm. The patent application filed by Boyao Energy Technology Co., Ltd., entitled "High-strength and Tough Large-Diameter Wind Turbine Bolt with Beyer / Marsh Dual-Phase Structure and Preparation Method Thereof", discloses a preparation method that includes "heating at 910-950℃ and quenching to 180-240℃; then performing partitioning at a partitioning temperature of 280-340℃". This patent requires "quenching and partitioning", and the heat treatment process is complex. At the same time, the patent does not mention the most important fatigue resistance of the bolt.

[0004] In summary, traditional 42CrMo bolt steel can no longer meet the manufacturing and performance requirements of bolts with higher strength levels (such as grade 12.9) and larger specifications. Existing patents disclose complex processes for bolt steel and its manufacturing methods, employing complex heat treatment methods such as quenching and tempering, or salt bath medium-temperature quenching, and rapid cooling with strong air, which are not conducive to energy conservation and emission reduction. At the same time, these complex processes are difficult to operate for large-specification bolts. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a hot-rolled air-cooled bainitic steel, its preparation method, and its application. This bainitic steel has a simple heat treatment process, excellent mechanical properties, and can be used to manufacture large-size bolts.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] On the one hand, the present invention provides a hot-rolled air-cooled bainitic steel, the chemical composition of which, by mass percentage, comprises: C 0.18%–0.28%, Mn 1.6%–2.4%, Si 0.8%–1.5%, Cr 0.2%–0.6%, Mo 0.1%–0.2%, V 0.02%–0.08%, Ti 0–0.015%, Al(t) 0.01%–0.02%, with the remainder being Fe and residual trace impurities.

[0008] In the technical solution of this invention, Al(t) represents dissolved aluminum.

[0009] In some specific embodiments, the mass percentage of C is 0.18%, 0.20%, 0.22%, 0.24%, 0.26%, or 0.28%.

[0010] In some specific embodiments, the mass percentage of Mn is 1.6%, 1.8%, 2.0%, 2.2%, or 2.4%.

[0011] In some specific embodiments, the mass percentage of Si is 0.8%, 1.0%, 1.2%, 1.4%, or 1.5%.

[0012] In some specific embodiments, the mass percentage of Cr is 0.2%, 0.4%, or 0.6%.

[0013] In some specific embodiments, the mass percentage of V is 0.02%, 0.04%, 0.06%, or 0.08%.

[0014] In some specific embodiments, the mass percentage of Ti is 0.011%, 0.012%, 0.013%, 0.014%, or 0.15%.

[0015] In the technical solution of the present invention, the residual trace impurities include N, P, S, O and H; wherein, the content of N is ≤60ppm, the mass percentage of P is ≤0.01%, the mass percentage of S is ≤0.01%, the content of [O] is ≤20ppm, and the content of [H] is ≤1.5ppm; wherein, [O] represents dissolved oxygen; and [H] represents all hydrogen.

[0016] In the technical solution of the present invention, the chemical composition of the hot-rolled air-cooled bainitic steel is as follows:

[0017] (1) Low carbon content is used to ensure the excellent strength and toughness of steel. Too low carbon content is not less than the improvement of strength, while too high carbon content is not conducive to the improvement of toughness.

[0018] (2) The combination of Mn and Si ensures the formation of carbide-free bainite under hot rolling and air cooling conditions. If the content of both is too low, ferrite or pearlite structure will be formed. If the content is too high, compositional segregation will occur, and brittle martensite structure will be easily formed.

[0019] (3) The proportion of bainite formed in round steel of different diameters is controlled by using Cr and Mo. If the content of both is too low, ferrite or pearlite structure will be formed in the core of large round steel. If the content of both is too high, the alloy cost will increase. At the same time, the proportion of bainite formed is too low, which is not conducive to improving toughness and plasticity.

[0020] (4) V and Ti elements are used to refine the grain size of round steel. At the same time, solid solution V and Ti can improve the hardenability of steel. A reasonable combination can reduce the addition of elements such as Cr and Mo. However, too much V and Ti will increase the cost. At the same time, too much Ti will induce the formation of coarse TiS and TiN, which is not good for toughness and fatigue.

[0021] (5) Controlling the content range of Al(t) and N is to form a small amount of fine AlN, which can consume excess N and promote the role of V and Ti in improving hardenability. However, if the content is too high, coarse AlN and TiN will be formed.

[0022] (6) Control the content range of P, S, O and H in order to improve the toughness, plasticity and fatigue performance of round steel.

[0023] In another aspect, the present invention provides a method for preparing the above-mentioned hot-rolled air-cooled bainitic steel, comprising the following steps:

[0024] 1) Cast the above-mentioned raw materials into round billets according to the proportions;

[0025] 2) Hold the above round billet at 1100-1200℃ for 30-60 minutes; hot roll it into round steel at a final rolling temperature of 850-950℃;

[0026] 3) Naturally air-cool to below 200℃ and then temper to obtain the hot-rolled air-cooled bainitic steel.

[0027] In the technical solution of the present invention, the raw materials used in the above composition are the types of raw materials commonly used in the art.

[0028] In a preferred embodiment, the diameter of the round billet in step 1) is not less than twice the diameter of the round steel in step 2), preferably 2 to 5 times.

[0029] Preferably, the diameter of the round steel in step 2) is 25–80 mm;

[0030] Preferably, the step of casting into a round billet includes: steelmaking in an electric furnace or converter, ladle refining and vacuum degassing, followed by ingot casting or continuous casting into a round billet;

[0031] In some specific embodiments, step 1) further includes the operation of slowly cooling the billet to room temperature;

[0032] In the technical solution of the present invention, by controlling the diameter of the round billet and round steel, the rolling deformation of the round steel can be further guaranteed, segregation and banded structure can be reduced, and the grains can be refined.

[0033] In a preferred embodiment, in step 3), the natural air cooling is carried out on a cooling bed; in the natural air cooling, the distance between adjacent round steel bars on the cooling bed is 1 to 2 times the diameter of the round steel bars;

[0034] Preferably, in step 3), the average cooling rate of the natural air cooling is: 0.5 to 2℃ / second between 850 and 500℃, and 0.05 to 0.5℃ / second between 500℃ and 200℃.

[0035] In the technical solution of the present invention, by controlling the distance between adjacent round steel bars during natural air cooling and the cooling rate of natural air cooling, it is possible to ensure the formation of bainitic structure during natural air cooling.

[0036] In a preferred embodiment, in step 3), the tempering temperature is 280–340°C, and the tempering time is 2–6 hours.

[0037] In some specific embodiments, the tempering is carried out in a walking beam tempering furnace or a bogie tempering furnace.

[0038] In the technical solution of this invention, tempering can stabilize the microstructure and eliminate residual stress.

[0039] In the technical solution of the present invention, the microstructure of the hot-rolled air-cooled bainitic steel has a volume fraction of 50% to 75% of carbide-free bainite, a volume fraction of 4% to 10% of retained austenite, and the remainder is martensite.

[0040] In the technical solution of the present invention, the hot-rolled air-cooled bainitic steel is tested according to the national standard GB / T10561-2023 "Standard Rating Chart Microscopic Detection Method for Determination of Non-metallic Inclusion Content in Steel", and the inclusion levels meet the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, and Class Ds ≤ 0.5.

[0041] In another aspect, the present invention provides the application of the above-mentioned hot-rolled air-cooled bainitic steel in the preparation of bainitic bolts.

[0042] Preferably, the bainitic bolt is an M24 to M76 type bolt.

[0043] In another aspect, the present invention provides a method for preparing bainitic bolts from the above-mentioned hot-rolled air-cooled bainitic steel, comprising the following steps:

[0044] (1) Cut the above-mentioned hot-rolled air-cooled bainitic steel into sections, straighten, machine, finish machine, and roll threads to manufacture bainitic bolts;

[0045] (2) Perform anti-corrosion surface treatment on the above-mentioned bainitic bolts.

[0046] In a preferred embodiment, step (2) includes zinc diffusion, dipping in a sealant, and drying.

[0047] Preferably, the zinc diffusion temperature is not higher than 380°C, and more preferably 360-380°C;

[0048] Preferably, the drying temperature is not higher than 360°C, and more preferably 280-360°C;

[0049] In the technical solution of the present invention, the above-mentioned anti-corrosion surface treatment can ensure that the corrosion resistance of the bainitic bolt is improved without damaging the bainitic structure in the bainitic bolt.

[0050] In the technical solution of this invention, the bainitic bolt, according to GB / T 228.1-2021 Metallic materials, tensile testing—Part 1: Room temperature test method, GB / T229-2020 Metallic materials, Charpy pendulum impact test method, and GB / T4337-2008 Metallic materials, fatigue test—rotational bending method, has the following properties: tensile strength ≥1400MPa, yield strength ≥1100MPa, meeting or exceeding the strength requirements of bolts of grade 12.9 to 14.9; simultaneously, elongation ≥12%, reduction of area ≥55%; room temperature impact toughness A KV ≥70J, -40℃ impact toughness A KV ≥40J, the room temperature fatigue strength is not less than half of the tensile strength, that is, the room temperature fatigue strength is ≥700MPa.

[0051] The beneficial effects of this invention are as follows:

[0052] (1) The hot-rolled air-cooled bainitic steel provided by the present invention has a simple preparation process. It adopts the method of natural air cooling and low-temperature tempering of hot-rolled round steel to form carbide-free bainite, retained austenite and martensite, avoiding quenching, quenching + medium and high temperature tempering, salt bath treatment and other processes, which can save heat treatment costs and reduce the emission of harmful gases.

[0053] (2) The hot-rolled air-cooled bainitic steel alloy provided by the present invention has a low total amount. It adopts multi-element compounding and limits the content of harmful elements. It adopts low-carbon Mn-Si system composition design, with a small amount of Cr and Mo, adopts composite micro-alloying of V and Ti, and uses Al to consume N, thereby improving hardenability and reducing the content of Cr and Mo.

[0054] (3) The hot-rolled air-cooled bainitic steel provided by this invention has superior comprehensive performance. Compared with traditional bolt steel, its low-temperature toughness and fatigue resistance are significantly improved, and its impact toughness at -40℃ is A KV With a strength of ≥40J and a room temperature fatigue strength of ≥700MPa, bolts can have a longer service life and improved reliability. Attached Figure Description

[0055] Figure 1 These are microstructure images of the hot-rolled air-cooled bainitic steel obtained in Example 1 of the present invention; wherein, the left image is a scanning electron microscope image and the right image is a transmission electron microscope image.

[0056] Figure 2 These are microstructure images of the hot-rolled air-cooled bainitic steel obtained in Example 5 of the present invention; the left image is a scanning electron microscope image, and the right image is a transmission electron microscope image.

[0057] Figure 3 This is a photograph of the bainitic bolt manufactured according to Embodiment 8 of the present invention.

[0058] Figure 4 This is a flowchart of the manufacturing process of hot-rolled air-cooled bainitic steel in this invention. Detailed Implementation

[0059] The following embodiments are merely some, not all, of the embodiments of the present invention. Therefore, the detailed descriptions of the embodiments provided below are not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0060] In this invention, unless otherwise specified, all equipment and raw materials are commercially available or commonly used in the industry. The methods described in the following embodiments are conventional methods in the art, unless otherwise specified.

[0061] Example 1: Hot-rolled air-cooled bainitic steel 1

[0062] The chemical composition of the hot-rolled air-cooled bainitic steel in this embodiment, by mass percentage, includes:

[0063] C 0.18%, Mn 2.2%, Si 1.2%, Cr 0.6%, Mo 0.2%, V 0.08%, Ti 0%, Al(t) 0.01%, N≤60ppm, P≤0.01%, S≤0.01%, [O]≤20ppm, [H]≤1.5ppm, with the remainder being Fe.

[0064] Preparation process as follows Figure 4 As shown, it includes the following steps:

[0065] 1) Prepare the ingredients according to the above composition, and after the raw materials are processed by electric furnace steelmaking, ladle refining and vacuum degassing, they are molded into round billets with a diameter of 100mm, and the round billets are slowly cooled to room temperature;

[0066] 2) Heat the above round billet to 1150℃, hold for 30 minutes, and then hot roll it into a round steel with a diameter of 25mm. The final rolling temperature is 850℃.

[0067] 3) The hot-rolled round bars are pushed into the cooling bed in a stepping motion, with a spacing of 25mm between each round bar. They are then naturally air-cooled to below 200℃. The average cooling rate is 2℃ / second between 850℃ and 500℃ and 0.5℃ / second between 500℃ and 200℃.

[0068] 4) The above round steel is put into a bogie-type tempering furnace for tempering at a tempering temperature of 300℃ and a tempering time of 6 hours to obtain hot-rolled air-cooled bainitic steel.

[0069] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured by optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffractometer. The proportion of carbide-free bainite in its microstructure was 75% (volume percentage), the proportion of retained austenite was 6% (volume percentage) and the remainder was martensite. Figure 1 The image shows the microstructure of the hot-rolled air-cooled bainitic steel in this embodiment; in the image, CFB represents carbide-free bainite, M represents martensite, and the upper right corner of the right image shows the diffraction spot of retained austenite.

[0070] The hot-rolled air-cooled bainitic steel prepared in this embodiment was tested according to the national standard GB10561-2023 "Standard Rating Chart Microscopic Detection Method for Determination of Non-metallic Inclusion Content in Steel". The inclusion levels met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, and Class Ds ≤ 0.5.

[0071] Example 2: Bainitic bolt 1

[0072] In this embodiment, bainitic bolts are prepared using hot-rolled air-cooled bainitic steel 1 from Example 1, and the process is as follows:

[0073] 1) Cut hot-rolled air-cooled bainitic steel into sections, straighten, machine, finish, and roll threads to manufacture M24 bainitic bolts;

[0074] 2) The above-mentioned bainitic bolts are subjected to anti-corrosion surface treatment, including zinc diffusion, Dacromet coating and drying. The zinc diffusion temperature is 380℃ and the drying temperature is 360℃.

[0075] Testing revealed that the strength of the aforementioned bainitic bolts meets the strength requirements for grade 12.9 bolts, specifically with a tensile strength of 1418 MPa, a yield strength of 1117 MPa, an elongation of 15%, a reduction of area of ​​60%, and a room temperature impact toughness of A. KV Impact toughness A at 92J and -40℃ KV The J is 45, and the room temperature fatigue strength is 750 MPa.

[0076] The mechanical property testing process refers to GB / T 228.1-2021 Metallic materials, tensile testing - Part 1: Test method at room temperature, GB / T229-2020 Metallic materials, Charpy pendulum impact test method and GB / T4337-2008 Metallic materials, fatigue test method, rotational bending method.

[0077] Example 3: Hot-rolled air-cooled bainitic steel 2

[0078] The chemical composition of the hot-rolled air-cooled bainitic steel in this embodiment, by mass percentage, includes: C 0.22%, Mn 2.2%, Si 1.4%, Cr 0.6%, Mo 0.2%, V 0.04%, Ti 0.01%, Al(t) 0.02%, N≤60ppm, P≤0.01%, S≤0.01%, [O]≤20ppm, [H]≤1.5ppm, with the remainder being Fe.

[0079] To manufacture, follow these steps:

[0080] 1) Prepare the raw materials according to the above composition, and after the raw materials are refined in a converter, refined in a ladle and degassed in a vacuum, they are continuously cast into round billets with a diameter of 150mm. The round billets are then slowly cooled to room temperature.

[0081] 2) Heat the above round billet to 1200℃, hold for 45 minutes, and then hot roll it into a round steel with a diameter of 38mm. The final rolling temperature is 900℃.

[0082] 3) The hot-rolled round bars are pushed into the cooling bed in a stepping motion, with a spacing of 40mm between each round bar. They are then naturally air-cooled to below 200℃, with an average cooling rate of 1.5℃ / second between 850℃ and 500℃ and an average cooling rate of 0.25℃ / second between 500℃ and 200℃.

[0083] 4) The above-mentioned round steel is put into a bogie-type tempering furnace for tempering. The tempering temperature is 320℃ and the tempering time is 4 hours to obtain hot-rolled air-cooled bainitic steel.

[0084] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured by optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffractometer. The proportion of carbide-free bainite in its microstructure was 65% (volume percentage), the proportion of retained austenite was 8% (volume percentage) and the remainder was martensite.

[0085] The hot-rolled air-cooled bainitic steel prepared in this embodiment was tested according to the national standard GB10561-2023 "Standard Rating Chart Microscopic Detection Method for Determination of Non-metallic Inclusion Content in Steel". The inclusion levels met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, and Class Ds ≤ 0.5.

[0086] Example 4: Bainitic Bolt 2

[0087] In this embodiment, bainitic bolts are prepared using hot-rolled air-cooled bainitic steel 2 from Example 3. The process is as follows:

[0088] 1) Cut hot-rolled air-cooled bainitic steel into sections, straighten, machine, finish, and roll threads to manufacture M36 bainitic bolts;

[0089] 2) The above-mentioned bainitic bolts are subjected to anti-corrosion surface treatment, including zinc diffusion, Dacromet coating and drying. The zinc diffusion temperature is 380℃ and the drying temperature is 320℃.

[0090] Testing revealed that the strength of the aforementioned bainitic bolts meets the strength requirements for grade 12.9 bolts, specifically with a tensile strength of 1448 MPa, a yield strength of 1185 MPa, an elongation of 16%, a reduction of area of ​​62%, and a room temperature impact toughness of A. KV Impact toughness A at 98J and -40℃ KV The J is 48, and the room temperature fatigue strength is 760 MPa.

[0091] The mechanical property testing process refers to GB / T 228.1-2021 Metallic materials, tensile testing - Part 1: Test method at room temperature, GB / T229-2020 Metallic materials, Charpy pendulum impact test method and GB / T4337-2008 Metallic materials, fatigue test method, rotational bending method.

[0092] Example 5: Hot-rolled air-cooled bainitic steel 3

[0093] The chemical composition of the hot-rolled air-cooled bainitic steel in this embodiment, by mass percentage, includes: C 0.25%, Mn 2.1%, Si 1.5%, Cr 0.4%, Mo 0.2%, V 0.02%, Ti 0.01%, Al(t) 0.01%, N≤60ppm, P≤0.01%, S≤0.01%, [O]≤20ppm, [H]≤1.5ppm, with the remainder being Fe.

[0094] To manufacture, follow these steps:

[0095] 1) Prepare the ingredients according to the above composition, and after the raw materials are processed by converter steelmaking, ladle refining and vacuum degassing, they are molded into round billets with a diameter of 200mm, and the round billets are slowly cooled to room temperature;

[0096] 2) The above-mentioned round billet is reheated to 1200℃, held for 45 minutes, and then hot rolled into a round steel with a diameter of 48mm at a final rolling temperature of 900℃.

[0097] 3) The hot-rolled round bars are pushed into the cooling bed in a stepping motion, with a spacing of 50mm between each round bar. They are then naturally air-cooled to below 200℃, with an average cooling rate of 1.0℃ / second between 850℃ and 500℃ and an average cooling rate of 0.15℃ / second between 500℃ and 200℃.

[0098] 4) The above-mentioned round steel is put into a bogie-type tempering furnace for tempering. The tempering temperature is 320℃ and the tempering time is 4 hours to obtain hot-rolled air-cooled bainitic steel.

[0099] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured by optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffractometer. The proportion of carbide-free bainite in its microstructure was 60% (volume percentage), the proportion of retained austenite was 7.4% (volume percentage) and the remainder was martensite. Figure 2 The image shown is a microstructure diagram of the hot-rolled air-cooled bainitic steel in this embodiment.

[0100] The hot-rolled air-cooled bainitic steel prepared in this embodiment was tested according to the national standard GB10561-2023 "Standard Rating Chart Microscopic Detection Method for Determination of Non-metallic Inclusion Content in Steel". The inclusion levels met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, and Class Ds ≤ 0.5.

[0101] Example 6: Bainitic Bolt 3

[0102] In this embodiment, bainitic bolts are prepared using hot-rolled air-cooled bainitic steel 3 from Example 5, and the process is as follows:

[0103] 1) Hot-rolled air-cooled bainitic steel is cut into 3 sections, straightened, machined, precision-machined, and thread-rolled to manufacture M45 bainitic bolts;

[0104] 2) The above-mentioned bainitic bolts are subjected to anti-corrosion surface treatment, including zinc diffusion, Dacromet coating and drying. The zinc diffusion temperature is 360℃ and the drying temperature is 280℃.

[0105] Testing revealed that the strength of the aforementioned bainitic bolts meets the strength requirements for grade 12.9 bolts, specifically with a tensile strength of 1445 MPa, a yield strength of 1128 MPa, an elongation of 15%, a reduction of area of ​​63.8%, and a room temperature impact toughness of A. KV Impact toughness A at 123J and -40℃ KV The J is 52, and the room temperature fatigue strength is 800 MPa.

[0106] The mechanical property testing process refers to GB / T 228.1-2021 Metallic materials, tensile testing - Part 1: Test method at room temperature, GB / T229-2020 Metallic materials, Charpy pendulum impact test method and GB / T4337-2008 Metallic materials, fatigue test method, rotational bending method.

[0107] Example 7: Hot-rolled air-cooled bainitic steel 4

[0108] The chemical composition of the hot-rolled air-cooled bainitic steel in this embodiment, by mass percentage, includes: C 0.28%, Mn 1.6%, Si 0.8%, Cr 0.2%, Mo 0.1%, V 0.02%, Ti 0.015%, Al(t) 0.015%, N≤60ppm, P≤0.01%, S≤0.01%, [O]≤20ppm, [H]≤1.5ppm, with the remainder being Fe.

[0109] To manufacture, follow these steps:

[0110] 1) Prepare the raw materials according to the above composition, and after the raw materials are refined in an electric furnace, refined in a ladle and degassed in a vacuum, they are continuously cast into round billets with a diameter of 200mm. The round billets are then slowly cooled to room temperature.

[0111] 2) The above round billet is reheated to 1100℃, held for 60 minutes, and then hot rolled into a round steel with a diameter of 58mm at a final rolling temperature of 950℃.

[0112] 3) The hot-rolled round bars are pushed into the cooling bed in a stepping motion, with a spacing of 60mm between each round bar. They are then naturally air-cooled to below 200℃. The average cooling rate is 0.6℃ / second between 850℃ and 500℃ and 0.08℃ / second between 500℃ and 200℃.

[0113] 4) The above round steel is put into a walking beam tempering furnace for tempering at a tempering temperature of 280°C and a tempering time of 6 hours to obtain hot-rolled air-cooled bainitic steel.

[0114] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured by optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffractometer. The proportion of carbide-free bainite in its microstructure was 50% (volume percentage), the proportion of retained austenite was 4% (volume percentage) and the remainder was martensite.

[0115] The hot-rolled air-cooled bainitic steel prepared in this embodiment was tested according to the national standard GB10561-2023 "Standard Rating Chart Microscopic Detection Method for Determination of Non-metallic Inclusion Content in Steel". The inclusion levels met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, and Class Ds ≤ 0.5.

[0116] Example 8: Bainitic bolt 4

[0117] In this embodiment, bainitic bolts are prepared using hot-rolled air-cooled bainitic steel 4 from Example 7, and the process is as follows:

[0118] 1) Hot-rolled air-cooled bainitic steel is cut into 4 sections, straightened, machined, precision-machined, and thread-rolled to manufacture M56 bainitic bolts;

[0119] 2) The above-mentioned bainitic bolts are subjected to anti-corrosion surface treatment, including zinc diffusion, Dacromet coating and drying. The zinc diffusion temperature is 360℃ and the drying temperature is 360℃.

[0120] Testing revealed that the strength of the aforementioned bainitic bolts meets the strength requirements for grade 14.9 bolts, specifically with a tensile strength of 1445 MPa, a yield strength of 1328 MPa, an elongation of 12%, a reduction of area of ​​55%, and a room temperature impact toughness of A. KV Impact toughness A at 70J and -40℃ KV The J is 40, and the room temperature fatigue strength is 750 MPa.

[0121] The mechanical property testing process refers to GB / T 228.1-2021 Metallic materials, tensile testing - Part 1: Test method at room temperature, GB / T229-2020 Metallic materials, Charpy pendulum impact test method and GB / T4337-2008 Metallic materials, fatigue test method, rotational bending method.

[0122] Figure 3 This is a photograph of the bainitic bolt manufactured in this embodiment.

[0123] Example 9: Hot-rolled air-cooled bainitic steel 5

[0124] The chemical composition of the hot-rolled air-cooled bainitic steel in this embodiment, by mass percentage, includes: C 0.23%, Mn 2.4%, Si 1.2%, Cr 0.5%, Mo 0.2%, V 0.08%, Ti 0.01%, Al(t) 0.01%, N≤60ppm, P≤0.01%, S≤0.01%, [O]≤20ppm, [H]≤1.5ppm, with the remainder being Fe.

[0125] To manufacture, follow these steps:

[0126] 1) Prepare the ingredients according to the above composition, and after the raw materials are processed by converter steelmaking, ladle refining and vacuum degassing, they are molded into round billets with a diameter of 200mm, and the round billets are slowly cooled to room temperature;

[0127] 2) The above round billet is reheated to 1200℃, held for 60 minutes, and then hot rolled into a round steel with a diameter of 78mm at a final rolling temperature of 900℃.

[0128] 3) The hot-rolled round bars are pushed into the cooling bed in a stepping motion, with a spacing of 80mm between each round bar. They are then naturally air-cooled to below 200℃, with an average cooling rate of 0.5℃ / second between 850℃ and 500℃ and an average cooling rate of 0.05℃ / second between 500℃ and 200℃.

[0129] 4) The above round steel is put into a walking beam tempering furnace for tempering at a tempering temperature of 340℃ and a tempering time of 4 hours to obtain hot-rolled air-cooled bainitic steel.

[0130] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured by optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffractometer. The proportion of carbide-free bainite in its microstructure was 65% (volume percentage), the proportion of retained austenite was 7% (volume percentage) and the remainder was martensite.

[0131] The hot-rolled air-cooled bainitic steel prepared in this embodiment was tested according to the national standard GB10561-2023 "Standard Rating Chart Microscopic Detection Method for Determination of Non-metallic Inclusion Content in Steel". The inclusion levels met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, and Class Ds ≤ 0.5.

[0132] Example 10: Ultra-high strength bainitic bolt 5

[0133] In this embodiment, bainitic bolts are prepared using hot-rolled air-cooled bainitic steel 5 from Example 9. The process is as follows:

[0134] 1) Cut hot-rolled air-cooled bainitic steel into 5-point segments, straighten, machine, finish, and roll threads to manufacture M76 bainitic bolts;

[0135] 2) The above-mentioned bainitic bolts are subjected to anti-corrosion surface treatment, including zinc diffusion, Dacromet coating and drying. The zinc diffusion temperature is 360℃ and the drying temperature is 300℃.

[0136] Testing revealed that the strength of the aforementioned bainitic bolts meets the strength requirements for grade 12.9 bolts, specifically with a tensile strength of 1402 MPa, a yield strength of 1127 MPa, an elongation of 16%, a reduction of area of ​​65.2%, and a room temperature impact toughness of A. KV Impact toughness A at 109J and -40℃ KV The J is 48, and the room temperature fatigue strength is 820 MPa.

[0137] The mechanical property testing process refers to GB / T 228.1-2021 Metallic materials, tensile testing - Part 1: Test method at room temperature, GB / T229-2020 Metallic materials, Charpy pendulum impact test method and GB / T4337-2008 Metallic materials, fatigue test method, rotational bending method.

[0138] Comparative Example 1

[0139] The preparation process of the hot-rolled air-cooled bainitic steel and bainitic bolts in this comparative example is the same as that in Examples 1 and 2. The difference is that the contents of C, Mn, Cr and Mo in the chemical composition of the hot-rolled air-cooled bainitic steel in this comparative example are different from those in Example 1. Specifically, C: 0.16%, Mn: 2.6%, Cr: 1.0%, and Mo: 0.3-0.5%.

[0140] In the microstructure of the hot-rolled air-cooled bainitic steel in this comparative example, the proportion of carbide-free bainite is 40%, the proportion of retained austenite is 3%, and the remainder is martensite. The room temperature impact toughness value A of the bainitic bolts is... KV 45-50J, impact toughness value A at -40℃ KV23-25J has a tensile strength of 1510MPa, but its room temperature fatigue strength is only 600MPa; toughness and fatigue performance deteriorate.

[0141] Comparative Example 2

[0142] The preparation process of the hot-rolled air-cooled bainitic steel and bainitic bolts in this comparative example is the same as that in Examples 3 and 4. The difference is that step 3) in the preparation process of the hot-rolled air-cooled bainitic steel is different. Step 3 in this comparative example is: the hot-rolled round steel is pushed into the cooling bed in a stepping motion, with a spacing of 40 mm between each round steel. It is then cooled by spraying to below 200°C. The average cooling rate is 10°C / second between 850°C and 500°C, and 2°C / second between 500°C and 200°C.

[0143] The microstructure of the hot-rolled, air-cooled bainitic steel prepared in this embodiment contains 30% carbide-free bainite, 4% retained austenite, and the remainder martensite. The room temperature impact toughness value A of the manufactured bolts is... KV 35-40J, impact toughness value A at -40℃ KV 20-27J, with a tensile strength of 1610MPa, but a room temperature fatigue strength of only 650MPa; toughness and fatigue performance deteriorate.

[0144] Comparative Example 3

[0145] The preparation process of the hot-rolled air-cooled bainitic steel and bainitic bolts in this comparative example is the same as that in Examples 5 and 6. The difference is that the contents of Ti, Al(t) and N in the chemical composition of the hot-rolled air-cooled bainitic steel in this comparative example are different from those in Example 5. Specifically, Ti: 0.05%, Al(t): 0.05%, and N: 120ppm.

[0146] The hot-rolled air-cooled bainitic steel in this comparative example contains a large number of TiN and AlN inclusions, and the room temperature fatigue strength of the bolts manufactured from it is 600 MPa, indicating a deterioration in fatigue performance.

[0147] Comparative Example 4

[0148] The preparation process of the hot-rolled air-cooled bainitic steel and bainitic bolts in this comparative example is the same as that in Examples 7 and 8, except that step 2) in the preparation process of the bainitic bolts is different. In this comparative example, the zinc diffusion temperature is 400°C and the drying temperature is 400°C.

[0149] The bolt manufactured in this comparative example has a tensile strength of 1334 MPa, a yield strength of 1212 MPa, an elongation of 10%, a reduction of area of ​​45%, and a room temperature impact toughness A. KV Impact toughness A at 35J and -40℃ KVThe strength is 12J, the room temperature fatigue strength is 550MPa, and the strength, toughness and fatigue performance deteriorate.

[0150] Comparative Example 5

[0151] The preparation process of the hot-rolled air-cooled bainitic steel and bainitic bolts in this comparative example is the same as that in Examples 9 and 10. The difference is that step 3) in the preparation process of the hot-rolled air-cooled bainitic steel is different. Step 3 in this comparative example is: the hot-rolled round steel is pushed into the cooling bed in a stepping motion, with a spacing of 10 mm between each round steel, and naturally air-cooled to below 200°C. The average cooling rate between 850°C and 500°C is 0.2°C / second, and the average cooling rate between 500°C and 200°C is 0.01°C / second.

[0152] In this comparative example, the hot-rolled air-cooled bainitic steel contains 85% bainite, 12% retained austenite, and the remainder is martensite. The bolts manufactured from this steel exhibit the following properties: tensile strength of 1280 MPa, yield strength of 927 MPa, and room temperature impact toughness A... KV Impact toughness A at 60J and -40℃ KV The strength is 33J, the room temperature fatigue strength is 520MPa, and the strength, toughness and fatigue performance deteriorate.

[0153] As demonstrated by the examples and comparative examples, the hot-rolled air-cooled bainitic steel and the high-strength bainitic bolts made from this round steel, obtained using the present invention, exhibit improved strength, plasticity, impact toughness, and fatigue performance. Furthermore, this patent offers advantages over domestic and international high-strength bolts in terms of alloy cost, performance uniformity of large-diameter bolts, strength stability, and low-temperature toughness.

[0154] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A hot-rolled, air-cooled bainitic steel, characterized in that, The chemical composition, by mass percentage, includes: C 0.18%~0.28%, Mn 1.6%~2.4%, Si 0.8%~1.5%, Cr 0.2%~0.6%, Mo 0.1%~0.2%, V 0.02%~0.08%, Ti 0%~0.015%, Al(t) 0.01%~0.02%, with the remainder being Fe and residual trace impurities; The method for preparing the hot-rolled air-cooled bainitic steel includes the following steps: 1) Cast the raw materials of the above composition into round billets according to the specified proportions; 2) Hold the above round billet at 1100~1200℃ for 30~60 minutes; hot roll it into round steel with a final rolling temperature of 850~950℃; 3) Allow the steel to air cool naturally to below 200°C, then temper it to obtain the hot-rolled air-cooled bainitic steel; In step 3), the average cooling rate of the natural air cooling is: 0.5~2℃ / second between 850~500℃, and 0.05~0.5℃ / second between 500℃~200℃. The properties of the bainitic bolts made from the hot-rolled and air-cooled bainitic steel are as follows: tensile strength ≥1400MPa, yield strength ≥1100MPa, elongation ≥12%, reduction of area ≥55%, and room temperature impact toughness A. KV ≥70 J, impact toughness A at -40℃ KV ≥40 J, room temperature fatigue strength not less than half of tensile strength, room temperature fatigue strength ≥700 MPa.

2. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, The residual trace impurities include N, P, S, O and H; wherein the content of N is ≤60 ppm, the mass percentage of P is ≤0.01%, the mass percentage of S is ≤0.01%, the content of [O] is ≤20 ppm and the content of [H] is ≤1.5 ppm.

3. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, The diameter of the round billet in step 1) is not less than twice the diameter of the round steel in step 2).

4. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, The diameter of the round billet in step 1) is 2 to 5 times the diameter of the round steel in step 2).

5. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, The diameter of the round steel in step 2) is 25~80 mm.

6. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, The steps for casting round billets include: steelmaking in an electric furnace or converter, ladle refining and vacuum degassing, followed by ingot casting or continuous casting to form round billets.

7. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, In step 3), the natural air cooling is carried out on a cooling bed; in the natural air cooling, the distance between adjacent round steel bars on the cooling bed is 1 to 2 times the diameter of the round steel bars.

8. The hot-rolled air-cooled bainitic steel according to claim 1, characterized in that, In step 3), the tempering temperature is 280~340℃; the tempering time is 2~6 hours.

9. The use of the hot-rolled air-cooled bainitic steel according to any one of claims 1-8 in the preparation of bainitic bolts.

10. The application according to claim 9, characterized in that, The bainitic bolts are M24 to M76 type bolts.

11. The method for preparing bainitic bolts from hot-rolled air-cooled bainitic steel according to any one of claims 1-8, characterized in that, Includes the following steps: (1) Cut the above hot-rolled air-cooled bainitic steel into sections, straighten, machine, finish machine, and roll threads to manufacture bainitic bolts; (2) Perform anti-corrosion surface treatment on the above-mentioned bainitic bolts.

12. The method according to claim 11, characterized in that, In step (2), the anti-corrosion surface treatment steps include zinc infiltration, dipping in a sealant, and drying.

13. The method according to claim 12, characterized in that, The zinc diffusion temperature shall not exceed 380°C.

14. The method according to claim 13, characterized in that, The zinc diffusion temperature is 360~380℃.

15. The method according to claim 12, characterized in that, The drying temperature shall not exceed 360°C.

16. The method according to claim 15, characterized in that, The drying temperature is 280~360℃.

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