Hot-rolled air-cooled bainite steel as well as preparation method and application thereof

Abstract

The invention discloses hot-rolled air-cooled bainite steel as well as a preparation method and application thereof. The hot-rolled air-cooled bainite steel comprises the following chemical components in percentage by mass: 0.18%-0.28% of C, 1.6%-2.4% of Mn, 0.8%-1.5% of Si, 0.2%-0.6% of Cr, 0.1%-0.2% of Mo, 0.02%-0.08% of V, 0-0.015% of Ti, 0.01%-0.02% of Al (t) and the balance of Fe and residual trace impurities. The preparation process of the hot-rolled air-cooled bainite steel is simple, carbide-free bainite, retained austenite and martensite are formed through natural air cooling and low-temperature tempering of the hot-rolled round steel, the processes of quenching, quenching, medium-high temperature tempering, salt bath treatment and the like are avoided, the heat treatment cost can be saved, and emission of harmful gas is reduced. In addition, the hot-rolled air-cooled bainite steel is excellent in mechanical property and can be used for manufacturing large-size bolts.

Description

Technical Field

[0001] The invention relates to the technical field of steel materials, and in particular to a hot-rolled air-cooled bainite circle and a preparation method and application thereof. Background Art

[0002] In recent years, with the development of economy and society, the demand for ultra-high strength and toughness bolts in the fields of rail transit, wind power, bridges, etc. has increased, especially for ultra-large bolt steel. At present, bolts of grade 10.9 or above are mainly quenched and tempered, that is, medium carbon steel is used, which is obtained by quenching + medium and high temperature tempering. For example, grade 10.9 bolts are generally made of 42CrMo steel through 880℃ quenching + 550℃ tempering to form tempered troostite, while 42CrMo steel is difficult to meet the manufacturing requirements of grade 12.9 or above bolts. In addition, the hardenability of 42CrMo steel is poor, and its maximum hardening diameter is only 42mm, which is not suitable for manufacturing larger bolts. In this context, the development of bolt steel suitable for higher strength levels, larger specifications and better performance has become a hot topic at home and abroad.

[0003] The patent "A high-homogeneity and high-hardenability steel, bar and manufacturing method for wind power bolts and its application" (202111121073.2) applied by Baoshan Iron & Steel Co., Ltd. discloses a high-homogeneity and hardenability steel for wind power bolts within 90mm. It adopts a medium-carbon and high-Cr composition design, supplemented by precious elements such as Mo and Ni, but the patent requires "water quenching + medium-high temperature tempering at 520-640℃". The heat treatment process used in the "Steel for high-strength bolts for wind power and high-strength bolts for wind power" (202211653246.X) applied by Aipu Parts Manufacturing (Suzhou) Co., Ltd. is: "S5. Low-temperature aging treatment of the bolt blank of S4: heating temperature 200-300℃, keeping warm for 6-8 hours, removing the bolt processing stress and reducing heat treatment deformation; S6. Austenitizing treatment of the aged bolts: sending them to the salt bath quenching equipment, heating the bolts to 920℃~940℃ and keeping warm for 2-3 hours for austenitizing treatment, heating In the process, carbon monoxide, hydrogen and nitrogen are introduced to protect the bolts from oxidation and decarburization during the heating process; S7, the bolts after S6 are transferred to the quenching salt tank for isothermal quenching, and the isothermal quenching is a two-stage quenching method to make its bainite content greater than 99.8%; S8, the quenched bolts are air-cooled to room temperature and sent to an ultrasonic cleaning machine for cleaning and air drying; S9, the cleaned bolts are hard chrome-plated to obtain high-strength bolts with higher surface hardness, good wear resistance and corrosion resistance. The overall heat treatment process of this patent is too complicated, requires salt bath medium-temperature quenching, and is environmentally poor. The production method disclosed in the patent "A high-strength and toughness non-quenched and tempered steel for wind power bolts and its production method" (202211055609.X) applied by Maanshan Iron and Steel Co., Ltd. includes "After spinning, the Stelmor cooling line uses strong wind to quickly cool to 450-500°C at a cooling rate of ≥30°C / s, and then adds an insulation cover to naturally cool to 400-440°C, roll it up, hook it, and then air cool it to room temperature." This patent requires strong wind to quickly cool at a cooling rate of ≥30°C / s, and is only suitable for wires with specifications of 6.5-25mm. The patent "High-strength and tough large-diameter wind power bolts with benzene / martz composite structure and preparation method" (202011357524.8) applied by Boyao Energy Technology Co., Ltd. discloses a preparation method including "heating at 910-950°C, quenching to 180°C-240°C; then partitioning, the partitioning temperature is 280°C-340°C". The patent requires "quenching and partitioning", and the heat treatment process is complicated. At the same time, the patent does not mention the most important fatigue resistance of the bolts.

[0004] In summary, it can be seen that the traditional 42CrMo bolt steel can no longer meet the manufacturing and performance requirements of bolts with higher strength levels (such as 12.9) and larger specifications. The bolt steel and its manufacturing method disclosed in the existing patents are complex in process, using complex heat treatment methods such as quenching and tempering, or salt bath medium temperature quenching, strong wind rapid cooling, etc., which is not conducive to energy conservation and emission reduction. At the same time, these complex processes are also difficult to operate for large-sized bolts. Summary of the invention

[0005] In view of the above technical problems, the present invention provides a hot-rolled air-cooled bainitic steel and a preparation method and application thereof. The bainitic steel has a simple heat treatment process and excellent mechanical properties, and can be used to manufacture bolts of large sizes.

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

[0007] On the one hand, the present invention provides a hot-rolled air-cooled bainitic steel, whose chemical composition includes, by mass percentage, C0.18%-0.28%, Mn 1.6%-2.4%, Si 0.8%-1.5%, Cr 0.2%-0.6%, Mo0.1%-0.2%, V 0.02%-0.08%, Ti 0-0.015%, Al(t) 0.01%-0.02%, and the rest is Fe and residual trace impurities.

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

[0009] In certain 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 certain 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 0.015%.

[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 total hydrogen.

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

[0017] (1) Low carbon 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, it will cause component segregation and easily form brittle martensite structure.

[0019] (3) The proportion of bainite formed in round steel with different diameters is regulated by combining Cr and Mo. If the content of both is too low, ferrite or pearlite will be formed in the core of large-sized 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 of V and Ti can improve the hardenability of steel. 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 also 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 composition is too high, coarse AlN and TiN will be formed.

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

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

[0024] 1) Casting the raw materials of the above composition into round billets according to proportion;

[0025] 2) keeping the round billet at 1100-1200° C. for 30-60 minutes; hot rolling the round billet into round steel with a final rolling temperature of 850-950° C.;

[0026] 3) Natural air cooling to below 200° C. and tempering to obtain the hot-rolled air-cooled bainite steel.

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

[0028] As a preferred embodiment, the diameter of the round billet in step 1) is not less than 2 times 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 to 80 mm;

[0030] Preferably, the step of casting into a round billet comprises: steelmaking in an electric furnace or a converter, refining outside the furnace and vacuum degassing, and then casting into a round billet by die casting or continuous casting;

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

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

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

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

[0035] In the technical solution of the present invention, by controlling the distance between adjacent round steels during natural air cooling and the cooling rate of natural air cooling, it is possible to ensure that a bainite structure is formed during natural air cooling.

[0036] As a preferred embodiment, in step 3), the tempering temperature is 280-340°C; the tempering time is 2-6 hours;

[0037] In some specific embodiments, the tempering is performed in a walking furnace or a trolley furnace.

[0038] In the technical solution of the present invention, the microstructure can be stabilized and the residual stress can be eliminated by tempering.

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

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

[0041] In another aspect, the present invention provides use of the hot-rolled air-cooled bainitic steel in preparing bainitic bolts.

[0042] Preferably, the bainite bolts are M24-M76 type bolts.

[0043] In another aspect, the present invention provides a method for preparing a bainite bolt by using the hot-rolled air-cooled bainite steel, comprising the following steps:

[0044] (1) cutting the hot-rolled air-cooled bainite steel into segments, straightening, machining, finishing, and thread rolling to produce bainite bolts;

[0045] (2) The above-mentioned bainite bolts are subjected to anti-corrosion surface treatment.

[0046] As a preferred embodiment, in step (2), the anti-corrosion surface treatment step includes zincizing, dipping sealant and drying;

[0047] Preferably, the temperature of the sherardizing is not higher than 380°C, preferably 360-380°C;

[0048] Preferably, the drying temperature is not higher than 360°C, 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 bainite bolt is improved while the bainite structure in the bainite bolt is not destroyed.

[0050] In the technical solution of the present invention, the bainite bolt is measured according to "GB / T 228.1-2021 Metal Material Tensile Test Part 1: Room Temperature Test Method", "GB / T229-2020 Metal Material Charpy Pendulum Impact Test Method" and "GB / T4337-2008 Metal Material Fatigue Test Rotational Bending Method" to obtain the following performance: tensile strength ≥1400MPa, yield strength ≥1100MPa, reaching or exceeding the strength requirements of 12.9 to 14.9 grade bolts; at the same time, elongation ≥12%, section shrinkage ≥55%; room temperature impact toughness A KV ≥70J, -40℃ impact toughness A KV ≥40J, room temperature fatigue strength is not less than half of the tensile strength, that is, room temperature fatigue strength ≥700MPa.

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

[0052] (1) The preparation process of the hot-rolled air-cooled bainitic steel provided by the present invention is simple. The hot-rolled round steel is naturally air-cooled + low-temperature tempered to form carbide-free bainite, residual austenite and martensite, thereby avoiding processes such as quenching, quenching + medium-high temperature tempering, and salt bath treatment, thereby saving heat treatment costs and reducing 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 a multi-element compound and limits the content of harmful elements. It adopts a low-carbon Mn-Si system component design, is matched with a small amount of Cr and Mo, adopts a composite microalloying of V and Ti, and utilizes Al to consume N, thereby improving hardenability and reducing the content of Cr and Mo.

[0054] (3) The hot-rolled air-cooled bainite steel provided by the present invention has better comprehensive performance. Compared with traditional bolt steel, its low-temperature toughness and fatigue resistance are significantly improved, and its -40℃ impact toughness A KV ≥40J, room temperature fatigue strength ≥700MPa, which can improve the service life and reliability of the bolts. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 3 is a microstructure diagram of the hot-rolled air-cooled bainite steel obtained in Example 1 of the present invention; wherein the left diagram is a scanning electron microscope photograph, and the right diagram is a transmission electron microscope photograph.

[0056] Figure 2 3 is a microstructure diagram of the hot-rolled air-cooled bainite steel obtained in Example 5 of the present invention; wherein the left diagram is a scanning electron microscope photograph, and the right diagram is a transmission electron microscope photograph.

[0057] Figure 3 This is a photo of a bainite bolt manufactured in Example 8 of the present invention.

[0058] Figure 4 It is a manufacturing process flow chart of the hot-rolled air-cooled bainitic steel in the present invention. DETAILED DESCRIPTION

[0059] The following embodiments are only some embodiments of the present invention, rather than all embodiments. Therefore, the detailed description in the embodiments of the present invention provided below is not intended to limit the scope of the present invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the protection scope of the present invention.

[0060] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments are all 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 bainite steel in this embodiment, measured 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, and the rest is Fe.

[0064] Preparation process Figure 4 As shown, the following steps are included:

[0065] 1) The raw materials are prepared according to the above composition, and after steelmaking in an electric furnace, refining outside the furnace and vacuum degassing, they are molded into round billets with a diameter of 100 mm, and the round billets are slowly cooled to room temperature;

[0066] 2) heating the round billet to 1150° C., keeping the temperature for 30 minutes, and then hot rolling it into round steel with a diameter of 25 mm, with the final rolling temperature being 850° C.;

[0067] 3) Push the hot-rolled round steel bars into the cooling bed step by step, with the spacing between each round steel bar being 25 mm, and naturally air-cooling to below 200°C, wherein the average cooling rate between 850°C and 500°C is 2°C / second, and the average cooling rate between 500°C and 200°C is 0.5°C / second;

[0068] 4) The round steel is placed in a trolley-type tempering furnace for tempering at a temperature of 300° C. for 6 hours to obtain hot-rolled air-cooled bainitic steel.

[0069] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured using an optical microscope, a scanning electron microscope, a transmission electron microscope and an 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 rest was martensite. Figure 1 The microstructure diagram of the hot-rolled air-cooled bainite steel in this embodiment is shown; in the figure, CFB is carbide-free bainite, M is martensite, and the upper right corner of the right figure is 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", and the inclusion level met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, Class Ds ≤ 0.5.

[0071] Example 2: Bainite bolt 1

[0072] In this embodiment, the hot-rolled and air-cooled bainite steel 1 in embodiment 1 is used to prepare bainite bolts, and the process is as follows:

[0073] 1) Cutting the hot-rolled air-cooled bainitic steel 1 into segments, straightening, machining, finishing, and thread rolling to produce M24 bainitic bolts;

[0074] 2) The bainite bolts are subjected to anti-corrosion surface treatment, including sherardizing, Dacromet dip coating and drying, the sherardizing temperature is 380°C, and the drying temperature is 360°C.

[0075] After testing, the strength of the above bainite bolts meets the strength requirements of 12.9 grade bolts, with a specific tensile strength of 1418MPa, a yield strength of 1117MPa, an elongation of 15%, a cross-sectional shrinkage of 60%, and a room temperature impact toughness of A. KV 92J, -40℃ impact toughness A KV The fatigue strength at room temperature is 45J and 750MPa.

[0076] The mechanical properties testing process refers to "GB / T 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method", "GB / T229-2020 Metal Materials Charpy Pendulum Impact Test Method" and "GB / T4337-2008 Metal Materials Fatigue Test 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 includes, by mass percentage, C0.22%, Mn2.2%, 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, and the rest is Fe.

[0079] Follow these steps to manufacture:

[0080] 1) The raw materials are mixed according to the above composition, and after converter steelmaking, refining outside the furnace and vacuum degassing, they are continuously cast into round billets with a diameter of 150 mm, and the round billets are slowly cooled to room temperature;

[0081] 2) heating the round billet to 1200° C., keeping the temperature for 45 minutes, and then hot rolling it into round steel with a diameter of 38 mm, with the final rolling temperature being 900° C.;

[0082] 3) Push the hot-rolled round steel bars into the cooling bed step by step, with the spacing between each round steel bar being 40 mm, and naturally air-cooling to below 200°C, wherein the average cooling rate between 850°C and 500°C is 1.5°C / second, and the average cooling rate between 500°C and 200°C is 0.25°C / second;

[0083] 4) The round steel is placed in a trolley-type tempering furnace for tempering at a temperature of 320° C. for 4 hours to obtain hot-rolled air-cooled bainitic steel.

[0084] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured using an optical microscope, a scanning electron microscope, a transmission electron microscope and an 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 rest 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", and the inclusion level met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, Class Ds ≤ 0.5.

[0086] Example 4: Bainite bolt 2

[0087] In this embodiment, the hot-rolled air-cooled bainite steel 2 in embodiment 3 is used to prepare bainite bolts, and the process is as follows:

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

[0089] 2) The bainite bolts are subjected to anti-corrosion surface treatment, including sherardizing, Dacromet dip coating and drying, the sherardizing temperature is 380°C, and the drying temperature is 320°C.

[0090] After testing, the strength of the above bainite bolts meets the strength requirements of 12.9 grade bolts, with a specific tensile strength of 1448MPa, a yield strength of 1185MPa, an elongation of 16%, a cross-sectional shrinkage of 62%, and a room temperature impact toughness of A. KV 98J, -40℃ impact toughness A KV It is 48J and the room temperature fatigue strength is 760MPa.

[0091] The mechanical properties testing process refers to "GB / T 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method", "GB / T229-2020 Metal Materials Charpy Pendulum Impact Test Method" and "GB / T4337-2008 Metal Materials Fatigue Test 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 includes, by mass percentage, 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, and the rest is Fe.

[0094] Follow these steps to manufacture:

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

[0096] 2) reheating the round billet to 1200° C., keeping the temperature for 45 minutes, and then hot rolling it into a round steel with a diameter of 48 mm, with a final rolling temperature of 900° C.;

[0097] 3) Push the hot-rolled round steel bars into the cooling bed step by step, with the spacing between each round steel bar being 50 mm, and naturally air-cooling to below 200°C, wherein the average cooling rate between 850°C and 500°C is 1.0°C / second, and the average cooling rate between 500°C and 200°C is 0.15°C / second;

[0098] 4) The round steel is placed in a trolley-type tempering furnace for tempering at a temperature of 320° C. for 4 hours to obtain hot-rolled air-cooled bainitic steel.

[0099] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured using an optical microscope, a scanning electron microscope, a transmission electron microscope and an 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 rest was martensite. Figure 2 Shown is the microstructure of the hot-rolled air-cooled bainite 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", and the inclusion level met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, Class Ds ≤ 0.5.

[0101] Example 6: Bainite bolt 3

[0102] In this embodiment, the hot-rolled air-cooled bainite steel 3 in embodiment 5 is used to prepare bainite bolts, and the process is as follows:

[0103] 1) Cut the hot-rolled air-cooled bainitic steel into three sections, straighten, machine, finish and thread roll to produce M45 bainitic bolts;

[0104] 2) The bainite bolts are subjected to anti-corrosion surface treatment, including sherardizing, Dacromet dip coating and drying, the sherardizing temperature is 360°C, and the drying temperature is 280°C.

[0105] After testing, the strength of the above bainite bolts meets the strength requirements of 12.9 grade bolts, with a specific tensile strength of 1445MPa, a yield strength of 1128MPa, an elongation of 15%, a cross-sectional shrinkage of 63.8%, and a room temperature impact toughness of A. KV 123J, -40℃ impact toughness A KV The fatigue strength at room temperature is 52J and 800MPa.

[0106] The mechanical properties testing process refers to "GB / T 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method", "GB / T229-2020 Metal Materials Charpy Pendulum Impact Test Method" and "GB / T4337-2008 Metal Materials Fatigue Test 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, measured 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, and the rest is Fe.

[0109] Follow these steps to manufacture:

[0110] 1) The raw materials are mixed according to the above composition, and after steelmaking in an electric furnace, refining outside the furnace and vacuum degassing, they are continuously cast into round billets with a diameter of 200 mm, and the round billets are slowly cooled to room temperature;

[0111] 2) reheating the round billet to 1100° C., keeping the temperature for 60 minutes, and then hot rolling it into a round steel with a diameter of 58 mm, with a final rolling temperature of 950° C.;

[0112] 3) Push the hot-rolled round steel bars into the cooling bed step by step, with the spacing between each round steel bar being 60 mm, and naturally air-cooling to below 200°C, wherein the average cooling rate between 850°C and 500°C is 0.6°C / second, and the average cooling rate between 500°C and 200°C is 0.08°C / second;

[0113] 4) The round steel is placed in a step-beam tempering furnace for tempering at a temperature of 280° C. for 6 hours to obtain hot-rolled air-cooled bainitic steel.

[0114] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured using an optical microscope, a scanning electron microscope, a transmission electron microscope and an 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 rest 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", and the inclusion level met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, Class Ds ≤ 0.5.

[0116] Example 8: Bainite bolt 4

[0117] In this embodiment, the hot-rolled air-cooled bainite steel 4 in embodiment 7 is used to prepare bainite bolts, and the process is as follows:

[0118] 1) Cut the hot-rolled air-cooled bainitic steel into 4 sections, straighten, machine, finish and thread roll to produce M56 bainitic bolts;

[0119] 2) The bainite bolts are subjected to anti-corrosion surface treatment, including sherardizing, Dacromet dip coating and drying, the sherardizing temperature is 360°C, and the drying temperature is 360°C.

[0120] After testing, the strength of the above bainite bolts meets the strength requirements of 14.9 grade bolts, with a specific tensile strength of 1445MPa, a yield strength of 1328MPa, an elongation of 12%, a cross-sectional shrinkage of 55%, and a room temperature impact toughness of A. KV 70J, -40℃ impact toughness A KV The fatigue strength at room temperature is 40J and 750MPa.

[0121] The mechanical properties testing process refers to "GB / T 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method", "GB / T229-2020 Metal Materials Charpy Pendulum Impact Test Method" and "GB / T4337-2008 Metal Materials Fatigue Test Rotational Bending Method".

[0122] Figure 3 This is a photo of a bainite 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 includes, by mass percentage, C0.23%, Mn2.4%, 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, and the rest is Fe.

[0125] Follow these steps to manufacture:

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

[0127] 2) reheating the round billet to 1200° C., keeping the temperature for 60 minutes, and then hot rolling it into a round steel with a diameter of 78 mm, with a final rolling temperature of 900° C.;

[0128] 3) Push the hot-rolled round steel bars into the cooling bed step by step, with the spacing between each round steel bar being 80 mm, and naturally air-cooling to below 200°C, wherein the average cooling rate between 850°C and 500°C is 0.5°C / second, and the average cooling rate between 500°C and 200°C is 0.05°C / second;

[0129] 4) The round steel is placed in a step-beam tempering furnace for tempering at a temperature of 340° C. for 4 hours to obtain hot-rolled air-cooled bainitic steel.

[0130] The hot-rolled air-cooled bainitic steel prepared in this embodiment was measured using an optical microscope, a scanning electron microscope, a transmission electron microscope and an 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 rest 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", and the inclusion level met the following requirements: Class A ≤ 1.5, Class B ≤ 0.5, Class C ≤ 1.0, Class D ≤ 1.0, Class Ds ≤ 0.5.

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

[0133] In this embodiment, the hot-rolled air-cooled bainite steel 5 in embodiment 9 is used to prepare bainite bolts, and the process is as follows:

[0134] 1) Cut the hot-rolled air-cooled bainitic steel into sections, straighten, machine, finish and thread roll to produce M76 bainitic bolts;

[0135] 2) The bainite bolts are subjected to anti-corrosion surface treatment, including sherardizing, Dacromet dip coating and drying, the sherardizing temperature is 360°C, and the drying temperature is 300°C.

[0136] After testing, the strength of the above bainite bolts meets the strength requirements of 12.9 grade bolts, with a specific tensile strength of 1402MPa, a yield strength of 1127MPa, an elongation of 16%, a cross-sectional shrinkage of 65.2%, and a room temperature impact toughness of A. KV 109J, -40℃ impact toughness A KV It is 48J and the room temperature fatigue strength is 820MPa.

[0137] The mechanical properties testing process refers to "GB / T 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method", "GB / T229-2020 Metal Materials Charpy Pendulum Impact Test Method" and "GB / T4337-2008 Metal Materials Fatigue Test Rotational Bending Method".

[0138] Comparative Example 1

[0139] The preparation process of the hot-rolled air-cooled bainitic steel and the bainitic bolt in this comparative example is the same as that in Example 1 and Example 2, except 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 bainite steel in this comparative example, the proportion of carbide-free bainite is 40%, the proportion of retained austenite is 3%, and the rest is martensite. KV 45-50J, -40℃ impact toughness value A KV23-25J, the tensile strength is 1510MPa, but the room temperature fatigue strength is only 600MPa; the 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 Example 3 and Example 4, except 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 steps, and the spacing between each round steel is 40 mm, and spray-cooled to below 200°C, wherein the average cooling rate between 850°C and 500°C is 10°C / second, and the average cooling rate between 500°C and 200°C is 2°C / second.

[0143] The microstructure of the hot-rolled air-cooled bainite steel prepared in this embodiment contains 30% carbide-free bainite, 4% retained austenite, and the rest is martensite. KV 35-40J, -40℃ impact toughness value A KV 20-27J, the tensile strength is 1610MPa, but the room temperature fatigue strength is only 650MPa; the 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 Example 5 and Example 6, except 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: 120 ppm.

[0146] The hot-rolled air-cooled bainite steel in this comparative example contained a large amount of TiN and AlN inclusions, and the room temperature fatigue strength of the manufactured bolts was 600 MPa, indicating deteriorated fatigue properties.

[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 Example 7 and Example 8, except that step 2) in the preparation process of the bainitic bolts is different. In this comparative example, the temperature of the sherardizing is 400°C and the drying temperature is 400°C.

[0149] The bolts manufactured in this comparative example have a tensile strength of 1334 MPa, a yield strength of 1212 MPa, an elongation of 10%, a cross-sectional shrinkage of 45%, and a room temperature impact toughness of A KV 35J, -40℃ impact toughness A KVIt is 12J, the room temperature fatigue strength is 550MPa, and the strength, toughness and fatigue performance are deteriorated.

[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 of Example 9 and Example 10, except 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 steps, with a spacing of 10 mm between each round steel, and naturally air-cooled to below 200°C, wherein 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 proportion of bainite in the hot-rolled air-cooled bainite steel is 85%, the proportion of retained austenite is 12%, and the rest is martensite. The performance of the manufactured bolts is tensile strength of 1280MPa, yield strength of 927MPa, and room temperature impact toughness A KV 60J, -40℃ impact toughness A KV It is 33J, the room temperature fatigue strength is 520MPa, and the strength, toughness and fatigue properties are deteriorated.

[0153] It can be seen from the examples and comparative examples that the hot-rolled air-cooled bainite steel obtained by the present invention and the high-strength bainite bolts made of the round steel have improved strength, plasticity, impact toughness and fatigue properties. In addition, the patent has advantages over domestic and foreign high-strength bolts in terms of alloy cost, performance uniformity of large-size bolts, strength stability and low-temperature toughness.

[0154] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A hot-rolled air-cooled bainitic steel, characterized in that: The chemical composition includes, by mass percentage, C 0.18% to 0.28%, Mn 1.6% to 2.4%, Si 0.8% to 1.5%, Cr 0.2% to 0.6%, Mo 0.1% to 0.2%, V 0.02% to 0.08%, Ti 0% to 0.015%, Al(t) 0.01% to 0.02%, and the rest is Fe and residual trace impurities.

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 ≤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.

3. The method for preparing hot-rolled air-cooled bainitic steel according to any one of claims 1-2, characterized in that: The following steps are involved: 1) Casting the raw materials of the composition into round billets according to proportion; 2) The round billet is kept at 1100-1200° C. for 30-60 minutes; hot-rolled into round steel, with the final rolling temperature being 850-950° C.; 3) Natural air cooling to below 200° C. and tempering to obtain the hot-rolled air-cooled bainite steel.

4. The preparation method according to claim 3, characterized in that: The diameter of the round billet in step 1) is not less than 2 times, preferably 2 to 5 times, the diameter of the round steel in step 2); Preferably, the diameter of the round steel in step 2) is 25 to 80 mm; Preferably, the step of casting into a round billet comprises: steelmaking in an electric furnace or a converter, refining outside the furnace and vacuum degassing, and then casting into a round billet by die casting or continuous casting.

5. The preparation method according to claim 3, characterized in that: In step 3), the natural air cooling is performed on a cooling bed; during the natural air cooling, the distance between adjacent round steels on the cooling bed is 1 to 2 times the diameter of the round steels; Preferably, in step 3), the average cooling rate of the natural air cooling is: the average cooling rate between 850 and 500°C is 0.5 to 2°C / second, and the average cooling rate between 500°C and 200°C is 0.05 to 0.5°C / second.

6. The preparation method according to claim 3, characterized in that: In step 3), the tempering temperature is 280-340° C.; the tempering time is 2-6 hours.

7. Use of the hot-rolled air-cooled bainitic steel according to any one of claims 1 to 2 in the preparation of bainitic bolts.

8. The use according to claim 7, characterized in that: The bainite bolts are M24-M76 type bolts.

9. The method for preparing bainite bolts by hot-rolled air-cooled bainite steel according to any one of claims 1-2, characterized in that: The following steps are involved: (1) cutting the hot-rolled air-cooled bainite steel into segments, straightening, machining, finishing, and thread rolling to produce bainite bolts; (2) The above-mentioned bainite bolts are subjected to anti-corrosion surface treatment.

10. The method according to claim 9, characterized in that In step (2), the anti-corrosion surface treatment steps include zinc infiltration, dip-coating of sealant and drying; Preferably, the temperature of the sherardizing is not higher than 380°C, preferably 360-380°C; Preferably, the drying temperature is not higher than 360°C, preferably 280-360°C.

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