Nb-V microalloyed steel for heavy-duty gear and preparation method of Nb-V microalloyed steel

Abstract

The invention relates to Nb-V microalloyed steel for a heavy-duty gear and a preparation method of the Nb-V microalloyed steel, and belongs to the technical field of metal materials. The preparation method comprises the following steps: S1, smelting, forging, normalizing, tempering, size processing and annealing raw materials of the steel for the Nb-V microalloyed heavy-duty gear in sequence to obtain a blank; s2, the obtained blank is subjected to carburizing treatment, and the steel for the heavy-duty gear is obtained; s3, the obtained steel for the heavy-duty gear is subjected to high-temperature tempering treatment; s4, the obtained steel for the heavy-duty gear is subjected to carbonitriding treatment; and S5, the obtained steel for the heavy-duty gear is subjected to cold treatment and low-temperature tempering treatment, and the Nb-V microalloyed steel for the heavy-duty gear is obtained. According to the Nb-V microalloyed steel for the heavy-duty gear, the depth of an effective hardened layer ranges from 5.90 mm to 6.10 mm, the surface hardness ranges from 69 HRC to 71 HRC, the tensile strength of the surface layer ranges from 2265 MPa to 2316 MPa, the yield strength of the surface layer ranges from 1708 MPa to 1756 MPa, the percentage elongation after fracture ranges from 5.1% to 5.4%, and the impact energy ranges from 97 J to 105 J.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials, and in particular relates to a Nb-V micro-alloyed heavy-duty gear steel and a preparation method thereof. Background Art

[0002] In the machinery manufacturing industry, heavy-duty gears are key transmission components, and their performance directly affects the operating stability and service life of the entire equipment. With the continuous advancement of industrial technology, the performance requirements for heavy-duty gear steel are increasing, especially the demand for its surface hardness and load-bearing capacity has increased significantly. Traditional heat treatment methods, such as single carburizing or nitriding, although they can improve the surface properties of heavy-duty gear steel to a certain extent, are often difficult to meet the comprehensive requirements of large carburized layer, high strength and high toughness at the same time, especially under heavy load conditions, which are prone to failure.

[0003] Invention patent CN113430461A discloses a Nb, V microalloyed gear steel and its preparation method, which uses the effect of Nb and V carbonitride precipitation particles pinning grain boundaries to refine grains, and ensures that the carburized surface has a high carbon content and hardness through a double carburizing process at 930±20℃, achieving tensile strength ≥1465MPa, yield strength ≥1340MPa, elongation ≥14%, and impact energy ≥95J. However, the double carburizing process still faces the problem of insufficient surface hardness and strength in the application of Nb-V microalloyed heavy-duty gear steel. Based on this, it is particularly important to develop a new heat treatment process for Nb-V microalloyed heavy-duty gear steel to achieve a perfect combination of high hardness and high strength and toughness, and to improve surface hardness and load-bearing capacity. This is of great significance and broad application prospects for improving the overall performance of mechanical equipment and reducing maintenance costs. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a Nb-V micro-alloyed heavy-duty gear steel and a preparation method thereof, which combines carburizing treatment and carbonitriding treatment to increase the surface carbon concentration, increase the volume fraction and average size of carbides, and improve the stability of the surface martensite by introducing nitrogen atoms to form nitrogen-containing martensite, thereby significantly improving the surface hardness and toughness of the Nb-V micro-alloyed heavy-duty gear steel.

[0005] The first object of the present invention is to provide a method for preparing Nb-V microalloyed heavy-duty gear steel, comprising the following steps:

[0006] S1. Smelting, forging, normalizing, tempering, dimension processing and annealing are performed on the Nb-V microalloyed heavy-duty gear steel raw material in sequence to obtain a blank; the chemical composition and mass percentage of the Nb-V microalloyed heavy-duty gear steel raw material are as follows: C 0.15%-0.17%, Cr 1.52%-1.65%, Si 0.24%-0.32%, Mn 0.62%-0.73%, Ni1.46%-1.59%, Mo 0.26%-0.29%, Nb 0.051%-0.074%, V0.077%-0.126%, P 0-0.010%, S 0-0.008%, and the balance is other inevitable impurities and iron;

[0007] S2, carburizing the blank obtained in S1 to obtain steel for heavy-duty gears;

[0008] S3, subjecting the heavy-duty gear steel obtained in S2 to high-temperature tempering treatment;

[0009] S4, performing carbonitriding treatment on the heavy-duty gear steel obtained in S3;

[0010] S5. Perform cold treatment and low temperature tempering treatment on the heavy-duty gear steel obtained in S4 to obtain the Nb-V micro-alloyed heavy-duty gear steel.

[0011] In one embodiment of the present invention, in S1, the smelting temperature is 1535°C-1550°C; the smelting equipment is an electric arc furnace or a vacuum induction furnace.

[0012] In one embodiment of the present invention, in S1, the forging ratio of the forging is not less than 3.2.

[0013] In one embodiment of the present invention, in S1, the normalizing temperature is 985°C-1010°C, and the time is 2h-3h; the tempering temperature is 670°C-695°C, and the time is 2h-4.5h; the annealing temperature is 200°C-400°C, and the time is 3h-4h.

[0014] In one embodiment of the present invention, in S2, the carburizing treatment is performed at a temperature of 995°C-1025°C, a carbon potential of 1.3wt%-1.5wt%, and a time of 13h-16h.

[0015] In one embodiment of the present invention, in S3, the high temperature tempering treatment is performed at a temperature of 660°C-695°C, for a time of 2.5h-6h, and for 2 times.

[0016] In one embodiment of the present invention, in S4, the carbonitriding treatment temperature is 895°C-950°C, the carbon potential is 1.1wt%-1.2wt%, the ammonia flow rate is 5L / min-6L / min, and the time is 6h-8h.

[0017] In one embodiment of the present invention, in S5, the temperature of the cold treatment is -85°C to -60°C, and the time is 2h-3h.

[0018] In one embodiment of the present invention, in S5, the low temperature tempering treatment is performed at a temperature of 180°C-250°C and for a time of 3h-8h.

[0019] The second object of the present invention is to provide a Nb-V micro-alloyed heavy-duty gear steel prepared by the method described above.

[0020] The technical solution of the present invention has the following advantages over the prior art:

[0021] (1) The preparation method of the present invention combines carburizing treatment with carbonitriding treatment, effectively increases the surface carbon concentration and introduces nitrogen atoms, and carbon and nitrogen combine with Nb and V elements in Nb-V microalloyed heavy-duty gear steel to promote the precipitation of Nb and V carbonitrides and increase the volume fraction and average size of Nb and V carbonitrides. The surface hardness is improved by solid solution strengthening of carbon and nitrogen and precipitation strengthening of Nb and V carbonitrides, solving the problem of low surface carbon concentration and hardness in a single carburizing process.

[0022] (2) The preparation method described in the present invention improves the plasticity and toughness of the hardened layer by carefully designing the composition of the Nb-V microalloyed heavy-duty gear steel and retaining a proper amount of austenite in combination with the cold treatment temperature. To this end, the austenite content in the hardened layer is precisely controlled to 7%-10% to extend the service life.

[0023] (3) The preparation method described in the present invention directly oil-cools to room temperature after carbonitriding treatment, which can simplify the process flow, improve production efficiency, and obtain a higher strength and toughness combination.

[0024] (4) The Nb and V contents of the Nb-V microalloyed heavy-duty gear steel described in the present invention are controlled within a certain range to promote the precipitation of Nb and V carbonitrides. The high-density Nb and V carbonitride precipitated particles are used to pin the grain boundaries to inhibit the grain growth during carburizing and carbonitriding, effectively increase the carburizing temperature and carbonitriding temperature, promote the diffusion of carbon atoms and nitrogen atoms, and obtain a large penetration depth in a short time, solving the problem that the penetration layer hardness is high but the penetration layer is shallow in a single nitriding process.

[0025] (5) The effective hardened layer depth of the Nb-V microalloyed heavy-duty gear steel described in the present invention is 5.90 mm-6.10 mm, the surface hardness is 69 HRC-71 HRC, the surface tensile strength is 2265 MPa-2316 MPa, the surface yield strength is 1708 MPa-1756 MPa, the elongation after fracture is 5.1%-5.4%, and the impact energy is 97 J-105 J. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0027] Figure 1 This is an optical microscope image of the Nb-V microalloyed heavy-duty gear steel prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The present invention is further described below in conjunction with specific embodiments so that those skilled in the art can better understand the present invention and implement it. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0029] In the present invention, unless otherwise explained, technical and scientific terms used in the present invention have the same meanings as commonly understood by those skilled in the art to which the present invention belongs.

[0030] In the present invention, unless otherwise stated, the term "and / or" used in the present invention includes any and all combinations of one or more of the associated listed items.

[0031] In the present invention, unless otherwise stated, the experimental methods used in the embodiments of the present invention are conventional methods unless otherwise stated, and the materials, reagents, etc. used are all commercially available unless otherwise stated.

[0032] Example 1

[0033] The Nb-V microalloyed heavy-duty gear steel and the preparation method thereof of the present embodiment specifically include the following steps:

[0034] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.051%, V 0.077%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0035] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0036] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0037] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 895°C, the carbon potential is 1.2wt%, the ammonia flow rate is 6L / min, the time is 8h, and the gear steel is directly oil-cooled to room temperature after the carbonitriding treatment;

[0038] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0039] Example 2

[0040] The Nb-V microalloyed heavy-duty gear steel and the preparation method thereof of the present embodiment specifically include the following steps:

[0041] S1. The raw materials are prepared according to the following mass percentage composition: C 0.17%, Cr 1.52%, Si 0.32%, Mn 0.73%, Ni 1.59%, Mo 0.26%, Nb 0.074%, V 0.126%, P 0.009%, S 0.007%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by a vacuum induction furnace at a smelting temperature of 1550°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.5; the normalizing temperature is 1010°C for 2h; the tempering temperature is 695°C for 2h; the annealing temperature is 400°C for 3h;

[0042] S2, carburizing the gear steel blank at a temperature of 1025°C, a carbon potential of 1.3wt%, and a time of 13h, and cooling to room temperature after carburizing;

[0043] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 695°C and keeping the temperature for 2.5 hours for the first tempering, and then heating to 695°C and keeping the temperature for 2.5 hours for the second tempering;

[0044] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 950°C, the carbon potential is 1.1wt%, the ammonia flow rate is 5L / min, and the time is 6h. After the carbonitriding treatment, the gear steel is directly oil-cooled to room temperature;

[0045] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -85°C and the time is 2h; the low-temperature tempering temperature is 250°C and the time is 3h.

[0046] Example 3

[0047] The Nb-V microalloyed heavy-duty gear steel and the preparation method thereof of the present embodiment specifically include the following steps:

[0048] S1. The raw materials are prepared according to the following mass percentage composition: C 0.16%, Cr 1.54%, Si 0.29%, Mn 0.70%, Ni 1.53%, Mo 0.27%, Nb 0.061%, V 0.094%, P 0.008%, S 0.007%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1540°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.3; the normalizing temperature is 995°C for 2h; the tempering temperature is 680°C for 3h; the annealing temperature is 300°C for 3h;

[0049] S2, carburizing the gear steel blank at a temperature of 1010°C, a carbon potential of 1.4wt%, and a time of 15h, and cooling to room temperature after carburizing;

[0050] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 675°C and keeping the temperature for 4 hours for the first tempering, and then heating to 675°C and keeping the temperature for 4 hours for the second tempering;

[0051] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 920°C, the carbon potential is 1.1wt%, the ammonia flow rate is 5L / min, and the time is 7h. After the carbonitriding treatment, the gear steel is directly oil-cooled to room temperature;

[0052] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -75°C and the time is 2h; the low-temperature tempering temperature is 220°C and the time is 5h.

[0053] Example 4

[0054] The Nb-V microalloyed heavy-duty gear steel and the preparation method thereof of the present embodiment specifically include the following steps:

[0055] S1. The raw materials are prepared according to the following mass percentage composition: C 0.16%, Cr 1.56%, Si 0.27%, Mn 0.71%, Ni 1.49%, Mo 0.28%, Nb 0.066%, V 0.117%, P 0.009%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by a vacuum induction furnace at a smelting temperature of 1550°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 1000°C for 2h; the tempering temperature is 680°C for 3h; the annealing temperature is 300°C for 3h;

[0056] S2, carburizing the gear steel blank at a temperature of 1015°C, a carbon potential of 1.3wt%, and a time of 14h, and cooling to room temperature after the carburizing treatment;

[0057] S3, the gear steel is subjected to two high temperature tempering treatments, i.e., the first tempering is performed by heating to 675°C and keeping the temperature for 3.5 hours, and the second tempering is performed by heating to 675°C and keeping the temperature for 3.5 hours;

[0058] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 930°C, the carbon potential is 1.1wt%, the ammonia flow rate is 5L / min, and the time is 7h. After the carbonitriding treatment, the gear steel is directly oil-cooled to room temperature;

[0059] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -80°C and the time is 2h; the low-temperature tempering temperature is 220°C and the time is 5h.

[0060] Comparative Example 1

[0061] The method is basically the same as Example 1, except that the mass percentage of Nb is adjusted to 0.039%, and specifically comprises the following steps:

[0062] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.039%, V 0.077%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0063] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0064] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0065] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 895°C, the carbon potential is 1.2wt%, the ammonia flow rate is 6L / min, the time is 8h, and the gear steel is directly oil-cooled to room temperature after the carbonitriding treatment;

[0066] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0067] Comparative Example 2

[0068] The method is basically the same as Example 1, except that the mass percentage of V is adjusted to 0.056%, and specifically comprises the following steps:

[0069] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.051%, V 0.056%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0070] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0071] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0072] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 895°C, the carbon potential is 1.2wt%, the ammonia flow rate is 6L / min, the time is 8h, and the gear steel is directly oil-cooled to room temperature after the carbonitriding treatment;

[0073] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0074] Comparative Example 3

[0075] The process is basically the same as Example 1, except that the temperature of the cold treatment is adjusted to -50°C, and specifically comprises the following steps:

[0076] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.051%, V 0.077%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0077] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0078] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0079] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 895°C, the carbon potential is 1.2wt%, the ammonia flow rate is 6L / min, the time is 8h, and the gear steel is directly oil-cooled to room temperature after the carbonitriding treatment;

[0080] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -50°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0081] Comparative Example 4

[0082] The method is basically the same as Example 1, except that the temperature of carbonitriding is adjusted to 870° C., and specifically includes the following steps:

[0083] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.051%, V 0.077%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0084] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0085] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0086] S4, performing carbonitriding treatment on the gear steel, wherein the carbonitriding treatment temperature is 870°C, the carbon potential is 1.2wt%, the ammonia flow rate is 6L / min, and the time is 8h. After the carbonitriding treatment, the gear steel is directly oil-cooled to room temperature;

[0087] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0088] Comparative Example 5

[0089] The method is basically the same as Example 1, except that the carbonitriding treatment is replaced by nitriding treatment, which specifically includes the following steps:

[0090] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.051%, V 0.077%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0091] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0092] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0093] S4. Perform nitriding treatment on the gear steel. The nitriding temperature is 895°C, the ammonia flow rate is 6L / min, and the time is 8h. After the nitriding treatment, the gear steel is directly oil-cooled to room temperature.

[0094] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0095] Comparative Example 6

[0096] The method is basically the same as Example 1, except that no carbonitriding treatment is performed between the high-temperature tempering treatment and the cold treatment. The gear steel is only heated to 895° C. and kept for 8 hours, and then directly oil-cooled to room temperature. Specifically, the following steps are included:

[0097] S1. The raw materials are prepared according to the following mass percentage composition: C 0.15%, Cr 1.65%, Si 0.24%, Mn 0.62%, Ni 1.46%, Mo 0.29%, Nb 0.051%, V 0.077%, P 0.010%, S 0.008%, and the rest are other inevitable impurities and iron, and are smelted into steel ingots by an electric arc furnace at a smelting temperature of 1535°C; the steel ingots are sequentially forged, normalized, tempered, dimensionally processed and annealed to form gear steel billets; the forging ratio is 3.2; the normalizing temperature is 985°C for 3h; the tempering temperature is 670°C for 4.5h; the annealing temperature is 200°C for 4h;

[0098] S2, carburizing the gear steel blank at a temperature of 995°C, a carbon potential of 1.5wt%, and a time of 16h, and cooling to room temperature after the carburizing treatment;

[0099] S3, the gear steel is subjected to two high temperature tempering treatments, that is, heating to 660°C and keeping the temperature for 6 hours for the first tempering, and then heating to 660°C and keeping the temperature for 6 hours for the second tempering;

[0100] S4, heat the gear steel to 895℃ and keep it at that temperature for 8h, then directly oil cool it to room temperature;

[0101] S5. The gear steel is subjected to cold treatment and low-temperature tempering treatment in sequence to obtain Nb-V micro-alloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3h; the low-temperature tempering temperature is 180°C and the time is 8h.

[0102] Test Example 1

[0103] The Nb-V microalloyed heavy-duty gear steel of Example 1 was characterized. Figure 1 As shown. Figure 1 It can be seen that the surface structure is composed of martensite, retained austenite and Nb, V carbonitrides. The content of retained austenite is 7.2%, and the effective hardened layer depth is 5.90mm, which effectively improves the surface hardness and toughness.

[0104] Test Example 2

[0105] The retained austenite content, effective hardened layer depth, surface hardness, surface tensile properties (tensile strength, yield strength and elongation after fracture) and impact properties of the Nb-V microalloyed heavy-duty gear steels of Examples 1-4 and Comparative Examples 1-6 were tested:

[0106] Retained austenite content: Tested in accordance with the standard "YB / T 5338-2019 X-ray diffractometer method for quantitative determination of austenite in steel";

[0107] Effective hardened layer: Tested in accordance with the standard GB / T 9450-2005 Determination and verification of the depth of carburizing and quenching hardened layer of steel parts;

[0108] Surface hardness: Tested in accordance with GB / T 4340.1-2009 Metallic Materials Vickers Hardness Test Part 1: Test Methods;

[0109] Surface tensile properties: Tested in accordance with GB / T 228.1-2021 Metallic material tensile test Part 1: Room temperature test method;

[0110] Impact performance: Tested in accordance with GB / T 229-2020 Metal Materials Charpy Pendulum Impact Test Method.

[0111] Table 1 shows the relevant properties of the Nb-V microalloyed heavy-duty gear steel finally measured:

[0112] Table 1

[0113]

[0114] As can be seen from Table 1, the Nb-V microalloyed heavy-duty gear steel obtained in the embodiment has high surface hardness, surface tensile properties and impact properties. The surface hardness is 69HRC-71HRC, the surface tensile strength is 2265MPa-2316MPa, the surface yield strength is 1708MPa-1756MPa, the elongation after fracture is 5.1%-5.4%, and the impact energy is 97J-105J.

[0115] Comparing Example 1 with Comparative Example 1, it can be seen that the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy-duty gear steel in Comparative Example 1 are all lower than those in Example 1. This is because the mass percentage of Nb in Comparative Example 1 is lower than that in Example 1, the effect of the precipitation of Nb and V carbonitride particles in inhibiting grain growth is weakened, and the content of retained austenite is reduced, resulting in a simultaneous decrease in hardness, strength, plasticity and toughness.

[0116] Comparison of Example 1 and Comparative Example 2 shows that the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy-duty gear steel in Comparative Example 2 are lower than those in Example 1. This is because the mass percentage of V in Comparative Example 2 is lower than that in Example 1, the effect of Nb and V carbonitride precipitation particles in inhibiting grain growth is weakened, and the content of retained austenite is reduced, resulting in a simultaneous decrease in hardness, strength, plasticity and toughness.

[0117] Comparison of Example 1 and Comparative Example 3 shows that the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy-duty gear steel in Comparative Example 3 are lower than those in Example 1. This is because the cold treatment temperature in Comparative Example 1 is higher than that in Example 1, resulting in a retained austenite content higher than 10%, which is not conducive to the improvement of the work hardening ability caused by dislocation entanglement and proliferation under higher stress levels, and is not conducive to the improvement of toughness and plasticity while reducing hardness and strength.

[0118] Comparison of Example 1 and Comparative Example 4 shows that the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy-duty gear steel in Comparative Example 4 are lower than those in Example 1. This is because the temperature of carbonitriding treatment in Comparative Example 4 is lower than that in Example 1, and carbon and nitrogen atoms are not fully diffused, resulting in a decrease in the content of retained austenite and the depth of the effective hardened layer, which in turn leads to a simultaneous decrease in hardness, strength, plasticity and toughness.

[0119] Comparison of Example 1 and Comparative Example 5 shows that the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy-duty gear steel in Comparative Example 5 are lower than those in Example 1. This is because the carbonitriding treatment is replaced by nitriding treatment in Comparative Example 5, and the carbon atoms are not fully diffused, resulting in a decrease in the content of retained austenite and the effective hardened layer depth, which in turn leads to a simultaneous decrease in hardness, strength, plasticity and toughness.

[0120] Comparing Examples 1-4 and Comparative Example 6, it can be seen that the surface hardness of the Nb-V microalloyed heavy-duty gear steel obtained in the examples is increased by 13.1%-16.4%, the tensile strength of the surface layer is increased by 13.4%-16.0%, the yield strength of the surface layer is increased by 21.8%-25.2%, the elongation after fracture is increased by 41.7%-50.0%, and the impact energy is increased by 21.3-31.3%. This is because Examples 1-4 combine carburizing treatment with carbonitriding treatment, effectively increase the surface carbon concentration and introduce nitrogen atoms, and carbon and nitrogen combine with Nb and V elements in the Nb-V microalloyed heavy-duty gear steel to promote the precipitation of Nb and V carbonitrides, and use the solid solution strengthening of carbon and nitrogen and the precipitation strengthening of Nb and V carbonitrides to improve the surface hardness, and accurately control the austenite content within the range of 7%-10%, ensuring a higher strength and toughness combination.

[0121] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A method for preparing Nb-V microalloyed heavy-duty gear steel, characterized in that: The following steps are involved: S1. The Nb-V microalloyed heavy-duty gear steel raw material is sequentially smelted, forged, normalized, tempered, dimensionally processed and annealed to obtain a blank; the chemical composition and mass percentage of the Nb-V microalloyed heavy-duty gear steel raw material are: C 0.15%-0.17%, Cr 1.52%-1.65%, Si 0.24%-0.32%, Mn 0.62%-0.73%, Ni 1.46%-1.59%, Mo 0.26%-0.29%, Nb 0.051%-0.074%, V0.077%-0.126%, P 0-0.010%, S 0-0.008%, and the balance is other inevitable impurities and iron; S2, carburizing the blank obtained in S1 to obtain steel for heavy-duty gears; S3, subjecting the heavy-duty gear steel obtained in S2 to high-temperature tempering treatment; S4, performing carbonitriding treatment on the heavy-duty gear steel obtained in S3; S5. Perform cold treatment and low temperature tempering treatment on the heavy-duty gear steel obtained in S4 to obtain the Nb-V micro-alloyed heavy-duty gear steel.

2. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S1, the smelting temperature is 1535°C-1550°C.

3. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S1, the forging ratio of the forging is not less than 3.

2.

4. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S1, the normalizing temperature is 985°C-1010°C, and the time is 2h-3h; the tempering temperature is 670°C-695°C, and the time is 2h-4.5h; the annealing temperature is 200°C-400°C, and the time is 3h-4h.

5. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S2, the carburizing treatment is carried out at a temperature of 995°C-1025°C, a carbon potential of 1.3wt%-1.5wt%, and a time of 13h-16h.

6. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S3, the high temperature tempering treatment is performed at a temperature of 660°C-695°C, for a time of 2.5h-6h, and for 2 times.

7. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S4, the carbonitriding treatment is performed at a temperature of 895°C-950°C, a carbon potential of 1.1wt%-1.2wt%, an ammonia flow rate of 5L / min-6L / min, and a time of 6h-8h.

8. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S5, the temperature of the cold treatment is -85°C to -60°C, and the time is 2h-3h.

9. The method for preparing Nb-V microalloyed heavy-duty gear steel according to claim 1, characterized in that: In S5, the low temperature tempering treatment is performed at a temperature of 180°C-250°C and for a time of 3h-8h.

10. A Nb-V microalloyed heavy-duty gear steel prepared by the method according to any one of claims 1 to 9.

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