Nb-v microalloyed heavy load gear steel and preparation method thereof

Abstract

The present application relates to a kind of Nb-V microalloyed heavy load gear steel and its preparation method, belong to metal material technical field.The preparation method of the present application includes the following steps: S1, Nb-V microalloyed heavy load gear steel raw material is sequentially melted, forged, normalized, tempered, size processing and annealing, obtain blank;S2, the blank obtained is carburized, obtains heavy load gear steel;S3, the heavy load gear steel obtained is high-temperature tempering treatment;S4, the heavy load gear steel obtained is carbonitriding treatment;S5, the heavy load gear steel obtained is cold treatment and low-temperature tempering treatment, obtains Nb-V microalloyed heavy load gear steel.The depth of the effective hardening layer of the Nb-V microalloyed heavy load gear steel is 5.90mm-6.10mm, surface hardness is 69HRC-71HRC, surface layer tensile strength is 2265MPa-2316MPa, surface layer yield strength is 1708MPa-1756MPa, elongation after break is 5.1%-5.4%, impact energy is 97J-105J.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal materials, and particularly relates to a Nb-V micro-alloyed heavy-load gear steel and a preparation method thereof. BACKGROUND

[0002] In the mechanical manufacturing industry, heavy-load gears as key transmission components directly affect the running stability and service life of the entire equipment. With the continuous progress of industrial technology, the performance requirements of heavy-load gear steels are increasingly improved, especially the demand for surface hardness and load-carrying capacity is significantly increased. Although traditional heat treatment methods, such as single carburizing or nitriding, can improve the surface performance of heavy-load gear steels to a certain extent, they often fail to meet the comprehensive requirements of large penetration layer, high strength and high toughness, especially under heavy-load working conditions, failure problems are prone to occur.

[0003] Invention patent CN113430461A discloses a Nb-V micro-alloyed gear steel and a preparation method thereof. The Nb and V carbonitride precipitated particles are used to pin the grain boundaries to refine the grains. Through the process of two times of carburizing at 930±20℃, the carburized surface layer has a high carbon content and hardness, realizing the tensile strength ≥1465MPa, yield strength ≥1340MPa, elongation ≥14%, and impact energy ≥95J. However, the two times of carburizing process still faces the problem of insufficient surface hardness and strength in the application of Nb-V micro-alloyed heavy-load gear steel. Therefore, it is particularly important to develop a new heat treatment process for Nb-V micro-alloyed heavy-load gear steel to realize the perfect combination of high hardness and high strength and toughness, and to improve the surface hardness and load-carrying capacity. This has important significance and broad application prospects for improving the overall performance of mechanical equipment and reducing maintenance costs. SUMMARY

[0004] To solve the above technical problems, the application provides a Nb-V micro-alloyed heavy-load gear steel and a preparation method thereof. The carburizing treatment and carbonitriding treatment are combined to improve the surface carbon concentration, increase the volume fraction and average size of carbides, and improve the surface martensite stability by introducing nitrogen atoms to form nitrogen-containing martensite, so that the surface hardness and strength of the Nb-V micro-alloyed heavy-load gear steel are obviously improved.

[0005] The first object of the application is to provide a preparation method of a Nb-V micro-alloyed heavy-load gear steel, comprising the following steps:

[0006] S1. Smelting, forging, normalizing, tempering, sizing, and annealing a Nb-V microalloyed heavy-duty gear steel raw material in sequence to obtain a billet; 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%, Ni 1.46%-1.59%, Mo 0.26%-0.29%, Nb 0.051%-0.074%, V 0.077%-0.126%, P 0-0.010%, S 0-0.008%, and the balance being other inevitable impurities and iron;

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

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

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

[0010] S5. Perform cold treatment and low-temperature tempering 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; and 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 twice.

[0016] In one embodiment of the present application, in S4, the temperature of the carbonitriding treatment is 895℃-950℃, 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 application, in S5, the temperature of the cold treatment is -85℃ to -60℃, and the time is 2h-3h.

[0018] In one embodiment of the present application, in S5, the temperature of the low-temperature tempering treatment is 180℃-250℃, and the time is 3h-8h.

[0019] A second object of the present application is to provide a Nb-V micro-alloyed heavy load gear steel prepared by the method.

[0020] The technical solution of the present application has the following advantages compared with the prior art:

[0021] (1) The preparation method of the present application combines carburizing treatment and carbonitriding treatment, effectively increases the surface layer carbon concentration and introduces nitrogen atoms, carbon and nitrogen combine with Nb and V elements in the Nb-V micro-alloyed heavy load gear steel, promotes the precipitation of Nb and V carbonitride, and increases the volume fraction and average size of Nb and V carbonitride. The surface layer hardness is improved by using the solid solution strengthening of carbon and nitrogen and the precipitation strengthening of Nb and V carbonitride, and the problem of low surface layer carbon concentration and hardness in single carburizing process is solved.

[0022] (2) The preparation method of the present application improves the plasticity and toughness of the hardened layer by carefully designing the composition of the Nb-V micro-alloyed heavy load gear steel and combining the temperature of the cold treatment to appropriately retain austenite. The austenite content in the hardened layer is precisely controlled at 7%-10% to prolong the service life.

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

[0024] (4) The Nb and V content of the Nb-V micro-alloyed heavy load gear steel of the present application is controlled within a certain range to promote the precipitation of Nb and V carbonitride. The effect of high-density Nb and V carbonitride precipitation particles pinning grain boundaries is used to inhibit the grain growth during carburizing and carbonitriding processes, effectively increasing the carburizing temperature and carbonitriding temperature, promoting the diffusion of carbon atoms and nitrogen atoms, and obtaining large case depth in a short time, solving the problem of shallow case depth although the case hardness is high in single nitriding process.

[0025] (5) The Nb-V micro-alloyed heavy load gear steel has an effective hardening layer depth of 5.90mm-6.10mm, a surface hardness of 69HRC-71HRC, a surface layer tensile strength of 2265MPa-2316MPa, a surface layer yield strength of 1708MPa-1756MPa, an elongation after fracture of 5.1%-5.4%, and an impact energy of 97J-105J. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to make the content of the present application more easily understood, the present application will be further described in detail below according to the specific embodiments of the present application and in conjunction with the drawings, in which:

[0027] Figure 1 An optical microscope image of the Nb-V micro-alloyed heavy load gear steel prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0028] The present application will be further described in conjunction with specific embodiments, so that those skilled in the art can better understand the present application and implement it. Obviously, the described embodiments are only some of the embodiments of the present application, not all. It should be understood that the specific embodiments are only used to explain the present application, but the embodiments are not limited to the present application.

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

[0030] In the present application, unless otherwise specified, the term "and / or" used in the embodiments of the present application includes any and all combinations of one or more related listed items.

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

[0032] Example 1

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

[0034] S1, the raw materials are allocated according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by an electric arc furnace, the melting temperature is 1535℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing, and is processed into a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985℃, and the time is 3h; the tempering temperature is 670℃, and the time is 4.5h; the annealing temperature is 200℃, and the time is 4h;

[0035] S2, the gear steel blank is subjected to carburizing treatment, the carburizing treatment temperature is 995℃, the carbon potential is 1.5wt%, and the time is 16h, and the gear steel blank is cooled to room temperature after the carburizing treatment;

[0036] S3, the gear steel is subjected to twice high-temperature tempering treatment, that is, heated to 660℃ for 6h for the first time and heated to 660℃ for 6h for the second time;

[0037] S4, the gear steel is subjected to carbonitriding treatment, the carbonitriding treatment temperature is 895℃, the carbon potential is 1.2wt%, the ammonia gas flow is 6L / min, and 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 sequentially subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro-alloyed heavy load gear steel; the cold treatment temperature is-60℃, and the time is 3h; the low-temperature tempering treatment temperature is 180℃, and the time is 8h.

[0039] Example 2

[0040] The Nb-V micro-alloyed heavy load gear steel and the preparation method thereof of the embodiment specifically include the following steps:

[0041] S1, the raw materials are allocated according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by an electric arc furnace, the melting temperature is 1535℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing, and is processed into a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985℃, and the time is 3h; the tempering temperature is 670℃, and the time is 4.5h; the annealing temperature is 200℃, and the time is 4h;

[0042] S2, the gear steel blank is subjected to carburizing treatment, the carburizing treatment temperature is 1025℃, the carbon potential is 1.3wt%, the time is 13h, and the gear steel blank is cooled to room temperature after the carburizing treatment;

[0043] S3, the gear steel is subjected to twice high-temperature tempering treatment, that is, the gear steel is heated to 695℃ for 2.5h for the first time and heated to 695℃ for 2.5h for the second time;

[0044] S4, the gear steel is subjected to carbonitriding treatment, the carbonitriding treatment temperature is 950℃, the carbon potential is 1.1wt%, the ammonia flow rate is 5L / min, the time is 6h, and the gear steel is directly oil-cooled to room temperature after the carbonitriding treatment;

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

[0046] Example 3

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

[0048] S1, the raw materials are proportioned according to the following mass percentage components: 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 is other inevitable impurities and iron, and the raw materials are smelted into a steel ingot by an electric arc furnace at a smelting temperature of 1540℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process the steel ingot into a gear steel blank; the forging ratio is 3.3; the normalizing temperature is 995℃, the time is 2h; the tempering temperature is 680℃, the time is 3h; and the annealing temperature is 300℃, the time is 3h;

[0049] S2, the gear steel blank is subjected to carburizing treatment, the carburizing treatment temperature is 1010℃, the carbon potential is 1.4wt%, the time is 15h, and the gear steel blank is cooled to room temperature after the carburizing treatment;

[0050] S3, the gear steel is subjected to twice high-temperature tempering treatment, that is, the gear steel is heated to 675℃ for 4h for the first time and heated to 675℃ for 4h for the second time;

[0051] S4, the gear steel is subjected to carbonitriding treatment, the carbonitriding treatment temperature is 920℃, the carbon potential is 1.1wt%, the ammonia flow rate is 5L / min, the time is 7h, and the gear steel is directly oil-cooled to room temperature after the carbonitriding treatment;

[0052] S5, the gear steel is sequentially subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro-alloyed heavy load gear steel; the temperature of the cold treatment is -75 DEG C, and the time is 2h; the temperature of the low-temperature tempering treatment is 220 DEG C, and the time is 5h.

[0053] Example 4

[0054] The Nb-V micro-alloyed heavy load gear steel and the preparation method thereof of the present embodiment specifically comprises the following steps:

[0055] S1, the raw materials are proportioned according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by a vacuum induction furnace at a temperature of 1550 DEG C; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process into a gear steel blank; the forging ratio is 3.2; the temperature of the normalizing is 1000 DEG C, and the time is 2h; the temperature of the tempering is 680 DEG C, and the time is 3h; the temperature of the annealing is 300 DEG C, and the time is 3h;

[0056] S2, the gear steel blank is subjected to carburizing treatment at a temperature of 1015 DEG C, a carbon potential of 1.3wt% and a time of 14h, and then cooled to room temperature;

[0057] S3, the gear steel is subjected to twice high-temperature tempering treatment, i.e. heated to 675 DEG C for 3.5h for the first time and then heated to 675 DEG C for 3.5h for the second time;

[0058] S4, the gear steel is subjected to carbonitriding treatment at a temperature of 930 DEG C, a carbon potential of 1.1wt%, an ammonia gas flow rate of 5L / min and a time of 7h, and then directly oil-cooled to room temperature;

[0059] S5, the gear steel is sequentially subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro-alloyed heavy load gear steel; the temperature of the cold treatment is -80 DEG C, and the time is 2h; the temperature of the low-temperature tempering treatment is 220 DEG C, and the time is 5h.

[0060] Comparative Example 1

[0061] The present embodiment 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 allocated according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by an electric arc furnace at a temperature of 1535℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985℃, and the time is 3h; the tempering temperature is 670℃, and the time is 4.5h; the annealing temperature is 200℃, and the time is 4h;

[0063] S2, the gear steel blank is subjected to carburizing treatment at a temperature of 995℃, a carbon potential of 1.5wt% and a time of 16h, and then cooled to room temperature;

[0064] S3, the gear steel is subjected to twice high-temperature tempering treatment, i.e., heated to 660℃ for 6h for the first time and then heated to 660℃ for 6h for the second time;

[0065] S4, the gear steel is subjected to carbonitriding treatment at a temperature of 895℃, a carbon potential of 1.2wt%, an ammonia gas flow of 6L / min and a time of 8h, and then directly oil-cooled to room temperature;

[0066] S5, the gear steel is sequentially subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro-alloyed heavy load gear steel; the cold treatment temperature is -60℃, and the time is 3h; the low-temperature tempering treatment temperature is 180℃, and the time is 8h.

[0067] Comparative Example 2

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

[0069] S1, the raw materials are allocated according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by an electric arc furnace at a temperature of 1535℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985℃, and the time is 3h; the tempering temperature is 670℃, and the time is 4.5h; the annealing temperature is 200℃, and the time is 4h;

[0070] S2, the gear steel blank is subjected to carburizing treatment, the carburizing treatment temperature is 995℃, the carbon potential is 1.5wt%, the time is 16h, and the gear steel blank is cooled to room temperature after the carburizing treatment;

[0071] S3, the gear steel is subjected to twice high-temperature tempering treatment, i.e. the first tempering is performed by heating to 660℃ and keeping for 6h, and the second tempering is performed by heating to 660℃ and keeping for 6h;

[0072] S4, the gear steel is subjected to carbonitriding treatment, the carbonitriding treatment temperature is 895℃, 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 the Nb-V micro-alloyed heavy-load gear steel; the cold treatment temperature is -60℃, and the time is 3h; and the low-temperature tempering treatment temperature is 180℃, and the time is 8h.

[0074] Comparative Example 3

[0075] The basic process is the same as that in Example 1, except that the cold treatment temperature is adjusted to -50℃, and the process specifically comprises the following steps:

[0076] S1, the raw materials are proportioned according to the following mass percentage components: 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 is other inevitable impurities and iron, and the raw materials are smelted into a steel ingot by an electric arc furnace at a smelting temperature of 1535℃; the steel ingot is subjected to forging, normalizing, tempering, size processing and annealing in sequence to process a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985℃, and the time is 3h; the tempering temperature is 670℃, and the time is 4.5h; and the annealing temperature is 200℃, and the time is 4h;

[0077] S2, the gear steel blank is subjected to carburizing treatment, the carburizing treatment temperature is 995℃, the carbon potential is 1.5wt%, the time is 16h, and the gear steel blank is cooled to room temperature after the carburizing treatment;

[0078] S3, the gear steel is subjected to twice high-temperature tempering treatment, i.e. the first tempering is performed by heating to 660℃ and keeping for 6h, and the second tempering is performed by heating to 660℃ and keeping for 6h;

[0079] S4, the gear steel is subjected to carbonitriding treatment, the carbonitriding treatment temperature is 895℃, 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 sequentially subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro-alloyed heavy load gear steel; the temperature of the cold treatment is -50℃, and the time is 3h; the temperature of the low-temperature tempering treatment is 180℃, and the time is 8h.

[0081] Comparative Example 4

[0082] The basic procedure is the same as that in Example 1, except that the temperature of the carbonitriding treatment is adjusted to 870℃, and the procedure specifically comprises the following steps:

[0083] S1, the raw materials are allocated according to the following mass percentage components: 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 is other unavoidable impurities and iron, and the steel ingot is melted by an electric arc furnace at a temperature of 1535℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process into a gear steel blank; the forging ratio is 3.2; the temperature of the normalizing is 985℃, and the time is 3h; the temperature of the tempering is 670℃, and the time is 4.5h; the temperature of the annealing is 200℃, and the time is 4h;

[0084] S2, the gear steel blank is subjected to carburizing treatment at a temperature of 995℃, a carbon potential of 1.5wt% and a time of 16h, and then cooled to room temperature;

[0085] S3, the gear steel is subjected to twice high-temperature tempering treatment, i.e. heated to 660℃ for 6h for the first time and then heated to 660℃ for 6h for the second time;

[0086] S4, the gear steel is subjected to carbonitriding treatment at a temperature of 870℃, a carbon potential of 1.2wt%, an ammonia gas flow rate of 6L / min and a time of 8h, and then directly oil-cooled to room temperature;

[0087] S5, the gear steel is sequentially subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro-alloyed heavy load gear steel; the temperature of the cold treatment is -60℃, and the time is 3h; the temperature of the low-temperature tempering treatment is 180℃, and the time is 8h.

[0088] Comparative Example 5

[0089] The basic procedure is the same as that in Example 1, except that the carbonitriding treatment is replaced by nitriding treatment, and the procedure specifically comprises the following steps:

[0090] S1, the raw materials are allocated according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by an electric arc furnace at a temperature of 1535 ℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985 ℃, and the time is 3 h; the tempering temperature is 670 ℃, and the time is 4.5 h; the annealing temperature is 200 ℃, and the time is 4 h;

[0091] S2, the gear steel blank is subjected to carburizing treatment at a temperature of 995 ℃, a carbon potential of 1.5 wt% and a time of 16 h, and then cooled to room temperature;

[0092] S3, the gear steel is subjected to twice high-temperature tempering treatment, i.e., heated to 660 ℃ for 6 h for the first time and then heated to 660 ℃ for 6 h for the second time;

[0093] S4, the gear steel is subjected to nitriding treatment at a temperature of 895 ℃, an ammonia gas flow rate of 6 L / min and a time of 8 h, and then directly oil-cooled to room temperature;

[0094] S5, the gear steel is sequentially subjected to cold treatment and low-temperature tempering treatment to obtain a Nb-V micro-alloyed heavy load gear steel; the cold treatment temperature is -60 ℃, and the time is 3 h; the low-temperature tempering treatment temperature is 180 ℃, and the time is 8 h.

[0095] Comparative Example 6

[0096] The same as Example 1, except that the carbonitriding treatment is not performed between the high-temperature tempering treatment and the cold treatment, and the gear steel is only heated to 895 ℃ for 8 h and then directly oil-cooled to room temperature, which specifically comprises the following steps:

[0097] S1, the raw materials are allocated according to the following mass percentage components: 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 is other inevitable impurities and iron, and the steel ingot is melted by an electric arc furnace at a temperature of 1535 ℃; the steel ingot is sequentially subjected to forging, normalizing, tempering, size processing and annealing to process a gear steel blank; the forging ratio is 3.2; the normalizing temperature is 985 ℃, and the time is 3 h; the tempering temperature is 670 ℃, and the time is 4.5 h; the annealing temperature is 200 ℃, and the time is 4 h;

[0098] S2. Carburizing the gear steel blank at a temperature of 995° C., a carbon potential of 1.5 wt %, and a time of 16 h, followed by cooling to room temperature.

[0099] S3, perform two high temperature tempering treatments on the gear steel, i.e., heat to 660°C and keep it for 6 hours for the first tempering, and then heat to 660°C and keep it for 6 hours for the second tempering;

[0100] S4. Heat the gear steel to 895℃ and keep it at this temperature for 8 hours, 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 microalloyed heavy-duty gear steel; the cold treatment temperature is -60°C and the time is 3 hours; the low-temperature tempering temperature is 180°C and the time is 8 hours.

[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 and V carbonitrides. The retained austenite content 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 Quantitative Determination of Austenite in Steel - X-ray Diffractometer Method";

[0107] Effective hardened layer: Refer to the standard test of "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 method;

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

[0110] Impact performance: tested according to the standard GB / T 229-2020 Metallic materials Charpy pendulum impact test method.

[0111] Table 1 shows the final measured properties of the Nb-V microalloyed heavy load gear steel:

[0112] Table 1

[0113]

[0114] As can be seen from Table 1, the Nb-V microalloyed heavy load gear steel obtained by the example has higher surface hardness, surface layer tensile properties and impact performance, the surface hardness is 69HRC-71HRC, the surface layer tensile strength is 2265MPa-2316MPa, the surface layer 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 and Comparative Example 1, it can be seen that the surface hardness, surface layer tensile strength, surface layer yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy load 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 Nb, V carbonitride precipitated particles weaken the effect of inhibiting grain growth, and the residual austenite content is reduced, resulting in simultaneous reduction of hardness, strength, plasticity and toughness.

[0116] Comparing Example 1 and Comparative Example 2, it can be seen that the surface hardness, surface layer tensile strength, surface layer yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy load gear steel in Comparative Example 2 are all 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 Nb, V carbonitride precipitated particles weaken the effect of inhibiting grain growth, and the residual austenite content is reduced, resulting in simultaneous reduction of hardness, strength, plasticity and toughness.

[0117] Comparing Example 1 and Comparative Example 3, it can be seen that the surface hardness, surface layer tensile strength, surface layer yield strength, elongation after fracture and impact energy of the Nb-V microalloyed heavy load gear steel in Comparative Example 3 are all lower than those in Example 1. This is because the temperature of cold treatment in Comparative Example 1 is higher than that in Example 1, resulting in a residual austenite content higher than 10%, which is not conducive to the improvement of work hardening ability caused by dislocation entanglement and proliferation under high stress level, and also not conducive to the improvement of toughness and plasticity while reducing hardness and strength.

[0118] As can be seen from the comparison between Comparative Example 1 and Comparative Example 4, the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V micro-alloyed heavy load gear steel in Comparative Example 4 are all lower than those in Example 1. This is because the temperature of the carbonitriding treatment in Comparative Example 4 is lower than that in Example 1, and the carbon and nitrogen atoms are not diffused sufficiently, resulting in a decrease in the content of residual austenite and the depth of effective hardening layer, and thus a simultaneous decrease in hardness, strength, plasticity and toughness.

[0119] As can be seen from the comparison between Comparative Example 1 and Comparative Example 5, the surface hardness, surface tensile strength, surface yield strength, elongation after fracture and impact energy of the Nb-V micro-alloyed heavy load gear steel in Comparative Example 5 are all lower than those in Example 1. This is because the carbonitriding treatment in Comparative Example 5 is replaced by nitriding treatment, and the carbon atoms are not diffused sufficiently, resulting in a decrease in the content of residual austenite and the depth of effective hardening layer, and thus a simultaneous decrease in hardness, strength, plasticity and toughness.

[0120] As can be seen from the comparison between Comparative Examples 1-4 and Comparative Example 6, the surface hardness of the Nb-V micro-alloyed heavy load gear steel obtained in the examples is increased by 13.1%-16.4% compared with that in Comparative Example 6, the surface tensile strength is increased by 13.4%-16.0%, the surface yield strength 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 the carburizing treatment and carbonitriding treatment are combined in Examples 1-4, which effectively increases the surface carbon concentration and introduces nitrogen atoms, and the carbon and nitrogen combine with the Nb and V elements in the Nb-V micro-alloyed heavy load gear steel, promoting the precipitation of Nb and V carbonitrides, and using the solid solution strengthening of carbon and nitrogen and the precipitation strengthening of Nb and V carbonitrides to increase the surface hardness, and precisely controlling the austenite content in the range of 7%-10% to ensure a higher combination of strength and toughness.

[0121] Obviously, the above examples are merely illustrative examples and are not intended to limit the embodiments. Based on the above description, other different forms of changes or variations can be made by those of ordinary skill in the art. Here, it is not necessary and impossible to exhaust all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A method of manufacturing a Nb-V microalloyed heavy load gear steel, characterized in that, It comprises the following steps: S1, the Nb-V micro alloying heavy load gear steel raw material is sequentially subjected to smelting, forging, normalizing, tempering, size processing and annealing to obtain a blank; the chemical component composition and mass percentage of the Nb-V micro alloying heavy load 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%, Ni 1.46%-1.59%, Mo 0.26%-0.29%, Nb 0.051%-0.074%, V 0.077%-0.126%, P 0-0.010%, S 0-0.008%, and the balance is other inevitable impurities and iron; S2, the blank obtained in S1 is subjected to carburizing treatment to obtain a heavy load gear steel; S3, the heavy load gear steel obtained in S2 is subjected to high-temperature tempering treatment; S4, the heavy load gear steel obtained in S3 is subjected to carbonitriding treatment; S5, the heavy load gear steel obtained in S4 is subjected to cold treatment and low-temperature tempering treatment to obtain the Nb-V micro alloying heavy load gear steel.

2. A method of producing the Nb-V micro-alloyed heavy load gear steel according to claim 1, characterized in that, In S1, the temperature of smelting is 1535-1550℃.

3. The method of manufacturing the Nb-V micro-alloyed heavy load gear steel according to claim 1, characterized in that, In S1, the forging ratio of forging is not less than 3.

2.

4. The method of producing the Nb-V microalloyed heavy load gear steel according to claim 1, characterized in that, In S1, the temperature of normalizing is 985-1010℃, the time is 2-3h; the temperature of tempering is 670-695℃, the time is 2-4.5h; the temperature of annealing is 200-400℃, the time is 3-4h.

5. The method of producing the Nb-V microalloyed heavy load gear steel according to claim 1, characterized in that, In S2, the temperature of carburizing treatment is 995-1025℃, the carbon potential is 1.3-1.5wt%, the time is 13-16h.

6. The method of producing the Nb-V micro-alloyed heavy load gear steel according to claim 1, characterized in that, In S3, the temperature of high-temperature tempering treatment is 660-695℃, the time is 2.5-6h, and the number of times is 2.

7. The method of producing the Nb-V micro-alloyed heavy load gear steel according to claim 1, characterized in that, In S4, the temperature of carbonitriding treatment is 895-950℃, the carbon potential is 1.1-1.2wt%, the ammonia flow rate is 5-6L / min, and the time is 6-8h.

8. The method of producing the Nb-V micro-alloyed heavy load gear steel according to claim 1, characterized in that, In S5, the temperature of cold treatment is -85 to -60℃, and the time is 2-3h.

9. The method of producing the Nb-V micro-alloyed heavy load gear steel according to claim 1, characterized in that, In S5, the temperature of low-temperature tempering treatment is 180-250℃, and the time is 3-8h.

10. The Nb-V micro alloying heavy load gear steel prepared by the method of any one of claims 1-9.

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