Carburizing bearing steel for axle box of high-speed motor train unit and preparation method of carburizing bearing steel

Abstract

The invention discloses alloy steel which comprises the following components in percentage by weight: 0.19 to 0.23 percent of C, 0.25 to 0.40 percent of Si, 0.50 to 0.70 percent of Mn, 0.45 to 0.65 percent of Cr, 0.16 to 2.00 percent of Ni, 0.20 to 0.30 percent of Mo, 0.25 to 0.40 percent of V, 0.02 to 0.04 percent of Nb, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S and the balance of Fe and inevitable impurities. The invention further discloses a preparation method. The preparation method comprises the steps of smelting, forging, carburizing, primary quenching, secondary quenching and tempering, primary cryogenic cooling and tempering, and secondary cryogenic cooling and tempering. In addition, the invention further discloses alloy steel which is prepared through the preparation method, and chemical components of the alloy steel are defined as before. The alloy steel is well matched in hardness, strength performance and impact performance, and can be used for manufacturing axle box bearings of high-speed motor train units.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and specifically relates to a carburized bearing steel for a high-speed train axle box with high strength and toughness and excellent contact fatigue performance and a preparation method thereof. Background Art

[0002] High-speed railways are an inevitable trend in the development of railway technology in the world. The axle box bearing of a high-speed train is one of the key components of the bogie system of a high-speed train. Its service life, reliability and other indicators directly affect the operation order and safety of the train. The axle box bearing of a high-speed train needs to withstand high-frequency cyclic loads during operation, which requires that the bearing steel must have characteristics such as long fatigue life, high wear resistance, high impact toughness and dimensional stability. The basis of these characteristics is the high cleanliness, high uniformity, high hardness, good strength-toughness matching and stability of material organization of the steel material.

[0003] At present, G20CrNi2Mo steel is commonly used for carburizing bearing steel in high-speed rail axle boxes. This steel obtains a matrix structure with lath martensite, bainite and a small amount of ferrite through carburizing, secondary quenching and tempering treatment. It allows a certain degree of fine needle martensite and a small amount of residual needle carbide to exist, so it has high hardness, excellent wear resistance and rolling contact fatigue performance. The current national standard GB / T 3203-2016 "Carburizing Bearing Steel" and the industry standard JB / T8881-2001 "Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts" stipulate the mechanical properties of this bearing steel: the tensile strength Rm is 980Mpa, and the impact energy is 63J; compared with the performance of foreign high-quality bearing steel, there is still a certain gap. The reason may be that the carbides (mainly carbide bands, carbide networks and carbide particles of bearing steel) and matrix grain size of domestic bearing steel are still quite different from those of foreign ones; this difference leads to a lower strength-toughness match of domestic bearing steel, thus affecting the contact fatigue performance and wear resistance of bearing steel.

[0004] In this regard, there have been many attempts to improve the existing technology. For example, the Chinese invention patent application with publication number CN 116815055A is "A low-cost, long-life bearing steel and its production method and heat treatment method". The bearing steel composition includes C 0.95% to 1.05%, Si 0.32% to 0.42%, Mn 0.38% to 0.58%, Cr 1.7% to 1.9%, Mo 0.4% to 0.7%, Nb 0.05% to 0.07%, V 0.015% to 0.025%, Ti ≤ 0.0015%, Alt 0.015~0.025%, P≤0.010%, S≤0.010%, TO≤0.0015%, N≤0.0065%, and the rest are Fe and other inevitable impurities; through the double quenching heat treatment process, the surface of the bearing steel is tempered bainite + tempered martensite + carbide + residual austenite, which has high wear resistance. The yield strength and surface hardness of this bearing steel are both high, but its impact energy is only about 20J, and its toughness is insufficient. In order to ensure high contact fatigue performance, the content of residual austenite in the matrix structure is about 20%, which increases the dimensional instability of the product under long-term use or in an environment with large temperature changes.

[0005] For example, the Chinese invention patent application with publication number CN 110863141A, "A method for alloying and heat treating a highly wear-resistant carburized bearing steel", discloses that the composition of the bearing steel includes C 0.17-0.25%, Si 0.15-0.4%, Mn 0.4-0.7%, Cr 0.35-0.65%, Ni 1.6-2.0%, Mo 0.2-0.3%, V 0.3-0.5%, and the balance is Fe; it improves the wear resistance of the material by adding a small amount of V alloy element to the existing carburized bearing steel and optimizing its heat treatment process. However, its impact energy is only 80J, and its toughness is not significantly improved compared with the traditional G20CrNi2Mo material, which is difficult to meet the future development needs of high-strength and toughness matching of bearing steel.

[0006] The Chinese invention patent application with publication number CN115449613 A, "A carburized bearing steel and its preparation method", discloses that the composition of the bearing steel includes C 0.12-0.15%, Mo 4.00-4.50%, Cr4.00-4.25%, V 1.13-1.33%, Ni 3.20-3.60%, Mn 0.15-0.35%, Si 0.10-0.25%, and the balance is iron and impurity elements; through five-stage vacuum heat treatment, the surface hardness of the bearing steel obtained is above 70HRC and the core fracture toughness can reach 65MPa.m 1 / 2 However, its process is complicated and costly, making it unsuitable for industrial development in basic industries.

[0007] The Chinese invention patent application with publication number CN117265394A, "A carburized bearing steel with ultra-high purity and high uniformity for railway freight cars and its production method", discloses that the composition of the carburized bearing steel includes: C 0.20%-0.22%, Si 0.25%-0.30%, Mn 0.55%-0.60%, Cr 0.53%-0.57%, Ni 1.80%-1.85%, Mo 0.23%-0.27%, Al 0.020%-0.035%, P≤0.010%, S<0.003%, Cu≤0.05%, As≤0.0150%, Sn≤0.0150%, Pb≤0.0020%, Sb≤0.0050%, Ca≤0.0010%, Ti≤0.0015%, [O]≤0.0010%, and the balance is Fe. The tensile strength of the carburized bearing steel is 1154-1267MPa, the elongation after fracture is 13.5%-14.5%, and the reduction of area is 53%-56%.

[0008] The Chinese invention patent application with publication number CN113584409A, “Carburized bearing steel for high-speed railway and its preparation method”, discloses that the chemical composition of the carburized bearing steel includes: C 0.19-0.23%, Si 0.25-0.40%, Mn 2.8-3.6%, Cr 0.90-1.20%, Ni 0.9-1.20%, Mo0.20-0.30%, Cu≤0.25%, Nb 0.005-0.20%, P≤0.030%, S≤0.030%, and the rest is Fe and impurities. After a heat treatment of quenching + tempering and isothermal quenching, the contact fatigue life of the carburized bearing steel is improved, and the maximum can reach ≥2.0×10 8 .

[0009] With the further increase in the speed of railway trains, higher requirements are placed on the strength matching, contact fatigue life, and high-precision machining dimensional stability of the axle box bearings of high-speed trains. Summary of the invention

[0010] In view of the problems existing in the prior art, the present invention provides a new alloy steel, and also provides a preparation method of the alloy steel. The alloy steel has good matching strength and toughness, long fatigue life, and good dimensional stability, and is particularly suitable for use as axle box bearings of high-speed EMUs at higher speeds. The present invention also provides a preparation method of the alloy steel, especially a heat treatment method.

[0011] Therefore, the present invention adopts the following technical scheme.

[0012] An alloy steel comprising the following components in weight percentage:

[0013] C: 0.19~0.23%, Si: 0.25~0.40%, Mn: 0.50~0.70%, Cr: 0.45~0.65%, Ni: 0.16~2.00%, Mo: 0.20~0.30%, V: 0.25~0.40%, Nb: 0.02~0.04%, P≤0.015%, S≤0.005%, and the rest are Fe and unavoidable impurities.

[0014] As a preferred embodiment, the present invention provides an alloy steel comprising the following components in weight percentage:

[0015] C: 0.20~0.22%, Si: 0.25~0.40%, Mn: 0.55~0.65%, Cr: 0.50~0.60%, Ni: 1.70~1.90%, Mo: 0.20~0.30%, V: 0.35~0.40%, Nb: 0.03~0.04%, P≤0.010%, S≤0.002%, and the rest are Fe and unavoidable impurities.

[0016] As a more preferred embodiment, the present invention provides an alloy steel comprising the following components in weight percentage:

[0017] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0018] The alloy steel of the present invention has various constituent elements that work synergistically, so that the alloy steel exhibits excellent performance as a whole. The V element has the ability to form strong carbides, so as to obtain a large number of fine, uniform, and dispersed alloy carbides, which effectively improves the hardness of the material; at the same time, the V element, Nb element, Mo element and other micro-alloying elements are used to refine the grains and improve the hardenability, thereby ensuring the good toughness of the material and making the material have a good match between strength and toughness.

[0019] Another object of the present invention is to provide a method for preparing alloy steel, comprising the following steps:

[0020] Smelting → forging → carburizing → primary quenching → secondary quenching + tempering → primary deep cooling + tempering → secondary deep cooling + tempering; among them,

[0021] Forging: The steel billet is forged at 1050-1150℃ to obtain bars;

[0022] Carburizing: Carburizing the bar in a conventional manner;

[0023] Primary quenching: quenching temperature 920±20℃, oil quenching;

[0024] Secondary quenching + tempering: quenching temperature 880±20℃, heat preservation 40~60min, oil quenching, tempering temperature 180±10℃;

[0025] One deep cooling + tempering: deep cooling temperature -80±10℃, keep warm for 1~2h; tempering temperature 180±20℃, keep warm for 1~2h; then air cool to room temperature;

[0026] Secondary deep cooling + tempering: deep cooling temperature -80±10℃, keep warm for 1~2h; tempering temperature 180±20℃, keep warm for 1~2h; then air cool to room temperature.

[0027] As a preferred embodiment, the present invention provides a method for preparing the alloy steel, comprising the following steps:

[0028] I. Smelting

[0029] Smelting the raw materials to obtain steel billets;

[0030] II. Forging

[0031] The steel billet is heated to 1050-1150°C for forging with a reduction ratio of 0.5-0.8, and naturally cooled to room temperature to obtain a bar;

[0032] III. Carburizing

[0033] The bar is carburized at 920±10℃ for 24-26h;

[0034] IV. Primary quenching

[0035] After carburizing, the bar is immediately immersed in vacuum quenching oil for a quenching process;

[0036] V. Secondary quenching + tempering

[0037] Heat the bar to 880±20℃, keep it warm for 45min, and then immediately immerse it in vacuum quenching oil for a second quenching; then heat it to 180±10℃, and keep it warm for 1~2h;

[0038] VI. One deep cooling + tempering

[0039] After step V is completed, the rod is immediately placed in a sealed environment at -80±10°C, kept warm for 1h to 2h, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 1 to 2h, and air-cooled to room temperature;

[0040] VII. Secondary deep cooling + tempering

[0041] The rod that has been cooled to room temperature is placed in a closed environment at -80±10℃ again, kept warm for 1h~2h, taken out, and naturally heated to room temperature; then heated to 180±10℃, kept warm for 1~2h, and air-cooled to room temperature.

[0042] Preferably, in step II, the reduction rate is 0.7.

[0043] Preferably, in steps IV and V, the temperature of the vacuum quenching oil is 30-50°C, more preferably 35-45°C.

[0044] In the above step I, conventional smelting methods and equipment in the art can be used, such as electroslag smelting, double vacuum smelting, refining outside the furnace or vacuum induction furnace smelting.

[0045] In the above-mentioned preparation process of the present invention, by increasing the quenching temperature, after two quenching + tempering, the carbides in the steel gradually dissolve into the austenite; at the same time, accompanied by the growth of austenite grains, the steel has a better combination of strength and toughness. After the second quenching and tempering, the matrix structure obtained is lath martensite, a small amount of residual austenite and a certain amount of carbides, and the strength and hardness of the steel are further improved. Two "deep cooling + tempering treatments" can refine the martensite laths and promote the precipitation and dispersion of second phases such as fine nano-carbides; at the same time, it promotes the gradual transformation of residual austenite into martensite and reduces the amount of residual austenite in the steel.

[0046] The third object of the present invention is to provide an alloy steel prepared by the above preparation method, and the alloy steel comprises the following components in weight percentage:

[0047] C: 0.19~0.23%, Si: 0.25~0.40%, Mn: 0.50~0.70%, Cr: 0.45~0.65%, Ni: 0.16~2.00%, Mo: 0.20~0.30%, V: 0.25~0.40%, Nb: 0.02~0.04%, P≤0.015%, S≤0.005%, and the rest are Fe and unavoidable impurities.

[0048] As a preferred embodiment, the present invention provides an alloy steel prepared by the above preparation method, and the alloy steel comprises the following components in weight percentage:

[0049] C: 0.20~0.22%, Si: 0.25~0.40%, Mn: 0.55~0.65%, Cr: 0.50~0.60%, Ni: 1.70~1.90%, Mo: 0.20~0.30%, V: 0.35~0.40%, Nb: 0.03~0.04%, P≤0.010%, S≤0.002%, and the rest are Fe and unavoidable impurities.

[0050] As a more preferred embodiment, the present invention provides an alloy steel prepared by the above preparation method, and the alloy steel comprises the following components in weight percentage:

[0051] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0052] A fourth object of the present invention is to provide an application of the alloy steel of the present invention in the preparation of axle box bearings for high-speed EMUs.

[0053] The present invention also provides a high-speed train axle box, wherein the bearings are made of the alloy steel described in the present invention.

[0054] In the specification of the present application, the term "about" has the meaning reasonably given to it by a person of ordinary skill in the art when used in conjunction with a specified numerical value or range, namely, indicating slightly higher or lower than the specified value or range: within ±10% of the specified value, ±9% of the specified value, ±8% of the specified value, ±7% of the specified value, ±6% of the specified value, ±5% of the specified value, ±4% of the specified value, ±3% of the specified value, ±2% of the specified value, or ±1% of the specified value.

[0055] Finally, the alloy steel of the present invention has fine martensite laths in the matrix structure, fine carbides are dispersed and evenly distributed, the amount of retained austenite is controlled within 1%, the hardness value can reach about 45HRC, the tensile strength Rm can reach about 1450MPa or even higher, the yield strength Rp0.2 can reach about 1240MPa, the impact energy Aku reaches about 110J, the elongation after fracture A can reach about 14% or even higher, and the cross-sectional shrinkage Z is about 63%. The above excellent performance is completely suitable for manufacturing axle box bearings of high-speed EMUs. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The present invention will be further described below in conjunction with the accompanying drawings.

[0057] Figure 1 The photograph shows the microscopic morphology of carbides in the alloy steel of Example 1 (5000×). The photograph shows uniform and finely dispersed carbides in martensite.

[0058] Figure 2 The photograph of FIG. 1 shows the martensitic microstructure of the alloy steel of Example 1 (40000×). The photograph shows the martensitic lath structure of the steel.

[0059] Figure 3The photograph shows the matrix structure (10000×) of the alloy steel of Example 2 after single quenching + tempering.

[0060] Figure 4 The photograph shows the matrix structure (10000×) of the alloy steel of Example 2 after a single cryogenic treatment.

[0061] Figure 5 The photograph shows the matrix structure (10000×) of the alloy steel of Example 2 after one deep cryogenic treatment + tempering.

[0062] Figure 6 The photograph shows the matrix structure (10000×) of the alloy steel of Example 2 after secondary deep cryogenic treatment.

[0063] Figure 7 The photograph shows the matrix structure (10000×) of the alloy steel of Example 2 after secondary deep cryogenic treatment + tempering.

[0064] Figure 8 The photograph shows the matrix grain size (1000×) of the alloy steel of Comparative Example 3. The photograph shows the original austenite grain size of the steel. DETAILED DESCRIPTION

[0065] The present invention is described below with reference to specific examples. It will be appreciated by those skilled in the art that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention in any way.

[0066] The experimental methods in the following examples are conventional methods unless otherwise specified. The raw materials, reagents, etc. used in the following examples are commercially available products unless otherwise specified.

[0067] Example 1 A kind of alloy steel

[0068] The alloy steel of this embodiment is prepared by the following steps:

[0069] I. Smelting

[0070] The raw materials are smelted in a vacuum induction furnace to obtain a 300×300mm billet;

[0071] II. Forging

[0072] The steel billet is heated to 1050-1100°C for forging with a reduction ratio of 0.7 to obtain a Φ90mm bar;

[0073] III. Carburizing

[0074] Transfer the rods into the carburizing furnace and carburize at 920±10℃ for 25h;

[0075] IV. Primary quenching

[0076] After carburizing, the bar is immediately immersed in vacuum quenching oil at about 40°C for a quenching process;

[0077] V. Secondary quenching + tempering

[0078] Take the bar out of the vacuum quenching oil, heat it to 880±20℃, keep it warm for 45min, and immediately immerse it in vacuum quenching oil at about 40℃ for a second quenching; then take the bar out of the vacuum quenching oil, heat it to 180±10℃, and keep it warm for 2h;

[0079] VI. One deep cooling + tempering

[0080] After the tempering in step V is completed, the rod is immediately placed in a closed environment at -80°C, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature.

[0081] VII. Secondary deep cooling + tempering

[0082] The rod that has been cooled to room temperature is placed in a closed environment at -80°C again, kept warm for 2 hours, taken out, naturally heated to room temperature, then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature to obtain alloy steel.

[0083] The alloy steel prepared in this embodiment has the following chemical composition:

[0084] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0085] After step VII is completed, take a sample, observe the matrix structure of the alloy steel with a scanning electron microscope and take photos. Figure 1 , Figure 2 shown.

[0086] like Figure 1 As shown, in the alloy steel prepared in this embodiment, fine carbides are evenly dispersed in the martensite. Figure 2 As shown, in the alloy steel prepared in this embodiment, fine lath-shaped martensite is distributed in the matrix composition, and austenite is almost not seen.

[0087] Example 2 A kind of alloy steel

[0088] The alloy steel was prepared again according to the same process as in Example 1. The alloy steel prepared in this example had the same chemical composition as in Example 1.

[0089] After step V (secondary quenching + tempering), take a sample, observe the matrix structure of the alloy steel with a scanning electron microscope and take photos. Figure 3 shown.

[0090] After deep cryogenic treatment in step VI (one deep cryogenic + tempering), take a sample, observe the matrix structure of the alloy steel with a scanning electron microscope and take photos, see Figure 4 shown.

[0091] After the tempering treatment in step VI (one deep cooling + tempering), take a sample, observe the matrix structure of the alloy steel with a scanning electron microscope and take photos, see Figure 5 shown.

[0092] After deep cooling treatment in step VII (secondary deep cooling + tempering), take a sample, observe the matrix structure of the alloy steel with a scanning electron microscope and take photos. Figure 6 shown.

[0093] After the tempering treatment in step VII (secondary deep cooling + tempering), take a sample, observe the matrix structure of the alloy steel with a scanning electron microscope and take photos, see Figure 7 shown.

[0094] like Figures 3 to 7 As shown in the figure, with the increase of cryogenic treatment and tempering times, the fine dispersed carbides tend to grow, but the degree of carbide dispersion increases, and the carbides gradually change from slightly pointed shapes after quenching to more regular spherical shapes. Figure 7 As shown in the figure, a small amount of retained austenite is mainly distributed between martensite laths, and the photo shows a bright white film. No large pieces of retained austenite are found in the steel. The content of retained austenite changes from 4.75% in the first quenching state to 0.82% after "first deep cooling + tempering" and 0.79% after "secondary deep cooling + tempering".

[0095] Example 3 A kind of alloy steel

[0096] The alloy steel of this embodiment is prepared by the following steps:

[0097] I. Smelting

[0098] The raw materials are smelted in a vacuum induction furnace to obtain a 300×300mm billet;

[0099] II. Forging

[0100] The steel billet was heated to 1050°C for forging with a reduction ratio of 0.7 to obtain a Φ90 mm bar;

[0101] III. Carburizing

[0102] Transfer the rods into the carburizing furnace and carburize at 920±10℃ for 25h;

[0103] IV. Primary quenching

[0104] After carburizing, the bar is immediately immersed in vacuum quenching oil at about 40°C for a quenching process;

[0105] V. Secondary quenching + tempering

[0106] Take the bar out of the vacuum quenching oil, heat it to 880±20℃, keep it warm for 60min, and immediately immerse it in vacuum quenching oil at about 40℃ for a second quenching; then take the bar out of the vacuum quenching oil, heat it to 180±10℃, and keep it warm for 2h;

[0107] VI. One deep cooling + tempering

[0108] After the tempering in step V is completed, the rod is immediately placed in a closed environment at -73°C, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature.

[0109] VII. Secondary deep cooling + tempering

[0110] The rod that has been cooled to room temperature is placed in a closed environment at -73°C again, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature to obtain alloy steel.

[0111] The alloy steel prepared in this embodiment has the following chemical composition:

[0112] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0113] Comparative Example 1 A kind of alloy steel

[0114] The alloy steel of this comparative example is prepared by the following steps:

[0115] I. Smelting

[0116] The same raw materials as those in Example 1 are smelted in a vacuum induction furnace to obtain a steel billet;

[0117] II. Forging

[0118] The steel billet is heated to 1050-1100°C for forging with a reduction ratio of 0.7 to obtain a Φ90mm bar;

[0119] III. Carburizing

[0120] Transfer the rod into the carburizing furnace, carburize at 920±10℃ for 25h, and cool naturally to room temperature;

[0121] IV.Quenching + tempering

[0122] Heat the bar to 880℃, keep it warm for 45min, immerse it in vacuum quenching oil at about 40℃ for quenching; then heat it to 180℃ and keep it warm for 2h;

[0123] V. Deep cooling + tempering

[0124] After the tempering in step IV is completed, the rod is immediately placed in a closed environment at -80°C, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature.

[0125] The alloy steel prepared in this comparative example has the following chemical composition:

[0126] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0127] Comparative Example 2 A kind of alloy steel

[0128] The alloy steel of this comparative example is prepared by the following steps:

[0129] I. Smelting

[0130] The same raw materials as those in Example 1 are smelted in a vacuum induction furnace to obtain a steel billet;

[0131] II. Forging

[0132] The steel billet is heated to 1050-1100°C for forging with a reduction ratio of 0.7 to obtain a Φ90mm bar;

[0133] III. Carburizing

[0134] Transfer the rod into the carburizing furnace, carburize at 920±10℃ for 25h, and cool naturally to room temperature;

[0135] IV.Quenching + tempering

[0136] Heat the bar to 900℃, keep it warm for 45min, immerse it in vacuum quenching oil at about 40℃ for quenching; then heat it to 180℃ and keep it warm for 2h;

[0137] V. Deep cooling + tempering

[0138] After the tempering in step IV is completed, the rod is immediately placed in a closed environment at -80°C, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature.

[0139] The alloy steel prepared in this embodiment has the following chemical composition:

[0140] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0141] Comparative Example 3 A kind of alloy steel

[0142] The alloy steel of this comparative example is prepared by the following steps:

[0143] I. Smelting

[0144] The same raw materials as those in Example 1 are smelted in a vacuum induction furnace to obtain a steel billet;

[0145] II. Forging

[0146] The steel billet is heated to 1050-1100°C for forging with a reduction ratio of 0.7 to obtain a Φ90mm bar;

[0147] III. Carburizing

[0148] Transfer the rod into the carburizing furnace, carburize at 920±10℃ for 25h, and cool naturally to room temperature;

[0149] IV.Quenching + tempering

[0150] Heat the bar to 910℃, keep it warm for 45min, and immediately immerse it in vacuum quenching oil at about 40℃ for quenching; then heat it to 180℃ and keep it warm for 2h;

[0151] V. Deep cooling + tempering

[0152] After the tempering in step IV is completed, the rod is immediately placed in a closed environment at -80°C, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 2 hours, and air-cooled to room temperature.

[0153] The alloy steel prepared in this comparative example has the following chemical composition:

[0154] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0155] After step V is completed, take a sample, observe the matrix structure of the alloy steel with an optical microscope and take photos. Figure 8 .

[0156] like Figure 8 As shown, after the "910°C quenching + tempering" treatment process of this comparative example, the original austenite grain size of the alloy steel reaches about level 10.

[0157] Comparative Example 4 A kind of alloy steel

[0158] The alloy steel of this comparative example is prepared by the following steps:

[0159] I. Smelting

[0160] The same raw materials as those in Example 1 were smelted in a vacuum induction furnace to obtain a 300×300 mm steel billet;

[0161] II. Forging

[0162] The steel billet is heated to 1050-1150°C for forging with a reduction ratio of 0.7 to obtain a φ90mm bar;

[0163] III. Carburizing

[0164] Transfer the rods into the carburizing furnace and carburize at 920±10℃ for 25h;

[0165] IV. Primary quenching

[0166] After carburizing, the bar is immersed in vacuum quenching oil at about 40°C for a quenching process;

[0167] V. Secondary quenching + tempering

[0168] Take the bar out of the vacuum quenching oil, heat it to 880±20℃, keep it warm for 45min, immerse it in vacuum quenching oil for the second quenching; then heat it to 180±10℃, keep it warm for 2h;

[0169] VI. Deep cooling + tempering

[0170] After the tempering in step V is completed, the rod is immediately placed in a closed environment at -80±10℃, kept warm for 2 hours, taken out, and naturally heated to room temperature; then heated to 180±10℃, kept warm for 2 hours, and air-cooled to room temperature.

[0171] The alloy steel prepared in this embodiment has the following chemical composition:

[0172] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0173] Comparative Example 5 A kind of alloy steel

[0174] The preparation steps and process conditions are basically the same as those in Example 1, except that the raw materials in step I are different from those in Example 1. The alloy steel prepared by this ratio finally has the following chemical composition:

[0175] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.10%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0176] Comparative Example 6 A kind of alloy steel

[0177] The preparation steps and process conditions are basically the same as those of Comparative Example 1, except that the raw materials in step I are different from those in Comparative Example 1. Finally, the alloy steel prepared in this comparative example has the following chemical composition:

[0178] C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.20%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

[0179] Test Case Alloy steel performance measurements of various embodiments and comparative examples

[0180] The hardness, tensile strength, yield strength, impact energy, elongation after fracture and reduction of area of ​​the alloy steel of each embodiment and comparative example were measured respectively. The results are shown in Table 1.

[0181] Table 1 Alloy steel performance test results of the embodiment and comparative example

[0182]

[0183] The data in Table 1 show:

[0184] 1. Although the alloy steel compositions of Examples 1 to 3 and Comparative Examples 1 to 4 are the same, the different processing techniques lead to different properties of the alloy steels:

[0185] 1) Comparative Examples 1 to 3 were subjected to only one quenching + tempering and one deep cooling + tempering, and the strength performance indicators of the alloy steel, such as hardness and tensile strength, were not as good as those of the alloy steels of Examples 1 to 3;

[0186] 2) Compared with Examples 1 to 3, as the quenching temperature increases, the strength performance indicators of the alloy steel, such as hardness and tensile strength, are improved accordingly;

[0187] 3) After the alloy steel is subjected to a primary quenching at about 920°C → a secondary quenching at 880°C plus low-temperature tempering → a deep cooling, the alloy steel of Comparative Example 4 is further improved in various properties compared with Comparative Examples 1 to 3, and the content of retained austenite is reduced from 4.75% in the initial quenching state to 0.82%;

[0188] 4) Compared with Example 1 and Comparative Example 4, after two quenching and two deep cooling, the austenite retention rate of the alloy steel is reduced from 0.82% to 0.79%, the surface hardness is increased from 44.4HRC to 45.2HRC, and the tensile strength is increased from 1440Mpa to 1450Mpa.

[0189] 2. The alloy steel preparation process conditions of Example 1 and Comparative Examples 5-6 are the same, but the chemical composition of the alloy steel is different due to different feed materials: the V element content of the alloy steel of Comparative Example 5 is 0.1%, the V element content of Comparative Example 6 is 0.2%, and the V element content of Example 1 is 0.4%. With the increase of the V element content, the surface hardness and impact energy of the alloy steel are significantly improved.

[0190] In summary, through reasonable component design and reasonable heat treatment process, the present application provides an alloy steel with well-matched hardness, strength and impact properties that can be used for axle box bearings of high-speed EMUs.

Claims

1. An alloy steel comprising the following components in weight percentage: C: 0.19~0.23%, Si: 0.25~0.40%, Mn: 0.50~0.70%, Cr: 0.45~0.65%, Ni: 0.16~2.00%, Mo: 0.20~0.30%, V: 0.25~0.40%, Nb: 0.02~0.04%, P≤0.015%, S≤0.005%, and the rest are Fe and unavoidable impurities.

2. The alloy steel according to claim 1, characterized in that: The composition includes the following weight percentages: C: 0.20~0.22%, Si: 0.25~0.40%, Mn: 0.55~0.65%, Cr: 0.50~0.60%, Ni: 1.70~1.90%, Mo: 0.20~0.30%, V: 0.35~0.40%, Nb: 0.03~0.04%, P≤0.010%, S≤0.002%, and the rest are Fe and unavoidable impurities.

3. The alloy steel according to claim 1, characterized in that: The composition includes the following weight percentages: C: 0.20%, Si: 0.30%, Mn: 0.60%, Cr: 0.52%, Ni: 1.80%, Mo: 0.25%, V: 0.40%, Nb: 0.03%, P ≤ 0.005%, S ≤ 0.002%, and the rest are Fe and unavoidable impurities.

4. A method for preparing alloy steel, the method comprising the following steps: Smelting → forging → carburizing → first quenching → second quenching + tempering → first deep cooling + tempering → second deep cooling + tempering; Among them, Forging: The steel billet is forged at 1050-1150°C to obtain bars; Carburizing: Carburizing the bar in a conventional manner; Primary quenching: quenching temperature 920±20℃, oil quenching; Secondary quenching + tempering: quenching temperature 880±20℃, heat preservation 40~60min, oil quenching, tempering temperature 180±10℃; One deep cooling + tempering: deep cooling temperature -80±10℃, keep warm for 1~2h; tempering temperature 180±20℃, keep warm for 1~2h; then air cool to room temperature; Secondary deep cooling + tempering: deep cooling temperature -80±10℃, keep warm for 1~2h; tempering temperature 180±20℃, keep warm for 1~2h; then air cool to room temperature.

5. The preparation method according to claim 4, comprising the steps of: I. Smelting Smelting the raw materials to obtain steel billets; II. Forging The steel billet is heated to 1050-1150°C for forging with a reduction ratio of 0.5-0.8, and naturally cooled to room temperature to obtain a bar; III. Carburizing The bar is carburized at 920±10℃ for 24-26h; IV. Primary quenching After carburizing, the bar is immediately immersed in vacuum quenching oil for a quenching process; V. Secondary quenching + tempering Heat the bar to 880±20℃, keep it warm for 45min, and then immediately immerse it in vacuum quenching oil for a second quenching; then heat it to 180±10℃, and keep it warm for 1~2h; VI. One deep cooling + tempering After step V is completed, the rod is immediately placed in a sealed environment at -80±10°C, kept warm for 1h to 2h, taken out, and naturally heated to room temperature; then heated to 180±10°C, kept warm for 1 to 2h, and air-cooled to room temperature; VII. Secondary deep cooling + tempering The rod that has been cooled to room temperature is placed in a closed environment at -80±10℃ again, kept warm for 1h~2h, taken out, and naturally heated to room temperature; then heated to 180±10℃, kept warm for 1~2h, and air-cooled to room temperature.

6. The preparation method according to claim 5, characterized in that: In the step II, the reduction ratio is 0.

7.

7. The preparation method according to claim 5, characterized in that: In the steps IV and V, the temperature of the vacuum quenching oil is 30-50°C, preferably 35-45°C.

8. An alloy steel prepared according to the preparation method according to any one of claims 4 to 5, and the chemical composition of the alloy steel is defined as any one of claims 1 to 3.

9. Use of the alloy steel according to any one of claims 1 to 3 or the alloy steel according to claim 8 in the preparation of axle box bearings for high-speed trains.

10. An axle box for a high-speed train, wherein the bearings are made of the alloy steel described in any one of claims 1 to 3 or claim 8.

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