Carburizing bearing steel and method for heat treating the same

Abstract

The present application relates to the technical field of machining heat treatment, in particular to a carburizing bearing steel and a heat treatment method thereof, wherein in the thickness direction of the carburizing bearing steel, the volume fraction of residual austenite at a distance of 0.1 mm from the surface of the carburizing bearing steel is 3-10%, the volume fraction of carbide is 20-35%, and the length of acicular martensite is 5-7 microns; the carbide with a diameter less than 1 micron accounts for 65-75% of the total number of carbides, and the carbide with a diameter less than 1 micron is spherical or granular, so that the carburizing bearing steel avoids excessive growth of surface carbide and has excellent corrosion resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of mechanical processing heat treatment, and particularly relates to a carburizing bearing steel and a heat treatment method thereof. BACKGROUND

[0002] As a new type of bearing and gear steel, high-alloy carburizing steel forms a gradient structure from the surface to the center after a series of heat treatments such as carburizing and quenching. The surface layer has high hardness, high wear resistance, and temperature resistance, while the center has good toughness and can withstand high load impact. Although the increase of surface carbon content helps to increase the content of surface carbide and significantly improve the surface hardness, high-alloy steel is prone to excessive residual austenite or coarse carbide, even network carbide under high-carbon concentration carburizing conditions, which reduces the hardness and fatigue strength. At the same time, too high carbon content also makes the material's corrosion resistance decrease sharply.

[0003] For key components such as gears and bearings, the morphology, distribution, size, and quantity of carburized carbides are the key to improving the service performance of the parts. High-alloy steel contains a large amount of carbide-forming elements with more than 30wt% alloy content. After carburizing, the carburized layer forms a large amount of carbides, and the optimization of carbide morphology becomes a big problem. Currently, the conventional heat treatment process of carburizing + quenching + low-temperature tempering is usually used to obtain martensite + carbide structure, so that the carbides are small, round and dispersed, the surface martensite structure is small, and the austenite content is reduced. However, this process is obviously not suitable for high-alloy carburizing steel. The existing carburizing heat treatment process for carburizing steel is carburizing + quenching + deep cooling + tempering. Chinese patent CN201811480006.8 discloses a low-pressure vacuum carburizing method for CSS-42L material, Chinese patent CN201811396268.6 discloses a nitriding method for third-generation carburizing steel after carburizing, and Chinese patent CN201611097539.9 discloses a softening method for the carburized region of CSS-42L gear steel after carburizing. The final workpiece still has strip-shaped, irregular, and even network carbides in the surface layer, so the effect of the carburizing process is not maximized, and the existing process lacks optimization of carbide structure.

[0004] In view of the problem that the surface hardness of high-alloy carburizing steel is improved by increasing the surface carbon concentration, but the surface carbide particles are excessively large and the corrosion resistance is reduced, the present application is proposed. The process improves the morphology and distribution of the carburized layer carbides and the organization composition, and at the same time realizes the spheroidization of the carbides and the reduction of the residual austenite content. The process parameters precisely control the local carburized layer to avoid the decrease of the center hardness, so as to maximize the hardness under the same carbon concentration or carburizing process, and avoid the excessive growth of surface carbides and the significant decrease of corrosion resistance.

[0005] Therefore, it is urgent to provide a high-alloy carburizing steel and a heat treatment method thereof which can avoid the excessive growth of surface carbides and improve the corrosion resistance. SUMMARY

[0006] The present application aims to solve the technical problem of how to provide a high-alloy carburizing steel and a heat treatment method thereof, which can avoid excessive growth of surface carbides and improve corrosion resistance.

[0007] To achieve the above-mentioned purpose, the present application provides a carburizing bearing steel, wherein the components and their weight percentages in the carburizing bearing steel are as follows:

[0008] The content of C is 0.10-0.16%;

[0009] The content of Cr is 13.75-15.00%;

[0010] The content of Co is 5.00-14.00%;

[0011] The content of Ni is 4.75-5.25%;

[0012] The content of Mo is 3.00-5.00%;

[0013] The content of Si is ≤0.05%;

[0014] The content of Mn is ≤0.05%;

[0015] The content of S is ≤0.01%;

[0016] The content of P is ≤0.015%;

[0017] The content of other impurity elements is ≤0.03% individually;

[0018] The total content of other impurity elements is ≤0.15%;

[0019] The balance is Fe;

[0020] The sum of the contents of Cr, Co, Mo and Ni is ≥25.00wt%;

[0021] In the thickness direction, at a distance of 0.1mm from the surface of the carburizing bearing steel, the volume fraction of residual austenite is 3-10%, the volume fraction of carbides is 20-35%, and the length of acicular martensite is 5-7μm;

[0022] Among them, the carbides with a diameter less than 1μm account for 65-75% of the total number of carbides, and the carbides with a diameter less than 1μm are spherical or granular.

[0023] The present application provides a heat treatment method of the above-mentioned carburizing bearing steel, wherein the heat treatment method comprises:

[0024] S1, carrying out carburizing treatment on the workpiece at 930-980 DEG C, the effective hardening layer depth of the carburized layer of the workpiece is 0.6-1.2 mm, the surface carbon concentration is 0.8-1.2 wt.%, the carburizing atmosphere is acetylene, after the carburizing is finished, the workpiece is cooled to 20-30 DEG C under the nitrogen gas pressure of 0.1-0.2 MPa, and the average cooling rate is 15-40 DEG C / min;

[0025] S2, placing the workpiece obtained in S1 into a vacuum heat treatment furnace to carry out vacuum spheroidizing;

[0026] S3, placing the workpiece obtained in S2 into an atmosphere quenching furnace to heat, taking out the workpiece after heating is finished and cooling under nitrogen gas;

[0027] S4, placing the workpiece obtained in S3 into a deep cooling box at-80~-120 DEG C to keep warm, and taking the workpiece to a forced air drying box to defrost after keeping warm is finished;

[0028] S5, placing the workpiece obtained in S4 into a vacuum heat treatment furnace to heat to 500-550 DEG C, keeping warm, taking out the workpiece after keeping warm is finished, and air cooling under vacuum environment;

[0029] S6, placing the workpiece obtained in S5 into a deep cooling box at-80~-120 DEG C to keep warm, and taking the workpiece to a forced air drying box to defrost after keeping warm is finished;

[0030] S7, placing the workpiece obtained in S6 into a vacuum heat treatment furnace to heat to 500-550 DEG C, keeping warm, taking out the workpiece after keeping warm is finished, and air cooling under vacuum environment, to obtain the carburizing bearing steel.

[0031] The beneficial effects of the present application are:

[0032] Compared with the prior art, the carburizing bearing steel provided by the present application has non-excessive growth of surface carbide and excellent corrosion resistance, the heat treatment method provided by the present application makes the surface layer rapidly precipitate carbide at a cooling rate greater than air cooling and less than quenching after the carburizing is finished, so that the growth of carbide is avoided; the spheroidizing process is added before quenching to make the surface layer carbide have sufficient time to precipitate and spheroidize again, so that the distribution of carbon elements is adjusted, the structure is prepared for quenching, the spheroidization of carbide and the reduction of residual austenite content are realized at the same time, the process parameters accurately control the local carburized layer, and the hardness of the core is prevented from decreasing. The process improves the morphology distribution and organizational structure of the carburized layer carbide, and achieves the purpose of maximizing the hardness under the same carbon concentration or carburizing process. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 It is a surface structure (×1000) schematic view of the embodiment 1 of the present application.

[0034] Figure 2 It is a surface structure (×1000) schematic view of the comparative example 1 of the present application.

[0035] Figure 3 The hardness gradient chart of Example 1 of the present application.

[0036] Figure 4 The hardness gradient chart of Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0037] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as exactly that endpoint. Any values that fall within common interpretations of the endpoints should be incorporated as if explicitly written herein. Every range recited is inclusive of the endpoints that are actually recited.

[0038] In the prior art, the effect of the carburizing process of the carburizing bearing steel is not maximized, and the carbide organization is not optimized.

[0039] In the present application, if the heat treatment process is improved, the carbide form of the carburizing bearing steel can be optimized and the corrosion resistance can be improved.

[0040] To achieve this goal, the present application attempts to optimize the organization structure and content of the carburizing bearing steel and the processing technology, and through the specific organization structure and content and heat treatment process, the above-mentioned purpose can be achieved.

[0041] The first aspect of the present application provides a carburizing bearing steel, wherein the components and their weight percentages in the carburizing bearing steel are as follows:

[0042] The content of C is 0.10-0.16%;

[0043] The content of Cr is 13.75-15.00%;

[0044] The content of Co is 5.00-14.00%;

[0045] The content of Ni is 4.75-5.25%;

[0046] The content of Mo is 3.00-5.00%;

[0047] The content of Si is ≤0.05%;

[0048] The content of Mn is ≤0.05%;

[0049] The content of S is ≤0.01%;

[0050] The content of P is ≤0.015%;

[0051] The content of other impurity elements is ≤0.03% individually;

[0052] The total content of other impurity elements is less than or equal to 0.15%;

[0053] The balance is Fe;

[0054] The sum of the contents of Cr, Co, Mo and Ni is greater than or equal to 25.00 wt%;

[0055] In the thickness direction, the volume fraction of residual austenite is 3-10% at a distance of 0.1 mm from the surface of the carburized bearing steel, the volume fraction of carbide is 20-35%, and the length of acicular martensite is 5-7 mu;

[0056] Among them, the carbides with a diameter less than 1 mu account for 65-75% of the total number of carbides, and the carbides with a diameter less than 1 mu are spherical or granular.

[0057] In the present application, the carburized bearing steel provided precipitates smaller carbides, the proportion of small carbide size increases, the proportion of carbides with a diameter less than 1 mu increases, the carbide morphology tends to be spherical and granular, the surface layer of the carburized bearing steel is refined, and through analysis of the XRD of the carburized bearing steel sample, it is found that the proportion of residual austenite on the surface layer decreases, the content of carbide increases, and the length of acicular martensite decreases from 3-grade martensite to 2-grade martensite.

[0058] According to the present application, the diameter of the spherical carbides with a diameter less than 1 mu is 0.1-0.8 mu;

[0059] The average aspect ratio of the granular carbides with a diameter less than 1 mu is 1-2:1.

[0060] According to the present application, the carburized bearing steel includes any one of CSS42, 15Cr14Co12Mo5NiW, 14Cr14Co13Mo4NiW.

[0061] The second aspect of the present application provides a heat treatment method of the above-mentioned carburized bearing steel, wherein the heat treatment method comprises:

[0062] S1, carburizing treatment is carried out on the workpiece at 930-980 DEG C, the effective hardening layer depth of the carburized layer of the workpiece is 0.6-1.2 mm, the surface carbon concentration is 0.8-1.2 wt.%, the carburizing atmosphere is acetylene, and after the carburizing is completed, the workpiece is cooled to 20-30 DEG C at a nitrogen gas pressure of 0.1-0.2 MPa, and the average cooling rate is 15-40 DEG C / min;

[0063] S2, the workpiece obtained in S1 is placed in a vacuum heat treatment furnace for vacuum spheroidizing;

[0064] S3, the workpiece obtained in S2 is placed in an atmosphere quenching furnace for heating, and after the heating is completed, the workpiece is taken out of the furnace and cooled by introducing nitrogen;

[0065] S4, the workpiece obtained in S3 is placed in a deep cooling box at -80~ -120 DEG C for heat preservation, after the heat preservation is completed, the workpiece is placed in a blast drying box for defrosting;

[0066] S5, the workpiece obtained in S4 is placed in a vacuum heat treatment furnace and heated to 500-550 DEG C, heat preservation, after the heat preservation is completed, the furnace is taken out, and air cooling is carried out in a vacuum environment;

[0067] S6, the workpiece obtained in S5 is placed in a deep cooling box at -80~ -120 DEG C for heat preservation, after the heat preservation is completed, the workpiece is placed in a blast drying box for defrosting;

[0068] S7, the workpiece obtained in S6 is placed in a vacuum heat treatment furnace and heated to 500-550 DEG C, heat preservation, after the heat preservation is completed, the furnace is taken out, and air cooling is carried out in a vacuum environment, to obtain the carburizing bearing steel.

[0069] In the application, S1 is a vacuum pulse carburizing process of multiple alternating strong carburization / diffusion of the workpiece, so that the surface carbon concentration is limited, wherein each pulse process is vacuum pumping first, then strong carburization by introducing acetylene, vacuum pumping again, and carbon diffusion by introducing nitrogen, which is adjusted according to the number, surface area and placement mode of the workpiece; the surface carbon concentration and carbon concentration gradient are adjusted by adjusting the strong carburization / diffusion time ratio, each pulse diffusion time / strong carburization time≥20, the time of the latter pulse≥the time of the former pulse, and the accelerated cooling process after carburizing avoids the growth of carbide and surface oxidation.

[0070] According to the application, the time of strong carburization by introducing acetylene is ≤120s.

[0071] According to the application, each pulse diffusion time / strong carburization time≥20, and the time of the latter pulse≥the time of the former pulse.

[0072] In the application, the spheroidizing process of S2 causes carbon atoms to precipitate and form fine and dispersed carbides, improves the morphology of carbides, and makes the carburized layer more uniform.

[0073] In the application, S3 is gas quenching, a large number of dispersed carbides exist in the carburized layer after spheroidizing treatment, which can hinder the growth of austenite grains, lay a foundation for obtaining good quenched structure and performance, promote the generation of acicular martensite, the dissolution of spherical carbides is more gentle, effectively inhibits the growth of austenite grains, reduces the overheating sensitivity of the material, and obtains finer martensite structure, the fine initial structure and uniform austenitizing make the thermal stress and organizational stress of each part of the workpiece smaller during quenching cooling, and the deformation and cracking risk of the workpiece is significantly reduced.

[0074] In this invention, the deep cooling and defrosting of S4 and S6, after spheroidizing pretreatment and uniform austenitization followed by quenching, result in more uniform martensite and more consistent composition and stability of the retained austenite. During the deep cooling process, the retained austenite can be transformed into martensite more synchronously and thoroughly, avoiding the problem of incomplete transformation caused by uneven microstructure.

[0075] In this invention, the high-temperature tempering of S5 and S7, and the spheroidized surface microstructure, make it easier to obtain fine acicular martensite after quenching and deep cooling. The precipitation and aggregation growth of carbides during tempering are slower and more uniform, resulting in finer and more uniform carbide particles distributed on the martensitic matrix. This microstructure is key to the excellent combination of strength and toughness.

[0076] According to the present invention, in S2, the vacuum environment is a vacuum degree of less than 20 Pa.

[0077] According to the present invention, in S2, the conditions for vacuum spheroidization include heating to 700-800°C, holding at that temperature in a vacuum environment for 2-4 hours, and then cooling to 600°C at a cooling rate of 3-5°C / min, followed by air cooling in a vacuum environment.

[0078] According to some preferred embodiments of the present invention, a second vacuum spheroidization is performed after S2, and the conditions for the second vacuum spheroidization are the same as those for the vacuum spheroidization in S2.

[0079] According to the present invention, in S3, the pressure of the nitrogen gas is 0.3-0.6 MPa.

[0080] According to the present invention, in S3, the heating conditions include: first heating to 500-600°C at a heating rate of 10-20°C / min, holding at that temperature for 30-90 min, and then heating to 1030-1070°C at a heating rate of 5-10°C / min, holding at that temperature for 30-90 min.

[0081] According to the present invention, in S3, the cooling conditions include: charging with nitrogen gas at 0.4-0.5 MPa, cooling at a rate of 25-50 °C / min, and cooling to 25-50 °C.

[0082] According to the present invention, in S4, the heat preservation time is 2-4 hours, and the defrosting conditions include: a defrosting temperature of 50-80°C and a defrosting heat preservation time of 20-40 minutes.

[0083] According to the present invention, in S6, the heat preservation time is 2-4 hours, and the defrosting conditions include: a defrosting temperature of 50-80°C and a defrosting heat preservation time of 20-40 minutes.

[0084] According to the present invention, in S5, the heat preservation time is 2-4 hours.

[0085] According to the application, the holding time in S7 is 2-4 h.

[0086] The cross-section gradient microhardness test method is detected according to GB / T 4340.1-2009 “Metallic Materials Vickers Hardness Test”. The Vickers hardness gradient test is carried out along the cross-section of the sample layer by a Vickers hardness tester, the load is 1 kg, the unit is HV, 25 points are selected for one test, three tests are carried out, and the average value is selected to ensure the accuracy of the data.

[0087] The hardened layer depth test method is detected according to GB / T9450-2005 “Determination and Checking of Hardened Layer Depth of Steel Carburizing Quenching”.

[0088] The metallographic structure test method is detected according to GB / T25744 “Steel Carburizing Quenching and Tempering Metallographic Test”.

[0089] The technical solutions of the present application will be further described in detail below in combination with the drawings and examples. Obviously, the examples described here are only part of the examples of the present application and are not used to limit the present application. Based on the examples in the present application, all other examples implemented by those skilled in the art without making creative improvements are within the protection scope of the present application.

[0090] Example 1

[0091] The test forging material is 15Cr14Co12Mo5Ni, and the weight percentage of each component is as follows: C: 0.15%; Cr: 14%; Co: 13%; Ni: 2%; Mo: 5%; Si: 0.02%; Mn: 0.03%; S: 0.005%; P: 0.001%; the content of each impurity element is ≤0.03%; the total content of other impurity elements is ≤0.15%; the balance is Fe; the total alloy content is 34wt%;

[0092] The heat treatment method is as follows:

[0093] S1, the workpiece is carburized at 980℃, the effective hardened layer depth of the carburized layer of the workpiece is 1.1mm, the surface carbon concentration is 0.9wt.%, the carburizing atmosphere is acetylene, the time for introducing acetylene for strong carburizing is ≤120s, the diffusion time / strong carburizing time of each pulse is ≥20, the time of the latter pulse is ≥ the time of the former pulse, after the carburizing is finished, the workpiece is cooled to 25℃ at a nitrogen gas pressure of 0.12MPa, and the average cooling rate is 35℃ / min.

[0094] S2, the workpiece obtained in S1 is placed in a vacuum heat treatment furnace, vacuumized to 10 Pa, heated to 760 DEG C at a heating rate of 150 DEG C / h, kept for 2 h, after the keeping, cooled to 600 DEG C at a cooling rate of 3 DEG C / min, rapidly cooled to room temperature by inputting 0.1 MPa nitrogen, heated to 760 DEG C at a heating rate of 150 DEG C / h, kept for 2 h, after the keeping, cooled to 600 DEG C at a cooling rate of 3 DEG C / min, rapidly cooled to room temperature by inputting 0.1 MPa nitrogen, and then taken out of the furnace.

[0095] S3, the workpiece obtained in S2 is placed in an atmosphere quenching furnace for heating, first heated to 600 DEG C at a heating rate of 15 DEG C / h, kept for 30 min, after the keeping, heated to a quenching temperature of 1070 DEG C at a heating rate of 10 DEG C / min, kept for 1 h at the quenching temperature, after the keeping, the workpiece is taken out of the heating chamber and pushed into a quenching cooling chamber, 0.5 MPa nitrogen is filled in, and the average cooling rate is 40 DEG C / min, and the workpiece is cooled to 25 DEG C, and the cooling is stopped.

[0096] S4, after the quenching in S3, the workpiece quenched in the atmosphere is placed in a deep cooling box, liquid nitrogen is inputted for cold treatment, the cold treatment temperature is -80 DEG C, and the workpiece is kept for 2 h after reaching the cold treatment temperature; after the deep cooling, the workpiece is placed in a 50 DEG C air blowing drying box for defrosting for 30 min.

[0097] S5, the defrosted workpiece obtained in S4 is taken into a vacuum heat treatment furnace for high temperature tempering, the furnace temperature is 500 DEG C, the keeping time is 2 h, after the tempering, the workpiece is taken out of the furnace and air cooled to 25 DEG C in a vacuum environment.

[0098] S6, the tempered workpiece obtained in S5 is subjected to a secondary deep cooling-defrosting process, the process and the specific parameters are according to step S4.

[0099] S7, the secondary deep cooling-defrosting workpiece obtained in S6 is subjected to a secondary tempering process, the process and the specific parameters are according to step S5, and the carburized bearing steel is obtained.

[0100] In the thickness direction of the carburized bearing steel, the volume ratio of residual austenite at a distance of 0.1 mm from the surface is 5%, the volume content of carbide is 30%, and the length of acicular martensite is 5 um;

[0101] Among them, the carbides with a diameter less than 1 um account for 75% of the total number of carbides, the carbides with a diameter less than 1 um are spherical or granular, the diameter of the spherical carbides with a diameter less than 1 um is 0.1-0.8 um;

[0102] The average aspect ratio of the granular carbides with a diameter less than 1 um is 1:1.

[0103] The maximum surface hardness is 780 HV.

[0104] Example 2

[0105] The heat treatment method according to Example 1, except that in step S1, the workpiece is carburized at 930℃, the effective hardened layer depth of the carburized layer of the workpiece is 0.6mm, the surface carbon concentration is 0.8wt.%, the carburizing atmosphere is acetylene, and after the carburizing is completed, the workpiece is cooled to 20℃ at a nitrogen gas pressure of 0.1MPa, and the cooling rate is 15℃ / min.

[0106] The carburized bearing steel prepared has a residual austenite ratio of 10% at a distance of 0.1mm from the surface in the thickness direction, a carbide volume content of 20%, and a needle-like martensite length of 7μm;

[0107] wherein the carbides with a diameter less than 1μm account for 65% of the total number of carbides, the carbides with a diameter less than 1μm are spherical or granular, the diameter of the spherical carbides with a diameter less than 1μm is 0.1-0.8μm;

[0108] The average aspect ratio of the granular carbides with a diameter less than 1μm is 1.2:1.

[0109] The maximum surface hardness is 740HV.

[0110] Example 3

[0111] The heat treatment method according to Example 1, except that in step S1, the workpiece is carburized at 930℃, the effective hardened layer depth of the carburized layer of the workpiece is 1.0mm, the surface carbon concentration is 1.2wt.%, the carburizing atmosphere is acetylene, and after the carburizing is completed, the workpiece is cooled to 30℃ at a nitrogen gas pressure of 0.2MPa, and the cooling rate is 40℃ / min.

[0112] The carburized bearing steel prepared has a residual austenite ratio of 8% at a distance of 0.1mm from the surface in the thickness direction, a carbide volume content of 28%, and a needle-like martensite length of 6μm;

[0113] wherein the carbides with a diameter less than 1μm account for 68% of the total number of carbides, the carbides with a diameter less than 1μm are spherical or granular; the diameter of the spherical carbides with a diameter less than 1μm is 0.1-0.8μm;

[0114] The average aspect ratio of the granular carbides with a diameter less than 1μm is 1.6:1.

[0115] The maximum surface hardness is 805HV.

[0116] Example 4

[0117] According to the heat treatment method of Example 1, except that step S2 is that the workpiece obtained in S1 is placed in a vacuum heat treatment furnace, vacuumized, heated to 760℃ at a heating rate of ≤150℃ / h, kept for 2h, after the end of keeping, cooled to 600℃ at a cooling rate of 3℃ / min, nitrogen gas of 0.1MPa is introduced to rapidly cool to room temperature, and then taken out of the furnace. No secondary spheroidization occurs.

[0118] The prepared carburized bearing steel has a residual austenite ratio of 7% at a distance of 0.1mm from the surface in the thickness direction, a carbide volume content of 25%, and a length of acicular martensite of 5μm;

[0119] The carbides with a diameter less than 1μm account for 70% of the total number of the carbides, and the carbides with a diameter less than 1μm are spherical or granular.

[0120] The length-width ratio of the granular carbides with a diameter less than 1μm is 1.3:1.

[0121] The maximum surface hardness is 760HV.

[0122] Comparative Example 1

[0123] According to the heat treatment method of Example 1, except that step S2 is not performed.

[0124] The prepared carburized bearing steel has a residual austenite ratio of 15% at a distance of 0.1mm from the surface in the thickness direction, a carbide volume content of 20%, and a length of acicular martensite of 7μm;

[0125] The carbides with a diameter less than 1μm account for 50% of the total number of the carbides, and the carbides with a diameter less than 1μm are spherical or granular.

[0126] The length-width ratio of the granular carbides with a diameter less than 1μm is 2.1:1.

[0127] The maximum surface hardness is 750HV.

[0128] Comparative Example 2

[0129] According to the heat treatment method of Example 1, except that in step S1, the workpiece is carburized at 860℃, the effective hardening layer depth of the carburized layer of the workpiece is 0.5mm, the surface carbon concentration is 0.6wt.%, the carburizing atmosphere is acetylene, and after the end of carburization, nitrogen gas is not introduced, but the workpiece is directly cooled to 25℃ in a vacuum environment.

[0130] The prepared carburized bearing steel has a residual austenite ratio of 20% at a distance of 0.1mm from the surface in the thickness direction, a carbide volume content of 18%, and a length of acicular martensite of 10μm;

[0131] wherein the total number of carbides having a diameter less than 1 μm is 45%, and the carbides having a diameter less than 1 μm are spherical or granular;

[0132] The average aspect ratio of the granular carbides having a diameter less than 1 μm is 3:1.

[0133] The maximum surface hardness is 680 HV.

[0134] Comparative Example 3

[0135] The heat treatment method of Example 1 is followed, except that in step S1, the workpiece is carburized at 1050°C, the effective hardening layer depth of the carburized layer of the workpiece is 1.1 mm, the surface carbon concentration is 1.2 wt.%, the carburizing atmosphere is acetylene, and after the carburization is completed, the workpiece is cooled to 25°C at a nitrogen gas pressure of 0.4 MPa, and the average cooling rate is 60°C / min.

[0136] The carburized bearing steel produced has a residual austenite ratio of 15% at a distance of 0.1 mm from the surface in the thickness direction, a carbide volume content of 32%, and a needle-like martensite length of 8 μm;

[0137] wherein the total number of carbides having a diameter less than 1 μm is 40%, the carbides having a diameter less than 1 μm are spherical or granular, and reticular carbides are present;

[0138] The average aspect ratio of the granular carbides having a diameter less than 1 μm is 2:1;

[0139] The reticular carbides exhibit a large number of carbides having an aspect ratio of ≥3, and are connected.

[0140] The maximum surface hardness is 740 HV.

[0141] The electrochemical corrosion performance of the surfaces of the materials of Examples 1-4 and Comparative Examples 1-3 is tested and characterized using polarization curves and alternating current impedance spectroscopy. The electrolyte is selected to be a 3.5 wt.% NaCl aqueous solution. Both tests use a conventional three-electrode system. The reference electrode is selected to be a saturated calomel electrode, the auxiliary electrode is selected to be a platinum sheet electrode, and the material to be tested is used as the working electrode. When the material produced in Example 1 of the present application is used, the corrosion potential of the carburized layer is increased from -0.53253 V of Comparative Example 1 to -0.40728 V of Example 1, and the corrosion current density also changes, decreasing monotonically from 3.3 x 10 -5 A / cm 2 to 1.1 x 10 -5 A / cm 2The high and low of the corrosion potential reflects the difficulty of the material corrosion, and the size of the corrosion current reflects the speed of the material corrosion rate. Generally speaking, the greater the corrosion potential, the smaller the corrosion current density, and the better the corrosion resistance of the material. Through experimental comparison and corrosion image observation, it can be found that the spheroidization treatment makes the final structure more uniform, reduces the phenomenon of local serious corrosion, at the same time, replaces the network carbide with dispersed carbide, eliminates the possibility of network carbide as a fast corrosion channel, reduces the risk of intergranular corrosion, and prevents the risk of brittle fracture or surface spalling caused by rapid corrosion along the network lines. Therefore, the corrosion performance deterioration risk can be effectively reduced under the premise of improving the hardness by the present application.

[0142] Drawings Figure 1 and Figure 2 It is shown that the present application provides a carburizing bearing steel, and the precipitated layer is smaller in size, the proportion of small carbide size is increased, the proportion of carbide with a diameter less than 1 μm is increased from 50% to 70%, the carbide morphology tends to be spherical granular, the surface XRD of the samples of the two processes is analyzed, and it is found that the proportion of residual austenite in the surface layer is reduced from 15% to 5%, the carbide content is increased from 20% to 30%, and the length of acicular martensite is reduced from 7 μm of 3-grade martensite to 5 μm of 2-grade martensite. Figure 3 The maximum hardness of the surface in the comparative example 1 is 750 HV, and the surface hardness of the workpiece treated by the present application is increased by 30 HV, the effective hardness of the carburized layer is increased, and the quality of the heat treatment of the 15Cr14Co12Mo5Ni high alloy carburizing steel is greatly improved. Figure 4 The maximum hardness of the surface in the comparative example 1 is 750 HV, and the surface hardness of the workpiece treated by the present application is increased by 30 HV, the effective hardness of the carburized layer is increased, and the quality of the heat treatment of the 15Cr14Co12Mo5Ni high alloy carburizing steel is greatly improved.

[0143] Through the comparison of the embodiment and the comparative example, it can be seen that the heat treatment method provided by the present application optimizes the heat treatment process of the high alloy carburizing steel carburized layer carbide morphology, so as to realize the dispersed distribution of the spherical carbide in the carburized layer and improve the carburizing heat treatment effect.

[0144] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art within the technical range disclosed by the present application can make equivalent replacement or change according to the technical scheme and the inventive concept of the present application, which should be covered within the protection scope of the present application.

Claims

1. A carburized bearing steel, characterized in that, The components and their weight percentages in the carburized bearing steel are as follows: The C content is 0.10-0.16%; The Cr content is 13.75-15.00%; The Co content is 5.00-14.00%; The Ni content is 4.75-5.25%; The Mo content is 3.00-5.00%; Si content ≤ 0.05%; Mn content ≤ 0.05%; S content ≤ 0.01%; P content ≤ 0.015%; The content of other impurity elements is ≤0.03%; The total content of other impurity elements is ≤0.15%; The balance is Fe; The sum of the contents of Cr, Co, Mo, and Ni is ≥25.00 wt%; In the thickness direction, at a distance of 0.1 mm from the surface of the carburized bearing steel, the volume fraction of retained austenite is 3-10%, the volume fraction of carbides is 20-35%, and the length of acicular martensite is 5-7 μm. Among them, carbides with a diameter of less than 1 μm account for 65-75% of the total number of carbides, and the carbides with a diameter of less than 1 μm are spherical or granular.

2. The carburized bearing steel according to claim 1, characterized in that, The diameter of the spherical carbides with a diameter of less than 1 μm is 0.1-0.8 μm; The average aspect ratio of the granular carbides with a diameter of less than 1 μm is 1-2:

1.

3. The carburized bearing steel according to claim 1, characterized in that, The carburized bearing steel includes any one of CSS42, 15Cr14Co12Mo5NiW, and 14Cr14Co13Mo4NiW.

4. A heat treatment method for carburized bearing steel according to any one of claims 1-3, characterized in that, The heat treatment method includes: S1. Carburize the workpiece at 930-980℃. The effective hardened layer depth of the carburized layer is 0.6-1.2mm, the surface carbon concentration is 0.8-1.2wt.%, the carburizing atmosphere is acetylene, and after carburizing, cool it to 20-30℃ with nitrogen pressure of 0.1-0.2MPa. The average cooling rate is 15-40℃ / min. S2. Place the workpiece obtained in S1 in a vacuum heat treatment furnace with a vacuum degree of less than 20 Pa for vacuum spheroidization. S3. Place the workpiece obtained in S2 into an atmosphere quenching furnace and heat it. After heating, remove it from the furnace and cool it by passing nitrogen gas through it. S4. Place the workpiece obtained in S3 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting. S5. Place the workpiece obtained in S4 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After the holding period, remove it from the furnace and air cool it in a vacuum environment. S6. Place the workpiece obtained in S5 in a cryogenic chamber at -80~-120℃ for heat preservation. After the heat preservation is completed, place the workpiece in a forced-air drying oven for defrosting. S7. Place the workpiece obtained in S6 in a vacuum heat treatment furnace and heat it to 500-550℃. Hold it at that temperature. After holding, remove it from the furnace and air cool it in a vacuum environment to obtain the carburized bearing steel.

5. The heat treatment method according to claim 4, characterized in that, In S2, the conditions for vacuum spheroidization include heating to 700-800℃, holding at that temperature in a vacuum environment for 2-4 hours, then cooling to 600℃ at a cooling rate of 3-5℃ / min, and air cooling in a vacuum environment.

6. The heat treatment method according to claim 4, characterized in that, A second vacuum spheroidization is performed after S2, and the conditions for the second vacuum spheroidization are the same as those for S2.

7. The heat treatment method according to claim 4, characterized in that, In S3, the pressure of the nitrogen gas is 0.3-0.6 MPa.

8. The heat treatment method according to claim 4, characterized in that, In S3, the heating conditions include: first heating to 500-600℃ at a heating rate of 10-20℃ / min, holding at that temperature for 30-90min, and then heating to 1030-1070℃ at a heating rate of 5-10℃ / min, holding at that temperature for 30-90min. In S3, the cooling conditions include: charging with nitrogen gas at 0.4-0.5 MPa, a cooling rate of 30-40 °C / min, and cooling to 25-50 °C.

9. The heat treatment method according to claim 4, characterized in that, In S4, the heat preservation time is 2-4 hours, and the defrosting conditions include: defrosting temperature of 50-80℃ and defrosting heat preservation time of 20-40 minutes. In S6, the heat preservation time is 2-4 hours, and the defrosting conditions include: defrosting temperature of 50-80℃ and defrosting heat preservation time of 20-40 minutes.

10. The heat treatment method according to claim 4, characterized in that, In S5, the heat preservation time is 2-4 hours; In S7, the heat preservation time is 2-4 hours.

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