A rapid spheroidizing annealing process for GCr15 bearing steel initially organized as martensite

Abstract

The application discloses a rapid spheroidizing annealing process of GCr15 bearing steel with initial martensite structure. The process comprises the following steps: step one, high-temperature tempering treatment: the GCr15 bearing steel with initial martensite structure is placed into a 680-720 DEG C electric resistance furnace, and is kept at the temperature for 15-90 min; step two, isothermal spheroidizing annealing treatment: the GCr15 bearing steel treated in step one is directly heated to 760-800 DEG C and kept for 10-60 min, so that incomplete austenitization is caused; then, the GCr15 bearing steel is cooled to 680-720 DEG C, kept for 30-120 min, and is furnace-cooled to 650 DEG C and discharged; and finally, the GCr15 bearing steel is air-cooled. The application has the advantages of high production quality, high energy utilization rate and high production efficiency.

Description

Technical Field

[0001] This invention relates to the field of heat treatment of bearing steel, and particularly to a rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure. Background Technology

[0002] Bearing steel possesses excellent wear resistance, high strength, and high hardness, making it widely used in aerospace, railway, and automotive industries. However, its high carbon content results in poor toughness and plasticity. Spheroidizing annealing can improve the machinability and toughness of bearing steel, while also preparing the microstructure for quenching. Refining the size of carbide and austenite grains is beneficial for improving the rolling contact fatigue life of bearing steel. Traditional spheroidizing annealing processes are divided into continuous spheroidizing annealing, isothermal spheroidizing annealing, and periodic spheroidizing annealing. Continuous spheroidizing annealing requires more than 20 hours, isothermal spheroidizing annealing generally requires 10-16 hours, while periodic spheroidizing annealing is difficult to use industrially due to its numerous steps. Therefore, traditional annealing processes often require long isothermal holding times (more than 10 hours) during spheroidizing annealing heat treatment, resulting in poor spheroidization rate, coarse carbides, and uneven distribution, leading to low production efficiency and high energy consumption.

[0003] Initial microstructure is one of the important factors affecting spheroidizing annealing. The initial microstructure of GCr15 before spheroidizing annealing is typically lamellar pearlite, obtained through air cooling after rolling or forging (resulting in a microstructure of lamellar pearlite and network cementite) followed by normalizing (to eliminate network cementite). Currently, there is considerable research on rapid spheroidizing annealing processes for GCr15 bearing steel with an initial microstructure of lamellar pearlite. For example, patent CN107058692A, "An Online Rapid Spheroidizing Annealing Process for GCr15 Bearing Steel After Hot Rolling," utilizes optimized spheroidizing treatment processes through reasonable selection of rolling temperature to control the precipitation of proeutectoid carbides during rolling via deformation induction, thus shortening the spheroidizing annealing time and improving energy efficiency. However, in actual forging or rolling processes, GCr15 bearing steel may produce non-equilibrium bainite or martensite. Some studies have found that using a non-equilibrium microstructure as the initial microstructure before spheroidization can shorten the spheroidization annealing time, eliminate the need for normalizing, and improve production efficiency. However, strict control of the cooling rate is required to prevent cracking of the parts. For example, patent CN114686655A invented a spheroidization annealing process for GCr15 bearing steel with an initial microstructure of medium-temperature bainitic structure. This process directly obtains a certain amount of undissolved carbides through bainitic transformation, and then performs isothermal spheroidization annealing using the principle of segregated eutectoid transformation, ultimately achieving online spheroidization of the carbides. However, its hardness after annealing is relatively high (28HRC≈270HB). Patent CN113337694A obtains a spheroidized annealed microstructure with fine granular carbides uniformly distributed on an equiaxed ferrite matrix by high-temperature tempering of ultra-high carbon bearing steel with a multiphase microstructure of nano-bainitic and martensite. However, its annealing time is still relatively long (4-8h). Therefore, this invention studies the spheroidizing annealing process for non-equilibrium martensite and designs a rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensite structure, which can obtain GCr15 bearing steel with ideal hardness in a shorter time. Summary of the Invention

[0004] This invention addresses the problem of non-equilibrium martensite structure appearing after forging or rolling of existing GCr15 bearing steel, and provides a rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic structure. This process combines short-time high-temperature tempering with isothermal spheroidizing annealing. The martensitic structure is pre-tempered at high temperature, causing a large number of carbides to precipitate, providing nucleation sites for subsequent segregated eutectoid transformation. Then, isothermal spheroidizing annealing further spheroidizes the structure, controlling the degree of austenitization and the amount of carbides by adjusting the austenitizing temperature and time. By adjusting the isothermal temperature and time, the precipitation and growth of carbides are controlled, ultimately obtaining a fine, spheroidized pearlite structure with ideal hardness. This achieves rapid spheroidizing annealing of GCr15 bearing steel with a martensitic initial structure. This invention, when applied to the field of bearing steel production technology, has advantages such as high production quality, high energy utilization, and high production efficiency.

[0005] To achieve the above objectives, the present invention is implemented according to the following technical solution:

[0006] A rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure, the process comprising the following steps:

[0007] Step 1, High-temperature tempering treatment: Place the GCr15 bearing steel with an initial martensitic structure into a resistance furnace at 680℃~720℃ and hold it at this temperature for 15~90min;

[0008] Step 2, Isothermal spheroidizing annealing: The GCr15 bearing steel treated in Step 1 is directly heated to 760℃~800℃ and held for 10~60min to induce incomplete austenitization; then cooled to 680℃~720℃, isothermally annealed for 30~120min, furnace cooled to 650℃ and then air cooled.

[0009] In the steps described, the equipment used in the process is a high-temperature box-type resistance furnace, and the diameter of the GCr15 steel wire is 8-12mm.

[0010] The GCr15 bearing steel contains the following components by mass percentage: C: 0.95%–1.10%, Cr: 1.30%–1.60%, Si: 0.15%–0.35%, Mn: 0.20%–0.40%, S≤0.02%, P≤0.02%, Ni≤0.30%, Cu≤0.25%, Mo≤0.10%, with the balance being Fe and unavoidable impurities.

[0011] The microstructure of the GCr15 bearing steel prepared by this invention is a fine pearlitic microstructure (i.e., a large number of fine carbides are distributed on a ferrite matrix), the spheroidization grade can reach level 2, and the average hardness is 195-205 HBW.

[0012] The essential features of this invention are:

[0013] The initial microstructure of conventional GCr15 bearing steel before spheroidizing annealing is lamellar pearlite. To shorten the spheroidizing annealing time, the effects of different initial microstructures on the spheroidizing annealing process and microstructure were investigated. Current technologies include a spheroidizing annealing process using medium-temperature bainite, which obtains a certain amount of undissolved carbides through a short-energy bainite transformation, followed by isothermal spheroidizing annealing using the principle of segregated eutectoid transformation, ultimately achieving online spheroidization of the carbides and improving the problem of lengthy spheroidizing annealing time.

[0014] This invention uses martensite as the initial microstructure before spheroidization. Martensite is supersaturated ferrite, and its microstructure is a metastable phase transformed from austenite through a diffusionless phase transformation. High-temperature tempering below Ac1 temperature allows a large number of carbides to precipitate and grow from the martensite in a short time, gradually transforming the microstructure into tempered sorbite. The large number of carbide particles pre-precipitated by high-temperature tempering transform into austenite and a large number of carbides when heated above Ac1. Then, it isothermally cooled below Ac1 temperature for a period of time to allow pearlite transformation. Utilizing the principle of segregated eutectoid transformation, the remaining large number of carbides in the microstructure at this time become nucleation sites for the precipitation of new carbides, causing the pearlite transformation to proceed in the form of segregated eutectoid transformation, rather than forming lamellar pearlite through co-growth, ultimately forming a fine spherical pearlite microstructure.

[0015] The beneficial effects of this invention are as follows:

[0016] 1. This invention employs a short-time high-temperature tempering + isothermal spheroidizing annealing process to spheroidize the martensitic structure of GCr15 bearing steel. The short-time high-temperature tempering precipitates a large number of carbides, which become nucleation sites for new carbide precipitation. The more nucleation sites there are, the easier it is for carbides to precipitate and spheroidize. This is beneficial for isothermal spheroidizing annealing, shortens the isothermal spheroidizing annealing time, accelerates the spheroidizing annealing process, and reduces the total spheroidizing annealing time to 2.5 hours. This shortens the production cycle (traditional spheroidizing annealing time is more than 10 hours), improves production efficiency, and reduces energy consumption.

[0017] 2. This process produces high-quality GCr15 bearing steel with fine, uniform, and rounded carbides in its microstructure. According to the spheroidizing annealing microstructure rating in the national standard (GB / T18254-2016), it can reach level 2, with a hardness of 195-205 HBW and a hardness not exceeding 207 HBW, which is within the range of GB / T18254-2016. This process can be used for spheroidizing annealing of non-equilibrium martensitic GCr15 bearing steel. It is suitable for spheroidizing annealing of blanks with small cross-sections, eliminating the need for normalizing treatment, shortening the annealing cycle, and saving energy. Attached Figure Description

[0018] Figure 1 This is a flowchart of the spheroidizing annealing method of the present invention;

[0019] Figure 2 This is a micrograph of the metallographic structure after processing in Example 1 of the present invention;

[0020] Figure 3 This is a micrograph of the processed metallographic structure of Example 2 of the present invention;

[0021] Figure 4 This is a micrograph of the metallographic structure after processing in Example 3 of the present invention;

[0022] Figure 5 This is a micrograph of the metallographic structure after processing in Comparative Example 1 of the present invention. Detailed Implementation

[0023] Specific embodiments of the present invention are given below. These specific embodiments are only used to further illustrate the present invention and do not limit the scope of protection of the claims of this application.

[0024] In this embodiment of the invention, GCr15 bearing steel is used as a 10mm diameter wire rod with the following composition: C 0.99%, Cr 1.44%, Si 0.22%, Mn 0.28%, S 0.007%, P 0.013%, Ni 0.06%, Cu 0.02%, Mo 0.01%, with the balance being Fe and unavoidable impurities. The GCr15 bearing steel is pretreated (austenitized at 850℃~1000℃ for 10~20min, followed by oil quenching) to obtain a martensitic structure, which is used as the initial structure before spheroidizing annealing in this embodiment of the invention. Then, spheroidizing annealing is performed. Specific spheroidizing annealing examples are as follows.

[0025] In the following embodiments, the resistance furnace used is the KSL-1500X-S high-temperature box resistance furnace produced by Hefei Kejing Materials Technology Co., Ltd., the metallographic microstructure is observed by an OLYMPUS-DSX510 metallographic microscope, and the hardness performance test is performed by a Shanghai Shangcai XHB-3000Z three-indenter digital display Brinell hardness tester.

[0026] In this embodiment of the invention, the national standard used to measure the spheroidization grade is the rating method in GB / T18254-2016 for high carbon chromium bearing steel.

[0027] Example 1

[0028] A rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure includes the following steps:

[0029] Step 1: High-temperature tempering treatment. Place the GCr15 bearing steel with an initial martensitic structure into a resistance furnace at 720℃ and hold it at this temperature for 30 minutes.

[0030] Step 2: Isothermal spheroidizing annealing treatment. The GCr15 bearing steel treated in Step 1 is directly heated to 770℃ and held for 30 minutes to induce incomplete austenitization; then cooled to 720℃ and isothermally annealed for 60 minutes, furnace cooled to 650℃ and then air cooled.

[0031] The metallographic structure of the GCr15 bearing steel prepared in this embodiment was observed using a Japanese OLYMPUS-DSX510 metallographic microscope. The obtained metallographic structure after spheroidization treatment is as follows: Figure 2 As shown, the carbide particles are dot-like and fine-grained pearlite, with no lamellar pearlite formation. According to the metallographic spheroidization rating in GB / T18254-2016 standard, it is grade 2, and the Brinell hardness is 200 HBW, which is within the range of GB / T18254-2016 standard. The improved spheroidizing annealing time is shortened to 2.5 hours.

[0032] Example 2

[0033] A rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure includes the following steps:

[0034] Step 1: High-temperature tempering treatment. Place the GCr15 bearing steel with an initial martensitic structure into a resistance furnace at 710℃ and hold it at this temperature for 30 minutes.

[0035] Step 2: Isothermal spheroidizing annealing treatment. The GCr15 bearing steel treated in Step 1 is directly heated to 770℃ and held for 30 minutes to induce incomplete austenitization; then cooled to 710℃ and isothermally annealed for 60 minutes, furnace cooled to 650℃ and then air cooled.

[0036] The metallographic structure of the GCr15 bearing steel prepared in this embodiment is as follows: Figure 3 As shown, the carbide particles are dot-like and fine-grained pearlite, with no lamellar pearlite formation. The spheroidization rating is 2, and the Brinell hardness is 203 HBW, which is within the range of GB / T18254-2016 standard.

[0037] Example 3

[0038] A rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure includes the following steps:

[0039] A spheroidizing annealing method for GCr15 bearing steel with an initial martensitic microstructure, the method comprising the following steps:

[0040] Step 1: High-temperature tempering treatment. Place the GCr15 bearing steel with an initial martensitic structure into a resistance furnace at 720℃ and hold it at this temperature for 30 minutes.

[0041] Step 2: Isothermal spheroidizing annealing treatment. The GCr15 bearing steel treated in Step 1 is directly heated to 780℃ and held for 30 minutes to induce incomplete austenitization. Then it is cooled to 710℃ and isothermal for 60 minutes. It is then furnace cooled to 650℃ and then air cooled.

[0042] The metallographic structure of the GCr15 bearing steel prepared in this embodiment is as follows: Figure 4As shown, the carbide particles are dot-like and fine-grained pearlite, with no lamellar pearlite formation. The spheroidization rating is 2, and the Brinell hardness is 205 HBW, which is within the range of GB / T18254-2016 standard.

[0043] Comparative Example 1

[0044] Using a conventional spheroidizing annealing process, GCr15 bearing steel with an initial microstructure of lamellar pearlite was placed in a resistance furnace at 790℃ and held for 20 minutes, then furnace cooled to 550℃ and removed from the furnace, and air cooled to room temperature.

[0045] The microstructure of the GCr15 bearing steel prepared in this comparative example after spheroidization treatment is as follows: Figure 5 As shown, most of the lamellar pearlite has been spheroidized, but a small amount of coarser lamellar pearlite has not been spheroidized. The spheroidization rating is 5, and the Brinell hardness is 215HBW, which is higher than the range of GB / T18254 2016 standard and does not meet the national standard application standard.

[0046] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

[0047] Matters not covered in this invention are common knowledge.

Claims

1. A rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure, characterized by: The process includes the following steps: Step 1, High-temperature tempering treatment: Place the GCr15 bearing steel with an initial martensitic structure into a resistance furnace at 680℃~720℃ and hold it at this temperature for 15~30 minutes. Step 2, Isothermal spheroidizing annealing: The GCr15 bearing steel treated in Step 1 is directly heated to 760℃~780℃ and held for 10~30 minutes to induce incomplete austenitization; then cooled to 680℃~720℃, isothermally annealed for 30~60 minutes, furnace cooled to 650℃ and then air cooled. The equipment used in the process is a high-temperature box-type resistance furnace; The diameter of the GCr15 steel wire is 8-12 mm; The GCr15 bearing steel contains the following components by mass percentage: C: 0.95%–1.10%, Cr: 1.30%–1.60%, Si: 0.15%–0.35%, Mn: 0.20%–0.40%, S≤0.02%, P≤0.02%, Ni≤0.30%, Cu≤0.25%, Mo≤0.10%, with the balance being Fe and unavoidable impurities.

2. The rapid spheroidizing annealing process for GCr15 bearing steel with an initial martensitic microstructure as described in claim 1, characterized in that: The resulting GCr15 bearing steel has a fine pearlitic structure, a spheroidization grade of 2, and an average hardness of 195~205 HBW.

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