A high-toughness backing bearing steel based on rare earth microalloying and its composite heat treatment process

Abstract

This invention provides a high-toughness backing bearing steel based on rare-earth microalloying and its composite heat treatment process, belonging to the field of bearing steel heat treatment and green manufacturing technology. The composite heat treatment process sequentially includes: performing rare-earth microalloying treatment and sulfur content control on molten steel to obtain a rare-earth microalloyed steel billet; forging to obtain a backing bearing ring blank; performing two-phase region critical spheroidizing annealing to obtain a first intermediate billet; performing austenitization and oil quenching to obtain a second intermediate billet; performing rare-earth induced micro-stress tempering to obtain a third intermediate billet; and finally applying magnetic field-assisted low-temperature long-aging treatment to obtain the high-toughness backing bearing steel. This invention controls the morphology of manganese sulfide through rare-earth microalloying, combines it with two-phase region annealing to eliminate grain boundary chain carbides, and utilizes alternating magnetic fields and rare-earth hydrogen traps to thoroughly capture diffused hydrogen and relax residual stress. This significantly improves the subsurface's resistance to whitening crack initiation and propagation while maintaining the high hardness of the bearing ring.

Description

Technical Field

[0001] This invention belongs to the field of bearing steel heat treatment and green manufacturing technology, specifically relating to a high-toughness backing bearing steel based on rare earth microalloying and its composite heat treatment process. Background Technology

[0002] Backing bearings are load-bearing components in the support rolls of large rolling mills. Their rings are typically made from high-carbon chromium bearing steel through smelting, forging, and heat treatment. To meet the basic requirements of high load-bearing capacity and wear resistance during rolling, a widely used manufacturing process in the industry typically includes: high-purity smelting of molten steel to reduce oxygen content and obtain a cast billet with excellent inclusion rating; high-temperature forging of the cast billet to form a ring blank; and sequential spheroidizing annealing, oil quenching, and low-temperature tempering treatment of the blank. Through this process, a finished ring with a microstructure consisting of tempered martensite matrix with carbides distributed on it is obtained, and the surface hardness can generally reach HRC60.

[0003] Although the aforementioned conventional manufacturing processes can impart high basic hardness to bearing rings and effectively control the content of macroscopic inclusions in the steel, the service life of backed bearing rings still faces severe challenges under actual heavy-load service conditions. In the early stages of service, atypical early microcracks are highly susceptible to initiation on the subsurface layer of the ring raceway. The propagation of these microcracks eventually leads to block spalling failure, severely impacting the continuous and stable operation of the system. Furthermore, with the increasing emphasis on green manufacturing and energy conservation in the industrial sector, developing long-life, highly reliable bearing components has become a crucial step in achieving green technology upgrades for equipment.

[0004] Therefore, how to further optimize the manufacturing process and significantly improve the fatigue spalling resistance and overall service life of the backing bearing steel has become a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a high-toughness backing bearing steel based on rare earth microalloying and its composite heat treatment process.

[0006] To achieve the above objectives, the technical solution adopted by this invention is as follows: a composite heat treatment process for high-toughness backing bearing steel based on rare earth microalloying, comprising the following steps: S1: Rare earth microalloying treatment and sulfur content control are applied to molten steel to obtain rare earth microalloyed steel billets by casting. S2: Forging a rare earth microalloyed steel billet to obtain a backing bearing ring blank; S3: Perform two-phase critical spheroidizing annealing on the backing bearing ring blank to obtain the first intermediate blank; S4: Austenitize the first intermediate billet and oil quench it to obtain the second intermediate billet; S5: The second intermediate billet is subjected to rare earth-induced micro-stress tempering treatment to obtain the third intermediate billet; wherein, the heating temperature of the rare earth-induced micro-stress tempering treatment is 200℃ to 220℃, and the holding time is 2h to 4h. S6: Apply magnetic field-assisted low-temperature long-aging treatment to the third intermediate billet to obtain high-toughness backing bearing steel; wherein, the magnetic field-assisted low-temperature long-aging treatment is held at a constant temperature range of 120℃ to 140℃ for 8h to 12h, and an alternating magnetic field is applied throughout the treatment process, with a magnetic field strength of 0.1T to 0.3T and a frequency of 30Hz to 50Hz.

[0007] Preferably, the implementation process of step S1 includes, in sequence, adjusting the sulfur content of molten steel, vacuum degassing of molten steel, rare earth microalloying treatment, and casting of molten steel; wherein, adjusting the sulfur content of molten steel controls the mass fraction of sulfur in molten steel to 0.003% to 0.008%; the rare earth microalloying treatment feeds cerium-lanthanum mixed rare earth wire into molten steel, and controls the sum of the residual mass fractions of cerium and lanthanum in molten steel after wire feeding to be 0.012% to 0.025%.

[0008] Preferably, the mass ratio of cerium to lanthanum in the cerium-lanthanum mixed rare earth wire is 1:1 to 2:1; the absolute pressure inside the furnace is controlled below 67 Pa by vacuum degassing of the molten steel.

[0009] Preferably, the implementation process of step S2 includes austenitizing heating of the billet, hot forging of the billet, and air cooling after forging; wherein, the heating temperature for austenitizing heating of the billet is controlled at 1150℃ to 1200℃; and the final forging temperature for hot forging of the billet is controlled at not less than 850℃.

[0010] Preferably, the implementation process of step S3 includes two-phase region heat preservation, subcritical isothermal heat preservation and cooling after annealing in sequence; wherein, the heating temperature of the two-phase region heat preservation is 800℃ to 810℃, and the heat preservation time is 2h to 4h; the subcritical isothermal heat preservation cools the billet to 730℃ to 750℃ at a slow cooling rate of no more than 30℃ / h, and then holds it isothermally for 6h to 8h.

[0011] Preferably, the implementation process of step S4 includes quenching austenitizing and holding, and oil quenching and cooling in sequence; wherein, the heating temperature of quenching austenitizing and holding is 850℃ to 870℃, and the holding time is calculated as 1.5min to 2.0min per millimeter of effective wall thickness of the ring; the oil quenching and cooling control the quenching oil temperature to be between 50℃ and 70℃.

[0012] Preferably, the implementation process of step S5 includes tempering and heat preservation followed by air cooling after tempering; air cooling after tempering is continuous cooling in a room temperature air environment until the overall temperature of the billet drops to room temperature.

[0013] Preferably, the implementation process of step S6 includes pre-aging, magnetic field-assisted low-temperature aging, and post-aging cooling in sequence; in the pre-aging process, the third intermediate blank is arranged in the uniform magnetic field area of ​​the magnetic field generating device, and the distance between adjacent blanks is not less than 50mm; the direction of the alternating magnetic field is parallel to the axial direction of the ring.

[0014] Preferably, during the cooling process after aging, the alternating magnetic field is removed, and the billet is placed in a room temperature air environment to cool to room temperature.

[0015] Another technical solution provided by the present invention is a high-toughness backing bearing steel based on rare earth microalloying, which is prepared by the above-mentioned composite heat treatment process.

[0016] The high-toughness backing bearing steel composite heat treatment process based on rare earth microalloying provided by this invention overcomes the defects of weak grain boundary bonding and uneven residual stress distribution in the micro-region of the material under the framework of high-purity smelting and quenching and tempering processes by reconstructing the overall hot working and post-hot working phase transformation control process. It solves the problem of atypical white etching microcracks and subsequent blocky spalling failure induced by the synergistic effect of micro-inclusions and discontinuous brittleness of grain boundaries on the subsurface of the raceway in the early stage of service of hot continuous rolling mill support roll backing bearing rings. It has the following beneficial effects: 1. By implementing rare-earth microalloying treatment and sulfur content control in molten steel, the morphology and distribution of manganese sulfide are altered. This is combined with critical spheroidizing annealing in the two-phase region to eliminate discontinuous chain carbides at the original austenite grain boundaries. The synergistic effect of rare-earth-induced micro-stress tempering and magnetic field-assisted low-temperature long-term aging treatment is utilized to relax residual stress in the micro-regions and trap diffusing hydrogen atoms. This invention maintains the high hardness of the high-toughness backing bearing ring while blocking the path of crack initiation and propagation during service, thus improving the subsurface's resistance to whitening crack initiation and propagation.

[0017] 2. The second intermediate billet is heated and held at a specific temperature through rare earth-induced micro-stress tempering, which promotes the decomposition of the cryptocrystalline martensite matrix and the precipitation of carbides. During this process, supersaturated carbon atoms in the martensite diffuse and precipitate out, forming dispersed ε-carbides. The precipitation process releases the martensite lattice distortion energy, thereby reducing the microscopic internal stress and macroscopic residual stress within the matrix. Simultaneously, rare earth atoms that have segregated at grain boundaries, dislocations, and other crystal defects during the previous high-temperature hot working stage exert a stable dragging effect on the slip and climb of dislocations during tempering, hindering the recovery process of martensite lath boundaries and the coarsening of carbide particles. Compared to conventional low-temperature tempering processes, this invention achieves internal stress relaxation to improve brittleness while controlling the decrease in matrix hardness within a controlled range through the pinning effect of rare earth atoms, resulting in a third intermediate billet with reduced internal residual stress and stable hardness.

[0018] 3. A magnetic field-assisted low-temperature long-term aging treatment involves continuously holding the third intermediate billet at a specific isothermal range while applying an alternating magnetic field throughout, trapping and fixing residual diffusible hydrogen atoms in the steel. During the holding process, rare earth atoms, due to their chemical affinity with hydrogen, form deep-trap hydrogen-capturing sites. The simultaneously applied low-frequency alternating magnetic field couples with the spin magnetic moments of the rare earth atoms, generating microscale lattice strain and vibrations around them. The magnetic field-induced microscale lattice vibrations lower the activation barrier for the diffusion of neighboring hydrogen atoms, thereby accelerating the kinetic process of the directional enrichment and capture of dissolved hydrogen atoms into rare earth traps. Compared to conventional aging methods without magnetic field assistance, this invention overcomes the limitation that hydrogen atom migration is only controlled by thermal diffusion, ensuring that diffusible hydrogen atoms are fixed within rare earth-related traps. Simultaneously, the prolonged isothermal state promotes the relaxation of residual stress in the micro-regions, resulting in a reduced sensitivity of the final high-toughness backed bearing ring product to hydrogen-induced delayed fracture and white etching cracks.

[0019] 4. Spatially, the rare-earth-induced micro-stress tempering treatment and the magnetic field-assisted low-temperature long-term aging treatment work in two ways. First, the rare-earth-induced micro-stress tempering treatment regulates martensitic decomposition within the bulk phase. Then, by relaxing the excessively high quenching macroscopic stress, it activates the trapping efficiency of the already agglomerated rare-earth atoms, pre-setting a high-density and uniformly distributed irreversible rare-earth-related hydrogen trap group within the matrix network, thus solving the bulk homogeneity problem of hydrogen trap distribution density. The subsequent magnetic field-assisted low-temperature long-term aging treatment targets the pre-set hydrogen trap sites, intervening in microscale lattice vibrations through localized magnetostrictive coupling effects, overcoming the dynamic limitations of hydrogen atom migration to dispersed trap sites. In terms of timing, the rare-earth-induced micro-stress tempering treatment provides a site carrier for the hydrogen atom trapping process by pre-constructing a deep trapping network and stripping away the stress concentration driving force. The magnetic field-assisted low-temperature long-term aging treatment is closely integrated, using external field energy to press the remaining diffusible hydrogen atoms into the pre-set traps, thereby improving hydrogen trap construction and hydrogen capture efficiency. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Figure 1 This is a flow chart of the composite heat treatment process for high-toughness backing bearing steel based on rare earth microalloying. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0023] Unless otherwise specified, all raw materials described below may be commercially available or prepared using conventional methods in the art. Exemplary commercial sources include: cerium-lanthanum mixed rare earth wire (lanthanum-cerium metal wire, La35±3% / Ce65±3%, purity 99%), thiourea (analytical grade), high-purity argon (volume fraction ≥99.99%), high-purity nitrogen (volume fraction ≥99.99%), weakly reducing protective atmosphere (carbon monoxide volume fraction 3%, high-purity nitrogen gas volume fraction 97%), and isothermal quenching mineral oil (Marquench, suitable for working temperatures of 50°C to 70°C).

[0024] Application Overview: To address the issue of cracking and spalling of the raceway subsurface layer in bearing rings during the early stages of service, conventional processes are insufficient to remove diffusing hydrogen and residual stress introduced during manufacturing. Furthermore, the tips of manganese sulfide inclusions easily become hydrogen atom enrichment sites under cyclic shear stress. On the other hand, the lack of effective pinning in grain boundary regions leads to the formation of discontinuously distributed chain-like carbides during tempering, significantly weakening grain boundary bonding strength. Simultaneously, the interface between these chain-like carbides and elongated manganese sulfide inclusions preferentially initiates and rapidly propagates intergranular microcracks under the combined effects of impact stress waves and hydrogen diffusion. Based on this, and in line with the development trends of energy conservation, environmental protection, and green manufacturing, this invention proposes eliminating grain boundary chain-like carbides and improving the morphology of manganese sulfide inclusions, while thoroughly capturing internal diffusing hydrogen and relaxing microscopic residual stress. This effectively blocks the path of crack initiation and propagation during service, significantly extending the service life of the component.

[0025] This invention actively alters the morphology and distribution of manganese sulfide by combining rare earth microalloying with targeted control of sulfur content. It eliminates grain boundary chain carbides and forms a dispersed granular pre-structure by implementing critical spheroidizing annealing in the two-phase region. In the final heat treatment stage after quenching, rare earth-induced micro-stress tempering and magnetic field-assisted low-temperature long-aging treatment are introduced in synergy to achieve irreversible hydrogen atom trapping and sufficient relaxation of residual stress in the micro-region. This significantly improves the secondary surface layer's resistance to the initiation and propagation of white etching cracks while maintaining the high hardness of the bearing rings.

[0026] Exemplary method: like Figure 1 As shown, a composite heat treatment process for high-toughness backing bearing steel based on rare earth microalloying includes the following steps: S1: Rare earth microalloying treatment and sulfur content control are applied to molten steel to obtain rare earth microalloyed steel billets by casting. S2: Forging a rare earth microalloyed steel billet to obtain a backing bearing ring blank; S3: Perform two-phase critical spheroidizing annealing on the backing bearing ring blank to obtain the first intermediate blank; S4: Austenitize the first intermediate billet and oil quench it to obtain the second intermediate billet; S5: The second intermediate billet is subjected to rare earth-induced micro-stress tempering treatment to obtain the third intermediate billet; S6: Apply magnetic field-assisted low-temperature long-aging treatment to the third intermediate billet to obtain high-toughness backing bearing steel.

[0027] Below, each step will be explained in detail based on the design principles.

[0028] Step S1, based on the microstructural control requirements for the morphology and grain boundary bonding of manganese sulfide inclusions in the bearing rings, involves rare earth microalloying treatment and sulfur content regulation of the molten bearing steel. Specifically, the implementation process of step S1, according to the changes in the state of the molten steel system, includes adjusting the sulfur content of the molten steel, vacuum degassing of the molten steel, rare earth microalloying treatment, and casting of the molten steel.

[0029] During the final stage of refining in the ladle refining furnace, the sulfur content of the molten steel is adjusted to stabilize it within a preset range, thus completing the sulfur content adjustment. Specifically, the sulfur content is controlled to 0.003% to 0.008%, the temperature is maintained at 1550℃ to 1650℃, the argon stirring flow rate is 50L / min to 200L / min, and the stirring time is 10min to 30min. If the sulfur content is below 0.003%, there is insufficient sulfur available for manganese sulfide formation, hindering heterogeneous manganese sulfide nucleation control. If the sulfur content is above 0.008%, the total amount of manganese sulfide inclusions in the molten steel is too high, failing to eliminate the negative impact of inclusions on the matrix properties. By adjusting the sulfur concentration in the molten steel, a chemical potential environment is provided for the combination reaction of rare earth elements with sulfur and oxygen. This allows the sulfur in the molten steel to be in a range where it can selectively combine with rare earth elements and participate in the heterogeneous nucleation of manganese sulfide during solidification, thus preventing the segregation of sulfur at grain boundaries.

[0030] Vacuum degassing is performed on molten steel after sulfur content adjustment to remove gaseous impurities. After treatment, the vacuum environment is broken, restoring the molten steel system to atmospheric pressure. Specifically, the absolute pressure inside the furnace during vacuum treatment is controlled below 67 Pa, the vacuum holding time is 15 to 30 minutes, argon is used as the venting medium, and the molten steel temperature is controlled between 1530℃ and 1630℃ after venting. When the absolute pressure inside the furnace is higher than 67 Pa or the vacuum holding time is shorter than 15 minutes, the removal of gaseous impurities such as hydrogen and oxygen in the molten steel is insufficient; when the vacuum holding time is longer than 30 minutes, the temperature drop of the molten steel is too large, and the loss of alloying elements is accelerated. Using argon to vent the molten steel avoids secondary oxidation and gas absorption, ensuring the purity of the molten steel. By reducing the system environmental pressure, dissolved hydrogen, nitrogen, and other interstitial atoms, as well as free oxygen atoms, escape from the molten steel, reducing the content of gaseous impurities in the molten steel, reducing the ineffective combination of rare earth elements and excess free oxygen, and ensuring that rare earth elements can preferentially participate in the nucleation and regulation process of target sulfides.

[0031] For molten steel that has undergone vacuum degassing and void-breaking treatment, a cerium-lanthanum mixed rare earth wire is fed in to uniformly dissolve the rare earth elements in the molten steel, thus completing the rare earth microalloying treatment. Specifically, the mass ratio of cerium to lanthanum in the cerium-lanthanum mixed rare earth wire is 1:1 to 2:1, the wire feeding speed is 100 m / min to 300 m / min, the sum of the residual mass fractions of cerium and lanthanum in the molten steel after wire feeding is controlled at 0.012% to 0.025%, the argon gas stirring flow rate is 80 L / min to 250 L / min, the stirring time is 15 min to 25 min, and the molten steel is allowed to stand for 5 min to 10 min after stirring. When the sum of the residual mass fractions of cerium and lanthanum is less than 0.012%, the total amount of rare earth elements in the molten steel is insufficient, making it impossible to achieve the morphology control of manganese sulfide and the grain boundary segregation effect. When the sum of the residual mass fractions of cerium and lanthanum is greater than 0.025%, the total amount of rare earth oxide inclusions in the molten steel increases, negatively impacting the matrix properties. Wire feeding speed and stirring parameters are used to ensure the uniform distribution of rare earth elements in the molten steel and avoid localized rare earth element concentration segregation. The chemical affinity of rare earth elements cerium and lanthanum with oxygen and sulfur in molten steel is higher than that of manganese with sulfur. Rare earth atoms preferentially combine with oxygen and sulfur in the molten steel to form high-melting-point, thermodynamically stable rare earth oxysulfides. Rare earth atoms dissolved in molten steel can segregate towards grain boundaries during solidification, reducing the equilibrium segregation concentration of sulfur at the grain boundaries and improving the grain boundary bonding strength.

[0032] For molten steel that has undergone rare earth microalloying treatment, continuous casting or ingot casting processes are used to transform the molten steel from a liquid state to a solid as-cast structure, forming a rare earth microalloyed steel billet. The casting temperature is controlled between 1520℃ and 1580℃, the continuous casting speed is controlled between 0.8 m / min and 2.0 m / min, and the ingot demolding time is controlled between 24 and 48 hours after casting. When the casting temperature is below 1520℃, the molten steel lacks fluidity and is prone to casting defects; when the casting temperature is above 1580℃, the grain coarsening trend of the billet intensifies, and the incidence of internal porosity defects increases. The continuous casting speed and ingot demolding time parameters are used to ensure the uniformity of the solidification structure of the billet and avoid component segregation and crack defects. In the solid-liquid two-phase region at the solidification front of molten steel, the rare earth oxygen sulfides that precipitate first serve as heterogeneous nucleation cores, inducing the growth of precipitated manganese sulfide on the basis of these cores, causing the manganese sulfide to transform from a long strip or chain shape into a fine spindle or near-spherical shape.

[0033] For example, the complete implementation process of step S1 includes placing the molten bearing steel to be treated in a ladle refining furnace. Under the protection of an argon atmosphere, at the end of the refining process, the mass fraction of sulfur in the molten steel is adjusted through the alloy feeding system of the ladle refining furnace. At the same time, argon gas is introduced through the permeable bricks at the bottom of the ladle for stirring, thus achieving targeted control of the sulfur content in the molten steel. The molten steel with the sulfur content controlled is transferred with the ladle to a vacuum degassing chamber for vacuum degassing. After the treatment is completed, argon gas is introduced into the vacuum degassing chamber to break the vacuum and restore the molten steel system to atmospheric pressure. The molten steel with the vacuum degassing completed is transferred with the ladle to a wire feeding station. A cerium-lanthanum mixed rare earth wire is fed into the molten steel through a wire feeder. After the wire feeding is completed, argon gas is introduced through the permeable bricks at the bottom of the ladle for stirring. After stirring, the molten steel is allowed to stand in the ladle. The molten steel that has undergone rare earth microalloying treatment is transferred to the tundish of the continuous casting machine and continuously cast through the continuous casting crystallizer, or transferred to the ingot mold for static casting to obtain a rare earth microalloyed steel billet.

[0034] Through step S1, the molten steel is transformed into a rare earth microalloyed steel billet. Sulfides are mixed with manganese sulfide in the form of rare earth oxysulfides as nucleation cores. Trace amounts of rare earth atoms are enriched in the grain boundary region of the steel billet. Gas impurities and macrosegregation inside the steel billet are under control.

[0035] Based on the as-cast microstructure of the rare-earth microalloyed steel billet obtained in step S1, step S2 involves thermoplastic deformation treatment of the rare-earth microalloyed steel billet to obtain a formed billet that matches the outer contour of the bearing backing ring. Specifically, the implementation process of step S2 includes, in order of microstructure change, austenitizing heating of the steel billet, hot forging of the steel billet, and air cooling after forging.

[0036] Rare earth microalloyed steel billets are placed in a heating environment and heated to the target temperature range at a preset heating rate, with a holding time corresponding to the desired duration. This process ensures complete austenitization and temperature homogenization within the billet, thus completing the austenitization heating process. The heating rate is controlled between 50℃ / h and 150℃ / h, the heating temperature between 1150℃ and 1200℃, and the holding time is calculated as 1 hour to 1.5 hours per 25mm of the billet's maximum effective thickness. The heating atmosphere is a weakly reducing protective atmosphere with a carbon monoxide volume fraction of 2% to 5%. Below 1150℃, austenitization within the billet is incomplete, resulting in excessively high resistance to plastic deformation and a tendency for forging cracks. Above 1200℃, austenite grains coarsen, and the melting tendency of low-melting-point phases at grain boundaries intensifies, reducing the post-forging microstructure and properties. During the heating process, the matrix temperature of the steel billet rises above the austenite transformation temperature, and ferrite and cementite undergo a diffusion-type phase transformation, transforming into a face-centered cubic austenite structure. During the heat preservation process, the austenite grains complete homogenization growth, and the internal temperature field and composition field of the steel billet tend to be uniform, reducing the structural inhomogeneity and deformation resistance during plastic deformation. The segregation of rare earth atoms at the austenite grain boundaries can inhibit the excessive coarsening of austenite grains and maintain the uniformity of austenite grain size.

[0037] For steel billets that have undergone austenitizing treatment, continuous compressive and shear stresses are applied to induce plastic deformation, shaping the billet into a form that matches the outer contour of the bearing backing ring. Simultaneously, the final forging temperature is controlled to be no lower than a preset lower limit, completing the hot forging of the billet. The forging process employs free forging or die forging, with a forging ratio controlled between 3 and 6, a final forging temperature controlled at no less than 850℃, a single-pass deformation controlled between 15% and 30%, and a total deformation of no less than 60%. When the final forging temperature is below 850℃, the billet enters the austenite-ferrite dual-phase region, significantly reducing plasticity and making it prone to internal cracks during deformation. When the forging ratio is below 3, the as-cast structure is not sufficiently fragmented, and internal porosity defects cannot be effectively welded. When the forging ratio is above 6, excessive distortion of the metal flow lines easily occurs, leading to excessive anisotropy. During the high-temperature plastic deformation process of the steel billet, internal casting defects such as porosity and voids undergo plastic closure and interface welding under triaxial compressive stress, increasing the density of the billet. The coarse dendritic structure in the as-cast state is broken by shear stress, forming fine deformed austenite grains. At the same time, the grains are oriented along the direction of metal flow, forming continuous forging flow lines. The dislocation proliferation and dynamic recrystallization behavior generated during the deformation process further refines the austenite grains and improves the uniformity of the billet structure.

[0038] The billet, after hot forging, is continuously cooled in room temperature air until its overall temperature drops to room temperature, completing post-forging air cooling. The cooling method is static air cooling, with the air velocity controlled between 0.5 m / s and 2 m / s. Local contact between the billet and water cooling or high thermal conductivity media is avoided during the cooling process. During air cooling, the high-temperature austenite structure undergoes a phase transformation as the temperature decreases, transforming into pearlite and ferrite. The slow air cooling process avoids excessive temperature gradients and thermal stress within the billet, reducing the risk of microcrack initiation during cooling. Rare earth atoms segregate towards grain boundaries during the phase transformation, suppressing the network precipitation of proeutectoid ferrite and maintaining the uniformity of the post-forging microstructure.

[0039] For example, the complete implementation process of step S2 includes placing the rare earth-containing microalloyed steel billet obtained in step S1 into a bogie-type heating furnace, introducing a weakly reducing protective atmosphere, raising the furnace temperature to 1150°C to 1200°C at a preset heating rate, and holding it at that temperature for the duration corresponding to the maximum effective thickness of the billet, so that the interior of the billet achieves complete austenitization and temperature homogenization. The heated billet is then transferred to the operating station of a forging press, where continuous plastic deformation is applied to the billet using free forging or die forging processes. The billet is formed according to a preset number of deformation passes, and the final forging temperature is controlled to be no less than 850°C to obtain a backing bearing ring blank. The forged backing bearing ring blank is then transferred to a ventilated cooling station and air-cooled in a still air environment until the overall temperature of the blank drops to room temperature.

[0040] Through step S2, the rare earth microalloyed steel billet is transformed into a backing bearing ring blank, the internal density is significantly improved, casting porosity and other defects are effectively eliminated, the macro forging flow lines are continuously distributed along the circumference of the ring, and the coarse dendritic structure in the cast state is transformed into a uniform and refined forging structure.

[0041] Step S3, based on the forged microstructure of the backing bearing ring blank obtained in Step S2, performs a two-phase critical spheroidizing annealing treatment on the backing bearing ring blank to eliminate the tendency of carbide network distribution in the forged microstructure. The implementation process of Step S3, in order of microstructure change, includes two-phase holding, subcritical isothermal holding, and cooling after annealing.

[0042] The blank of the bearing backing ring is placed in a heating environment and heated at a preset heating rate to the critical temperature range of the two-phase region where austenite and undissolved carbides coexist. It is then held at this temperature for the corresponding duration to allow partial austenitization of the matrix and selective dissolution of carbides, thus completing the two-phase region holding. Specifically, the heating rate should not exceed 80℃ / h, the heating temperature should be 800℃ to 810℃, the holding time should be 2h to 4h, and the heating atmosphere should be a high-purity nitrogen or argon atmosphere with a volume fraction of not less than 99.99%. The temperature uniformity within the furnace should be controlled within ±5℃. When the heating temperature is below 800℃, the austenitization ratio of the matrix is ​​insufficient, and the dissolution of lamellar or network carbides in the forged structure is incomplete. When the heating temperature is above 810℃, the austenitization ratio of the matrix is ​​too high, and the number of undissolved carbide particles is insufficient, failing to provide sufficient nucleation sites for carbide precipitation. When the heating rate exceeds 80℃ / h, the temperature difference between the inside and outside of the blank is too large, the microstructure transformation is asynchronous, and residual thermal stress and microstructure inhomogeneity are easily generated. When the matrix temperature rises to the two-phase region temperature range above the ferrite-austenite phase transformation critical point Ac1, the ferrite matrix undergoes a diffusion-type phase transformation and transforms into austenite, with some lamellar or network carbides dissolving into the newly formed austenite; rare earth atoms dissolved in the matrix segregate towards the austenite-carbide phase interface, reducing the interfacial energy barrier for further dissolution of carbides, inhibiting excessive dissolution of carbides, while retaining a sufficient amount of undissolved carbide particles and breaking the grain boundary carbide chain structure left during forging.

[0043] For the billet after the two-phase region is held at a preset slow cooling rate, it is cooled in the furnace to a subcritical temperature range below the ferrite-austenite phase transformation critical point Ac1. It is then isothermally held at this temperature for a corresponding duration to allow the undercooled austenite to undergo isothermal transformation and carbide granulation, thus completing the subcritical isothermal holding. Specifically, the slow cooling rate should not exceed 30℃ / h, the isothermal temperature should be between 730℃ and 750℃, and the isothermal holding time should be between 6 and 8 hours. When the slow cooling rate exceeds 30℃ / h, the rate of austenite undercooling is too rapid, resulting in uneven carbide nucleation and the formation of lamellar carbide structures. When the isothermal temperature is below 730℃, the austenite undercooling is too large, leading to excessively high transformation driving force and the formation of lamellar pearlite structures, making uniform carbide granulation impossible. When the isothermal temperature exceeds 750℃, the austenite stability is too high, resulting in insufficient carbon atom diffusion driving force, insufficient carbide precipitation, and inadequate granulation. When the isothermal holding time is less than 6 hours, the granulation transformation of carbides is incomplete, resulting in insufficient microstructure uniformity. When the isothermal holding time is longer than 8 hours, the precipitated carbide particles coarsen, reducing the refining effect of the quenched microstructure. During the subcritical isothermal holding process, the austenite temperature continuously decreases, the solid solubility of carbon in austenite decreases, and the supersaturation increases. Undissolved carbide particles and rare earth oxide sulfides in the steel can serve as a heterogeneous nucleation substrate for carbide precipitation, reducing the carbide nucleation work. During the isothermal holding process, carbon atoms in the supercooled austenite continuously diffuse, precipitating fine and dispersed granular carbides around the pre-set nucleation core. At the same time, austenite undergoes a eutectoid transformation, forming a ferrite matrix, resulting in a microstructure with uniformly distributed granular carbides on the ferrite matrix, eliminating carbide aggregation at the original austenite grain boundaries.

[0044] For the blanks that have undergone subcritical isothermal heat treatment, they are continuously cooled in the furnace to below the preset temperature. The blanks are then removed from the heating environment and cooled in room temperature air until their overall temperature matches room temperature, completing the post-annealing cooling process. The cooling termination temperature should not exceed 500℃. The cooling method after removal from the furnace is static air cooling, with the air velocity controlled between 0.5 m / s and 2 m / s. During cooling, local contact between the blank and high thermal conductivity media or water-cooled media should be avoided. If the cooling termination temperature exceeds 500℃, the blank will still be within the phase transformation temperature range after removal from the furnace. Inhomogeneous microstructure transformation is likely to occur during air cooling, generating additional microstructural and thermal stresses. During the post-annealing cooling process, the matrix temperature continuously decreases, and the remaining untransformed austenite completes the final phase transformation, resulting in a stable microstructure. The slow cooling process avoids excessive temperature gradients within the blank, eliminating internal stresses accumulated during the microstructure transformation. During the air cooling process after removal from the furnace, the overall temperature of the blank drops to room temperature, and the microstructure is completely fixed without any solid-state phase transformation.

[0045] For example, the complete implementation process of step S3 includes placing the backing bearing ring blank obtained in step S2 into a bogie-type annealing furnace, introducing a high-purity protective atmosphere with a volume fraction of not less than 99.99%, raising the furnace temperature to 800°C to 810°C at a rate not exceeding 80°C / h, and holding it at this temperature for 2 to 4 hours to achieve a uniform two-phase microstructure transformation inside the blank. After the holding period, the blank is slowly cooled in the furnace to 730°C to 750°C at a rate not exceeding 30°C / h, and isothermally held at this temperature for 6 to 8 hours to complete the granulation precipitation of carbides and the microstructure homogenization transformation. After the isothermal holding period, the blank is further cooled in the furnace to below 500°C, and then removed from the annealing furnace and placed in a still air environment to cool to room temperature.

[0046] Through step S3, the backing bearing ring blank is transformed into the first intermediate blank, with fine and dispersed granular carbides evenly distributed on the ferrite matrix, and no chain carbides aggregate at the original austenite grain boundaries. The forging residual stress inside the blank is fully eliminated, and the hardness is within the range suitable for machining.

[0047] Step S4, based on the granular carbide pre-structure of the first intermediate billet obtained in step S3, involves fully austenitizing and quenching the first intermediate billet to obtain a martensitic matrix structure through solid-state phase transformation. The implementation process of step S4, in order of material microstructure change, includes quenching austenitizing, holding, and oil quenching.

[0048] The first intermediate billet is placed in a heating environment and heated to the complete austenitizing temperature range at a preset heating rate. It is then held at this temperature for a duration corresponding to the effective wall thickness of the billet, allowing the matrix to undergo complete austenitizing transformation while simultaneously achieving uniform solid solution of carbon and alloying elements, thus completing the quenching austenitizing holding process. Specifically, the heating temperature is 850℃ to 870℃, the heating rate is 50℃ / h to 120℃ / h, and the holding time is calculated as 1.5 min to 2.0 min per millimeter of the effective wall thickness of the ring. The heating atmosphere is a high-purity nitrogen or argon protective atmosphere with a volume fraction of not less than 99.99%, and the temperature uniformity within the furnace is controlled within ±3℃. When the heating temperature is below 850℃, the matrix austenitizing is incomplete, the solid solution of carbon and alloying elements is insufficient, and the hardness of the matrix after quenching cannot reach the preset range. When the heating temperature is above 870℃, the austenite grains coarsen, resulting in coarse martensite structure after quenching and a decrease in matrix toughness. If the holding time is too short, the austenite composition will not be uniform enough, resulting in uneven microstructure and hardness distribution after quenching. If the holding time is too long, the austenite grains will coarsen excessively, reducing the overall performance of the matrix. When the matrix temperature rises to the complete austenitization temperature range above the ferrite-austenite phase transformation critical point Ac3, the ferrite matrix and granular carbides undergo a diffusion-type phase transformation, transforming entirely into austenite. Some of the dispersed granular carbides in the matrix dissolve into austenite, providing carbon content and solid solution strengthening effects of alloying elements to the matrix. Undissolved fine carbide particles pin the austenite grain boundaries, inhibiting austenite grain coarsening. Rare earth atoms dissolved in the grain boundaries further reduce the grain boundary migration rate, synergistically inhibiting austenite grain growth and obtaining a uniform and fine austenite grain structure.

[0049] For the billet that has undergone austenitizing and heat preservation treatment, it is rapidly transferred to a quenching oil medium at a preset temperature. Rapid cooling is then performed in the continuously agitated oil medium until the overall temperature of the billet matches that of the quenching oil medium, completing the oil quenching process. The quenching oil used is a special mineral oil for isothermal quenching, with the oil temperature controlled between 50℃ and 70℃. The time for transferring the billet to the quenching oil tank should not exceed 15 seconds, the agitation velocity of the quenching oil should be 0.5 m / s to 2 m / s, and the difference between the cooling termination temperature and the quenching oil temperature should not exceed 5℃. If the quenching oil temperature is below 50℃, the cooling rate of the oil medium is too fast, resulting in excessive internal thermal and structural stresses in the billet, which can easily lead to quenching cracks and excessive deformation. If the quenching oil temperature is above 70℃, the cooling rate of the oil medium is insufficient, and austenite cannot fully transform into martensite, resulting in excessively high residual austenite content and insufficient matrix hardness. If the billet transfer time exceeds 15 seconds, a pre-cooling phase transformation occurs on the billet surface, forming a non-target structure and reducing the uniformity of the quenched structure. When the billet is rapidly cooled in quenching oil, the austenite is supercooled to below the martensitic transformation initiation temperature Ms, undergoing a diffusionless shear transformation and transforming into a body-centered tetragonal martensitic structure. The rapid cooling process inhibits the long-range diffusion of carbon atoms, avoids the precipitation of non-target structures such as pearlite or bainite, and ensures the sufficiency of the martensitic transformation. Undissolved fine-grained carbides are uniformly distributed in the martensitic matrix, maintaining the hardness and wear resistance of the matrix. The segregation of rare earth atoms at the martensitic lath boundaries can refine the martensitic lath size and reduce the internal stress concentration of the quenched structure.

[0050] For example, the complete implementation process of step S4 includes placing the first intermediate billet obtained in step S3 into a trolley-type quenching furnace, introducing a high-purity protective atmosphere with a volume fraction of not less than 99.99%, raising the furnace temperature to 850°C to 870°C at a heating rate of 50°C / h to 120°C / h, and holding it at that temperature for 1.5 min to 2.0 min per millimeter of the effective wall thickness of the ring, so that the billet undergoes complete austenitization transformation and composition homogenization. After the holding period, the billet is quickly transferred to a quenching oil tank equipped with a circulating stirring system within 15 s. The oil temperature in the quenching oil tank is controlled at 50°C to 70°C, and the quenching oil is continuously circulated at a flow rate of 0.5 m / s to 2 m / s through the circulating stirring system until the overall temperature of the billet is consistent with the temperature of the quenching oil, thus completing the quenching and cooling process.

[0051] Through step S4, the first intermediate billet is transformed into the second intermediate billet. The microstructure of the second intermediate billet consists of cryptocrystalline martensite, a small amount of retained austenite, and uniformly distributed fine-grained undissolved carbides. The matrix hardness is in the high hardness range, and there are macroscopic and microscopic internal stresses introduced by the quenching process inside.

[0052] Step S5, based on the quenched martensitic microstructure of the second intermediate billet obtained in Step S4, applies rare earth-induced micro-stress tempering treatment to the second intermediate billet to regulate the martensite decomposition process and carbide precipitation behavior, eliminate macroscopic and microscopic internal stresses introduced during the quenching process, and maintain the stability of the matrix hardness. The implementation process of Step S5, in order of microstructure changes, includes tempering holding and air cooling after tempering.

[0053] The second intermediate billet is placed in a heating environment and heated to the target tempering temperature range at a preset heating rate. It is then held at this temperature for a fixed duration to allow for controlled decomposition of the martensitic matrix and precipitation of carbides, simultaneously relaxing the quenching stress and completing the tempering hold. Specifically, the heating temperature is 200℃ to 220℃, the heating rate is 30℃ / h to 80℃ / h, the holding time is 2h to 4h, and the heating atmosphere is a high-purity nitrogen or argon atmosphere with a volume fraction of not less than 99.99%. The temperature uniformity within the furnace is controlled within ±3℃. When the heating temperature is below 200℃, the diffusion capacity of carbon atoms in the martensitic matrix is ​​insufficient, carbide precipitation is incomplete, and the degree of quenching stress relief is insufficient. When the heating temperature is above 220℃, the recovery at the martensitic lath boundaries intensifies, the precipitated metastable carbides transform into stable cementite and coarsen, and the decrease in matrix hardness exceeds the controllable range. When the matrix temperature rises to the range of 200℃ to 220℃, the diffusion ability of supersaturated carbon atoms in martensite is significantly enhanced, causing them to precipitate from the martensite lattice and form diffusely distributed ε-carbides. ε-carbides are metastable low-carbon iron-carbon compounds with a close-packed hexagonal crystal structure. The carbon atom precipitation process releases the martensite lattice distortion energy, reducing the microscopic internal stress and macroscopic residual stress within the matrix. Rare earth atoms that have segregated at grain boundaries, dislocations, and other crystal defects during the early high-temperature austenitization stage exert a stable drag effect on the slip and climb of dislocations during this tempering process, hindering the recovery process of martensite lath boundaries and the coarsening kinetics of carbide particles, thus controlling the matrix hardness while achieving internal stress relaxation.

[0054] The tempered billet is removed from the heating environment and continuously cooled in room temperature air until the overall temperature of the billet drops to room temperature, completing the tempering process. The cooling method is static air cooling, with the air velocity controlled between 0.5 m / s and 2 m / s. During cooling, localized contact between the billet and water-cooled media or highly thermally conductive solid media is avoided. During air cooling, the matrix temperature continuously decreases, the diffusion capacity of carbon atoms decays exponentially with decreasing temperature, the carbide precipitation process completely terminates, and the matrix microstructure tends to stabilize. Continuous air cooling avoids excessive temperature gradients and new thermal stresses within the billet, maintaining the uniformity and stability of the microstructure after tempering. The physical pinning effect of rare earth atoms on grain boundaries and dislocations, as well as their stabilizing effect on the micro-stress field around dislocations, are maintained.

[0055] For example, the complete implementation process of step S5 includes placing the second intermediate billet obtained in step S4 into a pit-type tempering furnace, introducing a high-purity protective atmosphere with a volume fraction of not less than 99.99%, raising the furnace temperature to 200°C to 220°C at a heating rate of 30°C / h to 80°C / h, and holding it at that temperature for 4 hours to allow the billet to complete the uniform martensite decomposition, carbide precipitation, and internal stress relaxation process. After the holding period, the billet is removed from the tempering furnace and placed in a still air environment to cool until the overall temperature of the billet drops to room temperature.

[0056] Through step S5, the second intermediate billet is transformed into the third intermediate billet. Dispersed ε-carbides and undissolved granular carbides are distributed on the tempered martensitic matrix. The residual stress of quenching inside the matrix is ​​significantly reduced, and the hardness is maintained within the preset range. The pinning network of rare earth atoms at crystal defects such as grain boundaries and dislocations and hydrogen capture sites are effectively activated.

[0057] Step S6, based on the tempered martensitic microstructure of the third intermediate billet obtained in step S5, applies a magnetic field-assisted low-temperature long-term aging treatment to the third intermediate billet to trap residual diffusible hydrogen atoms in the steel, further relaxing residual stress in the micro-regions of the matrix, and obtaining high-toughness backing bearing steel. Specifically, the implementation process of step S6, according to the changes in microstructure, includes pre-aging, magnetic field-assisted low-temperature aging, and post-aging cooling.

[0058] The third intermediate billet is placed in a constant-temperature furnace equipped with a magnetic field generator. The billets are then uniformly arranged, and the furnace temperature, magnetic field parameters, and protective atmosphere conditions are preset to ensure the overall temperature of the billets reaches the preset aging temperature and achieves homogenization, completing the pre-aging setup. Specifically, the furnace temperature is preset to 120℃ to 140℃, with temperature uniformity controlled within ±2℃. The protective atmosphere is high-purity argon with a volume fraction of not less than 99.99%. The billets are arranged within the uniform magnetic field area of ​​the magnetic field generator, with a minimum distance of 50mm between adjacent billets. The temperature difference between the billet and the furnace is controlled within ±3℃. Maintaining consistency between the preset furnace temperature and the subsequent aging temperature avoids significant temperature fluctuations after loading, ensuring the stability of the temperature field during the aging process. The spacing between the billets and the defined magnetic field area ensure the uniformity of magnetic field strength and temperature across all parts of the billet, preventing localized differences in processing effects. The billet is subjected to overall temperature homogenization at a preset temperature, which makes the diffusion environment of hydrogen atoms in the matrix in a stable thermally activated state, and the magnetic moments of rare earth atoms dissolved in the matrix in an active state that can be effectively coupled with the alternating external magnetic field.

[0059] The third intermediate billet, after pre-treatment, is continuously held at a preset low-temperature isothermal range while an alternating magnetic field with preset parameters is applied throughout the process to achieve magnetic field-assisted low-temperature aging. Specifically, the isothermal furnace temperature is controlled between 120℃ and 140℃, the holding time is 8 to 12 hours, the alternating magnetic field strength is 0.1T to 0.3T, the alternating magnetic field frequency is 30Hz to 50Hz, and the magnetic field direction is parallel to the ring axis. A high-purity argon protective atmosphere is continuously introduced into the furnace during the holding process. When the isothermal temperature is below 120℃, the thermal activation diffusion ability of hydrogen atoms in the matrix is ​​insufficient, preventing them from migrating to the target sites within the preset time. When the isothermal temperature is above 140℃, the coarsening trend of the precipitated ε-carbides in the martensitic matrix intensifies, the matrix hardness decreases, and the trapping stability of rare earth hydrogen traps decreases. When the magnetic field strength is below 0.1T, sufficient magnetostrictive coupling effect cannot be excited, and the acceleration effect on hydrogen atom diffusion is not significant. When the magnetic field strength is above 0.3T, excessive alternating magnetostrictive stress amplitude will cause micro-region fatigue damage at micro-defects in the billet, forming new stress concentration sites. When the holding time is less than 8h, the diffusion migration and trapping process of hydrogen atoms is incomplete. When the holding time is more than 12h, there is no additional hydrogen trapping effect gain, and the production efficiency decreases. During the low-temperature isothermal holding process, diffusible hydrogen atoms dissolved in the interstitial positions of the matrix lattice undergo continuous diffusion under thermal activation. Rare earth atoms cerium and lanthanum have high chemical affinity with hydrogen and can form rare earth hydrides with negative enthalpy of formation, constituting deep trap-type hydrogen trapping sites. Hydrogen atoms diffusing to these sites are irreversibly trapped and fixed. The low-frequency alternating magnetic field applied throughout the process can couple with the spin magnetic moments of rare-earth atoms, generating microscale lattice vibrations around the rare-earth atoms through local magnetostriction. These vibrations lower the activation energy barrier for the diffusion of neighboring hydrogen atoms, significantly accelerating the dynamic process of hydrogen atom enrichment into rare-earth traps. Simultaneously, the prolonged low-temperature holding process promotes the continuous release of residual stress in incompletely relaxed micro-regions within the matrix, reducing the driving force for hydrogen enrichment at stress concentration sites.

[0060] For billets that have undergone synergistic aging treatment, the alternating magnetic field is immediately removed, and the billets are removed from the furnace and cooled to room temperature in air. After aging, they are cooled again. Since the aging temperature is only 120℃ to 140℃, air cooling within this extremely low temperature range will not generate macroscopic thermal stresses inside the billet that could lead to deformation or cracking. After the alternating magnetic field is removed, the magnetic field coupling effect of rare earth atoms disappears, and the trapping state of rare earth hydrogen atoms remains stable, preventing the trapped hydrogen atoms from escaping. During air cooling, the matrix temperature decreases rapidly, and the diffusion capacity of hydrogen atoms decreases exponentially with decreasing temperature. The migration and trapping process of hydrogen atoms completely terminates, and the microstructure, hardness, and dimensional accuracy of the matrix remain stable. No new solid-state phase transformations or carbide coarsening occur, significantly improving production efficiency while ensuring stable product performance.

[0061] For example, the complete implementation process of step S6 includes placing the third intermediate billet obtained in step S5 into a well-type constant-temperature aging furnace equipped with a low-frequency alternating magnetic field generator. The billets are evenly arranged along the furnace axis, with a spacing of not less than 50 mm between adjacent billets, so that all billets are within the uniform magnetic field area of ​​the magnetic field generator. A protective atmosphere of high-purity argon gas with a volume fraction of not less than 99.99% is continuously introduced into the furnace to raise the furnace temperature to 120°C to 140°C and hold it at that temperature until the overall temperature of the billet is uniformly consistent with the furnace temperature. After the billet temperature stabilizes, the magnetic field generator is activated to apply an alternating magnetic field with a magnetic field strength of 0.1T to 0.3T and a frequency of 50Hz to the billet. The magnetic field direction is parallel to the axial direction of the ring. The billet is held at a constant temperature of 120°C to 140°C for 8 to 12 hours. After the holding time is completed, the magnetic field generator is immediately turned off to remove the alternating magnetic field. The billet is then removed from the furnace and placed in a room temperature air environment to cool to room temperature.

[0062] Through step S6, the third intermediate billet is transformed into high-toughness backing bearing steel. Most of the diffusible hydrogen atoms are irreversibly fixed in rare earth-related hydrogen traps, the residual stress in the matrix micro-region is further relaxed, the microstructure maintains the stable state of dispersed ε-carbides and undissolved granular carbides distributed on the tempered martensite matrix, and the overall hardness and size of the bearing ring remain stable.

[0063] Example 1: This example provides a composite heat treatment process, the specific process of which is as follows: S1: Place the molten bearing steel in a ladle refining furnace. Under argon atmosphere protection, adjust the sulfur content in the molten steel to 0.005% at the end of refining. Simultaneously, introduce argon gas at a flow rate of 120 L / min through the permeable brick at the bottom of the ladle and stir for 20 minutes to complete the sulfur content adjustment. Transfer the molten steel to a vacuum degassing chamber, control the absolute pressure inside the furnace to 60 Pa, maintain the vacuum for 20 minutes, introduce argon gas to break the vacuum, and control the temperature of the molten steel to 1580℃ after breaking the vacuum to complete the vacuum degassing of the molten steel. Transfer the molten steel to... At the wire feeding station, a mixed rare earth wire with a cerium-lanthanum mass ratio of 1:1.86 is fed in at a speed of 200 m / min using a wire feeder. The sum of the residual mass fractions of cerium and lanthanum in the molten steel is controlled to be 0.018%. After wire feeding, argon gas with a flow rate of 150 L / min is introduced and stirred for 20 min. The mixture is then allowed to stand for 8 min to complete the rare earth microalloying treatment. The molten steel is then transferred to the tundish of the continuous casting machine. The casting temperature is controlled at 1550℃ and the continuous casting speed is 1.2 m / min to complete the casting of the molten steel and obtain a rare earth microalloyed steel billet.

[0064] S2: The rare earth microalloyed steel billet is placed in a bogie-type heating furnace and a weakly reducing protective atmosphere of 3% carbon monoxide and the balance of high-purity nitrogen is introduced. The furnace temperature is raised to 1180℃ at a heating rate of 100℃ / h. Based on a maximum effective thickness of 200mm for the steel billet, the temperature is held for 8 hours to complete the austenitization heating of the steel billet. The heated steel billet is transferred to a forging press and plastic deformation is carried out using a free forging process. The forging ratio is controlled at 4, the deformation amount per pass is 20%, the total deformation amount is 70%, and the final forging temperature is controlled at 880℃ to complete the hot forging of the steel billet and obtain the blank of the backing bearing ring. The blank is transferred to a ventilation cooling station and cooled to room temperature in a still air environment with a wind speed of 1m / s to complete the post-forging air cooling.

[0065] S3: Place the blank of the backing bearing ring in a bogie-type annealing furnace, introduce a protective atmosphere of high-purity nitrogen with a volume fraction of 99.99%, raise the furnace temperature to 805℃ at a heating rate of 60℃ / h, hold for 3h to complete the two-phase region holding; after the holding is completed, slowly cool with the furnace to 740℃ at a rate of 20℃ / h, hold isothermally for 7h to complete the subcritical isothermal holding; after the isothermal holding is completed, cool with the furnace to 480℃, remove the blank, and cool to room temperature in a still air environment with a wind speed of 1m / s to complete the annealing and cooling, and obtain the first intermediate blank.

[0066] S4: Place the first intermediate billet in a trolley-type quenching furnace, introduce a protective atmosphere of 99.99% high-purity argon gas, and raise the furnace temperature to 860℃ at a heating rate of 80℃ / h. Hold the temperature for 180 minutes with an effective wall thickness of 100mm to complete the quenching austenitizing holding. After the holding period, transfer the billet to a quenching oil tank equipped with a circulating stirring system within 10s. Control the quenching oil temperature at 60℃ and allow the quenching oil to flow continuously at a flow rate of 1m / s. Cool the billet until the temperature is consistent with the oil temperature to complete the oil quenching cooling and obtain the second intermediate billet.

[0067] S5: Place the second intermediate billet in a pit-type tempering furnace, introduce a protective atmosphere of high-purity argon gas with a volume fraction of 99.99%, raise the furnace temperature to 210℃ at a heating rate of 50℃ / h, hold for 4h to complete the tempering and holding; after the holding is completed, take out the billet, cool it to room temperature in a still air environment with a wind speed of 1m / s, and air cool after tempering to obtain the third intermediate billet.

[0068] S6: The third intermediate billet is placed in a well-type constant temperature aging furnace equipped with a low-frequency alternating magnetic field generator. The billets are evenly arranged along the furnace axis with a spacing of 60mm between adjacent billets, all within the uniform magnetic field area. A protective atmosphere of 99.99% high-purity argon gas is continuously introduced into the furnace to raise the furnace temperature to 130℃ and hold it until the billet temperature is uniform with the furnace temperature, completing the pre-aging setup. After the billet temperature stabilizes, the magnetic field generator is activated to apply an axial alternating magnetic field with a strength of 0.1T and a frequency of 50Hz to the billet. The billet is held at a constant temperature of 130℃ for 10 hours to complete the magnetic field-assisted low-temperature aging. After the holding period, the magnetic field generator is immediately turned off to remove the alternating magnetic field. The billet is then removed from the furnace and cooled to room temperature in a still air environment. After aging, it is cooled to obtain high-toughness backing bearing steel.

[0069] Example 2: The difference between this example and Example 1 is that the magnetic field strength in the magnetic field-assisted low-temperature aging in step S6 is replaced with 0.2T; the remaining steps and parameters are exactly the same as in Example 1.

[0070] Example 3: The difference between this example and Example 1 is that the magnetic field strength in the magnetic field-assisted low-temperature aging in step S6 is replaced with 0.3T; the remaining steps and parameters are exactly the same as in Example 1.

[0071] Example 4: The difference between this example and Example 2 is that the tempering temperature in step S5 is replaced with 200℃; the remaining steps and parameters are exactly the same as in Example 2.

[0072] Example 5: The difference between this example and Example 2 is that the tempering temperature in step S5 is replaced with 220°C; the remaining steps and parameters are exactly the same as in Example 2.

[0073] Comparative Example 1: The difference between this comparative example and Example 2 is that the magnetic field strength in the magnetic field-assisted low-temperature aging in step S6 is replaced with 0.32T; the remaining steps and parameters are exactly the same as in Example 2.

[0074] Comparative Example 2: The difference between this comparative example and Example 2 is that the magnetic field strength in the magnetic field-assisted low-temperature aging in step S6 is replaced with 0.08T; the remaining steps and parameters are exactly the same as in Example 2.

[0075] Comparative Example 3: The difference between this comparative example and Example 2 is that the tempering temperature in step S5 is replaced with 222°C; the remaining steps and parameters are exactly the same as in Example 2.

[0076] Comparative Example 4: The difference between this comparative example and Example 2 is that the tempering temperature in step S5 is replaced with 198°C; the remaining steps and parameters are exactly the same as in Example 2.

[0077] Comparative Example 5: This comparative example provides a composite heat treatment process, the specific process of which is as follows: S1: The sulfur mass fraction of molten steel is controlled at 0.005%. After conventional refining and vacuum degassing, it is cast into bearing steel billets. S2: The steel billet is heated to austenitize at a temperature of 1180℃ and held for 8 hours. It is then hot-forged to a final forging temperature of 880℃ and air-cooled after forging to obtain the blank of the backing bearing ring. S3: Perform spheroidizing annealing on the blank, heat to 790℃ and hold for 6 hours, then slowly cool in the furnace to below 500℃ and air cool to obtain the annealed blank. S4: Quenching and austenitizing the annealed billet by holding at 860℃ for 180 min, followed by oil quenching at 60℃ to obtain the quenched billet. S5: The quenched billet is subjected to low-temperature tempering at 160℃ for 4 hours, followed by air cooling to obtain the finished backing bearing ring.

[0078] Experimental Example 1: The test objects in this experimental example are the high-toughness backing bearing steels prepared in Examples 1 to 5 and Comparative Examples 1 to 5 above, and the following tests were performed respectively: 1. Determination of subsurface fracture toughness KIC value: A universal testing machine was used, and the test was performed according to GB / T4161-2007 "Metallic Materials - Test Method for Plane Strain Fracture Toughness KIC". The test environment was room temperature (25℃). Compact tensile specimens were cut from the subsurface layer of each raceway (1.0 mm ± 0.1 mm from the surface depth), with the specimen orientation in the CR direction. Three parallel specimens were tested in each group, and the average value was taken.

[0079] 2. Hydrogen-induced whitening crack susceptibility assessment: Electrolytic hydrogen charging and a rotary bending fatigue testing machine were used for testing. The test environment was room temperature (25℃). The race was immersed in a sulfuric acid-thiourea solution with pH=2.5 for electrolytic hydrogen charging, with a current density of 5 mA / cm². 2 The hydrogen charging time was 24 hours. Immediately after hydrogen charging, a rotational bending fatigue test was conducted with a stress ratio of R=-1 and a maximum cyclic stress of 800MPa. The cycle number at which the first surface crack with a length exceeding 50μm appeared was recorded (denoted as N50). The higher this value, the stronger the resistance to whitening crack initiation.

[0080] 3. Room temperature impact absorption energy determination: A Charpy pendulum impact testing machine was used, and the test was performed in accordance with GB / T229-2020 "Metallic Materials Charpy Pendulum Impact Test Method". The test environment was room temperature of 25℃. Standard Charpy V-notch impact specimens were cut from the center of the wall thickness of each ring, with the notch direction perpendicular to the ring axis. Three parallel specimens were tested in each group, and the average value of the results was taken.

[0081] 4. Rockwell hardness test: A Rockwell hardness tester was used, and the test was performed in accordance with GB / T230.1-2018 "Metallic materials, Rockwell hardness test - Part 1: Test method". The test environment was room temperature of 25℃. An HRC scale was used, and 5 points were evenly selected along the circumference of each ring end face for testing. The average value of the results was taken.

[0082] The results are shown in Table 1: Table 1 Performance test results of high-toughness backing bearing steel Example 1 22.4 14.3 28.5 60.3 Example 2 25.6 21.8 35.2 60.1 Example 3 24.1 18.6 32.1 59.8 Example 4 23.7 17.9 31.8 60.8 Example 5 24.9 20.4 34.5 59.0 Comparative Example 1 18.5 7.8 22.0 60.5 Comparative Example 2 19.7 9.3 24.2 59.6 Comparative Example 3 19.2 10.8 24.8 61.4 Comparative Example 4 20.5 12.1 26.1 58.3 Comparative Example 5 16.8 5.2 18.5 61.2 Comparing the test data of Examples 1, 2, and 3, the various toughness indicators first increased and then decreased with increasing magnetic field strength, reaching a peak performance at 0.2T. This is because at a magnetic field strength of 0.1T, only a weak magnetostrictive coupling effect can be excited, which cannot effectively reduce the activation energy barrier for hydrogen atom diffusion. The migration of hydrogen atoms is still dominated by thermal diffusion. Within the 10-hour aging period, most of the diffusible hydrogen in the bulk phase cannot complete the migration and enrichment into the rare earth hydrogen trap, resulting in insufficient hydrogen trapping efficiency and thus performance lower than the optimal value. When the magnetic field strength increases to 0.2T, the alternating magnetic field and the inherent magnetic square of rare earth atoms form a sufficient coupling effect. The micro-lattice vibrations generated around the rare earth atoms can just reduce the hydrogen atom diffusion. The activation barrier is dispersed without introducing additional alternating stress, which significantly accelerates the enrichment dynamics of hydrogen atoms into rare earth traps and ensures the irreversible capture of hydrogen atoms by the traps, thus achieving peak performance. When the magnetic field strength is further increased to 0.3T, the excessive magnetostrictive vibration introduces excessive alternating stress amplitude at the interface between rare earth oxysulfide inclusions and the matrix, causing dislocation pile-up and micro-region plastic deformation at the interface, forming new hydrogen enrichment sites and stress concentration sources, which partially offsets the capture effect of rare earth hydrogen traps, thus causing a decline in various performance characteristics.

[0083] Comparing the test data of Examples 4, 2, and 5, the toughness indicators first increased and then decreased with increasing tempering temperature, reaching a peak performance at 210℃. This is because, at a tempering temperature of 200℃, the diffusion ability of supersaturated carbon atoms in martensite is insufficient, and only a small amount of unevenly distributed ε-carbides can precipitate. The degree of elimination of quenching residual stress is limited. Due to the failure to effectively relax the excessively high quenching residual stress, the resulting large stress field partially offsets the hydrogen trapping ability of in-situ segregated rare earth atoms, resulting in the pinning effect of rare earth atoms and the hydrogen trapping effect not being fully utilized. The matrix still has high brittleness, so the performance is lower than the optimal value. When the tempering temperature rises to 210℃, it is exactly in the optimal temperature range for the dispersion and precipitation of ε-carbides. Carbon atoms are fully desoluble from martensite and form uniformly dispersed ε-carbides, and the quenching residual stress is eliminated. Sufficient relaxation allows the residual stress from quenching to relax fully and appropriately, stripping away the additional driving force that suppresses the effectiveness of rare earth traps. This enables the in-situ segregated rare earth atoms to fully exert their dragging effect and hydrogen capture function to suppress the recovery of martensitic lath boundaries and carbide coarsening, while also constructing high-density irreversible hydrogen traps, achieving the optimal balance between hardness and toughness. Therefore, all properties reach their peak. When the tempering temperature is further increased to 220℃, the recovery of martensitic lath boundaries intensifies, and some metastable ε-carbides begin to transform into stable θ-cementite and coarsen. The dragging effect of rare earth atoms can no longer suppress the softening of the microstructure and the weakening of grain boundaries. The hardness and toughness of the matrix decline simultaneously, resulting in a decrease in all properties.

[0084] From a temporal perspective, the 210℃ tempering treatment first completes the controllable decomposition of martensite and the diffuse precipitation of ε-carbides, fully relaxing the residual stress in the matrix and effectively relaxing the macroscopic internal stress of the matrix, eliminating the interference of stress concentration on hydrogen enrichment. This thoroughly awakens the high-density, uniformly distributed irreversible hydrogen traps formed by the previously agglomerated rare earth atoms throughout the entire matrix, providing sufficient sites for subsequent hydrogen atom capture. The subsequent 0.2T magnetic field-assisted aging treatment, targeting the pre-set hydrogen traps, reduces the hydrogen atom diffusion activation energy through the magnetostrictive coupling effect, accelerating the migration and irreversible fixation of diffusible hydrogen in the bulk phase to the pre-set traps. Through this sequential coordination, the efficiency of hydrogen trap construction and hydrogen capture is maximized.

[0085] From a spatial perspective, tempering temperature enables matrix microstructure control and rare earth atom distribution optimization within the bulk phase, addressing issues of matrix brittleness, internal stress, and bulk phase uniformity of hydrogen trap density. Magnetic field strength enables microscale hydrogen atom migration control, acting on the local lattice of individual rare earth trap sites to overcome the kinetic limitations of hydrogen atom migration to these sites. If only the optimal tempering temperature is used without a matching magnetic field strength, hydrogen atom diffusion kinetics are insufficient, and adequate trapping cannot be achieved within the aging period. Conversely, if only the optimal magnetic field strength is used without a matching tempering temperature, the matrix hydrogen trap density is insufficient, and internal stress concentrates. Even with accelerated hydrogen atom migration, hydrogen will still accumulate at stress concentration sites, leading to cracking.

[0086] Regarding the magnetic field strength, the magnetic field strength of Comparative Example 2 was 0.08T, only 0.02T lower than that of Example 1 (0.1T). The subsurface KIC value decreased by 17.4% and the N50 value decreased by 45.5% compared to Example 1, indicating a significant performance decline. This is because the magnetostrictive coupling effect of rare-earth atoms induced by the magnetic field has a minimum critical threshold of 0.1T. Below this threshold, the magnetic field energy cannot break through the minimum energy barrier for effective coupling of rare-earth atomic magnetic moments, and cannot generate lattice vibrations that effectively reduce the activation energy of hydrogen diffusion. The migration of hydrogen atoms remains entirely controlled by thermal diffusion, and effective enrichment into rare-earth traps cannot be completed within the aging period. The resistance to hydrogen-induced cracking directly drops to near-no-magnetic-field levels. The magnetic field strength of Comparative Example 1 was 0.32T, only 0.02T higher than that of Example 3 (0.3T). The subsurface KIC value decreased by 18.3% and the N50 value decreased by 50% compared to Example 3, resulting in a precipitous performance drop. This is because there is an upper limit critical threshold of 0.3T for magnetic field strength. When this threshold is exceeded, the stress amplitude generated by alternating magnetostriction exceeds the bonding strength threshold between rare earth inclusions and the matrix interface, causing irreversible micro-region fatigue damage and dislocation pile-up at the interface, forming new crack nucleation sites and hydrogen enrichment sources. This negative effect completely offsets the positive benefits of hydrogen capture, and the damage is irreversible. Therefore, even if the critical value of 0.02T is exceeded, the performance deteriorates sharply.

[0087] Regarding the tempering temperature, the tempering temperature of Comparative Example 4 was 198℃, only 2℃ lower than the 200℃ of Example 4. The KIC value of the subsurface layer decreased by 13.5% and the N50 value decreased by 32.4% compared to Example 4, resulting in a precipitous drop in performance. This is because there is a minimum critical activation temperature of 200℃ for the precipitation of ε-carbides from supersaturated carbon atoms in martensite. Below this temperature, the diffusion activation energy of carbon atoms is insufficient, and they cannot effectively precipitate from the martensite lattice to form dispersed ε-carbides. The residual stress from quenching cannot be effectively relaxed, and the extremely high micro-stress concentration inside the matrix dominates the hydrogen enrichment process, completely masking and suppressing the original hydrogen trapping function of rare earth atoms. The matrix still maintains a highly brittle quenched martensite state. Therefore, even with a decrease of only 2℃, the strength-toughness matching mechanism of the system completely fails. The tempering temperature of Comparative Example 3 was 222℃, only 2℃ higher than the 220℃ of Example 5. The KIC value of the subsurface layer decreased by 17.7% and the N50 value decreased by 40.7% compared to Example 5. The hardness fell below the acceptable lower limit, and the performance showed a precipitous decline. This is because 220℃ is the critical temperature for the transformation of metastable ε-carbide to stable θ-cementite. Above this temperature, ε-carbide undergoes irreversible phase transformation and rapid coarsening, martensite lath boundaries undergo violent recovery, dislocation density decreases significantly, and the dragging effect of rare earth atoms can no longer suppress the transformation process. The matrix undergoes irreversible softening and grain boundary weakening, and the strength-toughness balance system completely collapses. Therefore, even with an increase of only 2℃, the performance deteriorates sharply.

[0088] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A composite heat treatment process for high-toughness backing bearing steel based on rare earth microalloying, characterized in that, Includes the following steps: S1: Rare earth microalloying treatment and sulfur content control are applied to molten steel to obtain rare earth microalloyed steel billets by casting. S2: Forging a rare earth microalloyed steel billet to obtain a backing bearing ring blank; S3: Perform two-phase critical spheroidizing annealing on the backing bearing ring blank to obtain a first intermediate blank; the two-phase critical spheroidizing annealing includes two-phase holding, subcritical isothermal holding and cooling after annealing; the two-phase holding involves heating the backing bearing ring blank to 800°C to 810°C at a heating rate not exceeding 80°C / h and holding it at that temperature for 2h to 4h; the subcritical isothermal holding involves cooling the blank to 730°C to 750°C at a slow cooling rate not exceeding 30°C / h and holding it isothermally for 6h to 8h. S4: Austenitize the first intermediate billet and oil quench it to obtain the second intermediate billet; S5: The second intermediate billet is subjected to rare earth-induced micro-stress tempering treatment to obtain the third intermediate billet; wherein, the heating temperature of the rare earth-induced micro-stress tempering treatment is 200℃ to 220℃, and the holding time is 2h to 4h. S6: Apply magnetic field-assisted low-temperature long-aging treatment to the third intermediate billet to obtain high-toughness backing bearing steel; wherein, the magnetic field-assisted low-temperature long-aging treatment is continuously held at a constant temperature range of 120℃ to 140℃ for 8h to 12h, and an alternating magnetic field is applied throughout the treatment process, wherein the magnetic field strength of the alternating magnetic field is 0.1T to 0.3T and the frequency is 30Hz to 50Hz.

2. The composite heat treatment process according to claim 1, characterized in that, The implementation process of step S1 includes adjusting the sulfur content of molten steel, vacuum degassing of molten steel, rare earth microalloying treatment, and casting of molten steel. Among them, the sulfur content adjustment of molten steel controls the mass fraction of sulfur in molten steel to 0.003% to 0.008%. The rare earth microalloying treatment feeds cerium-lanthanum mixed rare earth wire into molten steel, and controls the sum of the residual mass fractions of cerium and lanthanum in molten steel after wire feeding to be 0.012% to 0.025%.

3. The composite heat treatment process according to claim 2, characterized in that, The mass ratio of cerium to lanthanum in the cerium-lanthanum mixed rare earth wire is 1:1 to 2:1; the absolute pressure inside the vacuum degassing furnace for the molten steel is below 67 Pa.

4. The composite heat treatment process according to claim 1, characterized in that, The implementation process of step S2 includes austenitizing heating of the billet, hot forging of the billet, and air cooling after forging; wherein, the heating temperature of the austenitizing heating of the billet is controlled between 1150℃ and 1200℃; and the final forging temperature of the hot forging of the billet is controlled at not less than 850℃.

5. The composite heat treatment process according to claim 1, characterized in that, The implementation process of step S4 includes quenching austenitizing and holding, and oil quenching and cooling in sequence; wherein, the heating temperature of the quenching austenitizing and holding is 850℃ to 870℃, and the holding time is calculated as 1.5min to 2.0min per millimeter of effective wall thickness of the ring; the oil quenching and cooling controls the quenching oil temperature to be between 50℃ and 70℃.

6. The composite heat treatment process according to claim 1, characterized in that, The implementation process of step S5 includes tempering and heat preservation followed by air cooling after tempering; the air cooling after tempering is continuous cooling in a room temperature air environment until the overall temperature of the billet drops to room temperature.

7. The composite heat treatment process according to claim 1, characterized in that, The implementation process of step S6 includes pre-aging, magnetic field-assisted low-temperature aging, and post-aging cooling. In the pre-aging process, the third intermediate billet is arranged in the uniform magnetic field area of ​​the magnetic field generating device, and the distance between adjacent billets is not less than 50mm. The direction of the alternating magnetic field is parallel to the axial direction of the ring.

8. The composite heat treatment process according to claim 7, characterized in that, During the post-aging cooling process, the alternating magnetic field is removed, and the billet is placed in a room temperature air environment to cool to room temperature.

9. A high-toughness backing bearing steel based on rare earth microalloying, characterized in that, It is prepared by any one of the composite heat treatment processes according to claims 1 to 8.

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