A method of heat treating a workpiece

Abstract

The application provides a heat treatment method of a workpiece, relates to the technical field of heat treatment, and comprises four treatment procedures. In the first heat treatment procedure, an initial state workpiece is annealed to obtain an intermediate state workpiece a, so as to refine grains. In the second and third heat treatment procedures, the intermediate state workpiece a is subjected to heat treatment at different temperatures in the second and third temperature intervals in sequence, so as to perform homogenization annealing treatment on the workpiece. In the third heat treatment procedure, the workpiece is subjected to heat treatment at different temperatures to obtain an intermediate state workpiece c, so as to precipitate TiN at low temperature and dissolve TiN at high temperature, hinder abnormal growth of TiN, and set the highest temperature of the third temperature interval to be lower than the highest temperature of the second temperature interval, thereby avoiding abnormal growth of TiN in the third temperature interval. In the fourth treatment procedure, the intermediate state workpiece c is rolled, so as to diffuse and precipitate TiN inclusions. The heat treatment method of the workpiece avoids abnormal growth of TiN inclusions.

Description

Technical Field

[0001] This application relates to the technical field of heat treatment, and more particularly to a heat treatment method for a workpiece. Background Technology

[0002] Non-metallic inclusions in bearing steel have a significant impact on the material's fatigue failure. The detrimental effects of large-sized hard TiN inclusions on bearing steel are becoming increasingly apparent. Related studies have shown that TiN inclusions have a far greater impact on the fatigue life of bearing steel than oxide inclusions of the same size.

[0003] In existing technologies, the control of TiN inclusions in bearing steel mainly focuses on two aspects: one is to suppress the precipitation of large-sized TiN inclusions during the solidification process of molten steel by controlling the content of Ti and N elements in the original material; the other is to add elements such as Ce to the bearing steel to suppress the precipitation of TiN inclusions during the solidification process of molten steel.

[0004] However, existing bearing steels suffer from abnormal growth of TiN inclusions during uniform diffusion annealing. Summary of the Invention

[0005] This application provides a heat treatment method for workpieces that improves the problem of abnormal growth of TiN inclusions in bearing steel during uniform diffusion annealing.

[0006] This application provides a heat treatment method for a workpiece, which includes the following steps:

[0007] In the first heat treatment process, the initial state workpiece is annealed at a first temperature to obtain the intermediate state workpiece a;

[0008] In the second heat treatment process, intermediate workpiece a is subjected to heat treatment at different temperatures in a second temperature range to obtain intermediate workpiece b.

[0009] The third heat treatment process involves heat treating the intermediate workpiece b at different temperatures within a third temperature range to obtain the intermediate workpiece c.

[0010] The fourth processing step involves rolling the intermediate workpiece c, which exits the furnace at the third temperature range, to obtain the final workpiece.

[0011] Among them, the first temperature is higher than the eutectoid transformation temperature of the initial state workpiece; the lowest temperature in the second and third temperature ranges is higher than the first temperature; and the highest temperature in the second temperature range is higher than the highest temperature in the third temperature range.

[0012] In the above-mentioned heat treatment method for the workpiece, optionally, the first heat treatment step, annealing the initial state workpiece at a first temperature to obtain the intermediate state workpiece a, includes:

[0013] The initial state workpiece is kept at a first temperature to obtain the intermediate state workpiece a1;

[0014] Intermediate workpiece a1 is cooled with the furnace to the first furnace cooling temperature to obtain intermediate workpiece a.

[0015] In the above-mentioned heat treatment method for the workpiece, optionally, the second heat treatment step involves heat treating the intermediate workpiece a at different temperatures in a second temperature range to obtain the intermediate workpiece b, including:

[0016] The second temperature range includes a second low temperature and a second high temperature. Intermediate workpiece a is kept at the second low temperature to obtain intermediate workpiece b1.

[0017] Intermediate workpiece b1 is kept at a second high temperature to obtain intermediate workpiece b2;

[0018] Intermediate workpiece b2 is cooled in the furnace to the second furnace cooling temperature to obtain intermediate workpiece b.

[0019] In the aforementioned heat treatment method for the workpiece, optionally, the third heat treatment step involves holding the intermediate workpiece b at different temperatures within a third temperature range to obtain the intermediate workpiece c, including:

[0020] The third temperature range includes the third low temperature and the third high temperature; the second furnace cooling temperature is equal to the third low temperature. Intermediate workpiece b is kept at the third low temperature to obtain intermediate workpiece c1, and intermediate workpiece c1 is kept at the third high temperature to obtain intermediate workpiece c.

[0021] In the above-mentioned heat treatment method for the workpiece, the first temperature range may be greater than or equal to 880°C and less than or equal to 920°C.

[0022] And / or, the range of the first furnace cooling temperature is greater than or equal to 600°C and less than or equal to 650°C.

[0023] And / or, the range of the second low temperature is greater than or equal to 840°C and less than or equal to 860°C.

[0024] And / or, the range of the second high temperature is greater than or equal to 1200°C and less than or equal to 1250°C.

[0025] And / or, the range of the third low temperature is greater than or equal to 1050℃ and less than or equal to 1100℃.

[0026] And / or, the range of the third high temperature is greater than or equal to 1180℃ and less than or equal to 1200℃.

[0027] In the above-mentioned heat treatment method for the workpiece, it is optional that the heating rate range from the initial temperature to the first temperature is greater than 0°C and less than or equal to 150°C.

[0028] And / or, the heating rate range from the first furnace cold temperature to the second low temperature is greater than 0℃ / h and less than or equal to 150℃ / h.

[0029] And / or, the heating rate range from the second low temperature to the second high temperature is greater than 0℃ / h and less than or equal to 150℃ / h.

[0030] And / or, the heating rate range from the third low temperature to the third high temperature is greater than 0℃ / h and less than or equal to 150℃ / h.

[0031] In the above-mentioned heat treatment method for the workpiece, it is optional that the initial state workpiece is held at the first temperature for a period of 30 minutes or less than or equal to 60 minutes.

[0032] And / or, the holding time of the intermediate workpiece a at the second low temperature is greater than or equal to 30 min and less than or equal to 60 min.

[0033] And / or, the holding time of the intermediate workpiece a1 at the second high temperature is greater than or equal to 8h and less than or equal to 12h.

[0034] And / or, the holding time of the intermediate workpiece b at the third low temperature is greater than or equal to 30 min and less than or equal to 60 min.

[0035] And / or, the holding time of the intermediate workpiece b1 at the third high temperature is greater than or equal to 8 hours and less than or equal to 12 hours.

[0036] In the above-mentioned heat treatment method for the workpiece, the initial state of the workpiece may include bearing steel.

[0037] In the above-mentioned heat treatment method for workpieces, the initial state workpiece includes pearlite, cementite, and titanium nitride; the intermediate state workpiece a includes pearlite, cementite, and titanium nitride; the intermediate state workpiece b includes austenite and titanium nitride; the intermediate state workpiece c includes austenite and titanium nitride; and the final state workpiece includes pearlite, cementite, and titanium nitride.

[0038] In the above-mentioned heat treatment method for the workpiece, it is optional that the grain size of the pearlite in the intermediate state workpiece a is smaller than the grain size of the pearlite in the initial state workpiece.

[0039] This application provides a heat treatment method for a workpiece, comprising four processing steps. In the first heat treatment step, the initial state workpiece is annealed to obtain an intermediate state workpiece a, which refines the grains and eliminates the internal stress of the initial state workpiece to prevent cracking during subsequent processing. In the second heat treatment step, the intermediate state workpiece a is heat-treated at different temperatures in a second temperature range to obtain an intermediate state workpiece b, achieving homogenized annealing of the workpiece. In the third heat treatment step, the intermediate state workpiece b is heat-treated at different temperatures in a third temperature range. By setting the highest temperature in the second temperature range higher than the highest temperature in the third temperature range, TiN in the intermediate state workpiece b precipitates at low temperatures and dissolves back at high temperatures in the third temperature range, thus avoiding abnormal growth of TiN in the third temperature range and reducing the size of TiN nonmetallic inclusions. In the fourth processing step, the intermediate state workpiece c is rolled to disperse the precipitation of TiN inclusions. This heat treatment method for the workpiece avoids abnormal growth of TiN inclusions in the intermediate state workpiece c. Attached Figure Description

[0040] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0041] Figure 1 This is a schematic diagram of the steps of the heat treatment method for a workpiece provided in the embodiments of this application;

[0042] Figure 2 A bar chart comparing the number of TiN inclusions with a particle size greater than or equal to 2 μm and less than 3 μm per 3000 square millimeters in different embodiments and comparative examples of the heat treatment method for the workpiece provided in this application.

[0043] Figure 3 A bar chart comparing the number of TiN inclusions with a particle size greater than or equal to 3 μm and less than 5 μm per 3000 square millimeters in different embodiments and comparative examples of the heat treatment method for the workpiece provided in this application.

[0044] Figure 4 A bar chart comparing the number of TiN inclusions with a particle size greater than or equal to 5 μm and less than 9 μm per 3000 square millimeters in different embodiments and comparative examples of the heat treatment method for the workpiece provided in this application.

[0045] Figure 5 A bar chart comparing the number of TiN inclusions with a particle size greater than or equal to 9 μm per 3000 square millimeters in different embodiments and comparative examples of the heat treatment method for the workpiece provided in this application.

[0046] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0047] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0048] First, let me explain the terms used in this application:

[0049] Ostwald ripening refers to the phenomenon in the later stages of precipitation in a supersaturated solid solution, where the precipitate particles are not of the same size. As smaller particles dissolve and larger particles continue to grow, the average particle size increases.

[0050] Ferrite: Ferrite is an interstitial solid solution of carbon dissolved in the body-centered cubic α-Fe crystal lattice. Ferrite exists at relatively high temperatures, hence it is called high-temperature ferrite. Pure iron has a body-centered cubic crystal lattice below 912℃. α-Fe has a body-centered cubic crystal structure with very small interstitial spaces, resulting in extremely poor carbon solubility. The maximum carbon solubility is reached at 727℃, reaching 0.0218%. The carbon solubility gradually decreases with decreasing temperature, reaching approximately 0.0057% at 600℃ and approximately 0.0008% at room temperature.

[0051] Cementite: a metallic compound formed by iron and carbon, containing 6.69% carbon and having a melting point of 1227°C.

[0052] Austenite: Ferrite will transform into austenite at temperatures between 912℃ and 1394℃, changing from a body-centered cubic lattice structure to a face-centered cubic lattice structure.

[0053] Pearlite is a microstructure consisting of alternating layers of ferrite and cementite that precipitates simultaneously with the eutectoid transformation of austenite. Pearlite is a mechanical mixture of ferrite and cementite.

[0054] The inventors of this application discovered during their research that non-metallic inclusions in bearing steel have a significant impact on the material's fatigue failure. Production practice shows that the harmful effects of large-sized hard TiN inclusions on high-quality bearing steel are becoming increasingly apparent. The impact of TiN inclusions on the fatigue life of bearing steel is far greater than that of oxide inclusions of the same size; a 6μm TiN inclusion has a similar negative impact on bearing fatigue life as a 25μm oxide inclusion.

[0055] In existing technologies, the control of TiN inclusions in bearing steel mainly focuses on two aspects: one is to suppress the precipitation of large-sized TiN inclusions during the solidification process of molten steel by controlling the content of Ti and N elements in the original material. Currently, through relevant measures, the Ti content in bearing steel can be stably controlled below 0.0015% and the N content below 0.0030%, i.e., ultra-low titanium or ultra-low nitrogen bearing steel; the other is to add elements such as Ce to bearing steel to suppress the precipitation of TiN inclusions during the solidification process of molten steel.

[0056] However, current high-end bearing steels have reached ultra-pure levels, making it difficult and costly to further reduce impurities such as O, Ti, and N in the raw steel material. Adding elements like Ce to molten steel can lead to turbulence and the introduction of larger inclusions. Furthermore, during billet heating, the size of TiN inclusions changes due to the Oswald curing process. Smaller TiN inclusions move between larger ones, creating a concentration gradient of Ti and N. Ti and N elements around smaller inclusions diffuse towards larger ones, causing the smaller inclusions to dissolve or even disappear, while the larger inclusions coarsen and grow. Although the solid solubility of TiN inclusions in steel is low and the coarsening rate is small, high-end bearing steel requires ultra-high temperature (≥1200℃) and ultra-long time (≥24 hours) homogenization diffusion annealing processes. Therefore, during high-temperature diffusion annealing, TiN inclusion particles also undergo significant coarsening.

[0057] In view of this, this application provides a heat treatment method for a workpiece, which includes four processing steps. In the first heat treatment step, the initial state workpiece is annealed to obtain an intermediate state workpiece a, so as to refine the grains and eliminate the internal stress of the initial state workpiece to prevent cracking during subsequent processing. In the second heat treatment step, the intermediate state workpiece a is heat-treated at different temperatures in a second temperature range to obtain an intermediate state workpiece b, thereby achieving homogenized annealing of the workpiece. In the third heat treatment step, the intermediate state workpiece b is heat-treated at different temperatures in a third temperature range. By setting the highest temperature in the second temperature range to be higher than the highest temperature in the third temperature range, TiN in the intermediate state workpiece b precipitates at low temperatures and dissolves back at high temperatures in the third temperature range. This avoids abnormal growth of TiN in the third temperature range and reduces the size of TiN non-metallic inclusions. In the fourth processing step, the intermediate state workpiece c is rolled to disperse the precipitation of TiN inclusions. This heat treatment method for the workpiece avoids abnormal growth of TiN inclusions in the intermediate state workpiece c.

[0058] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will now be described with reference to the accompanying drawings.

[0059] Reference Figure 1 As shown, this application provides a heat treatment method for a workpiece, which includes the following steps:

[0060] S100: First heat treatment process, annealing the initial state workpiece at a first temperature to obtain intermediate state workpiece a;

[0061] S200: The second heat treatment process involves heat treating the intermediate workpiece a at different temperatures in a second temperature range to obtain the intermediate workpiece b.

[0062] S300: The third heat treatment process, which involves heat treating the intermediate workpiece b at different temperatures in the third temperature range to obtain the intermediate workpiece c.

[0063] S400: The fourth processing step, rolling the intermediate workpiece c that has been taken out of the furnace in the third temperature range to obtain the final workpiece.

[0064] Among them, the first temperature is higher than the eutectoid transformation temperature of the initial state workpiece; the lowest temperature in the second and third temperature ranges is higher than the first temperature; and the highest temperature in the second temperature range is higher than the highest temperature in the third temperature range.

[0065] For example, in the first heat treatment process, the initial state workpiece is annealed, wherein the initial state workpiece includes titanium nitride (TiN). By annealing the initial state workpiece, the initial state workpiece undergoes a grain nucleation and growth process to refine the grains of the initial state workpiece, resulting in an intermediate state workpiece a with refined grains. In this way, not only is grain refinement achieved, but the internal stress of the workpiece is also eliminated to prevent cracking during subsequent processing. In addition, after grain refinement, the intermediate state workpiece a has more grain boundaries, which can suppress the abnormal growth of TiN during subsequent heat treatment.

[0066] In the second heat treatment step, intermediate workpiece a is heat-treated at different temperatures within a second temperature range to obtain intermediate workpiece b. In the third heat treatment step, intermediate workpiece b is heat-treated at different temperatures within a third temperature range. By sequentially heat-treating intermediate workpiece a at different temperatures within the second and third temperature ranges, a segmented approach is achieved to homogenize the heat treatment of intermediate workpiece a. In the third heat treatment step, TiN in intermediate workpiece b precipitates as small TiN inclusions at lower temperatures and undergoes re-dissolution at higher temperatures. By setting the highest temperature in the second temperature range higher than the highest temperature in the third temperature range, and controlling the precipitation of fine TiN followed by TiN re-dissolution in the third temperature range, the migration rate of small TiN inclusions to large TiN inclusions is slowed down, abnormal growth of TiN in the third temperature range is avoided, and the size of the TiN non-metallic inclusions is reduced.

[0067] In the fourth processing step, the intermediate workpiece c is rolled to disperse the TiN inclusions and obtain the final workpiece, which contains fine, dispersed TiN inclusions.

[0068] As one feasible implementation, the initial state workpiece includes bearing steel.

[0069] As one feasible implementation, the initial state workpiece includes pearlite, cementite, and titanium nitride; the intermediate state workpiece a includes pearlite, cementite, and titanium nitride; the intermediate state workpiece b includes austenite and titanium nitride; the intermediate state workpiece c includes austenite and titanium nitride; and the final state workpiece includes pearlite, cementite, and titanium nitride.

[0070] For example, the initial state workpiece can be bearing steel, and it includes pearlite, cementite, and titanium nitride (TiN). The initial state workpiece is often obtained directly after casting, and the pearlite in the initial state workpiece is relatively coarse. During the holding stage of the first heat treatment process, a phase transformation occurs in the pearlite, transforming it into austenite, while the cementite undergoes re-dissolution. During the cooling process after the holding stage of the first heat treatment process, the cementite re-precipitates, and the austenite undergoes a re-nucleation and growth process, transforming into pearlite. Through re-austenitization, the intermediate state workpiece a includes pearlite with refined grains.

[0071] Intermediate workpiece a includes pearlite, cementite, and titanium nitride. During the second heat treatment process, both cementite and some titanium nitride undergo re-dissolution. By performing homogenization annealing on intermediate workpiece a, intermediate workpiece b, which includes austenite and titanium nitride, is obtained.

[0072] Intermediate workpiece b includes austenite and titanium nitride. In the third heat treatment process, by setting the highest temperature of the second temperature range to be higher than the highest temperature of the third temperature range, intermediate workpiece b first precipitates some TiN when it is in the second temperature range to the third temperature range. Then, intermediate workpiece b is subjected to homogenization annealing treatment to obtain intermediate workpiece c, which includes austenite and titanium nitride.

[0073] The final state workpiece consists of pearlite, cementite, and titanium nitride. In the fourth processing step, the intermediate state workpiece c is rolled, causing cementite and titanium nitride to reprecipitate, and austenite to undergo a pearlite transformation. Furthermore, since rolling can be completed in a short time, after the fourth processing step, titanium nitride is dispersed in the pearlite.

[0074] As one feasible implementation, the grain size of the pearlite in the intermediate state workpiece a is smaller than the grain size of the pearlite in the initial state workpiece.

[0075] For example, the first temperature is higher than the eutectoid transformation temperature of the initial state workpiece; by annealing the initial state workpiece at the first temperature, during the holding stage, pearlite transforms into austenite, cementite undergoes re-dissolution, and during the cooling stage, austenite recrystallizes, austenite re-nucleates and grows, austenite transforms into pearlite, and cementite precipitates. In this way, refined pearlite can be obtained through the first heat treatment process. Therefore, the grain size of the pearlite in the intermediate state workpiece a is smaller than the grain size of the pearlite in the initial state workpiece.

[0076] As one feasible implementation, the first heat treatment step, which involves annealing the initial state workpiece at a first temperature to obtain the intermediate state workpiece a, includes:

[0077] The initial state workpiece is kept at a first temperature to obtain the intermediate state workpiece a1;

[0078] Intermediate workpiece a1 is cooled with the furnace to the first furnace cooling temperature to obtain intermediate workpiece a.

[0079] As one feasible implementation, the heating rate range from the initial temperature to the first temperature is greater than 0℃ / h and less than or equal to 150℃ / h.

[0080] As one feasible implementation, the range of the first temperature is greater than or equal to 880°C and less than or equal to 920°C.

[0081] As one feasible implementation, the range of the first furnace cooling temperature is greater than or equal to 600°C and less than or equal to 650°C.

[0082] As one feasible implementation, the initial state workpiece is held at the first temperature for a period of 30 minutes or less than or equal to 60 minutes.

[0083] For example, in the first heat treatment process, the initial state workpiece is first held at a first temperature, wherein the first temperature is higher than the eutectoid transformation temperature of the initial state workpiece. When the initial state workpiece is bearing steel, the bearing steel includes pearlite, cementite, and titanium nitride. The first temperature is 150°C-200°C above the eutectoid transformation temperature of the bearing steel, so as to dissolve the cementite in the bearing steel back into austenite.

[0084] By setting the heating rate range from the initial temperature to the first temperature to be greater than 0℃ / h and less than or equal to 150℃ / h, the cracking problem caused by the rapid heating rate of the initial workpiece is avoided.

[0085] By setting the first temperature to greater than or equal to 880°C, complete redissolution of cementite into austenite is ensured; by setting the first temperature to less than or equal to 920°C, redissolution of titanium nitride is avoided.

[0086] For example, after the initial state workpiece is held at a first temperature, an intermediate state workpiece a1 is obtained. Subsequently, the intermediate state workpiece a1 is cooled in the furnace. During the furnace cooling process, cementite recrystallizes, austenite recrystallizes, austenite nucleates and grows again, and austenite transforms into pearlite. This is beneficial for obtaining bearing steel with uniform chemical composition and microstructure, eliminating the internal stress of the initial state workpiece, and preventing deformation and cracking of the workpiece in subsequent processing.

[0087] For example, by setting the holding time of the initial workpiece at the first temperature to be greater than or equal to 30 minutes, complete dissolution of cementite into austenite is ensured. By setting the holding time of the initial workpiece at the first temperature to be less than or equal to 60 minutes, while ensuring the dissolution of cementite into austenite, the internal stress and work hardening of the initial workpiece are eliminated, preventing deformation and cracking during subsequent processing. By setting the range of the first furnace cooling temperature to be greater than or equal to 600°C and less than or equal to 650°C, austenite recombination and growth are facilitated, thus transforming austenite into pearlite and obtaining pearlite with refined grains.

[0088] As one feasible implementation, the second heat treatment step involves subjecting the intermediate workpiece a to heat treatment at different temperatures within a second temperature range to obtain the intermediate workpiece b, including:

[0089] The second temperature range includes a second low temperature and a second high temperature. Intermediate workpiece a is kept at the second low temperature to obtain intermediate workpiece b1.

[0090] Intermediate workpiece b1 is kept at a second high temperature to obtain intermediate workpiece b2;

[0091] Intermediate workpiece b2 is cooled in the furnace to the second furnace cooling temperature to obtain intermediate workpiece b.

[0092] For example, in the second heat treatment process, the intermediate workpiece a is first heated to a second low temperature and held at the second low temperature for a short time to obtain intermediate workpiece b1. Then, the intermediate workpiece b1 is heated to a second high temperature and held at the temperature for a long time, during which cementite and part of titanium nitride dissolve back to obtain intermediate workpiece b2, which includes austenite and titanium nitride.

[0093] In some embodiments, the heating rate range from the first furnace cold temperature to the second low temperature is greater than 0°C / h and less than or equal to 150°C / h.

[0094] In some embodiments, the heating rate range from the second low temperature to the second high temperature is greater than 0°C / h and less than or equal to 150°C / h.

[0095] By setting the heating rate range from the first cold temperature to the second low temperature to be greater than 0℃ / h and less than or equal to 150℃ / h, and setting the heating rate range from the second low temperature to the second high temperature to be greater than 0℃ / h and less than or equal to 150℃ / h, the cracking problem caused by the rapid heating rate of the intermediate workpiece a is avoided.

[0096] In some embodiments, the holding time of the intermediate workpiece a at the second low temperature is greater than or equal to 30 min and less than or equal to 60 min.

[0097] In some embodiments, the holding time of the intermediate workpiece a1 at the second high temperature is greater than or equal to 8 hours and less than or equal to 12 hours.

[0098] By setting the holding time of intermediate workpiece a at the second low temperature to be greater than or equal to 30 min and less than or equal to 60 min, the intermediate workpiece a was ensured to be heated evenly, thus preventing cracking of intermediate workpiece a during subsequent heating. By setting the holding time of intermediate workpiece a1 at the second high temperature to be greater than or equal to 8 h and less than or equal to 12 h, the heat treatment of intermediate workpiece a1 was homogenized.

[0099] In some embodiments, the range of the second low temperature is greater than or equal to 840°C and less than or equal to 860°C.

[0100] In some embodiments, the range of the second high temperature is greater than or equal to 1200°C and less than or equal to 1250°C.

[0101] By setting the second low temperature to greater than or equal to 840°C and less than or equal to 860°C, the intermediate workpiece a is uniformly heated at the second low temperature without undergoing a phase transformation reaction, thus avoiding the introduction of other impurity phases. By setting the range of the second high temperature to greater than or equal to 1200°C and less than or equal to 1250°C, the cementite and TiN in the intermediate workpiece a1 dissolve back at the second high temperature, thereby achieving high-temperature homogenization annealing treatment for the intermediate workpiece a1.

[0102] By setting the second heat treatment process to two temperature ranges, short-term heat preservation at the second low temperature and long-term heat preservation at the second high temperature, the cracking problem that may occur when the intermediate workpiece a is directly heated to the second high temperature is avoided, and the cementite and titanium nitride are ensured to dissolve back into austenite, thus achieving the homogenization annealing treatment of the intermediate workpiece a.

[0103] As one feasible implementation method, the third heat treatment process involves holding the intermediate workpiece b at different temperatures within a third temperature range to obtain the intermediate workpiece c, including:

[0104] The third temperature range includes the third low temperature and the third high temperature; the second furnace cooling temperature is equal to the third low temperature. Intermediate workpiece b is kept at the third low temperature to obtain intermediate workpiece c1, and intermediate workpiece c1 is kept at the third high temperature to obtain intermediate workpiece c.

[0105] For example, in the third heat treatment process, the intermediate workpiece b is first cooled to a third low temperature and held at the third low temperature for a short time to obtain the intermediate workpiece c1. When the intermediate workpiece b is subjected to low-temperature heat treatment at the third low temperature, titanium nitride precipitates, and the intermediate workpiece b includes fine titanium nitride distributed in austenite, thus obtaining the intermediate workpiece c1. Subsequently, the intermediate workpiece c1 is heated to a third high temperature and held at that temperature for a long time, and the cementite and fine titanium nitride dissolve back into the austenite to obtain the intermediate workpiece c, which includes austenite and titanium nitride.

[0106] By controlling the precipitation of titanium nitride in intermediate workpiece b at a third low temperature and the re-dissolution of titanium nitride into austenite at a third high temperature, intermediate workpiece b is processed in stages in the third heat treatment process. Titanium nitride is introduced at low temperature and dissolved at high temperature, which avoids the Ostwald ripening phenomenon caused by the direct aggregation of titanium nitride in intermediate workpiece c1 during long-term heat treatment and avoids the abnormal growth of titanium nitride particles.

[0107] In some embodiments, the range of the third low temperature is greater than or equal to 1050°C and less than or equal to 1100°C.

[0108] In some embodiments, the range of the third high temperature is greater than or equal to 1180°C and less than or equal to 1200°C.

[0109] The precipitation of titanium nitride is ensured by setting the third low temperature to be greater than or equal to 1050℃ and less than or equal to 1100℃; the homogenization heat treatment of intermediate workpiece c1 is ensured by setting the range of the third high temperature to be greater than or equal to 1180℃ and less than or equal to 1200℃. At the same time, the third high temperature is lower than the second high temperature to avoid the aggregation of dissolved titanium nitride in austenite.

[0110] By setting the third heat treatment process into two temperature ranges, a short-term holding at the third low temperature promotes the precipitation of titanium nitride; and a long-term holding at the third high temperature ensures that the cementite is completely dissolved back into austenite, thus achieving homogenized heat treatment of intermediate workpiece b. At the same time, it avoids the Ostwald curing phenomenon caused by the direct aggregation of titanium nitride in intermediate workpiece c1 during long-term heat treatment.

[0111] In some embodiments, the holding time of the intermediate workpiece b at the third low temperature is greater than or equal to 30 min and less than or equal to 60 min.

[0112] In some embodiments, the holding time of the intermediate workpiece b1 at the third high temperature is greater than or equal to 8 hours and less than or equal to 12 hours.

[0113] By setting the holding time of intermediate workpiece b at the third low temperature to be greater than or equal to 30 min and less than or equal to 60 min, uniform heating of all parts of intermediate workpiece b is ensured, avoiding cracking of intermediate workpiece b during subsequent heating, while ensuring sufficient precipitation time for titanium nitride; by setting the holding time of intermediate workpiece b1 at the third high temperature to be greater than or equal to 8 h and less than or equal to 12 h, homogenized heat treatment of intermediate workpiece c1 is achieved.

[0114] In some embodiments, the heating rate range from the third low temperature to the third high temperature is greater than 0°C / h and less than or equal to 150°C / h.

[0115] By setting the heating rate range from the third low temperature to the third high temperature to be greater than 0℃ / h and less than or equal to 150℃ / h, the cracking problem caused by the rapid heating rate of the intermediate workpiece b is avoided.

[0116] In Example 1, the first heat treatment step involves heating the initial workpiece in a soaking pit to 880°C at a heating rate of 150°C / h and holding it there for 30 minutes to obtain an intermediate workpiece a1. The intermediate workpiece a1 is then cooled in the furnace to 600°C to obtain intermediate workpiece a. The second heat treatment step involves heating the intermediate workpiece a in a soaking pit to 850°C at a heating rate of 150°C / h and holding it there for 30 minutes to obtain an intermediate workpiece b1. The intermediate workpiece b1 is then rapidly heated in the soaking pit to 120°C. The intermediate workpiece b is obtained by first high-temperature diffusion annealing at 00℃ for 8 hours. The intermediate workpiece b is then cooled to 1050℃ in the furnace and held for 30 minutes to obtain intermediate workpiece c1. Subsequently, intermediate workpiece c1 is rapidly heated to 1180℃ in a soaking pit and held for 12 hours to obtain intermediate workpiece c. The intermediate workpiece c is then obtained by second high-temperature diffusion annealing. The intermediate workpiece c is rolled at 1180℃.

[0117] Reference Figures 2 to 4 As shown, in the statistical process of TiN inclusions, with a Ti mass fraction of 0.0012%, a N mass fraction of 0.0038%, and a statistical area of ​​3000 square millimeters, the number of TiN inclusions with a diameter greater than or equal to 2 μm and less than 3 μm was 87, the number of TiN inclusions with a diameter greater than or equal to 3 μm and less than 5 μm was 19, the number of TiN inclusions with a diameter greater than or equal to 5 μm and less than 9 μm was 2, and the number of TiN inclusions with a diameter greater than or equal to 9 μm was 0.

[0118] In Example 2, the first heat treatment step involves heating the initial workpiece to 920°C in a soaking pit at a heating rate of 150°C / h and holding it there for 60 minutes to obtain an intermediate workpiece a1. The intermediate workpiece a1 is then cooled in the furnace to 650°C to obtain intermediate workpiece a. The second heat treatment step involves heating the intermediate workpiece a to 850°C in a soaking pit at a heating rate of 150°C / h and holding it there for 60 minutes to obtain an intermediate workpiece b1. The intermediate workpiece b1 is then rapidly heated to 120°C in the soaking pit. The intermediate workpiece b is obtained by first high-temperature diffusion annealing at 50℃ for 12 hours. The intermediate workpiece b is then cooled to 1100℃ in the furnace and held for 60 minutes to obtain intermediate workpiece c1. Subsequently, intermediate workpiece c1 is rapidly heated to 1180℃ in a soaking pit and held for 12 hours to obtain intermediate workpiece c. The intermediate workpiece c is then obtained by second high-temperature diffusion annealing. The intermediate workpiece c is rolled at 1180℃.

[0119] Reference Figures 2 to 5 As shown, in the statistical process of TiN inclusions, with a Ti mass fraction of 0.0012%, a N mass fraction of 0.0038%, and a statistical area of ​​3000 square millimeters, the number of TiN inclusions with a diameter greater than or equal to 2 μm and less than 3 μm was 92, the number of TiN inclusions with a diameter greater than or equal to 3 μm and less than 5 μm was 17, the number of TiN inclusions with a diameter greater than or equal to 5 μm and less than 9 μm was 3, and the number of TiN inclusions with a diameter greater than or equal to 9 μm was 0.

[0120] In Example 3, the first heat treatment step involves heating the initial workpiece to 900°C in a soaking pit at a heating rate of 150°C / h and holding it there for 45 minutes to obtain an intermediate workpiece a1. The intermediate workpiece a1 is then cooled in the furnace to 620°C to obtain intermediate workpiece a. The second heat treatment step involves heating the intermediate workpiece a to 850°C in a soaking pit at a heating rate of 150°C / h and holding it there for 45 minutes to obtain an intermediate workpiece b1. The intermediate workpiece b1 is then rapidly heated to 1... The intermediate workpiece b is obtained by high-temperature diffusion annealing at 230℃ for 10 hours. The intermediate workpiece b is then cooled to 1080℃ in the furnace and held for 40 minutes to obtain intermediate workpiece c1. Subsequently, intermediate workpiece c1 is rapidly heated to 1190℃ in a soaking pit and held for 10 hours to obtain intermediate workpiece c. The intermediate workpiece c is then obtained by high-temperature diffusion annealing at 1190℃.

[0121] Reference Figures 2 to 4As shown, in the statistical process of TiN inclusions, with a Ti mass fraction of 0.0012% and a N mass fraction of 0.0038%, and a statistical area of ​​3000 square millimeters, the number of TiN inclusions with a diameter greater than or equal to 2 μm and less than 3 μm was 79, the number of TiN inclusions with a diameter greater than or equal to 3 μm and less than 5 μm was 20, the number of TiN inclusions with a diameter greater than or equal to 5 μm and less than 9 μm was 2, and the number of TiN inclusions with a diameter greater than or equal to 9 μm was 1.

[0122] In the comparative example, refer to Figures 2 to 5 As shown, the initial state workpiece was heated to 850℃ in a soaking pit at a heating rate of 150℃ / h and held for 30-60 min. Then, the intermediate state workpiece was rapidly heated to 1200℃-1250℃ and held for 24 h. In the statistical analysis of TiN inclusions, based on a Ti mass fraction of 0.0012%, a N mass fraction of 0.0038%, and a statistical area of ​​3000 square millimeters, the number of TiN inclusions with a diameter greater than or equal to 2 μm and less than 3 μm was 104, the number of TiN inclusions with a diameter greater than or equal to 3 μm and less than 5 μm was 36, the number of TiN inclusions with a diameter greater than or equal to 5 μm and less than 9 μm was 14, and the number of TiN inclusions with a diameter greater than or equal to 9 μm was 5.

[0123] Through the above Examples 1-3 and Comparative Examples, the heat treatment method of the workpiece of this application significantly reduces the abnormal growth of titanium nitride during the high-temperature homogenization process.

[0124] It is understood that bearing steel also includes impurity phases, but the content of impurity phases is much lower than that of any of the pearlite, cementite and titanium nitride mentioned in this application in bearing steel, and the impurity phases do not have a substantial impact on the heat treatment process of bearing steel.

[0125] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this application are indicated by the following claims.

[0126] It should be understood that this application is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this application is limited only by the appended claims.

Claims

1. A heat treatment method for a workpiece, characterized in that, The heat treatment method for the workpiece includes the following steps: In the first heat treatment process, the initial state workpiece is annealed at a first temperature to obtain the intermediate state workpiece a; In the second heat treatment process, the intermediate workpiece a is subjected to heat treatment at different temperatures in a second temperature range to obtain the intermediate workpiece b; thus achieving homogenization annealing treatment of the workpiece. In the third heat treatment process, the intermediate workpiece b is subjected to heat treatment at different temperatures in a third temperature range to obtain the intermediate workpiece c; in the third temperature range, TiN precipitates in the intermediate workpiece b at low temperatures and dissolves back at high temperatures. The fourth processing step involves rolling the intermediate workpiece c, which has been removed from the furnace at the third temperature range, to obtain the final workpiece. Wherein, the first temperature is higher than the eutectoid transformation temperature of the initial state workpiece; the lowest temperature of both the second temperature range and the third temperature range is higher than the first temperature; the highest temperature of the second temperature range is higher than the highest temperature of the third temperature range; The first heat treatment step, wherein annealing the initial state workpiece at a first temperature to obtain the intermediate state workpiece a, includes: The initial state workpiece is kept at the first temperature to obtain the intermediate state workpiece a1; The intermediate workpiece a1 is cooled in the furnace to the first furnace cooling temperature to obtain the intermediate workpiece a; The second heat treatment step, wherein the intermediate workpiece a is subjected to heat treatment at different temperatures in a second temperature range to obtain intermediate workpiece b, includes: The second temperature range includes a second low temperature and a second high temperature. The intermediate workpiece a is kept at the second low temperature to obtain the intermediate workpiece b1. The intermediate workpiece b1 is kept at the second high temperature to obtain the intermediate workpiece b2; The intermediate workpiece b2 is cooled in the furnace to the second furnace cooling temperature to obtain the intermediate workpiece b; The third heat treatment process, which involves holding the intermediate workpiece b at different temperatures within a third temperature range to obtain the intermediate workpiece c, includes: The third temperature range includes a third low temperature and a third high temperature; the second furnace cooling temperature is equal to the third low temperature, the intermediate workpiece b is kept at the third low temperature to obtain the intermediate workpiece c1, and the intermediate workpiece c1 is kept at the third high temperature to obtain the intermediate workpiece c. The range of the first furnace cooling temperature is greater than or equal to 600°C and less than or equal to 650°C; The range of the third low temperature is greater than or equal to 1050°C and less than or equal to 1100°C; The initial state workpiece is bearing steel.

2. The heat treatment method for a workpiece according to claim 1, characterized in that, The range of the first temperature is greater than or equal to 880°C and less than or equal to 920°C; And / or, the range of the second low temperature is greater than or equal to 840°C and less than or equal to 860°C; And / or, the range of the second high temperature is greater than or equal to 1200°C and less than or equal to 1250°C; And / or, the range of the third high temperature is greater than or equal to 1180°C and less than or equal to 1200°C.

3. The heat treatment method for a workpiece according to claim 2, characterized in that, The heating rate range from the initial temperature to the first temperature is greater than 0°C / h and less than or equal to 150°C / h; And / or, the heating rate range from the first furnace cooling temperature to the second low temperature is greater than 0°C / h and less than or equal to 150°C / h; And / or, the heating rate range from the second low temperature to the second high temperature is greater than 0°C / h and less than or equal to 150°C / h; And / or, the heating rate range from the third low temperature to the third high temperature is greater than 0°C / h and less than or equal to 150°C / h.

4. The heat treatment method for a workpiece according to claim 2, characterized in that, The initial state workpiece is held at the first temperature for a period of time that is greater than or equal to 30 min and less than or equal to 60 min. And / or, the holding time of the intermediate workpiece a at the second low temperature is greater than or equal to 30 min and less than or equal to 60 min; And / or, the holding time of the intermediate workpiece a1 at the second high temperature is greater than or equal to 8 h and less than or equal to 12 h; And / or, the intermediate workpiece b is kept at the third low temperature for a period of 30 min or less than or equal to 60 min. And / or, the intermediate workpiece b1 is kept at the third high temperature for a period of 8 hours or less than or equal to 12 hours.

5. The heat treatment method for a workpiece according to claim 1, characterized in that, The initial state workpiece includes pearlite, cementite, and titanium nitride; the intermediate state workpiece a includes pearlite, cementite, and titanium nitride; the intermediate state workpiece b includes austenite and titanium nitride; the intermediate state workpiece c includes austenite and titanium nitride; and the final state workpiece includes pearlite, cementite, and titanium nitride.

6. The heat treatment method for a workpiece according to claim 5, characterized in that, The pearlite grain size of the intermediate state workpiece a is smaller than that of the pearlite grain size of the initial state workpiece.

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