A method for producing a lath martensite and / or bainite-based hypereutectoid steel

Abstract

The present application belongs to the technical field of steel material heat treatment, and particularly relates to a preparation method of a lath martensite and / or bainite-based hypereutectoid steel. A granular pearlite organization hypereutectoid steel without net-like carbide is subjected to pearlitization, rapid austenitization and subsequent heat treatment to obtain a lath martensite and / or bainite-based hypereutectoid steel, which contains a uniform organization with lath martensite and / or bainite as the main matrix, and between which, residual austenite mainly in the form of sheets can be distributed, and on which, unsolved globular carbides are uniformly distributed. Not only the uniformity of the organization is improved, but also the residual austenite is stabilized, so that the toughness, wear resistance and fatigue performance can be greatly improved on the premise of ensuring high strength and high hardness. The technology is suitable for cold / hot working die steel, bearing steel, high-speed steel, spring steel, gear steel and other hypereutectoid steels.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for steel materials, specifically relating to a method for preparing lath martensitic and / or bainitic hypereutectoid steel. Background Technology

[0002] Hypereutectoid steels, with their high carbon content, possess excellent hardness, strength, wear resistance, and rolling contact fatigue properties, making them widely used in tooling, aerospace, precision robotics, medical, mining, railway, and large-scale infrastructure projects. Currently, the main heat treatment processes for industrially used hypereutectoid steels, such as tool steels (cutting steel, die steel, and measuring tool steel) and bearing steels, include normalizing, spheroidizing annealing, and quenching-tempering or isothermal quenching. To further enhance the performance of steels to meet increasingly demanding service environments, researchers have improved upon these traditional heat treatment processes, such as the thermomechanical deformation-isothermal diffusion cyclic heat treatment process in Chinese patent CN 116497197 A. Other improvements include incorporating cold / hot deformation before quenching, cold treatment after quenching, aging after tempering, and multiple tempering processes. The main objectives are to refine grains, refine carbides, and improve microstructure uniformity, maximizing ductility, toughness, and wear resistance while maintaining high strength and hardness to cope with more complex service environments such as high speed, heavy load, impact, and high temperature.

[0003] The microstructure obtained after spheroidizing annealing is a polygonal ferrite matrix with spherical carbides distributed on it. The subsequent quenching-tempering process yields a microstructure composed of martensite, retained austenite, and spherical carbides. Due to the uneven distribution of carbides after spheroidizing annealing, cryptocrystalline martensite forms in areas with a higher distribution of spherical carbides after quenching, while crystalline martensite forms in areas with a lower distribution. Furthermore, since cryptocrystalline martensite has a lower carbon content than crystalline martensite, there is a significant difference in hardness between the two types of martensite, which easily leads to stress concentration, resulting in crack initiation and propagation, thus hindering the improvement of impact toughness and rolling contact fatigue performance. To address this, Chinese patent CN 115558769 A discloses a heat treatment method to improve the carbide distribution and microstructure uniformity of high-carbon chromium bearing steel, which involves heating the spheroidized annealed workpiece at a certain heating rate to... Ac 1 The following insulation platform undergoes uniform or oscillating insulation, and is then heated at a certain rate to [temperature missing]. Ac 1 The traditional quenching process is performed at 60~100℃. Although this method improves the uneven distribution of carbides and reduces the size of cryptocrystalline martensite and crystalline martensite regions, thereby improving the uniformity of the microstructure and increasing fatigue life, a considerable number of crystalline martensite regions still exist.

[0004] Crystalline martensite mainly consists of lamellar martensite and retained austenite distributed among them, while cryptocrystalline martensite mainly consists of lath martensite and retained austenite distributed among them. The substructure of lath martensite is generally dislocation-based, exhibiting a better strength-toughness balance than lamellar martensite. The substructure of lamellar martensite is generally twin-based; twins can hinder dislocation movement, resulting in twinned martensite having higher strength and hardness than lamellar martensite. However, lamellar martensite is prone to collision and compression during growth, often leading to significant internal stress and even cracks at their boundaries, which is detrimental to toughness. To further improve toughness, isothermal quenching was invented to prepare a matrix structure that mixes martensite and lower bainite; the lower bainite has slightly lower strength but better toughness, improving toughness while sacrificing a small amount of strength. However, isothermal quenching can also produce inhomogeneous structures similar to those of cryptocrystalline and crystalline martensite.

[0005] Retained austenite is a metastable phase in hypereutectoid steel. Under surface contact stress and other conditions, it may undergo martensitic transformation, causing volume expansion. Excessive transformation can lead to dimensional changes in the workpiece, significantly reducing the lifespan of precision components. In traditional heat treatment processes for hypereutectoid steel, a large amount of blocky retained austenite is distributed, especially in the crystalline martensite region. Notably, lamellar retained austenite exhibits higher stability than blocky retained austenite; lamellar retained austenite has a crack-deflecting effect, while blocky retained austenite, after transformation, is more likely to cause crack initiation and propagation. Therefore, how to prepare retained austenite with a predominantly lamellar morphology to improve the strength-toughness balance is a crucial technical problem that urgently needs to be solved.

[0006] To address the above issues, there is an urgent need to develop a novel heat treatment process for hypereutectoid steel to prepare a microstructure with lath martensite and / or bainite as the main matrix. This microstructure not only has a uniform distribution but also contains residual austenite with a predominantly lamellar morphology. This would improve toughness, wear resistance, fatigue performance, and dimensional stability while achieving high strength and hardness. Summary of the Invention

[0007] Based on the above factors, this application obtains a lath martensitic and / or martensitic hypereutectoid steel by pearlitizing, rapidly austenitizing, and subsequently heat-treating granular pearlitic hypereutectoid steel that meets the rating requirements of GB / T 18254-2016. This steel contains a uniform microstructure with lath martensite and / or bainite as the main matrix, interspersed with predominantly lamellar retained austenite or lamellar carbides, on which spherical carbides are relatively uniformly distributed. This not only improves the uniformity of the microstructure but also stabilizes the retained austenite, thereby significantly improving toughness, wear resistance, and fatigue resistance while ensuring high strength and high hardness. This technology is applicable to hypereutectoid steels such as cold / hot work die steels, bearing steels, high-speed steels, spring steels, and gear steels.

[0008] To achieve the above objectives, the first technical solution of this application discloses a method for preparing lath martensitic and / or bainitic hypereutectoid steel, comprising the following steps:

[0009] S1. Lamellar pearlitization: Hypereutectoid steel with granular pearlite structure that does not contain network carbides is first heated to the temperature range of the two-phase region where cementite and austenite coexist, and then held at the first temperature. It is then cooled to the two-phase region where ferrite and cementite coexist, and held at the second temperature. Finally, it is cooled to obtain steel with spherical carbides distributed in the lamellar pearlite matrix as the microstructure.

[0010] S2. Rapid austenitization: High-temperature austenitized steel is obtained by rapidly heating lamellar pearlite matrix steel to the temperature range of single-phase austenite and then holding it for a third time.

[0011] S3. Subsequent heat treatment: After the high-temperature austenitized steel is treated by any one of the following heat treatment methods: quenching-tempering, quenching-partitioning, isothermal quenching, or mixed martensite-bainite process, it is cooled to room temperature to obtain lath martensite and / or bainite-based hypereutectoid steel.

[0012] The raw materials of the hypereutectoid steel with granular pearlitic structure that does not contain network carbides contain at least one or a combination of Mn and Cr elements.

[0013] Furthermore, when the hypereutectoid steel with granular pearlite structure that does not contain network carbides contains only Mn, its content is ≥1.0wt%; when it contains only Cr, its content is ≥0.8wt%; when it contains both Mn and Cr, the content of Mn is 0.5wt%~8.0wt% and the content of Cr is 0.5wt%~3.0wt%.

[0014] Preferably, when the hypereutectoid steel with granular pearlite structure that does not contain network carbides contains only Mn, its content is 2.5wt%~3.0wt%; when it contains only Cr, its content is 1.0wt%~1.5wt%; when it contains both Mn and Cr, the content of Mn is 1.0wt%~2.0wt% and the content of Cr is 1.0wt%~1.5wt%.

[0015] Furthermore, the temperature range of the cementite and austenite two-phase region described in S1 is Ae1~Ae cm Between these times, the first heat preservation time is 10-120 minutes.

[0016] Furthermore, the two-phase region where ferrite and cementite coexist as described in S1 is the temperature range within which critical austenite undergoes pearlite transformation, and the second holding time is 1-72 hours.

[0017] Furthermore, the rapid heating rate described in S2 is ≥1℃ / s.

[0018] Furthermore, the single-phase austenite temperature range described in S2 is the critical austenite temperature range. Ae cm The temperature ranges from 20 to 150 ℃, and the third heat preservation time is 0-180 s.

[0019] Furthermore, step S3 may also incorporate a cold treatment process.

[0020] Furthermore, the lath martensitic and / or bainitic hypereutectoid steel obtained according to the above preparation method comprises at least nano- or submicron-sized lath martensitic and / or bainitic microstructures.

[0021] Furthermore, its microstructure may also include lamellar retained austenite and / or lamellar carbides.

[0022] Beneficial effects

[0023] (1) Compared with the traditional heat treatment process of hypereutectoid steel, the present invention has developed a new heat treatment process. By introducing pearlite transformation after spheroidizing annealing and combining it with rapid heating technology, high-temperature austenite with non-uniform chemical composition is formed. After cooling to room temperature or a certain temperature, the formation of crystalline martensite can be avoided. A uniform matrix structure mainly composed of lath martensite and / or bainite is successfully prepared, which improves the uniformity of microstructure, impact toughness and fatigue performance, and reduces wear rate.

[0024] (2) This invention utilizes the non-uniform distribution of the chemical composition of high-temperature austenite to refine martensite and / or bainite, thereby ensuring high strength and high hardness. It is worth noting that the morphology and content of retained austenite can be controlled by changing the pearlitization process parameters and the rapid heating process parameters, to obtain retained austenite with a predominantly lamellar morphology, and the volume fraction of retained austenite can be adjusted between 1% and 50%.

[0025] (3) Since the retained austenite is rich in manganese and the martensite and / or bainite is poor in manganese, the precipitation of carbides during tempering can be suppressed, carbon atoms can be diffused from martensite and / or bainite to the retained austenite, the stability of the retained austenite can be improved, and fatigue performance can be improved.

[0026] (4) This invention adds a pearlitization process step to the original heat treatment process of hypereutectoid steel, thereby achieving effective control of microstructure and improvement of mechanical properties. The heat treatment process of this invention is simple and easy to operate, adaptable to existing hypereutectoid steel production lines, and easy to promote and use. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the heat treatment process of the present invention;

[0028] Figure 2 This is a SEM image of the microstructure of the sample after spheroidizing annealing treatment in step (1) of Example 1;

[0029] Figure 3 This is a SEM image of the microstructure of the sample after pearlization in step (2) of Example 1;

[0030] Figure 4 The image shows the TEM-EDS image of the microstructure of the sample after pearlization in step (2) of Example 1.

[0031] Figure 5 This refers to the core-shell structure in the microstructure of the quenched sample in step (3) of Example 1;

[0032] Figure 6 The image shows the SEM image of the quenched sample after rapid austenitization in step (3) of Example 1.

[0033] Figure 7 This is a TEM image of the quenched sample after rapid austenitization in step (3) of Example 1;

[0034] Figure 8 The image shows the microstructure after quenching using the conventional process in Example 1 (SEM image).

[0035] Figure 9 The image shows the TEM-EDS image of the sample after rapid austenitization followed by quenching and cold treatment in Example 2.

[0036] Figure 10 This is a SEM image of the microstructure of the sample after pearlization in step (1) of Example 3;

[0037] Figure 11 This is a SEM image of the microstructure of the quenched sample after rapid austenitization in Example 3;

[0038] Figure 12 This is a SEM image of the microstructure after tempering using the conventional process in Example 3;

[0039] Figure 13 This is a SEM image of the microstructure after tempering using the process of the present invention in Example 3. Detailed Implementation

[0040] 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 a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0041] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of this application. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following drawings denote similar items; therefore, once an item is defined in one drawing, it need not be further discussed in subsequent drawings.

[0042] Unless otherwise specified, the technical terms in this specification have the same meaning as those generally understood by those skilled in the art; however, in case of any conflict, the definitions in this specification shall prevail.

[0043] The first embodiment of this application discloses a method for preparing lath martensitic and / or bainitic hypereutectoid steel, comprising the following steps:

[0044] S1. Lamellar pearlitization: Hypereutectoid steel with granular pearlite structure that does not contain network carbides is first heated to the temperature range of the two-phase region where cementite and austenite coexist, and then held at the first temperature. It is then cooled to the two-phase region where ferrite and cementite coexist, and held at the second temperature. Finally, it is cooled to obtain steel with spherical carbides distributed in the lamellar pearlite matrix as the microstructure.

[0045] S2. Rapid austenitization: High-temperature austenitized steel is obtained by rapidly heating lamellar pearlite matrix steel to the temperature range of single-phase austenite and then holding it for a third time.

[0046] S3. Subsequent heat treatment: After the high-temperature austenitized steel is treated by any one of the following heat treatment methods: quenching-tempering, quenching-partitioning, isothermal quenching, or mixed martensite-bainite process, it is cooled to room temperature to obtain lath martensite and / or bainite-based hypereutectoid steel.

[0047] In this embodiment, the hypereutectoid steel with granular pearlite structure that does not contain network carbides should first meet the following two conditions: ① it meets the granular pearlite structure specified in GB / T 18254-2016; ② it does not contain network carbides.

[0048] The aforementioned hypereutectoid steel with granular pearlite structure that does not contain network carbides can be obtained directly from commercial sources, or it can be obtained by using existing hypereutectoid steel (such as existing forged steel, ingots, high carbon chromium bearing steel, cold work die steel, measuring tool and cutting tool steel, etc., which meet the raw material composition standards of hypereutectoid steel, but have lamellar pearlite or martensite as the matrix microstructure) through spheroidizing annealing or high-temperature tempering; it can also be obtained by self-designed casting.

[0049] In a further embodiment, the specific steps of obtaining hypereutectoid steel from the prior art through spheroidizing annealing or high-temperature tempering are conventional steps in the art. For example, preferably, the spheroidizing annealing involves: heating the pre-finished steel to 1150~1300 ℃ and holding it at that temperature for 16~48 h for homogenization treatment, then forging and / or hot rolling the steel, followed by inspection of the microstructure. If the network carbide level does not meet the rating requirements of GB / T 18254-2016, the steel needs to be heated to... Ac cm The temperature is 10-50℃ (700-1000℃), held for 0.5-3 hours, and then cooled to room temperature. The resulting microstructure is pearlitic without network carbides. The pearlitic steel is then heated to the austenite transformation initiation temperature. Ac 1 Hold at 10-60℃ for 1-3 hours, then rapidly cool to the temperature at which austenite begins to transform. Ar 1The spheroidizing anneal is heated at 0-50℃ for 4-48 hours, then furnace-cooled to 450-650℃, and finally air-cooled to room temperature to obtain granular pearlite structure. Granular pearlite structure can also be obtained by continuous cooling spheroidizing annealing, cyclic spheroidizing annealing, or deformation spheroidizing annealing.

[0050] The high-temperature tempering process involves: heating the pre-finished steel to 1150-1300℃ and holding it at that temperature for 16-48 hours for homogenization treatment, followed by forging and / or hot rolling; heating the hot-formed steel to 750-1200℃ in the austenite single-phase region above the complete austenite transformation temperature and holding it for 0.5-3 hours, then cooling it to room temperature; then heating the steel to 300-700℃ and holding it for 3-76 hours for high-temperature tempering, and finally cooling it to room temperature.

[0051] like Figure 1 The invention discloses a conventional process for preparing hypereutectoid steel and the preparation process of this application. In the conventional process, hypereutectoid steel raw material is normalized and spheroidized annealed to obtain a granular pearlite structure, which is then directly quenched and tempered to obtain hypereutectoid steel. This hypereutectoid steel has a structure of spheroidal carbides distributed on a martensitic matrix, with a high proportion of lamellar twinned martensite in the matrix and blocky retained austenite, resulting in poor plasticity and toughness of the steel. In contrast, in the process of this invention, hypereutectoid steel with a granular pearlite structure is first obtained by normalizing and spheroidizing annealing, and then a lamellar pearlite matrix is ​​constructed through lamellar pearlitization. The microstructure is a matrix with a non-uniform alloy (Mn and / or Cr) distribution, on which spherical carbides are distributed. Then, rapid austenitization is carried out to obtain a non-uniform alloy high-temperature austenite. After subsequent heat treatment, a hypereutectoid steel with lath martensite and / or bainite matrix is ​​obtained. In this process, the alloy distribution in lath martensite and / or bainite is low, while the alloy distribution in retained lath austenite is high. This can suppress the precipitation of carbides during tempering, thereby promoting the diffusion of carbon atoms from martensite and / or bainite to retained austenite, improving the stability of retained austenite, and promoting the improvement of fatigue resistance.

[0052] In a further embodiment, since this application requires the construction of a matrix structure with a non-uniform alloy distribution during the lamellar pearlitization process, the raw material composition needs to include Mn and / or Cr elements. Although conventional hypereutectoid steels in the prior art also contain elements such as Mn and Cr, the content is generally low. Therefore, although they can also play a technical role in improving the performance of hypereutectoid steel, the effect is limited. To this end, this application has designed the following hypereutectoid steel composition, which can enable the construction of more non-uniform alloy distributions during the lamellar pearlitization and rapid austenitization processes, thereby improving the uniformity of the microstructure and impact toughness, and reducing the wear rate.

[0053] Specifically, when the hypereutectoid steel with granular pearlite structure that does not contain network carbides contains only Mn, its content is ≥1.0 wt%; when it contains only Cr, its content is ≥0.8 wt%; when it contains both Mn and Cr, the Mn content is 0.5 wt% ~ 8.0 wt%, and the Cr content is 0.5 wt% ~ 3.0 wt%. Preferably, when the hypereutectoid steel with granular pearlite structure that does not contain network carbides contains only Mn, its content is 2.5 wt% ~ 3.0 wt%; when it contains only Cr, its content is 1.0 wt% ~ 1.5 wt%; when it contains both Mn and Cr, the Mn content is 1.0 wt% ~ 2.0 wt%, and the Cr content is 1.0 wt% ~ 1.5 wt%.

[0054] In a further embodiment, the raw material also includes balance elements, such as Fe and impurity elements P, S, O, N, etc., which are unavoidable during the smelting process. Alternatively, it may further include one or more of the following elements: Si: 0 ~ 3.0 wt.%; Mo: 0 ~ 1.0 wt.%; Ni: 0 ~ 3.0 wt.%; V: 0 ~ 1.0 wt.%; Al: 0 ~ 2.0 wt.%; Nb: 0 ~ 0.5 wt.%; Ti: 0 ~ 1.0 wt.%; and more preferably, rare earth elements, such as Ce, Re, etc., may also be added.

[0055] In a further embodiment, the two-phase region where carbides and austenite coexist in step S1 is determined based on the bulk composition. Ae 1 ~Ae cm The temperature range in which cementite and critical austenite coexist is 700–1000 °C, and the first holding time is 10–60 min. More preferably, the two-phase region where ferrite and cementite coexist is determined based on the critical austenite composition, specifically the temperature range within which pearlite transformation occurs, which is 400–700 °C, and the second holding time is 1–72 h. It is worth noting that the holding time can be adjusted appropriately according to the sample size.

[0056] After being treated in step S1 and cooled to room temperature, the microstructure of the lamellar pearlitic matrix steel exhibits a non-uniform alloy distribution, consisting of lamellar cementite enriched with Mn and / or other alloying elements and ferrite depleted with Mn and / or other alloying elements; a small amount of granular pearlite consists of spherical cementite enriched with Mn and / or other alloying elements and ferrite depleted with Mn and / or other alloying elements, and spherical carbides are distributed on the matrix.

[0057] In a further embodiment, step S2 rapidly austenitizes the lamellar pearlite steel to obtain a high-temperature austenitic steel with a non-uniform distribution of Mn or other alloying elements. Specifically, the rapid heating refers to a heating rate ≥ 1℃ / s; the single-phase temperature range of the critical austenite refers to the temperature range within the critical austenite... Ae cm The above temperatures range from 20 to 150°C, which is equivalent to 720 to 1000°C. The third heat preservation time is 0 to 180 seconds.

[0058] Rapid austenitization ensures that the pearlite structure can transform into high-temperature austenite in a short time, while alloying elements such as Mn do not undergo significant long-range diffusion. Lamellar cementite transforms into Mn-rich high-temperature austenite, and ferrite transforms into Mn-depleted high-temperature austenite. Upon quenching to room temperature, the highly stable Mn-rich austenite is retained, while the less stable Mn-depleted austenite transforms into quenched martensite. Given the slow transformation kinetics of spherical undissolved carbides, these carbides hardly transform during rapid austenitization, and the remaining carbide particles ensure the wear resistance of the steel. Since the interface is a rapid diffusion pathway for Mn and other alloying elements, and the interface between the spherical undissolved carbides and the ferrite matrix is ​​a preferential nucleation site for austenite, a certain amount of Mn and other alloying elements can rapidly accumulate after nucleation at the two-phase interface. After quenching, the austenite is retained to room temperature, forming a core-shell structure of "martensite-austenite-carbide".

[0059] In a further embodiment, the high-temperature austenitizing steel in step S3 can be further heat-treated to obtain lath martensite and / or bainite-based hypereutectoid steel. During this process, the high-temperature austenitizing transformation forms a microstructure steel with a non-uniform chemical composition distribution, avoiding the formation of crystalline martensite. The alloy content in lath martensite and / or bainite is low, while the alloy content in the retained lath austenite is high, which can suppress the precipitation of carbides during tempering, thereby promoting the diffusion of carbon atoms from martensite and / or bainite to the retained austenite, improving the stability of the retained austenite, promoting the improvement of fatigue resistance, improving the uniformity of the microstructure and impact toughness, and reducing the wear rate.

[0060] It should be noted that after the lamellar pearlitization step, the microstructure consists of spherical carbides and lamellar pearlite. Then, during rapid austenitization, only the spherical carbides that have not been transformed into high-temperature austenite are retained after subsequent heat treatment and remain as spherical carbides. However, if some of the spherical carbides are transformed into high-temperature austenite during rapid austenitization, these transformed spherical carbides are retained at room temperature as martensite or retained austenite after subsequent heat treatment steps.

[0061] In a further embodiment, the subsequent heat treatment steps may include ① quenching-tempering (cooling to room temperature at a certain rate and then tempering), ② quenching-partitioning (cooling to the temperature between the start and end of martensite transformation and then performing partitioning treatment at that temperature or by raising it to a certain temperature), ③ isothermal quenching (cooling to the temperature between the start and end of bainite transformation and performing isothermal quenching), ④ any combination of martensite-bainite processes. Different heat treatment methods determine the core structure mainly contained in the microstructure of the final hypereutectoid steel.

[0062] For example, ①, it is preferable to cool the high-temperature austenitized steel to room temperature at a certain rate (e.g., 1 ~ 100 ℃ / s), then heat it to 100 ~ 500 ℃ and hold it at this temperature for 0 ~ 300 min, and then cool it to room temperature to temper the quenched steel, thereby obtaining hypereutectoid steel with lath martensite as the main matrix. The hypereutectoid steel matrix obtained at this time is mainly lath martensite + lamellar retained austenite (i.e., imitation pearlite), and a small amount or almost no lamellar martensite, with spherical carbides distributed on the matrix.

[0063] Further preferably, after cooling the steel with a microstructure consisting of precipitated spherical carbides and high-temperature critical austenite with non-uniform chemical element distribution to room temperature at a certain rate (e.g., 1~100 ℃ / s), the steel can be further cold-treated to reduce the content of retained austenite, i.e., isothermal treatment at -197~-10 ℃ for 1~5 hours; wherein, the tempering treatment may include multi-step tempering: after cooling the steel with a microstructure consisting of precipitated spherical carbides and high-temperature critical austenite with non-uniform chemical element distribution to room temperature at a certain rate (e.g., 1~100 ℃ / s), it is then heated to 100~500 ℃ and held at this temperature for 0~300 min, then cooled to room temperature, then heated again to 100~500 ℃ and held for 0~300 min, and then cooled to room temperature. This process is repeated several times to perform multi-step tempering treatment on the quenched steel.

[0064] For example, ②, it is preferable to cool the high-temperature austenitized steel to a two-phase region where ferrite and cementite coexist (400 ~ 700℃), hold it at that temperature for a sufficiently long time (1 ~ 48 h) to form a lamellar pearlite matrix, and then cool it to room temperature; then rapidly heat the fully lamellar pearlite steel from room temperature (1 ~ 200 ℃ / s) to the temperature range of the austenite single-phase region of austenite composition (750 ~ 1100 ℃), hold it at that temperature for 0 ~ 180 s, then cool the steel to the bainite transformation temperature range of 150 ~ 500 ℃, and hold it at that temperature for 0.5 min ~ 96 h to carry out the bainite transformation, and finally cool it to room temperature, and tempering treatment can also be performed. The hypereutectoid steel matrix obtained at this time is mainly bainite + lamellar retained austenite (i.e., pearlite), and may also contain a certain amount of lath martensite. Whether lath martensite is present depends on the steel composition and heat treatment process. Spherical carbides are distributed on the matrix.

[0065] For example, ③, it is preferable to cool the high-temperature austenitized steel to 50-200℃ and hold it for 0-5 min, then perform carbon fractionation at that temperature or by raising the temperature to 150-500℃ and holding it for 0.5-6 h, and then cool it to room temperature to obtain the hypereutectoid steel with lath martensite or bainite and retained austenite as the matrix. More preferably, a tempering treatment can be performed: the temperature is raised to 100-500℃ and held at this temperature for 0-300 min, then cooled to room temperature to temper the steel. The resulting hypereutectoid steel matrix is ​​mainly composed of lath martensite + lamellar retained austenite (i.e., pearlite-like), with a small amount or almost no lamellar martensite, and spherical carbides distributed on the matrix.

[0066] For example, in step ④, a preferred method is to cool the high-temperature austenitized steel to 300-500°C and hold it at that temperature for 0.5 min-48 h, then cool it to an even lower temperature of 100-300°C and hold it for 0.5-6 h for carbon fractionation, and then cool it to room temperature. The resulting hypereutectoid steel matrix is ​​mainly composed of lath martensite + bainite + lamellar retained austenite, with spherical carbides distributed on the matrix.

[0067] More preferably, in the preparation method of this application, after the rapid austenitization to form high-temperature austenite in step 2, a cryogenic treatment step may be included in the subsequent heat treatment process. The cryogenic treatment step can be performed before the subsequent heat treatment or throughout the subsequent heat treatment process. The cryogenic treatment step can be direct cooling to room temperature or deep cryogenic treatment, such as cooling to room temperature and then placing it in carbon dioxide dry ice or liquid nitrogen for a period of time before removing it and then restoring it to room temperature. The purpose of this step is to lower the temperature of the steel below room temperature, which can significantly reduce the content of retained austenite and improve the hardness and strength of the steel. The lower the temperature of the deep cryogenic treatment, the lower the content of retained austenite, and the higher the strength and hardness of the steel.

[0068] The technical effects of this application will be described in detail below with reference to specific embodiments and accompanying drawings.

[0069] Example 1

[0070] Lath martensitic hypereutectoid steel was prepared using pre-finished steel with the composition of Fe-0.95C-2.80Mn-0.95Si-0.21V (wt. %). The above composition is the preferred composition of this application.

[0071] This embodiment first obtains pre-finished steel. Steel with a composition of Fe-0.95C-2.80Mn-0.95Si-0.21V (wt.%) is subjected to vacuum induction melting and vacuum electroslag remelting to obtain ingots. The ingots are then homogenized by holding at 1200 ℃ in a vacuum heat treatment furnace for 36 h. Next, the steel undergoes a three-upsetting and three-drawing forging process to forge a billet with a cross-sectional size of 120mm × 80mm. The forging start temperature is above 1150 ℃, and the forging end temperature is above 800 ℃. Finally, it is air-cooled to room temperature to obtain a lamellar pearlite structure without network carbides. The billet is then heated to 765 ℃ and held for 2 h, followed by rapid cooling to 676 ℃ and holding for 6 h. Afterward, it is furnace-cooled to 500 ℃ and finally air-cooled to room temperature (spheroidizing annealing) to obtain a granular pearlite steel structure. Its SEM image is attached. Figure 2 As shown, it consists of a grayish-white spherical cementite matrix and a black ferrite matrix. Lath martensitic hypereutectoid steel is then prepared based on this matrix. The steps are as follows:

[0072] S1. Lamellar pearlitization: Granular pearlitic steel is first heated to 820℃ and held for 30 min (the temperature range of the two-phase region where cementite and austenite coexist), then cooled to 630℃ and held for 6 h (within the two-phase region where ferrite and cementite coexist), and finally quenched to room temperature to obtain lamellar pearlitic steel; For example... Figure 3 As shown (SEM image of steel with lamellar pearlite structure), and Figure 2The comparison shows that the annealed ferrite has been transformed into a fully lamellar pearlite matrix, with spherical carbide particles dispersed on it. The pearlite is composed of alternating gray cementite lamellars and black ferrite lamellars. Figure 4 The TEM-EDS results of this pearlite structure show the positional fraction of Mn in the lamellar cementite. U Mn =12%) is ferrite ( U Mn = 2%) is 6 times that of Mn, indicating that Mn diffuses from ferrite to cementite during the pearlitization process, and finally forms a matrix structure composed of Mn-rich cementite and Mn-poor ferrite.

[0073] S2. Rapid Austenitization: Lamellar pearlitic steel is placed in a salt bath furnace and rapidly heated (>50℃ / s) to 790℃ (single-phase austenite temperature range), held for 30 seconds. The resulting steel is a high-temperature austenitized steel. After quenching, the austenite is retained to room temperature, and a core-shell structure of martensite-austenite-carbide can be formed, such as... Figure 5 As shown, this structure plays an important role in improving friction and wear performance.

[0074] S3. Subsequent heat treatment (tempering heat treatment): The high-temperature austenitized steel is quenched to room temperature, and its microstructure is as follows: Figure 6 As shown, it is then reheated to 190 °C, tempered for 2 h, and then air-cooled to room temperature to obtain lath martensitic hypereutectoid steel; as Figure 7 As shown, the morphology of martensite is mainly lath-shaped with a high density of dislocations inside; meanwhile, the retained austenite is distributed in a plate-like morphology.

[0075] This embodiment demonstrates that the preparation process of lath martensite and / or bainitic hypereutectoid steel obtained by the method of this application can reduce the proportion of lath martensite, resulting in a more uniform microstructure. It can also control the morphology of retained austenite to be predominantly lath-like, and the retained austenite and tempered martensite after tempering still retain the non-uniform distribution of Mn. Due to the strong interaction between C and Mn elements, carbon atoms tend to diffuse from martensite to austenite during tempering rather than precipitate as carbides, thus improving the stability of retained austenite. Furthermore, the retained austenite fraction after tempering is 21%, a 55% reduction compared to traditional processes, improving the dimensional stability of the workpiece.

[0076] Comparative Example 1: For high-carbon martensitic steel prepared by the traditional quenching-tempering process (the pre-finished steel is directly heated to 820 ℃ and held for 30 min, then oil-cooled to room temperature (quenching), then reheated to 190 ℃ and held for 2 h, then air-cooled (tempering)), the microstructure of the quenched state is shown in the attached figure. Figure 8As shown, due to the uneven distribution of carbides, there are large-sized carbide-poor regions and carbide-rich regions, resulting in two types of large-sized martensite in this steel: cryptocrystalline martensite and crystalline martensite, with a retained austenite content of 47%.

[0077] Table 1 shows a comparison of the properties of the hypereutectoid steels prepared in Example 1 and Comparative Example 1 of this application:

[0078] Table 1 Comparison of properties of hypereutectoid steels

[0079]

[0080] The steel prepared by the traditional process and the process of this invention has a similar micro-Vickers hardness, but the yield strength and tensile strength are increased by 72.4% and 15.7%, respectively. Friction and wear performance was tested under the same test parameters, and the volumetric wear rate was calculated from the mass loss before and after the experiment. It can be seen that the process of this invention can refine the martensitic laths, improve strength while maintaining almost no change in hardness, improve the uniformity of hardness in the microstructure, increase impact toughness by 59.7%, and reduce the volumetric wear rate by 63.6%.

[0081] Example 2

[0082] Lath martensitic hypereutectoid steel is prepared from pre-finished steel with the composition of Fe-0.82C-3.6Mn-0.61Si-0.02Mo (wt. %), and the above composition is the preferred composition of this application.

[0083] This embodiment first obtains pre-finished steel. The steel with the composition of Fe-0.82C-3.6Mn-0.61Si-0.02Mo (wt.%) is vacuum induction melted and vacuum electroslag remelted to obtain an ingot. The ingot is then homogenized by holding it at 1150 ℃ in a vacuum heat treatment furnace for 26 h. Then, the steel is forged into a forging billet with a cross-sectional size of 40mm×40mm through a three-upsetting and three-drawing forging process. The forging start temperature is above 1150 ℃ and the forging end temperature is above 800 ℃. Finally, it is air-cooled to room temperature to obtain a lamellar pearlite structure without network carbides. It is then heated to 750 ℃ ​​and held for 3 h, then rapidly cooled to 647 ℃ and held for 5 h, then furnace-cooled to 450 ℃, and finally air-cooled to room temperature (spheroidizing annealing) to obtain a granular pearlite structure.

[0084] S1. Lamellar pearlitization: The granular pearlite steel is first heated to 790 ℃ and held for 40 min (the temperature range of the two-phase region where cementite and austenite coexist), then cooled to 550 ℃ and held for 12 h (within the range of the two-phase region where ferrite and cementite coexist), and finally quenched to room temperature to obtain lamellar pearlite steel.

[0085] S2. Rapid austenitization: Placing lamellar pearlite steel in a salt bath furnace and rapidly heating it (> 1 ℃ / s) to 770 ℃ (single-phase austenite temperature range) and holding it for 20s, the steel at this time is high-temperature austenitized steel.

[0086] S3. Cold treatment: Quench the high-temperature austenitized steel to room temperature, place it in liquid nitrogen and keep it at that temperature for 1 hour, then take it out and place it in air at room temperature until it reaches room temperature.

[0087] S4. Subsequent heat treatment (tempering heat treatment): The steel that has been cooled to room temperature in S3 is reheated to 230 ℃, tempered for 2 hours, and then air-cooled to room temperature to obtain the high-carbon martensitic steel with lath martensite as the main matrix.

[0088] In this embodiment, the granular pearlitic steel, after lamellar pearlitization, consists of alternating layers of gray Mn-rich cementite lamellae and black Mn-depleted ferrite lamellae. The microstructure of the quenched steel after rapid austenitization consists of martensite, spherical carbides, and lamellar and blocky retained austenite. Further cold treatment for 1 hour, when the sample temperature drops to the critical temperature for martensite transformation, the unstable blocky retained austenite transforms into martensite. Furthermore, some of the lamellar retained austenite, due to its low Mn content, is insufficient to stabilize the retained austenite to the liquid nitrogen temperature and thus also partially transforms into retained austenite. Compared to untreated steel, cold treatment can reduce the retained austenite content of the quenched steel by 58.6% without changing the morphology of the lamellar retained austenite, and the distribution of Mn between the lamellar retained austenite and the martensitic matrix remains unchanged, as shown in the attached figure. Figure 9 As shown, the Mn content of the lamellar retained austenite is 16 at.%, and the Mn content of the lath martensite is 4 at.%.

[0089] After tempering, some unstable retained austenite decomposes, and its content is further reduced. The microstructure in the tempered state is mainly lath martensite, lamellar retained austenite and spherical carbides. The morphology of the retained austenite changes from blocky to lamellar in the traditional process, and no transition carbides are observed to precipitate on the lath martensite matrix between the lamellar retained austenite.

[0090] Table 2 shows a comparison of the properties of the steel prepared by this invention with and without cold treatment.

[0091] Table 2 Comparison of performance between cold-treated and non-cold-treated conditions

[0092]

[0093] After cold treatment, the residual austenite content of the steel decreased, while its strength and hardness increased. Friction and wear performance was tested under the same parameters. The volumetric wear rate after the dry sliding friction and wear experiment was measured by a three-dimensional profilometer. The volumetric wear rate of the cold-treated sample decreased by 23.0%, and the impact toughness increased by 88.4%. This demonstrates that cold treatment can effectively reduce the residual austenite content, increase hardness and strength, and reduce the volumetric wear rate.

[0094] Example 3

[0095] Lath martensitic hypereutectoid steel was prepared using GCr15, a high-carbon chromium bearing steel widely used in industry, as the pre-finished steel.

[0096] Chemical analysis was performed on spheroidized annealed GCr15 bars produced by a steel mill. The test results were: Fe-0.96C-0.34Mn-1.48Cr-0.30Si (wt. %). Its composition and spheroidized microstructure conform to the requirements of GB / T 18254-2016, and it does not contain network carbides, meeting the pre-finished steel standard of this application. Lath martensitic hypereutectoid steel was prepared from this steel using the preparation method of this application, specifically as follows:

[0097] S1. Lamellar pearlitization: Heating from room temperature to 880 ℃ and holding for 40 min, then cooling to 620 ℃ and holding for 3 h, and finally quenching to room temperature. At this point, the annealed ferrite matrix has been transformed into a fully lamellar pearlite matrix.

[0098] S2. Rapid austenitization: Placing lamellar pearlitic steel in a salt bath furnace and rapidly heating it (>10 ℃ / s) to 900 ℃ and holding it at that temperature for 1 s, the steel at this time is high-temperature austenitized steel.

[0099] S3. Subsequent heat treatment (tempering heat treatment): The high-temperature austenitized steel is quenched to room temperature, reheated to 300 ℃, tempered for 1 h, and then air-cooled to room temperature to obtain the high-carbon martensitic steel with lath martensite as the main matrix.

[0100] The microstructure of the pearlitized steel in this embodiment is shown in the attached figure. Figure 10 As shown, the matrix is ​​lamellar pearlite with spherical carbide particles dispersed on it. The pearlite consists of alternating layers of gray cementite and black ferrite. After rapid austenitization and short-time holding in S2, Mn- and Cr-rich lamellar carbides and Mn- and Cr-poor lath martensite (such as...) are obtained. Figure 11The image shown is a SEM image of the steel after quenching to room temperature. It indicates that the lamellar carbides and lath martensite inherit the non-uniform distribution of Mn and Cr in cementite and ferrite, respectively, from the lamellar pearlite. The lamellar carbides refine the width of the lath martensite to 50-200 nm, and a high density of dislocations is observed in the martensite, while the twin density is relatively low. The microstructure after S3 tempering consists of Mn- and Cr-enriched lamellar carbides and lath martensite, with fewer carbides precipitated in the martensite.

[0101] Comparative Example 2: Using GCr15 bearing steel from the same batch, a traditional quenching-tempering process was performed: the steel was held at 870 ℃ for 40 min, then oil-cooled to room temperature, and then heated to 300 ℃ and held for 1 h for tempering. The microstructure of the steel after tempering is shown in the attached figure. Figure 12 As shown, the microstructure of GCr15 bearing steel prepared by the process of this invention consists of crystalline martensite (53.6 vol. %), cryptocrystalline martensite (37.7 vol. %), and spherical carbides. Figure 13 As shown, it consists of crystalline martensite (9.6 vol. %), cryptocrystalline martensite (81.8 vol. %), lamellar and spherical carbides. Therefore, the process of the present invention can adjust the morphology of martensite from lamellar to lath-like, and adjust the morphology of carbides to lamellar, while using lamellar carbides to refine the martensite laths, thereby improving the strength-toughness ratio.

[0102] Table 3 shows a comparison of the properties of the hypereutectoid steels prepared in Example 3 and Comparative Example 2:

[0103] Table 3

[0104]

[0105] It can be seen that the bearing steel prepared by the process of the present invention can reduce the proportion of crystalline martensite, synergistically improve the mechanical and service properties of the steel, and improve the uniformity of the microstructure (66.8%) and impact toughness (122%) while maintaining the same strength level, and reduce the volume wear rate by 48.3%.

[0106] Example 4

[0107] Lath martensitic hypereutectoid steel was prepared using cold work die steel 9Mn2V as the pre-finished steel.

[0108] 9Mn2V, a cold work die steel, is a high-carbon, low-alloy tool steel that does not contain elements such as Cr and Ni. Chemical analysis of annealed 9Mn2V bars produced by a steel mill yielded the following results: Fe-0.85C-1.12Mn-0.23Si-0.21V (wt.%). The microstructure consists of spherical carbide particles dispersed in a ferrite matrix. Its composition and spheroidized structure conform to the requirements of GB / T18254-2016, and it does not contain network carbides (i.e., the die steel has undergone the granular pearlitization process described in this application). The specific preparation method is as follows:

[0109] S1. Lamellar pearlitization: The granular pearlitic steel is heated from room temperature to 780 ℃ and held for 30 min (the temperature range of the two-phase region where cementite and austenite coexist), then cooled to 600 ℃ and held for 12 h (within the range of the two-phase region where ferrite and cementite coexist), and finally quenched to room temperature to obtain the lamellar pearlitic steel.

[0110] S2. Rapid austenitization: Placing lamellar pearlitic steel in a salt bath furnace and rapidly heating it (> 50 ℃ / s) to 850 ℃ and holding it for 2 s, the steel at this time is high-temperature austenitized steel.

[0111] S3. Isothermal quenching: The high-temperature austenitized steel is rapidly quenched in a salt bath at 350 ℃ and held for 12 h to undergo partial bainitic transformation, thereby obtaining the high-carbon martensitic steel with bainite, lath martensite and lamellar retained austenite as the matrix.

[0112] The microstructure of 9Mn2V cold work die steel after isothermal quenching according to this invention consists of bainite, lath martensite, and lamellar retained austenite. This transforms the blocky retained austenite obtained by traditional isothermal quenching into lamellar austenite, improving the stability of the retained austenite and reducing the proportion of lamellar martensite and twins while increasing the proportion of lath martensite. Compared to traditional processes, while maintaining relatively unchanged hardness and strength, the toughness increases by 16.8%, and the wear rate decreases by 62.3%.

[0113] Example 5

[0114] Lath martensitic hypereutectoid steel was prepared using 9SiCr steel for measuring tools and cutting tools as the pre-finished steel.

[0115] Chemical analysis was performed on forged 9SiCr steel produced by a steel plant. The test results were: Fe-0.91C-0.39Mn-1.04Cr-1.35Si (wt. %), the microstructure was lamellar pearlite, and there were network carbide precipitates along the grain boundaries.

[0116] Because the steel has network carbide precipitation, it needs to be treated first to obtain steel without network carbide, and then the preparation method of this application can be used to obtain the final product. The specific steps are as follows:

[0117] S1. Normalizing: In order to eliminate network carbides, the steel is heated to 910℃ and held for 10 minutes, then air-cooled to room temperature to obtain a microstructure composed of complete lamellar pearlite, which does not contain network carbides.

[0118] S2. Granular pearlitization: The normalized steel is heated to 800℃ and held for 2 hours, then rapidly cooled to 700℃ and held for 5 hours, and then slowly cooled to about 500℃ and air-cooled to obtain a microstructure in which spherical carbide particles are dispersed in the ferrite matrix. The spheroidized structure conforms to the requirements of GB / T 18254-2016 and does not contain network carbides.

[0119] S3. Lamellar pearlitization: The granular pearlitic steel is heated from room temperature to 840 ℃ and held for 30 min (the temperature range of the two-phase region where cementite and austenite coexist), cooled to 490 ℃ and held for 24 h (within the range of the two-phase region where ferrite and cementite coexist), and finally quenched to room temperature to obtain lamellar pearlitic steel.

[0120] S4. Rapid austenitization: Placing lamellar pearlite steel in a salt bath furnace and rapidly heating it (> 50 ℃ / s) to 950 ℃ (single-phase austenite temperature range) and holding it at that temperature for 0-1s, the steel at this time is high-temperature austenitized steel.

[0121] S5. Carbon fractionation: After cooling the high-temperature austenitized steel to 110 ℃ and holding it for 30 s, the steel is then heated to 300 ℃ and held for 90 min for carbon fractionation. Finally, it is quenched to room temperature to obtain the high-carbon martensitic steel with lath martensite as the main matrix.

[0122] Compared with the traditional partitioning process, the microstructure of 9SiCr steel for measuring tools and cutting tools after being partitioned by the process of this invention has a lower proportion of lamellar martensite and twinned substructure, and a higher proportion of lamellar retained austenite. While maintaining the hardness and strength of the material relatively unchanged, the toughness is increased by 19.1% and the wear rate is reduced by 47.4%.

[0123] Example 6

[0124] Lath martensitic hypereutectoid steel was prepared using 90MnCrV8 (German standard) mold steel as the pre-finished steel.

[0125] Chemical analysis was performed on annealed 90MnCrV8 bars produced by a steel mill. The test results were: Fe-0.89C-2.06Mn-0.33Cr-0.32Si-0.09V (wt. %). The microstructure consisted of spherical carbide particles dispersed in a ferrite matrix. Its composition and spheroidized structure conformed to the requirements of GB / T 18254-2016 and did not contain network carbides.

[0126] S1. Lamellar pearlitization: The granular pearlite steel is heated from room temperature to 810 ℃ and held for 40 min (the temperature range of the two-phase region where cementite and austenite coexist), cooled to 580 ℃ and held for 24 h (within the range of the two-phase region where ferrite and cementite coexist), and finally quenched to room temperature to obtain a steel with a lamellar pearlite matrix.

[0127] S2. Rapid austenitization: Placing lamellar pearlite steel in a salt bath furnace and rapidly heating it (> 85 ℃ / s) to 820 ℃ (single-phase austenite temperature range) and holding it for 10 s, the steel at this time is high-temperature austenitized steel.

[0128] S3. Combination of Bainitic and Martensitic Processes: The high-temperature austenitized steel is cooled to 180°C at a rate >10°C / s and held for 30 seconds. Then, it is heated to 450°C and held for 0.5 hours to induce bainitic transformation, followed by oil cooling to room temperature. Finally, the steel is heated to 110°C and tempered for 2 hours, then air-cooled to room temperature to obtain the high-carbon martensitic steel with lath martensite and bainite as the main matrix.

[0129] The matrix structure of 90MnCrV8 mold steel after S3 treatment consists of nanoscale lath martensite and bainite, with retained austenite exhibiting a lamellar morphology. This refines the martensitic laths and bainitic ferrite, resulting in a finer microstructure compared to traditional martensite-bainite dual-phase microstructure preparation processes. Furthermore, the bainite transformation rate is accelerated, completing the transformation within the bainite transformation temperature range after only 0.5 h of holding time. Compared to the fully martensitic matrix structure prepared by traditional processes, the steel prepared by this invention exhibits improved toughness by 13.7% and reduced volumetric wear rate by 41.5%, while maintaining relatively unchanged hardness and strength.

[0130] Example 7

[0131] Lath martensitic hypereutectoid steel was prepared from pre-finished steel with the composition of Fe-0.82C-3.6Mn-0.61Si-0.02Mo (wt. %).

[0132] This embodiment first obtains pre-finished steel. Steel with a composition of Fe-0.82C-3.6Mn-0.61Si-0.02Mo (wt.%) is subjected to vacuum induction melting and vacuum electroslag remelting to obtain an ingot. The ingot is then homogenized by holding it at 1200 ℃ in a vacuum heat treatment furnace for 48 h. Next, the steel undergoes a three-upsetting and three-drawing forging process to forge a blank with a cross-sectional size of 45 mm × 45 mm. The forging start temperature is above 1150 ℃, and the forging end temperature is above 800 ℃. Finally, it is air-cooled to room temperature to obtain a lamellar pearlitic structure without network carbides (i.e., the pre-finished steel). Then, lath martensitic hypereutectoid steel is prepared, with the following steps:

[0133] S1. Granular pearlitization: After heating to 830℃ and holding for 15 minutes, the steel is directly quenched to room temperature to obtain a fully martensitic microstructure; then the steel is heated to 620℃ and held for 36 hours, and then quenched to room temperature to obtain a pre-finished steel with a granular pearlitic structure.

[0134] S2. Lamellar pearlitization: The granular pearlitic steel is heated from room temperature to 850 ℃ and held for 30 min (the temperature range of the two-phase region where cementite and austenite coexist), cooled to 600 ℃ and held for 12 h (within the range of the two-phase region where ferrite and cementite coexist), and finally quenched to room temperature to obtain lamellar pearlitic steel.

[0135] S3. Rapid austenitization: Placing lamellar pearlite steel in a salt bath furnace and rapidly heating it (> 100 ℃ / s) to 820 ℃ (single-phase austenite temperature range) and holding it for 30s, the steel at this time is high-temperature austenitized steel.

[0136] S4. Cold treatment: After quenching, the steel is placed in carbon dioxide dry ice for 1 hour and then placed in air at room temperature until it reaches room temperature.

[0137] S5. Subsequent heat treatment (tempering heat treatment): The cold-treated steel is reheated to 350 ℃ and tempered for 30 min, then air-cooled to room temperature to obtain the high-carbon martensitic steel with lath martensite as the main matrix.

[0138] In S1, when the martensite after the first quenching is reheated to 620℃ and held, the martensite undergoes recovery and recrystallization. Simultaneously, a large amount of carbides precipitate and coarsen, growing larger. After holding, an initial microstructure of granular pearlite composed of spherical carbides and polygonal ferrite is obtained. The microstructure of S4 is similar to that in Example 1, with a Mn content of 13 at.% in the lamellar retained austenite and a Mn content of 2 at.% in the intercalated lath martensite. The difference is that this steel has a slightly higher C content, a slightly higher content of retained austenite, and a slightly wider width. However, the martensite between the retained austenite remains lath martensite with a dislocation substructure. This results in a better strength-toughness balance than the lamellar martensite microstructure obtained by traditional quenching and tempering, and also improves wear resistance.

[0139] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing lath martensitic and / or bainitic hypereutectoid steel, characterized in that, Includes the following steps: S1. Lamellar pearlitization: The hypereutectoid steel with a granular pearlitic structure lacking network carbides is first heated to the two-phase region temperature Ae1~Ae1 where cementite and austenite coexist. cm The steel is subjected to a first heat treatment of 10-120 min within the range, followed by cooling to a second heat treatment of 1-72 h within the two-phase region where ferrite and cementite coexist, and then further cooling to obtain a steel with a microstructure of spherical carbides distributed in a lamellar pearlite matrix; wherein the two-phase region where ferrite and cementite coexist is the temperature range in which critical austenite undergoes pearlite transformation. S2. Rapid austenitization: Steel with a lamellar pearlite matrix is ​​rapidly heated to the single-phase austenite temperature range at a rate ≥1℃ / s, followed by a third holding period of 0-180s to obtain high-temperature austenitized steel; wherein the single-phase austenite temperature range is the critical austenite temperature range. Ae cm Above 20 ~ 150 ℃; S3. Subsequent heat treatment: After the high-temperature austenitized steel is treated by any one of the following heat treatment methods: quenching-tempering, quenching-partitioning, isothermal quenching, or mixed martensite-bainite process, it is cooled to room temperature to obtain lath martensite and / or bainite-based hypereutectoid steel. The raw materials of the hypereutectoid steel with granular pearlitic structure that does not contain network carbides contain at least one or a combination of Mn and Cr elements.

2. The preparation method according to claim 1, characterized in that, When the hypereutectoid steel with granular pearlite structure that does not contain network carbides contains only Mn, its content is ≥1.0wt%; when it contains only Cr, its content is ≥0.8wt%; when it contains both Mn and Cr, the content of Mn is 0.5wt%~8.0wt% and the content of Cr is 0.5wt%~3.0wt%.

3. The preparation method according to claim 2, characterized in that, When the hypereutectoid steel with granular pearlite structure that does not contain network carbides contains only Mn, its content is 2.5wt%~3.0wt%; when it contains only Cr, its content is 1.0wt%~1.5wt%; when it contains both Mn and Cr, the content of Mn is 1.0wt%~2.0wt% and the content of Cr is 1.0wt%~1.5wt%.

4. A lath martensitic and / or bainitic hypereutectoid steel prepared by any of the preparation methods described in claims 1 to 3, comprising at least nano- or submicron-sized lath martensite and / or bainite microstructure.

5. The hypereutectoid steel according to claim 4, characterized in that, Its microstructure also includes lamellar retained austenite and / or lamellar carbides.

Images