Heat treatment method for improving impact toughness of hot work die steel for die casting

Abstract

The heat treatment method for improving the impact toughness of the hot work die steel for die casting comprises the following steps that S1, a workpiece to be subjected to heat treatment is subjected to heat preservation for t1 under the condition of T1, after heat preservation is finished, the temperature is increased to T2, heat preservation for t2 is conducted, then the workpiece is reheated to T3 lower than T2, and heat preservation for t3 is conducted; s2, the workpiece is cooled to the surface temperature T4 at v1, then cooled to the core part T5 at v2 larger than v1, cooled to the core part T6 at v3 smaller than v2, and then cooled to the room temperature at v4 larger than v3; s3, the workpiece is subjected to heat preservation for t4 under the condition of T7; s4, the workpiece is heated to T8, heat preservation t5 is conducted, then the workpiece is cooled to T9, and heat preservation t6 is conducted; and S5, the workpiece is subjected to heat preservation for t7 under the condition of T11 and then cooled to the room temperature, and then tempering treatment is conducted. By optimizing the post-forging multi-stage heat treatment process, the spheroidizing period is shortened, the spheroidizing structure uniformity and the carbide spheroidizing rate are improved, the rejection rate is reduced, the product impact toughness and the quality stability are improved, and good application and popularization prospects are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of metal materials, in particular to a heat treatment method for improving the impact toughness of hot working die steel for die casting. Background Art

[0002] Hot work die steel, an alloy tool steel with chromium (Cr), molybdenum (Mo), and vanadium (V) as core alloying elements, is widely used in the manufacture of dies for high-temperature applications such as die casting, forging, and extrusion. Hot work die steels, primarily H11 (4Cr5MoSiV), H13 (4Cr5MoSiV1), and DIEVAR (4Cr5Mo2V), are core materials for die-casting molds. They must withstand the cyclical erosion, thermal stress, and mechanical stresses of high-temperature molten metal during service, requiring them to possess excellent thermal fatigue resistance, high-temperature strength, and impact toughness. With the rapid development of technologies such as integrated die-casting for new energy vehicles, die-casting molds are trending towards larger, more integrated, higher-performance, and longer-lasting molds.

[0003] For example, Chinese patent publication number CN116397177A relates to a method for preparing high-toughness, high-uniformity, high-purity, large-size hot-working die steel for die casting. By optimizing electrode billet smelting, electroslag remelting, forging, and heat treatment, and strictly controlling the production process, the method achieves the goal of producing qualified large-size hot-working die flat steel. Electrode billets are smelted from molten iron and scrap steel to significantly improve their purity; the electroslag remelting process is optimized to improve inclusion removal and reduce casting segregation; segregation is improved through ultra-high temperature diffusion and three-way forging processes, thereby increasing density and uniformity; post-forging isothermal normalizing controls carbide network and austenite grain size; solution treatment and spheroidizing annealing control carbide dissolution and precipitation to improve uniformity; ensuring uniform distribution of alloying elements in the steel and sufficient deformation of the billet core, combined with post-forging cooling and isothermal normalizing to eliminate carbide network and refine austenite grains; and the rational application of solution treatment and spheroidizing annealing processes ensures carbide uniformity and spheroidization uniformity.

[0004] For example, Chinese patent publication number CN111593257A discloses a method for preparing hot-working die steel with high toughness and thermal stability. By optimizing alloy composition and rigorously controlling the production process, the specific process route includes: electric furnace smelting, ladle refining, vacuum refining, electrode billet casting, remelting in a protective atmosphere electroslag furnace, forging, ultrafine treatment, and spheroidizing annealing. The patent also discloses a hot-working die steel with high toughness and thermal stability produced using this method. The annealed microstructure of this product is uniform, with a well-developed spheroidized structure and a fine, uniform grain distribution. The quenched and tempered microstructure is also uniform. Fine precipitations of Mo and Cr carbides act as dispersion strengthening during use, improving the material's performance and imparting high toughness and isotropy. This product can be widely used in the manufacture of hot extrusion dies, mandrels, hammer dies for die forging hammers, forging press dies, and precision forging machine dies. It is particularly suitable for high-end die-casting dies for aluminum, copper, and their alloys.

[0005] However, there are many problems with conventional heat treatment of large-scale modules: (1) The stability of the core structure is poor: the increase in the thickness of the large-size module leads to a significant decrease in the cooling rate of the core, making it difficult to suppress the transformation of bainite or pearlite, and forming coarse non-martensite structure (such as granular bainite). At the same time, during the slow cooling process, the Cr-rich type (M 23 C6) and Mo-rich (M2C) carbides tend to precipitate in large quantities along grain boundaries, while MC carbides coarsen due to their insolubility. These structural defects reduce the impact toughness of the core, which is only 50% to 70% of that of the surface layer.

[0006] (2) The existing conventional processes are relatively limited: although a single high-temperature solution treatment can partially dissolve carbides, the temperature gradient in the core of large-sized modules is large, and the insoluble carbides (such as VC) are not completely dissolved; the cooling process adopts a direct water quenching method, which has a certain improvement on the cooling rate of the core, but greatly increases the risk of surface cracking; the conventional single ultra-fine + spheroidizing annealing treatment has limited improvement on the organizational properties. The carbides in the original organization have a low spheroidization rate, uneven distribution, and large size differences, which restricts the final strength and toughness performance matching.

[0007] In view of this, improvements should be proposed to the existing technology. Summary of the Invention

[0008] The main purpose of the present invention is to provide a heat treatment method for improving the impact toughness of hot working die steel for die casting. By optimizing the multi-stage heat treatment process after forging, the method shortens the spheroidization cycle, improves the uniformity of the spheroidization structure and the carbide spheroidization rate, reduces the scrap rate, improves the impact toughness and quality stability of the product, and has good prospects for promotion and application.

[0009] According to one aspect of the present invention, a heat treatment method for improving the impact toughness of hot working die steel for die casting is provided, which comprises the following steps: S1, cyclic ultrafine treatment: the workpiece to be heat treated is kept at T1 for t1, after which the temperature is raised to T2 and kept at t2, and then the workpiece is reheated to T3 which is lower than T2 and kept at t3; S2, gradient controlled cooling treatment: cool the workpiece at v1 to a surface temperature of T4, then cool it at v2 greater than v1 to a core temperature of T5, then cool it at v3 less than v2 to a core temperature of T6, and then cool it to room temperature at v4 greater than v3; S3, pre-precipitation treatment: the workpiece is kept at temperature T7 for t4; S4, two-stage spheroidizing annealing treatment: heat the workpiece to T8 and hold at t5, then cool it to T9 and hold at t6; S5, comprehensive performance control processing: the workpiece is placed in T 11 After keeping warm for t7 under the conditions, cool to room temperature and then carry out tempering treatment.

[0010] According to one embodiment of the present invention, in step S1, the temperature range of T1 is 970~1020℃, t1=t 1基础保温 +Extend 40~50min for every 40~60mm increase in workpiece thickness, t 1基础保温 is 1.5~2.5h; the temperature range of T2 is 1100~1150℃, t2 is 0.5~1.5h; the temperature range of T3 is 970~1020℃, t3= t 3基础保温 +Extend 20~30min for every 40~60mm increase in workpiece thickness, t 3基础保温 0.5~1.5h.

[0011] According to one embodiment of the present invention, in step S2, v1 is 5~10℃ / min, and the temperature range of T4 is 900~950℃; v2 is 30~50℃ / min, and the temperature range of T5 is 500~550℃; v3 is 3~5℃ / min, and the temperature range of T6 is 400~450℃; v4 is 30~50℃ / min.

[0012] According to one embodiment of the present invention, in step S2, cooling to T4 and cooling to T6 is performed by air cooling; cooling to T5 and cooling to room temperature is performed by water cooling.

[0013] According to one embodiment of the present invention, in step S2, the temperature of the water-cooled medium is controlled at 20-40° C., and circulating agitation is maintained, and the water flow rate is controlled at 0.8-1.5 m / s.

[0014] According to one embodiment of the present invention, in step S3, the temperature range of T7 is 500-550°C, t4= t4基础保温 +Extend 40~50min for every 40~60mm increase in workpiece thickness, t 4基础保温 The holding time is 3~5h. After the insulation is completed, air cooling is adopted for cooling. The cooling rate of air cooling is 3~5℃ / min.

[0015] According to one embodiment of the present invention, in step S4, the temperature range of T8 is 840-880°C, t5 is 4-8h, and the cooling rate to T9 is 10-15°C / min; the temperature range of T9 is 760-810°C, t6 is 8-12h; after the insulation is completed, the furnace is cooled to T 10 The following is air-cooled, T 10 The temperature range is ≤350℃.

[0016] According to one embodiment of the present invention, in step S5, T 11 The temperature range is 950~1050℃, t7 is 1~2h, and the cooling rate to room temperature is ≥50℃ / min.

[0017] According to one embodiment of the present invention, in step S5, three tempering treatments are performed, with the tempering temperatures being 580-600° C., 600-620° C., and 560-580° C., respectively, and the holding time being 2-4 hours.

[0018] According to one embodiment of the present invention, the thickness of the hot working die steel is ≥300 mm.

[0019] According to an embodiment of the present invention, a heat treatment method for improving the impact toughness of hot working die steel for die casting is developed. By optimizing the multi-stage heat treatment process after forging, the spheroidization cycle is shortened, the uniformity of the spheroidized structure and the carbide spheroidization rate are improved, the scrap rate is reduced, the impact toughness and quality stability of the product are improved, and the method has good prospects for promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some implementation cases of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0021] Figure 1 A process flow chart of a heat treatment method for improving the impact toughness of hot working die steel for die casting according to an exemplary embodiment of the present invention is shown; Figure 2 A heat treatment process curve of a heat treatment method for improving the impact toughness of hot working die steel for die casting according to an exemplary embodiment of the present invention is shown; Figure 3A comparison diagram of grain size OM images of Example 1 and Comparative Example 1 according to an exemplary embodiment of the present invention is shown; Figure 4 A comparison diagram of spheroidized tissue images of Example 2 and Comparative Example 2 according to an exemplary embodiment of the present invention is shown; Figure 5 A comparison diagram of spheroidized tissue images of Example 3 and Comparative Example 3 according to an exemplary embodiment of the present invention is shown. DETAILED DESCRIPTION

[0022] The detailed description of the following embodiments is intended to illustrate the principles of the present invention by way of example, but is not intended to limit the scope of the present invention. The present invention may be implemented in many different forms and is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

[0023] The present invention provides these embodiments in order to make this disclosure thorough and complete, and to fully convey the scope of the present invention to those skilled in the art. It should be noted that: unless otherwise specifically stated, the relative arrangement of parts and steps, the composition of materials, numerical expressions and numerical values ​​set forth in these embodiments should be interpreted as merely exemplary, and not as limiting.

[0024] It should be noted that, in the description of the present invention, unless otherwise specified, "plurality" means greater than or equal to two. Terms such as "upper," "lower," "left," "right," "inner," and "outer" indicating directions or positional relationships are intended solely to facilitate and simplify the description of the present invention and do not indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on the present invention. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0025] It should also be noted that, in the description of this invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood broadly. For example, they can refer to fixed connections, removable connections, or integral connections; they can be directly connected or indirectly connected through an intermediary. Those skilled in the art will understand the specific meanings of these terms in this invention depending on the specific circumstances. When a specific device is described as being located between a first device and a second device, there may or may not be an intervening device between the specific device and the first or second device.

[0026] All terms used herein have the same meanings as understood by one of ordinary skill in the art to which this disclosure belongs, unless otherwise specifically defined. It should also be understood that terms defined in, for example, common dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant art, and should not be interpreted in an idealized or highly formal sense, unless explicitly defined herein.

[0027] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, these technologies, methods, and equipment should be considered part of the specification.

[0028] like Figure 1 As shown, the present invention provides a heat treatment method for improving the impact toughness of hot working die steel for die casting, which comprises the following steps: S1, cyclic ultrafine treatment: the workpiece to be heat treated is kept at T1 for t1, after which the temperature is raised to T2 and kept at t2, and then the workpiece is reheated to T3 which is lower than T2 and kept at t3; S2, gradient controlled cooling treatment: cool the workpiece at v1 to a surface temperature of T4, then cool it at v2 greater than v1 to a core temperature of T5, then cool it at v3 less than v2 to a core temperature of T6, and then cool it to room temperature at v4 greater than v3; S3, pre-precipitation treatment: the workpiece is kept at T7 for t4; S4, two-stage spheroidizing annealing treatment: heat the workpiece to T8 and hold at t5, then cool it to T9 and hold at t6; S5, comprehensive performance control processing: the workpiece is placed in T 11 After keeping warm for t7 under the conditions, cool to room temperature and then carry out tempering treatment.

[0029] Specifically, the workpiece to be heat treated is a hot working die steel module for die casting that has been stress relief annealed after forging.

[0030] In a heat treatment method for improving the impact toughness of hot-working die steel for die casting according to an embodiment of the present invention, by optimizing the multi-stage post-forging heat treatment process, the spheroidization cycle is shortened, the uniformity of the spheroidized structure and the carbide spheroidization rate are improved, the scrap rate is reduced, and the impact toughness and quality stability of the product are improved. This solves the problem that when treating H13 (4Cr5MoSiV1) die-casting modules with a thickness of 300 mm or more in conventional heat treatment processes, coarse bainite or pearlite structure is easily formed in the core, and the carbide distribution is poor, resulting in low impact toughness. The method has good prospects for promotion and application.

[0031] like Figure 2 As shown, in some specific embodiments, in step S1, the temperature range of T1 is 970~1020℃, t1=t 1基础保温+Extend 40~50min for every 40~60mm increase in workpiece thickness, t 1基础保温 is 1.5~2.5h; the temperature range of T2 is 1100~1150℃, t2 is 0.5~1.5h; the temperature range of T3 is 970~1020℃, t3= t 3基础保温 +Extend 20~30min for every 40~60mm increase in workpiece thickness, t 3基础保温 0.5~1.5h.

[0032] The use of cyclic ultrafine treatment can fully dissolve coarse and insoluble carbides and refine the original austenite grains. Holding the steel at a lower temperature for a longer time allows the Cr-rich and Mo-rich carbides in the steel to fully dissolve. Raising the temperature to a higher temperature and holding it for a shorter time increases the dissolution rate of large MC-type insoluble carbides while preventing the grain size from increasing rapidly. After the first high-temperature ultrafine treatment, a second low-temperature ultrafine treatment is carried out. The low-temperature ultrafine treatment is then repeated, utilizing multiple austenite to martensite transformations to form high-density dislocations and subgrain boundaries, increase the grain nucleation rate, and refine the grains. t1 and t3 are based on the basic holding time and are appropriately extended according to the increase in workpiece thickness.

[0033] Based on the above embodiment, in step S2, v1 is 5~10℃ / min, the temperature range of T4 is 900~950℃; v2 is 30~50℃ / min, the temperature range of T5 is 500~550℃; v3 is 3~5℃ / min, the temperature range of T6 is 400~450℃; v4 is 30~50℃ / min.

[0034] During the ultrafine treatment cooling process, large-sized modules experience significant differences in cooling rates between the surface and core. Faster cooling rates can easily lead to cracking due to excessive internal stress, while insufficient cooling rates often result in the formation of bainite or pearlite in the core, leading to significant differences in microstructure and performance across the cross-section. To avoid these issues, the cooling process after ultrafine treatment is controlled.

[0035] In some specific embodiments, in step S2, cooling to T4 and cooling to T6 is performed by air cooling; cooling to T5 and cooling to room temperature is performed by water cooling.

[0036] Based on the above embodiment, in step S2, the temperature of the water-cooled medium is controlled at 20-40° C., and circulating stirring is maintained, and the water flow rate is controlled at 0.8-1.5 m / s.

[0037] After the ultrafine treatment and holding period, the workpiece is removed from the furnace and air-cooled to a surface temperature of 900-950°C. This prevents the initial temperature from being too high during rapid cooling, which could lead to excessive thermal stress within the workpiece. Water cooling is then employed, with the cooling medium temperature controlled between 20-40°C and agitated, with a flow rate of 0.8-1.5 m / s. The workpiece is water-cooled at a rate of 30-50°C / min to a core temperature of 500-550°C, with the water then removed. This avoids carbide precipitation-sensitive areas and reduces the risk of large-scale intergranular carbide precipitation during cooling. Air cooling is then performed to a core temperature of 400-450°C, minimizing the temperature difference between the workpiece surface and core, thereby reducing the risk of cracking during cooling in large modular workpieces. The workpiece is then water-cooled to room temperature at a rate of 30-50°C / min, rapidly passing through the bainite formation temperature range and preventing the formation of bainite in the core.

[0038] In some specific embodiments, the workpiece is subjected to a pre-precipitation treatment after the cyclic ultrafine treatment and the gradient controlled cooling treatment. In step S3, the temperature range of T7 is 500-550°C, t4= t 4基础保温 +Extend 40~50min for every 40~60mm increase in workpiece thickness, t 4基础保温 The holding time is 3~5h. After the insulation is completed, air cooling is adopted for cooling. The cooling rate of air cooling is 3~5℃ / min.

[0039] The high Cr and Mo content in Cr-Mo-V hot working die steel promotes the decomposition of a large amount of Cr-rich M 23 C6 carbides precipitate uniformly and dispersedly from the matrix, while forming a small amount of Mo-rich M2C carbides. This provides more carbide nucleation sites for subsequent spheroidizing annealing, increasing the number of carbides in the annealed structure, making their distribution more uniform and significantly improving the spheroidization rate, providing an excellent original structure for the final heat treatment.

[0040] Based on the above embodiment, in step S4, the temperature range of T8 is 840~880℃, t5 is 4~8h, and the cooling rate to T9 is 10~15℃ / min; the temperature range of T9 is 760~810℃, t6 is 8~12h; after the insulation is completed, the furnace is cooled to T 10 The following is air-cooled, T 10 The temperature range is ≤350°C, allowing carbides to fully precipitate and spheroidize, reducing the spacing and size differences between carbide particles. After the holding period, the carbide is cooled to below 350°C before being removed from the furnace and air-cooled. This two-stage annealing treatment achieves a spheroidized carbide structure with a high spheroidization rate (≥90%), fine size (≤2μm), and uniform distribution, ensuring excellent machinability and uniformity of the final heat-treated structure.

[0041] In some specific embodiments, in step S5, T 11The temperature range is 950~1050℃, t7 is 1~2h, and the cooling rate to room temperature is ≥50℃ / min to ensure that the workpiece is completely austenitized and the carbides are fully dissolved. The cooling rate to room temperature is ≥50℃ / min to ensure that lath martensite structure is obtained.

[0042] Based on the above embodiment, in step S5, three tempering treatments are performed, and the tempering temperatures are 580-600°C, 600-620°C, and 560-580°C, respectively, and the holding time is 2-4 hours. The three tempering treatments are performed to fully precipitate the nano-scale carbides and evenly disperse them in the matrix, and to ensure that the retained austenite is basically decomposed, with the retained austenite content being ≤1.5%, to avoid phase change during use, which may cause a significant decrease in mold precision or cracking.

[0043] Based on the above embodiment, the thickness of the hot working die steel is ≥300 mm.

[0044] In some specific embodiments, the water cooling in step S2 is intermittent, with each cooling time lasting 5 to 15 minutes. This short cooling and re-red cycle effectively controls material thermal stress, avoiding the risk of cracking caused by excessive temperature differences between the surface and the core. It also promotes phase transformation uniformity, reduces hardness and strength differences, and optimizes microstructure properties through the auto-tempering effect. The overall processing cycle is shortened, improving production efficiency. Combined with intermittent cycles, energy consumption can be reduced. Resource waste caused by excessive cooling is avoided, extending equipment life. In some specific embodiments, thermal insulation includes placing the workpiece in an inert gas atmosphere for 1 to 2 hours.

[0045] Based on the above examples, water cooling involves using a mixed medium of circulating water and polymer solutions (such as polyacrylic acid and polyvinyl alcohol). Circulating water reduces fresh water consumption, minimizes wastewater discharge, and improves resource utilization. The polymer solution enhances the wettability and heat transfer efficiency of the medium by adjusting its concentration and molecular structure, effectively regulating the cooling rate and preventing material cracking or deformation. The mixed medium combines the environmental friendliness of a circular economy with the process flexibility of polymer regulation. While reducing water consumption and wastewater treatment costs, it also achieves comprehensive cost optimization by increasing yield and efficiency, and improves material surface quality (for example, reducing oxide scale). Furthermore, the polymer solution inhibits bubble formation during the cooling process, improving cooling uniformity. This makes it suitable for cooling large-section forgings, achieving both performance and cost optimization.

[0046] The present invention is further described below through specific examples.

[0047] Example 1 S1. Circular Ultrafine Treatment: The forged workpiece (300mm thick) is held at 1000°C for 6.5 hours. After the holding period, the temperature is raised at 150°C / hour to 1130°C and held for 1 hour. After the first high-temperature ultrafine treatment, the workpiece is reheated to 1000°C and held for 3.5 hours.

[0048] S2. Gradient Controlled Cooling: After the heat preservation step S1 is completed, the workpiece is removed from the furnace and air-cooled to a surface temperature of 900°C. Water cooling is then applied, with the cooling medium at 25°C and agitation maintained. The water flow rate is controlled at 1.2 m / s. The workpiece is water-cooled at a rate of 40°C / min to a core temperature of 520°C, then air-cooled to a core temperature of 415°C. The workpiece is then water-cooled at a rate of 40°C / min to room temperature.

[0049] S3. Pre-precipitation treatment: keep the workpiece at 550°C for 8.5 hours and cool it by air cooling at a cooling rate of 4°C / min.

[0050] S4. Two-stage spheroidizing annealing: Heat the workpiece to 860°C, hold for 6 hours, then slowly cool to 770°C at a rate of 10°C / h, hold for 10 hours, and then cool to 300°C before air cooling.

[0051] S5. Comprehensive performance control treatment: The workpiece was held at 1030°C for 2 hours, then cooled to room temperature at a cooling rate of 60°C / min. It was then tempered three times at 590°C, 610°C, and 580°C, each for 4 hours. Air cooling was used after each hold. The final workpiece hardness was 45 HRC, and the core V-notch impact energy was 22 J.

[0052] Comparative Example 1 In Comparative Example 1 of the present invention, the thickness of the forged workpiece is the same as that of Example 1. Step S1 of the heat treatment method is different from that of Example 1, and the remaining steps are the same as those of Example 1, as follows: S1. Circular Ultrafine Treatment: The forged workpiece (300mm thick) was held at 1000°C for 6.5 hours. After this, the temperature was raised at 150°C / hour to 1130°C and held for 1 hour. The final workpiece hardness was 45HRC, and the core V-notch impact energy was 17J. Figure 3 The figure is a comparison of the grain size OM images of Example 1 and Comparative Example 1. The average grain diameter of Example 1 is 20 μm, while the average grain diameter of Comparative Example 1 is 30 μm due to the lack of low-temperature ultrafine treatment in step S1, resulting in a lower final core impact energy than that of Example 1.

[0053] Example 2 S1. Circular Ultrafine Treatment: The forged workpiece (350mm thick) was kept at 1020°C for 7.25 hours. After the holding period, the temperature was raised at 150°C / h to 1150°C and kept for 1.25 hours. After the first high-temperature ultrafine treatment, the workpiece was reheated to 1020°C and kept for 3.75 hours.

[0054] S2. Gradient Controlled Cooling: After the heat preservation step S1 is completed, the workpiece is removed from the furnace and air-cooled to a surface temperature of 910°C. Water cooling is then applied, with the cooling medium at 25°C and agitation maintained. The water flow rate is controlled at 1.3 m / s. The workpiece is water-cooled at a rate of 45°C / min to a core temperature of 510°C, then air-cooled to a core temperature of 430°C. The workpiece is then water-cooled at a rate of 45°C / min to room temperature.

[0055] S3. Pre-precipitation treatment: the workpiece is kept at 535°C for 9.25 hours and cooled by air cooling at a cooling rate of 3.5°C / min.

[0056] S4. Two-stage spheroidizing annealing: Heat the workpiece to 870°C, hold for 7 hours, then slowly cool to 780°C at a rate of 10°C / h, hold for 11 hours, and then cool to 300°C before air cooling.

[0057] S5. Comprehensive performance control treatment: The workpiece was held at 1030°C for 2 hours, then cooled to room temperature at a cooling rate of 60°C / min. It was then tempered three times at 600°C, 615°C, and 580°C, each for 4 hours. Air cooling was used after each hold. The final workpiece hardness was 44.5 HRC, and the core V-notch impact energy was 23 J.

[0058] Comparative Example 2 In Comparative Example 2 of the present invention, the thickness of the forged workpiece is the same as that of Example 2, the heat treatment method lacks step S3 compared with Example 2, and the remaining steps are the same as those of Example 2. The final workpiece hardness is 45HRC, and the core V-notch impact energy is 14J. Figure 4 This figure compares the spheroidized microstructure images of Example 2 and Comparative Example 2. Due to the lack of pre-precipitation treatment in step S3, Comparative Example 2 has fewer carbide nucleation sites during the spheroidizing annealing process. Consequently, the number of carbides in the spheroidized microstructure is small and their uniformity is poor. In particular, the number of white, Mo-rich M2C carbides is significantly reduced compared to Example 2, and their size is larger and their distribution is uneven.

[0059] Example 3 S1. Circular Ultrafine Treatment: The forged workpiece (320mm thick) was held at 980°C for 6.8 hours. After the holding period, the temperature was raised at 150°C / h to 1120°C and held for 1.1 hours. After the first high-temperature ultrafine treatment, the workpiece was reheated to 980°C and held for 3.6 hours.

[0060] S2. Gradient Controlled Cooling: After the heat preservation step S1 is completed, the workpiece is removed from the furnace and air-cooled to a surface temperature of 925°C. Water cooling is then applied, with the cooling medium at 25°C and agitation maintained. The water flow rate is controlled at 1.2 m / s. The workpiece is water-cooled at a rate of 40°C / min to a core temperature of 530°C, then air-cooled to a core temperature of 410°C. The workpiece is then water-cooled at a rate of 40°C / min to room temperature.

[0061] S3. Pre-precipitation treatment: keep the workpiece at 520°C for 8.8 hours and cool it by air cooling at a cooling rate of 4°C / min.

[0062] S4. Two-stage spheroidizing annealing: Heat the workpiece to 860°C and hold for 7 hours. Slowly cool the workpiece to 760°C at a rate of 10°C / h and hold for 11 hours. Cool the workpiece to 300°C and air cool it out of the furnace.

[0063] S5. Comprehensive performance control treatment: The workpiece was held at 1030°C for 2 hours, then cooled to room temperature at a cooling rate of 70°C / min. Three tempering treatments were then performed at 590°C, 615°C, and 570°C, each for 4 hours. Air cooling was used after each hold. The final workpiece hardness was 45.5 HRC, and the core V-notch impact energy was 22.5 J.

[0064] Comparative Example 3 In Comparative Example 3 of the present invention, the thickness of the forged workpiece is the same as that of Example 3, step S4 in the heat treatment method is different from that of Example 3, and the remaining steps are the same as those of Example 3, as follows: S4. Two-stage spheroidizing annealing: Heat the workpiece to 900°C and hold for 10 hours. Slowly cool the workpiece at a rate of 35°C / hour to 760°C and hold for 6 hours. Cool the workpiece to 300°C and air cool. The final workpiece hardness is 45HRC, and the core V-notch impact energy is 16J. Figure 5 This is a comparison of the spheroidized microstructure images of Example 3 and Comparative Example 3. In Comparative Example 3, the initial heating temperature in step S4 was too high, resulting in a low number of residual carbides and fewer nucleation sites during the spheroidization process. Furthermore, the slow cooling rate was too rapid, making spheroidization difficult to complete. The spheroidized microstructure of Example 3 exhibits a greater number of carbides, a more even distribution, and a higher degree of spheroidization, resulting in a significantly improved core impact energy.

[0065] The main chemical components of the hot work die steel described in the present invention are, by mass percentage, C: 0.3%-0.4%, Si: 0.3%-0.8%, Mn: 0.2%-0.5%, Cr: 5.0%-6.0%, Mo: 1.0%-1.5%, V: 0.8%-1.2%, with the balance being Fe and unavoidable impurities. After undergoing cyclic ultra-fine refinement, gradient controlled cooling, pre-precipitation, two-stage spheroidizing annealing, and comprehensive performance control, these large-scale (thickness ≥ 300 mm) hot work die steel forgings for die casting achieve a core V-notch impact energy consistently exceeding 20 J while maintaining a hardness of 44-46 HRC.

[0066] The heat treatment method disclosed herein is applicable to most mainstream Cr-Mo-V hot-working die steels. Compared with conventional heat treatment methods for hot-working die steel used in die casting, this method optimizes the multi-stage post-forging heat treatment process, shortening the spheroidization cycle, improving spheroidized microstructure uniformity and carbide spheroidization rate, reducing scrap rates, and enhancing product impact toughness and quality stability. This method offers promising prospects for widespread application.

[0067] The present invention optimizes the cooling process after ultrafine treatment. Compared with direct water cooling, the gradient controlled cooling technology avoids the formation of bainite structure in the core of the workpiece due to slow cooling rate or the precipitation of large amounts of carbides along the grains, while reducing the risk of cracking in large-scale workpieces due to rapid cooling. Conventional post-forging heat treatment uses ultrafine treatment + spheroidizing annealing. The present invention adds a pre-precipitation treatment before spheroidizing annealing to further improve the uniformity of the spheroidizing annealing structure and the carbide spheroidization rate. Ultimately, simply by optimizing the multi-stage post-forging heat treatment process, the cross-sectional microstructure uniformity of large-scale hot working die steel for die casting is significantly improved, and the core impact toughness is enhanced.

[0068] The above are exemplary embodiments disclosed in the present invention. The order in which the above embodiments of the present invention are disclosed is for description only and does not represent the advantages and disadvantages of the embodiments. However, it should be noted that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples. Various changes and modifications may be made without departing from the scope defined by the claims. The functions, steps and / or actions of the method claims according to the disclosed embodiments described herein do not need to be performed in any particular order. In addition, although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as plural unless expressly limited to the singular.

[0069] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to limit the scope of the disclosure of the present invention (including the claims) to these examples. Within the spirit of the present invention, the technical features of the above embodiments or different embodiments may be combined, and many other variations exist in different aspects of the present invention as described above, which are not provided in detail for the sake of clarity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heat treatment method for improving the impact toughness of hot working die steel for die casting, characterized in that: The following steps are involved: S1, cyclic ultrafine treatment: the workpiece to be heat treated is kept at a temperature of t1 under the condition of T1, and after the end of the heat preservation, the temperature is raised to T2 and kept at a temperature of t2, and then the workpiece is reheated to T3 which is lower than T2 and kept at a temperature of t3; S2, gradient controlled cooling treatment: cool the workpiece at v1 to a surface temperature of T4, then cool it at v2 greater than v1 to a core temperature of T5, then cool it at v3 less than v2 to a core temperature of T6, and then cool it to room temperature at v4 greater than v3; S3, pre-precipitation treatment: the workpiece is kept at T7 for t4; S4, two-stage spheroidizing annealing treatment: heat the workpiece to T8 and hold at t5, then cool it to T9 and hold at t6; S5, comprehensive performance control processing: the workpiece is placed in T 11 After keeping warm for t7 under the conditions, cool to room temperature and then carry out tempering treatment.

2. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S1, the temperature range of T1 is 970~1020℃, and t1 = t 1基础保温 +Extend 40~50min for every 40~60mm increase in workpiece thickness, t 1基础保温 is 1.5~2.5h; the temperature range of T2 is 1100~1150℃, the t2 is 0.5~1.5h; the temperature range of T3 is 970~1020℃, the t3= t 3基础保温 +Extend 20~30min for every 40~60mm increase in workpiece thickness, t 3基础保温 0.5~1.5h.

3. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S2, the v1 is 5~10℃ / min, and the temperature range of T4 is 900~950℃; the v2 is 30~50℃ / min, and the temperature range of T5 is 500~550℃; the v3 is 3~5℃ / min, and the temperature range of T6 is 400~450℃; and the v4 is 30~50℃ / min.

4. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S2, the temperature is cooled to T4 and T6 by air cooling; and cooled to T5 and room temperature by water cooling.

5. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 4, characterized in that: In step S2, the temperature of the water-cooled medium is controlled at 20-40°C, and circulating stirring is maintained, and the water flow rate is controlled at 0.8-1.5 m / s.

6. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S3, the temperature range of T7 is 500-550°C, and t4 = t 4基础保温 +Extend 40~50min for every 40~60mm increase in workpiece thickness, t 4基础保温 After the heat preservation is completed, the mixture is cooled by air cooling at a cooling rate of 3-5°C / min.

7. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S4, the temperature range of T8 is 840-880℃, the temperature of t5 is 4-8h, and the cooling rate to T9 is 10-15℃ / min; the temperature range of T9 is 760-810℃, the temperature of t6 is 8-12h; after the insulation is completed, the furnace is cooled to T 10 After being air-cooled out of the oven, the T 10 The temperature range is ≤350℃.

8. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S5, the T 11 The temperature range is 950~1050℃, the t7 is 1~2h, and the cooling rate to room temperature is ≥50℃ / min.

9. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: In step S5, the tempering treatment is performed three times, with the tempering temperatures being 580-600° C., 600-620° C., and 560-580° C., respectively, and the holding time being 2-4 hours.

10. The heat treatment method for improving the impact toughness of hot working die steel for die casting according to claim 1, characterized in that: The thickness of the hot working die steel is ≥300 mm.

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