Method for improving impact toughness of annealed structure of hot work die steel

Abstract

Disclosed in the present invention is a method for improving the impact toughness of an annealed structure of hot work die steel. The method comprises the following steps: taking a workpiece to be machined, wherein said workpiece is die steel of which the annealed structure after forging is a non-equilibrium mixed-grain structure; and sequentially carrying out high-temperature solution treatment, primary cooling, isothermal annealing, normalizing, secondary cooling, and isothermal spheroidizing annealing on said workpiece to obtain a finished product, wherein the primary cooling and the secondary cooling are both cooling to a metastable austenite temperature range. According to the present invention, a material of which the annealed structure is a non-equilibrium mixed-grain structure is optimized in a heat treatment process, and an optimization process is provided, to further eliminate the non-equilibrium structure of a martensite, prevent structural heredity, eliminate the mixed-grain structure, and obtain a desired structure in which carbides are dispersively distributed on uniform and fine ferrite matrixes, thereby further improving the impact toughness of the material, and improving the use performance.

Description

A method for improving the impact toughness of annealed structure of hot working die steel Technical Field

[0001] The invention relates to the technical field of die steel, and in particular to a method for improving the impact toughness of annealed structure of hot working die steel. Background Art

[0002] Die steel is used to manufacture molds such as cold stamping dies, hot forging dies, and die casting dies. The higher the toughness and more uniform the annealed structure of the die steel, the more expensive it is. To reduce costs, some manufacturers choose to wholesale cheaper die steel for secondary processing. This not only further improves the die steel's performance and facilitates subsequent production, but also reduces procurement costs.

[0003] However, cheap mold steels often have unstable organization and performance as delivered, and large quality dispersion. Figure 1 shows the original annealed micromorphology of a domestically produced 4Cr5MoSiV1 material (the original organization characteristics at room temperature were obtained using metallographic microscopy (OM) and EBSD, respectively). As can be seen from Figure 1, the original organization of this mold steel exhibits a lath-shaped non-equilibrium mixed crystal organization at room temperature, and under high temperature conditions of 1030°C, its austenite also exhibits mixed crystals when observed using a high-temperature laser confocal microscope (CLSM).

[0004] Since the non-equilibrium mixed crystal structure has significant organizational heredity and is difficult to eliminate in subsequent heat treatment, it will seriously affect the service characteristics of the steel and greatly reduce its working life. Conventional heat treatment methods cannot enable this type of mold steel to obtain more excellent physical properties. The heat treatment method needs to be improved to eliminate the non-equilibrium mixed crystal structure in the steel.

[0005] Currently, the main method for improving the annealed microstructure of hot-working die steel is to refine the original microstructure grains to improve material properties. However, since the original annealed microstructure is a non-equilibrium microstructure with a severe mixed crystal state (with microstructure inheritance), the annealed ferrite and austenite have a specific KS orientation relationship. During heating, ferrite transforms to austenite based on the KS relationship, causing the austenite to return to its original mixed crystal state and become unable to be refined. Therefore, its microstructure is difficult to improve under conventional heat treatment processes.

[0006] Therefore, for 4Cr5MoSiV1 materials with severe non-equilibrium mixed crystal structure, it is necessary to develop a new heat treatment process that can eliminate its organizational heredity.

[0007] Summary of the Invention

[0008] The purpose of the present invention is to provide a method for improving the impact toughness of the annealed structure of hot working die steel, so as to solve the technical problem in the prior art that it is very difficult to improve the grain refinement of the 4Cr5MoSiV1 material whose original annealed structure is a non-equilibrium structure, especially the severe mixed crystal state (with structural heredity).

[0009] In order to solve the above technical problems, the present invention specifically provides the following technical solutions:

[0010] The present invention provides a method for improving the impact toughness of annealed structure of hot working die steel, comprising the following steps:

[0011] A workpiece to be processed is obtained, wherein the workpiece is a die steel having a non-equilibrium mixed crystal structure in an annealed state after forging;

[0012] The workpiece is subjected to high-temperature solution treatment, first cooling, and isothermal annealing in sequence to block the tissue inheritance of the non-equilibrium tissue in the workpiece to obtain an intermediate piece;

[0013] Wherein, the high temperature solution treatment is: heating the initial martensite structure in the workpiece to an austenitizing temperature until the workpiece is austenitized;

[0014] The first cooling step is to quickly cool the workpiece to a metastable austenite temperature range.

[0015] The isothermal annealing is performed by holding the temperature for 5-15 hours, so that the initial martensite structure is transformed into the ferrite structure with a transformation rate greater than or equal to 98%, thereby fully eliminating the non-equilibrium structure in the workpiece and cutting off the tissue inheritance of the non-equilibrium structure;

[0016] After isothermal annealing, the intermediate piece is sequentially subjected to normalizing, secondary cooling, and isothermal spheroidizing annealing to refine the grains in the intermediate piece to obtain a finished piece;

[0017] The second cooling is to quickly cool the intermediate piece to a metastable austenite temperature zone.

[0018] As a preferred solution of the present invention, the workpiece is 4Cr5MoSiV1 hot working die steel whose annealed structure presents a lath-shaped non-equilibrium mixed crystal structure.

[0019] As a preferred solution of the present invention, the metastable austenite temperature zone is 550-600°C.

[0020] As a preferred embodiment of the present invention, the high temperature solution treatment heats the workpiece to an initial martensite structure and to an austenitizing temperature by heating the workpiece multiple times;

[0021] Wherein, the high temperature solution treatment comprises the following steps:

[0022] The workpiece is heated for the first time to 600-650° C. and kept at this temperature for 0.5-1 hour;

[0023] After the first heating and holding is completed, the workpiece is heated for a second time to 800-850° C. and held for 0.5-1 hour;

[0024] After the second heating and holding is completed, the workpiece is heated for the third time to 1010° C.-1050° C. and held for 0.5 h;

[0025] The speeds of the first heating, the second heating and the third heating are all ≤300°C / h.

[0026] As a preferred embodiment of the present invention, the isothermal annealing is a one-stage isothermal annealing or a two-stage isothermal annealing.

[0027] As a preferred embodiment of the present invention, the isothermal annealing step comprises the following steps:

[0028] The workpiece after the first cooling is directly placed in a heat treatment furnace heated to 720-890° C., kept at this temperature for 5-15 hours, and then cooled to room temperature to obtain the intermediate piece.

[0029] As a preferred embodiment of the present invention, the two-stage isothermal annealing comprises the following steps:

[0030] The primary workpiece is kept at a temperature range of 860-890° C. for 2-5 hours for a first annealing step;

[0031] The primary workpiece is kept at a temperature range of 720-760° C. for 3-10 hours, subjected to a second-stage annealing, and cooled to room temperature to obtain the intermediate workpiece.

[0032] As a preferred embodiment of the present invention, the normalizing comprises the following steps:

[0033] Raise the temperature of the intermediate piece to 600-650°C and keep it at this temperature for 0.5-1h;

[0034] Raise the temperature of the intermediate piece to 940-980° C. and keep it warm for 1-10 hours;

[0035] The intermediate piece is rapidly cooled to 550-600° C. in the metastable austenite region by air cooling or wind cooling.

[0036] As a preferred embodiment of the present invention, the isothermal spheroidizing annealing comprises the following steps:

[0037] The intermediate piece after the second cooling is kept at a temperature range of 860-890° C. for 3-6 hours;

[0038] The intermediate piece is cooled to 720-760° C., kept at this temperature for 8-15 hours, and then cooled to room temperature to obtain the finished piece.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] (1) The present invention adopts high-temperature solution treatment to heat the initial martensite structure in the mold steel whose annealed structure after forging is a non-equilibrium mixed crystal structure to an austenitizing temperature, directly austenitizes the non-equilibrium structure in the mold steel, and then cools the mold steel to a corresponding temperature and performs heat preservation annealing, so that the initial martensite structure is transformed into a ferrite structure, and the transformation rate is greater than or equal to 98%, so as to fully eliminate the non-equilibrium structure in the workpiece and cut off the tissue inheritance of the non-equilibrium structure;

[0041] (2) The present invention adopts air cooling or wind cooling in both the high-temperature solution treatment and normalizing stages to cool the workpiece to the metastable austenite region, and then directly sends it to the annealing furnace for annealing, which can avoid the formation of network carbides and non-equilibrium structure, and is time-saving and efficient at the same time, so as to achieve uniform grain refinement, and provide a good organizational foundation for improving material performance and quality.

[0042] (3) The present invention subsequently adopts a composite heat treatment process of normalizing, cooling to the metastable austenite temperature zone and then directly annealing to refine the grains in the intermediate part, so that the austenite in the mold steel part presents a uniform fine-grained structure and the grain size reaches level 8 or above, thereby improving the quality of the mold steel and meeting production requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0044] FIG1 shows the micromorphology of the original annealed state of 4Cr5MoSiV1 provided by the present invention: (a) room temperature OM (Optical Microscope) image; (b) room temperature EBSD (Electron Backscatter Diffraction) image; (c) 1030° C. CLSM (High Temperature Confocal Laser Scanning Microscopy) image;

[0045] FIG2 is a microstructure diagram of the original annealed state of 4Cr5MoSiV1 shown in Example 1 of the present invention;

[0046] FIG3 is a microstructure diagram of 4Cr5MoSiV1 subjected to high temperature solid solution treatment and annealing as shown in Example 1 of the present invention;

[0047] FIG4 is a microstructure diagram of the finished product shown in Example 1 provided by the present invention;

[0048] FIG5 is a flow chart of a method for improving the impact toughness of annealed structure of hot working die steel provided by the present invention. DETAILED DESCRIPTION

[0049] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0050] The present invention provides a method for improving the impact toughness of the annealed structure of hot working die steel, as shown in Figure 5, comprising sequentially treating the workpiece with high-temperature solution treatment, isothermal annealing, normalizing, and isothermal spheroidizing annealing to obtain a finished part.

[0051] The present invention employs a high-temperature solution heat treatment followed by annealing. After the high-temperature solution heat treatment, the workpiece is cooled to the metastable austenite region to eliminate non-equilibrium structures in the material. This not only prevents the formation of network carbides but also prevents the formation of non-equilibrium structures such as bainite and martensite. The ferrite transformation rate exceeds 98%, which is beneficial for improving subsequent microstructure and performance. Furthermore, a rapid cooling process is employed to cool the workpiece to the metastable austenite region after normalizing, saving time and energy. The grain size is refined from mixed crystals to above 8, further completely eliminating non-equilibrium structures and completely severing microstructure inheritance, resulting in even better results.

[0052] Since the heating rate is related to the transformation of the structure, if the heating rate is too fast, it will be detrimental to the transformation of the initial martensite structure to the austenite structure, especially at the core of the workpiece, and will also cause greater thermal stress in the workpiece. However, the temperature span during the heating process is generally large, and it takes a long time to complete the heating process, which results in a long solution treatment time and low economic benefits.

[0053] Moreover, since this mixed crystal structure retains its genetic characteristics during the subsequent quenching, cooling and tempering processes, it seriously affects the service life of the material. Simply increasing the heating temperature and extending the holding time will easily lead to coarse grains.

[0054] In order to solve the problems in the existing high-temperature solution treatment, the present invention provides a high-temperature solution treatment method, comprising the following steps:

[0055] The workpiece is heated for the first time to 600-650°C and kept at this temperature for 0.5-1h;

[0056] After the first heating and holding is completed, the workpiece is heated for a second time to 800-850° C. and held for 0.5-1 hour;

[0057] After the second heating and holding is completed, the workpiece is heated for the third time, heated to 1010℃-1050℃, and held for 0.5h; the best time is to hold the core for 0.5h, which can also be determined according to the thermal conductivity of the material and the experience of the technicians.

[0058] After the third heating and holding is completed, the workpiece is cooled to a metastable austenite temperature zone by air cooling or wind cooling.

[0059] Among them, the heating rate is ≤300℃ / h.

[0060] The present invention adopts a three-stage heating method and performs staged heat preservation after a single heating, so that even if the heating rate reaches 300°C / h, the temperature uniformity inside and outside the workpiece during the heating process can be improved, the heat conduction efficiency can be improved, the heating time can be greatly shortened, and the economic benefits can be improved.

[0061] The present invention adopts a high-temperature solid solution process to promote the dissolution of carbides into the matrix, reduce the pinning effect of carbides on the grain boundaries at the original grain boundaries, help refine the austenite and thus eliminate the heredity of non-equilibrium tissues. In addition, it also helps the re-precipitation and spheroidization of carbides in the subsequent isothermal annealing process, and prevents the formation of network carbides.

[0062] The present invention can adopt one-stage isothermal annealing or two-stage isothermal annealing.

[0063] After high-temperature solution treatment, the workpiece temperature is lowered to the metastable austenite temperature range of 550-600°C using air or gas cooling to effectively avoid the formation of network carbides. Direct annealing in an annealing furnace can effectively improve production efficiency and reduce working hours. During isothermal annealing, a holding time of 5-15 hours can achieve a ferrite transformation rate of over 98%, completely severing the tissue inheritance of the non-equilibrium structure.

[0064] Specifically, the isothermal annealing step includes the following steps: directly placing the workpiece after the first cooling into a heat treatment furnace heated to 720-890° C., keeping the temperature for 5-15 hours, and then cooling to room temperature to obtain the intermediate piece.

[0065] Compared with the one-stage isothermal annealing, preferably, the isothermal annealing adopts two-stage isothermal spheroidizing annealing at 860±20°C and 740±20°C. This process improves the transformation efficiency compared with the one-stage isothermal annealing and can reduce the deformation and cracking tendency of the workpiece.

[0066] After annealing, the workpiece is cooled to room temperature. At this point, the non-equilibrium structure in the workpiece is eliminated and the structure inheritance is completely cut off, but the mixed crystal problem has not been completely solved. Therefore, the intermediate part needs to be normalized, cooled for the second time, and isothermal spheroidizing annealing in order to refine the grains in the intermediate part and obtain the finished part.

[0067] The existing technology of eliminating non-equilibrium structure by full annealing is mainly applicable to hypoeutectoid steel, while the 4Cr5MoSiV1 of the present invention is a hypereutectoid steel. If full annealing is adopted, a large amount of network carbides will be formed during the slow cooling process, which will seriously affect the performance of the steel.

[0068] The present invention adopts high-temperature solid solution (air cooling / wind cooling) + annealing to eliminate the non-equilibrium structure and avoid the formation of network carbides.

[0069] Preferably, normalizing comprises the steps of:

[0070] The intermediate piece is heated for the fourth time to 600-650° C. and kept at this temperature for 0.5-1 hour;

[0071] After the fourth heating and holding, the intermediate piece is heated for the fifth time to 940-980° C. and held for 1-10 hours. The specific holding time is determined by the segregation of the structure. If the segregation is serious, a long holding time is used.

[0072] After the fifth heating and holding process is completed, the intermediate piece is cooled to a metastable austenite temperature zone by air cooling or wind cooling to obtain a secondary workpiece.

[0073] According to the present invention, the subsequent annealing process is directly performed after the metastable austenite region is cooled after normalizing, which can effectively avoid the generation of non-equilibrium structure and improve aging.

[0074] The invention finally adopts isothermal spheroidizing annealing. Specifically, the isothermal spheroidizing annealing includes the following steps:

[0075] Keeping the secondary workpiece at a temperature range of 860-890° C. for 3-6 hours;

[0076] The secondary workpiece is cooled to 720-760°C, kept at this temperature for 8-15 hours, and then air-cooled to room temperature to obtain the finished product. The specific steps of the heat treatment are as follows:

[0077] Step 1: Inspect the workpiece and observe the structure using an optical microscope or scanning electron microscope to determine the structure characteristics. For materials with non-equilibrium mixed crystal structure, perform the following steps;

[0078] Step 2: High temperature solution treatment: heat the workpiece to 600-650℃ at a heating rate of ≤300℃ / h and keep it at that temperature for 0.5-1h;

[0079] Then heat the workpiece to 800-850℃ at a heating rate of ≤300℃ / h and keep it warm for 0.5-1h;

[0080] Heat the workpiece to 1010-1050°C at a heating rate of ≤300°C / h, and keep the temperature at the core for 0.5h;

[0081] Then air cool or wind cool to the metastable austenite temperature zone;

[0082] Step 3: Isothermal annealing: Place the workpiece processed in step 2 directly into a heat treatment furnace heated to 720-760°C and keep warm for 2-5 hours;

[0083] Then raise the temperature of the heat treatment furnace to 860-890℃ and keep it warm for 3-10h;

[0084] Step 4: Normalizing treatment: Take the workpiece obtained in step 3, raise the temperature of the workpiece to 600-650℃ at a heating rate of ≤300℃ / h, and keep it at this temperature for 0.5-1h;

[0085] Then heat the workpiece to 940-980℃ at a heating rate of ≤300℃ / h and keep it warm for 1-10h;

[0086] Then air cool or wind cool to the metastable austenite temperature zone;

[0087] Step 5: Isothermal spheroidizing annealing: Place the workpiece processed in step 4 directly into a heat treatment furnace that has been heated to 860-890°C and keep it warm for 3-6 hours, then cool it to 720-760°C with the furnace and keep it warm for 8-15 hours, and air cool it to room temperature to obtain the finished product.

[0088] The above process is verified by using specific examples below.

[0089] Example 1

[0090] 1. Prepare the workpiece:

[0091] Annealed 4Cr5MoSiV1 hot working die steel with martensitic non-equilibrium mixed crystal structure was used as the workpiece. 4Cr5MoSiV1 hot working die steel (chemical composition is in accordance with the standard GB / T 1299-2014 "Tool Steel") was used.

[0092] 2. The original structure characteristics are lamellar non-equilibrium mixed crystal structure + carbide, as shown in Figure 2.

[0093] 3. Heat treatment

[0094] Step 1: Heat the workpiece to 650°C at a heating rate of 300°C / h and keep it for 0.5h, then heat the workpiece to 850°C at a heating rate of 300°C / h and keep it for 0.5h, then heat the workpiece to 1030°C at a heating rate of 300°C / h and keep it for 1h, and then air-cool it to the metastable austenite temperature zone of 550-600°C;

[0095] Step 2: Place the workpiece processed in step 1 directly into a heat treatment furnace that has been heated to 860°C and kept warm for 2 hours. Cool it down to 740°C after 15 minutes and keep warm for 12 hours. Then air cool it to room temperature.

[0096] Step 4: The workpiece treated in step 2 is heated to 650°C at a heating rate of 300°C / h and kept at that temperature for 0.5h, then heated to 980°C at a heating rate of 300°C / h and kept at that temperature for 0.5h, and then air-cooled to the metastable austenite temperature range of 550-600°C;

[0097] Step 5: directly placing the workpiece treated in step 4 into a heat treatment furnace heated to 860° C. and holding the temperature for 2 hours, cooling the temperature to 740° C. after 15 minutes and holding the temperature for 12 hours, and then air-cooling the workpiece to room temperature to obtain Example 1;

[0098] Step 6: Perform a quenching + tempering experiment on the workpiece of Example 1.

[0099] Characterization:

[0100] 1. Test the materials after high temperature solution treatment and isothermal annealing;

[0101] Results: Figure 3 shows the microstructure of 4Cr5MoSiV1 die steel after high temperature solution treatment, first cooling and isothermal annealing;

[0102] Conclusion: As can be seen from the figure, after annealing, the matrix structure of the material presents ferrite mixed crystals, which blocks the inheritance of the lamellar non-equilibrium structure. However, the mixed crystals still exist, with large grains of more than 120μm and small grains of 10-25μm.

[0103] 2. Test the finished product

[0104] Results: Figure 4 shows the microstructure of 4Cr5MoSiV1 die steel after the workpiece was subjected to high-temperature solution treatment, first cooling, isothermal annealing, normalizing, second cooling and isothermal spheroidizing annealing.

[0105] Conclusion: As can be seen from the figure, after high-temperature solution treatment, first cooling, and isothermal annealing, the non-equilibrium structural inheritance in the workpiece is basically cut off. In the subsequent treatment process, the grains are refined, making the finished part uniform and fine-grained, with a grain size of more than 8 (according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals");

[0106] 3. Test its hardness and impact toughness

[0107] Results: Hardness ≥ 45HRC, impact absorption energy KV2 ≥ 10.8J;

[0108] Conclusion: The material has high hardness and impact toughness, and is of high quality (the grade is assessed in accordance with GB / T 42083-2022 "Heat Treatment of 4Cr5MoSiV1 Hot Working Die Steel").

[0109] Through the method for improving the impact toughness of the annealed structure of the hot working die steel in this embodiment, a heat treatment process is developed for materials with poor annealed structure as supplied, especially those with non-equilibrium mixed crystal structure, and an optimized process is proposed to transform the initial martensite structure into a ferrite structure, eliminate the non-equilibrium mixed crystal structure in the martensite, cut off the tissue heredity, and finally obtain an ideal structure with uniform and fine carbides dispersed on the ferrite matrix, thereby significantly improving the impact performance, improving the quality and grade of the material, and improving the performance in use.

[0110] The above embodiments are merely exemplary embodiments of the present application and are not intended to limit the scope of the present application. The scope of protection of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and scope of protection of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the scope of protection of the present application.

Claims

1. A method for improving the impact toughness of annealed structure of hot working die steel, characterized in that: The steps include: Taking a workpiece to be processed, the workpiece is a die steel having a non-equilibrium mixed crystal structure in an annealed state after forging; The workpiece is subjected to high temperature solution treatment, first cooling and isothermal annealing in sequence to block the tissue inheritance of the non-equilibrium tissue in the workpiece to obtain an intermediate piece; Wherein, the high temperature solution treatment is: heating the initial martensite structure in the workpiece to an austenitizing temperature until the workpiece is austenitized; The first cooling is to quickly cool the workpiece to a metastable austenite temperature zone; The isothermal annealing is: keeping the temperature for 5-15 hours, so that the initial martensite structure is transformed into the ferrite structure, and the transformation rate is greater than or equal to 98%, so as to fully eliminate the non-equilibrium structure in the workpiece and cut off the tissue inheritance of the non-equilibrium structure; After isothermal annealing, the intermediate piece is subjected to normalizing, secondary cooling and isothermal spheroidizing annealing in sequence to refine the grains in the intermediate piece to obtain a finished piece; The second cooling is to quickly cool the intermediate piece to a metastable austenite temperature zone.

2. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 1, characterized in that: The workpiece is 4Cr5MoSiV1 hot working die steel whose annealed structure presents a lath-shaped non-equilibrium mixed crystal structure.

3. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 1, characterized in that: The metastable austenite temperature zone is 550-600°C.

4. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 1, characterized in that: The high temperature solution treatment heats the workpiece to an initial martensitic structure and to an austenitizing temperature by multiple temperature increases; Wherein, the high temperature solution treatment comprises the following steps: The workpiece is heated for the first time to 600-650° C. and kept at this temperature for 0.5-1 h; After the first heating and insulation is completed, the workpiece is heated for a second time to 800-850° C. and kept warm for 0.5-1h; After the second heating and heat preservation is completed, the workpiece is heated for the third time to 1010° C.-1050° C. and kept warm for 0.5 h; The speeds of the first heating, the second heating and the third heating are all ≤300°C / h.

5. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 1, characterized in that: The isothermal annealing is one-stage isothermal annealing or two-stage isothermal annealing.

6. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 5, characterized in that: The isothermal annealing step comprises the following steps: The workpiece after the first cooling is directly placed in a heat treatment furnace heated to 720-890° C., kept at this temperature for 5-15 hours, and then cooled to room temperature to obtain the intermediate piece.

7. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 5, characterized in that: The two-stage isothermal annealing comprises the following steps: The primary workpiece is kept at a temperature range of 860-890° C. for 2-5 hours for a first annealing step; The primary workpiece is kept at a temperature range of 720-760° C. for 3-10 hours, subjected to a second-stage annealing, and cooled to room temperature to obtain the intermediate workpiece.

8. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 1, characterized in that: The normalizing comprises the following steps: The intermediate piece is heated to 600-650°C and kept at this temperature for 0.5-1h; Raise the temperature of the intermediate piece to 940-980° C. and keep it warm for 1-10 hours; The intermediate piece is rapidly cooled by air cooling or wind cooling to a temperature of 550-600° C. in the metastable austenite region.

9. The method for improving the impact toughness of annealed structure of hot working die steel according to claim 1, characterized in that: The isothermal spheroidizing annealing comprises the following steps: The intermediate piece after the second cooling is kept at a temperature range of 860-890° C. for 3-6 hours; The intermediate piece is cooled to 720-760° C., kept at this temperature for 8-15 hours, and cooled to room temperature to obtain the finished piece.

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