A short process heat treatment process for large plastic mold steel

Abstract

This invention discloses a short-process heat treatment process for large plastic mold steel, designed for the production of hot work die steel, comprising the following steps: S1, after forging, the module is air-cooled in still air until the surface temperature drops to 350°C (T1); S2, the module cooled to 350°C is placed in a heating furnace, the furnace is first heated to a constant 350°C, and the module is held at that temperature for 4 hours; then the furnace is heated to 650°C, and the module is held at that temperature (T2); finally, the furnace is heated to 880°C. Afterwards, the module is kept at a constant temperature for time T3; S3, after the holding time is completed, water quenching is performed, with intermittent air cooling cycles during the process, and intermittent quenching is carried out by alternating water quenching and air cooling; S4, after intermittent quenching and cooling, the heating furnace is heated to a constant temperature of 200℃ and the module is kept at a constant temperature for 4 hours. Then the heating furnace is heated to t1℃ and the module is kept at a constant temperature for time T4. Then the heating furnace is heated to t2℃ and the module is kept at a constant temperature for time T5. Finally, after the holding time T5 is completed, the module is taken out of the furnace and air-cooled.

Description

Technical Field

[0001] This invention relates to the field of hot work die steel production, specifically a short-process heat treatment process for large plastic mold steel. Background Technology

[0002] Large plastic mold steels (such as P20, 718, 1.2738, etc.) are widely used in the molding of large plastic products such as car bumpers, dashboards, and appliance housings. The blank thickness of such molds is often 500 to 1200 mm. After heat treatment, the mold cross-section is required to have uniform hardness, fine structure, good machinability, and high comprehensive mechanical properties.

[0003] The traditional manufacturing and heat treatment process for large plastic mold steel is as follows: smelting → ingot casting → forging → post-forging heat treatment (normalizing + spheroidizing annealing) → rough machining → quenching and tempering heat treatment (quenching + high-temperature tempering) → finish machining. Post-forging heat treatment is crucial for determining the final microstructure uniformity and hardness, but it is a complex and time-consuming process. Current technology involves air cooling to room temperature after forging, followed by furnace heating to 870–900℃ for normalizing, and then prolonged spheroidizing annealing at an isothermal temperature of approximately 760℃. The overall holding time is often as long as 30–60 hours, and the entire post-forging heat treatment cycle can last 40–80 hours, resulting in high overall energy consumption. Furthermore, the core of large-section forgings cools more slowly than the outer layers, making it prone to developing coarse grains and banded structures after normalizing. These structures are difficult to eliminate completely during subsequent spheroidizing annealing, leading to uneven carbide morphology and distribution. In addition, when entering the tempering process, due to the large size of the mold blank, the temperature difference between the inside and outside is large during quenching and cooling. This not only easily causes deformation or even cracking, but also easily causes a large difference in hardness between the surface and the core (usually the hardness fluctuation exceeds 3HRC), making it difficult to meet the requirements of high-end molds for the uniformity of cross-sectional hardness.

[0004] To address these issues, existing processes attempt to simplify or adjust the heat treatment sequence, such as omitting the independent spheroidizing annealing step and directly tempering the forged module. However, because the microstructure is not refined and homogenized, it easily leads to large dispersion in the final hardness, resulting in unstable machinability of the module. Other methods attempt to utilize forging residual heat for online quenching or normalizing to shorten the process and save energy. However, when applied to large blanks, the temperature difference across the cross-section and the inheritance of high-temperature microstructure easily cause mixed grains and abnormal growth of austenite grains, ultimately resulting in defects such as low impact toughness and high hardness of the module, making subsequent processing difficult. Some studies have also attempted to improve grain characteristics by adding a low-temperature normalizing or short-time annealing before tempering, but the overall process remains long, and the time-saving effect is relatively limited.

[0005] In summary, existing heat treatment processes for large plastic mold steel generally suffer from problems such as long processes, high energy consumption, and long cycles. Furthermore, after process simplification, defects such as uneven cross-sectional properties and increased risk of quenching cracks are more likely to occur. Therefore, the industry urgently needs a heat treatment process that is short in process, energy-efficient, and can ensure uniform hardness of large cross-section modules and good machinability. Summary of the Invention

[0006] In view of the shortcomings of the existing heat treatment process for large plastic mold steel mentioned in the background technology, a short-process heat treatment process for large plastic mold steel is proposed. This process can significantly shorten the overall process cycle while ensuring the uniformity of cross-sectional hardness, fineness of microstructure, and good machinability of the module.

[0007] The present invention provides a short-process heat treatment process for large plastic mold steel, comprising the following steps: Step S1, Post-forging controlled cooling: After forging, the module is immediately subjected to forced air cooling for time T1. After the air cooling is completed, it continues to be air cooled in still air until the surface temperature of the module drops to 350°C. Step S2, segmented heating: First, place the module, which has been cooled to 350°C, into the heating furnace. Then, raise the furnace temperature to a constant 350°C and keep the module at that temperature for 4 hours. The heating furnace was then heated to 650°C, and the module was kept at that temperature for time T2. Finally, the heating furnace is heated to 880℃, and the module is kept at that temperature for time T3. Step S3, quenching and cooling: After the module is held at 880℃, it is taken out of the furnace and subjected to water quenching. During the water quenching process, fixed air cooling cycles are intermittently inserted to perform intermittent quenching by alternating water quenching and air cooling. Step S4, double tempering: After intermittent quenching and cooling, the module is placed in the heating furnace. The furnace temperature is first raised to a constant 200°C, and then the module is kept at that temperature for 4 hours. The heating furnace was then heated to t1℃, and the module was kept at that temperature for time T4. The heating furnace was then heated to t2℃, and the module was kept at that temperature for T5 time. Finally, after the T5 heat preservation is completed, the module is taken out of the furnace and air-cooled.

[0008] As a further improvement of the present invention, the mold steel is made of P20, 718 or 1.2738 steel, and the thickness d of the module section is in the range of 500mm-1200mm.

[0009] As a further improvement of the present invention, the wind speed of the forced air cooling in step S1 is 5-15m / s.

[0010] As a further improvement of the present invention, in step S1, the blowing time T1 = 0.6 min / mm × d, where d is the cross-sectional thickness of the module in mm.

[0011] As a further improvement of the present invention, in step S2, the average temperature rise rate of the heating furnace from 350°C to 650°C and the average temperature rise rate of the heating furnace from 650°C to 880°C are both less than or equal to 100°C / h.

[0012] As a further improvement of the present invention, in step S2, the heat preservation time T2 = 2.5 min / mm × d, and the heat preservation time T3 = 1.8 min / mm × d, where d is the cross-sectional thickness of the module in mm.

[0013] As a further improvement of the present invention, in step S3, the water temperature of the quenching water is ≤40℃, and the stirring and cooling tower circulation system for the quenching water is turned on in the quenching container to maintain the temperature standard of the quenching water.

[0014] As a further improvement of the present invention, after the intermittent quenching process of alternating water quenching and air cooling in step S3, an axial flow fan is used to continue cooling until the surface temperature of the module drops to 180°C. The air velocity used by the axial flow fan is 5-15 m / s. The alternating intermittent quenching process is divided into intervals according to the module cross-sectional thickness d, and is executed according to the following table: ; As a further improvement of the present invention, in step S4, the average temperature rise rate of the heating furnace is less than or equal to 60℃ / h; the heat preservation time T4 = 2.5min / mm×d, the heat preservation time T5 = 1.5min / mm×d, where d is the cross-sectional thickness of the module in mm.

[0015] As a further improvement of the present invention, in step S4, the temperatures t1 and t2 are determined according to the steel type as shown in the table below: ; Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention differs from the conventional process of "independent normalizing + tempering or long-term isothermal spheroidizing annealing" used in existing processes. Instead, it directly uses forced air cooling after the module is forged to normalize, accelerating the cooling process and reducing the time required for each step. During the segmented heating process, the heating process before reaching the tempering heat treatment temperature is rationally divided into steps. A fine non-equilibrium structure is established during the isothermal process at 350℃. Then, high-temperature tempering and hydrogen diffusion are performed at 650℃. Next, the carbides are dispersed and spheroidized and uniformly distributed during the 650℃ holding period. Finally, uniform and fine initial grains are obtained during austenitization at 880℃. Multiple pre-tempering heat treatments are completed in a single heating cycle, making the various process steps in the post-forging heat treatment more compact and compressing the overall production cycle.

[0016] 2. This invention provides a precisely matched two-stage tempering regime (selection of t1 and t2) for three different compositions of plastic mold steel: P20, 718, and 1.2738. First, preheating and holding at a low temperature of 200℃ ensures uniform temperature inside and outside the module, preparing it for subsequent microstructure transformation. Then, two stages of high-temperature tempering at different temperatures precisely control the precipitation and aggregation morphology of carbides, stably obtaining modules with a hardness range of 28–38 HRC, and controlling the hardness fluctuation of the entire cross-section within 2 HRC. This effectively improves the problem of large differences in hardness between the surface and core of modules produced by existing processes, meeting different processing and usage requirements.

[0017] 3. The stepped "water-cooling-air-cooling" alternating quenching cooling system of this invention ensures a sufficiently rapid cooling rate for the core of the module during the water-cooling immersion quenching stage to achieve the martensitic / bainitic transformation. After each short immersion quench, the air-cooling stage releases thermal stress and performs surface self-tempering, preventing quenching cracks and significantly reducing the tendency for surface quenching cracks in the module. This effectively addresses the problem of "cracking due to large internal and external temperature differences and the coupling of thermal stress and structural stress during the quenching of large cross-section modules" in existing processes. Furthermore, this invention provides a finely matched process table based on the module thickness range, providing precise production guidance for the quenching process of large plastic mold steels of different thicknesses, and has wide applicability. Detailed Implementation Specific Implementation Example 1:

[0019] This embodiment is for a P20 plastic mold steel forging module with a cross-sectional thickness d=600mm and a target hardness of 28-32HRC after forging heat treatment.

[0020] Step S1, Post-forging controlled cooling: After forging, the final forging temperature of the module was approximately 1020℃. Forced air cooling was immediately implemented using an axial flow fan for a duration of T1 = 0.6 min / mm × 600 mm = 360 min (6 h), with a wind speed of 10 m / s. After the forced air cooling was completed, air cooling continued in still air, and the module surface temperature dropped to 350℃ after approximately 2 hours.

[0021] Step S2, segmented heating: The module, air-cooled to a surface temperature of 350°C, is placed in a heating furnace. The furnace temperature is then raised to 350°C, and the module is held at this temperature for 4 hours to ensure that not only the surface temperature but also the core temperature of the module approaches 350°C. Next, the furnace temperature is raised to 650°C at an average temperature rise rate of 60°C / h over 5 hours, and the module is held at this temperature for T2 = 2.5 min / mm × 600 mm = 1500 min (25 h). Subsequently, the furnace temperature is raised to 880°C at an average temperature rise rate of 80°C / h over 2.875 hours, and the module is held at this temperature for T3 = 1.8 min / mm × 600 mm = 1080 min (18 h). Notably, during the initial temperature rise to 350°C, the furnace temperature rise rate is not limited.

[0022] Step S3, quenching and cooling: After the module was held at 880℃, it was quickly removed from the furnace and immersed in a water bath with a temperature of 40℃ or less for water cooling quenching. In this embodiment, the module cross-sectional thickness d=600mm, and intermittent quenching was performed according to the cooling regime corresponding to the range "500≤d≤600". The specific process was as follows: water cooling 15min → air cooling 5min → water cooling 15min → air cooling 5min → water cooling 8min → air cooling 5min → water cooling 8min → air cooling 5min → water cooling 5min → water cooling 5min → air cooling 5min → water cooling 5min → air cooling 5min → water cooling 5min, for a total time of 101min. After the intermittent quenching was completed, an axial flow fan was used to continue cooling the module surface at a wind speed of 5-15m / s until the measured module surface temperature dropped to 180℃. The module cooling process took about 1 hour. In particular, during the quenching process, the quenching water in the tank is kept running with a stirring and cooling tower circulation heat exchange system to ensure that the water temperature is always less than or equal to 40℃.

[0023] Step S4, double tempering: After the module is cooled by blowing air, it is placed in a heating furnace. The furnace temperature is raised to 200°C, and the module is held at that temperature for 4 hours. Furthermore, in this embodiment, the module material is P20 with a target hardness of 28-32 HRC. During the two-stage tempering, the furnace temperature t1 in stage one is selected as 530°C, and the furnace temperature t2 in stage two is selected as 580°C. First, the furnace temperature was raised from 200℃ to 530℃ at a rate of 60℃ / h over 5.5 hours, and the module was held at that temperature for T4 = 2.5 min / mm × 600 mm = 1500 min (25 h). Then, the furnace temperature was raised from 530℃ to 580℃ at a rate of 60℃ / h over approximately 0.83 hours, and the module was held at that temperature for T5 = 1.5 min / mm × 600 mm = 900 min (15 h). Finally, after the holding period, the module was removed from the furnace and air-cooled to room temperature.

[0024] The modules that underwent the aforementioned post-forging heat treatment were tested, and the results showed that the overall cross-sectional hardness of the modules was 28.7-30.2 HRC, with a hardness fluctuation of no more than 1.5 HRC; the metallographic structure consisted of uniform tempered sorbite and dispersed granular carbides, with a grain size of grade 8; ultrasonic testing revealed no defects or quenching cracks, meeting the requirements for steel used in automotive bumper molds. Compared to traditional processes, the post-forging heat treatment process described in this invention reduces the total time from the end of forging to the completion of tempering by approximately 42% for modules of similar size and material. Specific Implementation Example 2:

[0026] This embodiment is for a 718 plastic mold steel forging module with a cross-sectional thickness d=1100mm and a target hardness of 33-38HRC after forging heat treatment.

[0027] Step S1, Post-forging controlled cooling: After forging, the final forging temperature of the module was approximately 1050℃. Forced air cooling was immediately implemented using an axial flow fan for a duration of T1 = 0.6 min / mm × 1100 mm = 660 min (6 h), with a wind speed of 12 m / s. After the air cooling was completed, air cooling continued in still air, and the module surface temperature dropped to 350℃ after approximately 3.5 h.

[0028] Step S2, segmented heating: The module, air-cooled to a surface temperature of 350°C, is placed in a heating furnace. The furnace temperature is then raised to 350°C, and the module is held at this temperature for 4 hours to ensure that not only the surface temperature but also the core temperature of the module approaches 350°C. Next, the furnace temperature is raised to 650°C at an average temperature rise rate of 50°C / h over 6 hours, and the module is held at this temperature for T2 = 2.5 min / mm × 1100 mm = 2750 min (approximately 45.8 h). Subsequently, the furnace temperature is raised to 880°C at an average temperature rise rate of 70°C / h over approximately 3.286 hours, and the module is held at this temperature for T3 = 1.8 min / mm × 1100 mm = 1980 min (33 h). Notably, during the initial temperature rise to 350°C in the above process, the temperature rise rate is not limited.

[0029] Step S3, quenching and cooling: After the module is held at 880℃, it is quickly removed from the furnace and immersed in a water bath with a water temperature of less than or equal to 40℃ for water cooling quenching. In this embodiment, the module cross-sectional thickness d=1100mm, and intermittent quenching is performed according to the cooling regime corresponding to the range of "1000<d≤1100". The specific process is as follows: water cooling for 22min → air cooling for 5min → water cooling for 18min → air cooling for 5min → water cooling for 15min → air cooling for 5min → water cooling for 12min → air cooling for 5min → water cooling for 10min → air cooling for 5min → water cooling for 8min → air cooling for 5min → water cooling for 8min → air cooling for 5min → water cooling for 7min → air cooling for 5min → water cooling for 7min → air cooling for 5min → water cooling for 7min → air cooling for 7min → water cooling for 7min, for a total time of 183min. After intermittent quenching, an axial flow fan was used to continue cooling the module surface at a speed of 5-15 m / s until the measured module surface temperature dropped to 180℃. The module cooling process took approximately 1 hour. Notably, during the quenching process, the quenching water in the tank was continuously agitated and circulated by a cooling tower to ensure that the water temperature remained below or equal to 40℃.

[0030] Step S4, double tempering: After the module is cooled by blowing air, it is placed in a heating furnace. The furnace temperature is raised to 200°C, and the module is held at that temperature for 4 hours. Furthermore, in this embodiment, the module material is 718 stainless steel with a target hardness of 33-38 HRC. During the two-stage tempering, the furnace temperature t1 in stage one is selected as 535°C, and the furnace temperature t2 in stage two is selected as 590°C. First, the furnace temperature was raised from 200℃ to 535℃ at an average temperature rise rate of 50℃ / h, taking 6.7 hours, and the module was held at that temperature for T4 = 2.5 min / mm × 1100 mm = 2750 min (approximately 45.8 h). Then, the furnace temperature was raised from 535℃ to 590℃ at an average temperature rise rate of 50℃ / h, taking 1.1 hours, and the module was held at that temperature for T5 = 1.5 min / mm × 1100 mm = 1650 min (27.5 h). Finally, after the holding period, the module was removed from the furnace and air-cooled to room temperature.

[0031] The modules that underwent the aforementioned post-forging heat treatment were tested, and the results showed that the overall cross-sectional hardness of the modules was 34.2-35.8 HRC, with a hardness fluctuation of no more than 1.6 HRC; the metallographic structure consisted of uniform tempered sorbite and dispersed granular carbides, with a grain size of 7.5; ultrasonic testing revealed no defects or quenching cracks, meeting the requirements for steel used in large household appliance housing molds. Compared with traditional processes, the post-forging heat treatment process described in this invention reduces energy consumption by approximately 36% for modules of the same size and material. Specific Implementation Example 3:

[0033] This embodiment is for a 1.2738 plastic mold steel forging module with a cross-sectional thickness d=900mm and a target hardness of 31-37HRC after forging heat treatment.

[0034] Step S1, Post-forging controlled cooling: After forging, the final forging temperature of the module was approximately 1030℃. Forced air cooling was immediately implemented using an axial flow fan for a duration of T1 = 0.6 min / mm × 900 mm = 540 min (9 h), with a wind speed of 10 m / s. After the forced air cooling was completed, air cooling continued in still air, and the module surface temperature dropped to 350℃ after approximately 3 hours.

[0035] Step S2, segmented heating: The module, air-cooled to a surface temperature of 350°C, is placed into a heating furnace. The furnace temperature is then raised to 350°C, and the module is held at this temperature for 4 hours to ensure that not only the surface temperature but also the core temperature of the module approaches 350°C. Next, the furnace temperature is raised to 650°C at a rate of 55°C / h, taking approximately 5.45 hours, and the module is held at this temperature for T2 = 2.5 min / mm × 900 mm = 2250 min (37.5 h). Subsequently, the furnace temperature is raised to 880°C at a rate of 75°C / h, taking approximately 3.07 hours, and the module is held at this temperature for T3 = 1.8 min / mm × 900 mm = 1620 min (27 h). Notably, during the initial temperature rise to 350°C, the furnace temperature rise rate is not limited.

[0036] Step S3, quenching and cooling: After the module was held at 880℃, it was quickly removed from the furnace and immersed in a water bath with a temperature of 40℃ or less for water quenching. In this embodiment, the module cross-sectional thickness d=900mm, and intermittent quenching was performed according to the cooling regime corresponding to the range "800<d≤900". The specific process was as follows: water cooling 22min → air cooling 5min → water cooling 15min → air cooling 5min → water cooling 12min → air cooling 5min → water cooling 10min → air cooling 5min → water cooling 8min → air cooling 5min → water cooling 7min → air cooling 5min → water cooling 7min → air cooling 5min → water cooling 7min → air cooling 5min → water cooling 7min, for a total time of 147min. After intermittent quenching, an axial flow fan was used to continue cooling the module surface at a wind speed of 5-15m / s until the measured module surface temperature dropped to 180℃. The module cooling process took approximately 1 hour. In particular, during the quenching process, the quenching water in the tank is kept running with a stirring and cooling tower circulation heat exchange system to ensure that the water temperature is always less than or equal to 40℃.

[0037] Step S4, double tempering: After the module is cooled by blowing air, it is placed in a heating furnace. The furnace temperature is raised to 200°C, and the module is held at that temperature for 4 hours. Furthermore, in this embodiment, the module material is 1.2738, with a target hardness of 31-37 HRC. During the two-stage tempering, the furnace temperature t1 in stage one is selected as 545°C, and the furnace temperature t2 in stage two is selected as 600°C. First, the furnace temperature was raised from 200℃ to 545℃ at a rate of 55℃ / h, taking approximately 6.27 hours, and the module was held at this temperature for T4 = 2.5 min / mm × 900 mm = 2250 min (37.5 h). Then, the furnace temperature was raised from 545℃ to 600℃ at a rate of 55℃ / h, taking 1 hour, and the module was held at this temperature for T5 = 1.5 min / mm × 900 mm = 1350 min (22.5 h). Finally, after the holding period, the module was removed from the furnace and air-cooled to room temperature.

[0038] The modules that underwent the aforementioned post-forging heat treatment were tested, and the results showed that the overall cross-sectional hardness of the modules was 32.1-33.7 HRC, with a hardness fluctuation of no more than 1.6 HRC; the metallographic structure consisted of uniform tempered sorbite and dispersed granular carbides, with a grain size of 7.5; ultrasonic testing revealed no defects or quenching cracks, meeting the requirements for steel used in automotive instrument panel molds. Compared with traditional processes, the post-forging heat treatment process provided in this invention shortens the production cycle by approximately 38% for modules of similar size and material.

[0039] Specifically, including the above embodiments, the heat treatment process for large plastic mold steel with a short process provided in this invention is applicable to module cross-section thickness d, which is greater than 500 mm and less than or equal to 1200 mm; the forced air cooling wind speed in step S1 is between 5 m / s and 15 m / s; the average temperature rise rate of the heating furnace in step S2 is less than or equal to 100 °C / h; and the average temperature rise rate of the heating furnace in step S4 is less than or equal to 60 °C / h.

Claims

1. A short-process heat treatment process for large plastic mold steel, characterized in that, Includes the following steps: Step S1, Post-forging controlled cooling: After forging, the module is immediately subjected to forced air cooling for time T1. After the air cooling is completed, it continues to be air cooled in still air until the surface temperature of the module drops to 350°C. Step S2, segmented heating: First, place the module, which has been cooled to 350°C, into the heating furnace. Then, raise the furnace temperature to a constant 350°C and keep the module at that temperature for 4 hours. The heating furnace was then heated to 650°C, and the module was kept at that temperature for time T2. Finally, the heating furnace is heated to 880℃, and the module is kept at that temperature for time T3. Step S3, quenching and cooling: After the module is held at 880℃, it is taken out of the furnace and subjected to water quenching. During the water quenching process, fixed air cooling cycles are intermittently inserted to perform intermittent quenching by alternating water quenching and air cooling. Step S4, double tempering: After intermittent quenching and cooling, the module is placed in the heating furnace. The furnace temperature is first raised to a constant 200°C, and then the module is kept at that temperature for 4 hours. The heating furnace was then heated to t1℃, and the module was kept at that temperature for time T4. The heating furnace was then heated to t2℃, and the module was kept at that temperature for T5 time. Finally, after the T5 heat preservation is completed, the module is taken out of the furnace and air-cooled.

2. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: The mold steel is made of P20, 718 or 1.2738 steel, and the thickness d of the module section ranges from 500mm to 1200mm.

3. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: The forced air cooling speed in step S1 is 5-15 m / s.

4. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: In step S1, the blowing time T1 = 0.6 min / mm × d, where d is the cross-sectional thickness of the module in mm.

5. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: In step S2, the average temperature rise rate of the heating furnace from 350°C to 650°C and the average temperature rise rate of the heating furnace from 650°C to 880°C are both less than or equal to 100°C / h.

6. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: In step S2, the heat preservation time T2 = 2.5 min / mm × d, and the heat preservation time T3 = 1.8 min / mm × d, where d is the cross-sectional thickness of the module in mm.

7. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: In step S3, the water temperature of the quenching water is ≤40℃, and the stirring and cooling tower circulation system of the quenching water in the quenching container is turned on to maintain the temperature standard of the quenching water.

8. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that, After the intermittent quenching process involving alternating water quenching and air cooling as described in step S3, axial flow fans are used to continue cooling until the module surface temperature drops to 180°C. The airflow speed of the axial flow fans is 5-15 m / s. The alternating intermittent quenching process is divided into intervals based on the module cross-sectional thickness d and executed according to the following table: ； 。 9. The heat treatment process for large plastic mold steel with a short process according to claim 1, characterized in that: In step S4, the average temperature rise rate of the heating furnace is less than or equal to 60℃ / h; the holding time T4 = 2.5min / mm×d, and the holding time T5 = 1.5min / mm×d, where d is the cross-sectional thickness of the module in mm.

10. The heat treatment process for large plastic mold steel with a short process according to claim 9, characterized in that, In step S4, the temperatures t1 and t2 are determined according to the steel grade as shown in the table below: 。

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