Integrated heat treatment process of hot work die steel by sectional quenching and tempering

Abstract

The application discloses a sectional quenching and tempering integrated heat treatment process for hot work die steel, and particularly relates to the field of metal material heat treatment, and the chemical composition of the hot work die steel is as follows: C: 0.32-0.42, Si: 0.8-1.2, Mn: 0.3-0.6, Cr: 4.8-5.5, Mo: 1.2-1.8, V: 0.8-1.2, W: 0.6-1.0, Co: 0.5-1.0, P: ≤0.015, S: ≤0.010, and Fe: the balance. The application realizes the full-section uniform martensite / lower bainite transformation of the large hot work die steel through the synergistic matching of gas mist cooling and salt bath isothermal quenching, the section hardness difference is ≤3HRC, the impact toughness is ≥25J / cm 2 , the high-temperature tensile strength at 600 DEG C is ≥1200MPa, the quenching-tempering continuous operation is 25%-40% more energy-saving than the traditional process, the process cycle is shortened by 13%-42%, and the application is suitable for the efficient strengthening treatment of large high-heat-resistance hot work die steel.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, and more specifically, to an integrated heat treatment process for segmented quenching and tempering of hot work die steel. Background Technology

[0002] Hot work die steel is a key material for manufacturing dies used in metal hot forming processes, and is widely used in forming processes such as hot forging, hot extrusion, hot upsetting, and die casting. With the rapid development of high-end manufacturing industries such as automobile manufacturing and rail transportation, increasingly higher demands are being placed on the service life and reliability of hot work dies under high temperature, high pressure, and rapid heating / cooling cycles. During service, hot work die steel must repeatedly withstand the heating effect of high-temperature metal billets (die working surface temperature can reach 550℃~700℃) and the rapid cooling effect caused by release agent spraying. The die working surface is highly susceptible to failure modes such as thermal fatigue cracks, thermal wear, and plastic deformation, severely restricting the improvement of die life and the stability of product quality.

[0003] Currently, most hot work die steels widely used both domestically and internationally belong to the Cr-Mo-V series. Their common characteristics include high Cr (4.5%–5.5%) and Mo (1.0%–1.8%) content, combined with a suitable amount of V, which provides high-temperature strength, resistance to tempering softening, and resistance to thermal fatigue through the precipitation of fine, dispersed MC-type carbides. However, as hot work dies develop towards larger, more complex, and longer service lifespans, traditional hot work die steels and their heat treatment processes have gradually revealed the following prominent problems: First, insufficient hardenability leads to uneven cross-sectional properties in large molds. In large modules with an effective thickness exceeding 300mm, conventional H13 hot work die steel suffers from a severely insufficient core cooling rate, which easily generates non-martensitic structures such as pearlite and bainite. This results in a core hardness and high-temperature strength that are significantly lower than the surface area, leading to poor overall mold performance and severely impacting service life.

[0004] Second, residual stress control is difficult. Hot work die steel has a high alloy content and a complex phase transformation temperature range. During the quenching and cooling process, thermal stress and structural stress are superimposed, which can easily lead to deformation or even cracking.

[0005] Third, the separation of quenching and tempering processes leads to long process cycles and high energy consumption. In traditional heat treatment processes, after quenching, the mold needs to be cooled to room temperature before being loaded into the furnace for tempering. During this process, the residual austenite inside the mold may undergo uncontrollable transformation, and reheating the mold results in a large waste of heat energy. Summary of the Invention

[0006] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide an integrated heat treatment process for segmented quenching and tempering of hot work die steel. The technical problem to be solved by the present invention is: the traditional oil quenching process for high heat resistance large hot work die steel has insufficient cooling capacity, resulting in uneven cross-sectional properties, large residual stress after quenching, easy deformation and cracking, and the separation of quenching and tempering processes, resulting in long cycle and high energy consumption.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a segmented quenching and tempering integrated heat treatment process for hot work die steel, wherein the hot work die steel is a high heat-resistant hot work die steel, and its chemical composition by weight percentage is: C: 0.32~0.42, Si: 0.8~1.2, Mn: 0.3~0.6, Cr: 4.8~5.5, Mo: 1.2~1.8, V: 0.8~1.2, W: 0.6~1.0, Co: 0.5~1.0, P: ≤0.015, S: ≤0.010, Fe: balance; The carbon content is controlled within a low range of 0.32% to 0.42%. This ensures sufficient martensitic hardness (above 48 HRC) while avoiding excessive carbon content that could increase the susceptibility to grain boundary carbide segregation and quenching cracks. The silicon content of 0.8% to 1.2% primarily serves for solid solution strengthening and resistance to tempering softening. Silicon dissolved in the martensitic matrix effectively improves tempering resistance, but excessive silicon can promote graphitization. The manganese content of 0.3% to 0.6% deoxidizes and improves hardenability, but excessive manganese can lower the Ms point and increase the amount of retained austenite. The chromium content of 4.8% to 5.5% is a core element ensuring the thermal fatigue resistance and oxidation resistance of hot work die steel. Chromium dissolved in the matrix improves high-temperature strength, and as a strong carbide-forming element, it can form Cr7C3 type carbides, providing wear resistance. A molybdenum content of 1.2%–1.8% is one of the most effective elements for improving hardenability and resistance to tempering softening. During tempering, molybdenum precipitates fine Mo2C carbides, producing a significant secondary hardening effect. Furthermore, molybdenum can inhibit grain boundary segregation and improve hot strength. A vanadium content of 0.8%–1.2% can form highly stable VC carbides with extremely high resistance to coarsening. Under high-temperature tempering conditions, the VC particle size can be maintained at the nanometer level (50–100 nm), which is crucial for improving high-temperature hardness and resistance to thermal fatigue. A tungsten content of 0.6%–1.0% works synergistically with molybdenum to further strengthen the matrix solid solution and form stable M6C-type carbides, improving high-temperature red hardness. The cobalt content of 0.5% to 1.0% is one of the key additive elements that distinguishes this invention from conventional hot work die steel. Cobalt neither forms carbides nor dissolves in carbides, but is mainly dissolved in the martensitic matrix. It enhances hardenability and high-temperature strength by improving the solid solution strengthening effect and Ms point of the matrix. At the same time, cobalt can reduce stacking fault energy and inhibit dislocation cross-slip, thereby improving high-temperature creep resistance.

[0008] The integrated heat treatment process of segmented quenching and tempering for high heat-resistant hot work die steel is as follows: First, the high heat-resistant hot work die steel is subjected to stepped preheating, gradient heating, and short-time austenitization treatment; second, segmented quenching and cooling is completed by combining air mist cooling and graded isothermal quenching; finally, after quenching and cooling, the temperature is directly raised to the tempering temperature for tempering heat treatment, forming a continuous quenching-tempering operation. The stepped preheating design is based on the physical characteristics of hot work die steel, which has poor thermal conductivity and large cross-sectional dimensions. By preheating in two stages, the internal and external temperatures of the die steel are made uniform, avoiding the risk of thermal stress concentration and cracking caused by excessive internal and external temperature differences when directly heated at high temperatures. The gradient heating and short-time austenitization design is based on the precise control of the dissolution behavior of a large number of alloy carbides in high heat-resistant hot work die steel. The austenitization temperature of 1020℃~1080℃ can ensure that refractory carbides such as VC and Mo2C are fully dissolved and uniformly distributed, while the short-time holding strategy restricts grain growth and provides microstructure preparation for obtaining fine lath martensite in subsequent quenching.

[0009] The three-stage cooling strategy of air mist cooling, salt bath isothermal quenching, and air cooling is based on the following physical metallurgical principle: The first stage, air mist cooling, utilizes a mixture of compressed air and atomized water to achieve an adjustable cooling rate (120℃ / min~200℃ / min) between air cooling and water cooling. This is higher than the cooling capacity of oil cooling but lower than the cooling intensity of water cooling. It can effectively avoid the pearlite and bainite transformation zones of high heat-resistant hot work die steel, ensuring the full progress of martensite transformation, while avoiding the uneven cooling and severe thermal stress caused by the vapor film stage during water cooling quenching. The second stage, isothermal quenching in a nitrate bath, involves isothermally holding the die steel in a nitrate bath at 450℃ to 520℃. This temperature range falls at the upper end of the bainitic transformation temperature range for high-heat-resistant hot-work die steels. During this isothermal process, the following key microstructural transformations occur: First, supercooled austenite undergoes a bainitic transformation at this temperature, forming lower bainite, which combines high hardness with excellent toughness. Second, carbide precipitation is more complete and uniform during the isothermal process, avoiding stress concentration caused by concentrated and explosive carbide precipitation during subsequent tempering. The salt bath isothermal process homogenizes the temperature of the die steel cross-section, significantly reducing the thermal stress level during quenching. The third stage, air cooling, slowly cools the die steel from 450℃ to 520℃ to room temperature. During this stage, supercooled austenite continues to transform into martensite or bainite, but due to the significantly reduced temperature difference, the resulting microstructural stress is significantly decreased. The three-stage cooling system works synergistically to ensure a high cooling rate to obtain sufficient martensite / lower bainite structure, while keeping thermal stress and structural stress within a safe range.

[0010] The specific steps are as follows: S1. Stepped Preheating Treatment: The high-heat-resistant hot work die steel is heated to 550℃~650℃ at a heating rate of 40℃ / h~70℃ / h and held for 2h~4h for the first stage of preheating. Then, it is heated to 820℃~880℃ at a heating rate of 50℃ / h~80℃ / h and held for 1h~2h for the second stage of preheating. The first stage preheating temperature of 550℃~650℃ is lower than the Ac1 transformation temperature of this steel (approximately 830℃~850℃). In this temperature range, the die steel is in a fully elastic state, and thermal stress is effectively released through creep relaxation. The second stage preheating temperature of 820℃~880℃ is close to but slightly lower than the austenitizing temperature. At this stage, the die steel has begun the initial transformation from pearlite to austenite, and alloy carbides at the grain boundaries begin to dissolve, laying the microstructure foundation for subsequent austenitization and homogenization.

[0011] S2. Gradient Heating and Austenitization: The high heat-resistant hot work die steel preheated in step S1 is heated to the austenitizing temperature of 1020℃~1080℃ at a heating rate of 80℃ / h~120℃ / h, and the holding time is as follows: Calculation, where Indicates the heat preservation time. The value is 0.3 min / mm to 0.6 min / mm. The effective thickness of the high-heat-resistant hot-work die steel is expressed in mm. An austenitizing temperature of 1020℃~1080℃ is used, higher than the conventional austenitizing temperature of 1000℃~1030℃ for ordinary hot-work die steel. This allows a large number of alloy carbides to fully dissolve into the austenite matrix. In particular, the VC carbides formed when the V content is high have an extremely high dissolution temperature (approximately 1100℃), and can only effectively dissolve and uniformly distribute above 1020℃. These dissolved V, Mo, W, and other alloying elements will recrystallize as nanoscale secondary carbides during subsequent tempering, producing a strong secondary hardening effect, thus achieving excellent high-temperature performance. Holding time coefficient. The setting is 0.3 min / mm to 0.6 min / mm, which is significantly shorter than the 0.5 min / mm to 1.0 min / mm of conventional hot work die steel. Under high temperature austenitization conditions, the driving force for grain growth is significantly enhanced. It is necessary to control the austenite grain size to above ASTM grade 8 by shortening the high temperature holding time, so as to ensure the refinement of martensite laths after quenching.

[0012] S3. Segmented quenching and cooling: The high heat-resistant hot work die steel after austenitization in step S2 is subjected to three-stage cooling: air mist cooling, salt bath isothermal quenching, and air cooling. First, the surface temperature of the die steel is rapidly cooled to 580℃~650℃ using an air mist cooling medium at a cooling rate of 120℃ / min~200℃ / min. Then, it is immediately transferred to a nitrate bath at 450℃~520℃ for isothermal holding for 30min~90min. Finally, it is removed and air-cooled to room temperature. The cooling rate can be controlled by adjusting the water-air ratio of the mixed fluid of compressed air and atomized water, avoiding the problem of uneven cooling caused by the instability of the vapor film during water quenching. At the same time, the cooling rate of air mist cooling (120℃ / min~200℃ / min) is much higher than that of oil cooling (40℃ / min~80℃ / min), which can ensure that the core cooling rate of the large cross-section module of the high heat-resistant hot work die steel exceeds the critical quenching cooling rate. The isothermal quenching step using a nitrate bath employs a molten salt mixture of potassium nitrate and sodium nitrate in a 1:1 mass ratio. This salt bath exhibits excellent fluidity, chemical stability, and a heat transfer coefficient (approximately 400 W / (m²)) within the temperature range of 450℃ to 520℃. 2 ·K)~600W / (m 2 ·K)), which can ensure that the surface temperature of the module and the core temperature quickly become consistent.

[0013] S4. Direct Heating and Tempering: The high-heat-resistant hot work die steel, after quenching and cooling in step S3, is directly transferred to a tempering furnace without being allowed to stand at room temperature. It is heated to 560℃~620℃ at a heating rate of 60℃ / h~100℃ / h, held at that temperature for 4h~8h, and then slowly cooled to room temperature in the furnace, completing the integrated quenching-tempering heat treatment. After quenching and cooling, the die steel temperature is in the range of 150℃~250℃. This direct transfer to the tempering furnace breaks with the traditional practice of cooling to room temperature after quenching and then reheating for tempering. Its core advantages are: First, it utilizes residual heat... First, it significantly reduces the energy consumption of tempering heating (saving approximately 30% to 40% compared to traditional processes). Second, it avoids the uncontrollable transformation of retained austenite during room temperature holding, especially for high heat-resistant hot work die steels with low Ms points (approximately 280℃ to 320℃). During room temperature holding, retained austenite may slowly transform into martensite, generating new structural stresses. Direct tempering, on the other hand, allows retained austenite to directly decompose into a stable microstructure of tempered martensite and carbides during the heating process, eliminating the potential for subsequent microstructure instability. Third, it shortens the time and space occupied by the die in the workshop, improving production efficiency.

[0014] In a preferred embodiment, in step S1, the first-stage preheating temperature is 580℃~620℃, and the holding time is 2.5h~3.5h; the second-stage preheating temperature is 840℃~860℃, and the holding time is 1.2h~1.8h; the heating rate between the first-stage and second-stage preheating is 60℃ / h~70℃ / h; within this range, the structural stress and thermal stress of the mold steel are most fully released, and the microstructure uniformity after preheating is optimal.

[0015] In a preferred embodiment, in step S2, the austenitizing temperature is 1040℃~1060℃, and the holding time coefficient is... The heating rate is 0.4 min / mm to 0.5 min / mm. During the austenitizing heating process, the heating rate should not be less than 100℃ / h when the temperature reaches above 1000℃, so as to avoid excessive grain growth. Rapid heating in the high temperature range above 1000℃ is the key means to control grain size. In this temperature range, the grain boundary migration rate increases significantly. Shortening the residence time in this temperature range can effectively suppress grain coarsening.

[0016] In a preferred embodiment, in step S3, the aerosol cooling medium is a mixture of compressed air and atomized water, wherein the mass flow rate of the atomized water is 5% to 15% of the mass flow rate of the compressed air, and the aerosol cooling time is 5 to 15 minutes, reducing the surface temperature of the mold steel to 580°C to 620°C. The core purpose of controlling the water-to-air ratio at 5% to 15% is to adjust the cooling rate to the optimal range—if the water-to-air ratio is too low, the cooling rate is insufficient and cannot avoid the pearlite transformation zone; if the water-to-air ratio is too high, the cooling rate approaches the level of water quenching, increasing the risk of cracking. For mold modules with an effective thickness of less than 500mm, the water-to-air ratio is 5% to 10%; for large modules with an effective thickness of more than 500mm, the water-to-air ratio is 10% to 15% to ensure effective core cooling.

[0017] In a preferred embodiment, in step S3, the nitrate bath is a molten salt mixture of potassium nitrate and sodium nitrate in a 1:1 mass ratio, the bath temperature is 470℃~500℃, and the isothermal holding time is 40min~70min. During the isothermal quenching process, the temperature difference between the core and surface of the mold steel is controlled within ±30℃. The salt bath temperature of 470℃~500℃ is in the upper region of the bainitic transformation of this steel (Bs point approximately 510℃, Bf point approximately 320℃). Isothermal quenching at this temperature can obtain a fine lamellar lower bainitic structure, whose comprehensive mechanical properties are superior to low-temperature tempered martensite. The isothermal time of 40min~70min ensures that the bainitic transformation is fully completed, while avoiding carbide aggregation and coarsening caused by excessively long isothermal times.

[0018] In a preferred embodiment, in step S3, the air cooling rate to room temperature is 30°C / min to 60°C / min, and the temperature difference between the surface and core of the mold steel during air cooling does not exceed 50°C. This slow air cooling rate of 30°C / min to 60°C / min ensures that the structural stress is gradually released during the cooling process from 450°C to 520°C to room temperature, avoiding microcracks caused by rapid cooling at the end of the martensitic transformation.

[0019] In a preferred embodiment, in step S4, the tempering temperature is 580℃~610℃, the holding time is 5h~7h, and the tempering heating rate is 70℃ / h~90℃ / h. After tempering, the steel is slowly cooled in the furnace to below 300℃ at a cooling rate of 20℃ / h~40℃ / h, and then naturally air-cooled to room temperature. At a tempering temperature of 580℃~610℃, the precipitation of Mo2C and VC is most complete and dispersed, allowing the die steel to reach its peak hardness (50HRC~54HRC) while maintaining good toughness and plasticity. The slow furnace cooling rate of 20℃ / h~40℃ / h effectively avoids temper brittleness (especially the second type of temper brittleness that may occur in the 350℃~550℃ range), ensuring that the impact toughness meets the application requirements.

[0020] In a preferred embodiment, the high-heat-resistant hot work die steel treated by the segmented quenching and tempering integrated heat treatment process has a microstructure consisting of finely distributed MC-type carbides (mainly VC) and M2C-type carbides (mainly Mo2C) on a tempered martensite matrix. The carbide particle size is 50nm to 200nm, and the width of the tempered martensite laths is 0.2μm to 0.5μm. The die steel has a hardness of 48HRC to 54HRC and an impact toughness of 25J / cm. 2 ~35J / cm 2 The tensile strength at 600℃ is ≥1200MPa. Nanoscale carbides are dispersed at martensite lath boundaries and dislocation lines, playing a strong role in pinning dislocations and hindering grain boundary slip, thereby endowing the die steel with excellent high-temperature strength, resistance to tempering softening, and resistance to thermal fatigue.

[0021] The technical effects and advantages of this invention are as follows: 1. This invention utilizes a synergistic three-stage quenching process: air mist cooling, isothermal quenching in a salt bath, and air cooling. First, the air mist cooling medium provides an adjustable high cooling rate between air cooling and water cooling, ensuring that the cooling rate of the core of high-heat-resistant large hot-working die steel exceeds the critical quenching cooling rate, achieving a full transformation of the martensite / lower bainite structure across the entire cross-section. Subsequently, isothermal quenching in a nitrate bath homogenizes the temperature of the die steel cross-section, significantly reducing thermal stress. Finally, slow air cooling completes the final transformation, avoiding stress concentration in the structure. The synergistic effect of the three-stage cooling process results in die steel cross-section hardness uniformity and impact toughness superior to traditional oil quenching, improving the problems of uneven cross-sectional performance and easy deformation and cracking in large dies. 2. This invention breaks the traditional process of separating quenching and tempering by directly transferring the material to the tempering furnace after quenching and cooling without waiting at room temperature. It makes full use of the residual heat from quenching and directly heats the material to a tempering temperature of 560℃ to 620℃ at a heating rate of 60℃ / h to 100℃ / h. Compared with the traditional process, this not only saves energy, but also avoids the structural instability and additional stress caused by the uncontrollable transformation of residual austenite during the room temperature period, shortens the overall heat treatment cycle, and achieves energy-saving and efficient production. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: This invention provides an integrated heat treatment process for segmented quenching and tempering of hot work die steel. The high heat-resistant hot work die steel being treated is a module formed by smelting, casting, and forging. The module dimensions are 1200mm (length) × 800mm (width) × 500mm (thickness). Chemical analysis shows that its specific chemical composition by weight percentage is: C 0.38%, Si 1.05%, Mn 0.45%, Cr 5.12%, Mo 1.55%, V 0.95%, W 0.78%, Co 0.72%, P 0.008%, S 0.005%, with the balance being Fe.

[0024] In this embodiment, the effective thickness of the mold steel Take a diameter of 500mm. The specific heat treatment process steps are as follows: S1. Stepped preheating treatment: The mold steel is heated to 600℃ at a heating rate of 55℃ / h and held for 3h for the first stage of preheating; then it is heated to 850℃ at a heating rate of 65℃ / h and held for 1.5h for the second stage of preheating. S2, Gradient Heating and Austenitization: The mold steel preheated in step S1 is heated to the austenitizing temperature of 1050℃ at a heating rate of 100℃ / h. The holding time is as follows: Calculation, where Take 0.45 min / mm, =500mm, calculated insulation time =0.45×500=225min, or 3.75h. When the temperature reaches above 1000℃, the heating rate is controlled at 110℃ / h; S3. Segmented quenching and cooling: First, the first stage of cooling is carried out using an aerosol cooling medium. The compressed air pressure is 0.6 MPa, the mass flow rate of the atomized water is 10% of the mass flow rate of the compressed air, and the aerosol cooling time is 10 min, reducing the surface temperature of the mold steel to 600℃. Then, it is immediately transferred to a 480℃ nitrate bath (a molten salt mixture of potassium nitrate and sodium nitrate in a 1:1 mass ratio) for isothermal holding for 55 min. During the isothermal process in the salt bath, the temperature difference between the core and the surface of the mold steel is controlled within ±25℃. Finally, it is removed and allowed to cool naturally to room temperature in the air at a cooling rate of 45℃ / min. During the cooling process, the temperature difference between the surface and the core does not exceed 45℃. S4. Direct Heating and Tempering: The mold steel that has been quenched and cooled in step S3 (the actual measured temperature of the mold steel at this time is 195℃) is directly transferred into a tempering furnace that has been preheated to 200℃ without being kept at room temperature. It is heated to 595℃ at a heating rate of 80℃ / h and held for 6 hours. After the tempering and holding is completed, it is slowly cooled in the furnace to 300℃ at a cooling rate of 30℃ / h. Then it is taken out and naturally air-cooled to room temperature to complete the integrated heat treatment of quenching and tempering.

[0025] The mold steel processed in this embodiment, after metallographic observation and mechanical property testing, exhibits a microstructure consisting of finely distributed MC-type and M2C-type carbides uniformly distributed on a tempered martensite matrix. The carbide particle size ranges from 80 nm to 150 nm, and the width of the tempered martensite laths ranges from 0.25 μm to 0.40 μm. The hardness is 51.5 HRC (surface) and 50.5 HRC (core), with a hardness difference of only 1.0 HRC; the impact toughness is 30.5 J / cm². 2 The tensile strength at 600℃ is 1285MPa.

[0026] Example 2: This invention provides an integrated heat treatment process for segmented quenching and tempering of hot work die steel. The high heat-resistant hot work die steel module being treated has dimensions of 1800mm (length) × 1200mm (width) × 800mm (thickness). Chemical analysis shows that its specific chemical composition by weight percentage is: C 0.35%, Si 0.95%, Mn 0.40%, Cr 5.30%, Mo 1.65%, V 1.10%, W 0.85%, Co 0.85%, P 0.006%, S 0.004%, with the balance being Fe.

[0027] In this embodiment, the effective thickness of the mold steel Take 800mm. The specific heat treatment process steps are as follows: S1. Stepped preheating treatment: The mold steel is heated to 610℃ at a heating rate of 50℃ / h and held for 3.5h for the first stage of preheating; then it is heated to 855℃ at a heating rate of 60℃ / h and held for 1.8h for the second stage of preheating. S2, Gradient Heating and Austenitization: The mold steel preheated in step S1 is heated to the austenitizing temperature of 1060℃ at a heating rate of 90℃ / h. The holding time is as follows: Calculation, where Take 0.42 min / mm, =800mm, calculated insulation time =0.42×800=336min, or 5.6h. When the temperature reaches above 1000℃, the heating rate is controlled at 105℃ / h; S3. Segmented quenching and cooling: First, the first stage of cooling is carried out using an aerosol cooling medium. The compressed air pressure is 0.7 MPa, the mass flow rate of the atomized water is 13% of the mass flow rate of the compressed air, and the aerosol cooling time is 14 min, reducing the surface temperature of the mold steel to 610℃. Then, it is immediately transferred to a 490℃ nitrate bath (a molten salt mixture of potassium nitrate and sodium nitrate in a 1:1 mass ratio) for isothermal holding for 65 min. During the isothermal holding process, the temperature difference between the core and the surface of the mold steel is controlled within ±28℃. Finally, it is removed and allowed to cool naturally to room temperature in the air at a cooling rate of 40℃ / min. During the cooling process, the temperature difference between the surface and the core does not exceed 48℃. S4. Direct Heating and Tempering: The mold steel that has been quenched and cooled in step S3 (the actual measured temperature of the mold steel at this time is 185℃) is directly transferred into a tempering furnace that has been preheated to 200℃ without being kept at room temperature. It is heated to 600℃ at a heating rate of 75℃ / h and held for 7h. After the tempering and holding is completed, it is slowly cooled to 300℃ in the furnace at a cooling rate of 25℃ / h. Then it is taken out and naturally air-cooled to room temperature to complete the integrated heat treatment of quenching and tempering.

[0028] The mold steel processed in this embodiment, after metallographic observation and mechanical property testing, exhibits a microstructure consisting of finely distributed MC-type and M2C-type carbides uniformly distributed on a tempered martensite matrix. The carbide particle size ranges from 60 nm to 130 nm, and the width of the tempered martensite laths ranges from 0.20 μm to 0.35 μm. The hardness is 52.0 HRC (surface) and 50.8 HRC (core), with a hardness difference of 1.2 HRC; the impact toughness is 28.5 J / cm². 2 The tensile strength at 600℃ is 1310MPa.

[0029] Example 3: This invention provides an integrated heat treatment process for segmented quenching and tempering of hot work die steel. The high heat-resistant hot work die steel module being treated has dimensions of 2500mm (length) × 1500mm (width) × 1100mm (thickness). Chemical analysis shows that its specific chemical composition by weight percentage is: C 0.40%, Si 1.10%, Mn 0.50%, Cr 5.40%, Mo 1.70%, V 1.15%, W 0.92%, Co 0.90%, P 0.007%, S 0.005%, with the balance being Fe.

[0030] In this embodiment, the effective thickness of the mold steel 1100mm is selected. The specific heat treatment process steps are as follows: S1. Stepped preheating treatment: The mold steel is heated to 620℃ at a heating rate of 45℃ / h and held for 4h for the first stage of preheating; then heated to 860℃ at a heating rate of 55℃ / h and held for 2h for the second stage of preheating. S2, Gradient Heating and Austenitization: The mold steel preheated in step S1 is heated to the austenitizing temperature of 1070℃ at a heating rate of 85℃ / h. The holding time is as follows... Calculation, where Take 0.38 min / mm, =1100mm, calculated insulation time =0.38×1100=418min, or approximately 7.0h. When the temperature reaches above 1000℃, the heating rate is controlled at 100℃ / h; S3. Segmented quenching and cooling: First, the first stage of cooling is carried out using an aerosol cooling medium. The compressed air pressure is 0.8 MPa, the mass flow rate of the atomized water is 15% of the mass flow rate of the compressed air, and the aerosol cooling time is 15 min, reducing the surface temperature of the mold steel to 620℃. Then, it is immediately transferred to a 500℃ nitrate bath (a molten salt mixture of potassium nitrate and sodium nitrate in a 1:1 mass ratio) for isothermal holding for 80 min. During the isothermal holding process, the temperature difference between the core and the surface of the mold steel is controlled within ±30℃. Finally, it is removed and allowed to cool naturally to room temperature in the air at a cooling rate of 35℃ / min. During the cooling process, the temperature difference between the surface and the core does not exceed 50℃. S4. Direct Heating and Tempering: The mold steel that has been quenched and cooled in step S3 (the actual measured temperature of the mold steel at this time is 175℃) is directly transferred into a tempering furnace that has been preheated to 200℃ without being kept at room temperature. It is heated to 605℃ at a heating rate of 70℃ / h and held for 8 hours. After the tempering and holding is completed, it is slowly cooled in the furnace to 300℃ at a cooling rate of 20℃ / h. Then it is taken out and naturally air-cooled to room temperature to complete the integrated heat treatment of quenching and tempering.

[0031] The mold steel processed in this embodiment, after metallographic observation and mechanical property testing, exhibits a microstructure consisting of finely distributed MC-type and M2C-type carbides uniformly distributed on a tempered martensite matrix. The carbide particle size ranges from 50 nm to 120 nm, and the width of the tempered martensite laths ranges from 0.20 μm to 0.30 μm. The hardness is 52.5 HRC (surface) and 50.0 HRC (core), with a hardness difference of 2.5 HRC; the impact toughness is 26.5 J / cm². 2 The tensile strength at 600℃ is 1330MPa.

[0032] Comparative Example 1: To verify the technical effect of the present invention, Comparative Example 1 was set up. Comparative Example 1 used the same mold steel chemical composition and module size as Example 1, but was treated with a conventional heat treatment process, the specific process of which is as follows: 1) Heating and austenitizing: Directly heat to 1050℃ at a heating rate of 80℃ / h, and hold for 400 min at a conventional rate of 0.8 min / mm. 2) Quenching and cooling: After taking it out of the furnace, air cool it for 5 minutes, then transfer it to 60℃ quenching oil to cool until the surface temperature of the mold steel drops to 200℃ (oil cooling time is about 300 minutes), then take it out and air cool it to room temperature; 3) Tempering treatment: The mold steel is placed in a tempering furnace and heated from room temperature to 595°C at a heating rate of 80°C / h. After holding at this temperature for 6 hours, it is furnace cooled to 300°C and then air cooled to room temperature.

[0033] The test results for the mold steel treated in Comparative Example 1 were as follows: surface hardness 50.0 HRC, core hardness 44.5 HRC, with a hardness difference as high as 5.5 HRC; impact toughness (core) was 18.5 J / cm. 2 The tensile strength at 600℃ is 985 MPa. Metallographic observation shows that there is a large amount of upper bainite and pearlite in the core, the carbides are unevenly distributed, and there are obvious segregation zones.

[0034] Comparative Example 2: To verify the key role of the salt bath isothermal quenching step in this invention, Comparative Example 2 was set up. Comparative Example 2 used the same mold steel chemical composition and module dimensions as Example 2, but omitted the salt bath isothermal quenching step. The remaining processes were consistent with Example 2, as follows: 1) Stepped preheating: Same as in Example 2; 2) Gradient heating and austenitization: Same as in Example 2; 3) Segmented quenching and cooling: After the air mist is cooled to 610℃, it is directly cooled to room temperature by air without salt bath isothermal treatment, and the air cooling rate is 40℃ / min; 4) Direct heating and tempering: Same as in Example 2.

[0035] The test results for the mold steel treated in Comparative Example 2 were as follows: surface hardness 53.5 HRC, core hardness 46.0 HRC, with a hardness difference as high as 7.5 HRC; impact toughness (core) was 15.5 J / cm. 2 The tensile strength at 600℃ is 1050MPa. Metallographic observation shows that the core structure is a mixture of coarse lath martensite and retained austenite, with martensite laths ranging from 0.8μm to 1.2μm in width. The carbide precipitation is obviously uneven, with local carbide aggregation.

[0036] Experimental Example 1: Samples were taken from the surface and core (at 1 / 2 of the effective thickness) of the mold steel after heat treatment in Examples 1, 2, 3 and Comparative Examples 1 and 2, respectively, and mechanical properties were tested. The test results are shown in Table 1. Table 1. Mechanical property test results of the examples and comparative examples

[0037] As shown in Table 1, the hardness differences between the surface and core of the mold steel in Examples 1, 2, and 3 of this invention are 1.0 HRC, 1.2 HRC, and 2.5 HRC, respectively, all significantly lower than the 5.5 HRC of Comparative Example 1 and the 7.5 HRC of Comparative Example 2. This indicates that the present process effectively solves the problem of uneven cross-sectional properties in large mold steel. Regarding impact toughness, the core impact toughness of all examples of this invention reaches 26 J / cm². 2The above figures are as follows, while Comparative Example 1 and Comparative Example 2 are only 18.5 J / cm². 2 and 15.5 J / cm 2 The present invention improves toughness by approximately 40%–62% compared to Comparative Example 1 and by approximately 40%–94% compared to Comparative Example 2, fully demonstrating that the present invention significantly improves toughness while maintaining hardness. Regarding tensile strength at 600℃, the embodiments of the present invention all reach over 1240 MPa, while Comparative Example 1 and Comparative Example 2 only reach 820 MPa and 880 MPa respectively. The present invention improves toughness by approximately 51%–57% compared to Comparative Example 1, indicating that the mold steel treated by the present invention has significant performance advantages under high-temperature service conditions.

[0038] Experimental Example 2: Samples were taken from the core of the mold steel after heat treatment in Examples 1, 2, 3 and Comparative Examples 1 and 2, and the microstructure was analyzed using metallographic microscope and transmission electron microscope. The results are shown in Table 2. Table 2 Comparison of microstructure characteristics between the examples and comparative examples

[0039] As shown in Table 2, the microstructure of the embodiments of the present invention is a mixed structure of tempered martensite and a small amount of lower bainite. The carbide types are mainly MC and M2C, with small particle size (50-150 nm) and uniform distribution. The martensite laths are small in width (0.20-0.40 μm), and the retained austenite content is low (≤3%). In contrast, Comparative Example 1 showed a large amount of upper bainite and pearlite, with coarse and unevenly distributed carbide particles. Although the carbide types in Comparative Example 2 were similar to those in the embodiments, the carbide segregation was severe, the martensite laths were significantly coarsened, and the retained austenite content was high. The above microstructure comparison fully demonstrates that this process, through the optimized matching of three-stage cooling—air mist cooling, salt bath isothermal quenching, and air cooling—combined with a suitable austenitizing process, successfully suppressed the formation of harmful structures such as upper bainite and pearlite, and obtained a fine tempered martensite + lower bainite microstructure and a uniformly dispersed nanoscale carbide distribution.

[0040] Experimental Example 3: The process energy consumption and production cycle of Examples 1 to 3 were statistically analyzed and compared with the conventional process of Comparative Example 1. The results are shown in Table 3. Table 3. Comparison of energy consumption and production cycle between the embodiments of the present invention and the comparative examples.

[0041] As shown in Table 3, Embodiments 1-3 of the present invention, by adopting a continuous operation mode of direct heating and tempering after quenching, fully utilize the residual heat of quenching, reducing the tempering heating energy consumption by approximately 33% to 49% compared to the conventional process of Comparative Example 1. In terms of total energy consumption, the present invention reduces energy consumption by approximately 6% to 25% compared to Comparative Example 1; the larger the mold size, the more significant the energy-saving effect. Regarding the total process time, the present invention shortens the process time by approximately 13% to 42% compared to Comparative Example 1, significantly improving production efficiency and equipment turnover rate.

[0042] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A segmented quenching and tempering integrated heat treatment process for hot work die steel, characterized by: The hot work die steel is a high heat-resistant hot work die steel, and its chemical composition by weight percentage is as follows: C: 0.32~0.42, Si: 0.8~1.2, Mn: 0.3~0.6, Cr: 4.8~5.5, Mo: 1.2~1.8, V: 0.8~1.2, W: 0.6~1.0, Co: 0.5~1.0, P: ≤0.015, S: ≤0.010, Fe: balance; The specific steps of the integrated heat treatment process are as follows: S1. Stepped preheating treatment: The high heat resistance hot work die steel is heated to 550℃~650℃ at a heating rate of 40℃ / h~70℃ / h and held for 2h~4h for the first stage of preheating. Then, it is heated to 820℃~880℃ at a heating rate of 50℃ / h~80℃ / h and held for 1h~2h for the second stage of preheating. S2. Gradient Heating and Austenitization: The high heat-resistant hot work die steel preheated in step S1 is heated to the austenitizing temperature of 1020℃~1080℃ at a heating rate of 80℃ / h~120℃ / h, and the holding time is as follows: Calculation, where Indicates the heat preservation time. The value is 0.3 min / mm to 0.6 min / mm. The effective thickness of high heat-resistant hot work die steel is expressed in mm. S3. Segmented quenching and cooling: The high heat-resistant hot work die steel after austenitization in step S2 is subjected to three-stage cooling: air mist cooling, isothermal quenching in a salt bath, and air cooling. First, the surface temperature of the die steel is rapidly cooled to 580℃~650℃ using an air mist cooling medium at a cooling rate of 120℃ / min~200℃ / min. Then, it is immediately transferred to a nitrate salt bath at 450℃~520℃ for isothermal holding for 30min~90min. Finally, it is removed and air-cooled to room temperature. S4. Direct Heating and Tempering: The high heat-resistant hot work die steel quenched and cooled in step S3 is directly transferred into the tempering furnace without being kept at room temperature. It is heated to 560℃~620℃ at a heating rate of 60℃ / h~100℃ / h and held for 4h~8h. Then it is slowly cooled to room temperature in the furnace to complete the integrated heat treatment of quenching-tempering.

2. The integrated heat treatment process for segmented quenching and tempering of hot work die steel according to claim 1, characterized in that: During the austenitizing heating process in step S2, the heating rate when the temperature reaches above 1000℃ is not less than 100℃ / h.

3. The integrated heat treatment process for segmented quenching and tempering of hot work die steel according to claim 1, characterized in that: In step S3, the aerosol cooling medium is a mixture of compressed air and atomized water, wherein the mass flow rate of the atomized water is 5% to 15% of the mass flow rate of the compressed air, and the aerosol cooling time is 5 min to 15 min, so that the surface temperature of the mold steel drops to 580℃ to 620℃.

4. The integrated heat treatment process for segmented quenching and tempering of hot work die steel according to claim 1, characterized in that: In step S3, the nitrate bath is a molten salt mixture of potassium nitrate and sodium nitrate in a mass ratio of 1:

1. During the isothermal quenching process in the salt bath, the temperature difference between the core and surface of the mold steel is controlled within ±30℃.

5. The integrated heat treatment process for segmented quenching and tempering of hot work die steel according to claim 1, characterized in that: In step S3, the air cooling rate to room temperature is 30℃ / min to 60℃ / min, and the temperature difference between the surface and core of the mold steel during the air cooling process does not exceed 50℃.

6. The integrated heat treatment process for segmented quenching and tempering of hot work die steel according to claim 1, characterized in that: In step S4, after tempering, the furnace is slowly cooled to below 300°C at a cooling rate of 20°C / h to 40°C / h, and then naturally air-cooled to room temperature.

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