A heat treatment process for improving the strength and toughness of die steels

Abstract

The application discloses a heat treatment process for improving the strength and toughness of die steel and belongs to the technical field of metal heat treatment. The heat treatment process for improving the strength and toughness of die steel adopts die steel material with specific component proportion, including carbon, chromium, molybdenum, vanadium, tungsten, nickel, niobium, niobium carbide and trace boron elements, and significantly improves the hardness and toughness of the die steel through sequentially implementing austenitizing, quenching, deformation heat treatment, multi-stage deep cooling treatment, pulse magnetic field assisted tempering, gradient surface hardening and nano precipitation regulation treatment. Especially through the synergistic effect of various advanced heat treatment methods, the martensite structure is effectively refined, the carbide precipitation is strengthened, the surface hardness and the core toughness are improved, the comprehensive performance with both strength and toughness is realized, and the heat treatment process is suitable for the fields of high-performance injection molds and hot-working molds.

Description

Technical Field

[0001] This invention relates to the field of metal heat treatment technology, and more specifically, to a heat treatment process for improving the strength and toughness of mold steel. Background Technology

[0002] Mold steel is widely used in metal forming, injection molding, die casting, and other industrial fields. Its service performance directly affects the service life of molds and the quality of finished products. In existing technologies, to improve the overall performance of mold steel, its strength and toughness are usually improved by optimizing chemical composition and controlling heat treatment processes. However, traditional mold steel often suffers from high fracture sensitivity and insufficient impact toughness under high hardness conditions, making it difficult to simultaneously meet the dual requirements of high strength and high toughness.

[0003] Currently, commonly used heat treatment methods include austenitization, quenching, and conventional tempering. However, due to insufficient control of carbides, excessive retained austenite, or coarse microstructure, mold steel materials often suffer from low toughness or unstable fatigue performance. Meanwhile, localized strengthening methods such as laser surface hardening, while improving hardness, easily create stress concentration areas, affecting overall service life.

[0004] With the development of high-precision forming and complex mold applications, future mold steel materials need to possess comprehensive characteristics of "high strength, high toughness, and high stability." Therefore, research into integrated, multi-method synergistically controlled heat treatment processes, such as deformation composite treatment, multi-stage deep cryogenic treatment, pulsed magnetic field assistance, gradient surface strengthening, and nanoprecipitation regulation, has become a development trend. These technologies can regulate martensite morphology, carbide precipitation behavior, and grain boundary strengthening mechanisms at the microstructure level, thereby significantly improving toughness and thermal stability without sacrificing hardness, meeting the performance requirements of complex molds under extreme working conditions. Summary of the Invention

[0005] The purpose of this invention is to provide a heat treatment process for improving the strength and toughness of mold steel. Through the synergistic effect of multiple advanced heat treatment methods, it effectively refines the martensitic structure, strengthens carbide precipitation, and improves surface hardness and core toughness, thereby achieving comprehensive performance with both strength and toughness.

[0006] A heat treatment process for improving the strength and toughness of mold steel includes the following steps: S1. Composition of mold steel: Carbon 0.45-0.55%, Chromium 4.7-5.5%, Molybdenum 2.0-2.6%, Vanadium 1.6-2.0%, Tungsten 1.0-1.2%, Nickel 0.3-0.5%, Niobium 0.1-0.3%, Niobium carbide 0.06-0.1%, Boron 0.001-0.003%, Balance: Iron; S2. Perform austenitizing treatment on the mold steel in step S1; S3. The mold steel obtained in step S2 is subjected to quenching treatment. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3. S5. Perform multi-stage deep cryogenic treatment on the mold steel obtained in step S4. S6. Perform pulsed magnetic field-assisted tempering on the mold steel obtained in step S5. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6. S8. Perform nanoprecipitation control treatment on the mold steel obtained in step S7.

[0007] Preferably, the specific steps of the austenitizing treatment in step S2 are as follows: heat the mold steel in step S1 to 840-1000℃ and hold for 30-50 minutes.

[0008] Preferably, the specific steps of the quenching treatment in step S3 are as follows: the mold steel obtained in step S2 is subjected to oil quenching, the quenching medium is mineral oil, the oil quenching temperature is controlled at 24-40℃, the cooling rate is 10-20℃ / s, and the cooling time is 4-8 minutes.

[0009] Preferably, the specific steps of the deformation heat treatment composite process in step S4 are as follows: the mold steel obtained in step S3 is immediately subjected to 13-17% warm rolling deformation, and the deformation temperature is controlled at 40-60℃ above the Ms point.

[0010] Preferably, the specific steps of the multi-stage cryogenic treatment in step S5 are as follows: the mold steel obtained in step S4 is immediately subjected to cryogenic treatment at -80-60°C for 100-140 minutes, and then subjected to liquid nitrogen immersion treatment at -200-192°C for 8-12 minutes.

[0011] Preferably, the specific steps of pulsed magnetic field assisted tempering in step S6 are as follows: the mold steel obtained in step S5 is tempered at a tempering temperature of 500-540℃ for 100-140 minutes, and an alternating magnetic field of 0.4-0.6T with a frequency of 8-12Hz is applied during the tempering process.

[0012] Preferably, the specific steps of the gradient surface hardening treatment in step S7 are as follows: The mold steel obtained in step S6 is placed in a nitrogen atmosphere with a nitrogen flow rate of 10-20 L / min. The surface layer is hardened using a laser quenching process with a power of 2-4 kW and a scanning speed of 6-10 mm / s. Self-cooling is achieved by utilizing the thermal conductivity and convection cooling of the workpiece itself. The core is subjected to sub-temperature quenching at a temperature of 750-770℃ for 30-50 minutes. It is first isothermally cooled to 300-340℃ and held at this temperature for 20-40 minutes, and then air-cooled to room temperature.

[0013] Preferably, the specific steps of the nanoprecipitation control treatment in step S8 are as follows: the mold steel obtained in step S7 is subjected to two-stage aging treatment, first held at 440-460℃ for 3-5 hours, and then held at 370-390℃ for 7-9 hours to prepare a tough mold steel.

[0014] Compared with the prior art, the advantages of this invention are: (1) Dual-scale alloy strengthening mechanism, taking into account both hardenability and toughness control Through the synergistic design of high proportions of vanadium and niobium carbide (NbC), the dispersed precipitated nano-MC type carbides achieve intragranular strengthening. Nb element effectively inhibits grain growth under the grain boundary pinning effect, while nickel element improves grain boundary toughness, thus achieving dual strengthening of the matrix and grain boundaries.

[0015] (2) The combined use of deformation heat treatment and multi-stage deep cryogenic treatment effectively refines the martensitic structure and significantly reduces the amount of retained austenite. Ultra-fine lath martensite microstructure is obtained by warm rolling deformation above Ms point. Subsequently, the conversion rate of residual austenite is increased to over 90% through two-stage deep cryogenic treatment (-80℃ constant temperature + liquid nitrogen immersion), which takes into account both dimensional stability and internal stress release, and effectively improves the balance of strength and toughness.

[0016] (3) Pulsed magnetic field-assisted tempering promotes directional precipitation of carbides and improves the uniformity of the structure. Applying an alternating magnetic field during tempering significantly improves the carbide precipitation rate and directional alignment, suppresses grain boundary segregation, enhances the isotropy and fatigue crack resistance of the material, and solves the problems of softening and embrittlement during tempering of traditional high-hardness steel.

[0017] (4) Laser-sub-temperature composite gradient quenching to construct a hard outer shell-tough core gradient performance structure High surface hardness is achieved through laser hardening, while sub-temperature hardening combined with isothermal bainite control constructs the core toughness zone, forming a gradient distribution structure, which effectively improves the compatibility between wear resistance and fracture toughness of mold steel.

[0018] (5) Nanoprecipitation regulates and enhances the secondary strengthening effect, stabilizes the tissue, and prolongs the service life. A two-stage aging process was used to control the size and distribution of vanadium carbide and other nano-precipitates, stabilizing the precipitate particle size at <20nm, increasing dislocation density and enhancing tempering stability, and significantly improving high-temperature dimensional stability and thermal fatigue life. Detailed Implementation

[0019] Example 1: S1. Composition of mold steel: carbon 0.45%, chromium 4.7%, molybdenum 2.0%, vanadium 1.6%, tungsten 1.0%, nickel 0.3%, niobium 0.1%, niobium carbide 0.06%, boron 0.001%, balance iron; S2. Austenitizing treatment of the mold steel in step S1: Heat the mold steel in step S1 to 840℃ and hold for 30 minutes; S3. Quenching treatment of the mold steel obtained in step S2: The mold steel obtained in step S2 is subjected to oil quenching. The quenching medium is mineral oil. The oil quenching temperature is controlled at 24℃, the cooling rate is 10℃ / s, and the cooling time is 4 minutes. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3: Immediately perform 13% warm rolling deformation on the mold steel obtained in step S3, and control the deformation temperature at 40°C above Ms point. S5. Perform multi-stage cryogenic treatment on the mold steel obtained in step S4: Immediately subject the mold steel obtained in step S4 to cryogenic treatment at -80°C for 100 minutes, and then subject it to liquid nitrogen immersion treatment at -200°C for 8 minutes. S6. Perform pulsed magnetic field assisted tempering on the mold steel obtained in step S5: Temper the mold steel obtained in step S5 at a tempering temperature of 500℃ for 100 minutes, and apply an alternating magnetic field of 0.4T with a frequency of 8Hz during the tempering process. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6: Place the mold steel obtained in step S6 in a nitrogen atmosphere with a nitrogen flow rate of 10L / min. Use laser quenching process to harden the surface layer with a power of 2kW and a scanning speed of 6mm / s. Utilize the workpiece's own thermal conductivity and convection cooling to achieve self-cooling. The core is subjected to sub-temperature quenching at a temperature of 750℃ for 30 minutes. First, isothermally cool to 300℃ and hold at this temperature for 20 minutes, then air cool to room temperature. S8. Nanoprecipitation conditioning treatment of the mold steel obtained in step S7: The mold steel obtained in step S7 is subjected to two-stage aging treatment, first held at 440℃ for 3h, and then held at 370℃ for 7h to prepare a tough mold steel.

[0020] Example 2: S1. Composition of mold steel: carbon 0.475%, chromium 4.9%, molybdenum 2.15%, vanadium 1.7%, tungsten 1.05%, nickel 0.35%, niobium 0.15%, niobium carbide 0.07%, boron 0.0015%, balance iron; S2. Austenitizing treatment of the mold steel in step S1: Heat the mold steel in step S1 to 880℃ and hold for 35 minutes. S3. Quenching treatment of the mold steel obtained in step S2: The mold steel obtained in step S2 is subjected to oil quenching. The quenching medium is mineral oil. The oil quenching temperature is controlled at 28℃, the cooling rate is 12.5℃ / s, and the cooling time is 5 minutes. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3: Immediately perform 14% warm rolling deformation on the mold steel obtained in step S3, and control the deformation temperature at 45°C above Ms point. S5. Perform multi-stage cryogenic treatment on the mold steel obtained in step S4: Immediately subject the mold steel obtained in step S4 to cryogenic treatment at -75°C for 110 minutes, and then subject it to liquid nitrogen immersion treatment at -198°C for 9 minutes. S6. Perform pulsed magnetic field assisted tempering on the mold steel obtained in step S5: Temper the mold steel obtained in step S5 at a tempering temperature of 510℃ for 110 minutes, and apply an alternating magnetic field of 0.45T with a frequency of 9Hz during the tempering process. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6: Place the mold steel obtained in step S6 in a nitrogen atmosphere with a nitrogen flow rate of 12.5 L / min. Use laser quenching process to harden the surface layer with a power of 2.5 kW and a scanning speed of 7 mm / s. Utilize the workpiece's own thermal conductivity and convection cooling to achieve self-cooling. The core is subjected to sub-temperature quenching at a temperature of 755℃ for 35 minutes. First, isothermally cool to 310℃ and hold at this temperature for 25 minutes, then air cool to room temperature. S8. Nanoprecipitation conditioning treatment of the mold steel obtained in step S7: The mold steel obtained in step S7 is subjected to two-stage aging treatment, first held at 445℃ for 3.5h, and then held at 375℃ for 7.5h to prepare a tough mold steel.

[0021] Example 3: S1. Composition of mold steel: carbon 0.5%, chromium 5.1%, molybdenum 2.3%, vanadium 1.8%, tungsten 1.1%, nickel 0.4%, niobium 0.2%, niobium carbide 0.08%, boron 0.002%, balance iron; S2. Austenitizing treatment of the mold steel in step S1: Heat the mold steel in step S1 to 920℃ and hold for 40 minutes; S3. Quenching treatment of the mold steel obtained in step S2: The mold steel obtained in step S2 is subjected to oil quenching. The quenching medium is mineral oil. The oil quenching temperature is controlled at 32℃, the cooling rate is 15℃ / s, and the cooling time is 6 minutes. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3: Immediately subject the mold steel obtained in step S3 to 15% warm rolling deformation, with the deformation temperature controlled at 50°C above Ms point. S5. Perform multi-stage cryogenic treatment on the mold steel obtained in step S4: Immediately subject the mold steel obtained in step S4 to cryogenic treatment at -70°C for 120 minutes, and then subject it to liquid nitrogen immersion treatment at -196°C for 10 minutes. S6. Perform pulsed magnetic field assisted tempering on the mold steel obtained in step S5: Temper the mold steel obtained in step S5 at a tempering temperature of 520℃ for 120 minutes, and apply an alternating magnetic field of 0.5T with a frequency of 10Hz during the tempering process. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6: Place the mold steel obtained in step S6 in a nitrogen atmosphere with a nitrogen flow rate of 15L / min. Use laser quenching process to harden the surface layer with a power of 3kW and a scanning speed of 8mm / s. Utilize the workpiece's own thermal conductivity and convection cooling to achieve self-cooling. The core is subjected to sub-temperature quenching at a temperature of 760℃ for 40 minutes. First, isothermally cool to 320℃ and hold at this temperature for 30 minutes, then air cool to room temperature. S8. Nanoprecipitation conditioning treatment of the mold steel obtained in step S7: The mold steel obtained in step S7 is subjected to two-stage aging treatment, first held at 450℃ for 4h, and then held at 380℃ for 8h, to prepare a tough mold steel.

[0022] Example 4: S1. Composition of mold steel: carbon 0.525%, chromium 5.3%, molybdenum 2.45%, vanadium 1.9%, tungsten 1.15%, nickel 0.45%, niobium 0.25%, niobium carbide 0.09%, boron 0.0025%, balance iron; S2. Austenitizing treatment of the mold steel in step S1: Heat the mold steel in step S1 to 960℃ and hold for 45 minutes; S3. Quenching treatment of the mold steel obtained in step S2: The mold steel obtained in step S2 is subjected to oil quenching. The quenching medium is mineral oil. The oil quenching temperature is controlled at 36℃, the cooling rate is 17.5℃ / s, and the cooling time is 7 minutes. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3: Immediately subject the mold steel obtained in step S3 to 16% warm rolling deformation, with the deformation temperature controlled at 55°C above Ms point. S5. Perform multi-stage cryogenic treatment on the mold steel obtained in step S4: Immediately subject the mold steel obtained in step S4 to cryogenic treatment at -65°C for 130 minutes, and then subject it to liquid nitrogen immersion treatment at -194°C for 11 minutes. S6. Perform pulsed magnetic field assisted tempering on the mold steel obtained in step S5: Temper the mold steel obtained in step S5 at a tempering temperature of 530℃ for 130 minutes, and apply an alternating magnetic field of 0.55T with a frequency of 11Hz during the tempering process. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6: Place the mold steel obtained in step S6 in a nitrogen atmosphere with a nitrogen flow rate of 17.5 L / min. Use laser quenching process to harden the surface layer with a power of 3.5 kW and a scanning speed of 9 mm / s. Utilize the workpiece's own thermal conductivity and convection cooling to achieve self-cooling. The core is subjected to sub-temperature quenching at a temperature of 765℃ for 45 minutes. First, isothermally cool to 330℃ and hold at this temperature for 35 minutes, then air cool to room temperature. S8. Nanoprecipitation conditioning treatment of the mold steel obtained in step S7: The mold steel obtained in step S7 is subjected to two-stage aging treatment, first held at 455℃ for 4.5h, and then held at 385℃ for 8.5h, to prepare a tough mold steel.

[0023] Example 5: S1. Composition of mold steel: carbon 0.55%, chromium 5.5%, molybdenum 2.6%, vanadium 2.0%, tungsten 1.2%, nickel 0.5%, niobium 0.3%, niobium carbide 0.1%, boron 0.003%, balance iron; S2. Austenitizing treatment of the mold steel in step S1: Heat the mold steel in step S1 to 1000℃ and hold for 50 minutes; S3. Quenching treatment of the mold steel obtained in step S2: The mold steel obtained in step S2 is subjected to oil quenching. The quenching medium is mineral oil. The oil quenching temperature is controlled at 40℃, the cooling rate is 20℃ / s, and the cooling time is 8 minutes. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3: Immediately perform 17% warm rolling deformation on the mold steel obtained in step S3, and control the deformation temperature at 60°C above Ms point. S5. Perform multi-stage cryogenic treatment on the mold steel obtained in step S4: Immediately subject the mold steel obtained in step S4 to cryogenic treatment at -60°C for 140 minutes, and then subject it to liquid nitrogen immersion treatment at -192°C for 12 minutes. S6. Perform pulsed magnetic field assisted tempering on the mold steel obtained in step S5: Temper the mold steel obtained in step S5 at a tempering temperature of 540℃ for 140 minutes, and apply an alternating magnetic field of 0.6T with a frequency of 12Hz during the tempering process. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6: Place the mold steel obtained in step S6 in a nitrogen atmosphere with a nitrogen flow rate of 20L / min. Use laser quenching process to harden the surface layer with a power of 4kW and a scanning speed of 10mm / s. Utilize the workpiece's own thermal conductivity and convection cooling to achieve self-cooling. The core is subjected to sub-temperature quenching at a temperature of 770℃ for 50 minutes. First, isothermally cool to 340℃ and hold at this temperature for 40 minutes, then air cool to room temperature. S8. Nanoprecipitation conditioning treatment of the mold steel obtained in step S7: The mold steel obtained in step S7 is subjected to two-stage aging treatment, first held at 460℃ for 5h, and then held at 390℃ for 9h, to prepare a tough mold steel.

[0024] Performance testing Hardness testing (surface hardness, core hardness) Samples of the mold steel obtained in Examples 1-5 were cut to standard dimensions, with a size of 10×10×10mm. Their hardness was tested using a Wilson VH3100 Vickers hardness tester with a test load of 10kgf and a loading time of 15s. The test results are shown in the table below:

[0025] Impact toughness test Samples of the mold steel obtained in Examples 1-5 were cut to standard dimensions, 10×10×55mm, with a 2mm U-shaped notch machined in the center. Their impact toughness was tested using a JBS-300B fully automatic impact testing machine at room temperature. The test results are shown in the table below.

Claims

1. A heat treatment process for improving the strength and toughness of mold steel, characterized in that, Includes the following steps: S1. Composition ratio of mold steel: carbon 0.45-0.55%, chromium 4.7-5.5%, molybdenum 2.0-2.6%, vanadium 1.6-2.0%, tungsten 1.0-1.2%, nickel 0.3-0.5%, niobium 0.1-0.3%, niobium carbide 0.06-0.1%, boron 0.001-0.003%, balance iron; S2. Perform austenitizing treatment on the mold steel in step S1; S3. The mold steel obtained in step S2 is subjected to quenching treatment. S4. Perform a deformation heat treatment composite process on the mold steel obtained in step S3. Immediately perform 13-17% warm rolling deformation on the mold steel obtained in step S3, and control the deformation temperature at 40-60℃ above Ms point. S5. Perform multi-stage cryogenic treatment on the mold steel obtained in step S4. Immediately perform cryogenic treatment at -80-60℃ for 100-140 minutes, and then perform liquid nitrogen immersion treatment at -200-192℃ for 8-12 minutes. S6. The mold steel obtained in step S5 is subjected to pulsed magnetic field assisted tempering. The mold steel obtained in step S5 is tempered at a tempering temperature of 500-540℃ and a tempering time of 100-140 minutes. During the tempering process, an alternating magnetic field of 0.4-0.6T with a frequency of 8-12Hz is applied. S7. Perform gradient surface hardening treatment on the mold steel obtained in step S6. Place the mold steel obtained in step S6 in a nitrogen atmosphere with a nitrogen flow rate of 10-20 L / min. Use laser quenching process to harden the surface layer with a power of 2-4 kW and a scanning speed of 6-10 mm / s. Utilize the workpiece's own thermal conductivity and convection cooling to achieve self-cooling. The core is subjected to sub-temperature quenching at a temperature of 750-770℃ for 30-50 minutes. First, isothermally cool to 300-340℃ and hold at this temperature for 20-40 minutes, then air cool to room temperature. S8. The mold steel obtained in step S7 is subjected to nanoprecipitation regulation treatment. The mold steel obtained in step S7 is subjected to two-stage aging treatment. First, it is kept at 440-460℃ for 3-5h, and then kept at 370-390℃ for 7-9h to prepare a tough mold steel.

2. The heat treatment process for improving the strength and toughness of mold steel according to claim 1, characterized in that: The specific steps of the austenitizing treatment in step S2 are as follows: heat the mold steel in step S1 to 840-1000℃ and hold for 30-50 minutes.

3. The heat treatment process for improving the strength and toughness of mold steel according to claim 1, characterized in that: The specific steps of the quenching process in step S3 are as follows: the mold steel obtained in step S2 is subjected to oil quenching, the quenching medium is mineral oil, the oil quenching temperature is controlled at 24-40℃, the cooling rate is 10-20℃ / s, and the cooling time is 4-8 minutes.

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