A pre-hardening heat treatment method for P20 die steel

Abstract

The present application relates to a kind of P20 die steel pre-hardening heat treatment method, it is related to die steel heat treatment technical field, the technical problem that the poor section hardness uniformity after the pre-hardening treatment of existing large thickness P20 plastic die steel, insufficient heart hardness, easy deformation cracking is solved;Including the P20 die steel that is completed after being forged and carrying out austenitic heating is quenched, water-space intermittent cooling and oil cooling double-liquid grading quenching process are used in quenching, the P20 die steel after cold oil is sectioned isothermal tempering;After the P20 die steel of isothermal tempering is controlled, it is discharged from furnace, and the composite cooling mode of controlled cooling furnace cooling and air cooling is used in controlled cooling discharge from furnace;After controlled cooling is completed, the surface of P20 die steel is cleaned;With guaranteeing overall hardness and each performance meet industry standard, still can make the effect that the hardness difference of P20 die steel surface and heart is ≤±1.5HRC, heart hardness is ≥28HRC.

Description

Technical Field

[0001] This invention relates to the field of mold steel heat treatment technology, and specifically to a pre-hardening heat treatment method for P20 mold steel. Background Technology

[0002] P20 plastic mold steel is a pre-hardened plastic mold steel specified in ASTM A681 standard. It has a moderate carbon content and is enriched with alloying elements such as Cr, Mo, and Mn. After pre-hardening treatment, it is supplied in a pre-hardened state with HRC 28-32. Users can directly machine it without secondary heat treatment, which can shorten the mold manufacturing cycle by 40-50%. It is one of the most widely used basic materials in the field of plastic mold manufacturing.

[0003] As plastic molds become larger and more precise, the market demand for thicker P20 plastic mold steel (≥400mm) is increasing. However, existing pre-hardening heat treatment processes are mainly designed for medium-thin (<400mm) workpieces, employing a single quenching and tempering process of "850℃~880℃ oil quenching and 550℃~630℃ tempering." This cannot meet the heat treatment requirements of thicker workpieces, leading to a series of technical challenges in the production process, such as: 1. Poor hardness uniformity and insufficient core hardness: Thick P20 mold steel has poor thermal conductivity and limited hardenability. During the quenching process, the cooling rate of the surface and the core is different. The surface hardness can reach HRC 30 or above, while the core hardness is only HRC 22-26. The hardness difference of the cross section exceeds ±3HRC, which cannot meet the overall performance requirements of large molds. 2. Prone to deformation and cracking defects: During the cooling process, the internal thermal stress and structural stress of the workpiece are unevenly distributed. In particular, the rapid cooling during the quenching stage can easily cause stress concentration, leading to workpiece deformation and cracking. The defect rate is as high as 15-20%, which seriously affects the dimensional accuracy and service life of the mold. Therefore, developing a pre-hardening heat treatment process that can solve the above-mentioned defects and is suitable for thick P20 plastic mold steel, achieving uniform pre-hardening and low defect rate, has become an urgent need in the current mold steel heat treatment field. Summary of the Invention

[0004] The purpose of this invention is to provide a pre-hardening heat treatment method for P20 mold steel, which solves the technical problems of poor cross-sectional hardness uniformity, insufficient core hardness, and easy deformation and cracking of existing thick (≥400mm) P20 plastic mold steel after pre-hardening treatment.

[0005] This invention provides a pre-hardening heat treatment method for P20 mold steel. The thickness of the P20 plastic mold steel is ≥400mm. The chemical composition and mass percentage of the P20 plastic mold steel are as follows: C: 0.34wt%~0.40wt%, Si: 0.40wt%~0.60wt%, Mn: 0.90wt%~1.00wt%, P≤0.025wt%, S≤0.010wt%, Cr: 1.80wt%~1.90wt%, Mo: 0.40wt%~0.47wt%, Cu≤0.25wt%, V: 0.08wt%~0.12wt%, Ni: 0.15wt%~0.20wt%, with the remainder being Fe and unavoidable impurity elements. The pre-hardening heat treatment method includes the following steps: Step 1: Quench the P20 mold steel after forging and austenitic heating. The quenching adopts a water-space intermittent cooling and oil-cooling dual-liquid stage quenching process. The temperature of the P20 mold steel before quenching is controlled at 720℃~760℃, and the final temperature after oil cooling is controlled at 150℃~250℃. Step 2: The P20 mold steel after cooling oil is subjected to segmented isothermal tempering, with the tempering rate controlled at 30℃ / h~50℃ / h throughout the process; Step 3: The P20 mold steel after isothermal tempering is subjected to controlled-speed cooling before being removed from the furnace. The controlled-speed cooling is achieved by a combination of controlled-speed furnace cooling and air cooling. Step four: After the controlled cooling is completed, the surface of the P20 mold steel is cleaned.

[0006] Furthermore, the water-space quenching process in step one includes three stages. The first stage uses air cooling, with an air cooling time of 3 to 5 seconds, cooling to 250°C to 300°C. The second stage uses air cooling, with an air cooling time of 1.5 to 3 seconds, cooling to 200°C to 250°C. The third stage uses water cooling, with a water cooling time of 10 to 15 minutes, cooling to 200°C to 250°C before oil cooling.

[0007] Furthermore, in the oil cooling process of step one, the oil cooling time is controlled at 10S / mm to 20S / mm.

[0008] Furthermore, the isothermal tempering process in step two consists of first heating to 520℃~540℃ and holding for 3min / mm~4min / mm; then heating to 570℃~590℃ and holding for 1min / mm~2min / mm.

[0009] Furthermore, in the controlled-speed cooling process of step three, the furnace cooling stage is as follows: the furnace is cooled to below 200°C, and the cooling rate is controlled at 20°C / h ~ 30°C / h; the air cooling stage is as follows: after the furnace is cooled to below 200°C, the workpiece is taken out and air-cooled to room temperature.

[0010] Furthermore, the workpiece temperature is detected every hour during the furnace cooling stage.

[0011] Due to the application of the above-mentioned technical solution, the present invention has the following advantages compared with the prior art: This invention discloses a pre-hardening heat treatment method for P20 mold steel. Through technological innovations such as chemical composition micro-alloying, graded quenching, and segmented isothermal tempering, it solves the technical problems of poor cross-sectional hardness uniformity, insufficient core hardness, easy deformation and cracking, and poor microstructure stability in thick P20 plastic mold steel after pre-hardening treatment. This method achieves high-quality pre-hardening treatment for thick P20 plastic mold steel, while simplifying the process, shortening the production cycle, reducing production costs, and adapting to the needs of industrialized large-scale production. Attached Figure Description

[0012] The following sections will describe some specific embodiments of the invention in detail by way of example and not limitation, with reference to the accompanying drawings. The same reference numerals in the drawings denote the same or similar parts or portions. Those skilled in the art should understand that these drawings are not necessarily drawn to scale. In the drawings: Figure 1 This is a schematic diagram of the hardness test of a 400mm thick P20 plastic mold steel cross section in Embodiment 1 of the present invention; Figure 2 This is a microstructure diagram of P20 plastic mold steel with a thickness of 400mm under 500× in Embodiment 1 of the present invention; Figure 3 This is a grain size diagram of P20 plastic mold steel with a thickness of 400mm in Embodiment 1 of the present invention at 100×. Figure 4 This is a schematic diagram of the hardness test of a 600mm thick P20 plastic mold steel cross section in Embodiment 3 of the present invention; Figure 5 This is a microstructure diagram of P20 plastic mold steel with a thickness of 600mm under 500× in Embodiment 3 of the present invention; Figure 6 This is a grain size diagram of P20 plastic mold steel with a thickness of 600mm in Embodiment 3 of the present invention at 100×. Detailed Implementation

[0013] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. 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.

[0014] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0015] This invention provides a pre-hardening heat treatment method for P20 mold steel. The P20 plastic mold steel has a thickness of ≥400mm and its chemical composition and mass percentage are as follows: C: 0.34wt%~0.40wt%, Si: 0.40wt%~0.60wt%, Mn: 0.90wt%~1.00wt%, P≤0.025wt%, S≤0.010wt%, Cr: 1.80wt%~1.90wt%, Mo: 0.40wt%~0.47wt%, Cu≤0.25wt%, V: 0.08wt%~0.12wt%, Ni: 0.15wt%~0.20wt%, with the remainder being Fe and unavoidable impurity elements. The main function of adding Ni is to improve the hardenability and hardness uniformity of mold steel. The main functions of adding V are: 1. Because V is a strong carbide element, it forms dispersed fine carbides, pins the austenite grain boundaries, inhibits grain growth, and plays a role in refining the grains; 2. It improves tempering resistance and increases the tempering temperature, thereby making stress relief more complete during the tempering process.

[0016] The pre-hardening heat treatment method includes the following steps: Step 1: The P20 plastic mold steel prepared from the above-mentioned chemical composition by mass percentage is forged. After forging, it is heated to complete the austenitization transformation inside the P20 mold steel before quenching. Before the austenitization transformation after forging, it is directly tempered and dehydrogenated. The hydrogen content after treatment is ≤1.5ppm, oxygen content ≤20ppm, and nitrogen content ≤70ppm. Dehydrogenation treatment can prevent hydrogen-induced cracking. Inclusion and grain size requirements: Non-metallic inclusions A, B, C, D, and DS types (fine / coarse series) are all ≤1.0 grade, grain size ≥6 grade, with no microcracks or Widmanstätten structure. During forging, the forging ratio is ensured to break up the coarse grains in the core and forge together internal defects. UT flaw detection shows no defects larger than φ2mm.

[0017] After dehydrogenation treatment, heating is performed to complete the austenitization transformation within the P20 mold steel. The austenitization temperature can be determined through numerical simulation using JMatPro software combined with actual experiments. During the austenitization transformation process, a stepped preheating and segmented austenitization heating process is adopted to gradually increase the workpiece temperature, reduce the temperature gradient between the surface and the core, and avoid thermal stress caused by excessively rapid heating, thus laying the foundation for subsequent uniform quenching.

[0018] The quenching process employs a dual-liquid staged quenching process combining water-space intermittent cooling and oil cooling. The temperature of the P20 mold steel before quenching is controlled at 720℃~760℃. The water-space intermittent quenching process consists of three stages. The first stage uses air cooling, with a cooling time of 3S / mm~5S / mm, cooling to 250℃~300℃. The second stage uses air cooling, with a cooling time of 1.5S / min~3S / min, cooling to 200℃~250℃. The third stage uses water cooling, with a cooling time of 10min~15min, cooling to 200℃~250℃ before oil cooling. After oil cooling, the final temperature is controlled at 150℃~250℃, with the oil cooling time controlled at 10S / mm~20S / mm.

[0019] In this invention, quenching is performed vertically, and the furnace is loaded horizontally. Before quenching, surface cracks on the workpiece are removed, and forging dimensions and surface quality are strictly controlled. The quenching water temperature is controlled at 720℃~760℃ to enhance the quenching cooling intensity. Water-space intermittent cooling allows for rapid water cooling to the martensitic transformation temperature, ensuring minimal or no soft tissue formation in the workpiece core, which is predominantly composed of bainite and martensite. After water cooling to the designated temperature, oil cooling is immediately switched. Utilizing the excellent uniformity of oil cooling, the cooling rate of the workpiece surface and core tends to be consistent, improving hardness uniformity. During oil cooling, precise control of the cooling time allows for gradual transformation of the internal structure of the workpiece, avoiding stress concentration caused by excessively rapid martensitic transformation and reducing the risk of cracking. Therefore, by employing a dual-liquid staged quenching process of water-space intermittent cooling and oil cooling, the internal thermal and structural stresses of the workpiece are minimized while ensuring quenching hardness, suppressing deformation and cracking defects, and simultaneously ensuring core hardenability to avoid insufficient core hardness.

[0020] Step two involves rapidly transferring the P20 mold steel, after cooling in oil, into a tempering furnace. A segmented heating and prolonged holding isothermal tempering process is employed, replacing the traditional single-temperature tempering. The isothermal tempering process is as follows: first, heat to 520℃~540℃ and hold for 3min / mm~4min / mm; then heat to 570℃~590℃ and hold for 1min / mm~2min / mm. The tempering rate is controlled at 30℃ / h~50℃ / h throughout the process to avoid excessively rapid heating and the generation of secondary stress. Isothermal tempering allows the quenched martensite to fully transform into tempered sorbite, eliminating residual quenching stress. Prolonged holding ensures uniform carbide precipitation, further improving hardness uniformity and balancing the material's hardness and toughness.

[0021] Step 3: After isothermal tempering, the P20 mold steel is cooled using a combination of controlled-speed furnace cooling and air cooling to avoid stress concentration caused by excessively rapid cooling. Furnace cooling stage: The workpiece is cooled in the furnace to below 200℃, with the cooling rate controlled at 20℃ / h ~ 30℃ / h, which further eliminates residual stress and improves dimensional stability. During the furnace cooling stage, the workpiece temperature is monitored every 1 hour to ensure a stable cooling rate and prevent cracks caused by sudden temperature changes. Air cooling stage: After furnace cooling to below 200℃, the workpiece is removed and air-cooled to room temperature.

[0022] Step four: After the controlled cooling is completed, the P20 mold steel is cleaned of surface oxide scale and impurities to ensure the surface quality of the workpiece, thus completing the entire pre-hardening heat treatment process.

[0023] In summary, compared with the prior art, the present invention has significant technical advantages and economic benefits, and the specific beneficial effects are as follows: 1. Solving the core problem of poor hardness uniformity and achieving core hardness standards: Through the synergistic effect of microalloying improvement, stepped heating, and graded quenching, the temperature gradient between the workpiece surface and core is effectively reduced, ensuring sufficient austenitization and uniform cooling. After processing, P20 mold steel with a thickness of 400 mm to 700 mm can meet the overall HRC30 to 34 and various performance standards of the industry. In addition, the surface and core hardness difference of P20 mold steel can be ≤±1.5HRC, and the core hardness can be ≥28HRC, which is far superior to existing processes and meets the overall performance requirements of large molds.

[0024] 2. Significantly reduce deformation and cracking defects and improve product qualification rate: By adopting water-space intermittent cooling and oil-cooled staged quenching, combined with controlled-speed tempering and controlled-speed cooling, the internal thermal stress and structural stress of the workpiece are reduced to the maximum extent, reducing the product defect rate from 15-20% of the existing process to below 3%, avoiding cracking problems caused by secondary quenching, significantly improving dimensional stability and reducing production costs.

[0025] 3. Optimize microstructure and improve overall material performance: Through segmented isothermal tempering, the workpiece obtains a uniform and fine bainite structure with a small amount of tempered sorbite, free of pearlite, ferrite, and microcracks; the material has a tensile strength ≥850MPa, elongation ≥12%, and polishing precision Ra 0.2-0.4μm, significantly improving machinability, polishing performance, and toughness; the microstructure stability is greatly improved, and it is not prone to hardness decay or dimensional deformation after long-term use. The mold can be adapted to the use requirements of 100,000-500,000 cycles, and the wear resistance can be further improved through nitriding treatment, expanding the application range.

[0026] 4. Simplified process, suitable for industrial-scale production: This process eliminates the traditional two normalizing processes after forging, and directly performs tempering and dehydrogenation after forging, shortening the production cycle by 10-15% compared to existing processes; the process steps are clear and the parameters are controllable, requiring no new special equipment, and can achieve large-scale production using existing heat treatment equipment, with significantly reduced energy consumption and auxiliary material consumption; the process is environmentally friendly, with no toxic or harmful emissions, and conforms to the national green industry development trend.

[0027] 5. Product quality is controllable and all testing indicators meet the standards: Thick P20 plastic mold steel processed by this process can achieve SEP1921-Ee level or above in UT flaw detection (bottom hole diameter <1mm). Non-metallic inclusions, grain size, gas content, etc. all meet internal control and customer requirements. The product quality is stable and controllable and can completely replace imported similar products.

[0028] The present invention will be further described in detail below with reference to specific embodiments. The scope of protection of the present invention is not limited to the following embodiments. All equivalent transformations made based on the technical solutions of the present invention shall fall within the scope of protection of the present invention. Example

[0029] This embodiment provides a pre-hardening heat treatment method for P20 mold steel with a thickness of 400mm. The prototype specifications are: 400mm (thickness) × 1410mm (width) × maximum length, with an ingot size of 18t. The chemical composition and mass percentage of the P20 plastic mold steel are as follows: C: 0.36 wt%, Si: 0.50 wt%, Mn: 0.95 wt%, P: 0.006 wt%, S: 0.008 wt%, Cr: 1.85 wt%, Mo: 0.43 wt%, Cu: 0.04 wt%, V: 0.10 wt%, Ni: 0.18 wt%, with the remainder being Fe and unavoidable impurity elements. Step 1: The P20 plastic mold steel prepared from the above-mentioned chemical composition by mass percentage is forged, and then heated to complete the austenitization transformation inside the P20 mold steel before quenching; after dehydrogenation treatment after forging, the hydrogen content is 1.2 ppm, the oxygen content is 18 ppm, and the nitrogen content is 65 ppm.

[0030] The temperature of P20 mold steel before quenching is controlled at 750℃. The first stage of air cooling takes 4 seconds per minute, cooling to 280℃. The second stage of air cooling takes 2 seconds per minute, cooling to 240℃. The third stage of water cooling takes 13 minutes, cooling to 200℃ before oil cooling. The final temperature after oil cooling is controlled at 180℃. The oil cooling time is controlled at 1 second per minute.

[0031] Step 2: The P20 mold steel after cooling oil is subjected to segmented isothermal tempering, with the tempering rate controlled at 40℃ / h throughout the process. The isothermal tempering process is as follows: first, heat to 530℃ and hold for 3 min / mm; then heat to 580℃ and hold for 1 min / mm.

[0032] Step 3: The P20 mold steel after isothermal tempering is subjected to controlled-rate cooling before being removed from the furnace. This controlled-rate cooling employs a combined cooling method of controlled-rate furnace cooling and air cooling. In the controlled-rate cooling process, during the furnace cooling stage: the workpiece is cooled to 200°C at a rate controlled at 25°C / h; during the air cooling stage: after the furnace temperature drops below 200°C, the workpiece is removed and air-cooled to room temperature. The workpiece temperature is monitored every hour during the furnace cooling stage.

[0033] Step four: After the controlled cooling is completed, the surface of the P20 mold steel is cleaned.

[0034] Hardness testing Equipment: Portable Rockwell hardness tester; Test method: GB / T230.1 Rockwell hardness test method for alloys Test results: Overall HRC 31.5~32.3, cross-sectional hardness difference 1.5 HRC, core hardness HRC 30; tissue testing Equipment: Olympus BX51M metallurgical microscope (Japan); Test method: GB / T 13298 Method for examining the microstructure of metals. Test results: Bainite and a small amount of tempered sorbite, grain size 7~8.5, no microcracks or Widmanstätten structure; Performance testing Equipment: Pendulum impact testing machine, tensile testing machine; Test method: GB / T 228 Metallic materials - Tensile testing at room temperature Performance: longitudinal tensile strength 1050MPa, yield strength 890MPa, elongation 14.5%, impact energy 20J; Flaw detection Equipment: Ultrasonic flaw detector; Testing standard: SEP1921-84 UT flaw detection can reach SEP1921-Ee level or above, with a bottom hole diameter of <1mm. Example

[0035] This embodiment provides a pre-hardening heat treatment method for P20 mold steel with a thickness of 400mm. The prototype specifications are: 400mm (thickness) × 1410mm (width) × maximum length, with an ingot size of 18t. The chemical composition and mass percentage of the P20 plastic mold steel are as follows: C: 0.36 wt%, Si: 0.50 wt%, Mn: 0.95 wt%, P: 0.006 wt%, S: 0.008 wt%, Cr: 1.85 wt%, Mo: 0.43 wt%, Cu: 0.04 wt%, V: 0.08 wt%, Ni: 0.15 wt%, with the remainder being Fe and unavoidable impurity elements. Step 1: The P20 plastic mold steel prepared from the above-mentioned chemical composition by mass percentage is forged, and then heated to complete the austenitization transformation inside the P20 mold steel before quenching; after dehydrogenation treatment after forging, the hydrogen content is 1.2 ppm, the oxygen content is 18 ppm, and the nitrogen content is 65 ppm.

[0036] The temperature of P20 mold steel before quenching is controlled at 750℃. The first stage of air cooling takes 4 seconds per minute, cooling to 280℃. The second stage of air cooling takes 2 seconds per minute, cooling to 240℃. The third stage of water cooling takes 13 minutes, cooling to 200℃ before oil cooling. The final temperature after oil cooling is controlled at 180℃. The oil cooling time is controlled at 1 second per minute.

[0037] Step 2: The P20 mold steel after cooling oil is subjected to segmented isothermal tempering, with the tempering rate controlled at 40℃ / h throughout the process. The isothermal tempering process is as follows: first, heat to 540℃ and hold for 2 min / mm; then heat to 590℃ and hold for 1.5 min / mm.

[0038] Step 3: The P20 mold steel after isothermal tempering is subjected to controlled-rate cooling before being removed from the furnace. This controlled-rate cooling employs a combined cooling method of controlled-rate furnace cooling and air cooling. In the controlled-rate cooling process, during the furnace cooling stage: the workpiece is cooled to 200°C at a rate controlled at 25°C / h; during the air cooling stage: after the furnace temperature drops below 200°C, the workpiece is removed and air-cooled to room temperature. The workpiece temperature is monitored every hour during the furnace cooling stage.

[0039] Step four: After the controlled cooling is completed, the surface of the P20 mold steel is cleaned.

[0040] Hardness testing Equipment: Portable Rockwell hardness tester; Test method: GB / T230.1 Rockwell hardness test method for alloys Test results: Overall HRC 29.4-32.1, cross-sectional hardness difference 1.3 HRC, core hardness HRC 28.5; tissue testing Equipment: Olympus BX51M metallurgical microscope (Japan); Test method: GB / T 13298 Method for examining the microstructure of metals. Test results: Tempered bainite, grain size grade 6.5; Performance testing Equipment: Pendulum impact testing machine, tensile testing machine; Test method: GB / T 228 Metallic materials - Tensile testing at room temperature Performance: longitudinal tensile strength 1045MPa, yield strength 872MPa, elongation 14.5%, impact energy 12.2J; Flaw detection Equipment: Ultrasonic flaw detector; Testing standard: SEP1921-84 UT flaw detection can reach SEP1921-Ee level or above, with a bottom hole diameter of <1mm. Example

[0041] The content of this embodiment is basically the same as that of embodiment 1. The difference is that this embodiment provides a pre-hardening heat treatment method for P20 mold steel with a thickness of 600mm. The trial production specifications are: 600mm (thickness) × 1410mm (width) × maximum length (length), and the ingot size is 21t. The temperature of P20 mold steel before quenching is controlled at 760℃.

[0042] Hardness testing Equipment: Portable Rockwell hardness tester; Test method: GB / T230.1 Rockwell hardness test method for alloys Test results: Overall HRC 28~31, cross-sectional hardness difference ≤±1.5HRC, core hardness HRC 28.5; tissue testing Equipment: Olympus BX51M metallurgical microscope (Japan); Test method: GB / T 13298 Method for examining the microstructure of metals. Test results: Bainite and tempered sorbite, no soft tissue; Performance testing Equipment: Pendulum impact testing machine, tensile testing machine; Test method: GB / T 228 Metallic materials - Tensile testing at room temperature Performance: longitudinal tensile strength 1045MPa, yield strength 872MPa, elongation 14.5%, impact energy 12.2J; Flaw detection Equipment: Ultrasonic flaw detector; Testing standard: SEP1921-84 Flaw detection: SEP1921-Ee level or above, no defects.

[0043] Comparative Example 1 The content of this comparative example is basically the same as that of Example 1, except that the chemical composition of P20 plastic mold steel lacks microalloyed V and Ni.

[0044] Test results: Hardness: Overall HRC28-34, cross-sectional hardness difference 4HRC, core hardness HRC27.5-29.5; Structure: Grain size grade 5; Properties: Tensile strength 987 MPa, yield strength 815 MPa, elongation 10.2%, impact energy: 9.5 J Flaw detection: UT flaw detection showed no defects larger than φ2mm, and the rating was higher than SEP1921.

[0045] Comparative Example 2 The content of this comparative example is basically the same as that of Example 1, except that: step two only involves a single tempering, the tempering temperature is 600℃, and the holding time is 3min / mm.

[0046] Test results: Hardness: Overall HRC28-34.5, cross-sectional hardness difference 4.5HRC, core hardness HRC27-28.5; Microstructure: Grain size 7~8.5; Properties: Tensile strength 969 MPa, yield strength 825 MPa, elongation 10%, impact energy: 9.5 J Flaw detection: UT flaw detection showed no defects larger than φ2mm, and the rating was higher than SEP1921.

[0047] Comparative Example 3 The content of this comparative example is basically the same as that of Example 1, except that: in step one, quenching is done using only oil quenching, and the oil quenching temperature is controlled at 860℃.

[0048] Test results: Hardness: Overall HRC28-32.5, cross-sectional hardness difference 4HRC, core hardness HRC27-29.5; Structure: Grain size grade 5 Properties: Tensile strength 951 MPa, yield strength 805 MPa, elongation 15%, impact energy: 11.5 J Flaw detection: UT flaw detection shows no defects larger than φ2mm, and the rating is higher than SEP1921.

[0049] Comparative Example 4 The content of this comparative example is basically the same as that of Example 1, except that: the first step of quenching only adopts the water-space intermittent cooling process. The first stage of air cooling time is 4S / mm, and the air cooling temperature is 280℃; the second stage of air cooling time is 2S / min, and the air cooling temperature is 240℃; the third stage of water cooling time is 13min, and the cooling temperature is 200℃.

[0050] Test results: Hardness: Overall HRC29-33.5, cross-sectional hardness difference 4.5 HRC, core hardness HRC29-30.5; Grain size: Grade 6.5 Properties: Tensile strength 1020MPa, yield strength 887MPa, elongation 9.5%, impact energy: 10.5J Flaw detection: UT flaw detection shows no defects larger than φ2mm, and the rating is higher than SEP1921.

[0051] The above embodiments are only for illustrating the technical concept and features of the present invention. Their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for pre-hardening heat treatment of P20 mold steel, characterized in that, The thickness of the P20 plastic mold steel is ≥400mm. The chemical composition and mass percentage of the P20 plastic mold steel are as follows: C: 0.34wt%~0.40wt%, Si: 0.40wt%~0.60wt%, Mn: 0.90wt%~1.00wt%, P≤0.025wt%, S≤0.010wt%, Cr: 1.80wt%~1.90wt%, Mo: 0.40wt%~0.47wt%, Cu≤0.25wt%, V: 0.08wt%~0.12wt%, Ni: 0.15wt%~0.20wt%, with the remainder being Fe and unavoidable impurity elements. The pre-hardening heat treatment method includes the following steps: Step 1: Quench the P20 mold steel after forging and austenitic heating. The quenching adopts a water-space intermittent cooling and oil-cooling dual-liquid stage quenching process. The temperature of the P20 mold steel before quenching is controlled at 720℃~760℃, and the final temperature after oil cooling is controlled at 150℃~250℃. Step 2: The P20 mold steel after cooling oil is subjected to segmented isothermal tempering, with the tempering rate controlled at 30℃ / h~50℃ / h throughout the process; Step 3: The P20 mold steel after isothermal tempering is subjected to controlled-speed cooling before being removed from the furnace. The controlled-speed cooling is achieved by a combination of controlled-speed furnace cooling and air cooling. Step four: After the controlled cooling is completed, the surface of the P20 mold steel is cleaned.

2. The pre-hardening heat treatment method for P20 mold steel according to claim 1, characterized in that, The water-space intermittent quenching process in step one includes three stages. The first stage uses air cooling, with a cooling time of 3 to 5 seconds, cooling to 250°C to 300°C. The second stage uses air cooling, with a cooling time of 1.5 to 3 seconds, cooling to 200°C to 250°C. The third stage uses water cooling, with a cooling time of 10 to 15 minutes, cooling to 200°C to 250°C before oil cooling.

3. The pre-hardening heat treatment method for P20 mold steel according to claim 1 or 2, characterized in that, In the oil cooling process of step one, the oil cooling time is controlled at 10S / mm to 20S / mm.

4. The pre-hardening heat treatment method for P20 mold steel according to claim 3, characterized in that, The isothermal tempering process in step two consists of first heating to 520℃~540℃ and holding for 3min / mm~4min / mm; then heating to 570℃~590℃ and holding for 1min / mm~2min / mm.

5. The pre-hardening heat treatment method for P20 mold steel according to claim 4, characterized in that, In the controlled-speed cooling process of step three, the furnace cooling stage is as follows: the furnace is cooled to below 200°C, and the cooling rate is controlled at 20°C / h ~ 30°C / h; the air cooling stage is as follows: after the furnace is cooled to below 200°C, the workpiece is taken out and air-cooled to room temperature.

6. The pre-hardening heat treatment method for P20 mold steel according to claim 5, characterized in that, During the furnace cooling stage, the workpiece temperature is monitored every hour.

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