A heat treatment method for a large mirror surface plastic mold steel

Abstract

The application provides a heat treatment method of large mirror surface plastic die steel, and relates to the technical field of steel material heat treatment processing, and comprises the following steps: S1, quenching and heat preservation treatment and cooling treatment are sequentially performed on the 1.2083 flat steel part to be treated, wherein the quenching and heat preservation temperature is 990-1010 DEG C, the quenching and heat preservation time is calculated according to the thickness of the flat steel part, and the heat preservation time is 2.5-3.5 h per 100 mm; the cooling treatment is that the flat steel part after heat preservation is air-cooled to 770 DEG C-830 DEG C, then water-air alternating cooling is performed to 370 DEG C-430 DEG C, and finally air cooling is performed to 130 DEG C-160 DEG C; S2, the flat steel part obtained in step S1 is subjected to once tempering treatment; S3, the flat steel part obtained in step S2 is subjected to twice tempering treatment; the application solves the technical problem that the large mirror surface plastic die steel is prone to quenching cracking, and the obtained product has high quality and various indexes far higher than the standard requirements.

Description

Technical Field

[0001] This invention relates to the field of heat treatment processing technology for steel materials, and in particular to a heat treatment method for large mirror-finish plastic mold steel. Background Technology

[0002] 1.2083 steel is a high-quality mirror-finish plastic mold steel, and also a high-carbon, high-chromium martensitic stainless steel. Its chemical composition mainly includes 0.4% carbon, 13% chromium, and small amounts of molybdenum, manganese, and other elements. It has good polishability, corrosion resistance, and high hardness. The thickness of flat steel is generally below 200mm. With the increasing size of industrial products, the size of 1.2083 steel is also gradually increasing, and the market demand is gradually expanding to a thickness of (200-360)mm * width of (650-800)mm. 1.2083 steel requires high hardness, with a hardness of 35-41HRC. It also requires uniform hardness across the entire cross-section. To achieve this hardness, 1.2083 steel needs to undergo quenching heat treatment. The quenching heat treatment method has a decisive influence on its performance. If the quenching heat treatment method is improper, cracking is likely to occur during heat treatment, and the core hardness will be low, resulting in a low pass rate.

[0003] For 1.2083 steel with a flat bar thickness of 200mm or less, oil quenching is generally used for heat treatment. When oil quenching is used, if the thickness of the flat bar exceeds 200mm, the core hardness will be insufficient and uneven. When the size of the flat bar increases to a thickness of (200-360)mm and a width of (650-800)mm, the oil quenching heat treatment process can no longer produce qualified products, and the water quenching heat treatment process is very likely to cause quenching cracks, making the steel unusable.

[0004] In summary, in order to overcome the shortcomings and deficiencies of the existing technologies, it is necessary to innovatively adopt a new quenching heat treatment method to meet the product technical requirements and produce qualified products. Summary of the Invention

[0005] The purpose of this invention is to provide a heat treatment method for large mirror-finish plastic mold steel, especially a quenching heat treatment method for 1.2083 large-size forged flat steel. This method solves the technical problem of easy quenching cracking of large mirror-finish plastic mold steel, and the resulting product has high quality and various indicators far exceed the standard requirements.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a heat treatment method for large mirror-finish plastic mold steel, comprising the following steps:

[0008] S1. The 1.2083 flat steel parts to be processed are subjected to quenching and heat preservation treatment and cooling treatment in sequence. The quenching and heat preservation temperature is 990-1010℃, and the quenching and heat preservation time is calculated according to the thickness of the flat steel parts, with 2.5-3.5h per 100mm. The cooling treatment is to air cool the heat-preserved flat steel parts to 770℃-830℃, then water and air alternate cooling to 370-430℃, and finally air cooling to 130-160℃.

[0009] S2, perform a tempering treatment on the flat steel part obtained in step S1;

[0010] S3, perform a second tempering treatment on the flat steel parts obtained in step S2;

[0011] The specifications of the flat steel parts are: thickness of 200-360mm and width of 650-800mm.

[0012] Furthermore, the smelting method for the flat steel part includes sequential electric furnace smelting and electroslag remelting.

[0013] Furthermore, the chemical composition of the flat steel component, by weight percentage, includes: C: 0.38-0.44%, Si: ≤1.00%, Mn: ≤1.00%, P: ≤0.020%, S: ≤0.010%, Cr: 12.5-13.5%, Ni: ≤0.25%, Cu: ≤0.20%, Mo: 0.10-0.20%, V: 0.08-0.15%, H: ≤0.00015%, with the balance being Fe and unavoidable impurities.

[0014] Further, in step S1, the flat steel piece to be processed is first kept at 330-370℃ for 1 hour, then heated to 630-670℃ at a heating rate of ≤60℃ / h and kept at that temperature for 3 hours, and finally heated to the quenching and holding temperature of 990-1010℃ at a heating rate of ≤80℃ / h.

[0015] Furthermore, in step S1, the water-air alternating cooling involves immersing the flat steel piece in water for 30-40 minutes, then immersing it in water for 4-7 minutes and air cooling it, followed by immersing it in water for 8-12 minutes and air cooling it for 4-7 minutes. This process is repeated until the flat steel piece reaches a reheat temperature of 370-430℃ after immersing in water for 4-7 minutes and air cooling it.

[0016] Furthermore, in step S2, the holding temperature for the first tempering is 500-560℃, and the holding time is calculated based on the thickness of the flat steel piece, with a holding time of 6.5-7.5h per 100mm.

[0017] Furthermore, in step S3, the holding temperature for the secondary tempering is 470-530℃, and the holding time is calculated based on the thickness of the flat steel piece, with a holding time of 6.5-7.5h per 100mm.

[0018] Furthermore, in step S2, the flat steel piece is first kept at 230-270℃ for 3-5 hours, and then heated to 500-560℃ at a heating rate of ≤70℃ / h.

[0019] And / or, in step S3, the flat steel piece is first kept at 280-320℃ for 3 hours, and then heated to 470-530℃ at a heating rate of ≤70℃ / h.

[0020] Furthermore, in steps S2 and S3, after the flat steel piece is tempered and kept warm, it is cooled by air cooling at a rate of ≤30℃ / h.

[0021] And / or, the temperature measurement point of the flat steel piece is at the center of the large flat surface of the flat steel piece.

[0022] Furthermore, after tempering or heat treatment using the above-mentioned methods, the flat steel part has a hardness of 35-41 HRC, a core hardness of 36-40 HRC, and meets the level 4 requirement for ultrasonic flaw detection.

[0023] The beneficial effects of the heat treatment method for large mirror-finish plastic mold steel provided by this invention are as follows:

[0024] 1. The present invention applies appropriate quenching heating temperature and holding time: Higher temperature and longer heating time are conducive to the formation of austenite in flat steel; the present invention selects appropriate heating temperature of 1000±10℃ and heating time to avoid excessively high quenching temperature may lead to grain growth, and lower quenching temperature may lead to incomplete martensite formation, resulting in undesirable metallographic structure, and ultimately avoids coarse grains or insufficient hardness in the product.

[0025] 2. To reduce the risk of quenching cracks, the flat steel is pre-cooled to 800±30℃ by air cooling after being held at 1000±10℃, followed by alternating water and air cooling. This invention, by pre-cooling the 1.2083 steel to 800±30℃ by air cooling before quenching, can minimize the risk of edge cracking caused by rapid water cooling at 1000±10℃.

[0026] 3. This invention adopts a water-air alternating cooling method to ensure that the flat steel does not crack during quenching and to ensure that quenched martensite is obtained. Water-air alternating cooling can effectively avoid the "C-curve nose" when the steel is rapidly cooled at high temperature, thereby obtaining fine martensite and avoiding the transformation to bainite and pearlite. During air cooling for 4-7 minutes, the heat in the core of the flat steel is conducted to the lower surface temperature, reducing the temperature difference between the core and the surface of the flat steel and avoiding thermal stress cracking during intense cooling.

[0027] 4. After alternating water and air cooling, when the flat steel temperature is 400±30℃, it is air-cooled to 130-160℃ at the center of the large surface area before being tempered in the furnace. Air cooling to 130-160℃ allows the flat steel to undergo martensitic transformation below Ms point 230℃. The cooling rate slows down in the low-temperature stage, thus avoiding stress cracking and obtaining a good quenched martensitic structure. This cooling method ensures that the core of the 1.2083 steel is fully cooled without quenching cracking, resulting in a good quenched structure and qualified hardness. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 The diagram shows the quenching heat treatment process of 1.2083 flat steel provided in Embodiments 1 and 2 of the present invention;

[0030] Figure 2 This is a process diagram of the first tempering of 1.2083 flat steel provided in Embodiments 1 and 2 of the present invention;

[0031] Figure 3 The process diagram for the second tempering of 1.2083 flat steel provided in Embodiments 1 and 2 of the present invention;

[0032] Figure 4 A schematic diagram showing the hardness testing location and hardness value of a 1.2083 flat steel flat surface provided for the testing example of Embodiment 1 of the present invention;

[0033] Figure 5 A schematic diagram showing the hardness testing locations and hardness values ​​of two end faces of a 1.2083 flat steel provided for the testing example of Embodiment 1 of the present invention;

[0034] Figure 6 A schematic diagram showing the hardness testing location and hardness value after 400mm of the riser end of a 1.2083 flat steel was cut off, as provided in the testing example of Embodiment 1 of the present invention;

[0035] Figure 7 Metallographic diagram of 1.2083 flat steel after heat treatment (quenching + tempering) provided for the detection example of Embodiment 1 of the present invention;

[0036] Figure 8 A schematic diagram showing the hardness testing location and hardness value of a 1.2083 flat steel flat surface provided for the testing example of Embodiment 2 of the present invention;

[0037] Figure 9 A schematic diagram showing the hardness testing locations and hardness values ​​of two end faces of a 1.2083 flat steel bar provided for the testing example of Embodiment 2 of the present invention;

[0038] Figure 10 A schematic diagram showing the hardness testing location and hardness value after 400mm of the riser end of a 1.2083 flat steel was cut off, which is a test example provided in Embodiment 2 of the present invention;

[0039] Figure 11 The detection location and hardness value of the 1.2083 flat steel provided in Comparative Example 2 of this invention;

[0040] Figure 12 The detection location and hardness value of the 1.2083 flat steel provided in Comparative Example 3 of this invention;

[0041] Figure 13 The detection location and hardness value of the 1.2083 flat steel provided in Comparative Example 4 of this invention;

[0042] Figure 14 The detection location and hardness value of the 1.2083 flat steel provided in Comparative Example 5 of this invention;

[0043] Figure 15 The detection location and hardness value of the 1.2083 flat steel provided in Comparative Example 6 of this invention;

[0044] Figure 16 The detection location and hardness value of the 1.2083 flat steel provided in Comparative Example 7 of this invention;

[0045] Figure 17 This is a diagram illustrating the annealing process of flat steel after forging, as provided by the present invention. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0047] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0048] In the flat steel production process, the most critical step is heat treatment, which (quenching + tempering) is also the technical challenge of this invention. In order to meet the hardness requirements while avoiding cracking during quenching heat treatment, this invention innovatively uses a new heat treatment method (quenching + tempering).

[0049] This invention provides a heat treatment method for large mirror-finish plastic mold steel, comprising the following steps:

[0050] S1. The 1.2083 flat steel parts to be processed are subjected to quenching and heat preservation treatment and cooling treatment in sequence. The quenching and heat preservation temperature is 990-1010℃ (e.g., 992℃, 994℃, 996℃, 998℃, 1000℃, 1002℃, 1004℃, 1006℃, 1008℃), and the quenching and heat preservation time is calculated according to the thickness of the flat steel parts, with 2.5-3.5 hours of heat preservation per 100mm. The cooling treatment involves air cooling the heat-preserved flat steel parts to 770℃-830℃. For example, 775℃, 780℃, 785℃, 790℃, 795℃, 800℃, 805℃, 810℃, 815℃, 820℃, 825℃), then water and air alternately cool to 370-430℃ (such as 375℃, 380℃, 385℃, 390℃, 395℃, 400℃, 405℃, 410℃, 415℃, 420℃, 425℃), and finally air cool to 130-160℃ (such as 135℃, 140℃, 145℃, 150℃, 155℃);

[0051] S2, perform a tempering treatment on the flat steel part obtained in step S1;

[0052] S3, perform a second tempering treatment on the flat steel parts obtained in step S2;

[0053] The specifications of the flat steel parts are as follows: thickness of 200-360mm (e.g., 220mm, 240mm, 260mm, 280mm, 300mm, 320mm, 340mm) and width of 650-800mm (e.g., 680mm, 700mm, 720mm, 740mm, 760mm, 780mm).

[0054] This invention involves first air-cooling the flat steel parts from the furnace to 770-830℃ after holding at 990-1010℃, followed by subsequent cooling. This reduces thermal stress and prevents quenching cracks. Air-cooling the flat steel parts to too low a temperature after holding at 990-1010℃ results in a slow cooling rate below the austenite transformation temperature of 800℃, preventing martensitic transformation and leading to localized bainitic and pearlitic transformations. This fails to produce a martensitic quenched structure, ultimately resulting in low hardness. Conversely, air-cooling to too high a temperature cannot prevent quenching cracks at the edges during subsequent water cooling.

[0055] After air cooling, the present invention continues to perform water-air alternating cooling to ensure that the flat steel does not crack during quenching and to ensure that quenched martensite is obtained. Water-air alternating cooling can ensure that the steel effectively avoids the "C-curve nose" during rapid cooling at high temperature, thereby obtaining fine martensite and avoiding transformation to bainite and pearlite. During air cooling for 4-7 minutes, the heat in the core of the flat steel is conducted to the lower surface temperature, reducing the temperature difference between the core and the surface of the flat steel and avoiding thermal stress cracking during intense cooling.

[0056] After alternating water and air cooling, when the flat steel temperature is 400±30℃, it is air-cooled to 130-160℃ at the center of the large face before being tempered in the furnace. Air cooling to 130-160℃ allows the flat steel to undergo martensitic transformation below the Ms point of 230℃. The cooling rate slows down in the low-temperature stage, thus avoiding stress cracking and obtaining a good quenched martensitic structure. This cooling method ensures that the core of the 1.2083 steel is fully cooled without quenching cracking, resulting in a good quenched structure and qualified hardness.

[0057] If air cooling is used after water-air alternating cooling, the cooling rate is too slow, which is not conducive to the overall cooling of the flat steel and easily leads to bainite or pearlite transformation, which is not conducive to obtaining martensitic structure. Air cooling after water-air alternating cooling provides a cooling rate between that of water and air cooling, avoiding the severe cracking caused by water cooling and the bainite or pearlite transformation caused by the slow cooling rate of air cooling. Here, air cooling ensures that cracking is not prevented while successfully obtaining martensitic structure. The air cooling rate in this invention is defined by the fan speed, which is set at 1450 r / min, with a range of 1300-1600 r / min.

[0058] As an optional embodiment of the present invention, the smelting method of the flat steel part includes electric furnace smelting and electroslag remelting performed sequentially.

[0059] As an optional embodiment of the present invention, the chemical composition of the flat steel component, by weight percentage, includes: C: 0.38-0.44%, Si: ≤1.00%, Mn: ≤1.00%, P: ≤0.020%, S: ≤0.010%, Cr: 12.5-13.5%, Ni: ≤0.25%, Cu: ≤0.20%, Mo: 0.10-0.20%, V: 0.08-0.15%, H: ≤0.00015%. C, Si, Mn, and Cr are the main components of this flat steel component, while other components are residual components. The lower the content of residual components, the better. Preferably, Si: 0.40-0.60%, Mn: 0.70-0.90%.

[0060] As an optional embodiment of the present invention, in step S1, the flat steel piece to be processed is first kept at 330-370℃ (e.g., 335℃, 340℃, 345℃, 350℃, 355℃, 360℃, 365℃) for 1 hour, then heated to 630-670℃ (e.g., 635℃, 640℃, 645℃, 650℃, 655℃, 660℃, 665℃) at a heating rate of ≤60℃ / h (e.g., 30℃ / h, 40℃ / h, 50℃ / h, 55℃ / h, etc.) and kept at that temperature for 3 hours, and finally heated to 990-1010℃ at a heating rate of ≤80℃ / h (e.g., 70℃ / h, 60℃ / h, 50℃ / h, 40℃ / h, etc.).

[0061] As an optional embodiment of the present invention, in step S1, the water-air alternating cooling involves immersing the flat steel piece in water for 30-40 minutes, then immersing it in water for 4-7 minutes and air cooling it, followed by immersing it in water for 8-12 minutes and air cooling it for 4-7 minutes. This process is repeated until the flat steel piece is air cooled for 4-7 minutes after immersion in water, at which point the temperature is controlled at 370-430°C.

[0062] In the aforementioned water-air alternating cooling method, if the water cooling time is too long, the cooling will be too intense, easily leading to quenching cracks; if the water cooling time is too short, the cooling rate will be too slow, easily encountering the "C-curve nose" and resulting in bainite or pearlite, affecting the quenching effect. If the air cooling time is too long, the heat in the core of the flat steel will return to the surface, and the temperature cannot drop rapidly, resulting in a slow cooling rate and easily encountering the "C-curve nose," leading to bainite or pearlite, affecting the quenching effect; if the air cooling time is too short, the heat in the core of the flat steel will not have returned to the surface, which is equivalent to continuous water cooling, resulting in too intense cooling and easily leading to quenching cracks. The preferred embodiment of this invention adopts a water-air alternating cooling method with a longer initial water cooling time and a shorter subsequent water cooling time. After the first long water cooling, the temperature of the flat steel has dropped rapidly, so the second water cooling time cannot be too long, otherwise the cooling will be too intense, easily leading to quenching cracks.

[0063] As an optional embodiment of the present invention, in step S2, the holding temperature of the first tempering is 500-560℃ (e.g., 505℃, 510℃, 515℃, 520℃, 525℃, 530℃, 535℃, 540℃, 545℃, 550℃, 555℃), and the holding time is calculated according to the thickness of the flat steel piece, with a holding time of 6.5-7.5h per 100mm.

[0064] As an optional embodiment of the present invention, in step S3, the holding temperature of the secondary tempering is 470-530℃ (e.g., 475℃, 480℃, 485℃, 490℃, 495℃, 500℃, 505℃, 510℃, 515℃, 520℃, 525℃), and the holding time is calculated according to the thickness of the flat steel piece, with a holding time of 6.5-7.5h per 100mm.

[0065] As an optional embodiment of the present invention, in step S2, the flat steel piece is first kept at 230-270℃ (e.g., 235℃, 240℃, 245℃, 250℃, 255℃, 260℃, 265℃) for 3-5 hours, and then heated to 500-560℃ at a heating rate of ≤70℃ / h.

[0066] And / or, in step S3, the flat steel piece is first kept at 280-320℃ (e.g., 285℃, 290℃, 295℃, 300℃, 305℃, 310℃, 315℃) for 3 hours, and then heated to 470-530℃ at a heating rate of ≤70℃ / h.

[0067] The invention employs a double tempering process because flat steel parts exhibit high hardness and brittleness after quenching. Double tempering adjusts the hardness to 35-41 HRC and the core hardness to 36-40 HRC. The second tempering not only adjusts hardness but also effectively removes stress. The temperature for the second tempering is set 20-30°C lower than the first tempering temperature (e.g., 22°C, 24°C, 26°C, 28°C). This is because the purpose of the second tempering is to adjust hardness and remove stress. If the second tempering temperature is the same as the first tempering temperature, the hardness will be significantly reduced, resulting in substandard hardness. If the second tempering temperature is set too low compared to the first tempering temperature, the stress-relief effect will be insignificant, hindering the effective removal of stress.

[0068] As an optional embodiment of the present invention, after the flat steel piece is tempered in steps S2 and S3, it is cooled by air cooling at a rate of ≤30℃ / h; and / or, the temperature measurement point of the flat steel piece is at the center of the large flat surface of the flat steel piece.

[0069] The flat steel parts described in this invention, after being quenched and tempered or treated by the heat treatment method of this invention, have a hardness of 35-41 HRC and a core hardness of 36-40 HRC, which are uniform and consistent. Ultrasonic flaw detection meets level 4.

[0070] The flat steel parts heat-treated in this invention can be obtained by any conventional method in the art, including smelting, casting, forging, or rolling. For example, they can be obtained through the following process: electric furnace (LF+VD) → casting → annealing → electroslag remelting → hot charging → forging into finished products → annealing → flaw detection. After quenching and tempering treatment according to this invention, they are then milled → ultrasonically tested → inspected to obtain fully qualified delivered steel. The annealing process after forging is as follows, see details below. Figure 17 Specifically:

[0071] The forged material, heated from 250-350℃, is heated to 820-840℃ at a rate of ≤100℃ / h, held at that temperature for 3.5-4.5h / 100mm, then cooled to 660-680℃ at a rate of ≤30℃ / h, held at that temperature for 5.5-6.5h / 100mm, and finally cooled to ≤400℃ at a rate of ≤30℃ / h before being removed from the furnace.

[0072] The present invention will now be described in further detail with reference to specific embodiments.

[0073] The main chemical composition and content (wt%) of the 1.2083 flat steel parts in the following examples and comparative examples are as follows: C: 0.4%, Si: 0.5%, Mn: 0.75%, P: 0.015%, S: 0.005%, Cr: 13%, Mo: 0.15%, V: 0.10%, Ni: 0.10%, Cu: 0.10%, H: 0.00008%.

[0074] Example 1

[0075] This embodiment provides a heat treatment method for a 1.2083 flat steel component. The flat steel component is 360mm thick, 800mm wide, and 3000mm long. The heat treatment method for this flat steel component includes the following steps:

[0076] 1. Quenching process, process flow is as follows Figure 1 .

[0077] The flat steel is held at approximately 350℃ for 1 hour, then heated to approximately 650℃ at a rate of 60℃ / h and held for 3 hours. Finally, it is heated to approximately 1000℃ at a rate of 80℃ / h and held for 11 hours.

[0078] Air cooling is performed until the temperature at the center of the flat steel's large surface area reaches approximately 820℃. Then, alternating water and air cooling is applied. The specific steps of alternating water and air cooling are as follows: the flat steel is immersed in water for 35 minutes, then removed and air-cooled for 5 minutes. This process is repeated for 10 minutes in water followed by 5 minutes in air until the flat steel's temperature is controlled at approximately 400℃ at the center of the large surface area. Finally, it is air-cooled until the temperature at the center of the large surface area reaches approximately 150℃.

[0079] 2. Single tempering, process flow is as follows: Figure 2 .

[0080] The flat steel is held at around 270℃ for 4 hours, then heated to around 560℃ at a rate of 70℃ / h and held for 25.2 hours. Finally, it is cooled to room temperature by air cooling at a rate of 30℃ / h.

[0081] 3. Secondary tempering, process flow is as follows: Figure 3 .

[0082] The flat steel after the first tempering treatment is held at about 300℃ for 3 hours, then heated to about 530℃ at a heating rate of 70℃ / h, held for 25.2 hours, and then cooled to room temperature by air cooling at a rate of 30℃ / h.

[0083] Example 2

[0084] This embodiment provides a heat treatment method for a 1.2083 flat steel component. The flat steel component is 220mm thick, 660mm wide, and 5000mm long. The heat treatment method for this flat steel component includes the following steps:

[0085] 1. Quenching process, process flow is as follows Figure 1 .

[0086] The flat steel is held at approximately 350℃ for 1 hour, then heated to approximately 650℃ at a rate of 60℃ / h and held for 3 hours. Finally, it is heated to approximately 1000℃ at a rate of 80℃ / h and held for 7 hours.

[0087] Air cooling is performed until the temperature at the center of the flat steel's large surface area reaches approximately 820℃. Then, alternating water and air cooling is applied. The specific steps of alternating water and air cooling are as follows: the flat steel is immersed in water for 35 minutes, then removed and air-cooled for 5 minutes. This process is repeated for 10 minutes in water followed by 5 minutes in air until the flat steel's temperature is controlled at approximately 400℃ at the center of the large surface area. Finally, it is air-cooled until the temperature at the center of the large surface area reaches approximately 150℃.

[0088] 2. Single tempering, process flow is as follows: Figure 2 .

[0089] The flat steel is held at around 270℃ for 4 hours, then heated to around 560℃ at a rate of 70℃ / h and held for 15.4 hours. Finally, it is cooled to room temperature by air cooling at a rate of 30℃ / h.

[0090] 3. Secondary tempering, process flow is as follows: Figure 3 .

[0091] The flat steel after the first tempering treatment is held at about 300℃ for 3 hours, then heated to about 530℃ at a heating rate of 70℃ / h, held for 15.4 hours, and then cooled to room temperature by air cooling at a rate of 30℃ / h.

[0092] Comparative Example 1

[0093] The difference between this comparative example and Example 1 is that the air cooling to the center of the large flat surface at 770-830℃ is omitted. After quenching and holding, the flat steel is directly subjected to alternating water and air cooling. Other processes are the same as in Example 1. Because the flat steel was not pre-cooled by air cooling, the cooling was too intense, causing quenching cracks and rendering the flat steel unusable.

[0094] Comparative Example 2

[0095] The difference between this comparative example and Example 1 is that the air cooling to the center of the large flat surface, which was 770-830°C, was changed to 700°C, and then water-air alternating cooling was performed. The other processes are the same as in Example 1.

[0096] The slow cooling rate from 1000℃ to 700℃ caused the transformation of bainite and pearlite. Subsequent alternating water and air cooling resulted in insufficient and uneven hardening, ultimately leading to substandard hardness in the flat steel. For testing locations and hardness values, see [link to relevant documentation]. Figure 11 .

[0097] Comparative Example 3

[0098] The difference between this comparative example and Example 1 is that the air cooling in the quenching process is replaced with air cooling, while the other processes are the same as in Example 1.

[0099] Because air cooling was replaced by air cooling, the flat steel cooled more slowly in the later stages, resulting in bainite and pearlite transformation. This ultimately led to the flat steel having a lower hardness than expected, failing to meet the 35-41 HRC standard. The testing locations and hardness values ​​are detailed below. Figure 12 .

[0100] Comparative Example 4

[0101] The difference between this comparative example and Example 1 is that the quenching holding temperature of 1000℃ in the quenching process is replaced with 950℃, while the other processes are the same as in Example 1.

[0102] Because the quenching holding temperature was changed from 1000℃ to 950℃, the austenite transformation was incomplete. Insufficient microstructure transformation during quenching resulted in a lower hardness, failing to meet the 35-41HRC range. The hardness is unqualified. See [link to testing location and hardness value] for details. Figure 13 .

[0103] Comparative Example 5

[0104] The difference between this comparative example and Example 1 is that the temperature after water-air alternating cooling in the quenching process is controlled at 450℃ and 350℃ respectively, while the other processes are the same as in Example 1.

[0105] If the temperature after alternating water and air cooling in the quenching process is controlled at 450℃, then 450℃ is too high. Subsequent air cooling starting from 450℃ will result in a slow cooling rate, which can easily lead to pearlite transformation. Insufficient cooling will cause the flat steel to have a low quenching hardness, ultimately resulting in the flat steel failing to meet hardness standards after tempering. For testing locations and hardness values, see [link to relevant documentation]. Figure 14 .

[0106] If the temperature after water-air alternating cooling in the quenching process is controlled at 350℃, then 350℃ is too low. The temperature of the flat steel is too low after water-air alternating cooling, which is prone to quenching cracks, resulting in the flat steel cracking and being scrapped (hardness not tested).

[0107] Comparative Example 6

[0108] The difference between this comparative example and Example 1 is that the temperature of the flat steel parts during the quenching process is controlled at 200℃ and 100℃ respectively, while the other processes are the same as in Example 1.

[0109] If the air-cooling temperature of the flat steel parts during the quenching process is controlled at 200℃, then 200℃ is too high, resulting in insufficient martensitic transformation and insufficient quenching hardness. Ultimately, this leads to low hardness and failure to meet standards after tempering. For testing locations and hardness values, see [link to relevant documentation]. Figure 15 .

[0110] If the temperature of the air-cooled flat steel parts during the quenching process is controlled at 100℃, then 100℃ is too low and it is easy to cause the final cooling temperature to be too low, resulting in cracking during martensitic transformation and scrapping of the flat steel (hardness not tested).

[0111] Comparative Example 7

[0112] The difference between this comparative example and Example 1 is that the water cooling time in the alternating water and air cooling process during the quenching process is uniformly controlled to 10 minutes and the air cooling time is controlled to 5 minutes. Other processes are the same as in Example 1.

[0113] In the quenching process, the water cooling time during alternating water and air cooling is uniformly controlled to 10 minutes, and the air cooling time is controlled to 5 minutes. Other processes are the same as in Example 1. However, if the first water cooling time is too short, the cooling rate and intensity will be insufficient, easily leading to bainite or pearlite transformation, resulting in poor quenching effect and causing the flat steel to have a low hardness and fail to meet standards. The detection location and hardness value are shown below. Figure 16 Furthermore, if the cooling time is uniformly controlled to 10 minutes each time and the air cooling time is controlled to 5 minutes, then water cooling needs to be repeated many times, which increases the difficulty of operation and is not conducive to workers' operation.

[0114] Detection example

[0115] The Rockwell hardness test was used for hardness testing, and the national standard number is GB / T230.3.

[0116] Example 1: Hardness and microstructure testing of flat steel after heat treatment (quenching + tempering):

[0117] 1. The dimensions of the flat steel after heat treatment (quenching + tempering) are: thickness 360mm, width 800mm, and length 3000mm. The surface hardness and the hardness of both ends of the product were tested according to technical requirements. The surface hardness meets the range of 35-41 HRC, and the core hardness is 36-40 HRC. Specific test data are as follows:

[0118] 1.1. Hardness was tested at 25 points on the large flat surface of the flat steel. The test locations and hardness values ​​are detailed below. Figure 4 The lowest hardness was 37.8 HRC and the highest hardness was 38.7 HRC, both within the range of 35-41 HRC, meeting the requirements and passing the hardness test; the hardness range was 0.9 HRC, indicating good hardness uniformity.

[0119] 1 . 2. Hardness was tested at 11 points on each of the two end faces of the flat steel (sprue end face and riser end face). The test locations and hardness values ​​are detailed below. Figure 5 The lowest hardness is 38.1 HRC and the highest hardness is 38.7 HRC, both within the range of 35-41 HRC, which meets the requirements. The hardness test is qualified, and the hardness range is 0.6 HRC, indicating good hardness uniformity.

[0120] 1.3 To check whether the flat steel has been fully quenched and whether the core hardness is up to standard, 400mm of the riser end is sawn off, and then the hardness is tested on the sawn surface. The test location and hardness value are shown in [reference needed]. Figure 6 The lowest hardness was 37.6 HRC and the highest hardness was 38.5 HRC, both within the range of 35-41 HRC, which meets the requirements. The hardness test was qualified, and the hardness range was 0.9 HRC, indicating good hardness uniformity.

[0121] Based on the above test results, it can be seen that after the flat steel in Example 1 is heat-treated (quenched + tempered), the hardness is 35 - 41 HRC, and the hardness of the core of the flat steel is uniform, reaching 36 - 40 HRC.

[0122] 2. Metallographic structure

[0123] The structure of the 1.2083 flat steel after tempering is tempered sorbite. The martensite obtained by quenching gradually grows into fine cementite particles during tempering, that is, the mixture composed of recrystallized ferrite and cementite is tempered sorbite. The metallographic structure photos are as Figure 7 .

[0124] 3. Appearance and ultrasonic flaw detection of flat steel

[0125] There is no quenching crack in the flat steel. Ultrasonic flaw detection is carried out according to GB / T6402 - 2008, and the flaw detection meets level 4, and the flaw detection is qualified.

[0126] The above test data shows that the flat steel is heated to 990 - 1010 °C, air-cooled out of the furnace until the temperature at the center of the large plane of the flat steel is 770 - 830 °C, and then water-air alternate cooling is adopted. After the water-air alternate cooling is completed, it is air-cooled until the temperature at the center of the large surface of the flat steel is 130 - 160 °C and then put into the furnace for tempering. This can ensure that the flat steel is fully quenched and avoid the occurrence of quenching cracks, and the quality is controllable. After the flat steel in Example 1 is heat-treated (quenched + tempered), the product hardness is qualified and the hardness uniformity is good.

[0127] 2. Hardness test of the flat steel in Example 2 after heat treatment (quenched + tempered):

[0128] 2.1 The hardness of 25 points is detected on the large plane of the 220mm * 660mm * 5000mm flat steel. The detection positions and hardness values are shown in Figure 8 , among which, the lowest hardness is 37.9 HRC, the highest hardness is 39.0 HRC, both are within the range of 35 - 41 HRC, meeting the requirements, and the hardness test is qualified; the hardness range is 1.1 HRC, and the hardness uniformity is good.

[0129] 2.2 The hardness of 11 points is detected on each of the two end faces (water inlet end face, riser end face) of the 220mm * 660mm * 5000mm flat steel. The detection positions and hardness values are shown in Figure 9 , among which, the lowest hardness is 38.3 HRC, the highest hardness is 39.0 HRC, both are within the range of 35 - 41 HRC, meeting the requirements, the hardness test is qualified, and the hardness range is 0.7 HRC, and the hardness uniformity is good.

[0130] 2.3 In order to detect whether the 220mm * 600mm * 5000mm flat steel is fully quenched and whether the core hardness is qualified, 400mm is sawed off at the riser end, and then the hardness is detected on the sawed surface. The detection positions and hardness values are shown in Figure 10The lowest hardness was 37.8 HRC and the highest hardness was 38.7 HRC, both within the range of 35-41 HRC, which meets the requirements. The hardness test was qualified, and the hardness range was 0.9 HRC, indicating good hardness uniformity.

[0131] 3. Inspection results of flat steel samples 1-7 after heat treatment (quenching + tempering):

[0132] Comparative Example 1 was scrapped due to quenching cracks and was not tempered, so its hardness was not tested.

[0133] Comparative Examples 2-7: Hardness testing locations and hardness values ​​are shown in the diagram. Figures 11-16 The hardness values ​​of the steel are unevenly distributed across the entire cross section, mostly outside the range of 35-41 HRC and with most values ​​below 35 HRC. The core hardness values ​​are all below 36 HRC.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A heat treatment method for large mirror-finish plastic mold steel, characterized in that, Includes the following steps: S1. The 1.2083 flat steel parts to be processed are subjected to quenching and heat preservation treatment and cooling treatment in sequence. The quenching and heat preservation temperature is 990-1010℃, and the quenching and heat preservation time is calculated according to the thickness of the flat steel parts, with 2.5-3.5h per 100mm. The cooling treatment is to air cool the heat-preserved flat steel parts to 770℃-830℃, then alternately cool them with water and air to 370-430℃, and finally air cool them to 130-160℃. S2, perform a tempering treatment on the flat steel part obtained in step S1; S3, perform a second tempering treatment on the flat steel parts obtained in step S2; The specifications of the flat steel parts are: thickness 200-360mm, width 650-800mm; In step S1, the water-air alternating cooling involves immersing the flat steel piece in water for 30-40 minutes, then immersing it in water for 4-7 minutes and air cooling it for 4-7 minutes, then immersing it in water for 8-12 minutes and air cooling it for 4-7 minutes, and repeating this process until the flat steel piece reaches a reheat temperature of 370-430℃ after immersing it in water for 4-7 minutes and air cooling it for 8-12 minutes and air cooling it for 4-7 minutes. In step S2, the holding temperature for the first tempering is 500-560℃, and the holding time is calculated based on the thickness of the flat steel piece, with a holding time of 6.5-7.5 hours per 100mm. In step S3, the holding temperature for the secondary tempering is 470-530℃, and the holding time is calculated based on the thickness of the flat steel piece, with a holding time of 6.5-7.5h per 100mm.

2. The heat treatment method for large mirror-finish plastic mold steel according to claim 1, characterized in that, The smelting method for the flat steel parts includes electric furnace smelting and electroslag remelting performed sequentially.

3. The heat treatment method for large mirror-finish plastic mold steel according to claim 1, characterized in that, The chemical composition of the flat steel component, by weight percentage, includes: C: 0.38-0.44%, Si: ≤1.00%, Mn: ≤1.00%, P: ≤0.020%, S: ≤0.010%, Cr: 12.5-13.5%, Ni: ≤0.25%, Cu: ≤0.20%, Mo: 0.10-0.20%, V: 0.08-0.15%, H: ≤0.00015%, with the balance being Fe and unavoidable impurities.

4. The heat treatment method for large mirror-finish plastic mold steel according to claim 1, characterized in that, In step S1, the flat steel piece to be processed is first kept at 330-370℃ for 1 hour, then heated to 630-670℃ at a heating rate of ≤60℃ / h and kept at that temperature for 3 hours, and finally heated to the quenching and holding temperature of 990-1010℃ at a heating rate of ≤80℃ / h.

5. The heat treatment method for large mirror-finish plastic mold steel according to claim 1, characterized in that, In step S2, the flat steel piece is first kept at 230-270℃ for 3-5 hours, and then heated to 500-560℃ at a heating rate of ≤70℃ / h. And / or, in step S3, the flat steel piece is first kept at 280-320℃ for 3 hours, and then heated to 470-530℃ at a heating rate of ≤70℃ / h.

6. The heat treatment method for large mirror-finish plastic mold steel according to claim 1, characterized in that, In steps S2 and S3, after the flat steel parts are tempered and kept warm, they are cooled by air cooling at a rate of ≤30℃ / h. And / or, the temperature measurement point of the flat steel piece is at the center of the large flat surface of the flat steel piece.

7. The heat treatment method for large mirror-finish plastic mold steel according to any one of claims 1-6, characterized in that, The flat steel component, after being treated by the heat treatment method described in any one of claims 1-6, has a hardness of 35-41 HRC and a core hardness of 36-40 HRC, and meets the level 4 requirement for ultrasonic flaw detection.

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