Heat treatment method for large mirror plastic die steel

Abstract

The present invention relates to the technical field of heat treatment processing of steel materials. Provided is a heat treatment method for large mirror plastic die steel. The heat treatment method comprises the following steps: S1, subjecting a 1.2083 flat steel part to be treated to a quenching heat preservation treatment and a cooling treatment in sequence, wherein the quenching heat preservation temperature is 990-1010°C, and the quenching heat preservation time is calculated according to the thickness of the flat steel part: heat preservation is performed for 2.5-3.5 h per 100 mm; and the cooling treatment comprises subjecting the flat steel part obtained after heat preservation to air cooling to 770-830°C, then to alternate water-air cooling to 370-430°C, and finally to air cooling to 130-160°C; S2, subjecting the flat steel part obtained in step S1 to a primary tempering treatment; and S3, subjecting the flat steel part obtained in step S2 to a secondary tempering treatment. The present invention solves the technical problem of large mirror plastic die steel being prone to quenching cracking, and the obtained product is of a high quality, with all the indexes thereof being far higher than standard requirements.

Description

A heat treatment method for large mirror plastic mold steel Technical Field

[0001] The invention relates to the technical field of heat treatment processing of steel materials, in particular to a heat treatment method for large-scale mirror-surface plastic mold steel. Background Art

[0002] 1.2083 steel is a high-quality, mirror-finish plastic mold steel and a high-carbon, high-chromium martensitic stainless steel. Its chemical composition primarily consists of 0.4% carbon, 13% chromium, and small amounts of molybdenum, manganese, and other elements. It exhibits excellent polishability, corrosion resistance, and high hardness. Flat steel thicknesses are generally below 200mm. With the increasing size of industrial products, 1.2083 steel sizes are also gradually increasing, with market demand gradually expanding to sizes ranging from 200-360mm thick to 650-800mm wide. 1.2083 steel requires a high hardness of 35-41HRC, with uniform hardness across the entire cross-section. Achieving this hardness requires quenching heat treatment, a process that significantly impacts its performance. Improper quenching can easily lead to cracking during quenching, resulting in a low core hardness and a low pass rate.

[0003] Oil quenching is generally used for 1.2083 steel with a flat steel thickness of less than 200mm. However, oil quenching can result in insufficient and uneven hardness at the core of the flat steel when the flat steel thickness exceeds 200mm. When the flat steel size increases to a thickness of (200-360)mm and a width of (650-800)mm, oil quenching is no longer sufficient to produce qualified products, while water quenching is highly susceptible to quenching cracking, rendering the steel scrapped.

[0004] In summary, in order to solve the defects and shortcomings of the above-mentioned existing technologies, a new quenching heat treatment method must be innovatively adopted to meet the product technical requirements and produce qualified products. Summary of the Invention

[0005] The present invention aims to provide a heat treatment method for large-scale mirror-finish plastic mold steel, and in particular to provide a quenching heat treatment method for large-size 1.2083 forged flat steel. This method solves the technical problem of large-scale mirror-finish plastic mold steel being prone to quenching cracking, and the obtained product is of high quality, with various indicators far exceeding standard requirements.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

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

[0008] S1, sequentially performing quenching and holding treatments and cooling treatments on the 1.2083 flat steel piece to be processed, wherein the quenching and holding temperature is 990-1010°C, and the quenching and holding time is calculated according to the thickness of the flat steel piece, and the holding time is 2.5-3.5 hours per 100 mm; the cooling treatment comprises air cooling the flat steel piece after holding to 770-830°C, then alternating water and air cooling to 370-430°C, and finally air cooling to 130-160°C;

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

[0010] S3, performing secondary tempering treatment on the flat steel piece obtained in step S2;

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

[0012] Furthermore, the smelting method of the flat steel piece includes electric furnace smelting and electroslag remelting performed in sequence.

[0013] Furthermore, the chemical composition of the flat steel piece includes, by weight percentage: 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%, and the balance is Fe and unavoidable impurities.

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

[0015] Furthermore, in step S1, the water-air alternating cooling is to cool the flat steel piece in water for 30-40 minutes, then remove it from the water and air cool it for 4-7 minutes, then cool it in water for 8-12 minutes, then air cool it for 4-7 minutes, and cycle in this manner of cooling it in water for 8-12 minutes, then air cool it for 4-7 minutes until the flat steel piece returns to a temperature of 370-430°C after 4-7 minutes of air cooling.

[0016] Furthermore, in step S2, the holding temperature of the primary tempering is 500-560°C, and the holding time is calculated according to the thickness of the flat steel piece, that is, 6.5-7.5 hours per 100 mm.

[0017] Furthermore, in step S3, the holding temperature of the secondary tempering is 470-530° C., and the holding time is calculated according to the thickness of the flat steel piece, and is 6.5-7.5 hours per 100 mm.

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

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

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

[0022] Furthermore, the hardness of the flat steel piece after quenching and tempering or treatment with the above-mentioned heat treatment method is 35-41HRC, the hardness of the core is 36-40HRC, and the ultrasonic flaw detection meets level 4.

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

[0024] 1. The present invention applies appropriate quenching heating temperature and holding time: Under higher temperature and long time heating, it is 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 that may cause grain growth, and low quenching temperature that may cause incomplete martensite formation, which in turn leads to undesirable metallographic structure, and ultimately avoids coarse grains or insufficient hardness of the product.

[0025] 2. To reduce the risk of quenching cracking, the flat steel is pre-cooled to 800±30°C in air after being held at 1000±10°C, followed by alternating water-air cooling. This method of pre-cooling the 1.2083 steel to 800±30°C before quenching minimizes cracking at the edges of the flat steel caused by rapid cooling during rapid water cooling at 1000±10°C.

[0026] 3. The present invention adopts a water-air alternating cooling method to ensure that the flat steel does not suffer from quenching cracking during quenching while ensuring that quenched martensite is obtained; water-air alternating cooling can ensure that the steel effectively avoids the "C curve nose" when rapidly cooled in the high temperature stage, thereby obtaining fine martensite, thereby avoiding the transformation to bainite and pearlite; when air-cooled for 4-7 minutes, the heat of the flat steel core is transferred to the lower surface temperature, reducing the temperature difference between the flat steel core and the surface temperature, and avoiding thermal stress cracking during intense cooling.

[0027] 4. After alternating water and air cooling, when the flat steel temperature is 400±30℃, air cool it to 130-160℃ at the center of the large surface of the flat steel before tempering 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 is slowed down at low temperatures, 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 and quench cracking does not occur, thus obtaining a good quenched structure and qualified hardness. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] FIG1 is a diagram of a quenching heat treatment process for 1.2083 flat steel provided in Examples 1 and 2 of the present invention;

[0030] FIG2 is a diagram of the first tempering process of 1.2083 flat steel provided in Examples 1 and 2 of the present invention;

[0031] FIG3 is a diagram of the second tempering process of 1.2083 flat steel provided in Examples 1 and 2 of the present invention;

[0032] FIG4 is a schematic diagram of the hardness test position and hardness value of a large flat surface of 1.2083 flat steel provided in the test example of Example 1 of the present invention;

[0033] FIG5 is a schematic diagram of the hardness test positions and hardness values ​​of two end faces of 1.2083 flat steel provided in the test example of Example 1 of the present invention;

[0034] FIG6 is a schematic diagram of the hardness testing position and hardness value of a 1.2083 flat steel riser end after 400 mm has been removed, provided in a test example of Example 1 of the present invention;

[0035] FIG7 is a metallographic structure diagram of a 1.2083 flat steel heat-treated (quenched and tempered) according to a test example of Example 1 of the present invention;

[0036] FIG8 is a schematic diagram of the hardness test position and hardness value of a large flat surface of 1.2083 flat steel provided in a test example of Example 2 of the present invention;

[0037] FIG9 is a schematic diagram of the hardness test positions and hardness values ​​of two end faces of 1.2083 flat steel provided in the test example of Example 2 of the present invention;

[0038] FIG10 is a schematic diagram of the hardness testing position and hardness value of a 1.2083 flat steel riser end after 400 mm is removed, provided in a test example of Example 2 of the present invention;

[0039] FIG11 shows the detection position and hardness value of 1.2083 flat steel provided in Comparative Example 2 of the present invention;

[0040] FIG12 shows the detection position and hardness value of 1.2083 flat steel provided in Comparative Example 3 of the present invention;

[0041] FIG13 shows the detection position and hardness value of 1.2083 flat steel provided in Comparative Example 4 of the present invention;

[0042] FIG14 shows the detection position and hardness value of 1.2083 flat steel provided in Comparative Example 5 of the present invention;

[0043] FIG15 shows the detection position and hardness value of 1.2083 flat steel provided in Comparative Example 6 of the present invention;

[0044] FIG16 shows the detection position and hardness value of 1.2083 flat steel provided in Comparative Example 7 of the present invention;

[0045] FIG17 is a diagram showing the annealing process of the flat steel after forging provided by the present invention. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. The process parameters for which specific conditions are not specified in the following examples are generally in accordance with conventional conditions.

[0047] The endpoints of the ranges and any values ​​disclosed herein 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 endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

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

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

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

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

[0052] S3, performing secondary tempering treatment on the flat steel piece obtained in step S2;

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

[0054] The flat steel parts of the present invention are first removed from the furnace and air-cooled to 770°C-830°C after a holding temperature of 990-1010°C, followed by a subsequent cooling process. This reduces thermal stress and prevents quenching cracking in the flat steel parts. Air-cooling to too low a temperature after a holding temperature of 990-1010°C will prevent martensite transformation from occurring due to the temperature being below the austenite transformation temperature of 800°C and the air-cooling rate being too slow. This will result in localized bainite and pearlite transformation, preventing the formation of a martensite quenched structure and ultimately resulting in low hardness. Air-cooling to too high a temperature after removal from the furnace will also fail to prevent quenching cracking at the corners of the steel parts 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 suffer from quenching cracking during quenching while ensuring that quenched martensite is obtained; water-air alternating cooling can ensure that the steel effectively avoids the "C curve nose" when rapidly cooled in the high temperature stage, thereby obtaining fine martensite, thereby avoiding transformation to bainite and pearlite; when air cooling is performed for 4-7 minutes, the heat in the core of the flat steel is transferred 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-air cooling, the flat steel is cooled to 400±30°C, then air-cooled to 130-160°C at the center of the large surface before being placed in a furnace for tempering. Air-cooling to 130-160°C allows the flat steel to undergo martensitic transformation below the Ms point of 230°C. The cooling rate is slowed at low temperatures, thus preventing stress cracking and achieving a well-developed quenched martensitic structure. This cooling method ensures that the core of the 1.2083 steel is fully cooled, preventing quench cracking and achieving a well-developed quenched structure and acceptable hardness.

[0057] If air cooling is used after alternating water-air cooling, the cooling rate is too slow, which is not conducive to the overall cooling of the flat steel and is prone to bainite or pearlite transformation, which is not conducive to obtaining a martensitic structure. Air cooling is adopted after alternating water-air cooling. The air cooling rate is between water cooling and air cooling. It will not cause cracking due to intense water cooling, nor will it cause bainite or pearlite transformation due to a too slow air cooling rate. Here, air cooling can ensure that there is no cracking and that the martensitic structure transformation can be obtained smoothly. The air cooling rate of the present invention is defined by the fan speed, which is set at 1450r / min and ranges from 1300-1600r / min.

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

[0059] As an optional embodiment of the present invention, the chemical composition of the flat steel piece, by weight, 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 primary components of the flat steel piece, while the remaining components are residual, the content of which is preferably controlled as low as possible. Preferably, Si: 0.40-0.60%, and 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 heated to 330-370°C (such as 335°C, 340°C, 345°C, 350°C, 355°C, 360°C, 365°C) for 1 hour, then heated to 630-670°C (such as 635°C, 640°C, 645°C, 650°C, 655°C, 660°C, 665°C) at a heating rate of ≤60°C / h (such as 30°C / h, 40°C / h, 50°C / h, 55°C / h, etc.) and heated for 3 hours, and finally heated to 990-1010°C at a heating rate of ≤80°C / h (such as 70°C / h, 60°C / h, 50°C / h, 40°C / h, etc.).

[0061] As an optional embodiment of the present invention, in step S1, the water-air alternating cooling is to cool the flat steel piece in water for 30-40 minutes, then remove it from the water and air cool it for 4-7 minutes, then cool it in water for 8-12 minutes, then air cool it for 4-7 minutes, and the cycle is repeated in the manner of cooling in water for 8-12 minutes, then air cooling for 4-7 minutes, until the flat steel piece returns to a temperature of 370-430°C after 4-7 minutes of air cooling.

[0062] If the water-air alternating cooling time is too long, the cooling will be too intense and quenching cracks will be easily generated; if the water-cooling time is too short, the cooling rate will be too slow and the "C-curve nose" will be easily encountered, resulting in the appearance of bainite or pearlite, which will affect the quenching effect. If the air-cooling time is too long, the heat in the core of the flat steel will be returned to the surface, and the temperature cannot be reduced quickly; if the cooling rate is too slow, the "C-curve nose" will be easily encountered, resulting in the appearance of bainite or pearlite, which will affect the quenching effect; if the air-cooling time is too short, the heat in the core of the flat steel will not be returned to the surface, which is equivalent to being water-cooled all the time, and the cooling will be too intense and quenching cracks will be easily generated. The preferred solution of the present invention selects a method of performing water-air alternating cooling with a relatively long initial water-cooling time and a relatively short subsequent water-cooling time. After the flat steel has been cooled for a long time for the first time, the temperature has dropped rapidly, so the second water-cooling time cannot be too long, otherwise the cooling will be too intense and quenching cracks will be easily generated.

[0063] As an optional embodiment of the present invention, in step S2, the holding temperature of the primary tempering is 500-560°C (such as 505°C, 510°C, 515°C, 520°C, 525°C, 530°C, 535°C, 540°C, 545°C, 550°C, 555°C), and the holding time is calculated according to the thickness of the flat steel piece, and the holding time is 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°C (such as 475°C, 480°C, 485°C, 490°C, 495°C, 500°C, 505°C, 510°C, 515°C, 520°C, 525°C), and the holding time is calculated according to the thickness of the flat steel piece, and the holding time is 6.5-7.5 hours per 100 mm.

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

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

[0067] The present invention performs double tempering because the flat steel piece has high hardness and brittleness after quenching. The double tempering can adjust its hardness to 35-41HRC and the core hardness to 36-40HRC. Among them, the secondary tempering can not only adjust the hardness but also fully remove stress. The temperature setting of the secondary tempering is 20-30°C lower than the primary tempering temperature (for example, 22°C, 24°C, 26°C, 28°C). This is because the purpose of the secondary tempering is to adjust the hardness and partially remove stress. If the secondary tempering temperature is the same as the primary tempering temperature, the hardness will be greatly reduced, resulting in unqualified hardness. If the secondary tempering temperature is set much lower than the primary tempering temperature, the stress relief effect is not obvious, which is not conducive to fully removing stress.

[0068] As an optional embodiment of the present invention, in steps S2 and S3, the flat steel piece is cooled at an air cooling rate of ≤30°C / h after tempering; and / or the temperature of the flat steel piece is measured at the center of the large plane surface of the flat steel piece.

[0069] The hardness of the flat steel piece after quenching and tempering or after being treated by the heat treatment method of the present invention is 35-41HRC, the hardness of the core is 36-40HRC, and it is uniform and consistent, and the ultrasonic flaw detection meets level 4.

[0070] The flat steel pieces subjected to heat treatment in the present invention can be obtained by smelting, casting, forging, or rolling using any conventional method in the art. For example, they can be obtained through the following process: electric furnace (LF+VD) → casting → annealing → electroslag remelting → hot melt → forging → annealing → flaw detection. After the quenching and tempering treatment of the present invention, they are then milled → ultrasonic flaw detection → inspection to obtain fully qualified delivery steel. The annealing process after forging is as follows, as shown in Figure 17, specifically:

[0071] The forged material at 250-350℃ is heated to 820-840℃ at a heating rate of ≤100℃ / h, kept warm for 3.5-4.5h / 100mm, then cooled to 660-680℃ at a rate of ≤30℃ / h, kept warm for 5.5-6.5h / 100mm, then cooled to ≤400℃ at a rate of ≤30℃ / h and taken out of the furnace.

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

[0073] The main chemical components and their contents (wt%) of the 1.2083 flat steel pieces in the following examples and comparative examples are: 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%, and H: 0.00008%.

[0074] Example 1

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

[0076] 1. Quenching process, the process flow is shown in Figure 1.

[0077] The flat steel was kept at about 350°C for 1 hour, then heated to about 650°C at a heating rate of 60°C / h, kept warm for 3 hours, and finally heated to about 1000°C at a heating rate of 80°C / h, kept warm for 11 hours.

[0078] Air cooling is performed until the temperature at the center of the large flat surface of the flat steel reaches approximately 820°C, followed by alternating water-air cooling. The specific steps of alternating water-air cooling are as follows: cool the flat steel in water for 35 minutes, then remove it from the water and air cool it for 5 minutes, then cool it in water for 10 minutes, then air cool it for 5 minutes, and repeat this cycle of cooling in water for 10 minutes and then air cooling for 5 minutes until the temperature at the center of the large flat surface is controlled at approximately 400°C after removing it from the water and air cooling for 5 minutes, and then air cool it to approximately 150°C at the center of the large flat surface.

[0079] 2. Primary tempering, the process flow is shown in Figure 2.

[0080] The flat steel was kept at about 270°C for 4 hours, then heated to about 560°C at a heating rate of 70°C / h, kept at that temperature for 25.2 hours, and then cooled to room temperature at an air cooling rate of 30°C / h.

[0081] 3. Secondary tempering, the process flow is shown in Figure 3.

[0082] The flat steel after primary tempering treatment was kept at about 300℃ for 3h, then heated to about 530℃ at a heating rate of 70℃ / h, kept at this temperature for 25.2h, and then cooled to room temperature at an air cooling rate of 30℃ / h.

[0083] Example 2

[0084] This embodiment provides a heat treatment method for a 1.2083 flat steel piece, wherein the flat steel piece is a flat steel piece with a thickness of 220 mm, a width of 660 mm, and a length of 5000 mm. The heat treatment method for the flat steel piece includes the following steps:

[0085] 1. Quenching process, the process flow is shown in Figure 1.

[0086] The flat steel was kept at about 350°C for 1 hour, then heated to about 650°C at a heating rate of 60°C / h, kept warm for 3 hours, and finally heated to about 1000°C at a heating rate of 80°C / h, kept warm for 7 hours.

[0087] Air cooling is performed until the temperature at the center of the large flat surface of the flat steel reaches approximately 820°C, followed by alternating water-air cooling. The specific steps of alternating water-air cooling are as follows: cool the flat steel in water for 35 minutes, then remove it from the water and air cool it for 5 minutes, then cool it in water for 10 minutes, then air cool it for 5 minutes, and repeat this cycle of cooling in water for 10 minutes and then air cooling for 5 minutes until the temperature at the center of the large flat surface is controlled at approximately 400°C after removing it from the water and air cooling for 5 minutes, and then air cool it to approximately 150°C at the center of the large flat surface.

[0088] 2. Primary tempering, the process flow is shown in Figure 2.

[0089] The flat steel was kept at about 270°C for 4 hours, then heated to about 560°C at a heating rate of 70°C / h, kept at that temperature for 15.4 hours, and then cooled to room temperature at an air cooling rate of 30°C / h.

[0090] 3. Secondary tempering, the process flow is shown in Figure 3.

[0091] The flat steel after primary tempering treatment was kept at about 300℃ for 3h, then heated to about 530℃ at a heating rate of 70℃ / h, kept at this temperature for 15.4h, and then cooled to room temperature at an air cooling rate of 30℃ / h.

[0092] Comparative Example 1

[0093] This comparative example differs from Example 1 in that air cooling to a temperature of 770-830°C at the center of the large flat surface is omitted, and water-air alternating cooling is performed directly after quenching and holding. The other processes are the same as Example 1. Since the flat steel was not pre-cooled by air cooling, the cooling was too intense, resulting in quenching cracks in the flat steel, which was scrapped.

[0094] Comparative Example 2

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

[0096] The cooling rate from 1000℃ to 700℃ was too slow, resulting in bainite and pearlite transformation. Then water-air alternating cooling was performed, which resulted in insufficient quenching hardness and uneven hardness, ultimately causing the flat steel hardness to fail the test. The detection position and hardness value are shown in Figure 11.

[0097] Comparative Example 3

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

[0099] Since air cooling was replaced by air cooling, the cooling speed of the flat steel was slow in the later stage, and bainite and pearlite transformation occurred, which eventually resulted in the flat steel hardness being low and not meeting the 35-41HRC standard. The hardness was unqualified. The test position and hardness value are shown in Figure 12.

[0100] Comparative Example 4

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

[0102] Since the quenching and holding temperature of 1000℃ was replaced by 950℃, the austenite transformation was insufficient and the microstructure transformation during quenching was insufficient, resulting in low hardness, which did not meet the requirements of 35-41HRC. The hardness was unqualified. The test position and hardness value are shown in Figure 13.

[0103] Comparative Example 5

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

[0105] If the temperature after water-air alternating cooling in the quenching process is controlled at 450℃, then the temperature of 450℃ is too high. The subsequent air cooling from 450℃ will result in a slow cooling rate, which is prone to pearlite transformation. Insufficient cooling will cause the quenching hardness of the flat steel to be low, and ultimately cause the hardness of the flat steel after tempering to be unqualified. The detection position and hardness value are shown in 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 after water-air alternating cooling is too low, which can easily lead to quenching cracks and cause the flat steel to crack and be 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 piece during the quenching process is controlled at 200° C. and 100° C. respectively, and the other processes are the same as those in Example 1.

[0109] If the temperature of the flat steel piece is controlled at 200℃ during the quenching process, 200℃ is too high, resulting in insufficient martensite transformation and insufficient quenching hardness, which ultimately leads to low hardness after tempering and unqualified. The detection position and hardness value are shown in Figure 15.

[0110] If the temperature of the flat steel piece is controlled at 100℃ during the quenching process, 100℃ is too low, which may easily lead to a too low final cooling temperature, resulting in cracking due to martensitic transformation, and causing the flat steel to be scrapped (hardness was not tested).

[0111] Comparative Example 7

[0112] The difference between this comparative example and Example 1 is that the water cooling time in the water-air alternating cooling process in 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 the water-air alternating cooling process is uniformly controlled to 10 minutes, and the air cooling time is controlled to 5 minutes. The other processes are the same as in Example 1. However, due to the initial water cooling time being too short, the cooling rate and intensity are insufficient, which easily causes bainite or pearlite transformation, resulting in poor quenching effect, resulting in low hardness of the flat steel and unqualified. The test positions and hardness values ​​are shown in Figure 16. Furthermore, if each cooling time is uniformly controlled to 10 minutes and the air cooling time is controlled to 5 minutes, the water cooling process must be repeated many times, which increases the difficulty of the operation and is not conducive to workers.

[0114] Test example

[0115] The hardness test adopts the Rockwell hardness test method, and the national standard is GB / T230.3.

[0116] Example 1 Hardness and microstructure detection 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, length 3000mm. According to the technical requirements, the surface hardness and the hardness of both ends of the product are tested. The surface hardness test is in line with 35-41HRC, and the core hardness is 36-40HRC. The specific test data are as follows:

[0118] 1.1. The hardness of 25 points on the large flat surface of the flat steel is tested. The test positions and hardness values ​​are shown in Figure 4. Among them, the lowest hardness is 37.8HRC and the highest hardness is 38.7HRC, both within the range of 35-41HRC, meeting the requirements and passing the hardness test; the hardness extreme difference is 0.9HRC, and the hardness uniformity is good.

[0119] 1 . 2. The hardness of 11 points on each of the two end faces of the flat steel (nozzle end face and riser end face) was tested. The test positions and hardness values ​​are shown in Figure 5. Among them, the lowest hardness is 38.1HRC and the highest hardness is 38.7HRC, both within the range of 35-41HRC, meeting the requirements. The hardness test is qualified, and the extreme hardness difference is 0.6HRC, with good hardness uniformity.

[0120] 1.3. In order to detect whether the flat steel is hardened and whether the core hardness is qualified, the riser end is sawn off 400mm, and then the hardness is tested on the sawn surface. The test position and hardness value are shown in Figure 6. Among them, the lowest hardness is 37.6HRC and the highest hardness is 38.5HRC, both within the range of 35-41HRC, meeting the requirements. The hardness test is qualified, and the hardness extreme difference is 0.9HRC, and the hardness uniformity is good.

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

[0122] 2. Metallographic structure

[0123] After tempering, the microstructure of 1.2083 flat steel is tempered bainite. The martensite obtained by quenching gradually grows into fine cementite particles during tempering. This is a mixture of recrystallized ferrite and cementite, known as tempered bainite. A metallographic microstructure photograph is shown in Figure 7.

[0124] 3. Flat steel appearance and ultrasonic flaw detection

[0125] The flat steel has no quenching cracking and is tested by ultrasonic flaw detection according to GB / T6402-2008. The flaw detection meets level 4 and is qualified.

[0126] The above test data shows that the flat steel is heated to 990-1010°C, air-cooled to 770-830°C at the center of the large flat surface, and then cooled using alternating water-air cooling. After the alternating water-air cooling is completed, the flat steel is air-cooled to 130-160°C at the center of the large flat surface before being tempered in the furnace. This ensures that the flat steel is fully hardened and avoids quenching cracks, ensuring controllable quality. After the flat steel heat treatment (quenching + tempering) in Example 1, the product has qualified hardness and good hardness uniformity.

[0127] 2. Hardness test of flat steel after heat treatment (quenching + tempering) in Example 2:

[0128] 2.1 The hardness of 25 points was tested on a large flat surface of 220mm*660mm*5000mm flat steel. The test positions and hardness values ​​are shown in Figure 8. Among them, the lowest hardness was 37.9HRC and the highest hardness was 39.0HRC, both within the range of 35-41HRC, meeting the requirements and passing the hardness test; the extreme hardness difference was 1.1HRC, and the hardness uniformity was good.

[0129] 2.2 The hardness of 11 points on each of the two end faces (nozzle end face and riser end face) of the 220mm*660mm*5000mm flat steel was tested. The test positions and hardness values ​​are shown in Figure 9. Among them, the lowest hardness is 38.3HRC and the highest hardness is 39.0HRC, both within the range of 35-41HRC, meeting the requirements. The hardness test is qualified, and the extreme hardness difference is 0.7HRC, with good hardness uniformity.

[0130] 2.3 In order to detect whether the 220mm*600mm*5000mm flat steel is hardened and whether the core hardness is qualified, the riser end is sawn off 400mm, and then the hardness is tested on the sawn surface. The test position and hardness value are shown in Figure 10. Among them, the lowest hardness is 37.8HRC and the highest hardness is 38.7HRC, both within the range of 35-41HRC, meeting the requirements. The hardness test is qualified, and the hardness extreme difference is 0.9HRC, and the hardness uniformity is good.

[0131] 3. Comparative Example 1-7 flat steel after heat treatment (quenching + tempering):

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

[0133] Schematic diagrams of the hardness test positions and hardness values ​​of comparative examples 2-7 are shown in Figures 11 to 16. The hardness values ​​of the entire cross-section of the steel are unevenly distributed, most of which are not within the range of 35-41HRC and most of the hardness values ​​are lower than 35HRC. The core hardness values ​​are all lower than 36HRC.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 plastic mold steel, characterized in that: The steps include: S1, sequentially performing quenching and holding treatments and cooling treatments on the 1.2083 flat steel piece to be processed, wherein the quenching and holding temperature is 990-1010°C, and the quenching and holding time is calculated according to the thickness of the flat steel piece, and the holding time is 2.5-3.5 hours per 100 mm; the cooling treatment comprises air cooling the flat steel piece after holding to 770-830°C, then alternating water and air cooling to 370-430°C, and finally air cooling to 130-160°C; S2, performing a tempering treatment on the flat steel piece obtained in step S1; S3, performing secondary tempering treatment on the flat steel piece obtained in step S2; The specifications of the flat steel piece are: thickness of 200-360 mm and width of 650-800 mm.

2. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: The smelting method of the flat steel piece includes electric furnace smelting and electroslag remelting performed in sequence.

3. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: The chemical composition of the flat steel piece includes, by weight percentage, 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%, and the balance is Fe and unavoidable impurities.

4. The heat treatment method for large mirror 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°C for 1 hour, then heated to 630-670°C at a heating rate of ≤60°C / h and kept at this temperature for 3 hours, and finally heated to the quenching and holding temperature of 990-1010°C at a heating rate of ≤80°C / h.

5. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: In step S1, the water-air alternating cooling is to cool the flat steel piece in water for 30-40 minutes, then remove it from the water and air cool it for 4-7 minutes, then cool it in water for 8-12 minutes, then air cool it for 4-7 minutes, and cycle in this manner of cooling it in water for 8-12 minutes, then air cool it for 4-7 minutes until the flat steel piece returns to a temperature of 370-430°C after 4-7 minutes of air cooling.

6. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: In the step S2, the holding temperature of the primary tempering is 500-560°C, and the holding time is calculated according to the thickness of the flat steel piece, and the holding time is 6.5-7.5 hours per 100 mm.

7. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: In the step S3, the holding temperature of the secondary tempering is 470-530°C, and the holding time is calculated according to the thickness of the flat steel piece, that is, 6.5-7.5 hours per 100 mm.

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

9. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: 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°C / h; And / or, the temperature measuring point of the flat steel piece is at the center of the large plane surface of the flat steel piece.

10. The heat treatment method for large-scale mirror-finish plastic mold steel according to any one of claims 1 to 9, characterized in that: The hardness of the flat steel piece after quenching and tempering or treatment by the heat treatment method according to any one of claims 1 to 9 is 35-41HRC, the core hardness is 36-40HRC, and the ultrasonic flaw detection meets level 4.

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