Heat treatment method of large mirror plastic die steel

Abstract

The invention provides a heat treatment method for large mirror surface plastic die steel, and relates to the technical field of steel material heat treatment processing, the heat treatment method comprises the following steps: S1, a 1.2083 flat steel part to be treated is subjected to quenching heat preservation treatment and cooling treatment in sequence, the quenching heat preservation temperature is 990-1010 DEG C, the quenching heat preservation time is calculated according to the thickness of the flat steel part, and the cooling treatment is carried out according to the thickness of the flat steel part; heat preservation is conducted for 2.5-3.5 h every 100 mm; according to the cooling treatment, the flat steel part obtained after heat preservation is subjected to air cooling to be cooled to 770 DEG C-830 DEG C, then water-air alternate cooling is conducted to 370 DEG C-430 DEG C, and finally air cooling is conducted to 130 DEG C-160 DEG C; s2, the flat steel part obtained in the step S1 is subjected to primary tempering treatment; s3, the flat steel part obtained in the step S2 is subjected to secondary tempering treatment; the technical problem that large mirror surface plastic die steel is prone to quenching cracking is solved, the obtained product is high in quality, and all indexes are far higher than standard requirements.

Description

Technical Field

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

[0002] 1.2083 steel is a high-quality mirror plastic mold steel, and also a high-carbon high-chromium martensitic stainless steel. Its chemical composition mainly includes 0.4% carbon, 13% chromium and a small amount of molybdenum, manganese and other elements. It has good polishing, corrosion resistance, and high hardness. The thickness of the flat steel is generally below 200mm. With the large-scale industrial products, the size of 1.2083 steel is gradually large-scale, and the market demand size is gradually expanded to thickness (200-360) mm*width (650-800) mm. 1.2083 steel requires a higher hardness, which must meet 35-41HRC. At the same time, the hardness of the entire cross-section of the steel is required to be uniform. To achieve this hardness, 1.2083 steel needs to be quenched and heat treated. The quenching heat treatment method has a decisive influence on its performance. If the quenching heat treatment method is improper, it is easy to crack during quenching, and the hardness of the core is low, and the qualified rate is low.

[0003] The quenching heat treatment method for 1.2083 steel with a flat steel thickness of less than 200mm is generally oil quenching. When the flat steel thickness exceeds 200mm during oil quenching, the core hardness is insufficient and the hardness is uneven. When the flat steel size is expanded to a thickness of (200-360) mm* 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 and make the steel scrapped.

[0004] In summary, in order to solve the defects and shortcomings of the above-mentioned prior art, 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 purpose of the present invention is to provide a heat treatment method for large-scale mirror plastic mold steel, in particular to provide a quenching heat treatment method for 1.2083 large-size forged flat steel. The method solves the technical problem that large-scale mirror plastic mold steel is prone to quenching cracking, and the obtained product is of high quality, and various indicators are far higher than the 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 plastic mold steel, comprising the following steps:

[0008] S1, the 1.2083 flat steel piece to be processed is subjected to quenching and heat preservation treatment and cooling treatment in sequence, wherein the quenching and heat preservation temperature is 990-1010°C, the quenching and heat preservation time is calculated according to the thickness of the flat steel piece, and the heat preservation time is 2.5-3.5h per 100mm; the cooling treatment is to air cool the heat-insulated flat steel piece to 770°C-830°C, then alternately cool it to 370-430°C by water and air, and finally air cool it 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 200-360mm and width of 650-800mm.

[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 a 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 take it out of the water and air cool it for 4-7 minutes, then cool it in water for 8-12 minutes and then air cool it for 4-7 minutes, and the cycle is repeated in the manner of cooling in water for 8-12 minutes and then air cooling for 4-7 minutes until the flat steel piece returns to a temperature of 370-430°C 4-7 minutes after taking it out of the water and air cooling it.

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

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

[0018] Furthermore, in step S2, the flat steel piece is first kept at 230-270°C for 3-5h, 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 the steps S2 and S3, after the flat steel piece is tempered and heat-insulated, 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 after treatment by 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 mirror plastic mold steel provided by the present invention are:

[0024] 1. The present invention uses appropriate quenching heating temperature and holding time: Under higher temperature and long-term heating, it is beneficial to the formation of austenite in flat steel; the present invention selects appropriate heating temperature 1000±10℃ and heating time to avoid excessive quenching temperature that may cause grain growth, and low quenching temperature that causes incomplete martensite production, 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 precooled to 800±30℃ by air after being held at 1000±10℃, and then alternately cooled by water and air. The present invention precools the 1.2083 steel to 800±30℃ by air before quenching, which can avoid the cracking of the corners of the flat steel caused by too fast cooling of the corners during rapid water cooling at 1000±10℃.

[0026] 3. The present invention adopts 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 "C curve nose" is effectively avoided when the steel is 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 core of the flat steel is transferred to the lower temperature of the surface, 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 water-air alternating cooling, when the temperature of the flat steel is 400±30℃, air cool it to 130-160℃ at the center of the large surface of the flat steel and then put it into the furnace for tempering. Air cooling to 130-160℃ can make the flat steel undergo martensitic transformation below Ms point 230℃, slow down the cooling rate at low temperature stage, thus avoiding stress cracking of the organization and obtaining a good quenched martensitic organization. This cooling method can ensure that the core of 1.2083 steel is fully cooled without quenching cracking, and can obtain a good quenched organization and qualified hardness. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

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

[0034] Figure 6 A schematic diagram of the hardness testing position and hardness value of a 1.2083 flat steel riser end after 400 mm has been cut off, provided for the testing example of Example 1 of the present invention;

[0035] Figure 7 The metallographic structure diagram of 1.2083 flat steel heat treatment (quenching + tempering) provided for the test example of Example 1 of the present invention;

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

[0037] Fig. 9 A schematic diagram of the hardness test positions and hardness values ​​of two end faces of a 1.2083 flat steel provided for the test example of Embodiment 2 of the present invention;

[0038] Fig.10 A schematic diagram of the hardness testing position and hardness value of a 1.2083 flat steel riser end after 400 mm has been cut off, provided for the testing example of Embodiment 2 of the present invention;

[0039] Fig.11 The detection position and hardness value of the 1.2083 flat steel provided in Comparative Example 2 of the present invention;

[0040] Fig.12 The detection position and hardness value of the 1.2083 flat steel provided in Comparative Example 3 of the present invention;

[0041] Fig.13 The detection position and hardness value of the 1.2083 flat steel provided in Comparative Example 4 of the present invention;

[0042] Fig.14 The detection position and hardness value of the 1.2083 flat steel provided in Comparative Example 5 of the present invention;

[0043] Fig.15 The detection position and hardness value of the 1.2083 flat steel provided in Comparative Example 6 of the present invention;

[0044] Fig.16 The detection position and hardness value of the 1.2083 flat steel provided in Comparative Example 7 of the present invention;

[0045] Fig.17 This is a diagram of the annealing process of the flat steel after forging provided by the present invention. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment 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 only 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 ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0047] The endpoints and any values ​​of the ranges disclosed in the present 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0048] In the flat steel production process, the most critical process is heat treatment, and heat treatment (quenching + tempering) is also the technical difficulty of the present invention. In order to meet the hardness requirements and avoid 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 plastic mold steel, comprising the following steps:

[0050] S1, the 1.2083 flat steel piece to be processed is subjected to quenching and heat preservation treatment and cooling treatment in sequence, wherein the quenching and heat preservation 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), and the quenching and heat preservation time is calculated according to the thickness of the flat steel piece, and the heat preservation time is 2.5-3.5h per 100mm; the cooling treatment is to air cool the heat-insulated flat steel piece to 770°C-830°C ( Such as 775℃, 780℃, 785℃, 790℃, 795℃, 800℃, 805℃, 810℃, 815℃, 820℃, 825℃), then water-air alternating 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 as follows: 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] After the flat steel piece of the present invention is heat-insulated at 990-1010°C, it is first air-cooled to 770°C-830°C, and then subjected to subsequent cooling treatment, which can reduce thermal stress and avoid quenching cracking of the flat steel piece. After the heat-insulated at 990-1010°C, if it is air-cooled to too low a temperature, the temperature will be lower than the austenite transformation temperature of 800°C, and the air cooling rate will be too slow to cause martensite transformation, thereby partially causing bainite and pearlite transformation, and no martensite quenching structure can be obtained, resulting in low hardness. If it is air-cooled to too high a temperature after being heat-insulated, quenching cracking at the corners during subsequent water cooling of the steel piece cannot be avoided.

[0055] After air cooling, the present invention continues to perform water-air alternating cooling to ensure that the flat steel does not undergo quenching cracking during quenching while ensuring that quenched martensite is obtained; water-air alternating cooling can ensure that the "C curve nose" is effectively avoided when the steel is rapidly cooled at a 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 conducted to the lower surface temperature, reducing the temperature difference between the core and the surface of the flat steel, thereby avoiding thermal stress cracking during intense cooling.

[0056] After water-air alternating cooling, when the temperature of the flat steel is 400±30℃, it is air-cooled to 130-160℃ at the center of the large surface of the flat steel and then put into the furnace for tempering. Air cooling to 130-160℃ can make the flat steel undergo martensitic transformation below Ms point 230℃, slow down the cooling rate at low temperature stage, thus avoiding stress cracking of the organization and obtaining a good quenched martensitic organization. This cooling method can ensure that the core of 1.2083 steel is fully cooled without quenching cracking, and can obtain a good quenched organization 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 bainite or pearlite transformation is prone to occur, which is not conducive to obtaining a martensitic structure. After water-air alternating cooling, air cooling is adopted, and the speed of air cooling is between water cooling and air cooling. It will not cause cracking due to intense water cooling, nor will bainite or pearlite transformation occur due to too slow air cooling. Here, air cooling can ensure that there will be no cracking and the martensitic structure transformation can be obtained smoothly. The air cooling speed of the present invention is defined by the speed of the fan, and the speed of the fan is set at 1450r / min, ranging 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 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%. In the flat steel piece, C, Si, Mn, and Cr are the main components, and the other components are residual components, and the lower the content of the residual components is, the better. Preferably, Si: 0.40-0.60%, Mn: 0.70-0.90%.

[0060] As an optional embodiment of the present invention, in the 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 kept 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 take it out of the water and air cool it for 4-7 minutes, then cool it in water for 8-12 minutes and then air cool it for 4-7 minutes, and the cycle is repeated in the manner of cooling in water for 8-12 minutes and then air cooling for 4-7 minutes until the temperature of the flat steel piece is controlled to be 370-430°C after 4-7 minutes of air cooling.

[0062] If the water-to-air alternating cooling time is too long, the cooling is too intense and quenching cracks are likely to occur; if the water-to-air alternating cooling time is too short, the cooling rate is too slow and it is likely to hit the "C curve nose" and cause bainite or pearlite to appear, 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 be reduced quickly; if the cooling rate is too slow, it is likely to hit the "C curve nose" and cause bainite or pearlite to appear, affecting the quenching effect; if the air-cooling time is too short, the heat in the core of the flat steel has not yet returned to the surface, which is equivalent to being water-cooled all the time, and the cooling is too intense and quenching cracks are likely to occur. The preferred embodiment of the present invention selects a method of performing water-to-air alternating cooling in which the initial water-cooling time is relatively long and the subsequent water-cooling time is relatively short. 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 is too intense and quenching cracks are likely to occur.

[0063] As an optional embodiment of the present invention, in the step S2, the insulation 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 insulation time is calculated according to the thickness of the flat steel piece, and the insulation time is 6.5-7.5h per 100mm.

[0064] As an optional embodiment of the present invention, in step S3, the insulation 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 insulation time is calculated according to the thickness of the flat steel piece, and the insulation time is 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°C (such as 235°C, 240°C, 245°C, 250°C, 255°C, 260°C, 265°C) for 3-5h, 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 two temperings because the flat steel piece has high hardness and brittleness after quenching. The hardness can be adjusted to 35-41HRC and the core hardness can be adjusted to 36-40HRC by performing two temperings. Among them, the secondary tempering can not only adjust the hardness, but also fully remove the stress. The temperature setting of the secondary tempering is 20-30°C lower than the primary tempering temperature (such as 22°C, 24°C, 26°C, 28°C). This is because the purpose of the secondary tempering is to adjust the hardness and remove the 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 too much lower than the primary tempering temperature, the stress removal effect is not obvious, which is not conducive to fully removing the stress.

[0068] As an optional embodiment of the present invention, in the 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 piece subjected to heat treatment in the present invention can be obtained by smelting, casting, forging or rolling by any conventional method in the art, for example, it can be obtained by the following process: electric furnace (LF+VD) → casting rod → annealing → electroslag remelting → hot delivery → forging → annealing → flaw detection, after quenching and tempering treatment of the present invention, milling → ultrasonic flaw detection → inspection to obtain fully qualified delivery steel. The annealing process after forging is as follows, see Fig.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 is further described in detail below with reference to specific embodiments.

[0073] The main chemical components and contents (wt%) of the 1.2083 flat steel pieces in the following embodiments 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%, 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 a flat steel piece with a thickness of 360 mm, a width of 800 mm, and a length of 3000 mm. The heat treatment method for the flat steel piece comprises the following steps:

[0076] 1. Quenching process, process flow see Figure 1 .

[0077] The flat steel is 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 is about 820°C, and then water-air alternating cooling is performed. The specific steps of water-air alternating cooling are: cool the flat steel in water for 35 minutes, then cool it in water for 5 minutes, then cool it in water for 10 minutes, then cool it in air for 5 minutes, and then cool it in water for 10 minutes and then cool it in air for 5 minutes, and then cycle the process of cooling in water for 10 minutes and then cooling it in air for 5 minutes until the temperature of the flat steel is controlled at about 400°C at the center of the large area of ​​the flat steel after 5 minutes of air cooling, and then air cooling it to about 150°C at the center of the large area of ​​the flat steel.

[0079] 2. One tempering, process flow see 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, process flow see Figure 3 .

[0082] The flat steel after primary tempering is kept at about 300°C for 3h, then heated to about 530°C at a heating rate of 70°C / h, kept at this temperature for 25.2h, and then cooled to room temperature at an air cooling rate of 30°C / 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 comprises the following steps:

[0085] 1. Quenching process, process flow see Figure 1 .

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

[0087] Air cooling is performed until the temperature at the center of the large flat surface of the flat steel is about 820°C, and then water-air alternating cooling is performed. The specific steps of water-air alternating cooling are: cool the flat steel in water for 35 minutes, then cool it in water for 5 minutes, then cool it in water for 10 minutes, then cool it in air for 5 minutes, and then cool it in water for 10 minutes and then cool it in air for 5 minutes, and then cycle the process of cooling in water for 10 minutes and then cooling it in air for 5 minutes until the temperature of the flat steel is controlled at about 400°C at the center of the large area of ​​the flat steel after 5 minutes of air cooling, and then air cooling it to about 150°C at the center of the large area of ​​the flat steel.

[0088] 2. One tempering, process flow see 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, process flow see Figure 3 .

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

[0092] Comparative Example 1

[0093] The difference between this comparative example and Example 1 is 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 directly performed after quenching and heat preservation, and other processes are the same as Example 1. Since the flat steel is not pre-cooled by air cooling, the cooling is too intense, and quenching cracks occur in the flat steel, resulting in the scrapping of the flat steel.

[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 Example 1.

[0096] The cooling rate from 1000℃ to 700℃ is too slow, resulting in the transformation of bainite and pearlite. Then water-air alternating cooling is performed, and the quenching hardness is insufficient and uneven, which eventually causes the flat steel hardness to fail to meet the requirements. The detection position and hardness value are shown in Fig.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] As 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. See the test position and hardness value for details. Fig.12 .

[0100] Comparative Example 4

[0101] The difference between this comparative example and Example 1 is that the quenching and 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 not sufficient, and the microstructure transformation during quenching was not sufficient, resulting in low hardness, which did not meet the requirements of 35-41HRC. The hardness was unqualified. See the test position and hardness value for details. Fig.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° C. and 350° C. respectively, and the other processes are the same as those in Example 1.

[0105] If the temperature after water-air alternating cooling in the quenching process is controlled at 450℃, then the temperature at 450℃ is too high. If air cooling is subsequently started from 450℃, the cooling rate will be slow, and pearlite transformation is likely to occur. Insufficient cooling will cause the quenching hardness of the flat steel to be low, and ultimately the hardness of the flat steel after tempering will be unqualified. See the test position and hardness value for details. Fig.14 .

[0106] If the temperature after water-air alternating cooling in the quenching process is controlled at 350°C, then 350°C is too low, and the temperature of the flat steel is too low after water-air alternating cooling, 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 of Example 1.

[0109] The temperature of the flat steel piece is controlled at 200℃ during the quenching process. However, 200℃ is too high, which leads to 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 Fig.15 .

[0110] If the temperature of the flat steel piece is controlled at 100°C by air cooling during the quenching process, 100°C 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. The other processes are the same as those in Example 1.

[0113] 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. The other processes are the same as in Example 1. However, due to the short first water cooling time, insufficient cooling speed and intensity, bainite or pearlite transformation is likely to occur, resulting in poor quenching effect, resulting in low hardness of the flat steel and unqualified. The detection position and hardness value are shown in FIG. Fig.16 And each time is uniformly controlled to 10 minutes, and the air cooling time is controlled to 5 minutes, so water cooling must be repeated many times, which increases the difficulty of operation and is not conducive to workers' operation.

[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. Test the hardness of 25 points on the large flat surface of the flat steel. See the test positions and hardness values ​​for details. Figure 4 Among them, the lowest hardness is 37.8HRC and the highest hardness is 38.7HRC, both are within the range of 35-41HRC, meet the requirements, and the hardness test is qualified; the extreme hardness difference is 0.9HRC, and the hardness uniformity is good.

[0119] 1 . 2. Test the hardness of 11 points on each of the two ends of the flat steel (the nozzle end and the riser end). See the test positions and hardness values ​​for details. Figure 5 Among them, the lowest hardness is 38.1HRC and the highest hardness is 38.7HRC, both are within the range of 35-41HRC, meet the requirements, pass the hardness test, and the extreme hardness difference is 0.6HRC, with good hardness uniformity.

[0120] 1.3. In order to check 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 are within the range of 35-41HRC, which meets the requirements, the hardness test is qualified, and the extreme hardness 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] 1.2083 flat steel has a tempered martensite structure after tempering. The martensite obtained by quenching gradually grows into fine cementite particles during tempering, that is, a mixture of recrystallized ferrite and cementite, namely tempered martensite. Figure 7 .

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

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

[0126] The above test data show that the flat steel is heated to 990-1010°C, air-cooled to 770-830°C at the center of the large flat surface of the flat steel, and then cooled by water and air alternation. After the water and air alternation cooling is completed, the flat steel is air-cooled to 130-160°C at the center of the large surface of the flat steel and then tempered in the furnace. It can ensure that the flat steel is fully quenched and avoids the occurrence of quenching cracks, and the quality is controllable. Example 1 After the flat steel is heat treated (quenching + tempering), the product hardness is qualified and the hardness uniformity is good.

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

[0128] 2.1 Test the hardness of 25 points on a large flat surface of 220mm*660mm*5000mm flat steel. See the test position and hardness value for details. Figure 8 Among them, the lowest hardness is 37.9HRC and the highest hardness is 39.0HRC, both are within the range of 35-41HRC, meet the requirements, and the hardness test is qualified; the extreme hardness difference is 1.1HRC, and the hardness uniformity is good.

[0129] 2.2 Test 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. The test positions and hardness values ​​are shown in Fig. 9 Among them, the lowest hardness is 38.3HRC and the highest hardness is 39.0HRC, both are within the range of 35-41HRC, which meets the requirements, the hardness test is qualified, and the extreme hardness difference is 0.7HRC, and the hardness uniformity is good.

[0130] 2.3 In order to test 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 Fig.10Among them, the lowest hardness is 37.8HRC and the highest hardness is 38.7HRC, both are within the range of 35-41HRC, which meets the requirements, the hardness test is qualified, and the extreme hardness 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] Comparative Example 2-7 Hardness test position and hardness value diagram Figure 11-Figure 16 The hardness values ​​of the entire cross-section of the steel are unevenly distributed, most of them are not in the range of 35-41HRC, and most of the hardness values ​​are lower than 35HRC, and 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 by 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, the 1.2083 flat steel piece to be processed is subjected to quenching and heat preservation treatment and cooling treatment in sequence, wherein the quenching and heat preservation temperature is 990-1010°C, the quenching and heat preservation time is calculated according to the thickness of the flat steel piece, and the heat preservation time is 2.5-3.5h per 100mm; the cooling treatment is to air cool the heat-insulated flat steel piece to 770°C-830°C, then alternately cool it to 370-430°C by water and air, and finally air cool it 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 200-360mm and width of 650-800mm.

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 comprises 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 for 3 hours, and finally heated to a 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 take it out of the water and air cool it for 4-7 minutes, then cool it in water for 8-12 minutes and then air cool it for 4-7 minutes, and cycle in the manner of cooling in water for 8-12 minutes and then air cooling for 4-7 minutes until the flat steel piece returns to a temperature of 370-430°C 4-7 minutes after it is out of the water and air cooled.

6. The heat treatment method for large mirror plastic mold steel according to claim 1, characterized in that: In the step S2, the insulation temperature of the primary tempering is 500-560°C, and the insulation time is calculated according to the thickness of the flat steel piece, and the insulation 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 insulation temperature of the secondary tempering is 470-530°C, and the insulation time is calculated according to the thickness of the flat steel piece, and the insulation time 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 3-5h, 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 the steps S2 and S3, after the flat steel piece is tempered and heat-insulated, 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 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 after being treated 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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