Hot working die steel and preparation method and device thereof

Abstract

The invention discloses a hot working die steel and a preparation method and device thereof. The raw material components of the hot working die steel include, by mass percentage, C 0.33% to 0.41%, Si 0.80% to 1.10%, Mn 0.20% to 0.5%, S≤0.002%, P≤0.015%, Cr 5.00% to 5.50%, Mo1.2% to 1.50%, V 0.40% to 0.60%, impurity elements less than or equal to 0.04% and the balance Fe. The hot working die steel prepared by the invention has a qualified rate of spheroidized structure, banded structure, etc. of 99%, a qualified rate of flaw detection of ≥98%, and various performance indicators are superior to the prior art.

Description

Technical Field

[0001] The invention relates to the technical field of metal materials, and in particular to hot working die steel and a preparation method and device thereof. Background Art

[0002] CTH13 steel has the advantages of high strength, tempering stability, good toughness and thermal fatigue resistance, and is the preferred material to replace H13. Since the working environment of hot working molds is at a high temperature of more than 600℃, creep softening will inevitably occur (after long-term use, the matrix metal of the mold material itself will recover and transform, carbides will gather and grow, and the hardness and strength will decrease compared with the initial state of installation and use, and the performance will gradually deteriorate). Therefore, compared with cold working molds and plastic molds, the life of hot working molds is always unsatisfactory. The optimization and improvement of alloys for hot working die steel at home and abroad have never stopped. For example, the improvement and optimization of H13 steel abroad has formed new steel grades such as Dievar, DH21, and TQ1. The improvement direction is mostly to reduce Cr and Si and increase Mo; there is also an alloying idea of ​​reducing C and increasing Si and Mn; adding micro-alloying elements such as Nb and rare earth to improve its performance.

[0003] Large-scale, highly isotropic and long life are the development trends of die-casting molds. In order to meet such development requirements, on the one hand, it is necessary to improve the traditional process and adopt high-purity, high-uniformity spheroidized structure control technology, high production standards, and fine material supply. On the other hand, it is necessary to strengthen the development of new steel grades to obtain more stable matrix and carbide at high temperatures above 600°C. Summary of the invention

[0004] The purpose of the present invention is to provide a hot working die steel and a method and device for preparing the same. It is necessary to study the purity of molten steel and steel ingots, the uniformity and volatility of the composition, the uniformity of the spheroidized structure, and the size and distribution of the precipitated phase. From the perspective of key technologies for spheroidized structure control and heat treatment of steel, the uniformity and refinement of the spheroidized structure of die-casting die steel are solved. Combined with the service performance evaluation and failure behavior research of die-casting die steel, the key process for the stable production of high-end die-casting die steel is proposed. Finally, a high-end hot working die steel with mature technology, stable quality, high pass rate, less waste, and a stable yield rate at a high level is developed. To achieve the above purpose, the present invention provides the following technical solutions:

[0005] A hot working die steel, wherein the raw material components of the hot working die steel include, by mass percentage, C 0.33%-0.41%, Si 0.80%-1.10%, Mn 0.20%-0.5%, S≤0.002%, P≤0.015%, Cr 5.00%-5.50%, Mo1.2%-1.50%, V 0.40%-0.60%, impurity elements less than or equal to 0.04%, and the balance Fe.

[0006] Furthermore, the hardness of the hot working die steel is HRC 44-46, and the V-type impact strength is greater than or equal to 12J.

[0007] Furthermore, the grain size of the hot working die steel is greater than or equal to 7;

[0008] The spheroidized tissues are AS1-AS9; the banded tissues are BS1-BS4.

[0009] A method for preparing hot working die steel, the method comprising:

[0010] The hot working die steel molten steel is refined in EBT, LF furnace and VD furnace in sequence, and then cast to obtain a steel billet, wherein after the LF furnace refining is completed, niobium iron alloy and zirconium iron alloy are added to the molten steel; the raw material component content of the hot working die steel includes, by mass percentage, C 0.33% to 0.41%, Si 0.80% to 1.10%, Mn 0.20% to 0.5%, S≤0.002%, P≤0.015%, Cr 5.00% to 5.50%, Mo1.2% to 1.50%, V 0.40% to 0.60%, impurity elements less than or equal to 0.04% and the balance Fe;

[0011] The steel billet is electroslag remelted under gas protection to obtain an electroslag ingot and an anti-oxidation coating is applied to the surface of the electroslag ingot;

[0012] The electroslag ingot coated with an anti-oxidation coating is homogenized and subjected to multiple upsetting and forging to obtain a forging;

[0013] After the forging is air-cooled to 250-300° C. in the center of the large surface of the forging, annealing and ultra-fine heat treatment are performed to obtain hot working die steel.

[0014] Furthermore, the addition amounts of the ferroniobium alloy and the ferrozirconium alloy are respectively:

[0015] The mass ratio of ferroniobium alloy to molten steel is 0.1:1000-0.15:1000;

[0016] The mass ratio of ferrozirconium alloy: molten steel is 0.2:1000-0.3:1000.

[0017] Further, the niobium content in the ferroniobium alloy is 65-70%;

[0018] The ferrozirconium alloy is sponge ferrozirconium, and the zirconium content is 95-99%.

[0019] Furthermore, the components of the anti-oxidation coating include metal oxides containing manganese and microcrystalline glass.

[0020] Furthermore, the process conditions of the upsetting forging include a start forging temperature of ≥1050°C, a stop forging temperature of ≥850°C; the number of upsetting forging times of ≥3 times; and a height-to-diameter ratio of the upsetting forging lengthening of 2.2-2.5.

[0021] Furthermore, the homogenization process conditions include 1240±5° C. and greater than or equal to 30 hours.

[0022] Furthermore, after the forging is cooled to 200-250° C. in the center of the large surface of the forging, annealing and ultra-fine heat treatment are performed, including:

[0023] After the forging is air-cooled to 250-300℃ in the center of the large surface of the forging, it is air-cooled until the center of the large surface of the module is 200-250℃;

[0024] Hot-charge the forgings after air cooling and air cooling into the annealing furnace for soft annealing;

[0025] The annealed forgings are then subjected to ultra-fine treatment.

[0026] Furthermore, the softening annealing process includes hot charging the forgings after air cooling and air cooling to 250-300°C and keeping it warm for 6-8 hours; heating it to 830-850°C for a second time and keeping it warm for 7.5+0.25Q hours, where Q is the charge; and then cooling it to below 250°C, with a cooling rate of less than or equal to 30°C / h.

[0027] Furthermore, the ultra-fine treatment includes solution cooling and spheroidizing annealing.

[0028] A preparation device for hot working die steel, the device comprising:

[0029] The refining module is used to refine the molten steel in EBT, LF furnace and VD furnace in sequence and then cast the steel billet. After the LF furnace refining is completed, ferroniobium alloy and ferrozirconium alloy are added to the molten steel.

[0030] A first acquisition module is used for electroslag remelting the steel billet under gas protection, obtaining an electroslag ingot and coating an anti-oxidation coating on the surface of the electroslag ingot;

[0031] The second acquisition module is used to homogenize the electroslag ingot with an anti-oxidation coating on its surface and perform multiple upsetting and forging to obtain forgings;

[0032] The third acquisition module is used to air-cool the forging to 250-300° C. in the center of the large surface of the forging, and then perform annealing and ultra-fine heat treatment to obtain hot working die steel.

[0033] Technical effects and advantages of the present invention:

[0034] The products produced by the present invention have a qualified rate of spheroidized structure, banded structure, etc. of 99%, a qualified rate of flaw detection of ≥98%, and various performance indicators are superior to the prior art.

[0035] Produced in accordance with relevant technical agreements, the smelting adopts the "electric furnace + AOD + LF + mold casting + electroslag remelting" process technology. Under the premise of ensuring that the chemical composition meets the technical requirements and process internal control requirements, and adopting the core technology of this project, high-temperature homogenization treatment can basically eliminate the small-particle eutectic carbide in the ingot, partially dissolve the large-particle eutectic carbide, and significantly improve the composition segregation, thereby making the carbides in the steel small and uniform, and the lateral impact performance is significantly improved.

[0036] On the basis of ensuring that various components, performance and ultrasonic flaw detection meet the technical requirements, the module flaw detection and various performance qualification rates are guaranteed to be ≥98%.

[0037] Other features and advantages of the present invention will be described in the following description, and partly become obvious from the description, or be understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a heating homogenization process diagram in a specific embodiment of the present invention;

[0039] Figure 2 is an annealing process diagram in a specific embodiment of the present invention;

[0040] Figure 3 This is a temperature rise process diagram before solution cooling in a specific embodiment of the present invention;

[0041] Figure 4 It is a process diagram of spheroidizing annealing after solution cooling in a specific embodiment of the present invention;

[0042] Figure 5a This is a microscopic image of a forging without homogenization treatment in a specific embodiment of the present invention;

[0043] Figure 5b This is a microscopic image of a forging after homogenization treatment in a specific embodiment of the present invention;

[0044] Figure 6a This is a microscopic image of a forging without air cooling treatment in a specific embodiment of the present invention;

[0045] Figure 6b This is a microscopic image of a forging after air cooling treatment in a specific embodiment of the present invention;

[0046] Figure 7a This is a microscopic image of a forging without ultrafine heat treatment in a specific embodiment of the present invention;

[0047] Figure 7b This is a microscopic image of a forging after ultrafine heat treatment in a specific embodiment of the present invention;

[0048] Figure 8a This is a microscopic image of a forging in a specific embodiment of the present invention that is not treated by the method of the present invention;

[0049] Figure 8b This is a microscopic picture of a forging after using the method of the present invention in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0050] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] In order to solve the deficiencies of the prior art, the present invention discloses a hot working die steel. The raw material component contents of the hot working die steel include, by mass percentage, C 0.33%-0.41%, Si 0.80%-1.10%, Mn 0.20%-0.5%, S≤0.002%, P≤0.015%, Cr 5.00%-5.50%, Mo1.2%-1.50%, V 0.40%-0.60%, impurity elements less than or equal to 0.04% and the balance Fe.

[0052] The hardness of hot working die steel is HRC 44-46, and the V-type impact strength is greater than or equal to 12J.

[0053] The grain size of hot working die steel is greater than or equal to 7;

[0054] The spheroidized tissues are AS1-AS9; the banded tissues are BS1-BS4.

[0055] The preparation method of the hot working die steel comprises the following steps:

[0056] Step 1: Refining the molten steel of hot working die steel in EBT, LF furnace and VD furnace in sequence, and then casting to obtain a steel billet, wherein after the LF furnace refining is completed, adding niobium iron alloy and zirconium iron alloy to the molten steel; the component content of the hot working die steel includes, by mass percentage, C 0.33% to 0.41%, Si 0.80% to 1.10%, Mn 0.20% to 0.5%, S≤0.002%, P≤0.015%, Cr 5.00% to 5.50%, Mo1.2% to 1.50%, V 0.40% to 0.60%, impurity elements less than or equal to 0.04% and the balance Fe.

[0057] In a specific embodiment of the present invention, pig iron and alloy materials are batched according to the component content of hot working die steel, and melted and smelted in an EBT furnace. The process is as follows: the P and S contents in the materials are controlled during batching to ensure that the decarburization amount during electric furnace smelting is ≥0.20%, and the oxygen blowing temperature is to ensure sufficient degassing and removal of inclusions. After satisfying the condition of P≤0.008%, high-temperature steel is tapped, and high-temperature slag is removed after tapping. The slag amount entering the LF furnace is controlled to be about 300kg (slag thickness ≤30mm), so that the LF furnace can fully achieve low SiO 2 The calcium-aluminum slag system creates conditions.

[0058] The steel tapping temperature is ≥1650℃. The conditions for entering the LF furnace are as follows: temperature ≥1550℃, chemical composition C and Cr meet the lower limit of the specification, and Si ≤0.20%.

[0059] LF furnace: After the LF furnace enters the tank, 100m of aluminum wire is fed under the thin slag for strong deoxidation. According to the requirements of the final slag composition control, refined slag, lime and steel slag are added in batches according to the thickness of the slag entering the tank. After alloying, sampling is fully analyzed, Al is controlled at 0.05-0.06%, and the chemical composition is adjusted to meet the internal control requirements. After steel is tapped, the slag is removed to a slag thickness of 60-100mm. During the slag removal process, pay attention to the argon flow rate to prevent the molten steel from being exposed and inhaled.

[0060] After LF furnace refining is completed, the electrode rod is fed with Al wire at 0.10-0.15% to adjust Al.

[0061] At the end of the LF furnace and before the LF furnace is hung, niobium iron alloy and zirconium iron alloy are added, and the added amounts are: the mass ratio of niobium iron alloy to molten steel is 0.1:1000-0.15:1000; the mass ratio of zirconium iron alloy to molten steel is 0.2:1000-0.3:1000. The niobium content in the niobium iron alloy is 70%; the zirconium iron alloy is sponge ferrozirconium, and the zirconium content is 99%.

[0062] VD furnace: Before VD treatment, feed calcium silicon wire 250m / furnace, and keep the time ≥20min under the ultimate vacuum ≤67Pa. Ladle pouring temperature T=1530-1540℃. Ensure the vacuum degree and ultimate vacuum holding time, blow argon with large flow rate and strong stirring under the ultimate vacuum degree, fully contact the steel and slag, and improve the Al content in the steel. 2 O 3 The mass transfer kinetic conditions of inclusions can reduce the level of type B and type D oxide inclusions and improve the cleanliness of steel.

[0063] Pouring: Soft argon blowing time ≥ 15min, pouring when the temperature reaches the ladle temperature. Use argon protection during pouring to control the gas content in the steel. Control the chemical composition stability of the steel. The chemical composition of high-quality hot working die steel is strictly implemented in accordance with internal control standards to control the chemical composition within a small fluctuation range.

[0064] Step 2: The steel billet is electroslag remelted under gas protection to obtain an electroslag ingot, comprising the following steps:

[0065] The slag used can be used after being baked at 700-800℃ for ≥6 hours. The slag is made of slag system CaF 2 :Al 2 O 3 =70:30(%). Since the ALD constant melting rate protective atmosphere electroslag furnace does not use carbon electrodes to polarize the slag, but directly uses metal electrodes for remelting, all slag materials must be used, and fluorite (CaF 2 ) other than self-prepared slag.

[0066] Using slag system CaF 2 :Al 2 O 3 =70:30(%), 5-10 kg MgO can be added. Strictly implement the process parameters of different crystallizers and different electrode rods.

[0067] Use this steel or similar steel grades as base pads.

[0068] The outlet water temperature of the normal smelting crystallizer is controlled at 40-58℃.

[0069] Determine the amount of Al powder / particles and Ca-Si powder according to the composition of the electrode rod.

[0070] According to the needs of different electroslag ingots, different melting speed ranges are controlled respectively. The melting speed of Ф550mm (2.5t) ingot is controlled to be ≤7.5kg / min, the melting speed of Ф650mm ingot is controlled to be ≤8.5kg / min, the melting speed of Ф730mm (7t) ingot is controlled to be 10.5-9.0kg / min, and the melting speed of Ф850mm (10t) ingot is controlled to be ≤13kg / min.

[0071] An anti-oxidation coating is applied to the surface of the electroslag ingot; the thickness of the anti-oxidation coating is 1-2 mm; the components of the anti-oxidation coating include metal oxide containing manganese and microcrystalline glass.

[0072] Step 3: The electroslag ingot coated with the anti-oxidation coating is homogenized and then subjected to multiple upsetting and forging to obtain a forging;

[0073] In a specific embodiment of the present invention, Figure 1 As shown, the homogenization process comprises the following steps:

[0074] The electroslag ingot with anti-oxidation coating on the surface is first heated to 600-650℃, kept warm for 5-7h, then heated to 830-870℃ at a heating rate of less than 50℃ / h, kept warm for 5-7h, then heated to 900-1300℃ at a heating rate of less than 100℃ / h, kept warm for 5-7h, and then heated to 1235-1245℃ at a heating rate of less than 100℃ / h, and kept warm for 30 hours or more.

[0075] When heating the electroslag ingot with anti-oxidation coating on the surface, pay attention to ensure uniform material temperature and prevent yin and yang sides.

[0076] In a specific embodiment of the present invention, the electroslag ingot coated with an anti-oxidation coating after homogenization is subjected to multiple upsetting and forging, comprising the following steps:

[0077] a. The starting forging temperature of the electroslag ingot with anti-oxidation coating on the surface after homogenization treatment is ≥1050℃, and the stopping forging temperature is ≥850℃; above the starting forging temperature, the material deformation resistance is low, ensuring sufficient thermal deformation; stopping forging temperature, to prevent low-temperature uneven deformation and forging cracking, requires upsetting and drawing times ≥3 times.

[0078] b. After the electroslag ingot is taken out of the furnace, it is upset to 1 / 2 height (the length of the whole electroslag ingot), that is, half the height of the steel ingot (if the ingot is not long enough, it can be properly stretched to make the height-to-diameter ratio ≥ 2.0), and then stretched to a rough piece with a height-to-diameter ratio of about 2.2-2.5 and returned to the furnace. The calcination temperature of the rough piece is 1240±5℃, and the heat preservation time is not less than 30 hours.

[0079] c. After the remelting, the rough pieces are upset to 1 / 2 height, and then forged into finished products and forging blanks. When the module thickness exceeds 500mm, FM forging is adopted. FM forging method refers to the forging method without Mannesmann effect. The deformation of FM method is essentially to use the asymmetric deformation of the upper flat anvil and the lower platform. The friction resistance of the lower platform to the forging is large. The deformation is carried out gradually from top to bottom, so that the tensile stress is moved to the contact surface between the deformed body and the lower platform, and the compressive stress in the core of the forging is increased.

[0080] d. The heating temperature of the last fire is 1180±10℃, and the deformation of the last fire is ≥40%.

[0081] e. Accurately control the size and shape, pay attention to the stop forging temperature. The final forging temperature for finishing is ≥800℃.

[0082] f. Clean the surface iron oxide in time during the forging process to ensure the surface quality. At the end of forging, it is required to hot cut the head and tail, cut off the rotten materials and burrs at both ends, prevent the end face from tearing, and reduce the risk of cracking during water cooling after forging. Before the homogenization treatment using this method, the cracking ratio of the forgings was about 20-30%. After the homogenization treatment using this method, no cracking occurred, and the product qualification rate reached 100%. After forging, water cooling requires a dedicated person to monitor the temperature on site with a temperature measuring device to ensure timely loading and annealing in the furnace to prevent cracking.

[0083] like Figure 5a and 5b The micromorphology of the forgings without homogenization treatment and after homogenization treatment is shown respectively. According to the comparison, the internal quality of the forgings is improved after homogenization treatment, and the spheroidized structure is more uniform, thereby improving the impact toughness performance. The use of gradient heating and long-term homogenization treatment above 1240°C and repeated upsetting and drawing forging can improve the isotropy of the steel, the uniformity of the composition and spheroidized structure, and the grain refinement, which is conducive to improving the ability to resist crack initiation, thereby significantly improving the service life of CTH13. The use of gradient heating and forging process (the above process), reducing the S and P content, protective atmosphere electroslag remelting, high-temperature homogenization to eliminate the segregation of spheroidized structure, the impact performance and wear resistance of the forgings after high-temperature homogenization treatment are significantly better than those of the conventional H13 materials used as the research basis, with high purity, uniform spheroidized structure, fine grains, high strength and high toughness, suitable for harsh die-casting mold working environment.

[0084] Table 1. Performance data of forgings after homogenization and without homogenization

[0085]

[0086] According to the performance data of the forgings after homogenization treatment and without homogenization treatment shown in Table 1, the hardness and banding of the forgings meet the requirements, but the spheroidized structure of the forgings after homogenization treatment is AS3-AS4, and the spheroidized structure of the forgings without homogenization treatment is AS8-AS9. Although both are within the agreement range, the homogenization treatment can improve the spheroidized structure to a better AS3-AS4 (agreement range AS1-AS9); the impact of the forgings after homogenization treatment is 16.5-18.5J, with an average of 17.4J, and the impact of the forgings without homogenization treatment is 12-15J, with an average of 13.45J (agreement: average value ≥16J, minimum value ≥12J), indicating that the impact of the forgings after homogenization treatment is significantly better than that of the forgings without homogenization treatment.

[0087] Step 4: After the forging is air-cooled to 250-300°C in the center of the large surface of the forging, it is air-cooled until the center of the large surface of the module is 200-250°C; the forging after air cooling and air cooling treatment is hot loaded into the annealing furnace for softening annealing.

[0088] In a specific embodiment of the present invention, the forging is air-cooled, and the air-cooling comprises the following steps:

[0089] The forgings with a temperature of about 200-250℃ in the center of the large surface are heated to 250-300℃ and kept warm for 6-8 hours; the temperature is raised to 830-850℃ for the second time at a heating rate of more than 10h, and kept warm for 7.5+0.25Q hours, where Q is the furnace load in tons; then the temperature is cooled to below 250℃ at a cooling rate of less than or equal to 30℃ / h to obtain the forged module. In order to cool quickly, prevent the precipitation of carbides at the grain boundaries, lay the foundation for spheroidizing annealing, and optimize the microscopic spheroidized structure; air cooling after forging requires a dedicated person to monitor the temperature of the forgings on site with a temperature measuring device to ensure timely furnace annealing to prevent cracking. Air cooling to about 250-300℃ in the center of the large surface of the module (the core temperature is expected to be about 500℃), the product room plans the air cooling time after forging according to the center temperature of the large surface of the module.

[0090] like Figure 6a and 6b The microstructures of forgings without and with air cooling treatment are shown respectively. According to the comparison, the internal quality of forgings after air cooling treatment is improved, and the spheroidized structure is more uniform. Through this process, the problems of optimizing the purity of materials, inadequate elimination of chain carbides in the annealed state, uneven particle size and distribution of spheroidal carbides, and large fluctuations in transverse core impact toughness are solved, and high-quality die steel that meets the quality requirements of long-life hot working dies is produced.

[0091] After air cooling, it is changed to air cooling until the center area of ​​the large surface of the forging is about 200-250℃ (the on-site air cooling time is determined according to the software simulation results), and then hot-charged into the annealing furnace for softening annealing. Figure 2 As shown, the annealing process is as follows:

[0092] The forgings with a temperature of about 200-250°C in the center area of ​​the large surface are heated to 250-300°C and kept warm for 6-8 hours; the temperature is heated to 830-850°C for a second time at a heating rate greater than 10h, and kept warm for 7.5+0.25Q hours, where Q is the furnace load in tons; then the temperature is cooled to below 250°C at a cooling rate less than or equal to 30°C / h to obtain the forged module.

[0093] Step 5: After annealing, the forgings are subjected to ultra-fine treatment to obtain hot working die steel forging modules. When loading the furnace, the modules should be placed sideways to avoid deformation. The gap between every two modules is ≥300mm. The ultra-fine treatment includes solution cooling and spheroidizing annealing. During this process, a dedicated person is required to monitor the temperature of the forgings on site with a temperature measuring device to ensure the effect of solution cooling and timely loading the furnace for annealing to prevent cracking.

[0094] In a specific embodiment of the present invention, the solid solution cooling adopts medium cooling to prevent the risk of explosion during the cooling process; the cooling rate is accelerated to prevent the precipitation of grain boundary carbides as much as possible to form a unidirectional martensitic spheroidized structure and reduce the bainite content, wherein the medium is a water-based quenching medium, including a polyalkylene glycol (PAG) polymer quenching medium, and its cooling rate is between water and oil.

[0095] In a specific embodiment of the present invention, the solution cooling process comprises the following steps:

[0096] like Figure 3 As shown, the forged module is heated to 790-810°C and kept warm for more than or equal to 6 hours; the temperature is secondarily raised to 920-940°C at a heating rate of more than 2h, and kept warm for more than or equal to 1D+1 hour, where D is the thickness or diameter of the forged module in decimeters;

[0097] After the temperature reaches 930℃, it is taken out of the furnace and air-cooled to about 850℃ in the center of the large surface of the module; solid solution cooling is carried out, and the medium is quickly cooled to 200-250℃ in the center of the module and the medium is taken out. The determination of the center temperature is controlled by the time of entering the medium. The on-site personnel will formulate the post-forging medium cooling time according to the center temperature of the large surface of the module. After the medium is taken out and the temperature is returned to the highest point of 230℃ by air cooling, and then the temperature is lowered to 120-150℃ in the center of the large surface of the module, the furnace is immediately loaded and the following spheroidizing annealing process curve is continued.

[0098] In a specific embodiment of the present invention, Figure 4 As shown, the process of the spheroidizing annealing process includes the following steps:

[0099] The module after solution cooling treatment is heated to 200-250°C and kept at this temperature for 5 hours; the temperature is raised to 520-540°C at a heating rate of more than 3 hours, kept at this temperature for 8-10 hours, and then air-cooled to room temperature;

[0100] The temperature is raised to 810-830°C for three times at a heating rate of 10h or more, and the temperature is kept for 8.5+0.25Q hours or more, wherein Q is the furnace load in tons; the temperature is lowered to 700-720°C for two times at a cooling rate of 15-30°C / h and kept for 15h; the temperature is lowered to ≤300°C for three times at a cooling rate of 25°C / h or less, and then taken out of the furnace to obtain a hot working die steel forging module.

[0101] Ultrafine heat treatment is performed to improve the ultrafine heat treatment process, improve the microspheroidization structure of the ultra-large module, and increase the impact energy qualification rate. In a specific embodiment of the present invention, Figure 7a and 7bThe microstructures of forgings without and after ultrafine heat treatment are shown respectively. According to the comparison, the internal quality of the forgings is improved after ultrafine heat treatment, and the spheroidized structure is more uniform, thereby improving the impact toughness performance.

[0102] Table 2 Performance data of forgings after ultrafine heat treatment and without ultrafine heat treatment

[0103]

[0104] According to the performance data of forgings after and without ultrafine heat treatment shown in Table 2, the spheroidized structure is improved from the original AS9-AS10 (without ultrafine heat treatment) to AS3-AS4 (with ultrafine heat treatment), which is conducive to the full dissolution of the austenitization process, thereby reducing the number and size of undissolved carbides. The impact of forgings after ultrafine heat treatment is 15-18J, with an average value of ≥16J, and the impact of forgings without ultrafine heat treatment, solution cooling and annealing is 11-14J, with an average value of <16J (agreement: average value ≥16J, minimum value ≥12J), indicating that the impact of forgings after ultrafine heat treatment is significantly improved.

[0105] Step 6: The hot working die steel forging module obtained after ultra-fine heat treatment is sampled and tested according to the protocol requirements.

[0106] The present invention has no particular limitation on the parameters and steps of the sampling and testing, and the parameters and steps of the sampling and testing well known to those skilled in the art may be used, and those skilled in the art may select and adjust them according to actual production conditions, product requirements and quality requirements.

[0107] The present invention also discloses a hot working die steel preparation device, characterized in that the device comprises:

[0108] The refining module is used to refine the molten steel in EBT, LF furnace and VD furnace in sequence and then cast the steel billet. After the LF furnace refining is completed, ferroniobium alloy and ferrozirconium alloy are added to the molten steel.

[0109] A first acquisition module is used for electroslag remelting the steel billet under gas protection, obtaining an electroslag ingot and coating an anti-oxidation coating on the surface of the electroslag ingot;

[0110] The second acquisition module is used to perform homogenization treatment and multiple upsetting and forging on the electroslag ingot with an anti-oxidation coating on the surface to obtain a forging. The electroslag ingot with an anti-oxidation coating on the surface is subjected to multiple upsetting and forging and homogenization treatment to obtain a forging;

[0111] The third acquisition module is used to air-cool the forging to 250-300° C. in the center of the large surface of the forging, and then perform annealing and ultra-fine heat treatment to obtain hot working die steel.

[0112] Regarding the device in the above embodiment, the specific manner in which each module performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0113] In a specific embodiment of the present invention, according to the above-mentioned method for preparing the hot working die steel forging module, 3 furnaces of CTH13 special high-end hot working die steel were produced in 2021 using the method described in the present invention, and the inspection results were all qualified: the spheroidized structure was AS3-AS5 (protocol: AS1-AS9) (Table 5), impact 17-22J (protocol: average value ≥16J, minimum value ≥12J), hardness (HRC) 45-46 (Table 3), CTH13 non-metallic inclusions (Table 4) all met the requirements of the agreement and were high-quality tool and die steel.

[0114] Table 3 Hardness, impact

[0115]

[0116] Table 4CTH13 non-metallic inclusions

[0117]

[0118] Table 5 Spheroidized structure, banding and grain size of CTH13

[0119] Grain size Spheroidized tissue Ribbon Measured 9 AS3 BS2 protocol ≥7 AS1-AS9 BS1-BS4

[0120] according to Figure 8a and 8b The microscopic morphologies of CTH3 mold steel that was not treated by the method of the present invention (prior art) (no homogenization treatment, air cooling and spheroidizing annealing process and no homogenization treatment) and that was prepared by the treatment method of the present invention are shown. According to the comparison, it can be seen that the internal quality of CTH3 mold steel is improved after being treated by the method of the present invention, and the spheroidized organizational structure is more uniform, thereby improving the impact toughness performance.

[0121] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing hot working die steel, It is characterized in that The method comprises, The hot working die steel molten steel is refined in EBT, LF furnace and VD furnace in sequence, and then cast to obtain a steel billet, wherein after the LF furnace refining is completed, niobium iron alloy and zirconium iron alloy are added to the molten steel; the raw material component content of the hot working die steel includes, by mass percentage, C 0.33% to 0.41%, Si 0.80% to 1.10%, Mn 0.20% to 0.5%, S≤0.002%, P≤0.015%, Cr 5.00% to 5.50%, Mo1.2% to 1.50%, V 0.40% to 0.60%, impurity elements less than or equal to 0.04% and the balance Fe; The steel billet is electroslag remelted under gas protection to obtain an electroslag ingot and an anti-oxidation coating is applied to the surface of the electroslag ingot; The electroslag ingot coated with an anti-oxidation coating is subjected to homogenization treatment and multiple upsetting and forging to obtain a forging; the homogenization process conditions include 1240±5°C for greater than or equal to 30 hours; After the forging is air-cooled to 250-300°C in the center of the large surface of the forging, annealing and ultra-fine heat treatment are performed to obtain hot working die steel, wherein: After the forging is air-cooled to 250-300°C in the center of the large surface of the forging, annealing and ultra-fine heat treatment are performed to obtain hot working die steel, including: After the forging is air-cooled to 250-300℃ in the center of the large surface of the forging, it is then air-cooled until the center of the large surface of the module is 200-250℃; The forgings after air cooling and air cooling are hot charged into the annealing furnace for softening annealing; the softening annealing process comprises heating the forgings after air cooling and air cooling to 250-300°C and keeping the temperature for 6-8 hours; heating the forgings to 830-850°C for a second time and keeping the temperature for 7.5+0.25Q hours, wherein Q is the charge; and then cooling the temperature to below 250°C, with a cooling rate of less than or equal to 30°C / h; The annealed forgings are then subjected to ultra-fine treatment.

2. A method for preparing hot working die steel according to claim 1, It is characterized in that The addition amounts of the ferroniobium alloy and the ferrozirconium alloy are respectively: The mass ratio of ferroniobium alloy to molten steel is 0.1:1000-0.15:1000; The mass ratio of ferrozirconium alloy: molten steel is 0.2:1000-0.3:1000.

3. A method for preparing hot working die steel according to claim 1 or 2, It is characterized in that The niobium content in ferroniobium alloy is 65-70%; The ferrozirconium alloy is sponge ferrozirconium, and the zirconium content is 95-99%.

4. The method for preparing hot working die steel according to claim 1, It is characterized in that The components of the anti-oxidation coating include metal oxide containing manganese and microcrystalline glass.

5. The method for preparing hot working die steel according to claim 1, It is characterized in that The process conditions of the upsetting forging include: the start forging temperature is ≥1050°C, the stop forging temperature is ≥850°C; the number of upsetting forging is ≥3 times; and the height-to-diameter ratio of the upsetting forging is 2.2-2.

5.

6. The method for preparing hot working die steel according to claim 1, It is characterized in that The ultra-fine treatment includes solution cooling and spheroidizing annealing.

7. A device for implementing the method for preparing hot working die steel according to any one of claims 1 to 6, It is characterized in that The device comprises, A refining module is used to refine the molten steel in EBT, LF furnace and VD furnace in sequence, and then cast the molten steel to obtain a steel billet, wherein after the LF furnace refining is completed, ferroniobium alloy and ferrozirconium alloy are added to the molten steel; the raw material component content of the hot working die steel includes, by mass percentage, C0.33% to 0.41%, Si 0.80% to 1.10%, Mn 0.20% to 0.5%, S≤0.002%, P≤0.015%, Cr 5.00% to 5.50%, Mo1.2% to 1.50%, V 0.40% to 0.60%, impurity elements less than or equal to 0.04% and the balance Fe; A first acquisition module is used for electroslag remelting the steel billet under gas protection, obtaining an electroslag ingot and coating an anti-oxidation coating on the surface of the electroslag ingot; The second acquisition module is used to homogenize the electroslag ingot with an anti-oxidation coating on the surface and perform multiple upsetting and forging to obtain a forging; the homogenization process conditions include 1240±5°C and greater than or equal to 30 hours; The third acquisition module is used to air-cool the forging to 250-300°C in the center of the large surface of the forging, and then perform annealing and ultra-fine heat treatment to obtain hot working die steel, wherein: After the forging is air-cooled to 250-300°C in the center of the large surface of the forging, annealing and ultra-fine heat treatment are performed to obtain hot working die steel, including: After the forging is air-cooled to 250-300℃ in the center of the large surface of the forging, it is then air-cooled until the center of the large surface of the module is 200-250℃; The forgings after air cooling and air cooling are hot charged into the annealing furnace for softening annealing; the softening annealing process comprises heating the forgings after air cooling and air cooling to 250-300°C and keeping the temperature for 6-8 hours; heating the forgings to 830-850°C for a second time and keeping the temperature for 7.5+0.25Q hours, wherein Q is the charge; and then cooling the temperature to below 250°C, with a cooling rate of less than or equal to 30°C / h; The annealed forgings are then subjected to ultra-fine treatment.

8. A hot working die steel, It is characterized in that The hot working die steel is prepared by the preparation method according to any one of claims 1 to 6, wherein the raw material component contents of the hot working die steel include, by mass percentage, C 0.33% to 0.41%, Si 0.80% to 1.10%, Mn 0.20% to 0.5%, S≤0.002%, P≤0.015%, Cr 5.00% to 5.50%, Mo1.2% to 1.50%, V 0.40% to 0.60%, impurity elements less than or equal to 0.04%, and the balance Fe.

9. The hot working die steel according to claim 8, It is characterized in that The hardness of hot working die steel is HRC 44-46, and the V-type impact strength is greater than or equal to 12J.

10. The hot working die steel according to claim 8, It is characterized in that The grain size of hot working die steel is greater than or equal to 7; The spheroidized tissues are AS1-AS9; the banded tissues are BS1-BS4.

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