High-purity and high-toughness hot work die steel and preparation method thereof

Abstract

The invention relates to a preparation method of high-purity and high-toughness hot work die steel, which comprises the following steps: S1, electric furnace smelting; s2, refining in a ladle refining furnace; s3, refining in a vacuum refining furnace; s4, performing die casting on an electrode blank; s5, annealing the electrode blank; 6, electroslag remelting is conducted, specifically, an electrode blank is adopted, surface polishing is conducted, a calcium fluoride, aluminum oxide, magnesium oxide and silicon dioxide quaternary slag system is adopted for smelting, the starting point melting speed value of the electroslag smelting steady-state stage is 7-10 kg / min, the end point melting speed value of the electroslag smelting steady-state stage is 5.0-8.0 kg / min, an electroslag ingot is obtained, and then power-off furnace cooling is conducted for 80-100 min; s7, forging is conducted; s8, pretreatment after forging; and S9, solid solution and spheroidizing final treatment. According to the invention, on the basis of common hot work die steel H13, the prepared product has excellent high hardness, high purity and high toughness through fine adjustment of chemical components and special smelting, forging and heat treatment processes. The anti-cracking performance of a mold made of the material is greatly improved, the service life of the mold is prolonged, and meanwhile the use cost of the mold is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of mold steel materials and metallurgical preparation technology, specifically, to a high-purity, high-toughness hot work mold steel and its preparation method. This mold steel is suitable for manufacturing large, precision, complex, and long-life critical industrial molds, especially for die-casting molds for engines and motors, molding molds for new energy vehicle bodies and battery boxes, and high-end molds for automotive body panels, meeting the stringent performance requirements of mold materials in the high-end equipment manufacturing field. Background Technology

[0002] With the rapid development of modern industry, molds, as core equipment for parts processing and forming, have been applied in numerous key industries, including automobiles, electronics, instrumentation, home appliances, aerospace, building materials, motors, and communication equipment. Industry statistics show that approximately 60% to 80% of parts in these fields rely on mold processing and forming, earning molds the reputation of being the "mother of industry." Their technological level directly determines the product quality, production efficiency, and manufacturing costs of downstream industries. By 2025, the global mold market had exceeded US$200 billion, with China contributing over 35% of the share, firmly holding the top position globally and becoming the core driving force for the development of the global mold industry. In terms of industrial spillover effects, the mold industry has a significant pulling effect on related industries. Industry estimates suggest that the ratio of the mold industry's output to its upstream and downstream related industries is approximately 1:100, meaning that for every 100 million yuan increase in the output value of the mold industry, it can directly or indirectly drive a 10 billion yuan increase in the output value of related industries, making it a crucial supporting force for promoting the transformation and upgrading of the manufacturing industry.

[0003] Currently, the global mold industry's total industrial output value continues to grow. With its well-developed industrial system and huge market demand, my country has become one of the world's leading countries in the total production of mold steel, with a large number of mold steel production enterprises, forming a large-scale industrial base.

[0004] H13 steel, as one of the most in-demand and widely used hot work die steels, possesses good comprehensive mechanical properties. However, its purity and high toughness still cannot meet the requirements for high-end molds. Therefore, optimizing the composition and improving the preparation process of H13 steel to develop a high-purity, high-toughness, high-performance hot work die steel has become an urgent need to ensure the independent development of high-end manufacturing. The research and industrialization of such high-performance hot work die steel can not only replace imported materials while meeting customer usage conditions, further increasing the domestic substitution rate of high-end die steel (currently, the import substitution rate of high-end products is about 65%), but also help customers significantly reduce procurement costs (domestic high-end die steel prices are 30%~40% lower than imports), shorten production cycles, and maximize profits. At the same time, it is also conducive to promoting the upgrading of my country's die steel industry structure, enhancing the core competitiveness of my country's die steel industry, and better adapting to the development opportunities of the global die market of $200 billion and the high-frequency iteration needs of emerging industries such as new energy vehicles in China. Summary of the Invention

[0005] The first objective of this invention is to provide a high-purity, high-toughness hot work die steel and its preparation method. Based on the commonly used hot work die steel H13, through fine-tuning of the chemical composition and the adoption of special smelting, forging, and heat treatment processes, the prepared product exhibits excellent high hardness, high purity, and high toughness. Dies made from this material show significantly improved crack resistance, extending their service life while substantially reducing their operating costs.

[0006] To achieve the above-mentioned objectives, this invention provides a method for preparing high-purity, high-toughness hot work die steel, comprising the following steps:

[0007] S1. Electric Furnace Smelting: Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in the electric furnace. When the molten steel temperature is ≥1630℃, oxidation slag is removed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the molten steel temperature is ≥1620℃, aluminum wire is added for deoxidation, and then the steel is tapped.

[0008] The composition of the hot work die steel is as follows (by mass percentage): C 0.35%~0.40%, Mn 0.30%~0.50%, Si 0.70%~0.90%, S≤0.001%, P≤0.010%, Cr 4.90%~5.10%, Mo 1.20%~1.50%, V 0.80%~1.10%, with the remainder being Fe. The residual gas content is as follows: H≤1.5ppm, O≤13ppm, N≤90ppm; residual harmful element Cu≤0.10%, Ti, Sn, and Pb are all ≤0.002%.

[0009] S2. Ladle refining: After electric furnace smelting, the molten steel is transferred to a ladle and hoisted onto the refining furnace base for refining.

[0010] S3. Vacuum refining furnace refining: Empty refining furnace refining: Vacuum refining furnace four-stage inlet pump, final vacuum degree ≤35Pa, vacuum time maintained for more than 18 minutes in both stages, residual gas nitrogen target value ≤80ppm; after degassing, take samples for analysis, and after the composition is qualified, blow argon gas into the hoisting bag.

[0011] S4, In-mold cast electrode blank;

[0012] S5. Annealing of electrode blank;

[0013] S6. Electroslag remelting: The electrode billet obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 7~10 kg / min and the ending melting rate is 5.0~8.0 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 80~100 minutes before being sent to the forging process.

[0014] S7, Forging;

[0015] S8. Pre-treatment after forging;

[0016] S9, solution treatment + spheroidization final treatment.

[0017] Specifically, in step S2, the refining process in the ladle refining furnace is as follows: after smelting in the electric furnace, the molten steel is transferred to the ladle, hoisted onto the refining furnace base, and slag materials CaO, CaF2, and C~Si powder are added to reduce and create primary white slag. Based on the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation is completed, the white slag is removed, and then the molten steel is reheated to a temperature ≥1620℃. CaO, CaF2, C~Si powder, and red bricks are added to create secondary white slag.

[0018] Specifically, in step S4, the specific process of casting the electrode blank is as follows: preheat the ingot mold to 40~60℃, then fill the ingot mold with argon gas for 3~5 minutes for each ingot mold, then remove the argon gas pipe, cover the ingot mold with a lid, and then cast.

[0019] Furthermore, in step S4, the entire casting process is protected by argon gas with an asbestos cloth covering the casting, and the argon gas flow rate is 15~26 m³ / h. 3 The casting time is 4-8 minutes, the electrode blank diameter is 450-910 mm, and the die is removed after 3-5 hours.

[0020] Specifically, in step S5, the specific steps for annealing the electrode blank are as follows: annealing the electrode blank after demolding in step S4 at an annealing temperature of 850~870℃, holding time of 1~1.5min / mm, and furnace cooling to 300~380℃ before removing it from the furnace.

[0021] Specifically, in step S7, the forging process is as follows: the electroslag ingot obtained in step S6 is heated to 1290~1310℃ and held for 40~50h for diffusion homogenization, then furnace cooled to 1240~1260℃ and held for 6~10h, and then the ingot is taken out of the furnace in one firing to complete the Z, X, Y three-axis upsetting and drawing of the billet and drawing it along the Z direction to the finished size.

[0022] Specifically, in step S8, the post-forging pretreatment process is as follows: the forging billet obtained in step S7 is air-cooled to a surface temperature of 680~720℃, and then immersed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: the billet is water-cooled once to a surface temperature of ≤250℃; then it is loaded into a heat treatment furnace, heated to 680~720℃, and held for 20~30h for tempering treatment.

[0023] Specifically, in step S9, the solution treatment and spheroidizing final treatment is performed as follows: the workpiece obtained in step S8 is placed in a heating furnace and heated to 1010~1040℃. After holding at this temperature for 10~20 hours, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled until the maximum surface temperature is ≤250℃. After cooling, the workpiece is placed in an annealing furnace, heated to 820~860℃, held for 10~20 hours, and then furnace cooled to below 500℃ before being air cooled to room temperature.

[0024] Furthermore, in step S9, the furnace cooling rate is 10~20℃ / h.

[0025] The second objective of this invention is to provide a high-purity, high-toughness hot work die steel, prepared using the above-described method. The composition of the hot work die steel, by mass percentage, is: C 0.35%~0.40%, Mn 0.30%~0.50%, Si 0.70%~0.90%, S≤0.001%, P≤0.010%, Cr 4.90%~5.10%, Mo 1.20%~1.50%, V 0.80%~1.10%, with the remainder being Fe. The residual gas content is: H≤1.5ppm, O≤13ppm, N≤90ppm; residual harmful element Cu≤0.10%, Ti, Sn, and Pb are all ≤0.002%.

[0026] The present invention provides a high-purity, high-toughness hot work die steel and its preparation method, which have the following advantages over the prior art:

[0027] (1) Fine-tuning the C and Si composition design concept is the basis for materials to obtain high toughness;

[0028] (2) The optimization of processes such as raw material quality control, secondary refining and secondary degassing of electrode billets provides excellent metallurgical physicochemical conditions for removing inclusions in steel, thus ensuring the purity of the electrode billet.

[0029] (3) Optimization of the electroslag remelting slag system (adding appropriate amounts of magnesium oxide and silicon dioxide) and matching with a reasonable smelting process (arc starting process, steady-state process, feeding process), under the basic condition of argon protection, further isolate air and suppress the generation of silicon burning and aluminum increase, fundamentally reduce the content of total inclusions B and Ds inclusions in the molten steel, and ensure the purity of electroslag ingots.

[0030] (4) Appropriately lowering the Si content in the design suppresses segregation during the smelting process of electrode blanks and electroslag ingots, and alleviates dendritic segregation in the material. Matching the ultra-high temperature homogenization process significantly reduces the compositional segregation of the material, ensuring the uniformity of the material's microstructure.

[0031] (5) On the basis of uniform chemical composition, the three-dimensional forging process of one-fire is applied to greatly refine the austenite grain size. While the grain is strengthened and toughened, it also lays the foundation for the distribution and particle size control of subsequent carbides.

[0032] (6) The implementation of the one-stage spheroidizing process, combined with a suitable cooling rate, suppresses the coarsening of carbide particles.

[0033] In summary, precise control of inclusions and carbides throughout the entire process from smelting to heat treatment is essential for obtaining high-purity, high-toughness hot work die steel. Attached Figure Description

[0034] Figure 1 Metallographic image of the spheroidized annealed microstructure of a high-purity, high-toughness hot work die steel as described in Example 1;

[0035] Figure 2 Metallographic image of the grain size of a high-purity, high-toughness hot work die steel in Example 1;

[0036] Figure 3 Metallographic image of the spheroidized annealed microstructure of a high-purity, high-toughness hot work die steel in Example 2;

[0037] Figure 4 Metallographic image of the grain size of a high-purity, high-toughness hot work die steel in Example 2;

[0038] Figure 5 Metallographic image of the spheroidized annealed microstructure of a high-purity, high-toughness hot work die steel in Example 3;

[0039] Figure 6 Metallographic image of the grain size of a high-purity, high-toughness hot work die steel in Example 3;

[0040] Figure 7 Metallographic image of the spheroidized annealed microstructure of a high-purity, high-toughness hot work die steel as shown in Example 4;

[0041] Figure 8 Metallographic image of the grain size of a high-purity, high-toughness hot work die steel in Example 4;

[0042] Figure 9 Metallographic image of the spheroidized annealed microstructure of a high-purity, high-toughness hot work die steel in Example 5;

[0043] Figure 10 Metallographic image of the grain size of a high-purity, high-toughness hot work die steel as shown in Example 5. Detailed Implementation

[0044] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0045] Example 1

[0046] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 1:

[0047] Table 1

[0048]

[0049] Its preparation method includes the following steps;

[0050] S1, Electric furnace smelting:

[0051] Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in an electric furnace at a temperature of 1670℃. Oxidation slag removal is performed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the temperature of the molten steel reaches 1640℃, aluminum wire is added for deoxidation, and then the steel is tapped.

[0052] S2, Ladle refining furnace refining:

[0053] After smelting in the electric arc furnace, the molten steel is transferred to a ladle, hoisted onto the refining furnace base, and slag materials CaO, CaF2, and C~Si powder are added for reduction to create primary white slag. Based on the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation is completed, the white slag is removed, and then the steel is reheated to a temperature of 1670℃. CaO, CaF2, C~Si powder, and red bricks are added for secondary white slag creation.

[0054] S3, Vacuum refining furnace refining:

[0055] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤35Pa. The vacuum is maintained for at least 15 minutes in both stages, and the residual nitrogen target value is ≤80ppm. After degassing, samples are taken for analysis. Once the composition is qualified, argon gas is blown into the hoisting bag.

[0056] S4. In-mold cast electrode blank:

[0057] The ingot molds were preheated to 50°C, then argon gas was introduced into each mold for 4 minutes. The argon gas pipes were then removed, and the molds were covered before casting. Throughout the casting process, the castings were protected with argon gas shielding using an asbestos cloth, with an argon gas flow rate of 20 m³ / h. 3 / h, casting time is 6min, electrode blank diameter is 550mm, demolding is performed after 4h;

[0058] S5. Electrode blank annealing:

[0059] The electrode blank after demolding in step S4 is annealed at a temperature of 860℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0060] S6, Electroslag Remelting:

[0061] The electrode blank obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 8.5 kg / min and the ending melting rate is 6.5 kg / min, resulting in an electroslag ingot. After cooling in the electric furnace for 100 minutes, it is sent to the forging process.

[0062] S7, Forging:

[0063] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50 hours for diffusion homogenization. Then it is furnace cooled to 1250℃ and held for 8 hours. After that, it is upsetting and drawing in the Z, X and Y directions in one firing and then drawing along the Z direction to the finished size.

[0064] S8. Post-forging pretreatment

[0065] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then placed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: it is water-cooled once to a surface temperature of 230°C; then it is placed in a heat treatment furnace, heated to 700°C, held for 20 hours, and then furnace-cooled to 500°C before being taken out and air-cooled.

[0066] S9, Solution treatment + spheroidizing final treatment:

[0067] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1030°C. After holding at this temperature for 15 hours, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled to a maximum surface temperature of 220°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840°C, held for 15 hours, and then furnace cooled to below 500°C before being air cooled to room temperature. The furnace cooling rate is 10~20°C / h.

[0068] Example 2

[0069] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 2:

[0070] Table 2

[0071]

[0072] Its preparation method includes the following steps;

[0073] S1, Electric furnace smelting:

[0074] Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in an electric furnace at a temperature of 1670℃. Oxidation slag removal is performed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the temperature of the molten steel reaches 1640℃, aluminum wire is added for deoxidation, and then the steel is tapped.

[0075] S2, Ladle refining furnace refining:

[0076] After smelting in the electric arc furnace, the molten steel is transferred to a ladle, hoisted onto the refining furnace base, and reduced with slag materials CaO, CaF2, and C-Si powder to create primary white slag. Based on the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation, the white slag is removed, and then the steel is reheated to a temperature of 1670℃. CaO, CaF2, C-Si powder, and red bricks are added to create secondary white slag.

[0077] S3, Vacuum refining furnace refining:

[0078] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤35Pa. The vacuum is maintained for at least 15 minutes in both stages, and the residual nitrogen target value is ≤80ppm. After degassing, samples are taken for analysis. Once the composition is qualified, argon gas is blown into the hoisting bag.

[0079] S4. In-mold cast electrode blank:

[0080] The ingot molds were preheated to 50°C, then argon gas was introduced into each mold for 4 minutes. The argon gas pipes were then removed, and the molds were covered before casting. Throughout the casting process, the castings were protected with argon gas shielding using an asbestos cloth, with an argon gas flow rate of 20 m³ / h. 3 / h, casting time is 6min, electrode blank diameter is 550mm, demolding is performed after 4h;

[0081] S5. Electrode blank annealing:

[0082] The electrode blank after demolding in step S4 is annealed at a temperature of 860℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0083] S6, Electroslag Remelting:

[0084] The electrode blank obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 8.5 kg / min and the ending melting rate is 6.5 kg / min, resulting in an electroslag ingot. After cooling in the electric furnace for 100 minutes, it is sent to the forging process.

[0085] S7, Forging:

[0086] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50 hours for diffusion homogenization. Then it is furnace cooled to 1250℃ and held for 8 hours. After that, it is upsetting and drawing in the Z, X and Y directions in one firing and then drawing along the Z direction to the finished size.

[0087] S8. Post-forging pretreatment

[0088] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then immersed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: it is water-cooled once to a surface temperature of 220°C; then it is loaded into a heat treatment furnace, heated to 700°C, held for 20 hours, and then furnace-cooled to 500°C before being air-cooled.

[0089] S9, Solution treatment + spheroidizing final treatment:

[0090] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1030°C. After holding at this temperature for 15 hours, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled to a maximum surface temperature of 210°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840°C, held for 15 hours, and then furnace cooled to below 500°C before being air cooled to room temperature. The furnace cooling rate is 10~20°C / h.

[0091] Example 3

[0092] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 3:

[0093] Table 3

[0094]

[0095] Its preparation method includes the following steps;

[0096] S1, Electric furnace smelting:

[0097] Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in an electric furnace at a temperature of 1670℃. Oxidation slag removal is performed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the temperature of the molten steel reaches 1640℃, aluminum wire is added for deoxidation, and then the steel is tapped.

[0098] S2, Ladle refining furnace refining:

[0099] After smelting in the electric arc furnace, the molten steel is transferred to a ladle, hoisted onto the refining furnace base, and reduced with slag materials CaO, CaF2, and C-Si powder to create primary white slag. Based on the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation, the white slag is removed, and then the steel is reheated to a temperature of 1670℃. CaO, CaF2, C-Si powder, and red bricks are added to create secondary white slag.

[0100] S3, Vacuum refining furnace refining:

[0101] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤35Pa. The vacuum is maintained for at least 15 minutes in both stages, and the residual nitrogen target value is ≤80ppm. After degassing, samples are taken for analysis. Once the composition is qualified, argon gas is blown into the hoisting bag.

[0102] S4. In-mold cast electrode blank:

[0103] The ingot molds were preheated to 50°C, then argon gas was introduced into each mold for 4 minutes. The argon gas pipes were then removed, and the molds were covered before casting. Throughout the casting process, the castings were protected with argon gas shielding using an asbestos cloth, with an argon gas flow rate of 20 m³ / h. 3 / h, casting time is 6min, electrode blank diameter is 550mm, demolding is performed after 4h;

[0104] S5. Electrode blank annealing:

[0105] The electrode blank after demolding in step S4 is annealed at a temperature of 860℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0106] S6, Electroslag Remelting:

[0107] The electrode blank obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 8.5 kg / min and the ending melting rate is 6.5 kg / min, resulting in an electroslag ingot. After cooling in the electric furnace for 100 minutes, it is sent to the forging process.

[0108] S7, Forging:

[0109] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50 hours for diffusion homogenization. Then it is furnace cooled to 1250℃ and held for 8 hours. After that, it is upsetting and drawing in the Z, X and Y directions in one firing and then drawing along the Z direction to the finished size.

[0110] S8. Post-forging pretreatment

[0111] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then immersed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: it is water-cooled once to a surface temperature of 215°C; then it is loaded into a heat treatment furnace, heated to 700°C, held for 20 hours, and then furnace-cooled to 500°C before being air-cooled.

[0112] S9, Solution treatment + spheroidizing final treatment:

[0113] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1030°C. After holding at this temperature for 15 hours, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled to a maximum surface temperature of 225°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840°C, held for 15 hours, and then furnace cooled to below 500°C before being air cooled to room temperature. The furnace cooling rate is 10~20°C / h.

[0114] Example 4

[0115] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 4:

[0116] Table 4

[0117]

[0118] Its preparation method includes the following steps;

[0119] S1, Electric furnace smelting:

[0120] Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in an electric furnace at a temperature of 1670℃. Oxidation slag removal is performed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the temperature of the molten steel reaches 1640℃, aluminum wire is added for deoxidation, and then the steel is tapped.

[0121] S2, Ladle refining furnace refining:

[0122] After smelting in the electric arc furnace, the molten steel is transferred to a ladle, hoisted onto the refining furnace base, and reduced with slag materials CaO, CaF2, and C-Si powder to create primary white slag. Based on the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation, the white slag is removed, and then the steel is reheated to a temperature of 1670℃. CaO, CaF2, C-Si powder, and red bricks are added to create secondary white slag.

[0123] S3, Vacuum refining furnace refining:

[0124] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤35Pa. The vacuum is maintained for at least 15 minutes in both stages, and the residual nitrogen target value is ≤80ppm. After degassing, samples are taken for analysis. Once the composition is qualified, argon gas is blown into the hoisting bag.

[0125] S4. In-mold cast electrode blank:

[0126] The ingot molds were preheated to 50°C, then argon gas was introduced into each mold for 4 minutes. The argon gas pipes were then removed, and the molds were covered before casting. Throughout the casting process, the castings were protected with argon gas shielding using an asbestos cloth, with an argon gas flow rate of 20 m³ / h. 3 / h, casting time is 6min, electrode blank diameter is 550mm, demolding is performed after 4h;

[0127] S5. Electrode blank annealing:

[0128] The electrode blank after demolding in step S4 is annealed at a temperature of 860℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0129] S6, Electroslag Remelting:

[0130] The electrode blank obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 8.5 kg / min and the ending melting rate is 6.5 kg / min, resulting in an electroslag ingot. After cooling in the electric furnace for 100 minutes, it is sent to the forging process.

[0131] S7, Forging:

[0132] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50 hours for diffusion homogenization. Then it is furnace cooled to 1250℃ and held for 8 hours. After that, it is upsetting and drawing in the Z, X and Y directions in one firing and then drawing along the Z direction to the finished size.

[0133] S8. Post-forging pretreatment

[0134] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then immersed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: it is water-cooled once to a surface temperature of 220°C; then it is loaded into a heat treatment furnace, heated to 700°C, held for 20 hours, and then furnace-cooled to 500°C before being air-cooled.

[0135] S9, Solution treatment + spheroidizing final treatment:

[0136] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1030°C. After holding at this temperature for 15 hours, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled to a maximum surface temperature of 230°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840°C, held for 15 hours, and then furnace cooled to below 500°C before being air cooled to room temperature. The furnace cooling rate is 10~20°C / h.

[0137] Example 5

[0138] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 5:

[0139] Table 5

[0140]

[0141] Its preparation method includes the following steps;

[0142] S1, Electric furnace smelting:

[0143] Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in an electric furnace at a temperature of 1670℃. Oxidation slag removal is performed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the temperature of the molten steel reaches 1640℃, aluminum wire is added for deoxidation, and then the steel is tapped.

[0144] S2, Ladle refining furnace refining:

[0145] After smelting in the electric arc furnace, the molten steel is transferred to a ladle, hoisted onto the refining furnace base, and reduced with slag materials CaO, CaF2, and C-Si powder to create primary white slag. Based on the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation, the white slag is removed, and then the steel is reheated to a temperature of 1670℃. CaO, CaF2, C-Si powder, and red bricks are added to create secondary white slag.

[0146] S3, Vacuum refining furnace refining:

[0147] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤35Pa. The vacuum is maintained for at least 15 minutes in both stages, and the residual nitrogen target value is ≤80ppm. After degassing, samples are taken for analysis. Once the composition is qualified, argon gas is blown into the hoisting bag.

[0148] S4. In-mold cast electrode blank:

[0149] The ingot molds were preheated to 50°C, then argon gas was introduced into each mold for 4 minutes. The argon gas pipes were then removed, and the molds were covered before casting. Throughout the casting process, the castings were protected with argon gas shielding using an asbestos cloth, with an argon gas flow rate of 20 m³ / h. 3 / h, casting time is 6min, electrode blank diameter is 550mm, demolding is performed after 4h;

[0150] S5. Electrode blank annealing:

[0151] The electrode blank after demolding in step S4 is annealed at a temperature of 860℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0152] S6, Electroslag Remelting:

[0153] The electrode blank obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 8.5 kg / min and the ending melting rate is 6.5 kg / min, resulting in an electroslag ingot. After cooling in the electric furnace for 100 minutes, it is sent to the forging process.

[0154] S7, Forging:

[0155] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50 hours for diffusion homogenization. Then it is furnace cooled to 1250℃ and held for 8 hours. After that, it is upsetting and drawing in the Z, X and Y directions in one firing and then drawing along the Z direction to the finished size.

[0156] S8. Post-forging pretreatment

[0157] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then placed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: it is water-cooled once to a surface temperature of 230°C; then it is placed in a heat treatment furnace, heated to 700°C, held for 20 hours, and then furnace-cooled to 500°C before being taken out and air-cooled.

[0158] S9, Solution treatment + spheroidizing final treatment:

[0159] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1030°C. After holding at this temperature for 15 hours, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled to a maximum surface temperature of 235°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840°C, held for 15 hours, and then furnace cooled to below 500°C before being air cooled to room temperature. The furnace cooling rate is 10~20°C / h.

[0160] To verify the mechanical properties of the plastic mold steel provided by the preparation method of the present invention, the inventors took samples of the mold steel obtained in Examples 1 to 5 for microstructure analysis, such as... Figures 1 to 10 As shown; the performance test data are shown in Tables 6 and 7.

[0161] Table 6

[0162]

[0163] Table 7

[0164]

[0165] Therefore,

[0166] 1. As shown in the figure, the mold material exhibits a uniform microstructure in the spheroidized annealed state, with fine spherical secondary carbides evenly distributed on the ferrite matrix. Rated according to NADCA#207-2011 standards, the grade is ≤AS5. This spheroidized structure facilitates complete dissolution during the austenitization process, reducing the quantity and size of undissolved carbides. Further dispersion precipitation during tempering enhances impact toughness.

[0167] 2. During the quenching process, most alloying element carbides in this mold material fully dissolve into the fine austenite matrix, and then precipitate from the matrix during tempering. Due to the increased specific surface area of ​​the austenite grains from grain refinement, a large amount of Mo, Cr, and V carbides can be uniformly dispersed and precipitated within and at grain boundaries during tempering. The reduced number of carbide particles per unit grain surface helps reduce stress concentration and the initiation and propagation of microcracks, inhibiting the generation and diffusion of intergranular and transgranular fractures. This achieves the strengthening and toughening effect of the mold material. Simultaneously, the material's reasonable elemental ratio, high compositional uniformity, high purity, and fine austenite grains enable it to maintain good toughness while achieving high strength and hardness. The transverse impact energy of a single Charpy V-notch in the mold material's core is ≥22J, and the average Charpy V-notch transverse impact energy is ≥23J. The transverse impact energy of a single unnotched Charpy is ≥340J, and the average unnotched transverse impact energy is ≥355J.

[0168] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A method for preparing high-purity, high-toughness hot work die steel, characterized in that, Includes the following steps, S1. Electric Furnace Smelting: Based on the chemical composition of the steel grade to be smelted, scrap steel and alloy materials are prepared in advance. Scrap steel is smelted in the electric furnace. When the molten steel temperature is ≥1630℃, oxidation slag is removed. After slag removal, alloy materials are added and completely melted. Then, lime and fluorite are added to form alkaline slag. The molten steel in the electric furnace is dephosphorized and desulfurized. When the molten steel temperature is ≥1620℃, aluminum wire is added for deoxidation, and then the steel is tapped. The composition of the hot work die steel is as follows (by mass percentage): C 0.35%~0.40%, Mn 0.30%~0.50%, Si 0.70%~0.90%, S≤0.001%, P≤0.010%, Cr 4.90%~5.10%, Mo 1.20%~1.50%, V 0.80%~1.10%, with the remainder being Fe. The residual gas content is as follows: H≤1.5ppm, O≤13ppm, N≤90ppm; residual harmful element Cu≤0.10%, Ti, Sn, and Pb are all ≤0.002%. S2. Ladle refining: After electric furnace smelting, the molten steel is transferred to a ladle and hoisted onto the refining furnace base for refining. S3. Vacuum refining furnace refining: Empty refining furnace refining: Vacuum refining furnace four-stage inlet pump, final vacuum degree ≤35Pa, vacuum time maintained for more than 18 minutes in both stages, residual gas nitrogen target value ≤80ppm; after degassing, take samples for analysis, and after the composition is qualified, blow argon gas into the hoisting bag. S4, In-mold cast electrode blank; S5. Annealing of electrode blank; S6. Electroslag remelting: The electrode billet obtained in step S5 is polished and smelted using a quaternary slag system of calcium fluoride, alumina, magnesium oxide and silicon dioxide. The starting melting rate of the steady-state stage of electroslag smelting is 7~10 kg / min and the ending melting rate is 5.0~8.0 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 80~100 minutes before being sent to the forging process. S7, Forging; S8. Pre-treatment after forging; S9, solution treatment + spheroidization final treatment.

2. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 1, characterized in that, In step S2, the specific process of refining in the ladle refining furnace is as follows: after smelting in the electric furnace, the molten steel is transferred to the ladle, hoisted onto the refining furnace base, and slag materials CaO, CaF2, and C~Si powder are added to reduce and create primary white slag. According to the deviation between the actual composition and the target composition of the molten steel in the refining furnace base, corresponding alloy materials are added to complete the composition fine-tuning, desulfurization, and deoxidation operations. After the operation is completed, the white slag is removed, and then the molten steel is reheated to a temperature ≥1620℃. CaO, CaF2, C~Si powder, and red bricks are added to create secondary white slag.

3. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 1, characterized in that, In step S4, the specific process of casting the electrode blank is as follows: preheat the ingot mold to 40~60℃, then fill the ingot mold with argon gas for 3~5 minutes for each ingot mold, then remove the argon gas pipe, cover the ingot mold with a lid and then cast.

4. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 3, characterized in that, In step S4, the casting is protected by argon gas with an asbestos cloth covering it throughout the entire casting process. The argon gas flow rate is 15~26 m³ / h. 3 The casting time is 4-8 minutes, the electrode blank diameter is 450-910 mm, and it is demolded after 3-5 hours.

5. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 1, characterized in that, In step S5, the specific steps for annealing the electrode blank are as follows: anneal the electrode blank after demolding in step S4 at an annealing temperature of 850~870℃, a holding time of 1~1.5min / mm, and furnace cooling to 300~380℃ before removing it from the furnace.

6. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 1, characterized in that, In step S7, the specific forging process is as follows: the electroslag ingot obtained in step S6 is heated to 1290~1310℃ and held for 40~50h for diffusion homogenization. Then, it is furnace cooled to 1240~1260℃ and held for 6~10h. Then, it is taken out of the furnace and the Z, X, Y three-axis upsetting and drawing is completed in one firing and the billet is drawn along the Z direction to the finished size.

7. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 1, characterized in that, In step S8, the specific process of post-forging pretreatment is as follows: the forging billet obtained in step S7 is air-cooled to a surface temperature of 680~720℃, and then immersed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: the billet is water-cooled once to a surface temperature of ≤250℃; then it is loaded into a heat treatment furnace, heated to 680~720℃, and held for 20~30h for tempering treatment.

8. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 1, characterized in that, In step S9, the specific operation of the solution treatment and spheroidization final treatment is as follows: the workpiece obtained in step S8 is placed in a heating furnace and heated to 1010~1040℃. After holding at this temperature for 10~20h, it is cooled by direct water cooling to the bottom. After water cooling, it is air cooled until the maximum surface temperature is ≤250℃. After cooling, the workpiece is placed in an annealing furnace, heated to 820~860℃, held for 10~20h, and then furnace cooled to below 500℃ before being air cooled to room temperature.

9. The method for preparing a high-purity, high-toughness hot work die steel as described in claim 8, characterized in that, In step S9, the furnace cooling rate is 10~20℃ / h.

10. A high-purity, high-toughness hot work die steel, characterized in that, The hot work die steel is prepared by the preparation method described in any one of claims 1 to 9. The composition content of the hot work die steel is as follows, by mass percentage: C 0.35%~0.40%, Mn 0.30%~0.50%, Si 0.70%~0.90%, S≤0.001%, P≤0.010%, Cr 4.90%~5.10%, Mo 1.20%~1.50%, V 0.80%~1.10%, with the remainder being Fe. The residual gas content is as follows: H≤1.5ppm, O≤13ppm, N≤90ppm; residual harmful element Cu≤0.10%, Ti, Sn, and Pb are all ≤0.002%.

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