High-hardness and high-thermal-stability hot work die steel and preparation method thereof

Abstract

The invention relates to a preparation method of high-hardness and high-thermal-stability 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: annealing the demolded electrode blank in the step S4 at the annealing temperature of 710-760 DEG C and the heat preservation time of 1-1.5 min / mm, cooling the furnace to 300-380 DEG C, and discharging the electrode blank out of the furnace; s6, electroslag remelting is conducted, specifically, the electrode blank obtained in the step S5 is adopted, surface polishing is conducted, and smelting is conducted; 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, component design is carried out by adopting a high-C, low-Si and high-Mo component design concept, and special smelting, forging and heat treatment processes are added, so that the prepared product has excellent high hardness, high hardness stability and high fatigue resistance. The mold made of the material can better meet the requirements of modern industrial production for high-end molds.

Description

Technical Field

[0001] This invention relates to a mold steel and its preparation method, particularly a hot work mold steel with high hardness and high thermal stability and its preparation method. Background Technology

[0002] With the development of modern industry, the application of molds is becoming increasingly widespread. In products such as automobiles, electronics, instruments, home appliances, aerospace, building materials, motors, and communication equipment, approximately 60%-80% of the parts rely on mold processing for shaping, hence the name "mother of industry." It is estimated that the ratio of mold development to related industries is approximately 1:100, meaning that 100 million yuan in mold development can drive 10 billion yuan in related industries. Summary of the Invention

[0004] The first objective of this invention is to provide a method for preparing high-hardness, high-thermal-stability hot-work die steel. Based on the commonly used hot-work die steel H13, the method employs a composition design concept of high C, low Si, and high Mo, and adds special smelting, forging, and heat treatment processes. This results in a product with excellent high hardness, high hardness stability, and high fatigue resistance. Dies made from this material can better meet the requirements of modern industrial production for high-end dies.

[0005] To achieve the above-mentioned objectives, this invention provides a high-hardness, high-thermal-stability hot work die steel and its preparation method, comprising the following steps:

[0006] S1, electric furnace smelting;

[0007] The composition of hot work die steel is as follows (by mass percentage): C 0.46%–0.52%, Mn 0.35%–0.55%, Si 0.20%–0.40%, S≤0.001%, P≤0.010%, Cr 4.30%–4.60%, Mo 2.70%–3.00%, V 0.50%–0.70%, 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%.

[0008] S2. Ladle refining: After electric furnace smelting, the molten steel is transferred to a ladle and hoisted onto the refining furnace base for refining.

[0009] S3. Vacuum refining furnace refining: The vacuum refining furnace has a four-stage inlet pump, the final vacuum degree is ≤40Pa, and the vacuum time is maintained for more than 15 minutes in both stages. The target value of residual gas nitrogen is ≤80ppm. After degassing, samples are taken for analysis. After the composition is qualified, argon gas is blown into the hoisting bag.

[0010] S4, In-mold cast electrode blank;

[0011] S5. Electrode blank annealing: Anneal the electrode blank after demolding in step S4 at an annealing temperature of 710-760℃ and a holding time of 1-1.5 min / mm. Then, furnace cool to 300-380℃ before removing from the furnace.

[0012] 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.

[0013] S7, Forging;

[0014] S8. Pre-treatment after forging;

[0015] S9, solution treatment + spheroidization final treatment.

[0016] Specifically, in step S1, the electric furnace smelting process is as follows: according to the chemical composition content of the steel grade, 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.

[0017] Specifically, in step S2, after the molten steel is smelted in the electric furnace, it is transferred to a 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.

[0018] Furthermore, in step S2, after the composition fine-tuning, desulfurization, and deoxidation operations are completed, the white slag is removed, and then the steel is reheated to a temperature ≥1620℃. CaO, CaF2, C-Si powder, and red bricks are added to create a secondary white slag.

[0019] Specifically, in step S4, the specific steps for casting the electrode blank are as follows: preheat the ingot mold to 40-60°C, 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.

[0020] 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. 3The casting time is 4-8 minutes, the electrode blank diameter is 450-910 mm, and it is demolded after 3-5 hours.

[0021] Specifically, in step S7, the forging operation is as follows: the electroslag ingot obtained in step S6 is heated to 1290-1310℃ and held for 40-50h for diffusion homogenization. Then, the first heat is used for upsetting and drawing in both Z and X directions, and the second heat is used for upsetting and drawing in both Y and Z directions to prepare an intermediate billet. The intermediate billet is returned to the furnace at a temperature of 1180-1200℃ and held for ≥3h. It is then forged to the finished size.

[0022] Specifically, in step S8, the post-forging pretreatment is performed as follows: the forged billet obtained in step S7 is air-cooled to a surface temperature of 680-720°C, and then placed 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°C; then it is placed in a heat treatment furnace, heated to 1000-1020°C, held for 15-20 hours, air-cooled to a surface temperature of 680-720°C, air-cooled to a surface temperature of ≤500°C, and then placed in a furnace at 700-750°C for 25-40 hours for normalizing and 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 1000-1020°C. After holding at this temperature, it is cooled using a direct water-air alternating cooling method. After the first water cooling, it is air-cooled to a maximum surface temperature of 450-500°C. After the second water cooling, it is air-cooled to a maximum surface temperature of 200-250°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840-880°C, held for 15-30 hours, furnace-cooled to 670-720°C, held for 25-50 hours for spheroidizing annealing, and then furnace-cooled to below 500°C before being air-cooled to room temperature.

[0024] The second objective of this invention is to provide a high-hardness, high-thermal-stability hot work die steel, prepared by the method described above. The composition of the hot work die steel, by mass percentage, is: C 0.46%–0.52%, Mn 0.35%–0.55%, Si 0.20%–0.40%, S≤0.001%, P≤0.010%, Cr 4.30%–4.60%, Mo 2.70%–3.00%, V 0.50%–0.70%, 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%.

[0025] The present invention provides a high-hardness, high-thermal-stability hot work die steel and its preparation method, which have the following advantages over the prior art:

[0026] (1) The high C and high Mo composition design concept is the basis for ensuring the high hardness and high resistance to tempering softening of the material.

[0027] (2) The optimization of raw material quality control, electrode billet smelting and casting processes provides a complete process method for removing B-type and Ds inclusions in steel, ensuring the purity of the electrode billet; and the optimization of matching electroslag remelting slag system (adding silica).

[0028] (3) Optimization of the electroslag remelting slag system (adding an appropriate amount of silicon dioxide) and matching with a reasonable smelting process, fully suppressing the phenomenon of silicon burning and aluminum increase, fundamentally suppressing the production of inclusions B and Ds, and ensuring the purity of electroslag ingots.

[0029] (4) The low Si content design suppresses segregation during the smelting process of electrode blanks and electroslag ingots, significantly alleviating dendritic segregation in high C and high Mo materials. Matching this with an ultra-high temperature homogenization process further reduces material compositional segregation, ensuring the uniformity of the material's microstructure.

[0030] (5) Based on the uniform chemical composition, the three-dimensional forging process and normalizing process are applied to greatly refine the austenite grain size and ensure a high austenite grain size level. Attached Figure Description

[0031] Figure 1 Metallographic image of the banded structure of a high-hardness, high-thermal-stability hot work die steel of Example 1;

[0032] Figure 2 Metallographic image of the grain size of a high-hardness, high-thermal-stability hot work die steel of Example 1;

[0033] Figure 3 Metallographic image of the spheroidized annealed microstructure of a high-hardness, high-thermal-stability hot work die steel of Example 1;

[0034] Figure 4 Metallographic image of the banded structure of a high-hardness, high-thermal-stability hot work die steel in Example 2;

[0035] Figure 5 Metallographic image of grain size of a high-hardness, high-thermal-stability hot work die steel in Example 2;

[0036] Figure 6 Metallographic image of the spheroidized annealed microstructure of a high-hardness, high-thermal-stability hot work die steel in Example 2;

[0037] Figure 7 Metallographic image of the banded structure of a high-hardness, high-thermal-stability hot work die steel in Example 3;

[0038] Figure 8Metallographic image of grain size of a high-hardness, high-thermal-stability hot work die steel in Example 3;

[0039] Figure 9 Metallographic image of the spheroidized annealed microstructure of a high-hardness, high-thermal-stability hot work die steel in Example 3;

[0040] Figure 10 Metallographic image of the banded structure of a high-hardness, high-thermal-stability hot work die steel in Example 4;

[0041] Figure 11 Metallographic image of the grain size of a high-hardness, high-thermal-stability hot work die steel in Example 4;

[0042] Figure 12 Metallographic image of the spheroidized annealed microstructure of a high-hardness, high-thermal-stability hot work die steel in Example 4;

[0043] Figure 13 Metallographic image of the banded structure of a high-hardness, high-thermal-stability hot work die steel in Example 5;

[0044] Figure 14 Metallographic image of grain size of a high-hardness, high-thermal-stability hot work die steel in Example 5;

[0045] Figure 15 Metallographic image of the spheroidized annealed microstructure of a high-hardness, high-thermal-stability hot work die steel, as shown in Example 5. Detailed Implementation

[0046] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0047] Example 1

[0048] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 1:

[0049] Table 1

[0050]

[0051] Its preparation method includes the following steps;

[0052] S1, Electric furnace smelting:

[0053] 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.

[0054] S2, Ladle refining furnace refining:

[0055] 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.

[0056] S3, Vacuum refining furnace refining:

[0057] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤40Pa. The vacuum is maintained for more than 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.

[0058] S4. In-mold cast electrode blank:

[0059] 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;

[0060] S5. Electrode blank annealing:

[0061] The electrode blank after demolding in step S4 is annealed at a temperature of 760℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0062] S6, Electroslag Remelting:

[0063] 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 kg / min and the ending melting rate is 6 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 100 min before being sent to the forging process.

[0064] S7, Forging:

[0065] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50h for diffusion homogenization. Then, it is upset and drawn in the Z and X directions in the first heat, and upset and drawn in the Y and Z directions in the second heat to prepare an intermediate billet. The intermediate billet is returned to the furnace at 1200℃ and held for 3h. It is then forged to the finished size.

[0066] S8. Post-forging pretreatment

[0067] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then subjected to extreme cooling in water to suppress the precipitation of grain boundary carbides: a first water cooling to a surface temperature of 230°C; then it is placed in a heat treatment furnace, heated to 1020°C, held for 20 hours, removed from the furnace and air-cooled to a surface temperature of 700°C, and then air-cooled to a surface temperature of 450°C. Finally, it is placed in a furnace at 720°C and held for 35 hours for normalizing and tempering treatment.

[0068] S9, Solution treatment + spheroidizing final treatment:

[0069] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020°C. After holding at that temperature, it is cooled by direct water-air alternating cooling. After the first water cooling, it is air-cooled to a maximum surface temperature of 480°C. After the second 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 860°C, held for 20 hours, furnace-cooled to 700°C, held for 35 hours for spheroidizing annealing, and then furnace-cooled to 450°C before being air-cooled to room temperature.

[0070] Example 2

[0071] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 2:

[0072] Table 2

[0073]

[0074]

[0075] Its preparation method includes the following steps;

[0076] S1, Electric furnace smelting:

[0077] 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.

[0078] S2, Ladle refining furnace refining:

[0079] 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.

[0080] S3, Vacuum refining furnace refining:

[0081] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤40Pa. The vacuum is maintained for more than 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.

[0082] S4. In-mold cast electrode blank:

[0083] 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;

[0084] S5. Electrode blank annealing:

[0085] The electrode blank after demolding in step S4 is annealed at a temperature of 760℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0086] S6, Electroslag Remelting:

[0087] 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 kg / min and the ending melting rate is 6 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 100 min before being sent to the forging process.

[0088] S7, Forging:

[0089] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50h for diffusion homogenization. Then, it is upset and drawn in the Z and X directions in the first heat, and upset and drawn in the Y and Z directions in the second heat to prepare an intermediate billet. The intermediate billet is returned to the furnace at 1200℃ and held for 3h. It is then forged to the finished size.

[0090] S8. Post-forging pretreatment

[0091] The forged billet obtained in step S7 is air-cooled to a surface temperature of 700°C, and then subjected to extreme cooling in water to suppress the precipitation of grain boundary carbides: a first water cooling to a surface temperature of 240°C; then it is placed in a heat treatment furnace, heated to 1020°C, held for 20 hours, and then air-cooled to a surface temperature of 705°C, followed by air cooling to a surface temperature of 460°C. Finally, it is placed in a furnace at 720°C and held for 35 hours for normalizing and tempering treatment.

[0092] S9, Solution treatment + spheroidizing final treatment:

[0093] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020°C. After holding at that temperature, it is cooled by direct water-air alternating cooling. After the first water cooling, it is air-cooled to a maximum surface temperature of 470°C. After the second 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 860°C, held for 20 hours, furnace-cooled to 700°C, held for 35 hours for spheroidizing annealing, and then furnace-cooled to 450°C before being air-cooled to room temperature.

[0094] Example 3

[0095] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 3:

[0096] Table 3

[0097]

[0098] Its preparation method includes the following steps;

[0099] S1, Electric furnace smelting:

[0100] 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.

[0101] S2, Ladle refining furnace refining:

[0102] 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.

[0103] S3, Vacuum refining furnace refining:

[0104] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤40Pa. The vacuum is maintained for more than 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.

[0105] S4. In-mold cast electrode blank:

[0106] 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;

[0107] S5. Electrode blank annealing:

[0108] The electrode blank after demolding in step S4 is annealed at a temperature of 760℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0109] S6, Electroslag Remelting:

[0110] 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 kg / min and the ending melting rate is 6 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 100 min before being sent to the forging process.

[0111] S7, Forging:

[0112] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50h for diffusion homogenization. Then, it is upset and drawn in the Z and X directions in the first heat, and upset and drawn in the Y and Z directions in the second heat to prepare an intermediate billet. The intermediate billet is returned to the furnace at 1200℃ and held for 3h. It is then forged to the finished size.

[0113] S8. Post-forging pretreatment

[0114] The forged billet obtained in step S7 is air-cooled to a surface temperature of 690°C, and then subjected to extreme cooling in water to suppress the precipitation of grain boundary carbides: a first water cooling to a surface temperature of 250°C; then it is placed in a heat treatment furnace, heated to 1020°C, held for 20 hours, and then air-cooled to a surface temperature of 710°C, followed by air cooling to a surface temperature of 480°C. Finally, it is placed in a furnace at 720°C and held for 35 hours for normalizing and tempering treatment.

[0115] S9, Solution treatment + spheroidizing final treatment:

[0116] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020°C. After holding at that temperature, it is cooled by direct water-air alternating cooling. After the first water cooling, it is air-cooled to a maximum surface temperature of 490°C. After the second 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 860°C, held for 20 hours, furnace-cooled to 700°C, held for 35 hours for spheroidizing annealing, and then furnace-cooled to 450°C before being air-cooled to room temperature.

[0117] Example 4

[0118] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 4:

[0119] Table 4

[0120]

[0121] Its preparation method includes the following steps;

[0122] S1, Electric furnace smelting:

[0123] 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.

[0124] S2, Ladle refining furnace refining:

[0125] 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.

[0126] S3, Vacuum refining furnace refining:

[0127] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤40Pa. The vacuum is maintained for more than 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.

[0128] S4. In-mold cast electrode blank:

[0129] 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;

[0130] S5. Electrode blank annealing:

[0131] The electrode blank after demolding in step S4 is annealed at a temperature of 760℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0132] S6, Electroslag Remelting:

[0133] 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 kg / min and the ending melting rate is 6 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 100 min before being sent to the forging process.

[0134] S7, Forging:

[0135] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50h for diffusion homogenization. Then, it is upset and drawn in the Z and X directions in the first heat, and upset and drawn in the Y and Z directions in the second heat to prepare an intermediate billet. The intermediate billet is returned to the furnace at 1200℃ and held for 3h. It is then forged to the finished size.

[0136] S8. Post-forging pretreatment

[0137] The forged billet obtained in step S7 is air-cooled to a surface temperature of 710°C, and then subjected to extreme cooling in water to suppress the precipitation of grain boundary carbides: a first water cooling to a surface temperature of 240°C; then it is placed in a heat treatment furnace, heated to 1020°C, held for 20 hours, removed from the furnace and air-cooled to a surface temperature of 710°C, and then air-cooled to a surface temperature of 480°C. Finally, it is placed in a furnace at 720°C and held for 35 hours for normalizing and tempering treatment.

[0138] S9, Solution treatment + spheroidizing final treatment:

[0139] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020°C. After holding at that temperature, it is cooled by direct water-air alternating cooling. After the first water cooling, it is air-cooled to a maximum surface temperature of 490°C. After the second water cooling, it is air-cooled to a maximum surface temperature of 250°C. After cooling, the workpiece is placed in an annealing furnace, heated to 860°C, held for 20 hours, furnace-cooled to 700°C, held for 35 hours for spheroidizing annealing, and then furnace-cooled to 450°C before being air-cooled to room temperature.

[0140] Example 5

[0141] A type of hot work die steel, by mass fraction, is basically composed of the elements shown in Table 5:

[0142] Table 5

[0143]

[0144] Its preparation method includes the following steps;

[0145] S1, Electric furnace smelting:

[0146] 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.

[0147] S2, Ladle refining furnace refining:

[0148] 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.

[0149] S3, Vacuum refining furnace refining:

[0150] The vacuum refining furnace uses a four-stage inlet pump to achieve a final vacuum of ≤40Pa. The vacuum is maintained for more than 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.

[0151] S4. In-mold cast electrode blank:

[0152] 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;

[0153] S5. Electrode blank annealing:

[0154] The electrode blank after demolding in step S4 is annealed at a temperature of 760℃ and a holding time of 1.5 min / mm, and then furnace cooled to 340℃ before being removed from the furnace.

[0155] S6, Electroslag Remelting:

[0156] 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 kg / min and the ending melting rate is 6 kg / min to obtain an electroslag ingot. Then, the electric furnace is stopped and cooled for 100 min before being sent to the forging process.

[0157] S7, Forging:

[0158] The electroslag ingot obtained in step S6 is heated to 1295℃ and held for 50h for diffusion homogenization. Then, it is upset and drawn in the Z and X directions in the first heat, and upset and drawn in the Y and Z directions in the second heat to prepare an intermediate billet. The intermediate billet is returned to the furnace at 1200℃ and held for 3h. It is then forged to the finished size.

[0159] S8. Post-forging pretreatment

[0160] The forged billet obtained in step S7 is air-cooled to a surface temperature of 710°C, and then subjected to extreme cooling in water to suppress the precipitation of grain boundary carbides: a first water cooling to a surface temperature of 245°C; then it is placed in a heat treatment furnace, heated to 1020°C, held for 20 hours, and then air-cooled to a surface temperature of 720°C, followed by air cooling to a surface temperature of 440°C. Finally, it is placed in a furnace at 720°C and held for 35 hours for normalizing and tempering treatment.

[0161] S9, Solution treatment + spheroidizing final treatment:

[0162] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020°C. After holding at that temperature, it is cooled by direct water-air alternating cooling. After the first water cooling, it is air-cooled to a maximum surface temperature of 490°C. After the second water cooling, it is air-cooled to a maximum surface temperature of 240°C. After cooling, the workpiece is placed in an annealing furnace, heated to 860°C, held for 20 hours, furnace-cooled to 700°C, held for 35 hours for spheroidizing annealing, and then furnace-cooled to 450°C before being air-cooled to room temperature.

[0163] 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 15 As shown; the performance test data are shown in Tables 6 and 7.

[0164] Table 6

[0165]

[0166] Table 7

[0167]

[0168] Therefore,

[0169] 1, such as Figure 1-15 As shown, 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.

[0170] 2. During the quenching process, increasing the carbon content of most alloying elements in this mold material significantly improves its strength and wear resistance. Simultaneously, a suitable heat treatment process allows alloying carbides to dissolve into the matrix and precipitate diffusely from the matrix during tempering. These carbides, primarily Mo, Cr, and V, play a crucial role in dispersion strengthening during use. The significant increase in Mo content enhances the material's red hardness and high-temperature strength, maintaining a hardness of 45-46 HRC even at a tempering temperature of 635℃. Furthermore, the material's reasonable elemental ratio, high compositional uniformity, high purity, and fine austenite grains ensure good toughness despite its high hardness. The transverse impact energy of a single Charpy V-notch in the mold material's core is ≥15 J, and the average unnotched transverse impact energy is ≥16 J.

[0171] 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 a high-hardness, high-thermal-stability hot work die steel, characterized in that, Includes the following steps, S1, electric furnace smelting; The composition of hot work die steel is as follows (by mass percentage): C 0.46%–0.52%, Mn 0.35%–0.55%, Si 0.20%–0.40%, S≤0.001%, P≤0.010%, Cr 4.30%–4.60%, Mo 2.70%–3.00%, V 0.50%–0.70%, 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: The vacuum refining furnace has a four-stage inlet pump, the final vacuum degree is ≤40Pa, and the vacuum time is maintained for more than 15 minutes in both stages. The target value of residual gas nitrogen is ≤80ppm. After degassing, samples are taken for analysis. After the composition is qualified, argon gas is blown into the hoisting bag. S4, In-mold cast electrode blank; S5. Electrode blank annealing: Anneal the electrode blank after demolding in step S4 at an annealing temperature of 710-760℃ and a holding time of 1-1.5 min / mm. Then, furnace cool to 300-380℃ before removing from the furnace. 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-hardness, high-thermal-stability hot work die steel as described in claim 1, characterized in that, In step S1, the specific process of electric furnace smelting is as follows: according to the chemical composition content 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.

3. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 1, characterized in that, In step S2, the molten steel after electric furnace smelting 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 produce 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.

4. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 3, characterized in that, In step S2, after the composition fine-tuning, desulfurization and deoxidation operations are completed, the white slag is removed, and then the steel is reheated to a temperature ≥1620℃. CaO, CaF2, C-Si powder and red bricks are added to create white slag for a second time.

5. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 1, characterized in that, In step S4, the specific steps for casting the electrode blank are as follows: preheat the ingot mold to 40-60°C, 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.

6. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 5, 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.

7. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 1, characterized in that, In step S7, the specific forging operation is as follows: the electroslag ingot obtained in step S6 is heated to 1290-1310℃ and held for 40-50h for diffusion homogenization. Then, the first heat is used for upsetting and drawing in both Z and X directions, and the second heat is used for upsetting and drawing in both Y and Z directions to prepare an intermediate billet. The intermediate billet is returned to the furnace at a temperature of 1180-1200℃ and held for ≥3h. It is then forged to the finished size.

8. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 1, characterized in that, In step S8, the specific operation of the 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 placed in water for extreme cooling control to suppress the precipitation of grain boundary carbides: water-cooled once to a surface temperature of ≤250℃; then placed in a heat treatment furnace, heated to 1000-1020℃, held for 15-20 hours, taken out of the furnace and air-cooled to a surface temperature of 680-720℃, air-cooled to a surface temperature of ≤500℃, and then placed in a furnace at 700-750℃ for 25-40 hours for normalizing and tempering treatment.

9. The method for preparing a high-hardness, high-thermal-stability hot work die steel as described in claim 1, characterized in that, In step S9, the specific operation of the solution treatment and spheroidizing final treatment is as follows: the workpiece obtained in step S8 is placed in a heating furnace and heated to 1000-1020°C. After holding at this temperature, it is cooled by direct water-air alternating cooling. After the first water cooling, it is air-cooled to a maximum surface temperature of 450-500°C. After the second water cooling, it is air-cooled to a maximum surface temperature of 200-250°C. After cooling, the workpiece is placed in an annealing furnace, heated to 840-880°C, held for 15-30 hours, furnace-cooled to 670-720°C, held for 25-50 hours for spheroidizing annealing, and then furnace-cooled to below 500°C before being air-cooled to room temperature.

10. A high-hardness, high-thermal-stability 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 of the hot work die steel is as follows (by mass percentage): C 0.46%–0.52%, Mn 0.35%–0.55%, Si 0.20%–0.40%, S≤0.001%, P≤0.010%, Cr 4.30%–4.60%, Mo 2.70%–3.00%, V 0.50%–0.70%, 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%.

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