A high-strength and high-toughness hot forging die steel and its preparation method

Abstract

This invention discloses a high-strength, high-toughness hot forging die steel and its preparation method. The chemical composition and mass percentage of the die steel are as follows: C: 0.48–0.55%, Si: 0.20–0.40%, Mn: 0.30–0.50%, Cr: 3.60–4.00%, Mo: 3.60–4.00%, V: 0.30–0.60%, W: 0.30–0.60%, Nb: 0.05–0.15%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable impurities. The preparation method includes smelting, electroslag remelting under a protective atmosphere, high-temperature homogenization, forging, ultrafine treatment, and spheroidizing annealing. By optimizing the alloy composition, this invention improves the tempering stability and thermal stability of the die steel while ensuring high strength, high hardness, and high toughness, meeting the requirements for complex, precision, and long-life hot forging dies.

Description

Technical Field

[0001] This invention belongs to the field of mold steel technology, specifically relating to a high-strength, high-toughness hot forging die steel and its preparation method. Background Technology

[0002] Hot forging dies are essential process equipment in aerospace, automotive, machinery, and defense industries, and represent one of the typical application areas for hot work die steel. Abroad, the cost of hot forging dies accounts for 8-15% of forging costs, while in China, it can reach as high as 30-40%. The quality of the steel used for hot forging dies directly affects the production cost of forgings.

[0003] High-heat-resistant hot forging dies must withstand temperatures of 600–700°C on their surfaces during use, while the die cavity endures triaxial compressive stress and experiences intense friction between the cavity surface and the hot blank. With the upgrading of the manufacturing industry, the strength of hot-forged parts is continuously increasing, and hot forging equipment is constantly being upgraded. This places increasingly higher demands on the comprehensive performance of die materials, including strength, toughness, thermal stability, and resistance to thermal fatigue. Improper material selection will inevitably lead to early failures such as die cavity collapse, exceeding tolerances, and die cracking, affecting production progress and increasing production costs.

[0004] H13 steel is a representative of medium-heat-resistant hot work die steels. Due to its good strength, toughness, wear resistance, and thermal fatigue resistance, it is widely used in hot forging dies, hot extrusion dies, and non-ferrous metal die casting dies. Its optimal service temperature is below 550℃, which does not meet the requirements for producing high-strength and high-wear-resistant forgings. 3Cr2W8V is a representative of high-heat-resistant hot work die steels. It has high high-temperature strength and strong thermal stability, and is widely used in press forging dies, extrusion dies, and ferrous metal die casting dies. Its optimal service temperature is below 650℃. However, it has low toughness and poor thermal fatigue performance, and is prone to die chipping and brittle fracture. It has high requirements for die design, machining accuracy, and assembly clearance, which limits its application to some extent.

[0005] Therefore, developing a high-strength and high-toughness hot forging die steel with excellent comprehensive performance and suitable for working at temperatures above 600℃, and its preparation method, is of great practical significance. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a high-strength, high-toughness hot forging die steel and its preparation method. By optimizing the alloy composition, while ensuring high strength, high hardness, and high toughness, the tempering stability and thermal stability of the die steel are improved, thus meeting the requirements for the use of complex, precision, and long-life hot forging dies.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0008] A high-strength, high-toughness hot forging die steel has the following chemical composition and mass percentage: C: 0.48–0.55%, Si: 0.20–0.40%, Mn: 0.30–0.50%, Cr: 3.60–4.00%, Mo: 3.60–4.00%, V: 0.30–0.60%, W: 0.30–0.60%, Nb: 0.05–0.15%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable impurities.

[0009] The steel composition design for high-strength, high-toughness hot forging dies in this invention conforms to the principle of reducing Cr and increasing Si to reduce segregation, improve thermal fatigue resistance and thermal stability, and appropriately reduces V content while increasing W and Nb content to improve high-temperature strength, wear resistance and tempering stability. The mechanism of action of some elements is as follows:

[0010] (1) C: C content is the only factor affecting the hardenability of steel, and a relatively high C content is the basis for ensuring the strength, hardness and wear resistance of the material. The C content in the steel of this invention is controlled within the range of 0.48% to 0.55%.

[0011] (2) Cr: In the steel of this invention, the main function of Cr is to ensure the hardenability of the steel. Higher hardenability is a necessary prerequisite for producing large-section mold materials. 23 Secondary C6 carbides can improve the high-temperature strength and wear resistance of steel, but their stability is not high, and they gradually coarsen as the tempering temperature increases, reducing the tempering hardness. Therefore, the Cr content is controlled within the range of 3.60–4.00%.

[0012] (3) Mo and W: In the steel of this invention, the main functions of Mo and W are to enhance the secondary hardening effect during tempering and improve hot strength and red hardness. W is better than Mo in improving wear resistance and thermal stability, but the effect of increasing W content on reducing the toughness of mold steel is more obvious. Therefore, when the mold material requires high strength and also has requirements for toughness, a high Mo and low W composition design is usually adopted. Thus, the Mo content is controlled in the range of 3.60 to 4.00%, and the W content is controlled in the range of 0.30 to 0.60%.

[0013] (4) V and Nb: Both V and Nb are strong carbide-forming elements. The secondary precipitated MC-type carbides are relatively stable and do not easily aggregate and grow, thus improving the solid solution strengthening effect of steel. Unmelted V and Nb carbides are dispersed in the matrix, which has a strong pinning effect on grain boundaries and a significant grain refinement effect, which is excellent for improving the high strength, toughness and tempering stability of die steel. Excessive V and Nb are prone to forming eutectic carbides between dendrites in the as-cast structure, increasing the tendency for segregation. To ensure the forgeability and compositional uniformity of die steel, the V content is controlled within the range of 0.30-0.60%, and the Nb content is controlled within the range of 0.05-0.15%.

[0014] (5) Si: Si is not the main strengthening element of the steel of this invention. If the Si content is too high, the banded segregation will be more serious, causing material anisotropy and reducing the material's plasticity and toughness. Therefore, the Si content is controlled within the range of 0.20 to 0.40%.

[0015] (6) Mn: Mn is not the main strengthening element of the steel of this invention. It is usually added as a deoxidizer. It has a strong affinity with S to form MnS with certain plasticity, which reduces the risk of hot brittleness. The Mn content is controlled in the range of 0.30 to 0.50%.

[0016] (7) P: P increases the cold brittleness of die steel and severely reduces the plasticity and cold bending properties of steel. The P content should be reduced as much as possible. The P content of high-grade quality steel is controlled at ≤0.025%. The steel of this invention aims to obtain high-strength and high-toughness hot work die steel, which places higher demands on the control of P content. The P content range is ≤0.020%.

[0017] (8) S: S increases the hot brittleness of mold steel and reduces its toughness and ductility. Non-metallic inclusions such as MnS extend along the deformation direction, reducing the transverse mechanical properties of the steel. Electroslag remelted steel has high requirements for S content, and the S content in the steel of this invention is controlled within the range of ≤0.01%.

[0018] The present invention also provides a method for preparing the above-mentioned high-strength and high-toughness hot forging die steel, which includes the following steps:

[0019] (1) Smelting: The ingredients are smelted, then refined and degassed under vacuum, and then cast into electrode rods;

[0020] (2) Protective atmosphere electroslag remelting: The electrode rod is surface-polished or shot-blasted, and then subjected to protective atmosphere electroslag remelting. The resulting electroslag ingot is stress-relieved annealed at a heating temperature of 650-700℃ and a holding time of 2-4h.

[0021] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1220~1270℃ and a holding time of (0.25~0.35)×D hours, where D is the diameter of the electroslag ingot in cm, and then furnace cooled to a forging temperature of 1140~1180℃.

[0022] (4) Forging: The initial forging temperature is 1080~1140℃, the final forging temperature is 850~880℃, and after forging, the temperature is cooled by air to 300~400℃, and then placed in a slow cooling pit or buried in sand to slow cool to ≤200℃;

[0023] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1150~1180℃, holding for 60~90 minutes, and air cooling to room temperature;

[0024] (6) Spheroidizing annealing: The annealing temperature is 830±10℃, held for 2 to 3 hours, cooled in the furnace to 670±10℃, held for 3 to 5 hours, cooled in the furnace to ≤500℃, and then air-cooled to obtain the spheroidizing annealing.

[0025] Furthermore, in step (1), an electric arc furnace or a medium-frequency furnace is used for smelting.

[0026] Furthermore, in step (2), the slag system composition for protective atmosphere electroslag remelting is as follows: calcium fluoride 50-55%, alumina 18-22%, calcium oxide 22-26%, magnesium oxide 3-6%, and silicon oxide 0.5-2%.

[0027] Furthermore, in step (4), the electroslag ingot is forged using a three-stage forging and three-stage drawing method with a multi-directional forging process. The forging temperature is ≥1000℃, the initial forging ratio is ≥2, and the total forging ratio is ≥12.

[0028] After spheroidizing annealing, samples were taken from the core of the forging and subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1120±10℃, and the holding time was 0.5 hours. The samples were then oil-quenched to room temperature. The tempering temperature was 630±10℃, and the tempering time was 2 hours each time. The samples were tempered twice, and the samples were cooled to room temperature after each tempering. After quenching and tempering treatment, the tempering hardness of the die steel was 49~52HRC. The samples were then machined into standard V-nose Charpy impact test specimens of 10mm×10mm×55mm, with a transverse impact energy ≥9J.

[0029] The mold steel provided by this invention, after being heat-treated and kept at 650℃ for 5 hours, has a hardness value that decreases by less than 4 HRC, and after being kept at 650℃ for 10 hours, has a hardness value that decreases by less than 7 HRC.

[0030] The beneficial effects of this invention are as follows:

[0031] (1) By optimizing the alloy composition, this invention further improves the toughness of the steel plate while ensuring high strength and high tempering stability, which can better meet the requirements of hot forging dies.

[0032] (2) The steel for hot forging dies provided by the present invention has a hardness value that decreases by less than 4 HRC after being kept at 650℃ for 5 hours and by less than 7 HRC after being kept at 650℃ for 10 hours; it is suitable for preparing hot forging dies for producing high-strength ferrous metal forgings at a working temperature of 600-650℃. Attached Figure Description

[0033] Figure 1 This is a metallographic image (transverse view) of the annealed microstructure of Embodiment 1 of the present invention.

[0034] Figure 2 This is a SEM image (horizontal sample) of the annealed microstructure in Embodiment 1 of the present invention.

[0035] Figure 3 This is a metallographic image (longitudinal sample) of the annealed banded segregation in Embodiment 1 of the present invention. Detailed Implementation

[0036] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0037] Example 1

[0038] The preparation method of high-strength and high-toughness hot forging die steel in this embodiment includes the following steps:

[0039] (1) Smelting: The ingredients are smelted in an electric arc furnace, then refined and degassed under vacuum, and cast into electrode rods. The chemical composition of the electrode rods by mass percentage is: C: 0.48%, Si: 0.32%, Mn: 0.50%, Cr: 3.71%, Mo: 3.85%, V: 0.45%, W: 0.30%, Nb: 0.12%, P: 0.018%, S: 0.0053%, with the balance being Fe and unavoidable impurities.

[0040] (2) Electroslag remelting under protective atmosphere: The electrode rod is shot blasted and then electroslag remelted under protective atmosphere. The slag system ratio is: 50% calcium fluoride, 21% alumina, 25% calcium oxide, 3.5% magnesium oxide, and 0.5% silicon oxide. Electroslag ingots with a diameter of φ500mm are prepared. The obtained electroslag ingots are stress-relieved and annealed at a heating temperature of 650℃ and a holding time of 4h.

[0041] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1270°C and a holding time of 12.5 hours, followed by furnace cooling to a forging temperature of 1180°C;

[0042] (4) Forging: The electroslag ingot is forged using a three-up and three-draw multi-directional forging method. The forging heating temperature is 1180℃, the initial forging temperature is 1140℃, and the final forging temperature is 880℃. The forging billet is a φ240mm bar. After forging, it is air-cooled to 300~400℃ and placed in a slow cooling pit to ≤200℃. The upsetting temperature is ≥1000℃, the initial upsetting ratio is ≥2, and the total forging ratio is ≥12.

[0043] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1160℃, holding for 80 minutes, and air cooling to room temperature;

[0044] (6) Spheroidizing annealing: The annealing temperature is 830℃, held for 3 hours, cooled in the furnace to 680℃, held for 5 hours, cooled in the furnace to 480℃, and then air-cooled to obtain the spheroidizing annealing.

[0045] After spheroidizing annealing, samples were taken from the core of the forging and then subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1130℃, the holding time was 0.5 hours, and the sample was oil quenched to room temperature. The tempering temperature was 620℃, and the tempering time was 2 hours each time. The sample was tempered twice, and after each tempering, the sample was cooled to room temperature.

[0046] After tempering, the mold steel has a tempering hardness of 51.8 HRC. It is machined into a standard V-nose Charpy impact test specimen of 10mm×10mm×55mm with a transverse impact energy of 9.3J. After holding at 650℃ for 5h, the hardness value decreases by 3.5 HRC, and after holding at 650℃ for 10h, the hardness value decreases by 6.7 HRC.

[0047] The metallographic image (transverse sample) of the annealed microstructure of the high-strength, high-toughness hot forging die steel provided in this embodiment is as follows: Figure 1 As shown; SEM image of the annealed microstructure (horizontal sample) is shown below. Figure 2 As shown; Metallographic image of annealed banded segregation (vertical sample) as follows. Figure 3 As shown. From Figures 1-3 It can be seen that the high-strength and high-toughness hot forging die steel of this embodiment has SA2 grade banded segregation, AS3 grade annealed structure, good carbide dispersion and spheroidization effect, and high structure uniformity.

[0048] Example 2

[0049] The preparation method of high-strength and high-toughness hot forging die steel in this embodiment includes the following steps:

[0050] (1) Smelting: The ingredients are smelted in an electric arc furnace, then refined and degassed under vacuum, and cast into electrode rods. The chemical composition of the electrode rods by mass percentage is: C: 0.50%, Si: 0.38%, Mn: 0.39%, Cr: 3.85%, Mo: 3.92%, V: 0.52%, W: 0.35%, Nb: 0.08%, P: 0.017%, S: 0.0058%, with the balance being Fe and unavoidable impurities.

[0051] (2) Electroslag remelting under protective atmosphere: The electrode rod is surface-polished and then electroslag remelted under protective atmosphere. The slag system ratio is: calcium fluoride 55%, alumina 18%, calcium oxide 22%, magnesium oxide 3.5%, silicon oxide 1.5%. Electroslag ingots with a diameter of φ500mm are prepared. The obtained electroslag ingots are stress-relieved and annealed at a heating temperature of 680℃ and a holding time of 3h.

[0052] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1250°C and a holding time of 15 hours, followed by furnace cooling to a forging temperature of 1160°C.

[0053] (4) Forging: The electroslag ingot is forged using a three-up and three-draw multi-directional forging method. The forging heating temperature is 1160℃, the initial forging temperature is 1120℃, and the final forging temperature is 870℃. The forging billet is a φ200mm bar. After forging, it is air-cooled to 300~400℃ and then slowly cooled to ≤200℃ by sand burying. The upsetting temperature is ≥1000℃, the initial upsetting ratio is ≥2, and the total forging ratio is ≥12.

[0054] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1180℃, holding for 60 minutes, and air cooling to room temperature;

[0055] (6) Spheroidizing annealing: The annealing temperature is 830℃, held for 2.5 hours, cooled to 660℃ in the furnace, held for 4 hours, cooled to 495℃ in the furnace, and then air-cooled to obtain the product.

[0056] After spheroidizing annealing, samples were taken from the core of the forging and then subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1120℃, the holding time was 0.5 hours, and the sample was oil quenched to room temperature. The tempering temperature was 635℃, and the tempering time was 2 hours each time. The sample was tempered twice, and after each tempering, the sample was cooled to room temperature.

[0057] After tempering, the mold steel has a tempering hardness of 50.3 HRC. It is machined into a standard V-nose Charpy impact test specimen of 10mm×10mm×55mm with a transverse impact energy of 9.6J. After holding at 650℃ for 5h, the hardness value decreases by 3.8 HRC, and after holding at 650℃ for 10h, the hardness value decreases by 6.4 HRC.

[0058] Example 3

[0059] The preparation method of high-strength and high-toughness hot forging die steel in this embodiment includes the following steps:

[0060] (1) Smelting: The ingredients are smelted in an electric arc furnace, then refined and degassed under vacuum, and cast into electrode rods. The chemical composition of the electrode rods by mass percentage is: C: 0.55%, Si: 0.40%, Mn: 0.45%, Cr: 4.00%, Mo: 3.78%, V: 0.38%, W: 0.52%, Nb: 0.15%, P: 0.019%, S: 0.0061%, with the balance being Fe and unavoidable impurities.

[0061] (2) Protective atmosphere electroslag remelting: The electrode rod is surface-polished and then subjected to protective atmosphere electroslag remelting. The slag system ratio is: calcium fluoride 52%, alumina 22%, calcium oxide 22%, magnesium oxide 3%, silicon oxide 1%. Electroslag ingots with a diameter of φ500mm are prepared. The obtained electroslag ingots are stress-relieved annealed at a heating temperature of 700℃ and a holding time of 2h.

[0062] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1240℃ and a holding time of 16 hours, followed by furnace cooling to a forging temperature of 1140℃.

[0063] (4) Forging: The electroslag ingot is forged using a three-up and three-draw multi-directional forging method. The forging heating temperature is 1140℃, the initial forging temperature is 1100℃, and the final forging temperature is 850℃. The forging billet is a φ140mm bar. After forging, it is air-cooled to 300~400℃ and then placed in a slow cooling pit to ≤200℃. The upsetting temperature is ≥1000℃, the initial upsetting ratio is ≥2, and the total forging ratio is ≥12.

[0064] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1150℃, holding for 90 minutes, and air cooling to room temperature;

[0065] (6) Spheroidizing annealing: The annealing temperature is 835℃, held for 3 hours, cooled in the furnace to 665℃, held for 5 hours, cooled in the furnace to 485℃, and then air-cooled to obtain the spheroidizing annealing.

[0066] After spheroidizing annealing, samples were taken from the core of the forging and then subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1110℃, the holding time was 0.5 hours, and the sample was oil quenched to room temperature. The tempering temperature was 625℃, and the tempering time was 2 hours each time. The sample was tempered twice, and after each tempering, the sample was cooled to room temperature.

[0067] After tempering, the mold steel has a tempered hardness of 49.7 HRC. It is machined into a standard V-nose Charpy impact test specimen of 10mm×10mm×55mm, with a transverse impact energy of 9.5J. After holding at 650℃ for 5 hours, the hardness value decreases by 3.2 HRC, and after holding at 650℃ for 10 hours, the hardness value decreases by 6.3 HRC.

[0068] Example 4

[0069] The preparation method of high-strength and high-toughness hot forging die steel in this embodiment includes the following steps:

[0070] (1) Smelting: The ingredients are smelted in a medium frequency furnace, then refined and degassed under vacuum, and cast into electrode rods. The chemical composition of the electrode rods by mass percentage is: C: 0.54%, Si: 0.35%, Mn: 0.30%, Cr: 3.82%, Mo: 3.68%, V: 0.58%, W: 0.60%, Nb: 0.13%, P: 0.019%, S: 0.0068%, with the balance being Fe and unavoidable impurities.

[0071] (2) Electroslag remelting under protective atmosphere: The electrode rod is shot blasted and then electroslag remelted under protective atmosphere. The slag system ratio is: calcium fluoride 54%, alumina 18%, calcium oxide 22%, magnesium oxide 4.5%, silicon oxide 1.5%. Electroslag ingots with a diameter of φ500mm are prepared. The obtained electroslag ingots are stress-relieved and annealed at a heating temperature of 700℃ and a holding time of 2h.

[0072] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1250°C and a holding time of 15 hours, followed by furnace cooling to a forging temperature of 1150°C.

[0073] (4) Forging: The electroslag ingot is forged using a three-up and three-draw multi-directional forging method. The forging heating temperature is 1150℃, the initial forging temperature is 1110℃, and the final forging temperature is 860℃. The forging billet is a φ220mm bar. After forging, it is air-cooled to 300~400℃ and then placed in a slow cooling pit to ≤200℃. The upsetting temperature is ≥1000℃, the initial upsetting ratio is ≥2, and the total forging ratio is ≥12.

[0074] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1150℃, holding for 80 minutes, and air cooling to room temperature;

[0075] (6) Spheroidizing annealing: The annealing temperature is 825℃, held for 2.5 hours, cooled to 670℃ in the furnace, held for 4 hours, cooled to 490℃ in the furnace, and then air-cooled to obtain the product.

[0076] After spheroidizing annealing, samples were taken from the core of the forging and then subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1130℃, the holding time was 0.5 hours, and the sample was oil quenched to room temperature. The tempering temperature was 630℃, and the tempering time was 2 hours each time. The sample was tempered twice, and after each tempering, the sample was cooled to room temperature.

[0077] After tempering, the mold steel has a tempering hardness of 51.3 HRC. It is machined into a standard V-nose Charpy impact test specimen of 10mm×10mm×55mm with a transverse impact energy of 9.8J. After holding at 650℃ for 5h, the hardness value decreases by 3.6 HRC, and after holding at 650℃ for 10h, the hardness value decreases by 6.3 HRC.

[0078] Example 5

[0079] This embodiment provides a high-strength, high-toughness hot forging die steel and its preparation method, including the following steps:

[0080] (1) Smelting: The ingredients are smelted in a medium frequency furnace, then refined and degassed under vacuum, and cast into electrode rods. The chemical composition of the electrode rods by mass percentage is: C: 0.49%, Si: 0.28%, Mn: 0.43%, Cr: 3.69%, Mo: 3.83%, V: 0.42%, W: 0.45%, Nb: 0.05%, P: 0.019%, S: 0.0063%, with the balance being Fe and unavoidable impurities.

[0081] (2) Electroslag remelting under protective atmosphere: The electrode rod is shot blasted and then electroslag remelted under protective atmosphere. The slag system ratio is: calcium fluoride 51%, alumina 19%, calcium oxide 22%, magnesium oxide 6%, silicon oxide 2%. Electroslag ingots with a diameter of φ500mm are prepared. The obtained electroslag ingots are stress-relieved and annealed at a heating temperature of 680℃ and a holding time of 3h.

[0082] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1220°C and a holding time of 17.5 hours, followed by furnace cooling to a forging temperature of 1140°C.

[0083] (4) Forging: The electroslag ingot is forged using a three-up and three-draw multi-directional forging method. The forging heating temperature is 1140℃, the initial forging temperature is 1100℃, and the final forging temperature is 870℃. The forging billet is a φ160mm bar. After forging, it is air-cooled to 300~400℃ and then slowly cooled to ≤200℃ by sand burying. The upsetting temperature is ≥1000℃, the initial upsetting ratio is ≥2, and the total forging ratio is ≥12.

[0084] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1160℃, holding for 1 hour, and air cooling to room temperature;

[0085] (6) Spheroidizing annealing: The annealing temperature is 840℃, held for 2 hours, cooled in the furnace to 675℃, held for 3 hours, cooled in the furnace to 500℃, and then air-cooled to obtain the spheroidizing annealing.

[0086] After spheroidizing annealing, samples were taken from the core of the forging and then subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1120℃, the holding time was 0.5 hours, and the sample was oil quenched to room temperature. The tempering temperature was 640℃, and the tempering time was 2 hours each time. The sample was tempered twice, and after each tempering, the sample was cooled to room temperature.

[0087] After tempering, the mold steel has a tempering hardness of 49.2 HRC. It is machined into a standard V-nose Charpy impact test specimen of 10mm×10mm×55mm with a transverse impact energy of 10.2J. After holding at 650℃ for 5h, the hardness value decreases by 3.3 HRC, and after holding at 650℃ for 10h, the hardness value decreases by 6.5 HRC.

[0088] Example 6

[0089] This embodiment provides a high-strength, high-toughness hot forging die steel and its preparation method, including the following steps:

[0090] (1) Smelting: The ingredients are smelted in a medium frequency furnace, then refined and degassed under vacuum, and cast into electrode rods. The chemical composition of the electrode rods by mass percentage is: C: 0.51%, Si: 0.20%, Mn: 0.36%, Cr: 3.95%, Mo: 3.96%, V: 0.32%, W: 0.40%, Nb: 0.06%, P: 0.017%, S: 0.0056%, with the balance being Fe and unavoidable impurities.

[0091] (2) Electroslag remelting under protective atmosphere: The electrode rod is shot blasted and then electroslag remelted under protective atmosphere. The slag system ratio is: calcium fluoride 52%, alumina 22%, calcium oxide 22%, magnesium oxide 3.5%, silicon oxide 0.5%. Electroslag ingots with a diameter of φ500mm are prepared. The obtained electroslag ingots are stress-relieved and annealed at a heating temperature of 650℃ and a holding time of 4h.

[0092] (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1260°C and a holding time of 14 hours, followed by furnace cooling to a forging temperature of 1160°C.

[0093] (4) Forging: The electroslag ingot is forged using a three-up and three-draw multi-directional forging method. The forging heating temperature is 1160℃, the initial forging temperature is 1080℃, and the final forging temperature is 860℃. The forging billet is a φ100mm bar. After forging, it is air-cooled to 300~400℃ and then placed in a slow cooling pit to ≤200℃. The upsetting temperature is ≥1000℃, the initial upsetting ratio is ≥2, and the total forging ratio is ≥12.

[0094] (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1170℃, holding for 70 minutes, and air cooling to room temperature;

[0095] (6) Spheroidizing annealing: The annealing temperature is 830℃, held for 3 hours, cooled in the furnace to 660℃, held for 5 hours, cooled in the furnace to 490℃, and then air-cooled to obtain the spheroidizing annealing.

[0096] After spheroidizing annealing, samples were taken from the core of the forging and then subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1100℃, the holding time was 0.5 hours, and the sample was oil quenched to room temperature. The tempering temperature was 630℃, and the tempering time was 2 hours each time. The sample was tempered twice, and after each tempering, the sample was cooled to room temperature.

[0097] After tempering, the mold steel has a tempering hardness of 50.6 HRC. It is machined into a standard V-nose Charpy impact test specimen of 10mm×10mm×55mm with a transverse impact energy of 9.9J. After holding at 650℃ for 5h, the hardness value decreases by 3.5 HRC, and after holding at 650℃ for 10h, the hardness value decreases by 6.4 HRC.

Claims

1. A high-strength, high-toughness steel for hot forging dies, characterized in that, The chemical composition and mass percentage of the steel used for hot forging dies are as follows: C: 0.48–0.55%, Si: 0.20–0.40%, Mn: 0.30–0.50%, Cr: 3.60–4.00%, Mo: 3.60–4.00%, V: 0.30–0.60%, W: 0.30–0.60%, Nb: 0.05–0.15%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable impurities; The mold is made of steel that has been quenched and tempered to a hardness of 49-52 HRC and has a transverse impact energy of ≥9J. After the steel used for the mold is heat-treated, the hardness value decreases by less than 4 HRC when it is kept at 650℃ for 5 hours, and by less than 7 HRC when it is kept at 650℃ for 10 hours. The method for preparing the mold steel includes the following steps: (1) Smelting: The ingredients are smelted, then refined and degassed under vacuum, and then cast into electrode rods; (2) Protective atmosphere electroslag remelting: The electrode rod is surface-polished or shot-blasted, and then subjected to protective atmosphere electroslag remelting. The resulting electroslag ingot is stress-relieved annealed at a heating temperature of 650-700℃ and a holding time of 2-4h. (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1220~1270℃ and a holding time of (0.25~0.35)×D hours, where D is the diameter of the electroslag ingot in cm, and then furnace cooled to a forging temperature of 1140~1180℃. (4) Forging: The initial forging temperature is 1080~1140℃, the final forging temperature is 850~880℃, and after forging, it is air-cooled to 300~400℃, and then placed in a slow cooling pit or buried in sand for slow cooling to ≤200℃; (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1150~1180℃, holding for 60~90 minutes, and air cooling to room temperature; (6) Spheroidizing annealing: The annealing heating temperature is 830±10℃, and the holding temperature is 2 to 3 hours. Then, the furnace is cooled to 670±10℃, and the holding temperature is 3 to 5 hours. After the furnace is cooled to ≤500℃, the furnace is removed and air-cooled. After spheroidizing annealing, samples were taken from the core of the forging and subjected to quenching and tempering treatment. The sample size was 15mm×15mm×60mm. The heating temperature was 1120±10℃, and the holding time was 0.5 hours. The samples were then oil quenched to room temperature. The tempering temperature was 630±10℃, and the tempering time was 2 hours each time. The samples were tempered twice, and the samples were cooled to room temperature after each tempering. After quenching and tempering treatment, the tempering hardness of the mold steel was 49~52HRC. The samples were then machined into standard V-nose Charpy impact test specimens of 10mm×10mm×55mm with a transverse impact energy ≥9J.

2. The method for preparing high-strength, high-toughness hot forging die steel according to claim 1, characterized in that, The preparation method includes the following steps: (1) Smelting: The ingredients are smelted, then refined and degassed under vacuum, and then cast into electrode rods; (2) Protective atmosphere electroslag remelting: The electrode rod is surface-polished or shot-blasted, and then subjected to protective atmosphere electroslag remelting. The resulting electroslag ingot is stress-relieved annealed at a heating temperature of 650-700℃ and a holding time of 2-4h. (3) High-temperature homogenization: The electroslag ingot is subjected to high-temperature homogenization treatment, with a heating temperature of 1220~1270℃ and a holding time of (0.25~0.35)×D hours, where D is the diameter of the electroslag ingot in cm, and then furnace cooled to a forging temperature of 1140~1180℃. (4) Forging: The initial forging temperature is 1080~1140℃, the final forging temperature is 850~880℃, and after forging, it is air-cooled to 300~400℃, and then placed in a slow cooling pit or buried in sand for slow cooling to ≤200℃; (5) Ultrafine treatment: The obtained forging is subjected to ultrafine treatment, with normalizing heating temperature of 1150~1180℃, holding for 60~90 minutes, and air cooling to room temperature; (6) Spheroidizing annealing: The annealing temperature is 830±10℃, held for 2 to 3 hours, cooled in the furnace to 670±10℃, held for 3 to 5 hours, cooled in the furnace to ≤500℃, and then air-cooled to obtain the product.

3. The method for preparing high-strength, high-toughness hot forging die steel according to claim 2, characterized in that, In step (1), an electric arc furnace or a medium-frequency furnace is used for smelting.

4. The method for preparing high-strength, high-toughness hot forging die steel according to claim 2, characterized in that, In step (2), the slag system composition for protective atmosphere electroslag remelting is as follows: calcium fluoride 50-55%, alumina 18-22%, calcium oxide 22-26%, magnesium oxide 3-6%, and silicon oxide 0.5-2%.

5. The method for preparing high-strength, high-toughness hot forging die steel according to claim 2, characterized in that, In step (4), the electroslag ingot is forged using a three-stage forging and three-stage pulling multi-directional forging method. The forging temperature is ≥1000℃, the initial forging ratio is ≥2, and the total forging ratio is ≥12.

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