Hot forging die steel and manufacturing method thereof

Abstract

The invention discloses hot-forging die steel and a manufacturing method thereof. The hot-forging die steel comprises the following chemical components in percentage by mass: 0.45-0.55% of C; 0.50 to 1.00% of Si; 0.50 to 1.00 percent of Mn; 2.50% to 3.50% of Cr; 3.00% to 4.00% of W; the content of V is 1.50 to 2.50 percent; 0.50% to 1.50% of Mo; 0.10% to 0.20% of Nb; and the balance of Fe and inevitable impurities. According to the hot forging die steel, the optimal composition proportion relation between the content of the alloy elements C, Si, Mn, Cr, W, Mo, V and Nb and the obdurability, red hardness and abrasion resistance of the material is built, the defects of the traditional three types of die steel are overcome, and the hot forging die steel has excellent comprehensive mechanical properties such as thermal fatigue resistance, tempering softening resistance and cracking resistance and can meet the use requirement of a high-strength hot forging die.

Description

Technical Field

[0001] This invention relates to the field of nickel-based alloy smelting technology, specifically to a hot forging die steel and its manufacturing method. Background Technology

[0002] Hot forging dies are tools used to force metal to form at high temperatures through impact and pressure. Due to their high precision and uniform deformation, they are widely used in the automotive and machinery manufacturing industries. During operation, the die cavity of a hot forging die comes into contact with the high-temperature metal, with local temperatures reaching 500–700°C, and sometimes even around 1000°C. Simultaneously, it undergoes repeated heating and cooling, making the working surface of the die prone to thermal fatigue cracks. Furthermore, the friction between the hot metal and the die cavity surface during forced deformation causes the die to wear easily and its hardness to decrease. Among the many factors influencing die failure, poor die material quality and improper heat treatment processes are the most significant causes, accounting for approximately 70% of all failures. Appropriate selection of hot forging die steel is essential for improving the quality of hot forgings and reducing production costs. To ensure the service life of the die, hot work die steel must possess high thermal strength, high thermal fatigue resistance, good toughness, and wear resistance. Meanwhile, with the increasing capabilities of forging machinery, the increasing complexity of workpiece shapes, and especially the growing difficulty in processing materials such as titanium alloys, high-alloy steels, stainless steels, and heat-resistant steels, dies are becoming larger and more high-performance. Therefore, the requirements for the performance of steel used in hot forging dies are also becoming increasingly stringent.

[0003] Currently, common hot forging die steels in my country are divided into three categories: The first category is medium-carbon low-alloy hot work die steel, represented by 5CrMnMo and 5CrNiMo. The alloying principle of this type of steel is similar to that of medium-carbon alloy structural steel. It belongs to low heat resistance and high toughness steel, with low hot strength, thermal stability and hardenability, and is prone to cracking and collapse. The second category is chromium-based medium-alloy hot work die steel, represented by H11 and H13. The total alloy content of this type of steel is about 8%. It belongs to hypereutectoid steel. Due to selective crystallization in the later stage of solidification, eutectic carbides exist between dendrites. If the subsequent forging and heat treatment processes are not properly controlled, structural defects such as severe banded segregation and carbide precipitation along grains will occur, resulting in insufficient heat resistance, softening resistance and crack resistance, which will affect the die life. The third category consists of improved hot work die steels, represented by 3Cr3Mo3VNb (HM3) and 3Cr3Mo3W2V (HM1). These improved hot work die steels are characterized by increasing the content of alloys such as Mo, W, and Nb and reducing the C content on the basis of H13 type hot work die steels. This ensures that the die has a certain degree of plasticity and toughness, while improving high-temperature strength, tempering stability, and resistance to thermal fatigue. However, the material has poor wear resistance and cannot adapt to the increasingly demanding development of the hot forging industry.

[0004] Therefore, it is necessary to develop and design new hot forging die steels to meet the performance requirements of high-strength hot forging dies. Summary of the Invention

[0005] The purpose of this invention is to propose a new hot forging die steel to meet the performance requirements of high-strength hot forging dies.

[0006] In view of this, the solution of the present invention is as follows:

[0007] The first aspect of the present invention is to provide a hot forging die steel having the following chemical composition by mass percentage: C 0.45~0.55%; Si 0.50~1.00%; Mn 0.50-1.00%; Cr 2.50-3.50%; W 3.00~4.00%; V 1.50~2.50%; Mo 0.50~1.50%; Nb 0.10~0.20%; the remainder being Fe and unavoidable impurities.

[0008] A second aspect of the present invention is to provide a method for manufacturing the hot forging die steel described in the first aspect, comprising the steps of: S1. The steel raw material is put into the refining furnace for smelting, and after adjusting it to meet the target composition requirements, it is cast to obtain the electrode base material, and the first stress-relieving annealing is carried out. S2. The electrode base material is loaded into an electroslag furnace for remelting to obtain steel ingots, and then subjected to a second stress-relief annealing. S3. Heat the steel ingot obtained in S2 to 1200~1300℃ for high-temperature homogenization treatment; S4. The steel ingot obtained from S3 is cooled in the furnace to 1150~1250℃, and then upsetting and blanking is carried out using a fast forging machine. After reserving a forging ratio of not less than 1.5, it is then finished into finished products using a radial forging machine. S5. The forging obtained in step S4 is air-cooled to a surface temperature of 300~400℃, and then placed in an annealing furnace for holding for more than 2 hours; subsequently, the temperature is increased to 800~900℃ at a rate of 30~60℃ / h and held at that temperature. After the annealing process, the forging is furnace-cooled to ≤300℃ at a rate of 10~30℃ / h and then air-cooled to obtain the annealed forging. S6. Heat the forging from step S5 to 1050~1150℃ and hold it at that temperature. After that, cool it in oil to 150~250℃ and then put it into a tempering furnace for tempering at 550~650℃ to obtain qualified hot forging die steel forgings.

[0009] Furthermore, in step S1, the steel raw materials include ferroalloys, blast furnace iron, and CrMo scrap steel;

[0010] And / or, the smelting process controls the phosphorus content to be below 50 ppm, the sulfur content to be below 50 ppm, the nitrogen content to be below 150 ppm, and the oxygen content to be below 25 ppm.

[0011] Further, in step S1, the first stress-relief annealing is to heat to 800~900℃, hold at 100mm / 4h for the effective diameter of the electrode base material, and then slowly cool to below 200℃ and air cool.

[0012] And / or, in step S2, the second stress-relief annealing process is to heat to 850±25℃, hold at 100mm / 3h for the effective diameter of the steel ingot, and slowly cool to below 200℃ before air cooling.

[0013] Furthermore, in step S2, ternary pre-melted slag is used for purification.

[0014] Furthermore, in step S3, the high-temperature homogenization process is maintained at an effective diameter of 100mm for 2.5~3 hours.

[0015] Furthermore, in step S4, the upsetting ratio in the upsetting process is ≥1.6, and the total forging ratio in step S4 is ≥6; Furthermore, in step S5, the holding time of the annealing furnace holding process is calculated based on a workpiece diameter of 100 mm / h, and if it is less than 2 hours, it is calculated as 2 hours. And / or, the time for the heat preservation process at 800~900℃ is calculated as 6 hours per 100mm diameter workpiece; Furthermore, in step S6, the time of the heat preservation process at 1050~1150℃ is calculated based on a workpiece diameter of 50mm / h; And / or, the tempering holding time is based on a workpiece diameter of 25 mm / h, and the number of tempering cycles is not less than 2.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The hot forging die steel provided by this invention overcomes the weaknesses of the three traditional types of die steel by constructing the optimal composition ratio between the content of alloying elements C, Si, Mn, Cr, W, Mo, V, and Nb and the material's strength, toughness, red hardness, and wear resistance. It has excellent comprehensive mechanical properties such as resistance to thermal fatigue, resistance to tempering softening, and resistance to cracking, and can meet the requirements for high-strength hot forging dies.

[0018] The hot forging die steel manufactured by the present invention exhibits no obvious banded segregation, no eutectic carbides, and a stable microstructure after heat treatment that meets the HS1~HS4 premium grade specified in the NADCA#207 standard. At a hardness of 54±1HRC, the unnotched impact energy (KW2) of the 7*10mm transverse core reaches over 240J.

[0019] The hot forging die steel manufacturing method of the present invention provides a service life that is 1.5 to 3 times longer than that of HM1, HM3, H11, and H13 type die steels in high alloy steel, stainless steel, and heat-resistant steel die forging applications. Attached Figure Description

[0020] Figure 1 This is a photograph of the banded segregation of the hot forging die steel described in Embodiment 1 of the present invention.

[0021] Figure 2 This is a photograph of the eutectic carbides in the annealed microstructure of the hot forging die steel described in Embodiment 1 of the present invention.

[0022] Figure 3 This is a photograph of the heat-treated microstructure of the hot forging die steel described in Embodiment 1 of the present invention.

[0023] Figure 4 This is a photograph of the banded segregation of the hot forging die steel described in Embodiment 2 of the present invention.

[0024] Figure 5 This is a photograph of the eutectic carbides in the annealed microstructure of the hot forging die steel described in Embodiment 2 of the present invention.

[0025] Figure 6 This is a photograph of the heat-treated microstructure of the hot forging die steel described in Embodiment 2 of the present invention. Detailed Implementation

[0026] The technical solution of the present invention will now be clearly and completely described in conjunction with preferred embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Through in-depth research, the inventors of this invention discovered that by controlling the steel ingot within a specific range of chemical composition, and through specific heating, forging, and heat treatment, qualified mold steel can be obtained to meet the requirements of high-strength hot forging dies.

[0028] This invention provides a hot forging die steel with the following chemical composition and mass percentages: C 0.45~0.55%; Si 0.50~1.00%; Mn 0.50-1.00%; Cr 2.50-3.50%; W 3.00~4.00%; V 1.50~2.50%; Mo 0.50~1.50%; Nb 0.10~0.20%; the remainder being Fe and unavoidable impurities. This steel is an improvement on HM1 and HM3. Increased carbon content enhances wear resistance. Vanadium refines grain size in tool steel, reduces overheating sensitivity, and increases tempering stability and wear resistance, thus extending tool life. Tungsten primarily enhances tempering stability, red hardness, hot strength, and wear resistance; to a certain extent, tungsten can replace molybdenum, making it more economical. By comprehensively considering the roles of alloying elements in the steel, increasing tungsten and vanadium content, and decreasing molybdenum content, a hot forging die steel with optimal performance is obtained.

[0029] According to the present invention described above, a method for manufacturing hot forging die steel is provided, comprising the following steps: 1. The ferroalloy is melted in an alloy melting furnace; the blast furnace hot metal and CrMo scrap steel are dephosphorized in an electric furnace. 2. The molten steel from the alloy melting furnace and electric furnace is added to the LF refining furnace for desulfurization, degassing, and inclusion removal, and the composition is adjusted to the required level; then VD degassing treatment is carried out to control the nitrogen and oxygen content; 3. Argon gas is used to protect the casting of the electrode base material; then it is placed in an annealing furnace and heated to 800~900℃ for the first stress-relief annealing, and then slowly cooled to no more than 200℃ before being removed from the furnace and air-cooled to obtain a high-quality electrode base material; 4. The electrode base material is remelted in a gas-protected electroslag furnace to obtain steel ingots, and then heated to 850±25℃ in an annealing furnace for a second stress-relief annealing. After that, it is slowly cooled to no more than 200℃ and then air-cooled to obtain steel ingots with good surface quality. 5. The steel ingot obtained in step 4 is subjected to high-temperature homogenization treatment at 1200~1300℃, and the temperature is maintained for 2.5~3h with an effective diameter of 100mm to improve the original chemical composition and microstructure segregation of the steel ingot. 6. Cool the steel ingot from step 5 to 1150~1250℃ in the furnace, use a fast forging machine to upset the billet, reserve a forging ratio of not less than 1.5, and then use a radial forging machine to form the finished product, with an upsetting ratio ≥1.6 and a total forging ratio ≥6; 7. Air cool the forging obtained in step 6 to a surface temperature of 300~400℃, then place it in an annealing furnace and hold it for more than 2 hours; then raise the temperature to 800~900℃ at a rate of 30~60℃ / h and hold it thereafter, and finally furnace cool it to ≤300℃ at a rate of 10~30℃ / h before removing it from the furnace and air cooling it to obtain the annealed forging; 8. Heat the forging obtained in step 7 to 1050~1150℃ and hold it at that temperature. After that, cool it down to 150~250℃ and then put it into a tempering furnace for tempering treatment at 550~650℃ to obtain qualified hot forging die steel forgings.

[0030] In the above embodiments, the qualified hot forging die steel obtained by the manufacturing method has no obvious banded segregation, no eutectic carbides, and the heat-treated microstructure is stable, reaching the HS1~HS4 premium grade specified in the NADCA#207 standard. At a hardness of 54±1HRC, the unnotched impact energy (KW2) of the 7*10mm transverse core reaches over 240J. Furthermore, in high-alloy steel, stainless steel, and heat-resistant steel die forging applications, its service life is 1.5 to 3 times that of HM1, HM3, and H11, H13 type die steels.

[0031] In a preferred embodiment, the ferroalloy includes low-carbon ferrochrome, ferromolybdenum, ferrovanadium, ferrotungsten, and ferroniobium, and the CrMo steel can be selected from CrMo scrap steel; preferably, the phosphorus content after dephosphorization treatment is not higher than 0.025%.

[0032] In a preferred embodiment, in step 2, the sulfur content is controlled to be below 50 ppm, the nitrogen content to be below 150 ppm, and the oxygen content to be below 25 ppm.

[0033] In a preferred embodiment, in step 3, the annealing process is carried out at 800~900℃ for 4 hours with an effective diameter of 100mm, followed by slow cooling.

[0034] In a preferred embodiment, in step 4, before electroslag remelting, the electrode base material is shot-blasted to ensure the purity of the molten steel. This surface shot-blasting is used to obtain electrode base material with good surface quality and to prevent surface oxide scale and foreign inclusions from contaminating the molten steel. To obtain steel ingots with good internal quality, ternary pre-melted slag is used for purification.

[0035] In a preferred embodiment, in step 4, the annealing process is carried out at 850±25℃ with an effective diameter of 100mm / 3h.

[0036] In a preferred embodiment, step 6 involves using an upper upsetting cap and a lower flat plate for upsetting on a fast forging mill, with an upsetting ratio ≥2. Octagonal ingots are forged on the upper and lower flat anvils, with a forging ratio of not less than 1.5 reserved according to the forging size, and then transferred to a radial forging mill to form finished products.

[0037] In a preferred embodiment, in step 7, the holding temperature in the annealing furnace is controlled at 300~400℃ and the holding time is T(h) = workpiece diameter (1~3)D / 100mm, and the minimum holding time is not less than 2h; the holding time of the 800~900℃ holding process is T(h) = workpiece diameter (4~8)D / 100mm.

[0038] In a preferred embodiment, in step 8, the holding time T (h) of the heat preservation process at 1050~1150℃ is equal to the workpiece diameter (1.5~2.5D) / 100mm. The workpiece is cooled to 150~250℃ using oil cooling. Then, it is placed in a tempering furnace for tempering treatment at 550~650℃. The holding time T (h) is equal to the workpiece diameter (3~6)D / 100mm. The tempering is performed at least twice, and each tempering is air-cooled to room temperature.

[0039] In a preferred embodiment, gas protection is used in any smelting step of the preparation method, and argon gas is optimized.

[0040] The following are preferred embodiments of the present invention, used to verify the above scheme and technical effects. Unless otherwise specified, the equipment used is commonly used in the metallurgical field, and the means used are commonly used in the metallurgical field.

[0041] Example 1

[0042] A hot forging die steel, the chemical composition and mass percentage of which are: C 0.48%; Si 0.70%; Mn 0.80%; Cr 2.90%; W 3.3%; V 1.90%; Mo 0.80%; Nb 0.14%; the remainder being Fe and unavoidable impurities.

[0043] The manufacturing method and steps for the above-mentioned large-size die-casting mold steel are as follows: 1. Low-carbon ferrochrome, ferromolybdenum, ferrovanadium, ferrotungsten, and ferroniobium are melted in an alloy melting furnace; blast furnace hot metal and CrMo scrap are melted and dephosphorized in an electric furnace to achieve a phosphorus content of 0.003%. The molten steel from the alloy melting furnace and electric furnace is then transferred to an LF refining furnace for desulfurization to 0.003%, degassing, inclusion removal, and adjustment to the required composition. Subsequently, VD degassing treatment is performed to achieve a nitrogen content of 120*10. -6 Oxygen content below 20*10 -6 Argon gas was used to protect the casting of the electrode base material; then it was placed in an annealing furnace and heated to 830±25℃, and held at 100mm / 4h for an effective diameter. After that, it was slowly cooled to 160℃ and then air-cooled to obtain a high-quality electrode base material. 2. The surface of the above electrode base material is shot blasted, and then it is loaded into an electroslag furnace for remelting. The remelting process is under argon protection throughout, and ternary pre-melted slag is used for purification. The remelted steel ingot is heated to 840±25℃ in a natural gas bogie annealing furnace, held at 100mm effective diameter for 3 hours, and then slowly cooled to 170℃ before being air-cooled. 3. The above steel ingots are subjected to high-temperature homogenization treatment at 1240℃, and held at 100mm effective diameter for 2.8h. Then, the ingots are furnace cooled to 1210℃ and upset on a 45MN fast forging mill with an upper upsetting cap and a lower flat plate. The upsetting ratio is 1.8. The octagonal ingots of 380mm are forged on the upper and lower flat anvils and then transferred to a 16MN radial forging mill to form φ300mm round steel forgings. 4. Air-cool the above-mentioned forging blank to a surface temperature of 380℃, then place it in a natural gas trolley annealing furnace at 360℃ for 3 hours; subsequently, raise the temperature to 825℃ at a rate of 48℃ / h and hold for 18 hours; after holding, furnace cool at a rate of 22℃ / h to ≤260℃ and then air-cool. The annealed forging is obtained. 5. Heat the above forgings to 1120℃ and hold for 6 hours, then cool to 160℃ using oil cooling; then place them in a tempering furnace and temper at 580℃ for 12 hours, then air cool to room temperature; then place them in a tempering furnace and temper at 560℃ for 12 hours, then air cool to room temperature to obtain forgings with qualified performance.

[0044] The hot forging die steel forging manufactured in this embodiment has a specification of φ300mm, and the banded segregation meets the SEP1614 standard SB2 level. Figure 1 The eutectic carbide, observed at 100x magnification, is of order zero ( ). Figure 2 The heat treatment microstructure meets the HS3 premium grade specified in NADCA#207 standard. Figure 3 At a hardness of 54±1HRC, the impact energy of the 7*10mm unnotched core reaches 262J.

[0045] Example 2

[0046] A hot forging die steel, the chemical composition and mass percentage of which are: C 0.52%; Si 0.60%; Mn 0.90%; Cr 3.20%; W 3.6%; V 2.10%; Mo 1.20%; Nb 0.18%; the remainder being Fe and unavoidable impurities.

[0047] The steps of the above-mentioned method for manufacturing large-size die-casting mold steel are as follows:

[0048] 1. Low-carbon ferrochrome, ferromolybdenum, ferrovanadium, ferrotungsten, and ferroniobium are melted in an alloy melting furnace; blast furnace hot metal and CrMo scrap are melted and dephosphorized in an electric furnace to a phosphorus content of 0.002%; the molten steel from the alloy melting furnace and electric furnace is then added to an LF refining furnace for desulfurization to 0.001%, degassing, inclusion removal, and adjustment to the required composition; subsequently, VD degassing is performed to a nitrogen content of 118*10. -6 Oxygen content below 22*10 -6 Argon gas was used to protect the casting of the electrode base material; then it was placed in an annealing furnace and heated to 820±25℃, and held at 100mm / 4h for an effective diameter. After that, it was slowly cooled to 170℃ and then air-cooled to obtain a high-quality electrode base material.

[0049] 2. The surface of the above electrode base material is shot blasted, and then it is loaded into an electroslag furnace for remelting. The remelting process is under argon protection throughout, and ternary pre-melted slag is used for purification. The remelted steel ingot is heated to 845±25℃ in a natural gas bogie annealing furnace, held at 100mm effective diameter for 3 hours, and then slowly cooled to 190℃ before being air-cooled.

[0050] 3. The above steel ingots are subjected to high-temperature homogenization treatment at 1260℃, and held at 100mm effective diameter for 2.6h. Then, they are furnace cooled to 1190℃ and upset on a 45MN fast forging mill using an upper upsetting cap and a lower flat plate, with an upsetting ratio of 2.0. 320mm octagonal ingots are forged on the upper and lower flat anvils and then transferred to a 16MN radial forging mill to form φ250mm round steel forgings.

[0051] 4. Air-cool the above-mentioned forging blank to a surface temperature of 360℃, then place it in a natural gas trolley annealing furnace at 370℃ for 2.5 hours; subsequently, raise the temperature to 835℃ at a rate of 48℃ / hour and hold for 10 hours; after holding, furnace cool at a rate of 23℃ / hour to ≤250℃ and then air-cool. The annealed forging is obtained.

[0052] 5. Heat the above forgings to 1060℃ and hold for 5 hours, then cool them to 170℃ using oil cooling. Subsequently, place them in a tempering furnace and temper them at 560℃ for 8 hours, then air cool them to room temperature.

[0053] The hot forging die steel forging manufactured in this embodiment has a specification of φ200mm, and the banded segregation meets the SEP1614 standard SB1 level. Figure 4 The eutectic carbide, observed at 100x magnification, is of order zero ( ). Figure 5 The heat treatment microstructure meets the HS1 premium grade specified in NADCA#207 standard. Figure 6 At a hardness of 54±1HRC, the impact energy of the 7*10mm unnotched core reaches 293J.

[0054] Example 3

[0055] A hot forging die steel, the chemical composition and mass percentage of which are: C 0.50%; Si 0.72%; Mn 0.70%; Cr 2.90%; W 3.1%; V 2.00%; Mo 1.30%; Nb 0.15%; the remainder being Fe and unavoidable impurities.

[0056] The steps of the above-mentioned method for manufacturing large-size die-casting mold steel are as follows:

[0057] 1. Low-carbon ferrochrome, ferromolybdenum, ferrovanadium, ferrotungsten, and ferroniobium are melted in an alloy melting furnace; blast furnace hot metal and CrMo scrap are melted and dephosphorized in an electric furnace to a phosphorus content of 0.005%; the molten steel from the alloy melting furnace and electric furnace is then added to an LF refining furnace for desulfurization to 0.003%, degassing, inclusion removal, and adjustment to the required composition; subsequently, VD degassing treatment is performed to a nitrogen content of 110*10. -6 Oxygen content below 17*10-6 Argon gas was used to protect the casting of the electrode base material; then it was placed in an annealing furnace and heated to 860±25℃, and held at 100mm / 4h for an effective diameter. After that, it was slowly cooled to 190℃ and then air-cooled to obtain a high-quality electrode base material.

[0058] 2. The surface of the above electrode base material is shot blasted, and then it is loaded into an electroslag furnace for remelting. The remelting process is under argon protection throughout, and ternary pre-melted slag is used for purification. The remelted steel ingot is heated to 855±25℃ in a natural gas bogie annealing furnace, held at 100mm effective diameter for 3 hours, and then slowly cooled to 150℃ before being air-cooled.

[0059] 3. The above steel ingots are subjected to high-temperature homogenization treatment at 1230℃, and held at 100mm effective diameter for 2.8h. Then, they are furnace cooled to 1170℃ and upset on a 45MN fast forging mill using an upper upsetting cap and a lower flat plate, with an upsetting ratio of 2.0. 300mm octagonal ingots are forged on the upper and lower flat anvils and then transferred to a 16MN radial forging mill to produce 200mm square steel forgings.

[0060] 4. Air-cool the above-mentioned forging blank to a surface temperature of 370℃, then place it in a natural gas trolley annealing furnace at 380℃ for 2.5 hours; subsequently, raise the temperature to 845℃ at a rate of 45℃ / hour and hold for 15 hours; after holding, furnace cool at a rate of 24℃ / hour to ≤270℃ and then air-cool. The annealed forging is obtained.

[0061] 5. Heat the above forgings to 1080℃ and hold for 5 hours, then cool to 150℃ using oil cooling; then place them in a tempering furnace and temper at 570℃ for 10 hours, then air cool to room temperature; then place them in a tempering furnace and temper at 560℃ for 10 hours, then air cool to room temperature to obtain forgings with qualified performance.

[0062] The hot forging die steel forging manufactured in this embodiment has a specification of φ250mm, and the banded segregation meets the SEP1614 standard SB1 level; the eutectic carbides are grade zero when observed at 100x magnification; the heat-treated microstructure meets the HS1 excellent grade specified in the NADCA#207 standard. At a hardness of 54±1HRC, the unnotched impact energy of the 7*10mm transverse core reaches 271J KW2.

[0063] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A hot-forging die steel, characterized by, Its chemical composition by mass percentage is as follows: C 0.45~0.55%; Si 0.50~1.00%; Mn 0.50-1.00%; Cr 2.50-3.50%; W 3.00~4.00%; V 1.50~2.50%; Mo 0.50~1.50%; Nb 0.10~0.20%; the remainder being Fe and unavoidable impurities.

2. The method for manufacturing the hot forging die steel according to claim 1, characterized in that, include: S1. The steel raw material is put into the refining furnace for smelting, and after adjusting it to meet the target composition requirements, it is cast to obtain the electrode base material, and the first stress-relieving annealing is carried out. S2. The electrode base material is loaded into an electroslag furnace for remelting to obtain steel ingots, and then subjected to a second stress-relief annealing. S3. Heat the steel ingot obtained in S2 to 1200~1300℃ for high-temperature homogenization treatment; S4. The steel ingot obtained from S3 is cooled in the furnace to 1150~1250℃, and then upsetting and blanking is carried out using a fast forging machine. After reserving a forging ratio of not less than 1.5, it is then finished into finished products using a radial forging machine. S5. The forging obtained in step S4 is air-cooled to a surface temperature of 300~400℃, and then placed in an annealing furnace for holding for more than 2 hours; subsequently, the temperature is increased to 800~900℃ at a rate of 30~60℃ / h and held at that temperature. After the annealing process, the forging is furnace-cooled to ≤300℃ at a rate of 10~30℃ / h and then air-cooled to obtain the annealed forging. S6. Heat the forging from step S5 to 1050~1150℃ and hold it at that temperature. After that, cool it in oil to 150~250℃ and then put it into a tempering furnace for tempering at 550~650℃ to obtain qualified hot forging die steel forgings.

3. The manufacturing method according to claim 2, characterized in that, In step S1, the steel raw materials include ferroalloys, blast furnace iron, and CrMo scrap steel; And / or, the smelting process controls the phosphorus content to be below 50 ppm, the sulfur content to be below 50 ppm, the nitrogen content to be below 150 ppm, and the oxygen content to be below 25 ppm.

4. The manufacturing method according to claim 2, characterized in that, In step S1, the first stress-relief annealing is to heat the temperature to 800~900℃, hold it at 100mm / 4h for the effective diameter of the electrode base material, and then slowly cool it to below 200℃ and air cool it. And / or, in step S2, the second stress-relief annealing process is to heat to 850±25℃, hold at 100mm / 3h for the effective diameter of the steel ingot, and slowly cool to below 200℃ before air cooling.

5. The manufacturing method according to claim 2, characterized in that, In step S2, ternary pre-melted slag is used for purification.

6. The manufacturing method according to claim 2, characterized in that, In step S3, the high-temperature homogenization process is maintained at an effective diameter of 100mm for 2.5~3 hours.

7. The manufacturing method according to claim 2, characterized in that, In step S4, the upsetting ratio in the upsetting process is ≥1.6, and the total forging ratio in step S4 is ≥6.

8. The manufacturing method according to claim 2, characterized in that, In step S5, the holding time of the annealing furnace holding process is calculated based on a workpiece diameter of 100 mm / h, and if it is less than 2 hours, it is calculated as 2 hours. And / or, the time for the heat preservation process at 800~900℃ is calculated as 6h for 100mm diameter workpiece.

9. The manufacturing method according to claim 2, characterized in that, In step S6, the time for the heat preservation process at 1050~1150℃ is calculated based on a workpiece diameter of 50mm / h; And / or, the tempering holding time is 25 mm / h for the workpiece diameter, and the number of tempering cycles is not less than 2.

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