Die steel and its preparation method

Abstract

The present invention provides a die steel and a preparation method thereof. The die steel is prepared by means of electroslag remelting smelting, homogenizing heating, air blowing cooling and spheroidizing annealing processes, and its chemical components by mass percentage include: C: 0.38 - 0.42%, Si: 0.30 - 0.45%, Mn: 0.70 - 0.85%, Cr: 2.55 - 2.80%, Mo: 2.30 - 2.50%, V: 0.80 - 1.00%, Ni: 0.30 - 0.50%, Co: 2.50 - 3.00%, and the balance is Fe and impurities. By optimizing the contents of C and Cr, adding elements such as Mo, Ni and Co, comprehensively considering the rules of dissolution of carbides of different elements into austenite during quenching and their effects on refining grains, and adopting a special process for preparation, the die steel of the present invention has high hardness, heat fatigue resistance, and is not prone to thermal fatigue cracks during use, and can improve the service life of forging dies and stamping dies.

Description

Technical Field

[0001] The present invention relates to the field of steel alloys, and particularly to a die steel. The present invention also relates to a method for preparing the above die steel. Background Art

[0002] The most widely used and representative steel grade for hot working dies is H13 steel, grade: 4Cr5MoSiV1. This steel has high hardenability and thermal cracking resistance, contains relatively high contents of carbon and vanadium, has good wear resistance, relatively weakened toughness, good heat resistance, good strength and hardness at relatively high temperatures, high wear resistance and toughness, excellent comprehensive mechanical properties and high tempering stability. However, when used for forging and stamping dies, due to the relatively high temperature of the workpiece being processed (1000°C - 790°C) and the relatively low temperature of the coolant, it is extremely easy to generate fatigue cracks on the die surface under the influence of alternating heat and cold. Summary of the Invention

[0003] In view of this, the present invention provides a die steel to improve hardness and heat fatigue resistance and increase the service life of forging dies and stamping dies.

[0004] A die steel is prepared by electroslag remelting, homogenizing heating, air cooling and spheroidizing annealing processes. Its chemical components by mass percentage include: C: 0.38 - 0.42%, Si: 0.30 - 0.45%, Mn: 0.70 - 0.85%, Cr: 2.55 - 2.80%, Mo: 2.30 - 2.50%, V: 0.80 - 1.00%, Ni: 0.30 - 0.50%, Co: 2.50 - 3.00%, and the balance is Fe and impurities.

[0005] By optimizing the contents of C and Cr, adding elements such as Mo, Ni and Co, comprehensively considering the law of dissolution of different element carbides into austenite during quenching and their role in refining grains, and preparing with a special process, the die steel has high hardness, heat fatigue resistance, and is not prone to thermal fatigue cracks during use, and can increase the service life of forging dies and stamping dies. The service life is increased by 50 - 80% compared with the existing H13 hot working die steel.

[0006] Further, the impurities include P, and P: ≤0.02%.

[0007] Further, the impurities include S, and S: ≤0.010%.

[0008] Further, the impurities include N, and N: ≤0.005%.

[0009] Further, the liquid phase carbide content of the die steel is ≤0.001%.

[0010] Meanwhile, the present invention also provides a method for preparing die steel, comprising the following steps: Electroslag remelting and smelting: melting alloy raw materials and pouring them into a ladle, subjecting them to LF refining outside the furnace, vacuum degassing in a VD furnace, casting into ingots, and performing electroslag remelting and smelting; Homogenization heating: annealing the ingots obtained by electroslag remelting and then heating them in a chamber furnace at a heating temperature of 1260 - 1310 °C. After heating, perform blooming and rounding, and after stress relief annealing, heat to 1030 - 1130 °C and forge and roll into steel; After air cooling the steel, put it into a heating furnace for spheroidizing annealing, and cool it in the furnace to 490 - 510 °C and then take it out of the furnace.

[0011] Further, the holding time at the stage of homogenization heating at 1260 - 1310 °C is 15 - 20 hours.

[0012] Further, the spheroidizing annealing includes: First step: putting the steel into a heating furnace and heating it at a rate of 90 - 100 °C / h to 730 - 800 °C; Second step: heating the steel at a rate of 90 - 100 °C / h to 850 - 880 °C; Third step: quickly cooling the steel at a rate of ≥50 °C / h to 800 °C; Fourth step: slowly cooling the steel to 660 - 700 °C; Fifth step: slowly cooling the steel to 520 - 570 °C.

[0013] Further, the holding time in the first step of the spheroidizing annealing is 6 - 8 h, the holding time in the second step is 5.5 - 7 h, the holding time in the fourth step is 9 - 11 h, and the holding time in the fifth step is 3.5 - 4.5 h.

[0014] Function of C element: C element is one of the constituent elements of carbides. Part of the elements dissolve in the matrix to improve the matrix strength. The content of C element is not higher than 0.42% so that less or no liquid segregation carbides are generated during the solidification of molten steel; The C element is not lower than 0.38% so that appropriate hardness can be achieved after heat treatment.

[0015] Function of Mo element: Mo element is a medium carbide forming element. Under non-equilibrium cooling conditions, the carbides formed by Mo element undergo a phase change to produce metastable M2C carbides. The flaky and fan-shaped M2C decomposes into fine M6C + MC during forging heating and holding after solidification and cooling, and it is easy to be evenly distributed, increasing the toughness of the steel and improving its hot plasticity. Improving the stability of carbides and the strength and wear resistance of the steel, the Mo content in the present invention is 2.30 - 2.50%.

[0016] Function of Cr element: When the content of Cr element is not less than 2.55%, it can greatly reduce the carbon concentration in pearlite and the limiting solubility of carbon in austenite, promote the precipitation of carbides, improve the strength and hardness of steel, slow down the decomposition rate of austenite, and significantly improve the hardenability of steel; when the content of Cr element is not higher than 2.80%, the content of large particle M23C6 carbides can be less than 4%, so as to improve the toughness of steel.

[0017] Function of V element: V element is a strong carbide-forming element. In steel, it combines with C element to form MC carbides with high hardness and strength, which is the decisive organizational factor for the wear resistance of steel. The dissolved V element can greatly strengthen the secondary hardening of steel, while the retained carbide VC can greatly increase the wear resistance of steel. Therefore, the content of V element in this invention is designed to be 0.80 - 1.00%. The content of V element is not less than 0.80% to form a sufficient number of MC carbides. When the content of V is lower than 0.80%, the number of MC carbides decreases correspondingly, resulting in a reduction in the wear resistance of steel. The maximum content of V is not higher than 1.00% to prevent the mold from cracking due to liquid segregation carbides during use.

[0018] Si element strengthens ferrite, enhances the secondary hardening ability of steel heat treatment, reduces the critical cooling rate of steel, and improves the hardenability of steel. In this invention, Si is controlled at 0.30 - 0.45%.

[0019] Mn element improves the toughness, strength, hardness and wear resistance of steel. In this invention, Mn is controlled at 0.70 - 0.85%.

[0020] Co element forms a continuous solid solution with iron. During use, Co inhibits and delays the precipitation and aggregation of carbides of other elements, and significantly improves the thermal strength and high-temperature hardness of steel. In this invention, Co is controlled at 2.50 - 3.00%.

[0021] Ni element is infinitely soluble in iron, expands the austenite region of iron, lowers the A3 point, stabilizes austenite, reduces the diffusion rate of each element, improves the hardenability, increases the strength of steel, and improves the fatigue resistance of steel. In this invention, Ni is controlled at 0.30 - 0.50%.

[0022] P is a harmful element in the alloy, which increases the cold brittleness of the alloy, deteriorates the welding performance, reduces plasticity, and deteriorates the cold bending performance. Therefore, the preferred P content in this invention is ≤0.02%.

[0023] S is a non-metallic inclusion-forming element. To improve and eliminate the harm of low-melting non-metallic inclusions formed by S and other elements such as Fe, an appropriate amount of Mn is controlled to form MnS with S. However, MnS extends and distributes in the rolling direction, reducing the toughness in the rolling direction. It is desirable that the S content is as low as possible. In this invention, it is required that S ≤0.010%.

[0024] The N element is similar to the C element and forms nitrides with alloying elements. Nitrides generally precipitate as primary precipitates at the beginning of solidification. During the subsequent solidification process, they are extremely likely to grow under the action of thermodynamics and kinetics. However, as the second phase, nitrides enhance the strength and wear resistance of steel. To control the size and quantity of the nitride second phase, in this invention, the N content is controlled to be ≤0.005%. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention. In the drawings:

[0026] Figure 1 It is a flow chart of the preparation method of the die steel of this invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] It should be noted that, without conflict, the embodiments in this invention and the features in the embodiments can be combined with each other.

[0028] The experimental methods in the following embodiments are all conventional methods unless otherwise specified. In addition, except as specifically described in this embodiment, the various terms and processes involved in this embodiment can be understood according to the general knowledge and conventional methods in the prior art.

[0029] A kind of die steel is prepared by electroslag remelting, homogenizing heating, air blowing cooling and spheroidizing annealing processes, and its chemical components by mass percentage include: C: 0.38 - 0.42%, Si: 0.30 - 0.45%, Mn: 0.70 - 0.85%, Cr: 2.55 - 2.80%, Mo: 2.30 - 2.50%, V: 0.80 - 1.00%, Ni: 0.30 - 0.50%, Co: 2.50 - 3.00%, and the balance is Fe and impurities.

[0030] The preparation of the die steel of this invention undergoes electroslag remelting. Electroslag remelting is a method of smelting using the resistance heat generated when an electric current passes through the molten slag as the heat source. Its main purpose is to purify the metal and obtain an ingot with a clean, uniform and dense structure. The steel after electroslag remelting has high purity, low sulfur content, few non-metallic inclusions, a smooth surface of the ingot, is clean, uniform and dense, and has a uniform metallographic structure and chemical composition. The as-cast mechanical properties of electroslag steel can reach or exceed the indicators of forgings of the same steel grade.

[0031] Homogenization heating is to heat the alloy ingot to a temperature close to the non-equilibrium solidus temperature and hold it for a long time. Through the diffusion of alloying element atoms, the inhomogeneity of chemical composition and structure within the crystal can be eliminated or reduced, the liquid segregation carbides can be dissolved, the internal structure of the ingot can be improved, the residual stress of the ingot can be eliminated, the machining performance of the ingot can be improved. At the same time, the plasticity can be increased, the deformation resistance can be reduced, and the hot working process performance of the alloy can be improved.

[0032] Blowing air cooling is to provide pre-treated martensite structure for spheroidizing annealing when the hot-worked steel does not pass through the pearlite phase region and bainite phase region during the phase transformation process, preventing the uneven distribution of cementite in the subsequent structure and affecting the mechanical properties. At the same time, the grains of the hot-worked structure are refined through extremely strong cooling.

[0033] Spheroidizing annealing is annealing to spheroidize the carbides in the steel to obtain a structure with spherical or granular carbides evenly distributed on the ferrite matrix. Spheroidizing annealing is mainly used for eutectoid steel and hypereutectoid steel to obtain a spheroidized structure similar to granular pearlite (since it is not necessarily eutectoid composition, it is called spheroidized structure), thereby reducing hardness and improving machining performance. However, the C content of the present invention is 0.38%-0.42%, belonging to hypoeutectoid steel. The purpose of spheroidizing annealing for this is to further refine the grains of the recrystallized hot-worked structure and to form a spheroidized annealing structure with evenly distributed cementite in the structure after water-air alternation.

[0034] By optimizing the C and Cr contents, adding Mo, Ni and Co elements, comprehensively considering the law of dissolution of different element carbides into austenite during quenching and their effect of refining grains, and preparing by the above process, the present invention has high hardness, heat fatigue resistance, and is not prone to thermal fatigue cracks during use. After the steel is produced by the above process and detected according to GB / T18254, the liquid segregation carbide grade at 1 / 4 of the diameter is <1 level. Using the die steel of the present invention can improve the service life of the die. The service life is increased by 50-80% compared with the existing H13 hot work die steel.

[0035] The die steel of the present invention allows a certain amount of inevitable impurities, and the impurities may include at least one of P, S, and N. Preferably, P: ≤0.02%, S: ≤0.010%, N: ≤0.008%.

[0036] Furthermore, the liquid segregation carbide content of the die steel of the present invention is ≤0.001%.

[0037] At the same time, the present invention also provides a preparation method of die steel, which specifically includes the following steps:

[0038] Electroslag remelting: The alloy and returned materials are melted in a neutral crucible and poured into a ladle. After refining by LF (Ladle Furnace) outside the furnace and vacuum degassing by VD (Vacuum Degasser) furnace, N≤0.005%. It is cast into electrode bar ingots and undergoes electroslag remelting.

[0039] Homogenization heating: The electrode bar ingots after electroslag remelting are annealed and then heated in a chamber furnace. The heating temperature is 1260 - 1310 °C, and the preferred holding time is 15 - 20 hours. Using the above longer holding time can make the crystal distribution inside the ingot alloy more uniform. After heating, it is bloomed and rounded by a blooming mill, and after stress relief annealing, it is heated to 1030 - 1130 °C. After being formed into steel by a precision forging machine or rolling mill, it is subjected to air blowing cooling and refinement treatment.

[0040] Air blowing cooling: The steel formed by a precision forging machine or rolling mill is heated in a chamber furnace for 0.5 - 3 hours, and a blower with a capacity of 300m 3 / h cools the steel to 10 - 30 °C above the Ms point.

[0041] Spheroidizing annealing: First step: Put the steel into a heating furnace and heat it at a rate of 90 - 100 °C / h to 730 - 800 °C, and the preferred holding time is 6 - 8h; here, this steel grade can be uniformly heated below the austenite start transformation temperature, so that this steel grade is uniformly heated and the generation of thermal stress caused by too fast heating temperature rise can be prevented. Second step: Heat the steel at a rate of 90 - 100 °C / h to 850 - 880 °C, and the preferred holding time is 5.5 - 7h; here, this steel grade can be uniformly heated to the end temperature of austenite transformation, so that this steel grade is completely converted into austenite. Third step: Rapidly cool the steel at a rate of ≥50 °C / h to 800 °C without holding, so that this steel grade enters the start temperature of phase transformation. Fourth step: Slowly cool the steel to 660 - 700 °C and hold, and the preferred holding time is 9 - 11 hours; during this process, it is slowly cooled at a rate of ≤15 °C per hour, so that this steel grade completes phase transformation spheroidizing annealing and forms a pearlite spheroidized structure. Fifth step: Slowly cool to 520 - 570 °C, and the preferred holding time is 3.5 - 4.5h; before discharging from the furnace, it is cooled at a rate of ≤15 °C per hour, so that the surface and core of this steel grade are cooled evenly and the generation of internal and external thermal stress cracking can be prevented. After spheroidizing annealing is completed, open all heat dissipation ports and cool in the furnace to 490 - 510 °C and then discharge. Using the above stepped annealing method, the stepped temperature first rises and then decreases, which further refines the grains of the steel structure recrystallization and makes the structure after air blowing cooling form a spheroidized annealing structure with uniformly distributed cementite.

[0042] The following describes the specific implementation solutions of the present invention in detail.

[0043] Example 1

[0044] This embodiment relates to a die steel, comprising the following components by mass percentage: C: 0.41%, Si: 0.40%, Mn: 0.73%, P: 0.012%, S: 0.003%, Cr: 2.78%, Mo: 2.33%, V: 0.90%, N: 0.005%, Ni: 0.35%, Co: 2.75%, and the balance being iron and unavoidable impurities.

[0045] Its preparation method comprises the following steps:

[0046] Using a neutral crucible to melt the alloy and return materials and pour them into a ladle, subjecting to LF refining outside the furnace, VD vacuum degassing, casting into R165 electrode bars, and then subjecting to electroslag remelting or continuous cooling and ingot-pulling electroslag remelting to obtain φ400 electroslag ingots.

[0047] After annealing, the steel ingot is heated in a chamber furnace at 1260 °C for 17 hours for homogenization. After heating, it is rounded into a φ320 intermediate billet by a quick forging machine. After stress relief annealing, it is heated at 1080 °C, formed by a precision forging machine or rolling mill, blown and cooled to 190 °C, and then spheroidized annealed.

[0048] Further, the spheroidizing annealing process comprises the following steps:

[0049] The first step: putting the steel into a heating furnace, heating it at a rate of 90 - 100 °C / h to 730 - 800 °C, and holding for 6 - 8 h; the second step: heating the steel at a rate of 90 - 100 °C / h to 850 - 880 °C, and holding for 5.5 - 7 h; the third step: quickly cooling the steel at a rate of ≥50 °C / h to 800 °C; the fourth step: slowly cooling the steel to 660 °C and holding for 11 hours; the fifth step: slowly cooling to 520 °C and holding for 4 hours: opening all heat dissipation ports, and cooling with the furnace to 500 °C and then taking out of the furnace.

[0050] The product is oil quenched at 1030 °C and tempered at 535 °C. After tempering, the hardness reaches 51 HRC, the grain size reaches grade 10, and the impact toughness is 278 J.

[0051] Example Two

[0052] This embodiment relates to a die steel, comprising the following components by mass percentage: C: 0.41%, Si: 0.35%, Mn: 0.74%, P: 0.012%, S: 0.003%, Cr: 2.69%, Mo: 2.34%, V: 0.87%, N: 0.005%, Ni: 0.35%, Co: 2.75%, and the balance being iron and unavoidable impurities.

[0053] Its preparation method comprises the following steps:

[0054] The alloy and the return materials are melted in a neutral crucible and poured into a ladle. After refining by LF furnace and vacuum degassing by VD furnace, they are cast into R150 electrode bars, and then remelted by electroslag remelting or continuously cooled and drawn ingot type electroslag remelting into φ360 electroslag ingots.

[0055] The steel ingot is annealed and then heated in a chamber furnace at 1260°C for 15 hours for homogenization. After heating, it is rounded into a φ260 intermediate billet by a quick forging machine. After stress relief annealing, it is heated at 1080°C and formed into finished products by a precision forging machine or a rolling mill, then blown and cooled to 190°C and then spheroidized annealed.

[0056] Further, the spheroidizing annealing process includes the following steps:

[0057] The first step: Put the steel into a heating furnace and heat it at a rate of 90 - 100°C / h to 780°C and hold for 6 - 8 hours; The second step: Heat the steel at a rate of 90 - 100°C / h to 850 - 880°C and hold for 5.5 - 7 hours; The third step: Rapidly cool the steel at a rate of ≥50°C / h to 800°C; The fourth step: Slowly cool the steel to 680°C and hold for 9 - 11 hours; The fifth step: Slowly cool to 550°C and hold for 4 hours: Open all heat dissipation ports and cool in the furnace to 500°C and then take out of the furnace.

[0058] The product is oil quenched at 1050°C and tempered at 525°C. After tempering, the hardness reaches 52HRC, the grain size reaches grade 10, and the impact toughness is 265J.

[0059] Example 3

[0060] This example relates to a die steel, including the following components by mass percentage: C: 0.38%, Si: 0.30%, Mn: 0.70%, Cr: 2.55%, Mo: 2.30%, V: 1.00%, Ni: 0.50%, Co: 3.00%, and the balance is iron and inevitable impurities.

[0061] Its preparation method includes the following steps:

[0062] The alloy and the return materials are melted in a neutral crucible and poured into a ladle. After refining by LF furnace and vacuum degassing by VD furnace, they are cast into R150 electrode bars, and then remelted by electroslag remelting or continuously cooled and drawn ingot type electroslag remelting into φ315 electroslag ingots.

[0063] The steel ingot is annealed and then heated in a chamber furnace at 1260°C for 15 hours for homogenization. After heating, it is rounded into a φ190 intermediate billet by a precision forging machine. After stress relief annealing, it is heated at 1080°C and formed into finished products by a precision forging machine or a rolling mill, then blown and cooled to 190°C and then spheroidized annealed.

[0064] Further, the spheroidizing annealing process includes the following steps:

[0065] First stage: Put the steel into a heating furnace and heat it up to 800 °C at a rate of 90 - 100 °C / h, and keep it warm for 6 - 8 h; Second stage: Heat the steel at a rate of 90 - 100 °C / h to 850 - 880 °C and keep it warm for 5.5 - 7 h; Third stage: Rapidly cool the steel to 800 °C at a rate of ≥50 °C / h; Fourth stage: Slowly cool the steel to 700 °C and keep it warm for 9 hours; Fifth stage: Slowly cool it to 550 °C for 4 hours: Open all the heat dissipation ports and cool it in the furnace to 500 °C and then take it out of the furnace.

[0066] The product is oil quenched at 1030 °C and tempered at 550 °C. After tempering, the hardness reaches 49 HRC, the grain size reaches grade 10, and the impact toughness is 300 J.

[0067] Example 4

[0068] This example relates to a die steel, including the following components by mass percentage: C: 0.42%, Si: 0.45%, Mn: 0.85%, Cr: 2.80%, Mo: 2.50%, V: 0.80%, Ni: 0.30%, Co: 2.50%, and the balance is iron and inevitable impurities.

[0069] Its preparation method includes the following steps:

[0070] Use a neutral crucible to melt the alloy and return materials and pour them into a ladle. After refining outside the LF furnace and vacuum degassing in the VD furnace, cast it into an R165 electrode bar, and then remelt it by electroslag remelting or continuously cooled ingot-drawing type electroslag remelting to a φ450 electroslag ingot.

[0071] The steel ingot is annealed and then heated in a chamber furnace at 1260 °C for 15 hours for homogenization. After heating, it is rounded into a φ360 intermediate billet by a quick forging machine. After stress relief annealing, it is heated at 1080 °C and formed into a finished product by a precision forging machine or rolling mill, blown and cooled to 190 °C, and then spheroidized annealed.

[0072] Furthermore, the spheroidizing annealing process includes the following steps:

[0073] First stage: Put the steel into a heating furnace and heat it up to 800 °C at a rate of 90 - 100 °C / h, and keep it warm for 6 - 8 h; Second stage: Heat the steel at a rate of 90 - 100 °C / h to 850 - 880 °C and keep it warm for 5.5 - 7 h; Third stage: Rapidly cool the steel to 800 °C at a rate of ≥50 °C / h; Fourth stage: Slowly cool the steel to 700 °C and keep it warm for 9 hours; Fifth stage: Slowly cool it to 550 °C for 4 hours: Open all the heat dissipation ports and cool it in the furnace to 500 °C and then take it out of the furnace.

[0074] The product is oil quenched at 1050°C, tempered at 550°C, and after tempering, the hardness reaches 50 HRC, the grain size reaches grade 10, and the impact toughness is 280 J.

[0075] Comparative Example 1

[0076] This comparative example relates to a die steel with the same element content as in Example 1.

[0077] Its preparation method includes the following steps:

[0078] The alloy and return materials are melted in a neutral crucible and poured into a ladle, refined outside the LF furnace, vacuum degassed in the VD furnace, cast into an R165 electrode bar, and then remelted by electroslag remelting or continuously cooled and drawn ingot type electroslag remelting into a φ400 electroslag ingot.

[0079] After annealing, the steel ingot is heated in a chamber furnace at a heating temperature of 1150°C for a holding time of 4 h. After heating, it is rounded into a φ320 intermediate blank by a quick forging machine, stress relieved, heated at 1080°C, formed by a precision forging machine or rolling mill, blown and cooled to 190°C, and then spheroidized annealed.

[0080] Further, the spheroidizing annealing process includes the following steps:

[0081] First step: Put the steel into a heating furnace and heat it at a rate of 90 - 100°C / h to 730 - 800°C, and hold for 6 - 8 h; Second step: Heat the steel at a rate of 90 - 100°C / h to 850 - 880°C and hold for 5.5 - 7 h; Third step: Rapidly cool the steel at a rate of ≥50°C / h to 800°C; Fourth step: Slowly cool the steel to 660°C and hold for 11 hours; Fifth step: Slowly cool to 520°C for 4 hours: Open all heat dissipation ports and cool in the furnace to 500°C and then take out of the furnace.

[0082] The product is oil quenched at 1030°C, tempered at 535°C, and after tempering, the hardness reaches 51 HRC, the grain size reaches grade 10, and the impact toughness is 200 J.

[0083] Comparative Example 2

[0084] This comparative example relates to a die steel with the same element content as in Example 1.

[0085] Its preparation method includes the following steps:

[0086] This embodiment relates to a die steel, comprising the following components by mass percentage: C: 0.41%, Si: 0.35%, Mn: 0.74%, P: 0.012%, S: 0.003%, Cr: 2.69%, Mo: 2.34%, V: 0.87%, N: 0.005%, Ni: 0.35%, Co: 2.75%, and the balance being iron and unavoidable impurities.

[0087] Its preparation method comprises the following steps:

[0088] Using a neutral crucible to melt the alloy and return materials and pour them into a ladle, subjecting to LF furnace external refining, VD furnace vacuum degassing, casting into an R150 electrode bar, and then subjecting to electroslag remelting or continuous cooling ingot-pulling type electroslag remelting to obtain a φ360 electroslag ingot.

[0089] After annealing, the steel ingot is heated in a chamber furnace at a heating temperature of 1260 °C for 15 hours for homogenization. After heating, it is rounded into a φ260 intermediate billet by a quick forging machine, stress-relieved annealed, heated at 1080 °C, formed into finished products by a precision forging machine or rolling mill, and then spheroidized annealed.

[0090] Furthermore, the spheroidizing annealing process comprises the following steps:

[0091] First step: Put the steel into a heating furnace and heat it at a rate of 90 - 100 °C / h to 780 °C, and hold for 6 - 8 h; Second step: Heat the steel at a rate of 90 - 100 °C / h to 850 - 880 °C, and hold for 5.5 - 7 h; Third step: Rapidly cool the steel at a rate of ≥50 °C / h to 800 °C; Fourth step: Slowly cool the steel to 680 °C and hold for 9 - 11 hours; Fifth step: Slowly cool to 550 °C for 4 hours: Open all heat dissipation ports and cool in the furnace to 500 °C and then take out of the furnace.

[0092] The product is oil quenched at 1050 °C, tempered at 525 °C, and after tempering, the hardness reaches 52 HRC, the grain size reaches grade 9, and the impact toughness is 210 J.

[0093] Comparative Example 3

[0094] This comparative example relates to a die steel. Except for the V element in the element ratio, the other contents are the same as those in Example 1. The preparation method is the same as that in Example 1. The content of the V element is 1.50%.

[0095] The product is oil quenched at 1030 °C, tempered at 535 °C, and after tempering, the hardness reaches 52 HRC, the grain size reaches grade 10, and the impact toughness is 220 J.

[0096] The above examples and comparative examples are tested, and the test results of each example and comparative example are shown in the following table.

[0097] Table 1

[0098] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Impact toughness / J 278 265 300 280 200 210 220 Hardness / HRC 51 52 49 50 51 52 52 Grain size Grade 10 Grade 10 Grade 10 Grade 10 Grade 10 Grade 9 Grade 10

[0099] By comparing the above-mentioned examples and comparative examples, the impact toughness of the examples is significantly better than that of the comparative examples. Example 1 and Comparative Example 1 illustrate that the homogenization heating temperature is 1260 - 1310 °C, and the preferred holding time is 15 - 20 hours. Adopting the above-mentioned longer holding time can improve the chemical composition segregation inside the ingot alloy, and can dissolve a small amount of liquid segregation carbides formed in the non-equilibrium state during smelting at high temperature.

[0100] Example 1 and Comparative Example 2 illustrate that air blowing cooling enables the steel after hot working to not pass through the pearlite phase region and the bainite phase region during the phase transformation process, providing a pre-treated martensite structure for spheroidizing annealing, preventing uneven distribution of cementite in the subsequent structure and affecting mechanical properties. At the same time, the structure grains after hot working are refined through extremely strong cooling.

[0101] Example 1 and Comparative Example 3 illustrate that after the V content is higher than 1.00%, liquid segregation carbides are generated, and the impact toughness after heat treatment is significantly reduced.

[0102] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0103] In addition, it should be noted that, in the case of no contradiction, the various specific technical features described in the above specific embodiments can be combined in any appropriate way. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

Claims

1. A preparation method of die steel, characterized in that: the die steel is prepared by electroslag remelting smelting, homogenization heating, air blowing cooling and spheroidizing annealing processes; the chemical components of the die steel by mass percentage include: C: 0.38 - 0.42%, Si: 0.30 - 0.45%, Mn: 0.70 - 0.85%, Cr: 2.55 - 2.80%, Mo: 2.30 - 2.50%, V: 0.80 - 1.00%, Ni: 0.30 - 0.50%, Co: 2.50 - 3.00%, and the balance is Fe and impurities; the preparation method includes the following steps: Electroslag remelting smelting: Melting the alloy raw materials and pouring them into a ladle, refining outside the LF furnace, vacuum degassing in the VD furnace, casting into an ingot, and carrying out electroslag remelting smelting; Homogenization heating: After annealing the ingot smelted by electroslag remelting, heating it with a chamber furnace, the heating temperature is 1260 - 1310 °C, after heating, blooming and rounding, and then heating to 1030 - 1130 °C after stress relief annealing, and forging and rolling into steel; After air blowing cooling the steel, put it into a heating furnace for spheroidizing annealing, and cool it in the furnace to 490 - 510 °C and then take it out of the furnace.

2. The preparation method of die steel according to claim 1, characterized in that: the impurities include P, P: ≤ 0.02%.

3. The preparation method of die steel according to claim 1, characterized in that: the impurities include S, S: ≤ 0.010%.

4. The preparation method of die steel according to claim 1, characterized in that: the impurities include N, N: ≤ 0.005%.

5. The preparation method of die steel according to any one of claims 1 - 4, characterized in that: the content of liquid segregation carbide in the die steel ≤ 0.001%.

6. The preparation method of die steel according to claim 5, characterized in that: the holding time at the homogenization heating temperature of 1260 - 1310 °C is 15 - 20 hours.

7. The preparation method of die steel according to claim 5, characterized in that: the spheroidizing annealing includes: The first step: Put the steel into a heating furnace and heat it at a rate of 90 - 100 °C / h to 730 - 800 °C; The second step: Heat the steel at a rate of 90 - 100 °C / h to 850 - 880 °C; The third step: Rapidly cool the steel at a rate of ≥ 50 °C / h to 800 °C; The fourth step: Slowly cool the steel to 660 - 700 °C; The fifth step: Slowly cool the steel to 520 - 570 °C.

8. The preparation method of die steel according to claim 7, characterized in that: the holding time of the first step of the spheroidizing annealing is 6 - 8 h, the holding time of the second step is 5.5 - 7 h, the holding time of the fourth step is 9 - 11 h, and the holding time of the fifth step is 3.5 - 4.5 h.

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