Large-specification hot work die steel forge piece and preparation method and application thereof

Abstract

The invention relates to the technical field of alloy material casting, in particular to a large-specification hot work die steel forge piece and a preparation method and application thereof. The percentage of the chemical components of the large-specification hot work die steel forge piece meets the condition that Si + Mo is larger than or equal to 0.60% and smaller than or equal to 0.72%. 1.75% < = Mn + Cr + V < = 1.98%. By setting reasonable chemical components, good conditions and basis are provided for material performance and forging of the large-size hot work die steel forge piece, and the large-size hot work die steel forge piece with excellent performance and high yield is successfully prepared by combining a simple and convenient preparation process. By means of the technical scheme, the problems that in the prior art, the preparation technology of the 55NiCrMoV7 die steel large forge piece is complex, the condition is harsh, and the yield is low are effectively solved.

Description

Technical Field

[0001] The invention relates to the technical field of alloy material casting, and in particular to a large-size hot-working die steel forging and a preparation method and application thereof. Background Art

[0002] 55NiCrMoV7 steel is a hot working die steel. It has a high Cr content and is characterized by high hardenability, heat crack resistance, fatigue resistance and impact resistance. It is widely used in the manufacturing of hot working stamping dies such as heavy machinery, marine equipment, and special vehicle manufacturing. The use environment of 55NiCrMoV7 die steel is extremely harsh. It is in direct contact with hot billets during the working process. This requires it to not only overcome the thermal fatigue stress formed by alternating hot and cold, but also withstand high-intensity impact loads. Therefore, the quality requirements must be met: first, it has good wear resistance and thermal stability; second, it has high high temperature strength and good toughness; third, it has a relatively uniform organization and performance distribution.

[0003] Due to the size and weight characteristics of large 55NiCrMoV7 die steel forgings, the existing technology is prone to internal defects and surface defects in the forgings when preparing large 55NiCrMoV7 die steel forgings, resulting in forging failure and cracking. Based on this, the development of a 55NiCrMoV7 die steel large forging preparation technology has practical significance for broadening the application field of 55NiCrMoV7 die steel.

[0004] At present, the solution to solve the defects of 55NiCrMoV7 die steel large forgings is to optimize the forging process, such as controlling the forging temperature, deformation speed and deformation degree, preheating the forgings, etc. to achieve uniform metal deformation and reduce cracks. However, such an operation not only requires very fine control of temperature and time, but also requires multiple forging processes. The process is complex and the conditions are strict. The yield rate of large forgings can only reach about 65%.

[0005] Based on this, developing a preparation process for large forgings of 55NiCrMoV7 die steel with simple process and high yield is of practical significance for the application of 55NiCrMoV7 die steel. Summary of the invention

[0006] In view of the problems of complex preparation process, strict conditions and low yield rate of 55NiCrMoV7 die steel large forgings in the prior art, the present invention provides a large-size hot working die steel forging and its preparation method and application. The present invention provides good conditions and basis for the material properties and forging of large-size hot working die steel forgings by setting reasonable chemical composition, and successfully produces a large-size hot working die steel forging with excellent performance and high yield rate by combining with a simple preparation process, effectively solving the above-mentioned problems existing in 55NiCrMoV7 die steel large forgings, and providing a new design idea for the preparation of 55NiCrMoV7 die steel large forgings.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: The first aspect of the present invention provides a large-size hot working die steel forging, wherein the mass percentage of the chemical composition of the large-size hot working die steel forging is: C: 0.52%-0.58%, Si: 0.15%-0.35%, Mn: 0.70%-0.90%, Cr: 0.85%-1.15%, Ni: 1.55%-1.72%, Mo: 0.38%-0.49%, V: 0.05%-0.12%, S: ≤0.01%, P: ≤0.02% and the balance of Fe and unavoidable impurities; and 0.60%≤[Si]+[Mo]≤0.72%; 1.75%≤[Mn]+[Cr]+[V]≤1.98%, wherein [Si] represents the Si content in the large-sized hot working die steel forgings, [Mo] represents the Mo content in the large-sized hot working die steel forgings, [Mn] represents the Mn content in the large-sized hot working die steel forgings, [Cr] represents the Cr content in the large-sized hot working die steel forgings, and [V] represents the V content in the large-sized hot working die steel forgings.

[0008] Compared with the prior art, the present invention provides good conditions and foundation for the material properties and forging of large-scale hot working die steel forgings by setting reasonable chemical composition. The present invention not only improves the high-temperature oxidation resistance, mechanical strength and tempering resistance of large-scale hot working die steel forgings by limiting the mass percentage of Si and Mo; it also improves the toughness-brittleness transition temperature of the large-scale hot working die steel forgings, significantly improves the hardness of large-scale forgings, and avoids internal cracking. By limiting the mass percentage of Mn, Cr and V, on the one hand, the hardening depth of large-scale forgings is increased, the hot brittleness is reduced, and the forging and plasticity of the forgings are improved; on the other hand, the thermal strength of large-scale forgings can be improved, the elongation and cross-sectional shrinkage can be reduced, and surface defects of forgings can be avoided. At the same time, the addition of V can strengthen the grain boundaries and intergranular spaces of large-scale forgings, improve the hardness and mechanical strength of forgings, and can also be used as a grain refiner to reduce the grain size, thereby improving the toughness of large-scale forgings and reducing the occurrence of forging cracking.

[0009] The second aspect of the present invention provides a method for preparing the large-size hot working die steel forging, comprising the following steps: Step 1: Analyze the chemical composition of the returned material and prepare the new material according to the chemical composition requirements; Step 2, melting the returned material, when the returned material is melted to 70wt%-80wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1530°C-1580°C and a vacuum degree of 1Pa-2Pa to obtain a refined molten steel; adjusting the elements of the refined molten steel at 1550°C-1570°C to obtain a standard molten steel; Step 3: In an inert atmosphere, the standard molten steel is cast, and the temperature is lowered to 1000° C.-1100° C. to obtain an alloy ingot; the alloy ingot is heated to 1330° C.-1350° C. for high temperature homogenization treatment, cooled to 1210° C.-1260° C. for heat preservation, and forged twice in sequence to obtain a first alloy forging; Step 4: Cool the first alloy forging to 780°C-840°C, keep it warm, and then perform tempering at 615°C-700°C after oil cooling to obtain a second alloy forging; heat the second alloy forging to 910°C-960°C, perform normalizing, and air cool to obtain a large-size hot working die steel forging.

[0010] Compared with the prior art, the present invention successfully prepares large-sized hot working die steel forgings with excellent performance through refining, high-temperature homogenization treatment, pier forging, tempering treatment and normalizing treatment. The use of returned materials as the main raw material greatly reduces the inclusion content in the metal raw material, avoids internal defects caused by excessive impurities in the forging, and combines with high-temperature equalization treatment to further reduce carbide aggregation, refine the internal structure, and effectively solve the cracking problem caused by uneven internal stress. After two pier forgings, the overall structure of the forging is homogenized, which is conducive to the subsequent heat treatment to fully forge the core and eliminate internal stress to the greatest extent. Tempering and normalizing treatments can effectively remove internal stress of forgings, improve organizational stability, and prevent cracking of large-sized forgings.

[0011] Preferably, in step one, the mass percentage of the chemical composition of the return material is: C: 0.50%-0.58%, Si: 0.15%-0.30%, Mn: 0.70%-0.80%, Cr: 0.95%-1.15%, Ni: 1.55%-1.72%, Mo: 0.38%-0.42%, V: 0.05%-0.10%, S: ≤0.03%, P: ≤0.03% and the remainder of Fe and unavoidable impurities.

[0012] Preferably, in step 2, the refining insulation time is 2h-4h.

[0013] Preferably, in step three, the holding time of the high temperature homogenization treatment is 1h-2h.

[0014] Preferably, in step three, the insulation time is 20 min-30 min.

[0015] Preferably, in step three, the forging deformation amount of the first forging in the two forgings is 25%-30%, and the forging deformation amount of the second forging is 30%-35%.

[0016] Preferably, in the high temperature homogenization treatment, the temperature is raised to 1330°C-1350°C by programmed heating, and the heating rate is 25°C / min-30°C / min.

[0017] Preferably, in step 4, the temperature is lowered to 780°C-840°C by programmed cooling, and the cooling rate is 20°C / min-25°C / min.

[0018] Preferably, in step 4, the insulation time is 40 min-60 min.

[0019] Preferably, in step four, the tempering treatment is a five-time tempering treatment, and the specific operation includes the following steps: the first tempering temperature is 615°C-625°C, and the first tempering insulation time is 25min-40min; the second tempering temperature is 635°C-645°C, and the second tempering insulation time is 25min-40min; the third tempering temperature is 655°C-665°C, and the third tempering insulation time is 25min-40min; the fourth tempering temperature is 675°C-685°C, and the fourth tempering insulation time is 25min-40min; the fifth tempering temperature is 690°C-700°C, and the fifth tempering insulation time is 25min-40min.

[0020] The present invention utilizes multi-temperature and multi-time tempering to effectively eliminate the internal stress of the forging and improve the structural stability of large-size forgings.

[0021] Preferably, in step four, the normalizing treatment is five normalizing treatments, and the specific operation includes the following steps: the first normalizing temperature is 910℃-920℃, and the first normalizing insulation time is 15min-20min; the second normalizing temperature is 921℃-930℃, and the second normalizing insulation time is 15min-20min; the third normalizing temperature is 931℃-940℃, and the third normalizing insulation time is 15min-20min; the fourth normalizing temperature is 941℃-950℃, and the fourth normalizing insulation time is 15min-20min; the fifth normalizing temperature is 951℃-960℃, and the fifth normalizing insulation time is 15min-20min.

[0022] Gradient normalizing treatment can form a certain temperature gradient inside the forging, so that large-size forgings have good mechanical properties and crack resistance.

[0023] The third aspect of the present invention provides an application of the large-sized hot working die steel forgings or the large-sized hot working die steel forgings prepared by the preparation method of the large-sized hot working die steel forgings in the field of developing new hot working stamping die materials.

[0024] In summary, the present invention uses 55NiCrMoV7 die steel return material as the main raw material, and obtains a large-size hot working die steel forging with excellent performance by limiting the mass percentage of chemical composition and the preparation process. The large-size hot working die steel forging has a simple preparation process, easy operation and high preparation yield. The present invention effectively solves the problems of complex preparation process, strict conditions and low yield of large 55NiCrMoV7 die steel forgings in the prior art. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Example 1 This embodiment provides a large-size hot working die steel forging, which specifically includes the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.54%, Si: 0.27%, Mn: 0.83%, Cr: 0.94%, Ni: 1.64%, Mo: 0.41%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.54%, Si: 0.26%, Mn: 0.78%, Cr: 0.94%, Ni: 1.63%, Mo: 0.40%, V: 0.08%, S: 0.011%, P: 0.013% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted by 75wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1560°C and a vacuum degree of 1Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1560°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3, under an inert atmosphere, casting the standard molten steel, cooling to 1065° C., to obtain an alloy ingot; heating the alloy ingot to 1340° C. at a heating rate of 25° C. / min for high-temperature homogenization treatment for 1.5 h, cooling to 1240° C. for insulation for 25 min, and performing two pull-up forgings in sequence, wherein the forging deformation of the first pull-up forging is 25%, and the forging deformation of the second pull-up forging is 35%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then tempered at 615°C-700°C after oil cooling to obtain a second alloy forging; the second alloy forging is heated to 910°C-960°C for normalizing, and air-cooled to obtain a large-size hot working die steel forging.

[0027] Among them, the specific operation of tempering treatment includes the following steps:

[0028] The specific operation of normalizing treatment includes the following steps:

[0029] Example 2 This embodiment provides a large-size hot working die steel forging, which specifically includes the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.55%, Si: 0.27%, Mn: 0.76%, Cr: 0.85%, Ni: 1.72%, Mo: 0.41%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.55%, Si: 0.26%, Mn: 0.76%, Cr: 0.85%, Ni: 1.63%, Mo: 0.40%, V: 0.08%, S: 0.011%, P: 0.013% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted to 80wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1560°C and a vacuum degree of 2Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1570°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3: In an inert atmosphere, the standard molten steel is cast, and the temperature is reduced to 1000° C. to obtain an alloy ingot; the alloy ingot is heated to 1330° C. at a heating rate of 30° C. / min for high-temperature homogenization treatment for 2 hours, cooled to 1210° C. for insulation for 30 minutes, and forged twice in sequence, wherein the forging deformation of the first forging is 30%, and the forging deformation of the second forging is 30%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then tempered at 615°C-700°C after oil cooling to obtain a second alloy forging; the second alloy forging is heated to 910°C-960°C for normalizing, and air-cooled to obtain a large-size hot working die steel forging.

[0030] Among them, the specific operation of tempering treatment includes the following steps:

[0031] The specific operation of normalizing treatment includes the following steps:

[0032] Example 3 This embodiment provides a large-size hot working die steel forging, which specifically includes the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.54%, Si: 0.27%, Mn: 0.83%, Cr: 0.94%, Ni: 1.64%, Mo: 0.41%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.54%, Si: 0.16%, Mn: 0.83%, Cr: 0.94%, Ni: 1.55%, Mo: 0.41%, V: 0.11%, S: 0.01%, P: 0.015% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted by 75wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1550°C and a vacuum degree of 2Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1565°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3, under an inert atmosphere, casting the standard molten steel, cooling to 1065° C., to obtain an alloy ingot; heating the alloy ingot to 1340° C. at a heating rate of 30° C. / min for high-temperature homogenization for 1.5 h, cooling to 1235° C. for insulation for 25 min, and performing two pull-up forgings in sequence, wherein the forging deformation of the first pull-up forging is 25%, and the forging deformation of the second pull-up forging is 35%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then tempered at 615°C-700°C after oil cooling to obtain a second alloy forging; the second alloy forging is heated to 910°C-960°C for normalizing, and air-cooled to obtain a large-size hot working die steel forging.

[0033] Among them, the specific operation of tempering treatment includes the following steps:

[0034] The specific operation of normalizing treatment includes the following steps:

[0035] Example 4 This embodiment provides a large-size hot working die steel forging, which is different from the embodiment 1 in that the gradient tempering treatment is modified to a tempering treatment at 660° C. for 2.5 hours, specifically including the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.54%, Si: 0.27%, Mn: 0.83%, Cr: 0.94%, Ni: 1.64%, Mo: 0.41%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.54%, Si: 0.26%, Mn: 0.78%, Cr: 0.94%, Ni: 1.63%, Mo: 0.40%, V: 0.08%, S: 0.011%, P: 0.013% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted by 75wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1560°C and a vacuum degree of 1Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1560°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3, under an inert atmosphere, casting the standard molten steel, cooling to 1065° C., to obtain an alloy ingot; heating the alloy ingot to 1340° C. at a heating rate of 25° C. / min for high-temperature homogenization treatment for 1.5 h, cooling to 1240° C. for insulation for 25 min, and performing two pull-up forgings in sequence, wherein the forging deformation of the first pull-up forging is 25%, and the forging deformation of the second pull-up forging is 35%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then oil-cooled at 660°C for 2.5 hours to obtain a second alloy forging; the second alloy forging is heated to 910°C-960°C for normalizing, air-cooled, and a large-size hot working die steel forging is obtained.

[0036] The specific operation of normalizing treatment includes the following steps:

[0037] Example 5 This embodiment provides a large-size hot-working die steel forging, which differs from Embodiment 1 in that the gradient normalizing treatment is replaced by normalizing treatment at 935° C. for 100 min, specifically including the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.54%, Si: 0.27%, Mn: 0.83%, Cr: 0.94%, Ni: 1.64%, Mo: 0.41%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.54%, Si: 0.26%, Mn: 0.78%, Cr: 0.94%, Ni: 1.63%, Mo: 0.40%, V: 0.08%, S: 0.011%, P: 0.013% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted by 75wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1560°C and a vacuum degree of 1Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1560°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3, under an inert atmosphere, casting the standard molten steel, cooling to 1065° C., to obtain an alloy ingot; heating the alloy ingot to 1340° C. at a heating rate of 25° C. / min for high-temperature homogenization treatment for 1.5 h, cooling to 1240° C. for insulation for 25 min, and performing two pull-up forgings in sequence, wherein the forging deformation of the first pull-up forging is 25%, and the forging deformation of the second pull-up forging is 35%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then tempered at 615°C-700°C after oil cooling to obtain a second alloy forging; the second alloy forging is heated to 935°C and normalized for 100 minutes, and air-cooled to obtain a large-size hot working die steel forging.

[0038] Among them, the specific operation of tempering treatment includes the following steps:

[0039] Comparative Example 1 This comparative example provides a large-size hot-working die steel forging, which is different from Example 1 in that the Si+Mo content is 0.8%, and specifically includes the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.54%, Si: 0.28%, Mn: 0.83%, Cr: 0.94%, Ni: 1.64%, Mo: 0.52%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.54%, Si: 0.26%, Mn: 0.78%, Cr: 0.94%, Ni: 1.63%, Mo: 0.40%, V: 0.08%, S: 0.011%, P: 0.013% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted by 75wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1560°C and a vacuum degree of 1Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1560°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3, under an inert atmosphere, casting the standard molten steel, cooling to 1065° C., to obtain an alloy ingot; heating the alloy ingot to 1340° C. at a heating rate of 25° C. / min for high-temperature homogenization treatment for 1.5 h, cooling to 1240° C. for insulation for 25 min, and performing two pull-up forgings in sequence, wherein the forging deformation of the first pull-up forging is 25%, and the forging deformation of the second pull-up forging is 35%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then tempered at 615°C-700°C after oil cooling to obtain a second alloy forging; the second alloy forging is heated to 910°C-960°C for normalizing, and air-cooled to obtain a large-size hot working die steel forging.

[0040] Among them, the specific operation of tempering treatment includes the following steps:

[0041] The specific operation of normalizing treatment includes the following steps:

[0042] Comparative Example 2 This comparative example provides a large-size hot-working die steel forging, which differs from Example 1 in that Mn+Cr+V is 2.06%, and specifically includes the following contents: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.54%, Si: 0.27%, Mn: 0.85%, Cr: 1.10%, Ni: 1.64%, Mo: 0.41%, V: 0.11%, S: 0.006%, P: 0.008% and the balance of Fe and unavoidable impurities; The method for preparing the large-size hot working die steel forging comprises the following steps: Step 1: Analyze the chemical composition of the returned material, and prepare the new material according to the chemical composition requirements; wherein the mass percentage of the chemical composition of the returned material is: C: 0.54%, Si: 0.26%, Mn: 0.78%, Cr: 0.94%, Ni: 1.63%, Mo: 0.40%, V: 0.08%, S: 0.011%, P: 0.013% and the balance of Fe and unavoidable impurities; Step 2, melting the returned material, when the returned material is melted by 75wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1560°C and a vacuum degree of 1Pa, and keeping it warm for 3h to obtain a refined molten steel; at 1560°C, adding metal material to the refined molten steel according to the designed ratio, adjusting the elements of the refined molten steel to reach the composition content of the large-size hot working die steel forgings, and obtaining a standard molten steel; Step 3, under an inert atmosphere, casting the standard molten steel, cooling to 1065° C., to obtain an alloy ingot; heating the alloy ingot to 1340° C. at a heating rate of 25° C. / min for high-temperature homogenization treatment for 1.5 h, cooling to 1240° C. for insulation for 25 min, and performing two pull-up forgings in sequence, wherein the forging deformation of the first pull-up forging is 25%, and the forging deformation of the second pull-up forging is 35%, to obtain a first alloy forging; Step 4: The first alloy forging is gradient cooled to 800°C at a cooling rate of 20°C / min, kept warm, and then tempered at 615°C-700°C after oil cooling to obtain a second alloy forging; the second alloy forging is heated to 910°C-960°C for normalizing, and air-cooled to obtain a large-size hot working die steel forging.

[0043] Among them, the specific operation of tempering treatment includes the following steps:

[0044] The specific operation of normalizing treatment includes the following steps:

[0045] In order to further reflect the technical effect of the present invention, the present invention conducted relevant performance tests on the large-size hot working die steel forgings obtained in Examples 1-5 and Comparative Examples 1-2, specifically including the following contents: preparing 1000 forgings and counting their yield rates (with no surface cracks and no internal cracks in ultrasonic testing as the finished product standard); conducting room temperature tensile tests with reference to the national standard GB / T228.1-2010; using an HRC-150 Rockwell hardness tester to detect the hardness of the sample; and conducting an impact test with reference to the NADCA#207-90 standard. The test results are shown in Table 1.

[0046] Table 1 Performance test results of large-size hot working die steel forgings obtained in Examples 1-5 and Comparative Examples 1-2

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A large-size hot working die steel forging, characterized in that: The mass percentage of the chemical composition of the large-size hot-working die steel forging is: C: 0.52%-0.58%, Si: 0.15%-0.35%, Mn: 0.70%-0.90%, Cr: 0.85%-1.15%, Ni: 1.55%-1.72%, Mo: 0.38%-0.49%, V: 0.05%-0.12%, S: ≤0.01%, P: ≤0.02% and the balance of Fe and unavoidable impurities; and 0.60%≤[Si]+[Mo]≤0.72%; 1.75%≤[Mn]+[Cr]+[V]≤1.98%, wherein [Si] represents the Si content in the large-sized hot working die steel forgings, [Mo] represents the Mo content in the large-sized hot working die steel forgings, [Mn] represents the Mn content in the large-sized hot working die steel forgings, [Cr] represents the Cr content in the large-sized hot working die steel forgings, and [V] represents the V content in the large-sized hot working die steel forgings.

2. The method for preparing large-size hot-working die steel forgings according to claim 1, characterized in that: The steps include: Step 1: Analyze the chemical composition of the returned material and prepare the new material according to the chemical composition requirements; Step 2, melting the returned material, when the returned material is melted to 70wt%-80wt%, adding new material, melting, and obtaining a mixed molten steel; refining the mixed molten steel at 1530°C-1580°C and a vacuum degree of 1Pa-2Pa to obtain a refined molten steel; adjusting the elements of the refined molten steel at 1550°C-1570°C to obtain a standard molten steel; Step 3: In an inert atmosphere, the standard molten steel is cast, and the temperature is lowered to 1000° C.-1100° C. to obtain an alloy ingot; the alloy ingot is heated to 1330° C.-1350° C. for high temperature homogenization treatment, cooled to 1210° C.-1260° C. for heat preservation, and forged twice in sequence to obtain a first alloy forging; Step 4: Cool the first alloy forging to 780°C-840°C, keep it warm, and then perform tempering at 615°C-700°C after oil cooling to obtain a second alloy forging; heat the second alloy forging to 910°C-960°C, perform normalizing, and air cool to obtain a large-size hot working die steel forging.

3. The method for preparing large-size hot-working die steel forgings according to claim 2, characterized in that: In step 1, the mass percentage of the chemical composition of the return material is: C: 0.50%-0.58%, Si: 0.15%-0.30%, Mn: 0.70%-0.80%, Cr: 0.95%-1.15%, Ni: 1.55%-1.72%, Mo: 0.38%-0.42%, V: 0.05%-0.10%, S: ≤0.03%, P: ≤0.03% and the remainder of Fe and unavoidable impurities.

4. The method for preparing large-size hot-working die steel forgings according to claim 2, characterized in that: In step 2, the refining holding time is 2h-4h; and / or In step 3, the holding time of the high temperature homogenization treatment is 1h-2h; and / or In step 3, the insulation time is 20min-30min; and / or In step 3, the forging deformation amount of the first forging in the two forgings is 25%-30%, and the forging deformation amount of the second forging is 30%-35%.

5. The method for preparing large-size hot-working die steel forgings according to claim 2 or 4, characterized in that: In the high temperature homogenization treatment, the temperature is raised to 1330°C-1350°C by programmed heating, and the heating rate is 25°C / min-30°C / min.

6. The method for preparing large-size hot-working die steel forgings according to claim 2, characterized in that: In step 4, the temperature is lowered to 780°C-840°C by programmed cooling at a rate of 20°C / min-25°C / min; and / or In step 4, the insulation time is 40min-60min.

7. The method for preparing large-size hot-working die steel forgings according to claim 2, characterized in that: In step four, the tempering treatment is a five-time tempering treatment, and the specific operation includes the following steps: the first tempering temperature is 615°C-625°C, and the first tempering insulation time is 25min-40min; the second tempering temperature is 635°C-645°C, and the second tempering insulation time is 25min-40min; the third tempering temperature is 655°C-665°C, and the third tempering insulation time is 25min-40min; the fourth tempering temperature is 675°C-685°C, and the fourth tempering insulation time is 25min-40min; the fifth tempering temperature is 690°C-700°C, and the fifth tempering insulation time is 25min-40min.

8. The method for preparing large-size hot-working die steel forgings according to claim 2, characterized in that: In step four, the normalizing treatment is five normalizing treatments, and the specific operation includes the following steps: the first normalizing temperature is 910℃-920℃, and the first normalizing insulation time is 15min-20min; the second normalizing temperature is 921℃-930℃, and the second normalizing insulation time is 15min-20min; the third normalizing temperature is 931℃-940℃, and the third normalizing insulation time is 15min-20min; the fourth normalizing temperature is 941℃-950℃, and the fourth normalizing insulation time is 15min-20min; the fifth normalizing temperature is 951℃-960℃, and the fifth normalizing insulation time is 15min-20min.

9. An application of the large-sized hot working die steel forging as claimed in claim 1 or the large-sized hot working die steel forging prepared by the preparation method of the large-sized hot working die steel forging as claimed in any one of claims 2 to 8 in the field of developing new hot working stamping die materials.

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