Cold work die steel and method of making

Abstract

This application relates to a cold work die steel and its preparation method, belonging to the field of steel manufacturing technology. The chemical composition of the die steel, by mass fraction, is: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities. By optimizing the steel composition system, inclusions, harmful gases, and phosphorus and sulfur content are effectively reduced. Pure smelting and electroslag remelting technologies are employed to improve material purity; temperature-controlled upsetting and drawing processes are used in forging, combined with isothermal spheroidizing annealing, to achieve carbide refinement and microstructure homogenization; subsequent two-stage tempering is used to control the hardness and toughness balance. This mold steel combines excellent wear resistance and impact resistance, and can replace traditional high-end cold work mold steel, significantly improving the service life and stability of mold steel while reducing overall costs. It is suitable for the manufacture of high-end molds such as precision stamping and cold heading.

Description

Technical Field

[0001] This application relates to the field of steel manufacturing technology, and in particular to a cold work die steel and its preparation method. Background Technology

[0002] Cold work die steel is a key tool material used in manufacturing molds for room temperature applications. It is widely used in the processing of molds for cold drawing, cold heading, cold extrusion, embossing, and roll forming. Cold work die steel is responsible for forming high-precision parts for automobiles, home appliances, and electronic communications, playing a core role in improving the efficiency and quality of industrial production.

[0003] While current mainstream high-carbon, high-chromium cold work die steels (such as Cr12MoV and D2) have high hardness, the uneven distribution of coarse eutectic carbides in their as-cast microstructure leads to insufficient toughness and premature die failure. Foreign improved steel grades (such as DC53 and ASSAB88) have improved die steel toughness by adding high-valence molybdenum, but their high cost hinders widespread adoption. Domestically, some molybdenum-reducing alternatives have lowered costs, but improper carbon content adjustments have resulted in a significant decrease in wear resistance, failing to meet high-end performance requirements. Summary of the Invention

[0004] This application provides a cold work die steel and its preparation method to solve the following technical problem: how to achieve a breakthrough in the unified high toughness and high wear resistance of cold work die steel while reducing the amount of precious metals used. In a first aspect, embodiments of this application provide a cold work die steel, the chemical composition of which, by mass fraction, is: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities.

[0005] Optionally, the microstructure of the mold steel, by volume fraction, is: tempered martensite: 85%~90%, retained austenite: 8%~13%, and the remainder being carbides.

[0006] Optionally, the mold steel meets at least one of the following properties: a first tempering hardness of 61HRC~63HRC, a second tempering hardness of 60HRC~62HRC, and an impact toughness of 70J~100J.

[0007] Secondly, embodiments of this application provide a method for preparing the mold steel described in the first aspect, the method comprising: An electroslag ingot with the following chemical composition was obtained: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities; The electroslag ingot is sequentially heated and held, forged, isothermal annealed, quenched and tempered to obtain mold steel.

[0008] Optionally, the heating and heat preservation temperature is 1100℃~1180℃, and the heating and heat preservation time is 2.5h~4.0h.

[0009] Optionally, the initial forging temperature is 1070℃~1120℃, and the final forging temperature is ≥890℃.

[0010] Optionally, the forging adopts an upsetting-drawing process, and the total forging ratio is 5~10.

[0011] Optionally, the isothermal annealing temperature is 820℃~880℃, and the isothermal annealing holding time is 2h~4h.

[0012] Optionally, the cooling rate of the isothermal annealing is 20℃ / h~40℃ / h, and the furnace exit temperature of the isothermal annealing is ≤350℃.

[0013] Optionally, the quenching medium is quenching oil, and the quenching temperature is 1030℃~1060℃.

[0014] Optionally, the tempering includes primary tempering and secondary tempering; The temperature for the first tempering is 450℃~550℃, and the temperature for the second tempering is 350℃~450℃.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a cold work die steel with the following chemical composition by mass fraction: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities. By reconstructing the alloy element ratio system and innovating the preparation process, a synergistic leap in the toughness and wear resistance of the cold work die steel is achieved while reducing dependence on precious metals. At the composition design level, high-hardness vanadium carbides partially replace traditional molybdenum carbides to perform wear-resistant strengthening functions, while Ti elements induce the precipitation of nano-sized nitrogen carbides to refine the grains; the precise ratio of Si and Mn elements significantly improves the hardenability and tempering stability of the mold steel matrix, providing support for the balance of toughness and plasticity. At the process level, impurity elements are suppressed to below the critical threshold through a dual purification technology of pure smelting and electroslag remelting, blocking the brittle fracture initiation path from the source; temperature-controlled forging and isothermal spheroidizing annealing processes work together to break down coarse carbides and induce the formation of spheroidal structures, eliminating the hidden danger of stress concentration; the unique two-stage tempering process induces a secondary hardening effect at high temperature to form diffuse strengthening, and stabilizes the residual austenite and releases micro-stress at low temperature, ultimately constructing an ideal microstructure of "diffuse distribution of hard phase + continuous penetration of toughened matrix".

[0016] In summary, by optimizing the alloy composition to break the dependence on precious metals and relying on process innovation to achieve precise control of microstructure, mold steel can simultaneously achieve high toughness and high wear resistance within a low Mo cost framework, thus breaking the long-standing industrial dilemma of "cost reduction inevitably leads to performance loss" in the field of cold work mold steel. Attached Figure Description

[0017] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0019] Figure 1 This is a flowchart illustrating a cold work die steel and its preparation method, provided as an embodiment of this application. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0021] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges between 1 and 6 (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6). Unless otherwise specified, the terms "including" and "contains" used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship. "And / or" indicates that multiple situations can exist individually or simultaneously. Expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0022] In a first aspect, embodiments of this application provide a cold work die steel, the chemical composition of which, by mass fraction, is: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities.

[0023] The positive effects of limiting the C mass fraction to 0.60%~0.70%: C is a key chemical component determining the performance of high-toughness and high-wear-resistance cold work die steel. C can form high-hardness carbides with alloying elements such as V, Mo, and Cr, significantly improving the wear resistance of die steel. Simultaneously, C dissolved in martensite can effectively enhance the strength and hardenability of die steel. However, excessively high C mass fractions in die steel can lead to a decrease in plasticity and impact toughness. Therefore, the C mass fraction needs to be controlled within a reasonable range to balance high strength, high wear resistance, and good toughness. For example, the C mass fraction can be 0.60, 0.62, 0.64, 0.66, 0.68, 0.70, etc.

[0024] According to Steven's equilibrium C formula, as shown below:

[0025]

[0026] In the formula, the symbols of each element represent their mass fraction in steel, C p The calculated carbon parameter, C, is used to characterize the degree of alloying. s This represents the actual carbon content in the steel. A reflects the degree to which the actual carbon content meets the carbon requirements for the formation of carbides by alloying elements. A=1 indicates that the carbon content just meets the requirements for the precipitation of secondary hardening carbides; when A<1, there is a surplus of alloying elements; when A>1, the excess carbon will form M3C type carbides with a high iron content.

[0027] The positive effects of limiting the Si mass fraction to 0.80%~1.00% include: Si dissolved in the matrix of die steel can produce solid solution strengthening, which can significantly improve the elastic limit, yield strength, and tensile strength of die steel. Furthermore, Si can inhibit martensite decomposition during tempering, thereby effectively improving the tempering stability of die steel. For example, the Si mass fraction can be 0.20%, 0.85%, 0.90%, 0.95%, 1.00%, etc.

[0028] The positive effects of limiting the Mn mass fraction to 0.40%~0.60% are as follows: During the austenitization process of mold steel, most of the Mn dissolves into the mold steel matrix, which can increase the matrix alloy content and enhance the solid solution strengthening effect, thereby improving the strength of the mold steel. However, Mn has a significant positive segregation tendency and easily accumulates at eutectic grain boundaries to form intergranular carbides, leading to a decrease in the toughness of the mold steel. Therefore, it is necessary to control the Mn mass fraction within a certain range to achieve the goal of stably enhancing the matrix strength of the mold steel and maintaining the stability of the overall performance of the mold steel. For example, the Mn mass fraction can be 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, etc.

[0029] The positive effects of limiting the Cr mass fraction to 7.50%~8.50% include: Cr is a strong carbide-forming element, which can significantly improve the hardenability of die steel. Cr combines with C to form chromium carbides, which can effectively improve the wear resistance of die steel. However, Cr is also a major forming element of network carbides. If the mass fraction of Cr in die steel is too high, network carbides are easily formed, which in turn impairs the toughness of the die steel. For example, the mass fraction of Cr can be 1.50%, 8.00%, 8.50%, etc.

[0030] The positive effects of limiting the Mo mass fraction to 0.80%~1.20% include: Precisely controlling Mo within this range significantly reduces the cost of precious metals while fully activating Mo's diverse functions. As a strong carbide-forming element, Mo combines with C to form a high-hardness M2C type carbide dispersion phase, directly improving the wear resistance of the mold steel matrix. Simultaneously, Mo exerts a solid solution strengthening effect, enhancing austenite stability and hardening depth, ensuring a fully martensitic microstructure in the core of large-section mold steels. More importantly, Mo inhibits carbide coarsening during tempering, maintaining the persistence of the secondary hardening peak, providing hard phase support for a high-toughness matrix, and overcoming the bottleneck of cost reduction versus performance trade-offs. For example, the Mo mass fraction can be 0.80%, 0.90%, 1.00%, 1.10%, 1.20%, etc.

[0031] The positive effects of limiting the mass fraction of V to 0.85%~1.05% include: V can form highly dispersed hard carbides in mold steel, significantly improving the wear resistance of mold steel and refining the matrix grains of mold steel. During heat treatment, V plays a grain boundary pinning role, effectively inhibiting high-temperature grain coarsening, thereby improving the toughness of mold steel. For example, the mass fraction of V can be 0.85%, 0.95%, 1.05%, etc.

[0032] The positive effects of limiting the Ti mass fraction to 0.015%~0.025% include: adding Ti to mold steel can increase its critical point. TiC, formed by Ti and C, is extremely stable and difficult to dissolve within the conventional austenitizing temperature range of heat treatment, effectively pinning austenite grain boundaries and inhibiting grain growth. Furthermore, microalloying of Ti is expected to further improve the high-temperature microstructure stability of mold steel during tempering. For example, the Ti mass fraction can be 0.015%, 0.020%, 0.025%, etc.

[0033] The positive effects of limiting the sulfur (S) mass fraction to ≤0.010%: S can impair the machinability of die steel and easily cause overheating or burning during hot working. Therefore, controlling the S mass fraction at a low level helps improve the machinability and mechanical properties of die steel, especially suppressing the overheating tendency during continuous forging in radial forging mills. Furthermore, S precipitation at grain boundaries reduces impact toughness; therefore, controlling the S mass fraction can also effectively improve the toughness of die steel. For example, the S mass fraction can be 0.002%, 0.004%, 0.006%, 0.008%, 0.010%, etc.

[0034] The positive effects of limiting the phosphorus (P) mass fraction to ≤0.015%: P is a harmful element in die steel; an increase in the P mass fraction significantly increases the brittleness and reduces the impact toughness of the die steel. Therefore, controlling the P mass fraction in die steel to a lower level than that of traditional cold work die steel helps improve the impact toughness of the die steel. For example, the P mass fraction can be 0.005%, 0.010%, 0.015%, etc.

[0035] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is: The sum of the percentages of all components in a composition should equal 100%, and the content ranges of several components should meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100. Furthermore, the specific content of Fe is made up to 100% by the actual detected values ​​of the other chemical components mentioned above, together with any unlisted active elements and / or impurity elements, and Fe must constitute the absolute proportion as a matrix element.

[0036] In some embodiments, the microstructure of the mold steel, by volume fraction, is: tempered martensite: 85%~90%, retained austenite: 8%~13%, and the remainder being carbides.

[0037] Tempered martensite forms a continuous, strong, and toughened matrix framework, providing high hardness while effectively preventing crack propagation through its lath-like fine structure. Retained austenite, distributed as a toughening phase within the matrix, absorbs impact energy through stress-induced phase transformation, significantly enhancing the crack resistance of the die steel. Dispersed carbides act as a hard, wear-resistant skeleton, bearing frictional loads. The synergistic effect of these three components ultimately forms a multi-layered protection mechanism: a high-hardness matrix resisting plastic deformation, a toughening phase buffering stress, and a hard phase resisting abrasive cutting. This overcomes the bottleneck of traditional die steels where hardness and toughness are difficult to achieve simultaneously. For example, the volume fraction of tempered martensite can be 85%, 86%, 87%, 88%, 89%, 90%, etc.; the volume fraction of retained austenite can be 8%, 9%, 10%, 11%, 12%, 13%, etc.

[0038] In some embodiments, the mold steel satisfies at least one of the following properties: a first tempering hardness of 61HRC~63HRC, a second tempering hardness of 60HRC~62HRC, and an impact toughness of 70J~100J.

[0039] The hardness after primary tempering is between 61 HRC and 63 HRC, indicating that the die steel has achieved sufficient secondary hardening during the high-temperature tempering stage. This effect is mainly dominated by the strengthening effect of dispersed vanadium and molybdenum carbides, providing the die steel with the core ability to resist indentation deformation, thus ensuring a stable foundation for high wear resistance performance. For example, the hardness after primary tempering can be 61 HRC, 62 HRC, or 63 HRC. The hardness after secondary tempering is between 60 HRC and 62 HRC, confirming that the microstructure stress distribution of the die steel can be precisely controlled through low-temperature tempering. While maintaining the performance gains brought by secondary hardening, the microscopic distortion energy inside the die steel is effectively eliminated, thus achieving a balance between wear resistance and brittle fracture resistance, ultimately achieving a comprehensive performance balance of strength and toughness. For example, the hardness after secondary tempering can be 60 HRC, 61 HRC, or 62 HRC. The impact toughness is between 70 J and 100 J, demonstrating a significant breakthrough in the die steel's ability to suppress crack propagation. This performance stems from the synergistic mechanism of fine-grain strengthening and toughening through retained austenite phase transformation, enabling the die steel to effectively resist chipping and cracking under impact loads, thereby significantly extending tool life. For example, impact toughness can be 70J, 80J, 90J, 100J, etc.

[0040] Figure 1 This is a flowchart illustrating a cold work die steel and its preparation method, provided as an embodiment of this application.

[0041] Please see Figure 1 Secondly, this application provides a method for preparing the mold steel described in the first aspect, the method comprising: S1. Obtain an electroslag ingot with the following chemical composition: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities; S2. The electroslag ingot is sequentially heated and held, forged, isothermal annealed, quenched and tempered to obtain mold steel.

[0042] Steel ingots are melted using a vacuum induction furnace to precisely control the alloy composition within the target range. The steel ingots are then used as consumable electrodes for electroslag remelting. After the molten steel has completely cooled and solidified, an electroslag ingot conforming to the design dimensions is obtained.

[0043] In some embodiments, the heating and heat preservation temperature is 1100℃~1180℃, and the heating and heat preservation time is 2.5h~4.0h.

[0044] The heating and holding temperature is between 1100℃ and 1180℃ to precisely match the carbide dissolution kinetics requirements of high-alloy steel. This temperature range allows strong carbide-forming elements such as Cr, Mo, and V to fully dissolve in the austenitic matrix, effectively eliminating as-cast dendrite segregation and compositional inhomogeneity. Simultaneously, this process avoids the critical temperature for abnormal grain growth, providing a homogeneous billet with high plasticity and low deformation resistance for subsequent forging, thus laying the foundation for microstructure refinement in the die steel. For example, the heating and holding temperatures can be 1100℃, 1120℃, 1140℃, 1160℃, or 1180℃. The heating and holding time is between 2.5h and 4.0h. Through sufficient cross-scale diffusion time control, complete dissolution and compositional homogenization of carbides from the core to the surface of the die steel can be achieved, especially promoting the decomposition and atomic migration of large-sized vanadium carbides. This process simultaneously incubates dynamic recrystallization nuclei, effectively shortening the incubation period for subsequent forging deformation-induced fine grain formation, thereby improving the final microstructure uniformity of the die steel to a stable level required by the process. For example, the heating and holding time can be 2.5h, 3.0h, 3.5h, 4.0h, etc.

[0045] In some embodiments, the initial forging temperature is 1070℃~1120℃, and the final forging temperature is ≥890℃.

[0046] The initial forging temperature is between 1070℃ and 1120℃ to precisely match the stability domain of the austenitic single-phase microstructure of the die steel, imparting optimal high-temperature thermoplasticity to the electroslag ingot. This ensures a smooth forging process with large deformation and effectively prevents the initiation of hot working cracks. Simultaneously, this initial forging temperature range creates high-temperature mechanical conditions for the fragmentation of eutectic carbides in the core. In particular, heavy impact deformation within the plastic peak window of 950℃ to 1050℃ can efficiently drive strain energy transfer to the core of the electroslag ingot, achieving forced fragmentation of coarse carbides. For example, the initial forging temperature can be 1070℃, 1080℃, 1090℃, 1100℃, 1110℃, or 1120℃. The final forging temperature is ≥890℃, which can avoid the risks of microcracks and stress concentration caused by deformation of the die steel in the two-phase region. The final forging temperature maintains the recrystallization activity of the die steel, ensuring timely grain refinement after forging and thus preventing cracking due to a sudden drop in thermoplasticity during the final forging stage. Therefore, this process provides a damage-free microstructure basis for subsequent heat treatment. For example, the final forging temperature can be 890℃, 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, etc.

[0047] In some embodiments, the forging employs an upsetting-drawing process, and the total forging ratio is 5 to 10.

[0048] An upsetting-drawing process is employed, with the total forging ratio controlled between 5 and 10. This process, through multi-directional large deformation, thoroughly breaks down the coarse cast eutectic carbide network in the steel ingot, eliminating compositional segregation. The high forging ratio drives strain energy to penetrate deeply into the core of the ingot, achieving dispersed carbide distribution and equiaxed grains, thus laying the microstructure foundation for high toughness and high wear resistance in die steel. For example, the total forging ratio can be 5, 6, 7, 8, 9, or 10.

[0049] In some embodiments, the isothermal annealing temperature is 820℃~880℃, and the isothermal annealing holding time is 2h~4h.

[0050] Isothermal annealing is performed at temperatures between 820℃ and 880℃, with holding times ranging from 2 hours to 4 hours. This process precisely matches the thermodynamic conditions required for carbide spheroidization, promoting the complete dissolution and re-precipitation of lamellar carbides into uniform spherical structures. Sufficient holding time ensures complete microstructural transformation of the core of large-section mold steel, effectively eliminating hardness gradients and providing a pre-treated matrix with low internal stress for subsequent quenching. For example, isothermal annealing temperatures can be 820℃, 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, etc.; and isothermal annealing holding times can be 2 hours, 3 hours, 4 hours, etc.

[0051] In some embodiments, the cooling rate of the isothermal annealing is 20℃ / h to 40℃ / h, and the furnace exit temperature of the isothermal annealing is ≤350℃.

[0052] By controlling the mold steel to be slowly cooled to below 350℃ at a cooling rate between 20℃ / h and 40℃ / h before being unloaded from the furnace, this stepped cooling process effectively avoids the formation of brittle lamellar structures in the pearlite transformation zone. Low-temperature unloading suppresses the risk of cracking due to thermal stress during air cooling. For example, the cooling rate for isothermal annealing can be 20℃ / h, 25℃ / h, 30℃ / h, 35℃ / h, 40℃ / h, etc.; the unloading temperature for isothermal annealing can be 200℃, 250℃, 300℃, 350℃, etc.

[0053] In some embodiments, the quenching medium is quenching oil, and the quenching temperature is 1030℃~1060℃.

[0054] Quenching uses quenching oil as the medium, and the quenching temperature is between 1030℃ and 1060℃. This temperature range precisely matches the austenitization saturation point of the mold steel, promoting the full dissolution of carbides such as Cr, Mo, and V, achieving a high degree of uniformity in austenite composition and a stable microstructure. The moderate cooling rate provided by the quenching oil ensures that the mold steel generates sufficient high-hardness martensite while retaining an appropriate amount of tough residual austenite through thermodynamic hysteresis, thereby simultaneously improving the hardness and impact toughness of the mold steel. This temperature range design also effectively avoids grain coarsening caused by overheating of the mold steel, creating an ideal initial microstructure for the subsequent tempering process. For example, the quenching temperature can be 1030℃, 1040℃, 1050℃, 1060℃, etc.

[0055] In some embodiments, the tempering includes primary tempering and secondary tempering; The temperature for the first tempering is 450℃~550℃, and the temperature for the second tempering is 350℃~450℃.

[0056] Tempering includes primary and secondary tempering, with the tempering temperature primarily determined by the secondary precipitation hardening effect of carbides such as Cr, Mo, and V. This process promotes the formation of tempered martensite in the die steel, thereby increasing its hardness and effectively reducing quenching residual stress. Ultimately, the die steel achieves a stable microstructure with excellent properties.

[0057] The primary tempering temperature is between 450℃ and 550℃ to trigger a significant secondary hardening effect. This process promotes the dispersion and precipitation of V and Mo carbides from the supersaturated martensite in the die steel, while simultaneously transforming the austenite remaining after quenching into martensite. As a result, the die steel maintains high hardness while significantly improving its impact resistance, thus constructing a strong and toughened matrix structure. For example, the primary tempering temperature can be 450℃, 470℃, 490℃, 510℃, 530℃, or 550℃. The secondary tempering temperature is between 350℃ and 450℃. This temperature range improves the stability of the retained austenite, preventing it from further transforming into martensite at room temperature and reducing the toughness of the die steel. Secondary tempering eliminates the micro-stress accumulated in the die steel by the previous heat treatment, while retaining the nano-reinforcing phase formed by secondary hardening. Ultimately, the die steel achieves an optimal balance between hardness and toughness, thus possessing both good crack resistance and durable wear resistance. For example, the temperature for secondary tempering can be 350℃, 370℃, 390℃, 410℃, 430℃, 450℃, etc.

[0058] The product prepared by the method for preparing cold work die steel is the aforementioned cold work die steel. Since the method for preparing cold work die steel adopts some or all of the technical solutions of the embodiments of cold work die steel, it has at least all the beneficial effects brought about by the technical solutions of the aforementioned embodiments, which will not be elaborated here.

[0059] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0060] The chemical composition (mass percentage / %) of the examples and comparative examples is shown in Table 1.

[0061] Table 1

[0062] Example 1 The steel ingots were smelted in an induction furnace and then electroslag remelted to obtain the electroslag ingots with the chemical composition described in Example 1 of Table 1; wherein, the electroslag ingots are conical ingots with a bottom diameter of 400 mm; The electroslag ingot was heated to 1160℃ and held for 3 hours before forging. The initial forging temperature was 1080℃ and the final forging temperature was 930℃. It was then upset and drawn, with a forging ratio of 9, to forge the electroslag ingot into a steel plate with a thickness of 80mm. The steel plate was then isothermally annealed at 850℃ for 3 hours, followed by slow cooling to 340℃ at a cooling rate of 30℃ / h before being removed from the furnace. The steel plate was then quenched at 1050℃. Finally, the steel plate was tempered, with a first tempering temperature of 530℃ and a first annealing holding time of 3 hours, and a second tempering temperature of 400℃ and a second annealing holding time of 2.5 hours, yielding the die steel.

[0063] Example 2 The steel ingots were smelted in an induction furnace and then electroslag remelted to obtain the electroslag ingots with the chemical composition described in Example 2 of Table 1; wherein, the electroslag ingots are conical ingots with a bottom diameter of 400 mm; The electroslag ingot was heated to 1150℃ and held for 3.1 hours before forging. The initial forging temperature was 1110℃ and the final forging temperature was 920℃. Then, it was upset and drawn, with a forging ratio of 6, to forge the electroslag ingot into a steel plate with a thickness of 100mm. The steel plate was then isothermally annealed at 860℃ for 3.5 hours, followed by slow cooling to 330℃ at a cooling rate of 28℃ / h before being removed from the furnace. The steel plate was then quenched at 1040℃. Finally, the steel plate was tempered, with a first tempering temperature of 550℃ and a first annealing holding time of 3 hours, and a second tempering temperature of 400℃ and a second annealing holding time of 2.5 hours, yielding the die steel.

[0064] Example 3 The steel ingots were smelted in an induction furnace and then electroslag remelted to obtain the electroslag ingots with the chemical composition described in Example 3 of Table 1; wherein, the electroslag ingots are conical ingots with a bottom diameter of 400 mm; The electroslag ingot was heated to 1140℃ and held for 3.2 hours before forging. The initial forging temperature was 1090℃ and the final forging temperature was 910℃. Then, it was upset and drawn, with a forging ratio of 7, to forge the electroslag ingot into a steel plate with a thickness of 90mm. The steel plate was then isothermally annealed at 850℃ for 3.3 hours, followed by slow cooling to 350℃ at a cooling rate of 31℃ / h before being removed from the furnace. The steel plate was then quenched at 1050℃. Finally, the steel plate was tempered, with a first tempering temperature of 530℃ and a first annealing holding time of 3 hours, and a second tempering temperature of 410℃ and a second annealing holding time of 2.5 hours, yielding the die steel.

[0065] Example 4 The steel ingots were smelted in an induction furnace and then electroslag remelted to obtain the electroslag ingots with the chemical composition described in Example 4 of Table 1; wherein the electroslag ingots are conical ingots with a bottom diameter of 400 mm; The electroslag ingot was heated to 1160℃ and held for 3.1 hours before forging. The initial forging temperature was 1105℃ and the final forging temperature was 920℃. Then, it was upset and drawn, with a forging ratio of 8, to forge the electroslag ingot into a steel plate with a thickness of 80mm. The steel plate was then isothermally annealed at 840℃ for 3.5 hours, followed by slow cooling to 330℃ at a cooling rate of 32℃ / h before being removed from the furnace. The steel plate was then quenched at 1050℃. Finally, the steel plate was tempered, with a first tempering temperature of 520℃ and a first annealing holding time of 3 hours, and a second tempering temperature of 400℃ and a second annealing holding time of 2.5 hours, yielding the die steel.

[0066] Example 5 The steel ingots were smelted in an induction furnace and then electroslag remelted to obtain the electroslag ingots with the chemical composition described in Example 5 of Table 1; wherein the electroslag ingots are conical ingots with a bottom diameter of 400 mm; The electroslag ingot was heated to 1150℃ and held for 3.2 hours before forging. The initial forging temperature was 1085℃ and the final forging temperature was 930℃. Then, it was upset and drawn, with a forging ratio of 8, to forge the electroslag ingot into a steel plate with a thickness of 90mm. The steel plate was then isothermally annealed at 860℃ for 2.5 hours, followed by slow cooling to 300℃ at a cooling rate of 29℃ / h before being removed from the furnace. The steel plate was then quenched at 1040℃. Finally, the steel plate was tempered, with a first tempering temperature of 540℃ and a first annealing holding time of 3 hours, and a second tempering temperature of 390℃ and a second annealing holding time of 2.5 hours, yielding the die steel.

[0067] Comparative Example 1 The DC53 steel was purchased from Daido Steel in Japan. It was heat-treated material, but the heat treatment process is unknown.

[0068] Comparative Example 2 The steel ingots were smelted in an induction furnace and then electroslag remelted to obtain the electroslag ingots with the chemical composition described in Comparative Example 2 in Table 1; wherein the electroslag ingots are conical ingots with a bottom diameter of 400 mm. The electroslag ingot was heated to 1150℃ and held for 3.2 hours before forging. The initial forging temperature was 1080℃ and the final forging temperature was 910℃. Then, it was upset and drawn, with a forging ratio of 7, to forge the electroslag ingot into a steel plate with a thickness of 90mm. The steel plate was then isothermally annealed at 850℃ for 3 hours, followed by slow cooling to 330℃ at a cooling rate of 30℃ / h before being removed from the furnace. The steel plate was then quenched at 1040℃. Finally, the steel plate was tempered, with a first tempering temperature of 550℃ and a first annealing holding time of 3 hours, and a second tempering temperature of 400℃ and a second annealing holding time of 2.5 hours, yielding the die steel.

[0069] The performance of the embodiments and comparative examples is shown in Table 2.

[0070] Table 2

[0071] The data tables above provide a clear comparison of the differences between various embodiments and comparative examples. The following conclusions can be drawn: As can be seen from the data in Table 2, the mold steel provided in this application has a hardness of 61.4 HRC to 62.9 HRC after one tempering, a hardness of 60.9 HRC to 61.8 HRC after two tempering, and an impact toughness of 75 J / cm. 2 ~94J / cm 2 The weight loss due to grinding is 1.576 × 10⁻⁶. -2 g ~ 1.617 × 10 -2 g.

[0072] Examples 1-5 and Comparative Examples 1-2 demonstrate that Examples 1-5, employing an alloy composition system combined with a proprietary forging-heat treatment process, successfully achieved a synergistic improvement in the performance of cold work die steel. With a significantly reduced Mo mass fraction, Examples 1-5 all exhibited high hardness, ultra-high impact toughness, and excellent wear resistance. While Comparative Example 1 maintained a certain level of hardness, its impact toughness was severely insufficient; Comparative Example 2, lacking titanium microalloying, suffered a precipitous deterioration in toughness and showed no improvement in wear resistance. This verifies that the embodiments of this application achieve the goal of cost reduction and efficiency improvement simultaneously through compositional restructuring and process innovation.

[0073] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: This invention provides a cold work die steel that, through innovative design of the alloy composition system, significantly reduces dependence on precious metal elements while achieving precise control of microstructure and synergistic optimization of strength and toughness. Composition optimization effectively suppresses the coarsening tendency of eutectic carbides and refines the grain structure, laying the microstructural foundation for the high toughness of the die steel. Combined with a proprietary heat treatment process, the performance potential of the die steel is further stimulated, resulting in a breakthrough improvement in impact toughness and simultaneous enhancement of wear resistance. Ultimately, this die steel significantly reduces alloy costs while ensuring excellent mechanical properties, successfully overcoming the high cost constraint of high-molybdenum cold work die steel, and providing a material solution for high-end die manufacturing that combines excellent service performance with economic advantages.

[0074] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A cold work die steel, characterized in that, The chemical composition of the mold steel, by mass fraction, is as follows: C: 0.60%~0.70%, Si: 0.80%~1.00%, Mn: 0.40%~0.60%, Cr: 7.50%~8.50%, Mo: 0.80%~1.20%, V: 0.85%~1.05%, Ti: 0.015%~0.025%, S≤0.010%, P≤0.015%, with the remainder being Fe and unavoidable impurities.

2. The mold steel according to claim 1, characterized in that, The microstructure of the mold steel, by volume fraction, is: tempered martensite: 85%~90%, retained austenite: 8%~13%, and the remainder is carbides.

3. The mold steel according to claim 1, characterized in that, The mold steel meets at least one of the following properties: hardness after primary tempering is 61HRC~63HRC, hardness after secondary tempering is 60HRC~62HRC, and impact toughness is 70J~100J.

4. A method for preparing mold steel according to any one of claims 1 to 3, characterized in that, The method includes: An electroslag ingot having the chemical composition described in any one of claims 1 to 3 is obtained; The electroslag ingot is sequentially heated and held, forged, isothermal annealed, quenched and tempered to obtain mold steel.

5. The method according to claim 4, characterized in that, The heating and heat preservation temperature is 1100℃~1180℃, and the heating and heat preservation time is 2.5h~4.0h.

6. The method according to claim 4, characterized in that, The initial forging temperature is 1070℃~1120℃, and the final forging temperature is ≥890℃; and / or, The forging process employs an upsetting-drawing process, and the total forging ratio is 5-10.

7. The method according to claim 4, characterized in that, The isothermal annealing temperature is 820℃~880℃, and the isothermal annealing holding time is 2h~4h.

8. The method according to claim 4, characterized in that, The cooling rate of the isothermal annealing is 20℃ / h~40℃ / h, and the furnace exit temperature of the isothermal annealing is ≤350℃.

9. The method according to claim 4, characterized in that, The quenching medium is quenching oil, and the quenching temperature is 1030℃~1060℃.

10. The method according to claim 4, characterized in that, The tempering includes primary tempering and secondary tempering; The temperature for the first tempering is 450℃~550℃, and the temperature for the second tempering is 350℃~450℃.

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