Cold work die steel and preparation method thereof

Abstract

The cold work die steel comprises the following components in percentage by mass: 0.95 to 1.05 percent of C, 0.80 to 1.2 percent of Si, 0.40 to 0.50 percent of Mn, 7.00 to 9.0 percent of Cr, 2.0 to 2.5 percent of Mo, 0.25 to 0.35 percent of V, 0.50 to 0.80 percent of W, 0.005 to 0.01 percent of Y, less than or equal to 0.25 percent of Ni, less than or equal to 0.15 percent of Cu, less than or equal to 0.025 percent of P, less than or equal to 0.008 percent of S and the balance of iron and inevitable impurities. Wherein the matrix structure of the cold work die steel comprises martensite and lower bainite, and the volume fraction of the lower bainite is 10-20%. According to the cold work die steel and the preparation method thereof, by adding the heavy rare earth element Y, the effects of purifying a matrix and improving inclusions of the rare earth element are exerted, and the toughness and impact performance of steel are improved; meanwhile, strong carbide is added to form an element W, decarburization is reduced, and wear resistance is improved; the element W can prevent tempering softening, and the high-temperature hardness of the tempered steel is improved; and meanwhile, the total content of the C element and the W element is reasonably controlled, and the carbide distribution state of the cold work die steel is optimized, so that the cold work die steel has better toughness.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy smelting, and in particular relates to a cold working die steel and a preparation method thereof. Background Art

[0002] In the automotive industry, cold working dies are widely used in the production of automotive parts. Especially in the context of the rapid development of the new energy vehicle industry, more stringent requirements are placed on the performance, quality, specifications and varieties of cold working dies. Cold working dies are often subjected to complex mechanical forces such as extrusion, stretching, impact, friction, fatigue, etc. during use. Therefore, the performance requirements for die steel are extremely strict, especially its strength, hardness, plasticity and wear resistance.

[0003] Although traditional cold working die steels have good hardness and wear resistance to varying degrees, they often have certain deficiencies in impact toughness, which limits their application in high-strength stamping and complex shape processing. For example, existing cold working die steels are prone to cracking, breakage or excessive wear when subjected to high-strength impact and repeated loading, which shortens the service life of the die. This performance limitation makes traditional cold working die steels face a major technical bottleneck when meeting the application requirements under high-load and high-impact working conditions.

[0004] Therefore, it is necessary to provide a new solution to the above technical problems. Summary of the invention

[0005] The purpose of the present invention is to provide a cold working die steel and a preparation method thereof, wherein the cold working die steel has high hardness, excellent wear resistance, good impact toughness and fatigue resistance, and can effectively solve the performance bottleneck problem of traditional cold working die steel in high-strength stamping, complex shape processing and high-temperature environment.

[0006] To achieve the above purpose, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a cold working die steel, which comprises the following components in percentage by mass: C 0.95-1.05%, Si 0.80-1.2%, Mn 0.40-0.50%, Cr 7.00-9.0%, Mo 2.0-2.5%, V 0.25-0.35%, W 0.50-0.80%, Y 0.005-0.01%, Ni≤0.25%, Cu≤0.15%, P≤0.025%, S≤0.008%, and the remainder is iron and unavoidable impurities; wherein the cold working die steel matrix structure comprises martensite and lower bainite, and the volume fraction of the lower bainite is 10-20%.

[0008] In one or more embodiments, the total content of C and W in the cold working die steel is 1.50-1.80 wt %.

[0009] In one or more embodiments, the cold working die steel includes a composite eutectic carbide formed by a composite of WC and a eutectic carbide; and / or the cold working die steel includes a Y2O2S composite.

[0010] In one or more embodiments, the cold-working die steel matrix structure includes martensite laths, and the size of the martensite laths is 10-30 μm.

[0011] In a second aspect, the present invention provides a method for preparing the aforementioned cold working die steel, comprising:

[0012] The raw materials are added into a smelting furnace in proportion for smelting, and after smelting, LF refining and VD vacuum degassing are performed to obtain molten steel; the molten steel is cast to obtain an ingot; the ingot is forged and annealed to obtain a forged billet; the forged billet is hot rolled and isothermal spheroidizing annealed to obtain a steel billet; the steel billet is subjected to a composite heat treatment to form a martensite and lower bainite composite structure in the steel billet matrix to obtain the cold working die steel; wherein the composite heat treatment The treatment includes: heating the steel billet after isothermal spheroidizing annealing to 450-550°C and keeping it warm for 15-30 minutes; heating it to 800-900°C and keeping it warm for 25-35 minutes after equalization; then heating it to 1020-1050°C and keeping it warm for 30-50 minutes; finally cooling it to 230-260°C in a molten nitrate medium and keeping it warm for 2-4 hours, air cooling it to room temperature, then heating it to 550-600°C and keeping it warm for 4-8 hours, and then air cooling it to room temperature.

[0013] In one or more embodiments, the VD vacuum degassing treatment includes: after aluminum deoxidation at 1450-1550°C, a first-stage soft argon blowing is performed for 2 minutes, and then a second-stage soft argon blowing is performed for 4-6 minutes. During the second-stage soft argon blowing, yttrium-iron alloy wire is fed, and the wire feeding is completed before the end of the soft argon blowing, and the wire feeding rate is 2-4 m / s; wherein, the flow rate of argon during the wire feeding process is 90-100 L / min, and the flow rate of argon after the wire feeding is completed is 70-80 L / min.

[0014] In one or more embodiments, the yttrium content of the yttrium-iron alloy wire is ≥65wt%, the solid dissolved oxygen content is ≤0.005wt%, and the wire diameter is ≤5mm.

[0015] In one or more embodiments, the molten steel is cast, comprising: preheating the ingot mold to 250-400°C, casting the molten steel into the ingot mold at a casting temperature of 1450-1550°C, keeping it warm for 1 hour, demolding and cooling it in a heat-insulating pit, and removing it from the pit after 48 hours.

[0016] In one or more embodiments, the ingot is subjected to forging and annealing treatment, including: adopting segmented heating during the forging process, with the start forging temperature of 1050-1100°C, the final forging temperature ≥850°C, air cooling to room temperature after final forging, and a forging ratio greater than 3; after forging, annealing treatment is performed, the annealing temperature is 860-900°C, the annealing time is 6 hours, and then furnace cooling to room temperature; wherein the time interval between the forging and annealing treatment is ≤1h.

[0017] In one or more embodiments, the forging blank is subjected to hot rolling and isothermal spheroidizing annealing treatment, including: adopting segmented heating during hot rolling, the starting rolling temperature is 1080-1150°C, the final rolling temperature is ≥850°C, the rolling passes are ≥10, the total reduction is >70%, and air cooling is performed to room temperature after hot rolling; then isothermal spheroidizing annealing treatment is performed, the forging blank is heated to 780°C and kept for more than 5 hours, then the temperature is increased to 860°C and kept for more than 8 hours, then the temperature is reduced to 780°C and kept for more than 12 hours, then furnace cooled to 500°C, and finally air cooled to room temperature; wherein the time interval between the hot rolling and isothermal spheroidizing annealing treatment is ≤1h.

[0018] Compared with the prior art, the cold working die steel and the preparation method thereof provided by the present invention improve the toughness and impact performance of the steel by adding heavy rare earth element Y, which plays the role of rare earth element in purifying the matrix and improving inclusions; at the same time, strong carbide forming element W is added to reduce decarburization and improve wear resistance; and the W element can prevent tempering softening and improve the high temperature hardness of tempered steel; at the same time, the total content of C element and W element is reasonably controlled to obtain a better carbide distribution state than traditional Cr8 type cold working die steel, so that it has better toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 This is an optical microscopic image of the structure of the hot-rolled plate after spheroidizing annealing in Example 1 of the present invention;

[0021] Figure 2 This is a scanning electron microscope image of the structure after the final heat treatment in Example 1 of the present invention. DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0023] It should be noted that in the following description, the "%" indicating the amount is a weight basis unless otherwise specified. Unless otherwise specified, all numbers used in this specification and claims to indicate characteristic dimensions, quantities and physical properties should be understood to be modified by the term "about" in all cases. Therefore, unless otherwise specified, the numerical parameters listed in the above specification and the attached claims are approximate values, and those skilled in the art can use the teachings disclosed herein to seek to obtain the desired properties and appropriately change these approximate values. The use of numerical ranges expressed as endpoints includes all numbers within the range and any range within the range, for example, 1 to 5 includes 1, 1.2, 1.4, 1.55, 2, 2.75, 3, 3.80, 4 and 5, etc.

[0024] With the rapid development of the automotive industry, especially the new energy vehicle industry, higher performance requirements have been put forward for cold working die steel. Cold working dies are widely used in the production of automotive parts, especially in processes such as high-strength stamping and complex shape processing, and are subjected to extremely harsh working environments. In this context, although the existing cold working die steel can provide a certain hardness and wear resistance, it still has certain problems such as insufficient toughness, poor fatigue resistance, and short die life when facing working conditions such as high-strength impact and complex shape processing. These problems limit the application scope of traditional cold working die steel and affect its performance in the field of high-end manufacturing.

[0025] Cold working die steel can be roughly divided into low alloy cold working die steel, medium alloy cold working die steel and high carbon high chromium cold working die steel. Among them, high carbon high chromium cold working die steel has good wear resistance, but insufficient toughness. Medium and low alloy cold working die steel has high toughness, but poor wear resistance. In order to meet the high performance requirements of cold working die steel, countries around the world have successively developed a new type of cold working die material: Cr8 type cold working die steel material, such as SKD11, DC53, ASSAB88, VascoDie, etc. This kind of cold working die steel reduces the C and Cr content, the number of large eutectic carbides is reduced, and the remaining fine eutectic carbides are distributed more evenly, so that the material has both high hardness and good impact toughness.

[0026] DC53 steel is a typical Cr8 type low-medium alloy cold working die steel. With its good toughness, wear resistance and high tempering hardness, it has been widely used in the cold working die industry. The corresponding cold working die steel grade produced in my country is Cr8Mo2SiV, with reference to the standard GB / T 34564.1-2017. Cr8Mo2SiV has a certain gap with DC53 in terms of eutectic carbide control and matrix structure refinement, making its impact toughness lower than DC53.

[0027] A lot of research and exploration work has been done on the composition design of Cr8Mo2SiV die steel, including further reducing the C and Cr contents to reduce the content of M7C3 type ledeburite carbides, while increasing the content of strong carbide-forming elements such as Mo and V to increase secondary carbides, ensure the total amount of carbides, and improve wear resistance; adding non-carbide-forming elements such as Ni and Si to refine the grains, strengthen the matrix, and enhance toughness. In terms of heat treatment process, cyclic quenching process, pre-quenching and tempering process, multiple tempering process, deep cryogenic process and complex phase heat treatment process are adopted, all of which aim to regulate the type, structure and distribution of carbides and matrix organizations to achieve an excellent match between hardness, wear resistance and impact toughness.

[0028] Based on the above research, the present invention proposes to utilize the composite microalloying of heavy rare earth Y and W, and at the same time, with the help of a composite heat treatment process, the rare earth elements are used to form submicron rare earth composite inclusions with O and S, and the morphology and distribution of primary carbides are optimized while the pure steel liquid acts, and secondary carbides are formed with the help of W to strengthen the matrix, and then a composite structure of lower bainite and martensite is formed through a composite heat treatment process, ultimately achieving an excellent match of hardness, wear resistance and impact resistance of cold working die steel.

[0029] The present invention breaks through the performance bottleneck existing in the prior art by optimizing the alloy composition and microstructure of cold-working die steel, thereby improving the comprehensive performance of cold-working die steel and meeting more demanding industrial needs. In order to solve the problem of unstable performance of existing cold-working die steel under high-strength stamping, complex shape processing and high temperature conditions. The present invention not only solves the deficiencies of existing cold-working die steel in strength, toughness and wear resistance, but also significantly improves the service life and processing efficiency of the die, and is particularly suitable for high-demand industrial applications such as high-strength stamping and complex shape processing.

[0030] The technical ideas of the present invention are mainly concentrated in two aspects: one is to optimize the alloy composition, and the other is to improve the structure and performance of the steel through fine organization design and composite heat treatment process. First, by accurately controlling the content of carbon, chromium, molybdenum, tungsten, vanadium and other elements in the steel, the alloy ratio is optimized, aiming to improve the hardness, wear resistance and heat resistance of the cold working die steel, while enhancing its impact toughness and fatigue resistance. In particular, the addition of elements such as tungsten (W) and yttrium (Y) effectively improves the fatigue resistance and impact toughness of the cold working die steel, enabling it to adapt to more complex working environments.

[0031] Secondly, the matrix structure of the steel is a composite structure of martensite and lower bainite, in which the presence of lower bainite significantly improves the toughness of the steel and reduces the occurrence of impact cracks. During the heat treatment of the steel, the composite heat treatment process is used to ensure that the steel can form an optimized microstructure under different temperature conditions to further improve its strength, hardness and comprehensive mechanical properties.

[0032] The cold working die steel in one embodiment of the present invention comprises the following components by mass percentage: C 0.95-1.05%, Si 0.80-1.2%, Mn 0.40-0.50%, Cr 7.00-9.0%, Mo 2.0-2.5%, V 0.25-0.35%, W 0.50-0.80%, Y 0.005-0.01%, Ni≤0.25%, Cu≤0.15%, P≤0.025%, S≤0.008%, and the balance is iron and unavoidable impurities; wherein the cold working die steel matrix structure comprises martensite and lower bainite, and the volume fraction of the lower bainite is 10-20%.

[0033] The present invention introduces rare earth elements Y and W for composite microalloying. On the one hand, this design can form submicron and nanometer inclusions through Y, O and S elements, thereby inducing heterogeneous nucleation of eutectic carbides, thereby achieving a more uniform and stable carbide distribution. On the other hand, W forms WC (tungsten carbide) with carbon, and composites with eutectic carbides to form composite eutectic carbides, thereby breaking the eutectic carbide network in the cold working die steel, which helps to improve the hardness and wear resistance of the cold working die steel.

[0034] The composite structure of martensite and lower bainite in the cold-working die steel matrix of the present invention can balance the performance indicators such as hardness, toughness and wear resistance of the steel.

[0035] Martensite is a high-hardness phase that provides excellent hardness and wear resistance in cold-working die steel. Martensite is usually obtained by rapid quenching. Due to its special crystal structure, it has extremely high hardness and good wear resistance, so it is very suitable for use in high-stress, wear-prone environments. The hardness of martensite is usually greater than 60HRC, which is suitable for most cold-working die processing.

[0036] Lower bainite is a phase transformed from martensite, which has better toughness than martensite, but lower hardness. Lower bainite has a finer lath structure, which can improve the impact toughness and brittle fracture resistance of steel through its good plasticity and toughness. Lower bainite can not only alleviate the brittleness of martensite, but also improve the fatigue strength and impact resistance of mold steel, which helps cold working molds withstand high impact loads during complex forming processes.

[0037] Cold working dies are often faced with high-speed impact and high-stress working environments. Under these conditions, the die steel needs good impact resistance to avoid brittle fracture. Lower bainite has good plasticity and toughness, and can absorb more impact energy under high stress, thereby improving the impact resistance of the die and reducing the risk of die fracture.

[0038] The main reason for designing the volume fraction of lower bainite to be 10-20% is that this range can effectively balance the hardness, toughness and wear resistance of the mold steel, and meet the application requirements of cold working molds in high-intensity impact and wear environments. In the range of 10-20%, lower bainite can effectively increase the impact toughness of steel without reducing its hardness. Too low a lower bainite content may result in insufficient hardness of the steel, making it difficult to cope with high-intensity impact and wear; while too high a lower bainite content may reduce the hardness, resulting in insufficient mold wear resistance, which in turn affects the service life of the mold.

[0039] The C content in the cold working die steel provided by the present invention is 0.95-1.05%. As one of the constituent elements of carbide, part of the C element is dissolved in the matrix to improve the matrix strength; sufficient carbon content is used to ensure wear resistance, but too high carbon content will also reduce the impact toughness of the steel. Controlling its usage within the above range is conducive to ensuring that the die steel reaches appropriate hardness, wear resistance and toughness after heat treatment.

[0040] The Si content in the cold working die steel provided by the invention is 0.80-1.20%. The addition of Si element can strengthen the ferrite matrix, enhance the secondary hardening ability of steel heat treatment, and refine the secondary carbides precipitated by tempering, and improve the impact toughness.

[0041] The Mn content in the cold working die steel provided by the present invention is 0.40-0.50%. The Mn element is usually added as a deoxidizer, and a higher Mn content can enter the alloy carbide, which is beneficial to improve the wear resistance through the carbide, and improve the hardenability of the die steel together with alloy elements such as C and Cr. However, if the Mn content is too high, MnS inclusions are likely to appear, so its amount is controlled within the above range.

[0042] The Cr content in the cold working die steel provided by the present invention is 7.00-9.00%. The Cr element can promote the precipitation of chromium-based carbides, and has a favorable effect on the toughness and hardenability of the die steel. However, if the Cr content is too high, Cr7C3 type network eutectic carbides are easily generated, so the amount is controlled within the above range so that the performance is not affected by the formation of coarse carbides due to the eutectic reaction.

[0043] The Mo content in the cold working die steel provided by the present invention is 2.0-2.5%. Mo is a carbide-forming element, which is beneficial to improving the stability of carbides, and Mo solid solution in the matrix can play a role of solid solution strengthening, thereby improving the strength and wear resistance of the die steel; the Mo content should not be too high, because on the one hand, the cost increases, and on the other hand, it is easy to cause uneven precipitation and distribution of carbides, so the amount is controlled within the above range.

[0044] The cold working die steel provided by the present invention has a V content of 0.25-0.35%. V is a strong carbide-forming element and an important factor in determining the wear resistance of steel. The VC formed by it can effectively prevent grain growth even when heated at a relatively high austenitizing temperature; at the same time, it plays a role in refining the die steel structure and grains, thereby improving the strength and toughness of the die steel. However, too high a V content is likely to increase costs.

[0045] The Y content in the cold working die steel provided by the present invention is 0.005-0.01%. In the present invention, Y atoms can be segregated at the grain boundary, reduce the stacking fault of the alloy matrix, and are conducive to the refinement of grains and the influence of the distribution of grain boundary characteristics; moreover, the Y2O2S composite inclusions formed by Y can serve as heterogeneous nucleation sites of eutectic carbides, increase the nucleation rate of eutectic carbides, and are conducive to the breaking and refinement of eutectic carbides; in addition, the Y element can also purify the molten steel and improve the type and distribution of inclusions in the steel. However, as a heavy rare earth element, the Y element content cannot be too high, which is easy to produce excessive inclusions, and at the same time, it is not conducive to the smooth running of the smelting process. In addition, due to the high stability of the oxide, the Y2O2S composite inclusions can also improve the oxidation resistance of the steel and reduce the oxidation corrosion phenomenon of the steel in a high temperature or high humidity environment. This is particularly important for cold working die steel in harsh environments, especially in molds that need to work at high temperature and high pressure for a long time.

[0046] The cold working die steel provided by the present invention has a W content of 0.50-0.80%. As a strong carbide-forming element, W can form W6C carbides to prevent grain growth and improve the hardness of the steel; the addition of W can prevent decarburization of steel parts, and the formation of sufficient carbides can improve the wear resistance of steel parts; and W can prevent high-temperature softening and improve the high-temperature hardness of steel parts. However, too high a W content can easily lead to an increase in the cost of die steel, and the formation of excessive W-containing carbides is not conducive to improving impact toughness.

[0047] The total content of C and W in the cold working die steel provided by the present invention is 1.50-1.80wt%. The present invention controls the total content of C and W within the above range, which is conducive to regulating the distribution state of carbides and obtaining better toughness. If the total content of C and W exceeds 1.80wt%, it will lead to excessive carbides, resulting in increased brittleness of the steel, affecting impact toughness and fatigue resistance. At the same time, too much carbide will also increase the solid solution strengthening effect of tungsten, which may make the steel difficult to process or cause uneven structure during heat treatment, resulting in performance fluctuations. If the content of C and W is less than 1.50wt%, it is impossible to form sufficient hardening phases (such as carbides), resulting in insufficient hardness and wear resistance of the steel, and unable to meet the requirements under high load and harsh processing conditions. The mold is prone to rapid wear and surface failure during long-term work.

[0048] The sulfur content in the cold working die steel provided by the present invention is less than or equal to 0.008%. The sulfur element can cause hot brittleness of the steel and reduce the ductility and toughness of the steel. In the present invention, the sulfur content is controlled to be less than 0.008%, and the lower the content, the better.

[0049] The cold working die steel provided by the present invention has a P content of ≤0.025%. The P element is prone to form micro segregation when the molten steel solidifies, which reduces the toughness of the steel and increases its brittleness. In the present invention, the P content is controlled to be below 0.025%, and the lower the content, the better.

[0050] In an exemplary embodiment, the cold working die steel provided by the present invention includes a composite eutectic carbide formed by a composite of WC and eutectic carbides. Eutectic carbides are carbides formed in steel by different alloying elements (such as Cr, Mo, etc.) and carbon, and their formation usually helps the steel to maintain good wear resistance and thermal stability at high hardness.

[0051] In the heat treatment of cold working die steel, too many single carbides (such as single WC) may cause steel embrittlement because these carbides tend to have large sizes and aggregate in the matrix. Composite carbides (such as a composite of WC and eutectic carbides) can be evenly distributed in the matrix to form smaller and more uniform particles, improve the plasticity and toughness of the steel, and avoid brittle fracture.

[0052] The composite form of WC and eutectic carbides can exist stably during the heat treatment of steel (especially during high-temperature quenching and annealing), providing sufficient strength while avoiding uneven annealing or recrystallization of carbides during heat treatment, thereby enhancing thermal stability.

[0053] In an exemplary embodiment, the cold working die steel matrix structure provided by the present invention includes martensite laths, and the size of the martensite laths is 10 to 30 μm. The size of the martensite laths in the cold working die steel matrix structure is designed to be 10 to 30 μm, mainly by optimizing the microstructure of martensite to balance hardness, toughness and wear resistance, and meet the requirements of high strength, high toughness and good wear resistance of cold working dies in practical applications.

[0054] The formation of martensitic structure is a phase transformation to obtain a high-hardness martensitic structure during rapid cooling or quenching. The size of martensitic laths directly affects its mechanical properties. If the martensitic laths are too small (for example, less than 10μm), although the hardness of the steel can be improved, it may also lead to a decrease in the toughness and impact toughness of the steel, and then the mold is prone to brittle fracture when subjected to external force impact, which is not conducive to the use of cold working molds under complex working conditions such as high-strength stamping, extrusion and stretching.

[0055] On the other hand, if the size of the martensitic lath is too large (for example, greater than 30 μm), softened areas will appear between the large-sized laths in the material, resulting in a decrease in the overall strength of the material, and the coarse structure of the lath will lead to stress concentration and crack formation, and reduce the fatigue resistance of the mold. Although this coarse martensitic lath structure may slightly improve the wear resistance of the mold, it will significantly affect the service life of the mold under complex mechanical conditions. Therefore, controlling the size of the martensitic lath within the range of 10 to 30 μm can take into account both high hardness and good toughness, improve the impact resistance of the steel, avoid brittleness and stress concentration, and improve the reliability and service life of the mold.

[0056] The present invention also provides a method for preparing the aforementioned cold working die steel, and the preparation method specifically comprises the following steps:

[0057] S1: Add raw materials into a smelting furnace in proportion for smelting, and then perform LF refining and VD vacuum degassing to obtain molten steel.

[0058] It should be noted that the metal and alloy components in the raw materials are dissolved at high temperature to form molten steel. Through smelting, the uniformity of the steel composition can be ensured, especially the precise control of key alloying elements (such as carbon, tungsten, chromium, etc.). The smelted molten steel will undergo LF (Ladle Furnace) refining and VD vacuum degassing. LF refining can remove impurities in the molten steel, such as harmful elements such as oxygen, sulfur, and nitrogen, and adjust the composition of the molten steel to meet the final formula requirements. VD vacuum degassing further removes dissolved gases (such as hydrogen, nitrogen, etc.) in the molten steel and improves the quality of the molten steel. The purpose of refining and degassing is to improve the purity of the molten steel, reduce the content of inclusions and gases, and thus improve the mechanical properties of the steel, especially the resistance to hydrogen embrittlement and crack propagation.

[0059] In an exemplary embodiment, the VD vacuum degassing treatment includes: after aluminum deoxidation at 1450-1550°C, a first-stage soft argon blowing is performed for 2 minutes, and then a second-stage soft argon blowing is performed for 4-6 minutes. During the second-stage soft argon blowing, yttrium-iron alloy wire is fed, and the wire feeding is completed before the end of the soft argon blowing, and the wire feeding rate is 2-4 m / s; wherein, the flow rate of argon during the wire feeding process is 90-100 L / min, and the flow rate of argon after the wire feeding is completed is 70-80 L / min.

[0060] Aluminum has a strong reducing ability and can reduce oxides in molten steel to metallic aluminum, remove excess oxygen in the molten steel, and prevent oxides from negatively affecting the performance of steel. Aluminum deoxidation can reduce pores and inclusions in molten steel and improve the density and uniformity of the final product. Especially in molten steel with high alloy components, the control of oxygen content is particularly important because oxides and bubbles may cause brittleness or other performance problems of steel. Aluminum deoxidation can significantly improve the purity and processing efficiency of molten steel.

[0061] Soft argon blowing is to blow argon into the molten steel to promote gas removal and uniform distribution of alloy components. The blowing of argon helps to remove dissolved gases such as hydrogen, nitrogen and oxygen in the molten steel to prevent them from forming bubbles or inclusions during the subsequent cooling process. The first stage of soft argon blowing (2 minutes) is mainly for preliminary gas removal of the molten steel, while the second stage of soft argon blowing (4 to 6 minutes) is for more in-depth degassing. At the same time, the adjustment of argon flow rate and the timing of feeding alloys need to be precisely controlled. Through fine argon flow control (90 to 100L / min to 70 to 80L / min), it is possible to maintain appropriate degassing efficiency while avoiding excessive disturbance of the molten steel caused by excessive blowing of argon.

[0062] Feeding yttrium-iron alloy wire is an important step in the second stage of soft argon blowing treatment. Yttrium (Y), as a rare earth element, can effectively react with oxygen, sulfur and other elements in the molten steel to form stable oxides or sulfides, and play a role in removing impurities in the molten steel. Yttrium can also form yttrium compounds in the molten steel, improve the deoxidation effect of the molten steel, and play an important role in the subsequent microstructure of cold working die steel. For example, the addition of yttrium can promote the uniform distribution of alloying elements and improve the mechanical properties of steel, especially in terms of resistance to hydrogen embrittlement. In addition, the addition of yttrium can also act as a gas trap to adsorb undesirable dissolved gases such as hydrogen and nitrogen, thereby improving the overall quality of cold working die steel.

[0063] The precise control of the wire feeding rate (2-4 m / s) and the argon gas flow rate has an important influence on ensuring the stable dissolution and uniform distribution of the yttrium alloy. Too fast or too slow wire feeding rate will affect the dissolution efficiency and uniformity of the yttrium-iron alloy in the molten steel, resulting in fluctuations in the final molten steel composition. By controlling the argon gas flow rate and the wire feeding rate, the addition process of the yttrium alloy to the molten steel can be precisely regulated to ensure that the composition and quality of the molten steel meet the requirements.

[0064] Specifically, the yttrium content of the yttrium-iron alloy wire is ≥65wt%, the dissolved oxygen content is ≤0.005wt%, and the wire diameter is ≤5mm. If the rare earth content is too low, the deoxidation effect of yttrium is not significant, and a high oxygen content may exist in the molten steel, which will lead to unstable performance of the steel and even defects such as pores and cracks. Therefore, the yttrium content needs to be maintained above 65wt%. This range can ensure that the yttrium-iron alloy has sufficient deoxidation ability and stability in the molten steel, and effectively avoid the formation of excessive oxides. Too much rare earth elements may lead to excessive accumulation of rare earth elements in the molten steel, forming too many rare earth compounds, affecting the fluidity and casting quality of the molten steel.

[0065] The content of dissolved oxygen directly affects the purity of molten steel and the quality of the final product. Too much dissolved oxygen will lead to excessive oxides in the molten steel, forming pores or inclusions, which will increase the brittleness of the steel. Especially when used in high temperature and high pressure environments, problems such as hydrogen embrittlement and oxygen embrittlement are prone to occur. The main function of yttrium is to capture oxygen through oxidation reaction. Therefore, the dissolved oxygen content in yttrium-iron alloy wire is controlled to ≤0.005wt% to ensure that its deoxidation ability reaches the best state.

[0066] The wire diameter of yttrium-iron alloy wire has an important influence on its dissolution rate, distribution uniformity and alloying effect. A wire diameter that is too thick will cause the alloy wire to dissolve too slowly, making it difficult to quickly and evenly introduce rare earth elements into the molten steel. Especially at high temperatures of the molten steel, the dissolution process needs to be rapid and uniform, otherwise it may lead to uneven composition and affect the performance of the final steel. A wire diameter that is too thin may result in too small a feed amount, which cannot meet the alloying requirements of the yttrium element in the molten steel and cannot effectively play a deoxidizing role.

[0067] S2: Casting the molten steel to obtain an ingot.

[0068] It should be noted that after the molten steel is refined, it is cast to obtain an ingot. The size of the ingot is usually large, so that there is enough material for processing in the subsequent processing and the performance stability of the final product can be guaranteed. During the casting process, the molten steel is cooled and formed through the mold. This process determines the initial shape of the steel, including grain size, composition uniformity, etc., which is crucial for subsequent processing.

[0069] In an exemplary embodiment, the casting of the molten steel specifically includes: preheating the steel ingot mold to 250-400°C, casting the molten steel into the steel ingot mold at a casting temperature of 1450-1550°C, keeping it warm for 1 hour, demoulding and cooling it in a heat-insulating pit, and removing it from the pit after 48 hours.

[0070] During the casting of molten steel, the ingot mold must first be preheated to ensure that the mold can smoothly receive the high-temperature molten steel and avoid severe temperature stress. The purpose of preheating to 250-400°C is to make the mold temperature close to the casting temperature of the molten steel, thereby reducing the thermal stress when the molten steel contacts the mold and preventing the occurrence of mold cracks. At the same time, the appropriate preheating temperature can also prevent the molten steel from cooling too quickly during the pouring process, thereby affecting the internal quality of the ingot and avoiding defects such as cold shut and pores.

[0071] When casting molten steel, the casting temperature is controlled between 1450 and 1550°C. The selection of this temperature range can ensure that the molten steel maintains fluidity and can ensure that the molten steel can smoothly fill the entire ingot mold during the pouring process. Too low a temperature will increase the viscosity of the molten steel, reduce its fluidity, and cause casting defects, while too high a temperature may cause the molten steel to over-oxidize or evaporate too many alloy elements, affecting the composition stability and mechanical properties of the final product.

[0072] After the molten steel is poured into the ingot mold, it is usually necessary to carry out a certain period of insulation treatment. The insulation for 1 hour is to allow the molten steel to be evenly distributed in the mold and stably solidify. During the insulation period, the molten steel begins to gradually solidify from the surface and form a preliminary solid zone, which helps to reduce the internal stress of the ingot and ensure the integrity of the casting. After 1 hour of insulation, the ingot will be demolded and placed in a insulation pit for cooling. At this time, the temperature of the ingot is still high, so slow cooling through the insulation pit can effectively avoid excessive temperature differences between the inside and outside of the ingot, thereby reducing the impact of thermal stress on the casting. The cooling method of the insulation pit helps the uniform solidification of the ingot and the release of internal stress, preventing cracks or deformation.

[0073] S3: Forging and annealing the ingot to obtain a forged billet.

[0074] It should be noted that forging is to plastically deform the ingot through high-temperature heating and mechanical force to achieve the desired shape and density. Forging can remove large-sized pores in the ingot and improve the density and uniformity of the material. Annealing is to eliminate the internal stress that may be introduced during the forging process, further refine the grains, and improve the mechanical properties of the steel. Through this step, the internal structure of the billet is improved, laying the foundation for subsequent heat treatment and hot rolling.

[0075] In an exemplary embodiment, the ingot is subjected to forging and annealing treatment, including: adopting segmented heating during the forging process, with the start forging temperature of 1050-1100°C, the final forging temperature ≥850°C, air cooling to room temperature after final forging, and a forging ratio greater than 3; after forging is completed, annealing treatment is performed, the annealing temperature is 860-900°C, the annealing time is 6 hours, and then furnace cooling to room temperature; wherein the time interval between the forging and annealing treatments is ≤1h.

[0076] The forging process adopts a segmented heating method, and the forging temperature is 1050-1100℃. This temperature range is to ensure that the steel ingot has good plasticity and machinability during forging. When the forging temperature is too low, the steel may become brittle and hard, not easy to plastically deform, and may even cause cracks; when the forging temperature is too high, the grains of the steel will be too coarse, resulting in a decrease in subsequent mechanical properties. Therefore, the forging temperature of 1050-1100℃ can provide sufficient plasticity while avoiding the adverse effects caused by overheating.

[0077] The final forging temperature during the forging process is set to ≥850℃. This temperature is selected to obtain a good grain refinement effect during the forging process. A too low final forging temperature may cause the hardness of the steel to be too high, making the forging process difficult to carry out; while a too high final forging temperature is likely to cause coarse grains, affecting the strength and toughness of the steel. Therefore, a final forging temperature ≥850℃ helps ensure that the steel obtains a more ideal organization and achieves the best mechanical properties. The forging process requires a large degree of plastic deformation (forging ratio greater than 3), and a sufficient forging ratio is achieved to ensure that the inclusions and grains inside the steel are effectively refined, thereby improving the comprehensive performance of the die steel. A forging ratio that is too low will result in the inability to effectively eliminate the internal defects of the steel, affecting the final quality.

[0078] After forging, annealing treatment is carried out to further improve the structure and performance of the material. The annealing temperature is 860-900℃. The choice of this temperature range is to reduce the hardness of the material while releasing the stress inside the steel and promoting further refinement of the steel grains. Too high annealing temperature may cause grain coarsening and affect the strength of the steel; while too low annealing temperature may result in poor annealing effect and failure to fully release internal stress. The annealing time is 6 hours. Sufficient annealing time helps the steel to obtain a uniform structure at high temperature, remove internal residual stress, and improve the processing performance of the steel.

[0079] After annealing, the temperature of the steel is gradually reduced by furnace cooling to room temperature, avoiding thermal stress caused by rapid cooling, which is crucial to ensure the structural stability and mechanical properties of the steel. Too rapid cooling may cause excessive internal temperature differences, which in turn may cause cracks or uneven hardness distribution. Therefore, furnace cooling to room temperature helps to homogenize the overall structure of the steel.

[0080] It should be noted that the time interval between forging and annealing is ≤1h, which helps to ensure the structural stability of the steel. If the time between forging and annealing is too long, the grains of the steel may become coarse due to cooling, which will affect the effect of subsequent annealing. The shorter time interval ensures that the steel can enter the annealing stage in time after forging, thus avoiding grain coarsening or re-accumulation of internal stress.

[0081] S4: hot rolling and isothermal spheroidizing annealing are performed on the forged billet to obtain a steel billet.

[0082] It should be noted that hot rolling is to deform the forging billet under pressure by high-temperature heating, so as to change the size and shape of the steel to achieve the required specifications and mechanical properties. Hot rolling can further refine the grains and improve the surface quality of the steel. Isothermal spheroidizing annealing is to spheroidize the carbides in the steel by controlling the heating and holding time, thereby improving the plasticity and toughness of the steel. Isothermal spheroidizing annealing is usually carried out at a lower temperature, which can effectively refine the carbides and improve the processability of the steel. Through this step, the hard phase carbides in the steel are refined, the wear resistance of the steel is enhanced, and a good foundation is provided for composite heat treatment.

[0083] In an exemplary embodiment, the forging blank is subjected to hot rolling and isothermal spheroidizing annealing treatment, specifically comprising: adopting segmented heating during the hot rolling process, with a starting rolling temperature of 1080-1150°C, a final rolling temperature ≥850°C, a rolling pass ≥10, a total reduction >70%, and air cooling to room temperature after hot rolling; then performing isothermal spheroidizing annealing treatment, heating the forging blank to 780°C and keeping it for more than 5 hours, then heating it to 860°C and keeping it for more than 8 hours, then cooling it to 780°C and keeping it for more than 12 hours, then furnace cooling it to 500°C, and finally air cooling it to room temperature; wherein the time interval between the hot rolling and isothermal spheroidizing annealing treatment is ≤1h.

[0084] Preferably, the number of rolling passes during hot rolling is ≥15, and the total reduction is >75%; the isothermal spheroidizing annealing after hot rolling includes: heating the forging billet to 780°C and keeping it for 6 to 8 hours, then heating it to 860°C and keeping it for 9 to 11 hours, then cooling it to 780°C and keeping it for 12 to 15 hours, then cooling it to 500°C with the furnace, and finally air cooling it to room temperature.

[0085] The hot rolling process uses segmented heating, and the start rolling temperature is set in the range of 1080-1150℃, which can ensure that the steel has sufficient plasticity during rolling. If the start rolling temperature is too low, the steel will not have enough plasticity, which may cause cracks or uneven thickness during rolling; while too high a temperature may cause coarse grains, thus affecting the mechanical properties of the final product. Therefore, the start rolling temperature of 1080-1150℃ is a reasonable choice, which can ensure that the steel has good processability and provide appropriate conditions for subsequent rolling.

[0086] During the hot rolling process, the final rolling temperature is set to ≥850℃. This temperature requirement ensures the grain refinement of the steel and prevents grain growth, thereby obtaining better organizational stability. If the final rolling temperature is too low, the plasticity of the steel is poor, which may cause surface defects during the rolling process; when the temperature is too high, the grains may grow excessively, reducing the strength of the steel. The rolling pass requirement is ≥10, and the total reduction is >70%. The purpose of this design is to fully refine the steel structure through multiple rollings and increase the compression ratio to obtain a more uniform organizational structure. A larger reduction helps to eliminate casting defects in steel and improve the mechanical properties of steel, especially in improving the toughness and wear resistance of steel.

[0087] After hot rolling, the steel needs to be air-cooled to room temperature. The purpose of air cooling is to control the internal stress caused by excessive temperature difference during the cooling process of the steel. It also helps to stabilize the internal structure of the steel and create a good foundation for subsequent annealing treatment.

[0088] Next, the isothermal spheroidizing annealing treatment stage is entered. This process is particularly critical for the control of steel structure. The first step of isothermal spheroidizing annealing is to heat the forging billet to 780℃ and keep it warm for more than 5 hours. The purpose is to make the ferrite and cementite in the steel evenly distributed, so as to achieve finer grains and better structural stability. Subsequently, the temperature is raised to 860℃ and kept warm for more than 8 hours. This heating process makes the carbides in the steel completely globalized, further improving the machinability of the steel. Spheroidizing annealing is an effective means to improve the plasticity of steel and reduce work hardening, especially in the subsequent hot working process, refining the spheroidized structure can reduce the difficulty and energy consumption of processing.

[0089] Then, the temperature is lowered to 780℃ and kept at this temperature for more than 12 hours to ensure the thoroughness of the spheroidization process and make the steel structure more uniform and avoid uneven carbide or ferrite areas. After the furnace is cooled to 500℃, the process of air cooling to room temperature helps the steel to cool evenly and avoid cooling stress, thereby maintaining the inherent stability of the steel. The role of the entire isothermal spheroidizing annealing process is to make the carbides of the steel more uniform and refined by controlling the heating, insulation and cooling processes, thereby enhancing its mechanical properties, especially improving its wear resistance, impact resistance and service life.

[0090] The time interval between hot rolling and isothermal spheroidizing annealing is ≤1h. After forging, the steel should enter the hot rolling and annealing stage as soon as possible. Too long a time interval may cause adverse changes in the structure of the steel, such as grain coarsening or internal stress accumulation, thus affecting the effect of subsequent treatment. Therefore, shortening the time interval can ensure that the structure of the steel remains stable throughout the hot working process.

[0091] S5: performing a composite heat treatment on the steel billet to form a composite structure of martensite and lower bainite in the steel billet matrix to obtain the cold working die steel.

[0092] Composite heat treatment is a key step in the present invention, which directly affects the organizational structure and mechanical properties of the final cold working die steel. In this process, the steel billet is first heated to 450-550°C for 15-30 minutes, and then further heated to 800-900°C after temperature equalization, and then kept warm for 25-35 minutes. The purpose of heating at this stage is to evenly distribute the carbides in the steel and promote the austenitization of the steel, in preparation for subsequent high temperature treatment.

[0093] Next, the billet is heated to 1020-1050°C and kept at this temperature for 30-50 minutes to promote the stabilization of austenite and enable effective reaction between carbides and alloy elements to form a suitable metal matrix structure.

[0094] After high temperature treatment, the steel billet is rapidly cooled to 230-260°C, kept in a molten nitrate medium (50% KNO3+50% NaNO2) for 2-4 hours, and then air-cooled to room temperature. The cooling method of this step adopts nitrate medium cooling because nitrate has high thermal conductivity, which can cool the steel to a certain temperature quickly and reduce the uneven distribution of stress on the steel surface. The temperature and time design at this time can promote the composite formation of martensite and bainite in the steel, and further improve the impact toughness and wear resistance of the steel.

[0095] Finally, the billet is heated to 550-600℃ again and kept at this temperature for 4-8 hours before being air-cooled to room temperature. The heating and heat preservation process at this time helps to improve the structural stability of the steel, especially the formation of martensitic laths, further improving its hardness and toughness.

[0096] The preparation method of the cold working die steel provided by the present invention makes the carbides evenly dispersed in the matrix through hot working (forging and hot rolling) and two annealings. In addition, Y and W are used to refine the grain size of ferrite in the spheroidized annealed state and the original austenite grain size during high-temperature quenching, thereby realizing the refinement of the martensite lath after the composite heat treatment. Finally, through the composite heat treatment, 10-20% of lower bainite is made in the matrix, which greatly improves the impact toughness. Then, with the help of martensite refinement and WC to improve the hardness of the matrix, an improved cold working die steel with excellent comprehensive performance can be obtained. The hardness of the die steel after heat treatment is greater than 60HRC, the longitudinal impact energy without groove is greater than 70J, and the ratio of transverse to longitudinal impact energy is greater than 0.75. It has good wear resistance and can greatly improve the service life of the die.

[0097] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0098] Example 1

[0099] Chemical composition of cold working die steel: C: 0.98%, Si: 0.90%, Mn: 0.45%, Cr: 8.0%, Mo: 2.1%, V: 0.30%, W: 0.71%, Y: 0.009%, Ni ≤ 0.25%, Cu ≤ 0.15%, P ≤ 0.025%, S ≤ 0.008%, the balance is iron and unavoidable impurities.

[0100] (1) Smelting: After the above-mentioned ingredients are prepared, the steel is smelted in an electric furnace. After smelting, the molten steel needs to be subjected to LF refining outside the furnace and VD vacuum degassing. During the VD degassing process, soft argon blowing is performed at 1450°C, and the total soft argon blowing time is controlled within 8 minutes. Aluminum deoxidation is performed before soft argon blowing, and then after 2 minutes of soft blowing, heavy rare earth alloy wire feeding (yttrium-iron intermediate alloy wire is used, and the yttrium content is 68%, the solid dissolved oxygen content in the wire is ≤0.005wt%, and the wire diameter is 3mm) is started. The wire feeding is completed before the end of argon blowing, and the wire feeding rate is controlled at 3m / s. The flow rate of argon during the wire feeding process is 95L / min, and the flow rate of argon after the wire feeding is completed is controlled at 75L / min;

[0101] (2) Casting: Preheat the ingot mold to 350°C, pour the molten steel into the ingot mold at a casting temperature of 1500°C, keep it warm for 1 hour, demould and cool it in the pit, and remove it from the pit after 48 hours;

[0102] (3) Forging: adopt segmented heating, the starting forging temperature is 1080℃, the final forging temperature is 920℃, and the final forging is air-cooled to room temperature. The forging ratio is greater than 3;

[0103] (4) Full annealing: annealing temperature is 880°C, annealing time is 6 hours, and then furnace cooling to room temperature;

[0104] (5) Hot rolling: adopting segmented heating, the starting rolling temperature is 1100°C, the final rolling temperature is 910°C, and the rolling is carried out in multiple passes with a small reduction, 15 rolling passes, and a total reduction of 75%. After hot rolling, the steel is air-cooled to room temperature;

[0105] (6) Isothermal spheroidizing annealing: heat the forging to 780°C and keep it for 8 h, then heat it to 860°C and keep it for 10 h, then cool it to 780°C and keep it for 15 h, then cool it to 500°C in the furnace, and finally air cool it to room temperature;

[0106] (7) Composite heat treatment: The steel billet after isothermal spheroidization is heated to 500°C and kept at this temperature for 20 minutes for preheating. After uniform temperature, the temperature is raised to 850°C and kept at this temperature for 30 minutes for secondary preheating. Finally, the temperature is raised to 1030°C and kept at this temperature for 40 minutes. Subsequently, the steel billet is cooled to 250°C in a molten nitrate (50% KNO3+50% NaNO2) medium and kept at this temperature for 2 hours. Subsequently, the steel billet is air-cooled to room temperature. The steel billet is then heated to 550°C and kept at this temperature for 4 hours. Subsequently, the steel billet is air-cooled to room temperature.

[0107] Figure 1 This is an optical microscopic image of the structure of the hot-rolled plate after spheroidizing annealing in Example 1; Figure 2 This is a scanning electron microscope image of the structure after the final heat treatment in Example 1.

[0108] Example 2

[0109] The difference from Example 1 is that the W content is 0.65%, the Y content is 0.005%, and the other components and preparation process are the same as Example 1.

[0110] Comparative Example 1

[0111] The difference from Example 1 is that W is not added to the chemical composition, and the other components and preparation process are the same as Example 1.

[0112] Comparative Example 2

[0113] The difference from Example 1 is that no heavy rare earth Y is added to the chemical composition, and the other components and preparation process are the same as Example 1.

[0114] Comparative Example 3

[0115] The difference from Example 1 is that W and Y are not added to the chemical composition, and the other components and preparation process are the same as Example 1.

[0116] Comparative Example 4

[0117] The difference from Example 1 is that the W content added in the chemical composition is 0.95% (mass fraction), and the other components and preparation process are the same as Example 1.

[0118] Comparative Example 5

[0119] The difference from Example 1 is that the Y content added in the chemical composition is 0.02% (mass fraction), and the other components and preparation process are the same as Example 1.

[0120] Comparative Example 6

[0121] The difference from Example 1 is that the rare earth added in the chemical composition is mixed light rare earth (the total content of La and Ce is 0.008%), and the other components and preparation process are the same as Example 1.

[0122] Comparative Example 7

[0123] The difference from Example 1 is that the VD degassing process adopts soft argon blowing for 8 minutes and then feeding the heavy rare earth alloy wire. The composition and other preparation processes are the same as Example 1.

[0124] Comparative Example 8

[0125] The difference from Example 1 is that after forging, the steel sheet is directly hot-rolled without complete annealing, and the composition and other preparation processes are the same as those in Example 1.

[0126] Comparative Example 9

[0127] The difference from Example 1 is that after hot rolling, no isothermal spheroidizing annealing is performed, and secondary spheroidizing annealing is performed, and the composition and other preparation processes are the same as those of Example 1. That is, the forging billet is heated to 850°C, kept at this temperature for 8 hours, and then cooled to room temperature with the furnace, and then heated to 860°C, kept at this temperature for 10 hours, and then cooled to room temperature with the furnace.

[0128] Comparative Example 10

[0129] The difference from Example 1 is that the isothermal spheroidizing annealing process is followed by a quenching and high-temperature tempering process, that is, the isothermal spheroidized alloy is heated to 500°C, kept warm for 20 minutes for preheating, then heated to 850°C and kept warm for 30 minutes for secondary preheating, finally heated to 1030°C, kept warm for 40 minutes, air-cooled to room temperature, then heated to 550°C and kept warm for 2 hours, and then air-cooled to room temperature.

[0130] Table 1 - Performance test results of cold working die steel obtained in various embodiments and comparative examples

[0131]

[0132]

[0133] The cold working die steels prepared in Examples 1 to 2 and Comparative Examples 1 to 9 were subjected to performance tests, and the test results are shown in Table 1. As can be seen from Table 1, when W and heavy rare earth Y are not added, the hardness and isotropy of the cold working die steel are significantly reduced (Comparative Examples 1 to 3). When the W and Y addition content exceeds a certain value, it is not conducive to the improvement of the longitudinal impact energy, and it will also cause the anisotropy of the cold working die steel to increase (Comparative Examples 4 to 5). The use of La and Ce mixed light rare earths to replace heavy rare earth Y has no obvious effect on improving the longitudinal impact energy of cold working die steel (Comparative Example 6).

[0134] From the process point of view, the coordination of argon blowing and wire feeding process in the VD furnace is crucial. Comparative Example 7 shows that when rare earth wire is fed after argon blowing, the anisotropy of cold working die steel increases significantly due to the uneven distribution of rare earth. Combined with Example 1, Comparative Example 8 and Comparative Example 9, full annealing after forging and isothermal annealing after rolling are both conducive to the improvement of the transverse impact energy of cold working die steel, mainly because these two processes can improve the band distribution of primary carbides. Comparative Example 10 adopts conventional quenching and high temperature tempering process. Since the martensitic matrix is ​​obtained, the longitudinal and transverse impact energy are significantly reduced.

[0135] In summary, the cold working die steel and the preparation method thereof provided by the present invention improve the toughness and impact performance of the steel by adding the heavy rare earth element Y, so that the rare earth element can purify the matrix and improve the inclusions; at the same time, the strong carbide forming element W is added to reduce decarburization and improve the wear resistance; and the W element can prevent tempering softening and improve the high temperature hardness of the tempered steel; at the same time, the total content of the C element and the W element is reasonably controlled to obtain a better carbide distribution state than the traditional Cr8 type cold working die steel, so that it has better toughness.

[0136] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0137] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A cold working die steel, characterized in that: In terms of mass percentage, it includes the following ingredients: C 0.95~1.05%, Si 0.80~1.2%, Mn 0.40~0.50%, Cr 7.00~9.0%, Mo 2.0~2.5%, V 0.25~0.35%, W 0.50~0.80%, Y 0.005~0.01%, Ni≤0.25%, Cu≤0.15%, P≤0.025%, S≤0.008%, the balance is iron and unavoidable impurities; Wherein, the cold-working die steel matrix structure includes martensite and lower bainite, and the volume fraction of the lower bainite is 10-20%.

2. The cold working die steel according to claim 1, characterized in that: The total content of C and W in the cold working die steel is 1.50-1.80wt%.

3. The cold working die steel according to claim 1, characterized in that: The cold working die steel includes a composite eutectic carbide formed by a composite of WC and eutectic carbide; and / or The cold working die steel includes a Y2O2S compound.

4. The cold working die steel according to claim 1, characterized in that: The cold-working die steel matrix structure comprises martensite laths, and the size of the martensite laths is 10 to 30 μm.

5. A method for preparing cold working die steel as claimed in any one of claims 1 to 4, characterized in that: include: Adding raw materials into a smelting furnace in proportion for smelting, and then performing LF refining and VD vacuum degassing to obtain molten steel; Casting the molten steel to obtain an ingot; Forging and annealing the ingot to obtain a forged billet; hot rolling and isothermal spheroidizing annealing treatment are performed on the forged billet to obtain a steel billet; Performing a composite heat treatment on the steel billet to form a composite structure of martensite and lower bainite in the steel billet matrix to obtain the cold working die steel; The composite heat treatment comprises: heating the steel billet after isothermal spheroidizing annealing to 450-550°C and keeping it warm for 15-30 minutes; heating it to 800-900°C and keeping it warm for 25-35 minutes after equalization; then heating it to 1020-1050°C and keeping it warm for 30-50 minutes; finally cooling it to 230-260°C in a molten nitrate medium and keeping it warm for 2-4 hours, air cooling it to room temperature, then heating it to 550-600°C and keeping it warm for 4-8 hours, and then air cooling it to room temperature.

6. The method for preparing cold working die steel according to claim 5, characterized in that: The VD vacuum degassing process comprises: At 1450-1550°C, after aluminum deoxidation, a first-stage soft argon blowing is carried out for 2 minutes, and then a second-stage soft argon blowing is carried out for 4-6 minutes. During the second-stage soft argon blowing, yttrium-iron alloy wire is fed and the wire feeding is completed before the end of the soft argon blowing. The wire feeding rate is 2-4 m / s; wherein, the flow rate of argon during the wire feeding process is 90-100 L / min, and the flow rate of argon after the wire feeding is completed is 70-80 L / min.

7. The method for preparing cold working die steel according to claim 6, characterized in that: The yttrium content of the yttrium-iron alloy wire is ≥65wt%, the solid solution oxygen content is ≤0.005wt%, and the wire diameter is ≤5mm.

8. The method for preparing cold working die steel according to claim 5, characterized in that: Casting the molten steel comprises: Preheat the ingot mold to 250-400℃, pour the molten steel into the ingot mold at a casting temperature of 1450-1550℃, keep it warm for 1h, demould and cool it in the insulation pit, and take it out of the pit after 48h.

9. The method for preparing cold working die steel according to claim 5, characterized in that: The ingot is subjected to forging and annealing treatment, comprising: The forging process adopts segmented heating, the starting forging temperature is 1050~1100℃, the final forging temperature is ≥850℃, and the final forging is air-cooled to room temperature. The forging ratio is greater than 3. After forging, annealing is performed at a temperature of 860-900°C for 6 hours, followed by furnace cooling to room temperature. Wherein, the time interval between the forging and annealing treatment is ≤1h.

10. The method for preparing cold working die steel according to claim 5, characterized in that: The forging blank is subjected to hot rolling and isothermal spheroidizing annealing treatment, comprising: The hot rolling process adopts segmented heating, the starting rolling temperature is 1080-1150℃, the final rolling temperature is ≥850℃, the rolling pass number is ≥10, the total reduction is >70%, and the hot rolling is followed by air cooling to room temperature; Then, isothermal spheroidizing annealing treatment is carried out, wherein the forging blank is heated to 780°C and kept at this temperature for more than 5 hours, then heated to 860°C and kept at this temperature for more than 8 hours, then cooled to 780°C and kept at this temperature for more than 12 hours, then furnace cooled to 500°C, and finally air cooled to room temperature; Wherein, the time interval between the hot rolling and the isothermal spheroidizing annealing treatment is ≤1h.

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