Die steel having excellent temper softening resistance and method for manufacturing the same

Abstract

The present application provides a die steel with excellent temper softening resistance and its manufacturing method, the steel plate composition is calculated according to percentage by weight: C: 0.50%~0.60%, Si: 0.30%~0.50%, Mn: 0.30%~0.50%, P≤0.015%, S≤0.015%, Cr: 2.40%~3.40%, Mo: 3.50%~4.00%, V: 1.50%~2.00%, Ti: 0.20%~0.30%, Sr: 0.20%~0.30%, Zn: 0.30%~0.40%, wherein, V / Cr=0.585~0.625, Mo / Zn=10.0~11.7, Ti+Sr=0.45%~0.55%, the balance is Fe and inevitable impurities.The production method of the steel plate includes smelting, continuous casting, slab heating, rolling, straightening, slow cooling, heat treatment.The die steel plate of the present application has nanoscale uniformly distributed carbides even after long time tempering, the peak value of carbide particle size is distributed in the range of 25~50nm, accounting for 36%~38%, without large size carbides and uniformly dispersed distribution.It has good temper softening resistance, excellent high temperature wear resistance and high thermal conductivity, and meanwhile, the strength and toughness are matched, and the efficient and economical production process is adopted.

Description

Technical Field

[0001] This invention belongs to the field of metal material production technology, and in particular relates to a mold steel with excellent resistance to tempering softening and its preparation method. Background Technology

[0002] Because molds operate in environments affected by high temperatures, liquids, wear, and various stresses, they are prone to cracking, fatigue, wear, and failure. Therefore, high requirements are placed on mold steel materials. Since the microstructure and properties of steel are significantly influenced by heating temperature and time, microstructural defects are easily introduced. Furthermore, high temperatures cause softening and deformation, affecting wear resistance and thermal fatigue performance. Therefore, developing mold steel raw materials with high resistance to tempering softening and high-temperature wear has become a research and development direction for the mold steel industry. In addition, high thermal conductivity is also a key physical property of mold steel. If mold steel with low thermal conductivity cannot maintain a uniform temperature distribution over a large area during high-temperature operation, resulting in excessively high local temperatures, deterioration of heat resistance, and eventual failure, the material will fail.

[0003] Many domestic institutions have done a great deal of work in developing new mold steels, improving product quality, optimizing production processes, and extending mold life. Patent application number 202010836798.9, entitled "A Low-Cost Mold Steel with High Thermal Conductivity at High Temperature and Its Preparation Method," discloses a mold steel with the following composition percentages: C: 0.28%–0.34%, Cr: 0.38%–0.45%, Si: 0.68%–0.74%, Mn: 0.40%–0.48%, Mo: 1.05%–1.15%, V ≤ 0.08%, with the remainder being iron and unavoidable impurities. The thermal conductivity is improved by adjusting the contents of Cr, Si, Mn, Mo, and V. A low Mo content maintains high thermal conductivity at high temperatures. While reducing the content of elements such as Si and Cr can worsen thermal conductivity, it is difficult to guarantee important properties such as hardenability, thermal stability, strength, hardness, and wear resistance. Patent application No. 202011051518.X, entitled "An Ultra-High Strength Nanocrystalline 4Cr5MoWSi Mold Steel and Its Preparation Method," discloses a mold steel with the following composition percentages: C: 0.34%–0.42%, Cr: 4.2%–5.8%, W: 0.6%–1.4%, Mo: 0.6%–1.4%, Nb: 0.03%–0.05%, Ce: 0.01%–0.03%, Si: 0.4%–0.8%, V: 0.2%–0.4%, with the balance being Fe. After holding at 1020℃–1100℃ for a period of time, it is rapidly cooled to room temperature to obtain a nanoplate precursor. The strain rate is 0.5–2 s at 800–880℃. -1Within a certain range, thermal deformation with a total strain greater than or equal to 70% transforms the nano-precursor into a nanostructure. Further aging treatment yields nanocrystalline mold steel with ultra-high strength, good toughness, and high-temperature resistance. However, the prepared bulk nanocrystalline material has a size of 150×800×10mm, which is relatively small and does not conform to the development trend of thick-gauge, large-section molds. Furthermore, it is difficult to guarantee thermal stability and high-temperature wear resistance. Patent application number 202111171611.9, entitled "An Anti-Fatigue Hot Working Mold Steel," discloses a mold steel with the following composition percentages: Mo: 2.0%–3.2%, Cr: 3.3%–5.0%, S≤0.03%, P≤0.013%, C: 0.28%–0.5%, Fe: 70%–85%, Mn: 0.5%–1.8%, Ni: 0.2%–0.5%, Nb: 0.1%–0.15%, W: 0.07%, Co: 2%–3%. Hot work die steels obtained through processes such as electroslag remelting and forging possess high fatigue resistance, heat resistance, wear resistance, toughness, and corrosion resistance. However, it is difficult to guarantee the uniformity of the microstructure and thermal stability of the material's thickness and cross-sectional properties. Patent application number 200710171693.0, entitled "High Thermal Stability and High Strength Hot Work Die Steel," discloses a die steel with the following composition percentages: C: 0.3%–0.6%, Si: 0.5%–0.7%, Mn: 10.5%–14.5%, Cr: 2.0%–6.0%, Mo: 1.5%–3.5%, V: 0.5%–2.0%, P: 0.01%–0.02%, S < 0.005%. The greatest advantage of this die steel is its good thermal stability; it can maintain high hardness at 700℃, with a hardness above HRC45. The room temperature impact toughness of this material is greater than 300 J. However, it is difficult to guarantee the material's high-temperature wear resistance and high-temperature thermal conductivity. Uneven heat dissipation during service may lead to localized overheating and failure. Patent application number 201410194383.0, entitled "An Ultra-High Strength Mold Steel with Good Corrosion Resistance and Toughness," describes a composition with the following mass fractions: C: 0.08%–0.32%, Si: ≤0.8%, Mn: ≤0.5%, Cr: 5%–10%, Ni: 6.0%–8.0%, Co: 1.3%–1.8%, W: 0.9%–1.1%, V: 0.2%–0.5%, Nb: 0.08%–0.15%, N: ≤0.002%, O: ≤0.0015%, Mo: 0.9%–1.4%, Ti: 0.05%–0.4%, and S: 0.011%–0.025%. This mold steel, after forging, can be widely used in the automotive industry, where high strength and toughness are required. However, forging costs are high, yield is low, and it is difficult to guarantee the impact performance of this type of plastic mold steel.Patent application number 200610116358.6, entitled "High Heat Strength Hot Work Die Steel Material," discloses a hot work die steel with the following composition percentages: Cr: 3.5%–4.0%, Mo: 2.0%–2.5%, V: 1.0%–1.5%, W: 1.0%–1.5%, Mn: 0.1%–0.5%, Ni: 0.1%–0.25%, C: 0.3%–0.35%, Si: 0.1%–0.5%, S: 0.005%–0.01%, P: 0.01%–0.02%. This die steel exhibits high service hardness, ranging from 48 to 54 HRC. Its room temperature impact toughness is greater than 300 J, and it possesses excellent thermal fatigue performance. However, it cannot guarantee high-temperature wear resistance and high-temperature thermal conductivity. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to prepare a mold steel with excellent resistance to tempering softening and its preparation method. The mold steel of the present invention, through the compositional design of alloying elements, maintains a uniform distribution of nanoscale carbides even after prolonged tempering. Simultaneously, the process design enables the mold steel of the present invention to possess excellent resistance to tempering softening, excellent resistance to high-temperature wear, high thermal conductivity, and high hardness, while also achieving a balance between strength and toughness, uniform performance, and employing an efficient and economical production process, effectively improving the service life of molds.

[0005] The objective of this invention is achieved as follows:

[0006] A mold steel with excellent resistance to tempering softening comprises, by weight percentage: C: 0.50%–0.60%, Si: 0.30%–0.50%, Mn: 0.30%–0.50%, P≤0.015%, S≤0.015%, Cr: 2.40%–3.40%, Mo: 3.50%–4.00%, V: 1.50%–2.00%, Ti: 0.20%–0.30%, Sr: 0.20%–0.30%, Zn: 0.30%–0.40%, with the remainder being Fe and unavoidable impurities.

[0007] Furthermore, in the mold steel, the V / Cr ratio is 0.585 to 0.625.

[0008] Furthermore, in the mold steel, the Mo / Zn ratio is 10.0 to 11.7.

[0009] Furthermore, in the mold steel, Ti+Sr is 0.45% to 0.55%.

[0010] Furthermore, the mold steel plate has a room temperature hardness of 47-49 HRC, a thickness section hardness difference of ≤2 HRC, and a hardness of 43-44 HRC after being kept at 600℃ for 24 hours and a hardness of 34-35 HRC after being kept at 650℃ for 24 hours.

[0011] Furthermore, the unnotched room temperature transverse impact toughness of the mold steel plate is 210–220 J; the wear volume at 700℃ is (7.8–7.9) × 10⁻⁶. 7 μm 3 At 700℃, the wear track depth is 26–28 μm, and the wear track width is 0.85–0.95 mm; the thermal conductivity at 700℃ is (38–39) W·m. -1 ·K -1 The thickness of the mold steel plate is 50-150mm.

[0012] Furthermore, the peak particle size distribution of carbide in the mold steel plate is in the range of 15-25 nm, accounting for 29%-30%. After being kept at 650℃ for 24 hours, the peak particle size distribution of carbide is in the range of 25-50 nm, accounting for 36%-38%.

[0013] The rationale for the design of the components in this invention is as follows:

[0014] 1) C: In the mold steel of this invention, part of the carbon is dissolved in the matrix to play a solid solution strengthening role, while part interacts with alloying elements and precipitates in the form of alloy carbides. The carbon content can directly affect the strength, ductility, toughness, high-temperature strength, and thermal stability of the steel of this invention. Especially during the tempering process, the carbides decomposed from martensite and retained austenite are dispersed on the α phase, thereby improving the comprehensive properties of the mold steel, such as strength and toughness. The present invention appropriately increases the carbon content, which on the one hand allows more strong carbide-forming elements and weak carbide-forming elements such as Mn to be distributed from the steel matrix to the carbides, thereby reducing the large lattice distortion caused by the solid solution of these elements in the steel matrix and reducing the adverse effects of these elements on the thermal conductivity of the steel; on the other hand, it improves the strength and hardness of the steel, compensates for the decrease in matrix hardness caused by the reduction of Si and Cr elements, and ensures the wear resistance of the steel; and on the other hand, it avoids increasing the cold brittleness and aging sensitivity of the steel of this invention, ensuring the weldability and corrosion resistance of the steel. Therefore, this invention selects to add C: 0.50% to 0.60%, which works together with other alloying elements. Through the preparation method of this invention, even after long-term tempering, uniformly distributed nanoscale carbides can be obtained, giving this mold steel excellent comprehensive performance, relatively stable microstructure and thermal stability.

[0015] 2) Si: In this invention, Si can improve hardenability and matrix strength, which is beneficial for secondary hardening. It can also improve the dispersion of alloy carbides precipitated during high-temperature tempering of this type of mold steel, making them more uniformly distributed. Si restricts the migration rate of carbon in the steel, providing preferential precipitation sites for alloying elements during high-temperature tempering, so that fine and dispersed carbides are precipitated only at high temperatures, thus improving the thermal stability of the mold steel of this invention. In addition, it can diffuse into ε-carbides, improving the stability of ε-carbides through solid solution, and delaying the appearance of cementite during tempering. This process can effectively shorten the temperature range of first-type temper brittleness, thereby adjusting the strength and toughness combination of tempered martensite. This invention appropriately reduces the Si content to avoid severe segregation, which would cause anisotropy in the steel and adversely affect the uniformity of the steel's microstructure. On the other hand, the outer electron structure of Si differs significantly from that of Fe, resulting in greater lattice distortion when dissolved in steel. This enhanced lattice distortion stress field would increase interference with the movement of electrons and phonons, leading to increased thermal conductivity. Therefore, this invention requires controlling the Si content to be between 0.30% and 0.50%.

[0016] 3) Mn: In this invention, Mn plays a role in solid solution strengthening. Although its solid solution strengthening effect is weaker than that of C, it has little impact on the plasticity of steel and hardly reduces the ductility of steel. Secondly, it improves hardenability, and it is the element that most significantly improves hardenability. Thirdly, it can advance the secondary hardening temperature and promote the dissolution of carbides. During the austenitizing heating process, more carbides dissolve into the matrix and combine with other strong carbide elements to form carbides, thereby increasing the peak value of secondary hardening. However, excessive Mn content has adverse effects on toughness and high-temperature performance, and it easily leads to element enrichment and segregation, resulting in uneven composition and microstructure of the matrix material and large differences in the overall performance of the final steel plate. In addition, the outer electronic structure of Mn differs significantly from that of Fe, and its solid solution in steel produces greater lattice distortion. The enhanced lattice distortion stress field will increase interference with the movement of electrons and phonons, thus increasing thermal conductivity. Therefore, this invention adds an appropriate amount of Mn, eliminating the need for Ni. On the one hand, it stabilizes austenite, and on the other hand, it allows more carbides to dissolve into the matrix during high-temperature solid solution, combining with other strong carbide elements to form carbides and fully exerting the beneficial effects. On the other hand, it avoids adverse effects on the thermal conductivity of the steel. Therefore, this invention selects an Mn content of 0.30% to 0.50%.

[0017] 4) P and S: S is distributed in the steel in the form of MnS. During hot rolling, MnS elongates along the rolling direction, significantly reducing the transverse mechanical properties of sulfur-containing free-cutting steel and exacerbating the anisotropy of the steel. Simultaneously, it causes voids within the matrix, becoming channels for oxidation to penetrate deeper, thus reducing the thermal stability of this type of die steel. While P can moderately increase the hardness of ferrite and improve the surface finish and machinability of parts, it is prone to segregation at austenite grain boundaries, weakening the interatomic bonding force at the grain boundaries of the matrix material, resulting in high temper brittleness. Phosphorus segregation at grain boundaries can also cause brittle fracture inheritance. Furthermore, excessive S and P can affect the homogeneity and purity of the steel. Considering the steelmaking cost and its impact on steel, this invention selects to add P ≤ 0.015% and S ≤ 0.015%.

[0018] 5) Cr: Cr can improve the hardenability of steel and has a secondary strengthening effect, promote alloying, delay the transformation of pearlite and bainite, and increase the hardness and wear resistance of steel without making it brittle, ensuring the manufacturing and production of large-section molds. Cr easily combines with carbon to form various hard carbides. These carbides are distributed in the steel matrix and play an important role in improving the hardness, wear resistance, and thermal stability of the mold steel of this invention. If the Cr content is too high, during high-temperature tempering or long-term tempering, other alloying elements in the carbides will be replaced by Cr, forming coarse and soft high-chromium carbides, thereby reducing the thermal strength and thermal stability of the mold steel. In addition, if the Cr content is high, the amount dissolved in the matrix will be greater, causing a greater degree of lattice distortion in the steel matrix and reducing the thermal conductivity of the steel. Therefore, this invention adds an appropriate amount of Cr to ensure that, after solidification in the matrix, it can significantly enhance the hardenability of the steel, ensuring the manufacturing and production of large-section molds; to ensure the thermal strength and thermal stability of the mold steel of this invention; and to ensure the thermal conductivity of the steel. Furthermore, the addition of an appropriate amount of Cr in this invention can form a very dense Cr2O3 oxide film on the surface, improving the corrosion resistance of the steel. The combined effect of alloying elements such as Zn can increase the diffusion rate of Cr in the steel and reduce its aggregation and segregation in the core of the steel plate. Therefore, this invention selects a Cr content of 2.40% to 3.40%.

[0019] 6) Mo: In this invention, Mo plays two roles: First, it dissolves into the matrix to enhance the strength and hardness of the mold steel through solid solution strengthening. Mo dissolved in the matrix tends to agglomerate around dislocations, reducing the degree of collective lattice distortion and forming Cotillard or Suzuki atmospheres that pin dislocations, hindering dislocation activation, increasing the yield strength of the mold steel, and even creating a yield plateau. During tempering, due to the pinning effect of these atmospheres, dislocations in the α phase are difficult to aggregate, merge, or cancel each other out, resulting in a delayed subgrain temperature and significantly hindering the recovery and even recrystallization of the α phase, thus increasing the tempering stability of the material. Second, it precipitates MC and M2C carbides during subsequent tempering. These two types of carbides are small in size and diffusely distributed, contributing greatly to the secondary hardening effect. However, excessive Mo causes Mo2C to transform into M6C, precipitating along the original austenite grain boundaries and martensite lath boundaries. Large M6C particles can cause material embrittlement and reduce toughness, but too little Mo will result in an insignificant secondary hardening effect. The present invention appropriately increases the Mo content, which on the one hand improves the hardenability of the mold steel and ensures the uniformity of the microstructure and properties in the thickness direction of the large-section mold steel; on the other hand, it reduces the amount of M. 23 The formation of C6 and M7C3 carbides, and the formation of more stable MC and M2C type simple carbides, ensure the hot strength and thermal stability of the mold steel. On the one hand, it can reduce the degree of decarburization and oxidation; on the other hand, it can ensure the secondary hardening effect without reducing the toughness of the mold steel of the present invention. Therefore, the present invention selects a Mo content of 3.50% to 4.00%.

[0020] 7) V: The role of V is twofold: firstly, it dissolves in the matrix to provide solid solution strengthening; secondly, it combines with C in the material to form alloy carbides. In this invention, V can form high-melting-point VC, which is difficult to melt during heat treatment. Therefore, during the austenitization process, it can pin grain boundaries and prevent austenite grain growth, thereby effectively reducing the matrix grain size and achieving fine-grain strengthening. These carbides are uniformly dispersed in the material and have the characteristics of high melting point, high hardness, strong stability, and low growth rate. They can effectively enhance the wear resistance of the material. During high-temperature tempering, they can precipitate from the matrix, promoting secondary hardening and improving the high-temperature stability of the material, thus reducing the overheating sensitivity. In addition, they can improve the material's resistance to temper softening, mainly because VC has high thermal stability. Even under service conditions of 700℃, it is not easy to agglomerate and grow, tending to be fine and dispersed, giving the material excellent resistance to temper softening. However, excessive V can reduce the plasticity and toughness of the material. Therefore, this invention adds V at 1.50% to 2.00%.

[0021] This invention controls the V / Cr ratio to be between 0.585 and 0.625. Since V-containing carbides maintain dimensional stability better than Cr-containing carbides during tempering, these fine V-containing carbides can more effectively hinder dislocation movement, improving the high-temperature thermal stability of the die steel. Furthermore, the V / Cr ratio and carbon content in this invention promote the precipitation of V-containing MC-type carbides in the steel. These fine MC-type carbides are dimensionally stable and have a better pinning effect on dislocations. During tempering, a large number of nano-sized carbides can precipitate, significantly pinning dislocations and improving its resistance to tempering softening. This invention, by setting the V / Cr ratio, promotes the precipitation of MC and delays the precipitation of M. 23 The precipitation and transformation of C6 effectively hinder the coarsening and transformation of fine carbides, improving the thermal stability of the material. Furthermore, the material is subject to high-temperature wear during service. Below 600°C, a slight oxidation wear mechanism occurs, and its wear resistance depends on the properties of its oxides and the matrix. Although the Cr content in this invention is low, the combined effect of Cr with elements such as V and Mo gives the material matrix high hardness, enabling it to withstand oxidation wear. Above 600°C, the mold steel begins to soften. At this point, the friction-reducing effect of the friction oxides requires support from the matrix structure. The mold steel of this invention exhibits excellent resistance to tempering softening and thermal stability, providing better support for the friction oxide layer. It can form a single-layer friction oxide layer with uniform thickness, effectively protecting the matrix structure and giving the mold steel of this invention excellent high-temperature wear resistance.

[0022] 8) Ti: Ti can effectively improve the internal structure of the mold steel of this invention, refine the grains, and make the coarse network structure more uniform and fine. After refining the grains and strengthening the second phase, the strength and hardness of the steel are improved to a certain extent, the mechanical properties are enhanced, and the wear resistance and fatigue resistance of the mold steel are improved. If too little Ti is added, the grain refinement effect will not be obvious, while if too much Ti is added, the structure will easily become coarse and loose due to dynamic recrystallization, which is not conducive to improving the strength of the steel. Therefore, this invention selects to add Ti of 0.20% to 0.30% to maximize the improvement of the structure and properties of the steel.

[0023] 9) Sr: Sr is chemically reactive and can significantly refine the grains of the mold steel of this invention, making the coarse network structure more uniform and fine. During non-equilibrium solidification, the mold steel of this invention easily forms metastable lamellar eutectic carbides, distributed in a fan shape between equiaxed grains. These lamellar eutectic carbides can be decomposed by hot working such as rolling. If the decomposition is incomplete, some will remain and be inherited into the final steel plate structure, which is detrimental to mechanical properties and thermal stability. Adding an appropriate amount of Sr in this invention can significantly reduce sharp corners, alleviate the segregation of eutectic carbides, improve the microstructure, increase strength and mechanical properties, enhance wear resistance, and mitigate frictional wear and fatigue effects caused by harsh working environments such as high temperature and alternating stress, thus extending the service life of the mold steel of this invention. Therefore, the amount of Sr added in this invention is 0.20%–0.30%.

[0024] In this invention, Sr and Ti work together to ensure that Sr remains stable even at high temperatures and is not easily oxidized. This effectively enhances the hardness and strength of steel, increases ductility, and improves wear resistance. At the same time, the beneficial effects of Ti and Sr are utilized, and even after long-term tempering at high temperatures, uniform and fine grains can still be obtained, improving the material's resistance to tempering softening. Therefore, this invention controls Ti+Sr = 0.45% to 0.55% to avoid adverse effects caused by excessive amounts.

[0025] 10) Zn: The addition of an appropriate amount of Zn in this invention serves three purposes: First, it increases the adhesion between the base metal and the oxide film, as it has a "pinning" effect on the base metal, ensuring that the base metal retains high strength and stability at high temperatures. It also increases the diffusion rate of Cr in the steel, facilitating the formation of Cr2O3 on the steel surface and improving the material's oxidation resistance. Second, it improves the morphology of eutectic carbides in the as-cast microstructure of this type of mold steel, breaking down the network of eutectic carbides and dispersing them uniformly in a spherical shape. Third, it reduces the enrichment and segregation of easily segregating elements such as P and Mn in the base material, releasing a large amount of distortion energy stored within the material. This weakens the effect of P and Mn on grain boundaries, resulting in a more uniform distribution, improving the material's impact toughness and the uniformity of its microstructure, and extending the mold's service life. Therefore, the Zn content in this invention is 0.30%–0.40%.

[0026] This invention adds an appropriate amount of Mo, which gives the mold steel higher red hardness and resistance to tempering softening. The tempered structure produces a large number of M2C-type carbides. These fine M2C carbides hinder dislocation movement and grain boundary migration at high temperatures, resulting in better stability. Even after prolonged high-temperature tempering, the carbides only geomorphize in place, with minimal growth characteristics, thus improving the material's high-temperature strength. However, Mo in this invention is prone to point segregation. To fully utilize its beneficial effects and avoid the adverse effects of point segregation on composition and stress concentration, this invention controls the Mo / Zn ratio to be 10.0–11.7, resulting in a more uniform distribution of elements and microstructure. This makes the mold steel more suitable for manufacturing large-section molds and producing large, complex-shaped parts, extending mold life and preventing cracking failure.

[0027] The second technical solution of the present invention is to provide a method for manufacturing mold steel with excellent resistance to tempering softening, including smelting, continuous casting, slab heating, rolling, straightening, slow cooling and heat treatment.

[0028] Slab heating

[0029] The continuously cast slab is heated to 1260℃~1270℃ and held in the soaking zone for 4~5 hours. The mold steel of the present invention has a high alloy content, which usually requires a higher slab heating temperature and a longer holding time to allow the alloying elements to fully dissolve in the matrix, improve the non-uniformity of the slab composition, reduce compositional segregation, and thus reduce subsequent microstructure segregation, allowing large-size eutectic carbides to dissolve.

[0030] Rolling

[0031] The initial rolling temperature is controlled at 1060℃~1080℃, and the reduction rate of the first two passes during steel plate rolling is 17%~20%. Using a large reduction rate can increase the deformation penetration depth, allowing coarse columnar crystals to break down into fine, uniform grains, thus welding together central structural defects. The final rolling temperature is 1000℃~1020℃, and the reduction rate of the final rolling pass is controlled at 17%~20% to obtain steel plates with a thickness of 50~150mm. The steel plate exits the rolling mill at a temperature of 300~400℃. Immediately after exiting the mill, the plates are stacked using a "bottom-lay, top-cover" method for slow cooling, with a slow cooling time of 36~48 hours.

[0032] Heat treatment

[0033] a. High-temperature normalizing + high-temperature tempering pretreatment

[0034] The steel plate is heated to 1080–1100℃ and held for 3–4 hours, then air-cooled to room temperature. Subsequently, the steel plate is heated to 750–770℃ and held for 2–3 hours, then air-cooled to room temperature. This invention employs a high-temperature normalizing + high-temperature tempering pretreatment. At a high temperature, most of the carbides dissolve in the austenite, leaving only a small amount of isolated coarse carbides. Banded carbides are essentially eliminated, resulting in uniformly distributed, fine-sized spherical carbides. This improves the fracture toughness of the material without affecting hardness, which is beneficial for enhancing the fatigue crack propagation resistance and thermal fatigue cracking resistance of the mold steel of this invention. It provides a stable and uniform microstructure for the final state steel plate and improves its resistance to tempering softening.

[0035] b. Poisoning

[0036] The steel plate is heated to 1030–1050°C and held for 2–3 hours, then air-cooled to room temperature. The mold steel of this invention has a high carbon and alloy content, resulting in a strong ability to form carbides. Conventional heat treatment temperatures are too low to eliminate banded carbides. However, the high-temperature normalizing + tempering pretreatment and secondary normalizing process of this invention promotes the complete diffusion of carbides at grain boundaries into the austenite grains at high temperatures, thus eliminating banded carbides. The steel plate in this invention undergoes two γ-M phase transformations, resulting in a more uniform microstructure, finer grains, and fine, uniformly dispersed carbides. This microstructure increases dislocation slip resistance, preparing the microstructure for tempering heat treatment and increasing carbide dispersion after tempering, significantly improving the thermal stability and wear resistance of this mold steel.

[0037] c. Two tempering processes

[0038] The first tempering temperature is 580–610℃, held for 2–3 hours, and then air-cooled to room temperature. A second tempering heat treatment is then performed at 610–640℃, held for 2–3 hours, and then air-cooled to room temperature. The primary function of the first tempering in this invention is twofold: first, to reduce the internal stress generated after normalizing, thus stabilizing the microstructure; and second, to decompose the retained austenite. If there is too much undecomposed retained austenite, the quenched martensite from the austenite decomposition will become crack initiation sites during the use of the mold steel, leading to early cracking and causing the material to age prematurely and become unusable. The purpose of the secondary tempering process is threefold: first, to allow the products of the primary tempering transformation to undergo further tempering, eliminating residual stress, homogenizing the entire microstructure, and improving the material's toughness; second, to add a second high-temperature tempering process, which allows more alloying elements to precipitate from the solid solution, reducing lattice distortion. This not only weakens the influence of alloying elements on the thermal conductivity of the steel but also consumes more carbon elements dissolved in the matrix, reducing the influence of carbon elements on the iron lattice distortion and contributing to improved thermal conductivity; and third, to produce a uniform tempered martensite microstructure with a large number of fine precipitates dispersed in the matrix. These precipitates have high thermal stability, which is beneficial for improving the material's high-temperature strength and resistance to tempering softening. This allows the mold steel of this invention to maintain strength while possessing good toughness after tempering. Furthermore, these precipitates are hard phases, which can effectively improve the material's room temperature and high-temperature wear resistance.

[0039] The beneficial effects of this invention are as follows:

[0040] This invention employs a low-Si, high-Mo design philosophy, adding Mo, Cr, V, and Zn in specific proportions, with Ti and Sr working synergistically. It avoids the addition of precious metals such as W and Ni, resulting in mold steel with excellent resistance to tempering softening. Even after prolonged tempering, it retains a uniformly distributed nanoscale carbide structure. Furthermore, the process design ensures this mold steel exhibits good high-temperature wear resistance and high thermal conductivity. The production process involves hot metal pretreatment, smelting, LF, RH, continuous casting, slab heating, rolling, straightening, slow cooling, high-temperature normalizing + high-temperature tempering pretreatment, normalizing, and double tempering. This process ensures the mold steel of this invention possesses high hardness, a good balance of strength and toughness, and uniform performance.

[0041] 1. The mold steel of the present invention has excellent resistance to tempering softening. The peak size of carbide particles in the final state steel plate is distributed in the range of 15-25nm, accounting for 29%-30%. After being held at 650℃ for 24h, the peak size of carbide particles is distributed in the range of 25-50nm, accounting for 36%-38%. Even after long-term tempering, it still has uniformly distributed nanoscale carbides, with no large-sized carbides and uniformly dispersed.

[0042] 2. The mold steel of this invention has good high-temperature wear resistance and high thermal conductivity; the wear volume at 700℃ is (7.8~7.9)×10. 7 μm 3At 700℃, the wear track depth is 26–28 μm, and the wear track width is 0.85–0.95 mm; it also exhibits good high-temperature thermal conductivity, with a thermal conductivity of (38–39) W·m at 700℃. -1 ·K -1 .

[0043] 3. The mold steel of this invention has high hardness, strength and toughness matching and performance uniformity. After one tempering, the core hardness of the steel plate is 50-52 HRC, the room temperature hardness of the final steel plate is 47-49 HRC, and the hardness difference of the thickness section is ≤2 HRC. The transverse impact toughness of the unnotched core of the final steel plate at room temperature is 210-220 J. It has good resistance to tempering softening. After being held at 600℃ for 24 hours, the hardness remains at 43-44 HRC, and after being held at 650℃ for 24 hours, the hardness remains at 34-35 HRC. Detailed Implementation

[0044] The present invention will be further illustrated below through examples.

[0045] According to the component ratio of the technical solution, the embodiments of the present invention perform hot metal pretreatment, smelting, LF, RH, continuous casting, slab heating, rolling, straightening, slow cooling, high temperature normalizing + high temperature tempering pretreatment, normalizing, and two tempering.

[0046] Slab heating

[0047] Heat the continuously cast slab to 1260℃~1270℃ and hold it in the soaking zone for 4~5 hours;

[0048] Rolling

[0049] The initial rolling temperature is controlled at 1060℃~1080℃, the reduction rate of the first two passes during steel plate rolling is 17%~20%, the final rolling temperature is 1000℃~1020℃, and the reduction rate of the final rolling pass is controlled at 17%~20%, resulting in a steel plate thickness of 50~150mm.

[0050] High-temperature normalizing + high-temperature tempering pretreatment

[0051] The steel plate is heated to 1080℃~1100℃, held for 3~4 hours, removed from the furnace and air-cooled to room temperature. Then the steel plate is heated to 750℃~770℃, held for 2~3 hours, removed from the furnace and air-cooled to room temperature.

[0052] Zheng Huo

[0053] Heat the steel plate to 1030℃~1050℃, hold it at that temperature for 2~3 hours, and then air cool it to room temperature.

[0054] Two temperings

[0055] The first tempering temperature is 580℃~610℃, the holding time is 2~3h, and then air-cooled to room temperature. The second tempering temperature is 610~640℃, the holding time is 2~3h, and then air-cooled to room temperature.

[0056] Furthermore, the manufacturing method of the mold steel includes a stripping process after rolling, with a stripping temperature of 300-400°C. After stripping, the steel is immediately stacked using a "bottom-up, top-down" method for slow cooling, with a slow cooling time of 36-48 hours.

[0057] The following embodiments are merely some preferred embodiments of the present invention and do not limit the scope and technical means of the invention in any way. Table 1 shows the chemical composition of the embodiments of the present invention; Table 2 shows the slab heating, rolling and cooling processes of the embodiments; Table 3 shows the heat treatment process of the embodiments; Table 4 shows the average hardness of the core of the steel plate after one tempering in the embodiments; Table 5 shows the transverse impact toughness of the core at room temperature in each embodiment; Table 6 shows the Rockwell hardness of the final state steel plate in each embodiment; Table 7 shows the thermal stability of the final state steel plate in each embodiment; Table 8 shows the final state carbide particle size distribution of the core in the embodiments; Table 9 shows the carbide particle size distribution of the core sample after being held at 650℃ for 24 hours in the embodiments; Table 10 shows the high temperature wear performance of the embodiments; and Table 11 shows the high temperature thermal conductivity of the embodiments.

[0058] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0059]

[0060] Table 2. Slab heating, rolling, and cooling processes in the examples.

[0061]

[0062] Table 3 Heat treatment process of the embodiments

[0063]

[0064] Table 4 Average hardness of steel plate core after one tempering

[0065] Example First tempering hardness / HRC 1 50.3 2 51.0 3 50.5 4 51.5 5 51.8 6 52.0 7 50.0 8 51.4 9 50.9 10 51.7

[0066] Table 5. Room Temperature Core Lateral Impact Toughness of Each Example

[0067]

[0068] Note: The impact specimen measures 10×7×55mm (unnotched).

[0069] Table 6 Rockwell hardness properties of steel plates in their final state

[0070]

[0071] Table 7 Thermal stability of the final state steel plates in each embodiment

[0072]

[0073] Table 8. Particle size distribution of final-state carbides in the core of each embodiment.

[0074]

[0075] Table 9. Carbide particle size distribution of the core sample after being kept at 650℃ for 24 hours in the examples.

[0076]

[0077] Note: A 20×20×20mm sample was cut from the core of the sample in the example. Fifty fields of view were randomly selected at 5000x magnification to statistically analyze the carbide particle size distribution.

[0078] Table 10 High-Temperature Wear Performance of Examples

[0079] Example <![CDATA[Wear volume at 700 °C / 10 7 μm 3 > wear depth at 700℃ / μm Grinding mark width at 700℃ / mm 1 7.88 26.7 0.87 2 7.85 27.1 0.90 3 7.84 27.5 0.92 4 7.89 26.9 0.85 5 7.82 26.5 0.93 6 7.80 28.0 0.95 7 7.83 27.7 0.88 8 7.81 27.5 0.89 9 7.90 26.0 0.94 10 7.86 27.0 0.91

[0080] Note: Core samples from each embodiment were used for testing. The grinding material was high-hardness SiC, the load was 10N, the test time was 1 hour, and the sliding distance was 360m.

[0081] Table 11 High-Temperature Thermal Conductivity of Examples

[0082] Example <![CDATA[Thermal conductivity at 700 °C / W·m -1 ·K -1 > 1 38.5 2 38.2 3 38.9 4 38.0 5 38.7 6 38.3 7 38.0 8 38.8 9 38.4 10 38.6

[0083] As can be seen from the above, the peak carbide particle size distribution in the final state of the mold steel plate of the present invention is in the range of 15-25 nm, accounting for 29%-30%. After being held at 650℃ for 24 hours, the peak carbide particle size distribution is in the range of 25-50 nm, accounting for 36%-38%. Even after long-term tempering, it still has uniformly distributed nanoscale carbides, with no large-sized carbides and uniformly dispersed distribution; the wear volume at 700℃ is (7.8-7.9)×10 7 μm 3 At 700℃, the wear track depth is 26–28 μm, and the wear track width is 0.85–0.95 mm; it also exhibits good high-temperature thermal conductivity, with a thermal conductivity of (38–39) W·m at 700℃. -1 ·K -1 After one tempering, the core hardness of the steel plate is 50-52 HRC, and the room temperature hardness of the final state steel plate is 47-49 HRC, with a thickness section hardness difference of ≤2 HRC; the room temperature transverse impact toughness of the unnotched core of the final state steel plate is 210-220 J; it has good resistance to tempering softening, and after holding at 600℃ for 24 hours, the hardness remains at 43-44 HRC, and after holding at 650℃ for 24 hours, the hardness remains at 34-35 HRC.

[0084] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A mold steel with excellent resistance to temper softening, characterized in that, By weight percentage, it comprises the following components: C: 0.50%~0.60%, Si: 0.30%~0.50%, Mn: 0.30%~0.50%, P≤0.015%, S≤0.015%, Cr: 2.40%~3.40%, Mo: 3.50%~4.00%, V: 1.50%~2.00%, Ti: 0.20%~0.30%, Sr: 0.20%~0.30%, Zn: 0.30%~0.40%, with the remainder being Fe and unavoidable impurities; The manufacturing method of the aforementioned mold steel with excellent resistance to tempering softening includes smelting, continuous casting, slab heating, rolling, and heat treatment. (1) Slab heating Heat the continuously cast slab to 1260℃~1270℃ and hold it in the soaking zone for 4~5 hours; (2) Rolling The initial rolling temperature is controlled at 1060℃~1080℃, and the reduction rate of the first two passes during steel plate rolling is 17%~20%. The final rolling temperature is controlled at 1000℃~1020℃, and the reduction rate of the final rolling pass is controlled at 17%~20%. (3) Heat treatment a. High-temperature normalizing + high-temperature tempering pretreatment The steel plate is heated to 1080~1100℃, held for 3~4 hours, removed from the furnace and air-cooled to room temperature. Then the steel plate is heated to 750~770℃, held for 2~3 hours, removed from the furnace and air-cooled to room temperature. b. Poisoning Heat the steel plate to 1030~1050℃, hold it at that temperature for 2~3 hours, and then air cool it to room temperature after removing it from the furnace. c. Two tempering processes The first tempering temperature is 580~610℃, the holding time is 2~3 hours, and then air-cooled to room temperature. The second tempering temperature is 610~640℃, the holding time is 2~3 hours, and then air-cooled to room temperature.

2. The die steel with excellent resistance to tempering softening as described in claim 1, characterized in that, In the mold steel, the V / Cr ratio is 0.585~0.

625.

3. The die steel with excellent resistance to tempering softening as described in claim 1, characterized in that, In the mold steel, the Mo / Zn ratio is 10.0~11.

7.

4. The die steel with excellent resistance to tempering softening as described in claim 1, characterized in that, The mold steel contains 0.45% to 0.55% Ti+Sr.

5. The die steel with excellent resistance to tempering softening as described in claim 1, characterized in that, The mold steel has a room temperature hardness of 47~49HRC, a thickness section hardness difference of ≤2HRC, and a hardness of 43~44HRC after being kept at 600℃ for 24 hours and 34~35HRC after being kept at 650℃ for 24 hours.

6. The die steel with excellent resistance to tempering softening as described in claim 1, characterized in that, The unnotched mold steel has a room temperature transverse impact toughness of 210~220J; the wear volume at 700℃ is (7.8~7.9)×10⁻⁶. 7 μm 3 At 700℃, the wear track depth is 26~28μm, and the wear track width is 0.85~0.95mm; the thermal conductivity at 700℃ is (38~39) W·m. -1 ·K -1 The thickness of the mold steel plate is 50~150mm.

7. The die steel with excellent resistance to tempering softening as described in claim 1, characterized in that, The manufacturing method of the mold steel includes a stripping process after rolling. The stripping temperature is 300~400℃. After stripping, the steel is immediately stacked in a "bottom-up, top-down" manner for slow cooling, which takes 36~48 hours.

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