Die steel with uniformly distributed and spherical inclusions and its preparation method

Abstract

This invention proposes a mold steel with uniformly distributed inclusions and a low deformation rate, and its manufacturing method. The steel plate composition, by weight percentage, is as follows: C: 0.45%–0.55%, Si: 0.20%–0.30%, Mn: 0.20%–0.40%, P≤0.015%, S≤0.015%, Cr: 3.0%–4.0%, Mo: 3.0%–3.5%, V: 0.70%–0.80%, Ni: 0.80%–0.90%, Sc: 0.02%–0.03%, Zn: 0.03%–0.05%, wherein Ni / V = 1.125–1.145, Sc+Zn = 0.06%–0.07%, and the balance is Fe and unavoidable impurities. The steel plate production method includes smelting, continuous casting, slab heating, rolling, straightening, slow cooling, normalizing + annealing pretreatment, and quenching and tempering. The number of inclusions per unit area in the microstructure of the mold steel of this invention is (192-205) inclusions / mm. 2 The average particle size is 1.75μm to 1.85μm, fine and uniformly dispersed. The ratio of the maximum diameter to the minimum diameter of the inclusions is 1.69 to 1.78, approximately spherical. The thickness of the mold steel of this invention is 30 to 180 mm; the hardness of the steel plate is 46 to 48 HRC, and the hardness difference between the thickness and the cross-section is ≤2 HRC; the unnotched room temperature transverse impact toughness of the core is 220 to 230 J; the wear volume at 800℃ is (25 to 26) × 10⁻¹⁰. ‑3 mm 3 It has high thermal conductivity at high temperatures, with a thermal conductivity of (35~36) W·m at 800℃. ‑1 ·K ‑1 .

Description

Technical Field

[0001] This invention belongs to the field of metal material production technology, and particularly relates to a mold steel with uniformly distributed and spherical inclusions and its preparation method. Background Technology

[0002] The automotive industry faces challenges related to energy utilization, environmental protection, and safety. Due to environmental and energy conservation needs, lightweighting of automobiles is an effective method for achieving energy conservation and emission reduction. With the development of lightweighting, ultra-high-strength steel has gradually been developed and applied. However, the increased strength and decreased elongation of high-strength steel significantly reduce its formability, making cold stamping technology unsuitable for the processing requirements and production needs of high-strength steel sheets. Therefore, it is necessary to develop die steel suitable for hot stamping technology. This type of die steel, through a reasonable alloy composition ratio, improves thermal conductivity while ensuring a series of basic mechanical properties such as strength and hardness. Simultaneously, the material surface possesses a certain degree of corrosion resistance, ensuring increased sheet material speed during the hot stamping and quenching stage, resulting in improved microstructure strength of the parts. Furthermore, it can improve production efficiency and die lifespan.

[0003] Many domestic organizations have done a great deal of work in developing new types of mold steel, improving product quality, optimizing production processes, and extending mold life. Patent application CN202110261649.9, entitled "A Low-Vanadium Nitrogen-Containing Hot Work Mold Steel and Its Preparation Method," discloses a low-vanadium nitrogen-containing hot work mold steel with the following composition percentages: C: 0.3%-0.4%, Si: 0.2%-0.6%, Mn: 0.2%-0.5%, Cr: 4.5%-5.5%, Mo: 1.1%-1.7%, V: 0.4%-0.6%, N: 0.02%-0.07%, Ce: 0.005%-0.03%, Mg: 0.001%-0.006%, with the balance being Fe. The process adopted is as follows: (1) smelting steel according to the set composition, adding nitrogen by vapor phase nitriding, and casting; (2) homogenization at 1200-1250℃; (3) forging; (4) normalizing at 1000-1100℃; (5) spheroidizing annealing; (6) quenching by holding at 1000-1050℃ and oil cooling; (7) tempering twice at 530-620℃ for 2-6 hours. Reducing the V and Si content, increasing the appropriate amount of N, and synergistically adding trace amounts of rare earth and magnesium can improve the cleanliness of the steel, improve the distribution of carbides, and improve the performance of mold steel. However, it is difficult to guarantee the uniformity of cross-sectional hardness, corrosion resistance and wear resistance. 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 has high service hardness, ranging from 48-54 HRC, room temperature impact toughness greater than 300 J, and excellent thermal fatigue performance, but corrosion resistance cannot be guaranteed. 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 type of 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 202111273495.1, entitled "A Method for Soft Nitriding Heat Treatment of Mold Steel with High Hardness and Impact Energy," discloses a method for soft nitriding heat treatment of mold steel with high hardness and impact energy, including the following steps: Step 1: Pre-treating the surface of the mold steel to be nitrided using ultrasonic pressurized shot peening to form a nanocrystalline layer on the surface of the mold steel. The beneficial effects are: by pre-processing a nanocrystalline layer on the surface of the mold steel before nitriding, and simultaneously adding rare earth elements to accelerate nitriding during the nitriding process, the nitriding rate of the mold steel can be accelerated from both the mold steel itself and external conditions, effectively shortening the nitriding time and improving the overall efficiency of the soft nitriding process. Furthermore, the nanocrystalline layer significantly reduces the nitriding temperature while increasing the nitriding rate, effectively reducing the process difficulty of soft nitriding of the mold steel, avoiding the impact of traditional high-temperature nitriding on the properties of the mold steel, and greatly improving the surface hardness and impact resistance of the soft nitrided mold steel. While ensuring the surface hardness and toughness of the mold steel, the core hardness and toughness are difficult to guarantee, and its fatigue resistance and other properties are hard to prove. 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 contents of elements like Si and Cr can worsen thermal conductivity, it is difficult to guarantee important properties such as hardenability, thermal stability, strength, hardness, and wear resistance. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention aims to provide a mold steel with uniformly distributed and spherical inclusions and its preparation method. This invention, through the rational addition and interaction of alloying elements, alters the type, morphology, size, and distribution of oxides, sulfides, and carbides in molten steel, thereby modifying inclusions in the modified steel and achieving spheroidization, miniaturization, and dispersion of inclusions. Because the inclusions in the steel of this invention are uniformly distributed, small in size, and have a near-spherical morphology, their distribution, size, and morphology change minimally after slab heating, rolling, and heat treatment. This results in a mold steel with excellent comprehensive properties such as high thermal conductivity, excellent wear resistance, and pitting corrosion resistance. Simultaneously, the efficient and economical production process effectively improves the service life of molds, making it suitable for the ever-expanding mold market and increasingly demanding application requirements. It reaches the level of internationally advanced mold materials and has broad application prospects.

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

[0006] By weight percentage, it comprises the following components: C: 0.45%–0.55%, Si: 0.20%–0.30%, Mn: 0.20%–0.40%, P≤0.015%, S≤0.015%, Cr: 3.0%–4.0%, Mo: 3.0%–3.5%, V: 0.70%–0.80%, Ni: 0.80%–0.90%, Sc: 0.02%–0.03%, Zn: 0.03%–0.05%, with the remainder being Fe and unavoidable impurities.

[0007] Furthermore, the Ni / V ratio of the mold steel is 1.125 to 1.145.

[0008] Furthermore, the Sc+Zn content of the mold steel is 0.06% to 0.07%.

[0009] Furthermore, the thickness of the mold steel plate is 30-180mm.

[0010] Furthermore, the number of inclusions per unit area in the mold steel is (192-205) inclusions / mm. 2 The average particle size is 1.75μm to 1.85μm, and the inclusions are small and uniformly dispersed. The ratio of the maximum diameter to the minimum diameter of the inclusions is 1.69 to 1.78, and they are approximately spherical.

[0011] Furthermore, the mold steel plate has a hardness of 46-48 HRC, a thickness-section hardness difference of ≤2 HRC, and an unnotched room temperature transverse impact toughness of 220-230 J.

[0012] Furthermore, the wear volume of the mold steel at 800℃ is (25~26)×10. -3 mm 3 It has high thermal conductivity at high temperatures, with a thermal conductivity of (35~36) W·m at 800℃. -1 ·K -1 .

[0013] The rationale for the steel composition design of this invention is as follows:

[0014] C: In the mold steel of this invention, a portion of the carbon is dissolved in the matrix, playing a solid solution strengthening role, while another portion interacts with alloying elements and precipitates in the form of alloy carbides. The carbon content directly affects the strength, ductility, toughness, high-temperature strength, and thermal stability of the steel of this invention. Especially during tempering, 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. By appropriately increasing the carbon content in this invention, on the one hand, more strong carbide-forming elements and weak carbide-forming elements such as Mn are 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, compensating for the decrease in matrix hardness caused by the reduction of Mn and Cr elements, and ensuring 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, the present invention selects to add C: 0.45% to 0.55%, so that the mold steel has excellent comprehensive performance, relatively stable microstructure and wear resistance.

[0015] Si: In this invention, Si improves hardenability and matrix strength, which is beneficial for secondary hardening. It also increases 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. This results in the precipitation of fine, dispersed carbides only at high temperatures, improving the thermal stability of the mold steel of this invention. Furthermore, 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 balance 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 0.20% and 0.30%.

[0016] 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 greatly 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, resulting in increased thermal conductivity. Therefore, this invention adds an appropriate amount of Mn, which on the one hand stabilizes austenite and allows more carbides to dissolve into the matrix during high-temperature solid solution, combining with other strong carbide elements to form carbides and fully exert the beneficial effects, and on the other hand avoids adverse effects on the thermal conductivity of the steel. Therefore, the Mn content selected in this invention is 0.20% to 0.40%.

[0017] P and S: S is distributed in 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 ferrite hardness 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 affect the homogeneity and purity of the steel. Considering the steelmaking cost and its impact on steel, this invention selects P ≤ 0.015% and S ≤ 0.015%.

[0018] Cr: It 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, thus ensuring the manufacturing and production of large-section molds. Cr easily combines with carbon to form various carbides, which 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 hot strength 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 hot 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 steel's resistance to pitting corrosion. 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 3.0% to 4.0%.

[0019] Mo: In this invention, Mo plays two roles. First, it dissolves into the matrix to enhance the strength and hardness of the die steel through solid solution strengthening. Mo dissolved in the matrix will agglomerate around dislocations to reduce the degree of collective lattice distortion and form Cotillard or Suzuki atmospheres to pin dislocations, hindering dislocation activation, increasing the yield strength of the die steel, and even forming a yield plateau. During tempering, due to the pinning effect of the atmospheres, dislocations in the α phase are difficult to aggregate, merge, or cancel each other, resulting in a delayed temperature of subgrain formation, which significantly hinders the recovery and even recrystallization of the α phase, 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, which precipitates along the original austenite grain boundaries and martensite lath boundaries. Large M6C particles can cause material embrittlement and reduce toughness. Furthermore, these M6C particles are easily re-dissolved into the matrix during solution treatment, leading to a significant increase in Mo elements existing in solid solution form. Since Mo and Fe atoms have large atomic size differences, the solid-solution Mo will generate a large polar potential, increasing electron scattering and thus reducing thermal conductivity. However, too little Mo will result in insignificant secondary hardening effects. Therefore, this invention selects a Mo content of 3.0% to 3.5%.

[0020] The role of vanadium (V) is twofold: firstly, it acts as a solid solution strengthener in the matrix; secondly, it combines with carbon (C) to form alloy carbides within the material. In this invention, vanadium (VC) with a high melting point is formed, making it difficult to melt during heat treatment. Therefore, during austenitization, it pins grain boundaries, inhibits austenite grain growth, and effectively reduces the matrix grain size, achieving a fine-grain strengthening effect. These carbides are uniformly dispersed throughout the material and possess characteristics of high melting point, high hardness, strong stability, and resistance to growth, effectively enhancing the material's wear resistance. During high-temperature tempering, they precipitate from the matrix, promoting secondary hardening and improving the material's high-temperature stability, thereby reducing its overheat sensitivity. Furthermore, they improve the material's resistance to temper softening, mainly because VC has high thermal stability and does not easily aggregate or grow, even under service conditions of 700°C, tending towards a fine and dispersed distribution, giving the material excellent resistance to temper softening. However, excessive vanadium can reduce the material's ductility and toughness; therefore, this invention adds 0.70%–0.80% vanadium.

[0021] Ni: Appropriate amounts of Ni can improve the strength and toughness of steel, and enhance its hardenability. It can increase the passivation tendency of iron-chromium alloys, improve the corrosion resistance of steel in reducing media, and provide rust prevention and oxidation resistance at high temperatures. In this invention, within a certain temperature range, it can also interact with Cr to form a well-protected spinel oxide film, such as a NiO·Cr2O3 composite oxide film, effectively improving the corrosion resistance and high-temperature oxidation resistance of this type of mold steel. Furthermore, during the tempering process of the mold steel in this invention, Ni elements accumulate around carbides, thereby hindering the continued diffusion of carbon atoms in the ferrite surrounding the carbides, increasing the activation energy for carbide coarsening, inhibiting carbide growth, reducing the decrease in hardness of the steel, improving its high-temperature wear resistance, reducing stress concentration, and making the mold surface less prone to cracking. However, excessive Ni content increases costs, and when Ni dissolves in the matrix, it reduces the lattice constant of the matrix, causing lattice distortion, thereby reducing the diffusion rate of carbon in the matrix, hindering phase transformation kinetics, and delaying the dissolution process of undissolved carbides into austenite. Therefore, in this invention, Ni is 0.80% to 0.90%.

[0022] This invention controls the Ni / V ratio to be between 1.125 and 1.145. V-containing carbides exhibit excellent dimensional stability 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 Ni / V ratio and carbon content in this invention promote the precipitation of V-containing MC-type carbides. These fine MC-type carbides are dimensionally stable and provide better pinning effect on dislocations. During tempering, a large number of nano-sized carbides precipitate, significantly pinning dislocations and improving the steel's resistance to tempering softening. This invention, by setting the Ni / V ratio, promotes MC precipitation and delays M precipitation. 23The precipitation and transformation of C6 effectively hinder the coarsening and transformation process of fine carbides, thereby improving the thermal stability of the material. The Ni / V ratio set in this invention can improve high-temperature wear performance. Even under service conditions of 700℃ to 800℃, carbides are not prone to agglomeration and growth, tending to be fine and dispersed. It can also compensate for the adverse effects of V on the plasticity and toughness of the mold steel of this invention.

[0023] Sc: The addition of an appropriate amount of Sc element in this invention has the following effects: First, during the solidification process of molten steel, rare earth inclusions are formed, which can serve as non-uniform nucleation cores in the solidification process of liquid metal, promoting the reduction of grain size and improving the solidification structure; Second, as a deoxidizing and desulfurizing agent, it reacts with harmful impurity elements in molten steel to generate compounds, reducing their segregation at grain boundaries, thereby achieving the purpose of purifying molten steel; Third, it changes the type, morphology, size, and distribution of oxides, sulfides, and carbides in molten steel, modifying inclusions in modified steel, realizing the spheroidization, miniaturization, and dispersion of inclusions, reducing the average deformation rate of inclusions, enhancing the steel's resistance to pitting corrosion, and to a certain extent reducing the potential difference between the steel matrix and inclusions. Since pitting corrosion is an electrochemical catalytic process, the potential... The smaller the potential difference, the lower the corrosion rate; fourth, it can achieve microalloying by dissolving a small amount of Sc in steel, thereby changing the austenite grain size and the precipitation behavior of precipitated phases in austenite and ferrite, thus improving wear resistance, corrosion resistance, and fatigue resistance; fifth, it homogenizes the microstructure, which is beneficial to improving the pitting corrosion resistance of the mold steel of this invention. Inhomogeneous microstructures have a certain potential difference, which easily leads to the formation of corrosion microcells, thereby reducing the corrosion resistance of the steel. In homogenized microstructures, the potential difference is small, making it difficult for corrosion microcells to form. Sc is a surface-active element that easily adsorbs onto growing crystal nuclei, inhibiting austenite grain growth and refining the grains, resulting in a uniform final state steel plate and improving the corrosion resistance of the material. In summary, this invention selects to add Sc at a concentration of 0.02% to 0.03%.

[0024] 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. Because Zn has a "pinning" effect on the base metal, it ensures the base metal retains high strength and stability at high temperatures. It also increases the diffusion rate of Cr and Ni in the steel, facilitating the formation of a Cr2O3 and NiO·Cr2O3 composite oxide film on the steel surface, thus improving the material's oxidation and pitting corrosion 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 into a uniform, 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 0.03%–0.05%.

[0025] Because molten steel has poor wettability on nucleation particles of rare earth inclusions, the resulting rare earth inclusions tend to collide and grow under the eddy current of the molten steel. Most large inclusions float to the surface of the molten steel or are adsorbed onto the inner wall, while most small inclusions either do not float or, after floating to the surface, are not captured by surface scum. Therefore, the collision growth and floating tendency of inclusions should be controlled within a reasonable range. At this range, the size of inclusions remaining in the molten steel is more suitable. Therefore, this invention controls Sc+Zn = 0.06%~0.07%. Sc and Zn work together to improve the microstructure of the die steel in both the cast and rolled states, preventing the collision growth tendency of inclusions from exceeding their floating tendency, thus preventing large inclusions from floating. This makes the rare earth inclusions more spherical, reducing the number of inclusions with high deformation rates and increasing the number of inclusions with low deformation rates. Compared with the number, size and deformability of inclusions, the fewer the number of inclusions, the smaller the size and the smaller the deformation rate, the lower the sensitivity of the inclusions to pitting corrosion. By using the combined action of Sc and Zn to control the pitting corrosion, the anti-pitting corrosion performance of the mold steel matrix of the present invention is enhanced.

[0026] The second technical solution of the present invention is to provide a method for manufacturing mold steel with uniformly distributed and spherical inclusions, including the process of smelting, continuous casting, slab heating, rolling, straightening, slow cooling, normalizing + annealing pretreatment, and quenching and tempering.

[0027] The continuously cast slab is heated to 1240℃~1280℃ 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 high slab heating temperature and a long holding time to allow the alloying elements to be fully dissolved in the matrix, improve the non-uniformity of the slab composition, reduce compositional segregation, and thus reduce subsequent microstructure segregation, and dissolve large-size eutectic carbides.

[0028] The initial rolling temperature is controlled at 1070℃~1100℃, and the reduction rate for the first three passes during steel plate rolling is 18%~21%. Using a high reduction rate increases the deformation penetration depth, allowing coarse columnar crystals to break down into fine, uniform grains, welding together central structural defects, and obtaining steel plates with a thickness of 30~180mm. The steel plates are then cooled at a temperature of 300~400℃. Immediately after cooling, they are stacked using a "bottom-up, top-down" method for slow cooling for 36~48 hours.

[0029] The steel plate is then subjected to a normalizing and annealing pretreatment. It is heated to 1040–1060°C, held for 3–4 hours, and then air-cooled to room temperature. Subsequently, the steel plate is heated to 870–890°C, held for 2–3 hours, and then furnace-cooled to 300–350°C, followed by air-cooling to room temperature. This invention's normalizing and annealing pretreatment significantly improves the strength and toughness of this mold steel in the subsequent tempering temperature range. Simultaneously, it eliminates undissolved carbides and banded carbides distributed along grain boundaries, increases the alloying density in austenite, refines the grains, and ensures uniform distribution. This microstructure increases dislocation slip resistance and carbide dispersion after tempering, significantly improving fatigue strength, wear resistance, and corrosion resistance.

[0030] The steel is then subjected to quenching and tempering heat treatment. First, a quenching heat treatment is performed, heating the steel plate to 1050–1070°C and holding it for 2–3 hours. After removal from the furnace, it is oil-cooled to room temperature. Then, a tempering heat treatment is performed, heating the steel plate to 600–620°C and holding it for 2–3 hours, followed by air cooling 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 normalizing + annealing pretreatment and quenching process of this invention promotes the complete diffusion of carbides at grain boundaries into the austenite grains at high temperatures, eliminating banded carbides. This results in a more uniform microstructure, finer grains, and fine, uniformly dispersed carbides. This microstructure increases dislocation slip resistance, significantly improving the thermal stability and wear resistance of this mold steel. The purpose of tempering is threefold: first, to eliminate residual stress, homogenize the microstructure, and improve the material's toughness; second, to allow alloying elements to precipitate from the solid solution, reducing lattice distortion. This not only weakens the impact of alloying elements on the thermal conductivity of steel but also consumes more carbon elements dissolved in the matrix, reducing the influence of carbon elements on iron lattice distortion and contributing to improved thermal conductivity; and third, to create 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 toughness. Furthermore, these precipitates are hard phases, which can effectively improve the material's room temperature and high-temperature wear resistance.

[0031] The beneficial effects of this invention are as follows:

[0032] This invention employs a low-Si, high-Mn design philosophy, with Ni and V added in specific proportions, and Sc and Zn working together. No precious metals such as W and Co are added, resulting in a mold steel with fine, uniformly dispersed, and spherical inclusions. Furthermore, through process design, this mold steel exhibits high thermal conductivity, excellent wear resistance, and pitting corrosion resistance. It is produced using a process involving smelting, continuous casting, slab heating, rolling, straightening, slow cooling, normalizing + annealing pretreatment, and tempering.

[0033] 1. The number of inclusions per unit area (192-205) / mm² in the microstructure of the mold steel of this invention. 2 The average particle size is 1.75 μm to 1.85 μm, with fine and uniformly dispersed inclusions. The ratio of the maximum diameter to the minimum diameter is [value missing].

[0034] With a thickness of 1.69 to 1.78, and an approximate spherical shape, this type of mold steel possesses excellent comprehensive properties, including high thermal conductivity, excellent wear resistance, and pitting corrosion resistance, due to the uniform distribution, small size, and spherical shape of the inclusions.

[0035] 2. The mold steel of the present invention has high hardness, strength and toughness matching and performance uniformity. The room temperature hardness of the final steel plate is 46 to 48 HRC and the hardness difference of the thickness section is ≤2 HRC. The room temperature transverse impact toughness of the unnotched core of the final steel plate is 220 to 230 J.

[0036] 3. The mold steel of this invention has good high-temperature wear resistance, with a wear volume of (25~26)×10 at 800℃. - 3 mm 3 It has high thermal conductivity at high temperatures, with a thermal conductivity of (35~36) W·m at 800℃. -1 ·K -1 .

[0037] 4. It exhibits good resistance to pitting corrosion. In a 3.5% NaCl solution, at a pH of 7–8 and a solution temperature of (55±1)℃ without degassing, the pitting potential is (-0.30 to -0.31) V, and the pitting current is (3.014 to 3.025) × 10⁻⁶. -4 / A·cm -2 . Detailed Implementation

[0038] The present invention will be further illustrated below through examples.

[0039] According to the component ratio of the technical solution, the embodiments of the present invention carry out alloying smelting, continuous casting, slab heating, rolling, straightening, slow cooling, normalizing + annealing pretreatment, and tempering.

[0040] Slab heating

[0041] The continuously cast slab is heated to 1240℃~1280℃ and held in the soaking zone for 4~5 hours.

[0042] Rolling

[0043] The initial rolling temperature is controlled at 1070℃~1100℃, and the reduction rate of the first three passes during steel plate rolling is 18%~21%.

[0044] Normalizing + Annealing Pretreatment

[0045] Heat the steel plate to 1040-1060℃, hold for 3-4 hours, remove from the furnace and air cool to room temperature. Then heat the steel plate to 870-890℃, hold for 2-3 hours, furnace cool to 300-350℃, remove from the furnace and air cool to room temperature.

[0046] Conditioning

[0047] First, perform quenching heat treatment by heating the steel plate to 1050-1070℃ and holding it for 2-3 hours. Then, remove it from the furnace and oil-cool it to room temperature. Next, perform tempering heat treatment by heating the steel plate to 600-620℃ and holding it for 2-3 hours. Finally, air-cool it to room temperature.

[0048] Furthermore, the mold steel plate is produced at a temperature of 300-400℃, and immediately after production, it is stacked using a "bottom-up, top-down" method for slow cooling, with a slow cooling time of 36-48 hours.

[0049] 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 lists the components involved in each embodiment; Table 2 lists the slab heating, rolling, and cooling processes of each embodiment; Table 3 lists the heat treatment processes of each embodiment; Table 4 lists the Rockwell hardness properties of the final state steel plates of each embodiment; Table 5 lists the room temperature transverse impact toughness of the core of each embodiment; Table 6 lists the number of inclusions, average particle size, and diameter ratio of each embodiment in the rolled state; Table 7 lists the high-temperature wear performance of each embodiment; Table 8 lists the high-temperature thermal conductivity of each embodiment; and Table 9 lists the electrochemical corrosion performance of each embodiment in 3.5% NaCl solution.

[0050] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0051]

[0052] Table 2. Slab heating, rolling, and cooling processes in the examples.

[0053]

[0054] Table 3 Heat treatment process of the embodiments

[0055]

[0056] Table 4 Rockwell hardness properties of steel plates in their final state

[0057]

[0058] Table 5. Room Temperature Core Lateral Impact Toughness of Each Example

[0059]

[0060] Note: The impact specimen measures 10×7×55mm (unnotched).

[0061] Table 6. Number of inclusions, average particle size, and diameter ratio under rolling conditions in each embodiment.

[0062]

[0063] Note: at 300×250μm 2 Randomly photographed the core sample of the steel plate in the example under different fields of view, and statistically analyzed the area, quantity, and size of inclusions in 30 fields of view.

[0064] Table 7 High-Temperature Wear Performance of Examples

[0065] Example <![CDATA[Wear volume at 800 °C / 10 -3 mm 3 > 1 25.6 2 25.7 3 25.2 4 25.0 5 25.9 6 25.1 7 25.8 8 26.0 9 25.3 10 25.4

[0066] Note: Core specimens of each embodiment were cut, with a size of φ20mm×8mm. The grinding material was 45 steel, the test temperature was 800℃, the grinding wheel speed was 250r / min, the load was 450N, and the test time was 6h. The weight loss of the specimen was recorded.

[0067] Table 8 High-temperature thermal conductivity of the examples / W·m -1 ·K -1

[0068] Example 100℃ 200℃ 300℃ 400℃ 500℃ 600℃ 700℃ 800℃ 1 46.5 45.6 44.7 43.5 42.2 42.3 38.8 35.2 2 46.8 45.7 44.6 43.7 42.4 42.5 39.0 35.8 3 46.3 45.4 44.3 43.1 42.3 42.1 39.2 35.5 4 46.9 46.0 44.1 43.2 42.1 42.2 39.3 35.9 5 47.0 45.9 45.0 43.9 42.8 42.9 38.6 35.4 6 46.4 45.3 44.2 43.3 42.0 42.2 38.7 35.0 7 46.2 45.2 44.4 43.6 42.7 42.8 38.5 35.7 8 46.7 45.5 44.5 43.4 42.5 42.6 39.4 36.0 9 46.0 45.8 44.9 44.0 42.9 43.0 39.5 35.3 10 46.6 45.0 44.0 43.8 43.0 42.7 38.9 35.6

[0069] Table 9. Electrochemical corrosion performance of the examples in 3.5% NaCl solution

[0070] Example Pitting potential / V <![CDATA[Pitting current / (×10 -4 / A·cm -2 ) <!-- 10 -->]]> 1 -0.304 3.016 2 -0.307 3.018 3 -0.300 3.020 4 -0.302 3.022 5 -0.308 3.024 6 -0.309 3.014 7 -0.310 3.019 8 -0.305 3.023 9 -0.306 3.025 10 -0.303 3.021

[0071] Note: In the example, a 10×10×2mm sample was cut from the core of the steel plate along the rolling direction. The 10×10mm working surface was ground to 2000 grit. The control potential was started from -1500mV and scanned from negative to positive at a rate of 1mV / s. The process ended when the voltage reached 0V.

[0072] As can be seen from the above, the mold steel of the present invention has fine, uniformly dispersed spherical inclusions, which gives it good pitting corrosion resistance, excellent high-temperature wear resistance, and high thermal conductivity, while also achieving a good balance of strength and toughness. Furthermore, it employs an efficient and economical production process, effectively improving the service life of the mold and making it suitable for the ever-expanding mold market. The number of inclusions per unit area in the microstructure of the mold steel of the present invention is (192-205) inclusions / mm². 2 The average particle size is 1.75μm to 1.85μm, fine and uniformly dispersed. The ratio of the maximum diameter to the minimum diameter of the inclusions is 1.69 to 1.78, approximately spherical. The steel plate hardness is 46 to 48 HRC, with a thickness-section hardness difference ≤ 2 HRC; the unnotched room temperature transverse impact toughness of the core is 220 to 230 J; the wear volume at 800℃ is (25 to 26) × 10⁻⁶. -3 mm3 It has high thermal conductivity at high temperatures, with a thermal conductivity of (35~36) W·m at 800℃. -1 ·K -1 .

[0073] 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 method for manufacturing die steel with uniformly distributed and spherical inclusions, comprising smelting, continuous casting, slab heating, rolling, straightening, slow cooling, normalizing + annealing pretreatment, and quenching and tempering, characterized in that, Slab heating Heat the continuously cast slab to 1240℃~1280℃ and hold it in the soaking zone for 4~5 hours; Rolling The initial rolling temperature is controlled at 1070℃~1100℃, and the reduction rate of the first three passes during steel plate rolling is 18%~21%; Normalizing + Annealing Pretreatment Heat the steel plate to 1040~1060℃, hold for 3~4 hours, remove from the furnace and air cool to room temperature. Then heat the steel plate to 870~890℃, hold for 2~3 hours, furnace cool to 300~350℃, remove from the furnace and air cool to room temperature. Conditioning First, perform quenching heat treatment, heating the steel plate to 1050~1070℃ and holding it for 2~3 hours. Then, remove it from the furnace and oil cool it to room temperature. Next, perform tempering heat treatment, heating the steel plate to 600~620℃ and holding it for 2~3 hours. Finally, air cool it to room temperature. The mold steel, by weight percentage, comprises the following components: C: 0.45%~0.55%, Si: 0.20%~0.30%, Mn: 0.20%~0.40%, P≤0.015%, S≤0.015%, Cr: 3.0%~4.0%, Mo: 3.0%~3.5%, V: 0.70%~0.80%, Ni: 0.80%~0.90%, Sc: 0.02%~0.03%, Zn: 0.03%~0.05%, with the remainder being Fe and unavoidable impurities; the Ni / V ratio of the mold steel is 1.125~1.

145.

2. The method for manufacturing die steel with uniformly distributed and spherical inclusions according to claim 1, characterized in that, The Sc+Zn content of the mold steel is 0.06%~0.07%.

3. The method for manufacturing die steel with uniformly distributed and spherical inclusions according to claim 1, characterized in that, The thickness of the mold steel plate is 30~180mm.

4. The method for manufacturing die steel with uniformly distributed and spherical inclusions according to claim 1, characterized in that, The number of inclusions per unit area in the microstructure of the mold steel is (192~205) inclusions / mm. 2 The average particle size is 1.75μm~1.85μm, which is fine and uniformly dispersed. The ratio of the maximum diameter to the minimum diameter of the inclusions is 1.69~1.78, which is approximately spherical.

5. The method for manufacturing die steel with uniformly distributed and spherical inclusions according to claim 1, characterized in that, The mold steel plate has a hardness of 46~48HRC and a thickness-section hardness difference of ≤2HRC; the unnotched room temperature transverse impact toughness of the core is 220~230J.

6. A method for manufacturing die steel with uniformly distributed and spherical inclusions according to claim 1, characterized in that, The wear volume of the mold steel at 800℃ is (25~26)×10. -3 mm 3 It has high thermal conductivity at high temperatures, with a thermal conductivity of (35~36) W·m at 800℃. -1 ·K -1 .

7. The method for manufacturing die steel with uniformly distributed and spherical inclusions according to claim 1, characterized in that, The rolled steel plates are taken off the production line at a temperature of 300~400℃. Immediately after taking off the production line, they are stacked and slow-cooled using the "bottom layer, top layer" method, with a slow-cooling time of 36~48 hours.

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