Die steel with low carbide coarsening degree and low residual stress and preparation method thereof

Abstract

The present invention provides a die steel with low carbide coarsening degree and low residual stress and a manufacturing method thereof. The steel plate has the following components by weight: C: 0.32%-0.42%, Si: 1.10%-1.30%, Mn: 1.10%-1.30%, P≤0.015%, S≤0.015%, Cr: 3.0%-3.4%, Mo: 3.3%-3.5%, V: 1.10%-1.40%, Co: 0.80%-1.00%, Sn: 0.11%-0.14%, Ba: 0.05%-0.15%, Zn: 0.10%-0.20%, wherein Mo / Co=3.5-4.2, Ba+Zn=0.20%-0.30%, (Ba+Zn) / Sn=1.8-2.2, and the balance is Fe and unavoidable impurities. The steel plate production method includes smelting, continuous casting, slab heating, rolling, straightening, slow cooling, quenching, cyclic cryogenic treatment, and tempering. After being held at 580°C for 40 hours, the mold steel of the present invention exhibits an average carbide size that is 4.9 to 5.1 nm larger than the average carbide size in the final microstructure, exhibits low carbide coarsening, and exhibits excellent thermal stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material production, and in particular relates to a die steel with low carbide coarsening degree and low residual stress and a preparation method thereof. Background Art

[0002] Aluminum alloys, through the die casting method, can be used to produce complex, high-strength components, thus finding important applications in a variety of fields, including automotive manufacturing and aerospace. Die-casting die steel, a key tool in aluminum alloy production, has undergone years of development and holds a crucial position in industrial production. During the aluminum alloy casting process, die-casting die steel must withstand thermomechanical stresses and friction loads, requiring the steel to possess excellent thermal fatigue properties, thermal stability, and resistance to aluminum thermite melting. The molds also require machining and molding, and excellent cutting performance. Furthermore, residual stress in the mold can lead to stress concentration cracking and failure during service, easily initiating pitting corrosion and affecting the mold surface quality. Therefore, regulating the residual stress in the mold steel is crucial.

[0003] Many domestic institutions have made significant efforts to develop new mold steels, improve product quality, optimize production processes, and increase mold life. Patent application number 202010804851.7, "A Nanobainite Hot Work Die Steel and Its Preparation Method," discloses a nanobainite hot work die steel, belonging to the field of mold steel technology. The steel comprises, by mass percentage, the following: C: 0.32%-0.45%, Si: 0.80%-1.5%, Mn: 0.20%-0.50%, Cr: 4.75%-5.05%, Mo: 1.10%-1.75%, V: 0.8%-1.00%, P < 0.02%, S < 0.01%, with the remainder being Fe and unavoidable impurities. The hot work die steel has a nanobainite microstructure. The invention also discloses a method for preparing the nanobainite hot work die steel. The beneficial effects are as follows: 1. Quenching and tempering pretreatment improves the irregular shape and distribution of carbides in the post-forging structure; 2. The high Si content inhibits the precipitation of carbides during austempering, ensuring the formation of a nano-bainite structure; 3. The unnotched impact energy of the secondary tempered specimen of the nano-bainite hot-working die steel is no less than 500 J, the tensile strength is no less than 1900 MPa, and the hardness is no less than 52 HRC; 4. The preparation process is simple and easy, conducive to industrial production and high preparation efficiency. However, it is difficult to guarantee high-temperature performance. The patent application number 202010408706.7, "A H13 hot working die steel and its preparation method", discloses a H13 hot working die steel and its preparation method, which belongs to the field of mold technology. Its components include the following components in parts by weight: C: 0.36%-0.42%; Mn: 0.39%-0.46%; Si: 0.18%-0.23%; Ni: 0.06%-0.12%; Cr: 4.50%-5.00%; Cu: 0.05%-0.10%; Mo: 2.20%-2.70%; V: 0.50%-0.80%; S≤0.002%; the experimental steel is obtained by mixing and smelting the components, electroslag remelting, and heating the experimental steel for spheroidizing annealing. Compared with conventional H13 hot-work die steel, the H13 hot-work die steel of this invention increases the Mo content and reduces the Si, Cr, and V contents, thereby improving the material's toughness, high-temperature strength, and thermal fatigue resistance. The preparation process is simple and requires relatively mild conditions. However, uniformity of microstructure and properties cannot be guaranteed.Patent application number 202010429396.7, "A Hot Working Die Steel and Its Preparation Method," provides a hot working die steel and its preparation method, wherein the chemical composition of the hot working die steel is as follows: C: 0.20%-0.32%, Si: ≤0.5%, Mn: ≤0.5%, Cr: 1.5%-2.8%, Mo: 1.5%-2.5%, W: 0.5%-1.2%, Ni: 0.5%-1.6%, V: 0.15%-0. 7%, Nb: 0.01%-0.1%, the balance being iron, with an alloying degree of 5%-7%. The hot work die steel has a tensile strength of 560-700 MPa at 700°C and a room temperature hardness of 32 to 38 HRC after being held at 700°C for 3-5 hours. The hot work die steel also has an elongation of 14%-16%, a reduction of area of ​​48%-65%, and a room temperature impact toughness of 52-63 J at room temperature, demonstrating excellent thermal stability and room temperature plasticity and toughness. However, residual stress cannot be guaranteed. Patent application number 201811220791.3, "A High-Red Hardness Die Steel and Its Preparation Method," discloses a high-red hardness die steel and its preparation method. The chemical composition, by weight percentage, is as follows: C: 0.28%-0.30%; Si: 2.20%-2.50%; Mn: 1.40%-2.00%; Cr: 4.50%-4.70%; Mo: 0.80%-1.00%; V: 0.30%-0.50%; P ≤ 0.03%, S ≤ 0.03%, with the remainder being Fe and unavoidable impurities. The ratio of these alloying elements must also satisfy the following mathematical relationship: C = 1 / 30Cr + 1 / 25(Si + Mn). This invention utilizes a novel chemical composition ratio to achieve an economical hot-working die steel with improved key performance indicators, such as impact toughness and tempering resistance. However, excessive Si content can easily lead to anisotropy. Patent application number 201911298932.8, "A manufacturing process for hot working die steel for high-performance die-casting molds," provides a manufacturing process for hot working die steel for high-performance die-casting molds, which relates to the field of mold steel technology. The manufacturing process of the hot working die steel includes the following steps: (1) smelting; (2) high-temperature homogenization treatment; (3) forging; (4) annealing treatment; (5) tempering treatment; (6) nitriding treatment; and (7) tempering treatment. The hot working die steel is composed of the following components in weight percentage: C: 0.33%-0.46%, Si: 1.5%-2.3%, Mn: 0.32%-0.45%, Mo: 1.6%-2.3%, W: 2.6%-3.6%, Nb: 0.16%-0.27%, Cr: 2.8%-3.7%, Sr: 0.21%-0.53%, Ti: 0.16%-0.31%, Ce: 0.05%-0.09%, S≤0.003%, P≤0.03%, and the balance is Fe.The hot-working die steel produced by this invention has high surface hardness and wear resistance, good impact toughness, excellent fatigue resistance and strength, effectively extending the service life of hot-working die steel for die-casting molds. Although the nitriding process imparts high surface hardness and wear resistance, it is difficult to maintain core hardness. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to prepare a mold steel with a stable and uniform structure, low carbide coarsening degree and low residual stress after tempering. The alloy composition adopts the design concept of low Cr and high Mo, Mo and Co are added in proportion, Ba, Zn and Sn work together, and no precious metal elements such as Ni and W are added. After further process design, the mold steel of the present invention has a stable and uniform structure, low carbide coarsening degree and low residual stress after tempering; and has excellent cutting performance, thermal melting loss, high fatigue resistance and thermal stability, while taking into account strength and toughness matching. It also adopts an efficient and economical production process to effectively improve the service life of the mold, which is suitable for the mold market with an ever-expanding application range and the ever-increasing application requirements, reaching the level of international advanced mold materials and having broad application prospects.

[0005] The object of the invention is achieved like this:

[0006] Disclosed is a die steel with low carbide coarsening degree and low residual stress, comprising the following components, measured by weight percentage: C: 0.32%-0.42%, Si: 1.10%-1.30%, Mn: 1.10%-1.30%, P≤0.015%, S≤0.015%, Cr: 3.0%-3.4%, Mo: 3.3%-3.5%, V: 1.10%-1.40%, Co: 0.80%-1.00%, Sn: 0.11%-0.14%, Ba: 0.05%-0.15%, Zn: 0.10%-0.20%, and the remainder being Fe and unavoidable impurities.

[0007] Furthermore, the mold steel has a Mo / Co ratio of 3.5 to 4.2.

[0008] Furthermore, in the mold steel, Ba+Zn is 0.20% to 0.30%.

[0009] Furthermore, the mold steel has a (Ba+Zn) / Sn ratio of 1.8 to 2.2.

[0010] Furthermore, the thickness of the mold steel plate is 20 to 150 mm; the room temperature cross-sectional hardness is 45 to 47 HRC, and the thickness-section hardness difference is ≤ 2 HRC; the final unnotched core transverse impact toughness of the steel plate at room temperature is 245 to 255 J; after the mold steel is kept at 580° C. for 40 hours, the average size of carbides in the structure is 4.9 to 5.1 nm larger than the average size of carbides in the final structure.

[0011] Furthermore, the mold steel was subjected to a thermal cycle of repeated heating and cooling in flowing room temperature water between 600°C and 25°C, with a cycle period of 70s. After 2000 cycles, the main crack length was 94.2-94.3 μm and the hardness was 38.4-39.5 HRC.

[0012] Furthermore, the mold steel is cut using YT5 carbide as a tool. When cutting at a cutting rate of 130 m / min for 15 min, the tool flank wear is 0.08 to 0.09 mm. When cutting at a cutting rate of 170 m / min for 15 min, the tool flank wear is 0.17 to 0.18 mm.

[0013] The reasons for designing the steel composition of the present invention are as follows:

[0014] C: In the mold steel of this invention, a portion of the carbon dissolves in the matrix, acting as a solid solution strengthening agent, while a portion interacts with alloying elements and precipitates as alloy carbides. The carbon content directly impacts the strength, plasticity, high-temperature strength, and thermal stability of the steel. In particular, during the tempering process, carbides decomposed from martensite and retained austenite disperse in the α phase, thereby improving the overall properties of the mold steel, including strength and toughness. The addition of an appropriate amount of carbon improves the steel's strength and hardness, compensating for the reduced matrix hardness caused by the reduction of Cr, thereby ensuring the steel's wear resistance. It also prevents increased cold brittleness and aging sensitivity, ensuring the steel's weldability and corrosion resistance. Therefore, the present invention selects the addition of 0.32% to 0.42% C, resulting in this mold steel exhibiting excellent overall performance, a relatively stable microstructure, superior heat loss resistance, and thermal stability.

[0015] Si: Si in the present invention has the following effects: first, it can improve the hardenability and matrix strength, which is beneficial to secondary hardening, and can increase the dispersion of alloy carbides precipitated during the high-temperature tempering process of this type of mold steel, and inhibit its growth and coarsening at high temperature; second, the Si content designed by the present invention can increase the C content of retained austenite, inhibit the precipitation of cementite during the tempering process, reduce the carbides produced by the decomposition of supercooled austenite, and thus improve the stability of the retained austenite; third, Si will have a solid solution strengthening effect on austenite, the shear strength of supercooled austenite is enhanced, and the Ms point is reduced; fourth, at high temperatures, Si will promote the aggregation of C at dislocations to form Coriolis gas clusters. After lowering the temperature, Si will inhibit the diffusion of C, thereby increasing the shear strength of supercooled austenite at medium and low temperatures. On the other hand, during the tempering transformation process, Si will be enriched around the carbides, which will hinder the diffusion of C atoms in the matrix, thereby hindering the aggregation and growth of carbides, improving the tempering stability and thermal stability of the mold steel of the present invention, and improving the material's resistance to high-temperature aluminum liquid melting loss. In addition, it can diffuse into ε-carbide, improving the stability of ε in a solid solution manner, and delaying the appearance of cementite during the tempering process. This process can effectively shorten the temperature range of the first type of temper brittleness, thereby adjusting the strength and toughness combination of tempered martensite. The present invention adds an appropriate amount of Si element. On the one hand, it avoids the occurrence of more serious segregation, causing the steel to produce anisotropy and adversely affecting the uniformity of the steel's structure; on the other hand, it enhances the stability of the retained austenite, thereby improving the toughness of the steel and effectively hindering the initiation and expansion of cracks. Therefore, the present invention needs to control the Si content to 1.10% to 1.30%.

[0016] Mn: In the present invention, Mn firstly acts as a solid solution strengthener. Although its solid solution strengthening effect is weaker than that of C, its effect on steel plasticity is minimal, hardly reducing steel ductility. Secondly, it improves hardenability, being the element that most significantly improves hardenability. Thirdly, it can advance the secondary hardening temperature and promote the dissolution of carbides. During the austenitization heating process, more carbides dissolve into the matrix and combine with other strong carbide elements to form carbides, thereby increasing the secondary hardening peak. However, excessive Mn content can adversely affect toughness and high-temperature performance, and can easily lead to element enrichment and segregation, resulting in uneven matrix material composition and structure, leading to significant variations in the final steel plate's overall performance. Therefore, in the present invention, an appropriate amount of Mn is added. This not only stabilizes austenite, but also allows more carbides to dissolve into the matrix during high-temperature solid solution, combining with other strong carbide elements to form carbides, fully realizing the beneficial effects while avoiding adverse effects on the material's toughness and thermal fatigue resistance. Therefore, in the present invention, the Mn content is selected to be 1.10% to 1.30%.

[0017] 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-free-cutting steel and exacerbating the anisotropy of the steel. It also creates cavities within the matrix, which serve as channels for oxidation to develop vertically, reducing the thermal stability of the die steel. While phosphorus can moderately increase ferrite hardness, improving the surface finish and cutting performance of parts, it tends to segregate at austenite grain boundaries, weakening the interatomic bonding at these grain boundaries and causing the material to exhibit high temper brittleness. Phosphorus segregation at grain boundaries can also cause inherited brittle fractures. Furthermore, excessive amounts of S and P can affect the homogeneity and purity of the steel. Taking into account steelmaking costs and their impact on steel, the present invention selects the addition of P ≤ 0.015% and S ≤ 0.015%.

[0018] Cr: It improves the steel's hardenability and provides secondary strengthening, promoting alloying and delaying the transformation of pearlite and bainite. This increases the steel's hardness and wear resistance without brittleness, ensuring the manufacture and production of large-area molds. Cr readily combines with carbon to form various carbides, which are distributed within the steel matrix and play a significant role in enhancing the hardness, wear resistance, and thermal stability of the mold steel. If the Cr content is too high, other alloying elements in the carbide will be replaced by Cr during high-temperature or long-term tempering, forming coarse and soft high-chromium carbides, which will reduce the hot strength of the mold steel. Therefore, the present invention adds an appropriate amount of Cr. On the one hand, it ensures that after solid solution in the matrix, it can significantly enhance the permeability of the steel, ensuring the manufacture and production of large-cross-sectional molds. On the other hand, it ensures that the secondary carbides precipitated during tempering of the mold steel of the present invention are mainly MC carbides with high thermal stability, thereby inhibiting the aggregation and growth of carbides during high-temperature service. Pinning dislocations delays the recovery of the martensite matrix, thereby improving the high-temperature performance of the material. On the other hand, the material can be enriched on the matrix surface during corrosion, forming multi-component alloy oxides, filling microcracks and pores in the rust layer, thereby increasing the density of the rust layer and enhancing the material's resistance to thermal melting loss. The combined action 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, the present invention selects a Cr content of 3.0% to 3.4%.

[0019] Mo: In the present invention, the role of Mo is, first, to dissolve into the matrix and enhance the strength and hardness of the die steel by solid solution strengthening. Mo dissolved in the matrix will segregate around dislocations to reduce the degree of collective lattice distortion, and form Coriolis gas clusters or Suzuki gas clusters to pin dislocations, hindering the movement of dislocations, increasing the yield strength of the die steel, and even forming a yield platform. During tempering, due to the pinning effect of the gas clusters, dislocations in the α phase are difficult to aggregate, merge, or cancel, which delays the temperature of the subgrains, significantly hinders the recovery and even recrystallization of the α phase, and increases the tempering stability of the material; second, MC and M2C carbides are precipitated during subsequent tempering. These two carbides are small in size and dispersed in distribution, making a great contribution to the secondary hardening effect. The Mo content in the present invention is relatively high, which can make more M6C carbides exist in the steel. M6C carbides are formed by the transformation of Mo-rich M2C during high-temperature tempering. Therefore, M6C is more stable and the ΔG of this phase is smaller. The higher precipitation temperature and stability of M6C carbides can improve the thermal stability of the mold steel of this invention. However, excessive Mo content can increase the size of M6C carbide particles, embrittle the material, and reduce toughness. However, too little Mo content can lead to insignificant secondary hardening effects. Therefore, the present invention selects a Mo content of 3.3% to 3.5%.

[0020] V: The V element acts as a solid solution in the matrix, providing solid solution strengthening. It also combines with carbon in the material to form alloy carbides. In the present invention, V forms carbides with a high melting point, making them difficult to melt during heat treatment. Therefore, during austenitization, they pin grain boundaries and prevent austenite grain growth, effectively reducing the matrix grain size and achieving grain refinement. These carbides are uniformly dispersed throughout the material, possessing high melting points, high hardness, strong stability, and resistance to growth. They effectively enhance the material's wear resistance. During high-temperature tempering, they precipitate from the matrix, promoting secondary hardening and improving high-temperature stability, thereby reducing the material's sensitivity to overheating. Furthermore, they improve the material's resistance to temper softening. This is primarily due to the high thermal stability of V, which prevents aggregation and growth even under service conditions of 700°C, maintaining a fine, dispersed distribution, resulting in excellent resistance to temper softening. However, excessive V can reduce the material's plasticity and toughness, so the present invention adds V at 1.10% to 1.40%.

[0021] Co: Co is a non-carbide-forming element. It does not combine with carbon to form carbides in die steel, and rarely dissolves in other carbides. It mainly dissolves in the matrix, exerting a strong solid-solution strengthening effect. During quenching, Co can improve the structural stability of the steel when heated at high temperatures, allowing for higher quenching temperatures to achieve higher quenching hardness while maintaining a relatively small grain size. Co can reduce the solubility of elements such as Cr and Mo in the α phase, increase the activity of carbon in the α phase, and promote an increase in the nucleation rate of alloy carbides during secondary tempering, thereby promoting carbide precipitation. It can also hinder carbide growth during high-temperature and long-term tempering. Therefore, Co effectively increases the secondary hardening peak, improving the hot work die steel's resistance to temper softening, high-temperature hardness strength, and high-temperature creep resistance. This ensures that the die steel of the present invention maintains a high surface hardness after repeated scouring, making it more resistant to erosion and beneficially improving its resistance to aluminum liquid damage. However, Co is a precious metal and the alloying cost is extremely high. Excessive Co makes the mold steel difficult to process and has a strong decarburization tendency, affecting the operation and use of some instruments. Therefore, the present invention adds Co: 0.80% to 1.00%.

[0022] The present invention controls Mo / Co to be 3.5 to 4.2. First, during the tempering heat treatment process of the present invention, Co can promote the dispersion and precipitation of Mo carbides, thereby playing a precipitation strengthening role. Second, more fine-sized and dispersed MC and M2C carbides are formed, avoiding the formation of large-particle M6C carbides that can embrittle the material, which helps to improve the thermal stability of the material, thereby improving the heat loss resistance of the mold steel of the present invention. Third, Mo and Co work together in proportion to give full play to their respective advantages, reduce the content of retained austenite during the cold treatment process of the present invention, reduce the degree of carbide coarsening during the tempering process, reduce the roundness of the carbides, make them close to circular, and disperse and evenly distribute them, which is beneficial to the comprehensive performance of the material.

[0023] Sn: The Sn element in this invention improves the material's cutting performance. Due to its low melting point, Sn remains molten within the cutting temperature range. During cutting, molten Sn adsorbs onto the surfaces of the cutting tool and die steel, reducing the interfacial atomic bonding strength, embrittles the metal, and reduces the strength and plasticity of the material in contact with the cutting tool. Furthermore, molten Sn acts as a lubricant between the cutting tool, the chips, and the die steel, reducing friction and preventing metal adhesion during cutting. Secondly, it improves the heat loss resistance of the die steel. Its accumulation in the rust layer during corrosion significantly improves the protective properties of the rust layer, forming a continuous oxide layer that inhibits further corrosion. Furthermore, Sn can offset the harmful effects of nitrogen and slow the formation of σ phase. Too little Sn can hinder cutting performance and heat loss resistance, while too much Sn can reduce the thermoplasticity of the steel. Therefore, the present invention incorporates 0.11% to 0.14% Sn.

[0024] Sn is easily segregated at the grain boundaries in the steel of the present invention, which can reduce the thermoplasticity of the steel. The main manifestations are: first, the tin segregated at the grain boundaries reduces the surface energy of the grain boundaries, reduces the bonding force between each grain, and the micropores between the grain boundaries gradually nucleate and grow; second, the segregation of tin at the grain boundaries blocks the movement of the grain boundaries; third, the segregation of tin separates the connection between the micropores at the grain boundaries, hinders grain boundary migration and dynamic recrystallization, and the formation of micropores reduces the plasticity of the steel. In order to give full play to the beneficial effects of Sn and avoid its segregation at the grain boundaries, the present invention controls (Ba+Zn) / Sn=1.8~2.2. The first effect is to effectively fix the Sn element in part of the steel and suppress its concentration at the grain boundaries; second, it allows Ba and Zn to work together to reduce the system energy and reach a sub-temperature state, improve the stability of the grain boundaries, enhance the bonding strength of the grain boundaries, and make it difficult for cracks to generate and expand along the grain boundaries.

[0025] Ba: The addition of an appropriate amount of Ba in the present invention has the following effects: first, it makes the inclusions remaining in the steel small, dispersed and distributed in a spherical shape, and reduces the damage of the inclusions to the performance of the mold steel of the present invention. To a certain extent, it can reduce the potential difference between the steel matrix and the inclusions, thereby reducing the corrosion rate; second, during the cooling process, since barium is a strong surface active element in the molten steel, it can change the surface energy of the grains, thereby refining the structure of the steel after cooling, playing a role in fine grain strengthening, thereby obtaining a final steel plate with uniform structure, avoiding aluminum sticking in the uneven structure area of ​​the mold steel during die casting of high-temperature aluminum liquid, and reacting to form FexAly brittle The intermediate compound breaks and flakes under the impact of high-pressure molten aluminum, resulting in pitting on the mold surface. Furthermore, the impact of molten aluminum causes more severe aluminum sticking, leading to material failure and scrap. Thirdly, the barium-containing precipitates, which are present at grain boundaries or phase interfaces during cooling, improve the grain boundary structure, increase the degree of lattice mismatch, and strengthen the microalloying of grain boundaries, thereby improving wear resistance, corrosion resistance, and fatigue resistance. Fourthly, the barium-containing precipitates are enriched at grain boundaries, hindering the movement of grain boundaries and dislocations during grain growth and plastic deformation, thus providing a pinning effect, achieving a good strengthening effect and effectively suppressing the adverse effects of Sn. Therefore, in the present invention, Ba is added at a rate of 0.05% to 0.15%.

[0026] Zn: Adding an appropriate amount of Zn in the present invention has three effects. First, it increases the adhesion between the base metal and the oxide film. Because it "pins" the base metal, it ensures that the base metal maintains high strength and stability at high temperatures, improves the material's thermal melt loss resistance, increases the diffusion rate of Cr in the steel, and helps form a Cr2O3 composite oxide film on the steel surface, improving the material's oxidation and corrosion resistance. Second, it improves the morphology of eutectic carbides in the as-cast structure of the mold steel, breaking the network of eutectic carbides and dispersing them into spherical, uniform distribution. Third, it reduces the enrichment and segregation of easily segregated elements such as Sn and Mn in the base material, releasing the large amount of distortion energy stored within the material, weakening the effects of Sn and Mn on grain boundaries and making their distribution more uniform. This improves the material's impact toughness and uniformity of microstructure and properties, thereby extending the service life of the mold. Therefore, the Zn content in the present invention is 0.10% to 0.20%.

[0027] The present invention controls Ba+Zn=0.20%-0.30%. Ba and Zn work together to improve the microstructure morphology of the mold steel in the cast and rolled state, avoid the collision and growth tendency of inclusions being greater than their floating tendency, resulting in large-sized inclusions being unable to float, and making the inclusions more spherical. The fewer the number of inclusions, the smaller the size, and the closer the inclusions are to spherical shape, the lower the corrosion-inducing sensitivity of the inclusions, and the stronger the corrosion resistance of the steel matrix. Secondly, the grain boundary structure is further improved, the lattice mismatch degree is increased, the microalloying effect of the grain boundary is strengthened, and the adverse effect of the Sn element is effectively suppressed. Thirdly, during the grain growth and plastic deformation process, the movement of the grain boundary and dislocation is hindered, which plays a pinning role, thereby obtaining a better strengthening effect. The use of the control method of the combined action of Sc and Zn improves the uniformity of the microstructure and performance of the mold steel of the present invention, has excellent thermal stability and heat loss resistance, and helps to improve the cutting performance and corrosion resistance.

[0028] The second technical solution of the present invention is to provide a manufacturing method for mold steel with low carbide coarsening degree and low residual stress, including the process of molten iron pretreatment, smelting, LF, RH, continuous casting, slab heating, rolling, straightening, slow cooling, quenching, cyclic deep cold treatment, and tempering.

[0029] Slab heating

[0030] The continuous casting slab is heated to 1220°C to 1260°C and kept warm in the soaking section for 4 to 5 hours. The mold steel of the present invention has a high alloy content and usually requires a higher slab heating temperature. The longer holding time allows the alloy elements to be fully dissolved in the matrix, improves the unevenness of the slab composition, reduces composition segregation, and further reduces subsequent organizational segregation, allowing large-sized eutectic carbides to dissolve.

[0031] Rolling

[0032] The starting rolling temperature is controlled between 1020°C and 1060°C. The reduction ratio for the first two passes is 17% to 20%, and the final pass is 18% to 21%. Using a high reduction ratio increases the depth of deformation penetration, breaking up coarse columnar crystals to form fine, uniform grains, and welding together central structural defects, resulting in 20-150mm thick steel plates. The plate rolling temperature is kept between 300°C and 400°C. Immediately after rolling off the line, the steel plates are stacked in a "bottom-up, top-down" stacking system for a cooling period of 36-48 hours.

[0033] Quenching

[0034] Then, a quenching heat treatment is performed. The steel plate is heated to 1030-1040°C, kept at this temperature for 3-4 hours, and quickly oil-cooled to room temperature after being taken out of the furnace. Although a quenching temperature that is too low and a short holding time can result in smaller grains and reduce the amount of retained austenite, it will reduce the solid solution amount of alloying elements, affecting the strength and hardness of the mold steel of the present invention; although a quenching temperature that is too high and a long holding time can increase the solid solution amount of alloying elements, it will lead to coarse grains and an increase in the amount of retained austenite, affecting the toughness and plasticity of the steel. Therefore, the quenching process of the present invention can enable the mold steel to obtain a uniform and refined microstructure, prepare the structure for subsequent heat treatment, and make the material have excellent thermal fatigue resistance, wear resistance and a long service life.

[0035] Cyclic cold treatment

[0036] Then put the steel plate into liquid nitrogen at -150℃~-160℃, keep it for 9~10h, then take it out and let it recover to room temperature, then put the steel plate into liquid nitrogen at -150℃~-160℃, keep it for 1~2h, then take it out and let it recover to room temperature. The effect of the cyclic cold treatment of the present invention is that the carbon atoms in the martensite structure can more effectively fix the position of the iron atoms in the crystal structure, thereby weakening the residual stress generated by the organizational transformation. The toughness of the martensite obtained by deep cold treatment is stronger, so the generation of residual stress is further suppressed, thereby improving the hardness and wear resistance of the steel; secondly, it promotes the transformation of the residual austenite after quenching into martensite. The specific volume of austenite is much smaller than that of martensite. In the process of austenite transforming into martensite, the organizational volume will increase and generate corresponding organizational stress. Thermal stress and organizational stress lead to the fragmentation of martensite, thereby reducing the grain size and making the material organizational distribution more uniform. In this process, the residual stress generated in the material will be released, thereby weakening the The residual stress of the material makes the residual stress of the mold steel of the present invention after cyclic deep cryogenic treatment be -152.4MPa~-155.6MPa. At this time, the untransformed retained austenite will be refined and evenly distributed in the form of a film around the matrix. The refined and evenly distributed martensite and retained austenite can prepare the organization for tempering, further weakening the residual stress of the final steel plate; thirdly, the carbon atoms in the martensite are precipitated from the lattice and segregated around the defects, becoming the nucleation core of carbides in the subsequent tempering process, resulting in a large number of carbides precipitated, thereby increasing the hardness of the mold steel; fourthly, the carbides can be refined, and the dislocations and twin boundaries in the martensite are increased through cyclic deep cryogenic treatment. During the tempering process, the carbon atoms precipitate and form fine carbides with Cr, Mo, V, etc.

[0037] Tempering

[0038] Finally, a tempering heat treatment is performed, heating the steel plate to 610-620°C, holding the temperature for 2-3 hours, and then air-cooling it to room temperature. Through the composition and process design of the present invention, the mold steel can precipitate more small and evenly distributed carbides after tempering, which helps reduce the coarsening rate of the precipitated carbides, improve material stability, continue to eliminate residual stress, homogenize the entire structure, and improve material toughness, thereby significantly improving the material's mechanical properties, heat loss resistance, and thermal fatigue performance.

[0039] The beneficial effects of the present invention are:

[0040] The present invention adopts the design concept of low Cr and high Mo, Mo and Co are added in proportion, Ba, Zn and Sn work together, and no precious metal elements such as Ni and W are added. This makes the mold steel of the present invention have a stable and uniform structure, low carbide coarsening degree and low residual stress after tempering. At the same time, through process design, this mold steel has excellent cutting performance, thermal melting loss resistance, high fatigue resistance and thermal stability.

[0041] 1. The mold steel of the present invention has high hardness, strength-toughness matching and performance uniformity. The average hardness of the core in the quenched state is 51-52HRC, and the average hardness of the core after cyclic cold treatment is 53-54HRC. The room temperature cross-sectional hardness of the final steel plate is 45-47HRC, and the thickness-cross-sectional hardness difference is ≤2HRC; the transverse impact toughness of the unnotched core of the final steel plate at room temperature is 245-255J.

[0042] 2. The residual stress of the mold steel of the present invention is relatively low after cyclic cold treatment, which is -152.4MPa to -155.6MPa, and is converted from tensile stress to compressive stress. At this time, the carbon content in the martensite in the structure is relatively low, only 0.115% to 0.125%, which prepares the structure for the dispersion and precipitation of carbides during the tempering process. At the same time, the mold steel can obtain refined and evenly distributed martensite and retained austenite after cyclic cold treatment, which can further weaken the residual stress of the final steel plate after tempering.

[0043] 3. After the mold steel of the present invention is kept at 580°C for 40 hours, the average size of the carbides in the structure is 4.9 to 5.1 nm larger than the average size of the carbides in the final structure, and the degree of carbide coarsening is low. The mold steel has excellent thermal stability.

[0044] 4. The mold steel of the present invention is corroded in an ADC12 aluminum alloy solution at 650°C for 60 minutes, with a mass loss of 6.9%-7.0%, and has good heat-loss resistance. The mold steel is repeatedly heated and cooled in a cycle of 70 seconds under an upper and lower temperature limit of 600°C and a water flow temperature of 25°C. After 2000 cycles, the main crack length is 94.2-94.3 μm and the hardness is 38.4-39.5 HRC.

[0045] 5. The mold steel of the present invention uses YT5 cemented carbide as a cutting tool. When cutting at a cutting rate of 130 m / min for 15 min, the tool flank wear is 0.08-0.09 mm. When cutting at a cutting rate of 170 m / min for 15 min, the tool flank wear is 0.17-0.18 mm, and the mold steel has excellent cutting performance. DETAILED DESCRIPTION

[0046] The present invention will be further described below by way of examples.

[0047] The embodiment of the present invention carries out the process production of molten iron pretreatment, smelting, LF, RH, continuous casting, slab heating, rolling, straightening, slow cooling, quenching, cyclic deep cryogenic treatment, and tempering according to the component ratio of the technical solution.

[0048] Slab heating

[0049] Heat the continuous casting slab to 1220℃~1260℃ and keep it warm in the soaking section for 4~5h;

[0050] Rolling

[0051] The starting rolling temperature is controlled at 1020℃~1060℃, the reduction rate of the first two passes of steel plate rolling is 17%~20%, and the reduction rate of the final rolling pass is 18%~21%;

[0052] Heat treatment

[0053] (1) Quenching

[0054] Heat the steel plate to 1030-1040℃, keep it warm for 3-4 hours, and then quickly cool it to room temperature after it is taken out of the furnace.

[0055] (2) Cyclic cryogenic treatment

[0056] Place the steel plate in liquid nitrogen at -150℃~-160℃, keep it for 9~10 hours, then take it out and let it return to room temperature on its own. Place the steel plate in liquid nitrogen at -150℃~-160℃ again, keep it for 1~2 hours, then take it out and let it return to room temperature on its own.

[0057] (3) Tempering

[0058] Heat the steel plate to 610-620℃, keep it warm for 2-3 hours, then take it out of the furnace and air cool it to room temperature.

[0059] Furthermore, the die steel plate has a temperature of 300-400° C. and is immediately stacked and cooled in a "laying down and covering up" manner after it comes off the line, with a cooling time of 36-48 hours.

[0060] Furthermore, the average hardness of the core of the mold steel in the quenched state is 51-52 HRC; the average hardness of the core after cyclic cold treatment is 53-54 HRC; the residual stress after cyclic cold treatment is -152.4 MPa to -155.6 MPa, and the carbon content in the martensite is 0.115% to 0.125%, which prepares the organization for the dispersion and precipitation of carbides during tempering.

[0061] Table 1 shows the chemical composition of each embodiment, Table 2 shows the heating, rolling and cooling processes of the slab of the embodiment, Table 3 shows the heat treatment process of the embodiment, Table 4 shows the average hardness of the core of the embodiment under different conditions, Table 5 shows the Rockwell hardness of the final steel plate, Table 6 shows the transverse impact toughness of the core of the embodiment at room temperature, Table 7 shows the residual stress of the embodiment after cyclic deep cryogenic treatment and the carbon content of martensite in the structure, Table 8 shows the average growth size of carbides in the structure of the embodiment after keeping warm at 580°C for 40 hours, Table 9 shows the heat loss resistance of the embodiment, Table 10 shows the heat fatigue resistance of the embodiment, and Table 11 shows the cutting performance of the embodiment at different cutting rates.

[0062] Table 1 Chemical composition (wt%) of various embodiments of the present invention

[0063] Example C Si Mn P S Cr Mo V Co Sn Ba Zn 1 0.35 1.15 1.25 0.009 0.010 3.25 3.44 1.30 0.93 0.115 0.09 0.14 2 0.39 1.19 1.28 0.007 0.006 3.18 3.36 1.26 0.85 0.120 0.07 0.18 3 0.40 1.20 1.22 0.010 0.009 3.35 3.30 1.35 0.80 0.110 0.10 0.13 4 0.33 1.28 1.15 0.013 0.011 3.00 3.33 1.40 0.84 0.135 0.14 0.12 5 0.36 1.27 1.10 0.015 0.012 3.07 3.48 1.15 0.98 0.125 0.15 0.10 6 0.38 1.25 1.20 0.012 0.015 3.10 3.40 1.20 0.91 0.128 0.08 0.17 7 0.32 1.13 1.17 0.011 0.013 3.36 3.42 1.10 0.90 0.140 0.11 0.19 8 0.41 1.10 1.13 0.014 0.008 3.40 3.50 1.18 1.00 0.138 0.12 0.15 9 0.42 1.23 1.30 0.008 0.007 3.30 3.32 1.24 0.82 0.130 0.05 0.20 10 0.37 1.30 1.26 0.006 0.014 3.20 3.45 1.37 0.95 0.116 0.08 0.16

[0064] Table 2 Slab heating, rolling and cooling processes of various embodiments of the present invention

[0065]

[0066] Table 3 Heat treatment process of various embodiments of the present invention

[0067]

[0068] Table 4 Average hardness of the core of each embodiment of the present invention under different conditions

[0069] Example Quenched state / HRC Cyclic cryogenic treatment / HRC 1 51.6 53.7 2 51.9 53.9 3 51.3 53.2 4 51.2 53.3 5 51.0 53.0 6 51.7 53.8 7 52.0 54.0 8 51.8 53.6 9 51.4 53.5 10 51.1 53.4

[0070] Table 5 Rockwell hardness properties of the final steel plates of the present invention

[0071]

[0072] Table 6 Room temperature core transverse impact toughness of various embodiments of the present invention

[0073]

[0074] Note: The impact specimen size is 10×7×55mm (no notch)

[0075] Table 7 Residual stress and carbon content of martensite in each embodiment of the present invention after cyclic cryogenic treatment

[0076] Example Residual stress / MPa Carbon content of martensite in the structure / wt% 1 -154.5 0.119 2 -154.0 0.120 3 -153.8 0.115 4 -155.6 0.125 5 -155.0 0.123 6 -152.4 0.122 7 -153.0 0.121 8 -153.5 0.117 9 -152.9 0.118 10 -152.7 0.116

[0077] Table 8 Average carbide growth size in the microstructure of each embodiment of the present invention after being kept at 580℃ for 40h

[0078] Example Average carbide growth size / nm 1 4.98 2 5.06 3 5.00 4 4.96 5 4.90 6 5.09 7 5.03 8 5.10 9 4.94 10 5.05

[0079] Table 9 Heat loss resistance of various embodiments of the present invention

[0080]

[0081] Note: Example test size The temperature of ADC12 aluminum liquid is 650℃, the angular velocity of the sample rotating in the aluminum liquid is 120r / min, and the linear velocity is 0.05m / s.

[0082] Table 10 Thermal fatigue resistance of various embodiments of the present invention

[0083]

[0084] Note: A V-notch with a depth of 1.5 mm is machined on one end of the specimen. 600°C and 25°C (water flow temperature) are used as the upper and lower limit temperatures during service. The cycle period is 70 seconds, and the maximum temperature residence time is 2 seconds.

[0085] Table 11 Cutting performance of the embodiment at different cutting rates

[0086]

[0087] Note: The sample size is φ100mm×500mm, the test tool is YT5 carbide, the feed rate f=0.2mm / r, the cutting depth ap=1.0mm, and the cutting time is 15min.

[0088] The mold steel of the present invention balances cost and performance, exhibiting a stable and uniform microstructure, low carbide coarsening and residual stress after tempering, and excellent cutting performance, thermal melting loss resistance, high fatigue resistance, and thermal stability. The production process is efficient and economical, meeting market demands. After being held at 580°C for 40 hours, the average carbide size in the microstructure of the mold steel of the present invention is 4.9 to 5.1 nm larger than the average carbide size in the final microstructure. The room temperature cross-sectional hardness of the mold steel is 45 to 47 HRC, with a thickness-cross-sectional hardness difference of ≤2 HRC. The final unnotched core transverse impact toughness at room temperature is 245 to 255 J. After 2000 cycles of repeated heating and cooling in flowing ambient water between 600°C and 25°C, with a cycle period of 70 seconds, the main crack length is 94.2 to 94.3 μm, and the hardness is 38.4 to 39.5 HRC. YT5 carbide is used as the cutting tool. When cutting at a cutting rate of 130m / min for 15min, the tool flank wear is 0.08-0.09mm. When cutting at a cutting rate of 170m / min for 15min, the tool flank wear is 0.17-0.18mm.

[0089] In order to describe the present invention, the present invention has been appropriately and fully illustrated through the examples above. The above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the scope of protection of the present invention, and the patent protection scope of the present invention should be defined by the claims.

Claims

1. A die steel with low carbide coarsening and low residual stress, characterized in that: Calculated by weight percentage, it includes the following components: C: 0.32%~0.42%, Si: 1.10%~1.30%, Mn: 1.10%~1.30%, P≤0.015%, S≤0.015%, Cr: 3.0%~3.4%, Mo: 3.3%~3.5%, V: 1.10%~1.40%, Co: 0.80%~1.00%, Sn: 0.11%~0.14%, Ba: 0.05%~0.15%, Zn: 0.10%~0.20%, the rest are Fe and unavoidable impurities; A method for manufacturing die steel with low carbide coarsening and low residual stress, comprising smelting, continuous casting, slab heating, rolling, straightening, slow cooling, quenching, cyclic cryogenic treatment, and tempering. Slab heating Heat the continuous casting slab to 1220℃~1260℃ and keep it warm in the soaking section for 4~5h; Rolling The starting rolling temperature is controlled at 1020℃~1060℃, the reduction rate of the first two passes of steel plate rolling is 17%~20%, and the reduction rate of the final rolling pass is 18%~21%; Heat treatment (1) Quenching Heat the steel plate to 1030~1040℃, keep it warm for 3~4 hours, and then quickly cool it to room temperature after it is taken out of the furnace; (2) Cyclic cryogenic treatment Place the steel plate in liquid nitrogen at -150℃~-160℃, keep it for 9~10 hours, then take it out and let it return to room temperature. Place the steel plate in liquid nitrogen at -150℃~-160℃ again, keep it for 1~2 hours, then take it out and let it return to room temperature. (3) Tempering Heat the steel plate to 610~620℃, keep it warm for 2~3h, then take it out of the furnace and air cool it to room temperature.

2. The mold steel with low carbide coarsening degree and low residual stress according to claim 1, characterized in that: The Mo / Co ratio of the mold steel is 3.5-4.

2.

3. The mold steel with low carbide coarsening degree and low residual stress according to claim 1, characterized in that: The Ba+Zn content of the mold steel is 0.20% to 0.30%.

4. The mold steel with low carbide coarsening degree and low residual stress according to claim 1, characterized in that: The mold steel (Ba+Zn) / Sn is 1.8~2.

2.

5. The die steel with low carbide coarsening degree and low residual stress according to claim 1, characterized in that: The mold steel plate has a thickness of 20-150 mm, a cross-section hardness of 45-47 HRC at room temperature, and a thickness-cross-section hardness difference of ≤ 2 HRC; The final unnotched steel plate has a core transverse impact toughness of 245-255 J at room temperature. After the mold steel is kept at 580° C. for 40 h, the average size of carbides in the microstructure is 4.9-5.1 nm larger than that in the final microstructure.

6. The die steel with low carbide coarsening degree and low residual stress according to claim 1, characterized in that: The mold steel was subjected to a thermal cycle of repeated heating and cooling in flowing room temperature water between 600°C and 25°C, with a cycle period of 70 seconds. After 2000 cycles, the main crack length was 94.2~94.3μm and the hardness was 38.4~39.5HRC.

7. The die steel with low carbide coarsening and low residual stress according to claim 1, characterized in that: The mold steel is cut using YT5 carbide as a tool. When cutting at a cutting rate of 130 m / min for 15 min, the tool flank wear is 0.08-0.09 mm. When cutting at a cutting rate of 170 m / min for 15 min, the tool flank wear is 0.17-0.18 mm.

8. The die steel with low carbide coarsening and low residual stress according to claim 1, characterized in that: The die steel plate has a temperature of 300-400°C off the production line, and is immediately stacked and slowly cooled in a "bottom-laying and top-covering" manner after it comes off the production line, with a slow cooling time of 36-48 hours.

9. The die steel with low carbide coarsening and low residual stress according to claim 1, characterized in that: The average hardness of the core of the mold steel in the quenched state is 51-52 HRC; the average hardness of the core after cyclic cryogenic treatment is 53-54 HRC; the residual stress after cyclic cryogenic treatment is -152.4 MPa--155.6 MPa, and the carbon content in the martensite is 0.115 wt%-0.125 wt%.

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