High-temperature oxidation corrosion resistant die steel and preparation method thereof

Abstract

The invention discloses high-temperature oxidation corrosion resistant die steel and a preparation method thereof. The die steel comprises the following components in percentage by weight: 0.54%-0.81% of C, 1.04%-1.25% of Si, 0.75%-1.09% of Mn, 18.3%-21.7% of Cr, 2.70%-5.12% of Al, Mo, Ti, Cu, B, Ca, Mg, La, Ce, P and the balance of Fe and impurities (0.04 lt). (La + Ce) / (B + P) lt; 1.76, 3.23 lt, 1.26, 1.26, 1.26; (Al + Ti + Mg) / (C + B) lt; 9.45, 2.90 lt, 9.45, 2.90 lt; (Mn + Ca) / Slt; and 6.45. The die steel has excellent high-temperature oxidation corrosion resistance, workability and comprehensive mechanical properties.

Description

Technical Field

[0001] This invention relates to mold steel and its preparation method, specifically to a high-temperature oxidation and corrosion resistant mold steel and its preparation method. Background Technology

[0002] Modern industries such as automotive and plastics are placing increasingly stringent demands on the comprehensive performance of tool and die steels. These demands include machinability to adapt to high-speed, precision, and automation trends; service life resistant to high-temperature oxidation and chloride-containing corrosion; and mechanical properties and wear resistance against hard particle abrasion. While increasing the alloy content can improve material properties, it can also lead to increased hardness, making machining difficult, or poor thermal conductivity, causing tool overheating. A balance between performance, machinability, and cost is difficult to achieve with high-alloy-content tool and die steels.

[0003] Adding lead, selenium, or tellurium can improve machinability, but the high toxicity of these elements necessitates special protection, increases costs, and fails to meet environmental requirements. High lead content can lead to segregation and large inclusions during casting, reducing cutting efficiency; lead-containing steel is unsuitable for drilling and tapping (chips easily clog holes). YB191 and GB8731 free-cutting steels have low alloy content and insufficient overall performance, making them difficult to directly reference high-alloy mold steels. Molds for plastic / glass manufacturing require resistance to lubricants or chloride ion oxidation corrosion, such as the high chromium content of GB1298, GB1299, and ASTM D2 materials; adding molybdenum, vanadium, and nickel, similar to stainless steel (such as the Cr17 series of GB1220), results in high hardness but poor machinability.

[0004] Existing improvement solutions have limitations. For example, patent application number 201210406748.2 adds cobalt to increase corrosion resistance; patent application number 202111316559.1 homogenizes the hardness of medium and large plates; and patent application number 201910804468.9 adds copper or modifiers to improve the machinability of materials. These solutions are effective for specific problems, but it is difficult to balance process costs, alloy costs, and performance. Patent application number 202310480986.6 proposes to use medium alloy content combined with heat treatment processes such as normalizing and low-temperature annealing to improve uniformity; patent application number 201910853253.6 points out that medium and low alloy content combined with bottom injection feeding can reduce wide-face cracks; and patent application number 202211727530.7 proposes dispersion treatment to improve uniformity. However, the materials targeted usually have low alloy content and are difficult to cope with high-temperature oxidation and corrosion conditions.

[0005] To improve the performance of materials in high-temperature oxidative corrosion environments, a series of products such as 4Cr16Mo(N / Ni) have been developed based on stainless steel materials. However, the addition of high nickel and cobalt content has led to higher production costs, or the need to add nitrogen during the smelting process, which places high demands on equipment and can easily reduce the material's pass rate.

[0006] The working principles of elements in different steel compositions conflict. For example, there is controversy regarding nitrogen content. Patent application number 202111103042.4 argues that N has a significant effect, while patent application number 202110755633.3 argues for reducing N content. While these methods improve the material's weldability, surface strength, elongation anisotropy, dimensional accuracy, and machinability, they also have drawbacks such as requiring complex multi-step processes (LD / LF / ESR), the need for pickling which can pollute the environment, and the difficulty in balancing process complexity, performance, and uniformity. Different technical solutions focus on different aspects of comprehensive mold steel applications, emphasizing different trade-offs between various properties and processes. This is especially true for molds in the plastics, glass, or powder metallurgy industries, which face multiple challenges such as oxidation corrosion, high temperatures, wear, and stress, making it difficult for current technical solutions to comprehensively address all these issues. Summary of the Invention

[0007] Objectives of this invention: The objective of this invention is to provide a high-temperature oxidation-corrosion resistant mold steel that is easy to machine and resistant to oxidation and corrosion. Another objective of this invention is to provide a method for preparing such a high-temperature oxidation-corrosion resistant mold steel. This invention also provides an aluminum-containing, highly uniform, oxidation-corrosion resistant mold steel with good toughness, red hardness, and easy-to-machine performance. This invention provides a method for preparing an aluminum-containing, highly uniform, oxidation-corrosion resistant mold steel. This invention also provides a highly uniform, easy-to-machine, high-temperature oxidation resistant mold steel with uniform wear resistance and good machinability. This invention provides a method for preparing a highly uniform, easy-to-machine, high-temperature oxidation resistant mold steel.

[0008] Technical solution: The present invention provides a high-temperature oxidation and corrosion resistant mold steel, comprising the following components by weight percentage: C: 0.54~0.81%, Si: 1.04~1.25%, Mn: 0.75~1.09%, Cr: 18.30~21.70%, Mo: 0.08~0.12%, Al: 2.70~5.12%, Ti: 0.06~0.09%, Cu: 0.02~0.04%, B: 0.02~0.06%, Ca: 0. 0.005~0.007%, Mg: 0.05~0.08%, La: 0.02~0.04%, Ce: 0.02~0.05%, P: 0.031~0.041%, S: 0.17~0.26%, with the balance being Fe and unavoidable impurities, and satisfying 0.04<(La+Ce) / (B+P)<1.76, 3.23<(Al+Ti+Mg) / (C+B)<9.45, 2.90<(Mn+Ca) / S<6.45.

[0009] C: Carbon is one of the key elements in this invention. It can function alone or in combination with other alloying elements and heat treatment processes. Even steels with the same carbon content can exhibit significant differences in performance, requiring comprehensive control of carbon. In this invention, carbon's primary mechanism of action is solid solution strengthening, increasing the wear resistance of the matrix and compensating for the excessive ferrite formation due to the high chromium content. Carbon can also stabilize the small amount of austenite or participate in austenite in this invention, especially through processes similar to carbon partitioning, which can improve the material's high-temperature performance. Finally, carbon atoms dissolved in the matrix as interstitial atoms can hinder dislocation movement, increasing matrix strength. Improving wear resistance as carbides is also an important way carbon functions, and carbides dispersed in the alpha phase can improve the material's toughness. The various alloying elements added in this invention, such as vanadium, titanium, and chromium, can combine with carbon to form complex composite carbides, avoiding the disadvantage of single carbides easily growing and coarsening at high temperatures. However, excessively high carbon content can easily lead to the formation of liquid carbides during smelting, and it also affects the weldability of the material, making it unsuitable for manufacturing complex structural parts. A carbon content of 0.54–0.81% is preferred, resulting in mold steels that exhibit good wear resistance and toughness while also possessing good weldability.

[0010] Silicon (Si): As a non-alloying element, silicon is abundant in molten iron or steel produced by ore smelting, making it relatively inexpensive. Its role is multifaceted: it deoxidizes during smelting and improves material properties. In the ferrite-based steel of this invention, small amounts of austenite or retained austenite in the matrix are potential crack initiations, and silicon can improve the stability of the relevant microstructure. During tempering, silicon can inhibit cementite precipitation and reduce the tendency of carbide aggregation and growth, thereby improving material toughness. Silicon also plays a role in increasing the tempering transformation activation energy, reducing the diffusion rate of carbon in ferrite by about 10%, thus improving the tempering stability of the material. When this material is used as a plastic mold, it will come into contact with certain corrosive media, and silicon can improve the material's corrosion resistance and provide good protection against chloride ions. Finally, silicon, together with carbon, can improve the hardenability and solid solution strengthening effect of steel, increasing material strength. However, excessive silicon can easily lead to the formation of banded structures, weakening the transverse properties of the steel; therefore, its content needs to be controlled. The present invention preferably uses a silicon content of 1.04~1.25%, which achieves excellent corrosion resistance and strength while also having high stability.

[0011] Mn: As a weak carbide-forming element, manganese can partially replace other alloying elements in carbides, such as Cr. 23In C6 or Fe3C, Cr and Fe increase strength by enhancing the distortion energy of carbides. In this invention, manganese does not form individual carbides and delays the precipitation of other carbides during cooling. It also promotes the dissolution of carbides during heating, thus refining the carbides and preventing coarsening of carbides due to high carbon content. Manganese has solid solution strengthening properties, improving the strength and hardness of the matrix. While its solid solution strengthening effect is less than that of carbon, its negative impact on material plasticity is smaller, allowing it to be added together with carbon to maintain the ductility of steel. Manganese improves the hardenability of steel; in this application, its improvement is more significant than that of elements such as chromium, reducing the amount of chromium required and thus lowering costs. Manganese has a strong affinity for sulfur, reducing the formation of FeS. The resulting MnS and other elements facilitate chip breakage and detachment during subsequent turning, improving the material's machinability. However, excessive manganese can reduce the material's thermal conductivity, leading to heat accumulation and adverse effects during use. Furthermore, the probability of core segregation in ingots increases with increasing manganese content. A manganese content of 0.75–1.09% is preferred, enabling the prepared low-cost mold steel to achieve good hardness and hardenability without reducing plasticity, while also possessing excellent machinability.

[0012] Cr: Chromium is an important carbide-forming element, which can form MC, M6C, M7C3 and M in the matrix. 23 Various carbides, including C6, contain high-temperature stability and hardness MC, which can reduce the coarsening of carbides in steel at high temperatures. Furthermore, the dispersed and small-sized distribution of MC effectively pins dislocations and delays matrix recovery, thereby improving the high-temperature performance of the material. 23 C6 atoms are relatively large and have low hardness, tending to form at grain boundaries and easily becoming the source of cracks. This invention adds chromium along with manganese, molybdenum, and titanium, using a substitutional solid solution to replace Cr atoms and avoid M... 23 C6 is simplified to reduce system energy and improve stability. Chromium, along with elements such as manganese, can improve the hardenability of materials, and chromium can enhance the material's resistance to tempering softening. During oxidation corrosion, chromium can accumulate on the surface and form oxides on the matrix surface, hindering further contact between the matrix and oxidizing or corrosive media, thereby improving the material's resistance to oxidation corrosion. The higher chromium content in this invention application can also form oxides in cracks or pores and fill them, further enhancing the material's resistance to oxidation corrosion under cyclic stress. However, excessive chromium will form low-hardness ferrite. Adding it in combination with molybdenum, manganese, titanium, and rare earth elements can increase the matrix hardness. Excessive chromium may cause an increase in the stability of the brittle transition of steel. Preferably, 18.3~21.7 wt% chromium is used to obtain excellent oxidation corrosion resistance, as well as good hardness and stability, while avoiding an increase in the brittle transition temperature.

[0013] Mo: In this application, molybdenum's affinity for carbon is intermediate between that of titanium and chromium. It forms carbides before chromium, reducing the risk of coarse carbides caused by high chromium content. The molybdenum-containing carbides in this application mainly precipitate as flakes in the solidification stage after melting, in the form of M2C. However, metastable M2C is detrimental to material stability; therefore, it needs to be combined with subsequent forging and heat treatment processes to break it down and convert it into finer, more dispersed M6C or MC, thereby improving the material's thermal stability and toughness. In addition to forming carbides, molybdenum also penetrates the matrix through solid solution strengthening, improving the material's strength and hardness, compensating for the reduction in matrix hardness caused by high chromium and aluminum content. Solid-solution molybdenum atoms tend to cluster around dislocations, pinning them in the form of Cochrane gas clusters, hindering their movement and thus improving material strength. Furthermore, molybdenum can form molybdate ions in corrosive environments. These ions dissolve and accumulate on the material surface, forming a protective film. Together with chromium- or silicon-containing oxides, they inhibit the penetration of chloride ions and increase the pitting potential, thereby improving the material's corrosion resistance. However, metallic molybdenum is expensive, and excessively high molybdenum content can lead to a decrease in the material's thermal conductivity. A molybdenum content of 0.08–0.12 wt% is preferred, as it improves the material's thermal stability and toughness while achieving good strength and corrosion resistance.

[0014] Aluminum (Al): By forming an oxide layer on the substrate surface, aluminum isolates the substrate from further contact with oxidizing atmospheres and other media, and avoids the drawback of easy flaking of single chromium oxides, thus significantly improving the high-temperature oxidation resistance of materials. Furthermore, aluminum has a strong deoxidizing and nitrogen-fixing effect; during smelting, it forms AlN with nitrogen and other elements in steel, fixing nitrogen while also refining the grain size. Aluminum regulates chemical bonds in the matrix, weakening the bond strength between iron atoms and between iron and carbon atoms, increasing carbon activity, promoting the precipitation of high-hardness particles, and thus improving the material's red hardness. Aluminum can form a certain amount of dispersed intermetallic compounds in the matrix, which can improve the material's yield strength and high-temperature strength. Deoxidation and nitrogen fixation, as well as promoting particle precipitation, are all beneficial for grain refinement, improving material toughness and thus improving its processing plasticity, especially contributing to the improvement of hot working performance. In terms of machining, the high hardness of alumina in steel means that direct contact with the cutting tool will have adverse effects; therefore, the total amount of aluminum needs to be controlled, and it should be added together with titanium and other elements to modify the oxide properties. Excessive aluminum content can cause ferrite to grow rapidly at high temperatures, which can damage the material's uniformity and reduce its toughness. It is preferable to use 2.70 to 5.12 wt% aluminum, which can maintain the material's machinability while achieving good hot working properties and excellent high-temperature oxidation resistance.

[0015] Ti: Titanium plays a multifaceted role in this invention. Titanium can alter the morphology of oxides in steel, forming Ti-Al-Mn composite oxides with manganese and other materials, resulting in lower hardness and improved machinability. Titanium can also enter silicate inclusions, reducing their hardness and thus minimizing tool wear at high speeds, which is particularly significant for this invention with high silicon content. Titanium is a strong carbide-nitride forming element; the resulting Ti(C,N) oxides have high hardness and are widely dispersed, effectively improving the hardness and wear resistance of die steel. Titanium's tendency to form strong carbides reduces the coarsening of chromium carbides during processing and decarburization during use, thereby improving its stability. Titanium carbide precipitated during forging and rolling reduces grain boundary bonding, pins dislocations, and hinders grain recrystallization and growth, refining grains and improving material uniformity. However, excessive titanium content can cause nodules during smelting; a titanium content of 0.06~0.09 wt% is preferred, achieving good stability and hardness while improving material machinability.

[0016] Cu: In this invention, copper plays a crucial role in improving the machinability of the material. Copper enhances machinability through two main mechanisms. First, copper forms a composite phase with carbon. Since copper is not a carbide-forming element, this composite phase has low hardness and can act as a lubricant during cutting, encapsulating the cutting edge to form a solid lubricating film. This prevents prolonged contact between the cutting edge and the substrate, reducing tool wear. Second, copper increases the thermal conductivity of the material, promoting heat dissipation at the contact points between the tool and the material, thus preventing performance degradation due to temperature increases. The addition of copper improves the corrosion resistance of the material, especially in sulfur-containing environments. Copper enriched within the rust layer forms cuprous sulfide with sulfur, blocking further contact between the corrosive medium and the substrate. This, combined with the protection of molybdenum against chlorine-containing corrosive media, enhances the material's service life under complex corrosive conditions. Copper can combine with aluminum to form intermetallic compounds, providing secondary hardening to the material. Copper has low solubility in ferrite, and some copper precipitates to strengthen the matrix, increasing the material's yield strength. However, excessive copper can cause copper cracks. It is preferable to use 0.02 to 0.04 wt% copper, which can improve the material's machinability while obtaining excellent corrosion resistance and high strength.

[0017] B: Boron plays multiple roles in materials. Boron can fix nitrogen in steel to form BN. A small amount of BN can improve the machinability of the material because the bonding force between the

[0001] crystal planes of its hexagonal crystal structure is weak, making the cutting chips easier to break off. This overall structure also allows BN to act as a lubricant to reduce friction between the cutting edge and the matrix. However, the structural characteristics of BN also determine that if its quantity is too large or its size is too large, it will reduce the plasticity of the material, especially its high-temperature plasticity. Therefore, boron needs to be added together with titanium to avoid forming a single nitride. In addition, boron tends to segregate at grain boundaries, which can play a role in refining grains, especially for large-sized mold steels, where its effect on refining grains in casting and hot-rolled or forged structures is more obvious. Boron in part as an element can improve the hardenability of the material and at the same time improve the weldability of the material, saving high-cost metals such as molybdenum. Therefore, boron has specific functions whether in compound or elemental form. It readily forms compounds with aluminum and other elements, necessitating control of the relative content of aluminum and boron. Balancing and testing the effects of elements with similar physical properties is crucial. A balance between machinability and weldability can be achieved when (Al+Ti+Mg) / (C+B+O) < 9.45. Excessive boron, with a proportion greater than 3.23, weakens both grain boundary and matrix strength. This invention preferably uses 0.02~0.06 wt% boron to achieve good machinability, hardenability, and weldability simultaneously.

[0018] Ca: Calcium can improve the machinability of materials. Firstly, calcium improves machinability by modifying aluminum inclusions: after adding calcium, aluminum inclusions transform from high-hardness alumina to relatively lower-hardness calcium aluminate, calcium aluminum feldspar, or various calcium aluminum composite oxides. These oxides soften during turning and adhere to the cutting edge, forming a protective layer to reduce tool wear, with a more pronounced effect during high-speed machining. Besides modifying oxides, calcium can also combine with sulfur to form CaS, which similarly improves machinability, exhibiting a similar mechanism to manganese. Therefore, both are needed in combination. When the content of alloys such as chromium is high, (Mn+Ca) / S>2.90 has a significant effect. In this invention, the carbon content is high. During the solidification process of molten steel, some precipitated carbides are generated at the solid-liquid interface, which are potential crack initiators and difficult to eliminate through rolling and forging. Calcium can lower the decomposition temperature of these precipitated carbides, and with heat treatment, they decompose to eliminate the hazard. Calcium can also purify molten steel and improve its isotropic properties. However, excessive calcium content can affect the smelting and casting processes, requiring increased smelting time and hindering production efficiency. A calcium content of 0.005–0.007 wt% is preferred, achieving both good isotropic properties and excellent machinability.

[0019] Mg: Magnesium has a strong affinity for oxygen. Combined with the high aluminum content in this application, they can jointly form a large number of fine magnesium-aluminum oxides in the steel. These oxides have a lower hardness than alumina, which is beneficial to improving the material's machinability and toughness. In this respect, magnesium and titanium are similar. When the mass ratio of Al+Ti+Mg to C+B+O is small, the effect is not obvious. When it is greater than 3.23, the effect begins to appear. However, when the magnesium content is too high, it is easy to cause burn-off during the smelting process. Magnesium has a weaker affinity for sulfur than for oxygen, but it can work together with oxygen. Before the addition of magnesium, some MnS nucleates and grows with alumina as the core. After the addition of magnesium, sulfides use Mg-Al-O as nucleation sites, surrounded by manganese sulfides containing a small amount of magnesium. The newly formed sulfides are shorter and more dispersed, effectively improving the transverse impact toughness of the material, while also increasing the C-type chip content in the cutting chips. A small amount of magnesium can reduce the surface free energy of carbides, increasing the ratio of matrix / carbide interfacial energy to matrix / matrix interfacial energy, thus enabling the refinement of carbides. Excessive magnesium content increases smelting costs and fails to further enhance carbide refinement. A magnesium content of 0.05–0.08 wt% is preferred to achieve good toughness and machinability.

[0020] La and Ce: Lanthanum and cerium are adjacent elements in the periodic table and have similar physicochemical properties. The segregation of lanthanum and cerium at grain boundaries can suppress the segregation of low-melting-point impurities such as phosphorus, sulfur, lead, and tin, thereby purifying grain boundaries and improving the high-temperature plasticity of the material. The addition of lanthanum and cerium can also transform easily corroded sulfides into rare-earth-containing sulfides with better corrosion resistance, reducing the material's pitting corrosion sensitivity, decreasing the passivation current density, expanding the passivation zone, making the rust layer formed on the material more uniform, and slowing down the rate of rust layer thickness increase, thus improving the material's corrosion resistance. Lanthanum and cerium can also refine grains, especially cerium, whose CeO and other forms can act as nuclei to promote nucleation, refining grains during eutectic processes, and hindering the growth of borides, thereby improving the material's toughness and weldability. The effect is more prominent when (La+Ce) / (B+P)>0.04. However, if the content is too high, it will increase the smelting cost and easily generate a second phase at the grain boundary, which will reduce the toughness of the material. It is preferable to use 0.02~0.04 wt% lanthanum and 0.02~0.05 wt% cerium to obtain good plasticity and corrosion resistance while ensuring the toughness of the material.

[0021] Phosphorus has a dual effect. During the solidification of molten steel, adding a small amount of phosphorus can slow down the growth rate of columnar crystals, increase the content of equiaxed crystals, and refine the grains, thereby improving the mechanical properties of the steel. In complex alternating wet and dry environments, phosphorus can make the rust layer on the material surface denser, reduce the anode area, and thus improve the corrosion resistance of the material. Furthermore, phosphorus, aluminum, and silicon belong to the same third period and are adjacent to each other, with similar electronic structures. They have a similar effect on increasing the activity of carbon in cementite, which can inhibit the precipitation of cementite during cooling, resulting in fine and dispersed carbides. However, a high aluminum and silicon content is detrimental to the casting process, and using a small amount of phosphorus to replace aluminum and silicon can mitigate this harm. Phosphorus can also increase the work hardening rate of the material, making the roots of the stripped steel chips easier to break off, thereby improving the machinability and surface finish of the material. However, excessive phosphorus tends to agglomerate at grain boundaries, causing brittle fracture. Therefore, it needs to be added together with boron. Boron preferentially agglomerates at grain boundaries, mitigating the harmful effects of phosphorus. Lanthanum and cerium have a modifying effect on phosphorus inclusions, forming a small amount of composite inclusions to reduce the harmful effects of phosphorus. However, this effect is limited for excessively high phosphorus contents. This invention preferably uses a phosphorus content of 0.031~0.041%, combined with rare earth elements and boron: (La+Ce) / (B+P)>0.04, which improves the material's corrosion resistance while achieving good machinability.

[0022] S: Sulfur can effectively improve the machinability of materials. Its mechanism of action differs from that of phosphorus, and sulfur has a smaller impact on the hardness of the matrix itself. Sulfur combines with manganese and other substances to form MnS, which has a high melting point and good high-temperature plasticity. During rolling and forging, it can elongate longitudinally, disrupting the continuity of the matrix, thus making it easier for cutting chips to break off from the root during cutting. Sulfur has a fixing effect on carbon in steel. During cooling, it can form small amounts of sulfur-carbon compounds such as TiS and Ti4C2S2, which have higher stability than TiC and can improve the high-temperature stability of the material. While sulfur fixes carbon, titanium also fixes sulfur. When the sulfur content is too high, it is easy to combine with iron to form FeS, which tends to cause hot brittleness and affects the transverse properties of the material. Therefore, in addition to titanium, it is necessary to add manganese and calcium to limit it. When (Mn+Ca) / S>2.90, the probability of sulfur combining with iron can be reduced, thereby reducing hot brittleness and extending the service life of the material. However, excessive sulfur content can easily disrupt the balance between elements, resulting in a large number of inclusions that affect material properties. A sulfur content of 0.17~0.26 wt% is preferred, which can improve material stability while obtaining good machinability.

[0023] The present invention discloses a method for preparing a high-temperature oxidation and corrosion resistant mold steel, comprising the following steps:

[0024] (1) Vacuum induction melting: The raw materials are sequentially filled into the vacuum induction furnace, and the vacuum inside the furnace is evacuated to a level lower than 10 Pa. Melting is carried out under vacuum or inert gas protection. Elements are added to adjust the composition of the molten steel. After stirring, the steel is tapped and cast using continuous casting or ingot casting. After slow cooling, steel ingots are obtained.

[0025] (2) Electroslag remelting: After removing impurities from the steel ingot obtained in step (1), an Al2O3-TiO2-MgO ternary slag system is used. The slag is added in batches, and after remelting and cooling, an electroslag ingot is obtained. Al2O3 in the slag maintains a high aluminum content and balance between elements, and also improves the resistivity of the molten slag. TiO2 reduces the burning loss of titanium in the steel and can adjust the fluidity of the slag liquid. MgO can reduce the burning loss of magnesium in the steel and cover the surface of the slag pool to prevent the molten slag from absorbing oxygen. The slag is added in batches to avoid the slag liquid solidifying and extinguishing the electric arc.

[0026] (3) Forging or rolling;

[0027] (4) Heat treatment: The forged or rolled billet formed in step (3) is annealed, and then tempered or quenched and tempered to obtain high-temperature oxidation and corrosion resistant mold steel.

[0028] Furthermore, when casting in step (1) using continuous casting, the tapping temperature is 1517~1537℃; the tapping temperature is controlled at a higher temperature to avoid lumps forming at the tapping point.

[0029] Furthermore, when casting by mold casting in step (1), the mold is baked at 300~350℃; this removes moisture and improves the fluidity of the molten steel while reducing stress during the casting process.

[0030] Furthermore, before the temperature of the billet obtained from casting in step (1) drops to 400°C, it is placed in a heating furnace for slow cooling or in a heat-insulating pit for slow cooling to avoid thermal stress cracking.

[0031] Furthermore, in step (2), the amount of electrolytic aluminum added in batches is 0.5~0.6wt% of the billet weight; the Al2O3 in the slag cannot completely guarantee the uniformity of aluminum in the finished product.

[0032] Further, in step (3), the electroslag ingot is slowly heated to 830-850 ℃ at 35~45℃ / h, and then rapidly heated to the initial forging temperature of 1075~1165℃ or the initial rolling temperature of 1065~1150℃ at 149~163℃ / h, with the final forging temperature or final rolling temperature being 980~1030℃. In this invention, the aluminum and chromium content is relatively high, which easily leads to coarse structures during high-temperature periods. Therefore, slow heating in the medium-low temperature range and rapid heating in the high-temperature range are adopted to reduce the residence time of the material in the high-temperature range. The high alloy content results in greater material hardness, and the electroslag ingot is not softened enough if the heating temperature is low. Therefore, the initial forging temperature is 1075~1165 ℃ or the initial rolling temperature is 1065~1150 ℃.

[0033] Furthermore, in step (3), the initial reduction during forging is less than 30%, the final forging ratio is 3.5~4.5, and the furnace heating time during forging or rolling is less than 2.5h; the material has a high content of carbon and carbide-forming elements, which may form coarse carbides, so a larger forging ratio is adopted, and the high aluminum and high carbon content makes it easy to crack, which determines that it is difficult to directly use heavy hammer forging. The furnace return time is less than 2.5h to avoid coarse structure, metastable M2C and other carbides, high silicon easily produces banded structure, and high sulfur content leads to more inclusions, which determines that horizontal forging should be strengthened during forging, and light hammer fast hitting should be used to break carbides and avoid cracking.

[0034] Furthermore, the annealing temperature in step (4) is 865~900℃, held for 1~3 hours, and then cooled with the furnace.

[0035] Furthermore, in step (4), the quenching and tempering treatments are performed with a quenching temperature of 1015~1045℃ and a holding time of 1~3h. After air cooling or wind cooling, the heat treatment is performed immediately with a tempering temperature of 225~355℃ and a holding time of 2.5~5.5h. The heat treatment is repeated multiple times.

[0036] This invention also includes an aluminum-containing, highly uniform, oxidation- and corrosion-resistant mold steel, comprising the following components by weight percentage: C: 0.47~0.53%, Si: 0.91~1.12%, Mn: 0.31~0.67%, Cr: 17.30~20.90%, Al: 5.36~7.12%, W: 0.21~0.53%, V: 0.02~0.05%, Co: 0.01~0.02%, Zr: 0.05~0.11%. %, Zn: 0.05~0.07%, Mg: 0.01~0.02%, Y: 0.005~0.008%, Ca: 0.002~0.005%, P<0.03%, S<0.03%, As<0.02%, with the balance being Fe and unavoidable impurities, and simultaneously satisfying 0.04<(Mn+Zn) / (Al+Si)<0.12, 0.53<(Ca+Zn+Mg) / (Y+Zr)<1.73.

[0037] C: Carbon, dissolved in the matrix, pins dislocations, enhancing the matrix's strength and hardness, and plays a crucial role in ferrite formation. Carbon also forms carbides with chromium, tungsten, vanadium, etc., which, after shaping, crushing, and appropriate heat treatment, are dispersed in the α-phase, improving the material's wear resistance, toughness, and other comprehensive properties. The addition of carbide-forming elements helps form composite carbides, avoiding the tendency of single carbides to coarsen. The effects of carbon on non-carbide or weak carbide-forming elements are also closely related; cobalt enters the carbides through substitution, improving their properties, while silicon refines the carbides by influencing carbon diffusion, improving the material's overall performance. Excessive carbon content easily forms network carbides, causing uneven hardness and compromising material strength. Excessive carbon is also prone to decarburization at high temperatures, resulting in uneven surface and internal composition and hardness, and the decarburization process affects the material's high-temperature oxidation and corrosion resistance. A carbon content of 0.47~0.53 wt% is preferred, ensuring material uniformity and high-temperature oxidation and corrosion resistance while achieving good hardness, strength, and wear resistance.

[0038] Silicon (Si): Silicon can improve the wear resistance of materials by inhibiting the precipitation of cementite during tempering, increasing the dispersion of carbides in the matrix, and inhibiting carbide growth at high temperatures. This allows the material to undergo secondary hardening under impact wear, thus improving wear resistance. Silicon can also adjust the carbide type, promoting the transformation of eutectic carbides into finer, harder MC and M6C. Cracks caused by thermal fatigue or stress fatigue usually originate at the subsurface of the material and extend to the surface. Silicon can effectively prevent this process and reduce surface spalling failure caused by crack convergence. At high temperatures, silicon promotes the formation of Cotillard atmospheres at dislocation entanglements, increasing material strength. During the cooling process from high to low temperatures, silicon can effectively improve the hardenability of the material, which is beneficial to improving the uniformity of the material during quenching. At lower temperatures, silicon can improve the tempering stability of the material by hindering carbon diffusion, reducing the adverse effects of increased brittleness during tempering caused by manganese. During the smelting process, silicon can act as a deoxidizer. In the application of materials, silicon can form an oxide layer, improving the material's resistance to oxidation and corrosion. Excessive silicon content easily segregates, forming banded or columnar crystals, which impairs the material's lateral properties. This invention controls segregation by adding zinc and breaks down columnar crystals in conjunction with the forming process. A silicon content of 0.91~1.12 wt% is preferred, improving the material's wear resistance and hardenability while achieving good resistance to oxidation and corrosion and tempering stability.

[0039] Mn: Manganese has a solid solution strengthening effect, and its combined action with carbon significantly compensates for the low hardness caused by the high content of ferrite-forming elements. Although the solid solution strengthening effect of manganese in this invention is weaker than that of carbon, it does not significantly weaken the plasticity and ductility of the mold steel. As a weak carbide-forming element, manganese typically does not form individual carbides, but can be dissolved in cementite through substitution to form alloy cementite with low free energy, thus enhancing material stability. Manganese can also improve the hardenability of the material and increase the uniformity of the steel. During secondary hardening, manganese can promote the decomposition of carbides, providing conditions for the combination of various strong carbide-forming elements with carbon, thereby improving the secondary hardening effect and enhancing the wear resistance of the material. Manganese has a strong affinity for sulfur, which can reduce the formation of FeS and mitigate the harmful effects of sulfur. However, excessive manganese can easily lead to coarse grains, reducing material properties and increasing the probability of temper brittleness. A manganese content of 0.31~0.67wt% is preferred, which promotes the improvement of material uniformity and stability while obtaining higher hardness and wear resistance.

[0040] Chromium (Cr) significantly improves the oxidation and corrosion resistance of materials. At high temperatures, chromium oxide (chromium oxide) forms with good density, hindering further contact between oxidizing media and the matrix. In reducing media, such as chlorine-containing media, chromium has a lower corrosion potential and stronger passivation ability compared to iron. Higher chromium content shifts the critical passivation potential of the material in the negative direction, thus improving its corrosion resistance. Chromium-formed corrosion products accumulate on the surface, blocking microcracks and reducing further contact between corrosive media and the matrix. Chromium is an important carbide-forming element, forming MC-type carbides or complex carbides with vanadium, hindering carbide coarsening during prolonged high-temperature processing or service environments, pinning dislocations, and improving material strength and wear resistance. Chromium is an important hardenability-enhancing element; combined with higher addition levels, it can improve the uniformity of materials with larger dimensions, and chromium's ferrite-forming tendency can improve the dimensional stability of the material. The chromium added in this invention tends to accumulate and segregate in the core due to its high content. Therefore, zinc is added simultaneously to increase the diffusion rate of chromium and reduce its segregation. Excessive chromium content can easily lead to a decrease in the tempering stability of the material. A chromium content of 17.3~20.9 wt% is preferred, which improves the dimensional stability of the material while obtaining good resistance to oxidation and corrosion and strength.

[0041] Aluminum (Al) plays a multifaceted role. During smelting, it has a strong deoxidizing effect and simultaneously fixes free nitrogen in steel, improving the material's cleanliness and hardness. In forming, aluminum improves the material's hot-formability and reduces losses during hot working. During heat treatment and product use, carbon alters the composition of chemical bonds in the matrix, weakening the bonding forces between carbon and iron and other atoms, increasing carbon activity, thereby promoting the precipitation of hard phase particles, improving the material's red hardness, and enhancing wear resistance through secondary hardening. Aluminum's improvement in red hardness and hot strength helps reduce the use of cobalt, tungsten, or molybdenum, thus lowering costs. Aluminum also tends to form a continuous, dense oxide layer on the material surface, isolating the oxidizing medium from further contact with the matrix and improving the material's resistance to high-temperature oxidation. For oxidizing and corrosive media containing moisture, chromium alone is insufficient to provide a complete protective layer; chromium and aluminum need to work together, providing a "third element effect" to form a composite oxide layer that improves the material's resistance to high-temperature oxidation and corrosion. Further improvements in high-temperature oxidation and corrosion resistance are needed. This invention enhances the adhesion between the protective layer and the substrate by adding yttrium, preventing significant detachment during thermal cycling and expansion / contraction. The ferrite formed by high aluminum and chromium is broken into smaller pieces by carbides, which can absorb stress in the steel, reduce cracks, and improve toughness. The reduction in cracks, especially microcracks during fatigue, decreases the contact between the substrate and corrosive media, thus improving the material's corrosion resistance. Excessive aluminum content can easily cause nodule formation during smelting and casting, and the product's microstructure is prone to coarse segregation at high temperatures. A preferred aluminum content of 5.36–7.12 wt% improves the material's resistance to oxidation and corrosion while achieving good toughness and red hardness.

[0042] Tungsten is a strong carbide-forming element, capable of forming various types of carbides such as M6C, M2C, and MC. Especially fine-sized, high-hardness, and dispersed M2C and MC effectively improve the hardness, wear resistance, and thermal strength of materials. High-melting-point carbides act as nucleation sites, promoting grain refinement and compensating for the tendency for coarse microstructure caused by high aluminum and chromium content. The combined effect of tungsten-containing carbides and chromium-titanium carbides restricts grain growth, hinders dislocation movement, and improves material toughness. It also prevents the coarsening of single chromium carbides, improving the tempering stability of the material. Furthermore, tungsten can enhance the tensile strength and creep rupture strength of the matrix through solid solution strengthening and the formation of the Laves phase, with its effect being more pronounced at high temperatures than that of molybdenum. Small amounts of tungsten can improve the performance of chromium-aluminum-silicon-based passivation films, especially in chlorine-containing environments. The tungstate formed by tungsten reduces chloride ion adsorption on the surface and forms a composite compound with iron ions to plug micro-cracks and other defects on the passivation film, improving the material's corrosion resistance. Excessive tungsten content can easily form coarse skeletal eutectic carbides, which is detrimental to the mechanical properties of the material. In addition, tungsten is expensive. Using a tungsten content of 0.21~0.53wt% can improve the hardness and thermal strength of the material while achieving good tempering stability and corrosion resistance.

[0043] Vanadium plays a multifaceted role. On one hand, it strengthens ferrite through substitutional solid solution, tending to agglomerate near dislocation lines and intertwine with dislocations to form substructures, increasing the microstructure's resistance to recovery and thus improving tempering stability and cross-sectional microstructure uniformity. On the other hand, vanadium can form high-melting-point carbides. Their difficulty in melting during heat treatment allows them to pin grain boundaries, hindering grain growth and achieving a fine-grained strengthening effect. Vanadium-formed carbides are primarily VC (volatile carbon), characterized by high hardness and diffuse distribution. High-temperature tempering is necessary for greater precipitation, improving the material's wear resistance. The high thermal stability of VC reduces the material's thermal sensitivity; its low tendency to aggregate and grow further can prevent the formation of coarse carbides, thereby improving toughness, high-temperature strength, and thermal fatigue resistance. Under cyclic impact wear conditions, vanadium-containing carbides help form a high-hardness etched layer on the material surface, improving surface wear resistance. In addition to its interaction with carbon, vanadium also combines with nitrogen, reducing the material's aging tendency by fixing nitrogen in the steel. Adding excessive vanadium may lead to the formation of coarse carbides and reduce the amount of carbon dissolved in the matrix, thus reducing the hardness of the matrix. It is preferable to use 0.02~0.05wt% vanadium, which can improve the stability of the material while obtaining good hardness and toughness.

[0044] Co (Co) can dissolve in the matrix, forming a continuous solid solution with iron. This increases the matrix hardness while hindering the diffusion of carbon and other elements, thus suppressing the precipitation and growth of carbides, especially chromium-molybdenum carbides, resulting in a more dispersed carbide distribution. The fine, dispersed carbides impede the growth of matrix grains, effectively increasing the secondary hardening peak of the material, thereby enhancing its resistance to tempering softening and its structural stability. Furthermore, when the material prepared by this invention is applied to metal die casting or plastic injection molding, the cyclic heating and cooling process is similar to long-term tempering. Cobalt's impediment to the diffusion of carbon atoms in ferrite allows them to accumulate more on the surface in a solid solution form, increasing surface hardness, improving wear resistance, and extending service life. However, cobalt is expensive. Therefore, some manganese and zinc are added to replace cobalt to refine the structure of carbides and other materials. It is preferred to use 0.01~0.02wt% cobalt, and at the same time, 0.04<(Mn+Zn) / (Al+Si)<0.12 is added to save cobalt, improve the hardness of the material, and obtain good stability and wear resistance.

[0045] Zirconium (Zr) can form various compounds with carbon, nitrogen, and oxygen, with zirconium oxide (ZrO) preferentially precipitating and competing with alumina inclusions for formation, thus reducing the defects such as cracks caused by alumina inclusions. Its combination with carbon increases the variety of carbides, which helps avoid the coarseness of single carbides and improves material toughness. Zirconium, together with vanadium, fixes nitrogen, effectively reducing the material's aging tendency. Zirconium can also modify sulfide inclusions, reducing the number of elongated sulfides, decreasing the difference in transverse and longitudinal properties, and improving material uniformity. Zirconium also regulates silicate inclusions, making them smaller and blunting sharp heads or tails into near-circular shapes, reducing their probability of acting as crack initiation sites. This provides a basis for adding more silicon in this invention. During processing, zirconium, together with zinc, promotes chip curling, improving the material's machinability. However, excessively high zirconium content reduces the material's high-temperature ductility; a zirconium content of 0.05~0.11 wt% is preferred, achieving good plasticity and machinability while improving material uniformity.

[0046] Y: The role of rare earth yttrium is multifaceted. Due to its small radius, yttrium can enhance matrix strength through solid solution substitution with iron atoms. After yttrium is dissolved in the matrix, the corresponding solution temperature rises, promoting the incorporation of more alloying elements into austenite and other structures at high temperatures. This results in more dispersed precipitates, further improving material strength. Some yttrium tends to agglomerate at grain boundaries, purifying them and improving both strength and toughness. Yttrium can combine with oxygen to form yttrium oxide, which helps reduce the oxygen content in steel. Furthermore, when yttrium oxide is located at grain boundaries, it can hinder the formation of network carbides, especially for strip-shaped Cr. 23C6 exhibits a more pronounced disrupting effect, reducing the quantity and extent of banded structures and improving structural uniformity. Yttrium and zirconium work together; the former breaks down carbides, while the latter increases the variety of carbides, jointly preventing the formation of a network of coarse carbides and improving material toughness. Besides oxides, yttrium can also react with magnesium and sulfur oxides to form complex sulfur oxides, which act as nucleation sites during smelting, contributing to fine-grained strengthening. Furthermore, yttrium-containing oxides float rapidly in molten steel, helping to shorten smelting time and improve production efficiency while simultaneously desulfurizing. However, excessively high yttrium content can easily generate large oxide inclusions, causing stress concentration cracking. A yttrium content of 0.005–0.008 wt% is preferred, achieving good toughness while improving material strength.

[0047] Zinc (Zn) plays a multifaceted role. Higher aluminum content can easily lead to coarse microstructures at high temperatures. However, the combined effect of zinc and aluminum increases the undercooling of molten steel, reduces coarse carbides, and increases the nucleation rate, thereby refining the microstructure during steel crystallization and improving material toughness. Due to the increased nucleation rate and refined microstructure during crystallization, the amount of liquid phase surrounding dendrites during non-equilibrium solidification decreases, resulting in less component segregation and a reduction in the content of liquid carbides. This increases microstructure uniformity and reduces the likelihood of coarse liquid carbides formed due to higher carbon and carbide-forming element content. Zinc also enhances the material's resistance to oxidation and corrosion. While this can be achieved through separate galvanizing, it is highly polluting, involves additional processes, and is not suitable for materials such as mold steel. This invention incorporates zinc uniformly into the material during the smelting process. Under high-temperature oxidation and corrosion conditions, zinc enters the oxide layer, reducing the diffusion rate of iron within the oxide layer, thus improving the material's resistance to oxidation and corrosion. Zinc can be added alone or in combination with magnesium to modify inclusions by hindering dislocation slip and climb, reducing the material's crack susceptibility, and improving its toughness. Maintaining the (Ca+Zn+Mg) / (Y+Zr) ratio above 0.53 alters the wettability of inclusions with molten steel and increases the size of inclusions, making them easier to float. Excessive zinc content reduces material strength; a zinc content of 0.05~0.07 wt% is preferred to obtain good toughness, resistance to oxidation and corrosion, and excellent uniformity.

[0048] Mg: Magnesium can combine with various elements to exert its effects. Magnesium and zinc can form intermetallic compounds, which, when distributed near grain boundaries, can hinder grain growth and coarsening, thus improving material strength. Intermetallic compounds can also, to some extent, slow down the corrosion rate at grain boundaries through their excellent corrosion resistance. Magnesium has a strong affinity for oxygen, forming manganese aluminum spinel during steel melting and solidification. This spinel has high hardness, small size, and diffuse distribution, making it more beneficial for improving the mechanical properties of materials than strip-shaped alumina. Magnesium also has a strong affinity for sulfur, forming complex sulfides with manganese and promoting the transformation of sulfides from elongated type II inclusions to ellipsoidal type I or III inclusions. This reduces the length of chips generated during cutting, and their shape changes from elongated or spiral to C-shaped, indicating improved machinability. Magnesium has a certain attraction to phosphorus. In the steel prepared in this invention, magnesium oxide contained a certain amount of phosphorus, nine times higher than the phosphorus in the matrix. Therefore, magnesium can reduce material segregation by fixing phosphorus, improving the material's plasticity and toughness. Magnesium is not a strong carbide-forming element, but its radius is similar to that of chromium and vanadium, and it tends to agglomerate at interfaces. Therefore, it can form complex carbides with chromium and vanadium. In addition, as a surface-active element, it can change the interfacial energy, adjust the interfacial tension of different phases, and refine the carbides, thereby improving the material strength. However, excessive magnesium can easily produce large-sized inclusions. It is preferable to use 0.01~0.02wt% magnesium, which can improve machinability while maintaining good strength and uniformity.

[0049] Ca: Calcium has an arsenic-removing effect, which can improve the purity of steel, expand the range of raw material selection, and reduce production costs. This is of great significance to the present invention, which uses a vacuum induction melting process. Calcium can also adjust the liquid carbides in steel by lowering their decomposition temperature, promoting faster decomposition during heat treatment and avoiding defects such as cracking caused by stress concentration. Calcium has a modifying effect on inclusions, promoting the transformation of hard alumina inclusions into softer calcium-aluminum composite inclusions, reducing the hardness by about 40 HV compared to before addition. When calcium is added in combination with zinc and magnesium, the hardness reduction can reach more than 800 HV, thereby greatly improving the machinability of the material. The addition of calcium can also transform sulfides from elongated to spindle-shaped. Spindle-shaped inclusions have lower hardness, allowing them to adhere to the tool surface to form a lubricating layer, reducing tool wear. Furthermore, they can improve the isotropic properties of the material. Calcium, when added alone, requires a relatively high amount to achieve a significant effect and must be strictly matched with the smelting process. This invention adopts a multi-element composite addition method, controlling (Ca+Zn+Mg) / (Y+Zr) within 1.73, and the calcium content between 0.002 and 0.005 wt%. This avoids the significant drawbacks caused by excessive amounts of a single element and controls costs. At the same time, it modifies inclusions, adjusts the microstructure and distribution of carbides, and obtains good machinability.

[0050] S, P, As: Sulfur, phosphorus, and arsenic are generally considered hazardous elements, and their content should be as low as possible. However, due to limitations in raw material and process costs, complete removal is neither feasible nor possible. Therefore, this invention controls their content at a low level and simultaneously fixes them by adding rare earth elements and magnesium and calcium, or modifies any inclusions they form. This reduces their hazards while fully utilizing sulfur to improve machinability and phosphorus to enhance matrix hardness and surface finish. Considering both performance and cost, this invention controls their content to S < 0.03 wt%, P < 0.03 wt%, and As < 0.02 wt%.

[0051] The present invention discloses a method for preparing an aluminum-containing, highly uniform, oxidation- and corrosion-resistant mold steel, comprising the following steps:

[0052] (1) Vacuum induction melting: Raw materials are put into the vacuum induction furnace in batches, and the vacuum is drawn until the vacuum degree inside the furnace is lower than 10Pa. Melting is carried out under vacuum or inert gas protection. Elements are added to adjust the composition of the molten steel. After stirring, the steel is tapped and cast to obtain steel ingots.

[0053] (2) Electroslag remelting: The steel ingot obtained in step (1) is removed from impurities and then electroslag remelted. It is then cooled to room temperature or sent to form a billet before cooling to 300°C.

[0054] (3) Forging or rolling, followed by slow cooling in the furnace;

[0055] (4) Heat treatment: The material after the forming process in step (3) is quenched and then tempered after cooling.

[0056] Further, in step (1), the order of feeding materials is iron, chromium, manganese, tungsten, and cobalt. After degassing, 1 / 2 to 3 / 4 of vanadium, aluminum, zinc, yttrium, and carbon are added. After melting and clearing, the remaining raw materials, including calcium, are added, with calcium added in the form of aluminum foil or thin iron sheet.

[0057] Furthermore, in step (1), the steel is tapped and cast 5-8 minutes after the last batch of material is fed. If the time is too short, dephosphorization, desulfurization, and dearsenic removal will be insufficient, while if the time is too long, harmful phenomena such as arsenic reversion will occur.

[0058] Furthermore, during the forging process in step (3), the material is subjected to a uniform temperature treatment of 990~1010℃ before forging, and then heated to the initial forging temperature of 1145~1195℃ at a rate of 45~55℃ / h, and held for 1~5.5h. The final forging temperature is 985~1005℃. Due to the high aluminum and chromium content, ferrite structure is easily formed and it is easy to grow and coarsen rapidly when the temperature is greater than 1010℃. The material has poor hot formability at 1100℃.

[0059] Furthermore, in step (3) during forging, upsetting is performed first, followed by light hammer forging. During the forging process, the billet is placed in a heating furnace and kept at 1055 ~ 1105℃; columnar crystals and eutectic carbides are fully crushed.

[0060] Furthermore, the initial rolling temperature for step (3) is 1130~1185℃, and the final rolling temperature is 970~990℃.

[0061] Further, in step (4), the heat treatment quenching temperature is 1065~1105℃, and the holding temperature is 2~5h; the tempering temperature is 580~640℃, and the holding temperature is 2.5~4.5h, and the tempering is repeated 1~3 times.

[0062] Furthermore, the holding time in step (4) is coordinated with the material size. For materials with a round billet diameter or a square billet thickness of less than 250 mm, the quenching holding time is 2~3 h and the tempering holding time is 2.5~3.5 h. For round billets with a diameter of 250 mm~500 mm or square billets with a thickness between 250 mm~450 mm, the quenching holding time is 3~4 h and the tempering holding time is 3~4 h. For materials with a round billet diameter greater than 500 mm or a square billet thickness greater than 450 mm, the quenching holding time is 4~5 h and the tempering holding time is 3.5~4.5 h.

[0063] Furthermore, in step (4), the quenching process employs atomized cooling. The pressure of the atomizing nozzle is 0.35~0.50MPa, forming droplets with a diameter of 35~90μm, and atomizing 2~3 times. Due to the high carbide formation and coarse structure tendency, a faster cooling rate is required in the quenching cooling stage. Direct water cooling is prone to cracking, especially in cases of relatively complex shapes. Oil cooling is prone to contamination, while air cooling is too slow and difficult to avoid core segregation. Therefore, atomized cooling is adopted. During the cooling process, if the nozzle pressure is too low, the fluidity will be insufficient; if the pressure is too high, the droplets will be too fine and the impact on the material surface will be too small, weakening the cooling effect.

[0064] This invention also includes a highly uniform, free-machining, high-temperature oxidation-resistant mold steel, comprising the following components by weight percentage: C: 0.43~0.51%, Si: 0.89~1.08%, Mn: 0.69~1.01%, Cr: 19.33~24.45%, Mo: 0.04~0.08%, W: 0.01~0.02%, Al: 4.95~6.63%, Ti: 0.02~0.05%, Cu: 0.01~0. 0.02%, Bi: 0.002~0.007%, P: 0.02~0.03%, S: 0.07~0.11%, Zn: 0.04~0.06%, B: 0.01~0.02%, Ca: 0.010~0.015%, and simultaneously satisfying 0.07<(C+B) / (Al+Ti)<0.11, 6.38<(Ca+Mn+Bi) / S<14.74, with the balance being Fe and unavoidable impurities.

[0065] C: Carbon plays a multifaceted role, acting both alone and in combination with other elements. Carbon can dissolve in austenite and ferrite, and in this invention, ferrite tends to form, effectively compensating for the insufficient hardness and thermal strength of the matrix. Carbon can combine with various elements such as chromium and titanium to form carbides. Dispersed carbides effectively hinder dislocation slip, improving material strength and toughness. Some high-hardness carbides effectively improve wear resistance, and the interpenetration between different carbides and inclusions prevents the coarseness of a single microstructure from affecting the overall mechanical properties of the material. For large-sized molds, direct replacement is not advisable when partial wear occurs. By controlling the carbon content, the material achieves good weldability with low heat input, providing a solution for repairing worn parts through welding, extending service life and reducing production costs. However, excessively high carbon content can lead to material embrittlement; a carbon content of 0.43~0.51wt% is preferred, achieving good weldability while improving material hardness and wear resistance.

[0066] Silicon (Si) enhances the tempering resistance of materials by reducing the diffusion rate of carbon in ferrite, thereby decreasing the aggregation of carbides during tempering and making the precipitated carbides more dispersed, thus increasing the secondary hardening peak. The materials prepared according to the present invention are used in plastic and glass forming scenarios where the environment undergoes alternating hot and cold cycles, similar to the tempering process. Silicon can diffuse into ε-carbides, improving their stability through solid solution, delaying carbide transformation and the appearance of cementite, thus balancing the strength and toughness of the material. Silicon is a highly effective and inexpensive deoxidizer, contributing to improved material cleanliness. In chloride-containing environments, silicon can also increase the pitting potential, replacing some molybdenum elements, thus improving corrosion resistance while reducing costs. In high-temperature oxidizing media, silicon can form oxides, supplementing oxide layers such as chromium and aluminum, improving the oxidation resistance of the material. However, excessive silicon content may lead to coarse columnar grains; a silicon content of 0.89~1.08 wt% is preferred, achieving good stability and oxidation corrosion resistance while improving material hardness.

[0067] Mn (manganese): Manganese significantly improves the hardenability of materials, which is beneficial for obtaining highly uniform materials during heat treatment, and has a significant effect on large-sized parts. Manganese also enhances the strength of the matrix. It substitutes into the ferrite or austenite matrix, combining with interstitial carbon atoms to improve the hardness and strength of the material through solid solution. Compared to carbon, manganese has less impact on the ductility and plasticity of the material, thus improving the overall mechanical properties. Although manganese is a weak carbide-forming element, it can enter cementite through substitution to form iron-manganese composite cementite, which has higher stability and helps to stabilize complex alloy systems. The addition of manganese hinders carbide growth, resulting in finer and more dispersed precipitated carbides, thereby improving the toughness of the material. Finally, manganese has a strong affinity for sulfur, forming MnS while reducing the formation of FeS. This improves the machinability of the material and reduces grain boundary cracking under stress. However, the effect of manganese alone is not significant enough; it needs to be combined with bismuth, etc. The effect is more significant when (Mn+Ca+Bi) / S>6.38. However, excessive sulfur can easily cause segregation in the core. It is preferable to use 0.69~1.01wt% manganese, which can improve the stability of the material while obtaining good uniformity, stability and strength.

[0068] Chromium (Cr) plays a crucial role in several aspects. Firstly, it improves the hardenability and microstructure uniformity of materials. As a ferrite stabilizing element, it enhances dimensional stability, making the material suitable for manufacturing large-scale molds or parts. The chromium content significantly influences the effects of other elements. For instance, it combines with carbon to form carbides, especially secondary carbides precipitated during tempering. These carbides pin dislocations, delaying recovery and improving high-temperature performance. The highly stable MC-type carbides play a significant role in promoting material stability. Chromium also interacts with zinc to improve its diffusion efficiency and prevent segregation. In high-temperature oxidizing media, chromium forms a passivation layer to isolate the medium from further contact with the substrate. Furthermore, it complements silicon oxides, further enhancing the material's oxidation resistance. In corrosive media, the addition of chromium shifts the critical passivation potential negatively, improving corrosion resistance. However, excessive chromium will increase the temper brittleness of the material. It is preferable to use 19.33~24.45wt% chromium, which can improve the uniformity of material structure and dimensional stability while obtaining excellent resistance to high-temperature oxidation corrosion.

[0069] Mo (Mo): As a carbide-forming element, molybdenum (Mo) forms carbides that undergo phase transitions in the non-equilibrium cooling and solidification environment of actual production processes. Therefore, it needs to be combined with appropriate heat treatment processes to promote the metastable M2C carbides containing molybdenum to break down into finer, more dispersed, and more stable MC or M6C type carbides during forming and heat treatment, thereby improving the material's toughness and wear resistance. Besides forming carbides, molybdenum also tends to agglomerate around dislocations, reducing lattice distortion in the system, improving material stability, and pinning dislocations in the form of Cocteau clusters, thus increasing the material's yield strength. Furthermore, molybdenum can form a protective film on the material surface in the form of molybdate ions, increasing the pitting potential of the material, inhibiting the damage caused by chloride ions, and working with silicon to improve the material's corrosion resistance. Molybdenum has a strong affinity for impurities and can purify grain boundaries. However, molybdenum is expensive, and excessive addition can easily lead to temper brittleness. A concentration of 0.04~0.08 wt% molybdenum is preferred, as it improves the material's wear resistance while achieving excellent corrosion resistance and stability.

[0070] Tungsten plays a crucial role at various stages. As a strong carbide-forming element, tungsten forms WC and W₂C during the smelting stage, acting as nucleation sites to promote heterogeneous nucleation and improve material toughness through fine-grain strengthening. During the forming stage, some fan-shaped tungsten eutectic carbides require crushing with a larger forging ratio to enhance material toughness. In the heat treatment stage, it forms fine, dispersed carbides during tempering, improving the secondary hardening strength of the material. Furthermore, tungsten-containing carbides have higher thermal conductivity than chromium-containing carbides, facilitating heat diffusion and reducing precision loss due to localized heat accumulation, which is beneficial for the preparation and use of large-scale molds or parts. Tungsten can also form intermetallic compounds such as the Laves phase with molybdenum, improving the tempering stability and red hardness of the material while enhancing creep strength and extending its service life under high-temperature conditions. Tungsten can also form ferrous tungstate and other substances with iron, covering the material surface, hindering the development of localized corrosion, and improving the material's corrosion resistance. However, tungsten is expensive, and excessive tungsten can make the material too hard and difficult to process. This invention prefers 0.01 to 0.02 wt% tungsten, which improves the corrosion resistance and hardness of the material while obtaining higher toughness.

[0071] Aluminum (Al): Aluminum improves the processing plasticity of materials, especially enhancing the hot working performance of large-format materials. It helps to break up coarse structures caused by high chromium content during forming and improves potential segregation caused by silicon, thereby improving material properties and uniformity. Aluminum also influences the formation of chemical bonds in materials. When (C+B) / (Al+Ti) > 0.07, aluminum promotes the dispersed precipitation of high-hardness particles by increasing the activity of carbon, and can replace cobalt to improve the red hardness and hot strength of materials. Aluminum is an important deoxidizing element, effectively reducing inclusions in steel and simultaneously fixing nitrogen, which can refine grains and improve material purity. In oxidizing environments, aluminum can form an oxide layer that adheres to the material surface, improving oxidation resistance along with chromium oxides. The combined addition of aluminum and zinc can improve the as-cast microstructure of materials, reduce coarse carbides that increase the undercooling of molten steel, and thus improve the toughness of materials through grain refinement. However, excessive aluminum is detrimental to the smelting and casting process. It is preferable to use 4.95~6.63wt% aluminum, which can improve the thermal strength of the material while also providing excellent oxidation resistance and high hardness.

[0072] Titanium (Ti) combines with carbon to form titanium carbide, which is widely distributed and has high thermal stability. At high temperatures, it can inhibit grain growth and coarsening, and hinder dislocation activation and slip, thus improving material toughness. Titanium carbide also improves wear resistance through its high hardness. It can also intersect other carbides to prevent or reduce the formation of network carbides, or integrate into other carbides to form composite carbides, improving material strength and stability. Besides combining with carbon, titanium can also combine with nitrogen to form titanium nitride, thereby reducing the size and quantity of aluminum nitride and preventing excessively large aluminum nitride from affecting material toughness. Furthermore, titanium can form composite spinel inclusions with aluminum and manganese, or Ti4C2S2 with sulfur, which has lower hardness and melting point than alumina alone, reducing tool wear during machining and improving machinability. Titanium can also modify silicate inclusions to form composite inclusions, reducing sharp edges and thus lowering the probability of cracking due to stress concentration. The addition of titanium also refines the grains in the weld heat-affected zone, thereby improving the repairability of large-scale molds or parts and reducing costs. However, excessive titanium is detrimental to the casting process; a concentration of 0.02~0.05wt% is preferred, which improves the material's wear resistance and toughness while also achieving good machinability.

[0073] Bismuth (B) enhances the machinability of materials. When B exists in elemental form in steel, its dispersed distribution and softening during cutting create stress concentration, facilitating chip breakage and removal, thus improving machinability. Furthermore, B can modify inclusions by accumulating near them or forming composite inclusions, transforming sulfur-containing inclusions from rod-shaped or elongated forms to spherical shapes. This improves the isotropic mechanical properties of the material and promotes inclusion fracture under cutting stress, further enhancing machinability. This invention uses a lower B content, adjusting the sulfur content and the number of sulfur-containing inclusions to concentrate them within or around inclusions, reducing the probability of B existing in elemental form. The focus is on utilizing B's modification effect on inclusions to improve machinability. Excessive bismuth loss during smelting and excessive amounts can decrease the material's thermal strength; therefore, a B content of 0.002–0.007 wt% is preferred for optimal machinability.

[0074] Copper (Cu) is beneficial for improving the corrosion resistance of materials. This invention activates the anode with copper, promoting the passivation of the cathode and thus reducing the degree of corrosion. In liquid corrosive media, copper can also deposit on the material surface in the form of copper ions and combine with sulfur to form cuprous sulfide, weakening the electrochemical catalysis of sulfur ions. The use of higher sulfur content to improve machinability is significant. Copper partially integrated into the matrix increases hardness through displacement solid solution. Copper is an important precipitation hardening element, and its precipitation is related to manganese and sulfur content. Manganese sulfide can act as nucleation sites to promote copper precipitation. After heat treatment, some copper precipitates as nano-copper-rich phase particles, improving material toughness through precipitation. The role of copper in precipitation strengthening and the underlying principle are similar to those of molybdenum. This invention uses copper to replace some molybdenum, reducing costs and decreasing the probability of temper brittleness. The precipitated copper-rich phase acts as a lubricant during machining and promotes heat dissipation, improving cutting efficiency. However, excessive copper content can promote the growth of inclusions such as manganese sulfide, which can impair mechanical properties. It is preferable to use 0.01 to 0.02 wt% copper, which can improve the corrosion resistance of the material while obtaining excellent processing performance and toughness.

[0075] Phosphorus, along with silicon, enhances the strength and hardness of materials through solid solution, particularly improving the hardness of ferrite. This significantly mitigates the drawback of a soft matrix caused by excessive ferrite due to high chromium and aluminum content. Phosphorus also enhances work hardening, promotes chip breakage and exfoliation, improves machinability, and contributes to a smoother surface finish. Under corrosive conditions, phosphorus segregates through vacancy-solute atom diffusion and precipitates as phosphate ions on the material surface, inhibiting anodic reactions and thus improving corrosion resistance. Phosphorus also refines grain size and inhibits cementite precipitation, thereby improving mechanical properties. However, excessive phosphorus can segregate at grain boundaries. Therefore, it is necessary to add boron and calcium to purify the grain boundaries while controlling the total amount of phosphorus. A phosphorus content of 0.02–0.03 wt% is preferred to achieve excellent corrosion resistance and hardness while improving machinability.

[0076] S: Sulfur can significantly improve the machinability of materials. Sulfur combines with manganese to form MnS, and the compressive stress caused by the tool during cutting concentrates near MnS, promoting easy chip breakage and detachment through the notch effect. MnS also improves machining efficiency by forming a thin film on the rake face, effectively hindering thermal diffusion and adhesive wear between the tool and chip elements while lubricating them. MnS has a higher melting point than the matrix and good high-temperature plasticity. During pressure forming, it extends axially, reducing the uniformity of mechanical properties in all directions. It needs to be added together with copper or bismuth to promote the transformation of MnS from long strips or rods to spindle or quasi-circular shapes, improving material uniformity. Sulfur can also fix carbon by forming compounds, improving the high-temperature stability of materials. However, excessive sulfur can lead to hot brittleness. A sulfur content of 0.07~0.11wt% is preferred to improve material stability while obtaining good machinability.

[0077] Zinc (Zn): During the smelting stage, zinc improves the wettability of inclusions and molten steel, promoting the flotation of inclusions and thus improving material purity. During solidification, zinc improves the morphology of eutectic carbides, reduces the content of network carbides, promotes the transformation of coarse carbides into spherical shapes, and refines grains, thereby improving material toughness. During stress deformation, the addition of zinc prompts the matrix phase to respond to the fracture process through slip stretching, improving both toughness and strength. Zinc improves elemental segregation and enrichment, particularly helping to improve the enrichment of elements such as silicon and manganese at grain boundaries, thus improving material uniformity and providing conditions for obtaining large-size materials. Zinc can fill iron oxides, which are often porous and lack pores, making them a weak point in the material's resistance to oxidation and corrosion. Zinc filling reduces the diffusion rate of external atoms into the matrix and enhances the adhesion between the matrix and the oxide layer, thereby improving the material's resistance to oxidation and corrosion. However, excessive zinc can easily cause uneven material properties. It is preferable to use 0.04~0.06wt% zinc, which can improve the toughness of the material while obtaining good uniformity and resistance to oxidation and corrosion.

[0078] B: Boron can combine with nitrogen in steel to form boron nitride with a hexagonal crystal structure. Its bonding force along the z-axis is weak, making it prone to fracture under stress, thus providing conditions for chip breakage during machining. Besides boron nitride, boron can also form M2B-type borides with metallic elements. These borides are widely distributed and have higher thermal stability than carbides, pinning dislocations and hindering crack propagation, thereby strengthening mechanical properties. Borides also exhibit a much smaller tendency to grow and coarsen at high temperatures or during thermal fatigue compared to carbides, which is beneficial for extending material service life. In addition to forming individual borides, the proximity of boron atoms to carbon and nitrogen atoms in atomic numbers and sizes allows boron to substitute for carbon and nitrogen atoms in solid solutions, increasing their hardness and improving wear resistance through work hardening. Boron can also cleave carbides, refine grains, and improve material plasticity and toughness, which is extremely advantageous for manufacturing large-scale molds or parts. Meanwhile, the small size of boron atoms promotes their segregation at grain boundaries. While absorbing hydrogen atoms and purifying the grain boundaries, it also inhibits nucleation, improving the hardenability of the material. Compared to chromium and nickel, it has less impact on weldability, which is beneficial for product repair. However, excessive boron can easily lead to the formation of a network of borides, which, similar to network carbides, is detrimental to the properties of steel. This invention controls the (C+B) / (Al+Ti) ratio to below 0.11, preferably 0.01~0.02 wt% boron, thereby improving wear resistance while obtaining good chip-cutting performance.

[0079] Ca: The role of calcium in the technical solution of this invention is multifaceted. Calcium is an important deoxidizer, capable of combining with oxygen under extremely low oxygen partial pressures. It serves as an important supplement to deoxidizers such as aluminum and silicon. Further oxygen removal promotes the transformation of manganese sulfide from rod-shaped to spindle-shaped, improving the material's chip-cutting performance. Calcium can also directly alter the morphology of manganese by forming sulfides together. The effect is more significant when (Ca+Mn+Bi) / S > 6.38, resulting in composite inclusions with a large contact angle and low wettability to the matrix, tending to form spherical shapes and improving the mechanical uniformity of the material in all directions. When (Ca+Mn+Bi) / S exceeds 14.74, the improvement is limited. Calcium can also improve chip-cutting performance by directly combining with other inclusions: calcium combines with alumina inclusions to form dot-like inclusions such as calcium aluminate, whose hardness is much lower than that of alumina, which helps reduce tool wear and improve chip-cutting efficiency. During the cutting process, the inclusions modified by calcium form a composite oxide film on the rake face, which lubricates the material and reduces thermal adhesion wear between the tool and the workpiece. Calcium can also improve the fluidity of molten steel during smelting, purify grain boundaries during solidification, reduce sulfur segregation at grain boundaries, decrease the amount of network carbide precipitation at grain boundaries, lower the decomposition temperature of liquid carbides, and improve material toughness. However, the susceptibility of calcium to burn-off means that higher addition amounts increase process complexity. A calcium content of 0.010–0.015 wt% is preferred to achieve good toughness while improving machinability.

[0080] The present invention discloses a method for preparing a highly uniform, free-machining, high-temperature oxidation-resistant mold steel, comprising the following steps:

[0081] (1) Vacuum induction melting: The raw materials are sequentially filled into the vacuum induction furnace, the vacuum is drawn to a vacuum degree of less than 10 Pa and maintained for more than 5 minutes, the mixture is stirred thoroughly, the steel is tapped and cast, and the ingot is slowly cooled to obtain the ingot.

[0082] (2) Electroslag remelting: After removing impurities from the ingot obtained in step (1), the ingot is electroslag remelted under argon protection and then slowly cooled to obtain an electroslag ingot.

[0083] (3) Forging and forming: The electroslag ingots from step (2) are subjected to pre-forging homogenization treatment, followed by forging and forming treatment, stress-relief annealing or slow cooling to room temperature in the furnace;

[0084] (4) Heat treatment: Quenching and tempering are performed. The quenching temperature is 1055~1095℃ and the holding time is 1.1~1.3min / mm. After cooling, tempering is performed. The tempering temperature is 600~635℃ and the holding time is 2.5~3min / mm. After cooling, high uniformity high temperature oxidation resistant mold steel is obtained.

[0085] Furthermore, after the feeding is completed in step (1), the mixture is stirred and the steel is tapped within 8 minutes; excessively long smelting time can easily cause bismuth and calcium elements to burn off.

[0086] Furthermore, in step (1), the steel is tapped and cast under vacuum or argon protection to prevent elements such as hydrogen, oxygen, and nitrogen from re-entering the molten steel and remaining there as the steel solidifies.

[0087] Furthermore, the homogenization treatment temperature in step (3) is 1195~1295℃, the holding time is 2.0~2.3min / mm based on the diameter or thickness of the material, and the holding time is 5~28h; while promoting the reduction of element segregation in the core through diffusion, it avoids the edge parts from being coarsened due to excessively long high temperature time.

[0088] Further, in step (3), the initial forging temperature is 1130~1185℃, the final forging temperature is 970~990℃, and the total forging ratio is 7~9.

[0089] Further, in step (3), the forging process involves first upsetting and then drawing. The initial upsetting reduction is less than 20%, and then the reduction is increased to 50%. The drawing is carried out in a cross forging process along a fixed direction and a vertical direction. Increasing the reduction improves the forging efficiency. This process is carried out alternately to avoid the extension of defects such as cracks generated during forging.

[0090] Furthermore, the time for heat preservation during the forging process in step (3) is less than 3 hours.

[0091] Further, in step (4), the temperature is increased to the quenching temperature at a rate of 25~65℃ / h; controlling the heating rate promotes the shrinkage of the long length of the large-sized slender sulfides, which eventually break apart at part of the necking position, promoting their transformation into a spindle or quasi-spherical shape and improving the material's machinability.

[0092] Furthermore, step (4) employs atomized cooling with a nozzle pressure of 0.10~0.20 MPa. This avoids cracking caused by excessive cooling speed while increasing the cooling speed to be higher than air cooling, thus promoting the transformation of molybdenum-containing carbides from metastable M2C to stable and diffusely distributed MC or M6C.

[0093] The principle of this invention: High-temperature oxidation and corrosion resistant mold steel achieves excellent oxidation and corrosion resistance through the rational configuration of the content of major elements such as aluminum, chromium, molybdenum, and silicon. The effects of elements such as phosphorus and sulfur are controlled by manganese, magnesium, and boron, and further modified with rare earth elements to obtain good machinability. Finally, by combining the composition, smelting process, and forming process, a mold steel that is easy to process, resistant to high-temperature oxidation and corrosion, and possesses excellent comprehensive mechanical properties is prepared.

[0094] Aluminum-containing high-uniformity oxidation and corrosion resistant mold steel achieves its advantages by balancing the cost and effects of chemical composition and elements. It uses lower-cost elements such as aluminum to replace or partially replace high-cost elements such as cobalt, tungsten, or molybdenum. Combined with relatively simple equipment and heat treatment processes, it strengthens the material by forming multiple carbide composites and improves the uniformity of the material with elements such as zinc and magnesium. Ultimately, it produces a low-cost mold steel with excellent comprehensive mechanical properties, high hardness and wear resistance, and excellent oxidation and corrosion resistance.

[0095] High-uniformity, free-machining, high-temperature oxidation-resistant mold steel, through a rational configuration of its chemical composition, balancing the effects of various elements, and precise forming and heat treatment processes, has become a material suitable for manufacturing large-scale molds or parts. It exhibits good machinability, excellent resistance to high-temperature oxidation and corrosion, and superior comprehensive mechanical properties and uniform wear resistance, making it a promising material for industries such as glass, plastics, and metallurgy.

[0096] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0097] 1. High-temperature oxidation and corrosion resistant mold steel achieves excellent oxidation and corrosion resistance through the reasonable configuration of the content of major elements such as aluminum, chromium, molybdenum and silicon; the effects of elements such as phosphorus and sulfur are controlled by manganese, magnesium and boron, and further modified by rare earth elements, resulting in good machinability; the combination of composition control with smelting and forming processes makes it excellent in terms of high-temperature oxidation and corrosion resistance and comprehensive mechanical properties, and it can be used in plastics, glass, powder metallurgy or high-temperature corrosion complex working conditions.

[0098] 2. Aluminum-containing high-uniformity oxidation and corrosion resistant mold steel uses low-cost elements such as aluminum to replace or partially replace high-cost elements such as cobalt, tungsten, or molybdenum. Combined with relatively simple equipment and heat treatment processes, it strengthens the material by forming a variety of carbide composites and improves the uniformity of the material with elements such as zinc and magnesium. In the end, it obtains a low-cost mold steel with excellent comprehensive mechanical properties, high hardness and wear resistance, and excellent oxidation and corrosion resistance.

[0099] 3. High-uniformity, free-machining, high-temperature oxidation-resistant mold steel: Through reasonable chemical composition, balancing the effects of various elements, and precise forming and heat treatment processes, a material suitable for manufacturing large-scale molds or parts has been obtained. It exhibits good machinability, excellent resistance to high-temperature oxidation and corrosion, and superior comprehensive mechanical properties and uniform wear resistance. Attached Figure Description

[0100] Figure 1 The forging comparison diagrams are of the mold steel prepared by the present invention, wherein (a) is the forging diagram of the mold steel prepared in Example 22, and (b) is the forging diagram of the mold steel prepared in Comparative Example 5. Detailed Implementation

[0101] The present invention will be further described below with reference to specific embodiments.

[0102] Example 1

[0103] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1, wherein (La+Ce) / (B+P)=0.78, (Al+Ti+Mg) / (C+B)=5.02, and (Mn+Ca) / S=6.44.

[0104] Table 1. Composition (wt%) of high-temperature oxidation and corrosion resistant mold steel

[0105]

[0106] A method for preparing high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0107] (1) Vacuum induction melting: After cleaning the raw materials, iron, manganese, molybdenum, chromium, copper, chromium and other materials that are not easily burned are sequentially filled into the vacuum induction furnace. Low power heating is used for degassing, and the vacuum is evacuated until the vacuum degree in the furnace is lower than 10 Pa. The low vacuum promotes the discharge of nitrogen and reduces the formation of nitrogen oxides. Before melting, the raw materials are baked with low power and melted under vacuum or inert gas protection. After the raw materials melt, the molten steel is stirred for 3 minutes using electromagnetic induction to promote degassing and inclusion floating. The vacuum is then maintained for 20 minutes for further degassing. Then, 2 / 3 of the aluminum, titanium and carbon are added and electromagnetic stirring is used to promote the floating of inclusions. The added elements are used to adjust the composition of the molten steel to avoid excessive metal content, which would increase costs or affect subsequent casting. After stirring, the steel is tapped and cast using continuous casting. To avoid nozzle clogging, the tapping temperature is controlled at 1517℃. After slow cooling, steel ingots are obtained.

[0108] (2) Electroslag remelting: The head and flash of the billet often contain a lot of inclusions, which can easily cause the current to break down the crystallizer during subsequent electroslag remelting. They need to be removed by grinding or cutting. Since the mold steel contains carbon, chromium, aluminum and other elements that can easily make the structure coarse, the billet is not suitable for direct use. It needs to be treated by electroslag remelting to purify the molten steel, refine the grains and reduce the columnar crystal region. Before electroslag remelting, it needs to be baked to remove steam. Before charging the furnace, the arc igniter is placed in the center of the pad. The arc igniter needs to be cleaned and baked to remove oil, sewage and steam to avoid introducing impurities. 1 / 3 of the slag is placed around the arc igniter. The slag can be recycled and reused. The proportion of recycled slag exceeds 35%. After removing impurities from the steel ingot obtained in step (1), the Al2O3-TiO2-MgO ternary slag system is used. Al2O3 has the effect of improving the resistivity of the molten slag. At the same time, it can improve the acidity and alkalinity of the slag to maintain the sulfur content in the steel. During the smelting process, 0.5 wt% of electrolytic aluminum by weight of the billet is added in stages to ensure the uniformity of aluminum in the finished product. The slag also contains TiO2 and MgO to reduce element loss during burning and to adjust the slag viscosity and fluidity. MgO reduces magnesium loss from the steel and covers the slag pool surface to prevent oxygen absorption by the molten slag. After successful arc ignition, slag that has been baked for 3 hours to remove moisture is added in three stages to prevent the molten slag from solidifying and extinguishing the arc. The slag particle diameter is less than 16 mm. Heat control is implemented during the slag formation stage, following the principles of increasing current, stabilizing voltage, and rapid slag formation to ensure a smooth initial smelting process. This prevents unmelted solid slag from mixing with the molten steel and solidifying, improving the quality of the tail end of the electroslag ingot. After remelting and cooling, the electroslag ingot is obtained.

[0109] (3) The electroslag ingot obtained in step (2) is forged and rolled. During forging, due to the high aluminum and chromium content in the die steel, the material is prone to coarse structure during high-temperature holding. The temperature is slowly increased to 830℃ at 35℃ / h, and then rapidly increased to the initial forging temperature of 1075℃ at 149℃ / h. Due to the high content of carbide-forming elements in the material, coarse carbides are easily formed. Therefore, a forging ratio of 3.4 is adopted. The high aluminum, high carbon and metastable M2C carbides, the banded structure easily generated by high silicon and the inclusions caused by high sulfur content determine that horizontal forging needs to be strengthened during the forging process. Light hammer fast forging is adopted to crush carbides and avoid cracking. The reduction amount is less than 30% each time. The furnace heating time during forging is 2h to avoid coarse structure. The final forging temperature is 980℃. During rolling, the same heating rate as forging is used, except that the initial rolling temperature is 1150℃ and the final rolling temperature is also 980℃.

[0110] (4) Heat treatment: The forging and rolling blanks formed in step (3) are annealed at 865℃ for 3 hours; then air-cooled at 1045℃ for 1 hour, and air-cooled after holding at 275℃ for 2 hours, and then air-cooled after holding at 225℃ for 2.5 hours to obtain high-temperature oxidation and corrosion resistant mold steel.

[0111] Example 2

[0112] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.78, (Al+Ti+Mg) / (C+B)=4.55, and (Mn+Ca) / S=4.89. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0113] (1) Vacuum induction melting: continuous casting is adopted, and the tapping temperature is controlled at 1527℃, which is different from Example 1.

[0114] (2) Electroslag remelting: Same as in Example 1.

[0115] (3) The electrode ingot of step (2) is forged and rolled. Unlike Example 1, the forging process is carried out by slowly heating to 840°C at 40°C / h, and then rapidly heating to the initial forging temperature of 1090°C at 156°C / h, and the final forging temperature is 1000°C. The rolling process is carried out by slowly heating to 840°C at 40°C / h, and then rapidly heating to the initial rolling temperature of 1138°C at 156°C / h, and the final rolling temperature is 1000°C.

[0116] (4) Annealing is performed on the forged billet and rolled billet formed in step (3). The annealing process is 875℃×2.5h, followed by air cooling at 1015℃×3h, and then air cooling after heat preservation at 355℃×2h, and then air cooling after heat preservation at 225℃×2h to obtain high temperature oxidation corrosion resistant mold steel.

[0117] Example 3

[0118] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.79, (Al+Ti+Mg) / (C+B)=5.69, and (Mn+Ca) / S=3.60. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0119] (1) Vacuum induction melting: continuous casting is adopted, and the tapping temperature is controlled at 1537℃, which is different from Example 1.

[0120] (2) Electroslag remelting: Same as in Example 1.

[0121] (3) The electrode ingot of step (2) is forged and rolled. Unlike Example 1, during forging, the temperature is slowly increased to 850°C at 45°C / h, and then rapidly increased to the initial forging temperature of 1135°C at 163°C / h, and the final forging temperature is 1030°C. During rolling, the temperature is slowly increased to 850°C at 45°C / h, and then rapidly increased to the initial rolling temperature of 1100°C at 163°C / h, and the final rolling temperature is 1030°C.

[0122] (4) Annealing is performed on the forged billet and rolled billet formed in step (3). The annealing process is 900℃×1h, followed by air quenching at 1025℃×1h and tempering at 255℃×2.5h twice to obtain high temperature oxidation corrosion resistant mold steel.

[0123] Example 4

[0124] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.75, (Al+Ti+Mg) / (C+B)=7.06, and (Mn+Ca) / S=3.90. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0125] (1) Vacuum induction melting: Unlike Example 1, the casting method is mold casting. A half mold is used. Before casting, the half mold is baked at 300°C. After casting, it is kept in a vacuum chamber until it is completely solidified. When the billet is cooled to 450°C, it is put into the furnace to be slowly cooled to room temperature to obtain the ingot.

[0126] (2) Electroslag remelting: The ingot obtained in step (1) is deburred and the welding electrode is electroslag remelted, the same as in Example 1.

[0127] (3) The electrode ingot of step (2) is forged and rolled. Unlike Example 1, the forging process is carried out by slowly heating to 830°C at 35°C / h, and then rapidly heating to the initial forging temperature of 1165°C at 149°C / h, and the final forging temperature is 980°C. The rolling process is carried out by slowly heating to 830°C at 35°C / h, and then rapidly heating to the initial rolling temperature of 1075°C at 149°C / h, and the final rolling temperature is 980°C.

[0128] (4) Annealing is performed on the forged billet and rolled billet formed in step (3). The annealing process is 865℃×2.5h, followed by air quenching at 1045℃×1.5h and tempering at 230℃×2h twice to obtain high temperature oxidation corrosion resistant mold steel.

[0129] Example 5

[0130] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.78, (Al+Ti+Mg) / (C+B)=6.77, and (Mn+Ca) / S=4.13. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0131] (1) Vacuum induction melting: Unlike Example 4, the casting method is mold casting. A half mold is used. Before casting, the half mold is baked at 325°C. After casting, it is kept in a vacuum chamber until it is completely solidified. When the billet is cooled to 425°C, it is put into the furnace to be slowly cooled to room temperature to obtain the ingot.

[0132] (2) Electroslag remelting: The ingot obtained in step (1) is deburred and the welding electrode is electroslag remelted, the same as in Example 1.

[0133] (3) The electrode ingot from step (2) is forged and rolled. Unlike Example 1, during forging, the temperature is slowly increased to 840°C at 40°C / h, and then rapidly increased to the initial forging temperature of 1081°C at 156°C / h, with the final forging temperature being 1000°C. During rolling, the temperature is slowly increased to 840°C at 40°C / h, and then rapidly increased to the initial rolling temperature of 1065°C at 156°C / h, with the final rolling temperature being 1000°C.

[0134] (4) Annealing is performed on the forged billet and rolled billet formed in step (3). The annealing process is 875℃×1.5h, followed by air cooling at 1015℃×1h, tempering at 275℃×2h, and tempering at 225℃×2.5h to obtain high-temperature oxidation and corrosion resistant mold steel.

[0135] Example 6

[0136] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.98, (Al+Ti+Mg) / (C+B)=6.29, and (Mn+Ca) / S=4.21. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0137] (1) Vacuum induction melting: Unlike Example 4, the casting method is mold casting. A half mold is used. Before casting, the half mold is baked at 350°C. After casting, it is kept in a vacuum chamber until it is completely solidified. When the billet is cooled to 400°C, it is put into the furnace to be slowly cooled to room temperature to obtain the ingot.

[0138] (2) Electroslag remelting: The ingot obtained in step (1) is deburred and the welding electrode is electroslag remelted, the same as in Example 1.

[0139] (3) The electrode ingot from step (2) is forged and rolled. Unlike Example 1, during forging, the temperature is slowly increased to 850°C at 45°C / h, and then rapidly increased to the initial forging temperature of 1120°C at 163°C / h, with the final forging temperature being 1030°C. During rolling, the temperature is slowly increased to 850°C at 45°C / h, and then rapidly increased to the initial rolling temperature of 1100°C at 163°C / h, with the final rolling temperature being 1150°C.

[0140] (4) Annealing is performed on the forged billet and rolled billet formed in step (3). The annealing process is 900℃×1h, followed by air cooling at 1015℃×2.5h, tempering at 325℃×2h, and then tempering at 225℃×2.5h to obtain high-temperature oxidation and corrosion resistant mold steel.

[0141] Example 7

[0142] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.55, (Al+Ti+Mg) / (C+B)=4.57, and (Mn+Ca) / S=18.10. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0143] (1) Vacuum induction melting: Same as in Example 1, using continuous casting.

[0144] (2) Electroslag remelting: Same as in Example 1.

[0145] (3) The electrode ingot from step (2) is forged and rolled. Unlike Example 1, during forging, the temperature is slowly increased to 820°C at 30°C / h, and then rapidly increased to the initial forging temperature of 1140°C at 120°C / h, with the final forging temperature being 950°C. During rolling, the temperature is slowly increased to 820°C at 30°C / h, and then rapidly increased to the initial rolling temperature of 1140°C at 120°C / h, with the final rolling temperature being 950°C.

[0146] (4) Anneal the forged billet and rolled billet formed in step (3) for 855℃×1h, and then air quench at 1025℃×1h + tempering at 255℃×2.5h twice to obtain high temperature oxidation corrosion resistant mold steel.

[0147] Example 8

[0148] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=1.80, (Al+Ti+Mg) / (C+B)=4.68, and (Mn+Ca) / S=15.11. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0149] (1) Vacuum induction melting: Same as in Example 4, using the die casting method.

[0150] (2) Electroslag remelting: Same as in Example 1.

[0151] (3) The electrode ingot from step (2) is forged and rolled. Unlike Example 1, during forging, the temperature is slowly increased to 830°C at 35°C / h, and then rapidly increased to the initial forging temperature of 1120°C at 149°C / h, with the final forging temperature being 980°C. During rolling, the temperature is slowly increased to 830°C at 35°C / h, and then rapidly increased to the initial rolling temperature of 1100°C at 149°C / h, with the final rolling temperature being 980°C.

[0152] (4) Anneal the forged billet and rolled billet formed in step (3). The annealing process is to hold at 865℃ for 3 hours. Then, air cool at 1045℃ for 1 hour, and air cool after holding at 275℃ for 2 hours, and then air cool after holding at 225℃ for 2.5 hours to obtain high temperature oxidation and corrosion resistant mold steel.

[0153] Example 9

[0154] A high-temperature oxidation and corrosion resistant mold steel comprises the following components by weight percentage, as shown in Table 1: (La+Ce) / (B+P)=0.86, (Al+Ti+Mg) / (C+B)=6.31, and (Mn+Ca) / S=12.67. The preparation method of the high-temperature oxidation and corrosion resistant mold steel includes the following steps:

[0155] (1) Vacuum induction melting: Same as in Example 4, using the die casting method.

[0156] (2) Electroslag remelting: Same as in Example 1.

[0157] (3) The electrode ingot of step (2) is forged and rolled. Unlike Example 1, during forging, the temperature is slowly increased to 830°C at 55°C / h, and then rapidly increased to the initial forging temperature of 1100°C at 149°C / h, and the final forging temperature is 1050°C. During rolling, the temperature is slowly increased to 830°C at 55°C / h, and then rapidly increased to the initial rolling temperature of 1065°C at 149°C / h, and the final rolling temperature is 1050°C.

[0158] (4) Annealing is performed on the forged billet and rolled billet formed in step (3). The annealing process is 910℃×1.5h, followed by air cooling at 1015℃×1h, tempering at 275℃×2h, and tempering at 225℃×2.5h to obtain high-temperature oxidation and corrosion resistant mold steel.

[0159] Material property testing

[0160] The high-temperature oxidation and corrosion resistant mold steels obtained in Examples 1-6 and Comparative Examples 1-3 were subjected to material property tests according to national standards. The Rockwell hardness of the materials is shown in column HD of Table 2. The high-temperature oxidation resistance was tested at 1000℃, and the weight gain after 100 h of oxidation is shown in column HO of Table 2.

[0161] The materials from Examples 1-6 and Comparative Examples 1-3 were processed into cylindrical samples with a diameter of 10 mm and a length of 5 mm, and connected to copper wires. Except for the working surface, the remaining parts of the samples were isolated from the corrosive liquid. After polishing the sample surface, it was etched with a 3.5% NaCl aqueous solution. The dynamic polarization curves were tested in an electrochemical workstation to obtain the self-corrosion potential (V) and self-corrosion current (A / cm). 2 The values ​​are shown in columns CV and CA of Table 2, respectively.

[0162] Table 2. Process parameters and performance indicators of mold steels in Examples 1-6 and Comparative Examples 1-3

[0163]

[0164] In summary, Example 1 is the optimal embodiment of the method for preparing high-temperature oxidation and corrosion resistant mold steel of the present invention. Comparison shows that the oxidation and corrosion resistance of Comparative Examples 1, 2, and 3 is significantly deteriorated. Furthermore, the hardness of Examples 1-6 changes drastically with hot working temperature, while the hardness of Examples 1-6 fluctuates less with hot working temperature, which is beneficial for achieving a higher yield and exhibiting higher stability under complex working conditions. In addition, the corrosion resistance of Example 1 is significantly better than that of Comparative Examples 1-3.

[0165] Application examples

[0166] The material prepared by the method of this invention exhibits excellent comprehensive properties. Compared with Comparative Example 3 and commercial 4Cr13Ni, taking the material described in Example 1 as an example, the mold prepared using this material is used in the manufacture of glass lamps. During the pressing process, it exhibits good resistance to thermal fatigue. The machinability resulting from the synergy between the components results in excellent surface roughness of the mold, making the glass easy to demold and dimensionally accurate, reducing design tolerances by 15%. Furthermore, when insert injection molding is used between the glass and the plastic parts, this material exhibits good resistance to oxidation and corrosion.

[0167] Example 7

[0168] A high-uniformity, oxidation-resistant mold steel containing aluminum comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.07, (Ca+Zn+Mg) / (Y+Zr)=0.84. A comparative example is 2Cr25Ni20Si2 mold steel, commonly used in similar applications.

[0169] Table 3 Chemical composition (wt%) of aluminum-containing high-uniformity oxidation and corrosion resistant mold steel

[0170]

[0171] A method for preparing an aluminum-containing, highly uniform, oxidation- and corrosion-resistant mold steel includes the following steps. The preparation method of Comparative Example 4 is shown in the national standard GB / T 1299-2014.

[0172] (1) Vacuum induction melting: The materials are added in batches to ensure the accuracy of the chemical composition of the materials. First, the iron, chromium, manganese, tungsten and cobalt alloy materials are loaded into the furnace, and the refractory tungsten and cobalt are placed at the bottom. The vacuum is evacuated to 20 Pa and then the power is supplied to melt. When the vacuum degree is lower than 15 Pa, all the raw materials in the furnace are melted. The vacuum degree is further reduced to 10 Pa and maintained for more than 15 minutes to fully degas. Then, 2 / 3 of the vanadium, aluminum, zinc, yttrium and carbon are added. The power is increased and stirred. After melting, the remaining components including calcium are added and stirred to accelerate melting. The added calcium is wrapped in aluminum foil. The steel is tapped and cast in 5~8 minutes.

[0173] (2) Electroslag remelting: After removing impurities from the ingot obtained in step (1), it is remelted and sent to a slow cooling pit to cool to room temperature.

[0174] (3) Forging: The electrode ingot obtained in step (2) is heated to 1010℃ for homogenization treatment, and then heated to the initial forging temperature of 1145℃ at 45℃ / h and held for 1h before being taken out of the furnace for forging. First, it is upset, then lightly hammered forging with a reduction of 20%, and then forged with a reduction of 40%. During forging, the furnace temperature is lowered to 1055℃. After the first forging is completed, it is put into the furnace for holding and then forging is carried out. The final forging temperature is 985℃, and it is slowly cooled to room temperature with the furnace.

[0175] (4) Heat treatment: The formed material is processed into a round blank with a diameter of 240 mm. The quenching temperature is 1065℃ and held for 2 hours. Atomization cooling is adopted with a nozzle pressure of 0.35 MPa. Atomization cooling is performed twice. Then tempering treatment is carried out at a tempering temperature of 580℃ and a holding time of 2.5 hours. Tempering is performed once to obtain high uniformity oxidation and corrosion resistant mold steel.

[0176] Example 8

[0177] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.04, (Ca+Zn+Mg) / (Y+Zr)=0.87. Its preparation method includes the following steps.

[0178] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0179] (3) Forging: Unlike Example 7, the initial forging temperature was 1155℃, and the furnace was kept at that temperature for 1 hour before forging. The final forging temperature was 995℃, and the furnace was slowly cooled to room temperature.

[0180] (4) Heat treatment: Unlike Example 7, the quenching temperature was 1080℃ and held for 3 hours, followed by tempering treatment at 590℃ for 3.5 hours. The tempering was repeated twice to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0181] Example 9

[0182] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.06, (Ca+Zn+Mg) / (Y+Zr)=0.83. Its preparation method includes the following steps.

[0183] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0184] (3) Forging: Unlike Example 7, the initial forging temperature was 1165℃, and the furnace was kept at that temperature for 1 hour before forging. The final forging temperature was 1005℃, and the furnace was slowly cooled to room temperature.

[0185] (4) Heat treatment: Unlike Example 7, the formed material was processed into a round blank with a diameter of 350 mm, quenched at 1095 °C for 3 h, and then tempered at 600 °C for 3 h. The tempering was repeated twice to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0186] Example 10

[0187] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.07, (Ca+Zn+Mg) / (Y+Zr)=1.12. Its preparation method includes the following steps.

[0188] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0189] (3) Forging: Unlike Example 7, the initial forging temperature was 1175℃, and the furnace was kept at that temperature for 1 hour before forging. The final forging temperature was 985℃, and the furnace was slowly cooled to room temperature.

[0190] (4) Heat treatment: Unlike Example 7, the formed material was processed into a round blank with a diameter of 400 mm, quenched at 1105℃ and held for 3.5 h, and then tempered at 610℃ for 3.5 h. The tempering was repeated 3 times to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0191] Example 11

[0192] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.08, (Ca+Zn+Mg) / (Y+Zr)=0.94. Its preparation method includes the following steps.

[0193] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0194] (3) Forging: Unlike Example 7, the initial forging temperature was 1185℃, and the furnace was kept at that temperature for 1 hour before forging. The final forging temperature was 995℃, and the furnace was slowly cooled to room temperature.

[0195] (4) Heat treatment: Unlike Example 7, the formed material was processed into a round blank with a diameter of 450 mm, quenched at 1065℃ and held for 4 hours, and then tempered at 620℃ for 4 hours. The tempering was repeated 3 times to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0196] Example 12

[0197] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.09, (Ca+Zn+Mg) / (Y+Zr)=0.85. Its preparation method includes the following steps.

[0198] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0199] (3) Forging: Unlike Example 7, the initial forging temperature was 1195℃, and the furnace was kept at that temperature for 1 hour before forging. The final forging temperature was 1005℃, and the furnace was slowly cooled to room temperature.

[0200] (4) Heat treatment: Unlike Example 7, the formed material was processed into a round blank with a diameter of 500 mm, quenched at 1105℃ and held for 4 h, and then tempered at 630℃ for 3.5 h. The tempering was repeated 3 times to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0201] Example 13

[0202] A high-uniformity, oxidation-resistant mold steel containing aluminum comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.07, (Ca+Zn+Mg) / (Y+Zr)=0.98. Its preparation method includes the following steps.

[0203] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0204] (3) Forging: Unlike Example 7, the initial forging temperature was 1165℃, and the furnace was kept at that temperature for 1 hour before forging. The final forging temperature was 985℃, and the furnace was slowly cooled to room temperature.

[0205] (4) Heat treatment: Unlike Example 7, the formed material was processed into a round blank with a diameter of 250 mm, quenched at 1080°C for 2 hours, and then tempered at 580°C for 2.5 hours. The tempering was performed once to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0206] Example 14

[0207] A high-uniformity, oxidation-resistant mold steel containing aluminum comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.07, (Ca+Zn+Mg) / (Y+Zr)=0.98. Its preparation method includes the following steps.

[0208] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0209] (3) Forging: Unlike Example 7, the formed material is processed into a round billet with a diameter of 250 mm. It is forged at an initial forging temperature of 1165℃ and held for 1 hour before being taken out of the furnace for forging. The final forging temperature is 995℃, and it is slowly cooled to room temperature in the furnace.

[0210] (4) Heat treatment: Unlike Example 7, the quenching temperature was 1080℃ and held for 2.5h, followed by tempering treatment at 610℃ for 3h, and tempering was performed twice to obtain high uniformity oxidation and corrosion resistant mold steel.

[0211] Example 15

[0212] A high-uniformity, oxidation-resistant mold steel containing aluminum comprises the following components by weight percentage, as shown in Table 3: (Mn+Zn) / (Al+Si)=0.07, (Ca+Zn+Mg) / (Y+Zr)=0.98. Its preparation method includes the following steps.

[0213] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0214] (3) Forging: Unlike Example 7, the formed material is processed into a round billet with a diameter of 250 mm. It is forged at an initial forging temperature of 1165℃ and held for 1 hour before being taken out of the furnace for forging. The final forging temperature is 1005℃, and it is slowly cooled to room temperature in the furnace.

[0215] (4) Heat treatment: Unlike Example 7, the quenching temperature was 1080℃ and held for 3 hours, followed by tempering treatment at 640℃ for 3.5 hours. The tempering was repeated 3 times to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0216] Example 16

[0217] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3, wherein (Mn+Zn) / (Al+Si)=0.05 and (Ca+Zn+Mg) / (Y+Zr)=0.56. Its preparation method includes the following steps.

[0218] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0219] (3) Forging: Unlike Example 7, the formed material is processed into a round billet with a diameter of 510mm. It is forged at an initial forging temperature of 1175℃ and held for 1 hour before being taken out of the furnace for forging. The final forging temperature is 985℃, and it is slowly cooled to room temperature in the furnace.

[0220] (4) Heat treatment: Unlike Example 7, the quenching temperature was 1090℃ and held for 4 hours, followed by tempering treatment at 580℃ for 3.5 hours. The tempering was performed once to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0221] Example 17

[0222] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3, wherein (Mn+Zn) / (Al+Si)=0.05 and (Ca+Zn+Mg) / (Y+Zr)=0.56. Its preparation method includes the following steps.

[0223] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0224] (3) Forging: Unlike Example 7, the formed material is processed into a round billet with a diameter of 510mm. It is forged at an initial forging temperature of 1175℃ and held for 1 hour before being taken out of the furnace for forging. The final forging temperature is 995℃, and it is slowly cooled to room temperature in the furnace.

[0225] (4) Heat treatment: Unlike Example 7, the quenching temperature was 1090℃ and held for 4.5h, followed by tempering treatment at 610℃ for 4h, and tempering was performed twice to obtain high uniformity oxidation and corrosion resistant mold steel.

[0226] Example 18

[0227] A high-uniformity, oxidation-resistant, and corrosion-resistant aluminum-containing mold steel comprises the following components by weight percentage, as shown in Table 3, wherein (Mn+Zn) / (Al+Si)=0.05 and (Ca+Zn+Mg) / (Y+Zr)=0.56. Its preparation method includes the following steps.

[0228] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 7.

[0229] (3) Forging: Unlike Example 7, the formed material is processed into a round billet with a diameter of 510 mm. It is forged at an initial forging temperature of 1175℃ and held for 1 hour before being taken out of the furnace for forging. The final forging temperature is 1005℃, and it is slowly cooled to room temperature in the furnace.

[0230] (4) Heat treatment: Unlike Example 7, the quenching temperature was 1090℃ and held for 5 hours, followed by tempering treatment at 640℃ for 4.5 hours. The tempering was repeated 3 times to obtain a mold steel with high uniformity and resistance to oxidation and corrosion.

[0231] Material property testing

[0232] The materials obtained in Examples 7-18 and Comparative Example 4 were subjected to high-temperature oxidation weight gain and hardness tests in an atmospheric environment at 1000 °C for 100 h. The hardness test locations were the center, one-quarter of the way from the center to the surface, one-half of the way from the center to the surface, three-quarters of the way from the center to the surface, and the surface. The oxidation weight gain and average hardness data are detailed in Table 4.

[0233] Table 4. Material property test data of Examples 7-18 and Comparative Example 4

[0234]

[0235] In summary, Example 7 is the optimal embodiment of the method for preparing aluminum-containing high-uniformity oxidation-resistant and corrosion-resistant mold steel. Compared with Comparative Example 4, it has significant advantages in hardness, uniformity, and oxidation resistance.

[0236] Application examples

[0237] The aluminum-containing, highly uniform, oxidation- and corrosion-resistant mold steel prepared by this invention exhibits excellent comprehensive performance. Taking Example 7 as an example, the material prepared through the combination of components and a process matching the components results in a 35% reduction in cost and a 15% increase in service life for glass molds or steel rollers compared to the commonly used 2Cr25Ni20Si2 material. When preparing curved or arc-shaped glass under pressure, the material in this example demonstrates better wear resistance under higher pressure at the curved portion, reducing the number of cracks by approximately 12%.

[0238] Therefore, aluminum-containing, highly uniform, and oxidation-resistant mold steel exhibits a reasonable chemical composition and low cost. Combined with appropriate manufacturing processes, it can achieve excellent uniformity, hardness, and high-temperature oxidation-corrosion resistance. It can partially replace heat-resistant steel or stainless steel in applications involving direct contact with flames or oxidizing / corrosive media. It can also be used in the preparation of ceramics, glass, or plastics, reducing manufacturing costs while meeting application requirements.

[0239] Example 19

[0240] A high-uniformity, free-machining, high-temperature oxidation-resistant mold steel comprises the following components by weight percentage, wherein (C+B) / (Al+Ti)=0.09, (Ca+Mn+Bi) / S=10.03, as detailed in Table 5. Comparative Example 5 is a modified 2Cr25Ni20Si2 mold steel.

[0241] Table 5. Composition (wt%) of high-uniformity, free-machining, high-temperature oxidation-resistant mold steel

[0242]

[0243] A method for preparing a highly uniform, free-machining, high-temperature oxidation-resistant mold steel includes the following steps. The preparation process of Comparative Example 5 refers to the preparation method of 2Cr25Ni20Si2 in the national standard GB / T 1299-2014.

[0244] (1) Vacuum induction melting: Iron, chromium, manganese, tungsten and molybdenum alloy raw materials are sequentially filled into the furnace, low power is supplied for baking and vacuuming is started. When the vacuum degree drops below 50 Pa, the power is increased to melt the raw materials. Before the raw materials are completely melted, the vacuum degree is reduced to below 15 Pa. After the raw materials are completely melted, the vacuum degree is reduced to 10 Pa to fully exhaust the gas in the vacuum furnace to avoid oxygen forming too many oxide inclusions or causing too many rod-shaped MnS. The vacuum degree is reduced, 2 / 3 of aluminum and silicon are added for alloying and deoxidation and nitrogen fixation. After melting, sulfur-containing raw materials are added. After stirring, copper, carbon, titanium and aluminum are added. After melting, the vacuum degree is kept below 10 Pa for more than 5 minutes. Finally, the remaining raw materials such as zinc, calcium and bismuth are added. The power is increased for stirring and steel is prepared to be tapped and cast within 8 minutes. Argon gas is used for protection during the tapping and casting process. The steel ingot is slowly cooled to room temperature.

[0245] (2) Electroslag remelting: The steel ingot from step (1) is electroslag remelted and then sent to a slow cooling pit to be slowly cooled to room temperature to obtain an electroslag ingot.

[0246] (3) Forging and forming: The electroslag ingot obtained in step (2) is homogenized at 1195℃ for 28 hours, and then cooled to the initial forging temperature of 1130℃. It is first upset and then drawn. During the first upset, the reduction is no more than 20%. After the initial forging of the structure, the reduction is increased to 50% to improve efficiency. After upset, it is drawn. During the drawing process, it is first forged along a fixed direction, and then forged at a 90℃ angle to the original direction. The alternation is carried out, and chamfering and corner crack removal operations are added during the alternation. The final forging temperature is 970℃. It is forged 3 times. The reheating time in the furnace during each forging is 3 hours. The total forging ratio is 7. After forging, it is cooled to room temperature with the furnace to obtain the forging billet.

[0247] (4) Heat treatment: The forging billet from step (3) is processed into a round billet with a diameter of 530 mm and loaded into the furnace. The temperature is raised to the quenching temperature of 1055℃ at a rate of 25℃ / h and held for 9.7h. Then it is cooled by atomization. The pressure of the atomizing nozzle is 0.2MPa, adjusted to 0.1MPa after 30s, and then adjusted to 0.2MPa after 50s until it is cooled to room temperature. Then it is tempered at a tempering temperature of 600℃ and held for 22.5h. After cooling to room temperature, a high-uniformity high-temperature oxidation resistant mold steel is obtained.

[0248] Example 20

[0249] A high-uniformity, free-machining, high-temperature oxidation-resistant mold steel comprises the following components by weight percentage, wherein (C+B) / (Al+Ti)=0.09, (Ca+Mn+Bi) / S=8.95, as detailed in Table 5, and its preparation method includes the following steps.

[0250] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 19.

[0251] (3) Forging: Unlike Example 19, the electroslag ingot obtained in step (2) was homogenized at 1200°C for 25 hours, then cooled to the initial forging temperature of 1140°C and the final forging temperature of 975°C. It was forged 3 times, and the reheating time in the furnace was 3 hours each time during forging. The total forging ratio was 7. After forging, it was cooled to room temperature with the furnace to obtain the forged billet.

[0252] (4) Heat treatment: The forging billet from step (3) is processed into a round billet with a diameter of 500 mm and loaded into the furnace. The temperature is raised to the quenching temperature of 1065℃ at a rate of 30℃ / h, held for 9.2h, then atomized and cooled to room temperature before tempering. The tempering temperature is 615℃ and the holding time is 21h. After cooling to room temperature, a high uniformity high temperature oxidation resistant mold steel is obtained.

[0253] Example 21

[0254] A high-uniformity, free-machining, high-temperature oxidation-resistant mold steel comprises the following components by weight percentage, wherein (C+B) / (Al+Ti)=0.09, (Ca+Mn+Bi) / S=7.71, as detailed in Table 5, and its preparation method includes the following steps.

[0255] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 19.

[0256] (3) Forging: Unlike Example 19, the electroslag ingot obtained in step (2) was homogenized at 1225°C for 20 hours, then cooled to the initial forging temperature of 1155°C and the final forging temperature of 980°C. It was forged 4 times, and the reheating time in the furnace during each forging was 3 hours. The total forging ratio was 8. After forging, it was cooled to room temperature in the furnace to obtain the forging billet.

[0257] (4) Heat treatment: The forging billet from step (3) is processed into a round billet with a diameter of 450 mm and loaded into the furnace. The temperature is raised to the quenching temperature of 1075℃ at a rate of 50℃ / h and held for 9.0h. Then, it is atomized and cooled to room temperature and tempered at a tempering temperature of 630℃ for 19.5h. After cooling to room temperature, a high-uniformity high-temperature oxidation resistant mold steel is obtained.

[0258] Example 22

[0259] A high-uniformity, free-machining, high-temperature oxidation-resistant mold steel comprises the following components by weight percentage, wherein (C+B) / (Al+Ti)=0.08, (Ca+Mn+Bi) / S=7.48, as detailed in Table 5, and its preparation method includes the following steps.

[0260] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 19.

[0261] (3) Forging: Unlike Example 19, the electroslag ingot obtained in step (2) was homogenized at 1250°C for 15 hours, then cooled to the initial forging temperature of 1165°C and the final forging temperature of 980°C. It was forged 3 times, and the reheating time in the furnace was 3 hours each time. The total forging ratio was 8. After forging, it was cooled to room temperature in the furnace to obtain the forging billet.

[0262] (4) Heat treatment: The forging billet from step (3) is processed into a round billet with a diameter of 400 mm and loaded into the furnace. The temperature is raised to the quenching temperature of 1075℃ at a rate of 50℃ / h and held for 8.0h. Then, it is atomized and cooled to room temperature and tempered at a tempering temperature of 610℃ for 20h. After cooling to room temperature, a high-uniformity high-temperature oxidation resistant mold steel is obtained.

[0263] Example 23

[0264] A high-uniformity, free-machining, high-temperature oxidation-resistant mold steel comprises the following components by weight percentage, wherein (C+B) / (Al+Ti)=0.08, (Ca+Mn+Bi) / S=10.64, as detailed in Table 5, and its preparation method includes the following steps.

[0265] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 19.

[0266] (3) Forging: Unlike Example 19, the electroslag ingot obtained in step (2) was homogenized at 1275°C for 10 hours, then cooled to the initial forging temperature of 1175°C and the final forging temperature of 985°C. It was forged 3 times, and the reheating time in the furnace during each forging was 3 hours. The total forging ratio was 9. After forging, it was cooled to room temperature in the furnace to obtain the forging billet.

[0267] (4) Heat treatment: The forging billet from step (3) is processed into a round billet with a diameter of 310 mm and loaded into the furnace. The temperature is raised to the quenching temperature of 1095℃ at a rate of 60℃ / h and held for 6.2h. Then, it is atomized and cooled to room temperature and tempered at a temperature of 625℃ for 10.8h. After cooling to room temperature, a high-uniformity high-temperature oxidation resistant mold steel is obtained.

[0268] Example 24

[0269] A high-uniformity, free-machining, high-temperature oxidation-resistant mold steel comprises the following components by weight percentage, wherein (C+B) / (Al+Ti)=0.07, (Ca+Mn+Bi) / S=10.32, as detailed in Table 5, and its preparation method includes the following steps.

[0270] (1) Vacuum induction melting and (2) electroslag remelting: Same as in Example 19.

[0271] (3) Forging: Unlike Example 19, the electroslag ingot obtained in step (2) was homogenized at 1295°C for 5 hours, then cooled to the initial forging temperature of 1185°C and the final forging temperature of 990°C. It was forged 5 times, and the reheating time in the furnace during each forging was 3 hours. The total forging ratio was 9. After forging, it was cooled to room temperature in the furnace to obtain the forging billet.

[0272] (4) Heat treatment: The forging billet from step (3) is processed into a round billet with a diameter of 250 mm and loaded into the furnace. The temperature is raised to the quenching temperature of 1095℃ at a rate of 65℃ / h and held for 4.5h. Then, it is cooled by atomization. The pressure cycle of the atomizing nozzle is set to 0.1 MPa for 30 s and then 0.2 MPa for 20 s. It is cooled to room temperature for tempering. The tempering temperature is 625℃ and the holding time is 10.8h. After cooling to room temperature, a high-uniformity high-temperature oxidation resistant mold steel is obtained.

[0273] Material property testing

[0274] The materials obtained in Examples 19-24 and Comparative Example 5 were subjected to low-magnification testing according to GB / T226-2015 "Acid Etching Test Method for Low-Magnification Structure and Defects of Steel". The central porosity was all less than grade 2.5, significantly better than the grade 3.0 of Comparative Example 5. Hardness testing was performed from the center to the edge, obtaining the average hardness and the maximum hardness difference. The examples showed good hardness uniformity. The corrosion rate of the samples was determined by spraying with a 5% NaCl aqueous solution for 72 hours, and the corrosion rate was found to be no greater than 0.02 mm / a. The material performance test results are detailed in Table 6. Figure 1 As shown, Figure 1 Image a shows the forging process of the die steel prepared in Example 22. Figure 1 Image b shows the forging process of the mold steel prepared in Comparative Example 5. It can be concluded that the present invention adopts a forming process that matches the chemical composition, resulting in fewer and smaller defects such as cracks during the forging process.

[0275] Table 6. Material property test results obtained from Examples 19-24 and Comparative Example 5

[0276]

Claims

1. A high-temperature oxidation and corrosion resistant die steel, characterized in that, It includes the following components by weight percentage: C: 0.54~0.81%, Si: 1.04~1.25%, Mn: 0.75~1.09%, Cr: 18.3~21.7%, Mo: 0.08~0.12%, Al: 2.70~5.12%, Ti: 0.06~0.09%, Cu: 0.02~0.04%, B: 0.02~0.06%, Ca: 0.005~0.007%, M g: 0.05~0.08%, La: 0.02~0.04%, Ce: 0.02~0.05%, P: 0.031~0.041%, S: 0.17~0.26%, with the balance being Fe and unavoidable impurities, and satisfying 0.04<(La+Ce) / (B+P)<1.76, 3.23<(Al+Ti+Mg) / (C+B)<9.45, 2.90<(Mn+Ca) / S<6.

45.

2. A method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 1, characterized in that, Includes the following steps: (1) Vacuum induction melting: The raw materials are sequentially filled into the vacuum induction furnace, and the vacuum inside the furnace is evacuated to a level lower than 10 Pa. Melting is carried out under vacuum or inert gas protection. Elements are added to adjust the composition of the molten steel. After stirring, the steel is tapped and cast using continuous casting or ingot casting. After slow cooling, steel ingots are obtained. (2) Electroslag remelting: After removing impurities from the steel ingot obtained in step (1), the slag material is added in batches using the Al2O3-TiO2-MgO ternary slag system, and the ingot is obtained after remelting and cooling. (3) Forging or rolling; (4) Heat treatment: Anneal the forged or rolled billet after step (3) forming, and then heat treatment or quenching and tempering treatment.

3. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, The step (1) is cast by continuous casting, and the tapping temperature is 1517~1537℃.

4. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, When casting by mold casting in step (1), the mold is baked at 300~350℃.

5. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, Before the temperature of the billet obtained by casting in step (1) drops to 400℃, it is placed in a heating furnace for slow cooling or in a heat preservation pit for slow cooling.

6. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, In step (2), the amount of electrolytic aluminum added in batches is 0.5~0.6wt% of the billet weight.

7. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, In step (3), the electroslag ingot is slowly heated to 830-850℃ at 35-45℃ / h, and then rapidly heated to the initial forging temperature of 1075-1165℃ or the initial rolling temperature of 1065-1150℃ at 149-163℃ / h. The final forging temperature or final rolling temperature is 980-1030℃.

8. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 7, characterized in that, In step (3), the initial reduction during forging is less than 30%, the final forging ratio is 3.5~4.5, and the furnace heating time during forging or rolling is less than 2.5h.

9. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, The annealing temperature in step (4) is 865~900℃, held for 1~3 hours, and then cooled in the furnace.

10. The method for preparing high-temperature oxidation and corrosion resistant mold steel according to claim 2, characterized in that, In step (4), the quenching and tempering treatments are performed with a quenching temperature of 1015~1045℃ and a holding time of 1~3h. After air cooling or wind cooling, the heat treatment is performed immediately with a tempering temperature of 225~355℃ and a holding time of 2.5~5.5h. The heat treatment is repeated multiple times.

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