Patent Application
20240093339
The invention belongs to the field of materials, specifically, the technical field of stainless steel materials, in particular to high-entropy austenitic stainless steels and their preparation methods.
Stainless steels are widely used in many industrial fields such as aviation, aerospace, marine, and nuclear industries due to their good corrosion and oxidation resistance. However, excellent strength and high-plasticity are difficult to achieve simultaneously in all commercial stainless steels. For example, austenitic stainless steels have a good plasticity and low strength; ferritic stainless steels and martensitic stainless steels have slightly higher strength and a moderate plasticity; precipitation hardening stainless steels have the highest strength and poorest plasticity in all stainless steels. At present, the austenitic stainless steels commonly used in the market are 201-, 301-, 304-, 316-type, and other stainless steels with low strength and a good plasticity and their derived materials. The weight content of chemical elements in these austenitic stainless steels is: C≤0.15%, Si≤2.0%, Mn≤2.0% (in 201- and 202-type stainless steels: 5.0%≤Mn≤10.5%), P≤0.045%, S≤0.03%, N≤0.025%, 15.0%≤Cr≤28.0%, 3.5%≤Ni≤36.0% (the content of Cr and Ni is dependent on the type of steels). Others are trace doping elements such as Cu, Nb, W, Ta, B, and Al, as well as iron and other unavoidable impurities.
In recent years, researchers have used the means of severe plastic deformation, such as liquid nitrogen cold rolling, mechanical alloying, high-pressure torsion, and extrusion, to directly refine the grain size of a material to nanometer scale or indirectly refine the grain size by the subsequent martensite to austenite reversion, to effectively improve the strength of austenitic steels. Grain refinement has an obvious effect on improving strength, but it causes a great loss of plasticity and toughness of the material. In the field of material science, a new type of metal material has emerged in recent years, which is called high-entropy alloy or multi-principal element alloy. High-entropy alloys contain at least five principal elements, and the content of each element is between 5 and 35 at %. High-entropy alloys have excellent properties that are difficult to achieve in conventional alloys, such as high hardness, strength, oxidation resistance, corrosion resistance, fatigue resistance, high-temperature softening resistance, creep resistance, wear resistance, unique magnetism, and excellent low-temperature mechanical properties. C. T. Liu et al. introduced a design scheme for strengthening FeCoNiTiAl high-entropy alloys with L12-type multicomponent intermetallic nano-sized precipitates (MCINP). By introducing L12-type MCINP into the multi-principal alloy matrix, the strength of the material is greatly improved, and a good tensile uniform elongation is maintained [Science 362 (2018) 933-937]. In this method, the strength is improved by introducing high-density nano-sized precipitates. During the tensile deformation process, dislocations will shear the nano-sized precipitates, so that the stress concentration occurs on the grain boundary, thus maintaining its high-plasticity. Therefore, the introduction of fine and high-density nano-sized precipitates in the matrix alloy plays a critical role in simultaneously achieving good strength and toughness. However, the above-mentioned high-entropy alloy contains expensive strategic metal, cobalt, makes it difficult to apply in large quantities. Therefore, a new type of high-entropy austenitic stainless steel without element cobalt is necessary to be developed to fill the technical bottleneck of the existing commercial austenitic steels, whose strength and plasticity are difficult to be balanced, and to achieve a strength-plasticity combination far beyond that in the existing austenitic stainless steels.
The composition of high-entropy austenitic stainless steels in this invention is mainly designed based on the following ideas:
The addition of Cr element can improve the strength of stainless steels while ensuring their corrosion resistance, high Cr content is helpful for the application in the service environment of nuclear materials (supercritical water, liquid lead-bismuth alloy, etc.).
The addition of Ni element can widen the phase region formed by nano-sized precipitates and inhibit the formation of other harmful intermetallic compounds to avoid brittleness.
The appropriate addition of Al element can endow the alloy with good oxidation and corrosion resistance, and promote the generation of nano-sized precipitates so that the nano-sized precipitates and the matrix maintain a high degree of coherence.
The appropriate addition of Ti element can refine the grains and homogenize the structure, and form nano-sized precipitates with Ni and Al to improve the strength of stainless steels. Moreover, Ti replaces expensive elements such as Cu, Co, Nb, and Mo to reduce production costs without destroying the microstructure of nano-sized precipitates.
The volatilization of Mn in the melting process brings inconvenience to the preparation of the alloy, and causes great waste; the presence of Cu may cause segregation of the material and bring inhomogeneity to the structure. Therefore, the alloy in the invention needs to remove these two elements.
The variation rules of the content of each element in the alloy: the content of Ti and Al is determined by the content of the other three elements. When the content of Cr and Ni is higher than that of Ti and Al, the content of Ti and Al needs to be reduced appropriately. When the content of Cr and Ni is lower than that of Ti and Al, it is necessary to increase the content of Ti and Al. When the content of Cr element increases, it is necessary to increase the content of Ni element at the same time because, on the one hand, this can ensure that the matrix has an austenitic structure, on the other hand, this can ensure that there are enough Ni atoms to form nano-sized precipitates.
In order to solve the shortcomings of the existing techniques, the purpose of the invention is to provide high-strength, high-plasticity, and high-entropy austenitic stainless steels and their preparation methods. The technical methods used in the invention are as follows:
A high-entropy austenitic stainless steel is characterized in that the elemental composition in atomic percentage is as follows:
Preferably, according to the atomic percentage, the elemental composition is as follows:
Furthermore, the size of nano-sized precipitates in the high-entropy stainless steel is below 30 nm, and the number density of nano-sized precipitates is above 5.0×1021 m−3.
A preparation method for high-entropy austenitic stainless steels is introduced, the specific steps are as follows: mixing the raw materials according to the desired atomic content, obtaining an ingot by melting and casting in a vacuum argon arc furnace, performing a solution/homogenization treatment for the cast ingot, cold-rolling and recrystallizing (1) or hot-rolling, cold-rolling, and recrystallizing (2) the recrystallized ingot, and finally carrying out aging treatment to obtain a high-entropy austenitic stainless steel.
Furthermore, the cold rolling process of (1) is as follows: the reduction in thickness per pass is no more than 0.2 mm, and the total reduction in thickness is 60%-70%.
Furthermore, the process of hot rolling and cold rolling of (2) is as follows: carrying out hot rolling at 800-1150° C., the reduction in thickness per pass is no more than 0.5 mm, and the temperature is guaranteed to be within the range of 800-1150° C. during the hot rolling process. In case the temperature of the specimen during the hot rolling is lowered, the specimen can be placed back into a furnace and remained within the rolling temperature range for 5-15 min, and the total reduction in thickness should be 50%-60%, the cold rolling reduction in thickness per pass is no more than 0.2 mm, and the total reduction in thickness is 60%-70%.
Furthermore, the specific operation of recrystallization is as follows: keeping the ingot rolled by (1) or (2) at 1140-1160° C. for 1-3 min. If the volume of the ingot is large, the recrystallization time can be increased.
Preferably, the heating rate for the recrystallization process is 10-20° C./min.
Furthermore, vacuuming the argon arc furnace to a pressure below 5.0×10−3 Pa, filling argon into the arc furnace to reach a pressure of 5.0×103 Pa, and starting arc melting when the oxygen content and nitrogen content in the furnace are lower than 0.002% within 180 min;
Furthermore, removing oxygen with pure Ti before starting the arc melting.
Furthermore, the weight of pure Ti is 30-40 g, which is not used as raw material and does not participate in melting.
Preferably, the vacuum argon arc melting process is repeated at least four times.
Furthermore, the specific operation of the solution/homogenization treatment is as follows: heating the ingot to 1140-1160° C. under a vacuum better than 1.0×10−3 Pa, then remaining at this temperature for 1-2.5 h, and finally quenching the ingot in water or cooling the ingot in air.
Preferably, the heating rate of the solution/homogenization treatment is 10-20° C./min.
Furthermore, the specific operation of the aging treatment is as follows: ageing the recrystallized ingot at 500-600° C. for 0.5-1.5 h and then quenching in water or cooling in air.
Preferably, the heating rate of the aging treatment is 5-15° C./min.
The beneficial effects of the invention:
The invention provides a high-entropy austenitic stainless steel, whose composition is expressed by atomic percentage of each element: Cr: 5-30%; Ni: 5-50%; Ti: 1-15%; Al: 1-15%; the rest are Fe and other unavoidable impurity elements (C, N, O, etc.) introduced during melting or heat treatment. The component of stainless steel is so simple that only five alloying elements (Fe, Cr, Ni, Ti, and Al) are used. The invented stainless steel reduces some precious metals and trace doping elements and minimizes the addition of alloying elements. By adjusting the atomic proportion of each component, the number of nano-sized precipitates is maximized so that the prepared high-entropy austenitic stainless steels have both high strength and high plasticity.
The strength and corrosion resistance of the stainless steels are improved by adding Cr. The addition of Ni can be used to broaden the phase region for the formation of nano-sized precipitates and inhibit the generation of harmful intermetallic compounds. The addition of Al endows the material with high oxidation and corrosion resistance, and contributes to the formation of nano-sized precipitates. Ti element refines the grain and homogenizes the microstructure, and forms nano-sized precipitates with Ni and Al to improve the strength of stainless steels. At the same time, Ti replaces expensive Cu, Co, Nb, and Mo to reduce production costs and does not destroy the microstructure of nano-sized precipitates. The stainless steels take the Fe—Cr—Ni phase as the matrix. By controlling the content of Cr and Ni, the content of Ti and Al is adjusted to form nano-sized precipitates to strengthen the matrix. Among them, when the content of Cr element increases, it is necessary to increase the content of Ni element because, on the one hand, it is necessary to ensure that the matrix has an austenitic structure, on the other hand, it is necessary to ensure that enough Ni atoms form nano-sized precipitates. At the same time, the content of Ti and Al should be reduced appropriately to prevent the formation of brittle intermetallic compounds, and vice versa.
In a preferred scheme, the content of each element is 5-19% Cr; 5-29% Ni; 6-15% Ti; 5-15% Al; the rest is iron. On the premise of ensuring the strength and plasticity of the alloy, the amount of Cr and Ni is reduced, and the production cost is further reduced.
The invention also provides a preparation process for the high-strength, high-plasticity, and high-entropy austenitic stainless steels, which simplifies the heat treatment process, reduces the production cost and has a broad application prospect. The solution/homogenization treatment method in the invention can make the alloy elements fully dissolve into the austenite matrix so that the alloy composition is more uniform. The high-temperature and short-time recrystallization treatment can achieve uniform and coarse equiaxed grains, ensure the good plasticity of the alloy, and greatly improve the production efficiency and save the cost.
In one of the preferred schemes of the invention, the rolling process of hot rolling followed by cold rolling is helpful to solve the difficulties in rolling large ingots. Hot rolling can eliminate the cracks caused by rolling at the initial stage, help subsequent cold rolling and reduce the risk. Through the subsequent aging treatment, it is helpful to form the nano-sized strengthened precipitates, thereby improving the strength and plasticity of stainless steels. The stainless steels prepared by the preparation method of the invention are superior to most commercial stainless steel due to their good strength and plasticity and are suitable for most of the service fields of stainless steels.
In order to understand the invention more clearly, the invention is further described with reference to the following embodiments and drawings. The embodiments are only used to explain without limiting the invention in any way. In the embodiments, each raw reagent material is commercially available, and the unspecified experimental method and condition are those well known in the field, or according to those recommended by the instrument manufacturers.
This embodiment provides a high-entropy austenitic stainless steel with a chemical composition of Fe47Cr16Ni26Ti6Al5 (atomic ratio) or Fe48.56Cr15.39Ni28.24Ti5.32Al2.5 (weight ratio). The influence of the introduction of unavoidable and very few impurity elements (C, N, O, etc.) during the melting process and heat treatment process on the properties of the material can be ignored.
The preparation steps of the high-entropy austenitic stainless steels are as follows:
According to the design proportion of elements, the raw elemental materials with a purity≥99.9% were weighed and mixed. The argon arc furnace was vacuumed to a pressure below 5.0×10−3 Pa and then filled with argon to a pressure of 5.0×103 Pa. When the oxygen content and nitrogen content in the furnace were lower than 0.002% within 180 min, 30 g of pure Ti was melted first to remove oxygen and then the arc melting was started. A 60×10×5 mm3 flake ingot was obtained by the arc melting for 6 times.
The ingot was placed in a furnace for solution and homogenization treatment. The ingot was heated to 1150° C. at a rate of 15° C./min, remained at 1150° C. for 120 min, and then quenched in water. Cold rolling deformation was performed for the ingot after the solution/homogenization treatment. The rolling process was as follows: the reduction in thickness per pass was no more than 0.2 mm, and the total reduction in thickness was 66.7%. The rolled ingot was heated to 1150° C. at a rate of 10° C./min and recrystallized at 1150° C. for 1.5 min. The recrystallized ingot was heated to 600° C. at a rate of 10° C./min, remained at 600° C. for 1 h, and then quenched in water to complete the aging treatment.
The conventional Cu Kα radiation was used as the X-ray source for x-ray diffraction. The diffraction pattern was shown in
This embodiment provides a high-entropy austenitic stainless steel with a chemical composition of Fe47Cr16Ni26Ti6Al5 (atomic ratio) or Fe48.36Cr15.39Ni28.24Ti5.32Al2.5 (weight ratio). The influence of the introduction of unavoidable and very few impurity elements (C, N, O, etc.) during the melting process and heat treatment process on the properties of the material can be ignored.
The preparation steps of the high-entropy austenitic stainless steel are as follows:
According to the design proportion of elements, the raw elemental materials with a purity≥99.9% were weighed and mixed. The argon arc furnace was vacuumed to a pressure below 5.0×10−3 Pa and then filled with argon to a pressure of 5.0×103 Pa. When the oxygen content and nitrogen content in the furnace were lower than 0.002% within 180 min, 30 g of pure Ti was melted first to remove oxygen and then the arc melting was started. A 60×10×5 mm3 flake ingot was obtained by the arc melting for 6 times.
The ingot was placed in a furnace for solution and homogenization treatment. The ingot wash heated to 1150° C. at a rate of 15° C./min, remained at 1150° C. for 120 min, and then cooled in air. After the solution/homogenization treatment, hot rolling and the subsequent cold rolling were performed. The rolling process was as follows: the hot rolling temperature was 1150° C., and the temperature was guaranteed to be in the range of 800-1150° C. during the hot rolling process. If the temperature was reduced during the hot rolling, the ingot can be put back into the furnace and remained at the desired rolling temperature for 5-15 min. The reduction in thickness per rolling pass was no more than 0.5 mm, and the total reduction in thickness was 50% during the hot rolling. Then, cold rolling was performed. The reduction in thickness per rolling pass was no more than 0.2 mm, and the total reduction in thickness was 66.7%. The rolled ingot was heated to 1140° C. at a rate of 15° C./min and recrystallized at 1140° C. for 1.5 min. The recrystallized ingot was heated to 550° C. at a rate of 15° C./min, remained at 550° C. for 1.5 h, and then cooled in the air to complete the aging treatment.
The material was characterized by transmission electron microscopy. The transmission electron microscopy and element distribution image was shown in
This embodiment provides a high-entropy austenitic stainless steel with a chemical composition of Fe39Cr20Ni30Ti6Al5 (atomic ratio) or Fe40.33Cr19.25Ni32.6Ti5.32Al2.5 (weight ratio). The influence of the introduction of unavoidable and very few impurity elements (C, N, O, etc.) during the melting process and heat treatment process on the properties of the material can be ignored.
The preparation steps of the high-entropy austenitic stainless steel are as follows:
According to the design proportion of elements, the raw elemental materials with a purity≥99.9% were weighed and mixed. The argon arc furnace was vacuumed to a pressure below 5.0×10−3 Pa and then filled with argon to a pressure of 5.0×103 Pa. When the oxygen content and nitrogen content in the furnace were lower than 0.002% within 180 min, 35 g of pure Ti was melted first to remove oxygen and then the arc melting was started. A 60×10×5 mm3 flake ingot was obtained by the arc melting for 5 times.
The ingot was placed in a furnace for solution/homogenization treatment. The ingot was heated to 1150° C. at a rate of 15° C./min, remained at 1150° C. for 120 min, and then quenched in water. After the solution/homogenization treatment, cold rolling was performed. The cold rolling process was as follows: the reduction in thickness per rolling pass was no more than 0.2 mm, and the total reduction in thickness was 66.7%. The cold-rolled ingot was heated to 1145° C. at a rate of 18° C./min and recrystallized at 1145° C. for 1.5 min. The recrystallized ingot was heated to 600° C. at a rate of 12° C./min, remained at 600° C. for 1 h, and then quenched in water to complete the aging treatment.
This embodiment provides a high-entropy austenitic stainless steel with a chemical composition of Fe31Cr24Ni34Ti6Al5 (atomic ratio) or Fe32.08Cr23.12Ni36.98Ti5.32Al2.5 (weight ratio). The influence of the introduction of unavoidable and very few impurity elements (C, N, O, etc.) during the melting process and heat treatment process on the properties of the material can be ignored.
The preparation steps of the high-entropy austenitic stainless steel are as follows:
According to the design proportion of elements, the raw elemental materials with a purity≥99.9% were weighed and mixed. The argon arc furnace was vacuumed to a pressure below 5.0×10−3 Pa and then filled with argon to a pressure of 5.0×103 Pa. When the oxygen content and nitrogen content in the furnace were lower than 0.002% within 180 min, 40 g of pure Ti was melted first to remove oxygen and then the arc melting was started. A 60×10×5 mm3 flake ingot was obtained by the arc melting for 5 times.
The ingot was placed in a furnace for solution/homogenization treatment. The ingot was heated to 1150° C. at a rate of 15° C./min, remained at 1150° C. for 120 min, and then quenched in water. After the solution/homogenization treatment, cold rolling deformation was performed for the ingot. The cold rolling process was as follows: the reduction in thickness per pass was no more than 0.2 mm, and the total reduction in thickness was 66.7%. The cold-rolled ingot was heated to 1155° C. at a rate of 10° C./min and recrystallized at 1155° C. for 1.5 min. The recrystallized ingot was heated to 600° C. at a rate of 10° C./min, remained at 600° C. for 1 h, and then quenched in water to complete the aging treatment.
This embodiment of the invention provides a high-entropy austenitic stainless steel with a chemical composition of Fe42Cr16Ni28Ti7Al7 (atomic ratio) or Fe43.88Cr15.56Ni30.75Ti6.27Al3.53 (weight ratio). The influence of the introduction of unavoidable and very few impurity elements (C, N, O, etc.) during the melting process and heat treatment process on the properties of the material can be ignored.
The preparation steps of the high-entropy austenitic stainless steel are as follows:
According to the design proportion of elements, the raw elemental materials with a purity≥99.9% were weighed and mixed. The argon arc furnace was vacuumed to a pressure below 5.0×10−3 Pa and then filled with argon to a pressure of 5.0×103 Pa. When the oxygen content and nitrogen content in the furnace were lower than 0.002% within 180 min, 35 g of pure Ti was melted first to remove oxygen and then the arc melting was started. A 60×10×5 mm3 flake ingot was obtained by the arc melting for 6 times.
The ingot was placed in a furnace for solution and homogenization treatment. The ingot was heated 1150° C. at a rate of 15° C./min, remained at 1150° C. for 120 min, and then quenched in water. After the solution/homogenization treatment, cold rolling deformation was performed for the ingot. The cold rolling process was as follows: the reduction in thickness per pass was no more than 0.2 mm, and the total reduction in thickness was 66.7%. The cold-rolled ingot was heated to 1160° C. at a rate of 20° C./min and recrystallized at 1160° C. for 1.5 min. The recrystallized ingot was heated to 600° C. at a rate of 10° C./min, remained at 600° C. for 1 h, and then quenched in water to complete the aging treatment.
The embodiment of the invention provides a high-entropy austenitic stainless steel with a chemical composition of Fe49Cr16Ni28Ti4Al3 (atomic ratio) or Fe49.9Cr15.17Ni29.98Ti3.49A1.48 (weight ratio). The influence of the introduction of unavoidable and very few impurity elements (C, N, O, etc.) during the melting process and heat treatment process on the properties of the material can be ignored.
The preparation steps of the high-entropy austenitic stainless steel are as follows:
According to the design proportion of elements, the raw elemental materials with a purity≥99.9% were weighed and mixed. The argon arc furnace was vacuumed to a pressure below 5.0×10−3 Pa and then filled with argon to a pressure of 5.0×103 Pa. When the oxygen content and nitrogen content in the furnace were lower than 0.002% within 180 min, 30 g of pure Ti was melted first to remove oxygen and then the arc melting was started. A 60×10×5 mm3 flake ingot was obtained by the arc melting for 6 times.
The ingot was placed in a furnace for solution and homogenization treatment. The ingot was heated to 1150° C. at a rate of 15° C./min, remained at 1150° C. for 120 min, and then quenched in water. After solution/homogenization treatment, cold rolling deformation was performed for the ingot. The cold rolling process was as follows: the reduction in thickness per pass was no more than 0.2 mm, and the total reduction in thickness was 66.7%. The cold-rolled ingot was heated to 1150° C. at a rate of 10° C./min and recrystallized at 1150° C. for 1.5 min. The recrystallized ingot was heated to 600° C. at a rate of 10° C./min, remained at 600° C. for 1 h, and then quenched in water to complete the aging treatment.
The statistical analysis results of yield strength ReL, ultimate tensile strength Rm, fracture elongation E, and yield ratio (ReL/Rm) of the materials prepared by Embodiments 1-6 are shown in Table 1. Each sample in the table was tested three times, and sample was randomly selected.
It can be seen from Table 1 that the yield strength, tensile strength, and fracture elongation of the high-entropy austenitic stainless steels prepared by Embodiments 1-3 of the invention maintain a high level, and the yield ratio is within a reasonable range of 0.67-0.73. In Embodiments 4-5, brittle intermetallic compounds are formed due to the excessive content of Ti and Al. As a result, although the strength is improved, the plasticity is significantly degraded. In Embodiment 6, the precipitation strengthening cannot be maximized due to the insufficient content of Ti and Al, which results in low strength.
Obviously, the above embodiments are only examples for a clear explanation, not a limitation on the implementation methods. For ordinary technical personnel in their field, other different forms of change or modification can be made based on the above description. There is no need and no way to exhaust all the implementation methods. The obvious changes or modifications thus extended are still within the protection scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202111396824.1 | Nov 2021 | CN | national |
This application is the national phase entry of International Application No. PCT/CN2022/128626, filed on Oct. 31, 2022, which is based upon and claims priority to Chinese Patent Application No. 202111396824.1, filed on Nov. 23, 2021, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2022/128626 | 10/31/2022 | WO |
