A rare earth steel and its microstructure optimization method
By applying electromagnetic fields and mechanical vibrations during the solidification and annealing process of rare earth steel, and by optimizing the microstructure of rare earth steel in combination with electromagnetic fields, the problems of coarse grains and inclusions in rare earth steel were solved, the refinement of equiaxed grains and the uniformity of microstructure were achieved, and the performance of steel was improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- INNER MONGOLIA METAL MATERIAL RES INST
- Filing Date
- 2022-08-08
- Publication Date
- 2026-05-26
Smart Images
Figure CN116213682B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of metal material preparation technology, and in particular to a rare earth steel and its microstructure optimization method. Background Technology
[0002] The grain size of solidified metal has a significant impact on the properties of ingots or castings, especially in the production of high-quality steel, where the microstructure inherited from the grain size and uniformity of the as-cast microstructure has a particularly pronounced effect on its properties. When molten metal is poured into an ingot mold, different shapes of solidification structures form from the solidification surface to the center during the solidification process: a fine-grained surface region, a columnar grain region, and an equiaxed grain region in the center. The fine-grained surface region has a denser microstructure, resulting in better mechanical properties. The columnar grain region also has a denser microstructure and is less prone to micro-shrinkage porosity than equiaxed grains, but the interface between two rows of adjacent columnar grains developing perpendicular to the mold wall exhibits poor strength and plasticity, making it prone to cracking along this weak surface during forging and rolling. Equiaxed grains, on the other hand, have more uniform properties in all directions, lack weak interfaces, and the interlocking of grains with different orientations makes crack propagation less likely. Therefore, uniform, fine equiaxed grains are often desirable in production.
[0003] Currently, one effective method for obtaining uniform, fine equiaxed grains in existing technologies is to add rare earth elements to steel. This purifies the molten steel and refines the microstructure. However, the amount of rare earth elements added to steel is usually trace, and they are easily burned off during smelting, making it difficult to disperse them quickly and uniformly in the steel. Furthermore, during the purification process, rare earth elements can form rare earth compounds with impurities such as O and S in the molten steel. If these compounds are not removed promptly during smelting, they will exist in the molten steel as inclusions and act as initial nucleation sites during solidification, leading to coarse grains. Therefore, the degree of optimization of steel microstructure by simply using rare earth elements is limited.
[0004] Therefore, there is an urgent need to provide a method for optimizing the microstructure of rare earth steel, which can effectively optimize the microstructure of rare earth steel, so that it forms fine equiaxed crystals and avoids the formation of coarse columnar crystals. Summary of the Invention
[0005] The purpose of this invention is to provide a rare earth steel and a method for optimizing its microstructure. The rare earth steel ingot prepared by the microstructure optimization method provided by this invention has refined columnar crystal grains, made the equiaxed crystal grains small, and resulted in a uniform microstructure.
[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:
[0007] This invention provides a method for optimizing the microstructure of rare earth steel, comprising the following steps:
[0008] (1) The raw materials of rare earth steel are smelted to obtain molten steel;
[0009] (2) The molten steel obtained in step (1) is cast and then solidified to obtain rare earth steel ingots; an electromagnetic field and mechanical vibration are applied simultaneously during the solidification process; the electromagnetic field is applied from the beginning to the end of solidification, and the mechanical vibration is applied for 6 to 10 minutes.
[0010] (3) Anneal the rare earth steel ingot obtained in step (2) to obtain rare earth steel; an electromagnetic field is applied during the annealing process.
[0011] Preferably, the raw materials in step (1) include base steel raw materials and rare earth raw materials; the rare earth raw materials are rare earth-manganese master alloys.
[0012] Preferably, the amount of rare earth-manganese master alloy added is calculated according to formula ①;
[0013] m 稀土-锰中间合金 =(Rare earth mass content setting value in rare earth steel × mass of rare earth steel to be smelted) / [Rare earth content of rare earth-manganese master alloy × empirical value of rare earth yield of rare earth steel × (1-κ)] Formula ①;
[0014] The rare earth content in the rare earth steel is set to 0.02-0.12%; the empirical value of the rare earth recovery rate of the rare earth steel is 65-80%; κ is the correction coefficient for the rare earth recovery rate, and the value of κ is 0.0082-0.0087.
[0015] Preferably, in step (2), the direction of the electromagnetic field is parallel to the surface of the molten steel, the current of the electromagnetic field is 180~320A, and the frequency of the electromagnetic field is 3~5Hz.
[0016] Preferably, in step (2), the direction of the mechanical vibration is perpendicular to the surface of the molten steel, the frequency of the mechanical vibration is 6 to 10 Hz, the amplitude of the mechanical vibration is 0.3 to 1 mm, and the excitation force of the mechanical vibration is 5 to 10 kN.
[0017] Preferably, the annealing process in step (3) includes: first heating to 350-450°C at a heating rate of 3-5°C / min and holding for 4-8 minutes, then heating to 650-720°C at a heating rate of 2-4°C / min and holding for 25-40 minutes.
[0018] Preferably, the cooling method for the annealing process in step (3) is furnace cooling to 200°C and then air cooling to room temperature.
[0019] Preferably, in step (3), the electromagnetic field is applied for 10 to 15 minutes after the temperature rises to 650 to 720°C during the annealing process.
[0020] Preferably, in step (3), the current of the electromagnetic field is 160~300A and the frequency of the electromagnetic field is 2~4Hz.
[0021] This invention also provides rare earth steel prepared by the microstructure optimization method described in the above technical solution.
[0022] This invention provides a method for optimizing the microstructure of rare earth steel, comprising the following steps: smelting the raw materials for rare earth steel to obtain molten steel; casting the obtained molten steel and then solidifying it to obtain a rare earth steel ingot; simultaneously applying an electromagnetic field and mechanical vibration during the solidification process; the electromagnetic field is applied from the start to the completion of solidification, and the mechanical vibration is applied for 6-10 minutes; annealing the obtained rare earth steel ingot to obtain rare earth steel; and applying an electromagnetic field during the annealing process. This invention, by applying an electromagnetic field and mechanical vibration during the solidification process and controlling the timing and duration of their application, enables the alloying elements to diffuse rapidly under the influence of the electromagnetic field throughout the solidification process, avoiding element segregation and improving microstructure uniformity. Simultaneously, applying mechanical vibration can suppress the formation of coarse grains in the early stages of solidification, shorten dendrite spacing, and refine the microstructure. Applying an electromagnetic field during annealing can increase the diffusion rate of alloying elements, lower the annealing temperature, and shorten the annealing time, further contributing to obtaining rare earth steel with fine grains and a uniform microstructure. Experimental results show that the rare earth steel prepared by the preparation method provided by the present invention has a metallographic micrograph showing that there are no coarse rare earth inclusions, coarse grains and dendrites in the rare earth steel, and the structure is uniform and dense.
[0023] The tissue optimization method provided by this invention is simple and easy to implement, with easily controllable parameters, energy-saving and environmentally friendly, and low in cost. Attached Figure Description
[0024] Figure 1 Optical metallographic micrograph of the equiaxed grains in the core of the rare earth steel ingot prepared in Example 1 of this invention;
[0025] Figure 2 This is an optical metallographic micrograph of the equiaxed grains in the core of a steel ingot prepared without electromagnetic field or mechanical vibration, as shown in Comparative Example 1 of the present invention.
[0026] Figure 3 Optical metallographic micrograph of columnar crystal grains at the edge of a rare earth steel ingot prepared in Example 1 of this invention;
[0027] Figure 4 This is an optical metallographic micrograph of columnar crystal grains at the edge of a steel ingot prepared without electromagnetic field or mechanical vibration, as shown in Comparative Example 1 of this invention. Detailed Implementation
[0028] This invention provides a method for optimizing the microstructure of rare earth steel, comprising the following steps:
[0029] (1) The raw materials of rare earth steel are smelted to obtain molten steel;
[0030] (2) The molten steel obtained in step (1) is cast and then solidified to obtain rare earth steel ingots; an electromagnetic field and mechanical vibration are applied simultaneously during the solidification process; the electromagnetic field is applied from the beginning to the end of solidification, and the mechanical vibration is applied for 6 to 10 minutes.
[0031] (3) Anneal the rare earth steel ingot obtained in step (2) to obtain rare earth steel; an electromagnetic field is applied during the annealing process.
[0032] This invention involves smelting rare earth steel raw materials to obtain molten steel.
[0033] In this invention, the raw materials preferably include base steel and rare earth materials.
[0034] This invention does not specifically limit the type or source of the raw materials for the base steel; the materials can be formulated according to the grade or designed composition of the base steel. In this invention, the preferred grades of the base steel include 30MnCrNiMo steel, Mn18Cr2 steel, 35CrNiMnV steel, or H13 steel.
[0035] In this invention, the rare earth raw material is preferably a rare earth-manganese master alloy; the rare earth-manganese master alloy preferably comprises 30-70% rare earth by mass. By selecting a rare earth-manganese master alloy as the rare earth raw material and controlling the rare earth content within the above range, this invention can improve the oxidation of rare earth and obtain a higher melting point, while reducing rare earth burn-off.
[0036] This invention does not have any special requirements regarding the type of rare earth element; any rare earth element well-known to those skilled in the art can be used. In this invention, the rare earth element preferably includes cerium, lanthanum, or yttrium, and more preferably cerium.
[0037] The present invention does not impose any particular limitation on the preparation method of the rare earth-manganese master alloy; any method for preparing master alloys known to those skilled in the art can be used.
[0038] In this invention, the amount of rare earth-manganese master alloy added is preferably calculated according to formula ①;
[0039] m 稀土-锰中间合金 =(Rare earth mass content setting value in rare earth steel × mass of rare earth steel to be smelted) / [Rare earth content of rare earth-manganese master alloy × empirical value of rare earth yield of rare earth steel × (1-κ)] Formula ①;
[0040] In this invention, the rare earth content in the rare earth steel is set to 0.02-0.12% by mass; the empirical value of the rare earth yield in the rare earth steel ranges from 65-80%; κ is a correction coefficient for the rare earth yield, and the value of κ ranges from 0.0082-0.0087. By calculating the amount of rare earth-manganese master alloy added according to the above formula ①, this invention can make the rare earth content in the rare earth steel closer to the set value, obtain a higher rare earth yield, and thus refine the microstructure of the steel using rare earth elements.
[0041] In this invention, the particle size of the rare earth-manganese master alloy is preferably ≤0.5μm, more preferably 0.1μm~0.5μm. In this invention, when the particle size of the rare earth-manganese master alloy does not meet the above requirements, the rare earth-manganese master alloy is preferably subjected to mechanical crushing and / or ball milling. This invention does not have special limitations on the operation of mechanical crushing and ball milling, as long as the required particle size of the rare earth-manganese master alloy can be obtained. By controlling the particle size of the rare earth-manganese master alloy within the above range, this invention allows the rare earth-manganese master alloy to melt rapidly and mix uniformly in molten steel, which is more conducive to refining the steel grains and effectively improving the mechanical properties of the steel.
[0042] In this invention, the rare earth-manganese master alloy is preferably dried before use. The drying process is not particularly demanding; any drying method well-known to those skilled in the art that effectively removes moisture is acceptable. By drying the rare earth-manganese master alloy, this invention avoids introducing impurities such as hydrogen and oxygen into the steel during its use.
[0043] In this invention, the rare earth-manganese master alloy is preferably coated with pure iron sheet before use. By coating the rare earth-manganese master alloy with pure iron sheet, this invention avoids the oxidation of rare earth elements before use, thereby preventing the introduction of impurities such as oxygen into the steel.
[0044] In this invention, the preferred melting method is to first melt the base steel raw material, and then add the rare earth raw material for smelting. This feeding sequence reduces the rare earth burn-off rate.
[0045] In this invention, the preferred melting temperature is 1610~1640℃. This invention does not have a specific limitation on the melting time, as long as it ensures that the molten steel is homogeneous after melting.
[0046] In this invention, the melting atmosphere is preferably a vacuum with an inert gas introduced; the vacuum degree is preferably 5 × 10⁻⁶. -3 ~5×10 -2 Pa.
[0047] After obtaining the molten steel, the molten steel is cast and then solidified to obtain rare earth steel ingots.
[0048] In this invention, the casting temperature is preferably 1570~1620℃, more preferably 1600℃. By controlling the casting temperature within the above range, this invention enables the molten steel to have high fluidity, allowing it to quickly fill the mold during casting and reducing casting defects such as incomplete filling and shrinkage cavities.
[0049] In this invention, the mold used for casting is preferably a cast iron crucible. By selecting a cast iron crucible as the casting mold, this invention utilizes the fact that cast iron has a lower coefficient of thermal expansion than steel, making it easier to demold the steel after it solidifies.
[0050] In this invention, an electromagnetic field and mechanical vibration are applied simultaneously during the solidification process.
[0051] In this invention, the direction of the electromagnetic field is preferably parallel to the surface of the molten steel; the current of the electromagnetic field is preferably 180~320A, more preferably 200~300A, and most preferably 220~250A; the frequency of the electromagnetic field is preferably 3~5Hz, more preferably 4Hz. By controlling the parameters of the electromagnetic field within the above ranges, this invention is more conducive to improving the diffusion rate of alloying elements, avoiding element segregation during solidification, suppressing the formation of columnar crystals and thus facilitating the acquisition of fine equiaxed crystals, effectively improving the microstructure uniformity of rare earth steel, and further improving the mechanical properties of rare earth steel.
[0052] In this invention, the direction of the mechanical vibration is preferably perpendicular to the surface of the molten steel; the frequency of the mechanical vibration is preferably 6~10Hz, more preferably 7~9Hz, and most preferably 8Hz; the amplitude of the mechanical vibration is preferably 0.3~1mm, more preferably 0.5~0.8mm; and the excitation force of the mechanical vibration is preferably 5~10kN, more preferably 6~9kN, and most preferably 7~8kN. By controlling the parameters of the mechanical vibration within the above ranges, this invention can effectively break up coarse grains and coarse dendrites formed during the solidification process of molten steel, suppress the formation of columnar crystals, and thus facilitate the acquisition of fine equiaxed crystals, effectively refining the microstructure of rare earth steel and further improving its mechanical properties.
[0053] In this invention, the electromagnetic field is applied for a period from the start of solidification to the completion of solidification, preferably 30-50 minutes; the mechanical vibration is applied for a period of 6-10 minutes, preferably 7-9 minutes. By controlling the duration of the electromagnetic field and mechanical vibration within the above ranges, this invention enables the electromagnetic field and mechanical vibration to work together in the early stage of solidification, effectively preventing elemental segregation in the semi-solid state during the initial solidification stage and controlling the initial formation and growth of grains, thereby significantly refining the grains. In the later stage of solidification, when the rare earth steel is already in a solid state, continuing to treat it with the electromagnetic field can make solute atoms uniformly dispersed in the microstructure, further improving the microstructure uniformity of the rare earth steel.
[0054] In this invention, the solidification atmosphere is preferably a vacuum with an inert gas introduced; the vacuum degree is preferably 1×10⁻⁶. -2 ~6×10 -2 Pa.
[0055] After obtaining the rare earth steel ingot, the present invention performs annealing treatment on the rare earth steel ingot to obtain rare earth steel.
[0056] In this invention, an electromagnetic field is applied during the annealing process.
[0057] In this invention, the annealing treatment preferably includes: first heating to 350-450°C at a heating rate of 3-5°C / min and holding for 4-8 minutes, then heating to 650-720°C at a heating rate of 2-4°C / min and holding for 25-40 minutes; more preferably, it includes: first heating to 400°C at a heating rate of 4°C / min and holding for 6 minutes, then heating to 700°C at a heating rate of 3°C / min and holding for 30 minutes. By controlling the annealing process and its parameters within the above range, this invention can eliminate the casting stress of rare earth steel, refine the grains, and make the microstructure more uniform.
[0058] In this invention, the electromagnetic field is preferably applied starting when the temperature rises to 650-720°C during the annealing process and lasts for 10-15 minutes; more preferably, it is applied starting when the temperature rises to 680-700°C during the annealing process and lasts for 12-14 minutes. By applying an electromagnetic field during the annealing process and controlling its timing and duration, this invention enables alloying elements to diffuse rapidly under the influence of the electromagnetic field, achieving a homogenized microstructure. Simultaneously, the application of the electromagnetic field can lower the holding temperature and shorten the holding time during the annealing process, preventing the grains of rare earth steel from coarsening at higher temperatures.
[0059] In this invention, the current of the electromagnetic field during the annealing process is preferably 160~300A, more preferably 200~250A; the frequency of the electromagnetic field is preferably 2~4Hz, more preferably 3Hz. By controlling the parameters of the electromagnetic field during the annealing process within the above range, this invention enables the rapid diffusion of alloying elements in the rare earth steel ingot and suppresses grain coarsening, thereby making it more conducive to refining the grains and achieving a uniform microstructure of the rare earth steel.
[0060] This invention does not have special requirements for the direction of the electromagnetic field during the annealing process, as long as the rare earth steel ingot is within the electromagnetic field.
[0061] In this invention, the annealing atmosphere is preferably a vacuum purged with an inert gas; the vacuum degree is preferably 1×10⁻⁶. -1 ~1Pa.
[0062] In this invention, the preferred cooling method for the annealing treatment is furnace cooling to 200°C and then air cooling to room temperature.
[0063] This invention also provides rare earth steel prepared by the microstructure optimization method described in the above technical solution.
[0064] The columnar crystals in the rare earth steel provided by this invention are significantly refined, and the equiaxed crystals have small grains and a uniform structure.
[0065] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.
[0066] Example 1
[0067] This embodiment provides a method for optimizing the microstructure of rare earth steel, specifically comprising the following steps:
[0068] (1) The raw materials of rare earth steel are smelted to obtain molten steel;
[0069] The raw materials for rare earth steel are the base steel (30MnCrNiMo) and rare earth raw materials (a cerium-manganese master alloy with a rare earth cerium content of 70%). The bulk rare earth raw materials are mechanically crushed to obtain particles smaller than 10mm. A planetary ball mill is used to grind the particles, and the ball mill jar is vacuum-treated to a vacuum degree of 1×10⁻⁶. -1 At a pressure of Pa, an inert gas is introduced, and the vacuum level is maintained at 5 × 10⁻⁶. -1 The ball mill was operated at a revolution speed of 260 r / min and a rotation speed of 560 r / min for 90 min, with the planetary disk running alternately in forward and reverse directions at 10-min intervals. The ground powder was sieved, and powder with a particle size ≤0.5 μm was selected, dried, and stored for later use. The rare earth steel composition was prepared according to a rare earth content of 0.080%, and the amount of rare earth-manganese intermediate alloy added was: (0.080%×100Kg) / [70%×80%×(1-κ)]=0.1440~0.1441Kg (κ ranges from 0.0082 to 0.0087). The mass of the added rare earth-manganese intermediate alloy was found to be 0.1441Kg.
[0070] 100 kg of rare earth steel raw material was smelted in a vacuum medium-frequency induction furnace, and the vacuum was evacuated to a degree of 10. - 3 Pa, then an inert gas is introduced to maintain the vacuum at 5 × 10⁻⁶. -3Pa; the melting temperature is 1610℃. The raw materials are added in order of ease of burning. First, the raw materials of the base steel are added, then the rare earth raw materials are added. The rare earth-manganese intermediate alloy powder is wrapped in pure iron sheet and equipped with pure iron counterweights. Finally, the molten steel is obtained after all the metals have melted and been kept at the temperature for 8 minutes.
[0071] (2) The molten steel obtained in step (1) is cast and then solidified to obtain rare earth steel ingots; during the solidification process, electromagnetic field and mechanical vibration are applied simultaneously.
[0072] The casting temperature is 1570℃. The casting mold is a cast iron crucible with an inverted cone shape inside. A sieve plate is placed on the top, and the molten metal enters the cast iron crucible after being filtered through the sieve plate.
[0073] A transverse electromagnetic field parallel to the surface of the molten steel is set on the cast iron crucible, and a two-dimensional vibration table is installed at the bottom to apply mechanical vibration perpendicular to the surface of the molten steel. After the molten steel is poured, the transverse electromagnetic field and the longitudinal vibration table are turned on at the same time. The electromagnetic field current is controlled at 200A and the frequency is controlled at 3Hz. The mechanical vibration frequency is 6Hz, the amplitude is 0.5mm, the excitation force is 6kN, and the mechanical vibration time is 6min. The electromagnetic field is kept on until the molten steel solidifies.
[0074] During the process from casting to steel solidification, inert gas is introduced to maintain a vacuum of 1×10⁻⁶. -2 After the steel solidifies, inert gas is continuously introduced until the internal pressure is balanced with the external pressure. This process is maintained for 30 minutes, and then the steel ingot is obtained by removing it from the furnace.
[0075] (3) Anneal the rare earth steel ingot obtained in step (2) to obtain rare earth steel; an electromagnetic field is applied during the annealing process.
[0076] Among them, a vacuum heat treatment furnace is used to anneal rare earth steel ingots, with a vacuum degree of 1×10⁻⁶. -1 The annealing process is as follows: ① First, heat to 400℃ at a heating rate of 3℃ / min and hold for 4 min; ② Then, heat to 680℃ at a heating rate of 3℃ / min, turn on the electromagnetic field and hold for 12 min, turn off the electromagnetic field and continue holding for 18 min (hold at annealing temperature of 680℃ for 30 min); ③ Cool with the furnace, maintaining a vacuum of 1×10⁻⁶. -1 Pa, the furnace temperature is cooled to below 200℃ and then air-cooled to room temperature to obtain rare earth steel.
[0077] Example 2
[0078] This embodiment provides a method for optimizing the microstructure of rare earth steel, which includes the following steps:
[0079] (1) The raw materials of rare earth steel are smelted to obtain molten steel;
[0080] The raw materials for rare earth steel are the base steel (30MnCrNiMo) and rare earth raw materials (a cerium-manganese master alloy with a rare earth cerium content of 50%). The bulk rare earth raw materials are mechanically crushed to obtain particles smaller than 10mm. These particles are then ground using a planetary ball mill, and the mill jar is subjected to vacuum treatment to achieve a vacuum degree of 2×10⁻⁶. -1 At a pressure of Pa, an inert gas is introduced, and the vacuum level is maintained at 5 × 10⁻⁶. -1 The ball mill was operated at a revolution speed of 280 r / min and a rotation speed of 580 r / min for 100 min, with the planetary disk running alternately in forward and reverse directions at 12-min intervals. The ground powder was sieved, and powder with a particle size ≤0.5 μm was selected, dried, and stored for later use. The rare earth steel composition was prepared according to a rare earth content of 0.080%, and the amount of rare earth-manganese intermediate alloy added was: (0.080%×100Kg) / [50%×725×(1-κ)]=0.2240~0.2242Kg (κ ranges from 0.0082 to 0.0087). The mass of the added rare earth-manganese intermediate alloy was found to be 0.2242Kg.
[0081] 100 kg of rare earth steel raw material was smelted in a vacuum medium-frequency induction furnace, and the vacuum was evacuated to a degree of 10. - 3 Pa, then an inert gas is introduced to maintain the vacuum at 5 × 10⁻⁶. -3 Pa; the melting temperature is 1620℃. The raw materials are added in order of their burning loss. First, the base steel raw materials are added, then the rare earth raw materials are added. The rare earth-manganese intermediate alloy powder is wrapped in pure iron sheet and equipped with pure iron counterweights. Finally, the molten steel is obtained after all the metal has melted and been kept at the temperature for 8 minutes.
[0082] (2) The molten steel obtained in step (1) is cast and then solidified to obtain rare earth steel ingots; during the solidification process, electromagnetic field and mechanical vibration are applied simultaneously.
[0083] The casting temperature is 1590℃. The casting mold is a cast iron crucible with an inverted cone shape inside. A sieve plate is placed on the top, and the molten metal enters the cast iron crucible after being filtered through the sieve plate.
[0084] A transverse electromagnetic field parallel to the surface of the molten steel is set on the cast iron crucible, and a two-dimensional vibration table is installed at the bottom to apply mechanical vibration perpendicular to the surface of the molten steel. After the molten steel is poured, the transverse electromagnetic field and the longitudinal vibration table are turned on at the same time. The electromagnetic field current is controlled at 230A and the frequency is controlled at 4Hz. The mechanical vibration frequency is 6Hz, the amplitude is 0.8mm, the excitation force is 6kN, and the mechanical vibration time is 6min. The electromagnetic field is kept on until the molten steel solidifies.
[0085] During the process from casting to steel solidification, inert gas is introduced to maintain a vacuum of 1×10⁻⁶. -2 After the steel solidifies, inert gas is continuously introduced until the internal pressure is balanced with the external pressure. This process is maintained for 30 minutes, and then the steel ingot is obtained by removing it from the furnace.
[0086] (3) Anneal the rare earth steel ingot obtained in step (2) to obtain rare earth steel; an electromagnetic field is applied during the annealing process.
[0087] Among them, a vacuum heat treatment furnace is used to anneal rare earth steel ingots, with a vacuum degree of 1×10⁻⁶. -1 The annealing process is as follows: ① First, heat to 400℃ at a heating rate of 4℃ / min and hold for 4 min; ② Then, heat to 680℃ at a heating rate of 3℃ / min, turn on the electromagnetic field and hold for 12 min, turn off the electromagnetic field and continue holding for 20 min (hold at annealing temperature of 680℃ for 32 min); ③ Cool with the furnace, maintaining a vacuum of 1×10⁻⁶. -1 Pa, the furnace temperature is cooled to below 200℃ and then air-cooled to room temperature to obtain rare earth steel.
[0088] Example 3
[0089] This embodiment provides a method for optimizing the microstructure of rare earth steel, specifically comprising the following steps:
[0090] (1) The raw materials of rare earth steel are smelted to obtain molten steel;
[0091] The raw materials for rare earth steel are the base steel (30MnCrNiMo) and rare earth raw materials (a cerium-manganese master alloy with a rare earth cerium content of 30%). The bulk rare earth raw materials are mechanically crushed to obtain particles smaller than 10mm. These particles are then ground using a planetary ball mill, and the mill jar is subjected to vacuum treatment to achieve a vacuum degree of 1×10⁻⁶. -1 At a pressure of Pa, an inert gas is introduced, and the vacuum level is maintained at 5 × 10⁻⁶. -1 The ball mill was operated at a revolution speed of 300 r / min and a rotation speed of 600 r / min for 110 min, with the planetary disk running alternately in forward and reverse directions at 10-min intervals. The ground powder was sieved, and powder with a particle size ≤0.5 μm was selected, dried, and stored for later use. The rare earth steel composition was prepared according to a rare earth content of 0.080%, and the amount of rare earth-manganese intermediate alloy added was: (0.080%×100Kg) / [30%×60%×(1-κ)]=0.4481~0.4483Kg (κ ranges from 0.0082 to 0.0087). The mass of the added rare earth-manganese intermediate alloy was found to be 0.4481Kg.
[0092] 100 kg of rare earth steel raw material was smelted in a vacuum medium-frequency induction furnace, and the vacuum was evacuated to a degree of 10. - 3 Pa, then an inert gas is introduced to maintain the vacuum at 5 × 10⁻⁶. -3 Pa; the melting temperature is 1630℃. The raw materials are added in order of ease of burning. First, the raw materials of the base steel are added, then the rare earth raw materials are added. The rare earth-manganese intermediate alloy powder is wrapped in pure iron sheet and equipped with pure iron counterweights. Finally, the molten steel is obtained after all the metals have melted and been kept at the temperature for 8 minutes.
[0093] (2) The molten steel obtained in step (1) is cast and then solidified to obtain rare earth steel ingots; during the solidification process, electromagnetic field and mechanical vibration are applied simultaneously.
[0094] The casting temperature is 1620℃. The casting mold is a cast iron crucible with an inverted cone shape inside. A sieve plate is placed on the top, and the molten metal enters the cast iron crucible after being filtered through the sieve plate.
[0095] A transverse electromagnetic field parallel to the surface of the molten steel is set on the cast iron crucible, and a two-dimensional vibration table is installed at the bottom to apply mechanical vibration perpendicular to the surface of the molten steel. After the molten steel is poured, the transverse electromagnetic field and the longitudinal vibration table are turned on at the same time. The electromagnetic field current is controlled at 300A and the frequency is controlled at 5Hz. The mechanical vibration frequency is 9Hz, the amplitude is 0.8mm, the excitation force is 10kN, and the mechanical vibration time is 6min. The electromagnetic field is kept on until the molten steel solidifies.
[0096] During the process from casting to steel solidification, inert gas is introduced to maintain a vacuum of 1×10⁻⁶. -2 After the steel solidifies, inert gas is continuously introduced until the internal pressure is balanced with the external pressure. This process is maintained for 30 minutes, and then the steel ingot is obtained by removing it from the furnace.
[0097] (3) Anneal the rare earth steel ingot obtained in step (2) to obtain rare earth steel; an electromagnetic field is applied during the annealing process.
[0098] Among them, a vacuum heat treatment furnace is used to anneal rare earth steel ingots, with a vacuum degree of 1×10⁻⁶. -1 The annealing process is as follows: ① First, heat to 400℃ at a heating rate of 3℃ / min and hold for 4 min; ② Then, heat to 680℃ at a heating rate of 3℃ / min, turn on the electromagnetic field and hold for 12 min, turn off the electromagnetic field and continue holding for 18 min (hold at annealing temperature of 680℃ for 30 min); ③ Cool with the furnace, maintaining a vacuum of 1×10⁻⁶. -1 Pa, the furnace temperature is cooled to below 200℃ and then air-cooled to room temperature to obtain rare earth steel.
[0099] Comparative Example 1
[0100] The method for preparing rare earth steel provided in this comparative example omits the simultaneous application of electromagnetic field and mechanical vibration during the solidification process in step (2) of Example 1, and also omits the application of electromagnetic field during the annealing process in step (3). The remaining technical features are the same as those in Example 1.
[0101] The rare earth steels prepared in Examples 1-3 and the rare earth-free steel obtained in Comparative Example 1 were subjected to surface cleaning and "tail removal" treatment. Then, the core microstructure of the treated rare earth steels from Examples 1 and Comparative Example 1 was observed using an optical microscope. The resulting optical metallographic micrographs are shown below. Figure 1 and Figure 2 As shown. Simultaneously, the edge microstructure of the rare earth steels obtained in Example 1 and Comparative Example 1 after treatment was observed using an optical microscope. The resulting optical metallographic micrographs are shown below. Figure 3 and Figure 4 As shown.
[0102] Depend on Figures 1-2 It can be seen that the rare earth steel prepared by the microstructure optimization method of this invention has significantly refined equiaxed grains, with no columnar crystals in the core, and the equiaxed grains are fine and the microstructure is uniform; while Figure 2 and Figure 1 The grain size is significantly larger and uneven at the same magnification.
[0103] Depend on Figures 3-4 It can be seen that the rare earth steel prepared by the microstructure optimization method of this invention exhibits significantly refined edge columnar grains and a uniform microstructure; while Figure 4 and Figure 2 The grain size is significantly larger and uneven at the same magnification.
Claims
1. A method for optimizing the microstructure of rare earth steel, comprising the following steps: (1) The raw materials of rare earth steel are smelted to obtain molten steel; (2) The molten steel obtained in step (1) is cast and then solidified to obtain rare earth steel ingots; an electromagnetic field and mechanical vibration are applied simultaneously during the solidification process; the electromagnetic field is applied from the beginning to the end of solidification, and the mechanical vibration is applied for 6 to 10 minutes. (3) Anneal the rare earth steel ingot obtained in step (2) to obtain rare earth steel; an electromagnetic field is applied during the annealing process. The raw materials in step (1) include the raw materials of the base steel and rare earth raw materials; the rare earth raw materials are rare earth-manganese master alloys. The amount of rare earth-manganese master alloy added is calculated according to formula ①; m 稀土-锰中间合金 = (Set value of rare earth mass content in rare earth steel × Mass of rare earth steel to be smelted) / [Rare earth content of rare earth-manganese master alloy × Empirical value of rare earth yield of rare earth steel × (1-κ)] Formula ①; in, The rare earth mass content in rare earth steel is set to 0.02~0.12%; the empirical value of rare earth recovery rate of rare earth steel is 65~80%; κ is the correction coefficient of rare earth recovery rate, and the value of κ is 0.0082~0.0087. In step (2), the direction of the mechanical vibration is perpendicular to the surface of the molten steel, the frequency of the mechanical vibration is 6~10Hz, the amplitude of the mechanical vibration is 0.3~1mm, and the excitation force of the mechanical vibration is 5~10kN. The annealing process in step (3) includes: first heating to 350-450℃ at a heating rate of 3-5℃ / min and holding for 4-8 minutes, then heating to 650-720℃ at a heating rate of 2-4℃ / min and holding for 25-40 minutes. In step (3), the electromagnetic field is applied starting when the temperature rises to 650-720°C during the annealing process and lasts for 10-15 minutes.
2. The tissue optimization method as described in claim 1, characterized in that, In step (2), the direction of the electromagnetic field is parallel to the surface of the molten steel, the current of the electromagnetic field is 180~320A, and the frequency of the electromagnetic field is 3~5Hz.
3. The tissue optimization method as described in claim 1, characterized in that, The cooling method for the annealing process in step (3) is furnace cooling to 200°C and then air cooling to room temperature.
4. The tissue optimization method as described in claim 1, characterized in that, In step (3), the current of the electromagnetic field is 160~300A and the frequency of the electromagnetic field is 2~4Hz.
5. A rare earth steel prepared by the microstructure optimization method according to any one of claims 1 to 4.