A method for improving high temperature resistance of cast steel

Abstract

The application discloses a cast steel with high-temperature resistance, characterized in that the cast steel contains the following chemical components in percentage by weight: C: 0.3-0.4%, Si: 1.0-1.5%, Mn: <=2.0%, Mo <=0.5%, Ni: 18-22%, Cr: 24-30%, V: 0.3-0.5%, Nb: 0.5-1.2%, and the rest is Fe. In the application, the content ratio of each component is adjusted, and specific contents of V and Nb are added, so that the high-temperature creep resistance of the material is improved, especially at a high temperature of 1000 DEG C, the material still has excellent creep resistance, the creep limit at 1000 DEG C reaches 75 MPa, the endurance strength reaches 90 MPa, and the average oxidation weight gain after oxidation for 100 hours is 0.015 g / m 2 ·h.

Description

[0001] This patent is a divisional application of invention number 202311250034.1, entitled "A cast steel with high creep resistance and its preparation method". Technical Field

[0002] This invention relates to the field of alloy steel preparation technology, and specifically to a method for improving the high-temperature resistance of cast steel. Background Technology

[0003] Creep refers to the slow plastic deformation of a material over time when subjected to a constant external force below its yield strength. It is an important mechanism for structural failure in high-temperature materials. Creep reflects the rheological properties of a material under load, i.e., its flow after loading; moreover, creep also reflects the stability of the material under temperature changes, making it one of the important indicators of a material's high-temperature resistance.

[0004] Once creep occurs, the properties of a material deteriorate over time, eventually leading to failure. Many metal components operate in high-temperature and high-pressure environments; if their performance declines or even their structure fails, it can result in catastrophic consequences and huge economic losses. Therefore, higher requirements are placed on the safety and reliability of these components. Numerous scholars both domestically and internationally have conducted a series of studies on the factors influencing creep in metallic materials. Results show that temperature, stress, and characteristics such as alloy precipitates, porosity, grain size, composition, crystal structure, diffusion, and dislocation movement are all important factors affecting creep.

[0005] When manufacturing thin parts, the narrow cavity can amplify the impact of material feeding performance on casting defects, leading to a decrease in product molding rate. Therefore, on the one hand, we can improve the high-temperature resistance of materials by enhancing their resistance to oxidation and creep, thereby improving their stability and reliability under thermo-chemical-mechanical coupling factors. On the other hand, we can start by adjusting the composition of the material to adapt to the casting of thin parts, thus manufacturing parts that meet the requirements of the engine's exhaust system.

[0006] Therefore, in order to improve the creep resistance of medium carbon high alloy steel, based on existing heat-resistant cast steel materials, it is necessary to develop a medium carbon high alloy cast steel material with better high-temperature creep resistance, which can meet the requirements of longer service life, higher reliability, and thin-film casting performance, so as to expand the application range of heat-resistant cast steel in high-temperature fields. Summary of the Invention

[0007] The purpose of this invention is to provide a cast steel with high high temperature resistance.

[0008] Another objective of this invention is to provide a method for improving the high-temperature resistance of cast steel, effectively ensuring that the cast steel material has excellent high-temperature oxidation resistance, while significantly improving the high-temperature creep resistance of the material.

[0009] The objective of this invention is achieved through the following technical solution: A cast steel with high temperature resistance, characterized in that: the chemical components in the cast steel, by weight percentage, are: C: 0.3~0.4%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~22%, Cr: 24~30%, V: 0.3~0.5%, Nb: 0.5~1.2%, with the remainder being Fe.

[0010] Preferably, the chemical components in the cast steel, by weight percentage, are: C: 0.3~0.35%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~20%, Cr: 26~28%, V: 0.4~0.5%, Nb: 0.7~1.0%, with the remainder being Fe.

[0011] C (carbon) can expand the γ-phase region and is also a constituent element of carbides. With increasing temperature, the decomposition and segregation of carbides reduce the creep resistance of the prepared cast steel material. Therefore, it is essential to control the type, content, distribution, and morphology of carbides. Because the Cr content added in this invention is relatively high, C will compete with oxygen for Cr to form chromium carbides, resulting in chromium-depleted regions near grain boundaries. This reduces the chromium content near the grain boundaries, affecting the material's oxidation resistance. Furthermore, it increases solidification shrinkage and affects casting performance, making defects such as shrinkage cavities more likely. However, if the C content is too low, the flow properties will deteriorate. At 1600℃, the spiral test length for C ≤ 0.2% is ≤ 500 mm, which is unfavorable for filling molten steel in narrow cavities. Therefore, the C content is controlled within the range of 0.25% to 0.40%.

[0012] Ni, as the main additive element for controlling the austenite region, is a key alloying element for ensuring the formation of a γ-phase microstructure at room temperature. Controlling the microstructure to a single γ-phase to improve stability has become a consensus in heat-resistant steels. However, adding large amounts of Ni increases material costs and degrades casting performance. Under the composition specified in this invention, when the Ni content exceeds 23%, the solidification temperature range of the material significantly increases. During solidification, a wide, pasty region exists between the completely solid and completely liquid phases, resulting in pasty solidification. When used in thin-walled, narrow-cavity molds, the feeding channels are narrow, and the crystals in the pasty region grow in a dendritic manner. The volume shrinkage of the final solidified portion is blocked by the intricate dendrites, preventing complete solution replenishment and causing defects such as shrinkage cavities and porosity, reducing the density of the casting. However, when the Ni content is below 18%, its compatibility with Cr is insufficient, leading to a decrease in oxidation resistance at temperatures above 800℃. Therefore, the Ni content is ultimately controlled within the range of 18% to 22%.

[0013] The main function of Cr is to improve the high-temperature oxidation resistance of materials. It can combine with Ni to form a NiO·Cr2O3 composite oxide film on the surface, preventing corrosive gases such as oxygen, sulfur, and nitrogen from diffusing into the steel. Furthermore, Cr is a ferrite-forming element and can work with C and Ni to regulate the size of the γ-phase region. Nb and Cr have a significant impact on the liquidus temperature of the alloy. When the Cr content exceeds 30%, it leads to an increased range of austenite and liquid phase two-phase regions and more defects. When the content is below 23%, the combination with elements such as C and Ni is poor. Therefore, by adjusting the ratio of Nb and Cr, the range of the austenite and liquid phase two-phase region can be reduced, thereby narrowing the solidification temperature range, bringing the material closer to the eutectic composition point, improving the material's sequential solidification characteristics, and reducing casting defects. Therefore, considering both oxidation resistance and casting performance requirements, the Cr content is controlled within the range of 23.0% to 30.0%.

[0014] V and Nb enhance the creep resistance of the alloy. However, when V increases above 1.0% or Nb increases above 2.0%, the matrix hardness increases significantly, leading to a deterioration in machinability. Under the wear of high-hardness carbides, the machining efficiency decreases. When V decreases below 0.3% or Nb decreases below 0.5%, the improvement in creep resistance above 850℃ is not significant. Therefore, the final ranges for V and Nb are controlled at 0.3%~0.5% and 0.5%~1.2%, respectively. Within these ranges, the high-temperature creep resistance of the material is significantly enhanced.

[0015] Si can assist Cr in forming a dense oxide on the material surface, improving its antioxidant properties. At the same time, Si can also lower the melting point and improve fluidity. More importantly, SiC formed by Si and C can effectively hinder the slip of dislocations around it, directly improving the material's creep resistance. However, in this invention, when the Si content exceeds 1.5%, columnar crystals are easily formed, increasing the tendency for hot cracking. Therefore, in this invention, the Si content is controlled at 1.0%~1.5%.

[0016] Furthermore, the aforementioned high-temperature resistant cast steel is obtained through alloy design, smelting, casting, annealing, solution treatment, and aging treatment. The annealing treatment involves heating at a rate of 100~120℃ / h to 800~820℃, holding for 1~4h, and then cooling in the furnace to 400℃ before being removed from the furnace and air-cooled.

[0017] Furthermore, the solution treatment temperature is 1200~1250℃, the time is 1~3h, and then water quenching is performed for 2~5min, with the outlet water temperature below 100℃.

[0018] Furthermore, the aging treatment is carried out at a temperature of 700~800℃ for 4~10 hours, and then cooled in the furnace to 400℃ before being removed from the furnace and air-cooled.

[0019] A method for improving the high-temperature resistance of cast steel includes alloy design, smelting, casting, annealing, solution treatment, and aging treatment. The method is characterized in that the alloy design is based on Cr-Mo heat-resistant alloy steel, adjusting the chemical composition and proportions of the alloy steel, specifically: C: 0.3~0.4%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~22%, Cr: 24~30%, V: 0.3~0.5%, Nb: 0.5~1.2%, with the remainder being Fe.

[0020] Furthermore, the smelting process involves weighing pure iron, low-carbon ferrochrome, nickel plate, ferromolybdenum, ferroniobium, ferrovanadium, carbon blocks, etc., according to the nominal mass fraction of each element in the designed alloy composition. The smelting temperature is controlled at 1650℃~1750℃, and the smelting process is carried out with sufficient electromagnetic stirring and mechanical stirring.

[0021] Furthermore, the pouring temperature is 1550~1600℃, and the pouring rate is 2~5kg / s.

[0022] Furthermore, the annealing process involves heating at a rate of 100-120℃ / h to 800-820℃, holding for 1-4 hours, and then cooling in the furnace to 400℃ before being removed from the furnace and air-cooled.

[0023] Furthermore, the solution treatment temperature is 1200~1250℃, the time is 1~3h, and then water quenching is performed for 2~5min, with the outlet water temperature below 100℃.

[0024] Furthermore, the aging treatment is carried out at a temperature of 700~800℃ for 4~10 hours, and then cooled in the furnace to 400℃ before being removed from the furnace and air-cooled.

[0025] A method for improving the high-temperature resistance of cast steel, characterized by the following steps: (1) Alloy design: Based on Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted, specifically C: 0.3~0.4%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~22%, Cr: 24~30%, V: 0.3~0.5%, Nb: 0.5~1.2%, and the remainder is Fe; (2) Smelting and casting: According to the proportion of each alloy component designed, weigh pure iron, low carbon ferrochrome, nickel plate, ferromolybdenum, ferroniobium, ferrovanadium and carbon blocks, and smelt them. The smelting temperature is controlled at 1650℃~1750℃. The smelting process is carried out with full electromagnetic stirring and mechanical stirring. (3) Pouring: The temperature is 1550~1600℃, the pouring rate is 2~5kg / s, and the pouring is allowed to stand after pouring; (4) Annealing treatment: The cast blank is heated to 800-820℃ at a heating rate of 100-120℃ / h and held for 1-4 hours. After the holding time, it is cooled to 400℃ in the furnace and then removed from the furnace and air-cooled. (5) Solution treatment: After annealing, solution treatment is performed at a temperature of 1200~1250℃ for 1~3h. Then, water quenching is performed for 2~5min, and the water temperature is below 100℃. (6) Aging treatment: After the solution treatment is completed, aging treatment is carried out at a temperature of 700~800℃ for 4~10h. Then, the furnace is cooled to 400℃ and then the furnace is removed and air-cooled.

[0026] In addition, elements such as Cr, Mo, V, and Nb react with C to form Cr 23Carbides such as C6, Cr7C3, Mo2C, VC, and NbC do not dissolve when heated to 900℃~1000℃. This invention adjusts the proportions of each element to regulate the types and contents of the generated carbides, and at a solution temperature of 1200~1250℃, allows the alloying elements to fully dissolve and diffuse uniformly in the matrix. These carbides anchor in the grain boundaries, reducing the dislocation climb rate and preventing grain boundary creep at high temperatures, thereby improving the high-temperature creep resistance of the material. Subsequent high-temperature aging treatment at 700~800℃ generates a second phase in the matrix, forming a dispersed precipitate phase, further improving the creep resistance of the cast steel.

[0027] The present invention has the following technical effects: This invention improves the high-temperature creep resistance of the material by adjusting the proportions of its components and adding specific amounts of V and Nb. In particular, it maintains excellent creep resistance even at temperatures up to 1000℃, achieving a creep limit of 75 MPa and a creep rupture strength of 90 MPa at 1000℃. The average weight gain after 100 hours of oxidation is 0.015 g / m³. 2 ·h. Detailed Implementation

[0028] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0029] Example 1 A method for improving the high-temperature resistance of cast steel is carried out according to the following steps: (1) Alloy design: Based on the Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted. Specifically, C: 0.35%, Si: 1.2%, Mn: 1.5%, Mo: 0.2%, Ni: 20%, Cr: 24%, V: 0.45%, Nb: 0.8%, and the remainder is Fe; (2) Smelting and casting: According to the proportion of each alloy component designed, weigh pure iron, low carbon ferrochrome, nickel plate, ferromolybdenum, ferroniobium, ferrovanadium and carbon blocks, and smelt them. The smelting temperature is controlled at 1700℃. The smelting process is carried out with full electromagnetic stirring and mechanical stirring. (3) Pouring: The temperature is 1580℃, the pouring rate is 4kg / s, and the mixture is left to stand after pouring; (4) Annealing treatment: The cast blank is heated to 810℃ at a heating rate of 110℃ / h and held for 3 hours. After the holding time, it is cooled to 400℃ in the furnace and then removed from the furnace and air-cooled. (5) Solution treatment: After annealing, solution treatment is performed at a temperature of 1220℃ for 2 hours, followed by water quenching for 4 minutes and the water temperature is below 100℃. (6) Aging treatment: After the solution treatment is completed, aging treatment is carried out at a temperature of 750℃ for 6 hours. Then, the furnace is cooled to 400℃ and then removed from the furnace and air-cooled.

[0030] Creep limit and creep strength are important indicators for evaluating the creep resistance of materials.

[0031] Creep limit refers to the tensile strength at which a material does not undergo brittle deformation under long-term high-temperature load.

[0032] Duration strength refers to the maximum stress value that causes a material to fracture within a certain temperature and a specified duration.

[0033] Based on Example 1, by changing the ratio of V and Nb, corresponding cast steel materials were prepared, and the corresponding performance data of the materials at 800℃ were tested. The results are shown in Table 1. The creep limit and endurance strength were tested with reference to "GB / T2039-2012 Metallic Materials Uniaxial Tensile Creep Test Method", and the oxidation resistance was tested with reference to "GB / T 13303-91 Determination of Oxidation Resistance of Steel".

[0034] Table 1:

[0035] With other conditions remaining constant, the effects of changes in V and Nb on the creep resistance of the material are shown in the table above. When the addition amounts of V and Nb are between 0.3~0.5% and 0.5~1.2%, respectively, the material exhibits excellent creep resistance and relatively good oxidation resistance at 800℃. When the addition amounts of V and Nb are between 0.4~0.5% and 0.7~1.0%, respectively, the material exhibits even better creep resistance and oxidation resistance.

[0036] Comparative Example 1 Unlike Example 1, the solution treatment temperature was 1180°C and the holding time was 2 hours, while the remaining steps and parameters remained the same as in Example 1.

[0037] Example 2 A method for improving the high-temperature resistance of cast steel is carried out according to the following steps: (1) Alloy design: Based on Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted. Specifically, C: 0.3%, Si: 1.0%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18%, Cr: 24%, V: 0.3%, Nb: 0.5%, and the remainder is Fe; (2) Smelting and casting: According to the proportion of each alloy component designed, weigh pure iron, low carbon ferrochrome, nickel plate, ferromolybdenum, ferroniobium, ferrovanadium and carbon blocks, and smelt them. The smelting temperature is controlled at 1650℃. The smelting process is carried out with full electromagnetic stirring and mechanical stirring. (3) Pouring: The temperature is 1550℃, the pouring rate is 5kg / s, and the mixture is left to stand after pouring; (4) Annealing treatment: The cast blank is heated to 820℃ at a heating rate of 120℃ / h and held for 1 hour. After the holding time, it is cooled to 400℃ in the furnace and then removed from the furnace and air-cooled. (5) Solution treatment: After annealing, solution treatment is performed at a temperature of 1200℃ for 3 hours, followed by water quenching for 5 minutes and the water temperature is below 100℃. (6) Aging treatment: After the solution treatment is completed, aging treatment is carried out at a temperature of 800℃ for 4 hours. Then, the furnace is cooled to 400℃ and then the furnace is removed and air-cooled.

[0038] Example 3 A method for improving the high-temperature resistance of cast steel is carried out according to the following steps: (1) Alloy design: Based on Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted, specifically C: 0.4%, Si: 1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 22%, Cr: 30%, V: 0.5%, Nb: 1.2%, and the remainder is Fe; (2) Smelting and casting: According to the proportion of each alloy component designed, weigh pure iron, low carbon ferrochrome, nickel plate, ferromolybdenum, ferroniobium, ferrovanadium and carbon blocks, and smelt them. The smelting temperature is controlled at 1750℃. The smelting process is carried out with full electromagnetic stirring and mechanical stirring. (3) Pouring: The temperature is 1600℃, the pouring rate is 2kg / s, and the mixture is left to stand after pouring; (4) Annealing treatment: The cast blank is heated to 800℃ at a heating rate of 100℃ / h and held for 4 hours. After the holding time, it is cooled to 400℃ in the furnace and then removed from the furnace and air-cooled. (5) Solution treatment: After annealing, solution treatment is performed at a temperature of 1250℃ for 1 hour, followed by water quenching for 2 minutes, with the water temperature below 100℃. (6) Aging treatment: After the solution treatment is completed, aging treatment is carried out at a temperature of 700℃ for 10 hours. Then, the furnace is cooled to 400℃ and then removed from the furnace and air-cooled.

[0039] Table 2 shows the creep resistance limits of the cast steel materials prepared in each embodiment and comparative example under given pressure, temperature and time conditions.

[0040] Table 2:

[0041] Lower solution temperatures affect the re-dissolution and diffusion of alloying elements, leading to a decrease in creep resistance. Higher solution temperatures, on the other hand, promote the full re-dissolution of alloying elements and their uniform diffusion within the matrix. These elements anchor at grain boundaries, reducing dislocation climb rates and preventing grain boundary creep at high temperatures, thus lowering the creep rate and improving the material's high-temperature creep resistance and oxidation resistance.

Claims

1. A cast steel with high-temperature resistance, characterized in that: The chemical components in the cast steel, by weight percentage, are: C: 0.3~0.4%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~22%, Cr: 24~30%, V: 0.3~0.5%, Nb: 0.5~1.2%, with the remainder being Fe.

2. The cast steel with high-temperature resistance as described in claim 1, characterized in that: The chemical components in the cast steel, by weight percentage, are: C: 0.3~0.35%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~20%, Cr: 26~28%, V: 0.4~0.5%, Nb: 0.7~1.0%, with the remainder being Fe.

3. A cast steel with high-temperature resistance as described in claim 1 or 2, characterized in that: The cast steel is produced through alloy design, smelting, casting, annealing, solution treatment and aging treatment. The annealing treatment is carried out at a heating rate of 100~120℃ / h, heating to 800~820℃, holding for 1~4h, and after holding, cooling to 400℃ in the furnace and then air cooling.

4. The cast steel with high-temperature resistance as described in claim 3, characterized in that: The solution treatment is performed at a temperature of 1200~1250℃ for 1~3 hours, followed by water quenching for 2~5 minutes, with the outlet water temperature below 100℃.

5. A cast steel with high-temperature resistance as described in claim 3 or 4, characterized in that: The aging treatment is performed at a temperature of 700-800℃ for 4-10 hours, and then the furnace is cooled to 400℃ before being removed and air-cooled.

6. A method for improving the high-temperature resistance of cast steel, comprising alloy design, smelting, casting, annealing, solution treatment, and aging treatment, characterized in that: The alloy design is based on Cr-Mo heat-resistant alloy steel, with adjustments to the chemical composition and proportions of the alloy steel. Specifically, the composition is: C: 0.3~0.4%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo≤0.5%, Ni: 18~22%, Cr: 24~30%, V: 0.3~0.5%, Nb: 0.5~1.2%, with the remainder being Fe.

7. The method for improving the high-temperature resistance of cast steel as described in claim 6, characterized in that: The annealing process involves heating at a rate of 100-120℃ / h to 800-820℃, holding for 1-4 hours, and then cooling in the furnace to 400℃ before being removed from the furnace and air-cooled.

8. A method for improving the high-temperature resistance of cast steel as described in claim 6 or 7, characterized in that: The solution treatment is performed at a temperature of 1200~1250℃ for 1~3 hours, followed by water quenching for 2~5 minutes, with the outlet water temperature below 100℃.

9. A method for improving the high-temperature resistance of cast steel as described in any one of claims 6-8, characterized in that: The aging treatment is performed at a temperature of 700-800℃ for 4-10 hours, and then the furnace is cooled to 400℃ before being removed and air-cooled.

10. A method for improving the high-temperature resistance of cast steel, characterized in that, Follow these steps: (1) Alloy design: Based on the Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted. Specifically, C: 0.35%, Si: 1.2%, Mn: 1.5%, Mo: 0.2%, Ni: 20%, Cr: 24%, V: 0.45%, Nb: 0.8%, and the remainder is Fe; (2) Smelting and casting: According to the proportion of each alloy component designed, weigh pure iron, low carbon ferrochrome, nickel plate, ferromolybdenum, ferroniobium, ferrovanadium and carbon blocks, and smelt them. The smelting temperature is controlled at 1700℃. The smelting process is carried out with full electromagnetic stirring and mechanical stirring. (3) Pouring: The temperature is 1580℃, the pouring rate is 4kg / s, and the mixture is left to stand after pouring; (4) Annealing treatment: The cast blank is heated to 810℃ at a heating rate of 110℃ / h and held for 3 hours. After the holding time, it is cooled to 400℃ in the furnace and then removed from the furnace and air-cooled. (5) Solution treatment: After annealing, solution treatment is performed at a temperature of 1220℃ for 2 hours, followed by water quenching for 4 minutes and the water temperature is below 100℃. (6) Aging treatment: After the solution treatment is completed, aging treatment is carried out at a temperature of 750℃ for 6 hours. Then, the furnace is cooled to 400℃ and then removed from the furnace and air-cooled.

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