A heat-resistant pressure-bearing cast steel, its preparation method and application

Abstract

This invention relates to a heat-resistant pressure-bearing cast steel, its preparation method, and its application. The composition of the heat-resistant pressure-bearing cast steel is as follows: C: 0.15-0.20%, Si≤0.60%, Mn: 0.50-0.90%, P≤0.010%, S≤0.010%, Cr: 1.20-1.50%, Mo: 0.80-1.20%, V: 0.20-0.40%, Nb: 0.30-0.60%, Y: 0.25-0.45%, with the remainder being Fe and unavoidable impurities. Furthermore, the composition of the heat-resistant pressure-bearing cast steel satisfies the following relationship: F1=2.77[Nb]+5.62[Y], 2.54≤F1≤3.68; where [Nb] and [Y] represent the weight percentages of Nb and Y, respectively. The cast steel is preferably subjected to a quenching and tempering heat treatment process and is suitable for use in high-temperature pressure-bearing components such as heat exchangers and reaction vessels.

Description

Technical Field

[0001] This invention relates to a heat-resistant pressure-bearing cast steel, its preparation method, and its application, belonging to the technical field of cast steel. The cast steel provided by this invention has excellent high-temperature mechanical properties and a wide range of applications. At the same time, this invention provides a method for preparing the cast steel. Background Technology

[0002] High-temperature pressure vessels, such as reactors and heat exchangers, have high requirements for the high-temperature mechanical properties of steel. In the past, cast steel was used to manufacture high-temperature pressure vessels. However, as the working conditions of high-temperature pressure vessels become increasingly harsh, the high-temperature mechanical properties of cast steel often fail to meet the requirements of high-temperature and pressure-bearing conditions. Therefore, industry technicians now tend to use heat-resistant stainless steels such as martensitic, austenitic, and ferritic stainless steels as the material for high-temperature pressure vessels.

[0003] While using heat-resistant stainless steel as the material for high-temperature pressure vessels can certainly meet performance requirements, this type of stainless steel typically contains high levels of precious elements such as Cr and Ni, often totaling up to 20%, and in some materials even reaching 40%. Cr and Ni are expensive and have limited reserves. The extensive use of Cr and Ni in this type of material is not conducive to cost control, nor to resource conservation. Furthermore, this type of steel usually requires rolling and welding processes to finally obtain high-temperature pressure-bearing components, making its manufacturing process complex and costly.

[0004] If cast steel can be used as the material for high-temperature pressure vessels, its advantages in raw material cost, resource cost, and process cost are obvious. Cast steel has a very low Cr content, and many cast steels do not even contain Ni, resulting in low raw material costs and avoiding the large-scale use of precious metals. In particular, cast steel can be directly cast in one piece, without the rolling, cutting, and welding steps of traditional stainless steel, which also reduces process costs.

[0005] Therefore, in order to fully utilize the aforementioned advantages of cast steel and realize the application of heat-resistant cast steel to replace heat-resistant stainless steel in pressure-bearing components, improving the high-temperature mechanical properties of heat-resistant cast steel has become an urgent problem to be solved. Summary of the Invention

[0006] This invention provides a heat-resistant pressure-bearing cast steel, its preparation method, and its applications. The cast steel designed by this invention exhibits excellent high-temperature mechanical properties, making it suitable for use as a material for high-temperature pressure vessels. It can be widely applied to high-temperature pressure-bearing components such as heat exchangers and reactors. Compared to traditional heat-resistant stainless steel, it offers significant advantages in raw material cost, resource advantage, and process cost. This allows for a large-scale replacement of heat-resistant stainless steel with heat-resistant cast steel, substantially reducing material costs while meeting high-temperature mechanical performance requirements. Furthermore, this invention also provides a method for preparing the aforementioned heat-resistant pressure-bearing cast steel.

[0007] The technical objective of this invention is achieved through the following means.

[0008] The purpose of this invention is to provide a heat-resistant pressure-bearing cast steel with the following composition: C: 0.15-0.20%, Si≤0.60%, Mn: 0.50-0.90%, P≤0.010%, S≤0.010%, Cr: 1.20-1.50%, Mo: 0.80-1.20%, V: 0.20-0.40%, Nb: 0.40-0.80%, Y: 0.35-0.65%, with the remainder being Fe and unavoidable impurities. Furthermore, the composition of the heat-resistant pressure-bearing cast steel satisfies the following relationship: F1=2.77[Nb]+5.62[Y], 2.54≤F1≤3.68; where [Nb] and [Y] represent the weight percentages of Nb and Y, respectively.

[0009] As mentioned earlier, traditional heat-resistant pressure-bearing cast steels lack sufficient high-temperature mechanical properties, limiting their widespread use in demanding operating conditions. Through extensive experimental research, the inventors of this invention discovered that adding a certain amount of Nb and Y, and rationally controlling their content within a specific range, can significantly improve the high-temperature mechanical properties of cast steel. This broadens its application scope, enabling the replacement of heat-resistant stainless steel with heat-resistant cast steel in high-temperature pressure-bearing components. It also reduces raw material costs, avoids the large-scale use of expensive resources, simplifies the manufacturing process, and lowers process costs.

[0010] The role of each element in the heat-resistant pressure-bearing cast steel of this invention is described below.

[0011] Carbon: Carbon improves the hardenability of steel and is one of the important strengthening elements in steel. It enhances the strength of steel through solid solution strengthening and precipitation strengthening. By controlling the carbon content at a level of 0.15-0.20%, it is possible to ensure that cast steel has the optimal balance between hardenability and toughness.

[0012] Silicon: Silicon is a deoxidizing element in steel. Silicon also has the function of solid solution strengthening. However, if the silicon content is too high, it will lead to a decrease in the toughness of cast steel. This invention controls the silicon content to be below 0.6%.

[0013] Manganese: Manganese plays a role in solid solution strengthening in steel. It can improve both strength and toughness of cast steel. However, excessive manganese will lead to component segregation, affecting the casting quality of cast steel and deteriorating its performance. The manganese content in this invention is controlled at 0.50-0.90%.

[0014] Phosphorus and sulfur: Phosphorus and sulfur are unavoidable impurity elements in steel. Excessive content will drastically deteriorate the performance of cast steel. In this invention, the phosphorus and sulfur content is controlled below 0.01%. The lower the phosphorus and sulfur content, the better, provided that the cost is acceptable.

[0015] Chromium: Chromium is an important strengthening element in steel. Controlling its content within a reasonable range is crucial for ensuring excellent high-temperature strength. Too low a chromium content results in insufficient strengthening, while too high a chromium content not only increases costs but also reduces toughness. This invention controls the chromium content to be between 1.20% and 1.50%.

[0016] Molybdenum: Molybdenum can refine grains and is an important element for improving the hardenability of steel and the room temperature and high temperature strength of cast steel. Too low a molybdenum content results in insufficient strengthening effect and significant temper brittleness in the steel, while too high a molybdenum content leads to saturation of the strength-enhancing effect and a decrease in plasticity. This invention controls the molybdenum content to be between 0.80% and 1.20%.

[0017] Vanadium: Vanadium can refine the grain structure, improve strength and toughness, and also plays a role in ensuring high-temperature strength. Vanadium is a strong carbide-forming element, and it readily combines with carbon to form vanadium carbide. Its fine and dispersed distribution in the cast steel structure can effectively ensure its mechanical properties at both room temperature and high temperature. In this invention, the vanadium content is controlled at 0.20-0.40%.

[0018] Niobium and Yttrium: The inventors discovered that adding appropriate amounts of niobium and yttrium to the cast steel system of this invention can significantly improve the high-temperature strength of the cast steel. The inventors determined the optimal addition amounts of niobium and yttrium through experiments, which are 0.30-0.60% for niobium and 0.25-0.45% for yttrium. If the content of niobium or yttrium is too low, the improvement effect on the high-temperature strength of the cast steel is not significant. However, if the content of niobium or yttrium is too high, not only will the effect of improving the high-temperature strength reach its peak, but it will also begin to have a negative impact on the high-temperature strength and lead to an increase in the cost of cast steel.

[0019] Even more unexpectedly, the inventors discovered during the experiments that simply meeting the aforementioned niobium and yttrium addition amounts did not always guarantee satisfactory high-temperature strength. Coordinated control of the niobium and yttrium content was also necessary. Through repeated experimentation, the inventors found that by controlling the niobium and yttrium content relationship within the aforementioned range to meet certain conditions, cast steel with excellent high-temperature strength could be obtained. Therefore, the inventors experimentally summarized and planned the crucial niobium and yttrium content relationship for this invention: F1 = 2.77[Nb] + 5.62[Y], 2.54 ≤ F1 ≤ 3.68; where [Nb] and [Y] represent the weight percentages of Nb and Y, respectively.

[0020] Furthermore, the inventors' research and experiments revealed that when the composition of the heat-resistant pressure-bearing cast steel also satisfies the following relationship, it is particularly advantageous for obtaining superior heat-resistant pressure-bearing cast steel: F2=0.29[Cr]+0.63[Mo]+1.42[V]+3.12[Nb]+6.25[Y], 4.45≤F2≤5.32; where [Cr], [Mo], [V], [Nb], and [Y] represent the weight percentages of Cr, Mo, V, Nb, and Y, respectively.

[0021] Furthermore, the high-temperature mechanical properties of the heat-resistant pressure-bearing cast steel satisfy: R p0.2,600℃ ≥285MPa, R p0.2,650℃ With a strength of ≥225MPa, it can meet the requirements for long-term stable operation at 650℃.

[0022] As a non-limiting description, the room temperature mechanical properties of the heat-resistant pressure-bearing cast steel are: R p0.2 ≥450 MPa, R m ≥600MPa, elongation ≥15%, room temperature KV2 ≥30J.

[0023] As a non-limiting description, the heat-resistant pressure-bearing cast steel is preferably heat-treated cast steel. As an example, the heat treatment is quenching and tempering; wherein the quenching temperature is 920-960℃ and the tempering temperature is 680-740℃.

[0024] This invention also provides a method for preparing heat-resistant pressure-bearing cast steel, comprising the following steps: melting and casting to obtain cast steel that meets the aforementioned composition and content requirements, and then subjecting the cast steel to quenching and tempering heat treatment. Preferably, the quenching temperature is 920-960℃, the tempering temperature is 680-740℃, and the quenching medium is air, water, or oil, and the tempered steel is air-cooled to room temperature.

[0025] The present invention also provides the use of the aforementioned heat-resistant pressure-bearing cast steel or the heat-resistant pressure-bearing cast steel prepared by the aforementioned method in high-temperature pressure vessels; as a non-limiting description, the high-temperature pressure vessel can be a heat exchanger, a reaction vessel, or other high-temperature pressure-bearing components.

[0026] The present invention has the following technical effects.

[0027] This invention optimizes and adjusts the composition of cast steel by controlling the content of various elements, particularly by adding appropriate amounts of niobium and yttrium, and synergistically controlling the content relationship between niobium and yttrium. This results in a pressure-bearing cast steel with excellent high-temperature mechanical properties, capable of stable operation for extended periods at 650°C. Compared to commonly used heat-resistant stainless steel, the heat-resistant pressure-bearing cast steel obtained by this invention has similar performance, a simpler composition, and a lower content of expensive metals, which helps to reduce costs and conserve precious metal resources. Furthermore, the cast steel can be directly formed without the need for complex subsequent forming processes such as continuous casting, rolling, and welding, saving process costs. This enables the use of heat-resistant cast steel to replace heat-resistant stainless steel in pressure equipment, significantly expanding the application scope of inexpensive cast steel in high-temperature pressure-bearing fields. Detailed Implementation

[0028] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following detailed explanation is provided in conjunction with specific experimental examples.

[0029] According to the composition and element content relationship of the present invention, molten steel is smelted and cast into steel ingots. The P and S contents are controlled at 0.008±0.001%. The composition test results are shown in Table 1. In Table 1, F1=2.77[Nb]+5.62[Y], F2=0.29[Cr]+0.63[Mo]+1.42[V]+3.12[Nb]+6.25[Y]. The ingot size is 1000mm×500mm×350mm.

[0030] Table 1: Composition of each cast steel, %, balance is Fe.

[0031]

[0032] All cast steels underwent a tempering treatment of water quenching followed by holding at 930℃ for 1.5 hours, then holding at 700℃ for 3.5 hours, followed by air cooling. Afterwards, the room temperature strength, elongation, toughness, and high-temperature strength of each cast steel were tested. Room temperature strength and elongation were tested according to GB / T228.1-2021, room temperature toughness according to GB / T229-2020, and high-temperature strength according to GB / T228.2-2015. The test results are recorded in Table 2.

[0033] Table 2: Various mechanical properties of cast steel.

[0034]

[0035] The above embodiments and comparative examples will be further analyzed and explained below with reference to Tables 1 and 2.

[0036] The composition and element content relationships F1 of test numbers 1-6 in Table 2 all meet the requirements of this invention, and their final room temperature mechanical properties meet the standards, and their high temperature mechanical properties can meet R. p0.2,600℃ ≥285MPa, R p0.2,650℃ The invention requires a strength of ≥225 MPa; therefore, the above-mentioned test numbers are embodiments of the present invention. Specifically, the elemental content relationship F2 of test numbers 1, 3, and 4 satisfies the preferred range of 4.45~5.32 of the present invention. The tests confirm that the above-mentioned test numbers, while meeting the room temperature mechanical properties requirements, exhibit superior high-temperature mechanical properties and can meet the R... p0.2,600℃ ≥320MPa, R p0.2,650℃ The high-temperature strength performance level is ≥260MPa; therefore, Examples 1, 3, and 4 are preferred embodiments of the present invention. At least one of Nb, Y, and F1 in test numbers 7-21 fails to meet the requirements of the invention. Test results confirm that although its room temperature mechanical properties meet the standard, its high-temperature mechanical properties are poor and cannot meet the requirements of the present invention. p0.2,600℃ ≥285MPa, R p0.2,650℃ The requirement of ≥225MPa, test number 7-21 is a comparative example of the present invention.

[0037] Below, we will analyze the above comparison ratio in detail.

[0038] Comparative Example 7 is a comparative example of Example 2. With other components the same as in Example 2, the niobium content was increased. The adjusted niobium content is still within the range of the present invention. However, the niobium and yttrium content relationship F1 is higher than the requirements of the invention. The results show that, since F1 does not meet the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0039] Comparative Example 8 is a comparative example of Example 5. With other components the same as in Example 5, the niobium content was reduced. The adjusted niobium content is still within the range of the present invention, but the niobium-yttrium content relationship F1 is lower than the requirements of the invention. The results show that, because F1 does not meet the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0040] Comparative Example 9 is a comparative example of Example 4. With other components the same as in Example 4, the yttrium content was increased. The adjusted yttrium content is still within the range of the present invention. However, the niobium-yttrium content relationship F1 is higher than the requirements of the invention. The results show that, because F1 does not meet the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0041] Comparative Example 10 is a comparative example of Example 3. With other components the same as in Example 3, the yttrium content was reduced. The adjusted yttrium content is still within the range of the present invention, but the niobium-yttrium content relationship F1 is lower than the requirements of the invention. The results show that, because F1 does not meet the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0042] Comparative Example 11 is a comparative example of Example 4. With other components the same as in Example 4, the niobium content was increased. The adjusted niobium and yttrium content relationship F1 is still within the scope of the present invention, but the niobium content is higher than the requirements of the invention. The results show that, due to the niobium content not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0043] Comparative Example 12 is a comparative example of Example 4. With other components the same as in Example 4, the yttrium content was reduced. The adjusted niobium and yttrium content relationship F1 is still within the scope of the present invention, but the yttrium content is lower than the requirements of the invention. The results show that, due to the yttrium content not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0044] Comparative Example 13 is a comparative example of Example 1. With other components the same as in Example 1, the niobium content was reduced. The adjusted niobium and yttrium content relationship F1 is still within the scope of the present invention, but the niobium content is lower than the requirements of the invention. The results show that, due to the niobium content not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0045] Comparative Example 14 is a comparative example of Example 1. With other components the same as in Example 1, the yttrium content was increased. The adjusted niobium and yttrium content relationship F1 is still within the scope of the present invention, but the yttrium content is higher than the requirements of the invention. The results show that, due to the yttrium content not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0046] Comparative Example 15 is a comparative example of Example 5. With other components being the same as in Example 5, the niobium content was reduced, and the adjusted niobium and yttrium content relationship F1 was not within the scope of this invention, and the niobium content was lower than the requirements of the invention. The results showed that, due to the niobium content and F1 not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C was low and could not meet the requirements of the invention.

[0047] Comparative Example 16 is a comparative example of Example 5. With other components the same as in Example 5, the yttrium content was reduced. The adjusted niobium and yttrium content relationship F1 is also outside the scope of this invention, and the yttrium content is lower than the requirements of the invention. The results show that, due to the yttrium content and F1 not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0048] Comparative Example 17 is a comparative example of Example 2. With other components the same as in Example 2, the niobium content was increased. The adjusted niobium and yttrium content relationship F1 is also outside the scope of this invention, and the niobium content is higher than the requirements of the invention. The results show that, due to the niobium content and F1 not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0049] Comparative Example 18 is a comparative example of Example 2. With other components the same as in Example 2, the yttrium content was increased. The adjusted niobium and yttrium content relationship F1 is also outside the scope of this invention, and the yttrium content is higher than the requirements of the invention. The results show that, due to the yttrium content and F1 not meeting the requirements of the invention, the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0050] Comparative Example 19 is a comparative example of Example 6. With other components being the same as in Example 6, niobium was used instead of yttrium. The adjusted niobium and yttrium content relationship F1 is also outside the scope of this invention. However, the niobium content is higher than the requirements of the invention and does not contain yttrium, and F1 does not meet the requirements of the invention. The results show that, because the niobium content does not meet the requirements of the invention, does not contain yttrium, and F1 does not meet the requirements of the invention, the yield strength of this cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention.

[0051] Comparative Example 20 is a comparative example of Example 6. With other components the same as in Example 6, yttrium was used instead of niobium. The adjusted niobium and yttrium content relationship F1 is also not within the scope of this invention. Since the yttrium content is higher than the requirements of the invention and does not contain niobium and F1 does not meet the requirements of the invention, the results show that the yield strength of the cast steel at 600°C and 650°C is low and cannot meet the requirements of the invention because the yttrium content does not meet the requirements of the invention, does not contain niobium, and F1 does not meet the requirements of the invention.

[0052] Comparative Example 21 is a comparative example of Example 3. While other components are the same as in Example 3, niobium and yttrium are omitted. Therefore, the content relationship F1 of niobium and yttrium is necessarily outside the scope of this invention. The results show that, due to the absence of niobium and yttrium, the yield strength of the cast steel at 600°C and 650°C is particularly low, which cannot meet the requirements of the invention and cannot meet the needs of high-temperature working conditions.

[0053] In summary, it is clear that by adding niobium and yttrium in combination and synergistically controlling their content relationship, this invention can ensure the production of cast steel with excellent high-temperature strength properties. The cast steel is suitable for use as high-temperature pressure-bearing components, such as heat exchangers, reactors, and other high-temperature pressure vessels. It has the characteristics of excellent high-temperature performance, low raw material cost, and simple preparation process, and can replace expensive and costly heat-resistant stainless steel.

[0054] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0055] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A heat-resistant pressure-bearing cast steel, characterized in that, The composition of the heat-resistant pressure-bearing cast steel is as follows: C: 0.15-0.20%, Si≤0.60%, Mn: 0.50-0.90%, P≤0.010%, S≤0.010%, Cr: 1.20-1.50%, Mo: 0.80-1.20%, V: 0.20-0.40%, Nb: 0.30-0.60%, Y: 0.25-0.45%, with the remainder being Fe and unavoidable impurities. Furthermore, the composition of the heat-resistant pressure-bearing cast steel satisfies the following relationship: F1=2.77[Nb]+5.62[Y], 2.54≤F1≤3.68; F2=0.29[Cr]+0.63[Mo]+1.42[V]+3.12[Nb]+6.25[Y], 4.45≤F2≤5.32; Where [Cr], [Mo], [V], [Nb], and [Y] represent the weight percentages of Cr, Mo, V, Nb, and Y, respectively.

2. The heat-resistant pressure-bearing cast steel according to claim 1, characterized in that, The high-temperature mechanical properties of the heat-resistant pressure-bearing cast steel meet the following requirements: R p0.2,600℃ ≥285MPa, R p0.2,650℃ ≥225MPa.

3. The heat-resistant pressure-bearing cast steel according to claim 1, characterized in that, The heat-resistant pressure-bearing cast steel is a heat-treated cast steel.

4. The heat-resistant pressure-bearing cast steel according to claim 3, characterized in that, The heat treatment is quenching followed by tempering.

5. The heat-resistant pressure-bearing cast steel according to claim 4, characterized in that, The quenching temperature is 920-960℃, and the tempering temperature is 680-740℃.

6. A method for preparing a heat-resistant pressure-bearing cast steel according to any one of claims 1-5, characterized in that, The steel is smelted and cast to obtain cast steel that meets the requirements of composition and content, and then subjected to heat treatment of quenching and tempering.

7. The method for preparing a heat-resistant pressure-bearing cast steel according to claim 6, characterized in that, The quenching temperature is 920-960℃, and the tempering temperature is 680-740℃.

8. The method for preparing heat-resistant pressure-bearing cast steel according to claim 6, characterized in that, The quenching medium is air, water, or oil, and the tempering is followed by air cooling to room temperature.

9. The use of heat-resistant pressure-bearing cast steel prepared by the method of preparing heat-resistant pressure-bearing cast steel according to any one of claims 1-5 or according to any one of claims 6-8 in high-temperature pressure vessels, wherein the high-temperature pressure vessel is a heat exchanger or a reaction vessel.

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