High-toughness pressure-bearing cast steel and preparation method and application thereof

Abstract

The application relates to a high-strength and high-toughness pressure-bearing cast steel and a preparation method and application thereof. The high-strength and high-toughness pressure-bearing cast steel is composed of C: 0.10-0.15%, Si: <=0.40%, Mn: 0.40-0.70%, P: <=0.020%, S: <=0.010%, Cr: 0.20-0.50%, Mo: 0.40-0.60%, V: 0.15-0.30%, Ti: 0.04-0.08%, N: 0.012-0.023%, Ca: 0.02-0.04%, Sr: 0.005-0.015%, and the rest is Fe and inevitable impurities, and the composition of the high-strength and high-toughness pressure-bearing cast steel satisfies the following relations: F1=([Ca]-1.25[S]) / 40, F2=[Sr] / 88, F3=[Ti] / 48, F1 / F2=4.0-6.5, and F3 / (F1+F2)=1.5-4.5; wherein, [Ca], [S], [Sr] and [Ti] respectively represent the weight percentage of Ca, S, Sr and Ti. The cast steel is prepared through the processes of smelting, casting, quenching and tempering. The cast steel is suitable for normal-temperature pressure containers (such as water storage tanks and gas storage tanks).

Description

Technical Field

[0001] This invention relates to a high-strength and high-toughness 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 strength and toughness matching and a wide range of applications. At the same time, this invention provides the preparation method and application of the cast steel. Background Technology

[0002] Ambient temperature pressure vessels are widely used in daily life, such as water tanks and gas tanks (coal gas cylinders). In recent years, the performance requirements for ambient temperature pressure vessels have become increasingly stringent. Alloy steel and cast steel can both be used as steels for ambient temperature pressure vessels. Alloy steel has good performance but is expensive, while cast steel is cheaper and has a simpler forming process, making it an ideal material for ambient temperature pressure vessels. However, cast steel has poor overall strength and toughness, and cannot meet the requirements for large-scale, high-pressure applications.

[0003] As has been discovered in existing technologies, Ti and N, as inexpensive strengthening elements, have been used in many types of steel. Similarly, applying Ti and N as strengthening agents to cast steel can also achieve significant strength improvements. However, TiN typically exhibits sharp morphologies such as angular edges. While these sharp morphologies can increase strength, they drastically degrade the steel's toughness, leading to insufficient pressure resistance in room-temperature pressure vessel environments and a predisposition to cracking failures. This limits the application of Ti and N as inexpensive and effective strengthening elements in room-temperature pressure-bearing cast steel.

[0004] Therefore, how to introduce inexpensive strengthening elements Ti and N into pressure-bearing cast steel to improve its strength without deteriorating its toughness has become the research objective of this invention. Summary of the Invention

[0005] This invention provides a high-strength and high-toughness pressure-bearing cast steel, its preparation method, and its applications. The cast steel designed by this invention utilizes inexpensive elements Ti and N for effective strengthening. Furthermore, by adding trace amounts of modifying elements while controlling the dosage, the morphology of TiN is modified, resulting in a spherical or near-spherical morphology of the modified TiN, avoiding the appearance of sharp-shaped TiN and thus ensuring sufficient toughness. This pressure-bearing cast steel is suitable for use in ambient temperature pressure vessels, such as large high-pressure liquid storage tanks, high-pressure water storage tanks, gas tanks, etc. Simultaneously, this invention also provides a method for preparing this pressure-bearing cast steel.

[0006] The technical objective of this invention is achieved through the following means.

[0007] The purpose of this invention is to provide a high-strength and high-toughness pressure-bearing cast steel with the following composition: C: 0.10-0.15%, Si≤0.40%, Mn: 0.40-0.70%, P≤0.020%, S≤0.010%, Cr: 0.20-0.50%, Mo: 0.40-0.60%, V: 0.15-0.30%, Ti: 0.04-0.08%, N: 0.012-0.023%, Ca: 0.02-0.04%, Sr: 0. The high-strength and high-toughness pressure-bearing cast steel contains 0.005-0.015% Fe and unavoidable impurities, and the composition of the high-strength and high-toughness pressure-bearing cast steel satisfies the following relationship: F1=([Ca]-1.25[S]) / 40, F2=[Sr] / 88, F3=[Ti] / 48, F1 / F2=4.0-6.5, F3 / (F1+F2)=1.5-4.5; where [Ca], [S], [Sr], and [Ti] represent the weight percentages of Ca, S, Sr, and Ti, respectively.

[0008] As described in the background section, Ti and N are recognized as inexpensive and effective strengthening elements. However, TiN exhibits a sharp, angular morphology, which drastically deteriorates toughness, limiting its application as strengthening elements in pressure-bearing cast steel. Based on the aforementioned problems, the inventors of this invention conducted in-depth research and experimentation, discovering that adding a certain amount of Ca and Sr, and controlling the content of Ca (removing the portion bound to S), Sr, and Ti within a specific range, can effectively modify the morphology of TiN through Ca and Sr, transforming it from a sharp, angular shape to a spherical or near-spherical morphology. Ultimately, this improves the strength of the pressure-bearing cast steel without compromising its toughness, resulting in a high-strength, high-toughness pressure-bearing cast steel that combines both strength and toughness. This makes the cast steel, with its advantages in both material and processing costs, an ideal choice for room-temperature pressure vessels.

[0009] The role of each element in the high-strength and high-toughness pressure-bearing cast steel of this invention is described below.

[0010] 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 0.10-0.15%, it is possible to ensure that cast steel has an excellent balance of strength and toughness.

[0011] 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 0.40%.

[0012] 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 0.40-0.70%.

[0013] Phosphorus and sulfur: Both phosphorus and sulfur are unavoidable impurity elements in steel. Excessive phosphorus and sulfur content will drastically deteriorate the performance of cast steel. In this invention, the phosphorus and sulfur content is controlled below 0.01%, and the lower the phosphorus and sulfur content, the better, provided the cost is acceptable. In particular, sulfur (S) readily combines with Ca, one of the modifying elements in this invention. Therefore, this invention preferably controls S to below 0.007%, and particularly preferably below 0.005%.

[0014] Chromium: Chromium is an important strengthening element in steel. Controlling its content within a reasonable range is crucial for ensuring excellent 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 0.20% and 0.50%.

[0015] Molybdenum: Molybdenum can refine grains and is an important element for improving the hardenability and strength of steel. Too low a molybdenum content results in insufficient strengthening and significant temper brittleness in the steel, while too high a content leads to saturation of the strength-enhancing effect and a decrease in plasticity. This invention controls the molybdenum content to be between 0.40% and 0.60%.

[0016] Vanadium: Vanadium can refine the grain structure, improving strength and toughness. Vanadium is a strong carbide-forming element, particularly prone to combining with carbon to form vanadium carbide, which is finely and dispersedly distributed in the structure of cast steel, effectively strengthening the cast steel. This invention controls the vanadium content to be 0.15-0.30%.

[0017] Titanium and Nitrogen: Titanium is an important strengthening element in this invention. Its combination with nitrogen to form TiN has a significant effect on improving the strength of cast steel. If the content is too low, it will not have an effective strengthening effect, while if the content is too high, the strengthening effect will be saturated, and the plasticity and toughness will be severely deteriorated. Considering the actual strengthening effect and toughness performance, the content of titanium and nitrogen needs to be controlled. This invention controls the titanium content to 0.04-0.08% and the nitrogen content to 0.012-0.023%. In addition, in order to ensure the combination of titanium and nitrogen, it is necessary to control [N]=[Ti] / 3.43, that is, nitrogen and titanium exist in an equiatomic ratio (approximately equiatomic ratio).

[0018] Calcium and Strontium: Calcium and strontium are elements intentionally added in this invention. As mentioned earlier, the inventors discovered that calcium and strontium can effectively improve the morphology of TiN, transforming it from a sharp, angular shape to a spherical or near-spherical shape. This avoids the deteriorating effect of sharp-shaped TiN on toughness, ensuring high strength and toughness in pressure-bearing cast steel without compromising toughness. Furthermore, the inventors found that controlling the content of calcium and strontium at certain levels, and simultaneously controlling the relative amounts of calcium, strontium, and titanium, is crucial for effectively modifying the TiN morphology without causing deterioration in other properties. Regarding the calcium content, it is also necessary to consider removing the portion that combines with sulfur and cannot play a modifying role. If the calcium and strontium content is too low, the modification effect is insufficient, failing to effectively spheroidize the TiN morphology and ensuring adequate toughness. Conversely, if the calcium or strontium content is too high, it causes a sharp deterioration in both strength and toughness, making it impossible to obtain pressure-bearing cast steel with both high strength and toughness. To achieve a balance between strength and toughness, the calcium content of this invention is controlled at 0.02-0.04%, and the strontium content is controlled at 0.005-0.015%.

[0019] As mentioned above, in addition to controlling the calcium and strontium content, it is also necessary to control the content relationship of calcium, strontium, and titanium to ultimately obtain the high-strength and high-toughness pressure-bearing cast steel described in this invention. Through repeated experiments and explorations, the inventors of this invention have discovered the crucial role of the aforementioned elemental content relationship in ensuring strength and toughness. Specifically, F1 = ([Ca] - 1.25[S]) / 40, F2 = [Sr] / 88, F3 = [Ti] / 48, where [Ca], [S], [Sr], and [Ti] represent the weight percentages of Ca, S, Sr, and Ti, respectively. The inventors found that when F1 / F2 = 4.0-6.5 and F3 / (F1+F2) = 1.5-4.5 are satisfied, the pressure-bearing cast steel can balance both strength and toughness.

[0020] As a further improvement, the 40*F1 is 0.01-0.03%, thereby ensuring that there is enough unbound Ca to play a modifying role in TiN.

[0021] As a further improvement to this well design, the room temperature mechanical properties of the high-strength, high-toughness pressure-bearing cast steel satisfy: R m For pressures above 720 MPa, R p0.2 It has an impact strength of 425 MPa or higher and an impact toughness KV2 of 30 J or higher.

[0022] As a non-limiting description, the high-strength and tough pressure-bearing cast steel is a heat-treated cast steel, preferably using a quenching and tempering heat treatment process. The quenching temperature is preferably 950-1000℃, the tempering temperature is preferably 650-720℃, and the quenching medium can be air, water, or oil. After tempering, it is preferably air-cooled to room temperature.

[0023] The present invention also provides a method for preparing high-strength and tough pressure-bearing cast steel, the steps of which are: melting and casting to obtain cast steel that meets the requirements of composition and content relationship, and then subjecting the cast steel to heat treatment of quenching and tempering.

[0024] As a non-limiting description, Ca and Sr in the cast steel composition can be added to the ladle at the end of the melting process before casting, or they can be added during the casting process. This is mainly because Ca and Sr are relatively reactive and easily burned off; therefore, Ca and Sr cannot be added too early.

[0025] As a particularly preferred technical solution, Ca and Sr in the cast steel composition are added along with the flow during the casting process. Since Ca and Sr are relatively reactive and easily burned off, Ca and Sr can be added along with the molten steel during the casting process. Ca and Sr can be added in the form of pure metals or in the form of intermediate alloys, and can be added in the form of granules, powders, or wires.

[0026] The present invention also provides an application of the aforementioned high-strength and high-toughness pressure-bearing cast steel or the high-strength and high-toughness pressure-bearing cast steel prepared by the aforementioned preparation method in a room temperature pressure vessel, which can be a pressure-bearing component such as a high-pressure water tank or an air storage tank.

[0027] The present invention has the following technical effects.

[0028] This invention optimizes the composition of cast steel by adding Ti and N as strengthening elements to significantly improve its strength. Furthermore, Ca and Sr are introduced as modifying elements for TiN. By controlling the Ca and Sr contents and comprehensively regulating the relationship between their contents, the TiN morphology transforms from sharp, angular to spherical or near-spherical, maintaining excellent toughness while improving strength. This results in a high-strength, high-toughness pressure-bearing cast steel. The pressure-bearing cast steel of this invention exhibits excellent strength and toughness, meeting the performance requirements of large-size, high-pressure, room-temperature pressure-bearing components. This cast steel offers high strength and toughness while maintaining low raw material costs, and the molding process for room-temperature pressure-bearing components prepared using this cast steel is simple, which helps reduce processing costs. Attached Figure Description

[0029] Figure 1 This describes the morphology of TiN in the microstructure of the cast steel in Example 1 of this invention. Detailed Implementation

[0030] 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.

[0031] According to the composition and element content relationship of the present invention, molten steel is smelted and cast to obtain cast steel. Ca and Sr are added with the molten steel during the casting process, and the content of P is controlled at 0.015±0.001%. The composition test results are shown in Table 1. In Table 1, let: F1=([Ca]-1.25[S]) / 40, F2=[Sr] / 88, F3=[Ti] / 48, F1 / F2 is denoted as A, and F3 / (F1+F2) is denoted as B; where [Ca], [S], [Sr], and [Ti] represent the weight percentage content of Ca, S, Sr, and Ti, respectively, and the ingot size is 1500mm×400mm×400mm.

[0032] Table 1: Composition of each cast steel, %, balance is Fe.

[0033]

[0034] All cast steels underwent a tempering treatment of water quenching after holding at 975℃ for 3 hours, followed by air cooling after holding at 680℃ for 3 hours. Afterwards, the room temperature strength and toughness of each cast steel were tested. The room temperature strength was tested according to GB / T228.1-2021, and the room temperature toughness was tested according to GB / T229-2020. The test results are recorded in Table 2.

[0035] Table 2: Properties of cast steel.

[0036]

[0037] The above embodiments and comparative examples will be further analyzed and explained below with reference to Tables 1 and 2.

[0038] The composition and element content relationships F1 / F2 and F3 / (F1+F2) 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 satisfy R. m For pressures above 720 MPa, R p0.2 The invention requires an impact strength of 425 MPa or higher and an impact toughness KV2 of 30 J or higher. Therefore, the above test numbers are embodiments of the present invention. Figure 1 The image shows the morphology of TiN in the microstructure of cast steel in Example 1. It can be seen that, with the addition of Ca and Sr and control of the contents of Ca, Sr and Ti, TiN exhibits a spherical or near-spherical morphology.

[0039] At least one of Ti, Ca, Sr, F1 / F2 (hereinafter referred to as A), and F3 / (F1+F2) (hereinafter referred to as B) in test serial numbers 7-18 failed to meet the requirements of the invention. The test results confirmed that their room temperature toughness did not meet the requirements of the invention, and the room temperature strength of some test serial numbers also failed to meet the requirements of the invention. Therefore, test serial numbers 7-18 belong to the comparative examples of the present invention.

[0040] The following is a detailed analysis of the above comparative examples.

[0041] Comparative Example 7 is a comparative example of Example 1. While other components are the same as in Example 1, the calcium content is reduced. The adjusted calcium content is not within the scope of this invention, and A and B are also not within the scope of this invention. The results show that, although the room temperature strength of the cast steel is acceptable, its room temperature toughness is poor and cannot meet the requirements of this invention because the calcium content and A and B do not meet the requirements of this invention.

[0042] Comparative Example 8 is a comparative example of Example 2. With other components the same as in Example 2, the calcium content was increased. The adjusted calcium content is not within the scope of the present invention, and A and B are also not within the scope of the present invention. The results show that because the calcium content and A and B do not meet the requirements of the present invention, especially the excessively high calcium content, the room temperature strength and room temperature toughness of the cast steel deteriorated sharply and could not meet the requirements of the present invention.

[0043] Comparative Example 9 is a comparative example of Example 3. With other components being the same as in Example 3, the strontium content was reduced. The adjusted strontium content is not within the scope of the present invention, and A is also not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because the strontium content and A do not meet the requirements of the present invention.

[0044] Comparative Example 10 is a comparative example of Example 4. With other components being the same as in Example 4, the strontium content was increased. The adjusted strontium content is not within the scope of the present invention, and A is also not within the scope of the present invention. The results show that, due to the strontium content and A not meeting the requirements of the present invention, especially the excessively high strontium content, the room temperature strength and room temperature toughness of the cast steel deteriorated sharply and could not meet the requirements of the present invention.

[0045] Comparative Example 11 is a comparative example of Example 5. While other components are the same as in Example 5, the titanium content is reduced. The adjusted titanium content is not within the scope of the present invention, and B is also not within the scope of the present invention. The results show that because the titanium content and B do not meet the requirements of the present invention, especially the excessively low titanium content leading to a reduction in TiN, the room temperature strength and room temperature toughness of the cast steel deteriorate sharply and cannot meet the requirements of the present invention.

[0046] Comparative Example 12 is a comparative example of Example 5. While other components are the same as in Example 5, the titanium content is increased. The adjusted titanium content is not within the scope of this invention. The results show that due to the excessively high titanium content, there is not enough N to form TiN, and the strengthening effect tends to saturate. At the same time, the excessively high Ti content seriously deteriorates the room temperature toughness of the cast steel, resulting in the room temperature toughness of the cast steel failing to meet the requirements of the invention.

[0047] Comparative Example 13 is a comparative example of Example 1. With other components the same as in Example 1, the calcium content was reduced. The adjusted calcium content is still within the scope of the present invention, but A is not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because A does not meet the requirements of the present invention.

[0048] Comparative Example 14 is a comparative example of Example 2. With other components the same as in Example 2, the calcium content was increased. The adjusted calcium content is still within the scope of the present invention, but B is not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because B does not meet the requirements of the present invention.

[0049] Comparative Example 15 is a comparative example of Example 3. With other components being the same as in Example 3, the strontium content was reduced. The adjusted strontium content is still within the scope of the present invention, but A is not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because A does not meet the requirements of the present invention.

[0050] Comparative Example 16 is a comparative example of Example 4. With other components being the same as in Example 4, the strontium content was increased. The adjusted strontium content is still within the scope of the present invention, but A is not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because A does not meet the requirements of the present invention.

[0051] Comparative Example 17 is a comparative example of Example 4. With other components the same as in Example 4, the titanium content was increased (the nitrogen content was adjusted accordingly according to the relationship [N]=[Ti] / 3.43). The adjusted titanium and nitrogen contents are still within the scope of the present invention, but B is not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because B does not meet the requirements of the present invention.

[0052] Comparative Example 18 is a comparative example of Example 5. With other components the same as in Example 5, the titanium content was reduced (the nitrogen content was adjusted accordingly according to the relationship [N]=[Ti] / 3.43). The adjusted titanium and nitrogen contents are still within the scope of the present invention, but B is not within the scope of the present invention. The results show that, although the room temperature strength of the cast steel is acceptable, the room temperature toughness is poor and cannot meet the requirements of the present invention because B does not meet the requirements of the present invention.

[0053] In summary, it is clear that this invention, based on the addition of Ti and N for strengthening, further adds a certain amount of Ca and Sr to modify the morphology of TiN, avoiding the deteriorating effect of the sharp morphology of TiN on the toughness of cast steel. While using inexpensive strengthening elements Ti and N to improve strength, it maintains excellent toughness and obtains a pressure-bearing cast steel with excellent strength and toughness, which is suitable for various room temperature pressure vessels.

[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 high-strength and high-toughness pressure-bearing cast steel, characterized in that, The composition of the high-strength and high-toughness pressure-bearing cast steel is as follows: C: 0.10-0.15%, Si≤0.40%, Mn: 0.40-0.70%, P≤0.020%, S≤0.010%, Cr: 0.20-0.50%, Mo: 0.40-0.60%, V: 0.15-0.30%, Ti: 0.04-0.08%, N: 0.012-0.023%, Ca: 0.02-0.04%, Sr: 0.005-0.015%, with the remainder being Fe and unavoidable impurities. Furthermore, the composition of the high-strength and high-toughness pressure-bearing cast steel satisfies the following relationship: F1=([Ca]-1.25[S]) / 40, F2=[Sr] / 88, F3=[Ti] / 48, F1 / F2=4.0-6.5, F3 / (F1+F2)=1.5-4.5; Wherein, [Ca], [S], [Sr], and [Ti] represent the weight percentages of Ca, S, Sr, and Ti, respectively.

2. The high-strength and high-toughness pressure-bearing cast steel according to claim 1, characterized in that, 40*F1 is 0.01-0.03%.

3. The high-strength and high-toughness pressure-bearing cast steel according to claim 1, characterized in that, [N] = [Ti] / 3.43, where [N] and [Ti] represent the weight percentages of N and Ti, respectively.

4. The high-strength and high-toughness pressure-bearing cast steel according to claim 1, characterized in that, The room temperature mechanical properties of the high-strength and high-toughness pressure-bearing cast steel meet the following requirements: R m For pressures above 720 MPa, R p0.2 It has an impact strength of 425 MPa or higher and an impact toughness KV2 of 30 J or higher.

5. The high-strength and high-toughness pressure-bearing cast steel according to claim 1, characterized in that, The high-strength and high-toughness pressure-bearing cast steel is a heat-treated cast steel.

6. The high-strength and high-toughness pressure-bearing cast steel according to claim 5, characterized in that, The heat treatment is quenching followed by tempering.

7. A method for preparing high-strength and high-toughness pressure-bearing cast steel according to any one of claims 1-6, 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.

8. The method for preparing high-strength and high-toughness pressure-bearing cast steel according to claim 7, characterized in that, Ca and Sr are added during the casting process of cast steel.

9. The application of the high-strength and high-toughness pressure-bearing cast steel according to any one of claims 1-6 or the high-strength and high-toughness pressure-bearing cast steel prepared by the preparation method according to any one of claims 7-8 in a room temperature pressure vessel, wherein the room temperature pressure vessel is a water storage tank or a gas storage tank.

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