A pressure-bearing cast steel with high toughness and a preparation method and application thereof

Abstract

The application relates to a pressure-bearing cast steel with high strength and toughness and a preparation method and application thereof. The 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: 0.03-0.07%, N: 0.009-0.02%, Mg: 0.008-0.02%, Ba: 0.003-0.013%, and the rest is Fe and inevitable impurities. The composition of the pressure-bearing cast steel with high strength and toughness satisfies the following relations: F1=([Mg]-0.75[S]) / 24, F2=[Ba] / 137, F3=[Ti] / 48, F1 / F2=5.7-8.0, and F3 / (F1+F2)=1.5-2.8; wherein, [Mg], [S], [Ba] and [Ti] represent the weight percentage of Mg, S, Ba and Ti respectively. The cast steel is prepared through the processes of smelting, casting, quenching and tempering. The cast steel is suitable for normal-temperature pressure vessels (such as water storage tanks and gas storage tanks).

Description

Technical Field

[0001] This invention relates to a pressure-bearing cast steel with both high strength and toughness, its preparation method, and its application. It belongs 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 pressure-bearing cast steel with both high strength and toughness, 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 pressure-bearing cast steel with both high strength and toughness, the composition of which is 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: 0.03-0.07%, N: 0.009-0.02%, Mg: 0.008-0.02%, Ba: 0. 0.003-0.013%, the remainder being Fe and unavoidable impurities, and the composition of the high-strength and tough pressure-bearing cast steel satisfies the following relationship: F1=([Mg]-0.75[S]) / 24, F2=[Ba] / 137, F3=[Ti] / 48, F1 / F2=5.7-8.0, F3 / (F1+F2)=1.5-2.8; where [Mg], [S], [Ba], and [Ti] represent the weight percentages of Mg, S, Ba, 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 Mg and Ba, while controlling the content of Mg (removing the portion bound to S), Ba, and Ti within a specific range, can effectively modify the morphology of TiN through Mg and Ba, 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 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 following describes the role of each element in the high-strength and tough pressure-bearing cast steel of this invention.

[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 Mg, 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 0.03-0.07% and the nitrogen content to 0.009-0.02%. 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] Magnesium and Barium: Magnesium and barium are elements intentionally added in this invention. As mentioned earlier, the inventors discovered that magnesium and barium 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 that the toughness is not worsened while introducing TiN for strengthening, thus guaranteeing a pressure-bearing cast steel with both strength and toughness. Furthermore, the inventors found that it is necessary to control the content of magnesium and barium at certain levels, and simultaneously control the content of magnesium, barium, and titanium to meet certain relationships, in order to effectively modify the morphology of TiN without causing deterioration of other properties. Regarding the magnesium content, it is also necessary to consider removing the portion that combines with sulfur and cannot play a modifying role. If the content of magnesium and barium is too low, the modification effect is insufficient, and the TiN morphology cannot be effectively spheroidized, failing to ensure that the toughness meets the standard. If the content of magnesium or barium is too high, it will cause a sharp deterioration in strength and toughness, making it impossible to obtain a pressure-bearing cast steel with both strength and toughness. To achieve a balance between strength and toughness, the magnesium content of this invention is controlled at 0.008-0.02%, and the barium content is controlled at 0.003-0.013%.

[0019] As mentioned above, in addition to controlling the magnesium and barium content, it is also necessary to control the content relationship of magnesium, barium, 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 = ([Mg] - 0.75[S]) / 24, F2 = [Ba] / 137, F3 = [Ti] / 48, where [Mg], [S], [Ba], and [Ti] represent the weight percentages of Mg, S, Ba, and Ti, respectively. The inventors found that when F1 / F2 = 5.7-8.0 and F3 / (F1+F2) = 1.5-2.8 are satisfied, the pressure-bearing cast steel can achieve a balance between strength and toughness.

[0020] As a further improvement, the 24*F1 is 0.005-0.015%, thereby ensuring that there is enough unbound Mg 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 and tough pressure-bearing cast steel satisfy: R m For pressures above 700 MPa, R p0.2 It has an impact strength of over 400MPa and an impact toughness of over 30J KV2.

[0022] As a non-limiting description, the high-strength and tough pressure-bearing cast steel is 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℃, the quenching medium can be air, water or oil, and after tempering, it is preferably air-cooled to room temperature.

[0023] The present invention also provides a method for preparing pressure-bearing cast steel with both strength and toughness, 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, Mg and Ba 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 Mg and Ba are relatively reactive and easily burn-off; therefore, they cannot be added too early.

[0025] As a particularly preferred technical solution, Mg and Ba in the cast steel composition are added along with the flow during the casting process. Since Mg and Ba are relatively reactive and easily burned off, Mg and Ba can be added along with the molten steel during the casting process. Mg and Ba 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-toughness pressure-bearing cast steel or the pressure-bearing cast steel prepared by the aforementioned high-toughness pressure-bearing cast steel preparation method in a room temperature pressure vessel, which can be a pressure-bearing component such as a high-pressure water tank or a gas 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, Mg and Ba are introduced as modifying elements for TiN. By controlling the Mg and Ba contents and comprehensively regulating the relationship between Mg, Ba, and Ti contents, the sharp angular morphology of TiN is transformed into a spherical or near-spherical morphology. This improves strength while maintaining excellent toughness, ultimately resulting in a high-strength and 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. Moreover, the molding process for room-temperature pressure-bearing components prepared using 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 3 of the present 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. Mg and Ba are added with the molten steel during the casting process, and the content of P is controlled at 0.015±0.001%. The composition detection results are shown in Table 1. In Table 1, let: F1=([Mg]-0.75[S]) / 24, F2=[Ba] / 137, F3=[Ti] / 48, F1 / F2 is denoted as A, and F3 / (F1+F2) is denoted as B; where [Mg], [S], [Ba], and [Ti] represent the weight percentage content of Mg, S, Ba, and Ti, respectively, and the ingot size is 1800mm×500mm×500mm.

[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 980℃ for 2 hours, followed by holding at 700℃ for 1.5 hours and then air cooling. 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 700 MPa, R p0.2 The invention requires an impact strength of 400 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 3. It can be seen that, with the addition of Mg and Ba and the control of the Mg, Ba and Ti contents, TiN exhibits a spherical or near-spherical morphology.

[0039] At least one of Ti, Mg, Ba, 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 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.

[0042] Comparative Example 8 is a comparative example of Example 2. With other components the same as in Example 2, the titanium content was increased. 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 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, because the B content does not meet the requirements of the present invention and the excessively high Ti content seriously deteriorates the room temperature toughness of the cast steel, the room temperature toughness of the cast steel cannot meet the requirements of the present invention.

[0043] Comparative Example 9 is a comparative example of Example 3. With other components the same as in Example 3, the magnesium content was increased. The adjusted magnesium 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, due to the magnesium content and A and B not meeting the requirements of the present invention, especially the excessively high magnesium 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.

[0044] Comparative Example 10 is a comparative example of Example 4. With other components the same as in Example 4, the magnesium content was reduced. The adjusted magnesium 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, 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 magnesium content and A and B do 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 barium content is reduced. The adjusted barium 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 barium content and A and B do not meet the requirements of this invention.

[0046] Comparative Example 12 is a comparative example of Example 6. With other components the same as in Example 6, the barium content was increased. The adjusted barium 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 because the barium content and A do not meet the requirements of the present invention, especially the excessively high barium 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.

[0047] Comparative Example 13 is a comparative example of Example 1. With other components the same as in Example 1, 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.

[0048] Comparative Example 14 is a comparative example of Example 2. With other components the same as in Example 2, 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.

[0049] Comparative Example 15 is a comparative example of Example 3. With other components the same as in Example 3, the magnesium content was reduced. The adjusted magnesium content is still within the scope of the present invention, but A and B are 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 and B do 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 magnesium content was increased. The adjusted magnesium content is still within the scope of the present invention, but A and B are 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 and B do not meet the requirements of the present invention.

[0051] Comparative Example 17 is a comparative example of Example 5. While other components are the same as in Example 5, the barium content is increased. The adjusted barium 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, its room temperature toughness is poor and it cannot meet the requirements of the present invention because A does not meet the requirements of the present invention.

[0052] Comparative Example 18 is a comparative example of Example 6. While other components are the same as in Example 6, the barium content is reduced. The adjusted barium 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.

[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 Mg and Ba 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, resulting in 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 pressure-bearing cast steel possessing both high strength and toughness, characterized in that, The composition of the high-strength and tough 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: 0.03-0.07%, N: 0.009-0.02%, Mg: 0.008-0.02%, Ba: 0.003-0.013%, with the remainder being Fe and unavoidable impurities. Furthermore, the composition of the high-strength and tough pressure-bearing cast steel satisfies the following relationship: F1=([Mg]-0.75[S]) / 24, F2=[Ba] / 137, F3=[Ti] / 48, F1 / F2=5.7-8.0, F3 / (F1+F2)=1.5-2.8; Wherein, [Mg], [S], [Ba], and [Ti] represent the weight percentages of Mg, S, Ba, and Ti, respectively.

2. The pressure-bearing cast steel with both high strength and toughness according to claim 1, characterized in that, 24*F1 is 0.005-0.015%.

3. The pressure-bearing cast steel with both high strength and toughness 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 pressure-bearing cast steel with both high strength and toughness according to claim 1, characterized in that, The room temperature mechanical properties of the aforementioned high-strength and tough pressure-bearing cast steel satisfy: R m For pressures above 700 MPa, R p0.2 It has an impact strength of over 400MPa and an impact toughness of over 30J KV2.

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

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

7. A method for preparing a pressure-bearing cast steel with both high strength and toughness 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 a pressure-bearing cast steel with both high strength and toughness according to claim 7, characterized in that, Mg and Ba are added during the casting process of cast steel.

9. The application of the high-strength and tough pressure-bearing cast steel according to any one of claims 1-6 or the high-strength and tough 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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