High-strength invar steel and method for manufacturing same

Abstract

The application relates to the technical field of inconel steel production, in particular to high-strength inconel steel and a preparation method thereof. The inconel steel is composed of the following chemical components in percentage by weight: C: 0.04%-0.08%, Si:<0.5%, Mn: 0.2%-0.5%, P<=0.02%, S<=0.006%, Mo: 0.3%-0.85%, Ni: 35.5%-36.5%, Al: 1.3%-1.6%, Ti: 1.3%-1.7%, W: 0.3%-0.5%, Nb: 0.4%-0.6%, and the rest is Fe and impurity elements. The inconel steel manufactured by the application has a tensile strength of greater than or equal to 920 MPa, a yield strength of greater than or equal to 580 MPa, an elongation of greater than or equal to 8, a hardness of greater than or equal to 340 HV, and an expansion coefficient of 2.7-3.2*10-6 / DEG C. The application effectively solves the problems of coarse columnar crystal, low strength and easy crack of inconel alloy, improves the strength and crack sensitivity of the inconel steel through alloying and electromagnetic stirring, can not only improve the strength of the alloy, but also stabilize the expansion coefficient of the alloy, reduces the segregation and shrinkage hole phenomenon, and further improves the strength of the alloy.

Description

Technical Field

[0001] This invention relates to the field of Invar steel production technology, specifically to a high-strength Invar steel and its preparation method. Background Technology

[0002] Invar alloy is a high-nickel iron-based alloy with a nickel content of 36%. It is a single austenitic phase, and its Chinese designation is 4J36. This alloy exhibits an extremely low coefficient of thermal expansion (1.0–1.5 × 10⁻⁶) below the Curie temperature (Tc = 230℃). -6 / ℃, hence it is also known as non-expanding steel. Due to its extremely low thermal expansion and good toughness, it has become an important component of precision alloys and is widely used in the parts of precision instruments and meters. It is an indispensable material in all sectors of the national economy, national defense construction, and people's daily lives.

[0003] With industrial development, higher demands are being placed on material quality. Invar alloys, due to their low expansion properties, have attracted attention in many fields, requiring them to not only possess an extremely low coefficient of expansion but also ensure sufficient strength. However, the high nickel content results in a single-phase austenitic microstructure (Ms < -150℃) at room temperature, leading to lower strength (Rel ≤ 500 MPa, Rm ≤ 700 MPa), making them unsuitable as high-strength load-bearing components and severely limiting their applications. Traditional Invar alloys cannot meet these requirements; therefore, improving the strength of Invar alloys while maintaining their low expansion characteristics has become a pressing technical challenge in this field. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the present invention provides a high-strength Invar steel and its preparation method, which can improve the strength of Invar steel and improve its crack susceptibility while maintaining the low expansion characteristics.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] A high-strength Invar steel is composed of the following chemical composition in weight percentages:

[0007] C: 0.04%–0.08%, Si: <0.5%, Mn: 0.2%–0.5%, P≤0.02%, S≤0.006%, Mo: 0.3%–0.85%, Ni: 35.5%–36.5%, Al: 1.3%–1.6%, Ti: 1.3%–1.7%, W: 0.3%–0.5%, Nb: 0.4%–0.6%, with the remainder being Fe and impurity elements.

[0008] The steel used in this product has a tensile strength ≥920MPa, yield strength ≥580MPa, elongation ≥8%, hardness ≥340HV, and coefficient of thermal expansion 2.7~3.2×10⁻⁶. -6 / ℃.

[0009] The reasons for using the above-mentioned components and their weight percentages in this invention are explained in detail below:

[0010] C:

[0011] Carbon (C) is a crucial element determining the properties of metallurgical materials. It can enhance the strength and hardness of Invar alloys through solid solution strengthening and precipitation strengthening. In solid solution strengthening, C dissolves in the matrix to form a solid solution, increasing the alloy's strength. In precipitation strengthening, C can combine with other elements to form carbides. Carbides have high hardness, increasing the alloy's hardness and thus improving its wear resistance. Appropriate amounts of C can also improve the heat treatment performance of Invar alloys, giving them better thermal stability and mechanical properties during heat treatment. Therefore, the C content in this invention is controlled between 0.04% and 0.08%.

[0012] Si:

[0013] Silicon (Si) is a metallurgical reducing agent and deoxidizer, primarily dissolved in the ferrite phase. Increasing the tensile strength of materials also raises their elastic limit, thus improving the corrosion resistance of Invar alloys. Silicon compounds possess excellent corrosion resistance, effectively resisting environmental erosion such as oxidation and corrosion. The presence of Si reduces the coefficient of thermal expansion of Invar alloys, improving their thermal and dimensional stability. However, excessively high silicon content will reduce the material's plasticity. Therefore, the Si content in this invention is controlled to be <0.5%.

[0014] Mn:

[0015] Mn is commonly used as a deoxidizer and desulfurizer in metallurgy, and it is also an element that stabilizes austenite and improves hardenability. Appropriate amounts of Mn can improve the machinability of Invar alloys, making them easier to process, form, and weld. However, excessive Mn can lead to coarsening of the alloy grains, reducing its plasticity and toughness. Therefore, the Mn content in this invention is controlled at 0.2% to 0.5%.

[0016] Ti:

[0017] Ti's main role in Invar is grain refinement. Through the dispersed precipitation of its carbonitride deposits and the solid solution of Ti, it greatly improves the strength and toughness of the steel, and also has a strong precipitation strengthening effect. Simultaneously, it enhances the alloy's corrosion resistance and wear resistance. However, excessive Ti content easily leads to the formation of coarse TiN, thus affecting the steel's impact plasticity and fatigue resistance. Ti also increases the thermal expansion of Invar alloys. Therefore, in this invention, the mass percentage of Ti is controlled between 1.3% and 1.7%.

[0018] Mo:

[0019] Mo dissolves extensively in austenite at high temperatures, strongly refining the grain size and thus enhancing the mechanical properties of the alloy, resulting in higher hardness and strength. Furthermore, Mo improves the wear resistance of Invar alloys, giving them better wear resistance under high temperature and high stress conditions and extending their service life. Mo also helps improve the alloy's thermal stability and corrosion resistance, making it perform better in harsh environments. Due to its strong strengthening effect, the Mo content is controlled between 0.3% and 0.85%.

[0020] S, P:

[0021] S can improve the machinability of Invar alloys, making them easier to process and shape. However, it is prone to forming MnS inclusions with manganese. High S content can cause thermal brittleness and reduce the impact performance of the alloy.

[0022] P is generally considered a harmful element, which tends to segregate at grain boundaries, reducing the toughness of steel and increasing the tendency for intergranular fracture.

[0023] Therefore, the sulfur (S) content should be controlled below 0.006%, and the phosphorus (P) content should be controlled below 0.020%.

[0024] Ni:

[0025] Ni is the most important element in Invar alloy. The addition of Ni gives Invar alloy a low coefficient of thermal expansion; the coefficient of thermal expansion is lowest when the Ni content is around 36%. Therefore, in this invention, the Ni content is controlled between 35.5% and 36.5%.

[0026] Al:

[0027] The important role of Al in Invar alloys is to reduce the alloy's density, increase its strength and hardness, and improve its high-temperature performance. Invar alloys are high-temperature alloys commonly used in the manufacture of aero-engine components. Adding Al effectively reduces the alloy's density, making it lighter, while simultaneously increasing its strength and hardness, and enhancing its heat resistance and corrosion resistance. Al also helps improve the high-temperature oxidation and thermal expansion properties of Invar alloys, making them more stable and reliable in high-temperature environments. Therefore, the Al content in this invention is controlled at 1.3% to 1.6%.

[0028] W, Nb:

[0029] Solid solution strengthening of Invar alloys severely affects their coefficient of thermal expansion, making it unsuitable for this method. The addition of W and Nb elements combines with C in the steel to form NbC and WC carbide inclusions that are dispersed throughout the matrix, achieving excellent grain refinement and strengthening effects. This not only strengthens the alloy but also acts as a deoxidizing and purifying agent. Furthermore, the carbides themselves have a relatively low coefficient of thermal expansion, thus having minimal impact on the overall alloy thermal expansion coefficient.

[0030] Therefore, in this invention, the W element content is controlled at 0.3% to 0.5%, and the Nb element content is controlled at 0.4% to 0.6%.

[0031] The preparation method of the above-mentioned high-strength Invar steel specifically includes the following steps:

[0032] 1) Vacuum electric furnace smelting:

[0033] The vacuum degree is maintained below 0.5 Pa during smelting, the oxygen content is controlled below 50 ppm when smelting is completed, and the furnace exit temperature is controlled below 1550℃.

[0034] 2) Alloying process:

[0035] In the alloying process, refining slag is added first, followed by the addition of ferrosilicon for deoxidation;

[0036] Argon gas is blown and stirred at the bottom, with the argon gas flow rate controlled at 7-10 m³ / h. 3 / min, while argon purging protection throughout the entire alloying process;

[0037] W and Nb with a purity of 99% are made into powder with a particle size of <200 mesh. 2-3 kg / t of W and 2.5-5 kg / t of Nb are added at 13-16 min and 25-28 min respectively at the beginning of smelting, and then smelting is continued for more than 15 min.

[0038] 3) Continuous casting process:

[0039] During continuous casting, the tundish temperature is maintained at 1450–1500℃. An electromagnetic stirrer is added at the crystallizer, and the billet casting speed is controlled at 0.8–1.4 m / min. The electromagnetic stirrer operates at a frequency of 40 Hz and a current of 500 A, using three-phase AC power.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] 1. This invention employs a composition design incorporating W and Nb microalloying elements. Solid solution strengthening of Invar alloys severely affects their coefficient of thermal expansion, making them unsuitable for this method. The addition of W and Nb alloys allows them to combine with the carbon element in the alloy, forming micro-carbide alloys that enhance the alloy's strength. Since the W and Nb alloys are added to the Invar alloy in fine powder form, they act as precipitation strengthening. Adding alloying elements that can form stable compounds to metallic materials, under certain conditions, causes the resulting second-phase compounds to precipitate from the solid solution and disperse diffusely throughout the microstructure, thereby effectively improving the material's strength.

[0042] 2. Simultaneously, the addition of electromagnetic stirring further disperses the carbide alloy, resulting in grain refinement and strengthening. The grain boundaries of polycrystalline materials strongly impede dislocation movement at room temperature, thereby increasing the material's strength. The finer the grains and the greater the grain boundary area, the higher the strength, forming grain refinement strengthening. This not only improves the alloy's strength but also stabilizes its coefficient of thermal expansion and reduces segregation and shrinkage cavities, further enhancing the alloy's strength. This method can significantly improve the strength of Invar alloys while having minimal impact on their coefficient of thermal expansion, thus expanding the application range of Invar alloys.

[0043] In summary, the Invar steel manufactured using this invention has a tensile strength ≥920MPa, a yield strength ≥580MPa, an elongation ≥8%, a hardness ≥340HV, and a coefficient of thermal expansion of 2.7~3.2×10⁻⁶. -6 / ℃. This invention effectively solves the problems of coarse columnar crystals, low strength, and easy cracking in Invar alloys. This invention improves the strength of Invar steel and reduces its crack sensitivity by adding electromagnetic stirring to the alloy. It can not only improve the strength of the alloy, but also stabilize the coefficient of expansion of the alloy, and reduce segregation and shrinkage cavities, thereby further improving the strength of the alloy. Attached Figure Description

[0044] Figure 1 This is a 100x magnification metallographic image of the present invention.

[0045] Figure 2 This is a 200x magnification metallographic structure diagram of the present invention.

[0046] Figure 3 This is a 500x magnification metallographic image of the present invention. Detailed Implementation

[0047] This invention discloses a high-strength Invar steel and its preparation method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0048]

Example

[0049] Each embodiment of the present invention is manufactured according to the following process, specifically including the following steps:

[0050] 1. Vacuum electric furnace smelting:

[0051] Remove sulfur and phosphorus as soon as possible during the smelting process, maintain the vacuum degree below 0.5 Pa during smelting, control the oxygen content below 50 ppm when smelting is completed, and control the furnace exit temperature below 1550℃.

[0052] 2. Alloying process:

[0053] Refining slag is added first during the alloying process. Then, ferrosilicon is added for deoxidation, and argon is blown into the bottom for stirring at a flow rate of 7-10 m³ / h. 3 / min. Simultaneously, argon purging is used for protection throughout the entire alloying process.

[0054] W and Nb with a purity of 99% were made into powder with a mesh size of <200. 2.5 kg / t of W and 3 kg / t of Nb were added at 15 min and 27 min respectively at the beginning of smelting, and then smelting was continued for another 15 min.

[0055] 3. Continuous casting process: During the continuous casting process, the temperature of the tundish is maintained between 1450 and 1500℃, electromagnetic stirring is added at the crystallizer, and the billet pulling speed is 1m / min.

[0056] Invar alloy reinforced with W and Nb alloys was cast into a 5t consumable electrode. The steel ingot was purified by electroslag remelting. During the electroslag remelting process, electromagnetic stirring was applied to further enhance the casting quality.

[0057] Equation for solidification and nucleation of equiaxed-columnar crystals:

[0058]

[0059] Where: ne crystal nucleus density (l / m) 3 ), v represents (m / s), and the supercooled nucleation rate Nnu(1 / (m 3The nucleation rate Nfrag(1 / (m) caused by dendrite fragmentation) 3 ·s).

[0060] Electromagnetic stirring was applied to the crystallizer at a frequency of 40 Hz and a current of 500 A, using three-phase alternating current. Calculations showed that applying electromagnetic stirring effectively promoted the formation of equiaxed crystals, thus strengthening the crystal structure.

[0061] Solid solution strengthening of Invar alloys can severely affect their coefficient of thermal expansion, making them unsuitable for this method. However, the addition of W and Nb alloys allows them to combine with the carbon element in the alloy, forming tiny carbides that enhance the alloy's strength. Since W and Nb alloys are added to Invar alloys in the form of fine powder, they also act as precipitation strengthening. Adding alloying elements that can form stable compounds to metallic materials, under certain conditions, causes the resulting second-phase compounds to precipitate from the solid solution and disperse throughout the microstructure, thereby effectively improving the material's strength.

[0062] Simultaneously, the addition of electromagnetic stirring further disperses the carbide alloy, resulting in grain refinement and strengthening. The grain boundaries of polycrystalline materials strongly impede dislocation movement at room temperature, thereby increasing the material's strength. The finer the grains and the greater the grain boundary area, the higher the strength, forming grain refinement strengthening. This not only improves the alloy's strength but also stabilizes its coefficient of thermal expansion and reduces segregation and shrinkage cavities, further enhancing the alloy's strength. This method can significantly improve the strength of Invar alloys while having minimal impact on their coefficient of thermal expansion, thus expanding the application range of Invar alloys.

[0063] The chemical composition (Wt%) of the embodiments of the present invention is shown in Table 1; the technical performance of the embodiments and comparative examples of the present invention is shown in Table 2.

[0064] Table 1. Comparison of chemical composition (wt%) of embodiments of the present invention

[0065]

[0066] Table 2 Summary of Performance Tests for Embodiments and Comparative Examples of the Invention

[0067]

[0068] As shown in Tables 1-2, the high-strength Invar steel manufactured using this invention has a tensile strength ≥920MPa, a yield strength ≥580MPa, an elongation ≥8%, a hardness ≥340HV, and a coefficient of thermal expansion of 2.7~3.2×10⁻⁶. -6 / ℃. Compared to existing Invar steel, this invention improves the strength and crack susceptibility of Invar steel while maintaining its low expansion characteristics. This invention effectively solves the problems of coarse columnar grains, low strength, and susceptibility to cracking in Invar alloys. By adding electromagnetic stirring to the alloy, this invention improves the strength and crack susceptibility of Invar steel, not only increasing the alloy's strength but also stabilizing its expansion coefficient and reducing segregation and shrinkage cavities, further enhancing the alloy's strength.

[0069] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-strength Invar steel, characterized in that, It is composed of the following chemical components in weight percentage: C: 0.04%–0.08%, Si: <0.5%, Mn: 0.2%–0.5%, P≤0.02%, S≤0.006%, Mo: 0.3%–0.85%, Ni: 35.5%–36.5%, Al: 1.3%–1.6%, Ti: 1.3%–1.7%, W: 0.3%–0.5%, Nb: 0.4%–0.6%, with the remainder being Fe and impurity elements.

2. The high-strength Invar steel according to claim 1, characterized in that, The Invar steel has a tensile strength ≥920MPa, yield strength ≥580MPa, elongation ≥8%, hardness ≥340HV, and coefficient of thermal expansion 2.7~3.5×10⁻⁶. -6 / ℃.

3. A method for preparing high-strength Invar steel as described in claim 1 or 2, characterized in that, Specifically, the steps include the following: 1) Vacuum electric furnace smelting: The vacuum degree is maintained below 0.5 Pa during smelting, the oxygen content is controlled below 50 ppm when smelting is completed, and the furnace exit temperature is controlled below 1550℃. 2) Alloying process: In the alloying process, refining slag is added first, followed by the addition of ferrosilicon for deoxidation; Argon blowing and stirring at the bottom, while argon protection is applied throughout the alloying process; W and Nb with a purity of 99% are made into powder with a particle size of <200 mesh. 2-3 kg / t of W and 2.5-5 kg / t of Nb are added at 13-16 min and 25-28 min respectively at the beginning of smelting, and then smelting is continued for more than 15 min. 3) Continuous casting process: During continuous casting, the tundish temperature is maintained at 1450–1500℃, electromagnetic stirring is added at the crystallizer, and the billet casting speed is controlled at 0.8–1.4 m / min.

4. The method for preparing high-strength Invar steel according to claim 3, characterized in that, In step 2), the argon gas flow rate for stirring is controlled at 7-10 m³ / h. 3 / min.

5. The method for preparing high-strength Invar steel according to claim 3, characterized in that, In step 3), the electromagnetic stirring frequency is 40Hz, the current is 500A, and three-phase AC power is used for stirring.

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