A method for improving mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel by grain boundary engineering treatment

Abstract

The application aims to provide a method for improving mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel through grain boundary engineering, and belongs to the field of austenitic heat-resistant steel. The method is based on the idea of grain boundary engineering, and builds heat-resistant steel with special grain boundary structure by plastic deformation + high-temperature annealing of Fe-22Cr-25Ni type austenitic heat-resistant steel. The method inhibits the grain boundary precipitation behavior of M 23 C6 from the perspective of kinetics, reduces the high-energy defect density in the material, controls the distribution state of M 23 C6 on the grain boundary, and improves the microstructure stability. Through the grain boundary engineering treatment of cold rolling + annealing, the grains are fully recrystallized, are obviously refined compared with the solid solution sample, and more twin crystals are obtained in the grains. After aging at 700 DEG C, the grain boundary precipitated phase of the grain boundary engineering treatment sample is less distributed, the microstructure stability is improved, the yield strength, tensile strength and elongation are increased, and the mechanical properties are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of austenitic heat-resistant steel, and particularly relates to a method for improving the mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel through grain boundary engineering. Background Art

[0002] To reduce carbon emissions and improve the environment, maximizing the thermal efficiency of coal-fired power generation units is one of the most effective options. Therefore, selecting heat-resistant steel with excellent performance is imperative. Currently, the materials used for superheaters and reheaters in coal-fired boilers are primarily austenitic heat-resistant steels and nickel-based alloys. Ultra-high-temperature boilers operate at temperatures up to 700°C, but the performance of ferritic heat-resistant steels deteriorates rapidly at these temperatures, damaging the equipment and making them unsuitable for use in superheaters and reheaters. In recent years, rising nickel prices have made nickel-based superalloys relatively expensive compared to other heat-resistant steels, hindering their widespread application. Austenitic heat-resistant steel is considered a promising candidate. Its high alloying degree and high Cr and Ni contents provide excellent oxidation and corrosion resistance. Furthermore, the addition of alloying elements such as Mo, W, Nb, V, and Cu enhances the austenitic matrix through solid solution strengthening and precipitation strengthening. Sanicro25 and C-HRA-5 are two of the most promising Fe-22Cr-25Ni austenitic heat-resistant steels. They offer excellent structural stability, high creep strength, thermal fatigue resistance, and weldability, while also offering the advantages of lower cost over nickel-based alloys. Sanicro25 was developed by Sandvik of Sweden, while C-HRA-5 was designed and developed by China Taiyuan Iron and Steel Corporation (TISCO) in conjunction with the General Iron and Steel Research Institute.

[0003] Fe-22Cr-25Ni type austenitic heat-resistant steel usually contains high Cr and C. At 500~900℃, M 23 C6 (M = Fe, Cr, Ni, etc.) is easy to precipitate quickly, M 23 The appropriate amount of C6 phase precipitation is beneficial to stabilizing the grain boundary, but excessive M 23 For example, recently, Harbin Boiler Plant and Xi'an Thermal Engineering Institute found that the grain boundary M of Sanicro 25 steel weakened with the aging temperature and aging time. 23 The amount and volume fraction of C6 increase accordingly, and the precipitated phase at the grain boundary is distributed in a chain-like manner, resulting in a significant decrease in room temperature impact toughness [Materials Science&Engineering A, 2018, 737: 40-46]. 23C6 coarsening has become a common problem affecting the safe service life of high-Cr (Fe-22Cr-25Ni) austenitic heat-resistant steels. For face-centered cubic (FCC) metals with low stacking fault energy, grain boundary engineering (GBE) can be used to control the deformation and heat treatment parameters during the deformation process, improve grain boundary mobility, and promote the formation of special low-ΣCSL grain boundaries, ultimately improving material properties.

[0004] For example, Chinese patents CN201410444680.6 and CN201510022326.9 disclose grain boundary engineering processes for 304 and 316 stainless steels. The processes involve first keeping the steel at 1050-1150°C and then cooling it with water, followed by a 3-15% machining deformation, and finally annealing and keeping the steel in water before cooling it with water. This process can produce stainless steel with higher corrosion resistance.

[0005] Chinese patent 202210459032.2 conducted a two-step high-temperature solution treatment on C-HRA-5 austenitic heat-resistant steel samples and found that it could improve its intergranular corrosion resistance. Chinese patent CN202010035476.4 discloses a grain boundary engineering process for adjusting the η phase distribution in nickel-based high-temperature alloys. However, there are few reports on the use of grain boundary engineering to treat austenitic heat-resistant steel. The ΣCSL grain boundary is mainly composed of Σ3 n Grain boundary composition, Σ3 n Grain boundaries are composed of twins and grain boundaries with a specific orientation relationship to the twins. The stacking fault energy strongly influences their formation. As the stacking fault energy decreases, twinning behavior is more likely to initiate. The smaller the stacking fault energy, the lower the energy required to generate a stacking fault per unit area. This increases the probability of atomic layers misaligning and forming stacking faults during annealing, and the likelihood of twin boundaries forming. Summary of the Invention

[0006] The present invention aims to provide a method for improving the mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel by grain boundary engineering treatment. By performing appropriate plastic deformation + high-temperature annealing on the austenitic heat-resistant steel, a heat-resistant steel with a special grain boundary structure is constructed to improve the organizational stability.

[0007] The present invention is based on the concept of "grain boundary engineering" and constructs a heat-resistant steel with a special grain boundary structure by performing appropriate plastic deformation + high temperature annealing on Fe-22Cr-25Ni type austenitic heat-resistant steel. 23 The grain boundary precipitation behavior of C6 is optimized by optimizing the distribution of grain boundary characteristics, reducing the high energy defect density inside the material, and regulating M 23 The distribution of C6 at grain boundaries and improved tissue stability.

[0008] The present invention adopts the following technical solutions:

[0009] A method for improving the mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel by grain boundary engineering treatment comprises the following steps:

[0010] (1) The material used is Fe-22Cr-25Ni type austenitic heat-resistant steel, and the chemical composition by weight percentage is:

[0011] C: 0.04-0.10%, Si≤0.4%, Mn≤0.6%, P≤0.025%, S≤0.015%, Cr: 21.5-23.5%, Ni: 23.5-26.5%, Co: 1.0-2.0%, Cu: 2.5-3.5%, W: 3.0-4.0%, Nb: 0.40-0.60%, N: 0.20-0.30%, B: 0.002-0.008, the balance is Fe and other inevitable impurity elements;

[0012] Smelt in a 50kg vacuum induction furnace according to the alloy composition ratio, cast into ingots under vacuum, and remove from the mold after air cooling;

[0013] (2) After the riser is removed from the smelted ingot, solution treatment is performed at 1150-1200°C, and after holding for 2-4 hours, high-temperature forging is performed, and the final forging temperature is not less than 900°C;

[0014] (3) The forged slab is kept at 800-850℃ for 0.5 h and then hot rolled. The hot rolling temperature is not less than 700℃ and multiple hot rolling is performed.

[0015] (4) Solution treatment was performed at 1180 °C for 30 min. The sample after solution treatment was water-cooled and then cold rolled;

[0016] (5) The sample obtained by cold rolling deformation is kept at 1150-1180°C for 10-120 minutes, and then water-cooled to obtain a sample treated with grain boundary engineering;

[0017] (6) The solid solution sample prepared in step (2) was used as a comparison sample and the sample obtained in step (5) was subjected to aging treatment at 700°C for 1000 hours and then water-cooled;

[0018] (7) Perform room temperature tensile testing on the sample obtained in step (6).

[0019] Furthermore, in step (1), an ingot of 120×100×500 mm is cast.

[0020] Furthermore, in step (3), the steel plate is hot rolled into a steel plate with a thickness of 2-4 mm.

[0021] Furthermore, the cold rolling deformation in step (4) is 25-30%.

[0022] The beneficial effects of the present invention are as follows:

[0023] Compared with existing austenitic heat-resistant steel, the outstanding advantages of the present invention are:

[0024] Through the grain boundary engineering treatment of cold rolling and annealing, the grains are fully recrystallized, with significant refinement compared to the solution-treated samples, and more twins are formed within the grains. After aging at 700°C, the grain boundary precipitates in the GBE-treated samples are reduced, improving microstructural stability, increasing yield strength, tensile strength, and elongation, and enhancing mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is the reverse pole figure of the solid solution state;

[0026] Figure 2 is the distribution diagram of solid solution grain boundaries;

[0027] Figure 3 This is the inverse pole figure after 30% cold deformation;

[0028] Figure 4 This is the grain boundary distribution diagram after 30% cold deformation;

[0029] Figure 5 The inverse pole figure is 30% cold deformation + 1150℃ annealing for 10min;

[0030] Figure 6 This is the grain boundary distribution diagram of 30% cold deformation + 1150℃ annealing for 10min;

[0031] Figure 7 Metallographic photographs of the control sample and the austenitic heat-resistant steel treated with grain boundary engineering after aging at 700℃ for 0 h and 1000 h; (a) and (c) are the solid solution state samples after aging at 700℃ for 0 h and 1000 h; (b) and (d) are the grain boundary engineering samples after aging at 700℃ for 0 h and 1000 h.

[0032] Figure 8 is the stress-strain curve of C-HRA-5 steel in solid solution state;

[0033] Figure 9 The stress-strain curve of C-HRA-5 steel after 30% cold deformation and annealing at 1150℃ for 10 min;

[0034] Figure 10 This is a comparison chart of the tensile strength of the control sample and the austenitic heat-resistant steel treated with grain boundary engineering. DETAILED DESCRIPTION

[0035] Example 1: Preparation of Grain Boundary Engineering Samples

[0036] Heat-resistant steel was smelted in a 50 kg vacuum induction furnace according to the alloy composition shown in Table 1. Ingots were cast under vacuum and air-cooled before being removed from the mold. The melted ingots were solution treated at 1180°C after cutting the riser, held at 800°C for 3 hours, and then subjected to high-temperature forging with a final forging temperature of no less than 900°C. The forged slabs were held at 800°C for 0.5 hours before being hot rolled at no less than 700°C in multiple passes to a thickness of 4 mm. Solution treatment was then performed at 1180°C for 30 minutes. The solution-treated samples were water-cooled and then cold-rolled by 30%. Subsequently, the 30% cold-rolled C-HRA-5 steel samples were held at 1150°C for 10 minutes and then water-cooled to obtain grain boundary engineered austenitic heat-resistant steel. The solution-treated and grain boundary treated samples were then aged at 700°C for 0, 24, and 1000 hours, respectively, before being water-cooled. The aged specimens were cut into dog-bone tensile specimens using a wire cutting machine and polished to smoothness using 120-grit SiC sandpaper before tensile testing.

[0037] Table 1

[0038]

[0039] Example 2: Microstructure of Grain Boundary Engineering Sample

[0040] like Figures 1 to 6 The following is an EBSD photo of the metallographic microstructure of austenitic heat-resistant steel after 30% cold deformation. Figure 5 and Figure 6 ) The grains are obviously refined compared with those of the solid solution state sample.

[0041] Depend on Figure 1 and Figure 2 It can be seen that the grains of the solution treated samples are all equiaxed, but the grain size is uneven, and some areas have small grain clusters. According to statistics, the proportion of twin boundaries is 58.7%. Figure 3 and Figure 4 As can be seen in the figure, after 30% cold rolling, the grains deformed along the rolling direction, and stress concentrations occurred at the grain boundaries. Notably, areas of small grain clusters experienced even greater stress concentrations. After annealing at 1150°C for 10 minutes, the cold-deformed grains fully recrystallized. Compared to the solution-treated specimen, the grains treated with grain boundary engineering were smaller and more uniform, with twin boundaries comprising 61.4%.

[0042] Example 3: Precipitation distribution of grain boundary engineered austenitic heat-resistant steel after aging

[0043] as follows Figure 7The metallographic microstructures of austenitic heat-resistant steel in different conditions aged at 700°C for 0, 24, and 1000 h are shown. With increasing aging time, the number of precipitates gradually increases, first forming at grain boundaries and then within the grains. The dispersed distribution of precipitates within the grains contributes to precipitation strengthening. Furthermore, the precipitates at the grain boundaries of the samples treated with grain boundary engineering are more discontinuous than those in the solid solution state, which contributes to improved elongation.

[0044] Example 4: Mechanical properties of grain boundary engineered austenitic heat-resistant steel

[0045] The changes in mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel are analyzed by stress-strain curve, such as Figures 8 to 10 shown. Figure 8 The tensile strengths of the solution-state samples aged at 700°C for 0 h, 24 h, and 1000 h were 785 MPa, 814 MPa, and 839 MPa, respectively; the yield strengths were 417 MPa, 475 MPa, and 495 MPa, respectively; and the elongations were 40.45%, 36%, and 32.8%, respectively. The figure shows that the tensile strength and yield strength of the solution-state increase with increasing aging time, while the elongation decreases. Figure 9 The figure shows the stress-strain curves of grain boundary engineering treated samples at different aging times. The tensile strengths at 0 h, 24 h, and 1000 h are 803 MPa, 842 MPa, and 868 MPa, respectively; the yield strengths are 432 MPa, 478 MPa, and 500 MPa, respectively; and the elongations are 43.2%, 37.1%, and 35.9%, respectively. Figure 10 The tensile strength of the two samples is compared. Mechanical properties tests show that the tensile strength and toughness of the austenitic heat-resistant steel are appropriately improved after grain boundary engineering treatment, indicating that grain boundary engineering treatment improves the mechanical properties of austenitic heat-resistant steel.

Claims

1. A method for improving the mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel by grain boundary engineering, characterized by: The steps include: (1) The material used is Fe-22Cr-25Ni type austenitic heat-resistant steel, and the chemical composition by weight percentage is: C: 0.04-0.10%, Si≤0.4%, Mn≤0.6%, P≤0.025%, S≤0.015%, Cr: 21.5-23.5%, Ni: 23.5-26.5%, Co: 1.0-2.0%, Cu: 2.5-3.5%, W: 3.0-4.0%, Nb: 0.40-0.60%, N: 0.20-0.30%, B: 0.002-0.008, and the balance is Fe and other unavoidable impurity elements; Smelt in a 50kg vacuum induction furnace according to the alloy composition ratio, cast into ingots under vacuum, and remove from the mold after air cooling; (2) After the riser is removed from the smelted ingot, solution treatment is performed at 1150-1200°C, and after holding for 2-4 hours, high-temperature forging is performed, and the final forging temperature is not less than 900°C; (3) The forged slab is kept at 800-850℃ for 0.5 h and then hot rolled. The hot rolling temperature is not less than 700℃ and multiple hot rolling is performed. (4) performing a solution treatment at 1180°C for 30 min, water cooling the sample after the solution treatment and then cold rolling the sample, wherein the cold rolling deformation is 25-30%; (5) The sample obtained by cold rolling deformation is kept at 1150-1180°C for 10-120 minutes, and then water-cooled to obtain a sample treated with grain boundary engineering; (6) The solid solution sample prepared in step (2) was used as a comparison sample and the sample obtained in step (5) was subjected to aging treatment at 700°C for 1000 hours and then water-cooled; (7) Perform room temperature tensile testing on the sample obtained in step (6).

2. The method for improving the mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel by grain boundary engineering according to claim 1, characterized in that: In step (1), an ingot of 120×100×500 mm is cast.

3. The method for improving the mechanical properties of Fe-22Cr-25Ni type austenitic heat-resistant steel by grain boundary engineering according to claim 1, characterized in that: In step (3), the steel plate is hot rolled into a steel plate with a thickness of 2-4 mm.

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