A high-strength, corrosion-resistant, dual-phase medium-entropy stainless steel and heat treatment method thereof

Abstract

The present invention provides a high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof. The composition design is based on the traditional stainless steel Fe-Cr composition, and high content of Co, Ni, Mo, and Cu are jointly added to increase the mixed entropy value of the alloy. And the content of the added elements and the corresponding heat treatment process are regulated, so that the content ratio of BCC and FCC phases in the organization reaches 50%, and a duplex medium-entropy stainless steel with a new composition system is obtained. The duplex medium-entropy stainless steel of the present invention is not hot-rolled, only solidified and heat-treated, and the tensile and yield strengths reach 763MPa and 479MPa respectively, and the elongation reaches 24%. The tensile strength, plasticity, sulfuric acid resistance, and NaCl solution corrosion resistance of the stainless steel are significantly better than the 2205 duplex stainless steel with mature industrial applications prepared after the same smelting and optimized solution treatment, and can be widely used in the application field of duplex stainless steel materials with high requirements for material strength, plasticity and corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of metal materials, and in particular to a high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof. Background Art

[0002] Stainless steel is the most important type of high-alloy corrosion-resistant steel in the national economy. For decades, researchers have done a lot of work on the modification of the microstructure and performance of high-Cr stainless steel by adding a small amount of elements such as Ni, Co, Mo, Mn, Cu, Al, and Ti alone or in combination, and developed austenitic stainless steel, duplex stainless steel, and single-phase BCC structure ferritic stainless steel. Among them, duplex stainless steel with 50% ferrite and 50% austenite each has higher plasticity and toughness than ferritic stainless steel, as well as higher strength and resistance to intergranular corrosion and chloride stress corrosion than austenitic stainless steel. It has become one of the important varieties of stainless steel research and is widely used in nuclear power, construction, food machinery and other fields. However, traditional steel theory generally believes that excessive addition of alloying elements will lead to the formation of a large number of complex brittle intermetallic compounds, which is very harmful to toughness and plasticity. Therefore, research on adding a large number of complex alloying elements to stainless steel at the same time is rare.

[0003] In recent years, this composition design theory has been greatly challenged by the research results in the field of high entropy alloys. Researchers have found that complex high entropy / medium entropy alloys can still obtain simple solid solution phases after solidification. Some components have extremely high toughness and excellent corrosion resistance, which has become an important hotspot in the design of new alloy materials. 60 Cr 18 Cu3(MnCoNiMoAlTi) 19 The pitting-resistant multi-component precipitation-hardened medium-entropy stainless steel has a mixed entropy value between 1.0R and 1.5R, which belongs to the category of medium-entropy alloys. The ferritic stainless steel with a single-phase BCC solid solution structure has a hardness of up to 600HV and good corrosion resistance after solid solution and aging treatment, but its plasticity has not been reported. It can be seen that further optimizing the composition on the basis of the reported research on medium-entropy stainless steel, developing and designing a duplex medium-entropy stainless steel composition with high strength, high plasticity and excellent corrosion resistance, is of urgent significance and engineering value to improve the variety and performance of existing stainless steel varieties. Summary of the invention

[0004] The present invention provides a high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof. The composition design is based on the traditional stainless steel Fe-Cr composition, and the mixed entropy value of the alloy is increased by jointly adding high-content Co, Ni, Mo, and Cu four elements. And the content of the added alloy elements and the corresponding heat treatment method are precisely regulated, so that the content ratio of the BCC and FCC phases in the organization reaches 50%, and a duplex medium-entropy stainless steel with a new composition system is obtained. Compared with the 2205 duplex stainless steel prepared under the same solidification and heat treatment conditions and widely used in industry, the duplex medium-entropy stainless steel of the present invention has significantly better tensile strength, plasticity, and sulfuric acid and NaCl solution corrosion resistance, and can be widely used in duplex stainless steel material application fields with high requirements for material strength, plasticity and corrosion resistance.

[0005] In order to achieve the above requirements, the technical solution of the present invention is:

[0006] A high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof, characterized in that the stainless steel comprises the following components in atomic percentage: Fe: 58%-62%, Cr: 16%-18%, Co: 5%-7%, Ni: 5%-7%, Mo: 5%-7%, Cu: 4%-5%.

[0007] A high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof, characterized in that: after solidification, the stainless steel is subjected to solution treatment in a temperature range of 1080-1180°C, so that the content ratio of the BCC and FCC phases in its structure can be adjusted to 50%, the tensile and yield strengths can reach up to 763MPa and 479MPa, the elongation can reach 24%, and it exhibits excellent corrosion resistance in sulfuric acid and NaCl solutions.

[0008] Preferably, the high-strength, corrosion-resistant duplex medium-entropy stainless steel can be prepared by conventional smelting methods such as a vacuum arc furnace, and the heat treatment can be carried out in an ordinary box furnace or a vacuum heat treatment furnace.

[0009] The distinguishing features and beneficial effects of the present invention and the existing duplex stainless steel are as follows:

[0010] (1) The present invention discloses a high-strength, corrosion-resistant, dual-phase medium-entropy stainless steel and a heat treatment method thereof. In addition to Fe and Cr, the composition elements thereof are combined with high-content Ni, Mo, Co, and Cu elements in nearly equal molar ratios, so that the mixed entropy of the alloy reaches 1.25R. And through the precise configuration of the content of each element and the heat treatment process, the content ratio of the BCC and FCC phases in the structure can be adjusted to 50%. Therefore, the invented alloy belongs to a dual-phase medium-entropy stainless steel, and due to the high entropy effect of the alloy, there is no brittle intermetallic compound second phase in the structure.

[0011] (2) The present invention discloses a high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof, which are distinguished from existing duplex stainless steels on the market in that the combined addition of high content of Mo, Cu, Co, and Ni elements improves the corrosion resistance and solid solution strengthening effect of the alloy. At the same time, after the medium-entropy stainless steel is solution treated, a nano-precipitation phase that is not common in traditional duplex stainless steel appears in the BCC phase, and a large number of dislocation lines and dislocation walls exist in the FCC phase. On the one hand, dislocation slip can improve plasticity, and on the other hand, nano-precipitation strengthening can improve strength. Therefore, the high-strength, corrosion-resistant duplex medium-entropy stainless steel of the present invention achieves a good match between strength and toughness. After solidification and heat treatment, the sample is significantly better than the traditional 2205 stainless steel prepared under the same conditions in terms of tensile strength, yield strength, elongation and corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The X-ray diffraction patterns of the high-strength, corrosion-resistant duplex medium-entropy stainless steel in the embodiment in the solidified state and after solution treatment at different temperatures of 1080°C, 1130°C, and 1180°C;

[0013] Figure 2 (a), (b), and (c) are the metallographic structures of the high-strength, corrosion-resistant duplex medium-entropy stainless steel in the embodiment after solid solution at 1080°C, 1130°C, and 1180°C, respectively;

[0014] Figure 3 The engineering stress-strain curves of the high-strength and corrosion-resistant duplex medium-entropy stainless steel and 2205 stainless steel after solution treatment by optimized process in the embodiment;

[0015] Figure 4 The precipitated phase and dislocation in the TEM structure of the high-strength and corrosion-resistant dual-phase medium-entropy stainless steel in the embodiment;

[0016] Figure 5 The corrosion results of the high-strength and corrosion-resistant duplex medium-entropy stainless steel in the embodiment when immersed in a 45wt.% sulfuric acid solution at 60°C;

[0017] Figure 6 (a) and (b) are the surface pitting corrosion morphologies of the high-strength and corrosion-resistant duplex medium-entropy stainless steel and 2205 stainless steel in the embodiment after the potentiodynamic polarization experiment. Specific embodiments

[0018] The present invention is further described below in conjunction with specific embodiments. A high-strength, corrosion-resistant duplex medium-entropy stainless steel and a heat treatment method thereof, characterized in that it is composed of the following components in molar percentage: Fe: 58%-62%, Cr: 16%-18%, Co: 5%-7%, Ni: 5%-7%, Mo: 5%-7%, Cu: 4%-5%, and is prepared by vacuum arc melting. The ingredients are prepared according to the molar percentage, and an electronic balance with an induction capacity of 0.1 mg is used to weigh 30 grams of single Fe, Cr, Co, Ni, Mo, and Cu blocks with a purity greater than 99.9% respectively, and the blocks are cleaned and dried with alcohol, placed in the furnace chamber of a vacuum arc melting furnace, and evacuated to 5×10 -3 Pa, and filled with 99.9% pure argon protection, so that the pressure in the furnace is -0.05MPa relative to the standard atmospheric pressure. The arc is struck at about 50A, and the current is gradually increased to 180A. The melting time is 120 seconds. The current is turned off and the alloy is cooled. The alloy ingot is turned over by a manipulator and melted repeatedly. This is repeated 4-5 times to ensure uniform melting of the alloy elements. Finally, the cast alloy is obtained by cooling to room temperature in a copper crucible, and the cast alloy is solution treated at 1080℃, 1130℃, and 1180℃ respectively, and then quickly cooled with water.

[0019] Figure 1 The XRD diffraction patterns of high-strength and corrosion-resistant dual-phase medium-entropy stainless steel in solid state and after solution treatment at different temperatures are shown in Figure 2. Figure 2 It is the corresponding metallographic structure diagram. It can be seen that the alloy structure always contains only FCC phase and BCC phase, and with the increase of solid solution temperature, the diffraction peak of BCC phase becomes higher, the content becomes more, and the content ratio with FCC phase finally approaches 1:1. Figure 3 The tensile results of high-strength and corrosion-resistant duplex medium-entropy stainless steel after being kept at 1100℃ for 8 hours and 1180℃ for 4 hours are shown. It can be seen that after the duplex medium-entropy stainless steel is solution treated at 1180℃ for 4 hours, the yield strength reaches 479MPa, the tensile strength reaches 763MPa, and the elongation reaches about 24%. Although the FCC and BCC content ratio of the 2205 duplex stainless steel prepared under the same smelting process can reach 1:1 after being treated by the optimized solution process at 1075℃ for 2 hours, its tensile strength, yield strength and elongation are significantly lower than the duplex medium-entropy stainless steel disclosed in the present invention.

[0020] Figure 4This is a TEM image of the junction of FCC and BCC phases of high-strength corrosion-resistant dual-phase medium-entropy stainless steel after 1180°C solid solution treatment. In the figure, there are a large number of dislocation lines and dislocation walls in the dark FCC phase, and they can pass through the grain boundary to reach the white BCC phase. There are a large number of nano-precipitated phases in the BCC phase. Dislocation slip can improve plasticity, and nano-precipitation strengthening can improve strength. Therefore, the high-strength corrosion-resistant dual-phase medium-entropy stainless steel of the present invention can achieve good strength-toughness matching through this mechanism. Figure 5 , Figure 6 The corrosion comparison results are as follows: the high-strength and corrosion-resistant duplex medium-entropy stainless steel after solution treatment at 1180℃ for 4h, the 2205 stainless steel prepared under the same smelting conditions, and the hot-rolled 2205 stainless steel purchased from the market after solution treatment at 1075℃ for 2h. The FCC and BCC contents in the microstructures of the three groups of comparison samples are close to 1:1. Figure 5 As shown, after being immersed in a 45wt.% sulfuric acid solution at 60°C for the same 6 hours, the corrosion resistance of the high-strength, corrosion-resistant duplex medium-entropy stainless steel of the present invention is significantly higher, and no obvious corrosion is found on the surface, while the 2205 comparison samples all have obvious surface rust, among which the hot-rolled 2205 purchased on the market has slightly better corrosion resistance than the 2205 melted in the laboratory, indicating that the corrosion resistance of the high-strength, corrosion-resistant duplex medium-entropy stainless steel disclosed in the present invention can be further improved by rolling treatment in the future. Figure 6 (a) and (b) are the surface pitting corrosion morphologies of the duplex medium entropy stainless steel disclosed in the present invention and the 2205 duplex stainless steel smelted under the same conditions after the potentiodynamic polarization experiment. Both samples were optimized by solution treatment before the potentiodynamic polarization experiment. It can be seen that the surface pitting pits of the duplex medium entropy stainless steel sample of the present invention are smaller and less, and the pitting corrosion resistance of NaCl solution is also better than that of the 2205 duplex stainless steel which is currently the most widely used in commercial applications.

Claims

1. A high-strength, corrosion-resistant duplex medium-entropy stainless steel, characterized in that The molar percentages of the alloy components are: Fe: 58%-62%, Cr: 16%-18%, Co: 5%-7%, Ni: 5%-7%, Mo: 5%-7%, Cu: 4%-5%.

2. A method for heat treatment of high-strength, corrosion-resistant duplex medium-entropy stainless steel, characterized in that: The medium entropy stainless steel in claim 1 is subjected to solution treatment at a temperature range of 1080-1180° C. for 2-8 hours, thereby adjusting the ratio of the BCC hard phase to the FCC soft phase in its structure to 1:

1.

3. The heat treatment method of a high-strength, corrosion-resistant duplex medium-entropy stainless steel according to claim 2, characterized in that After the medium entropy stainless steel is solution treated, nano-precipitated phases and dislocation lines favorable to plasticity exist in the BCC and FCC phases, respectively, and the solid solution strengthening effect achieves a good match between its strength and toughness, with a yield strength of 479 MPa, a tensile strength of 763 MPa, and an elongation of 24%.

4. The heat treatment method of a high-strength, corrosion-resistant duplex medium-entropy stainless steel according to claim 2, characterized in that The combined addition of Cr and Mo, Cu, Co, and Ni elements in the medium entropy stainless steel enables the duplex medium entropy stainless steel to have corrosion resistance in both sulfuric acid and NaCl solutions.

5. The heat treatment method of a high-strength, corrosion-resistant duplex medium-entropy stainless steel according to claim 2, characterized in that The medium entropy stainless steel is prepared by conventional melting and casting methods including vacuum arc melting.

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