Method for improving microstructure stability of austenitic stainless steel at extremely low temperature

Abstract

The application belongs to the field of stainless steel, and particularly relates to a method for improving the ultra-low temperature structure stability of austenitic stainless steel, which comprises the following steps: (1) smelting to obtain a qualified stainless steel liquid; (2) continuously casting the stainless steel liquid to obtain a continuous casting billet; (3) performing homogenization treatment on the continuous casting billet and then performing rolling to obtain a steel plate; and (4) performing solid solution treatment on the steel plate. The method for improving the ultra-low temperature structure stability of austenitic stainless steel effectively improves the ultra-low temperature structure stability of the austenitic stainless steel by special component design and simultaneous control of key process points in the processes of continuous casting, homogenization annealing and solid solution treatment.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel, and specifically relates to a method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures. Background Technology

[0002] Under the national policy of achieving carbon peaking and carbon neutrality, green and low-carbon industries such as solar, wind, hydrogen, and natural gas are developing rapidly in order to transform the energy consumption structure. Since many emerging energy sources require storage and transportation in extremely low-temperature environments below -163℃, austenitic stainless steel sheets with excellent low-temperature toughness and high strength are currently widely chosen. However, due to the extremely high requirements for safe service in cryogenic liquid gases, even higher demands are placed on the material's structural stability under extremely low-temperature conditions.

[0003] Austenitic stainless steel sheets produced using traditional compositions and processes have the following problems during production and service:

[0004] 1) The composition design is unreasonable and cannot guarantee that the full austenitic structure will be maintained under extremely low temperature conditions;

[0005] 2) The production process is difficult to control and is prone to producing residual ferrite, resulting in poor microstructure stability;

[0006] 3) Martensitic phase transformation is prone to occur during the processing and forming process, resulting in poor local plasticity and risk of failure during extremely low temperature service. Summary of the Invention

[0007] In order to overcome the above-mentioned shortcomings of existing austenitic stainless steels for ultra-low temperature environments, the present invention provides a method for improving the microstructure stability of austenitic stainless steel at ultra-low temperatures.

[0008] Specifically, the method for improving the low-temperature microstructure stability of austenitic stainless steel according to the present invention includes:

[0009] (1) Smelting to obtain stainless steel liquid with qualified composition;

[0010] (2) The stainless steel liquid is continuously cast to obtain a continuously cast billet;

[0011] (3) The continuously cast billet is homogenized and then rolled to obtain a steel plate;

[0012] (4) Solution treatment of steel plate.

[0013] The above-mentioned method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures, wherein the stainless steel liquid comprises, by weight percentage: C 0.025-0.035%, Si 0.2-0.4%, Mn 2-3%, Cr 16-17%, Ni 11-11.5%, N 0.1-0.15%, with the balance being Fe and other unavoidable impurity elements.

[0014] The above-mentioned method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures, wherein the content of each component in the stainless steel liquid satisfies the following formula: 3.2×(Cr%+1.5×Si%)-2.5×(Ni%+0.5×Mn%+30×C%+30×N%)-24.7≤0.

[0015] The above-mentioned method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures requires a cooling water strength of 160-180 L / t during continuous casting.

[0016] The above-mentioned method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures involves homogenization treatment at a temperature of 1240–1260°C and a holding time of 12–13 hours.

[0017] The above-mentioned method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures involves rapidly immersing the continuously cast billet in water for cooling after homogenization treatment.

[0018] The above-mentioned method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures involves a solution treatment temperature of 1120–1150℃ and a holding time of 4–5 min / mm thickness.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects:

[0020] (1) The residual ferrite content of the cold plate can be controlled to 0%;

[0021] (2) Cold-rolled plates can still maintain a fully austenitic structure at temperatures of -163℃ and lower.

[0022] (3) After the cold plate is used to make the storage tank, no martensitic phase transformation occurs. Detailed Implementation

[0023] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0024] When a range of values ​​is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0025] The main factors affecting the microstructure stability at extremely low temperatures include the base composition, solidification process, heating, and processing. Therefore, the core of improving microstructure stability lies in precisely controlling the microstructure throughout the entire processing flow, based on composition design, to achieve excellent matching between each step, thus enabling precise control of microstructure stability. The overall concept of this invention is as follows:

[0026] 1) Optimize the composition design to achieve austenite stability under extremely low temperature conditions without significantly increasing costs;

[0027] 2) By optimizing the continuous casting cooling process, the solidification heat transfer conditions are improved, and the residual ferrite content of the billet is reduced;

[0028] 3) Add a homogenization annealing process to the continuously cast billet to make Cr and Ni elements diffuse evenly at high temperature, eliminate non-equilibrium residual ferrite, and improve the uniformity of the microstructure.

[0029] 4) Optimize the solution treatment process to prevent deformation and martensitic phase transformation during processing.

[0030] Based on this, the present invention provides a method for improving the microstructural stability of austenitic stainless steel at extremely low temperatures, comprising:

[0031] (1) Smelting to obtain stainless steel liquid with qualified composition;

[0032] (2) The stainless steel liquid is continuously cast to obtain a continuously cast billet;

[0033] (3) The continuously cast billet is homogenized and then rolled to obtain a steel plate;

[0034] (4) Solution treatment of steel plate.

[0035] Preferably, by weight percentage, the molten stainless steel comprises: C 0.025–0.035%, Si 0.2–0.4%, Mn 2–3%, Cr 16–17%, Ni 11–11.5%, N 0.1–0.15%, with the balance being Fe and other unavoidable impurity elements. More preferably, the content of each component in the molten stainless steel satisfies the following formula: 3.2 × (Cr% + 1.5 × Si%) - 2.5 × (Ni% + 0.5 × Mn% + 30 × C% + 30 × N%) - 24.7 ≤ 0.

[0036] The reasons for limiting the content range of the main alloying elements will be explained in detail below.

[0037] C is an austenite stabilizing element. In this invention, the C content is appropriately increased to increase the Ni equivalent, thereby improving the microstructure stability. However, since the M23C6 carbide formed by C and Cr is prone to accumulate at the grain boundaries, reducing the main element forming corrosion resistance, the C content is limited to 0.025-0.035%.

[0038] As a deoxidizer added during smelting, Si is effectively reduced in this invention, thereby improving microstructure stability. Therefore, the Si content is limited to 0.2% to 0.4%.

[0039] The main function of manganese (Mn) is to stabilize the austenitic phase and increase the solubility of nitrogen (N) in steel. Since this invention appropriately increases the N content, the Mn content must be above 2% to achieve a fully austenitic microstructure and match the N content. However, excessive addition can easily form MnS inclusions, reducing corrosion resistance. Therefore, the Mn content is limited to 2-3%.

[0040] Cr is the most important alloying element for forming the passivation film on stainless steel. However, in order to improve the stability of the microstructure, the Cr content is appropriately reduced in this invention; therefore, the Cr content is limited to 16-17%.

[0041] Ni is the most critical element for improving the stability of microstructure at extremely low temperatures, but adding too much Ni will lead to excessively high costs. Therefore, the Ni content is limited to 11-11.5%.

[0042] Nitrogen (N) can improve the stability of austenite and suppress the formation of intermetallic phases. In this invention, the N content is further increased to 0.1% or more. However, excessive N will reduce toughness. Therefore, the N content is limited to 0.1% to 0.15%.

[0043] Theoretically, through compositional optimization design, the solidification microstructure should be an all-austenitic structure. However, the actual solidification process is non-equilibrium solidification. On the one hand, during the crystallization and precipitation of austenite solid phase in the liquid phase, the cooling process causes uneven molecular diffusion in the precipitated solid phase, resulting in a compositional gradient at the solid-liquid interface and causing component segregation. During solidification, due to component segregation, more high-temperature ferrite tends to precipitate in the Cr and Mo ferrite-forming element agglomeration regions. Although the precipitation temperature of ferrite is lower than that of austenite, it will still precipitate in the liquid phase.

[0044] With increasing cooling water intensity, the central ferrite content decreases significantly. The diffusion process is not only difficult to occur in the solid phase but is also suppressed in the liquid phase, causing the alloy to enter the so-called "diffusion-free crystallization" stage and reducing the tendency for compositional segregation. Therefore, increasing the cooling water intensity to 160-180 L / t, ensuring sufficient cooling intensity, and improving heat transfer conditions can significantly reduce the residual ferrite content.

[0045] Although controlling the cooling rate during solidification significantly reduces the ferrite content, it still cannot meet the requirement of zero ferrite phase. Therefore, it is necessary to achieve high-temperature diffusion of the ferrite phase in the solid state through a high-temperature, long-term holding section to ensure uniform diffusion of Cr and Ni elements, eliminate non-equilibrium residual ferrite, and improve the uniformity of the microstructure. After loading the continuously cast billet into the furnace, it is heated to 1240–1260℃ and held for 12–13 hours. After being removed from the furnace, it is quickly cooled in water to prevent the precipitation of the ferrite phase.

[0046] Furthermore, the temperature for solution treatment of the rolled steel plate is 1120–1150℃, and the holding time is 4–5 min / mm thickness, thereby preventing deformation and martensitic phase transformation during processing.

[0047] It should be noted that other process parameters not disclosed in the smelting, continuous casting, homogenization and solution treatment processes of this invention are all carried out in accordance with the prior art, and this invention does not make specific limitations on them.

[0048] The method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures in this invention effectively improves the microstructure stability of austenitic stainless steel at extremely low temperatures by using special composition design and controlling key process points in continuous casting, homogenization annealing, and solution treatment.

[0049] Example

[0050] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments, unless otherwise specified, are performed according to conventional methods and conditions.

[0051] The process flow used in Examples 1-6 and Comparative Example (304L) was smelting → continuous casting → homogenization treatment → rolling → solution treatment. The chemical composition of the stainless steel prepared in each example and comparative example is shown in Table 1, the process parameters used in the preparation process are shown in Table 2, and the summary of the microstructural stability evaluation of the stainless steel is shown in Table 3.

[0052] Table 1. Chemical composition (weight percentage) of the stainless steel prepared in the Examples and Comparative Examples.

[0053]

[0054] Table 2 Comparison of the preparation processes of stainless steel in the Examples and Comparative Examples

[0055]

[0056] Table 3 shows the evaluation of the microstructure stability of the stainless steel prepared in the examples and comparative examples:

[0057]

[0058] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for improving the microstructural stability of austenitic stainless steel at extremely low temperatures, characterized in that, include: (1) Smelting to obtain stainless steel liquid with qualified composition; The stainless steel liquid, by weight percentage, comprises: C 0.025-0.035%, Si 0.2-0.4%, Mn 2-3%, Cr 16-17%, Ni 11-11.5%, N 0.1-0.15%, with the balance being Fe and other unavoidable impurity elements; (2) The stainless steel liquid is continuously cast to obtain a continuously cast billet; The strength of the cooling water during continuous casting is 160-180 L / t; (3) After homogenization treatment, the continuously cast billet is quickly cooled in water and then rolled to obtain a steel plate; The homogenization treatment temperature is 1240–1260℃, and the holding time is 12–13 hours. (4) The steel plate is subjected to solution treatment at a temperature of 1120 to 1150°C and a holding time of 4 to 5 min / mm thickness.

2. The method for improving the microstructure stability of austenitic stainless steel at extremely low temperatures according to claim 1, characterized in that, The content of each component in the stainless steel liquid satisfies the following formula: 3.2×(Cr%+1.5×Si%)-2.5×(Ni%+0.5×Mn%+30×C%+30×N%)-24.7≤0.

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