Low temperature steel and method of heat treatment and use thereof

Abstract

The application discloses a low-temperature steel and a heat treatment method and application thereof. The low-temperature steel contains, in mass, C: 0.01%-0.02%, Mn: 4.8%-5.6%, Si: 0.05%-0.15%, Ti: 0.005%-0.025%, S: ≤0.003%, P: ≤0.008%, and the balance of Fe and impurity elements. Through the alloy design idea of replacing nickel with manganese and the optimization of other element compositions, the material obtains excellent low-temperature toughness by using reasonable rolling and heat treatment processes, and the cost is greatly reduced compared with 5Ni steel with the same service temperature.

Description

Technical Field

[0001] This invention relates to a low-temperature steel and its heat treatment method and application, and more particularly to a low-temperature steel with a low-temperature performance of -120℃ and its heat treatment method and application. Background Technology

[0002] Low-temperature steel is a general term for steels that are used at temperatures below 0°C and possess sufficient notch toughness. Its development has a history of nearly a century, significantly driving advancements in science and technology in the field of cryogenics. Low-temperature steels can be broadly classified into four categories: low-carbon aluminum-killed steel, low-temperature high-strength steel, nickel-based low-temperature steel, and austenitic stainless steel.

[0003] Nickel-based low-temperature steels use Ni as the main alloying element, significantly reducing the ductile-brittle transition temperature through a high Ni content. Depending on the service temperature, the Ni content of these low-temperature steels can be 1.5%, 3.5%, 5%, and 9%, with Charpy impact test temperatures of -80℃, -110℃, -120℃, and -196℃, and yield strengths exceeding 275MPa, 345MPa, 390MPa, and 575MPa, respectively. Among these, 5Ni and 9Ni steels are the most widely used.

[0004] In typical low-temperature steels, Ni is the most fundamental and important alloying element. It can form α or γ solid solutions with Fe, improving the strength-toughness balance of the ferrite phase while also enhancing toughness by stabilizing the austenite phase, thus significantly lowering the ductile-brittle transition temperature. However, the large addition of Ni increases material costs. Mn, another beneficial element in low-temperature steel, can lower the ductile-brittle transition temperature and improve low-temperature toughness, and it has a significant price advantage compared to Ni. If an appropriate amount of Mn can replace some or all of the Ni, the alloying cost of low-temperature steel can be greatly reduced, improving its economic viability. Summary of the Invention

[0005] Purpose of the invention: The present invention aims to provide a low-temperature steel with excellent low-temperature performance of -120℃, its heat treatment method and application.

[0006] Technical solution: The low-temperature steel of the present invention contains, by mass, C: 0.01%-0.02%, Mn: 4.8%-5.6%, Si: 0.05%-0.15%, Ti: 0.005%-0.025%, S: ≤0.003%, P: ≤0.008%, and the balance Fe and impurity elements; it has a martensitic-austenitic dual-phase composite structure, wherein the volume fraction of austenite is 9%-16%.

[0007] Carbon (C) can enhance strength through solid solution strengthening or precipitation strengthening, and it can also stabilize the austenite phase. However, to lower the ductile-brittle transition temperature of the material, the C content should be minimized. Furthermore, C is detrimental to the weldability of the material. Therefore, this invention controls the C content to a low level of 0.01%-0.02%.

[0008] Mn is the main alloying element in the low-temperature steel described in this invention. Mn is a ferrite-strengthening element and also an austenite-stabilizing element. Therefore, Mn can replace Ni, which is more expensive, to a certain extent. To achieve excellent low-temperature toughness without adding Ni, the Mn content needs to be higher than the range for low-alloy steels. However, excessively high Mn content will exacerbate segregation, increase smelting difficulty, and raise material costs. This invention controls the Mn content at 4.8%-5.6%.

[0009] In steelmaking, silicon (Si) acts as a deoxidizing element. An appropriate amount of Si can inhibit the segregation of Mn and P, while excessive O content and Mn and P segregation both impair low-temperature toughness. However, Si can also increase the ductile-brittle transition temperature; therefore, its content needs to be controlled to prevent it from becoming too high for low-temperature steels. This invention controls the Si content to 0.05%-0.15%.

[0010] In this invention, the addition of trace amounts of Ti can hinder grain boundary migration at high temperatures through the precipitation of fine and dispersed second phases, thereby refining the grains and improving mechanical properties. The amount added is controlled within the range of 0.005%-0.025%.

[0011] S readily combines with Mn to form MnS, and P tends to segregate at grain boundaries and reduce the grain boundary's resistance to crack propagation. To improve the low-temperature toughness of materials, S and P need to be controlled to a minimum.

[0012] Preferably, the steel plate has a thickness of 5-80mm, a yield strength of 440-490MPa, a tensile strength of 670-695MPa, an elongation after fracture of 26-30%, and a Charpy impact absorption energy of 140-190J at -120℃.

[0013] Further preferred, by mass, it contains C: 0.016%, Mn: 5.3%, Si: 0.15%, Ti: 0.014%, S: 0.002%, P: 0.006%, and the balance Fe and impurity elements; the volume percentage of austenite in the duplex composite structure is 14%.

[0014] Further preferred, by mass, it contains C: 0.01%, Mn: 4.8%, Si: 0.25%, Ti: 0.005%, S: 0.003%, P: 0.008%, and the balance Fe and impurity elements; the volume percentage of austenite in the duplex composite structure is 16%.

[0015] Further preferred, by mass, it contains C: 0.02%, Mn: 5.6%, Si: 0.05%, Ti: 0.025%, S: 0.001%, P: 0.005%, and the balance Fe and impurity elements; the volume percentage of austenite in the duplex composite structure is 9%.

[0016] The heat treatment method for low-temperature steel according to the present invention includes the following steps:

[0017] (1) Quenching: Heat the steel plate to 820-880℃ and hold for 30 minutes, then cool it to room temperature with water.

[0018] (2) Tempering: Heat the above steel plate to 640-660℃ and hold for 120 minutes, then air cool to room temperature.

[0019] Among them, the untreated steel plates are prepared using conventional methods for low-temperature steel, such as steelmaking, continuous casting, and rolling processes.

[0020] Preferably, in step (1), the steel plate is heated to 830-850°C.

[0021] Preferably, the steel plate in step (2) forms 15% volumetric austenite in reverse transformation during the heat preservation process.

[0022] The mechanism of the preparation method in this invention is as follows:

[0023] The billet is heated to austenitize, and the alloying elements are homogenized through diffusion. Excessive heating temperature or holding time will result in overly coarse high-temperature austenite grains, while excessively low heating temperature or short holding time is detrimental to the homogenization of alloying element distribution. Therefore, this invention controls the heating temperature between 820-880℃. This temperature range is within the austenite phase region, and a holding time of 30 minutes is sufficient for complete austenitization and sufficiently uniform alloying element distribution. Excessively high temperature or excessively long holding time will cause coarse grains and reduce toughness. Austenitization is accelerated by water cooling (preferably at a cooling rate of not less than 5℃ / s) to suppress carbide precipitation, and quenching yields a quenched martensite structure. In the tempering heat treatment process, the steel plate is heated to 640-660℃, which falls within the austenite-ferrite two-phase region of the composition of this invention. Holding at this temperature for 120 minutes allows for the formation of approximately 15% volume fraction of reverse-transformed austenite, enriched with alloying elements to achieve sufficient thermal stability. This enables the steel to maintain a face-centered cubic structure without phase transformation even at -120℃. Another function of tempering is to restore the martensitic structure. After heat treatment, a multiphase structure of tempered martensite + reverse-transformed austenite is obtained, exhibiting excellent low-temperature toughness.

[0024] The cryogenic steel described in this invention is used in storage and transportation containers (e.g., storage tanks), pipelines, and transportation vehicles (ships, vehicles, aircraft, etc.) for the preparation of cryogenic gases.

[0025] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0026] This invention, through the alloy design concept of replacing nickel with manganese and optimizing the composition of other elements, and by adopting reasonable rolling and heat treatment processes, enables the material to obtain excellent low-temperature toughness, and significantly reduces the cost compared with 5Ni steel at the same service temperature. Attached Figure Description

[0027] Figure 1 This is an EBSD microstructure characterization diagram of the low-temperature steel in Example 1 of the present invention. Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to the embodiments.

[0029] A 30mm thick low-temperature steel sheet was prepared with the following chemical composition and mass fractions: 0.016% C, 5.3% Mn, 0.15% Si, 0.014% Ti, 0.002% S, 0.006% P, with the balance being Fe and impurities. The steel sheet was heated to 840℃ and held for 30 minutes, then water-cooled to room temperature. The steel sheet was then heated to 650℃ and held for 120 minutes, followed by air cooling to room temperature. Figure 1 It can be seen that the steel plate has a dual-phase composite structure of martensite and austenite (red area), with an austenite volume fraction of 14%, a yield strength of 475 MPa, a tensile strength of 680 MPa, an elongation after fracture of 28%, and a Charpy impact energy of 175 J at -120℃.

[0030] Example 2

[0031] A 5mm thick low-temperature steel has the following chemical composition and mass fractions: 0.01% C, 4.8% Mn, 0.25% Si, 0.005% Ti, 0.003% S, 0.008% P, with the balance being Fe and impurities. The steel plate was heated to 830℃ and held for 30 minutes, then water-cooled to room temperature. It was then heated to 660℃ and held for 120 minutes, followed by air cooling to room temperature. The steel plate exhibits a martensitic + austenitic dual-phase composite microstructure, with an austenite volume fraction of 16%. It has a yield strength of 440 MPa, a tensile strength of 670 MPa, an elongation after fracture of 30%, and a Charpy impact absorption energy of 190 J at -120℃.

[0032] Example 3

[0033] A low-temperature steel with a thickness of 80 mm has the following chemical composition and mass fractions: 0.02% C, 5.6% Mn, 0.05% Si, 0.025% Ti, 0.001% S, 0.005% P, with the balance being Fe and impurities. The steel plate was heated to 860℃ and held for 30 min, then water-cooled to room temperature. It was then heated to 640℃ and held for 90 min, followed by air cooling to room temperature. The steel plate exhibits a martensitic + austenitic dual-phase composite microstructure, with an austenite volume fraction of 9%. It has a yield strength of 490 MPa, a tensile strength of 670 MPa, an elongation after fracture of 26%, and a Charpy impact energy of 140 J at -120℃.

Claims

1. A low-temperature steel, characterized in that, The material, by mass, contains C: 0.01%-0.02%, Mn: 4.8%-5.6%, Si: 0.05%-0.15%, Ti: 0.005%-0.025%, S: ≤0.003%, P: ≤0.008%, and the balance Fe and impurity elements; the low-temperature steel has a martensitic-austenitic dual-phase composite structure, wherein the volume fraction of austenite is 9%-16%; the steel plate thickness is 5-80mm, the yield strength is 440-490MPa, the tensile strength is 670-695MPa, the elongation after fracture is 26%-30%, and the impact absorption energy at -120℃ is 140-190J; the low-temperature steel is obtained through the following heat treatment: (1) Quenching: Heat the steel plate to 820-880℃ and hold for 30 minutes, then cool it to room temperature with water; (2) Tempering: Heat the above steel plate to 640-660℃ and hold for 120 minutes, then air cool to room temperature.

2. The low-temperature steel according to claim 1, characterized in that, By mass, it contains C: 0.016%, Mn: 5.3%, Si: 0.15%, Ti: 0.014%, S: 0.002%, P: 0.006%, and the balance Fe and impurity elements; the volume percentage of austenite in the duplex composite structure is 14%.

3. The low-temperature steel according to claim 1, characterized in that, By mass, it contains C: 0.02%, Mn: 5.6%, Si: 0.05%, Ti: 0.025%, S: 0.001%, P: 0.005%, and the balance Fe and impurity elements; the volume percentage of austenite in the dual-phase composite structure is 9%.

4. A heat treatment method for low-temperature steel according to claim 1, characterized in that, Includes the following steps: (1) Quenching: Heat the steel plate to 820-880℃ and hold for 30 minutes, then cool it to room temperature with water; (2) Tempering: Heat the above steel plate to 640-660℃ and hold for 120 minutes, then air cool to room temperature.

5. The heat treatment method according to claim 4, characterized in that, Step (1) Heat the steel plate to 830-850℃.

6. The heat treatment method according to claim 4, characterized in that, In step (2), the steel plate forms a 15% volumetric austenite in reverse transformation during the heat preservation process.

7. The application of the cryogenic steel according to claim 1 in the manufacture of cryogenic gas storage and transportation containers, pipelines, and transportation vehicles.

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