An ultra-low temperature steel and a method of heat treatment thereof

Abstract

The application discloses an ultralow-temperature steel, which comprises the following components in percentage by mass: 0.46% to 0.49% of C, 24.6% to 24.9% of Mn, 3.0% to 3.3% of Cr, and the balance of Fe and inevitable impurity elements. The application also discloses a heat treatment method of the ultralow-temperature steel. The application does not add expensive Ni elements, and the composition and process are simple and easy to implement, so that the ultralow-temperature steel can replace S316 stainless steel and be used in ultralow-temperature service environments such as liquid hydrogen or liquid helium storage and transportation containers.

Description

Technical Field

[0001] This invention relates to the field of metallurgy, and more particularly to an ultra-low temperature steel and its heat treatment method. Background Technology

[0002] Materials requiring excellent mechanical properties even at cryogenic temperatures as low as -269°C, such as those used in liquid hydrogen or liquid helium containers, typically employ austenitic stainless steels like S316. S316 and similar austenitic stainless steels contain 10–14% Ni and 16–18% Cr, with a C content not exceeding 0.1% and a Mn content not exceeding 2%. This compositional design results in an austenitic microstructure with a yield strength exceeding 205 MPa, a tensile strength exceeding 515 MPa, an elongation after fracture exceeding 45%, and good cryogenic toughness, exhibiting a lateral expansion exceeding 0.53 mm at -269°C. The excellent strength-ductility-toughness balance of stainless steel stems from its sufficiently stable austenitic microstructure. Ni is a typical austenite-forming element; while Cr is not an austenite-forming element, it can lower the martensitic transformation temperature. In fact, the primary goal of this compositional design for stainless steel is to achieve excellent corrosion resistance rather than cryogenic mechanical properties.

[0003] Stainless steel possesses a good balance of strength, plasticity, and toughness, but its high cost is due to the addition of a large amount of nickel. When used as a structural material in ultra-low temperature environments (-269℃), high mechanical properties are required, but corrosion resistance is not. Therefore, as an ultra-low temperature structural material, it is necessary to redesign the alloy composition to reduce material costs while ensuring mechanical properties, which is more conducive to industrial applications. "Replacing nickel with manganese" is an important direction for optimizing the composition of ultra-low temperature steel, and matching manufacturing processes also need to be developed. Summary of the Invention

[0004] Purpose of the invention: To address the shortcomings and defects of existing technologies, this invention provides an ultra-low temperature steel and its heat treatment method, which does not add expensive Ni elements, has a simple composition and process that is easy to implement, and can replace S316 stainless steel for use in ultra-low temperature service environments such as liquid hydrogen or liquid helium storage and transportation containers.

[0005] Technical solution: The present invention provides an ultra-low temperature steel comprising the following components in the indicated mass percentages: 0.46% to 0.49% C, 24.6% to 24.9% Mn, 3.0% to 3.3% Cr, with the balance being Fe and unavoidable impurity elements.

[0006] The heat treatment method for ultra-low temperature steel of the present invention includes the following steps:

[0007] 1) Heating: The furnace atmosphere temperature is 1020℃~1070℃, and the heating time coefficient calculated in millimeters is 1.5min / mm~2.0min / mm;

[0008] 2) Isothermal treatment: After the steel is heated to the target temperature, it is subjected to isothermal treatment for 50 to 65 minutes.

[0009] 3) Cooling: The steel is immediately cooled in water after being taken out of the furnace, and the water temperature is 41℃~136℃.

[0010] The thickness of the ultra-low temperature steel ranges from 6mm to 30mm.

[0011] Among them, the austenitic grain size of the ultra-low temperature steel is grade 4 to 5, the yield strength is 260 MPa to 290 MPa, the tensile strength is 660 MPa to 710 MPa, the elongation after fracture is 63% to 69%, and the lateral expansion in the Charpy impact test at -269℃ is 1.9 mm to 2.4 mm.

[0012] Preferably, the ultra-low temperature steel comprises the following components in the following mass percentages: 0.49% C, 24.7% Mn, 3.0% Cr, with the balance being Fe and unavoidable impurity elements.

[0013] The ultra-low temperature steel has a thickness of 17mm. It is heated in a furnace at an atmosphere temperature of 1055℃ for 31 minutes. After reaching the set temperature, it is isothermaled for 57 minutes. The steel is then immediately cooled in water after being taken out of the furnace, with an outlet water temperature of 113℃.

[0014] Preferably, the ultra-low temperature steel comprises the following components in the following mass percentages: 0.47% C, 24.6% Mn, 3.3% Cr, with the balance being Fe and unavoidable impurity elements.

[0015] The ultra-low temperature steel is 30mm thick and is heated in a furnace at an atmosphere temperature of 1070℃ for 60 minutes. After reaching the set temperature, it is isothermaled for 65 minutes. The steel is then immediately cooled in water after being taken out of the furnace, with an outlet water temperature of 136℃.

[0016] Preferably, the ultra-low temperature steel comprises the following components in the following mass percentages: 0.46% C, 24.9% Mn, 3.2% Cr, with the balance being Fe and unavoidable impurity elements.

[0017] The ultra-low temperature steel has a thickness of 6mm. It is heated in a heating furnace at an atmosphere temperature of 1020℃ for 9 minutes. After reaching the set temperature, it is isothermaled for 50 minutes. The steel is then immediately cooled in water after being taken out of the furnace, with an outlet water temperature of 41℃.

[0018] Beneficial Effects: Compared with existing technologies, this invention has the following significant advantages: The cryogenic steel of this invention exhibits excellent strength-plasticity-toughness matching in a cryogenic environment of -269℃, replacing austenitic stainless steel for cryogenic storage and transportation containers for media such as liquid hydrogen or liquid helium, achieving the effect of saving nickel and reducing material costs. This invention uses C, Mn, and Cr as alloying elements, without adding expensive Ni, making its composition and process simple and easy to implement. Its mechanical properties are superior to existing typical S316 stainless steel, exhibiting excellent impact toughness at -269℃, and can replace S316 stainless steel for cryogenic service environments such as liquid hydrogen or liquid helium storage and transportation containers. Attached Figure Description

[0019] Figure 1 This is a micrograph of the austenitic structure of the ultra-low temperature steel of the present invention. Detailed Implementation

[0020] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0021] The ultra-low temperature steel of the present invention comprises the following components in the following mass percentages: 0.46% to 0.49% C, 24.6% to 24.9% Mn, 3.0% to 3.3% Cr, with the balance being Fe and unavoidable impurity elements.

[0022] The heat treatment method for ultra-low temperature steel of the present invention is characterized by comprising the following steps:

[0023] 1) Heating: The furnace atmosphere temperature is 1020℃~1070℃, and the heating time coefficient calculated in millimeters is 1.5min / mm~2.0min / mm;

[0024] 2) Isothermal treatment: After the steel is heated to the target temperature, it is subjected to isothermal treatment for 50 to 65 minutes.

[0025] 3) Cooling: The steel is immediately cooled in water after being taken out of the furnace, and the water temperature is 41℃~136℃.

[0026] The thickness of the ultra-low temperature steel of this invention is 6mm to 30mm. The austenitic grain size of the ultra-low temperature steel is grade 4 to 5, the yield strength is 260MPa to 290MPa, the tensile strength is 660MPa to 710MPa, the elongation after fracture is 63% to 69%, and the lateral expansion in the Charpy impact test at -269℃ is 1.9mm to 2.4mm.

[0027] Mn and C are also austenite stabilizing elements, and this invention uses these two elements to replace the traditional Ni and Cr. The preferred Mn content in this invention is 24.6%–24.9%, which translates to a nickel equivalent of 12%, comparable to the Ni content of S316 stainless steel. Furthermore, this invention retains a certain amount of Cr, preferably 3.0%–3.3%, which is beneficial for improving austenite stability and preserving certain atmospheric corrosion resistance. Cr is not an expensive element and therefore does not significantly increase costs. This invention designs a novel ultra-low temperature steel by replacing the traditional Ni-Cr alloy system in low-temperature stainless steel with a C-Mn-Cr alloy system.

[0028] The carbon content design of this invention is related to the heat treatment process. For the austenitic structure of the ultra-low temperature steel of this invention, a high-temperature heat treatment was performed at an isothermal temperature of 1020–1070°C for 50–65 min. After the high-temperature isothermal heat treatment, the dislocation density in the microstructure decreased to a very low level, and the contribution of dislocation strengthening to strength was negligible. The austenitic grain size was 4–5 grades, and after further considering twinning, the contribution of grain boundary strengthening was approximately 70 MPa. For the austenitic structure, the solid solution strengthening effect of substitutional atoms such as Mn and Cr is almost zero; solid solution strengthening is mainly achieved through interstitial atoms such as C. In this invention, the strength contribution coefficient per unit mass fraction of C is 305 MPa / %, and with a preferred C content of 0.46–0.49%, the solid solution strengthening contribution is approximately 145 MPa. Furthermore, the lattice friction stress is approximately 55 MPa. The sum of these three factors contributes approximately 270 MPa to the strength, which is consistent with the actual yield strength value of this invention.

[0029] The high work hardening capacity of austenitic microstructure results in high tensile strength and elongation after fracture. The austenitic structure of this invention exhibits sufficient stability, remaining unchanged even at -269°C, and the dislocation density within the microstructure is reduced through high-temperature heat treatment, thus providing excellent impact toughness at low temperatures. This invention demonstrates a yield strength of 260–290 MPa, a tensile strength of 660–710 MPa, an elongation after fracture of 63–69%, and a Charpy impact test lateral expansion of 1.9–2.4 mm at -269°C. Its comprehensive mechanical properties surpass those of typical existing S316 stainless steel.

[0030] In implementing the heat treatment process of this invention, in addition to controlling the target temperature, it is also necessary to control the heating time and cooling regime. Austenitic structures have slower heat transfer, requiring a longer heating time compared to low-alloy steels; the heating time coefficient, calculated per millimeter thickness, is 1.5–2.0 min / mm. After isothermal treatment, the steel is immediately immersed in water after exiting the furnace to prevent the precipitation of carbides at grain boundaries at high temperatures. The steel is cooled to room temperature as much as possible. Considering factors such as steel plate thickness and water temperature variations, the actual outlet water temperature is 41–136°C.

[0031] Example 1:

[0032] The cryogenic steel of this embodiment comprises the following components by mass percentage: 0.49% C, 24.7% Mn, 3.0% Cr, with the balance being Fe and unavoidable impurity elements. The cryogenic steel has a thickness of 17 mm and is heated in a furnace at an atmosphere temperature of 1055°C for 31 minutes. After reaching the set temperature, it is isothermated for 57 minutes. The steel is immediately immersed in water for cooling after being removed from the furnace, with an outlet water temperature of 113°C.

[0033] Testing revealed that the austenitic grain size of the ultra-low temperature steel in this embodiment is grade 4.5, the yield strength is 285 MPa, the tensile strength is 695 MPa, the elongation after fracture is 64.5%, and the lateral expansion in the Charpy impact test at -269℃ is 2.3 mm.

[0034] Example 2:

[0035] The cryogenic steel of this embodiment comprises the following components by mass percentage: 0.47% C, 24.6% Mn, 3.3% Cr, with the balance being Fe and unavoidable impurity elements. The cryogenic steel has a thickness of 30 mm and is heated in a furnace at an atmosphere temperature of 1070°C for 60 minutes. After reaching the set temperature, it is isothermated for 65 minutes. The steel is immediately immersed in water for cooling after being removed from the furnace, with an outlet water temperature of 136°C.

[0036] Testing revealed that the austenitic grain size of the ultra-low temperature steel in this embodiment is grade 4, the yield strength is 260 MPa, the tensile strength is 660 MPa, the elongation after fracture is 69%, and the lateral expansion in the Charpy impact test at -269℃ is 2.4 mm.

[0037] Example 3:

[0038] The cryogenic steel of this embodiment comprises the following components by mass percentage: 0.46% C, 24.9% Mn, 3.2% Cr, with the balance being Fe and unavoidable impurity elements. The cryogenic steel has a thickness of 6 mm and is heated in a furnace at an atmosphere temperature of 1020°C for 9 minutes. After reaching the set temperature, it is isothermated for 50 minutes. Immediately after being removed from the furnace, the steel is cooled in water at an outlet temperature of 41°C.

[0039] According to the test results, the austenitic grain size of the ultra-low temperature steel in this embodiment is grade 5, the yield strength is 290MPa, the tensile strength is 710MPa, the elongation after fracture is 63%, and the lateral expansion in the Charpy impact test at -269℃ is 1.9mm.

Claims

1. A type of ultra-low temperature steel, characterized in that: It includes the following components in the following mass percentages: 0.46%~0.49% C, 24.6%~24.9% Mn, 3.0%~3.3% Cr, with the balance being Fe and unavoidable impurity elements; The heat treatment method for this ultra-low temperature steel includes the following steps: 1) Heating: The furnace atmosphere temperature is 1020℃~1070℃, and the heating time coefficient calculated in millimeters is 1.5min / mm~2.0min / mm; 2) Isothermal treatment: After the steel is heated to the target temperature, it is subjected to isothermal treatment for 50 to 65 minutes. 3) Cooling: The steel is immediately cooled in water after being taken out of the furnace, and the water temperature is 41℃~136℃; The thickness of the ultra-low temperature steel is 6mm to 30mm.

2. The ultra-low temperature steel according to claim 1, characterized in that: The austenitic grain size of the ultra-low temperature steel is grade 4 to 5, the yield strength is 260 MPa to 290 MPa, the tensile strength is 660 MPa to 710 MPa, the elongation after fracture is 63% to 69%, and the lateral expansion in the Charpy impact test at -269℃ is 1.9 mm to 2.4 mm.

3. The ultra-low temperature steel according to claim 1, characterized in that: It comprises the following components in the following mass percentages: 0.49% C, 24.7% Mn, 3.0% Cr, with the balance being Fe and unavoidable impurity elements.

4. The heat treatment method for ultra-low temperature steel according to claim 3, characterized in that: The ultra-low temperature steel has a thickness of 17mm. It is heated in a furnace at an atmosphere temperature of 1055℃ for 31 minutes. After reaching the set temperature, it is isothermaled for 57 minutes. The steel is then immediately cooled in water after being taken out of the furnace, with an outlet water temperature of 113℃.

5. The ultra-low temperature steel according to claim 1, characterized in that: It comprises the following components in the following mass percentages: 0.47% C, 24.6% Mn, 3.3% Cr, with the balance being Fe and unavoidable impurity elements.

6. The heat treatment method for ultra-low temperature steel according to claim 5, characterized in that: The ultra-low temperature steel is 30mm thick and is heated in a furnace at an atmosphere temperature of 1070℃ for 60 minutes. After reaching the set temperature, it is isothermaled for 65 minutes. The steel is then immediately cooled in water after being taken out of the furnace, with an outlet water temperature of 136℃.

7. The ultra-low temperature steel according to claim 1, characterized in that: It comprises the following components by mass percentage: 0.46% C, 24.9% Mn, 3.2% Cr, with the balance being Fe and unavoidable impurity elements.

8. The heat treatment method for ultra-low temperature steel according to claim 7, characterized in that: The ultra-low temperature steel has a thickness of 6mm. It is heated in a heating furnace at an atmosphere temperature of 1020℃ for 9 minutes. After reaching the set temperature, it is isothermaled for 50 minutes. The steel is then immediately cooled in water after being taken out of the furnace, with an outlet water temperature of 41℃.

Images