Ultra-low temperature steel and method of rolling and use thereof

Abstract

This invention discloses a cryogenic steel, its rolling method, and its applications. This cryogenic steel uses C, Mn, Mo, and Si as alloying elements, exhibiting an austenitic structure with suitable stacking fault energy. It possesses high strength and high ductility at -269℃, making it suitable as a high-strength material for cryogenic environments such as superconducting coil armor and liquid helium container materials. Compared to stainless steel under the same operating conditions, it has higher strength and does not contain Ni, thus improving the material's economic efficiency.

Description

Technical Field

[0001] This invention relates to an ultra-low temperature steel, its rolling method and application, and more particularly to an ultra-low temperature steel with an ultra-low temperature performance of -269℃, its rolling method and application. Background Technology

[0002] Storage and transportation containers for cryogenic media such as liquefied natural gas, liquid hydrogen, and liquid helium are conventionally made of high-nickel steel, such as Invar steel (36% Ni), 316 stainless steel (12% Ni), and 9% Ni steel. Superconducting coil armor or liquid helium containers require high-strength materials capable of operating at -269°C. While stainless steel has high ductility, its strength is low, typically requiring deformation strengthening to improve it. Furthermore, stainless steel contains a large amount of Ni, resulting in high alloy costs. When used as a structural material at -269°C, high mechanical properties are required, not necessarily corrosion resistance. Therefore, as a cryogenic structural material, it is necessary to redesign the alloy composition to improve mechanical properties while reducing material costs, which is more beneficial for practical applications. "Replacing nickel with manganese" is an important direction for optimizing the design of materials such as stainless steel, requiring targeted design of composition and processes based on the performance requirements of different operating environments. Summary of the Invention

[0003] Purpose of the invention: The present invention aims to provide an ultra-low temperature steel with excellent ultra-low temperature performance, its rolling method and application.

[0004] Technical solution: The ultra-low temperature steel of the present invention contains, by mass, 0.75% to 0.85% C, 22% to 26% Mn, 0.5% to 1.0% Mo, 0.1% to 0.4% Si, with the balance being Fe and unavoidable impurity elements; the steel plate thickness is 10 to 20 mm, the microstructure is austenitic, and the grain size is 4 to 5.

[0005] Preferably, the austenite stacking fault energy of the ultra-low temperature steel of the present invention at a temperature of -269°C is 42×C + 3.1×Mn - 15×C. 2 -0.026×Mn 2 -0.063×C×Mn-58, yield strength not less than 1500MPa, elongation after fracture not less than 30%.

[0006] Further preferably, the austenite stacking fault energy is 20–28 mJ / m. 2 .

[0007] Preferably, the ultra-low temperature steel of the present invention contains, by weight, 0.75% C, 22% Mn, 0.5% Mo, 0.1% Si, with the balance being Fe and unavoidable impurity elements;

[0008] Or it contains 0.79% C, 24% Mn, 0.7% Mo, 0.3% Si, with the balance being Fe and unavoidable impurity elements;

[0009] Alternatively, it may contain 0.85% C, 26% Mn, 1.0% Mo, 0.4% Si, with the balance being Fe and unavoidable impurity elements.

[0010] The chemical composition design principle of the ultra-low temperature steel described in this invention is as follows:

[0011] While martensitic steel boasts high strength, it exhibits low ductility. Furthermore, martensitic steel becomes embrittled at low temperatures, resulting in reduced toughness and ductility; therefore, its service temperature is typically no lower than -60°C. Even 9% Ni steel, known for its excellent low-temperature performance, has a service temperature no lower than -196°C. Austenitic steel, on the other hand, exhibits superior low-temperature performance compared to martensitic steel, possessing advantages such as high ductility and non-magnetic properties. Its minimum service temperature can reach ultra-low temperatures as low as -269°C, as exemplified by SUS316 austenitic stainless steel with approximately 12% Ni content. However, austenitic stainless steel is expensive due to the large amount of Ni added. Mn can inhibit the transformation of austenite to martensite and can be used as a Ni substitute to improve the stability of the austenitic structure. Mn's effect on austenite stability is approximately half that of Ni; therefore, the optimal mass percentage of Mn in the ultra-low temperature steel of this invention is 22%–26%.

[0012] C has a strong austenite stabilizing effect and is an effective element for improving austenite stability. Furthermore, C can hinder dislocation movement and increase strength. From the perspective of austenite stabilization, the addition of C can increase the stacking fault energy of austenite, causing austenite to twinnize under strain instead of undergoing a martensitic transformation. The strain-induced twinning mechanism of austenite can significantly improve plasticity. The stacking fault energy of austenite is related to composition and temperature. At -269℃, the austenite stacking fault energy of the ultra-low temperature steel of this invention is mainly determined by the mass percentages of C and Mn, specifically expressed as: 42×C + 3.1×Mn - 15×C 2 -0.026×Mn 2 -0.063×C×Mn-58. The preferred stacking fault energy in this invention is 20–28 mJ / m. 2 When the mass percentage of Mn is 22% to 26%, the mass percentage of C is 0.75% to 0.85% accordingly.

[0013] To suppress the precipitation of coarse cementite at grain boundaries under high carbon content conditions, this invention adds 0.5% to 1.0% by mass of Mo to improve plasticity. In addition to the above chemical composition, this invention optimizes the types and contents of other added elements. Among them, Si, a commonly used deoxidizing element in steelmaking, can also produce a certain degree of solid solution strengthening in finished steel plates; however, Si tends to segregate at grain boundaries, reducing plasticity, and its content needs to be controlled. This invention preferably controls the mass percentage of Si to 0.1% to 0.4%.

[0014] The rolling method for ultra-low temperature steel according to the present invention includes the following steps:

[0015] (1) Heating: Heating billets with the same composition and a thickness 15 times that of steel plates are carried out at a temperature of 1100-1150℃ and a total furnace time of 1.5-1.7 min / mm × billet thickness;

[0016] (2) Descaling: Remove the iron oxide scale from the heated billet. The high-pressure water pressure is 22-24 MPa.

[0017] (3) Rolling: The initial rolling temperature is 1005~1030℃, and the final rolling temperature is 800~830℃;

[0018] (4) Cooling: The rolled steel plate is cooled rapidly with an inlet water temperature of 730-765℃, an outlet water temperature of 130-240℃, and a cooling rate of 27-48℃ / s.

[0019] The heating process utilizes a walking beam furnace.

[0020] Preferably, the liquidus temperature for preparing the billet is 1369–1391 °C.

[0021] The rolling process mechanism of the ultra-low temperature steel plate described in this invention is as follows:

[0022] To achieve sufficient grain refinement and strain accumulation through thermomechanical rolling, and thus sufficient strength in the finished steel plate, a sufficient compression ratio is required in the selection of slab thickness, i.e., the ratio of slab thickness (H) to finished steel plate thickness (h) (H / h). This invention controls the compression ratio at 15.

[0023] The liquidus temperature of the ultra-low temperature steel of this invention is 1369-1391℃, which is about 140℃ lower than that of low alloy steel. Therefore, it is necessary to control the billet heating temperature to avoid overheating. This invention controls the heating temperature at 1100-1150℃.

[0024] Because the thermal conductivity of the ultra-low temperature steel of this invention is only about one-third that of ordinary low-alloy steel, it is necessary to ensure that slabs of different thicknesses have sufficient furnace heating time to ensure complete austenitization. This invention controls the furnace time at (1.5–1.7 min / mm) × H, where H is the slab thickness. For example, when the slab thickness is 210 mm, the furnace time is 315–357 min.

[0025] After being heated in a walking beam furnace, the slabs are removed from the furnace and then pass through a descaling box to remove iron oxide scale. High-pressure water at a pressure of 22-24 MPa is used to ensure the descaling effect.

[0026] The slab is descaled in a descaling box before rolling. The initial rolling temperature is 1005–1030℃, and the final rolling temperature is controlled at a relatively low level, i.e., 800–830℃. Thermomechanical rolling refines the austenite grains, accumulates sufficient strain, and improves the strength of the finished steel plate. Sufficient slab thickness is beneficial for improving the thermomechanical rolling effect.

[0027] After rolling, the steel plate enters an ultra-fast cooling system for high-pressure water cooling. The inlet water temperature is 730–765℃, and the plate is cooled to an outlet water temperature of 130–240℃ at a cooling rate of 27–48℃ / s. The accelerated cooling serves two purposes: firstly, it preserves the accumulated thermomechanical rolling strain and improves the strength of the steel plate; secondly, it inhibits carbide precipitation and improves the plasticity of the steel plate.

[0028] It should be noted that the actual content of alloying elements during material manufacturing fluctuates within a small range near the design range, which is an unavoidable fluctuation in normal industrial production. Although this invention clearly specifies the content range of each element and also specifies the operating range of heat treatment process parameters, these deviations will not significantly affect the effectiveness of this invention within a reasonable range.

[0029] The cryogenic steel described in this invention is used in the manufacture of cryogenic storage and transportation containers (such as storage tanks, ships, vehicles, spacecraft, etc.), pipes, and superconducting coil armor.

[0030] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0031] This steel plate uses C, Mn, Mo, and Si as alloying elements, forming an austenitic structure with suitable stacking fault energy. It exhibits high strength and high ductility at -269℃, making it suitable as a high-strength material for cryogenic environments such as superconducting coil armor and liquid helium container materials. Compared to stainless steel materials used under the same conditions, it has higher strength and does not contain Ni, resulting in better economic efficiency. Attached Figure Description

[0032] Figure 1 This is a micrograph of the austenitic structure of the ultra-low temperature steel plate of Example 1 of the present invention. Detailed Implementation

[0033] The technical solution of the present invention will be further described below with reference to the embodiments.

[0034] Example 1

[0035] An ultra-low temperature steel plate has a chemical composition of 0.75C, 22Mn, 0.5Mo, and 0.1Si by mass percentage, with the balance being Fe and unavoidable impurity elements. The slab thickness is 210mm, and the rolled steel plate thickness is 14mm. The slab is heated in a walking beam furnace to a target temperature of 1140℃ for a total furnace time of 315min. A descaling box is used to remove the iron oxide scale from the heated slab after exiting the furnace, with a high-pressure water pressure of 22MPa. The initial rolling temperature is 1020℃, and the final rolling temperature is 825℃. The rolled steel plate is accelerated cooled using an ultra-fast cooling system, with an inlet water temperature of 760℃, an outlet water temperature of 180℃, and a cooling rate of 42℃ / s. The steel plate has an austenitic microstructure, and the austenite stacking fault energy at -269℃ is 20mJ / m. 2 Yield strength 1525MPa, elongation after fracture 39%.

[0036] Example 2

[0037] An ultra-low temperature steel plate has a chemical composition of 0.79C, 24Mn, 0.7Mo, and 0.3Si by mass percentage, with the balance being Fe and unavoidable impurity elements. The slab thickness is 150mm, and the rolled steel plate thickness is 10mm. The slab is heated in a walking beam furnace to a target temperature of 1150℃ for a total furnace time of 255min. A descaling box is used to remove the iron oxide scale from the heated slab after exiting the furnace, with a high-pressure water pressure of 24MPa. The initial rolling temperature is 1005℃, and the final rolling temperature is 800℃. The rolled steel plate is accelerated cooled using an ultra-fast cooling system, with an inlet water temperature of 730℃, an outlet water temperature of 130℃, and a cooling rate of 48℃ / s. The steel plate has an austenitic microstructure, with an austenite stacking fault energy of 24mJ / m at -269℃. 2 Yield strength 1560MPa, elongation after fracture 34%.

[0038] Example 3

[0039] An ultra-low temperature steel plate has a chemical composition of 0.85C, 26Mn, 1.0Mo, and 0.4Si by mass percentage, with the balance being Fe and unavoidable impurity elements. The slab thickness is 300mm, and the rolled steel plate thickness is 20mm. The slab is heated in a walking beam furnace to a target temperature of 1100℃ for a total furnace time of 485min. A descaling box is used to remove the iron oxide scale from the heated slab after exiting the furnace, with a high-pressure water pressure of 22MPa. The initial rolling temperature is 1030℃, and the final rolling temperature is 830℃. The rolled steel plate is accelerated cooled using an ultra-fast cooling system, with an inlet water temperature of 765℃, an outlet water temperature of 240℃, and a cooling rate of 27℃ / s. The steel plate has an austenitic microstructure, with an austenitic stacking fault energy of 28mJ / m at -269℃. 2 Yield strength 1540MPa, elongation after fracture 36%.

Claims

1. A type of ultra-low temperature steel, characterized in that, By mass, it contains 0.75%~0.85% C, 22%~26% Mn, 0.5%~1.0% Mo, and 0.1%~0.4% Si, with the balance being Fe and unavoidable impurity elements; the austenite stacking fault energy of the ultra-low temperature steel at -269℃ is 42×C+3.1×Mn-15×C. 2 -0.026×Mn 2 -0.063×C×Mn-58, yield strength not less than 1500MPa, elongation after fracture not less than 30%; The rolling method for ultra-low temperature steel includes the following steps: (1) Heating: The billet with the same composition and a thickness of 15 times that of the steel plate is heated to a temperature of 1100~1150℃ and a total furnace time of 1.5~1.7min / mm×bill thickness; the thickness of the steel plate is 10~20mm. (2) Descaling: Remove the iron oxide scale from the heated billet. The high-pressure water pressure is 22~24MPa. (3) Rolling: The initial rolling temperature is 1005~1030℃, and the final rolling temperature is 800~830℃; (4) Cooling: The rolled steel plate is cooled rapidly with an inlet water temperature of 730~765℃, an outlet water temperature of 130~240℃, and a cooling rate of 27~48℃ / s.

2. The ultra-low temperature steel according to claim 1, characterized in that, The steel plate is 10-20mm thick, with an austenitic microstructure and a grain size of 4-5.

3. The ultra-low temperature steel according to claim 1, characterized in that, The stacking fault energy of the austenite is 20~28 mJ / m 2 .

4. The characteristic of the method according to claim 1 is that, by mass, it contains 0.75% C, 22% Mn, 0.5% Mo, 0.1% Si, and the balance is Fe and unavoidable impurity elements.

5. The characteristic of the method according to claim 1 is that, by mass, it contains 0.79% C, 24% Mn, 0.7% Mo, 0.3% Si, and the balance is Fe and unavoidable impurity elements.

6. The characteristic of the method according to claim 1 is that, by mass, it contains 0.85% C, 26% Mn, 1.0% Mo, 0.4% Si, and the balance is Fe and unavoidable impurity elements.

7. A method for rolling ultra-low temperature steel according to claim 1, characterized in that, Includes the following steps: (1) Heating: The billet with the same composition and a thickness of 15 times that of the steel plate is heated to a temperature of 1100~1150℃ and a total furnace time of 1.5~1.7min / mm×bill thickness; the thickness of the steel plate is 10~20mm. (2) Descaling: Remove the iron oxide scale from the heated billet. The high-pressure water pressure is 22~24MPa. (3) Rolling: The initial rolling temperature is 1005~1030℃, and the final rolling temperature is 800~830℃; (4) Cooling: The rolled steel plate is cooled rapidly with an inlet water temperature of 730~765℃, an outlet water temperature of 130~240℃, and a cooling rate of 27~48℃ / s.

8. The steel rolling method according to claim 7, characterized in that, The liquidus temperature for preparing the billet is 1369~1391℃.

9. The application of the cryogenic steel of claim 1 in the manufacture of cryogenic storage and transportation containers, pipelines, and superconducting coil armor.

Images