Cryogenic steels and their heat treatment processes and applications

Abstract

The present invention discloses a cryogenic steel and its heat treatment process and application, the cryogenic steel having a composition of 0.41%-0.45% C, 23.5%-24.5% Mn, 3.5%-3.7% Cr, 0.35%-0.45% Cu, 0.55%-0.65% Ni, 0.20%-0.24% V, 0.20%-0.24% Mo, 0.15%-0.25% Si, 0.02%-0.04% Al, and the balance being Fe and unavoidable impurities, has a fully austenitic structure, and has a stacking fault energy of 18-21 mJ·m at −269°C. -2 The above cryogenic steel, through a specific heat treatment process and in accordance with the composition of the components, has excellent cryogenic resistance, corrosion resistance and hydrogen damage resistance, and is applicable to the storage and transportation of cryogenic media such as liquefied natural gas and liquid hydrogen, and is suitable for complex environments such as land, marine and aviation.

Description

[Technical Field]

[0001] The present invention relates to a cryogenic steel and its heat treatment process and application, and more particularly to a cryogenic steel that can withstand cryogenic temperatures of -269°C and has excellent corrosion resistance and hydrogen damage resistance, and its heat treatment process and application. [Background technology]

[0002] Storage and transportation vessels for cryogenic media such as liquefied ethylene, liquefied natural gas, liquid hydrogen, and liquid helium are typically manufactured using high-nickel steels, such as Invar alloy (36% Ni), 316 stainless steel (12% Ni), and 9% Ni steel. However, these steels are economically unsuitable for practical use. Austenitic stainless steels can also be used for storage and transportation vessels for some cryogenic media, but their low strength and large expansion coefficients prevent them from meeting the requirements for extreme low-temperature environments, such as temperatures of -269°C, limiting their applications. In recent years, permanganized steels have also emerged as low-temperature materials. However, previously designed permanganized steels are limited to low-temperature environments down to -196°C and lack the required corrosion resistance and hydrogen embrittlement susceptibility. Summary of the Invention

[0003] The present invention aims to provide a cryogenic steel with excellent low-temperature performance, corrosion resistance, and hydrogen damage resistance, as well as a heat treatment process and application thereof, in response to the drawbacks of conventional cryogenic steels, which have only a single performance, are economical, and are subject to application regulations.

[0004] Regarding the technical solution, as a first aspect of the present invention, the cryogenic steel of the present invention contains, in mass%, C: 0.41% to 0.45%, Mn: 23.5% to 24.5%, Cr: 3.5% to 3.7%, Cu: 0.35% to 0.45%, Ni: 0.55% to 0.65%, V: 0.20% to 0.24%, Mo: 0.20% to 0.24%, Si: 0.15% to 0.25%, Al: 0.02% to 0.04%, and the balance being Fe and unavoidable impurities.

[0005] The above cryogenic steel has a fully austenitic structure and a stacking fault energy of 18 to 21 mJ·m at -269°C. -2 is.

[0006] Most preferably, the cryogenic steel contains, in mass %, C: 0.43%, Mn: 24.0%, Cr: 3.6%, Cu: 0.40%, Ni: 0.60%, V: 0.22%, Mo: 0.22%, Si: 0.20%, Al: 0.03%, with the balance being Fe and unavoidable impurities.

[0007] The mechanism for designing the components of the cryogenic steel is as follows.

[0008] An austenitic structure has well-balanced strength, plasticity, toughness, and a lower service temperature. Adding a high content of Ni can produce an austenitic structure, such as 316 austenitic stainless steel with a 12% Ni content, but at the expense of higher alloy costs. Mn can inhibit the transformation of austenite to martensite, so it can be used as a substitute for Ni to obtain an austenitic structure. The effect of Mn on the stability of austenite is approximately half that of Ni, so the most preferred Mn content in the cryogenic steel of the present invention is 24.0%.

[0009] C has a strong austenite stabilizing effect and is an element that enhances austenite stability, and C can inhibit dislocation movement and increase yield strength. However, if the C content is too high, the amount of carbides increases and toughness decreases, so the C content of the cryogenic steel according to the present invention is most preferably 0.43%.

[0010] Cr is a ferrite-forming element, but it is advantageous in improving austenite stability by lowering the martensite transformation temperature. If the Cr content is too high, carbide precipitation will progress and toughness will decrease. Therefore, the Cr content in the cryogenic steel according to the present invention is most preferably 3.6%.

[0011] The present invention adds 0.43% C, 24.0% Mn, and 3.6% Cr to obtain a highly stable austenitic structure, and even if subjected to a 0.35 true strain prior to subzero treatment at -269°C, no martensite structure appears. At the same time, the austenite is transformed to a stacking fault energy of 18 to 21 mJ m at -269°C. -2 The combined mechanism of dislocation and twinning achieves a balance between strength and toughness, and the cryogenic steel of the present invention has a lateral expansion of ≧1.35 mm in a Charpy impact test at −269°C.

[0012] To improve the corrosion resistance of the cryogenic steel of the present invention, it is preferable to add 0.40% Cu to the composition. However, Cu increases the tendency of cracking on the steel sheet surface, which is unfavorable for material manufacturing. To avoid the adverse effects of Cu, it is preferable to add 0.60% Ni in the present invention, ensuring that the Ni / Cu content ratio is ≥ 1. The Ni / Cu content ratio is preferably approximately 1.5, which effectively suppresses the precipitation of low-melting-point Cu at grain boundaries and improves the surface quality of the steel sheet. In addition to Cu, Cr and Ni also have the effect of improving corrosion resistance. By designing the combined addition of corrosion-resistant elements, the corrosion rate of the cryogenic steel of the present invention in a natural exposure test is ≤ 0.04 mm / a.

[0013] In order to obtain higher hydrogen damage resistance when manufacturing a liquid hydrogen tank from the cryogenic steel according to the present invention, it is preferable to add 0.22% V and 0.22% Mo at the same time as obtaining an austenite matrix structure. By combining with the corresponding heat treatment process, dispersed precipitate phases of V and Mo carbides are formed, which act as hydrogen trap sites and inhibit the diffusion of hydrogen atoms. -6 Quantitatively expressed by strain rate tensile testing, the sensitivity of the cryogenic steel according to the present invention to a hydrogen gas atmosphere is ≦0.15.

[0014] In addition to the above chemical components, the present invention optimizes the types and contents of other elements added. While Si can induce solid solution strengthening to a certain extent, its concentration at grain boundaries weakens the grain boundaries, increasing brittleness along the grain boundaries and reducing plasticity. Therefore, in the present invention, the Si content is preferably controlled to 0.20%. Al can also improve weldability as a deoxidizing element during the manufacturing process, but excessive addition of Al can easily form coarse precipitates, impairing toughness. Therefore, in the present invention, the Al content is preferably controlled to 0.03%.

[0015] More specifically, the above cryogenic steel further contains the following components:

[0016] The alloy contains, by mass%, C: 0.43%, Mn: 23.9%, Cr: 3.6%, Cu: 0.40%, Ni: 0.61%, V: 0.22%, Mo: 0.20%, Si: 0.18%, Al: 0.03%, and the balance being Fe and unavoidable impurity elements; Or, it contains C: 0.41%, Mn: 23.5%, Cr: 3.7%, Cu: 0.45%, Ni: 0.55%, V: 0.20%, Mo: 0.23%, Si: 0.25%, Al: 0.02%, and the balance is Fe and unavoidable impurity elements, Alternatively, it contains 0.45% C, 24.5% Mn, 3.5% Cr, 0.35% Cu, 0.65% Ni, 0.24% V, 0.24% Mo, 0.15% Si, and 0.04% Al, with the remainder being Fe and unavoidable impurity elements.

[0017] As a second aspect of the present invention, the heat treatment process for the cryogenic steel described above comprises: (1) A step of heating a steel plate rolled to a predetermined standard to 1040°C to 1060°C and keeping the temperature; (2) After the heat retention of the steel plate is completed, a step of cooling to 840 ° C to 850 ° C, the preferred set temperature being 845 ° C; (3) performing a water cooling process.

[0018] The above heat treatment process may further include a straightening operation.

[0019] The heat treatment process of the above cryogenic steel is as follows:

[0020] Taking a steel plate with a nominal thickness of 20 mm as an example, the plate is first heated to 1040°C to 1060°C and held at that temperature for 25 to 35 minutes to homogenize the austenitic alloying elements. Then, during furnace cooling to 840°C to 850°C, V and Mo are dispersed and precipitated as carbides, improving hydrogen damage resistance. If the furnace cooling temperature is too high, V and Mo are difficult to precipitate. If the furnace cooling temperature is too low, V and Mo can precipitate, but Cr and Mn carbides also precipitate at the grain boundaries, reducing the grain boundary bonding strength. Water cooling after the steel plate is removed from the furnace provides a stable austenitic matrix structure while suppressing the precipitation of Cr and Mn carbides at the austenite grain boundaries, ensuring grain boundary bonding strength and improving toughness.

[0021] Specifically, the steel plate in step (1) is manufactured through steelmaking, continuous casting, and rolling processes, and then the above-mentioned heat treatment is performed. The heat-retention time can be adjusted according to the specific specifications of the steel plate. For example, a rolled steel plate with a nominal thickness of 20 mm requires heat retention for 25 to 35 minutes.

[0022] By setting up a heat treatment process, the operational restrictions of the previous processing steps can be reduced, the performance of the steel sheet can be maintained, the performance of the steel sheet can be made uniform, and post-processing steps such as pickling and lapping can also be omitted.

[0023] In addition, the actual content of alloying elements during the material manufacturing process will fluctuate within a limited, small range near the design range, and this fluctuation is usually unavoidable during industrial production. In the present invention, the content range of each element is clearly defined, and similarly, the operating range of the parameters of the heat treatment process is limited, but within a reasonable range of variation, it will not significantly affect the effects of the present invention.

[0024] In a third aspect of the present invention, the cryogenic steel is applied to the storage and transportation of liquefied ethylene, liquefied natural gas, liquefied hydrogen or liquid helium, and specifically is manufactured into a storage and transportation vessel or a transportation line and applied to land, marine or aviation environments.

[0025] In terms of beneficial effects, the present invention has the following significant advantages over the prior art:

[0026] 1. This cryogenic steel has excellent cryogenic resistance, corrosion resistance and hydrogen damage resistance. In the Charpy impact test at -269°C, the lateral expansion is ≥ 1.35mm, the corrosion rate in the natural exposure test is ≤ 0.04mm / a, and the sensitivity to hydrogen gas atmosphere is ≤ 0.15.

[0027] 2. By matching the heat treatment process with the composition of cryogenic steel, the performance of the steel plate can be further improved, the operational restrictions of the previous processing steps can be reduced, the impact of the entire process flow on the performance of the steel plate can be reduced, the performance of the steel plate can be made uniform, and the post-processing steps can be omitted.

[0028] 3. This cryogenic steel has a wide range of applications. It can be used in the storage and transportation of various cryogenic media such as liquefied ethylene, liquefied natural gas, liquid hydrogen, and liquid helium, and is suitable for a variety of complex environments, including land, marine, and aviation. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a microstructure diagram of a cryogenic steel according to the present invention. [Figure 2] 1 is a microstructure diagram of the cryogenic steel according to the present invention after it has been subjected to a 0.35 true strain in advance and then to a sub-zero treatment at −269° C. DETAILED DESCRIPTION OF THE INVENTION

[0030] The technical solution of the present invention will be further illustrated below with reference to examples.

[0031] Example 1 [Table 1]

[0032] Referring to the alloying element compositions in Table 1, the manufacturing process of the cryogenic steel according to the present invention is as follows:

[0033] (1) In steelmaking, the end temperature of the converter blowing was 1630℃, and the melting components were controlled using an LF refining furnace.

[0034] (2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, and the thickness of the slab was 260 mm. The final temperature of the slab was 620°C, and it was slowly cooled in a heat-retaining pit.

[0035] (3) Regarding rolling, the raw material was reheated to 1203°C and then rolled. The rolling start temperature was 1040°C, and the finish rolling temperature was 920°C. After finishing the rolling, it was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

[0036] (4) Regarding heat treatment, a steel plate having a nominal thickness of 20 mm was heated to 1045°C, kept at that temperature for 30 minutes, and then cooled in a furnace to 844°C. After the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel according to the present invention.

[0037] Example 2 Referring to the alloying element compositions in Table 1, the manufacturing process of the cryogenic steel according to the present invention is as follows:

[0038] (1) In steelmaking, the end temperature of the converter blowing was 1650℃, and the melting components were controlled using an LF refining furnace.

[0039] (2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, and the thickness of the slab was 260 mm. The final temperature of the slab was 580°C, and it was placed in a heat retention pit for slow cooling.

[0040] (3) Regarding rolling, the raw material was reheated to 1210°C and then rolled. The rolling start temperature was 1030°C, and the finish rolling temperature was 907°C. After finishing the rolling, it was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

[0041] (4) Regarding heat treatment, a steel plate having a nominal thickness of 20 mm was heated to 1060°C, kept at that temperature for 32 minutes, and then cooled in a furnace to 840°C. After the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel according to the present invention.

[0042] Example 3 Referring to the alloying element compositions in Table 1, the manufacturing process of the cryogenic steel according to the present invention is as follows:

[0043] (1) In steelmaking, the end temperature of the converter blowing was 1645℃, and the melting components were controlled using an LF refining furnace.

[0044] (2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, and the thickness of the slab was 260 mm. The final temperature of the slab was 593°C, and it was placed in a heat retention pit and slowly cooled.

[0045] (3) Regarding rolling, the material was reheated to 1215°C and then rolled. The rolling start temperature was 1042°C, and the finish rolling temperature was 940°C. After finishing the rolling, the material was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

[0046] (4) Regarding heat treatment, a steel plate having a nominal thickness of 20 mm was heated to 1050°C, kept at that temperature for 30 minutes, and then cooled in a furnace to 840°C. After the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel according to the present invention.

[0047] Comparative Example 1 With reference to the alloying element compositions in Table 1, the manufacturing process of the steel plate according to this comparative example is as follows.

[0048] (1) In steelmaking, the end temperature of the converter blowing was 1660℃, and the melting components were controlled using an LF refining furnace.

[0049] (2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, and the thickness of the slab was 260 mm. The final temperature of the slab was 570°C, and it was placed in a heat-retaining pit for slow cooling.

[0050] (3) Regarding rolling, the material was reheated to 1190°C and then rolled. The rolling start temperature was 1045°C, and the finish rolling temperature was 922°C. After finishing the rolling, the material was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

[0051] (4) Regarding heat treatment, a steel plate having a nominal thickness of 20 mm was heated to 1055°C, kept at that temperature for 35 minutes, and then cooled in a furnace to 843°C. After the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel according to this comparative example.

[0052] Comparative Example 2 With reference to the alloying element compositions in Table 1, the manufacturing process of the steel plate according to this comparative example is as follows.

[0053] (1) In steelmaking, the end temperature of the converter blowing was 1643℃, and the melting components were controlled using an LF refining furnace.

[0054] (2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, and the thickness of the slab was 260 mm. The final temperature of the slab was 624°C, and it was placed in a heat retention pit and slowly cooled.

[0055] (3) Regarding rolling, the raw material was reheated to 1212°C and then rolled. The rolling start temperature was 1041°C, and the finish rolling temperature was 944°C. After finishing the rolling, it was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

[0056] (4) Regarding heat treatment, a steel plate having a nominal thickness of 20 mm was heated to 1040°C and kept at that temperature for 30 minutes, then cooled in a furnace to 846°C, and after the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel according to this comparative example.

[0057] Comparative Example 3 With reference to the alloying element compositions in Table 1, the manufacturing process of the steel plate according to this comparative example is as follows.

[0058] (1) In steelmaking, the end temperature of the converter blowing was 1660℃, and the melting components were controlled using an LF refining furnace.

[0059] (2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, and the thickness of the slab was 260 mm. The final temperature of the slab was 640°C, and it was placed in a heat retention pit and slowly cooled.

[0060] (3) Regarding rolling, the raw material was reheated to 1205°C and then rolled. The rolling start temperature was 1038°C, and the finish rolling temperature was 922°C. After finishing the rolling, it was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

[0061] (4) Regarding heat treatment, a steel plate having a nominal thickness of 20 mm was heated to 1045°C and kept at that temperature for 37 minutes, then furnace-cooled to 845°C, and after the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel according to this comparative example.

[0062] Comparative Example 4 This example differs from Example 1 in that the steel sheet is furnace-cooled to 980°C in the heat treatment process.

[0063] Comparative Example 5 This example differs from Example 1 in that the steel sheet is furnace-cooled to 713°C in the heat treatment process.

[0064] Comparative Example 6 This example differs from Example 1 in that in the heat treatment process, the steel sheet is furnace-cooled to 847°C and then air-cooled after being removed from the furnace.

[0065] Example 4: Evaluation of steel plate performance 1. Low temperature performance About the test method The lateral expansion amount and test method for Charpy impact test refer to the national standard GB / T 229 "Method of Charpy hammer impact test for metallic materials."

[0066] 2. Corrosion resistance About the test method The corrosion test is carried out in the atmosphere, and the test method is based on the national standard GB / T 14165 "General requirements for atmospheric corrosion testing of metals and alloys - field testing".

[0067] 3. Hydrogen damage resistance About the test method A tensile delayed fracture test was conducted in a hydrogen atmosphere at room temperature, and the test method was based on the national standard GB / T 15970.7 "Corrosion of Metals and Alloys - Stress Corrosion Test Part 7: Slow Strain Rate Test."

[0068] 4. Test results [Table 2]

[0069] From Table 2, it can be seen that the chemical compositions and heat treatment processes of Examples 1 to 3 meet the design of the present invention and achieve good performance effects, with a lateral expansion in the Charpy impact test at -269°C of ≥ 1.35 mm, a corrosion rate in the natural exposure test of ≤ 0.4 mm / a, and a sensitivity to hydrogen gas atmosphere of ≤ 0.15.

[0070] Regarding low-temperature performance, in terms of the composition, Comparative Example 1 had a similar heat treatment process to Examples 1 to 3, but the C, Mn, and Cr contents in the chemical composition were reduced, resulting in decreased austenite stability and decreased toughness. In terms of the heat treatment process, Comparative Example 5 had the same chemical composition as Example 1, but was furnace-cooled to an excessively low temperature in the heat treatment process, resulting in excessive carbide precipitation during furnace cooling and decreased toughness. On the other hand, Comparative Example 6 had the same chemical composition as Example 1, but was cooled to air cooling instead of furnace cooling during the heat treatment process, resulting in excessive carbide precipitation during air cooling and decreased toughness. Furthermore, in terms of the microstructure, when the cryogenic low-temperature steel according to the present invention was subjected to a 0.35 true strain in advance and then to sub-zero treatment at −269°C, no martensite structure appeared in the microstructure, remaining an austenite structure, and dislocation and twin strain mechanisms were present ( FIG. 2 ).

[0071] Regarding corrosion resistance, in terms of components, Comparative Example 2 had a similar heat treatment process to Examples 1 to 3, but the content of Cu and Ni in the chemical components was reduced, which reduced the corrosion-resistant elements and resulted in a decrease in corrosion resistance.

[0072] Regarding hydrogen damage resistance, in Comparative Example 3, the heat treatment process was similar in terms of composition compared to Examples 1 to 3, but the V and Mo contents in the chemical composition were reduced, which reduced the dispersed precipitation of V and Mo and increased the sensitivity to hydrogen gas atmosphere. In terms of the heat treatment process, in Comparative Example 4, the chemical composition was the same as in Example 1, but the heat treatment process resulted in furnace cooling to an excessively high temperature, which reduced the dispersed precipitation of V and Mo and increased the sensitivity to hydrogen gas atmosphere.

[0073] As described above, the excellent performance of the cryogenic steel according to the present invention can be obtained by designing the composition of alloying elements and then matching it with the corresponding heat treatment process.

Claims

1. The composition, in mass%, includes C: 0.41% to 0.45%, Mn: 23.5% to 24.5%, Cr: 3.5% to 3.7%, Cu: 0.35% to 0.45%, Ni: 0.55% to 0.65%, V: 0.20% to 0.24%, Mo: 0.20% to 0.24%, Si: 0.15% to 0.25%, Al: 0.02% to 0.04%, and the balance being Fe and unavoidable impurity elements; It has a fully austenitic structure and a stacking fault energy of 18 to 21 mJ·m −2 at −269°C. Cryogenic steel characterized by:

2. The composition, in mass%, includes C: 0.43%, Mn: 24.0%, Cr: 3.6%, Cu: 0.40%, Ni: 0.60%, V: 0.22%, Mo: 0.22%, Si: 0.20%, Al: 0.03%, and the balance is Fe and unavoidable impurity elements.

2. The cryogenic steel according to claim 1 .

3. The composition, in mass%, includes C: 0.43%, Mn: 23.9%, Cr: 3.6%, Cu: 0.40%, Ni: 0.61%, V: 0.22%, Mo: 0.20%, Si: 0.18%, Al: 0.03%, and the balance is Fe and unavoidable impurity elements.

2. The cryogenic steel according to claim 1 .

4. The composition, in mass%, includes C: 0.41%, Mn: 23.5%, Cr: 3.7%, Cu: 0.45%, Ni: 0.55%, V: 0.20%, Mo: 0.23%, Si: 0.25%, Al: 0.02%, and the balance is Fe and unavoidable impurity elements.

2. The cryogenic steel according to claim 1 .

5. The composition, in mass%, includes C: 0.45%, Mn: 24.5%, Cr: 3.5%, Cu: 0.35%, Ni: 0.65%, V: 0.24%, Mo: 0.24%, Si: 0.15%, Al: 0.04%, and the balance is Fe and unavoidable impurity elements.

2. The cryogenic steel according to claim 1 .

6. A heat treatment process for producing the cryogenic steel according to any one of claims 1 to 5, comprising: (1) heating a steel plate rolled to a predetermined standard to 1040°C to 1060°C and maintaining the temperature; (2) After the heat retention of the steel plate is completed, a step of cooling the steel plate to 840°C to 850°C; (3) performing a water cooling process. A heat treatment process for producing cryogenic steel, characterized by:

7. 6. Application of the cryogenic steel according to any one of claims 1 to 5 to the storage and transportation of liquefied ethylene, liquefied natural gas, liquid hydrogen or liquid helium.

8. The cryogenic steel is made into a storage vessel or a transportation line for use in a land, marine or aviation environment.

8. The application according to claim 7 .

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