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 a cryogenic temperature 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 usually manufactured from high nickel content steels such as Invar alloy (36% Ni), 316 stainless steel (12% Ni), and 9% Ni steel, but these steels are economically inefficient and therefore have relatively limited practical applications. Austenitic stainless steels can also be used as steels for storage and transportation vessels for some cryogenic media, but they have low strength and large expansion coefficients, and cannot meet the requirements for use in extreme low temperature environments such as -269°C, so their applications are relatively limited. In recent years, permanganese steels have also been considered as low-temperature materials, but the performance of conventionally designed permanganese steels can only be used in low temperature environments down to -196°C, and they do not meet the requirements for use in terms of corrosion resistance and hydrogen embrittlement susceptibility. Summary of the Invention

[0003] Regarding the object of the invention, the present invention aims to provide a cryogenic steel having 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 the conventional low temperature steels, which have only a single performance, are economical, and have limitations in application.

[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%-0.45%, Mn: 23.5%-24.5%, Cr: 3.5%-3.7%, Cu: 0.35%-0.45%, Ni: 0.55%-0.65%, V: 0.20%-0.24%, Mo: 0.20%-0.24%, Si: 0.15%-0.25%, Al: 0.02%-0.04%, and the balance being Fe and unavoidable impurities.

[0005] The cryogenic steels mentioned above have a fully austenitic structure and a stacking fault energy of 18 to 21 mJ m at -269°C. -2 It 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 of component design of the above cryogenic steel is as follows.

[0008] Austenitic structures have well-balanced strength, plasticity, toughness and lower service temperature. Adding high content of Ni can obtain austenitic structures, for example 316 austenitic stainless steel with Ni content of 12%, but the alloy cost is high. Mn can suppress the transformation of austenite to martensite, so it can be used as an alternative element to Ni to obtain austenitic structures. The effect of Mn on the stability of austenite is about half that of Ni, so the content of Mn in the cryogenic steel according to the present invention is most preferably 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 the 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 it is subjected to a 0.35 true strain beforehand and then subzero treatment at -269°C, no martensite structure appears. At the same time, the austenite is transformed into a steel with a stacking fault energy of 18 to 21 mJ m at -269°C. -2 The present invention achieves a balance between strength and toughness through the combined mechanism of dislocation + twin crystals, 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] In order to improve the corrosion resistance of the cryogenic steel according to the present invention, it is preferable to add 0.40% Cu to the components. However, Cu increases the crack tendency of the steel sheet surface, which is disadvantageous to the manufacturing of the material. In order 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, and the Ni / Cu content ratio is preferably about 1.5, which can effectively inhibit the precipitation of low-melting point Cu at the grain boundary and improve the surface quality of the steel sheet. In addition to Cu, Cr and Ni also have the effect of improving the corrosion resistance. Through the design of the combined addition of corrosion-resistant elements, the natural exposure test corrosion rate of the cryogenic steel according to the present invention is ≦0.04 mm / a.

[0013] In order to obtain a higher hydrogen damage resistance performance when manufacturing a liquid hydrogen tank from the cryogenic steel according to the present invention, the present invention preferably adds 0.22% V and 0.22% Mo while obtaining an austenite matrix structure. By matching with the corresponding heat treatment process, a carbide dispersion precipitate phase of V and Mo is formed, which acts as a hydrogen trap site and inhibits 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 type and content of other elements added. Here, Si can cause solid solution strengthening to a certain extent, but Si weakens the grain boundary by concentrating at the grain boundary, and in addition to increasing the brittleness along the grain boundary, Si also reduces plasticity, so in the present invention, it is preferable to control the Si content to 0.20%. Al can improve the weldability of joints as a deoxidizing element in the manufacturing process, but if added in excess, it is likely to form coarse precipitates and impair toughness, so in the present invention, it is preferable to control the Al content to 0.03%.

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

[0016] The composition contains, by mass%, 0.43% C, 23.9% Mn, 3.6% Cr, 0.40% Cu, 0.61% Ni, 0.22% V, 0.20% Mo, 0.18% Si, 0.03% Al, and the remainder being Fe and unavoidable impurities. Or, 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 being Fe and unavoidable impurities; Or, it contains 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.

[0017] As a second aspect of the present invention, the heat treatment process of the above cryogenic steel 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 step.

[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] For example, a steel plate with a nominal thickness of 20 mm is first heated to 1040°C to 1060°C and kept at that temperature for 25 to 35 minutes to homogenize the austenite alloy elements. Then, in the process of furnace cooling the steel plate to 840°C to 850°C, V and Mo are dispersed and precipitated in the form of carbides, improving hydrogen damage resistance. If the furnace cooling temperature is too high, V and Mo are difficult to precipitate, and 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 binding force. By water cooling after the steel plate is removed from the furnace, a stable austenite matrix structure is obtained, while the precipitation of Cr and Mn carbides at the grain boundaries of austenite is suppressed, ensuring the grain boundary binding force and improving toughness.

[0021] Specifically, the steel plate in step (1) is manufactured through the processes of steel making, continuous casting, and rolling, and then the above-mentioned heat treatment is performed, and 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 the heat treatment process, the operational restrictions of the previous processing steps can be reduced, the performance of the steel plate can be maintained and uniformed, and post-processing steps such as pickling and lapping can be omitted.

[0023] In addition, the actual content of alloying elements in 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 the operation range of the parameters of the heat treatment process is also 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, liquid hydrogen or liquid helium, specifically, to be manufactured into a storage and transportation vessel or a transportation line and applied to a land, marine or aviation environment.

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

[0026] 1. The cryogenic steel has excellent cryogenic resistance, corrosion resistance and hydrogen damage resistance. The lateral expansion in the Charpy impact test at -269°C 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. The heat treatment process is matched with the composition of cryogenic steel, which further improves the performance of the steel plate and reduces the operation restrictions of the previous processing steps, reduces the impact of the entire process flow on the performance of the steel plate, makes the performance of the steel plate uniform, and can omit the post-processing steps.

[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 such as land, marine, and aviation. [Brief description of the drawings]

[0029] [Figure 1] FIG. 2 is a microstructure diagram of the cryogenic steel according to the present invention. [Diagram 2] FIG. 2 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 PREFERRED EMBODIMENTS

[0030] The technical solutions of the present invention are further illustrated below with reference to examples.

[0031] Example 1 [Table 1]

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

[0033] (1) Regarding steelmaking, the end temperature of the converter blowing was 1630°C, and the molten metal 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 cooled slowly in a heat retention pit.

[0035] (3) Regarding rolling, the raw material was reheated to 1203°C and then rolled. The rolling start temperature was 1040°C, the finish rolling temperature was 920°C, and 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 cooled in a furnace to 844°C. After the steel plate was taken out of the furnace, it was water-cooled to produce the cryogenic steel according to the present invention.

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

[0038] (1) Regarding steelmaking, the end temperature of the converter blowing was 1650°C, and the molten metal 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 raw material was 260 mm. The final temperature of the raw material was 580°C, and it was slowly cooled in a heat retention pit.

[0040] (3) Regarding rolling, the raw material was reheated to 1210°C and then rolled. The rolling start temperature was 1030°C, the finish rolling temperature was 907°C, and 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, cooled in a furnace to 840°C, and then water-cooled after being taken out of the furnace, to produce the cryogenic steel according to the present invention.

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

[0043] (1) Regarding steelmaking, the end temperature of the converter blowing was 1645°C, and the molten metal 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 cooled slowly in a heat retention pit.

[0045] (3) Regarding rolling, the raw material was reheated to 1215°C and then rolled. The rolling start temperature was 1042°C, the finish rolling temperature was 940°C, and after finishing the rolling, it 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 cooled in a furnace to 840°C. After the steel plate was taken out of 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) Regarding steelmaking, the end temperature of the converter blowing was 1660°C, and the molten metal 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 cooled slowly in a heat retention pit.

[0050] (3) Regarding rolling, the raw material was reheated to 1190°C and then rolled. The rolling start temperature was 1045°C, the finish rolling temperature was 922°C, and after finishing the rolling, it 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) Regarding steelmaking, the end temperature of the converter blowing was 1643°C, and the molten metal components were controlled using an LF refining furnace.

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

[0055] (3) Regarding rolling, the raw material was reheated to 1212°C and then rolled. The rolling start temperature was 1041°C, the finish rolling temperature was 944°C, and after finishing 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. 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) Regarding steelmaking, the end temperature of the converter blowing was 1660°C, and the molten metal components were controlled using an LF refining furnace.

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

[0060] (3) Regarding rolling, the raw material was reheated to 1205°C and then rolled. The rolling start temperature was 1038°C, the finish rolling temperature was 922°C, and after finishing 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 cooled in a furnace to 845°C. 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 embodiment 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 taken out of the furnace.

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

[0066] 2. Corrosion resistance About the test method The corrosion test is carried out in the atmosphere, and the test method refers to the national standard GB / T 14165 "Atmospheric corrosion test of metals and alloys - General requirements for on-site 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 good performance effects are obtained, with the lateral expansion in the -269°C Charpy impact test being ≧1.35mm, the corrosion rate in the natural exposure test being ≦0.4mm / a, and the sensitivity to hydrogen gas atmosphere being ≦0.15.

[0070] Regarding low temperature performance, in terms of the components, Comparative Example 1 has a similar heat treatment process compared to Examples 1 to 3, but the contents of C, Mn, and Cr in the chemical components were reduced, so that the austenite stability and toughness were reduced. In terms of the heat treatment process, Comparative Example 5 has the same chemical components compared to Example 1, but the heat treatment process caused furnace cooling to an excessively low temperature, so that carbides were excessively precipitated during furnace cooling, and the toughness was reduced. On the other hand, Comparative Example 6 has the same chemical components compared to Example 1, but the furnace cooling method in the heat treatment process was changed to air cooling, so that carbides were excessively precipitated during air cooling, and the toughness was reduced. In addition, in terms of the microstructure, when the cryogenic steel according to the present invention is subjected to 0.35 true strain in advance and then subjected to -269°C sub-zero treatment, the martensite structure does not appear in the microstructure, but remains an austenite structure, and dislocation and twin distortion mechanisms are generated (FIG. 2).

[0071] Regarding corrosion resistance, in terms of the components, although the heat treatment process of Comparative Example 2 was similar to that of Examples 1 to 3, the content of Cu and Ni in the chemical components was reduced, so that the corrosion-resistant elements were reduced and the corrosion resistance was reduced.

[0072] With regard to hydrogen damage resistance, in terms of the components, Comparative Example 3 had a similar heat treatment process as Examples 1 to 3, but the V and Mo contents in the chemical components were reduced, which reduced the dispersed precipitation of V and Mo and increased the sensitivity to the hydrogen gas atmosphere. In terms of the heat treatment process, Comparative Example 4 had the same chemical components as Example 1, but the heat treatment process involved furnace cooling to an excessively high temperature, which reduced the dispersed precipitation of V and Mo and increased the sensitivity to the 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 the alloy elements and matching it with the corresponding heat treatment process.

Claims

1. The components, in mass%, are 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. A cryogenic steel characterized by:

2. It is an all-austenitic structure.

2. The cryogenic steel according to claim 1 .

3. Stacking fault energy at -269°C is 18 to 21 mJ·m －２ That is, 2. The cryogenic steel according to claim 1 .

4. The composition, in mass%, is 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. The cryogenic steel according to any one of claims 1 to 3.

5. The composition, in mass%, is 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. The cryogenic steel according to any one of claims 1 to 3.

6. The composition, in mass%, is 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. The cryogenic steel according to any one of claims 1 to 3.

7. The composition, in mass%, is 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. The cryogenic steel according to any one of claims 1 to 3.

8. A heat treatment process for cryogenic steel according to any one of claims 1 to 7, comprising the steps of: (1) heating a steel plate rolled to a predetermined standard to 1040°C to 1060°C and keeping the temperature constant; (2) After the heat retention of the steel plate is completed, the steel plate is cooled to 840°C to 850°C; (3) performing a water cooling process. A heat treatment process for cryogenic steels, characterized in that

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

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

10. The application according to claim 9 .

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