JP7760077B2 - Cryogenic steels and their heat treatment processes and applications - Google Patents

Description

The present invention relates to cryogenic steel, its heat treatment process, and applications, and in particular to cryogenic steel that can withstand cryogenic temperatures of -269°C and has excellent corrosion resistance and hydrogen damage resistance, as well as its heat treatment process and applications.

Storage and transportation vessels for cryogenic media such as liquefied ethylene, liquefied natural gas, liquid hydrogen, and liquid helium are typically manufactured from high-nickel steels, such as Invar alloy (36% Ni), 316 stainless steel (12% Ni), and 9% Ni steel. However, these steels are not economically viable and therefore have relatively limited practical applications. Austenitic stainless steels can also be used for storage and transportation vessels for some cryogenic media, but their low strength and large expansion coefficients mean they cannot meet the requirements for extreme low-temperature environments, such as -269°C, limiting their applications. In recent years, permanganized steel has also emerged as a low-temperature material. However, the performance of previously designed permanganized steels is limited to low-temperature environments down to -196°C, and they also do not meet the requirements for corrosion resistance and hydrogen embrittlement susceptibility.

Objective of the invention: The present invention aims to provide an ultra-low temperature steel with excellent low temperature performance, corrosion resistance, and hydrogen damage resistance, as well as a heat treatment process and applications for it, in response to the limited performance, poor economics, and application regulations of conventional low temperature steels.

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 impurity elements.

The cryogenic steel has a fully austenitic structure and a stacking fault energy of 18 to 21 mJ·m ⁻² at −269° C.

Most preferably, the above-mentioned cryogenic steel contains, in mass %, the following components: 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.

The chemical composition design mechanism for the above cryogenic steel is as follows:

Austenitic structures have well-balanced strength, plasticity, toughness, and lower service temperatures. Adding high amounts of Ni can produce austenitic structures, 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 and can be used as a substitute for Ni to produce austenitic structures. Because the effect of Mn on austenite stability is approximately half that of Ni, the most preferred Mn content in the cryogenic steel of the present invention is 24.0%.

C has a strong austenite stabilizing effect and is an element that enhances austenite stability. C also inhibits dislocation movement and can 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 type according to the present invention is most preferably 0.43%.

Cr is a ferrite-forming element, but it is also beneficial in improving austenite stability by lowering the martensitic transformation temperature. Too much Cr promotes carbide precipitation and reduces toughness. Therefore, the most preferable Cr content in the cryogenic steel of the present invention is 3.6%.

The present invention adds 0.43% C, 24.0% Mn, and 3.6% Cr to obtain a highly stable austenitic structure, and even when subjected to a 0.35 true strain prior to sub-zero treatment at -269°C, no martensite structure appears. At the same time, the stacking fault energy of the austenite at -269°C is controlled to 18-21 mJ·m ^-2 , achieving a balance between strength and toughness through the combined mechanism of dislocations and twinning. The cryogenic steel of the present invention achieves a lateral expansion of ≥ 1.35 mm in a Charpy impact test at -269°C.

To improve the corrosion resistance of the cryogenic steel of the present invention, it is preferable to add 0.40% Cu to the components. However, Cu increases the tendency for cracking on the steel sheet surface, which is disadvantageous for material manufacturing. To avoid the adverse effects of Cu, it is preferable to add 0.60% Ni in the present invention, ensuring a Ni/Cu content ratio of 1 or greater. 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.

In order to achieve higher hydrogen damage resistance 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 combining with the corresponding heat treatment process, dispersed precipitate phases of V and Mo carbides are formed, which act as hydrogen trapping sites and inhibit the diffusion of hydrogen atoms. Quantitatively expressed by a ^10-6 strain rate tensile test, the sensitivity of the cryogenic steel according to the present invention to a hydrogen gas atmosphere is ≦0.15.

In addition to the above chemical components, the present invention optimizes the types and contents of other added elements. While Si can cause 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 this invention, it is preferable to control the Si content to 0.20%. Al can also improve joint weldability as a deoxidizing element during the manufacturing process, but excessive addition can easily lead to the formation of coarse precipitates, impairing toughness. Therefore, in this invention, it is preferable to control the Al content to 0.03%.

More specifically, the above-mentioned cryogenic steel further contains the following components:

The alloy 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, and 0.03% Al, with the balance being Fe and unavoidable impurity elements,
Alternatively, the alloy contains 0.41% C, 23.5% Mn, 3.7% Cr, 0.45% Cu, 0.55% Ni, 0.20% V, 0.23% Mo, 0.25% Si, and 0.02% Al, with the balance being Fe and unavoidable impurities;
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 balance being Fe and unavoidable impurity elements.

In a second aspect of the present invention, the heat treatment process for the cryogenic steel described above comprises:
(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 sheet is completed, a step of cooling to 840°C to 850°C, with a preferred set temperature being 845°C;
(3) performing a water cooling process.

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

The heat treatment process for the above cryogenic steel is as follows:

Taking a steel plate with a nominal thickness of 20 mm as an example, it is first heated to 1040-1060°C and held at that temperature for 25-35 minutes to homogenize the austenitic alloy elements. Then, as the steel plate is furnace-cooled to 840-850°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 less likely to precipitate. If the furnace cooling temperature is too low, V and Mo can precipitate, but Cr and Mn carbides will also precipitate at the grain boundaries, reducing grain boundary bonding strength. Water-cooling the steel plate after it has been removed from the furnace results in 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.

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

By setting up a heat treatment process, operational restrictions in the previous processing steps are reduced, the performance of the steel plate is maintained and uniformed, and post-processing steps such as pickling and lapping are also eliminated.

The actual content of alloying elements during material manufacturing processes will fluctuate within a small, limited range near the design range, and this variation is usually unavoidable during industrial production. In this invention, the range of content of each element is clearly defined, and similarly, the operating range of the heat treatment process parameters is limited. However, this is within a reasonable range of variation and does not significantly affect the effects of the invention.

In a third aspect of the present invention, the above-mentioned cryogenic steel is applied to the storage and transportation of liquefied ethylene, liquefied natural gas, liquid hydrogen, or liquid helium, and specifically, is manufactured into storage and transportation vessels or transportation lines for use in land, marine, or aviation environments.

In terms of beneficial effects, the present invention has the following significant advantages over conventional technology:

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

2. By matching the heat treatment process to 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 post-processing steps can be omitted.

3. This cryogenic steel has a wide range of potential applications, and can be used to store and transport a variety of cryogenic media, including liquefied ethylene, liquefied natural gas, liquid hydrogen, and liquid helium, and is suitable for a variety of complex environments, including land, marine, and aviation.

1 is a microstructure diagram of a cryogenic steel according to the present invention. 1 is a microstructure diagram of the cryogenic steel according to the present invention after it has been subjected to a true strain of 0.35° C. and then to a sub-zero treatment at −269° C.

The technical solution of the present invention will be further explained below with reference to examples.

Example 1

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

(1) For steelmaking, the end temperature of the converter blowing was 1630°C, and the melt composition was controlled using an LF refining furnace.

(2) Continuous casting was carried out using a vertical bending type continuous slab casting machine, with a thickness of 260 mm. The final temperature of the slab was 620°C, and it was placed in a heat retention pit for slow cooling.

(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.

(4) Regarding heat treatment, a steel plate with a nominal thickness of 20 mm was heated to 1045°C, held at that temperature for 30 minutes, and then furnace-cooled 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.

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:

(1) For steelmaking, the end temperature of the converter blowing was 1650°C, and the melt composition was controlled using an LF refining furnace.

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

(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.

(4) Regarding heat treatment, a steel plate with a nominal thickness of 20 mm was heated to 1,060°C, held at that temperature for 32 minutes, and then furnace-cooled 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.

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:

(1) For steelmaking, the end temperature of the converter blowing was 1645°C, and the melt composition was controlled using an LF refining furnace.

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

(3) Regarding rolling, the raw 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, it was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

(4) Regarding heat treatment, a steel plate with a nominal thickness of 20 mm was heated to 1050°C, held at that temperature for 30 minutes, and then furnace-cooled 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.

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.

(1) For steelmaking, the end temperature of the converter blowing was 1,660°C, and the melt composition was controlled using an LF refining furnace.

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

(3) Regarding rolling, the raw 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, it was water-cooled to room temperature. The nominal thickness of the rolled steel plate was 20 mm.

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

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.

(1) For steelmaking, the end temperature of the converter blowing was 1,643°C, and the melt composition was controlled using an LF refining furnace.

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

(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.

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

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.

(1) For steelmaking, the end temperature of the converter blowing was 1,660°C, and the melt composition was controlled using an LF refining furnace.

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

(3) Regarding rolling, the raw material was reheated to 1205°C before rolling. 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.

(4) Regarding heat treatment, a steel plate with a nominal thickness of 20 mm was heated to 1,045°C and maintained at that temperature for 37 minutes, then furnace-cooled to 845°C. After the steel plate was removed from the furnace, it was water-cooled to produce the cryogenic steel for this comparative example.

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.

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.

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.

Example 4: Evaluation of steel sheet performance 1. Low temperature performance Test method The lateral expansion amount of the Charpy impact test was measured in accordance with the national standard GB/T 229, "Metallic materials: Charpy hammer impact test method."

2. Corrosion resistance performance Test method: 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 test on metals and alloys - field test".

3. Hydrogen Damage Resistance 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."

4. Test results

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 of ≥ 1.35 mm in a -269°C Charpy impact test, a corrosion rate of ≤ 0.4 mm/a in a natural exposure test, and a hydrogen gas atmosphere sensitivity of ≤ 0.15.

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

Regarding corrosion resistance, in terms of composition, Comparative Example 2 had a similar heat treatment process to Examples 1 to 3, but the content of Cu and Ni in the chemical composition was reduced, resulting in a reduction in corrosion-resistant elements and a decrease in corrosion resistance.

Regarding hydrogen damage resistance, in terms of composition, Comparative Example 3 had a similar heat treatment process compared to Examples 1 to 3, but the V and Mo content in the chemical composition was reduced, which reduced the dispersed precipitation of V and Mo and increased sensitivity to hydrogen gas atmospheres. In terms of heat treatment process, Comparative Example 4 had the same chemical composition as 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 sensitivity to hydrogen gas atmospheres.

As a result, the excellent performance of the cryogenic steel of the present invention is achieved by designing the composition of alloying elements and then matching it with the corresponding heat treatment process.

Claims (8)

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: 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 . 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 . 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 . 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 . 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: 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. 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 .