Steel plate for liquefied hydrogen storage tanks, method for manufacturing the same, and liquefied hydrogen storage tank

Abstract

The present invention provides a steel sheet for liquefied hydrogen storage tanks that exhibits cryogenic toughness, a liquefied hydrogen storage tank, and a method for manufacturing the steel sheet for liquefied hydrogen storage tanks. [Solution] A steel sheet for liquefied hydrogen storage tanks having a chemical composition satisfying the following: C: 0.010% by mass or more and 0.070% by mass or less, Si: 0.01% by mass or more and 0.30% by mass or less, Mn: 0.70% by mass or less, P: 0.0050% by mass or less, S: 0.0030% by mass or less, Al: 0.005% by mass or more and 0.055% by mass or less, Ni: 8.0% by mass or more and 9.5% by mass or less, and N: 0.0050% by mass or less, with the remainder being Fe and unavoidable impurities, the proportion of martensite and bainite structures in the total structure being 80% by area or more, containing retained austenite, and having a parameter X represented by a predetermined formula (1) of 0.29 or more.

Description

Technical Field

[0001] The present disclosure relates to a steel plate for a liquefied hydrogen storage tank, a method for manufacturing the same, and a liquefied hydrogen storage tank.

Background Art

[0002] Examples of clean energy sources include natural gas (LNG). Patent Document 1 shows a high Ni steel plate having excellent low-temperature toughness, which is used for the main body of a tank for storing the LNG. In Patent Document 1, a steel containing a steel having a predetermined chemical composition and the balance being Fe and inevitable impurities is hot-rolled and then quenched from a quenching temperature of 800°C or higher and 820°C or lower, an intermediate heat treatment step of holding at a heating temperature of 690°C or higher and 710°C or lower and then cooling to a cooling end temperature of 200°C or lower at an average cooling rate of 5°C / second or higher, and a tempering step of tempering at a tempering temperature of 570°C or higher and 600°C or lower are included in this order. In the intermediate heat treatment step and the tempering step, a parameter H represented by a predetermined formula (1) is set to 1.73×10 -6 or more and 1.96×10 -6 or less, and a method for manufacturing a Ni-containing steel plate is shown.

[0003] Further, Patent Document 2 shows a low-temperature nickel-containing steel material used for a storage tank for LNG, which has predetermined components, the ratio of the surface hardness to the t / 4 hardness is 1.10 or less, and the retained austenite at a position 1.5 mm from the surface is 3.0% by volume or more and 20.0% by volume or less, the yield strength is 590 MPa or more and 800 MPa or less, the tensile strength is 690 MPa or more and 830 MPa or less, and the average value of the JIS No. 4 Charpy impact absorption energy (vE-196) at -196°C is 150 J or more.

[0004] In addition to the above-mentioned LNG as a fuel, in recent years, considering the environment, it has been considered to use hydrogen as a fuel for power generation, automobiles, etc., and the demand for hydrogen is increasing.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-039065 [Patent Document 2] Japanese Patent Publication No. 2018-123417 [Overview of the project] [Problems that the invention aims to solve]

[0006] As described above, with the increasing demand for hydrogen, facilities such as tanks for storing hydrogen are required. Facilities such as tanks for storing hydrogen are required to exhibit toughness at lower temperatures than the steel materials for LNG storage tanks shown in Patent Documents 1 and 2, i.e., cryogenic toughness. This disclosure has been made in view of the above circumstances, and its purpose is to provide a steel sheet for liquefied hydrogen storage tanks that exhibits cryogenic toughness, a liquefied hydrogen storage tank formed from the steel sheet for liquefied hydrogen storage tanks, and a method for manufacturing the steel sheet for liquefied hydrogen storage tanks. [Means for solving the problem]

[0007] One aspect of the present invention is: The chemical composition is C: 0.010 mass% or more and 0.070 mass% or less, Si: 0.01% by mass or more and 0.30% by mass or less, Mn: 0.70% by mass or less, P: 0.0050% by mass or less, S: 0.0030% by mass or less, Al: 0.005% by mass or more and 0.055% by mass or less, Ni: 8.0% by mass or more and 9.5% by mass or less, N: 0.0050% by mass or less It satisfies the requirement, and the remainder consists of Fe and unavoidable impurities. The proportion of martensitic and bainite tissue in the total tissue is 80% or more by area, and it contains retained austenite. It is a steel sheet for a liquefied hydrogen storage tank in which the parameter X represented by the following formula (1) is 0.29 or more.

Number

[0008] Aspect 2 of the present invention is It is the steel sheet for a liquefied hydrogen storage tank according to Aspect 1, in which the parameter X is 0.31 or more.

[0009] Aspect 3 of the present invention is It is a liquefied hydrogen storage tank formed of the steel sheet for a liquefied hydrogen storage tank according to Aspect 1 or 2.

[0010] Aspect 4 of the present invention is It is a method for manufacturing a steel sheet for a liquefied hydrogen storage tank according to Aspect 1 or 2, The chemical composition is C: 0.010 mass % or more and 0.070 mass % or less, Si: 0.01 mass % or more and 0.30 mass % or less, Mn: 0.70 mass % or less, P: 0.0050 mass % or less, [[ID=I48]] S: 0.0030 mass % or less, Al: 0.005 mass % or more and 0.055 mass % or less, Ni: 8.0 mass % or more and 9.5 mass % or less, and N: 0.0050 mass % or less <o000088>using a steel material satisfying the above and the balance being composed of Fe and inevitable impurities, Performing hot rolling with a total reduction rate of 80% or more to obtain a rolled material (hot rolling step), A method for manufacturing a steel plate for a liquefied hydrogen storage tank includes performing a heat treatment (heat treatment step) in which the following steps (a) to (c) are sequentially performed on the above-mentioned rolled material. (a) Performing quenching by holding the above-mentioned rolled material at a temperature of 720°C or higher and 820°C or lower for 0.5 minutes or longer and 40 minutes or shorter and then water-cooling it (quenching step) (b) After the quenching step, performing two-phase region quenching by holding it at a temperature of 610°C or higher and 690°C or lower for 5 minutes or longer and 60 minutes or shorter and then water-cooling it (two-phase region quenching step) (c) After the two-phase region quenching step, performing tempering by holding it at a temperature of 510°C or higher and 600°C or lower for 5 minutes or longer and 60 minutes or shorter and then performing water-cooling or air-cooling (tempering step)

Advantages of the Invention

[0011] According to the present disclosure, it is possible to provide a steel plate for a liquefied hydrogen storage tank exhibiting extremely low temperature toughness, a liquefied hydrogen storage tank formed of the steel plate for a liquefied hydrogen storage tank, and a method for manufacturing the steel plate for a liquefied hydrogen storage tank.

Brief Description of the Drawings

[0012] [[ID=ID=19]] [[ID=ID=20]] [Figure 1] FIG. 1 is a diagram showing the shape of a test piece for a Charpy impact test used in an example. [Figure 2] FIG. 2 is a graph showing the relationship between parameter X and the Charpy impact test results (three-sided slit Charpy absorption energy at 4K).

Embodiments for Carrying Out the Invention

[0013] The inventors diligently conducted research to provide a steel sheet for liquefied hydrogen storage tanks that exhibits cryogenic toughness (hereinafter sometimes simply referred to as "steel sheet"), a liquefied hydrogen storage tank formed from the steel sheet, and a method for manufacturing the steel sheet. As a result, they found that a steel sheet exhibits cryogenic toughness if the parameter X, expressed by a predetermined formula (1), is above a certain level and includes factors such as the amount of retained austenite in the structure of the steel sheet, the amount of solid-solution carbon in the retained austenite, the grain size of crystal grains surrounded by large-angle grain boundaries with an orientation difference of 25° or more as determined by EBSD (hereinafter sometimes simply referred to as "grain size"), and the Mn content in the steel. They also found that controlling the heat treatment conditions of the rolled material is important to obtain a steel sheet with parameter X above a certain level.

[0014] First, we will explain the steel sheet of this disclosure, particularly parameter X. Through our investigations, we first determined that the following factors influence the cryogenic toughness at approximately 4K, well below -200°C: the amount of retained austenite, the amount of dissolved carbon in the retained austenite, the grain size measured by a predetermined method, and the Mn content in the steel, all of which are part of the chemical composition and structure of the steel sheet. We then found that parameter X, which includes these factors, correlates with the absorbed energy of the three-slit Charpy impact test at 4K (hereinafter simply referred to as "absorbed energy" in this specification), which is an indicator of cryogenic toughness. Furthermore, we found that to achieve the desired cryogenic toughness, an absorbed energy of 29J or more, parameter X needs to be 0.29 or higher. Parameter X is preferably 0.31 or higher, more preferably 0.33 or higher, and even more preferably 0.35 or higher. While a higher parameter X is preferable, if it is too high, for example, the yield strength cannot be ensured; therefore, the upper limit of parameter X may be 5.0.

number

[0015] In this disclosure, it is sufficient that the above parameter X is 0.29 or greater, and the ranges of each factor, namely the amount of retained austenite in the microstructure, the amount of solid-solution carbon in the retained austenite, the grain size, and the Mn content in the steel, are not limited. For example, each factor may be within the following ranges. First, when retained austenite is present in the microstructure, it acts as resistance to brittle cracks, increasing fracture resistance. Therefore, a higher amount of retained austenite is preferable, for example, 5 volume% or more. The upper limit of the amount of retained austenite is not particularly limited, but it may be, for example, 15 volume% or less.

[0016] An increase in the amount of dissolved carbon in retained austenite stabilizes it, making it more susceptible to fracture resistance. From these perspectives, the amount of dissolved carbon in retained austenite should be, for example, 0.435 mass% or more. The upper limit of the amount of dissolved carbon in retained austenite is not particularly limited, but it may be, for example, 0.7 mass% or less.

[0017] It is known that the finer the grain size, the more effective it is in improving toughness. There is a positive correlation between the toughness value and the reciprocal of the square root of the grain size. Therefore, to ensure excellent toughness, it is desirable for the grain size to be fine. From the viewpoint of reliably increasing toughness, it is possible to set the grain size to, for example, 15 μm or less. While a smaller grain size is preferable, considering the manufacturing conditions, the lower limit of the grain size may be 1 μm.

[0018] The microstructure of the steel sheet disclosed herein has a total proportion of martensitic and bainite structures of 80 area or more. The remaining microstructure, other than martensitic and bainite structures, includes retained austenite as described above. The remaining microstructure may also include structures other than retained austenite, such as cementite.

[0019] To obtain a steel sheet that satisfies the above-mentioned parameter X range, it is preferable to use a steel material having a predetermined chemical composition, hot-roll it at a predetermined total reduction ratio to obtain a rolled material, and then heat-treat the rolled material by combining predetermined heat treatment process conditions, namely the conditions of the quenching (also called "Q treatment") process, the two-phase quenching (also called "L treatment") process, and the tempering (also called "T treatment") process. The above manufacturing conditions will be explained later.

[0020] 1. Chemical composition of steel plate First, the chemical composition of the steel sheet in this disclosure will be described. This chemical composition is also the chemical composition of steel materials and rolled materials.

[0021] [C: 0.010 mass% or more and 0.070 mass% or less] Carbon (C) is an element that increases the strength of steel, ensuring the desired strength. In addition, C is present in retained austenite and improves its stability. From these viewpoints, the amount of C should be 0.010 mass% or more, preferably 0.020 mass% or more, and more preferably 0.040 mass% or more. On the other hand, if the amount of C is excessive, the strength becomes excessive and the toughness decreases. In particular, it leads to a significant increase in the strength of the martensitic structure and a decrease in toughness. Therefore, the amount of C should be 0.070 mass% or less, preferably 0.060 mass% or less.

[0022] [Si: 0.01 mass% or more and 0.30 mass% or less] Si acts as a deoxidizing element and can also increase the strength of steel. From these viewpoints, the Si content is set to 0.01 mass% or more, preferably 0.03 mass% or more, more preferably 0.08 mass% or more, and even more preferably 0.17 mass% or more. On the other hand, Si is an element that causes embrittlement during tempering, and if the Si content is excessive, it tends to lead to a deterioration of the strength-toughness balance. Therefore, the Si content is set to 0.30 mass% or less, preferably 0.28 mass% or less, and more preferably 0.27 mass% or less.

[0023] [Mn: 0.70% by mass or less] Mn is an element that increases the strength of steel, but it is also an element that increases susceptibility to tempering embrittlement. If there is an excess of Mn, the steel becomes embrittle during tempering due to heat treatment, and its toughness decreases. Therefore, it is desirable to have a low amount of Mn. From this viewpoint, the amount of Mn should be 0.70 mass% or less, preferably 0.68 mass% or less, and more preferably 0.67 mass% or less. Furthermore, by further reducing the amount of Mn, toughness, especially cryogenic toughness, can be further improved. From this viewpoint, in order to obtain a steel sheet with even better cryogenic toughness, the amount of Mn should be even more preferably 0.50 mass% or less, even more preferably 0.30 mass% or less, and particularly preferably 0.10 mass% or less. The lower limit of the amount of Mn is not particularly limited and may be less than 0.01 mass% (which may be below the detection limit) and may be greater than 0 mass%.

[0024] [P: 0.0050% by mass or less] P is an element that is inevitably present as an impurity. If P is present in large quantities, it embrittles the grain boundaries during tempering, leading to a deterioration of the strength-toughness balance. Therefore, the amount of P is 0.0050 mass% or less, preferably 0.0040 mass% or less, and more preferably 0.0030 mass% or less. While a lower amount of P is preferable, it is present at an unavoidable impurity level and can exceed 0 mass%. Excessive reduction leads to increased steelmaking costs, so the lower limit of the amount of P can be around 0.0001 mass%.

[0025] [S: 0.0030% by mass or less] S is an element that is inevitably present as an impurity. If there is a large amount of S, it will embrittle the grain boundaries during tempering, leading to a deterioration of the strength-toughness balance. Therefore, the amount of S is 0.0030 mass% or less, preferably 0.0025 mass% or less, and more preferably 0.0015 mass% or less. The less S there is, the better, but it is present at an unavoidable impurity level and can be greater than 0 mass%. Excessive reduction will lead to an increase in steelmaking costs, so the lower limit of the amount of S can be around 0.0001 mass%.

[0026] [Al: 0.005 mass% or more and 0.055 mass% or less] Al acts as a deoxidizing element. If the amount of Al is too low, the oxygen concentration in the steel increases, the amount of coarse Al-based inclusions increases, and cryogenic toughness cannot be ensured. Therefore, the amount of Al should be 0.005 mass% or more, preferably 0.006 mass% or more, and more preferably 0.007 mass% or more. On the other hand, if the amount of Al is too high, coarse oxides are formed, leading to a deterioration of the strength-toughness balance. Therefore, the amount of Al should be 0.055 mass% or less, preferably 0.052 mass% or less, and more preferably 0.050 mass% or less.

[0027] [Ni: 8.0 mass% or more and 9.5 mass% or less] Ni is an element that causes matrix softening (solid solution softening) at low temperatures, thereby improving toughness. Ni is also an element that enhances stability when present in retained austenite. From these viewpoints, the amount of Ni is 8.0 mass% or more, preferably 8.3 mass% or more, more preferably 8.5 mass% or more, even more preferably 8.7 mass% or more, and even more preferably 9.0 mass% or more. On the other hand, Ni is expensive, and an excess amount of Ni significantly increases the price of steel. Therefore, the amount of Ni is 9.5 mass% or less, preferably 9.4 mass% or less.

[0028] [N: 0.0050% by mass or less] N is an element that is inevitably present as an impurity, and it dissolves in the matrix, reducing toughness through strain aging. Therefore, the amount of N should be kept below 0.0050 mass%. Preferably, the amount of N is 0.0040 mass% or less. On the other hand, while it is preferable for the amount of N in steel to be as low as possible, it is an element that is difficult to completely remove industrially and exists at the impurity level. Therefore, the amount of N can be greater than 0 mass%, and even greater than 0.0001 mass%.

[0029] In one preferred embodiment, the remainder consists of Fe and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) introduced depending on the conditions of the raw materials, materials, manufacturing equipment, etc., is acceptable. Note that there are elements such as P, S, and N, which are generally preferable in smaller amounts and therefore unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, when we refer to "unavoidable impurities" that constitute the remainder, we mean the concept excluding elements whose composition range is separately defined.

[0030] In addition to these elements, one or more of Cr, Mo, and Cu may be present as unavoidable impurities in amounts of less than approximately 0.01% by mass for each element.

[0031] The steel plates of this disclosure may have, for example, a thickness in the range of 5 mm to 40 mm. This disclosure also includes a liquefied hydrogen storage tank formed from the steel plates of this disclosure.

[0032] 2. Characteristics of steel plates The steel sheet of this disclosure exhibits cryogenic toughness as described above. Cryogenic toughness means that, as described in the examples below, when a Charpy impact test (also referred to as "3-sided slit Charpy impact test at 4K") is performed at 4K (-269°C) using a 3-sided slit test specimen, the absorbed energy is 29 J or more. Preferably, the absorbed energy is 30 J or more.

[0033] 3. Manufacturing method of steel plates Next, the method for manufacturing the steel sheet of this disclosure will be described. The inventors have found that by using a steel material having a predetermined chemical composition, hot-rolling it at a predetermined total reduction ratio to obtain a rolled material, and then performing heat treatment on the rolled material using a combination of predetermined heat treatment process conditions, namely the conditions for Q treatment, L treatment, and T treatment, to control the morphology of the steel structure, a Ni-containing steel sheet that satisfies the above-mentioned range of parameter X and exhibits cryogenic toughness can be obtained.

[0034] According to the manufacturing method of this disclosure, when a rolled material obtained by hot rolling at a predetermined total reduction ratio is quenched (Q treatment is performed), it transforms from an austenitic structure to a martensitic structure. Subsequently, during two-phase heat treatment (L treatment), the martensitic structure partially undergoes reverse transformation to generate an austenitic structure. After the L treatment, the structure is frozen by water cooling. At this time, a portion of the austenitic structure becomes a martensitic structure, but a portion remains as an austenitic structure, retaining as retained austenite. After that, the amount of solid dissolved carbon in the retained austenite structure is increased at the T treatment temperature to stabilize it.

[0035] In the manufacturing method of this disclosure, in particular, to satisfy the range of parameter X, it is preferable to first refine the austenite grains by making the total reduction ratio in the hot rolling process 80% or more and lowering the Q treatment temperature, and then to refine the martensite structure that is subsequently formed, thereby refining the grain size. Furthermore, in satisfying the range of parameter X described above, it is preferable to increase the amount of retained austenite, which acts as resistance to brittle cracks and can increase fracture resistance, by setting the L treatment temperature to an appropriate range that is neither too low nor too high. In addition, in satisfying the range of parameter X described above, it is preferable to lengthen the T treatment time and / or use water cooling as the cooling method in the T treatment. This is preferable to maintain the amount of solid-solution carbon in the retained austenite at a certain level or higher, suppress the transformation of the retained austenite, and increase the amount of retained austenite. This also makes it possible to increase the resistance to brittle cracks by retained austenite, increase fracture resistance, and as a result, obtain a Ni-containing steel sheet that exhibits cryogenic toughness. The following describes each process.

[0036] (Manufacturing process for rolled materials) First, the steelmaking raw materials are melted in a conventional melting furnace such as a converter, and a slab is obtained as a base steel (steel material) that satisfies the chemical composition of this disclosure, for example, by a continuous casting method. The obtained base steel is heated to 950-1250°C, a temperature at which hot rolling is possible, by conventional methods, and then hot rolling (AR: As-Roll) is performed with a total reduction ratio of 80% or more to obtain a rolled material of the desired plate thickness. By making the total reduction ratio 80% or more, microstructure refinement can be achieved. The total reduction ratio is preferably 83% or more, more preferably 85% or more. The above "total reduction ratio" is the value (%) obtained by [(Tt) / T] × 100, where T (mm) is the plate thickness before hot rolling and t (mm) is the plate thickness after finish rolling (25 mm in the example described later).

[0037] (Heat treatment process for rolled material) The rolled material described above is subjected to the following steps (a) to (c) in sequence. The following describes each step (a) to (c) in the heat treatment process. (a) A quenching ("Q treatment") process in which the rolled material is held at a temperature of 720°C to 820°C for 0.5 minutes to 40 minutes, and then water-cooled. (b) A two-phase zone quenching ("L treatment") process, in which the material is held at a temperature of 610°C to 690°C for 5 to 60 minutes after the quenching process, and then water-cooled. (c) After the two-phase quenching process, a tempering process ("T process") is performed, in which the material is held at a temperature of 510°C to 600°C for 5 to 60 minutes, followed by water cooling or air cooling.

[0038] (a) Quenching (Q treatment) process The rolled material is held at a temperature of 720°C to 820°C (also referred to as the "holding temperature in Q treatment" or "Q treatment temperature") for 0.5 minutes to 40 minutes, and then water-cooled. In this disclosure, by lowering the Q treatment temperature compared to conventional methods, the austenite grains become finer, and the martensite structure that subsequently forms becomes finer. In addition, by shortening the holding time at the Q treatment temperature (not making it too long), the coarsening of the austenite grains is prevented. As a result, the grain size of the martensite structure becomes finer, and consequently, the grain size of the steel structure, which is mainly composed of martensite, becomes finer, and brittle fracture is suppressed.

[0039] If the holding temperature in the Q treatment is too high, the austenite grains will coarseen, and the martensite structure that is subsequently formed will also coarseen. Therefore, the holding temperature in the Q treatment should be 820°C or lower, preferably 815°C or lower. On the other hand, the Q treatment aims to form a martensite structure by performing quenching. Therefore, it is necessary to perform heat treatment at Ac3 or higher. Therefore, the holding temperature in the Q treatment should be 720°C or higher, preferably 725°C or higher.

[0040] The holding time at the Q treatment temperature is 0.5 minutes or more, more preferably more than 0.5 minutes, even more preferably 1 minute or more, and even more preferably more than 1 minute. Furthermore, if the holding time at the Q treatment temperature exceeds 40 minutes, the austenite grains become coarser. Therefore, the holding time is 40 minutes or less, preferably 30 minutes or less, more preferably less than 25 minutes, even more preferably less than 20 minutes, and even more preferably less than 10 minutes.

[0041] Methods of water cooling in the Q treatment include direct immersion in a water tank. Water cooling refers to a cooling rate, for example, an average cooling rate of 2°C / s or more from the holding temperature to 500°C.

[0042] Furthermore, it is preferable to perform multiple heating stages as part of the Q treatment. By performing multiple heating stages, the microstructure can be uniformly refined, and toughness can be improved. When performing multiple heating stages in this manner, it is sufficient that the final heating stage satisfies the conditions for the Q treatment described above.

[0043] (b) Two-phase zone quenching (L treatment) process After the quenching process, the material is held at a temperature of 610°C to 690°C (also referred to as the "holding temperature in L treatment" or "L treatment temperature"), which is the α+γ two-phase coexistence region, for 5 to 60 minutes, and then water-cooled. In this disclosure, the amount of retained austenite can be increased by setting the L treatment temperature within an appropriate temperature range. By increasing the amount of retained austenite in the microstructure, the resistance to brittle cracks due to retained austenite can be increased, thereby raising the fracture resistance.

[0044] The L treatment promotes the formation of retained austenite and increases the amount of alloying elements (e.g., Ni) in the retained austenite. If the holding temperature in the L treatment is too high, the amount of alloying elements in the retained austenite decreases, accelerating the transformation and reducing the total amount of retained austenite. Therefore, the holding temperature in the L treatment should be 690°C or lower, preferably 680°C or lower. On the other hand, it is necessary to generate a certain amount of austenite in the martensite structure, but if the holding temperature in the L treatment is too low, the amount of austenite that undergoes reverse transformation from the structure as it was after the Q treatment decreases, reducing the total amount of austenite. Therefore, the holding temperature in the L treatment should be 610°C or higher, preferably 620°C or higher. Furthermore, the holding time at the L treatment temperature should be 5 minutes or more from the viewpoint of ensuring uniformity of the structure. On the other hand, from an economic standpoint, the upper limit of the holding time at the L treatment temperature is 60 minutes.

[0045] Methods of water cooling in L treatment include direct immersion in a water tank. Water cooling refers to a cooling rate, for example, an average cooling rate of 2°C / s or more from the holding temperature to 500°C.

[0046] (c) Tempering process (T treatment) After the two-phase quenching process, the material is held at a temperature of 510°C to 600°C for 5 to 60 minutes. The retained austenite may be transformed by the plastic field around the crack, and it is desirable to maintain a high level of dissolved carbon in the austenite. In this disclosure, the amount of dissolved carbon in the retained austenite can be maintained at 0.435 mass% or higher by extending the T treatment time and / or by using water cooling. In particular, extending the T treatment time allows for sufficient diffusion of carbon, which can increase the amount of dissolved carbon in the retained austenite.

[0047] If the holding temperature during the T treatment is too high, the amount of dissolved carbon in the retained austenite decreases. Therefore, the holding temperature during the T treatment should be 600°C or lower, preferably 590°C or lower. On the other hand, if the cementite formation temperature is around 540°C and the holding temperature during the T treatment is low, cementite will form in the retained austenite, significantly reducing the stability of the retained austenite and adversely affecting toughness. Furthermore, it will also lead to a decrease in the amount of retained austenite. Therefore, the holding temperature during the T treatment should be 510°C or higher, preferably 520°C or higher. The holding time at the T treatment temperature should be 5 minutes or more from the viewpoint of ensuring and maintaining a sufficiently uniform amount of dissolved carbon in the retained austenite in the structure (e.g., 0.435% by mass or more). From an economic standpoint, the holding time at the T treatment temperature should be 60 minutes or less.

[0048] After holding at the T treatment temperature, water cooling or air cooling is performed. Methods of water cooling in the T treatment include direct immersion in a water bath. Water cooling means that the cooling rate, for example, the average cooling rate from the holding temperature to 500°C, is 2°C / s or higher. Air cooling means that the cooling rate, for example, the average cooling rate from the holding temperature to 500°C, is less than 2°C / s. Water cooling is preferable to air cooling in order to prevent the diffusion of carbon in the retained austenite during cooling and the formation of cementite.

[0049] In all of these processes, "holding" at each temperature includes not only cases where the holding temperature is constant, but also cases where it fluctuates within a predetermined temperature range.

[0050] In conventional techniques, the heat treatment conditions for Q, L, and T, including the L and T treatment temperatures, were not adequately considered. Therefore, 9% Ni steel produced by conventional techniques had a low amount of retained austenite, or the stability of retained austenite was not defined by controlling, for example, the amount of carbon in the retained austenite, making it difficult to ensure toughness at lower temperatures (4K). In contrast, the manufacturing method of the present disclosure described above makes it possible to obtain steel sheets suitable for liquefied hydrogen storage tanks that exhibit cryogenic toughness. [Examples]

[0051] The present invention will be described in more detail below with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that is consistent with the spirit described above and below, and all such modifications are included within the technical scope of the present invention.

[0052] 1. Manufacturing of steel plates The raw materials for steelmaking were melted in conventional furnaces such as converters, and slabs were obtained as steel materials (base steel) by continuous casting. The steel materials (base steel) with the chemical composition shown in Table 1 were heated to 1050°C, and then subjected to hot rolling (AR: As-Roll) with a total reduction ratio of 90% to obtain rolled materials (also called "hot-rolled plates") A and B with a target plate thickness of 25 mm.

[0053] Using the obtained rolled materials A and B, heat treatment was performed sequentially according to (1) to (3) below to obtain steel plates No. 1 to 17. The chemical composition of the steel plates obtained by heat treatment is the same as that of the rolled materials. Table 2-1 shows the heat treatment conditions for rolled material A when steel plates No. 1 to 4 were obtained by heat treatment of rolled material A, and Table 2-2 shows the heat treatment conditions for rolled material B when steel plates No. 5 to 17 were obtained by heat treatment of rolled material B. (Heat treatment) (1) Q treatment (quenching): The material was heated to the temperature shown in Table 2-1 or Table 2-2, held at that temperature for the holding time shown in Table 2-1 or Table 2-2, and then water-cooled. (2) L treatment (α+γ two-phase region quenching): The temperature was heated to the temperature shown in Table 2-1 or Table 2-2, held at that temperature for the holding time shown in Table 2-1 or Table 2-2, and then water-cooled. (3) T treatment (tempering treatment): The material was heated to the temperature shown in Table 2-1 or Table 2-2, held at that temperature for the holding time shown in Table 2-1 or Table 2-2, and then cooled to 500°C or below using the cooling method shown in Table 2-1 or Table 2-2. Underlined values ​​in the table indicate values ​​that fall outside the specified range or do not meet the characteristic evaluation criteria (Charpy absorption energy value of 29 J or more).

[0054] [Table 1]

[0055] [Table 2-1]

[0056] [Table 2-2]

[0057] 2. Organizational Evaluation The amount of retained austenite and the amount of dissolved carbon in the retained austenite in the total microstructure of the obtained steel sheets were measured as follows. Furthermore, microstructural analysis by EBSD confirmed that in all steel sheets, the combined martensitic and bainite structures accounted for 80% or more of the area.

[0058] (Measurement of residual austenite content) The amount of retained austenite was determined by X-ray diffraction measurement. Measurements were taken centered at the t / 2 (t: plate thickness) position in the cross section perpendicular to the rolling width direction. The X-ray diffraction measurement conditions were as follows: X-ray diffraction measurement conditions • Equipment: RIGAK Corporation's 2D micro-X-ray diffractometer RINT-RAPIDII ·Tube:Co • Tube output: 40kV-30mA • Collimator: Φ300μm ·ω angle: 15°~25°(2° / sec) ·Φ angle: rotation (1° / sec)

[0059] The analysis software used was JADE-Pro, and the Whole Pattern Fitting (WPF) method (also known as the WPPD method) was employed for the analysis. A paper explaining this WPPD method is available from Hideho Toraya, "Profile Fitting Method, WPPD Method, and Rietveld Method," Bulletin of the Japan Institute of Metals, Vol. 28, No. 3 (1989), pp. 189-194.

[0060] (Measurement of the amount of solid-solution carbon in retained austenite) The lattice constant u(Å), obtained by measurement from the aforementioned X-ray diffraction, was substituted into the following equation (2) to calculate the value. Solute carbon content (mass%)=(u-3.578-0.00095Mn+0.0002Ni-0.0220N) / 0.033 (2) In equation (2), Mn, Ni, and N are the content (mass%) in the steel.

[0061] Note that equation (2) is derived from the lattice constant a described in the paper: DJ Dyson, B. Holmes, EFFECT OF ALLOYING ADDITIONS ON THE LATTICE PARAMETER OF AUSTENITE, Vol.208, No.5, (1970), pp.469-474. o This is based on a formula relating to [the subject], and the following formula is shown in the aforementioned paper. a o =3.5780+0.0330C+0.00095Mn-0.0002Ni+0.0006Cr+0.0220N+0.0056Al-0.0004Co+0.0015Cu+0.0031Mo+0.0051Nb+0.0039Ti+0.0018V+0.0018W

[0062] In the formula described in the above paper, Cr, Co, Cu, Mo, Nb, Ti, V, and W, which are not added to the steel sheet of this disclosure, were set to 0 mass%. Also, considering that Al is present as an inclusion and is not almost dissolved in the matrix, Al = 0 mass% was set. After deleting the terms related to these elements, the formula was rearranged to obtain the above formula (2).

[0063] (crystal grain size) Electron backscatter diffraction (EBSD) measurements were performed centered at the t / 2 (t: plate thickness) position on the cross-section perpendicular to the rolling width direction of the steel plate, and EBSD data was obtained. The EBSD measurement conditions were as follows. EBSD measurement conditions • Device: JEOL JSM-IT100 • Sample tilt angle: 70° ·Measurement area: 200×200μm • Step (pixel) size: 0.3 μm • Phases to consider: ferrite, austenite

[0064] The obtained EBSD data was analyzed using the analysis software OIM Analysis (manufactured by TSL Solutions Co., Ltd.). Specifically, points with a Confidence Index of 0.100 or less were removed from the data points, and grain boundaries with a crystal orientation of 25 degrees or more from adjacent pixels were defined as large-angle grain boundaries for analysis. The average equivalent circle diameter calculated from this was used as the grain size.

[0065] The microstructure of the steel sheet of this disclosure, which contains approximately 9 mass% Ni, is mainly martensite, with bainite structures optionally included. Therefore, the grain size of the microstructure was organized using grain size determined from a single Bain group. As mentioned above, the microstructure of the steel sheet of this disclosure is mainly martensite, and even when bainite structures are included, the grain size was determined by organizing them into single Bain groups without distinguishing between martensite and bainite structures. However, retained austenite (FCC) was excluded from the analysis. For a variant (V1) generated from the same austenite grain in a KS relationship, the orientation difference within the same Bain group as variant (V1) is approximately 10 to 20 degrees. The orientation difference with a different Bain group is 30 degrees or more, and the orientation difference should be defined between 20 and 30 degrees. Therefore, in this disclosure, grain boundaries with a crystal orientation of 25° or more with respect to adjacent pixels were defined as large-angle grain boundaries for analysis.

[0066] The value of parameter X was determined from the amount of retained austenite, the amount of dissolved carbon in the retained austenite, the grain size, and the Mn content obtained from the above measurements. The results are shown in Table 3-1 for steel sheets obtained by heat-treating rolled material A, and in Table 3-2 for steel sheets obtained by heat-treating rolled material B.

[0067] 3. Evaluation of properties (cryogenic toughness) The cryogenic toughness of the obtained steel plates was evaluated by performing Charpy impact tests at 4K using three-sided slit Charpy impact test specimens.

[0068] The preparation of the three-sided slit Charpy impact test specimen was carried out in accordance with the following literature. Specifically, the test specimen was taken from the t / 2 position of the steel plate so that the longitudinal side of the specimen was parallel to the plate width direction (C direction), and a Charpy impact test specimen was prepared by making slits on three sides of the steel plate as shown in Figure 1. Reference: Akio Deguchi, Masuo Tada, and Hiroshi Yajima, "A Study on the Arrest Performance and New Impact Test of 9% Ni Steel Plates," Transactions of the Japan Society of Naval Architects and Engineers, 1990(167), 271-277. https: / / doi.org / 10.2534 / jjasnaoe1968.1990.271

[0069] The Charpy impact test was conducted based on the following reference: Toshio Ogata, Keijiro Hiraga, Hisashi Nagai, and Keisuke Ishikawa, "A Simple Charpy Impact Test Method at Liquid Helium Temperature" (1983), Iron and Steel, 69(6), 641-646. https: / / doi.org / 10.2355 / tetsutohagane1955.69.6_641. In detail, the test apparatus consisted of a 30 kgfm Charpy impact tester, a liquid helium tank, two vacuum-insulated transfer tubes, and a helium gas cylinder to maintain the pressure inside the liquid helium tank.

[0070] For tests conducted in the liquid helium temperature range (4K), the Charpy test specimens were placed in insulating capsules and used as Charpy test specimens.

[0071] A Charpy impact test was conducted at 4K using the aforementioned Charpy specimen. In the Charpy impact test, liquid helium was introduced into an insulating capsule to cool the Charpy specimen. After confirming that the sample had cooled sufficiently using a thermocouple attached to the sample, the Charpy impact test was performed to determine the Charpy absorbed energy. Cooling continued until after the impact test was completed. In this case, since the Charpy specimen was placed in a cooling capsule during the 4K test, the value obtained by subtracting the absorbed energy of the cooling capsule was used as the Charpy absorbed energy. A determined absorbed energy of 29 J or higher was evaluated as indicating cryogenic toughness.

[0072] The Charpy absorption energy values ​​obtained from the above measurements are shown in Tables 3-1 and 3-2. Figure 2 shows a graph illustrating the relationship between parameter X and the three-sided slit Charpy absorption energy.

[0073] [Table 3-1]

[0074] [Table 3-2]

[0075] As shown in Tables 3-1, 3-2, and Figure 2, steel plates No. 1, 5, 6, 15, and 16 did not meet the specified requirements and did not exhibit cryogenic toughness.

[0076] In detail, steel plates No. 1, 5, and 6 had a high holding temperature during the L treatment, resulting in a lower amount of retained austenite, and the parameter X values ​​were 0.24, 0.12, and 0.11, respectively, which are less than 0.29. As a result, the absorbed energy in the three-sided slit Charpy impact test was small, and they did not exhibit cryogenic toughness.

[0077] Steel plate No. 15 had a low holding temperature during the L treatment, resulting in a small amount of retained austenite undergoing reverse transformation. Consequently, the parameter X value was 0.28, which is less than 0.29. As a result, the absorbed energy in the three-sided slit Charpy impact test was small, and it did not exhibit cryogenic toughness.

[0078] Steel plate No. 16 had a low holding temperature during the T treatment, which reduced the amount of dissolved carbon and destabilized the retained austenite, resulting in a lower value for parameter X (0.22, less than 0.29). As a result, it exhibited low absorbed energy in the three-sided slit Charpy impact test and did not show cryogenic toughness.

[0079] In contrast, the steel sheets of the present disclosure Nos. 2-4, 7-14, and 17 exhibited cryogenic toughness by controlling retained austenite, the amount of solid-solution carbon in retained austenite, grain size, and Mn content, with parameter X being 0.29 or higher in all cases.

[0080] In particular, No. 2 met all the requirements specified in this disclosure, and by increasing the duration of the T treatment, the amount of dissolved carbon was increased, and the value of parameter X was sufficiently raised, resulting in excellent cryogenic toughness. In No. 3, water cooling was performed during the T treatment, which ensured the amount of dissolved carbon in the retained austenite and allowed parameter X to be kept within the specified range, resulting in even better cryogenic toughness. In No. 4, the grain size was reduced by lowering the Q treatment temperature, and water cooling was performed during the T treatment, which increased the amount of retained austenite and allowed parameter X to be sufficiently raised, resulting in even better cryogenic toughness.

[0081] In samples No. 7 and 8, the Q treatment temperature was lowered and the treatment time was shortened to reduce the grain size, and water cooling was performed in the T treatment, which increased the amount of retained austenite. As a result, the value of parameter X could be sufficiently increased, and excellent cryogenic toughness was observed.

[0082] In samples No. 9 and 10, the Q treatment temperature was lowered and the treatment time was shortened to reduce the grain size, and the amount of retained austenite was increased by performing the L treatment under specified conditions. As a result, the value of parameter X could be sufficiently increased, and excellent cryogenic toughness was observed.

[0083] In Nos. 11, 12, and 13, the Q treatment temperature was lowered and the treatment time was shortened to reduce the grain size, and the amount of dissolved carbon was increased by performing the T treatment under specified conditions. As a result, the value of parameter X could be sufficiently increased, and excellent cryogenic toughness was observed.

[0084] In No. 14, the Q treatment temperature was lowered to reduce the grain size, and the T treatment temperature was extended to increase the amount of dissolved carbon. As a result, the value of parameter X was sufficiently increased, and excellent cryogenic toughness was observed.

[0085] In No. 17, all the requirements specified in this disclosure were met, and in particular, by increasing the T treatment temperature within the specified range, the amount of solid-solution carbon was increased and cementite formation in the retained austenite was suppressed, thereby increasing the amount of retained austenite. As a result, the value of parameter X could be sufficiently increased, resulting in excellent cryogenic toughness.

[0086] This disclosure provides a Ni-containing steel sheet exhibiting cryogenic toughness, a method for manufacturing the same, and a liquefied hydrogen storage tank formed from the steel sheet. The realization of this cryogenic tough Ni-containing steel sheet and the liquefied hydrogen storage tank formed from the steel sheet can contribute to a stable supply of hydrogen gas.

Claims

1. The chemical composition is C: 0.010% by mass or more and 0.070% by mass or less, Si: 0.01% by mass or more and 0.30% by mass or less, Mn: 0.70% by mass or less, P: 0.0050% by mass or less, S: 0.0030% by mass or less, Al: 0.005% by mass or more and 0.055% by mass or less, Ni: 8.0% by mass or more and 9.5% by mass or less, N: 0.0050% by mass or less It satisfies the following conditions, and the remainder consists of Fe and unavoidable impurities. The proportion of martensitic and bainite tissue in the total tissue is 80% or more by area, and it contains retained austenite. A steel plate for liquefied hydrogen storage tanks, wherein the parameter X, represented by the following formula (1), is 0.29 or greater. [Math 1] Here, a: Amount of retained austenite (volume %) b: Amount of dissolved carbon in retained austenite (mass%) c: The grain size (10) of crystal grains surrounded by large-angle grain boundaries with an orientation difference of 25° or more, as determined by EBSD. －６ m) d: Mn content in steel (mass %) It represents.

2. The steel plate for a liquefied hydrogen storage tank according to claim 1, wherein parameter X is 0.31 or greater.

3. A liquefied hydrogen storage tank formed from a steel plate for liquefied hydrogen storage tanks according to claim 1 or 2.

4. A method for manufacturing a steel plate for a liquefied hydrogen storage tank according to claim 1 or 2, The chemical composition is C: 0.010% by mass or more and 0.070% by mass or less, Si: 0.01% by mass or more and 0.30% by mass or less, Mn: 0.70% by mass or less, P: 0.0050% by mass or less, S: 0.0030% by mass or less, Al: 0.005% by mass or more and 0.055% by mass or less, Ni: 8.0% by mass or more and 9.5% by mass or less, N: 0.0050% by mass or less The steel material used satisfies the following conditions, with the remainder consisting of Fe and unavoidable impurities. Obtaining rolled material by hot rolling with a total reduction ratio of 80% or more, and A method for manufacturing steel sheets for liquefied hydrogen storage tanks, comprising performing the following heat treatments sequentially on the rolled material described above (a) to (c). (a) The rolled material is quenched by holding it at a temperature of 720°C to 820°C for 0.5 minutes to 40 minutes, and then water-cooling it. (b) After the quenching, perform two-phase quenching by holding at a temperature of 610°C to 690°C for 5 minutes to 60 minutes and then water-cooling. (c) After the two-phase quenching, the material is held at a temperature of 510°C to 600°C for 5 to 60 minutes, and then subjected to a tempering treatment by water cooling or air cooling.

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