Steel materials for transporting or storing liquid ammonia

Abstract

To provide a steel for ammonia transport or storage, developed in light of current situations, applied to structural members of large structures such as plants and tanks, which are used in liquid ammonia environments and require resistance to ammonia SCC, and demonstrating excellent ammonia SCC resistance.SOLUTION: A steel for liquid ammonia transport or storage contains, in mass%, C: 0.50% or less, Si: 0.01 to 1.00%, Mn: 0.10 to 3.00%, P: 0.030% or less, and S: 0.0100% or less, and further contains a specific amount of one or more kinds selected from Zr, Hf, Ta, Ga, Te, Sr, Se, Pb, As, Bi, Ba, La, Sm, Pr, Nd, Ce, Sc, Ag, Pt, and Au, with the balance being Fe and inevitable impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to steel materials suitable for transporting or storing liquid ammonia, which are suitable for structural members of large structures such as pipelines, plants, and tanks used in liquid ammonia environments. [Background technology]

[0002] Ammonia is a compound that is widely manufactured and distributed, mainly as a raw material for basic chemicals such as nitric acid and fertilizers. On the other hand, ammonia is difficult to handle, and stress corrosion cracking (hereinafter also called ammonia SCC) is known to occur in carbon steel pipes, storage tanks, tank cars, and line pipes that handle liquid ammonia. Therefore, conventionally, for structures used in liquid ammonia environments, steel materials with low susceptibility to stress corrosion cracking (SCC) have been applied, and operational measures have been taken to suppress ammonia-induced SCC. For example, it is empirically known that the occurrence of ammonia SCC is correlated with the strength of the material. When using carbon steel, efforts are made to suppress ammonia SCC by setting an upper limit on its strength and by applying stress relief annealing to the welded joints. Furthermore, in a liquid ammonia environment, water coexisting with the liquid ammonia exhibits an effect that suppresses the occurrence of stress corrosion cracking. For this reason, preventative measures are sometimes taken to add water at a level that does not impair the quality of the liquid ammonia. Incidentally, in recent years, global demand for liquid ammonia has been increasing due to the expanding range of applications, leading to a trend towards larger equipment and cost reductions in distribution and manufacturing. Consequently, it has become difficult to implement the above-mentioned measures to suppress and prevent ammonia-induced spring crashes (SCCs). For example, applying stress relief annealing to welded joints would increase the manufacturing process, making its application impractical, especially in large-scale equipment. Furthermore, adding water to liquid ammonia requires careful control of the water concentration, which becomes increasingly difficult as equipment size increases. Moreover, for high-purity liquid ammonia, which has seen increasing demand in recent years, it's simply not possible to implement preventative measures through water addition. Therefore, there is a need for the development of steel materials with excellent ammonia SCC resistance that are suitable for use in structural components of plants and tanks that handle liquid ammonia.

[0003] As a technology relating to steel materials used in a liquid ammonia environment, for example, Patent Document 1 discloses a method for producing high-tensile steel with excellent ammonia cracking resistance, characterized in that after hot-rolling a slab, it is heated to the austenitization temperature for microstructure control, cooled at a cooling rate of less than or equal to air cooling, then further heated and quenched to a two-phase region temperature (Ac1 to Ac3), and subsequently tempered, thereby securing desired properties, namely tensile strength and yield strength, and thus achieving ammonia cracking resistance.

[0004] Furthermore, Patent Document 2 describes a method for producing steel with excellent resistance to sulfide stress corrosion cracking and ammonia stress corrosion cracking, characterized by rolling a steel billet with PCM ≤ 0.24% to a predetermined plate thickness, then heating it to a temperature of 1100-1300°C before the quenching treatment, forming a decarburized layer of 0.5 mm or more with a C ≤ 0.05% on the surface of the steel sheet, and then performing quenching and tempering treatment, resulting in a tempered steel with excellent resistance to sulfide stress corrosion cracking and ammonia stress corrosion cracking, with a tempered finish of 60 kgf / mm². 2 A method for manufacturing high-strength steel sheets is disclosed.

[0005] Furthermore, Patent Document 3 discloses a method for manufacturing a steel sheet for ammonia tanks, comprising the steps of: surface decarburizing the steel sheet material so that the carbon content within 0.3 mm from the surface is 50% or less of the base material carbon content; and heating the surface decarburized steel sheet to a quenching temperature and then cooling the decarburized surface so that the cooling rate is 150°C / sec or less in a temperature range of 800 to 500°C. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-9571 [Patent Document 2] Japanese Patent Application Publication No. 61-279631 [Patent Document 3] Japanese Patent Application Publication No. 58-67830 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the steel materials obtained by the manufacturing methods disclosed in Patent Documents 1 to 3 ensure ammonia-resistant SCC by controlling the surface structure. Therefore, in actual construction, if the steel materials from Patent Documents 1 to 3 are subjected to heat processing, the surface structure may be altered. For this reason, it cannot be said that sufficient ammonia-resistant SCC is necessarily obtained.

[0008] The present invention has been developed in view of the above-mentioned circumstances, and aims to provide a steel material for transporting or storing liquid ammonia that has excellent ammonia SCC resistance, applicable to structural members of large structures such as plants and tanks that are used in a liquid ammonia environment and require ammonia SCC resistance. [Means for solving the problem]

[0009] The inventors of this invention have diligently conducted research to solve the above problems.

[0010] First, the inventors investigated in detail the mechanism of ammonia SCC generation in a liquid ammonia environment and obtained the following findings. In a liquid ammonia environment, the following corrosion reactions occur: Anodic reaction: 2Fe → 2Fe 2+ +4e - Cathode reaction: O2 + 2NH4 + +4e - →2OH - +2NH3 However, an inert oxide film is formed on the steel surface as a result of the above corrosion reaction. Therefore, under normal circumstances, the total amount of reaction in the above corrosion reaction is not large. Consequently, a liquid ammonia environment is not inherently a harsh corrosive environment. However, when new surfaces are formed on the steel surface due to residual stress in the steel or externally applied stress, a selective iron dissolution reaction proceeds, using these new surfaces, which lack an oxide film, as anodes, forming cracks. Since cracks become areas of stress concentration, coating failure and corrosion reactions at the crack tips progress at an accelerating rate, ultimately leading to fracture of the steel. Once cracks occur due to SCC, the lifespan of the steel is determined by its resistance to SCC crack propagation. Therefore, in order to ensure the ammonia resistance to SCC of steel, it is necessary to form a strong oxide film on the steel surface to prevent the formation of new surfaces, thereby preventing the occurrence of SCC and increasing the resistance to SCC crack propagation.

[0011] Therefore, based on the above findings, the inventors diligently conducted research toward developing a steel material that exhibits excellent resistance to ammonia-induced scalding (SCC) in an ammonia environment.

[0012] As a result, it has been found that in order to improve the ammonia SCC resistance, it is effective to contain appropriate amounts of Zr, Hf, Ga, Ta, Te, Sr, Se, Pb, As, Bi, Ba, La, Sm, Pr, Nd, Ce, Sc, Ag, Pt, and Au, and it has been found that it is possible to provide a steel material for liquid ammonia transportation or storage suitable for structural members of large structures such as pipelines, plants, and tanks used in an ammonia environment.

[0013] The present invention has been completed through further studies based on the above findings. That is, the gist configuration of the present invention is as follows. [1] In mass%, C: 0.50% or less, Si: 0.01 to 1.00%, Mn: 0.10 to 3.00%, P: 0.030% or less, S: 0.0100% or less, containing, and further Zr: 0.001 to 0.100%, Hf: 0.001 to 0.100%, Ta: 0.001 to 0.200%, Ga: 0.001 to 0.300%, Te: 0.001 to 0.500%, Sr: 0.001 to 0.500%, Se: 0.001 to 0.500%, Pb: 0.001 to 0.500%, As: 0.001 to 0.500%, Bi: 0.001 to 0.500%, Ba: 0.001 to 0.500%, La: 0.001 to 0.100%, Sm: 0.001 to 0.100%, Pr: 0.001 to 0.100%, Nd: 0.001 to 0.100%, Ce: 0.001 to 0.100%, Sc: 0.001 to 0.100%, Ag: 0.001 to 0.010%, Pt: 0.001~0.010%, Au: 0.001~0.010% A steel material for transporting or storing liquid ammonia, characterized by having a component composition that contains one or more selected from among, with the remainder being Fe and unavoidable impurities. [2] The steel material for transporting or storing liquid ammonia according to [1], characterized in that the component composition further contains, by mass%, one or more selected from groups A to G below. Note Group A: Ti: 0.100% or less, Group B: Sb: 0.50% or less, Sn: 0.50% or less, Cu:3.00% or less, Ni: 3.00% or less, Cr:3.00% or less, One or more types selected from among them, Group C: Ca: 0.0100% or less, Y: 0.100% or less, Mg: 0.0200% or less, One or more types selected from among them, Group D: Co: 0.50% or less, Mo: 1.00% or less W: 1.00% or less, V: 0.200% or less, Nb: 0.200% or less, One or more types selected from among them, Group E: Al: 0.300% or less, Group F: B: 0.0300% or less, Group G: N: 0.0100% or less [3] The steel material for transporting or storing liquid ammonia according to [1] or [2], characterized in that the maximum value of the Vickers hardness HV0.1 in the surface layer of the steel material is 265 or less. [Effects of the Invention]

[0014] According to the present invention, it is possible to obtain a steel material for transporting or storing liquid ammonia that has excellent resistance to ammonia stress corrosion cancellation (SCC), and is suitable for use as structural members in large structures such as pipelines, plants, and tanks used in liquid ammonia environments. Furthermore, since the steel material of the present invention ensures ammonia SCC resistance by containing an appropriate amount of specific components, there is no risk of the ammonia SCC resistance being impaired due to alteration of the surface structure, surface structure control is unnecessary, and it can be manufactured without heat treatment such as tempering after hot rolling. Moreover, when the steel material of the present invention is applied to, for example, a liquid ammonia storage tank, it can be used for a longer period than conventional methods without stress relief annealing of the welded joint, which is extremely advantageous from an industrial perspective. [Modes for carrying out the invention]

[0015] Embodiments of the present invention will be described below. Note that all units in the composition of the steel material are in "mass%", and unless otherwise specified, they will be simply referred to as "%".

[0016] C: 0.50% or less Carbon (C) is an effective element for ensuring the strength of steel. Therefore, in this invention, it is preferable to have a C content of 0.01% or more. More preferably, it is 0.02% or more. On the other hand, if the C content exceeds 0.50%, the workability and weldability deteriorate significantly, and the ammonia SCC resistance decreases as the strength increases. For this reason, the C content should be 0.50% or less. Preferably, it is 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less.

[0017] Si: 0.01~1.00% Si is an element that acts as a deoxidizing agent for molten steel. To ensure this effect, a Si content of 0.01% or more is required. Preferably, the Si content is 0.02% or more, more preferably 0.03% or more, and even more preferably 0.05% or more. On the other hand, if the Si content exceeds 1.00%, ductility decreases, toughness deteriorates, and inclusions increase, which increases the risk of SCC (Steel Chain Crust) occurring as the initiation point of cracks. For this reason, the Si content should be 1.00% or less, preferably 0.80% or less, more preferably 0.70% or less, and even more preferably 0.60% or less.

[0018] Mn: 0.10~3.00% Mn is an element that improves strength and toughness. Here, if the Mn content is less than 0.10%, the effect is not sufficient, so the Mn content should be 0.10% or more, preferably 0.20% or more, and more preferably 0.50% or more. On the other hand, if the Mn content exceeds 3.00%, the weldability deteriorates, and the risk of ammonia SCC generation increases due to the increase in strength, so the Mn content should be 3.00% or less, preferably 2.00% or less.

[0019] P:0.030% or less Since phosphorus (P) is a harmful element that degrades toughness and weldability, the P content should be 0.030% or less. Preferably, it should be 0.025% or less. There is no particular lower limit, but it is preferable that the P content be 0.001% or more because excessive dephosphorization leads to increased costs.

[0020] S: 0.0100% or less Since sulfur (S) is a harmful element that degrades the toughness and weldability of steel, it is desirable to reduce its content as much as possible. In particular, if the S content exceeds 0.0100%, the deterioration of the toughness of the base material and the toughness of the weld becomes significant. For this reason, the S content should be 0.0100% or less. Preferably, it should be 0.0080% or less, and more preferably 0.0060% or less. There is no particular lower limit, but it is preferable that the S content be 0.0001% or more because excessive desulfurization leads to increased costs.

[0021] In addition to the above components, the following elements (Zr: 0.001~0.100%, Hf: 0.001~0.100%, Ta: 0.001~0.200%, Ga: 0.001~0.300%, Te: 0.001~0.500%, Sr: 0.001~0.500%, Se: 0.001~0.500%, Pb: 0.001~0.500%, As: 0.001~0.500%) Bi:0.001~0.500%, Ba:0.001~0.500%, La:0.001~0.100%, Sm:0.001~0.100%, Pr:0.001~0.100%, Nd:0.001~0.100%, Ce:0.001~0.100%, Sc:0.001~0.100%, Ag:0.001~0.010%, Pt:0.001~0.010%, You need to add one or more Au (0.001~0.010%). The steel material of the present invention is not particularly limited in terms of the combination of constituent components, but may contain one or more elements from Zr: 0.001~0.100%, Hf: 0.001~0.100%, Ta: 0.001~0.200%, Te: 0.001~0.500%, Sr: 0.001~0.500%, Se: 0.001~0.500%, Pb: 0.001~0.500%, As: 0.001~0.500%, Bi: 0.001~0.500%, Ba: 0.001~0.500%, By simultaneously containing one or more elements from the following categories: La: 0.001-0.100%, Sm: 0.001-0.100%, Pr: 0.001-0.100%, Nd: 0.001-0.100%, Ce: 0.001-0.100%, and Sc: 0.001-0.100%, further improvements in ammonia resistance and SCC (Scaling, Carbon Dehydration, Chloride) can be expected.

[0022] Zr:0.001~0.100%, Hf:0.001~0.100%, Ta:0.001~0.200% Zr, Hf, and Ta are important elements for obtaining sufficient ammonia SCC resistance. That is, all of these elements have the function of forming a strong oxide film on the surface of steel materials in a liquid ammonia environment. Thereby, it suppresses the occurrence of local corrosion that serves as the starting point of stress corrosion cracking, suppresses the progress of the selective anodic dissolution reaction at the crack tip, and reduces the ammonia SCC susceptibility of the steel material. Such an effect is manifested by containing these elements at 0.001% or more, so it is made to contain 0.001% or more. On the other hand, when the content of Zr and Hf exceeds 0.100%, the effect saturates and the cost increases. Therefore, the Zr content and the Hf content are each set to 0.100% or less, and preferably 0.080% or less. When the content of Ta exceeds 0.200%, the effect saturates and the cost increases. Therefore, the Ta content is set to 0.200% or less, and preferably 0.180% or less.

[0023] Examples of the above strong oxide film in a liquid ammonia environment include Zr(OH)4, Hf(OH)4, and Ta(OH)5, and the film thickness of these oxide films is preferably 5 nm or more and 500 nm or less.

[0024] Ga: 0.001 - 0.300% Ga is an important element for obtaining sufficient ammonia SCC resistance. That is, Ga has the property of being easily released as Ga 3+ ions from the steel surface. The released Ga 3+ ions are hydroxide ions (OH -It reacts rapidly with ) to form Ga(OH)3. As the crack deepens, Ga(OH)3 is formed and deposited in the crack. This protects the crack, suppresses the progress of selective anodic dissolution reaction in the crack, and as a result improves ammonia SCC resistance. This effect is achieved by including 0.001% or more of Ga, so it should be included at a concentration of 0.001% or more. On the other hand, if Ga is included in excess, the toughness of the weld metal decreases and costs increase. Therefore, the Ga content should be 0.300% or less, preferably 0.200% or less. The film thickness of Ga(OH)3 is preferably 5 nm to 500 nm.

[0025] Te:0.001~0.500%, Sr:0.001~0.500%, Se:0.001~0.500%, Pb:0.001~0.500%, As:0.001~0.500%, Bi:0.001~0.500%, Ba:0.001~0.500% Te, Sr, Se, Pb, As, Bi, and Ba are important elements for obtaining sufficient ammonia SCC resistance. These elements enhance the corrosion resistance of steel and suppress the accelerating corrosion reaction that occurs when the pH drops excessively due to selective anodic dissolution at the crack tip. To suppress the accelerating corrosion reaction, it is preferable that these elements are concentrated in the steel within a 1 mm range from the surface, meaning that there is at least one area where these elements are present (concentrated) at a composition of 0.01% or more. A composition of 0.01% or more refers to an area of ​​1 μm × 1 μm or larger. This effect is achieved by including 0.001% or more of these elements; therefore, if they are included, the content should be 0.001% or more. However, including large amounts of any of these elements degrades weldability and toughness, and increases costs. The content of these elements should be 0.500% or less, preferably 0.400% or less, and more preferably 0.350% or less.

[0026] La:0.001~0.100%, Sm:0.001~0.100%, Pr:0.001~0.100%, Nd:0.001~0.100%, Ce:0.001~0.100%, Sc:0.001~0.100% La, Sm, Pr, Nd, Ce, and Sc are important elements for obtaining sufficient ammonia-resistant SCC (Scatter Cracking and Crystallinity) properties. Specifically, they prevent the formation of large subsurface inclusions by controlling sulfide morphology, thereby improving SCC crack propagation resistance. By controlling sulfide morphology, the particle size of all large subsurface inclusions can be reduced to 100 μm or less. This effect is achieved by including 0.001% or more of these elements, so the content should be 0.001% or more. However, including large amounts of any of these elements will degrade the toughness of the weld and increase costs. Therefore, the content of these elements should be 0.100% or less, preferably 0.080% or less. These elements are part of the elements called REM (rare earth elements).

[0027] Ag:0.001~0.010%, Pt:0.001~0.010%, Au:0.001~0.010% Ag, Pt, and Au are important elements for obtaining sufficient resistance to ammonia-induced corrosion cracking (SCC). Specifically, these elements dissolve in the liquid ammonia environment as the steel corrodes, forming an inert film on the steel surface. This suppresses the selective anodic dissolution reaction at the crack tip, thereby reducing the steel's susceptibility to ammonia-induced SCC. This effect is achieved by including at least 0.001% of these elements; therefore, if included, the concentration should be 0.001% or higher. However, including large amounts of any of these elements degrades toughness and increases costs. Therefore, the content of these elements should be kept below 0.010%.

[0028] Examples of the inert films formed in a liquid ammonia environment include AgOH, Au(OH)3, and Pt(OH)2, and the thickness of these inert films is preferably between 5 nm and 500 nm.

[0029] The above explains the basic essential components, but the following elements may be included as needed. Specifically, one or more elements selected from groups A to G below may be included as appropriate.

[0030] Group A: Ti: 0.100% or less Ti is an element that further improves resistance to ammonia-induced stress corrosion cracking (SCC) and may be included as needed. Specifically, Ti forms a strong oxide film on the surface of steel materials in a liquid ammonia environment. This suppresses the occurrence of localized corrosion, which is the starting point for stress corrosion cracking, and also suppresses the progress of selective anodic dissolution reactions at the crack tip, thereby reducing the steel's susceptibility to ammonia-induced SCC. To obtain such effects, if Ti is included, it is preferable to include 0.001% or more, and more preferably 0.020% or more. On the other hand, since the effect saturates above 0.100%, if Ti is included, the Ti content should be 0.100% or less, and preferably 0.080% or less.

[0031] Examples of the strong oxide film in a liquid ammonia environment include Ti(OH)4, and the thickness of these oxide films is preferably between 5 nm and 500 nm.

[0032] Group B: One or more elements selected from Sb: 0.50% or less, Sn: 0.50% or less, Cu: 3.00% or less, Ni: 3.00% or less, Cr: 3.00% or less. Sb, Sn, Cu, Ni, and Cr are elements that further improve ammonia SCC resistance, and one or more of these elements may be included. All of these elements enhance the corrosion resistance of steel materials and have the function of suppressing the corrosion reaction that progresses at an accelerating rate when the pH drops excessively as a result of selective anodic dissolution at the crack tip. To obtain such an effect, it is preferable to include 0.01% or more of these elements, and more preferably 0.02% or more. However, including large amounts of any of these elements will degrade weldability and toughness, and lead to increased costs. Therefore, when including these elements, the Sb content should be 0.50% or less, and preferably 0.35% or less. The Sn content should be 0.50% or less, and preferably 0.35% or less. The Cu content should be 3.00% or less, and preferably 2.00% or less. The Ni content should be 3.00% or less, and preferably 2.00% or less. The Cr content should be 3.00% or less, preferably 2.00% or less.

[0033] Group C: One or more elements selected from Ca: 0.0100% or less, Y: 0.100% or less, Mg: 0.0200% or less. Ca, Y, and Mg may be included in one or more quantities to ensure the toughness of the weld. To achieve this effect, it is preferable to include 0.0001% or more of Ca, 0.001% or more of Y, and 0.001% or more of Mg. However, including large amounts of any of these elements will lead to deterioration of the toughness of the weld and an increase in cost. Therefore, when including these elements, the Ca content should be 0.0100% or less, the Y content 0.100% or less, and the Mg content 0.0200% or less. Preferably, the Ca content should be 0.0080% or less, the Y content 0.080% or less, and the Mg content 0.0180% or less.

[0034] Group D: One or more elements selected from Co: 0.50% or less, Mo: 1.00% or less, W: 1.00% or less, V: 0.200% or less, Nb: 0.200% or less. Co, Mo, W, V, and Nb are elements that improve the hardenability of steel materials, and one or more of these may be included as needed to ensure the desired strength of the steel material. To obtain such effects, it is preferable to include 0.01% or more of Co, 0.01% or more of Mo, 0.01% or more of W, 0.001% or more of V, and 0.001% or more of Nb. However, if Co, Mo, and W are included in large quantities, the toughness and weldability of the steel deteriorate, leading to increased costs. Therefore, when these elements are included, the Co content should be 0.50% or less, and the Mo and W content should be 1.00% or less. Also, the effects of V and Nb saturate when their content exceeds 0.200%. Preferably, the Co content should be 0.40% or less, and the Mo and W content should be 0.90% or less. Also, the effects of V and Nb saturate when their content exceeds 0.200%. Therefore, when these elements are included, the V and Nb content should be 0.200% or less. The V and Nb content is preferably 0.180% or less.

[0035] Group E: Al: 0.300% or less Al is an effective deoxidizing agent and may be included as needed to improve deoxidation efficiency. To obtain such an effect, it is preferable to include 0.010% or more. However, if the Al content exceeds 0.300%, it will reduce the toughness of the steel. Therefore, when including Al, its content should be 0.300% or less, and preferably 0.250% or less.

[0036] Group F: B: 0.0300% or less B is an element that improves the hardenability of steel materials and may be included as needed to ensure the desired strength of the steel material. To obtain such an effect, it is preferable to include 0.0001% or more of B, and more preferably 0.0003% or more. However, if the B content exceeds 0.0300%, it will lead to a significant deterioration of the toughness of the steel material. Therefore, when including B, it is preferable to keep the content at 0.0300% or less, and at 0.0200% or less.

[0037] Group G: N: 0.0100% or less Since nitrogen (N) is a harmful element that reduces toughness, it is desirable to reduce its content as much as possible. In particular, when the N content exceeds 0.0100%, the reduction in toughness of the steel material becomes significant. Therefore, when N is included, the N content should be 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0070% or less. On the other hand, in order to avoid an excessive increase in refining costs in the steelmaking process, the N content should preferably be 0.0005% or more, and more preferably 0.0010% or more.

[0038] Other components are Fe and unavoidable impurities, where unavoidable impurities refer to less than 0.01% O (oxygen) or less than 0.001% of the above-mentioned components.

[0039] Furthermore, from the perspective of further improving ammonia-resistant SCC performance, it is effective to set the maximum Vickers hardness HV0.1 value in the surface layer of the steel material to 265 or less.

[0040] Maximum Vickers hardness (HV0.1) value in the surface layer of steel: 265 or less The hardened phase in the surface layer of steel, resulting from dislocations and precipitates, promotes ammonia-mediated scaling cell carcinogenesis (SCC) by 1) promoting the formation of coarse slip surfaces and 2) acting as a preferred site for anodic dissolution. This is true even for minute hardened phases present in the surface layer of steel.

[0041] In other words, to further improve the ammonia-resistant SCC (Sclerotic Crude Carbide) properties of steel, it is effective to suppress the increase in hardness at the surface of the steel due to localized hardening phases. By suppressing the increase in hardness at the surface of the steel, it is possible to suppress the progress of selective iron dissolution reactions with the steel surface as the anode, thereby suppressing the initiation and propagation of cracks.

[0042] Therefore, the maximum value of the Vickers hardness HV0.1 in the surface layer of the steel material is preferably 265 or less. More preferably 250 or less. Even more preferably 230 or less, and most preferably 210 or less. There is no particular limit to the lower limit, but it is preferably 140 or more. Here, the maximum Vickers hardness HV0.1 in the surface layer of the steel material is the maximum value of the Vickers hardness measured at 10 points in the rolling direction of the steel material, at a depth of 0.5 mm from the surface of the steel material in the cross-section in the rolling direction of the steel material, in accordance with JIS Z 2244 (2009), under the conditions of a test force of 0.1 kgf (0.9807 N) and a pitch of 1 mm.

[0043] By using steel materials having the compositional components described above and possessing the hardness described above, it is possible to prevent cracks from occurring and propagating due to the selective iron dissolution reaction that uses the steel surface without an oxide film as the anode when stress is applied in ammonia.

[0044] The term "liquid ammonia" as used above may include cases where some gaseous ammonia is present.

[0045] The steel material of the present invention is not particularly limited in its structure, but since increased surface strength reduces resistance to steel crystal corrosion (SCC), it is preferable that martensite accounts for 50% or less of the total structure within a 1 mm radius from the surface of the steel material.

[0046] Next, the method for manufacturing steel materials according to the present invention will be described. However, the manufacturing method of the present invention is not limited to the method described below.

[0047] [Methods for manufacturing steel materials] The steel material of the present invention can be manufactured by melting steel adjusted to the above-described component composition using known refining processes such as converters, electric furnaces, and vacuum degassing, forming a steel material (slab) by continuous casting or ingot-part rolling, and then reheating this steel material as needed before hot rolling to produce steel plates or structural steel.

[0048] Here, when hot-rolling the above-mentioned steel material (slab) to the desired dimensions and shape, it is preferable to reheat the slab at a reheating temperature of 900 to 1350°C. If the slab reheating temperature is below 900°C, the deformation resistance is high, making hot rolling difficult. On the other hand, if the slab reheating temperature exceeds 1350°C, a partially molten phase is formed on the surface of the steel, resulting in surface marks, scale loss, and increased fuel consumption.

[0049] Furthermore, in the steel manufacturing method according to the present invention, instead of the process of reheating and hot-rolling the steel material (slab), it is possible to hot-roll the steel material (slab) produced by continuous casting or ingot-partial rolling without cooling it to a temperature range of less than 900°C and without reheating. Alternatively, the hot-rolled steel sheet obtained after hot-rolling may be subjected to reheating, pickling, and cold-rolling to obtain a cold-rolled steel sheet of a predetermined thickness.

[0050] In hot rolling, it is preferable to set the finish rolling completion temperature to 650°C or higher. If the finish rolling completion temperature is below 650°C, the rolling load increases due to the increased deformation resistance, making it difficult to carry out the rolling. Furthermore, it is preferable to set the finish rolling completion temperature to 950°C or lower. If the rolling completion temperature exceeds 950°C, a sufficient reduction ratio in the non-recrystallized temperature range cannot be secured, resulting in a decrease in the strength and toughness of the final steel sheet.

[0051] Cooling after hot rolling can be done by either air cooling or accelerated cooling, but accelerated cooling is preferable if higher strength is to be obtained. When using accelerated cooling, it is preferable to set the average cooling rate to 2 to 100°C / s and the cooling stop temperature to 700 to 400°C. That is, if the average cooling rate is less than 2°C / s and / or the cooling stop temperature is greater than 700°C, the effect of accelerated cooling is small and sufficient high strength may not be achieved. On the other hand, if the average cooling rate is greater than 100°C / s and / or the cooling stop temperature is less than 400°C, the toughness of the steel may decrease or distortion may occur in the shape of the steel. The average cooling rate referred to here is the average cooling rate from the cooling start temperature to the cooling stop temperature in accelerated cooling.

[0052] In this invention, from the viewpoint of ammonia SCC resistance, it is not necessary to perform heat treatment on the steel sheet after it has cooled following rolling. However, if distortion occurs in the steel sheet, heat treatment can be performed to correct it, in which case it is preferable to heat it to 200 to 700°C. In that case, it is preferable to hold the heating for 300 seconds or more.

[0053] Furthermore, in this invention, the temperature in the manufacturing conditions is always the average temperature of the steel plate. The average temperature of the steel plate is determined from the plate thickness, surface temperature, cooling conditions, etc., by simulation calculations or the like. For example, the average temperature of the steel plate can be determined by calculating the temperature distribution in the thickness direction using the finite difference method.

[0054] As described above, the steel material obtained in the present invention can be used as a structural member for large structures such as plants and tanks, which are used in a liquid ammonia environment and require ammonia SCC resistance, for transporting or storing liquid ammonia. [Examples]

[0055] Next, embodiments of the present invention will be described. However, the present invention is not limited to these embodiments.

[0056] Steel with the component compositions shown in Tables 1-1, 1-2, 1-3, and 1-4 (the remainder being Fe and unavoidable impurities) was melted in a converter and cast into steel slabs by continuous casting. After reheating these steel slabs, they were hot-rolled and immediately water-cooled after the completion of hot rolling. Test specimens were taken from the resulting steel plates and the following ammonia SCC resistance evaluation tests were performed.

[0057] [Ammonia SCC Resistance Evaluation Test] From the steel material obtained as described above, a test specimen measuring 20 mm in width, 120 mm in length, and 3.0 mm in thickness was taken. Next, the test specimen was bent into a U-shape in the longitudinal direction with an inner radius of 15 mm, and then immersed for 168 hours in a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia while applying an anode voltage of 2.0 V (vs. Pt). After immersion, a cross-section of the test specimen was cut out, and the maximum crack depth (distance from the surface of the test specimen to the tip of the crack) present in the cross-section was measured, and the ammonia SCC resistance was evaluated according to the following criteria. A score of ○ or ◎ indicated sufficient ammonia SCC resistance. The results are shown in Tables 2-1 and 2-2. ◎ (Pass, particularly excellent): Maximum crack depth is less than 100 μm ○ (Pass): Maximum crack depth is 100 μm or more and less than 300 μm × (Fail): Maximum crack depth is 300 μm or more, or fracture occurs. Among the examples of the invention, steels No. 22-24, 26-28, 30-32, 35, and 38-42, which simultaneously contain one or more elements from Zr, Hf, and Ta, one or more elements from Te, Sr, Se, Pb, As, Bi, and Ba, and one or more elements from La, Sm, Pr, Nd, Ce, and Sc, showed further improved ammonia SCC resistance (rated as excellent).

[0058] [Vickers hardness test] The Vickers hardness HV0.1 of the surface layer of steel material is measured using the following procedure. The steel material is cut so that the symmetrical surface is parallel to the rolling direction and perpendicular to the surface of the steel material. Next, at a depth of 0.5 mm from the surface of the cut steel material (rolling direction section (L section)), the Vickers hardness was measured at 10 points in the rolling direction of the steel material, in accordance with JIS Z 2244 (2009), under the conditions of a test force of 0.1 kgf (0.9807 N) and a pitch of 1 mm. The results are shown together in Tables 2-1 and 2-2.

[0059] [Table 1-1]

[0060] [Table 1-2]

[0061] [Table 1-3]

[0062] [Table 1-4]

[0063] [Table 2-1]

[0064] [Table 2-2]

[0065] As shown in Tables 2-1 and 2-2, in all of the inventive examples, excellent ammonia SCC resistance was obtained even without heat treatment such as tempering after hot rolling. In contrast, in all of the comparative examples, sufficient ammonia SCC resistance was not obtained.

Claims

1. In mass percent, C: 0.50% or less, Si: 0.01-1.00%, Mn: 0.10-3.00%, P: 0.030% or less, S: 0.0100% or less, It contains, and further Zr: 0.001 to 0.100%, Hf: 0.001-0.100%, Ta: 0.001-0.200%, Ga: 0.001-0.300%, Te: 0.001 to 0.500%, Sr: 0.001-0.500%, Se: 0.001 to 0.500%, Pb: 0.001 to 0.500%, As: 0.001 to 0.500%, Bi: 0.001-0.500%, Ba: 0.001-0.500%, La: 0.001 to 0.100%, Sm: 0.001-0.100%, Pr: 0.001 to 0.100%, Nd: 0.001 to 0.100%, Ce: 0.001 to 0.100%, Sc:0.001~0.100%, Ag: 0.001-0.010%, Pt: 0.001-0.010%, Au: 0.001-0.010% A steel material for transporting or storing liquid ammonia, characterized by having a component composition that contains one or more selected from among, with the remainder being Fe and unavoidable impurities.

2. In mass%, C: 0.50% or less, Si: 0.01-1.00%, Mn: 0.10-3.00%, P: 0.030% or less, S: 0.0100% or less, It contains, and further, Hf: 0.001-0.100%, Ta: 0.001-0.200%, Ga: 0.001-0.300%, Te: 0.001 to 0.500%, Sr: 0.001-0.500%, Se: 0.001 to 0.500%, Pb: 0.001 to 0.500%, As: 0.001 to 0.500%, Bi: 0.001-0.500%, Ba: 0.001-0.500%, Ag: 0.001-0.010%, Pt: 0.001-0.010%, Au: 0.001-0.010% It contains one or more selected from among, with the remainder being Fe and unavoidable impurities, and has a component composition thereof. The aforementioned component composition is further characterized by containing, by mass%, one or more groups selected from the following groups A to G, as a steel material for transporting or storing liquid ammonia. Note Group A: Ti: 0.100% or less, Group B: Sb: 0.50% or less, Sn: 0.50% or less, Cu: 3.00% or less, Ni: 3.00% or less, Cr: 3.00% or less, One or more types selected from among them, Group C: Ca: 0.0100% or less, Mg: 0.0200% or less, One or more types selected from among them, Group D: Co: 0.50% or less, Mo: 1.00% or less W: 1.00% or less, V: 0.200% or less, Nb: 0.200% or less, One or more types selected from among them, Group E: Al: 0.300% or less, Group F: B: 0.0300% or less, Group G: N: 0.0100% or less

3. The steel material for transporting or storing liquid ammonia according to claim 1 or 2, characterized in that the maximum value of the Vickers hardness HV0.1 in the surface layer of the steel material is 265 or less.

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