JP7160213B2 - Steel material for transportation and storage of liquid ammonia, and method for producing steel material for transportation and storage of liquid ammonia - Google Patents

Description

The present invention provides steel materials for transporting and storing liquid ammonia suitable for structural members of large structures such as pipelines, plants and tanks used in a liquid ammonia environment, and steel materials for transporting and storing liquid ammonia. It relates to a manufacturing method.

Ammonia is a compound that is widely manufactured and distributed mainly as basic chemicals such as nitric acid and as a raw material for fertilizers. On the other hand, ammonia is difficult to handle, and stress corrosion cracking due to ammonia (hereinafter also referred to as ammonia SCC (Stress Corrosion Cracking)) occurs especially in carbon steel pipes, storage tanks, tank cars, and line pipes that handle liquid ammonia. known to do.

For this reason, conventionally, for structures used in a liquid ammonia environment, application of steel materials with low susceptibility to stress corrosion cracking and operational measures to suppress ammonia SCC have been taken.

For example, it is empirically known that the generation of ammonia SCC has a correlation with the strength of the material. When carbon steel is used, ammonia SCC is suppressed by setting an upper limit on its strength and by subjecting welds to stress-relief annealing.

In addition, in a liquid ammonia environment, water coexisting with liquid ammonia exhibits an effect of suppressing the occurrence of stress corrosion cracking. For this reason, a precautionary measure is sometimes taken to add water at a level that does not affect the quality of the liquid ammonia.

By the way, in recent years, against the background of the expansion of the use of liquid ammonia, the demand is increasing worldwide, and there is a desire to increase the size of facilities and reduce the cost in distribution and manufacturing. Along with this, it is difficult to take measures to suppress and prevent ammonia SCC as described above.

For example, applying stress-relief annealing to welds increases the number of manufacturing processes, so its application is not realistic, especially in large-scale facilities. Also, when adding water to liquid ammonia, it is necessary to appropriately manage the concentration of water in the liquid ammonia. Concentration control becomes more difficult as the equipment becomes larger. Furthermore, with regard to high-purity liquid ammonia, for which demand has been increasing in recent years, it is impossible to take preventive measures by adding water in the first place.

Therefore, there is a demand for the development of a steel material having excellent ammonia SCC resistance suitable for application to structural members such as plants and tanks handling liquid ammonia.

As a technology related to steel materials used in a liquid ammonia environment, for example, Patent Document 1 discloses, in weight %, C: 0.15% or less, Si: 0.15 to 0.40%, Mn: 0.80 to 2 .00%, P: 0.020% or less, S: 0.005% or less, Alsol 0.015 to 0.050%, Cu: 0.35% or less, Ni: 1.00% or less, Cr : 0.50% or less, Mo: 0.25% or less, V: 0.05% or less, Nb: 0.05% or less, and Ti: 0.05% or less, and the balance is Fe After hot rolling the slab consisting of and inevitable impurities, it is heated to the austenitizing temperature, cooled at a cooling rate lower than air cooling, further heated and quenched to the two-phase region temperature (Ac1 to Ac3), and then tempered. Disclosed is a method for producing high-strength steel with excellent ammonia cracking resistance characterized by:

In addition, in Patent Document 2, C: 0.06 to 0.14%, Si: 0.50% or less, Mn: 0.30 to 1.80%, P: 0.025% or less, S : 0.020% or less, V: 0.01 to 0.10%, Al: 0.010 to 0.10%, N: _0.0050 % or less, the balance being iron and unavoidable impurities, and PCM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+ _5B (%), after rolling a billet with PCM ≤ 0.24% to a predetermined plate thickness, before quenching and heating Heating to a temperature of 1100 to 1300 ° C., forming a decarburized layer with a thickness of 0.5 mm or more where C ≤ 0.05% on the surface of the steel plate, and then performing quenching and tempering treatment. A method for manufacturing a tempered 60 kgf/mm class ² high-strength steel sheet with excellent stress corrosion cracking resistance and ammonia stress corrosion cracking resistance is disclosed.

Furthermore, in Patent Document 3, in a method for manufacturing a steel plate for an ammonia tank, a step of decarburizing the surface so that the C content within 0.3 mm from the surface of the steel plate material is 50% or less of the C content of the base material. , after heating the surface decarburized steel sheet to a quenching temperature, the decarburized surface is cooled at a cooling rate of 150°C/sec or less in a temperature range of 800 to 500°C. Disclosed is a method for producing a tempered steel sheet characterized by excellent ammonia cracking resistance.

JP-A-5-9571 JP-A-61-279631 JP-A-58-67830

However, the steel materials obtained by the production methods disclosed in Patent Documents 1 to 3 guarantee ammonia SCC resistance by controlling the surface texture. Therefore, in actual construction, when the steel materials of Patent Documents 1 to 3 are subjected to heat processing, the surface structure may be altered. Therefore, it cannot be said that sufficient ammonia SCC resistance is necessarily obtained.

In addition, in all of the manufacturing methods of Patent Documents 1 to 3, it is essential to perform heat treatment such as tempering after the hot rolling process, and the load in the manufacturing process including the manufacturing cost and lead time is extremely high. growing. Furthermore, size restrictions on heat treatment facilities are disadvantageous in terms of supplying members that constitute large-scale structures.

INDUSTRIAL APPLICABILITY The present invention has been developed in view of the above-mentioned current situation, and is suitable for application to structural members of large structures such as plants and tanks used in a liquid ammonia environment. However, it is an object of the present invention to provide a steel material for transporting and storing liquid ammonia excellent in ammonia SCC resistance, and a method for manufacturing the steel material for transporting and storing liquid ammonia.

The inventors of the present invention conducted various studies in order to solve the above problems.

First, the present inventors made a detailed study of the ammonia SCC generation mechanism in a liquid ammonia environment, and obtained the following findings.

In a liquid ammonia environment, the following corrosive reactions occur.
Anode reaction: 2Fe→2Fe ²⁺ +4e ⁻
Cathodic reaction: O ₂ +2NH ₄ ⁺ +4e ⁻ →2OH ⁻ +2NH ₃
However, an inert oxide film is formed on the surface of the steel material due to the above-described corrosion reaction. For this reason, the total reaction amount of the above corrosion reaction is normally not large. Therefore, a liquid ammonia environment is not inherently a severely corrosive environment.

However, when a new surface is generated on the surface of the steel material due to residual stress in the steel material or stress applied from the outside, a selective iron dissolution reaction proceeds with the new surface where the oxide film does not exist as an anode site, and cracks are formed.

Since the crack becomes a stress concentration part, film breakage and corrosion reaction progress at an accelerated rate at the tip of the crack, eventually leading to fracture of the steel material. It should be noted that once a crack occurs due to SCC, the life of the steel material is determined by the resistance (arrest property) of the steel material to SCC crack propagation. Therefore, in order to ensure the SCC resistance of steel materials, it is necessary to increase the SCC crack propagation resistance. Specifically, there is a need to reduce anodic dissolution susceptibility at crack tips.

Therefore, based on the above findings, the present inventors have made intensive studies to develop a steel material that exhibits excellent ammonia SCC resistance in a liquid ammonia environment.

As a result, the inventors have found that adding appropriate amounts of Cu and Sb is effective in improving the ammonia SCC resistance.

The progress of ammonia SCC is a type of localized corrosion phenomenon caused by film breakage at crack tips and progress of corrosion reaction as described above. Therefore, even if the anodic dissolution susceptibility at the tip of the crack is reduced on average, if there is non-uniformity in the anodic dissolution susceptibility near the tip, the non-uniformity will serve as a driving force, and local corrosion will progress. , does not lead to suppression of the progress of ammonia SCC. That is, it was found that by limiting the degree of segregation (local enrichment) of Cu and Sb in the plate thickness direction to a certain level or less, the anode dissolution sensitivity in the steel material can be made uniform, and the ammonia SCC resistance can be greatly improved. did.

The present invention was completed after further studies based on the above findings. That is, the gist and configuration of the present invention are as follows.
[1] % by mass,
C: 0.50% or less,
Si: 0.01 to 1.00%,
Mn: 0.10-3.00%,
P: 0.030% or less,
S: 0.0100% or less,
N: 0.0005 to 0.0100%,
Al: 0.001-0.10%
and further Cu: 0.010 to 0.50% and Sb: 0.010 to 0.50%
containing one or two selected from among, with the balance having a component composition consisting of Fe and unavoidable impurities,
(Cu+Sb) steel for transporting and storing liquid ammonia, having a degree of segregation of less than 15;
Here, the (Cu+Sb) segregation degree is defined by the following formula (1).
[(Cu + Sb) segregation degree] = [(Cu + Sb) concentration in segregation part] / [average (Cu + Sb) concentration] (1)
[2] The component composition is further mass %,
Sn: 0.01 to 0.50%,
Ni: 0.01 to 3.00%,
Cr: 0.01-3.00%
The steel material for transporting and storing liquid ammonia according to [1], containing one or more selected from among.
[3] The component composition is further mass %,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0200% and REM: 0.001-0.200%
The steel material for transporting and storing liquid ammonia according to [1] or [2], containing one or more selected from among.
[4] The component composition is further mass %,
Ti: 0.005 to 0.100%,
Zr: 0.005 to 0.100%,
Nb: 0.005-0.100% and V: 0.005-0.100%
The steel material for transporting and storing liquid ammonia according to any one of [1] to [3], containing one or more selected from among.
[5] The component composition is further mass %,
Co: 0.01-0.50%
The steel material for transporting and storing liquid ammonia according to any one of [1] to [4] containing
[6] The component composition is further mass %,
B: 0.0001 to 0.0300%
The steel material for transporting and storing liquid ammonia according to any one of [1] to [5] containing
[7] A steel material is produced by continuously casting steel having the chemical composition described in any one of [1] to [6] at a casting speed of 0.3 to 2.8 m/min,
The steel material is reheated under the conditions of reheating temperature: 900 to 1350 ° C., residence time in the temperature range of 800 to 950 ° C. during reheating: 8 to 150 minutes,
A method for producing steel materials for transportation and storage of liquid ammonia, comprising hot-rolling a reheated steel material at a final rolling temperature of 650°C or higher.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain a steel material for transporting and storing liquid ammonia, which is suitable for structural members of large structures such as pipelines, plants and tanks used in a liquid ammonia environment. In addition, the steel material of the present invention can be manufactured without heat treatment such as tempering after hot rolling, which is advantageous in terms of manufacturability. Furthermore, when the steel material of the present invention is applied to, for example, a storage tank for liquid ammonia, it can be used for a longer period of time than before without subjecting the weld to stress-relief annealing. is extremely advantageous.

FIG. 1 is a schematic diagram for explaining the surface area to be measured when determining the degree of (Cu+Sb) segregation.

Embodiments of the present invention will be described below. In addition, all the units in the component composition are "% by mass", and hereinafter, they are indicated simply by "%" unless otherwise specified.

C: 0.50% or less C is an effective element for ensuring the strength of steel. Therefore, in the present invention, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more. On the other hand, when the C content exceeds 0.50%, the workability and weldability deteriorate significantly. Therefore, the C content should be 0.50% or less. It is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.20% or less.

Si: 0.01-1.00%
Si is an element effective in improving ammonia SCC resistance. That is, Si elutes as the steel corrodes in the liquid ammonia environment and forms an inert SiO ₂ coating on the steel surface. This 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 exhibited by setting the Si content to 0.01% or more. The Si content is preferably 0.02% or more, more preferably 0.03% or more, and still more preferably 0.05% or more. On the other hand, when the Si content exceeds 1.00%, toughness and weldability deteriorate. Therefore, the Si content is 1.00% or less, preferably 0.80% or less, more preferably 0.70% or less, and even more preferably 0.60% or less. In addition, in the SCC crack propagation process of ammonia, since the crack grows deep, the oxygen concentration at the tip of the crack decreases and the SiO ₂ film is not sufficiently formed. Therefore, in order to stably improve the ammonia SCC resistance, it is necessary to control the degree of (Cu+Sb) segregation, which will be described later, in addition to the content of Cu or Sb.

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 is 0.10% or more, preferably 0.20% or more, and 0.50% or more. is more preferred. On the other hand, if the Mn content exceeds 3.00%, the weldability deteriorates, so the Mn content is 3.00% or less, preferably 2.00% or less.

P: 0.030% or less P deteriorates toughness and weldability, so the P content is made 0.030% or less. Preferably, it is 0.025% or less.

S: 0.0100% or less S is a harmful element that deteriorates the toughness and weldability of steel, so it is desirable to reduce it as much as possible. In particular, when the S content exceeds 0.0100%, the toughness of the base material and the toughness of the weld zone are greatly deteriorated. Therefore, the S content should be 0.0100% or less. It is preferably 0.0080% or less, more preferably 0.0060% or less.

N: 0.0005 to 0.0100%
N is a harmful element that lowers the toughness, so it is desirable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0100%, the toughness is greatly reduced. Therefore, the amount of N is set to 0.0100% or less. Preferably it is 0.0080%. More preferably, it is 0.0070%. On the other hand, in order to avoid an excessive increase in refining costs in the steelmaking process, the amount of N is set to 0.0005% or more, preferably 0.0010% or more.

Al: 0.001-0.10%
Al is an element added as a deoxidizing agent, and the amount of Al is set to 0.001% or more, preferably 0.003% or more. However, when the amount of Al exceeds 0.10%, the toughness of the steel is lowered. Therefore, the Al content is set to 0.10% or less, preferably 0.08% or less.

One or two selected from Cu: 0.010 to 0.50% and Sb: 0.010 to 0.50% Cu and Sb are important elements for improving ammonia SCC resistance, One or two of these must be contained. That is, Cu and Sb quickly concentrate on the surface as poorly soluble metal Cu and metal Sb, respectively, as the steel material is eluted from the anode in a liquid ammonia environment. Surface enrichment of these refractory metals results in reduced anodic dissolution susceptibility at the crack tip. As a result, the anodic reaction at the crack tip of stress corrosion cracking is suppressed, and the crack growth rate decreases. In order to obtain such an effect, it is necessary to set the Cu content to 0.010% or more when Cu is contained, and the Sb content to 0.010% or more when Sb is contained. be. On the other hand, if Cu and Sb are contained excessively, weldability and toughness deteriorate, which is disadvantageous from the viewpoint of cost. Therefore, the Cu content and the Sb content are each set to 0.50% or less. Each is preferably 0.40% or less, more preferably 0.30% or less.

Although the basic components have been described above, the following elements may be included as appropriate, if necessary.

One or more selected from Sn: 0.01 to 0.50%, Ni: 0.01 to 3.00%, and Cr: 0.01 to 3.00% Sn, Ni, and Cr are ammonia-resistant SCC It is an element that further improves the properties, and one or more of these elements may be contained. All of these elements are elements that increase the acid resistance of steel materials, and as a result of selective anodic dissolution at crack tips, when the pH drops excessively, they work to suppress the corrosion reaction that progresses at an accelerated rate. have. Such an effect is exhibited by containing these elements in an amount of 0.01% or more. Therefore, the content is preferably 0.01% or more, and more preferably 0.02% or more. However, if any element is included in a large amount, the weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost. Therefore, when these elements are contained, the Sn content is preferably 0.50% or less, more preferably 0.35% or less. The Ni content is preferably 3.00% or less, more preferably 2.00% or less. The Cr content is preferably 3.00% or less, more preferably 2.00% or less.

One or more selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200% and REM: 0.001 to 0.200% Ca, Mg and REM are all welded For the purpose of ensuring the toughness of the part, one or more of these may be contained. However, if any element is included in a large amount, the toughness of the weld zone deteriorates and the cost increases. Therefore, when these elements are contained, the contents are Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.001 to 0.200%. is preferred.

One selected from Ti: 0.005 to 0.100%, Zr: 0.005 to 0.100%, Nb: 0.005 to 0.100% and V: 0.005 to 0.100% Above Ti, Zr, Nb and V may be contained in one or more of them in order to ensure the desired strength. However, if any element is included in a large amount, it deteriorates toughness and weldability. Therefore, when these elements are contained, the content is preferably in the range of 0.005 to 0.100%, more preferably in the range of 0.005 to 0.050%.

Co: 0.01-0.50%
Co is an element that increases the strength of steel materials, and may be contained as necessary. In order to obtain such effects, it is preferable to contain 0.01% or more of Co. However, if the Co content exceeds 0.50%, toughness and weldability deteriorate. Therefore, when Co is contained, the content is preferably in the range of 0.01 to 0.50%, more preferably in the range of 0.01 to 0.30%.

B: 0.0001 to 0.0300%
B is an element that improves the hardenability of the steel material, and may be contained as necessary for the purpose of ensuring the strength of the steel material. In order to obtain such an effect, the B content is preferably 0.0001% or more, more preferably 0.0003% or more. However, when the B content exceeds 0.0300%, a large deterioration in toughness is caused. Therefore, when B is contained, the content is preferably 0.0300% or less, more preferably 0.0020% or less.

Components other than the above are Fe and unavoidable impurities.

The chemical composition of the steel material for transporting and storing liquid ammonia of the present invention has been described above. In the steel material for transporting and storing liquid ammonia of the present invention, it is extremely important to control the degree of (Cu+Sb) segregation as follows. is.

(Cu+Sb) segregation degree: less than 15 The segregation of Cu and Sb increases the difference in anodic dissolution resistance between the segregated portion and the non-segregated portion. This difference in resistance to anodic dissolution serves as a driving force for selective anodic dissolution, and promotes crack propagation of ammonia SCC, which is anodic dissolution-dominated. That is, in the crack propagation process of the ammonia SCC, the crack grows deep, so the oxygen concentration at the crack tip decreases and the SiO ₂ film is not sufficiently formed. Therefore, in order to ensure excellent ammonia SCC resistance in a chemical composition containing Cu and Sb, in addition to containing Cu or Sb described above, the segregation of Cu and Sb is suppressed to increase the susceptibility to anode dissolution in the steel material. It is important to limit the difference. From this point of view, the (Cu+Sb) segregation degree is set to less than 15. Preferably less than 14. It is more preferably 12 or less. Although the lower limit is not particularly limited, it is preferably 1.5 or more.

The degree of (Cu + Sb) segregation referred to here means the line of an electron probe microanalyzer (hereinafter referred to as EPMA) in a cross section cut parallel to the rolling direction of the steel material (cross section perpendicular to the surface of the steel material). It is the ratio of the (Cu+Sb) concentration in the segregation part to the average (Cu+Sb) concentration obtained by analysis. Here, the (Cu+Sb) concentration is the sum of the Cu concentration and the Sb concentration. Specifically, when the thickness of the steel material is t (mm) and the width (direction perpendicular to the rolling direction and thickness direction of the steel material) is W (mm), first , The rolling direction of the steel material in a section parallel to the rolling direction of the steel material and perpendicular to the surface of the steel material (cross section perpendicular to the surface of the steel material): 1 mm, all plate thickness directions (excluding the outermost surface layer 0.05 mm on the front and back surfaces of the plate): (t EPMA line analysis of Cu and Sb is performed in a surface area of -0.1) mm under the conditions of a beam diameter of 20 μm and a pitch of 50 μm in the thickness direction of the steel material. In addition, the EPMA surface analysis of Cu and Sb is an arbitrary 1 mm × (t-0.1) mm surface area in two cross sections at a position of W / 4 and a position of 3 W / 4 from one width end. (See FIG. 1). Then, the maximum value of the (Cu+Sb) concentration (mass concentration), which is the sum of the concentrations of Cu and Sb, is obtained for each measurement line, and the average value of these is taken as the (Cu+Sb) concentration of the segregation part. The value obtained by dividing the (Cu + Sb) concentration in this segregation part by the average (Cu + Sb) concentration, which is the arithmetic mean value of the sum of the Cu and Sb concentrations of all the measured values of the measurement line, is the (Cu + Sb) segregation degree. That is, the (Cu+Sb) segregation degree is defined by the following formula (1).
[(Cu + Sb) segregation degree] = [(Cu + Sb) concentration in segregation part] / [average (Cu + Sb) concentration] (1)
As described above, the steel material for transporting and storing liquid ammonia of the present invention suppresses the segregation of Cu and Sb from the viewpoint of ensuring SCC resistance. It is extremely important to control the degree of segregation to a predetermined value or less. Here, the (Cu+Sb) segregation degree varies greatly depending on the manufacturing conditions even if the chemical composition is the same. Therefore, in order to suppress the segregation of Cu and Sb, it is very important to appropriately control the method of manufacturing the steel material, which will be described later.

Next, a preferred method for producing the steel material for transporting and storing liquid ammonia of the present invention will be described.

The steel material of the present invention is produced by melting steel adjusted to the above-described chemical composition using a known refining process such as a converter, an electric furnace, and vacuum degassing, and then by a continuous casting method or an ingot making-blooming rolling method. It can be manufactured by using a steel material (slab), then reheating this steel material as necessary, and then hot-rolling it into a steel plate or shaped steel.

Here, in the case of continuous casting, the casting speed (drawing speed) is preferably 0.3 to 2.8 m/min in the present invention. If the casting speed is less than 0.3 m/min, the operating efficiency will be poor, so it is preferably 0.3 m/min or more, more preferably 0.4 m/min or more. On the other hand, if the casting speed exceeds 2.8 m/min, the surface temperature becomes uneven, and the supply of molten steel to the inside of the slab becomes insufficient, which promotes the segregation of Cu and Sb. min or less is preferable. From the viewpoint of suppressing the segregation of Cu and Sb, the casting speed is more preferably 2.6 m/min or less, and even more preferably 2.2 m/min or less.

In addition, a light reduction method in which a cast piece having an unsolidified layer in the final stage of solidification is cast while being gradually reduced by a group of reduction rolls at a total reduction amount and reduction rate corresponding to the sum of the amount of solidification shrinkage and the amount of thermal shrinkage. It is preferable to By performing the soft reduction method, the center segregation of Cu and Sb at the center in the thickness direction of the slab can be reduced by performing the soft reduction on the slab at the final stage of solidification having an unsolidified layer. can. Therefore, the degree of (Cu+Sb) segregation can be controlled to a low level even in the finally obtained steel material, which is effective in improving the ammonia SCC resistance. The conditions for the light reduction are not particularly limited. It is preferable to apply a reduction of 5 to 2.0 mm/min.

Next, when the steel material (steel slab) is hot-rolled into a desired size and shape, it is preferable to reheat the slab at a reheating temperature of 900 to 1350°C. If the slab reheating temperature is less than 900° C., the deformation resistance is large and hot rolling becomes difficult. On the other hand, if the slab reheating temperature exceeds 1350° C., a partial melting phase occurs on the surface of the steel material, resulting in surface scratches, scale loss, and increased fuel consumption.

In addition, since the degree of segregation of Cu and Sb is affected by the heating conditions, it is preferable to appropriately control the heating conditions from the viewpoint of ensuring the ammonia SCC resistance. Specifically, in the slab reheating temperature range of 800 to 950 ° C., Cu and Sb diffuse rapidly, and the residence time in this temperature range up to 800 to 950 ° C. A range of up to 150 min is preferable. If the residence time is less than 8 minutes, the diffusion may be insufficient and it may become difficult to ensure the ammonia SCC resistance. More preferably, it is 15 min or more. Further, if the heating residence time exceeds 150 minutes, the selective oxidation of iron on the steel surface will proceed excessively, and as a result, a new segregation phase of Cu and Sb will be newly formed near the surface layer, and the ammonia resistance will be reduced. Since the SCC property may deteriorate, it is preferably 150 min or less, more preferably 120 min or less. More preferably, it is 100 min or less.

In the steel material manufacturing method according to the present invention, instead of the process of reheating the steel material (slab) and hot rolling, the steel material (slab) manufactured by the continuous casting method or the ingot casting-slabbing rolling method ) can be hot-rolled without cooling to a temperature range of less than 900° C. and without reheating. In this case, since Cu and/or Sb can easily diffuse in the steel material (slab) before hot rolling, the segregation that occurs when the slab is cooled does not occur and poses no problem. Therefore, in this case, there is no need to take measures to reduce the segregation of Cu and/or Sb, such as keeping the steel material (slab) in a specific temperature range for a specific period of time when reheating the steel material (slab). Alternatively, a hot-rolled steel sheet obtained after hot rolling may be reheated, pickled, and cold-rolled to obtain a cold-rolled steel sheet having a predetermined thickness.

In hot rolling, it is preferable to set the finishing temperature of finish rolling to 650° C. or higher. If the finish rolling end temperature is less than 650°C, the rolling load increases due to an increase in deformation resistance, making it difficult to carry out rolling. In addition, it is preferable that finish rolling finish temperature shall be 950 degrees C or less. If the rolling end temperature exceeds 950° C., the rolling reduction in the non-recrystallization temperature range cannot be sufficiently ensured, and the strength and toughness of the finally obtained steel sheet are lowered.

Cooling after hot rolling may be performed by either air cooling or accelerated cooling, but accelerated cooling is preferred when higher strength is desired. Here, when accelerated cooling is performed, it is preferable to set the cooling rate to 2 to 100°C/s and the cooling stop temperature to 700 to 400°C. That is, when the cooling rate is less than 2° C./s and/or the cooling stop temperature is more than 700° C., the effect of accelerated cooling is small, and sufficient high strength may not be achieved. On the other hand, if the cooling rate exceeds 100° C./s and/or the cooling stop temperature is lower than 400° C., the toughness of the steel material may decrease or the shape of the steel material may be distorted.

In the present invention, from the viewpoint of ammonia SCC resistance, it is not necessary to heat-treat the steel sheet that has been cooled after rolling. If the steel sheet is distorted, heat treatment can be applied to correct the distortion. In this case, heating to 200 to 700° C. is preferable.

Moreover, in the present invention, the temperature in the manufacturing conditions is the steel sheet average temperature. The steel plate average temperature is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the average temperature of the steel sheet can be obtained by calculating the temperature distribution in the sheet thickness direction using the finite difference method.

Next, examples of the present invention will be described. In addition, the present invention is not limited only to these examples.

A steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and continuously cast under the conditions shown in Table 2 to obtain a steel slab. In the continuous casting process, No. 11, No. 53 and no. In 59, a light reduction was performed. Specifically, a rolling reduction of 0.5 to 2.0 mm/min was applied in a state in which the solid fraction at the center in the thickness direction of the slab was 0.3 to 0.7. In contrast, no soft reduction was performed in other cases. After reheating these steel slabs to 1130° C., they were held under the conditions shown in Table 2 and subjected to hot rolling at a final rolling temperature of 800° C. to obtain steel materials with a thickness of 25 mm. The cooling after hot rolling was water cooling (accelerated cooling) at a cooling rate of 10°C/s and a cooling stop temperature of 550°C.

Then, the degree of (Cu+Sb) segregation in the obtained steel material was obtained by the method described above. In addition, from the viewpoint of SCC crack propagation characteristics, the ammonia SCC resistance was evaluated by a slow strain rate test (SSRT) method. SSRT is a test in which strain is applied to the steel material at a constant strain rate, and the SCC generation process is not considered. Therefore, it can be said that this is an SCC characteristic evaluation test method that reflects the crack propagation characteristic of SCC. Specifically, the procedure was as follows.

A steel material is processed into a round bar of 130 mm × 6.35 mmφ, both ends are threaded, and each 12.7 mm from the center of the round bar is processed to 3.81 mmφ from the center toward both ends, and the length is 25.4 mm. A parallel part was provided. This test material was subjected to ultrasonic degreasing in acetone for 5 minutes and attached to the SSRT tester. A solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia was filled into the cell covering the test material, and the strain rate was 1 × 10 ^-6 /s under a dry air atmosphere under conditions of filling and unfilling, respectively. to add distortion. Then, the ratio of total elongation until breakage ({total elongation when filled with solution/total elongation when not filled with solution}×100) was calculated, and ammonia SCC resistance was evaluated according to the following criteria. If it is ◯ or ⊚, it is judged to have sufficient ammonia SCC resistance.
⊚ (excellent): 90% or more ○ (good): 80% or more and less than 90% × (insufficient): less than 80% Table 2 shows the obtained results.

As shown in Table 2, all invention examples have excellent ammonia SCC resistance. On the other hand, all of the comparative examples have insufficient ammonia SCC resistance, and are unsuitable as steel materials for transporting and storing liquid ammonia.

Claims (7)

in % by mass,
C: 0.50% or less,
Si: 0.01 to 1.00%,
Mn: 0.10-3.00%,
P: 0.030% or less,
S: 0.0100% or less,
N: 0.0005 to 0.0100%,
Al: 0.001-0.10%
and further Cu: 0.010 to 0.50% and Sb: 0.010 to 0.50%
containing one or two selected from among, with the balance having a component composition consisting of Fe and unavoidable impurities,
(Cu+Sb) steel for transporting and storing liquid ammonia, having a degree of segregation of less than 15;
Here, the (Cu+Sb) segregation degree is defined by the following formula (1).
[(Cu + Sb) segregation degree] = [(Cu + Sb) concentration in segregation part] / [average (Cu + Sb) concentration] (1) The component composition is further mass %,
Sn: 0.01 to 0.50%,
Ni: 0.01 to 3.00%,
Cr: 0.01-3.00%
The steel material for transporting and storing liquid ammonia according to claim 1, containing one or more selected from The component composition is further mass %,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0200% and REM: 0.001-0.200%
3. The steel material for transporting and storing liquid ammonia according to claim 1 or 2, containing one or more selected from among. The component composition is further mass %,
Ti: 0.005 to 0.100%,
Zr: 0.005 to 0.100%,
Nb: 0.005-0.100% and V: 0.005-0.100%
The steel material for transporting and storing liquid ammonia according to any one of claims 1 to 3, containing one or more selected from. The component composition is further mass %,
Co: 0.01-0.50%
The steel material for transporting and storing liquid ammonia according to any one of claims 1 to 4, containing The component composition is further mass %,
B: 0.0001 to 0.0300%
The steel material for transporting and storing liquid ammonia according to any one of claims 1 to 5, containing A steel material is produced by continuously casting steel having the chemical composition according to any one of claims 1 to 6 at a casting speed of 0.3 to 2.8 m / min,
The steel material is reheated under the conditions of reheating temperature: 900 to 1350 ° C., residence time in the temperature range of 800 to 950 ° C. during reheating: 8 to 150 minutes,
A method for producing steel materials for transportation and storage of liquid ammonia, comprising hot-rolling a reheated steel material at a final rolling temperature of 650°C or higher.

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