KR20220084138A - Steel for transport and storage of liquid ammonia, and method for manufacturing steel for transport and storage of liquid ammonia - Google Patents

Abstract

It is preferable to apply to structural members of large structures such as plants or tanks used in a liquid ammonia environment, and is advantageous in terms of manufacturability as well as steel materials for transporting and storing liquid ammonia excellent in ammonia SCC resistance, and for transporting and storing liquid ammonia An object of the present invention is to provide a method for manufacturing a steel material for storage.
Steel materials for transport and storage of liquid ammonia, having a predetermined component composition and a (Cu+Sb) segregation degree 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)

Description

Steel for transport and storage of liquid ammonia, and method for manufacturing steel for transport and storage of liquid ammonia

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

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

For this reason, conventionally, about the structure used in a liquid ammonia environment, the application of the steel material with low susceptibility to stress corrosion cracking, and the operational measure which suppresses ammonia SCC has been taken.

For example, the generation of ammonia SCC is known empirically to be correlated with the strength of the material. In use of carbon steel, suppression of ammonia SCC is aimed at by setting an upper limit to the intensity|strength, and performing stress relief annealing with respect to a welding part.

Moreover, in a liquid ammonia environment, the water which coexists with liquid ammonia shows the effect|action which suppresses the generation|occurrence|production of a stress corrosion cracking. For this reason, there are cases where a precautionary measure is taken to add water at a level that does not impair the quality of the liquid ammonia.

However, in recent years, against the background of the expansion of the use of liquid ammonia, its demand is increasing worldwide, and the enlargement of equipment and cost reduction in distribution and manufacturing are oriented. In connection with this, it is becoming difficult to implement the above measures for suppressing ammonia SCC and preventive measures.

For example, since stress relief annealing to a welding part increases a manufacturing process, especially in a large-scale facility, the application cannot be said to be realistic. In addition, in addition of water to liquid ammonia, it is necessary to manage the water concentration in liquid ammonia appropriately. With the enlargement of the facility, the concentration control becomes difficult. In addition, with respect to high-purity liquid ammonia, which is in high demand in recent years, it is not possible to take preventive measures by adding water in the first place.

Therefore, the development of steel materials excellent in ammonia SCC resistance suitable for application to structural members such as plants and tanks handling liquid ammonia is desired.

As a technology related to steel materials used in a liquid ammonia environment, for example, in Patent Document 1, 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, the slab containing the remainder Fe and unavoidable impurities is hot-rolled, heated to an austenitization temperature, cooled at a cooling rate of air cooling or less, and further There is disclosed a method for producing high-tensile steel having excellent ammonia cracking resistance, characterized by heating and quenching at a two-phase temperature (Ac1 to Ac3) followed by a tempering treatment.

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 by weight% : 0.010 to 0.10%, N: 0.0050% or less, the remainder consists of iron and unavoidable impurities, and P _CM = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo When set to /15+V/10+5B (%), a steel piece having _PCM ≤ 0.24% is rolled to a predetermined plate thickness, and then heated to a temperature of 1100 to 1300° C. before quenching treatment heating, C ≤ A decarburization layer having a thickness of 0.05% and a thickness of 0.5 mm or more is formed on the surface portion of the steel sheet, followed by quenching and tempering treatment. ) A method of manufacturing a 60 kgf/mm class ² high-tensile steel sheet is disclosed.

In addition, Patent Document 3 discloses, in a method for manufacturing a steel sheet for an ammonia tank, a step of surface decarburizing so that the C content within 0.3 mm from the surface of the steel sheet material is 50% or less of the base material C amount, and the surface decarburization steel sheet, Manufacturing a tempered steel sheet having excellent ammonia cracking resistance, characterized in that it has a step of heating to a quenching temperature and then cooling the decarburization surface at a cooling rate of 150°C/sec or less in a temperature range of 800 to 500°C A method is disclosed.

Japanese Laid-Open Patent Publication No. 5-9571 Japanese Laid-Open Patent Publication No. 61-279631 Japanese Laid-Open Patent Publication No. 58-67830

However, the steel materials obtained by the manufacturing method disclosed by patent documents 1-3 guarantee ammonia SCC resistance by controlling the surface structure. Therefore, in actual construction, when the steel materials of patent documents 1 - 3 are subjected to heat processing, there is a possibility that the surface structure changes in quality. For this reason, it cannot necessarily be said that sufficient ammonia SCC resistance is obtained.

Moreover, 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 manufacturing cost and lead time becomes very large. Moreover, it becomes disadvantageous in supplying the member which comprises a large-sized structure by the size restriction|limiting of a heat processing facility.

The present invention has been developed in view of the current situation, and is preferably applied to structural members of large structures such as plants or tanks used in liquid ammonia environments, and liquid ammonia with excellent ammonia SCC resistance advantageous in terms of manufacturability. An object of the present invention is to provide a method for manufacturing a steel for transport and storage, and a steel for transport and storage of liquid ammonia.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the present inventors repeated various examinations.

First, the present inventors acquired the following knowledge, as a result of examining in detail the generation|occurrence|production mechanism of ammonia SCC in a liquid ammonia environment.

In a liquid ammonia environment, the following corrosion reactions occur.

Anode reaction: 2Fe → 2Fe ²⁺ + 4e ^-

Cathodic reaction: O ₂ + 2NH ₄ ⁺ + 4e ^- → 2OH ^- + 2NH ₃

However, an inactive oxide film is formed on the steel material surface with said corrosion reaction. For this reason, normally, the total reaction amount of the said corrosion reaction is not much. Thus, the liquid ammonia environment is not inherently a demanding corrosive environment.

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

Since the crack becomes a stress concentration zone, the film breakage at the crack tip and the corrosion reaction proceed acceleratingly, ultimately leading to fracture of the steel material. In addition, when a crack is generated once by SCC, it is the resistance (arrest property) with respect to SCC crack propagation of steel materials that determines the life of steel materials. Therefore, in order to ensure the SCC resistance of steel materials, it is necessary to increase the SCC crack propagation resistance. Specifically, it is necessary to reduce the anode dissolution susceptibility at the crack tip.

Then, based on said knowledge, the present inventors repeated earnest research toward development of the steel material which shows the outstanding ammonia SCC resistance in a liquid ammonia environment.

As a result, in order to improve ammonia SCC resistance, it discovered that it is effective to add Cu and Sb in appropriate amounts.

In addition, progress of ammonia SCC is a kind of local corrosion phenomenon which arises by the film breakage at the crack tip and advancing of a corrosion reaction, as mentioned above. Therefore, even if the anode dissolution susceptibility at the tip of the crack is reduced on average, if there is non-uniformity in the anode dissolution susceptibility in the vicinity of the tip, the non-uniformity is used as a driving force, and local corrosion proceeds, suppressing the progress of ammonia SCC does not reach That is, by limiting the degree of segregation (localized thickening) in the plate thickness direction of Cu and Sb to a certain level or less, the anode dissolution sensitivity in the steel can be uniformed, and the ammonia SCC resistance can be significantly improved. .

This invention was completed after adding further examination based on said knowledge. That is, the configuration of the gist 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;

N: 0.0005 to 0.0100%;

Al: 0.001 to 0.10%

contains, and further

Cu: 0.010 to 0.50% and

Sb: 0.010 to 0.50%

It contains 1 type or 2 types selected from, and has a component composition which the remainder consists of Fe and an unavoidable impurity,

(Cu+Sb) Steel for transport and storage of liquid ammonia with a segregation degree 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 in mass%,

Sn: 0.01 to 0.50%,

Ni: 0.01 to 3.00%,

Cr: 0.01 to 3.00%

The steel for transport and storage of liquid ammonia according to [1], containing at least one selected from among.

[3] The component composition is further in mass%,

Ca: 0.0001 to 0.0100%,

Mg: 0.0001 to 0.0200% and

REM: 0.001 to 0.200%

The steel for transport and storage of liquid ammonia according to [1] or [2], which contains at least one selected from among.

[4] The component composition is further in mass%,

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%

The steel material for transport and storage of liquid ammonia according to any one of [1] to [3], containing at least one selected from among them.

[5] The component composition is further in mass%,

Co: 0.01 to 0.50%

The steel material for transport and storage of liquid ammonia according to any one of [1] to [4], comprising:

[6] The component composition is further in mass%,

B: 0.0001 to 0.0300%

The steel material for transport and storage of liquid ammonia according to any one of [1] to [5], comprising:

[7] A steel material is produced by continuously casting steel having the component composition according to 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 a reheating temperature: 900 to 1350 ° C, and a residence time in a temperature range of 800 to 950 ° C at the time of reheating: 8 to 150 min;

A method of manufacturing steel for transport and storage of liquid ammonia in which the reheated steel material is hot-rolled at the finish rolling end temperature: 650 ℃ or higher.

According to the present invention, it is possible to obtain a steel material for transporting and storing liquid ammonia, which is preferable to be applied to a structural member of a large structure such as a pipeline, a plant or a tank used in a liquid ammonia environment. Moreover, since the steel materials of this invention can be manufactured even if it does not perform heat processing, such as tempering, after hot rolling, it is advantageous also from the point of manufacturability. In addition, when the steel of the present invention is applied to, for example, a liquid ammonia storage tank, it is very advantageous in industry because it can be used for a longer period than in the prior art even without stress relief annealing to the welded part.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining the surface area|region to be measured at the time of calculating|requiring a (Cu+Sb) segregation degree.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described. In addition, all units in a component composition are "mass %", and, unless otherwise indicated, below, it shows simply by "%".

C: 0.50% or less

C is an element effective for securing the strength of steel. Therefore, in this invention, it is preferable to contain 0.01 % or more. More preferably, it is 0.02 % or more. On the other hand, when C content exceeds 0.50 %, workability and weldability will deteriorate significantly. For this reason, the C content is made 0.50% or less. Preferably it is 0.40 % or less, More preferably, it is 0.30 % or less, More preferably, it is 0.20 % or less.

Si: 0.01 to 1.00%

Si is an element effective for improving ammonia SCC resistance. That is, Si elutes with corrosion of steel materials in a liquid ammonia environment, and forms an inactive SiO2 film on _the steel materials surface. Thereby, the progress of the selective anode dissolution reaction at the crack tip is suppressed, and the ammonia SCC sensitivity of steel materials is reduced. Such an effect is expressed by making Si content into 0.01 % or more. It is preferable that Si content is 0.02 % or more, It is more preferable that it is 0.03 % or more, It is still more preferable that it is 0.05 % or more. On the other hand, when Si content exceeds 1.00 %, toughness and weldability will deteriorate. For this reason, Si content shall be 1.00 % or less, 0.80 % or less is preferable, 0.70 % or less is more preferable, It is still more preferable that it is 0.60 % or less. Moreover, in the SCC crack propagation process of ammonia, since a crack advances deeply, the oxygen concentration at the crack tip falls and _a SiO2 film is not fully formed. Therefore, in order to stably improve ammonia SCC resistance, in addition to Cu or Sb content, it is necessary to control the (Cu+Sb) segregation degree described later.

Mn: 0.10 to 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 made 0.10% or more, preferably 0.20% or more, and more preferably 0.50% or more. On the other hand, when Mn content exceeds 3.00 %, since weldability will deteriorate, Mn content shall be 3.00 % or less, and it is preferable that it is 2.00 % or less.

P: 0.030% or less

Since P deteriorates toughness and weldability, P content is made into 0.030 % or less. Preferably it is 0.025 % or less.

S: 0.0100% or less

Since S is a harmful element which deteriorates the toughness and weldability of steel, it is preferable to reduce it as much as possible. In particular, when the S content exceeds 0.0100%, deterioration of the base metal toughness and the weld joint toughness becomes large. Therefore, the S content is made 0.0100% or less. Preferably it is 0.0080 % or less, More preferably, it is 0.0060 % or less.

N: 0.0005 to 0.0100%

Since N is a harmful element which reduces toughness, it is preferable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0100%, the decrease in toughness becomes large. Therefore, the amount of N is made 0.0100% or less. Preferably it is 0.0080 %. More preferably, it is 0.0070 %. On the other hand, in order to avoid excessive increase of the refining cost in the steelmaking process, the amount of N is made into 0.0005% or more, and it is preferable that it is 0.0010% or more.

Al: 0.001 to 0.10%

Al is an element added as a deoxidizer, and the amount of Al is 0.001% or more, and is preferably 0.003% or more. However, when the amount of Al exceeds 0.10 %, the toughness of steel will fall. For this reason, the Al content shall be 0.10 % or less, and it is preferable that it is 0.08 % or less.

One or two types 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, and it is necessary to contain one or two of them. That is, Cu and Sb are rapidly concentrated on the surface as metal Cu and metal Sb of poor solubility, respectively, with the anode elution of steel materials in a liquid ammonia environment. As a result of the surface thickening of these sparingly soluble metals, the anode dissolution susceptibility at the crack tip is lowered. As a result, the anode reaction at the crack tip of the stress corrosion cracking is suppressed, and the crack propagation rate is lowered. In order to obtain such an effect, when containing Cu, it is necessary to make Cu content into 0.010 % or more, and when containing Sb, it is necessary to make Sb content into 0.010 % or more, respectively. On the other hand, when Cu and Sb are contained excessively, weldability and toughness will deteriorate, and it will become disadvantageous also from a viewpoint of cost. For this reason, Cu content and Sb content shall be 0.50 % or less, respectively. Preferably it is 0.40 % or less, respectively, More preferably, it is each 0.30 % or less.

As mentioned above, although the basic component was demonstrated, you may contain the following elements suitably as needed.

Sn: 0.01 to 0.50%, Ni: 0.01 to 3.00%, Cr: at least one selected from 0.01 to 3.00%

Sn, Ni, and Cr are elements that further improve ammonia SCC resistance, and may contain one or more of them. All of these elements are elements that increase the acid resistance of steel materials, and have a function of suppressing an accelerated corrosion reaction when the pH is excessively lowered as a result of selective anode dissolution at the crack tip. Since such an effect is expressed by containing 0.01% or more of these elements, it is preferable to contain it 0.01% or more, and it is more preferable to contain it 0.02% or more. However, when any element is also contained in a large amount, weldability and toughness are deteriorated, and it becomes disadvantageous also from a viewpoint of cost. Therefore, when containing these elements, 0.50 % or less is preferable and, as for content of Sn, 0.35 % or less is more preferable. 3.00 % or less is preferable and, as for content of Ni, 2.00 % or less is more preferable. 3.00 % or less is preferable and, as for content of Cr, 2.00 % or less is more preferable.

Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: at least one selected from 0.001 to 0.200%

All of Ca, Mg, and REM may contain one or more of these for the purpose of securing the toughness of the weld zone. However, when any element is contained in a large amount, the toughness deterioration of a welding part and an increase in cost will be caused. Accordingly, when these elements are contained, the content thereof is preferably within the range of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.001 to 0.200%.

Ti: 0.005 to 0.100%, Zr: 0.005 to 0.100%, Nb: 0.005 to 0.100%, and V: at least one selected from 0.005 to 0.100%

Ti, Zr, Nb, and V may contain one or two or more of them in order to secure a desired strength. However, if any element is contained in a large amount, toughness and weldability will deteriorate. Therefore, when containing these elements, it is preferable to set all the content into the range of 0.005 to 0.100%, More preferably, it is the range of 0.005 to 0.050%.

Co: 0.01 to 0.50%

Co is an element which raises the intensity|strength of steel materials, and you may contain it as needed. In order to obtain such an effect, it is preferable to contain 0.01% or more of Co. However, when Co content exceeds 0.50 %, toughness and weldability will deteriorate. Therefore, when containing Co, it is preferable to make the content into 0.01 to 0.50% of range, More preferably, it is 0.01 to 0.30% of range.

B: 0.0001 to 0.0300%

B is an element which improves the hardenability of steel materials, and you may contain it as needed for the purpose of ensuring the intensity|strength of steel materials. In order to acquire such an effect, it is preferable to contain 0.0001 % or more of B, and it is more preferable to make it contain 0.0003 % or more. However, when the B content exceeds 0.0300%, significant deterioration of toughness is caused. Therefore, when containing B, it is preferable to set it as 0.0300 % or less, and, as for the content, it is more preferable to set it as 0.0020 % or less.

Components other than the above are Fe and unavoidable impurities.

As mentioned above, although the component composition of the steel material for transport and storage of liquid ammonia of the present invention has been described, in the steel material for transport and storage of liquid ammonia of the present invention, it is very important to control the (Cu+Sb) segregation degree as follows.

(Cu+Sb) Segregation degree: less than 15

Segregation of Cu and Sb increases the difference in anode solubility between the segregated portion and the non-segregated portion. This difference in anode solubility becomes a driving force for selective anode dissolution, and promotes crack propagation of ammonia SCC, which is an anode dissolution dominant type. That is, in the crack propagation process of ammonia SCC, since a crack advances deeply, the oxygen concentration at the crack tip falls, and _a SiO2 film is not fully formed. Therefore, in order to secure excellent ammonia-resistant SCC characteristics in the component composition containing Cu and Sb, in addition to the above-mentioned containing Cu or Sb, the segregation of Cu and Sb is suppressed to suppress the difference in anode dissolution susceptibility in the steel. it is important From such a viewpoint, (Cu+Sb) segregation degree is set to less than 15. Preferably less than 14. More preferably, it is 12 or less. Although it does not specifically limit about a minimum, It is preferable to set it as 1.5 or more.

In addition, the (Cu+Sb) segregation degree referred to herein is a cross section cut parallel to the rolling direction of the steel (a cross section perpendicular to the surface of the steel material). It is the ratio of the (Cu+Sb) concentration of the segregated portion to the average (Cu+Sb) concentration obtained by Here, the (Cu+Sb) concentration is the sum of the Cu concentration and the Sb concentration. (Cu+Sb) The method of obtaining the segregation degree is specifically, when the thickness of the steel is t (mm) and the width (the direction perpendicular to the rolling direction and thickness direction of the steel) is W (mm), first, Rolling direction of steel with a cross-section parallel to the rolling direction and perpendicular to the steel surface (cross-section perpendicular to the steel surface): 1 mm, overall plate thickness direction (except for 0.05 mm of the outermost layer of the front and back surfaces): (t-0.1 ) mm WHEREIN: EPMA line analysis of Cu and Sb is performed under the conditions of beam diameter: 20 micrometers and pitch: 50 micrometers in the thickness direction of steel materials. In addition, the EPMA surface analysis of Cu and Sb is an arbitrary 1 mm x (t-0.1) mm surface area in two cross sections of a position of W/4 and a position of 3W/4 from one width edge part. (see FIG. 1). Then, the maximum value of the (Cu+Sb) concentration (mass concentration) that is the sum of the concentrations of Cu and Sb for each measurement line is obtained, and the average value thereof is taken as the (Cu+Sb) concentration of the segregation portion. A value obtained by dividing the (Cu+Sb) concentration of this segregation portion by the average (Cu+Sb) concentration, which is the arithmetic mean of the sum of Cu and Sb concentrations of all measured values of the measurement line, is defined as 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 transport and storage of liquid ammonia of the present invention suppresses the segregation of Cu and Sb from the viewpoint of ensuring SCC resistance, that is, shows the degree of segregation of Cu and Sb (Cu+Sb ) It is very important to control the segregation level below a predetermined value. Here, the degree of (Cu+Sb) segregation largely changes depending on the production conditions even if the component composition is the same. For this reason, in order to suppress segregation of Cu and Sb, it is very important to control appropriately the manufacturing method of steel materials mentioned later.

Next, a preferable manufacturing method of the steel for transport and storage of liquid ammonia of the present invention will be described.

The steel of the present invention is prepared by melting the steel adjusted to the above-mentioned composition by using a known refining process such as a converter, electric furnace, vacuum degassing, etc. Then, this steel raw material is reheated as needed and then hot rolled, whereby it can be manufactured into a steel plate or a section steel or the like.

Here, in the case of continuous casting, in the present invention, it is preferable to set the casting speed (drawing speed) to 0.3 to 2.8 m/min. If the casting speed is less than 0.3 m/min, since operation efficiency deteriorates, it is preferable that it is 0.3 m/min or more, and it is more preferable that it is 0.4 m/min or more. On the other hand, when the casting speed exceeds 2.8 m/min, surface temperature non-uniformity occurs, and the molten steel supply to the inside of the slab becomes insufficient, and segregation of Cu and Sb is promoted, so that it is preferably 2.8 m/min or less. From the viewpoint of suppressing the segregation of Cu and Sb, the casting speed is more preferably 2.6 m/min or less, and still more preferably 2.2 m/min or less.

In addition, it is preferable to perform a light reduction method in which a cast slab at the end of solidification having an unsolidified layer is cast while gradually rolling down by a reduction roll group with a total reduction amount and reduction speed equivalent to the sum of the amount of solidification shrinkage and heat shrinkage. . By performing the light pressure reduction method, the center segregation of Cu and Sb in the thickness direction center part of the cast steel can be reduced by performing light pressure reduction with respect to the slab at the end of solidification having an unsolidified layer. For this reason, also in the steel materials finally obtained, the (Cu+Sb) segregation degree can be controlled to a low level, and it is effective in improving ammonia SCC resistance. Conditions under light pressure are not particularly limited, but in the case of performing light pressure reduction, for example, in a state where the solid phase ratio of the central portion in the thickness direction of the cast steel is 0.3 to 0.7, applying a reduction of 0.5 to 2.0 mm/min desirable.

Next, when hot-rolling the said steel raw material (steel slab) to a desired dimensional shape, it is preferable to set the slab reheating temperature to 900-1350 degreeC, and to reheat. When the slab reheating temperature is less than 900°C, the deformation resistance is large, and hot rolling becomes difficult. On the other hand, when slab reheating temperature exceeds 1350 degreeC, since a partial molten phase generate|occur|produces on the steel material surface, a surface mark will generate|occur|produce, or a scale loss and a fuel unit will increase.

Moreover, since the segregation degree of Cu and Sb is affected by heating conditions, it is preferable from a viewpoint of ensuring ammonia SCC resistance to control heating conditions appropriately. Specifically, in the slab reheating temperature range of 800 to 950°C, Cu and Sb diffuse rapidly, and the residence time in the 800 to 950°C temperature range (sum of the residence time in the above temperature range) is preferably in the range of 8 to 150 min. If the residence time is less than 8 min, diffusion becomes insufficient and there is a fear that it becomes difficult to secure ammonia SCC resistance. Therefore, it is preferably 8 min or more, and more preferably 10 min or more. More preferably, it is 15 min or more. Moreover, when the heating residence time exceeds 150 min, selective oxidation of iron on the steel surface proceeds excessively, and as a result, similarly, segregation phases of Cu and Sb are newly formed in the vicinity of the surface layer, and ammonia SCC resistant Since there exists a possibility that a performance may deteriorate, it is preferable that it is 150 min or less, and it is more preferable that it is 120 min or less. More preferably, it is 100 min or less.

Further, in the method for manufacturing a steel material according to the present invention, instead of the process of reheating the steel material (slab) and hot rolling, the steel material (slab) produced by the continuous casting method or the ingot-ingot rolling method is heated to less than 900 ° C. It is possible to perform hot rolling without cooling to a temperature range and without reheating as it is. In this case, since Cu and/or Sb can be easily diffused in the steel material (slab) before hot rolling, segregation as in the case of cooling the slab does not occur and does not pose a problem. For this reason, in this case, the treatment for reducing the segregation of Cu and/or Sb, such as making the steel raw material (slab) stay for a specific time in the specific temperature range prescribed|regulated when reheating, is unnecessary. Moreover, it is good also considering reheating processing, pickling, and cold rolling to the hot-rolled steel plate obtained after hot rolling, and making it a cold-rolled steel plate of a predetermined|prescribed plate thickness.

In hot rolling, it is preferable to set the finish rolling end temperature 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 the deformation resistance, making it difficult to perform rolling. In addition, it is preferable that the finish rolling end temperature shall be 950 degrees C or less. When the rolling end temperature exceeds 950°C, the reduction in the non-recrystallization temperature region cannot be sufficiently ensured, and the strength and toughness of the finally obtained steel sheet fall.

Although either method of air cooling and accelerated cooling may be sufficient as cooling after hot rolling, it is preferable to perform accelerated cooling when higher intensity|strength is desired. Here, when performing accelerated cooling, it is preferable to make a cooling rate into 2-100 degreeC/s, and to make cooling stop temperature into 700-400 degreeC. That is, when the cooling rate is less than 2°C/s, and/or when the cooling stop temperature is more than 700°C, the effect of accelerated cooling is small, and sufficient strength enhancement may not be achieved in some cases. On the other hand, if the cooling rate is more than 100°C/s and/or the cooling stop temperature is less than 400°C, the toughness of the steel may be lowered or the shape of the steel may be deformed.

In the present invention, from the viewpoint of ammonia SCC resistance, it is not necessary to heat-treat the cooled steel sheet after rolling. In addition, when a deformation|transformation generate|occur|produces in a steel plate, it is possible to perform heat processing for the purpose of correction, and in that case, it is preferable to heat to 200-700 degreeC.

Moreover, in this invention, let all the temperatures in manufacturing conditions be the steel plate average temperature. The average steel sheet temperature is obtained by simulation calculation or the like from sheet thickness, surface temperature, cooling conditions, and the like. For example, the average temperature of a steel plate is calculated|required by calculating the temperature distribution in a plate|board thickness direction using a difference method.

Example 1

Next, an embodiment of the present invention will be described. In addition, this invention is not limited only to these Examples.

Steel of the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a converter, and was made into a steel slab by continuous casting under the conditions shown in Table 2. Continuous casting process WHEREIN: In No.11, No.53, and No.59, light pressure reduction was implemented. Specifically, a reduction of 0.5 to 2.0 mm/min was applied in a state where the solidity ratio of the central portion in the thickness direction of the cast steel was 0.3 to 0.7. On the other hand, in other cases, light pressure reduction was not performed. After reheating these steel slabs to 1130 degreeC, it hold|maintained on the conditions shown in Table 2, finish-rolling completion|finish temperature: 800 degreeC hot-rolled, and the steel materials of plate|board thickness:25 mm were obtained. In addition, cooling after hot rolling was made into water cooling (accelerated cooling) of cooling rate:10 degreeC/s, and cooling stop temperature:550 degreeC.

And the (Cu+Sb) segregation degree in the obtained steel materials was calculated|required by the said method. In addition, from the viewpoint of the 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 always applied to steel at a constant strain rate, and the process of occurrence of SCC is not considered. Therefore, it can be said that it is an SCC characteristic evaluation test method that reflects the crack propagation characteristics of SCC. Specifically, it carried out in the following procedure.

의 환봉으로 가공하고, 양단에 비틀어 끊는 가공을 실시함과 함께, 환봉의 중심부로부터 양단을 향하여 12.7 ㎜ 씩을 3.81 ㎜

로 가공하여, 길이 25.4 ㎜ 의 평행부를 형성하였다. 본 시험재를, 아세톤 중에서 초음파 탈지를 5 분간 실시하고, SSRT 시험기에 장착하였다. 시험재를 덮는 셀 중으로, 카르바민산암모늄 12.5 g 과 액체 암모니아 1 L 를 혼합한 용액을, 충전한 조건과 충전하지 않는 조건에서, 각각 건조 공기 분위기하, 1 × 10^-6/s 의 변형 속도로 변형을 가하였다. 그리고, 파단에 이르기까지의 전연신의 비율 (｛용액을 충전했을 때의 전연신/용액을 충전하지 않을 때의 전연신｝× 100) 을 산출하고, 이하의 기준으로 내 암모니아 SCC 성을 평가하였다. 또한, ○ 혹은 ◎ 이면, 충분한 내 암모니아 SCC 특성을 가지고 있다고 판정된다.Steel, 130 mm × 6.35 mm

At the same time, the round bar is processed by twisting at both ends, and 12.7 mm from the center of the round bar toward both ends is 3.81 mm each.

was processed to form parallel portions with a length of 25.4 mm. This test material was ultrasonically degreased in acetone for 5 minutes, and was mounted on an SSRT tester. In the cell covering the test material, a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia was charged under a dry air atmosphere and a strain rate of 1 × 10 ^-6 /s, respectively, under the conditions of charging and non-charging. was transformed with Then, the ratio of total elongation to breakage (w total elongation when filled with solution / total elongation when not filled with solution x 100) was calculated, and the ammonia SCC resistance was evaluated based on the following criteria. Moreover, it is judged that it has sufficient ammonia-resistant SCC characteristic as it is (circle) or (double-circle).

◎ (Excellent): 90% or more

○ (Good): 80% or more and less than 90%

× (Insufficient): less than 80%

The obtained result is shown in Table 2.

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

Claims (7)

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;
N: 0.0005 to 0.0100%;
Al: 0.001 to 0.10%
contains, and further
Cu: 0.010 to 0.50% and
Sb: 0.010 to 0.50%
It contains 1 type or 2 types selected from, and has a component composition which the remainder consists of Fe and an unavoidable impurity,
(Cu+Sb) Steel for transport and storage of liquid ammonia with a segregation degree 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 method of claim 1,
The component composition is further in mass%,
Sn: 0.01 to 0.50%,
Ni: 0.01 to 3.00%,
Cr: 0.01 to 3.00%
Steel for transport and storage of liquid ammonia containing at least one selected from among. 3. The method according to claim 1 or 2,
The component composition is further in mass%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0200% and
REM: 0.001 to 0.200%
Steel for transport and storage of liquid ammonia containing at least one selected from among. 4. The method according to any one of claims 1 to 3,
The component composition is further in mass%,
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%
Steel for transport and storage of liquid ammonia containing at least one selected from among. 5. The method according to any one of claims 1 to 4,
The component composition is further in mass%,
Co: 0.01 to 0.50%
Steel for transport and storage of liquid ammonia containing 6. The method according to any one of claims 1 to 5,
The component composition is further in mass%,
B: 0.0001 to 0.0300%
Steel for transport and storage of liquid ammonia containing A steel material is produced by continuously casting steel having the component 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 a reheating temperature: 900 to 1350 ° C, and a residence time in a temperature range of 800 to 950 ° C at the time of reheating: 8 to 150 min;
A method of manufacturing steel for transport and storage of liquid ammonia in which the reheated steel material is hot-rolled at the finish rolling end temperature: 650 ℃ or higher.

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